Incentives - Peer-to-Peer Computing: Applications, Architecture, Protocols, and Challenges - page 74

Chapter 5
Incentives
5.1 Introduction
The highly flexible features of P2P computing such as a dynamic popula-
tion (users come and go asynchronously at will), dynamic topologies (it is im-
practical, if not impossible, to enforce a fixed communication structure), and
anonymity, come at a significant cost—autonomy, by its very nature, is not
always in harmony with tight cooperation. Consequently, ine cient or lack of
cooperation could lead to undesirable effects in P2P computing. Among them
the most critical one is “free-riding” [Feldman and Chuang, 2005,Ramaswamy
and Liu, 2003] behavior. Loosely speaking, free-riding occurs when some users
do not follow the presumed altruistic cooperation rules such as sharing files
voluntarily, sharing bandwidth voluntarily, or sharing energy voluntarily, so
as to benefit the whole community.
Such altruistic sharing actions, presumably, would bring indirect and intan-
gible (and even remote) returns to the users. For instance, if everyone shares
files voluntarily, every user would eventually benefit from the high availability
of a large and diverse set of selections. Unfortunately, there are some users
that do not believe or buy in to such utopia-like concepts and would, then,
“rationally” choose to just enjoy the benefits derived from the community,
but not contribute their own resources.
To deter or avoid free-riding behaviors, the P2P community has to provide
some incentives—returns for resource expenditure that are, more often than
not, tangible and immediate [Golle et al., 2001]. Such incentives would then
motivate an otherwise selfish user to rationally choose to cooperate because
such cooperation would bring tangible and immediate benefits. To mention an
analogy, in human society, getting pay for our work is a tangible and immediate
incentive to motivate us to devote our energy, which could otherwise be spent
on other activities. Indeed, it is important for the incentive to be tangible
so that a user can perform a cost-benefit analysis—if benefit outweighs cost,
the user would then take a cooperative action [Krishnana et al., 2003]. It is
also important for the incentive to be immediate (though this is a relative
concept) because any resource is associated with an opportunity cost in that
if immediate return cannot be obtained from a cooperative action, then the
user might want to save the effort for some other private tasks.
75