Signaling Composite Candidate Path of SR Policy using BGP-LS
draft-li-idr-bgpls-sr-policy-composite-path-09
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Changwang Lin , Weiqiang Cheng , Zafar Ali , Aravind Babu MahendraBabu , Ran Chen | ||
| Last updated | 2025-11-06 | ||
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draft-li-idr-bgpls-sr-policy-composite-path-09
Network Working Group C. Lin
Internet-Draft New H3C Technologies
Intended status: Standards Track W. Cheng
Expires: 10 May 2026 China Mobile
Z. Ali
A. MahendraBabu
Cisco Systems, Inc
R. Chen
ZTE Corporation
6 November 2025
Signaling Composite Candidate Path of SR Policy using BGP-LS
draft-li-idr-bgpls-sr-policy-composite-path-09
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, and each
candidate path is either dynamic, explicit or composite. This
document specifies the extensions to BGP Link State (BGP-LS) to carry
composite candidate path information in the advertisement of an SR
policy.
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
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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.
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Internet-Draft SR Policy Composite Path in BGP-LS November 2025
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. BGP-LS Extensions for Composite Candidate Path . . . . . . . 3
3.1. Composite Candidate Path TLV . . . . . . . . . . . . . . 3
3.2. Per-Flow Forwarding Class TLV . . . . . . . . . . . . . . 4
4. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
As described in [RFC9552], BGP Link State (BGP-LS) provides a
mechanism by which link-state and TE information can be collected
from networks and shared with external components using the BGP
routing protocol.
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].
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, each
with explicit candidate paths and/or dynamic candidate paths with
potentially different optimization objectives and constraints, for a
load-balanced steering of packet flows over its constituent SR
Policies.
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[I-D.jiang-idr-sr-policy-composite-path] defines extensions for BGP
to distribute SR policies carrying composite candidate path
information. While as defined in Section 3.6 of
[I-D.ietf-pce-multipath], PCEP signals the Composite Candidate Path.
[RFC9857] describes a mechanism to collect the SR policy information
that is locally available in a node and advertise it into BGP-LS
updates. This document extends it to provide some extra information
to carry composite candidate path information in the BGP-LS
advertisement.
2. Terminology
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. BGP-LS Extensions for Composite Candidate Path
[RFC9552] defines the BGP-LS NLRI that can be a Node NLRI, a Link
NLRI or a Prefix NLRI. The corresponding BGP-LS attribute is a Node
Attribute, a Link Attribute or a Prefix Attribute. [RFC9857]
describes a mechanism to collect the SR Policy information that is
locally available in a node and advertise it into BGP Link State
(BGP-LS) updates. This section defines a new sub-TLV which is
carried in the optional non-transitive BGP Attribute "LINK_STATE
Attribute" defined in [RFC9552].
3.1. Composite Candidate Path TLV
Segment Routing Policy (SR Policy) architecture is specified in
[RFC9256]. A SR Policy can comprise of one or more candidate paths,
and each candidate path is either dynamic, explicit or composite. A
composite candidate path can comprise of one or more constituent SR
policies. 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.
The Composite Candidate Path TLV is used to report the constituent SR
policy(s) of a composite candidate path. It is carried in the
optional non-transitive BGP- LS Attribute defined in [RFC9552] and is
associated with the SR Policy Candidate Path NLRI type. Only a
single instance of this TLV is advertised for a given candidate path.
If multiple instances are present, then the first valid (i.e., not
determined to be malformed as per section 8.2.2 of [RFC9552]) one is
used and the rest are ignored.The TLV has following format:
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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 (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
* Type: to be assigned by IANA.
* Length: the total length of the value field not including Type and
Length fields.
* Reserved: 32 bits reserved and MUST be set to 0 on transmission
and MUST be ignored on receipt.
* Color: 4 octets that indicates the color of the constituent SR
Policy.
* Weight: 4 octet field that indicates the weight associated with
the SID-List for weighted load-balancing. Refer Section 2.2 and
2.11 of [RFC9256].
* Sub-TLVs: variable and contains any other optional attributes
associated with the Composite Candidate Path. Currently, the sub-
TLV only defines the Per-Flow Forwarding Class TLV.
3.2. Per-Flow Forwarding Class TLV
Per-Flow Candidate Path builds on top of the concept of the Composite
Candidate Path. Each Path in a Per-Flow Candidate Path is assigned a
3-bit forward class value, which allows Quality of Service (QoS)
classified traffic to be steered depending on the forward class. The
Per-Flow Forwarding Class TLV is an optional sub-TLV of the Composite
Candidate Path TLV.Only a single instance of this sub-TLV is
advertised for a given candidate path. If multiple instances are
present, then the first valid (i.e., not determined to be malformed
as per section 8.2.2 of [RFC9552]) one is used and the rest are
ignored.
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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 | FC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
* Type (16 bits): TBD1 for "PER-FLOW-FORWARD-CLASS" TLV.
* Length (16 bits): 4.
* Reserved: 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.
4. Operations
The document does not bring new operation beyond the description of
operations defined in [RFC9552] and [RFC9857]. The existing
operations defined in [RFC9552] and [RFC9857] can apply to this
document directly.
Typically but not limit to, the BGP-LS messages carrying composite
candidate path information along with the SR policy are distributed
to a controller.
After configuration, the composite candidate path information will be
advertised by BGP update messages. The operation of advertisement is
the same as defined in [RFC9552] and [RFC9857], as well as the
reception.
5. Security Considerations
Procedures and protocol extensions defined in this document do not
affect the security considerations discussed in [RFC9857].
6. IANA Considerations
This document defines a new TLV in the BGP-LS NLRI and Attribute
TLVs:
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+=======+===========================+===============+
| Value | Description | Reference |
+=======+===========================+===============+
| TBA | Composite Candidate Path | This document |
+-------+---------------------------+---------------+
| TBA | Per-Flow Forwarding Class | This document |
+-------+---------------------------+---------------+
Table 1
7. References
7.1. 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>.
[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>.
[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>.
[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
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[I-D.ietf-pce-multipath]
Koldychev, M., Sivabalan, S., Saad, T., Beeram, V. P.,
Bidgoli, H., Yadav, B., Peng, S., Mishra, G. S., and S.
Sidor, "Path Computation Element Communication Protocol
(PCEP) Extensions for Signaling Multipath Information",
Work in Progress, Internet-Draft, draft-ietf-pce-
multipath-16, 17 October 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
multipath-16>.
[I-D.jiang-idr-sr-policy-composite-path]
Wenying, J., Lin, C., and R. Chen, "BGP Extensions of SR
Policy for Composite Candidate Path", Work in Progress,
Internet-Draft, draft-jiang-idr-sr-policy-composite-path-
03, 20 October 2025,
<https://datatracker.ietf.org/doc/html/draft-jiang-idr-sr-
policy-composite-path-03>.
[RFC9552] Talaulikar, K., "Distribution of Link-State and Traffic
Engineering Information Using BGP", RFC 9552,
DOI 10.17487/RFC9552, January 2024,
<https://www.rfc-editor.org/info/rfc9552>.
Authors' Addresses
Changwang Lin
New H3C Technologies
Email: linchangwang.04414@h3c.com
Weiqiang Cheng
China Mobile
Email: chengweiqiang@chinamobile.com
Zafar Ali
Cisco Systems, Inc
Email: zali@cisco.com
Aravind Babu MahendraBabu
Cisco Systems, Inc
Email: aramahen@cisco.com
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Ran Chen
ZTE Corporation
Nanjing
China
Email: chen.ran@zte.com.cn
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