Multiagent Systems and Societies of Agents

1. Multiagent Systems and Societies of Agents Authors: Michael N. Huhns and Larry M. Stephens Speaker: Lin Xu (part I) and Shabbir Syed (part II) CSCE 976, April 3rd 2002

2. Outline • Introduction • Agent communications • Coordination • Dimensions of meaning • Message types • Communication levels • Speech acts, KQML, KIF, Ontology, other • Agent interaction protocols • Societies of agents • Conclusions

3. Introduction How to analyze, describe, and design environments in which agents can operate effectively and interact with each other productively. • Communication protocols [Xu Lin] Enable agents to exchange and understand messages • Interaction protocols [Shabbir Syed] Enable agents to have conversations, which are structured exchanges of messages

4. Communication protocols • Enable agents to exchange and understand messages • The messages can be exchanged between two agents: • Propose a course of action • Accept a course of action • Reject a course of action • Retract a course of action • Disagree with a proposed course of action • Counter-propose a course of action

5. Interaction protocols • Enable agents to have conversations, which are structured exchanges of messages • Negotiation can occur between Agent1 and Agent2 • Agent1 proposes a course of action to Agent2 • Agent2 evaluates the proposal and • Sends acceptance to Agent1 or • Sends counterproposal to Agent1 or • Sends disagreement to Agent1 or • Sends rejection to Agent1

6. Motivation Centralized solutions are generally more efficient, why should we interested in distribution system? • Easier to understand and easier to develop, when the problem being solved is itself distributed. • Lead to computational algorithms that might not have been discovered with a centralized approach. • A centralized approach is impossible. • Respect real conditions: privacy of agents, distribution

7. Characteristics of Multiagent Environments • Provide an infrastructure specifying communication and interaction protocols • Typically open and have no centralized design • Contain agents that are autonomous and distributed, and may be self-interested or cooperative

8. Agent Communications An agent is an active object with the ability to perceive, reason, and act • An agent has explicitly represented knowledge and a mechanism for operating on or drawing inferences from its knowledge • An agent has the ability to communicate (receiving messages and sending messages)

9. Communications • Coordination • Dimensions of meaning • Message types • Communication levels • Examples: • Speech acts • KQML • KIF • Ontologies • Other…

10. Coordination A property of a system of agents performing some activity in a shared environment • Avoid extraneous activity by reducing resource contention • Avoid livelock and deadlock • Maintain applicable safety conditions Cooperation: Among non-antagonistic agents Negotiation: Among competitive/self-interested agents

11. Ways for coordinating behavior and activities among agents

12. How well a system behaves as a unit? • How it can maintain global coherence without explicit global control • Be able to determine on their own goals they share with other agents • Determine common task • Avoid unnecessary conflicts • Pool knowledge and evidence • Some organization among the agents is needed

13. Dimensions of meaning • Three aspects to the formal study of communication: • Syntax: how the symbols of communication are structured • Semantics: what the symbol denote • Pragmatics: how the symbol are interpreted • Meaning is a combination of semantics and pragmatics

14. Different dimensions of meaning associated with communication • Descriptive vs. Prescriptive • Personal vs. Conventional meaning • Subjective vs. Objective meaning • Speaker’s vs. Hearers’s vs. Society’s Perspective • Semantics vs. Pragmatics • Contextually • Coverage • Identity • Cardinality

15. Message types Two basic message types: assertions and queries • Basic agent: accept assertions • Passive role (answer questions): accept a query, send a reply, accept information • Active role: issue queries, make assertions, accept assertion • Peer: assume both active and passive role in dialog

16. Message types Two basic message types: assertions and queries Dialogue vs. Function Active Master Passive Slave Both Both

17. Communication levels Communication protocols are typically specified at several levels: • Lowest level: specifies the method of interconnection • Middle level: specifies the format, or syntax, of the information being transferred. • Top level: specifies the meaning, or semantics, of the information.

18. Communication levels (cont’d) There are both binary and n-ary communication protocols: • Binary: a single sender and a single receiver • N-ary: a single sender and multiple receivers A protocol is specified by a data structure with 5 fields: • Sender • Receiver(s) • Language in the protocol • Encoding and decoding functions • Actions to be taken by the receiver(s)

19. Speech Act (I) • A popular basis for analyzing human communication is speech act theory • Speech act theory views human natural language as actions • Spoken human communication is used as the model for communication among computational agents

20. Speech Act (II) A speech act has three aspects: • Locution: the physical utterance by the speaker. • Illocution: the intended meaning of the utterance by the speaker. • Perlocution: the action that results from the locution. Speech act theory helps define the type of message by using the concept of illocutionary force, which constraints the semantics of the communication act itself

21. Knowledge Query and Manipulation Language (KQML) [Finin 94] KQML is a protocol for exchanging information and knowledge.

22. The basic KQML Information for understanding the content of the message is includes in the communication itself (KQML-performative :sender <word> :receiver <word> :language <word> :ontology <word> :content <expression> …) Syntax is Lisp-like :--)

23. Nested KQML message

24. Seven basic categories of KQML • Basic query performatives • Multiresponse query performatives • Response performatives • Generic informational performatives • Generator performatives • Capability-definition performatives • Networking performatives

25. Issues • The sender and receiver must understand the agent communication language • The ontology must be created and be accessible to the agents that are communicating • KQML must operate within a communication infrastructure that allows agents to locate each other • KQML is still a work in progress and its semantics have not been completely defined [1987]

26. Knowledge Interchange Format (KIF)[Genesereth?] • A logic language proposed as a standard to describe facts in expert systems, database, intelligent agents, etc. • Specifically designed to serve as an “interlingua” or mediator in the translation of other languages • KIF is a prefix version of first order predicate calculus with extensions to support non-monotonic reasoning and definitions. It also can be used to describe procedures.

27. Ontologies [Fikes et al.] • A specification of objects, concepts, and relationships in an area of interest • The classes and relationships must be represented in the ontology • An agent must represent its knowledge in the vocabulary of a specified ontology

28. Other communication protocols • Speech Act, KQML, KIF, Ontology in no way preclude other means by which agents can interact, communicate, and be interconnected • Once communication protocols are defined and agreed upon by a set of agents, higher level protocols can be readily implemented  Interaction Protocols

29. Questions?If not, let’s start the discussion