Disable MTU feedback in IPv6 if the sender writes data that must be fragmented.

Discussed extensively with KAME.  The API author's intent isn't clear at this
point, so rather than remove the code entirely, #if 0 out and put a big
comment in for now. The IPV6_RECVPATHMTU sockopt is available if the
application wants to be notified of the path MTU to optimize packet sizes.

Thanks to JINMEI Tatuya <jinmei@isl.rdc.toshiba.co.jp> for putting up
with my incessant badgering on this issue, and fenner for pointing out
the API issue and suggesting solutions.

1 files changed, 14 insertions, 0 deletions

diff --git a/sys/netinet6/ip6_output.c b/sys/netinet6/ip6_output.c
index 03ef3bc95db2..22be481afd87 100644
--- a/sys/netinet6/ip6_output.c
+++ b/sys/netinet6/ip6_output.c
@@ -1035,8 +1035,10 @@ passout:
 		struct ip6_frag *ip6f;
 		u_int32_t id = htonl(ip6_randomid());
 		u_char nextproto;
+#if 0
 		struct ip6ctlparam ip6cp;
 		u_int32_t mtu32;
+#endif
 		int qslots = ifp->if_snd.ifq_maxlen - ifp->if_snd.ifq_len;
 
 		/*
@@ -1048,12 +1050,24 @@ passout:
 		if (mtu > IPV6_MAXPACKET)
 			mtu = IPV6_MAXPACKET;
 
+#if 0
+		/*
+		 * It is believed this code is a leftover from the
+		 * development of the IPV6_RECVPATHMTU sockopt and 
+		 * associated work to implement RFC3542.
+		 * It's not entirely clear what the intent of the API
+		 * is at this point, so disable this code for now.
+		 * The IPV6_RECVPATHMTU sockopt and/or IPV6_DONTFRAG
+		 * will send notifications if the application requests.
+		 */
+
 		/* Notify a proper path MTU to applications. */
 		mtu32 = (u_int32_t)mtu;
 		bzero(&ip6cp, sizeof(ip6cp));
 		ip6cp.ip6c_cmdarg = (void *)&mtu32;
 		pfctlinput2(PRC_MSGSIZE, (struct sockaddr *)&ro_pmtu->ro_dst,
 		    (void *)&ip6cp);
+#endif
 
 		len = (mtu - hlen - sizeof(struct ip6_frag)) & ~7;
 		if (len < 8) {