How To Configure the Firewall VoIP Support Service (SIP ALG)

Some more detail about SIP and SIP ALG
The Session Initiation Protocol (SIP), in conjunction with the Session Description Protocol(SDP), is an application layer protocol that is able to negotiate end-to-end sessions for users on the Internet. Such sessions can be used to carry various forms of multimedia data including voice traffic via RTP (VoIP). SIP utilises an infrastructure of SIP Proxy servers to allow Internet endpoints (SIP User Agents) to locate each other and negotiate session
parameters. SIP supports four basic functions in order to establish, modify and terminate :
- Location of an Internet end point
- Signal of a desire to communicate
- Negotiation of session parameters to establish the session
- Tear down of an established session
A SIP Application Layer Gateway (ALG) provides functionality to allow VoIP traffic to pass both from the private to public and public to private side of the firewall when using Network  Address & Port Translation (NAPT). The SIP-ALG inspects and modifies SIP traffic to allow SIP traffic to pass through the firewall so that person-to-person SIP sessions may be established. More exactly, NAPT mapping is provided from the users on the private side of the firewall to the SIP Proxy on the public side of the firewall by altering the IP addresses,UDP port numbers, and SIP/SDP messages in SIP packets.
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