[PATCH][TCP] tcp_vegas.c: (tcp_vegas_cong_avoid) fix a bug in disabling slow start by gamma parameter
From: Xiaoliang (David) Wei <hidden>
Date: 2007-10-09 07:10:53
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 2007-10-0822:44:46.000000000 -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. */