Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -3494,7 +3494,7 @@
 
 	TCPSTAT_INC(tcps_rttupdated);
 	tp->t_rttupdated++;
-	if (tp->t_srtt != 0) {
+	if (tp->t_srtt != 0 && tp->t_rxtshift <= TCP_RTT_INVALIDATE) {
 		/*
 		 * srtt is stored as fixed point with 5 bits after the
 		 * binary point (i.e., scaled by 8).  The following magic
Index: sys/netinet/tcp_timer.h
===================================================================
--- sys/netinet/tcp_timer.h
+++ sys/netinet/tcp_timer.h
@@ -119,6 +119,15 @@
 
 #define	TCPTV_DELACK	( hz/10 )		/* 100ms timeout */
 
+/* Begin Isilon */
+/*
+ * If we exceed this number of retransmits for a single segment, we'll consider
+ * the current srtt measurement no longer valid and will recalculate from
+ * scratch starting with the next ACK.
+ */
+#define TCP_RTT_INVALIDATE (TCP_MAXRXTSHIFT / 4)
+/* End Isilon */
+
 #ifdef	TCPTIMERS
 static const char *tcptimers[] =
     { "REXMT", "PERSIST", "KEEP", "2MSL", "DELACK" };
Index: sys/netinet/tcp_timer.c
===================================================================
--- sys/netinet/tcp_timer.c
+++ sys/netinet/tcp_timer.c
@@ -845,20 +845,16 @@
 	    (tp->t_rxtshift == 3))
 		tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP|TF_SACK_PERMIT);
 	/*
-	 * 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 we backed off this far, notify the L3 protocol that we're having
+	 * connection problems.
 	 */
-	if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
+	if (tp->t_rxtshift > TCP_RTT_INVALIDATE) {
 #ifdef INET6
 		if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
 			in6_losing(tp->t_inpcb);
 		else
 #endif
 			in_losing(tp->t_inpcb);
-		tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
-		tp->t_srtt = 0;
 	}
 	tp->snd_nxt = tp->snd_una;
 	tp->snd_recover = tp->snd_max;