diff --git a/share/man/man4/tcp.4 b/share/man/man4/tcp.4
--- a/share/man/man4/tcp.4
+++ b/share/man/man4/tcp.4
@@ -31,7 +31,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd November 30, 2023
+.Dd January 17, 2023
 .Dt TCP 4
 .Os
 .Sh NAME
@@ -504,6 +504,9 @@
 specific connection.
 This is needed to help with connection establishment
 when a broken firewall is in the network path.
+.It Va ecn.option
+Reflect back the number of received bytes with a particular ECN marking
+by using the Accurate ECN TCP option on each outgoing packet.
 .It Va fast_finwait2_recycle
 Recycle
 .Tn TCP
diff --git a/sys/netinet/tcp.h b/sys/netinet/tcp.h
--- a/sys/netinet/tcp.h
+++ b/sys/netinet/tcp.h
@@ -121,6 +121,10 @@
 #define	   TCPOLEN_SIGNATURE		18
 #define	TCPOPT_FAST_OPEN	34
 #define	   TCPOLEN_FAST_OPEN_EMPTY	2
+#define	TCPOPT_ACCECN0		0xAC
+#define	TCPOPT_ACCECN1		0XAE
+#define	   TCPOLEN_ACCECN_EMPTY		2
+#define	   TCPOLEN_ACCECN_COUNTER	3
 
 #define	MAX_TCPOPTLEN		40	/* Absolute maximum TCP options len */
 
@@ -431,12 +435,12 @@
 	/* Accurate ECN counters. */
 	u_int32_t	tcpi_delivered_ce;
 	u_int32_t	tcpi_received_ce;		/* # of CE marks received */
-	u_int32_t	__tcpi_delivered_e1_bytes;
-	u_int32_t	__tcpi_delivered_e0_bytes;
-	u_int32_t	__tcpi_delivered_ce_bytes;
-	u_int32_t	__tcpi_received_e1_bytes;
-	u_int32_t	__tcpi_received_e0_bytes;
-	u_int32_t	__tcpi_received_ce_bytes;
+	u_int32_t	tcpi_delivered_e1_bytes;
+	u_int32_t	tcpi_delivered_e0_bytes;
+	u_int32_t	tcpi_delivered_ce_bytes;
+	u_int32_t	tcpi_received_e1_bytes;
+	u_int32_t	tcpi_received_e0_bytes;
+	u_int32_t	tcpi_received_ce_bytes;
 
 	u_int32_t	tcpi_total_tlp;		/* tail loss probes sent */
 	u_int64_t	tcpi_total_tlp_bytes;	/* tail loss probe bytes sent */
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
@@ -48,6 +48,24 @@
 uint16_t tcp_ecn_syncache_respond(uint16_t, struct syncache *);
 int	 tcp_ecn_get_ace(uint16_t);
 
+static inline void hton24(u_char **p, uint32_t v)
+{
+	*(*p)++ = (u_char)(v >> 16);
+	*(*p)++ = (u_char)(v >> 8);
+	*(*p)++ = (u_char)(v);
+}
+
+static inline uint32_t ntoh24(u_char *p)
+{
+	uint32_t v;
+
+	v  = (uint32_t)(p[0] << 16);
+	v |= (uint32_t)(p[1] << 8);
+	v |= (uint32_t)(p[2] << 0);
+	return v;
+}
+
+
 #endif /* _KERNEL */
 
 #endif /* _NETINET_TCP_ECN_H_ */
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
@@ -110,13 +110,17 @@
     CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(tcp_ecn_maxretries), 0,
     "Max retries before giving up on ECN");
 
+VNET_DEFINE(int, tcp_ecn_option) = 0;
+SYSCTL_INT(_net_inet_tcp_ecn, OID_AUTO, option,
+    CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(tcp_ecn_option), 0,
+    "Use AccECN TCP option");
+
 /*
  * Process incoming SYN,ACK packet
  */
 void
 tcp_ecn_input_syn_sent(struct tcpcb *tp, uint16_t thflags, int iptos)
 {
-
 	if (V_tcp_do_ecn == 0)
 		return;
 	if ((V_tcp_do_ecn == 1) ||
@@ -149,7 +153,9 @@
 		case (0|TH_CWR|0):
 			tp->t_flags2 |= TF2_ACE_PERMIT;
 			tp->t_flags2 &= ~TF2_ECN_PERMIT;
-			tp->t_scep = 5;
+			if (V_tcp_ecn_option)
+				tp->t_flags |= TF_ACCECN_OPT;
+			tp->t_ae.scep = 5;
 			TCPSTAT_INC(tcps_ecn_shs);
 			TCPSTAT_INC(tcps_ace_nect);
 			break;
@@ -157,7 +163,9 @@
 		case (TH_AE|0|0):
 			tp->t_flags2 |= TF2_ACE_PERMIT;
 			tp->t_flags2 &= ~TF2_ECN_PERMIT;
-			tp->t_scep = 5;
+			if (V_tcp_ecn_option)
+				tp->t_flags |= TF_ACCECN_OPT;
+			tp->t_ae.scep = 5;
 			TCPSTAT_INC(tcps_ecn_shs);
 			TCPSTAT_INC(tcps_ace_ect0);
 			break;
@@ -165,7 +173,9 @@
 		case (0|TH_CWR|TH_ECE):
 			tp->t_flags2 |= TF2_ACE_PERMIT;
 			tp->t_flags2 &= ~TF2_ECN_PERMIT;
-			tp->t_scep = 5;
+			if (V_tcp_ecn_option)
+				tp->t_flags |= TF_ACCECN_OPT;
+			tp->t_ae.scep = 5;
 			TCPSTAT_INC(tcps_ecn_shs);
 			TCPSTAT_INC(tcps_ace_ect1);
 			break;
@@ -173,7 +183,9 @@
 		case (TH_AE|TH_CWR|0):
 			tp->t_flags2 |= TF2_ACE_PERMIT;
 			tp->t_flags2 &= ~TF2_ECN_PERMIT;
-			tp->t_scep = 6;
+			if (V_tcp_ecn_option)
+				tp->t_flags |= TF_ACCECN_OPT;
+			tp->t_ae.scep = 6;
 			/*
 			 * reduce the IW to 2 MSS (to
 			 * account for delayed acks) if
@@ -196,16 +208,16 @@
 		 */
 		switch (iptos & IPTOS_ECN_MASK) {
 		case (IPTOS_ECN_NOTECT):
-			tp->t_rcep = 0b010;
+			tp->t_ae.rcep = 0b010;
 			break;
 		case (IPTOS_ECN_ECT0):
-			tp->t_rcep = 0b100;
+			tp->t_ae.rcep = 0b100;
 			break;
 		case (IPTOS_ECN_ECT1):
-			tp->t_rcep = 0b011;
+			tp->t_ae.rcep = 0b011;
 			break;
 		case (IPTOS_ECN_CE):
-			tp->t_rcep = 0b110;
+			tp->t_ae.rcep = 0b110;
 			break;
 		}
 	}
@@ -248,6 +260,8 @@
 		case (TH_AE|TH_CWR|TH_ECE):
 			tp->t_flags2 |= TF2_ACE_PERMIT;
 			tp->t_flags2 &= ~TF2_ECN_PERMIT;
+			if (V_tcp_ecn_option)
+				tp->t_flags |= TF_ACCECN_OPT;
 			TCPSTAT_INC(tcps_ecn_shs);
 			/*
 			 * Set the AccECN Codepoints on
@@ -258,16 +272,16 @@
 			 */
 			switch (iptos & IPTOS_ECN_MASK) {
 			case (IPTOS_ECN_NOTECT):
-				tp->t_rcep = 0b010;
+				tp->t_ae.rcep = 0b010;
 				break;
 			case (IPTOS_ECN_ECT0):
-				tp->t_rcep = 0b100;
+				tp->t_ae.rcep = 0b100;
 				break;
 			case (IPTOS_ECN_ECT1):
-				tp->t_rcep = 0b011;
+				tp->t_ae.rcep = 0b011;
 				break;
 			case (IPTOS_ECN_CE):
-				tp->t_rcep = 0b110;
+				tp->t_ae.rcep = 0b110;
 				break;
 			}
 			break;
@@ -294,18 +308,31 @@
 		TCPSTAT_INC(tcps_ecn_rcvect1);
 		break;
 	}
-
 	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)
-				tp->t_rcep += 1;
+			switch (iptos & IPTOS_ECN_MASK) {
+			case IPTOS_ECN_CE:
+				tp->t_flags2 |= TF2_ACO_CE;
+				tp->t_ae.rceb += tlen;
+				tp->t_ae.rcep++;
+				break;
+			case IPTOS_ECN_ECT0:
+				tp->t_flags2 |= TF2_ACO_E0;
+				tp->t_ae.re0b += tlen;
+				break;
+			case IPTOS_ECN_ECT1:
+				tp->t_flags2 |= TF2_ACO_E1;
+				tp->t_ae.re1b += tlen;
+				break;
+			}
 			if (tp->t_flags2 & TF2_ECN_PERMIT) {
 				delta_cep = (tcp_ecn_get_ace(thflags) + 8 -
-					    (tp->t_scep & 7)) & 7;
+					    (tp->t_ae.scep & 7)) & 7;
 				if (delta_cep < pkts)
 					delta_cep = pkts -
 					    ((pkts - delta_cep) & 7);
-				tp->t_scep += delta_cep;
+				tp->t_ae.scep += delta_cep;
+				tp->t_ae.dcep = delta_cep;
 			} else {
 				/*
 				 * process the final ACK of the 3WHS
@@ -320,16 +347,16 @@
 					/* FALLTHROUGH */
 				case 0b100:
 					/* ECT0 SYN or SYN,ACK */
-					tp->t_scep = 5;
+					tp->t_ae.scep = 5;
 					break;
 				case 0b110:
 					/* CE SYN or SYN,ACK */
-					tp->t_scep = 6;
+					tp->t_ae.scep = 6;
 					tp->snd_cwnd = 2 * tcp_maxseg(tp);
 					break;
 				default:
 					/* mangled AccECN handshake */
-					tp->t_scep = 5;
+					tp->t_ae.scep = 5;
 					break;
 				}
 				tp->t_flags2 |= TF2_ECN_PERMIT;
@@ -338,7 +365,7 @@
 			/* RFC3168 ECN handling */
 			if ((thflags & (TH_SYN | TH_ECE)) == TH_ECE) {
 				delta_cep = 1;
-				tp->t_scep++;
+				tp->t_ae.scep++;
 			}
 			if (thflags & TH_CWR) {
 				tp->t_flags2 &= ~TF2_ECN_SND_ECE;
@@ -419,21 +446,21 @@
 	 */
 	if (tp->t_flags2 & TF2_ACE_PERMIT) {
 		*thflags &= ~(TH_AE|TH_CWR|TH_ECE);
-		if (tp->t_rcep & 0x01)
+		if (tp->t_ae.rcep & 0x01)
 			*thflags |= TH_ECE;
-		if (tp->t_rcep & 0x02)
+		if (tp->t_ae.rcep & 0x02)
 			*thflags |= TH_CWR;
-		if (tp->t_rcep & 0x04)
+		if (tp->t_ae.rcep & 0x04)
 			*thflags |= TH_AE;
 		if (!(tp->t_flags2 & TF2_ECN_PERMIT)) {
 			/*
 			 * here we process the final
 			 * ACK of the 3WHS
 			 */
-			if (tp->t_rcep == 0b110) {
-				tp->t_rcep = 6;
+			if (tp->t_ae.rcep == 0b110) {
+				tp->t_ae.rcep = 6;
 			} else {
-				tp->t_rcep = 5;
+				tp->t_ae.rcep = 5;
 			}
 			tp->t_flags2 |= TF2_ECN_PERMIT;
 		}
@@ -446,7 +473,6 @@
 		if (tp->t_flags2 & TF2_ECN_SND_ECE)
 			*thflags |= TH_ECE;
 	}
-
 	return ipecn;
 }
 
@@ -468,13 +494,17 @@
 			/* FALLTHROUGH */
 		case SCF_ACE_1:
 			tp->t_flags2 |= TF2_ACE_PERMIT;
-			tp->t_scep = 5;
-			tp->t_rcep = 5;
+			if (V_tcp_ecn_option)
+				tp->t_flags |= TF_ACCECN_OPT;
+			tp->t_ae.scep = 5;
+			tp->t_ae.rcep = 5;
 			break;
 		case SCF_ACE_CE:
 			tp->t_flags2 |= TF2_ACE_PERMIT;
-			tp->t_scep = 6;
-			tp->t_rcep = 6;
+			if (V_tcp_ecn_option)
+				tp->t_flags |= TF_ACCECN_OPT;
+			tp->t_ae.scep = 6;
+			tp->t_ae.rcep = 6;
 			break;
 		/* undefined SCF codepoint */
 		default:
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
@@ -994,6 +994,8 @@
 	}
 
 	tp = intotcpcb(inp);
+	to.to_ae = &tp->t_ae;
+
 	switch (tp->t_state) {
 	case TCPS_TIME_WAIT:
 		/*
@@ -1520,7 +1522,7 @@
 	int acked, ourfinisacked, needoutput = 0;
 	sackstatus_t sack_changed;
 	int rstreason, todrop, win, incforsyn = 0;
-	uint32_t tiwin;
+	uint32_t tiwin, old_sceb;
 	uint16_t nsegs;
 	char *s;
 	struct inpcb *inp = tptoinpcb(tp);
@@ -1534,6 +1536,7 @@
 	thflags = tcp_get_flags(th);
 	tp->sackhint.last_sack_ack = 0;
 	sack_changed = SACK_NOCHANGE;
+	to.to_ae = &tp->t_ae;
 	nsegs = max(1, m->m_pkthdr.lro_nsegs);
 
 	NET_EPOCH_ASSERT();
@@ -1605,9 +1608,15 @@
 	/*
 	 * Parse options on any incoming segment.
 	 */
+	old_sceb = tp->t_ae.sceb;
 	tcp_dooptions(&to, (u_char *)(th + 1),
 	    (th->th_off << 2) - sizeof(struct tcphdr),
 	    (thflags & TH_SYN) ? TO_SYN : 0);
+	if ((to.to_flags & TOF_ACCE_CE) &&
+	    (tp->t_ae.dcep != 0) &&
+	    ((tp->t_ae.sceb - old_sceb) == 0))
+		tp->t_ae.scep -= tp->t_ae.dcep;
+
 
 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 	if ((tp->t_flags & TF_SIGNATURE) != 0 &&
@@ -3443,7 +3452,7 @@
 void
 tcp_dooptions(struct tcpopt *to, u_char *cp, int cnt, int flags)
 {
-	int opt, optlen;
+	int opt, optlen, tmp;
 
 	to->to_flags = 0;
 	for (; cnt > 0; cnt -= optlen, cp += optlen) {
@@ -3536,6 +3545,48 @@
 			to->to_tfo_len = optlen - 2;
 			to->to_tfo_cookie = to->to_tfo_len ? cp + 2 : NULL;
 			break;
+		case TCPOPT_ACCECN0:
+		case TCPOPT_ACCECN1:
+			to->to_flags |= TOF_ACCECNOPT;
+			if (optlen >= (TCPOLEN_ACCECN_EMPTY +
+					1 * TCPOLEN_ACCECN_COUNTER)) {
+				tmp = ntoh24(cp + TCPOLEN_ACCECN_EMPTY + 0);
+				if (opt == TCPOPT_ACCECN0) {
+					to->to_flags |= TOF_ACCE_E0;
+					tmp -= (to->to_ae->se0b & 0xFFFFFF);
+					if (tmp > 0)
+						to->to_ae->se0b += tmp;
+				} else {
+					to->to_flags |= TOF_ACCE_E1;
+					tmp -= (to->to_ae->se1b & 0xFFFFFF);
+					if (tmp > 0)
+						to->to_ae->se1b += tmp;
+				}
+			}
+			if (optlen >= (TCPOLEN_ACCECN_EMPTY +
+					2 * TCPOLEN_ACCECN_COUNTER)) {
+				to->to_flags |= TOF_ACCE_CE;
+				tmp = ntoh24(cp + TCPOLEN_ACCECN_EMPTY + 3);
+				tmp -= (to->to_ae->sceb & 0xFFFFFF);
+				if (tmp > 0)
+					to->to_ae->sceb += tmp;
+			}
+			if (optlen >= (TCPOLEN_ACCECN_EMPTY +
+					3 * TCPOLEN_ACCECN_COUNTER)) {
+				tmp = ntoh24(cp + TCPOLEN_ACCECN_EMPTY + 6);
+				if (opt == TCPOPT_ACCECN0) {
+					to->to_flags |= TOF_ACCE_E1;
+					tmp -= (to->to_ae->se1b & 0xFFFFFF);
+					if (tmp > 0)
+						to->to_ae->se1b += tmp;
+				} else {
+					to->to_flags |= TOF_ACCE_E0;
+					tmp -= (to->to_ae->se0b & 0xFFFFFF);
+					if (tmp > 0)
+						to->to_ae->se0b += tmp;
+				}
+			}
+			break;
 		default:
 			continue;
 		}
diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -596,10 +596,14 @@
 		 * Note: this may not work when tcp headers change
 		 * very dynamically in the future.
 		 */
-		if ((((tp->t_flags & TF_SIGNATURE) ?
+		if ((min(TCP_MAXOLEN,
+		    (((tp->t_flags & TF_SIGNATURE) ?
 			PADTCPOLEN(TCPOLEN_SIGNATURE) : 0) +
 		    ((tp->t_flags & TF_RCVD_TSTMP) ?
 			PADTCPOLEN(TCPOLEN_TIMESTAMP) : 0) +
+		    ((tp->t_flags & TF_ACCECN_OPT) ?
+			PADTCPOLEN(TCPOLEN_ACCECN_EMPTY +
+			    3 * TCPOLEN_ACCECN_COUNTER) : 0))) +
 		    len) >= tp->t_maxseg)
 			goto send;
 		/*
@@ -876,9 +880,32 @@
 		if (tp->t_flags & TF_SIGNATURE)
 			to.to_flags |= TOF_SIGNATURE;
 #endif /* TCP_SIGNATURE */
-
+		/*
+		 * AccECN option
+		 * Don't send on <SYN>, only on <SYN,ACK> or
+		 * when doing an AccECN session
+		 */
+		if (tp->t_flags & TF_ACCECN_OPT) {
+			to.to_flags |= TOF_ACCECNOPT;
+			to.to_ae = &tp->t_ae;
+			to.to_flags |=  ((tp->t_flags2 & TF2_ACO_E0) ? TOF_ACCE_E0 : 0) |
+					((tp->t_flags2 & TF2_ACO_E1) ? TOF_ACCE_E1 : 0) |
+					((tp->t_flags2 & TF2_ACO_CE) ? TOF_ACCE_CE : 0);
+			if (flags & TH_SYN)
+				to.to_flags |= TOF_ACCE_SYN;
+			if (tp->t_flags & TF_ACKNOW)
+				to.to_flags |= TOF_ACCE_ACKNOW;
+		}
 		/* Processing the options. */
 		hdrlen += optlen = tcp_addoptions(&to, opt);
+		if (to.to_flags & TOF_ACCECNOPT) {
+			if ((to.to_flags & TOF_ACCE_E0) == 0)
+				tp->t_flags2 &= ~TF2_ACO_E0;
+			if ((to.to_flags & TOF_ACCE_E1) == 0)
+				tp->t_flags2 &= ~TF2_ACO_E1;
+			if ((to.to_flags & TOF_ACCE_CE) == 0)
+				tp->t_flags2 &= ~TF2_ACO_CE;
+		}
 		/*
 		 * If we wanted a TFO option to be added, but it was unable
 		 * to fit, ensure no data is sent.
@@ -1919,6 +1946,78 @@
 			optlen += total_len;
 			break;
 			}
+		case TOF_ACCECNOPT:
+			{
+			int tmp = 0;
+			int max_len = TCP_MAXOLEN - optlen;
+			if (max_len < TCPOLEN_ACCECN_EMPTY) {
+				to->to_flags &= ~TOF_ACCECNOPT;
+				continue;
+			}
+			if (max_len < (TCPOLEN_ACCECN_EMPTY +
+					1 * TCPOLEN_ACCECN_COUNTER)) {
+				if (to->to_flags & TOF_ACCE_SYN) {
+					*optp++ = TCPOPT_ACCECN0;
+					optlen += TCPOLEN_ACCECN_EMPTY;
+					*optp++ = TCPOLEN_ACCECN_EMPTY;
+					continue;
+				} else {
+					to->to_flags &= ~TOF_ACCECNOPT;
+					continue;
+				}
+			}
+			*optp++ = (to->to_flags & TOF_ACCE_E1) ?
+				    TCPOPT_ACCECN1 : TCPOPT_ACCECN0;
+			if (max_len >= (TCPOLEN_ACCECN_EMPTY +
+					3 * TCPOLEN_ACCECN_COUNTER)) {
+				tmp = TCPOLEN_ACCECN_EMPTY +
+					3 * TCPOLEN_ACCECN_COUNTER;
+			} else
+			if (max_len >= (TCPOLEN_ACCECN_EMPTY +
+					2 * TCPOLEN_ACCECN_COUNTER)) {
+				tmp = TCPOLEN_ACCECN_EMPTY +
+					2 * TCPOLEN_ACCECN_COUNTER;
+			} else
+			if (max_len >= (TCPOLEN_ACCECN_EMPTY +
+					1 * TCPOLEN_ACCECN_COUNTER)) {
+				tmp = TCPOLEN_ACCECN_EMPTY +
+					1 * TCPOLEN_ACCECN_COUNTER;
+			}
+			*optp++ = tmp;
+			optlen += tmp;
+			if (to->to_flags & TOF_ACCE_E1) {
+				hton24(&optp, to->to_ae->re1b);
+			} else {
+				hton24(&optp, to->to_ae->re0b);
+				to->to_flags &= ~TOF_ACCE_E0;
+			}
+			if (max_len < (TCPOLEN_ACCECN_EMPTY +
+					2 * TCPOLEN_ACCECN_COUNTER)) {
+				to->to_flags &= ~TOF_ACCE_E1;
+				continue;
+			}
+			hton24(&optp, to->to_ae->rceb);
+			to->to_flags &= ~TOF_ACCE_CE;
+			if (max_len < (TCPOLEN_ACCECN_EMPTY +
+					3 * TCPOLEN_ACCECN_COUNTER)) {
+				to->to_flags &= ~TOF_ACCE_E1;
+				continue;
+			}
+			/*
+			 * TCP option sufficient to hold full AccECN option
+			 * but only send changed counters normally,
+			 * full counters on ACKNOW
+			 */
+			if (to->to_flags & TOF_ACCE_E1) {
+				hton24(&optp, to->to_ae->re0b);
+				to->to_flags &= ~TOF_ACCE_E0;
+				to->to_flags &= ~TOF_ACCE_E1;
+				continue;
+			} else {
+				hton24(&optp, to->to_ae->re1b);
+				continue;
+			}
+			}
 		default:
 			panic("%s: unknown TCP option type", __func__);
 			break;
diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -1805,7 +1805,6 @@
 #ifdef INVARIANTS
 	int thflags = tcp_get_flags(th);
 #endif
-
 	KASSERT(tp != NULL || m != NULL, ("tcp_respond: tp and m both NULL"));
 	NET_EPOCH_ASSERT();
 
@@ -2013,9 +2012,24 @@
 		if (tp->t_flags & TF_SIGNATURE)
 			to.to_flags |= TOF_SIGNATURE;
 #endif
+		/* AccECN option */
+		if (tp->t_flags & TF_ACCECN_OPT) {
+			to.to_flags |= TOF_ACCECNOPT;
+			to.to_ae = &tp->t_ae;
+			to.to_flags |= ((tp->t_flags2 & TF2_ACO_E0) ? TOF_ACCE_E0 : 0) |
+					((tp->t_flags2 & TF2_ACO_E1) ? TOF_ACCE_E1 : 0) |
+					((tp->t_flags2 & TF2_ACO_CE) ? TOF_ACCE_CE : 0);
+		}
 		/* Add the options. */
 		tlen += optlen = tcp_addoptions(&to, optp);
-
+		if (to.to_flags & TOF_ACCECNOPT) {
+			if ((to.to_flags & TOF_ACCE_E0) == 0)
+				tp->t_flags2 &= ~TF2_ACO_E0;
+			if ((to.to_flags & TOF_ACCE_E1) == 0)
+				tp->t_flags2 &= ~TF2_ACO_E1;
+			if ((to.to_flags & TOF_ACCE_CE) == 0)
+				tp->t_flags2 &= ~TF2_ACO_CE;
+		}
 		/* Update m_len in the correct mbuf. */
 		optm->m_len += optlen;
 	} else
@@ -2330,6 +2344,14 @@
 	tcp_log_tcpcbinit(tp);
 #endif
 	tp->t_pacing_rate = -1;
+	if (V_tcp_do_lrd)
+		tp->t_flags |= TF_LRD;
+	tp->t_ae.re0b = 1;
+	tp->t_ae.re1b = 1;
+	tp->t_ae.rceb = 0;
+	tp->t_ae.se0b = 1;
+	tp->t_ae.se1b = 1;
+	tp->t_ae.sceb = 0;
 	if (tp->t_fb->tfb_tcp_fb_init) {
 		if ((*tp->t_fb->tfb_tcp_fb_init)(tp, &tp->t_fb_ptr)) {
 			refcount_release(&tp->t_fb->tfb_refcnt);
diff --git a/sys/netinet/tcp_syncache.c b/sys/netinet/tcp_syncache.c
--- a/sys/netinet/tcp_syncache.c
+++ b/sys/netinet/tcp_syncache.c
@@ -1810,6 +1810,7 @@
 #ifdef INET6
 	struct ip6_hdr *ip6 = NULL;
 #endif
+	struct accecn ae;
 
 	NET_EPOCH_ASSERT();
 
@@ -1949,6 +1950,20 @@
 				/* don't send cookie again when retransmitting response */
 				sc->sc_tfo_cookie = NULL;
 			}
+			if (V_tcp_ecn_option)
+				to.to_flags |= TOF_ACCE_SYN;
+		}
+		if (V_tcp_ecn_option &&
+		    (sc->sc_flags & SCF_ECN_MASK) &&
+		    ((sc->sc_flags & SCF_ECN_MASK) != SCF_ECN)) {
+			to.to_flags |= TOF_ACCECNOPT;
+			to.to_flags |= TOF_ACCE_E0 |
+					TOF_ACCE_E1 |
+					TOF_ACCE_CE;
+			ae.re0b = 1;
+			ae.re1b = 1;
+			ae.rceb = 0;
+			to.to_ae = &ae;
 		}
 		if (sc->sc_flags & SCF_TIMESTAMP) {
 			to.to_tsval = sc->sc_tsoff + tcp_ts_getticks();
diff --git a/sys/netinet/tcp_timer.c b/sys/netinet/tcp_timer.c
--- a/sys/netinet/tcp_timer.c
+++ b/sys/netinet/tcp_timer.c
@@ -789,6 +789,15 @@
 #endif
 			in_losing(inp);
 	}
+	/*
+	 * Disable AccECN option when
+	 * retransmitting after multiple
+	 * timeouts.
+	 */
+	if ((tp->t_rxtshift >= V_tcp_ecn_maxretries) &&
+	    (tp->t_flags2 & TF2_ACE_PERMIT) &&
+	    (tp->t_flags & TF_ACCECN_OPT))
+		tp->t_flags &= ~TF_ACCECN_OPT;
 	tp->snd_nxt = tp->snd_una;
 	tp->snd_recover = tp->snd_max;
 	/*
diff --git a/sys/netinet/tcp_usrreq.c b/sys/netinet/tcp_usrreq.c
--- a/sys/netinet/tcp_usrreq.c
+++ b/sys/netinet/tcp_usrreq.c
@@ -1614,15 +1614,23 @@
 	 * AccECN related counters.
 	 */
 	if ((tp->t_flags2 & (TF2_ECN_PERMIT | TF2_ACE_PERMIT)) ==
-	    (TF2_ECN_PERMIT | TF2_ACE_PERMIT))
+	    (TF2_ECN_PERMIT | TF2_ACE_PERMIT)) {
 		/*
 		 * Internal counter starts at 5 for AccECN
 		 * but 0 for RFC3168 ECN.
 		 */
-		ti->tcpi_delivered_ce = tp->t_scep - 5;
-	else
-		ti->tcpi_delivered_ce = tp->t_scep;
-	ti->tcpi_received_ce = tp->t_rcep;
+		ti->tcpi_delivered_ce = tp->t_ae.scep - 5;
+		ti->tcpi_received_ce = tp->t_ae.rcep - 5;
+	} else {
+		ti->tcpi_delivered_ce = tp->t_ae.scep;
+		ti->tcpi_received_ce = tp->t_ae.rcep;
+	}
+	ti->tcpi_received_e0_bytes = tp->t_ae.re0b - 1;
+	ti->tcpi_received_e1_bytes = tp->t_ae.re1b - 1;
+	ti->tcpi_received_ce_bytes = tp->t_ae.rceb;
+	ti->tcpi_delivered_e0_bytes = tp->t_ae.se0b - 1;
+	ti->tcpi_delivered_e1_bytes = tp->t_ae.se1b - 1;
+	ti->tcpi_delivered_ce_bytes = tp->t_ae.sceb;
 }
 
 /*
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
@@ -129,6 +129,18 @@
 	int32_t		lost_bytes;	/* number of rfc6675 IsLost() bytes */
 };
 
+struct accecn {
+	uint32_t	rcep;		/* Number of received CE marked pkts */
+	uint32_t	scep;		/* Synced number of delivered CE pkts */
+	uint32_t	dcep;		/* delta of CE marks for rollback */
+	uint32_t	re0b;		/* Number of ECT0 marked data bytes */
+	uint32_t	re1b;		/* Number of ECT1 marked data bytes */
+	uint32_t	rceb;		/* Number of CE marked data bytes */
+	uint32_t	se0b;		/* Synced number of delivered ECT0 bytes */
+	uint32_t	se1b;		/* Synced number of delivered ECT1 bytes */
+	uint32_t	sceb;		/* Synced number of delivered CE bytes */
+};
+
 #define SEGQ_EMPTY(tp) TAILQ_EMPTY(&(tp)->t_segq)
 
 STAILQ_HEAD(tcp_log_stailq, tcp_log_mem);
@@ -431,8 +443,7 @@
 	int	t_dupacks;		/* consecutive dup acks recd */
 	int	t_lognum;		/* Number of log entries */
 	int	t_loglimit;		/* Maximum number of log entries */
-	uint32_t t_rcep;		/* Number of received CE marked pkts */
-	uint32_t t_scep;		/* Synced number of delivered CE pkts */
+	struct accecn	t_ae;		/* AccECN related byte counters */
 	int64_t	t_pacing_rate;		/* bytes / sec, -1 => unlimited */
 	struct tcp_log_stailq t_logs;	/* Log buffer */
 	struct tcp_log_id_node *t_lin;
@@ -798,7 +809,7 @@
 #define	TF_TSO		0x01000000	/* TSO enabled on this connection */
 #define	TF_TOE		0x02000000	/* this connection is offloaded */
 #define	TF_CLOSED	0x04000000	/* close(2) called on socket */
-#define	TF_UNUSED1	0x08000000	/* unused */
+#define	TF_ACCECN_OPT	0x08000000	/* AccECN is using TCP options */
 #define	TF_LRD		0x10000000	/* Lost Retransmission Detection */
 #define	TF_CONGRECOVERY	0x20000000	/* congestion recovery mode */
 #define	TF_WASCRECOVERY	0x40000000	/* was in congestion recovery */
@@ -853,7 +864,9 @@
 #define	TF2_MBUF_QUEUE_READY	0x00020000 /* Inputs can be queued */
 #define	TF2_DONT_SACK_QUEUE	0x00040000 /* Don't wake on sack */
 #define	TF2_CANNOT_DO_ECN	0x00080000 /* The stack does not do ECN */
-
+#define	TF2_ACO_E0		0x00100000 /* EE0 counter changed */
+#define	TF2_ACO_E1		0x00200000 /* EE1 counter changed */
+#define	TF2_ACO_CE		0x00400000 /* ECE counter changed */
 /*
  * Structure to hold TCP options that are only used during segment
  * processing (in tcp_input), but not held in the tcpcb.
@@ -864,14 +877,21 @@
  */
 struct tcpopt {
 	u_int32_t	to_flags;	/* which options are present */
-#define	TOF_MSS		0x0001		/* maximum segment size */
-#define	TOF_SCALE	0x0002		/* window scaling */
-#define	TOF_SACKPERM	0x0004		/* SACK permitted */
-#define	TOF_TS		0x0010		/* timestamp */
-#define	TOF_SIGNATURE	0x0040		/* TCP-MD5 signature option (RFC2385) */
-#define	TOF_SACK	0x0080		/* Peer sent SACK option */
-#define	TOF_FASTOPEN	0x0100		/* TCP Fast Open (TFO) cookie */
-#define	TOF_MAXOPT	0x0200
+#define	TOF_MSS		0x00000001	/* maximum segment size */
+#define	TOF_SCALE	0x00000002	/* window scaling */
+#define	TOF_SACKPERM	0x00000004	/* SACK permitted */
+#define	TOF_TS		0x00000010	/* timestamp */
+#define	TOF_SIGNATURE	0x00000040	/* TCP-MD5 signature option (RFC2385) */
+#define	TOF_SACK	0x00000080	/* Peer sent SACK option */
+#define	TOF_FASTOPEN	0x00000100	/* TCP Fast Open (TFO) cookie */
+#define	TOF_ACCECNOPT	0x00000200	/* AccECN Option */
+#define	TOF_MAXOPT	0x00000400
+					/* Keep internal flags above TOF_MAXOPT */
+#define	TOF_ACCE_SYN	0x80000000	/* send empty option */
+#define	TOF_ACCE_CE	0x40000000	/* CE counter changed */
+#define	TOF_ACCE_E0	0x20000000	/* E0 counter changed */
+#define	TOF_ACCE_E1	0x10000000	/* E1 counter changed */
+#define	TOF_ACCE_ACKNOW	0x08000000	/* send full option */
 	u_int32_t	to_tsval;	/* new timestamp */
 	u_int32_t	to_tsecr;	/* reflected timestamp */
 	u_char		*to_sacks;	/* pointer to the first SACK blocks */
@@ -881,7 +901,8 @@
 	u_int8_t	to_wscale;	/* window scaling */
 	u_int8_t	to_nsacks;	/* number of SACK blocks */
 	u_int8_t	to_tfo_len;	/* TFO cookie length */
-	u_int32_t	to_spare;	/* UTO */
+	struct accecn	*to_ae;		/* pointer to AccECN byte counters */
+	u_int32_t       to_spare;       /* UTO */
 };
 
 /*
@@ -1283,6 +1304,7 @@
 VNET_DECLARE(int, tcp_do_sack);
 VNET_DECLARE(int, tcp_do_tso);
 VNET_DECLARE(int, tcp_ecn_maxretries);
+VNET_DECLARE(int, tcp_ecn_option);
 VNET_DECLARE(int, tcp_initcwnd_segments);
 VNET_DECLARE(int, tcp_insecure_rst);
 VNET_DECLARE(int, tcp_insecure_syn);
@@ -1329,6 +1351,7 @@
 #define	V_tcp_do_sack			VNET(tcp_do_sack)
 #define	V_tcp_do_tso			VNET(tcp_do_tso)
 #define	V_tcp_ecn_maxretries		VNET(tcp_ecn_maxretries)
+#define	V_tcp_ecn_option		VNET(tcp_ecn_option)
 #define	V_tcp_initcwnd_segments		VNET(tcp_initcwnd_segments)
 #define	V_tcp_insecure_rst		VNET(tcp_insecure_rst)
 #define	V_tcp_insecure_syn		VNET(tcp_insecure_syn)