Apply by doing:
        cd /usr/src
        patch -p0 < 027_rtt.patch

And then rebuild your kernel.

Index: sys/netinet/tcp_input.c
===================================================================
RCS file: /cvs/src/sys/netinet/tcp_input.c,v
retrieving revision 1.158.2.2
retrieving revision 1.158.2.3
diff -u -p -r1.158.2.2 -r1.158.2.3
--- sys/netinet/tcp_input.c	26 May 2004 20:02:06 -0000	1.158.2.2
+++ sys/netinet/tcp_input.c	11 Jan 2005 04:40:29 -0000	1.158.2.3
@@ -2901,8 +2901,13 @@ tcp_xmit_timer(tp, rtt)
 	short delta;
 	short rttmin;
 
-	tcpstat.tcps_rttupdated++;
 	--rtt;
+	if (rtt < 0)
+		rtt = 0;
+	if (rtt > TCP_RTT_MAX)
+		rtt = TCP_RTT_MAX;
+
+	tcpstat.tcps_rttupdated++;
 	if (tp->t_srtt != 0) {
 		/*
 		 * srtt is stored as fixed point with 3 bits after the
Index: sys/netinet/tcp_var.h
===================================================================
RCS file: /cvs/src/sys/netinet/tcp_var.h,v
retrieving revision 1.61
retrieving revision 1.61.2.1
diff -u -p -r1.61 -r1.61.2.1
--- sys/netinet/tcp_var.h	2 Mar 2004 12:51:12 -0000	1.61
+++ sys/netinet/tcp_var.h	11 Jan 2005 04:40:30 -0000	1.61.2.1
@@ -325,6 +325,7 @@ tcp_reass_unlock(struct tcpcb *tp)
 #define	TCP_RTT_SHIFT		3	/* shift for srtt; 3 bits frac. */
 #define	TCP_RTTVAR_SCALE	4	/* multiplier for rttvar; 2 bits */
 #define	TCP_RTTVAR_SHIFT	2	/* multiplier for rttvar; 2 bits */
+#define TCP_RTT_MAX		(1<<9)	/* maximum rtt */
 
 /*
  * The initial retransmission should happen at rtt + 4 * rttvar.