BGP Flow-Spec Redirect to SR Segment List Action
draft-ll-idr-flowspec-redirect-sidlist-01
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Yao Liu , Changwang Lin , Shengnan Yue | ||
| Last updated | 2026-06-16 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
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| Stream | Stream state | (No stream defined) | |
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draft-ll-idr-flowspec-redirect-sidlist-01
Network Working Group Y. Liu
Internet-Draft ZTE Corporation
Intended status: Standards Track C. Lin
Expires: 18 December 2026 New H3C Technologies
S. Yue
China Mobile
16 June 2026
BGP Flow-Spec Redirect to SR Segment List Action
draft-ll-idr-flowspec-redirect-sidlist-01
Abstract
BGP Flow Specification (Flow-spec) provides a mechanism for
distributing traffic filtering and policy-based forwarding rules.
Existing works enables traffic steering into an SR Policy. However,
in Artificial Intelligence (AI) network scenarios, "elephant flows"
may require deterministic forwarding over specific segment lists
within an SR Policy candidate path.
This document specifies a new BGP Flow-spec Redirect action that
identifies a specific segment list within 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
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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 18 December 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Redirect to Segment List Action . . . . . . . . . . . . . . . 4
2.1. Action SubTLV Format . . . . . . . . . . . . . . . . . . 4
2.1.1. Flags Field . . . . . . . . . . . . . . . . . . . . . 5
2.1.2. SR Policy Identification . . . . . . . . . . . . . . 5
2.1.3. Candidate Path Identification . . . . . . . . . . . . 6
2.1.4. Segment List Identification . . . . . . . . . . . . . 6
3. Operational Considerations . . . . . . . . . . . . . . . . . 6
3.1. Validation and Installation . . . . . . . . . . . . . . . 6
3.2. Fallback Behavior . . . . . . . . . . . . . . . . . . . . 7
3.3. Recovery Behavior . . . . . . . . . . . . . . . . . . . . 7
4. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
8. Informative References . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
BGP Flow Specification (Flow-spec) [RFC8955] [RFC8956] is an
extension to BGP that allows for the dissemination of traffic flow
specification rules. BGP Flow-spec Version 2 (FSv2) is defined in
[I-D.ietf-idr-flowspec-v2].
[RFC9256] details the concepts of Segment Routing (SR) Policy and
steering into an SR Policy. An SR Policy consists of one or more
candidate paths, each comprising one or more segment lists. As
specified in [RFC9256], if a set of segment lists is associated with
the active path of the policy, then the steering is per flow and
weighted-ECMP (W-ECMP) based according to the relative weight of each
segment list.
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Traffic generated by AI training and sample transmission exhibits
elephant flow characteristics. Compared to ordinary traffic,
elephant flows are characterized by high per-flow bandwidth, and long
duration.
However, the W-ECMP mechanism is fundamentally hash-based: the weight
of each segment list determines the probability that a given flow is
hashed to it. This algorithm is inherently agnostic to the actual
bandwidth or size of individual flows. While this statistical
approach works reasonably for mice flows, it leads to significant
load imbalance in elephant flow scenarios, where a single long-lived
high-bandwidth flow can persistently consume the capacity of the
segment list it happens to be assigned to.
Current BGP-FS supports steering traffic into an SR Policy.
[I-D.ietf-idr-ts-flowspec-srv6-policy] enables steering into an SR
Policy using the address in the Redirect-to-IP action
[I-D.ietf-idr-flowspec-redirect-ip] as the endpoint and the color in
the Color Extended Community [RFC9012] as the color of the target SR
Policy. [I-D.ietf0-idr-srv6-flowspec-path-redirect] proposes a
scheme that steers traffic into an SR Policy through a Binding SID.
[I-D.li-idr-flowspec-sr-policy] defines a FSv2 Redirect to SR Policy
action directly using Policy Color and endpoint as the identifier.
This document extends BGP-FS to allow traffic to be redirected to a
specific segment list within an SR Policy candidate path.
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.
1.2. Terminology
This document uses terminology from [RFC8955], [RFC9256], and
[RFC9857].
* SR Policy: Segment Routing Policy, identified by <headend, color,
endpoint>.
* Candidate Path: A specific path option for an SR Policy, with an
associated preference value.
* Segment List: A specific SID stack that realizes a path; may have
an associated weight for load balancing.
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* Segment List ID: A 32-bit identifier assigned to a segment list,
unique within the scope of a candidate path [RFC9857].
* Elephant Flow: A long-lived, high-bandwidth traffic flow typical
in AI/ML training data transfers and storage replication flows.
2. Redirect to Segment List Action
This action instructs the headend to redirect matching traffic into a
specific segment list of a specific candidate path of an SR Policy.
This action is encoded as a Flow Specification v2 (FSv2) Action
SubTLV carried in a Wide Community container
[I-D.ietf-idr-wide-bgp-communities].
2.1. Action SubTLV Format
The Redirect to Segment List action is encoded as a single FSv2
Action SubTLV.
Sub-Type: TBD1 (2 octets)
Length: 2 octets, indicating the total number of octets of the Value
field.
Value: Variable, formatted as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | AFI | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Color (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Endpoint (4 or 16 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol-Origin (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator( 20octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Discriminator (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Segment List ID (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Redirect to Segment List Action SubTLV Format
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2.1.1. Flags Field
The Flags field is 1 octet and defined as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|F|R| Reserved |
+-+-+-+-+-+-+-+-+
Figure 2
F (Fallback Enabled - 1 bit):
When set (1), the headend MUST perform the fallback procedure
described in Section 3.2 if the designated segment list or candidate
path becomes invalid or ceases to be the active candidate path. When
clear (0), the headend MUST NOT fall back.
R (Recovery - 1 bit):
When set (1), indicates that if the designated segment list later
recovers, the headend SHOULD revert to pinning the traffic flow onto
it. When clear (0), the headend SHOULD NOT automatically revert.
Reserved (6 bits):
MUST be set to 0 on transmission and ignored on receipt.
2.1.2. SR Policy Identification
Color (4 octets):
The Color value of the target SR Policy, an unsigned non-zero 32-bit
integer.
AFI (2 octets):
Address Family Identifier of the Endpoint. Values are taken from
IANA's "Address Family Numbers" registry. This field determines the
length of the Endpoint field:
- 1 (IPv4): Endpoint = 4 octets
- 2 (IPv6): Endpoint = 16 octets
Other AFI values are reserved and MUST NOT be sent; if received, the
entire Action SubTLV MUST be treated as malformed (Section 4).
Endpoint (variable):
The endpoint address of the SR Policy, encoded according to the AFI
field.
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2.1.3. Candidate Path Identification
Protocol-Origin (4 octets):
The Protocol-Origin value of the candidate path. (See [RFC9256]
Section 2.3.)
Originator (20 octets):
The Originator of the candidate path as specified in [RFC9256]
Section 2.4.
Discriminator (4 octets):
The Discriminator of the candidate path, as defined in [RFC9256]
Section 2.5.
2.1.4. Segment List Identification
Segment List ID (4 octets):
The identifier of the segment list within the candidate path. The
value zero is reserved and MUST NOT be used; if received, the Action
SubTLV MUST be treated as malformed.
3. Operational Considerations
3.1. Validation and Installation
Upon receipt of a Flow Specification route containing this Action
SubTLV, the headend MUST perform the following steps in order:
1. Resolve the SR Policy using <Color, Endpoint>.
2. Locate the candidate path within that SR Policy that exactly
matches <Protocol-Origin, Originator, Discriminator>.
3. Verify that the candidate path is the active candidate path of
the SR Policy (as in [RFC9256] Section 2.9) .
4. Locate the segment list within that candidate path whose Segment
List ID matches the value in the SubTLV.
5. Verify that the segment list is valid as per [RFC9256] Section 5.
If any of these steps fail, the Action SubTLV is considered invalid,
and the Flow Specification rule continues to be processed as if this
Action SubTLV were not present. An implementation SHOULD log the
validation failure.
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If all steps succeed, the headend installs a forwarding rule that
redirects matching traffic exclusively to the designated segment
list. Traffic SHOULD NOT be load-balanced to other segment lists
within the same candidate path.
3.2. Fallback Behavior
After successful installation, if the F-Flag is set and any of the
following events occur:
* The designated segment list becomes invalid,
* The designated candidate path becomes invalid,
* The designated candidate path is no longer the active candidate
path of the SR Policy.
Then the headend MUST revert to ordinary SR Policy forwarding
behavior as defined in [RFC9256].
An implementation SHOULD generate an alert to indicate that explicit
path pinning is no longer in effect and the reason for fallback.
3.3. Recovery Behavior
If the R-Flag is set and the designated segment list later recovers
(becomes valid again) while the designated candidate path remains
active, the headend SHOULD revert to pinning the traffic flow onto
the recovered segment list.
An implementation MAY apply a configurable revert timer to avoid
flapping. During the reversion process, traffic MUST NOT be dropped.
The transition MAY be performed via make-before-break.
If the R-Flag is clear, the headend SHOULD NOT automatically revert.
A controller MAY withdraw and re-advertise the Flow Specification
route to re-establish pinning.
4. Error Handling
A Redirect to SR Policy Segment List SubTLV is considered malformed
if:
* The Length field does not match the expected size derived from the
AFI and fixed fields.
* The AFI is not 1 (IPv4) or 2 (IPv6).
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* The Segment List ID is zero.
* The Originator field does not comply with the encoding rules
(e.g., non-zero high-order bits for an IPv4 address).
* The Protocol-Origin field has non-zero high-order 24 bits.
Malformed SubTLVs MUST be handled according to [RFC7606] as treat-as-
withdraw. The implementation SHOULD log the error.
5. IANA Considerations
IANA is requested to assign a new Action Sub-Type value from the "BGP
FSv2 Action types" registry (established by
[I-D.ietf-idr-flowspec-v2]) for the action defined in this document.
Sub-Type Name Reference
--------------------------------------------------------
TBD1 Redirect to SR Policy Segment List [this document]
6. Security Considerations
The security considerations of [RFC8955], [RFC9256], and
[I-D.ietf-idr-flowspec-v2] apply.
The ability to pin traffic to a specific candidate path and segment
list introduces additional risk. A malicious controller could direct
all elephant flows to a single segment list, causing link overload
and congestion collapse. Operators SHOULD:
* Implement per-session policies to restrict which BGP peers are
allowed to send BGP-FS routes with this action.
* Limit the rate of BGP-FS updates to protect headend resources.
7. Normative References
[I-D.ietf-idr-flowspec-v2]
Hares, S., Eastlake, D. E., Yadlapalli, C., and S.
Maduschke, "BGP Flow Specification Version 2", Work in
Progress, Internet-Draft, draft-ietf-idr-flowspec-v2-04,
28 April 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-idr-flowspec-v2-04>.
[I-D.ietf-idr-wide-bgp-communities]
Raszuk, R., Haas, J., Lange, A., Decraene, B., Amante, S.,
and P. Jakma, "BGP Community Container Attribute", Work in
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Progress, Internet-Draft, draft-ietf-idr-wide-bgp-
communities-12, 17 March 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
wide-bgp-communities-12>.
[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>.
[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, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8955] Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
Bacher, "Dissemination of Flow Specification Rules",
RFC 8955, DOI 10.17487/RFC8955, December 2020,
<https://www.rfc-editor.org/info/rfc8955>.
[RFC8956] Loibl, C., Ed., Raszuk, R., Ed., and S. Hares, Ed.,
"Dissemination of Flow Specification Rules for IPv6",
RFC 8956, DOI 10.17487/RFC8956, December 2020,
<https://www.rfc-editor.org/info/rfc8956>.
[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012,
DOI 10.17487/RFC9012, April 2021,
<https://www.rfc-editor.org/info/rfc9012>.
[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>.
8. Informative References
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[I-D.ietf0-idr-srv6-flowspec-path-redirect]
Van de Velde, G., Patel, K., Li, Z., Zhuang, S., and H.
Chen, "Flowspec Indirection-id Redirect for SRv6", Work in
Progress, Internet-Draft, draft-ietf0-idr-srv6-flowspec-
path-redirect-15, 19 March 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf0-idr-
srv6-flowspec-path-redirect-15>.
[I-D.ietf-idr-flowspec-redirect-ip]
Haas, J., Henderickx, W., and A. Simpson, "BGP Flow-Spec
Redirect-to-IP Action", Work in Progress, Internet-Draft,
draft-ietf-idr-flowspec-redirect-ip-16, 21 May 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
flowspec-redirect-ip-16>.
[I-D.ietf-idr-ts-flowspec-srv6-policy]
Wenying, J., Liu, Y., Zhuang, S., Mishra, G. S., and S.
Chen, "Traffic Steering using BGP FlowSpec with SR
Policy", Work in Progress, Internet-Draft, draft-ietf-idr-
ts-flowspec-srv6-policy-10, 18 March 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-ts-
flowspec-srv6-policy-10>.
[I-D.li-idr-flowspec-sr-policy]
Li, Z. and liusong, "BGP Flowspec Redirects to SR Policy",
Work in Progress, Internet-Draft, draft-li-idr-flowspec-
sr-policy-03, 26 February 2026,
<https://datatracker.ietf.org/doc/html/draft-li-idr-
flowspec-sr-policy-03>.
Authors' Addresses
Yao Liu
ZTE Corporation
Email: liu.yao71@zte.com.cn
Changwang Lin
New H3C Technologies
Email: linchangwang.04414@h3c.com
Shengnan Yue
China Mobile
Email: yueshengnan@chinamobile.com
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