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Date: Thu, 1 Feb 2007 13:03:49 -0800
From: "Xiaoliang (David) Wei" <davidwei79@...il.com>
To: netdev@...r.kernel.org
Subject: A potential bug in tcp_vegas.c
Hi gurus,
I am testing tcp_vegas.c in Linux with the NS-2 TCP-Linux.
It seems that the ssthresh is not correctly reset to 2 in the
"tcp_vegas_cong_avoid" function.
The problem might lead to very unfair behavior among Vegas flows, when
some flows exit slow start due to loss, not delay.
Please see the detailed effect at:
http://www.cs.caltech.edu/%7Eweixl/technical/ns2linux/known_linux/#vegas
(the section of "Setting of Slow-Start-Threshold")
A patch I wrote (for Linux 2.6.19-2) is attached.
Basically, the branch that sets ssthresh to be 2 should be outside the
condition of (cwnd<=ssthresh).
--- tcp_vegas.c.old 2007-02-01 00:33:55.000000000 -0800
+++ tcp_vegas.c 2007-02-01 00:39:49.000000000 -0800
@@ -265,26 +265,25 @@
*/
diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd;
- if (tp->snd_cwnd <= tp->snd_ssthresh) {
- /* Slow start. */
- if (diff > gamma) {
- /* Going too fast. Time to slow down
- * and switch to congestion avoidance.
- */
- tp->snd_ssthresh = 2;
+ if (diff > gamma && tp->snd_ssthresh > 2 ) {
+ /* Going too fast. Time to slow down
+ * and switch to congestion avoidance.
+ */
+ tp->snd_ssthresh = 2;
- /* Set cwnd to match the actual rate
- * exactly:
- * cwnd = (actual rate) * baseRTT
- * Then we add 1 because the integer
- * truncation robs us of full link
- * utilization.
- */
- tp->snd_cwnd = min(tp->snd_cwnd,
- (target_cwnd >>
- V_PARAM_SHIFT)+1);
+ /* Set cwnd to match the actual rate
+ * exactly:
+ * cwnd = (actual rate) * baseRTT
+ * Then we add 1 because the integer
+ * truncation robs us of full link
+ * utilization.
+ */
+ tp->snd_cwnd = min(tp->snd_cwnd,
+ (target_cwnd >>
+ V_PARAM_SHIFT)+1);
- }
+ } else if (tp->snd_cwnd <= tp->snd_ssthresh) {
+ /* Slow start. */
tcp_slow_start(tp);
} else {
/* Congestion avoidance. */
Thanks.
-David
--
Xiaoliang (David) Wei Graduate Student, CS@...tech
http://davidwei.org
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