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authorXiaoliang (David) Wei <davidwei79@gmail.com>2007-10-29 20:24:36 -0700
committerDavid S. Miller <davem@sunset.davemloft.net>2007-10-29 22:37:25 -0700
commitc940587bf603b4295a7f5e9ff8fed123368a1ef7 (patch)
treeeb442c1632039667cdad235cdcc29cb5ca2976fc /net
parent5c81833c2f74304142be6dde5688bf444316da21 (diff)
[TCP] vegas: 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 <davidwei79@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net')
-rw-r--r--net/ipv4/tcp_vegas.c37
1 files changed, 18 insertions, 19 deletions
diff --git a/net/ipv4/tcp_vegas.c b/net/ipv4/tcp_vegas.c
index b49dedcda52..007304e9984 100644
--- a/net/ipv4/tcp_vegas.c
+++ b/net/ipv4/tcp_vegas.c
@@ -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) {
- /* 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);
+ 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. */
tcp_slow_start(tp);
} else {
/* Congestion avoidance. */