Index: share/man/man4/cc_dctcp.4
===================================================================
--- share/man/man4/cc_dctcp.4
+++ share/man/man4/cc_dctcp.4
@@ -28,7 +28,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 29, 2019
+.Dd May 17, 2020
 .Dt CC_DCTCP 4
 .Os
 .Sh NAME
@@ -91,6 +91,11 @@
 .It Va slowstart
 A flag if the congestion window should be reduced by one half after slow start.
 Valid settings 0 and 1, default 0.
+.It Va ect1
+Controls if a DCTCP session should use IP ECT(1) marking when sending out
+segments (default), or ECT(0) marking like regular RFC3168 sessions.
+Changes to this setting will only affect new sessions, existing sessions will
+retain their previous marking value.
 .El
 .Sh SEE ALSO
 .Xr cc_chd 4 ,
Index: sys/netinet/cc/cc_dctcp.c
===================================================================
--- sys/netinet/cc/cc_dctcp.c
+++ sys/netinet/cc/cc_dctcp.c
@@ -64,6 +64,8 @@
 #define	V_dctcp_shift_g	    VNET(dctcp_shift_g)
 VNET_DEFINE_STATIC(uint32_t, dctcp_slowstart) = 0;
 #define	V_dctcp_slowstart   VNET(dctcp_slowstart)
+VNET_DEFINE_STATIC(uint32_t, dctcp_ect1) = 1;
+#define	V_dctcp_ect1	    VNET(dctcp_ect1)
 
 struct dctcp {
 	uint32_t bytes_ecn;	  /* # of marked bytes during a RTT */
@@ -300,8 +302,11 @@
 
 	dctcp_data = ccv->cc_data;
 
-	if (CCV(ccv, t_flags2) & TF2_ECN_PERMIT)
+	if (CCV(ccv, t_flags2) & TF2_ECN_PERMIT) {
 		dctcp_data->save_sndnxt = CCV(ccv, snd_nxt);
+		if (V_dctcp_ect1)
+			CCV(ccv, t_flags2) |= TF2_ECN_USE_ECT1;
+	}
 }
 
 /*
@@ -465,4 +470,9 @@
     &VNET_NAME(dctcp_slowstart), 0, &dctcp_slowstart_handler, "IU",
     "half CWND reduction after the first slow start");
 
+SYSCTL_UINT(_net_inet_tcp_cc_dctcp, OID_AUTO, ect1, 
+    CTLFLAG_VNET | CTLTYPE_UINT | CTLFLAG_RW | CTLFLAG_NEEDGIANT,
+    &VNET_NAME(dctcp_ect1), 0,
+    "Send DCTCP segments with ÍP ECT(1) or ECT(0)");
+
 DECLARE_CC_MODULE(dctcp, &dctcp_cc_algo);
Index: sys/netinet/tcp_output.c
===================================================================
--- sys/netinet/tcp_output.c
+++ sys/netinet/tcp_output.c
@@ -1165,13 +1165,20 @@
 		if (len > 0 && SEQ_GEQ(tp->snd_nxt, tp->snd_max) &&
 		    (sack_rxmit == 0) &&
 		    !((tp->t_flags & TF_FORCEDATA) && len == 1)) {
+			int ect;
+			if (tp->t_flags2 & TF2_ECN_USE_ECT1) {
+				ect = IPTOS_ECN_ECT1;
+				TCPSTAT_INC(tcps_ecn_ect1);
+			} else {
+				ect = IPTOS_ECN_ECT0;
+				TCPSTAT_INC(tcps_ecn_ect0);
+			}
 #ifdef INET6
 			if (isipv6)
-				ip6->ip6_flow |= htonl(IPTOS_ECN_ECT0 << 20);
+				ip6->ip6_flow |= htonl(ect << 20);
 			else
 #endif
-				ip->ip_tos |= IPTOS_ECN_ECT0;
-			TCPSTAT_INC(tcps_ecn_ect0);
+				ip->ip_tos |= ect;
 		}
 
 		/*
Index: sys/netinet/tcp_var.h
===================================================================
--- sys/netinet/tcp_var.h
+++ sys/netinet/tcp_var.h
@@ -434,6 +434,7 @@
 #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_USE_ECT1	0x00000200 /* Use ECT(1) marking on session */
 
 /*
  * Structure to hold TCP options that are only used during segment