The "200 OK" message returned from the UAC will need to be manipulated too as above. Thus a NAT handling code must be included in the section on_reply_route[]. A flag was set (7) on the INVITE message to indicate that this transaction is to or from a client behind a NAT device. In the reply route we will check this flag and if it is set we will have to fix the contact, and have the RTP forced to Media Relay server. It is important to emphasize that the "200 OK" message contains an SDP header describing the session parameters agreed.