<s>
IP	B-Protocol
in	I-Protocol
IP	I-Protocol
is	O
an	O
IP	B-Protocol
tunneling	I-Protocol
protocol	O
that	O
encapsulates	O
one	O
IP	B-Protocol
packet	O
in	O
another	O
IP	B-Protocol
packet	O
.	O
</s>
<s>
To	O
encapsulate	O
an	O
IP	B-Protocol
packet	O
in	O
another	O
IP	B-Protocol
packet	O
,	O
an	O
outer	O
header	O
is	O
added	O
with	O
Source	O
IP	B-Protocol
,	O
the	O
entry	O
point	O
of	O
the	O
tunnel	O
,	O
and	O
Destination	O
IP	B-Protocol
,	O
the	O
exit	O
point	O
of	O
the	O
tunnel	O
.	O
</s>
<s>
If	O
the	O
packet	O
size	O
,	O
including	O
the	O
outer	O
header	O
,	O
is	O
greater	O
than	O
the	O
Path	O
MTU	B-Protocol
,	O
the	O
encapsulator	O
fragments	O
the	O
packet	O
.	O
</s>
<s>
Outer	O
IP	B-Protocol
header	O
has	O
the	O
following	O
fields	O
:	O
</s>
<s>
For	O
the	O
Outer	O
IP	B-Protocol
Header	O
,	O
a	O
new	O
number	O
is	O
generated	O
.	O
</s>
<s>
The	O
value	O
is	O
set	O
to	O
4	O
for	O
IP	B-Protocol
in	I-Protocol
IP	I-Protocol
.	O
</s>
<s>
This	O
field	O
is	O
the	O
IP	B-Protocol
checksum	O
of	O
outer	O
header	O
.	O
</s>
<s>
This	O
field	O
in	O
general	O
is	O
NOT	O
copied	O
from	O
the	O
inner	O
IP	B-Protocol
header	O
.	O
</s>
<s>
This	O
field	O
is	O
used	O
to	O
fill	O
the	O
datagram	O
so	O
that	O
IP	B-Protocol
Payload	O
starts	O
on	O
a	O
32	O
bit	O
boundary	O
.	O
</s>
