for layer 3 protocols that does not support path MTU discovery

(I mean, IPv4) do not try to use rmx_mtu on routing table.
this symptom was introduced by rmx_mtu initialization (necessary for IPv6
path MTU discovery) in net/route.c.  now prior behavior is recovered.
From: Hugh Graham <hugh@openbsd.org>

there are several question about mssdflt semantics, though:

Question 1: with the current code, mssdflt does not override rmx_mtu value
(mssdflt overrides interface mtu only).  should we override rmx_mtu by
mssdflt as well?

Question 2: with the current code, mssdflt overrides mss computed from
if mtu, only when the destination is IPv4 non-local.  is it safe enough?
we may want to use mssdflt, whenever we are uncertain.
mss = if mtu - hdrsiz;
if (IPv4 non-local destination)
	mss = min(mss, mssdflt);

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index d84f958..c5663d4 100644 (file)
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: tcp_input.c,v 1.57 2000/02/21 21:42:13 provos Exp $   */
+/*     $OpenBSD: tcp_input.c,v 1.58 2000/04/14 04:20:57 itojun Exp $   */
 /*     $NetBSD: tcp_input.c,v 1.23 1996/02/13 23:43:44 christos Exp $  */
 
 /*
@@ -2787,37 +2787,49 @@ tcp_mss(tp, offer)
                    tp->t_rttmin, TCPTV_REXMTMAX);
        }
        /*
-        * if there's an mtu associated with the route, use it
+        * if there's an mtu associated with the route and we support
+        * path MTU discovery for the underlying protocol family, use it.
         */
-       if (rt->rt_rmx.rmx_mtu)
-#ifdef INET6
-       {
-         /*
-          * One may wish to lower MSS to take into account options,
-          * especially security-related options.
-          */
-         if (tp->pf == AF_INET6) 
-           mss = rt->rt_rmx.rmx_mtu - sizeof(struct tcpipv6hdr);
-         else
-#endif /* INET6 */
-               mss = rt->rt_rmx.rmx_mtu - sizeof(struct tcpiphdr);
+       if (rt->rt_rmx.rmx_mtu) {
+               /*
+                * One may wish to lower MSS to take into account options,
+                * especially security-related options.
+                */
+               mss = rt->rt_rmx.rmx_mtu - sizeof(struct tcphdr);
+               switch (tp->pf) {
 #ifdef INET6
-       }
-#endif /* INET6 */
-       else
+               case AF_INET6:
+                       mss -= sizeof(struct ip6_hdr);
+                       break;
+#endif
+#ifdef notdef  /* no IPv4 path MTU discovery yet */
+               case AF_INET:
+                       mss -= sizeof(struct ip);
+                       break;
+#endif
+               default:
+                       /* the family does not support path MTU discovery */
+                       mss = 0;
+                       break;
+               }
+       } else
+               mss = 0;
+#else
+       mss = 0;
 #endif /* RTV_MTU */
-       {
-         /*
-          *  ifp may be null and rmx_mtu may be zero in certain
-          *  v6 cases (e.g., if ND wasn't able to resolve the 
-          *  destination host.
-          */
+       if (mss == 0) {
+               /*
+                * ifp may be null and rmx_mtu may be zero in certain
+                * v6 cases (e.g., if ND wasn't able to resolve the 
+                * destination host.
+                */
                mss = ifp ? ifp->if_mtu - sizeof(struct tcpiphdr) : 0;
-#ifdef INET6
-               if (tp->pf == AF_INET)
-#endif /* INET6 */
-               if (!in_localaddr(inp->inp_faddr))
-                       mss = min(mss, tcp_mssdflt);
+               switch (tp->pf) {
+               case AF_INET:
+                       if (!in_localaddr(inp->inp_faddr))
+                               mss = min(mss, tcp_mssdflt);
+                       break;
+               }
        }
        /*
         * The current mss, t_maxseg, is initialized to the default value.