Supplement of BGP-LS Distribution for SR Policies and State

Internet-Draft	BGP-LS SR Policy	March 2023
Liu & Peng	Expires 10 September 2023	[Page]

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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-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.¶

[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 SR Policy CP level. For example, there's a B Flag in the SR Candidate Path State TLV indicating the CP is in an administrative shut state when set.¶

Currently, a few segment list-related information is not included in [I-D.ietf-idr-bgp-ls-sr-policy]. One is the information to indicate that the segment list is a backup path as described in [I-D.ietf-pce-multipath]. And the segment list may be shut by the administrator, this information may also needed and reported via BGP-LS.¶

This document supplements some additional information of the segment list 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 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |D|E|C|V|R|F|A|T|M|S|B|         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Flag Field of SR Segment List TLV

S-Flag: Indicates the segment list is in administrative shut state when set.¶
B-Flag: Indicates the segment list is a backup path described in [I-D.ietf-pce-multipath] when set, otherwise it is the primary path.¶

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-00, 9 March 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-ls-sr-policy-00>.
[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-07, 14 November 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-pce-multipath-07>.
[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>.