Source Selectable Path Diversity via Routing Deflections

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Source Selectable Path Diversity via Routing
Deflections

• 
  Presenter Sean
• Authors Xiaowei Yang
• University of California, Irvine
• xwy_at_ics.uci.edu
• Sigcomm 2006

David Wetherall University of
Washington djw_at_cs.washington.edu
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Deflections

• Refraction?

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Why Routing Deflection
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Why Routing Deflection
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Outlines

• Introduction
• Deflection Rules
• Tag Architecture
• Inter-domain Rules
• Evaluation
• Conclusion

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Outlines

• Introduction
• Deflection Rules
• Tag Architecture
• Inter-domain Rules
• Evaluation
• Conclusion

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Introduction

• Why source routing
• End-system partially or fully specify the paths
  taken by their packets
• Goals improve the reliability and performance of
  networks
• Lower latency
• Greater bandwidth availability
• Failure mask
• How provides path diversity, reduce the
  dependence on a single network path with
  undesirable characteristics
• Problems with source routing
• Map for preferred routes is not ready
• Conflict with the ISP policy based routing today
• Security threat

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Approach

• Using deflection routing to construct diverse
  path
• Routers forward packets off the shortest path
  when it is not available
• Three deflection rules enabling routers to
  independently deflect packets with loop-free, and
  ISP policies compatible
• Built on top of shortest path routing machinery
• Incrementally deployable
• Tag routing architecture
• Users use tag to express their routing preferred.
  
• ISPs make the routing decision using the Tag as
  the routing selector

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Outlines

• Introduction
• Deflection Rules
• Tag Architecture
• Inter-domain Rules
• Evaluation
• Conclusion

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Deflection Rules

• Deflection set set of neighbors that a router
  can use to reach particular destinations
• Two main concerns for deflection
• Correctness of deflection
• A safety condition loop-free
• A liveness condition reach the destination
• Effectiveness through evaluation
• Notions
• ni
• cost(ni, dest) -gt cost(ni) shortest path cost

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Rule 1 (One Hop Down)
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Rule 1 (One Hop Down)

• Loop free
• Cost after the first hop strictly decrease
• Generalization of shortest path
• Special case ECMP, Equal Cost Multiple Path
• Implementation need to know the cost of
  neighbors
• Distance vector protocol directly achieve
• Link-state protocol need multiple round of
  computation

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Rule 2 (Two Hops Down)
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Rule 2 (Two Hops Down)

• Loop free
• Still a generalization of shortest path
• Cost strictly decrease every two hops
• No two-node sequence can repeat, no link-level
  loop
• One node can be visited more than once
• Implementation
• Overload is slighter higher than Rule 1.
• Know the costs of neighbors incoming link
  information

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Rule 3 (Two Hops Forward)
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Rule 3 (Two Hops Forward)

• Loop free
• Eliminate the immediate backtracking.
• More flexible than Rule 2
• Implementation
• Implementation complexity is slight increase
  comparing with Rule 2.
• Need to compute costs for nodes neighbors in
  G\li

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Outlines

• Introduction
• Deflection Rules
• Tag Architecture
• Inter-domain Rules
• Evaluation
• Conclusion

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Tag Architecture

• Each packet carries a tag that determines the
  path it takes through the network from the
  present location to the destination
• Tag properties
• Tags must be consistent in their path selections
  to the same extent as existing Internet routes
• Tags are opaque and lack global meaning except
  that we require a value of zero to correspond to
  the default Internet path
• Different tags should select a diverse set of
  network paths.
• Two approach
• Shim encoding
• IP tag encoding

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Shim Encoding
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IP Header and IP Tag Encoding
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IP Tag Encoding

• Use IP identifier to encode the tag
• Use rarely used portion of the TTL space as the
  indication to use the tag selection
• Internet paths rarely exceed 40 hops
• Common initial TTL values 30, 32, 60, 64, 128,
  255
• Rarely used TTL space 128 215
• Modification 160-200-gtuse tag,
• 200 entire path
• gt200 only the end of path
• 160ltTTLlt200 only the beginning of the path
• Other tag selection turn off
• True incremental deployment
• Disadvantages
• Not completely backwards-compatible
• Reroute when TTLs within the tag selection range
• Traceroute cannot be used
• Not compatible with other proposals with IP ID
  overloaded

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Mapping Tags to Deflections

• Pick up the selection set S by a rule, S K
• Number pseudo-randomly from 0 to K
• 0 default shortest cost neighbor
• Choose next-hop pseudo-randomly
• Pseudo-randomly choose a small prime number P gt K
• Tag value T
• Selection N (T mod P) mod K
• Outer mod produce a number in the right range
• Inner mod produce further degree of freedom
  (valuable)

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Outlines

• Introduction
• Deflection Rules
• Tag Architecture
• Inter-domain Rules
• Evaluation
• Conclusion

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Inter-domain Rules

• Deflection built on top of BGP and IGP
• Cost is decided based on BGP and IGP
• Problems deflection can change the default
  egress point and hence cost metric for a dest may
  change unexpectedly when the packet is deflected
• Solution extending the cost function
• cost(n, dest) -gt cost(n, nexthop(n, dest))
• Benefits

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Outlines

• Introduction
• Deflection Rules
• Tag Architecture
• Inter-domain Rules
• Evaluation
• Conclusion

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Evaluation

• Goal Simulate the tag architecture and
  deflection rules to characterize the kinds of
  path diversity that they provide
• Desirable results a high degree of path
  diversity is desirable to increase the ability of
  a source to avoid faulty links or nodes on their
  default paths
• Characterize path diversity in three respects
• The deflection paths that exist between
  particular source and destination nodes
• The ability to route around particular nodes or
  links deemed faulty
• The ability to switch peering points

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Methodology

• Custom simulator
• Three input topologies
• Real network (Abilene and GEANT) (small)
• Measured ISP topologies from Rocketfuel (Sprint,
  Ebone, Tiscali, Exodus and Abovenet) (larger)
• Topologies randomly generated with Brite
  power-law, uniform degree distribution
• Barabasi Albert (BA)
• Waxman model (Waxman)
• Three output metrics
• The number of neighbors in the deflection set at
  each router
• The fraction of shortest paths that can be
  re-routed to by-pass a faulty link or node
• How often a source can arrive at an egress that
  is not its lowest-cost exit

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of Deflection Neighbors
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of Deflection Path
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Node Difference
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of Faulty-Avoiding Node Pairs
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Faction of Peer-Switching Nodes
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of Tags to Bypass A Fault
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Outlines

• Introduction
• Deflection Rules
• Tag Architecture
• Inter-domain Rules
• Evaluation
• Conclusion

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Conclusions

• Present a practical tag-based routing system that
  provides the benefits of source-controlled routes
  
• Scalable, compatible with ISP policies, and
  easily incrementally deployable.
• Provide a high-level of path diversity
• Results show that a source can avoid most single
  node of link faults by trying only a handful of
  tags, with better results for larger networks
• The most interesting part is defining the routing
  deflections rules
• Loop-free (safety)
• Destination reachable (liveness)

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Questions

• Make use of BGP Have the disadvantage of BGP?
• Dynamics of BGP
• Loop again?
• Effect of delayed update of the cost?
• Instability?
• Routing oscillations?
• Convergence time?

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Thanks and Questions
sean_at_wayne.edu and visit http//www.cs.wayne.edu/
sean
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of Deflection Neighbors
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of Deflection Neighbors
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of Deflection Neighbors
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of Deflection Path
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of Deflection Path
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of Deflection Path
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Node Difference
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Node Difference
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Node Difference
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of Faulty-Avoiding Node Pairs
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of Faulty-Avoiding Node Pairs
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of Faulty-Avoiding Node Pairs
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Faction of Peer-Switching Nodes
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Faction of Peer-Switching Nodes
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Faction of Peer-Switching Nodes
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of Tags to Bypass A Fault
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of Tags to Bypass A Fault
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of Tags to Bypass A Fault