+ 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;
+
+ /*
+ * Persistance timer into zero window.
+ * Force a byte to be output, if possible.
+ */
+ case TCPT_PERSIST:
+ tcpstat.tcps_persisttimeo++;
+ tcp_setpersist(tp);
+ tp->t_force = 1;
+ (void) tcp_output(tp);
+ tp->t_force = 0;
+ break;
+
+ /*
+ * Keep-alive timer went off; send something
+ * or drop connection if idle for too long.
+ */
+ case TCPT_KEEP:
+ tcpstat.tcps_keeptimeo++;
+ if (tp->t_state < TCPS_ESTABLISHED)
+ goto dropit;
+ if (tp->t_inpcb->inp_socket->so_options & SO_KEEPALIVE &&
+ tp->t_state <= TCPS_CLOSE_WAIT) {
+ if (tp->t_idle >= tcp_keepidle + tcp_maxidle)
+ goto dropit;
+ /*
+ * Send a packet designed to force a response
+ * if the peer is up and reachable:
+ * either an ACK if the connection is still alive,
+ * or an RST if the peer has closed the connection
+ * due to timeout or reboot.
+ * Using sequence number tp->snd_una-1
+ * causes the transmitted zero-length segment
+ * to lie outside the receive window;
+ * by the protocol spec, this requires the
+ * correspondent TCP to respond.
+ */
+ tcpstat.tcps_keepprobe++;
+#ifdef TCP_COMPAT_42
+ /*
+ * The keepalive packet must have nonzero length
+ * to get a 4.2 host to respond.
+ */
+ tcp_respond(tp, tp->t_template,
+ tp->rcv_nxt - 1, tp->snd_una - 1, 0);
+#else
+ tcp_respond(tp, tp->t_template,
+ tp->rcv_nxt, tp->snd_una - 1, 0);
+#endif
+ tp->t_timer[TCPT_KEEP] = tcp_keepintvl;
+ } else
+ tp->t_timer[TCPT_KEEP] = tcp_keepidle;
+ break;
+ dropit:
+ tcpstat.tcps_keepdrops++;
+ tp = tcp_drop(tp, ETIMEDOUT);
+ break;