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Scalable TCP: Improving Performance in Highspeed Wide Area Networks

Scalable TCP: Improving Performance in Highspeed Wide Area Networks. Paper by Tom Kelly Presented by Christoph Jechlitschek. The Problem. TCP performs badly with very large congestion windows Converges slowly Underutilizes available link capacity

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Scalable TCP: Improving Performance in Highspeed Wide Area Networks

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  1. Scalable TCP: Improving Performance in Highspeed Wide Area Networks Paper by Tom Kelly Presented by Christoph Jechlitschek

  2. The Problem • TCP performs badly with very large congestion windows • Converges slowly • Underutilizes available link capacity • Example: A 1 Gbps flow needs 28 minutes to recover from a single packet loss

  3. TCP congestion window update • cwnd  cwnd + 1/cwnd; - if no loss was detected • cwnd  cwnd/2; - if a loss was detected

  4. TCP congestion window

  5. Scalable TCP congestionwindow update • cwnd  cwnd + 0.01; - if no loss was detected • cwnd  cwnd – 0.125 * cwnd; - if a loss was detected

  6. Scalable TCP congestion window

  7. Analysis • Generalized form of update function: cwnd  cwnd + a; if no loss detected cwnd  cwnd – b * cwnd; if loss detected • What makes a = 0.01 and b = 0.125 a good choice?

  8. Legacy connections • Traditional connections: - are not designed to use large windows - limited amount of buffers • Legacy window size lwnd = 16 • Legacy loss rate pl = 5.86 * 10-3

  9. Response curve • Relates a and b • Choosing one fixes the other

  10. Instantaneous rate variation • Avoid large instantaneous rate variations • Coefficient of variance depends on b only • Therefore b should be as small as possible

  11. Convergence • Converge to a new bound as fast as possible • Large a and b speed up convergence • Contradicts with earlier statement

  12. Decision • Balance variation with converge time • Choose a = 0.01 and b = 0.125

  13. Experiment setup

  14. Gigabit kernel modifications • Increases size of kernel interface queues • Avoids unnecessary packet copying • Removes debug counters

  15. Performance test 1 • 4 sender/receiver pairs transfer 2 GB files • Count number of transfers in 1200 seconds

  16. Performance test 1 results

  17. Performance test 2 • Simulate Web traffic using standard TCP • Background bulk transfer with Scalable TCP

  18. Performance test 2 results

  19. Conclusion • Simple sender-side modifications greatly improved performance • No modification to receiver needed • Negligible effect on standard TCP connections • Incrementally deployable

  20. Questions?

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