



Interdomain Routing Working Group                                L. Gong
Internet-Draft                                              China Mobile
Intended status: Standards Track                                 J. Dong
Expires: 23 April 2026                               Huawei Technologies
                                                                  C. Lin
                                                    New H3C Technologies
                                                                 R. Chen
                                                         ZTE Corporation
                                                         20 October 2025


BGP Extensions for Inter-Domain Flexible Algorithm with Segment Routing
                            over IPv6 (SRv6)
             draft-gong-idr-inter-domain-srv6-flex-algo-00

Abstract

   IGP Flexible Algorithm (Flex-Algo) provides a mechanism for IGP nodes
   to compute the best paths according to a set of constraints in both
   the topology and computation metrics.  Segment Routing over IPv6
   (SRv6) can be used as one of the data plane of IGP Flex-Algo.

   In SRv6 networks, services may be carried across multiple network
   domains which are under the same administration.  For some services,
   it is expected that the an end-to-end inter-domain path can be
   computed according to the same constraints (such as low latency)
   defined by Flex-Algo.

   This document describes BGP extensions to advertise SRv6 locators as
   IPv6 prefixes, together with the associated algorithm across the AS
   boundary.

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 23 April 2026.



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Copyright Notice

   Copyright (c) 2025 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The Scenario  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Flex-Algo Extended Community  . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   IGP Flexible Algorithm (Flex-Algo) [RFC9350] provides a mechanism for
   IGP nodes to compute the best paths according to a set of constraints
   in the topology and computation metrics.  Segment Routing over IPv6
   (SRv6) [RFC8986] can be used as one data plane of IGP Flex-Algo.

   In SRv6 networks, services may be carried across multiple network
   domains which are under the same administration.  For some services,
   it is expected that the an end-to-end inter-domain path can be
   computed according to the same constraints (such as low latency)
   defined by Flex-Algo.

   This document defines the BGP extensions to advertise SRv6 locators
   as IPv6 prefixes, together with the associated algorithm across the
   AS boundary.








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2.  The Scenario

   This section shows a typical scenario of the inter-domain algorithm-
   specific IPv6 route distribution in Figure 1.  AS1 and AS2 belong to
   the same network operator, the service from CE1 to CE2 requires an
   inter-domain path from PE1 to PE2 based on specific constraints (such
   as low latency).  In each domain, the intra-domain low latency path
   can be computed based on flexible algorithm.  In both AS1 and AS2,
   Flex-Algo 128 is defined to compute the best path using the latency
   metrics.  In AS2, the SRv6 locator of PE2 which is associated with
   Flex-Algo 128 is advertised in IGP.  To make the SRv6 locator of PE2
   reachable in AS1, the SRv6 locator of PE2 is advertised as an IPv6
   prefix by ASBR2 to ASBR1 in BGP IPv6 Unicast [RFC2545].  However, the
   associated algorithm is not carried in the Update message.  When the
   BGP route is redestributed into IGP in AS1, the SRv6 locator of PE2
   will be treated as a IPv6 prefix without algorithm information.  As a
   result, the IGP nodes in AS1 will only compute the shortest path to
   the SRv6 locator of PE2 using the default IGP metric, then inter-
   domain path computed based on Flex-Algo 128 is not possible.


              +--------------+        +--------------+
              |              |        |              |
              |     [P1]     |        |     [P3]     |
        CE1--[PE1]       [ASBR1]---[ASBR2]        [PE2]--CE2
              |     [P2]     |        |     [P4]     |
              |              |        |              |
              +--------------+        +--------------+
                   AS1                     AS2

       Flex-Algo 128                 Flex-Algo 128
         Metric-type: latency           Metric-type: latency

       Figure 1. Inter-domain Flex-Algo specific route distribution

   One possible approach to achieve inter-domain algorithm-specific path
   computation is to configure route-policy on ASBR1 to apply Flex-Algo
   128 to specific prefixes received from ASBR2.  This requires manual
   configuration of route-policy for each prefix which is algorithm-
   specific SRv6 locators, hus it is complex and error-prone.  A
   preferable approach is to add the associated algorithm information in
   the BGP routes advertised across the AS boundary, so that when the
   BGP route is redistributed into IGP, the associated algorithm could
   be obtained automatically.







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3.  Flex-Algo Extended Community

   A new extended community is defined to carry the algorithm
   information associated with the prefix carried in the IPv6 Unicast
   NLRI.  The Flex-Algo extended community is a transitive Opaque
   Extended Community which is used to carry the Algorithm ID associated
   with the IPv6 prefixes advertised in BGP Update message.  The
   encoding of the Flex-Algo Extended Community is shown as below:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Type High =0x03| Type Low =TBD |      Flags    |   Algorithm   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Reserved                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value of the extended Type field is 0x03, which indicates it is a
   transitive opaque extended community.  The value of the low-order
   octet of the extended Type field is TBA.

   Flags: 1 octet, carries the flags related to an Algorithm.  No flag
   is defined in this document.

   Algorithm: 1 octet, it can be the algorithm as defined in the "IGP
   Algorithm Types" registry [RFC8665], or Flexible Algorithms as
   defined in [RFC9350].

   Reserved: 4-octet reserved for future use.

4.  IANA Considerations

   IANA is requested to assign a code point for Flex-Algo Extended
   Community from the "Transitive Opaque Extended Community Sub-Types"
   subregistry of the "Border Gateway Protocol (BGP) Extended
   Communities" registry:

        Sub-type Value     Name                            Reference
        ----------------------------------------------------------------
        TBA                Flex-Algo Extended Community    This document

5.  Security Considerations

   This document does not introduce any additional security
   considerations than those described in [RFC4271] and [RFC4272].






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6.  Acknowledgements

   The authors would like to thank XXX for the review and valuable
   discussion.

7.  References

7.1.  Normative References

   [RFC2545]  Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
              Extensions for IPv6 Inter-Domain Routing", RFC 2545,
              DOI 10.17487/RFC2545, March 1999,
              <https://www.rfc-editor.org/info/rfc2545>.

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

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <https://www.rfc-editor.org/info/rfc4272>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

   [RFC9350]  Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
              and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
              DOI 10.17487/RFC9350, February 2023,
              <https://www.rfc-editor.org/info/rfc9350>.

7.2.  Informative References

Authors' Addresses

   Liyan Gong
   China Mobile
   China
   Email: gongliyan@chinamobile.com



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   Jie Dong
   Huawei Technologies
   China
   Email: jie.dong@huawei.com


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


   Ran Chen
   ZTE Corporation
   China
   Email: chen.ran@zte.com.cn



































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