On the effectiveness of service differentiation based resource-provision incentive mechanisms in dynamic and autonomous P2P networks

Abstract

Intrinsically, P2P (Peer-to-Peer) networks are anonymous, dynamic and autonomous, which has the following implications: users can change their identities with near zero cost (cheap pseudonyms); most interactions should be one-time (that is, each peer has no idea about other peers’ behavior history, except their current behaviors); and all behaviors and actions are all endogenous, voluntarily chosen and determined by independent and rational peers. On the other hand, service differentiation based incentive mechanisms are proposed in P2P networks, which basically could be provided with two ways: punish defect behavior (punishment-based scheme), or reward cooperative behavior (reward-based scheme). Then the naturally resulted question is that: under the above P2P networking environment, how to effectively design service differentiation based resource-provision incentive mechanisms? Our contributions are threefold. First, we found that the traditional service differentiation based incentive schemes cannot successfully encourage peers to contribute resource to the whole system, irrespective of punishment-based and reward-based schemes. Then, if peers can voluntarily join the system, and small entry fee is set for participation, we obtained that the performance of punishment-based scheme (first providing high-level service plus punishment) is always better than that of reward-based scheme (first providing low-level service plus reward). Finally, unlike the existing result that was based on persistent users’ identities and truly repeated interactions, we illustrate that punisher’s average payoff in punishment-based scheme is almost same as the ideal but unfeasible case of reward-based scheme: rewarder could selectively reward other rewarders, and reward cost is zero.

Introduction

P2P systems are self-organizing and distributed resource-sharing networks. By pooling together the resources of many autonomous users, they are able to provide an inexpensive and highly scalable platform for distributed computing, storage or data-sharing, etc. Note that there are two extreme cases in resource management: resource allocation (allocation of the existing resource) and resource provision (provision of resource shared by all participants). In the first case, the designer should decide whether and what percentage of a good (with given predefined capacity) each peer should consume. In the second case, the designer’s task is to entice independent participant to provide resource (with its’ right share). In this paper, we focus on the latter case. Furthermore, in its simplest case, each peer acquires and contributes exactly the same amount of resource. Even for this extremely simple case, stimulating participants to voluntarily provide resource is still a very challenging task in anonymous, dynamic and autonomous P2P systems.

In P2P systems, it is imperative for peers to voluntarily contribute resources (e.g., storage, bandwidth and content, etc.). However, intuitively, each peer would prefer to “free ride” on the contribution of other peers by consuming available resources and services without contributing anything, and thus avoid the corresponding costs. It was reported that nearly 70% of Gnutella users share nothing with other users (these users simply free-ride on other users who share information), and nearly 50% of all file search responses come from the top 1% of information sharing nodes [1]. In following-up study (5 yr later), it was found that 85% of users share nothing [2], which implies the free-riding problem had got worse in the intervening years.

Generally, lack of cooperation is one of the key problems that confronts today’s P2P systems. Incentive mechanisms play a crucial role to encourage cooperation among autonomous nodes. Specifically, a simple rule-based (differential services based) incentive mechanism is preliminarily advocated by [3], to encourage the resource provision in PlanetLab (http://www.planet-lab.org/), the most popular shared network testbed. However, the above work did not investigate deeply how to effectively design service differentiation based resource-provision incentive mechanisms in anonymous, dynamic and autonomous P2P systems.

General P2P networks should be anonymous, dynamic and autonomous. By anonymous it means that, ideally, users would like to be anonymous, and not accountable for their actions, that is, users could change their identities with near zero cost (cheap pseudonyms). By dynamic it means that most interactions among peers are one-time, that is, each peer has no idea about other peers’ behavior history, except their current behaviors. By autonomous it means that there is no central management entity to assign peers to different classes. Specifically, each strategy and action are voluntarily chosen and determined by independent, rational and autonomous peers, and all behaviors are all endogenous, that is, no exogenous organization can effectively enforce the punishment and/or reward.

Under the above networking environment, basically in the simplest case, we could define two categories of services: high-level and low-level. Naturally, from practical viewpoint, the following two service differentiation based incentive schemes are possible:

• Initially, all peers will be served with high-level class, and then, according to peers’ behaviors, voluntary punishers lower the defectors’ service level provided by those punishers, which is called punishment-based scheme in our paper;

• Initially, all peers will be served with lower-level class, and then, according to peers’ behaviors, voluntary rewarders promote the cooperators’ service level provided by those rewarders, which is called reward-based scheme. Note that, in reward-based incentive scheme, the reason why low-level service should be initially provided, lies in that, in dynamic and autonomous P2P networks, no exogenous organization exists to enforce the punishment and/or reward, like reimbursing the cooperative behaviors with some out-of-band resource (or money), thus, rewarders have to set aside some resource for their rewarding behaviors in future, which lead to the fact that those peers have to provide relative lower-level service initially.

Then, under the above anonymous, dynamic and autonomous P2P environments, the naturally resulted question is: for the above two service differentiation schemes, do they both perform well? or which is better to incentivize peers’ cooperative (or reciprocative) behavior? This paper attempts to answer the above question based on evolutionary game model. Our contributions are threefold. First, we prove that, when there exists punishment (or reward) cost, traditional service differentiation based incentive schemes might not work, that is, cannot stimulate peers to provide resource. Then, if peers could voluntarily join the system, and small entry fee is set for participation, we got that the performance of first providing high-level service plus punishment scheme is better than that of first providing low-level service plus reward scheme. Finally, unlike the existing conclusion that was based on the persistent users’ identities and truly repeated interactions, we illustrate that punisher’s average payoff in punishment-based scheme is almost same as the ideal but unfeasible case of reward-based scheme: rewarder could selectively reward other rewarders, and reward cost is zero.

The paper is organized as follows. Section 2 briefly describes the related work of service differentiation based incentive mechanisms and their differences from our work. Section 3 provides the architecture of service differentiation based incentive mechanisms. Specifically, we briefly summarize the features of anonymous, dynamic and autonomous P2P environment, and then analyze the reason why, in the above environment, traditional service differentiation based incentive mechanisms could not stimulate peers to provide resource to the whole system. Based on the public goods game, we propose the analytical models of punishment-based and reward-based incentive mechanisms for resource provision, and discuss system design of the entry fee enabled and service-differentiation based incentive mechanisms which aim at stimulating peers to contribute resource in community-based P2P applications with the feature of “contributing while consuming”. In Section 4, we theoretically analyze the dynamics of the system, through inferring the fixation probability between pair of pure strategies. The simulation and theoretical results in Section 5 illustrates that the voluntary principle plus small entry fee in punishment based scheme can effectively encourage peers to provide resource. Section 6 briefly, discusses the related problems in the entry fee enabled and service-differentiation based incentive mechanisms. Finally, we briefly conclude this paper.