File-sharing and P2P networks

File-sharing services allow participants to provide access to files on their personal computers, such as music or video. In turn, the user can browse the service to find and down-load material of interest. The structure is generally that of a peer-to-peer (P2P) network with no central server.

The first major file-sharing service was Napster. This was a P2P network but had a central server that provided the searchable list of files and locations—but not the files themselves, which were downloaded from users’ PCs. Napster was forced to close in 2001 by legal action from copyright holders (see intellectual property and com-puting). A new but unrelated for-pay service opened later under the same name.

Because of the legal vulnerability of centralized-list P2P services, a new model was developed, typified by Gnutella. This is a fully P2P model with both indexing and data decentralized in nodes throughout the network. As of mid-2006, Gnutella and similar services such as Kazaa had an estimated 10 million users.

BitTorrent

Many services today use the popular BitTorrent file-shar-ing protocol. A BitTorrent client (either the program of that name or another compatible one) can transmit or receive any type of data. To share a file, the client creates a “tor-rent”—a small file that contains metadata describing the file and an assignment to a “tracker.” The tracker is another computer (node) that coordinates the distribution of the file. Although this sounds complicated and a request takes longer to set up than an ordinary HTTP connection, the advantage is that once set up, downloading is efficiently managed even for files for which there is high demand. The downloading client connects to multiple clients that provide pieces of the desired file. Because of its efficiency, BitTorrent allows for distribution of substantial amounts of data at low cost, particularly since the system “scales up” automatically without having to provide extra resources. BitTorrent is currently being used for a variety of legally distributed material, including video, sound, and textual content (see blogs and blogging, podcasting, and rss).

Legal Issues

Because of their frequent use to share copyrighted music, video, or other material, a variety of organizations of copy-right owners have sued file-sharing services and/or their users. The biggest problem for the courts is to determine whether there is “substantial non-infringing use”—that is, the service is being used to exchange legal data. Some file-sharing services have been accused of dis-tributing malware (viruses or spyware) or of being used to distribute material that is illegal per se (such as child pornography). In response to litigation threats, file-sharing services have tended to become more decentralized, and some have features that increase anonymity of users (see anonymity and the Internet) or use encryption.