<s>
The	O
TCP	B-Protocol
window	I-Protocol
scale	I-Protocol
option	I-Protocol
is	O
an	O
option	O
to	O
increase	O
the	O
receive	O
window	O
size	O
allowed	O
in	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
above	O
its	O
former	O
maximum	O
value	O
of	O
65,535	O
bytes	O
.	O
</s>
<s>
The	O
throughput	O
of	O
a	O
TCP	O
communication	O
is	O
limited	O
by	O
two	O
windows	B-Application
:	O
the	O
congestion	B-Protocol
window	I-Protocol
and	O
the	O
receive	O
window	O
.	O
</s>
<s>
The	O
congestion	B-Protocol
window	I-Protocol
tries	O
not	O
to	O
exceed	O
the	O
capacity	O
of	O
the	O
network	O
(	O
congestion	O
control	O
)	O
;	O
the	O
receive	O
window	O
tries	O
not	O
to	O
exceed	O
the	O
capacity	O
of	O
the	O
receiver	O
to	O
process	O
data	O
(	O
flow	O
control	O
)	O
.	O
</s>
<s>
If	O
,	O
for	O
example	O
,	O
a	O
sender	O
receives	O
an	O
ack	O
which	O
acknowledges	O
byte	O
4000	O
and	O
specifies	O
a	O
receive	O
window	O
of	O
10000	O
(	O
bytes	O
)	O
,	O
the	O
sender	O
will	O
not	O
send	O
packets	O
after	O
byte	O
14000	O
,	O
even	O
if	O
the	O
congestion	B-Protocol
window	I-Protocol
allows	O
it	O
.	O
</s>
<s>
TCP	B-Protocol
window	I-Protocol
scale	I-Protocol
option	I-Protocol
is	O
needed	O
for	O
efficient	O
transfer	O
of	O
data	O
when	O
the	O
bandwidth-delay	O
product	O
(	O
BDP	O
)	O
is	O
greater	O
than	O
64KB	O
.	O
</s>
<s>
For	O
instance	O
,	O
if	O
a	O
T1	B-Protocol
transmission	O
line	O
of	O
1.5Mbit/second	O
was	O
used	O
over	O
a	O
satellite	O
link	O
with	O
a	O
513millisecond	O
round-trip	O
time	O
(	O
RTT	O
)	O
,	O
the	O
bandwidth-delay	O
product	O
is	O
bits	O
or	O
about	O
96,187	O
bytes	O
.	O
</s>
<s>
Because	O
some	O
firewalls	O
do	O
not	O
properly	O
implement	O
TCP	B-Protocol
Window	I-Protocol
Scaling	O
,	O
it	O
can	O
cause	O
a	O
user	O
's	O
Internet	O
connection	O
to	O
malfunction	O
intermittently	O
for	O
a	O
few	O
minutes	O
,	O
then	O
appear	O
to	O
start	O
working	O
again	O
for	O
no	O
reason	O
.	O
</s>
<s>
TCP	B-Protocol
Window	I-Protocol
Scaling	O
is	O
implemented	O
in	O
Windows	B-Application
since	O
Windows	B-Application
2000	I-Application
.	O
</s>
<s>
It	O
is	O
enabled	O
by	O
default	O
in	O
Windows	B-Application
Vista	I-Application
/	O
Server	O
2008	O
and	O
newer	O
,	O
but	O
can	O
be	O
turned	O
off	O
manually	O
if	O
required	O
.	O
</s>
<s>
Windows	B-Application
Vista	I-Application
and	O
Windows	B-Device
7	I-Device
have	O
a	O
fixed	O
default	O
TCP	O
receive	O
buffer	O
of	O
64kB	O
,	O
scaling	O
up	O
to	O
16MB	O
through	O
"	O
autotuning	O
"	O
,	O
limiting	O
manual	O
TCP	O
tuning	O
over	O
long	O
fat	O
networks	O
.	O
</s>
<s>
Linux	B-Operating_System
kernels	I-Operating_System
(	O
from	O
2.6.8	O
,	O
August	O
2004	O
)	O
have	O
enabled	O
TCP	B-Protocol
Window	I-Protocol
Scaling	O
by	O
default	O
.	O
</s>
<s>
The	O
configuration	O
parameters	O
are	O
found	O
in	O
the	O
/proc	B-Application
filesystem	I-Application
,	O
see	O
pseudo-file	O
and	O
its	O
companions	O
and	O
(	O
more	O
information	O
:	O
,	O
section	O
sysctl	B-Application
)	O
.	O
</s>
<s>
Default	O
setting	O
for	O
FreeBSD	B-Operating_System
,	O
OpenBSD	B-Operating_System
,	O
NetBSD	B-Device
and	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
is	O
to	O
have	O
window	B-Protocol
scaling	I-Protocol
(	O
and	O
other	O
features	O
related	O
to	O
RFC	O
1323	O
)	O
enabled	O
.	O
</s>
<s>
To	O
verify	O
their	O
status	O
,	O
a	O
user	O
can	O
check	O
the	O
value	O
of	O
the	O
""	O
variable	O
via	O
the	O
sysctl	B-Application
command	O
:	O
</s>
<s>
To	O
ensure	O
that	O
it	O
is	O
set	O
at	O
boot	O
time	O
,	O
add	O
the	O
following	O
line	O
to	O
/etc/sysctl.conf	O
:	O
</s>
