<s>
The	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
commonly	O
known	O
as	O
TCP/IP	B-Protocol
,	O
is	O
a	O
framework	O
for	O
organizing	O
the	O
set	O
of	O
communication	O
protocols	O
used	O
in	O
the	O
Internet	O
and	O
similar	O
computer	B-Architecture
networks	I-Architecture
according	O
to	O
functional	O
criteria	O
.	O
</s>
<s>
The	O
foundational	O
protocols	O
in	O
the	O
suite	O
are	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
,	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
,	O
and	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
provides	O
end-to-end	B-Application
data	I-Application
communication	I-Application
specifying	O
how	O
data	O
should	O
be	O
packetized	O
,	O
addressed	O
,	O
transmitted	O
,	O
routed	B-Protocol
,	O
and	O
received	O
.	O
</s>
<s>
This	O
functionality	O
is	O
organized	O
into	O
four	O
abstraction	B-Architecture
layers	I-Architecture
,	O
which	O
classify	O
all	O
related	O
protocols	O
according	O
to	O
each	O
protocol	O
's	O
scope	O
of	O
networking	O
.	O
</s>
<s>
An	O
implementation	O
of	O
the	O
layers	O
for	O
a	O
particular	O
application	O
forms	O
a	O
protocol	B-Protocol
stack	I-Protocol
.	O
</s>
<s>
From	O
lowest	O
to	O
highest	O
,	O
the	O
layers	O
are	O
the	O
link	B-Protocol
layer	I-Protocol
,	O
containing	O
communication	O
methods	O
for	O
data	O
that	O
remains	O
within	O
a	O
single	O
network	O
segment	O
(	O
link	O
)	O
;	O
the	O
internet	B-Protocol
layer	I-Protocol
,	O
providing	O
internetworking	B-Architecture
between	O
independent	O
networks	O
;	O
the	O
transport	B-Architecture
layer	I-Architecture
,	O
handling	O
host-to-host	O
communication	O
;	O
and	O
the	O
application	B-Application
layer	I-Application
,	O
providing	O
process-to-process	O
data	O
exchange	O
for	O
applications	O
.	O
</s>
<s>
The	O
technical	O
standards	O
underlying	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
and	O
its	O
constituent	O
protocols	O
are	O
maintained	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
predates	O
the	O
OSI	O
model	O
,	O
a	O
more	O
comprehensive	O
reference	O
framework	O
for	O
general	O
networking	O
systems	O
.	O
</s>
<s>
Initially	O
referred	O
to	O
as	O
the	O
DOD	O
Internet	O
Architecture	O
Model	O
,	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
has	O
its	O
roots	O
in	O
research	O
and	O
development	O
sponsored	O
by	O
the	O
Defense	O
Advanced	O
Research	O
Projects	O
Agency	O
(	O
DARPA	O
)	O
in	O
the	O
late	O
1960s	O
.	O
</s>
<s>
After	O
DARPA	O
initiated	O
the	O
pioneering	O
ARPANET	O
in	O
1969	O
,	O
Steve	O
Crocker	O
established	O
a	O
"	O
Networking	O
Working	O
Group	O
"	O
which	O
developed	O
a	O
host-host	O
protocol	O
,	O
the	O
Network	B-Protocol
Control	I-Protocol
Program	I-Protocol
(	O
NCP	B-Protocol
)	O
.	O
</s>
<s>
In	O
the	O
early	O
1970s	O
,	O
DARPA	O
started	O
work	O
on	O
several	O
other	O
data	O
transmission	O
technologies	O
,	O
including	O
mobile	O
packet	B-Protocol
radio	O
,	O
packet	B-Protocol
satellite	O
service	O
,	O
local	O
area	O
networks	O
,	O
and	O
other	O
data	B-Architecture
networks	I-Architecture
in	O
the	O
public	O
and	O
private	O
domains	O
.	O
</s>
<s>
In	O
1972	O
,	O
Bob	O
Kahn	O
joined	O
the	O
DARPA	O
Information	O
Processing	O
Technology	O
Office	O
,	O
where	O
he	O
worked	O
on	O
both	O
satellite	O
packet	B-Protocol
networks	O
and	O
ground-based	O
radio	O
packet	B-Protocol
networks	O
,	O
and	O
recognized	O
the	O
value	O
of	O
being	O
able	O
to	O
communicate	O
across	O
both	O
.	O
</s>
<s>
In	O
the	O
spring	O
of	O
1973	O
,	O
Vinton	O
Cerf	O
joined	O
Kahn	O
to	O
work	O
on	O
open-architecture	B-Architecture
interconnection	O
models	O
with	O
the	O
goal	O
of	O
designing	O
the	O
next	O
protocol	O
generation	O
for	O
the	O
ARPANET	O
.	O
</s>
<s>
They	O
drew	O
on	O
the	O
experience	O
from	O
the	O
ARPANET	O
research	O
community	O
and	O
the	O
International	B-Protocol
Networking	I-Protocol
Working	I-Protocol
Group	I-Protocol
,	O
which	O
Cerf	O
chaired	O
.	O
</s>
<s>
By	O
the	O
summer	O
of	O
1973	O
,	O
Kahn	O
and	O
Cerf	O
had	O
worked	O
out	O
a	O
fundamental	O
reformulation	O
,	O
in	O
which	O
the	O
differences	O
between	O
local	O
network	O
protocols	O
were	O
hidden	O
by	O
using	O
a	O
common	O
internetwork	B-Architecture
protocol	O
,	O
and	O
,	O
instead	O
of	O
the	O
network	O
being	O
responsible	O
for	O
reliability	O
,	O
as	O
in	O
the	O
existing	O
ARPANET	O
protocols	O
,	O
this	O
function	O
was	O
delegated	O
to	O
the	O
hosts	O
.	O
</s>
<s>
The	O
new	O
protocol	O
was	O
implemented	O
as	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Program	I-Protocol
in	O
1974	O
.	O
</s>
<s>
Initially	O
,	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Program	I-Protocol
(	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
did	O
not	O
then	O
exist	O
as	O
a	O
separate	O
protocol	O
)	O
provided	O
only	O
a	O
reliable	O
byte	O
stream	O
service	O
to	O
its	O
users	O
,	O
not	O
datagram	B-Protocol
.	O
</s>
<s>
As	O
experience	O
with	O
the	O
protocol	O
grew	O
,	O
collaborators	O
recommended	O
division	O
of	O
functionality	O
into	O
layers	O
of	O
distinct	O
protocols	O
,	O
allowing	O
users	O
direct	O
access	O
to	O
datagram	B-Protocol
service	I-Protocol
.	O
</s>
<s>
Advocates	O
included	O
Danny	O
Cohen	O
,	O
who	O
needed	O
it	O
for	O
his	O
packet	B-Application
voice	I-Application
work	O
;	O
Jonathan	O
Postel	O
of	O
the	O
University	O
of	O
Southern	O
California	O
's	O
Information	O
Sciences	O
Institute	O
,	O
who	O
edited	O
the	O
Request	O
for	O
Comments	O
(	O
RFCs	O
)	O
,	O
the	O
technical	O
and	O
strategic	O
document	O
series	O
that	O
has	O
both	O
documented	O
and	O
catalyzed	O
Internet	O
development	O
;	O
and	O
the	O
research	O
group	O
of	O
Robert	O
Metcalfe	O
at	O
Xerox	O
PARC	O
.	O
</s>
<s>
Postel	O
stated	O
,	O
"	O
We	O
are	O
screwing	O
up	O
in	O
our	O
design	O
of	O
Internet	B-Protocol
protocols	I-Protocol
by	O
violating	O
the	O
principle	O
of	O
layering.	O
"	O
</s>
<s>
Encapsulation	B-Protocol
of	O
different	O
mechanisms	O
was	O
intended	O
to	O
create	O
an	O
environment	O
where	O
the	O
upper	O
layers	O
could	O
access	O
only	O
what	O
was	O
needed	O
from	O
the	O
lower	O
layers	O
.	O
</s>
<s>
In	O
version	O
3	O
of	O
TCP	O
,	O
written	O
in	O
1978	O
,	O
Cerf	O
,	O
Cohen	O
and	O
Postel	O
split	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Program	I-Protocol
into	O
two	O
distinct	O
protocols	O
,	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
as	O
connectionless	B-Protocol
layer	O
and	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
as	O
a	O
reliable	O
connection-oriented	B-Protocol
service	I-Protocol
.	O
</s>
<s>
The	O
design	O
of	O
the	O
network	O
included	O
the	O
recognition	O
that	O
it	O
should	O
provide	O
only	O
the	O
functions	O
of	O
efficiently	O
transmitting	O
and	O
routing	B-Protocol
traffic	O
between	O
end	O
nodes	O
and	O
that	O
all	O
other	O
intelligence	O
should	O
be	O
located	O
at	O
the	O
edge	O
of	O
the	O
network	O
,	O
in	O
the	O
end	O
nodes	O
.	O
</s>
<s>
This	O
design	O
is	O
known	O
as	O
the	O
end-to-end	B-Application
principle	I-Application
.	O
</s>
<s>
Using	O
this	O
design	O
,	O
it	O
became	O
possible	O
to	O
connect	O
other	O
networks	O
to	O
the	O
ARPANET	O
that	O
used	O
the	O
same	O
principle	O
,	O
irrespective	O
of	O
other	O
local	O
characteristics	O
,	O
thereby	O
solving	O
Kahn	O
's	O
initial	O
internetworking	B-Architecture
problem	O
.	O
</s>
<s>
A	O
popular	O
expression	O
is	O
that	O
TCP/IP	B-Protocol
,	O
the	O
eventual	O
product	O
of	O
Cerf	O
and	O
Kahn	O
's	O
work	O
,	O
can	O
run	O
over	O
"	O
two	O
tin	O
cans	O
and	O
a	O
string.	O
"	O
</s>
<s>
Years	O
later	O
,	O
as	O
a	O
joke	O
,	O
the	O
IP	B-Protocol
over	I-Protocol
Avian	I-Protocol
Carriers	I-Protocol
formal	O
protocol	O
specification	O
was	O
created	O
and	O
successfully	O
tested	O
.	O
</s>
<s>
During	O
development	O
of	O
the	O
protocol	O
the	O
version	O
number	O
of	O
the	O
packet	B-Protocol
routing	B-Protocol
layer	O
progressed	O
from	O
version	O
1	O
to	O
version	O
4	O
,	O
the	O
latter	O
of	O
which	O
was	O
installed	O
in	O
the	O
ARPANET	O
in	O
1983	O
.	O
</s>
<s>
It	O
became	O
known	O
as	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
as	O
the	O
protocol	O
that	O
is	O
still	O
in	O
use	O
in	O
the	O
Internet	O
,	O
alongside	O
its	O
current	O
successor	O
,	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
.	O
</s>
<s>
In	O
1975	O
,	O
a	O
two-network	O
IP	B-Protocol
communications	I-Protocol
test	O
was	O
performed	O
between	O
Stanford	O
and	O
University	O
College	O
London	O
.	O
</s>
<s>
A	O
computer	O
called	O
a	O
router	B-Protocol
is	O
provided	O
with	O
an	O
interface	O
to	O
each	O
network	O
.	O
</s>
<s>
It	O
forwards	O
network	B-Protocol
packets	I-Protocol
back	O
and	O
forth	O
between	O
them	O
.	O
</s>
<s>
Originally	O
a	O
router	B-Protocol
was	O
called	O
gateway	B-Application
,	O
but	O
the	O
term	O
was	O
changed	O
to	O
avoid	O
confusion	O
with	O
other	O
types	O
of	O
gateways	B-Application
.	O
</s>
<s>
In	O
March	O
1982	O
,	O
the	O
US	O
Department	O
of	O
Defense	O
declared	O
TCP/IP	B-Protocol
as	O
the	O
standard	O
for	O
all	O
military	O
computer	B-Architecture
networking	I-Architecture
.	O
</s>
<s>
The	O
migration	O
of	O
the	O
ARPANET	O
from	O
NCP	B-Protocol
to	O
TCP/IP	B-Protocol
was	O
officially	O
completed	O
on	O
flag	B-General_Concept
day	I-General_Concept
January	O
1	O
,	O
1983	O
,	O
when	O
the	O
new	O
protocols	O
were	O
permanently	O
activated	O
.	O
</s>
<s>
In	O
1985	O
,	O
the	O
Internet	O
Advisory	O
Board	O
(	O
later	O
Internet	O
Architecture	O
Board	O
)	O
held	O
a	O
three-day	O
TCP/IP	B-Protocol
workshop	O
for	O
the	O
computer	O
industry	O
,	O
attended	O
by	O
250	O
vendor	O
representatives	O
,	O
promoting	O
the	O
protocol	O
and	O
leading	O
to	O
its	O
increasing	O
commercial	O
use	O
.	O
</s>
<s>
In	O
1985	O
,	O
the	O
first	O
Interop	O
conference	O
focused	O
on	O
network	O
interoperability	O
by	O
broader	O
adoption	O
of	O
TCP/IP	B-Protocol
.	O
</s>
<s>
IBM	O
,	O
AT&T	O
and	O
DEC	O
were	O
the	O
first	O
major	O
corporations	O
to	O
adopt	O
TCP/IP	B-Protocol
,	O
this	O
despite	O
having	O
competing	O
proprietary	O
protocols	O
.	O
</s>
<s>
In	O
IBM	O
,	O
from	O
1984	O
,	O
Barry	O
Appelman	O
's	O
group	O
did	O
TCP/IP	B-Protocol
development	O
.	O
</s>
<s>
They	O
navigated	O
the	O
corporate	O
politics	O
to	O
get	O
a	O
stream	O
of	O
TCP/IP	B-Protocol
products	O
for	O
various	O
IBM	O
systems	O
,	O
including	O
MVS	B-Application
,	O
VM	B-Application
,	O
and	O
OS/2	B-Application
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
several	O
smaller	O
companies	O
,	O
such	O
as	O
FTP	O
Software	O
and	O
the	B-Operating_System
Wollongong	I-Operating_System
Group	I-Operating_System
,	O
began	O
offering	O
TCP/IP	B-Protocol
stacks	I-Protocol
for	O
DOS	B-Device
and	O
Microsoft	B-Application
Windows	I-Application
.	O
</s>
<s>
The	O
first	O
VM/CMS	B-Application
TCP/IP	B-Protocol
stack	I-Protocol
came	O
from	O
the	O
University	O
of	O
Wisconsin	O
.	O
</s>
<s>
Some	O
of	O
the	O
early	O
TCP/IP	B-Protocol
stacks	I-Protocol
were	O
written	O
single-handedly	O
by	O
a	O
few	O
programmers	O
.	O
</s>
<s>
Jay	O
Elinsky	O
and	O
of	O
IBM	O
Research	O
wrote	O
TCP/IP	B-Protocol
stacks	I-Protocol
for	O
VM/CMS	B-Application
and	O
OS/2	B-Application
,	O
respectively	O
.	O
</s>
<s>
The	O
spread	O
of	O
TCP/IP	B-Protocol
was	O
fueled	O
further	O
in	O
June	O
1989	O
,	O
when	O
the	O
University	O
of	O
California	O
,	O
Berkeley	O
agreed	O
to	O
place	O
the	O
TCP/IP	B-Protocol
code	O
developed	O
for	O
BSD	B-Operating_System
UNIX	I-Operating_System
into	O
the	O
public	O
domain	O
.	O
</s>
<s>
Various	O
corporate	O
vendors	O
,	O
including	O
IBM	O
,	O
included	O
this	O
code	O
in	O
commercial	O
TCP/IP	B-Protocol
software	O
releases	O
.	O
</s>
<s>
Microsoft	O
released	O
a	O
native	O
TCP/IP	B-Protocol
stack	I-Protocol
in	O
Windows	O
95	O
.	O
</s>
<s>
This	O
event	O
helped	O
cement	O
TCP/IP	B-Protocol
'	O
s	O
dominance	O
over	O
other	O
protocols	O
on	O
Microsoft-based	O
networks	O
,	O
which	O
included	O
IBM	O
's	O
Systems	B-Architecture
Network	I-Architecture
Architecture	I-Architecture
(	O
SNA	O
)	O
,	O
and	O
on	O
other	O
platforms	O
such	O
as	O
Digital	O
Equipment	O
Corporation	O
's	O
DECnet	B-Protocol
,	O
Open	O
Systems	O
Interconnection	O
(	O
OSI	O
)	O
,	O
and	O
Xerox	B-Protocol
Network	I-Protocol
Systems	I-Protocol
(	O
XNS	O
)	O
.	O
</s>
<s>
Nonetheless	O
,	O
for	O
a	O
period	O
in	O
the	O
late	O
1980s	O
and	O
early	O
1990s	O
,	O
engineers	O
,	O
organizations	O
and	O
nations	O
were	O
polarized	B-Protocol
over	I-Protocol
the	I-Protocol
issue	I-Protocol
of	I-Protocol
which	I-Protocol
standard	I-Protocol
,	O
the	O
OSI	O
model	O
or	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
would	O
result	O
in	O
the	O
best	O
and	O
most	O
robust	O
computer	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
The	O
technical	O
standards	O
underlying	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
and	O
its	O
constituent	O
protocols	O
have	O
been	O
delegated	O
to	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
.	O
</s>
<s>
The	O
characteristic	O
architecture	O
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
is	O
its	O
broad	O
division	O
into	O
operating	O
scopes	O
for	O
the	O
protocols	O
that	O
constitute	O
its	O
core	O
functionality	O
.	O
</s>
<s>
The	O
defining	O
specification	O
of	O
the	O
suite	O
is	O
RFC	O
1122	O
,	O
which	O
broadly	O
outlines	O
four	O
abstraction	B-Architecture
layers	I-Architecture
.	O
</s>
<s>
As	O
such	O
a	O
model	O
of	O
networking	O
,	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
predates	O
the	O
OSI	O
model	O
,	O
a	O
more	O
comprehensive	O
reference	O
framework	O
for	O
general	O
networking	O
systems	O
.	O
</s>
<s>
The	O
end-to-end	B-Application
principle	I-Application
has	O
evolved	O
over	O
time	O
.	O
</s>
<s>
Real-world	O
needs	O
for	O
firewalls	B-Application
,	O
network	O
address	O
translators	O
,	O
web	O
content	O
caches	O
and	O
the	O
like	O
have	O
forced	O
changes	O
in	O
this	O
principle	O
.	O
</s>
<s>
That	O
is	O
,	O
it	O
must	O
be	O
careful	O
to	O
send	O
well-formed	O
datagrams	B-Protocol
,	O
but	O
must	O
accept	O
any	O
datagram	B-Protocol
that	O
it	O
can	O
interpret	O
(	O
e.g.	O
,	O
not	O
object	O
to	O
technical	O
errors	O
where	O
the	O
meaning	O
is	O
still	O
clear	O
)	O
.	O
"	O
</s>
<s>
Encapsulation	B-Protocol
is	O
used	O
to	O
provide	O
abstraction	O
of	O
protocols	O
and	O
services	O
.	O
</s>
<s>
Encapsulation	B-Protocol
is	O
usually	O
aligned	O
with	O
the	O
division	O
of	O
the	O
protocol	B-Protocol
suite	I-Protocol
into	O
layers	O
of	O
general	O
functionality	O
.	O
</s>
<s>
The	O
data	O
is	O
further	O
encapsulated	B-Protocol
at	O
each	O
level	O
.	O
</s>
<s>
The	O
application	B-Application
layer	I-Application
is	O
the	O
scope	O
within	O
which	O
applications	O
,	O
or	O
processes	B-Operating_System
,	O
create	O
user	O
data	O
and	O
communicate	O
this	O
data	O
to	O
other	O
applications	O
on	O
another	O
or	O
the	O
same	O
host	O
.	O
</s>
<s>
The	O
applications	O
make	O
use	O
of	O
the	O
services	O
provided	O
by	O
the	O
underlying	O
lower	O
layers	O
,	O
especially	O
the	O
transport	B-Architecture
layer	I-Architecture
which	O
provides	O
reliable	B-Protocol
or	I-Protocol
unreliable	I-Protocol
pipes	O
to	O
other	O
processes	B-Operating_System
.	O
</s>
<s>
The	O
communications	O
partners	O
are	O
characterized	O
by	O
the	O
application	O
architecture	O
,	O
such	O
as	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
and	O
peer-to-peer	O
networking	O
.	O
</s>
<s>
This	O
is	O
the	O
layer	O
in	O
which	O
all	O
application	B-Application
protocols	I-Application
,	O
such	O
as	O
SMTP	B-Protocol
,	O
FTP	O
,	O
SSH	O
,	O
HTTP	B-Protocol
,	O
operate	O
.	O
</s>
<s>
Processes	B-Operating_System
are	O
addressed	O
via	O
ports	O
which	O
essentially	O
represent	O
services	O
.	O
</s>
<s>
The	O
transport	B-Architecture
layer	I-Architecture
performs	O
host-to-host	O
communications	O
on	O
either	O
the	O
local	O
network	O
or	O
remote	O
networks	O
separated	O
by	O
routers	B-Protocol
.	O
</s>
<s>
UDP	O
is	O
the	O
basic	O
transport	B-Architecture
layer	I-Architecture
protocol	O
,	O
providing	O
an	O
unreliable	O
connectionless	B-Protocol
datagram	B-Protocol
service	I-Protocol
.	O
</s>
<s>
The	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
provides	O
flow-control	O
,	O
connection	O
establishment	O
,	O
and	O
reliable	O
transmission	O
of	O
data	O
.	O
</s>
<s>
The	O
internet	B-Protocol
layer	I-Protocol
exchanges	O
datagrams	B-Protocol
across	O
network	O
boundaries	O
.	O
</s>
<s>
It	O
is	O
therefore	O
also	O
the	O
layer	O
that	O
establishes	O
internetworking	B-Architecture
.	O
</s>
<s>
This	O
layer	O
defines	O
the	O
addressing	O
and	O
routing	B-Protocol
structures	O
used	O
for	O
the	O
TCP/IP	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
The	O
primary	O
protocol	O
in	O
this	O
scope	O
is	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
which	O
defines	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
Its	O
function	O
in	O
routing	B-Protocol
is	O
to	O
transport	O
datagrams	B-Protocol
to	O
the	O
next	O
host	O
,	O
functioning	O
as	O
an	O
IP	B-Protocol
router	I-Protocol
,	O
that	O
has	O
the	O
connectivity	O
to	O
a	O
network	O
closer	O
to	O
the	O
final	O
data	O
destination	O
.	O
</s>
<s>
The	O
link	B-Protocol
layer	I-Protocol
defines	O
the	O
networking	O
methods	O
within	O
the	O
scope	O
of	O
the	O
local	O
network	O
link	O
on	O
which	O
hosts	O
communicate	O
without	O
intervening	O
routers	B-Protocol
.	O
</s>
<s>
This	O
layer	O
includes	O
the	O
protocols	O
used	O
to	O
describe	O
the	O
local	O
network	O
topology	O
and	O
the	O
interfaces	O
needed	O
to	O
affect	O
the	O
transmission	O
of	O
Internet	B-Protocol
layer	I-Protocol
datagrams	B-Protocol
to	O
next-neighbor	O
hosts	O
.	O
</s>
<s>
The	O
protocols	O
of	O
the	O
link	B-Protocol
layer	I-Protocol
operate	O
within	O
the	O
scope	O
of	O
the	O
local	O
network	O
connection	O
to	O
which	O
a	O
host	O
is	O
attached	O
.	O
</s>
<s>
This	O
regime	O
is	O
called	O
the	O
link	O
in	O
TCP/IP	B-Protocol
parlance	O
and	O
is	O
the	O
lowest	O
component	O
layer	O
of	O
the	O
suite	O
.	O
</s>
<s>
The	O
link	O
includes	O
all	O
hosts	O
accessible	O
without	O
traversing	O
a	O
router	B-Protocol
.	O
</s>
<s>
In	O
principle	O
,	O
TCP/IP	B-Protocol
is	O
designed	O
to	O
be	O
hardware	O
independent	O
and	O
may	O
be	O
implemented	O
on	O
top	O
of	O
virtually	O
any	O
link-layer	B-Protocol
technology	O
.	O
</s>
<s>
This	O
includes	O
not	O
only	O
hardware	O
implementations	O
,	O
but	O
also	O
virtual	O
link	B-Protocol
layers	I-Protocol
such	O
as	O
virtual	B-Application
private	I-Application
networks	I-Application
and	O
networking	B-Protocol
tunnels	I-Protocol
.	O
</s>
<s>
The	O
link	B-Protocol
layer	I-Protocol
is	O
used	O
to	O
move	O
packets	B-Protocol
between	O
the	O
Internet	B-Protocol
layer	I-Protocol
interfaces	O
of	O
two	O
different	O
hosts	O
on	O
the	O
same	O
link	O
.	O
</s>
<s>
The	O
processes	B-Operating_System
of	O
transmitting	O
and	O
receiving	O
packets	B-Protocol
on	O
the	O
link	O
can	O
be	O
controlled	O
in	O
the	O
device	B-Application
driver	I-Application
for	O
the	O
network	B-Protocol
card	I-Protocol
,	O
as	O
well	O
as	O
in	O
firmware	B-Application
or	O
by	O
specialized	O
chipsets	B-Device
.	O
</s>
<s>
These	O
perform	O
functions	O
,	O
such	O
as	O
framing	O
,	O
to	O
prepare	O
the	O
Internet	B-Protocol
layer	I-Protocol
packets	B-Protocol
for	O
transmission	O
,	O
and	O
finally	O
transmit	O
the	O
frames	O
to	O
the	O
physical	B-Application
layer	I-Application
and	O
over	O
a	O
transmission	O
medium	O
.	O
</s>
<s>
The	O
TCP/IP	B-Protocol
model	I-Protocol
includes	O
specifications	O
for	O
translating	O
the	O
network	O
addressing	O
methods	O
used	O
in	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
to	O
link-layer	B-Protocol
addresses	O
,	O
such	O
as	O
media	B-Protocol
access	I-Protocol
control	I-Protocol
(	O
MAC	O
)	O
addresses	O
.	O
</s>
<s>
All	O
other	O
aspects	O
below	O
that	O
level	O
,	O
however	O
,	O
are	O
implicitly	O
assumed	O
to	O
exist	O
,	O
and	O
are	O
not	O
explicitly	O
defined	O
in	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
.	O
</s>
<s>
The	O
link	B-Protocol
layer	I-Protocol
in	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
has	O
corresponding	O
functions	O
in	O
Layer	O
2	O
of	O
the	O
OSI	O
model	O
.	O
</s>
<s>
Internetworking	B-Architecture
requires	O
sending	O
data	O
from	O
the	O
source	O
network	O
to	O
the	O
destination	O
network	O
.	O
</s>
<s>
This	O
process	O
is	O
called	O
routing	B-Protocol
and	O
is	O
supported	O
by	O
host	O
addressing	O
and	O
identification	O
using	O
the	O
hierarchical	O
IP	B-Protocol
addressing	I-Protocol
system	O
.	O
</s>
<s>
The	O
internet	B-Protocol
layer	I-Protocol
provides	O
an	O
unreliable	O
datagram	B-Protocol
transmission	O
facility	O
between	O
hosts	O
located	O
on	O
potentially	O
different	O
IP	B-Protocol
networks	I-Protocol
by	O
forwarding	O
datagrams	B-Protocol
to	O
an	O
appropriate	O
next-hop	O
router	B-Protocol
for	O
further	O
relaying	O
to	O
its	O
destination	O
.	O
</s>
<s>
The	O
internet	B-Protocol
layer	I-Protocol
has	O
the	O
responsibility	O
of	O
sending	O
packets	B-Protocol
across	O
potentially	O
multiple	O
networks	O
.	O
</s>
<s>
With	O
this	O
functionality	O
,	O
the	O
internet	B-Protocol
layer	I-Protocol
makes	O
possible	O
internetworking	B-Architecture
,	O
the	O
interworking	O
of	O
different	O
IP	B-Protocol
networks	I-Protocol
,	O
and	O
it	O
essentially	O
establishes	O
the	O
Internet	O
.	O
</s>
<s>
The	O
internet	B-Protocol
layer	I-Protocol
does	O
not	O
distinguish	O
between	O
the	O
various	O
transport	B-Architecture
layer	I-Architecture
protocols	O
.	O
</s>
<s>
IP	O
carries	O
data	O
for	O
a	O
variety	O
of	O
different	O
upper	B-Protocol
layer	I-Protocol
protocols	I-Protocol
.	O
</s>
<s>
These	O
protocols	O
are	O
each	O
identified	O
by	O
a	O
unique	O
protocol	B-Protocol
number	I-Protocol
:	O
for	O
example	O
,	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
ICMP	O
)	O
and	O
Internet	B-Protocol
Group	I-Protocol
Management	I-Protocol
Protocol	I-Protocol
(	O
IGMP	B-Protocol
)	O
are	O
protocols	O
1	O
and	O
2	O
,	O
respectively	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
Protocol	I-Protocol
is	O
the	O
principal	O
component	O
of	O
the	O
internet	B-Protocol
layer	I-Protocol
,	O
and	O
it	O
defines	O
two	O
addressing	O
systems	O
to	O
identify	O
network	O
hosts	O
and	O
to	O
locate	O
them	O
on	O
the	O
network	O
.	O
</s>
<s>
The	O
original	O
address	O
system	O
of	O
the	O
ARPANET	O
and	O
its	O
successor	O
,	O
the	O
Internet	O
,	O
is	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
.	O
</s>
<s>
It	O
uses	O
a	O
32-bit	O
IP	B-Protocol
address	I-Protocol
and	O
is	O
therefore	O
capable	O
of	O
identifying	O
approximately	O
four	O
billion	O
hosts	O
.	O
</s>
<s>
This	O
limitation	O
was	O
eliminated	O
in	O
1998	O
by	O
the	O
standardization	O
of	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
which	O
uses	O
128-bit	O
addresses	O
.	O
</s>
<s>
IPv6	B-Protocol
production	O
implementations	O
emerged	O
in	O
approximately	O
2006	O
.	O
</s>
<s>
The	O
transport	B-Architecture
layer	I-Architecture
establishes	O
basic	O
data	O
channels	O
that	O
applications	O
use	O
for	O
task-specific	O
data	O
exchange	O
.	O
</s>
<s>
Connectivity	O
at	O
the	O
transport	B-Architecture
layer	I-Architecture
can	O
be	O
categorized	O
as	O
either	O
connection-oriented	B-Protocol
,	O
implemented	O
in	O
TCP	O
,	O
or	O
connectionless	B-Protocol
,	O
implemented	O
in	O
UDP	O
.	O
</s>
<s>
The	O
protocols	O
in	O
this	O
layer	O
may	O
provide	O
error	B-Error_Name
control	I-Error_Name
,	O
segmentation	B-Architecture
,	O
flow	O
control	O
,	O
congestion	B-Protocol
control	I-Protocol
,	O
and	O
application	O
addressing	O
(	O
port	B-Protocol
numbers	I-Protocol
)	O
.	O
</s>
<s>
For	O
the	O
purpose	O
of	O
providing	O
process-specific	O
transmission	O
channels	O
for	O
applications	O
,	O
the	O
layer	O
establishes	O
the	O
concept	O
of	O
the	O
network	B-Protocol
port	I-Protocol
.	O
</s>
<s>
For	O
many	O
types	O
of	O
services	O
,	O
these	O
port	B-Protocol
numbers	I-Protocol
have	O
been	O
standardized	O
so	O
that	O
client	B-Protocol
computers	I-Protocol
may	O
address	O
specific	O
services	O
of	O
a	O
server	O
computer	O
without	O
the	O
involvement	O
of	O
service	B-Protocol
discovery	I-Protocol
or	O
directory	O
services	O
.	O
</s>
<s>
Because	O
IP	O
provides	O
only	O
a	O
best-effort	O
delivery	O
,	O
some	O
transport-layer	B-Architecture
protocols	I-Architecture
offer	O
reliability	O
.	O
</s>
<s>
TCP	O
is	O
a	O
connection-oriented	B-Protocol
protocol	I-Protocol
that	O
addresses	O
numerous	O
reliability	O
issues	O
in	O
providing	O
a	O
reliable	O
byte	O
stream	O
:	O
</s>
<s>
The	O
newer	O
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
)	O
is	O
also	O
a	O
reliable	O
,	O
connection-oriented	B-Protocol
transport	O
mechanism	O
.	O
</s>
<s>
It	O
also	O
provides	O
multihoming	B-Protocol
support	O
,	O
in	O
which	O
a	O
connection	O
end	O
can	O
be	O
represented	O
by	O
multiple	O
IP	B-Protocol
addresses	I-Protocol
(	O
representing	O
multiple	O
physical	O
interfaces	O
)	O
,	O
such	O
that	O
if	O
one	O
fails	O
,	O
the	O
connection	O
is	O
not	O
interrupted	O
.	O
</s>
<s>
The	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
is	O
a	O
connectionless	B-Protocol
datagram	B-Protocol
protocol	O
.	O
</s>
<s>
Reliability	O
is	O
addressed	O
through	O
error	B-Error_Name
detection	I-Error_Name
using	O
a	O
checksum	O
algorithm	O
.	O
</s>
<s>
UDP	O
is	O
typically	O
used	O
for	O
applications	O
such	O
as	O
streaming	O
media	O
(	O
audio	O
,	O
video	O
,	O
Voice	B-Application
over	I-Application
IP	I-Application
etc	O
.	O
)	O
</s>
<s>
where	O
on-time	O
arrival	O
is	O
more	O
important	O
than	O
reliability	O
,	O
or	O
for	O
simple	O
query/response	O
applications	O
like	O
DNS	B-Protocol
lookups	O
,	O
where	O
the	O
overhead	O
of	O
setting	O
up	O
a	O
reliable	O
connection	O
is	O
disproportionately	O
large	O
.	O
</s>
<s>
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
is	O
a	O
datagram	B-Protocol
protocol	O
that	O
is	O
used	O
over	O
UDP	O
and	O
is	O
designed	O
for	O
real-time	O
data	O
such	O
as	O
streaming	O
media	O
.	O
</s>
<s>
The	O
applications	O
at	O
any	O
given	O
network	O
address	O
are	O
distinguished	O
by	O
their	O
TCP	O
or	O
UDP	B-Protocol
port	I-Protocol
.	O
</s>
<s>
The	O
TCP/IP	B-Protocol
model	I-Protocol
's	O
transport	O
or	O
host-to-host	B-Architecture
layer	I-Architecture
corresponds	O
roughly	O
to	O
the	O
fourth	O
layer	O
in	O
the	O
OSI	O
model	O
,	O
also	O
called	O
the	O
transport	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
QUIC	B-Protocol
is	O
rapidly	O
emerging	O
as	O
an	O
alternative	O
transport	B-Architecture
protocol	I-Architecture
.	O
</s>
<s>
Whilst	O
it	O
is	O
technically	O
carried	O
via	O
UDP	B-Protocol
packets	I-Protocol
it	O
seeks	O
to	O
offer	O
enhanced	O
transport	O
connectivity	O
relative	O
to	O
TCP	O
.	O
</s>
<s>
HTTP/3	B-Protocol
works	O
exclusively	O
via	O
QUIC	B-Protocol
.	O
</s>
<s>
The	O
application	B-Application
layer	I-Application
includes	O
the	O
protocols	O
used	O
by	O
most	O
applications	O
for	O
providing	O
user	O
services	O
or	O
exchanging	O
application	O
data	O
over	O
the	O
network	O
connections	O
established	O
by	O
the	O
lower	O
level	O
protocols	O
.	O
</s>
<s>
This	O
may	O
include	O
some	O
basic	O
network	O
support	O
services	O
such	O
as	O
routing	B-Protocol
protocols	I-Protocol
and	O
host	O
configuration	O
.	O
</s>
<s>
Examples	O
of	O
application	B-Application
layer	I-Application
protocols	O
include	O
the	O
Hypertext	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
HTTP	B-Protocol
)	O
,	O
the	O
File	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
FTP	O
)	O
,	O
the	O
Simple	B-Protocol
Mail	I-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
SMTP	B-Protocol
)	O
,	O
and	O
the	O
Dynamic	B-Protocol
Host	I-Protocol
Configuration	I-Protocol
Protocol	I-Protocol
(	O
DHCP	B-Protocol
)	O
.	O
</s>
<s>
Data	O
coded	O
according	O
to	O
application	B-Application
layer	I-Application
protocols	O
are	O
encapsulated	B-Protocol
into	O
transport	B-Architecture
layer	I-Architecture
protocol	O
units	O
(	O
such	O
as	O
TCP	O
streams	O
or	O
UDP	O
datagrams	B-Protocol
)	O
,	O
which	O
in	O
turn	O
use	O
lower	B-Protocol
layer	I-Protocol
protocols	I-Protocol
to	O
effect	O
actual	O
data	O
transfer	O
.	O
</s>
<s>
The	O
TCP/IP	B-Protocol
model	I-Protocol
does	O
not	O
consider	O
the	O
specifics	O
of	O
formatting	O
and	O
presenting	O
data	O
and	O
does	O
not	O
define	O
additional	O
layers	O
between	O
the	O
application	O
and	O
transport	B-Architecture
layers	I-Architecture
as	O
in	O
the	O
OSI	O
model	O
(	O
presentation	O
and	O
session	O
layers	O
)	O
.	O
</s>
<s>
According	O
to	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
,	O
such	O
functions	O
are	O
the	O
realm	O
of	O
libraries	B-Library
and	O
application	B-Application
programming	I-Application
interfaces	I-Application
.	O
</s>
<s>
The	O
application	B-Application
layer	I-Application
in	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
is	O
often	O
compared	O
to	O
a	O
combination	O
of	O
the	O
fifth	O
(	O
session	O
)	O
,	O
sixth	O
(	O
presentation	O
)	O
,	O
and	O
seventh	O
(	O
application	O
)	O
layers	O
of	O
the	O
OSI	O
model	O
.	O
</s>
<s>
Application	B-Application
layer	I-Application
protocols	O
are	O
often	O
associated	O
with	O
particular	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
applications	O
,	O
and	O
common	O
services	O
have	O
well-known	O
port	B-Protocol
numbers	I-Protocol
reserved	O
by	O
the	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
HyperText	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
uses	O
server	O
port	O
80	O
and	O
Telnet	B-Protocol
uses	O
server	O
port	B-Protocol
23	I-Protocol
.	O
</s>
<s>
Clients	B-Protocol
connecting	O
to	O
a	O
service	O
usually	O
use	O
ephemeral	B-Protocol
ports	I-Protocol
,	O
i.e.	O
,	O
port	B-Protocol
numbers	I-Protocol
assigned	O
only	O
for	O
the	O
duration	O
of	O
the	O
transaction	O
at	O
random	O
or	O
from	O
a	O
specific	O
range	O
configured	O
in	O
the	O
application	O
.	O
</s>
<s>
At	O
the	O
application	B-Application
layer	I-Application
,	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
distinguishes	O
between	O
user	O
protocols	O
and	O
support	O
protocols	O
.	O
</s>
<s>
Support	O
protocols	O
provide	O
services	O
to	O
a	O
system	O
of	O
network	B-Architecture
infrastructure	I-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
FTP	O
is	O
a	O
user	O
protocol	O
and	O
DNS	B-Protocol
is	O
a	O
support	O
protocol	O
.	O
</s>
<s>
Although	O
the	O
applications	O
are	O
usually	O
aware	O
of	O
key	O
qualities	O
of	O
the	O
transport	B-Architecture
layer	I-Architecture
connection	O
such	O
as	O
the	O
endpoint	O
IP	B-Protocol
addresses	I-Protocol
and	O
port	B-Protocol
numbers	I-Protocol
,	O
application	B-Application
layer	I-Application
protocols	O
generally	O
treat	O
the	O
transport	B-Architecture
layer	I-Architecture
(	O
and	O
lower	O
)	O
protocols	O
as	O
black	B-Device
boxes	I-Device
which	O
provide	O
a	O
stable	O
network	O
connection	O
across	O
which	O
to	O
communicate	O
.	O
</s>
<s>
The	O
transport	B-Architecture
layer	I-Architecture
and	O
lower-level	O
layers	O
are	O
unconcerned	O
with	O
the	O
specifics	O
of	O
application	B-Application
layer	I-Application
protocols	O
.	O
</s>
<s>
Routers	B-Protocol
and	O
switches	B-Protocol
do	O
not	O
typically	O
examine	O
the	O
encapsulated	B-Protocol
traffic	O
,	O
rather	O
they	O
just	O
provide	O
a	O
conduit	O
for	O
it	O
.	O
</s>
<s>
However	O
,	O
some	O
firewall	B-Application
and	O
bandwidth	O
throttling	O
applications	O
use	O
deep	B-Application
packet	I-Application
inspection	I-Application
to	O
interpret	O
application	O
data	O
.	O
</s>
<s>
An	O
example	O
is	O
the	O
Resource	B-Protocol
Reservation	I-Protocol
Protocol	I-Protocol
(	O
RSVP	B-Protocol
)	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
evolved	O
through	O
research	O
and	O
development	O
funded	O
over	O
a	O
period	O
of	O
time	O
.	O
</s>
<s>
the	O
application	B-Application
layer	I-Application
,	O
the	O
presentation	O
layer	O
and	O
the	O
session	O
layer	O
,	O
are	O
not	O
distinguished	O
separately	O
in	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
which	O
only	O
has	O
an	O
application	B-Application
layer	I-Application
above	O
the	O
transport	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
While	O
some	O
pure	O
OSI	O
protocol	O
applications	O
,	O
such	O
as	O
X.400	B-Protocol
,	O
also	O
combined	O
them	O
,	O
there	O
is	O
no	O
requirement	O
that	O
a	O
TCP/IP	B-Protocol
protocol	I-Protocol
stack	I-Protocol
must	O
impose	O
monolithic	O
architecture	O
above	O
the	O
transport	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
the	O
NFS	O
application	B-Application
protocol	I-Application
runs	O
over	O
the	O
External	B-Protocol
Data	I-Protocol
Representation	I-Protocol
(	O
XDR	O
)	O
presentation	O
protocol	O
,	O
which	O
,	O
in	O
turn	O
,	O
runs	O
over	O
a	O
protocol	O
called	O
Remote	B-Operating_System
Procedure	I-Operating_System
Call	I-Operating_System
(	O
RPC	O
)	O
.	O
</s>
<s>
Different	O
authors	O
have	O
interpreted	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
differently	O
,	O
and	O
disagree	O
whether	O
the	O
link	B-Protocol
layer	I-Protocol
,	O
or	O
any	O
aspect	O
of	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
,	O
covers	O
OSI	B-Application
layer	I-Application
1	I-Application
(	O
physical	B-Application
layer	I-Application
)	O
issues	O
,	O
or	O
whether	O
TCP/IP	B-Protocol
assumes	O
a	O
hardware	O
layer	O
exists	O
below	O
the	O
link	B-Protocol
layer	I-Protocol
.	O
</s>
<s>
Several	O
authors	O
have	O
attempted	O
to	O
incorporate	O
the	O
OSI	O
model	O
's	O
layers	O
1	O
and	O
2	O
into	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
since	O
these	O
are	O
commonly	O
referred	O
to	O
in	O
modern	O
standards	O
(	O
for	O
example	O
,	O
by	O
IEEE	O
and	O
ITU	O
)	O
.	O
</s>
<s>
This	O
often	O
results	O
in	O
a	O
model	O
with	O
five	O
layers	O
,	O
where	O
the	O
link	B-Protocol
layer	I-Protocol
or	O
network	B-Protocol
access	I-Protocol
layer	I-Protocol
is	O
split	O
into	O
the	O
OSI	O
model	O
's	O
layers	O
1	O
and	O
2	O
.	O
</s>
<s>
The	O
IETF	O
has	O
repeatedly	O
stated	O
that	O
Internet	B-Protocol
Protocol	I-Protocol
and	O
architecture	O
development	O
is	O
not	O
intended	O
to	O
be	O
OSI-compliant	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
session	O
and	O
presentation	O
layers	O
of	O
the	O
OSI	O
suite	O
are	O
considered	O
to	O
be	O
included	O
in	O
the	O
application	B-Application
layer	I-Application
of	O
the	O
TCP/IP	B-Protocol
suite	I-Protocol
.	O
</s>
<s>
The	O
functionality	O
of	O
the	O
session	O
layer	O
can	O
be	O
found	O
in	O
protocols	O
like	O
HTTP	B-Protocol
and	O
SMTP	B-Protocol
and	O
is	O
more	O
evident	O
in	O
protocols	O
like	O
Telnet	B-Protocol
and	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
.	O
</s>
<s>
Session-layer	O
functionality	O
is	O
also	O
realized	O
with	O
the	O
port	B-Protocol
numbering	I-Protocol
of	O
the	O
TCP	O
and	O
UDP	B-Protocol
protocols	I-Protocol
,	O
which	O
are	O
included	O
in	O
the	O
transport	B-Architecture
layer	I-Architecture
of	O
the	O
TCP/IP	B-Protocol
suite	I-Protocol
.	O
</s>
<s>
Functions	O
of	O
the	O
presentation	O
layer	O
are	O
realized	O
in	O
the	O
TCP/IP	B-Protocol
applications	O
with	O
the	O
MIME	B-Protocol
standard	O
in	O
data	O
exchange	O
.	O
</s>
<s>
Another	O
difference	O
is	O
in	O
the	O
treatment	O
of	O
routing	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
The	O
OSI	O
routing	B-Protocol
protocol	I-Protocol
IS-IS	B-Protocol
belongs	O
to	O
the	O
network	O
layer	O
,	O
and	O
does	O
not	O
depend	O
on	O
CLNS	B-Protocol
for	O
delivering	O
packets	B-Protocol
from	O
one	O
router	B-Protocol
to	O
another	O
,	O
but	O
defines	O
its	O
own	O
layer-3	O
encapsulation	B-Protocol
.	O
</s>
<s>
In	O
contrast	O
,	O
OSPF	B-Protocol
,	O
RIP	B-Protocol
,	O
BGP	B-Protocol
and	O
other	O
routing	B-Protocol
protocols	I-Protocol
defined	O
by	O
the	O
IETF	O
are	O
transported	O
over	O
IP	O
,	O
and	O
,	O
for	O
the	O
purpose	O
of	O
sending	O
and	O
receiving	O
routing	B-Protocol
protocol	I-Protocol
packets	B-Protocol
,	O
routers	B-Protocol
act	O
as	O
hosts	O
.	O
</s>
<s>
As	O
a	O
consequence	O
,	O
include	O
routing	B-Protocol
protocols	I-Protocol
in	O
the	O
application	B-Application
layer	I-Application
.	O
</s>
<s>
Some	O
authors	O
,	O
such	O
as	O
Tanenbaum	O
in	O
Computer	B-Architecture
Networks	I-Architecture
,	O
describe	O
routing	B-Protocol
protocols	I-Protocol
in	O
the	O
same	O
layer	O
as	O
IP	O
,	O
reasoning	O
that	O
routing	B-Protocol
protocols	I-Protocol
inform	O
decisions	O
made	O
by	O
the	O
forwarding	O
process	O
of	O
routers	B-Protocol
.	O
</s>
<s>
IETF	O
protocols	O
can	O
be	O
encapsulated	B-Protocol
recursively	O
,	O
as	O
demonstrated	O
by	O
tunnelling	B-Protocol
protocols	I-Protocol
such	O
as	O
Generic	B-Protocol
Routing	I-Protocol
Encapsulation	I-Protocol
(	O
GRE	O
)	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
does	O
not	O
presume	O
any	O
specific	O
hardware	O
or	O
software	O
environment	O
.	O
</s>
<s>
It	O
only	O
requires	O
that	O
hardware	O
and	O
a	O
software	O
layer	O
exists	O
that	O
is	O
capable	O
of	O
sending	O
and	O
receiving	O
packets	B-Protocol
on	O
a	O
computer	B-Architecture
network	I-Architecture
.	O
</s>
<s>
A	O
minimal	O
implementation	O
of	O
TCP/IP	B-Protocol
includes	O
the	O
following	O
:	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
,	O
Address	O
Resolution	O
Protocol	O
(	O
ARP	O
)	O
,	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
ICMP	O
)	O
,	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
,	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
,	O
and	O
Internet	B-Protocol
Group	I-Protocol
Management	I-Protocol
Protocol	I-Protocol
(	O
IGMP	B-Protocol
)	O
.	O
</s>
<s>
In	O
addition	O
to	O
IP	O
,	O
ICMP	O
,	O
TCP	O
,	O
UDP	O
,	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
requires	O
Neighbor	B-Protocol
Discovery	I-Protocol
Protocol	I-Protocol
(	O
NDP	O
)	O
,	O
ICMPv6	B-Protocol
,	O
and	O
Multicast	B-Protocol
Listener	I-Protocol
Discovery	I-Protocol
(	O
MLD	O
)	O
and	O
is	O
often	O
accompanied	O
by	O
an	O
integrated	O
IPSec	B-Protocol
security	O
layer	O
.	O
</s>
