



RTGWG                                                           J. Zhang
Internet-Draft                                                  W. Cheng
Intended status: Informational                              China Mobile
Expires: 3 September 2026                                   2 March 2026


     Requirements and Gap Analysis of Multicast in AI Data Centers
         draft-zhang-rtgwg-multicast-requirements-gaps-aidc-00

Abstract

   Multicast has the potential to be applied in Artificial Intelligence
   Data Centers (AIDCs) to improve the efficiency of point-to-multipoint
   data transmission during large language model training and inference.
   This document identifies key requirements of multicast in AIDCs, and
   analyzes the gaps between these requirements and the capabilities of
   existing multicast technologies.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 3 September 2026.

Copyright Notice

   Copyright (c) 2026 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to 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 are
   provided without warranty as described in the Revised BSD License.



Zhang & Cheng           Expires 3 September 2026                [Page 1]

Internet-Draft              Abbreviated Title                 March 2026


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Multicast Requirements  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Bidirectional Interactivity . . . . . . . . . . . . . . .   3
     2.2.  High Reliability  . . . . . . . . . . . . . . . . . . . .   4
     2.3.  High Dynamics . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Sparseness  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.5.  Simplicity  . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Gap Analysis  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Typical Multicast Technologies  . . . . . . . . . . . . .   6
     3.2.  Gap Analysis Against Requirements . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Artificial Intelligence (AI) Data Centers (AIDCs) serve as the key
   infrastructure for AI large language model (LLM) training and
   inference, where point-to-multipoint (P2MP) communication patterns
   are pervasive and critical to overall system efficiency.  Network
   multicast leverages in-network data replication to achieve efficient
   distribution of identical data, reducing processing overhead and
   network bandwidth consumption of the sender, thereby enhancing the
   efficiency of P2MP data transmission.  Multicast is a promising
   technique for deployment in AIDCs.  The typical use cases of
   multicast in AIDCs are as follows:

   *  Token dispatch in Mixture-of-Experts (MoE) models: MoE is a
      mainstream architecture for LLMs.  During execution, input tokens
      are dispatched to multiple expert nodes based on routing
      decisions.  This token dispatch process naturally follows the
      multicast communication pattern
      [I-D.zzhang-bier-optimized-use-in-aidc].

   *  Data broadcast in AllReduce operations: In distributed training of
      LLMs, AllReduce is a core collective communication operation for
      data parallelism and tensor parallelism.  Although AllReduce can
      be implemented in multiple ways, decomposing it into Reduce and
      Broadcast phases is a basic approach, where the Broadcast phase
      can benefit from efficient network multicast.





Zhang & Cheng           Expires 3 September 2026                [Page 2]

Internet-Draft              Abbreviated Title                 March 2026


   *  Multi-replica checkpoint storage: To avoid loss of training
      progress due to failures, training programs periodically save
      model states, i.e., checkpoints, to multiple storage nodes.
      Multicast is a promising technique in this scenario, which is
      supposed to improve the efficiency of transmitting massive data to
      multiple replicas [I-D.liu-multicast-for-computing-storage].

   Despite these potential opportunities, existing multicast
   technologies are not originally designed to address the specific
   characteristics of AIDC networks.  AIDC networks are defined by
   ultra-high bandwidth (often 400 Gbps or greater), microsecond-level
   latency, and high reliability that demands near-zero packet loss.
   These core performance characteristics necessitate corresponding
   qualities in multicast technologies, including interactivity,
   reliability, and simplicity.  Furthermore, emerging multicast use
   cases in AIDCs, such as MoE token dispatch, also introduce specific
   requirements, including high dynamics and membership sparseness.

   This document identifies the key requirements for multicast in AIDCs
   and analyzes the limitations of existing multicast technologies in
   meeting these requirements.

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.  Multicast Requirements

2.1.  Bidirectional Interactivity

   AI workloads are highly sensitive to packet loss.  In LLM training,
   packet loss without a reliability acknowledgment mechanism can
   corrupt model parameters, leading to degraded model quality or even
   training failures.  Moreover, congestion control is required to
   actively avoid congestion and packet loss.  Therefore, networks in
   AIDCs are required to support closed-loop control, such
   acknowledgment and congestion control, to meet the high-performance
   and high-reliability requirements of AI workloads.

   Traditional IP multicast only supports best-effort P2MP data
   delivery, while multicast in AIDCs should support bidirectional
   interaction, including both efficient P2MP data forwarding and
   multipoint-to-point (MP2P) feedback forwarding.  The core
   interactivity demands are as follows:



Zhang & Cheng           Expires 3 September 2026                [Page 3]

Internet-Draft              Abbreviated Title                 March 2026


   *  P2MP forwarding: Multicast should support efficient P2MP
      forwarding, which is the fundamental requirement of multicast.

   *  MP2P forwarding: Multicast in AIDCs should natively support
      efficient MP2P forwarding, particularly for feedback signals
      generated from receivers to senders such as acknowledgments (ACKs)
      to confirm successful data reception and negative acknowledgments
      (NACKs) to report packet loss, as well as congestion notification
      signals.

   *  MP2P packet aggregation: In large-scale AIDCs with numerous
      receivers, if each receiver sends feedback packets to a sender
      independently, it can lead to excessive reverse traffic to the
      sender, resulting in network congestion, increased latency, and
      amplified jitter.  To address this issue, multicast in AIDCs
      should support MP2P packet aggregation.  Network devices serving
      as rendezvous points should aggregate multiple feedback packets
      from different receivers into a single packet and send it to the
      sender.

2.2.  High Reliability

   Maintaining uninterrupted tasks for long periods is crucial for LLM
   training.  However, hardware is prone to failures, and as the scale
   of training networks increases, the likelihood of network failures
   rises due to an increasing number of switches, network interface
   cards, and optical modules
   [I-D.cheng-rtgwg-ai-network-reliability-problem].  Therefore,
   multicast in AIDCs should provide high reliability to ensure service
   continuity.  The specific requirements are as follows:

   *  Fast failure detection: The multicast should support fast
      detection of link failures and node failures and efficient
      detection of gray failures, which are the prerequisite for any
      subsequent recovery action.

   *  Fast failure recovery: Upon failure detection, the multicast
      should support fast recovery mechanisms to restore multicast
      traffic rapidly.  It is unacceptable to rely solely on global
      control-plane convergence and multicast tree reconstruction for
      slow recovery time.

   *  Minimized failure domain: The recovery mechanism should confine
      the impact of a failure to the smallest possible set of receivers.
      Local link or node failures should only affect the faulty segment,
      without spreading to the entire multicast tree or other service
      branches.




Zhang & Cheng           Expires 3 September 2026                [Page 4]

Internet-Draft              Abbreviated Title                 March 2026


2.3.  High Dynamics

   AI workloads, especially those using sparse architectures like MoE,
   have highly dynamic communication patterns.  MoE-based AI training
   and inference uses token dispatch, where gating networks select
   expert nodes per token at microsecond timescales, dynamically
   determining real-time multicast receiver sets with no fixed groups.
   This ultra-fast selection leaves no time for traditional multicast to
   establish, update, or tear down trees, leading to delays, packet
   loss, or AI task failure [I-D.zhang-rtgwg-llmmoe-multicast].
   Therefore, multicast in AIDC should meet high dynamics requirements,
   and the key points are as follows:

   *  Fast change of multicast members: Multicast should be able to
      adapt to the dynamic change of multicast members in microsecond
      timescales.

   *  Low overhead for dynamic change: The dynamic change of multicast
      members should generate minimal overhead in both the control plane
      and data plane.  Excessive signaling or processing overhead during
      dynamic change will increase transmission latency and reduce the
      efficiency of AI workloads.

2.4.  Sparseness

   Multicast in AIDCs frequently involves multicast groups where only a
   small fraction of the total nodes in the cluster are multicast
   members, a characteristic closely tied to the sparse activation
   mechanism of modern AI models such as MoE.  For example, DeepSeekV3
   uses 256 experts and activates 9 experts at a time.  Multicast
   technologies that are designed for dense groups are inefficient for
   this sparse mode.  The multicast should be efficient when the group
   size is small relative to the network size, and meet the following
   sparseness requirements:

   *  Efficient sparse member identification: Multicast technologies
      should support efficient identification of sparse multicast
      members.  The methods for identifying multicast members should
      avoid unnecessary scanning or signaling of non-member nodes, and
      be efficient for forwarding.

   *  Low overhead for sparse state maintenance: The maintenance of
      multicast member state should be lightweight and low-overhead,
      adapting to the sparse characteristics of AIDC multicast groups.
      It should avoid maintaining redundant state information for non-
      member nodes, reducing state maintenance burden and ensuring that
      state updates do not introduce additional latency that affects AI
      task efficiency.



Zhang & Cheng           Expires 3 September 2026                [Page 5]

Internet-Draft              Abbreviated Title                 March 2026


2.5.  Simplicity

   Simplicity is a foundational architectural principle for multicast in
   AIDCs, directly enabling the microsecond-timescale low-latency
   transmission in large-scale AIDC networks.  Complexity in the control
   or data plane manifests as variable latency, unpredictable jitter,
   and an inability to meet the strict performance bounds of AI
   workloads.  Therefore, multicast in AIDCs should be governed by the
   following overarching simplicity requirements:

   *  Control plane simplicity: The multicast control plane should be
      architecturally simple to maintain core functions like multicast
      routing, minimizing signaling interaction overhead and control
      processes.  It should avoid complex state synchronization and
      protocol negotiation processes, to reduce network operation and
      maintenance complexity.

   *  Data plane simplicity: The multicast data plane needs to be highly
      efficient and simple, including efficient member identification
      and forwarding adapting to sparse and dynamic multicast
      characteristics, and optimized packet processing mechanisms.
      These ensure minimal forwarding and processing overhead, meeting
      the low-latency transmission requirements of AI workloads.

3.  Gap Analysis

   To address the gaps between multicast requirements in AIDCs and
   existing technologies, typical multicast technologies are first
   introduced, followed by an analysis of their capabilities against key
   requirements.

3.1.  Typical Multicast Technologies

   Protocol Independent Multicast (PIM) is a widely deployed multicast
   routing protocol that operates independently of underlying unicast
   routing protocols.  It supports dense mode (PIM-DM) [RFC3973] and
   sparse mode (PIM-SM) [RFC7761].  PIM-SM builds unidirectional shared
   trees rooted at a Rendezvous Point per group and it optionally
   creates shortest-path trees per source.

   Multipoint extensions for Label Distribution Protocol (mLDP)
   [RFC6388] constructs the P2MP or multipoint-to-multipoint (MP2MP)
   Label Switched Paths (LSPs) in Multiprotocol Label Switching (MPLS)
   networks without interacting with or relying upon any other multicast
   tree construction protocol.






Zhang & Cheng           Expires 3 September 2026                [Page 6]

Internet-Draft              Abbreviated Title                 March 2026


   Segment Routing Point-to-Multipoint (SR-P2MP)
   [I-D.ietf-pim-sr-p2mp-policy] enables creation of P2MP trees for
   efficient multi-point packet delivery in a Segment Routing (SR)
   domain.  It requires the routing module of the controller or ingress
   node to calculate and determine the path of the multicast traffic,
   and the data plane can reuse existing SR unicast forwarding
   mechanisms.

   Bit Indexed Explicit Replication (BIER) [RFC8279] is a stateless
   multicast technology that eliminates the need for explicit tree
   construction.  Instead, the set of intended receivers is encoded as a
   BitString within the packet header.  Intermediate BIER Forwarding
   Routers (BFRs) replicate packets based on the BitString, without
   maintaining any per-flow or per-tree state.

3.2.  Gap Analysis Against Requirements

   The support of typical multicast technologies for multicast
   requirements in AIDCs is summarized in Table 1.

   +==========+=============+===========+========+==========+==========+
   |Technology|Interactivity|Reliability|Dynamics|Sparseness|Simplicity|
   +==========+=============+===========+========+==========+==========+
   |   PIM    |      No     |    Poor   |  Poor  |   Good   |   Poor   |
   +----------+-------------+-----------+--------+----------+----------+
   |   mLDP   |      No     |    Poor   |  Poor  |   Good   |   Poor   |
   +----------+-------------+-----------+--------+----------+----------+
   | SR-P2MP  |      No     |    Good   |Moderate|   Good   | Moderate |
   +----------+-------------+-----------+--------+----------+----------+
   |   BIER   |      No     |    Good   |  Good  |   Poor   |   Good   |
   +----------+-------------+-----------+--------+----------+----------+

                           Table 1: Gap Analysis

   Interactivity: Traditional multicast technologies can support best-
   effort P2MP data delivery, but none of them can natively support the
   reverse MP2P forwarding or aggregation to achieve bidirectional
   interactivity.

   Reliability: The reliability of PIM and mLDP basically relies on
   routing convergence and multicast tree reconstruction.  Although some
   fast detection and recovery mechanisms [RFC9186][RFC9860][RFC7715]
   can be adopted to accelerate failure recovery, their tree-based
   architectures often keep the failure impact domain tree-level.  In
   contrast, BIER and SR-P2MP can effectively reuse unicast’s
   reliability capabilities such as Fast ReRouting, and control the
   failure domain within the damaged receivers, demonstrating good
   reliability.



Zhang & Cheng           Expires 3 September 2026                [Page 7]

Internet-Draft              Abbreviated Title                 March 2026


   Dynamics: PIM and mLDP adjust multicast trees via control signals,
   leading to slow convergence that struggles to handle high-frequency
   member changes.  SR-P2MP dynamically recalculates forwarding trees
   via a controller, which need global recalculating and result
   distribution.  BIER only requires updating the BitString in packets,
   enabling faster responses to member changes and exhibiting good
   dynamics.

   Sparseness: PIM, mLDP, and SR-P2MP can all adapt well to sparse
   scenarios, as they establish multicast trees or tunnels on demand,
   and multicast member identification is based on IP or other non-
   contiguous labels.  In contrast, BIER encodes the receiver set as a
   BitString, whose length is proportional to the number of nodes in the
   domain.  Even with sparse members, the full BitString must still be
   carried, leading to significant degradation in bandwidth overhead and
   forwarding efficiency.  This limits BIER’s applicability in AIDC
   sparse multicast scenarios.

   Simplicity: PIM and mLDP require the maintenance of complex multicast
   tree states and signaling mechanisms, resulting in high operational
   complexity and poor simplicity.  SR-P2MP reuses the SR unicast
   forwarding plane, with the control plane relying on a controller,
   leading to moderate complexity but still requiring additional tree
   management logic.  BIER, on the other hand, eliminates the need for
   explicit multicast tree construction, with no per-flow state at
   intermediate nodes, resulting in better simplicity.  Moreover,
   simplicity still needs further optimization to meet the ultra-high
   performance requirements of AI networks.

   In summary, the most critical common gap is the lack of native
   support for efficient, scalable bidirectional interactivity, which is
   the cornerstone for implementing closed-loop acknowledgement and
   congestion control.  Furthermore, no single multicast technology
   excels in all dimensions: some lack reliability, dynamics or
   simplicity (PIM, mLDP, SR-P2MP), others are inefficient for sparse
   groups (BIER).  Consequently, merely deploying or combining these
   existing technologies is insufficient to meet the stringent demands
   of AIDC workloads.  This gap analysis underscores the need for either
   a new architecture designed from the ground up for AIDCs or
   significant extensions to existing technologies.

4.  IANA Considerations

   TBD.







Zhang & Cheng           Expires 3 September 2026                [Page 8]

Internet-Draft              Abbreviated Title                 March 2026


5.  Security Considerations

   TBD.

6.  References

6.1.  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>.

   [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>.

6.2.  Informative References

   [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>.

   [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>.

   [RFC6388]  Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
              Thomas, "Label Distribution Protocol Extensions for Point-
              to-Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011,
              <https://www.rfc-editor.org/info/rfc6388>.

   [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>.

   [RFC9186]  Mirsky, G. and X. Ji, "Fast Failover in Protocol
              Independent Multicast - Sparse Mode (PIM-SM) Using
              Bidirectional Forwarding Detection (BFD) for Multipoint
              Networks", RFC 9186, DOI 10.17487/RFC9186, January 2022,
              <https://www.rfc-editor.org/info/rfc9186>.




Zhang & Cheng           Expires 3 September 2026                [Page 9]

Internet-Draft              Abbreviated Title                 March 2026


   [RFC9860]  Liu, Y., McBride, M., Zhang, Z., Xie, J., and C. Lin,
              "Multicast-Only Fast Reroute (MoFRR) Based on Topology
              Independent Loop-Free Alternate (TI-LFA) Fast Reroute",
              RFC 9860, DOI 10.17487/RFC9860, October 2025,
              <https://www.rfc-editor.org/info/rfc9860>.

   [RFC7715]  Wijnands, IJ., Ed., Raza, K., Atlas, A., Tantsura, J., and
              Q. Zhao, "Multipoint LDP (mLDP) Node Protection",
              RFC 7715, DOI 10.17487/RFC7715, January 2016,
              <https://www.rfc-editor.org/info/rfc7715>.

   [I-D.ietf-pim-sr-p2mp-policy]
              Parekh, R., Voyer, D., Filsfils, C., Bidgoli, H., and Z.
              J. Zhang, "Segment Routing Point-to-Multipoint Policy",
              Work in Progress, Internet-Draft, draft-ietf-pim-sr-p2mp-
              policy-22, 4 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pim-sr-
              p2mp-policy-22>.

   [I-D.zzhang-bier-optimized-use-in-aidc]
              Zhang, Z. J., Xu, X., Zhang, Z., Tantsura, J., and A.
              Mahale, "Optimized Use of BIER in AIML Data Centers", Work
              in Progress, Internet-Draft, draft-zzhang-bier-optimized-
              use-in-aidc-00, 20 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-zzhang-bier-
              optimized-use-in-aidc-00>.

   [I-D.zhang-rtgwg-llmmoe-multicast]
              Zhang, Z., Duan, W., and X. Xu, "Multicast usage in LLM
              MoE", Work in Progress, Internet-Draft, draft-zhang-rtgwg-
              llmmoe-multicast-01, 20 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-zhang-rtgwg-
              llmmoe-multicast-01>.

   [I-D.liu-multicast-for-computing-storage]
              Liu, Y. and X. Geng, "Multicast for Computing and
              Storage", Work in Progress, Internet-Draft, draft-liu-
              multicast-for-computing-storage-00, 10 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-liu-
              multicast-for-computing-storage-00>.

   [I-D.cheng-rtgwg-ai-network-reliability-problem]
              Cheng, W., Lin, C., wangwenxuan, and B. Xu, "Reliability
              in AI Networks Gap Analysis, Problem Statement, and
              Requirements", Work in Progress, Internet-Draft, draft-
              cheng-rtgwg-ai-network-reliability-problem-03, 6 June
              2025, <https://datatracker.ietf.org/doc/html/draft-cheng-
              rtgwg-ai-network-reliability-problem-03>.



Zhang & Cheng           Expires 3 September 2026               [Page 10]

Internet-Draft              Abbreviated Title                 March 2026


Authors' Addresses

   Junye Zhang
   China Mobile
   China
   Email: zhangjunye@chinamobile.com


   Weiqiang Cheng
   China Mobile
   China
   Email: chengweiqiang@chinamobile.com







































Zhang & Cheng           Expires 3 September 2026               [Page 11]
