PIM

Summary

The PIM Working Group session at IETF 125 focused on the progression of several core drafts, including the long-standing draft-ietf-pim-rfc1112bis and various multicast auto-discovery documents. A significant portion of the meeting was dedicated to new proposals, specifically native IPv4 multicast over IPv6-only cores and the use of multicast for Large Language Model (LLM) synchronization. The session also included updates on operational lessons learned, interop testing results for BIER/MVPN, and dynamic tunneling mechanisms for the "middle mile."

Key Discussion Points

Working Group Status and Administration

Chairs: Stig Venaas and Mike McBride.
Charter Alignment: AD Gunter Van de Velde emphasized that new work must strictly align with the WG charter or require a charter update. He noted that recent documents had some friction during the approval process due to lack of explicit charter justification in shepherd write-ups.
Draft Updates:
- draft-ietf-pim-rfc1112bis is nearing publication request after extensive directorate reviews.
- draft-ietf-pim-gaap and draft-ietf-pim-ipv6-zeroconf-assignment have passed Working Group Last Call (WGLC) and are awaiting final review incorporations.
- draft-ietf-pim-multicast-over-srv6 and draft-ietf-pim-flex-algo have been officially adopted.

Lessons learned

Presenter: Mike McBride
Document: draft-ietf-pim-multicast-lessons-learned
Discussion: The latest revision adds a section on IGMP/MLD snooping operational issues (e.g., black-holing when a querier disappears).
Feedback:
- Dave Thaler suggested adding information on Socket APIs and the concept of Scope IDs for multicast.
- Prasidh (Cisco) recommended adding a section on Quality of Service (QoS) and data plane considerations, noting that multicast is more taxing on the data plane than unicast.

RFC8059-9798-bis

Presenter: Prasad Miriyala
Document: draft-ietf-pim-rfc8059-9798bis
Discussion: This bis document merges RFC 8059 and RFC 9798 to move the LISP PIM Join Attributes from Experimental to Standards Track. IANA coordination is underway to ensure the original code points are reassigned to this new document.

Native IPv4 multicast in IPv6 Core using PIM

Presenter: Stig Venaas
Technical Concept: Leveraging RFC 8950 (formerly 5549) logic to forward IPv4 multicast over a core where routers have no IPv4 addresses on interfaces. The PIM Join uses an IPv6 header, but the encoded group and source addresses are IPv4.
Discussion:
- David Lamparter noted that existing implementations often check address families and might drop mixed-family packets, supporting the need for a PIM Hello Option to signal capability.
- Mankamana Mishra clarified that while the transport is IPv6, the state remains IPv4 end-to-end.
- Humen (Nokia) questioned the benefit versus dual-stack, while David Lamparter clarified that saving IPv4 address space on links is a major driver for operators.
Poll: A show of hands was taken regarding interest in adopting this work. 9 participants were in favor, 0 against.

RFC1112bis update

Presenter: Toerless Eckert
Document: draft-ietf-pim-rfc1112bis
Discussion: Incorporated feedback from SECDIR, INTDIR, IOTDIR, and RTGDIR. Key updates involve refining terminology (ASM/SSM/Host Group) to ensure clarity for both IPv4 and IPv6.
IoT Considerations: Toerless is investigating if RFC 9685 (IoT-related) acts as a replacement for MLD in some contexts and how that should be referenced.

llmsyn-multicast

Presenter: Sunny Zhang (ZTE)
Use Case: Synchronizing Large Language Models (70GB to 1TB) across distributed GPU clouds. Multicast is proposed to solve the "cold start" latency and storage IO bottlenecks.
Discussion:
- Jeff Tantsura argued that for AI inference (Mixture of Experts - MOE), multicast trees must be pre-established because signaling latency is too high for runtime decisions.
- Toerless Eckert and Jeff Tantsura debated the difference between bulk model distribution (MBONED/PIM) versus real-time training/inference traffic.
- The chairs and ADs discussed whether this work belongs in PIM, MBONED, or RTGWG, as it may require PIM extensions but involves higher-level architectural decisions.

Dynamic Internet Multicast Tunneling

Presenter: Lenny Giuliano
Concept: Using a BGP Extended Community to discover Automatic Multicast Tunneling (AMT) relays for the "middle mile." This allows routers to dynamically find the correct relay for a specific source without manual configuration or global DNS-based relay discovery limitations.
Discussion: Stig Venaas noted that this allows for finding the relay closest to the receiver, which optimizes the use of native multicast paths.

EANTC-Interop2026

Presenter: Humen (Nokia)
Summary: Report on Bit Index Explicit Replication (BIER) and Next-Generation MVPN interop testing.
Findings: Successful interop between Nokia, Huawei, and HPE. Key technical issues resolved included Next Protocol bit mismatches (1 vs 2) related to upstream vs. downstream label assignment. Nokia implemented a configurable "knob" to allow flexibility in processing different Next Protocol values to facilitate multi-vendor compatibility.

Decisions and Action Items

Decision: The Working Group expressed strong interest in adopting the "Native IPv4 multicast in IPv6 Core using PIM" proposal. The chairs will take the adoption call to the mailing list.
Action Item: Mike McBride to incorporate Dave Thaler’s (Socket APIs/Scope IDs) and Prasidh’s (QoS/Data Plane) feedback into draft-ietf-pim-multicast-lessons-learned.
Action Item: Toerless Eckert to address the final IANA early review comments and TSVDIR feedback for draft-ietf-pim-rfc1112bis.

Next Steps

Initiate WGLC for draft-ietf-pim-rfc1112bis once the final directorate comments are integrated.
Progress the LISP multicast attributes (draft-ietf-pim-rfc8059-9798bis) toward WGLC following the update to the IANA section.
The LLM synchronization discussion will likely continue in a cross-working group context, including RTGWG and MBONED.

Automatic IETF Minutes

PIM