Waaay back I had the idea of sending data over the audio/mic jack, and made a crummy implementation that could only send text messages (very unreliable) http://seiferteric.com/?p=319 . This was on hackaday many moons ago.
This time I wanted to send data using your monitor and a webcam, but how could I send data reliably without too much work? QR Code! So the idea this time is to use a tun device, read packets from it, convert to QR Code and display on screen. Then the other side will see it with the cam and decode and inject the packet into it’s tun device.
I ran into a few issues along the way. First, you need some way to acknowledge that the other side read my message, so I added a simple header of the form SEQ/ACK where I increment the sequence with each new qrcode and increment the acknowledge when I