IAX is a VoIP protocol that can be used for any type of streaming media including video, but is mainly designed for IP voice calls.
IAX uses a single User Datagram Protocol (UDP) data stream between endpoints for both the session signaling and the media payloads. Thus it uses only a single UDP port number, typically 4569. This feature provides benefits for traversing network address translators on network boundaries, as it simplifies firewall configuration. Other VoIP protocols typically use independent streams for signaling and media, such as the Session Initiation Protocol (SIP), H.323, and the Media Gateway Control Protocol (MGCP), which carry media with the Real-time Transport Protocol (RTP).
IAX is a binary-encoded protocol. New extension features must have a new numeric code allocated. Historically, this was modeled after the internal data passing of Asterisk modules.
IAX supports trunking, multiplexing channels over a single link. When trunking, data from multiple sessions 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.
IAX2 supports native encryption of both control and media streams using AES-128.