- allow the user to force the TCP mss below the fail-safe 216 with a low

interface MTU.
- break a tcp_output() -> tcp_mtudisc() -> tcp_output() infinite recursion
when the TCP mss ends up larger than the interface MTU (when the if_mtu is
smaller than the tcp header).  connections will still stall
feedback from itojun@, claudio@ and provos and testing from beck@

3 files changed, 17 insertions, 13 deletions

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index f0cffa74bb5..57c2a850dae 100644
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: tcp_input.c,v 1.164 2004/04/20 20:05:29 markus Exp $	*/
+/*	$OpenBSD: tcp_input.c,v 1.165 2004/04/26 18:12:25 frantzen Exp $	*/
 /*	$NetBSD: tcp_input.c,v 1.23 1996/02/13 23:43:44 christos Exp $	*/
 
 /*
@@ -3068,13 +3068,15 @@ tcp_mss(tp, offer)
 	 * If we compute a larger value, return it for use in sending
 	 * a max seg size option, but don't store it for use
 	 * unless we received an offer at least that large from peer.
-	 * However, do not accept offers under 216 bytes.
+	 * However, do not accept offers under 216 bytes unless the
+	 * interface MTU is actually that low.
 	 */
 	if (offer > 0)
 		tp->t_peermss = offer;
 	if (tp->t_peermss)
 		mss = min(mss, tp->t_peermss);
-	mss = max(mss, 216);		/* sanity - at least max opt. space */
+	/* sanity - at least max opt. space */
+	mss = max(mss, min(216, ifp->if_mtu - iphlen - sizeof(struct tcphdr)));
 
 	/*
 	 * maxopd stores the maximum length of data AND options
diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index ceeac82cdb9..fd9e97cb04b 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: tcp_output.c,v 1.65 2004/02/16 21:51:03 markus Exp $	*/
+/*	$OpenBSD: tcp_output.c,v 1.66 2004/04/26 18:12:25 frantzen Exp $	*/
 /*	$NetBSD: tcp_output.c,v 1.16 1997/06/03 16:17:09 kml Exp $	*/
 
 /*
@@ -1190,7 +1190,7 @@ out:
 			 * initiate retransmission, so it is important to
 			 * not do so here.
 			 */
-			tcp_mtudisc(tp->t_inpcb, 0);
+			tcp_mtudisc(tp->t_inpcb, -1);
 			return (0);
 		}
 		if ((error == EHOSTUNREACH || error == ENETDOWN) &&
diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
index 27e9bd25203..c46ec66bc9a 100644
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: tcp_subr.c,v 1.77 2004/03/02 12:51:12 markus Exp $	*/
+/*	$OpenBSD: tcp_subr.c,v 1.78 2004/04/26 18:12:25 frantzen Exp $	*/
 /*	$NetBSD: tcp_subr.c,v 1.22 1996/02/13 23:44:00 christos Exp $	*/
 
 /*
@@ -1018,8 +1018,10 @@ tcp_mtudisc(inp, errno)
 {
 	struct tcpcb *tp = intotcpcb(inp);
 	struct rtentry *rt = in_pcbrtentry(inp);
+	int change = 0;
 
 	if (tp != 0) {
+		int orig_maxseg = tp->t_maxseg;
 		if (rt != 0) {
 			/*
 			 * If this was not a host route, remove and realloc.
@@ -1029,18 +1031,18 @@ tcp_mtudisc(inp, errno)
 				if ((rt = in_pcbrtentry(inp)) == 0)
 					return;
 			}
-
-			if (rt->rt_rmx.rmx_mtu != 0) {
-				/* also takes care of congestion window */
-				tcp_mss(tp, -1);
-			}
+			if (orig_maxseg != tp->t_maxseg ||
+			    (rt->rt_rmx.rmx_locks & RTV_MTU))
+				change = 1;
 		}
+		tcp_mss(tp, -1);
 
 		/*
-		 * Resend unacknowledged packets.
+		 * Resend unacknowledged packets
 		 */
 		tp->snd_nxt = tp->snd_una;
-		tcp_output(tp);
+		if (change || errno > 0)
+			tcp_output(tp);
 	}
 }