Transcript IP Fast Reroute with Interface Specific Forwarding Srihari Nelakuditi

IP Fast Reroute with
Interface Specific Forwarding
Srihari Nelakuditi
University of South Carolina, Columbia
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What is Interface Specific Forwarding?
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Interface-independent forwarding
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Interface-specific forwarding
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destination  next-hop
Each line card has a copy of the same FIB
<incoming interface, destination>  next-hop
Different forwarding entries at each line card
Forwarding operation remains the same
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ISF Enables Local Rerouting
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Infer failures based on interface and destination
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Precompute interface-specific forwarding tables
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Find the farthest keylink whose failure would cause a
packet to arrive at the unusual interface along the
reverse shortest path to the destination
Avoid the keylink in choosing next hop for a destination
Failure Inferencing based Fast Rerouting
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IP fast reroute without explicit routing/tunneling
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Illustration: No Failure Scenario
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Illustration: Local Rerouting without ISF
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Illustration: Local Rerouting with ISF
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ISF Table Computation
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Infer failed links from packet’s arrival at an interface
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K dj i keylink whose failure causes packet to d arrive at i from j
A link uv is a candidate keylink if
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K dj i is the farthest one from i among candidate keylinks
Avoid keylink in choosing the destination’s next hop
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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
Fjdi next hops to d from i when packet arrives at i from j
Fjdi  Rid ( E \ K dj i )
Failure inferencing is not done per packet
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ISF table entries computed upon link state updates
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Illustration: ISF Table Computation
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K BC A  {}
K BD A  {}
K BE A  {B-E}
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K BF A  {E-F}
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
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 interfacespecific forwarding tables
Link up after
suppression interval
Link state update
Recompute interfacespecific forwarding tables
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Handling both Link and Node Failures
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Infer keynodes instead of keylinks
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A node u is a candidate keynode if
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Keynode is the farthest one from i among candidates
When no route to destination without a node
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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
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
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FIFR can handle asymmetric link weights
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By forcing packets to take reverse shortest path
Provided links are bidirectional
Keynode computation based on rSPF
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A node u is a candidate keynode if
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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
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FIFR applicable to networks with broadcast links
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Adjacent failures
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A broadcast link is modeled with point to point links
from/to the designated router
Broadcast link failure treated as that of designated router
Non-adjacent failures
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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
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When link weights are symmetric
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When link weights are asymmetric
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Ordered FIB updating compatible with FIFR
Requires stricter ordering than symmetric case
May need destination-specific ordering
Continuous loop-free forwarding with FIFR
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Summary of FIFR
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Fast reroute under any single failures
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May cause loops under multiple failures
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Without changing/encapsulating IP datagram
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
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Two forwarding entries per destination
O(|E|log2|V|) to compute forwarding entries
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FIFR Discussion
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Local repair for multicast
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Contrasting implicit FIFR and explicit not-via
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Encapsulate multicast packet in a unicast packet
addressed to the next hop along the multicast tree
FIFR can not handle partitioning failures
FIFR may have more no routes under SRLG failures
FIFR with one additional bit
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Can protect against single failures
Loop-free under multiple failures
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