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Fast Reroute Using Alternative Shortest Path <draft-tian-frr-alt-shortest-path-01.txt>

Fast Reroute Using Alternative Shortest Path <draft-tian-frr-alt-shortest-path-01.txt>. Albert J. Tian, tian@redback.com Naiming Shen, naiming@redback.com. Overview. A new way to calculate repair path using alternative shortest path which provides 100% repair coverage

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Fast Reroute Using Alternative Shortest Path <draft-tian-frr-alt-shortest-path-01.txt>

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  1. Fast Reroute Using Alternative Shortest Path<draft-tian-frr-alt-shortest-path-01.txt> Albert J. Tian, tian@redback.com Naiming Shen, naiming@redback.com

  2. Overview • A new way to calculate repair path using alternative shortest path which provides 100% repair coverage • Proposed an “explicit path with loose segments” model to characterize the repair paths • Asserting that a valid loose segments in a repair path will not be changed by the failure of the protected link or node

  3. Repair Path Termination Point • Link Protection: terminate at nexthop • Node Protection: • If nexthop N is not a primary egress, terminate at the next-nexthop • If nexthop N is a primary egress but is not the only primary egress, terminate at another primary egress • If nexthop N is the only primary egress and there is an alternative egress, terminate at one of the alternative egresses. • If nexthop N is the only egress, terminate at the nexthop and attempt link protection

  4. Repair Path Calculation • Use alternative shortest path as repair path • Link protection: take out the link being protected and recalculate shortest path to nexthop • Node protection: take out the node being protected and recalculate shortest paths to termination points(usually next-nexthops)

  5. Repair Path Implementation • Pure IP solution: • IP TE Route Switched Path (RSP) <draft-shen-ip-te-rsp-01.txt> • MPLS based solutions: • MPLS Source Route with Domain Wide Label <draft-tian-mpls-lsp-source-route-01.txt> • RSVP-TE <RFC3209> • RSVP-TE with loose segment optimization <draft-tian-rsvp-loose-seg-opt-00.txt>

  6. Loose Segments Optimization • General: reduce the number segments and simplify implementation • For unicast traffic protection, when the nexthop is not the only primary egress, the last loose segment can be optimized out • Gave out an algorithm to identify loose segments in an alternative shortest path to simplify repair path implementation

  7. Example Y 1 3 1 I 1 X Z E 1 2

  8. Data Plane • Reroute link protection: • Assign traffic to each link, and create repair path for that link • In case of link failure, switch traffic assigned to the link to repair path • Reroute node protection: • Assign traffic to each next-nexthop, and create repair path for each next-nexthop • In case of node failure, switch traffic assigned to each next-nexthop affected by the node failure to repair paths

  9. Standardization • None

  10. Repair Coverage • 100% repair coverage

  11. Control Plane Complexity Link Protection L is the number of links: • L SPFs needed to compute repair paths for L links • If link metrics are all symmetrical, no additional SPF for loose segment optimization • One additional reverse metric SPF for loose segment optimization if some link metrics are asymmetrical

  12. Control Plane Complexity Node Protection N is the number of nexthops • N SPFs: Repair paths without loose segment optimization • 2*N SPFs: Repair paths with loose segment optimization when link metrics are symmetrical • 3*N SPFs: Repair paths with loose segment optimization when some link metrics are asymmetrical

  13. Data Plane Complexity • Require a mechanism that can support arbitrary repair paths • Simple source routed: MPLS Source Route with Domain Wide Label • RSVP based: • IP TE Route Switched Path (RSP) • RSVP-TE <RFC3209> • RSVP-TE with loose segment optimization

  14. Comparison • Commonalities: • Explicit Path with Loose Segments • Properties of the last loose segment and conditions under which it can be optimized out • Inter-area/inter-domain repair paths • Differences: • Support arbitrary repair path hence 100% coverage • Repair paths terminate at nexthop or next-nexthop resulting in much less repair paths to manage • Can protect multicast traffic(with other extensions) • What can be borrowed: • Handling of LAN and pseudo nodes • Loop free transition

  15. Repair Path Classification • Classification: • SL: Downstream, ECMP • LL/SLL/LSL/SLSL: Tunnel approach • SSL (subset): Uturn • Arbitrary: LSP Source Route, RSVP-TE (possibly with loose segment optimization), IP-TE-RSP • A common abstraction can help separate repair path calculation from implementation

  16. Conclusion • A solution for IP fast reroute that provide 100% coverage • Request to become a working group document

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