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Fast Reroute Using Alternative Shortest Path
<draft-tian-frr-alt-shortest-path-01.txt>
Albert J. Tian, [email protected]
Naiming Shen, [email protected]
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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
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Repair Path Termination Point• Link Protection: terminate at nexthop• Node Protection:
a) If nexthop N is not a primary egress, terminate at the next-nexthop
b) If nexthop N is a primary egress but is not the only primary egress, terminate at another primary egress
c) If nexthop N is the only primary egress and there is an alternative egress, terminate at one of the alternative egresses.
d) If nexthop N is the only egress, terminate at the nexthop and attempt link protection
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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)
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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>
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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
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Example
Y
ZX 1
13
E2
1
I 1
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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
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Standardization
• None
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Repair Coverage
• 100% repair coverage
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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
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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
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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
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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
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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
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Conclusion
• A solution for IP fast reroute that provide 100% coverage
• Request to become a working group document