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This paper describes a general approach to constructing cooperative services that span multiple administrative domains. In such environments, protocols must tolerate both Byzantine behaviors when broken, misconfigured, or malicious nodes arbitrarily deviate from their specification and rational behaviors when selfish nodes deviate from their specification to increase their local benefit. The paper makes three contributions: (1) It introduces the BAR (Byzantine, Altruistic, Rational) model as a foundation for reasoning about cooperative services; (2) It proposes a general three-level architecture to reduce the complexity of building services under the BAR model; and (3) It describes an implementation of BAR-B, the first cooperative backup service to tolerate both Byzantine users and an unbounded number of rational users. At the core of BAR-B is an asynchronous replicated state machine that provides the customary safety and liveness guarantees despite nodes exhibiting both Byzantine and rational behaviors. Our prototype provides acceptable performance for our application: our BAR-tolerant state machine executes 15 requests per second, and our BAR-B backup service can back up 100 MB of data in under 4 minutes.

This paper describes a general approach to constructing cooperative services that span multiple administrative domains. In such environments, protocols must tolerate both Byzantine behaviors when broken, misconfigured, or malicious nodes arbitrarily deviate from their specification and rational behaviors when selfish nodes deviate from their specification to increase their local benefit. The paper makes three contributions: (1) It introduces the BAR (Byzantine, Altruistic, Rational) model as a foundation for reasoning about cooperative services; (2) It proposes a general three-level architecture to reduce the complexity of building services under the BAR model; and (3) It describes an implementation of BAR-B, the first cooperative backup service to tolerate both Byzantine users and an unbounded number of rational users. At the core of BAR-B is an asynchronous replicated state machine that provides the customary safety and liveness guarantees despite nodes exhibiting both Byzantine and rational behaviors. Our prototype provides acceptable performance for our application: our BAR-tolerant state machine executes 15 requests per second, and our BAR-B backup service can back up 100 MB of data in under 4 minutes.

Allowing read operations to return stale data with low probability has been proposed as a means to increase availability in quorums systems. Existing solutions that allow stale reads cannot tolerate an adversarial scheduler that can maliciously delay messages between servers and clients in the system and for such a scheduler existing solutions cannot enforce a bound on the staleness of data read. This paper considers the possibility of increasing system availability while at the same time tolerating a malicious scheduler and guaranteeing an upper bound on the staleness of data. We characterize the conditions under which this increase is possible and show that it depends on the ratio of the write frequency to the servers' failure frequency. For environments with a relatively large failure frequency compared to write frequency, we propose K-quorums that can provide higher availability than the strict quorum systems and also guarantee bounded staleness. We also propose a definition of k-atomicity and present a protocol to implement a k-atomic register using k-quorums.

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