How does this iOS app communicate over the USB port?

Question

This app promises to turn your iOS device into a second monitor and uses the standard USB cable to connect to the computer, as opposed to the network which all other similar app

Answer 1

The information, code and tools to accomplish generic USB port communication is available to those who join the Apple MFi program. You have to do the paperwork and get licensed by Apple.

http://developer.apple.com/programs/mfi/

The libraries necessary are included in the app and then the usual streaming libraries and code do the rest.

Answer 2

Dean told about it on his blog.

Duet uses Peertalk, an open source library allowing to pass TCP connections through the USB connection without being part of the MFI program.