



IDR Working Group                                                 Y. Liu
Internet-Draft                                                   S. Peng
Intended status: Standards Track                                     ZTE
Expires: 17 April 2026                                             Z. Li
                                                            China Mobile
                                                         14 October 2025


      Supplement of BGP-LS Distribution for SR Policies and State
              draft-lp-idr-bgp-ls-sr-policy-supplement-03

Abstract

   This document supplements additional information of the segment list
   in the BGP-LS advertisement for SR Policy state information.  Two new
   flags and a new sub-TLV are introduced in the SR Segment List TLV of
   BGP-LS SR Policy Candidate Path NLRI.

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
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   This Internet-Draft will expire on 17 April 2026.

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
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   provided without warranty as described in the Revised BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  BGP-LS Extensions for Distributing Segment List States  . . .   3
     3.1.  New Flags in SR Segment List TLV  . . . . . . . . . . . .   3
     3.2.  MPLS Label Sub-TLV  . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   SR Policy architecture details are specified in [RFC9256].  An SR
   Policy comprises one or more candidate paths (CP) of which at a given
   time one and only one may be active.  Each CP in turn may have one or
   more SID-List of which one or more may be active; when multiple are
   active then traffic is load balanced over them.

   [I-D.ietf-idr-bgp-ls-sr-policy] describes a mechanism to collect the
   SR policy information that is locally available in a node and
   advertise it into BGP Link State (BGP-LS) updates.  Various TLVs are
   defined to enable the headend to report the state at the candidate
   path level and the segment list level.

   Currently, a few segment-list-related information is not yet included
   in [I-D.ietf-idr-bgp-ls-sr-policy]:

   *  Whether the segment list is a backup path.
      [I-D.ietf-pce-multipath] proposes extensions to PCEP to specify
      the protection relationship among segment lists within the
      candidate path.  There would be segment lists in the CP acting as
      backup for one or more primary segment lists, the backup lists
      only carry rerouted traffic after the protected path fails.

   *  Whether the segment list is in administrative shut state.  For the
      candidate path, there's already an S-Flag in the SR Candidate Path
      State TLV in [I-D.ietf-idr-bgp-ls-sr-policy] indicating the CP is
      in an administrative shut state.  In some usecases, the segment
      list may also be shut by an administrator for traffic engineering
      or power saving purpose, e.g, the network administrator may shut
      certain segment list when the load on the SR Policy is light.
      This information may also be needed and reported via BGP-LS.





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   Besides, [RFC8662] describes how Entropy labels (ELs) are to be
   applied to SR-MPLS to improve load-balancing.  Multiple <ELI, EL>
   pairs may be inserted in the SR-MPLS label stack.  A typical use case
   is that the ingress router (e.g., the headend node) computes a hash
   based on several fields from a given packet and places the result in
   the EL(s).  The values and the positions of the ELs inserted in the
   SID-lists may be required by the controller when the headend reports
   the state of SR Policies via BGP-LS.  However, carrying MPLS labels
   in BGP-LS that are not SR-MPLS SIDs are not yet supported.

   This document supplements some additional information of the segment
   list state as mentioned above in the BGP-LS advertisement for SR
   Policy state information.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119] [RFC8174]
   when, and only when, they appear in all capitals, as shown here.

3.  BGP-LS Extensions for Distributing Segment List States

3.1.  New Flags in SR Segment List TLV

   SR Segment List TLV is defined in [I-D.ietf-idr-bgp-ls-sr-policy] to
   report the SID-List(s) of a candidate path.  As show in Figure 1,this
   document introduces two new flags in the flag field of SR Segment
   List TLV, where,

                         0                   1
                         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        | | | | | | | | | |S|B|         |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 1: New Flags in the Flag Field of SR Segment List TLV

   *  S-Flag: Indicates the segment list is in administrative shut state
      when set.  The segment list may be shut by the administrator via
      CLI or other methods, and it is out of the scope of this document.

   *  B-Flag: Indicates that the segment list is a pure backup path as
      specified in [I-D.ietf-pce-multipath] section 4.4 when set.  When
      B-Flag is clear, it indicates it is the primary path that carries
      normal traffic.





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3.2.  MPLS Label Sub-TLV

   The MPLS Label sub-TLV is defined in this section to carry the
   generic MPLS Label information.  The MPLS Label Sub-TLV is an
   optional sub-TLV of SR Segment List TLV, and it may be used as the
   sub-TLV of other TLVs, for the latter case, the detailed usage is out
   of the scope of this document.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  MPLS Label                                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 2: MPLS Label Sub-TLV

   Type: TBA

   Length: The value is 4 octets.

   MPLS Label: a 4-octet-field carrying the MPLS Label.

   When receiving the BGP-LS advertisement for the SR Policy Candidate
   Path NLRI with SR Segment List TLV carrying the MPLS Label sub-TLV,
   it indicates that there's a MPLS Label inserted in the SID LIST.  The
   MPLS Label sub-TLV can appear multiple times in the SR Segment List
   TLV.  For example, if there're two adjacent MPLS Label sub-TLVs in
   the SR Segment List TLV, with the value of the MPLS Label in the
   first MPLS Label sub-TLV set as 7 (the reserved label value for
   ELI[RFC6790]), it indcates that a <ELI, EL> pair has been inserted in
   the SID list, and the second MPLS Label sub-TLV carries the
   corresponding EL value.

4.  IANA Considerations

   This document requests bit 9 and bit 10 in the flag field of "SR
   Segment List TLV" [I-D.ietf-idr-bgp-ls-sr-policy] under the "BGP-LS
   Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
   TLVs" registry.

       Bit     Description                                Reference
      ------------------------------------------------------------------
        9     Administrative Shut State Flag(S-Flag)      This document
       10     Backup Path State Flag(B-Flag)              This document





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   This document requests a new type sub-TLV of "SR Segment List TLV"
   [I-D.ietf-idr-bgp-ls-sr-policy] under the "BGP-LS Node Descriptor,
   Link Descriptor, Prefix Descriptor, and Attribute TLVs" registry.

       Type     Description                                Reference
      ------------------------------------------------------------------
       TBA     MPLS Label Sub-TLV                         This document


5.  Security Considerations

   Procedures and protocol extensions defined in this document do not
   affect the security considerations discussed in
   [I-D.ietf-idr-bgp-ls-sr-policy].

6.  References

6.1.  Normative References

   [I-D.ietf-idr-bgp-ls-sr-policy]
              Previdi, S., Talaulikar, K., Dong, J., Gredler, H., and J.
              Tantsura, "Advertisement of Segment Routing Policies using
              BGP Link-State", Work in Progress, Internet-Draft, draft-
              ietf-idr-bgp-ls-sr-policy-17, 6 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
              ls-sr-policy-17>.

   [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

   [I-D.ietf-pce-multipath]
              Koldychev, M., Sivabalan, S., Saad, T., Beeram, V. P.,
              Bidgoli, H., Yadav, B., Peng, S., Mishra, G. S., and S.
              Sidor, "Path Computation Element Communication Protocol
              (PCEP) Extensions for Signaling Multipath Information",
              Work in Progress, Internet-Draft, draft-ietf-pce-
              multipath-14, 5 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pce-
              multipath-14>.




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   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8662]  Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
              Shakir, R., and J. Tantsura, "Entropy Label for Source
              Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
              DOI 10.17487/RFC8662, December 2019,
              <https://www.rfc-editor.org/info/rfc8662>.

   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

Authors' Addresses

   Yao Liu
   ZTE
   Nanjing
   China
   Email: liu.yao71@zte.com.cn


   Shaofu Peng
   ZTE
   Nanjing
   China
   Email: peng.shaofu@zte.com.cn


   Zhenqiang Li
   China Mobile
   Email: lizhenqiang@chinamobile.com











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