+ tcpstat.tcps_rexmttimeo++;
+ rexmt = ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1;
+ rexmt *= tcp_backoff[tp->t_rxtshift];
+ TCPT_RANGESET(tp->t_rxtcur, rexmt, TCPTV_MIN, TCPTV_REXMTMAX);
+ tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
+ /*
+ * If losing, let the lower level know and try for
+ * a better route. Also, if we backed off this far,
+ * our srtt estimate is probably bogus. Clobber it
+ * so we'll take the next rtt measurement as our srtt;
+ * move the current srtt into rttvar to keep the current
+ * retransmit times until then.
+ */
+ if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
+ in_losing(tp->t_inpcb);
+ tp->t_rttvar += (tp->t_srtt >> 2);
+ tp->t_srtt = 0;
+ }
+ tp->snd_nxt = tp->snd_una;
+ /*
+ * 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;
+ }
+ (void) tcp_output(tp);
+ break;