A Survey of Self-Stabilizing Spanning-Tree Construction Algorithms

Self-stabilizing systems can automatically recover from arbitrary state perturbations in finite time. They are therefore well-suited for dynamic, failure prone environments. Spanning-tree construction in distributed systems is a fundamental task which forms the basis for many other network algorithms (like token circulation or routing).This paper surveys self-stabilizing algorithms that construct a spanning tree within a network of processing entities. Lower bounds and related work are also discussed

A Survey of Self-Stabilizing Spanning-Tree Construction Algorithms

Self-stabilizing systems can automatically recover from arbitrary state perturbations in finite time. They are therefore well-suited for dynamic, failure prone environments. Spanning-tree construction in distributed systems is a fundamental task which forms the basis for many other network algorithms (like token circulation or routing).This paper surveys self-stabilizing algorithms that construct a spanning tree within a network of processing entities. Lower bounds and related work are also discussed

Fair Exchange with Guardian Angels

In this paper we propose a new probabilistic Fair Exchange Protocol which requires no central Trusted Third Party. Instead, it relies on a virtually distributed and decentralized Trusted Third Party which is formalized as a Guardian Angel: a kind of Observer e.g. a tamper proof security device. We thus introduce a network model with Pirates and Guardian Angels which is well suited for Ad Hoc networks. In this setting we reduce the Fair Exchange Problem to a Synchronization Problem in which honest parties need to eventually decide whether or not a protocol succeeded in a synchronous way through a hostile network which does not guaranty that sent messages will be eventually received. This problem can be of independent interest in order to add reliability of protocol termination in secure channels

Distributed eventual leader election in the crash-recovery and general omission failure models.

102 p.Distributed applications are present in many aspects of everyday life. Banking, healthcare or transportation are examples of such applications. These applications are built on top of distributed systems. Roughly speaking, a distributed system is composed of a set of processes that collaborate among them to achieve a common goal. When building such systems, designers have to cope with several issues, such as different synchrony assumptions and failure occurrence. Distributed systems must ensure that the delivered service is trustworthy.Agreement problems compose a fundamental class of problems in distributed systems. All agreement problems follow the same pattern: all processes must agree on some common decision. Most of the agreement problems can be considered as a particular instance of the Consensus problem. Hence, they can be solved by reduction to consensus. However, a fundamental impossibility result, namely (FLP), states that in an asynchronous distributed system it is impossible to achieve consensus deterministically when at least one process may fail. A way to circumvent this obstacle is by using unreliable failure detectors. A failure detector allows to encapsulate synchrony assumptions of the system, providing (possibly incorrect) information about process failures. A particular failure detector, called Omega, has been shown to be the weakest failure detector for solving consensus with a majority of correct processes. Informally, Omega lies on providing an eventual leader election mechanism

Atomic broadcast:a fault-tolerant token based algorithm and performance evaluations

Within only a couple of generations, the so-called digital revolution has taken the world by storm: today, almost all human beings interact, directly or indirectly, at some point in their life, with a computer system. Computers are present on our desks, computer systems control the antilock braking system and the stability control in cars, they collect usage statistics in elevators in order to anticipate maintenance and repair operations. Computer systems also operate critical systems, such as nuclear power plants, airplane control systems or space rockets. Furthermore, computer systems are not only omnipresent, but also increasingly networked. As the use of computer systems has increased dramatically over the past decades, the needs and expectations associated with these systems have also increased. In particular, one of the critical points of a system is its availability (the fraction of the time during which the system provides a service to the users): the costs and negative publicity of a system outage (be it a commercial web site or a stock exchange for example) are often considerable. Fault tolerance is one of the approaches to designing a highly-available system: a fault tolerant system is designed in such a way that the failure of one of the components of the system does not compromise the functionality of the system as a whole. Replication is one of the common fault tolerance techniques. Instead of having a single machine (a replica) providing a service, the system is composed of several replicas running the service and connected through a network. If one of the replicas fails, the service is still provided by the remaining replicas. The replication technique is interesting as it can be achieved by using software running on commodity hardware, thus avoiding the high cost of special purpose hardware. Replication, although intuitive to understand, is complex to implement in practice, as the replicas have to interact in order to ensure the consistency of the system as a whole. Group communication simplifies the replication problem, by hiding issues such as the communication between the replicas, the crashes of one or several replicas and the synchronization of the replicas. In this thesis, we start by comparing two replication techniques – group communication and quorum systems – and identifying in which case either technique should be used. Atomic broadcast (a group communication primitive at the heart of this work) allows replicas to broadcast messages to each other and then deliver them in the same total order, even if replicas broadcast messages quasi simultaneously. Atomic broadcast is especially useful for replication: since all replicas deliver messages in the same order, their state is kept consistent. After the comparison between the replication techniques, we present an atomic broadcast algorithm designed to perform well when the system is heavily loaded and that allows to quickly detect crashed replicas (by minimizing the consequences of wrongly suspecting a non-crashed replica). The presentation of the algorithm includes simulation results comparing the performance of the new algorithm to previously proposed atomic broadcast algorithms. The second part of the thesis focuses on the experimental performance evaluation of the new algorithm in several settings. We start by comparing four atomic broadcast algorithms in a local area network. We then compare three of the four algorithms in a wide area network, with sites in Switzerland, Japan and France, and where the round trip time between the sites varies between 4 and 300 ms. Finally, we evaluate the impact of the size of the system (the number if replicas) on the performance of the algorithms