SR Policies Extensions for Path Segment and Bidirectional Path in BGP-LS

3.1. SR Path Segment Sub-TLV

This section defines the SR Path Segment sub-TLV to describe a Path Segment, and it can be included in the Segment List sub-TLV as defined in [I-D.ietf-idr-te-lsp-distribution] . An SR Path Segment sub-TLV can be associated with an SR path specified by a Segment List sub-TLV. Multiple Path Segment MAY be included in a Segment List for different use cases. When all the SID Lists within a candidate path share the same Path Segment ID, the Path Segment can be used to collect the aggregated information of the candidate path. The format of Path Segment TLV is shown 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           |             Length              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |              Flag           |             Reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Path Segment ID (4 or 16 octets)            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 //   Sub-TLVs (variable)                                       //
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 3. Path Segment sub-TLV

Where,¶

• Type: to be assigned by IANA.¶
• Length: the total length of the value field not including Type and Length fields.¶
• Flags: 2 octet field that indicates attribute and status of the Path Segment. The following bit positions are defined. Other bits SHOULD be cleared by originator and MUST be ignored by receiver.¶

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

Where:¶

• • D-Flag : Indicates the dataplane for the BSIDs, it is set when Path Segment ID is a 16-octet SRv6 SID unset when the Path Segment ID is a 4-octet SR/MPLS label value.¶
  • B-Flag: This flag, when set, indicates the presence of the SRv6 Endpoint Behavior and SID Structure encoding specified in [I-D.ietf-idr-bgpls-srv6-ext]. It MUST be ignored when D-flag is unset. They indicate the SRv6 Endpoint behavior and SID structure for the Path Segment ID value in the TLV.¶
  • L-Flag: Local flag. Set when the Path Segment has local significance on an SR node.¶

• RESERVED: 2 octets. SHOULD be set to 0 by originator and MUST be ignored by receiver.¶
• Path Segment ID: It indicates the Path Segment ID value based on the status flags.¶

The SRv6 Endpoint Behavior TLV (1250) and the SRv6 SID Structure TLV (1252) defined in [I-D.ietf-idr-bgpls-srv6-ext] are used as sub-TLVs of the SR Path Segment Sub-TLV to optionally indicate the SRv6 Endpoint behavior and SID structure for the Binding SID value in the TLV when the Path Segment is an SRv6 Path Segment.¶

3.2. Reverse Segment List Sub-TLV

In some scenarios like mobile backhaul transport network, there are requirements to support bidirectional path. In SR, a bidirectional path can be represented as a binding of two unidirectional SR paths [I-D.ietf-spring-mpls-path-segment]. An SR policy carrying SR bidirectional path information is expressed in Figure 2. [I-D.ietf-idr-sr-policy-path-segment] defines a new sub-TLV to describe a reversed SR path of an SID list.¶

This section defines a Reverse Segment List sub-TLV to specify a reverse SR path associated with the path specified by the Segment List, and it reuses the format of SR Segment List TLV defined in [I-D.ietf-idr-te-lsp-distribution]:¶

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Flags            |           RESERVED            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             MTID              |   Algorithm   |    RESERVED   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Weight (4 octets)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   sub-TLVs (variable)                                        //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 5. Reverse Segment List Sub-TLV

All fields, except the type are defined in [I-D.ietf-idr-te-lsp-distribution], and this TLV reuses it directly. The Type of this TLV is TBA.¶

The SR Segment sub-TLV [I-D.ietf-idr-te-lsp-distribution] MUST be included as an ordered set of sub-TLVs within the SR Segment List TLV when the SID-List is not empty. A SID-List may be empty in certain cases (e.g. for a dynamic path) where the headend has not yet performed the computation and hence not derived the segments required for the path; in such cases, the SR Segment List TLV SHOULD NOT include any SR Segment sub-TLVs [I-D.ietf-idr-te-lsp-distribution].¶

Note: currently, only one reverse SID list is supported, so the weight field CAN be ignored when processing. However, multiple reverse SID list MAY be supported in the future, and the use case of supporting this still need to be discussed.¶

9.1. Normative References

[I-D.ietf-idr-bgpls-srv6-ext]

Dawra, G., Filsfils, C., Talaulikar, K., Chen, M., Bernier, D., and B. Decraene, "BGP Link State Extensions for SRv6", Work in Progress, Internet-Draft, draft-ietf-idr-bgpls-srv6-ext-09, 10 November 2021, <https://www.ietf.org/archive/id/draft-ietf-idr-bgpls-srv6-ext-09.txt>.

[I-D.ietf-idr-segment-routing-te-policy]

Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., Jain, D., and S. Lin, "Advertising Segment Routing Policies in BGP", Work in Progress, Internet-Draft, draft-ietf-idr-segment-routing-te-policy-14, 10 November 2021, <https://www.ietf.org/archive/id/draft-ietf-idr-segment-routing-te-policy-14.txt>.

[I-D.ietf-idr-te-lsp-distribution]

Previdi, S., Talaulikar, K., Dong, J., Chen, M., Gredler, H., and J. Tantsura, "Distribution of Traffic Engineering (TE) Policies and State using BGP-LS", Work in Progress, Internet-Draft, draft-ietf-idr-te-lsp-distribution-16, 22 October 2021, <https://www.ietf.org/archive/id/draft-ietf-idr-te-lsp-distribution-16.txt>.

[I-D.ietf-spring-mpls-path-segment]

Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler, "Path Segment in MPLS Based Segment Routing Network", Work in Progress, Internet-Draft, draft-ietf-spring-mpls-path-segment-07, 20 December 2021, <https://www.ietf.org/archive/id/draft-ietf-spring-mpls-path-segment-07.txt>.

[I-D.ietf-spring-segment-routing-policy]

Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", Work in Progress, Internet-Draft, draft-ietf-spring-segment-routing-policy-14, 25 October 2021, <https://www.ietf.org/archive/id/draft-ietf-spring-segment-routing-policy-14.txt>.

[I-D.ietf-spring-srv6-path-segment]

Li, C., Cheng, W., Chen, M., Dhody, D., and Y. Zhu, "Path Segment for SRv6 (Segment Routing in IPv6)", Work in Progress, Internet-Draft, draft-ietf-spring-srv6-path-segment-03, 27 November 2021, <https://www.ietf.org/archive/id/draft-ietf-spring-srv6-path-segment-03.txt>.

[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>.

[RFC7752]

Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and S. Ray, "North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP", RFC 7752, DOI 10.17487/RFC7752, March 2016, <https://www.rfc-editor.org/info/rfc7752>.

[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>.

9.2. Informative References

[I-D.gandhi-spring-udp-pm]

Gandhi, R., Filsfils, C., Voyer, D., Salsano, S., Ventre, P. L., and M. Chen, "UDP Path for In-band Performance Measurement for Segment Routing Networks", Work in Progress, Internet-Draft, draft-gandhi-spring-udp-pm-02, 14 September 2018, <https://www.ietf.org/archive/id/draft-gandhi-spring-udp-pm-02.txt>.

[I-D.ietf-idr-sr-policy-path-segment]

Li, C., Li, Z., Yin, Y., Cheng, W., and K. Talaulikar, "SR Policy Extensions for Path Segment and Bidirectional Path", Work in Progress, Internet-Draft, draft-ietf-idr-sr-policy-path-segment-05, 23 January 2022, <https://www.ietf.org/archive/id/draft-ietf-idr-sr-policy-path-segment-05.txt>.

[I-D.ietf-mpls-bfd-directed]

Mirsky, G., Tantsura, J., Varlashkin, I., and M. Chen, "Bidirectional Forwarding Detection (BFD) Directed Return Path for MPLS Label Switched Paths (LSPs)", Work in Progress, Internet-Draft, draft-ietf-mpls-bfd-directed-18, 20 August 2021, <https://www.ietf.org/archive/id/draft-ietf-mpls-bfd-directed-18.txt>.