PhD Ludivine Orriols
email: ludivine.orriols@isped.u-bordeaux2.fr

MD Alexandra Foubert-Samier

MSc Blandine Gadegbeku

MD Bernard Delorme

MSc Aurore Tricotel

MD Pierre Philip

MD, PhD Nicholas Moore

PhD Emmanuel Lagarde

Prescription of antiepileptics and the risk of road traffic crash

Keywords: road traffic crashes, antiepileptics, epidemiology Word count: introduction: 317, discussion: 880

Objective: Studies assessing the impact of epilepsy and its medication on the risk of road traffic crashes have shown inconsistent results. The aim in this study was to assess this risk using French databases. Methods: Data from three French national databases were extracted and matched: the national health care insurance database, police reports and the national police database of injurious crashes. Only antiepileptics prescribed predominantly in epilepsy were studied (phenobarbital, phenytoin, ethosuximide, valproic acid, vigabatrin, tiagabin, levitiracetam, zonisamide and lacosamide). A case-control analysis comparing responsible and non-responsible drivers and a case-crossover analysis were performed.

Results: 72,685 drivers involved in an injurious crash in France between July 2005 and May 2008, were included. Drivers exposed to prescribed antiepileptic medicines (n=251) had an increased risk of being responsible for a crash (OR 1.74 [1.29-2.34]). The association was also significant for the most severe epileptic patients (n=99; OR=2.20 [1.31-3.69]). Casecrossover analysis found no association between crash risk and treatment prescription.

Conclusion:

Patients with prescription of antiepileptic drugs should be cautioned about their potential risk of road traffic crash. This risk is however more likely to be related to seizures that to the effect of antiepileptic medicines.

INTRODUCTION

The impact of epilepsy on the risk of road traffic crashes is still debated. Some studies have found an increased risk of road traffic crash in drivers with epilepsy, [START_REF] Hansotia | The effect of epilepsy or diabetes mellitus on the risk of automobile accidents[END_REF][START_REF] Lings | Increased driving accident frequency in Danish patients with epilepsy[END_REF] another one showed an increased risk of more severe crashes only. [START_REF] Taylor | Risk of accidents in drivers with epilepsy[END_REF] However, in a recent cohort study, patients with epilepsy were not at increased risk of experiencing a road traffic crash. [START_REF] Kwon | Motor vehicle accidents, suicides, and assaults in epilepsy: A population-based study[END_REF] Antiepileptic medicines are used to treat general or partial seizures and to prevent the risk of relapse, but also in numerous other diseases such as bipolar disorders, [START_REF] Yatham | Newer anticonvulsants in the treatment of bipolar disorder[END_REF] social phobia and generalised anxiety disorder. [START_REF] Mula | The role of anticonvulsant drugs in anxiety disorders: a critical review of the evidence[END_REF] Most antiepileptic medicines have however central side effects that may impair driving ability, such as drowsiness, confusion, dizziness, and visual disturbances. The first generation of antiepileptic medicines includes phenobarbital and phenytoin, which have gradually been supplanted by the second generation, carbamazepine and valproate. The new antiepileptic medicines (or third generation) appeared on the market in the 1990's (e.g. lamotrigine, vigabatrin, gabapentin…). Finally, some benzodiazepines, such as clonazepam, are used in the treatment of epilepsy for their anticonvulsant properties.

Experimental studies show that phenobarbital seems to have the most deleterious effects on behaviour and cognition. [START_REF] Kwan | Neuropsychological effects of epilepsy and antiepileptic drugs[END_REF] Available data on the third-generation drugs suggest a more favourable profile than older treatments, although differences between them remain to be explored. [START_REF] Aldenkamp | Newer antiepileptic drugs and cognitive issues[END_REF][START_REF] Sabers | Newer anticonvulsants: comparative review of drug interactions and adverse effects[END_REF] As with other benzodiazepines, clonazepam is associated with a high rate of drowsiness and decreased attention. [START_REF] Kwan | Neuropsychological effects of epilepsy and antiepileptic drugs[END_REF] In spite of these potential effects on driving skills, available data suggest antiepileptic treatment to favour road safety by controlling the risk of seizure. In a retrospective cohort study, Hansotia and Broste found that the lack of antiepileptic treatment increased the risk of crashes among drivers who had a history of seizures. [START_REF] Hansotia | Epilepsy and traffic safety[END_REF] In a case-control study, changes in antiepileptic treatment (antiepileptic medicines being reduced or switched) decreased the risk of road traffic crashes due to seizures. [START_REF] Krauss | Risk factors for seizure-related motor vehicle crashes in patients with epilepsy[END_REF] The aim of this study was to assess the risk of road traffic crashes among drivers with prescribed antiepileptic treatment.

METHODS

Data from three French nationwide databases were matched: the national health care insurance database, police reports, and the national police database of injurious crashes.

Drivers were included in the study on the basis of their national ID number, extracted from police reports by an automatic procedure. The ID was used to link drivers to medicine reimbursement data around the date of the crash. Police reports were matched to records in the national police database of injury-related crashes, using a probabilistic linkage method. [START_REF] Orriols | On behalf of the CESIR research group. Prescribed medicines and the risk of road traffic crashes: a French registry-based study[END_REF]

Data sources

National police database of injurious crashes. Police personnel collect details about injurious road traffic crashes and store all information about the crash, vehicles, and persons involved in this database. They also conduct investigations from hospital records about the severity of the driver's injuries: unhurt, slightly injured, seriously injured (hospitalised >24 h), or killed (died within 30 days following the crash). All drivers involved in an injurious road traffic crash are required to be tested for the presence of alcohol, using a breath test. If this test is positive (≥0.5 g/l), the driver refuses the test, or the severity of the crash makes it impossible to administer the test, then the blood alcohol concentration is measured. If the breath test is negative, the driver is registered as not being under the influence of alcohol. Health care insurance database. The national health care insurance system database covers the entire population of France (64 million people in 2008). A record is added each time a reimbursed prescription medicine is dispensed to an outpatient at a pharmacy, including national ID number, date of dispensing, and the seven-digit code that identifies medicines. Data on long-term chronic diseases are also registered in this database, with the ICD-10 code (International Classification of Diseases, Tenth Revision) as well as the start and end dates of the disease. In France, patients are fully reimbursed for health care expenses related to 30 recognized long-term chronic diseases.

Epilepsy

In order to allocate epileptic patients to the administrative status of "long-term chronic disease", several severity criteria are investigated: impact on cognition, on family or on professional life, frequency and type of seizures, resistance to treatments or side effects linked to treatment. Thus, we used this status as a proxy for epilepsy severity, indicating patients with the most severe symptoms. [START_REF]Epilepsies graves[END_REF] In France, this represents around 10% of epileptic patients (Haute Autorité de Santé, 2007). Patients were considered as suffering from severe epilepsy on the day of the crash if the date of the crash was included between the start and end dates of the ICD-10 G40 code long-term chronic disease.

Medicines and exposure periods

Antiepileptics. In order to study antiepileptics predominantly prescribed in epilepsy and not in other conditions, we used data from a survey on medicine prescriptions in France (EPPM, IMS Health) that describes indications for each medicine. We included in the analyses medicines used in more than 75% of cases in epilepsy. These antiepileptic medicines were further studied according to the three following groups: first generation (phenobarbital and phenytoin), second generation (valproic acid and ethosuximide) and third generation (vigabatrin, tiagabin, levitiracetam, zonisamide and lacosamide). Patients receiving more than one single antiepileptic drug were defined as under polytherapy.

In France, no more than 30 days' worth of treatment with antiepileptics may be dispensed by pharmacies, so 30 days of exposure were attributed following each dispensing.

Concomitant exposure. In France, a classification of medicines that potentially affect driving abilities have classified into four levels of risk (Castot and Delorme, 2009; Arrêté, 2008; Afssaps, 2009). [START_REF] Castot | Medicinal products and driving". Medicinal products and driving: how to assess the risk ? P2T Congress Marseille[END_REF][15][16] Consequently, analyses were adjusted for the use of other medicines grouped according to this classification system.

Exposure was considered to have started the day after the drug was dispensed. In order to ensure that the medicines were not those prescribed as a treatment for injuries sustained in the crash, medicines dispensed on the day of the crash were not considered in the analysis.

Analysis

Inclusion of subjects. Subjects whose police report did not contain their national ID number were not included. We compared the included subjects with the non-included ones with respect to age, gender, severity of injury, vehicle type, location of the crash, the type of police personnel who filled in the police report, alcohol level, and responsibility status Descriptive analysis. Frequencies of exposures to antiepileptics were compared according to individual and crash characteristics in a multivariate logistic regression analysis.

Responsibility analysis. The principle of responsibility analysis is to compare exposure probabilities on the day of the crash between the drivers deemed responsible for the crash (cases) and those not responsible (controls). Responsibility levels related to the crash were determined by a method adapted from Robertson and Drummer. [START_REF] Robertson | Responsibility analysis: a methodology to study the effects of drugs in driving[END_REF] This method, recently used in France using data from the national police database of fatal crashes, takes into account six factors likely to reduce driver responsibility: road, vehicle and driving conditions, type of crash, traffic rule obedience, and difficulty of task involved. A score is assigned to each driver for each of these factors, from 1 (favourable for driving) to 4 (not favourable for driving). [START_REF] Laumon | Cannabis intoxication and fatal road crashes in France: population based case-control study[END_REF] The logistic regression model included terms for age, gender, socioeconomic category, month of the crash, time of the crash, location of the crash, type of vehicle, severity of injury, blood alcohol level, concomitant medications, long-term chronic diseases, and severe epilepsy.

Case-crossover analysis. The case-crossover analysis consisted of a pair-matched analytical approach to compare exposure during a period immediately before the crash (case period) with exposure during an earlier period (control period) for the same subject. [START_REF] Maclure | The case-crossover design: a method for studying transient effects on the risk of acute events[END_REF] We compared antiepileptic exposure during the three months before the crash with antiepileptic exposure during an earlier three-month control period. Odds ratios were estimated by conditional logistic regression. Data were analysed using the SAS ® statistical software package, version 9.0 (SAS Institute, Cary, NC).

This study was approved by the French Data Protection Authority.

RESULTS

Extraction and matching procedures led to the inclusion of 72,685 drivers who were involved in road traffic crashes (34,896 were responsible for causing the crash and 37,789 were not).

These accounted for 18.5% of the 392,169 drivers registered in the national police database of injurious traffic crashes. The severity of injury was the main factor associated with the probability of being part of the study. The inclusion rate was approximately the same for responsible and non-responsible drivers (18.8% and 18.3%, respectively). 251 drivers (0.35%) were exposed to at least one antiepileptic on the day of the crash. The multivariate descriptive analysis showed that exposure to antiepileptics was higher among men, drivers aged over 45 years and unemployed drivers. Antiepileptic exposure was more likely among drivers involved in single-vehicle crashes. Exposure to antiepileptic medicines was not associated with blood alcohol concentration (Table 1).

Multivariate responsibility analysis measured an odds ratio of 1.74 [1.29-2.34] for drivers exposed to antiepileptics. 99 drivers (0.14%) were registered on the day of the crash as with ICD-10 G40 code long-term chronic disease. The odds ratio was 2.20 [1.31-3.69] for these drivers compared with all other drivers. Among these 99 drivers with severe epilepsy, 36 (36.4%) had their last antiepileptic prescription at least 31 days before the crash suggesting they were not exposed to the medicine on the day of the crash. The odds ratio for responsible crash in this subsample was slightly the same as in the 63 drivers exposed to an antiepileptic on the day of the crash (OR=2.91 [1.25-6.77] and OR=2.02 [1.01-4.04], respectively).

Multivariate responsibility analysis showed a significant association between exposure to second and third generation of antiepileptics and responsibility (OR=2.15 [1.56-2.97] and OR=3.01 [1.40-6.44], respectively) (Table 2).

The odds ratio was 1.67 [0.98-2.83] for drivers exposed to several antiepileptics on the day of the crash (n=87) and 1.81 [1.27-2.57] for drivers under monotherapy (n=164), compared to non-users.

The case-crossover analysis found no association between the risk of crash and exposure to antiepileptics (OR=0.68 [0.42-1.10]).

DISCUSSION

The analysis of medicines dispensed to 72,685 drivers involved in injurious road traffic crashes in France between July 2005 and May 2008 found that drivers with prescribed antiepileptic medicines have an increased risk of being responsible for a crash (adjusted OR 1.74 [1.29-2.34]. Drivers with severe epilepsy had a two-fold increased responsibility risk (OR=2.20 [1.31-3.69]).

The responsibility analysis lends real strength to the study because it allows for comparisons of cases and controls who share the characteristic of being drivers. In a previous study on the impact of illegal drug consumption, using the same national police database but limited to fatal collisions, [START_REF] Laumon | Cannabis intoxication and fatal road crashes in France: population based case-control study[END_REF] the same method was used to determine responsibility and was approved by an expert evaluation of responsibility. [START_REF] Laumon | Cannabis intoxication and fatal road crashes in France: population based case-control study[END_REF] The strong dose-effect relationship found in our study between alcohol level and responsibility is a further indirect validation of the method.

Responsibility levels were computed independently of the use of alcohol and illicit drugs because of their potential interactions with medical drugs.

The results showed that drivers who had been severely injured were more likely to be included in the study than those who had sustained slight injuries or who had died. Injured drivers were more likely to be admitted to hospital, and therefore their health care numbers were more likely to be noted in police reports. Consequently, our study sample slightly overrepresented drivers who had been involved in more serious crashes.

In our study, severe epilepsy referred to a long-term chronic disease for which patients are fully reimbursed for health care expenses related to the disease. Patients who may benefit from this system are those suffering from resistant epilepsy and/or for whom the epileptic disease has an impact on their socio-professional life. In France, a renewable one-year temporary driving licence can be delivered to epileptic patients after advice from a medical commission.

A study showed that when a seizure arises while driving, 55% of epileptic patients crash. [START_REF] Gastaut | The risk of automobile accidents with seizures occurring while driving: relation to seizure type[END_REF] A retrospective case-control study found that patients who had seizure-free intervals of more than one year had a 93% reduced odds for crashing compared with patients with shorter intervals. [START_REF] Krauss | Risk factors for seizure-related motor vehicle crashes in patients with epilepsy[END_REF] In a longitudinal study, 222 of 1,089 epileptic patients (20.4%), followed up between 1965 and 1991, had a seizure at the wheel and this percentage was even higher in drivers who had at least one seizure a month (36.2%). [START_REF] Beaussart | Epileptic drivers--a study of 1,089 patients[END_REF] A literature review on medical risks in epilepsy concluded that frequent seizures are a risk factor for accidents resulting in injuries, and that seizure-related fatalities are much more important in chronic epilepsy than in newonset cases. [START_REF] Tomson | Medical risks in epilepsy: a review with focus on physical injuries, mortality, traffic accidents and their prevention[END_REF] It is thus likely that drivers suffering from severe epilepsy in our study represent chronic epileptic patients with more frequent seizures. However, no information on the number or frequency of seizures was available in our study. An accident analysis report showed that crashes due to seizures are more likely to involve a single vehicle. [START_REF] Van Der Lugt | Traffic accidents caused by epilepsy[END_REF] We consistently observed that drivers under antiepileptic treatment were more frequently involved in single-vehicle crashes.

A study showed that 59.4% of the patients who had seizures at the wheel also suffered from psychological disorders. [START_REF] Beaussart | Epileptic drivers--a study of 1,089 patients[END_REF] In a recent cohort study, comorbidities were significantly higher in those with epilepsy compared with those without epilepsy, including alcohol abuse, drug abuse, psychoses, depression or other neurologic disorders. [START_REF] Kwon | Motor vehicle accidents, suicides, and assaults in epilepsy: A population-based study[END_REF] Our analyses were adjusted for blood alcohol level, prescribed medicines used in anxiety and depression disorders and other long-term chronic diseases including some mental, behavioural and nervous system disorders.

Exposure to medications was ascertained from computerised records of reimbursed prescriptions filled in at the pharmacy, avoiding any recall bias. We did not know whether the medicines were actually ingested and had no information on serum drug level. A retrospective cohort study conducted in the US showed that epilepsy patients without regular driver's licences were characterised by more non-compliance with antiepileptic treatment. [START_REF] Hansotia | Epilepsy and traffic safety[END_REF] Another study showed that 20% of patients who had a crash during a seizure crashed immediately after missing an antiepileptic dose. [START_REF] Krauss | Risk factors for seizure-related motor vehicle crashes in patients with epilepsy[END_REF] Non compliance could not be explored in our study because exposure was estimated solely from dispensing data. Further studies should be designed to measure antiepileptic blood concentrations in patients involved in road traffic crashes.

Our results showed no significant association for the first generation of antiepileptics.

Nowadays, these medicines are less and less prescribed. Patients undergoing these treatments may thus be long-time users who do not need to switch for newer drugs as their epilepsy is well controlled. Patients using these drugs may thus be less likely to experience seizures at the wheel. The increased responsibility risk for users of second and third generation of antiepileptics is probably more related to epilepsy than to the medicines themselves. The difficulty in such studies is to disentangle the impact of the medical condition and of medication. The absence of an association between the risk of crash and treatment prescription in the case-crossover analysis indicates that there is no acute effect of the treatment, suggesting that the risk is linked to seizures and probably not due to medicines. This hypothesis is reinforced by the results showing that the odds ratio was approximately the same in the subgroup of drivers suffering from severe epilepsy and not exposed to an antiepileptic medicine on the day of the crash (OR=2.91 [1.25-6.77]) compared with exposed drivers (OR=2.02 [1.01-4.04]). In addition, in our study, patients under polypharmacy were not at increased risk of being responsible for a crash. Data reported from epidemiological studies, also suggests that the risk of crash is associated with the lack of treatment or changes in treatment (antiepileptic medicines being reduced or switched). [START_REF] Hansotia | Epilepsy and traffic safety[END_REF][START_REF] Krauss | Risk factors for seizure-related motor vehicle crashes in patients with epilepsy[END_REF] Patients with prescription of antiepileptic drugs should however be cautioned about their potential risk of road traffic crash.

  Police reports. Police personnel are required to fill out a report for each injurious traffic crashes occurring in the country. Police reports are scanned and archived as image files. For approximately 28% of drivers involved, the national ID number is recorded in the report. All the 210,818 available police reports in France from July 2005 to May 2008 were gathered.

Table 1 . Exposure to antiepileptics on the day of the crash according to characteristics of drivers and accidents

 1 p<0.05, **p<0.001, ***p<0.0001 (multivariate analysis), NS= Not significant

		Exposed to antiepileptics
	N	n (%)
	72,685	251 (0.35)

*

Table 2 . Odds ratios for responsible road traffic crashes in users of antiepileptics by generation

 2 Odds ratios adjusted for age, gender, socioeconomic category, month, time of day, location, vehicle type, alcohol level, injury severity, concomitant exposure, severe epilepsy and long-term chronic diseases

	Antiepileptics	n	OR [95% CI] a	OR [95% CI] b
	All	251	1.69 [1.31-2.18]*** 1.74 [1.29-2.34]**
	1st generation c	71	1.00 [0.63-1.59]	0.91 [0.53-1.56]
	2nd generation d	167	2.15 [1.56-2.97]*** 2.10 [1.45-3.03]***
	3rd generation e	34	3.01 [1.40-6.44]*	3.05 [1.15-8.08]*
	Reference: not exposed to medicines considered

a Crude odds ratios b c Phenobarbital and phenytoin d Valproic acid and ethosuximide e Vigabatrin, tiagabin, levitiracetam, zonisamide and lacosamide * p<0.05, ** p< 0.001, ***p<0.0001

Acknowledgments

We thank the CESIR research group for its collaborative support: in addition to the authors, Marta Avalos (Inserm U897, Université Bordeaux 2), Fabienne Bazin (Inserm U657), Sylvie Blazejewski (CIC 0005, Bordeaux), Benjamin Contrand (Inserm U897), Geneviève Durrieu (Service de pharmacologie médicale et clinique, CHU Toulouse), Pierre-Olivier Girodet (CIC 0005, Bordeaux), Marcel Goldberg (Inserm U687-UVSQ), Bernard Laumon (Ifsttar), Dominique Lauque (CHU Toulouse), Nathalie Lecoules (CHU Toulouse), Laurence Memes (CIC 0005, Bordeaux), Louis Merle (CHU Limoges), Jean-Louis Montastruc (Service de pharmacologie médicale et clinique, CRPV, Inserm U1027, Université de Toulouse, CHU Toulouse), Pernelle Noize (Inserm U657), Nathalie Orsoni (CHU Limoges), Antoine Pariente (Inserm U657, CIC 0005, Bordeaux), Régis Ribéreau-Gayon (CHU Bordeaux), Louis-Rachid Salmi (Inserm U897, CHU Bordeaux) Frantz Thiessard (LESIM). We acknowledge the French National Health Insurance (CNAMTS), the Inter-Departmental Observatory on Road Safety (ONISR) and Agira-TransPV for providing healthcare and road traffic crash data, as well as the public health Research Federative Institute (IFR 99). The CESIR-A project was funded by the Afssaps, the French National Research Agency (ANR, DAA n° 0766CO204), the French Medical Research Foundation (Equipe FRM), the French Direction Générale de la Santé (DGS) and the French National Medical Research Institute (Equipe INSERM). Employees of Afssaps participated in data collection, interpretation and review of the manuscript.


Luigi Alfredo Grieco
email: a.grieco@poliba.it

Mahdi Ben Alaya

Thierry Monteil

Khalil Drira
email: khalil@laas.fr

Architecting Information Centric ETSI-M2M systems

The European Telecommunications Standards Institute (ETSI) released a set of specifications to define a restful architecture for enabling seamless service provisioning across heterogeneous Machine-to-Machine (M2M) systems. The current version of this architecture is strongly centralized, thus requiring new enhancements to its scalability, fault tolerance, and flexibility. To bridge this gap, herein it is presented an Overlay Service Capability Layer, based on Information Centric Networking design. Key features, example use cases and preliminary performance assessments are also discussed to highlight the potential of our approach.

I. INTRODUCTION

Machine-to-Machine (M2M) communication technologies promise to inter-connect up to 50 billion devices by 2020, thus paving the way to advanced pervasive applications in several domains [START_REF] Boswarthick | M2M Communications: A Systems Approach[END_REF]. Current M2M market, unfortunately, is very fragmented. Many vertical M2M solutions have been designed independently for different applications, which inevitably hinder a large-scale M2M deployment [START_REF]Standardized M2M Software Development Platform[END_REF].

To this end, the European Telecommunications Standards Institute (ETSI) come up with a set of specifications which provides a restful architecture able to standardize the way heterogeneous devices can offer services and access to them seamlessly [START_REF] Boswarthick | M2M Communications: A Systems Approach[END_REF]. The building blocks of an ETSI M2M system are: devices, gateways, and networks. A device is a machine equipped with a set of resources/services that can be made accessible to the rest of the system. Many devices may also bind to the same gateway in order to make their resources visible outside their local domain. Finally, the resources available at many gateways and devices are exposed at a wide area scope through an ETSI M2M network. The standard defines a functional architecture with a generic set of service capabilities for M2M on top of connectivity layers deployed in M2M networks, gateways, and devices. In this abstract functional architecture each physical equipment is represented as an instance of a Service Capability Layer (SCL). Each device, gateways, and the network will find a corresponding entity in the SCL, so that the abstract model will be a collection of Device SCL (SCL), Gateway SCL (GSCL), and Network SCL (NSCL). Moreover, each SCL instance is responsible for a subset of resources modeled and named according a recursive hierarchical tree.

At the present stage, the ETSI M2M architecture is very centralized and the system critically depends on the NSCL (see also Fig. 1). In fact, the NSCL is in charge of handling: (1) mutual authentications; (2) resource discovery operations; (3) subscriptions; and (4) data relaying. In other words, in M2M networks with millions of device the NSCL has to be able to take part to all the active data sessions by relaying both signaling information (i.e., subscriptions) and data exchanged between remote endpoints (i.e., devices and gateways). This design choice might impair the scalability of the system and its fault tolerance. Furthermore, the overall scale of the system could be limited by a too centralized architecture because each single event that happens in the M2M infrastructure has to be relayed by the NSCL. Moreover, even if the NSCL can be implemented in a distributed way, its scale and costs would be much higher if the capabilities of GSCL and DSCL are not fully exploited. For these reasons, recent studies [START_REF]Standardized M2M Software Development Platform[END_REF] advocate extensions to the M2M standard that could lead to more scalable and distributed operations based on a meshed overlays of SCL instances. Also the oneM2M standard (which embraces ETSI M2M) is following the same direction [START_REF]oneM2M Functional Architecture[END_REF].

At the same time, the management of a meshed overlay that connect many SCL instances requires new challenges to afford in order to ensure effective service discovery and access to M2M resources. This problem becomes even more relevant if we consider that distributed caching mechanisms can be used in ETSI M2M systems.

A satisfactorily answer to such challenges can be found in the emerging Information Centric Networking (ICN) paradigm, which grounds its foundation on name based content discovery mechanisms and, more in general, on name oriented networking primitives in distributed networks of caches [START_REF] Xylomenos | A survey of information-centric networking research[END_REF]- [START_REF] Ravindran | Information-centric networking based homenet[END_REF]. Many ICN architectures are available nowadays, all having the common ambition to move a step beyond the current host centric Internet design towards the Future Internet. Also, ICN architectures are natively capable to embrace, merge, and extend most of the technical solutions conceived in peer-topeer systems literature so far [START_REF] Xylomenos | A survey of information-centric networking research[END_REF]. The Information-Centric Networking Research Group (ICNRG) within the Internet Research Task Force (IRTF) is currently defining unifying baseline terminology, evaluation methodology, and design rules for ICN [START_REF] Pentikousis | Information Centric Networking: baseline scenarios[END_REF].

Since the resources of ETSI M2M systems are named according to a recursive hierarchical tree, we propose herein to design an Overlay Service Capability Layer (OSCL) in accordance to the Named Data Networking (NDN) architecture 1 [START_REF] Jacobson | Networking named content[END_REF], which, among the available ICN platforms, is the leading one based on hierarchical names. Moreover, the way NDN has been conceived will allow native support to multicast applications, seamless wireless connectivity, content level security, and in network caching mechanisms, thus improving the scalability, fault tolerance, and flexibility of ETSI M2M systems. Noticeably, the OSCL we propose could be implemented also in ICN architectures other than NDN, provided that they allow an efficient management of hierarchical content names.

II. NDN OVERVIEW

The NDN architecture is based on the Content Centric Networking (CCN) rationale [START_REF] Jacobson | Networking named content[END_REF] that assumes hierarchical content names, receiver initiated sessions, content level security schema, and in-network caching mechanisms. The NDN architecture is born to natively support mobile applications and multicast data dissemination while, at the same time, to improve content distribution services across heterogeneous platforms. The entire NDN paradigm is based on two packets: Interest and Data. When a consumer of data needs a given information it just sends an Interest message in which it is specified the hierarchical name of the requested item. This Interest message is routed towards one or multiple nodes that are in possess of the requested content, which is then encapsulated in a Data message and sent back as an answer to the consumer. Along its path to the requester, the Data message can be stored in the cache of relaying nodes, that will become able to answer to future requests for the same content, thus lowering the load on the servers that store permanent copies of the asked item. Furthermore, if a NDN router receives multiple requests for the same content that have been issued by different nodes, it will forward only the first Interest packet and it will suppress next ones by keeping note of their arrival faces, thus enabling a native multicast dissemination model. Accordingly, three data structures are used in NDN: the Content Store (CS), representing the cache memory; the Forwaring Information Base (FIB) that plays the same role as in classic IP routers; and the Pending Interest table (PIT) in which it is possible to keep track of the arrival faces of the Interest messages received in the past and for which a Data packet has not been yet received. 1 www.named-data.org

III. OVERLAY SERVICE CAPABILITY LAYER

This paper proposes an OSCL to increase scalability, robustness, and flexibility of ETSI-M2M systems. From a networking perspective, the OSCL is a meshed overlay that inter-connects many SCL instances to allow the provisioning of M2M services without the compulsory inter-mediation of the NSCL.

In the ESTI M2M standard, all devices and gateways should be mutually authenticated at the NSCL. In absence of mobility, this operation is executed only once per device and allows the M2M system to handle future resource discovery operations. This kind of discovery is centralized and it uses the NSCL as a relaying point of all signaling messages. On the opposite, in our proposal, it will be used as a fallback solution only when the OSCL fails to handle a discovery operation.

Similarly, after a resource has been discovered, the NSCL is in charge to relay subscriptions to that resource (e.g., the sensing service of an electricity metering application) and all the data sent to subscribers (e.g., a remote energy monitoring application). Contrariwise, we propose to leverage OSCL capabilities to access to resources in a distributed way.

In the ETSI M2M standard, the resources are named according to a hierarchical scheme. To provide an example, a possible resource name for the last reading of a sensor, in the ETSI M2M standard, could be: Gscl1/applications/meter app/containers/meter data/ content instances/latest. Accordingly, we propose to adopt the NDN architecture to handle the services offered by the OSCL. Thanks to this choice a named resource can be located by simply issuing an Interest message containing its name. This message will be routed within the OSCL under the control of the NDN strategy layer and it will trigger at least one Data message containing the locator of the target resource. Many routing algorithms are nowadays available for NDN based on different kind of assumptions [START_REF] Xylomenos | A survey of information-centric networking research[END_REF]. The choice is application specific and can be left open in order to allow customized M2M deployments. It is worth to remark that Interest routing operations could fail for many reasons (e.g., stale information in the FIB of the nodes, resource not reachable due to a not connected topology graph, research scope too limited to reach the target resource). Under such circumstances, the classic centralized resource discovery already available in the ETSI M2M standard is used as a backup solution.

After the resource discovery has been accomplished, a QoS Monitoring phase is executed. If the resource was discovered using the distributed approach, then a path in the OSCL already exists that could be used for subsequent subscriptions and data exchange phases. The QoS Monitoring phase will measure the key performance indicators of the path (e.g., packet loss ratio, throughput, delay) and if the quality of the path will result to be poor a new link will be established in the OSCL to enable a direct access to the target resource. The link will be set up using the locator information provided by the discovery phase. Of course, if the distributed discovery failed and, as a consequence, the centralized one was used, a new link will be immediately set up in the OSCL. We remark that, similarly to [START_REF] Li | Merts: A more efficient real-time traffic support scheme for content centric networking[END_REF], we extend the Interest header, and related in network handling operations, in order to solicit the transmission of more than one Data message by the target resource.

In summary, the Overlay Service Capability Layer (OSCL) enables the following enhancements (see also Fig. 2): Distributed Service Discovery. It is used to discover a resource in the M2M system based on ICN primitives. This phase, if successful, opens at least one path in the OSCL towards the discovered resource. Also, it returns both the URI of the resource and its network locator, e.g., IP address and Port. These two additional information are useful to handle alias and to establish a new link in the OSCL (if required), respectively.

P2P Subscription & Data Exchange. Thanks to the discovery phase, subscriptions to a target resource and subsequent exchange of information can be handled without the centralized control of the NSCL, i.e., a multihop path in the OSCL can be used.

QoS Monitoring. The path activated using the Distributed Service Discovery is composed by a sequence of logical links and nodes within the OSCL. It can be used to access to the remote resource. At the same time, the QoS this path is able to provide could not be enough for effectively supporting P2P Subscription & Data Exchange phases. As a matter of fact, some nodes within the overlay could not be able to effectively act as network relayers and thus worsening the QoS due to additional delays and packet losses. For this reason, the OSCL we envisage also include a Distributed QoS Monitoring service that establish a new link with the remote resource when the path being used is not suitable anymore for a given M2M application. Notice that all these new features are optional so that the legacy interoperability with existing ETSI M2M deployments would be ensured together with the possibility of a graceful upgrade of currently available systems.

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IV. EXAMPLE USE CASES

In this section we describe two basic uses cases to demonstrate the main features of the proposed approach. The first one is made of the M2M Network, one gateway, and one device (see Fig. 1). The gateway and the device host an electricity metering application and an energy monitoring application, respectively. The monitoring application needs to read the data sensed by the metering process. Before this becomes possible, if OSCL is not used, it is necessary to accomplish the following actions: (1) the gateway and the device mutually authenticate to the NSCL; (2) the electricity metering application registers to the GSCL and then creates a data container.; (3) the monitoring application registers to the DSCL; (4) the monitoring application discovers the name of the gateway by querying the NSCL and then discovers the name of the application by querying the GSCL. Both queries are routed by the NSCL. At the end of this process the monitoring application knows the URI of the metering application (e.g., GSCL/applications/electricity meter); (5) the monitoring application issues a subscription targeted to that URI. This subscription is routed by the NSCL towards the GSCL; (6) after the GSCL accepts the subscription, every time the electricity metering application pushes a new event to the data container (i.e., a new measurement) a notification is sent to the monitoring application (through the NSCL).

By applying the OSCL to this use case, it becomes possible to increase the scalability of the ETSI M2M system. In fact, as shown in Fig. 3, the NSCL is no longer in charge of relaying the data sensed from the metering application. Also, from the message sequence chart in Fig. 3, it is possible to note as the authentication, discovery, subscription, and data exchange operations do not require significant modifications to ETSI messages: they only entail different endpoints in the exchange of packets. A slightly more complex scenario is depicted in Fig. 4, where two more gateways are present with respect to the previous case. This second scenario allows us to detail how NDN messages (i.e., Interest and Data) are routed through multiple GSCL instances in the OSCL. Fig. 5 shows that the Interest message opens a path towards the target resource during the discovery phase. This path is then used to transport both signaling and application data (encapsulated in Data messages) between two remote SCL instances in the OSCL, without any mandatory intervention of the NSCL. It is also worth to note that relaying data through a multi-hop path reduces the number of logical links established in the overlay per each SCL instance thus further increasing the scalability of the system.

V. PERFORMANCE EVALUATION

To provide a preliminary assessment of the properties of the OSCL, we have considered a complex scenario composed by N SCL instances (with N ranging from 2 5 to 2 16 ) and assigned to the QoS Monitoring function the role of not allowing the creation of paths longer than D hops (with D ranging from 3 to 10). This requirement is particularly relevant in order to shorten the maximum packet delay in the data sessions. To this end, as soon as a new couple of nodes needs to communicate, a straight link in the OSCL is created among them, if and only if no path already exists shorter than D hops. In particular, we have considered a long random sequence of couples of nodes wishing to establish a data session and we observed how the degree (i.e., average number of links per node) of the OSCL evolves over the time, subject to the constraint imposed by the QoS Monitoring function. At the present stage, we have not considered the impact of caching because it will affect the average delay and load rather then the properties of the topology. Research in this direction is in progress. Based on this scenario, results indicate that the OSCL average degree can be approximated by O( D √ 2N ln N ). This expression has been derived using graph theoretical arguments [START_REF] Loguinov | Graph-theoretic analysis of structured peer-to-peer systems: routing distances and fault resilience[END_REF] and validated using computer simulations (to this end we devised a system level simulator in Matlab2 ). It provides a way to predict the relaying overhead of SCL instances, which is closely coupled to the number of links per node in the OSCL topology, and to finely tune D based on the scale (N ) of the M2M system and the expected capabilities of its components.

VI. CONCLUSION AND FUTURE RESEARCH

An ICN approach to M2M systems has been presented in this paper in order to enhance the ETSI M2M standard. As future works we plan to: (i) evaluate the performance of the resulting OSCL in realistic settings; (ii) afford the problem of an efficient name space design; (iii) investigate topology optimization mechanisms; (iv) add the possibility to express goals in content names.

Fig. 1 .

 1 Fig. 1. A Simple ETSI M2M scenario.

Fig. 2 .

 2 Fig. 2. OSCL operations: simplified flow chart.

Fig. 3 .

 3 Fig. 3. Message Sequence Chart with OSCL. Straight lines represent the ETSI M2M messages used also by the OSCL. Dashed lines represent the new messages introduced with the OSCL. The gray lines are the ETSI M2M messages that are no longer used thanks to the OSCL.

Fig. 4 .

 4 Fig. 4. An extended ETSI M2M scenario with P2P capabilities.

Fig. 5 .

 5 Fig. 5. Message Sequence Chart with OSCL and ICN messages.

M2M Gateway M2M Device NSCL Resource Tree M2M Network GSCL Resource Tree DSCL Resource Tree Electricity Meter

	Nscl
	Scls
		Gscl
		Dscl
	Applications
	Containers
	(…)
	Gscl	Dscl
	Scls	Scls
	Nscl	Nscl
	Applications	Applications
	Electricity_Measurement_App	Electricity_Monitoring_App
	Containers	Containers
	(…)	(…)

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ACKNOWLEDGMENTS

This work was supported by the INSA of Toulouse (FR) and the PON project RES NOVAE, funded by the Italian MIUR and by the European Union (European Social Fund).