From aa-asterisk.org.uk wiki
The Inter-Asterisk Exchange is a protocol developed for the Asterisk PBX. The protocol is designed to be used to link Asterisk servers together but can also interoperate with other servers (e.g.) by way of the libiax library.
Version 1 is now officially deprecated as of version 1.2 and should no longer be used.
Known as IAX2 or just IAX. The current supported version of the protocol.
The protocol itself has now been made into RFC 5456. However it does appear that this RFC does not document the IAX2 protocol completely as it currently stands in the latest version of Asterisk.