References of "Leduc, Guy"
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See detailResource Trading Agents for Adaptive Active Network Applications
Yamamoto, Lidia; Leduc, Guy ULg

in Networking and Information Systems Journal (2000), 3(4-5), 743-768

Adaptive applications seem to be the only realistic answer to the increasing diversity and decentralisation in networks. In order to accommodate fluctuations in network conditions, adaptive applications ... [more ▼]

Adaptive applications seem to be the only realistic answer to the increasing diversity and decentralisation in networks. In order to accommodate fluctuations in network conditions, adaptive applications need to obtain information about resource availability. Using mobile agent technology applied to active networks, new models for adaptive applications can be envisaged. In this article we review the state of the art in agent and active network techniques for adaptive applications, and describe our work on an agent model for trading resources inside an active network node. We apply the model to the case of an audio mixing application. Our simulation results indicate that the model allows the application to make efficient use of the available resources, and to share resources according to user preferences. [less ▲]

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See detailCycle-based TCP-Friendly Algorithm
Ait-Hellal, Omar; Yamamoto, Lidia; Leduc, Guy ULg

in Proceedings of IEEE Globecom'99 (1999, December)

Several TCP-Friendly algorithms have been recently proposed to support multimedia applications. These algorithms try to mimic the congestion control behavior of TCP. However, the oscillatory (bursty ... [more ▼]

Several TCP-Friendly algorithms have been recently proposed to support multimedia applications. These algorithms try to mimic the congestion control behavior of TCP. However, the oscillatory (bursty) nature of TCP traffic is widely known to be unsuitable for most typical real-time applications. Adopting such behavior would result in annoying QoS oscillations for the users of such real-time applications. In the present paper we describe a new TCP-Friendly algorithm based on the TCP cycle estimation. We show through simulations that the proposed algorithm is able to smooth the oscillations while keeping fairness towards TCP. [less ▲]

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See detailA scheduler for delay-based service differentiation among AF classes
Tufail, Mudassir; Jennes, Geoffroy; Leduc, Guy ULg

in Tsang, D.; Kühn, P. (Eds.) Broadband Communications - Convergence of Network Technologies (1999, November)

In the Differentiated Services (DS) framework, service differentiation is performed among the aggregates (collection of one or more microflows) rather than among the microflows (data stream pertaining to ... [more ▼]

In the Differentiated Services (DS) framework, service differentiation is performed among the aggregates (collection of one or more microflows) rather than among the microflows (data stream pertaining to a single connection). We analyse three quality metrics namely bandwidth, loss and delay (that might be used for defining a service differentiation at a DS node) on two criteria 1) service differentiation should be respected at all loads and 2) service provision at aggregate level should scale down to microflow level without being microflow aware. We find that bandwidth requires microflow aware management, loss lacks in simplicity (though it satisfies the criterion # 2), and delay is the right candidate. Ensuring better delays at an aggregate level also means ensuring better delays for all the included micro flows, and additionally it is easier to define a scheduler that can adapt itself to the relative loads of the aggregates so that relative delays between aggregate are preserved at all loads. Our objective is to provide relative quantification service in DiffServ by a delay-based scheduler while satisfying both criteria. Delay is also a meaningful QoS parameter for both interactive real-time applications and TCP applications, since the mean TCP throughput is roughly inversely proportional to the RTT. We, therefore, develop a scheduler for Assured Forwarding (AF) PHB where service differentiation among aggregates is based on delays. We provide simulation results that prove that relative delays among aggregates are perfectly respected at all loads. [less ▲]

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See detailAttaining per flow QoS with Class-based Differentiated Services
Tufail, Musassir; Jennes, Geoffroy; Leduc, Guy ULg

in Onvural, R.; Civanlar, S.; Luciani, J. (Eds.) Internet II: Quality of Service and Future Directions (1999, September)

The Differentiated Services (DiffServ or DS) framework takes an edge over IntServ because it is scalable and lesser complex. On the other hand, the application level end-to-end quality o f service, in ... [more ▼]

The Differentiated Services (DiffServ or DS) framework takes an edge over IntServ because it is scalable and lesser complex. On the other hand, the application level end-to-end quality o f service, in DiffServ, may get compromised because: 1) network resources are not allocated at microflow level (a data stream pertaining to a single connection) but at aggregate level (collection of one or more microflows), 2) the DiffServ working group does not specify algorithms for PHBs but their output behavio urs and 3) end-to-end quality is function of Service Level Agreements (SLAs) between the adjacent domains transited by the connection and a large diversity in SLAs is quite evident as each DS domain would have different service provision policies. We focus, in this paper, on the first two issues. Our goal is to ha ve DiffServ deployed with all its simplicity and still be able to provide application level end-to-end qu ality of service. For that, we study a PHB for AF classes. A PHB comprises a packet scheduler and a packe t accept/discard algorithm. For packet scheduler, we use the Extended-VirtualClock (Ex-VC) algorithm. Ex-VC perfo rms delay-based service differentiation among the aggregates while selecting a packet for service. The reasons for having delay-based definition for service differentiation are: it is adaptable to load per aggr egate and it does not need to be microflow aware. Other definitions like bandwidth and loss may also be u sed but the former requires microflow aware management and the latter lacks in simplicity. For packet accept/discard algorithm, we use RED when all packets have the same drop precedence level and DI-RO (Deterministic for In-RED for Out) when packets are policed at the ingress DS node and packets viol ating the Service Level Agreement (SLA) are marked OUT. In DI-RO, IN packets are always accepted (except buffer overflow) whereas OUT packets are accepted probabilistically. [less ▲]

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See detailA RED discard strategy for ATM networks and its performance evaluation with TCP/IP Traffic
Rosolen, Vincent; Bonaventure, Olivier; Leduc, Guy ULg

in Computer Communication Review (1999), 29(3), 23-43

In ATM UBR networks supporting TCP traffic, optimal efficiency can only be envisaged if switches adopt a discard mechanism that operates at the packet level rather than the cell level. In this paper, we ... [more ▼]

In ATM UBR networks supporting TCP traffic, optimal efficiency can only be envisaged if switches adopt a discard mechanism that operates at the packet level rather than the cell level. In this paper, we define a variant of the RED discard strategy suitable for ATM switches. An interesting feature of this ATM-RED is that it has a similar per-VC implementation complexity as the Early Packet Discard (EPD) algorithm. To study the efficiency of the ATM-RED discard strategy, we compare its performance with plain the UBR, EPD and Fair Buffer Acceptance (FBA) discard strategies by means of simulation with TCP/IP traffic. We give comparative results with respect to different performance criteria such as goodput and fairness in various environments, such as end-to-end ATM networks and IP-based networks with an ATM backbone, in both single-bottlenecked and GFC topologies. [less ▲]

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See detailEditorial - Réseaux et protocoles
Garavel, Hubert; Leduc, Guy ULg; Groz, Roland

in Technique et Science Informatiques (1999), 18(6),

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See detailModel-based verification of a security protocol for conditional access to services
Leduc, Guy ULg; Bonaventure, Olivier; Koerner, Eckhart et al

in Formal Methods In System Design (1999), 14(2), 171-191

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 ... [more ▼]

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. [less ▲]

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See detailImpact of cell discard strategies on TCP/IP in ATM UBR networks
Rosolen, Vincent; Bonaventure, Olivier; Leduc, Guy ULg

in Performance Modelling and Evaluation of ATM Networks (1998, July)

In ATM UBR networks supporting TCP traffic, optimal efficiency can only be envisaged if switches adopt a discard mechanism that operates at the packet level rather than the cell level. In this paper, we ... [more ▼]

In ATM UBR networks supporting TCP traffic, optimal efficiency can only be envisaged if switches adopt a discard mechanism that operates at the packet level rather than the cell level. In this paper, we investigate the performance of the EPD, RED and FBA strategies by means of simulation, and give comparative results with respect to different environments and performance criteria. We show that implementing such a strategy in a LAN brings little added value, while the benefit is more obvious in a WAN access network with asymmetrical bandwidths. Specifically, the improvement not only concerns goodput but also fairness among competing TCP connections, and depends upon the Maximum Segment Size used by the sources. [less ▲]

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See detailA formal definition of time in LOTOS
Léonard, Luc; Leduc, Guy ULg

in Formal Aspects of Computing (1998), 10(3), 248-266

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 ... [more ▼]

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 elsewhere. 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 proper-ties 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. To our knowledge, this is the first in-depth study of a language that combines data types and real-time behaviours. [less ▲]

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See detailA formal definition of time in LOTOS (Full version)
Léonard, Luc; Leduc, Guy ULg

in Formal Aspects of Computing (1998), 10E

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 ... [more ▼]

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 elsewhere. 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. To our knowledge, this is the first in-depth study of a language that combines data types and real-time behaviours. [less ▲]

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See detailA Computer Aided Design of a Secure Registration Protocol
Germeau, François; Leduc, Guy ULg

in Formal Description Techniques and Protocol Specification, Testing and Verification (1997, November)

We use the formal language LOTOS to specify a registration protocol between a user and a Trusted Third Party, that requires mutual authentication. We explain how a model-based verification method can be ... [more ▼]

We use the formal language LOTOS to specify a registration protocol between a user and a Trusted Third Party, that requires mutual authentication. We explain how a model-based verification method can be used to verify its robustness to attacks by an intruder. This method is also used to find a simpler protocol that remains secure. [less ▲]

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See detailModel-based Design and Verification of Security Protocols using LOTOS
Germeau, François; Leduc, Guy ULg

in Design and Formal Verification of Security Protocols (1997, September)

We explain how the formal language LOTOS can be used to specify security protocols and cryptographic operations. We describe how to model security properties as safety properties and how a model-based ... [more ▼]

We explain how the formal language LOTOS can be used to specify security protocols and cryptographic operations. We describe how to model security properties as safety properties and how a model-based verification method can be used to verify the robustness of a protocol against attacks of an intruder. We illustrate our technique on a concrete registration protocol. We find a simpler protocol that remains secure, and a more sophisticated protocol that allows a better distinction between intruder's attacks and ordinary errors. [less ▲]

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See detailCFIP'97 Ingénierie des protocoles
Leduc, Guy ULg

Book published by Hermès (1997)

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See detailQoS specification of ODP binding objects
Février, Arnaud; Najm, Elie; Leduc, Guy ULg et al

in Telektronikk (1997), 93(1), 42-49

We present a QoS oriented notation suitable for the ODP framework. In particular, we focus on a computational view of objects: we consider systems described as configurations of interacting objects and we ... [more ▼]

We present a QoS oriented notation suitable for the ODP framework. In particular, we focus on a computational view of objects: we consider systems described as configurations of interacting objects and we deal with two types of communications: message passing and flows. In message passing, signals (from one object to another) are conveyed through the (implicit) underlying infrastructure. This form of interaction is suitable for client/server applications where no strong real time or ordering constraints are needed from the communication infrastructure. In contract, in a flow type of communication, signals are conveyed through third party (binding) objects that may be explicitly called for in order to ensure specific QoS requirements needed by specific applications. 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. [less ▲]

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See detailAn introduction to ET-LOTOS for the description of time-sensitive systems
Léonard, Luc; Leduc, Guy ULg

in Computer Networks & ISDN Systems (1997), 29(3), 271-292

Enhanced Timed-LOTOS, called ET-LOTOS, is an extension of LOTOS allowing the modelling of time-sensitive systems (i.e. systems whose behaviour is influenced by the passing of time). It is the basis of the ... [more ▼]

Enhanced Timed-LOTOS, called ET-LOTOS, is an extension of LOTOS allowing the modelling of time-sensitive systems (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. [less ▲]

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See detailSpecification and verification of a TTP protocol for the conditional access to services
Leduc, Guy ULg; Bonaventure, Olivier; Koerner, Eckhart et al

in Formal Methods and their Applications: Telecommunications, VLSI and Real-Time Computerized Control Systems (1996, October)

We use the formal language LOTOS to specify the Equicrypt protocol and verify its robustness to attacks by an intruder. We use the model-based CADP verification tools from the Eucalyptus toolbox to ... [more ▼]

We use the formal language LOTOS to specify the Equicrypt protocol and verify its robustness to attacks by an intruder. We use the model-based CADP verification tools from the Eucalyptus toolbox to discover some successful attacks against this protocol. [less ▲]

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See detailE-LOTOS Core Language
Jeffrey, Alan; Leduc, Guy ULg

Report (1996)

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See detailCompositional Specification of ODP Binding Objects
Février, Arnaud; Najm, Elie; Leduc, Guy ULg et al

in Information Network and Data Communication (1996, June)

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 ... [more ▼]

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. [less ▲]

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See detailTowards an Integrated Solution for Multimedia Communications
Mathy, Laurent ULg; Leduc, Guy ULg; Danthine, André ULg et al

in Bulletin Scientifique de l'Association des Ingénieurs Electriciens sortis de l'Institut Electrotechnique Montefiore (1996)

During the last few years, we have been witnessing tremendous changes in the communication environment. Those changes are not only due to the availability of higher data rates in the communication ... [more ▼]

During the last few years, we have been witnessing tremendous changes in the communication environment. Those changes are not only due to the availability of higher data rates in the communication networks but also to the appearance of new application types such as multimedia applications. After a brief introduction to multimedia communications, we will present the research activities in multimedia we have undertaken during the last few years. They focus on two aspects of multimedia communications: a new semantics for expressing Quality of Service (QoS), and a support for group communications. [less ▲]

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See detailAn Extended LOTOS for the design of Real-Time Systems
Léonard, Luc; Leduc, Guy ULg

in Design and Analysis of Real Time Systems (1995, November)

We give a brief presentation of ET-LOTOS. ET-LOTOS extends with quantative time the formal description technique LOTOS. ET-LOTOS serves as basis for the time extension part of E-LOTOS, the new standard ... [more ▼]

We give a brief presentation of ET-LOTOS. ET-LOTOS extends with quantative time the formal description technique LOTOS. ET-LOTOS serves as basis for the time extension part of E-LOTOS, the new standard for LOTOS currently developed within ISO (ISO/IEC JTC1/SC21). [less ▲]

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