| As network environments become increasingly hostile, even well secured distributed systems are likely to suffer at least one compromised subcomponent. During the past several years, Byzantine fault-tolerant replication has emerged as a promising technique for constructing intrusion-tolerant systems that function correctly even when an attacker controls part of the system. Prior to our work, Byzantine replication systems were based on flat (nonhierarchical) protocols and offered high performance only when deployed on local-area networks.;This dissertation presents the first two hierarchical Byzantine fault-tolerant replication architectures that scale to systems that span multiple wide-area sites. Our first architecture, Steward, confines the effects of a malicious replica to its local site, reduces message complexity of wide-area communication, and allows read-only queries to be performed locally within a site for the price of additional commodity hardware. Our second architecture improves upon Steward by providing customizability of the fault tolerance approach used within each site and on the wide area and by including new optimizations. Prototype implementations are evaluated in several network topologies and compared with the previous state of the art. The experimental results show an order of magnitude improvement over flat Byzantine replication protocols in typical wide-area deployments. |
