Mobius 2.3 is an extensible dependability, security, and performance modeling environment for large-scale discrete-event systems. It provides multiple model formalisms and solution techniques, facilitating the representation of each part of a system in the formalism that is most appropriate for it, and the application of the solution method or methods best-suited to estimating the system's behavior. Since its initial release in 2001, many advances have been made in Moumlbius's design and implementation that have strengthened its place in the modeling and analysis community. With almost a decade of widespread academic and industrial use, Moumlbius has proven itself to be useful in a wide variety of modeling situations. This paper documents the current feature set of Mobius 2.3, emphasizing recent significant enhancements.

Viruses that infect smartphones are emerging as a new front in the fight against computer viruses. In this paper, we model the propagation of mobile phone viruses in order to study their impact on the dependability of mobile phones. We propose response mechanisms and use the models to obtain insight on the effectiveness of these virus mitigation techniques. In particular, we consider the effects of multimedia messaging system (MMS) viruses that spread by sending infected messages to other phones. The virus model is implemented using the Mobius software tool and is highly parameterized, enabling representation of a wide range of potential MMS virus behavior. Using the model, we present the results of four illustrative MMS virus scenarios simulated with and without response mechanisms. By measuring the propagation rate and the extent of virus penetration in the simulation phone population, we quantitatively compare the effectiveness of mobile phone virus response mechanisms.

UNLABELLED: Möbius has found numerous applications in computational biology to build and solve stochastic models of biological processes. It provides the user with a modeling workflow and several sophisticated features that are not available in the simulation tools commonly used by computational biologists. AVAILABILITY: Möbius is free for academic users. It can be downloaded from www.mobius.uiuc.edu

MOTIVATION: The stochastic kinetics of a well-mixed chemical system, governed by the chemical Master equation, can be simulated using the exact methods of Gillespie. However, these methods do not scale well as systems become more complex and larger models are built to include reactions with widely varying rates, since the computational burden of simulation increases with the number of reaction events. Continuous models may provide an approximate solution and are computationally less costly, but they fail to capture the stochastic behavior of small populations of macromolecules. RESULTS: In this article we present a hybrid simulation algorithm that dynamically partitions the system into subsets of continuous and discrete reactions, approximates the continuous reactions deterministically as a system of ordinary differential equations (ODE) and uses a Monte Carlo method for generating discrete reaction events according to a time-dependent propensity. Our approach to partitioning is improved such that we dynamically partition the system of reactions, based on a threshold relative to the distribution of propensities in the discrete subset. We have implemented the hybrid algorithm in an extensible framework, utilizing two rigorous ODE solvers to approximate the continuous reactions, and use an example model to illustrate the accuracy and potential speedup of the algorithm when compared with exact stochastic simulation. AVAILABILITY: Software and benchmark models used for this publication can be made available upon request from the authors.

Abstract Applications that are part of a mission‐critical information system need to maintain a usable level of key services through ongoing cyber‐attacks. In addition to the well‐publicized denial of service (DoS) attacks, these networked and distributed applications are increasingly threatened by sophisticated attacks that attempt to corrupt system components and violate service integrity. While various approaches have been explored to deal with DoS attacks, corruption‐inducing attacks remain largely unaddressed. We have developed a collection of mechanisms based on redundancy, Byzantine fault tolerance, and adaptive middleware that help distributed, object‐based applications tolerate corruption‐inducing attacks. In this paper, we present the ITUA architecture, which integrates these mechanisms in a framework for auto‐adaptive intrusion‐tolerant systems, and we describe our experience in using the technology to defend a critical application that is part of a larger avionics system as an example. We also motivate the adaptive responses that are key to intrusion tolerance, and explain the use of the ITUA architecture to support them in an architectural framework. Copyright © 2006 John Wiley & Sons, Ltd.

An increasing number of computer systems are designed to be distributed across both local and wide-area networks, performing a multitude of critical information-sharing and computational tasks. Malicious attacks on such systems are a growing concern, where attackers typically seek to degrade quality of service by intrusions that exploit vulnerabilities in networks, operating systems, and application software. Accordingly, designers are seeking improved techniques for validating such systems with respect to specified survivability requirements. In this regard, we describe a model-based validation effort that was undertaken as part of a unified approach to validating a networked intrusion-tolerant information system. Model-based results were used to guide the system's design as well as to determine whether a given survivability requirement was satisfied.

Functional Interface (AFI). Models and solution techniques interact with one another through the use of the standard interface, allowing them to interact with M OBIUS framework components, not formalism components. This permits novel combinations of modeling techniques. The AFI uses abstract classes to implement the framework components. The most basic model in the M OBIUS framework is an atomic model, and is made up of state variables that hold information about the state of a model and actions that are used for changing model state. Stochastic activity networks (SANs) and the stochastic process algebra PEPA are example atomic models that have been successfully implemented in the M OBIUS tool.

Building dependable distributed systems from commercial off-the-shelf components is of growing practical importance. For both cost and production reasons, there is interest in approaches and architectures that facilitate building such systems. The AQuA architecture is one such approach; its goal is to provide adaptive fault tolerance to CORBA applications by replicating objects. The AQuA architecture allows application programmers to request desired levels of dependability during applications' runtimes. It provides fault tolerance mechanisms to ensure that a CORBA client can always obtain reliable services, even if the CORBA server object that provides the desired services suffers from crash failures and value faults. AQuA includes a replicated dependability manager that provides dependability management by configuring the system in response to applications' requests and changes in system resources due to faults. It uses Maestro/Ensemble to provide group communication services. It contains a gateway to intercept standard CORBA IIOP messages to allow any standard CORBA application to use AQuA. It provides different types of replication schemes to forward messages reliably to the remote replicated objects. All of the replication schemes ensure strong, data consistency among replicas. This paper describes the AQuA architecture and presents, in detail, the active replication pass-first scheme. In addition, the interface to the dependability manager and the design of the dependability manager replication are also described. Finally, we describe performance measurements that were conducted for the active replication pass-first scheme, and we present results from our study of fault detection, recovery, and blocking times.

Building large-scale distributed object-oriented systems that provide multidimensional quality of service (QoS) in terms of fault tolerance, scalability, and performance is challenging. In order to meet this challenge, we need an architecture that can ensure that applications' requirements can be met while providing reusable technologies and software solutions. This paper describes techniques, based on the AQuA architecture, that enhance the applications' dependability and scalability by introducing two types of group members and a novel passive replication scheme. In addition, we describe how to make the management structure itself dependable by using the passive replication scheme. Finally, we provide performance measurements for the passive replication scheme.

The Mobius framework is an environment for supporting multiple modeling formalisms and solution techniques. Models expressed in formalisms that are compatible with the framework are translated into equivalent models using Mobius framework components. This translation preserves the structure of the models, allowing efficient solutions. The framework is implemented in the tool by a well-defined abstract functional interface. Models and solution techniques interact with one another through the use of the standard interface, allowing them to interact with Mobius framework components, not formalism components. This permits novel combinations of modeling techniques, and will be a catalyst for new research in modeling techniques. This paper describes our approach, focusing on the "atomic model". We describe the formal description of the Mobius components as well as their implementations in our software tool.

This paper presents a new method for multiscale segmentation of volume images. The segmentation is achieved using a recent nonlinear transform which leads to well-characterized regions at different spatial and intensity scales. The detected three-dimensional regions are closed and are homogeneous relative to their surround. A pyramid is generated containing the region information extracted across a range of homogeneity scales. The pyramid represents the multiscale volumetric structure. Experimental results are given for magnetic resonance data as well as video sequences.