Distributed transactional reads: the strong, the quick, the fresh & the impossible

International audienceThis paper studies the costs and trade-offs of providing transactional consistent reads in a distributed storage system. We identify the following dimensions: read consistency, read delay (latency), and data freshness. We show that there is a three-way trade-off between them, which can be summarised as follows: (i) it is not possible to ensure at the same time order-preserving (e.g., causally-consistent) or atomic reads, Minimal Delay, and maximal freshness; thus, reading data that is the most fresh without delay is possible only in a weakly-isolated mode; (ii) to ensure atomic or order-preserving reads at Minimal Delay imposes to read data from the past (not fresh); (iii) however, order-preserving minimal-delay reads can be fresher than atomic; (iv) reading atomic or order-preserving data at maximal freshness may block reads or writes indefinitely. Our impossibility results hold independently of other features of the database, such as update semantics (totally ordered or not) or data model (structured or unstructured). Guided by these results, we modify an existing protocol to ensure minimal-delay reads (at the cost of freshness) under atomic-visibility and causally-consistent semantics. Our experimental evaluation supports the theoretical results

An Efficient Token-Based Algorithm for Distributed Mutual Exclusion

In this paper, we present an efficient token-based algorithm for distributed mutual exclusion. The algorithm uses a distributed queue, which is not necessarily FIFO, to enqueue the request messages of the nodes for entry into the critical section (CS). These request messages are timestamped using roughly synchronized clocks. On the average, the algorithm requires 2 to 3 number of messages per critical section entry over a wide range of workloads and a few additional number of messages at lower loads for large networks. This is in contrast to the existing algorithms in the literature which require around O(n) number of messages at higher or lower loads, where n is the number of nodes in the system. The algorithm is deadlockfree and starvation-free. 1 Introduction In distributed computing, many problems require that a shared object be allocated to a number of requesting processes in mutually exclusive manner. Hencae, the mutual exclusion problem plays a vital role in the design of distr..

Security and performance analysis of the SEAP authentication protocol in MANETs

This paper describes and analyses a secure and efficient authentication protocol (SEAP) designed for mobile ad-hoc networks (MANETs). The SEAP protocol is a server coordination-based pairwise symmetric key management protocol which works on a hierarchical network architecture and supports dynamic membership. Besides, the protocol uses a passive external membership granting server (MGS) to provide stronger security. To ensure efficiency, the protocol uses only symmetric key cryptography, and to achieve storage scalability, it makes use of a pseudo random function (PRF). The protocol is also robust against authentication server (AS) compromise. We have formally proved the correctness of the security guarantees provided by the SEAP protocol using the strand space verification model. We have evaluated the proposed protocol using simulations on the QualNet simulator, and experimentations on a MANET testbed. Both simulation and experimental study demonstrate the effectiveness of the proposed protocol as compared to the other existing approaches

Efficient Destination Discovery using Geographical Gossiping in MANETs

Abstract — Due to dynamic topology of Mobile ad hoc networks (MANETS), early designs of routing protocols incur a large number of discovery packets while trying to discover a route to a destination node in the network. To reduce the number of discovery packets, geographical information assisted routing protocols came into picture. In case of geographical ad hoc routing protocols, there is no need to discover a route to a destination node. But, they need to discover the fresh location of a destination node to deliver data packets to the destination location. Geographical information assisted ad hoc routing protocols reduce discovery packet overhead using past information about the destination node such as location, velocity and direction of motion. When a source node does not have any information about a destination node, the existing geographical routing protocols use flooding techniques or location database server to know the present location of the destination. A flooding technique or a location database server induces large number of control packets in the network. To reduce the number of control packets during location discovery, we propose a novel geographical gossiping technique for MANETs. The technique basically uses two types of gossiping viz. selective and random gossiping. We have evaluated the performance of the proposed technique using qualnet simulator and compared its performance with flooding technique and probability based gossiping technique. The simulation results clearly show that our technique has considerably reduced control packet overhead compared to flooding and probability based gossiping technique. Index Terms — Ad hoc networks, geographical routing, gossiping, location discovery, flooding I