Fabíola Greve

Knowledge Connectivity vs. Synchrony Requirements for Fault-Tolerant Agreement in Unknown Networks

In self-organizing systems, such as mobile ad-hoc and peer-to-peer networks, consensus is a fundamental building block to solve agreement problems. It contributes to coordinate actions of nodes distributed in an ad-hoc manner in order to take consistent decisions. It is well known that in classical environments, in which entities behave asynchronously and where identities are known, consensus cannot be solved in the presence of even one process crash. It appears that self-organizing systems are even less favorable because the set and identity of participants are not known. We define necessary and sufficient conditions under which fault-tolerant consensus become solvable in these environments. Those conditions are related to the synchrony requirements of the environment, as well as the connectivity of the knowledge graph constructed by the nodes in order to communicate with their peers.

Byzantine Consensus with Unknown Participants

Consensus is a fundamental building block used to solve many practical problems that appear on reliable distributed systems. In spite of the fact that consensus is being widely studied in the context of classical networks, few studies have been conducted in order to solve it in the context of dynamic and self-organizing systems characterized by unknown networks. While in a classical network the set of participants is static and known, in a scenario of unknown networks, the set and number of participants are previously unknown. This work goes one step further and studies the problem of Byzantine Fault-Tolerant Consensus with Unknown Participants , namely BFT-CUP. This new problem aims at solving consensus in unknown networks with the additional requirement that participants in the system can behave maliciously. This paper presents a solution for BFT-CUP that does not require digital signatures. The algorithms are shown to be optimal in terms of synchrony and knowledge connectivity among participants in the system.

Eventual Leader Election in Evolving Mobile Networks

Many reliable distributed services rely on an eventual leader election to coordinate actions. The eventual leader detector has been proposed as a way to implement such an abstraction. It ensures that, eventually, each process in the system will be provided by an unique leader, elected among the set of correct processes in spite of crashes and uncertainties. A number of eventual leader election protocols were suggested. Nonetheless, as far as we are aware of, no one of these protocols tolerates a free pattern of node mobility. This paper proposes a new protocol for this scenario of dynamic and mobile unknown networks.

Eventually Strong Failure Detector with Unknown Membership

The distributed computing scenario is rapidly evolving for integrating self-organizing and dynamic wireless networks. Unreliable failure detectors (FDs) are classical mechanisms that provide information about process failures and can help systems to cope with the high dynamics of these networks. A number of failure detection algorithms have been proposed so far. Nonetheless, most of them assume a global knowledge about the membership as well as a fully communication connectivity; additionally, they are time-based, requiring that eventually some bound on the message transmission will permanently hold. These assumptions are no longer appropriate to the new scenario. This paper presents a new FD protocol that implements a new class of detectors, namely ⋄ SM, which adapts the properties of the ⋄ S class to a dynamic network with an unknown membership. It has the interesting feature of being time-free, so that it does not rely on timers to detect failures; moreover, it tolerates the mobility of nodes and message losses.

An Unreliable Failure Detector for Unknown and Mobile Networks

This paper presents an asynchronous implementation of a failure detector for unknown and mobile networks. Our approach does not rely on timers. Neither the composition nor the number of nodes in the system are known. Our algorithm can implement failure detectors of class

A failure detector for wireless networks with unknown membership

\diamondsuit S

What model and what conditions to implement unreliable failure detectors in dynamic networks

when behavioral properties and connectivity conditions are satisfied by the underlying system.

Solving the group priority inversion problem in a timed asynchronous system

The distributed computing scenario is rapidly evolving for integrating self-organizing and dynamic wireless networks. Unreliable failure detectors are classical mechanisms which provide information about process failures and can help systems to cope with the high dynamism of these networks. A number of failure detection algorithms has been proposed so far; nonetheless, most of them assume a global knowledge about the membership as well as a fully communication connectivity; additionally, they are timer-based, requiring that eventually some bound on the message transmission will hold. These assumptions are no longer appropriate to the new scenario. This paper presents a new failure detector protocol which implements a new class of detectors, namely ⋄SM, which adapts the properties of the ⋄S class to a dynamic network with an unknown membership. It has the interesting feature to be time-free, so that it does not rely on timers to detect failures; moreover, it tolerates mobility of nodes and message losses.

Primary component asynchronous group membership as an instance of a generic agreement framework

Failure detectors are classical mechanisms which provide information about process failures and can help systems to cope with the high dynamics of self-organizing, unstructured and mobile wireless networks. Unreliable failure detectors of class ◊S are of special interest because they meet the weakest assumptions able to solve fundamental problems on the design of dependable systems. Unfortunately, a negative result states that no failure detector of that class can be implemented in a network of an unknown membership; but full membership knowledge as well as fully communication connectivity are no longer appropriate assumptions to the new scenario of dynamic networks. In this paper, we provide a discussion about the conditions and model able to implement failure detectors in dynamic networks and define a new class, namely ◊SM, which adapts the properties of the ◊S class to a dynamic network with an unknown membership.

Consensus Based on Strong Failure Detectors: A Time and Message-Efficient Protocol

Considers the priority inversion problem in an actively replicated system. Priority inversion was originally defined in the context of nonreplicated systems. Therefore, we first introduce the concept of group priority inversion, which extends the concept of (local) priority inversion to the context of a group of processors that perform an actively replicated processing. We then present the properties of a request scheduling protocol to enforce a total ordering for the processing of requests while avoiding group priority inversions. These properties have been implemented in a protocol that relies on a timed asynchronous system model equipped with a failure detector of the class /spl diams/S. The proposed solution allows us to replicate a critical server while ensuring that the processing of all the incoming requests is consistent (mechanisms for solving the atomic broadcast problem) and predictable (mechanisms for solving the group priority inversion problem). Thus, the described request scheduling protocol is a key component which can be used to develop fault-tolerant real-time applications in a timed asynchronous system.