idr Q. Xiong Internet-Draft H. Fu Intended status: Standards Track ZTE Corporation Expires: 18 April 2026 Z. Du China Mobile C. Lin New H3C Technologies 15 October 2025 BGP SR Policy Extensions for Computing-Aware Traffic Steering (CATS) draft-xiong-idr-cats-sr-policy-00 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 18 April 2026. Copyright Notice Copyright (c) 2025 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 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 Xiong, et al. Expires 18 April 2026 [Page 1] Internet-Draft BGP SR Policy Extensions for CATS October 2025 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-sub-TLV . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 7.2. Informative References . . . . . . . . . . . . . . . . . 7 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 Xiong, et al. Expires 18 April 2026 [Page 2] Internet-Draft BGP SR Policy Extensions for CATS October 2025 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. Xiong, et al. Expires 18 April 2026 [Page 3] Internet-Draft BGP SR Policy Extensions for CATS October 2025 +------+ :<------| 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: Xiong, et al. Expires 18 April 2026 [Page 4] Internet-Draft BGP SR Policy Extensions for CATS October 2025 SR Policy SAFI NLRI: 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-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. Xiong, et al. Expires 18 April 2026 [Page 5] Internet-Draft BGP SR Policy Extensions for CATS October 2025 * RESERVED: 1 octet of reserved bits. 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. 4.2. CSCI-ID Sub-sub-TLV The format of CSCI-ID Sub-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-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 To be discussed in future versions of this document. 6. IANA Considerations TBD. Xiong, et al. Expires 18 April 2026 [Page 6] Internet-Draft BGP SR Policy Extensions for CATS October 2025 7. References 7.1. 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-15, 15 September 2025, . [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, . [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, . [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, . 7.2. 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, . [RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . Xiong, et al. Expires 18 April 2026 [Page 7] Internet-Draft BGP SR Policy Extensions for CATS October 2025 Authors' Addresses Quan Xiong ZTE Corporation Email: xiong.quan@zte.com.cn Huakai Fu ZTE Corporation Email: fu.huakai@zte.com.cn Zongpeng Du China Mobile Email: duzongpeng@chinamobile.com Changwang Lin New H3C Technologies Email: linchangwang.04414@h3c.com Xiong, et al. Expires 18 April 2026 [Page 8]