[TCP]: sysctl to allow TCP window > 32767 sans wscale

Back in the dark ages, we had to be conservative and only allow 15-bit
window fields if the window scale option was not negotiated.  Some
ancient stacks used a signed 16-bit quantity for the window field of
the TCP header and would get confused.

Those days are long gone, so we can use the full 16-bits by default
now.

There is a sysctl added so that we can still interact with such old
stacks

Signed-off-by: Rick Jones <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: Rick Jones

Documentation/networking/ip-sysctl.txt

@@ -355,6 +355,13 @@ somaxconn - INTEGER
 	Defaults to 128.  See also tcp_max_syn_backlog for additional tuning
 	for TCP sockets.
 
+tcp_workaround_signed_windows - BOOLEAN
+	If set, assume no receipt of a window scaling option means the
+	remote TCP is broken and treats the window as a signed quantity.
+	If unset, assume the remote TCP is not broken even if we do
+	not receive a window scaling option from them.
+	Default: 0
+
 IP Variables:
 
 ip_local_port_range - 2 INTEGERS

include/linux/sysctl.h

@@ -402,6 +402,7 @@ enum
 	NET_IPV4_IPFRAG_MAX_DIST=112,
  	NET_TCP_MTU_PROBING=113,
 	NET_TCP_BASE_MSS=114,
+	NET_IPV4_TCP_WORKAROUND_SIGNED_WINDOWS=115,
 };
 
 enum {

include/net/tcp.h

@@ -224,6 +224,7 @@ extern int sysctl_tcp_tso_win_divisor;
 extern int sysctl_tcp_abc;
 extern int sysctl_tcp_mtu_probing;
 extern int sysctl_tcp_base_mss;
+extern int sysctl_tcp_workaround_signed_windows;
 
 extern atomic_t tcp_memory_allocated;
 extern atomic_t tcp_sockets_allocated;

net/ipv4/sysctl_net_ipv4.c

@@ -680,7 +680,14 @@ ctl_table ipv4_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec,
 	},
-
+        {
+		.ctl_name	= NET_IPV4_TCP_WORKAROUND_SIGNED_WINDOWS,
+		.procname	= "tcp_workaround_signed_windows",
+		.data		= &sysctl_tcp_workaround_signed_windows,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec
+	},
 	{ .ctl_name = 0 }
 };
 

net/ipv4/tcp_output.c

@@ -45,6 +45,11 @@
 /* People can turn this off for buggy TCP's found in printers etc. */
 int sysctl_tcp_retrans_collapse = 1;
 
+/* People can turn this on to  work with those rare, broken TCPs that
+ * interpret the window field as a signed quantity.
+ */
+int sysctl_tcp_workaround_signed_windows = 0;
+
 /* This limits the percentage of the congestion window which we
  * will allow a single TSO frame to consume.  Building TSO frames
  * which are too large can cause TCP streams to be bursty.
@@ -177,12 +182,18 @@ void tcp_select_initial_window(int __space, __u32 mss,
 		space = (space / mss) * mss;
 
 	/* NOTE: offering an initial window larger than 32767
-	 * will break some buggy TCP stacks. We try to be nice.
-	 * If we are not window scaling, then this truncates
-	 * our initial window offering to 32k. There should also
-	 * be a sysctl option to stop being nice.
+	 * will break some buggy TCP stacks. If the admin tells us
+	 * it is likely we could be speaking with such a buggy stack
+	 * we will truncate our initial window offering to 32K-1
+	 * unless the remote has sent us a window scaling option,
+	 * which we interpret as a sign the remote TCP is not
+	 * misinterpreting the window field as a signed quantity.
 	 */
-	(*rcv_wnd) = min(space, MAX_TCP_WINDOW);
+	if (sysctl_tcp_workaround_signed_windows)
+		(*rcv_wnd) = min(space, MAX_TCP_WINDOW);
+	else
+		(*rcv_wnd) = space;
+
 	(*rcv_wscale) = 0;
 	if (wscale_ok) {
 		/* Set window scaling on max possible window
@@ -241,7 +252,7 @@ static u16 tcp_select_window(struct sock *sk)
 	/* Make sure we do not exceed the maximum possible
 	 * scaled window.
 	 */
-	if (!tp->rx_opt.rcv_wscale)
+	if (!tp->rx_opt.rcv_wscale && sysctl_tcp_workaround_signed_windows)
 		new_win = min(new_win, MAX_TCP_WINDOW);
 	else
 		new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));