IDR Working Group Y. Liu
Internet-Draft China Mobile
Intended status: Standards Track C. Lin
Expires: October 30, 2026 New H3C Technologies
R. Chen
ZTE
T. Zhang
BUPT
G. Zeng
Huawei
April 30, 2026
BGP Extension for SRv6 Policy Segment List optimization
draft-liu-idr-sr-segment-list-optimize-06
Abstract
In some use cases, an SRv6 policy's segment 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 document specifies a BGP extension to indicate whether the
endpoint's node SID needs to be included or excluded when installing
the SRv6 Policy. This optimization can improve the forwarding
efficiency of data packets when End SID and Service SID are present.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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reference material or to cite them other than as "work in progress."
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Table of Contents
1. Introduction...................................................3
2. Terminology....................................................3
3. Extension......................................................4
4. Operation......................................................4
5. Use Case.......................................................5
5.1. Data packet Processing to VPN.............................6
5.2. OAM Packet Processing to the Egress Node..................7
6. IANA Considerations............................................7
7. Security Considerations........................................8
8. References.....................................................8
8.1. Normative References......................................8
8.2. Informative References....................................9
9. Acknowledgments................................................9
Appendix A. Cross WG Information.................................10
A.1. Link to Spring WG........................................10
A.2. Interaction with PCE WG..................................10
A.3. Interaction with SRV6ops Recommendations.................10
Authors' Addresses...............................................12
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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). [RFC9830] document
specifies how BGP may distribute SR Policy candidate paths.
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. [I-D. draft-ietf-spring-srv6-policy-sid-
list-opt] specifies procedures to indicate whether the endpoint's
node SID needs to be included or excluded when installing the SRv6
Policy.
This document specifies a BGP extension to indicate whether the
endpoint's node SID needs to be included or excluded when installing
the SRv6 Policy. This optimization can improve the forwarding
efficiency of data packets when End SID and Service SID are present.
2. Terminology
The following terminologies are used in this document.
SR: Segment Routing
SRv6: SR for IPv6
SRH: Segment Routing Header
SID: Segment Identifier
CE: Customer Edge
PE: Provider Edge
VPN: Virtual Private Network
PSP: Penultimate Segment Pop
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3. Extension
IFN-flag (Install Final Node-sid flag) is proposed in the Candidate
Path Administrative Flags Sub-TLV specified in [I.D draft-lin-idr-
sr-policy-admin-flags]. The bit position for the flag is to be
defined by IANA.
A flag is introduced in the Candidate Path Administrative Flags Sub-
TLV [I.D draft-lin-idr-sr-policy-admin-flags] to advertise the IFN-
flag property:
Value: TBD
Description: IFN-flag (Install Final Node-sid flag)
where:
- IFN-flag (Bit TBD): indicate the endpoint node SID is included
in installing SID list(s) of the Candidate Path (CP) when set.
* 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.
- The unassigned bits in the Flags field MUST be set to zero
upon transmission and MUST be ignored upon receipt.
4. Operation
When the controller distributes the SRv6 Policy configuration to the
head node through BGP, set the IFN-flag is proposed in the Candidate
Path Administrative Flags Sub-TLV specified in [I.D draft-lin-idr-
sr-policy-admin-flags].
After receiving the SRv6 Policy configuration with the IFN-Flag (set
to zero) of the Candidate Path Administrative Flags, the ingress
node will not simultaneously arrange the End SID and Service SID of
the egress node into the SRH.SegmentList of packet.
For data packets forwarded to VPN through this SRv6 Policy, the
SRH.SegmentList will not encapsulate the End SID corresponding to
the egress node in the SID list of SRv6 Policy.
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If the forwarding path does not include the service SID of the
egress node, then the End SID of the egress node should be
encapsulated in SRH.SegmentList.
For OAM detection packets of the SR policy, the SRH.SegmentList is
encapsulated according to the SID list of the SR policy, only
encapsulating node SIDs.
5. Use Case
Taking Figure 1 as an example, describe how SRv6 data packets and
OAM packets are forwarded in the SRv6 network based on the optimized
Segment List arrangement mechanism.
+------------+
| Controller |
+------------+
/ \
/ \
/ \
+---+ +---+ +---+ +---+ +---+ +---+
|CE1|----|PE1|-----| P1|-----| P2|-----|PE2|---|CE2|
+---+ +---+ +---+ +---+ +---+ +---+
2::2 3::3 4::4 5::5
End.DT4 SID: 5::100
Figure 1
CE1 and CE2 are VPN access devices that connect to the IPv6 backbone
network through PE. PE1 has a locator 2::/64. P1 has a locator
3::/64. P2 has an End SID 4::4 with PSP Flavor. PE2 has a locator
5::/64 and a VPN SID 5::100. The traffic from CE1 to CE2 is
forwarded along the path PE1->P1->P2->PE2.
P2 needs to perform the PSP behavior to remove the SRH extension
header.
The controller calculates the SRv6 forwarding path from PE1 to PE2
based on the collected topology and configuration information, and
distributes the SRv6 Policy to PE1 through BGP. The Endpoint address
is 5::5 of PE2. There is only one candidate path. The candidate path
contains a Segment list <3::3, 4::4, 5::5> with IFN-Flag set zero in
in the Candidate Path Administrative Flags Sub-TLV.
PE2 advertises a BGP VPN route to PE1, and the next hop of the BGP
route is the endpoint address 5::5. After receiving the BGP route,
PE1 iterates to the SRv6 Policy using the color and the next hop of
the route.
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There are two types of packets sent from PE1 to PE2: data packets
and OAM packets.
5.1. Data packet Processing to VPN
After PE1 receives the data packet from CE1 to CE2, it looks up the
VPN instance routing table and iterates to SRv6 Policy.
PE1 adds the SRH extension header to the packet and encapsulates the
Segment List of the SRv6 Policy. The Segment List in the SRH
extension header is encapsulated as <3::3, 4::4, 5::100>, and the SL
is set to 2.
The Segment List in SRH is shown in Figure 2.
+--------+
Segment List[0] | 5::100 | ==> PE2's End.DT4 SID
+--------+
Segment List[1] | 4::4 |
+--------+
Segment List[2] | 3::3 |
+--------+
Figure 2
The segment list optimization method proposed in this document is
suitable for both SRv6 SID compressed [RFC9800] and non-compressed
scenarios. If the END SID and VPN SID of the egress node share a
common Locator-Block with a sequence of consecutive nodes, the SIDs
of the egress node can also be arranged in a compressed Segment
List.
In order to improve compression efficiency and reduce the overhead
of SRv6 packet header, the compressed Segment List can only contain
the compressed VPN SID.
As shown in Figure 3, PE1, P1, P2, and PE3 share the common Locator-
block A:0:0:0/64 (represented by LB in Figure3).
+---+ +---+ +---+ +---+ +---+ +---+
|CE1|----|PE1|-----| P1|-----| P2|-----|PE2|---|CE2|
+---+ +---+ +---+ +---+ +---+ +---+
LB:2:1:: LB:2:2:: LB:2:3:: LB:2:4::
End.DT4 SID: LB:2:100::
Figure 3
The compressed Segment List optimized in SRH is shown in Figure 4.
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+-----------+-------+-------+-------+------+
| A:0:0:0 | 2:2 | 2:3 | 2:100| 0 |
+-----------+-------+-------+-------+------+
64bits 16bits 16bits 16bits
Figure 4
5.2. OAM Packet Processing to the Egress Node
If the head node enables OAM function and detects a fault in the
SRv6 Policy forwarding path, PE1 will send OAM detection messages to
PE2, such as BFD packets.
The OAM detection message sends by PE1 encapsulate the segment list
corresponding to the SRv6 Policy. Since the message does not need to
be sent to VPN, the Segment List of the SRH extension header is
encapsulated as <3::3, 4::4, 5::5>.
The Segment List in SRH is shown in Figure 5.
+--------+
Segment List[0] | 5::5 | ==> PE2's End SID
+--------+
Segment List[1] | 4::4 |
+--------+
Segment List[2] | 3::3 |
+--------+
Figure 5
6. IANA Considerations
IFN-flag (Install Final Node-sid flag) is proposed in the Candidate
Path Administrative Flags Sub-TLV specified in [I.D draft-lin-idr-
sr-policy-admin-flags].
- IFN-Flag (Bit TBD): indicate the endpoint node SID is included in
installing SID list(s) of the Candidate Path (CP) when set.
* 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.
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7. Security Considerations
The security considerations of BGP [RFC4271] and BGP SR policy
[RFC9830] apply to this document.
The Candidate Path Administrative Flags Sub-TLV, as defined in the
relevant draft, introduces a flag to advertise the IFN-flag
property. This flag indicates whether the endpoint's Node SID should
be included or excluded during the installation of the SRv6 Policy,
which ultimately influences packet forwarding behavior.
Consequently, when configuring, querying, or reporting the IFN-flag
via BGP, it is essential to implement safeguards to protect this
mission-critical or commercially sensitive information.
8. References
8.1. Normative References
[I-D. draft-lin-idr-sr-policy-admin-flags] C, Lin, "BGP SR Policy
Extensions for Administrative Flags ", Work in Progress,
Internet-Draft, draft-lin-idr-sr-policy-admin-flags-04, 28
February 2026,<https://datatracker.ietf.org/doc/html/
draft-lin-idr-sr-policy-admin-flags-04>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI
10.17487/RFC4271, January 2006, <https://www.rfc-
editor.org/info/rfc4271>.
[RFC8400] Chen, H., Liu, A., Saad, T., Xu, F., and L. Huang,
"Extensions to RSVP-TE for Label Switched Path (LSP)
Egress Protection", RFC 8400, DOI 10.17487/RFC8400, June
2018, <https://www.rfc-editor.org/info/rfc8400>.
[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>.
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[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>.
[RFC9830] Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
D. Jain, "Advertising Segment Routing Policies in BGP",
RFC 9830, DOI 10.17487/RFC9830, September 2025,
<https://www.rfc-editor.org/info/rfc9830>.
8.2. Informative References
[I-D. draft-ietf-spring-srv6-policy-sid-list-opt] Z. Ali, C. Lin, Y.
Liu, R. Chen and C. Li, "SRv6 Policy SID List
Optimization", Work in Progress, Internet-Draft, draft-
ietf-spring-srv6-policy-sid-list-opt, 25 November
2025,<https://datatracker.ietf.org/doc/html/draft-ietf-
spring-srv6-policy-sid-list-opt-00>.
[I-D. draft-ietf-srv6ops-srv6-deployment] McBride, M., Liu, Y., Li,
Z., Durmus, M., Erdogan, E., Mishra, G., and Horn, J.,
"SRv6 Deployment Options", Work in Progress, Internet-
Draft, draft-ietf-srv6ops-srv6-deployment, 25 February
2026,<https://datatracker.ietf.org/doc/html/draft-ietf-
srv6ops-srv6-deployment-02>.
[I-D. draft-all-pce-srv6-policy-sid-list-optimization] Z. Ali, C.
Lin, Y. Liu, R. Chen 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, 02 March
2026, <https://datatracker.ietf.org/doc/html/draft-all-
pce-srv6-policy-sid-list-optimization-04>.
[RFC9800] Cheng, W., Ed., Filsfils, C., Li, Z., Decraene, B., and F.
Clad, Ed., "Compressed SRv6 Segment List Encoding",RFC
9800, DOI 10.17487/RFC9800, June 2025, <https://www.rfc-
editor.org/info/rfc9800>.
9. Acknowledgments
TBD
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Appendix A. Cross WG Information
This section describes cross-working group information for use
during the IETF review process. This section will be removed by the
RFC editor prior to publication.
A.1. Link to Spring WG
This document implements the procedures in Section 3.2 of [I-D.
draft-ietf-spring-srv6-policy-sid-list-opt] via BGP. This spring
feature requires both this document and [I-D. draft-lin-idr-sr-
policy-admin-flags] to be implemented.
The work is split into two drafts due to their distinct technical
scopes and design hierarchy:
* [I-D. draft-lin-idr-sr-policy-admin-flags] defines a foundational
extension for managing the administrative state (e.g., Up/Down) of
SR Policies, applicable universally.
* This document specifies a specialized extension for the path
optimization features described in [I-D. draft-ietf-spring-srv6-
policy-sid-list-opt].
This work separation ensures deployment flexibility, allowing
independent implementation of basic operational control and advanced
optimization logic. It also reflects a clean layered architecture,
reducing complexity and promoting modular protocol design.
If only one of the two IDR drafts is implemented, the spring
requirements for a complete and operable SRv6 Policy control
plane cannot be fully satisfied.
A.2. Interaction with PCE WG
The optimization feature implemented in BGP by this document also
could be implemented in PCEP according to Section 3.1 of [I-D.
draft-ietf-spring-srv6-policy-sid-list-opt].
[I-D. draft-all-pce-srv6-policy-sid-list-optimization] specifies a
PCEP extension to indicate whether the endpoint's node SID needs to
be included or excluded when installing the SRv6 Policy.
A.3. Interaction with SRV6ops Recommendations
For SRv6 deployment recommendations of the optimization feature
implemented in BGP by this document, please refer to Section 5.2 of
[I-D. draft-ietf-srv6ops-srv6-deployment].
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This optimization feature could be distributed to external
components using BGP-LS extension defined in [I-D. draft-ali-idr-
srv6-policy-sl-opt-distribution].
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Authors' Addresses
Yisong Liu
China Mobile
Email: liuyisong@chinamobile.com
Changwang Lin
New H3C Technologies
Email: linchangwang.04414@h3c.com
Ran Chen
ZTE Corporation
Email: chen.ran@zte.com.cn
Tiankui Zhang
BUPT
Email: zhangtiankui@bupt.edu.cn
Guanming Zeng
Huawei
Email: zengguanming@huawei.com
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