SIP ALG destination address translation
Incoming calls are directed from a SIP phone on the Internet to the interface of the FortiGate connected to the Internet. To receive these calls you must add a security policy to accept SIP sessions from the Internet. The security policy requires a firewall virtual IP. SIP INVITE messages from the Internet connect to the external IP address of the virtual IP. The SIP ALG uses the destination address translation defined in the virtual IP to translated the addresses in the SIP message to addresses on the private network.
When a 200 OK response message arrives from the private network, the SIP ALG translates the addresses in the message to Internet addresses and opens pinholes for media sessions from the private network to the Internet.
When the ACK message is received for the 200 OK, it is also intercepted by the SIP ALG. If the ACK message contains SDP information, the SIP ALG checks to determine if the IP addresses and port numbers are not changed from the previous INVITE. If they are, the SIP ALG deletes pinholes and creates new ones as required. The ALG also monitors the Via:, Contact:, and Record-Route: SIP fields and opens new pinholes as required.