key material), little or no manual configuration is needed to maintain the system.

Multiple administrative domains. The participating nodes are not owned and controlled by a single organization. In general, each node is owned and operated by an independent individual who voluntarily joins the system.

P2P systems have several distinctive characteristics that make them interesting:

Low barrier to deployment. Because P2P systems require little or no dedicated infrastructure, the upfront investment needed to deploy a P2P service tends to be low when compared to client-server systems.

Organic growth. Because the resources are contributed by participating nodes, a P2P system can grow almost arbitrarily without requiring a “fork-lift upgrade” of existing infrastructure, for example, the replacement of a server with more powerful hardware.

Resilience to faults and attacks. P2P systems tend to be resilient to faults because there are few if any nodes that are critical to the system’s operation. To attack or shut down a P2P system, an attacker must target a large proportion of the nodes simultaneously.

Abundance and diversity of resources. Popular P2P systems have an abundance of resources that few organizations would be able to afford individually. The resources tend to be diverse in terms of their hardware and software architecture, network attachment, power supply, geographic location and jurisdiction. This diversity reduces their vulnerability to correlated failure, attack, and even censorship.

As with other technologies (for example, cryptography), the properties of P2P systems lend themselves to desirable and undesirable use. For instance, P2P systems’ resilience may help citizens avoid censorship by a totalitarian regime; at the same time, it can be abused to try and hide criminal activity from law enforcement agencies. The scalability of a P2P system can be used to disseminate a critical software update efficiently at a planetary scale, but can also be used to facilitate the illegal distribution of copyrighted content.

Despite having acquired a negative
reputation for some of its initial pur-
poses, P2P technologies are increas-
ingly being used for legal applications
with enormous business potential, and
there is consensus about their ability to
lower the barrier for the introduction of
innovative technologies. Nevertheless,
P2P technology faces many challenges.
The decentralized nature of P2P sys-
tems raises concerns about manage-
ability, security, and law enforcement.
Moreover, P2P applications are affect-
ing the traffic experienced by Internet
service providers (ISPs) and threaten to
disrupt the current Internet econom-
ics. In this article, we briefly sketch im-
portant highlights of the technology,
its applications, and the challenges it
faces.

applications

Here, we discuss some of the most successful P2P systems and also mention promising P2P systems that have not yet received as much attention.

Sharing and distributing files. Presently, the most popular P2P applications are file sharing (for example, eDonkey) and bulk data distribution (for example, Bit Torrent).

Both types of systems can be viewed as successors of Napster. In Napster, users shared a subset of their disk files with other participants, who were able to search for keywords in the file names. Users would then download any of the files in the query results directly from the peer that shared it.

Much of the content shared by Napster users was music, which led to copyright infringement lawsuits. Napster was found guilty and had to shut down its services. Simultaneously, a series of similar P2P systems appeared, most notably Gnutella and Fast Track (better known by one of its client applications, Kazaa). Gnutella, unlike Napster, has no centralized components and is not operated by any single entity (perhaps in part to make it harder to prosecute).

The desire to reduce the download
time for very large files lead to the de-
sign of BitTorrent, 10 which enables a
large set of users to download bulk data
quickly and efficiently. The system uses
spare upload bandwidth of concurrent
downloaders and peers who already
have the complete file (either because
they are data sources or have finished
the download) to assist other down-
loaders in the system. Unlike file-shar-
ing applications, BitTorrent and other
P2P content distribution networks do
not include a search component, and
users downloading different content
are unaware of each other, since they
form separate networks. The protocol
is widely used for disseminating data,
software, or media content.