diff --git a/sys/netinet/tcp_ecn.h b/sys/netinet/tcp_ecn.h
--- a/sys/netinet/tcp_ecn.h
+++ b/sys/netinet/tcp_ecn.h
@@ -40,6 +40,9 @@
 #include <netinet/tcp_var.h>
 #include <netinet/tcp_syncache.h>
 
+#define TH_ACE_SHIFT 6
+#define MAX_ACE_DELTA 3
+
 void	 tcp_ecn_input_syn_sent(struct tcpcb *, uint16_t, int);
 void	 tcp_ecn_input_parallel_syn(struct tcpcb *, uint16_t, int);
 int	 tcp_ecn_input_segment(struct tcpcb *, uint16_t, int, int, int);
@@ -48,7 +51,6 @@
 void	 tcp_ecn_syncache_socket(struct tcpcb *, struct syncache *);
 int	 tcp_ecn_syncache_add(uint16_t, int);
 uint16_t tcp_ecn_syncache_respond(uint16_t, struct syncache *);
-int	 tcp_ecn_get_ace(uint16_t);
 
 #endif /* _KERNEL */
 
diff --git a/sys/netinet/tcp_ecn.c b/sys/netinet/tcp_ecn.c
--- a/sys/netinet/tcp_ecn.c
+++ b/sys/netinet/tcp_ecn.c
@@ -98,6 +98,12 @@
 #include <netinet/tcpip.h>
 #include <netinet/tcp_ecn.h>
 
+
+static inline
+int  tcp_ecn_get_ace(uint16_t);
+static inline
+void tcp_ecn_set_ace(uint16_t *, uint32_t);
+
 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, ecn,
     CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
     "TCP ECN");
@@ -299,8 +305,14 @@
 
 	if (tp->t_flags2 & (TF2_ECN_PERMIT | TF2_ACE_PERMIT)) {
 		if (tp->t_flags2 & TF2_ACE_PERMIT) {
-			if ((iptos & IPTOS_ECN_MASK) == IPTOS_ECN_CE)
+			if ((iptos & IPTOS_ECN_MASK) == IPTOS_ECN_CE) {
 				tp->t_rcep += 1;
+				if (tp->t_rcepdelta < UCHAR_MAX) {
+					tp->t_rcepdelta++;
+				}
+				if (tp->t_rcepdelta >= MAX_ACE_DELTA)
+					tp->t_flags |= TF_ACKNOW;
+			}
 			if (tp->t_flags2 & TF2_ECN_PERMIT) {
 				delta_cep = (tcp_ecn_get_ace(thflags) + 8 -
 					    (tp->t_scep & 7)) & 7;
@@ -420,13 +432,8 @@
 	 * Reply with proper ECN notifications.
 	 */
 	if (tp->t_flags2 & TF2_ACE_PERMIT) {
-		*thflags &= ~(TH_AE|TH_CWR|TH_ECE);
-		if (tp->t_rcep & 0x01)
-			*thflags |= TH_ECE;
-		if (tp->t_rcep & 0x02)
-			*thflags |= TH_CWR;
-		if (tp->t_rcep & 0x04)
-			*thflags |= TH_AE;
+		tcp_ecn_set_ace(thflags, tp->t_rcep);
+		tp->t_rcepdelta = 0;
 		if (!(tp->t_flags2 & TF2_ECN_PERMIT)) {
 			/*
 			 * here we process the final
@@ -601,16 +608,15 @@
 	return thflags;
 }
 
-int
+static inline int
 tcp_ecn_get_ace(uint16_t thflags)
 {
-	int ace = 0;
-
-	if (thflags & TH_ECE)
-		ace += 1;
-	if (thflags & TH_CWR)
-		ace += 2;
-	if (thflags & TH_AE)
-		ace += 4;
-	return ace;
+	return ((thflags & (TH_AE|TH_CWR|TH_ECE)) >> TH_ACE_SHIFT);
 }
+
+static inline void
+tcp_ecn_set_ace(uint16_t *thflags, uint32_t t_rcep)
+{
+	*thflags &= ~(TH_AE|TH_CWR|TH_ECE);
+	*thflags |= ((t_rcep & 0x07) << TH_ACE_SHIFT);
+}
\ No newline at end of file
diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h
--- a/sys/netinet/tcp_var.h
+++ b/sys/netinet/tcp_var.h
@@ -458,6 +458,7 @@
 	uint32_t t_dsack_pack;		/* dsack packets we have eceived */
 	uint8_t t_tmr_granularity;	/* Granularity of all timers srtt etc */
 	uint8_t t_rttupdated;		/* number of times rtt sampled */
+	uint8_t t_rcepdelta;		/* Number of received CE marks not sent */
 	/* TCP Fast Open */
 	uint8_t t_tfo_client_cookie_len; /* TFO client cookie length */
 	uint32_t t_end_info_status;	/* Status flag of end info */