The Problem with BGP

1
The Problem with BGP

• Craig Labovitz, Abha Ahuja,
• Abhijit Abose, Farnam Jahanian

2
At Last NANOG(http//www.nanog.org/mtg-9910/conve
rge.html)

• Presented experimental results from two year
  study which measured 150,000 BGP faults injected
  into peering sessions at several IXPs
• Found
• Internet averages 3 minutes to converge after
  failover
• Some multihomed failovers (short to long ASPath)
  require 15 minutes

3
BGP Convergence Times
4
End-to-End Impact Failover
5
The Problem with Distance Vector

• Distance vector protocols (e.g. RIP) suffer
  routing table loops
• Counting-to-infinity
• Routing table loops
• Bouncing problem
• BGP uses path vector to solve problems seen
  with RIP and other Bellman-Ford derived protocols

6
Counting to Infinity
B
2
A
1
R
A 2 R 1
B 2 R 123
235
R 527
R 729
7
Taming Infinity

• RIP solved counting to infinity problem by
  re-defining infinity.
• Added speedups poison reverse, split horizon,
  triggered updates.
• Strictly increasing O(N)
• ASPath limits infinity to the width of the
  Internet (an ASPath through all your neighbors)
• Monotonically increasing
• Upper bound?

8
Convergence Example
9
N gt 4?
AS6453
AS2497
6453 1239 5696 237
AS6113
2497 5696 237
6113 2914 237
AS6461
6461 5696 237
AS1239
1239 5696 237
AS5696
5696 237
AS2914
2914 237
AS237
237
AS701
701 6461 5696 237
AS5000
5000 237
AS1
AS1673
1 5696 237
1673 5696 237
10
The Problem with BGP

• If we assume
• unbounded delay on BGP processing and propagation
• Full BGP mesh BGP peers
• Constrained shortest path first selection
  algorithm
• BGP is O(N!), where N number of default-free BGP
  speakers

There exists possible ordering of messages such
that BGP will explore all possible ASPaths of all
possible lengths
11
BGP and RIP

• RIP precisely monotonically increasing. Can
  explore metrics (1N)
• BGP monotonically increasing. Multiple (N!) ways
  to represent a path metric of N.
• BGP solved RIP routing table loop problem by
  making it exponentially worse

2117 5696 2129 2117 1 5696 2129 2117 2041 3508
3508 4540 7037 1239 5696 2129 2117 1 2041 3508
3508 4540 7037 1239 5696 2129 2117 2041 3508 3508
4540 7037 1239 6113 5696 2129 2117 1 2041 3508
3508 4540 7037 1239 6113 5696 2129
12
BGP Best Case

• What is the best we can expect from BGP?
• Implementation of MinRouteAdver timer leads to 30
  second rounds
• Time complexity is O(n-3)30 seconds
• State/Computational complexity O(n)
• At its best, BGP performs as well as RIP2 (but
  uses exponentially more memory in the process)

13
MinRouteAdver

• Minimum interval between successive updates sent
  to a peer for a given prefix
• Allow for greater efficiency/packing of updates
• Rate throttle
• Applied only to announcements (at least according
  to BGP RFC)
• Applied on (prefix destination, peer) basis, but
  implemented on (peer) basis

14
MinRouteAdver

• 30(N-3) delay due to creation mutual
  dependencies. Provide proof that N-3 rounds
  necessarily created during bounded BGP
  MinRouteAdver convergence
• Rounds due to
• Ambiguity in the BGP RFC and lack receiver loop
  detection
• Inclusion of BGP withdrawals with MinRouteAdver
  (in violation of RFC)

15
Simulation Results
16
More Info

• Submitted for publication, tech report available
  soon
• http//www.merit.edu/ipma