M. Ümit Uyar

Finite state machine based formal methods in protocol conformance testing: from theory to implementation

Abstract Recent evolution of communication networks has led toward increasingly complex communication protocols to interconnect heterogeneous systems. In order for such systems to function properly, communication protocols require formal methodologies for verification, implementation and testing. In this paper, the use of formal methods for protocol conformance testing is presented. Four major formal techniques for conformance test generation reported in the literature are compared: transition tours, distinguishing sequences, characterizing sequences and unique input/output sequences. The implementation of each approach as a test generation methodology is illustrated. The result is the capability to generate abstract test suites that require a minimum number of steps to execute while maintaining complete coverage of the state transitions (or test purposes) of the protocol. The impact of the formal methodologies on the practical aspects of conformance testing, including the test system implementation and automatic test script generation, is discussed. The experience based on testing various protocol implementations from multiple manufacturers at AT&T Bell Laboratories is described. The protocols tested include X.25 and ISDN Q.921 and Q.931 for basic and primary rate interfaces. In order to standardize various efforts on conformance testing, the ISO and the CCITT have developed principles to specify abstract and executable test suites, and methodologies to design testbed architectures. The relationship between such standards and the formal methods for test generation is explored.

Genetic algorithms for self-spreading nodes in MANETs

We present a force-based genetic algorithm for self-spreading mobile nodes uniformly over a geographical area. Wireless mobile nodes adjust their speed and direction using a genetic algorithm, where each mobile node exchanges its genetic information of speed and direction encoded in its chromosomes with the neighboring nodes. Simulation experiments show encouraging results for the performance of our force-based genetic algorithm with respect to normalized area coverage..

Test Generation in the Presence of Conflicting Timers

The UD’s and CCNY’s ongoing research to generate conformance tests for the Army network protocol MIL-STD 188–220 addressed test generation when multiple timers are running simultaneously. A test sequence may become unrealizable if there are conflicting conditions based on a protocol’s timers. This problem is handled in the hitherto generated tests by manually expanding a protocol’s extended FSM based on the set of conflicting timers, resulting in test sequences that are far from minimum-length. Similar inconsistencies, but based on arbitrary linear variables, are present in the extended FSMs modeling VHDL specifications. This paper presents an efficient solution to the conflicting timers problem that eliminates the redundancies of manual state expansion. CCNY’ s inconsistency removal algorithms are applied to a new model for testing protocols with multiple timers, in which complex timing dependencies are captured by simple linear expressions. This test generation technique is expected to significantly shorten the test sequences without compromising their fault coverage.

Modeling VHDL specifications as consistent EFSMs

The dramatic increase in the complexity of digital systems has led to the use of formal description languages such as VHDL. This paper presents the preliminary results on properties of VHDL specifications that allow for automatic test generation without the exponential growth. In general, VHDL specifications can be modeled as extended finite machines (EFSMs). A class of EFSMs, called consistent EFSMs, allows for the finite state machines (FSM) based test generation techniques to be directly applied to VHDL specifications. An algorithm to identify the consistent EFMSs is introduced.

Self organization for area coverage maximization and energy conservation in mobile ad hoc networks

Mobile Ad hoc Networks (manets) are widely used for a large number of strategic applications from military to commercial tasks including disaster area discovery, mine field clearing, and transportation systems. In realistic applications, it is not feasible to deploy mobile nodes manually or using a centralized controller. We provide a nature-inspired approach to achieve self-organization of mobile nodes over unknown terrains. In this framework, each mobile node uses a genetic algorithm as a self-distribution mechanism to decide its next speed and movement direction to obtain a uniform distribution. We present a formal analysis of the effectiveness of our genetic algorithm and introduce an inhomogeneous Markov chain model to prove its convergence. The experiment results from our simulation software and our vmware-based testbed show that our nature-inspired algorithm delivers promising results for uniform distribution of mobile nodes over unknown terrains.

Formal design and testing of MIL-STD 188-220A based on Estelle

This paper describes the Estelle specification of MIL-STD 188-280A intranet layer as well as a methodology for generating test sequences for checking the conformance of a protocol implementation to its specification. The methodology for deriving test cases from an Estelle specification, which serves as input to test generation techniques, is presented. A Chinese postman tour is used to determine a minimum-cost tour of the transition graph for various transition types. Finally, the paper discusses several controllability and optimization issues that need to be addressed in test cases generation for the intranet and datalink layers of MIL-STD 188-220A.

Analysis of Emergent Behavior for GA-based Topology Control Mechanism for Self-Spreading Nodes in MANETs

We introduce a genetic algorithm based MANET topology control mechanism to be used in decision making process of adaptive and autonomic systems at run time. A mobile node adapts its speed and direction using limited information collected from local neighbors operating in an unknown geographical terrain. We represent the genetic operators (i.e., selection, crossover and mutation) as a dynamical system model to describe the behavior of a single node’s decision mechanism. In this dynamical system model each mobile node is viewed as a stochastic variable. We build a homogeneous Markov chain to study the convergent nature of multiple mobile nodes running our algorithm, called FGA. Each state in our chain represents a configuration of the nodes in a MANET for a given instant. The homogeneous Markov chain model of our FGA is shown to be ergodic; its convergence is demonstrated using Dobrushin’s contraction coefficients. We also observe that the nodes with longer communication ranges utilize more information about their neighborhood to make better decisions, require less movement and converge faster, whereas smaller communication ranges utilize limited information, take more time to escape local optima, and, hence, consume more energy.

On stability analysis of virtual backbone in mobile ad hoc networks

Service discovery architectures and cluster-assisted routing protocols in mobile ad-hoc networks (MANETs) heavily use formation and maintenance of a virtual backbone (VB), where the most stable mobile nodes with higher node degree are dynamically selected as the backbone nodes. In this paper we present a novel analytic model for VB stability in MANETs. The model employs the dynamics of node movements, where link creation/failure is modeled via a random walk with probabilistic state-transition matrix. The backbone formation algorithm gives preference to the nodes with the smaller number of link changes and the higher degree. Therefore, the link arrivals and departures determine the probability (and thus the expected time) for a mobile node to leave, join, or remain in the backbone, i.e., the stability of a dynamic structure of VB.

Generation of Feasible Test Sequences for EFSM Models

A method that enables the generation of realizable test sequences from a class of EFSMs is presented. If the interdependencies among the variables used in the actions and the conditions of EFSMs are not considered during test generation, the test sequences may be unrealizable in a test laboratory. Algorithms for the detection and elimination of inconsistencies from the EFSM models are presented. Once inconsistencies are eliminated, realizable test sequences can be generated from the resulting consistent EFSM by using the methods available for FSM models.

Testing protocols modeled as FSMs with timing parameters

An optimization method is introduced for generating minimum-length test sequences taking into account timing constraints for FSM models of communication protocols. Due to active timers in many of todays protocols, the number of consecutive self-loops that can be traversed in a given state before a timeout occurs is limited. A test sequence that does not consider timing constraints will likely be unrealizable in a test laboratory, thereby potentially resulting in the incorrect failing of valid implementations (or, vice versa). The solution uses a series of augmentations for a protocols directed graph representation. The resulting test sequence is proven to be of minimum-length while not exceeding the tolerable limit of consecutive self-loops at each state. Although UIO sequences are used for state verification method, the results also are applicable to test generation that uses distinguishing or characterizing sequences.