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
@@ -43,7 +43,7 @@
 
 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	 tcp_ecn_input_segment(struct tcpcb *, uint16_t, int, int, int);
 uint16_t tcp_ecn_output_syn_sent(struct tcpcb *);
 int	 tcp_ecn_output_established(struct tcpcb *, uint16_t *, int, bool);
 void	 tcp_ecn_syncache_socket(struct tcpcb *, struct syncache *);
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
@@ -271,9 +271,9 @@
  * TCP ECN processing.
  */
 int
-tcp_ecn_input_segment(struct tcpcb *tp, uint16_t thflags, int iptos)
+tcp_ecn_input_segment(struct tcpcb *tp, uint16_t thflags, int tlen, int pkts, int iptos)
 {
-	int delta_ace = 0;
+	int delta_cep = 0;
 
 	if (tp->t_flags2 & (TF2_ECN_PERMIT | TF2_ACE_PERMIT)) {
 		switch (iptos & IPTOS_ECN_MASK) {
@@ -292,9 +292,12 @@
 			if ((iptos & IPTOS_ECN_MASK) == IPTOS_ECN_CE)
 				tp->t_rcep += 1;
 			if (tp->t_flags2 & TF2_ECN_PERMIT) {
-				delta_ace = (tcp_ecn_get_ace(thflags) + 8 -
-					    (tp->t_scep & 0x07)) & 0x07;
-				tp->t_scep += delta_ace;
+				delta_cep = (tcp_ecn_get_ace(thflags) + 8 -
+					    (tp->t_scep & 7)) & 7;
+				if (delta_cep < pkts)
+					delta_cep = pkts -
+					    ((pkts - delta_cep) & 7);
+				tp->t_scep += delta_cep;
 			} else {
 				/*
 				 * process the final ACK of the 3WHS
@@ -326,7 +329,7 @@
 		} else {
 			/* RFC3168 ECN handling */
 			if ((thflags & (TH_SYN | TH_ECE)) == TH_ECE) {
-				delta_ace = 1;
+				delta_cep = 1;
 				tp->t_scep++;
 			}
 			if (thflags & TH_CWR) {
@@ -341,7 +344,7 @@
 		cc_ecnpkt_handler_flags(tp, thflags, iptos);
 	}
 
-	return delta_ace;
+	return delta_cep;
 }
 
 /*
diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -1627,7 +1627,9 @@
 	/*
 	 * TCP ECN processing.
 	 */
-	if (tcp_ecn_input_segment(tp, thflags, iptos))
+	if (tcp_ecn_input_segment(tp, thflags, tlen,
+	    tcp_packets_this_ack(tp, th->th_ack),
+	    iptos))
 		cc_cong_signal(tp, th, CC_ECN);
 
 	/*
diff --git a/sys/netinet/tcp_stacks/rack.c b/sys/netinet/tcp_stacks/rack.c
--- a/sys/netinet/tcp_stacks/rack.c
+++ b/sys/netinet/tcp_stacks/rack.c
@@ -13528,8 +13528,10 @@
 			rack_cc_after_idle(rack, tp);
 		}
 		tp->t_rcvtime = ticks;
-		/* Now what about ECN? */
-		if (tcp_ecn_input_segment(tp, ae->flags, ae->codepoint))
+		/* Now what about ECN of a chain of pure ACKs? */
+		if (tcp_ecn_input_segment(tp, ae->flags, 0,
+			tcp_packets_this_ack(tp, ae->ack),
+			ae->codepoint))
 			rack_cong_signal(tp, CC_ECN, ae->ack, __LINE__);
 #ifdef TCP_ACCOUNTING
 		/* Count for the specific type of ack in */
@@ -14320,7 +14322,9 @@
 	 * TCP ECN processing. XXXJTL: If we ever use ECN, we need to move
 	 * this to occur after we've validated the segment.
 	 */
-	if (tcp_ecn_input_segment(tp, thflags, iptos))
+	if (tcp_ecn_input_segment(tp, thflags, tlen,
+	    tcp_packets_this_ack(tp, th->th_ack),
+	    iptos))
 		rack_cong_signal(tp, CC_ECN, th->th_ack, __LINE__);
 
 	/*
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
@@ -551,6 +551,12 @@
 #endif
 
 #define	BYTES_THIS_ACK(tp, th)	(th->th_ack - tp->snd_una)
+static int inline
+tcp_packets_this_ack(struct tcpcb *tp, tcp_seq ack)
+{
+	return ((ack - tp->snd_una) / tp->t_maxseg +
+		((((ack - tp->snd_una) % tp->t_maxseg) != 0) ? 1 : 0));
+}
 
 /*
  * Flags for the t_oobflags field.