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

Contributions on agreement in dynamic distributed systems

139 p.This Ph.D. thesis studies the agreement problem in dynamic distributed systems by integrating both the classical fault-tolerance perspective and the more recent formalism based on evolving graphs. First, we developed a common framework that allows to analyze and compare models of dynamic distributed systems for eventual leader election. The framework extends a previous proposal by Baldoni et al. by including new dimensions and levels of dynamicity. Also, we extend the Time-Varying Graph (TVG) formalism by introducing the necessary timeliness assumptions and the minimal conditions to solve agreement problems. We provide a hierarchy of time-bounded, TVG-based, connectivity classes with increasingly stronger assumptions and specify an implementation of Terminating Reliable Broadcast for each class. Then we define an Omega failure detector, W, for the eventual leader election in dynamic distributed systems, together with a system model, , which is compatible with the timebounded TVG classes. We implement an algorithm that satisfy the properties of W in M. According to our common framework, M results to be weaker than the previous proposed dynamic distributed system models for eventual leader election. Additionally we use simulations to illustrate this fact and show that our leader election algorithm tolerates more general (i.e., dynamic) behaviors, and hence it is of application in a wider range of practical scenarios at the cost of a moderate overhead on stabilization times

Models of leader elections and their applications

New research about cyber-physical systems is rapidly changing the way we think about critical infrastructures such as the power grid. Changing requirements for the generation, storage, and availability of power are all driving the development of the smart-grid. Many smart-grid projects disperse power generation across a wide area and control devices with a distributed system. However, in a distributed system, the state of processes is hard to determine due to isolation of memory. By using information flow security models, we reason about a process\u27s beliefs of the system state in a distributed system. Information flow analysis aided in the creation of Markov models for the expected behavior of a cyber controller in a smart-grid system using a communication network with omission faults. The models were used as part of an analysis of the distributed system behavior when there are communication faults. With insight gained from these models, existing congestion management techniques were extended to create a feedback mechanism, allowing the cyber-physical system to better react to issues in the communication network --Abstract, page iii

The Failure Detector Abstraction

This paper surveys the failure detector concept through two dimensions. First we study failure detectors as building blocks to simplify the design of reliable distributed algorithms. More specifically, we illustrate how failure detectors can factor out timing assumptions to detect failures in distributed agreement algorithms. Second, we study failure detectors as computability benchmarks. That is, we survey the weakest failure detector question and illustrate how failure detectors can be used to classify problems. We also highlights some limitations of the failure detector abstraction along each of the dimensions

Byzantine fault-tolerant agreement protocols for wireless Ad hoc networks

Tese de doutoramento, Informática (Ciências da Computação), Universidade de Lisboa, Faculdade de Ciências, 2010.The thesis investigates the problem of fault- and intrusion-tolerant consensus in resource-constrained wireless ad hoc networks. This is a fundamental problem in distributed computing because it abstracts the need to coordinate activities among various nodes. It has been shown to be a building block for several other important distributed computing problems like state-machine replication and atomic broadcast. The thesis begins by making a thorough performance assessment of existing intrusion-tolerant consensus protocols, which shows that the performance bottlenecks of current solutions are in part related to their system modeling assumptions. Based on these results, the communication failure model is identified as a model that simultaneously captures the reality of wireless ad hoc networks and allows the design of efficient protocols. Unfortunately, the model is subject to an impossibility result stating that there is no deterministic algorithm that allows n nodes to reach agreement if more than n2 omission transmission failures can occur in a communication step. This result is valid even under strict timing assumptions (i.e., a synchronous system). The thesis applies randomization techniques in increasingly weaker variants of this model, until an efficient intrusion-tolerant consensus protocol is achieved. The first variant simplifies the problem by restricting the number of nodes that may be at the source of a transmission failure at each communication step. An algorithm is designed that tolerates f dynamic nodes at the source of faulty transmissions in a system with a total of n 3f + 1 nodes. The second variant imposes no restrictions on the pattern of transmission failures. The proposed algorithm effectively circumvents the Santoro- Widmayer impossibility result for the first time. It allows k out of n nodes to decide despite dn 2 e(nk)+k2 omission failures per communication step. This algorithm also has the interesting property of guaranteeing safety during arbitrary periods of unrestricted message loss. The final variant shares the same properties of the previous one, but relaxes the model in the sense that the system is asynchronous and that a static subset of nodes may be malicious. The obtained algorithm, called Turquois, admits f < n 3 malicious nodes, and ensures progress in communication steps where dnf 2 e(n k f) + k 2. The algorithm is subject to a comparative performance evaluation against other intrusiontolerant protocols. The results show that, as the system scales, Turquois outperforms the other protocols by more than an order of magnitude.Esta tese investiga o problema do consenso tolerante a faltas acidentais e maliciosas em redes ad hoc sem fios. Trata-se de um problema fundamental que captura a essência da coordenação em actividades envolvendo vários nós de um sistema, sendo um bloco construtor de outros importantes problemas dos sistemas distribuídos como a replicação de máquina de estados ou a difusão atómica. A tese começa por efectuar uma avaliação de desempenho a protocolos tolerantes a intrusões já existentes na literatura. Os resultados mostram que as limitações de desempenho das soluções existentes estão em parte relacionadas com o seu modelo de sistema. Baseado nestes resultados, é identificado o modelo de falhas de comunicação como um modelo que simultaneamente permite capturar o ambiente das redes ad hoc sem fios e projectar protocolos eficientes. Todavia, o modelo é restrito por um resultado de impossibilidade que afirma não existir algoritmo algum que permita a n nós chegaram a acordo num sistema que admita mais do que n2 transmissões omissas num dado passo de comunicação. Este resultado é válido mesmo sob fortes hipóteses temporais (i.e., em sistemas síncronos) A tese aplica técnicas de aleatoriedade em variantes progressivamente mais fracas do modelo até ser alcançado um protocolo eficiente e tolerante a intrusões. A primeira variante do modelo, de forma a simplificar o problema, restringe o número de nós que estão na origem de transmissões faltosas. É apresentado um algoritmo que tolera f nós dinâmicos na origem de transmissões faltosas em sistemas com um total de n 3f + 1 nós. A segunda variante do modelo não impõe quaisquer restrições no padrão de transmissões faltosas. É apresentado um algoritmo que contorna efectivamente o resultado de impossibilidade Santoro-Widmayer pela primeira vez e que permite a k de n nós efectuarem progresso nos passos de comunicação em que o número de transmissões omissas seja dn 2 e(n k) + k 2. O algoritmo possui ainda a interessante propriedade de tolerar períodos arbitrários em que o número de transmissões omissas seja superior a . A última variante do modelo partilha das mesmas características da variante anterior, mas com pressupostos mais fracos sobre o sistema. Em particular, assume-se que o sistema é assíncrono e que um subconjunto estático dos nós pode ser malicioso. O algoritmo apresentado, denominado Turquois, admite f < n 3 nós maliciosos e assegura progresso nos passos de comunicação em que dnf 2 e(n k f) + k 2. O algoritmo é sujeito a uma análise de desempenho comparativa com outros protocolos na literatura. Os resultados demonstram que, à medida que o número de nós no sistema aumenta, o desempenho do protocolo Turquois ultrapassa os restantes em mais do que uma ordem de magnitude.FC

A Survey on Fault Tolerance Techniques for Wireless Vehicular Networks

Future intelligent transportation systems (ITS) hold the promise of supporting the operation of safety-critical applications, such as cooperative self-driving cars. For that purpose, the communications among vehicles and with the road-side infrastructure will need to fulfil the strict real-time guarantees and challenging dependability requirements. These safety requisites are particularly important in wireless vehicular networks, where road traffic presents several threats to human life. This paper presents a systematic survey on fault tolerance techniques in the area of vehicular communications. The work provides a literature review of publications in journals and conferences proceedings, available through a set of different search databases (IEEE Xplore, Web of Science, Scopus and ScienceDirect). A systematic method, based on the preferred reporting items for systematic reviews and meta-analyses (PRISMA) Statement was conducted in order to identify the relevant papers for this survey. After that, the selected articles were analysed and categorised according to the type of redundancy, corresponding to three main groups (temporal, spatial and information redundancy). Finally, a comparison of the core features among the different solutions is presented, together with a brief discussion regarding the main drawbacks of the existing solutions, as well as the necessary steps to provide an integrated fault-tolerant approach to the future vehicular communications systems

The Failure Detector Abstraction

A failure detector is a fundamental abstraction in distributed computing. This paper surveys this abstraction through two dimensions. First we study failure detectors as building blocks to simplify the design of reliable distributed algorithms. In particular, we illustrate how failure detectors can factor out timing assumptions to detect failures in distributed agreement algorithms. Second, we study failure detectors as computability benchmarks. That is, we survey the weakest failure detector question and illustrate how failure detectors can be used to classify problems. We also highlight some limitations of the failure detector abstraction along each of the dimensions

The Alpha of Indulgent Consensus

This paper presents a simple framework unifying a family of consensus algorithms that can tolerate process crash failures and asynchronous periods of the network, also called indulgent consensus algorithms. Key to the framework is a new abstraction we introduce here, called Alpha, and which precisely captures consensus safety. Implementations of Alpha in shared memory, storage area network, message passing and active disk systems are presented, leading to directly derived consensus algorithms suited to these communication media. The paper also considers the case where the number of processes is unknown and can be arbitrarily larg