Marin Bertier

Gossiping personalized queries

This paper presents P3Q, a fully decentralized gossip-based protocol to personalize query processing in social tagging systems. P3Q dynamically associates each user with social acquaintances sharing similar tagging behaviours. Queries are gossiped among such acquaintances, computed on the fly in a collaborative, yet partitioned manner, and results are iteratively refined and returned to the querier. Analytical and experimental evaluations convey the scalability of P3Q for top-k query processing. More specifically, we show that on a 10,000-user delicious trace, with little storage at each user, the queries are accurately computed within reasonable time and bandwidth consumption. We also report on the inherent ability of P3Q to cope with users updating profiles and departing.

Build One, Get One Free: Leveraging the Coexistence of Multiple P2P Overlay Networks

Many different P2P overlay networks providing various functionalities, targeting specific applications, have been proposed in the past five years. It is now reasonable to consider that multiple overlays may be deployed over a large set of nodes so that the most appropriate overlay might be chosen depending on the application. A physical peer may then host several instances of logical peers belonging to different overlay networks. In this paper, we show that the coexistence of a structured P2P overlay and an unstructured one may be leveraged so that, by building one, the other is automatically constructed as well. More specifically, we show that the randomness provided by an unstructured gossip-based overlay can be used to build the routing table of a structured P2P overlay and the randomness in the numerical proximity links in the structured networks provides the random peer sampling required by gossip-based unstructured overlays. In this paper, we show that maintaining the leaf set of Pastry and the proximity links of an unstructured overlay is enough to build the complete overlays. Simulation results comparing our approach with both a Pastry-like system and a gossip-based unstructured overlay show that we significantly reduce the overlay maintenance overhead without sacrificing the performance.

The GOSSPLE anonymous social network

While social networks provide news from old buddies, you can learn a lot more from people you do not know, but with whom you share many interests. We show in this paper how to build a network of anonymous social acquaintances using a gossip protocol we call Gossple, and how to leverage such a network to enhance navigation within Web 2.0 collaborative applications, a la LastFM and Delicious. Gossple nodes (users) periodically gossip digests of their interest profiles and compute their distances (in terms of interest) with respect to other nodes. This is achieved with little bandwidth and storage, fast convergence, and without revealing which profile is associated with which user. We evaluate Gossple on real traces from various Web 2.0 applications with hundreds of PlanetLab hosts and thousands of simulated nodes.

GCP: gossip-based code propagation for large-scale mobile wireless sensor networks

Wireless sensor networks (WSN) have recently received an increasing interest. They are now expected to be deployed for long periods of time, thus requiring software updates. Updating the software code automatically on a huge number of sensors is a challenging task, especially when all participating sensors are embedded on mobile entities. In this paper, we investigate an approach to automatically update software in mobile sensor-based applications when no localization mechanism is available. We leverage the peer-to-peer cooperation paradigm to achieve a good trade-off between reliability and scalability of code propagation. More specifically, we present the design and evaluation of GCP (Gossip-based Code Propagation), a distributed software update algorithm for mobile wireless sensor networks. GCP relies on two different mechanisms, piggybacking and forwarding control, to balance the load among sensors without sacrificing on the propagation speed. We compare GCP against traditional dissemination approaches. Simulation results based on both synthetic and realistic work-loads show that GCP achieves a good convergence speed while balancing the load evenly between sensors.

On gossip and populations

Gossip protocols are simple, robust and scalable and have been consistently applied to many (mostly wired) distributed systems. Nevertheless, most validation in this area has been empirical so far and there is a lack of a theoretical counterpart to characterize what can and cannot be computed with gossip protocols. Population protocols, on the other hand, benefit from a sound theoretical framework but little empirical evaluation. In this paper, we establish a correlation between population and gossip-based protocols. We propose a classification of gossip-based protocols, based on the nature of the underlying peer sampling service. First, we show that the class of gossip protocols, where each node relies on an arbitrary sample, is equivalent to population protocols. Second, we show that gossip-based protocols, relying on a more powerful peer sampling service providing peers using a clearly identified set of other peers, are equivalent to community protocols, a modern variant of population protocols. Leveraging the resemblances between population and gossip protocols enables to provide a theoretical framework for distributed systems where global behaviors emerge from a set of local interactions, both in wired and wireless settings. The practical validations of gossip-protocols provide empirical evidence of quick convergence times of such algorithms and demonstrate their practical relevance. While existing results in each area can be immediately applied, this also leaves the space to transfer any new results, practical or theoretical, from one domain to the other.

On the Deterministic Tracking of Moving Objects with a Binary Sensor Network

This paper studies the problem of associating deterministically a track revealed by a binary sensor network with the trajectory of a unique moving anonymous object, namely the Multiple Object Tracking and Identification(MOTI) problem. In our model, the network is represented by a sparse connected graph where each vertex represents a binary sensor and there is an edge between two sensors if an object can pass from a sensed region to another without activating any other remaining sensor. The difficulty of MOTI lies in the fact that trajectories of two or more objects can be so close (track merging) that the corresponding tracks on the sensor network can no longer be distinguished, thus confusing the deterministic association between an object trajectory and a track. The paper presents several results. We first show that MOTI cannot be solved on a general graph of ideal binary sensors even by an omniscient external observer if all the objects can freely move on the graph. Then, we describe some restrictions that can be imposed a priorieither on the graph, on the object movements or both, to make MOTI problem always solvable. We also discuss the consequences of our results and present some related open problems.

Analysis of Deterministic Tracking of Multiple Objects Using a Binary Sensor Network

Let consider a set of anonymous moving objects to be tracked in a binary sensor network. This article studies the problem of associating deterministically a track revealed by the sensor network with the trajectory of an unique anonymous object, namely the multiple object tracking and identification (MOTI) problem. In our model, the network is represented by a sparse connected graph where each vertex represents a binary sensor and there is an edge between two sensors if an object can pass from one sensed region to another one without activating any other sensor. The difficulty of MOTI lies in the fact that the trajectories of two or more objects can be so close that the corresponding tracks on the sensor network can no longer be distinguished (track merging), thus confusing the deterministic association between an object trajectory and a track. The article presents several results. We first show that MOTI cannot be solved on a general graph of ideal binary sensors even by an omniscient external observer if all the objects can freely move on the graph. Then we describe restrictions that can be imposed a priori either on the graph, on the object movements, or on both, to make the MOTI problem always solvable. In the absence of an omniscient observer, we show how our results can lead to the definition of distributed algorithms that are able to detect when the system is in a state where MOTI becomes unsolvable.

Energy aware self-organizing density management in wireless sensor networks

Energy consumption is the most important factor that determines sensor node lifetime. The optimization of wireless sensor network lifetime targets not only the reduction of energy consumption of a single sensor node but also the extension of the entire network lifetime. We propose a simple and adaptive energy-conserving topology management scheme, called SAND (Self-Organizing Active Node Density). SAND is fully decentralized and relies on a distributed probing approach and on the redundancy resolution of sensors for energy optimizations, while preserving the data forwarding and sensing capabilities of the network. We present the SANDs algorithm, its analysis of convergence, and simulation results. Simulation results show that, though slightly increasing path lengths from sensor to sink nodes, the proposed scheme improves significantly the network lifetime for different neighborhood densities degrees, while preserving both sensing and routing fidelity.

D2HT: The Best of Both Worlds, Integrating RPS and DHT

Distributed Hash Tables (DHTs) and Random Peer Sampling (RPS) provide important and complementary services in the area of P2P overlay networks. DHTs achieve efficient lookup while RPS enables nodes to build and maintain connectivity in the presence of high churn. Clearly, many applications, e.g. in the area of search, would greatly benefit if both these services were available together at a reasonable cost. This paper integrates a structured P2P overlay and a Random Peer Sampling service through gossip protocols. This system called D2HT, leverages the small-world nature of DHTs and relies on two cohabiting gossip protocols maintaining the close and long-range links respectively. The long links are chosen according to a harmonic distribution, following the Kleinberg small-world model. This approach exhibits several benefits: (i) The resulting DHT is highly dynamic and self-stabilizing, changes are tracked for free through the gossip nature of the protocol. This removes the need for complex, usually disjoint, and expensive join and repair procedures. Yet, it achieves reasonable routing performance with respect to standard DHTs; (ii) The resulting peer sampling service provides a biased sampling following a harmonic distribution: this improves the routing without jeopardizing the quality of the RPS. The set of long-range links which are a source of RPS can be used independently by others applications for free. They change continuously, achieving well-balanced routing across the nodes. We perform extensive simulations and compare the performances of D2HT with Cyclon, HRing, Symphony and Pastry to demonstrate the gains achieved by the approach proposed in this paper.

Beyond the Clouds: How Should Next Generation Utility Computing Infrastructures Be Designed?

To accommodate the ever-increasing demand for Utility Computing (UC) resources while taking into account both energy and economical issues, the current trend consists in building even larger data centers in a few strategic locations. Although, such an approach enables to cope with the actual demand while continuing to operate UC resources through centralized software system, it is far from delivering sustainable and efficient UC infrastructures. In this scenario, we claim that a disruptive change in UC infrastructures is required in the sense that UC resources should be managed differently, considering locality as a primary concern. To this aim, we propose to leverage any facilities available through the Internet in order to deliver widely distributed UC platforms that can better match the geographical dispersal of users as well as the unending resource demand. Critical to the emergence of such locality-based UC (LUC) platforms is the availability of appropriate operating mechanisms. We advocate the implementation of a unified system driving the use of resources at an unprecedented scale by turning a complex and diverse infrastructure into a collection of abstracted computing facilities that is both easy to operate and reliable. By deploying and using such a LUC Operating System on backbones, our ultimate vision is to make possible to host/operate a large part of the Internet by its internal structure itself: a scalable and nearly infinite set of resources delivered by any computing facilities forming the Internet, starting from the larger hubs operated by ISPs, governments, and academic institutions to any idle resources that may be provided by end users.