Consensus Beyond Thresholds: Generalized Byzantine Quorums Made Live

Existing Byzantine fault-tolerant (BFT) consensus protocols address only threshold failures, where the participating nodes fail independently of each other, each one fails equally likely, and the protocol's guarantees follow from a simple bound on the number of faulty nodes. With the widespread deployment of Byzantine consensus in blockchains and distributed ledgers today, however, more sophisticated trust assumptions are needed. This paper presents the first implementation of BFT consensus with generalized quorums. It starts from a number of generalized trust structures motivated by practice and explores methods to specify and implement them efficiently. In particular, it expresses the trust assumption by a monotone Boolean formula (MBF) with threshold operators and by a monotone span program (MSP), a linear-algebraic model for computation. An implementation of HotStuff BFT consensus using these quorum systems is described as well and compared to the existing threshold model. Benchmarks with HotStuff running on up to 40 replicas demonstrate that the MBF specification incurs no significant slowdown, whereas the MSP expression affects latency and throughput noticeably due to the involved computations.Comment: To appear in the proceedings of SRDS 202

Forschungsbericht Universität Mannheim 2006 / 2007

Sie erhalten darin zum einen zusammenfassende Darstellungen zu den Forschungsschwerpunkten und Forschungsprofilen der Universität und deren Entwicklung in der Forschung. Zum anderen gibt der Forschungsbericht einen Überblick über die Publikationen und Forschungsprojekte der Lehrstühle, Professuren und zentralen Forschungseinrichtungen. Diese werden ergänzt um Angaben zur Organisation von Forschungsveranstaltungen, der Mitwirkung in Forschungsausschüssen, einer Übersicht zu den für Forschungszwecke eingeworbenen Drittmitteln, zu den Promotionen und Habilitationen, zu Preisen und Ehrungen und zu Förderern der Universität Mannheim. Darin zeigt sich die Bandbreite und Vielseitigkeit der Forschungsaktivitäten und deren Erfolg auf nationaler und internationaler Ebene

Solving consensus using structural failure models

Abstract Failure models characterise the expected component failures in fault-tolerant computing. In the context of distributed systems, a failure model usually consists of two parts: a functional part specifying in what way individual processing entities may fail and a structural part specifying the potential scope of failures within the system. Such models must be expressive enough to cover all relevant practical situations, but must also be simple enough to allow uncomplicated reasoning about fault-tolerant algorithms. Usually, an increase in expressiveness complicates formal reasoning, but enables more accurate models that allow to improve the assumption coverage and resilience of solutions. In this paper, we introduce the structural failure model class DiDep that allows to specify directed dependent failures, which, for example, occur in the area of intrusion tolerance and security. DiDep is a generalisation of previous classes for undirected dependent failures, namely the general adversary structures, the fail-prone systems, and the core and survivor sets, which we show to be equivalent. We show that the increase in expressiveness of DiDep does not significantly penalise the simplicity of corresponding models by giving an algorithm that transforms any Consensus algorithm for undirected dependent failures into a Consensus algorithm for a DiDep model. We characterise the improved resilience obtained with DiDep and show that certain models even allow to circumvent the famous FLP impossibility result