PCE Working Group Z. Ali Internet-Draft R. Venkateswaran Intended status: Standards Track S. Sidor Expires: 24 April 2025 Cisco 21 October 2024 Path Computation Element Communication Protocol (PCEP) extensions for SRv6 Policy SID List Optimization draft-ali-pce-srv6-policy-sid-list-optimization-00 Abstract In some use cases, an SRv6 policy's SID list ends with the policy endpoint's node SID, and the traffic steered (over policy) already ensures that it is taken to the policy endpoint. In such cases, the SID list can be optimized by excluding the endpoint Node SID when installing the policy. This draft specifies a PCEP extension to indicate whether the endpoint's node SID needs to be included or excluded when installing the SRv6 Policy. 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. 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 24 April 2025. Ali, et al. Expires 24 April 2025 [Page 1] Internet-Draft Path Computation Element Communication P October 2024 Copyright Notice Copyright (c) 2024 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 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. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Overview of PCEP Extensions . . . . . . . . . . . . . . . . . 3 4.1. New flag in SRv6-PCE-CAPABILITY sub-TLV . . . . . . . . . 4 4.2. New flag in the LSP-EXTENDED-FLAG TLV . . . . . . . . . . 4 5. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 4 6. Backward compatibility . . . . . . . . . . . . . . . . . . . 5 7. Security Considerations . . . . . . . . . . . . . . . . . . . 5 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 9.1. Normative References . . . . . . . . . . . . . . . . . . 5 9.2. Informative References . . . . . . . . . . . . . . . . . 6 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction Segment Routing (SR) [RFC8402] allows a node to steer a packet flow along any path. A Segment Routing Policy (SR Policy) [RFC8402] is an ordered list of segments that represent a source-routed policy. The headend node is said to steer a flow into an SR Policy. The packets steered into an SR Policy have an ordered list of segments associated with that SR Policy written into them. Segment Routing Policy Architecture [RFC9256] updates [RFC8402] as it details the concepts of SR Policy and steering into an SR Policy. [RFC8986] describes the representation and processing of this ordered list of segments for Segment Routing over IPv6 (SRv6). [RFC9603] specifies PCEP extensions to support SR for the IPv6 data plane. Ali, et al. Expires 24 April 2025 [Page 2] Internet-Draft Path Computation Element Communication P October 2024 A PCE computes the SRv6 TE Policy SID list from the headend to the endpoint. The computed SID list may end with the policy endpoint's Node SID or the penultimate hop adjacency SID. If the computed SID list ends with the policy endpoint's node SID and the overlay SID in the steered traffic (over policy) already ensures that the traffic is taken to the policy endpoint with the same intent, the SRv6 policy endpoint device needs to process back-to-back local node SIDs. Examples of overlay SID containing the local node SID are a service SID, a binding SID for transit policies, an EPE SID, etc. From a compression efficiency viewpoint, carrying back-to-back end-point node SID is inefficient. The SID list in the packet can be optimized by excluding the end-point node SID when installing the policy. End- point node SID exclusion improves the compression efficiency and makes packet processing more efficient for the policy endpoint. Excluding the policy endpoint's node SID is possible in most use cases, but not all. For example, if the SRv6 policy is used to carry MPLS traffic, as described in [I-D.draft-agrawal-spring-srv6-mpls- interworking], it is not possible to exclude the policy endpoint's node SID. Specifically, the endpoint's node SID inclusion or exclusion is a policy attribute. This draft specifies a PCEP extension to include or exclude the node SID when installing the SRv6 Policy. 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. Terminology This document uses the following terms defined in [RFC5440]: PCC, PCE, PCEP, PCEP Peer. SR: Segment Routing. SID: Segment Identifier. SRv6: Segment Routing over IPv6 data plane. 4. Overview of PCEP Extensions Ali, et al. Expires 24 April 2025 [Page 3] Internet-Draft Path Computation Element Communication P October 2024 4.1. New flag in SRv6-PCE-CAPABILITY sub-TLV I-flag (endpoint node SID Inclusion capability flag) is proposed in the SRv6-PCE-CAPABILITY sub-TLV defined in [RFC9603]. The bit position for the flag in the SRv6 Capability Flag Field registry is to be defined by IANA. I (endpoint node SID Inclusion capability flag) - 1 bit (Bit Position TBD1): * If set to 1, it indicates support for the I-flag in the LSP- EXTENDED-FLAG TLV. 4.2. New flag in the LSP-EXTENDED-FLAG TLV I-flag (endpoint node SID Inclusion flag) is proposed in the LSP- EXTENDED-FLAG TLV introduced [RFC9357]. The bit position for the flag is to be defined by IANA. I (endpoint node SID Inclusion) - 1 bit (Bit Position TBD2): * If set to 1, the endpoint node SID MUST be included when installing the SR Policy SID list(s) used to carry the data traffic. * If set to 0, the endpoint node SID MUST NOT be included when installing the SR Policy SID list(s) used to carry the data traffic. 5. Operation A PCEP speaker indicates its ability to support I-flag in LSP- EXTENDED-FLAG TLV during the PCEP initialization phase by setting the I-flag in the SRv6-PCE-CAPABILITY sub-TLV in the Open message. A PCEP peer indicates the inclusion or exclusion of the endpoint's Node SID in I-flag in LSP-EXTENDED-FLAG TLV. A PCEP peer MUST NOT set the I-flag flag if capability was not advertised by both peers. If the PCEP peers are capable of supporting the I-flag and the I-flag in the LSP-EXTENDED-FLAG TLV is set, the PCC MUST include the endpoint node SID when installing the Policy sid list(s) used to carry data traffic. Ali, et al. Expires 24 April 2025 [Page 4] Internet-Draft Path Computation Element Communication P October 2024 If the PCEP peers are capable of supporting the I-flag and the I-flag in the LSP-EXTENDED-FLAG TLV is not set, the PCC MUST NOT include the endpoint node SID when installing the Policy sid list(s) used to carry data traffic. I-flag value in LSP-EXTENDED-FLAG TLV MUST NOT change for a given SR Policy Candidate Path during its lifetime. Local policy at PCC MAY override the I-flag. PCE ignores the I-flag received from the PCC when computing the path and computes the SRv6 TE Policy SID list from the headend to the endpoint. PCE MAY use the I-flag value for debugging purposes. 6. Backward compatibility If at least one PCEP peer is not capable of supporting the I-flag, the endpoint Node SID inclusion/exclusion SHOULD be set based on local policy at the PCC. 7. Security Considerations TBA 8. IANA Considerations TBA 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, March 2009, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Ali, et al. Expires 24 April 2025 [Page 5] Internet-Draft Path Computation Element Communication P October 2024 [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, . [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, . [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, . [RFC9357] Xiong, Q., "Label Switched Path (LSP) Object Flag Extension for Stateful PCE", RFC 9357, DOI 10.17487/RFC9357, February 2023, . [RFC9603] Li, C., Ed., Kaladharan, P., Sivabalan, S., Koldychev, M., and Y. Zhu, "Path Computation Element Communication Protocol (PCEP) Extensions for IPv6 Segment Routing", RFC 9603, DOI 10.17487/RFC9603, July 2024, . 9.2. Informative References Appendix A. Acknowledgements The authors would like to thank Ketan Talaulikar for the review comments. Authors' Addresses Zafar Ali Cisco Email: zali@cisco.com Rajesh M Venkateswaran Cisco Email: melarco@cisco.com Samuel Sidor Cisco Ali, et al. Expires 24 April 2025 [Page 6] Internet-Draft Path Computation Element Communication P October 2024 Email: ssidor@cisco.com Ali, et al. Expires 24 April 2025 [Page 7]