The blockchain technology promises innovation by moving away from conventional centralized architectures, where trust is placed in a small number of actors, to a decentralized environment where a collection of actors must work together to maintain consensus in the overall system. Blockchain offers security and pseudo-anonymity to its adopters, through the use of various cryptographic methods. While much attention has focused on creating new applications that make use of this technology, equal importance must be given to studying naturally occurring phenomena in existing blockchain ecosystems and mitigating their effects where harmful. In this dissertation, we develop a novel open-source log-to-file system that provides the ability to record information relevant to events as they take place in live blockchain networks. Specifically, our open-source software facilitates in-situ measurements on full nodes in the live Bitcoin and Bitcoin Cash blockchain networks. This measurement framework sheds new light on many phenomena that were previously unknown or scarcely studied. First, we examine the presence and impact of churn, namely nodes joining and leaving, on the behavior of the Bitcoin network. Our data analysis over a two-month period shows that a large number of Bitcoin nodes churn at least once. We perform statistical distribution fitting to this churn and emulate it in our measurement nodes to evaluate the impact of churn on the performance of the Bitcoin protocol. From our experiments, we find that blocks received by churning nodes experience as much as five times larger propagation delay than those received by non-churning nodes. We introduce and evaluate a novel synchronization scheme to mitigate such effects on the performance of the protocol. Our empirical evaluation shows that blocks received by churning nodes that synchronize their mempools with peers have roughly half the delay in propagation experienced by those that do not synchronize their mempools. We next evaluate and compare the performance of three block relay protocols, namely the default protocol, and the more recent compact block and Graphene protocols. This evaluation is conducted over full nodes running the Bitcoin Unlimited client (which is used in conjunction with the Bitcoin Cash network). We find that in most scenarios, the Graphene block relay protocol outperforms the other two in terms of the block propagation delay and the amount of total communication associated with block relay. An exception is when nodes churn frequently and spend a significant fraction of time off the network, in which case the compact block relay protocol performs best. In-depth analyses reveal subtle inefficiencies of the protocols. Thus, in the case of frequent churns, the Graphene block relay protocol performs as many as two extra round-trips of communication to recover information necessary to reconstruct blocks. Likewise, an inspection of the compact block relay protocol indicates that the full transactions included in the initial block message are either unnecessary or insufficient for the successful reconstruction of blocks. Finally, we investigate the occurrence of orphan transactions which are those whose parental income sources are missing at the time that they are processed. These transactions typically languish in a local buffer until they are evicted or all their parents are discovered, at which point they may be propagated further. Our data reveals that slightly less than half of orphan transactions end up being included in the blockchain. Surprisingly, orphan transactions tend to have fewer parents on average than non-orphan transactions, and their missing parents have a lower fee, a larger size, and a lower transaction fee per byte than all other received transactions. Moreover, the network overhead incurred by these orphan transactions can be significant when using the default orphan memory pool size (i.e., 100 transactions), although this overhead can be made negligible if the pool size is simply increased to 1,000 transactions. In summary, this dissertation demonstrates the importance of characterizing the inner behavior of the peer-to-peer network underlying a blockchain. While our results primarily focus on the Bitcoin network and its variants, this work provides foundations that should prove useful for studying and characterizing other blockchains.

Content distribution networks deliver content like videos, apps, and music to users through servers deployed in multiple datacenters to increase availability and delivery speed of content. The motivation of this work is to create a content distribution network that maintains a consumer’s rights and access to works they have purchased indefinitely. If a user purchases content from a traditional content distribution network, they lose access to the content when the service is no longer available. The system uses a peer to peer network for content distribution along with a blockchain for digital rights management. This combination may give users indefinite access to purchased works. The system benefits content rights owners because they can sell their content in a lower cost manner by distributing costs among the community of peers.

Detta arbete undersöker hur skalbarheten i ett peer-to-peer-nätverk (P2P) byggt med voronoi-based overlay network (VON) som topologi, påverkas av entitetshantering och felhantering, när det används i nätverkslösnigen till ett realtidsspel med många spelare. Under arbetet skapades en testplattform som används under ett experiment för att utvärdera huruvida aspekter såsom ansvarsuppdelning och nodkraschhantering påverkar antalet meddelanden som behöver skickas och därigenom skalbarheten i nätverket. Experimentet undersöker flera fall, med olika mycket betoning på entitetshantering och felhantering och resultaten visar att nätverket behåller sin skalbarhet och att totala antalet meddelanden som skickas håller sig mestadels opåverkad mellan fallen, trots hanteringen. / <p>Det finns övrigt digitalt material (t.ex. film-, bild- eller ljudfiler) eller modeller/artefakter tillhörande examensarbetet som ska skickas till arkivet.</p><p>There are other digital material (eg film, image or audio files) or models/artifacts that belongs to the thesis and need to be archived.</p>

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No / With the rapid growth of the scale, complexity, and heterogeneity of Peer-to-Peer (P2P) systems, it has become a great challenge to deal with the peer's network-oblivious traffic and self-organization problems. A potential solution is to deploy servers in appropriate locations. However, due to the unique features and requirements of P2P systems, the traditional placement models cannot yield the desirable service performance. To fill this gap, we propose an efficient server placement model for P2P live streaming systems. Compared to the existing solutions, this model takes the Internet Service Provider (ISP) friendly problem into account and can reduce the cross-network traffic among ISPs. Specifically, we introduce the peers' contribution into the proposed model, which makes it more suitable for P2P live streaming systems. Moreover, we deploy servers based on the theoretical solution subject to practical data and apply them to practical live streaming applications. The experimental results show that this new model can reduce the amount of cross-network traffic and improve the system efficiency, has a better adaptability to Internet environment, and is more suitable for P2P systems than the traditional placement models.

In the recent decade, peer-to-peer (P2P) technology has greatly enhanced the scalability of multimedia streaming on the Internet by enabling efficient cooperation among end-users. However, existing streaming applications are plagued by the problems of long playback latency and long churn-induced delays. First of all, many streaming applications, such as IPTV and video conferencing, have rigorous constraints on end-to-end delays. Moreover, churn-induced delays, including delays from channel switching and streaming recovery, in current P2P streaming applications are typically in the scale of 10-60 seconds, which is far below the favorable user experience as in cable TV systems. These two issues in terms of playback latency and churn-induced delays have hindered the extensive commercial deployment of P2P systems. Motivated by this, in this dissertation, we focus on reducing delays in P2P live streaming systems. Specifically, we propose solutions for reducing delays in P2P live streaming systems in four problem spaces: (1) minimizing the maximum end-to-end delay in P2P streaming; (2) minimizing the average end-to-end delay in P2P streaming; (3) minimizing the average delay in multi-channel P2P streaming; and (4) reducing churn-induced delays. We devise a streaming scheme to minimize the maximum end-to-end streaming delay under a mesh-based overlay network paradigm. We call this problem, the MDPS problem. We formulate the MDPS problem and prove its NP-completeness. We then present a polynomial-time approximation algorithm, called Fastream-I, for this problem, and show that the performance of Fastream-I is bounded by a ratio of O(SQRT(log n)), where n is the number of peers in the system. We also develop a distributed version of Fastream-I that can adapt to network dynamics. Our simulation study reveals the effectiveness of Fastream-I, and shows a reasonable message overhead. While Fastream-I yields the minimum maximum end-to-end streaming delay (within a factor of O(SQRT(log n)), in many P2P settings, users may desire the minimum average end-to-end P2P streaming delay. Towards this, we devise a streaming scheme which optimizes the bandwidth allocation to achieve the minimum average end-to-end P2P streaming delay. We call this problem, the MADPS problem. We first develop a generic analytical framework for the MADPS problem. We then present Fastream-II as a solution to the MADPS problem. The core part of Fastream-II is a fast approximation algorithm, called APX-Fastream-II, based on primal-dual schema. We prove that the performance of APX-Fastream-II is bounded by a ratio of 1+w, where w is an adjustable input parameter. Furthermore, we show that the flexibility of w provides a trade-off between the approximation factor and the running time of Fastream-II. The third problem space of the dissertation is minimizing the average delay in multi-channel P2P streaming systems. Toward this, we present an algorithm, called Fastream-III. To reduce the influence from frequent channel-switching behavior, we build Fastream-III for the view-upload decoupling (VUD) model, where the uploaded content from a serving node is independent of the channel it views. We devise an approximation algorithm based on primal-dual schema for the critical component of Fastream-III, called APX-Fastream-III. In contrast to APX-Fastream-II, APX-Fastream-III addresses the extra complexity in the multichannel scenario and maintains the approximation bound by a ratio of 1+w. Besides playback lag, delays occurring in P2P streaming may arise from two other factors: node churn and channel switching. Since both stem from the re-connecting request in churn, we call them churn-induced delays. Optimizing churn-induced delays is the dissertation's fourth problem space. Toward this, we propose NAP, a novel agent-based P2P scheme, that provides preventive connections to all channels. Each channel in NAP selects powerful peers as agents to represent the peers in the channel to minimize control and message overheads. Agents distill the bootstrapping peers with superior bandwidth and lifetime expectation to quickly serve the viewer in the initial period of streaming. We build a queueing theory model to analyze NAP. Based on this model, we numerically compare NAP's performance with past efforts. The results of the numerical analysis reveal the effectiveness of NAP. / Ph. D.

The threat of surveillance by governmental and industrial parties is more eminent than ever. As communication moves into the digital domain, the advances in automatic assessment and interpretation of enormous amounts of data enable tracking of millions of people, recording and monitoring their private life with an unprecedented accurateness. The knowledge of such an all-encompassing loss of privacy affects the behavior of individuals, inducing various degrees of (self-)censorship and anxiety. Furthermore, the monopoly of a few large-scale organizations on digital communication enables global censorship and manipulation of public opinion. Thus, the current situation undermines the freedom of speech to a detrimental degree and threatens the foundations of modern society. Anonymous and censorship-resistant communication systems are hence of utmost importance to circumvent constant surveillance. However, existing systems are highly vulnerable to infiltration and sabotage. In particular, Sybil attacks, i.e., powerful parties inserting a large number of fake identities into the system, enable malicious parties to observe and possibly manipulate a large fraction of the communication within the system. Friend-to-friend (F2F) overlays, which restrict direct communication to parties sharing a real-world trust relationship, are a promising countermeasure to Sybil attacks, since the requirement of establishing real-world trust increases the cost of infiltration drastically. Yet, existing F2F overlays suffer from a low performance, are vulnerable to denial-of-service attacks, or fail to provide anonymity. Our first contribution in this thesis is concerned with an in-depth analysis of the concepts underlying the design of state-of-the-art F2F overlays. In the course of this analysis, we first extend the existing evaluation methods considerably, hence providing tools for both our and future research in the area of F2F overlays and distributed systems in general. Based on the novel methodology, we prove that existing approaches are inherently unable to offer acceptable delays without either requiring exhaustive maintenance costs or enabling denial-of-service attacks and de-anonymization. Consequentially, our second contribution lies in the design and evaluation of a novel concept for F2F overlays based on insights of the prior in-depth analysis. Our previous analysis has revealed that greedy embeddings allow highly efficient communication in arbitrary connectivity-restricted overlays by addressing participants through coordinates and adapting these coordinates to the overlay structure. However, greedy embeddings in their original form reveal the identity of the communicating parties and fail to provide the necessary resilience in the presence of dynamic and possibly malicious users. Therefore, we present a privacy-preserving communication protocol for greedy embeddings based on anonymous return addresses rather than identifying node coordinates. Furthermore, we enhance the communication’s robustness and attack-resistance by using multiple parallel embeddings and alternative algorithms for message delivery. We show that our approach achieves a low communication complexity. By replacing the coordinates with anonymous addresses, we furthermore provably achieve anonymity in the form of plausible deniability against an internal local adversary. Complementary, our simulation study on real-world data indicates that our approach is highly efficient and effectively mitigates the impact of failures as well as powerful denial-of-service attacks. Our fundamental results open new possibilities for anonymous and censorship-resistant applications. / Die Bedrohung der Überwachung durch staatliche oder kommerzielle Stellen ist ein drängendes Problem der modernen Gesellschaft. Heutzutage findet Kommunikation vermehrt über digitale Kanäle statt. Die so verfügbaren Daten über das Kommunikationsverhalten eines Großteils der Bevölkerung in Kombination mit den Möglichkeiten im Bereich der automatisierten Verarbeitung solcher Daten erlauben das großflächige Tracking von Millionen an Personen, deren Privatleben mit noch nie da gewesener Genauigkeit aufgezeichnet und beobachtet werden kann. Das Wissen über diese allumfassende Überwachung verändert das individuelle Verhalten und führt so zu (Selbst-)zensur sowie Ängsten. Des weiteren ermöglicht die Monopolstellung einiger weniger Internetkonzernen globale Zensur und Manipulation der öffentlichen Meinung. Deshalb stellt die momentane Situation eine drastische Einschränkung der Meinungsfreiheit dar und bedroht die Grundfesten der modernen Gesellschaft. Systeme zur anonymen und zensurresistenten Kommunikation sind daher von ungemeiner Wichtigkeit. Jedoch sind die momentanen System anfällig gegen Sabotage. Insbesondere ermöglichen es Sybil-Angriffe, bei denen ein Angreifer eine große Anzahl an gefälschten Teilnehmern in ein System einschleust und so einen großen Teil der Kommunikation kontrolliert, Kommunikation innerhalb eines solchen Systems zu beobachten und zu manipulieren. F2F Overlays dagegen erlauben nur direkte Kommunikation zwischen Teilnehmern, die eine Vertrauensbeziehung in der realen Welt teilen. Dadurch erschweren F2F Overlays das Eindringen von Angreifern in das System entscheidend und verringern so den Einfluss von Sybil-Angriffen. Allerdings leiden die existierenden F2F Overlays an geringer Leistungsfähigkeit, Anfälligkeit gegen Denial-of-Service Angriffe oder fehlender Anonymität. Der erste Beitrag dieser Arbeit liegt daher in der fokussierten Analyse der Konzepte, die in den momentanen F2F Overlays zum Einsatz kommen. Im Zuge dieser Arbeit erweitern wir zunächst die existierenden Evaluationsmethoden entscheidend und erarbeiten so Methoden, die Grundlagen für unsere sowie zukünftige Forschung in diesem Bereich bilden. Basierend auf diesen neuen Evaluationsmethoden zeigen wir, dass die existierenden Ansätze grundlegend nicht fähig sind, akzeptable Antwortzeiten bereitzustellen ohne im Zuge dessen enorme Instandhaltungskosten oder Anfälligkeiten gegen Angriffe in Kauf zu nehmen. Folglich besteht unser zweiter Beitrag in der Entwicklung und Evaluierung eines neuen Konzeptes für F2F Overlays, basierenden auf den Erkenntnissen der vorangehenden Analyse. Insbesondere ergab sich in der vorangehenden Evaluation, dass Greedy Embeddings hoch-effiziente Kommunikation erlauben indem sie Teilnehmer durch Koordinaten adressieren und diese an die Struktur des Overlays anpassen. Jedoch sind Greedy Embeddings in ihrer ursprünglichen Form nicht auf anonyme Kommunikation mit einer dynamischen Teilnehmermengen und potentiellen Angreifern ausgelegt. Daher präsentieren wir ein Privätssphäre-schützenden Kommunikationsprotokoll für F2F Overlays, in dem die identifizierenden Koordinaten durch anonyme Adressen ersetzt werden. Des weiteren erhöhen wir die Resistenz der Kommunikation durch den Einsatz mehrerer Embeddings und alternativer Algorithmen zum Finden von Routen. Wir beweisen, dass unser Ansatz eine geringe Kommunikationskomplexität im Bezug auf die eigentliche Kommunikation sowie die Instandhaltung des Embeddings aufweist. Ferner zeigt unsere Simulationstudie, dass der Ansatz effiziente Kommunikation mit kurzen Antwortszeiten und geringer Instandhaltungskosten erreicht sowie den Einfluss von Ausfälle und Angriffe erfolgreich abschwächt. Unsere grundlegenden Ergebnisse eröffnen neue Möglichkeiten in der Entwicklung anonymer und zensurresistenter Anwendungen.

Les Systèmes multi-agents ou SMA proposent une approche originale de conception de systèmes intelligents et coopératifs. Ils se caractérisent par la distribution du contrôle global du système et par la présence d'agents autonomes évoluant dans un environnement partagé et dynamique. De plus, il existe plusieurs interdépendances entre les buts des agents, leurs capacités et les ressources qu'ils utilisent, donc afin d'éviter d'éventuels conflits, de favoriser la synergie des activités des agents et de partager les ressources de l’environnement commun, il est important que les agents coordonnent leurs actions. Nous nous sommes intéressés, dans le cadre de cette thèse, au déploiement des systèmes multi-agents sur une architecture pair à pair (Peer-to-Peer ou P2P) et ceci dans le but d’établir la communication entre ces agents et les relier entre eux. Cependant, du fait de la nature dynamique des systèmes P2P où chaque pair peut apparaître et disparaître à tout moment, des nouveaux problèmes se posent pour la coordination d’agents nécessitant des mécanismes de coordination adaptés au contexte spécifique des P2P. Pour cela, nous avons proposé une méthode de formation de groupe comme solution à ces problèmes. Nous nous sommes intéressés ensuite à l’application des systèmes multi-agents à architecture P2P au domaine d’apprentissage collaboratif en ligne où des apprenants contribuent aux apprentissages du groupe, et en retour, le groupe contribue à ceux des apprenants et c’est la cohérence du collectif qui permet d’atteindre l’objectif. Cependant, l’apprentissage collaboratif à distance implique des nouveaux rôles pour l’enseignant ainsi que pour les apprenants. Il est donc essentiel de définir ces rôles pour identifier les besoins qui en découlent pour pouvoir intégrer à l’outil informatique des fonctionnalités afin de satisfaire ces besoins. En effet, il est essentiel de fournir aux enseignants et apprenants la possibilité d’avoir des informations sur la progression de leur apprentissage ainsi que sur les niveaux de collaboration et de sociabilité de chaque apprenant et du groupe. Enfin, nous avons proposé, comme application de nos travaux, un système appelé COLYPAN (COllaborative Learning sYstem for Project mANagment) conçu pour l’apprentissage à distance et de façon collaborative de la gestion des projets. / Multi-Agents systems (MAS) propose an original approach to design intelligent and cooperative systems. They are characterized by the distribution of the overall system control and the presence of autonomous agents operating in a shared and dynamic environment. In addition, there are many interdependencies between: agents' goals, their abilities and used resources. So, in order to avoid possible conflicts, promote synergy of agents activities and share resources of the common environment, it is important that the agents coordinate their actions. We are interested, in the context of this thesis, in the deployment of multi-agents systems on Peer-to-Peer (P2P) networks in order to establish communication between these agents. However, because of the dynamic nature of P2P systems where each peer may appear and disappear at any time, new problems arise concerning the coordination of agents. Thus, coordination mechanisms adapted to the specific context of P2P are required. For that, we have proposed a group formation method to solve these problems. Then, we were interested in the usage of multi-agents systems with P2P architecture in the field of collaborative e-learning. In such applications, each learner contributes in the learning process of the group, and in return, the group contributes in the learning process of its members. The consistency of the whole group allows to achieve the goal. However, collaborative e-learning implies new roles for teachers as well as for learners. It is therefore essential to define these roles in order to identify the users needs and integrate, in the platform, the functionalities that allows us to satisfy such needs. Indeed, it is essential to provide teachers and learners with the opportunity to obtain information about the progress of their learning processes as well as the level of collaboration and sociability of each learner in the group. Finally, we have proposed, as an application of our work, a system called COLYPAN (COllaborative Learning sYstem for Project management) designed for the collaborative e-Learning project management.

Serviços para entrega de conteúdo multimídia tem se tornado cada vez mais comuns com o advento de conexões mais rápidas à Internet. Porém, esse crescimento na base de usuários consumidores desse tipo de serviços, levam a gargalo de desempenho devido à restrições de recursos de infraestrutura. Para tentar reduzir tais restrições, análises na forma como o conteúdo pode ser entregue foram realizadas e novos mecanismos de entrega de conteúdo surgiram. Um desses mecanismos envolve o uso de redes P2P - Par a Par, no qual o usuário é consumidor e provedor de conteúdo para seus pares. Sistemas IPTV utilizando protocolos P2P estão se tornando populares pelo mundo, devido à descentralização da infraestrutura necessária para entrega do conteúdo, e redução do gargalo de desempenho. Um dos protocolos de rede P2P mais comum é BitTorrent, que é utilizado amplamente pelo mundo, devido à seu método descentralizado de compartilhar os dados. Dos trabalhos desenvolvidos utilizando este método, houve uma necessidade de avaliar os requisitos de segurança para garantir o funcionamento do sistema sem comprometer a infraestrutura ou o usuário. Porém, existem problemas de segurança em aberto, e o objetivo deste trabalho é o de analisar a privacidade do usuário enquanto utiliza o sistema. Para tanto, este trabalho propõe o P2Priv-TV, um mecanismo que garante que o conteúdo que um determinado usuário irá assistir não será de conhecimento dos demais usuários da rede. Este mecanismo é avaliado por meio de emulação para determinar sua viabilidade e eficácia. / There are many video streaming services available to users due to broadband connections popularity. These kind of service has increasing numbers of users and that may lead to performance bottlenecks because of shortage of infrastructure resources. To avoid shortages, there are many studies to think in a new way to deliver content and this lead to new deliver methods. One of these methods is the use of P2P (peer to peer) Networks, where the user is content consumer and provider to other peers. BitTorrent is the most popular P2P protocol widely used in the world and there are IPTV systems using this protocol as content delivery mechanism. P2P IPTV systems were developed and security issues were analyzed to ensure that users and system do not been compromised. However, there are other issues, and the goal of this work is to analyze user privacy in the system. This work presents the P2Priv-TV, a mechanism that ensure a user can consume a content without another user in the system knows about it. This mechanism will be analyzed using emulation to analyze feasibility and effectiveness.