[Serusers] rtpproxy must be located on the same machine with SIP server??

Hi, All: Please forgive me for such stupid questions, Whether it is a must that rtpprox must be located on the same machine with the SER. I want to enable the SER to support both the video and audio port modification when SIP UA　are behind the NAT. To do that what files should I modify(modules/nathelper/ nathelper.c???), but I don't know the protocol between the rtpproxy and SER. It is very kind of you to help me on that! Last day I used the SER on the iptel.org to test the nathelp module, my twu UA is located behind the NAT. one UA is registered on my SER server, the other on iptel.org'ser. After establishing connection, I checked the message log, I found the SER modified the SDP's c IP value to iptel.org's address(195.37.77.99). If I am right, the ser can know whether both UA are located within the same NAT clouds by examining the source IP of SIP message, why does it modified its SDP c value under such situations. Thanks for your kind help and advices! Best Regards. Sun Zongjun