The Impact of Policy and Topology on Internet Routing Convergence NANOG 20 October 23, 2000

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The Impact of Policy and Topology on Internet
Routing ConvergenceNANOG 20October 23, 2000
Abha Ahuja InterNap ahuja_at_umich.edu
Craig Labovitz Microsoft Research labovit_at_microsof
t.com
In collaboration with Roger Wattenhofer,
Srinivasan Venkatachary, Madan Musuvathi
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Background

• In NANOG 19, we showed BGP exhibits poor
  convergence behavior
• Measured convergence times of up to 20 minutes
  for BGP path changes/failures
• Factorial (N!) theoretic upper bound on BGP
  convergence complexity (explore all paths of all
  possible lengths)
• Open question In practice, what topological and
  policy factors impact convergence delay ?

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This Talk

• Goal Understand BGP convergence behavior under
  real topologies/policies
• Given a physical topology and ISP policies, can
  we estimate the time required for convergence?
• Do convergence behaviors of ISPs differ?
• How does steady-state topology compare to paths
  explored during failure?
• Can we change policies/topology to improve BGP
  convergence times?

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Experiments

• Analyzed secondary paths between between 20
  source/destination AS pairs
• Inject and monitor BGP faults
• Survey providers to determine policies behind
  paths
• To provide intuition, we will focus on faults
  injected into three ISPs at Mae-West
• Observed faults via fourth ISP (in Japan)
• Three ISPs roughly map onto tier1, tier2, tier3
  providers
• Results from these three ISPs representative of
  all data

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Comparing ISP Convergence Latencies

• CDF of faults injected into three Mae-West
  providers and observed at Japanese ISP
• Significant variations between providers
• Not related to geography

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Observed Fault Injection Topologies
ISP 4
MAE-WEST

• In steady-state, topologies between ISP1, ISP2,
  ISP3 similar all direct BGP peers of ISP4. Does
  not explain variation on previous slide

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Factors Impacting BGP Propagation

• Topology and policy impact graph (usually DAG)
• Each AS router adds between 0-45 seconds of
  MinRouteAdver Delay
• iBGP/Route Reflector
• MinRouteAdver and path race conditions affect
  which routes chosen as backup routes

iBGP
D
C
B
A
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ISP1-ISP4 Paths During Failure
ISP 4
Steady State
FAULT
R1
ISP 1

• Only one back up path (length 3)

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ISP2-ISP4 Paths During Failure
ISP 4
Steady State
FAULT
R2
ISP 2
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ISP3-ISP4 Paths During Failure
ISP 4
Steady State
FAULT
R3
ISP 3
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Why the Different Levels of Complexity?

• Provider relationship taxonomy
• Transit relationships
• customer/provider
• customer sends their customer routes
• provider sends default-free routing info (or
  default)
• Peer relationships
• Bilateral exchange of customer routes
• Back-up transit
• peer relationship becomes transit relationship
  based on failure
• These relationships constrain topology (no N!
  states) and determine number of possible backup
  paths

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Convergence in the Real World
3
customer
peer
2
1
4
X
5
Longest path 3 4 5 2 1
Possible paths for node 3 2 1 x 4 2 1 x (4 5 2
1 x)
Possible paths for node 4 2 1 x 3 2 1 x 5 2 1 x
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Convergence in the Real World
Hierarchy eliminates some states
3
customer
peer
2
1
4
X
5
Tier 1?
Longest path 3 4 5 2 1
Possible paths for node 3 2 1 x 4 5 2 1 x
Possible paths for node 4 3 2 1 x 5 2 1 x
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Policy and Convergence

• Strict hierarchical relationships eliminate
  exploring some extra states
• Policy controls the number of possible paths to
  explore.
• But turns out the number of paths does not matter

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Relationship Between Backup Paths and Convergence
Longest Observed ASPath Between AS Pair

• Convergence related to length longest possible
  backup ASPath between two nodes

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So, what does all of this mean for convergence
time?

• Convergence time is related to the length of the
  longest path that needs to be explored
• Before fail-over, need to withdraw all
  alternative paths
• This is bounded O(n) by length of the longest
  alternative path in the system
• This longest path is related to policy

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Towards Millisecond BGP Convergence

• Three possible solutions
• Entirely new protocol
• Turn off MinRouteAdver timer
• Tag BGP updates
• Provide hint so nodes can detect bogus state
  information

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Further Information
C. Labovitz, R. Wattenhofer, A. Ahuja, S.
Venkatachary, The Impact of Topology and Policy
on Delayed Internet Routing Convergence. MSR
Technical Report (number pending). June,
2000. C. Labovitz, A. Ahuja, A. Bose, F.
Jahanian, Internet Delayed Routing Convergence.
To appear in Proceedings of ACM SIGCOMM. August,
2000. Send email to ipma-support_at_merit.edu for
more information or to participate in the policy
survey