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In this paper we address the issue of content availability in p2p file sharing systems. Content availability is a public good: the copying of a file by one peer does not prevent another peer also from copying it; but contributing files to the common pool is costly. The asymptotic analysis of certain public good models for p2p file sharing suggests that when the aim is to maximize social welfare, a fixed contribution scheme in terms of the number of files shared per unity of time can be asymptotically optimal as the number of participants n grows to infinity. However, the enforcement of such an incentive scheme is not straightforward in a realistic p2p system, where no trusted software or central entity accounting for peers' transactions can be assumed and peers are free to change their identity with no cost. We present a realistic version of the fixed contribution scheme, which does not require the use of system memory but relies only on the time peers are consuming resources to ensure that they contribute adequately. We describe the functionality that should be supported for enforcement and discuss the additional incentive issues that arise in this context, proposing some practical solutions to address them. We also formulate a suitable economic model to estimate the efficiency-loss of the proposed mechanism (compared to the one achieved using the theoretically optimal schemes under complete and incomplete information) and provide some insights for the correct tuning of its basic parameters. Our first results indicate that the proposed mechanism constitutes a good compromise between economic efficiency and implementability and should lead to some interesting and practical solutions for providing incentives for content availability in p2p systems.

We consider how the OO notion of subtyping relates to lotos testing theory. In particular, we investigate which of the standard lotos preorders is a suitable instantiation of behavioural subtyping and argue that each of the main preorders, trace preorder, trace extension, reduction and extension, is in some way de cient. Then, in the light of pre and post condition based models of OO subtyping, we rework the basic interpretation applied to lotos behaviour descriptions. We argue that this re-interpretation enables reduction to be used as an instantiation of behavioural subtyping.

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In this paper we explore the incentive and architectural issues that arise in Consortia of Peer-to-Peer Wireless Local Area Networks. A P2P WLAN Consortium (PWC) is a community of WLAN Administrative Domains (ADs) that offer network access to each other's registered users. The great benefit of ubiquitous access that these roaming members enjoy compensates for their AD's cost of providing access to visitors. Existing roaming schemes utilize central authorities or bilateral contracts to control the parties' behavior. In contrast, a PWC forms a P2P community in which participating ADs are autonomous entities. They make independent decisions concerning the amount of resources (e.g. access bandwidth) they contribute. As a result, similarly to existing P2P systems, a PWC will suffer from abusive behavior (free riding) if no incentive mechanisms exist to ensure that ADs offer the amount of resources that is economically justified. We explore the use of flexible rules on reciprocity to guide domain policies. We develop a suitable economic model and derive rules that would bring the system to a near-optimum equilibrium by forcing peers to contribute in order to consume. We then discuss certain key implementation issues related to the selection of critical parameters and rules enforcement in such a distributed environment.

Abstract. We investigate assume-guarantee reasoning for global specifications consisting of conjunctions of local specifications. We present a sound and complete assume-guarantee rule that permits reasoning about individual modules for local specifications and draws conclusions on global specifications. We illustrate our approach with an example from the field of network congestion control, where different agents are responsible for controlling packet flow across a shared infrastructure. In this context, we derive an assume-guarantee rule for system stability, and show that this rule is valuable to reason about any number of agents, any initial flow configuration, and any topology of bounded degree.

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