



idr                                                             Q. Xiong
Internet-Draft                                                     H. Fu
Intended status: Standards Track                         ZTE Corporation
Expires: 4 December 2026                                           Z. Du
                                                            China Mobile
                                                                  C. Lin
                                                    New H3C Technologies
                                                             2 June 2026


  BGP SR Policy Extensions for Computing-Aware Traffic Steering (CATS)
                   draft-xiong-idr-cats-sr-policy-01

Abstract

   An SR (Segment Routing) Policy is a set of candidate paths, each
   consisting of one or more segment lists.  The CATS (Computing-Aware
   Traffic Steering) can steer traffic between clients of a service and
   sites offering the service.  This document proposes the BGP SR policy
   extensions for distributing CATS services.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 4 December 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   and restrictions with respect to this document.  Code Components



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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
     2.1.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   2
     2.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   3.  BGP SR Policy for CATS  . . . . . . . . . . . . . . . . . . .   3
   4.  CATS Service Identifier in SR Policy  . . . . . . . . . . . .   4
     4.1.  CS-ID Sub-TLV . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  CSCI-ID Sub-TLV . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   explicitly indicates the forwarding path for packets at the 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 policies to the headend, [RFC9830]
   specifies a mechanism by using BGP.

   The CATS (Computing-Aware Traffic Steering) as per
   [I-D.ietf-cats-framework] can steer traffic between clients of a
   service and sites offering the service.  Segment Routing (SR) can be
   used as an encapsulation solution for CATS data plane from an Ingress
   CATS-Router to an Egress CATS-Router while using an anycast IP
   address as the Computing-aware Service ID (CS-ID) associated with a
   service.  And the CATS Service Contact Instance ID (CSCI-ID) is
   representing a specific service contact instance which serves the
   service request.  This document proposes the BGP SR policy extensions
   for distributing CATS services.

2.  Conventions Used in This Document

2.1.  Abbreviations






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2.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  BGP SR Policy for CATS

   As per [I-D.ietf-cats-framework], a standalone C-PS can be a
   functional component of a centralized controller.  And C-PS will
   collect the metric information from C-SMA and C-NMA and also
   determine the best paths to forward traffic.  When the SR is used as
   the data plane encapsulation for CATS from an Ingress CATS-Router to
   an Egress CATS-Router, the C-PS or the controller may distribute SR
   policies to the CATS Ingress CATS-Router.

   The Figure 1 shows an example of BGP SR Policy for CATS service from
   CATS-Forwarder 1 as ingress node to CATS-Forwarder 2 as egress node.
   The SR policy is configured with policy color 100 and NLRI is mapping
   to the CATS service which is refereed as CS-ID 1.  Two service sites
   with service contact instances represented with CSCI-ID 1 and CSCI-ID
   2 are connected to the CATS-Forwarder 2 from the interfaces with
   Endpoint SID End.DX-1 and End.DX-2.  The SR policy may be distributed
   to carry the identifiers of CATS services.

























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                             +------+
                     :<------| C-PS |
                     :       |      |<------+             Service Site 1
BGP SR Policy:       :       +------+       |    End.DX-1   +---------+
NLRI: CS-ID 1        :          ^           |           +---|CS-ID 1  |
Policy Color: 100    :          |           |           |   |CSCI-ID 1|
Endpoint SID:        :          |  +----------------+   |   +---------+
   End.DX-1/End.DX-2 :          |  |    C-SMA       |---| Service Site 2
                     :          |  +----------------+   |   +---------+
                     :          |  |CATS-Forwarder 2|   +---|CS-ID 1  |
                     :          |  +----------------+       |CSCI-ID 2|
          +--------+ :          |            |   End.DX-2   +---------+
          | Client | :  Network |  +----------------------+
          +--------+ :  metrics |  | +-------+            |
               |     :          +----| C-NMA |            |
               |     :             | +-------+            |
          +----------------+       |    |                 |
          |CATS-Forwarder 1|<-----------+                 |
          |                |-------|                      |
          +----------------+       |       Underlay       |
                                   |     Infrastructure   |
                                   |                      |
                                   +----------------------+

                Figure 1: Example of BGP SR Policy for CATS

4.  CATS Service Identifier in SR Policy

   As per [I-D.ietf-cats-framework], CS-ID is representing a service in
   CATS, which the clients use to access it.  CSCI-ID is representing a
   specific service contact instance.  As defined in [RFC9830], the SR
   policy with CS-ID and CSCI-ID encoding structure is shown in Figure 2
   as follows:


















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

            Figure 2: SR policy with CS-ID and CSCI-ID Encoding

4.1.  CS-ID Sub-TLV

   The format of CS-ID Sub-TLV is shown in Figure 3 as follows:

       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    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                           CS-ID                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                           sub-TLVs                            ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        Figure 3: CS-ID Sub-TLV

   where:

   *  Type: TBD.

   *  Length: variable.

   *  Flags: 1 octet of flags.  None are defined at this stage.  Flags
      SHOULD be set to zero on transmission and MUST be ignored on
      receipt.



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   *  RESERVED: 1 octet of reserved bits.  It SHOULD be set to zero on
      transmission and MUST be ignored on receipt.

   *  CS-ID: indicates the identifier associated with the CATS service.
      It is 4 octets which carry a 32-bit unsigned non-zero number in SR
      networks and 16 octets which carry a 128-bit unsigned non-zero
      number in SRv6 networks.

   *  sub-TLVs: indicates the CSCI-ID Sub-TLVs that can be carried.

4.2.  CSCI-ID Sub-TLV

   The format of CSCI-ID Sub-TLV is shown in Figure 4 as follows:

       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    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                           CSCI-ID                             ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 4: CSCI-ID Sub-TLV

   where:

   *  Type: TBD.

   *  Length: variable.

   *  Flags: 1 octet of flags.  None are defined at this stage.  Flags
      SHOULD be set to zero on transmission and MUST be ignored on
      receipt.

   *  RESERVED: 1 octet of reserved bits.  SHOULD be set to zero on
      transmission and MUST be ignored on receipt.

   *  CSCI-ID: indicates the identifier for a specific service contact
      instance.  It is 4 octets which carry a 32-bit unsigned non-zero
      number in SR networks and 16 octets which carry a 128-bit unsigned
      non-zero number in SRv6 networks.

5.  Security Considerations

   This document defines new BGP Tunnel Encapsulation Attribute sub-TLVs
   for CATS, which do not introduce any new security considerations
   beyond those already listed in [RFC9256] and [RFC9830].




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6.  IANA Considerations

   IANA is requested to assign a new sub-TLV code point from "BGP Tunnel
   Encapsulation Attribute sub-TLVs" registry in the "Border Gateway
   Protocol (BGP) Tunnel Encapsulation" registry group.

              +======+========+===============+
              | Type | Name   |   Reference   |
              +======+========+===============+
              | TBD1 | CS-ID  | this document |
              +------+--------+---------------+

   This document requests creation of a new registry called "CSCI-ID
   Sub-TLVs" .
   The allocation policy of this registry is "Specification Required"
   according to [RFC8126].

   The following initial Sub-TLV codepoints are assigned by this
   document:

              +======+========+===============+
              | Type | Name   |   Reference   |
              +======+========+===============+
              | TBD2 |CSCI-ID | this document |
              +------+--------+---------------+

7.  Contributors

   The following people have substantially contributed to this document:


   Qinghua Shao
   ZTE Corporation
   Email: shao.qinghua@zte.com.cn

8.  References

8.1.  Normative References

   [I-D.ietf-cats-framework]
              Li, C., Du, Z., Boucadair, M., Contreras, L. M., and J.
              Drake, "A Framework for Computing-Aware Traffic Steering
              (CATS)", Work in Progress, Internet-Draft, draft-ietf-
              cats-framework-24, 2 April 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cats-
              framework-24>.





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   [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/rfc/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/rfc/rfc9256>.

   [RFC9830]  Previdi, S., Filsfils, C., Talaulikar, K., Ed., Mattes,
              P., and D. Jain, "Advertising Segment Routing Policies in
              BGP", RFC 9830, DOI 10.17487/RFC9830, September 2025,
              <https://www.rfc-editor.org/rfc/rfc9830>.

8.2.  Informative References

   [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/rfc/rfc2119>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [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/rfc/rfc8174>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/rfc/rfc8986>.

Authors' Addresses

   Quan Xiong
   ZTE Corporation
   Email: xiong.quan@zte.com.cn


   Huakai Fu
   ZTE Corporation
   Email: fu.huakai@zte.com.cn




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   Zongpeng Du
   China Mobile
   Email: duzongpeng@chinamobile.com


   Changwang Lin
   New H3C Technologies
   Email: linchangwang.04414@h3c.com











































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