Peer-To-Peer (P2P) networks are distributed systems that lack any hierarchical organization or centralized control. Peers form self-organizing networks that are overlayed on the Internet Protocol (IP) networks, offering a mix of various features such as robust wide-area routing architecture, efficient search of data items, selection of nearby peers, redundant storage, permanence, hierarchical naming, trust and authentication, anonymity, massive scalability and fault tolerance. These systems go beyond services offered by client-server systems by having symmetry in roles where a client may also be a server. It allows access to its resources by other systems and supports resource-sharing, which requires fault-tolerance, self-organization and massive scalability properties. Unlike Grid systems, P2P networks do not arise from the collaboration between established and connected groups of systems and without a more reliable set of resources to share. The core operation in a P2P network is the efficient location of data items. In this chapter, we will look at two widely-known P2P networks, Chord and Freenet.
Chord  is a distributed lookup protocol that addresses the problem of efficiently locating the node that stores a particular data item in a structured P2P application. Given a key, it maps the key onto a node. A key is associated with each data item and key/data item pair is stored at the node to which the key maps. Chord is designed to adapt efficiently as nodes join and leave the network dynamically. Freenet  is an unstructured P2P network application that allows the publication, replication and retrieval of data while protecting the anonymity of both the authors and the readers. It operates as a network of identical nodes that collectively pool their storage space to store data files and cooperate to route requests to the most likely physical location of data. The files are referred to in a location-independent manner, and are dynamically replicated in locations near requestors and deleted from locations where there is no interest. It is infeasible to discover the true origin or destination of a file passing through the network, and difficult for a node operator to be held responsible for the actual physical contents of her node.
- 1.Clarke, Ian, Oskar Sandberg, Brandon Wiley, and Theodore W. Hong. 2001. Freenet: A distributed anonymous information storage and retrieval system. In Designing privacy enhancing technologies, 46–66. Berlin: Springer.Google Scholar
- 2.Lua, Eng Keong, Jon Crowcroft, Marcelo Pias, Ravi Sharma, and Steven Lim. 2005. A survey and comparison of peer-to-peer overlay network schemes. IEEE Communications Surveys & Tutorials 7 (2): 72–93.Google Scholar
- 3.Stoica, Ion, Robert Morris, David Liben-Nowell, David R. Karger, M. Frans Kaashoek, Frank Dabek, and Hari Balakrishnan. 2003. Chord: A scalable peer-to-peer lookup protocol for internet applications. IEEE/ACM Transactions on Networking (TON) 11 (1): 17–32.Google Scholar
- 4.Zhang, Hui, Ashish Goel, and Ramesh Govindan. 2002. Using the small-world model to improve freenet performance. In INFOCOM 2002. Twenty-first annual joint conference of the IEEE computer and communications societies. Proceedings. IEEE, vol. 3, 1228–1237. IEEE.Google Scholar