<s>
A	O
protocol-based	B-Protocol
intrusion	I-Protocol
detection	I-Protocol
system	I-Protocol
(	O
PIDS	O
)	O
is	O
an	O
intrusion	O
detection	O
system	O
which	O
is	O
typically	O
installed	O
on	O
a	O
web	B-Application
server	I-Application
,	O
and	O
is	O
used	O
in	O
the	O
monitoring	O
and	O
analysis	O
of	O
the	O
protocol	O
in	O
use	O
by	O
the	O
computing	O
system	O
.	O
</s>
<s>
A	O
typical	O
use	O
for	O
a	O
PIDS	O
would	O
be	O
at	O
the	O
front	O
end	O
of	O
a	O
web	B-Application
server	I-Application
monitoring	O
the	O
HTTP	B-Protocol
(	O
or	O
HTTPS	B-Protocol
)	O
stream	O
.	O
</s>
<s>
Because	O
it	O
understands	O
the	O
HTTP	B-Protocol
relative	O
to	O
the	O
web	O
server/system	O
it	O
is	O
trying	O
to	O
protect	O
it	O
can	O
offer	O
greater	O
protection	O
than	O
less	O
in-depth	O
techniques	O
such	O
as	O
filtering	O
by	O
IP	B-Protocol
address	I-Protocol
or	O
port	B-Protocol
number	I-Protocol
alone	O
,	O
however	O
this	O
greater	O
protection	O
comes	O
at	O
the	O
cost	O
of	O
increased	O
computing	O
on	O
the	O
web	B-Application
server	I-Application
.	O
</s>
<s>
Where	O
HTTPS	B-Protocol
is	O
in	O
use	O
then	O
this	O
system	O
would	O
need	O
to	O
reside	O
in	O
the	O
"	O
shim	O
"	O
or	O
interface	O
between	O
where	O
HTTPS	B-Protocol
is	O
un-encrypted	O
and	O
immediately	O
prior	O
to	O
it	O
entering	O
the	O
Web	O
presentation	B-Application
layer	I-Application
.	O
</s>
