Index: sys/netinet/tcp_output.c
===================================================================
--- sys/netinet/tcp_output.c
+++ sys/netinet/tcp_output.c
@@ -675,6 +675,12 @@
 
 send:
 	SOCKBUF_LOCK_ASSERT(&so->so_snd);
+        if (len > 0) {
+                if (len >= tp->t_maxseg)
+                        tp->t_flags |= TF_MAXSEGSNT;
+                else
+                        tp->t_flags &= ~TF_MAXSEGSNT;
+        }
 	/*
 	 * Before ESTABLISHED, force sending of initial options
 	 * unless TCP set not to do any options.
@@ -1219,8 +1225,12 @@
 	 *
 	 * NB: Don't set DF on small MTU/MSS to have a safe fallback.
 	 */
-	if (V_path_mtu_discovery && tp->t_maxopd > V_tcp_minmss)
+	if (V_path_mtu_discovery && tp->t_maxopd > V_tcp_minmss) {
 		ip->ip_off |= htons(IP_DF);
+		tp->t_flags |= TF_PMTUD;
+	} else {
+		tp->t_flags &= ~TF_PMTUD;
+	}
 
 	if (tp->t_state == TCPS_SYN_SENT)
 		TCP_PROBE5(connect__request, NULL, tp, ip, tp, th);
Index: sys/netinet/tcp_timer.c
===================================================================
--- sys/netinet/tcp_timer.c
+++ sys/netinet/tcp_timer.c
@@ -66,6 +66,9 @@
 #include <netinet/tcp_fsm.h>
 #include <netinet/tcp_timer.h>
 #include <netinet/tcp_var.h>
+#ifdef INET6
+#include <netinet6/tcp6_var.h>
+#endif
 #include <netinet/tcpip.h>
 #ifdef TCPDEBUG
 #include <netinet/tcp_debug.h>
@@ -119,6 +122,13 @@
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, keepcnt, CTLFLAG_RW, &tcp_keepcnt, 0,
     "Number of keepalive probes to send");
 
+int	tcp_pmtud_black_hole_detect = 0 ;
+SYSCTL_INT(_net_inet_tcp, OID_AUTO, pmtud_blackhole_detection, CTLTYPE_INT|CTLFLAG_RW,
+    &tcp_pmtud_black_hole_detect, 0, "Path MTU Discovery Black Hole Detection");
+
+int	tcp_pmtud_black_hole_mss = 1200 ;
+SYSCTL_INT(_net_inet_tcp, OID_AUTO, pmtud_blackhole_mss, CTLTYPE_INT|CTLFLAG_RW,
+    &tcp_pmtud_black_hole_mss, 0, "Path MTU Discovery Black Hole Detection lowered MSS");
 	/* max idle probes */
 int	tcp_maxpersistidle;
 
@@ -539,6 +549,9 @@
 
 	ostate = tp->t_state;
 #endif
+#ifdef INET6
+        int isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV4) == 0;
+#endif /* INET6 */
 	INP_INFO_RLOCK(&V_tcbinfo);
 	inp = tp->t_inpcb;
 	/*
@@ -640,6 +653,63 @@
 		rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
 	TCPT_RANGESET(tp->t_rxtcur, rexmt,
 		      tp->t_rttmin, TCPTV_REXMTMAX);
+		if (tcp_pmtud_black_hole_detect && (tp->t_state == TCPS_ESTABLISHED)) {
+			int optlen = 0;
+			if (((tp->t_flags & (TF_PMTUD|TF_MAXSEGSNT)) == (TF_PMTUD|TF_MAXSEGSNT)) &&
+				(tp->t_rxtshift == 2)) {
+				/*
+				 * Enter Path MTU Black-hole Detection mechanism:
+				 * - Disable Path MTU Discovery (IP "DF" bit).
+				 * - Reduce MTU to lower value than what we negociated with peer.
+				 */
+				/* Disable Path MTU Discovery for now */
+				tp->t_flags &= ~TF_PMTUD;
+				/* Record that we may have found a black hole */
+				tp->t_flags |= TF_BLACKHOLE;
+				optlen = tp->t_maxopd - tp->t_maxseg;
+				/* Keep track of previous MSS */
+				tp->t_pmtud_saved_maxopd = tp->t_maxopd;
+				/* Reduce the MSS to intermediary value */
+				if (tp->t_maxopd > tcp_pmtud_black_hole_mss) {
+					tp->t_maxopd = tcp_pmtud_black_hole_mss;
+				} else {
+					tp->t_maxopd = 	/* use the default MSS */
+#ifdef INET6
+						isipv6 ? V_tcp_v6mssdflt :
+#endif /* INET6 */
+							tcp_mssdflt;
+				}
+				tp->t_maxseg = tp->t_maxopd - optlen;
+
+				/*
+				 * Reset the slow-start flight size
+				 * as it may depend on the new MSS
+				 */
+				if (CC_ALGO(tp)->conn_init != NULL)
+					CC_ALGO(tp)->conn_init(tp->ccv);
+			}
+			/*
+			 * If further retransmissions are still unsuccessful with a lowered MTU,
+			 * maybe this isn't a Black Hole and we restore the previous MSS and
+			 * blackhole detection flags.
+			 */
+			else {
+				if ((tp->t_flags & TF_BLACKHOLE) && (tp->t_rxtshift > 4)) {
+					tp->t_flags |= TF_PMTUD;
+					tp->t_flags &= ~TF_BLACKHOLE;
+					optlen = tp->t_maxopd - tp->t_maxseg;
+					tp->t_maxopd = tp->t_pmtud_saved_maxopd;
+					tp->t_maxseg = tp->t_maxopd - optlen;
+					/*
+					 * Reset the slow-start flight size as it
+					 * may depend on the new MSS
+					 */
+					if (CC_ALGO(tp)->conn_init != NULL)
+						CC_ALGO(tp)->conn_init(tp->ccv);
+				}
+			}
+		}
+
 	/*
 	 * Disable RFC1323 and SACK if we haven't got any response to
 	 * our third SYN to work-around some broken terminal servers
Index: sys/netinet/tcp_var.h
===================================================================
--- sys/netinet/tcp_var.h
+++ sys/netinet/tcp_var.h
@@ -200,8 +200,9 @@
 	u_int	t_keepcnt;		/* number of keepalives before close */
 
 	u_int	t_tsomax;		/* tso burst length limit */
+        u_int   t_pmtud_saved_maxopd;   /* MSS saved before performing PMTU-D BlackHole detection */
 
-	uint32_t t_ispare[8];		/* 5 UTO, 3 TBD */
+	uint32_t t_ispare[7];		/* 5 UTO, 3 TBD */
 	void	*t_pspare2[4];		/* 1 TCP_SIGNATURE, 3 TBD */
 	uint64_t _pad[6];		/* 6 TBD (1-2 CC/RTT?) */
 };
@@ -238,6 +239,10 @@
 #define	TF_ECN_SND_ECE	0x10000000	/* ECN ECE in queue */
 #define	TF_CONGRECOVERY	0x20000000	/* congestion recovery mode */
 #define	TF_WASCRECOVERY	0x40000000	/* was in congestion recovery */
+#define TF_BLACKHOLE    0x80000000      /* Path MTU Discovery Black Hole detection */
+#define TF_PMTUD        0x100000000     /* Perform Path MTU Discovery for this connection */
+#define TF_MAXSEGSNT    0x200000000     /* last segment sent was a full segment */
+
 
 #define	IN_FASTRECOVERY(t_flags)	(t_flags & TF_FASTRECOVERY)
 #define	ENTER_FASTRECOVERY(t_flags)	t_flags |= TF_FASTRECOVERY