Jeremie Leguay

Opportunistic content distribution in an urban setting

This paper investigates the feasibility of a city-wide content distribution architecture composed of short range wireless access points. We look at how a target group of intermittently and partially connected mobile nodes can improve the diffusion of information within the group by leveraging fixed and mobile nodes that are exterior to the group. The fixed nodes are data sources, and the external mobile nodes are data relays, and we examine the trade off between the use of each in order to obtain high satisfaction within the target group, which consists of data sinks. We conducted an experiment in Cambridge, UK, to gather mobility traces that we used for the study of this content distribution architecture. In this scenario, the simple fact that members of the target group collaborate leads to a delivery ratio of 90%. In addition, the use of external mobile nodes to relay the information slightly increases the delivery ratio while significantly decreasing the delay.

The Accordion Phenomenon: Analysis, Characterization, and Impact on DTN Routing

We analyze the dynamics of a mobility dataset collected in a pipelined disruption-tolerant network (DTN), a particular class of intermittently-connected wireless networks characterized by a one-dimensional topology. First, we collected and investigated traces of contact times among a thousand participants of a rollerblading tour in Paris. The dataset shows extreme dynamics in the mobility pattern of a large number of nodes. Most strikingly, fluctuations in the motion of the rollerbladers cause a typical accordion phenomenon - the topology expands and shrinks with time, thus influencing connection times and opportunities between participants. Second, we show through an analytical model that the accordion phenomenon, through the variation of the average node degree, has a major impact on the performance of epidemic dissemination. Finally, we test epidemic dissemination and other existing forwarding schemes on our traces, and argue that routing should adapt to the varying, though predictable, nature of the network. To this end, we propose DA-SW (density-aware spray-and-wait), a measurement-oriented variant of the spray-and-wait algorithm that tunes, in a dynamic fashion, the number of a message copies disseminated in the network. We show that DA-SW leads to performance results that are close to the best case (obtained with an oracle).

Characterizing pairwise inter-contact patterns in delay tolerant networks

A good understanding of contact patterns in delay tolerant networks (DTNs) is elemental to the design of effective routing or content distribution schemes. Prior work has typically focused on inter-contact time patterns in the aggregate. In this paper, we argue that pairwise inter-contact patterns are a more refined and efficient tool for characterizing DTNs. First, we provide a detailed statistical analysis of pairwise contact and inter-contact times in three reference DTN data sets. We characterize heterogeneities in contact times and inter-contact times, and find that the empirical distributions of inter-contact times tend to be well fitted by log-normal curves, with exponential curves also fitting a significant portion of the distributions. Second, we investigate analytically the relationship between pairwise and aggregate inter-contact times. In particular, we consider both the exponential and log-normal cases and show analytically how the aggregation of pairwise inter-contacts may lead to aggregate inter-contacts with power laws of various degrees.

DISCO: Distributed multi-domain SDN controllers

Software-Defined Networking (SDN) is now envisioned for Wide Area Networks (WAN) and constrained overlay networks. Such networks require a resilient, scalable and easily extensible SDN control plane. In this paper, we propose DISCO, an extensible DIstributed SDN COntrol plane able to cope with the distributed and heterogeneous nature of modern overlay networks. A DISCO controller manages its own network domain and communicates with other controllers to provide end-to-end network services. This east-west communication is based on a lightweight and highly manageable control channel. We implemented DISCO on top of the Floodlight OpenFlow controller and the AMQP protocol and we evaluated it through an inter-domain topology disruption use case.

Relieving the wireless infrastructure: When opportunistic networks meet guaranteed delays

Major wireless operators are nowadays facing network capacity issues in striving to meet the growing demands of mobile users. At the same time, 3G-enabled devices increasingly benefit from ad hoc radio connectivity (e.g., Wi-Fi). In this context of hybrid connectivity, we propose Push-and-track, a content dissemination framework that harnesses ad hoc communication opportunities to minimize the load on the wireless infrastructure while guaranteeing tight delivery delays. It achieves this through a control loop that collects user-sent acknowledgements to determine if new copies need to be reinjected into the network through the 3G interface. Push-and-Track includes multiple strategies to determine how many copies of the content should be injected, when, and to whom. The short delay-tolerance of common content, such as news or road traffic updates, make them suitable for such a system. Based on a realistic large-scale vehicular dataset from the city of Bologna composed of more than 10,000 vehicles, we demonstrate that Push-and-Track consistently meets its delivery objectives while reducing the use of the 3G network by over 90%.

Fixed point opportunistic routing in delay tolerant networks

We propose in this work a single copy and multi-hop opportunistic routing scheme for sparse delay tolerant networks (DTNs). The scheme uses as only input the estimates of the average inter-contact times between the nodes in the network. Defined as the fixed point of a recursive process, it aims at minimizing delivery time in case of independent exponential pairwise inter-contacts. The two properties of loop-free forwarding and polynomial convergence make the scheme workable for routing in DTNs. The routing performances of the scheme are evaluated on three publicly available reference data sets. Comparisons with well known single-copy schemes, including MED and the two hop relay strategy, consistently demonstrate improvements for both delivery ratio and delay.

Fast track article: Push-and-track: Saving infrastructure bandwidth through opportunistic forwarding

Major wireless operators are nowadays facing network capacity issues in striving to meet the growing demands of mobile users. At the same time, 3G-enabled devices increasingly benefit from ad hoc radio connectivity (e.g., WiFi). In this context of hybrid connectivity, we propose Push-and-track, a content dissemination framework that harnesses ad hoc communication opportunities to minimize the load on the wireless infrastructure while guaranteeing tight delivery delays. It achieves this through a control loop that collects user-sent acknowledgements to determine if new copies need to be re-injected into the network through the 3G interface. Push-and-Track is flexible and can be applied to a variety of scenarios, including periodic message flooding and floating data. For the former, this paper examines multiple strategies to determine how many copies of the content should be injected, when, and to whom; for the latter, it examines the achievable offload ratio depending on the freshness constraints. The short delay-tolerance of common content, such as news or road traffic updates, make them suitable for such a system. Use cases with a long delay-tolerance, such as software updates, are an even better fit. Based on a realistic large-scale vehicular dataset from the city of Bologna composed of more than 10,000 vehicles, we demonstrate that Push-and-Track consistently meets its delivery objectives while reducing the use of the 3G network by about 90%.

Density-Aware Routing in Highly Dynamic DTNs: The RollerNet Case

We analyze the dynamics of a mobility data set collected in a pipelined disruption-tolerant network (DTN), a particular class of intermittently-connected wireless networks characterized by a 1-D topology. First, we collected and investigated traces of contact times among thousands of participants of a rollerblading tour in Paris. The data set shows extreme dynamics in the mobility pattern of a large number of nodes. Most strikingly, fluctuations in the motion of the rollerbladers cause a typical accordion phenomenon - the topology expands and shrinks with time, thus influencing connection times and opportunities between participants. Second, we show through an analytical model that the accordion phenomenon, through the variation of the average node degree, has a major impact on the performance of epidemic dissemination. Finally, we test epidemic dissemination and other existing forwarding schemes on our traces, and conclude that routing should adapt to the varying, though predictable, nature of the network. To this end, we propose DA-SW (Density-Aware Spray-and-Wait), a measurement-oriented variant of the spray-and-wait algorithm that tunes, in a dynamic fashion, the number of a message copies to be disseminated in the network. The particularity of DA-SW is that it relies on a set of abaci that represents the three phases of the accordion phenomenon: aggregation, expansion, and stabilization. We show that DA-SW leads to performance results that are close to the best case (obtained with an oracle).

ITETRIS: a modular simulation platform for the large scale evaluation of cooperative ITS applications

Cooperative ITS systems are expected to improve road traffic safety and efficiency, and provide infotainment services on the move, through the dynamic exchange of messages between vehicles, and between vehicles and infrastructure nodes. The complexity of cooperative ITS systems and the interrelation between its components requires their extensive testing before deployment. The lack of simulation platforms capable to test, with high modelling accuracy, cooperative ITS systems and applications in large scale scenarios triggered the implementation of the EU-funded iTETRIS simulation platform. iTETRIS is a unique open source simulation platform characterized by a modular architecture that allows integrating two widely adopted traffic and wireless simulators, while supporting the implementation of cooperative ITS applications in a language-agnostic fashion. This paper presents in detail the iTETRIS simulation platform, and describes its architecture, standard compliant implementation, operation and new functionalities. Finally, the paper demonstrates iTETRIS large scale cooperative ITS evaluation capabilities through the implementation and evaluation of cooperative traffic congestion detection and bus lane management applications. The detailed description and implemented examples provide valuable information on how to use and exploit iTETRIS simulation potential.

An efficient service oriented architecture for heterogeneous and dynamic wireless sensor networks

The purpose of this work is to bridge the gap between high-end networked devices and wireless networks of ubiquituous and resource-constrained sensors and actuators by extensively applying Service-Oriented Architecture (SOA) patterns. We present a multi-level approach that implements existing SOA standards on higher tiers, and propose a novel protocol stack, WSN-SOA, which brings the benefits of SOA to low capacity nodes without the overhead of XML-based technologies. This solution fully supports network dynamicity, auto-configuration, service discovery, device heterogeneity and interoperability with legacy architectures. As a proof-of-concept, we have studied a surveillance scenario in which the detection of an intruder, conducted within the range of a network of wireless sensors (e.g., MICAz from Crossbow), leads to the automatic triggering of tracking activities by a Linux-powered network camera and of alerts and video streams toward a control room.