Nicolas C. Nicolaou

Efficient vector-based forwarding for underwater sensor networks

Underwater Sensor Networks (UWSNs) are significantly different from terrestrial sensor networks in the following aspects: low bandwidth, high latency, node mobility, high error probability, and 3-dimensional space. These new features bring many challenges to the network protocol design of UWSNs. In this paper, we tackle one fundamental problem in UWSNs: robust, scalable, and energy efficient routing. We propose vector-based forwarding (VBF), a geographic routing protocol. In VBF, the forwarding path is guided by a vector from the source to the target, no state information is required on the sensor nodes, and only a small fraction of the nodes is involved in routing. To improve the robustness, packets are forwarded in redundant and interleaved paths. Further, a localized and distributed self-adaptation algorithm allows the nodes to reduce energy consumption by discarding redundant packets. VBF performs well in dense networks. For sparse networks, we propose a hop-by-hop vector-based forwarding (HH-VBF) protocol, which adapts the vector-based approach at every hop. We evaluate the performance of VBF and HH-VBF through extensive simulations. The simulation results show that VBF achieves high packet delivery ratio and energy efficiency in dense networks and HH-VBF has high packet delivery ratio even in sparse networks.

Fault-tolerant semifast implementations of atomic read/write registers

This paper investigates time-efficient implementations of atomic read-write registers in message-passing systems where the number of readers can be unbounded. In particular we study the case of a single writer, multiple readers, and S servers, such that the writer, any subset of the readers, and up to t servers may crash. A recent result of Dutta et al. [P. Dutta, R. Guerraoui, R.R. Levy, A. Chakraborty, How fast can a distributed atomic read be? In: Proceedings of the 23rd ACM Symposium on Principles of Distributed Computing, 2004, pp. 236-245] shows how to obtain fast implementations in which both reads and writes complete in one communication round-trip, under the constraint that the number of readers is less than St-2, where t

On the Efficiency of Atomic Multi-reader, Multi-writer Distributed Memory

This paper considers quorum-replicated, multi-writer, multi-reader (MWMR) implementations of survivable atomic registers in a distributed message-passing system with processors prone to failures. Previous implementations in such settings invariably required two rounds of communication between readers/writers and replica owners. Hence the question arises whether it is possible to have single round read and/or write operations in this setting. We thus devise an algorithm, called Sfw , that exploits a new technique called server side ordering ( SSO ), which ---unlike previous approaches--- places partial responsibility for the ordering of write operations on the replica owners (the servers). With SSO, fast write operations are introduced for the very first time in the MWMR setting. We prove that our algorithm preserves atomicity in all permissible executions. While algorithm SFW shows that in principle fast writes are possible, we also show that under certain conditions the MWMR model imposes inherent limitations on any quorum-based fast write implementation of a safe read/write register and potentially even restricts the number of writer participants in the system. In this case our algorithm achieves near optimal efficiency.

State-Wide Elections, Optical Scan Voting Systems, and the Pursuit of Integrity

In recent years, two distinct electronic voting technologies have been introduced and extensively utilized in election procedures: direct recording electronic systems and optical scan (OS) systems. The latter are typically deemed safer, as they inherently provide a voter-verifiable paper trail that enables hand-counted audits and recounts that rely on direct voter input. For this reason, OS machines have been widely deployed in the United States. Despite the growing popularity of these machines, they are known to suffer from various security vulnerabilities that, if left unchecked, can compromise the integrity of elections in which the machines are used. This article studies general auditing procedures designed to enhance the integrity of elections conducted with optical scan equipment and, additionally, describes the specific auditing procedures currently in place in the State of Connecticut. We present an abstract view of a typical OS voting technology and its relationship to the general election process. With this in place, we lay down a ldquotemporal-resourcerdquo adversarial model, providing a simple language for describing the disruptive power of a potential adversary. Finally, we identify how audit procedures, injected at various critical stages before, during, and after an election, can frustrate such adversarial interference and so contribute to election integrity. We present the implementation of such auditing procedures for elections in the State of Connecticut utilizing the Premiere (Diebold) AccuVote OS; these audits were conducted by the UConn VoTeR Center, at the University of Connecticut, on request of the Office of the Secretary of the State. We discuss the effectiveness of such procedures in every stage of the process and we present results and observations gathered from the analysis of past election data.

On the Robustness of (Semi) Fast Quorum-Based Implementations of Atomic Shared Memory

This paper studies a trade-off between fault-tolerance and latency in implementations of atomic read/write objects in message-passing systems. In particular, considering fastor semifastquorum-basedimplementations, that is, implementations where allor respectively mostread and write operations complete in a single communication round-trip, it is shown that such implementations are not robustdue to the fact that they necessarily require a quorum system with a common intersection between its quorums. To trade speed for fault-tolerance, the notion of weak-semifastimplementations is introduced. Here more than a single complete slow (two round-trip) read operation is allowed for each write operation (semifast implementations allow only one such slow read). A quorum-based algorithm is given next and it is formally shown that it constitutes a weak-semifast implementation of atomic registers. The algorithm uses the notion of Quorum Viewsto facilitate the characterization of all possible object timestamp distributions that a read operation may witness during its first communication round-trip. Noteworthy is that the algorithm allows fast read operations even if they are concurrent with other read and write operations. Finally, experimental results were gathered by simulating the algorithm using the NS-2 network simulator. The results show that under realistic conditions, less than 13% of read operations are slow, thus the overwhelming majority of operations take a single communication round-trip.

At-most-once semantics in asynchronous shared memory

This paper investigates the feasibility of implementing at-most-once access semantics in a model where a collection of actions is to be performed by failure-prone, asynchronous shared-memory processes. We introduce the At-Most-Once problem for performing a set of n jobs using m processors, and we define the notion of efficiency for such protocols, called effectiveness, that allows the classification of algorithms solving the problem. The effectiveness for an at-most-once implementation is the number of jobs safely completed by the implementation, expressed as a function of the number of jobs n, the number of processes m, and the number of process crashes f. We prove a lower bound of n--f on the effectiveness of any algorithm. We then present two process solutions that offer a trade off between work and space complexity. Finally, we generalize a two-process solution for the multi-process setting using a hierarchical algorithm that achieves effectiveness of n--log m†o(n), coming reasonably close, asymptotically, to the corresponding lower bound.

Implementing distributed shared memory for dynamic networks

Atomically consistent memory services provide resiliency in dynamic settings.

Towards Feasible Implementations of Low-Latency Multi-writer Atomic Registers

This work explores implementations of multiwriter/multi-reader (MWMR) atomic registers in asynchronous, crash-prone, message-passing systems with the focus on low latency and computational feasibility. The efficiency of atomic read/write register implementations is traditionally measured in terms of the latency of read and write operations. To reduce operation latency researchers focused on the communication costs, expressed as the number of communication round-trips (or rounds), often ignoring the computation costs. In this paper we consider efficiency of a register implementation in terms of both communication and computation costs. As of this writing, algorithm SFW is the sole known MWMR algorithm that allows single round read and write operations. The algorithm uses collections of intersecting sets (quorums), and to enable single round operations, SFW relies on the evaluation of certain predicates. We formulate a new combinatorial problem that captures the computational burden of evaluating the predicates in algorithm SFW and we show that it is NP-Complete. To make the evaluation of the predicates feasible, we present a polynomial log-approximation algorithm for this problem and we show how to use it with algorithm SFW. Then we present a new algorithm, called CWFR, that allows fast operations independently of the underlying quorum system construction. The algorithm implements two-round writes and allows reads to complete in a single round. We conclude with experimental evaluations of our algorithms obtained from simulations in NS2.

Formalizing and Implementing Distributed Ledger Objects

Despite the hype about blockchains and distributed ledgers, formal abstractions of these objects are scarce1. To face this issue, in this paper we provide a proper formulation of a distributed ledger object. In brief, we de ne a ledger object as a sequence of records, and we provide the operations and the properties that such an object should support. Implemen- tation of a ledger object on top of multiple (possibly geographically dispersed) computing devices gives rise to the distributed ledger object. In contrast to the centralized object, dis- tribution allows operations to be applied concurrently on the ledger, introducing challenges on the consistency of the ledger in each participant. We provide the de nitions of three well known consistency guarantees in terms of the operations supported by the ledger object: (1) atomic consistency (linearizability), (2) sequential consistency, and (3) eventual consistency. We then provide implementations of distributed ledgers on asynchronous message passing crash- prone systems using an Atomic Broadcast service, and show that they provide eventual, sequen- tial or atomic consistency semantics respectively. We conclude with a variation of the ledger the validated ledger which requires that each record in the ledger satis es a particular validation rule.

Implementing Atomic Data through Indirect Learning in Dynamic Networks

Developing middleware services for dynamic distributed systems, e.g., ad-hoc networks, is a challenging task given that such services deal with dynamically changing membership and asynchronous communication. Algorithms developed for static settings are often not usable in such settings because they rely on (logical) all-to-all node connectivity through routing protocols, which may be unfeasible or prohibitively expensive to implement in highly dynamic settings. This paper explores the indirect learning, via periodic gossip, approach to information dissemination within a dynamic, distributed data service implementing atomic read/write memory service. The indirect learning scheme is used to improve the liveness of the service in the settings with uncertain connectivity. The service is formally proved to guarantee atomicity in all executions. Conditional performance analysis of the new service is presented, where this analysis has the potential of being generalized to other similar dynamic algorithms. Under the assumption that the network is connected, and assuming reasonable timing conditions, the bounds on the duration of read/write operations of the new service are calculated. Finally, the paper proposes a deployment strategy where indirect learning leads to an improvement in communication costs relative to a previous solution that assumes all-to-all connectivity.