Secure and Seamless Session Management in Mobile and Heterogeneous

Environment

Ali Hammami and No¨emie Simoni

Telecom ParisTech, LTCI, UMR 5141 CNRS, 46 Rue Barrault, F75634 Paris Cedex13, France

Keywords:

Security as a Service, Device as a Service, Mobility and Heterogeneity, Secure and Unique Session, Security

Continuity, Token, SIP+, Virtual Private Device Network.

Abstract:

The Next Generation Network and Services (NGN/NGS) environment becomes more and more heterogeneous

and mobile. Furthermore, today user seeks to access his services within a secured session ensuring the conti-

nuity and the quality of service. This rapid evolution and requirements raise the issue of guarantying the con-

tinuity of user-centric session in an advanced mobility context. This work targets particularly access control

and security aspects based on Service Oriented Architecture in mobile and heterogeneous environments. To

address the aforementioned challenges, we propose a secure and seamless session management solution that is

based on several concepts and mechanisms. First, this solution ensures security management that overcomes

session security and uniqueness challenges by gathering ubiquitous, mutualisable, autonomous and stateless

service components. Second, we present a multiple and heterogeneous terminal composition by proposing

a Virtual Private Device Network (VPDN) concept that is based on secure and auto-managed components.

Finally, in addition to these proposed architecture components and concepts, we introduce SIP+ in order to

ensure the security continuity within a seamless session during user mobility.

1 INTRODUCTION

The emergence and evolution of Next Generation

Networks (NGN) have raised several challenges

mainly in terms of heterogeneity, mobility and secu-

rity. In fact, the user is able, in such environment,

to have access to many networks, via multiple de-

vices, with a vast choice of services offered by dif-

ferent providers. Furthermore, end-users claim to be

constantly connected anywhere, anytime and anyhow.

Besides, they want to have a secure access to their

services through a dynamic, seamless and continuous

session according to their preferences.

In order to meet these challenges, a new vision

of services called Next Generation Services (NGS),

which addresses heterogeneity, mobility and user-

centric issues, should be adopted. This approach

based on Service Oriented Architecture conceives

each resource (terminal, access network and service)

as a service that intervenes in a dynamic service ses-

sion. In addition, it provides many advantages in

terms of reusability, dynamicity, ﬂexibility, interop-

erability and mutualization.

However, security is still a crucial issue in such

open mobile environment and dynamic session that

involves multiple services. Indeed, in order to inte-

grate security aspects to this novel service architec-

ture, security should not be ensured in a static and

centralized way, it should be provided as a service.

Thus, we aim through our proposal based on security

as a service approach to ensure a secure service ac-

cess and to guard security continuity within a seam-

less session.

To achieve this objective, we have to consider mo-

bility aspects involved in our NGN/NGS context. In

fact, in a user-centric vision, the user wants to access

his personalized services that are provided by various

service providers (service mobility), while switch-

ing between different terminals (user mobility) or ac-

cess networks (terminal mobility). All these mobil-

ity types must be ensured while preserving a contin-

uous user session (session mobility). In this paper,

we focus particularly on combining security and user

mobility aspects. This type of mobility consists of

the fact that a user can change his terminal during the

same session according to his preferences. To ensure

a secure usage and change of the different user ter-

minal, we introduce the device-as-a-service approach

that consists of considering each user terminal as a

service component belonging to Virtual Private De-

vice Network (VPDN). In this way, we show how the

user can use and change his terminal within a secure

282

Hammami A. and Simoni N..

Secure and Seamless Session Management in Mobile and Heterogeneous Environment.

DOI: 10.5220/0004071602820289

In Proceedings of the International Conference on Security and Cryptography (SECRYPT-2012), pages 282-289

ISBN: 978-989-8565-24-2

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

and continuous session.

In this paper, we propose then a secure and seam-

less session management solution applied in mobile

and heterogeneous environment. This solution per-

mits to answer principally the following questions:

How to guarantee a secure service access within a

continuous session? How to manage user terminals

and ensure secure usage of these terminals during his

session? And how to ensure security continuity dur-

ing user mobility?

The remainder of the paper is organized as fol-

lows. In section 2, we present an overview of ex-

isting approaches that invoke security and mobil-

ity aspects and use the Session Initiation Protocol

(SIP)(Rosenberget al., 2002) to support these aspects.

In section 3, we describe our proposition. In this sec-

tion, we deﬁne ﬁrst our security management solu-

tion based on security service components and a to-

ken used for single sign seamless session. Then, we

introduce the VPDN concept. Next, we address user

mobility aspect by proposing a token based SIP+ pro-

tocol to ensure the continuity of security. Section 4

shows the feasibility of our proposition. Finally, sec-

tion 5 presents the conclusion and perspectives for fu-

ture work.

2 RELATED WORK

Mobility management, within a seamless session

while ensuring a secured access in continuous and

simpliﬁed way, is still a major research issue. We

present and discuss, in this part, some research works

related to the different aspects involved in this context

and that are based on SIP the most popular signalling

protocol for this issue.

(Schulzrinne and Wedlund, 2000), is the very ini-

tial paper that shows how mobility management can

be supported by SIP in order to provide all common

forms of mobility, including terminal, session, per-

sonal and service mobility for SIP-based applications.

Particularly, we are interested to session, and personal

mobility. As it is deﬁned by authors, session mobility

allows a user to maintain a media session even while

changing.terminals. They describe the way when it

is supported by SIP using REFER and INVITE mes-

sages. Personal mobility, as it is deﬁned in this paper,

allows to address a single user located at different ter-

minals by the same logical address. This type of mo-

bility is ensured by a SIP forking proxies making the

user reached at any of his devices. These different

types of mobility are addressed without taking into

account security aspects.

(Zhang et al., 2009), propose SIP security mecha-

nisms that support seamless mobility only during the

handover of mobile terminals among different access

networks (terminal mobility). This solution does not

address user mobility that consists of changing termi-

nal with ensuring service continuity. Moreover, au-

thors focus on security aspects for network and trans-

port layer to secure SIP signaling and data transmis-

sion of SIP services. But, they do not consider non-

functional security aspects such as identiﬁcation, au-

thentication and authorization.

As deﬁned in (ETSI, 2010), IMS is an Overlay

Session/ Control Architecture that acts as a session

middleware in NGN. IMS uses basically SIP for con-

trolling sessions. This protocol supports terminal and

user mobility. However, when dealing with user mo-

bility, a new session is established with the second

terminal and services offered by the prime terminal

become inaccessible.

In (Vim et al., 2010), authors deal with session

mobility which allows user to maintain his session

and ensures service continuity even while changing

terminals. They discuss two ways for supporting ses-

sion mobility: network- and user equipment- based

approach. In network based approach, network ini-

tiates a session transfer while user equipment initi-

ates a session transfer in user equipment based ap-

proach. They show the feasibility of session mobility

control according to the former approach in IMS. This

work focuses only on continuity of media services,

and does not consider continuity of security services

that ensures a secure and simpliﬁed access to any ser-

vice during a seamless session.

None of the cited work has treated mobility im-

pact on security aspects, namely, access control. Most

of these recent work deal only with functional aspect

of mobility independently of security aspects. Even

there is some work that introduce security to mobile

environment, they consider this aspect only on net-

work level. Therefore, a new solution that integrates

security to service, equipment (terminal) and user lev-

els, while supporting mobility, is needed. Thus, we

propose, through this paper, an innovative security

management approach that guarantees a secure termi-

nal and service access for each end-user. Moreover,

our proposal supports mobility, particularly, user mo-

bility, in a seamless way while keeping all user re-

sources, namely his terminals considered as service

platforms, accessible in a secured way during his ses-

sion. So, for example, when a user wants to continue

a session begun on his mobile phone on his laptop,

he can remain using the GPS service offered by his

mobile phone after he changes the terminal. For this

purpose, we introduce the VPDN concept. Finally,

we propose mechanisms to ensure security continuity

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within a secure and seamless session.

3 PROPOSITION

In this section, we describe our proposal that is com-

posed of three complementary parts. First, we present

our security management solution that is based on se-

curity service components and security token that is

exchanged between these different components. Sec-

ond, we introduce our VPDN concept that permits to

manage the set of user terminals and to provide a se-

cure usage and change of these terminals. Finally,

we treat security continuity within a seamless session

during user mobility. Therefore, we propose an ex-

tension of the signalling protocol SIP+ that supports

the token transfer.

3.1 Security Management

3.1.1 Security Service Components

As NGN users require continuity of service in a het-

erogeneous, mobile and open environment, security

as a service concept can be an efﬁcient approach to

facilitate this requirement. In fact, in order to ensure

security, continuity and uniqueness of the session, and

to meet the needs of mobility (terminal, user, ses-

sion and service mobility), security should not be seen

with its classic image. It must be provided as a service

ensured by a set of security components offering the

user a secured and simpliﬁed access to his services.

Therefore, we adopt in our architecture design

a Service Oriented Architecture (SOA). This SOA

approach provides many advantages in terms of

reusability, dynamicity, ﬂexibility, interoperability

and evolution of services. Nevertheless, according to

our NGN/ NGS context, our proposition of security

services is not limited to the aforementionedSOA ser-

vice characteristics.

Indeed, our approach consists of further architec-

tural aspects. First, we consider the mutualization as-

pect that reﬂects ability of a service to be not only

reusable, but also shareable. To satisfy this aspect,

SEs (Service Elements) should be stateless such that

they can be used and shared among multiple sessions.

Second, SEs should be autonomous that implies in-

dependence between the different SEs. This means

that SE should not need other SEs to ensure its func-

tionalities. Finally, we consider the Self-management

aspect that represents the Self-monitoring of the QoS

contract.

Taking into account these different aspects and

characteristics, our proposed security management is

based on three main components: Securityware, Se-

curity Agent and Security Data store.

The Security Service Provider (Securityware),

shown in Figure 1, is responsible of the security man-

agement and control. Thus, our Security Service

Provider proposition gathers all the security compo-

nents as following:

• Identiﬁcation Service: it enables the recognition

of a user by the system . User can have one or

more identiﬁer types (e.g. login/password, One

Time Password, smart card, biometry, or certiﬁ-

cate);

• Authentication Service: it determines if an iden-

tity is actually what it claims to be. It aims to au-

thenticate user only one time per session (unique

authentication) for all requested services;

• Authorization Service: it occurs at the opening

session time, when a user requests a set of ser-

vices, to allow (or deny) him to use each service

component. Indeed, authorization service evalu-

ates the effective rights. Permission is granted ac-

cording to rights associated to the user role;

• Token Service: it generates and updates the to-

ken;

• Session Service: it ensures the session creation

and activation and the session identiﬁer (Ses-

sion ID) generation (upon successful authentica-

tion). This service is responsible for managing

and maintaining a secure end-to-end session;

• VPDN Service: it manages the set of user termi-

nals (VPDN) and it generates the VPDN identiﬁer

(VPDN ID).

Figure 1: Securityware Architecture Components.

In order to facilitate and simplify the management of

security and access control, we propose also a Secu-

rity Agent that is deployed in each service platform

or terminal which is considered simultaneously as a

service platform and as a service. The Security Agent

intercepts requests for the protected resource (service

or terminal). It sends then a request to the Security-

ware asking for decisions regarding the required ser-

vices. So, the Security Agent secures access to ser-

vice platforms by checking authorizations due to the

Securityware.

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Finally, the Security Datastore contains all the in-

formation needed by the Securityware, namely, user

proﬁle, device proﬁle, VPDN proﬁle, session proﬁle

and service proﬁle.

3.1.2 Token Role and Structure

In our proposed security management solution, the to-

ken is used for access control, namely, authentication

and for maintaining a seamless session. It is created

and updated by the Token Service during the session

and VPDN creation.

Within the Securityware, the unique authentica-

tion is based on the token. It is considered as a ﬂexi-

ble and powerful mechanism for exchanging security

context between different service platforms or termi-

nals (domains). This token represents a small collec-

tion of information that is transmitted to user (i.e. ter-

minal security agent) when the session is ﬁrst created.

As user visits different service platforms that are pro-

tected by security agents, the token is propagated to

these platforms and is used to retrieve the User ID.

Indeed, this token is retrieved at each subsequent re-

quest from the terminal such that the service platform

(which may be the terminal itself) can recognize re-

quests from the same user. Thus, the unique session is

maintained automatically due to the token exchange.

The proposed token structure and data model are

illustrated in Figure 2. Despite the fact that tokens

have common structure, they can be different in usage

and in the way they are generated.

Some tokens can also contain optional values. The

common token structure of different tokens contains

the following attributes and elements:

• Token Type: is an optional attribute;

• Token ID: is a token attribute that represents its

unique identiﬁer;

• Token Value: is an optional element that can be

used for authentication.

Session information:

• Session ID: is the unique end-to-end session

identiﬁer. It is an attribute of the token that is ex-

changed between the Security Agent in the termi-

nal and the Securityware;

• Session Name: is a local value attribute in our

Securityware that differentiates between sessions

of the same user;

• Session Duration: indicates the session validity

period.

User information:

• User ID: is the user identiﬁer in the current ses-

sion. This attribute can be a random value gener-

ated by the Securityware. It can be a value chosen

from the user characteristics such as IP address or

VPDN information:

• VPDN ID: is the identiﬁer of the set of termi-

nals deployed by the user during his session. This

identiﬁer remains unique even when the token is

regenerated for a long duration session.

• Domain: is an element that identiﬁes the domain

that the generated token is related.

• secure: is a boolean value that indicates if the

transport channel secure.

Figure 2: Token structure and format.

In our context, we deﬁne two types of token that

have a common data structure and contain different

proﬁles and different creation processes:

• Authentication Token: In this type of token, the

Token Value element contains a value that can be

used as an authentication value. It is calculated by

applying, as an example, HMAC function to user

credentials.

• VPDN Token: this token extends the authentica-

tion token and contains the VPDN ID in addition

to session and user information.

3.2 The Virtual Private Device Network

3.2.1 What is the VPDN?

The VPDN (Virtual Private Device Network) repre-

sents a network of terminals which are present in the

PAN (Personal Area Network) of the user and used

during his service session. It permits to aggregate and

manage this set of terminals in order to facilitate ter-

minal change while preserving service continuity.

Within the VPDN, each terminal is considered as

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Figure 3: Token Creation.

a service component that deﬁnes a set of personal-

ization and adaptation features and uses the real time

user proﬁle to manage his sessions based on his pref-

erences and mobility. Moreover, each terminal ac-

commodates a range of services that it can also be

considered as a service platform or a service provider.

In fact, it offers a set of terminal services (e.g. display

service, keypad service, touch screen service) and it

permits to deploy other services such as applicative

services (e.g. location service).

Consequently, the terminal plays two roles: it is a

service component managed by the VPDN (device as

a service) and it represents a service platform.

3.2.2 VPDN Creation

In this part, we explain the way that a VPDN is cre-

ated and a terminal can be added to it. This process in-

vokes, principally, the three major security functions:

identiﬁcation, authentication and authorization. It im-

plies also the Token Creation (Figure 3).

Hence, we describe the overall process involving

security aspects.

We consider particularly login/password identiﬁ-

cation type. After user registration, the Identiﬁcation

Service creates a login and a password. When us-

ing the terminal, the user should be identiﬁed. Then,

the Identiﬁcation Service checks if his login exists al-

ready in the Security Data-store. If the response is

positive, the terminal security agent receives from the

Securityware a response meaning that the user already

has an identiﬁer.

After the identiﬁcation phase, the system asks the

user to be authenticated. Then, the user types his pass-

word. The request is directed to the Securityware. Af-

terward, the Authentication Service veriﬁes the cre-

dentials (login/password) provided by the user com-

pared with information stored in the Security Data-

store.

Once the user is successfully authenticated, ses-

sion initialization is performed by the Session Ser-

vice. It generates the current session attributes namely

the ”Session ID” and the ”Session Duration”. Then,

a token is generated by the Token Service. It contains

the necessary session and user information. In fact,

it encapsulates the ”Session ID”, ”Session Duration”

and ”User ID” attributes. This latter attribute is re-

trieved from the Authentication Service. Afterward,

the token is transmitted by the Securityware to the Se-

curity Agent in the terminal. This transmission takes

place in a secure channel.

Due to this token, the user will be able then to

use, depending on his rights, any service from dif-

ferent service platforms without being asked, in this

unique session, to enter his credentials again.

Next, we have to verify that the user is authorized

to use this terminal and to create a VPDN. So, an au-

thorization request related to the use of terminal is

sent by the Security Agent to the Securityware. The

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Authorization Service generates a positive or negative

response based on user rights associated to his role

and on privileges related to the resource (terminal).

Then, the response is sent to the Security Agent.

Once the user is authorized, the VPDN Service

generates the ”VPDN ID” which is the identiﬁer of

the set of terminals deployed by the user during his

session forming the VPDN.

The Token Service retrieves the VPDN identiﬁer

and updates the token. The terminal is registered in

the VPDN proﬁle.

3.3 Security Continuity: Token based

SIP+

Due to NGN evolution, end users claim to access his

services while using multiple terminals. They de-

sire to have a dynamic session while considering their

preferences and their QoS requirements. In addition,

they want to maintain their sessions continuous and

unique despite their mobility.

To reach these objectives and to guarantee the se-

curity, continuity and end-to-endQoS requirementsin

the user session, we propose token based SIP+ (Souli-

mani et al., 2011) that should overcomes these chal-

lenges regarding user mobility. It is an extended pro-

tocol from Session Initiation Protocol (SIP) that con-

siders the terminal as a service platform. This pro-

tocol permits the user session creation, modiﬁcation

and termination based on service composition includ-

ing the terminal.

To satisfy the user requirements when he needs to

change his terminal (user mobility), SIP+ is able to

maintain an end-to-end session continuity and to en-

sure QoS managementand security continuity. There-

fore, we should extend information carried by SIP

methods to add the necessary information related to

security aspects and QoS management for each ser-

vice component involved in the session. This infor-

mation is carried by REFER SIP+ and INVITE SIP+

messages. The user mobility process begins by send-

ing a REFER message that informs the Securityware

that the user wants to transfer his session to another

terminal. Then, the Securityware send an INVITE

SIP+ message to the second terminal. This message

is composed of a header and a body that contains the

security token and requested QoS criteria such as re-

liability, availability, delay and capacity. All added

information are in XML format.

4 FEASABILITY

In this section, we describe ﬁrst a user mobility sce-

nario that invokes all the concepts and mechanisms

that we have deﬁned previously. Then we describe the

implementation of our different security management

components.

4.1 Scenario: User Mobility

In this subsection, we explain how the aforemen-

tioned security components, concepts and models are

able to maintain a seamless, continuous and secure

user session despite possible variations in user pref-

erences and locations, and different mobility modes

such as user mobility. The user mobility represents

user ability to switch between terminals during his

session and to access his services from anywhere at

anytime.

For more clarity, we describe a user mobility sce-

nario (Figure 4) on which we apply our proposed se-

curity components and SIP+ Invite and Refer mes-

sages. Thus, we consider the following use case.

Alice is at home, she wants to use her laptop.

After the VPDN creation (initiation) process which

consists of identiﬁcation, authentication, authoriza-

tion and Token generation phases, Alice can use her

laptop (terminal1), open her session and compose her

services after she is authorized.

The laptop plays two roles. First, it is considered

as a service platform (SP1) that provides a set of ser-

vices protected by a Security Agent. We consider in

this scenario the display service (SE11) and the mail

service (SE12). The former is a terminal service but

the latter is an applicative service deployed in laptop.

Second, this laptop represents a service component

belonging to the VPDN.

Alice uses her laptop with its display compo-

nent (SE11). She can then receive her mails (SE12)

through this terminal. In this case, the VPDN of Alice

is formed only by her laptop:

VPDN (Alice) = Laptop

We suppose that Alice leaves her house to go to

the birthday ceremony of her friend by car. She de-

sires to switch her session from her laptop to her

Smartphone while keeping it continuous and secure.

Then, the Security Agent1 (laptop) sends a ”Sip+ Re-

fer” request to the Securityware. The Securityware

inspects and veriﬁes whether terminal2 (Smartphone)

belongs to the PAN of Alice, which is considered as

her trusted devices (terminals), and opens a secure

channel between two terminals.(ﬁgure4 , (1), (2) and

(3)).

As we consider the VPDN as a peer-to-peer net-

work, each terminal is auto-managed and acts as a

peer. Thus, the Security Agent1 is able to send an ”In-

vite SIP+” request directly to terminal2 (Smartphone)

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Figure 4: Security Continuity during User Mobility.

carrying the token that contains the VPDN identi-

ﬁer(ﬁgure4, (4)). The Security Agent2 intercepts the

request to verify due to the Securityware if terminal2

is part of the VPDN of Alice and if she is authorized

to access his session and compose services(ﬁgure4,

(5)). The Securityware inspects the token and session

validity. If the answer is positive, an acquittal is sent

by the Securityware and the VPDN proﬁle is updated.

(ﬁgure4, (6) and (7)). For the VPDN proﬁle update,

see the two cases below.

Alice passes from her laptop to her Smartphone

while maintaining her session (user mobility). The

terminal change implies invocation of the component

(SE21) which corresponds to the display service of

the Smartphone.

Depending on her needs, Alice changes her ser-

vice composition to build an application using a new

logic of service.

She can then continue her communication with

her friend all along her drive. Following the user mo-

bility, we can consider two cases:

• 1st Case:

Terminal 1 (Laptop) remains active. In this case,

terminal2 (Smartphone) could be seen as a ser-

vice platform that provides (SE21) and (SE22)

services. When switching his terminal, user actu-

ally preserves the same service composition with

a set of services provided by terminal 2. For ex-

ample, Alice used, with terminal1, the display ser-

vice (SE11), and she needs, when using terminal

2, the service (SE21) which is functionally equiv-

alent to (SE11). Thus, the Security Agent checks,

with the Securityware, if Alice has the right or not

to access this service considering his role. In this

case, the VPDN becomes:

VPDN(Alice) = {Laptop, Smartphone} (1)

• 2nd Case:

Terminal 1 (Laptop) state becomes unavailable

and it must send ”bye” to Securityware. In this

case, terminal1 could be seen as a service compo-

nent that is changed by terminal 2(Smartphone) in

the VPDN. Thus, the Security Agent only checks

token validity, and the VPDN becomes:

VPDN(Alice) = {Smartphone} (2)

4.2 Implementation

In order to prove the feasibility and to validate our

proposition, we use the test-bed architecture of our

UBIS project shown in ﬁgure 9. In the terminal level

of this architecture, we found the service platform

Userware, deﬁned on the terminal and that contains

applicative services and management services (e.g.

the Security Agent). For the transport network, the

VirtuOR solution is used. It permits to have differ-

ent virtual entities such as IPv4 and IPv6 routers, ac-

cess points, SIP servers, etc. Due to this solution,

we can obtain a transport network platform. In the

core network, we use an advanced version of Fokus

Open IMS for the control layer to support SIP+ sig-

naling messages. In the service level, a service plat-

form named Servicewarethat contains applicativeser-

vices and management services developed by UBIS

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Figure 5: Token XML pseudo-code.

equip. This platform is implemented using GlassFish

and Sailﬁn. We explain below how we implement

and deploy our different security management com-

ponents in this architecture.

First, we use Java language to write the proposed

security architecture components, namely, Security-

ware and Security Agent. For our Securityware, we

use EJB technology to develop autonomous, loosely

connected and stateless services. These services are

deployed in SailFin Application Server that supports

various APIs such as JMS, JNDI, JDBC and Sip

servlet. Our proposed Securityware extends actu-

ally some security parts involved in OpenSSO project.

For our Security Agents, they are deployed in each

terminal or service platform. In order to support

SIP+, we deploy then Converged Application Con-

tainer which is composed of SIP and HTTP servlets

and will permits to switch from HTTP to SIP. We note

that all transactions between different components are

secured using SSL protocol.

Second, our Security Datastore is an LDAP direc-

tory (openDS) that contains our used information and

is connected to the Securityware. Finally, we present,

in Figure 10, the XML based pseudo-code of our pro-

posed token.

5 CONCLUSIONS

In this paper, we have proposed a security as a service

solution that ensures a secure and seamless session

management in mobile and heterogeneous environ-

ment. This solution relies basically on security ser-

vice components and token mechanism. We have in-

troduced also VPDN concept that manages a secured

access and usage of user terminals according to his

preferences and locations.

In addition, we have proposed a SIP+ extension

that supports token exchange between different com-

ponents. Then, we have used this token-based SIP+ to

ensure user mobility while keeping security continu-

ity within a seamless session. Finally, we have proved

the feasibility of our solution. As future work, we aim

to secure SIP+ messages including the token and to

evaluate the performance of our solution against pos-

sible attacks and vulnerabilities. Furthermore, our fu-

ture work will not be limited to terminal level but we

will consider also service level to ensure consequently

session mobility.

ACKNOWLEDGEMENTS

The authors would like to thank Hassane AISSAOUI.

They would also like to thank all the participants

in the UBIS project, ﬁnanced by the French ANR

VERSO 2008 in which is situated this work.

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