Internet-Draft	BGP Extensions for Segment List	March 2023
Liu & Peng	Expires 10 September 2023	[Page]

Workgroup:: IDR Working Group
Internet-Draft:: draft-lp-idr-sr-path-protection-04
Published:: 9 March 2023
Intended Status:: Standards Track
Expires:: 10 September 2023
Authors:: Y. Liu

ZTE

S. Peng

ZTE

BGP Extensions of SR Policy for Segment List Identification and Protection

Abstract

This document proposes extensions of BGP in order to provide identification and protection information of segment lists when delivering SR policy via BGP.¶

Status of This Memo

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Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

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1. Introduction

Segment Routing [RFC8402] allows a headend node to steer a packet flow along any path. [RFC9256] details the concept of SR Policy and steering into an SR Policy. An SR Policy is a set of candidate paths, each consisting of one or more segment lists. The headend of an SR Policy may learn multiple candidate paths for an SR Policy.¶

Candidate path can be used for path protection, that is, the lower preference candidate path may be designated as the backup for a specific or all (active) candidate path(s). Backup candidate path provide protection only when all the segment lists in the active CP are invalid.If a candidate path is associated with a set of Segment-Lists, each Segment-List is associated with weight for weighted load balancing.¶

The protection mechanism for SR Policy is not flexible enough. For example, there're two active segment lists(SL1, SL2) in the primary candidate path CP1, SL1 and SL2 can together carry 80 Gbps. If SL1 fails, CP1 are still the primary path, but the bandwith of CP1 is probably not enough. If there's a backup segment list for SL1, e.g, SL3, in CP1, traffic will be load-balanced between SL3 and SL2 after SL1 fails.¶

The pcep extensions for segment list identification and protection relationship among segment lists specification are proposed in [I-D.ietf-pce-multipath].¶

[I-D.ietf-idr-segment-routing-te-policy] specifies BGP extensions for the advertisement of SR Policy. This document proposes extensions of BGP in order to provide identification as well as the protection information of segment lists when delivering SR policy via BGP.¶

This document proposes extensions of BGP in order to provide identification and protection information of segment lists when delivering SR policy via BGP.¶

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 Extensions for Advertising Segment List

2.1. Extensions of Segment List sub-TLV

Segment List sub-TLV is introduced in [I-D.ietf-idr-segment-routing-te-policy] and it includes the elements of the paths (i.e., segments).¶

This document introduces a one-bit flag in the RESERVED field.¶

    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            |B|  RESERVED   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                           sub-TLVs                          //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Segment List sub-TLV

B-Flag(Backup Flag): one bit. When set to 0, it indicates that the segment list acts as the active member in the candidate path. When set to 1, it indicates that the segment list acts as the backup path in the candidate path.¶

Using segment lists for path protection can be compatible with using candidate paths. When a path fails, the backup segment list within the same candidate path is used preferentially for path protection. If the backup list is also invalid, then other candidate path can be enabled for protection.¶

2.2. List Identifier Sub-TLV

This document introduces a new sub-sub-tlv of Segment List sub-TLV, where,¶

    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     |           RESERVED            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      List Identifier                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                      Optional sub-TLVs                        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: List Identifier Sub-TLV

Type: 1 octet. TBD.¶
Length: 1 octet, specifies the length of the value field not including Type and Length fields.¶
RESERVED: 2 octet of reserved bits. SHOULD be unset on transmission and MUST be ignored on receipt.¶
List Identifier: 4 octets. It is the identifier of the corresponding segment list, so that the segment list can be operated according to the specified Segment List identifier.¶
This sub-TLV is optional and it MUST NOT appear more than once inside the Segment List sub-TLV.¶

2.2.1. List Protection Sub-TLV

The List Protection Info sub-TLV is an optional sub-TLV of List Identifier sub-TLV, where:¶

    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     |           RESERVED            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Backup  List ID 1                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ...                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Backup  List ID N                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: List Protection Info Sub-TLV

Type: 1 octet. TBD.¶
Length: 1 octet, specifies the length of the value field not including Type and Length fields.¶
RESERVED: 2 octet of reserved bits. SHOULD be unset on transmission and MUST be ignored on receipt.¶
Backup List ID: 4 octets. It is the List Identifier of the backup segment list that protects this segment list. If there're multiple backup paths, the list ID of each path should be included in the TLV.¶

As defined in [I-D.ietf-idr-segment-routing-te-policy], the SR Policy encoding structure is as follows:¶

      SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
      Attributes:
         Tunnel Encaps Attribute (23)
            Tunnel Type: SR Policy
                Binding SID
                Preference
                Priority
                Policy Name
                Explicit NULL Label Policy (ENLP)
                Segment List
                    Weight
                    Segment
                    Segment
                    ...
                Segment List
                    ...
                ...

The new SR Policy encoding structure with List Identifier sub-TLV is shown as below:¶

        SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
        Attributes:
       Tunnel Encaps Attribute (23)
         Tunnel Type: SR Policy
             Binding SID
             SRv6 Binding SID
             Preference
             Priority
             Policy Name
             Policy Candidate Path Name
             Explicit NULL Label Policy (ENLP)
             Segment List
                 List Identifier
                   List Protection Info
                 Weight
                 Segment
                 Segment
                 ...
             Segment List
                 ...
             ...

3. IANA Considerations

3.1. New Registry: Flag Field of Segment List sub-TLV

This document introduces a one-bit flag field in the Segment List sub-TLV [I-D.ietf-idr-segment-routing-te-policy] for the Backup Flag (B-Flag).¶

3.2. Existing Registry: BGP Tunnel Encapsulation Attribute sub-TLVs

This document defines a new sub-TLV in the registry "SR Policy List Sub-TLVs" [I-D.ietf-idr-segment-routing-te-policy] to be assigned by IANA:¶

        Codepoint   Description                           Reference
        -------------------------------------------------------------
        TBD         List Identifier Sub-TLV               This document

3.3. New Registry: List Identifier Sub-TLVs

This document requests the creation of a new registry called "List Identifier Sub-TLVs" under the "BGP Tunnel Encapsulation" registry. Following initial Sub-TLV codepoint are assigned by this document.¶

        Codepoint   Description                           Reference
        -------------------------------------------------------------
        TBD         List Protection Sub-TLV               This document

[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-20, 27 July 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-idr-segment-routing-te-policy-20>.
[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>.

Authors' Addresses

Yao Liu

ZTE

Nanjing

China

Email: liu.yao71@zte.com.cn

Shaofu Peng

ZTE