View-oriented group communication is an important and widely used building block for many distributed applications. Much current research has been dedicated to specifying the semantics and services of view-oriented Group Communication Systems (GCSs). However, the guarantees of different GCSs are formulated using varying terminologies and modeling techniques, and the specifications vary in their rigor. This makes it difficult to analyze and compare the different systems. This paper provides a comprehensive set of clear and rigorous specifications, which may be combined to represent the guarantees of most existing GCSs. In the light of these specifications, over thirty published GCS specifications are surveyed. Thus, the specifications serve as a unifying framework for the classification, analysis and comparison of group communication systems. The survey also discusses over a dozen different applications of group communication systems, shedding light on the usefulness of the p...

ly, the remote procedure call problem, which an RPC protocol undertakes to solve, consists of emulating LPC using message passing. LPC has a number of "properties" -- a single procedure invocation results in exactly one execution of the procedure body, the result returned is reliably delivered to the invoker, and exceptions are raised if (and only if) an error occurs. Given a completely reliable communication environment, which never loses, duplicates, or reorders messages, and given client and server processes that never fail, RPC would be trivial to solve. The sender would merely package the invocation into one or more messages, and transmit these to the server. The server would unpack the data into local variables, perform the desired operation, and send back the result (or an indication of any exception that occurred) in a reply message. The challenge, then, is created by failures. Were it not for the possibility of process and machine crashes, an RPC protocol capable of overcomi...

Operating Systems]: Organization and Design---distributed systems General Terms: Protocols, Performance, Reliability Additional Key Words and Phrases: Flow control, membership, reliable delivery, token passing, total ordering, virtual synchrony Earlier versions of the Totem single-ring protocol appeared in the Proceedings of the IEE International Conference on Information Engineering, Singapore (December 1991) and in the Proceedings of the IEEE 13th International Conference on Distributed Computing Systems, Pittsburgh, PA (May 1993). This research was supported by NSF Grant No. NCR-9016361, ARPA Contract No. N00174-93K -0097, and Rockwell CMC/State of California MICRO Grant No. 92-101. Authors' Addresses: Y. Amir, Computer Science Department, The Hebrew University of Jerusalem, Israel; L. E. Moser and P. M. Melliar-Smith, Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106; D. A. Agarwal, Lawrence Berkele

Secure group communication is an increasingly popular research area having received much attention in recent years. The fundamental challenge revolves around secure and efficient group key management. While centralized methods are often appropriate for key distribution in large groups, many collaborative group settings require distributed key agreement techniques. This work investigates a novel approach to group key agreement by blending binary key trees with Diffie-Hellman key exchange. The resultant protocol suite is very simple, secure and fault-tolerant. Moreover, its efficiency surpasses that of prior art.

Building a wide area group communication system is a challenge. This paper presents the design and protocols of the Spread wide area group communication system. Spread integrates two low-level protocols: one for local area networks called Ring, and one for the wide area network connecting them, called Hop. Spread decouples the dissemination and local reliability mechanisms from the global ordering and stability protocols. This allows many optimizations useful for wide area network settings. Spread is operational and publicly available on the Web. 1. Introduction There exist some fundamental difficulties with high-performance group communication over wide-area networks. These difficulties include: . The characteristics (loss rates, amount of buffering) and performance (latency, bandwidth) vary widely in different parts of the network. . The packet loss rates and latencies are significantly higher and more variable then on LANs. . It is not as easy to implement efficient reliability...

Group communication services are becoming accepted as effective building blocks for the construction of fault-tolerant distributed applications. Many specifications for group communication services have been proposed. However, there is still no agreement about what these specifications should say, especially in cases where the services are partitionable, that is, where communication failures may lead to simultaneous creation of groups with disjoint memberships, such that each group is unaware of the existence of any other group. In this paper

Database replication is traditionally seen as a way to increase the availability and performance of distributed databases. Although a large number of protocols providing data consistency and fault-tolerance have been proposed, few of these ideas have ever been used in commercial products due to their complexity and performance implications. Instead, current products allow inconsistencies and often resort to centralized approaches which eliminates some of the advantages of replication. As an alternative, we propose a suite of replication protocols that addresses the main problems related to database replication. On the one hand, our protocols maintain data consistency and the same transactional semantics found in centralized systems. On the other hand, they provide flexibility and reasonable performance. To do so, our protocols take advantage of the rich semantics of group communication primitives and the relaxed isolation guarantees provided by most databases. This allows us to eliminate the possibility of deadlocks, reduce the message overhead and increase performance. A detailed simulation study shows the feasibility of the approach and the flexibility with which different types of bottlenecks can be circumvented.

The Horus project seeks to develop a communication system addressing the requirements of a wide variety of distributed applications. Horus implements the group communications model providing (among others) unreliable or reliable FIFO, causal, or total group multicasts. It is extensively layered and highly reconfigurable allowing applications to only pay for services they use. This architecture enables groups with different communication needs to coexist in a single system. The approach permits experimentation with new communication properties and incremental extension of the system, and enables us to support a variety of application-oriented interfaces. Our initial experiments show good performance. 1 Introduction In the last several years, we have seen a growing use of group communication primitives in distributed and/or parallel applications. Physically distributed systems (such as stock markets and factories) employ group communications to disseminate information to large numbers o...

In systems based on the client-server model, a single server may serve many clients and the heavy load on the server may cause the response time to be adversely affected. In such circumstances, replicating data or servers may improve performance. Replication may also improve the availability of information when processors crash or the network partitions. Existing replication methods are often needlessly expensive. They sometimes use pointto -point communication when multicast communication is available; they typically pay the full price of end-to-end acknowledgments for all of the participants for every update; they may claim locks, and therefore, may be vulnerable to faults that can unnecessarily block the system for long periods of time. This thesis presents a new architecture and algorithms for replication over a partitioned network. The architecture is structured into two layers: a replication server and a group communication layer. Each of the replication servers maintains a priva...

Group communication systems are proven tools upon which to build fault-tolerant systems. As the demands for fault-tolerance increase and more applications require reliable distributed computing over wide area networks, wide area group communication systems are becoming very useful. However, building a wide area group communication system is a challenge. This paper presents the design of the transport protocols of the Spread wide area group communication system. We focus on two aspects of the system. First, the value of using overlay networks for application level group communication services. Second, the requirements and design of effective low latency link protocols used to construct wide area group communication. We support our claims with the results of live experiments conducted over the Internet. Keywords---Group Communication, Overlay Networks, Reliable Multicast, Wide Area Networks, TCP/IP. 1 Introduction There exist some fundamental difficulties with highperformance group co...