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 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 5 August 2026. Chen, et al. Expires 5 August 2026 [Page 1] Internet-Draft Validity of SR Policy Candidate Path February 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 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. 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. Chen, et al. Expires 5 August 2026 [Page 2] Internet-Draft Validity of SR Policy 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. 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 by various means including: via configuration, PCEP, or BGP. The detailed protocol extension will be described in a separate document. Chen, et al. Expires 5 August 2026 [Page 3] Internet-Draft Validity of SR Policy Candidate Path February 2026 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 Chen, et al. Expires 5 August 2026 [Page 4] Internet-Draft Validity of SR Policy Candidate Path February 2026 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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . 8.2. Informative References Chen, et al. Expires 5 August 2026 [Page 5] Internet-Draft Validity of SR Policy Candidate Path February 2026 [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, . 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 Chen, et al. Expires 5 August 2026 [Page 6]