FastTrack is a closed source C++ based file sharing protocol used by KaZaA and its clones. It is semi-centralized: Searches are propagated among so-called "superpeers" (very high-bandwidth users). Each superpeer maintains an index of the files by all users connected to it. All transactions are encrypted, which makes the protocol resistant to reverse engineering.
A major attempt to accomplish this was the giFT project. The project, started in 2001, was originally called "Kazaatux" as an attempt to bring KaZaA to Linux, but renamed to "giFT" when a mysterious group referred to as "the givers" released details about the FastTrack protocol. This briefly allowed giFT users full access to the FastTrack network, until the protocol was modified again in mid-2000. There is now a working KaZaA plugin for giFT again, called giFT-FastTrack. It is not developed by the giFT team.