Supplement of BGP-LS Distribution for SR Policies and State

Internet-Draft	BGP-LS SR Policy	February 2025
Liu, et al.	Expires 24 August 2025	[Page]

Abstract

This document supplements additional information of the segment list in the BGP-LS advertisement for SR Policy state information. Two new flags are introduced in 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.¶

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This Internet-Draft will expire on 24 August 2025.¶

Copyright Notice

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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 a B 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.¶

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

1.1. 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.¶

2. BGP-LS Extensions for Distributing Segment List States

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.¶

3. 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

5. References

5.1. 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-14, 17 February 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-ls-sr-policy-14>.
[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>.

5.2. Informative References

[I-D.ietf-pce-multipath]: Koldychev, M., Sivabalan, S., Saad, T., Beeram, V. P., Bidgoli, H., Yadav, B., Peng, S., and G. S. Mishra, "PCEP Extensions for Signaling Multipath Information", Work in Progress, Internet-Draft, draft-ietf-pce-multipath-12, 8 October 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-pce-multipath-12>.
[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>.
[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>.