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Scalable TCP: Improving Performance in Highspeed Wide Area Networks

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Scalable TCP: Improving Performance in Highspeed Wide Area Networks. Paper by Tom Kelly ... TCP performs badly with very large congestion windows. Converges slowly ... – PowerPoint PPT presentation

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Title: Scalable TCP: Improving Performance in Highspeed Wide Area Networks


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 congestion window 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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