Improving File Sharing Experience with Incentive Based Coalitions - Agents and Artificial Intelligence - page 287

2. To split the file size Z , size ( Z )= T , between the coalition members, p i ∈

S .
(a) We estimate the minimum amount of time needed to transfer Z as a function
of the known bandwidth limits. Following this, the minimum amount of time
can be estimated as follows:
t i = T/
i∈S

min( b i i ,b out ) (7)

i

This estimated time is the same initially for all the peers in the coalition S 2 .
(b) Once we know the estimation of the time for each peer, we can carry out a
partitioning of the file taking into account the capacities of each peer. Every
peer will have to transfer a number of bytes b i i ∗ t i . The file is divided into

blocks of this size that are assigned to the peers.
3. To inform each coalition member of the size of the block to be transferred. p 0
communicates to each peer member of S the number of bytes to be transferred.

3 The Incentive Mechanism

As we have already mentioned our incentive mechanism is based on providing a better
quality of service to the peers that participate in the coalitions. In order to achieve
this, we define a Responsiveness Bonus , Rb , for every peer. This value reflects the
peer's overall contribution to the system (i.e. how much work it has carried out for the
other peers in the system). In accordance with the above model, in the proposed payoff
division each peer obtains a utility which is proportional to the resources that it supplies.
Therefore, the peer p i that supplies a greater bandwidth (uploading peers) will obtain a
greater utility, and this utility will increase its Rb . Conversely, the value of Rb i should
be reduced when p i acts as a downloading peer and does not contribute.We consider
that an auditing authority is responsible for storing and updating the Rb , using proper
methods to control the concurrency.
So the value of Rb i will be calculated as a heuristic function of x i that can be adjusted
with data from the real system behaviour or from simulation results. This uses the x i
values obtained by the uploading peers of the coalition as uploading points, Up i .For
the downloading peer of the coalition its downloading points, Dp i , are calculated as
the average of the utility obtained by the uploading peers of the coalition. Each peer
p i accumulates Dp i and Up i points by adding the points obtained in each coalition
formation process in which it participates 3 .

Up i = Up i + x i (8)

2 b ou i will be used to download files from the P2P sharing file system, in the case p i does not
have the file previously and it works for another node in the coalition (it will be 0 if the peer
had the file).

3

Since a new peer that joins the coalition formation system will have its uploading and down-
loading points set to 0, we allow the peers to download a minimum amount set to a parameter
MinDownload .