Spring Working Group Z. Ali
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track C. Lin
Expires: 29 May 2026 New H3C Technologies
Y. Liu
China Mobile
R. Chen
ZTE Corporation
C. Li
Huawei Technologies
25 November 2025
SRv6 Policy SID List Optimization
draft-ietf-spring-srv6-policy-sid-list-opt-00
Abstract
Segment Routing (SR) allows a node to steer a packet flow along any
path. SR Policy is an ordered list of segments (i.e., instructions)
that represent a source-routed policy. The packets steered into an
SR Policy carry an ordered list of segments associated with that SR
Policy. An SR Policy can be instantiated SR-MPLS and SRv6 data
planes.
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 procedures 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.
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Internet-Drafts are working documents of the Internet Engineering
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working documents as Internet-Drafts. The list of current Internet-
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This Internet-Draft will expire on 29 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. SID List Optimization During Policy Creation . . . . . . . . 4
3.1. Policy Creation when using PCEP . . . . . . . . . . . . . 4
3.2. Policy Creation when using BGP . . . . . . . . . . . . . 4
4. SID List Optimization Advertisement . . . . . . . . . . . . . 5
5. OAM Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Operational Considerations . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 6
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Terminology
This document uses the following terms defined in [RFC5440]: PCC,
PCE, PCEP.
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SR: Segment Routing.
SID: Segment Identifier.
SRv6: Segment Routing over IPv6 data plane.
2. 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. An SR Policy can be
instantiated SR-MPLS and SRv6 data planes. [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.
The SRv6 Policy 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.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 procedures needed to include or exclude the node
SID when installing the SRv6 Policy.
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3. SID List Optimization During Policy Creation
3.1. Policy Creation when using PCEP
The following procedure is applicable to both PCC-initiated Mode and
PCE-initiated Mode.
A PCE always computes the SRv6 TE Policy SID list from the headend to
the endpoint (node SID).
PCC and PCE exchange capabilities during the PCEP initialization
phase to indicate the support for the SID list optimization.
If the PCEP peers are capable of supporting the SID list
optimization, the PCE indicates the inclusion or exclusion of the
last SID in the ERO as follows:
* If the computed SID list ends with the policy endpoint's Node SID
and the traffic being steered over policy does not take the
traffic to the policy endpoint, the PCE MUST signal the
installation of the last node SID to the PCC. In this case, the
PCC MUST install the last SID (the endpoint node SID) when
installing the SRv6 Policy sid list(s) used to carry data traffic.
In this case, the PCE considers the the policy endpoint's Node SID
in the MSD consideration procedure. However, in the case when the
installation of the last SID is not required, the PCE does not
consider the the policy endpoint's Node SID in the MSD
consideration procedure. Specifically, suppose the size of the
SRv6 TE Policy SID list computed by PCE is L. In the case when
the the last SID is not installed by PCC, the PCE uses the sid-
list length L-1 in the headend MSD consideration procedure. This
is because the endpoint node SID is suppressed.
* If the computed SID list does not ends with the policy endpoint's
Node SID, e.g., penultimate hop Adjacency SID, the PCC MUST
install the last SID when installing the SRv6 Policy sid list(s)
used to carry data traffic.
Protocol extension details can be found in [I-D.all-pce-srv6-policy-
sid-list-optimization].
3.2. Policy Creation when using BGP
As defined in [RFC9256], an SR Policy is associated with one or more
candidate paths. An SR Policy Controller [RFC9256] defines the set
of policies and advertises them to policy headend routers (typically
ingress routers). These policy advertisements use the BGP SR Policy
SAFI [I-D.ietf-idr-sr-policy-safi].
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When the SR Policy Controller completes computation of the SID list
path:
* If the computed SID list ends with the policy endpoint's Node SID
and the traffic being steered over policy does not take the
traffic to the policy endpoint, the SR Policy Controller
installation of the last node SID to SRv6 Policy headend routers.
In this case, the headend router MUST install the last SID (the
endpoint node SID) when installing the SRv6 Policy sid list(s)
used to carry data traffic.
* If the computed SID list does not ends with the policy endpoint's
Node SID, e.g., penultimate hop Adjacency SID, the SR Policy
headend router MUST install this SID when installing the SRv6
Policy SID list(s) for data traffic.
Protocol extension details can be found in [I-D.liu-idr-sr-segment-
list-optimize] and [I-D.lin-idr-sr-policy-admin-flags].
4. SID List Optimization Advertisement
[I-D.ietf-idr-bgp-ls-sr-policy] describes a mechanism to distribute
SR policy information to external components using BGP-LS. SR policy
information can be used by external components for path computation,
re-optimization, service placement, network visualization, etc.
BGP-LS needs to be extended to indicate whether the endpoint's node
SID is included or excluded in installing SID list(s) of the
Candidate Path (CP) of an SRv6 Policy.
Protocol extension details can be found in [I-D.ali-idr-srv6-policy-
sl-opt-distribution].
5. OAM Considerations
The SID list optimization outlined in this draft applies to the case
where service traffic, like L3VPN, L2VPN, etc. is steered over
policy. It does not apply to the probe packets used for OAM
(Operational and Management) of the SRv6 policy in the transport
network. When performing OAM operation over the policy, e.g., ping,
traceroute, BFD, STAMP, etc. over policy, the initiator always
includes the last SID of the policy segment list.
Please note that both data traffic and OAM packets are routed by the
locator of the policy endpoint node. Therefore, the data traffic and
OAM packets always follow the same route in the network.
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6. Operational Considerations
This document specifies the detailed construction of the SRv6 Policy
with inclusion or exclusion of end-point node SID and its operational
mechanisms. Therefore, the manageability considerations of [RFC9256]
apply.
Support of the inclusion or exclusion of end-point node SID
capability defined in this document SHOULD be configurable. The
control of this capability for the candidate path of an SRv6 Policy
can be achieved via BGP, PCEP, YANG, or CLI. This capability itself
does not alter packet forwarding behavior but rather optimizes the
segment list orchestration for service packets, thereby enhancing
forwarding performance.
A YANG model for the configuration and operation of SR Policy has
been defined in [SR-POLICY-YANG]. In later versions, the capability
to control the inclusion or exclusion of end-point node SID will be
added under the candidate-path node of SRv6 Policy.
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
None
9. 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
10. References
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10.1. 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,
<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>.
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10.2. Informative References
[I-D.ali-idr-srv6-policy-sl-opt-distribution]
Ali, Z., Lin, C., Liu, Y., and R. Chen, "SRv6 Policy SID
List Optimization Advertisement", Work in Progress,
Internet-Draft, draft-ali-idr-srv6-policy-sl-opt-
distribution-01, 7 July 2025,
<https://datatracker.ietf.org/doc/html/draft-ali-idr-srv6-
policy-sl-opt-distribution-01>.
[I-D.all-pce-srv6-policy-sid-list-optimization]
Ali, Z., Lin, C., Liu, Y., Chen, R., and C. Li, "Path
Computation Element Communication Protocol (PCEP)
extensions for SRv6 Policy SID List Optimization", Work in
Progress, Internet-Draft, draft-all-pce-srv6-policy-sid-
list-optimization-03, 5 November 2025,
<https://datatracker.ietf.org/doc/html/draft-all-pce-srv6-
policy-sid-list-optimization-03>.
[I-D.ietf-idr-bgp-ls-sr-policy]
Previdi, S., Talaulikar, K., Dong, J., Gredler, H., and J.
Tantsura, "Advertisement of Segment Routing Policies using
BGP Link-State", Work in Progress, Internet-Draft, draft-
ietf-idr-bgp-ls-sr-policy-17, 6 March 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
ls-sr-policy-17>.
[I-D.ietf-idr-sr-policy-safi]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
D. Jain, "Advertising Segment Routing Policies in BGP",
Work in Progress, Internet-Draft, draft-ietf-idr-sr-
policy-safi-13, 6 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
policy-safi-13>.
[I-D.ietf-spring-srv6-mpls-interworking]
Agrawal, S., Filsfils, C., Voyer, D., Dawra, G., Li, Z.,
and S. Hegde, "SRv6 and MPLS interworking", Work in
Progress, Internet-Draft, draft-ietf-spring-srv6-mpls-
interworking-01, 7 July 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
srv6-mpls-interworking-01>.
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[I-D.lin-idr-sr-policy-admin-flags]
Lin, C., Liu, Y., Chen, R., and A. Karboubi, "BGP SR
Policy Extensions for Administrative Flags", Work in
Progress, Internet-Draft, draft-lin-idr-sr-policy-admin-
flags-03, 5 November 2025,
<https://datatracker.ietf.org/doc/html/draft-lin-idr-sr-
policy-admin-flags-03>.
[I-D.liu-idr-sr-segment-list-optimize]
Liu, Y., Lin, C., and R. Chen, "BGP Extension for SRv6
Policy Segment List optimization", Work in Progress,
Internet-Draft, draft-liu-idr-sr-segment-list-optimize-01,
20 February 2025, <https://datatracker.ietf.org/doc/html/
draft-liu-idr-sr-segment-list-optimize-01>.
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.com
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