Multi-Agent Systems Lecture 3 University

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Multi-Agent SystemsLecture 3University
Politehnica of Bucarest2005-2006Adina Magda
Floreaadina_at_cs.pub.rohttp//turing.cs.pub.ro/bl
ia_06
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Agent communicationLecture outline

• The nature of communication
• Indirect communication
• Direct communication
• Agent Communication Languages
• Content languages
• Ontologies
• Theory of speech acts
• KQML
• FIPA and FIPA-ACL
• Interaction protocols

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1. The nature of communication

• 1.1 Human communication
• Communication is the intentional exchange of
  information brought about by the production and
  perception of signs drawn from a shared system of
  conventional signs (AIMA, RussellNorvig) ?
  language
• Action (communicative act) intentional stance
• 1.2 Component steps of communication
• Speaker Hearer
• ? Intention ? Perception
• ? Generation ? Analysis
• ? Synthesis ? Disambiguation
• ? Incorporation

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• 1.3 Artificial Communication
• low-level language vs high-level languages
• direct communication vs. indirect communication
• Agent communication/ MAS communication
• low-level communication simple signals, traces,
  low-level languages
• high-level communication - cognitive agents,
  mostly seen as intentional systems
• Communication in MAS
• Implies interaction
• The environment provides a computational
  infrastructure where interactions among agents
  take place.
• The infrastructure includes protocols for agents
  to communicate and protocols for agents to
  interact

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• Communication protocols enables agents to
  exchange and understand messages
• Interaction protocols enable agents to have
  conversations, i.e., structured exchanges of
  messages
• Aim ? Communication enables agents to
• coordinate their actions and behavior, a
  property of a MAS performing some activity in a
  shared environment
• attempt to change state of the other agents
• attempt to make the other agents perform some
  actions

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2. Indirect communication

• 2.1 Signal propagation - Manta, A. Drogoul 1993
• An agent sends a signal, which is broadcast into
  the environment, and whose intensity decreases as
  the distance decreases
• At a point x, the signal may have one of the
  following intensities
• V(x)V(x0)/dist(x,x0) or V(x)V(x0)/dist(x,x0)2

?
Reactive agents

• 2.2 Trails - L. Steels, 1995
• agents drop "radioactive crumbs" making trails
• an agent following a trail makes the trail faint
  until it disappears

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• 2.3 Blackboard systems, Barbara Hayes-Roth, 1985
• Blackboard a common area (shared memory) in
  which agents can exchange information, data,
  knowledge
• Agents initiates communication by writing info on
  the blackboard
• Agents are looking for new info, they may filter
  it
• Agents must register with a central site to
  receive an access authorization to the blackboard
• Blackboard a distributed knowledge computation
  paradigm
• Agents Knowledge sources (KS)

Blackboard
Cognitive agents
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3. Direct communication

• Sending messages
• method invocation Actors
• exchange of partial plans coordination of
  cooperative agents
• ACL Agent Communication Languages
• Need to communicate knowledge ? knowledge
  representation
• Need to understand the message in a context ?
  ontologies
• Communication is seen as an action -
  communicative acts

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3.1 Agent Communication Languages

• Concepts (distinguish ACLs from RPC, RMI or
  CORBA, ORB)
• An ACL message describes a desired state in a
  declarative language, rather than a procedure or
  method invocation
• ACLs handle propositions, rules, and actions
  instead of objects with no associated semantics -
  KR
• ACLs are mainly based on BDI theories BDI agents
  attempt to communicate their BDI states or
  attempt to alter interlocutor's BDI state
  Cognitive Agents
• ACLs are based on Speech Act Theory
  Communicative Acts
• ACLs refer to shared Ontologies
• Agent behavior and strategy drive communication
  and lead to conversations - Protocols

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• Origins of ACLs
• Knowledge Sharing Effort - DARPA, 1990
• External Interface Group - interaction between
  KBS - KQML
• Interlingua - common language of KB - KIF
• Shared, Reusable Knowledge Bases - Ontolingua

• Communication primitives
• and protocols
• Content languages
• KIF
• Prolog
• Clips
• SQL
• FIPA-SL, FIPA-KIF
• Ontologies
• DAML
• OWL

DARPA Agent Markup Language (August 2000). The
goal of the DAML effort is to develop a
language and tools to facilitate the concept of
the Semantic Web.
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• 3.2 Content languages for ACL
• (Knowledge representation)
• Description Logics (DL) - a formalism for
  expressing concepts and their interrelationships.
• In DL, concepts are organized into IS-A
  hierarchies.
• Concepts are specifications such that given an
  individual (object instance) a DL system can
  recognize the individual and determine which
  concepts it belongs to.
• DL systems also perform subsumption checking.
• Knowledge Interchange Format (KIF) - is based
  on first order predicate logic and has a
  LISP-like prefix syntax.
• KIF is capable of expressing facts and rules.
• KIF provides constructs for describing
  procedures, i.e. programs to (possibly) be
  executed by an agent.

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• Knowledge Interchange Format (KIF)
• Facts
• (salary 015-46-3946 john 72000)
• (salary 026-40-9152 michael 36000)
• (salary 415-32-4707 sam 42000)
• Asserted relation
• (gt ( (width chip1) (length chip1))
• ( (width chip2) (length chip2)))
• Rule
• (gt (and (real-number ?x)
• (even-number ?n))
• (gt (expt ?x ?n) 0))
• Procedure
• (progn (fresh-line t)
• (print "Hello!")
• (fresh-line t))

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• 3.3 Ontologies
• Ontology a specification of objects, concepts,
  and relationships in a particular domain
• It comprises a vocabulary, a domain theory and a
  conceptual schemata to describe organization and
  interpretation
• Lexicalized ontologies (WordNet, EuroWordNet,
  BalkanNet, FrameNet, MikroKosmos).
• Ontologies for knowledge representation

Person
Pupil
Stud
Empl
Sun_E
IBM_E
Joe
Alice
Person
Woman
Man
Empl
Stud
Joe
Alice
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Ontology Languages

• Wide variety of languages for Explicit
  Specification
• Graphical notations
• Semantic networks

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Ontology Languages

• Wide variety of languages for Explicit
  Specification
• Graphical notations
• Topic Maps

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Ontology Languages

• Wide variety of languages for Explicit
  Specification
• Graphical notations
• UML

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Ontology Languages

• Wide variety of languages for Explicit
  Specification
• Graphical notations
• RDF

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Ontology Languages

• Wide variety of languages for Explicit
  Specification
• Logic based
• Description Logics (e.g., OIL, DAMLOIL, OWL)
• Rules (e.g., RuleML, LP/Prolog)
• First Order Logic (e.g., KIF)

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Many languages use object oriented model based
on

• Objects/Instances/Individuals
• Elements of the domain of discourse
• Equivalent to constants in FOL
• Types/Classes/Concepts
• Sets of objects sharing certain characteristics
• Equivalent to unary predicates in FOL
• Relations/Properties/Roles
• Sets of pairs (tuples) of objects
• Equivalent to binary predicates in FOL
• Such languages are/can be
• Well understood
• Formally specified
• (Relatively) easy to use
• Amenable to machine processing

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Web Schema Languages

• Existing Web languages extended to facilitate
  content description
• XML ? XML Schema (XMLS)
• RDF ? RDF Schema (RDFS)
• XMLS not an ontology language
• Changes format of DTDs (document schemas) to be
  XML
• Adds an extensible type hierarchy
• Integers, Strings, etc.
• Can define sub-types, e.g., positive integers
• RDFS is recognisable as an ontology language
• Classes and properties
• Sub/super-classes (and properties)
• Range and domain (of properties)

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RDF and RDFS

• RDF stands for Resource Description Framework
• It is a W3C candidate recommendation
  (http//www.w3.org/RDF)
• RDF is graphical formalism ( XML syntax
  semantics)
• for representing metadata
• for describing the semantics of information in a
  machine- accessible way
• RDFS extends RDF with schema vocabulary, e.g.
• Class, Property
• type, subClassOf, subPropertyOf
• range, domain

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Problems with RDFS

• RDFS too weak to describe resources in sufficient
  detail
• No localised range and domain constraints
• Cant say that the range of hasChild is person
  when applied to persons and elephant when applied
  to elephants
• No existence/cardinality constraints
• Cant say that all instances of person have a
  mother that is also a person, or that persons
  have exactly 2 parents
• No transitive, inverse or symmetrical properties
• Cant say that isPartOf is a transitive property,
  that hasPart is the inverse of isPartOf or that
  touches is symmetrical
• Difficult to provide reasoning support
• No native reasoners for non-standard semantics
• May be possible to reason via axiomatisation

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Web Ontology Language Requirements

• Desirable features identified for Web Ontology
  Language
• Extends existing Web standards
• Such as XML, RDF, RDFS
• Easy to understand and use
• Should be based on familiar KR idioms
• Formally specified
• Of adequate expressive power
• Possible to provide automated reasoning support

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OWL - Web Ontology Language

• W3C
• OWL - designed for use by applications that need
  to process the content of information instead of
  just presenting information to humans.
• OWL facilitates greater machine interpretability
  of Web content than that supported by XML, RDF,
  and RDF Schema (RDF-S) by providing additional
  vocabulary along with a formal semantics.
• OWL has three increasingly-expressive
  sublanguages
• OWL Lite supports a classification hierarchy and
  simple constraints.
• OWL DL supports maximum expressiveness while
  retaining computational completeness (all
  conclusions are guaranteed to be computed) and
  decidability (all computations will finish in
  finite time).
• OWL Full supports maximum expressiveness and the
  syntactic freedom of RDF with no computational
  guarantees.

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OWL - Web Ontology Language

• Ontology header
• ltowlOntology rdfabout""gt
• ltrdfscommentgtAn example OWL
  ontologylt/rdfscommentgt
• ltowlpriorVersion rdfresource"http//www.w
  3.org/TR/2003/CR-owl-guide-20030818/wine"/gt
• ltowlimports rdfresource"http//www.w3.org
  /TR/2003/PR-owl-guide-20031215/food"/gt
• ltrdfslabelgtWine Ontologylt/rdfslabelgt
• Simple Named Classes
• Class, rdfssubClassOf
• ltowlClass rdfID"Winery"/gt
• ltowlClass rdfID"Region"/gt
• ltowlClass rdfID"ConsumableThing"/gt

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• 3.4 Theory of Speech Acts
• J. Austin - How to do things with words, 1962,
  J. Searle - Speech acts, 1969
• A speech act has 3 aspects
• locution physical utterance by the speaker
• illocution the intended meaning of the
  utterance by the speaker (performative)
• prelocution the action that results from the
  locution
• Alice told Tom "Would you please close the
  door"
• locution illocution content
• prelocution door closed (hopefully!)
• Illocutionary aspect - several categories
• Assertives, which inform the door is shut
• Directives, which request shut the door, can
  pelicans fly?
• Commissives, which promise something I will shut
  the door
• Permissive, which gives permission for an act
  you may shut the door
• Prohibitives, which ban some act do not shut the
  door
• Declaratives, which causes events I name you
  king of Ruritania
• Expressives, which express emotions and
  evaluations I am happy

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• 3.5 KQML - Knowledge Query and Manipulation
  Language
• A high-level, message-oriented communication
  language and protocol for information exchange,
  independent of content syntax (KIF, SQL,
  Prolog,) and application ontology
• KQML separates
• semantics of the communication protocol (domain
  independent)
• semantics of the message (domain dependent)
• 3 (conceptual) layers

Core of KQML - identity of the network
protocol with which to deliver the message -
speech act or performative Optional - content
language - ontology
Describes low level communication parameters -
identity of sender and receiver - an unique id
associated with the communication
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• Syntax S-expressions used in LISP
• KQML performatives are classified
• Queries - These performatives are used to send
  questions for evaluation somewhere.
• Generative - Used for controlling and initiating
  the exchange of messages.
• Response - Used by a agent in order reply to
  queries.
• Informational - Informational performatives are
  used to transfer information.
• Capability definition - Allows an agent to learn
  about the capabilities of other agents and to
  announce its own to the agent community.
• Networking - Networking performatives make it
  possible to pass directives to underlying
  communication layers.
• Example
• (ask-one sender joe
• receiver ibm-stock
• reply-with ibm-stock
• language PROLOG
• ontology NYSE-TICKS
• content (price ibm ?price) )

(tell sender willie receiver
joe reply-with block1 language
KIF ontology BlockWorld content (AND (Block
A)(Block B) (On A B)) )
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1. Query performatives ask-one, ask-all,
ask-if, stream-all,...
(stream-all sender willie receiver
ibm-stock content (price ?VL ?price ) )
A
ask-one(P)
B
tell(P)
B
A
stream-all(P)
B
tell(P1)
A
tell(P2)
eos
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2. Generative performatives standby, ready,
next, rest, discard, generate,...
3. Response performatives reply, sorry ...
B
A
4. Generic informational performatives tell,
untell, insert, delete, ...
5. Capability performatives advartise,
subscribe, recommend...
A
B
6. Network performatives register, unregister,
forward, route, ...
Facilitator
In fact, KQML contains only 2 types of
illocutionary acts assertives and directives
facilitator and network-related performatives (no
necessarily speech acts)
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• Facilitator agent
• an agent that performs various useful
  communication services
• maintaining a registry of service names (Agent
  Name Server)
• forwarding messages to named services
• routing messages based on content
• matchmaking between information providers and
  clients
• providing mediation and translation services

point-to-point
B
A
A
B
B
A
A
B
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• Semantics of KQML (Labrou Finin)
• Use preconditions and postconditions that govern
  the use of a performative the final state for
  the successful performance of the performative
• Uses propositional attitudes belief, knowledge,
  desire, intentions
• Preconditions the necessary states for an agent
  to send a performative and for the receiver to
  accept it and successfully process it if the
  precondition does not hold, the most likely
  response is error or sorry
• Postconditions - describe the state of the sender
  after successful utterance of a performative and
  of the receiver after the receipt and processing
  of a message
• Completion condition - the final state after a
  conversation has taken place and that the
  intention associated with the performative that
  started the conversation has been fulfilled
• Propositional attitudes
• Bel(A,P) Know(A,S) Want(A,S) Int(A,S)
• Instances of action
• Proc(A,M) SendMsg(A,B,M)

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tell(A,B,X) A states to B that A believes the
content X to be true, Bel(A,X) Pre(A) Bel(A,X)
? Know(A, Want(B, Know(B, Bel(A,X)))) Pre(B)
Int(B, Know(B, Bel(A,X))) or
?Bel(A,X) Post(A) Know(A, Know(B, Bel(A,X)))
no unsolicited information Post(B)
Know(B, Bel(A,X)) Completion Know(B,
Bel(A,X)) advertise(A,B,M) A states to B that A
can and will process the message M from B, if it
receives one Int(A, Proc(A,M)) commisive
act Pre(A) Int(Proc(A,M)) Pre(B)
NONE Post(A) Know(A, Know(B, Int(A,
Proc(A,M))) Post(B) Know(B, Int(A,
Proc(A,M))) Completion Know(B, Int(A,
Proc(A,M)))
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• 3.6 FIPA and FIPA - ACL
• Foundation for Intelligent Physical Agents,
  1996
• Goal of FIPA make available specifications that
  maximize interoperability across agent-based
  systems
• FIPA Committees ACL, agent specification,
  agent-software interaction
• As KQML, FIPA ACL is based on speech act theory
  it sees messages as communication acts (CA)
  syntax similar to KQML
• Differs in the names of CAs, set of CAs, and
  semantics

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• FIPA standard define normative specifications
  for
• agent management (or agent platform services)
• white pages via an Agent Name Server (ANS)
• yellow pages via a Directory Facilitator (DF)
• registration in a given DF defines a domain
  (agent community)
• an Agent Platform defines a logical "place"
  containing an ANS, DF, management tools, and a
  collection of agents
• interplatform communication takes place via an
  Agent Communication Channel (ACC), which defaults
  to CORBA IIOP
• agent communication language - FIPA ACL
• based on speech acts
• has a formal semantics
• also included are several predefined protocols
  (e.g., contract-net negotiation and auction
  protocols), and the concept of application-specifi
  c protocols

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• agent-software integration
• defines Agent Request Broker and Wrapper roles
• allows an agent system to integrate non-agent
  software
• details of how wrapper communicates with wrapped
  software are left to the implementor (a Wrapper
  is an agent, and communicates with other agents
  via ACL)
• several reference applications for
• personal travel assistance
• personal assistant
• network provisioning and management
• audio/video entertainment and broadcasting

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• FIPA has extended these specifications, including
  work on
• agent management support for mobility
  (identifying the relationship between this work
  and MASIF is explicitly targeted)
• an ontology service, supporting
• - translation of terms between different
  ontologies
• - downloading meanings of terms, axioms, and
  relationships between terms
• - querying for relationships between ontologies
• uploading and updating of ontologies
• additional applications, e.g., product design and
  manufacturing agents
• FIPA does not currently constrain the low-level
  implementation of agents to any great extent,
  nor, except for defining agent platform services,
  does it constrain the infrastructure a great
  deal.

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FIPA - ACL
FIPA communicative acts Informatives -
query_if, subscribe, inform, inform_if, confirm,
disconfirm, not_understood Task distribution -
request, request_whenever, cancel, agree, refuse,
failure Negotiation - cfp, propose,
accept_proposal, reject_proposal
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• FIPA-SL
• (inform sender Agent1
• receiver Agent2
• content (price good2 150)
• in-reply-to round-1
• reply-with bid03
• language S1
• ontology hp-auction
• reply-by 10
• protocol offer
• conversation-id conv-1 )

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• FIPA-SL
• (request sender (agent-identifier name i)
• receiver (set (agent-identifer
  name j)
• content ((action (agent-identifier name
  j)
• (deliver box7 (loc 10 15))))
• protocol fipa-request
• language fipa-sl
• reply-with order56 )
• (agree sender (agent-identifier name j)
• receiver (set (agent-identifer
  name i)
• content ((action (agent-identifier name
  j)
• (deliver box7 (loc 10 15))) (priority
  order56 low))
• protocol fipa-request
• language fipa-sl
• in-reply-to order56 )

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• FIPA - Semantics
• SL (Semantic Language) - a quantified,
  multi-modal logic, with modal operators
• Allows to represent
• beliefs
• uncertain beliefs
• desires
• intentions
• B ? - belief D ? - desire U ? - uncertain belief
• PG ? - intention
• Bif ? - express whether an agent has a definite
  opinion one way or another about the truth or
  falsity of ?
• Uif ? - the agent is uncertain about ?

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• FIPA - Semantics
• The semantics of a CA is specified as a set of
  SL's formulae that describe
• Feasibility preconditions - the necessary
  conditions for the sender - the sender is not
  obliged to perform the CA
• Rational effect - the effect that an agent can
  expect to occur as a result of performing the
  action it also typically specifies conditions
  that should hold true of the recipient
• The receiving agent is not required to ensure
  that the expected effect comes about
• The sender can not assume that the rational
  effect will necessary follow
• ltA, inform(B, ?)gt
• Pre BA ? ? ?BA (BifB ? ? UifB ?)
• Post BB ?

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• KQML and FIPA ACL
• The two ACLs are essentially the same
• Although FIPA ACL requires agents to have a
  limited knowledge of SL, both the ACLs do not
  have fixed semantics.
• The FIPA ACL does not provide for facilitator
  agents.
• This is a major drawback as using facilitator is
  one of the best way to overcome different systems
  using different content language and providing
  matchmaking service.
• KQML provides for brokering and recommendation
  service, whereas FIPA ACL don't really take this
  into account.

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4. Interaction protocols

• Interaction protocols enable agents to have
  conversations, i.e., structured exchanges of
  messages
• Finite automata
• Conversations in KQML
• Petri nets
• FIPA IP standards
• FIPA-query, FIPA-request, FIPA-contract-net, ...

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4.1 Finite state automata
COOL, Barbuceanu,95
ABltltask(do P)
BAltltaccept(do P)
proposeS(P)
BAltltrefuse(do P)
acceptR(P)
rejectR(P)
BAltltresult(do P)
BAltltfail(do P)
counterR(P)
counterS(P)
Winograd, Flores, 1986
rejectS(P)
acceptS(P)
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• 4.2 Conversations in KQML
• Use Definite Clause Grammars (DCG) formalism for
  the specification of conversation policies for
  KQML performatives
• DCGs extend Context Free Grammars in the
  following way
• non-terminals may be compound terms
• the body of the rule may contain procedural
  attachments, written as "" and "" that express
  extra conditions that must be satisfied for the
  rule to be valid
• Ex noun(N) ? W, RootForm(W,N),
  is_noun(N)
• S ? s(Conv, P, S, R, inR, Rw, IO, Content),
  member(P, advertise, ask-if
• s(Conv, ask-if, S, R, inR, Rw, IO, Content) ?
• ask-if, S, R, inR, Rw, IO, Content
• ask-if, S, R, inR, Rw, IO, Content, OI is
  inv(IO),
• r(Conv, ask-if, S, R, _, Rw, OI, Content)
• r(Conv, ask-if, R, S, _, inR, IO, Content) ?
• tell, S, R, inR, Rw, IO, Content
• problem(Conv, R, S, inR, _, IO)

Labrou, Finin, 1998
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4.3 Petri nets
Ferber, 1997
Petri net oriented graph with 2 type of
nodesplaces and transitions there are moving
tokens through the net - representation of
dynamic aspect of processes. Tokens are moved
from place to place, following firing rules. A
transition T is enabled if all the input places P
of T posses a token (several other rules may be
defined). A marking is a distribution of tokens
over places. Colored Petri-nets
B does not want to do(P)
A wants to do P, A cannot do P
B is willing to do(P)
Request do(P)
Refuse do(P)
Accept/request do(P)
Success
Completed(P)
Fail to do(P)
Impossible to do(P)
Notification of end(P)
FB
FA1
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Failure
FA2
Satisfaction
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• References
• M. Huhns, L. Stephens. Multiagent systems and
  societies of agents. In Multiagent Systems - A
  Modern Approach to Distributed Artificial
  Intelligence, G. Weiss (Ed.), The MIT Press,
  2001, p.79-120.
• M. Wooldrige. Reasoning about Rational Agents.
  The MIT Press, 2000, Chapter 7
• Y. Labrou, T. Finin. Semantics and conversations
  for an agent communication language. In Readings
  in Agents, M. Huhns M. Singh (Eds.), Morgan
  Kaufmann, 1998, p.235-242.
• J. Ferber - Multi-Agent Systems. Addison-Wesley,
  1999, Chapter 6
• T. Finnin, R. Fritzson - KQML as an agent
  communication language. In Proc. of the Third
  International Conference on Information and
  Knowledge Management (CIKM'94), ACM Press, 1994.
• M. Singh. Agent communication languages
  Rethinking the principles. IEEE Computer, Dec.
  1998, p.40-47.
• Y. Labrou, T. Finnin, Y. Peng. Agent
  communication languages The current Landscape.
  IEEE Computer, March/April 1999, p. 45-52.
• FIPA97. "Agent Communication Language"
  Specification FIPA, 11/28/97

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• Web References
• DARPA KSE http//www-ksl.stanford.edu/knowledge-s
  haring/
• KQML http//www.cs.umbc.edu/kqml/
• KIF http//logic.stanford.edu/kif/
• Ontolingua http//www-ksl-svc.stanford.edu5915/
  serviceframe-editor
• FIPA http//www.fipa.org/
• DAML http//www.daml.org/
• OWL http//www.w3.org/TR/owl-guide/
• References for Ontologies (due to prof. Stefan
  Trausan)
• Constandache, G.G., Stefan Trausan-Matu,
  Ontologia si hermeneutica calculatoarelor, Ed.
  Tehnica, 2001
• Gruber, T., What is an Ontology,
  http//www.kr.org/top/definitions.html
• J. Sowa, Ontologia si reprezentarea
  cunostintelor, în (Constandache si Trausan-Matu,
  2001)
• http//www.w3.org/2001/sw/WebOnt/
• http//www.cs.man.ac.uk/horrocks/Slides/index.htm
  l

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