M Kabir-Querrec

S Mocanu

J.-M Thiriet

Power Utility Automation Cybersecurity: IEC 61850 Specification of an Intrusion Detection Function

The IEC 61850 standard defines a global framework for designing power utility automation systems. The main goal of IEC 61850 being interoperability, it brings information and tools for both system modelling and communication architecture. But cybersecurity measures and propositions are scarce. They should be a priority. To help fill this lack of cybersecurity, we specify a fully IEC 61850-compatible intrusion detection function. This paper explains the procedure of defining functions and necessary model objects consistent with the standard requirements. We then detail our intrusion detection function.

INTRODUCTION

Context

Information and Communication Technologies have been pervading monitoring and control systems for a few decades, especially in substations of the electric power transmission and distribution grid. They allow reading measurements remotely, retrieving logs and sending commands. Such communication was primarily characterized by proprietary protocols and closed networks, hence bringing security through obscurity and through isolation. Aspirations for vendor interoperability along with ever growing complexity of technologies led to standardization of networks and protocols used within substations, resulting particularly into the IEC 61850 standard -Communication networks and systems for power utility automation (TC57). This standard is key to substation automation systems (SAS) but seriously lacks digital security measures when substations are interconnected with more global and open networks, which makes them exposed to cyber incidents, whether intentional or not. Cyber vulnerabilities are threats to the safety of the SAS and thus to the safety and the reliability of the whole power grid.

Scope of the paper

The purpose of this paper is to propose a specification of a cybersecurity function compatible with the IEC 61850 standard, especially with the data object model it defines. This would allow for a future integration directly into Intelligent Electronic Devices (IEDs), as a function of its own along with other more common functions such as "distance protec-tion" or "synchronized CB switching" (Circuit Breaker). Hence our cybersecurity function must be designed following the scheme given in the standard. All definitions and statements stipulated in the IEC 61850 standard make up an abstract model, meaning they are independent of any actual implementation or technology. And so is the specification of the cybersecurity function presented in this paper. It is about defining the objects and concepts that the standard lacks to address the problem of managing intrusion detection.

This paper is organized as follows: we first briefly explain the key concepts introduced by the IEC 61850 standard and relevant to this work. The next section is about IEC 61850 functions specification process. Section 3 is an overview of several papers related to the cybersecurity of substation automation system and the understanding and modelling of IEC 61850 concepts. The presentation of the proposed cybersecurity function comes in section 4. Conclusion and a discussion about possible further work end the paper.

IEC 61850 STANDARD

Our work must be considered in the scope of the international standard IEC 61850. Developing standard communication protocols for Substation Automation Systems (SAS) has long been considered essential by the whole power utility community. They would support interoperability, which is a common goal for electric utilities, equipment vendors and standardization bodies. As stated in the introductory part of the standard (IEC 61850 -1), in-teroperability is the ability for all the IEDs of a SAS "to operate on the same network or communication path sharing information and commands". From the user point of view interoperability also means interchangeability, that is replacing an IED by another one whoever the manufacturer is, without changing anything in the other elements in the system.

Another important objective of SAS standardization is to develop communication protocols able to support future evolution of technologies. Logical nodes are organized in thirteen specific categories. The names of LNs are standardized as follows: Group designator / Three-letter abbreviation of function / Instance ID. For instance XCBR1 belongs to the "Switchgear" group and models a switch with circuit breaker capability, first instance (IEC 61850 -7.4). Note that in this article we deliberately do not respect this standardized naming to keep new object names understandable and ensure that our function description is easy to follow. For instance CYAN will stand for "CYbersecurity ANalyzer" and CYComChkSingle for "CYbersecurity Communication Checker for a Single frame". The standard defines logical node classes regarding substation common functionalities, giving mandatory and optional LN information. Part 7.3 also provides Common Data Classes describing necessary data: name, type, trigger option, value range, etc…

Data Object Model

Information exchange

In the context of the IEC 61850 series, only LNs exchange data and therefore a function exchanging data must be composed of at least one LN. Information exchange between two LNs is defined by the PICOM concept, Piece of Information for COMmunication. By definition, a PICOM is "a given data element or block of data on a given logical path with a given communication attributes" (Cigré 2001). To keep consistency with the data object model described above, PICOM can be spoken of as a class stipulating information transfer between a source LN and a sink LN. This class has many attributes characterizing either the information content (such as data type, name, value…) or the communication requirements (such as source and sink names, priority of transmission, data integrity…). Some of those attributes must be covered by any message when others are to be considered at configuration time or for data flow calculations. PICOMs are related to the application layer and do not represent the actual format and structure of the data over the physical network.

A device is fully described by its functions (LNs) and its transmissions (PICOMs). For unambiguous machine-to-machine communication, PICOM identifiers and attributes shall be understandable by every machine involved in the substation automation system. Hence defining new functions must comprise the task of PICOMs' specification along with the logical components (LNs and LDs). The standard defines message types based on a grouping of the performance related PICOM attributes:  Type 1 -Fast messages,  Type 2 -Medium speed messages,  Type 3 -Low speed messages,  Type 4 -Raw speed messages,  Type 5 -File transfer functions,  Type 6 -Time synchronization,  Type 7 -Command messages with access control.

Choosing adequate message types will improve the whole SAS performance.

Communication architecture

Parts 7-2, 8-1 and 9-2 of the standard propose the communication architecture shown in Figure 2. This data flow method is another major contribution of this standard. At the supervision level, the Electrical Network Monitoring and Control System (ENMCS) is connected to the IEDs over the station bus that operates MMS (Manufacturing Message Specification). This protocol is based on the TCP/IP layers (Fig. 3) and allows traditional supervision operations: read, write and report. Horizontal communication between baylevel IEDs is allowed via GOOSE (Generic Object Oriented Substation Event) messages, directly mapped on the Ethernet data link layer to meet the time performance required for such data exchanges, that is 4ms end-to-end transfer time. Such messages work on a publisher/subscriber mechanism on broadcast MAC addresses. Sampled Values (SV) are used for transmitting digitized power quantities from process systems, mainly current and voltage values.

Defining new functions

The task of defining SAS functions and the data elements they make use of has to follow some rules to ensure the interoperability between all the components of the global system [START_REF] Xu | Analysis on IEC 61850 Interoperability Support[END_REF]). It consists of three steps: describe the function and its decomposition into LNs, then describe each of the LN and the exchanged PICOMs, and finally detail the PICOMs. An IEC 61850 function is then defined by:  Task description: a formal description of the function task and its context of execution,  Starting criteria: the specific reason of the function initiation,  Result or impact: the output of the function,  Performance: total requested response time must be guaranteed, possible additional criteria may be accuracy of synchronization...,  Function decomposition: decomposition into LNs,  Interactions with other functions: data exchanged with other functions, will result in definition of PICOMs. A similar scheme has to be followed for describing LNs:  Introduction: task description and context of execution,  Starting criteria,  Input and output by PICOMs,  Operation mode: safe behavior in case of degraded data exchange must be provided,  Performance: based on performance attributes of PICOMs.

To fully specify a LN, this description is not enough. The LN data have to be explicitly defined, that means that the following questions have to be answered: Of which type is this data object? What are its data attributes? Of which type is each of those attributes? Etc… This is a stepwise process summarized in diagram in Figure 4. The idea is to use the data elements defined by the standard when appropriate and to enrich the model with new data elements required to fulfill the task of the function. To build a new functionality, the first step is to decompose the function into subfunctions. If a LN of the standard meets the characteristics of the subfunction, use it as is. Otherwise, if adding existing or new data to an existing LN is not sufficient, then create a new LN. The same flow can be run to define all the data of a LN along with their existing and new data attributes.

Once the data model description has been completed, the logical connections still have to be written. They depict the information exchange between LNs and hence between functions. Then, the new functionality is fully specified. As mentioned earlier, PICOM description provides information about semantics, logical point-to-point connection, performance requirements and type of data.

CYBERSECURITY FOR SMART-GRIDS SUBSTATIONS

Intrusion and anomaly detection

The purpose of intrusion detection is to monitor a network or a system activity to detect attempts to gain unauthorized access or to cause damages. Intrusion detection systems (IDS) are either host-based or network-based. The first category is a software module running on a device of the network and analyzing incoming and outgoing packets. Networkbased IDSs are independent devices placed at strategic points of the subnet. They perform analysis on the traffic to and from all agents of the network. IDSs and firewalls are not to be confused. While the latter limit access at the entry point of a network segment to prevent intrusion, IDSs check for an attack from inside it and generate alarms. There are two types of intrusion detection approaches:  Signature-based: to detect known attacks only,  Anomaly intrusion detection: to detect known and unknown attacks by quantifying how much the observed behavior deviates from normality. The anomaly-based detection was first introduced in 1985 by Dorothy E. Denning. In [START_REF] Denning | An Intrusion-detection model[END_REF] she proposed an intrusion detection model based on the idea that security violations can be detected by monitoring system audit records for abnormal patterns of system usage. She described a complete model emphasizing the importance of the metric and statistical analysis applied to each particular activity profile. Her idea was to introduce a system-independent model in the context of computer sciences. Using such a model in the particular case of industrial applications with embedded systems would mean to define these attributes even more carefully in accordance with the context, especially metrics and statistical models.

IDS dedicated to industrial control systems

A possible way of implementing an anomaly detection system is by analyzing whether the system is in an acceptable or a critical state. This approach can be correlated to Fault Detection research field where measurable variables are monitored to estimate if the system is approaching some critical state. Such works are presented in [START_REF] Bigham | Anomaly detection in electricity cyber infrastructures[END_REF]) and [START_REF] Fovino | Critical State-Based Filtering System for Securing SCADA Network Protocols[END_REF], although the latter makes use of traffic analysis to update and forecast an image of the system.

A more common approach is communicationbased IDS. It can be implemented by specifying normal behavior as a baseline and then checking whether the actual network traffic agrees with this baseline. Snort is an open-source software for IP networks which uses such rules. [START_REF] Premaratne | An intrusion detection system for IEC 61850 automated substations[END_REF]) presents an IDS dedicated to IEC 61850 automated substations, specifying Snort rules. But using only rule-based detection seems to mean designing either a very generic IDS which seems dangerous or an IDS totally dedicated to a single equipment of a defined infrastructure which is properly inconvenient in case of deployment.

As a countermeasure to the non-adaptivity of IDS to potential normal evolution of system usages, [START_REF] Sekar | Specification-based anomaly detection: A new approach for detecting network intrusions[END_REF] combines specification of rules with probabilistic techniques. The concept is very valuable but may be difficult to implement for embedded system networks with real-time constraints.

Combining the two methods is also the strategy adopted in [START_REF] Cheung | Using model-based intrusion detection for SCADA networks[END_REF]) which proposes a three-level IDS for SCADA networks. The rule approach is for checking protocol characteristics and network access patterns when the probabilistic one is concerned with service availability.

All the methods mentioned and others have in common to be hosted by dedicated equipment. As highlighted in [START_REF] Premaratne | An intrusion detection system for IEC 61850 automated substations[END_REF]) IEDs have very low computational power and are not able to carry out cybersecurity functions as efficiently as those developed more recently. The authors of [START_REF] Premaratne | An intrusion detection system for IEC 61850 automated substations[END_REF]) hence recommend implementing the IEC 61850 IDS they designed within the gateway or if not feasible to connect the IDS to a port mirror of the gateway to ensure effective monitoring of the whole incoming traffic. The "critical state-based filtering system" described in [START_REF] Fovino | Critical State-Based Filtering System for Securing SCADA Network Protocols[END_REF]) is a part of the firewall through which all command packets from SCADA flow to the slaves. In [START_REF] Vollmer | Fuzzy logic based anomaly detection for embedded network security cyber sensor[END_REF]), the IDS is implemented into an embedded network security cyber sensor at the entry point of the process network whose security is to be monitored. However it is worth working on the idea of an IED-compatible anomaly detection function for future more capable gears because a hostbased IDS has the advantage of operating independently even if other IEDs are corrupted.

From this perspective, we propose in this work to design an anomaly detection function specific to the IEC 61850 protocol stack and compliant with the standardized data object model. It would thus be able to operate independently as an autonomous device or would be embedded as a module in an IED.

Anomaly detection for IEC 61850 SAS

SAS specificities make necessary to think up dedicated intrusion detection methods:  As stated above, unlike conventional computers IEDs are generally embedded systems hence having limited computational resources.  Industrial applications often rely on real-time operations with highly constrained timerequirements, as emphasized in the 2011 Alstom Network Protection and Automation Guide (Alstom Grid 2011).  SAS run dedicated communication protocols such as Modbus TCP/IP, DNP3 and IEC 61850 communication stack.  Control of transport and distribution power grid provided by SAS relies on a fixed network topology and known mechanisms specific to the application domain. In particular communication mechanisms are well defined and highly constrained. Advantages can be taken out of this knowledge to attain the objective of a tailored IDS. Some examples of such an approach can be found in literacy. In [START_REF] Cheung | Using model-based intrusion detection for SCADA networks[END_REF]), a thorough study of Modbus specifications leads to enunciation of system behavior rules. Similarly, the author of [START_REF] Diallo | Détection d'intrusion dans les systèmes industriels: Suricata et le cas de Modbus[END_REF]) describes a Suricata-based IDS for Modbus industrial processes. The network-based anomaly detection system for substation automation presented in [START_REF] Hong | Integrated Anomaly Detection for Cyber Security of the Substations[END_REF]) identifies malicious multicast messages matching rules for known attacks. These rules were defined based on the GOOSE and SV specifications, given by the IEC 61850 standard. The choice of a network-based anomaly detection system seems relevant to deal with multicast packets. The device would just need to subscribe to all GOOSE and SV messages to parse them.

AN IEC 61850 CYBERSECURITY FUNCTION

Scope

A "System security management" function can be found in the standard. It allows the control and supervision of the security of the system against unauthorized access and loss of activity. It monitors and provides all activities regarding security violations. The core element of this function is the logical node named Generic Security Application (GSAL) whose role is to monitor violations regarding authorization, access control, service privileges and inactive associations. These two elements seem focused on authentication and privileges which is out of our scope. For this reason and also to keep things simple, we chose to make our anomaly detection function an independent one for now.

The objective here is not to propose a classical IDS adapted for use in a SAS but to specify an anomaly detection function based on IEC 61850 stipulations, that is detecting incoherencies regarding the standard. To do so, all the specificities of IEC 61850 SAS have to be taken into account and IEC 61850 function definition rules have to be diligently followed.

The presented intrusion detection function task is quite traditional but must be expressed according to the IEC 61850. Figure 5 shows a model of an IDS implemented as a dedicated 61850 IED in a SAS. LNs of the model are correlated to corresponding IDS modules. Following the function definition procedure given by the standard that we detailed earlier, this "Anomaly detection" function can be described as follows: Our anomaly detection function is built from many cybersecurity-dedicated LNs we define below. These new LNs names start with "CY". Let us focus on GOOSE messages to describe the anomaly detection processing regarding communication only. We won't describe checker module dedicated to the application model, in grey in Figure 6 diagram. The approach regarding SV and MMS would be similar. LNs of the station level (Fig. 6) are classical IEC 61850 LNs.

New data elements: excerpt

Following the flowchart of Figure 4, we have fully specified our anomaly detection function covering all the data layers from LNs to basic types. While the purpose of this article is not to describe this function in detail, let us depict the bay-level LNs involved in the processing of a single frame: CYSN, CYComChkSingle and CYAL.

As shown in Table 1, CYSN mainly consists of the data object RecInfo of type Message Information (MSG). There was no existing LN for sniffing so we created CYSN. Note that CYSN input is not a PICOM as normally expressed for LNs but it pulls all incoming messages out of the reception buffer. The way it is done is a local issue, out of the scope of the IEC 61850 standard.

MSG is a new data class we made up for our purpose. Its main data attributes are GoFr (Goose-Frame -a packed list of the frame items), recT (re-ceptionTime -TimeStamp) and lgth (frame length -INT). The GoFr attribute of type "Packed List" is new. The "Packed List" type is defined by the standard as an "ordered list of types. Defined where the type is used."

The specifications of logical nodes are given in the IEC 61850 standard as tables comparable to Table 1. All LN instances have a common structure with Common Logical Node Information, mandatory whatever LN it is, the following categories of data existing when relevant: Control Information, Status Information, Settings Information, Configuration-Description-Extension Information. Specifications of other objects of the model are given following similar templates. Hence specifying our cybersecurity function has resulted in such tables.

CYComChkSingle status information related attributes are of security violation counting common data class (SEC): PktStructDflt (Packet structure default), NumDataDflt (Number of data items wrong), AddrDflt (Source or destination address wrong), SqNumDflt (Inconsistency in sequence number incrementation), StNumDflt (Inconsistency in sequence number incrementation).

As an example of information exchange, here are the PICOMs transmitted by source LN CYAL, we can consider:  Events to sink LNs CALH, IHMI, ITCI, ITMI.  Diagnostic data to sink LNs IHMI, ITCI, ITMI.

For sake of clarity these PICOM connections do not appear in Figure 6. Next step of our work is naturally implementing our function and test it. For these practical tasks, our laboratory is equipped with a platform dedicated to cybersecurity and interoperability of industrial control systems. Devices running on this test bench comprise IEDs (IEC 61850 but not only), PLCs, HMIs, supervision softwares and embedded boards for prototyping. It would also be interesting to realize the same work of specification and then implementation of other cybersecurity functions such as mapping of the communication architecture, resource availability, encryption, etc…

  One of the main contributions of the IEC 61850 standard is an object-based data model for functions and services, which brings functional decomposition and information modeling. The figure below illustrates this data object model. An application function is decomposed into the smallest entities called Logical Nodes (LN). LNs are characterized by their data and methods, and exchange data with each other.

Figure 1 .

 1 Figure 1. IEC 61850 Data Object Modeling All the LNs required to complete a specific function are aggregated along with relevant data sets into a virtual device called Logical Device (LD). A real Physical Device (PD), such as an IED, can be composed of many LDs.Logical nodes are organized in thirteen specific categories. The names of LNs are standardized as follows: Group designator / Three-letter abbreviation of function / Instance ID. For instance XCBR1 belongs to the "Switchgear" group and models a switch with circuit breaker capability, first instance (IEC 61850 -7.4). Note that in this article we deliberately do not respect this standardized naming to keep new object names understandable and ensure

Figure 2 .

 2 Figure 2. 61850 communication architecture (Nachar 2013)

Figure 4 .

 4 Figure 4. IEC 61850 function extension rules.

Figure 5 .

 5 Figure 5. Model of an intrusion/anomaly detection function as a dedicated IED.



  Task: The anomaly detection function allows detecting anomalies in the system behavior. It analyzes input frames for feature extraction to check them for compliance with the communication model (syntax and semantics, and exchanges) and the system model. Results of this analysis are recorded and alarms are generated in case of deviance from the normality.  Starting criteria: Reception of a frame.  Result or impact: Results of the analysis are stored in logs and alarms are generated if need be.  Performance: Total processing time (starting time + internal process time + overall transfer time per PICOM + delay time) and hence percentage of analyzed packets according to the throughput. PICOMs of type 2 and 3 would fit alarm generating and analysis report transmission. Also memory allocation should be well supported by the processor.  Function decomposition: As given in Figure 6.  Interactions with other functions: Network management, Event management, Alarm management, Archiving, System security management, Access security management… 4.2 Function decomposition

Figure 6 .

 6 Figure 6. Decomposition of the anomaly detection function into interacting LNs on different levels.

Table 1 .

 1 LN class: Sniffer of LN group CY (Cybersecurity). Name: CYSN.As an answer to the serious lack of cybersecurity considerations which IEC 61850 standard suffers from, this article presented an anomaly detection function. It has been designed by strictly following extension rules from the standard, thus ensuring consistency with the IEC 61850 model to not compromise the interoperability it aims to bring. LNs, DATA, attribute types, etc… were created to enrich the model with the necessary model objects.

		CYSN (Sniffer)
	Data Attr.	Attribute	Explanation	M/O
	Name	Type
	Common Logical Node Information
			LN shall inherit all Man-	M
			datory Data from Common
			LN Class
	Status Information
	RecInfo	MSG	Reception Information: In-	M
		(Message	formation about message
		Information)	reception
	5 CONCLUSION AND FURTHER WORK


Stéphane Mocanu
email: stephane.mocanu@grenoble-inp.fr

Maëlle Kabir-Querrec
email: maelle.kabir-querrec@grenoble-inp.fr

Jean-Marc Thiriet
email: thiriet@grenoble-inp.fr

Eric Savary
email: eric.savary@euro-system.fr

Cybersécurité des sous-stations électriques IEC 61850

Introduction

La notion de cybersécurité couvre divers aspects selon les menaces auxquelles l'on souhaite répondre :

• La confidentialité : empêcher la diffusion d'informations, • L'intégrité : empêcher les modifications des configurations et contenus par des utilisateurs non autorisés,

• L'authentification : s'assurer que seules les entités (utilisateurs, services…) autorisées usent de leurs privilèges de lecture, d'actions…, • La disponibilité / la non-répudiation : empêcher qu'une entité autorisée soit confrontée à un DoS (Denial of Service),

• La journalisation : enregistrement des événements pour analyse ultérieure.

Les vulnérabilités d'un système d'automation d'une sous-station électrique sont cependant différentes de celles auxquelles est confronté réseau d'entreprise traditionnel. Premaratne et al.

[Premaratne_2008] distinguent deux objectifs principaux d'éventuels attaquants : perturber le service (compromet la disponibilité) et accéder aux informations confidentielles pour usage malveillant (faille dans la confidentialité).Toutefois d'autres types d'attaques peuvent être utilisés pour servir ces deux buts.

On peut souligner trois différences principales entre un réseau dédié à des opérations administratives ou d'ingénierie et un réseau industriel. Tout d'abord contrairement des ordinateurs conventionnels, les IEDs sont généralement des systèmes embarqués disposant de ressources computationnelles limitées. La seconde différence est la criticité temporelle : comme souligné dans l'édition 2011 du Guide Alstom pour la protection et l'automation des réseaux (Alstom Network Protection & Automation Guide), les applications industrielles reposent sur des opérations temps-réel avec des contraintes de temps très strictes. La dernière différence mais non la moindre concerne les protocoles de communication. Par exemple Modbus, DNP3 ou la pile de protocoles CEI-61850 sont des bus de terrain spécifiquement conçus pour des infrastructures et les IDS conventionnels ne gèrent pas ces protocoles.

Ainsi, les différences de technologies et d'usages rendent évidente la nécessité de techniques de détection d'intrusion dédiées aux systèmes d'automation des sous-stations.

Contexte

Un IDS conforme à la norme CEI-61850

Un procédé industriel repose en général sur une topologie de réseau fixe des mécanismes connus spécifiques au domaine d'application. En particulier les mécanismes de communication sont bien définis et en général plus contraints que ceux de réseaux conventionnels tels qu'internet. On peut tirer avantage de cette connaissance pour concevoir un système de détection d'intrusion (IDS) ou d'anomalies sur mesure. C'est l'approche adoptée par Cheung et al. Les communications MMS sont un autre problème puisqu'elles utilisent des adresses de destination précises. Elles sont utilisées à l'interface des niveaux de supervision et de baie. C'est pourquoi il a été proposé d'implémenter l'IDS directement dans la passerelle au point d'entrée du réseau de la baie [Premaratne_2010] ou d'utiliser un port miroir. L'autre argument en faveur de cette implémentation est la puissance computationnelle limitée des IEDs. L'expérimentation sur la plateforme dont le GIPSA-lab s'est dotée depuis l'automne 2014 (G-ICS, GreEn-ER Industrial Control systems Sandbox) nous aidera à répondre à cette question.

Quant au design de l'IDS lui-même, nous le voulons conforme au modèle orienté objet décrit précédemment. Cela afin de rendre possible une éventuelle future intégration de ce module de cybersécurité directement dans les IEDs comme une fonction à part entière. Notre fonction de cybersécurité doit donc être conçue en suivant la procédure donnée dans le standard : décrire d'abord la fonction et sa décomposition en LNs, puis décrire l'ensemble des LNs ainsi que les PICOMs échangés (Piece of Information for COMmunication, classe du modèle de données décrivant le transfert d'information entre LNs). La description détaillée des PICOMs achève la spécification de la fonction. Les procédures de spécifications sont données dans la partie 5 du standard.

La spécification de cette fonction de cybersécurité inclut les objets (fonctions, LNs, PICOMs) existants dans la norme et en propose de nouveaux quand nécessaire. Le travail en cours a pour objectif de proposer un modèle complet de LNs et PICOMs décrivant précisément la fonction de détection d'anomalies numériques CEI-61850.

Un autre objectif de ce travail de recherche est d'écrire un modèle du système basé sur la théorie des automates, ce qui pourra s'avérer utile pour l'estimation des performances et pour une future implémentation. De même il serait bon d'identifier les scenarios d'attaques et leurs conséquences, des méthodes inspirées de la sûreté de fonctionnement sont envisagées (Graphes d'attaque, méthodes d'analyse qualitative). ICIAFS 2008, 4th International Conference on , pp.194,199

Glossaire

GOOSE

Forts

  de ce constat, le consortium formé d'Euro-System, bureau d'études et d'intégration dans l'Automatisme et l'Informatique Industrielle du bassin grenoblois, et du GIPSA-lab, laboratoire d'Automatique et de Traitement du Signal de Grenoble (Grenoble Image Parole Signal Automatique) a décidé de démarrer une thèse sur le thème de la cybersécurité des systèmes de contrôle des postes électriques. Ces travaux de recherche s'inscrivent dans le cadre du standard international CEI-61850 normalisant les réseaux et systèmes de communication des sous-stations [CEI-61850]. L'un des objectifs de cette norme est de permettre l'interopérabilité, définie dans la partie introductive (CEI-61850 -1) comme la capacité pour tous les IEDs du SAS "d'opérer sur le même réseau ou vecteur de communication en partageant données et commandes". L'un des apports majeurs de ce standard est la définition d'un modèle de données orienté objet qui permet une décomposition fonctionnelle des services et une modélisation hiérarchisée de l'information. La figure ci-dessous permet de visualiser ce modèle.

Fig. 1 .

 1 Fig. 1. Modélisation des données orientée objet définie par la norme CEI-61850

Fig. 2 .

 2 Fig. 2. Architecture de communication définie par la CEI-61850 (M. Nachar, Euro-System)

  [Cheung_2007] : une étude approfondie des spécifications Modbus a permis d'énoncer des règles de comportement du système pour un IDS de règles. L'IDS réseau pour SAS présenté dans [Hong_2014] identifie les messages multicast malveillants conformes à des signatures d'attaques connues. Ces règles ont été établies à partir des spécifications des protocoles GOOSE et SMV définies dans la norme CEI-61850. Contrairement à Hong et al. [Hong_2014], nous voulons caractériser le comportement normal des systèmes et réseaux de communication d'une sous-station CEI-61850 et quantifier à quel point le système réel diverge de ce modèle. Cependant le choix fait dans cet article d'un IDS basé réseau semble le plus adapté pour gérer les messages multicast. Il suffira que l'équipement s'abonne à l'ensemble des GOOSE et SVM pour les parser.

  Communication networks and systems in substations Cheung S., Dutertre B., Fong M., Lindqvist U., Skinner K., Valdes A. (2007), Using model-based intrusion detection for scada networks, In Proceedings of the SCADA Security Scientific Symposium, Miami Beach, Florida, USA Hong Junho, Liu Chen-Ching, Govindarasu M. (July 2014), Integrated Anomaly Detection for Cyber Security of the Substations, Smart Grid, IEEE Transactions on , vol.5, no.4, pp.1643,1653 Premaratne, U.; Samarabandu, J.; Sidhu, T.; Beresh, B.; Jian-Cheng Tan (Dec. 2008), Evidence Theory based Decision Fusion for Masquerade Detection in IEC61850 Automated Substations, Information and Automation for Sustainability,

		Generic Object Oriented Substation Event
	IED	Intelligent Electronic Device
	MMS	Manufacturing Message Specification
	LD	Logical Device
	LN	Logical Node
	PICOM	Piece of Information for COMmunication
	SAS	Substation Automation System
	SMV	Sampled Measured Values
	Références
	Alstom Network Protection and Automation Guide, 2011
	CEI 61850


Dr Johanna Choumert
email: j.choumert.nkolo@surveybe.com

Dr Pascale Phélinas
email: pascale.phelinas@ird.fr

Catherine Araujo Bonjean

Keywords: Argentina, Hedonic price, Lease, Contracts, Soybean. JEL codes Q13, Q15, R11

We study the determinants of rental prices of farmland in the Argentinean Pampas. In particular, we examine the value of lease contract characteristics within a hedonic price framework, while controlling for other potential sources of variation. Using first-hand data for 255 parcels, our results indicate that both short-term contracts and contracts with sowing pools push rental prices upwards. We also find that soybean yields have a significant impact on land rental rates. These results suggest that if Argentina intends to protect the enormous natural advantage it has for agricultural production, it should consider strictly regulating land rental contracts.

Introduction

Argentinian land markets have experienced great changes in the past decades, notably with the expansion of biotech genetically modified (GM) crops. Since the introduction of GM soybeans to Argentina in 1996, there has been a dramatic rise in production. 1 As a result, all production of soybeans in Argentina is genetically modified and is mainly concentrated in the Pampas region. Today, Argentina is the third largest exporting country, after the USA and Brazil [START_REF] Filomeno | State Capacity and Intellectual Property Regimes: Lessons from South American Soybean Agriculture[END_REF][START_REF] Leguizamón | Modifying Argentina: GM Soy and Socio-Environmental Change[END_REF][START_REF] Urcola | Land Tenancy, Soybean, Actors and Transformations in the Pampas: A District Balance[END_REF].

Within this context, the rental price of farmland has increased dramatically. In the Pampas, the central agricultural producing area of Argentina, land rental prices have doubled since 2001 (F. [START_REF] Bert | Agent-Based Modelling of a Rental Market for Agricultural Land in the Argentine Pampas[END_REF]. This upward movement was driven by a dramatic increase in land demand, brought about by record profits in GM soybean cultivation. Strong cost savings associated with high yields, high prices in international markets and economic reforms during the 1990s, contributed to increasing soybean producers' incomes.

Technical changes in GM soybean production have encouraged economies of scale in farming, leading to increased farm sizes. Between 1988 and[START_REF] Moss | Leasing Contract Choice: Do Transaction Characteristics Matter?[END_REF], the average farm size rose by 25%, according to agricultural censuses. Because of the high cost of land, the race for land to produce the very profitable GM soybean has been dominated by the acquisition of user's rights, rather than ownership of land. As a result, the total area of land farmed under tenancy contracts has increased sharply, compared to land farmed by landowners. Today, in the soybean growing regions of Argentina more than 60% of cultivated land is under some sort of tenancy contract. Fixed rental contracts account for 90% of total leased land, whereas share contracts represent only 10%. Among fixed rental arrangements, contracts for a single agricultural season are expanding at a high rate as well. 2 The growing disconnect between land ownership and land cultivation, compounded by the shortening of lease contracts, has important implications for land use sustainability. A large number of studies suggests that rented land is managed differently according to the type of lease contract, which may affect the selection of crops, the choice of technological packages, and the use of conservation practices such as crop rotation [START_REF] Abdulai | Land Tenure Differences and Investment in Land Improvement Measures: Theoretical and Empirical Analyses[END_REF][START_REF] Fraser | Land Tenure and Agricultural Management: Soil Conservation on Rented and Owned Fields in Southwest British Columbia[END_REF][START_REF] Myyrä | Exploring Long-Term Land Improvements under Land Tenure Insecurity[END_REF][START_REF] Soule | Land Tenure and the Adoption of Conservation Practices[END_REF]. Indeed, the increased number of short-term cash lease contracts in Argentina has been correlated with a land allocation strategy favoring soybean monoculture and the massive use of agrochemicals. (F. E. [START_REF] Bert | An Agent Based Model to Simulate Structural and Land Use Changes in Agricultural Systems of the Argentine Pampas[END_REF] have shown that, given the level of the rental prices, tenanted land must be allocated to soybean if producers want to be sufficiently profitable. How to promote rational use of the land in order to guarantee the preservation of the environment has become a central issue in Argentina. Some of the literature attempts to explain agricultural land rents with hedonic models. The impact of farmland characteristics (such as quality of soils, localization of parcels, surface, and productive potential) on land-lease rates was highlighted, as well as the impact of public intervention, and more recently, environmental considerations. However, the extent to which lease contract properties are correlated to land rent prices is an intriguing question which has not been explored, but deserves attention. Obviously, contract design must affect the land-lease price because it determines the duration of payments, the level of risk taken, and sometimes the allocation of land to different uses.

The objective of this paper is to shed light on the effects of contract characteristics on farmland rental rates, while controlling for other potential sources of variation, such as land characteristics and expected returns. We assume that the contractual conditions are capitalized into land rents, as suggested by [START_REF] Palmquist | Land as a Differentiated Factor of Production: A Hedonic Model and Its Implications for Welfare Measurement[END_REF]. The great variety of contractual land arrangements in Argentina offers a good opportunity to evaluate the preferences of landowners and producers associated with specific contracts. Our results, based on a first-hand survey among producers, lead to a better understanding of the determinants of land rental prices.

To reach our research objectives, we use a hedonic price model. The structure of this paper is as follows. Section 2 discusses the design of lease contracts in the Pampas region of Argentina. Section 3 presents the hedonic literature on farmland rental prices. Section 4 contains the empirical analysis, followed by the conclusion.

Lease transactions in a GM soybean area of Argentina

The context of land lease in Argentina

The actors of the Argentinian land market are of two types: physical persons and societies; they can be lessors or lessees. Landowners are usually physical persons. Tenants might be farmers who already own land but seek to extend their production. Those farmers are the most important group, both in terms of number and area sown. Tenants can also be societies. Indeed, GM soybean cultivation has been accompanied with deep changes in the organization of the production. New forms of associations between farmers emerged, namely "pooles de siembra", to manage and finance soybean production (V. A. [START_REF] Hernandez | Ruralidad Globalizada Y El Paradigma de Los Agronegocios En Las Pampas Gringas[END_REF]. These sowing pools are agricultural societies consisting of farmers who gather to lease tracts of land as well as services for the main farming operations (planting, spraying and harvesting) and sometimes for transport.

The legal framework for agricultural land leases in Argentina is defined by the law n° 13246 passed in 1948. This law stipulated, among other things, a minimum term of three years for leasing contracts, the respective obligations of landlords and tenants, and stated that an "irrational use of the soil that causes its erosion or exhaustion is forbidden." This law was revised in 1980 and this updated version is nowadays in force. The most significant modification was the introduction of the de facto limited contract, which covers only one current vegetative cycle (that is one year or less). The revised law also states that rents must be expressed in currency units (local or foreign), but does not prescribe any form of price control mechanism. Landowners and tenants are free to negotiate a rate agreeable to both parties. In the same vein, no continuation rights are legally guaranteed, meaning there is no obligation for a landlord to renew a contract with a tenant. Therefore, tenants cannot claim rights on land they may have cultivated for a long period, and landlords have no tenure insecurity associated with leasing land. Despite this legal framework, most land contracts are neither registered nor approved by the land law, and are frequently oral agreements. The terms and conditions are freely agreed upon by the parties, but officially unknown. However, information about the cost of rent for any given type of soil is usually known by most farmers. The Argentinian "modelo sojero" is characterized by intensive social interactions taking place through producers associations [START_REF] Goulet | Vers Un Modèle de Développement et D'identités Professionnelles Agricoles Globalisés ?. Dynamiques D'innovation Autour Du Semis Direct En Argentine et En France[END_REF]. Information is easily obtained from other farmers or landlords, making the land market fairly transparent. Additionally, high competition among tenants, who are often farmers looking to enlarge their farms, ensures that rental prices, like in pure spot markets, are negotiated competitively, and result from individual decisions and profit considerations that reflect all the characteristics of the lease contract. The land market therefore appears quite open and dynamic. We can reasonably assume that the price equilibrates the demand and supply of leased land and clears the market.

Data on land-lease in the Pampas

Because data on land-lease transactions are not available in Argentina, we focus on a sub landrental market in two provinces within the Pampas region, Buenos Aires and Santa Fé, where we collected information. Within these two provinces, a portion of territory of 110 thousand hectares was randomly selected and each plot of land pertaining to this selected area had been listed. Information about the owner or the producer, the type of productive activities, and the tenure modes were collected. 3 This rich database allowed us to select a simple random sample comprising of 186 farmers cultivating 542 parcels, of which 321 were leased. The survey was undertaken during July and August of 2011, collecting information about each plot's size, location, type of land, cropping pattern, productive assets, type of lease contract, and method of payment.4 Detailed information was collected on the identity of contracting parties and some of the landlords' characteristics, such as living place, relationship to the tenant, and status with respect to agricultural activity (active farmer, retired farmer, non-farmer investor). We analyse data for 255 of the parcels, after removing those without a complete set of information.

Landlords in Argentina tend to be retired farmers or farmers that are no longer involved in farming activities. In our sample, nearly 75% of parcels are rented by landlords who are non-producers. Their children and/or heirs have left agriculture and migrated to urban centers. Most landlords describe themselves as too small to handle the technical changes or the risk of farming themselves, or they feel technically outdated and unable to compete with younger farmers for additional land [START_REF] Gras | Changing Patterns in Family Farming: The Case of the Pampa Region, Argentina[END_REF][START_REF] Urcola | Land Tenancy, Soybean, Actors and Transformations in the Pampas: A District Balance[END_REF]. Other reasons are a lack of capital and ability to finance the purchase of inputs (seeds, agrochemicals, labor, agricultural services, and so forth). Their objective is to obtain the highest quasi riskless short-term return on land capital, and indeed, the level of rental prices is high enough to provide a comfortable standard of living [START_REF] Urcola | Land Tenancy, Soybean, Actors and Transformations in the Pampas: A District Balance[END_REF]. These characteristics largely explain the progressive disappearance of share contracts and the lack of concern about tenants' practices that deviate from good agronomic routines.

As far as rent payment is concerned, a specific system is working according to the "use and customs." The survey data indicate that the dominant form of landlord-tenant contracts is a lease wherein the tenant pays the landlord a fixed amount, expressed in quintals of soybean per hectare rented (instead of its monetary equivalent), no matter the length of the contract. Since soybean prices exhibit high intra-seasonal fluctuations, payment in kind enables the landlord to reap the best possible price from his "rent" payment, either by storing the output to sell at a future higher price or by transporting it to distant markets that offer a better price. With a share lease, the landlord receives a stipulated percentage of the production. 5In the two provinces under study, we find three main types of lease contract: a regular, or multiyear, fixed-lease contract, with a term of at least three years; a one-cultivation-cycle fixed contract, where land is leased for a single growing season and sometimes for a single crop; and a multi-year share contract. The primary contractual arrangement remains the fixed lease, at 92% of plots, while share contracts still occur, though to a much lesser extent, at 6% of plots. There are many reasons that explain farmers' preference for a fixed land lease, and are well documented in the existing literature on contract choice. Both risk and transaction costs are usually considered.

First, given the socio-economic characteristics of landlords in Argentina, the marked preference for fixed rent contracts reflects owners' disinterest in production and management decisions and/or difficulties in monitoring day-to-day farming operations. Fixed-rent contracts also protect landlords from suboptimal work effort from tenants, as well as underreporting of the harvest. Transaction costs, either in time or in money (if the landlord has to pay supervisors), are thus saved. There is evidence in the literature suggesting that transaction costs are an important factor in the choice of a fixed-rent arrangement (D. W. [START_REF] Allen | The Role of Risk in Contract Choice[END_REF][START_REF] Jacoby | Incentives, Supervision, and Sharecropper Productivity[END_REF]. On the other side, fixed-rent contracts tend to provide managerial autonomy to the tenant, who gains freedom in decision-making and ensures that his productive efforts and managerial ability will not be shared with the landlord.

Second, fixed-rent contracts protect landlords from market uncertainty, as well as from output fluctuations arising from exogenous shocks, such as climatic accident (flood, dryness) or pest invasion. A large strand of empirical evidence suggests that landlords who want to avoid risk prefer fixed-rent contracts (D. Allen and Lueck 1992a;[START_REF] Cheung | Transaction Costs, Risk Aversion, and the Choice of Contractual Arrangements[END_REF]W. Huffman and Just 2004;[START_REF] Stiglitz | Incentives and Risk Sharing in Sharecropping[END_REF].

Third, in a context where input and capital markets are well developed, or when the wealth level of the tenant is high enough, which is the case in the Pampas region of Argentina, a share arrangement is unlikely to occur [START_REF] Rainey | Arkansas Landlord Selection of Land-Leasing Contract Type and Terms[END_REF]).

However, fixed-rent contracts create incentives for tenants to use land unsustainably in order to increase income, for instance by planting soybean crop after soybean crop, or by applying huge volumes of herbicides, practices that deplete the soil over time. If landlords have long-term interests in farming activities, then natural-resource conservation will be taken into account. One of the most problematic issues is how to monitor tenants' agronomic practices and ensure that overuse and misuse of land do not result in degradation and exhaustion of the soil. Landlords who want to ensure that a tenant does not alter the long-term productive potential of the land have a higher probability of offering a crop-share contract, according to many studies (D. Allen and Lueck 1992b; W. E. Huffman and Fukunaga 2008).

Among fixed-rent contracts, 28% are short term. That share is rapidly increasing, driven by the boom in soybean prices, which has increased expectations about future returns from soybean cultivation. In that context, it is rational to roll over short-term contracts and negotiate an increase in rent each time, rather than engaging for longer terms and risking a lower income from renting. Shortterm contracts are thus particularly suited for periods of rising prices. They also introduce increased flexibility in the seasonal decisions about what to produce and in what volumes. This allows producers to respond very quickly to a changing economic environment and large price fluctuations.

Capturing the effects of lease contract using a hedonic pricing framework

The rental price of an agricultural parcel represents the equilibrium relationship between supply and demand for land. Rents differ because parcels do not have the same characteristics and they are localized in different places. The hedonic price method connects the rental value of the plots to their characteristics. It allows for calculating the weight of each feature in the rent paid to landowners.

While [START_REF] Rosen | Hedonic Prices and Implicit Markets: Product Differentiation in Pure Competition[END_REF]) is the seminal article on hedonic pricing for housing, we refer to [START_REF] Palmquist | Land as a Differentiated Factor of Production: A Hedonic Model and Its Implications for Welfare Measurement[END_REF] for a hedonic price model of the rental prices of farmland. 67 In his model, which is a standard hedonic equation, rents are explained by characteristics of farmlands such as:

𝑅 = 𝑅(𝑧 1 , … , 𝑧 𝑛 ),
where R is the rental price of each parcel and z is a vector of n characteristics of parcels.

There are many empirical contributions, mostly from OECD countries [START_REF] Donoso | A Hedonic Price Model of Argentinean Land Prices[END_REF][START_REF] Herriges | The Implicit Value of Corn Base Acreage[END_REF][START_REF] Huttel | Is There a Term Structure in Land Lease Rates?[END_REF][START_REF] März | Analysing Farmland Rental Rates Using Bayesian Geoadditive Quantile Regression[END_REF]. [START_REF] Herriges | The Implicit Value of Corn Base Acreage[END_REF] analyze the capitalization of a U.S. commodity program into farmland rents using Iowa rental survey data. [START_REF] Huttel | Is There a Term Structure in Land Lease Rates?[END_REF] investigate, among other variables, the impact of the length of contracts (in years) on farmland rental values in Germany. More directly related to our study, [START_REF] Donoso | A Hedonic Price Model of Argentinean Land Prices[END_REF] investigate rental rates in the Pampas Region in Argentina using survey data for 86 parcels and focusing on soil erosion.

To the best of our knowledge, no hedonic study has connected farmland rental values and contractual arrangements, with the exception of [START_REF] Huttel | Is There a Term Structure in Land Lease Rates?[END_REF]. To a broader extent, literature on any correlation between these variables is scant. There are, however, some exceptions. In the Netherlands, based on a survey asking land agents and landowners to rank the value of tenanted land, [START_REF] Slangen | Land Lease Contracts: Properties and the Value of Bundles of Property Rights[END_REF] show that the value of land under tenancy depends on the type of contractual arrangement, with the shortest contract having the highest value. [START_REF] Moss | Leasing Contract Choice: Do Transaction Characteristics Matter?[END_REF] study the bidding behaviour of a panel of Illinois producers regarding different lease types. In an experimental approach, farmers were asked to make bids using the three different contractual arrangements (share, cash, and hybrid). The results indicate that the potential return to management drives more aggressive bidding behaviour for cash leases compared to hybrid or share leases.

Empirical analysis

Sample characteristics

There are major differences between analysing farmland values and rental prices, and these shall motivate the choice of variables in the hedonic model. Quoting (Palmquist and Danielson 1989, 55) "When people rent land, their only interest will be in the current productive capabilities of the land, although the lease may require them to protect the interests of the landowner. The value of land as an asset depends on the present value of future rents. The land may be used for different purposes in the future, so different characteristics may be relevant. These characteristics would then influence asset value but not rental value. For example, proximity of farmland to a major population center might increase land values even though it did not increase agricultural productivity. In the same vein, a characteristic that is of value in agricultural use, such as soil productivity, may be discounted in the asset price if that characteristic is not as highly valued in some alternative use (for example, commercial use) that is anticipated in the near future." The variables used in our model are presented inErreur ! Source du renvoi introuvable. and their summary statistics in Table 2. The average rental price per hectare of land is $1,239, ranging from $160 to $2,571. The average surface of plots is 119 hectare. In our study, 42% of the plots are located in the province of Buenos Aires and 58% in the Province of Santa Fé. Although these two provinces are quite representative of Pampas agriculture, the former is more urbanized. Indeed, its capital city Junín is closer to the city of Buenos Aires than San Justo (Santa Fé). In the latter, livestock is more developed. As noted by (Choumert and Phélinas 2015b), farmland parcels in the province of Buenos Aires are more valued than those situated in Santa Fé, notably because the province benefits from better infrastructure (such as roads), greater commercial and residential development, and better accessibility to major markets. As far as agricultural potential is concerned, the two districts differ more in the quality of their soils than in their climate. The territorial scan carried out by the project CLARIS LPB showed that 80% of Junin's soils are suitable for agriculture and have a very high productive potential whereas the proportion of land for agricultural use is only 44% in San Justo, among which, only 29% have a high productive capacity (V. [START_REF] Hernandez | Confronting Farmers' Perceptions of Climatic Vulnerability with Observed Relationships between Yields and Climate Variability in Central Argentina[END_REF]. Nonetheless, both provinces share the same climatic environment with mean length of dry spells as well as mean length of wet spells being nearly the same (V. [START_REF] Hernandez | Confronting Farmers' Perceptions of Climatic Vulnerability with Observed Relationships between Yields and Climate Variability in Central Argentina[END_REF]. These results are consistent with previous studies conducted in the Pampas region [START_REF] Magrin | Changes in Climate and Crop Production During the 20th Century in Argentina[END_REF][START_REF] Van Dam | Large-Scale Bioenergy Production from Soybeans and Switchgrass in Argentina[END_REF].

The sample of plots contains various land types (arable land, grassland), which are rented at different prices. However, most contracted land is used for grain or soybean production. In a majority of plots, farmers grow soybeans (72%). Wheat is grown on 20% of plots and corn on 23%. Only 15% of plots have been rented for cattle ranching or dairy production. 8 We introduce measures of yields for the three crops and a binary variable for livestock activity. The measure of soybean yields allows us to take the expected return of land cultivation into account.

In our sample, family, neighbours, and local tenants dominate the land rental market, whereas sowing pools, which are agricultural trusts seeking to lease tracts of land temporarily, appear to be a minor actor. This is in line with [START_REF] Urcola | Land Tenancy, Soybean, Actors and Transformations in the Pampas: A District Balance[END_REF], where they analyse the land leasing market in Balcarce (province of Buenos Aires).

Empirical analysis

In line with the literature on hedonic models, we estimate two functional forms, that is log-lin and log-log.9 Plots are heterogeneous goods. This heterogeneity can create heteroscedasticity in the residuals of the estimation of the hedonic price function. We actually detect heteroscedasticity in our model; hence, we estimate a model with robust standard errors. Given that multicollinearity is a frequent concern in hedonic studies, we verify variance inflation factors (VIF) to detect potential collinearity of the regressors, and find none, as the maximum VIF is 16.

Results

The results are shown in Erreur ! Source du renvoi introuvable.. Our hedonic analysis explains around 60% of rental price variations. Marginal effects are presented in Table 4. As expected, rental prices are higher in the Buenos Aires province. Renting a plot in this province is substantially more expensive than renting one in Santa Fé. This result is in line with the literature on land rental prices which demonstrates that the productive potential of the land is one of the main factor that affect land rents. Indeed previous studies, such as (V. [START_REF] Hernandez | Confronting Farmers' Perceptions of Climatic Vulnerability with Observed Relationships between Yields and Climate Variability in Central Argentina[END_REF], have highlighted the productive and soil quality differentials between those provinces. This result also supports that localization is an important factor explaining land prices and corroborates the idea that better access to infrastructure and proximity to major markets are capitalized in rental prices.

Farmland rental values fluctuate according to the profitability of what is being produced on a rented plot, which, in turn, determine the income that can be generated from the parcel. As expected, we observe a variation in rental prices as a function of soybean yield, which is a good proxy for the expected market return of the plot, with soybeans being the most profitable crop. Similarly, allocating land to cattle production exerts a negative influence on farmland rents. Lower output prices and/or lower profitability explain this result.

The rental price per hectare tends to be negatively correlated with the surface of parcels. This is a standard result in hedonic studies. It also supports the inverse farm size-productivity relationship and its land market expression [START_REF] Barrett | On Price Risk and the Inverse Farm Size-Productivity Relationship[END_REF][START_REF] Carter | Identification of the Inverse Relationship between Farm Size and Productivity: An Empirical Analysis of Peasant Agricultural Production[END_REF].

The identity of contracting parties matters. The level of the rent is significantly affected by the nature of the relationships between the parties (familial or vicinity) and the geographical distance between landlords and tenants. Rental rates are much lower when the lease contract is concluded between family members or within the local network of neighbours and acquaintances, than they are when the lease contract is concluded with societies, such as sowing pools. The latter are charged the highest rental price. Several factors drive these results. First, sowing pools have a strong motivation to secure access to land, without which they have no business. Since these societies usually lease services for the main farming operations (planting, spraying and harvesting) from professional service providers who are frequently agronomists, the land benefits from the best technical expertise and is expected to provide a high return. In addition, societies, which often operate in a more dispersed geographic area, including foreign countries, call upon the services of brokers to find leasing opportunities [START_REF] Urcola | Land Tenancy, Soybean, Actors and Transformations in the Pampas: A District Balance[END_REF]. This might result in a higher cost for the whole process. They also might be less informed about land rental rates in an area. Second, such results are in line with most previous studies on farmland transactions showing that transactions with family members are discounted because they are easier to enforce and the probability of morally hazardous behaviour is lower. The informational advantage of the local network, driven by long-term relationships, also plays a significant role [START_REF] Kostov | Do Buyers' Characteristics and Personal Relationships Affect Agricultural Land Prices?[END_REF][START_REF] Otsuka | Theories of Share Tenancy: A Critical Survey[END_REF][START_REF] Rainey | Arkansas Landlord Selection of Land-Leasing Contract Type and Terms[END_REF]. However, [START_REF] Bryan | Do Landlord-Tenant Relationships Influence Rental Contracts for Farmland or the Cash Rental Rate?[END_REF] do not find any impact of family relations on the magnitude of rental rates. Third, the channels through which leasing opportunities are found differ according to the physical distance separating the contracting parties. Informal local networks allow local landlords to exploit informational advantages to learn about their tenants' skill, effort, reputation and trustworthiness. Usually, landlords and tenants who live in the same area already know each other before they enter into a contractual agreement. There is, accordingly, a tendency for the resulting rent to be lower, compared to the rent agreed between contracting parties living in a different district or department.

As far as the length of the contract is concerned, we found evidence that rental prices of short-term cash contracts lay well above rental prices for multi-year cash contracts, whereas prices for share contracts appear to be the lowest. These results largely reflect differences in crop mix according to the type of contractual arrangement. There is a tendency for soybeans to be grown continuously in areas under short-term contract. Therefore, the significantly higher rent that is associated with this type of contract captures the short-term expected profits associated with soybean cultivation, which benefits both the landlord and the tenant. For the tenant, an intensive use of the soil means higher income and therefore higher capacity to pay a higher rent.

Unfortunately, insofar as the rent level is tied to the intensity of the land use, short-term contracts create disincentives to good agricultural practices or investment in soil conservation measures. Longterm productivity can be seriously diminished because of overplanting soybeans, since the benefits of chemical application have a short duration. An increase in the current year's income therefore comes at the expense of future income.

Longer cash leases are less valued than short cash leases, but not by much. This indicates that longer-term leases, which are agreed upon at the start of the lease period, and usually not re-negotiated or adjusted annually, might not reflect the current level of return in agricultural production exactly, since post-harvest commodity prices are unknown when rental agreements are made. This expectation error might be particularly strong in periods of increasing prices, as was the case in 2011, the year our field survey was conducted. A longer contractual period might also indicate mutual trust between the landlord and the tenant. The resulting lower rent could be considered as a market translation of lower enforcement costs. This argument is supported by the theory of incentive contracting (Huffmann and Just, 2004;Allen and Lueck, 1992a).

Share contracts exhibit the lowest rent, a result that is consistent with the existing theoretical literature on land-lease contracts. This result strongly supports the hypothesis that landlords who enter this type of contracts have requirements regarding land management, herbicide application, and rotation of soybeans with cereals, at the cost of a lower lease price. There is a large body of empirical evidence showing that share tenancy is preferred when landlords have the ability to monitor tenants, in order to avoid potential degradation of the land asset, among other things. Immediate income is clearly sacrificed for soil conservation. Another explanation for this result might come from differences in tenants' entrepreneurial abilities, which are not easily observable. Those endowed with higher skills may seek fixed-rent contracts that allow them to perceive a higher return on their effort. As a result, less skilled tenants might be self-selected for share contracts.

Conclusion

High international prices combined with technological advances raised returns on Argentinian agricultural production, thereby increasing the demand for farmland. As a result, land rentals increased sharply during the last decades.

In this paper we have studied the rental farmland market in the Argentinian Pampas. In particular, we question the importance of contractual arrangements in the context of rapid GM soybean development. We used a hedonic pricing model, which includes three main types of leasing contracts and controls for other characteristics of the plots leased. We provide empirical evidence that contractual arrangements are capitalized in land rental values, all things being equal.

The one-cultivation-cycle contract has the highest rental value, and the land area under this type of contract is expected to increase. Since productive orientation is clearly related to land rental practices, continuous soybean cultivation is likely to expand. Indeed, the technically recommended crop rotation is rarely followed when the land is under short-term contract. Declining ecological conditions could be a serious consequence of the expansion of this type of contract. In particular, long-term negative effects on land's productive capacity are expected, due to excessive planting of soybeans.

Since a growing share of farmland in Argentina is cultivated by tenants, our results suggest that policy recommendations to minimize the adverse environmental impacts of changes in land tenure are needed. If Argentina intends to protect its enormous natural advantage for agricultural production, a strict regulation of land rental contracts, with respect to length of term and land management, should be considered. Another alternative would be to support the relative profitability of other crops through export taxes and/or input cost reduction.

Table 1 .

 1 Description of variables

	VARIABLES	DESCRIPTION

Table 2 .

 2 Descriptive statistics

	Variable	Mean	Std. Dev.	Min	Max
	Rent	1238.89	499.65	160	2571
	Ln_Rent	7.03	0.45	5.08	7.85
	Buenos Aires	0.42	0.49	0	1
	Santa Fe	0.58	0.49	0	1
	Surface_plot	118.93	131.74	7	998
	Wheat_yield	0.83	1.83	0	10
	Corn_yield	1.50	3.20	0	13.33
	Soybean_yield	1.97	1.42	0	4.83
	Livestock	0.15	0.36	0	1
	Tenant_farming	0.66	0.47	0	1
	Sharecropping	0.06	0.24	0	1
	Short_contract	0.28	0.45	0	1
	Relation_parent	0.22	0.41	0	1
	Relation_neighbor	0.29	0.46	0	1
	Relation_in_dep	0.36	0.48	0	1
	Relation_out_dep	0.11	0.32	0	1
	Relation_society	0.02	0.12	0	1
	N = 255

Table 3 .

 3 Results of the hedonic analysis

		(1)	(2)	(3)	(4)
	VARIABLES	ln_rent	ln_rent	ln_rent	ln_rent
	Buenos Aires	0.607***	0.616***	0.607***	0.617***
		(0.0405)	(0.0417)	(0.0404)	(0.0421)
	Surface_plot	-0.000251	-0.000435**
		(0.000172)	(0.000182)
	Wheat_yield	0.0130	0.0163	0.0120	0.0152
		(0.00975)	(0.0103)	(0.00986)	(0.0104)
	Corn_yield	0.00219	0.00132	0.00196	0.00101
		(0.00729)	(0.00763)	(0.00725)	(0.00753)
	Soybean_yield	0.0308**	0.0419**	0.0300**	0.0413**
		(0.0149)	(0.0165)	(0.0149)	(0.0166)
	Livestock	-0.247***		-0.258***
		(0.0825)		(0.0818)
	Tenant_farming	-0.0544*	-0.0881**	-0.0542*	-0.0895**
		(0.0324)	(0.0346)	(0.0325)	(0.0351)
	Sharecropping	-0.217*	-0.219*	-0.218*	-0.219*
		(0.125)	(0.123)	(0.125)	(0.122)
	Short_contract	-	-	-	-
	Relation_parent	-0.579***	-0.525***	-0.574***	-0.509***
		(0.120)	(0.0913)	(0.121)	(0.0920)
	Relation_neighbor	-0.399***	-0.328***	-0.396***	-0.314***
		(0.0969)	(0.0530)	(0.0988)	(0.0544)
	Relation_in_dep	-0.496***	-0.442***	-0.491***	-0.425***
		(0.103)	(0.0649)	(0.105)	(0.0645)
	Relation_out_dep	-0.441***	-0.358***	-0.444***	-0.353***
		(0.107)	(0.0637)	(0.110)	(0.0670)
	Relation_society	-	-	-	-
	ln_Surface_plot			-0.0247	-0.0477*
				(0.0243)	(0.0246)
	Constant	7.294***	7.212***	7.372***	7.356***
		(0.0906)	(0.0381)	(0.110)	(0.0913)
	Observations	255	255	255	255
	R-squared	0.588	0.555	0.585	0.549

Table 4 .

 4 Marginal prices for Model 1

Significant variables Variation of the rental price per hectare due for having the characteristic (dummy) or to a one unit increase (continuous variable)

			Variation for the
			average rental price
			per hectare
		%	USD per hectare
	Buenos Aires	83.49	1034.37
	Soybean_yield	3.08	38.16
	Livestock	-21.89	-271.14
	Tenant_farming	-5.29	-65.60
	Sharecropping	-19.51	-241.67
	Relation_parent	-43.95	-544.54
	Relation_neighbor	-32.90	-407.61
	Relation_in_dep	-39.10	-484.45
	Relation_out_dep	-35.66	-441.80

The Argentinian model of GM soybean production is called "modelo sojero". For further information on its precise characteristics and it driving factors, seeBárcena et al.,

[START_REF] Fraser | Land Tenure and Agricultural Management: Soil Conservation on Rented and Owned Fields in Southwest British Columbia[END_REF][START_REF] Burachik | Experience from Use of GMOs in Argentinian Agriculture, Economy and Environment[END_REF][START_REF] Carreño | Tradeoffs between Economic and Ecosystem Services in Argentina during 50 Years of Land-Use Change[END_REF][START_REF] Choumert | Determinants of Agricultural Land Values in Argentina[END_REF][START_REF] De La Fuente | Soybean Weed Community Composition and Richness between 1995 and 2003 in the Rolling Pampas (Argentina)[END_REF][START_REF] Gavier-Pizarro | Expansion and Intensification of Row Crop Agriculture in the Pampas and Espinal of Argentina Can Reduce Ecosystem Service Provision by Changing Avian Density[END_REF][START_REF] Leguizamón | Modifying Argentina: GM Soy and Socio-Environmental Change[END_REF] Urcola et al., 2015. 2 It is difficult to assess land under tenancy since 2002 because, for different reasons, the results of the last Rural National Census conducted in 2008 are not published. The figure we mention comes from estimates given by different authors[START_REF] Delvenne | The 'soy-Ization' of Argentina: The Dynamics of the 'globalized' Privatization Regime in a Peripheral Context[END_REF][START_REF] Manciana | Large-Scale Acquisition of Land Rights for Agricultural or Natural Resource-Based Use: ARGENTINA[END_REF]. They are consistent with the share of land under different tenancy arrangements we found in our survey.

This preliminary work was conducted within the context of two research programs implemented earlier. The first one is a European program on climate, named CLARIS LPB, and the second one is a project financed by the French Agency for Research (ANR), named INTERRA.

Since tenants sometimes contract with more than on landlord, the sample unit is a contract, not an individual tenant.

In both cases, the level of the rent has been given in quintals of soybean by respondents, so that the rent for share and fixed contracts is expressed in the same unit. To obtain a value, we assumed that producers face the same price for output and used the mean of soybean prices for 2011 calculated from the time series given by the professional trade magazine "Margenes Agropecuarios."

See[START_REF] Palmquist | A Hedonic Study of the Effects of Erosion Control and Drainage on Farmland Values[END_REF], for a model on farmland values. For an overview of the empirical literature, see[START_REF] Choumert | Determinants of Agricultural Land Values in Argentina[END_REF][START_REF] Maddison | A Hedonic Analysis of Agricultural Land Prices in England and Wales[END_REF]

Another strand of the literature is focused on farmland rental market in the Pampas and use agent based modelling. This literature goes beyond the scope of our analysis. For further reading see (F.[START_REF] Bert | Agent-Based Modelling of a Rental Market for Agricultural Land in the Argentine Pampas[END_REF] F. E. Bert et al. 2014; F. E. Bert et al. 2011).

Note that farmers can grow several types of crops on a plot within an agricultural cycle.

A linear functional form would imply a constant implicit marginal price, that is independent of the level of characteristics.

Acknowledgment

The research leading to these results has received funding from the Agence Nationale de la Recherche (ANR) under the program SYSTERRA with reference ANR-09-STRA-04.

This work was supported by the LABEX IDGM+ (ANR-10-LABX-14-01) within the program "Investissements d'Avenir" operated by the French National Research Agency (ANR). http://www.cerdi.org/


Julie Vidal
email: julievidal.doc@gmail.com

Sylvie Thouësny
email: sylvie.thouesny@icall-research.net

Francesca Helm

Linda Bradley

Marta Guarda

Computer-mediated synchronous and asynchronous corrective feedback provided by trainee teachers to learners of French: a preliminary study

Keywords: corrective feedback, multimodality, synchronous, asynchronous, trainee teacher

In this paper, we investigate whether trainee teachers' practices, with respect to multimodal feedback, differ from current research, and to what extent it may affect students' language development. More specifically, the goal of the present study is threefold: (1) it observes how trainee teachers responded, whether synchronously, asynchronously, or a combination of both, to their students' incorrect language while or after interacting orally with them in French via a videoconference platform, (2) it considers the trainee teachers' beliefs regarding the efficacy of their feedback in light of semi-structured interviews, and (3) it explores the students' responses to corrective feedback received in synchronous and asynchronous settings through recorded videoconferencing sessions and interviews. The data set used for this preliminary study is drawn from a multimodal learning and teaching corpus, the InteractionS and Multimodality in lAnguagE Learning (ISMAEL) Project, a large collection of multimodal interactions and productions occurring between French trainee teachers in France and learners of French at university level in Ireland. Results show that asynchronous written feedback might strengthen the reception of oral synchronous feedback, thus leading to internalisation.

Introduction

Corrective feedback, defined as "any indication to a learner that his or her use of the target language is incorrect" (Lightbown & Spada, 1999, p. 172), may be provided to students synchronously or asynchronously, where the main contrast "lies in the timing of the feedback -namely, whether it is immediate or delayed", respectively (Shintani, 2015, p. 17). Corrective feedback may also be implicit or explicit. As specified by [START_REF] Ellis | Implicit and explicit corrective feedback and the acquisition of L2 grammar[END_REF], "[i]n the case of implicit feedback, there is no overt indicator that an error has been committed, whereas in explicit feedback types, there is" (pp. 340-341).

Advanced technology nowadays enables teachers to provide corrective feedback in several modes. Multimodality is defined by [START_REF] Siegel | New time for multimodality? Confronting the accountability culture[END_REF] as "the social practice of making meaning by combining multiple semiotic resources" (p. 671). Multimodal corrective feedback, in the context of this preliminary study, implies feedback provided to students both orally during interactive sessions and in writing in an end-of-session report, in which each feedback items could be accompanied with video excerpts from the sessions and audio recordings.

After briefly presenting the data, this paper focuses on how feedback emitted by French trainee teachers either synchronously or asynchronously was received by learners of French.

Methodology

Participants and context

The ISMAEL corpus includes multimodal interactions and productions of 19 firstyear university students in Ireland learning French as a foreign language [START_REF] Blin | Creating and sharing a language learning and teaching corpus of multimodal interactions: ethical challenges and methodological implications[END_REF]. They participated once a week during 6 weeks in 45-minute videoconferences with 12 French trainee teachers based in France. Of relevance to the present study are the recordings of 8 videoconferencing sessions involving 4 trainee teachers and 7 students, for which we have the former's immediate and delayed feedback, as well as most post semi-structured interviews for both.

Data coding and analysis

The data was coded in terms of feedback emission and reception. Our classification with respect to emission of synchronous and asynchronous corrective feedback (Table 1) is arranged from implicit to explicit and is adapted from the work of Ellis ( 2009), [START_REF] Lyster | Corrective feedback and learner uptake[END_REF][START_REF] Lyster | Corrective feedback and learner uptake[END_REF][START_REF] Zourou | Towards a typology of corrective feedback moves in an asynchronous distance language learning environment[END_REF]. The grid of feedback reception is built from our own data observation (Table 2).

Results and discussion

Trainee teachers' responses to students' incorrect forms

Table 3 shows that trainee teacher Adèle, for instance, identified 29 + 13 errors while interacting with her students, thus providing 42 synchronous opportunities for learners to notice their incorrect forms, from which 29 incorrect forms were further detailed in her written report. In addition, Adèle described 3 errors in her written report, for which she did not provide any synchronous explanations. In general terms, trainee teachers provided more synchronous -or a combination of synchronous and asynchronous -corrective feedback rather than asynchronous only.

Table 3. Incorrect forms identified by trainee teachers

Trainee teachers' beliefs regarding feedback strategies

The interviews with the trainee teachers reveal that their beliefs with respect to feedback strategies are not always in alignment with their actual practices. For instance, Victor claimed that he deliberately chose not to offer any synchronous feedback while interacting with his student. Yet, we count as much as 83.24% of synchronous feedback compared to 16.66% of exclusive asynchronous feedback. This statement is in line with numerous studies which have found that teachers' beliefs with respect to corrective feedback in educational settings and their actual practices were not always aligned (e.g. [START_REF] Junqueira | I just want to do it right, but it's so hard": a novice teacher's written feedback beliefs and practices[END_REF][START_REF] Roothooft | The relationship between adult EFL teachers' oral feedback practices and their beliefs[END_REF]. For instance, [START_REF] Roothooft | The relationship between adult EFL teachers' oral feedback practices and their beliefs[END_REF] observes that while teachers trust it is important not to disrupt the students' flow of conversation and prefer to leave an error uncorrected, they are convinced that providing oral corrective feedback is an essential stage in language learning; they seem to be unaware of the amount of feedback they actually provide to students while interacting with them.

Students' responses to corrective feedback

Table 4 displays the tendency of how feedback was received while emitted synchronously. For instance, trainee teachers placed 17 markers (E0) -indicating their intention of asynchronous feedback -, which naturally were not perceived by students (R0). Although it is worth mentioning that no feedback was provided at levels E3, E4, and E5, and that almost half of the feedback was provided at level E2 with only 20.83% of feedback received and properly re-employed (R6), our intention is not to infer that to ensure feedback is received 100 percent of the time (R6), they must be emitted at level E7. As [START_REF] Lantolf | Dynamic assessment in the classroom: Vygotskian praxis for second language development[END_REF] argue, "if the instructional aim is simply to help learners arrive at a correct response, then explicit feedback is certainly an efficient means. However, [...] if the intention is to promote development then process must be foregrounded, as in the [zone of proximal development]" (p. 17).

Table 4. Synchronous feedback emission/reception

In line with sociocultural perspectives, we noted that some corrective feedback (25 in total) was provided as a discussion, where meaning was negotiated, rather than as standalone pieces of information on one particular linguistic aspect. One discussion, for instance, evolved around the word bailler (to yawn). Mélissa, a trainee teacher, asked Ana (student #1) for clarification as she did not understand the student's request. Ana, understanding the trainee teacher's recast, repeated herself (E2/R6). Mélissa asked this time for clarification in writing using the chat modality. Ana understood the task but requested help with respect to spelling (E2/ R4). The trainee teacher further requested clarification before seeing what the student meant and proposing the correct form, whose suggestion was straight away accepted by Ana and successfully re-employed (E6/R6). At this stage, Alejandra (student #2) seemed to grasp the meaning of the word with a smile and a timid "yes" (E6/R3). The trainee teacher then went on with Alejandra and offered her the opportunity to repeat and use the word, though this did not happen, as the student only confirmed she understood (E2/R3). While in general, and more particularly in this example, feedback emission (when provided as a set of interactions) tends to go from implicit to explicit, our certitude that the feedback is actually received and at what level is, however, not predictable.

With respect to asynchronous feedback, our dataset shows that more than half of the feedback provided were at level E2, the rest being mostly offered at level E6/E7 (E2=65.55%; E6=15.2%; E7=7.6%; positive feedback related to negative synchronous feedback=1.9%). From Ana's post interview (student #1), we know that the verb bailler (to yawn), further explained in her end-ofsession written report, is now fully internalised, thus confirming her reception at level R6 while interacting synchronously. However, the level of reception for Alejandra (student #2) could not be further investigated as this linguistic point was not part of her report.

Conclusion

It is worth recalling that the findings of this preliminary study are drawn from the data of a small sample, thus implying that further research is necessary to shed a clearer light on the relation between emission and reception of synchronous and asynchronous feedback.

Although trainee teachers' beliefs with respect to feedback and their actual practices were not always aligned, they tried to make the most of the situation to students notice their incorrect forms. They all provided synchronous oral corrective feedback during the sessions and gave a more detailed account to some of the errors in their end-of-session written reports.

Either provided synchronously or asynchronously, feedback was mostly emitted at a very implicit or explicit level. Only 1 in 5 implicit feedback was re-employed at the time of the interactions. However, it is worth mentioning that a feedback labeled with a lower reception level does not infer that it was not received, it just indicates that we do not know whether or not it was noticed for later re-use, for instance, in the same or subsequent sessions; this would need further investigation.

Finally, students in general -Ana in particular -were pleased with the additional explanations provided in the end-of-session written reports. While being post interviewed, students talked about their incorrect forms occurring in their reports, which clearly shows that the correct forms for most of them are now fully internalised.

Table 1 .

 1 Feedback emission

Table 2 .

 2 Feedback reception

See Guichon, Betrancourt, and Prié (2012).

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