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Index: head/sys/netinet/in_pcb.h
===================================================================
--- head/sys/netinet/in_pcb.h
+++ head/sys/netinet/in_pcb.h
@@ -122,6 +122,7 @@
* Flags for inc_flags.
*/
#define INC_ISIPV6 0x01
+#define INC_IPV6MINMTU 0x02
#define inc_fport inc_ie.ie_fport
#define inc_lport inc_ie.ie_lport
Index: head/sys/netinet/tcp_input.c
===================================================================
--- head/sys/netinet/tcp_input.c
+++ head/sys/netinet/tcp_input.c
@@ -1051,6 +1051,8 @@
#ifdef INET6
if (isipv6) {
inc.inc_flags |= INC_ISIPV6;
+ if (inp->inp_inc.inc_flags & INC_IPV6MINMTU)
+ inc.inc_flags |= INC_IPV6MINMTU;
inc.inc6_faddr = ip6->ip6_src;
inc.inc6_laddr = ip6->ip6_dst;
} else
Index: head/sys/netinet/tcp_subr.c
===================================================================
--- head/sys/netinet/tcp_subr.c
+++ head/sys/netinet/tcp_subr.c
@@ -2865,6 +2865,9 @@
KASSERT(inc != NULL, ("tcp_maxmtu6 with NULL in_conninfo pointer"));
+ if (inc->inc_flags & INC_IPV6MINMTU)
+ return (IPV6_MMTU);
+
if (!IN6_IS_ADDR_UNSPECIFIED(&inc->inc6_faddr)) {
in6_splitscope(&inc->inc6_faddr, &dst6, &scopeid);
if (fib6_lookup_nh_ext(inc->inc_fibnum, &dst6, scopeid, 0,
Index: head/sys/netinet/tcp_usrreq.c
===================================================================
--- head/sys/netinet/tcp_usrreq.c
+++ head/sys/netinet/tcp_usrreq.c
@@ -1584,6 +1584,42 @@
if (inp->inp_vflag & INP_IPV6PROTO) {
INP_WUNLOCK(inp);
error = ip6_ctloutput(so, sopt);
+ /*
+ * In case of the IPV6_USE_MIN_MTU socket option,
+ * the INC_IPV6MINMTU flag to announce a corresponding
+ * MSS during the initial handshake.
+ * If the TCP connection is not in the front states,
+ * just reduce the MSS being used.
+ * This avoids the sending of TCP segments which will
+ * be fragmented at the IPv6 layer.
+ */
+ if ((error == 0) &&
+ (sopt->sopt_dir == SOPT_SET) &&
+ (sopt->sopt_level == IPPROTO_IPV6) &&
+ (sopt->sopt_name == IPV6_USE_MIN_MTU)) {
+ INP_WLOCK(inp);
+ if ((inp->inp_flags &
+ (INP_TIMEWAIT | INP_DROPPED))) {
+ INP_WUNLOCK(inp);
+ return (ECONNRESET);
+ }
+ inp->inp_inc.inc_flags |= INC_IPV6MINMTU;
+ tp = intotcpcb(inp);
+ if ((tp->t_state >= TCPS_SYN_SENT) &&
+ (inp->inp_inc.inc_flags & INC_ISIPV6)) {
+ struct ip6_pktopts *opt;
+
+ opt = inp->in6p_outputopts;
+ if ((opt != NULL) &&
+ (opt->ip6po_minmtu ==
+ IP6PO_MINMTU_ALL)) {
+ if (tp->t_maxseg > TCP6_MSS) {
+ tp->t_maxseg = TCP6_MSS;
+ }
+ }
+ }
+ INP_WUNLOCK(inp);
+ }
}
#endif /* INET6 */
#if defined(INET6) && defined(INET)

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