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

• 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

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

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)

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>

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

Example

Y

ZX 1

13

E2

1

I 1

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

Standardization

• None

Repair Coverage

• 100% repair coverage

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

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

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

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

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

Conclusion

• A solution for IP fast reroute that provide 100% coverage

• Request to become a working group document