| RFC 9798 | PIM Join Attributes for LISP Multicast | May 2025 |
| Govindan & Venaas | Experimental | [Page] |
This document specifies an update to the PIM Receiver RLOC 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.¶
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.¶
Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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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 (root-EID, G) data packets are to be LISP-encapsulated into (root-RLOC, G) multicast packets. [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). We use the term root-EID or root-RLOC to refer to the source of the multicast tree rooted at the EID or 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 [RFC9300], such as EID, RLOC, ITR, and ETR.¶
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.¶
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.¶
Three distinct types of overlay to underlay group mappings are possible:¶
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.¶
No changes are proposed 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.¶
The definition of the "Receiver RLOC" field of the Receiver ETR RLOC attribute [RFC8059] is updated as follows:¶
- 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 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.¶
When the ETR determines to use the multicast underlay:¶
When the ITR receives a PIM Join/Prune message:¶
This document has no IANA actions.¶
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.¶
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.¶