BGP Extensions of SR Policy for Composite Candidate Path
draft-jiang-idr-sr-policy-composite-path-04
This document is an Internet-Draft (I-D).
Anyone may submit an I-D to the IETF.
This I-D is not endorsed by the IETF and has no formal standing in the
IETF standards process.
| Document | Type | Active Internet-Draft (idr WG) | |
|---|---|---|---|
| Authors | Jiang Wenying , Changwang Lin , Ran Chen | ||
| Last updated | 2026-02-06 | ||
| Replaces | draft-li-idr-sr-policy-composite-path | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Candidate for WG Adoption | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-jiang-idr-sr-policy-composite-path-04
IDR W. Jiang
Internet Draft China Mobile
Intended status: Standards Track C. Lin
Expires: August 08, 2026 New H3C Technologies
R. Chen
ZTE Corporation
February 06, 2026
BGP Extensions of SR Policy for Composite Candidate Path
draft-jiang-idr-sr-policy-composite-path-04
Abstract
Segment Routing is a source routing paradigm that explicitly
indicates the forwarding path for packets at the ingress node. An
SR Policy is associated with one or more candidate paths. A
candidate path is either dynamic, explicit or composite. 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
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 August 08, 2026.
Copyright Notice
Copyright (c) 2026 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
Jiang, et al. Expires August 08, 2026 [Page 1]
Internet-Draft BGP SR Policy Composite Candidate Path February 2026
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
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.........................5
3. Procedures.....................................................6
4. Error Handling.................................................7
5. Security Considerations........................................7
6. IANA Considerations............................................8
7. References.....................................................8
7.1. Normative References......................................8
7.2. Informative References....................................9
Authors' Addresses...............................................10
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.
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
candidate path construct enables combination of SR Policies, for a
load-balanced steering of packet flows over its constituent SR
Policies.
This document defines extensions to Border Gateway Protocol (BGP) to
distribute SR policies carrying composite candidate path
information. After BGP distributions valid information about the
composite path, the SR Policy Module (SRPM) will instantiate valid
SR Policies.
Jiang, et al. Expires August 08, 2026 [Page 2]
Internet-Draft BGP SR Policy Composite Candidate Path February 2026
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
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:
Jiang, et al. Expires August 08, 2026 [Page 3]
Internet-Draft BGP SR Policy Composite Candidate Path February 2026
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.
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:
Jiang, et al. Expires August 08, 2026 [Page 4]
Internet-Draft BGP SR Policy Composite Candidate Path February 2026
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.
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.
Jiang, et al. Expires August 08, 2026 [Page 5]
Internet-Draft BGP SR Policy Composite Candidate Path February 2026
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.
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.
Jiang, et al. Expires August 08, 2026 [Page 6]
Internet-Draft BGP SR Policy Composite Candidate Path February 2026
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 NRP 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
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.
Jiang, et al. Expires August 08, 2026 [Page 7]
Internet-Draft BGP SR Policy Composite Candidate Path February 2026
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.
[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>.
Jiang, et al. Expires August 08, 2026 [Page 8]
Internet-Draft BGP SR Policy Composite Candidate Path February 2026
[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>.
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>.
Jiang, et al. Expires August 08, 2026 [Page 9]
Internet-Draft BGP SR Policy Composite Candidate Path February 2026
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
Jiang, et al. Expires August 08, 2026 [Page 10]