Peer-to-peer (P2P) applications have become a popular method for obtaining digital content. Recent research has shown that the amount of time spent downloading from a poor performing peer effects the total download duration. Current peer selection strategies attempt to limit the amount of time spent downloading from a poor performing peer, but they do not use both advanced knowledge and service capacity after the connection has been made to aid in peer selection. Advanced knowledge has traditionally been obtained from methods that add additional overhead to the P2P network, such as polling peers for service capacity information, using round trip time techniques to calculate the distance between peers, and by using tracker peers. This work investigated the creation of a new download strategy that replaced the random selection of peers with a method that selects server peers based on historic service capacity and ISP in order to further reduce the amount of time needed to complete a download session. Peer-to-peer (P2P) applications have become a popular method for obtaining digital content. Recent research has shown that the amount of time spent downloading from a poor performing peer effects the total download duration. Current peer selection strategies attempt to limit the amount of time spent downloading from a poor performing peer, but they do not use both advanced knowledge and service capacity after the connection has been made to aid in peer selection. Advanced knowledge has traditionally been obtained from methods that add additional overhead to the P2P network, such as polling peers for service capacity information, using round trip time techniques to calculate the distance between peers, and by using tracker peers. This work investigated the creation of a new download strategy that replaced the random selection of peers with a method that selects server peers based on historic service capacity and ISP in order to further reduce the amount of time needed to complete a download session. The results of this new historic based peer selection strategy have shown that there are benefits in using advanced knowledge to select peers and only replacing the worst performing peers. This new approach showed an average download duration improvement of 16.6% in the single client simulation and an average cross ISP traffic reduction of 55.17% when ISPs were participating in cross ISP throttling. In the multiple clients simulation the new approach showed an average download duration improvement of 53.31% and an average cross ISP traffic reduction of 88.83% when ISPs were participating in cross ISP throttling. This new approach also significantly improved the consistency of the download duration between download sessions allowing for the more accurate prediction of download times.

The time it takes to download a file in a peer-to-peer (P2P) overlay network is dependent on several factors. These factors include the quality of the network between peers (e.g. packet loss, latency, and link failures), distance, peer selection technique, and packet loss due to Internet Service Providers (ISPs) engaging in traffic shaping. Recent research shows that P2P download time is adversely impacted by the presence of distant peers, particularly when traffic goes across an ISP that could be engaging in P2P traffic throttling activities. It has also been observed that additional delays are introduced when distant candidate nodes for exchanging data are included during the formation of a P2P network overlay. Researchers have shifted their attention to the mechanism for peer selection. They started questioning the random technique because it ignores the location of nodes in the topology of the underlying physical network. Therefore, selecting nodes for interaction in a distributed system based on their position in the network continues to be an active area of research. The goal of this work was to reduce the cumulative file download time and variance for the majority of participating peers in a P2P network by using a peer selection mechanism that favors nearby nodes. In this proposed proximity strategy, the Internet address space is separated by IP blocks that belong to different Autonomous Systems (AS). IP blocks are further broken up into subsets named zones. Each zone is given a landmark (a.k.a. beacon), for example routers or DNS servers, with a known geographical location. At the time peers joined the network, peers were grouped into zones based on their geographical distance to the selected beacons. Peers that end up in the same zone were put at the top of the list of available nodes for interactions during the formation of the overlay. Experiments were conducted to compare the proposed proximity based peer selection strategy to the random peer selection strategy. The results indicate that the proximity technique outperforms the random approach for peer selection in a network with low packet loss and latency and also in a more realistic network subject to packet loss, traffic shaping and long distances. However, this improved performance came at the cost of additional memory (230 megabytes) and to a lesser extent some additional CPU cycles to run the additional subroutines needed to group peers into zones. The framework and algorithms developed for this work made it possible to implement a fully functioning prototype that implements the proximity strategy. This prototype enabled high fidelity testing with a real client implementation in real networks including the Internet. This made it possible to test without having to rely exclusively on event-driven simulations to prove the hypothesis.

Smoking rates among senior students have been related to smoking initiation in younger students. Opportunities to select smoking friends may be one explanation, however our understanding of this process has been limited by cross-sectional designs. <br ><br /> The purpose of this longitudinal study was to determine whether senior student smoking rates a) predict smoking initiation in younger elementary school students, controlling for individual exposure to family and friends who smoke and b) are related to the selection of smoking friends, increasing risk of smoking initiation as a result. <br ><br /> This study involved secondary data analysis of 2798 students from 84 Ontario elementary schools involved with the Third Waterloo Smoking Prevention Project (WSPP3). Grade 8 students completed a questionnaire at baseline to obtain the percentage of senior students who smoke in each school. Students in grade 6 completed a similar questionnaire at baseline, and were surveyed again in grades 7 and 8. Multilevel regression analyses were used to examine school and individual characteristics simultaneously. <br ><br /> Each 5% increase in the senior student smoking rate at a school increased the risk that a non-smoking grade 6 student would try smoking more than once by grade 8 (OR 1. 05) and that a non-smoking grade 6 student with no smoking friends would gain a smoking friend by grade 7 (OR 1. 10). Students who remained non-smokers in grade 7 but gained a smoking friend were more likely to try smoking more than once by grade 8 (OR 4. 31). <br ><br /> In schools where a high proportion of senior students smoked, younger students were more likely to initiate smoking, and gain a smoking friend. Anti-smoking policies and interventions may be more urgently required in these schools to lower senior student smoking rates and reduce initiation among younger students. Tailoring the intensity and content of programs to match the needs of schools is one way to potentially maximize effectiveness.

The download duration of peer-to-peer overlay networks is highly dependent upon the client's selection of candidate node-servers and the algorithms used in that process. Recent findings suggest that as node-server network capacity increases the deviation from the average total download time can vary as much as 300 percent between selection algorithms. This work investigated the current selection algorithms based upon chunk size, parallel connections, permanent connection, and time based switching. The time based switching algorithm is a variation of the chunk based algorithm. Time based switching enables a client to randomly select a new node-server regardless of connection speed at predetermined time slots. Simulations indicate a 41% percent decrease in download time when compared to chunk based switching. The effects of inserting chokepoints in the time based switching algorithm were investigated. This work investigated improving a client's download performance by preemptively releasing a client from a poor performing node-server. To achieve this, the client will gather a peer-to-peer network overlay capacity from a global catalog. This information will be used to seed a client choke algorithm. Clients will then be able to continually update a local capacity average based upon past download sessions. This local average will be used to make a comparison between the current download session and the previously calculated average. A margin has been introduced to allow the client to vary from the average calculated capacity. The client will perform comparisons against chokepoints and make performance decisions to depart a node-server that does not meet minimum capacity standards. Experimental results in this research demonstrated the effectiveness of applying a choking algorithm to improve upon client download duration as well as increasing the accuracy of download duration estimates. In the single downloader scenario, the choke based algorithm improved performance up to 44% in extreme congestion and a more modest 13% under normal conditions. The multiple client scenarios yielded on average a 1% decrease in client download duration along with a 44% increase download homogeneity. Furthermore, the results indicate that a client based choking algorithm can decrease overall peer-to-peer network congestion buy improving upon client selection of node-servers.

Smoking rates among senior students have been related to smoking initiation in younger students. Opportunities to select smoking friends may be one explanation, however our understanding of this process has been limited by cross-sectional designs. <br ><br /> The purpose of this longitudinal study was to determine whether senior student smoking rates a) predict smoking initiation in younger elementary school students, controlling for individual exposure to family and friends who smoke and b) are related to the selection of smoking friends, increasing risk of smoking initiation as a result. <br ><br /> This study involved secondary data analysis of 2798 students from 84 Ontario elementary schools involved with the Third Waterloo Smoking Prevention Project (WSPP3). Grade 8 students completed a questionnaire at baseline to obtain the percentage of senior students who smoke in each school. Students in grade 6 completed a similar questionnaire at baseline, and were surveyed again in grades 7 and 8. Multilevel regression analyses were used to examine school and individual characteristics simultaneously. <br ><br /> Each 5% increase in the senior student smoking rate at a school increased the risk that a non-smoking grade 6 student would try smoking more than once by grade 8 (OR 1. 05) and that a non-smoking grade 6 student with no smoking friends would gain a smoking friend by grade 7 (OR 1. 10). Students who remained non-smokers in grade 7 but gained a smoking friend were more likely to try smoking more than once by grade 8 (OR 4. 31). <br ><br /> In schools where a high proportion of senior students smoked, younger students were more likely to initiate smoking, and gain a smoking friend. Anti-smoking policies and interventions may be more urgently required in these schools to lower senior student smoking rates and reduce initiation among younger students. Tailoring the intensity and content of programs to match the needs of schools is one way to potentially maximize effectiveness.

The purpose of this dissertation is to explore the influences of industry dynamic factors (e.g., peer selections) on a client’s subsequent decision to select the type of auditor (e.g., Big N versus non-Big N), following auditor turnover. More specifically, drawing on social norms and social learning theories, I develop testable implications and investigate whether and how industry dynamics have an incremental power in explaining auditor choice beyond traditional firm-specific variables documented in prior research. Using a large sample from years 1988 – 2012, I find that clients are more likely to imitate their industry peers’ prior selections to select the type of their succeeding auditors, consistent with the implications of social learning theory. I also find that clients in industries with stronger industry norms, as measured by a greater proportion of clients audited by Big N auditors in an industry, are more likely to select Big N auditors as their succeeding auditors, consistent with the implications of social norms theory. To my best knowledge, this is the first study to explore the impact of social dynamics measured at the industry level on auditor selection and provide large-sample evidence on the relations between industry dynamics and auditor selection at the firm level. Findings of this study provide insights into the dynamic process of auditor selection in which companies do not make auditor-selection decisions in isolation of one another as often posited in existing literature, contribute to the research on the determinants of auditor choice by incorporating industry dynamics into an agent-principal model, and provide a more comprehensive view of the phenomenon of auditor selection.

Due to its ability to deliver scalable and fault-tolerant solutions, applications based on the peer-to-peer (P2P) paradigm are used by millions of users on the internet. Recently, wireless mesh networks (WMNs) have attracted a lot of interest from both academia and industry, because of their potential to provide flexible and alternative broadband wireless internet connectivity. However, due to various reasons such as unstable wireless link characteristics and multi-hop forwarding operation, the performance of current P2P systems is rather low in WMNs. This dissertation studies the technological challenges involved while deploying P2P systems over WMNs. We study the benefits of location-awareness and resource replication to the P2P overlay while targeting efficient resource lookup in WMNs. We further propose a cross-layer information exchange between the P2P overlay and the WMN in order to reduce resource lookup delay by augmenting the overlay routing table with physical neighborhood and resource lookup history information. Aiming to achieve throughput maximization and fairness in P2P systems, we model the peer selection problem as a mathematical optimization problem by using a set of mixed integer linear equations. A study of the model reveals the relationship between peer selection, resource replication and channel assignment on the performance of P2P systems over WMNs. We extend the model by formulating the P2P download problem as chunk scheduling problem. As a novelty, we introduce constraints to model the capacity limitations of the network due to the given routing and channel assignment strategy. Based on the analysis of the model, we propose a new peer selection algorithm which incorporates network load information and multi-path routing capability. By conducting testbed experiments, we evaluate the achievable throughput in multi-channel multi-radio WMNs. We show that the adjacent channel interference (ACI) problem in multi-radio systems can be mitigated, making better use of the available spectrum. Important lessons learned are also outlined in order to design practical channel and channel bandwidth assignment algorithms in multi-channel multi-radio WMNs.

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.

La consommation d’alcool (CA) à l’adolescence est souvent associée à des conséquences négatives. L’un de ses meilleurs prédicteurs est la CA des amis. Deux mécanismes peuvent l’expliquer, soit la socialisation et la sélection des pairs. Les études comparant les deux mécanismes ont des résultats incohérents, peut-être dus à des variations et des limites méthodologiques. D’autres études sont donc nécessaires et l’analyse de modérateurs possibles est encouragée. L’attachement des adolescents pourrait en être un, vu son association avec les relations interpersonnelles. Objectifs. À l’aide de modèles à décalage croisé, cette étude vise à analyser, à l’aide d’une cohorte de participants de 14 ans au premier temps de mesure, (1) les relations bidirectionnelles entre la CA et la CA excessive des participants et celles autorapportées des amis et (2) le rôle modérateur de l’attachement dans ces relations. Méthode. Les participants proviennent d’une étude longitudinale Québécoise, où 182 d’entre eux et jusqu’à cinq de leurs amis ont rapporté annuellement leur CA et leur CA excessive lorsque les participants avaient entre 14 et 16 ans. Les participants ont complété les échelles« Évitement des relations sociales » et « Préoccupation d’être aimé » du Questionnaire des styles d’attachement à 15 ans. Résultats et conclusion. Pour les deux types de consommation, les coefficients des relations entre la CA du participant à 14 ou 15 ans et celle de ses amis un an plus tard étaient plus grands que les coefficients des relations entre la CA des amis à 14 ou 15 ans et celle du participant un an plus tard. Cela supporte le mécanisme de sélection plutôt que celui de socialisation. Deux modérations utilisant l’échelle d’évitement avec le modèle de CA étaient significatives, indiquant que les coefficients correspondant à la sélection (de 14 à 15 ans) et à la socialisation (de 15 à 16 ans) étaient plus faibles pour les adolescents évitant. Cela suggère que les adolescents non évitant sont plus sensible à l’influence des pairs. / Alcohol use (AU) in adolescence is often linked with immediate or long-term negative consequences. Among the best-known predictors of adolescent AU is friend use. Two mechanisms can explain this association: socialization and peer selection. Studies comparing both mechanisms have had inconsistent results, perhaps because of methodological variations or limitations. Thus, further studies are needed and examination of possible moderator is advised. Adolescent attachment is an interesting potential moderator as it is closely linked with interpersonal relationships. Study aims. Thus, using cross-lagged models to account for bidirectional associations and independent assessments of adolescent and friend alcohol use, this study aims to examine in a cohort of 14-year-old children followed over two years (1) the relations between participant and friend self-reported alcohol use and binge drinking frequency and (2) the moderating role of attachment style in these relations. Method. Participants come from a Quebec longitudinal study where 182 adolescents and up to five of their friends reported annually on their AU and binge drinking (BD) frequency between 14 and 16 years. Participants completed the “Avoidance of social relations” and “Preoccupation with being loved” scales of the Attachment Style Questionnaire at 15. Results and conclusion. Results show that the cross-lagged path coefficients between participant AU at 14 or 15 and friend use a year later were stronger than the ones between friend use at 14 or 15 and participant use a year later for both the AU and BD models. As such, the selection mechanism prevails over the socialization one between 14 and 16. Two significant moderation using the avoidance scale in the AU models were found, showing that selection (14-15 years old) and socialization (15-16 years old) weights were weaker for avoidant adolescents, suggesting that adolescents with low avoidance are more susceptible to peer influence related to alcohol use.