333 research outputs found

    Adaptive Redundancy Management for Durable P2P Backup

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    We design and analyze the performance of a redundancy management mechanism for Peer-to-Peer backup applications. Armed with the realization that a backup system has peculiar requirements -- namely, data is read over the network only during restore processes caused by data loss -- redundancy management targets data durability rather than attempting to make each piece of information availabile at any time. In our approach each peer determines, in an on-line manner, an amount of redundancy sufficient to counter the effects of peer deaths, while preserving acceptable data restore times. Our experiments, based on trace-driven simulations, indicate that our mechanism can reduce the redundancy by a factor between two and three with respect to redundancy policies aiming for data availability. These results imply an according increase in storage capacity and decrease in time to complete backups, at the expense of longer times required to restore data. We believe this is a very reasonable price to pay, given the nature of the application. We complete our work with a discussion on practical issues, and their solutions, related to which encoding technique is more suitable to support our scheme

    A HOLISTIC REDUNDANCY- AND INCENTIVE-BASED FRAMEWORK TO IMPROVE CONTENT AVAILABILITY IN PEER-TO-PEER NETWORKS

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    Peer-to-Peer (P2P) technology has emerged as an important alternative to the traditional client-server communication paradigm to build large-scale distributed systems. P2P enables the creation, dissemination and access to information at low cost and without the need of dedicated coordinating entities. However, existing P2P systems fail to provide high-levels of content availability, which limit their applicability and adoption. This dissertation takes a holistic approach to device mechanisms to improve content availability in large-scale P2P systems. Content availability in P2P can be impacted by hardware failures and churn. Hardware failures, in the form of disk or node failures, render information inaccessible. Churn, an inherent property of P2P, is the collective effect of the users’ uncoordinated behavior, which occurs when a large percentage of nodes join and leave frequently. Such a behavior reduces content availability significantly. Mitigating the combined effect of hardware failures and churn on content availability in P2P requires new and innovative solutions that go beyond those applied in existing distributed systems. To addresses this challenge, the thesis proposes two complementary, low cost mechanisms, whereby nodes self-organize to overcome failures and improve content availability. The first mechanism is a low complexity and highly flexible hybrid redundancy scheme, referred to as Proactive Repair (PR). The second mechanism is an incentive-based scheme that promotes cooperation and enforces fair exchange of resources among peers. These mechanisms provide the basis for the development of distributed self-organizing algorithms to automate PR and, through incentives, maximize their effectiveness in realistic P2P environments. Our proposed solution is evaluated using a combination of analytical and experimental methods. The analytical models are developed to determine the availability and repair cost properties of PR. The results indicate that PR’s repair cost outperforms other redundancy schemes. The experimental analysis was carried out using simulation and the development of a testbed. The simulation results confirm that PR improves content availability in P2P. The proposed mechanisms are implemented and tested using a DHT-based P2P application environment. The experimental results indicate that the incentive-based mechanism can promote fair exchange of resources and limits the impact of uncooperative behaviors such as “free-riding”

    CHORUS Deliverable 2.2: Second report - identification of multi-disciplinary key issues for gap analysis toward EU multimedia search engines roadmap

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    After addressing the state-of-the-art during the first year of Chorus and establishing the existing landscape in multimedia search engines, we have identified and analyzed gaps within European research effort during our second year. In this period we focused on three directions, notably technological issues, user-centred issues and use-cases and socio- economic and legal aspects. These were assessed by two central studies: firstly, a concerted vision of functional breakdown of generic multimedia search engine, and secondly, a representative use-cases descriptions with the related discussion on requirement for technological challenges. Both studies have been carried out in cooperation and consultation with the community at large through EC concertation meetings (multimedia search engines cluster), several meetings with our Think-Tank, presentations in international conferences, and surveys addressed to EU projects coordinators as well as National initiatives coordinators. Based on the obtained feedback we identified two types of gaps, namely core technological gaps that involve research challenges, and “enablers”, which are not necessarily technical research challenges, but have impact on innovation progress. New socio-economic trends are presented as well as emerging legal challenges

    SoS: self-organizing substrates

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    Large-scale networked systems often, both by design or chance exhibit self-organizing properties. Understanding self-organization using tools from cybernetics, particularly modeling them as Markov processes is a first step towards a formal framework which can be used in (decentralized) systems research and design.Interesting aspects to look for include the time evolution of a system and to investigate if and when a system converges to some absorbing states or stabilizes into a dynamic (and stable) equilibrium and how it performs under such an equilibrium state. Such a formal framework brings in objectivity in systems research, helping discern facts from artefacts as well as providing tools for quantitative evaluation of such systems. This thesis introduces such formalism in analyzing and evaluating peer-to-peer (P2P) systems in order to better understand the dynamics of such systems which in turn helps in better designs. In particular this thesis develops and studies the fundamental building blocks for a P2P storage system. In the process the design and evaluation methodology we pursue illustrate the typical methodological approaches in studying and designing self-organizing systems, and how the analysis methodology influences the design of the algorithms themselves to meet system design goals (preferably with quantifiable guarantees). These goals include efficiency, availability and durability, load-balance, high fault-tolerance and self-maintenance even in adversarial conditions like arbitrarily skewed and dynamic load and high membership dynamics (churn), apart of-course the specific functionalities that the system is supposed to provide. The functionalities we study here are some of the fundamental building blocks for various P2P applications and systems including P2P storage systems, and hence we call them substrates or base infrastructure. These elemental functionalities include: (i) Reliable and efficient discovery of resources distributed over the network in a decentralized manner; (ii) Communication among participants in an address independent manner, i.e., even when peers change their physical addresses; (iii) Availability and persistence of stored objects in the network, irrespective of availability or departure of individual participants from the system at any time; and (iv) Freshness of the objects/resources' (up-to-date replicas). Internet-scale distributed index structures (often termed as structured overlays) are used for discovery and access of resources in a decentralized setting. We propose a rapid construction from scratch and maintenance of the P-Grid overlay network in a self-organized manner so as to provide efficient search of both individual keys as well as a whole range of keys, doing so providing good load-balancing characteristics for diverse kind of arbitrarily skewed loads - storage and replication, query forwarding and query answering loads. For fast overlay construction we employ recursive partitioning of the key-space so that the resulting partitions are balanced with respect to storage load and replication. The proper algorithmic parameters for such partitioning is derived from a transient analysis of the partitioning process which has Markov property. Preservation of ordering information in P-Grid such that queries other than exact queries, like range queries can be efficiently and rather trivially handled makes P-Grid suitable for data-oriented applications. Fast overlay construction is analogous to building an index on a new set of keys making P-Grid suitable as the underlying indexing mechanism for peer-to-peer information retrieval applications among other potential applications which may require frequent indexing of new attributes apart regular updates to an existing index. In order to deal with membership dynamics, in particular changing physical address of peers across sessions, the overlay itself is used as a (self-referential) directory service for maintaining the participating peers' physical addresses across sessions. Exploiting this self-referential directory, a family of overlay maintenance scheme has been designed with lower communication overhead than other overlay maintenance strategies. The notion of dynamic equilibrium study for overlays under continuous churn and repairs, modeled as a Markov process, was introduced in order to evaluate and compare the overlay maintenance schemes. While the self-referential directory was originally invented to realize overlay maintenance schemes with lower overheads than existing overlay maintenance schemes, the self-referential directory is generic in nature and can be used for various other purposes, e.g., as a decentralized public key infrastructure. Persistence of peer identity across sessions, in spite of changes in physical address, provides a logical independence of the overlay network from the underlying physical network. This has many other potential usages, for example, efficient maintenance mechanisms for P2P storage systems and P2P trust and reputation management. We specifically look into the dynamics of maintaining redundancy for storage systems and design a novel lazy maintenance strategy. This strategy is algorithmically a simple variant of existing maintenance strategies which adapts to the system dynamics. This randomized lazy maintenance strategy thus explores the cost-performance trade-offs of the storage maintenance operations in a self-organizing manner. We model the storage system (redundancy), under churn and maintenance, as a Markov process. We perform an equilibrium study to show that the system operates in a more stable dynamic equilibrium with our strategy than for the existing maintenance scheme for comparable overheads. Particularly, we show that our maintenance scheme provides substantial performance gains in terms of maintenance overhead and system's resilience in presence of churn and correlated failures. Finally, we propose a gossip mechanism which works with lower communication overhead than existing approaches for communication among a relatively large set of unreliable peers without assuming any specific structure for their mutual connectivity. We use such a communication primitive for propagating replica updates in P2P systems, facilitating management of mutable content in P2P systems. The peer population affected by a gossip can be modeled as a Markov process. Studying the transient spread of gossips help in choosing proper algorithm parameters to reduce communication overhead while guaranteeing coverage of online peers. Each of these substrates in themselves were developed to find practical solutions for real problems. Put together, these can be used in other applications, including a P2P storage system with support for efficient lookup and inserts, membership dynamics, content mutation and updates, persistence and availability. Many of the ideas have already been implemented in real systems and several others are in the way to be integrated into the implementations. There are two principal contributions of this dissertation. It provides design of the P2P systems which are useful for end-users as well as other application developers who can build upon these existing systems. Secondly, it adapts and introduces the methodology of analysis of a system's time-evolution (tools typically used in diverse domains including physics and cybernetics) to study the long run behavior of P2P systems, and uses this methodology to (re-)design appropriate algorithms and evaluate them. We observed that studying P2P systems from the perspective of complex systems reveals their inner dynamics and hence ways to exploit such dynamics for suitable or better algorithms. In other words, the analysis methodology in itself strongly influences and inspires the way we design such systems. We believe that such an approach of orchestrating self-organization in internet-scale systems, where the algorithms and the analysis methodology have strong mutual influence will significantly change the way future such systems are developed and evaluated. We envision that such an approach will particularly serve as an important tool for the nascent but fast moving P2P systems research and development community

    CHORUS Deliverable 2.1: State of the Art on Multimedia Search Engines

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    Based on the information provided by European projects and national initiatives related to multimedia search as well as domains experts that participated in the CHORUS Think-thanks and workshops, this document reports on the state of the art related to multimedia content search from, a technical, and socio-economic perspective. The technical perspective includes an up to date view on content based indexing and retrieval technologies, multimedia search in the context of mobile devices and peer-to-peer networks, and an overview of current evaluation and benchmark inititiatives to measure the performance of multimedia search engines. From a socio-economic perspective we inventorize the impact and legal consequences of these technical advances and point out future directions of research

    Empirical and Analytical Perspectives on the Robustness of Blockchain-related Peer-to-Peer Networks

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    Die Erfindung von Bitcoin hat ein großes Interesse an dezentralen Systemen geweckt. Eine häufige Zuschreibung an dezentrale Systeme ist dabei, dass eine Dezentralisierung automatisch zu einer höheren Sicherheit und Widerstandsfähigkeit gegenüber Angriffen führt. Diese Dissertation widmet sich dieser Zuschreibung, indem untersucht wird, ob dezentralisierte Anwendungen tatsächlich so robust sind. Dafür werden exemplarisch drei Systeme untersucht, die häufig als Komponenten in komplexen Blockchain-Anwendungen benutzt werden: Ethereum als Infrastruktur, IPFS zur verteilten Datenspeicherung und schließlich "Stablecoins" als Tokens mit Wertstabilität. Die Sicherheit und Robustheit dieser einzelnen Komponenten bestimmt maßgeblich die Sicherheit des Gesamtsystems in dem sie verwendet werden; darüber hinaus erlaubt der Fokus auf Komponenten Schlussfolgerungen über individuelle Anwendungen hinaus. Für die entsprechende Analyse bedient sich diese Arbeit einer empirisch motivierten, meist Netzwerklayer-basierten Perspektive -- angereichert mit einer ökonomischen im Kontext von Wertstabilen Tokens. Dieses empirische Verständnis ermöglicht es Aussagen über die inhärenten Eigenschaften der studierten Systeme zu treffen. Ein zentrales Ergebnis dieser Arbeit ist die Entdeckung und Demonstration einer "Eclipse-Attack" auf das Ethereum Overlay. Mittels eines solchen Angriffs kann ein Angreifer die Verbreitung von Transaktionen und Blöcken behindern und Netzwerkteilnehmer aus dem Overlay ausschließen. Des weiteren wird das IPFS-Netzwerk umfassend analysiert und kartografiert mithilfe (1) systematischer Crawls der DHT sowie (2) des Mitschneidens von Anfragenachrichten für Daten. Erkenntlich wird hierbei, dass die hybride Overlay-Struktur von IPFS Segen und Fluch zugleich ist, da das Gesamtsystem zwar robust gegen Angriffe ist, gleichzeitig aber eine umfassende Überwachung der Netzwerkteilnehmer ermöglicht wird. Im Rahmen der wertstabilen Kryptowährungen wird ein Klassifikations-Framework vorgestellt und auf aktuelle Entwicklungen im Gebiet der "Stablecoins" angewandt. Mit diesem Framework wird somit (1) der aktuelle Zustand der Stablecoin-Landschaft sortiert und (2) ein Mittel zur Verfügung gestellt, um auch zukünftige Designs einzuordnen und zu verstehen.The inception of Bitcoin has sparked a large interest in decentralized systems. In particular, popular narratives imply that decentralization automatically leads to a high security and resilience against attacks, even against powerful adversaries. In this thesis, we investigate whether these ascriptions are appropriate and if decentralized applications are as robust as they are made out to be. To this end, we exemplarily analyze three widely-used systems that function as building blocks for blockchain applications: Ethereum as basic infrastructure, IPFS for distributed storage and lastly "stablecoins" as tokens with a stable value. As reoccurring building blocks for decentralized applications these examples significantly determine the security and resilience of the overall application. Furthermore, focusing on these building blocks allows us to look past individual applications and focus on inherent systemic properties. The analysis is driven by a strong empirical, mostly network-layer based perspective; enriched with an economic point of view in the context of monetary stabilization. The resulting practical understanding allows us to delve into the systems' inherent properties. The fundamental results of this thesis include the demonstration of a network-layer Eclipse attack on the Ethereum overlay which can be leveraged to impede the delivery of transaction and blocks with dire consequences for applications built on top of Ethereum. Furthermore, we extensively map the IPFS network through (1) systematic crawling of its DHT, as well as (2) monitoring content requests. We show that while IPFS' hybrid overlay structure renders it quite robust against attacks, this virtue of the overlay is simultaneously a curse, as it allows for extensive monitoring of participating peers and the data they request. Lastly, we exchange the network-layer perspective for a mostly economic one in the context of monetary stabilization. We present a classification framework to (1) map out the stablecoin landscape and (2) provide means to pigeon-hole future system designs. With our work we not only scrutinize ascriptions attributed to decentral technologies; we also reached out to IPFS and Ethereum developers to discuss results and remedy potential attack vectors

    On Personal Storage Systems: Architecture and Design Considerations

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    Actualment, els usuaris necessiten grans quantitats d’espai d’emmagatzematge remot per guardar la seva informació personal. En aquesta dissertació, estudiarem dues arquitectures emergents de sistemes d’emmagatzematge d’informació personal: els Núvols Personals (centralitzats) i els sistemes d’emmagatzematge social (descentralitzats). A la Part I d'aquesta tesi, contribuïm desvelant l’operació interna d’un Núvol Personal d’escala global, anomenat UbuntuOne (U1), incloent-hi la seva arquitectura, el seu servei de metadades i les interaccions d’emmagatzematge de dades. A més, proporcionem una anàlisi de la part de servidor d’U1 on estudiem la càrrega del sistema, el comportament dels usuaris i el rendiment del seu servei de metadades. També suggerim tota una sèrie de millores potencials al sistema que poden beneficiar sistemes similars. D'altra banda, en aquesta tesi també contribuïm mesurant i analitzant la qualitat de servei (p.e., velocitat, variabilitat) de les transferències sobre les REST APIs oferides pels Núvols Personals. A més, durant aquest estudi, ens hem adonat que aquestes interfícies poden ser objecte d’abús quan són utilitzades sobre els comptes gratuïts que normalment ofereixen aquests serveis. Això ha motivat l’estudi d’aquesta vulnerabilitat, així com de potencials contramesures. A la Part II d'aquesta dissertació, la nostra primera contribució és analitzar la qualitat de servei que els sistemes d’emmagatzematge social poden proporcionar en termes de disponibilitat de dades, velocitat de transferència i balanceig de la càrrega. El nostre interès principal és entendre com fenòmens intrínsecs, com les dinàmiques de connexió dels usuaris o l’estructura de la xarxa social, limiten el rendiment d’aquests sistemes. També proposem nous mecanismes de manegament de dades per millorar aquestes limitacions. Finalment, dissenyem una arquitectura híbrida que combina recursos del Núvol i dels usuaris. Aquesta arquitectura té com a objectiu millorar la qualitat de servei del sistema i deixa als usuaris decidir la quantitat de recursos utilitzats del Núvol, o en altres paraules, és una decisió entre control de les seves dades i rendiment.Los usuarios cada vez necesitan espacios mayores de almacenamiento en línea para guardar su información personal. Este reto motiva a los investigadores a diseñar y evaluar nuevas infraestructuras de almacenamiento de datos personales. En esta tesis, nos centramos en dos arquitecturas emergentes de almacenamiento de datos personales: las Nubes Personales (centralización) y los sistemas de almacenamiento social (descentralización). Creemos que, pese a su creciente popularidad, estos sistemas requieren de un mayor estudio científico. En la Parte I de esta disertación, examinamos aspectos referentes a la operación interna y el rendimiento de varias Nubes Personales. Concretamente, nuestra primera contribución es desvelar la operación interna e infraestructura de una Nube Personal de gran escala (UbuntuOne, U1). Además, proporcionamos un estudio de la actividad interna de U1 que incluye la carga diaria soportada, el comportamiento de los usuarios y el rendimiento de su sistema de metadatos. También sugerimos mejoras sobre U1 que pueden ser de utilidad en sistemas similares. Por otra parte, en esta tesis medimos y caracterizamos el rendimiento del servicio de REST APIs ofrecido por varias Nubes Personales (velocidad de transferencia, variabilidad, etc.). También demostramos que la combinación de REST APIs sobre cuentas gratuitas de usuario puede dar lugar a abusos por parte de usuarios malintencionados. Esto nos motiva a proponer mecanismos para limitar el impacto de esta vulnerabilidad. En la Parte II de esta tesis, estudiamos la calidad de servicio que pueden ofrecer los sistemas de almacenamiento social en términos de disponibilidad de datos, balanceo de carga y tiempos de transferencia. Nuestro interés principal es entender la manera en que fenómenos intrínsecos, como las dinámicas de conexión de los usuarios o la estructura de su red social, limitan el rendimiento de estos sistemas. También proponemos nuevos mecanismos de gestión de datos para mejorar esas limitaciones. Finalmente, diseñamos y evaluamos una arquitectura híbrida para mejorar la calidad de servicio de los sistemas de almacenamiento social que combina recursos de usuarios y de la Nube. Esta arquitectura permite al usuario decidir su equilibrio entre control de sus datos y rendimiento.Increasingly, end-users demand larger amounts of online storage space to store their personal information. This challenge motivates researchers to devise novel personal storage infrastructures. In this thesis, we focus on two popular personal storage architectures: Personal Clouds (centralized) and social storage systems (decentralized). In our view, despite their growing popularity among users and researchers, there still remain some critical aspects to address regarding these systems. In the Part I of this dissertation, we examine various aspects of the internal operation and performance of various Personal Clouds. Concretely, we first contribute by unveiling the internal structure of a global-scale Personal Cloud, namely UbuntuOne (U1). Moreover, we provide a back-end analysis of U1 that includes the study of the storage workload, the user behavior and the performance of the U1 metadata store. We also suggest improvements to U1 (storage optimizations, user behavior detection and security) that can also benefit similar systems. From an external viewpoint, we actively measure various Personal Clouds through their REST APIs for characterizing their QoS, such as transfer speed, variability and failure rate. We also demonstrate that combining open APIs and free accounts may lead to abuse by malicious parties, which motivates us to propose countermeasures to limit the impact of abusive applications in this scenario. In the Part II of this thesis, we study the storage QoS of social storage systems in terms of data availability, load balancing and transfer times. Our main interest is to understand the way intrinsic phenomena, such as the dynamics of users and the structure of their social relationships, limit the storage QoS of these systems, as well as to research novel mechanisms to ameliorate these limitations. Finally, we design and evaluate a hybrid architecture to enhance the QoS achieved by a social storage system that combines user resources and cloud storage to let users infer the right balance between user control and QoS
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