IDR Y. Liu Internet-Draft ZTE Corporation Intended status: Standards Track 5 August 2026 Expires: 6 February 2027 BGP-LS Extension for SR Policy Packet Spray State draft-liu-idr-bgp-ls-sr-policy-spray-state-00 Abstract This document proposes an extension to BGP-LS that allows a headend node, when reporting SR Policy state information via BGP-LS, to carry a new state flag at the candidate path level. This flag indicates whether the candidate path is currently being used for packet spraying. The information can be consumed by external controllers for path optimization, operations management or other purposes. 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/. 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Liu Expires 6 February 2027 [Page 1] Internet-Draft BGP-LS SR Policy Spray Flag August 2026 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2 3. BGP-LS Extensions . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Headend Advertisement Rules . . . . . . . . . . . . . . . 3 3.2. Receiver Processing Rules . . . . . . . . . . . . . . . . 3 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . . . . 4 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 6.1. Normative References . . . . . . . . . . . . . . . . . . 4 6.2. Informative References . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction Segment Routing (SR) architecture provides flexible traffic engineering forwarding mechanisms through SR Policy [RFC9256]. An SR Policy consists of one or more candidate paths, and each candidate path may contain one or more segment lists. As in [RFC9256], only one candidate path is selected as the active path for each SR Policy, and traffic is load-balanced across the multiple segment lists of the active candidate path based on their respective weights. [RFC9857] defines the mechanism for advertising SR Policies via BGP- LS, in which the SR Candidate Path State TLV is used to report the operational status and attributes at the candidate path level. Based on local policy, a headend node may decide to enable packet spraying across the multiple segment lists of an active candidate path. External controllers may need to be aware of this operational state for path optimization, troubleshooting or other purposes. This document extends the SR Candidate Path State TLV [RFC9857] by allocating a new PS-Flag (Packet Spray Flag). This flag is used by the headend to report whether the candidate path is being used for packet spraying. 2. 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. Liu Expires 6 February 2027 [Page 2] Internet-Draft BGP-LS SR Policy Spray Flag August 2026 3. BGP-LS Extensions This document allocates 1 bit from the reserved bits of the Flags field in the SR Candidate Path State TLV (Type 1202) as the PS-Flag (Packet Spray Flag). * When the PS-Flag is set to 1, it indicates that the candidate path is currently being used for packet spraying. The headend has decided, based on local policy, to perform packet spraying across the multiple segment lists of this candidate path. * When the PS-Flag is set to 0, it indicates that the candidate path is currently not being used for packet spraying, or the headend does not support this extension. How the headend decides whether to enable packet spraying on a candidate path is out of the scope of this document. 3.1. Headend Advertisement Rules When advertising SR Policy state via BGP-LS, the headend node SHOULD set the PS-Flag according to the following rules: * *Active candidate path only:* The PS-Flag is meaningful only when the candidate path is currently the active candidate path (i.e., the A-Flag is set). For non-active candidate paths, the PS-Flag SHOULD be set to 0. * *Multiple segment lists only:* If the active candidate path contains only one segment list, packet spraying is not possible, and the PS-Flag SHOULD be set to 0. * If the active candidate path contains multiple segment lists and the headend has enabled packet spraying, the PS-Flag SHOULD be set to 1. 3.2. Receiver Processing Rules Upon receiving a BGP-LS UPDATE containing an SR Candidate Path State TLV, the receiver (e.g., a controller) SHOULD process the PS-Flag according to the following rules: * *Validity check -- A-Flag:* The receiver SHOULD first check the A-Flag of the candidate path. If the A-Flag is 0 (i.e., the candidate path is not active), the receiver SHOULD ignore the PS- Flag. Liu Expires 6 February 2027 [Page 3] Internet-Draft BGP-LS SR Policy Spray Flag August 2026 * *Validity check -- segment list count:* If the number of segment lists is less than 2, the receiver SHOULD ignore the PS-Flag. * If the receiver finds that the PS-Flag is set to 1 but the candidate path's A-Flag is 0 or the segment list count is less than 2, the receiver MAY log the event for trouble shooting purpose. If a headend's spraying state flaps frequently, the PS-Flag will cause corresponding BGP-LS updates to be generated. Implementations SHOULD apply update throttling or dampening mechanisms to avoid excessive BGP-LS update churn. 4. IANA Considerations This document requests a bit in the flag field of "SR Candidate Path State TLV" [RFC9857] under the "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" registry. +=====+==========================+===========+ | Bit | Description | Reference | +=====+==========================+===========+ | TBA | PS-Flag: The CP is being | This | | | used for packet spraying | document | +-----+--------------------------+-----------+ Table 1: New Flag Allocation for SR Candidate Path State TLV 5. Security Considerations Procedures and protocol extensions defined in this document do not affect the security considerations discussed in [RFC9857]. The setting of the PS-Flag may indirectly expose the headend node's local policy information (e.g., whether spraying is enabled). Operators SHOULD assess the risk of such information disclosure according to their own security policies and use appropriate measures to protect such information. 6. References 6.1. 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, . Liu Expires 6 February 2027 [Page 4] Internet-Draft BGP-LS SR Policy Spray Flag August 2026 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . 6.2. Informative References [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, . Author's Address Yao Liu ZTE Corporation Email: liu.yao71@zte.com.cn Liu Expires 6 February 2027 [Page 5]