<s>
The	O
protocol	B-Protocol
stack	I-Protocol
or	O
network	B-Protocol
stack	I-Protocol
is	O
an	O
implementation	O
of	O
a	O
computer	B-Architecture
networking	I-Architecture
protocol	B-Protocol
suite	I-Protocol
or	O
protocol	B-Protocol
family	I-Protocol
.	O
</s>
<s>
This	O
modularization	B-Architecture
simplifies	O
design	O
and	O
evaluation	O
.	O
</s>
<s>
Because	O
each	O
protocol	O
module	B-Architecture
usually	O
communicates	O
with	O
two	O
others	O
,	O
they	O
are	O
commonly	O
imagined	O
as	O
layers	B-Architecture
in	O
a	O
stack	O
of	O
protocols	O
.	O
</s>
<s>
User	O
applications	O
usually	O
deal	O
only	O
with	O
the	O
topmost	O
layers	B-Architecture
.	O
</s>
<s>
Imagine	O
three	O
computers	O
:	O
A	O
,	O
B	O
,	O
and	O
C	O
.	O
A	O
and	O
B	O
both	O
have	O
radio	O
equipment	O
and	O
can	O
communicate	O
via	O
the	O
airwaves	O
using	O
a	O
suitable	O
network	B-Application
protocol	O
(	O
such	O
as	O
IEEE	O
802.11	O
)	O
.	O
</s>
<s>
B	O
and	O
C	O
are	O
connected	O
via	O
a	O
cable	O
,	O
using	O
it	O
to	O
exchange	O
data	O
(	O
again	O
,	O
with	O
the	O
help	O
of	O
a	O
protocol	O
,	O
for	O
example	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
)	O
.	O
</s>
<s>
However	O
,	O
neither	O
of	O
these	O
two	O
protocols	O
will	O
be	O
able	O
to	O
transport	B-Architecture
information	O
from	O
A	O
to	O
C	O
,	O
because	O
these	O
computers	O
are	O
conceptually	O
on	O
different	O
networks	O
.	O
</s>
<s>
An	O
inter-network	B-Architecture
protocol	O
is	O
required	O
to	O
connect	O
them	O
.	O
</s>
<s>
It	O
is	O
easier	O
to	O
leave	O
the	O
base	O
protocols	O
alone	O
,	O
and	O
design	O
a	O
protocol	O
that	O
can	O
work	O
on	O
top	O
of	O
any	O
of	O
them	O
(	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
is	O
an	O
example	O
)	O
.	O
</s>
<s>
The	O
inter-network	B-Architecture
protocol	O
will	O
communicate	O
with	O
each	O
of	O
the	O
base	O
protocol	O
in	O
their	O
simpler	O
language	O
;	O
the	O
base	O
protocols	O
will	O
not	O
talk	O
directly	O
to	O
each	O
other	O
.	O
</s>
<s>
On	O
this	O
computer	O
,	O
the	O
lower	O
layer	O
handlers	O
will	O
pass	O
the	O
packet	O
up	O
to	O
the	O
inter-network	B-Architecture
protocol	O
,	O
which	O
,	O
on	O
recognizing	O
that	O
B	O
is	O
not	O
the	O
final	O
destination	O
,	O
will	O
again	O
invoke	O
lower-level	O
functions	O
.	O
</s>
<s>
This	O
time	O
,	O
the	O
cable	O
protocol	O
is	O
used	O
to	O
send	O
the	O
data	O
to	O
C	O
.	O
There	O
,	O
the	O
received	O
packet	O
is	O
again	O
passed	O
to	O
the	O
upper	O
protocol	O
,	O
which	O
(	O
with	O
C	O
being	O
the	O
destination	O
)	O
will	O
pass	O
it	O
on	O
to	O
a	O
higher	O
protocol	O
or	O
application	B-Application
on	O
C	O
.	O
</s>
<s>
In	O
practical	O
implementation	O
,	O
protocol	B-Protocol
stacks	I-Protocol
are	O
often	O
divided	O
into	O
three	O
major	O
sections	O
:	O
media	O
,	O
transport	B-Architecture
,	O
and	O
applications	O
.	O
</s>
<s>
A	O
particular	O
operating	B-General_Concept
system	I-General_Concept
or	O
platform	O
will	O
often	O
have	O
two	O
well-defined	O
software	O
interfaces	O
:	O
one	O
between	O
the	O
media	O
and	O
transport	B-Architecture
layers	I-Architecture
,	O
and	O
one	O
between	O
the	O
transport	B-Architecture
layers	I-Architecture
and	O
applications	O
.	O
</s>
<s>
The	O
media-to-transport	O
interface	O
defines	O
how	O
transport	B-Architecture
protocol	I-Architecture
software	O
makes	O
use	O
of	O
particular	O
media	O
and	O
hardware	O
types	O
and	O
is	O
associated	O
with	O
a	O
device	B-Application
driver	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
this	O
interface	O
level	O
would	O
define	O
how	O
TCP/IP	B-Protocol
transport	B-Architecture
software	O
would	O
talk	O
to	O
the	O
network	B-Protocol
interface	I-Protocol
controller	I-Protocol
.	O
</s>
<s>
Examples	O
of	O
these	O
interfaces	O
include	O
ODI	B-General_Concept
and	O
NDIS	B-Application
in	O
the	O
Microsoft	B-Application
Windows	I-Application
and	O
DOS	B-Device
environment	O
.	O
</s>
<s>
The	O
application-to-transport	O
interface	O
defines	O
how	O
application	B-Application
programs	O
make	O
use	O
of	O
the	O
transport	B-Architecture
layers	I-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
this	O
interface	O
level	O
would	O
define	O
how	O
a	O
web	B-Application
browser	I-Application
program	O
would	O
talk	O
to	O
TCP/IP	B-Protocol
transport	B-Architecture
software	O
.	O
</s>
<s>
Examples	O
of	O
these	O
interfaces	O
include	O
Berkeley	B-Protocol
sockets	I-Protocol
and	O
System	O
V	O
STREAMS	B-Operating_System
in	O
Unix-like	B-Operating_System
environments	O
,	O
and	O
Winsock	B-Application
for	O
Microsoft	B-Application
Windows	I-Application
.	O
</s>
<s>
"	O
Certain	O
protocols	O
are	O
designed	O
with	O
the	O
specific	O
purpose	O
of	O
bridging	O
differences	O
at	O
the	O
lower	O
layers	B-Architecture
,	O
so	O
that	O
common	O
agreements	O
are	O
not	O
required	O
there	O
.	O
</s>
<s>
As	O
a	O
practical	O
matter	O
,	O
real	O
interoperation	O
is	O
achieved	O
by	O
the	O
definition	O
and	O
use	O
of	O
effective	O
spanning	O
layers	B-Architecture
.	O
</s>
<s>
In	O
the	O
Internet	B-Protocol
protocol	I-Protocol
stack	O
,	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Suite	I-Protocol
constitutes	O
a	O
spanning	O
layer	O
that	O
defines	O
a	O
best-effort	O
service	O
for	O
global	O
routing	O
of	O
datagrams	B-Protocol
at	O
Layer	B-Application
3	I-Application
.	O
</s>
<s>
The	O
Internet	B-Protocol
is	O
the	O
community	O
of	O
interoperation	O
based	O
on	O
this	O
spanning	O
layer	O
.	O
</s>
