Validity of SR Policy Candidate Path
draft-chen-spring-sr-policy-cp-validity-06
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| Authors | Ran Chen , Yisong Liu , Ketan Talaulikar , Detao Zhao , Zafar Ali | ||
| Last updated | 2026-02-01 (Latest revision 2025-10-20) | ||
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draft-chen-spring-sr-policy-cp-validity-06
SPRING Working Group R. Chen
Internet-Draft ZTE Corporation
Intended status: Standards Track Y. Liu
Expires: 5 August 2026 China Mobile
K. Talaulikar
Cisco Systems, Inc.
D. Zhao
ZTE Corporation
Z. Ali
Cisco Systems, Inc.
1 February 2026
Validity of SR Policy Candidate Path
draft-chen-spring-sr-policy-cp-validity-06
Abstract
An SR Policy comprises one or more candidate paths (CP) of which at a
given time one and only one may be active (i.e., installed in
forwarding and usable for steering of traffic). Each CP in turn may
have one or more SID-List of which one or more may be active; when
multiple SID-List are active then traffic is load balanced over them.
However, a candidate path is valid when at least one SID-List is
active. This candidate path validity criterion cannot meet the needs
of some scenarios.
This document defines the new candidate path validity criterion.
Status of This Memo
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This Internet-Draft will expire on 5 August 2026.
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Copyright Notice
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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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Validity of a Candidate Path . . . . . . . . . . . . . . . . 3
4. Use Cases for Candidate Path Validity . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
SR Policy architecture are specified in [RFC9256]. An SR Policy
comprises one or more candidate paths (CP) of which at a given time
one and only one may be active (i.e., installed in forwarding and
usable for steering of traffic). Each CP in turn may have one or
more SID-List of which one or more may be active; when multiple SID-
List are active then traffic is load balanced over them. However, a
candidate path is valid when at least one SID-List is active. This
candidate path validity criterion cannot meet the needs of some
scenarios.
This document defines the new candidate path validity criterions
based on [RFC9256]. For the segment list invalidation rules, refer
to [RFC9256] and [I-D.liu-spring-sr-policy-flexible-path-selection].
This document does not change the segment list invalidation rules.
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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. Motivation
The candidate path validity criterion defined in [RFC9256] can't meet
the needs of the following scenarios:
+----------------------+
+---------| SL1(Weight 1, 100MB) |
+----------------+ | +----------------------+
| CP1 (200MB) |------+
+----------------+ | +----------------------+
+---------| SL2(Weight 1, 100MB) |
+----------------------+
Figure 1
The SR Policy POL1 has two candidate paths: CP1 and CP2, and CP1 is
the active candidate path (it is valid and has the highest
Preference). The two segment lists (SL1 and SL2) of CP1 are
installed as the forwarding instantiation of the SR Policy POL1.
Each segment list is assumed to have a maximum capacity of 100Mbps.
CP1 carries a total of 200Mbps of traffic. Within POL1, flow-based
hashing is performed across each SL based on its relative weight.
With an equal weight assigned to each SL, the fraction of flows
steered into each SL is 50%, meaning each SL carries 100 Mbps of
traffic.
At this time, if one of the segment lists is determined to be invalid
by the rule defined in [RFC9256], the remaining Segment List cannot
carry the full 200Mbps of traffic due to its capacity limit.
However, the CP1 remains the active candidate path according to
[RFC9256], as a candidate path is valid as long as it has at least
one valid Segment List.
3. Validity of a Candidate Path
A headend MAY be informed about the validity control parameters of a
candidate path for an SR Policy <Color, Endpoint> by various means
including: via configuration, PCEP, or BGP. The detailed protocol
extension will be described in a separate document.
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This document defines the following validity control parameters under
candidate Path to control the validity judgment of candidate Path:
* Minimum valid SL count: 8-bit value, The value is 1-0xff.
Indicates the minimum number of valid segment Lists under the
active candidate path. When the number of valid segment Lists
under candidate path is greater than or equal to this field, the
candidate path is considered valid.
0xff indicates that the candidate path is considered valid only if
all the segment Lists are valid.
* Minimum Cumulative SL Weight: 32-bit value, The value is
0-0xffffffff.
Indicates the minimum value of the sum of the weights of the valid
segment List under the active candidate Path.
When the sum of the weights of the valid segment Lists under the
candidate path is greater than or equal to this field, the
candidate Path is considered valid.
0 indicates no requirement for weight.
0xffffffff indicates that the candidate path is considered valid
only if all the segment Lists are valid.
Candidate path is considered valid only if both validity control
parameters are satisfied.
4. Use Cases for Candidate Path Validity
The following scenarios illustrate how the validity control
parameters of a candidate path defined in Section 3 address the
capacity and validity issues described in Section 2.
* Minimum valid SL count: Following the scenario in Section 2, where
the aggregate traffic load is 200 Mbps and each SL has a capacity
of 100 Mbps, an operator can configure a "Minimum valid SL count"
of 2. In this case, the candidate path (CP) is rendered invalid
as soon as any single Segment List (SL) becomes invalid. This
prevents the CP from remaining active when its capacity is
insufficient to carry the full traffic load.
* Minimum Cumulative SL Weight: Alternatively, an operator can
assign a weight of 1 to both SL1 and SL2 to reflect their
identical 100 Mbps capacity. By setting the "Minimum Cumulative
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SL Weight" to 2, the operator ensures the candidate path remains
active only when the aggregate capacity meets the 200 Mbps demand.
If one SL becomes invalid, the sum of the weights of the remaining
valid SLs becomes 1, falling below the threshold. Consequently,
CP1 is declared invalid, thus preventing the steering of 200 Mbps
of traffic into a single 100 Mbps link.
5. IANA Considerations
This document makes no request of IANA.
6. Security Considerations
The security considerations of segment routing in [RFC9256] are
applicable to this document.
7. Acknowledgements
The authors would like to thank Joel Halpern, Samuel Sidor ,
Changwang Lin, Alvaro Retana and Imtiyaz Mohammad for their review
and discussion of this document.
8. References
8.1. 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>.
[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>.
8.2. Informative References
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[I-D.liu-spring-sr-policy-flexible-path-selection]
Liu, Y., Lin, C., Peng, S., Chen, R., Ali, Z., Mishra, G.
S., and Y. Qiu, "Flexible Candidate Path Selection of SR
Policy", Work in Progress, Internet-Draft, draft-liu-
spring-sr-policy-flexible-path-selection-13, 21 January
2026, <https://datatracker.ietf.org/doc/html/draft-liu-
spring-sr-policy-flexible-path-selection-13>.
Authors' Addresses
Ran Chen
ZTE Corporation
Nanjing
China
Email: chen.ran@zte.com.cn
Yisong Liu
China Mobile
Beijing
China
Email: liuyisong@chinamobile.com
Ketan Talaulikar
Cisco Systems, Inc.
Email: ketant.ietf@gmail.com
Detao Zhao
ZTE Corporation
Nanjing
China
Email: zhao.detao@zte.com.cn
Zafar Ali
Cisco Systems, Inc.
Email: zali@cisco.com
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