Network Working Group S. M. K. Varigonda Internet-Draft Samsung R&D Bangalore Intended status: Informational V. R. Vallem Expires: 12 November 2025 Tejas Networks 11 May 2025 OSPF Extensions for Precomputed Fast Reroute draft-kumarvarigonda-ospf-precomputed-frr-00 Abstract This document proposes an enhancement to OSPF (Open Shortest Path First) that enables routers to precompute backup loop-free alternate (LFA) paths for fast reroute (FRR) in case of primary path failures. This mechanism improves convergence time and network stability by eliminating the delay associated with on-demand SPF recalculation during failure events. 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 12 November 2025. 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 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. Varigonda & Vallem Expires 12 November 2025 [Page 1] Internet-Draft OSPF FRR May 2025 Table of Contents 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Design Overview . . . . . . . . . . . . . . . . . . . . . . . 2 3. Detailed Use Case and Diagram . . . . . . . . . . . . . . . . 2 4. Precomputation Process . . . . . . . . . . . . . . . . . . . 3 5. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 3 6. Proposed Opaque LSA Format: Precomputed Backup Path Advertisement . . . . . . . . . . . . . . . . . . . . . . 4 7. Router Information Opaque LSA . . . . . . . . . . . . . . . . 4 8. Comparison with Existing Mechanisms . . . . . . . . . . . . . 4 9. Interaction with BFD . . . . . . . . . . . . . . . . . . . . 5 10. Scalability Considerations . . . . . . . . . . . . . . . . . 5 11. Security Considerations . . . . . . . . . . . . . . . . . . . 5 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 5 15. Appendix A: Example CLI Behavior . . . . . . . . . . . . . . 5 16. Appendix B: Future Work . . . . . . . . . . . . . . . . . . . 6 17. Appendix C: Change Log . . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Terminology SPT - Shortest Path Tree FRR - Fast Reroute LFA - Loop-Free Alternate iSPF - Incremental SPF 2. Design Overview Each OSPF router computes its primary SPT as per standard operation. In addition, the router precomputes alternate paths to selected destinations, simulating failure scenarios (e.g., primary next-hop failure). These alternate paths are validated for loop-freeness using LFA criteria and stored in a local cache for immediate use. 3. Detailed Use Case and Diagram Consider a simple OSPF topology where R1 is connected to R2, R3, and R4. The destination prefix 10.0.0.0/24 is reachable via R2 under normal conditions. The enhancement involves precomputing a backup path via R3. Varigonda & Vallem Expires 12 November 2025 [Page 2] Internet-Draft OSPF FRR May 2025 +-----+ +-----+ | R2 |--------| R5 | +-----+ +-----+ / \ / \ +-----+ +-----+ | R1 |--------------------| R3 | +-----+ +-----+ \ / \ / +-----+ +-----+ | R4 |--------| R6 | +-----+ +-----+ Steps: 1. R1 computes shortest path to 10.0.0.0/24 via R2. 2. R1 simulates failure of R1–R2 link and recomputes SPF excluding that link. 3. R3 is validated as an LFA-capable neighbor and cached as a backup next-hop. 4. On failure detection (e.g., via BFD), R1 switches to the cached path via R3. 4. Precomputation Process For each destination: a. Identify alternate neighbors not used in the primary path. b. Simulate primary link failure. c. Run SPF on the altered topology. d. Validate for loop-freeness. e. Store valid backup path in a local cache. 5. Protocol Extensions No OSPF LSA format changes are required. Optionally, a new Router Information LSA TLV may advertise precomputed FRR capability. Varigonda & Vallem Expires 12 November 2025 [Page 3] Internet-Draft OSPF FRR May 2025 6. Proposed Opaque LSA Format: Precomputed Backup Path Advertisement In implementations where it is desirable to advertise backup paths to neighbors or controllers, an Opaque LSA format may be introduced. TLV Type: TBD2 Value: List of destinations and corresponding backup next-hops 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=TBD2 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Prefix (IPv4/IPv6) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Backup Next-Hop Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 7. Router Information Opaque LSA A TLV may indicate support for precomputed backup paths. TLV Type: TBD1 Length: 1 byte Value: 0x01 indicates support 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=TBD1 | Length=1 | Value=0x01 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 8. Comparison with Existing Mechanisms - OSPF LFA (RFC 5286): on-the-fly LFA, not proactive - NotVia (RFC 6981): complex, encapsulation required - TI-LFA (RFC 8665): segment-routing-based, not applicable to classic OSPF - Proposed: simple, proactive, no encapsulation or SR dependencies Varigonda & Vallem Expires 12 November 2025 [Page 4] Internet-Draft OSPF FRR May 2025 9. Interaction with BFD Bidirectional Forwarding Detection (BFD) can be used in conjunction with this proposal to detect failures quickly. Upon BFD session failure, the router can immediately switch to the precomputed alternate path. 10. Scalability Considerations The number of precomputed backup entries should be limited to avoid overwhelming memory. Incremental SPF (iSPF) techniques SHOULD be used. Alternate paths SHOULD be computed selectively, and aging mechanisms MAY be applied. 11. Security Considerations This mechanism does not introduce new protocol messages and inherits OSPF's security properties. However, LSAs used in precomputation MUST be validated to prevent path manipulation by attackers. Authentication and origin validation are RECOMMENDED. 12. IANA Considerations This document makes no requests of IANA at this time. 13. Acknowledgements The authors thank members of the IETF OSPF Working Group for their feedback and suggestions on earlier versions of this draft. 14. Contributors Srinivasa Mohan Kumar Varigonda Veerendranatha Reddy Vallem 15. Appendix A: Example CLI Behavior router ospf 1 fast-reroute precompute backup-paths enable timers lfa reevaluation 300 Varigonda & Vallem Expires 12 November 2025 [Page 5] Internet-Draft OSPF FRR May 2025 16. Appendix B: Future Work - Support for SR-MPLS and SRv6 backup path precomputation. - Incorporation of topology constraints in alternate path selection. 17. Appendix C: Change Log draft-kumarvarigonda-ospf-precomputed-frr-00 - Initial version including use case, diagram, and precomputation process. - Added example Opaque LSA encoding and scalability discussion. - Included proposed backup path advertisement format. - Added comparison with existing mechanisms section. Authors' Addresses Srinivasa Mohan Kumar Varigonda Samsung R&D Bangalore Email: sri.mohan@samsung.com Veerendranatha Reddy Vallem Tejas Networks Email: vallemr@tejasnetworks.com Varigonda & Vallem Expires 12 November 2025 [Page 6]