Ok here's what I have done. You have to set a NAT detected flag from ser.cfg after a successful nat_uac_test - ie. if nat_uac_test(..) { setflag(6) } (Thanks to Klaus for the tip!!) In nathelper.c, force_rtp_proxy_f checks if flag 6 (am harcoding 6 into the code at the moment) is set. If yes, it tells RTP proxy not to save the IP:port which is in the SDP body. So rtpproxy will have to wait for atleast one packet from this UA before it knows the correct natted IP:port to which RTP packets from the other end is to be sent. If this nat flag is not set, nathelper will forward the IP:port from SDP body to rtpproxy. Since rtpproxy knows the right IP:port for this end, it will forward any RTP packets coming from the other end without waiting for any UDP packets to come from this end. There is another problem with the existing nathelper + rtpproxy which I did not mention in my previous email. rtpproxy expects BOTH ends to be sending and receiving on the same port (symmetric RTP) - otherwise it does not work. But if a UA is not behind NAT, it should work with or without symmetric RTP support (regardless of whether the other end is natted or not). I have tried to solve this as follows - first rtpproxy tries to match IP:port of the sender with the ones (addr1 and addr2) it has in its record. If it finds a match, it will forward the packet to the other IP:port. However if no match is found and addr1 and addr2 are not NULL, rtpproxy tries to do a match using only the IP address (ie. port number is not taken into consideration). If the sender's IP address matches with one of the two stored addresses (but port number does not match), forward the packet to the other IP:port. I have made the necessary modifications - but will test it on Monday. Regards, Dhiraj -----Original Message----- From: Klaus Darilion [mailto:darilion@ict.tuwien.ac.at] Sent: Fri 12/12/2003 20:31 To: serdev(a)iptel.org Cc: Bhuyan,D,Dhiraj,XVR3 R; jiri(a)iptel.org; sobomax(a)portaone.com Subject: RE: nat + multiple RTP proxy ...just some thoughts from me ... You also must change the behaviour of the rtpproxy - rtpproxy should be enhanced to be symmetrical only in one direction: imagine the following scenario: ua1 ----NAT----ser+rtpproxy-------ua2 ua1 is behind NAT, ua2 not: rtpproxy should relay rtp packets from ua2 to the ip-address:port it received the packets from ua1 (therefore it has to receive at least one packet from ua1 before relaying from ua2->ua1). RTP packets from ua1 to the rtpproxy should be forwarded the ip-address:port announced in the SDP from ua2 - this can be done immediately. Therefore, I suggest to make decision if a UA is NATed or not in the ser script, but enhance the force_rtp_proxy() and use a parameter, e.g. force_rtp_proxy(0) should tell the rtp-proxy to be symmetrical to this client, whereas force_rtp_proxy(1) will tell the rtp-proxy to relay to this client using the ip-address:port in the SDP maybe it will be useful to use two ports at the rtpproxy (one for each client) to easily distinguish the two clients.