After some more testing, I found that the problem is RTSP uses RTCP in order to control and check if the streamer is still available. The RTCP  UDP packets are not making their way through NAT. because of random or mismatched ports.

Instead of using the DMZ to solve the problem, I am using port forwarding (1024-65535) to forward UDP packets to the decoder and things are working OK.

The ideal solution would be a good ALG RTSP "helper".

@Kevin_Z

Thanks for responding.

Please allow me to give you a little background info before I answer your question. Until now, I have always had the IPTV decoder connected directly to the ISP’s router to avoid any problems. The ISP (Movistar) provides Internet, IPTV and VOIP services on separate vlans which terminate and are bridged together in the ISP’s router and appear on the router’s LAN ports. My AX55 is connected to one of the LAN ports and provides all my network services except TV and telephone. This means using double NAT which has never caused a problem with anything (no games here!).

I have never liked the idea of double NAT so I decided to configure the ISP’s router to work in bridge mode. In Movistar’s bridge mode, only the Internet vlan is actually bridged and passed through the router (untagged in the process), leaving IPTV and VOIP still in the router. The AX55 must then be configured for a PPPoE connection in order to handle the internet services.

With IPTV, things get a little more complicated. The AX55’s PPPoE connection must be configured with a secondary connection (using a static IP) in order to connect to the Movistar router for IPTV traffic. Several static routes must also be added in the AX55 so all IPTV related traffic can be properly routed back through the Movistar router instead of through the default gateway to the internet.

After connecting the decoder to the AX55’s network, everything worked except for VOD and ”Timeshifting” of a live channel. It was then determined that the ISP uses RTSP for these functions, creating in the process a UDP stream. This is where things are breaking down. A solution was found by putting the decoder in the DMZ. After more packet analysis and still not being able to pinpoint the problem, the final solution (and for now) was found to be the use of Port Forwarding (ports 1024-65535) for UDP packets.

One final test was made restoring the Movistar router to it’s native router mode and going back to double NAT. With the decoder connected to the AX55 and using the same port forwarding, everything works OK. This leads me to conclude that double NAT is not the problem but that any NAT is the problem.

Enabling and disabling the AX55 option for ALG RTSP (under Security in the web interface) made no difference during my tests.