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

There is a sysctl added so that we can still interact with such old
Signed-off-by: default avatarRick Jones <>
Signed-off-by: default avatarDavid S. Miller <>
parent c1b1bce8
......@@ -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
......@@ -402,6 +402,7 @@ enum
enum {
......@@ -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;
......@@ -680,7 +680,14 @@ ctl_table ipv4_table[] = {
.mode = 0644,
.proc_handler = &proc_dointvec,
.procname = "tcp_workaround_signed_windows",
.data = &sysctl_tcp_workaround_signed_windows,
.maxlen = sizeof(int),
.mode = 0644,
.proc_handler = &proc_dointvec
{ .ctl_name = 0 }
......@@ -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);
(*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);
new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));
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