Index: head/sys/netinet/tcp_input.c
===================================================================
--- head/sys/netinet/tcp_input.c
+++ head/sys/netinet/tcp_input.c
@@ -1545,8 +1545,10 @@
 	 * TCP ECN processing.
 	 */
 	if (tp->t_flags2 & TF2_ECN_PERMIT) {
-		if (thflags & TH_CWR)
+		if (thflags & TH_CWR) {
 			tp->t_flags2 &= ~TF2_ECN_SND_ECE;
+			tp->t_flags |= TF_ACKNOW;
+		}
 		switch (iptos & IPTOS_ECN_MASK) {
 		case IPTOS_ECN_CE:
 			tp->t_flags2 |= TF2_ECN_SND_ECE;
Index: head/sys/netinet/tcp_stacks/rack.c
===================================================================
--- head/sys/netinet/tcp_stacks/rack.c
+++ head/sys/netinet/tcp_stacks/rack.c
@@ -7701,8 +7701,10 @@
 	 * this to occur after we've validated the segment.
 	 */
 	if (tp->t_flags2 & TF2_ECN_PERMIT) {
-		if (thflags & TH_CWR)
+		if (thflags & TH_CWR) {
 			tp->t_flags2 &= ~TF2_ECN_SND_ECE;
+			tp->t_flags |= TF_ACKNOW;
+		}
 		switch (iptos & IPTOS_ECN_MASK) {
 		case IPTOS_ECN_CE:
 			tp->t_flags2 |= TF2_ECN_SND_ECE;