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> Chapter 33 - VoIP Solutions: SIP > How the SIP ALG performs NAT > SIP NAT scenario: source address translation (source NAT)

SIP NAT scenario: source address translation (source NAT)

The following figures show a source address translation scenario involving two SIP phones on different networks, separated by a FortiGate. In the scenario, SIP Phone A sends an INVITE request to SIP Phone B and SIP Phone B replies with a 200 OK response and then the two phones start media streams with each other.

To simplify the diagrams, some SIP messages are not included (for example, the Ringing and ACK response messages) and some SIP header lines and SDP profile lines have been removed from the SIP messages.

SIP source NAT scenario part 1: INVITE request sent from Phone A to Phone B

For the replies to SIP packets sent by Phone A to be routable on Phone Bs network, the FortiGate uses source NAT to change their source address to the address of the WAN1 interface. The SIP ALG makes similar changes the source addresses in the SIP headers and SDP profile. For example, the original INVITE request from Phone A includes the address of Phone A (10.31.101.20) in the from header line. After the INVITE request passes through the FortiGate, the address of Phone A in the From SIP header line is translated to 172.20.120.122, the address of the FortiGate WAN1 interface. As a result, Phone B will reply to SIP messages from Phone A using the WAN1 interface IP address.

The FortiGate also opens a pinhole so that it can accept media sessions sent to the WAN1 IP address using the port number in the m= line of the INVITE request and forward them to Phone A after translating the destination address to the IP address of Phone A.

Phone B sends the 200 OK response to the INVITE message to the WAN1 interface. The SDP profile includes the port number that Phone B wants to use for its media stream. The FortiGate forwards 200 OK response to Phone A after translating the addresses in the SIP and SDP lines back to the IP address of Phone A. The SIP ALG also opens a pinhole on the Internal interface that accepts media stream sessions from Phone A with destination address set to the IP address of Phone B and using the port that Phone B added to the SDP m= line.

SIP source NAT scenario part 2: 200 OK returned and media streams established