LSR Working Group P. Psenak Internet-Draft J. Horn Intended status: Standards Track L. Ginsberg Expires: 3 January 2025 Cisco Systems 2 July 2024 IGP Reverse Metric Algorithm draft-ppsenak-lsr-igp-reverse-spf-algo-00 Abstract IANA has set up a subregistry called "IGP Algorithm Type" under the "Interior Gateway Protocol (IGP) Parameters" registry. This draft introduces a new algorithm type which utilizes the cost in the reverse direction on each link. This document also discusses using this new algorithm type in combination with IGP Flexible Algorithm to compute constraint-based paths. 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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Expires 3 January 2025 [Page 1] Internet-Draft IGP Reverse Metric Algorithm July 2024 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 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Use Case Example . . . . . . . . . . . . . . . . . . . . . . 3 4. Flexible Algorithm Usage of Reverse Shortest Path First Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 3 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 5.1. IGP Algorithm Types Registry . . . . . . . . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . 4 7. Normative References . . . . . . . . . . . . . . . . . . . . 4 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction Existing IGP Algorithm types as defined in the IANA IGP Algorithm Types registry created by [RFC8665] utilize metrics in the forward direction on each link. But there are use cases (discussed later in this document) where utilizing the metric in the reverse direction on each link is desirable. This document defines a new IGP Algorithm type which uses link metrics in the reverse direction. An IGP Flexible Algorithm as specified in [RFC9350] computes a constraint-based path. It supports various metric types, including native Interior Gateway Protocol (IGP) metric, Min Unidirectional Link Delay, Traffic Engineering Default Metric, all defined in [RFC9350]. In addition, Bandwidth Metric and User defined metrics are defined in [I-D.ietf-lsr-flex-algo-bw-con]. Existing IGP Flexible Algorithm use cases specify an IGP Algorithm Type where all these link metric types are used by Flexible Algorithm in the forward direction, e.g., the direction in which the link is added to the topology during the computation. This document extends IGP Flexible Algorithm to be able to use the IGP Algorithm Type which specifies using the link metric in a reverse direction. Usage of the Reverse Shortest Path First (RSPF) algorithm outside of the IGP Flexible Algorithm is outside of the scope of this draft. Psenak, et al. Expires 3 January 2025 [Page 2] Internet-Draft IGP Reverse Metric Algorithm July 2024 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. 3. Use Case Example A multicast tree is typically established from the receivers (R) towards the multicast source (S). When determining the path for traffic originating from S towards R, a shortest path from R to S is often used. When the metric on all links is symmetric, e.g. equal in both directions, this provides the correct outcome. With the introduction of IGP Flexible Algorithms, additional metrics become available for calculating the shortest path. One commonly employed metric is latency, which often exhibits asymmetry. Multicast traffic is frequently required to take the path with the minimal latency. In the presence of the asymmetric link latency, the usage of the reverse link latency is required for multicast tree calculation. Failure to use the reverse link latency can result in multicast traffic not taking the least latency path across the network. 4. Flexible Algorithm Usage of Reverse Shortest Path First Algorithm Flexible Algorithm Definition Sub-TLV [RFC9350] defines the Calc-Type field, that carries the calculation-type used by the Flexible Algorithm. Values are defined in IANA "IGP Algorithm Types" registry defined under the "Interior Gateway Protocol (IGP) Parameters" registry. We define a new IGP Algorithm Type - Reverse Shortest Path First (SPF) algorithm based on link metric. It is equivalent to the IGP Algorithm Type 0, but the link metric is taken from the reverse direction of the link. If the winning Flexible Algorithm Definition signals the Reverse SPF algorithm calculation-type, defines the metric-type other than IGP metric and such metric is not advertised for the link in the reverse direction in a topology for which the computation is done, such link MUST be pruned from the computation. A metric of value 0 MUST NOT be assumed in such a case. This is the application of the rule (5), as specified in section 13 of [RFC9350], in a reverse direction. Psenak, et al. Expires 3 January 2025 [Page 3] Internet-Draft IGP Reverse Metric Algorithm July 2024 5. IANA Considerations 5.1. IGP Algorithm Types Registry This document defines a new algorithm type in the IANA "IGP Algorithm Types" registry defined under the "Interior Gateway Protocol (IGP) Parameters" registry. The "value" is suggested - to be assigned by IANA. Value Description ----- ------------------------------ 2 Shortest Path First (SPF) algorithm based on reverse link metric. It's equivalent to standard (SPF) algorithm (IGP Algorithm Type 0), but uses the metric from the reverse direction of the link. 6. Security Considerations This document inherits security considerations from the [RFC9350] 7. Normative References [I-D.ietf-lsr-flex-algo-bw-con] Hegde, S., Britto, W., Shetty, R., Decraene, B., Psenak, P., and T. Li, "Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints", Work in Progress, Internet- Draft, draft-ietf-lsr-flex-algo-bw-con-12, 19 May 2024, . [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, . [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, . Psenak, et al. Expires 3 January 2025 [Page 4] Internet-Draft IGP Reverse Metric Algorithm July 2024 [RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI 10.17487/RFC9350, February 2023, . Authors' Addresses Peter Psenak Cisco Systems Apollo Business Center Mlynske nivy 43 82109 Bratislava Slovakia Email: ppsenak@cisco.com Jakub Horn Cisco Systems Milpitas, CA 95035 United States of America Email: jakuhorn@cisco.com Les Ginsberg Cisco Systems Milpitas, CA 95035 United States of America Email: ginsberg@cisco.com Psenak, et al. Expires 3 January 2025 [Page 5]