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Message-ID: <7335583a0710090010w4ec27015h33d5f058e0f1a3ab@mail.gmail.com>
Date: Tue, 9 Oct 2007 00:10:51 -0700
From: "Xiaoliang (David) Wei" <davidwei79@...il.com>
To: netdev@...r.kernel.org
Subject: [PATCH][TCP] tcp_vegas.c: (tcp_vegas_cong_avoid) fix a bug in disabling slow start by gamma parameter
TCP Vegas implementation has a bug in the process of disabling
slow-start with gamma parameter. The bug may lead to extreme
unfairness in the presence of early packet loss. See details in:
http://www.cs.caltech.edu/~weixl/technical/ns2linux/known_linux/index.html#vegas
Switch the order of "if (tp->snd_cwnd <= tp->snd_ssthresh)" statement
and "if (diff > gamma)" statement to eliminate the problem.
Signed-off-by: Xiaoliang (David) Wei <http://www.davidwei.org>
---
linux-2.6.22.9-fixedvegas/net/ipv4/tcp_vegas.c | 35 ++++++++++++-------------
1 files changed, 17 insertions(+), 18 deletions(-)
diff -uprN linux-2.6.22.9/net/ipv4/tcp_vegas.c
linux-2.6.22.9-fixedvegas/net/ipv4/tcp_vegas.c
--- linux-2.6.22.9/net/ipv4/tcp_vegas.c 2007-09-26 11:03:01.000000000 -0700
+++ linux-2.6.22.9-fixedvegas/net/ipv4/tcp_vegas.c 2007-10-08
22:44:46.000000000 -0700
@@ -266,26 +266,25 @@ static void tcp_vegas_cong_avoid(struct
*/
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. */
-
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