Index: sys/netinet/icmp_var.h
===================================================================
--- sys/netinet/icmp_var.h
+++ sys/netinet/icmp_var.h
@@ -56,6 +56,7 @@
 	u_long	icps_bmcasttstamp;	/* b/mcast tstamp requests dropped */
 	u_long	icps_badaddr;		/* bad return address */
 	u_long	icps_noroute;		/* no route back */
+	u_long	icps_pmtuchg;		/* path MTU changes */
 };
 
 #ifdef _KERNEL
Index: sys/netinet/ip.h
===================================================================
--- sys/netinet/ip.h
+++ sys/netinet/ip.h
@@ -213,6 +213,7 @@
 #define	IPFRAGTTL	60		/* time to live for frags, slowhz */
 #define	IPTTLDEC	1		/* subtracted when forwarding */
 #define	IP_MSS		576		/* default maximum segment size */
+#define IP_MMTU		68		/* Minimal MTU from RFC 1191 */
 
 /*
  * This is the real IPv4 pseudo header, used for computing the TCP and UDP
Index: sys/netinet/ip_output.c
===================================================================
--- sys/netinet/ip_output.c
+++ sys/netinet/ip_output.c
@@ -85,6 +85,8 @@
 #include <netinet/ip_options.h>
 #include <netinet/ip_mroute.h>
 
+#include <netinet/tcp_var.h>
+
 #include <netinet/udp.h>
 #include <netinet/udp_var.h>
 
@@ -105,6 +107,10 @@
 	&mbuf_frag_size, 0, "Fragment outgoing mbufs to this size");
 #endif
 
+VNET_DEFINE(int, rfc1191);
+SYSCTL_INT(_net_inet_ip, 0, rfc1191, CTLFLAG_VNET | CTLFLAG_RW,
+    &VNET_NAME(rfc1191), 0, "Enable RFC1191 PMTU support");
+
 static void	ip_mloopback(struct ifnet *, const struct mbuf *, int);
 
 extern int in_mcast_loop;
@@ -304,6 +310,46 @@
 	ro->ro_flags |= (nh_flags & NHF_GATEWAY) ? RT_HAS_GW : 0;
 }
 
+/*
+ * Calculates IPv4 path MTU for @dst based on the hostcache data and a fallback
+ * nexthop MTU value @nh_mtu.
+ *
+ * Note that @nh_mtu is a valid fall-back MTU if there is no PMTU entry in the
+ * hostcache. It does not need to necessarily come from nh_object, as is the
+ * case when this function is called from ip_output() for a broadcast
+ * destination.
+ *
+ * Returns the calculated MTU.
+ */
+static uint32_t
+ip_calcpmtu(const struct in_addr dst, uint32_t nh_mtu, u_int proto)
+{
+	uint32_t mtu = 0;
+	struct in_conninfo inc;
+
+	bzero(&inc, sizeof(inc));
+	inc.inc_faddr = dst;
+
+	/* TCP is known to react to pmtu changes so skip hc,
+	 * XXX: Either add list of known good protocols or an inpcb flag filled
+	 * by the PMTU aware protocol.
+	 */
+	if (proto != IPPROTO_TCP)
+		mtu = tcp_hc_getmtu(&inc);
+
+	if (mtu)
+		mtu = min(mtu, nh_mtu);
+	else
+		mtu = nh_mtu;
+	/* XXX: below is an extremely unlikely case, because it is a minimal
+	 * unfragmented packet size (RFC 791).
+	 */
+	if (__predict_false(mtu < IP_MMTU))
+		mtu = IP_MMTU;
+
+	return mtu;
+}
+
 /*
  * IP output.  The packet in mbuf chain m contains a skeletal IP
  * header (with len, off, ttl, proto, tos, src, dst).
@@ -326,7 +372,6 @@
 	struct ifnet *ifp = NULL;	/* keep compiler happy */
 	struct mbuf *m0;
 	int hlen = sizeof (struct ip);
-	int mtu = 0;
 	int error = 0;
 	int vlan_pcp = -1;
 	struct sockaddr_in *dst, sin;
@@ -335,7 +380,7 @@
 	struct in_addr src;
 	int isbroadcast;
 	uint16_t ip_len, ip_off;
-	uint32_t fibnum;
+	uint32_t fibnum, mtu = 0;
 #if defined(IPSEC) || defined(IPSEC_SUPPORT)
 	int no_route_but_check_spd = 0;
 #endif
@@ -550,6 +595,9 @@
 		    in_ifaddr_broadcast(dst->sin_addr, ia)));
 	}
 
+	/* Determine the path MTU. */
+	if (V_rfc1191)
+		mtu = ip_calcpmtu(ip->ip_dst, mtu, ip->ip_p);
 	/* Catch a possible divide by zero later. */
 	KASSERT(mtu > 0, ("%s: mtu %d <= 0, ro=%p (nh_flags=0x%08x) ifp=%p",
 	    __func__, mtu, ro,
Index: sys/netinet/ip_var.h
===================================================================
--- sys/netinet/ip_var.h
+++ sys/netinet/ip_var.h
@@ -180,6 +180,7 @@
 struct sockopt;
 struct inpcbinfo;
 
+VNET_DECLARE(int, rfc1191);
 VNET_DECLARE(int, ip_defttl);			/* default IP ttl */
 VNET_DECLARE(int, ipforwarding);		/* ip forwarding */
 VNET_DECLARE(int, ipsendredirects);
@@ -195,6 +196,7 @@
 VNET_DECLARE(int, drop_redirect);
 extern struct	pr_usrreqs rip_usrreqs;
 
+#define	V_rfc1191	VNET(rfc1191)
 #define	V_ip_id			VNET(ip_id)
 #define	V_ip_defttl		VNET(ip_defttl)
 #define	V_ipforwarding		VNET(ipforwarding)