Index: sys/netinet/in_pcb.h
===================================================================
--- sys/netinet/in_pcb.h
+++ sys/netinet/in_pcb.h
@@ -122,8 +122,9 @@
  * Flags for inc_flags.
  */
 #define	INC_ISIPV6	0x01
+#define	INC_IPV6MINMTU	0x02
 
-#define	inc_isipv6	inc_flags	/* temp compatibility */
+#define	inc_isipv6	(inc_flags & INC_ISIPV6)	/* temp compatibility */
 #define	inc_fport	inc_ie.ie_fport
 #define	inc_lport	inc_ie.ie_lport
 #define	inc_faddr	inc_ie.ie_faddr
Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -1051,6 +1051,8 @@
 #ifdef INET6
 		if (isipv6) {
 			inc.inc_flags |= INC_ISIPV6;
+			if (inp->inp_inc.inc_flags & INC_IPV6MINMTU)
+				inc.inc_flags |= INC_IPV6MINMTU;
 			inc.inc6_faddr = ip6->ip6_src;
 			inc.inc6_laddr = ip6->ip6_dst;
 		} else
Index: sys/netinet/tcp_subr.c
===================================================================
--- sys/netinet/tcp_subr.c
+++ sys/netinet/tcp_subr.c
@@ -2850,6 +2850,9 @@
 
 	KASSERT(inc != NULL, ("tcp_maxmtu6 with NULL in_conninfo pointer"));
 
+	if (inc->inc_flags & INC_IPV6MINMTU)
+		return (IPV6_MMTU);
+
 	if (!IN6_IS_ADDR_UNSPECIFIED(&inc->inc6_faddr)) {
 		in6_splitscope(&inc->inc6_faddr, &dst6, &scopeid);
 		if (fib6_lookup_nh_ext(inc->inc_fibnum, &dst6, scopeid, 0,
Index: sys/netinet/tcp_usrreq.c
===================================================================
--- sys/netinet/tcp_usrreq.c
+++ sys/netinet/tcp_usrreq.c
@@ -1580,6 +1580,33 @@
 		if (inp->inp_vflag & INP_IPV6PROTO) {
 			INP_WUNLOCK(inp);
 			error = ip6_ctloutput(so, sopt);
+			if ((error == 0) &&
+			    (sopt->sopt_dir == SOPT_SET) &&
+			    (sopt->sopt_level == IPPROTO_IPV6) &&
+			    (sopt->sopt_name == IPV6_USE_MIN_MTU)) {
+				INP_WLOCK(inp);
+				if ((inp->inp_flags &
+				    (INP_TIMEWAIT | INP_DROPPED))) {
+					INP_WUNLOCK(inp);
+					return (ECONNRESET);
+				}
+				inp->inp_inc.inc_flags |= INC_IPV6MINMTU;
+				tp = intotcpcb(inp);
+				if ((tp->t_state >= TCPS_SYN_SENT) &&
+				    (inp->inp_inc.inc_flags & INC_ISIPV6)) {
+					struct ip6_pktopts *opt;
+
+					opt = inp->in6p_outputopts;
+					if ((opt != NULL) &&
+					    (opt->ip6po_minmtu ==
+					    IP6PO_MINMTU_ALL)) {
+						if (tp->t_maxseg > TCP6_MSS) {
+							tp->t_maxseg = TCP6_MSS;
+						}
+					}
+				}
+				INP_WUNLOCK(inp);
+			}
 		}
 #endif /* INET6 */
 #if defined(INET6) && defined(INET)