+ /*
+ * If timing a segment in this window, stop the timer.
+ */
+ tp->t_rtt = 0;
+ /*
+ * Close the congestion window down to one segment
+ * (we'll open it by one segment for each ack we get).
+ * Since we probably have a window's worth of unacked
+ * 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_ssthresh = win * tp->t_maxseg;
+ }