[TCP]: Add tcp_default_win_scale sysctl.

Documentation/networking/ip-sysctl.txt

@@ -340,6 +340,11 @@ tcp_bic_fast_convergence - BOOLEAN
 	more rapidly.
 	Default: 1
 
+tcp_default_win_scale - INTEGER
+	Sets the minimum window scale TCP will negotiate for on all
+	conections.
+	Default: 7
+
 ip_local_port_range - 2 INTEGERS
 	Defines the local port range that is used by TCP and UDP to
 	choose the local port. The first number is the first, the 

include/linux/sysctl.h

@@ -336,6 +336,7 @@ enum
  	NET_TCP_BIC=102,
  	NET_TCP_BIC_FAST_CONVERGENCE=103,
 	NET_TCP_BIC_LOW_WINDOW=104,
+	NET_TCP_DEFAULT_WIN_SCALE=105,
 };
 
 enum {

include/net/tcp.h

@@ -610,6 +610,7 @@ extern int sysctl_tcp_nometrics_save;
 extern int sysctl_tcp_bic;
 extern int sysctl_tcp_bic_fast_convergence;
 extern int sysctl_tcp_bic_low_window;
+extern int sysctl_tcp_default_win_scale;
 
 extern atomic_t tcp_memory_allocated;
 extern atomic_t tcp_sockets_allocated;
@@ -1751,6 +1752,9 @@ static inline void tcp_select_initial_window(int __space, __u32 mss,
 		if (*rcv_wscale && sysctl_tcp_app_win && space>=mss &&
 		    space - max((space>>sysctl_tcp_app_win), mss>>*rcv_wscale) < 65536/2)
 			(*rcv_wscale)--;
+
+		*rcv_wscale = max((__u8)sysctl_tcp_default_win_scale,
+				  *rcv_wscale);
 	}
 
 	/* Set initial window to value enough for senders,

net/ipv4/sysctl_net_ipv4.c

@@ -665,6 +665,14 @@ ctl_table ipv4_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec,
 	},
+	{
+		.ctl_name	= NET_TCP_DEFAULT_WIN_SCALE,
+		.procname	= "tcp_default_win_scale",
+		.data		= &sysctl_tcp_default_win_scale,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 	{ .ctl_name = 0 }
 };
 

net/ipv4/tcp.c

@@ -276,6 +276,8 @@ kmem_cache_t *tcp_timewait_cachep;
 
 atomic_t tcp_orphan_count = ATOMIC_INIT(0);
 
+int sysctl_tcp_default_win_scale = 7;
+
 int sysctl_tcp_mem[3];
 int sysctl_tcp_wmem[3] = { 4 * 1024, 16 * 1024, 128 * 1024 };
 int sysctl_tcp_rmem[3] = { 4 * 1024, 87380, 87380 * 2 };