Doing a scalable, practical P2P file transfer is very non-trivial, even from a purely technical point of view (not touching the ugly politics of it all). The Gnutella people screwed it up for years. Further, most of the scalable P2P apps that are out there have been re-implementing each others' ideas and calling them different things (look particularly at what Freenet calls "CHK"--similar ideas are used in almost every P2P app around, except the simple-stupid ones). So don't add to the mess. As hard as it is, many people have already created good solutions, and you'll do well to find them.