620 research outputs found

    Analyzing the Interaction between Knowledge and Social Commitments in Multi-Agent Systems

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    Both knowledge and social commitments in Multi-Agent Systems (MASs) have long been under research independently, especially for agent communication. Plenty of work has been carried out to define their semantics. However, in concrete applications such as business settings and web-based applications, agents should reason about their knowledge and their social commitments at the same time, particularly when they are engaged in conversations. In fact, studying the interaction between knowledge and social commitments is still in its beginnings. Therefore, in this thesis, we aim to provide a practical and formal framework that analyzes the interaction between knowledge and communicative social commitments in MASs from the semantics, model checking, complexity, soundness and completeness perspectives. To investigate such an interaction, we, first, combine CTLK (an extension of computation Tree Logic (CTL) with modality for reasoning about knowledge) and CTLC (an extension of CTL with modalities for reasoning about commitments and their fulfillments) in one new logic named CTLKC. By so doing, we identify some paradoxes in the new logic showing that simply combining current versions of commitment and knowledge logics results in a language of logic that violates some fundamental intuitions. Consequently, we propose CTLKC+, a new consistent logic of knowledge and commitments that fixes the identified paradoxes and allows us to reason about social commitments and knowledge simultaneously in a consistent manner. Second, we use correspondence theory for modal logics to prove the soundness and completeness of CTLKC+. To do so, we develop a set of reasoning postulates in CTLKC+ and correspond them to certain classes of frames. The existence of such correspondence allows us to prove that the logic generated by any subset of these postulates is sound and complete, with respect to the models that are based on the corresponding frames. Third, we address the problem of model checking CTLKC+ by transforming it to the problem of model checking GCTL∗ (a generalized version of Extended Computation Tree Logic (CTL∗) with action formulas) and ARCTL (the combination of CTL with action formulas) in order to respectively use the CWB-NC automata-based model checker and the extended NuSMV symbolic model checker. Moreover, we prove that the transformation techniques are sound. Fourth, we analyze the complexity of the proposed model checking techniques. The results of this analysis reveal that the complexity of our transformation procedures is PSPACE-complete for local concurrent programs with respect to the size of these programs and the length of the formula being checked. From the time perspective, we prove that the complexity of the proposed approaches is P-complete with regard to the size of the model and length of the formula. Finally, we implement our model checking approaches and report some experimental results by verifying the well-known NetBell payment protocol against some desirable properties

    Standpoint Logic: A Logic for Handling Semantic Variability, with Applications to Forestry Information

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    It is widely accepted that most natural language expressions do not have precise universally agreed definitions that fix their meanings. Except in the case of certain technical terminology, humans use terms in a variety of ways that are adapted to different contexts and perspectives. Hence, even when conversation participants share the same vocabulary and agree on fundamental taxonomic relationships (such as subsumption and mutual exclusivity), their view on the specific meaning of terms may differ significantly. Moreover, even individuals themselves may not hold permanent points of view, but rather adopt different semantics depending on the particular features of the situation and what they wish to communicate. In this thesis, we analyse logical and representational aspects of the semantic variability of natural language terms. In particular, we aim to provide a formal language adequate for reasoning in settings where different agents may adopt particular standpoints or perspectives, thereby narrowing the semantic variability of the vague language predicates in different ways. For that purpose, we present standpoint logic, a framework for interpreting languages in the presence of semantic variability. We build on supervaluationist accounts of vagueness, which explain linguistic indeterminacy in terms of a collection of possible interpretations of the terms of the language (precisifications). This is extended by adding the notion of standpoint, which intuitively corresponds to a particular point of view on how to interpret vague terminology, and may be taken by a person or institution in a relevant context. A standpoint is modelled by sets of precisifications compatible with that point of view and does not need to be fully precise. In this way, standpoint logic allows one to articulate finely grained and structured stipulations of the varieties of interpretation that can be given to a vague concept or a set of related concepts and also provides means to express relationships between different systems of interpretation. After the specification of precisifications and standpoints and the consideration of the relevant notions of truth and validity, a multi-modal logic language for describing standpoints is presented. The language includes a modal operator for each standpoint, such that \standb{s}\phi means that a proposition ϕ\phi is unequivocally true according to the standpoint ss --- i.e.\ ϕ\phi is true at all precisifications compatible with ss. We provide the logic with a Kripke semantics and examine the characteristics of its intended models. Furthermore, we prove the soundness, completeness and decidability of standpoint logic with an underlying propositional language, and show that the satisfiability problem is NP-complete. We subsequently illustrate how this language can be used to represent logical properties and connections between alternative partial models of a domain and different accounts of the semantics of terms. As proof of concept, we explore the application of our formal framework to the domain of forestry, and in particular, we focus on the semantic variability of `forest'. In this scenario, the problematic arising of the assignation of different meanings has been repeatedly reported in the literature, and it is especially relevant in the context of the unprecedented scale of publicly available geographic data, where information and databases, even when ostensibly linked to ontologies, may present substantial semantic variation, which obstructs interoperability and confounds knowledge exchange

    Model Checking Trust-based Multi-Agent Systems

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    Trust has been the focus of many research projects, both theoretical and practical, in the recent years, particularly in domains where open multi-agent technologies are applied (e.g., Internet-based markets, Information retrieval, etc.). The importance of trust in such domains arises mainly because it provides a social control that regulates the relationships and interactions among agents. Despite the growing number of various multi-agent applications, they still encounter many challenges in their formal modeling and the verification of agents’ behaviors. Many formalisms and approaches that facilitate the specifications of trust in Multi-Agent Systems (MASs) can be found in the literature. However, most of these approaches focus on the cognitive side of trust where the trusting entity is normally capable of exhibiting properties about beliefs, desires, and intentions. Hence, the trust is considered as a belief of an agent (the truster) involving ability and willingness of the trustee to perform some actions for the truster. Nevertheless, in open MASs, entities can join and leave the interactions at any time. This means MASs will actually provide no guarantee about the behavior of their agents, which makes the capability of reasoning about trust and checking the existence of untrusted computations highly desired. This thesis aims to address the problem of modeling and verifying at design time trust in MASs by (1) considering a cognitive-independent view of trust where trust ingredients are seen from a non-epistemic angle, (2) introducing a logical language named Trust Computation Tree Logic (TCTL), which extends CTL with preconditional, conditional, and graded trust operators along with a set of reasoning postulates in order to explore its capabilities, (3) proposing a new accessibility relation which is needed to define the semantics of the trust modal operators. This accessibility relation is defined so that it captures the intuition of trust while being easily computable, (4) investigating the most intuitive and efficient algorithm for computing the trust set by developing, implementing, and experimenting different model checking techniques in order to compare between them in terms of memory consumption, efficiency, and scalability with regard to the number of considered agents, (5) evaluating the performance of the model checking techniques by analyzing the time and space complexity. The approach has been applied to different application domains to evaluate its computational performance and scalability. The obtained results reveal the effectiveness of the proposed approach, making it a promising methodology in practice

    Specifying and verifying communities of Web services using argumentative agents

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    This thesis includes two main contributions: the first one is specifying the use of argumentative agents in the design and development of communities of Web services; the second is using a formal technique to verify communication protocols against given properties for these communities. Web services that provide a similar functionality are gathered into a single community, independently of their origins, locations, and ways of doing. Associating Web services with argumentative agents that are able to persuade and negotiate with others organizes these Web services in a better way so that they can achieve the goals they set in an efficient way. A community is led by a master component, which is responsible among others for attracting new Web services to the community, retaining existing Web services in the community, and identifying the Web services in the community that will participate in composite scenarios. Besides FIPA-ACL, argumentative dialogue games are also used for agent interaction. In this thesis, we use tableau-based model checking algorithm to verify our argumentative agent-base community of Web services negotiation protocol. This algorithm aims at verifying systems designed as a set of autonomous interacting agents. We provide the soundness, completeness, termination and complexity results. We also simulate our specification with Jadex BDI programming language and implement our verification with a modified and enhanced version of CWB-NC model checker. Keywords. Multi-agent systems, BDI agent architecture, model checking, agent oriented programming, FIPA-ACL, dialogue game, agent-based negotiation protocol, Jadex, CWB-NC

    DFKI publications : the first four years ; 1990 - 1993

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    Model Checking Logics of Social Commitments for Agent Communication

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    This thesis is about specifying and verifying communications among autonomous and possibly heterogeneous agents, which are the key principle for constructing effective open multi-agent systems (MASs). Effective systems are those that successfully achieve applicability, feasibility, error-freeness and balance between expressiveness and verification efficiency aspects. Over the last two decades, the MAS community has advocated social commitments, which successfully provide a powerful representation for modeling communications in the figure of business contracts from one agent to another. While modeling communications using commitments provides a fundamental basis for capturing flexible communications and helps address the challenge of ensuring compliance with specifications, the designers and business process modelers of the system as a whole cannot guarantee that an agent complies with its commitments as supposed to or at least not wantonly violate or cancel them. They may still wish to first formulate the notion of commitment-based protocols that regulate communications among agents and then establish formal verification (e.g., model checking) by which compliance verification in those protocols is possible. In this thesis, we address the aforementioned challenges by firstly developing a new branching-time temporal logic---called ACTL*c---that extends CTL* with modal operators for representing and reasoning about commitments and all associated actions. The proposed semantics for ACL (agent communication language) messages in terms of commitments and their actions is formal, declarative, meaningful, verifiable and semi-computationally grounded. We use ACTL*c to derive a new specification language of commitment-based protocols, which is expressive and suitable for model checking. We introduce a reduction method to formally transform the problem of model checking ACTL*c to the problem of model checking GCTL* so that the use of the CWB-NC model checker is possible. We prove the soundness of our reduction method and implement it on top of CWB-NC. To check the effectiveness of our reduction method, we report the verification results of the NetBill protocol and Contract Net protocol against some properties. In addition to the reduction method, we develop a new symbolic algorithm to perform model checking ACTL*c. To balance between expressiveness and verification efficiency, we secondly adopt a refined fragment of ACTL*c, called CTLC, an extension of CTL with modalities for commitments and their fulfillment. We extend the formalism of interpreted systems introduced to develop MASs with shared and unshared variables and considered agents' local states in the definition of a full-computationally grounded semantics for ACL messages using commitments. We present reasonable axioms of commitment and fulfillment modalities. In our verification technique, the problem of model checking CTLC is reduced into the problems of model checking ARCTL and GCTL* so that respectively extended NuSMV and CWB-NC (as a benchmark) are usable. We prove the soundness of our reduction methods and then implement them on top of the extended NuSMV and CWB-NC model checkers. To evaluate the effectiveness of our reduction methods, we verified the correctness of two business case studies. We finally proceed to develop a new symbolic model checking algorithm to directly verify commitments and their fulfillment and commitment-based protocols. We analyze the time complexity of CTLC model checking for explicit models and its space complexity for concurrent programs that provide compact representations. We prove that although CTLC extends CTL, their model checking algorithms still have the same time complexity for explicit models, and the same space complexity for concurrent programs. We fully implement the proposed algorithm on top of MCMAS, a model checker for the verification of MASs, and then check its efficiency and scalability using an industrial case study

    Reducing model checking commitments for agent communication to model checking ARCTL and GCTL*

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    Social commitments have been extensively and effectively used to represent and model business contracts among autonomous agents having competing objectives in a variety of areas (e.g., modeling business processes and commitment-based protocols). However, the formal verification of social commitments and their fulfillment is still an active research topic. This paper presents CTLC+ that modifies CTLC, a temporal logic of commitments for agent communication that extends computation tree logic (CTL) logic to allow reasoning about communicating commitments and their fulfillment. The verification technique is based on reducing the problem of model checking CTLC+ into the problem of model checking ARCTL (the combination of CTL with action formulae) and the problem of model checking GCTL* (a generalized version of CTL* with action formulae) in order to respectively use the extended NuSMV symbolic model checker and the CWB-NC automata-based model checker as a benchmark. We also prove that the reduction techniques are sound and the complexity of model checking CTLC+ for concurrent programs with respect to the size of the components of these programs and the length of the formula is PSPACE-complete. This matches the complexity of model checking CTL for concurrent programs as shown by Kupferman et al. We finally provide two case studies taken from business domain along with their respective implementations and experimental results to illustrate the effectiveness and efficiency of the proposed technique. The first one is about the NetBill protocol and the second one considers the Contract Net protocol

    Modeling and Verifying Probabilistic Social Commitments in Multi-Agent Systems

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    Interaction among autonomous agents in Multi-Agent Systems (MASs) is the key aspect for solving complex problems that an individual agent cannot handle alone. In this context, social approaches, as opposed to the mental approaches, have recently received a considerable attention in the area of agent communication. They exploit observable social commitments to develop a verifiable formal semantics by which communication protocols can be specified. However, existing approaches for defining social commitments tend to assume an absolute guarantee of correctness so that systems run in a certain manner. That is, social commitments have always been modeled with the assumption of certainty. Moreover, the widespread use of MASs increases the interest to explore the interactions between different aspects of the participating agents such as the interaction between agents’ knowledge and social commitments in the presence of uncertainty. This results in having a gap, in the literature of agent communication, on modeling and verifying social commitments in probabilistic settings. In this thesis, we aim to address the above-mentioned problems by presenting a practical formal framework that is capable of handling the problem of uncertainty in social commitments. First, we develop an approach for representing, reasoning about, and verifying probabilistic social commitments in MASs. This includes defining a new logic called the probabilistic logic of commitments (PCTLC), and a reduction-based model checking procedure for verifying the proposed logic. In the reduction technique, the problem of model checking PCTLC is transformed into the problem of model checking PCTL so that the use of the PRISM (Probabilistic Symbolic Model Checker) is made possible. Formulae of PCTLC are interpreted over an extended version of the probabilistic interpreted systems formalism. Second, we extend the work we proposed for probabilistic social commitments to be able to capture and verify the interactions between knowledge and commitments. Properties representing the interactions between the two aspects are expressed in a new developed logic called the probabilistic logic of knowledge and commitment (PCTLkc). Third, we develop an adequate semantics for the group social commitments, for the first time in the literature, and integrate it into the framework. We then introduce an improved version of PCTLkc and extend it with operators for the group knowledge and group social commitments. The new refined logic is called PCTLkc+. In each of the latter stages, we respectively develop a new version of the probabilistic interpreted systems over which the presented logic is interpreted, and introduce a new reduction-based verification technique to verify the proposed logic. To evaluate our proposed work, we implement the proposed verification techniques on top of the PRISM model checker and apply them on several case studies. The results demonstrate the usefulness and effectiveness of our proposed work

    Comodo: Collaborative Monitoring of Commitment Delegations

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    Understanding accountability in contract violations, e.g., whom is accountable for what, is a tedious, time-consuming, and costly task for human decision-making, especially when contractual responsibilities are delegated among parties. Intelligent software agents equipped with expert capabilities such as monitoring and diagnosis help save time and improve accuracy of diagnosis by formal reasoning upon electronic contracts. Such contracts are represented as commitment norms, a well studied artifact in multi-agent systems, which provide semantics for agent interactions. Due to the open and heterogeneous nature of multi-agent systems, commitments are often violated. When a commitment is violated, e.g., an exception occurs, agents need to collaborate to understand what went wrong and which agent is responsible. We propose Comodo: a framework for monitoring commitment delegations and detecting violations. We define a complete set of possible rational delegation schemes for commitments, identifying for each combination of delegations what critical situations may lead to an improper delegation and potentially to a commitment violation. Comodo provides a sound and complete distributed reasoning procedure that is able to find all improper delegations of a given commitment. We provide the complete implementation of Comodo using the Reactive Event Calculus, and present an e-commerce case study to demonstrate its workings. Due to its generic nature, we discuss the application of our approach to other distributed diagnosis problems in emergency healthcare, Internet of Things and smart environments, and security, privacy, and accountability in the context of socio-technical system
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