André Schiper

Lightweight causal and atomic group multicast

Reference LSR-ARTICLE-1991-001View record in Web of Science Record created on 2005-05-20, modified on 2016-08-08

Probabilistic broadcast for flooding in wireless mobile ad hoc networks

Although far from the optimal, flooding is an indispensable message dissemination technique for network-wide broadcast within mobile ad hoc networks (MANETs). As such, the plain flooding algorithm provokes a high number of unnecessary packet rebroadcasts, causing contention, packet collisions and ultimately wasting precious limited bandwidth. We explore the phase transition phenomenon observed in percolation theory and random graphs as a basis for defining probabilistic flooding algorithm. By considering ideal and realistic models, we acquire a better understanding of the factors that determine phase transition, the consequences of the passage to realistic MANET conditions and to what extent we may benefit from probabilistic flooding in real MANET networks.

Software-based replication for fault tolerance

Replication handled by software on off-the-shelf hardware costs less than using specialized hardware. Although an intuitive concept, replication requires sophisticated techniques for successful implementation. Group communication provides an adequate framework. We present a survey of the techniques developed since the mid-1980s to implement replicated services, emphasizing the relationship between replication techniques and group communication.

On the accuracy of MANET simulators

The deployment of wireless applications or protocols in the context of Mobile Ad-hoc NETworks (MANETs), often requires to step through a simulation phase. For the results of the simulation to be meaningful, it is important that the model on which is based the simulator matches as closely as possible the reality. In this paper we present the simulation results of a straightforward algorithm using several popular simulators (OPNET Modeler, NS-2, GloMoSim). The results tend to show that significant divergences exist between the simulators. This can be explained partly by the mismatching of the modelisation of each simulator and also by the different levels of detail provided to implement and configure the simulated scenarios.

Database replication techniques: a three parameter classification

Data replication is an increasingly important topic as databases are more and more deployed over clusters of workstations. One of the challenges in database replication is to introduce replication without severely affecting performance. Because of this difficulty, current database products use lazy replication, which is very efficient but can compromise consistency. As an alternative, eager replication guarantees consistency but most existing protocols have a prohibitive cost. In order to clarify the current state of the art and open up new avenues for research, this paper analyses existing eager techniques using three key parameters (server architecture, server interaction and transaction termination). In our analysis, we distinguish eight classes of eager replication protocols and, for each category, discuss its requirements, capabilities and cost. The contribution lies in showing when eager replication is feasible and in spelling out the different aspects a database replication protocol must account for.

The causal ordering abstraction and a simple way to implement it

Control in distributed systems is mainly introduced to reduce nondeterminism. This nondeterminism is due on the one hand to the asynchronous execution of the processes located on the various sites of the system, and on the other hand to the asynchronous nature of the communication channels. In order to limit the asynchronism due to communication channels, a new message ordering relation, known as causal ordering, has been introduced by Birman and Joseph. After giving some examples of this causal ordering, we propose a simple algorithm to implement it. This algorithm is based on message sequence numbering. A proof of the correctness of the algorithm is also given.

Early consensus in an asynchronous system with a weak failure detector

Summary. Consensus is one of the most fundamental problems in the context of fault-tolerant distributed computing. The problem consists, given a set Ω of processes having each an initial value vi, in deciding among Ω on a common value v. In 1985, Fischer, Lynch and Paterson proved that the consensus problem is not solvable in an asynchronous system subject to a single process crash. In 1991, Chandra and Toueg showed that, by augmenting the asynchronous system model with a well defined unreliable failure detector, consensus becomes solvable. They also give an algorithm that solves consensus using the ◊? failure detector. In this paper we propose a new consensus algorithm, also using the ◊? failure detector, that is more efficient than the Chandra-Toueg consensus algorithm. We measure efficiency by introducing the notion of latency degree, which defines the minimal number of communication steps needed to solve consensus. The Chandra-Toueg algorithm has a latency degree of 3 (it requires at least three communication steps), whereas our early consensus algorithm requires only two communication steps (latency degree of 2). We believe that this is an interesting result, which adds to our current understanding of the cost of consensus algorithms based on ◊?.

The Database State Machine Approach

Database replication protocols have historically been built on top of distributed database systems, and have consequently been designed and implemented using distributed transactional mechanisms, such as atomic commitment. We present the Database State Machine approach, a new way to deal with database replication in a cluster of servers. This approach relies on a powerful atomic broadcast primitive to propagate transactions between database servers, and alleviates the need for atomic commitment. Transaction commit is based on a certification test, and abort rate is reduced by the reordering certification test. The approach is evaluated using a detailed simulation model that shows the scalability of the system and the benefits of the reordering certification test.

A New Algorithm to Implement Causal Ordering

This paper presents a new algorithm to implement causal ordering. Causal ordering was first proposed in the ISIS system developed at Cornell University. The interest of causal ordering in a distributed system is that it is cheaper to realize than total ordering. The implementation of causal ordering proposed in this paper uses logical clocks of Mattern-Fidge (which define a partial order between events in a distributed system) and presents two advantages over the implementation in ISIS: (1) the information added to messages to ensure causal ordering is bounded by the number of sites in the system, and (2) no special protocol is needed to dispose of this added information when it has become useless. The implementation of ISIS presents however advantages in the case of site failures.

The Heard-Of model: computing in distributed systems with benign faults

Problems in fault-tolerant distributed computing have been studied in a variety of models. These models are structured around two central ideas: (1) degree of synchrony and failure model are two independent parameters that determine a particular type of system, (2) the notion of faulty component is helpful and even necessary for the analysis of distributed computations when faults occur. In this work, we question these two basic principles of fault-tolerant distributed computing, and show that it is both possible and worthy to renounce them in the context of benign faults: we present a computational model based only on the notion of transmission faults. In this model, computations evolve in rounds, and messages missed in a round are lost. Only information transmission is represented: for each round r and each process p, our model provides the set of processes that p “hears of” at round r (heard-of set), namely the processes from which p receives some message at round r. The features of a specific system are thus captured as a whole, just by a predicate over the collection of heard-of sets. We show that our model handles benign failures, be they static or dynamic, permanent or transient, in a unified framework. We demonstrate how this approach leads to shorter and simpler proofs of important results (non-solvability, lower bounds). In particular, we prove that the Consensus problem cannot be generally solved without an implicit and permanent consensus on heard-of sets. We also examine Consensus algorithms in our model. In light of this specific agreement problem, we show how our approach allows us to devise new interesting solutions.