A potential bug in tcp_vegas.c

["Xiaoliang (David) Wei" <davidwei79@gmail.com>]

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 in this file and 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 details at:

http://www.cs.caltech.edu/%7Eweixl/technical/ns2linux/known_linux/index.html#vegas
(the section of "Setting of Slow-Start-Threshold")

A fix I wrote (for Linux 2.6.19.2) is as follow:

--- 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@Caltech
http://davidwei.org
***********************************************