<s>
Inter-Asterisk	B-Protocol
eXchange	I-Protocol
(	O
IAX	B-Protocol
)	O
is	O
a	O
communications	O
protocol	O
native	O
to	O
the	O
Asterisk	B-Language
private	B-Device
branch	I-Device
exchange	I-Device
(	O
PBX	O
)	O
software	O
,	O
and	O
is	O
supported	O
by	O
a	O
few	O
other	O
softswitches	B-Device
,	O
PBX	O
systems	O
,	O
and	O
softphones	O
.	O
</s>
<s>
It	O
is	O
used	O
for	O
transporting	O
VoIP	B-Application
telephony	I-Application
sessions	O
between	B-Protocol
servers	I-Protocol
and	O
to	O
terminal	O
devices	O
.	O
</s>
<s>
The	O
original	O
IAX	B-Protocol
protocol	O
is	O
deprecated	B-General_Concept
and	O
has	O
been	O
superseded	O
by	O
a	O
second	O
version	O
,	O
commonly	O
called	O
IAX2	B-Protocol
.	O
</s>
<s>
The	O
IAX2	B-Protocol
protocol	O
was	O
published	O
as	O
an	O
informational	O
(	O
non-standards-track	O
)	O
RFC	O
5456	O
by	O
discretion	O
of	O
the	O
RFC	O
Editor	O
in	O
February	O
2010	O
.	O
</s>
<s>
IAX	B-Protocol
is	O
a	O
VoIP	B-Application
protocol	O
that	O
can	O
be	O
used	O
for	O
any	O
type	O
of	O
streaming	O
media	O
including	O
video	O
,	O
but	O
is	O
mainly	O
designed	O
for	O
IP	B-Application
voice	I-Application
calls	O
.	O
</s>
<s>
IAX	B-Protocol
uses	O
a	O
single	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
data	O
stream	O
between	O
endpoints	O
for	O
both	O
the	O
session	O
signaling	O
and	O
the	O
media	O
payloads	O
.	O
</s>
<s>
Thus	O
it	O
uses	O
only	O
a	O
single	O
UDP	O
port	B-Protocol
number	I-Protocol
,	O
typically	O
4569	O
.	O
</s>
<s>
This	O
feature	O
provides	O
benefits	O
for	O
traversing	O
network	B-Protocol
address	I-Protocol
translators	I-Protocol
on	O
network	O
boundaries	O
,	O
as	O
it	O
simplifies	O
firewall	B-Application
configuration	O
.	O
</s>
<s>
Other	O
VoIP	B-Application
protocols	O
typically	O
use	O
independent	O
streams	O
for	O
signaling	O
and	O
media	O
,	O
such	O
as	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	B-Application
)	O
,	O
H.323	B-Protocol
,	O
and	O
the	O
Media	B-Protocol
Gateway	I-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
MGCP	B-Protocol
)	O
,	O
which	O
carry	O
media	O
with	O
the	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
.	O
</s>
<s>
IAX	B-Protocol
is	O
a	O
binary-encoded	O
protocol	O
.	O
</s>
<s>
Historically	O
,	O
this	O
was	O
modeled	O
after	O
the	O
internal	O
data	O
passing	O
of	O
Asterisk	B-Language
modules	O
.	O
</s>
<s>
IAX	B-Protocol
supports	O
trunking	O
,	O
multiplexing	B-Architecture
channels	O
over	O
a	O
single	O
link	O
.	O
</s>
<s>
When	O
trunking	O
,	O
data	O
from	O
multiple	O
sessions	O
are	O
merged	O
into	O
a	O
single	O
stream	O
of	O
packets	B-Protocol
between	O
two	O
endpoints	O
,	O
reducing	O
the	O
IP	O
overhead	O
without	O
creating	O
additional	O
latency	O
.	O
</s>
<s>
This	O
is	O
advantageous	O
in	O
VoIP	B-Application
transmissions	O
,	O
in	O
which	O
IP	O
headers	O
use	O
a	O
large	O
percentage	O
of	O
bandwidth	O
.	O
</s>
<s>
IAX2	B-Protocol
supports	O
native	O
encryption	O
of	O
both	O
control	O
and	O
media	O
streams	O
using	O
AES-128	B-Algorithm
.	O
</s>
<s>
Both	O
versions	O
of	O
the	O
IAX	B-Protocol
protocol	O
were	O
created	O
by	O
Mark	O
Spencer	O
and	O
much	O
of	O
the	O
development	O
was	O
carried	O
out	O
in	O
the	O
Asterisk	B-Language
open-source	O
community	O
.	O
</s>
<s>
The	O
primary	O
goals	O
for	O
IAX	B-Protocol
are	O
to	O
minimize	O
bandwidth	O
used	O
in	O
media	O
transmissions	O
,	O
with	O
particular	O
attention	O
drawn	O
to	O
control	O
individual	O
voice	O
calls	O
,	O
and	O
to	O
provide	O
native	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
transparency	O
.	O
</s>
<s>
It	O
was	O
intended	O
to	O
be	O
easy	O
to	O
use	O
behind	O
firewalls	B-Application
.	O
</s>
<s>
Awkward	O
extensibility	O
:	O
Due	O
to	O
the	O
lack	O
of	O
a	O
generic	O
extension	O
mechanism	O
,	O
new	O
features	O
have	O
to	O
be	O
added	O
in	O
the	O
protocol	O
specification	O
,	O
which	O
makes	O
the	O
protocol	O
less	O
flexible	O
than	O
H.323	B-Protocol
,	O
SIP	B-Application
or	O
MGCP	B-Protocol
.	O
</s>
<s>
Vulnerability	O
:	O
Older	O
implementations	O
of	O
IAX2	B-Protocol
were	O
vulnerable	O
to	O
resource	O
exhaustion	O
DoS	O
attacks	O
that	O
are	O
available	O
to	O
the	O
public	O
.	O
</s>
<s>
Internet-facing	O
IAX2	B-Protocol
ports	O
are	O
considered	O
vulnerable	O
and	O
should	O
be	O
monitored	O
closely	O
.	O
</s>
<s>
These	O
issues	O
were	O
briefly	O
mentioned	O
in	O
the	O
IAX	B-Protocol
RFC	O
5456	O
on	O
page	O
94	O
.	O
</s>
<s>
This	O
flaw	O
does	O
not	O
exist	O
in	O
up-to-date	O
installations	O
of	O
Asterisk	B-Language
or	O
other	O
PBXes	O
.	O
</s>
