IAX protocol (Inter-Asterisk eXchange protocol) was designed to provide control and transmission of voip data between Asterisk servers. Nowadays it is also used for connections between clients and servers which support the protocol.

The present version of IAX is IAX2 since the first version of IAX is obsolete. IAX is a protocol designed and thought for VoIP connections (audio streaming) although it can support another type of data (for example video streaming)

The main goals of IAX are:

-Minimize bandwidth usage for both control and media transmissions with specific emphasis on individual voice calls
-Avoid NAT problems (Network Address Translation)
-Support the ability to transmit dialplan information

To reduce the bandwidth IAX or IAX2 use a binary protocol instead of a text protocol like SIP and that is why the size of IAX messages is less than SIP messages.

In order to avoid NAT problems, IAX or IAX2 uses UDP transport protocol, normally on port 4569, (IAX1 used port 5036), and both, signaling information and data go together using the same protocol (unlike SIP) Therefore, IAX has less NAT problems and it can pass through routers and firewalls in a better way.