<s>
Network	B-Protocol
address	I-Protocol
translation	I-Protocol
traversal	O
is	O
a	O
computer	O
networking	O
technique	O
of	O
establishing	O
and	O
maintaining	O
Internet	B-Protocol
Protocol	I-Protocol
connections	O
across	O
gateways	B-Application
that	O
implement	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
.	O
</s>
<s>
NAT	B-Protocol
traversal	I-Protocol
techniques	O
are	O
required	O
for	O
many	O
network	O
applications	O
,	O
such	O
as	O
peer-to-peer	O
file	O
sharing	O
and	O
voice	B-Application
over	I-Application
IP	I-Application
.	O
</s>
<s>
NAT	B-Protocol
devices	I-Protocol
allow	O
the	O
use	O
of	O
private	B-Protocol
IP	I-Protocol
addresses	I-Protocol
on	O
private	B-Protocol
networks	I-Protocol
behind	O
routers	O
with	O
a	O
single	O
public	O
IP	B-Protocol
address	I-Protocol
facing	O
the	O
Internet	O
.	O
</s>
<s>
The	O
internal	O
network	O
devices	O
communicate	O
with	O
hosts	O
on	O
the	O
external	O
network	O
by	O
changing	O
the	O
source	O
address	O
of	O
outgoing	O
requests	O
to	O
that	O
of	O
the	O
NAT	B-Protocol
device	I-Protocol
and	O
relaying	O
replies	O
back	O
to	O
the	O
originating	O
device	O
.	O
</s>
<s>
This	O
leaves	O
the	O
internal	O
network	O
ill-suited	O
for	O
hosting	O
servers	B-Application
,	O
as	O
the	O
NAT	B-Protocol
device	I-Protocol
has	O
no	O
automatic	O
method	O
of	O
determining	O
the	O
internal	O
host	O
for	O
which	O
incoming	O
packets	O
are	O
destined	O
.	O
</s>
<s>
However	O
,	O
applications	O
such	O
as	O
peer-to-peer	O
file	O
sharing	O
,	O
VoIP	B-Application
services	O
,	O
and	O
video	B-Device
game	I-Device
consoles	I-Device
require	O
clients	O
to	O
be	O
servers	B-Application
as	O
well	O
.	O
</s>
<s>
Furthermore	O
,	O
many	O
of	O
these	O
types	O
of	O
services	O
carry	O
IP	B-Protocol
address	I-Protocol
and	O
port	B-Protocol
number	I-Protocol
information	O
in	O
the	O
application	O
data	O
,	O
potentially	O
requiring	O
substitution	O
with	O
deep	B-Application
packet	I-Application
inspection	I-Application
.	O
</s>
<s>
Network	B-Protocol
address	I-Protocol
translation	I-Protocol
technologies	O
are	O
not	O
standardized	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
methods	O
used	O
for	O
NAT	B-Protocol
traversal	I-Protocol
are	O
often	O
proprietary	O
and	O
poorly	O
documented	O
.	O
</s>
<s>
Many	O
traversal	O
techniques	O
require	O
assistance	O
from	O
servers	B-Application
outside	O
of	O
the	O
masqueraded	O
network	O
.	O
</s>
<s>
NAT	B-Protocol
traversal	I-Protocol
techniques	O
usually	O
bypass	O
enterprise	O
security	O
policies	O
.	O
</s>
<s>
Enterprise	O
security	O
experts	O
prefer	O
techniques	O
that	O
explicitly	O
cooperate	O
with	O
NAT	O
and	O
firewalls	O
,	O
allowing	O
NAT	B-Protocol
traversal	I-Protocol
while	O
still	O
enabling	O
marshalling	O
at	O
the	O
NAT	O
to	O
enforce	O
enterprise	O
security	O
policies	O
.	O
</s>
<s>
IETF	O
standards	O
based	O
on	O
this	O
security	O
model	O
are	O
Realm-Specific	B-Protocol
IP	I-Protocol
(	O
RSIP	B-Protocol
)	O
and	O
middlebox	O
communications	O
(	O
MIDCOM	O
)	O
.	O
</s>
<s>
The	O
following	O
NAT	B-Protocol
traversal	I-Protocol
techniques	O
are	O
available	O
:	O
</s>
<s>
Socket	B-Application
Secure	I-Application
(	O
SOCKS	B-Application
)	O
is	O
a	O
technology	O
created	O
in	O
the	O
early	O
1990s	O
that	O
uses	O
proxy	O
servers	B-Application
to	O
relay	O
traffic	O
between	O
networks	O
or	O
systems	O
.	O
</s>
<s>
Traversal	B-Protocol
Using	I-Protocol
Relays	I-Protocol
around	I-Protocol
NAT	I-Protocol
(	O
TURN	O
)	O
is	O
a	O
relay	O
protocol	O
designed	O
specifically	O
for	O
NAT	B-Protocol
traversal	I-Protocol
.	O
</s>
<s>
Session	B-Protocol
Traversal	I-Protocol
Utilities	I-Protocol
for	I-Protocol
NAT	I-Protocol
(	O
STUN	B-Protocol
)	O
is	O
a	O
standardized	O
set	O
of	O
methods	O
and	O
a	O
network	O
protocol	O
for	O
NAT	O
hole	O
punching	O
.	O
</s>
<s>
Interactive	O
Connectivity	O
Establishment	O
(	O
ICE	O
)	O
is	O
a	O
complete	O
protocol	O
for	O
using	O
STUN	B-Protocol
and/or	O
TURN	O
to	O
do	O
NAT	B-Protocol
traversal	I-Protocol
while	O
picking	O
the	O
best	O
network	O
route	O
available	O
.	O
</s>
<s>
It	O
fills	O
in	O
some	O
of	O
the	O
missing	O
pieces	O
and	O
deficiencies	O
that	O
were	O
not	O
mentioned	O
by	O
STUN	B-Protocol
specification	O
.	O
</s>
<s>
UPnP	B-Application
Internet	B-Protocol
Gateway	I-Protocol
Device	I-Protocol
Protocol	I-Protocol
(	O
IGDP	O
)	O
is	O
supported	O
by	O
many	O
small	O
NAT	O
gateways	B-Application
in	O
home	O
or	O
small	O
office	O
settings	O
.	O
</s>
<s>
It	O
allows	O
a	O
device	O
on	O
a	O
network	O
to	O
ask	O
the	O
router	O
to	O
open	O
a	O
port	B-Protocol
.	O
</s>
<s>
NAT-PMP	B-Protocol
is	O
a	O
protocol	O
introduced	O
by	O
Apple	O
as	O
an	O
alternative	O
to	O
IGDP	O
.	O
</s>
<s>
PCP	B-Protocol
is	O
a	O
successor	O
of	O
NAT-PMP	B-Protocol
.	O
</s>
<s>
Application-level	O
gateway	B-Application
(	O
ALG	O
)	O
is	O
a	O
component	O
of	O
a	O
firewall	O
or	O
NAT	O
that	O
allows	O
for	O
configuring	O
NAT	B-Protocol
traversal	I-Protocol
filters	O
.	O
</s>
<s>
The	O
recent	O
proliferation	O
of	O
symmetric	O
NATs	O
has	O
reduced	O
NAT	B-Protocol
traversal	I-Protocol
success	O
rates	O
in	O
many	O
practical	O
situations	O
,	O
such	O
as	O
for	O
mobile	O
and	O
public	O
WiFi	O
connections	O
.	O
</s>
<s>
Hole	O
punching	O
techniques	O
,	O
such	O
as	O
STUN	B-Protocol
and	O
ICE	O
,	O
fail	O
in	O
traversing	O
symmetric	O
NATs	O
without	O
the	O
help	O
of	O
a	O
relay	O
server	O
,	O
as	O
is	O
practiced	O
in	O
TURN	O
.	O
</s>
<s>
Techniques	O
that	O
traverse	O
symmetric	O
NATs	O
by	O
attempting	O
to	O
predict	O
the	O
next	O
port	B-Protocol
to	O
be	O
opened	O
by	O
each	O
NAT	B-Protocol
device	I-Protocol
were	O
discovered	O
in	O
2003	O
by	O
Yutaka	O
Takeda	O
at	O
Panasonic	O
Communications	O
Research	O
Laboratory	O
and	O
in	O
2008	O
by	O
researchers	O
at	O
Waseda	O
University	O
.	O
</s>
<s>
Port	B-Protocol
prediction	O
techniques	O
are	O
only	O
effective	O
with	O
NAT	B-Protocol
devices	I-Protocol
that	O
use	O
known	O
deterministic	O
algorithms	O
for	O
port	B-Protocol
selection	O
.	O
</s>
<s>
This	O
predictable	O
yet	O
non-static	O
port	B-Protocol
allocation	O
scheme	O
is	O
uncommon	O
in	O
large	O
scale	O
NATs	O
such	O
as	O
those	O
used	O
in	O
4G	O
LTE	O
networks	O
and	O
therefore	O
port	B-Protocol
prediction	O
is	O
largely	O
ineffective	O
on	O
those	O
mobile	O
broadband	O
networks	O
.	O
</s>
<s>
IPsec	B-Protocol
virtual	B-Application
private	I-Application
network	I-Application
clients	O
use	O
NAT	B-Protocol
traversal	I-Protocol
in	O
order	O
to	O
have	O
Encapsulating	O
Security	O
Payload	O
packets	O
traverse	O
NAT	O
.	O
</s>
<s>
IPsec	B-Protocol
uses	O
several	O
protocols	O
in	O
its	O
operation	O
which	O
must	O
be	O
enabled	O
to	O
traverse	O
firewalls	O
and	O
network	B-Protocol
address	I-Protocol
translators	I-Protocol
:	O
</s>
<s>
Many	O
routers	O
provide	O
explicit	O
features	O
,	O
often	O
called	O
IPsec	B-Protocol
Passthrough	O
.	O
</s>
<s>
In	O
Windows	O
XP	O
,	O
NAT	B-Protocol
traversal	I-Protocol
is	O
enabled	O
by	O
default	O
,	O
but	O
in	O
Windows	O
XP	O
with	O
Service	O
Pack	O
2	O
it	O
has	O
been	O
disabled	O
by	O
default	O
for	O
the	O
case	O
when	O
the	O
VPN	B-Application
server	I-Application
is	O
also	O
behind	O
a	O
NAT	B-Protocol
device	I-Protocol
,	O
because	O
of	O
a	O
rare	O
and	O
controversial	O
security	O
issue	O
.	O
</s>
<s>
IPsec	B-Protocol
NAT-T	B-Protocol
patches	O
are	O
also	O
available	O
for	O
Windows	O
2000	O
,	O
Windows	O
NT	O
and	O
Windows	O
98	O
.	O
</s>
<s>
NAT	B-Protocol
traversal	I-Protocol
and	O
IPsec	B-Protocol
may	O
be	O
used	O
to	O
enable	O
opportunistic	B-Protocol
encryption	I-Protocol
of	O
traffic	O
between	O
systems	O
.	O
</s>
<s>
NAT	B-Protocol
traversal	I-Protocol
allows	O
systems	O
behind	O
NATs	O
to	O
request	O
and	O
establish	O
secure	O
connections	O
on	O
demand	O
.	O
</s>
<s>
Hosted	O
NAT	B-Protocol
traversal	I-Protocol
(	O
HNT	O
)	O
is	O
a	O
set	O
of	O
mechanisms	O
,	O
including	O
media	O
relaying	O
and	O
latching	O
,	O
that	O
is	O
widely	O
used	O
by	O
communications	O
providers	O
for	O
historical	O
and	O
practical	O
reasons	O
.	O
</s>
