<p>The ever increasing demand for video content directed the focus of researchfrom traditional server-based schemes to peer-to-peer systems for videodelivery. In such systems, video data is delivered to the users by utilizing theresources of the users themselves, leading to a potentially scalable solution.Users connect to each other, forming a p2p overlay network on top of theInternet and exchange the video segments among themselves. The performanceof a p2p system is characterized by its capability to deliver the videocontent to all peers without errors and with the smallest possible delay. Thisconstitutes a challenge since peers dynamically join and leave the overlay andalso contribute different amounts of resources to the system.The contribution of this thesis lies in two areas. The first area is theperformance evaluation of the most prominent p2p streaming architectures.We study the streaming quality in multiple-tree-based systems. We derivemodels to evaluate the stability of a multiple tree overlay in dynamic scenariosand the efficiency of the data distribution over the multiple trees. Then, westudy the data propagation in mesh-based overlays. We develop a generalframework for the evaluation of forwarding algorithms in such overlays anduse this framework to evaluate the performance of four different algorithms.The second area of the thesis is a study of streaming in heterogeneous p2poverlays. The streaming quality depends on the aggregate resources that peerscontribute to the system: low average contribution leads to low streamingquality. Therefore, maintaining high streaming quality requires mechanismsthat either prohibit non-contributing peers or encourage contribution. In thisthesis we investigate both approaches. For the former, we derive a model tocapture the evolution of available capacity in an overlay and propose simpleadmission control mechanisms to avoid capacity drainage. For the latter, inour last work, we propose a novel incentive mechanism that maximizes thestreaming quality in an overlay by encouraging highly contributing peers tooffer more of their resources.</p> / QC 20100506

Peer-to-peer (p2p) approaches are an increasingly effective way to deploy services. Popular examples include BitTorrent, Skype, and KaZaA. These approaches are attractive because they can be highly fault-tolerant, scalable, adaptive, and less expensive than a more centralized solution. Cooperation lies at the heart of these strengths. Yet, in settings where working together is crucial, a natural question is: "What if users stop cooperating?" After all, cooperative services are typically deployed over multiple administrative domains, and thus vulnerable to Byzantine failures and users who may act selfishly. This dissertation explores how to construct p2p systems to tolerate Byzantine participants while also incentivizing selfish participants to contribute resources. We describe how to balance obedience against choice in building a robust p2p live streaming system. Imposing obedience is desirable as it leaves little room for peers to attack or cheat the system. However, providing choice is also attractive as it allows us to engineer flexible and efficient solutions. We first focus on obedience by using Nash equilibria to drive the design of BAR Gossip, the first gossip protocol that is resilient to Byzantine and selfish nodes. BAR Gossip relies on verifiable pseudo-random partner selection to eliminate non-determinism, which can be used to game the system, while maintaining the robustness and rapid convergence of traditional gossip. A novel fair enough exchange primitive entices cooperation among selfish peers on short timescales, thereby avoiding the need for distributed reputation schemes. We next focus on tempering obedience with choice by using approximate equilibria to guide the construction of a novel p2p live streaming system. These equilibria allow us to design incentives to limit selfish behavior rigorously, yet provide sufficient flexibility to build practical systems. We show the advantages of using an [element of]-Nash equilibrium, instead of an exact Nash, to design and implement FlightPath, our live streaming system that uses bandwidth efficiently, absorbs flash crowds, adapts to sudden peer departures, handles churn, and tolerates malicious activity. / text

The ever increasing demand for video content directed the focus of researchfrom traditional server-based schemes to peer-to-peer systems for videodelivery. In such systems, video data is delivered to the users by utilizing theresources of the users themselves, leading to a potentially scalable solution.Users connect to each other, forming a p2p overlay network on top of theInternet and exchange the video segments among themselves. The performanceof a p2p system is characterized by its capability to deliver the videocontent to all peers without errors and with the smallest possible delay. Thisconstitutes a challenge since peers dynamically join and leave the overlay andalso contribute different amounts of resources to the system.The contribution of this thesis lies in two areas. The first area is theperformance evaluation of the most prominent p2p streaming architectures.We study the streaming quality in multiple-tree-based systems. We derivemodels to evaluate the stability of a multiple tree overlay in dynamic scenariosand the efficiency of the data distribution over the multiple trees. Then, westudy the data propagation in mesh-based overlays. We develop a generalframework for the evaluation of forwarding algorithms in such overlays anduse this framework to evaluate the performance of four different algorithms.The second area of the thesis is a study of streaming in heterogeneous p2poverlays. The streaming quality depends on the aggregate resources that peerscontribute to the system: low average contribution leads to low streamingquality. Therefore, maintaining high streaming quality requires mechanismsthat either prohibit non-contributing peers or encourage contribution. In thisthesis we investigate both approaches. For the former, we derive a model tocapture the evolution of available capacity in an overlay and propose simpleadmission control mechanisms to avoid capacity drainage. For the latter, inour last work, we propose a novel incentive mechanism that maximizes thestreaming quality in an overlay by encouraging highly contributing peers tooffer more of their resources. / QC 20100506

In the future, more data intensive applications, such as P2P auction networks, P2P job--search networks, P2P multi--player games, will require the capability to respond to more complex queries such as the nearest neighbor queries involving numerous data types. For the problem of answering nearest neighbor queries (NN query) for spatial region data in the P2P environment, a quadtree-based structure probably is a good choice. However, the quadtree stores the data in the leaf nodes, resulting in the load unbalance and expensive cost of any query. The MX--CIF quadtree can solve this problem. The MX--CIF quadtree has three properties: controlling efficiently the height of the tree, reducing load unbalance, and reducing the NNquery scope with controlling the value of the radius. Although the P2P MX--CIF quadtree can do the NN query efficiently, it still has some problems as follows: low accuracy of the nearest neighbor query, the expensive cost of the tree construction, the high search cost of the NN query, and load unbalance. In fact, the index structures for the region data can also work for the point data which can be considered as the degenerated case of the region data. Therefore, the KDB--tree which is a well-known algorithm for the point data can be used to reduce load unbalance, but it has the same problem as the quadtree. The data is stored only in the leaf nodes of the KDB--tree. In this thesis, we propose an Adjustable KDB--tree (AKDB--tree) to improve this situation for the P2P system. The AKDB--tree has five properties: reducing load unbalance, low cost of the tree construction, storing the data in the internal nodes and leaf nodes, high accuracy and low search cost of the NN query. The Chord system is a well--known structured P2P system in which the data search is performed by a hash function, instead of flooding used in most of the unstructured P2P system. Since the Chord system is a hash approach, it is easy to deal with peers joining/exiting. Besides, in order to combine AKDB--tree with the Chord system, we design the IDs of the nodes in the AKDB--tree. Each node is hashed to the Chord system by the ID. The IDs can be used to differentiate the edge node in the AKDB-tree is a vertical edge or a horizontal edge and the relative position of two nodes in the 2D space. And, we can calculate the related edge of a region in the 2D space according to the ID of the region. As discussed above, we make use of the property of IDs to reduce the search cost of the NN query by a wide margin. In our simulation study, we compare our method with the P2P MX--CIF quadtree by considering five performance measures under four different situations of the P2P MX--CIF quadtree. From our simulation results, for the NN query, our AKDB-tree can provide the higher accuracy and lower search cost than the P2P MX--CIF quadtree. For the problem of load, our AKDB-tree is more balance than the P2P MX--CIF quadtree. For the time of the tree construction, our AKDB-tree needs shorter time than the P2P MX--CIF quadtree.

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 méthodes d'apprentissage semi-supervisé constituent une catégorie de méthodes d'apprentissage automatique qui combinent points étiquetés et données non labellisées pour construire le classifieur. Dans la première partie de la thèse, nous proposons un formalisme d'optimisation général, commun à l'ensemble des méthodes d'apprentissage semi-supervisé et en particulier aux Laplacien Standard, Laplacien Normalisé et PageRank. En utilisant la théorie des marches aléatoires, nous caractérisons les différences majeures entre méthodes d'apprentissage semi-supervisé et nous définissons des critères opérationnels pour guider le choix des paramètres du noyau ainsi que des points étiquetés. Nous illustrons la portée des résultats théoriques obtenus sur des données synthétiques et réelles, comme par exemple la classification par le contenu et par utilisateurs des systèmes pair-à-pair. Cette application montre de façon édifiante que la famille de méthodes proposée passe parfaitement à l’échelle. Les algorithmes développés dans la deuxième partie de la thèse peuvent être appliquées pour la sélection des données étiquetées, mais également aux autres applications dans la recherche d'information. Plus précisément, nous proposons des algorithmes randomisés pour la détection rapide des nœuds de grands degrés et des nœuds avec de grandes valeurs de PageRank personnalisé. A la fin de la thèse, nous proposons une nouvelle mesure de centralité, qui généralise à la fois la centralité d'intermédiarité et PageRank. Cette nouvelle mesure est particulièrement bien adaptée pour la détection de la vulnérabilité de réseau. / Semi-supervised learning methods constitute a category of machine learning methods which use labelled points together with unlabeled data to tune the classifier. The main idea of the semi-supervised methods is based on an assumption that the classification function should change smoothly over a similarity graph. In the first part of the thesis, we propose a generalized optimization approach for the graph-based semi-supervised learning which implies as particular cases the Standard Laplacian, Normalized Laplacian and PageRank based methods. Using random walk theory, we provide insights about the differences among the graph-based semi-supervised learning methods and give recommendations for the choice of the kernel parameters and labelled points. We have illustrated all theoretical results with the help of synthetic and real data. As one example of real data we consider classification of content and users in P2P systems. This application demonstrates that the proposed family of methods scales very well with the volume of data. The second part of the thesis is devoted to quick detection of network central nodes. The algorithms developed in the second part of the thesis can be applied for the selections of quality labelled data but also have other applications in information retrieval. Specifically, we propose random walk based algorithms for quick detection of large degree nodes and nodes with large values of Personalized PageRank. Finally, in the end of the thesis we suggest new centrality measure, which generalizes both the current flow betweenness centrality and PageRank. This new measure is particularly well suited for detection of network vulnerability.

Une grande partie de l’impulsion de diverses technologies d’Internet par les systèmes Pair-à-Pair (Peer-to-Peer ou P2P) peut être vue comme une réaction au détriment du centrage de contenu sur les serveurs devant des clients passifs. Une des caractéristiques distinctives de tout système P2P est ce que nous appelons souvent connectivité directe de bout en bout entre pairs égaux. Le Pair-à-Pair a augmenté les débits des échanges entre des communautés dynamiques des utilisateurs qui tendent à augmenter rapidement. Nous parlons donc de systèmes distribués à large échelle dans lesquels l’information échangée, partagée et recherchée atteint des volumes de plus en plus impressionnants. Dans le cadre de cette thèse, nous nous intéressons essentiellement à la Recherche d'Information dans les systèmes de Recherche d’Information P2P (RIP2P) et plus précisément au problème d'agrégation des résultats dans de tels systèmes. Résoudre le problème d'agrégation en RIP2P de la même manière que sa résolution dans un cadre de Recherche d’Information Distribuée (RID) va manquer beaucoup d’intelligibilité. En effet, ça fait perdre de vue tout un contexte qui a changé en RIP2P, vu le facteur d'échelle et l’absence d’une vision globale sur le système, dans ces réseaux qui s'étendent naturellement à des milliers voire des millions de pairs. Ceci va impliquer notamment la suppression d'un serveur courtier inadéquat dans ce contexte et va soulever le problème de retrouver de nouvelles politiques pour agréger des résultats provenant de pairs hétérogènes dans une liste unique tout en reflétant les attentes de l'utilisateur. Toutes ces raisons nous ont incités à explorer un mécanisme d’agrégation basé sur les profils des utilisateurs déduits de leurs comportements passés suite à leurs interactions avec les résultats d’une requête. Dans cette thèse nos contributions portent sur deux axes complémentaires. D’abord, nous proposons une nouvelle vision d'agrégation de résultats dans un contexte large échelle. Dans ce cadre un modèle de profils et une approche de score hybride à base de profils sont proposés. Ensuite nous avons mis l’accent sur la mise en place d’un cadre d'évaluation de notre approche dans les systèmes à large échelle / A huge part of the impetus of various internet technologies through the Peer-to-Peer (Peer-to-Peer or P2P) system can be seen as a reaction to the content centering detriment on the servers in front of passive clients. One of the distinctive features of any P2P system is what we often call direct connectivity between equal peers. The Peer-to-Peer increased the exchange flows between dynamic communities of users which tend to grow rapidly. We talk, therefore, about large-scale distributed systems in which the exchanged, shared and sought information reaches a more and more impressive volumes. Solving the aggregation problem in P2PIR systems the same way as its resolution in Distributed Information Retrieval (DIR) will miss a lot of intelligibility. In fact, the context has changed in RIP2P, given the scale factor and the lack of a global vision of the system in these networks that extend naturally to thousands or even millions peers. This will involve the removal of a broker server that is inadequate in this context and will raise the problem of finding new policies to aggregate results coming from heterogeneous peers in a single list while reflecting the user’s expectations. All these reasons prompted us to explore an aggregation mechanism based on user profiles deduced from their past behavior due to their interaction with query results. Our contributions, in this thesis, focus on two complementary axes. First, we propose a new vision of results aggregation in a large scale system. In this context, a profiles model and a hybrid score profiles-based approach are proposed. Second, we focused on the development of an evaluation framework of our approach in large-scale systems. In this thesis, we are mainly interested in the Information Retrieval problem in P2P systems (P2PIR) and focusing more specifically on the problem of results’ aggregation in such systems

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.

O uso de Internet para a distribuição de fluxos de vídeo tem se mostrado uma tendência atual e traz consigo grandes desafios. O alicerce sobre qual a Internet está fundamentada, comutação por pacotes e arquitetura cliente-servidor, não proporciona as melhores condições para este tipo de serviço. A arquitetura P2P (peer-to-peer) vem sendo considerada como infraestrutura para a distribuição de fluxos de vídeo na Internet. A idéia básica da distribuição de vídeo com o suporte de P2P é a de que os vários nós integrantes da rede sobreposta distribuem e encaminham pedaços de vídeo de forma cooperativa, dividindo as tarefas, e colocando à disposição da rede seus recursos locais. Dentro deste contexto, é importante investigar o que ocorre com a qualidade do serviço de distribuição de vídeo quando a infraestrutura provida pelas redes P2P é contaminada por nós que não estejam dispostos a cooperar, já que a base desta arquitetura é a cooperação. Neste trabalho, inicialmente é feito um estudo para verificar o quanto a presença de nós não-cooperativos pode afetar a qualidade da aplicação de distribuição de fluxo de vídeo em uma rede P2P. Com base nos resultados obtidos, é proposto um mecanismo de incentivo à cooperação para que seja garantida uma boa qualidade de vídeo aos nós cooperativos e alguma punição aos nós não-cooperativos. Os testes e avaliações foram realizados utilizando-se o simulador PeerSim. / Using the Internet for video stream is becoming a trend, but it brings many challenges. The foundation upon which the Internet is based, packet switching and client-server architecture, is not suitable for this type of service. P2P (peer to peer) architecture is being considered as an infrastructure for video streams on the Internet. The basic idea is that the several members of the overlay network cooperate in the task of distributing and fowarding video chunks, making available their local resources to the network. Within this context, it is important to investigate what happens to the quality of service of the video distribution when the infrastructure provided by the P2P network is contaminated with free-riding nodes, which are not willing to cooperate, since the basis of this architecture is cooperation. In this work, study is initially carried out to check how the presence of uncooperative nodes can affect the quality of the distribution application of video streaming on a P2P network. Based on these results, a mechanism is proposed to encourage cooperation in order to be guaranteed a video with good quality to the cooperative nodes and some punishment for those uncooperative. The tests and evaluations were performed using the PeerSim simulator.

O uso de Internet para a distribuição de fluxos de vídeo tem se mostrado uma tendência atual e traz consigo grandes desafios. O alicerce sobre qual a Internet está fundamentada, comutação por pacotes e arquitetura cliente-servidor, não proporciona as melhores condições para este tipo de serviço. A arquitetura P2P (peer-to-peer) vem sendo considerada como infraestrutura para a distribuição de fluxos de vídeo na Internet. A idéia básica da distribuição de vídeo com o suporte de P2P é a de que os vários nós integrantes da rede sobreposta distribuem e encaminham pedaços de vídeo de forma cooperativa, dividindo as tarefas, e colocando à disposição da rede seus recursos locais. Dentro deste contexto, é importante investigar o que ocorre com a qualidade do serviço de distribuição de vídeo quando a infraestrutura provida pelas redes P2P é contaminada por nós que não estejam dispostos a cooperar, já que a base desta arquitetura é a cooperação. Neste trabalho, inicialmente é feito um estudo para verificar o quanto a presença de nós não-cooperativos pode afetar a qualidade da aplicação de distribuição de fluxo de vídeo em uma rede P2P. Com base nos resultados obtidos, é proposto um mecanismo de incentivo à cooperação para que seja garantida uma boa qualidade de vídeo aos nós cooperativos e alguma punição aos nós não-cooperativos. Os testes e avaliações foram realizados utilizando-se o simulador PeerSim. / Using the Internet for video stream is becoming a trend, but it brings many challenges. The foundation upon which the Internet is based, packet switching and client-server architecture, is not suitable for this type of service. P2P (peer to peer) architecture is being considered as an infrastructure for video streams on the Internet. The basic idea is that the several members of the overlay network cooperate in the task of distributing and fowarding video chunks, making available their local resources to the network. Within this context, it is important to investigate what happens to the quality of service of the video distribution when the infrastructure provided by the P2P network is contaminated with free-riding nodes, which are not willing to cooperate, since the basis of this architecture is cooperation. In this work, study is initially carried out to check how the presence of uncooperative nodes can affect the quality of the distribution application of video streaming on a P2P network. Based on these results, a mechanism is proposed to encourage cooperation in order to be guaranteed a video with good quality to the cooperative nodes and some punishment for those uncooperative. The tests and evaluations were performed using the PeerSim simulator.