Hi Chrigui, thanks for your feedback.
A related question was posted/answered here:
http://community.collanos.com/index.php/topic,16.msg36.html#msg36.
This should (at least partially) also answer your question. If it does not well enough, please elaborate a bit more and get back to me with more questions. I'll be happy to provide additional information.
With regards to the "back server" functionality, we do have currently two services running centrally: (1) The so-called "Rendezvous" Service (see also above link to the other post) is responsible for keeping dynamic lists of "who is currently online" and "where". (2) The so-called "Relay" Service is needed to help route traffic between any two peers, if one (or both) of the peers are behind firewalls with only Port 80 / HTTP protocol enabled.
BTW, a great new body of knowledge is the following book: Peer-to-Peer Systems and Applications (Lecture Notes in Computer Science) (Paperback)
by Ralf Steinmetz (Editor), Klaus Wehrle (Editor).