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Automatic Verification and Analysis of Complex Systems

General Information

This project addresses the rigorous mathematical analysis of models of complex safety critical computerized systems, such as aircrafts, trains, cars, or other artifacts, whose failure can endanger human life. Our aim is to raise the state of the art in automatic verification and analysis techniques (v&a) from its current level, where it is applicable only to isolated facets (concurrency, time, continuous control, stability, dependability, mobility, data structures, hardware constraints, modularity, levels of refinement), to a level allowing a comprehensive and holistic verification of such systems.

  • We will investigate the mathematical models and their interrelationship as they arise at the various levels of design of safety critical computerized systems. These models range from classical non-deterministic transition systems to probabilistic, real-time, and hybrid system models.
  • We will, in particular, deal with the complete range of time-models needed to support the development of concrete systems, and the transition between them. This includes the determination of safe sampling rates in plant control as well as the transition between virtual time models and physical execution time.
  • We will also address a wide range of system structures in this application domain. These range from models of distributed target architectures (such as hierarchical bus-structures connecting multiple electronic control units) to task models (depicting hierarchical task structures with communication and timing requirements) to specification models of electronic control units (such as a controller enforcing a speed-profile) to system models (e.g. of the electronic on-board train systems) to inter-system specification models (e.g. for coordination of train movements).

Today's v&a technology at best addresses isolated points in this space of models. The proposed project sets out to develop automatic verification techniques covering the entire space. It is our firm belief that by improving and better automating base verification methods (such as abstract interpretation, SAT solvers, symbolic model checking, abstraction techniques, linear programming, constraint solving, Lyapunov functions, automatic decision procedures for relevant first-order theories, automata based verification, heuristic search) and by integrating them in a deep and focused manner we will be able to achieve a dramatic increase in the complexity of models amenable to automatic verification. (By complexity we mean both system size as well as logical complexity as induced by the interferences between the various facets of the system.) By identifying new, as well as by exploiting well-established design and modeling paradigms (mathematically reflected by decomposition theorems that break down complex systems into manageable sub-systems and facets), the combination of the individual, complementing automated verification techniques will deliver synergies that we do not see in present verification systems.

Project Area R - Real-Time Systems

Real-time systems are systems that interact with their environment in such a way that for certain inputs the corresponding outputs have to occur within given time bounds. Many embedded systems, in particular those in safety critical applications, are of this type. The practical importance of this class of systems has led to a large body of research on the specification, verification and analysis of real-time systems.
Analysis and verification of real-time systems is based on computational models of such systems. Models with continuous time are close to the physical world and are well suited for dealing with the requirement and specification level; their automatic verification is performed using tools based on timed automata with real-valued clocks. To gain in efficiency also models with discrete time are considered; their automatic verification is performed using classical approaches to temporal logic model checking. At the level of machine real-time systems are modeled as set of tasks that need to be scheduled to meet the required deadlines.
While these real-time models have led to powerful analysis and verifications techniques, they turn out to be too idealistic or too simplistic to deal with the challenges of high-level specification languages, highly concurrent systems, and the intricacies of distributed computer architectures.
The AVACS subprojects R1--3 strive to overcome these restrictions by exploiting the structure of the real-time systems at all levels of abstraction ranging from specification level down to implementation level:

  • subproject R1 will develop verification techniques for real-time requirements of high-level specifications based on constraint manipulation, predicate abstraction, and decision procedures for high-level data types;
  • subproject R2 will develop task distribution and scheduling techniques taking the constraints of distributed computer architectures into account, in particular the combination of caches and pipelines;
  • subproject R3 considers highly concurrent real-time systems and develops model checking techniques that combine heuristic search with abstraction.

Project Area S - Coarse Grain System Structure

Project Area S focuses on techniques to support the global analysis of the interplay between different systems, or between subsystems, thus addressing both inter-system and system-level design. Typical instances in the train system domain relate to the interaction between trains and track-side elements such as RBC, s, or between the different components of the on-board electronic subsystems of a train. The overall challenge for Project Area S is thus the need to address complete systems, or even systems of systems, demanding dedicated techniques to cater for the sheer complexity of resulting models. Subproject S1 addresses this by automating compositional verification; it uses learning strategies and combinations of abstraction techniques, game theory, and SAT and model checking based verification methods to automatically synthesize assumptions, allowing to reduce global verification of (potentially only partially specified) system level designs to local verification tasks. The particular challenges of inter-system coordination protocols, which typically require dynamically changing communication structures and must address systems entering and leaving surveillance regions, are attacked in S2 with a combination of static analysis techniques, abstraction techniques, and symmetry arguments to reduce the verification of such coordination protocols to finite state verification problems. Subproject S3 provides formal proofs of system dependability requirements through stochastic model checking of system models, taking into account failure models and component failure behaviors, while deriving safety requirements for subsequent design steps.
By addressing complete systems and their interplay, Project Area S provides methods for reducing global verification tasks to smaller problem instances (S1, S2) and dependability analysis, which inherently requires global system analysis. Subprojects S1 and S2 complement each other, in that verification instances resulting from the reduction methods of S2 can be further decomposed using techniques of S1. Models used will reflect the particular complexity dimension attacked in each subproject, providing sufficiently rich structuring methods to asynchronously compose transition systems, supporting their dynamic creation and destruction and dynamic communication structures for S2, and using extended Markov Processes in S3. Later project phases will enrich the class of models, allowing to deal with real-time and hybrid component models, and extend decomposition theorems to such richer classes of models, thus paving the way for realizing the AVACS vision, where an overall verification methodology invokes such decomposition theorems, unfolding a proof tree for global problems to a level where verification tasks can be discharged with the verification methods developed in Project Areas R and H.


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