<s>
A	O
circuit-level	B-Protocol
gateway	I-Protocol
is	O
a	O
type	O
of	O
firewall	B-Application
.	O
</s>
<s>
Circuit-level	B-Protocol
gateways	I-Protocol
work	O
at	O
the	O
session	B-Application
layer	I-Application
of	O
the	O
OSI	O
model	O
,	O
or	O
as	O
a	O
"	O
shim-layer	O
"	O
between	O
the	O
application	B-Application
layer	I-Application
and	O
the	O
transport	B-Architecture
layer	I-Architecture
of	O
the	O
TCP/IP	B-Protocol
stack	I-Protocol
.	O
</s>
<s>
Information	O
passed	O
to	O
a	O
remote	O
computer	O
through	O
a	O
circuit-level	B-Protocol
gateway	I-Protocol
appears	O
to	O
have	O
originated	O
from	O
the	O
gateway	O
.	O
</s>
<s>
Firewall	B-Application
traffic	O
is	O
cleaned	O
based	O
on	O
particular	O
session	O
rules	O
and	O
may	O
be	O
controlled	O
to	O
acknowledged	O
computers	O
only	O
.	O
</s>
<s>
Circuit-level	O
firewalls	B-Application
conceal	O
the	O
details	O
of	O
the	O
protected	O
network	O
from	O
the	O
external	O
traffic	O
,	O
which	O
is	O
helpful	O
for	O
interdicting	O
access	O
to	O
impostors	O
.	O
</s>
<s>
Circuit-level	B-Protocol
gateways	I-Protocol
are	O
relatively	O
inexpensive	O
and	O
have	O
the	O
advantage	O
of	O
hiding	O
information	O
about	O
the	O
private	O
network	O
they	O
protect	O
.	O
</s>
