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Commit 15d99e02 authored by Rick Jones's avatar Rick Jones Committed by David S. Miller
Browse files

[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: default avatarRick Jones <rick.jones2@hp.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent c1b1bce8

Documentation/networking/ip-sysctl.txt

+7 −0
Original line number Diff line number Diff line
@@ -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

+1 −0
Original line number Diff line number Diff line
@@ -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

+1 −0
Original line number Diff line number Diff line
@@ -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

+8 −1
Original line number Diff line number Diff line
@@ -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

+17 −6
Original line number Diff line number Diff line
@@ -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.

	 */

	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));