Person:

Parkes, David

Establishing trust amongst agents is of central importance to the development of well-functioning multi-agent systems. For example, the anonymity of transactions on the Internet can lead to inefficiencies; e.g., a seller on eBay failing to ship a good as promised, or a user free-riding on a file-sharing network. Trust (or reputation) mechanisms can help by aggregating and sharing trust information between agents. Unfortunately these mechanisms can often be manipulated by strategic agents. Existing mechanisms are either very robust to manipulation (i.e., manipulations are not beneficial for strategic agents), or they are very informative (i.e., good at aggregating trust data), but never both. This paper explores this trade-off between these competing desiderata. First, we introduce a metric to evaluate the informativeness of existing trust mechanisms. We then show analytically that trust mechanisms can be combined to generate new hybrid mechanisms with intermediate robustness properties. We establish through simulation that hybrid mechanisms can achieve higher overall efficiency in environments with risky transactions and mixtures of agent types (some cooperative, some malicious, and some strategic) than any previously known mechanism.

Plug-in hybrid electric vehicles are expected to place a considerable strain on local electricity distribution networks, requiring charging to be coordinated in order to accommodate capacity constraints. We design a novel online auction protocol for this problem, wherein vehicle owners use agents to bid for power and also state time windows in which a vehicle is available for charging. This is a multi-dimensional mechanism design domain, with owners having non-increasing marginal valuations for each subsequent unit of electricity. In our design, we couple a greedy allocation algorithm with the occasional "burning" of allocated power, leaving it unallocated, in order to adjust an allocation and achieve monotonicity and thus truthfulness. We consider two variations: burning at each time step or on-departure. Both mechanisms are evaluated in depth, using data from a real-world trial of electric vehicles in the UK to simulate system dynamics and valuations. The mechanisms provide higher allocative efficiency than a fixed price system, are almost competitive with a standard scheduling heuristic which assumes non-strategic agents, and can sustain a substantially larger number of vehicles at the same per-owner fuel cost saving than a simple random scheme.

A common challenge in distributed work systems like P2P file-sharing communities, or ad-hoc routing networks, is to minimize the number of free-riders and incentivize contributions. Without any centralized monitoring it is difficult to distinguish contributors from free-riders. One way to address this problem is via accounting mechanisms which rely on voluntary reports by individual agents and compute a score for each agent in the network. In Seuken et al. [11], we have recently proposed a mechanism which removes any incentive for a user to manipulate the mechanism via misreports. However, we left the existence of sybil-proof accounting mechanisms as an open question. In this paper, we settle this question, and show the striking impossibility result that under reasonable assumptions no sybil-proof accounting mechanism exists. We show, that a significantly weaker form of K-sybil-proofness can be achieved against certain classes of sybil attacks. Finally, we explain how limited robustness to sybil manipulations can be achieved by using max-flow algorithms in accounting mechanism design.