Information Technology Reference
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as a whole exhibits good behaviour when self-interested nodes pursue self-interested
strategies ([7]).
Our approach proposes the application of a coalition formation scheme based on
game theory to P2P file sharing systems (in [1] we presented an early version of this
work). The main idea of the coalition formation scheme is the fact that peers which
contribute more get a better experience. We define a “responsiveness bonus” that reflects
the peer's overall contribution to the system, and we use the game theory utility concept
to calculate it. It is possible to form a coalition among peers with a re-distribution
of the number of bytes to be transferred. A peer that participates in a coalition lends
“bandwidth” to other peers of the coalition, in exchange for utility; and this utility
will increase its responsiveness bonus. The coalition formation scheme rewards the
peers with a higher responsiveness bonus (therefore giving them greater bandwidth to
download files), and penalises the ones that only consume resources, decreasing their
responsiveness bonus and consequently their bandwidth.
The proposed incentive mechanism encourages cooperative behaviour between the
peers preventing the free-riding problem. Using the game theory concept of “core”,
the peers forming the coalition get in return a fair utility in relation to the bandwidth
they supply (achieving fairness in bandwidth sharing); And in addition, it allows the
formation of coalitions of peers that help each other in downloading files, increasing
the performance of the P2P network.
2
Downloading with Coalitions
In this section, we describe the model of the environment in which the system is de-
ployed and the mechanism of coalition formation among peers. We firstly describe a
simplified situation, illustrating the advantages of forming coalitions for P2P down-
loads and the way of computing and dividing the utility or profit obtained by peer that
participates in the coalition. Secondly, and in more general terms, we describe the coali-
tion formation process and how the data and the bandwidth are distributed among the
coalition members.
2.1
P2P Network Type
For our work, we have selected a P2P system with a partially centralised architecture
and an unstructured network. The first characteristic is related to the degree of central-
isation of the peer's network, and the second with the fact that the network is created
in a non-deterministic way.When a peer wants to download a file, it directs its request
to a supernode and this searches the file in its index (a supernode is a peer that acts
as a central index for files shared for a subpart of the peer network). When the file is
located, supernode sends to the “requester” peer an indexed result with the set of nodes
that store the requested file. Then, the requester peer opens a direct connection with one
or more peers that hold the requested file, and proceeds to download it.
