<s>
A	O
Web	B-Protocol
cache	I-Protocol
(	O
or	O
HTTP	B-Protocol
cache	I-Protocol
)	O
is	O
a	O
system	O
for	O
optimizing	O
the	O
World	O
Wide	O
Web	O
.	O
</s>
<s>
It	O
is	O
implemented	O
both	O
client-side	B-Operating_System
and	O
server-side	B-Application
.	O
</s>
<s>
The	O
caching	B-General_Concept
of	O
multimedias	O
and	O
other	O
files	B-Operating_System
can	O
result	O
in	O
less	O
overall	O
delay	O
when	O
browsing	B-Application
the	O
Web	O
.	O
</s>
<s>
A	O
forward	O
cache	B-General_Concept
is	O
a	O
cache	B-General_Concept
outside	O
the	O
web	B-Application
server	I-Application
's	O
network	O
,	O
e.g.	O
</s>
<s>
in	O
the	O
client	O
's	O
web	B-Application
browser	I-Application
,	O
in	O
an	O
ISP	O
,	O
or	O
within	O
a	O
corporate	O
network	O
.	O
</s>
<s>
A	O
network-aware	O
forward	O
cache	B-General_Concept
only	O
caches	B-General_Concept
heavily	O
accessed	O
items	O
.	O
</s>
<s>
A	O
proxy	B-Application
server	I-Application
sitting	O
between	O
the	O
client	O
and	O
web	B-Application
server	I-Application
can	O
evaluate	O
HTTP	B-Protocol
headers	O
and	O
choose	O
whether	O
to	O
store	O
web	O
content	O
.	O
</s>
<s>
A	O
reverse	O
cache	B-General_Concept
sits	O
in	O
front	O
of	O
one	O
or	O
more	O
web	B-Application
servers	I-Application
,	O
accelerating	O
requests	O
from	O
the	O
Internet	O
and	O
reducing	O
peak	O
server	O
load	O
.	O
</s>
<s>
This	O
is	O
usually	O
a	O
content	B-Operating_System
delivery	I-Operating_System
network	I-Operating_System
(	O
CDN	O
)	O
that	O
retains	O
copies	O
of	O
web	O
content	O
at	O
various	O
points	O
throughout	O
a	O
network	O
.	O
</s>
<s>
The	O
Hypertext	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
HTTP	B-Protocol
)	O
defines	O
three	O
basic	O
mechanisms	O
for	O
controlling	O
caches	B-General_Concept
:	O
freshness	O
,	O
validation	O
,	O
and	O
invalidation	O
.	O
</s>
<s>
This	O
is	O
specified	O
in	O
the	O
header	O
of	O
HTTP	B-Protocol
response	O
messages	O
from	O
the	O
server	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Expires	O
response	O
header	O
gives	O
a	O
date	O
when	O
the	O
document	O
becomes	O
stale	O
,	O
and	O
the	O
Cache-Control	O
:	O
max-age	O
directive	O
tells	O
the	O
cache	B-General_Concept
how	O
many	O
seconds	O
the	O
response	O
is	O
fresh	O
for	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
response	O
has	O
a	O
Last-Modified	O
header	O
,	O
a	O
cache	B-General_Concept
can	O
make	O
a	O
conditional	O
request	O
using	O
the	O
If-Modified-Since	O
header	O
to	O
see	O
if	O
it	O
has	O
changed	O
.	O
</s>
<s>
The	O
ETag	B-Protocol
(	O
entity	B-Protocol
tag	I-Protocol
)	O
mechanism	O
also	O
allows	O
for	O
both	O
strong	O
and	O
weak	O
validation	O
.	O
</s>
<s>
Invalidation	O
is	O
usually	O
a	O
side	O
effect	O
of	O
another	O
request	O
that	O
passes	O
through	O
the	O
cache	B-General_Concept
.	O
</s>
<s>
Many	O
CDNs	O
and	O
manufacturers	O
of	O
network	O
equipment	O
have	O
replaced	O
this	O
standard	O
HTTP	B-Protocol
cache	I-Protocol
control	O
with	O
dynamic	O
caching	B-General_Concept
.	O
</s>
<s>
§	O
:	O
512	O
)	O
that	O
exempts	O
system	O
operators	O
from	O
copyright	O
liability	O
for	O
the	O
purposes	O
of	O
caching	B-General_Concept
.	O
</s>
<s>
This	O
is	O
a	O
list	O
of	O
server-side	B-Application
web	B-Protocol
caching	I-Protocol
software	O
.	O
</s>
