RFC 9798

RFC 9798: PIM Join/Prune Attributes for Locator/ID Separation Protocol (LISP) Environments Using Underlay Multicast 💖

Okay bestie, this one is from the IETF (Internet Engineering Task Force) and it's giving protocol nerd heaven 🤓✨. It was written by V. Govindan and S. Venaas, both at Cisco Systems, Inc., and it was published in June 2025 as RFC 9798 (ISSN 2070-1721). It's labeled Experimental, so it's not a full Internet Standards Track spec 🧪. It also updates RFC 8059, which means it tweaks some of that doc's details 🎀.

Abstract ✨

Okay so this doc tweaks the Receiver RLOC (Routing Locator) field of the PIM Join/Prune attribute 💅. The whole point is to help build multicast distribution trees when the source and the receivers live in different LISP sites and are connected with underlay IP multicast 🌈. This attribute lets the receiver site tell the control plane of the root Ingress Tunnel Router (ITR) which underlay multicast group it wants 🫶. And yes, this document updates RFC 8059 🦋.

Status of This Memo ✨

Bestie, this doc is not an Internet Standards Track spec. It's published so people can examine it, try out experimental implementations, and evaluate it 🧪.

It defines an Experimental Protocol for the Internet community 🔬. It's a product of the IETF and represents the consensus of the IETF community 💜. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG) 👑. Not every IESG-approved doc is eligible for any level of Internet Standard, so check Section 2 of RFC 7841 if you want the deets 🙈.

For the latest status, errata, and how to send feedback, head to https://www.rfc-editor.org/info/rfc9798 🌸.

Copyright Notice ✨

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

This doc is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as they're in effect on the date of publication. Please actually read these, because they describe your rights and restrictions for this document 📜. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions, and they're provided without warranty as described in the Revised BSD License ⚖️.

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 ✨

Okay so the construction of multicast distribution trees where the root and receivers sit in different LISP sites [RFC9300] is defined in [RFC6831] 🌸.

[RFC6831] says that (EID, G) data packets get LISP-encapsulated into (RLOC, G) multicast packets 📦. In this doc we use the term root-EID or root-RLOC to mean the source of the multicast tree that's rooted at the EID or RLOC 🦋. [RFC8059] defines PIM Join/Prune attribute extensions for building multicast distribution trees 💅. Please refer to Section 3 of [RFC6831] for the definitions of Endpoint ID (EID) and Routing Locator (RLOC) 📖. This document extends the Receiver ETR RLOC PIM Join/Prune attribute [RFC8059] so you can build underlay multicast trees for (root-RLOC, G) 🌈.

Specifically, the underlay multicast group assignment has to be done in consonance with the downstream Tunnel Router (xTR) nodes 🎯. That's how you avoid unnecessary replication or traffic hairpinning 🙅‍♀️.

The Receiver RLOC Attribute in [RFC8059] only covers the Ingress Replication case 😭. So this doc extends the scope of that PIM Join/Prune attribute to cover scenarios where the underlay uses multicast transport 💖. This scope extension complies with the base specification [RFC5384] ✨.

This document uses terminology from [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 🌟

Okay so when LISP-based multicast trees are built with IP multicast in the underlay, the mapping between the overlay group address and the underlay group address becomes a huge 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 is a trade-off between replication overhead and how flexible your address range assignment can be 🫠. It may be necessary in specific use cases, like Proxy Tunnel Routers or when you're 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 these 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]) gets updated like this 🌈:

   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 stay exactly the same 🧚.

When the ITR needs to track the list of ETRs that the PIM joins are coming from, 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 decides to use the multicast underlay, it does a few things 💅:

• 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 does these things 👑:

• 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 🚦.

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Part 1 of 2 💖 IANA Considerations, Security Considerations and the References are up next 🫶

Acknowledgements ✨

Okay bestie, big thank-you 🫶 to the people who helped make this doc slay. The authors give a huge shoutout to Dino Farinacci, Victor Moreno, Alvaro Retana, Aswin Kuppusami, Joe Clarke, and Peter Yee for their super valuable comments 💖. The authors also thank Sankaralingam T and Amit Kumar for their contributions to the document 🎀. And the authors thank Gunter Van de Velde for his valuable comments too 🌸.

Authors' Addresses 🦋

Meet the two icons who wrote it 💅 (contact info below, obviously):

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