BGP Extensions for Inter-Domain Flexible Algorithm with Segment Routing over IPv6 (SRv6)
draft-gong-idr-inter-domain-srv6-flex-algo-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Liyan Gong , Jie Dong , Changwang Lin , Ran Chen | ||
| Last updated | 2025-10-20 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
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| IESG | IESG state | I-D Exists | |
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draft-gong-idr-inter-domain-srv6-flex-algo-00
Interdomain Routing Working Group L. Gong
Internet-Draft China Mobile
Intended status: Standards Track J. Dong
Expires: 23 April 2026 Huawei Technologies
C. Lin
New H3C Technologies
R. Chen
ZTE Corporation
20 October 2025
BGP Extensions for Inter-Domain Flexible Algorithm with Segment Routing
over IPv6 (SRv6)
draft-gong-idr-inter-domain-srv6-flex-algo-00
Abstract
IGP Flexible Algorithm (Flex-Algo) provides a mechanism for IGP nodes
to compute the best paths according to a set of constraints in both
the topology and computation metrics. Segment Routing over IPv6
(SRv6) can be used as one of the data plane of IGP Flex-Algo.
In SRv6 networks, services may be carried across multiple network
domains which are under the same administration. For some services,
it is expected that the an end-to-end inter-domain path can be
computed according to the same constraints (such as low latency)
defined by Flex-Algo.
This document describes BGP extensions to advertise SRv6 locators as
IPv6 prefixes, together with the associated algorithm across the AS
boundary.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 23 April 2026.
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Copyright Notice
Copyright (c) 2025 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
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Scenario . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Flex-Algo Extended Community . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
IGP Flexible Algorithm (Flex-Algo) [RFC9350] provides a mechanism for
IGP nodes to compute the best paths according to a set of constraints
in the topology and computation metrics. Segment Routing over IPv6
(SRv6) [RFC8986] can be used as one data plane of IGP Flex-Algo.
In SRv6 networks, services may be carried across multiple network
domains which are under the same administration. For some services,
it is expected that the an end-to-end inter-domain path can be
computed according to the same constraints (such as low latency)
defined by Flex-Algo.
This document defines the BGP extensions to advertise SRv6 locators
as IPv6 prefixes, together with the associated algorithm across the
AS boundary.
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2. The Scenario
This section shows a typical scenario of the inter-domain algorithm-
specific IPv6 route distribution in Figure 1. AS1 and AS2 belong to
the same network operator, the service from CE1 to CE2 requires an
inter-domain path from PE1 to PE2 based on specific constraints (such
as low latency). In each domain, the intra-domain low latency path
can be computed based on flexible algorithm. In both AS1 and AS2,
Flex-Algo 128 is defined to compute the best path using the latency
metrics. In AS2, the SRv6 locator of PE2 which is associated with
Flex-Algo 128 is advertised in IGP. To make the SRv6 locator of PE2
reachable in AS1, the SRv6 locator of PE2 is advertised as an IPv6
prefix by ASBR2 to ASBR1 in BGP IPv6 Unicast [RFC2545]. However, the
associated algorithm is not carried in the Update message. When the
BGP route is redestributed into IGP in AS1, the SRv6 locator of PE2
will be treated as a IPv6 prefix without algorithm information. As a
result, the IGP nodes in AS1 will only compute the shortest path to
the SRv6 locator of PE2 using the default IGP metric, then inter-
domain path computed based on Flex-Algo 128 is not possible.
+--------------+ +--------------+
| | | |
| [P1] | | [P3] |
CE1--[PE1] [ASBR1]---[ASBR2] [PE2]--CE2
| [P2] | | [P4] |
| | | |
+--------------+ +--------------+
AS1 AS2
Flex-Algo 128 Flex-Algo 128
Metric-type: latency Metric-type: latency
Figure 1. Inter-domain Flex-Algo specific route distribution
One possible approach to achieve inter-domain algorithm-specific path
computation is to configure route-policy on ASBR1 to apply Flex-Algo
128 to specific prefixes received from ASBR2. This requires manual
configuration of route-policy for each prefix which is algorithm-
specific SRv6 locators, hus it is complex and error-prone. A
preferable approach is to add the associated algorithm information in
the BGP routes advertised across the AS boundary, so that when the
BGP route is redistributed into IGP, the associated algorithm could
be obtained automatically.
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3. Flex-Algo Extended Community
A new extended community is defined to carry the algorithm
information associated with the prefix carried in the IPv6 Unicast
NLRI. The Flex-Algo extended community is a transitive Opaque
Extended Community which is used to carry the Algorithm ID associated
with the IPv6 prefixes advertised in BGP Update message. The
encoding of the Flex-Algo Extended Community is shown as below:
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 High =0x03| Type Low =TBD | Flags | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value of the extended Type field is 0x03, which indicates it is a
transitive opaque extended community. The value of the low-order
octet of the extended Type field is TBA.
Flags: 1 octet, carries the flags related to an Algorithm. No flag
is defined in this document.
Algorithm: 1 octet, it can be the algorithm as defined in the "IGP
Algorithm Types" registry [RFC8665], or Flexible Algorithms as
defined in [RFC9350].
Reserved: 4-octet reserved for future use.
4. IANA Considerations
IANA is requested to assign a code point for Flex-Algo Extended
Community from the "Transitive Opaque Extended Community Sub-Types"
subregistry of the "Border Gateway Protocol (BGP) Extended
Communities" registry:
Sub-type Value Name Reference
----------------------------------------------------------------
TBA Flex-Algo Extended Community This document
5. Security Considerations
This document does not introduce any additional security
considerations than those described in [RFC4271] and [RFC4272].
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6. Acknowledgements
The authors would like to thank XXX for the review and valuable
discussion.
7. References
7.1. Normative References
[RFC2545] Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
Extensions for IPv6 Inter-Domain Routing", RFC 2545,
DOI 10.17487/RFC2545, March 1999,
<https://www.rfc-editor.org/info/rfc2545>.
[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>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[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>.
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
DOI 10.17487/RFC9350, February 2023,
<https://www.rfc-editor.org/info/rfc9350>.
7.2. Informative References
Authors' Addresses
Liyan Gong
China Mobile
China
Email: gongliyan@chinamobile.com
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Jie Dong
Huawei Technologies
China
Email: jie.dong@huawei.com
Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Ran Chen
ZTE Corporation
China
Email: chen.ran@zte.com.cn
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