Peer-to-Peer Networks

A peer-to-peer (P2P) network is one which does not distinguish between clients and servers: each "node" in the network is equal, and serves as both client and server in relation to other nodes. In practical terms, that means that when you want to retrieve data from the network, the nodes from which you retrieve your data are not only servers, but clients who are also retrieving data from other nodes. For example, you might download File X from Node A, who downloaded it from Node B and who is also currently downloading File Y from Node C.

P2P networks have the benefit of not relying on a single server or set of servers for the continued running of data exchange. As data is shared, it is duplicated in several places in the network and can be downloaded without the need for a central server.

Also, in a pure P2P network, a central server is not even involved in finding files for download.

Another benefit of P2P is that all clients/servers provide resources to the network, including storage space, computing power and bandwidth.

Different Kinds of P2P File Sharing

There are several kinds of P2P protocols.

- Gnutella is the P2P protocol used by popular P2P programs such as LimeWire and Morpheus. The network is decentralised, meaning that no one company or organisation controls the contents of the network. Multi-source downloading is supported. The Gnutella network gained popularity after the threat of closure for the Napster network, which is now no longer a P2P network, but a legal paid download site.

- FastTrack, which is used, most notably, by the program Kazaa, uses "supernodes" to improve scalability. These supernodes are automatically created when a powerful computer with a fast network connection runs FastTrack client software. The computer becomes a sort of temporary indexing server.

- eDonkey's most interesting feature is that any user can add a server to the network by downloading eDonkey network server software. Some eDonkey clients support complex Boolean searches. There are websites available which list active servers and sought-after files.

- BitTorrent is a P2P protocol which differs from Gnutella in that, when a file is requested, the client actually makes several small P2P requests, and downloads files in "chunks", in a random or rarest-first order rather than starting from the beginning of the file and downloading until it gets to the end. The benefit of this is that all parts of a file are equally available because they are downloaded by clients in a random order. Unfortunately, streaming torrent downloads are currently not possible because the end might be downloaded before the beginning. But VLC Media Player is able to play torrent downloads with one or two chunks missing. To use BitTorrent software, you need to download a torrent file corresponding to the file you want, and open it using your BitTorrent software.

- Soulseek is a P2P network (and P2P software: Soulseek for Windows, SoulSeeX for Mac, Museek for Linux) which relies on a central file list held on a server. It is mostly used for music because multi-source downloading is not permitted, so large file downloads take a long time. It is possible to view all the files being shared by a single user and to download whole albums in this way.

Legal Considerations

There are legitimate file sharing activities possible on P2P networks. For example, it is legal to share open-source software, creative commons works, and any files which are in the public domain. However, the majority of P2P users use the networks to share copyrighted material illegally, including music, video, movies and software.

Various high-profile lawsuits have been conducted against the providers of P2P networks and software, including Kazaa and Napster. The providers are accused of "permitting" and "facilitating" illegal activity. Decentralised networks, such as BitTorrent, are much more difficult to launch attacks against because there is no one node which can be shut down, and the networks are in many ways independent of their creators.

Some commentators have argued that illegal downloads have not had a negative effect on the music industry, despite complaints to the contrary. A study into the question, conducted by Felix Oberholzer-Gee and Koleman Strumpf, found that illegal music downloads have had no noticeable affect on music sales.

However, whether or not it has an effect on the music or any other industry, downloading copyrighted material is illegal. P2P networks should only be used for legitimate downloading.

Security Problems

Some P2P software, including LimeWire and Kazaa, has been, in the past, suspected of being bundled with spyware and adware. LimeWire has since addressed these accusations and no longer installs bundled software, but Kazaa is still thought to be a problem, and it is wise to check reviews of any P2P clients before downloading and installing them.

You should also be aware that many P2P networks are littered with viruses and other malware. Make sure that your antivirus and firewall software is up-to-date before initiating any downloads. Sometimes a file will just be accidentally misnamed; other times, a virus will be given the name of a popular file, or bad chunks or packets will be inserted into an otherwise healthy file.

For more information on security issues with P2P networks, see Tony Bradley's article on About.com.

By Iain Ford

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