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hese rarest pieces will be selected first to download from the connected peers. Due
to the limitations of the local knowledge each peer has, the pieces chosen to down-
load may not be the rarest in the entire torrent.
A simple alternative to the rarest-first policy is to use a random piece selection
policy for downloading. his policy was used by the original BitTorrent client when
it was first released, but due to the superiority of the rarest-first policy it is not in
use by any clients today.
6.1.3 The Incentive Mechanism
here are many policies at work in a BitTorrent client that govern how peers con-
nect to other peers. One of the most important is the incentive mechanism, or
tit-for-tat policy. It is responsible for choosing peers to upload to, with the goal of
ensuring that peers who upload (contribute) to the system are more likely to be able
to download. his game-theoretic method of encouraging sharing and fairness is
built into the system to discourage free-riding peers from not uploading.
Each client notifies other peers that have pieces the client needs of the client's
interest in downloading from the peer. A downloading client also monitors the
download rates it receives from its neighboring peers. he client then allows uploads
to the peers that are interested in downloading, and that it is receiving the highest
download rates from. his is referred to as unchoking the peer. here are a limited
number of unchoke slots available, so a downloading peer's neighbors will compete
for its unchoke slots by uploading to it. In addition, in order to explore the con-
nected peers and find better peers to upload to, the client will periodically choose
one peer at random to unchoke, which is known as optimisticunchoking .
Each peer will have to make these decisions based only on the limited local
knowledge it has of the system, specifically the peers it is connected to and their
current uploading rates to it. A poor policy for making this decision could lead
to networks that are inefficient in distributing pieces throughout the system, or
are easily susceptible to disconnection due to departing or failing peers. Either of
these problems would lead to a system which makes inefficient use of the uploading
bandwidth available to replicate the file.
6.1.4 Networks in the Torrents
Given the complex relations among peers, BitTorrent actually maintains four net-
works within a torrent. Previous studies have focused on only one or two of the
following networks, but we will investigate the properties and evolution of all four
networks.
Connection Network . his is the network of neighbors that each peer main-
tains. hese neighbors are chosen randomly by the tracker from the list of peers in
the torrent. Each peer creates connections to the peers returned by the tracker (up
to its limit on initiating connections) and also makes return connections to other
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