[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>

1 files changed, 18 insertions, 19 deletions

diff --git a/net/ipv4/tcp_vegas.c b/net/ipv4/tcp_vegas.c
index b49dedcda52d..007304e99842 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. */