Dependability in Aggregation by Averaging

Aggregation is an important building block of modern distributed applications, allowing the determination of meaningful properties (e.g. network size, total storage capacity, average load, majorities, etc.) that are used to direct the execution of the system. However, the majority of the existing aggregation algorithms exhibit relevant dependability issues, when prospecting their use in real application environments. In this paper, we reveal some dependability issues of aggregation algorithms based on iterative averaging techniques, giving some directions to solve them. This class of algorithms is considered robust (when compared to common tree-based approaches), being independent from the used routing topology and providing an aggregation result at all nodes. However, their robustness is strongly challenged and their correctness often compromised, when changing the assumptions of their working environment to more realistic ones. The correctness of this class of algorithms relies on the maintenance of a fundamental invariant, commonly designated as "mass conservation". We will argue that this main invariant is often broken in practical settings, and that additional mechanisms and modifications are required to maintain it, incurring in some degradation of the algorithms performance. In particular, we discuss the behavior of three representative algorithms Push-Sum Protocol, Push-Pull Gossip protocol and Distributed Random Grouping under asynchronous and faulty (with message loss and node crashes) environments. More specifically, we propose and evaluate two new versions of the Push-Pull Gossip protocol, which solve its message interleaving problem (evidenced even in a synchronous operation mode).Comment: 14 pages. Presented in Inforum 200

Self-Stabilizing Byzantine Resilient Topology Discovery and Message Delivery

Traditional Byzantine resilient algorithms use $2f + 1$ vertex disjoint paths to ensure message delivery in the presence of up to f Byzantine nodes. The question of how these paths are identified is related to the fundamental problem of topology discovery. Distributed algorithms for topology discovery cope with a never ending task, dealing with frequent changes in the network topology and unpredictable transient faults. Therefore, algorithms for topology discovery should be self-stabilizing to ensure convergence of the topology information following any such unpredictable sequence of events. We present the first such algorithm that can cope with Byzantine nodes. Starting in an arbitrary global state, and in the presence of f Byzantine nodes, each node is eventually aware of all the other non-Byzantine nodes and their connecting communication links. Using the topology information, nodes can, for example, route messages across the network and deliver messages from one end user to another. We present the first deterministic, cryptographic-assumptions-free, self-stabilizing, Byzantine-resilient algorithms for network topology discovery and end-to-end message delivery. We also consider the task of r-neighborhood discovery for the case in which $r$ and the degree of nodes are bounded by constants. The use of r-neighborhood discovery facilitates polynomial time, communication and space solutions for the above tasks. The obtained algorithms can be used to authenticate parties, in particular during the establishment of private secrets, thus forming public key schemes that are resistant to man-in-the-middle attacks of the compromised Byzantine nodes. A polynomial and efficient end-to-end algorithm that is based on the established private secrets can be employed in between periodical re-establishments of the secrets

A Distributed Presence Service over Epidemic Multicast

In a dynamic distributed system with a very large number of nodes, such as a cloud, it issometimes useful to discover the nodes that are up in the system at a given time. The number of those nodes changes continually along the operation time, as some nodes crash and some join the system. In this paper we introduce a presence service that was implemented over a gossip structure using an epidemic multicast protocol. Unlike other common presence services, our service is fully distributed. Due to epidemic dissemination and inherent redundancy provided by the multicast protocol, the service is resilient against message loss and link crashes. In a scenario we developed to evaluate the efficiency and scalability of our presence service, we show how presence notifications propagate to reach all nodes in the group and we also show how adjustments for the gossip configuration can benefit the efficiency and resilience of the notification dissemination. The results of the experimental evaluation show that following a distributed approach over epidemic communication leads to a resilient and scalable presence service.Key words: Presence service, epidemic protocols, resilience, fault tolerance, clouds

Multiplatform communication in access networks

Dizertační práce se zabývá výzkumem zaměřeným na detekce poruchy v bezdrátové přístupové síti pomocí distribuovaných stochastických algoritmů. Byla navržena a simulována nová metoda detekce poruchy na základě algoritmu push-sum. V rámci plnění cílů práce byla komparována statistická kredibilita reprezentanta průměrné rychlosti konvergence protokolu push-sum a vliv ztráty zprávy během výpočtu na robustnost systému uvedený protokol využívající. Na základě získaných poznatků byla prokázána schopnost navržené metody matematicky odvodit odchylky od reálného průměru hodnot v zadané topologii, a tím byla prokázána či vyvrácena existence abnormality v síti.Doctoral thesis deals with failure detection methods in wireless access network using distributed stochastic algorithms. A new method of detecting a fault based on the push-sum algorithm has been designed and simulated. Within the scope of the work objectives, the statistical credibility of the average push-sum protocol convergence rate representative and the effect of message loss during the calculation on the robustness of the system using this protocol were compared. Based on the acquired knowledge, the ability of the protocol to mathematically derive deviations from the real average of the values in the specified topology was demonstrated and thereby the existence of an abnormality in the network has been proved or refuted.

Adaptive Backbone and Link Correlation based Data Transmission Schemes for Wireless Sensor Networks

Masteroppgave informasjons- og kommunikasjonsteknologi - Universitetet i Agder, 2015One of the main challenges faced by modern wireless sensor networks (WSN) is rapid energy depletion of individual sensor nodes. In many applications, sensor nodes are deployed in outdoor environments and it is difficult to replace or recharge node batteries. Depending on network topology and the transmission schemes implemented, certain nodes could have higher energy consumption compared with other nodes. In highly unbalanced load distributions this could lead to early total energy depletion of a critical node in the topology resulting in what is known as an energy hole in the network. Considering these factors it is vital that the transmission schemes employed in WSNs are designed to address these energy constraints. At the same time transmission schemes should be able to achieve the expected levels of reliability and latency requirements of the applications. This thesis proposes one basic scheme (BS) and an advanced scheme (AS) considering aforementioned requirements for efficient and reliable data transmissions. AS scheme would be employed if 100% reliability is required in the network while BS could be employed otherwise. The proposed schemes utilize the concept of wireless link correlation to minimize the number of transmissions and hence to reduce the energy consumption. The BS proposes 3 main components, backbone selection criteria, hop count based back-off algorithm and the selective re-transmission (SR) phase . The backbone selection method extends the ideas from connected dominating set (CDS) to achieve more balanced load distribution and the hop count based back-off algorithm aims to reduce the number of intermediate re-transmissions. In addition,the selective re-transmission (SR) helps to improve the reliability of BS. The proposed schemes are implemented along with two other transmission schemes in MATLAB based environment and extensive simulations are carried out for performance evaluation and comparison. The results show that BS and AS are capable of achieving higher level of reliability with comparatively low levels of energy consumption and number of transmissions. Furthermore, BS and AS have better performance in weak correlations than their counter parts like Collective Flooding (CF). The AS is capable of reaching 100 % reliability in all scenarios for a slightly higher energy consumption compared with BS. The results reveal also the trade off between energy consumption and reliability in a WSN. Overall, the proposed schemes could contribute towards the energy conservation of the network while providing a higher reliability when required

A trading model and security regime for mobile e-commerce via ad hoc wireless networking

Ad hoc wireless networking offers mobile computer users the prospect of trading with others in their vicinity anywhere anytime. This thesis explores the potential for developing such trading applications. A notable difficulty in designing their security services is being unable to use trusted parties. No one can be guaranteed present in each ad hoc wireless network session. A side benefit is that their costs don't have to be paid for. A reference model is defined for ad hoc m-commerce and a threat model is for- mulated of its security vulnerabilities. They are used to elicit security objectives and requirements for such trading systems. Possible countermeasures to address the threats are critically analysed and used to design security services to mitigate them. They include a self-organised P2P identity support scheme using PGP cer- tificates; a distributed reputation system backed by sanctions; a group membership service based on membership vouchers, quorate decisions by some group members and partial membership lists; and a security warning scheme. Security analysis of the schemes shows that they can mitigate the threats to an adequate degree to meet the trading system's security objectives and requirements if users take due care when trading within it. Formal verification of the system shows that it satisfies certain safety properties

A framework for enabling energy efficient semantic views in wireless sensor networks for data intensive applications

Sensor networks have been envisioned to be a promising techniquefor data intensive applications such as disaster management andemergency response and are being designed and deployed for theseapplications. The effectiveness of sensor networksin providing information is determined by human's capacity torecognize and comprehend information from the raw data collected,and act accordingly.Finding relevantinformation from the large amount of data, however, becomes achallenging problem because user interests continues to grow asthe number and variety of sensors increase and users expect toreceive only the data they select to view. Transmitting usersirrelevant data during data processing not only overloads userswith unneeded data but also incurs unnecessary communicationoverhead. Furthermore, the user interests may be correlated when alarge number of users seek information from sensor networks. As aresult, a lot of redundant data transmission can be incurredduring processing in resource-constrained sensor networks. Dataaggregation, though effective in reducing data transmission foraggregated queries, doesn't take the correlation among userinterests into consideration during processing. Therefore,additional techniques need to be proposed to provide efficientinformation delivery for correlated user interests inresource-constrained sensor networks.To bridge the gap between data collected by sensors and the information interests of users, the concept of "semantic view" is proposed in this thesis. The semantic view is a powerful abstraction which allows the fusion of multi-sensor and multi-source data into a virtual data gathering and analysis infrastructure commensurate with the interest of an end user. The main challenge is to enable semantic views in an energy efficient manner in resource constrained sensor networks. To that end, a framework which consists of five protocols and algorithms, "Query Aware Sensing", "Probabilistic Query Dissemination", "Correlated Multi-query Processing", "Location Discovery using Out-of-Range information with multi-lateration" and "End-to-end pairwise key establishment" is presented. In the proposed framework, The ultimate goal is to develop an energy efficient and secure framework towards enabling semantic views in sensor networks for data intensive applications

How robust are gossip-based communication protocols?

Gossip-based communication protocols are often touted as being robust. Not surprisingly, such a claim relies on assumptions under which gossip protocols are supposed to operate. In this paper, we discuss and in some cases expose some of these assumptions and discuss how sensitive the robustness of gossip is to these assumptions. This analysis gives rise to a collection of new research challenges