Peer-to-Peer Middleware

Introduction Peer-to-Peer networking has a great potential to make a vast amount of resources accessible [19]. Several years ago, file sharing applications like Napster [15] and Gnutella [8] impressively demonstrated the possibilities for the first time. Because of their success, Peerto-Peer mistakenly became synonymous for file sharing. However, the fundamental Peer-toPeer concept is general and not limited to a specific application type. Thus, a broader field of applications can benefit from using Peer-to-Peer technology. Content delivery [11], media streaming [22], games [14], and collaboration tools [9] are examples of applications fields that use Peer-to-Peer networks today. Although Peer-to-Peer networking is still an emerging area, some Peer-to-Peer concepts are already applied successfully in different contexts. Good examples are Internet routers, which deliver IP packages along paths that are considered efficient. Theses routers form a decentralized, hierarchical network. They consider each others as peers, which collaborate in the routing process and in updating each other. Unlike centralized networks, they can compensate node failures and remain functional as a network. Another example of decentralized systems with analogies to Peer-to-Peer networks is the Usenet [31]. Considering those examples, many Peer-to-Peer concepts are nothing new. However, Peerto-Peer takes these concepts from the network to the application layer, where software defines purpose and algorithms of virtual (non-physical) Peer-to-Peer networks. Widely used Web-based services such as Google, Yahoo, Amazon, and eBay can handle a large number of users while maintaining a good degree of failure tolerance. These centralized systems offer a higher level of control, are easier to develop, and perform more predictable than decentralized systems. Thus, a pure Peer-to-Peer system would an inappropriate choice for applications demanding a certain degree of control, for example “who may access what”. Although Peer-to-Peer systems cannot replace centralized system, there are areas where they can complement them. For example, Peer-to-Peer systems encourage direct collaboration of users. If a centralized system in between is not required, this approach can be more efficient because the communication path is shorter. In addition, a Peer-to-Peer (sub-) system does not require additional server logic and is more resistant to server failures. For similar reasons, because they take workload and traffic off from servers to peers, Peer-to-Peer could reduce the required infrastructure of centralized systems. In this way, Peer-to-Peer networks could cut acquisition and running costs for server hardware. This becomes increasingly relevant when one considers the growing number of end users with powerful computers connected by high bandwidth links.