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...

Gossip-based protocols for group communication have attractive scalability and reliability properties. The probabilistic gossip schemes studied so far typically assume that each group member has full knowledge of the global membership and chooses gossip targets uniformly at random. The requirement of global knowledge impairs their applicability to very large-scale groups. In this paper, we present SCAMP (Scalable Membership protocol), a novel peer-to-peer membership protocol which operates in a fully decentralized manner and provides each member with a partial view of the group membership. Our protocol is self-organizing in the sense that the size of partial views naturally converges to the value required to support a gossip algorithm reliably. This value is a function of the group size, but is achieved without any node knowing the group size. We propose additional mechanisms to achieve balanced view sizes even with highly unbalanced subscription patterns. We present the design, theoretical analysis, and a detailed evaluation of the basic protocol and its refinements. Simulation results show that the reliability guarantees provided by SCAMP are comparable to previous schemes based on global knowledge. The scale of the experiments attests to the scalability of the protocol.

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...

Abstract. Gossip-based protocols have received considerable attention for broadcast applications due to their attractive scalability and reliability properties. The reliability of probabilistic gossip schemes studied so far depends on each user having knowledge of the global membership and choosing gossip targets uniformly at random. The requirement of global knowledge is undesirable in largescale distributed systems. In this paper, we present a novel peer-to-peer membership service which operates in a completely decentralized manner in that nobody has global knowledge of membership. However, membership information is replicated robustly enough to support gossip with high reliability. Our scheme is completely self-organizing in the sense that the size of local views naturally converges to the ‘right ’ value for gossip to succeed. This ‘right ’ value is a function of system size, but is achieved without any node having to know the system size. We present the design, theoretical analysis and preliminary evaluation of SCAMP. Simulations show that its performance is comparable to that of previous schemes which use global knowledge of membership at each node.

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...

We describe a novel scalable group membership service designed explicitly for wide area networks. Our membership service is scalable in the number of groups supported, in the number of members in each group, and in the topology each group spans. Our service also supplies the hooks needed to provide clients with full virtual synchrony semantics. Our service attains, on average, a low message overhead by agreeing on membership within a single message round. Furthermore, our service avoids notifying the application of obsolete membership views when the network is unstable, yet it converges when the network has stabilized.

We give a formal specification and an implementation for a partitionable group communication service in asynchronous distributed systems. Our specification is motivated by the requirements for building “partition-aware ” applications that can continue operating without blocking in multiple concurrent partitions and reconfigure themselves dynamically when partitions merge. The specified service guarantees liveness and excludes trivial solutions; it constitutes a useful basis for building realistic partition-aware applications; and it is implementable in practical asynchronous distributed systems where certain stability conditions hold.

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The increasing popularity and diversity of collaborative applications prompts a need for highly secure and reliable communication platforms for dynamic peer groups. Security mechanisms for such groups tend to be both expensive and complex and their integration with reliable group communication services presents a formidable challenge. This paper discusses some important integration issues, reports on our implementation experience and provides experimental results. Our approach utilizes distributed group key management developed by the Cliques project. We enhance it to handle processor and network faults (under a fail-stop or crash-and-recover model) and asynchronous membership events (such as joins, leaves, merges and network partitions). Our approach leverages the strong properties provided by the Spread group communication system, such as message ordering, clean failure semantics and a membership service. The result of this work is a secure group communications layer and an API that provide the application programmer with both standard group communication services and flexible security services. 1

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