<s>
An	O
application	B-Application
layer	I-Application
is	O
an	O
abstraction	B-Architecture
layer	I-Architecture
that	O
specifies	O
the	O
shared	O
communications	O
protocols	O
and	O
interface	B-Application
methods	O
used	O
by	O
hosts	B-Device
in	O
a	O
communications	O
network	O
.	O
</s>
<s>
An	O
application	B-Application
layer	I-Application
abstraction	O
is	O
specified	O
in	O
both	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Suite	I-Protocol
(	O
TCP/IP	B-Protocol
)	O
and	O
the	O
OSI	O
model	O
.	O
</s>
<s>
In	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
the	O
application	B-Application
layer	I-Application
contains	O
the	O
communications	O
protocols	O
and	O
interface	B-Application
methods	O
used	O
in	O
process-to-process	O
communications	O
across	O
an	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
computer	O
network	O
.	O
</s>
<s>
The	O
application	B-Application
layer	I-Application
only	O
standardizes	O
communication	O
and	O
depends	O
upon	O
the	O
underlying	O
transport	B-Architecture
layer	I-Architecture
protocols	O
to	O
establish	O
host-to-host	O
data	O
transfer	O
channels	O
and	O
manage	O
the	O
data	O
exchange	O
in	O
a	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
or	O
peer-to-peer	O
networking	O
model	O
.	O
</s>
<s>
Though	O
the	O
TCP/IP	B-Protocol
application	B-Application
layer	I-Application
does	O
not	O
describe	O
specific	O
rules	O
or	O
data	O
formats	O
that	O
applications	O
must	O
consider	O
when	O
communicating	O
,	O
the	O
original	O
specification	O
(	O
in	O
)	O
does	O
rely	O
on	O
and	O
recommend	O
the	O
robustness	O
principle	O
for	O
application	O
design	O
.	O
</s>
<s>
In	O
the	O
OSI	O
model	O
,	O
the	O
definition	O
of	O
the	O
application	B-Application
layer	I-Application
is	O
narrower	O
in	O
scope	O
.	O
</s>
<s>
The	O
OSI	O
model	O
defines	O
the	O
application	B-Application
layer	I-Application
as	O
only	O
the	O
interface	B-Application
responsible	O
for	O
communicating	O
with	O
host-based	O
and	O
user-facing	O
applications	O
.	O
</s>
<s>
OSI	O
then	O
explicitly	O
distinguishes	O
the	O
functionality	O
of	O
two	O
additional	O
layers	O
,	O
the	O
session	B-Application
layer	I-Application
and	O
presentation	B-Application
layer	I-Application
,	O
as	O
separate	O
levels	O
below	O
the	O
application	B-Application
layer	I-Application
and	O
above	O
the	O
transport	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
OSI	O
specifies	O
a	O
strict	O
modular	O
separation	O
of	O
functionality	O
at	O
these	O
layers	O
and	O
provides	O
protocol	B-Protocol
implementations	I-Protocol
for	O
each	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Suite	I-Protocol
compiles	O
these	O
functions	O
into	O
a	O
single	O
layer	O
.	O
</s>
<s>
Originally	O
the	O
OSI	O
model	O
consisted	O
of	O
two	O
kinds	O
of	O
application	B-Application
layer	I-Application
services	O
with	O
their	O
related	O
protocols	O
.	O
</s>
<s>
These	O
two	O
sublayers	O
are	O
the	O
common	O
application	B-Application
service	I-Application
element	O
(	O
CASE	O
)	O
and	O
specific	O
application	B-Application
service	I-Application
element	O
(	O
SASE	O
)	O
.	O
</s>
<s>
Generally	O
,	O
an	O
application	B-Application
layer	I-Application
protocol	O
is	O
realized	O
by	O
the	O
use	O
of	O
the	O
functionality	O
of	O
a	O
number	O
of	O
application	B-Application
service	I-Application
elements	O
.	O
</s>
<s>
Some	O
application	B-Application
service	I-Application
elements	O
invoke	O
different	O
procedures	O
based	O
on	O
the	O
version	O
of	O
the	O
session	O
service	O
available	O
.	O
</s>
<s>
The	O
common	O
application	B-Application
service	I-Application
element	O
sublayer	O
provides	O
services	O
for	O
the	O
application	B-Application
layer	I-Application
and	O
request	O
services	O
from	O
the	O
session	B-Application
layer	I-Application
.	O
</s>
<s>
It	O
provides	O
support	O
for	O
common	O
application	B-Application
services	I-Application
,	O
such	O
as	O
:	O
</s>
<s>
The	O
specific	O
application	B-Application
service	I-Application
element	O
sublayer	O
provides	O
application-specific	O
services	O
(	O
protocols	O
)	O
,	O
such	O
as	O
:	O
</s>
<s>
The	O
IETF	O
definition	O
document	O
for	O
the	O
application	B-Application
layer	I-Application
in	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Suite	I-Protocol
is	O
RFC	O
1123	O
.	O
</s>
<s>
Additional	O
notable	O
application-layer	B-Application
protocols	O
include	O
the	O
following	O
:	O
</s>
