Information Technology Reference
In-Depth Information
Ta b l e 1 .
No Coalition Mechanism Downloaded Bytes
No Coalition
Population 1 Population 2
Free Rider
157 Gb
123 Gb
Collaborative
156 Gb
92 Gb
Adaptive
91Gb
total
313Gb
306 Gb
No Coalitions (NC), where no incentive mechanism is considered and Coalitions (C),
which implements our proposal. After repeating the simulation experiments 100 times
we take the average to give the results. To compare how the incentive mechanisms and
the user behaviours affect the P2P system, two main metrics have been considered:
Downloaded Bytes and Average Time. In addition, we analyse the Work Progress; This
measure shows how the simulations evolve towards the final target.
Number of Downloaded Bytes.
Figure 2 shows the evolution of the downloaded bytes
distribution for No Coalitions mechanism for experiments run with Population 1 on the
left and Population 2 on the right. Similar figures for Coalitions in Figure 3. Down-
loaded bytes can be interpreted as the benefit obtained from the system. Next, we anal-
yse these results for each of the different populations.
In Population 1 all work is done by the collaborative users, since free-riders do not
collaborate. Figure 2 left shows the evolution of the distribution of the downloaded
bytes is round 50 % for both users during the simulation. This means, the collaborative
users do all the work, and the benefits are shared equally with the free riders. However,
when we run the Coalition mechanism, Figure 3 left, free riders are stopped, the per-
centage of bytes downloaded by free riders drastically decreases after the first 100 steps
of simulation. This demonstrates how the coalition formation prevents free-riders from
obtaining more bytes as simulation time advances, and so from fully using the system's
resources.
When Adaptive users are simulated, this is Population 2, distribution of downloaded
bytes are affected as shown in figures 2 right and 3 right. With respect to collaborative
and adaptive users, both are 30 % of the population, they do all the work and share
more or less equally the benefits with free riders. In figure 3 right the evolution of the
distribution of the downloaded bytes shows how free riders are again stopped, as in
Population 1, and this means that the benefit is shared between the collaborative and
adaptive users, these are those that are uploading files. In addition, collaborative users
increase the percentage of downloaded bytes during the simulation; However, adaptive
users first increase and after decrease the percentage. This is due to the behaviour of
adaptive users, which are penalised when they are not sharing enough.
Tables 1 and 4.2 summarise the downloaded bytes per populations and per behaviour.
In both tables it can be observed that the bytes downloaded by free riders are slightly
reduced in Population 2 with respect to Population 1, this is because there are 10% less
users in this population, the average bytes per user is very similar. With Population 1 it
can also be observed that the coalition mechanism reduces the total bytes downloaded
to 50,54%, with respect to No Coalitions, but 83,61% of this reduction is due to the
Free Riders detection. This shows again how the algorithm prevents free-riders from
