leecher in Tenen & Foxman 2014
eed to review some of the
fundamentals behind the *BitTorrent* protocol. At its bare minimum (as
it was described in the original specification by Bram Cohen) the
protocol involves a "seeder," someone willing to share something it its
entirety; a "leecher," someone downloading shared data; and a torrent
"tracker" that coordinates activity between seeders and
leechers.^[41](#fn-2025-41){#fnref-2025-41}^
Imagine a music album sharing agreement between three friends, where,
initially, only one holds a copy of some album: for example, Nirvana's
*Nevermind*. Under the centralized model of file sharing, the friend
holding the album would transmit two copies, one to each friend. The
power of *BitTorrent* comes from shifting the burden of sharing from a
single seeder (friend one) to a "swarm" of leechers (friends two and
three). On this model, the first leecher joining the network (friend
two, in our case) would begin to get his data from the seeder directly,
as before. But the second leecher would receive some bits from the
seeder and some from the first leecher, in a non-linear, asynchronous
fashion. In our example, we can imagine the remaining friend getting
some songs from the first friend and some from the second. The friend
who held the album originally now transmitted something less than two
full copies of the album, since the other two friends exchanged some
bits of information between themselves, lessening the load on the
original album holder.
When downloading from the *BitTorrent* network, a peer may receive some
bits from the beginning of the document, some from the middle, and some
from the end, in parts distributed among the members of the swarm. A
local application called the "client" is responsible for checking the
integrity of the pieces and for reassembling the them into a coherent
whole. A torrent "tracker" coordinates the activity between peers,
keeping track of who has what where. Having received the whole document,
a leecher can, in turn, become a seeder by sharing all of his downloaded
bits with the remaining swarm (who only have partial copies). The
leecher can also take the file offline, choosing not to share at
all.^[42](#fn-2025-42){#fnref-2025-42}^
The original protocol left torrent trackers vulnerable to charges of
aiding and abetting copyright
infringement.^[43](#fn-2025-43){#fnref-2025-43}^ Early in 2008, Cohen
extended *BitTorrent* to make use of "distributed sloppy hash tables"
(DHT) for storing peer locations without resorting to a central tracker.
Under these new guidelines, each peer would maintain a small routing
table pointing to a handful of nearby peer locations. In effect, DHT
placed additional responsibility on the swarm to become a tracker of
sorts, however "sloppy" and imperfect. By November of of 2009, *Pirate
Bay* announced its transition away from tracking entirely, in favor of
DHT and the related PEX and Magnetic Li
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