- * Determine a reasonable value for maxseg size.
- * If the route is known, use one that can be handled
- * on the given interface without forcing IP to fragment.
- * If bigger than an mbuf cluster (MCLBYTES), round down to nearest size
- * to utilize large mbufs.
- * If interface pointer is unavailable, or the destination isn't local,
- * use a conservative size (512 or the default IP max size, but no more
- * than the mtu of the interface through which we route),
- * as we can't discover anything about intervening gateways or networks.
- * We also initialize the congestion/slow start window to be a single
- * segment if the destination isn't local; this information should
- * probably all be saved with the routing entry at the transport level.
- *
- * This is ugly, and doesn't belong at this level, but has to happen somehow.
+ * Collect new round-trip time estimate
+ * and update averages and current timeout.
+ */
+tcp_xmit_timer(tp)
+ register struct tcpcb *tp;
+{
+ register short delta;
+
+ tcpstat.tcps_rttupdated++;
+ if (tp->t_srtt != 0) {
+ /*
+ * srtt is stored as fixed point with 3 bits after the
+ * binary point (i.e., scaled by 8). The following magic
+ * is equivalent to the smoothing algorithm in rfc793 with
+ * an alpha of .875 (srtt = rtt/8 + srtt*7/8 in fixed
+ * point). Adjust t_rtt to origin 0.
+ */
+ delta = tp->t_rtt - 1 - (tp->t_srtt >> TCP_RTT_SHIFT);
+ if ((tp->t_srtt += delta) <= 0)
+ tp->t_srtt = 1;
+ /*
+ * We accumulate a smoothed rtt variance (actually, a
+ * smoothed mean difference), then set the retransmit
+ * timer to smoothed rtt + 4 times the smoothed variance.
+ * rttvar is stored as fixed point with 2 bits after the
+ * binary point (scaled by 4). The following is
+ * equivalent to rfc793 smoothing with an alpha of .75
+ * (rttvar = rttvar*3/4 + |delta| / 4). This replaces
+ * rfc793's wired-in beta.
+ */
+ if (delta < 0)
+ delta = -delta;
+ delta -= (tp->t_rttvar >> TCP_RTTVAR_SHIFT);
+ if ((tp->t_rttvar += delta) <= 0)
+ tp->t_rttvar = 1;
+ } else {
+ /*
+ * No rtt measurement yet - use the unsmoothed rtt.
+ * Set the variance to half the rtt (so our first
+ * retransmit happens at 2*rtt)
+ */
+ tp->t_srtt = tp->t_rtt << TCP_RTT_SHIFT;
+ tp->t_rttvar = tp->t_rtt << (TCP_RTTVAR_SHIFT - 1);
+ }
+ tp->t_rtt = 0;
+ tp->t_rxtshift = 0;
+
+ /*
+ * the retransmit should happen at rtt + 4 * rttvar.
+ * Because of the way we do the smoothing, srtt and rttvar
+ * will each average +1/2 tick of bias. When we compute
+ * the retransmit timer, we want 1/2 tick of rounding and
+ * 1 extra tick because of +-1/2 tick uncertainty in the
+ * firing of the timer. The bias will give us exactly the
+ * 1.5 tick we need. But, because the bias is
+ * statistical, we have to test that we don't drop below
+ * the minimum feasible timer (which is 2 ticks).
+ */
+ TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
+ tp->t_rttmin, TCPTV_REXMTMAX);
+
+ /*
+ * We received an ack for a packet that wasn't retransmitted;
+ * it is probably safe to discard any error indications we've
+ * received recently. This isn't quite right, but close enough
+ * for now (a route might have failed after we sent a segment,
+ * and the return path might not be symmetrical).
+ */
+ tp->t_softerror = 0;
+}
+
+/*
+ * Determine a reasonable value for maxseg size.
+ * If the route is known, check route for mtu.
+ * If none, use an mss that can be handled on the outgoing
+ * interface without forcing IP to fragment; if bigger than
+ * an mbuf cluster (MCLBYTES), round down to nearest multiple of MCLBYTES
+ * to utilize large mbufs. If no route is found, route has no mtu,
+ * or the destination isn't local, use a default, hopefully conservative
+ * size (usually 512 or the default IP max size, but no more than the mtu
+ * of the interface), as we can't discover anything about intervening
+ * gateways or networks. We also initialize the congestion/slow start
+ * window to be a single segment if the destination isn't local.
+ * While looking at the routing entry, we also initialize other path-dependent
+ * parameters from pre-set or cached values in the routing entry.
projects / unix-history / blobdiff