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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