Diss. (sammanfattning) Uppsala : Univ., 2003. / Härtill 6 uppsatser.

Thesis (Ph.D.)--University of Wollongong, 2005. / Typescript. Includes bibliographical references: leaf 88-95.

Thesis (M.Geom.M.)--University of Melbourne, Dept. of Geomatics, 2007. / Typescript. Includes bibliographical references (leaves 100-104).

Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Massively Multiplayer Online Games are large virtual worlds co-inhabited by players over the Internet. As up to thousands of players can be simultaneously connected to the game, the server and network architectures are required to scale e ciently. The traditional client/server model results in a heavy nancial burden for operation of the server. Various alternative architectures have been proposed as a replacement for the traditional model, but the adoption of these alternatives are slow as they present their own set of challenges. The proposed hybrid system is based on many di erent architectures and peer-topeer concepts that were reviewed in the literature. It aims to provide a compromise for existing, commercially successful MMOGs to introduce peer-to-peer components into their systems with no requirement of modi cation to their server or client software. With the system's design presented, the middleware software is implemented and deployed in a real, controlled environment alongside an Ultima Online game server and its clients. The movement game mechanic was distributed amongst the peers while the others remained the responsiblity of the server. A number of performance experiments are performed to measure the e ects of the modi ed system over the original client/server system on bandwidth, latency, and hardware impact. The results revealed an increase in the server bandwidth usage by 35%, slave bandwidth usage by 17% and supernode bandwiwdth usage by 3111%. The latencies of distributed server mechanics were reduced by up to 94%, while the non-distributed latencies were increased by up to 6000%. These results suggested that a system with absolutely no modi cation to the server is unlikely to provide the desired bene ts. However, with 2 minor modi cations to the server, the middleware is able to reduce both server load and player latencies. The server bandwidth can be reduced by 39%, while the supernode's bandwidth is increased only by 1296%. The distributed latencies maintain their reduction while non-distributed latencies remain unchanged from the C/S system. / AFRIKAANSE OPSOMMING: Massiewe Multispeler Aanlyn Speletjies (MMAS) is groot virtuele w^erelde op die Internet wat bewoon word deur spelers. Aangesien duisende spelers gelyktydig kan inskakel op die speletjie word daar verwag van die bediener en netwerk argitektuur om e ektief te skaleer om die groot hoeveelhede spelers te kan hanteer. Die traditionele kli ent/bediener model lei tot 'n groot nansi ele las vir die operateur van die bediener. Verskeie alternatiewe argitekture is al voorgestel om die tradisionele model te vervang, maar die aanvaarding en in gebruik neem van hierdie alternatiewe (soos eweknie-netwerke) is 'n stadige proses met sy eie stel uitdagings. Die voorgestelde hibriede stelsel is gebaseer op baie verskillende argitektuur- en eweknie konsepte wat in die literatuur oorweeg is. Die doel is om 'n kompromie vir bestaande komersieel suksesvolle MMASs te verskaf om eweknie komponente te implementeer sonder om die die bediener- of kli ent sagteware aan te pas. Met hierdie stelsel se ontwerp word die middelware sagteware ge mplementeer en gebruik in 'n regte, dog gekontroleerde omgewing, tesame met 'n Ultima Online bediener en sy kli ente. Die beweging speletjie meganisme word versprei onder die eweknie netwerk en die ander meganismes bly die verantwoordelikheid van die bediener. 'n Aantal eksperimente is ingespan om die e ek van die hibriede stelsel te meet op die oorspronklike kli ent/bediener stelsel, in terme van bandwydte, vertraging en impak op hardeware. Die resultate toon 'n toename van 35% in bediener-, 17% in slaaf-, en 3111% in supernodus bandwydte gebruik. Die vertraging van verspreide bediener meganismes neem af met tot 94%, terwyl onverspreide vertragings toeneem met tot 6000%. Hierdie resultate wys dat 'n stelsel wat geen aanpassing maak aan die bediener sagteware onwaarskynlik die gewenste voordele sal lewer. Deur egter 2 klein aanpassings toe te laat tot die bediener, is dit moontlik vir die hibriede stelsel om data las van die bediener en die speler se vertraging te verminder. Die bediener bandwydte kan met 39% verminder word, terwyl die supernodus bandwydte slegs met 1296% toeneem. Die verpreide vertragings handhaaf hul vermindering, terwyl die onverspreide vertragings onveranderd bly van die C/S stelsel.

Millions of users world-wide are sharing content using Peer-to-Peer (P2P) networks, such as Skype and Bit Torrent. While such new innovations undoubtedly bring benefits, there are nevertheless some associated threats. One of the main hazards is that P2P worms can penetrate the network, even from a single node and then spread rapidly. Understanding the propagation process of such worms has always been a challenge for researchers. Different techniques, such as simulations and analytical models, have been adopted in the literature. While simulations provide results for specific input parameter values, analytical models are rather more general and potentially cover the whole spectrum of given parameter values. Many attempts have been made to model the worm propagation process in P2P networks. However, the reported analytical models to-date have failed to cover the whole spectrum of all relevant parameters and have therefore resulted in high false-positives. This consequently affects the immunization and mitigation strategies that are adopted to cope with an outbreak of worms. The first key contribution of this thesis is the development of a susceptible, exposed, infectious, and Recovered (SEIR) analytical model for the worm propagation process in a P2P network, taking into account different factors such as the configuration diversity of nodes, user behaviour and the infection time-lag. These factors have not been considered in an integrated form previously and have been either ignored or partially addressed in state-of-the-art analytical models. Our proposed SEIR analytical model holistically integrates, for the first time, these key factors in order to capture a more realistic representation of the whole worm propagation process. The second key contribution is the extension of the proposed SEIR model to the mobile M-SEIR model by investigating and incorporating the role of node mobility, the size of the worm and the bandwidth of wireless links in the worm propagation process in mobile P2P networks. The model was designed to be flexible and applicable to both wired and wireless nodes. The third contribution is the exploitation of a promising modelling paradigm, Agent-based Modelling (ABM), in the P2P worm modelling context. Specifically, to exploit the synergies between ABM and P2P, an integrated ABM-Based worm propagation model has been built and trialled in this research for the first time. The introduced model combines the implementation of common, complex P2P protocols, such as Gnutella and GIA, along with the aforementioned analytical models. Moreover, a comparative evaluation between ABM and conventional modelling tools has been carried out, to demonstrate the key benefits of ease of real-time analysis and visualisation. As a fourth contribution, the research was further extended by utilizing the proposed SEIR model to examine and evaluate a real-world data set on one of the most recent worms, namely, the Conficker worm. Verification of the model was achieved using ABM and conventional tools and by then comparing the results on the same data set with those derived from developed benchmark models. Finally, the research concludes that the worm propagation process is to a great extent affected by different factors such as configuration diversity, user-behaviour, the infection time lag and the mobility of nodes. It was found that the infection propagation values derived from state-of-the-art mathematical models are hypothetical and do not actually reflect real-world values. In summary, our comparative research study has shown that infection propagation can be reduced due to the natural immunity against worms that can be provided by a holistic exploitation of the range of factors proposed in this work.

Orientador: Paulo Licio de Geus / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Computação / Made available in DSpace on 2018-08-13T11:26:28Z (GMT). No. of bitstreams: 1 Quinellato_DouglasGielo_M.pdf: 2170090 bytes, checksum: 70af102166738a9e7bd99af678848faf (MD5) Previous issue date: 2009 / Resumo: As redes P2P ganharam bastante popularidade na ultima decada, consolidando-se como um dos serviços mais populares da internet, provendo uma arquitetura distribuída para o fornecimento de servi¸cos sem a necessidade de um host assumir o papel de servidor. A popularidade trouxe, entretanto, a necessidade de se desenvolver mecanismos para garantir o funcionamento perante os crescentes ataques 'a rede. Com a estabilidade dos algoritmos relacionados ao funcionamento das redes P2P foi possível um aumento no desenvolvimento destes mecanismos de segurança. Nesta dissertação e proposto um sistema de reputação para redes P2P de compartilhamento de arquivos, um mecanismo de seguran¸ca que visa impedir a proliferação de arquivos corrompidos. Tais sistemas funcionam gerenciando as opiniões emitidas pelos nós participantes da rede sobre os serviços prestados pelos outros nós. Estas opiniões podem ser sobre o nó que prestou o serviço ou sobre a qualidade do serviço prestado. As opiniões sobre um mesmo nó ou serviço avaliado são armazenadas e posteriormente agregadas atraves de uma função, formando a reputação destes. O mecanismo proposto baseia-se nas opiniões emitidas sobre a autenticidade os arquivos, utilizando a reputação dos nós para indicar a qualidade da opinião sendo emitida por eles. Essa verificação da qualidade da opinião visa aumentar a confiança na opinião utilizada com a adicão de um nível de verificação por motivos de eficiência, visto que implementar uma rede de confiança inteira é custosa. Foram realizadas simulaçõs para a verificação da eficácia da rede, realizando comparações tanto com uma rede sem nenhum sistema de reputação quanto com outros sistemas de reputação. / Abstract: P2P networks have earned a great deal of popularity over the last decade, consolidating itself as one of the most popular internet service, providing a distributed architecture for the furnishing of services without the need of a centralized server host. However, such popularity brought the necessity for security mechanisms in order to assure the network availability in spite of the attacks on the network. Stability in the algorithms related to the basic operation of the P2P networks made possible the rise on the development of security systems. In this dissertation it's proposed a reputation system for file sharing P2P networks, a security mechanism aimed at lowering the spread of corrupted files in the network. Such systems work by managing the opinions issued by the participants of the network about the received services from the other nodes. These opinions can be about the nodes, or about the quality of the services themselves. Opinions about the same service or node are them joined through the use of a mathematical model (function), calculating their reputation. The proposed reputation system is based on the reputation of the files, using the node reputation as a means to assess the quality of the opinion being issued. This check is made with the purpose of improving trust in the used opinion by adding one level of opinion checking. Only one level is used for efficiency, since implementing a full trust network is expensive. Simulations were used in order to assess the effectiveness of the proposed reputation system. The results are used in comparisons with the same simulation without the use of any reputation system, and with the results of other reputation systems found in the literature. / Mestrado / Segurança de Redes / Mestre em Ciência da Computação

Orientador: Edmundo Roberto Mauro Madeira / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Computação / Made available in DSpace on 2018-08-11T15:56:37Z (GMT). No. of bitstreams: 1 Siqueira_ThiagoSenadorde_M.pdf: 7328903 bytes, checksum: f944c1a0baac79f360047d39c4871f32 (MD5) Previous issue date: 2008 / Resumo: A computação sobre P2P tem surgido como uma solução alternativa e complementaràs grades computacionais. O uso da tecnologia P2P é capaz de prover a flexibilização e descentralização dos processos de execução e gerenciamento de workflows nas grades computacionais. Neste trabalho é apresentado um middleware P2P completamente descentralizado para a computação de workflows. O middleware coleta o poder de processamento compartilhado pelos peers para possibilitar a execução de workflows, modelados como DAGs, compostos por um conjunto de tarefas dependentes. Através do processo distribuído de escalonamento de tarefas e do mecanismo de tolerância a faltas baseado em leasing, o middleware atinge um nível alto de paralelismo na execução e eficiência na recuperação de execuções em ocorrência de faltas. O middleware é implementado em Java, juntamente com RMI e a biblioteca JXTA. Os resultados experimentais obtidos mostram a eficiência do middleware na execução distribuída dos workflows assim como a recuperação rápida de execução em cenários com faltas / Abstract: P2P Computing has been raised as an alternative and complementary solution to Grid Computing. The use of P2P technology is able to provide a flexible and decentralized execution and management of Grid workflows. In this work we present a completely decentralized P2P middleware for workflow computing. The middleware collects the shared processing power of the peers in order to execute workflows, modeled as DAG structures, composed of a set of dependent tasks. Through a distributed scheduling algorithm and a leasing-based fault tolerance mechanism, the middleware achieves high execution parallelism and efficient execution recovery in failure occurrences. The middleware is implemented in Java, through RMI and the JXTA library. The obtained experimental results show the efficiency of the middleware in the distributed execution of workflows as well as the fast execution recovery / Mestrado / Ciência da Computação / Mestre em Ciência da Computação

Les systèmes de recommandation essayent de déduire les intérêts de leurs utilisateurs afin de leurs suggérer des items pertinents. Ces systèmes offrent ainsi aux utilisateurs un service utile car ils filtrent automatiquement les informations non-pertinentes, ce qui évite le problème de surcharge d’information qui est courant de nos jours. C’est pourquoi les systèmes de recommandation sont aujourd’hui populaires, si ce n’est omniprésents dans certains domaines tels que le World Wide Web. Cependant, les intérêts d’un individu sont des données personnelles et privées, comme par exemple son orientation politique ou religieuse. Les systèmes de recommandation recueillent donc des données privées et leur utilisation répandue nécessite des mécanismes de protection de la vie privée. Dans cette thèse, nous étudions la protection de la confidentialité des intérêts des utilisateurs des systèmes de recommandation appelés systèmes de filtrage collaboratif (FC). Notre première contribution est Hide & Share, un nouveau mécanisme de similarité, respectueux de la vie privée, pour la calcul décentralisé de graphes de K-Plus-Proches-Voisins (KPPV). C’est un mécanisme léger, conçu pour les systèmes de FC fondés sur les utilisateurs et décentralisés (ou pair-à-pair), qui se basent sur les graphes de KPPV pour fournir des recommandations. Notre seconde contribution s’applique aussi aux systèmes de FC fondés sur les utilisateurs, mais est indépendante de leur architecture. Cette contribution est double : nous évaluons d’abord l’impact d’une attaque active dite « Sybil » sur la confidentialité du profil d’intérêts d’un utilisateur cible, puis nous proposons une contre-mesure. Celle-ci est 2-step, une nouvelle mesure de similarité qui combine une bonne précision, permettant ensuite de faire de bonnes recommandations, avec une bonne résistance à l’attaque Sybil en question. / Recommendation systems try to infer their users’ interests in order to suggest items relevant to them. These systems thus offer a valuable service to users in that they automatically filter non-relevant information, which avoids the nowadays common issue of information overload. This is why recommendation systems are now popular, if not pervasive in some domains such as the World Wide Web. However, an individual’s interests are personal and private data, such as one’s political or religious orientation. Therefore, recommendation systems gather private data and their widespread use calls for privacy-preserving mechanisms. In this thesis, we study the privacy of users’ interests in the family of recommendation systems called Collaborative Filtering (CF) ones. Our first contribution is Hide & Share, a novel privacy-preserving similarity mechanism for the decentralized computation of K-Nearest-Neighbor (KNN) graphs. It is a lightweight mechanism designed for decentralized (a.k.a. peer-to-peer) user-based CF systems, which rely on KNN graphs to provide recommendations. Our second contribution also applies to user-based CF systems, though it is independent of their architecture. This contribution is two-fold: first we evaluate the impact of an active Sybil attack on the privacy of a target user’s profile of interests, and second we propose a counter-measure. This counter-measure is 2-step, a novel similarity metric combining a good precision, in turn allowing for good recommendations,with high resilience to said Sybil attack.

The wide-spread coverage and ubiquitous presence of mobile networks has propelled the usage and adoption of mobile phones to an unprecedented level around the globe. The computing capabilities of these mobile phones have improved considerably, supporting a vast range of third party applications. Simultaneously, Peer-to-Peer (P2P) overlay networks have experienced a tremendous growth in terms of usage as well as popularity in recent years particularly in fixed wired networks. In particular, Distributed Hash Table (DHT) based Structured P2P overlay networks offer major advantages to users of mobile devices and networks such as scalable, fault tolerant and self-managing infrastructure which does not exhibit single points of failure. Integrating P2P overlays on the mobile network seems a logical progression; considering the popularities of both technologies. However, it imposes several challenges that need to be handled, such as the limited hardware capabilities of mobile phones and churn (i.e. the frequent join and leave of nodes within a network) intensive mobile networks offering limited yet expensive bandwidth availability. This thesis investigates the feasibility of extending P2P to mobile networks so that users can take advantage of both these technologies: P2P and mobile networks. This thesis utilises OverSim, a P2P simulator, to experiment with the performance of various P2P overlays, considering high churn and bandwidth consumption which are the two most crucial constraints of mobile networks. The experiment results show that Kademlia and EpiChord are the two most appropriate P2P overlays that could be implemented in mobile networks. Furthermore, Network Address Translation (NAT) is a major barrier to the adoption of P2P overlays in mobile networks. Integrating NAT traversal approaches with P2P overlays is a crucial step for P2P overlays to operate successfully on mobile networks. This thesis presents a general approach of NAT traversal for ring based overlays without the use of a single dedicated server which is then implemented in OverSim. Several experiments have been performed under NATs to determine the suitability of the chosen P2P overlays under NATed environments. The results show that the performance of these overlays is comparable in terms of successful lookups in both NATed and non-NATed environments; with Kademlia and EpiChord exhibiting the best performance. The presence of NATs and also the level of churn in a network influence the routing techniques used in P2P overlays. Recursive routing is more resilient to IP connectivity restrictions posed by NATs but not very robust in high churn environments, whereas iterative routing is more suitable to high churn networks, but difficult to use in NATed environments. Kademlia supports both these routing schemes whereas EpiChord only supports the iterating routing. This undermines the usefulness of EpiChord in NATed environments. In order to harness the advantages of both routing schemes, this thesis presents an adaptive routing scheme, called Churn Aware Routing Protocol (ChARP), combining recursive and iterative lookups where nodes can switch between recursive and iterative routing depending on their lifetimes. The proposed approach has been implemented in OverSim and several experiments have been carried out. The experiment results indicate an improved performance which in turn validates the applicability and suitability of ChARP in NATed environments.

This thesis investigates the application of autonomic management to a distributed storage system. Effects on performance and resource consumption were measured in experiments, which were carried out in a local area test-bed. The experiments were conducted with components of one specific distributed storage system, but seek to be applicable to a wide range of such systems, in particular those exposed to varying conditions. The perceived characteristics of distributed storage systems depend on their configuration parameters and on various dynamic conditions. For a given set of conditions, one specific configuration may be better than another with respect to measures such as resource consumption and performance. Here, configuration parameter values were set dynamically and the results compared with a static configuration. It was hypothesised that under non-changing conditions this would allow the system to converge on a configuration that was more suitable than any that could be set a priori. Furthermore, the system could react to a change in conditions by adopting a more appropriate configuration. Autonomic management was applied to the peer-to-peer (P2P) and data retrieval components of ASA, a distributed storage system. The effects were measured experimentally for various workload and churn patterns. The management policies and mechanisms were implemented using a generic autonomic management framework developed during this work. The motivation for both groups of experiments was to test management policies with the objective to avoid unsatisfactory situations with respect to resource consumption and performance. Such unsatisfactory situations occur when either the P2P layer or the data retrieval mechanism is configured statically. In a statically configured P2P system two unsatisfactory situations can be identified. The first arises when the frequency with which P2P node states are verified is low and membership churn is high. The P2P node state becomes inaccurate due to a high membership churn, leading to errors during the routing process and a reduction in performance. In this situation it is desirable to increase the frequency to increase P2P state accuracy. The converse situation arises when the frequency is high and churn is low. In this situation network resources are used unnecessarily, which may also reduce performance, making it desirable to decrease the frequency. In ASA’s data retrieval mechanism similar unsatisfactory situations can be identified with respect to the degree of concurrency (DOC). The DOC controls the eagerness with which multiple redundant replicas are retrieved. An unsatisfactory situation arises when the DOC is low and there is a large variation in the times taken to retrieve replicas. In this situation it is desirable to increase the DOC, because by retrieving more replicas in parallel a result can be returned to the user sooner. The converse situation arises when the DOC is high, there is little variation in retrieval time and there is a network bottleneck close to the requesting client. In this situation it is desirable to decrease the DOC, since the low variation removes any benefit in parallel retrieval, and the bottleneck means that decreasing parallelism reduces both bandwidth consumption and elapsed time for the user. The experimental evaluations of autonomic management show promising results, and suggest several future research topics. These include optimisations of the managed mechanisms, alternative management policies, different evaluation methods, and the application of developed management mechanisms to other facets of a distributed storage system. The findings of this thesis could be exploited in building other distributed storage systems that focus on harnessing storage on user workstations, since these are particularly likely to be exposed to varying, unpredictable conditions.