what-when-how
In Depth Tutorials and Information
peers that connect to it (up to its maximum neighbor limit). All neighbor connec-
tions are bi-directional, so the Connection network is undirected.
Interest Network.
his network represents the interest that peers have in other
peers. Each peer maintains a list of the pieces stored by its neighboring peers. A
peer is interested in any neighboring peer that has a piece it does not have, and
so this network is a subset of the Connection network. Since interest can be uni-
directional, the Interest network is directed.
Unchoked Network.
his network is formed by the incentive mechanism pres-
ent in BitTorrent. Each uploading peer assigns its limited number of unchoke slots
to certain neighboring peers in an effort to maximize the downloads that it receives
from them. Only peers that are interested in receiving an upload are unchoked, so
this network's transpose is a subset of the Interest network. Since unchoking can be
uni-directional, the Unchoked network is directed.
Download Network.
his network is formed by the peers that are download-
ing from other peers. Since a peer has to be unchoked before it can download, this
network is a subset of the Unchoked network's transpose. Since downloading can
be uni-directional, the Download network is directed.
We emphasize that the Connection and Unchoked networks are the most impor-
tant of these four networks. he Connection network forms the neighbor set for all of
the peers in the system, and is a superset of the other three networks. he Unchoked
network is necessary for the uploading and downloading of data from other peers,
and so it is very important for the scalability and efficiency of BitTorrent.
6.2 DataExchangeamongPeers:The
Piece-LevelExamination
We start our examination from the microscope piece selection, which decides the
order of pieces to download and plays an important role in achieving high-efficient
downloading. Each peer will have to make this decision based only on the local
knowledge it has of the system. An inadequate policy could lead to some pieces
becoming poorly replicated, and therefore almost unavailable, while others are
overly replicated, leading to starvation in areas of the system where new pieces are
needed. To understand how the policy for choosing pieces in BitTorrent affects the
system, it is necessary to examine the system-wide population of pieces available.
his microscopic information would help to understand the dynamics and evolu-
tion of the torrent, and especially the effectiveness of the policy used by BitTorrent
to ensure an even distribution of pieces.
We present a systematic measurement study on the distribution and evolution
of the piece population in BitTorrent. Our measurement is based on real BitTorrent
data gathered from both regular Internet and controlled PlanetLab torrents. he
data is collected by multiple administered clients distributed in different parts of
the network, which collectively offer a global view of the piece distribution.
