<s>
The	O
internet	B-Protocol
layer	I-Protocol
is	O
a	O
group	O
of	O
internetworking	B-Architecture
methods	O
,	O
protocols	O
,	O
and	O
specifications	O
in	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
that	O
are	O
used	O
to	O
transport	O
network	B-Protocol
packets	I-Protocol
from	O
the	O
originating	O
host	B-Device
across	O
network	B-Protocol
boundaries	I-Protocol
;	O
if	O
necessary	O
,	O
to	O
the	O
destination	O
host	B-Device
specified	O
by	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
internet	B-Protocol
layer	I-Protocol
derives	O
its	O
name	O
from	O
its	O
function	O
facilitating	O
internetworking	B-Architecture
,	O
which	O
is	O
the	O
concept	O
of	O
connecting	O
multiple	O
networks	O
with	O
each	O
other	O
through	O
gateways	B-Application
.	O
</s>
<s>
The	O
internet	B-Protocol
layer	I-Protocol
does	O
not	O
include	O
the	O
protocols	O
that	O
fulfill	O
the	O
purpose	O
of	O
maintaining	O
link	O
states	O
between	O
the	O
local	O
nodes	O
and	O
that	O
usually	O
use	O
protocols	O
that	O
are	O
based	O
on	O
the	O
framing	O
of	O
packets	B-Protocol
specific	O
to	O
the	O
link	O
types	O
.	O
</s>
<s>
Such	O
protocols	O
belong	O
to	O
the	O
link	B-Protocol
layer	I-Protocol
.	O
</s>
<s>
Internet-layer	B-Protocol
protocols	O
use	O
IP-based	O
packets	B-Protocol
.	O
</s>
<s>
A	O
common	O
design	O
aspect	O
in	O
the	O
internet	B-Protocol
layer	I-Protocol
is	O
the	O
robustness	O
principle	O
:	O
"	O
Be	O
liberal	O
in	O
what	O
you	O
accept	O
,	O
and	O
conservative	O
in	O
what	O
you	O
send	O
"	O
as	O
a	O
misbehaving	O
host	B-Device
can	O
deny	O
Internet	O
service	O
to	O
many	O
other	O
users	O
.	O
</s>
<s>
The	O
internet	B-Protocol
layer	I-Protocol
has	O
three	O
basic	O
functions	O
:	O
</s>
<s>
For	O
outgoing	O
packets	B-Protocol
,	O
select	O
the	O
next-hop	O
host	B-Device
(	O
gateway	B-Application
)	O
and	O
transmit	O
the	O
packet	B-Protocol
to	O
this	O
host	B-Device
by	O
passing	O
it	O
to	O
the	O
appropriate	O
link	B-Protocol
layer	I-Protocol
implementation	O
;	O
</s>
<s>
For	O
incoming	O
packets	B-Protocol
,	O
capture	O
packets	B-Protocol
and	O
pass	O
the	O
packet	B-Protocol
payload	O
up	O
to	O
the	O
appropriate	O
transport	B-Architecture
layer	I-Architecture
protocol	O
,	O
if	O
appropriate	O
.	O
</s>
<s>
In	O
Version	O
4	O
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IPv4	B-Protocol
)	O
,	O
during	O
both	O
transmit	O
and	O
receive	O
operations	O
,	O
IP	O
is	O
capable	O
of	O
automatic	O
or	O
intentional	O
fragmentation	B-Protocol
or	I-Protocol
defragmentation	I-Protocol
of	O
packets	B-Protocol
,	O
based	O
,	O
for	O
example	O
,	O
on	O
the	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
of	O
link	O
elements	O
.	O
</s>
<s>
However	O
,	O
this	O
feature	O
has	O
been	O
dropped	O
in	O
IPv6	B-Protocol
,	O
as	O
the	O
communications	O
end	O
points	O
,	O
the	O
hosts	O
,	O
now	O
have	O
to	O
perform	O
path	B-Protocol
MTU	I-Protocol
discovery	I-Protocol
and	O
assure	O
that	O
end-to-end	O
transmissions	O
do	O
n't	O
exceed	O
the	O
maximum	O
discovered	O
.	O
</s>
<s>
In	O
its	O
operation	O
,	O
the	O
internet	B-Protocol
layer	I-Protocol
is	O
not	O
responsible	O
for	O
reliable	B-Protocol
transmission	I-Protocol
.	O
</s>
<s>
This	O
means	O
that	O
the	O
network	O
makes	O
no	O
guarantees	O
about	O
the	O
proper	O
arrival	O
of	O
packets	B-Protocol
.	O
</s>
<s>
This	O
in	O
accordance	O
with	O
the	O
end-to-end	B-Application
principle	I-Application
and	O
a	O
change	O
from	O
the	O
previous	O
protocols	O
used	O
on	O
the	O
early	O
ARPANET	O
.	O
</s>
<s>
Since	O
packet	B-Protocol
delivery	O
across	O
diverse	O
networks	O
is	O
an	O
inherently	O
unreliable	O
and	O
failure-prone	O
operation	O
,	O
the	O
burden	O
of	O
providing	O
reliability	O
was	O
placed	O
with	O
the	O
end	O
points	O
of	O
a	O
communication	O
path	O
,	O
i.e.	O
,	O
the	O
hosts	O
,	O
rather	O
than	O
on	O
the	O
network	O
.	O
</s>
<s>
This	O
is	O
one	O
of	O
the	O
reasons	O
of	O
the	O
resiliency	O
of	O
the	O
Internet	O
against	O
individual	O
link	O
failures	O
and	O
its	O
proven	O
scalability	B-Architecture
.	O
</s>
<s>
The	O
function	O
of	O
providing	O
reliability	O
of	O
service	O
is	O
the	O
duty	O
of	O
higher	O
level	O
protocols	O
,	O
such	O
as	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
in	O
the	O
transport	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
In	O
IPv4	B-Protocol
,	O
a	O
checksum	B-Algorithm
is	O
used	O
to	O
protect	O
the	O
header	O
of	O
each	O
datagram	O
.	O
</s>
<s>
The	O
checksum	B-Algorithm
ensures	O
that	O
the	O
information	O
in	O
a	O
received	O
header	O
is	O
accurate	O
,	O
however	O
,	O
IPv4	B-Protocol
does	O
not	O
attempt	O
to	O
detect	O
errors	O
that	O
may	O
have	O
occurred	O
to	O
the	O
data	O
in	O
each	O
packet	B-Protocol
.	O
</s>
<s>
IPv6	B-Protocol
does	O
not	O
include	O
this	O
header	B-Algorithm
checksum	I-Algorithm
,	O
instead	O
relying	O
on	O
the	O
link	B-Protocol
layer	I-Protocol
to	O
assure	O
data	O
integrity	O
for	O
the	O
entire	O
packet	B-Protocol
including	O
the	O
checksum	B-Algorithm
.	O
</s>
<s>
The	O
primary	O
protocols	O
in	O
the	O
internet	B-Protocol
layer	I-Protocol
are	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
.	O
</s>
<s>
It	O
is	O
implemented	O
in	O
two	O
versions	O
,	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
.	O
</s>
<s>
The	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
ICMP	O
)	O
is	O
primarily	O
used	O
for	O
error	O
and	O
diagnostic	O
functions	O
.	O
</s>
<s>
Different	O
implementations	O
exist	O
for	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
.	O
</s>
<s>
The	O
Internet	B-Protocol
Group	I-Protocol
Management	I-Protocol
Protocol	I-Protocol
(	O
IGMP	B-Protocol
)	O
is	O
used	O
by	O
IPv4	B-Protocol
hosts	O
and	O
adjacent	O
IP	B-Protocol
multicast	I-Protocol
routers	O
to	O
establish	O
multicast	O
group	O
memberships	O
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
Security	I-Protocol
(	O
IPsec	B-Protocol
)	O
is	O
a	O
suite	O
of	O
protocols	O
for	O
securing	O
IP	B-Protocol
communications	I-Protocol
by	O
authenticating	O
and	O
encrypting	O
each	O
IP	O
packet	B-Protocol
in	O
a	O
data	O
stream	O
.	O
</s>
<s>
IPsec	B-Protocol
also	O
includes	O
protocols	O
for	O
key	O
exchange	O
.	O
</s>
<s>
IPsec	B-Protocol
was	O
originally	O
designed	O
as	O
a	O
base	O
specification	O
in	O
IPv6	B-Protocol
in	O
1995	O
,	O
and	O
later	O
adapted	O
to	O
IPv4	B-Protocol
,	O
with	O
which	O
it	O
has	O
found	O
widespread	O
use	O
in	O
securing	O
virtual	B-Application
private	I-Application
networks	I-Application
.	O
</s>
<s>
Because	O
the	O
internet	B-Protocol
layer	I-Protocol
of	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
is	O
easily	O
compared	O
directly	O
with	O
the	O
network	B-Application
layer	I-Application
(	O
layer	B-Application
3	I-Application
)	O
in	O
the	O
Open	O
Systems	O
Interconnection	O
(	O
OSI	O
)	O
protocol	O
stack	O
,	O
the	O
internet	B-Protocol
layer	I-Protocol
is	O
often	O
improperly	O
called	O
network	B-Application
layer	I-Application
.	O
</s>
