There’s a number of devices out there that extend HDMI over IP. You connect a video source to the transmitter, a display to the receiver, and link the two with a CAT5/5e/6 cable. These cables are much cheaper than HDMI cables, and can run longer distances.
[Daniel] didn’t care about extending HDMI, instead he wanted a low cost HDMI input for his PC. Capture cards are a bit expensive, so he decided to reverse engineer an IP HDMI extender.
After connecting a DVD player and TV, he fired up Wireshark and started sniffing the packets. The device was using IP multicast on two ports. One of these ports had a high bitrate, and contained JPEG headers. It looked like the video stream was raw MJPEG data.
The next step was to write a listener that could sniff the packets and spit the data into a JPEG file. After dealing with some quirks, JPEG images could be saved from the remote device. Some more code was needed to have the computer initiate the streaming, and to extract audio from the second port.
In the end, video capture with the low cost device was possible. [Daniel] also provided a bonus teardown of the device in his writeup.