Réseau superposé | EPFL Graph Search

thumb|Un réseau superposé et ses couches successives. Un réseau superposé, ou réseau overlay, est un réseau informatique bâti sur un autre réseau. Les nœuds du réseau superposé sont interconnectés par des liens logiques du réseau sous-jacent. La complexité du réseau sous-jacent n'est pas visible par le réseau superposé. Cette abstraction du réseau sous-jacent est une source d'inefficacité des flux, qui peuvent transiter plusieurs fois par les mêmes liens physiques. D'autre part, chaque couche réseau tend à réserver une partie de la capacité pour assurer la redondance. Exemples Un réseau IP utilisant des circuits virtuels ATM, eux-mêmes créés sur des liens SDH, eux-mêmes sur de la fibre noire, donne un exemple de réseau superposé. Depuis les années 2000 quelques logiciels de partage de fichiers en pair à pair utilisent la technologie superposée, à des fins de P2P anonyme. Tor (réseau), Freenet, I2P sont aussi des exemples de réseaux superposés. D'autres exemples sont ac

The Performance of Measurement-Based Overlay Networks

Sonja Buchegger, Paolo Scotton

The literature contains propositions for the use of overlay networks to supplement the normal IP routing functions with higher-level information in order to improve aspects of network behavior. We consider the use of such an overlay to optimize the end-to-end behavior of some special tra c ows. Measurements are used both to construct the virtual links of the overlay and to establish the link costs for use in a link-state routing protocol. The overlay attempts to forward certain packets over the least congested rather than the shortest path. We present simulation results showing that contrary to common belief overlay networks are not always bene cial and can be detrimental.

2002

Adding structure to Gnutella to improve search performance in a real-world deployment

Manfred Hauswirth, Mark Kornfilt

Gnutella is still one of the most popular P2P systems with millions of users. The advantages of Gnutella are its low maintenance overhead, its excellent robustness properties, and its query processing flexibility. Recent improvements, such as the introduction of ultrapeers and augmented node degrees also significantly reduced its excessive network bandwidth usage which was one of Gnutella's major drawbacks. Despite these improvements, Gnutella is still inefficient for rare queries in terms of low success rates and massive message propagation overhead. In this paper we augment the unstructured Gnutella network with a structured overlay network of ultrapeers based on the Kademlia DHT to address the problem of rare queries in Gnutella. We present the required query, maintenance, and ultrapeer election algorithms which use both overlays at their optimal efficiency, describe the protocols and architecture of our hybrid system, and present our implementation on the basis of the LimeWire Gnutella client and the Azureus Kademlia implementation. To demonstrate the advantages and efficiency of our hybrid approach we provide experimental results from large-scale experiments with hybrid ultrapeers running on PlanetLab which were connected to the live LimeWire Gnutella and Azureus Kademlia networks, with approximately 4 million (LimeWire) and 800 thousand (Azureus) connected users during the experiments.

2006

Schemes and architectures for wireless ad hoc networks and cooperative communication

Tarik Tabet

The focus of this thesis is on the study of decentralized wireless multi-hop networks. We are particularly interested in establishing bounds on the traffic-carrying capabilities of wireless ad hoc networks and conditions on the scalability of such networks with node mobility. This theoretical investigation brings forward challenges on the design of such networks. This leads to a second part of this thesis that considers the feasibility and the design of physical layer architectures and schemes for decentralized wireless multi-hop networks. In the first part of this thesis, bounds on the capacity of wireless ad hoc networks with two types of non-uniform traffic patterns are established. We focus on the impact of traffic patterns where local communications predominate and show the improvement in terms of per user-capacity over ad hoc networks with unbounded average communication distances. We then study the capacity of hybrid wireless networks, where long-distance relaying is performed by a fixed overlay network of base-stations. We investigate the scaling of capacity versus the number of nodes and the density of base-stations in the area of the network. It is shown that the gain in performance is mainly due to the reduction in the mean number of hops from source to destination. Then, we investigate the impact of mobility on the ad hoc network capacity. We propose a set of necessary and sufficient conditions under which the long-term averaged throughput in an ad hoc network can remain constant as the number of nodes increases. The main idea is to use a connectivity graph that does not represent the actual physical network, but rather the available communication resources. This graph also allows to translate the problem of maximizing the throughput in ad hoc networks to the multi-commodity flow problem and directly apply related results. In contrast to these macroscopic studies, in the second part we focus on a microscopic analysis of ad hoc wireless networks. We are interested in characterizing the performance of decentralized multiple-access and retransmission schemes for multi-hop wireless networks with the goal of drawing conclusions on cross-layer design. We investigate different transmission strategies in order to assess the tradeoff between spatial density of communications and the range of each transmission. We present tools for characterizing the spatial throughput as a function of topological parameters (e.g node population density) and system parameters (propagation, bandwith etc). The results of this work also show that coding and retransmissions provide means of reliable communication coupled with a completely decentralized multiple-access strategy. Finally, an efficient protocol for the delay-limited fading Automatic Retransmission reQuest (ARQ) single relay channel is considered for cooperative communications. The proposed protocol exploits two kinds of diversity: (i) space diversity available through the cooperative (relay) terminal, which retransmits the source's signals, (ii) ARQ diversity obtained by leveraging the retransmission delay to enhance the reliability. The performance characterization is in terms of the achievable diversity, multiplexing gain and delay tradeoff for a high signal-to-noise ratio (SNR) regime. Then, by letting the source's power level vary over the retransmission rounds, we show the benefits of power control on the diversity.

EPFL2007

Schemes and architectures for wireless ad hoc networks and cooperative communication

Tarik Tabet

EPFL2007