A Sufficient Condition for Gaining Belief in Byzantine Fault-Tolerant Distributed Systems

Existing protocols for byzantine fault tolerant distributed systems usually rely on the correct agents' ability to detect faulty agents and/or to detect the occurrence of some event or action on some correct agent. In this paper, we provide sufficient conditions that allow an agent to infer the appropriate beliefs from its history, and a procedure that allows these conditions to be checked in finite time. Our results thus provide essential stepping stones for developing efficient protocols and proving them correct.Comment: In Proceedings TARK 2023, arXiv:2307.0400

Immersive Composition for Sensory Rehabilitation: 3D Visualisation, Surround Sound, and Synthesised Music to Provoke Catharsis and Healing

There is a wide range of sensory therapies using sound, music and visual stimuli. Some focus on soothing or distracting stimuli such as natural sounds or classical music as analgesic, while other approaches emphasize the active performance of producing music as therapy. This paper proposes an immersive multi-sensory Exposure Therapy for people suffering from anxiety disorders, based on a rich, detailed surround-soundscape. This soundscape is composed to include the users’ own idiosyncratic anxiety triggers as a form of habituation, and to provoke psychological catharsis, as a non-verbal, visceral and enveloping exposure. To accurately pinpoint the most effective sounds and to optimally compose the soundscape we will monitor the participants’ physiological responses such as electroencephalography, respiration, electromyography, and heart rate during exposure. We hypothesize that such physiologically optimized sensory landscapes will aid the development of future immersive therapies for various psychological conditions, Sound is a major trigger of anxiety, and auditory hypersensitivity is an extremely problematic symptom. Exposure to stress-inducing sounds can free anxiety sufferers from entrenched avoidance behaviors, teaching physiological coping strategies and encouraging resolution of the psychological issues agitated by the sound

Stochastic Analysis of a Churn-Tolerant Structured Peer-to-Peer Scheme

We present and analyze a simple and general scheme to build a churn (fault)-tolerant structured Peer-to-Peer (P2P) network. Our scheme shows how to "convert" a static network into a dynamic distributed hash table(DHT)-based P2P network such that all the good properties of the static network are guaranteed with high probability (w.h.p). Applying our scheme to a cube-connected cycles network, for example, yields a $O(\log N)$ degree connected network, in which every search succeeds in $O(\log N)$ hops w.h.p., using $O(\log N)$ messages, where $N$ is the expected stable network size. Our scheme has an constant storage overhead (the number of nodes responsible for servicing a data item) and an $O(\log N)$ overhead (messages and time) per insertion and essentially no overhead for deletions. All these bounds are essentially optimal. While DHT schemes with similar guarantees are already known in the literature, this work is new in the following aspects: (1) It presents a rigorous mathematical analysis of the scheme under a general stochastic model of churn and shows the above guarantees; (2) The theoretical analysis is complemented by a simulation-based analysis that validates the asymptotic bounds even in moderately sized networks and also studies performance under changing stable network size; (3) The presented scheme seems especially suitable for maintaining dynamic structures under churn efficiently. In particular, we show that a spanning tree of low diameter can be efficiently maintained in constant time and logarithmic number of messages per insertion or deletion w.h.p. Keywords: P2P Network, DHT Scheme, Churn, Dynamic Spanning Tree, Stochastic Analysis

Runtime resource management for vision-based applications in mobile robots

Computer-vision (CV) applications are an important part of mobile robot automation, analyzing the perceived raw data from vision sensors and providing a rich amount of information on the surrounding environment. The design of a high-speed and energy-efficient CV application for a resource-constrained mobile robot, while maintaining a certain targeted level of accuracy in computation, is a challenging task. This is because such applications demand a lot of resources, e.g. computing capacity and battery energy, to run seamlessly in real time. Moreover, there is always a trade-off between accuracy, performance and energy consumption, as these factors dynamically affect each other at runtime. In this thesis, we investigate novel runtime resource management approaches to improve performance and energy efficiency of vision-based applications in mobile robots. Due to the dynamic correlation between different management objectives, such as energy consumption and execution time, both environmental and computational observations need to be dynamically updated, and the actuators are manipulated at runtime based on these observations. Algorithmic and computational parameters of a CV application (output accuracy and CPU voltage/frequency) are adjusted by measuring the key factors associated with the intensity of computations and strain on CPUs (environmental complexity and instantaneous power). Furthermore, we show how mechanical characteristics of the robot, i.e. the speed of movement in this thesis, can affect the computational behaviour. Based on this investigation, we add the speed of a robot, as an actuator, to our resource management algorithm besides the considered computational knobs (output accuracy and CPU voltage/frequency). To evaluate the proposed approach, we perform several experiments on an unmanned ground vehicle equipped with an embedded computer board and use RGB and event cameras as the vision sensors for CV applications. The obtained results show that the presented management strategy improves the performance and accuracy of vision-based applications while significantly reducing the energy consumption compared with the state-of-the-art solutions. Moreover, we demonstrate that considering simultaneously both computational and mechanical aspects in management of CV applications running on mobile robots significantly reduces the energy consumption compared with similar methods that consider these two aspects separately, oblivious to each other’s outcome

Cross-layer Peer-to-Peer Computing in Mobile Ad Hoc Networks

The future information society is expected to rely heavily on wireless technology. Mobile access to the Internet is steadily gaining ground, and could easily end up exceeding the number of connections from the fixed infrastructure. Picking just one example, ad hoc networking is a new paradigm of wireless communication for mobile devices. Initially, ad hoc networking targeted at military applications as well as stretching the access to the Internet beyond one wireless hop. As a matter of fact, it is now expected to be employed in a variety of civilian applications. For this reason, the issue of how to make these systems working efficiently keeps the ad hoc research community active on topics ranging from wireless technologies to networking and application systems. In contrast to traditional wire-line and wireless networks, ad hoc networks are expected to operate in an environment in which some or all the nodes are mobile, and might suddenly disappear from, or show up in, the network. The lack of any centralized point, leads to the necessity of distributing application services and responsibilities to all available nodes in the network, making the task of developing and deploying application a hard task, and highlighting the necessity of suitable middleware platforms. This thesis studies the properties and performance of peer-to-peer overlay management algorithms, employing them as communication layers in data sharing oriented middleware platforms. The work primarily develops from the observation that efficient overlays have to be aware of the physical network topology, in order to reduce (or avoid) negative impacts of application layer traffic on the network functioning. We argue that cross-layer cooperation between overlay management algorithms and the underlying layer-3 status and protocols, represents a viable alternative to engineer effective decentralized communication layers, or eventually re-engineer existing ones to foster the interconnection of ad hoc networks with Internet infrastructures. The presented approach is twofold. Firstly, we present an innovative network stack component that supports, at an OS level, the realization of cross-layer protocol interactions. Secondly, we exploit cross-layering to optimize overlay management algorithms in unstructured, structured, and publish/subscribe platforms

The Network of Online Stolen Data Markets: How Vendor Flows Connect Digital Marketplaces

In the face of market uncertainty, illicit actors on the darkweb mitigate risk by displacing their operations across digital marketplaces. In this study, we reconstruct market networks created by vendor displacement to examine how digital marketplaces are connected on the darkweb and identify the properties that drive vendor flows before and after a law enforcement disruption. Findings show that vendors’ movement across digital marketplaces creates a highly connected ecosystem; nearly all markets are directly or indirectly connected. These network characteristics remain stable following a law enforcement operation; prior vendor flows predict vendor movement before and after the interdiction. The findings inform work on collective patterns in offender decision-making and extend discussions of displacement into digital spaces