This paper concerns the transfer of files via a lossy communication channel. It formally specifies this file transfer service in a property-oriented way and investigates -- using two different techniques -- whether a given bounded retransmission protocol conforms to this service. This protocol is based on the well-known alternating bit protocol but allows for a bounded number of retransmissions of a chunk, i.e., part of a file, only. So, eventual delivery is not guaranteed and the protocol may abort the file transfer. We investigate to what extent real-time aspects are important to guarantee the protocol's correctness and use Spin and

Techniques for efficiently evaluating future time Linear Temporal Logic (abbreviated LTL) formulae on finite execution traces are presented. While the standard models of LTL are infinite traces, finite traces appear naturally when testing and/or monitoring real applications that only run for limited time periods. A finite trace variant of LTL is formally defined, together with an immediate executable semantics which turns out to be quite inefficient if used directly, via rewriting, as a monitoring procedure. Then three algorithms are investigated. First, a simple synthesis algorithm for monitors based on dynamic programming is presented; despite the e#- ciency of the generated monitors, they unfortunately need to analyze the trace backwards, thus making them unusable in most practical situations. To circumvent this problem, two rewritingbased practical algorithms are further investigated, one using rewriting directly as a means for online monitoring, and the other using rewriting to generate automata-like monitors, called binary transition tree finite state machines (and abbreviated BTT-FSMs). Both rewriting algorithms are implemented in Maude, an executable specification language based on a very e#cient implementation of term rewriting. The first rewriting algorithm essentially consists of a set of equations establishing an executable semantics of LTL, using a simple formula transforming approach. This algorithm is further improved to build automata on-the-fly via caching and reuse of rewrites (called memoization), resulting in a very e#cient and small Maude program that can be used to monitor program executions. The second rewriting algorithm builds on the first one and synthesizes provably minimal BTT-FSMs from LTL formulae, which can then be used to a...

Abstract. Abstraction reduces the problem of whether an infinite state system satisfies version. The most common abstractions are quotients of the original system. We present a simple method of defining quotient abstractions by means of equations collapsing the set of states. Our method yields the minimal quotient system together with a set of proof obligations that guarantee its executability and can be discharged with tools such as those in the Maude formal environment.

We consider symbolic on-the-fly verification methods for systems of finite-state machines that communicate by exchanging messages via unbounded and lossy FIFO queues. We propose a novel representation formalism, called simple regular expressions (SREs), for representing sets of states of protocols with lossy FIFO channels. We show that the class of languages representable by SREs is exactly the class of downward closed languages that arise in the analysis of such protocols. We give methods for (i) computing inclusion between SREs, (ii) an SRE representing the set of states reachable by executing a single transition in a system, and (iii) an SRE representing the set of states reachable by an arbitrary number of executions of a control loop of a program. All these operations are rather simple and can be carried out in polynomial time. With these techniques, one can straightforwardly construct an algorithm which explores the set of reachable states of a protocol, in order t...

We consider the problem of verifying automatically infinitestate systems that are systems of finite machines that communicate by exchanging messages through unbounded lossy fifo channels. In a previous work [1], we proposed an algorithmic approach based on constructing a symbolic representation of the set of reachable configurations of a system by means of a class of regular expressions (SREs). The construction of such a representation consists of an iterative computation with an acceleration technique which enhance the chance of convergence. This technique is based on the analysis of the effect of iterating control loops. In the work we present here, we experiment our approach and show how it can be effectively applied. For that, we developed a tool prototype based on the results in [1]. Using this tool, we provide a fully automatic verification of (the parameterized version of) the Bounded Retransmission Protocol, for arbitrary values of the size of the transmitted files, and the allowed number of retransmissions. ? Contact author. 1 1

Abstract: In this paper, we discuss the use of a model checker in combination with the speci cation method of Abstract State Machines (ASMs). A schema is introduced for transforming ASM models into the language of a model checker. We prove that the transformation preserves the semantics of ASMs and provide a theoretical framework for a transformation tool. Experience with model-checking the ASM model of the Production Cell demonstrates that this approach o ers e ective support for verifying ASM speci cations.

In this paper, we present our initial design and implementation of a declarative network verifier (DNV). DNV utilizes theorem proving, a well established verification technique where logic-based axioms that automatically capture network semantics are generated, and a userdriven proof process is used to establish network correctness properties. DNV takes as input declarative networking specifications written in the Network Datalog (NDlog) query language, and maps that automatically into logical axioms that can be directly used in existing theorem provers to validate protocol correctness. DNV is a significant improvement compared to existing use case of theorem proving which typically require several man-months to construct the system specifications. Moreover, NDlog, a high-level specification, whose semantics are precisely compiled into DNV without loss, can be directly executed as implementations, hence bridging specifications, verification, and implementation. To validate the use of DNV, we present case studies using DNV in conjunction with the PVS theorem prover to verify routing protocols, including eventual properties of protocols in dynamic settings.

ions: The abstraction module implements the method presented in [1] for computing abstractions of infinite state systems. For a given concrete system and a given abstraction function, it computes an abstraction of the concrete system compositionally and automatically. The process of generation of the abstract system does not depend on the assumed semantics of the parallel operator; it works for the synchronous as well as for the asynchronous computation model. The generated abstract system has the same structure as the concrete one and there is a clear correspondence between the transitions of both systems. This does not only allow to apply further abstractions and techniques to mitigate the state explosion problem but also facilitates the debugging of the concrete system. Indeed, traces of the abstract system can be transformed into concrete traces, which are then checked whether they are behaviors of the concrete system. The generated abstract system is represented optionally in the ...

The well-known Sliding Window protocol caters for the reliable and efficient transmission of data over unreliable channels that can lose, reorder and duplicate messages. Despite the practical importance of the protocol and its high potential for errors, it has never been formally verified for the general setting. We try to fill this gap by giving a fully formal specification and verification of an improved version of the protocol. The protocol is specified by a timed state machine in the language of the verification system PVS. This allows a mechanical check of the proof by the interactive proof checker of PVS. Our modelling is very general and includes such important features of the protocol as sending and receiving windows of arbitrary size, bounded sequence numbers and channels that may lose, reorder and duplicate messages.

We present a methodology for constructing abstractions and refining them by analyzing counter-examples. We also present a uniform verification method that combines abstraction, model-checking and deductive verification in a novel way. In particular, it allows and shows how to use the set of reachable states of the abstract system in a deductive proof even when the abstract model does not satisfy the specification and when it simulates the concrete system with respect to a weaker simulation notion than Milner's.