Alberto Lafuente

Fault-Tolerant Leader Election in Mobile Dynamic Distributed Systems

This paper addresses the leader election problem in dynamic distributed systems with mobile processes. To do so, it is assumed that the system alternates periods of good and bad behavior, in the line of the timed asynchronous model of Cristian and Fetzer. We extend the eventual leadership properties recently proposed by Larrea et al. for non-mobile dynamic systems, defining two new properties that take into account graph joins/fragmentations due to process mobility. We also propose a new leader election algorithm in a weak mobile dynamic distributed system model. Using a categorization framework, we compare our system model with a number of models proposed in the literature, showing that our leader election algorithm works in a model which is weaker than the rest.

Managing Intelligent Services for People with Disabilities and Elderly People

Ambient Supported Living systems for people with physical, sensory or cognitive restrictions have to guarantee that the environment is safe, fault tolerant and universally accessible. In addition it is necessary to overcome technological challenges, common to ubiquitous computing, such as the design of a middleware layer that ensures the interoperability of multiple wired and wireless networks and performs discovery actions. On top of that the system has to provide efficient support to the intelligent applications designed to assist people living there. In this paper we present the AmbienNet architecture designed to allow structured context information to be shared among the intelligent applications that support people with disabilities or elderly people living alone.

Architectures for ubiquitous environments

Ubiquitous (or pervasive) systems are characterized by component distribution and dynamic behavior. In a ubiquitous environment, such as a future automated home (also called a domotic environment), user devices explore the environment to discover resources and services to control. This paper analyzes architectures for ubiquitous systems based on the structure of the environment and the way discovery mechanisms (such as Jini and UPnP) operate. As a result, a framework to classify the architectures is provided. As an example, a UPnP-based prototype for a domotic environment is presented. We use standard devices, such as PDAs and cell phones, to allow users to access and control virtual resources. The system provides user management capabilities, persistence, and standard interfaces, while preserving full UPnP compatibility. The prototype can be easily transferred to future realistic domotic installations.

Communication-efficient failure detection and consensus in omission environments

Failure detectors have been shown to be a very useful mechanism to solve the consensus problem in the crash failure model, for which a number of communication-efficient algorithms have been proposed. In this paper we deal with the definition, implementation and use of communication-efficient failure detectors in the general omission failure model, where processes can fail by crashing and by omitting messages when sending and/or receiving. We first define a new failure detector class for this model in terms of completeness and accuracy properties. Then we propose an algorithm that implements a failure detector of the proposed class in a communication-efficient way, in the sense that only a linear number of links are used to send messages forever. We also explain how the well-known consensus algorithm of Chandra and Toueg can be adapted in order to use the proposed failure detector.

An architecture for the personalized control of domotic resources

This work presents an architecture for the management and personalized control of devices in a <i>domotic</i> environment (i.e., an automated home). The architecture allows for user and profile management from mobile hand-held devices such as PDAs and cell phones. The system provides for interaction from any device fitted with TCP/IP support and a web browser, so that the specific characteristics of the access device are not a conditioning factor. We have implemented a prototype to test out the application of the architecture in domotic environments. We have used virtual UPnP devices that emulate domotic devices on PCs, a local area network interconnecting the PCs to form a domotic network, and a PDA with Bluetooth and a cell phone with GPRS connection as wireless access devices. A key design goal was that the prototype could be easily transferred to future domotic installations with real devices, possibly interconnected by the home power line.

On the implementation of communication-optimal failure detectors

Several algorithms implementing failure detectors have been proposed in the literature. In particular, we have proposed a family of communication-efficient ⋄P algorithms, i.e., algorithms using n links to carry messages forever, being n the number of processes in the system. Moreover, we have recently proposed a ⋄P algorithm that uses only C links, being C the number of correct processes. In this paper, we show that C is the minimum number of links required to implement ⋄P. We also show that, assuming that there is at least one incorrect process, C is optimal not only for ⋄P but also for ⋄S and Ω. We revisit our Reliable Broadcast based communication-optimal ⋄P algorithm, and we show that, regarding QoS measures, it performs better than the communication-efficient algorithms.

Secure Failure Detection and Consensus in TrustedPals

We present a modular redesign of TrustedPals, a smart card-based security framework for solving Secure Multiparty Computation (SMC). Originally, TrustedPals assumed a synchronous network setting and allowed to reduce SMC to the problem of fault-tolerant consensus among smart cards. We explore how to make TrustedPals applicable in environments with less synchrony and show how it can be used to solve asynchronous SMC. Within the redesign we investigate the problem of solving consensus in a general omission failure model augmented with failure detectors. To this end, we give novel definitions of both consensus and the class oP of failure detectors in the omission model, which we call ◇P(om), and show how to implement ◇P(om) and have consensus in such a system with very weak synchrony assumptions. The integration of failure detection and consensus into the TrustedPals framework uses tools from privacy enhancing techniques such as message padding and dummy traffic.

Secure failure detection in TrustedPals

This paper presents a modular redesign of TrustedPals, a smartcard-based security framework for solving secure multiparty computation (SMC). TrustedPals allows to reduce SMC to the problem of fault-tolerant consensus between smartcards. Within the redesign we investigate the problem of solving consensus in a general omission failure model augmented with failure detectors. To this end, we give novel definitions of both consensus and the class of ⋄P failure detectors in the omission model and show how to implement ⋄P and have consensus in such a system with some weak synchrony assumptions. The integration of failure detection into the TrustedPals framework uses tools from privacy enhancing techniques such as message padding and dummy traffic.

A Connectivity Model for Agreement in Dynamic Systems

The consensus problem is a fundamental paradigm in distributed systems, because it captures the difficulty to solve other agreement problems. Many current systems evolve with time, e.g., due to node mobility, and consensus has been little studied in these systems so far. Specifically, it is not well established how to define an appropriate set of assumptions for consensus in dynamic distributed systems. This paper studies a hierarchy of three classes of time-varying graphs, and provides a solution for each class to the problem of Terminating Reliable Broadcast (TRB). The classes introduce increasingly stronger assumptions on timeliness, so that the trade-off between weakness versus implementability and efficiency can be analysed. Being TRB equivalent to consensus in synchronous systems, the paper extends this equivalence to dynamic systems.

Communication-optimal eventually perfect failure detection in partially synchronous systems ☆

Abstract Since Chandra and Toueg introduced the failure detector abstraction for crash-prone systems, several algorithms implementing failure detectors in partially synchronous systems have been proposed. Their performance can be measured by their Communication efficiency , defined as the number of links used forever. In this regard, in a communication-efficient algorithm only n links are used forever, n being the number of processes in the system. In this paper, we present communication optimality , a communication efficiency degree reached when only c links are used forever, c being the number of correct processes. We show that c is the minimum number of links used forever required to implement ◇ P and that c is also optimal for ◇ S and Ω when c n . Finally, we propose two communication-optimal ◇ P algorithms following respectively one-to-all and one-to-one communication patterns to manage suspicions, showing that there is a trade-off between detection latency and sporadic communication overhead.