<s>
network	B-Protocol
address	I-Protocol
translators	I-Protocol
(	O
NAT	B-Protocol
)	O
are	O
used	O
to	O
overcome	O
the	O
lack	O
of	O
IPv4	B-Protocol
address	O
availability	O
by	O
hiding	O
an	O
enterprise	O
or	O
even	O
an	O
operator	O
's	O
network	O
behind	O
one	O
or	O
few	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
The	O
devices	O
behind	O
the	O
NAT	B-Protocol
use	O
private	B-Protocol
IP	I-Protocol
addresses	I-Protocol
that	O
are	O
not	O
routable	O
in	O
the	O
public	O
Internet	O
.	O
</s>
<s>
The	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	B-Application
)	O
has	O
established	O
itself	O
as	O
the	O
de	O
facto	O
standard	O
for	O
voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
communication	O
.	O
</s>
<s>
In	O
order	O
to	O
establish	O
a	O
call	O
,	O
a	O
caller	O
sends	O
a	O
SIP	B-Application
message	O
,	O
which	O
contains	O
its	O
own	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
callee	O
is	O
supposed	O
to	O
reply	O
back	O
with	O
a	O
SIP	B-Application
message	O
destined	O
to	O
the	O
IP	B-Protocol
addresses	I-Protocol
included	O
in	O
the	O
received	O
SIP	B-Application
message	O
.	O
</s>
<s>
This	O
will	O
obviously	O
not	O
work	O
if	O
the	O
caller	O
is	O
behind	O
a	O
NAT	B-Protocol
and	O
is	O
using	O
a	O
private	B-Protocol
IP	I-Protocol
address	I-Protocol
.	O
</s>
<s>
Probably	O
the	O
single	O
biggest	O
mistake	O
in	O
SIP	B-Application
design	O
was	O
ignoring	O
the	O
existence	O
of	O
NATs	O
.	O
</s>
<s>
This	O
error	O
came	O
from	O
a	O
belief	O
in	O
IETF	O
leadership	O
that	O
IP	B-Protocol
address	I-Protocol
space	O
would	O
be	O
exhausted	O
more	O
rapidly	O
and	O
would	O
necessitate	O
global	O
upgrade	O
to	O
IPv6	B-Protocol
and	O
eliminate	O
the	O
need	O
for	O
NATs	O
.	O
</s>
<s>
The	O
SIP	B-Application
standard	O
has	O
assumed	O
that	O
NATs	O
do	O
not	O
exist	O
,	O
an	O
assumption	O
,	O
which	O
turned	O
out	O
to	O
be	O
a	O
failure	O
.	O
</s>
<s>
SIP	B-Application
simply	O
did	O
n't	O
work	O
for	O
the	O
majority	O
of	O
Internet	O
users	O
who	O
are	O
behind	O
NATs	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
it	O
became	O
apparent	O
that	O
the	O
standardization	O
life-cycle	O
is	O
slower	O
than	O
how	O
the	O
market	O
ticks	O
:	O
Session	B-Protocol
Border	I-Protocol
Controllers	I-Protocol
(	O
SBC	B-Protocol
)	O
were	O
born	O
,	O
and	O
began	O
to	O
fix	O
what	O
the	O
standards	O
failed	O
to	O
do	O
:	O
NAT	B-Protocol
traversal	I-Protocol
.	O
</s>
<s>
In	O
case	O
a	O
user	O
agent	O
is	O
located	O
behind	O
a	O
NAT	B-Protocol
then	O
it	O
will	O
use	O
a	O
private	B-Protocol
IP	I-Protocol
address	I-Protocol
as	O
its	O
contact	O
address	O
in	O
the	O
contact	O
and	O
via	O
headers	O
as	O
well	O
as	O
the	O
SDP	B-Language
part	O
.	O
</s>
<s>
There	O
are	O
different	O
NAT	B-Protocol
traversal	I-Protocol
solutions	O
such	O
as	O
STUN	B-Protocol
,	O
TURN	O
and	O
ICE	O
.	O
</s>
<s>
Which	O
solution	O
to	O
use	O
depends	O
on	O
the	O
behavior	O
of	O
the	O
NAT	B-Protocol
and	O
the	O
call	O
scenario	O
.	O
</s>
<s>
When	O
using	O
an	O
SBC	B-Protocol
to	O
solve	O
the	O
NAT	B-Protocol
traversal	I-Protocol
issues	O
the	O
most	O
common	O
approach	O
for	O
SBC	B-Protocol
is	O
to	O
act	O
as	O
the	O
public	O
interface	O
of	O
the	O
user	O
agents	O
.	O
</s>
<s>
This	O
is	O
achieved	O
by	O
replacing	O
the	O
user	O
agent	O
's	O
contact	O
information	O
with	O
those	O
of	O
the	O
SBC	B-Protocol
.	O
</s>
<s>
In	O
order	O
for	O
a	O
user	O
agent	O
to	O
be	O
reachable	O
through	O
the	O
public	O
interfaces	O
of	O
an	O
SBC	B-Protocol
,	O
the	O
SBC	B-Protocol
will	O
manipulate	O
the	O
registration	O
information	O
of	O
the	O
user	O
agent	O
.	O
</s>
<s>
The	O
user	O
includes	O
its	O
private	B-Protocol
IP	I-Protocol
address	I-Protocol
as	O
its	O
contact	O
information	O
in	O
the	O
REGISTER	B-Application
requests	O
.	O
</s>
<s>
The	O
SBC	B-Protocol
replaces	O
the	O
information	O
in	O
the	O
contact	O
header	O
with	O
its	O
own	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Calls	O
destined	O
to	O
the	O
user	O
will	O
then	O
be	O
directed	O
to	O
the	O
SBC	B-Protocol
.	O
</s>
<s>
In	O
order	O
for	O
the	O
SBC	B-Protocol
to	O
know	O
which	O
user	O
agent	O
is	O
actually	O
being	O
contacted	O
the	O
SBC	B-Protocol
can	O
keep	O
a	O
local	O
copy	O
of	O
the	O
user	O
agent	O
's	O
registration	O
.	O
</s>
<s>
The	O
local	O
copy	O
includes	O
the	O
private	B-Protocol
IP	I-Protocol
address	I-Protocol
and	O
the	O
user	O
's	O
SIP	B-Protocol
URI	I-Protocol
as	O
well	O
as	O
the	O
public	O
IP	B-Protocol
address	I-Protocol
included	O
in	O
the	O
IP	O
header	O
that	O
was	O
assigned	O
to	O
the	O
SIP	B-Application
message	O
by	O
the	O
NAT	B-Protocol
.	O
</s>
<s>
Alternatively	O
the	O
SBC	B-Protocol
can	O
store	O
this	O
information	O
in	O
the	O
forwarded	O
SIP	B-Application
messages	O
.	O
</s>
<s>
The	O
contact	O
information	O
include	O
the	O
user	O
's	O
private	B-Protocol
IP	I-Protocol
address	I-Protocol
and	O
SIP	B-Protocol
URI	I-Protocol
as	O
well	O
as	O
the	O
public	O
IP	B-Protocol
address	I-Protocol
in	O
the	O
IP	O
header	O
of	O
the	O
SIP	B-Application
message	O
.	O
</s>
<s>
When	O
the	O
registrar	O
receives	O
a	O
request	O
for	O
the	O
user	O
,	O
the	O
registrar	O
will	O
return	O
the	O
complete	O
contact	O
information	O
to	O
the	O
proxy	O
,	O
which	O
will	O
include	O
this	O
information	O
in	O
the	O
SIP	B-Application
message	O
.	O
</s>
<s>
The	O
SBC	B-Protocol
can	O
then	O
retrieve	O
this	O
information	O
from	O
the	O
SIP	B-Application
request	O
and	O
use	O
it	O
to	O
properly	O
route	O
the	O
request	O
to	O
the	O
user	O
.	O
</s>
<s>
As	O
the	O
SBC	B-Protocol
does	O
not	O
have	O
to	O
keep	O
local	O
registration	O
information	O
this	O
solution	O
is	O
simple	O
to	O
implement	O
and	O
does	O
not	O
require	O
memory	O
for	O
keeping	O
the	O
information	O
.	O
</s>
<s>
Further	O
,	O
requests	O
destined	O
to	O
the	O
user	O
agent	O
do	O
not	O
necessarily	O
have	O
to	O
traverse	O
the	O
SBC	B-Protocol
that	O
has	O
processed	O
the	O
user	O
agent	O
's	O
registration	O
messages	O
.	O
</s>
<s>
Any	O
SBC	B-Protocol
that	O
can	O
reach	O
the	O
user	O
agent	O
can	O
correctly	O
route	O
messages	O
destined	O
to	O
the	O
user	O
agent	O
based	O
on	O
the	O
information	O
included	O
in	O
the	O
SIP	B-Application
request	O
.	O
</s>
<s>
In	O
case	O
the	O
NAT	B-Protocol
used	O
in	O
front	O
of	O
the	O
user	O
agent	O
accepts	O
traffic	O
only	O
from	O
the	O
IP	B-Protocol
addresses	I-Protocol
which	O
the	O
user	O
agent	O
has	O
contacted	O
previously	O
then	O
only	O
the	O
SBC	B-Protocol
that	O
has	O
processed	O
the	O
user	O
agent	O
's	O
REGISTER	B-Application
requests	O
will	O
be	O
able	O
to	O
contact	O
the	O
user	O
agent	O
.	O
</s>
<s>
The	O
other	O
option	O
is	O
to	O
keep	O
a	O
local	O
copy	O
of	O
the	O
registration	O
information	O
which	O
can	O
,	O
however	O
,	O
increase	O
the	O
processing	O
requirements	O
on	O
the	O
SBC	B-Protocol
.	O
</s>
<s>
The	O
SBC	B-Protocol
will	O
have	O
to	O
manage	O
a	O
local	O
registration	O
database	O
.	O
</s>
<s>
Beside	O
the	O
memory	O
requirements	O
the	O
SBC	B-Protocol
will	O
have	O
to	O
replicate	O
this	O
information	O
to	O
a	O
backup	O
system	O
if	O
it	O
is	O
to	O
be	O
highly	O
available	O
.	O
</s>
<s>
This	O
will	O
further	O
increase	O
the	O
processing	O
requirements	O
on	O
the	O
SBC	B-Protocol
and	O
increase	O
the	O
bandwidth	O
consumption	O
.	O
</s>
<s>
When	O
receiving	O
a	O
message	O
from	O
a	O
user	O
agent	O
a	O
network	B-Protocol
address	I-Protocol
translator	I-Protocol
binds	O
the	O
private	B-Protocol
IP	I-Protocol
address	I-Protocol
of	O
the	O
user	O
agent	O
to	O
a	O
public	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
In	O
case	O
the	O
user	O
agent	O
does	O
not	O
send	O
or	O
receive	O
any	O
messages	O
for	O
a	O
period	O
of	O
time	O
longer	O
than	O
the	O
binding	O
period	O
then	O
the	O
NAT	B-Protocol
will	O
delete	O
the	O
binding	O
and	O
the	O
user	O
agent	O
will	O
no	O
longer	O
be	O
reachable	O
from	O
the	O
outside	O
.	O
</s>
<s>
This	O
is	O
achieved	O
by	O
sending	O
REGISTER	B-Application
requests	O
at	O
time	O
intervals	O
shorter	O
than	O
the	O
binding	O
period	O
.	O
</s>
<s>
As	O
REGISTER	B-Application
messages	O
have	O
to	O
be	O
usually	O
authenticated	O
,	O
having	O
to	O
deal	O
with	O
REGISTER	B-Application
messages	O
sent	O
at	O
a	O
high	O
frequency	O
would	O
impose	O
a	O
high	O
performance	O
hit	O
on	O
the	O
operator	O
's	O
infrastructure	O
.	O
</s>
<s>
When	O
a	O
user	O
agent	O
sends	O
the	O
first	O
REGISTER	B-Application
request	O
,	O
the	O
SBC	B-Protocol
forwards	O
the	O
REGISTER	B-Application
request	O
to	O
the	O
operator	O
's	O
registration	O
servers	O
.	O
</s>
<s>
Once	O
the	O
registration	O
was	O
successfully	O
authenticated	O
and	O
accepted	O
by	O
the	O
operator	O
,	O
the	O
SBC	B-Protocol
will	O
keep	O
a	O
local	O
copy	O
of	O
the	O
registration	O
information	O
.	O
</s>
<s>
Instead	O
of	O
forwarding	O
each	O
incoming	O
REGIETER	O
request	O
to	O
the	O
operator	O
's	O
registration	O
servers	O
,	O
the	O
SBC	B-Protocol
will	O
only	O
send	O
REGISTER	B-Application
requests	O
to	O
the	O
registration	O
servers	O
at	O
rather	O
large	O
time	O
intervals	O
(	O
in	O
the	O
range	O
of	O
hours	O
)	O
.	O
</s>
<s>
Registration	O
requests	O
arriving	O
from	O
the	O
user	O
agent	O
that	O
do	O
not	O
change	O
the	O
content	O
registration	O
information	O
will	O
be	O
replied	O
to	O
by	O
the	O
SBC	B-Protocol
itself	O
.	O
</s>
<s>
The	O
SBC	B-Protocol
will	O
also	O
inform	O
the	O
registration	O
server	O
once	O
the	O
local	O
registration	O
expires	O
or	O
changes	O
.	O
</s>
<s>
Similar	O
to	O
the	O
registration	O
case	O
,	O
the	O
SBC	B-Protocol
will	O
also	O
include	O
itself	O
in	O
the	O
path	O
of	O
INVITE	B-Application
and	O
other	O
request	O
messages	O
.	O
</s>
<s>
When	O
receiving	O
an	O
INVITE	B-Application
from	O
a	O
user	O
agent	O
behind	O
a	O
NAT	B-Protocol
,	O
the	O
SBC	B-Protocol
will	O
include	O
a	O
via	O
header	O
with	O
its	O
own	O
address	O
,	O
replace	O
the	O
information	O
in	O
the	O
contact	O
header	O
with	O
its	O
own	O
address	O
and	O
also	O
replace	O
the	O
address	O
information	O
in	O
the	O
SDP	B-Language
body	O
with	O
its	O
own	O
address	O
.	O
</s>
<s>
Thereby	O
,	O
all	O
SIP	B-Application
messages	O
and	O
media	O
packets	O
will	O
traverse	O
the	O
SBC	B-Protocol
.	O
</s>
<s>
While	O
NAT	B-Protocol
traversal	I-Protocol
of	O
SIP	B-Application
messages	O
may	O
appear	O
complicated	O
after	O
all	O
,	O
the	O
yet	O
more	O
complex	O
task	O
is	O
enabling	O
media	O
to	O
traverse	O
NATs	O
.	O
</s>
<s>
If	O
SIP	B-Application
devices	O
behind	O
NATs	O
advertise	O
their	O
IP	B-Protocol
addresses	I-Protocol
,	O
their	O
peers	O
on	O
the	O
other	O
side	O
of	O
NATs	O
cannot	O
route	O
traffic	O
to	O
them	O
.	O
</s>
<s>
The	O
solution	O
SBCs	O
came	O
with	O
simply	O
ignores	O
the	O
way	O
SIP	B-Application
works	O
.	O
</s>
<s>
Instead	O
of	O
sending	O
media	O
to	O
the	O
IP	B-Protocol
address	I-Protocol
and	O
port	O
number	O
advertised	O
in	O
the	O
SIP	B-Application
SDP	B-Language
bodies	O
,	O
SBCs	O
send	O
media	O
for	O
a	O
user	O
agent	O
symmetrically	O
back	O
to	O
where	O
the	O
agent	O
has	O
sent	O
its	O
own	O
media	O
from	O
.	O
</s>
<s>
This	O
symmetric	O
communication	O
typically	O
works	O
because	O
it	O
is	O
the	O
traffic	O
pattern	O
NAT	B-Protocol
manufactures	O
have	O
been	O
used	O
to	O
before	O
the	O
arrival	O
of	O
VoIP	B-Application
.	O
</s>
<s>
The	O
other	O
noticeable	O
disadvantage	O
is	O
"	O
triangular	O
routing	O
"	O
:	O
an	O
SBC	B-Protocol
must	O
relay	O
all	O
VoIP	B-Application
traffic	O
for	O
a	O
call	O
,	O
to	O
make	O
the	O
paths	O
caller-SBC	O
and	O
SBC-callee	O
symmetric	O
.	O
</s>
<s>
That	O
is	O
in	O
fact	O
quite	O
an	O
overhead	O
for	O
a	O
VoIP	B-Application
operator	O
.	O
</s>
<s>
With	O
the	O
most	O
common	O
codec	O
,	O
G.711	B-Algorithm
,	O
a	O
relayed	O
call	O
consumes	O
four	O
87.2	O
kbit/s	O
streams	O
:	O
two	O
outbound	O
,	O
two	O
inbound	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
SIP	B-Application
device	O
uses	O
voice	B-General_Concept
activity	I-General_Concept
detection	I-General_Concept
(	O
VAD	O
)	O
and	O
fails	O
to	O
send	O
any	O
voice	O
packets	O
initially	O
,	O
the	O
SBC	B-Protocol
will	O
not	O
learn	O
its	O
address	O
and	O
will	O
not	O
forward	O
incoming	O
media	O
to	O
it	O
as	O
well	O
.	O
</s>
