Luc Léonard

An introduction to ET-LOTOS for the description of time-sensitive systems

Abstract Enhanced Timed-LOTOS, called ET-LOTOS, is an extension of LOTOS allowing the modelling of time-sensitive system (i.e. systems whose behaviour is influenced by the passing of time). It is the basis of the timed extension of LOTOS currently developed by ISO (1995). The purpose of this paper is to present ET-LOTOS in a tutorial style and show its applicability. The detailed study of the formal semantics is addressed in another paper. A collection of small, but realistic, examples illustrates a wide variety of time-sensitive protocol mechanisms. These examples are used to introduce and justify the extensions of our language. Finally, the basics of the formal semantics are given and a comparison is made with other timed formalisms.

A Formal Definition of Time in LOTOS

Abstract. Enhanced Timed-LOTOS, denoted ET-LOTOS, is an extension of LOTOS that allows the modelling of real-time behaviours. It covers all the aspects of full LOTOS, including data types, it supports both a dense and a discrete time domain and can manipulate time values as any other data values. A tutorial on ET-LOTOS, showing many application examples, has already been published [LéL97]. The present paper adds to it by providing an in-depth presentation of its theoretical aspects. The complete semantics is given and explained, and its properties are studied. In particular, we prove that the semantics is consistent and that strong bisimulation is a congruence. This requires to deal carefully with the presence of negative premises in the operational semantics, which are necessary to express urgency. ET-LOTOS is also shown to be a conservative extension of LOTOS for guarded processes, and is the basis of the timed extension of LOTOS currently developed by ISO [ISO98]. To our knowledge, this is the first in-depth study of a language that combines data types and real-time behaviours.

Model-Based Verification of a Security Protocol for Conditional Access to Services

We use the formal language LOTOS to specify and verify the robustness of the Equicrypt protocol under design in the European OKAPI project for conditional access to multimedia services. We state some desired security properties and formalize them. We describe a generic intruder process and its modelling, and show that some properties are falsified in the presence of this intruder. The diagnostic sequences can be used almost directly to exhibit the scenarios of possible attacks on the protocol. Finally, we propose an improvement of the protocol which satisfies our properties.

Compositional Specification of ODP Binding Objects

A building blocks approach for the formal specification of binding objects in the ODP computational Model is presented. The formal notation that is used is based on LOTOS extended with two features — real time and gate passing. These features are among the extensions that are currently studied in the ISO standardisation Formal Description Techniques group. We apply our building blocks approach to the specification of a multicast, multimedia binding object.

QoS Enhancements and the New Transport Services

The transport protocols TCP and TP4 have been designed in the late seventies at a time when the network environment was essentially based on the switched telephone network and on leased-lines and when the packet switching was an emerging concept.