



IDR Working Group                                                 Y. Liu
Internet-Draft                                                   S. Peng
Intended status: Standards Track                                     ZTE
Expires: 30 August 2026                                            Z. Li
                                                            China Mobile
                                                        26 February 2026


      Supplement of BGP-LS Distribution for SR Policies and State
              draft-lp-idr-bgp-ls-sr-policy-supplement-06

Abstract

   This document supplements additional information of the segment list
   in the BGP-LS advertisement for SR Policy state information.  A new
   flag 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 30 August 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
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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 Flag in SR Segment List TLV . . . . . . . . . . . . .   3
     3.2.  MNA Sub-Stack Sub-TLV . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Manageability Considerations  . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  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.

   [RFC9857] 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, the following segment-list-related information is not yet
   included in [RFC9857]:

   *  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 [RFC9857] 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.

   *  The 32-bit MPLS LSE information, especially, the MPLS Network
      Actions (MNA) sub-stack.  Traditional MPLS LSE consists of 20-bit
      MPLS label, 3-bit TC, 1-bit S(bottom of stack indication) and
      8-bit TTL.  Accordingly, SR Segment List TLV [RFC9857] only
      supports carrying MPLS labels with the TC, S and TTL fields set to
      0 in SR Segment Sub-TLV.  However, [I-D.ietf-mpls-mna-hdr] defines
      the MPLS Network Actions sub-stack (NAS) solution for carrying



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      Network Actions and Ancillary Data in the MPLS label stack, unlike
      traditional MPLS LSE, some LSEs defined for MNA repurposed the TC
      and TTL field to carry additional information.  MNA such as
      Network Resource Partition (NRP) [I-D.ietf-mpls-mna-nrp-selector],
      IOAM [I-D.ietf-mpls-mna-ioam] may be inserted in the SID list in
      the format of LSEs.  The contents of the LSEs inserted in the SID-
      lists may be required by the controller when the headend reports
      the state of SR Policies via BGP-LS.

   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 Flag in SR Segment List TLV

   SR Segment List TLV is defined in [RFC9857] to report the SID-List(s)
   of a candidate path.  As show in Figure 1,this document introduces a
   new flag 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|           |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        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.

3.2.  MNA Sub-Stack Sub-TLV

   The MNA Sub-Stack Sub-TLV is defined in this section to carry the MNA
   Sub-Stack information.







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      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                       MNA Sub-Stack                           ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: MNA Sub-Stack Sub-TLV

   *  Type: TBA

   *  Length: Variable, the total length (in octets) of MNA Sub-Stack
      portion in octets, MUST be the multiple of 4.  The value indicates
      the number of the LSEs in this sub-TLV.

   *  MNA Sub-Stack: one or more 4-octet-field carrying the MNA Sub-
      Stack defined in [I-D.ietf-mpls-mna-hdr]

   The MNA Sub-Stack is an optional sub-TLV of SR Segment List TLV, and
   can appear more than once in the SR Segment List TLV.  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.

4.  IANA Considerations

   This document requests bit 9 in the flag field of "SR Segment List
   TLV" [RFC9857] 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

   This document requests a new type sub-TLV of "SR Segment List TLV"
   [RFC9857] under the "BGP-LS Node Descriptor, Link Descriptor, Prefix
   Descriptor, and Attribute TLVs" registry.

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










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5.  Manageability Considerations

   The considerations as specified in [RFC9552] apply to this document.
   In general, unknown and unsupported types MUST be preserved and
   propagated within both the NLRI and the BGP-LS Attribute.  The
   presence of unknown or unexpected TLVs MUST NOT result in the NLRI or
   the BGP-LS Attribute being considered malformed.

   If the receiver doesn't recognize the new sub-TLV type defined in
   this document, it SHOULD skip it and process the remaining part in
   the SR Segment List TLV normally.

6.  Security Considerations

   Procedures and protocol extensions defined in this document do not
   affect the security considerations discussed in [RFC9857].

7.  References

7.1.  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>.

   [RFC9552]  Talaulikar, K., Ed., "Distribution of Link-State and
              Traffic Engineering Information Using BGP", RFC 9552,
              DOI 10.17487/RFC9552, December 2023,
              <https://www.rfc-editor.org/info/rfc9552>.

   [RFC9857]  Previdi, S., Talaulikar, K., Ed., Dong, J., Gredler, H.,
              and J. Tantsura, "Advertisement of Segment Routing
              Policies Using BGP - Link State", RFC 9857,
              DOI 10.17487/RFC9857, October 2025,
              <https://www.rfc-editor.org/info/rfc9857>.

7.2.  Informative References

   [I-D.ietf-mpls-mna-hdr]
              Rajamanickam, J., Gandhi, R., Zigler, R., Song, H., and K.
              Kompella, "MPLS Network Action (MNA) Sub-Stack
              Specification including In-Stack Network Actions and
              Data", Work in Progress, Internet-Draft, draft-ietf-mpls-



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              mna-hdr-21, 24 February 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
              mna-hdr-21>.

   [I-D.ietf-mpls-mna-ioam]
              Gandhi, R., Mirsky, G., Li, T., Song, H., and B. Wen,
              "Supporting In Situ Operations, Administration and
              Maintenance Using MPLS Network Actions", Work in Progress,
              Internet-Draft, draft-ietf-mpls-mna-ioam-04, 20 November
              2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
              mpls-mna-ioam-04>.

   [I-D.ietf-mpls-mna-nrp-selector]
              Li, T., Beeram, V. P., Drake, J., Saad, T., and I. Meilik,
              "MPLS Network Actions for Network Resource Partition
              Selector", Work in Progress, Internet-Draft, draft-ietf-
              mpls-mna-nrp-selector-03, 23 December 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
              mna-nrp-selector-03>.

   [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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