Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -157,6 +157,11 @@
     &VNET_NAME(tcp_do_rfc6675_pipe), 0,
     "Use calculated pipe/in-flight bytes per RFC 6675");
 
+VNET_DEFINE(int, tcp_do_rfc6675_cwnd) = 0;
+SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc6675_cwnd, CTLFLAG_VNET | CTLFLAG_RW,
+    &VNET_NAME(tcp_do_rfc6675_cwnd), 0,
+    "Follow RFC 6675 for cwnd and sshthresh calculations");
+
 VNET_DEFINE(int, tcp_do_rfc3042) = 1;
 #define	V_tcp_do_rfc3042	VNET(tcp_do_rfc3042)
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3042, CTLFLAG_VNET | CTLFLAG_RW,
@@ -2582,7 +2587,8 @@
 						 * we have less than 1/2 the original window's
 						 * worth of data in flight.
 						 */
-						if (V_tcp_do_rfc6675_pipe)
+						if (V_tcp_do_rfc6675_pipe ||
+						    V_tcp_do_rfc6675_cwnd)
 							awnd = tcp_compute_pipe(tp);
 						else
 							awnd = (tp->snd_nxt - tp->snd_fack) +
@@ -2628,7 +2634,18 @@
 						TCPSTAT_INC(
 						    tcps_sack_recovery_episode);
 						tp->sack_newdata = tp->snd_nxt;
-						tp->snd_cwnd = maxseg;
+						/*
+						 * As per RFC6675,
+						 * ssthresh = cwnd = pipe / 2
+						 */
+						if (V_tcp_do_rfc6675_cwnd) {
+							int pipe;
+							pipe = tcp_compute_pipe(tp);
+							tp->snd_ssthresh =
+							    tp->snd_cwnd = pipe / 2;
+						} else
+							tp->snd_cwnd = maxseg;
+
 						(void) tp->t_fb->tfb_tcp_output(tp);
 						goto drop;
 					}
Index: sys/netinet/tcp_var.h
===================================================================
--- sys/netinet/tcp_var.h
+++ sys/netinet/tcp_var.h
@@ -730,6 +730,9 @@
 VNET_DECLARE(int, tcp_do_rfc6675_pipe);
 #define V_tcp_do_rfc6675_pipe	VNET(tcp_do_rfc6675_pipe)
 
+VNET_DECLARE(int, tcp_do_rfc6675_cwnd);
+#define V_tcp_do_rfc6675_cwnd   VNET(tcp_do_rfc6675_cwnd)
+
 int	 tcp_addoptions(struct tcpopt *, u_char *);
 int	 tcp_ccalgounload(struct cc_algo *unload_algo);
 struct tcpcb *