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[TCP] vegas: Fix a bug in disabling slow start by gamma parameter.

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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 <davidwei79@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Xiaoliang (David) Wei 2007-10-29 20:24:36 -07:00 committed by David S. Miller
parent 5c81833c2f
commit c940587bf6
1 changed files with 18 additions and 19 deletions
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37
net/ipv4/tcp_vegas.c
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@ -266,26 +266,25 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack,
*/
diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd;
if (tp->snd_cwnd <= tp->snd_ssthresh) {
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);
} else 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;
/* 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);
}
tcp_slow_start(tp);
} else {
/* Congestion avoidance. */