+ if (len) {
+ if (len == tp->t_maxseg)
+ goto send;
+ if ((idle || tp->t_flags & TF_NODELAY) &&
+ len + off >= so->so_snd.sb_cc)
+ goto send;
+ if (tp->t_force)
+ goto send;
+ if (len >= tp->max_sndwnd / 2)
+ goto send;
+ if (SEQ_LT(tp->snd_nxt, tp->snd_max))
+ goto send;
+ }
+
+ /*
+ * Compare available window to amount of window
+ * known to peer (as advertised window less
+ * next expected input). If the difference is at least two
+ * max size segments or at least 35% of the maximum possible
+ * window, then want to send a window update to peer.
+ */
+ if (win > 0) {
+ int adv = win - (tp->rcv_adv - tp->rcv_nxt);
+
+ if (so->so_rcv.sb_cc == 0 && adv >= 2 * tp->t_maxseg)
+ goto send;
+ if (100 * adv / so->so_rcv.sb_hiwat >= 35)
+ goto send;
+ }
+
+ /*
+ * TCP window updates are not reliable, rather a polling protocol
+ * using ``persist'' packets is used to insure receipt of window
+ * updates. The three ``states'' for the output side are:
+ * idle not doing retransmits or persists
+ * persisting to move a small or zero window
+ * (re)transmitting and thereby not persisting
+ *
+ * tp->t_timer[TCPT_PERSIST]
+ * is set when we are in persist state.
+ * tp->t_force
+ * is set when we are called to send a persist packet.
+ * tp->t_timer[TCPT_REXMT]
+ * is set when we are retransmitting
+ * The output side is idle when both timers are zero.
+ *
+ * If send window is too small, there is data to transmit, and no
+ * retransmit or persist is pending, then go to persist state.
+ * If nothing happens soon, send when timer expires:
+ * if window is nonzero, transmit what we can,
+ * otherwise force out a byte.
+ */
+ if (so->so_snd.sb_cc && tp->t_timer[TCPT_REXMT] == 0 &&
+ tp->t_timer[TCPT_PERSIST] == 0) {
+ tp->t_rxtshift = 0;
+ tcp_setpersist(tp);
+ }