Index: share/man/man4/tcp.4
===================================================================
--- share/man/man4/tcp.4
+++ share/man/man4/tcp.4
@@ -34,7 +34,7 @@
 .\"     From: @(#)tcp.4	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd January 8, 2022
+.Dd February 25, 2022
 .Dt TCP 4
 .Os
 .Sh NAME
@@ -711,6 +711,23 @@
 specific connection.
 This is needed to help with connection establishment
 when a broken firewall is in the network path.
+.It Va ecn.generalized
+Enable sending all segments as ECN capable transport,
+including SYN, SYN/ACK, and retransmissions.
+This may only be enabled when ECN support itself is also active.
+Disabling ECN support will disable this feature automatically.
+Settings:
+.Bl -tag -compact
+.It 0
+Regular RFC3168 operation.
+Send only new data segments as ECN capable transport.
+(default)
+.It 1
+Support generalized ECN (ECN++), and send all segments of an ECN-enabled
+session as ECN capable transport.
+Also control packets to non-established and non-listening ports are
+identically marked, if outgoing sessions would request ECN.
+.El
 .It Va pmtud_blackhole_detection
 Enable automatic path MTU blackhole detection.
 In case of retransmits of MSS sized segments,
Index: sys/netinet/tcp_ecn.h
===================================================================
--- sys/netinet/tcp_ecn.h
+++ sys/netinet/tcp_ecn.h
@@ -48,7 +48,7 @@
 int	 tcp_ecn_output_established(struct tcpcb *, uint16_t *, int);
 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_syncache_respond(uint16_t *, struct syncache *);
 
 #endif /* _KERNEL */
 
Index: sys/netinet/tcp_ecn.c
===================================================================
--- sys/netinet/tcp_ecn.c
+++ sys/netinet/tcp_ecn.c
@@ -111,9 +111,11 @@
 {
 	thflags &= (TH_CWR|TH_ECE);
 
-	if (((thflags & (TH_CWR | TH_ECE)) == TH_ECE) &&
+	if ((thflags == TH_ECE) &&
 	    V_tcp_do_ecn) {
 		tp->t_flags2 |= TF2_ECN_PERMIT;
+		if (V_tcp_ecn_generalized)
+			tp->t_flags2 |= TF2_ECN_PLUSPLUS;
 		TCPSTAT_INC(tcps_ecn_shs);
 	}
 }
@@ -133,6 +135,8 @@
 		if ((thflags & (TH_CWR | TH_ECE)) == (TH_CWR | TH_ECE)) {
 			tp->t_flags2 |= TF2_ECN_PERMIT;
 			tp->t_flags2 |= TF2_ECN_SND_ECE;
+			if (V_tcp_ecn_generalized)
+				tp->t_flags2 |= TF2_ECN_PLUSPLUS;
 			TCPSTAT_INC(tcps_ecn_shs);
 		}
 	}
@@ -214,7 +218,20 @@
 	 */
 	newdata = (len > 0 && SEQ_GEQ(tp->snd_nxt, tp->snd_max) &&
 		    !((tp->t_flags & TF_FORCEDATA) && len == 1));
-	if (newdata) {
+	if (newdata ||
+	    /*
+	     * Send ECN SYN segments as ECN-capable transport
+	     * when ecn.generalized is set. This can not be
+	     * futher simplified, as a fall-back to non-ECN
+	     * may occur.
+	     */
+	    ((tp->t_flags2 & TF2_ECN_PLUSPLUS) &&
+	     (((*thflags & (TH_SYN|TH_ACK|TH_ECE|TH_CWR)) ==
+			(TH_SYN|       TH_ECE|TH_CWR)) ||
+	      ((*thflags & (TH_SYN|TH_ACK|TH_ECE|TH_CWR)) ==
+			(TH_SYN|TH_ACK|       TH_CWR)) ||
+	      ((*thflags & (TH_SYN|TH_ACK|TH_ECE|TH_CWR)) ==
+			(TH_SYN|TH_ACK|TH_ECE       ))))) {
 		ipecn = IPTOS_ECN_ECT0;
 		TCPSTAT_INC(tcps_ecn_ect0);
 	}
@@ -243,6 +260,8 @@
 		switch (sc->sc_flags & SCF_ECN_MASK) {
 		case SCF_ECN:
 			tp->t_flags2 |= TF2_ECN_PERMIT;
+			if (V_tcp_ecn_generalized)
+				tp->t_flags2 |= TF2_ECN_PLUSPLUS;
 			break;
 		/* undefined SCF codepoint */
 		default:
@@ -278,20 +297,25 @@
  * Set up the ECN information for the <SYN,ACK> from
  * syncache information.
  */
-uint16_t
-tcp_ecn_syncache_respond(uint16_t thflags, struct syncache *sc)
+int
+tcp_ecn_syncache_respond(uint16_t *thflags, struct syncache *sc)
 {
-	if ((thflags & TH_SYN) &&
+	int ipecn = IPTOS_ECN_NOTECT;
+
+	if ((*thflags & TH_SYN) &&
 	    (sc->sc_flags & SCF_ECN_MASK)) {
 		switch (sc->sc_flags & SCF_ECN_MASK) {
 		case SCF_ECN:
-			thflags |= (0 | TH_ECE);
+			*thflags |= (0 | TH_ECE);
 			TCPSTAT_INC(tcps_ecn_shs);
+			if ((V_tcp_ecn_generalized &&
+				(*thflags & TH_ACK)))
+				ipecn = IPTOS_ECN_ECT0;
 			break;
 		/* undefined SCF codepoint */
 		default:
 			break;
 		}
 	}
-	return thflags;
+	return ipecn;
 }
Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -211,8 +211,10 @@
     "TCP ECN");
 
 VNET_DEFINE(int, tcp_do_ecn) = 2;
-SYSCTL_INT(_net_inet_tcp_ecn, OID_AUTO, enable, CTLFLAG_VNET | CTLFLAG_RW,
-    &VNET_NAME(tcp_do_ecn), 0,
+static int sysctl_net_inet_tcp_ecn_enable_check(SYSCTL_HANDLER_ARGS);
+SYSCTL_PROC(_net_inet_tcp_ecn, OID_AUTO, enable,
+    CTLFLAG_VNET | CTLFLAG_RW | CTLTYPE_UINT | CTLFLAG_NEEDGIANT,
+    &VNET_NAME(tcp_do_ecn), 0, &sysctl_net_inet_tcp_ecn_enable_check, "IU",
     "TCP ECN support");
 
 VNET_DEFINE(int, tcp_ecn_maxretries) = 1;
@@ -220,6 +222,13 @@
     &VNET_NAME(tcp_ecn_maxretries), 0,
     "Max retries before giving up on ECN");
 
+VNET_DEFINE(int, tcp_ecn_generalized) = 0;
+static int sysctl_net_inet_tcp_ecn_generalized_check(SYSCTL_HANDLER_ARGS);
+SYSCTL_PROC(_net_inet_tcp_ecn, OID_AUTO, generalized,
+    CTLFLAG_VNET | CTLFLAG_RW | CTLTYPE_UINT | CTLFLAG_NEEDGIANT,
+    &VNET_NAME(tcp_ecn_generalized), 0, &sysctl_net_inet_tcp_ecn_generalized_check, "IU",
+    "Send all packets as ECT");
+
 VNET_DEFINE(int, tcp_insecure_syn) = 0;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, insecure_syn, CTLFLAG_VNET | CTLFLAG_RW,
     &VNET_NAME(tcp_insecure_syn), 0,
@@ -4071,3 +4080,45 @@
 			return (4 * maxseg);
 	}
 }
+
+static int
+sysctl_net_inet_tcp_ecn_enable_check(SYSCTL_HANDLER_ARGS)
+{
+	uint32_t new;
+	int error;
+
+	new = V_tcp_do_ecn;
+	error = sysctl_handle_int(oidp, &new, 0, req);
+	if (error == 0 && req->newptr != NULL) {
+		if (new > 2)
+			error = EINVAL;
+		else {
+			V_tcp_do_ecn = new;
+			if (new == 0)
+				V_tcp_ecn_generalized = new;
+		}
+	}
+
+	return (error);
+}
+
+static int
+sysctl_net_inet_tcp_ecn_generalized_check(SYSCTL_HANDLER_ARGS)
+{
+	uint32_t new;
+	int error;
+
+	new = V_tcp_ecn_generalized;
+	error = sysctl_handle_int(oidp, &new, 0, req);
+	if (error == 0 &&  req->newptr != NULL) {
+		if (new > 1)
+			error = EINVAL;
+		else
+			if (!V_tcp_do_ecn && new == 1)
+				error = EINVAL;
+			else
+				V_tcp_ecn_generalized = new;
+	}
+
+	return (error);
+}
Index: sys/netinet/tcp_output.c
===================================================================
--- sys/netinet/tcp_output.c
+++ sys/netinet/tcp_output.c
@@ -1204,8 +1204,9 @@
 		flags |= tcp_ecn_output_syn_sent(tp);
 	}
 	/* Also handle parallel SYN for ECN */
-	if ((TCPS_HAVERCVDSYN(tp->t_state)) &&
-	    (tp->t_flags2 & TF2_ECN_PERMIT)) {
+	if ((tp->t_flags2 & TF2_ECN_PLUSPLUS) ||
+	    (TCPS_HAVERCVDSYN(tp->t_state) &&
+	    (tp->t_flags2 & TF2_ECN_PERMIT))) {
 		int ect = tcp_ecn_output_established(tp, &flags, len);
 		if ((tp->t_state == TCPS_SYN_RECEIVED) &&
 		    (tp->t_flags2 & TF2_ECN_SND_ECE))
Index: sys/netinet/tcp_stacks/rack.c
===================================================================
--- sys/netinet/tcp_stacks/rack.c
+++ sys/netinet/tcp_stacks/rack.c
@@ -18276,8 +18276,9 @@
 		flags |= tcp_ecn_output_syn_sent(tp);
 	}
 	/* Also handle parallel SYN for ECN */
-	if (TCPS_HAVERCVDSYN(tp->t_state) &&
-	    (tp->t_flags2 & TF2_ECN_PERMIT)) {
+	if ((tp->t_flags2 & TF2_ECN_PLUSPLUS) ||
+	    (TCPS_HAVERCVDSYN(tp->t_state) &&
+	    (tp->t_flags2 & TF2_ECN_PERMIT))) {
 		int ect = tcp_ecn_output_established(tp, &flags, len);
 		if ((tp->t_state == TCPS_SYN_RECEIVED) &&
 		    (tp->t_flags2 & TF2_ECN_SND_ECE))
Index: sys/netinet/tcp_subr.c
===================================================================
--- sys/netinet/tcp_subr.c
+++ sys/netinet/tcp_subr.c
@@ -2033,6 +2033,26 @@
 	}
 #endif
 
+	/*
+	 * Send out control packets with same IP ECN header
+	 * bits, as when an established or listening socket
+	 * would exist.
+	 */
+	if (V_tcp_ecn_generalized && ((V_tcp_do_ecn == 1) ||
+	    ((tp != NULL) && (tp->t_flags2 & TF2_ECN_PERMIT)))) {
+#ifdef INET6
+		if (isipv6)
+			ip6->ip6_flow |= htonl(IPTOS_ECN_ECT0 << 20);
+#endif /* INET6 */
+#if defined(INET6) && defined(INET)
+		else
+#endif
+#ifdef INET
+			ip->ip_tos |= IPTOS_ECN_ECT0;
+#endif /* INET */
+	}
+
+	m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
 #ifdef INET6
 	if (isipv6) {
 		if (port) {
Index: sys/netinet/tcp_syncache.c
===================================================================
--- sys/netinet/tcp_syncache.c
+++ sys/netinet/tcp_syncache.c
@@ -132,7 +132,7 @@
 static void	 syncache_drop(struct syncache *, struct syncache_head *);
 static void	 syncache_free(struct syncache *);
 static void	 syncache_insert(struct syncache *, struct syncache_head *);
-static int	 syncache_respond(struct syncache *, const struct mbuf *, int);
+static int	 syncache_respond(struct syncache *, const struct mbuf *, uint16_t);
 static struct	 socket *syncache_socket(struct syncache *, struct socket *,
 		    struct mbuf *m);
 static void	 syncache_timeout(struct syncache *sc, struct syncache_head *sch,
@@ -1816,14 +1816,14 @@
  * i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL.
  */
 static int
-syncache_respond(struct syncache *sc, const struct mbuf *m0, int flags)
+syncache_respond(struct syncache *sc, const struct mbuf *m0, uint16_t flags)
 {
 	struct ip *ip = NULL;
 	struct mbuf *m;
 	struct tcphdr *th = NULL;
 	struct udphdr *udp = NULL;
 	int optlen, error = 0;	/* Make compiler happy */
-	u_int16_t hlen, tlen, mssopt, ulen;
+	uint16_t hlen, tlen, mssopt, ulen;
 	struct tcpopt to;
 #ifdef INET6
 	struct ip6_hdr *ip6 = NULL;
@@ -1940,7 +1940,17 @@
 	th->th_win = htons(sc->sc_wnd);
 	th->th_urp = 0;
 
-	flags = tcp_ecn_syncache_respond(flags, sc);
+	int ect = tcp_ecn_syncache_respond(&flags, sc);
+#ifdef INET6
+	if (sc->sc_inc.inc_flags & INC_ISIPV6)
+		ip6->ip6_flow |= htonl(ect << 20);
+#endif
+#if defined(INET6) && defined(INET)
+	else
+#endif
+#ifdef INET
+		ip->ip_tos |= ect;
+#endif
 	tcp_set_flags(th, flags);
 
 	/* Tack on the TCP options. */
Index: sys/netinet/tcp_usrreq.c
===================================================================
--- sys/netinet/tcp_usrreq.c
+++ sys/netinet/tcp_usrreq.c
@@ -3068,6 +3068,10 @@
 		db_printf("%sTF2_ACE_PERMIT", comma ? ", " : "");
 		comma = 1;
 	}
+	if (t_flags2 & TF2_ECN_PLUSPLUS) {
+		db_printf("%sTF2_ECN_PLUSPLUS", comma ? ", " : "");
+		comma = 1;
+	}
 	if (t_flags2 & TF2_FBYTES_COMPLETE) {
 		db_printf("%sTF2_FBYTES_COMPLETE", comma ? ", " : "");
 		comma = 1;
Index: sys/netinet/tcp_var.h
===================================================================
--- sys/netinet/tcp_var.h
+++ sys/netinet/tcp_var.h
@@ -569,6 +569,7 @@
 #define	TF2_ECN_SND_CWR		0x00000040 /* ECN CWR in queue */
 #define	TF2_ECN_SND_ECE		0x00000080 /* ECN ECE in queue */
 #define	TF2_ACE_PERMIT		0x00000100 /* Accurate ECN mode */
+#define	TF2_ECN_PLUSPLUS	0x00000200 /* ECN++ session */
 #define TF2_FBYTES_COMPLETE	0x00000400 /* We have first bytes in and out */
 /*
  * Structure to hold TCP options that are only used during segment
@@ -1004,6 +1005,7 @@
 VNET_DECLARE(int, tcp_do_sack);
 VNET_DECLARE(int, tcp_do_tso);
 VNET_DECLARE(int, tcp_ecn_maxretries);
+VNET_DECLARE(int, tcp_ecn_generalized);
 VNET_DECLARE(int, tcp_initcwnd_segments);
 VNET_DECLARE(int, tcp_insecure_rst);
 VNET_DECLARE(int, tcp_insecure_syn);
@@ -1050,6 +1052,7 @@
 #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_generalized		VNET(tcp_ecn_generalized)
 #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)