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IAX server and client

IAX is the Inter-Asterisk eXchange protocol used by Asterisk to establish VoIP connections between servers and clients that use the IAX protocol and between Asterisk servers.

A newer version of this protocol is IAX2. It is meant to replace the old protocol. That is why, when referring to IAX, we will refer to the latest version of the protocol, IAX2. This is an alternative to SIP and H323, but it has not been the effort of a standards group, rather a community, collaborative development.

IAX is a signaling and media protocol. It operates in peer-to-peer mode, meaning that the connection is made between the two endpoints that are responsible for the protocol operations. When it was designed, the people working on it had the following main purposes: minimize bandwidth usage for control and for media and make it work behind NAT (Network Address Translation). NAT is the process in which an address is passing through a server or firewall that rewrites it. Usually, it is used for multiple computers across a network to share the same Internet connection, using only one public IP. As packets come to the server, it automatically rewrites the private address of each computer to the public IP, also keeping pieces of information about every packet.

IAX is a full-featured protocol and yet easy to implement. It works with any codec and any number of streams. This is a very powerful attribute, as it means it can be used as a transport for any type of data. Communication between endpoints takes place on port 4569, using a single UDP data stream. UDP is used for both signaling and data. Using in-band transmission it is possible for IAX to cross through firewalls and over NAT servers. IAX multiplexes the single UDP connection over multiple media streams. It is a binary protocol and designed in such a way to eliminate as much as possible the overhead especially for voice streams.

One advantage of IAX is that it supports trunking. In computer networking, this word means that multiple cables/wires/ports are being used in parallel to achieve faster speeds than with only one cable or port. Data from multiple calls is considered as a single set of packets, resulting in one IP datagram to carry data from more than one call. This reduces overhead a lot and does not create external unneeded latency. Considering the fact that IP overhead is a large part of the total bandwidth, this is a great improvement.

A client using IAX is nothing more than a host that initiates the conversation and a server is a host that receives the initial call setup request. The process is explained fully with the help of some rather complicated state machines that show in detail the mechanism of initiating (client) and accepting (server) a connection. We will not discuss about them here, as this is not the purpose of the book.

July 2014:
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Yate 4.0 released.
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Nov 2011:
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9 Mar 2011:
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Feb 2011:
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31 Jan 2011:
Yate 3.1 released.
Yate client support for Google Voice. Support for any country tones in tonegen.

20 Dec 2010:
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3 May 2010:
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Yate 2.1 launched. Can replace a Cisco PGW2200 to control a Cisco AS54xx.

6 Aug 2008:
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