Acquiring debug traces via tcpdump
Additionally to wireshark you can use tcpdump to get the traces we need for debugging. An example command-line would be:
tcpdump -i eth0 -s 0 -w rygel.pcap "(ip src 192.168.0.1 and ip dst 192.168.0.2) or (ip dst 192.168.0.1 and ip src 192.168.0.2)"
Run this command as root, substituting the ip addresses with the ones from the device and the machine Rygel runs on as well as eth0 with the network device Rygel is using. The filter makes sure that only the communication between your machine and the rendering device is captured.
The attached wizard script can help you in creating a correct tcpdump command-line. It needs ipcalc, ip and zenity.