A Formal Specification of Dynamic Protocols for Open Agent Systems

Open MAS can be viewed as instances of normative systems [46]. A feature of this type of system is that actuality, what is the case, and ideality, what ought to be the case, do not necessarily coincide. Therefore, it is essential to specify what is permitted, prohibited, and obligatory, and perhaps other more complex normative relations that may exist between the agents. Among these relations, considerable emphasis has been placed on the representation of institutional power [47]. This is a standard feature of any normative system whereby designated agents, when acting in specified roles, are empowered by an institution to create specific relations or states of affairs. Consider, for example, the case in which an agent is empowered by an institution to award a contract and thereby create a bundle of normative relations between the contracting parties.

Several approaches have been proposed in the literature for the specification of open MAS. The majority of these approaches offer ‘static’ specifications, that is, there is no support for run-time specification modification. In some open MAS, however, environmental, social or other conditions may favour, or even require, specifications that are modifiable during the system execution. Consider, for instance, the case of a malfunction of a large number of sensors in a sensor network, or the case of manipulation of a voting procedure due to strategic voting, or when an organisation conducts its business in an inefficient manner. Therefore, we present in this paper an infrastructure for ‘dynamic’ specifications, that is, specifications that are developed at design-time but may be modified at run-time by the members of a system. The presented infrastructure is an extension of our work on static specifications [5,7], and is motivated by ‘dynamic argument systems’ [11]. These are argument systems in which, at any point in the disputation, agents may start a meta level debate, that is, the rules of order become the current point of discussion, with the intention of altering these rules.

Our infrastructure for dynamic specifications allows agents to alter the specification of a protocol P during the protocol execution. P is considered an ‘object’ protocol; at any point in time during the execution of the object protocol the participants may start a ‘meta’ protocol in order to decide whether the object protocol specification should be modified. Moreover, the participants of the meta protocol may initiate a meta-meta protocol to decide whether to modify the specification of the meta protocol, or they may initiate a meta-meta-meta protocol to modify the specification of the meta-meta protocol, and so on.

Unlike existing approaches on dynamic specifications, we place emphasis on the procedure with which agents initiate a meta protocol. We distinguish between successful and unsuccessful attempts to initiate a meta protocol by identifying the conditions in which an agent has the institutional power to propose a specification change. We evaluate an agent’s proposal for specification change by modelling a dynamic specification as a metric space [13], and by taking into consideration the effects of accepting a proposal on system utility. We constrain the enactment of proposals that do not meet the evaluation criteria. Furthermore, we formalise procedures for role-assignment in a meta level, that is, we specify which agents may participate in a meta protocol, and the roles they may occupy in the meta protocol.

We employ a resource-sharing protocol to illustrate our infrastructure for dynamic specifications: the object protocol concerns resource-sharing while the meta protocols are voting protocols. In other words, at any time during a resource-sharing procedure the agents may vote to change the rules that govern the management of resources. The resource-sharing protocol was chosen for the sake of providing a concrete example. In general, the object protocol may be any protocol for open MAS, such as a protocol for coordination or e-commerce. Similarly, a meta protocol can be any procedure for decision-making over specification modification (argumentation, negotiation, and so on).

We encode dynamic MAS specifications in executable action languages. In this paper we employ the action language C + [35], a formalism with explicit transition system semantics. The C + language, when used with its associa

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