Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -1611,6 +1611,14 @@
 	 * XXX this is traditional behavior, may need to be cleaned up.
 	 */
 	if (tp->t_state == TCPS_SYN_SENT && (thflags & TH_SYN)) {
+		/* Handle parallel SYN for ECN */
+		if (!(thflags & TH_ACK) &&
+		    ((thflags & (TH_CWR | TH_ECE)) == (TH_CWR | TH_ECE)) &&
+		    ((V_tcp_do_ecn == 1) || (V_tcp_do_ecn == 2))) {
+			tp->t_flags2 |= TF2_ECN_PERMIT;
+			tp->t_flags2 |= TF2_ECN_SND_ECE;
+			TCPSTAT_INC(tcps_ecn_shs);
+		}
 		if ((to.to_flags & TOF_SCALE) &&
 		    (tp->t_flags & TF_REQ_SCALE)) {
 			tp->t_flags |= TF_RCVD_SCALE;
Index: sys/netinet/tcp_output.c
===================================================================
--- sys/netinet/tcp_output.c
+++ sys/netinet/tcp_output.c
@@ -1154,6 +1154,12 @@
 		} else
 			flags |= TH_ECE|TH_CWR;
 	}
+	/* Handle parallel SYN for ECN */
+	if ((tp->t_state == TCPS_SYN_RECEIVED) &&
+	    (tp->t_flags2 & TF2_ECN_SND_ECE)) {
+			flags |= TH_ECE;
+			tp->t_flags2 &= ~TF2_ECN_SND_ECE;
+	}
 
 	if (tp->t_state == TCPS_ESTABLISHED &&
 	    (tp->t_flags2 & TF2_ECN_PERMIT)) {
Index: sys/netinet/tcp_stacks/rack.c
===================================================================
--- sys/netinet/tcp_stacks/rack.c
+++ sys/netinet/tcp_stacks/rack.c
@@ -11070,6 +11070,14 @@
 		 * this is traditional behavior, may need to be cleaned up.
 		 */
 		if (tp->t_state == TCPS_SYN_SENT && (thflags & TH_SYN)) {
+			/* Handle parallel SYN for ECN */
+			if (!(thflags & TH_ACK) &&
+			    ((thflags & (TH_CWR | TH_ECE)) == (TH_CWR | TH_ECE)) &&
+			    ((V_tcp_do_ecn == 1) || (V_tcp_do_ecn == 2))) {
+				tp->t_flags2 |= TF2_ECN_PERMIT;
+				tp->t_flags2 |= TF2_ECN_SND_ECE;
+				TCPSTAT_INC(tcps_ecn_shs);
+			}
 			if ((to.to_flags & TOF_SCALE) &&
 			    (tp->t_flags & TF_REQ_SCALE)) {
 				tp->t_flags |= TF_RCVD_SCALE;
@@ -13523,6 +13531,12 @@
 		} else
 			flags |= TH_ECE | TH_CWR;
 	}
+	/* Handle parallel SYN for ECN */
+	if ((tp->t_state == TCPS_SYN_RECEIVED) &&
+	    (tp->t_flags2 & TF2_ECN_SND_ECE)) {
+		flags |= TH_ECE;
+		tp->t_flags2 &= ~TF2_ECN_SND_ECE;
+	}
 	if (tp->t_state == TCPS_ESTABLISHED &&
 	    (tp->t_flags2 & TF2_ECN_PERMIT)) {
 		/*