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improved. Specifically, Jia et al. proposed a new micropayment system, called
CPay (an improved version of PPay), which has one significant new feature.
The new feature is that the broker judiciously selects the most appropriate
peer to be the owner of a coin. Specifically, the owner of a coin should be
one that is expected to stay in the system for a long period of time. Thus,
the broker's potential burden of checking coin owners' transactions can be
considerably reduced.
Figueiredo et al. [Figueiredo et al., 2005] also considered a payment-based
system to entice cooperation among peers. Specifically, each peer requiring
message forwarding service from other peers needs to pay real money to these
peers. Thus, as a peer stays in the P2P network and provides forwarding ser-
vice to other peers, it can gain money. Indeed, such a monetary gain represents
an incentive to enhance the availability of a peer in the network.
Saito [Saito, 2003] proposed an Internet-based electronic currency called
i-WAT, which can be used by users in a P2P system for “purchasing” services.
Each i-WAT message is an electronic ticket signed using OpenPGP. Saito et
al. [Saito et al., 2005] then extended the i-WAT system by adding a new feature
called “multiplication over time,” which means that a requesting peer's debt
(in terms of i-WAT units) increases over time. This feature then encourages
service providing peers to stay in the system for a longer period of time so as
to defer the redemption of i-WAT tickets, thereby increasing the gains from
the requesting peers.
5.2.1.4
Cost of Sharing
Varian [Varian, 2003] reported a simple but insightful analytical study on
disincentives in P2P sharing. Table 5.1 lists the notation used in Varian's
analysis.
TABLE 5.1: Notation used in Varian's analysis on disincentives for P2P
sharing.
Symbol
Definition
p
unit price
v
value of the item as perceived by each peer
n
number of peers in the system
D
total cost of producing all the items
D
n
d =
average development cost
k
number of peers in each group that share an item
t
sharing cost incurred by each member of a group
c
cost imposed by the central authority to
those peers who participate in sharing
π
profit derived by the central authority
Note: From Varian, 2003.
In Varian's model, a single item (e.g., a single music file) is considered and
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