Re: Bad "bad" retransmit recovery in TCP

From: Andre Oppermann (andre_at_freebsd.org)
Date: 01/25/04

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    Date: Sun, 25 Jan 2004 23:49:14 +0100
To: Guohan Lu <lguohan00@mails.tsinghua.edu.cn>

    Guohan Lu wrote:
    >
    > hi,
    > I noticed in my TCP trace that there are many bad "bad" retransmit
    > recoveries. That is the packet get lost, the timeout happens. However,
    > when the lost packet are acknowledged, the cwnd of sender is not 2,
    > but resumes the value before timeout. This kind of behavior happens when
    > the RTT is very small, around several ms, no more than 10ms.
    > It seems to me that following code is run when the acknowledgement
    > for the lost packet is received.
    > if (tp->t_rxtshift == 1 && ticks < tp->t_badrxtwin) {
    > tp->snd_cwnd = tp->snd_cwnd_prev;
    > tp->snd_ssthresh = tp->snd_ssthresh_prev;
    > tp->snd_nxt = tp->snd_max;
    > tp->t_badrxtwin = 0; /* XXX probably not required */
    > }
    > I wonder if t_badrxtwin is not correctly set in this situation? (when
    > the rtt is very small)

    What version of FreeBSD are you running? A 'uname -a' would be
    great of great help.

    >From your description it sounds like the code which is supposed
    to recover from the case when just a returning ACK is lost (not
    the sent segment) is going in here. For some reason it looks
    like the code doesn't detect that a real retransmit of the segment
    happend.

    On the other hand it could be that you see fast recovery in action.
    A packet trace of your case would be helpful too to narrow down the
    problem. 

    -- 
Andre
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