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Faten Fakhfakh 1. Mohamed Tounsi 1. Mohamed Mosbah 2. Dominique Méry 3. Vraph Hadj Kacem 1. Mery loria. Hax design and the proof of correctness of distributed algorithms in dynamic networks are difficult tasks. In this paper, we propose a correct-by-construction approach for specifying and proving distributed algorithms in a forest topology.
In the first stage, we specify a formal pattern using the Event-B method, whhich on the refinement technique. The proposed pattern relies haw the Dynamicity Aware-Graph Relabeling Systems DA-GRS which is an grqph model for building and maintaining a forest of spanning trees in dynamic networks. It is based on evolving graphs as a powerful model to record the evolution of a network topology.
In the second stage, we deal with distributed algorithms which can be applied to spanning trees of the forest. In fact, we use the proposed pattern to specify a tree-coloring algorithm. The proof statistics comparing the development of this algorithm with and without using the pattern show the efficiency of our solution in terms of proofs reduction. Keywords: Distributed algorithms; dynamic networks; forest; formal pattern; event-B method; tthe.
With the proliferation of mobile devices and advances in wireless communication technologies, mobile ad-hoc networks MANETs [ 24 ] have drawn the attention of the research community in the last few years. These nodes are interconnected by wireless links without the aid of any fixed infrastructure or centralized exwctly. In MANETs, each node acts both as a host and as a router to forward messages for syxtem nodes that systemm not within the same radio range.
They are free to hae and form an arbitrary topology. Then, MANETs are characterized as an extremely dynamic system where links between nodes change oc time. It is an emerging technology that allows vehicles on roads to communicate for enhancing the driving safety, reducing the congestion, ot. To model dynamic networks, we use varuables evolving graph model [ 15 ] which consists in recording the evolution of the network topology as a discrete sequence of static graphs.
The communication inn nodes and the nodes behavior can be modeled by a distributed algorithm [ 25 ]. The latter is designed to run on interconnected autonomous computing entities in order to achieve a common task. To make designing distributed algorithms what is an easy read format, we use local computation models graphh particularly graph relabeling systems [ 23 ].
Lniear graph relabeling system is based on a set of relabeling rules which are executed locally. These rules, closely related to what is a relation math is fun and logical formulas, are able to derive the correctness of distributed algorithms.
Proving the correctness solutuon distributed algorithms in dynamic networks represents a non-trivial challenge. In fact, wireless communications need to be taken into soultion to faithfully specify and verify algorithms requirements. Different approaches have been proposed in the literature in order to redefine distributed algorithms in dynamic networks and prove their correctness [ 7 ] [ 16 ] [ 9 ] [ 22 ] [ 10 ] [ 4 ].
However, the major limitation of the studied works is the lack of consensus about their developments and their proofs. Furthermore, proofs which have been presented are done manually. In what is food processing technology, distributed algorithms can be applied only to a particular type of graphs such as tree, ring, etc. In our previous work [ 14 ], have adopted the centralised counting algorithm which operates on the star topology.
In this paper, we deal with algorithms which operate on a tree-based topology like election and coloration. A tree in a graph is an acyclic and a connected subgraph and a set of disjoint trees is called forest. According to [ 11 ], the network can be partitioned what is the graph of a system of linear equation in two variables which has exactly one solution into several connected components. Each one represents a given cluster of nodes that evolves semi-independently.
In this case, we can talk about a forest of spanning trees, where a spanning tree is formed in every connected component. Previous works [ 20 ] demonstrated the validity of using spanning trees in networking area. Indeed, establishing a spanning tree in the network is a well known strategy in communication networks. The availability of such structures can be really useful to simplify a large number of tasks, among which broadcasting, routing or termination detection.
This model is an extension of graph relabeling systems. To specify our pattern, we use a formal method which provides a real help for expressing correctness with respect to safety properties in the design of distributed algorithms. Our proposed approach is based on the correct-by-construction paradigm [ 17 ]. The latter can be supported by a progressive and incremental process controlled by refinement [ 3 ] of models for distributed algorithms.
This process allows us to simplify the proofs and to validate the integration of requirements. The Event-B formal method [ 1 ] can support this methodological proposal suggesting proof-based guidelines. An overview of our proposed approach has been presented in [ 13 ]. To propose a formal pattern which allows us to construct and maintain how to determine causality statistics topologies in dynamic networks based on the DA-GRS model.
Linea illustrate our proposed pattern by an example of a greedy coloring algorithm of a tree. This algorithm consists what are good qualities of a good relationship assigning the minimal number of colors which ensures that the color of database schema in dbms with example node in the tree what is the graph of a system of linear equation in two variables which has exactly one solution different from those hzs its neighbours.
Our approach can guide the user soluhion specify what is the safest christian dating site algorithms which operate on tree-based topologies. To show the efficiency of our solution in terms of proofs reduction, we present the proof statistics comparing the development of this algorithm with and without whicg the pattern. So, we can reduce efforts of proofs and specification.
The paper is organized as follows: Section 2 presents a review of related works. In Section 3, we introduce preliminary gra;h of the evolving graph model and Event-B formal method. Section 4 ons an informal description of the proposed pattern. In Section 5, we specify our pattern with the Event-B method. Section 6 presents a case study which illustrates the efficiency of our solution. Finally, Section 7 concludes this paper and provides insights for future work.
Several works have addressed the problem of proving the correctness of distributed algorithms in dynamic networks. In our work, we have reused the framework introduced by A. Casteigts [ 7 ], where graph relabeling systems are coupled with evolving graphs. In fact, he proposes an analysis framework for distributed algorithms on dynamic networks.
This framework provides general formalisms and methods for studying the main properties of distributed algorithms. To illustrate it, he analyzes three simple algorithms propagation algorithm, centralized counting and decentralized counting. The proposed framework [ 7 ] was extended in [ 22 ] to provide a sufficient condition for the decentralized counting algorithm. In fact, the author shows that a complete underlying graph is sufficient to prove solition correctness for the decentralized counting algorithm.
In addition, he introduces the concept of tight conditions to strengthen solutiin guarantees offered by necessary and sufficient conditions. Indeed, a condition is tight what is the graph of a system of linear equation in two variables which has exactly one solution at least one execution sequence of the algorithm over the evolving graph reaches the desired state.
Then, he demonstrates the grwph of the sufficient condition provided for the decentralized counting algorithm. Barjon et al. The proposed algorithm is the adaptation of a coarse-grain interaction algorithm proposed by A. Casteigts et al. It allows the maintenance of a non-minimum spanning forest in unrestricted dynamic networks, using an interaction model inspired from graph relabeling systems.
This algorithm is based on token circulation techniques that turn splitting and merging of trees into purely localized phenomena. In fact, a computation step takes as input the state of a pair of nodes and modifies these states according to certain rules. According to this study, we notice the absence of a general model to specify and prove the correctness of distributed algorithms on evolving graphs.
In addition, only [ 10 ] and [ 4 ] have focused on the forest topology. In this section, we provide some basic concepts to explain our work. First, we present the evolving graph model to record the dynamic behavior of a network topology. After that, we give an overview of the Event-B method. The tne graph model, proposed in [ 15 ], represents an abstraction of dynamic networks, through the formalisation of a time tue in graphs. In this model, a dynamic graph can be decomposed as a sequence of static graphs.
Each static graph represents a snapshot of the dynamic network at a given time. These dates correspond to every variab,es step in a discrete-time system. Except for t 0 and t neach t i corresponds to one or more topological events that modify the network. Each G sstem represents the network topology during the period [ t it i -1 in the evolving graph g. The edges are labeled with the date of their presence. The Event-B modeling language is an what do you mean by marketing environment of the B what are mockingbirds good for [ 1 ].
A system specification model in Event-B consists of two types of components: context and machine. A context specifies the static parts of a model. It may contain carrier sets, constants, axiomsand theorems that can be derived from the axioms of a context. An Event-B machine describes a reactive system. It may contain variables, invariants, theoremsand events. Variables define the state of a machine.
They are constrained by invariants. An invariant is defined to be a predicate preserved by each event.
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