BGP Extension for ECMP Group ID
draft-cheng-idr-ecmp-groupid-00

IDR Working Group                                               W. Cheng
Internet-Draft                                              China Mobile
Intended status: Standards Track                                  C. Lin
Expires: 1 May 2027                                 New H3C Technologies
                                                         28 October 2026

                    BGP Extension for ECMP Group ID
                    draft-cheng-idr-ecmp-groupid-00

Abstract

   In AI training scenarios, per-flow ECMP is the mainstream load
   balancing technology.  However, hash collision issues lead to
   imbalanced traffic distribution, adversely affecting application
   performance.

   This document defines a new BGP Extended Community, BGP_EXT-COMM-
   ECMP-GROUPID, which carries an ECMP Group ID associated with a route.
   The ECMP Group ID allows an ingress router to map flows destined for
   different remote addresses to different local ECMP subgroups,
   providing finer-grained load distribution and resolving load
   imbalance caused by hash collisions.

Status of This Memo

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   This Internet-Draft will expire on 1 May 2027.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Protocol Extension  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     5.2.  Informational References  . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   Currently, there are two granularities for network load balancing:
   per-flow ECMP and per-packet forwarding.

   The per-flow ECMP method employs flow characteristic-based hashing
   (typically using the five-tuple) to distribute traffic across
   multiple ECMP paths.  Its primary advantage is the elimination of
   packet reordering issues.  However, this method presents limitations
   when dealing with either a limited number of flows, or the presence
   of elephant flows.  In such cases, five-tuple-based hashing may lead
   to hash collisions, causing disproportionate mapping of oversized
   flows to the same path.  This results in suboptimal load balancing
   performance.

   The per-packet forwarding approach applies hashing to each individual
   packet.  Theoretically, it achieves optimal load-balancing
   granularity.  However, it introduces severe packet reordering within
   the same flow, necessitating additional mechanisms (e.g., reordering
   buffers or sequence tracking) to handle out-of-order delivery.

   In AI training scenarios, the current mainstream load balancing
   technology is per-flow ECMP.  However, hash collision issues lead to
   imbalanced traffic distribution, adversely affecting application
   performance.

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   To address this problem, an enhanced ECMP method that resolves load
   imbalance caused by hash collisions has been proposed in
   [I-D.cheng-rtgwg-enhanced-ecmp].  In one of the proposed methods, the
   ECMP egress interfaces on the ingress router are divided into
   subgroups, and flows are mapped to subgroups based on the remote
   egress group index carried in the route.  This requires the route to
   carry such an egress group index.

   This document defines a new BGP Extended Community to carry the ECMP
   Group ID associated with a route, as required by the enhanced ECMP
   mechanism.

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.  Protocol Extension

   This document defines a new extended community attribute type
   [RFC4360] to carry the ECMP ID associated with a BGP route [RFC4271].
   The ID comprises a 4-byte Router ID and a 2-byte Group-ID.  The
   format is as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    0x83       | Sub-Type(TBD) |           Router ID           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Router ID                   |        Group-ID               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1: ECMP GroupID Extended Community

   Sub-Type: BGP_EXT-COMM-ECMP-GROUPID (TBD)

   Value Structure:

   *  Local BGP RouterID (4 bytes)

   *  ECMP-Group-ID Value (2 bytes)

   When a route is advertised with the enhanced ECMP mechanism described
   in [I-D.cheng-rtgwg-enhanced-ecmp], the route MUST carry this
   extended community.  The Router ID field MUST be set to the local BGP

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   RouterID of the advertising device, and the ECMP-Group-ID field MUST
   be set to the egress group index assigned on the advertising device.
   A router that receives this extended community but does not support
   the enhanced ECMP mechanism MAY ignore it and perform normal ECMP
   forwarding.

3.  Security Considerations

   TBD.

4.  IANA Considerations

   Registry Name: Transitive BGP_EXT-COMM-ECMP-GROUPID Community Sub-
   Types

   TBD: BGP_EXT-COMM-ECMP-GROUPID

5.  References

5.1.  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>.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271,
              January 2006, <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <https://www.rfc-editor.org/info/rfc4360>.

   [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>.

5.2.  Informational References

   [I-D.cheng-rtgwg-enhanced-ecmp]
              Cheng, W. and C. Lin, "Enhanced ECMP for AI Cluster", Work
              in Progress, Internet-Draft, draft-cheng-rtgwg-enhanced-
              ecmp-01, 25 March 2026, <https://datatracker.ietf.org/doc/
              draft-cheng-rtgwg-enhanced-ecmp/>.

Authors' Addresses

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

   Changwang Lin
   New H3C Technologies
   Beijing
   China
   Email: linchangwang.04414@h3c.com

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