1. 1 IP Fast Reroute with Interface Specific Forwarding Srihari Nelakuditi University of South Carolina, Columbia

2. 2 What is Interface Specific Forwarding?  Interface-independent forwarding destination  next-hop Each line card has a copy of the same FIB  Interface-specific forwarding  next-hop Different forwarding entries at each line card  Forwarding operation remains the same

3. 3 ISF Enables Local Rerouting  Infer failures based on interface and destination Find the farthest keylink whose failure would cause a packet to arrive at the unusual interface along the reverse shortest path to the destination  Precompute interface-specific forwarding tables Avoid the keylink in choosing next hop for a destination  Failure Inferencing based Fast Rerouting IP fast reroute without explicit routing/tunneling

4. 4 F Illustration: No Failure Scenario BB CC DD EB FB AA CAE DA EE FE F F F F F F

5. 5 Illustration: Local Rerouting without ISF BB CC DD EB FB AA CA DA EA FA F F F

6. 6 Illustration: Local Rerouting with ISF BB CC DD EB FB AA CA DA EA FA F F F B- CC DD EC FD F F F F F

7. 7 ISF Table Computation  Infer failed links from packet ’ s arrival at an interface keylink whose failure causes packet to d arrive at i from j A link uv is a candidate keylink if  with uv, j is a next hop from i to d  without uv, edge ji is along the shortest path from u to d is the farthest one from i among candidate keylinks  Avoid keylink in choosing the destination ’ s next hop next hops to d from i when packet arrives at i from j  Failure inferencing is not done per packet ISF table entries computed upon link state updates

8. 8 Illustration: ISF Table Computation {B-E} {} B- CC DD EC FD {E-F} BB CC D- EB FB BB C- DD EB FB When no more than one link failure is suppressed in a network with symmetric weights, FIFR always forwards successfully to a destination if a path to it exists

9. 9 Operations under FIFR EventAdjacent nodesOther nodes Packet arrivalInterface-specific forwarding Link downInitiate local rerouting Link up before suppression interval Resume forwarding on the recovered link Link down beyond suppression interval Link state updateRecompute interface- specific forwarding tables Link up after suppression interval Link state updateRecompute interface- specific forwarding tables

10. 10 Handling both Link and Node Failures  Infer keynodes instead of keylinks A node u is a candidate keynode if  with u, j is a next hop from i to d  without u, edge ji is along the shortest path from the upstream node of u (w.r.t. the path from i to u) to d Keynode is the farthest one from i among candidates  When no route to destination without a node Node adjacent to the failure assumes link failure Non-adjacent nodes treat it as adjacent node failure May cause loops when destination is indeed not reachable  Protects against non-partitioning single failures

11. 11 Networks with Asymmetric Link Weights  FIFR can handle asymmetric link weights By forcing packets to take reverse shortest path Provided links are bidirectional  Keynode computation based on rSPF A node u is a candidate keynode if  with u, j is a next hop from i to d  without u, edge ij is along the shortest path from d to the upstream node of u (w.r.t the path from i to u) Keynode is the farthest one from i among candidates Works with both symmetric and asymmetric weights

12. 12 Networks with Broadcast Links  FIFR applicable to networks with broadcast links A broadcast link is modeled with point to point links from/to the designated router  Adjacent failures Broadcast link failure treated as that of designated router  Non-adjacent failures Not necessary to know the previous hop of a packet to compute interface-specific keynode per destination Failure inferencing can be done as before

13. 13 Ordered FIB updating  When link weights are symmetric Ordered FIB updating compatible with FIFR  When link weights are asymmetric Requires stricter ordering than symmetric case May need destination-specific ordering  Continuous loop-free forwarding with FIFR

14. 14 Summary of FIFR  Fast reroute under any single failures Without changing/encapsulating IP datagram  May cause loops under multiple failures With ISF, guaranteed-protection against single failures or loop-freedom under multiple failures but not both Blacklist-based Interface Specific Forwarding  Needs interface-specific forwarding Two forwarding entries per destination O(|E|log 2 |V|) to compute forwarding entries

15. 15 FIFR Discussion  Local repair for multicast Encapsulate multicast packet in a unicast packet addressed to the next hop along the multicast tree  Contrasting implicit FIFR and explicit not-via FIFR can not handle partitioning failures FIFR may have more no routes under SRLG failures  FIFR with one additional bit Can protect against single failures Loop-free under multiple failures