Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -164,6 +164,19 @@
     &VNET_NAME(tcp_do_initcwnd10), 0,
     "Enable RFC 6928 (Increasing initial CWND to 10)");
 
+SYSCTL_NODE(_net_inet_tcp, OID_AUTO, nonstandard, CTLFLAG_RW, 0,
+    "Nonstandard TCP extensions");
+
+VNET_DEFINE(int, tcp_nonstandard_allowed) = 0;
+SYSCTL_INT(_net_inet_tcp_nonstandard, OID_AUTO, allowed, CTLFLAG_VNET | CTLFLAG_RW,
+    &VNET_NAME(tcp_nonstandard_allowed), 0,
+    "Allow nonstandard TCP extensions");
+
+VNET_DEFINE(int, tcp_nonstandard_initcwnd) = 0;
+SYSCTL_INT(_net_inet_tcp_nonstandard, OID_AUTO, initcwnd, CTLFLAG_VNET | CTLFLAG_RW,
+    &VNET_NAME(tcp_nonstandard_initcwnd), 0,
+    "Slow-start flight size (initial congestion window)");
+
 VNET_DEFINE(int, tcp_do_rfc3465) = 1;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3465, CTLFLAG_VNET | CTLFLAG_RW,
     &VNET_NAME(tcp_do_rfc3465), 0,
@@ -361,6 +374,7 @@
 	 * RFC5681 Section 3.1 specifies the default conservative values.
 	 * RFC3390 specifies slightly more aggressive values.
 	 * RFC6928 increases it to ten segments.
+	 * Support for user specified value for initial flight size.
 	 *
 	 * If a SYN or SYN/ACK was lost and retransmitted, we have to
 	 * reduce the initial CWND to one segment as congestion is likely
@@ -368,6 +382,9 @@
 	 */
 	if (tp->snd_cwnd == 1)
 		tp->snd_cwnd = tp->t_maxseg;		/* SYN(-ACK) lost */
+	else if (V_tcp_nonstandard_allowed && V_tcp_nonstandard_initcwnd)
+		tp->snd_cwnd = min(V_tcp_nonstandard_initcwnd * tp->t_maxseg,
+		    max(2 * tp->t_maxseg, V_tcp_nonstandard_initcwnd * 1460));
 	else if (V_tcp_do_initcwnd10)
 		tp->snd_cwnd = min(10 * tp->t_maxseg,
 		    max(2 * tp->t_maxseg, 14600));
Index: sys/netinet/tcp_var.h
===================================================================
--- sys/netinet/tcp_var.h
+++ sys/netinet/tcp_var.h
@@ -611,6 +611,8 @@
 VNET_DECLARE(int, tcp_delack_enabled);
 VNET_DECLARE(int, tcp_do_rfc3390);
 VNET_DECLARE(int, tcp_do_initcwnd10);
+VNET_DECLARE(int, tcp_nonstandard_allowed);
+VNET_DECLARE(int, tcp_nonstandard_initcwnd);
 VNET_DECLARE(int, tcp_sendspace);
 VNET_DECLARE(int, tcp_recvspace);
 VNET_DECLARE(int, path_mtu_discovery);
@@ -623,6 +625,8 @@
 #define	V_tcp_delack_enabled	VNET(tcp_delack_enabled)
 #define	V_tcp_do_rfc3390	VNET(tcp_do_rfc3390)
 #define	V_tcp_do_initcwnd10	VNET(tcp_do_initcwnd10)
+#define	V_tcp_nonstandard_allowed   VNET(tcp_nonstandard_allowed)
+#define	V_tcp_nonstandard_initcwnd  VNET(tcp_nonstandard_initcwnd)
 #define	V_tcp_sendspace		VNET(tcp_sendspace)
 #define	V_tcp_recvspace		VNET(tcp_recvspace)
 #define	V_path_mtu_discovery	VNET(path_mtu_discovery)