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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" version="3" category="std" consensus="true" docName="draft-ietf-rtcweb-jsep-26"
ipr="trust200902"> indexInclude="true" ipr="trust200902" number="8829" prepTime="2021-01-19T20:18:44" scripts="Common,Latin" sortRefs="true" submissionType="IETF" symRefs="true" tocDepth="4" tocInclude="true" xml:lang="en">
  <link href="https://datatracker.ietf.org/doc/draft-ietf-rtcweb-jsep-26" rel="prev"/>
  <link href="https://dx.doi.org/10.17487/rfc8829" rel="alternate"/>
  <link href="urn:issn:2070-1721" rel="alternate"/>
  <front>
    <title abbrev="JSEP">JavaScript Session Establishment
    Protocol</title> Protocol (JSEP)</title>
    <seriesInfo name="RFC" value="8829" stream="IETF"/>
    <author fullname="Justin Uberti" initials="J." surname="Uberti">
      <organization>Google</organization>
      <organization showOnFrontPage="true">Google</organization>
      <address>
        <postal>
          <street>747 6th St S</street> Street South</street>
          <city>Kirkland</city>
          <region>WA</region>
          <code>98033</code>
          <country>USA</country>
          <country>United States of America</country>
        </postal>
        <email>justin@uberti.name</email>
      </address>
    </author>
    <author fullname="Cullen Jennings" initials="C." surname="Jennings">
      <organization>Cisco</organization>
      <organization showOnFrontPage="true">Cisco</organization>
      <address>
        <postal>
          <street>400 3rd Avenue SW</street>
          <city>Calgary</city>
          <region>AB</region>
          <code>T2P 4H2</code>
          <country>Canada</country>
        </postal>
        <email>fluffy@iii.ca</email>
      </address>
    </author>
    <author fullname="Eric Rescorla" initials="E.K." initials="E." surname="Rescorla" role="editor">
      <organization>Mozilla</organization>
      <organization showOnFrontPage="true">Mozilla</organization>
      <address>
        <postal>
          <street>331 Evelyn Ave</street>
          <city>Mountain View</city>
          <region>CA</region>
          <code>94041</code>
          <country>USA</country>
        </postal>
        <email>ekr@rtfm.com</email>
      </address>
    </author>
    <date />
    <area>RAI</area>
    <abstract>

      <t>This month="01" year="2021"/>
    <keyword>webrtc</keyword>
    <keyword>sdp</keyword>
    <keyword>negotiation</keyword>
    <keyword>signaling</keyword>
    <keyword>peerconnection</keyword>
    <keyword>api</keyword>
    <keyword>ice</keyword>
    <keyword>rtp</keyword>
    <keyword>offer</keyword>
    <keyword>answer</keyword>
    <abstract pn="section-abstract">
      <t indent="0" pn="section-abstract-1">This document describes the mechanisms for allowing a
      JavaScript application to control the signaling plane of a
      multimedia session via the interface specified in the W3C
      RTCPeerConnection API, API and discusses how this relates to existing
      signaling protocols.</t>
    </abstract>
  </front>
  <middle>
    <boilerplate>
      <section title="Introduction" anchor="sec.introduction">

      <t>This anchor="status-of-memo" numbered="false" removeInRFC="false" toc="exclude" pn="section-boilerplate.1">
        <name slugifiedName="name-status-of-this-memo">Status of This Memo</name>
        <t indent="0" pn="section-boilerplate.1-1">
            This is an Internet Standards Track document.
        </t>
        <t indent="0" pn="section-boilerplate.1-2">
            This document describes how the W3C WEBRTC RTCPeerConnection
      interface
      <xref target="W3C.webrtc"></xref> is used to control the setup,
      management and teardown of a multimedia session.</t>
      <section title="General Design product of JSEP"
      anchor="sec.general-design-of-jsep">

        <t>WebRTC call setup the Internet Engineering Task Force
            (IETF).  It represents the consensus of the IETF community.  It has
            received public review and has been designed to focus approved for publication by
            the Internet Engineering Steering Group (IESG).  Further
            information on controlling Internet Standards is available in Section 2 of
            RFC 7841.
        </t>
        <t indent="0" pn="section-boilerplate.1-3">
            Information about the media plane, leaving signaling plane behavior up current status of this document, any
            errata, and how to provide feedback on it may be obtained at
            <eref target="https://www.rfc-editor.org/info/rfc8829" brackets="none"/>.
        </t>
      </section>
      <section anchor="copyright" numbered="false" removeInRFC="false" toc="exclude" pn="section-boilerplate.2">
        <name slugifiedName="name-copyright-notice">Copyright Notice</name>
        <t indent="0" pn="section-boilerplate.2-1">
            Copyright (c) 2021 IETF Trust and the
        application as much persons identified as possible. The rationale the
            document authors. All rights reserved.
        </t>
        <t indent="0" pn="section-boilerplate.2-2">
            This document is that
        different applications may prefer subject to use different protocols,
        such as BCP 78 and the existing SIP call signaling protocol, or something
        custom IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<eref target="https://trustee.ietf.org/license-info" brackets="none"/>) in effect on the particular application, perhaps for a novel use
        case. In date of
            publication of this approach, the key information that needs to be
        exchanged is the multimedia session description, which
        specifies the necessary transport document. Please review these documents
            carefully, as they describe your rights and media configuration
        information necessary restrictions with
            respect to establish the media plane.</t>

        <t>With these considerations in mind, this document. Code Components extracted from this
            document describes
        the JavaScript Session Establishment Protocol (JSEP) that
        allows for full control must include Simplified BSD License text as described in
            Section 4.e of the signaling state machine from
        JavaScript. As Trust Legal Provisions and are provided without
            warranty as described above, JSEP assumes a model in which a
        JavaScript application executes inside a runtime containing
        WebRTC APIs (the "JSEP implementation"). The JSEP
        implementation is almost entirely divorced from the core
        signaling flow, which is instead handled by the JavaScript
        making use Simplified BSD License.
        </t>
      </section>
    </boilerplate>
    <toc>
      <section anchor="toc" numbered="false" removeInRFC="false" toc="exclude" pn="section-toc.1">
        <name slugifiedName="name-table-of-contents">Table of Contents</name>
        <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1">
          <li pn="section-toc.1-1.1">
            <t indent="0" pn="section-toc.1-1.1.1"><xref derivedContent="1" format="counter" sectionFormat="of" target="section-1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-introduction">Introduction</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.1.2">
              <li pn="section-toc.1-1.1.2.1">
                <t indent="0" keepWithNext="true" pn="section-toc.1-1.1.2.1.1"><xref derivedContent="1.1" format="counter" sectionFormat="of" target="section-1.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-general-design-of-jsep">General Design of two interfaces: (1) passing in local JSEP</xref></t>
              </li>
              <li pn="section-toc.1-1.1.2.2">
                <t indent="0" keepWithNext="true" pn="section-toc.1-1.1.2.2.1"><xref derivedContent="1.2" format="counter" sectionFormat="of" target="section-1.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-other-approaches-considered">Other Approaches Considered</xref></t>
              </li>
              <li pn="section-toc.1-1.1.2.3">
                <t indent="0" keepWithNext="true" pn="section-toc.1-1.1.2.3.1"><xref derivedContent="1.3" format="counter" sectionFormat="of" target="section-1.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-contradiction-regarding-bun">Contradiction regarding bundle-only "m=" sections</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.2">
            <t indent="0" pn="section-toc.1-1.2.1"><xref derivedContent="2" format="counter" sectionFormat="of" target="section-2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-terminology">Terminology</xref></t>
          </li>
          <li pn="section-toc.1-1.3">
            <t indent="0" pn="section-toc.1-1.3.1"><xref derivedContent="3" format="counter" sectionFormat="of" target="section-3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-semantics-and-syntax">Semantics and Syntax</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.3.2">
              <li pn="section-toc.1-1.3.2.1">
                <t indent="0" pn="section-toc.1-1.3.2.1.1"><xref derivedContent="3.1" format="counter" sectionFormat="of" target="section-3.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-signaling-model">Signaling Model</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.2">
                <t indent="0" pn="section-toc.1-1.3.2.2.1"><xref derivedContent="3.2" format="counter" sectionFormat="of" target="section-3.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-session-descriptions-and-st">Session Descriptions and remote
        session descriptions State Machine</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.3">
                <t indent="0" pn="section-toc.1-1.3.2.3.1"><xref derivedContent="3.3" format="counter" sectionFormat="of" target="section-3.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-session-description-format">Session Description Format</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.4">
                <t indent="0" pn="section-toc.1-1.3.2.4.1"><xref derivedContent="3.4" format="counter" sectionFormat="of" target="section-3.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-session-description-control">Session Description Control</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.3.2.4.2">
                  <li pn="section-toc.1-1.3.2.4.2.1">
                    <t indent="0" pn="section-toc.1-1.3.2.4.2.1.1"><xref derivedContent="3.4.1" format="counter" sectionFormat="of" target="section-3.4.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-rtptransceivers">RtpTransceivers</xref></t>
                  </li>
                  <li pn="section-toc.1-1.3.2.4.2.2">
                    <t indent="0" pn="section-toc.1-1.3.2.4.2.2.1"><xref derivedContent="3.4.2" format="counter" sectionFormat="of" target="section-3.4.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-rtpsenders">RtpSenders</xref></t>
                  </li>
                  <li pn="section-toc.1-1.3.2.4.2.3">
                    <t indent="0" pn="section-toc.1-1.3.2.4.2.3.1"><xref derivedContent="3.4.3" format="counter" sectionFormat="of" target="section-3.4.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-rtpreceivers">RtpReceivers</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.3.2.5">
                <t indent="0" pn="section-toc.1-1.3.2.5.1"><xref derivedContent="3.5" format="counter" sectionFormat="of" target="section-3.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ice">ICE</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.3.2.5.2">
                  <li pn="section-toc.1-1.3.2.5.2.1">
                    <t indent="0" pn="section-toc.1-1.3.2.5.2.1.1"><xref derivedContent="3.5.1" format="counter" sectionFormat="of" target="section-3.5.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ice-gathering-overview">ICE Gathering Overview</xref></t>
                  </li>
                  <li pn="section-toc.1-1.3.2.5.2.2">
                    <t indent="0" pn="section-toc.1-1.3.2.5.2.2.1"><xref derivedContent="3.5.2" format="counter" sectionFormat="of" target="section-3.5.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ice-candidate-trickling">ICE Candidate Trickling</xref></t>
                    <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.3.2.5.2.2.2">
                      <li pn="section-toc.1-1.3.2.5.2.2.2.1">
                        <t indent="0" pn="section-toc.1-1.3.2.5.2.2.2.1.1"><xref derivedContent="3.5.2.1" format="counter" sectionFormat="of" target="section-3.5.2.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ice-candidate-format">ICE Candidate Format</xref></t>
                      </li>
                    </ul>
                  </li>
                  <li pn="section-toc.1-1.3.2.5.2.3">
                    <t indent="0" pn="section-toc.1-1.3.2.5.2.3.1"><xref derivedContent="3.5.3" format="counter" sectionFormat="of" target="section-3.5.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ice-candidate-policy">ICE Candidate Policy</xref></t>
                  </li>
                  <li pn="section-toc.1-1.3.2.5.2.4">
                    <t indent="0" pn="section-toc.1-1.3.2.5.2.4.1"><xref derivedContent="3.5.4" format="counter" sectionFormat="of" target="section-3.5.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ice-candidate-pool">ICE Candidate Pool</xref></t>
                  </li>
                  <li pn="section-toc.1-1.3.2.5.2.5">
                    <t indent="0" pn="section-toc.1-1.3.2.5.2.5.1"><xref derivedContent="3.5.5" format="counter" sectionFormat="of" target="section-3.5.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ice-versions">ICE Versions</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.3.2.6">
                <t indent="0" pn="section-toc.1-1.3.2.6.1"><xref derivedContent="3.6" format="counter" sectionFormat="of" target="section-3.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-video-size-negotiation">Video Size Negotiation</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.3.2.6.2">
                  <li pn="section-toc.1-1.3.2.6.2.1">
                    <t indent="0" pn="section-toc.1-1.3.2.6.2.1.1"><xref derivedContent="3.6.1" format="counter" sectionFormat="of" target="section-3.6.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-creating-an-imageattr-attri">Creating an imageattr Attribute</xref></t>
                  </li>
                  <li pn="section-toc.1-1.3.2.6.2.2">
                    <t indent="0" pn="section-toc.1-1.3.2.6.2.2.1"><xref derivedContent="3.6.2" format="counter" sectionFormat="of" target="section-3.6.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-interpreting-imageattr-attr">Interpreting imageattr Attributes</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.3.2.7">
                <t indent="0" pn="section-toc.1-1.3.2.7.1"><xref derivedContent="3.7" format="counter" sectionFormat="of" target="section-3.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-simulcast">Simulcast</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.8">
                <t indent="0" pn="section-toc.1-1.3.2.8.1"><xref derivedContent="3.8" format="counter" sectionFormat="of" target="section-3.8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-interactions-with-forking">Interactions with Forking</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.3.2.8.2">
                  <li pn="section-toc.1-1.3.2.8.2.1">
                    <t indent="0" pn="section-toc.1-1.3.2.8.2.1.1"><xref derivedContent="3.8.1" format="counter" sectionFormat="of" target="section-3.8.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sequential-forking">Sequential Forking</xref></t>
                  </li>
                  <li pn="section-toc.1-1.3.2.8.2.2">
                    <t indent="0" pn="section-toc.1-1.3.2.8.2.2.1"><xref derivedContent="3.8.2" format="counter" sectionFormat="of" target="section-3.8.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-parallel-forking">Parallel Forking</xref></t>
                  </li>
                </ul>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.4">
            <t indent="0" pn="section-toc.1-1.4.1"><xref derivedContent="4" format="counter" sectionFormat="of" target="section-4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-interface">Interface</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.4.2">
              <li pn="section-toc.1-1.4.2.1">
                <t indent="0" pn="section-toc.1-1.4.2.1.1"><xref derivedContent="4.1" format="counter" sectionFormat="of" target="section-4.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-peerconnection">PeerConnection</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.4.2.1.2">
                  <li pn="section-toc.1-1.4.2.1.2.1">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.1.1"><xref derivedContent="4.1.1" format="counter" sectionFormat="of" target="section-4.1.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-constructor">Constructor</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.2">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.2.1"><xref derivedContent="4.1.2" format="counter" sectionFormat="of" target="section-4.1.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-addtrack">addTrack</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.3">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.3.1"><xref derivedContent="4.1.3" format="counter" sectionFormat="of" target="section-4.1.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-removetrack">removeTrack</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.4">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.4.1"><xref derivedContent="4.1.4" format="counter" sectionFormat="of" target="section-4.1.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-addtransceiver">addTransceiver</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.5">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.5.1"><xref derivedContent="4.1.5" format="counter" sectionFormat="of" target="section-4.1.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-onaddtrack-event">onaddtrack Event</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.6">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.6.1"><xref derivedContent="4.1.6" format="counter" sectionFormat="of" target="section-4.1.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-createdatachannel">createDataChannel</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.7">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.7.1"><xref derivedContent="4.1.7" format="counter" sectionFormat="of" target="section-4.1.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ondatachannel-event">ondatachannel Event</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.8">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.8.1"><xref derivedContent="4.1.8" format="counter" sectionFormat="of" target="section-4.1.8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-createoffer">createOffer</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.9">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.9.1"><xref derivedContent="4.1.9" format="counter" sectionFormat="of" target="section-4.1.9"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-createanswer">createAnswer</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.10">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.10.1"><xref derivedContent="4.1.10" format="counter" sectionFormat="of" target="section-4.1.10"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-sessiondescriptiontype">SessionDescriptionType</xref></t>
                    <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.4.2.1.2.10.2">
                      <li pn="section-toc.1-1.4.2.1.2.10.2.1">
                        <t indent="0" pn="section-toc.1-1.4.2.1.2.10.2.1.1"><xref derivedContent="4.1.10.1" format="counter" sectionFormat="of" target="section-4.1.10.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-use-of-provisional-answers">Use of Provisional Answers</xref></t>
                      </li>
                      <li pn="section-toc.1-1.4.2.1.2.10.2.2">
                        <t indent="0" pn="section-toc.1-1.4.2.1.2.10.2.2.1"><xref derivedContent="4.1.10.2" format="counter" sectionFormat="of" target="section-4.1.10.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-rollback">Rollback</xref></t>
                      </li>
                    </ul>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.11">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.11.1"><xref derivedContent="4.1.11" format="counter" sectionFormat="of" target="section-4.1.11"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-setlocaldescription">setLocalDescription</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.12">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.12.1"><xref derivedContent="4.1.12" format="counter" sectionFormat="of" target="section-4.1.12"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-setremotedescription">setRemoteDescription</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.13">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.13.1"><xref derivedContent="4.1.13" format="counter" sectionFormat="of" target="section-4.1.13"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-currentlocaldescription">currentLocalDescription</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.14">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.14.1"><xref derivedContent="4.1.14" format="counter" sectionFormat="of" target="section-4.1.14"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-pendinglocaldescription">pendingLocalDescription</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.15">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.15.1"><xref derivedContent="4.1.15" format="counter" sectionFormat="of" target="section-4.1.15"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-currentremotedescription">currentRemoteDescription</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.16">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.16.1"><xref derivedContent="4.1.16" format="counter" sectionFormat="of" target="section-4.1.16"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-pendingremotedescription">pendingRemoteDescription</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.17">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.17.1"><xref derivedContent="4.1.17" format="counter" sectionFormat="of" target="section-4.1.17"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-cantrickleicecandidates">canTrickleIceCandidates</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.18">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.18.1"><xref derivedContent="4.1.18" format="counter" sectionFormat="of" target="section-4.1.18"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-setconfiguration">setConfiguration</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.19">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.19.1"><xref derivedContent="4.1.19" format="counter" sectionFormat="of" target="section-4.1.19"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-addicecandidate">addIceCandidate</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.20">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.20.1"><xref derivedContent="4.1.20" format="counter" sectionFormat="of" target="section-4.1.20"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-onicecandidate-event">onicecandidate Event</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.4.2.2">
                <t indent="0" pn="section-toc.1-1.4.2.2.1"><xref derivedContent="4.2" format="counter" sectionFormat="of" target="section-4.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-rtptransceiver">RtpTransceiver</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.4.2.2.2">
                  <li pn="section-toc.1-1.4.2.2.2.1">
                    <t indent="0" pn="section-toc.1-1.4.2.2.2.1.1"><xref derivedContent="4.2.1" format="counter" sectionFormat="of" target="section-4.2.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-stop">stop</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.2.2.2">
                    <t indent="0" pn="section-toc.1-1.4.2.2.2.2.1"><xref derivedContent="4.2.2" format="counter" sectionFormat="of" target="section-4.2.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-stopped">stopped</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.2.2.3">
                    <t indent="0" pn="section-toc.1-1.4.2.2.2.3.1"><xref derivedContent="4.2.3" format="counter" sectionFormat="of" target="section-4.2.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-setdirection">setDirection</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.2.2.4">
                    <t indent="0" pn="section-toc.1-1.4.2.2.2.4.1"><xref derivedContent="4.2.4" format="counter" sectionFormat="of" target="section-4.2.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-direction">direction</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.2.2.5">
                    <t indent="0" pn="section-toc.1-1.4.2.2.2.5.1"><xref derivedContent="4.2.5" format="counter" sectionFormat="of" target="section-4.2.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-currentdirection">currentDirection</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.2.2.6">
                    <t indent="0" pn="section-toc.1-1.4.2.2.2.6.1"><xref derivedContent="4.2.6" format="counter" sectionFormat="of" target="section-4.2.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-setcodecpreferences">setCodecPreferences</xref></t>
                  </li>
                </ul>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.5">
            <t indent="0" pn="section-toc.1-1.5.1"><xref derivedContent="5" format="counter" sectionFormat="of" target="section-5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sdp-interaction-procedures">SDP Interaction Procedures</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.5.2">
              <li pn="section-toc.1-1.5.2.1">
                <t indent="0" pn="section-toc.1-1.5.2.1.1"><xref derivedContent="5.1" format="counter" sectionFormat="of" target="section-5.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-requirements-overview">Requirements Overview</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.5.2.1.2">
                  <li pn="section-toc.1-1.5.2.1.2.1">
                    <t indent="0" pn="section-toc.1-1.5.2.1.2.1.1"><xref derivedContent="5.1.1" format="counter" sectionFormat="of" target="section-5.1.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-usage-requirements">Usage Requirements</xref></t>
                  </li>
                  <li pn="section-toc.1-1.5.2.1.2.2">
                    <t indent="0" pn="section-toc.1-1.5.2.1.2.2.1"><xref derivedContent="5.1.2" format="counter" sectionFormat="of" target="section-5.1.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-profile-names-and-interoper">Profile Names and (2) interacting with Interoperability</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.5.2.2">
                <t indent="0" pn="section-toc.1-1.5.2.2.1"><xref derivedContent="5.2" format="counter" sectionFormat="of" target="section-5.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-constructing-an-offer">Constructing an Offer</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.5.2.2.2">
                  <li pn="section-toc.1-1.5.2.2.2.1">
                    <t indent="0" pn="section-toc.1-1.5.2.2.2.1.1"><xref derivedContent="5.2.1" format="counter" sectionFormat="of" target="section-5.2.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-initial-offers">Initial Offers</xref></t>
                  </li>
                  <li pn="section-toc.1-1.5.2.2.2.2">
                    <t indent="0" pn="section-toc.1-1.5.2.2.2.2.1"><xref derivedContent="5.2.2" format="counter" sectionFormat="of" target="section-5.2.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-subsequent-offers">Subsequent Offers</xref></t>
                  </li>
                  <li pn="section-toc.1-1.5.2.2.2.3">
                    <t indent="0" pn="section-toc.1-1.5.2.2.2.3.1"><xref derivedContent="5.2.3" format="counter" sectionFormat="of" target="section-5.2.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-options-handling">Options Handling</xref></t>
                    <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.5.2.2.2.3.2">
                      <li pn="section-toc.1-1.5.2.2.2.3.2.1">
                        <t indent="0" pn="section-toc.1-1.5.2.2.2.3.2.1.1"><xref derivedContent="5.2.3.1" format="counter" sectionFormat="of" target="section-5.2.3.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-icerestart">IceRestart</xref></t>
                      </li>
                      <li pn="section-toc.1-1.5.2.2.2.3.2.2">
                        <t indent="0" pn="section-toc.1-1.5.2.2.2.3.2.2.1"><xref derivedContent="5.2.3.2" format="counter" sectionFormat="of" target="section-5.2.3.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-voiceactivitydetection">VoiceActivityDetection</xref></t>
                      </li>
                    </ul>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.5.2.3">
                <t indent="0" pn="section-toc.1-1.5.2.3.1"><xref derivedContent="5.3" format="counter" sectionFormat="of" target="section-5.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-generating-an-answer">Generating an Answer</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.5.2.3.2">
                  <li pn="section-toc.1-1.5.2.3.2.1">
                    <t indent="0" pn="section-toc.1-1.5.2.3.2.1.1"><xref derivedContent="5.3.1" format="counter" sectionFormat="of" target="section-5.3.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-initial-answers">Initial Answers</xref></t>
                  </li>
                  <li pn="section-toc.1-1.5.2.3.2.2">
                    <t indent="0" pn="section-toc.1-1.5.2.3.2.2.1"><xref derivedContent="5.3.2" format="counter" sectionFormat="of" target="section-5.3.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-subsequent-answers">Subsequent Answers</xref></t>
                  </li>
                  <li pn="section-toc.1-1.5.2.3.2.3">
                    <t indent="0" pn="section-toc.1-1.5.2.3.2.3.1"><xref derivedContent="5.3.3" format="counter" sectionFormat="of" target="section-5.3.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-options-handling-2">Options Handling</xref></t>
                    <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.5.2.3.2.3.2">
                      <li pn="section-toc.1-1.5.2.3.2.3.2.1">
                        <t indent="0" pn="section-toc.1-1.5.2.3.2.3.2.1.1"><xref derivedContent="5.3.3.1" format="counter" sectionFormat="of" target="section-5.3.3.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-voiceactivitydetection-2">VoiceActivityDetection</xref></t>
                      </li>
                    </ul>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.5.2.4">
                <t indent="0" pn="section-toc.1-1.5.2.4.1"><xref derivedContent="5.4" format="counter" sectionFormat="of" target="section-5.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-modifying-an-offer-or-answe">Modifying an Offer or Answer</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.5">
                <t indent="0" pn="section-toc.1-1.5.2.5.1"><xref derivedContent="5.5" format="counter" sectionFormat="of" target="section-5.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-processing-a-local-descript">Processing a Local Description</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.6">
                <t indent="0" pn="section-toc.1-1.5.2.6.1"><xref derivedContent="5.6" format="counter" sectionFormat="of" target="section-5.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-processing-a-remote-descrip">Processing a Remote Description</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.7">
                <t indent="0" pn="section-toc.1-1.5.2.7.1"><xref derivedContent="5.7" format="counter" sectionFormat="of" target="section-5.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-processing-a-rollback">Processing a Rollback</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.8">
                <t indent="0" pn="section-toc.1-1.5.2.8.1"><xref derivedContent="5.8" format="counter" sectionFormat="of" target="section-5.8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-parsing-a-session-descripti">Parsing a Session Description</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.5.2.8.2">
                  <li pn="section-toc.1-1.5.2.8.2.1">
                    <t indent="0" pn="section-toc.1-1.5.2.8.2.1.1"><xref derivedContent="5.8.1" format="counter" sectionFormat="of" target="section-5.8.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-session-level-parsing">Session-Level Parsing</xref></t>
                  </li>
                  <li pn="section-toc.1-1.5.2.8.2.2">
                    <t indent="0" pn="section-toc.1-1.5.2.8.2.2.1"><xref derivedContent="5.8.2" format="counter" sectionFormat="of" target="section-5.8.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-media-section-parsing">Media Section Parsing</xref></t>
                  </li>
                  <li pn="section-toc.1-1.5.2.8.2.3">
                    <t indent="0" pn="section-toc.1-1.5.2.8.2.3.1"><xref derivedContent="5.8.3" format="counter" sectionFormat="of" target="section-5.8.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-semantics-verification">Semantics Verification</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.5.2.9">
                <t indent="0" pn="section-toc.1-1.5.2.9.1"><xref derivedContent="5.9" format="counter" sectionFormat="of" target="section-5.9"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-applying-a-local-descriptio">Applying a Local Description</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.10">
                <t indent="0" pn="section-toc.1-1.5.2.10.1"><xref derivedContent="5.10" format="counter" sectionFormat="of" target="section-5.10"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-applying-a-remote-descripti">Applying a Remote Description</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.11">
                <t indent="0" pn="section-toc.1-1.5.2.11.1"><xref derivedContent="5.11" format="counter" sectionFormat="of" target="section-5.11"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-applying-an-answer">Applying an Answer</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.6">
            <t indent="0" pn="section-toc.1-1.6.1"><xref derivedContent="6" format="counter" sectionFormat="of" target="section-6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-processing-rtp-rtcp">Processing RTP/RTCP</xref></t>
          </li>
          <li pn="section-toc.1-1.7">
            <t indent="0" pn="section-toc.1-1.7.1"><xref derivedContent="7" format="counter" sectionFormat="of" target="section-7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-examples">Examples</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.7.2">
              <li pn="section-toc.1-1.7.2.1">
                <t indent="0" pn="section-toc.1-1.7.2.1.1"><xref derivedContent="7.1" format="counter" sectionFormat="of" target="section-7.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-simple-example">Simple Example</xref></t>
              </li>
              <li pn="section-toc.1-1.7.2.2">
                <t indent="0" pn="section-toc.1-1.7.2.2.1"><xref derivedContent="7.2" format="counter" sectionFormat="of" target="section-7.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-detailed-example">Detailed Example</xref></t>
              </li>
              <li pn="section-toc.1-1.7.2.3">
                <t indent="0" pn="section-toc.1-1.7.2.3.1"><xref derivedContent="7.3" format="counter" sectionFormat="of" target="section-7.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-early-transport-warmup-exam">Early Transport Warmup Example</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.8">
            <t indent="0" pn="section-toc.1-1.8.1"><xref derivedContent="8" format="counter" sectionFormat="of" target="section-8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-security-considerations">Security Considerations</xref></t>
          </li>
          <li pn="section-toc.1-1.9">
            <t indent="0" pn="section-toc.1-1.9.1"><xref derivedContent="9" format="counter" sectionFormat="of" target="section-9"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-iana-considerations">IANA Considerations</xref></t>
          </li>
          <li pn="section-toc.1-1.10">
            <t indent="0" pn="section-toc.1-1.10.1"><xref derivedContent="10" format="counter" sectionFormat="of" target="section-10"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-references">References</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.10.2">
              <li pn="section-toc.1-1.10.2.1">
                <t indent="0" pn="section-toc.1-1.10.2.1.1"><xref derivedContent="10.1" format="counter" sectionFormat="of" target="section-10.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-normative-references">Normative References</xref></t>
              </li>
              <li pn="section-toc.1-1.10.2.2">
                <t indent="0" pn="section-toc.1-1.10.2.2.1"><xref derivedContent="10.2" format="counter" sectionFormat="of" target="section-10.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-informative-references">Informative References</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.11">
            <t indent="0" pn="section-toc.1-1.11.1"><xref derivedContent="Appendix A" format="default" sectionFormat="of" target="section-appendix.a"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sdp-abnf-syntax">SDP ABNF Syntax</xref></t>
          </li>
          <li pn="section-toc.1-1.12">
            <t indent="0" pn="section-toc.1-1.12.1"><xref derivedContent="" format="none" sectionFormat="of" target="section-appendix.b"/><xref derivedContent="" format="title" sectionFormat="of" target="name-acknowledgements">Acknowledgements</xref></t>
          </li>
          <li pn="section-toc.1-1.13">
            <t indent="0" pn="section-toc.1-1.13.1"><xref derivedContent="" format="none" sectionFormat="of" target="section-appendix.c"/><xref derivedContent="" format="title" sectionFormat="of" target="name-authors-addresses">Authors' Addresses</xref></t>
          </li>
        </ul>
      </section>
    </toc>
  </front>
  <middle>
    <section anchor="sec.introduction" numbered="true" toc="include" removeInRFC="false" pn="section-1">
      <name slugifiedName="name-introduction">Introduction</name>
      <t indent="0" pn="section-1-1">This document describes how the ICE state
        machine. The combination of W3C Web Real-Time Communication (WebRTC) RTCPeerConnection
      interface
      <xref target="W3C.webrtc" format="default" sectionFormat="of" derivedContent="W3C.webrtc"/> is used to control the JSEP implementation setup,
      management, and teardown of a multimedia session.</t>
      <section anchor="sec.general-design-of-jsep" numbered="true" toc="include" removeInRFC="false" pn="section-1.1">
        <name slugifiedName="name-general-design-of-jsep">General Design of JSEP</name>
        <t indent="0" pn="section-1.1-1">WebRTC call setup has been designed to focus on controlling
        the media plane, leaving signaling-plane behavior up to the
        JavaScript
        application as much as possible. The rationale is referred that
        different applications may prefer to throughout this document use different protocols,
        such as the existing SIP call signaling protocol, or something
        custom to the particular application, perhaps for a "JSEP endpoint".</t>

        <t>In novel use
        case. In this document, approach, the use of key information that needs to be
        exchanged is the multimedia session description, which
        specifies the transport and media configuration
        information necessary to establish the media plane.</t>
        <t indent="0" pn="section-1.1-2">With these considerations in mind, this document describes
        the JavaScript Session Establishment Protocol (JSEP), which
        allows for full control of the signaling state machine from
        JavaScript. As described above, JSEP assumes a model in which a
        JavaScript application executes inside a runtime containing
        WebRTC APIs (the "JSEP implementation"). The JSEP
        implementation is almost entirely divorced from the core
        signaling flow, which is instead handled by the JavaScript
        making use of two interfaces: (1) passing in local and remote
        session descriptions and (2) interacting with the Interactive
        Connectivity Establishment (ICE) state
        machine <xref target="RFC8445" format="default" sectionFormat="of" derivedContent="RFC8445"/>. The combination of the JSEP implementation and the
        JavaScript application is referred to throughout this document
        as a "JSEP endpoint".</t>
        <t indent="0" pn="section-1.1-3">In this document, the use of JSEP is described as if it
        always occurs between two JSEP endpoints. Note though Note, though, that in many
        cases it will actually be between a JSEP endpoint and some kind
        of server, such as a gateway or MCU. Multipoint Control Unit (MCU). This distinction is
        invisible to the JSEP endpoint; it just follows the
        instructions it is given via the API.</t>

        <t>JSEP's
        <t indent="0" pn="section-1.1-4">JSEP's handling of session descriptions is simple and
        straightforward. Whenever an offer/answer exchange is needed,
        the initiating side creates an offer by calling a createOffer() createOffer
        API. The application then uses that offer to set up its local
        config
        configuration via the setLocalDescription() setLocalDescription API. The offer is finally
        sent off to the remote side over its preferred signaling
        mechanism (e.g., WebSockets); upon receipt of that offer, the
        remote party installs it using the setRemoteDescription() setRemoteDescription
        API.</t>

        <t>To
        <t indent="0" pn="section-1.1-5">To complete the offer/answer exchange, the remote party uses
        the createAnswer() createAnswer API to generate an appropriate answer,
        applies it using the setLocalDescription() setLocalDescription API, and sends the
        answer back to the initiator over the signaling channel. When
        the initiator gets that answer, it installs it using the
        setRemoteDescription()
        setRemoteDescription API, and initial setup is complete. This
        process can be repeated for additional offer/answer
        exchanges.</t>

        <t>Regarding
        <t indent="0" pn="section-1.1-6">Regarding ICE
        <xref target="RFC8445"></xref>, target="RFC8445" format="default" sectionFormat="of" derivedContent="RFC8445"/>, JSEP decouples the ICE state
        machine from the overall signaling state machine, as the machine. The ICE
        state machine must remain in the JSEP implementation, implementation because
        only the implementation has the necessary knowledge of
        candidates and other transport information. Performing this
        separation provides additional flexibility in protocols that
        decouple session descriptions from transport. For instance, in
        traditional SIP, each offer or answer is self-contained,
        including both the session descriptions and the transport
        information. However,
        <xref target="I-D.ietf-mmusic-trickle-ice-sip" /> target="RFC8840" format="default" sectionFormat="of" derivedContent="RFC8840"/> allows SIP to
        be used with trickle Trickle ICE
        <xref target="I-D.ietf-ice-trickle" />, target="RFC8838" format="default" sectionFormat="of" derivedContent="RFC8838"/>, in which the session
        description can be sent immediately and the transport
        information can be sent when available. Sending transport
        information separately can allow for faster ICE and DTLS
        startup, since ICE checks can start as soon as any transport
        information is available rather than waiting for all of it.
        JSEP's decoupling of the ICE and signaling state machines
        allows it to accommodate either model.</t>

        <t>Through its abstraction of
        <t indent="0" pn="section-1.1-7">Although it abstracts signaling, the JSEP approach does
        require
        requires that the application to be aware of the signaling process.
        While the application does not need to understand the contents
        of session descriptions to set up a call, the application must
        call the right APIs at the right times, convert the session
        descriptions and ICE information into the defined messages of
        its chosen signaling protocol, and perform the reverse
        conversion on the messages it receives from the other side.</t>

        <t>One
        <t indent="0" pn="section-1.1-8">One way to make life easier for the application is to
        provide a JavaScript library that hides this complexity from
        the developer; said library would implement a given signaling
        protocol along with its state machine and serialization code,
        presenting a higher level higher-level call-oriented interface to the
        application developer. For example, libraries exist to adapt provide
        implementations of the JSEP API into an API suitable for a SIP or XMPP. <xref target="RFC3261" format="default" sectionFormat="of" derivedContent="RFC3261"/> and Extensible Messaging
        and Presence Protocol (XMPP) <xref target="RFC6120" format="default" sectionFormat="of" derivedContent="RFC6120"/> signaling
        protocols atop the JSEP API.
        Thus, JSEP
        provides greater control for the experienced developer without
        forcing any additional complexity on the novice developer.</t>
      </section>
      <section title="Other anchor="sec.other-approaches-consider" numbered="true" toc="include" removeInRFC="false" pn="section-1.2">
        <name slugifiedName="name-other-approaches-considered">Other Approaches Considered"
      anchor="sec.other-approaches-consider">

        <t>One Considered</name>
        <t indent="0" pn="section-1.2-1">One approach that was considered instead of JSEP was to
        include a lightweight signaling protocol. Instead of providing
        session descriptions to the API, the API would produce and
        consume messages from this protocol. While providing a more
        high-level API, this put more control of signaling within the
        JSEP implementation, forcing it to have to understand and
        handle concepts like signaling glare (see
        <xref target="RFC3264" />, Section 4).</t>

        <t>A sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-4" derivedContent="RFC3264"/>).</t>
        <t indent="0" pn="section-1.2-2">A second approach that was considered but not chosen was to
        decouple the management of the media control objects from
        session descriptions, instead offering APIs that would control
        each component directly. This was rejected based on the
        argument that requiring exposure of this level of complexity to
        the application programmer would not be beneficial; it would
        (1) result in an API where even a simple example would require a
        significant amount of code to orchestrate all the needed
        interactions, as well as creating
        interactions and (2) create a large API surface that
        needed
        would need to be agreed upon and documented.
        In addition, these API
        points could be called in any order, resulting in a more
        complex set of interactions with the media subsystem than the
        JSEP approach, which specifies how session descriptions are to
        be evaluated and applied.</t>

        <t>One
        <t indent="0" pn="section-1.2-3">One variation on JSEP that was considered was to keep the
        basic session description-oriented API, session-description-oriented API but to move the
        mechanism for generating offers and answers out of the JSEP
        implementation. Instead of providing createOffer/createAnswer
        methods within the implementation, this approach would instead
        expose a getCapabilities API API, which would provide the
        application with the information it needed in order to generate
        its own session descriptions. This increases the amount of work
        that the application needs to do; it needs to know how to
        generate session descriptions from capabilities, and especially
        how to generate the correct answer from an arbitrary offer and
        the supported capabilities. While this could certainly be
        addressed by using a library like the one mentioned above, it
        basically forces the use of said library even for a simple
        example. Providing createOffer/createAnswer avoids this
        problem.</t>
      </section>
    </section>
      <section title="Terminology" anchor="sec.terminology">

      <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", numbered="true" removeInRFC="false" toc="include" pn="section-1.3">
        <name slugifiedName="name-contradiction-regarding-bun">Contradiction regarding bundle-only "m=" sections</name>
        <t indent="0" pn="section-1.3-1">Since the approval of the WebRTC specification documents, the IETF has become
aware of an inconsistency between the document specifying JSEP and
      "OPTIONAL" in this the document are to be interpreted as described in specifying BUNDLE (this RFC and <xref target="RFC2119"></xref>.</t> target="RFC8843" format="default" sectionFormat="of" derivedContent="RFC8843"/>, respectively).  Rather than delaying publication further to come to a resolution, the documents are being published as they were originally approved.  The IETF intends to restart work on these technologies, and revised versions of these documents will be published as soon as a resolution becomes available.</t>
        <t indent="0" pn="section-1.3-2">The specific issue involves the handling of "m=" sections that are designated as bundle-only, as discussed in <xref target="sec.pc-constructor" format="default" sectionFormat="of" derivedContent="Section 4.1.1"/> of this RFC. Currently, there is divergence between JSEP and BUNDLE, as well as between these specifications and existing browser implementations:</t>
        <ul bare="false" empty="false" indent="3" spacing="normal" pn="section-1.3-3">
          <li pn="section-1.3-3.1">JSEP prescribes that said "m=" sections should use port zero and add an "a=bundle-only" attribute in initial offers, but not in answers or subsequent offers.</li>
          <li pn="section-1.3-3.2">BUNDLE prescribes that these "m=" sections should be marked as described in the previous point, but in all offers and answers.</li>
          <li pn="section-1.3-3.3">Most current browsers do not mark any "m=" sections with port zero and instead use the same port for all bundled "m=" sections; one follows the JSEP behavior.</li>
        </ul>
      </section>
    </section>
    <section title="Semantics anchor="sec.terminology" numbered="true" toc="include" removeInRFC="false" pn="section-2">
      <name slugifiedName="name-terminology">Terminology</name>
      <t indent="0" pn="section-2-1">The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
    "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>",
    "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>",
    "<bcp14>SHOULD NOT</bcp14>",
    "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are
    to be interpreted as described in BCP 14 <xref target="RFC2119" format="default" sectionFormat="of" derivedContent="RFC2119"/>
        <xref target="RFC8174" format="default" sectionFormat="of" derivedContent="RFC8174"/> when, and Syntax"
    anchor="sec.semantics-and-syntax"> only when, they appear in all capitals,
    as shown here.</t>
    </section>
    <section title="Signaling Model" anchor="sec.signaling-model">

        <t>JSEP anchor="sec.semantics-and-syntax" numbered="true" toc="include" removeInRFC="false" pn="section-3">
      <name slugifiedName="name-semantics-and-syntax">Semantics and Syntax</name>
      <section anchor="sec.signaling-model" numbered="true" toc="include" removeInRFC="false" pn="section-3.1">
        <name slugifiedName="name-signaling-model">Signaling Model</name>
        <t indent="0" pn="section-3.1-1">JSEP does not specify a particular signaling model or state
        machine, other than the generic need to exchange session
        descriptions in the fashion described by
        <xref target="RFC3264"></xref> target="RFC3264" format="default" sectionFormat="of" derivedContent="RFC3264"/> (offer/answer) in order for both
        sides of the session to know how to conduct the session. JSEP
        provides mechanisms to create offers and answers, as well as to
        apply them to a session. However, the JSEP implementation is
        totally decoupled from the actual mechanism by which these
        offers and answers are communicated to the remote side,
        including addressing, retransmission, forking, and glare
        handling. These issues are left entirely up to the application;
        the application has complete control over which offers and
        answers get handed to the implementation, and when.</t>
        <figure anchor="fig-sigModel" title="JSEP align="left" suppress-title="false" pn="figure-1">
          <name slugifiedName="name-jsep-signaling-model">JSEP Signaling Model">
          <artwork>
            <![CDATA[ Model</name>
          <artwork name="" type="ascii-art" align="left" alt="" pn="section-3.1-2.1">
      +-----------+                               +-----------+
      |  Web App  |<---  |&lt;--- App-Specific Signaling -->| --&gt;|  Web App  |
      +-----------+                               +-----------+
            ^                                            ^
            |  SDP                                       |  SDP
            V                                            V
      +-----------+                                +-----------+
      |   JSEP    |<-----------    |&lt;----------- Media ------------>| ------------&gt;|   JSEP    |
      |   Impl.   |                                |   Impl.   |
      +-----------+                                +-----------+
]]> </artwork>
        </figure>
      </section>
      <section title="Session anchor="sec.session-descriptions-and-state-machine" numbered="true" toc="include" removeInRFC="false" pn="section-3.2">
        <name slugifiedName="name-session-descriptions-and-st">Session Descriptions and State Machine"
      anchor="sec.session-descriptions-and-state-machine">

        <t>In Machine</name>
        <t indent="0" pn="section-3.2-1">In order to establish the media plane, the JSEP
        implementation needs specific parameters to indicate what to
        transmit to the remote side, as well as how to handle the media
        that is received. These parameters are determined by the
        exchange of session descriptions in offers and answers, and
        there are certain details to this process that must be handled
        in the JSEP APIs.</t>

        <t>Whether
        <t indent="0" pn="section-3.2-2">Whether a session description applies to the local side or
        the remote side affects the meaning of that description. For
        example, the list of codecs sent to a remote party indicates
        what the local side is willing to receive, which, when
        intersected with the set of codecs the remote side supports,
        specifies what the remote side should send. However, not all
        parameters follow this rule; some parameters are declarative declarative,
        and the remote side MUST must either accept them or reject them
        altogether. An example of such a parameter is the DTLS TLS
        fingerprints <xref target="RFC8122"></xref>, which target="RFC8122" format="default" sectionFormat="of" derivedContent="RFC8122"/>
        as used in the context of DTLS <xref target="RFC6347" format="default" sectionFormat="of" derivedContent="RFC6347"/>;
        these fingerprints are calculated based on
        the local certificate(s) offered, offered and are not subject to
        negotiation.</t>

        <t>In
        negotiation.
        </t>
        <t indent="0" pn="section-3.2-3">In addition, various RFCs put different conditions on the
        format of offers versus answers. For example, an offer may
        propose an arbitrary number of m= "m=" sections (i.e., media
        descriptions as described in
        <xref target="RFC4566" />, Section 5.14), sectionFormat="comma" section="5.14" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.14" derivedContent="RFC4566"/>), but an answer must
        contain the exact same number as the offer.</t>

        <t>Lastly,
        <t indent="0" pn="section-3.2-4">Lastly, while the exact media parameters are only known only
        after an offer and an answer have been exchanged, the offerer
        may receive ICE checks, and possibly media (e.g., in the case
        of a re-offer after a connection has been established) before
        it receives an answer. To properly process incoming media in
        this case, the offerer's media handler must be aware of the
        details of the offer before the answer arrives.</t>

        <t>Therefore,
        <t indent="0" pn="section-3.2-5">Therefore, in order to handle session descriptions properly,
        the JSEP implementation needs:
        <list style="numbers">

          <t>To
        </t>
        <ol spacing="normal" type="1" indent="adaptive" start="1" pn="section-3.2-6">
          <li pn="section-3.2-6.1" derivedCounter="1.">To know if a session description pertains to the local or
          remote side.</t>

          <t>To side.</li>
          <li pn="section-3.2-6.2" derivedCounter="2.">To know if a session description is an offer or an
          answer.</t>

          <t>To
          answer.</li>
          <li pn="section-3.2-6.3" derivedCounter="3.">To allow the offer to be specified independently of the
          answer.</t>
        </list>JSEP
          answer.</li>
        </ol>
        <t indent="0" pn="section-3.2-7">JSEP addresses this by adding both setLocalDescription
        and setRemoteDescription methods and having session description
        objects contain a type field indicating the type of session
        description being supplied. This satisfies the requirements
        listed above for both the offerer, who first calls
        setLocalDescription(sdp [offer]) and then later
        setRemoteDescription(sdp [answer]), as well as for and the
        answerer, who first calls setRemoteDescription(sdp [offer]) and
        then later setLocalDescription(sdp [answer]).</t>

        <t>During
        <t indent="0" pn="section-3.2-8">During the offer/answer exchange, the outstanding offer is
        considered to be "pending" at the offerer and the answerer, as
        it may either be either accepted or rejected. If this is a re-offer,
        each side will also have "current" local and remote
        descriptions, which reflect the result of the last offer/answer
        exchange. Sections
        <xref target="sec.pendinglocaldescription" />, format="counter" sectionFormat="of" derivedContent="4.1.14"/>,
        <xref target="sec.pendingremotedescription" />, format="counter" sectionFormat="of" derivedContent="4.1.16"/>,
        <xref target="sec.currentlocaldescription" />, format="counter" sectionFormat="of" derivedContent="4.1.13"/>, and
        <xref target="sec.currentremotedescription" />, format="counter" sectionFormat="of" derivedContent="4.1.15"/> provide more
        detail on pending and current descriptions.</t>

        <t>JSEP
        <t indent="0" pn="section-3.2-9">JSEP also allows for an answer to be treated as provisional
        by the application. Provisional answers provide a way for an
        answerer to communicate initial session parameters back to the
        offerer, in order to allow the session to begin, while allowing
        a final answer to be specified later. This concept of a final
        answer is important to the offer/answer model; when such an
        answer is received, any extra resources allocated by the caller
        can be released, now that the exact session configuration is
        known. These "resources" can include things like extra ICE
        components, TURN Traversal Using Relays around NAT (TURN) candidates, or video decoders. Provisional
        answers, on the other hand, do no such deallocation; as a
        result, multiple dissimilar provisional answers, with their own
        codec choices, transport parameters, etc., can be received and
        applied during call setup. Note that the final answer itself
        may be different than any received provisional answers.</t>

        <t>In
        <t indent="0" pn="section-3.2-10">In
        <xref target="RFC3264"></xref>, target="RFC3264" format="default" sectionFormat="of" derivedContent="RFC3264"/>, the constraint at the signaling
        level is that only one offer can be outstanding for a given
        session, but at the media stack JSEP level, a new offer can be
        generated at any point. For example, when using SIP for
        signaling, if one offer is sent, sent and is then cancelled canceled using a SIP
        CANCEL, another offer can be generated even though no answer
        was received for the first offer. To support this, the JSEP
        media layer can provide an offer via the createOffer() createOffer method
        whenever the JavaScript application needs one for the
        signaling. The answerer can send back zero or more provisional
        answers,
        answers and then finally end the offer-answer offer/answer exchange by sending a
        final answer. The state machine for this is as follows:</t>

        <t>
        <figure anchor="fig-state-machine"
          title="JSEP align="left" suppress-title="false" pn="figure-2">
          <name slugifiedName="name-jsep-state-machine">JSEP State Machine">
            <artwork>
              <![CDATA[ Machine</name>
          <artwork name="" type="ascii-art" align="left" alt="" pn="section-3.2-11.1">
                    setRemote(OFFER)               setLocal(PRANSWER)
                        /-----\                               /-----\
                        |     |                               |     |
                        v     |                               v     |
         +---------------+    |                +---------------+    |
         |               |----/                |               |----/
         |  have-        | setLocal(PRANSWER)  | have-         |
         |  remote-offer |------------------- >| &gt;| local-pranswer|
         |               |                     |               |
         |               |                     |               |
         +---------------+                     +---------------+
              ^   |                                   |
              |   | setLocal(ANSWER)                  |
setRemote(OFFER)  |                                   |
              |   V                  setLocal(ANSWER) |
         +---------------+                            |
         |               |                            |
         |               |<---------------------------+               |&lt;---------------------------+
         |    stable     |
         |               |<---------------------------+               |&lt;---------------------------+
         |               |                            |
         +---------------+          setRemote(ANSWER) |
              ^   |                                   |
              |   | setLocal(OFFER)                   |
setRemote(ANSWER) |                                   |
              |   V                                   |
         +---------------+                     +---------------+
         |               |                     |               |
         |  have-        | setRemote(PRANSWER) |have-          |
         |  local-offer  |------------------- >|remote-pranswer| &gt;|remote-pranswer|
         |               |                     |               |
         |               |----\                |               |----\
         +---------------+    |                +---------------+    |
                        ^     |                               ^     |
                        |     |                               |     |
                        \-----/                               \-----/
                    setLocal(OFFER)               setRemote(PRANSWER)
]]> </artwork>
        </figure>
        </t>

        <t>Aside
        <t indent="0" pn="section-3.2-12">Aside from these state transitions transitions, there is no other
        difference between the handling of provisional ("pranswer") and
        final ("answer") answers.</t>
      </section>
      <section title="Session anchor="sec.session-description-forma" numbered="true" toc="include" removeInRFC="false" pn="section-3.3">
        <name slugifiedName="name-session-description-format">Session Description Format"
      anchor="sec.session-description-forma">

        <t>JSEP's Format</name>
        <t indent="0" pn="section-3.3-1">JSEP's session descriptions use SDP Session Description Protocol (SDP) syntax for their
        internal representation. While this format is not optimal for
        manipulation from JavaScript, it is widely accepted, accepted and is
        frequently updated with new features; any alternate encoding of
        session descriptions would have to keep pace with the changes
        to SDP, at least until the time that this new encoding eclipsed
        SDP in popularity.</t>

        <t>However,
        <t indent="0" pn="section-3.3-2">However, to provide for future flexibility, the SDP syntax
        is encapsulated within a SessionDescription object, which can
        be constructed from SDP, SDP and be serialized out to SDP. If
        future specifications agree on a JSON format for session
        descriptions, we could easily enable this object to generate
        and consume that JSON.</t>

        <t>As
        <t indent="0" pn="section-3.3-3">As detailed below, most applications should be able to treat
        the SessionDescriptions produced and consumed by these various
        API calls as opaque blobs; that is, the application will not
        need to read or change them.</t>
      </section>
      <section title="Session anchor="sec.session-description-ctrl" numbered="true" toc="include" removeInRFC="false" pn="section-3.4">
        <name slugifiedName="name-session-description-control">Session Description Control"
      anchor="sec.session-description-ctrl">

        <t>In Control</name>
        <t indent="0" pn="section-3.4-1">In order to give the application control over various common
        session parameters, JSEP provides control surfaces which that tell
        the JSEP implementation how to generate session descriptions.
        This avoids the need for JavaScript to modify session
        descriptions in most cases.</t>

        <t>Changes
        <t indent="0" pn="section-3.4-2">Changes to these objects result in changes to the session
        descriptions generated by subsequent createOffer/Answer createOffer/createAnswer
        calls.</t>
        <section title="RtpTransceivers" anchor="sec.rtptransceivers">

          <t>RtpTransceivers anchor="sec.rtptransceivers" numbered="true" toc="include" removeInRFC="false" pn="section-3.4.1">
          <name slugifiedName="name-rtptransceivers">RtpTransceivers</name>
          <t indent="0" pn="section-3.4.1-1">RtpTransceivers allow the application to control the RTP
          media associated with one m= "m=" section. Each RtpTransceiver has
          an RtpSender and an RtpReceiver, which an application can use
          to control the sending and receiving of RTP media. The
          application may also modify the RtpTransceiver directly, for
          instance, by stopping it.</t>

          <t>RtpTransceivers
          <t indent="0" pn="section-3.4.1-2">RtpTransceivers generally have a 1:1 mapping with m= "m="
          sections, although there may be more RtpTransceivers than m= "m="
          sections when RtpTransceivers are created but not yet
          associated with a m= an "m=" section, or if RtpTransceivers have been
          stopped and disassociated from m= "m=" sections. An RtpTransceiver
          is said to be associated with an m= "m=" section if its mid
          media identification (mid) property is non-null; otherwise otherwise, it is said to be
          disassociated. The associated m= "m=" section is determined using
          a mapping between transceivers and m= "m=" section indices, formed
          when creating an offer or applying a remote offer.</t>

          <t>An
          <t indent="0" pn="section-3.4.1-3">An RtpTransceiver is never associated with more than one
          m=
          "m=" section, and once a session description is applied, a m= an "m="
          section is always associated with exactly one RtpTransceiver.
          However, in certain cases where a m= an "m=" section has been
          rejected, as discussed in
          <xref target="sec.subsequent-offers" /> format="default" sectionFormat="of" derivedContent="Section 5.2.2"/> below, that m= "m=" section
          will be "recycled" and associated with a new RtpTransceiver
          with a new mid MID value.</t>

          <t>RtpTransceivers
          <t indent="0" pn="section-3.4.1-4">RtpTransceivers can be created explicitly by the
          application or implicitly by calling setRemoteDescription
          with an offer that adds new m= "m=" sections.</t>
        </section>
        <section title="RtpSenders" anchor="sec.rtpsenders">

          <t>RtpSenders anchor="sec.rtpsenders" numbered="true" toc="include" removeInRFC="false" pn="section-3.4.2">
          <name slugifiedName="name-rtpsenders">RtpSenders</name>
          <t indent="0" pn="section-3.4.2-1">RtpSenders allow the application to control how RTP media
          is sent. An RtpSender is conceptually responsible for the
          outgoing RTP stream(s) described by an m= "m=" section. This
          includes encoding the attached MediaStreamTrack, sending RTP
          media packets, and generating/processing RTCP the RTP Control Protocol (RTCP) for the
          outgoing RTP streams(s).</t>
        </section>
        <section title="RtpReceivers" anchor="sec.rtpreceivers">

          <t>RtpReceivers anchor="sec.rtpreceivers" numbered="true" toc="include" removeInRFC="false" pn="section-3.4.3">
          <name slugifiedName="name-rtpreceivers">RtpReceivers</name>
          <t indent="0" pn="section-3.4.3-1">RtpReceivers allow the application to inspect how RTP
          media is received. An RtpReceiver is conceptually responsible
          for the incoming RTP stream(s) described by an m= "m=" section.
          This includes processing received RTP media packets, decoding
          the incoming stream(s) to produce a remote MediaStreamTrack,
          and generating/processing RTCP for the incoming RTP
          stream(s).</t>
        </section>
      </section>
      <section title="ICE" anchor="sec.ice">
        <section title="ICE anchor="sec.ice" numbered="true" toc="include" removeInRFC="false" pn="section-3.5">
        <name slugifiedName="name-ice">ICE</name>
        <section anchor="sec.ice-gather-overview" numbered="true" toc="include" removeInRFC="false" pn="section-3.5.1">
          <name slugifiedName="name-ice-gathering-overview">ICE Gathering Overview"
        anchor="sec.ice-gather-overview">

          <t>JSEP Overview</name>
          <t indent="0" pn="section-3.5.1-1">JSEP gathers ICE candidates as needed by the application.
          Collection of ICE candidates is referred to as a gathering
          phase, and this is triggered either by the addition of a new
          or recycled m= "m=" section to the local session description, description or by
          new ICE credentials in the description, indicating an ICE
          restart. Use of new ICE credentials can be triggered
          explicitly by the application, application or implicitly by the JSEP
          implementation in response to changes in the ICE
          configuration.</t>

          <t>When
          <t indent="0" pn="section-3.5.1-2">When the ICE configuration changes in a way that requires
          a new gathering phase, a 'needs-ice-restart' bit is set. When
          this bit is set, calls to the createOffer API will generate
          new ICE credentials. This bit is cleared by a call to the
          setLocalDescription API with new ICE credentials from either
          an offer or an answer, i.e., from either a local- locally or
          remote-initiated
          remotely initiated ICE restart.</t>

          <t>When
          <t indent="0" pn="section-3.5.1-3">When a new gathering phase starts, the ICE agent will
          notify the application that gathering is occurring through an a state
          change event. Then, when each new ICE candidate becomes available,
          the ICE agent will supply it to the application via an
          additional
          onicecandidate event; these candidates will also automatically be
          added to the current and/or pending local session
          description. Finally, when all candidates have been gathered,
          an
          a final onicecandidate event will be dispatched to signal that the
          gathering process is complete.</t>

          <t>Note
          <t indent="0" pn="section-3.5.1-4">Note that gathering phases only gather the candidates
          needed by new/recycled/restarting m= "m=" sections; other m= "m="
          sections continue to use their existing candidates. Also, if
          an m= "m=" section is bundled (either by a successful bundle
          negotiation or by being marked as bundle-only), then
          candidates will be gathered and exchanged for that m= "m=" section
          if and only if its MID item is a BUNDLE-tag, as described in
          <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation" />.</t> target="RFC8843" format="default" sectionFormat="of" derivedContent="RFC8843"/>.</t>
        </section>
        <section title="ICE anchor="sec.ice-candidate-trickling" numbered="true" toc="include" removeInRFC="false" pn="section-3.5.2">
          <name slugifiedName="name-ice-candidate-trickling">ICE Candidate Trickling"
        anchor="sec.ice-candidate-trickling">

          <t>Candidate Trickling</name>
          <t indent="0" pn="section-3.5.2-1">Candidate trickling is a technique through which a caller
          may incrementally provide candidates to the callee after the
          initial offer has been dispatched; the semantics of "Trickle
          ICE" are defined in
          <xref target="I-D.ietf-ice-trickle"></xref>. target="RFC8838" format="default" sectionFormat="of" derivedContent="RFC8838"/>. This process
          allows the callee to begin acting upon the call and setting
          up the ICE (and perhaps DTLS) connections immediately,
          without having to wait for the caller to gather all possible
          candidates. This results in faster media setup in cases where
          gathering is not performed prior to initiating the call.</t>

          <t>JSEP
          <t indent="0" pn="section-3.5.2-2">JSEP supports optional candidate trickling by providing
          APIs, as described above, that provide control and feedback
          on the ICE candidate gathering process. Applications that
          support candidate trickling can send the initial offer
          immediately and send individual candidates when they get the
          notified of a new candidate; applications that do not support
          this feature can simply wait for the indication that
          gathering is complete, and then create and send their offer,
          with all the candidates, at this that time.</t>

          <t>Upon
          <t indent="0" pn="section-3.5.2-3">Upon receipt of trickled candidates, the receiving
          application will supply them to its ICE agent. This triggers
          the ICE agent to start using the new remote candidates for
          connectivity checks.</t>
          <section title="ICE anchor="sec.ice-candidate-format" numbered="true" toc="include" removeInRFC="false" pn="section-3.5.2.1">
            <name slugifiedName="name-ice-candidate-format">ICE Candidate Format"
          anchor="sec.ice-candidate-format">

            <t>In Format</name>
            <t indent="0" pn="section-3.5.2.1-1">In JSEP, ICE candidates are abstracted by an
            IceCandidate object, and as with session descriptions, SDP
            syntax is used for the internal representation.</t>

            <t>The
            <t indent="0" pn="section-3.5.2.1-2">The candidate details are specified in an IceCandidate
            field, using the same SDP syntax as the
            "candidate-attribute" field defined in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 4.1. target="RFC8839" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.1" derivedContent="RFC8839"/>. Note that this
            field does not contain an "a=" prefix, as indicated in the
            following example:</t>
            <figure>
              <artwork>
                <![CDATA[
            <sourcecode name="" type="sdp" markers="false" pn="section-3.5.2.1-3">
candidate:1 1 UDP 1694498815 192.0.2.33 10000 typ host
]]>
</artwork>
            </figure>

            <t>The host</sourcecode>
            <t indent="0" pn="section-3.5.2.1-4">The IceCandidate object contains a field to indicate
            which ICE ufrag username fragment (ufrag) it is associated with, as defined in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 4.4. target="RFC8839" sectionFormat="comma" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.4" derivedContent="RFC8839"/>. This value is used
            to determine which session description (and thereby which
            gathering phase) this IceCandidate belongs to, which helps
            resolve ambiguities during ICE restarts. If this field is
            absent in a received IceCandidate (perhaps when
            communicating with a non-JSEP endpoint), the most recently
            received session description is assumed.</t>

            <t>The
            <t indent="0" pn="section-3.5.2.1-5">The IceCandidate object also contains fields to indicate
            which m= "m=" section it is associated with, which can be
            identified in one of two ways, ways: either by a m= an "m=" section
            index,
            index or by a MID. The m= "m=" section index is a zero-based
            index, with index N referring to the N+1th m= "m=" section in
            the session description referenced by this IceCandidate.
            The MID is a "media stream identification" value, as
            defined in
            <xref target="RFC5888"></xref>, Section 4, target="RFC5888" sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5888#section-4" derivedContent="RFC5888"/>, which provides a
            more robust way to identify the m= "m=" section in the session
            description, using the MID of the associated RtpTransceiver
            object (which may have been locally generated by the
            answerer when interacting with a non-JSEP endpoint that
            does not support the MID attribute, as discussed in
            <xref target="sec.applying-a-remote-desc" /> format="default" sectionFormat="of" derivedContent="Section 5.10"/> below). If the
            MID field is present in a received IceCandidate, it MUST <bcp14>MUST</bcp14> be
            used for identification; otherwise, the m= "m=" section index is
            used instead.</t>

            <t>When creating an IceCandidate object, JSEP
            implementations MUST populate each of the candidate, ufrag,
            m= section index, and MID fields. Implementations MUST also
            <t indent="0" pn="section-3.5.2.1-6">Implementations <bcp14>MUST</bcp14>
            be prepared to receive objects with some fields missing, as
            mentioned above.</t>
          </section>
        </section>
        <section title="ICE anchor="sec.ice-candidate-policy" numbered="true" toc="include" removeInRFC="false" pn="section-3.5.3">
          <name slugifiedName="name-ice-candidate-policy">ICE Candidate Policy"
        anchor="sec.ice-candidate-policy">

          <t>Typically, Policy</name>
          <t indent="0" pn="section-3.5.3-1">Typically, when gathering ICE candidates, the JSEP
          implementation will gather all possible forms of initial
          candidates - -- host, server reflexive, server-reflexive, and relay.
          However, in
          certain cases, applications may want to have more specific
          control over the gathering process, due to privacy or related
          concerns. For example, one may want to only use relay
          candidates, to leak as little location information as
          possible (keeping in mind that this choice comes with
          corresponding operational costs). To accomplish this, JSEP
          allows the application to restrict which ICE candidates are
          used in a session. Note that this filtering is applied on top
          of any restrictions the implementation chooses to enforce
          regarding which IP addresses are permitted for the
          application, as discussed in
          <xref target="I-D.ietf-rtcweb-ip-handling" />.</t>

          <t>There target="RFC8828" format="default" sectionFormat="of" derivedContent="RFC8828"/>.</t>
          <t indent="0" pn="section-3.5.3-2">There may also be cases where the application wants to
          change which types of candidates are used while the session
          is active. A prime example is where a callee may initially
          want to use only relay candidates, to avoid leaking location
          information to an arbitrary caller, but then change to use
          all candidates (for lower operational cost) once the user has
          indicated that they want to take the call. For this scenario, the
          JSEP implementation MUST <bcp14>MUST</bcp14> allow the candidate policy to be
          changed in mid-session, subject to the aforementioned
          interactions with local policy.</t>

          <t>To
          <t indent="0" pn="section-3.5.3-3">To administer the ICE candidate policy, the JSEP
          implementation will determine the current setting at the
          start of each gathering phase. Then, during the gathering
          phase, the implementation MUST NOT <bcp14>MUST NOT</bcp14> expose candidates
          disallowed by the current policy to the application, use them
          as the source of connectivity checks, or indirectly expose
          them via other fields, such as the raddr/rport attributes for
          other ICE candidates. Later, if a different policy is
          specified by the application, the application can apply it by
          kicking off a new gathering phase via an ICE restart.</t>
        </section>
        <section title="ICE anchor="sec.ice-candidate-pool" numbered="true" toc="include" removeInRFC="false" pn="section-3.5.4">
          <name slugifiedName="name-ice-candidate-pool">ICE Candidate Pool"
        anchor="sec.ice-candidate-pool">

          <t>JSEP Pool</name>
          <t indent="0" pn="section-3.5.4-1">JSEP applications typically inform the JSEP implementation
          to begin ICE gathering via the information supplied to
          setLocalDescription, as the local description indicates the
          number of ICE components which that will be needed and for which
          candidates must be gathered. However, to accelerate cases
          where the application knows the number of ICE components to
          use ahead of time, it may ask the implementation to gather a
          pool of potential ICE candidates to help ensure rapid media
          setup.</t>

          <t>When
          <t indent="0" pn="section-3.5.4-2">When setLocalDescription is eventually called, called and the
          JSEP implementation goes prepares to gather the needed ICE candidates,
          it SHOULD <bcp14>SHOULD</bcp14> start by checking if any candidates are available
          in the pool. If there are candidates in the pool, they SHOULD <bcp14>SHOULD</bcp14>
          be handed to the application immediately via the ICE
          candidate event. If the pool becomes depleted, either because
          a larger-than-expected number of ICE components is used, are used or
          because the pool has not had enough time to gather
          candidates, the remaining candidates are gathered as usual.
          This only occurs for the first offer/answer exchange, after
          which the candidate pool is emptied and no longer used.</t>

          <t>One
          <t indent="0" pn="section-3.5.4-3">One example of where this concept is useful is an
          application that expects an incoming call at some point in
          the future, and wants to minimize the time it takes to
          establish connectivity, to avoid clipping of initial media.
          By pre-gathering candidates into the pool, it can exchange
          and start sending connectivity checks from these candidates
          almost immediately upon receipt of a call. Note though Note, though, that
          by holding on to these pre-gathered candidates, which will be
          kept alive as long as they may be needed, the application
          will consume resources on the STUN/TURN servers it is
          using.</t>
        </section>
        <section title="ICE Versions">
          <t>While
          using. ("STUN" stands for "Session Traversal Utilities for NAT".)</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-3.5.5">
          <name slugifiedName="name-ice-versions">ICE Versions</name>
          <t indent="0" pn="section-3.5.5-1">While this specification formally relies on <xref
          target="RFC8445"></xref>, target="RFC8445" format="default" sectionFormat="of" derivedContent="RFC8445"/>, at the time of its publication, the
          majority of WebRTC implementations support the version
          of ICE described in <xref target="RFC5245"></xref>. target="RFC5245" format="default" sectionFormat="of" derivedContent="RFC5245"/>. The use of
          the "ice2" attribute defined in <xref target="RFC8445"></xref> target="RFC8445" format="default" sectionFormat="of" derivedContent="RFC8445"/>
          can be used to detect the version in use by a remote endpoint
          and to provide a smooth transition from the older specification
          to the newer one.  Implementations MUST <bcp14>MUST</bcp14> be able to accept remote
          descriptions that do not have the "ice2" attribute.</t>
        </section>
      </section>
      <section anchor="sec.imageattr" title="Video numbered="true" toc="include" removeInRFC="false" pn="section-3.6">
        <name slugifiedName="name-video-size-negotiation">Video Size Negotiation">

        <t>Video Negotiation</name>
        <t indent="0" pn="section-3.6-1">Video size negotiation is the process through which a
        receiver can use the "a=imageattr" SDP attribute
        <xref target="RFC6236" /> format="default" sectionFormat="of" derivedContent="RFC6236"/> to indicate what video frame sizes it
        is capable of receiving. A receiver may have hard limits on
        what its video decoder can process, or it may have some maximum
        set by policy. By specifying these limits in an "a=imageattr"
        attribute, JSEP endpoints can attempt to ensure that the remote
        sender transmits video at an acceptable resolution. However,
        when communicating with a non-JSEP endpoint that does not
        understand this attribute, any signaled limits may be exceeded,
        and the JSEP implementation MUST <bcp14>MUST</bcp14> handle this gracefully, e.g.,
        by discarding the video.</t>

        <t>Note
        <t indent="0" pn="section-3.6-2">Note that certain codecs support transmission of samples
        with aspect ratios other than 1.0 (i.e., non-square pixels).
        JSEP implementations will not transmit non-square pixels, pixels but
        SHOULD
        <bcp14>SHOULD</bcp14> receive and render such video with the correct aspect
        ratio. However, sample aspect ratio has no impact on the size
        negotiation described below; all dimensions are measured in
        pixels, whether square or not.</t>
        <section anchor="sec.creating-imageattr"
        title="Creating numbered="true" toc="include" removeInRFC="false" pn="section-3.6.1">
          <name slugifiedName="name-creating-an-imageattr-attri">Creating an imageattr Attribute">

          <t>The Attribute</name>
          <t indent="0" pn="section-3.6.1-1">The receiver will first intersect combine any known local limits
          (e.g., hardware decoder capababilities, capabilities or local policy) to
          determine the absolute minimum and maximum sizes it can
          receive. If there are no known local limits, the
          "a=imageattr" attribute SHOULD <bcp14>SHOULD</bcp14> be omitted. If these local
          limits preclude receiving any video, i.e., the degenerate
          case of no permitted resolutions, the "a=imageattr" attribute
          MUST
          <bcp14>MUST</bcp14> be omitted, and the m= "m=" section MUST <bcp14>MUST</bcp14> be marked as
          sendonly/inactive, as appropriate.</t>

          <t>Otherwise,
          <t indent="0" pn="section-3.6.1-2">Otherwise, an "a=imageattr" attribute is created with a
          "recv" direction, and the resulting resolution space formed
          from the aforementioned intersection is used to specify its
          minimum and maximum x= "x=" and y= "y=" values.</t>

          <t>The
          <t indent="0" pn="section-3.6.1-3">The rules here express a single set of preferences, and
          therefore, the "a=imageattr" q= "q=" value is not important. It
          SHOULD
          <bcp14>SHOULD</bcp14> be set to 1.0.</t>

          <t>The "1.0".</t>
          <t indent="0" pn="section-3.6.1-4">The "a=imageattr" field is payload type specific. When all
          video codecs supported have the same capabilities, use of a
          single attribute, with the wildcard payload type (*), is
          RECOMMENDED.
          <bcp14>RECOMMENDED</bcp14>. However, when the supported video codecs have
          different limitations, specific "a=imageattr" attributes MUST <bcp14>MUST</bcp14>
          be inserted for each payload type.</t>

          <t>As
          <t indent="0" pn="section-3.6.1-5">As an example, consider a system with a multiformat video
          decoder, which is capable of decoding any resolution from
          48x48 to 720p, 720p. In this case, the implementation would
          generate this attribute:</t>

          <t>a=imageattr:*
          <sourcecode type="sdp" markers="false" pn="section-3.6.1-6">
          a=imageattr:* recv [x=[48:1280],y=[48:720],q=1.0]</t>

          <t>This [x=[48:1280],y=[48:720],q=1.0]
</sourcecode>
          <t indent="0" pn="section-3.6.1-7">This declaration indicates that the receiver is capable of
          decoding any image resolution from 48x48 up to 1280x720
          pixels.</t>
        </section>
        <section anchor="sec.interpreting-imageattr"
        title="Interpreting numbered="true" toc="include" removeInRFC="false" pn="section-3.6.2">
          <name slugifiedName="name-interpreting-imageattr-attr">Interpreting imageattr Attributes">

          <t> Attributes</name>
          <t indent="0" pn="section-3.6.2-1">
          <xref target="RFC6236" /> format="default" sectionFormat="of" derivedContent="RFC6236"/> defines "a=imageattr" to be an
          advisory field. This means that it does not absolutely
          constrain the video formats that the sender can use, use but
          gives an indication of the preferred values.</t>

          <t>This
          <t indent="0" pn="section-3.6.2-2">This specification prescribes behavior that is more specific behavior. specific. When
          a MediaStreamTrack, which is producing video of a certain
          resolution (the "track resolution"), is attached to a an
          RtpSender, which is encoding the track video at the same or
          lower resolution(s) (the "encoder resolutions"), and a remote
          description is applied that references the sender and
          contains valid "a=imageattr recv" attributes, it MUST <bcp14>MUST</bcp14> follow
          the rules below to ensure that the sender does not transmit a
          resolution that would exceed the size criteria specified in
          the attributes. These rules MUST <bcp14>MUST</bcp14> be followed as long as the
          attributes remain present in the remote description,
          including cases in which the track changes its resolution, resolution or
          is replaced with a different track.</t>

          <t>Depending
          <t indent="0" pn="section-3.6.2-3">Depending on how the RtpSender is configured, it may be
          producing a single encoding at a certain resolution, resolution or, if
          simulcast
          <xref
          (<xref target="sec.simulcast" /> format="default" sectionFormat="of" derivedContent="Section 3.7"/>) has been negotiated, multiple
          encodings, each at their own specific resolution. In
          addition, depending on the configuration, each encoding may
          have the flexibility to reduce resolution when needed, needed or may
          be locked to a specific output resolution.</t>

          <t>For
          <t indent="0" pn="section-3.6.2-4">For each encoding being produced by the RtpSender, the set
          of "a=imageattr recv" attributes in the corresponding m= "m="
          section of the remote description is processed to determine
          what should be transmitted. Only attributes that reference
          the media format selected for the encoding are considered;
          each such attribute is evaluated individually, starting with
          the attribute with the highest "q=" value. If multiple
          attributes have the same "q=" value, they are evaluated in
          the order they appear in their containing m= "m=" section. Note
          that while JSEP endpoints will include at most one
          "a=imageattr recv" attribute per media format, JSEP endpoints
          may receive session descriptions from non-JSEP endpoints with
          m=
          "m=" sections that contain multiple such attributes.</t>

          <t>For
          <t indent="0" pn="section-3.6.2-5">For each "a=imageattr recv" attribute, the following rules
          are applied. If this processing is successful, the encoding
          is transmitted accordingly, and no further attributes are
          considered for that encoding. Otherwise, the next attribute
          is evaluated, in the aforementioned order. If none of the
          supplied attributes can be processed successfully, the
          encoding MUST NOT <bcp14>MUST NOT</bcp14> be transmitted, and an error SHOULD <bcp14>SHOULD</bcp14> be
          raised to the application.
          <list style="symbols">

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-3.6.2-6">
            <li pn="section-3.6.2-6.1">The limits from the attribute are compared to the
            encoder resolution. Only the specific limits mentioned
            below are considered; any other values, such as picture
            aspect ratio, MUST <bcp14>MUST</bcp14> be ignored. When considering a
            MediaStreamTrack that is producing rotated video, the
            unrotated resolution MUST <bcp14>MUST</bcp14> be used for the checks. This is
            required regardless of whether the receiver supports
            performing receive-side rotation (e.g., through CVO Coordination of
            Video Orientation (CVO)
            <xref target="TS26.114" />), format="default" sectionFormat="of" derivedContent="TS26.114"/>), as it significantly simplifies
            the matching logic.</t>

            <t>If logic.</li>
            <li pn="section-3.6.2-6.2">If the attribute includes a "sar=" (sample aspect ratio)
            value set to something other than "1.0", indicating that the
            receiver wants to receive non-square pixels, this cannot be
            satisfied and the attribute MUST NOT <bcp14>MUST NOT</bcp14> be used.</t>

            <t>If used.</li>
            <li pn="section-3.6.2-6.3">If the encoder resolution exceeds the maximum size
            permitted by the attribute, attribute and the encoder is allowed to
            adjust its resolution, the encoder SHOULD <bcp14>SHOULD</bcp14> apply downscaling
            in order to satisfy the limits. Downscaling MUST NOT <bcp14>MUST NOT</bcp14> change
            the picture aspect ratio of the encoding, ignoring any
            trivial differences due to rounding. For example, if the
            encoder resolution is 1280x720, 1280x720 and the attribute specified
            a maximum of 640x480, the expected output resolution would
            be 640x360. If downscaling cannot be applied, the attribute
            MUST NOT
            <bcp14>MUST NOT</bcp14> be used.</t>

            <t>If used.</li>
            <li pn="section-3.6.2-6.4">If the encoder resolution is less than the minimum size
            permitted by the attribute, the attribute MUST NOT <bcp14>MUST NOT</bcp14> be used;
            the encoder MUST NOT <bcp14>MUST NOT</bcp14> apply upscaling. JSEP implementations
            SHOULD
            <bcp14>SHOULD</bcp14> avoid this situation by allowing receipt of
            arbitrarily small resolutions, perhaps via fallback to a
            software decoder.</t>

            <t>If decoder.</li>
            <li pn="section-3.6.2-6.5">If the encoder resolution is within the maximum and
            minimum sizes, no action is needed.</t>
          </list></t> needed.</li>
          </ul>
        </section>
      </section>
      <section title="Simulcast" anchor="sec.simulcast">

        <t>JSEP anchor="sec.simulcast" numbered="true" toc="include" removeInRFC="false" pn="section-3.7">
        <name slugifiedName="name-simulcast">Simulcast</name>
        <t indent="0" pn="section-3.7-1">JSEP supports simulcast transmission of a MediaStreamTrack,
        where multiple encodings of the source media can be transmitted
        within the context of a single m= "m=" section. The current JSEP API
        is designed to allow applications to send simulcasted media but
        only to receive a single encoding. This allows for multi-user
        scenarios where each sending client sends multiple encodings to
        a server, which then, for each receiving client, chooses the
        appropriate encoding to forward.</t>

        <t>Applications
        <t indent="0" pn="section-3.7-2">Applications request support for simulcast by configuring
        multiple encodings on an RtpSender. Upon generation of an offer
        or answer, these encodings are indicated via SDP markings on
        the corresponding m= "m=" section, as described below. Receivers
        that understand simulcast and are willing to receive it will
        also include SDP markings to indicate their support, and JSEP
        endpoints will use these markings to determine whether
        simulcast is permitted for a given RtpSender. If simulcast
        support is not negotiated, the RtpSender will only use the
        first configured encoding.</t>

        <t>Note
        <t indent="0" pn="section-3.7-3">Note that the exact simulcast parameters are up to the
        sending application. While the aforementioned SDP markings are
        provided to ensure that the remote side can receive and demux
        multiple simulcast encodings, the specific resolutions and
        bitrates to be used for each encoding are purely a send-side
        decision in JSEP.</t>

        <t>JSEP
        <t indent="0" pn="section-3.7-4">JSEP currently does not provide a mechanism to configure
        receipt of simulcast. This means that if simulcast is offered
        by the remote endpoint, the answer generated by a JSEP endpoint
        will not indicate support for receipt of simulcast, and as such
        the remote endpoint will only send a single encoding per m= "m="
        section.</t>

        <t>In
        <t indent="0" pn="section-3.7-5">In addition, JSEP does not provide a mechanism to handle an
        incoming offer requesting simulcast from the JSEP endpoint.
        This means that setting up simulcast in the case where the JSEP
        endpoint receives the initial offer requires out-of-band
        signaling or SDP inspection. However, in the case where the
        JSEP endpoint sets up simulcast in its in initial offer, any
        established simulcast streams will continue to work upon
        receipt of an incoming re-offer. Future versions of this
        specification may add additional APIs to handle the incoming
        initial offer scenario.</t>

        <t>When
        <t indent="0" pn="section-3.7-6">When using JSEP to transmit multiple encodings from a an
        RtpSender, the techniques from
        <xref target="I-D.ietf-mmusic-sdp-simulcast" /> target="RFC8853" format="default" sectionFormat="of" derivedContent="RFC8853"/> and
        <xref target="I-D.ietf-mmusic-rid" /> target="RFC8851" format="default" sectionFormat="of" derivedContent="RFC8851"/> are used. Specifically,
        when multiple encodings have been configured for a an RtpSender,
        the m= "m=" section for the RtpSender will include an "a=simulcast"
        attribute, as defined in
        <xref target="I-D.ietf-mmusic-sdp-simulcast" />, Section 6.2, target="RFC8853" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8853#section-5.1" derivedContent="RFC8853"/>,
        with a "send" simulcast stream description that lists each
        desired encoding, and no "recv" simulcast stream description.
        The m= "m=" section will also include an "a=rid" attribute for each
        encoding, as specified in
        <xref target="I-D.ietf-mmusic-rid" />, Section 4; target="RFC8851" sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8851#section-4" derivedContent="RFC8851"/>; the use of
        RID identifiers
        Restriction Identifiers (RIDs, also called rid-ids or RtpStreamIds)
        allows the individual encodings to be
        disambiguated even though they are all part of the same m= "m="
        section.</t>
      </section>
      <section title="Interactions With Forking"
      anchor="sec.interactions-with-forking">

        <t>Some anchor="sec.interactions-with-forking" numbered="true" toc="include" removeInRFC="false" pn="section-3.8">
        <name slugifiedName="name-interactions-with-forking">Interactions with Forking</name>
        <t indent="0" pn="section-3.8-1">Some call signaling systems allow various types of forking
        where an SDP Offer may be provided to more than one device. For
        example, SIP
        <xref target="RFC3261"></xref> target="RFC3261" format="default" sectionFormat="of" derivedContent="RFC3261"/> defines both a "Parallel Search" "parallel search"
        and "Sequential Search". "sequential search". Although these are primarily signaling
        level signaling-level issues that are outside the scope of JSEP, they do have
        some impact on the configuration of the media plane that is
        relevant. When forking happens at the signaling layer, the
        JavaScript application responsible for the signaling needs to
        make the decisions about what media should be sent or received
        at any point of in time, as well as which remote endpoint it
        should communicate with; JSEP is used to make sure the media
        engine can make the RTP and media perform as required by the
        application. The basic operations that the applications can
        have the media engine do are:
        <list style="symbols">

          <t>Start are as follows:
        </t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-3.8-2">
          <li pn="section-3.8-2.1">Start exchanging media with a given remote peer, but keep
          all the resources reserved in the offer.</t>

          <t>Start offer.</li>
          <li pn="section-3.8-2.2">Start exchanging media with a given remote peer, and free
          any resources in the offer that are not being used.</t>
        </list></t> used.</li>
        </ul>
        <section title="Sequential Forking"
        anchor="sec.sequential-forking">

          <t>Sequential anchor="sec.sequential-forking" numbered="true" toc="include" removeInRFC="false" pn="section-3.8.1">
          <name slugifiedName="name-sequential-forking">Sequential Forking</name>
          <t indent="0" pn="section-3.8.1-1">Sequential forking involves a call being dispatched to
          multiple remote callees, where each callee can accept the
          call, but only one active session ever exists at a time; no
          mixing of received media is performed.</t>

          <t>JSEP
          <t indent="0" pn="section-3.8.1-2">JSEP handles sequential forking well, allowing the
          application to easily control the policy for selecting the
          desired remote endpoint. When an answer arrives from one of
          the callees, the application can choose to apply it either as
          a either
          (1) a provisional answer, leaving open the possibility of using a
          different answer in the future, future or apply it as a (2) a final
          answer, ending the setup flow.</t>

          <t>In
          <t indent="0" pn="section-3.8.1-3">In a "first-one-wins" situation, the first answer will be
          applied as a final answer, and the application will reject
          any subsequent answers. In SIP parlance, this would be ACK +
          BYE.</t>

          <t>In
          <t indent="0" pn="section-3.8.1-4">In a "last-one-wins" situation, all answers would be
          applied as provisional answers, and any previous call leg
          will be terminated. At some point, the application will end
          the setup process, perhaps with a timer; at this point, the
          application could reapply the pending remote description as a
          final answer.</t>
        </section>
        <section title="Parallel Forking"
        anchor="sec.parallel-forking">

          <t>Parallel anchor="sec.parallel-forking" numbered="true" toc="include" removeInRFC="false" pn="section-3.8.2">
          <name slugifiedName="name-parallel-forking">Parallel Forking</name>
          <t indent="0" pn="section-3.8.2-1">Parallel forking involves a call being dispatched to
          multiple remote callees, where each callee can accept the
          call,
          call and multiple simultaneous active signaling sessions can
          be established as a result. If multiple callees send media at
          the same time, the possibilities for handling this are
          described in
          <xref target="RFC3960"></xref>, Section 3.1. target="RFC3960" sectionFormat="comma" section="3.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3960#section-3.1" derivedContent="RFC3960"/>. Most SIP devices
          today only support exchanging media with a single device at a
          time,
          time and do not try to mix multiple early media audio
          sources, as that could result in a confusing situation. For
          example, consider having a European ringback tone mixed
          together with the North American ringback tone - -- the
          resulting sound would not be like either tone, tone and would
          confuse the user. If the signaling application wishes to only
          exchange media with one of the remote endpoints at a time,
          then from a media engine point of view, this is exactly like
          the sequential forking case.</t>

          <t>In
          <t indent="0" pn="section-3.8.2-2">In the parallel forking case where the JavaScript
          application wishes to simultaneously exchange media with
          multiple peers, the flow is slightly more complex, but the
          JavaScript application can follow the strategy that
          <xref target="RFC3960"></xref> describes target="RFC3960" format="default" sectionFormat="of" derivedContent="RFC3960"/> describes, using UPDATE. The
          UPDATE approach allows the signaling to set up a separate
          media flow for each peer that it wishes to exchange media
          with. In JSEP, this offer used in the UPDATE would be formed
          by simply creating a new PeerConnection (see
          <xref target="sec.peerconnection" />) format="default" sectionFormat="of" derivedContent="Section 4.1"/>) and making sure that
          the same local media streams have been added into this new
          PeerConnection. Then the new PeerConnection object would
          produce a an SDP offer that could be used by the signaling to
          perform the UPDATE strategy discussed in
          <xref target="RFC3960"></xref>.</t>

          <t>As target="RFC3960" format="default" sectionFormat="of" derivedContent="RFC3960"/>.</t>
          <t indent="0" pn="section-3.8.2-3">As a result of sharing the media streams, the application
          will end up with N parallel PeerConnection sessions, each
          with a local and remote description and their own local and
          remote addresses. The media flow from these sessions can be
          managed using setDirection (see
          <xref target="sec.transceiver-set-direction" />), format="default" sectionFormat="of" derivedContent="Section 4.2.3"/>), or the
          application can choose to play out the media from all
          sessions mixed together. Of course, if the application wants
          to only keep a single session, it can simply terminate the
          sessions that it no longer needs.</t>
        </section>
      </section>
    </section>
    <section title="Interface" anchor="sec.interface">

      <t>This anchor="sec.interface" numbered="true" toc="include" removeInRFC="false" pn="section-4">
      <name slugifiedName="name-interface">Interface</name>
      <t indent="0" pn="section-4-1">This section details the basic operations that must be present
      to implement JSEP functionality. The actual API exposed in the
      W3C API may have somewhat different syntax, syntax but should map easily
      to these concepts.</t>
      <section title="PeerConnection" anchor="sec.peerconnection"> concepts.
      </t>
      <section title="Constructor" anchor="sec.pc-constructor">

          <t>The anchor="sec.peerconnection" numbered="true" toc="include" removeInRFC="false" pn="section-4.1">
        <name slugifiedName="name-peerconnection">PeerConnection</name>
        <section anchor="sec.pc-constructor" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.1">
          <name slugifiedName="name-constructor">Constructor</name>
          <t indent="0" pn="section-4.1.1-1">The PeerConnection constructor allows the application to
          specify global parameters for the media session, such as the
          STUN/TURN servers and credentials to use when gathering
          candidates, as well as the initial ICE candidate policy and
          pool size, and also the bundle policy to use.</t>

          <t>If
          <t indent="0" pn="section-4.1.1-2">If an ICE candidate policy is specified, it functions as
          described in
          <xref target="sec.ice-candidate-policy" />, format="default" sectionFormat="of" derivedContent="Section 3.5.3"/>, causing the JSEP
          implementation to only surface the permitted candidates
          (including any implementation-internal filtering) to the
          application,
          application and only use those candidates for connectivity
          checks. The set of available policies is as follows:
          <list style="hanging">
            <t hangText="all:">All
          </t>
          <dl newline="false" spacing="normal" indent="3" pn="section-4.1.1-3">
            <dt pn="section-4.1.1-3.1">all:</dt>
            <dd pn="section-4.1.1-3.2">All candidates permitted by
            implementation policy will be gathered and used.</t>

            <t></t>
            <t hangText="relay:">All used.</dd>
            <dt pn="section-4.1.1-3.3">relay:</dt>
            <dd pn="section-4.1.1-3.4">All candidates except relay candidates
            will be filtered out. This obfuscates the location
            information that might be ascertained by the remote peer
            from the received candidates. Depending on how the
            application deploys and chooses relay servers, this could
            obfuscate location to a metro or possibly even global
            level.</t>
          </list></t>

          <t>The
            level.</dd>
          </dl>
          <t indent="0" pn="section-4.1.1-4">The default ICE candidate policy MUST <bcp14>MUST</bcp14> be set to "all" "all", as
          this is generally the desired policy, policy and also typically
          reduces the use of application TURN server resources
          significantly.</t>

          <t>If
          <t indent="0" pn="section-4.1.1-5">If a size is specified for the ICE candidate pool, this
          indicates the number of ICE components to pre-gather
          candidates for. Because pre-gathering pre‑gathering results in utilizing
          STUN/TURN server resources for potentially long periods of
          time, this must <bcp14>MUST</bcp14> only occur upon application request, and
          therefore the default candidate pool size MUST <bcp14>MUST</bcp14> be zero.</t>

          <t>The
          <t indent="0" pn="section-4.1.1-6">The application can specify its preferred policy regarding
          use of bundle, the multiplexing mechanism defined in
          <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation"> target="RFC8843" format="default" sectionFormat="of" derivedContent="RFC8843">
            </xref>. Regardless of policy, the application will always
          try to negotiate bundle onto a single transport, transport and will
          offer a single bundle group across all m= "m=" sections; use of
          this single transport is contingent upon the answerer
          accepting bundle. However, by specifying a policy from the
          list below, the application can control exactly how
          aggressively it will try to bundle media streams together,
          which affects how it will interoperate with a
          non-bundle-aware endpoint. When negotiating with a
          non-bundle-aware endpoint, only the streams not marked as
          bundle-only streams will be established.</t>

          <t>The
          <t indent="0" pn="section-4.1.1-7">The set of available policies is as follows:
          <list style="hanging">
            <t hangText="balanced:">The
          </t>
          <dl newline="false" spacing="normal" indent="3" pn="section-4.1.1-8">
            <dt pn="section-4.1.1-8.1">balanced:</dt>
            <dd pn="section-4.1.1-8.2">The first m= "m=" section of each type
            (audio, video, or application) will contain transport
            parameters, which will allow an answerer to unbundle that
            section. The second and any subsequent m= section "m=" sections of each
            type will be marked bundle-only. The result is that if
            there are N distinct media types, then candidates will be
            gathered for for N media streams. This policy balances
            desire to multiplex with the need to ensure that basic audio and
            video can still be negotiated in legacy cases. When acting
            as answerer, if there is no bundle group in the offer, the
            implementation will reject all but the first m= "m=" section of
            each type.</t>

            <t></t>
            <t hangText="max-compat:">All m= type.</dd>
            <dt pn="section-4.1.1-8.3">max-compat:</dt>
            <dd pn="section-4.1.1-8.4">All "m=" sections will contain
            transport parameters; none will be marked as bundle-only.
            This policy will allow all streams to be received by
            non-bundle-aware endpoints, endpoints but will require separate candidates
            to be gathered for each media stream.</t>

            <t></t>
            <t hangText="max-bundle:">Only stream.</dd>
            <dt pn="section-4.1.1-8.5">max-bundle:</dt>
            <dd pn="section-4.1.1-8.6">Only the first m= "m=" section will
            contain transport parameters; all streams other than the
            first will be marked as bundle-only. This policy aims to
            minimize candidate gathering and maximize multiplexing, at
            the cost of less compatibility with legacy endpoints. When
            acting as answerer, the implementation will reject any m= "m="
            sections other than the first m= "m=" section, unless they are
            in the same bundle group as that m= section.</t>
          </list></t>

          <t>As "m=" section.</dd>
          </dl>
          <t indent="0" pn="section-4.1.1-9">As it provides the best tradeoff trade-off between performance and
          compatibility with legacy endpoints, the default bundle
          policy MUST <bcp14>MUST</bcp14> be set to "balanced".</t>

          <t>The
          <t indent="0" pn="section-4.1.1-10">The application can specify its preferred policy regarding
          use of RTP/RTCP multiplexing
          <xref target="RFC5761"></xref> target="RFC5761" format="default" sectionFormat="of" derivedContent="RFC5761"/> using one of the following
          policies:
          <list style="hanging">
            <t hangText="negotiate:">The
          </t>
          <dl newline="false" spacing="normal" indent="3" pn="section-4.1.1-11">
            <dt pn="section-4.1.1-11.1">negotiate:</dt>
            <dd pn="section-4.1.1-11.2">The JSEP implementation will
            gather both RTP and RTCP candidates but also will offer
            "a=rtcp-mux", thus allowing for compatibility with either
            multiplexing or non-multiplexing endpoints.</t>
            <t hangText="require:">The endpoints.</dd>
            <dt pn="section-4.1.1-11.3">require:</dt>
            <dd pn="section-4.1.1-11.4">The JSEP implementation will only
            gather RTP candidates and will insert an "a=rtcp-mux-only"
            indication into any new m= "m=" sections in offers it generates.
            This halves the number of candidates that the offerer needs
            to gather. Applying a description with an m= "m=" section that
            does not contain an "a=rtcp-mux" attribute will cause an
            error to be returned.</t>
          </list></t>

          <t>The returned.</dd>
          </dl>
          <t indent="0" pn="section-4.1.1-12">The default multiplexing policy MUST <bcp14>MUST</bcp14> be set to "require".
          Implementations MAY <bcp14>MAY</bcp14> choose to reject attempts by the
          application to set the multiplexing policy to
          "negotiate".</t>
        </section>
        <section title="addTrack" anchor="sec.addTrack">

          <t>The anchor="sec.addTrack" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.2">
          <name slugifiedName="name-addtrack">addTrack</name>
          <t indent="0" pn="section-4.1.2-1">The addTrack method adds a MediaStreamTrack to the
          PeerConnection, using the MediaStream argument to associate
          the track with other tracks in the same MediaStream, so that
          they can be added to the same "LS" (Lip Synchronization) group when creating an
          offer or answer. Adding tracks to the same "LS" group
          indicates that the playback of these tracks should be
          synchronized for proper lip sync, as described in
          <xref target="RFC5888"></xref>, Section 7. addTrack target="RFC5888" sectionFormat="comma" section="7" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5888#section-7" derivedContent="RFC5888"/>.  addTrack attempts
          to minimize the number of transceivers as follows: If if the
          PeerConnection is in the "have-remote-offer" "have‑remote-offer" state, the track
          will be attached to the first compatible transceiver that was
          created by the most recent call to setRemoteDescription() setRemoteDescription and
          does not have a local track. Otherwise, a new transceiver
          will be created, as described in
          <xref target="sec.addTransceiver" />.</t> format="default" sectionFormat="of" derivedContent="Section 4.1.4"/>.</t>
        </section>
        <section title="removeTrack" anchor="sec.removeTrack">

          <t>The anchor="sec.removeTrack" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.3">
          <name slugifiedName="name-removetrack">removeTrack</name>
          <t indent="0" pn="section-4.1.3-1">The removeTrack method removes a MediaStreamTrack from the
          PeerConnection, using the RtpSender argument to indicate
          which sender should have its track removed. The sender's
          track is cleared, and the sender stops sending. Future calls
          to createOffer will mark the m= "m=" section associated with the
          sender as recvonly (if transceiver.direction is sendrecv) or
          as inactive (if transceiver.direction is sendonly).</t>
        </section>
        <section title="addTransceiver" anchor="sec.addTransceiver">

          <t>The anchor="sec.addTransceiver" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.4">
          <name slugifiedName="name-addtransceiver">addTransceiver</name>
          <t indent="0" pn="section-4.1.4-1">The addTransceiver method adds a new RtpTransceiver to the
          PeerConnection. If a MediaStreamTrack argument is provided,
          then the transceiver will be configured with that media type
          and the track will be attached to the transceiver. Otherwise,
          the application MUST <bcp14>MUST</bcp14> explicitly specify the type; this mode
          is useful for creating recvonly transceivers as well as for
          creating transceivers to which a track can be attached at
          some later point.</t>

          <t>At
          <t indent="0" pn="section-4.1.4-2">At the time of creation, the application can also specify
          a transceiver direction attribute, a set of MediaStreams
          which
          that the transceiver is associated with (allowing LS "LS" group
          assignments), and a set of encodings for the media (used for
          simulcast as described in
          <xref target="sec.simulcast" />).</t> format="default" sectionFormat="of" derivedContent="Section 3.7"/>).</t>
        </section>
        <section anchor="sec.onaddtrack" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.5">
          <name slugifiedName="name-onaddtrack-event">onaddtrack Event</name>
          <t indent="0" pn="section-4.1.5-1">The onaddtrack event is dispatched to the application when a new
          remote track has been signaled as a result of a setRemoteDescription
          call. The new track is supplied as a MediaStreamTrack object in the
          event, along with the MediaStream(s) the track is part of.
          </t>
        </section>
        <section title="createDataChannel"
        anchor="sec.createDataChannel">

          <t>The anchor="sec.createDataChannel" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.6">
          <name slugifiedName="name-createdatachannel">createDataChannel</name>
          <t indent="0" pn="section-4.1.6-1">The createDataChannel method creates a new data channel
          and attaches it to the PeerConnection. If no data channel
          currently exists for this PeerConnection, then a new
          offer/answer exchange is required. All data channels on a
          given PeerConnection share the same SCTP/DTLS association ("SCTP" stands
          for "Stream Control Transmission Protocol") and
          therefore the same m= "m=" section, so subsequent creation of data
          channels does not have any impact on the JSEP state.</t>

          <t>The
          <t indent="0" pn="section-4.1.6-2">The createDataChannel method also includes a number of
          arguments which that are used by the PeerConnection (e.g.,
          maxPacketLifetime) but are not reflected in the SDP and do
          not affect the JSEP state.</t>
        </section>
        <section title="createOffer" anchor="sec.createoffer">

          <t>The anchor="sec.ondatachannel" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.7">
          <name slugifiedName="name-ondatachannel-event">ondatachannel Event</name>
          <t indent="0" pn="section-4.1.7-1">The ondatachannel event is dispatched to the application when a
          new data channel has been negotiated by the remote side, which can
          occur at any time after the underlying SCTP/DTLS association has been
          established. The new data channel object is supplied in the event.
          </t>
        </section>
        <section anchor="sec.createoffer" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.8">
          <name slugifiedName="name-createoffer">createOffer</name>
          <t indent="0" pn="section-4.1.8-1">The createOffer method generates a blob of SDP that
          contains a
          <xref target="RFC3264"></xref> an offer per <xref target="RFC3264" format="default" sectionFormat="of" derivedContent="RFC3264"/> with the supported
          configurations for the session, including descriptions of the
          media added to this PeerConnection, the codec/RTP/RTCP codec, RTP, and RTCP
          options supported by this implementation, and any candidates
          that have been gathered by the ICE agent. An options
          parameter may be supplied to provide additional control over
          the generated offer. This options parameter allows an
          application to trigger an ICE restart, for the purpose of
          reestablishing connectivity.</t>

          <t>In
          <t indent="0" pn="section-4.1.8-2">In the initial offer, the generated SDP will contain all
          desired functionality for the session (functionality that is
          supported but not desired by default may be omitted); for
          each SDP line, the generation of the SDP will follow the
          process defined for generating an initial offer from the
          document
          specification that specifies defines the given SDP line. The exact
          handling of initial offer generation is detailed in
          <xref target="sec.initial-offers" /> format="default" sectionFormat="of" derivedContent="Section 5.2.1"/> below.</t>

          <t>In
          <t indent="0" pn="section-4.1.8-3">In the event createOffer is called after the session is
          established, createOffer will generate an offer to modify the
          current session based on any changes that have been made to
          the session, e.g., adding or stopping RtpTransceivers, or
          requesting an ICE restart. For each existing stream, the
          generation of each SDP line must <bcp14>MUST</bcp14> follow the process defined
          for generating an updated offer from the RFC that specifies
          the given SDP line. For each new stream, the generation of
          the SDP must <bcp14>MUST</bcp14> follow the process of generating an initial
          offer, as mentioned above. If no changes have been made, or
          for SDP lines that are unaffected by the requested changes,
          the offer will only contain the parameters negotiated by the
          last offer-answer offer/answer exchange. The exact handling of subsequent
          offer generation is detailed in
          <xref target="sec.subsequent-offers" />. format="default" sectionFormat="of" derivedContent="Section 5.2.2"/> below.</t>

          <t>Session
          <t indent="0" pn="section-4.1.8-4">Session descriptions generated by createOffer must <bcp14>MUST</bcp14> be
          immediately usable by setLocalDescription; if a system has
          limited resources (e.g. (e.g., a finite number of decoders),
          createOffer should <bcp14>SHOULD</bcp14> return an offer that reflects the current
          state of the system, so that setLocalDescription will succeed
          when it attempts to acquire those resources.</t>

          <t>Calling
          <t indent="0" pn="section-4.1.8-5">Calling this method may do things such as generating new
          ICE credentials, but it does not change the PeerConnection
          state, trigger candidate gathering, or cause media to start
          or stop flowing. Specifically, the offer is not applied, and
          does not become the pending local description, until
          setLocalDescription is called.</t>
        </section>
        <section title="createAnswer" anchor="sec.createanswer">

          <t>The anchor="sec.createanswer" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.9">
          <name slugifiedName="name-createanswer">createAnswer</name>
          <t indent="0" pn="section-4.1.9-1">The createAnswer method generates a blob of SDP that
          contains a
          <xref target="RFC3264"></xref> an SDP answer per <xref target="RFC3264" format="default" sectionFormat="of" derivedContent="RFC3264"/> with the supported
          configuration for the session that is compatible with the
          parameters supplied in the most recent call to
          setRemoteDescription, which MUST
          setRemoteDescription; setRemoteDescription <bcp14>MUST</bcp14> have been called prior to
          calling createAnswer. Like createOffer, the returned blob
          contains descriptions of the media added to this
          PeerConnection, the codec/RTP/RTCP options negotiated for
          this session, and any candidates that have been gathered by
          the ICE agent. An options parameter may be supplied to
          provide additional control over the generated answer.</t>

          <t>As
          <t indent="0" pn="section-4.1.9-2">As an answer, the generated SDP will contain a specific
          configuration that specifies how the media plane should be
          established; for each SDP line, the generation of the SDP
          must
          <bcp14>MUST</bcp14> follow the process defined for generating an answer from
          the document specification that specifies defines the given SDP line. The exact
          handling of answer generation is detailed in
          <xref target="sec.generating-an-answer" />. format="default" sectionFormat="of" derivedContent="Section 5.3"/> below.</t>

          <t>Session
          <t indent="0" pn="section-4.1.9-3">Session descriptions generated by createAnswer must <bcp14>MUST</bcp14> be
          immediately usable by setLocalDescription; like createOffer,
          the returned description should <bcp14>SHOULD</bcp14> reflect the current state of
          the system.</t>

          <t>Calling
          <t indent="0" pn="section-4.1.9-4">Calling this method may do things such as generating new
          ICE credentials, but it does not change the PeerConnection
          state, trigger candidate gathering, or or cause a media state
          change. Specifically, the answer is not applied, and does not
          become the current local description, until
          setLocalDescription is called.</t>
        </section>
        <section title="SessionDescriptionType"
        anchor="sec.sessiondescriptiontype">

          <t>Session anchor="sec.sessiondescriptiontype" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.10">
          <name slugifiedName="name-sessiondescriptiontype">SessionDescriptionType</name>
          <t indent="0" pn="section-4.1.10-1">Session description objects (RTCSessionDescription) may be
          of type "offer", "pranswer", "answer" "answer", or "rollback". These
          types provide information as to how the description parameter
          should be parsed, parsed and how the media state should be
          changed.</t>

          <t>"offer"
          <t indent="0" pn="section-4.1.10-2">"offer" indicates that a description should <bcp14>MUST</bcp14> be parsed as
          an offer; said description may include many possible media
          configurations. A description used as an "offer" may be
          applied anytime any time the PeerConnection is in a stable state, "stable" state or
          applied as an update to a previously supplied but unanswered
          "offer".</t>

          <t>"pranswer"
          <t indent="0" pn="section-4.1.10-3">"pranswer" indicates that a description should <bcp14>MUST</bcp14> be parsed
          as an answer, but not a final answer, and so should not <bcp14>MUST NOT</bcp14>
          result in the freeing of allocated resources. It may result
          in the start of media transmission, if the answer does not
          specify an inactive media direction. A description used as a
          "pranswer" may be applied as a response to an "offer", "offer" or as an
          update to a previously sent "pranswer".</t>

          <t>"answer"
          <t indent="0" pn="section-4.1.10-4">"answer" indicates that a description should <bcp14>MUST</bcp14> be parsed as
          an answer, the offer-answer offer/answer exchange should <bcp14>MUST</bcp14> be considered
          complete, and any resources (decoders, candidates) that are
          no longer needed can <bcp14>SHOULD</bcp14> be released. A description used as an
          "answer" may be applied as a response to an "offer", "offer" or as an
          update to a previously sent "pranswer".</t>

          <t>The
          <t indent="0" pn="section-4.1.10-5">The only difference between a provisional and final answer
          is that the final answer results in the freeing of any unused
          resources that were allocated as a result of the offer. As
          such, the application can use some discretion on whether an
          answer should be applied as provisional or final, final and can
          change the type of the session description as needed. For
          example, in a serial forking scenario, an application may
          receive multiple "final" answers, one from each remote
          endpoint. The application could choose to accept the initial
          answers as provisional answers, answers and only apply an answer as
          final when it receives one that meets its criteria (e.g. (e.g., a
          live user instead of voicemail).</t>

          <t>"rollback"
          <t indent="0" pn="section-4.1.10-6">"rollback" is a special session description type implying indicating
          that the state machine should <bcp14>MUST</bcp14> be rolled back to the previous
          stable
          "stable" state, as described in
          <xref target="sec.rollback" />. format="default" sectionFormat="of" derivedContent="Section 4.1.10.2"/>. The contents MUST <bcp14>MUST</bcp14> be
          empty.</t>
          <section title="Use anchor="sec.use-of-provisional-answer" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.10.1">
            <name slugifiedName="name-use-of-provisional-answers">Use of Provisional Answers"
          anchor="sec.use-of-provisional-answer">

            <t>Most Answers</name>
            <t indent="0" pn="section-4.1.10.1-1">Most applications will not need to create answers using
            the "pranswer" type. While it is good practice to send an
            immediate response to an offer, in order to warm up the
            session transport and prevent media clipping, the preferred
            handling for a JSEP application is to create and send a
            "sendonly" final answer with a null MediaStreamTrack
            immediately after receiving the offer, which will prevent
            media from being sent by the caller, caller and allow media to be
            sent immediately upon answer by the callee. Later, when the
            callee actually accepts the call, the application can plug
            in the real MediaStreamTrack and create a new "sendrecv"
            offer to update the previous offer/answer pair and start
            bidirectional media flow.  While this could also be done
            with a "sendonly" pranswer, pranswer followed by a "sendrecv"
            answer, the initial pranswer leaves the offer-answer offer/answer
            exchange open, which means that the caller cannot send an
            updated offer during this time.</t>

            <t>As time.  </t>
            <t indent="0" pn="section-4.1.10.1-2">As an example, consider a typical JSEP application that
            wants to set up audio and video as quickly as possible.
            When the callee receives an offer with audio and video
            MediaStreamTracks, it will send an immediate answer
            accepting these tracks as sendonly (meaning that the caller
            will not send the callee any media yet, and because the
            callee has not yet added its own MediaStreamTracks, the
            callee will not send any media either). It will then ask
            the user to accept the call and acquire the needed local
            tracks. Upon acceptance by the user, the application will
            plug in the tracks it has acquired, which, because ICE handshaking
            and DTLS handshaking have likely completed by this point, can
            start transmitting immediately. The application will also
            send a new offer to the remote side indicating call
            acceptance and moving the audio and video to be two-way
            media. A detailed example flow along these lines is shown
            in
            <xref target="sec.warmup-example"></xref>.</t>

            <t>Of target="sec.warmup-example" format="default" sectionFormat="of" derivedContent="Section 7.3"/>.</t>
            <t indent="0" pn="section-4.1.10.1-3">Of course, some applications may not be able to perform
            this double offer-answer offer/answer exchange, particularly ones that
            are attempting to gateway to legacy signaling protocols. In
            these cases, pranswer can still provide the application
            with a mechanism to warm up the transport.</t>
          </section>
          <section title="Rollback" anchor="sec.rollback">

            <t>In anchor="sec.rollback" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.10.2">
            <name slugifiedName="name-rollback">Rollback</name>
            <t indent="0" pn="section-4.1.10.2-1">In certain situations situations, it may be desirable to "undo" a
            change made to setLocalDescription or setRemoteDescription.
            Consider a case where a call is ongoing, ongoing and one side wants
            to change some of the session parameters; that side
            generates an updated offer and then calls
            setLocalDescription. However, the remote side, either
            before or after setRemoteDescription, decides it does not
            want to accept the new parameters, parameters and sends a reject
            message back to the offerer. Now, the offerer, and possibly
            the answerer as well, need needs to return to a stable "stable" state and
            the previous local/remote description. To support this, we
            introduce the concept of "rollback", which discards any
            proposed changes to the session, returning the state
            machine to the stable "stable" state. A rollback is performed by
            supplying a session description of type "rollback" with
            empty contents to either setLocalDescription or
            setRemoteDescription.</t>
          </section>
        </section>
        <section title="setLocalDescription"
        anchor="sec.setlocaldescription">

          <t>The anchor="sec.setlocaldescription" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.11">
          <name slugifiedName="name-setlocaldescription">setLocalDescription</name>
          <t indent="0" pn="section-4.1.11-1">The setLocalDescription method instructs the
          PeerConnection to apply the supplied session description as
          its local configuration. The type field indicates whether the
          description should be processed as an offer, provisional
          answer, final answer, or rollback; offers and answers are
          checked differently, using the various rules that exist for
          each SDP line.</t>

          <t>This
          <t indent="0" pn="section-4.1.11-2">This API changes the local media state; among other
          things, it sets up local resources for receiving and decoding
          media. In order to successfully handle scenarios where the
          application wants to offer to change from one media format to
          a different, incompatible format, the PeerConnection must <bcp14>MUST</bcp14> be
          able to simultaneously support use of both the current and
          pending local descriptions (e.g., support the codecs that
          exist in either description). This dual processing begins
          when the PeerConnection enters the "have-local-offer" state,
          and it continues until setRemoteDescription is called with
          either a (1) a final answer, at which point the PeerConnection can
          fully adopt the pending local description, description or a (2) a rollback,
          which results in a revert to the current local
          description.</t>

          <t>This
          <t indent="0" pn="section-4.1.11-3">This API indirectly controls the candidate gathering
          process. When a local description is supplied, supplied and the number
          of transports currently in use does not match the number of
          transports needed by the local description, the
          PeerConnection will create transports as needed and begin
          gathering candidates for each transport, using ones from the
          candidate pool if available.</t>

          <t>If
          <t indent="0" pn="section-4.1.11-4">If (1) setRemoteDescription was previously called with an
          offer, and (2) setLocalDescription is called with an answer
          (provisional or final), and (3) the media directions are
          compatible, and (4) media is available to send, this will result
          in the starting of media transmission.</t> transmission.
          </t>
        </section>
        <section title="setRemoteDescription"
        anchor="sec.setremotedescription">

          <t>The anchor="sec.setremotedescription" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.12">
          <name slugifiedName="name-setremotedescription">setRemoteDescription</name>
          <t indent="0" pn="section-4.1.12-1">The setRemoteDescription method instructs the
          PeerConnection to apply the supplied session description as
          the desired remote configuration. As in setLocalDescription,
          the type field of the description indicates how it should be
          processed.</t>

          <t>This
          <t indent="0" pn="section-4.1.12-2">This API changes the local media state; among other
          things, it sets up local resources for sending and encoding
          media.</t>

          <t>If
          media. </t>
          <t indent="0" pn="section-4.1.12-3">If (1) setLocalDescription was previously called with an
          offer, and (2) setRemoteDescription is called with an answer
          (provisional or final), and (3) the media directions are
          compatible, and (4) media is available to send, this will result
          in the starting of media transmission.</t>
        </section>
        <section title="currentLocalDescription"
        anchor="sec.currentlocaldescription">

          <t>The anchor="sec.currentlocaldescription" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.13">
          <name slugifiedName="name-currentlocaldescription">currentLocalDescription</name>
          <t indent="0" pn="section-4.1.13-1">The currentLocalDescription method returns the current
          negotiated local description - -- i.e., the local description
          from the last successful offer/answer exchange - -- in addition
          to any local candidates that have been generated by the ICE
          agent since the local description was set.</t>

          <t>A
          <t indent="0" pn="section-4.1.13-2">A null object will be returned if an offer/answer exchange
          has not yet been completed.</t>
        </section>
        <section title="pendingLocalDescription"
        anchor="sec.pendinglocaldescription">

          <t>The anchor="sec.pendinglocaldescription" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.14">
          <name slugifiedName="name-pendinglocaldescription">pendingLocalDescription</name>
          <t indent="0" pn="section-4.1.14-1">The pendingLocalDescription method returns a copy of the
          local description currently in negotiation - -- i.e., a local
          offer set without any corresponding remote answer - -- in
          addition to any local candidates that have been generated by
          the ICE agent since the local description was set.</t>

          <t>A
          <t indent="0" pn="section-4.1.14-2">A null object will be returned if the state of the
          PeerConnection is "stable" or "have-remote-offer".</t>
        </section>
        <section title="currentRemoteDescription"
        anchor="sec.currentremotedescription">

          <t>The anchor="sec.currentremotedescription" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.15">
          <name slugifiedName="name-currentremotedescription">currentRemoteDescription</name>
          <t indent="0" pn="section-4.1.15-1">The currentRemoteDescription method returns a copy of the
          current negotiated remote description - -- i.e., the remote
          description from the last successful offer/answer exchange - --
          in addition to any remote candidates that have been supplied
          via processIceMessage since the remote description was
          set.</t>

          <t>A
          <t indent="0" pn="section-4.1.15-2">A null object will be returned if an offer/answer exchange
          has not yet been completed.</t>
        </section>
        <section title="pendingRemoteDescription"
        anchor="sec.pendingremotedescription">

          <t>The anchor="sec.pendingremotedescription" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.16">
          <name slugifiedName="name-pendingremotedescription">pendingRemoteDescription</name>
          <t indent="0" pn="section-4.1.16-1">The pendingRemoteDescription method returns a copy of the
          remote description currently in negotiation - -- i.e., a remote
          offer set without any corresponding local answer - -- in
          addition to any remote candidates that have been supplied via
          processIceMessage since the remote description was set.</t>

          <t>A
          <t indent="0" pn="section-4.1.16-2">A null object will be returned if the state of the
          PeerConnection is "stable" or "have-local-offer".</t>
        </section>
        <section title="canTrickleIceCandidates"
        anchor="sec.cantrickle">

          <t>The anchor="sec.cantrickle" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.17">
          <name slugifiedName="name-cantrickleicecandidates">canTrickleIceCandidates</name>
          <t indent="0" pn="section-4.1.17-1">The canTrickleIceCandidates property indicates whether the
          remote side supports receiving trickled candidates. There are
          three potential values:
          <list style="hanging">
            <t hangText="null:">No
          </t>
          <dl newline="false" spacing="normal" indent="3" pn="section-4.1.17-2">
            <dt pn="section-4.1.17-2.1">null:</dt>
            <dd pn="section-4.1.17-2.2">No SDP has been received from the other
            side, so it is not known if it can handle trickle. This is
            the initial value before setRemoteDescription() setRemoteDescription is
            called.</t>
            <t hangText="true:">SDP
            called.</dd>
            <dt pn="section-4.1.17-2.3">true:</dt>
            <dd pn="section-4.1.17-2.4">SDP has been received from the other
            side indicating that it can support trickle.</t>
            <t hangText="false:">SDP trickle.</dd>
            <dt pn="section-4.1.17-2.5">false:</dt>
            <dd pn="section-4.1.17-2.6">SDP has been received from the other
            side indicating that it cannot support trickle.</t>
          </list></t>

          <t>As trickle.</dd>
          </dl>
          <t indent="0" pn="section-4.1.17-3">As described in
          <xref target="sec.ice-candidate-trickling" />, format="default" sectionFormat="of" derivedContent="Section 3.5.2"/>, JSEP
          implementations always provide candidates to the application
          individually, consistent with what is needed for Trickle ICE.
          However, applications can use the canTrickleIceCandidates
          property to determine whether their peer can actually do
          Trickle ICE, i.e., whether it is safe to send an initial
          offer or answer followed later by candidates as they are
          gathered. As "true" is the only value that definitively
          indicates remote Trickle ICE support, an application which that
          compares canTrickleIceCandidates against "true" will by
          default attempt Half Trickle on initial offers and Full
          Trickle on subsequent interactions with a Trickle
          ICE-compatible agent.</t>
        </section>
        <section title="setConfiguration"
        anchor="sec.setconfiguration">

          <t>The anchor="sec.setconfiguration" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.18">
          <name slugifiedName="name-setconfiguration">setConfiguration</name>
          <t indent="0" pn="section-4.1.18-1">The setConfiguration method allows the global
          configuration of the PeerConnection, which was initially set
          by constructor parameters, to be changed during the session.
          The effects of calling this method call depend on when it is invoked,
          and differ they will differ, depending on which specific parameters are
          changed:</t>

          <t>
            <list style="symbols">

              <t>Any
          changed: </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-4.1.18-2">
            <li pn="section-4.1.18-2.1">Any changes to the STUN/TURN servers to use affect the
              next gathering phase. If an ICE gathering phase has
              already started or completed, the 'needs-ice-restart' bit
              mentioned in
              <xref target="sec.ice-gather-overview" /> format="default" sectionFormat="of" derivedContent="Section 3.5.1"/> will be set.
              This will cause the next call to createOffer to generate
              new ICE credentials, for the purpose of forcing an ICE
              restart and kicking off a new gathering phase, in which
              the new servers will be used. If the ICE candidate pool
              has a nonzero size, size and a local description has not yet
              been applied, any existing candidates will be discarded,
              and new candidates will be gathered from the new
              servers.</t>

              <t>Any
              servers.</li>
            <li pn="section-4.1.18-2.2">Any change to the ICE candidate policy affects the
              next gathering phase. If an ICE gathering phase has
              already started or completed, the 'needs-ice-restart' bit
              will be set. Either way, changes to the policy have no
              effect on the candidate pool, because pooled candidates
              are not made available to the application until a
              gathering phase occurs, and so any necessary filtering
              can still be done on any pooled candidates.</t>

              <t>The candidates.</li>
            <li pn="section-4.1.18-2.3">The ICE candidate pool size MUST NOT <bcp14>MUST NOT</bcp14> be changed after
              applying a local description. If a local description has
              not yet been applied, any changes to the ICE candidate
              pool size take effect immediately; if increased,
              additional candidates are pre-gathered; if decreased, the
              now-superfluous candidates are discarded.</t>

              <t>The discarded.</li>
            <li pn="section-4.1.18-2.4">The bundle and RTCP-multiplexing policies MUST NOT <bcp14>MUST NOT</bcp14> be
              changed after the construction of the PeerConnection.</t>
            </list>
          </t>

          <t>This call PeerConnection.</li>
          </ul>
          <t indent="0" pn="section-4.1.18-3">Calling this method may result in a change to the state of the ICE
          Agent.</t>
          agent.</t>
        </section>
        <section title="addIceCandidate" anchor="sec.addicecandidate">

          <t>The anchor="sec.addicecandidate" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.19">
          <name slugifiedName="name-addicecandidate">addIceCandidate</name>
          <t indent="0" pn="section-4.1.19-1">The addIceCandidate method provides an update to the ICE
          agent via an IceCandidate object
          <xref
          (<xref target="sec.ice-candidate-format" />. format="default" sectionFormat="of" derivedContent="Section 3.5.2.1"/>). If the
          IceCandidate's candidate field is filled in, non-null, the IceCandidate
          is treated as a new remote ICE candidate, which will be added
          to the current and/or pending remote description according to
          the rules defined for Trickle ICE. Otherwise, the
          IceCandidate is treated as an end-of-candidates indication,
          as defined in
          <xref target="I-D.ietf-ice-trickle" />.</t>

          <t>In target="RFC8838" sectionFormat="comma" section="14" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8838#section-14" derivedContent="RFC8838"/>.</t>
          <t indent="0" pn="section-4.1.19-2">In either case, the m= "m=" section index, MID, and ufrag
          fields from the supplied IceCandidate are used to determine
          which m= "m=" section and ICE candidate generation the
          IceCandidate belongs to, as described in
          <xref target="sec.ice-candidate-format" /> format="default" sectionFormat="of" derivedContent="Section 3.5.2.1"/> above. In the case
          of an end-of-candidates indication, null values for the absence of both the
          m=
          "m=" section index and MID fields is are interpreted to mean that
          the indication applies to all m= "m=" sections in the specified
          ICE candidate generation. However, if both fields are absent null
          for a new remote candidate, this MUST <bcp14>MUST</bcp14> be treated as an
          invalid condition, as specified below.</t>

          <t>If
          <t indent="0" pn="section-4.1.19-3">If any IceCandidate fields contain invalid values, values or an
          error occurs during the processing of the IceCandidate
          object, the supplied IceCandidate MUST <bcp14>MUST</bcp14> be ignored and an
          error MUST <bcp14>MUST</bcp14> be returned.</t>

          <t>Otherwise,
          <t indent="0" pn="section-4.1.19-4">Otherwise, the new remote candidate or end-of-candidates
          indication is supplied to the ICE agent. In the case of a new
          remote candidate, connectivity checks will be sent to the new
          candidate.</t>
        </section>
      </section>
      <section title="RtpTransceiver" anchor="sec.transceiver">
        <section title="stop" anchor="sec.transceiver-stop">

          <t>The stop method stops an RtpTransceiver. This will cause
          future calls to createOffer
          candidate, assuming setLocalDescription has already been
          called to generate a zero port for initialize the
          associated m= section. See below for more details.</t> ICE gathering process.</t>
        </section>
        <section title="stopped" anchor="sec.transceiver-stopped">

          <t>The stopped property indicates whether anchor="sec.onicecandidate" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.20">
          <name slugifiedName="name-onicecandidate-event">onicecandidate Event</name>
          <t indent="0" pn="section-4.1.20-1">The onicecandidate event is dispatched to the transceiver application in two
          situations: (1) when the ICE agent has
          been stopped, either by discovered a call new allowed local
          ICE candidate during ICE gathering, as outlined in
          <xref target="sec.ice-gather-overview" format="default" sectionFormat="of" derivedContent="Section 3.5.1"/> and
          subject to stopTransceiver the restrictions discussed in
          <xref target="sec.ice-candidate-policy" format="default" sectionFormat="of" derivedContent="Section 3.5.3"/>, or by
          applying
          (2) when an answer that rejects ICE gathering phase completes. The event contains a single
          IceCandidate object, as defined in
          <xref target="sec.ice-candidate-format" format="default" sectionFormat="of" derivedContent="Section 3.5.2.1"/>.</t>
          <t indent="0" pn="section-4.1.20-2">In the associated m= section. In
          either of these cases, it first case, the newly discovered candidate is set to "true", reflected
          in the IceCandidate object, and otherwise all of its fields <bcp14>MUST</bcp14> be non-null.
          This candidate will also be set added to "false".</t>

          <t>A stopped RtpTransceiver does not send any outgoing RTP or
          RTCP or process any incoming RTP or RTCP. It cannot be
          restarted.</t>
        </section>
        <section title="setDirection"
        anchor="sec.transceiver-set-direction">

          <t>The setDirection method sets the direction of a
          transceiver, current and/or pending local
          description according to the rules defined for Trickle ICE.</t>
          <t indent="0" pn="section-4.1.20-3">In the second case, the event's IceCandidate object
          <bcp14>MUST</bcp14> have its candidate field set to null to indicate
          that the current gathering phase is complete, i.e., there will be no
          further onicecandidate events in this phase. However, the
          IceCandidate's ufrag field <bcp14>MUST</bcp14> be specified to
          indicate which ICE candidate generation is ending. The IceCandidate's
          "m=" section index and MID fields <bcp14>MAY</bcp14> be specified to indicate that
          the event applies to a specific "m=" section, or set to null to
          indicate it applies to all "m=" sections in the current ICE candidate
          generation. This event can be used by the application to generate an
          end-of-candidates indication, as defined in
          <xref target="RFC8838" sectionFormat="comma" section="13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8838#section-13" derivedContent="RFC8838"/>.</t>
        </section>
      </section>
      <section anchor="sec.transceiver" numbered="true" toc="include" removeInRFC="false" pn="section-4.2">
        <name slugifiedName="name-rtptransceiver">RtpTransceiver</name>
        <section anchor="sec.transceiver-stop" numbered="true" toc="include" removeInRFC="false" pn="section-4.2.1">
          <name slugifiedName="name-stop">stop</name>
          <t indent="0" pn="section-4.2.1-1">The stop method stops an RtpTransceiver. This will cause
          future calls to createOffer to generate a zero port for the
          associated "m=" section. See below for more details.</t>
        </section>
        <section anchor="sec.transceiver-stopped" numbered="true" toc="include" removeInRFC="false" pn="section-4.2.2">
          <name slugifiedName="name-stopped">stopped</name>
          <t indent="0" pn="section-4.2.2-1">The stopped property indicates whether the transceiver has
          been stopped, either by a call to stop or by
          applying an answer that rejects the associated "m=" section. In
          either of these cases, it is set to "true" and otherwise
          will be set to "false".</t>
          <t indent="0" pn="section-4.2.2-2">A stopped RtpTransceiver does not send any outgoing RTP or
          RTCP or process any incoming RTP or RTCP. It cannot be
          restarted.</t>
        </section>
        <section anchor="sec.transceiver-set-direction" numbered="true" toc="include" removeInRFC="false" pn="section-4.2.3">
          <name slugifiedName="name-setdirection">setDirection</name>
          <t indent="0" pn="section-4.2.3-1">The setDirection method sets the direction of a
          transceiver, which affects the direction property of the
          associated m= "m=" section on future calls to createOffer and
          createAnswer. The permitted values for direction are
          "recvonly", "sendrecv", "sendonly", and "inactive", mirroring
          the identically-named directional identically named direction attributes defined in
          <xref target="RFC4566"></xref>, Section 6.</t>

          <t>When target="RFC4566" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/>.</t>
          <t indent="0" pn="section-4.2.3-2">When creating offers, the transceiver direction is
          directly reflected in the output, even for re-offers. When
          creating answers, the transceiver direction is intersected
          with the offered direction, as explained in
          <xref target="sec.generating-an-answer" /> format="default" sectionFormat="of" derivedContent="Section 5.3"/> below.</t>

          <t>Note
          <t indent="0" pn="section-4.2.3-3">Note that while setDirection sets the direction property
          of the transceiver immediately (
          <xref (<xref target="sec.transceiver-direction" />), format="default" sectionFormat="of" derivedContent="Section 4.2.4"/>), this property
          does not immediately affect whether the transceiver's
          RtpSender will send or its RtpReceiver will receive. The
          direction in effect is represented by the currentDirection
          property, which is only updated when an answer is
          applied.</t>
        </section>
        <section title="direction" anchor="sec.transceiver-direction">

          <t>The anchor="sec.transceiver-direction" numbered="true" toc="include" removeInRFC="false" pn="section-4.2.4">
          <name slugifiedName="name-direction">direction</name>
          <t indent="0" pn="section-4.2.4-1">The direction property indicates the last value passed
          into setDirection. If setDirection has never been called, it
          is set to the direction the transceiver was initialized
          with.</t>
        </section>
        <section title="currentDirection"
        anchor="sec.transceiver-current-direction">

          <t>The anchor="sec.transceiver-current-direction" numbered="true" toc="include" removeInRFC="false" pn="section-4.2.5">
          <name slugifiedName="name-currentdirection">currentDirection</name>
          <t indent="0" pn="section-4.2.5-1">The currentDirection property indicates the last
          negotiated direction for the transceiver's associated m= "m="
          section. More specifically, it indicates the
          <xref target="RFC3264"></xref> directional
          direction attribute <xref target="RFC3264" format="default" sectionFormat="of" derivedContent="RFC3264"/> of the
          associated m= "m=" section in the last applied answer (including
          provisional answers), with "send" and "recv" directions
          reversed if it was a remote answer.  For example, if the
          directional
          direction attribute for the associated m= "m=" section in a
          remote answer is "recvonly", currentDirection is set to
          "sendonly".</t>

          <t>If
          <t indent="0" pn="section-4.2.5-2">If an answer that references this transceiver has not yet
          been applied, applied or if the transceiver is stopped,
          currentDirection is set to null.</t> "null".</t>
        </section>
        <section title="setCodecPreferences"
        anchor="sec.transceiver-set-codec-preferences">

          <t>The anchor="sec.transceiver-set-codec-preferences" numbered="true" toc="include" removeInRFC="false" pn="section-4.2.6">
          <name slugifiedName="name-setcodecpreferences">setCodecPreferences</name>
          <t indent="0" pn="section-4.2.6-1">The setCodecPreferences method sets the codec preferences
          of a transceiver, which in turn affect the presence and order
          of codecs of the associated m= "m=" section on future calls to
          createOffer and createAnswer. Note that setCodecPreferences
          does not directly affect which codec the implementation
          decides to send. It only affects which codecs the
          implementation indicates that it prefers to receive, via the
          offer or answer. Even when a codec is excluded by
          setCodecPreferences, it still may be used to send until the
          next offer/answer exchange discards it.</t>

          <t>The
          <t indent="0" pn="section-4.2.6-2">The codec preferences of an RtpTransceiver can cause
          codecs to be excluded by subsequent calls to createOffer and
          createAnswer, in which case the corresponding media formats
          in the associated m= "m=" section will be excluded. The codec
          preferences cannot add media formats that would otherwise not
          be present.</t>

          <t>The
          <t indent="0" pn="section-4.2.6-3">The codec preferences of an RtpTransceiver can also
          determine the order of codecs in subsequent calls to
          createOffer and createAnswer, in which case the order of the
          media formats in the associated m= "m=" section will follow the
          specified preferences.</t>
        </section>
      </section>
    </section>
    <section title="SDP anchor="sec.sdp-interaction-procedure" numbered="true" toc="include" removeInRFC="false" pn="section-5">
      <name slugifiedName="name-sdp-interaction-procedures">SDP Interaction Procedures"
    anchor="sec.sdp-interaction-procedure">

      <t>This Procedures</name>
      <t indent="0" pn="section-5-1">This section describes the specific procedures to be followed
      when creating and parsing SDP objects.</t>
      <section title="Requirements Overview"
      anchor="sec.requirements-overview">

        <t>JSEP anchor="sec.requirements-overview" numbered="true" toc="include" removeInRFC="false" pn="section-5.1">
        <name slugifiedName="name-requirements-overview">Requirements Overview</name>
        <t indent="0" pn="section-5.1-1">JSEP implementations must <bcp14>MUST</bcp14> comply with the specifications
        listed below that govern the creation and processing of offers
        and answers.</t>
        <section title="Usage Requirements"
        anchor="sec.usage-requirements">

          <t>All anchor="sec.usage-requirements" numbered="true" toc="include" removeInRFC="false" pn="section-5.1.1">
          <name slugifiedName="name-usage-requirements">Usage Requirements</name>
          <t indent="0" pn="section-5.1.1-1">All session descriptions handled by JSEP implementations,
          both local and remote, MUST <bcp14>MUST</bcp14> indicate support for the
          following specifications. If any of these are absent, this
          omission MUST <bcp14>MUST</bcp14> be treated as an error.
          <list style="symbols">

            <t>ICE,
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.1.1-2">
            <li pn="section-5.1.1-2.1">ICE, as specified in
            <xref target="RFC8445"></xref>, MUST target="RFC8445" format="default" sectionFormat="of" derivedContent="RFC8445"/>, <bcp14>MUST</bcp14> be used. Note that the
            remote endpoint may use a Lite lite implementation;
            implementations MUST <bcp14>MUST</bcp14> properly handle remote endpoints which
            do ICE-Lite.</t>

            <t>DTLS that
            use ICE-lite. The remote endpoint may also use
an older version of ICE; implementations <bcp14>MUST</bcp14> properly handle remote endpoints that use ICE
as specified in <xref target="RFC5245" format="default" sectionFormat="of" derivedContent="RFC5245"/>.</li>
            <li pn="section-5.1.1-2.2">DTLS
            <xref target="RFC6347" /> format="default" sectionFormat="of" derivedContent="RFC6347"/> or DTLS-SRTP
            <xref target="RFC5763"></xref>, MUST target="RFC5763" format="default" sectionFormat="of" derivedContent="RFC5763"/> <bcp14>MUST</bcp14> be used, as
            appropriate for the media type, as specified in
            <xref target="I-D.ietf-rtcweb-security-arch" /></t>
          </list></t>

          <t>The SDES target="RFC8827" format="default" sectionFormat="of" derivedContent="RFC8827"/>.</li>
          </ul>
          <t indent="0" pn="section-5.1.1-3">The SDP security descriptions mechanism for SRTP keying mechanism from
          <xref target="RFC4568" /> MUST NOT format="default" sectionFormat="of" derivedContent="RFC4568"/> <bcp14>MUST NOT</bcp14> be used, as discussed in
          <xref target="I-D.ietf-rtcweb-security-arch" />.</t> target="RFC8827" format="default" sectionFormat="of" derivedContent="RFC8827"/>.</t>
        </section>
        <section title="Profile anchor="sec.profile-names" numbered="true" toc="include" removeInRFC="false" pn="section-5.1.2">
          <name slugifiedName="name-profile-names-and-interoper">Profile Names and Interoperability"
        anchor="sec.profile-names">

          <t>For Interoperability</name>
          <t indent="0" pn="section-5.1.2-1">For media m= "m=" sections, JSEP implementations MUST <bcp14>MUST</bcp14> support
          the "UDP/TLS/RTP/SAVPF" profile specified in
          <xref target="RFC5764"></xref> target="RFC5764" format="default" sectionFormat="of" derivedContent="RFC5764"/> as well as the "TCP/DTLS/RTP/SAVPF"
          profile specified in <xref target="RFC7850"></xref>, target="RFC7850" format="default" sectionFormat="of" derivedContent="RFC7850"/> and MUST <bcp14>MUST</bcp14> indicate
          one of these profiles for each media m= "m=" line they produce in an offer.
          For data m= "m=" sections, implementations MUST <bcp14>MUST</bcp14> support the
          "UDP/DTLS/SCTP" profile as well as the "TCP/DTLS/SCTP" profile, profile and
          MUST
          <bcp14>MUST</bcp14> indicate one of these profiles for each data m= "m=" line they produce
          in an offer. The exact profile to use is determined by the protocol
          associated with the current default or selected ICE candidate, as
          described in
          <xref target="I-D.ietf-mmusic-ice-sip-sdp"></xref>, Section 3.2.1.2.
          </t>

          <t>Unfortunately, target="RFC8839" sectionFormat="comma" section="4.2.1.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-4.2.1.2" derivedContent="RFC8839"/>.</t>
          <t indent="0" pn="section-5.1.2-2">Unfortunately, in an attempt at compatibility, some
          endpoints generate other profile strings even when they mean
          to support one of these profiles. For instance, an endpoint
          might generate "RTP/AVP" but supply "a=fingerprint" and
          "a=rtcp-fb" attributes, indicating its willingness to support
          "UDP/TLS/RTP/SAVPF" or "TCP/DTLS/RTP/SAVPF". In order to
          simplify compatibility with such endpoints, JSEP
          implementations MUST <bcp14>MUST</bcp14> follow the following rules when
          processing the media m= "m=" sections in a received offer:</t>

          <t>
            <list style="symbols">

              <t>Any
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.1.2-3">
            <li pn="section-5.1.2-3.1">
              <t indent="0" pn="section-5.1.2-3.1.1">Any profile in the offer matching one of the following
              MUST
              <bcp14>MUST</bcp14> be accepted:
              <list style="symbols">

                <t>"RTP/AVP" (Defined in
                <xref target="RFC4566"></xref>, Section 8.2.2)</t>

                <t>"RTP/AVPF" (Defined in
                <xref target="RFC4585"></xref>, Section 9)</t>

                <t>"RTP/SAVP" (Defined in
                <xref target="RFC3711"></xref>, Section 12)</t>

                <t>"RTP/SAVPF" (Defined in
                <xref target="RFC5124"></xref>, Section 6)</t>

                <t>"TCP/DTLS/RTP/SAVP" (Defined
              </t>
              <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.1.2-3.1.2">
                <li pn="section-5.1.2-3.1.2.1">"RTP/AVP" (defined in
                <xref target="RFC7850"></xref>, Section 3.4)</t>

                <t>"TCP/DTLS/RTP/SAVPF" (Defined target="RFC4566" sectionFormat="comma" section="8.2.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-8.2.2" derivedContent="RFC4566"/>)</li>
                <li pn="section-5.1.2-3.1.2.2">"RTP/AVPF" (defined in
                <xref target="RFC7850"></xref>, Section 3.5)</t>

                <t>"UDP/TLS/RTP/SAVP" (Defined target="RFC4585" sectionFormat="comma" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4585#section-9" derivedContent="RFC4585"/>)</li>
                <li pn="section-5.1.2-3.1.2.3">"RTP/SAVP" (defined in
                <xref target="RFC3711" sectionFormat="comma" section="12" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3711#section-12" derivedContent="RFC3711"/>)</li>
                <li pn="section-5.1.2-3.1.2.4">"RTP/SAVPF" (defined in
                <xref target="RFC5124" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5124#section-6" derivedContent="RFC5124"/>)</li>
                <li pn="section-5.1.2-3.1.2.5">"TCP/DTLS/RTP/SAVP" (defined in
                <xref target="RFC7850" sectionFormat="comma" section="3.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7850#section-3.4" derivedContent="RFC7850"/>)</li>
                <li pn="section-5.1.2-3.1.2.6">"TCP/DTLS/RTP/SAVPF" (defined in
                <xref target="RFC7850" sectionFormat="comma" section="3.5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7850#section-3.5" derivedContent="RFC7850"/>)</li>
                <li pn="section-5.1.2-3.1.2.7">"UDP/TLS/RTP/SAVP" (defined in
                <xref target="RFC5764"></xref>, Section 9)</t>

                <t>"UDP/TLS/RTP/SAVPF" (Defined target="RFC5764" sectionFormat="comma" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5764#section-9" derivedContent="RFC5764"/>)</li>
                <li pn="section-5.1.2-3.1.2.8">"UDP/TLS/RTP/SAVPF" (defined in
                <xref target="RFC5764"></xref>, Section 9)</t>
              </list></t>

              <t>The target="RFC5764" sectionFormat="comma" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5764#section-9" derivedContent="RFC5764"/>)</li>
              </ul>
            </li>
            <li pn="section-5.1.2-3.2">The profile in any "m=" line in any generated answer
              MUST
              <bcp14>MUST</bcp14> exactly match the profile provided in the offer.</t>

              <t>Because offer.</li>
            <li pn="section-5.1.2-3.3">Because DTLS-SRTP is REQUIRED, <bcp14>REQUIRED</bcp14>, the choice of SAVP or
              AVP has no effect; support for DTLS-SRTP is determined by
              the presence of one or more "a=fingerprint" attribute. attributes.
              Note that lack of an "a=fingerprint" attribute will lead
              to negotiation failure.</t>

              <t>The failure.</li>
            <li pn="section-5.1.2-3.4">The use of AVPF or AVP simply controls the timing
              rules used for RTCP feedback. If AVPF is provided, provided or an
              "a=rtcp-fb" attribute is present, assume AVPF timing,
              i.e., a default value of "trr-int=0". Otherwise, assume
              that AVPF is being used in an AVP compatible AVP-compatible mode and use
              a value of "trr-int=4000".</t>

              <t>For "trr-int=4000".</li>
            <li pn="section-5.1.2-3.5">For data m= "m=" sections, implementations MUST <bcp14>MUST</bcp14> support
              receiving the "UDP/DTLS/SCTP", "TCP/DTLS/SCTP", or
              "DTLS/SCTP" (for backwards compatibility) profiles.</t>
            </list>
          </t>

          <t>Note profiles.</li>
          </ul>
          <t indent="0" pn="section-5.1.2-4">Note that re-offers by JSEP implementations MUST <bcp14>MUST</bcp14> use the
          correct profile strings even if the initial offer/answer
          exchange used an (incorrect) older profile string. This
          simplifies JSEP behavior, with minimal downside, as any
          remote endpoint that fails to handle such a re-offer will
          also fail to handle a JSEP endpoint's initial offer.</t>
        </section>
      </section>
      <section anchor="sec-create-offer" title="Constructing numbered="true" toc="include" removeInRFC="false" pn="section-5.2">
        <name slugifiedName="name-constructing-an-offer">Constructing an Offer">

        <t>When Offer</name>
        <t indent="0" pn="section-5.2-1">When createOffer is called, a new SDP description must <bcp14>MUST</bcp14> be
        created that includes the functionality specified in
        <xref target="I-D.ietf-rtcweb-rtp-usage"></xref>. target="RFC8834" format="default" sectionFormat="of" derivedContent="RFC8834"/>. The exact
        details of this process are explained below.</t>
        <section title="Initial Offers" anchor="sec.initial-offers">

          <t>When anchor="sec.initial-offers" numbered="true" toc="include" removeInRFC="false" pn="section-5.2.1">
          <name slugifiedName="name-initial-offers">Initial Offers</name>
          <t indent="0" pn="section-5.2.1-1">When createOffer is called for the first time, the result
          is known as the initial offer.</t>

          <t>The
          <t indent="0" pn="section-5.2.1-2">The first step in generating an initial offer is to
          generate session-level attributes, as specified in
          <xref target="RFC4566"></xref>, Section 5. target="RFC4566" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5" derivedContent="RFC4566"/>. Specifically:
          <list style="symbols">

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.2.1-3">
            <li pn="section-5.2.1-3.1">The first SDP line MUST <bcp14>MUST</bcp14> be "v=0", "v=0" as specified defined in
            <xref target="RFC4566"></xref>, Section 5.1</t>

            <t>The target="RFC4566" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.1" derivedContent="RFC4566"/>.</li>
            <li pn="section-5.2.1-3.2">The second SDP line MUST <bcp14>MUST</bcp14> be an "o=" line, line as specified defined
            in
            <xref target="RFC4566"></xref>, Section 5.2. target="RFC4566" sectionFormat="comma" section="5.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.2" derivedContent="RFC4566"/>.
 The value of
            the &lt;username&gt; field SHOULD <bcp14>SHOULD</bcp14> be "-". The sess-id MUST &lt;sess-id&gt; <bcp14>MUST</bcp14>
            be representable by a 64-bit signed integer, and the
           value MUST <bcp14>MUST</bcp14> be less than (2**63)-1.
	    2<sup>63</sup>-1.
            It is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that the
            sess-id
            &lt;sess-id&gt; be constructed by generating a 64-bit quantity with
            the highest bit set to zero and the remaining 63
            bits being cryptographically random. The value of the
            &lt;nettype&gt; &lt;addrtype&gt; &lt;unicast-address&gt;
            tuple SHOULD <bcp14>SHOULD</bcp14> be set to a non-meaningful address, such as IN
            IP4 0.0.0.0, to prevent leaking a local IP address in this
            field; this problem is discussed in
            <xref target="I-D.ietf-rtcweb-ip-handling" />. target="RFC8828" format="default" sectionFormat="of" derivedContent="RFC8828"/>. As mentioned in
            <xref target="RFC4566"></xref>, target="RFC4566" format="default" sectionFormat="of" derivedContent="RFC4566"/>, the entire o= "o=" line needs to
            be unique, but selecting a random number for
            &lt;sess-id&gt; is sufficient to accomplish this.</t>

            <t>The this.</li>
            <li pn="section-5.2.1-3.3">The third SDP line MUST <bcp14>MUST</bcp14> be a "s=" line, line as specified defined in
            <xref target="RFC4566"></xref>, Section 5.3; target="RFC4566" sectionFormat="comma" section="5.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.3" derivedContent="RFC4566"/>; to match the
            "o=" line, a single dash SHOULD <bcp14>SHOULD</bcp14> be used as the session
            name, e.g. e.g., "s=-". Note that this differs from the advice in

            <xref target="RFC4566" /> format="default" sectionFormat="of" derivedContent="RFC4566"/>, which proposes a single space, but
            as both "o=" and "s=" are meaningless in JSEP, having the
            same meaningless value seems clearer.</t>

            <t>Session clearer.</li>
            <li pn="section-5.2.1-3.4">Session Information ("i="), URI ("u="), Email Address
            ("e="), Phone Number ("p="), Repeat Times ("r="), and Time
            Zones ("z=") lines are not useful in this context and
            SHOULD NOT
            <bcp14>SHOULD NOT</bcp14> be included.</t>

            <t>Encryption included.</li>
            <li pn="section-5.2.1-3.5">Encryption Keys ("k=") lines do not provide sufficient
            security and MUST NOT <bcp14>MUST NOT</bcp14> be included.</t>

            <t>A included.</li>
            <li pn="section-5.2.1-3.6">A "t=" line MUST <bcp14>MUST</bcp14> be added, as specified in
            <xref target="RFC4566"></xref>, Section 5.9; target="RFC4566" sectionFormat="comma" section="5.9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.9" derivedContent="RFC4566"/>; both
            &lt;start-time&gt; and &lt;stop-time&gt; SHOULD <bcp14>SHOULD</bcp14> be set to
            zero, e.g. e.g., "t=0 0".</t>

            <t>An 0".</li>
            <li pn="section-5.2.1-3.7">An "a=ice-options" line with the "trickle" and "ice2"
            options MUST <bcp14>MUST</bcp14> be added, as specified in <xref
            target="I-D.ietf-ice-trickle"></xref>, Section 3 target="RFC8840" sectionFormat="comma" section="4.1.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8840#section-4.1.1" derivedContent="RFC8840"/> and
            <xref target="RFC8445"></xref>, Section 10.</t>

            <t>If target="RFC8445" sectionFormat="comma" section="10" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8445#section-10" derivedContent="RFC8445"/>.
</li>
            <li pn="section-5.2.1-3.8">If WebRTC identity is being used, an "a=identity" line
            <bcp14>MUST</bcp14> be added, as described in
            <xref target="I-D.ietf-rtcweb-security-arch" />, Section
            5.</t>
          </list></t>

          <t>The target="RFC8827" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8827#section-5" derivedContent="RFC8827"/>.</li>
          </ul>
          <t indent="0" pn="section-5.2.1-4">The next step is to generate m= "m=" sections, as specified in
          <xref target="RFC4566" />, Section 5.14. sectionFormat="comma" section="5.14" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.14" derivedContent="RFC4566"/>. An m= "m=" section is
          generated for each RtpTransceiver that has been added to the
          PeerConnection, excluding any stopped RtpTransceivers; this
          is done in the order the RtpTransceivers were added to the
          PeerConnection. If there are no such RtpTransceivers, no m= "m="
          sections are generated; more can be added later, as discussed
          in
          <xref target="RFC3264" />, Section 5.</t>

          <t>For sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-5" derivedContent="RFC3264"/>.</t>
          <t indent="0" pn="section-5.2.1-5">For each m= "m=" section generated for an RtpTransceiver,
          establish a mapping between the transceiver and the index of
          the generated m= "m=" section.</t>

          <t>Each m=
          <t indent="0" pn="section-5.2.1-6">Each "m=" section, provided it is not marked as bundle-only,
          MUST generate
          <bcp14>MUST</bcp14> contain a unique set of ICE credentials and gather its
          own
          a unique set of ICE candidates. Bundle-only m= "m=" sections
          MUST NOT
          <bcp14>MUST NOT</bcp14> contain any ICE credentials and MUST NOT <bcp14>MUST NOT</bcp14> gather any
          candidates.</t>

          <t>For
          <t indent="0" pn="section-5.2.1-7">For DTLS, all m= "m=" sections MUST <bcp14>MUST</bcp14> use any and all the certificate(s) certificates
          that have been specified for the PeerConnection; as a result,
          they MUST <bcp14>MUST</bcp14> all have the same
          <xref target="RFC8122"></xref> fingerprint value(s), value or values
          <xref target="RFC8122" format="default" sectionFormat="of" derivedContent="RFC8122"/>, or these
          value(s) MUST
          values <bcp14>MUST</bcp14> be session-level attributes.</t>

          <t>Each m=
          <t indent="0" pn="section-5.2.1-8">Each "m=" section should <bcp14>MUST</bcp14> be generated as specified in
          <xref target="RFC4566"></xref>, Section 5.14. target="RFC4566" sectionFormat="comma" section="5.14" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.14" derivedContent="RFC4566"/>. For the m= "m=" line
          itself, the following rules MUST <bcp14>MUST</bcp14> be followed:
          <list style="symbols">

            <t>If
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.2.1-9">
            <li pn="section-5.2.1-9.1">If the m= "m=" section is marked as bundle-only, then the
            port
            &lt;port&gt; value MUST <bcp14>MUST</bcp14> be set to 0. zero. Otherwise, the port &lt;port&gt; value is
            set to the port of the default ICE candidate for this m= "m="
            section, but given that no candidates are available yet,
            the "dummy" default port value of 9 (Discard) MUST <bcp14>MUST</bcp14> be used, as
            indicated in
            <xref target="I-D.ietf-ice-trickle"></xref>, Section
            5.1.</t>

            <t>To target="RFC8840" sectionFormat="comma" section="4.1.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8840#section-4.1.1" derivedContent="RFC8840"/>.</li>
            <li pn="section-5.2.1-9.2">To properly indicate use of DTLS, the &lt;proto&gt;
            field MUST <bcp14>MUST</bcp14> be set to "UDP/TLS/RTP/SAVPF", as specified in
            <xref target="RFC5764" />, Section 8.</t>

            <t>If sectionFormat="comma" section="8" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5764#section-8" derivedContent="RFC5764"/>.</li>
            <li pn="section-5.2.1-9.3">If codec preferences have been set for the associated
            transceiver, media formats MUST <bcp14>MUST</bcp14> be generated in the
            corresponding order, order and MUST <bcp14>MUST</bcp14> exclude any codecs not
            present in the codec preferences.</t>

            <t>Unless preferences.</li>
            <li pn="section-5.2.1-9.4">Unless excluded by the above restrictions, the media
            formats MUST <bcp14>MUST</bcp14> include the mandatory audio/video codecs as
            specified in
            <xref target="RFC7874"></xref>, Section 3, target="RFC7874" sectionFormat="comma" section="3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7874#section-3" derivedContent="RFC7874"/> and
            <xref target="RFC7742"></xref>, Section 5.</t>
          </list></t>

          <t>The m= target="RFC7742" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7742#section-5" derivedContent="RFC7742"/>.</li>
          </ul>
          <t indent="0" pn="section-5.2.1-10">The "m=" line MUST <bcp14>MUST</bcp14> be followed immediately by a "c=" line,
          as specified in
          <xref target="RFC4566"></xref>, Section 5.7. target="RFC4566" sectionFormat="comma" section="5.7" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.7" derivedContent="RFC4566"/>. Again, as no
          candidates are available yet, the "c=" line must <bcp14>MUST</bcp14> contain the
          "dummy"
          default value "IN IP4 0.0.0.0", as defined in
          <xref target="I-D.ietf-ice-trickle"></xref>, Section 5.1.</t>

          <t> target="RFC8840" sectionFormat="comma" section="4.1.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8840#section-4.1.1" derivedContent="RFC8840"/>.</t>
          <t indent="0" pn="section-5.2.1-11">
          <xref target="I-D.ietf-mmusic-sdp-mux-attributes" /> target="RFC8859" format="default" sectionFormat="of" derivedContent="RFC8859"/> groups
          SDP attributes into different categories. To avoid
          unnecessary duplication when bundling, attributes of category
          IDENTICAL or TRANSPORT MUST NOT <bcp14>MUST NOT</bcp14> be repeated in bundled m= "m="
          sections, repeating the guidance from
          <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation" />,
          Section 8.1. target="RFC8843" sectionFormat="comma" section="7.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-7.1.3" derivedContent="RFC8843"/>.
          This includes m= "m=" sections for which bundling has
          been negotiated and is still desired, as well as m= "m=" sections
          marked as bundle-only.</t>

          <t>The
          <t indent="0" pn="section-5.2.1-12">The following attributes, which are of a category other
          than IDENTICAL or TRANSPORT, MUST <bcp14>MUST</bcp14> be included in each m= "m="
          section:</t>

          <t>
            <list style="symbols">

              <t>An
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.2.1-13">
            <li pn="section-5.2.1-13.1">An "a=mid" line, as specified in
              <xref target="RFC5888"></xref>, Section 4. target="RFC5888" sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5888#section-4" derivedContent="RFC5888"/>. All MID values
              MUST
              <bcp14>MUST</bcp14> be generated in a fashion that does not leak user
              information, e.g., randomly or using a per-PeerConnection
              counter, and SHOULD <bcp14>SHOULD</bcp14> be 3 bytes or less, to allow them to
              efficiently fit into the RTP header extension defined in
              <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation">
              </xref>, Section 14. target="RFC8843" sectionFormat="comma" section="15.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-15.2" derivedContent="RFC8843"/>.
              Note that this does not set the
              RtpTransceiver mid property, as that only occurs when the
              description is applied. The generated MID value can be
              considered a "proposed" MID at this point.</t>

              <t>A point.</li>
            <li pn="section-5.2.1-13.2">A direction attribute which that is the same as that of the
              associated transceiver.</t>

              <t>For transceiver.</li>
            <li pn="section-5.2.1-13.3">For each media format on the m= "m=" line, "a=rtpmap" and "a=fmtp" lines, as specified in
              <xref target="RFC4566"></xref>, Section 6, target="RFC4566" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/> and
              <xref target="RFC3264"></xref>, Section 5.1.</t>

              <t>For target="RFC3264" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-5.1" derivedContent="RFC3264"/>.</li>
            <li pn="section-5.2.1-13.4">For each primary codec where RTP retransmission should
              be used, a corresponding "a=rtpmap" line indicating "rtx"
              with the clock rate of the primary codec and an "a=fmtp"
              line that references the payload type of the primary
              codec, as specified in
              <xref target="RFC4588"></xref>, Section 8.1.</t>

              <t>For target="RFC4588" sectionFormat="comma" section="8.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4588#section-8.1" derivedContent="RFC4588"/>.</li>
            <li pn="section-5.2.1-13.5">For each supported FEC Forward Error Correction (FEC) mechanism, "a=rtpmap" and
              "a=fmtp" lines, as specified in
              <xref target="RFC4566"></xref>, Section 6. target="RFC4566" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/>. The FEC
              mechanisms that MUST <bcp14>MUST</bcp14> be supported are specified in
              <xref target="I-D.ietf-rtcweb-fec"></xref>, Section 6, target="RFC8854" sectionFormat="comma" section="7" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8854#section-7" derivedContent="RFC8854"/>,
              and specific usage for each media type is outlined in
              Sections 4 <xref target="sec.interface" format="counter" sectionFormat="of" derivedContent="4"/> and 5.</t>

              <t>If <xref target="sec.sdp-interaction-procedure" format="counter" sectionFormat="of" derivedContent="5"/>.</li>
            <li pn="section-5.2.1-13.6">If this m= "m=" section is for media with configurable
              durations of media per packet, e.g., audio, an
              "a=maxptime" line, indicating the maximum amount of
              media, specified in milliseconds, that can be
              encapsulated in each packet, as specified in
              <xref target="RFC4566"></xref>, Section 6. target="RFC4566" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/>. This value is
              set to the smallest of the maximum duration values across
              all the codecs included in the m= section.</t>

              <t>If "m=" section.</li>
            <li pn="section-5.2.1-13.7">If this m= "m=" section is for video media, media and there are
              known limitations on the size of images which that can be
              decoded, an "a=imageattr" line, as specified in
              <xref target="sec.imageattr"></xref>.</t>

              <t>For target="sec.imageattr" format="default" sectionFormat="of" derivedContent="Section 3.6"/>.</li>
            <li pn="section-5.2.1-13.8">For each supported RTP header extension, an "a=extmap"
              line, as specified in
              <xref target="RFC5285"></xref>, Section 5. target="RFC5285" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5285#section-5" derivedContent="RFC5285"/>.
              The list of
              header extensions that SHOULD/MUST <bcp14>SHOULD</bcp14>/<bcp14>MUST</bcp14> be supported is
              specified in
              <xref target="I-D.ietf-rtcweb-rtp-usage"></xref>, Section
              5.2. target="RFC8834" sectionFormat="comma" section="5.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8834#section-5.2" derivedContent="RFC8834"/>. Any header extensions that require encryption MUST <bcp14>MUST</bcp14>
              be specified as indicated in
              <xref target="RFC6904"></xref>, Section 4.</t>

              <t>For target="RFC6904" sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc6904#section-4" derivedContent="RFC6904"/>.</li>
            <li pn="section-5.2.1-13.9">For each supported RTCP feedback mechanism, an
              "a=rtcp-fb" line, as specified in
              <xref target="RFC4585"></xref>, Section 4.2. target="RFC4585" sectionFormat="comma" section="4.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4585#section-4.2" derivedContent="RFC4585"/>. The list of
              RTCP feedback mechanisms that SHOULD/MUST <bcp14>SHOULD</bcp14>/<bcp14>MUST</bcp14> be supported is
              specified in
              <xref target="I-D.ietf-rtcweb-rtp-usage"></xref>, Section
              5.1.</t>

              <t>If target="RFC8834" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8834#section-5.1" derivedContent="RFC8834"/>.</li>
            <li pn="section-5.2.1-13.10">
              <t indent="0" pn="section-5.2.1-13.10.1">If the RtpTransceiver has a sendrecv or sendonly
              direction:
              <list style="symbols">

                <t>For
              </t>
              <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.2.1-13.10.2">
                <li pn="section-5.2.1-13.10.2.1">For each MediaStream that was associated with the
                transceiver when it was created via addTrack or
                addTransceiver, an "a=msid" line, as specified in
                <xref target="I-D.ietf-mmusic-msid"></xref>, Section 2, target="RFC8830" sectionFormat="comma" section="2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8830#section-2" derivedContent="RFC8830"/>,
                but omitting the "appdata" field.</t>
              </list></t>

              <t>If field.</li>
              </ul>
            </li>
            <li pn="section-5.2.1-13.11">If the RtpTransceiver has a sendrecv or sendonly
              direction, and the application has specified RID values a rid-id for an encoding,
              or has specified more than one encoding in the
              RtpSenders's parameters, an "a=rid" line for each
              encoding specified. The "a=rid" line is specified in
              <xref target="I-D.ietf-mmusic-rid"></xref>, target="RFC8851" format="default" sectionFormat="of" derivedContent="RFC8851"/>, and its
              direction MUST <bcp14>MUST</bcp14> be "send". If the application has chosen a
              RID value,
              rid-id, it MUST <bcp14>MUST</bcp14> be used as the rid-identifier;
              otherwise used;
              otherwise, a RID value MUST rid-id <bcp14>MUST</bcp14> be generated by the
              implementation. RID values MUST rid-ids <bcp14>MUST</bcp14> be generated in a fashion
              that does not leak user information, e.g., randomly or
              using a per-PeerConnection counter, counter (see guidance at the end
              of <xref target="RFC8852" sectionFormat="comma" section="3.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8852#section-3.3" derivedContent="RFC8852"/>), and SHOULD <bcp14>SHOULD</bcp14> be 3 bytes
              or less, to allow them to efficiently fit into the RTP
              header extension extensions defined in
              <xref target="I-D.ietf-avtext-rid"></xref>, Section 3. target="RFC8852" sectionFormat="comma" section="3.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8852#section-3.3" derivedContent="RFC8852"/>.
              If no encodings have been specified, or only one encoding is
              specified but without a RID value, rid-id, then no "a=rid" lines
              are generated.</t>

              <t>If generated.</li>
            <li pn="section-5.2.1-13.12">If the RtpTransceiver has a sendrecv or sendonly
              direction and more than one "a=rid" line has been
              generated, an "a=simulcast" line, with direction "send",
              as defined in
              <xref target="I-D.ietf-mmusic-sdp-simulcast"></xref>,
              Section 6.2. target="RFC8853" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8853#section-5.1" derivedContent="RFC8853"/>. The list associated set of RIDs MUST rid-ids <bcp14>MUST</bcp14>
              include all of the RID
              identifiers rid-ids used in the "a=rid" lines for this m=
              section.</t>

              <t>If "m="
              section.</li>
            <li pn="section-5.2.1-13.13">If (1) the bundle policy for this PeerConnection is set to
              "max-bundle",
              "max-bundle" and this is not the first m= section, "m=" section or (2)
              the bundle policy is set to "balanced", "balanced" and this is not
              the first m= "m=" section for this media type, an
              "a=bundle-only" line.</t>
            </list>
          </t>

          <t>The line.</li>
          </ul>
          <t indent="0" pn="section-5.2.1-14">The following attributes, which are of category IDENTICAL
          or TRANSPORT, MUST <bcp14>MUST</bcp14> appear only in "m=" sections which that either
          have a unique address or which are associated with the
          bundle-tag.
          BUNDLE-tag. (In initial offers, this means those "m="
          sections which that do not contain an "a=bundle-only"
          attribute.)</t>

          <t>
            <list style="symbols">

              <t>"a=ice-ufrag"
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.2.1-15">
            <li pn="section-5.2.1-15.1">"a=ice-ufrag" and "a=ice-pwd" lines, as specified in
              <xref target="I-D.ietf-mmusic-ice-sip-sdp"></xref>,
              Section 4.4.</t>

              <t>For target="RFC8839" sectionFormat="comma" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.4" derivedContent="RFC8839"/>.</li>
            <li pn="section-5.2.1-15.2">For each desired digest algorithm, one or more
              "a=fingerprint" lines for each of the endpoint's
              certificates, as specified in
              <xref target="RFC8122"></xref>, Section 5.</t>

              <t>An target="RFC8122" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8122#section-5" derivedContent="RFC8122"/>.</li>
            <li pn="section-5.2.1-15.3">An "a=setup" line, as specified in
              <xref target="RFC4145"></xref>, Section 4, target="RFC4145" sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4145#section-4" derivedContent="RFC4145"/> and clarified
              for use in DTLS-SRTP scenarios in
              <xref target="RFC5763"></xref>, Section 5. target="RFC5763" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5763#section-5" derivedContent="RFC5763"/>. The role value
              in the offer MUST <bcp14>MUST</bcp14> be "actpass".</t>

              <t>An "actpass".</li>
            <li pn="section-5.2.1-15.4">An "a=tls-id" line, as specified in
              <xref target="I-D.ietf-mmusic-dtls-sdp" />, Section
              5.2.</t>

              <t>An target="RFC8842" sectionFormat="comma" section="5.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8842#section-5.2" derivedContent="RFC8842"/>.</li>
            <li pn="section-5.2.1-15.5">An "a=rtcp" line, as specified in
              <xref target="RFC3605"></xref>, Section 2.1, target="RFC3605" sectionFormat="comma" section="2.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3605#section-2.1" derivedContent="RFC3605"/>, containing
              the dummy default value "9 IN IP4 0.0.0.0", because no candidates
              have yet been gathered.</t>

              <t>An gathered.</li>
            <li pn="section-5.2.1-15.6">An "a=rtcp-mux" line, as specified in
              <xref target="RFC5761"></xref>, Section 5.1.3.</t>

              <t>If target="RFC5761" sectionFormat="comma" section="5.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5761#section-5.1.3" derivedContent="RFC5761"/>.</li>
            <li pn="section-5.2.1-15.7">If the RTP/RTCP multiplexing policy is "require", an
              "a=rtcp-mux-only" line, as specified in
              <xref target="I-D.ietf-mmusic-mux-exclusive" />, Section
              4.</t>

              <t>An target="RFC8858" sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8858#section-4" derivedContent="RFC8858"/>.</li>
            <li pn="section-5.2.1-15.8">An "a=rtcp-rsize" line, as specified in
              <xref target="RFC5506"></xref>, Section 5.</t>
            </list>
          </t>

          <t>Lastly, target="RFC5506" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5506#section-5" derivedContent="RFC5506"/>.</li>
          </ul>
          <t indent="0" pn="section-5.2.1-16">Lastly, if a data channel has been created, a m= an "m=" section
          MUST
          <bcp14>MUST</bcp14> be generated for data. The &lt;media&gt; field MUST <bcp14>MUST</bcp14> be
          set to "application" "application", and the &lt;proto&gt; field MUST <bcp14>MUST</bcp14> be set
          to "UDP/DTLS/SCTP"
          <xref target="I-D.ietf-mmusic-sctp-sdp"></xref>. target="RFC8841" format="default" sectionFormat="of" derivedContent="RFC8841"/>. The "fmt" &lt;fmt&gt;
          value MUST <bcp14>MUST</bcp14> be set to "webrtc-datachannel" as specified in
          <xref target="I-D.ietf-mmusic-sctp-sdp"></xref>, Section
          4.1.</t>

          <t>Within target="RFC8841" sectionFormat="comma" section="4.2.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8841#section-4.2.2" derivedContent="RFC8841"/>.
</t>
          <t indent="0" pn="section-5.2.1-17">Within the data m= "m=" section, an "a=mid" line MUST <bcp14>MUST</bcp14> be
          generated and included as described above, along with an
          "a=sctp-port" line referencing the SCTP port number, as
          defined in
          <xref target="I-D.ietf-mmusic-sctp-sdp"></xref>, Section 5.1, target="RFC8841" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8841#section-5.1" derivedContent="RFC8841"/>;
          and, if appropriate, an "a=max-message-size" line, as defined
          in
          <xref target="I-D.ietf-mmusic-sctp-sdp"></xref>, Section
          6.1.</t>

          <t>As target="RFC8841" sectionFormat="comma" section="6.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8841#section-6.1" derivedContent="RFC8841"/>.</t>
          <t indent="0" pn="section-5.2.1-18">As discussed above, the following attributes of category
          IDENTICAL or TRANSPORT are included only if the data m= "m="
          section either has a unique address or is associated with the
          bundle-tag
          BUNDLE-tag (e.g., if it is the only m= "m=" section):
          <list style="symbols">

            <t>"a=ice-ufrag"</t>

            <t>"a=ice-pwd"</t>

            <t>"a=fingerprint"</t>

            <t>"a=setup"</t>

            <t>"a=tls-id"</t>
          </list></t>

          <t>Once
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.2.1-19">
            <li pn="section-5.2.1-19.1">"a=ice-ufrag"</li>
            <li pn="section-5.2.1-19.2">"a=ice-pwd"</li>
            <li pn="section-5.2.1-19.3">"a=fingerprint"</li>
            <li pn="section-5.2.1-19.4">"a=setup"</li>
            <li pn="section-5.2.1-19.5">"a=tls-id"</li>
          </ul>
          <t indent="0" pn="section-5.2.1-20">Once all m= "m=" sections have been generated, a session-level
          "a=group" attribute MUST <bcp14>MUST</bcp14> be added as specified in
          <xref target="RFC5888"></xref>. target="RFC5888" format="default" sectionFormat="of" derivedContent="RFC5888"/>. This attribute MUST <bcp14>MUST</bcp14> have
          semantics "BUNDLE", "BUNDLE" and MUST <bcp14>MUST</bcp14> include the mid identifiers of
          each m= "m=" section. The effect of this is that the JSEP
          implementation offers all m= "m=" sections as one bundle group.
          However, whether the m= "m=" sections are bundle-only or not
          depends on the bundle policy.</t>

          <t>The
          <t indent="0" pn="section-5.2.1-21">The next step is to generate session-level lip sync groups
          as defined in
          <xref target="RFC5888" />, Section 7. sectionFormat="comma" section="7" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5888#section-7" derivedContent="RFC5888"/>. For each MediaStream
          referenced by more than one RtpTransceiver (by passing those
          MediaStreams as arguments to the addTrack and addTransceiver
          methods), a group of type "LS" MUST <bcp14>MUST</bcp14> be added that contains
          the mid MID values for each RtpTransceiver.</t>

          <t>Attributes which
          <t indent="0" pn="section-5.2.1-22">Attributes that SDP permits to either be at either the session
          level or the media level SHOULD <bcp14>SHOULD</bcp14> generally be at the media
          level even if they are identical. This assists development
          and debugging by making it easier to understand individual
          media sections, especially if one of a set of initially
          identical attributes is subsequently changed. However,
          implementations MAY <bcp14>MAY</bcp14> choose to aggregate attributes at the
          session level level, and JSEP implementations MUST <bcp14>MUST</bcp14> be prepared to
          receive attributes in either location.</t>

          <t>Attributes
          <t indent="0" pn="section-5.2.1-23">Attributes other than the ones specified above MAY <bcp14>MAY</bcp14> be
          included, except for the following attributes attributes, which are
          specifically incompatible with the requirements of
          <xref target="I-D.ietf-rtcweb-rtp-usage"></xref>, target="RFC8834" format="default" sectionFormat="of" derivedContent="RFC8834"/> and MUST
          NOT <bcp14>MUST NOT</bcp14> be included:
          <list style="symbols">

            <t>"a=crypto"</t>

            <t>"a=key-mgmt"</t>

            <t>"a=ice-lite"</t>
          </list></t>

          <t>Note
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.2.1-24">
            <li pn="section-5.2.1-24.1">"a=crypto"</li>
            <li pn="section-5.2.1-24.2">"a=key-mgmt"</li>
            <li pn="section-5.2.1-24.3">"a=ice-lite"</li>
          </ul>
          <t indent="0" pn="section-5.2.1-25">Note that when bundle is used, any additional attributes
          that are added MUST <bcp14>MUST</bcp14> follow the advice in
          <xref target="I-D.ietf-mmusic-sdp-mux-attributes"></xref> target="RFC8859" format="default" sectionFormat="of" derivedContent="RFC8859"/> on
          how those attributes interact with bundle.</t>

          <t>Note
          <t indent="0" pn="section-5.2.1-26">Note that these requirements are in some cases stricter
          than those of SDP. Implementations MUST <bcp14>MUST</bcp14> be prepared to accept
          compliant SDP even if it would not conform to the
          requirements for generating SDP in this specification.</t>
        </section>
        <section title="Subsequent Offers"
        anchor="sec.subsequent-offers">

          <t>When anchor="sec.subsequent-offers" numbered="true" toc="include" removeInRFC="false" pn="section-5.2.2">
          <name slugifiedName="name-subsequent-offers">Subsequent Offers</name>
          <t indent="0" pn="section-5.2.2-1">When createOffer is called a second (or later) time, time or is
          called after a local description has already been installed,
          the processing is somewhat different than for an initial
          offer.</t>

          <t>If
          <t indent="0" pn="section-5.2.2-2">If the previous offer was not applied using
          setLocalDescription, meaning the PeerConnection is still in
          the "stable" state, the steps for generating an initial offer
          should
          <bcp14>MUST</bcp14> be followed, subject to the following restriction:
          <list style="symbols">

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.2.2-3">
            <li pn="section-5.2.2-3.1">The fields of the "o=" line MUST <bcp14>MUST</bcp14> stay the same except
            for the &lt;session-version&gt; field, which MUST <bcp14>MUST</bcp14> increment
            by one on each call to createOffer if the offer might
            differ from the output of the previous call to createOffer;
            implementations MAY <bcp14>MAY</bcp14> opt to increment
            &lt;session-version&gt; on every call. The value of the
            generated &lt;session-version&gt; is independent of the
            &lt;session-version&gt; of the current local description;
            in particular, in the case where the current version is N,
            an offer is created and applied with version N+1, and then
            that offer is rolled back so that the current version is
            again N, the next generated offer will still have version
            N+2.</t>
          </list></t>

          <t>Note
            N+2.</li>
          </ul>
          <t indent="0" pn="section-5.2.2-4">Note that if the application creates an offer by reading
          currentLocalDescription instead of calling createOffer, the
          returned SDP may be different than when setLocalDescription
          was originally called, due to the addition of gathered ICE
          candidates, but the &lt;session-version&gt; will not have
          changed. There are no known scenarios in which this causes
          problems, but if this is a concern, the solution is simply to
          use createOffer to ensure a unique
          &lt;session-version&gt;.</t>

          <t>If
          <t indent="0" pn="section-5.2.2-5">If the previous offer was applied using
          setLocalDescription, but a corresponding answer from the
          remote side has not yet been applied, meaning the
          PeerConnection is still in the "have-local-offer" state, an
          offer is generated by following the steps in the "stable"
          state above, along with these exceptions:
          <list style="symbols">

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.2.2-6">
            <li pn="section-5.2.2-6.1">The "s=" and "t=" lines MUST <bcp14>MUST</bcp14> stay the same.</t>

            <t>If same.</li>
            <li pn="section-5.2.2-6.2">If any RtpTransceiver has been added, added and there exists
            an m= "m=" section with a zero port in the current local
            description or the current remote description, that m= "m="
            section MUST <bcp14>MUST</bcp14> be recycled by generating an m= "m=" section for
            the added RtpTransceiver as if the m= "m=" section were being
            added to the session description (including a new MID
            value),
            value) and placing it at the same index as the m= "m=" section
            with a zero port.</t>

            <t>If port.</li>
            <li pn="section-5.2.2-6.3">If an RtpTransceiver is stopped and is not associated
            with an m= "m=" section, an m= "m=" section MUST NOT <bcp14>MUST NOT</bcp14> be generated for
            it. This prevents adding back RtpTransceivers whose m= "m="
            sections were recycled and used for a new RtpTransceiver in
            a previous offer/ answer exchange, as described above.</t>

            <t>If above.</li>
            <li pn="section-5.2.2-6.4">If an RtpTransceiver has been stopped and is associated
            with an m= "m=" section, and the m= "m=" section is not being
            recycled as described above, an m= "m=" section MUST <bcp14>MUST</bcp14> be
            generated for it with the port set to zero and all "a=msid"
            lines removed.</t>

            <t>For removed.</li>
            <li pn="section-5.2.2-6.5">For RtpTransceivers that are not stopped, the "a=msid"
            line(s) MUST
            line or lines <bcp14>MUST</bcp14> stay the same if they are present in the
            current description, regardless of changes to the
            transceiver's direction or track. If no "a=msid" line is
            present in the current description, "a=msid" line(s) MUST <bcp14>MUST</bcp14>
            be generated according to the same rules as for an initial
            offer.</t>

            <t>Each
            offer.</li>
            <li pn="section-5.2.2-6.6">Each "m=" and c=" "c=" line MUST <bcp14>MUST</bcp14> be filled in with the port,
            relevant RTP profile, and address of the default candidate for the
            m=
            "m=" section, as described in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp"></xref>,
            Section 3.2.1.2, target="RFC8839" sectionFormat="comma" section="4.2.1.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-4.2.1.2" derivedContent="RFC8839"/> and clarified in
            <xref target="sec.profile-names"/>. target="sec.profile-names" format="default" sectionFormat="of" derivedContent="Section 5.1.2"/>.
            If no RTP candidates have yet been gathered, dummy default
            values MUST <bcp14>MUST</bcp14> still be used, as described above.</t>

            <t>Each above.</li>
            <li pn="section-5.2.2-6.7">Each "a=mid" line MUST <bcp14>MUST</bcp14> stay the same.</t>

            <t>Each same.</li>
            <li pn="section-5.2.2-6.8">Each "a=ice-ufrag" and "a=ice-pwd" line MUST <bcp14>MUST</bcp14> stay the
            same, unless the ICE configuration has changed (either (e.g., changes to
            either the supported STUN/TURN servers, servers or the ICE
            candidate policy), policy) or the "IceRestart" IceRestart option (
            <xref
            (<xref target="sec.icerestart" /> format="default" sectionFormat="of" derivedContent="Section 5.2.3.1"/>) was specified.
            If the m= "m="
            section is bundled into another m= "m=" section, it still MUST
            NOT <bcp14>MUST NOT</bcp14> contain any ICE credentials.</t>

            <t>If credentials.</li>
            <li pn="section-5.2.2-6.9">If the m= "m=" section is not bundled into another m= "m="
            section, its "a=rtcp" attribute line MUST <bcp14>MUST</bcp14> be filled in with
            the port and address of the default RTCP candidate, as
            indicated in
            <xref target="RFC5761"></xref>, Section 5.1.3. target="RFC5761" sectionFormat="comma" section="5.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5761#section-5.1.3" derivedContent="RFC5761"/>. If no RTCP
            candidates have yet been gathered, dummy default values MUST <bcp14>MUST</bcp14> be
            used, as described in <xref target="sec.initial-offers" format="default" sectionFormat="of" derivedContent="Section 5.2.1"/> above.</li>
            <li pn="section-5.2.2-6.10">If the initial offer section above.</t>

            <t>If the m= "m=" section is not bundled into another m= "m="
            section, for each candidate that has been gathered during
            the most recent gathering phase (see
            <xref target="sec.ice-gather-overview"></xref>), target="sec.ice-gather-overview" format="default" sectionFormat="of" derivedContent="Section 3.5.1"/>), an
            "a=candidate" line MUST <bcp14>MUST</bcp14> be added, as defined in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp"></xref>, Section 4.1. target="RFC8839" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.1" derivedContent="RFC8839"/>.
            If candidate gathering for the section has completed, an
            "a=end-of-candidates" attribute MUST <bcp14>MUST</bcp14> be added, as described
            in
            <xref target="I-D.ietf-ice-trickle"></xref>, Section 9.3. target="RFC8840" sectionFormat="comma" section="8.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8840#section-8.2" derivedContent="RFC8840"/>.
            If the m= "m=" section is bundled into another m= "m=" section, both
            "a=candidate" and "a=end-of-candidates" MUST <bcp14>MUST</bcp14> be
            omitted.</t>

            <t>For
            omitted.
</li>
            <li pn="section-5.2.2-6.11">For RtpTransceivers that are still present, the "a=rid"
            lines MUST <bcp14>MUST</bcp14> stay the same.</t>

            <t>For same.</li>
            <li pn="section-5.2.2-6.12">For RtpTransceivers that are still present, any
            "a=simulcast" line MUST <bcp14>MUST</bcp14> stay the same.</t>
          </list></t>

          <t>If same.</li>
          </ul>
          <t indent="0" pn="section-5.2.2-7">If the previous offer was applied using
          setLocalDescription, and a corresponding answer from the
          remote side has been applied using setRemoteDescription,
          meaning the PeerConnection is in the "have-remote-pranswer"
          state or the "stable" states, state, an offer is generated based on the
          negotiated session descriptions by following the steps
          mentioned for the "have-local-offer" state above.</t>

          <t>In
          <t indent="0" pn="section-5.2.2-8">In addition, for each existing, non-recycled, non-rejected
          m=
          "m=" section in the new offer, the following adjustments are
          made based on the contents of the corresponding m= "m=" section in
          the current local or remote description, as appropriate:
          <list style="symbols">

            <t>The m=
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.2.2-9">
            <li pn="section-5.2.2-9.1">The "m=" line and corresponding "a=rtpmap" and "a=fmtp"
            lines MUST <bcp14>MUST</bcp14> only include media formats which that have not been
            excluded by the codec preferences of the associated
            transceiver,
            transceiver and MUST also <bcp14>MUST</bcp14> include all currently available
            formats. Media formats that were previously offered but are
            no longer available (e.g., a shared hardware codec) MAY <bcp14>MAY</bcp14> be
            excluded.</t>

            <t>Unless
            excluded.</li>
            <li pn="section-5.2.2-9.2">Unless codec preferences have been set for the
            associated transceiver, the media formats on the m= "m=" line
            MUST
            <bcp14>MUST</bcp14> be generated in the same order as in the most recent
            answer. Any media formats that were not present in the most
            recent answer MUST <bcp14>MUST</bcp14> be added after all existing formats.</t>

            <t>The formats.</li>
            <li pn="section-5.2.2-9.3">The RTP header extensions MUST <bcp14>MUST</bcp14> only include those that
            are present in the most recent answer.</t>

            <t>The answer.</li>
            <li pn="section-5.2.2-9.4">The RTCP feedback mechanisms MUST <bcp14>MUST</bcp14> only include those
            that are present in the most recent answer, except for the
            case of format-specific mechanisms that are referencing a
            newly-added
            newly added media format.</t>

            <t>The format.</li>
            <li pn="section-5.2.2-9.5">The "a=rtcp" line MUST NOT <bcp14>MUST NOT</bcp14> be added if the most recent
            answer included an "a=rtcp-mux" line.</t>

            <t>The line.</li>
            <li pn="section-5.2.2-9.6">The "a=rtcp-mux" line MUST <bcp14>MUST</bcp14> be the same as that in the
            most recent answer.</t>

            <t>The answer.</li>
            <li pn="section-5.2.2-9.7">The "a=rtcp-mux-only" line MUST NOT <bcp14>MUST NOT</bcp14> be added.</t>

            <t>The added.</li>
            <li pn="section-5.2.2-9.8">The "a=rtcp-rsize" line MUST NOT <bcp14>MUST NOT</bcp14> be added unless present
            in the most recent answer.</t>

            <t>An answer.</li>
            <li pn="section-5.2.2-9.9">An "a=bundle-only" line MUST NOT be added, line, as indicated defined in
            <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation" />,
            Section 6. target="RFC8843" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-6" derivedContent="RFC8843"/>,
            <bcp14>MUST NOT</bcp14> be added.
            Instead, JSEP implementations MUST <bcp14>MUST</bcp14> simply omit
            parameters in the IDENTICAL and TRANSPORT categories for
            bundled m= "m=" sections, as described in
            <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation" />,
            Section 8.1.</t>

            <t>Note target="RFC8843" sectionFormat="comma" section="7.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-7.1.3" derivedContent="RFC8843"/>.</li>
            <li pn="section-5.2.2-9.10">Note that if media m= "m=" sections are bundled into a data
            m=
            "m=" section, then certain TRANSPORT and IDENTICAL attributes
            may appear in the data m= "m=" section even if they would
            otherwise only be appropriate for a media m= "m=" section (e.g.,
            "a=rtcp-mux"). This cannot happen in initial offers because
            in the initial offer JSEP implementations always list media
            m=
            "m=" sections (if any) before the data m= "m=" section (if any),
            and at least one of those media m= "m=" sections will not have
            the "a=bundle-only" attribute. Therefore, in initial
            offers, any "a=bundle-only" m= "m=" sections will be bundled
            into a preceding non-bundle-only media m= section.</t>
          </list></t>

          <t>The "m=" section.</li>
          </ul>
          <t indent="0" pn="section-5.2.2-10">The "a=group:BUNDLE" attribute MUST <bcp14>MUST</bcp14> include the MID
          identifiers specified in the bundle group in the most recent
          answer, minus any m= "m=" sections that have been marked as
          rejected, plus any newly added or re-enabled m= "m=" sections. In
          other words, the bundle attribute must <bcp14>MUST</bcp14> contain all m= "m="
          sections that were previously bundled, as long as they are
          still alive, as well as any new m= "m=" sections.</t>

          <t>"a=group:LS"
          <t indent="0" pn="section-5.2.2-11">"a=group:LS" attributes are generated in the same way as
          for initial offers, with the additional stipulation that any
          lip sync groups that were present in the most recent answer
          MUST
          <bcp14>MUST</bcp14> continue to exist and MUST <bcp14>MUST</bcp14> contain any previously
          existing MID identifiers, as long as the identified m= "m="
          sections still exist and are not rejected, and the group
          still contains at least two MID identifiers. This ensures
          that any synchronized "recvonly" m= "m=" sections continue to be
          synchronized in the new offer.</t>
        </section>
        <section title="Options Handling"
        anchor="sec.options-handling1">

          <t>The anchor="sec.options-handling1" numbered="true" toc="include" removeInRFC="false" pn="section-5.2.3">
          <name slugifiedName="name-options-handling">Options Handling</name>
          <t indent="0" pn="section-5.2.3-1">The createOffer method takes as a parameter an
          RTCOfferOptions object. Special processing is performed when
          generating a an SDP description if the following options are
          present.</t>
          <section title="IceRestart" anchor="sec.icerestart">

            <t>If anchor="sec.icerestart" numbered="true" toc="include" removeInRFC="false" pn="section-5.2.3.1">
            <name slugifiedName="name-icerestart">IceRestart</name>
            <t indent="0" pn="section-5.2.3.1-1">If the "IceRestart" IceRestart option is specified, with a value of
            "true", the offer MUST <bcp14>MUST</bcp14> indicate an ICE restart by
            generating new ICE ufrag and pwd attributes, as specified
            in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp"></xref>,
            Section 3.4.1.1.1. target="RFC8839" sectionFormat="comma" section="4.4.3.1.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-4.4.3.1.1" derivedContent="RFC8839"/>. If this
            option is specified on an initial offer, it has no effect
            (since a new ICE ufrag and pwd are already generated).
            Similarly, if the ICE configuration has changed, this
            option has no effect, since new ufrag and pwd attributes
            will be generated automatically. This option is primarily
            useful for reestablishing connectivity in cases where
            failures are detected by the application.</t>
          </section>
          <section title="VoiceActivityDetection"
          anchor="sec.voiceactivitydetection1">

            <t>Silence anchor="sec.voiceactivitydetection1" numbered="true" toc="include" removeInRFC="false" pn="section-5.2.3.2">
            <name slugifiedName="name-voiceactivitydetection">VoiceActivityDetection</name>
            <t indent="0" pn="section-5.2.3.2-1">Silence suppression, also known as discontinuous
            transmission ("DTX"), can reduce the bandwidth used for
            audio by switching to a special encoding when voice
            activity is not detected, at the cost of some fidelity.</t>

            <t>If
            <t indent="0" pn="section-5.2.3.2-2">If the "VoiceActivityDetection" option is specified,
            with a value of "true", the offer MUST <bcp14>MUST</bcp14> indicate support for
            silence suppression in the audio it receives by including
            comfort noise ("CN") codecs for each offered audio codec,
            as specified in
            <xref target="RFC3389"></xref>, Section 5.1, target="RFC3389" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3389#section-5.1" derivedContent="RFC3389"/>, except for
            codecs that have their own internal silence suppression
            support. For codecs that have their own internal silence
            suppression support, the appropriate fmtp parameters for
            that codec MUST <bcp14>MUST</bcp14> be specified to indicate that silence
            suppression for received audio is desired. For example,
            when using the Opus codec
            <xref target="RFC6716" />, format="default" sectionFormat="of" derivedContent="RFC6716"/>, the "usedtx=1" parameter,
            specified in
            <xref target="RFC7587" />, format="default" sectionFormat="of" derivedContent="RFC7587"/>, would be used in the offer.</t>

            <t>If
            <t indent="0" pn="section-5.2.3.2-3">If the "VoiceActivityDetection" option is specified,
            with a value of "false", the JSEP implementation MUST NOT <bcp14>MUST NOT</bcp14>
            emit "CN" codecs. For codecs that have their own internal
            silence suppression support, the appropriate fmtp
            parameters for that codec MUST <bcp14>MUST</bcp14> be specified to indicate
            that silence suppression for received audio is not desired.
            For example, when using the Opus codec, the "usedtx=0"
            parameter would be specified in the offer. In addition, the
            implementation MUST NOT <bcp14>MUST NOT</bcp14> use silence suppression for media
            it generates, regardless of whether the "CN" codecs or
            related fmtp parameters appear in the peer's description.
            The impact of these rules is that silence suppression in
            JSEP depends on mutual agreement of both sides, which
            ensures consistent handling regardless of which codec is
            used.</t>

            <t>The
            <t indent="0" pn="section-5.2.3.2-4">The "VoiceActivityDetection" option does not have any
            impact on the setting of the "vad" value in the signaling
            of the client to mixer client-to-mixer audio level header extension
            described in
            <xref target="RFC6464"></xref>, Section 4.</t> target="RFC6464" sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc6464#section-4" derivedContent="RFC6464"/>.</t>
          </section>
        </section>
      </section>
      <section title="Generating anchor="sec.generating-an-answer" numbered="true" toc="include" removeInRFC="false" pn="section-5.3">
        <name slugifiedName="name-generating-an-answer">Generating an Answer"
      anchor="sec.generating-an-answer">

        <t>When Answer</name>
        <t indent="0" pn="section-5.3-1">When createAnswer is called, a new SDP description must <bcp14>MUST</bcp14> be
        created that is compatible with the supplied remote description
        as well as the requirements specified in
        <xref target="I-D.ietf-rtcweb-rtp-usage"></xref>. target="RFC8834" format="default" sectionFormat="of" derivedContent="RFC8834"/>. The exact
        details of this process are explained below.</t>
        <section title="Initial Answers" anchor="sec.initial-answers">

          <t>When anchor="sec.initial-answers" numbered="true" toc="include" removeInRFC="false" pn="section-5.3.1">
          <name slugifiedName="name-initial-answers">Initial Answers</name>
          <t indent="0" pn="section-5.3.1-1">When createAnswer is called for the first time after a
          remote description has been provided, the result is known as
          the initial answer. If no remote description has been
          installed, an answer cannot be generated, and an error MUST <bcp14>MUST</bcp14>
          be returned.</t>

          <t>Note
          <t indent="0" pn="section-5.3.1-2">Note that the remote description SDP may not have been
          created by a JSEP endpoint and may not conform to all the
          requirements listed in
          <xref target="sec-create-offer"></xref>. target="sec-create-offer" format="default" sectionFormat="of" derivedContent="Section 5.2"/>. For many cases, this
          is not a problem. However, if any mandatory SDP attributes
          are missing, missing or functionality listed as mandatory-to-use
          above is not present, this MUST <bcp14>MUST</bcp14> be treated as an error, error and
          MUST
          <bcp14>MUST</bcp14> cause the affected m= "m=" sections to be marked as
          rejected.</t>

          <t>The
          <t indent="0" pn="section-5.3.1-3">The first step in generating an initial answer is to
          generate session-level attributes. The process here is
          identical to that indicated in the initial offers section <xref target="sec.initial-offers" format="default" sectionFormat="of" derivedContent="Section 5.2.1"/> above, except that the "a=ice-options" line, with the
          "trickle" option as specified in
          <xref target="I-D.ietf-ice-trickle"></xref>, Section 3, target="RFC8840" sectionFormat="comma" section="4.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8840#section-4.1.3" derivedContent="RFC8840"/>
          and the "ice2" option as specified in
          <xref target="RFC8445"></xref>, Section 10, target="RFC8445" sectionFormat="comma" section="10" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8445#section-10" derivedContent="RFC8445"/>, is
          only included if such an option was present in the offer.</t>

          <t>The
          <t indent="0" pn="section-5.3.1-4">The next step is to generate session-level lip sync
          groups, as defined in
          <xref target="RFC5888" />, Section 7. sectionFormat="comma" section="7" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5888#section-7" derivedContent="RFC5888"/>. For each group of type
          "LS" present in the offer, select the local RtpTransceivers
          that are referenced by the MID values in the specified group,
          and determine which of them either reference a common local
          MediaStream (specified in the calls to
          addTrack/addTransceiver used to create them), them) or have no
          MediaStream to reference because they were not created by
          addTrack/addTransceiver. If at least two such RtpTransceivers
          exist, a group of type "LS" with the mid MID values of these
          RtpTransceivers MUST <bcp14>MUST</bcp14> be added. Otherwise Otherwise, the offered "LS"
          group MUST <bcp14>MUST</bcp14> be ignored and no corresponding group generated in
          the answer.</t>

          <t>As
          <t indent="0" pn="section-5.3.1-5">As a simple example, consider the following offer of a
          single audio and single video track contained in the same
          MediaStream. SDP lines not relevant to this example have been
          removed for clarity. As explained in
          <xref target="sec-create-offer" />, format="default" sectionFormat="of" derivedContent="Section 5.2"/>, a group of type "LS" has
          been added that references each track's RtpTransceiver.</t>

          <t>
            <figure>
              <artwork>
                <![CDATA[
          <sourcecode name="" type="sdp" markers="false" pn="section-5.3.1-6">
          a=group:LS a1 v1
          m=audio 10000 UDP/TLS/RTP/SAVPF 0
          a=mid:a1
          a=msid:ms1
          m=video 10001 UDP/TLS/RTP/SAVPF 96
          a=mid:v1
          a=msid:ms1
                ]]>
</artwork>
            </figure>
          </t>

          <t>If
          a=msid:ms1</sourcecode>
          <t indent="0" pn="section-5.3.1-7">If the answerer uses a single MediaStream when it adds its
          tracks, both of its transceivers will reference this stream,
          and so the subsequent answer will contain a "LS" group
          identical to that in the offer, as shown below:</t>

          <t>
            <figure>
              <artwork>
                <![CDATA[
          <sourcecode name="" type="sdp" markers="false" pn="section-5.3.1-8">
          a=group:LS a1 v1
          m=audio 20000 UDP/TLS/RTP/SAVPF 0
          a=mid:a1
          a=msid:ms2
          m=video 20001 UDP/TLS/RTP/SAVPF 96
          a=mid:v1
          a=msid:ms2
                ]]>
</artwork>
            </figure>
          </t>

          <t>However,
          a=msid:ms2</sourcecode>
          <t indent="0" pn="section-5.3.1-9">However, if the answerer groups its tracks into separate
          MediaStreams, its transceivers will reference different
          streams, and so the subsequent answer will not contain a "LS"
          group.</t>

          <t>
            <figure>
              <artwork>
                <![CDATA[
          <sourcecode name="" type="sdp" markers="false" pn="section-5.3.1-10">
          m=audio 20000 UDP/TLS/RTP/SAVPF 0
          a=mid:a1
          a=msid:ms2a
          m=video 20001 UDP/TLS/RTP/SAVPF 96
          a=mid:v1
          a=msid:ms2b
                ]]>
</artwork>
            </figure>
          </t>

          <t>Finally,
          a=msid:ms2b</sourcecode>
          <t indent="0" pn="section-5.3.1-11">Finally, if the answerer does not add any tracks, its
          transceivers will not reference any MediaStreams, causing the
          preferences of the offerer to be maintained, and so the
          subsequent answer will contain an identical "LS" group.</t>

          <t>
            <figure>
              <artwork>
                <![CDATA[
          <sourcecode name="" type="sdp" markers="false" pn="section-5.3.1-12">
          a=group:LS a1 v1
          m=audio 20000 UDP/TLS/RTP/SAVPF 0
          a=mid:a1
          a=recvonly
          m=video 20001 UDP/TLS/RTP/SAVPF 96
          a=mid:v1
          a=recvonly
                ]]>
</artwork>
            </figure>
          </t>

          <t>The
          <xref target="sec.detailed-example" />
          a=recvonly</sourcecode>
          <t indent="0" pn="section-5.3.1-13">The example later in this
          document <xref target="sec.detailed-example" format="default" sectionFormat="of" derivedContent="Section 7.2"/> shows a more involved case of "LS" group
          generation.</t>

          <t>The
          <t indent="0" pn="section-5.3.1-14">The next step is to generate m= sections an "m=" section for each m= "m="
          section that is present in the remote offer, as specified in
          <xref target="RFC3264"></xref>, Section 6. target="RFC3264" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-6" derivedContent="RFC3264"/>. For the purposes
          of this discussion, any session-level attributes in the offer
          that are also valid as media-level attributes are considered
          to be present in each m= "m=" section. Each offered m= "m=" section
          will have an associated RtpTransceiver, as described in
          <xref target="sec.applying-a-remote-desc" />. format="default" sectionFormat="of" derivedContent="Section 5.10"/>. If there are
          more RtpTransceivers than there are m= "m=" sections, the
          unmatched RtpTransceivers will need to be associated in a
          subsequent offer.</t>

          <t>For
          <t indent="0" pn="section-5.3.1-15">For each offered m= "m=" section, if any of the following
          conditions are true, the corresponding m= "m=" section in the
          answer MUST <bcp14>MUST</bcp14> be marked as rejected by setting the port &lt;port&gt; in the
          m=
          "m=" line to zero, as indicated in
          <xref target="RFC3264"></xref>, Section 6, target="RFC3264" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-6" derivedContent="RFC3264"/>, and further
          processing for this m= "m=" section can be skipped:
          <list style="symbols">

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.3.1-16">
            <li pn="section-5.3.1-16.1">The associated RtpTransceiver has been stopped.</t>

            <t>None of the stopped.</li>
            <li pn="section-5.3.1-16.2">There is no offered media formats are format that is both supported
            and, if applicable, allowed by codec preferences.</t>

            <t>The preferences.</li>
            <li pn="section-5.3.1-16.3">The bundle policy is "max-bundle", and this is not the
            first m= "m=" section or in the same bundle group as the first
            m= section.</t>

            <t>The
            "m=" section.</li>
            <li pn="section-5.3.1-16.4">The bundle policy is "balanced", and this is not the
            first m= "m=" section for this media type or in the same bundle
            group as the first m= "m=" section for this media type.</t>

            <t>This m= type.</li>
            <li pn="section-5.3.1-16.5">This "m=" section is in a bundle group, and the group's
            offerer tagged m= "m=" section is being rejected due to one of
            the above reasons. This requires all m= "m=" sections in the
            bundle group to be rejected, as specified in
            <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation" />,
            Section 7.3.3.</t>
          </list></t>

          <t>Otherwise, target="RFC8843" sectionFormat="comma" section="7.3.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-7.3.3" derivedContent="RFC8843"/>.</li>
          </ul>
          <t indent="0" pn="section-5.3.1-17">Otherwise, each m= "m=" section in the answer should <bcp14>MUST</bcp14> then be
          generated as specified in
          <xref target="RFC3264"></xref>, Section 6.1. target="RFC3264" sectionFormat="comma" section="6.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-6.1" derivedContent="RFC3264"/>. For the m= "m=" line
          itself, the following rules must <bcp14>MUST</bcp14> be followed:
          <list style="symbols">

            <t>The port followed:</t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.3.1-18">
            <li pn="section-5.3.1-18.1">The &lt;port&gt; value would normally be set to the port of the
            default ICE candidate for this m= "m=" section, but given that
            no candidates are available yet, the "dummy" port default &lt;port&gt; value of
            9 (Discard) MUST <bcp14>MUST</bcp14> be used, as indicated in
            <xref target="I-D.ietf-ice-trickle"></xref>, Section
            5.1.</t>

            <t>The target="RFC8840" sectionFormat="comma" section="4.1.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8840#section-4.1.1" derivedContent="RFC8840"/>.</li>
            <li pn="section-5.3.1-18.2">The &lt;proto&gt; field MUST <bcp14>MUST</bcp14> be set to exactly match the
            &lt;proto&gt; field for the corresponding m= "m=" line in the
            offer.</t>

            <t>If
            offer.</li>
            <li pn="section-5.3.1-18.3">If codec preferences have been set for the associated
            transceiver, media formats MUST <bcp14>MUST</bcp14> be generated in the
            corresponding order, regardless of what was offered, and
            MUST
            <bcp14>MUST</bcp14> exclude any codecs not present in the codec
            preferences.</t>

            <t>Otherwise,
            preferences.</li>
            <li pn="section-5.3.1-18.4">Otherwise, the media formats on the m= "m=" line MUST <bcp14>MUST</bcp14> be
            generated in the same order as those offered in the current
            remote description, excluding any currently unsupported
            formats. Any currently available media formats that are not
            present in the current remote description MUST <bcp14>MUST</bcp14> be added
            after all existing formats.</t>

            <t>In formats.</li>
            <li pn="section-5.3.1-18.5">In either case, the media formats in the answer MUST <bcp14>MUST</bcp14>
            include at least one format that is present in the offer, offer
            but MAY <bcp14>MAY</bcp14> include formats that are locally supported but not
            present in the offer, as mentioned in
            <xref target="RFC3264" />, Section 6.1. sectionFormat="comma" section="6.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-6.1" derivedContent="RFC3264"/>. If no common format
            exists, the m= "m=" section is rejected as described above.</t>
          </list></t>

          <t>The m= above.</li>
          </ul>
          <t indent="0" pn="section-5.3.1-19">The "m=" line MUST <bcp14>MUST</bcp14> be followed immediately by a "c=" line,
          as specified in
          <xref target="RFC4566"></xref>, Section 5.7. target="RFC4566" sectionFormat="comma" section="5.7" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.7" derivedContent="RFC4566"/>. Again, as no
          candidates are available yet, the "c=" line must <bcp14>MUST</bcp14> contain the
          "dummy"
          default value "IN IP4 0.0.0.0", as defined in
          <xref target="I-D.ietf-ice-trickle"></xref>, Section 5.1.</t>

          <t>If target="RFC8840" sectionFormat="comma" section="4.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8840#section-4.1.3" derivedContent="RFC8840"/>.</t>
          <t indent="0" pn="section-5.3.1-20">If the offer supports bundle, all m= "m=" sections to be
          bundled must <bcp14>MUST</bcp14> use the same ICE credentials and candidates; all
          m=
          "m=" sections not being bundled must <bcp14>MUST</bcp14> use unique ICE credentials
          and candidates. Each m= "m=" section MUST <bcp14>MUST</bcp14> contain the following
          attributes (which are of attribute types other than IDENTICAL
          and
          or TRANSPORT):
          <list style="symbols">

            <t>If
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.3.1-21">
            <li pn="section-5.3.1-21.1">If and only if present in the offer, an "a=mid" line, as
            specified in
            <xref target="RFC5888"></xref>, Section 9.1. target="RFC5888" sectionFormat="comma" section="9.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5888#section-9.1" derivedContent="RFC5888"/>. The "mid" MID
            value MUST <bcp14>MUST</bcp14> match that specified in the offer.</t>

            <t>A offer.</li>
            <li pn="section-5.3.1-21.2">A direction attribute, determined by applying the rules
            regarding the offered direction specified in
            <xref target="RFC3264" />, Section 6.1, sectionFormat="comma" section="6.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-6.1" derivedContent="RFC3264"/>, and then
            intersecting with the direction of the associated
            RtpTransceiver. For example, in the case where an m= "m="
            section is offered as "sendonly", "sendonly" and the local transceiver
            is set to "sendrecv", the result in the answer is a
            "recvonly" direction.</t>

            <t>For direction.</li>
            <li pn="section-5.3.1-21.3">For each media format on the m= "m=" line, "a=rtpmap" and "a=fmtp" lines, as specified in
            <xref target="RFC4566"></xref>, Section 6, target="RFC4566" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/> and
            <xref target="RFC3264"></xref>, Section 6.1.</t>

            <t>If target="RFC3264" sectionFormat="comma" section="6.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-6.1" derivedContent="RFC3264"/>.</li>
            <li pn="section-5.3.1-21.4">If "rtx" is present in the offer, for each primary codec
            where RTP retransmission should be used, a corresponding
            "a=rtpmap" line indicating "rtx" with the clock rate of the
            primary codec and an "a=fmtp" line that references the
            payload type of the primary codec, as specified in
            <xref target="RFC4588"></xref>, Section 8.1.</t>

            <t>For target="RFC4588" sectionFormat="comma" section="8.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4588#section-8.1" derivedContent="RFC4588"/>.</li>
            <li pn="section-5.3.1-21.5">For each supported FEC mechanism, "a=rtpmap" and
            "a=fmtp" lines, as specified in
            <xref target="RFC4566"></xref>, Section 6. target="RFC4566" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/>. The FEC
            mechanisms that MUST <bcp14>MUST</bcp14> be supported are specified in
            <xref target="I-D.ietf-rtcweb-fec"></xref>, Section 6, target="RFC8854" sectionFormat="comma" section="7" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8854#section-7" derivedContent="RFC8854"/>, and
            specific usage for each media type is outlined in Sections
            4
            <xref target="sec.interface" format="counter" sectionFormat="of" derivedContent="4"/> and 5.</t>

            <t>If <xref target="sec.sdp-interaction-procedure" format="counter" sectionFormat="of" derivedContent="5"/>.</li>
            <li pn="section-5.3.1-21.6">If this m= "m=" section is for media with configurable
            durations of media per packet, e.g., audio, an "a=maxptime"
            line, as described in
            <xref target="sec-create-offer" />.</t>

            <t>If format="default" sectionFormat="of" derivedContent="Section 5.2"/>.</li>
            <li pn="section-5.3.1-21.7">If this m= "m=" section is for video media, media and there are
            known limitations on the size of images which that can be
            decoded, an "a=imageattr" line, as specified in
            <xref target="sec.imageattr"></xref>.</t>

            <t>For target="sec.imageattr" format="default" sectionFormat="of" derivedContent="Section 3.6"/>.</li>
            <li pn="section-5.3.1-21.8">For each supported RTP header extension that is present
            in the offer, an "a=extmap" line, as specified in
            <xref target="RFC5285"></xref>, Section 5. target="RFC5285" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5285#section-5" derivedContent="RFC5285"/>. The list of
            header extensions that SHOULD/MUST <bcp14>SHOULD</bcp14>/<bcp14>MUST</bcp14> be supported is
            specified in
            <xref target="I-D.ietf-rtcweb-rtp-usage"></xref>, Section
            5.2. target="RFC8834" sectionFormat="comma" section="5.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8834#section-5.2" derivedContent="RFC8834"/>. Any header extensions that require encryption MUST <bcp14>MUST</bcp14> be
            specified as indicated in
            <xref target="RFC6904"></xref>, Section 4.</t>

            <t>For target="RFC6904" sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc6904#section-4" derivedContent="RFC6904"/>.</li>
            <li pn="section-5.3.1-21.9">For each supported RTCP feedback mechanism that is
            present in the offer, an "a=rtcp-fb" line, as specified in
            <xref target="RFC4585"></xref>, Section 4.2. target="RFC4585" sectionFormat="comma" section="4.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4585#section-4.2" derivedContent="RFC4585"/>. The list of
            RTCP feedback mechanisms that SHOULD/MUST <bcp14>SHOULD</bcp14>/<bcp14>MUST</bcp14> be supported is
            specified in
            <xref target="I-D.ietf-rtcweb-rtp-usage"></xref>, Section
            5.1.</t>

            <t>If target="RFC8834" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8834#section-5.1" derivedContent="RFC8834"/>.</li>
            <li pn="section-5.3.1-21.10">
              <t indent="0" pn="section-5.3.1-21.10.1">If the RtpTransceiver has a sendrecv or sendonly
            direction:
            <list style="symbols">

              <t>For
              </t>
              <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.3.1-21.10.2">
                <li pn="section-5.3.1-21.10.2.1">For each MediaStream that was associated with the
              transceiver when it was created via addTrack or
              addTransceiver, an "a=msid" line, as specified in
              <xref target="I-D.ietf-mmusic-msid"></xref>, Section 2, target="RFC8830" sectionFormat="comma" section="2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8830#section-2" derivedContent="RFC8830"/>,
              but omitting the "appdata" field.</t>
            </list></t>
          </list></t>

          <t>Each m= field.</li>
              </ul>
            </li>
          </ul>
          <t indent="0" pn="section-5.3.1-22">Each "m=" section which that is not bundled into another m=
          section, MUST "m="
          section <bcp14>MUST</bcp14> contain the following attributes (which are of
          category IDENTICAL or TRANSPORT):</t>

          <t>
            <list style="symbols">

              <t>"a=ice-ufrag"
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.3.1-23">
            <li pn="section-5.3.1-23.1">"a=ice-ufrag" and "a=ice-pwd" lines, as specified in
              <xref target="I-D.ietf-mmusic-ice-sip-sdp"></xref>,
              Section 4.4.</t>

              <t>For target="RFC8839" sectionFormat="comma" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.4" derivedContent="RFC8839"/>.</li>
            <li pn="section-5.3.1-23.2">For each desired digest algorithm, one or more
              "a=fingerprint" lines for each of the endpoint's
              certificates, as specified in
              <xref target="RFC8122"></xref>, Section 5.</t>

              <t>An target="RFC8122" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8122#section-5" derivedContent="RFC8122"/>.</li>
            <li pn="section-5.3.1-23.3">An "a=setup" line, as specified in
              <xref target="RFC4145"></xref>, Section 4, target="RFC4145" sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4145#section-4" derivedContent="RFC4145"/> and clarified
              for use in DTLS-SRTP scenarios in
              <xref target="RFC5763"></xref>, Section 5. target="RFC5763" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5763#section-5" derivedContent="RFC5763"/>. The role value
              in the answer MUST <bcp14>MUST</bcp14> be "active" or "passive". When the
              offer contains the "actpass" value, as will always be the
              case with JSEP endpoints, the answerer SHOULD <bcp14>SHOULD</bcp14> use the
              "active" role. Offers from non-JSEP endpoints MAY <bcp14>MAY</bcp14> send
              other values for "a=setup", in which case the answer MUST <bcp14>MUST</bcp14>
              use a value consistent with the value in the offer.</t>

              <t>An offer.</li>
            <li pn="section-5.3.1-23.4">An "a=tls-id" line, as specified in
              <xref target="I-D.ietf-mmusic-dtls-sdp" />, Section
              5.3.</t>

              <t>If target="RFC8842" sectionFormat="comma" section="5.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8842#section-5.3" derivedContent="RFC8842"/>.</li>
            <li pn="section-5.3.1-23.5">If present in the offer, an "a=rtcp-mux" line, as
              specified in
              <xref target="RFC5761"></xref>, Section 5.1.3. target="RFC5761" sectionFormat="comma" section="5.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5761#section-5.1.3" derivedContent="RFC5761"/>. Otherwise,
              an "a=rtcp" line, as specified in
              <xref target="RFC3605"></xref>, Section 2.1, target="RFC3605" sectionFormat="comma" section="2.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3605#section-2.1" derivedContent="RFC3605"/>, containing
              the dummy default value "9 IN IP4 0.0.0.0" (because no candidates
              have yet been gathered).</t>

              <t>If gathered).</li>
            <li pn="section-5.3.1-23.6">If present in the offer, an "a=rtcp-rsize" line, as
              specified in
              <xref target="RFC5506"></xref>, Section 5.</t>
            </list>
          </t>

          <t>If target="RFC5506" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5506#section-5" derivedContent="RFC5506"/>.</li>
          </ul>
          <t indent="0" pn="section-5.3.1-24">If a data channel m= "m=" section has been offered, a m= an "m="
          section MUST <bcp14>MUST</bcp14> also be generated for data. The &lt;media&gt;
          field MUST <bcp14>MUST</bcp14> be set to "application" "application", and the &lt;proto&gt; and
          &lt;fmt&gt; fields MUST <bcp14>MUST</bcp14> be set to exactly match the fields in
          the offer.</t>

          <t>Within
          <t indent="0" pn="section-5.3.1-25">Within the data m= "m=" section, an "a=mid" line MUST <bcp14>MUST</bcp14> be
          generated and included as described above, along with an
          "a=sctp-port" line referencing the SCTP port number, as
          defined in
          <xref target="I-D.ietf-mmusic-sctp-sdp"></xref>, Section 5.1, target="RFC8841" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8841#section-5.1" derivedContent="RFC8841"/>;
          and, if appropriate, an "a=max-message-size" line, as defined
          in
          <xref target="I-D.ietf-mmusic-sctp-sdp"></xref>, Section
          6.1.</t>

          <t>As target="RFC8841" sectionFormat="comma" section="6.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8841#section-6.1" derivedContent="RFC8841"/>.</t>
          <t indent="0" pn="section-5.3.1-26">As discussed above, the following attributes of category
          IDENTICAL or TRANSPORT are included only if the data m= "m="
          section is not bundled into another m= "m=" section:
          <list style="symbols">

            <t>"a=ice-ufrag"</t>

            <t>"a=ice-pwd"</t>

            <t>"a=fingerprint"</t>

            <t>"a=setup"</t>

            <t>"a=tls-id"</t>
          </list></t>

          <t>Note
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.3.1-27">
            <li pn="section-5.3.1-27.1">"a=ice-ufrag"</li>
            <li pn="section-5.3.1-27.2">"a=ice-pwd"</li>
            <li pn="section-5.3.1-27.3">"a=fingerprint"</li>
            <li pn="section-5.3.1-27.4">"a=setup"</li>
            <li pn="section-5.3.1-27.5">"a=tls-id"</li>
          </ul>
          <t indent="0" pn="section-5.3.1-28">Note that if media m= "m=" sections are bundled into a data m= "m="
          section, then certain TRANSPORT and IDENTICAL attributes may
          also appear in the data m= "m=" section even if they would
          otherwise only be appropriate for a media m= "m=" section (e.g.,
          "a=rtcp-mux").</t>

          <t>If
          <t indent="0" pn="section-5.3.1-29">If "a=group" attributes with semantics of "BUNDLE" are
          offered, corresponding session-level "a=group" attributes
          MUST
          <bcp14>MUST</bcp14> be added as specified in
          <xref target="RFC5888"></xref>. target="RFC5888" format="default" sectionFormat="of" derivedContent="RFC5888"/>. These attributes MUST <bcp14>MUST</bcp14> have
          semantics "BUNDLE", "BUNDLE" and MUST <bcp14>MUST</bcp14> include the all mid identifiers
          from the offered bundle groups that have not been rejected.
          Note that regardless of the presence of "a=bundle-only" in
          the offer, no m= all "m=" sections in the answer should <bcp14>MUST NOT</bcp14> have an
          "a=bundle-only" line.</t>

          <t>Attributes
          <t indent="0" pn="section-5.3.1-30">Attributes that are common between all m= "m=" sections MAY <bcp14>MAY</bcp14> be
          moved to session-level, the session level if explicitly defined to be valid at
          session-level.</t>

          <t>The
          the session level.</t>
          <t indent="0" pn="section-5.3.1-31">The attributes prohibited in the creation of offers are
          also prohibited in the creation of answers.</t>
        </section>
        <section title="Subsequent Answers"
        anchor="sec.subsequent-answers">

          <t>When anchor="sec.subsequent-answers" numbered="true" toc="include" removeInRFC="false" pn="section-5.3.2">
          <name slugifiedName="name-subsequent-answers">Subsequent Answers</name>
          <t indent="0" pn="section-5.3.2-1">When createAnswer is called a second (or later) time, time or
          is called after a local description has already been
          installed, the processing is somewhat different than for an
          initial answer.</t>

          <t>If
          <t indent="0" pn="section-5.3.2-2">If the previous answer was not applied using
          setLocalDescription, meaning the PeerConnection is still in
          the "have-remote-offer" state, the steps for generating an
          initial answer should <bcp14>MUST</bcp14> be followed, subject to the following
          restriction:
          <list style="symbols">

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.3.2-3">
            <li pn="section-5.3.2-3.1">The fields of the "o=" line MUST <bcp14>MUST</bcp14> stay the same except
            for the &lt;session-version&gt; field, which MUST <bcp14>MUST</bcp14> increment
            if the session description changes in any way from the
            previously generated answer.</t>
          </list></t>

          <t>If answer.</li>
          </ul>
          <t indent="0" pn="section-5.3.2-4">If any session description was previously supplied to
          setLocalDescription, an answer is generated by following the
          steps in the "have-remote-offer" state above, along with
          these exceptions:
          <list style="symbols">

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.3.2-5">
            <li pn="section-5.3.2-5.1">The "s=" and "t=" lines MUST <bcp14>MUST</bcp14> stay the same.</t>

            <t>Each same.</li>
            <li pn="section-5.3.2-5.2">Each "m=" and c=" "c=" line MUST <bcp14>MUST</bcp14> be filled in with the port
            and address of the default candidate for the m= "m=" section, as
            described in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp"></xref>,
            Section 3.2.1.2. target="RFC8839" sectionFormat="comma" section="4.2.1.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-4.2.1.2" derivedContent="RFC8839"/>. Note that in certain cases, the m= "m=" line protocol
            may not match that of the default candidate, because the m= "m=" line
            protocol value MUST <bcp14>MUST</bcp14> match what was supplied in the offer, as
            described above.</t>

            <t>Each above.</li>
            <li pn="section-5.3.2-5.3">Each "a=ice-ufrag" and "a=ice-pwd" line MUST <bcp14>MUST</bcp14> stay the
            same, unless the m= "m=" section is restarting, in which case
            new ICE credentials must <bcp14>MUST</bcp14> be created as specified in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp"></xref>,
            Section 3.4.1.1.1. target="RFC8839" sectionFormat="comma" section="4.4.1.1.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-4.4.1.1.1" derivedContent="RFC8839"/>. If the m= "m="
            section is bundled into another m= "m=" section, it still MUST
            NOT <bcp14>MUST NOT</bcp14> contain any ICE credentials.</t>

            <t>Each credentials.</li>
            <li pn="section-5.3.2-5.4">Each "a=tls-id" line MUST <bcp14>MUST</bcp14> stay the same same, unless the
            offerer's "a=tls-id" line changed, in which case a new
            "a=tls-id"
            tls-id value MUST <bcp14>MUST</bcp14> be created, as described in
            <xref target="I-D.ietf-mmusic-dtls-sdp" />, Section
            5.2.</t>

            <t>Each target="RFC8842" sectionFormat="comma" section="5.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8842#section-5.2" derivedContent="RFC8842"/>.</li>
            <li pn="section-5.3.2-5.5">Each "a=setup" line MUST <bcp14>MUST</bcp14> use an "active" or "passive"
            role value consistent with the existing DTLS association,
            if the association is being continued by the offerer.</t>

            <t>RTCP offerer.</li>
            <li pn="section-5.3.2-5.6">RTCP multiplexing must <bcp14>MUST</bcp14> be used, and an "a=rtcp-mux" line
            inserted if and only if the m= "m=" section previously used RTCP
            multiplexing.</t>

            <t>If
            multiplexing.</li>
            <li pn="section-5.3.2-5.7">If the m= "m=" section is not bundled into another m= "m=" section
            and RTCP multiplexing is not active, an "a=rtcp" attribute
            line MUST <bcp14>MUST</bcp14> be filled in with the port and address of the
            default RTCP candidate. If no RTCP candidates have yet been
            gathered, dummy default values MUST <bcp14>MUST</bcp14> be used, as described in
            <xref target="sec.initial-answers" format="default" sectionFormat="of" derivedContent="Section 5.3.1"/> above.</li>
            <li pn="section-5.3.2-5.8">If the
            initial answer section above.</t>

            <t>If the m= "m=" section is not bundled into another m= "m="
            section, for each candidate that has been gathered during
            the most recent gathering phase (see
            <xref target="sec.ice-gather-overview"></xref>), target="sec.ice-gather-overview" format="default" sectionFormat="of" derivedContent="Section 3.5.1"/>), an
            "a=candidate" line MUST <bcp14>MUST</bcp14> be added, as defined in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp"></xref>,
            Section 4.1. target="RFC8839" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.1" derivedContent="RFC8839"/>.
            If candidate gathering for the section has completed, an
            "a=end-of-candidates" attribute MUST <bcp14>MUST</bcp14> be added, as described
            in
            <xref target="I-D.ietf-ice-trickle"></xref>, Section 9.3. target="RFC8840" sectionFormat="comma" section="8.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8840#section-8.2" derivedContent="RFC8840"/>.
            If the m= "m=" section is bundled into another m= "m=" section, both
            "a=candidate" and "a=end-of-candidates" MUST <bcp14>MUST</bcp14> be
            omitted.</t>

            <t>For
            omitted.
</li>
            <li pn="section-5.3.2-5.9">For RtpTransceivers that are not stopped, the "a=msid"
            line(s) MUST <bcp14>MUST</bcp14> stay the same, regardless of changes to the
            transceiver's direction or track. If no "a=msid" line is
            present in the current description, "a=msid" line(s) MUST <bcp14>MUST</bcp14>
            be generated according to the same rules as for an initial
            answer.</t>
          </list></t>
            answer.</li>
          </ul>
        </section>
        <section title="Options Handling"
        anchor="sec.options-handling2">

          <t>The anchor="sec.options-handling2" numbered="true" toc="include" removeInRFC="false" pn="section-5.3.3">
          <name slugifiedName="name-options-handling-2">Options Handling</name>
          <t indent="0" pn="section-5.3.3-1">The createAnswer method takes as a parameter an
          RTCAnswerOptions object. The set of parameters for
          RTCAnswerOptions is different than those supported in
          RTCOfferOptions; the IceRestart option is unnecessary, as ICE
          credentials will automatically be changed for all m= "m=" sections
          where the offerer chose to perform ICE restart.</t>

          <t>The
          <t indent="0" pn="section-5.3.3-2">The following options are supported in
          RTCAnswerOptions.</t>
          <section title="VoiceActivityDetection"
          anchor="sec.voiceactivitydetection2">

            <t>Silence anchor="sec.voiceactivitydetection2" numbered="true" toc="include" removeInRFC="false" pn="section-5.3.3.1">
            <name slugifiedName="name-voiceactivitydetection-2">VoiceActivityDetection</name>
            <t indent="0" pn="section-5.3.3.1-1">Silence suppression in the answer is handled as
            described in
            <xref target="sec.voiceactivitydetection1"></xref>, target="sec.voiceactivitydetection1" format="default" sectionFormat="of" derivedContent="Section 5.2.3.2"/>, with
            one exception: if support for silence suppression was not
            indicated in the offer, the VoiceActivityDetection
            parameter has no effect, and the answer should <bcp14>MUST</bcp14> be generated
            as if VoiceActivityDetection was set to false. "false". This is done
            on a per-codec basis (e.g., if the offerer somehow offered
            support for CN but set "usedtx=0" for Opus, setting
            VoiceActivityDetection to true "true" would result in an answer
            with CN codecs and "usedtx=0"). The impact of this rule is
            that an answerer will not try to use silence suppression
            with any endpoint that does not offer it, making silence
            suppression support bilateral even with non-JSEP
            endpoints.</t>
          </section>
        </section>
      </section>
      <section title="Modifying anchor="sec.modifying-sdp" numbered="true" toc="include" removeInRFC="false" pn="section-5.4">
        <name slugifiedName="name-modifying-an-offer-or-answe">Modifying an Offer or Answer"
      anchor="sec.modifying-sdp">

        <t>The Answer</name>
        <t indent="0" pn="section-5.4-1">The SDP returned from createOffer or createAnswer MUST NOT <bcp14>MUST NOT</bcp14>
        be changed before passing it to setLocalDescription. If precise
        control over the SDP is needed, the aforementioned
        createOffer/createAnswer options or RtpTransceiver APIs MUST <bcp14>MUST</bcp14> be
        used.</t>

        <t>After
        <t indent="0" pn="section-5.4-2">After calling setLocalDescription with an offer or answer,
        the application MAY <bcp14>MAY</bcp14> modify the SDP to reduce its capabilities
        before sending it to the far side, as long as it follows the
        rules above that define a valid JSEP offer or answer. Likewise,
        an application that has received an offer or answer from a peer
        MAY
        <bcp14>MAY</bcp14> modify the received SDP, subject to the same constraints,
        before calling setRemoteDescription.</t>

        <t>As
        <t indent="0" pn="section-5.4-3">As always, the application is solely responsible for what it
        sends to the other party, and all incoming SDP will be
        processed by the JSEP implementation to the extent of its
        capabilities. It is an error to assume that all SDP is
        well-formed;
        well formed; however, one should be able to assume that any
        implementation of this specification will be able to process,
        as a remote offer or answer, unmodified SDP coming from any
        other implementation of this specification.</t>
      </section>
      <section title="Processing anchor="sec.processing-a-local-desc" numbered="true" toc="include" removeInRFC="false" pn="section-5.5">
        <name slugifiedName="name-processing-a-local-descript">Processing a Local Description"
      anchor="sec.processing-a-local-desc">

        <t>When Description</name>
        <t indent="0" pn="section-5.5-1">When a SessionDescription is supplied to
        setLocalDescription, the following steps MUST <bcp14>MUST</bcp14> be performed:
        <list style="symbols">

          <t>If
        </t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.5-2">
          <li pn="section-5.5-2.1">If the description is of type "rollback", follow the
          processing defined in
          <xref target="sec.processing-a-rollback" /> format="default" sectionFormat="of" derivedContent="Section 5.7"/> and skip the
          processing described in the rest of this section.</t>

          <t>Otherwise, section.</li>
          <li pn="section-5.5-2.2">
            <t indent="0" pn="section-5.5-2.2.1">Otherwise, the type of the SessionDescription is checked
          against the current state of the PeerConnection:
          <list style="symbols">

            <t>If
            </t>
            <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.5-2.2.2">
              <li pn="section-5.5-2.2.2.1">If the type is "offer", the PeerConnection state MUST <bcp14>MUST</bcp14> be
            either "stable" or "have-local-offer".</t>

            <t>If "have-local-offer".</li>
              <li pn="section-5.5-2.2.2.2">If the type is "pranswer" or "answer", the
            PeerConnection state MUST <bcp14>MUST</bcp14> be either "have-remote-offer" or
            "have-local-pranswer".</t>
          </list></t>

          <t>If
            "have-local-pranswer".</li>
            </ul>
          </li>
          <li pn="section-5.5-2.3">If the type is not correct for the current state,
          processing MUST <bcp14>MUST</bcp14> stop and an error MUST <bcp14>MUST</bcp14> be returned.</t>

          <t>The returned.</li>
          <li pn="section-5.5-2.4">The SessionDescription is then checked to ensure that its
          contents are identical to those generated in the last call to
          createOffer/createAnswer, and thus have not been altered, as
          discussed in
          <xref target="sec.modifying-sdp" />; format="default" sectionFormat="of" derivedContent="Section 5.4"/>; otherwise, processing
          MUST
          <bcp14>MUST</bcp14> stop and an error MUST <bcp14>MUST</bcp14> be returned.</t>

          <t>Next, returned.</li>
          <li pn="section-5.5-2.5">Next, the SessionDescription is parsed into a data
          structure, as described in
          <xref target="sec.parsing-a-desc" /> below.</t>

          <t>Finally, format="default" sectionFormat="of" derivedContent="Section 5.8"/> below.</li>
          <li pn="section-5.5-2.6">Finally, the parsed SessionDescription is applied as
          described in
          <xref target="sec.applying-a-local-desc" /> below.</t>
        </list></t> format="default" sectionFormat="of" derivedContent="Section 5.9"/> below.</li>
        </ul>
      </section>
      <section title="Processing anchor="sec.processing-a-remote-desc" numbered="true" toc="include" removeInRFC="false" pn="section-5.6">
        <name slugifiedName="name-processing-a-remote-descrip">Processing a Remote Description"
      anchor="sec.processing-a-remote-desc">

        <t>When Description</name>
        <t indent="0" pn="section-5.6-1">When a SessionDescription is supplied to
        setRemoteDescription, the following steps MUST <bcp14>MUST</bcp14> be performed:
        <list style="symbols">

          <t>If
        </t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.6-2">
          <li pn="section-5.6-2.1">If the description is of type "rollback", follow the
          processing defined in
          <xref target="sec.processing-a-rollback" /> format="default" sectionFormat="of" derivedContent="Section 5.7"/> and skip the
          processing described in the rest of this section.</t>

          <t>Otherwise, section.</li>
          <li pn="section-5.6-2.2">
            <t indent="0" pn="section-5.6-2.2.1">Otherwise, the type of the SessionDescription is checked
          against the current state of the PeerConnection:
          <list style="symbols">

            <t>If
            </t>
            <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.6-2.2.2">
              <li pn="section-5.6-2.2.2.1">If the type is "offer", the PeerConnection state MUST <bcp14>MUST</bcp14> be
            either "stable" or "have-remote-offer".</t>

            <t>If "have-remote-offer".</li>
              <li pn="section-5.6-2.2.2.2">If the type is "pranswer" or "answer", the
            PeerConnection state MUST <bcp14>MUST</bcp14> be either "have-local-offer" or
            "have-remote-pranswer".</t>
          </list></t>

          <t>If
            "have-remote-pranswer".</li>
            </ul>
          </li>
          <li pn="section-5.6-2.3">If the type is not correct for the current state,
          processing MUST <bcp14>MUST</bcp14> stop and an error MUST <bcp14>MUST</bcp14> be returned.</t>

          <t>Next, returned.</li>
          <li pn="section-5.6-2.4">Next, the SessionDescription is parsed into a data
          structure, as described in
          <xref target="sec.parsing-a-desc" /> format="default" sectionFormat="of" derivedContent="Section 5.8"/> below. If parsing fails
          for any reason, processing MUST <bcp14>MUST</bcp14> stop and an error MUST <bcp14>MUST</bcp14> be
          returned.</t>

          <t>Finally,
          returned.</li>
          <li pn="section-5.6-2.5">Finally, the parsed SessionDescription is applied as
          described in
          <xref target="sec.applying-a-remote-desc" /> below.</t>
        </list></t>
      </section>
      <section title="Processing format="default" sectionFormat="of" derivedContent="Section 5.10"/> below.</li>
        </ul>
      </section>
      <section anchor="sec.processing-a-rollback" numbered="true" toc="include" removeInRFC="false" pn="section-5.7">
        <name slugifiedName="name-processing-a-rollback">Processing a Rollback"
      anchor="sec.processing-a-rollback">

        <t>A Rollback</name>
        <t indent="0" pn="section-5.7-1">A rollback may be performed if the PeerConnection is in any
        state except for "stable". This means that both offers and
        provisional answers can be rolled back. Rollback can only be
        used to cancel proposed changes; there is no support for
        rolling back from a stable "stable" state to a previous stable "stable" state. If
        a rollback is attempted in the "stable" state, processing MUST <bcp14>MUST</bcp14>
        stop and an error MUST <bcp14>MUST</bcp14> be returned. Note that this implies that
        once the answerer has performed setLocalDescription with his its
        answer, this cannot be rolled back.</t>

        <t>The
        <t indent="0" pn="section-5.7-2">The effect of rollback MUST <bcp14>MUST</bcp14> be the same regardless of
        whether setLocalDescription or setRemoteDescription is
        called.</t>

        <t>In
        <t indent="0" pn="section-5.7-3">In order to process rollback, a JSEP implementation abandons
        the current offer/answer transaction, sets the signaling state
        to "stable", and sets the pending local and/or remote
        description (see Sections
        <xref target="sec.pendinglocaldescription" /> format="counter" sectionFormat="of" derivedContent="4.1.14"/> and
        <xref target="sec.pendingremotedescription" />) format="counter" sectionFormat="of" derivedContent="4.1.16"/>) to null. "null". Any
        resources or candidates that were allocated by the abandoned
        local description are discarded; any media that is received is
        processed according to the previous local and remote
        descriptions.</t>

        <t>A
        <t indent="0" pn="section-5.7-4">A rollback disassociates any RtpTransceivers that were
        associated with m= "m=" sections by the application of the
        rolled-back session description (see Sections
        <xref target="sec.applying-a-remote-desc" /> format="counter" sectionFormat="of" derivedContent="5.10"/> and
        <xref target="sec.applying-a-local-desc" />). format="counter" sectionFormat="of" derivedContent="5.9"/>).
        This means that
        some RtpTransceivers that were previously associated will no
        longer be associated with any m= "m=" section; in such cases, the
        value of the RtpTransceiver's mid property MUST <bcp14>MUST</bcp14> be set to null, "null",
        and the mapping between the transceiver and its m= "m=" section
        index MUST <bcp14>MUST</bcp14> be discarded. RtpTransceivers that were created by
        applying a remote offer that was subsequently rolled back MUST <bcp14>MUST</bcp14>
        be stopped and removed from the PeerConnection. However, a an
        RtpTransceiver MUST NOT <bcp14>MUST NOT</bcp14> be removed if a track was attached to
        the RtpTransceiver via the addTrack method. This is so that an
        application may call addTrack, then call setRemoteDescription
        with an offer, then roll back that offer, then call createOffer
        and have a m= an "m=" section for the added track appear in the
        generated offer.</t>
      </section>
      <section title="Parsing anchor="sec.parsing-a-desc" numbered="true" toc="include" removeInRFC="false" pn="section-5.8">
        <name slugifiedName="name-parsing-a-session-descripti">Parsing a Session Description"
      anchor="sec.parsing-a-desc">

        <t>The Description</name>
        <t indent="0" pn="section-5.8-1">The SDP contained in the session description object consists
        of a sequence of text lines, each containing a key-value
        expression, as described in
        <xref target="RFC4566" />, Section 5. sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5" derivedContent="RFC4566"/>. The SDP is read,
        line-by-line,
        line by line, and converted to a data structure that contains
        the deserialized information. However, SDP allows many types of
        lines, not all of which are relevant to JSEP applications. For
        each line, the implementation will first ensure that it is
        syntactically correct according to its defining ABNF, check
        that it conforms to the semantics used in
        <xref target="RFC4566" /> format="default" sectionFormat="of" derivedContent="RFC4566"/> and
        <xref target="RFC3264" /> semantics, format="default" sectionFormat="of" derivedContent="RFC3264"/>, and then either parse and
        store or discard the provided value, as described below.</t>

        <t>If
        <t indent="0" pn="section-5.8-2">If any line is not well-formed, well formed or cannot be parsed as
        described, the parser MUST <bcp14>MUST</bcp14> stop with an error and reject the
        session description, even if the value is to be discarded. This
        ensures that implementations do not accidentally misinterpret
        ambiguous SDP.</t>
        <section title="Session-Level Parsing"
        anchor="sec.session-level-parse">

          <t>First, anchor="sec.session-level-parse" numbered="true" toc="include" removeInRFC="false" pn="section-5.8.1">
          <name slugifiedName="name-session-level-parsing">Session-Level Parsing</name>
          <t indent="0" pn="section-5.8.1-1">First, the session-level lines are checked and parsed.
          These lines MUST <bcp14>MUST</bcp14> occur in a specific order, and with a
          specific syntax, as defined in
          <xref target="RFC4566" />, Section 5. sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5" derivedContent="RFC4566"/>. Note that while the
          specific line types (e.g. (e.g., "v=", "c=") MUST <bcp14>MUST</bcp14> occur in the
          defined order, lines of the same type (typically "a=") can
          occur in any order.</t>

          <t>The
          <t indent="0" pn="section-5.8.1-2">The following non-attribute lines are not meaningful in
          the JSEP context and MAY <bcp14>MAY</bcp14> be discarded once they have been
          checked.
          <list>

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8.1-3">
            <li pn="section-5.8.1-3.1">The "c=" line MUST <bcp14>MUST</bcp14> be checked for syntax syntax, but its value
            is only used for ICE mismatch detection, as defined in
            <xref target="RFC8445" />, Section 5.4. sectionFormat="comma" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8445#section-5.4" derivedContent="RFC8445"/>. Note that JSEP
            implementations should never encounter this condition
            because ICE is required for WebRTC.</t>

            <t>The WebRTC.</li>
            <li pn="section-5.8.1-3.2">The "i=", "u=", "e=", "p=", "t=", "r=", "z=", and "k="
            lines are not used by this specification; they MUST <bcp14>MUST</bcp14> be
            checked for syntax syntax, but their values are not used.</t>
          </list></t>

          <t>The otherwise used. </li>
          </ul>
          <t indent="0" pn="section-5.8.1-4">The remaining non-attribute lines are processed as
          follows:
          <list>

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8.1-5">
            <li pn="section-5.8.1-5.1">The "v=" line MUST <bcp14>MUST</bcp14> have a version of 0, as specified in
            <xref target="RFC4566" />, Section 5.1.</t>

            <t>The sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.1" derivedContent="RFC4566"/>.</li>
            <li pn="section-5.8.1-5.2">The "o=" line MUST <bcp14>MUST</bcp14> be parsed as specified in
            <xref target="RFC4566" />, Section 5.2.</t>

            <t>The sectionFormat="comma" section="5.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.2" derivedContent="RFC4566"/>.</li>
            <li pn="section-5.8.1-5.3">The "b=" line, if present, MUST <bcp14>MUST</bcp14> be parsed as specified
            in
            <xref target="RFC4566" />, Section 5.8, sectionFormat="comma" section="5.8" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.8" derivedContent="RFC4566"/>, and the bwtype and
            bandwidth values stored.</t>
          </list></t>

          <t>Finally, stored.</li>
          </ul>
          <t indent="0" pn="section-5.8.1-6">Finally, the attribute lines are processed. Specific
          processing MUST <bcp14>MUST</bcp14> be applied for the following session-level
          attribute ("a=") lines:
          <list style="symbols">

            <t>Any
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8.1-7">
            <li pn="section-5.8.1-7.1">Any "a=group" lines are parsed as specified in
            <xref target="RFC5888" />, Section 5, sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5888#section-5" derivedContent="RFC5888"/>, and the group's
            semantics and mids are stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.1-7.2">If present, a single "a=ice-lite" line is parsed as
            specified in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 4.3, target="RFC8839" sectionFormat="comma" section="5.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.3" derivedContent="RFC8839"/>, and a value
            indicating the presence of ice-lite is stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.1-7.3">If present, a single "a=ice-ufrag" line is parsed as
            specified in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 4.4, target="RFC8839" sectionFormat="comma" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.4" derivedContent="RFC8839"/>, and the ufrag value is stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.1-7.4">If present, a single "a=ice-pwd" line is parsed as
            specified in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 4.4, target="RFC8839" sectionFormat="comma" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.4" derivedContent="RFC8839"/>, and the password value is stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.1-7.5">If present, a single "a=ice-options" line is parsed as
            specified in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 4.6, target="RFC8839" sectionFormat="comma" section="5.6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.6" derivedContent="RFC8839"/>, and the set of specified options is stored.</t>

            <t>Any stored.</li>
            <li pn="section-5.8.1-7.6">Any "a=fingerprint" lines are parsed as specified in
            <xref target="RFC8122" />, Section 5, sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8122#section-5" derivedContent="RFC8122"/>, and the set of
            fingerprint and algorithm values is stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.1-7.7">If present, a single "a=setup" line is parsed as
            specified in
            <xref target="RFC4145" />, Section 4, sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4145#section-4" derivedContent="RFC4145"/>, and the setup value
            is stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.1-7.8">If present, a single "a=tls-id" line is parsed as
            specified in <xref target="I-D.ietf-mmusic-dtls-sdp" /> Section 5, target="RFC8842" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8842#section-5" derivedContent="RFC8842"/>, and
            the tls-id attribute value is stored.</t>

            <t>Any stored.</li>
            <li pn="section-5.8.1-7.9">Any "a=identity" lines are parsed and the identity
            values stored for subsequent verification, as specified in
            <xref target="I-D.ietf-rtcweb-security-arch" />, Section
            5.</t>

            <t>Any target="RFC8827" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8827#section-5" derivedContent="RFC8827"/>.</li>
            <li pn="section-5.8.1-7.10">Any "a=extmap" lines are parsed as specified in
            <xref target="RFC5285" />, Section 5, sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5285#section-5" derivedContent="RFC5285"/>, and their values are
            stored.</t>
          </list></t>

          <t>Other
            stored.</li>
          </ul>
          <t indent="0" pn="section-5.8.1-8">Other attributes that are not relevant to JSEP may also be
          present, and implementations SHOULD <bcp14>SHOULD</bcp14> process any that they
          recognize. As required by
          <xref target="RFC4566"></xref>, Section 5.13, target="RFC4566" sectionFormat="comma" section="5.13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.13" derivedContent="RFC4566"/>, unknown
          attribute lines MUST <bcp14>MUST</bcp14> be ignored.</t>

          <t>Once
          <t indent="0" pn="section-5.8.1-9">Once all the session-level lines have been parsed,
          processing continues with the lines in m= "m=" sections.</t>
        </section>
        <section title="Media anchor="sec.media-level-parse" numbered="true" toc="include" removeInRFC="false" pn="section-5.8.2">
          <name slugifiedName="name-media-section-parsing">Media Section Parsing"
        anchor="sec.media-level-parse">

          <t>Like Parsing</name>
          <t indent="0" pn="section-5.8.2-1">Like the session-level lines, the media section lines MUST <bcp14>MUST</bcp14>
          occur in the specific order and with the specific syntax
          defined in
          <xref target="RFC4566" />, Section 5.</t>

          <t>The sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5" derivedContent="RFC4566"/>.</t>
          <t indent="0" pn="section-5.8.2-2">The "m=" line itself MUST <bcp14>MUST</bcp14> be parsed as described in
          <xref target="RFC4566" />, Section 5.14, sectionFormat="comma" section="5.14" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.14" derivedContent="RFC4566"/>, and the media, port,
          proto, &lt;media&gt;, &lt;port&gt;,
          &lt;proto&gt;, and fmt &lt;fmt&gt; values stored.</t>

          <t>Following
          <t indent="0" pn="section-5.8.2-3">Following the "m=" line, specific processing MUST <bcp14>MUST</bcp14> be
          applied for the following non-attribute lines:
          <list style="symbols">

            <t>As
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8.2-4">
            <li pn="section-5.8.2-4.1">As with the "c=" line at the session level, the "c="
            line MUST <bcp14>MUST</bcp14> be parsed according to
            <xref target="RFC4566" />, Section 5.7, sectionFormat="comma" section="5.7" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.7" derivedContent="RFC4566"/>, but its value is
            not used.</t>

            <t>The used.</li>
            <li pn="section-5.8.2-4.2">The "b=" line, if present, MUST <bcp14>MUST</bcp14> be parsed as specified
            in
            <xref target="RFC4566" />, Section 5.8, sectionFormat="comma" section="5.8" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.8" derivedContent="RFC4566"/>, and the bwtype and
            bandwidth values stored.</t>
          </list></t>

          <t>Specific stored.</li>
          </ul>
          <t indent="0" pn="section-5.8.2-5">Specific processing MUST <bcp14>MUST</bcp14> also be applied for the following
          attribute lines:
          <list style="symbols">

            <t>If
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8.2-6">
            <li pn="section-5.8.2-6.1">If present, a single "a=ice-ufrag" line is parsed as
            specified in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 4.4, target="RFC8839" sectionFormat="comma" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.4" derivedContent="RFC8839"/>, and the ufrag value is stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.2-6.2">If present, a single "a=ice-pwd" line is parsed as
            specified in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 4.4, target="RFC8839" sectionFormat="comma" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.4" derivedContent="RFC8839"/>, and the password value is stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.2-6.3">If present, a single "a=ice-options" line is parsed as
            specified in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />, Section 4.6, target="RFC8839" sectionFormat="comma" section="5.6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.6" derivedContent="RFC8839"/>,
            and the set of specified options is stored.</t>

            <t>Any stored.</li>
            <li pn="section-5.8.2-6.4">Any "a=candidate" attributes MUST <bcp14>MUST</bcp14> be parsed as specified
            in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 4.1, target="RFC8839" sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.1" derivedContent="RFC8839"/>, and their values stored.</t>

            <t>Any stored.</li>
            <li pn="section-5.8.2-6.5">Any "a=remote-candidates" attributes MUST <bcp14>MUST</bcp14> be parsed as
            specified in
            <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 4.2, target="RFC8839" sectionFormat="comma" section="5.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.2" derivedContent="RFC8839"/>, but their values are ignored.</t>

            <t>If ignored.</li>
            <li pn="section-5.8.2-6.6">If present, a single "a=end-of-candidates" attribute
            MUST
            <bcp14>MUST</bcp14> be parsed as specified in
            <xref target="I-D.ietf-ice-trickle" />, Section 8.2, target="RFC8840" sectionFormat="comma" section="8.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8840#section-8.1" derivedContent="RFC8840"/>, and
            its presence or absence flagged and stored.</t>

            <t>Any stored.</li>
            <li pn="section-5.8.2-6.7">Any "a=fingerprint" lines are parsed as specified in
            <xref target="RFC8122" />, Section 5, sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8122#section-5" derivedContent="RFC8122"/>, and the set of
            fingerprint and algorithm values is stored.</t>
          </list></t>

          <t>If stored.</li>
          </ul>
          <t indent="0" pn="section-5.8.2-7">If the "m=" proto &lt;proto&gt; value indicates use of RTP, as described
          in
          <xref target="sec.profile-names" /> format="default" sectionFormat="of" derivedContent="Section 5.1.2"/> above, the following
          attribute lines MUST <bcp14>MUST</bcp14> be processed:
          <list style="symbols">

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8.2-8">
            <li pn="section-5.8.2-8.1">The "m=" fmt &lt;fmt&gt; value MUST <bcp14>MUST</bcp14> be parsed as specified in
            <xref target="RFC4566" />, Section 5.14, sectionFormat="comma" section="5.14" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.14" derivedContent="RFC4566"/>, and the individual
            values stored.</t>

            <t>Any stored.</li>
            <li pn="section-5.8.2-8.2">Any "a=rtpmap" or "a=fmtp" lines MUST <bcp14>MUST</bcp14> be parsed as
            specified in
            <xref target="RFC4566" />, Section 6, sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/>, and their values
            stored.</t>

            <t>If
            stored.</li>
            <li pn="section-5.8.2-8.3">If present, a single "a=ptime" line MUST <bcp14>MUST</bcp14> be parsed as
            described in
            <xref target="RFC4566" />, Section 6, sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/>, and its value
            stored.</t>

            <t>If
            stored.</li>
            <li pn="section-5.8.2-8.4">If present, a single "a=maxptime" line MUST <bcp14>MUST</bcp14> be parsed as
            described in
            <xref target="RFC4566" />, Section 6, sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/>, and its value
            stored.</t>

            <t>If
            stored.</li>
            <li pn="section-5.8.2-8.5">If present, a single direction attribute line (e.g. (e.g.,
            "a=sendrecv") MUST <bcp14>MUST</bcp14> be parsed as described in
            <xref target="RFC4566" />, Section 6, sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/>, and its value
            stored.</t>

            <t>Any
            stored.</li>
            <li pn="section-5.8.2-8.6">Any "a=ssrc" attributes MUST <bcp14>MUST</bcp14> be parsed as specified in
            <xref target="RFC5576" />, Section 4.1, sectionFormat="comma" section="4.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5576#section-4.1" derivedContent="RFC5576"/>, and their values
            stored.</t>

            <t>Any
            stored.</li>
            <li pn="section-5.8.2-8.7">Any "a=extmap" attributes MUST <bcp14>MUST</bcp14> be parsed as specified in

            <xref target="RFC5285" />, Section 5, sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5285#section-5" derivedContent="RFC5285"/>, and their values
            stored.</t>

            <t>Any
            stored.</li>
            <li pn="section-5.8.2-8.8">Any "a=rtcp-fb" attributes MUST <bcp14>MUST</bcp14> be parsed as specified
            in
            <xref target="RFC4585" />, Section 4.2., sectionFormat="comma" section="4.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4585#section-4.2" derivedContent="RFC4585"/>, and their values
            stored.</t>

            <t>If
            stored.</li>
            <li pn="section-5.8.2-8.9">If present, a single "a=rtcp-mux" attribute MUST <bcp14>MUST</bcp14> be
            parsed as specified in
            <xref target="RFC5761"></xref>, Section 5.1.3, target="RFC5761" sectionFormat="comma" section="5.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5761#section-5.1.3" derivedContent="RFC5761"/>, and its
            presence or absence flagged and stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.2-8.10">If present, a single "a=rtcp-mux-only" attribute MUST <bcp14>MUST</bcp14> be
            parsed as specified in
            <xref target="I-D.ietf-mmusic-mux-exclusive" />, Section 3, target="RFC8858" sectionFormat="comma" section="3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8858#section-3" derivedContent="RFC8858"/>,
            and its presence or absence flagged and stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.2-8.11">If present, a single "a=rtcp-rsize" attribute MUST <bcp14>MUST</bcp14> be
            parsed as specified in
            <xref target="RFC5506" />, Section 5, sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5506#section-5" derivedContent="RFC5506"/>, and its presence or
            absence flagged and stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.2-8.12">If present, a single "a=rtcp" attribute MUST <bcp14>MUST</bcp14> be parsed
            as specified in
            <xref target="RFC3605" />, Section 2.1, sectionFormat="comma" section="2.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3605#section-2.1" derivedContent="RFC3605"/>, but its value is
            ignored, as this information is superfluous when using
            ICE.</t>

            <t>If
            ICE.</li>
            <li pn="section-5.8.2-8.13">If present, "a=msid" attributes MUST <bcp14>MUST</bcp14> be parsed as
            specified in
            <xref target="I-D.ietf-mmusic-msid" />, Section 3.2, target="RFC8830" sectionFormat="comma" section="3.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8830#section-3.2" derivedContent="RFC8830"/>, and
            their values stored, ignoring any "appdata" field. If no "a=msid"
            attributes are present, a random msid-id value is generated for a
            "default" MediaStream for the session, if not already present, and
            this value is stored.</t>

            <t>Any stored.</li>
            <li pn="section-5.8.2-8.14">Any "a=imageattr" attributes MUST <bcp14>MUST</bcp14> be parsed as specified
            in
            <xref target="RFC6236" />, Section 3, sectionFormat="comma" section="3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc6236#section-3" derivedContent="RFC6236"/>, and their values
            stored.</t>

            <t>Any
            stored.</li>
            <li pn="section-5.8.2-8.15">Any "a=rid" lines MUST <bcp14>MUST</bcp14> be parsed as specified in
            <xref target="I-D.ietf-mmusic-rid"></xref>, Section 10, target="RFC8851" sectionFormat="comma" section="10" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8851#section-10" derivedContent="RFC8851"/>, and
            their values stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.2-8.16">If present, a single "a=simulcast" line MUST <bcp14>MUST</bcp14> be parsed
            as specified in
            <xref target="I-D.ietf-mmusic-sdp-simulcast"></xref>, target="RFC8853" format="default" sectionFormat="of" derivedContent="RFC8853"/>, and
            its values stored.</t>
          </list></t>

          <t>Otherwise, stored.</li>
          </ul>
          <t indent="0" pn="section-5.8.2-9">Otherwise, if the "m=" proto &lt;proto&gt; value indicates use of SCTP,
          the following attribute lines MUST <bcp14>MUST</bcp14> be processed:
          <list style="symbols">

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8.2-10">
            <li pn="section-5.8.2-10.1">The "m=" fmt  &lt;fmt&gt; value MUST <bcp14>MUST</bcp14> be parsed as specified in
            <xref target="I-D.ietf-mmusic-sctp-sdp" />, Section 4.3, target="RFC8841" sectionFormat="comma" section="4.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8841#section-4.3" derivedContent="RFC8841"/>,
            and the application protocol value stored.</t>

            <t>An stored.</li>
            <li pn="section-5.8.2-10.2">An "a=sctp-port" attribute MUST <bcp14>MUST</bcp14> be present, and it MUST <bcp14>MUST</bcp14>
            be parsed as specified in
            <xref target="I-D.ietf-mmusic-sctp-sdp" />, Section 5.2, target="RFC8841" sectionFormat="comma" section="5.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8841#section-5.2" derivedContent="RFC8841"/>,
            and the value stored.</t>

            <t>If stored.</li>
            <li pn="section-5.8.2-10.3">If present, a single "a=max-message-size" attribute MUST <bcp14>MUST</bcp14>
            be parsed as specified in
            <xref target="I-D.ietf-mmusic-sctp-sdp" />, Section 6, target="RFC8841" sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8841#section-6" derivedContent="RFC8841"/>, and
            the value stored. Otherwise, use the specified default.</t>
          </list></t>

          <t>Other default.</li>
          </ul>
          <t indent="0" pn="section-5.8.2-11">Other attributes that are not relevant to JSEP may also be
          present, and implementations SHOULD <bcp14>SHOULD</bcp14> process any that they
          recognize. As required by
          <xref target="RFC4566"></xref>, Section 5.13, target="RFC4566" sectionFormat="comma" section="5.13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.13" derivedContent="RFC4566"/>, unknown
          attribute lines MUST <bcp14>MUST</bcp14> be ignored.</t>
        </section>
        <section title="Semantics Verification">

          <t>Assuming numbered="true" toc="include" removeInRFC="false" pn="section-5.8.3">
          <name slugifiedName="name-semantics-verification">Semantics Verification</name>
          <t indent="0" pn="section-5.8.3-1">Assuming that parsing completes successfully, the parsed
          description is then evaluated to ensure internal consistency
          as well as proper support for mandatory features.
          Specifically, the following checks are performed:
          <list style="symbols">

            <t>For
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8.3-2">
            <li pn="section-5.8.3-2.1">
              <t indent="0" pn="section-5.8.3-2.1.1">For each m= "m=" section, valid values for each of the
            mandatory-to-use features enumerated in
            <xref target="sec.usage-requirements" /> MUST format="default" sectionFormat="of" derivedContent="Section 5.1.1"/> <bcp14>MUST</bcp14> be present.
            These values MAY either <bcp14>MAY</bcp14> be either present at the media level, level or
            inherited from the session level.
            <list style="symbols">

              <t>ICE
              </t>
              <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8.3-2.1.2">
                <li pn="section-5.8.3-2.1.2.1">ICE ufrag and password values, which MUST <bcp14>MUST</bcp14> comply with
              the size limits specified in
              <xref target="I-D.ietf-mmusic-ice-sip-sdp" />, Section 4.4.</t>

              <t>tls-id target="RFC8839" sectionFormat="comma" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.4" derivedContent="RFC8839"/>.</li>
                <li pn="section-5.8.3-2.1.2.2">A tls-id value, which MUST <bcp14>MUST</bcp14> be set according to
              <xref target="I-D.ietf-mmusic-dtls-sdp" />, Section 5. target="RFC8842" sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8842#section-5" derivedContent="RFC8842"/>. If
	       this is a re-offer or a response to a re-offer and the
	       tls-id value is different from that presently in use, the
	       DTLS connection is not being continued and the remote
	       description MUST <bcp14>MUST</bcp14> be part of an ICE restart, together with
	       new ufrag and password values.</t>

              <t>DTLS values.</li>
                <li pn="section-5.8.3-2.1.2.3">A DTLS setup value, which MUST <bcp14>MUST</bcp14> be set according to the
	       rules specified in
	       <xref target="RFC5763" />, Section 5 sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5763#section-5" derivedContent="RFC5763"/> and MUST <bcp14>MUST</bcp14> be
	       consistent with the selected role of the current DTLS
	       connection, if one exists and is being continued.</t>

              <t>DTLS continued.</li>
                <li pn="section-5.8.3-2.1.2.4">DTLS fingerprint values, where at least one
	       fingerprint MUST <bcp14>MUST</bcp14> be present.</t>
            </list></t>

            <t>All RID values present.</li>
              </ul>
            </li>
            <li pn="section-5.8.3-2.2">All rid-ids referenced in an "a=simulcast" line MUST <bcp14>MUST</bcp14>
	     exist as "a=rid" lines.</t>

            <t>Each m= lines.</li>
            <li pn="section-5.8.3-2.3">Each "m=" section is also checked to ensure that prohibited
	     features are not used.</t>

            <t>If used.</li>
            <li pn="section-5.8.3-2.4">If the RTP/RTCP multiplexing policy is "require", each
            m=
	     "m=" section MUST <bcp14>MUST</bcp14> contain an "a=rtcp-mux" attribute. If an m= "m="
	     section contains an "a=rtcp-mux-only" attribute, that
	     section MUST <bcp14>MUST</bcp14> also contain an "a=rtcp-mux" attribute.</t>

            <t>If attribute.</li>
            <li pn="section-5.8.3-2.5">If an m= "m=" section was present in the previous answer, the
	     state of RTP/RTCP multiplexing MUST <bcp14>MUST</bcp14> match what was
	     previously negotiated.</t>
          </list></t>

          <t>If negotiated.</li>
          </ul>
          <t indent="0" pn="section-5.8.3-3">If this session description is of type "pranswer" or
	   "answer", the following additional checks are applied:
          <list style="symbols">

            <t>The
          </t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8.3-4">
            <li pn="section-5.8.3-4.1">The session description must <bcp14>MUST</bcp14> follow the rules defined in
	     <xref target="RFC3264" />, Section 6, sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-6" derivedContent="RFC3264"/>, including the
	     requirement that the number of m= "m=" sections MUST <bcp14>MUST</bcp14> exactly
	     match the number of m= "m=" sections in the associated
            offer.</t>

            <t>For
	     offer.</li>
            <li pn="section-5.8.3-4.2">For each m= "m=" section, the media type and protocol values
            MUST
	     <bcp14>MUST</bcp14> exactly match the media type and protocol values in
	     the corresponding m= "m=" section in the associated offer.</t>
          </list></t>

          <t>If offer.</li>
          </ul>
          <t indent="0" pn="section-5.8.3-5">If any of the preceding checks failed, processing MUST <bcp14>MUST</bcp14>
	   stop and an error MUST <bcp14>MUST</bcp14> be returned.</t>
        </section>
      </section>
      <section title="Applying anchor="sec.applying-a-local-desc" numbered="true" toc="include" removeInRFC="false" pn="section-5.9">
        <name slugifiedName="name-applying-a-local-descriptio">Applying a Local Description"
      anchor="sec.applying-a-local-desc">

        <t>The Description</name>
        <t indent="0" pn="section-5.9-1">The following steps are performed at the media engine level
	 to apply a local description. If an error is returned, the
	 session MUST <bcp14>MUST</bcp14> be restored to the state it was in before
	 performing these steps.</t>

        <t>First, m=
        <t indent="0" pn="section-5.9-2">First, "m=" sections are processed. For each m= "m=" section, the
	 following steps MUST <bcp14>MUST</bcp14> be performed; if any parameters are out of
        bounds,
	 bounds or cannot be applied, processing MUST <bcp14>MUST</bcp14> stop and an error
        MUST
	 <bcp14>MUST</bcp14> be returned.
        <list style="symbols">

          <t>If
        </t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.9-3">
          <li pn="section-5.9-3.1">If this m= "m=" section is new, begin gathering candidates for
	   it, as defined in
	   <xref target="RFC8445" />, Section 5.1.1, sectionFormat="comma" section="5.1.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8445#section-5.1.1" derivedContent="RFC8445"/>, unless it is
	   definitively being bundled (either (1) this is an offer and the
          m=
	   "m=" section is marked bundle-only, bundle-only or it (2) it is an answer and the
          m=
	   "m=" section is bundled into into another m= section.)</t>

          <t>Or, "m=" section).</li>
          <li pn="section-5.9-3.2">Or, if the ICE ufrag and password values have changed,
	   trigger the ICE agent to start an ICE restart as described in
	   <xref target="RFC8445" />, Section 9, sectionFormat="comma" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8445#section-9" derivedContent="RFC8445"/>, and begin
	   gathering new candidates for the m= "m=" section. If this
	   description is an answer, also start checks on that media
          section.</t>

          <t>If
	   section.</li>
          <li pn="section-5.9-3.3">
            <t indent="0" pn="section-5.9-3.3.1">If the m= "m=" section proto &lt;proto&gt; value indicates use of RTP:
          <list style="symbols">

            <t>If
            </t>
            <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.9-3.3.2">
              <li pn="section-5.9-3.3.2.1">
                <t indent="0" pn="section-5.9-3.3.2.1.1">If there is no RtpTransceiver associated with this m= "m="
	     section, find one and associate it with this m= "m=" section
	     according to the following steps. Note that this situation
	     will only occur when applying an offer.
            <list style="symbols">

              <t>Find
                </t>
                <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.9-3.3.2.1.2">
                  <li pn="section-5.9-3.3.2.1.2.1">Find the RtpTransceiver that corresponds to this m= "m="
	       section, using the mapping between transceivers and m= "m="
	       section indices established when creating the offer.</t>

              <t>Set offer.</li>
                  <li pn="section-5.9-3.3.2.1.2.2">Set the value of this RtpTransceiver's mid property to
	       the MID of the m= section.</t>
            </list></t>

            <t>If "m=" section.</li>
                </ul>
              </li>
              <li pn="section-5.9-3.3.2.2">If RTCP mux is indicated, prepare to demux RTP and RTCP
	     from the RTP ICE component, as specified in
	     <xref target="RFC5761" />, Section 5.1.3.</t>

            <t>For sectionFormat="comma" section="5.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5761#section-5.1.3" derivedContent="RFC5761"/>.</li>
              <li pn="section-5.9-3.3.2.3">For each specified RTP header extension, establish a
	     mapping between the extension ID and URI, as described in
	     <xref target="RFC5285" />, Section 6.</t>

            <t>If sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5285#section-6" derivedContent="RFC5285"/>.</li>
              <li pn="section-5.9-3.3.2.4">If the MID header extension is supported, prepare to
	     demux RTP streams intended for this m= "m=" section based on the
	     MID header extension, as described in
	     <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation" />,
            Section 15.</t>

            <t>For target="RFC8843" sectionFormat="comma" section="15" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-15" derivedContent="RFC8843"/>.</li>
              <li pn="section-5.9-3.3.2.5">For each specified media format, establish a mapping
	     between the payload type and the actual media format, as
	     described in
	     <xref target="RFC3264" />, Section 6.1. sectionFormat="comma" section="6.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-6.1" derivedContent="RFC3264"/>. In addition,
	     prepare to demux RTP streams intended for this m= "m=" section
	     based on the media formats supported by this m= "m=" section, as
	     described in
	     <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation" />,
            Section 10.2.</t>

            <t>For target="RFC8843" sectionFormat="comma" section="9.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-9.2" derivedContent="RFC8843"/>.
 </li>
              <li pn="section-5.9-3.3.2.6">For each specified "rtx" media format, establish a
	     mapping between the RTX payload type and its associated
	     primary payload type, as described in
	     Sections <xref target="RFC4588" />, Sections 8.6 section="8.6" sectionFormat="bare" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4588#section-8.6" derivedContent="RFC4588"/> and 8.7.</t>

            <t>If <xref target="RFC4588" section="8.7" sectionFormat="bare" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4588#section-8.7" derivedContent="RFC4588"/> of <xref target="RFC4588" format="default" sectionFormat="of" derivedContent="RFC4588"/>.

 </li>
              <li pn="section-5.9-3.3.2.7">If the directional direction attribute is of type "sendrecv" or
	     "recvonly", enable receipt and decoding of media.</t>
          </list></t>
        </list></t>

        <t>Finally, media.</li>
            </ul>
          </li>
        </ul>
        <t indent="0" pn="section-5.9-4">Finally, if this description is of type "pranswer" or
	 "answer", follow the processing defined in
	 <xref target="sec.applying-an-answer" /> format="default" sectionFormat="of" derivedContent="Section 5.11"/> below.</t>
      </section>
      <section title="Applying anchor="sec.applying-a-remote-desc" numbered="true" toc="include" removeInRFC="false" pn="section-5.10">
        <name slugifiedName="name-applying-a-remote-descripti">Applying a Remote Description"
      anchor="sec.applying-a-remote-desc">

        <t>The Description</name>
        <t indent="0" pn="section-5.10-1">The following steps are performed to apply a remote
	 description. If an error is returned, the session MUST <bcp14>MUST</bcp14> be
	 restored to the state it was in before performing these
	 steps.</t>

        <t>If
        <t indent="0" pn="section-5.10-2">If the answer contains any "a=ice-options" attributes where
	 "trickle" is listed as an attribute, update the PeerConnection
        canTrickle
	 canTrickleIceCandidates property to be true. "true". Otherwise, set this property to
        false.</t>

        <t>The
	 "false".</t>
        <t indent="0" pn="section-5.10-3">The following steps MUST <bcp14>MUST</bcp14> be performed for attributes at the
	 session level; if any parameters are out of bounds, bounds or cannot
	 be applied, processing MUST <bcp14>MUST</bcp14> stop and an error MUST <bcp14>MUST</bcp14> be returned.

        <list style="symbols">

          <t>For

        </t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.10-4">
          <li pn="section-5.10-4.1">For any specified "CT" bandwidth value, set this value as the
	   limit for the maximum total bitrate for all m= "m=" sections, as
	   specified in
	   <xref target="RFC4566"></xref>, Section 5.8. target="RFC4566" sectionFormat="comma" section="5.8" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.8" derivedContent="RFC4566"/>. Within this
	   overall limit, the implementation can dynamically decide how
	   to best allocate the available bandwidth between m= "m=" sections,
	   respecting any specific limits that have been specified for
	   individual m= sections.</t>

          <t>For "m=" sections.</li>
          <li pn="section-5.10-4.2">For any specified "RR" or "RS" bandwidth values, handle as
	   specified in
	   <xref target="RFC3556"></xref>, Section 2.</t>

          <t>Any target="RFC3556" sectionFormat="comma" section="2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3556#section-2" derivedContent="RFC3556"/>.</li>
          <li pn="section-5.10-4.3">Any "AS" bandwidth value MUST (<xref target="RFC4566" sectionFormat="comma" section="5.8" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-5.8" derivedContent="RFC4566"/>)
           <bcp14>MUST</bcp14> be ignored, as the meaning
	   of this construct at the session level is not well
          defined.</t>
        </list></t>

        <t>For
	   defined. </li>
        </ul>
        <t indent="0" pn="section-5.10-5">For each m= "m=" section, the following steps MUST <bcp14>MUST</bcp14> be performed;
	 if any parameters are out of bounds, bounds or cannot be applied,
	 processing MUST <bcp14>MUST</bcp14> stop and an error MUST <bcp14>MUST</bcp14> be returned.
        <list style="symbols">

          <t>If
        </t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.10-6">
          <li pn="section-5.10-6.1">
            <t indent="0" pn="section-5.10-6.1.1">If the ICE ufrag or password changed from the previous
	   remote description:
          <list style="symbols">

            <t>If
            </t>
            <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.10-6.1.2">
              <li pn="section-5.10-6.1.2.1">If the description is of type "offer", the
	     implementation MUST <bcp14>MUST</bcp14> note that an ICE restart is needed, as
	     described in
	     <xref target="I-D.ietf-mmusic-ice-sip-sdp" />,
            Section 3.4.1.1.1</t>

            <t>If target="RFC8839" sectionFormat="comma" section="4.4.1.1.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-4.4.1.1.1" derivedContent="RFC8839"/>.</li>
              <li pn="section-5.10-6.1.2.2">If the description is of type "answer" or "pranswer",
	     then check to see if the current local description is an
	     ICE restart, and if not, generate an error. If the
	     PeerConnection state is "have-remote-pranswer", "have-remote-pranswer" and the ICE
	     ufrag or password changed from the previous provisional
	     answer, then signal the ICE agent to discard any previous
	     ICE check list checklist state for the m= "m=" section. Finally, signal
	     the ICE agent to begin checks.</t>
          </list></t>

          <t>If checks.
</li>
            </ul>
          </li>
          <li pn="section-5.10-6.2">If the current local description indicates an ICE restart,
          and either restart
	   but neither the ICE ufrag or nor the password has not changed from the
	   previous remote description, as description (as prescribed by
	   <xref target="RFC8445" />, Section 9, sectionFormat="comma" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8445#section-9" derivedContent="RFC8445"/>), generate an
          error.</t>

          <t>Configure
	   error.</li>
          <li pn="section-5.10-6.3">Configure the ICE components associated with this media
	   section to use the supplied ICE remote ufrag and password for
	   their connectivity checks.</t>

          <t>Pair checks.</li>
          <li pn="section-5.10-6.4">Pair any supplied ICE candidates with any gathered local
	   candidates, as described in
	   <xref target="RFC8445" />, Section 6.1.2, sectionFormat="comma" section="6.1.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8445#section-6.1.2" derivedContent="RFC8445"/>, and start
	   connectivity checks with the appropriate credentials.</t>

          <t>If credentials.</li>
          <li pn="section-5.10-6.5">If an "a=end-of-candidates" attribute is present, process
	   the end-of-candidates indication as described in
	   <xref target="I-D.ietf-ice-trickle" />, Section 11.</t>

          <t>If target="RFC8838" sectionFormat="comma" section="14" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8838#section-14" derivedContent="RFC8838"/>.</li>
          <li pn="section-5.10-6.6">
            <t indent="0" pn="section-5.10-6.6.1">If the m= "m=" section proto &lt;proto&gt; value indicates use of RTP:
          <list style="symbols">

            <t>If
            </t>
            <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.10-6.6.2">
              <li pn="section-5.10-6.6.2.1">If the m= "m=" section is being recycled (see
	      <xref target="sec.subsequent-offers"></xref>), dissociate target="sec.subsequent-offers" format="default" sectionFormat="of" derivedContent="Section 5.2.2"/>), disassociate
	      the currently associated RtpTransceiver by setting its mid
	      property to null, "null", and discard the mapping between the
	      transceiver and its m= "m=" section index.</t>

            <t>If index.</li>
              <li pn="section-5.10-6.6.2.2">
                <t indent="0" pn="section-5.10-6.6.2.2.1">If the m= "m=" section is not associated with any
	      RtpTransceiver (possibly because it was dissociated disassociated in the
	      previous step), either find an RtpTransceiver or create one
	      according to the following steps:
            <list style="symbols">

              <t>If
                </t>
                <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.10-6.6.2.2.2">
                  <li pn="section-5.10-6.6.2.2.2.1">If the m= "m=" section is sendrecv or recvonly, and there
		are RtpTransceivers of the same type that were added to
		the PeerConnection by addTrack and are not associated
		with any m= "m=" section and are not stopped, find the first
		(according to the canonical order described in
		<xref target="sec.initial-offers" />) format="default" sectionFormat="of" derivedContent="Section 5.2.1"/>) such
              RtpTransceiver.</t>

              <t>If
		RtpTransceiver.</li>
                  <li pn="section-5.10-6.6.2.2.2.2">If no RtpTransceiver was found in the previous step,
		create one with a recvonly direction.</t>

              <t>Associate direction.</li>
                  <li pn="section-5.10-6.6.2.2.2.3">Associate the found or created RtpTransceiver with the
              m=
		"m=" section by setting the value of the RtpTransceiver's
		mid property to the MID of the m= "m=" section, and establish
		a mapping between the transceiver and the index of the m= "m="
		section. If the m= "m=" section does not include a MID (i.e.,
		the remote endpoint does not support the MID extension),
		generate a value for the RtpTransceiver mid property,
		following the guidance for "a=mid" mentioned in
		<xref target="sec.initial-offers" />.</t>
            </list></t>

            <t>For format="default" sectionFormat="of" derivedContent="Section 5.2.1"/>.</li>
                </ul>
              </li>
              <li pn="section-5.10-6.6.2.3">For each specified media format that is also supported
	      by the local implementation, establish a mapping between
	      the specified payload type and the media format, as
	      described in
	      <xref target="RFC3264" />, Section 6.1. sectionFormat="comma" section="6.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-6.1" derivedContent="RFC3264"/>. Specifically, this
	      means that the implementation records the payload type to
	      be used in outgoing RTP packets when sending each specified
	      media format, as well as the relative preference for each
	      format that is indicated in their ordering. If any
	      indicated media format is not supported by the local
	      implementation, it MUST <bcp14>MUST</bcp14> be ignored.</t>

            <t>For ignored.</li>
              <li pn="section-5.10-6.6.2.4">For each specified "rtx" media format, establish a
	      mapping between the RTX payload type and its associated
	      primary payload type, as described in
	      <xref target="RFC4588" />, Section 4. sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4588#section-4" derivedContent="RFC4588"/>. If any referenced
	      primary payload types are not present, this MUST <bcp14>MUST</bcp14> result in
	      an error. Note that RTX payload types may refer to primary
	      payload types which that are not supported by the local media
	      implementation, in which case, case the RTX payload type MUST <bcp14>MUST</bcp14>
	      also be ignored.</t>

            <t>For ignored.</li>
              <li pn="section-5.10-6.6.2.5">For each specified fmtp parameter that is supported by
	      the local implementation, enable them on the associated
	      media formats.</t>

            <t>For formats.</li>
              <li pn="section-5.10-6.6.2.6">For each specified SSRC Synchronization Source (SSRC) that is signaled in the m= "m="
	      section, prepare to demux RTP streams intended for this m= "m="
	      section using that SSRC, as described in
	      <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation" />,
            Section 10.2.</t>

            <t>For target="RFC8843" sectionFormat="comma" section="9.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-9.2" derivedContent="RFC8843"/>.</li>
              <li pn="section-5.10-6.6.2.7">For each specified RTP header extension that is also
	      supported by the local implementation, establish a mapping
	      between the extension ID and URI, as described in
	      <xref target="RFC5285" />, Section 5. sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5285#section-5" derivedContent="RFC5285"/>. Specifically, this
	      means that the implementation records the extension ID to
	      be used in outgoing RTP packets when sending each specified
	      header extension. If any indicated RTP header extension is
	      not supported by the local implementation, it MUST <bcp14>MUST</bcp14> be
            ignored.</t>

            <t>For
	      ignored.</li>
              <li pn="section-5.10-6.6.2.8">For each specified RTCP feedback mechanism that is
	      supported by the local implementation, enable them on the
	      associated media formats.</t>

            <t>For formats.</li>
              <li pn="section-5.10-6.6.2.9">
                <t indent="0" pn="section-5.10-6.6.2.9.1">For any specified "TIAS" ("Transport
     Independent Application Specific Maximum") bandwidth value, set this value
	      as a constraint on the maximum RTP bitrate to be used when
	      sending media, as specified in
	      <xref target="RFC3890"></xref>. target="RFC3890" format="default" sectionFormat="of" derivedContent="RFC3890"/>. If a "TIAS" value is not
            present,
	      present but an "AS" value is specified, generate a "TIAS"
	      value using this formula:
            <list style="format">

              <t>TIAS
                </t>
                <artwork align="left" pn="section-5.10-6.6.2.9.2">
		    TIAS = AS * 1000 * 0.95 - (50 * 40 * 8)</t>
            </list>The 50 is based on 50 packets per second, the 40 is
            based on an estimate of total header size, the 8)
		  </artwork>
                <t indent="0" pn="section-5.10-6.6.2.9.3">
              The 1000 changes the unit from kbps to bps (as required
              by TIAS), and the 0.95 is to allocate 5% to RTCP. "TIAS"
              An estimate of header overhead is used then subtracted out, in
            preference which
              the 50 is based on 50 packets per second, the 40 is based on
              typical header size (in bytes), and the 8 converts bytes to bits.
              Note that "TIAS" is preferred over
	      "AS" because it provides more accurate
	      control of bandwidth.</t>

            <t>For
              </li>
              <li pn="section-5.10-6.6.2.10">For any "RR" or "RS" bandwidth values, handle as
	      specified in
	      <xref target="RFC3556"></xref>, Section 2.</t>

            <t>Any target="RFC3556" sectionFormat="comma" section="2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3556#section-2" derivedContent="RFC3556"/>.</li>
              <li pn="section-5.10-6.6.2.11">Any specified "CT" bandwidth value MUST <bcp14>MUST</bcp14> be ignored, as
	      the meaning of this construct at the media level is not
	      well defined.</t>

            <t>If defined.</li>
              <li pn="section-5.10-6.6.2.12">
                <t indent="0" pn="section-5.10-6.6.2.12.1">If the m= "m=" section is of type audio:
            <list style="symbols">

              <t>For "audio":
                </t>
                <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.10-6.6.2.12.2">
                  <li pn="section-5.10-6.6.2.12.2.1">For each specified "CN" media format, configure
		silence suppression for all supported media formats with
		the same clockrate, clock rate, as described in
		<xref target="RFC3389" />, Section 5, sectionFormat="comma" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3389#section-5" derivedContent="RFC3389"/>, except for formats
		that have their own internal silence suppression
		mechanisms. Silence suppression for such formats (e.g.,
		Opus) is controlled via fmtp parameters, as discussed in
		<xref target="sec.voiceactivitydetection1" />.</t>

              <t>For format="default" sectionFormat="of" derivedContent="Section 5.2.3.2"/>.</li>
                  <li pn="section-5.10-6.6.2.12.2.2">For each specified "telephone-event" media format,
		enable DTMF dual-tone multifrequency (DTMF) transmission for all supported media formats
		with the same clockrate, clock rate, as described in
		<xref target="RFC4733" />, Section 2.5.1.2. sectionFormat="comma" section="2.5.1.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4733#section-2.5.1.2" derivedContent="RFC4733"/>. If there are
		any supported media formats that do not have a
		corresponding telephone-event format, disable DTMF
		transmission for those formats.</t>

              <t>For formats.</li>
                  <li pn="section-5.10-6.6.2.12.2.3">For any specified "ptime" value, configure the
		available media formats to use the specified packet size
		when sending. If the specified size is not supported for
		a media format, use the next closest value instead.</t>
            </list></t>
          </list></t>
        </list></t>

        <t>Finally, instead.</li>
                </ul>
              </li>
            </ul>
          </li>
        </ul>
        <t indent="0" pn="section-5.10-7">Finally, if this description is of type "pranswer" or
	  "answer", follow the processing defined in
	  <xref target="sec.applying-an-answer" /> format="default" sectionFormat="of" derivedContent="Section 5.11"/> below.</t>
      </section>
      <section title="Applying anchor="sec.applying-an-answer" numbered="true" toc="include" removeInRFC="false" pn="section-5.11">
        <name slugifiedName="name-applying-an-answer">Applying an Answer"
      anchor="sec.applying-an-answer">

        <t>In Answer</name>
        <t indent="0" pn="section-5.11-1">In addition to the steps mentioned above for processing a
	  local or remote description, the following steps are performed
	  when processing a description of type "pranswer" or
	  "answer".</t>

        <t>For
        <t indent="0" pn="section-5.11-2">For each m= "m=" section, the following steps MUST <bcp14>MUST</bcp14> be performed:
        <list style="symbols">

          <t>If
        </t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.11-3">
          <li pn="section-5.11-3.1">If the m= "m=" section has been rejected (i.e. port (i.e., the &lt;port&gt; value is set to
	    zero in the answer), stop any reception or transmission of
	    media for this section, and, unless a non-rejected m= "m=" section
	    is bundled with this m= "m=" section, discard any associated ICE
	    components, as described in
	    <xref target="I-D.ietf-mmusic-ice-sip-sdp" />, Section 3.4.3.1.</t>

          <t>If target="RFC8839" sectionFormat="comma" section="4.4.3.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-4.4.3.1" derivedContent="RFC8839"/>.</li>
          <li pn="section-5.11-3.2">If the remote DTLS fingerprint has been changed or the
          tls-id
	     value of the "a=tls-id" attribute has changed, tear down the DTLS connection. This
	     includes the case when the PeerConnection state is
	     "have-remote-pranswer". If a DTLS connection needs to be torn
	     down but the answer does not indicate an ICE restart or, in
	     the case of "have-remote-pranswer", new ICE credentials, an
	     error MUST <bcp14>MUST</bcp14> be generated. If an ICE restart is performed
	     without a change in the tls-id value or fingerprint, then the same DTLS
	     connection is continued over the new ICE channel. Note that
	     although JSEP requires that answerers change the tls-id value
	     if and only if the offerer does, non-JSEP answerers are
	     permitted to change the tls-id value as long as the offer contained
	     an ICE restart. Thus, JSEP implementations which that process DTLS
	     data prior to receiving an answer MUST <bcp14>MUST</bcp14> be prepared to receive
	     either a ClientHello or data from the previous DTLS
          connection.</t>

          <t>If
	     connection.</li>
          <li pn="section-5.11-3.3">If no valid DTLS connection exists, prepare to start a
	     DTLS connection, using the specified roles and fingerprints,
	     on any underlying ICE components, once they are active.</t>

          <t>If active.</li>
          <li pn="section-5.11-3.4">
            <t indent="0" pn="section-5.11-3.4.1">If the m= "m=" section proto &lt;proto&gt; value indicates use of RTP:
          <list style="symbols">

            <t>If
            </t>
            <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.11-3.4.2">
              <li pn="section-5.11-3.4.2.1">If the m= "m=" section references RTCP feedback mechanisms
	       that were not present in the corresponding m= "m=" section in
	       the offer, this indicates a negotiation problem and MUST <bcp14>MUST</bcp14>
	       result in an error. However, new media formats and new RTP
	       header extension values are permitted in the answer, as
	       described in
	       <xref target="RFC3264" />, Section 7, sectionFormat="comma" section="7" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-7" derivedContent="RFC3264"/> and
	       <xref target="RFC5285" />, Section 6.</t>

            <t>If sectionFormat="comma" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5285#section-6" derivedContent="RFC5285"/>.</li>
              <li pn="section-5.11-3.4.2.2">If the m= "m=" section has RTCP mux enabled, discard the RTCP
	       ICE component, if one exists, and begin or continue muxing
	       RTCP over the RTP ICE component, as specified in
	       <xref target="RFC5761" />, Section 5.1.3. sectionFormat="comma" section="5.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5761#section-5.1.3" derivedContent="RFC5761"/>. Otherwise,
	       prepare to transmit RTCP over the RTCP ICE component; if no
	       RTCP ICE component exists, exists because RTCP mux was previously
	       enabled, this MUST <bcp14>MUST</bcp14> result in an error.</t>

            <t>If error.</li>
              <li pn="section-5.11-3.4.2.3">If the m= "m=" section has reduced-size Reduced-Size RTCP enabled,
	       configure the RTCP transmission for this m= "m=" section to use
            reduced-size
	       Reduced-Size RTCP, as specified in
	       <xref target="RFC5506" />.</t>

            <t>If format="default" sectionFormat="of" derivedContent="RFC5506"/>.</li>
              <li pn="section-5.11-3.4.2.4">If the directional direction attribute in the answer indicates
	       that the JSEP implementation should be sending media
	       ("sendonly" for local answers, "recvonly" for remote
	       answers, or "sendrecv" for either type of answer), choose
	       the media format to send as the most preferred media format
	       from the remote description that is also locally supported,
	       as discussed in Sections <xref target="RFC3264" />, Sections 6.1 section="6.1" sectionFormat="bare" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-6.1" derivedContent="RFC3264"/> and 7, <xref target="RFC3264" section="7" sectionFormat="bare" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-7" derivedContent="RFC3264"/> of <xref target="RFC3264" format="default" sectionFormat="of" derivedContent="RFC3264"/>, and start
	       transmitting RTP media using that format once the
	       underlying transport layers have been established. If an
	       SSRC has not already been chosen for this outgoing RTP
	       stream, choose a unique random one. If media is already being
	       transmitted, the same SSRC SHOULD <bcp14>SHOULD</bcp14> be used unless the
            clockrate
	       clock rate of the new codec is different, in which case a
	       new SSRC MUST <bcp14>MUST</bcp14> be chosen, as specified in
	       <xref target="RFC7160" />, Section 3.1.</t>

            <t>The sectionFormat="comma" section="4.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7160#section-4.1" derivedContent="RFC7160"/>.</li>
              <li pn="section-5.11-3.4.2.5">The payload type mapping from the remote description is
	       used to determine payload types for the outgoing RTP
	       streams, including the payload type for the send media
	       format chosen above. Any RTP header extensions that were
	       negotiated should be included in the outgoing RTP streams,
	       using the extension mapping from the remote description; if description. If the RID MID
         header extension has been negotiated, and RID
            values are specified, include it in the RID outgoing RTP
         streams, as indicated in
         <xref target="RFC8843" sectionFormat="comma" section="15" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-15" derivedContent="RFC8843"/>.
	       If the RtpStreamId or RepairedRtpStreamId header extension extensions have been
         negotiated and rid-ids have been established, include these header
         extensions in the outgoing RTP streams, as indicated in
	       <xref target="I-D.ietf-mmusic-rid"></xref>, Section 4.</t>

            <t>If target="RFC8851" sectionFormat="comma" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8851#section-4" derivedContent="RFC8851"/>.</li>
              <li pn="section-5.11-3.4.2.6">If the m= "m=" section is of type audio, "audio", and silence
	       suppression was (1) configured for the send media format as a
	       result of processing the remote description, description and is (2) also
	       enabled for that format in the local description, use
	       silence suppression for outgoing media, in accordance with
	       the guidance in
	       <xref target="sec.voiceactivitydetection1" />. format="default" sectionFormat="of" derivedContent="Section 5.2.3.2"/>.
               If these
	       conditions are not met, silence suppression MUST NOT <bcp14>MUST NOT</bcp14> be
	       used for outgoing media.</t>

            <t>If media.</li>
              <li pn="section-5.11-3.4.2.7">If simulcast has been negotiated, send the appropriate number of
	       Source RTP Streams as specified in
	       <xref target="I-D.ietf-mmusic-sdp-simulcast"></xref>,
            Section 6.2.2.</t>

            <t>If target="RFC8853" sectionFormat="comma" section="5.3.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8853#section-5.3.3" derivedContent="RFC8853"/>.</li>
              <li pn="section-5.11-3.4.2.8">If the send media format chosen above has a
	       corresponding "rtx" media format, format or a FEC mechanism has
	       been negotiated, establish a Redundancy redundancy RTP Stream stream with a
	       unique random SSRC for each Source RTP Stream, and start or
	       continue transmitting RTX/FEC packets as needed.</t>

            <t>If needed.</li>
              <li pn="section-5.11-3.4.2.9">If the send media format chosen above has a
	       corresponding "red" media format of the same clockrate, clock rate,
	       allow redundant encoding using the specified format for
	       resiliency purposes, as discussed in
	       <xref target="I-D.ietf-rtcweb-fec" />, Section 3.2. target="RFC8854" sectionFormat="comma" section="3.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8854#section-3.2" derivedContent="RFC8854"/>. Note
	       that unlike RTX or FEC media formats, the "red" format is
	       transmitted on the Source RTP Stream, not the Redundancy redundancy
	       RTP Stream.</t>

            <t>Enable stream.</li>
              <li pn="section-5.11-3.4.2.10">Enable the RTCP feedback mechanisms referenced in the
	       media section for all Source RTP Streams using the
	       specified media formats. Specifically, begin or continue
	       sending the requested feedback types and reacting to
	       received feedback, as specified in
	       <xref target="RFC4585" />, Section 4.2. sectionFormat="comma" section="4.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4585#section-4.2" derivedContent="RFC4585"/>. When sending RTCP
	       feedback, follow the rules and recommendations from
   <xref target="RFC8108"></xref> Section 5.4.1, target="RFC8108" sectionFormat="comma" section="5.4.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8108#section-5.4.1" derivedContent="RFC8108"/> to select
	       which SSRC to use.</t>

            <t>If use.</li>
              <li pn="section-5.11-3.4.2.11">If the directional direction attribute in the answer indicates
	       that the JSEP implementation should not be sending media
	       ("recvonly" for local answers, "sendonly" for remote
	       answers, or "inactive" for either type of answer) answer), stop
	       transmitting all RTP media, but continue sending RTCP, as
	       described in
	       <xref target="RFC3264" />, Section 5.1.</t>
          </list></t>

          <t>If sectionFormat="comma" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3264#section-5.1" derivedContent="RFC3264"/>.</li>
            </ul>
          </li>
          <li pn="section-5.11-3.5">
            <t indent="0" pn="section-5.11-3.5.1">If the m= "m=" section proto &lt;proto&gt; value indicates use of SCTP:
          <list style="symbols">

            <t>If
            </t>
            <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.11-3.5.2">
              <li pn="section-5.11-3.5.2.1">If an SCTP association exists, exists and the remote SCTP port
	       has changed, discard the existing SCTP association. This
	       includes the case when the PeerConnection state is
            "have-remote-pranswer".</t>

            <t>If
	       "have-remote-pranswer".</li>
              <li pn="section-5.11-3.5.2.2">If no valid SCTP association exists, prepare to initiate
            a
	       an SCTP association over the associated ICE component and
	       DTLS connection, using the local SCTP port value from the
	       local description, description and the remote SCTP port value from the
	       remote description, as described in
	       <xref target="I-D.ietf-mmusic-sctp-sdp" />, Section
            10.2.</t>
          </list></t>
        </list></t>

        <t>If target="RFC8841" sectionFormat="comma" section="10.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8841#section-10.2" derivedContent="RFC8841"/>.</li>
            </ul>
          </li>
        </ul>
        <t indent="0" pn="section-5.11-4">If the answer contains valid bundle groups, discard any ICE
	   components for the m= "m=" sections that will be bundled onto the
	   primary ICE components in each bundle, and begin muxing these
        m=
	   "m=" sections accordingly, as described in
	   <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation" />,
        Section 8.2.</t>

        <t>If target="RFC8843" sectionFormat="comma" section="7.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-7.4" derivedContent="RFC8843"/>.</t>
        <t indent="0" pn="section-5.11-5">If the description is of type "answer", "answer" and there are still
	   remaining candidates in the ICE candidate pool, discard
	   them.</t>
      </section>
    </section>
    <section title="Processing RTP/RTCP" anchor="sec.rtp.demux">

      <t>When anchor="sec.rtp.demux" numbered="true" toc="include" removeInRFC="false" pn="section-6">
      <name slugifiedName="name-processing-rtp-rtcp">Processing RTP/RTCP</name>
      <t indent="0" pn="section-6-1">When bundling, associating incoming RTP/RTCP with the proper
      m=
	 "m=" section is defined in
	 <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation" />, Section
      10.2. target="RFC8843" sectionFormat="comma" section="9.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8843#section-9.2" derivedContent="RFC8843"/>. When not bundling, the proper m= "m=" section is clear from the
	 ICE component over which the RTP/RTCP is received.</t>

      <t>Once
      <t indent="0" pn="section-6-2">Once the proper m= section(s) "m=" section or sections are known, RTP/RTCP is delivered
	 to the RtpTransceiver(s) associated with the m= "m=" section(s) and
	 further processing of the RTP/RTCP is done at the RtpTransceiver
	 level. This includes using the RID mechanism
	 <xref target="I-D.ietf-mmusic-rid" /> target="RFC8851" format="default" sectionFormat="of" derivedContent="RFC8851"/> and its associated RtpStreamId and
   RepairedRtpStreamId identifiers to distinguish between
	 multiple Encoded Streams, as well as encoded streams and determine which Source RTP
	 stream should be repaired by a given Redundancy redundancy RTP stream.</t>
    </section>
    <section title="Examples" anchor="sec.examples">

      <t>Note anchor="sec.examples" numbered="true" toc="include" removeInRFC="false" pn="section-7">
      <name slugifiedName="name-examples">Examples</name>
      <t indent="0" pn="section-7-1">Note that this example section shows several SDP fragments. To
      format in 72 columns,
	 accommodate RFC line-length restrictions, some of the lines in SDP lines have been split
	 into multiple lines, where leading whitespace indicates that a
	 line is a continuation of the previous line. In addition, some
	 blank lines have been added to improve readability but are not
	 valid in SDP.</t>

      <t>More
      <t indent="0" pn="section-7-2">More examples of SDP for WebRTC call flows, including examples
	 with IPv6 addresses, can be found in
	 <xref target="I-D.ietf-rtcweb-sdp"></xref>.</t>
      <section title="Simple Example" anchor="sec.simple-examples">

        <t>This target="I-D.ietf-rtcweb-sdp" format="default" sectionFormat="of" derivedContent="SDP4WebRTC"/>.</t>
      <section anchor="sec.simple-examples" numbered="true" toc="include" removeInRFC="false" pn="section-7.1">
        <name slugifiedName="name-simple-example">Simple Example</name>
        <t indent="0" pn="section-7.1-1">This section shows a very simple example that sets up a
	   minimal audio / video audio/video call between two JSEP endpoints without
	   using trickle Trickle ICE. The example in the following section
	   provides a more detailed example of what could happen in a JSEP
	   session.</t>

        <t>The
        <t indent="0" pn="section-7.1-2">The code flow below shows Alice's endpoint initiating the
	   session to Bob's endpoint. The messages from the JavaScript
	   application in Alice's browser to the JavaScript in Bob's
	   browser, abbreviated as AliceJS "AliceJS" and BobJS "BobJS", respectively, are
	   assumed to flow over some signaling protocol via a web server.
	   The JavaScript on both Alice's side and Bob's side waits for
	   all candidates before sending the offer or answer, so the
	   offers and answers are complete; trickle Trickle ICE is not used. The
	   user agents (JSEP implementations) in Alice Alice's and Bob's browsers,
	   abbreviated as AliceUA "AliceUA" and BobUA "BobUA", respectively, are both using the
	   default bundle policy of "balanced", "balanced" and the default RTCP mux
	   policy of "require".</t>

        <t>
          <figure>
            <artwork>
              <![CDATA[
        <artwork name="" type="ascii-art" align="left" alt="" pn="section-7.1-3">
//                  set up local media state
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   create new PeerConnection
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   addTrack with two tracks: audio and video
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   createOffer to get offer
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   setLocalDescription with offer
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   multiple onicecandidate events with candidates

//                  wait for ICE gathering to complete
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   onicecandidate event with null candidate
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   get |offer-A1| from pendingLocalDescription

//                  |offer-A1| is sent over signaling protocol to Bob
AliceJS->WebServer:
AliceJS-&gt;WebServer: signaling with |offer-A1|
WebServer->BobJS:
WebServer-&gt;BobJS:   signaling with |offer-A1|

//                  |offer-A1| arrives at Bob
BobJS->BobUA:
BobJS-&gt;BobUA:       create a PeerConnection
BobJS->BobUA:
BobJS-&gt;BobUA:       setRemoteDescription with |offer-A1|
BobUA->BobJS:
BobUA-&gt;BobJS:       ontrack events for audio and video tracks

//                  Bob accepts call
BobJS->BobUA:
BobJS-&gt;BobUA:       addTrack with local tracks
BobJS->BobUA:
BobJS-&gt;BobUA:       createAnswer
BobJS->BobUA:
BobJS-&gt;BobUA:       setLocalDescription with answer
BobUA->BobJS:
BobUA-&gt;BobJS:       multiple onicecandidate events with candidates

//                  wait for ICE gathering to complete
BobUA->BobJS:
BobUA-&gt;BobJS:       onicecandidate event with null candidate
BobJS->BobUA:
BobJS-&gt;BobUA:       get |answer-A1| from currentLocalDescription

//                  |answer-A1| is sent over signaling protocol
//                  to Alice
BobJS->WebServer:
BobJS-&gt;WebServer:   signaling with |answer-A1|
WebServer->AliceJS:
WebServer-&gt;AliceJS: signaling with |answer-A1|

//                  |answer-A1| arrives at Alice
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   setRemoteDescription with |answer-A1|
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   ontrack events for audio and video tracks

//                  media flows
BobUA->AliceUA:
BobUA-&gt;AliceUA:     media sent from Bob to Alice
AliceUA->BobUA:
AliceUA-&gt;BobUA:     media sent from Alice to Bob
]]> </artwork>
          </figure>
        </t>

        <t>The
        <t indent="0" pn="section-7.1-4">The SDP for |offer-A1| looks like:</t>

        <t>
          <figure>
            <artwork alt="offer-A1">
              <![CDATA[
        <sourcecode name="offer-A1" type="sdp" markers="false" pn="section-7.1-5">
v=0
o=- 4962303333179871722 1 IN IP4 0.0.0.0
s=-
t=0 0
a=ice-options:trickle ice2
a=group:BUNDLE a1 v1
a=group:LS a1 v1

m=audio 10100 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 203.0.113.100
a=mid:a1
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:97 0-15
a=fmtp:98 0-15
a=maxptime:120
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=msid:47017fee-b6c1-4162-929c-a25110252400
a=ice-ufrag:ETEn
a=ice-pwd:OtSK0WpNtpUjkY4+86js7ZQl
a=fingerprint:sha-256
              19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04:
              BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:actpass
a=tls-id:91bbf309c0990a6bec11e38ba2933cee
a=rtcp:10101 IN IP4 203.0.113.100
a=rtcp-mux
a=rtcp-rsize
a=candidate:1 1 udp 2113929471 203.0.113.100 10100 typ host
a=candidate:1 2 udp 2113929470 203.0.113.100 10101 typ host
a=end-of-candidates

m=video 10102 UDP/TLS/RTP/SAVPF 100 101 102 103
c=IN IP4 203.0.113.100
a=mid:v1
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 H264/90000
a=fmtp:101 packetization-mode=1;profile-level-id=42e01f
a=rtpmap:102 rtx/90000
a=fmtp:102 apt=100
=rtpmap:103
a=rtpmap:103 rtx/90000
a=fmtp:103 apt=101
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=msid:47017fee-b6c1-4162-929c-a25110252400
a=ice-ufrag:BGKk
a=ice-pwd:mqyWsAjvtKwTGnvhPztQ9mIf
a=fingerprint:sha-256
              19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04:
              BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:actpass
a=tls-id:91bbf309c0990a6bec11e38ba2933cee
a=rtcp:10103 IN IP4 203.0.113.100
a=rtcp-mux
a=rtcp-rsize
a=candidate:1 1 udp 2113929471 203.0.113.100 10102 typ host
a=candidate:1 2 udp 2113929470 203.0.113.100 10103 typ host
a=end-of-candidates
]]>
</artwork>
          </figure>
        </t>

        <t>The
a=end-of-candidates</sourcecode>
        <t indent="0" pn="section-7.1-6">The SDP for |answer-A1| looks like:</t>

        <t>
          <figure>
            <artwork alt="answer-A1">
              <![CDATA[
        <sourcecode name="answer-A1" type="sdp" markers="false" pn="section-7.1-7">
v=0
o=- 6729291447651054566 1 IN IP4 0.0.0.0
s=-
t=0 0
a=ice-options:trickle ice2
a=group:BUNDLE a1 v1
a=group:LS a1 v1

m=audio 10200 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 203.0.113.200
a=mid:a1
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:97 0-15
a=fmtp:98 0-15
a=maxptime:120
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=msid:61317484-2ed4-49d7-9eb7-1414322a7aae
a=ice-ufrag:6sFv
a=ice-pwd:cOTZKZNVlO9RSGsEGM63JXT2
a=fingerprint:sha-256
              6B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35:
              DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=setup:active
a=tls-id:eec3392ab83e11ceb6a0990c903fbb19
a=rtcp-mux
a=rtcp-rsize
a=candidate:1 1 udp 2113929471 203.0.113.200 10200 typ host
a=end-of-candidates

m=video 10200 UDP/TLS/RTP/SAVPF 100 101 102 103
c=IN IP4 203.0.113.200
a=mid:v1
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 H264/90000
a=fmtp:101 packetization-mode=1;profile-level-id=42e01f
a=rtpmap:102 rtx/90000
a=fmtp:102 apt=100
=rtpmap:103
a=rtpmap:103 rtx/90000
a=fmtp:103 apt=101
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=msid:61317484-2ed4-49d7-9eb7-1414322a7aae
]]>
</artwork>
          </figure>
        </t>
a=msid:61317484-2ed4-49d7-9eb7-1414322a7aae</sourcecode>
      </section>
      <section title="Detailed Example" anchor="sec.detailed-example">

        <t>This anchor="sec.detailed-example" numbered="true" toc="include" removeInRFC="false" pn="section-7.2">
        <name slugifiedName="name-detailed-example">Detailed Example</name>
        <t indent="0" pn="section-7.2-1">This section shows a more involved example of a session
	   between two JSEP endpoints. Trickle ICE is used in full trickle
	   mode, with a bundle policy of "max-bundle", an RTCP mux policy
	   of "require", and a single TURN server. Initially, both Alice
	   and Bob establish an audio channel and a data channel. Later,
	   Bob adds two video flows, flows -- one for his video feed, feed and one for
        screensharing,
	   screen sharing, both supporting FEC, and FEC -- with the video feed
	   configured for simulcast. Alice accepts these video flows, flows but
	   does not add video flows of her own, so they are handled as
	   recvonly. Alice also specifies a maximum video decoder
	   resolution.</t>

        <t>
          <figure>
            <artwork>
              <![CDATA[
        <artwork name="" type="ascii-art" align="left" alt="" pn="section-7.2-2">
//                  set up local media state
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   create new PeerConnection
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   addTrack with an audio track
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   createDataChannel to get data channel
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   createOffer to get |offer-B1|
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   setLocalDescription with |offer-B1|

//                  |offer-B1| is sent over signaling protocol to Bob
AliceJS->WebServer:
AliceJS-&gt;WebServer: signaling with |offer-B1|
WebServer->BobJS:
WebServer-&gt;BobJS:   signaling with |offer-B1|

//                  |offer-B1| arrives at Bob
BobJS->BobUA:
BobJS-&gt;BobUA:       create a PeerConnection
BobJS->BobUA:
BobJS-&gt;BobUA:       setRemoteDescription with |offer-B1|
BobUA->BobJS:
BobUA-&gt;BobJS:       ontrack event with audio track from Alice

//                  candidates are sent to Bob
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   onicecandidate (host) |offer-B1-candidate-1|
AliceJS->WebServer:
AliceJS-&gt;WebServer: signaling with |offer-B1-candidate-1|
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   onicecandidate (srflx) |offer-B1-candidate-2|
AliceJS->WebServer:
AliceJS-&gt;WebServer: signaling with |offer-B1-candidate-2|
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   onicecandidate (relay) |offer-B1-candidate-3|
AliceJS->WebServer:
AliceJS-&gt;WebServer: signaling with |offer-B1-candidate-3|

WebServer->BobJS:

WebServer-&gt;BobJS:   signaling with |offer-B1-candidate-1|
BobJS->BobUA:
BobJS-&gt;BobUA:       addIceCandidate with |offer-B1-candidate-1|
WebServer->BobJS:
WebServer-&gt;BobJS:   signaling with |offer-B1-candidate-2|
BobJS->BobUA:
BobJS-&gt;BobUA:       addIceCandidate with |offer-B1-candidate-2|
WebServer->BobJS:
WebServer-&gt;BobJS:   signaling with |offer-B1-candidate-3|
BobJS->BobUA:
BobJS-&gt;BobUA:       addIceCandidate with |offer-B1-candidate-3|

//                  Bob accepts call
BobJS->BobUA:
BobJS-&gt;BobUA:       addTrack with local audio
BobJS->BobUA:
BobJS-&gt;BobUA:       createDataChannel to get data channel
BobJS->BobUA:
BobJS-&gt;BobUA:       createAnswer to get |answer-B1|
BobJS->BobUA:
BobJS-&gt;BobUA:       setLocalDescription with |answer-B1|

//                  |answer-B1| is sent to Alice
BobJS->WebServer:
BobJS-&gt;WebServer:   signaling with |answer-B1|
WebServer->AliceJS:
WebServer-&gt;AliceJS: signaling with |answer-B1|
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   setRemoteDescription with |answer-B1|
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   ontrack event with audio track from Bob

//                  candidates are sent to Alice
BobUA->BobJS:
BobUA-&gt;BobJS:       onicecandidate (host) |answer-B1-candidate-1|
BobJS->WebServer:
BobJS-&gt;WebServer:   signaling with |answer-B1-candidate-1|
BobUA->BobJS:
BobUA-&gt;BobJS:       onicecandidate (srflx) |answer-B1-candidate-2|
BobJS->WebServer:
BobJS-&gt;WebServer:   signaling with |answer-B1-candidate-2|
BobUA->BobJS:
BobUA-&gt;BobJS:       onicecandidate (relay) |answer-B1-candidate-3|
BobJS->WebServer:
BobJS-&gt;WebServer:   signaling with |answer-B1-candidate-3|

WebServer->AliceJS:

WebServer-&gt;AliceJS: signaling with |answer-B1-candidate-1|
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   addIceCandidate with |answer-B1-candidate-1|
WebServer->AliceJS:
WebServer-&gt;AliceJS: signaling with |answer-B1-candidate-2|
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   addIceCandidate with |answer-B1-candidate-2|
WebServer->AliceJS:
WebServer-&gt;AliceJS: signaling with |answer-B1-candidate-3|
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   addIceCandidate with |answer-B1-candidate-3|

//                  data channel opens
BobUA->BobJS:
BobUA-&gt;BobJS:       ondatachannel event
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   ondatachannel event
BobUA->BobJS:
BobUA-&gt;BobJS:       onopen
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   onopen

//                  media is flowing between endpoints
BobUA->AliceUA:
BobUA-&gt;AliceUA:     audio+data sent from Bob to Alice
AliceUA->BobUA:
AliceUA-&gt;BobUA:     audio+data sent from Alice to Bob

//                  some time later later, Bob adds two video streams
//                  note,                  note: no candidates exchanged, because of bundle
BobJS->BobUA:
BobJS-&gt;BobUA:       addTrack with first video stream
BobJS->BobUA:
BobJS-&gt;BobUA:       addTrack with second video stream
BobJS->BobUA:
BobJS-&gt;BobUA:       createOffer to get |offer-B2|
BobJS->BobUA:
BobJS-&gt;BobUA:       setLocalDescription with |offer-B2|

//                  |offer-B2| is sent to Alice
BobJS->WebServer:
BobJS-&gt;WebServer:   signaling with |offer-B2|
WebServer->AliceJS:
WebServer-&gt;AliceJS: signaling with |offer-B2|
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   setRemoteDescription with |offer-B2|
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   ontrack event with first video track
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   ontrack event with second video track
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   createAnswer to get |answer-B2|
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   setLocalDescription with |answer-B2|

//                  |answer-B2| is sent over signaling protocol
//                  to Bob
AliceJS->WebServer:
AliceJS-&gt;WebServer: signaling with |answer-B2|
WebServer->BobJS:
WebServer-&gt;BobJS:   signaling with |answer-B2|
BobJS->BobUA:
BobJS-&gt;BobUA:       setRemoteDescription with |answer-B2|

//                  media is flowing between endpoints
BobUA->AliceUA:
BobUA-&gt;AliceUA:     audio+video+data sent from Bob to Alice
AliceUA->BobUA:
AliceUA-&gt;BobUA:     audio+video+data sent from Alice to Bob
]]> </artwork>
          </figure>
        </t>

        <t>The
        <t indent="0" pn="section-7.2-3">The SDP for |offer-B1| looks like:</t>

        <t>
          <figure>
            <artwork alt="offer-B1">
              <![CDATA[
        <sourcecode name="offer-B1" type="sdp" markers="false" pn="section-7.2-4">
v=0
o=- 4962303333179871723 1 IN IP4 0.0.0.0
s=-
t=0 0
a=ice-options:trickle ice2
a=group:BUNDLE a1 d1

m=audio 9 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 0.0.0.0
a=mid:a1
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:97 0-15
a=fmtp:98 0-15
a=maxptime:120
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=msid:57017fee-b6c1-4162-929c-a25110252400
a=ice-ufrag:ATEn
a=ice-pwd:AtSK0WpNtpUjkY4+86js7ZQl
a=fingerprint:sha-256
              29:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04:
              BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:actpass
a=tls-id:17f0f4ba8a5f1213faca591b58ba52a7
a=rtcp-mux
a=rtcp-mux-only
a=rtcp-rsize

m=application 0 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 0.0.0.0
a=mid:d1
a=sctp-port:5000
a=max-message-size:65536
a=bundle-only
]]>
</artwork>
          </figure>
        </t>

        <t>|offer-B1-candidate-1|
a=bundle-only</sourcecode>
        <t indent="0" pn="section-7.2-5">|offer-B1-candidate-1| looks like:</t>

        <t>
          <figure>
            <artwork alt="offer-B1-candidate-1">
              <![CDATA[
        <sourcecode name="offer-B1-candidate-1" type="sdp" markers="false" pn="section-7.2-6">
ufrag ATEn
index 0
mid   a1
attr  candidate:1 1 udp 2113929471 203.0.113.100 10100 typ host
]]>
</artwork>
          </figure>
        </t>

        <t>|offer-B1-candidate-2| host</sourcecode>
        <t indent="0" pn="section-7.2-7">|offer-B1-candidate-2| looks like:</t>

        <t>
          <figure>
            <artwork alt="offer-B1-candidate-2">
              <![CDATA[
        <sourcecode name="offer-B1-candidate-2" type="sdp" markers="false" pn="section-7.2-8">
ufrag ATEn
index 0
mid   a1
attr  candidate:1 1 udp 1845494015 198.51.100.100 11100 typ srflx
                raddr 203.0.113.100 rport 10100
]]>
</artwork>
          </figure>
        </t>

        <t>|offer-B1-candidate-3| 10100</sourcecode>
        <t indent="0" pn="section-7.2-9">|offer-B1-candidate-3| looks like:</t>

        <t>
          <figure>
            <artwork alt="offer-B1-candidate-3">
              <![CDATA[
        <sourcecode name="offer-B1-candidate-3" type="sdp" markers="false" pn="section-7.2-10">
ufrag ATEn
index 0
mid   a1
attr  candidate:1 1 udp 255 192.0.2.100 12100 typ relay
                raddr 198.51.100.100 rport 11100
]]>
</artwork>
          </figure>
        </t>

        <t>The 11100</sourcecode>
        <t indent="0" pn="section-7.2-11">The SDP for |answer-B1| looks like:</t>

        <t>
          <figure>
            <artwork alt="answer-B1">
              <![CDATA[
        <sourcecode name="answer-B1" type="sdp" markers="false" pn="section-7.2-12">
v=0
o=- 7729291447651054566 1 IN IP4 0.0.0.0
s=-
t=0 0
a=ice-options:trickle ice2
a=group:BUNDLE a1 d1

m=audio 9 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 0.0.0.0
a=mid:a1
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:97 0-15
a=fmtp:98 0-15
a=maxptime:120
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=msid:71317484-2ed4-49d7-9eb7-1414322a7aae
a=ice-ufrag:7sFv
a=ice-pwd:dOTZKZNVlO9RSGsEGM63JXT2
a=fingerprint:sha-256
              7B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35:
              DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=setup:active
a=tls-id:7a25ab85b195acaf3121f5a8ab4f0f71
a=rtcp-mux
a=rtcp-mux-only
a=rtcp-rsize

m=application 9 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 0.0.0.0
a=mid:d1
a=sctp-port:5000
a=max-message-size:65536
]]>
</artwork>
          </figure>
        </t>

        <t>|answer-B1-candidate-1|
a=max-message-size:65536</sourcecode>
        <t indent="0" pn="section-7.2-13">|answer-B1-candidate-1| looks like:</t>

        <t>
          <figure>
            <artwork alt="answer-B1-candidate-1">
              <![CDATA[
        <sourcecode name="answer-B1-candidate-1" type="sdp" markers="false" pn="section-7.2-14">
ufrag 7sFv
index 0
mid   a1
attr  candidate:1 1 udp 2113929471 203.0.113.200 10200 typ host
]]>
</artwork>
          </figure>
        </t>

        <t>|answer-B1-candidate-2| host</sourcecode>
        <t indent="0" pn="section-7.2-15">|answer-B1-candidate-2| looks like:</t>

        <t>
          <figure>
            <artwork alt="answer-B1-candidate-2">
              <![CDATA[
        <sourcecode name="answer-B1-candidate-2" type="sdp" markers="false" pn="section-7.2-16">
ufrag 7sFv
index 0
mid   a1
attr  candidate:1 1 udp 1845494015 198.51.100.200 11200 typ srflx
                raddr 203.0.113.200 rport 10200
]]>
</artwork>
          </figure>
        </t>

        <t>|answer-B1-candidate-3| 10200</sourcecode>
        <t indent="0" pn="section-7.2-17">|answer-B1-candidate-3| looks like:</t>

        <t>
          <figure>
            <artwork alt="answer-B1-candidate-3">
              <![CDATA[
        <sourcecode name="answer-B1-candidate-3" type="sdp" markers="false" pn="section-7.2-18">
ufrag 7sFv
index 0
mid   a1
attr  candidate:1 1 udp 255 192.0.2.200 12200 typ relay
                raddr 198.51.100.200 rport 11200
]]>
</artwork>
          </figure>
        </t>

        <t>The 11200</sourcecode>
        <t indent="0" pn="section-7.2-19">The SDP for |offer-B2| is shown below. In addition to the
	   new m= "m=" sections for video, both of which are offering FEC, FEC and
	   one of which is offering simulcast, note the increment of the
	   version number in the o= line, "o=" line; changes to the c= "c=" line,
	   indicating the local candidate that was selected, selected; and the
	   inclusion of gathered candidates as a=candidate lines.</t>

        <t>
          <figure>
            <artwork alt="offer-B2">
              <![CDATA[
        <sourcecode name="offer-B2" type="sdp" markers="false" pn="section-7.2-20">
v=0
o=- 7729291447651054566 2 IN IP4 0.0.0.0
s=-
t=0 0
a=ice-options:trickle ice2
a=group:BUNDLE a1 d1 v1 v2
a=group:LS a1 v1

m=audio 12200 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 192.0.2.200
a=mid:a1
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:97 0-15
a=fmtp:98 0-15
a=maxptime:120
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=msid:71317484-2ed4-49d7-9eb7-1414322a7aae
a=ice-ufrag:7sFv
a=ice-pwd:dOTZKZNVlO9RSGsEGM63JXT2
a=fingerprint:sha-256
              7B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35:
              DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=setup:actpass
a=tls-id:7a25ab85b195acaf3121f5a8ab4f0f71
a=rtcp-mux
a=rtcp-mux-only
a=rtcp-rsize
a=candidate:1 1 udp 2113929471 203.0.113.200 10200 typ host
a=candidate:1 1 udp 1845494015 198.51.100.200 11200 typ srflx
            raddr 203.0.113.200 rport 10200
a=candidate:1 1 udp 255 192.0.2.200 12200 typ relay
            raddr 198.51.100.200 rport 11200
a=end-of-candidates

m=application 12200 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 192.0.2.200
a=mid:d1
a=sctp-port:5000
a=max-message-size:65536

m=video 12200 UDP/TLS/RTP/SAVPF 100 101 102 103 104
c=IN IP4 192.0.2.200
a=mid:v1
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 H264/90000
a=fmtp:101 packetization-mode=1;profile-level-id=42e01f
a=rtpmap:102 rtx/90000
a=fmtp:102 apt=100
=rtpmap:103
a=rtpmap:103 rtx/90000
a=fmtp:103 apt=101
a=rtpmap:104 flexfec/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=msid:71317484-2ed4-49d7-9eb7-1414322a7aae
a=rid:1 send
a=rid:2 send
a=rid:3 send
a=simulcast:send 1;2;3

m=video 12200 UDP/TLS/RTP/SAVPF 100 101 102 103 104
c=IN IP4 192.0.2.200
a=mid:v2
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 H264/90000
a=fmtp:101 packetization-mode=1;profile-level-id=42e01f
a=rtpmap:102 rtx/90000
a=fmtp:102 apt=100
=rtpmap:103
a=rtpmap:103 rtx/90000
a=fmtp:103 apt=101
a=rtpmap:104 flexfec/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=msid:81317484-2ed4-49d7-9eb7-1414322a7aae
]]>
</artwork>
          </figure>
        </t>

        <t>The
a=msid:81317484-2ed4-49d7-9eb7-1414322a7aae</sourcecode>
        <t indent="0" pn="section-7.2-21">The SDP for |answer-B2| is shown below. In addition to the
	   acceptance of the video m= "m=" sections, the use of a=recvonly to
	   indicate one-way video, and the use of a=imageattr to limit the
	   received resolution, note the use of setup:passive to maintain
	   the existing DTLS roles.</t>

        <t>
          <figure>
            <artwork alt="answer-B2">
              <![CDATA[
        <sourcecode name="answer-B2" type="sdp" markers="false" pn="section-7.2-22">
v=0
o=- 4962303333179871723 2 IN IP4 0.0.0.0
s=-
t=0 0
a=ice-options:trickle ice2
a=group:BUNDLE a1 d1 v1 v2
a=group:LS a1 v1

m=audio 12100 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 192.0.2.100
a=mid:a1
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:97 0-15
a=fmtp:98 0-15
a=maxptime:120
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=msid:57017fee-b6c1-4162-929c-a25110252400
a=ice-ufrag:ATEn
a=ice-pwd:AtSK0WpNtpUjkY4+86js7ZQl
a=fingerprint:sha-256
              29:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04:
              BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:passive
a=tls-id:17f0f4ba8a5f1213faca591b58ba52a7
a=rtcp-mux
a=rtcp-mux-only
a=rtcp-rsize
a=candidate:1 1 udp 2113929471 203.0.113.100 10100 typ host
a=candidate:1 1 udp 1845494015 198.51.100.100 11100 typ srflx
            raddr 203.0.113.100 rport 10100
a=candidate:1 1 udp 255 192.0.2.100 12100 typ relay
            raddr 198.51.100.100 rport 11100
a=end-of-candidates

m=application 12100 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 192.0.2.100
a=mid:d1
a=sctp-port:5000
a=max-message-size:65536

m=video 12100 UDP/TLS/RTP/SAVPF 100 101 102 103
c=IN IP4 192.0.2.100
a=mid:v1
a=recvonly
a=rtpmap:100 VP8/90000
a=rtpmap:101 H264/90000
a=fmtp:101 packetization-mode=1;profile-level-id=42e01f
a=rtpmap:102 rtx/90000
a=fmtp:102 apt=100
=rtpmap:103
a=rtpmap:103 rtx/90000
a=fmtp:103 apt=101
a=imageattr:100 recv [x=[48:1920],y=[48:1080],q=1.0]
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli

m=video 12100 UDP/TLS/RTP/SAVPF 100 101 102 103
c=IN IP4 192.0.2.100
a=mid:v2
a=recvonly
a=rtpmap:100 VP8/90000
a=rtpmap:101 H264/90000
a=fmtp:101 packetization-mode=1;profile-level-id=42e01f
a=rtpmap:102 rtx/90000
a=fmtp:102 apt=100
=rtpmap:103
a=rtpmap:103 rtx/90000
a=fmtp:103 apt=101
a=imageattr:100 recv [x=[48:1920],y=[48:1080],q=1.0]
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
]]>
</artwork>
          </figure>
        </t> pli</sourcecode>
      </section>
      <section title="Early anchor="sec.warmup-example" numbered="true" toc="include" removeInRFC="false" pn="section-7.3">
        <name slugifiedName="name-early-transport-warmup-exam">Early Transport Warmup Example"
      anchor="sec.warmup-example">

        <t>This Example</name>
        <t indent="0" pn="section-7.3-1">This example demonstrates the early warmup early-warmup technique
	   described in
	   <xref target="sec.use-of-provisional-answer" />. format="default" sectionFormat="of" derivedContent="Section 4.1.10.1"/>. Here, Alice's
	   endpoint sends an offer to Bob's endpoint to start an
	   audio/video call. Bob immediately responds with an answer that
	   accepts the audio/video m= sections, "m=" sections but marks them as sendonly
	   (from his perspective), meaning that Alice will not yet send
	   media. This allows the JSEP implementation to start negotiating
	   ICE and DTLS immediately. Bob's endpoint then prompts him to
	   answer the call, and when he does, his endpoint sends a second
        offer
	   offer, which enables the audio and video m= "m=" sections, and
	   thereby bidirectional media transmission. The advantage of such
	   a flow is that as soon as the first answer is received, the
	   implementation can proceed with ICE and DTLS negotiation and
	   establish the session transport. If the transport setup
	   completes before the second offer is sent, then media can be
	   transmitted immediately by the callee immediately upon
	   answering the call, minimizing perceived post-dial-delay. post-dial delay. The
	   second offer/answer exchange can also change the preferred
	   codecs or other session parameters.</t>

        <t>This
        <t indent="0" pn="section-7.3-2">This example also makes use of the "relay" ICE candidate
	   policy described in
	   <xref target="sec.ice-candidate-policy" /> format="default" sectionFormat="of" derivedContent="Section 3.5.3"/> to minimize the ICE
	   gathering and checking needed.</t>

        <t>
          <figure>
            <artwork>
              <![CDATA[
        <artwork name="" type="ascii-art" align="left" alt="" pn="section-7.3-3">
//                  set up local media state
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   create new PeerConnection with "relay" ICE policy
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   addTrack with two tracks: audio and video
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   createOffer to get |offer-C1|
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   setLocalDescription with |offer-C1|

//                  |offer-C1| is sent over signaling protocol to Bob
AliceJS->WebServer:
AliceJS-&gt;WebServer: signaling with |offer-C1|
WebServer->BobJS:
WebServer-&gt;BobJS:   signaling with |offer-C1|

//                  |offer-C1| arrives at Bob
BobJS->BobUA:
BobJS-&gt;BobUA:       create new PeerConnection with "relay" ICE policy
BobJS->BobUA:
BobJS-&gt;BobUA:       setRemoteDescription with |offer-C1|
BobUA->BobJS:
BobUA-&gt;BobJS:       ontrack events for audio and video

//                  a relay candidate is sent to Bob
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   onicecandidate (relay) |offer-C1-candidate-1|
AliceJS->WebServer:
AliceJS-&gt;WebServer: signaling with |offer-C1-candidate-1|

WebServer->BobJS:

WebServer-&gt;BobJS:   signaling with |offer-C1-candidate-1|
BobJS->BobUA:
BobJS-&gt;BobUA:       addIceCandidate with |offer-C1-candidate-1|

//                  Bob prepares an early answer to warmup warm up the
//                  transport
BobJS->BobUA:
BobJS-&gt;BobUA:       addTransceiver with null audio and video tracks
BobJS->BobUA:
BobJS-&gt;BobUA:       transceiver.setDirection(sendonly) for both
BobJS->BobUA:
BobJS-&gt;BobUA:       createAnswer
BobJS->BobUA:
BobJS-&gt;BobUA:       setLocalDescription with answer

//                  |answer-C1| is sent over signaling protocol
//                  to Alice
BobJS->WebServer:
BobJS-&gt;WebServer:   signaling with |answer-C1|
WebServer->AliceJS:
WebServer-&gt;AliceJS: signaling with |answer-C1|

//                  |answer-C1| (sendonly) arrives at Alice
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   setRemoteDescription with |answer-C1|
AliceUA->AliceJS:
AliceUA-&gt;AliceJS:   ontrack events for audio and video

//                  a relay candidate is sent to Alice
BobUA->BobJS:
BobUA-&gt;BobJS:       onicecandidate (relay) |answer-B1-candidate-1|
BobJS->WebServer:
BobJS-&gt;WebServer:   signaling with |answer-B1-candidate-1|

WebServer->AliceJS:

WebServer-&gt;AliceJS: signaling with |answer-B1-candidate-1|
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   addIceCandidate with |answer-B1-candidate-1|

//                  ICE and DTLS establish while call is ringing

//                  Bob accepts call, starts media, and sends
//                  new offer
BobJS->BobUA:
BobJS-&gt;BobUA:       transceiver.setTrack with audio and video tracks
BobUA->AliceUA:
BobUA-&gt;AliceUA:     media sent from Bob to Alice
BobJS->BobUA:
BobJS-&gt;BobUA:       transceiver.setDirection(sendrecv) for both
                    transceivers
BobJS->BobUA:
BobJS-&gt;BobUA:       createOffer
BobJS->BobUA:
BobJS-&gt;BobUA:       setLocalDescription with offer

//                  |offer-C2| is sent over signaling protocol
//                  to Alice
BobJS->WebServer:
BobJS-&gt;WebServer:   signaling with |offer-C2|
WebServer->AliceJS:
WebServer-&gt;AliceJS: signaling with |offer-C2|

//                  |offer-C2| (sendrecv) arrives at Alice
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   setRemoteDescription with |offer-C2|
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   createAnswer
AliceJS->AliceUA:
AliceJS-&gt;AliceUA:   setLocalDescription with |answer-C2|
AliceUA->BobUA:
AliceUA-&gt;BobUA:     media sent from Alice to Bob

//                  |answer-C2| is sent over signaling protocol
//                  to Bob
AliceJS->WebServer:
AliceJS-&gt;WebServer: signaling with |answer-C2|
WebServer->BobJS:
WebServer-&gt;BobJS:   signaling with |answer-C2|
BobJS->BobUA:
BobJS-&gt;BobUA:       setRemoteDescription with |answer-C2|
]]> </artwork>
          </figure>
        </t>

        <t>The
        <t indent="0" pn="section-7.3-4">The SDP for |offer-C1| looks like:</t>

        <t>
          <figure>
            <artwork alt="offer-C1">
              <![CDATA[
        <sourcecode name="offer-C1" type="sdp" markers="false" pn="section-7.3-5">
v=0
o=- 1070771854436052752 1 IN IP4 0.0.0.0
s=-
t=0 0
a=ice-options:trickle ice2
a=group:BUNDLE a1 v1
a=group:LS a1 v1

m=audio 9 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 0.0.0.0
a=mid:a1
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:97 0-15
a=fmtp:98 0-15
a=maxptime:120
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=msid:bbce3ba6-abfc-ac63-d00a-e15b286f8fce
a=ice-ufrag:4ZcD
a=ice-pwd:ZaaG6OG7tCn4J/lehAGz+HHD
a=fingerprint:sha-256
              C4:68:F8:77:6A:44:F1:98:6D:7C:9F:47:EB:E3:34:A4:
              0A:AA:2D:49:08:28:70:2E:1F:AE:18:7D:4E:3E:66:BF
a=setup:actpass
a=tls-id:9e5b948ade9c3d41de6617b68f769e55
a=rtcp-mux
a=rtcp-mux-only
a=rtcp-rsize

m=video 0 UDP/TLS/RTP/SAVPF 100 101 102 103
c=IN IP4 0.0.0.0
a=mid:v1
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 H264/90000
a=fmtp:101 packetization-mode=1;profile-level-id=42e01f
a=rtpmap:102 rtx/90000
a=fmtp:102 apt=100
=rtpmap:103
a=rtpmap:103 rtx/90000
a=fmtp:103 apt=101
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=msid:bbce3ba6-abfc-ac63-d00a-e15b286f8fce
a=bundle-only
]]>
</artwork>
          </figure>
        </t>

        <t>|offer-C1-candidate-1|
a=bundle-only</sourcecode>
        <t indent="0" pn="section-7.3-6">|offer-C1-candidate-1| looks like:</t>

        <t>
          <figure>
            <artwork alt="offer-C1-candidate-1">
              <![CDATA[
        <sourcecode name="offer-C1-candidate-1" type="sdp" markers="false" pn="section-7.3-7">
ufrag 4ZcD
index 0
mid   a1
attr  candidate:1 1 udp 255 192.0.2.100 12100 typ relay
                raddr 0.0.0.0 rport 0
]]>
</artwork>
          </figure>
        </t>

        <t>The 0</sourcecode>
        <t indent="0" pn="section-7.3-8">The SDP for |answer-C1| looks like:</t>

        <t>
          <figure>
            <artwork alt="answer-C1">
              <![CDATA[
        <sourcecode name="answer-C1" type="sdp" markers="false" pn="section-7.3-9">
v=0
o=- 6386516489780559513 1 IN IP4 0.0.0.0
s=-
t=0 0
a=ice-options:trickle ice2
a=group:BUNDLE a1 v1
a=group:LS a1 v1

m=audio 9 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 0.0.0.0
a=mid:a1
a=sendonly
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:97 0-15
a=fmtp:98 0-15
a=maxptime:120
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=msid:751f239e-4ae0-c549-aa3d-890de772998b
a=ice-ufrag:TpaA
a=ice-pwd:t2Ouhc67y8JcCaYZxUUTgKw/
a=fingerprint:sha-256
              A2:F3:A5:6D:4C:8C:1E:B2:62:10:4A:F6:70:61:C4:FC:
              3C:E0:01:D6:F3:24:80:74:DA:7C:3E:50:18:7B:CE:4D
a=setup:active
a=tls-id:55e967f86b7166ed14d3c9eda849b5e9
a=rtcp-mux
a=rtcp-mux-only
a=rtcp-rsize

m=video 9 UDP/TLS/RTP/SAVPF 100 101 102 103
c=IN IP4 0.0.0.0
a=mid:v1
a=sendonly
a=rtpmap:100 VP8/90000
a=rtpmap:101 H264/90000
a=fmtp:101 packetization-mode=1;profile-level-id=42e01f
a=rtpmap:102 rtx/90000
a=fmtp:102 apt=100
=rtpmap:103
a=rtpmap:103 rtx/90000
a=fmtp:103 apt=101
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=msid:751f239e-4ae0-c549-aa3d-890de772998b
]]>
</artwork>
          </figure>
        </t>

        <t>|answer-C1-candidate-1|
a=msid:751f239e-4ae0-c549-aa3d-890de772998b</sourcecode>
        <t indent="0" pn="section-7.3-10">|answer-C1-candidate-1| looks like:</t>

        <t>
          <figure>
            <artwork alt="answer-C1-candidate-1">
              <![CDATA[
        <sourcecode name="answer-C1-candidate-1" type="sdp" markers="false" pn="section-7.3-11">
ufrag TpaA
index 0
mid   a1
attr  candidate:1 1 udp 255 192.0.2.200 12200 typ relay
                raddr 0.0.0.0 rport 0
]]>
</artwork>
          </figure>
        </t>

        <t>The 0</sourcecode>
        <t indent="0" pn="section-7.3-12">The SDP for |offer-C2| looks like:</t>

        <t>
          <figure>
            <artwork alt="offer-C2">
              <![CDATA[
        <sourcecode name="offer-C2" type="sdp" markers="false" pn="section-7.3-13">
v=0
o=- 6386516489780559513 2 IN IP4 0.0.0.0
s=-
t=0 0
a=ice-options:trickle ice2
a=group:BUNDLE a1 v1
a=group:LS a1 v1

m=audio 12200 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 192.0.2.200
a=mid:a1
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:97 0-15
a=fmtp:98 0-15
a=maxptime:120
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=msid:751f239e-4ae0-c549-aa3d-890de772998b
a=ice-ufrag:TpaA
a=ice-pwd:t2Ouhc67y8JcCaYZxUUTgKw/
a=fingerprint:sha-256
              A2:F3:A5:6D:4C:8C:1E:B2:62:10:4A:F6:70:61:C4:FC:
              3C:E0:01:D6:F3:24:80:74:DA:7C:3E:50:18:7B:CE:4D
a=setup:actpass
a=tls-id:55e967f86b7166ed14d3c9eda849b5e9
a=rtcp-mux
a=rtcp-mux-only
a=rtcp-rsize
a=candidate:1 1 udp 255 192.0.2.200 12200 typ relay
            raddr 0.0.0.0 rport 0
a=end-of-candidates

m=video 12200 UDP/TLS/RTP/SAVPF 100 101 102 103
c=IN IP4 192.0.2.200
a=mid:v1
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 H264/90000
a=fmtp:101 packetization-mode=1;profile-level-id=42e01f
a=rtpmap:102 rtx/90000
a=fmtp:102 apt=100
=rtpmap:103
a=rtpmap:103 rtx/90000
a=fmtp:103 apt=101
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=msid:751f239e-4ae0-c549-aa3d-890de772998b
]]>
</artwork>
          </figure>
        </t>

        <t>The
a=msid:751f239e-4ae0-c549-aa3d-890de772998b</sourcecode>
        <t indent="0" pn="section-7.3-14">The SDP for |answer-C2| looks like:</t>

        <t>
          <figure>
            <artwork alt="answer-C2">
              <![CDATA[
        <sourcecode name="answer-C2" type="sdp" markers="false" pn="section-7.3-15">
v=0
o=- 1070771854436052752 2 IN IP4 0.0.0.0
s=-
t=0 0
a=ice-options:trickle ice2
a=group:BUNDLE a1 v1
a=group:LS a1 v1

m=audio 12100 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 192.0.2.100
a=mid:a1
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:97 0-15
a=fmtp:98 0-15
a=maxptime:120
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=msid:bbce3ba6-abfc-ac63-d00a-e15b286f8fce
a=ice-ufrag:4ZcD
a=ice-pwd:ZaaG6OG7tCn4J/lehAGz+HHD
a=fingerprint:sha-256
              C4:68:F8:77:6A:44:F1:98:6D:7C:9F:47:EB:E3:34:A4:
              0A:AA:2D:49:08:28:70:2E:1F:AE:18:7D:4E:3E:66:BF
a=setup:passive
a=tls-id:9e5b948ade9c3d41de6617b68f769e55
a=rtcp-mux
a=rtcp-mux-only
a=rtcp-rsize
a=candidate:1 1 udp 255 192.0.2.100 12100 typ relay
            raddr 0.0.0.0 rport 0
a=end-of-candidates

m=video 12100 UDP/TLS/RTP/SAVPF 100 101 102 103
c=IN IP4 192.0.2.100
a=mid:v1
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 H264/90000
a=fmtp:101 packetization-mode=1;profile-level-id=42e01f
a=rtpmap:102 rtx/90000
a=fmtp:102 apt=100
=rtpmap:103
a=rtpmap:103 rtx/90000
a=fmtp:103 apt=101
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=msid:bbce3ba6-abfc-ac63-d00a-e15b286f8fce
]]>
</artwork>
          </figure>
        </t>
a=msid:bbce3ba6-abfc-ac63-d00a-e15b286f8fce</sourcecode>
      </section>
    </section>
    <section title="Security Considerations"
    anchor="sec.security-considerations">

      <t>The anchor="sec.security-considerations" numbered="true" toc="include" removeInRFC="false" pn="section-8">
      <name slugifiedName="name-security-considerations">Security Considerations</name>
      <t indent="0" pn="section-8-1">The IETF has published separate documents
	 <xref target="I-D.ietf-rtcweb-security-arch" />
      <xref target="I-D.ietf-rtcweb-security" /> target="RFC8827" format="default" sectionFormat="of" derivedContent="RFC8827"/>
        <xref target="RFC8826" format="default" sectionFormat="of" derivedContent="RFC8826"/> describing the security
	 architecture for WebRTC as a whole. The remainder of this section
	 describes security considerations for this document.</t>

      <t>While
      <t indent="0" pn="section-8-2">While formally the JSEP interface is an API, it is better to
	 think of it as an Internet protocol, with the application
	 JavaScript being untrustworthy from the perspective of the JSEP
	 implementation. Thus, the threat model of
	 <xref target="RFC3552" /> format="default" sectionFormat="of" derivedContent="RFC3552"/> applies. In particular, JavaScript can
	 call the API in any order and with any inputs, including
	 malicious ones. This is particularly relevant when we consider
	 the SDP which that is passed to setLocalDescription(). setLocalDescription. While correct
	 API usage requires that the application pass in SDP which that was
	 derived from createOffer() createOffer or createAnswer(), createAnswer, there is no
	 guarantee that applications do so. The JSEP implementation MUST <bcp14>MUST</bcp14>
	 be prepared for the JavaScript to pass in bogus data instead.</t>

      <t>Conversely,
      <t indent="0" pn="section-8-3">Conversely, the application programmer needs to be aware that
	 the JavaScript does not have complete control of endpoint
	 behavior. One case that bears particular mention is that editing
	 ICE candidates out of the SDP or suppressing trickled candidates
	 does not have the expected behavior: implementations will still
	 perform checks from those candidates even if they are not sent to
	 the other side. Thus, for instance, it is not possible to prevent
	 the remote peer from learning your public IP address by removing
      server reflexive
	 server-reflexive candidates. Applications which that wish to conceal
	 their public IP address should <bcp14>MUST</bcp14> instead configure the ICE agent to
	 use only relay candidates.</t>
    </section>
    <section title="IANA Considerations"
    anchor="sec.iana-considerations">

      <t>This anchor="sec.iana-considerations" numbered="true" toc="include" removeInRFC="false" pn="section-9">
      <name slugifiedName="name-iana-considerations">IANA Considerations</name>
      <t indent="0" pn="section-9-1">This document requires has no actions from IANA.</t>
    </section>
    <section title="Acknowledgements" anchor="sec.acknowledgements">

      <t>Harald Alvestrand, Taylor Brandstetter, Suhas Nandakumar, and
      Peter Thatcher provided significant text for this draft. Bernard
      Aboba, Adam Bergkvist, Dan Burnett, Ben Campbell, Alissa Cooper,
      Richard Ejzak, Stefan Hakansson, Ted Hardie, Christer Holmberg
      Andrew Hutton, Randell Jesup, Matthew Kaufman, Anant Narayanan,
      Adam Roach, Robert Sparks, Neil Stratford, Martin Thomson, Sean
      Turner, and Magnus Westerlund all provided valuable feedback on
      this proposal.</t> IANA actions.</t>
    </section>
  </middle>
  <back>
    <displayreference target="I-D.ietf-rtcweb-sdp" to="SDP4WebRTC"/>
    <references title="Normative References">
      <?rfc include='reference.I-D.ietf-avtext-rid'?>
      <?rfc include='reference.I-D.ietf-ice-trickle'?>
      <?rfc include='reference.I-D.ietf-mmusic-dtls-sdp'?>
      <?rfc include='reference.I-D.ietf-mmusic-ice-sip-sdp'?>
      <?rfc include='reference.I-D.ietf-mmusic-msid'?>
      <?rfc include='reference.I-D.ietf-mmusic-mux-exclusive'?>
      <?rfc include='reference.I-D.ietf-mmusic-rid'?>
      <?rfc include='reference.I-D.ietf-mmusic-sctp-sdp'?>
      <?rfc include='reference.I-D.ietf-mmusic-sdp-bundle-negotiation'?>
      <?rfc include='reference.I-D.ietf-mmusic-sdp-mux-attributes'?>
      <?rfc include='reference.I-D.ietf-mmusic-sdp-simulcast'?>
      <?rfc include='reference.I-D.ietf-rtcweb-fec'?>
      <?rfc include='reference.I-D.ietf-rtcweb-rtp-usage'?>
      <?rfc include='reference.I-D.ietf-rtcweb-security'?>
      <?rfc include='reference.I-D.ietf-rtcweb-security-arch'?>
      <?rfc include='reference.RFC.2119.xml'?>
      <?rfc include='reference.RFC.3261.xml'?>
      <?rfc include='reference.RFC.3264.xml'?>
      <?rfc include='reference.RFC.3552.xml'?>
      <?rfc include='reference.RFC.3605.xml'?>
      <?rfc include='reference.RFC.3890.xml'?>
      <?rfc include='reference.RFC.4145.xml'?>
      <?rfc include='reference.RFC.4566.xml'?>
      <?rfc include='reference.RFC.4585.xml'?>
      <?rfc include='reference.RFC.5124.xml'?>
      <?rfc include='reference.RFC.5285.xml'?>
      <?rfc include='reference.RFC.5761.xml'?>
      <?rfc include='reference.RFC.5888.xml'?>
      <?rfc include='reference.RFC.6236.xml'?>
      <?rfc include='reference.RFC.6347.xml'?>
      <?rfc include='reference.RFC.6716.xml'?>
      <?rfc include='reference.RFC.6904.xml'?>
      <?rfc include='reference.RFC.7160.xml'?>
      <?rfc include='reference.RFC.7587.xml'?>
      <?rfc include='reference.RFC.7742.xml'?>
      <?rfc include='reference.RFC.7850.xml'?>
      <?rfc include='reference.RFC.7874.xml'?>
      <?rfc include='reference.RFC.8108.xml'?>
      <?rfc include='reference.RFC.8122.xml'?>
      <?rfc include='reference.RFC.8445.xml'?>
      <?rfc include='reference.RFC.3711.xml'?>
    </references> pn="section-10">
      <name slugifiedName="name-references">References</name>
      <references title="Informative References">
      <?rfc include='reference.I-D.ietf-rtcweb-ip-handling'?>
      <?rfc include='reference.I-D.ietf-mmusic-trickle-ice-sip'?>
      <?rfc include='reference.I-D.ietf-rtcweb-sdp'?>
      <?rfc include='reference.RFC.3389.xml'?>
      <?rfc include='reference.RFC.3960.xml'?>
      <?rfc include='reference.RFC.4568.xml'?>
      <?rfc include='reference.RFC.4588.xml'?>
      <?rfc include='reference.RFC.4733.xml'?>
      <?rfc include='reference.RFC.5245.xml'?>
      <?rfc include='reference.RFC.5506.xml'?>
      <?rfc include='reference.RFC.5576.xml'?>
      <?rfc include='reference.RFC.5763.xml'?>
      <?rfc include='reference.RFC.5764.xml'?>
      <?rfc include='reference.RFC.6464.xml'?>
      <?rfc include='reference.RFC.6544.xml'?>
      <?rfc include='reference.RFC.3556.xml'?> pn="section-10.1">
        <name slugifiedName="name-normative-references">Normative References</name>
        <reference anchor="W3C.webrtc"
      target="https://www.w3.org/TR/2017/WD-webrtc-20170515/"> anchor="RFC2119" target="https://www.rfc-editor.org/info/rfc2119" quoteTitle="true" derivedAnchor="RFC2119">
          <front>
          <title>WebRTC 1.0: Real-time Communication Between
          Browsers</title>
            <title>Key words for use in RFCs to Indicate Requirement Levels</title>
            <author fullname="Adam Bergkvist" initials="A."
          surname="Bergkvist">
            <organization>Ericsson</organization> initials="S." surname="Bradner" fullname="S. Bradner">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="1997" month="March"/>
            <abstract>
              <t indent="0">In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized. This document defines these words as they should be interpreted in IETF documents.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="14"/>
          <seriesInfo name="RFC" value="2119"/>
          <seriesInfo name="DOI" value="10.17487/RFC2119"/>
        </reference>
        <reference anchor="RFC3261" target="https://www.rfc-editor.org/info/rfc3261" quoteTitle="true" derivedAnchor="RFC3261">
          <front>
            <title>SIP: Session Initiation Protocol</title>
            <author initials="J." surname="Rosenberg" fullname="J. Rosenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="Daniel C. Burnett" initials="D."
          surname="Burnett">
            <organization></organization> initials="H." surname="Schulzrinne" fullname="H. Schulzrinne">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="Cullen Jennings" initials="C."
          surname="Jennings">
            <organization>Cisco</organization> initials="G." surname="Camarillo" fullname="G. Camarillo">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="Anant Narayanan" initials="A."
          surname="Narayanan">
            <organization>Mozilla</organization> surname="Johnston" fullname="A. Johnston">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="Bernard Aboba" initials="B."
          surname="Aboba">
            <organization>Microsoft Corporation</organization> initials="J." surname="Peterson" fullname="J. Peterson">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="Taylor Brandstetter" initials="T."
          surname="Brandstetter">
            <organization>Google</organization> initials="R." surname="Sparks" fullname="R. Sparks">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Handley" fullname="M. Handley">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E." surname="Schooler" fullname="E. Schooler">
              <organization showOnFrontPage="true"/>
            </author>
            <date day="15" month="May" year="2017" /> year="2002" month="June"/>
            <abstract>
              <t indent="0">This document describes Session Initiation Protocol (SIP), an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants.  These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="World Wide Web Consortium WD"
        value="WD-webrtc-20170515" />
        <format target="https://www.w3.org/TR/2017/WD-webrtc-20170515/"
        type="HTML" /> name="RFC" value="3261"/>
          <seriesInfo name="DOI" value="10.17487/RFC3261"/>
        </reference>
        <reference anchor="TS26.114"
      target="http://www.3gpp.org/DynaReport/26114.htm"> anchor="RFC3264" target="https://www.rfc-editor.org/info/rfc3264" quoteTitle="true" derivedAnchor="RFC3264">
          <front>
          <title>3rd Generation Partnership Project; Technical
          Specification Group Services and System Aspects; IP
          Multimedia Subsystem (IMS); Multimedia Telephony; Media
          handling and interaction (Release 12)</title>
          <author>
            <organization>3GPP TS 26.114 V12.8.0</organization>
            <title>An Offer/Answer Model with Session Description Protocol (SDP)</title>
            <author initials="J." surname="Rosenberg" fullname="J. Rosenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="H." surname="Schulzrinne" fullname="H. Schulzrinne">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2014" month="December" />
        </front>
      </reference>
    </references>
    <section title="Appendix A" anchor="sec.appendix-a">

      <t>For year="2002" month="June"/>
            <abstract>
              <t indent="0">This document defines a mechanism by which two entities can make use of the syntax validation performed Session Description Protocol (SDP) to arrive at a common view of a multimedia session between them.  In the model, one participant offers the other a description of the desired session from their perspective, and the other participant answers with the desired session from their perspective.  This offer/answer model is most useful in
      <xref target="sec.parsing-a-desc" />, unicast sessions where information from both participants is needed for the following list complete view of ABNF
      definitions the session.  The offer/answer model is used:</t>
      <texttable anchor="sdp-abnf" title="SDP ABNF References">
        <ttcol align='left'>Attribute</ttcol>
        <ttcol align='left'>Reference</ttcol>
        <c>ptime</c>
        <c>
        <xref target="RFC4566" /> Section 9</c>
        <c>maxptime</c>
        <c>
        <xref target="RFC4566" /> Section 9</c>
        <c>rtpmap</c>
        <c>
        <xref target="RFC4566" /> Section 9</c>
        <c>recvonly</c>
        <c>
        <xref target="RFC4566" /> Section 9</c>
        <c>sendrecv</c>
        <c>
        <xref target="RFC4566" /> Section 9</c>
        <c>sendonly</c>
        <c>
        <xref target="RFC4566" /> Section 9</c>
        <c>inactive</c>
        <c>
        <xref target="RFC4566" /> Section 9</c>
        <c>framerate</c>
        <c>
        <xref target="RFC4566" /> Section 9</c>
        <c>fmtp</c>
        <c>
        <xref target="RFC4566" /> Section 9</c>
        <c>quality</c>
        <c>
        <xref target="RFC4566" /> Section 9</c>
        <c>rtcp</c>
        <c>
        <xref target="RFC3605" /> Section 2.1</c>
        <c>setup</c>
        <c>
        <xref target="RFC4145" /> Sections 3, 4, and 5</c>
        <c>connection</c>
        <c>
        <xref target="RFC4145" /> Sections 3, 4, and 5</c>
        <c>fingerprint</c>
        <c>
        <xref target="RFC8122" /> Section 5</c>
        <c>rtcp-fb</c>
        <c>
        <xref target="RFC4585" /> Section 4.2</c>
        <c>extmap</c>
        <c>
        <xref target="RFC5285" /> Section 7</c>
        <c>mid</c>
        <c>
        <xref target="RFC5888" /> Sections 4 and 5</c>
        <c>group</c>
        <c>
        <xref target="RFC5888" /> Sections 4 and 5</c>
        <c>imageattr</c>
        <c>
        <xref target="RFC6236" /> Section 3.1</c>
        <c>extmap (encrypt option)</c>
        <c>
        <xref target="RFC6904" /> Section 4</c>
        <c>candidate</c>
        <c>
        <xref target="I-D.ietf-mmusic-ice-sip-sdp" /> Section 4.1</c>
        <c>remote-candidates</c>
        <c>
        <xref target="I-D.ietf-mmusic-ice-sip-sdp" /> Section 4.2</c>
        <c>ice-lite</c>
        <c>
        <xref target="I-D.ietf-mmusic-ice-sip-sdp" /> Section 4.3</c>
        <c>ice-ufrag</c>
        <c>
        <xref target="I-D.ietf-mmusic-ice-sip-sdp" /> Section 4.4</c>
        <c>ice-pwd</c>
        <c>
        <xref target="I-D.ietf-mmusic-ice-sip-sdp" /> Section 4.4</c>
        <c>ice-options</c>
        <c>
        <xref target="I-D.ietf-mmusic-ice-sip-sdp" /> Section 4.6</c>
        <c>msid</c>
        <c>
        <xref target="I-D.ietf-mmusic-msid" /> Section 2</c>
        <c>rid</c>
        <c>
        <xref target="I-D.ietf-mmusic-rid" /> Section 10</c>
        <c>simulcast</c>
        <c>
        <xref target="I-D.ietf-mmusic-sdp-simulcast" /> Section 6.1</c>
        <c>tls-id</c>
        <c>
        <xref target="I-D.ietf-mmusic-dtls-sdp" /> Section 4</c>
      </texttable>
    </section>
    <section title="Change log" anchor="sec.change-log">

      <t>Note to used by protocols like the Session Initiation Protocol (SIP).  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3264"/>
          <seriesInfo name="DOI" value="10.17487/RFC3264"/>
        </reference>
        <reference anchor="RFC3552" target="https://www.rfc-editor.org/info/rfc3552" quoteTitle="true" derivedAnchor="RFC3552">
          <front>
            <title>Guidelines for Writing RFC Editor: Please remove this section before
      publication.</t>

      <t>Changes in draft-26:</t>

      <t>
        <list style="symbols">
          <t>Update guidance Text on generation of the m= proto value Security Considerations</title>
            <author initials="E." surname="Rescorla" fullname="E. Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="B." surname="Korver" fullname="B. Korver">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2003" month="July"/>
            <abstract>
              <t indent="0">All RFCs are required to be
          consistent with ice-sip-sdp.</t>
        </list>
      </t>

      <t>Changes in draft-25:</t>

      <t>
        <list style="symbols">

          <t>Remove MSID track ID from offers and answers.</t>

          <t>Add note about rejecting all m= sections in have a BUNDLE group.</t>

          <t>Update ICE references Security Considerations section. Historically, such sections have been relatively weak.  This document provides guidelines to RFC 8445 authors on how to write a good Security Considerations section.   This document specifies an Internet Best Current Practices for the Internet Community, and mention ice2.</t>
        </list>
      </t>

      <t>Changes requests discussion and suggestions for improvements.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="72"/>
          <seriesInfo name="RFC" value="3552"/>
          <seriesInfo name="DOI" value="10.17487/RFC3552"/>
        </reference>
        <reference anchor="RFC3605" target="https://www.rfc-editor.org/info/rfc3605" quoteTitle="true" derivedAnchor="RFC3605">
          <front>
            <title>Real Time Control Protocol (RTCP) attribute in draft-24:</t>

      <t>
        <list style="symbols">

          <t>Clarify that rounding Session Description Protocol (SDP)</title>
            <author initials="C." surname="Huitema" fullname="C. Huitema">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2003" month="October"/>
            <abstract>
              <t indent="0">The Session Description Protocol (SDP) is permitted when trying to maintain
          aspect ratio.</t>

          <t>Update tls-id handling used to match what is specified in
          dtls-sdp.</t>
        </list>
      </t>

      <t>Changes describe the parameters of media streams used in draft-23:</t>

      <t>
        <list style="symbols">

          <t>Clarify rollback handling, and treat it similarly to other
          setLocal/setRemote usages.</t>

          <t>Adopt multimedia sessions.  When a first-fit policy for handling session requires multiple remote
          a=imageattr attributes.</t>

          <t>Clarify ports, SDP assumes that a these ports have consecutive numbers.  However, when the session description with zero m= sections
          is legal.</t>
        </list>
      </t>

      <t>Changes in draft-22:</t>

      <t>
        <list style="symbols">

          <t>Clarify currentDirection versus direction.</t>

          <t>Correct session-id text so that it aligns with RFC
          3264.</t>

          <t>Clarify crosses a network address translation device that generated ICE candidate objects must have all
          four fields.</t>

          <t>Make rollback work from any state besides stable and
          regardless of whether setLocalDescription or
          setRemoteDescription is used.</t>

          <t>Allow modifying SDP before sending or after receiving
          either offers or answers (previously this was forbidden for
          answers).</t>

          <t>Provide rationale for several design choices.</t>
        </list>
      </t>

      <t>Changes in draft-21:</t>

      <t>
        <list style="symbols">

          <t>Change dtls-id to tls-id to match MMUSIC draft.</t>

          <t>Replace regular expression for proto field with a list and
          clarify that the answer must exactly match the offer.</t>

          <t>Remove text about how to error check on setLocal because
          local descriptions cannot be changed.</t>

          <t>Rework silence suppression support to always require that
          both sides agree to silence suppression or none is used.</t>

          <t>Remove instructions to parse "a=ssrc-group".</t>

          <t>Allow the addition of new codecs in answers and in
          subsequent offers.</t>

          <t>Clarify imageattr processing. Replace use of [x=0,y=0]
          with direction indicators.</t>

          <t>Document when early media can occur.</t>

          <t>Fix ICE default also uses port handling when bundle-only is
          used.</t>

          <t>Forbid duplicating IDENTICAL/TRANSPORT attributes when you
          are bundling.</t>

          <t>Clarify mapping, the number ordering of components to gather when bundle is
          involved.</t>

          <t>Explicitly state that PTs and SSRCs are to ports can be used for
          demuxing.</t>

          <t>Update guidance on "a=setup" line. This should now match destroyed by the MMUSIC draft.</t>

          <t>Update guidance on certificate/digest matching to conform
          to RFC8122.</t>

          <t>Update examples.</t>
        </list>
      </t>

      <t>Changes in draft-20:</t>

      <t>
        <list style="symbols">

          <t>Remove Appendix-B.</t>
        </list>
      </t>

      <t>Changes in draft-19:</t>

      <t>
        <list style="symbols">

          <t>Examples are now machine-generated for correctness, and
          use IETF-approved example IP addresses.</t>

          <t>Add early transport warmup example, and add missing
          attributes translation.  To handle this, we propose an extension attribute to existing examples.</t>

          <t>Only send "a=rtcp-mux-only" and "a=bundle-only" on new m=
          sections.</t>

          <t>Update references.</t>

          <t>Add coverage of a=identity.</t>

          <t>Explain SDP.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3605"/>
          <seriesInfo name="DOI" value="10.17487/RFC3605"/>
        </reference>
        <reference anchor="RFC3711" target="https://www.rfc-editor.org/info/rfc3711" quoteTitle="true" derivedAnchor="RFC3711">
          <front>
            <title>The Secure Real-time Transport Protocol (SRTP)</title>
            <author initials="M." surname="Baugher" fullname="M. Baugher">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="McGrew" fullname="D. McGrew">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Naslund" fullname="M. Naslund">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E." surname="Carrara" fullname="E. Carrara">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="K." surname="Norrman" fullname="K. Norrman">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2004" month="March"/>
            <abstract>
              <t indent="0">This document describes the lipsync group algorithm more thoroughly.</t>

          <t>Remove unnecessary list Secure Real-time Transport Protocol (SRTP), a profile of MTI specs.</t>

          <t>Allow codecs the Real-time Transport Protocol (RTP), which weren't offered to appear in answers can provide confidentiality, message authentication, and which weren't selected replay protection to appear in subsequent
          offers.</t>

          <t>Codec preferences now are applied on both initial and
          subsequent offers the RTP traffic and answers.</t>

          <t>Clarify a=msid handling for recvonly m= sections.</t>

          <t>Clarify behavior of attributes for bundle-only data
          channels.</t>

          <t>Allow media attributes to appear in data m= sections when
          all the media m= sections are bundle-only.</t>

          <t>Use consistent terminology for JSEP implementations.</t>

          <t>Describe how to handle failed API calls.</t>

          <t>Some cleanup on routing rules.</t>
        </list>
      </t>

      <t>Changes in draft-18:</t>

      <t>
        <list style="symbols">

          <t>Update demux algorithm and move it to an appendix in
          preparation control traffic for merging it into BUNDLE.</t>

          <t>Clarify why we can't handle an incoming offer to send
          simulcast.</t>

          <t>Expand IceCandidate object text.</t>

          <t>Further document use of ICE candidate pool.</t>

          <t>Document removeTrack.</t>

          <t>Update requirements to only accept RTP, the last generated
          offer/answer as an argument to setLocalDescription.</t>

          <t>Allow round pixels.</t>

          <t>Fix code around default timing when AVPF is not
          specified.</t>

          <t>Clean up terminology around m= line and m=section.</t>

          <t>Provide a more realistic example Real-time Transport Control Protocol (RTCP).   [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3711"/>
          <seriesInfo name="DOI" value="10.17487/RFC3711"/>
        </reference>
        <reference anchor="RFC3890" target="https://www.rfc-editor.org/info/rfc3890" quoteTitle="true" derivedAnchor="RFC3890">
          <front>
            <title>A Transport Independent Bandwidth Modifier for minimum decoder
          capabilities.</t>

          <t>Document behavior when rtcp-mux policy is require but
          rtcp-mux attribute not provided.</t>

          <t>Expanded discussion of RtpSender and RtpReceiver.</t>

          <t>Add RtpTransceiver.currentDirection and the Session Description Protocol (SDP)</title>
            <author initials="M." surname="Westerlund" fullname="M. Westerlund">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2004" month="September"/>
            <abstract>
              <t indent="0">This document
          setDirection.</t>

          <t>Require imageattr x=0, y=0 to indicate that there are no
          valid resolutions.</t>

          <t>Require defines a privacy-preserving MID/RID construction.</t>

          <t>Require support for RFC 3556 Session Description Protocol (SDP) Transport Independent Application Specific Maximum (TIAS) bandwidth modifiers.</t>

          <t>Update maxptime description.</t>

          <t>Note modifier that endpoints may encounter extra codecs in answers
          and subsequent offers from non-JSEP peers.</t>

          <t>Update references.</t>
        </list>
      </t>

      <t>Changes in draft-17:</t>

      <t>
        <list style="symbols">

          <t>Split createOffer and createAnswer sections to clearly
          indicate attributes which always appear and which only appear
          when does not bundled into another m= section.</t>

          <t>Add descriptions of RtpTransceiver methods.</t>

          <t>Describe how include transport overhead; instead an additional packet rate attribute is defined.  The transport independent bit-rate value together with the maximum packet rate can then be used to process RTCP feedback attributes.</t>

          <t>Clarify transceiver directions calculate the real bit-rate over the transport actually used. </t>
              <t indent="0"> The existing SDP bandwidth modifiers and their interaction with
          3264.</t>

          <t>Describe setCodecPreferences.</t>

          <t>Update RTP demux algorithm. Include RTCP.</t>

          <t>Update requirements values include the bandwidth needed for the transport and IP layers.  When using SDP with protocols like the Session Announcement Protocol (SAP), the Session Initiation Protocol (SIP), and the Real-Time Streaming Protocol (RTSP), and when a=rtcp is included, limiting the involved hosts has different transport overhead, for example due to cases where it different IP versions, the interpretation of what lower layer bandwidths are included is needed for backward compatibility.</t>

          <t>Clarify SAR handling.</t>

          <t>Updated addTrack matching algorithm.</t>

          <t>Remove a=ssrc requirements.</t>

          <t>Handle a=setup not clear.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3890"/>
          <seriesInfo name="DOI" value="10.17487/RFC3890"/>
        </reference>
        <reference anchor="RFC4145" target="https://www.rfc-editor.org/info/rfc4145" quoteTitle="true" derivedAnchor="RFC4145">
          <front>
            <title>TCP-Based Media Transport in reoffers.</t>

          <t>Discuss how RTX/FEC should be handled.</t>

          <t>Discuss how telephone-event should be handled.</t>

          <t>Discuss the Session Description Protocol (SDP)</title>
            <author initials="D." surname="Yon" fullname="D. Yon">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G." surname="Camarillo" fullname="G. Camarillo">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2005" month="September"/>
            <abstract>
              <t indent="0">This document describes how CN/DTX should be handled.</t>

          <t>Add missing references to ABNF table.</t>
        </list>
      </t>

      <t>Changes in draft-16:</t>

      <t>
        <list style="symbols">

          <t>Update addIceCandidate to indicate ICE generation and
          allow per-m= section end-of-candidates.</t>

          <t>Update fingerprint handling to use
          draft-ietf-mmusic-4572-update.</t>

          <t>Update text around express media transport over TCP using the Session Description Protocol (SDP).  It defines the SDP processing of RTP header extensions
          and payload formats.</t>

          <t>Add sections on simulcast, addTransceiver, 'TCP' protocol identifier, the SDP 'setup' attribute, which describes the connection setup procedure, and
          createDataChannel.</t>

          <t>Clarify text to ensure that the session ID is a positive
          63 bit integer.</t>

          <t>Clarify SDP processing for direction indication.</t>

          <t>Describe 'connection' attribute, which handles connection reestablishment.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4145"/>
          <seriesInfo name="DOI" value="10.17487/RFC4145"/>
        </reference>
        <reference anchor="RFC4566" target="https://www.rfc-editor.org/info/rfc4566" quoteTitle="true" derivedAnchor="RFC4566">
          <front>
            <title>SDP: Session Description Protocol</title>
            <author initials="M." surname="Handley" fullname="M. Handley">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="V." surname="Jacobson" fullname="V. Jacobson">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C." surname="Perkins" fullname="C. Perkins">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2006" month="July"/>
            <abstract>
              <t indent="0">This memo defines the Session Description Protocol (SDP).  SDP processing is intended for rtcp-mux-only.</t>

          <t>Specify how SDP describing multimedia sessions for the purposes of session version in o= line.</t>

          <t>Require announcement, session invitation, and other forms of multimedia session initiation.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4566"/>
          <seriesInfo name="DOI" value="10.17487/RFC4566"/>
        </reference>
        <reference anchor="RFC4585" target="https://www.rfc-editor.org/info/rfc4585" quoteTitle="true" derivedAnchor="RFC4585">
          <front>
            <title>Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)</title>
            <author initials="J." surname="Ott" fullname="J. Ott">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Wenger" fullname="S. Wenger">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="N." surname="Sato" fullname="N. Sato">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C." surname="Burmeister" fullname="C. Burmeister">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Rey" fullname="J. Rey">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2006" month="July"/>
            <abstract>
              <t indent="0">Real-time media streams that when doing an re-offer, use RTP are, to some degree, resilient against packet losses.  Receivers may use the capabilities base mechanisms of the new session are mostly required Real-time Transport Control Protocol (RTCP) to be report packet reception statistics and thus allow a subset of sender to adapt its transmission behavior in the
          previously negotiated session.</t>

          <t>Clarified ICE restart interaction with bundle-only.</t>

          <t>Remove support for changing SDP before calling
          setLocalDescription.</t>

          <t>Specify algorithm mid-term.  This is the sole means for demuxing RTP based on MID, PT, feedback and
          SSRC.</t>

          <t>Clarify rules for rejecting m= lines when bundle policy is
          balanced or max-bundle.</t>
        </list>
      </t>

      <t>Changes in draft-15:</t>

      <t>
        <list style="symbols">

          <t>Clarify text around codecs offered in subsequent
          transactions to refer to what's been negotiated.</t>

          <t>Rewrite LS handling text feedback-based error repair (besides a few codec-specific mechanisms).  This document defines an extension to indicate edge cases and that
          we're living with them.</t>

          <t>Require the Audio-visual Profile (AVP) that answerer reject m= lines when there are no
          codecs in common.</t>

          <t>Enforce max-bundle on offer processing.</t>

          <t>Fix TIAS formula enables receivers to handle bits vs. kilobits.</t>

          <t>Describe addTrack algorithm.</t>

          <t>Clean up references.</t>
        </list>
      </t>

      <t>Changes in draft-14:</t>

      <t>
        <list style="symbols">

          <t>Added discussion of RtpTransceivers + RtpSenders +
          RtpReceivers, provide, statistically, more immediate feedback to the senders and how they interact with
          createOffer/createAnswer.</t>

          <t>Removed obsolete OfferToReceiveX options.</t>

          <t>Explained how addIceCandidate can be used thus allows for
          end-of-candidates.</t>
        </list>
      </t>

      <t>Changes in draft-13:</t>

      <t>
        <list style="symbols">

          <t>Clarified which SDP lines can be ignored.</t>

          <t>Clarified how short-term adaptation and efficient feedback-based repair mechanisms to handle various received attributes.</t>

          <t>Revised how attributes should be generated for bundled m=
          lines.</t>

          <t>Remove unused references.</t>

          <t>Remove text advocating use of unilateral PTs.</t>

          <t>Trigger an ICE restart even if the ICE candidate policy is
          being made more strict.</t>

          <t>Remove implemented.  This early feedback profile (AVPF) maintains the 'public' ICE candidate policy.</t>

          <t>Move open issues into GitHub issues.</t>

          <t>Split local/remote description accessors into
          current/pending.</t>

          <t>Clarify a=imageattr handling.</t>

          <t>Add more detail on VoiceActivityDetection handling.</t>

          <t>Reference draft-shieh-rtcweb-ip-handling.</t>

          <t>Make it clear when an ICE restart should occur.</t>

          <t>Resolve changes needed in references.</t>

          <t>Remove MSID semantics.</t>

          <t>ice-options are now at session level.</t>

          <t>Default AVP bandwidth constraints for RTCP mux policy is now 'require'.</t>
        </list>
      </t>

      <t>Changes in draft-12:</t>

      <t>
        <list style="symbols">

          <t>Filled in sections on applying local and remote
          descriptions.</t>

          <t>Discussed downscaling preserves scalability to large groups.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4585"/>
          <seriesInfo name="DOI" value="10.17487/RFC4585"/>
        </reference>
        <reference anchor="RFC5124" target="https://www.rfc-editor.org/info/rfc5124" quoteTitle="true" derivedAnchor="RFC5124">
          <front>
            <title>Extended Secure RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/SAVPF)</title>
            <author initials="J." surname="Ott" fullname="J. Ott">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E." surname="Carrara" fullname="E. Carrara">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2008" month="February"/>
            <abstract>
              <t indent="0">An RTP profile (SAVP) for secure real-time communications and upscaling another profile (AVPF) to fulfill imageattr
          requirements.</t>

          <t>Updated what SDP can be modified by provide timely feedback from the application.</t>

          <t>Updated to latest datachannel SDP.</t>

          <t>Allowed multiple fingerprint lines.</t>

          <t>Switched back receivers to IPv4 for dummy candidates.</t>

          <t>Added additional clarity on ICE default candidates.</t>
        </list>
      </t>

      <t>Changes a sender are defined in draft-11:</t>

      <t>
        <list style="symbols">

          <t>Clarified handling RFC 3711 and RFC 4585, respectively.  This memo specifies the combination of both profiles to enable secure RTP CNAMEs.</t>

          <t>Updated what SDP lines should be processed or ignored.</t>

          <t>Specified how a=imageattr should be used.</t>
        </list>
      </t>

      <t>Changes in draft-10:</t>

      <t>
        <list style="symbols">

          <t>Described video size negotiation communications with imageattr.</t>

          <t>Clarified rejection of sections that do not have
          mux-only.</t>

          <t>Add handling of LS groups</t>
        </list>
      </t>

      <t>Changes in draft-09:</t>

      <t>
        <list style="symbols">

          <t>Don't return null feedback.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5124"/>
          <seriesInfo name="DOI" value="10.17487/RFC5124"/>
        </reference>
        <reference anchor="RFC5285" target="https://www.rfc-editor.org/info/rfc5285" quoteTitle="true" derivedAnchor="RFC5285">
          <front>
            <title>A General Mechanism for {local,remote}Description after
          close().</t>

          <t>Changed TCP/TLS RTP Header Extensions</title>
            <author initials="D." surname="Singer" fullname="D. Singer">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="H." surname="Desineni" fullname="H. Desineni">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2008" month="July"/>
            <abstract>
              <t indent="0">This document provides a general mechanism to UDP/DTLS in use the header extension feature of RTP profile names.</t>

          <t>Separate out bundle and mux policy.</t>

          <t>Added specific references (the Real-Time Transport Protocol).  It provides the option to FEC mechanisms.</t>

          <t>Added canTrickle mechanism.</t>

          <t>Added section on subsequent answers and, answer
          options.</t>

          <t>Added text defining set{Local,Remote}Description
          behavior.</t>
        </list>
      </t>

      <t>Changes use a small number of small extensions in draft-08:
      <list style="symbols">

        <t>Added new example section each RTP packet, where the universe of possible extensions is large and removed old examples registration is de-centralized.  The actual extensions in
        appendix.</t>

        <t>Fixed &lt;proto&gt; field handling.</t>

        <t>Added text describing a=rtcp attribute.</t>

        <t>Reworked handling of OfferToReceiveAudio use in a session are signaled in the setup information for that session.   [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5285"/>
          <seriesInfo name="DOI" value="10.17487/RFC5285"/>
        </reference>
        <reference anchor="RFC5761" target="https://www.rfc-editor.org/info/rfc5761" quoteTitle="true" derivedAnchor="RFC5761">
          <front>
            <title>Multiplexing RTP Data and
        OfferToReceiveVideo per discussion at IETF 90.</t>

        <t>Reworked trickle ICE handling Control Packets on a Single Port</title>
            <author initials="C." surname="Perkins" fullname="C. Perkins">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Westerlund" fullname="M. Westerlund">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2010" month="April"/>
            <abstract>
              <t indent="0">This memo discusses issues that arise when multiplexing RTP data packets and its impact RTP Control Protocol (RTCP) packets on m= a single UDP port. It updates RFC 3550 and c=
        lines per discussion at interim.</t>

        <t>Added max-bundle-and-rtcp-mux policy.</t>

        <t>Added description of maxptime handling.</t>

        <t>Updated ICE candidate pool default RFC 3551 to 0.</t>

        <t>Resolved open issues around AppID/receiver-ID.</t>

        <t>Reworked describe when such multiplexing is and expanded how changes to the ICE configuration
        are handled.</t>

        <t>Some reference updates.</t>

        <t>Editorial clarification.</t>
      </list></t>

      <t>Changes in draft-07:
      <list style="symbols">

        <t>Expanded discussion of VAD is not appropriate, and Opus DTX.</t>

        <t>Added a security considerations section.</t>

        <t>Rewrote it explains how the section on modifying SDP Session Description Protocol (SDP) can be used to require
        implementations signal multiplexed sessions.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5761"/>
          <seriesInfo name="DOI" value="10.17487/RFC5761"/>
        </reference>
        <reference anchor="RFC5888" target="https://www.rfc-editor.org/info/rfc5888" quoteTitle="true" derivedAnchor="RFC5888">
          <front>
            <title>The Session Description Protocol (SDP) Grouping Framework</title>
            <author initials="G." surname="Camarillo" fullname="G. Camarillo">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="H." surname="Schulzrinne" fullname="H. Schulzrinne">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2010" month="June"/>
            <abstract>
              <t indent="0">In this specification, we define a framework to clearly indicate whether any given
        modification is allowed.</t>

        <t>Clarified impact of IceRestart on CreateOffer group "m" lines in local-offer
        state.</t>

        <t>Guidance on whether attributes should be defined at the
        media level or Session Description Protocol (SDP) for different purposes.  This framework uses the session level.</t>

        <t>Renamed "default" bundle policy to "balanced".</t>

        <t>Removed default ICE candidate pool size "group" and clarify how it
        works.</t>

        <t>Defined a canonical order for assignment "mid" SDP attributes, both of MSTs which are defined in this specification.  Additionally, we specify how to m=
        lines.</t>

        <t>Removed discussion of rehydration.</t>

        <t>Added Eric Rescorla as a draft editor.</t>

        <t>Cleaned up references.</t>

        <t>Editorial cleanup</t>
      </list></t>

      <t>Changes in draft-06:
      <list style="symbols">

        <t>Reworked handling of m= line recycling.</t>

        <t>Added handling of BUNDLE and bundle-only.</t>

        <t>Clarified handling of rollback.</t>

        <t>Added text describing use the ICE Candidate Pool framework for two different purposes: for lip synchronization and its
        behavior.</t>

        <t>Allowed OfferToReceiveX to create multiple recvonly m=
        sections.</t>
      </list></t>

      <t>Changes in draft-05:
      <list style="symbols">

        <t>Fixed for receiving a media flow consisting of several issues identified media streams on different transport addresses.  This document obsoletes RFC 3388. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5888"/>
          <seriesInfo name="DOI" value="10.17487/RFC5888"/>
        </reference>
        <reference anchor="RFC6236" target="https://www.rfc-editor.org/info/rfc6236" quoteTitle="true" derivedAnchor="RFC6236">
          <front>
            <title>Negotiation of Generic Image Attributes in the createOffer/Answer
        sections during Session Description Protocol (SDP)</title>
            <author initials="I." surname="Johansson" fullname="I. Johansson">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="K." surname="Jung" fullname="K. Jung">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2011" month="May"/>
            <abstract>
              <t indent="0">This document review.</t>

        <t>Updated references.</t>
      </list></t>

      <t>Changes in draft-04:
      <list style="symbols">

        <t>Filled in sections on createOffer and createAnswer.</t>

        <t>Added SDP examples.</t>

        <t>Fixed references.</t>
      </list></t>

      <t>Changes in draft-03:
      <list style="symbols">

        <t>Added text describing relationship proposes a new generic session setup attribute to W3C specification</t>
      </list></t>

      <t>Changes in draft-02:
      <list style="symbols">
        <!-- make it possible to negotiate different image attributes such as image size.  A -->

        <t>Converted from nroff</t>
        <!-- B -->

        <t>Removed comparisons possible use case is to old approaches abandoned by make it possible for a \%low-end \%hand- held terminal to display video without the
        working group</t>
        <!-- C -->

        <t>Removed stuff that has moved need to W3C specification</t>
        <!-- D -->

        <t>Align SDP handling with W3C draft</t>
        <!-- E -->

        <t>Clarified section on forking.</t>
        <!-- F -->
        <!-- G -->
        <!-- H -->
        <!-- I -->
        <!-- J -->
        <!-- K -->
        <!-- L -->
      </list></t>

      <t>Changes in draft-01:
      <list style="symbols">

        <t>Added diagrams for architecture and state machine.</t>

        <t>Added sections on forking and rehydration.</t>

        <t>Clarified meaning rescale the image, something that may consume large amounts of "pranswer" and "answer".</t>

        <t>Reworked how ICE restarts memory and media directions are
        controlled.</t>

        <t>Added list of parameters processing power.  The document also helps to maintain an optimal bitrate for video as only the image size that can be changed is desired by the receiver is transmitted.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6236"/>
          <seriesInfo name="DOI" value="10.17487/RFC6236"/>
        </reference>
        <reference anchor="RFC6347" target="https://www.rfc-editor.org/info/rfc6347" quoteTitle="true" derivedAnchor="RFC6347">
          <front>
            <title>Datagram Transport Layer Security Version 1.2</title>
            <author initials="E." surname="Rescorla" fullname="E. Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="N." surname="Modadugu" fullname="N. Modadugu">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2012" month="January"/>
            <abstract>
              <t indent="0">This document specifies version 1.2 of the Datagram Transport Layer Security (DTLS) protocol.  The DTLS protocol provides communications privacy for datagram protocols.  The protocol allows client/server applications to communicate in a
        description.</t>

        <t>Updated suggested API and examples way that is designed to match latest
        thinking.</t>

        <t>Suggested API prevent eavesdropping, tampering, or message forgery.  The DTLS protocol is based on the Transport Layer Security (TLS) protocol and examples have been moved to an
        appendix.</t>
      </list></t>

      <t>Changes in draft -00:
      <list style="symbols">

        <t>Migrated from draft-uberti-rtcweb-jsep-02.</t>
      </list></t> provides equivalent security guarantees.  Datagram semantics of the underlying transport are preserved by the DTLS protocol.  This document updates DTLS 1.0 to work with TLS version 1.2.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6347"/>
          <seriesInfo name="DOI" value="10.17487/RFC6347"/>
        </reference>
        <reference anchor="RFC6716" target="https://www.rfc-editor.org/info/rfc6716" quoteTitle="true" derivedAnchor="RFC6716">
          <front>
            <title>Definition of the Opus Audio Codec</title>
            <author initials="JM." surname="Valin" fullname="JM. Valin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="K." surname="Vos" fullname="K. Vos">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="T." surname="Terriberry" fullname="T. Terriberry">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2012" month="September"/>
            <abstract>
              <t indent="0">This document defines the Opus interactive speech and audio codec. Opus is designed to handle a wide range of interactive audio applications, including Voice over IP, videoconferencing, in-game chat, and even live, distributed music performances.  It scales from low bitrate narrowband speech at 6 kbit/s to very high quality stereo music at 510 kbit/s.  Opus uses both Linear Prediction (LP) and the Modified Discrete Cosine Transform (MDCT) to achieve good compression of both speech and music.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6716"/>
          <seriesInfo name="DOI" value="10.17487/RFC6716"/>
        </reference>
        <reference anchor="RFC6904" target="https://www.rfc-editor.org/info/rfc6904" quoteTitle="true" derivedAnchor="RFC6904">
          <front>
            <title>Encryption of Header Extensions in the Secure Real-time Transport Protocol (SRTP)</title>
            <author initials="J." surname="Lennox" fullname="J. Lennox">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2013" month="April"/>
            <abstract>
              <t indent="0">The Secure Real-time Transport Protocol (SRTP) provides authentication, but not encryption, of the headers of Real-time Transport Protocol (RTP) packets.  However, RTP header extensions may carry sensitive information for which participants in multimedia sessions want confidentiality.  This document provides a mechanism, extending the mechanisms of SRTP, to selectively encrypt RTP header extensions in SRTP.</t>
              <t indent="0">This document updates RFC 3711, the Secure Real-time Transport Protocol specification, to require that all future SRTP encryption transforms specify how RTP header extensions are to be encrypted.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6904"/>
          <seriesInfo name="DOI" value="10.17487/RFC6904"/>
        </reference>
        <reference anchor="RFC7160" target="https://www.rfc-editor.org/info/rfc7160" quoteTitle="true" derivedAnchor="RFC7160">
          <front>
            <title>Support for Multiple Clock Rates in an RTP Session</title>
            <author initials="M." surname="Petit-Huguenin" fullname="M. Petit-Huguenin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G." surname="Zorn" fullname="G. Zorn" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2014" month="April"/>
            <abstract>
              <t indent="0">This document clarifies the RTP specification regarding the use of different clock rates in an RTP session.  It also provides guidance on how legacy RTP implementations that use multiple clock rates can interoperate with RTP implementations that use the algorithm described in this document.  It updates RFC 3550.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7160"/>
          <seriesInfo name="DOI" value="10.17487/RFC7160"/>
        </reference>
        <reference anchor="RFC7587" target="https://www.rfc-editor.org/info/rfc7587" quoteTitle="true" derivedAnchor="RFC7587">
          <front>
            <title>RTP Payload Format for the Opus Speech and Audio Codec</title>
            <author initials="J." surname="Spittka" fullname="J. Spittka">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="K." surname="Vos" fullname="K. Vos">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="JM." surname="Valin" fullname="JM. Valin">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2015" month="June"/>
            <abstract>
              <t indent="0">This document defines the Real-time Transport Protocol (RTP) payload format for packetization of Opus-encoded speech and audio data necessary to integrate the codec in the most compatible way.  It also provides an applicability statement for the use of Opus over RTP. Further, it describes media type registrations for the RTP payload format.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7587"/>
          <seriesInfo name="DOI" value="10.17487/RFC7587"/>
        </reference>
        <reference anchor="RFC7742" target="https://www.rfc-editor.org/info/rfc7742" quoteTitle="true" derivedAnchor="RFC7742">
          <front>
            <title>WebRTC Video Processing and Codec Requirements</title>
            <author initials="A.B." surname="Roach" fullname="A.B. Roach">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2016" month="March"/>
            <abstract>
              <t indent="0">This specification provides the requirements and considerations for WebRTC applications to send and receive video across a network.  It specifies the video processing that is required as well as video codecs and their parameters.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7742"/>
          <seriesInfo name="DOI" value="10.17487/RFC7742"/>
        </reference>
        <reference anchor="RFC7850" target="https://www.rfc-editor.org/info/rfc7850" quoteTitle="true" derivedAnchor="RFC7850">
          <front>
            <title>Registering Values of the SDP 'proto' Field for Transporting RTP Media over TCP under Various RTP Profiles</title>
            <author initials="S." surname="Nandakumar" fullname="S. Nandakumar">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2016" month="April"/>
            <abstract>
              <t indent="0">The Real-time Transport Protocol (RTP) specification establishes a registry of profile names for use by higher-level control protocols, such as the Session Description Protocol (SDP), to refer to the transport methods.  This specification describes the following new SDP transport protocol identifiers for transporting RTP Media over TCP: 'TCP/RTP/AVPF', 'TCP/RTP/SAVP', 'TCP/RTP/SAVPF', 'TCP/DTLS/RTP/SAVP', 'TCP/DTLS/RTP/SAVPF', 'TCP/TLS/RTP/AVP', and 'TCP/TLS/RTP/AVPF'.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7850"/>
          <seriesInfo name="DOI" value="10.17487/RFC7850"/>
        </reference>
        <reference anchor="RFC7874" target="https://www.rfc-editor.org/info/rfc7874" quoteTitle="true" derivedAnchor="RFC7874">
          <front>
            <title>WebRTC Audio Codec and Processing Requirements</title>
            <author initials="JM." surname="Valin" fullname="JM. Valin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C." surname="Bran" fullname="C. Bran">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2016" month="May"/>
            <abstract>
              <t indent="0">This document outlines the audio codec and processing requirements for WebRTC endpoints.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7874"/>
          <seriesInfo name="DOI" value="10.17487/RFC7874"/>
        </reference>
        <reference anchor="RFC8108" target="https://www.rfc-editor.org/info/rfc8108" quoteTitle="true" derivedAnchor="RFC8108">
          <front>
            <title>Sending Multiple RTP Streams in a Single RTP Session</title>
            <author initials="J." surname="Lennox" fullname="J. Lennox">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Westerlund" fullname="M. Westerlund">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="Q." surname="Wu" fullname="Q. Wu">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C." surname="Perkins" fullname="C. Perkins">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2017" month="March"/>
            <abstract>
              <t indent="0">This memo expands and clarifies the behavior of Real-time Transport Protocol (RTP) endpoints that use multiple synchronization sources (SSRCs).  This occurs, for example, when an endpoint sends multiple RTP streams in a single RTP session.  This memo updates RFC 3550 with regard to handling multiple SSRCs per endpoint in RTP sessions, with a particular focus on RTP Control Protocol (RTCP) behavior.  It also updates RFC 4585 to change and clarify the calculation of the timeout of SSRCs and the inclusion of feedback messages.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8108"/>
          <seriesInfo name="DOI" value="10.17487/RFC8108"/>
        </reference>
        <reference anchor="RFC8122" target="https://www.rfc-editor.org/info/rfc8122" quoteTitle="true" derivedAnchor="RFC8122">
          <front>
            <title>Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP)</title>
            <author initials="J." surname="Lennox" fullname="J. Lennox">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C." surname="Holmberg" fullname="C. Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2017" month="March"/>
            <abstract>
              <t indent="0">This document specifies how to establish secure connection-oriented media transport sessions over the Transport Layer Security (TLS) protocol using the Session Description Protocol (SDP).  It defines the SDP protocol identifier, 'TCP/TLS'.  It also defines the syntax and semantics for an SDP 'fingerprint' attribute that identifies the certificate that will be presented for the TLS session.  This mechanism allows media transport over TLS connections to be established securely, so long as the integrity of session descriptions is assured.</t>
              <t indent="0">This document obsoletes RFC 4572 by clarifying the usage of multiple fingerprints.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8122"/>
          <seriesInfo name="DOI" value="10.17487/RFC8122"/>
        </reference>
        <reference anchor="RFC8174" target="https://www.rfc-editor.org/info/rfc8174" quoteTitle="true" derivedAnchor="RFC8174">
          <front>
            <title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
            <author initials="B." surname="Leiba" fullname="B. Leiba">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2017" month="May"/>
            <abstract>
              <t indent="0">RFC 2119 specifies common key words that may be used in protocol  specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the  defined special meanings.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="14"/>
          <seriesInfo name="RFC" value="8174"/>
          <seriesInfo name="DOI" value="10.17487/RFC8174"/>
        </reference>
        <reference anchor="RFC8445" target="https://www.rfc-editor.org/info/rfc8445" quoteTitle="true" derivedAnchor="RFC8445">
          <front>
            <title>Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal</title>
            <author initials="A." surname="Keranen" fullname="A. Keranen">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C." surname="Holmberg" fullname="C. Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Rosenberg" fullname="J. Rosenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2018" month="July"/>
            <abstract>
              <t indent="0">This document describes a protocol for Network Address Translator (NAT) traversal for UDP-based communication.  This protocol is called Interactive Connectivity Establishment (ICE).  ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN).</t>
              <t indent="0">This document obsoletes RFC 5245.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8445"/>
          <seriesInfo name="DOI" value="10.17487/RFC8445"/>
        </reference>
        <reference anchor="RFC8826" target="https://www.rfc-editor.org/info/rfc8826" quoteTitle="true" derivedAnchor="RFC8826">
          <front>
            <title>Security Considerations for WebRTC</title>
            <author initials="E." surname="Rescorla" fullname="Eric Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8826"/>
          <seriesInfo name="DOI" value="10.17487/RFC8826"/>
        </reference>
        <reference anchor="RFC8827" target="https://www.rfc-editor.org/info/rfc8827" quoteTitle="true" derivedAnchor="RFC8827">
          <front>
            <title>WebRTC Security Architecture</title>
            <author initials="E." surname="Rescorla" fullname="Eric Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8827"/>
          <seriesInfo name="DOI" value="10.17487/RFC8827"/>
        </reference>
        <reference anchor="RFC8830" target="https://www.rfc-editor.org/info/rfc8830" quoteTitle="true" derivedAnchor="RFC8830">
          <front>
            <title>WebRTC MediaStream Identification in the Session Description Protocol</title>
            <author initials="H" surname="Alvestrand" fullname="Harald Alvestrand">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8830"/>
          <seriesInfo name="DOI" value="10.17487/RFC8830"/>
        </reference>
        <reference anchor="RFC8834" target="https://www.rfc-editor.org/info/rfc8834" quoteTitle="true" derivedAnchor="RFC8834">
          <front>
            <title>Media Transport and Use of RTP in WebRTC</title>
            <author initials="C." surname="Perkins" fullname="Colin Perkins">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Westerlund" fullname="Magnus Westerlund">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Ott" fullname="Jörg Ott">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8834"/>
          <seriesInfo name="DOI" value="10.17487/RFC8834"/>
        </reference>
        <reference anchor="RFC8838" target="https://www.rfc-editor.org/info/rfc8838" quoteTitle="true" derivedAnchor="RFC8838">
          <front>
            <title>Trickle ICE: Incremental Provisioning of Candidates for the Interactive Connectivity Establishment (ICE) Protocol</title>
            <author initials="E" surname="Ivov" fullname="Emil Ivov">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J" surname="Uberti" fullname="Justin Uberti">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="P" surname="Saint-Andre" fullname="Peter Saint-Andre">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8838"/>
          <seriesInfo name="DOI" value="10.17487/RFC8838"/>
        </reference>
        <reference anchor="RFC8839" target="https://www.rfc-editor.org/info/rfc8839" quoteTitle="true" derivedAnchor="RFC8839">
          <front>
            <title>Session Description Protocol (SDP) Offer/Answer Procedures for Interactive Connectivity Establishment (ICE)</title>
            <author initials="M" surname="Petit-Huguenin" fullname="Marc Petit-Huguenin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S" surname="Nandakumar" fullname="Suhas Nandakumar">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C" surname="Holmberg" fullname="Christer Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="A" surname="Keränen" fullname="Ari Keränen">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R" surname="Shpount" fullname="Roman Shpount">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8839"/>
          <seriesInfo name="DOI" value="10.17487/RFC8839"/>
        </reference>
        <reference anchor="RFC8840" target="https://www.rfc-editor.org/info/rfc8840" quoteTitle="true" derivedAnchor="RFC8840">
          <front>
            <title>A Session Initiation Protocol (SIP) Usage for Incremental Provisioning of Candidates for the Interactive Connectivity Establishment (Trickle ICE)</title>
            <author initials="E" surname="Ivov" fullname="Emil Ivov">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="T" surname="Stach" fullname="Thomas Stach">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E" surname="Marocco" fullname="Enrico Marocco">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C" surname="Holmberg" fullname="Christer Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8840"/>
          <seriesInfo name="DOI" value="10.17487/RFC8840"/>
        </reference>
        <reference anchor="RFC8841" target="https://www.rfc-editor.org/info/rfc8841" quoteTitle="true" derivedAnchor="RFC8841">
          <front>
            <title>Session Description Protocol (SDP) Offer/Answer Procedures for Stream Control Transmission Protocol (SCTP) over Datagram Transport Layer Security (DTLS) Transport</title>
            <author initials="C." surname="Holmberg" fullname="Christer Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Shpount" fullname="Roman Shpount">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Loreto" fullname="Salvatore Loreto">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G." surname="Camarillo" fullname="Gonzalo Camarillo">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8841"/>
          <seriesInfo name="DOI" value="10.17487/RFC8841"/>
        </reference>
        <reference anchor="RFC8842" target="https://www.rfc-editor.org/info/rfc8842" quoteTitle="true" derivedAnchor="RFC8842">
          <front>
            <title>Session Description Protocol (SDP) Offer/Answer Considerations for Datagram Transport Layer Security (DTLS) and Transport Layer Security (TLS)</title>
            <author initials="C." surname="Holmberg" fullname="Christer Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Shpount" fullname="Roman Shpount">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8842"/>
          <seriesInfo name="DOI" value="10.17487/RFC8842"/>
        </reference>
        <reference anchor="RFC8843" target="https://www.rfc-editor.org/info/rfc8843" quoteTitle="true" derivedAnchor="RFC8843">
          <front>
            <title>Negotiating Media Multiplexing Using the Session Description Protocol (SDP)</title>
            <author initials="C" surname="Holmberg" fullname="Christer Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="H" surname="Alvestrand" fullname="Harald Alvestrand">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C" surname="Jennings" fullname="Cullen Jennings">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8843"/>
          <seriesInfo name="DOI" value="10.17487/RFC8843"/>
        </reference>
        <reference anchor="RFC8851" target="https://www.rfc-editor.org/info/rfc8851" quoteTitle="true" derivedAnchor="RFC8851">
          <front>
            <title>RTP Payload Format Restrictions</title>
            <author initials="A.B." surname="Roach" fullname="Adam Roach" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8851"/>
          <seriesInfo name="DOI" value="10.17487/RFC8851"/>
        </reference>
        <reference anchor="RFC8852" target="https://www.rfc-editor.org/info/rfc8852" quoteTitle="true" derivedAnchor="RFC8852">
          <front>
            <title>RTP Stream Identifier Source Description (SDES)</title>
            <author initials="A.B." surname="Roach" fullname="Adam Roach"/>
            <author initials="S" surname="Nandakumar" fullname="Suhas Nandakumar"/>
            <author initials="P" surname="Thatcher" fullname="Peter Thatcher"/>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8852"/>
          <seriesInfo name="DOI" value="10.17487/RFC8852"/>
        </reference>
        <reference anchor="RFC8853" target="https://www.rfc-editor.org/info/rfc8853" quoteTitle="true" derivedAnchor="RFC8853">
          <front>
            <title>Using Simulcast in Session Description Protocol (SDP) and RTP Sessions</title>
            <author initials="B" surname="Burman" fullname="Bo Burman">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M" surname="Westerlund" fullname="Magnus Westerlund">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S" surname="Nandakumar" fullname="Suhas Nandakumar">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M" surname="Zanaty" fullname="Mo Zanaty">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8853"/>
          <seriesInfo name="DOI" value="10.17487/RFC8853"/>
        </reference>
        <reference anchor="RFC8854" target="https://www.rfc-editor.org/info/rfc8854" quoteTitle="true" derivedAnchor="RFC8854">
          <front>
            <title>WebRTC Forward Error Correction Requirements</title>
            <author initials="J." surname="Uberti" fullname="Justin Uberti">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8854"/>
          <seriesInfo name="DOI" value="10.17487/RFC8854"/>
        </reference>
        <reference anchor="RFC8858" target="https://www.rfc-editor.org/info/rfc8858" quoteTitle="true" derivedAnchor="RFC8858">
          <front>
            <title>Indicating Exclusive Support of RTP and RTP Control Protocol (RTCP) Multiplexing Using the Session Description Protocol (SDP)</title>
            <author initials="C." surname="Holmberg" fullname="Christer Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8858"/>
          <seriesInfo name="DOI" value="10.17487/RFC8858"/>
        </reference>
        <reference anchor="RFC8859" target="https://www.rfc-editor.org/info/rfc8859" quoteTitle="true" derivedAnchor="RFC8859">
          <front>
            <title>A Framework for Session Description Protocol (SDP) Attributes When Multiplexing</title>
            <author initials="S" surname="Nandakumar" fullname="Suhas Nandakumar">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8859"/>
          <seriesInfo name="DOI" value="10.17487/RFC8859"/>
        </reference>
      </references>
      <references pn="section-10.2">
        <name slugifiedName="name-informative-references">Informative References</name>
        <reference anchor="RFC3389" target="https://www.rfc-editor.org/info/rfc3389" quoteTitle="true" derivedAnchor="RFC3389">
          <front>
            <title>Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)</title>
            <author initials="R." surname="Zopf" fullname="R. Zopf">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2002" month="September"/>
          </front>
          <seriesInfo name="RFC" value="3389"/>
          <seriesInfo name="DOI" value="10.17487/RFC3389"/>
        </reference>
        <reference anchor="RFC3556" target="https://www.rfc-editor.org/info/rfc3556" quoteTitle="true" derivedAnchor="RFC3556">
          <front>
            <title>Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth</title>
            <author initials="S." surname="Casner" fullname="S. Casner">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2003" month="July"/>
            <abstract>
              <t indent="0">This document defines an extension to the Session Description Protocol (SDP) to specify two additional modifiers for the bandwidth attribute. These modifiers may be used to specify the bandwidth allowed for RTP Control Protocol (RTCP) packets in a Real-time Transport Protocol (RTP) session.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3556"/>
          <seriesInfo name="DOI" value="10.17487/RFC3556"/>
        </reference>
        <reference anchor="RFC3960" target="https://www.rfc-editor.org/info/rfc3960" quoteTitle="true" derivedAnchor="RFC3960">
          <front>
            <title>Early Media and Ringing Tone Generation in the Session Initiation Protocol (SIP)</title>
            <author initials="G." surname="Camarillo" fullname="G. Camarillo">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="H." surname="Schulzrinne" fullname="H. Schulzrinne">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2004" month="December"/>
            <abstract>
              <t indent="0">This document describes how to manage early media in the Session Initiation Protocol (SIP) using two models: the gateway model and the application server model.  It also describes the inputs one needs to consider in defining local policies for ringing tone generation.  This memo provides information for the Internet community.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3960"/>
          <seriesInfo name="DOI" value="10.17487/RFC3960"/>
        </reference>
        <reference anchor="RFC4568" target="https://www.rfc-editor.org/info/rfc4568" quoteTitle="true" derivedAnchor="RFC4568">
          <front>
            <title>Session Description Protocol (SDP) Security Descriptions for Media Streams</title>
            <author initials="F." surname="Andreasen" fullname="F. Andreasen">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Baugher" fullname="M. Baugher">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="Wing" fullname="D. Wing">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2006" month="July"/>
            <abstract>
              <t indent="0">This document defines a Session Description Protocol (SDP) cryptographic attribute for unicast media streams.  The attribute describes a cryptographic key and other parameters that serve to configure security for a unicast media stream in either a single message or a roundtrip exchange.  The attribute can be used with a variety of SDP media transports, and this document defines how to use it for the Secure Real-time Transport Protocol (SRTP) unicast media streams.  The SDP crypto attribute requires the services of a data security protocol to secure the SDP message.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4568"/>
          <seriesInfo name="DOI" value="10.17487/RFC4568"/>
        </reference>
        <reference anchor="RFC4588" target="https://www.rfc-editor.org/info/rfc4588" quoteTitle="true" derivedAnchor="RFC4588">
          <front>
            <title>RTP Retransmission Payload Format</title>
            <author initials="J." surname="Rey" fullname="J. Rey">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="Leon" fullname="D. Leon">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="A." surname="Miyazaki" fullname="A. Miyazaki">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="V." surname="Varsa" fullname="V. Varsa">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Hakenberg" fullname="R. Hakenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2006" month="July"/>
            <abstract>
              <t indent="0">RTP retransmission is an effective packet loss recovery technique for real-time applications with relaxed delay bounds.  This document describes an RTP payload format for performing retransmissions. Retransmitted RTP packets are sent in a separate stream from the original RTP stream.  It is assumed that feedback from receivers to senders is available.  In particular, it is assumed that Real-time Transport Control Protocol (RTCP) feedback as defined in the extended RTP profile for RTCP-based feedback (denoted RTP/AVPF) is available in this memo.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4588"/>
          <seriesInfo name="DOI" value="10.17487/RFC4588"/>
        </reference>
        <reference anchor="RFC4733" target="https://www.rfc-editor.org/info/rfc4733" quoteTitle="true" derivedAnchor="RFC4733">
          <front>
            <title>RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals</title>
            <author initials="H." surname="Schulzrinne" fullname="H. Schulzrinne">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="T." surname="Taylor" fullname="T. Taylor">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2006" month="December"/>
            <abstract>
              <t indent="0">This memo describes how to carry dual-tone multifrequency (DTMF) signalling, other tone signals, and telephony events in RTP packets. It obsoletes RFC 2833.</t>
              <t indent="0">This memo captures and expands upon the basic framework defined in RFC 2833, but retains only the most basic event codes.  It sets up an IANA registry to which other event code assignments may be added. Companion documents add event codes to this registry relating to modem, fax, text telephony, and channel-associated signalling events. The remainder of the event codes defined in RFC 2833 are conditionally reserved in case other documents revive their use.</t>
              <t indent="0">This document provides a number of clarifications to the original document.  However, it specifically differs from RFC 2833 by removing the requirement that all compliant implementations support the DTMF events.  Instead, compliant implementations taking part in out-of-band negotiations of media stream content indicate what events they support.  This memo adds three new procedures to the RFC 2833 framework: subdivision of long events into segments, reporting of multiple events in a single packet, and the concept and reporting of state events.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4733"/>
          <seriesInfo name="DOI" value="10.17487/RFC4733"/>
        </reference>
        <reference anchor="RFC5245" target="https://www.rfc-editor.org/info/rfc5245" quoteTitle="true" derivedAnchor="RFC5245">
          <front>
            <title>Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols</title>
            <author initials="J." surname="Rosenberg" fullname="J. Rosenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2010" month="April"/>
            <abstract>
              <t indent="0">This document describes a protocol for Network Address Translator (NAT) traversal for UDP-based multimedia sessions established with the offer/answer model.  This protocol is called Interactive Connectivity Establishment (ICE).  ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN).  ICE can be used by any protocol utilizing the offer/answer model, such as the Session Initiation Protocol (SIP).  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5245"/>
          <seriesInfo name="DOI" value="10.17487/RFC5245"/>
        </reference>
        <reference anchor="RFC5506" target="https://www.rfc-editor.org/info/rfc5506" quoteTitle="true" derivedAnchor="RFC5506">
          <front>
            <title>Support for Reduced-Size Real-Time Transport Control Protocol (RTCP): Opportunities and Consequences</title>
            <author initials="I." surname="Johansson" fullname="I. Johansson">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Westerlund" fullname="M. Westerlund">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2009" month="April"/>
            <abstract>
              <t indent="0">This memo discusses benefits and issues that arise when allowing Real-time Transport Protocol (RTCP) packets to be transmitted with reduced size.  The size can be reduced if the rules on how to create compound packets outlined in RFC 3550 are removed or changed.  Based on that analysis, this memo defines certain changes to the rules to allow feedback messages to be sent as Reduced-Size RTCP packets under certain conditions when using the RTP/AVPF (Real-time Transport Protocol / Audio-Visual Profile with Feedback) profile (RFC 4585). This document updates RFC 3550, RFC 3711, and RFC 4585.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5506"/>
          <seriesInfo name="DOI" value="10.17487/RFC5506"/>
        </reference>
        <reference anchor="RFC5576" target="https://www.rfc-editor.org/info/rfc5576" quoteTitle="true" derivedAnchor="RFC5576">
          <front>
            <title>Source-Specific Media Attributes in the Session Description Protocol (SDP)</title>
            <author initials="J." surname="Lennox" fullname="J. Lennox">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Ott" fullname="J. Ott">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="T." surname="Schierl" fullname="T. Schierl">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2009" month="June"/>
            <abstract>
              <t indent="0">The Session Description Protocol (SDP) provides mechanisms to describe attributes of multimedia sessions and of individual media streams (e.g., Real-time Transport Protocol (RTP) sessions) within a multimedia session, but does not provide any mechanism to describe individual media sources within a media stream.  This document defines a mechanism to describe RTP media sources, which are identified by their synchronization source (SSRC) identifiers, in SDP, to associate attributes with these sources, and to express relationships among sources.  It also defines several source-level attributes that can be used to describe properties of media sources.   [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5576"/>
          <seriesInfo name="DOI" value="10.17487/RFC5576"/>
        </reference>
        <reference anchor="RFC5763" target="https://www.rfc-editor.org/info/rfc5763" quoteTitle="true" derivedAnchor="RFC5763">
          <front>
            <title>Framework for Establishing a Secure Real-time Transport Protocol (SRTP) Security Context Using Datagram Transport Layer Security (DTLS)</title>
            <author initials="J." surname="Fischl" fullname="J. Fischl">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="H." surname="Tschofenig" fullname="H. Tschofenig">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E." surname="Rescorla" fullname="E. Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2010" month="May"/>
            <abstract>
              <t indent="0">This document specifies how to use the Session Initiation Protocol (SIP) to establish a Secure Real-time Transport Protocol (SRTP) security context using the Datagram Transport Layer Security (DTLS) protocol.  It describes a mechanism of transporting a fingerprint attribute in the Session Description Protocol (SDP) that identifies the key that will be presented during the DTLS handshake.  The key exchange travels along the media path as opposed to the signaling path.  The SIP Identity mechanism can be used to protect the integrity of the fingerprint attribute from modification by intermediate proxies.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5763"/>
          <seriesInfo name="DOI" value="10.17487/RFC5763"/>
        </reference>
        <reference anchor="RFC5764" target="https://www.rfc-editor.org/info/rfc5764" quoteTitle="true" derivedAnchor="RFC5764">
          <front>
            <title>Datagram Transport Layer Security (DTLS) Extension to Establish Keys for the Secure Real-time Transport Protocol (SRTP)</title>
            <author initials="D." surname="McGrew" fullname="D. McGrew">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E." surname="Rescorla" fullname="E. Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2010" month="May"/>
            <abstract>
              <t indent="0">This document describes a Datagram Transport Layer Security (DTLS) extension to establish keys for Secure RTP (SRTP) and Secure RTP Control Protocol (SRTCP) flows.  DTLS keying happens on the media path, independent of any out-of-band signalling channel present. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5764"/>
          <seriesInfo name="DOI" value="10.17487/RFC5764"/>
        </reference>
        <reference anchor="RFC6120" target="https://www.rfc-editor.org/info/rfc6120" quoteTitle="true" derivedAnchor="RFC6120">
          <front>
            <title>Extensible Messaging and Presence Protocol (XMPP): Core</title>
            <author initials="P." surname="Saint-Andre" fullname="P. Saint-Andre">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2011" month="March"/>
            <abstract>
              <t indent="0">The Extensible Messaging and Presence Protocol (XMPP) is an application profile of the Extensible Markup Language (XML) that enables the near-real-time exchange of structured yet extensible data between any two or more network entities.  This document defines XMPP's core protocol methods: setup and teardown of XML streams, channel encryption, authentication, error handling, and communication primitives for messaging, network availability ("presence"), and request-response interactions.  This document obsoletes RFC 3920.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6120"/>
          <seriesInfo name="DOI" value="10.17487/RFC6120"/>
        </reference>
        <reference anchor="RFC6464" target="https://www.rfc-editor.org/info/rfc6464" quoteTitle="true" derivedAnchor="RFC6464">
          <front>
            <title>A Real-time Transport Protocol (RTP) Header Extension for Client-to-Mixer Audio Level Indication</title>
            <author initials="J." surname="Lennox" fullname="J. Lennox" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E." surname="Ivov" fullname="E. Ivov">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E." surname="Marocco" fullname="E. Marocco">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2011" month="December"/>
            <abstract>
              <t indent="0">This document defines a mechanism by which packets of Real-time Transport Protocol (RTP) audio streams can indicate, in an RTP header extension, the audio level of the audio sample carried in the RTP packet.  In large conferences, this can reduce the load on an audio mixer or other middlebox that wants to forward only a few of the loudest audio streams, without requiring it to decode and measure every stream that is received.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6464"/>
          <seriesInfo name="DOI" value="10.17487/RFC6464"/>
        </reference>
        <reference anchor="RFC8828" target="https://www.rfc-editor.org/info/rfc8828" quoteTitle="true" derivedAnchor="RFC8828">
          <front>
            <title>WebRTC IP Address Handling Requirements</title>
            <author initials="J" surname="Uberti" fullname="Justin Uberti">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G" surname="Shieh" fullname="Guo-wei Shieh">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2021"/>
          </front>
          <seriesInfo name="RFC" value="8828"/>
          <seriesInfo name="DOI" value="10.17487/RFC8828"/>
        </reference>
        <reference anchor="I-D.ietf-rtcweb-sdp" quoteTitle="true" target="https://tools.ietf.org/html/draft-ietf-rtcweb-sdp-14" derivedAnchor="SDP4WebRTC">
          <front>
            <title>Annotated Example SDP for WebRTC</title>
            <author initials="S" surname="Nandakumar" fullname="Suhas Nandakumar">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C" surname="Jennings" fullname="Cullen Jennings">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="December" day="17" year="2020"/>
            <abstract>
              <t indent="0">The Web Real Time Communications (WebRTC) family of protocols defines mechanism for direct interactive rich communication using audio, video, and data between two peers' web browsers.  With in the WebRTC framework, the Session Description Protocol (SDP) is used for negotiating session capabilities between the peers.  Such a negotiation happens based on the SDP offer/answer exchange mechanism  This document provides an informational reference in describing the role of SDP and the offer/answer exchange mechanism for the most common WebRTC use cases.  This document makes no changes to the SDP offer/answer exchange mechanism.</t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-rtcweb-sdp-14"/>
          <format type="TXT" target="http://www.ietf.org/internet-drafts/draft-ietf-rtcweb-sdp-14.txt"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
        <reference anchor="TS26.114" target="https://www.3gpp.org/DynaReport/26114.htm" quoteTitle="true" derivedAnchor="TS26.114">
          <front>
            <title>3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; IP Multimedia Subsystem (IMS); Multimedia Telephony; Media handling and interaction (Release 16)</title>
            <seriesInfo name="3GPP TS" value="26.114 V16.3.0"/>
            <author>
              <organization showOnFrontPage="true">3GPP</organization>
            </author>
            <date year="2019" month="September"/>
          </front>
        </reference>
        <reference anchor="W3C.webrtc" target="https://www.w3.org/TR/2020/PR-webrtc-20201215/" quoteTitle="true" derivedAnchor="W3C.webrtc">
          <front>
            <title>WebRTC 1.0: Real-time Communication Between Browsers</title>
            <author fullname="Cullen Jennings" initials="C." surname="Jennings" role="editor">
              <organization showOnFrontPage="true">Cisco</organization>
            </author>
            <author fullname="Henrik Boström" asciiFullname="Henrik Bostrom" initials="H." surname="Boström" asciiSurname="Bostrom" role="editor">
              <organization showOnFrontPage="true">Google</organization>
            </author>
            <author fullname="Jan-Ivar Bruaroey" initials="J." surname="Bruaroey" role="editor">
              <organization showOnFrontPage="true">Mozilla</organization>
            </author>
            <date month="Dec" year="2020"/>
          </front>
          <refcontent>World Wide Web Consortium PR PR-webrtc-20201215</refcontent>
        </reference>
      </references>
    </references>
    <section anchor="sec.appendix-a" numbered="true" toc="include" removeInRFC="false" pn="section-appendix.a">
      <name slugifiedName="name-sdp-abnf-syntax">SDP ABNF Syntax</name>
      <t indent="0" pn="section-appendix.a-1">For the syntax validation performed in
      <xref target="sec.parsing-a-desc" format="default" sectionFormat="of" derivedContent="Section 5.8"/>, the following list of ABNF
      definitions is used:</t>
      <table anchor="sdp-abnf" align="center" pn="table-1">
        <name slugifiedName="name-sdp-abnf-references">SDP ABNF References</name>
        <thead>
          <tr>
            <th align="left" colspan="1" rowspan="1">Attribute</th>
            <th align="left" colspan="1" rowspan="1">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td align="left" colspan="1" rowspan="1">ptime</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC4566" sectionFormat="of" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">maxptime</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC4566" sectionFormat="of" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">rtpmap</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC4566" sectionFormat="of" section="6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-6" derivedContent="RFC4566"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">recvonly</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC4566" sectionFormat="of" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-9" derivedContent="RFC4566"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">sendrecv</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC4566" sectionFormat="of" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-9" derivedContent="RFC4566"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">sendonly</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC4566" sectionFormat="of" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-9" derivedContent="RFC4566"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">inactive</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC4566" sectionFormat="of" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-9" derivedContent="RFC4566"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">fmtp</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC4566" sectionFormat="of" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4566#section-9" derivedContent="RFC4566"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">rtcp</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC3605" sectionFormat="of" section="2.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3605#section-2.1" derivedContent="RFC3605"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">setup</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC4145" section="4" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4145#section-4" derivedContent="RFC4145"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">fingerprint</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8122" sectionFormat="of" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8122#section-5" derivedContent="RFC8122"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">rtcp-fb</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC4585" sectionFormat="of" section="4.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4585#section-4.2" derivedContent="RFC4585"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">extmap</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC5285" sectionFormat="of" section="7" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5285#section-7" derivedContent="RFC5285"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">mid</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC5888" section="4" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5888#section-4" derivedContent="RFC5888"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">group</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC5888" section="5" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5888#section-5" derivedContent="RFC5888"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">imageattr</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC6236" sectionFormat="of" section="3.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc6236#section-3.1" derivedContent="RFC6236"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">extmap (encrypt option)</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC6904" sectionFormat="of" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc6904#section-4" derivedContent="RFC6904"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">candidate</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8839" sectionFormat="of" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.1" derivedContent="RFC8839"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">remote-candidates</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8839" sectionFormat="of" section="5.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.2" derivedContent="RFC8839"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">ice-lite</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8839" sectionFormat="of" section="5.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.3" derivedContent="RFC8839"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">ice-ufrag</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8839" sectionFormat="of" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.4" derivedContent="RFC8839"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">ice-pwd</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8839" sectionFormat="of" section="5.4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.4" derivedContent="RFC8839"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">ice-options</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8839" sectionFormat="of" section="5.6" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8839#section-5.6" derivedContent="RFC8839"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">msid</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8830" sectionFormat="of" section="3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8830#section-3" derivedContent="RFC8830"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">rid</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8851" sectionFormat="of" section="10" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8851#section-10" derivedContent="RFC8851"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">simulcast</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8853" sectionFormat="of" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8853#section-5.1" derivedContent="RFC8853"/></td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">tls-id</td>
            <td align="left" colspan="1" rowspan="1">
              <xref target="RFC8842" sectionFormat="of" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8842#section-4" derivedContent="RFC8842"/></td>
          </tr>
        </tbody>
      </table>
    </section>
    <section anchor="sec.acknowledgements" numbered="false" toc="include" removeInRFC="false" pn="section-appendix.b">
      <name slugifiedName="name-acknowledgements">Acknowledgements</name>
      <t indent="0" pn="section-appendix.b-1"><contact fullname="Harald Alvestrand"/>, <contact fullname="Taylor       Brandstetter"/>, <contact fullname="Suhas Nandakumar"/>, and
      <contact fullname="Peter Thatcher"/> provided significant text for this
      document. <contact fullname="Bernard Aboba"/>, <contact fullname="Adam       Bergkvist"/>, <contact fullname="Jan-Ivar Bruaroey"/>,
      <contact fullname="Dan Burnett"/>, <contact fullname="Ben       Campbell"/>, <contact fullname="Alissa Cooper"/>,
      <contact fullname="Richard Ejzak"/>, <contact fullname="Stefan       Håkansson"/>, <contact fullname="Ted Hardie"/>, <contact fullname="Christer Holmberg"/>,
      <contact fullname="Andrew Hutton"/>, <contact fullname="Randell       Jesup"/>, <contact fullname="Matthew Kaufman"/>, <contact fullname="Anant Narayanan"/>,
      <contact fullname="Adam Roach"/>, <contact fullname="Robert Sparks"/>,
      <contact fullname="Neil Stratford"/>, <contact fullname="Martin       Thomson"/>, <contact fullname="Sean       Turner"/>, and <contact fullname="Magnus Westerlund"/> all provided valuable feedback on
      this document.</t>
    </section>
    <section anchor="authors-addresses" numbered="false" removeInRFC="false" toc="include" pn="section-appendix.c">
      <name slugifiedName="name-authors-addresses">Authors' Addresses</name>
      <author fullname="Justin Uberti" initials="J." surname="Uberti">
        <organization showOnFrontPage="true">Google</organization>
        <address>
          <postal>
            <street>747 6th Street South</street>
            <city>Kirkland</city>
            <region>WA</region>
            <code>98033</code>
            <country>United States of America</country>
          </postal>
          <email>justin@uberti.name</email>
        </address>
      </author>
      <author fullname="Cullen Jennings" initials="C." surname="Jennings">
        <organization showOnFrontPage="true">Cisco</organization>
        <address>
          <postal>
            <street>400 3rd Avenue SW</street>
            <city>Calgary</city>
            <region>AB</region>
            <code>T2P 4H2</code>
            <country>Canada</country>
          </postal>
          <email>fluffy@iii.ca</email>
        </address>
      </author>
      <author fullname="Eric Rescorla" initials="E." surname="Rescorla" role="editor">
        <organization showOnFrontPage="true">Mozilla</organization>
        <address>
          <email>ekr@rtfm.com</email>
        </address>
      </author>
    </section>
  </back>
</rfc>