PCE Working Group                                                 Z. AliInternet-Draft                                       Cisco Systems, Inc.Intended status: Standards Track                                  C. LinExpires: 10 May 2026                                New H3C Technologies                                                                  Y. Liu                                                            China Mobile                                                                 R. Chen                                                         ZTE Corporation                                                                   C. Li                                                     Huawei Technologies                                                         6 November 2025 Path Computation Element Communication Protocol (PCEP) extensions for                   SRv6 Policy SID List Optimization           draft-all-pce-srv6-policy-sid-list-optimization-03Abstract   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/.Ali, et al.                Expires 10 May 2026                  [Page 1]Internet-Draft  PCEP extensions for SRv6 Policy SID List   November 2025   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 10 May 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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4   4.  Overview of PCEP Extensions . . . . . . . . . . . . . . . . .   4     4.1.  New TLV in the SRPA Object  . . . . . . . . . . . . . . .   4     4.2.  A New flag in the SRPOLICY-POL-ATTRIBUTE TLV  . . . . . .   5     4.3.  New flag in SRv6-PCE-CAPABILITY sub-TLV . . . . . . . . .   5   5.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   5     5.1.  MSD Consideration . . . . . . . . . . . . . . . . . . . .   6   6.  Backward compatibility  . . . . . . . . . . . . . . . . . . .   7   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   7   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7     10.2.  Informative References . . . . . . . . . . . . . . . . .   9   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9Ali, et al.                Expires 10 May 2026                  [Page 2]Internet-Draft  PCEP extensions for SRv6 Policy SID List   November 20251.  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.   A PCE computes the SRv6 TE Policy SID list from the headend to the   endpoint.  As described in [I-D.draft-ali-spring-srv6-policy-sid-   list-optimization], 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-ietf-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.   The procedure specified in this draft are equally applicable to PCE   initiated LSPs as well as PCC inited LSPs.Ali, et al.                Expires 10 May 2026                  [Page 3]Internet-Draft  PCEP extensions for SRv6 Policy SID List   November 20252.  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 Extensions4.1.  New TLV in the SRPA Object   The draft specifies a new SRPOLICY-POL-ATTRIBUTE TLV for the SR   Policy Association object defined in [RFC9862].  The SRPOLICY-POL-   ATTRIBUTE TLV is optional.    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                             |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                    Figure 1: SRPOLICY-POL-ATTRIBUTE TLV   *  Type: TBD1 for "SRPOLICY-POL-ATTRIBUTE" TLV.   *  Length: 4.   *  Flags: The unassigned bits in the Flag octet MUST be set to zero      upon transmission and MUST be ignored upon receipt.Ali, et al.                Expires 10 May 2026                  [Page 4]Internet-Draft  PCEP extensions for SRv6 Policy SID List   November 20254.2.  A New flag in the SRPOLICY-POL-ATTRIBUTE TLV   This document specifies IFN-flag (Install Final Node-sid) bit in the   Flags field of "SRPOLICY-POL-ATTRIBUTE" TLV specified in this   document.  The flag is applicable only to SR policies with SRv6 data   plane.  The flag MUST be ignored for SR policies with SR-MPLS data   plane.   IFN (Install Final Node-sid) - 1 bit (Bit Position TBD2):   *  If set to 1, the endpoint node SID MUST be installed when      installing the SRv6 Policy SID list(s) used to carry the data      traffic.   *  If set to 0, the endpoint node SID MUST NOT be installed when      installing the SRv6 Policy SID list(s) used to carry the data      traffic.4.3.  New flag in SRv6-PCE-CAPABILITY sub-TLV   IFN (Install Final Node-sid 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.   IFN (Install Final Node-sid flag) - 1 bit (Bit Position TBD3):   *  If set to 1, it indicates support for the IFN-flag in the      SRPOLICY-POL-ATTRIBUTE TLV.5.  Operation   A PCE always computes the SRv6 TE Policy SID list from the headend to   the endpoint (node SID).   A PCEP speaker indicates its ability to support IFN-flag in the Flags   field of the SRPOLICY-POL-ATTRIBUTE TLV during the PCEP   initialization phase by setting the IFN-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 IFN-flag in the Flags field of the SRPOLICY-POL-ATTRIBUTE   TLV.   A PCEP peer MUST NOT set the IFN-flag flag if capability was not   advertised by both peers.Ali, et al.                Expires 10 May 2026                  [Page 5]Internet-Draft  PCEP extensions for SRv6 Policy SID List   November 2025   If the computed SID list ends with the policy endpoint's Node SID and   the traffic steered over policy already ensures that the traffic is   taken to the policy endpoint and the PCEP peers are capable of   supporting the IFN-flag, the PCE MUST set IFN-flag to 0.   If the computed SID list ends with the policy endpoint's Node SID and   the traffic steered over policy does not takes the traffic to the   policy endpoint and the PCEP peers are capable of supporting the IFN-   flag, the PCE MUST set IFN-flag to 1.   If the computed SID list ends with the penultimate hop adjacency SID,   and the PCEP peers are capable of supporting the IFN-flag, the PCE   MUST set IFN-flag to 1.   If the PCEP peers are capable of supporting the IFN-flag and the IFN-   flag in the Flags field of the SRPOLICY-POL-ATTRIBUTE TLV is set, the   PCC MUST install the endpoint node SID when installing the SRv6   Policy sid list(s) used to carry data traffic.   If the PCEP peers are capable of supporting the IFN-flag and the IFN-   flag in the Flags field of the SRPOLICY-POL-ATTRIBUTE TLV is not set,   the PCC MUST NOT install the endpoint node SID when installing the   SRv6 Policy sid list(s) used to carry data traffic.   IFN-flag value in the Flags field of the SRPOLICY-POL-ATTRIBUTE TLV   MUST NOT change for a given SRv6 Policy Candidate Path during its   lifetime.   Local policy at PCC MAY override the IFN-flag.   PCE ignores the IFN-flag received from the PCC when computing the   path and computes the SRv6 Policy SID list from the headend to the   endpoint.  PCE MAY use the IFN-flag value for debugging purposes.5.1.  MSD Consideration   In some cases, the SID list computed by the PCE exceeds the Maximum   Stack Depth (MSD) that the headend node is capable of supporting.  In   such cases, the PCE has to install transit policies to reduce the   sid-list to fit within the MSD capability of the headend node.  As   the SRv6 policy endpoint node suppression reduces the sid-list size,   the section describes the MSD consideration related to this draft.   Suppose the size of the SRv6 TE Policy SID list computed by PCE is L.   If the PCEP peers are capable of supporting the IFN-flag, and the PCE   sets the IFN-flag to 1, the PCE uses the full sid-list length (L) in   the headend MSD consideration procedure.  If the PCEP peers are   capable of supporting the IFN-flag and the PCE sets the IFN-flag toAli, et al.                Expires 10 May 2026                  [Page 6]Internet-Draft  PCEP extensions for SRv6 Policy SID List   November 2025   0, the PCE uses the sid-list length (L-1) in the headend MSD   consideration procedure.  This is because the endpoint node SID is   not installed in forwarding.  The MSD consideration procedure is   outside the scope of this document.6.  Backward compatibility   If at least one PCEP peer is not capable of supporting the IFN-flag,   the endpoint Node SID inclusion/exclusion SHOULD be set based on   local policy at the PCC.7.  Security Considerations   [RFC8754] defines the notion of an SR domain and use of SRH within   the SR domain.  Procedures for securing an SR domain are defined the   section 5.1 and section 7 of [RFC8754].  This document does not   impose any additional security challenges to be considered beyond   security threats described in [RFC8754], [RFC8679] and [RFC8986].8.  IANA Considerations   TBA9.  Contributors   The following people have contributed to this document:      Rajesh M Venkateswaran      Cisco Systems, Inc.      Email: rmelarco@cisco.com      Yuanxiang Qiu      New H3C Technologies      Email: qiuyuanxiang@h3c.com      Samuel Sidor      Cisco Systems, Inc.      Email: ssidor@cisco.com10.  References10.1.  Normative ReferencesAli, et al.                Expires 10 May 2026                  [Page 7]Internet-Draft  PCEP extensions for SRv6 Policy SID List   November 2025   [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>.   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation              Element (PCE) Communication Protocol (PCEP)", RFC 5440,              DOI 10.17487/RFC5440, March 2009,              <https://www.rfc-editor.org/info/rfc5440>.   [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>.   [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>.   [RFC8679]  Shen, Y., Jeganathan, M., Decraene, B., Gredler, H.,              Michel, C., and H. Chen, "MPLS Egress Protection              Framework", RFC 8679, DOI 10.17487/RFC8679, December 2019,              <https://www.rfc-editor.org/info/rfc8679>.   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,              <https://www.rfc-editor.org/info/rfc8754>.   [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/info/rfc8986>.   [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>.   [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,              <https://www.rfc-editor.org/info/rfc9603>.Ali, et al.                Expires 10 May 2026                  [Page 8]Internet-Draft  PCEP extensions for SRv6 Policy SID List   November 2025   [RFC9862]  Koldychev, M., Sivabalan, S., Sidor, S., Barth, C., Peng,              S., and H. Bidgoli, "Path Computation Element              Communication Protocol (PCEP) Extensions for Segment              Routing (SR) Policy Candidate Paths", RFC 9862,              DOI 10.17487/RFC9862, October 2025,              <https://www.rfc-editor.org/info/rfc9862>.10.2.  Informative ReferencesAppendix A.  Acknowledgements   The authors would like to thank Ketan Talaulikar and Andrew Stone for   the review comments.Authors' Addresses   Zafar Ali   Cisco Systems, Inc.   Email: zali@cisco.com   Changwang Lin   New H3C Technologies   Email: linchangwang.04414@h3c.com   Yisong Liu   China Mobile   Email: liuyisong@chinamobile.com   Ran Chen   ZTE Corporation   Email: chen.ran@zte.com.cn   Cheng Li   Huawei Technologies   Email: c.l@huawei.comAli, et al.                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