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Index: tcp_subr.c
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_subr.c,v
retrieving revision 1.248
diff -u -p -u -r1.248 tcp_subr.c
--- tcp_subr.c	8 Sep 2012 02:58:13 -0000	1.248
+++ tcp_subr.c	9 Apr 2013 19:44:17 -0000
@@ -173,11 +173,28 @@ int	tcp_do_timestamps = 1;	/* RFC1323 ti
 int	tcp_ack_on_push = 0;	/* set to enable immediate ACK-on-PUSH */
 int	tcp_do_ecn = 0;		/* Explicit Congestion Notification */
 #ifndef TCP_INIT_WIN
-#define	TCP_INIT_WIN	0	/* initial slow start window */
+#define	TCP_INIT_WIN	4	/* initial slow start window */
 #endif
 #ifndef TCP_INIT_WIN_LOCAL
 #define	TCP_INIT_WIN_LOCAL 4	/* initial slow start window for local nets */
 #endif
+/*
+ * Up to 5 we scale linearly, to reach 3 * 1460; then (iw) * 1460.
+ * This is to simulate current behavior for iw == 4
+ */
+int tcp_init_win_max[] = {
+	1460,
+	1460,
+	2920,
+	2920,
+	4380,
+	7300,
+	8750,
+	10220,
+	11680,
+	13140,
+	14600
+};
 int	tcp_init_win = TCP_INIT_WIN;
 int	tcp_init_win_local = TCP_INIT_WIN_LOCAL;
 int	tcp_mss_ifmtu = 0;
Index: tcp_usrreq.c
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_usrreq.c,v
retrieving revision 1.165
diff -u -p -u -r1.165 tcp_usrreq.c
--- tcp_usrreq.c	2 Jun 2012 21:36:47 -0000	1.165
+++ tcp_usrreq.c	9 Apr 2013 19:44:17 -0000
@@ -1600,6 +1600,27 @@ sysctl_tcp_congctl(SYSCTLFN_ARGS)
 }
 
 static int
+sysctl_tcp_init_win(SYSCTLFN_ARGS)
+{
+	int error;
+	u_int iw;
+	struct sysctlnode node;
+
+	iw = *(u_int *)rnode->sysctl_data;
+	node = *rnode;
+	node.sysctl_data = &iw;
+	node.sysctl_size = sizeof(iw);
+	error = sysctl_lookup(SYSCTLFN_CALL(&node));
+	if (error || newp == NULL)
+		return error;
+
+	if (iw >= __arraycount(tcp_init_win_max))
+		return EINVAL;
+	*(u_int *)rnode->sysctl_data = iw;
+	return 0;
+}
+
+static int
 sysctl_tcp_keep(SYSCTLFN_ARGS)
 {  
 	int error;
@@ -1732,7 +1753,7 @@ sysctl_net_inet_tcp_setup2(struct sysctl
 		       CTLFLAG_PERMANENT|CTLFLAG_READWRITE,
 		       CTLTYPE_INT, "init_win",
 		       SYSCTL_DESCR("Initial TCP congestion window"),
-		       NULL, 0, &tcp_init_win, 0,
+		       sysctl_tcp_init_win, 0, &tcp_init_win, 0,
 		       CTL_NET, pf, IPPROTO_TCP, TCPCTL_INIT_WIN, CTL_EOL);
 	sysctl_createv(clog, 0, NULL, NULL,
 		       CTLFLAG_PERMANENT|CTLFLAG_READWRITE,
@@ -1861,7 +1882,7 @@ sysctl_net_inet_tcp_setup2(struct sysctl
 		       CTLFLAG_PERMANENT|CTLFLAG_READWRITE,
 		       CTLTYPE_INT, "init_win_local",
 		       SYSCTL_DESCR("Initial TCP window size (in segments)"),
-		       NULL, 0, &tcp_init_win_local, 0,
+		       sysctl_tcp_init_win, 0, &tcp_init_win_local, 0,
 		       CTL_NET, pf, IPPROTO_TCP, TCPCTL_INIT_WIN_LOCAL,
 		       CTL_EOL);
 	sysctl_createv(clog, 0, NULL, NULL,
Index: tcp_var.h
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_var.h,v
retrieving revision 1.169
diff -u -p -u -r1.169 tcp_var.h
--- tcp_var.h	2 Feb 2012 19:43:08 -0000	1.169
+++ tcp_var.h	9 Apr 2013 19:44:17 -0000
@@ -613,8 +613,7 @@ struct syn_cache_head {
  * Compute the initial window for slow start.
  */
 #define	TCP_INITIAL_WINDOW(iw, segsz) \
-	(((iw) == 0) ? (min(4 * (segsz), max(2 * (segsz), 4380))) : \
-	 ((segsz) * (iw)))
+	min((iw) * (segsz), max(2 * (segsz), tcp_init_win_max[(iw)]))
 
 /*
  * TCP statistics.
@@ -797,6 +796,7 @@ extern	int tcp_minmss;		/* minimal seg s
 extern  int tcp_msl;		/* max segment life */
 extern	int tcp_init_win;	/* initial window */
 extern	int tcp_init_win_local;	/* initial window for local nets */
+extern	int tcp_init_win_max[11];/* max sizes for values of tcp_init_win_* */
 extern	int tcp_mss_ifmtu;	/* take MSS from interface, not in_maxmtu */
 extern	int tcp_compat_42;	/* work around ancient broken TCP peers */
 extern	int tcp_cwm;		/* enable Congestion Window Monitoring */