[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>

diff --git a/net/ipv4/tcp_vegas.c b/net/ipv4/tcp_vegas.c
index b49dedc..007304e 100644 (file)
--- 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. */