BitTorrent suffers from one fundamental problem: the long-term availability of content. This occurs on a massivescale with 38% of torrents becoming unavailable within the first month. In this paper we explore this problem by performing two large-scale measurement studies including 46K torrents and 29M users. The studies go significantly beyond any previous work by combining per-node, per-torrent and system-wide observations to ascertain the causes, characteristics and repercussions of file unavailability. The study confirms the conclusion from previous works that seeders have a significant impact on both performance and availability. However, we also present some crucial new findings: (i) the presence of seeders is not the sole factor involved in file availability, (ii) 23.5% of nodes that operate in seedless torrents can finish their downloads, and (iii) BitTorrent availability is discontinuous, operating in cycles of temporary unavailability. Due to our new findings, we consider it is important to revisit the solution space; to this end, we perform large-scale trace-based simulations to explore the potential of two abstract approaches.

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Peer-to-peer is a technology concept applied at different levels of the systems architecture. Its main characteristics are direct interaction and data exchange between peer systems. It is the basis for decentralised distributed computing. The concept is widely deployed in different contexts and no formal definition exists. This paper gives an overview of the different areas peerto-peer technology is used and introduces the main characteristics of peer-to-peer systems. It also discusses the issues and problems encountered when deploying peer-to-peer technology.