two-phase slow start

SCCS-vsn: sys/netinet/tcp_timer.c 7.10

diff --git a/usr/src/sys/netinet/tcp_timer.c b/usr/src/sys/netinet/tcp_timer.c
index 18d6a63..7723bab 100644 (file)
--- a/usr/src/sys/netinet/tcp_timer.c
+++ b/usr/src/sys/netinet/tcp_timer.c
@@ -3,7 +3,7 @@
  * All rights reserved.  The Berkeley software License Agreement
  * specifies the terms and conditions for redistribution.
  *
  * All rights reserved.  The Berkeley software License Agreement
  * specifies the terms and conditions for redistribution.
  *

- *     @(#)tcp_timer.c 7.9 (Berkeley) %G%
+ *     @(#)tcp_timer.c 7.10 (Berkeley) %G%

  */
 
 #include "param.h"
  */
 
 #include "param.h"


@@ -183,8 +183,30 @@ tcp_timers(tp, timer)
                 * data accumulated, this "slow start" keeps us from
                 * dumping all that data as back-to-back packets (which
                 * might overwhelm an intermediate gateway).
                 * data accumulated, this "slow start" keeps us from
                 * dumping all that data as back-to-back packets (which
                 * might overwhelm an intermediate gateway).

+                *
+                * There are two phases to the opening: Initially we
+                * open by one mss on each ack.  This makes the window
+                * size increase exponentially with time.  If the
+                * window is larger than the path can handle, this
+                * exponential growth results in dropped packet(s)
+                * almost immediately.  To get more time between 
+                * drops but still "push" the network to take advantage
+                * of improving conditions, we switch from exponential
+                * to linear window opening at some threshhold size.
+                * For a threshhold, we use half the current window
+                * size, truncated to a multiple of the mss.
+                *
+                * (the minimum cwnd that will give us exponential
+                * growth is 2 mss.  We don't allow the threshhold
+                * to go below this.)

                 */
                 */

+               {
+               u_int win = MIN(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
+               if (win < 2)
+                       win = 2;

                tp->snd_cwnd = tp->t_maxseg;
                tp->snd_cwnd = tp->t_maxseg;

+               tp->snd_ssthresh = win * tp->t_maxseg;
+               }

                (void) tcp_output(tp);
                break;
 
                (void) tcp_output(tp);
                break;