Understanding the Properties of the BitTorrent Overlay

In this paper, we conduct extensive simulations to understand the properties of the overlay generated by BitTorrent. We start by analyzing how the overlay properties impact the efficiency of BitTorrent. We focus on the average peer set size (i.e., average number of neighbors), the time for a peer to reach its maximum peer set size, and the diameter of the overlay. In particular, we show that the later a peer arrives in a torrent, the longer it takes to reach its maximum peer set size. Then, we evaluate the impact of the maximum peer set size, the maximum number of outgoing connections per peer, and the number of NATed peers on the overlay properties. We show that BitTorrent generates a robust overlay, but that this overlay is not a random graph. In particular, the connectivity of a peer to its neighbors depends on its arriving order in the torrent. We also show that a large number of NATed peers significantly compromise the robustness of the overlay to attacks. Finally, we evaluate the impact of peer exchange on the overlay properties, and we show that it generates a chain-like overlay with a large diameter, which will adversely impact the efficiency of large torrents

Swarming Overlay Construction Strategies

Swarming peer-to-peer systems play an increasingly instrumental role in Internet content distribution. It is therefore important to better understand how these systems behave in practice. Recent research efforts have looked at various protocol parameters and have measured how they affect system performance and robustness. However, the importance of the strategy based on which peers establish connections has been largely overlooked. This work utilizes extensive simulations to examine the default overlay construction strategy in BitTorrent systems. Based on the results, we identify a critical parameter, the maximum allowable number of outgoing connections at each peer, and evaluate its impact on the robustness of the generated overlay. We find that there is no single optimal value for this parameter using the default strategy. We then propose an alternative strategy that allows certain new peer connection requests to replace existing connections. Further experiments with the new strategy demonstrate that it outperforms the default one for all considered metrics by creating an overlay more robust to churn. Additionally, our proposed strategy exhibits optimal behavior for a well-defined value of the maximum number of outgoing connections, thereby removing the need to set this parameter in an ad-hoc manner

Swarming Overlay Construction Strategies

International audienceSwarming peer-to-peer systems play an increasingly instrumental role in Internet content distribution. It is therefore important to better understand how these systems behave in practice. Recent research efforts have looked at various protocol parameters and have measured how they affect system performance and robustness. However, the importance of the strategy based on which peers establish connections has been largely overlooked. This work utilizes extensive simulations to examine the default overlay construction strategy in BitTorrent systems. Based on the results, we identify a critical parameter, the maximum allowable number of outgoing connections at each peer, and evaluate its impact on the robustness of the generated overlay. We find that there is no single optimal value for this parameter using the default strategy. We then propose an alternative strategy that allows certain new peer connection requests to replace existing connections. Further experiments with the new strategy demonstrate that it outperforms the default one for all considered metrics by creating an overlay more robust to churn. Additionally, our proposed strategy exhibits optimal behavior for a well-defined value of the maximum number of outgoing connections, thereby removing the need to set this parameter in an ad-hoc manner

Adapting BitTorrent for Wireless Ad Hoc networks

International audienceBitTorrent is one of the Internet''s most efficient content distribution protocols. It is known to perform very well over the wired Internet where end-to-end performance is almost guaranteed. However, in wireless ad hoc networks, many constraints appear as the scarcity of resources and their shared nature, which make running BitTorrent in such an environment with its default configuration not lead to best performances. To these constraints it adds the fact that peers are both routers and end-users and that TCP-performance drops seriously with the number of hops. We show in this work that the neighbor selection mechanism in BitTorrent plays an important role in determining the performance of the protocol when deployed over a wireless ad hoc network. It is no longer efficient to choose and treat with peers independently of their location. A first solution is to limit the scope of the neighborhood. In this case, TCP connections are fast but there is no more diversity of pieces in the network: pieces propagate in a unique direction from the seed to distant peers. This prohibits peers from reciprocating data and leads to low sharing ratios and suboptimal utilization of network resources. To recover from these impairments, we propose BitHoc, an enhancement to BitTorrent, which aims to minimize the time to download the content and at the same time to enforce cooperation and fairness among peers. BitHoc considers a restricted neighborhood to reduce routing overhead and to improve throughput, while establishing few connections to remote peers to improve diversity of pieces. To support this, BitHoc modifies the choking algorithm and adds a new piece selection strategy. With the help of extensive NS-2 simulations, we show that these enhancements to BitTorrent significantly improve the file completion time while fully profiting from the incentives implemented in BitTorrent to enforce fair sharing

Network Coding for Wireless Mesh Networks: A Case Study

Network coding is a new transmission paradigm that proved its strength in optimizing the usage of network resources. In this paper, we evaluate the gain from using network coding for file sharing applications running on top of wireless mesh networks. With extensive simulations carried out on a simulator we developed specifically for this study, we confirm that network coding can improve the performance of the file sharing application, but not as in wired networks. The main reason is that nodes over wireless cannot listen to different neighbors simultaneously. Nevertheless, one can get more from network coding if the information transmission is made more diverse inside the network. We support this argument by varying the loss rate over wireless links and adding more sources

Approches pour une distribution de données à large échelle sur l'Internet

Dans cette thèse, nous étudions le problème de la distribution de contenu de grande taille sur Internet. Nous nous concentrons essentiellement sur deux services majeurs : la vidéo à la demande (VoD) et la réplication de fichiers (service de partage). Nous étudions en premier lieu comment fournir le service VoD à un grand nombre de clients dans un réseau dédié (par example Akamai). Nous proposons une nouvelle architecture très efficace à large échelle et développons un nouveau modèle analytique permettant le calcul du coût minimal pour transmettre la vidéo. Notre modèle analytique nous permet d'étude plusieurs scénarios intéressants comme le problème de dimensionnement d'un système VoD ou l'évaluation des choix architecturaux, tels que la transmission partielle de la vidéo par satellite. En second lieu, nous nous intéressons à la VoD dans un réseau pair-à-pair. Alors que la plupart des solutions actuelles utilise des arbres multicast pour transmettre la vidéo aux clients, nous montrons que nous pouvons atteindre une efficacité similaire et d'une façon plus simple en introduisant une nouvelle approche nommée PBA. Enfin, nous étudions la réplication des fichiers dans les réseaux pair-à-pair. Les solutions actuelles peuvent être classées en deux catégories, selon que les clients soient organisés en arbre ou en maille. Notre première contribution consiste à démontrer que les architectures en maille sont au moins aussi efficaces que celles en arbre, tout en restant plus simples et plus dynamiques. Nous élaborons ensuite une analyse complète des approches en maille en identifiant les principaux paramètres qui influent sur la performance du système.In this thesis, we address the problem of distributing large contents in the Internet. We focus on two interesting and very common services, video on demand (VoD) and file replication. We first investigate how to provide an efficient VoD service to a large number of clients in a dedicated overlay network (e.g. Akamai). Our contribution here is a new video distribution architecture that is highly scalable and very cost effective. The novelty of our architecture is an analytical cost model that allows us to find the minimum delivery cost of the video. Using the cost model, we study many interesting scenarios like the dimensioning problem of a VoD system from scratch or the evaluation of architectural choices. One architectural choice that we consider is the use of satellite to broadcast a part of the video. We then continue our work on VoD but in a P2P network (rather than overlay network). While previous work constructs multicast trees to deliver the video to clients, we prove that we can achieve a similar efficiency in a simpler way. To this purpose, we introduce a new approach, called PBA, where clients perform locally an algorithm to find available servants from which they retrieve the video. Finally, we study the use of P2P networks for file replication. Existing solutions for this service can be largely classified into tree-based and mesh-based approaches. Our first contribution here is to prove that mesh approaches can be at least as efficient as tree ones. Our second contribution is a complete analysis of mesh approaches where we identify the main parameters that that influence their performance.NICE-BU Sciences (060882101) / SudocSudocFranceF

Approches pour une distribution de données à large échelle sur l'Internet

Dans cette thèse, nous étudions le problème de la distribution de contenu de grande taille sur Internet. Nous nous concentrons essentiellement sur deux services majeurs : la vidéo à la demande (VoD) et la réplication de fichiers (service de partage). Nous étudions en premier lieu comment fournir le service VoD à un grand nombre de clients dans un réseau dédié (par example Akamai). Nous proposons une nouvelle architecture très efficace à large échelle et développons un nouveau modèle analytique permettant le calcul du coût minimal pour transmettre la vidéo. Notre modèle analytique nous permet d'étude plusieurs scénarios intéressants comme le problème de dimensionnement d'un système VoD ou l'évaluation des choix architecturaux, tels que la transmission partielle de la vidéo par satellite. En second lieu, nous nous intéressons à la VoD dans un réseau pair-à-pair. Alors que la plupart des solutions actuelles utilise des arbres multicast pour transmettre la vidéo aux clients, nous montrons que nous pouvons atteindre une efficacité similaire et d'une façon plus simple en introduisant une nouvelle approche nommée PBA. Enfin, nous étudions la réplication des fichiers dans les réseaux pair-à-pair. Les solutions actuelles peuvent être classées en deux catégories, selon que les clients soient organisés en arbre ou en maille. Notre première contribution consiste à démontrer que les architectures en maille sont au moins aussi efficaces que celles en arbre, tout en restant plus simples et plus dynamiques. Nous élaborons ensuite une analyse complète des approches en maille en identifiant les principaux paramètres qui influent sur la performance du système.In this thesis, we address the problem of distributing large contents in the Internet. We focus on two interesting and very common services, video on demand (VoD) and file replication. We first investigate how to provide an efficient VoD service to a large number of clients in a dedicated overlay network (e.g. Akamai). Our contribution here is a new video distribution architecture that is highly scalable and very cost effective. The novelty of our architecture is an analytical cost model that allows us to find the minimum delivery cost of the video. Using the cost model, we study many interesting scenarios like the dimensioning problem of a VoD system from scratch or the evaluation of architectural choices. One architectural choice that we consider is the use of satellite to broadcast a part of the video. We then continue our work on VoD but in a P2P network (rather than overlay network). While previous work constructs multicast trees to deliver the video to clients, we prove that we can achieve a similar efficiency in a simpler way. To this purpose, we introduce a new approach, called PBA, where clients perform locally an algorithm to find available servants from which they retrieve the video. Finally, we study the use of P2P networks for file replication. Existing solutions for this service can be largely classified into tree-based and mesh-based approaches. Our first contribution here is to prove that mesh approaches can be at least as efficient as tree ones. Our second contribution is a complete analysis of mesh approaches where we identify the main parameters that that influence their performance.NICE-BU Sciences (060882101) / SudocSudocFranceF