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

[szj]

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