Information Technology Reference
In-Depth Information
5
Related Work
Reviewing the bibliography, several approaches have been proposed to combat the free
riding problem. Karakaya [2] et al. have categorised them into three main types: firstly,
incentive mechanisms based on monetary payments: one party offering a service to an-
other is remunerated and inversely, resources consumed must be remunerated or paid
for. Secondly, mechanisms based on reputation: it keeps information about the peer rep-
utation, and peers with a good reputation are offered better services. Thirdly, incentive
mechanisms based on differential services or reciprocity-based: peers that contribute
more get a better quality of service [14] [5] [8] [12] [13]. Our approach could be in-
cluded in this category, although its foundation is different and innovative.
Although some of the above approaches [5] [8] [13] are based on differential ser-
vices, they do not promote a cooperative behaviour between peers that improves the
download performance in the P2P System. And, in addition they do not achieve fair
service differentiation between peers.
Those remaining, more similar to our approach, propose incentive mechanisms that
encourage collaboration among peers. For example, 2Fast [12] is based on creating
groups of peers that collaborate in downloading a file. However, compared to our pro-
posal, it does not enforce fairness among the collector and helper peers, and in addition
it is not specified how the helper may reclaim its contributed bandwidth in the future.
Bit torrent [14] is also based on collaboration between peers. Its “tit-for-tat” policy of
data sharing works right when the peers show a reciprocal interest in a particular file.
However, in bit-torrent, the peers' download bandwidth is limited to their upload capac-
ity, thereby reducing the achievable download performance. However, in our approach,
the system's download capacity is not reduced to its upload capacity; And, using the
Rb
does not force a “mutual reciprocity” mechanism (like “tit-for-tat”); and thus the
bandwidth contributed by a peer can be used in later downloads.
EMule [18] also promotes cooperation between peers. It uses a credit system to re-
ward frequent uploaders and alleviates the free-riding problem. However, credits are
exchanged between two specific peers, so content trading can happen only between
peers that have mutual interests, and in addition it does not enforce fairness in band-
width sharing.
Finally, the work of Ma et al. [10], also provides service differentiation based on
the amount of services that each node has provided to a P2P community, and it uses a
game theoretic framework. However, while we use a cooperative approach that proposes
coalition formation, they propose a mechanism that makes different requesting users bid
for resources, creating a dynamic competitive game.
6
Conclusions
In this paper we have presented a coalition formation based incentive mechanism for
P2P file sharing systems. It is based on game theory and takes into account the rational
and self-interested behaviour of the peers. In [1], the initial idea of applying this model
to this problem was presented. Now, we have formally demonstrated the fairness of
the model using game theory and, more concretely, the concept of “core”. In addition,
