Very good news: RakNet is now Open Source (modified BSD license). RakNet has a very broad spectrum of facilities that you need to build your networked graphics system. We used RakNet in the examples in the book (though they need minor updates which we will try to do in the next couple of weeks).
Unfortunately we had had to stop using RakNet for some of our activities because we had commercial partners (and the licensing terms didn't support mixed commercial/academic projects). So this is new licensing scheme will definitely help.