<s>
Context-based	B-Protocol
access	I-Protocol
control	I-Protocol
(	O
CBAC	O
)	O
is	O
a	O
feature	O
of	O
firewall	B-Application
software	I-Application
,	O
which	O
intelligently	O
filters	B-Protocol
TCP	B-Protocol
and	O
UDP	B-Protocol
packets	I-Protocol
based	O
on	O
application	O
layer	O
protocol	O
session	O
information	O
.	O
</s>
<s>
It	O
can	O
be	O
used	O
for	O
intranets	B-General_Concept
,	O
extranets	B-Architecture
and	O
internets	O
.	O
</s>
<s>
CBAC	O
can	O
be	O
configured	O
to	O
permit	O
specified	O
TCP	B-Protocol
and	O
UDP	B-Protocol
traffic	O
through	O
a	O
firewall	B-Application
only	O
when	O
the	O
connection	O
is	O
initiated	O
from	O
within	O
the	O
network	O
needing	O
protection	O
.	O
</s>
<s>
However	O
,	O
while	O
this	O
example	O
discusses	O
inspecting	O
traffic	O
for	O
sessions	O
that	O
originate	O
from	O
the	O
external	O
network	O
,	O
CBAC	O
can	O
inspect	O
traffic	O
for	O
sessions	O
that	O
originate	O
from	O
either	O
side	O
of	O
the	O
firewall	B-Application
.	O
</s>
<s>
This	O
is	O
the	O
basic	O
function	O
of	O
a	O
stateful	O
inspection	O
firewall	B-Application
.	O
</s>
<s>
Without	O
CBAC	O
,	O
traffic	O
filtering	O
is	O
limited	O
to	O
access	O
list	O
implementations	O
that	O
examine	O
packets	O
at	O
the	O
network	B-Application
layer	I-Application
,	O
or	O
at	O
most	O
,	O
the	O
transport	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
However	O
,	O
CBAC	O
examines	O
not	O
only	O
network	B-Application
layer	I-Application
and	O
transport	B-Architecture
layer	I-Architecture
information	O
but	O
also	O
examines	O
the	O
application-layer	O
protocol	O
information	O
(	O
such	O
as	O
FTP	B-Protocol
connection	O
information	O
)	O
to	O
learn	O
about	O
the	O
state	O
of	O
the	O
TCP	B-Protocol
or	O
UDP	B-Protocol
session	O
.	O
</s>
<s>
This	O
allows	O
support	O
of	O
protocols	O
that	O
involve	O
multiple	O
channels	O
created	O
as	O
a	O
result	O
of	O
negotiations	O
in	O
the	O
FTP	B-Protocol
control	O
channel	O
.	O
</s>
<s>
Most	O
of	O
the	O
multimedia	O
protocols	O
as	O
well	O
as	O
some	O
other	O
protocols	O
(	O
such	O
as	O
FTP	B-Protocol
,	O
RPC	O
,	O
and	O
SQL*Net	O
)	O
involve	O
multiple	O
control	O
channels	O
.	O
</s>
<s>
CBAC	O
inspects	O
traffic	O
that	O
travels	O
through	O
the	O
firewall	B-Application
to	O
discover	O
and	O
manage	O
state	O
information	O
for	O
TCP	B-Protocol
and	O
UDP	B-Protocol
sessions	O
.	O
</s>
<s>
This	O
state	O
information	O
is	O
used	O
to	O
create	O
temporary	O
openings	O
in	O
the	O
firewall	B-Application
's	O
access	O
lists	O
to	O
allow	O
return	O
traffic	O
and	O
additional	O
data	O
connections	O
for	O
permissible	O
sessions	O
(	O
sessions	O
that	O
originated	O
from	O
within	O
the	O
protected	O
internal	B-General_Concept
network	I-General_Concept
)	O
.	O
</s>
<s>
CBAC	O
works	O
through	O
deep	B-Application
packet	I-Application
inspection	I-Application
and	O
hence	O
Cisco	O
calls	O
it	O
'	O
IOS	O
firewall	B-Application
 '	O
in	O
their	O
Internetwork	O
Operating	O
System	O
(	O
IOS	O
)	O
.	O
</s>
