To find a file in BitTorrent, users access web sites
which act as global directories of available files. In Table
1, we show for the most popular of these web sites
the number of different files and the number of active file
transfers at a certain time. In december 2004, the top two
of these web sites went offline. In this paper we assume
Suprnova as the directory web site.
The Suprnova web site uses a mirroring system to balance
user requests across its mirror sites. The web pages
on Suprnova show for each available file the name and
size, the current numbers of downloaders and seeds, and
the name of the person who uploaded the file. To start
the download of a file, a user clicks on a link pointing
to .torrent meta-data file. These meta-data files are
not stored on Suprnova or its mirrors, but are distributed
among a number of .torrent file servers. In turn, each
.torrent file points to a tracker, which keeps a global
registry of all the downloaders and seeds of the corresponding
file. The tracker responds to a user’s request
with a list of some of the peers having (part of) the requested
file, with whom the user can establish direct connections
to barter for chunks of the file. One tracker can
supervise the simultaneous downloads of multiple files.
New content is injected into BitTorrent by uploading a
.torrent file to the Suprnova web site and creating a
seed with the first copy of the file. In order to reduce the
pollution level, new content is first manually inspected by
moderators, who weed out fake content, content with low
perceptual quality, and content with incorrect naming. A
normal user who injects content is called a moderated
submitter. To lower the burden on the moderators, a user
who frequently injects correct content is promoted to the
rank of unmoderated submitter, and is allowed to directly
add content. Unmoderated submitters can request a promotion
to moderator status to existing moderators.
Together, BitTorrent and Suprnova form a unique infrastructure
that uses mirroring of the web servers with
its directory structure, meta-data distribution for load balancing,
a bartering technique for fair resource sharing,
and a P2P moderation system to filter fake files.