<s>
6in4	B-Protocol
is	O
an	O
IPv6	B-Protocol
transition	I-Protocol
mechanism	I-Protocol
for	O
migrating	O
from	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
to	O
IPv6	B-Protocol
.	O
</s>
<s>
It	O
is	O
a	O
tunneling	B-Protocol
protocol	I-Protocol
that	O
encapsulates	O
IPv6	B-Protocol
packets	O
on	O
specially	O
configured	O
IPv4	B-Protocol
links	O
according	O
to	O
the	O
specifications	O
of	O
.	O
</s>
<s>
The	O
IP	B-Protocol
protocol	I-Protocol
number	I-Protocol
for	O
6in4	B-Protocol
is	O
41	O
,	O
per	O
IANA	O
reservation	O
.	O
</s>
<s>
The	O
6in4	B-Protocol
packet	O
format	O
consists	O
of	O
the	O
IPv6	B-Protocol
packet	O
preceded	O
by	O
an	O
IPv4	B-Protocol
packet	O
header	O
.	O
</s>
<s>
Thus	O
,	O
the	O
encapsulation	O
overhead	O
is	O
the	O
size	O
of	O
the	O
IPv4	B-Protocol
header	O
of	O
20	O
bytes	O
.	O
</s>
<s>
On	O
Ethernet	O
with	O
a	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
of	O
1500	O
bytes	O
,	O
IPv6	B-Protocol
packets	O
of	O
1480	O
bytes	O
may	O
therefore	O
be	O
transmitted	O
without	O
fragmentation	O
.	O
</s>
<s>
6in4	B-Protocol
tunneling	O
is	O
also	O
referred	O
to	O
as	O
proto-41	O
static	O
because	O
the	O
endpoints	O
are	O
configured	O
statically	O
.	O
</s>
<s>
Although	O
6in4	B-Protocol
tunnels	O
are	O
generally	O
manually	O
configured	O
,	O
the	O
utility	O
AICCU	B-Language
can	O
configure	O
tunnel	O
parameters	O
automatically	O
after	O
retrieving	O
information	O
from	O
a	O
Tunnel	O
Information	O
and	O
Control	O
Protocol	O
(	O
TIC	O
)	O
server	O
.	O
</s>
<s>
The	O
similarly	O
named	O
methods	O
6to4	B-Protocol
or	O
6over4	B-Protocol
describe	O
a	O
different	O
mechanism	O
.	O
</s>
<s>
The	O
6to4	B-Protocol
method	O
also	O
makes	O
use	O
of	O
proto-41	O
,	O
but	O
the	O
endpoint	O
IPv4	B-Protocol
address	O
information	O
is	O
derived	O
from	O
the	O
IPv6	B-Protocol
addresses	O
within	O
the	O
IPv6	B-Protocol
packet	O
header	O
,	O
instead	O
of	O
from	O
static	O
configuration	O
of	O
the	O
endpoints	O
.	O
</s>
<s>
When	O
an	O
endpoint	O
of	O
a	O
6in4	B-Protocol
tunnel	O
is	O
inside	O
a	O
network	O
that	O
uses	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
to	O
external	O
networks	O
,	O
the	O
DMZ	B-General_Concept
feature	O
of	O
a	O
NAT	O
router	O
may	O
be	O
used	O
to	O
enable	O
the	O
service	O
.	O
</s>
<s>
Some	O
NAT	B-Protocol
devices	I-Protocol
automatically	O
permit	O
transparent	O
operation	O
of	O
6in4	B-Protocol
.	O
</s>
<s>
Even	O
though	O
6in4	B-Protocol
tunnels	O
are	O
static	O
in	O
nature	O
,	O
with	O
the	O
help	O
of	O
for	O
example	O
the	O
heartbeat	O
protocol	O
one	O
can	O
still	O
have	O
dynamic	O
tunnel	O
endpoints	O
.	O
</s>
<s>
A	O
tool	O
such	O
as	O
AICCU	B-Language
can	O
then	O
update	O
the	O
endpoints	O
,	O
in	O
effect	O
making	O
the	O
endpoint	O
dynamic	O
while	O
still	O
using	O
the	O
6in4	B-Protocol
protocol	O
.	O
</s>
<s>
The	O
6in4	B-Protocol
protocol	O
has	O
no	O
security	O
features	O
,	O
thus	O
one	O
can	O
inject	O
IPv6	B-Protocol
packets	O
by	O
spoofing	O
the	O
source	O
IPv4	B-Protocol
address	O
of	O
a	O
tunnel	O
endpoint	O
and	O
sending	O
it	O
to	O
the	O
other	O
endpoint	O
.	O
</s>
<s>
This	O
problem	O
can	O
partially	O
be	O
solved	O
by	O
implementing	O
network	O
ingress	O
filtering	O
(	O
not	O
near	O
the	O
exit	O
point	O
but	O
close	O
to	O
the	O
true	O
source	O
)	O
or	O
with	O
IPsec	B-Protocol
.	O
</s>
<s>
The	O
mentioned	O
packet	O
injection	O
loophole	O
of	O
6in4	B-Protocol
was	O
exploited	O
for	O
a	O
research	O
benefit	O
in	O
a	O
method	O
called	O
IPv6	B-Protocol
Tunnel	O
Discovery	O
which	O
allowed	O
the	O
researchers	O
to	O
discover	O
operating	O
IPv6	B-Protocol
tunnels	O
around	O
the	O
world	O
.	O
</s>
