Designing and Implementing Cooperative Services

Cooperative services are an increasingly popular way to deploy applications. In these services, computers controlled by different entities are supposed to work together to achieve a common goal, such as file-sharing [6, 12], mesh routing [2], and peer-to-peer backup [1, 7]. The resulting systems can be more fault-tolerant, scalable, and less expensive than traditional client-server approaches. However, a cooperative service may never see these benefits if it does not tolerate Byzantine users who may disrupt the service or selfish ones who may try to use it without contributing their fair share. One of the key challenges in designing a cooperative service is ensuring that participants actually cooperate. Several deployed applications [6, 12] and research prototypes [7, 17] include incentives and punishments to encourage cooperation. However, selfish participants still frequently find ways to cheat. For example, in the KaZaA [12] network almost half the users falsify their contributions by using a hacked binary [13]. In BitTorrent [6], researchers have found ways to free-ride [19]. These vulnerabilities are symptoms of a deeper problem: almost no cooperative service rigorously shows that its incentives and punishments are sufficient to induce cooperation. In my research, I aim to build cooperative systems that tolerate the actions of Byzantine users while removing incentives for selfish users to cheat. In this adversarial environment, constructing systems that work well and are robust requires me to step back and forth between practice and theory, something that distinguishes my research from that of many others who design these systems. I believe that pursuing the practice-theory cycle—letting practice guide theory and vice versa—is necessary in building robust and practical systems. This approach requires me to be persistent, resourceful, and creative. Drawing upon these traits and my training so far, I plan to i) develop the theoretical framework necessary to reason about selfish and Byzantine behaviors in cooperative services, ii) design efficient and scalable mechanisms in this framework to address practical issues in building real services, and iii) implement large-scale prototypes that are competitive with if not better than existing less robust works.