Peer-to-peer (P2P) interaction refers to the ability of any two information appliances, such as computers, to connect directly with one another without going through any central intermediary.
The notion of P2P interaction is not unique to the Internet. Consider the following scenario. Someone who is interested in obtaining a recipe for strawberry rhubarb pie can pursue a couple of strategies: go to a library (or log onto the Web) and search a central repository of documents for a recipe, or ask one or more friends if they have a recipe. A person who followed the first strategy would be a “client” obtaining information from a “server” that stored all of the information at one location—the library or Web site. A person who followed the second strategy would be engaging in P2P interaction, bypassing any intermediary central collection of recipes. For any given purpose, it may be far more effective to pursue one type of strategy or the other.
The same two strategies are in effect on the Internet. The advent of the Web made it possible for some large Web sites to “serve” information to millions of “clients” who used Web browsers. Users can find recipes, buy books, and read the news on many well-known Web sites, such as Amazon.com, Yahoo! and AOL. More recently, there have been many applications on the Internet that also make it possible for users to pursue P2P strategies. These applications fall into three broad categories: file sharing, computer-resource sharing, and communication and collaboration.
First, there are applications that allow peers to share information (audio, text, video, and graphics files) directly with one another without going to a Web site. Perhaps the most popular such application was Napster, which allowed individuals to share music files on their computers with millions of their peers anywhere in the world. Some have argued that Napster is not a “true” P2P application, because it had an intermediary “server” that provided a directory of which peers had what songs on their computers. There are other applications, such as Gnutella and Freenet, that dispense with the intermediary directory server and are therefore indisputably P2P. With these applications, a user would send his request for information (a song or any other digital document) to a small subset of peers who were connected to the application; they in turn relayed the request to a further subset of their peers. Soon, the user would have a list of peers from around the world that had the information that she requested. Second, there are applications aimed at sharing computing resources among peers' computers, rather than sharing files. Clay Shirky, an expert on Internet applications, observes that “At a conservative estimate … the world's Net-connected PCs presently host an aggregate of 10 billion megahertz of processing power and 10 thousand terabytes of storage.” In many cases, these Internet-connected PCs have spare storage space on their hard drives. In addition, these PCs are not being used much of the time. In 1995, scientists at University of California–Berkeley proposed using the computing power of these PCs to help in the search for extraterrestrial civilizations. The SETI (Search for Extraterrestrial Intelligence) Project was collecting massive amounts of radio signals from the skies using radio telescopes; unfortunately, it did not have the vast computing resources required to analyze these signals to detect extraterrestrial life. To solve this problem, a program called SETI@home was designed. It worked as a screensaver that a user could download. Whenever the user's computer was idle, this screensaver would be launched, and would connect with the SETI@home data distribution server. It would download a small portion of the data, analyze it on the user's computer, then send the results to the server. As of October 2000, the project had received more than 200 million results from users, making up the largest computation ever performed in the world. Inspired by SETI's success, others are launching similar applications to help with computational challenges in genetic research, fighting AIDS, and financial data mining. While users volunteer their computing resources for the SETI@home project, companies such as Popular Power and Distributed Net offer to compensate users for the donation of their idle computing resources. Third, some applications help peers to communicate and collaborate directly with one another. In 1996, a young Israeli firm called Mirabilis launched the first such application, called ICQ (“I seek you”). It allowed users to instantly send messages between PCs connected to the Internet without going through any central server (such as an email server). American Online's Instant Messenger (AIM) program is a similar P2P application (AOL acquired ICQ in 1998), allowing users to communicate on a P2P basis with “buddies.” More recently, Ray Ozzie, who created Lotus Notes, developed Groove, a P2P collaboration environment that allows groups of individuals to create a shared space without the presence of a central network server. These environments allow not only for the sharing of files, but also for more intensive collaboration such as simultaneous editing and automatic updating of files, even when users are not all connected at the same time. Some applications incorporate more than one of the three P2P categories described above. For instance, programs such as Aimster allow AIM users to share files only with others on their “buddy” lists. Mojo Nation allows users to pay for downloading files with a digital “currency” called Mojo, which they can earn by sharing their computer's resources. The use of P2P applications may increase with the advent of millions of Internet-enabled information appliances, such as mobile phones and personal digital assistants (PDAs). Technologies such as Bluetooth and Jini will greatly facilitate the participation of these appliances in P2P applications. However, many of the stand-alone P2P applications have not yet developed successful business models. In addition, users need to contend with issues such as trust, accountability, security, intellectual property, and privacy, which are raised by P2P environments that bypass the central, reliable, and well-established Web “servers” that have been embraced over the past decade. —Noshir Contractor
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