SQUARE: Scalable Quorum-Based Atomic Memory with Local Reconfiguration

International audienceInternet applications require more and more resources to satisfy the unpredictable clients needs. Specifically, such applications must ensure quality of service despite bursts of load. Distributed dynamic self-organized systems present an inherent adaptiveness that can face unpredictable bursts of load. Nevertheless quality of service, and more particularly data consistency, remains hardly achievable in such systems since participants (i.e., nodes) can crash, leave, and join the system at arbitrary time. The atomic consistency guarantees that any read operation returns the last written value of a data and is generalizable to data composition. To guarantee atomicity in message-passing model, mutually intersecting sets (a.k.a.quorums) of nodes are used. The solution presented here, namely SQUARE, provides scalability, load-balancing, fault-tolerance, and self-adaptiveness, while ensuring atomic consistency. We specify our solution, prove it correct and analyse it through simulations. \\ Les applications utilisées via internet nécessitent de plus en plus de ressources afin de satisfaire les besoins imprévisibles des clients. De telles applications doivent assurer une certaine qualité de service en dépit des pics de charge. Les systèmes distribués dynamiques capable de s'auto-organiser ont une capacité intrinsèque pour supporter ces pics de charge imprévisibles. Cependant, la qualité de service et plus particulièrement la cohérence des données reste très difficile à assurer dans de tels systèmes. En effet, les participants, ou noeuds, peuvent rejoindre, quitter le système, et tomber en panne de façon arbitraire. La cohérence atomique assure que toute lecture renvoie la dernière valeur écrite et la relation de composition la préserve. Afin de garantir l'atomicité dans un modèle à passage de message, des ensembles de noeuds s'intersectant mutuellement (les quorums) sont utilisés. La solution présentée ici, appelée SQUARE, est exploitable à grande échelle, permet de balancer la charge, tolère les pannes et s'auto-adapte tout en assurant l'atomicité. Nous spécifions la solution, la prouvons correcte et la simulons pour en analyser les performances

Content Replication in Mobile Networks

Performance and reliability of content access in mobile networks is conditioned by the number and location of content replicas deployed at the network nodes. In this work, we design a practical, distributed solution to content replication that is suitable for dynamic environments and achieves load balancing. Simulation results show that our mechanism, which uses local measurements only, approximates well an optimal solution while being robust against network and demand dynamics. Also, our scheme outperforms alternative approaches in terms of both content access delay and access congestio

PAN: Providing Reliable Storage in Mobile Ad Hoc Networks with Probabilistic Quorum Systems

Reliable storage of data with concurrent read/write accesses (or query/update) is an ever recurring issue in distributed settings. In mobile ad hoc networks, the problem becomes even more challenging due to highly dynamic and unpredictable topology changes. It is precisely this unpredictability that makes probabilistic protocols very appealing for such environments. Inspired by the principles of probabilistic quorum systems, we present a Probabilistic quorum system for Ad hoc Networks (PAN), a collection of protocols for the reliable storage of data in mobile ad hoc networks. Our system behaves in a predictable way due to the gossip-based diffusion mechanism applied for quorum accesses, and the protocol overhead is reduced by adopting an asymmetric quorum construction. We present an analysis of our PAN system, in terms of both reliability and overhead, which can be used to fine tune protocol parameters to obtain the desired tradeoff between efficiency and fault tolerance. We confirm the predictability and tunability of PAN through simulations with ns-2

Consistency Management Among Replicas in Peer-to-Peer Mobile Ad Hoc Networks

Recent advances in wireless communication along with peer-to-peer (P2P) paradigm have led to increasing interest in P2P mobile ad hoc networks. In this paper, we assume an environment where each mobile peer accesses data items held by other peers which are connected by a mobile ad hoc network. Since peers\u27 mobility causes frequent network partitions, replicas of a data item may be inconsistent due to write operations performed by mobile peers. In such an environment, the global consistency of data items is not desirable by many applications. Thus, new consistency maintenance based on local conditions such as location and time need to be investigated. This paper attempts to classify different consistency levels according to requirements from applications and provides protocols to realize them. We report simulation results to investigate the characteristics of these consistency protocols in a P2P wireless ad hoc network environment and their relationship with the quorum sizes

Scalable and Robust Distributed Algorithms for Privacy-Preserving Applications

We live in an era when political and commercial entities are increasingly engaging in sophisticated cyber attacks to damage, disrupt, or censor information content and to conduct mass surveillance. By compiling various patterns from user data over time, untrusted parties could create an intimate picture of sensitive personal information such as political and religious beliefs, health status, and so forth. In this dissertation, we study scalable and robust distributed algorithms that guarantee user privacy when communicating with other parties to either solely exchange information or participate in multi-party computations. We consider scalability and robustness requirements in three privacy-preserving areas: secure multi-party computation (MPC), anonymous broadcast, and blocking-resistant Tor bridge distribution. We propose decentralized algorithms for MPC that, unlike most previous work, scale well with the number of parties and tolerate malicious faults from a large fraction of the parties. Our algorithms do not require any trusted party and are fully load-balanced. Anonymity is an essential tool for achieving privacy; it enables individuals to communicate with each other without being identified as the sender or the receiver of the information being exchanged. We show that our MPC algorithms can be effectively used to design a scalable anonymous broadcast protocol. We do this by developing a multi-party shuffling protocol that can efficiently anonymize a sequence of messages in the presence of many faulty nodes. Our final approach for preserving user privacy in cyberspace is to improve Tor; the most popular anonymity network in the Internet. A current challenge with Tor is that colluding corrupt users inside a censorship territory can completely block user\u27s access to Tor by obtaining information about a large fraction of Tor bridges; a type of relay nodes used as the Tor\u27s primary mechanism for blocking-resistance. We describe a randomized bridge distribution algorithm, where all honest users are guaranteed to connect to Tor in the presence of an adversary corrupting an unknown number of users. Our simulations suggest that, with minimal resource costs, our algorithm can guarantee Tor access for all honest users after a small (logarithmic) number of rounds