<s>
An	O
application	B-General_Concept
delivery	I-General_Concept
network	I-General_Concept
(	O
ADN	O
)	O
is	O
a	O
suite	O
of	O
technologies	O
that	O
,	O
when	O
deployed	O
together	O
,	O
provide	O
availability	O
,	O
security	O
,	O
visibility	O
,	O
and	O
acceleration	O
for	O
Internet	O
applications	O
such	O
as	O
websites	O
.	O
</s>
<s>
Gartner	O
defines	O
application	O
delivery	O
networking	O
as	O
the	O
combination	O
of	O
WAN	B-Protocol
optimization	I-Protocol
controllers	I-Protocol
(	O
WOCs	O
)	O
and	O
application	B-Protocol
delivery	I-Protocol
controllers	I-Protocol
(	O
ADCs	O
)	O
.	O
</s>
<s>
At	O
the	O
data	O
center	O
end	O
of	O
an	O
ADN	O
is	O
the	O
ADC	O
,	O
an	O
advanced	O
traffic	O
management	O
device	O
that	O
is	O
often	O
also	O
referred	O
to	O
as	O
a	O
web	O
switch	O
,	O
content	O
switch	O
,	O
or	O
multilayer	B-Application
switch	I-Application
,	O
the	O
purpose	O
of	O
which	O
is	O
to	O
distribute	O
traffic	O
among	O
a	O
number	O
of	O
servers	O
or	O
geographically	O
dislocated	O
sites	O
based	O
on	O
application	O
specific	O
criteria	O
.	O
</s>
<s>
In	O
the	O
branch	O
office	O
portion	O
of	O
an	O
ADN	O
is	O
the	O
WAN	B-Protocol
optimization	I-Protocol
controller	I-Protocol
,	O
which	O
works	O
to	O
reduce	O
the	O
number	O
of	O
bits	O
that	O
flow	O
over	O
the	O
network	O
using	O
caching	O
and	O
compression	O
,	O
and	O
shapes	O
TCP	B-Protocol
traffic	O
using	O
prioritization	O
and	O
other	O
optimization	O
techniques	O
.	O
</s>
<s>
Application	B-General_Concept
delivery	I-General_Concept
networks	I-General_Concept
are	O
also	O
offered	O
by	O
some	O
CDN	O
vendors	O
.	O
</s>
<s>
The	O
ADC	O
,	O
one	O
component	O
of	O
an	O
ADN	O
,	O
evolved	O
from	O
layer	B-Application
4-7	I-Application
switches	I-Application
in	O
the	O
late	O
1990s	O
when	O
it	O
became	O
apparent	O
that	O
traditional	O
load	O
balancing	O
techniques	O
were	O
not	O
robust	O
enough	O
to	O
handle	O
the	O
increasingly	O
complex	O
mix	O
of	O
application	O
traffic	O
being	O
delivered	O
over	O
a	O
wider	O
variety	O
of	O
network	O
connectivity	O
options	O
.	O
</s>
<s>
The	O
Internet	O
was	O
designed	O
according	O
to	O
the	O
end-to-end	B-Application
principle	I-Application
.	O
</s>
<s>
An	O
Application	B-General_Concept
Delivery	I-General_Concept
Network	I-General_Concept
(	O
ADN	O
)	O
enhances	O
the	O
delivery	O
of	O
applications	O
across	O
the	O
Internet	O
by	O
employing	O
a	O
number	O
of	O
optimization	O
techniques	O
.	O
</s>
<s>
In	O
theory	O
,	O
an	O
Application	B-General_Concept
Delivery	I-General_Concept
Network	I-General_Concept
(	O
ADN	O
)	O
is	O
closely	O
related	O
to	O
a	O
content	B-Operating_System
delivery	I-Operating_System
network	I-Operating_System
.	O
</s>
<s>
Application	B-General_Concept
Fluent	I-General_Concept
Network	I-General_Concept
(	O
AFN	O
)	O
is	O
based	O
on	O
the	O
concept	O
of	O
Application	O
Fluency	O
to	O
refer	O
to	O
WAN	B-Protocol
optimization	I-Protocol
techniques	O
applied	O
at	O
Layer	O
Four	O
to	O
Layer	O
Seven	O
of	O
the	O
OSI	O
model	O
for	O
networks	O
.	O
</s>
<s>
Application	B-General_Concept
Fluent	I-General_Concept
Network	I-General_Concept
is	O
an	O
addition	O
of	O
SDN	O
capabilities	O
.	O
</s>
<s>
The	O
acronym	O
'	O
AFN	O
 '	O
is	O
used	O
by	O
Alcatel-Lucent	O
Enterprise	O
to	O
refer	O
to	O
an	O
Application	B-General_Concept
Fluent	I-General_Concept
Network	I-General_Concept
.	O
</s>
<s>
Application	O
delivery	O
uses	O
one	O
or	O
more	O
layer	B-Application
4	I-Application
–	I-Application
7	I-Application
switches	I-Application
,	O
also	O
known	O
as	O
a	O
web	O
switch	O
,	O
content	O
switch	O
,	O
or	O
multilayer	B-Application
switch	I-Application
to	O
intelligently	O
distribute	O
traffic	O
to	O
a	O
pool	O
,	O
also	O
known	O
as	O
a	O
cluster	O
or	O
farm	O
,	O
of	O
servers	O
.	O
</s>
<s>
The	O
application	B-Protocol
delivery	I-Protocol
controller	I-Protocol
(	O
ADC	O
)	O
is	O
assigned	O
a	O
single	O
virtual	O
IP	O
address	O
(	O
VIP	O
)	O
that	O
represents	O
the	O
pool	O
of	O
servers	O
.	O
</s>
<s>
TCP	B-Protocol
Multiplexing	O
is	O
loosely	O
based	O
on	O
established	O
connection	B-General_Concept
pooling	I-General_Concept
techniques	O
utilized	O
by	O
application	O
server	O
platforms	O
to	O
optimize	O
the	O
execution	O
of	O
database	O
queries	O
from	O
within	O
applications	O
.	O
</s>
<s>
This	O
has	O
the	O
effect	O
of	O
reducing	O
the	O
overhead	O
imposed	O
by	O
establishing	O
and	O
tearing	O
down	O
the	O
TCP	B-Protocol
connection	O
with	O
the	O
server	O
,	O
improving	O
the	O
responsiveness	O
of	O
the	O
application	O
.	O
</s>
<s>
Some	O
ADN	O
implementations	O
take	O
this	O
technique	O
one	O
step	O
further	O
and	O
also	O
multiplex	O
HTTP	B-Protocol
and	O
application	O
requests	O
.	O
</s>
<s>
There	O
are	O
a	O
number	O
of	O
Request	O
for	O
Comments	O
(	O
RFCs	O
)	O
which	O
describe	O
mechanisms	O
for	O
improving	O
the	O
performance	O
of	O
TCP	B-Protocol
.	O
</s>
<s>
Many	O
ADN	O
implement	O
these	O
RFCs	O
in	O
order	O
to	O
provide	O
enhanced	O
delivery	O
of	O
applications	O
through	O
more	O
efficient	O
use	O
of	O
TCP	B-Protocol
.	O
</s>
<s>
There	O
are	O
two	O
types	O
of	O
compression	O
used	O
by	O
ADNs	O
today	O
:	O
industry	O
standard	O
HTTP	B-Protocol
compression	I-Protocol
and	O
proprietary	O
data	O
reduction	O
algorithms	O
.	O
</s>
<s>
HTTP	B-Protocol
compression	I-Protocol
is	O
asymmetric	O
and	O
transparent	O
to	O
the	O
client	O
.	O
</s>
<s>
Support	O
for	O
HTTP	B-Protocol
compression	I-Protocol
is	O
built	O
into	O
web	B-Application
servers	I-Application
and	O
web	O
browsers	O
.	O
</s>
<s>
All	O
commercial	O
ADN	O
products	O
currently	O
support	O
HTTP	B-Protocol
compression	I-Protocol
.	O
</s>
<s>
A	O
separate	O
class	O
of	O
devices	O
known	O
as	O
WAN	B-Protocol
Optimization	I-Protocol
Controllers	I-Protocol
(	O
WOC	O
)	O
provide	O
this	O
functionality	O
,	O
but	O
the	O
technology	O
has	O
been	O
slowly	O
added	O
to	O
the	O
ADN	O
portfolio	O
over	O
the	O
past	O
few	O
years	O
as	O
this	O
class	O
of	O
device	O
continues	O
to	O
become	O
more	O
application	O
aware	O
,	O
providing	O
additional	O
features	O
for	O
specific	O
applications	O
such	O
as	O
CIFS	B-Operating_System
and	I-Operating_System
SMB	I-Operating_System
.	O
</s>
<s>
Although	O
often	O
erroneously	O
assigned	O
to	O
the	O
application	O
layer	O
,	O
SSL	B-Protocol
is	O
the	O
most	O
common	O
method	O
of	O
securing	O
application	O
traffic	O
through	O
an	O
ADN	O
today	O
.	O
</s>
<s>
SSL	B-Protocol
uses	O
PKI	B-Architecture
to	O
establish	O
a	O
secure	O
connection	O
between	O
the	O
client	O
and	O
the	O
ADN	O
,	O
making	O
it	O
difficult	O
for	O
attackers	O
to	O
decrypt	O
the	O
data	O
in	O
transit	O
or	O
hijack	O
the	O
session	O
.	O
</s>
<s>
Because	O
the	O
ADN	O
is	O
designed	O
to	O
understand	O
applications	O
and	O
application	O
protocols	O
,	O
such	O
as	O
HTTP	B-Protocol
,	O
it	O
can	O
manipulate	O
certain	O
aspects	O
of	O
the	O
protocol	O
to	O
cloak	O
the	O
servers	O
in	O
the	O
pool	O
and	O
prevent	O
potentially	O
useful	O
information	O
regarding	O
the	O
software	O
and	O
hardware	O
infrastructure	O
from	O
being	O
exposed	O
.	O
</s>
<s>
This	O
is	O
usually	O
accomplished	O
by	O
rewriting	O
the	O
Server	O
field	O
in	O
an	O
HTTP	B-Protocol
response	O
.	O
</s>
<s>
A	O
second	O
typical	O
use	O
of	O
this	O
functionality	O
is	O
the	O
exploitation	O
of	O
the	O
ADN	O
's	O
ability	O
to	O
rewrite	O
the	O
URI	B-Protocol
portion	O
of	O
an	O
HTTP	B-Protocol
request	O
.	O
</s>
<s>
The	O
client	O
is	O
presented	O
with	O
a	O
URI	B-Protocol
and	O
VIP	O
that	O
are	O
known	O
only	O
to	O
the	O
ADN	O
,	O
and	O
upon	O
receiving	O
the	O
request	O
the	O
ADN	O
may	O
either	O
(	O
a	O
)	O
rewrite	O
the	O
URI	B-Protocol
and	O
send	O
a	O
302	O
redirect	O
or	O
(	O
b	O
)	O
transparently	O
translates	O
the	O
URI	B-Protocol
and	O
responds	O
to	O
the	O
client	O
as	O
if	O
the	O
URI	B-Protocol
were	O
the	O
right	O
one	O
in	O
the	O
first	O
place	O
.	O
</s>
<s>
In	O
recent	O
years	O
commercial	O
ADNs	O
have	O
begun	O
to	O
include	O
application	B-Protocol
firewall	I-Protocol
functionality	O
to	O
further	O
secure	O
applications	O
during	O
the	O
delivery	O
process	O
.	O
</s>
<s>
This	O
is	O
a	O
hotly	O
debated	O
subject	O
with	O
many	O
security	O
professionals	O
arguing	O
that	O
the	O
functionality	O
included	O
in	O
an	O
application	B-Protocol
firewall	I-Protocol
are	O
unnecessary	O
and	O
should	O
be	O
handled	O
by	O
the	O
application	O
while	O
others	O
consider	O
employing	O
as	O
much	O
security	O
as	O
possible	O
,	O
regardless	O
of	O
position	O
in	O
the	O
delivery	O
network	O
,	O
to	O
be	O
the	O
best	O
practice	O
.	O
</s>
<s>
Delayed	B-Protocol
binding	I-Protocol
,	O
also	O
called	O
TCP	B-Protocol
splicing	I-Protocol
,	O
is	O
the	O
postponement	O
of	O
the	O
connection	O
between	O
the	O
client	O
and	O
the	O
server	O
in	O
order	O
to	O
obtain	O
sufficient	O
information	O
to	O
make	O
a	O
routing	O
decision	O
.	O
</s>
<s>
ADNs	O
often	O
have	O
the	O
ability	O
to	O
filter	O
traffic	O
based	O
on	O
Access	O
Control	O
Lists	O
(	O
ACLs	O
)	O
,	O
Bogus	O
IP	O
ranges	O
(	O
Bogon	O
filtering	O
)	O
and	O
deep	B-Application
packet	I-Application
inspection	I-Application
pattern	O
matching	O
.	O
</s>
<s>
The	O
deep	B-Application
packet	I-Application
inspection	I-Application
capabilities	O
of	O
some	O
of	O
these	O
products	O
can	O
identify	O
traffic	O
by	O
application	O
type	O
and	O
can	O
be	O
used	O
to	O
analyze	O
,	O
block	O
,	O
shape	O
and	O
prioritize	O
traffic	O
.	O
</s>
