IDR W. Jiang
Internet Draft China Mobile
Intended status: Standards Track C. Lin
Expires: October 24, 2026 New H3C Technologies
R. Chen
ZTE Corporation
April 24, 2026
BGP Extensions of SR Policy for Composite Candidate Path
draft-jiang-idr-sr-policy-composite-path-08
Abstract
SR Policy Architecture [RFC9256] defines the concept of a Composite
Candidate Path. A regular SR Policy Candidate Path outputs traffic
to a set of Segment Lists, while an SR Policy Composite Candidate
Path outputs traffic recursively to a set of SR Policies on the same
headend. This document defines extensions to BGP to distribute SR
policies carrying composite candidate path information. So that
composite candidate paths can be installed when the SR policy is
applied.
Status of this Memo
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This Internet-Draft will expire on October 24, 2026.
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Table of Contents
1. Introduction...................................................2
1.1. Requirements Language.....................................3
2. Constituent SR Policy Attributes in SR Policy..................3
2.1. Constituent SR Policy Sub-TLV.............................4
2.2. Per-Flow Forwarding Class Sub-TLV.........................6
3. Procedures.....................................................7
4. Error Handling.................................................7
5. Security Considerations........................................8
6. IANA Considerations............................................8
7. References.....................................................8
7.1. Normative References......................................8
7.2. Informative References....................................9
Authors' Addresses...............................................11
Appendix A. Cross-WG Information.................................12
A.1 Spring WG.................................................12
A.2 PCE WG....................................................12
A.3 SRv6ops...................................................12
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.
SR Policy Architecture [RFC9256] defines the concept of a Composite
Candidate Path. A regular SR Policy Candidate Path outputs traffic
to a set of Segment Lists, while an SR Policy Composite Candidate
Path outputs traffic recursively to a set of SR Policies on the same
headend.
An SR Policy is associated with one or more candidate paths. A
composite candidate path acts as a container for grouping of SR
Policies. As described in section 2.2 in [RFC9256], the composite
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candidate path construct enables combination of SR Policies, for a
load-balanced steering of packet flows over its constituent SR
Policies.
Composite Candidate Paths can be distributed via the Path
Computation Element Communication Protocol (PCEP), as described in
section 3.6 of [I-D. draft-ietf-pce-multipath].
This document defines extensions to Border Gateway Protocol (BGP) to
distribute SR policies carrying composite candidate path
information. The SR policy in the composite path can be distributed
by BGP as described in [RFC9830], by PCEP as described in [RFC
9862], or through static configuration. This document does not alter
the existing deployment methods for SR policies; it only extends BGP
to support the distribution of composite candidate paths based on
[RFC9830].
1.1. 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.
2. Constituent SR Policy Attributes in SR Policy
As defined in section 2.2 of [RFC9830], the SR policy encoding
structure is as follows:
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
SRv6 Binding SID
Preference
Priority
Policy Name
Policy Candidate Path Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Segment
Segment
...
...
Figure 1: SR Policy Encoding
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As described in section 2.2 in [RFC9256], the endpoints of the
constituent SR Policies and the parent SR Policy MUST be identical,
and the colors of each of the constituent SR Policies and the parent
SR Policy MUST be different. Therefore a constituent SR Policy is
referenced only by color in the composite candidate path since its
headend and endpoint are identical to the parent SR policy.
SR policy with composite candidate path information is expressed as
below:
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
SRv6 Binding SID
Preference
Priority
Policy Name
Policy Candidate Path Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Segment
Segment
...
Constituent SR Policy
Color
Weight
Fowarding Class
...
Figure 2: SR policy with composite candidate path Encoding
SR Policy Architecture [RFC9256] defines the concept of a Composite
Candidate Path. A regular SR Policy Candidate Path outputs traffic
to a set of Segment Lists, while an SR Policy Composite Candidate
Path outputs traffic recursively to a set of SR Policies on the same
headend.
2.1. Constituent SR Policy Sub-TLV
The Constituent SR Policy sub-TLV is an optional sub-TLV of BGP
Tunnel Encapsulation Attribute, and MAY appear multiple times in the
SR Policy encoding. The ordering of Constituent SR Policy sub-TLVs
does not matter. The Constituent SR Policy sub-TLV MAY contain a
Weight sub-TLV.
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The Constituent SR Policy sub-TLV and the Segment List sub-TLV MUST
NOT appear in the same candidate path.
The Constituent SR Policy sub-TLV has the following format:
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 | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Color |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-TLVs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Constituent SR Policy sub-TLV format
where:
* Type: to be assigned by IANA.
* Length: The total length (not including the Type and Length
fields) of the sub-TLVs encoded within the Constituent SR
Policy sub-TLV in terms of octets.
* RESERVED: 1 octet of reserved bits. This field MUST be set to
Zero on transmission and MUST be ignored on receipt.
* Color: 4 octets that carry an unsigned non-zero integer value
indicating the Color of the Constituent SR Policy. As
described in section 2.2 in [RFC9256], the endpoints of the
constituent SR Policies and the parent SR Policy MUST be
identical, thus different constituent SR Policies can be
distinguished by Color.
* Weight: 4 octets carrying an unsigned integer value indicating
the weight associated with a segment list as described in
Section 2.11 of [RFC9256]. A weight value of zero is
invalid.
* sub-TLVs currently defined: An optional single Per-Flow
Forwarding Class sub-TLV which is defined in section 2.2 on
this document. The other Sub-TLVs in Constituent SR Policy Sub-
TLV are out of scope of this document.
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2.2. Per-Flow Forwarding Class Sub-TLV
Per-Flow Forwarding Path builds on top of the concept of the
Composite Candidate Path. Each Path in a Per-Flow Forwarding Path
is assigned a 3-bit Forward Class (FC) value, which allows QoS
classified traffic to be steered depending on the FC. The Per-Flow
FC sub-TLV is an optional sub-TLV of the Constituent SR Policy TLV.
The Per-flow FC sub-TLV is OPTIONAL and it MUST NOT appear more than
once inside the Constituent SR Policy sub-TLV.
The Per-flow FC sub-TLV has the following format:
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-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESERVED-2 | FC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 Per-Flow FC Sub-TLV
where:
* Type: to be assigned by IANA.
* Length: Specifies the length of the value field (not including
Type and Length fields) in terms of octets. The value MUST be
6.
* Flags: 1 octet of flags. No flags are defined in this
document. The Flags field MUST be set to zero on transmission
and MUST be ignored on receipt.
* RESERVED-1: 1 octet of reserved bits. This field MUST be set
to zero on transmission and MUST be ignored on receipt.
* Reserved-2(29 bits): This field MUST be set to zero on
transmission and MUST be ignored on receipt.
* FC (3 bits): Forward class value that is given by the QoS
classifier to traffic entering the given Candidate Path.
Different classes of traffic that enter the given Candidate
Path can be differentially steered into different Colors.
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3. Procedures
The document does not bring new operation beyond the description of
operations defined in [RFC9830]. The existing operations defined in
[RFC9830] can apply to this document directly.
Typically but not limit to, the SR policies carrying composite
candidate path information are configured by a controller.
After configuration, the SR policies carrying path composite
candidate path information will be advertised by BGP update
messages.
The operation of advertisement is the same as defined in [RFC9830],
as well as the reception.
Note the differences among several color TLVs. The Tunnel Egress
Endpoint and Color sub-TLVs of the Tunnel Encapsulation Attribute,
as defined in [RFC9012], are not utilized for SR Policy encodings;
see more details in Section 2.3 of [RFC9830].
The Color Extended Community (as defined in [RFC9012]) is used to
steer traffic into an SR Policy, as described in Section 8.8 of
[RFC9256] and Section 3 of [RFC9830].
The color of the Constituent SR Policy is identified by its color,
as described in Section 2.1.
4. Error Handling
The error handling of the BGP Update messages for BGP SR Policy SAFI
with the extensions defined in this document follows the procedures
in section 5 of [RFC9830].
The validation of the TLVs/sub-TLVs introduced in this document and
defined in their respective subsections of Section 2 MUST be
performed to determine if they are malformed or invalid.
The Constituent SR Policy sub-TLV and the Segment List sub-TLV MUST
NOT appear in the same candidate path. If Constituent SR Policy sub-
TLV does not match the above description, or its format is
considered malformed, the associated BGP SR Policy NLRI is
considered malformed and the "treat-as-withdraw" strategy of
[RFC7606] MUST be applied.
The Per-flow FC sub-TLV is optional and MUST NOT appear more than
once for one Constituent SR Policy sub-TLV. The Per-flow FC sub-TLV
is considered malformed if its format does not match the above
description. If the Per-flow FC sub-TLV appears more than once, or
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its format is considered malformed, the associated BGP SR Policy
NLRI is considered malformed and the "treat-as-withdraw" strategy of
[RFC7606] MUST be applied.
5. Security Considerations
The security considerations of BGP [RFC4271] and BGP SR policy
[RFC9830] apply to this document.
This document defines BGP extensions for distributing SR policies
that carry composite candidate path information. These functions
extend the risks associated with SR Policy into the dynamic realm.
Misconfiguration or errors in configuring an SR Policy Composite
Candidate Path may lead to packets being forwarded along unintended
paths for the affected routes.
6. IANA Considerations
This document defines a sub-TLV in the "BGP Tunnel Encapsulation
Attribute Sub-TLVs" registry under the "Border Gateway Protocol
(BGP) Tunnel Encapsulation" registry group.
+=======+===============================+===============+
| Value | Description | Reference |
+=======+===============================+===============+
| TBA | Constituent SR Policy Sub-TLV | This document |
+-------+-------------------------------+---------------+
This document creates a new registry called "Constituent SR Policy
sub-TLV" under the "Border Gateway Protocol (BGP) Tunnel
Encapsulation" registry group.
+=======+===============================+===============+
| Value | Description | Reference |
+=======+===============================+===============+
| TBA | Per-Flow FC Sub-TLV | This document |
+-------+-------------------------------+---------------+
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[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>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and
K.Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017
[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>.
[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>.
[RFC9857] Previdi, S., Talaulikar, K., Ed., Dong, J., Gredler, H.,
and J. Tantsura, "Advertisement of Segment Routing
Policies Using BGP - Link State", RFC 9857, DOI
10.17487/RFC9857, October 2025, <https://www.rfc-
editor.org/info/rfc9857>.
7.2. Informative References
[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>.
[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/rfc/rfc9862>.
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[I-D.ietf-pce-multipath] Koldychev, M., Sivabalan, S., Saad, T.,
Beeram, V. P., Bidgoli, H., Peng, S., and S. Sidor, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Signaling Multipath Information", Work in
Progress, Internet-Draft, draft-ietf-pce-multipath-23, 13
April 2026, <https://datatracker.ietf.org/doc/html/draft-
ietf-pce-multipath-23>.
[I-D.ietf-spring-sr-policy-group] Cheng, W., Wenying, J., Lin, C.,
Chen, R., and Y. Zhang, "SR Policy Group", Work in
Progress, Internet-Draft, draft-ietf-spring-sr-policy-
group-00, 13 January
2026,<https://datatracker.ietf.org/doc/html/draft-ietf-
spring-sr-policy-group-00>.
[I-D. draft-li-idr-bgpls-sr-policy-composite-path] Lin, C., Cheng,
W., Ali, Z., MahendraBabu, A., and Chen, R., " Signaling
Composite Candidate Path of SR Policy using BGP-LS", Work
in Progress, Internet-Draft, draft-li-idr-bgpls-sr-policy-
composite-path-10, 23 April
2026,<https://datatracker.ietf.org/doc/html/ draft-li-idr-
bgpls-sr-policy-composite-path-10>.
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Authors' Addresses
Wenying Jiang
China Mobile
Beijing
China
Email: jiangwenying@chinamobile.com
Changwang Lin
New H3C Technologies
Beijing
China
Email: linchangwang.04414@h3c.com
Ran Chen
ZTE Corporation
Email: chen.ran@zte.com.cn
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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 Spring WG
Section 2.2 of [RFC9256] defines the concept of a Composite
Candidate Path. Section 2.13 of [RFC9256] illustrates an information
model for hierarchical relationships between the SR Policy
constructs.
A Parent SR Policy is the composite candidate path that acts as a
container for grouping SR Policies which meet different service
optimization objectives and constraints and have the same
destination endpoint. [I-D. draft-ietf-spring-sr-policy-group]
defines illustrates some use cases for parent SR Policy and SR
Policy Group to simplify deployment and provide best practice cases
for operators.
A.2 PCE WG
Composite Candidate Paths can be distributed via the Path
Computation Element Communication Protocol (PCEP), as described in
section 3.6 of [I-D.draft-ietf-pce-multipath].
A.3 SRv6ops
[RFC9857] describes a mechanism to collect the SR policy information
that is locally available in a node and advertise it into BGP-LS
updates. [I-D.draft-li-idr-bgpls-sr-policy-composite-path] extends
it to provide some extra information to carry composite candidate
path information in the BGP-LS advertisement.
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