BGP SR Policy Extensions for Segment List Identifier

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Table of Contents

1. Introduction

Segment routing (SR) [RFC8402] is a source routing paradigm that explicitly indicates the forwarding path for packets at an ingress node. The ingress node steers packets into a specific path according to the Segment Routing Policy (SR Policy) as defined in [RFC9256]. In order to distribute SR Policy Candidate Paths to the headend (i.e., the node where the instructions for source routing are written into the packet), [RFC9830] specifies a mechanism by using BGP.¶

However, there is no identifier for segment lists in BGP SR Policy, which may cause inconvenience for other mechanisms to designate segment lists distributed by BGP.¶

Consider a scenario where a network controller distributes SR Policy Candidate Paths to the headend nodes, which need to collect traffic forwarding statistics per segment list. When a headend node reports each statistic to the controller, it needs to specify the segment list to which the statistic belongs. Due to the lack of an identifier, the headend node usually reports all SIDs in the associated segment list along with the statistic, and then the controller needs to compare the SIDs one by one to recognize which segment list it is. The advertisement of all SIDs in the segment list consumes a lot of octets, and the comparison of SIDs can be complicated.¶

Consider a second example where a network controller distributes SR Policy Candidate Paths using BGP, and then uses YANG [RFC7950] /NETCONF [RFC6241] to set some configurations of the segment lists that are not suitable to be carried in BGP. The controller needs to specify which segment list these configurations belong to when it issues them. In this case, an identifier of the segment list can also be helpful.¶

An identifier of a segment list may also serve as a user-friendly attribute for debugging and troubleshooting purposes, such as displaying an invalid segment list when its associated BFD session is down.¶

Section 5.7.4 of [RFC9857] defines an SR Segment List Identifier sub-TLV for reporting the identifier associated with a specific SID list. This document defines extensions to BGP SR Policy to specify the identifier of an SR Segment List. Note that the Segment List ID is scoped to a Candidate Path. It does not uniquely identify a segment list across an SR Policy or headend node, nor does it imply that the SID sequence is immutable. A controller may keep the same Segment List ID while updating the SIDs within the segment list. Any external reference that uses this identifier MUST also specify the associated Candidate Path.¶

2. Segment List Identifier in SR Policy

As defined in Section 2.2 of [RFC9830], the SR Policy Encoding structure is as follows:¶

SR Policy SAFI NLRI: <Distinguisher, Color, Endpoint>
Attributes:
   Tunnel Encapsulation Attribute (23)
      Tunnel Type: SR Policy (15)
          Binding SID
          SRv6 Binding SID
          Preference
          Priority
          SR Policy Name
          SR Policy Candidate Path Name
          Explicit NULL Label Policy (ENLP)
          Segment List
              Weight
              Segment
              Segment
              ...
          ...

SR policy with segment list identifier is expressed as below:¶

SR Policy SAFI NLRI: <Distinguisher, Color, Endpoint>
Attributes:
   Tunnel Encapsulation Attribute (23)
      Tunnel Type: SR Policy (15)
          Binding SID
          SRv6 Binding SID
          Preference
          Priority
          SR Policy Name
          SR Policy Candidate Path Name
          Explicit NULL Label Policy (ENLP)
          Segment List
              Weight
              Segment List ID
              Segment
              Segment
              ...
          ...

The segment list identifier can be advertised using the Segment List ID sub-TLV, as defined in Section 2.1.¶

The notation used in Figure 1 and Figure 2 is a logical representation of the BGP attribute hierarchy. It is not a new encoding specification but a common convention to illustrate the nesting of TLVs and sub-TLVs. The top-level line represents the NLRI, and each indented line represents a TLV or sub-TLV contained within the parent. Angle brackets (<...>) enclose a tuple of fields that together form an NLRI. Ellipses (...) indicate that additional instances of the preceding item type or additional sub-TLVs may be present. This notation is consistent with the SR Policy encoding conventions used in [RFC9830] Section 2.2. The actual TLV encoding follows the TLV rules defined in the BGP Tunnel Encapsulation Attribute specification [RFC9012].¶

 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    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Segment List ID                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5. Security Considerations

The protocol extensions defined in this document do not affect the base BGP security model. The security requirements and mechanisms described in [RFC9830] also apply to this document. SR operates within a trusted SR domain [RFC8402] and its security considerations also apply to BGP sessions when carrying SR Policy information.¶

The Segment List ID sub-TLV is an optional sub-TLV that specifies an identifier associated with a segment list. The scope of this identifier is the SR Policy Candidate Path. The Segment List ID uniquely identifies a segment list within an SR Policy Candidate Path.¶

The Segment List ID is assigned by the originator (often a controller), distributed via BGP, and used as an identifier for the segment list. Since this identifier may expose mission-critical or commercially sensitive network information, it introduces a confidentiality risk.¶

It is the responsibility of the network operator to ensure that only trusted nodes (that include both routers and controller applications) within the SR domain are configured to receive such information.¶

The management of the Segment List ID itself (e.g., via YANG/NETCONF) is out of scope for this document. As mentioned in Section 1, the identifier may be used to cross-reference configurations delivered via other mechanisms such as NETCONF, but the definition of such management interfaces is left to future documents.¶

6. Implementation Status

[Note to the RFC Editor - remove this section before publication, as well as remove the reference to [RFC7942].]¶

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶

According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶

7. IANA Considerations

This document defines a new Sub-TLV in the registry "SR Policy Segment List Sub-TLVs" [RFC9830] [IANA-SR-POLICY-SEGMENT-LIST].¶

Value     Description                   Reference
-------------------------------------------------------
19        Segment List ID sub-TLV       This document

8. Acknowledgments

The authors would like to acknowledge Hao Li, Haiyang Zhang, Jie Dong, Yongqing Zhu, Nat Kao, Allan Michael, Yisong Liu, Ran Chen, Libin Liu, Lancheng Qin, Xinxin Yi, Wei Wang, Jiangbo Wang, Wisdomtan, Zheng Zhang, Yujia Gao, Alex Lee, David Wright, Hongwei Li, Quan Xiong, Xiao Min and Xiaolan Wang for their review and discussion of this document.¶

The authors would also like to thank Susan Hares for her shepherd review and helpful comments to improve this document.¶

Thanks to Henning Rogge for the RTGDIR, and Alexey Melnikov for the SECDIR Early review.¶

The authors would like to thank Gunter Van de Velde, Mohamed Boucadair, Eric Vyncke, Mike Bishop, Mahesh Jethanandani, Roman Danyliw, Deb Cooley, and Jim Guichard for the IESG review.¶

Appendix A. Cross WG Information

[Note to the RFC Editor - remove this section before publication, as well as remove the reference to [I-D.ietf-pce-multipath].]¶

This section describes cross-working group information for the IETF review process.¶

Authors' Addresses