



Internet Engineering Task Force (IETF)                   H. Bidgoli, Ed.
Request for Comments: 9739                                         Nokia
Category: Standards Track                                      S. Venaas
ISSN: 2070-1721                                                M. Mishra
                                                     Cisco Systems, Inc.
                                                                Z. Zhang
                                                        Juniper Networks
                                                              M. McBride
                                             Futurewei Technologies Inc.
                                                              March 2025


            Protocol Independent Multicast Light (PIM Light)

Abstract

   This document specifies Protocol Independent Multicast Light (PIM
   Light) and the PIM Light Interface (PLI).  A PLI does not need a PIM
   Hello message to accept PIM Join/Prune messages, and it can signal
   multicast states over networks that cannot support full PIM neighbor
   discovery, such as Bit Index Explicit Replication (BIER) networks
   that connect two or more PIM domains.  This document outlines the PIM
   Light protocol and procedures to ensure loop-free multicast traffic
   between two or more PIM Light routers.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9739.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  PIM Light Interface
     3.1.  Message Types Supported by PIM Light
     3.2.  Considerations for the Absence of Hello Message
       3.2.1.  Join Attribute
       3.2.2.  DR Election
       3.2.3.  PIM Assert
     3.3.  PLI Configuration
     3.4.  Failures in PLR Domain
     3.5.  Reliable Transport Mechanism for PIM Light
     3.6.  PIM Variants Not Supported
   4.  IANA Considerations
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   This document specifies procedures for Protocol Independent Multicast
   Light (PIM Light) and the PIM Light Interface (PLI).  The PLI is a
   new type of PIM interface that allows signaling of PIM Join/Prune
   packets without full PIM neighbor discovery.  A PLI is useful in
   scenarios where multicast states need to be signaled over networks or
   media that cannot support full PIM neighborship between routers or,
   alternatively, where full PIM neighborship is not desired.  These
   types of networks and media are called "PIM Light domains" within
   this document.  Lack of full PIM neighborship will remove some PIM
   functionality as explained in Section 3.2 of this document.  PIM
   Light only supports the PIM - Sparse Mode (PIM-SM) protocol,
   including PIM Source-Specific Multicast (PIM-SSM), as per [RFC7761].
   This document details procedures and considerations needed for PIM
   Light and the PLI to ensure efficient routing of multicast groups for
   specific deployment environments.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses terminology from "Protocol Independent Multicast -
   Sparse Mode (PIM-SM): Protocol Specification (Revised)" [RFC7761].

3.  PIM Light Interface

   Section 4.3.1 of [RFC7761] describes PIM neighbor discovery via Hello
   messages.  Section 4.5 of [RFC7761] notes that if a router receives a
   Join/Prune message from a particular IP source address and it has not
   seen a PIM Hello message from that source address, then the Join/
   Prune message SHOULD be discarded without further processing.

   In certain scenarios, it is desirable to establish multicast states
   between two routers without forming a PIM neighborship.  This can be
   necessary for various reasons, such as signaling multicast states
   upstream between multiple PIM domains over a network that is not
   optimized for PIM or that does not necessitate PIM neighbor
   establishment.  An example is a Bit Index Explicit Replication (BIER)
   [RFC8279] network connecting multiple PIM domains, where PIM Join/
   Prune messages are tunneled via BIER as specified in [BIER-PIM].

   A PLI accepts Join/Prune messages from an unknown PIM router without
   requiring a PIM Hello message from the router.  The absence of Hello
   messages on a PLI means there is no mechanism to discover neighboring
   PIM routers or their capabilities or to execute basic algorithms such
   as Designated Router (DR) election [RFC7761].  Consequently, the PIM
   Light router does not create any general-purpose state for
   neighboring PIM routers and only processes Join/Prune messages from
   downstream routers in its multicast routing table.  Processing these
   Join/Prune messages will introduce multicast states in a PIM Light
   router.

   Due to these constraints, a PLI should be deployed in very specific
   scenarios where PIM-SM is not suitable.  The applications or the
   networks on which PLIs are deployed MUST ensure that there is no
   multicast packet duplication, such as multiple upstream routers
   sending the same multicast stream to a single downstream router.  For
   example, an implementation should ensure that DR election is done on
   upstream redundant PIM routers that are at the edge of the PIM Light
   domain to ensure that a single DR forwards the PIM Join message from
   the receiver to the source.

3.1.  Message Types Supported by PIM Light

   The "PIM Message Types" registry [IANA-PIM-Mess-Types] lists the
   message types supported by PIM.  PIM Light only supports the
   following message types in that registry:

   *  type 1 (Register)

   *  type 2 (Register Stop)

   *  type 3 (Join/Prune)

   *  type 8 (Candidate RP Advertisement)

   *  type 13.0 (PIM Packed Null-Register)

   *  type 13.1 (PIM Packed Register-Stop)

   *  Any future PIM message types where the destination is a unicast IP
      address

   No other message types are supported by PIM Light; other message
   types MUST NOT be processed if received on a PLI.

3.2.  Considerations for the Absence of Hello Message

   Because Hello messages are not processed in a PIM Light domain, the
   considerations in the subsections below should be taken into account.

3.2.1.  Join Attribute

   Since a PLI does not use PIM Hello messages, it also does not support
   the Join Attribute option in PIM Hello as specified in [RFC5384].  As
   such, PIM Light is unaware of its neighbor's capability to process
   Join Attributes and SHOULD NOT send a Join message containing a Join
   Attribute.

   There are two cases in which a PLI can support a Join Attribute:

   *  The neighbors on the PLI are known via configuration to be capable
      of processing the attribute.

   *  Internet-Drafts and RFCs may dictate that certain Join Attributes
      are allowed to be used without explicit configuration of the PLI
      in certain scenarios.  The details are left to those Internet-
      Drafts and RFCs.

3.2.2.  DR Election

   Due to the absence of Hello messages, DR election is not supported on
   a PIM Light router.  The network design must ensure DR election
   occurs within the PIM domain, assuming the PIM Light domain
   interconnects PIM domains.

   For instance, in a BIER domain connecting two PIM domains as in the
   figure below, a PLI can be used between BIER edge routers solely for
   multicast state communication and transmit only PIM Join/Prune
   messages.  If there are redundant PIM routers at the edge of the BIER
   domain, they MUST establish PIM adjacency as per [RFC7761] to prevent
   multicast stream duplication and to ensure DR election at the edge of
   the BIER domain.  For example, DR election could be between router D
   and F in the figure below.  When the Join or Prune message arrives
   from a PIM domain to the downstream BIER edge router, it can be
   forwarded over the BIER tunnel to the upstream BIER edge router only
   via the DR.

                    Bier edge router       Bier edge router
           |--PIM domain--|--BIER domain (PLI)--|--PIM domain--|
 Source--( A )----------( B ) ---- ( C ) ---- ( D )----------( E )--Host
           |       PIM Adj|         | |         |PIM Adj       |
           |------------( E )-------| |-------( F )------------|
                                          (DR election)

3.2.3.  PIM Assert

   In scenarios where multiple PIM routers peer over a shared LAN or a
   point-to-multipoint medium, more than one upstream router may have
   valid forwarding state for a packet, which can potentially cause
   packet duplication.  PIM Assert is used to select a single
   transmitter when such duplication is detected.  According to
   Section 4.6 of [RFC7761], PIM Assert should only be accepted from a
   known PIM neighbor.

   In PIM Light implementations, care must be taken to avoid duplicate
   streams arriving from multiple upstream PIM Light routers to a single
   downstream PIM Light router.  If network design constraints prevent
   this, the implemented network architecture must take measures to
   avoid traffic duplication.  For example, in a scenario with PIM Light
   over a BIER domain, a downstream IBBR (Ingress BIER Border Router) in
   a BIER domain can identify the nearest EBBRs (Egress BIER Border
   Routers) to the source using the Shortest Path First (SPF) algorithm
   with post-processing as described in Appendix A.1 of [BIER-PIM].  If
   the downstream IBBR identifies two EBBRs, it can select one using a
   unique IP selection algorithm, such as choosing the EBBR with the
   lowest or highest IP address.  If the selected EBBR goes offline, the
   downstream router can use the next EBBR based on the IP selection
   algorithm, which is beyond the scope of this document.

3.3.  PLI Configuration

   Since a PLI doesn't require PIM Hello Messages and PIM neighbor
   adjacency is not checked for arriving Join/Prune messages, there
   needs to be a mechanism to enable PLIs on interfaces.  Join/Prune
   messages not received from a PIM neighbor MUST be dropped unless PLI
   is enabled on the interface.  In some cases, a PLI may be enabled
   automatically via an underlying mechanism on a logical interface.
   For example, in a BIER domain, a logical interface can connect two or
   more BIER edge routers as per [BIER-PIM].

3.4.  Failures in PLR Domain

   Because Hello messages are not processed on the PLI, PLI failures may
   not be discovered in a PIM Light domain, and multicast routes will
   not be pruned toward the source on the PIM Light domain.  This
   results in the upstream routers continuously sending multicast
   streams until the outgoing interface (OIF) expires.

   Other protocols can be used to detect these failures in the PIM Light
   domain, and they can be implementation specific.  As an example, the
   interface on which PIM Light is configured can be protected via
   Bidirectional Forwarding Detection (BFD) or similar technology.  If
   BFD to the far-end PLI goes down and the PIM Light router is upstream
   and has an OIF for a multicast route (S,G), PIM must remove that PLI
   from its OIF list.

   In another example, the PLI is configured automatically between the
   BIER Edge Routers (BERs) as in the figure below.  When the Downstream
   BIER Edge Router (DBER) is no longer reachable on the Upstream BIER
   Edge Router (UBER), the UBER (which is also a PIM Light router) can
   prune the (S,G) advertised toward the source on the PIM domain to
   stop the transmission of the multicast stream.

                         UBER                 DBER
           |--PIM domain--|--BIER domain (PLI)--|--PIM domain--|
 Source--( A )----------( B ) ---- ( C ) ---- ( D )----------( E )--Host
                  <--Prune (S,G)          <failure on D>

3.5.  Reliable Transport Mechanism for PIM Light

   [RFC6559] defines a reliable transport mechanism called PIM Over
   Reliable Transport (PORT) for PIM transmission of Join/Prune
   messages, using either TCP or SCTP as the transport protocol.  Both
   TCP and SCTP use destination port number 8471.  SCTP is explained in
   [RFC9260] and is used as a second option for PORT.  [RFC6559]
   mentions that when a router is configured to use PIM over TCP on a
   given interface, it MUST include the PIM-over-TCP-Capable Hello
   Option in its Hello messages for that interface.  The same is true
   for SCTP; the router MUST include the PIM-over-SCTP-Capable Hello
   Option in its Hello messages on that interface.

   These Hello options contain a Connection ID, which is an IPv4 or IPv6
   address used to establish the SCTP or TCP connection.  For PORT using
   TCP, the Connection ID is used to determine which peer is doing an
   active transport open to the neighbor and which peer is doing passive
   transport open, as per Section 4 of [RFC6559].  When the router is
   using SCTP, the Connection ID is not used to determine the active and
   passive peer since SCTP can handle call collision.

   Because PIM Light lacks Hello messages, the PLI can be configured
   with the Connection ID (i.e., the IPv4 or IPv6 address used to
   establish the SCTP or TCP connection).  For PIM Light using the TCP
   PORT option, each end of the PLI must be explicitly and correctly
   configured as being either active transport open or passive transport
   open to ensure that call collision is avoided.

3.6.  PIM Variants Not Supported

   The following PIM variants are not supported with PIM Light and not
   covered by this document:

   *  PIM - Dense Mode (PIM-DM) [RFC3973]

   *  Bidirectional PIM (BIDIR-PIM) [RFC5015]

4.  IANA Considerations

   This document has no IANA actions.

5.  Security Considerations

   Since PIM Light does not require PIM Hello messages and does not
   verify PIM neighbor adjacency for incoming Join/Prune messages, for
   security reasons, it is crucial that implementations ensure that only
   Join/Prune messages arriving at a configured PLI are processed.  Any
   Join/Prune messages received on an interface that is not configured
   as a PLI MUST be discarded and not processed.  Additionally, as a
   secondary line of defense, route policies SHOULD be implemented to
   process only the Join/Prune messages associated with the desired
   (S,G) pairs, while all other (S,G) pairs MUST be discarded and not
   processed.

   Furthermore, because PIM Light can be used for signaling PIM-SM Join/
   Prune messages, the security considerations outlined in [RFC7761] and
   [RFC4607] SHOULD be considered where appropriate.

   Per Section 6.1.1 of [RFC7761], only forged Join/Prune messages
   should be considered as a potential attack vector, as PIM Light does
   not process Hello or Assert messages.  In addition, as detailed in
   Section 6.3 of [RFC7761], the authentication mechanisms described in
   [RFC5796] can be applied to PIM Light via IPsec Encapsulating
   Security Payload (ESP) or, optionally, the Authentication Header
   (AH).

6.  References

6.1.  Normative References

   [IANA-PIM-Mess-Types]
              IANA, "PIM Message Types",
              <https://www.iana.org/assignments/pim-parameters>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
              <https://www.rfc-editor.org/info/rfc4607>.

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
              <https://www.rfc-editor.org/info/rfc5015>.

   [RFC5384]  Boers, A., Wijnands, I., and E. Rosen, "The Protocol
              Independent Multicast (PIM) Join Attribute Format",
              RFC 5384, DOI 10.17487/RFC5384, November 2008,
              <https://www.rfc-editor.org/info/rfc5384>.

   [RFC5796]  Atwood, W., Islam, S., and M. Siami, "Authentication and
              Confidentiality in Protocol Independent Multicast Sparse
              Mode (PIM-SM) Link-Local Messages", RFC 5796,
              DOI 10.17487/RFC5796, March 2010,
              <https://www.rfc-editor.org/info/rfc5796>.

   [RFC6559]  Farinacci, D., Wijnands, IJ., Venaas, S., and M.
              Napierala, "A Reliable Transport Mechanism for PIM",
              RFC 6559, DOI 10.17487/RFC6559, March 2012,
              <https://www.rfc-editor.org/info/rfc6559>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,
              <https://www.rfc-editor.org/info/rfc8279>.

   [RFC9260]  Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
              Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
              June 2022, <https://www.rfc-editor.org/info/rfc9260>.

6.2.  Informative References

   [BIER-PIM] Bidgoli, H., Ed., Xu, F., Kotalwar, J., Wijnands, I.,
              Mishra, M., and Z. Zhang, "PIM Signaling Through BIER
              Core", Work in Progress, Internet-Draft, draft-ietf-bier-
              pim-signaling-13, 3 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bier-
              pim-signaling-13>.

   [RFC3973]  Adams, A., Nicholas, J., and W. Siadak, "Protocol
              Independent Multicast - Dense Mode (PIM-DM): Protocol
              Specification (Revised)", RFC 3973, DOI 10.17487/RFC3973,
              January 2005, <https://www.rfc-editor.org/info/rfc3973>.

Acknowledgments

   The authors would like to thank Zheng (Sandy) Zhang and Tanmoy Kundu
   for their suggestions and contributions to this document.

Authors' Addresses

   Hooman Bidgoli (editor)
   Nokia
   March Road
   Ottawa Ontario K2K 2T6
   Canada
   Email: hooman.bidgoli@nokia.com


   Stig Venaas
   Cisco Systems, Inc.
   Tasman Drive
   San Jose, CA 95134
   United States of America
   Email: stig@cisco.com


   Mankamana Mishra
   Cisco Systems, Inc.
   Tasman Drive
   San Jose, CA 95134
   United States of America
   Email: mankamis@cisco.com


   Zhaohui Zhang
   Juniper Networks
   Boston, MA
   United States of America
   Email: zzhang@juniper.net


   Mike McBride
   Futurewei Technologies Inc.
   Santa Clara, CA
   United States of America
   Email: michael.mcbride@futurewei.com
