6559743
A refinement logic for the fork calculus
The Fork Calculus FC presents a theory of communicating systems in family with CCS, but it differs in the way that processes are put in parallel. In CCS there is a binary parallel operator |, whereas FC contains a unary fork operator. We provide FC with an operational semantics, together with a congruence relation between processes. Further, a refinement logic for program specification and design is presented. In this logic it is possible to freely mix programming constructs with specification constructs, thereby allowing us to define a compositional proof system. The proof rules of this system are applied to a non-trivial example.
12983032
Practical verification of embedded software
Using a new verification algorithm called the compositional backward technique, the authors demonstrate that they can exhaustively verify even the largest industrial applications-comprising more than 1,000 components-in a few minutes on a standard PC.
4574615
As Cheap as Possible: Efficient Cost-Optimal Reachability for Priced Timed Automata
In this paper we present an algorithm for efficiently computing optimal cost of reaching a goal state in the model of Linearly Priced Timed Automata (LPTA). The central contribution of this paper is a priced extension of so-called zones. This, together with a notion of facets of a zone, allows the entire machinery for symbolic reachability for timed automata in terms of zones to be lifted to cost-optimal reachability using priced zones. We report on experiments with a cost-optimizing extension of Uppaal on a number of examples.
14315022
Scaling up Uppaal Automatic Verification of Real-Time Systems Using Compositionality and Abstraction
To combat the state-explosion problem in automatic verification, we present a method for scaling up the real-time verification tool UPPAAL by complementing it with methods for abstraction and compositionality. We identify a notion of timed ready simulation which we show is a sound condition for preservation of safety properties between real-time systems, and in addition is a precongruence with respect to parallel composition. Thus, it supports both abstraction and compositionality. We furthermore present a method for automatically testing for the existence of a timed ready simulation between real-time systems using the UPPAAL tool.
8987497
Efficient Timed Reachability Analysis Using Clock Difference Diagrams
One of the major problems in applying automatic verification tools to industrial-size systems is the excessive amount of memory required during the state-space exploration of a model. In the setting of real-time, this problem of state-explosion requires extra attention as information must be kept not only on the discrete control structure but also on the values of continuous clock variables. 
 
In this paper, we exploit Clock Difference Diagrams, CDD's, a BDD-like data-structure for representing and effectively manipulating certain nonconvex subsets of the Euclidean space, notably those encountered during verification of timed automata. 
 
A version of the real-time verification tool Uppaal using CDD's as a compact data-structure for storing explored symbolic states has been implemented. Our experimental results demonstrate significant spacesavings: for eight industrial examples, the savings are in average 42% with moderate increase in runtime. 
 
We further report on how the symbolic state-space exploration itself may be carried out using CDD's.
1457644
Time-Optimal Test Cases for Real-Time Systems
Testing is the primary software validation technique used by industry today, but remains ad hoc, error prone, and very expensive. A promising improvement is to automatically generate test cases from formal models of the system under test.
26883600
Distributed Redundancy or Cluster Solution? An Experimental Evaluation of Two Approaches for Dependable Mobile Internet Services
Third generation mobile networks are offering the user access to Internet services. The Session Initiation Protocol (SIP) is being deployed for establishing, modifying, and terminating those multimedia sessions. Mobile operators put high requirements on their infrastructure, in particular – and in focus of this paper – on availability and reliability of call control. One approach to minimize the impact of server failures is to implement redundant servers and to replicate the state between them in a timely manner. In this paper, we study two concepts for such a fault-tolerant architecture in their application to the highly relevant use-case of SIP call control. The first approach is implemented as a distributed set of servers gathered in a so-called pool, with fail-over functionality assigned to the pool access protocols. The second is a cluster-based solution that normally implies that the servers are confined in a local network, but on the other hand the latter solution is completely transparent to clients accessing the service deployed in the cluster. To evaluate these two approaches, both were implemented in an experimental testbed mimicking SIP call control scenarios in 3rd generation mobile networks. An approach for measurement of various dependability and performance parameters in this experimental setting is developed and concluded with a set of preliminary results.
30882911
Complexity of Layered Binary Search Trees with Relaxed Balance
When search trees are made relaxed, balance constraints are weakened such that updates can be made without immediate rebalancing. This can lead to a speed-up in some circumstances. However, the weakened balance constraints also make it more challenging to prove complexity results for relaxed structures.In our opinion, one of the simplest and most intuitive presentations of balanced search trees has been given via layered trees. We show that relaxed layered trees are among the best of the relaxed structures. More precisely, rebalancing is worst-case logarithmic and amortized constant per update, and restructuring is worst-case constant per update.
12699441
Fully abstract models for a process language with refinement
We study the use of sets of labelled partial orders (pomsets) as denotational models for process algebras. More specifically, we study their capability to capture degrees of nonsequentiality of processes. We present four full abstractness results. The operational equivalences are based on maximal action-sequences and step-sequences — defined for a very simple process language and its extensions with a refinement combinator (change of atomicity). The denotational models are all expressed as abstractions of a standard association of sets of labelled partial orders with processes.
3019314
Amortized constant relaxed rebalancing using standard rotations
Abstract. The idea of relaxed balance is to uncouple the rebalancing in search trees from the updating in order to speed up request processing in main-memory databases. This paper defines the first relaxed binary search tree with amortized constant rebalancing using only standard single or double rotations.
2070879
Injectivity of Composite Functions
The problem of deciding injectivity of functions is addressed. The functions under consideration are compositions of more basic functions for which information about injectivity properties is available. We present an algorithm which will often be able to prove that such a composite function is injective. This algorithm constructs a set of propositional Horn clause axioms from the function specification and the available information about the basic functions. The existence of a proof of injectivity is then reduced to the problem of propositional Horn clause deduction. Dowling and Gallier have designed several very fast algorithms for this problem, the efficiency of which our algorithm inherits. The proof of correctness of the algorithm amounts to showing soundness and completeness of the generated Horn clause axioms.