IP Fast Reroute with Interface Specific Forwarding

1
IP Fast Reroute withInterface Specific Forwarding

• Srihari Nelakuditi
• University of South Carolina, Columbia

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What is Interface Specific Forwarding?

• Interface-independent forwarding
• destination ? next-hop
• Each line card has a copy of the same FIB
• Interface-specific forwarding
• ltincoming interface, destinationgt ? next-hop
• Different forwarding entries at each line card
• Forwarding operation remains the same

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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

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Illustration No Failure Scenario
A A
C AE
D A
E E
F E
B B
C C
D D
E B
F B
5
Illustration Local Rerouting without ISF
A A
C A
D A
E A
F A
B B
C C
D D
E B
F B
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Illustration Local Rerouting with ISF
A A
C A
D A
E A
F A
B -
C C
D D
E C
F D
B B
C C
D D
E B
F B
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ISF Table Computation

• Infer failed links from packets 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 destinations 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

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Illustration ISF Table Computation
B -
C C
D D
E C
F D
B B
C -
D D
E B
F B
B B
C C
D -
E B
F B
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
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Operations under FIFR
Event Adjacent nodes Other nodes
Packet arrival Interface-specific forwarding Interface-specific forwarding
Link down Initiate local rerouting
Link up before suppression interval Resume forwarding on the recovered link
Link down beyond suppression interval Link state update Recompute interface-specific forwarding tables
Link up after suppression interval Link state update Recompute interface-specific forwarding tables
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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

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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

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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

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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

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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(Elog2V) to compute forwarding entries

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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