IAX2 协议

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　　The IAX revision 2 protocol is used by the Asterisk VOIP PBX and FreeSwitch Softswitch as an alternative to SIP, H.323,
　　etc. when connecting to other devices that support IAX (a limited list
　　at the moment, but growing very rapidly). However meanwhile other
　　software like Yate SofaSwitch and OPAL have added IAX2 support.
　　AX2 is not the result of a standards group, rather a collaborative, community based effort (Note: IAX2 is now: RFC 5456)
　　IAX2 uses a single UDP port 4569, and thus works well in NAT
　　environments (the obsolete IAX1 protocol used port 5036). IAX2 uses ONLY
　　one udp port for both control and data traffic. As outlined in point 4