Hi, All: I just tested the SER+RTPProxy. My ser and rtpproxy are located on the same machine with public ip address. my SIP UA is behind the NAT device(Symmetric NAT). When UA registered to SER, the SER send the response to the port specified in the Via head parameters, not the real port receiving the Registration request. After I put the force_rport() in the ser.cfg, all will works well. I wonder why the SER doesn't send the response back to the receiving port. Whether that conflicts the RFC3261 chapter 19. I am glad to hear your advice and help Best Regards Sun Zongjun