Index: stable/10/sys/netinet/tcp_input.c
===================================================================
--- stable/10/sys/netinet/tcp_input.c
+++ stable/10/sys/netinet/tcp_input.c
@@ -184,7 +184,7 @@
 
 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, ecn, CTLFLAG_RW, 0, "TCP ECN");
 
-VNET_DEFINE(int, tcp_do_ecn) = 0;
+VNET_DEFINE(int, tcp_do_ecn) = 2;
 SYSCTL_VNET_INT(_net_inet_tcp_ecn, OID_AUTO, enable, CTLFLAG_RW,
     &VNET_NAME(tcp_do_ecn), 0,
     "TCP ECN support");
Index: stable/10/sys/netinet/tcp_output.c
===================================================================
--- stable/10/sys/netinet/tcp_output.c
+++ stable/10/sys/netinet/tcp_output.c
@@ -1109,7 +1109,7 @@
 	 * resend those bits a number of times as per
 	 * RFC 3168.
 	 */
-	if (tp->t_state == TCPS_SYN_SENT && V_tcp_do_ecn) {
+	if (tp->t_state == TCPS_SYN_SENT && V_tcp_do_ecn == 1) {
 		if (tp->t_rxtshift >= 1) {
 			if (tp->t_rxtshift <= V_tcp_ecn_maxretries)
 				flags |= TH_ECE|TH_CWR;