Error Resilient TCP for High Latency Networks

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Error Resilient TCP for High Latency Networks
Omesh Tickoo Networks Lab Retreat Sep. 20, 2004
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TCP Over Wireless Problems

• TCP Congestion Control
• Loss ? Congestion
• Response ? Reduce sending rate
• Question ? What happens when the loss is not
  congestion related?
• Example Wireless Transmission Errors
• Answer ? TCP Reduces Sending Rate
• Not really good

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Solutions

• Main Goal
• Do not reduce rate on non-congestion losses
• Solution 1
• Differentiate between congestion and transmission
  loss
• Just retransmit on transmission loss
• Not really easy/accurate esp. for long links
• Solution 2 Tweak TCP to either
• Lose very little to transmission errors
• Lose nothing to transmission errors

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Lose Very Little To Transmission Errors

• Transmission errors act on bits
• A bit error in a packet renders the packet
  useless
• So ? The number of bits in a packet determine its
  susceptibility to errors
• Then ? Control the packet size
• Tell me more ? Ask Vijay

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Lose Nothing To Transmission Errors

• Protect each packet by adding redundancy
• Send each packet more than once
• Smarter method ? Use FEC
• Problem ? How much FEC to use?
• Tradeoffs
• Protection v/s Overhead

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How Much Redundancy

• Factor 1 (Bottleneck bandwidth)
• The bottleneck bandwidth is shared by the
  redundancy and Data Packets.
• Data bandwidth decreases with increasing
  Redundancy
• Factor 2 (FEC Bound)
• Effective loss rate decreases with increasing
  redundancy
• TCP Formula ? Throughput increases with
  decreasing loss rate
• Need to find the optimal point

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TCP Bound
BN
BA
0
n-k
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FEC Bound
BN
BB
(1-p)BN
0
n-k
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Optimal Operating Point
BN
BB
Bmax
(1-p)BN
BA
0
n-k
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Prelim Results
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Static FEC SimulationsIssues Identified

• Problem 1
• Window Arrest at low redundancy levels.

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Solution Adaptive MSS

• Change (Reduce) MSS so that FEC is accommodated
  provide initial thrust.

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Static FEC SimulationsIssues Identified

• Problem 2 TCP Timeouts cause low utilization

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Timeouts Burst Losses
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Protection Against Burst Losses Reactive FEC

• Scheme in design
• Use multiple reactive FEC retransmissions in
  response to loss indications.
• Protect against loss of retransmissions
• Encouraging early results