[Serusers] Theory behind a media proxy and NATed UA

The problem with NAT is because the IP address of the SDP body is a non routeable IP. So how does a media proxy works? I read that a media proxy put his address at the sdp body and both UA talks with the media proxy instead of each other. At the time a NAT user sends an INVITE, there is no corresponding port on the NAT. So How does a media proxy contact the NATed UA? Isn't this the same problem the called part will have to contact the NATed party? I read a couple of papers about it, but in all of them it is said that both ends talk to the media proxy and that is all. No explanation about how things work. I know it works because I am using mediaproxy. If anyone could point me how this works or where I can find a document explaining.... Thanks, Felipe -- Master Student - Electrical Engineering Department Computer Engineering and Telecommunications Research Group Universidade Federal de Minas Gerais - Brazil "For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life." John 3:16