IAX is the Inter-Asterisk eXchange protocol native to Asterisk PBX and supported by a number of other softswitches and PBXs. It is used for enabling VoIP connections between servers beside client–server communication.
IAX now most commonly refers to IAX2, the second version of the IAX protocol. The original IAX protocol has been deprecated in favor of IAX2.
The IAX2 protocol was published as an informational (non-standards-track) RFC 5456 by discretion of the RFC Editor in February 2010.[1]
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IAX2 is a VoIP protocol that carries both signaling and media on the same port. The commands and parameters are sent in binary format and any extension has to have a new numeric code allocated. Historically this was modeled after the internal data passing of Asterisk modules .
IAX2 uses a single UDP data stream (usually on port 4569) to communicate between endpoints, multiplexing signaling and media flow. IAX2 easily traverses firewalls and network address translators. This is in contrast to SIP, H.323 and MGCP which use an out-of-band RTP stream to deliver information.
IAX2 supports trunking, multiplexing channels over a single link. When trunking, data from multiple calls are merged into a single stream of packets between two endpoints, reducing the IP overhead without creating additional latency. This is advantageous in VoIP transmissions, in which IP headers use a large percentage of bandwidth.
The IAX and IAX2 protocols were created by Mark Spencer for Asterisk for VoIP signaling. The protocol sets up internal sessions and these sessions can use whichever codec they want for voice transmission. The Inter-Asterisk Exchange protocol essentially provides control and transmission of streaming media over IP (Internet Protocol) networks. IAX is extremely flexible and can be used with any type of streaming media including video, however it is mainly designed for control of IP voice calls.
The primary goals for IAX were to minimize bandwidth used in media transmissions, with particular attention drawn to control and individual voice calls, and to provide native support for NAT (Network Address Translation) transparency. Another goal is to be easy to use behind firewalls.
The basic structure of IAX is that it multiplexes signaling and multiple media streams over a single UDP (user datagram protocol) stream between two computers. IAX is a binary protocol, designed to reduce overhead especially in regard to voice streams. Bandwidth efficiency in some places is sacrificed in exchange for bandwidth efficiency for individual voice calls. One UDP stream is easier to set up for users that are behind a firewall.
An additional benefit to having a single stream is the added security, which can be implemented very easily. Furthermore, in countries where ISPs are filtering VoIP, IAX can be easily hidden.
This flaw no longer exists in up-to-date installations of Asterisk and other PBXes.[2]