Ohad Rodeh

The Horus and Ensemble projects: accomplishments and limitations

The Horus and Ensemble efforts culminated a multi-year Cornell research program in process group communication used for fault-tolerance, security and adaptation. Our intent was to understand the degree to which a single system could offer flexibility and yet maintain high performance, to explore the integration of fault tolerance with security and real-time mechanisms, and to increase trustworthiness of our solutions by applying formal methods. Here, we summarize the accomplishments of the effort and evaluate the successes and failures of the approach.

Using AVL Trees for Fault Tolerant Group Key Management

Abstract.In this paper we describe an efficient algorithm for the management of group keys for group communication systems. Our algorithm is based on the notion of key graphs, previously used for managing keys in large Internet-protocol multicast groups. The standard protocol requires a centralized key server that has knowledge of the full key graph. Our protocol does not delegate this role to any one process. Rather, members enlist in a collaborative effort to create the group key graph. The key graph contains n keys, of which each member learns log2n of them. We show how to balance the key graph, a result that is applicable to the centralized protocol. We also show how to optimize our distributed protocol, and provide a performance study of its capabilities.

Efficient update diffusion in byzantine environments

We present a protocol for diffusion of updates among replicas in a distributed system where up to b replicas may suffer Byzantine failures. Our algorithm ensures that no correct replica accepts spurious updates introduced by faulty replicas, by requiring that a replica accepts an update only after receiving it from at least b+1 distinct replicas (or directly from the update source). Our algorithm diffuses updates more efficiently than previous such algorithms and, by exploiting additional information available in some practical settings, sometimes more efficiently than known lower bounds predict.

The architecture and performance of security protocols in the ensemble group communication system: Using diamonds to guard the castle

Ensemble is a Group Communication System built at Cornell and the Hebrew universities. It allows processes to create process groups within which scalable reliable fifo-ordered multicast and point-to-point communication are supported. The system also supports other communication properties, such as causal and total multicast ordering, flow control, and the like. This article describes the security protocols and infrastructure of Ensemble. Applications using Ensemble with the extensions described here benefit from strong security properties. Under the assumption that trusted processes will not be corrupted, all communication is secured from tampering by outsiders. Our work extends previous work performed in the Horus system (Ensembles predecessor) by adding support for multiple partitions, efficient rekeying, and application-defined security policies. Unlike Horus, which used its own security infrastructure with nonstandard key distribution and timing services, Ensembles security mechanism is based on off-the shelf authentication systems, such as PGP and Kerberos. We extend previous results on group rekeying, with a novel protocol that makes use of diamondlike data structures. Our Diamond protocol allows the removal of untrusted members within milliseconds. In this work we are considering configurations of hundreds of members, and further assume that member trust policies are symmetric and transitive. These assumptions dictate some of our design decisions.

Secure reliable multicast protocols in a WAN

Summary. A secure reliable multicast protocol enables a process to send a message to a group of recipients such that all correct destinations receive the same message, despite the malicious efforts of fewer than a third of the total number of processes, including the sender. This has been shown to be a useful tool in building secure distributed services, albeit with a cost that typically grows linearly with the size of the system. For very large networks, for which this is prohibitive, we present two approaches for reducing the cost: First, we show a protocol whose cost is on the order of the number of tolerated failures. Secondly, we show how relaxing the consistency requirement to a probabilistic guarantee can reduce the associated cost, effectively to a constant.