Information Technology Reference
In-Depth Information
Dp
i
=
Dp
i
+
s
x
s
/
|
S
|
(9)
∈
S
Rb
i
is a value included in the interval
[0
..
1]
. The correction of the bandwidth is only ap-
plied to the download bandwidth
Rb
i
b
ou
i
(it makes no sense to correct the upload band-
width, because we would be decreasing the upload capacity of the collaborative peers).
Initially the
Rb
i
of the peers (uploading/downloading) is 1
4
. A higher responsiveness
bonus (
Rb
i
closer to 1) will mean that
p
i
will be able to fill all its reserved bandwidth,
since it can add more peers to the coalition in order to complete its bandwidth, reducing
the download time. Otherwise, an
Rb
i
closer to
0
will limit the possibility of adding
peers to the coalition (in fact, in some cases it will avoid creating any coalition for
the download). In this way, our incentive mechanism promotes cooperation taking into
account the selfish behaviour of the peers.
Rb
i
(
Up
i
,Dp
i
,Fs
i
)=
⎧
⎨
1
if
(
Up
i
−
Dp
i
)
≥
0
0
if
(
Up
i
−
Dp
i
)
<
0
∧
Up
i
=0
∧
Fs
i
=0
(10)
1
if
(
Up
i
−
Dp
i
)
<
0
∧
Up
i
=0
∧
Fs
i
>
0
⎩
Up
i
·γ
Dp
i
if
(
Up
i
− Dp
i
)
<
0
∧ Up
i
>
0
The equation 10 computes
Rb
i
in relation to
Dp
i
,
Up
i
,
γ
and
Fs
i
(the number of files
shared by peer i).
If
Up
i
-
Dp
i
≥
0
, it means that the peer is contributing to the system more than it is
consuming from it, and so
Rb
i
=1
. If,
Up
i
=0
, the peer has not contributed anything
to the system and, if, in addition, the number of shared files is 0 obviously the peer
is a free-rider and its
Rb
i
must be 0. Conversely, if the peer has not contributed to the
system, but the number of shared files is not 0, it means that the peer wants to contribute
to the system but its shared files have still not been downloaded by other peers; So its
Rb
i
must stay at 1. Finally, if the peer has contributed to the system, but less than what
it has been consuming from it, its
Rb
i
will be proportional to
Up
i
/Dp
i
5
.Thevariable
γ
allows us to regulate this formula in order to increase or decrease the proportional
relation between the benefit,
Up
i
, and the penalty,
Dp
i
.
4
Performance Evaluation
In this section we describe the simulations we performed and the corresponding results.
In order to simulate our coalition formation model for P2P file sharing, we have defined
and implemented a generic P2P simulator for service oriented networks. The simulation
tool is presented in detail in [16]. Additionally, it should be noted that we are dealing
with situations which are different from traditional system simulators, since, we are also
trying to model the user behaviour. For this reason, and in order to model the user as
close to reality as possible, the peers are classified in three categories according to their
4
Otherwise their bandwidths would be reduced from the beginning, and the download times of
the files would be higher (compared to the scheme without coalitions).
5
This value is always
<
1.
