A Study of Multiple IP Link Failure

1
A Study of Multiple IP Link Failure

• Fang Yu
• fyu_at_eecs.berkeley.edu

2
Motivation

• Baltimore Tunnel Fire 18 July 2001
• Keynote Systems says the July 19 Internet
  slowdown was not caused by the spreading of Code
  Red. Rather, a train wreck in a Baltimore tunnel
  that knocked out a major UUNet cable caused it.
• The fire severed two OC-192 links between
  Vienna, VA and New York, NY as well as an OC-48
  link from, D.C. to Chicago. Metromedia routed
  traffic around the fiber break, relying heavily
  on switching centers in Chicago, Dallas, and
  D.C.
• Traffic slowdowns were also seen in Seattle, Los
  Angeles and Atlanta, possibly resulting from
  re-routing around the affected
• The accident caused certain connections 10 times
  slower than normal, such as the ones between
  Washington, D.C., and San Diego backbones.

R. Katz, CS294-3 Distributed Service
Architectures in Converged Networks
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Why Multiple IP Link Failure?
IP Layer
Multiple Link Failure Does Occur!
J. Strand Optical Network with Modification
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Methodology

• Use SSFNet network simulator with OSPF v2
  extension (RFC 2328)
• Use a 24-node 54-link IP network from SSFNet
• Evaluation matrix
• Convergence Time
• Number of Route Changes
• Loop
• record the duration of each loop
• sum up as total loop time
• Invalid Routes
• record the duration of the each route containing
  failed link
• sum up as total Invalid route time
• Unreachable Routes
• Record the total number of routes failed due to
  network partitioning

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A Case Study of Two-Link Failure

• Two links fail simultaneously from time 50s to
  150s.
• OSPF detects the first link failure at time 81.03
  second, converges at time 86.04 second
• OSPF detects the first link up at time 156.08
  second, converges at time 161.1 second

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LSA Update Messages and Route Pathology Caused at
Link Down Time
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OSPF Convergence Time and Number of Route Change

• There are dramatic differences between failure
  cases
• 3-link failure converges slower than node failure
  although node failure brings down an average of
  4.5 links!
• Neighbor node has some what synchronized clock ?
  detect multiple link failure almost at the same
  time
• 55 cases, node failure generates less route
  changes than multiple failure cases

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Route Pathology

• Node failure causes a lot of loop routes
• Multiple Link failure cause more invalid routes

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Summary of Observations

• Multiple link failure is more problematic than
  node failure
• Each node failure will bring down
• An average of 4.5 links
• All the connections originated from it or
  destined to it.
• 2-link failure and 3-link failure cause
• average 1/2 or 2/3 less link failures than a node
  failure
• 55 of the cases, it will create more route
  changes and 10 times more invalid routes during
  OSPF re-route time.
• Different combinations of multiple link failure
  have dramatic different impact on OSPF
• Reason The multiple link failure wont bring
  down nodes, so OSPF has to re-route a larger
  number of connections compared to node failure

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Future Work

• Study on the real IP network
• E.g. ATT Common IP Backbone
• Study of correlated IP link failure based on
  optical topology
• IP link failure are not randomly correlated
• A Fiber cut will cause more link failures
• Propose multi-layer routing scheme to effectively
  deploy IP layer on Optical network
• Avoid severe multiple IP link failure scenarios
• Minimize the re-route duration and route
  pathology under failure