



Internet Engineering Task Force (IETF)                       V. Govindan
Request for Comments: 9798                                     S. Venaas
Updates: 8059                                        Cisco Systems, Inc.
Category: Experimental                                         June 2025
ISSN: 2070-1721


  PIM Join/Prune Attributes for Locator/ID Separation Protocol (LISP)
                 Environments Using Underlay Multicast

Abstract

   This document specifies an update to the Receiver RLOC (Routing
   Locator) field of the PIM Join/Prune attribute that supports the
   construction of multicast distribution trees where the source and
   receivers are located in different Locator/ID Separation Protocol
   (LISP) sites and are connected using underlay IP multicast.  This
   attribute allows the receiver site to signal the underlay multicast
   group to the control plane of the root Ingress Tunnel Router (ITR).
   This document updates RFC 8059.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see 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/rfc9798.

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Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  The Case for Extending the Received ETR RLOC Attribute of RFC
           8059
     2.1.  Flexible Mapping of Overlay to Underlay Group Ranges
     2.2.  Multicast Address Range Constraints
   3.  Updates to RFC 8059
     3.1.  Scope
     3.2.  Receiver ETR RLOC Attribute
     3.3.  Using the Receiver RLOC Attribute
   4.  IANA Considerations
   5.  Security Considerations
   6.  Normative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The construction of multicast distribution trees where the root and
   receivers are located in different LISP sites [RFC9300] is defined in
   [RFC6831].

   [RFC6831] specifies that (EID, G) data packets are to be LISP-
   encapsulated into (RLOC, G) multicast packets.  In this document, we
   use the term root-EID or root-RLOC to refer to the source of the
   multicast tree rooted at the EID or RLOC.  [RFC8059] defines PIM
   Join/Prune attribute extensions to construct multicast distribution
   trees.  Please refer to Section 3 of [RFC6831] for the definition of
   the terms Endpoint ID (EID) and Routing Locator (RLOC).  This
   document extends the Receiver ETR RLOC PIM Join/Prune attribute
   [RFC8059] to facilitate the construction of underlay multicast trees
   for (root-RLOC, G).

   Specifically, the assignment of the underlay multicast group needs to
   be done in consonance with the downstream Tunnel Router (xTR) nodes
   needed to avoid unnecessary replication or traffic hairpinning.

   Since the Receiver RLOC Attribute defined in [RFC8059] only addresses
   the Ingress Replication case, this document extends the scope of that
   PIM Join/Prune attribute to include scenarios where the underlay uses
   multicast transport.  The scope extension complies with the base
   specification [RFC5384].

   This document uses terminology defined in [RFC6831], such as EID,
   RLOC, ITR and ETR.

1.1.  Requirements Language

   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.

2.  The Case for Extending the Received ETR RLOC Attribute of RFC 8059

   When LISP-based multicast trees are constructed using IP multicast in
   the underlay, the mapping between the overlay group address and the
   underlay group address becomes a crucial engineering decision.

2.1.  Flexible Mapping of Overlay to Underlay Group Ranges

   Three distinct types of overlay to underlay group mappings are
   possible:

   *  Many-to-one mapping: Many (root-EID, G) flows originating from an
      RLOC can be mapped to a single underlay multicast (root-RLOC, G-u)
      flow.

   *  One-to-many mapping: Conversely a single same overlay flow can be
      mapped to two or more flows -- e.g., (root-RLOC, G-u1) and (root-
      RLOC, G-u2) -- to cater to the requirements of downstream xTR
      nodes.

   *  One-to-one mapping: Every (root-EID, G) flow is mapped to a unique
      (root-RLOC, G-u) flow.

2.2.  Multicast Address Range Constraints

   Under certain conditions, different subsets of xTRs subscribing to
   the same overlay multicast stream may be constrained to use distinct
   underlay multicast mapping ranges.

   This introduces a trade-off between replication overhead and the
   flexibility of address range assignment, which may be necessary in
   specific use cases like Proxy Tunnel Routers or when using nodes with
   limited hardware resources as explained below.

   Inter-site Proxy Tunnel Routers (PxTR):
     When multiple LISP sites are interconnected through a LISP-based
     transit, the site border node (i.e., PxTR) connects the site-facing
     interfaces with the external LISP core.  In such cases, different
     ranges of multicast group addresses may be used for constructing
     (S-RLOC, G) trees within the LISP site and in the external LISP
     core.  This distinction is desirable for various operational
     reasons.

   Hardware resource restrictions:
     Platform limitations may necessitate engineering decisions to
     restrict multicast address ranges in the underlay due to hardware
     resource constraints.

3.  Updates to RFC 8059

3.1.  Scope

   There are no changes to the syntax or semantics of the Transport
   Attribute defined in [RFC8059].

   The scope of the updates to [RFC8059] is limited to the case where
   the "Transport" field of the Transport Attribute is set to zero
   (multicast) only.

3.2.  Receiver ETR RLOC Attribute

   The definition of the "Receiver RLOC" field of the Receiver ETR RLOC
   attribute (see Section 5.1 of [RFC8059]) is updated as follows:

   OLD:

   |  Receiver RLOC:  The RLOC address on which the receiver ETR wishes
   |     to receive the unicast-encapsulated flow.

   NEW:

   |  Receiver RLOC:  The RLOC address on which the receiver ETR wishes
   |     to receive the encapsulated flow.  A unicast IP Receiver RLOC
   |     address is used for unicast-encapsulated flows.  Alternately, a
   |     multicast IP Receiver RLOC address is used for multicast-
   |     encapsulated flows.  A multicast IP address MUST be used only
   |     when the underlay network of the LISP core supports IP
   |     multicast transport.

   The definitions of the other fields of the Receiver ETR RLOC
   Attribute remain unchanged.

   When the ITR needs to track the list of ETRs from which the PIM joins
   are received, the ITR MUST use the source IP address field of the
   incoming PIM Join/Prune message.  The source IP address of the PIM
   Join/Prune MUST be an ETR RLOC IP address.

3.3.  Using the Receiver RLOC Attribute

   When the ETR determines to use the multicast underlay:

   *  It chooses an underlay multicast group that it can join.  This is
      a matter of local decision, which is beyond the scope of this
      document.

   *  It identifies the upstream LISP site where the underlay multicast
      tree needs to be rooted.

   *  It constructs the PIM Join/Prune message as specified in
      [RFC8059].  Only the Receiver RLOC attribute is encoded as above.


   When the ITR receives a PIM Join/Prune message:

   *  It allocates a new entry in the outgoing interface list [RFC6831]
      for every unique underlay multicast mapping.

   *  The ITR MAY apply local policy to perform any kind of rate-
      limiting on the number of copies it needs to make in the underlay.
      Such actions are beyond the scope of this document.


4.  IANA Considerations

   This document has no IANA actions.

5.  Security Considerations

   An attack vector arises where an attacker sends numerous PIM Join
   messages with different group addresses.  This could interfere with
   legitimate multicast traffic if the group addresses overlap.
   Additionally, resource exhaustion may occur if replication is
   requested for a large number of groups, potentially resulting in
   significant resource consumption.  To mitigate these risks, PIM
   authentication mechanisms [RFC5796] could be employed to validate
   join requests.  Furthermore, implementations may consider explicit
   tracking mechanisms to manage joins more effectively.  Configurable
   controls could be introduced, allowing for a maximum permissible
   number of groups for each ETR RLOC used as the source of overlay
   joins.  These controls would limit the impact of such attacks and
   ensure that resource allocation is managed appropriately.

6.  Normative References

   [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>.

   [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>.

   [RFC6831]  Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas, "The
              Locator/ID Separation Protocol (LISP) for Multicast
              Environments", RFC 6831, DOI 10.17487/RFC6831, January
              2013, <https://www.rfc-editor.org/info/rfc6831>.

   [RFC8059]  Arango, J., Venaas, S., Kouvelas, I., and D. Farinacci,
              "PIM Join Attributes for Locator/ID Separation Protocol
              (LISP) Environments", RFC 8059, DOI 10.17487/RFC8059,
              January 2017, <https://www.rfc-editor.org/info/rfc8059>.

   [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>.

   [RFC9300]  Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A.
              Cabellos, Ed., "The Locator/ID Separation Protocol
              (LISP)", RFC 9300, DOI 10.17487/RFC9300, October 2022,
              <https://www.rfc-editor.org/info/rfc9300>.

Acknowledgements

   The authors would like to thank Dino Farinacci, Victor Moreno, Alvaro
   Retana, Aswin Kuppusami, Joe Clarke, and Peter Yee for their valuable
   comments.  The authors also thank Sankaralingam T and Amit Kumar for
   their contributions to the document.  The authors thank Gunter Van de
   Velde for his valuable comments.

Authors' Addresses

   Vengada Prasad Govindan
   Cisco Systems, Inc.
   Email: venggovi@cisco.com


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