We analyze the ability of peer to peer networks to deliver a complete file among the peers. Early on we motivate a broad generalization of network behavior organizing it into one of two successive phases. According to this view the network has two main states: first centralized - few sources (roots) hold the complete file, and next distributed - peers hold some parts (chunks) of the file such that the entire network has the whole file, but no individual has it. In the distributed state we study two scenarios, first, when the peers are patient'', i.e, do not leave the system until they obtain the complete file; second, peers are impatient'' and almost always leave the network before obtaining the complete file.
A lot of work has been devoted to the area of file sharing in P2P networks. Many experimental papers provide practical strategies and preliminary results concerning the behavior of these kind of networks. @cite_10 for instance, the authors essentially describe properties like liveness and downloading rate by means of extended experiments and simulations under several assumptions.
@cite_10
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Popular content such as software updates is requested by a large number of users. Traditionally, to satisfy a large number of requests, lager server farms or mirroring are used, both of which are expensive. An inexpensive alternative are peer-to-peer based replication systems, where users who retrieve the file, act simultaneously as clients and servers. In this paper, we study BitTorrent, a new and already very popular peer-to-peer application that allows distribution of very large contents to a large set of hosts. Our analysis of BitTorrent is based on measurements collected on a five months long period that involved thousands of peers. We assess the performance of the algorithms used in BitTorrent through several metrics. Our conclusions indicate that BitTorrent is a realistic and inexpensive alternative to the classical server-based content distribution.