Information Technology Reference
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•Peer-Approved: This is a reputation-based scheme in that each user
is associated with a rating which is computed using metrics such as the
number of requests successfully served by the user. A user can download
files from any owner who has a lower or the same rating. Thus, to gain
access to more files in the system, a user has to actively provide service
to other users so as to increase the rating.
•Service Quality: This is also a reputation-based scheme similar to
Peer-Approved. The major difference is that a file owner provides dif-
ferentiated service qualities to users with different ratings.
Theoretical analysis [Ranganathan et al., 2003] indicates that the Peer-
Approved policy with a logarithmic benefit function (in terms of number of ac-
cessible files) can lead to the optimal equilibrium where every user contributes
fully to the system. Simulation results also suggest that Peer-Approved gen-
erates performance (in terms of total number of files shared) comparable to
that of Token Exchange, which entails a higher di culty in practical imple-
mentation as it requires a payment system.
Becker and Clement [Becker and Clement, 2004] also suggested an inter-
esting analysis of the sharing behaviors using variants of the classical 2-player
Prisoner's Dilemma. Specifically, the P2P file sharing process is divided into
three different stages: introduction, growth, and settlement. In the introduc-
tion stage, the P2P network usually consists of just a few altruistic users
who are eager to make the network viable. Thus, sharing of files is a trusted
social norm. The payoffs of the two possible actions (supply files or not sup-
ply files) are depicted in Figure 5.3. Here, we have the payoffs ranking as:
R > T > S > P (note: T: Temptation, R: Reward, S: Sucker, P: Punishment).
Consequently, the Nash equilibrium profile is: (Supply, Supply). Notice that
the payoffs ranking in the original Prisoner's Dilemma is: T > R > P > S,
and as such, the Nash equilibrium is the action profile in the lower right corner
of the table.
Player 2
Action: Supply
Action: No Supply
1
2
g
g
Player 1
R
T
Action: Supply
1
g
R
S
S
P
Action: No Supply
2
g
T
P
FIGURE 5.3: Payoff table in the introduction stage [Becker and Clement,
2004].
In the growth stage, we can expect that more and more non-cooperative
users join the network. For these users, the payoffs ranking becomes: T > R >
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