peer-to-peer and agent-based computingAgent communication

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Plan for next two lectures• Communication• Agent Communication Languages• Communication Protocols

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Agent Communication• For agents to communicate they must agree on:

– The syntax of individual messages (i.e., the language)– The sequence of messages (i.e., the protocol)– N.B.1: Languages are distinct from protocols!– N.B.2: We need both!

• We shall look at proposals for– Agent communication languages (ACLs)– Description of protocols

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Agent Communication (Cont’d)• Agents are different from distributed processes, as

far as their message exchanges:– Individual messages may contain sophisticated

information or knowledge • E.g., a message with an explanation of a diagnosis

– Protocols are more complex • E.g., a many-party argumentation over fishing rights

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How to do Things With Words• J.L. Austin

– How To Do Things with Words (1962)http://en.wikipedia.org/wiki/J._L._Austin

– Speech acts can change the state of the world– E.g., “I pronounce you man and wife”

• Locutionary act– Act of making an utterance– E.g., “Shut the door!”.

• Illocutionary act– Action performed in saying something– E.g., “He told me to shut the door”.

• Perlocution – The effect of the act– E.g., “He caused me to shut the door”.

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Background: Speech Acts• John Searle (1969), Speech Acts

– Home page: http://socrates.berkeley.edu/~jsearle/– Extended Austin’s theory:

• Properties required for a speech act to succeed– Wikipedia’s entry:

http://en.wikipedia.org/wiki/Speech_Act• For an imperative:

– Normal I/O conditions state that the hearer is able to hear the request, the act was performed in normal circumstances.

– Preparatory conditions state what must be true for this speech act to be effective, e.g., the hearer must be capable of the action requested, and the speaker must believe the hearer can do it.

– Sincerity conditions state, for example, that the speaker really does want the action performed.

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Searle: Types of Speech Acts• Representatives:

– Commits speaker to the truth of an expressed proposition

• Directives: – Attempt to get the hearer to do something (an

imperative)• Commissives:

– Commits the speaker to some course of action• Expressive:

– Express some psychological state• Declaratives:

– Causes a change in an institutional state of affairs

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Agent Communication Languages• Agents exchange messages expressed in an Agent

Communication Language (ACL)– Parties must agree (or be aware) of the syntax of

individual messages • Most ACLs identify distinct kinds of messages

– They represent the intention of the speaker• Examples:

– Indicatives (representatives) – “The cat is on the mat.”– Imperatives (directives) – “Put the cat on the mat!”– Interrogatives – “Where is the cat?”

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Message Structure• Based on communicative acts (CAs), i.e., the

performatives:– Illocutionary verbs, i.e., they describe the speaker’s

intention– ACL = CA + MSG

• ACLs provide a mechanism for context: – sender, receiver, ontology, protocol, etc.

Message expressed in some Agent Communication Language

Sender: Agent-nameReceiver: Agent-name

Message content expressed in a content language, refers to an ontology

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KQML• Knowledge Query Manipulation Language

– Wikipedia’s entry:http://en.wikipedia.org/wiki/

Knowledge_Query_and_Manipulation_Language• Developed in the early 1990s • Originally conceived as an interface to KBSs

– “Repurposed” as an ACL… • Web site:

http://www.cs.umbc.edu/kqml/

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KQML ACL• KQML messages are of the form

• For example:

(performative :sender S :receiver R :language L :ontology O :in-reply-to I1 :reply-with I2 :content K)

(ask-one :sender shopper :receiver amazon-agent :language prolog :ontology book-ontology :in-reply-to advert1234 :reply-with q321 :content book(‘How to do things with words’,‘Austin’,Price))

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KQML (Informal) Semantics• In 1997 KQML had 35 “performatives”, including:

ask-if S wants to know if K is in R’s KBask-all S wants to know all instantiations of K that are trueask-one S wants one instantiation of K that is true in R’s KBtell K is in S’s KBinsert S asks R to add K to its KBadvertise S wants R to know that S can process Kbroker-one S wants R to provide one response to K (some agent other

than R is going to provide the response)broker-all S wants R to provide all responses to K (some agents

other than R are going to provide the responses)recommend-one S wants to learn of an agent who can respond to Krecommend-all S wants to learn of all agents who can respond to K

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KQML Facilitation

(advertise :sender amazon-agent :receiver facilitator42 :language KQML :ontology KQML-ontology :content (ask-one :sender amazon-agent :receiver facilitator42 :language prolog :ontology book-ontology :reply-with advert1234 :content book(Title,Author,Price) ))

• KQML performatives divided into groups:– Discourse: ask-one/all, ask-if, tell, insert,…– Facilitation: advertise, broker-one/all, recommend-one/all– Mechanics: low-level stuff like IP address registration

• Facilitator agent to handle facilitation performatives.– Usually involving nested KQML messages:

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Recommend-Style Facilitation• Conversation for recommend-style facilitation:

1.Agent A advertises to broker B that it can supply K2.Originator O requests recommendation from B for a

supplier of K3.B sends O advertisement that A can supply K4.O asks A a query Q concerning K.5.A tells O some information I in response to Q.

Advantage: O can cache advert message for later reuse

O A

B1. advertise K

3. advertise K

2. recommend K?

4. ask Q

5. tell I

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Broker-Style Facilitation• Alternative style of brokerage: “middleperson”

1.A advertises to B that it can supply K, as before.2.O requests B to act as broker for a query Q.3.B determines that Q concerns K, so asks Q to A.4.A tells B a response I to Q.5.B tells I to O.

Advantage: agent O only needs to know the broker

O A

B

1. advertise K

5. tell I

2. broker Q3. ask Q

4. tell I

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FIPA ACL• The Foundation for Intelligent Physical Agents

– Standards organisation promoting agent-based technology– Goal: interoperability of standards with other technologies– Web site: http://www.fipa.org

• The FIPA ACL– Open standard described at

http://www.fipa.org/specs/fipa00061/SC00061G.pdf – Based on ARCOL (Sadek) & KQML

• Content – content languages FIPA SL, constraint language, KIF, RDF.

• FIPA interaction protocols – Conversations to achieve some interaction or effect; e.g. auction,

contract net, negotiation.

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FIPA: Message StructureFIPA-ACL messages consist of the following

parameters:

Parameter Descriptionperformative type of communicative actsender identity of agent sending the messagereceiver identity of intended recipient of message

reply-to id of agent to which subsequent messages should be directed

content object of the communicative actionlanguage syntax of the content of the message

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FIPA: Message Structure (Cont’d)

Parameter Descriptionencoding specific encoding of content expressionontology semantics (meaning) of contentprotocol interaction protocol of which message is partconversation-id unique identifier of the conversation

reply-with identifier (local to this conversation) for the response to this message

in-reply-to identifier of the message this is a response toreply-by deadline for the reply

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Message Structure (Cont’d)• Only mandatory element is performative

– However, most messages will also have at least the sender, receiver and content

• If an agent is unable to process one or more element of a message, it must respond with not-understood– An agent that complies with FIPA ACL must at least

support not-understood

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(request :sender (:name dominicagent@whitestein.com:8080) :receiver (:name rexhotel@tcp://hotelrex.com:6600) :ontology personaltravelassistant :language FIPA-SL :protocol fipa-request :content (action movenpickhotel@tcp://movenpick.com:6600 (bookhotel (:arrival 25/11/2000) (:departure 05/12/2000) ...)))

Sample Message• A request for a hotel booking:

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Another example: Inform• Summary:

– Sender informs receiver that something is true• Message Content: a fact• Description – the sending agent:

1.Believes that some fact is true,2.Intends that the receiving agent also comes to believe

that the fact is true, and,3.Does not already believe that receiver has any

knowledge of the truth of the fact.

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• Sending agent is sincere– Has (somehow) generated the intention that the receiver

should know the fact.• Last property concerns semantic soundness of the

act:– If an agent knows already that some state of the world

holds (that the receiver knows fact p), THEN– It cannot rationally adopt an intention to bring about

that state of the world– A rational agent X should never adopt an intention to tell

Y something that X knows that Y already knows…

Another example: Inform (Cont’d)

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Another example: Inform (Cont’d)• From the receiver's viewpoint, receiving an inform

message entitles it to believe that:– The sender believes the fact contained in the message,

and,– The sender wishes the receiver to believe that fact also

• Whether or not receiver adopts the belief in the

fact will be determined by the receiver’s trust in the sincerity and reliability of the sender

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Other Performatives• FIPA Communicative Act Library Specification• Includes:

– request an agent to perform some action– query (ask a question)– propose a deal during negotiation– accept-proposal to commit to an agreement– reject-proposal to terminate negotiation without

agreement– etc.

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FIPA ACL and JADE• JADE: FIPA-compliant platform• Incorporates FIPA ACL

– Functionalities to assemble and parse messages in FIPA-ACL

– Pushing a standard onto industry