Index: sys/netinet/tcp_subr.c
===================================================================
--- sys/netinet/tcp_subr.c
+++ sys/netinet/tcp_subr.c
@@ -1541,11 +1541,6 @@
 					 * in the route to the suggested new
 					 * value (if given) and then notify.
 					 */
-					bzero(&inc, sizeof(inc));
-					inc.inc_faddr = faddr;
-					inc.inc_fibnum =
-					    inp->inp_inc.inc_fibnum;
-
 				    	mtu = ntohs(icp->icmp_nextmtu);
 					/*
 					 * If no alternative MTU was
@@ -1560,14 +1555,18 @@
 						mtu = V_tcp_minmss +
 						    sizeof(struct tcpiphdr);
 					/*
-					 * Only cache the MTU if it
-					 * is smaller than the interface
-					 * or route MTU.  tcp_mtudisc()
-					 * will do right thing by itself.
+					 * Only process the offered MTU if it
+					 * is smaller than the current one.
 					 */
-					if (mtu <= tcp_maxmtu(&inc, NULL))
+					if (mtu < tp->t_maxopd +
+					    sizeof(struct tcpiphdr)) {
+						bzero(&inc, sizeof(inc));
+						inc.inc_faddr = faddr;
+						inc.inc_fibnum =
+						    inp->inp_inc.inc_fibnum;
 						tcp_hc_updatemtu(&inc, mtu);
-					tcp_mtudisc(inp, mtu);
+						tcp_mtudisc(inp, mtu);
+					}
 				} else
 					inp = (*notify)(inp,
 					    inetctlerrmap[cmd]);