The ability to administratively change the MTU of an interface presents

a few new wrinkles for MTU discovery which tcp_output() had better
be prepared to handle.  ip_output() is also modified to do something
helpful in this case, since it has already calculated the information
we need.

sys/netinet/ip_output.c

@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)ip_output.c	8.3 (Berkeley) 1/21/94
- * $Id: ip_output.c,v 1.22 1995/07/02 16:45:07 joerg Exp $
+ *	$Id: ip_output.c,v 1.23 1995/07/26 18:05:13 wollman Exp $
  */
 
 #include <sys/param.h>
@@ -331,6 +331,20 @@ sendit:
 	 */
 	if (ip->ip_off & IP_DF) {
 		error = EMSGSIZE;
+#ifdef MTUDISC
+		/*
+		 * This case can happen if the user changed the MTU
+		 * of an interface after enabling IP on it.  Because
+		 * most netifs don't keep track of routes pointing to
+		 * them, there is no way for one to update all its
+		 * routes when the MTU is changed.
+		 */
+		if ((ro->ro_rt->rt_flags & (RTF_UP | RTF_HOST))
+		    && !(ro->ro_rt->rt_rmx.rmx_locks & RTV_MTU)
+		    && (ro->ro_rt->rt_rmx.rmx_mtu > ifp->if_mtu)) {
+			ro->ro_rt->rt_rmx.rmx_mtu = ifp->if_mtu;
+		}
+#endif /* MTUDISC */
 		ipstat.ips_cantfrag++;
 		goto bad;
 	}

sys/netinet/tcp_output.c

@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)tcp_output.c	8.4 (Berkeley) 5/24/95
- *	$Id: tcp_output.c,v 1.13 1995/09/20 21:00:58 wollman Exp $
+ *	$Id: tcp_output.c,v 1.14 1995/09/22 20:05:58 wollman Exp $
  */
 
 #include <sys/param.h>
@@ -689,6 +689,18 @@ out:
 			tcp_quench(tp->t_inpcb, 0);
 			return (0);
 		}
+#ifdef MTUDISC
+		if (error == EMSGSIZE) {
+			/*
+			 * ip_output() will have already fixed the route
+			 * for us.  tcp_mtudisc() will, as its last action,
+			 * initiate retransmission, so it is important to
+			 * not do so here.
+			 */
+			tcp_mtudisc(tp->t_inpcb, 0);
+			return 0;
+		}
+#endif /* MTUDISC */.
 		if ((error == EHOSTUNREACH || error == ENETDOWN)
 		    && TCPS_HAVERCVDSYN(tp->t_state)) {
 			tp->t_softerror = error;

sys/netinet/tcp_subr.c

@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)tcp_subr.c	8.2 (Berkeley) 5/24/95
- *	$Id: tcp_subr.c,v 1.18 1995/10/10 17:45:40 wollman Exp $
+ *	$Id: tcp_subr.c,v 1.19 1995/10/12 17:37:24 wollman Exp $
  */
 
 #include <sys/param.h>
@@ -429,10 +429,6 @@ tcp_notify(inp, error)
 	sowwakeup(so);
 }
 
-#ifdef MTUDISC
-static void tcp_mtudisc __P((struct inpcb *, int));
-#endif /* MTUDISC */
-
 void
 tcp_ctlinput(cmd, sa, ip)
 	int cmd;
@@ -481,7 +477,7 @@ tcp_quench(inp, errno)
  * since we know the packet we just sent was dropped.
  * This duplicates some code in the tcp_mss() function in tcp_input.c.
  */
-static void
+void
 tcp_mtudisc(inp, errno)
 	struct inpcb *inp;
 	int errno;

sys/netinet/tcp_timewait.c

@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)tcp_subr.c	8.2 (Berkeley) 5/24/95
- *	$Id: tcp_subr.c,v 1.18 1995/10/10 17:45:40 wollman Exp $
+ *	$Id: tcp_subr.c,v 1.19 1995/10/12 17:37:24 wollman Exp $
  */
 
 #include <sys/param.h>
@@ -429,10 +429,6 @@ tcp_notify(inp, error)
 	sowwakeup(so);
 }
 
-#ifdef MTUDISC
-static void tcp_mtudisc __P((struct inpcb *, int));
-#endif /* MTUDISC */
-
 void
 tcp_ctlinput(cmd, sa, ip)
 	int cmd;
@@ -481,7 +477,7 @@ tcp_quench(inp, errno)
  * since we know the packet we just sent was dropped.
  * This duplicates some code in the tcp_mss() function in tcp_input.c.
  */
-static void
+void
 tcp_mtudisc(inp, errno)
 	struct inpcb *inp;
 	int errno;

sys/netinet/tcp_var.h

@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)tcp_var.h	8.4 (Berkeley) 5/24/95
- * 	$Id: tcp_var.h,v 1.19 1995/10/10 17:45:43 wollman Exp $
+ * 	$Id: tcp_var.h,v 1.20 1995/10/12 17:37:25 wollman Exp $
  */
 
 #ifndef _NETINET_TCP_VAR_H_
@@ -349,6 +349,9 @@ void	 tcp_init __P((void));
 void	 tcp_input __P((struct mbuf *, int));
 void	 tcp_mss __P((struct tcpcb *, int));
 int	 tcp_mssopt __P((struct tcpcb *));
+#ifdef MTUDISC
+void tcp_mtudisc __P((struct inpcb *, int));
+#endif /* MTUDISC */
 struct tcpcb *
 	 tcp_newtcpcb __P((struct inpcb *));
 void	 tcp_notify __P((struct inpcb *, int));