Inter-Asterisk eXchange (IAX) is a communications protocol native to the

Asterisk The asterisk ( ), from Late Latin , from Ancient Greek , ''asteriskos'', "little star", is a typographical symbol. It is so called because it resembles a conventional image of a heraldic star. Computer scientists and mathematicians often voc ...

private branch exchange A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone syst ...

(PBX) software, and is supported by a few other softswitches, PBX systems, and softphones. It is used for transporting VoIP telephony sessions between servers and to terminal devices. The original IAX protocol is deprecated and has been superseded by a second version, commonly called IAX2. The IAX2 protocol was published as an informational (non-standards-track) RFC 5456 by discretion of the RFC Editor in February 2010.

Basic properties

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 In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

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 The Session Initiation Protocol (SIP) is a signaling protocol used for initiating, maintaining, and terminating communication sessions that include voice, video and messaging applications. SIP is used in Internet telephony, in private IP telepho ...

(SIP),

H.323 H.323 is a recommendation from the ITU Telecommunication Standardization Sector (ITU-T) that defines the protocols to provide audio-visual communication sessions on any packet network. The H.323 standard addresses call signaling and control, m ...

, 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 In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - a ...

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.

Origin

Both versions of the IAX protocol were created by Mark Spencer and much of the development was carried out in the Asterisk open-source community.

Goals

The primary goals for IAX are to minimize bandwidth used in media transmissions, with particular attention drawn to control individual voice calls, and to provide native network address translation (NAT) transparency. It was intended to be easy to use behind firewalls.

Drawbacks

* Awkward extensibility: Due to the lack of a generic extension mechanism, new features have to be added in the protocol specification, which makes the protocol less flexible than

, SIP or MGCP. * Vulnerability: Older implementations of IAX2 were vulnerable to resource exhaustion DoS attacks that are available to the public. While no solutions existed for these issues, the best practices included limiting UDP port access to specific trusted IP addresses. Internet-facing IAX2 ports are considered vulnerable and should be monitored closely. The fuzzer used to detect these application vulnerabilities was posted on milw0rm and is included in the ''VoIPer'' development tree. These issues were briefly mentioned in the IAX RFC 5456 on page 94. This flaw does not exist in up-to-date installations of Asterisk or other PBXes.

References

External links