A burst from NewReno when a partial ACK is received

From: Noritoshi Demizu (demizu_at_dd.iij4u.or.jp)
Date: 06/24/05

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    Date: Fri, 24 Jun 2005 14:07:26 +0900 (JST)
To: freebsd-net@freebsd.org

    I'm using FreeBSD current for my experiences.
    I observed bursts sent by NewReno when a partial ACK is received.
    I have two packet traces of such bursts. One of such bursts is
    analyzed at http://www.demizu.org/~noritosi/memo/2005/0623/ .

    I think tcp_newreno_partial_ack() in tcp_input.c rev 1.275 has a bug
    in calculating a new value of snd_cwnd at the tail of the function.
    Currently, snd_cwnd is re-calculated as following:

      L.3113: tp->snd_cwnd -= (th->th_ack - tp->snd_una - tp->t_maxseg);

    Since snd_cwnd is u_long, if snd_cwnd < SEG.ACK - SND.UNA - MSS,
    snd_cwnd becomes awfully huge and a burst of data can be sent.

    To fix this problem, I'd like to suggest the patch below.

    Thanks.

    Regards,
    Noritoshi Demizu

    Index: tcp_input.c
    ===================================================================
    RCS file: /home/cvsup/FreeBSD/ncvs/src/sys/netinet/tcp_input.c,v
    retrieving revision 1.275
    diff -u -r1.275 tcp_input.c
    --- tcp_input.c 1 Jun 2005 12:03:18 -0000 1.275
    +++ tcp_input.c 23 Jun 2005 06:20:28 -0000
    @@ -3110,7 +3112,11 @@
              * Partial window deflation. Relies on fact that tp->snd_una
              * not updated yet.
              */
    - tp->snd_cwnd -= (th->th_ack - tp->snd_una - tp->t_maxseg);
    + if (tp->snd_cwnd > th->th_ack - tp->snd_una)
    + tp->snd_cwnd -= th->th_ack - tp->snd_una;
    + else
    + tp->snd_cwnd = 0;
    + tp->snd_cwnd += tp->t_maxseg;
     }
     
     /*
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