Index: netinet/tcp_input.c
===================================================================
--- netinet/tcp_input.c	(revision 184855)
+++ netinet/tcp_input.c	(working copy)
@@ -2765,6 +2765,12 @@
 	if (maxmtu == 0)
 		return;
 
+	if (tp->t_usermss) {
+		if (maxmtu > (tp->t_usermss + min_protoh))
+			maxmtu = tp->t_usermss + min_protoh;
+		if (offer > tp->t_usermss)
+			offer = tp->t_usermss;
+	}
 	/* What have we got? */
 	switch (offer) {
 		case 0:
Index: netinet/tcp_var.h
===================================================================
--- netinet/tcp_var.h	(revision 184855)
+++ netinet/tcp_var.h	(working copy)
@@ -210,6 +210,7 @@
 	void	*t_pspare[3];		/* toe usrreqs / toepcb * / congestion algo / vimage / 1 general use */
 	struct toe_usrreqs *t_tu;       /* offload operations vector */
 	void	*t_toe;			/* TOE pcb pointer */
+	u_int	t_usermss;		/* User requested t_maxseg */
 };
 
 #define IN_FASTRECOVERY(tp)	(tp->t_flags & TF_FASTRECOVERY)
Index: netinet/tcp_output.c
===================================================================
--- netinet/tcp_output.c	(revision 184855)
+++ netinet/tcp_output.c	(working copy)
@@ -649,6 +649,8 @@
 		if (flags & TH_SYN) {
 			tp->snd_nxt = tp->iss;
 			to.to_mss = tcp_mssopt(&tp->t_inpcb->inp_inc);
+			if (tp->t_usermss && to.to_mss > tp->t_usermss)
+				to.to_mss = tp->t_usermss;
 			to.to_flags |= TOF_MSS;
 		}
 		/* Window scaling. */
Index: netinet/tcp_syncache.c
===================================================================
--- netinet/tcp_syncache.c	(revision 184855)
+++ netinet/tcp_syncache.c	(working copy)
@@ -115,6 +115,7 @@
 	struct		in_conninfo sc_inc;	/* addresses */
 	int		sc_rxttime;		/* retransmit time */
 	u_int16_t	sc_rxmits;		/* retransmit counter */
+	u_int16_t	sc_usermss;		/* User-requested mss cap */
 
 	u_int32_t	sc_tsreflect;		/* timestamp to reflect */
 	u_int32_t	sc_ts;			/* our timestamp to send */
@@ -1017,6 +1018,7 @@
 	struct mbuf *ipopts = NULL;
 	u_int32_t flowtmp;
 	int win, sb_hiwat, ip_ttl, ip_tos, noopt;
+	u_int16_t usermss;
 	char *s;
 #ifdef INET6
 	int autoflowlabel = 0;
@@ -1050,6 +1052,7 @@
 	win = sbspace(&so->so_rcv);
 	sb_hiwat = so->so_rcv.sb_hiwat;
 	noopt = (tp->t_flags & TF_NOOPT);
+	usermss = tp->t_usermss;
 
 	/* By the time we drop the lock these should no longer be used. */
 	so = NULL;
@@ -1188,6 +1191,7 @@
 	sc->sc_iss = arc4random();
 	sc->sc_flags = 0;
 	sc->sc_flowlabel = 0;
+	sc->sc_usermss = usermss;
 
 	/*
 	 * Initial receive window: clip sbspace to [0 .. TCP_MAXWIN].
@@ -1252,8 +1256,11 @@
 #endif
 	if (to->to_flags & TOF_SACKPERM)
 		sc->sc_flags |= SCF_SACK;
-	if (to->to_flags & TOF_MSS)
+	if (to->to_flags & TOF_MSS) {
 		sc->sc_peer_mss = to->to_mss;	/* peer mss may be zero */
+		if (sc->sc_usermss && sc->sc_peer_mss > sc->sc_usermss)
+			sc->sc_peer_mss = sc->sc_usermss;
+	}
 	if (noopt)
 		sc->sc_flags |= SCF_NOOPT;
 
@@ -1321,6 +1328,8 @@
 
 	/* Determine MSS we advertize to other end of connection. */
 	mssopt = tcp_mssopt(&sc->sc_inc);
+	if (sc->sc_usermss && mssopt > sc->sc_usermss)
+		mssopt = sc->sc_usermss;
 	if (sc->sc_peer_mss)
 		mssopt = max( min(sc->sc_peer_mss, mssopt), tcp_minmss);
 
@@ -1723,6 +1732,8 @@
 
 	sc->sc_rxmits = 0;
 	sc->sc_peer_mss = tcp_sc_msstab[mss];
+	if (sc->sc_usermss && sc->sc_peer_mss > sc->sc_usermss)
+		sc->sc_peer_mss = sc->sc_usermss;
 
 	tcpstat.tcps_sc_recvcookie++;
 	return (sc);
Index: netinet/tcp_usrreq.c
===================================================================
--- netinet/tcp_usrreq.c	(revision 184855)
+++ netinet/tcp_usrreq.c	(working copy)
@@ -1357,11 +1357,18 @@
 				return (error);
 
 			INP_WLOCK_RECHECK(inp);
-			if (optval > 0 && optval <= tp->t_maxseg &&
-			    optval + 40 >= tcp_minmss)
-				tp->t_maxseg = optval;
-			else
-				error = EINVAL;
+			if ((so->so_state & SS_ISCONNECTED) == 0) {
+				if (optval > 0 && optval + 40 >= tcp_minmss)
+					tp->t_usermss = optval;
+				else
+					error = EINVAL;
+			} else {
+				if (optval > 0 && optval <= tp->t_maxseg &&
+				    optval + 40 >= tcp_minmss)
+					tp->t_maxseg = optval;
+				else
+					error = EINVAL;
+			}
 			INP_WUNLOCK(inp);
 			break;
 
@@ -1394,7 +1401,10 @@
 			error = sooptcopyout(sopt, &optval, sizeof optval);
 			break;
 		case TCP_MAXSEG:
-			optval = tp->t_maxseg;
+			if ((so->so_state & SS_ISCONNECTED) == 0)
+				optval = tp->t_usermss;
+			else
+				optval = tp->t_maxseg;
 			INP_WUNLOCK(inp);
 			error = sooptcopyout(sopt, &optval, sizeof optval);
 			break;