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 October 13, 2014
+.Dd October 27, 2015
 .Dt TCP 4
 .Os
 .Sh NAME
@@ -454,6 +454,17 @@
 For this reason, we use 200ms of slop and a near-0
 minimum, which gives us an effective minimum of 200ms (similar to
 .Tn Linux ) .
+.It Va initcwnd_segs
+Enable the ability to specify initial congestion window in number of segments.
+The default value is 10 as suggested by RFC 6928.
+Changing the value on fly would not affect connections using congestion window
+from the hostcache.
+Caution:
+This regulates the burst of packets allowed to be sent in the first RTT.
+The value should be relative to the link capacity.
+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 rfc3042
 Enable the Limited Transmit algorithm as described in RFC 3042.
 It helps avoid timeouts on lossy links and also when the congestion window
Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -159,10 +159,10 @@
 SYSCTL_NODE(_net_inet_tcp, OID_AUTO, experimental, CTLFLAG_RW, 0,
     "Experimental TCP extensions");
 
-VNET_DEFINE(int, tcp_do_initcwnd10) = 1;
-SYSCTL_INT(_net_inet_tcp_experimental, OID_AUTO, initcwnd10, CTLFLAG_VNET | CTLFLAG_RW,
-    &VNET_NAME(tcp_do_initcwnd10), 0,
-    "Enable RFC 6928 (Increasing initial CWND to 10)");
+VNET_DEFINE(int, tcp_initcwnd_segs) = 10;
+SYSCTL_INT(_net_inet_tcp, OID_AUTO, initcwnd_segs, CTLFLAG_VNET | CTLFLAG_RW,
+    &VNET_NAME(tcp_initcwnd_segs), 0,
+    "Slow-start flight size (initial congestion window) in number of segments");
 
 VNET_DEFINE(int, tcp_do_rfc3465) = 1;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3465, CTLFLAG_VNET | CTLFLAG_RW,
@@ -361,6 +361,7 @@
 	 * 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
@@ -368,9 +369,9 @@
 	 */
 	if (tp->snd_cwnd == 1)
 		tp->snd_cwnd = tp->t_maxseg;		/* SYN(-ACK) lost */
-	else if (V_tcp_do_initcwnd10)
-		tp->snd_cwnd = min(10 * tp->t_maxseg,
-		    max(2 * tp->t_maxseg, 14600));
+	else if (V_tcp_initcwnd_segs)
+		tp->snd_cwnd = min(V_tcp_initcwnd_segs * tp->t_maxseg,
+		    max(2 * tp->t_maxseg, V_tcp_initcwnd_segs * 1460));
 	else if (V_tcp_do_rfc3390)
 		tp->snd_cwnd = min(4 * tp->t_maxseg,
 		    max(2 * tp->t_maxseg, 4380));
Index: sys/netinet/tcp_var.h
===================================================================
--- sys/netinet/tcp_var.h
+++ sys/netinet/tcp_var.h
@@ -610,7 +610,7 @@
 VNET_DECLARE(int, tcp_minmss);
 VNET_DECLARE(int, tcp_delack_enabled);
 VNET_DECLARE(int, tcp_do_rfc3390);
-VNET_DECLARE(int, tcp_do_initcwnd10);
+VNET_DECLARE(int, tcp_initcwnd_segs);
 VNET_DECLARE(int, tcp_sendspace);
 VNET_DECLARE(int, tcp_recvspace);
 VNET_DECLARE(int, path_mtu_discovery);
@@ -622,7 +622,7 @@
 #define	V_tcp_minmss		VNET(tcp_minmss)
 #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_initcwnd_segs	VNET(tcp_initcwnd_segs)
 #define	V_tcp_sendspace		VNET(tcp_sendspace)
 #define	V_tcp_recvspace		VNET(tcp_recvspace)
 #define	V_path_mtu_discovery	VNET(path_mtu_discovery)