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HighSpeed TCP for Large Congestion Windows by Sally Floyd Notes

  • Modifies TCP congestion control mechanism for use with large congestion windows
  • TCP constrains congestion windows that can be achieved in realistic environments
  • Standard TCP average congestion window is
    1.2/sqrt(p) segments
    where p is loss.
  • 83000 segments = 1 Gb
  • Do not ramp up more quickly than allowed by slow start. It would necessitate more explicit feedback from routers along path.
  • Simpilify to packet loss rates of at most 10^-2
  • In standard tcp: need w/1.5 rtts between loss events.
  • Implement:
    Low_Window: threshold of which response function to use - highspeed or standard
    High_Window: average congestion window to be achieved.
    High_P: the tcp packet drop rate.
  • Need that response function gives straight line on log-log scale. Gives response curve of:
    W = (p/Low_P)^S Low_Window
    Where Low_P corresponds to the packet drop rate of Low_Window.
    S = (log High_Window - log Low_Window) / (log High_P - log Low_P )
    Eg for Low_P = 10^-3, Low_Window = 38 and High_Window = 83000, High_P = 10^-7, W = 0.12 / p ^0.835
  • Appropiate response function is one that gives a plausible packet drop rate for a connection throughput of 10gb/s.
  • Fairness: Considered through ratio of window sizes for different p
  • Need AIMD parameters of a(w) and b(w)
  • Response to ACK: w -> w + a(w)/w
  • Congestion: w -> ( 1 - b(w) ) w
  • Standard TCP: a(w) = 1, b(w) = 0.5 for all values of w.
  • When w = High_Window, we have loss rate High_P. so,
  • a(w) = High_Window^2 * High_P * 2 * b(w)/( 2 - b(w) )
  • High_Decrease: decrease parameter for b(w) when w = High_Window.
    Eg. High_Decrease = 0.1, High_Window = 83000, High_P = 10^-7 => b(83000) = 0.1, ie a decrease of 10% after a congestion event. Also a(83000) = 72, if increase of 72 segments or under 0.1% of W.
  • Need b(w) when w > Low_Window:
  • b(w) = ( High_Decrease - 0.5 ) ( log(w) - log(W) / ( log( W_1) - log(W) ) ) + 0.5
  • Giving
    a(w) = w^2 * p(w) * 2 * b(w) / ( 2 - b(w) )

 

Thu, 7 August, 2003 3:25 Previous PageNext Page
 
 
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2001-2003, Yee-Ting Li, email: ytl@hep.ucl.ac.uk, Tel: +44 (0) 20 7679 1376, Fax: +44 (0) 20 7679 7145
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