Index: head/share/man/man4/tcp.4
===================================================================
--- head/share/man/man4/tcp.4
+++ head/share/man/man4/tcp.4
@@ -538,6 +538,12 @@
 Start with small values for lower-capacity links.
 Large bursts can cause buffer overruns and packet drops if routers have small
 buffers or the link is experiencing congestion.
+.It Va newcwd
+Enable the New Congestion Window Validation mechanism as described in RFC 7661.
+This gently reduces the congestion window during periods, where TCP is
+application limited and the network bandwidth is not utilized completely.
+That prevents self-inflicted packet losses once the application starts to
+transmit data at a higher speed.
 .It Va rfc6675_pipe
 Calculate the bytes in flight using the algorithm described in RFC 6675, and
 is also a prerequisite to enable Proportional Rate Reduction.
Index: head/sys/netinet/tcp_input.c
===================================================================
--- head/sys/netinet/tcp_input.c
+++ head/sys/netinet/tcp_input.c
@@ -150,6 +150,11 @@
     &VNET_NAME(drop_synfin), 0,
     "Drop TCP packets with SYN+FIN set");
 
+VNET_DEFINE(int, tcp_do_newcwv) = 0;
+SYSCTL_INT(_net_inet_tcp, OID_AUTO, newcwv, CTLFLAG_VNET | CTLFLAG_RW,
+    &VNET_NAME(tcp_do_newcwv), 0,
+    "Enable New Congestion Window Validation per RFC7661");
+
 VNET_DEFINE(int, tcp_do_rfc6675_pipe) = 0;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc6675_pipe, CTLFLAG_VNET | CTLFLAG_RW,
     &VNET_NAME(tcp_do_rfc6675_pipe), 0,
@@ -297,7 +302,9 @@
 
 	tp->ccv->nsegs = nsegs;
 	tp->ccv->bytes_this_ack = BYTES_THIS_ACK(tp, th);
-	if (tp->snd_cwnd <= tp->snd_wnd)
+	if ((!V_tcp_do_newcwv && (tp->snd_cwnd <= tp->snd_wnd)) ||
+	    (V_tcp_do_newcwv && (tp->snd_cwnd <= tp->snd_wnd) &&
+	     (tp->snd_cwnd < (tcp_compute_pipe(tp) * 2))))
 		tp->ccv->flags |= CCF_CWND_LIMITED;
 	else
 		tp->ccv->flags &= ~CCF_CWND_LIMITED;
Index: head/sys/netinet/tcp_stacks/rack.c
===================================================================
--- head/sys/netinet/tcp_stacks/rack.c
+++ head/sys/netinet/tcp_stacks/rack.c
@@ -1663,7 +1663,9 @@
 			tp->ccv->bytes_this_ack = max;
 		}
 	}
-	if (tp->snd_cwnd <= tp->snd_wnd)
+	if ((!V_tcp_do_newcwv && (tp->snd_cwnd <= tp->snd_wnd)) ||
+	    (V_tcp_do_newcwv && (tp->snd_cwnd <= tp->snd_wnd) &&
+	     (tp->snd_cwnd < (ctf_flight_size(tp, rack->r_ctl.rc_sacked) * 2))))
 		tp->ccv->flags |= CCF_CWND_LIMITED;
 	else
 		tp->ccv->flags &= ~CCF_CWND_LIMITED;
Index: head/sys/netinet/tcp_var.h
===================================================================
--- head/sys/netinet/tcp_var.h
+++ head/sys/netinet/tcp_var.h
@@ -767,6 +767,7 @@
 VNET_DECLARE(int, tcp_do_autorcvbuf);
 VNET_DECLARE(int, tcp_do_autosndbuf);
 VNET_DECLARE(int, tcp_do_ecn);
+VNET_DECLARE(int, tcp_do_newcwv);
 VNET_DECLARE(int, tcp_do_rfc1323);
 VNET_DECLARE(int, tcp_do_rfc3042);
 VNET_DECLARE(int, tcp_do_rfc3390);
@@ -789,6 +790,7 @@
 VNET_DECLARE(struct inpcbhead, tcb);
 VNET_DECLARE(struct inpcbinfo, tcbinfo);
 
+#define	V_tcp_do_newcwv			VNET(tcp_do_newcwv)
 #define	V_drop_synfin			VNET(drop_synfin)
 #define	V_path_mtu_discovery		VNET(path_mtu_discovery)
 #define	V_tcb				VNET(tcb)