Availability Analysis of Network-Attack-Resilient Byzantine Fault Tolerant Systems

Byzantine Fault Tolerant (BFT) systems are used in applications that need to maintain high availability even in the presence of failures or compromises. BFT systems are typically designed to tolerate a preconfigured threshold number of faulty replicas (f), where the $f$ faulty replicas may behave arbitrarily (potentially maliciously). However, as BFT system designs become more complex, it is important to evaluate the impact of such designs on the overall system availability with respect to standard crash faults. In this paper, we analyze three BFT systems that are designed to withstand both system compromises and network attacks: Spire, Confidential Spire, and Decoupled Spire. These network-attack-resilient BFT systems utilize a combination of on-premises and data center replicas to reduce cost and simplify deployment for system operators. We develop an availability model that accounts for differences between the availability of on- premises and data center replicas and analyze Spire, Confidential Spire, and Decoupled Spire under this model. Key system design insights from the analysis are that network-attack-resilient BFT configurations can meet the availability requirements of critical systems, as they provide higher availability than Classical BFT with less available servers. We show also that network-attack- resilient BFT configurations with fewer geographic sites provide higher availability, and that Confidential Spire has the highest availability, followed by Spire and then Decoupled Spire.