Chapter 22 VoIP Solutions: SIP : FortiGate VoIP solutions: SIP : How the SIP ALG performs NAT : Additional SIP NAT scenarios : Different source and destination NAT for SIP and RTP
  
Different source and destination NAT for SIP and RTP
This is a more complex scenario that a SIP service provider may use. It can also be deployed in large-scale SIP environments where RTP has to be processed by the FortiGate unit and the RTP server IP has to be translated differently than the SIP server IP.
Figure 301: Different source and destination NAT for SIP and RTP
In this scenario, shown in Figure 301, assume there is a SIP server and a separate media gateway. The SIP server is configured so that the SIP phone (219.29.81.20) will connect to 217.233.90.60. The media gateway (RTP server: 219.29.81.10) will connect to 217.233.90.65.
What happens is as follows:
1. The SIP phone connects to the SIP VIP. The FortiGate ALG translates the SIP contact header to the SIP server: 219.29.81.20 > 217.233.90.60 (> 10.0.0.60).
2. The SIP server carries out RTP to 217.233.90.65.
3. The FortiGate ALG opens pinholes, assuming that it knows the ports to be opened.
4. RTP is sent to the RTP-VIP (217.233.90.65.) The FortiGate ALG translates the SIP contact header to 192.168.0.21.