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Understanding RTT Unfairness

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• Some background Information
  • From Padhye's analysis:

    If TCP that uses the following increase and decrease mechanism:

    1 new ACK: CWND = CWND + ------ CWND 1 TD ACK: CWND = CWND - --- CWND 2

    Then the throughput of the TCP is given (approximately when p is small) by: (see: click here )

    ------- 1 / 3 B(p) = ----- \ / ----- RTT \/ 2bp

    Assuming b = 1, we have:

    ------ ------- 1 / 3 1 / 1.5 B(p) = ----- \ / ---- = ----- \ / ----- RTT \/ 2p RTT \/ p 1 1.2 B(p) = ----- * ------- . . . . . . . . . . . . (1) RTT ---- \/ p

  • Notice that:

    The Throughput of TCP is increased when the RTT is decreased !!!

    This alone will tell us that the performance of TCP will be biased against long RTT connections

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• Dynamics of TCP throughput

  • In the congestion avoidance phase, the TCP throughput changes as follows when there are no packet losses:

    W packets W+1 W+2 |<--------------->|<--------------->|<--------------->| RTT RTT RTT W W+1 W+2 TPut = ----- TPut = ----- TPut = ----- RTT RTT RTT \ / \ / \ / \ / \ / \ / 1 1 Difference = ----- Difference = ----- RTT RTT