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2608Threshold/rate halving in Linux TCP

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  • Bert Van Lee
    May 9, 2001

      On trying to set up a high-speed connection between two Linux machines A and
      B each running kernel 2.4.3 over a 8 Mbit/s link with a RTT of about 640 ms,
      I noticed that quite early the TCP sender must have changed to congestion
      avoidance. The delay/BW limiting was done using a third Linux machine L with
      the Nistnet network emulation software on it. There is a 100 Mbit/s Ethernet
      between A and L and between L and B.

      When the speed reached about 160 kByte/s or 1.3 Mbit/s, the exponential
      build-up stopped and the speed increased linearly up till about 7 Mbit/s ,
      after which the rate was halved to 4 Mbit/s, again linear build up to 7,
      halving to 4 etc. The build-up from 4 to 7 takes about 140 SECONDS.

      I allowed SACK, RTT measurement, Window scaling.

      Set wmem/rmem params in .../core and .../ipv4. Used default send and recv
      buffers of 1 MByte, 2 MByte max. TCPs both report window scaling by 4 bits,
      i.e. x16, which is a healthy sign.

      My questions are:

      1. where can I specify the threshold for congestion avoidance, as I would
      like the transmitting TCP to continue to 8 (or 7 or so...) Mbit/s before
      doing so? I could not find any setting in .../core or .../ipv4???

      2. Can I somehow avoid the rate-halving that supposedly happens because
      sstresh is halved, as this is not what I want, since the link is a 8 MByte/s
      link and there is no congestion at the other end, just a limitation of

      3. Other question: as far as I am concerned the sending TCP may start
      blazing at 7 Mbit/s, rather than start with 2 segments only. This appears to
      be non-RFC, right? Is there any action started to allow this, e.g. withing
      T/TCP which I do not know well?

      Thank you,

      Eric Verlind

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