  BitTorrent suffers from one fundamental problem: the long-term availability
of content. This occurs on a massive-scale 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.
