Chapter 6 agent communications--agent communications

Chapter 6
Agent Communication
Marc-Philippe Huget

Introduction
 Agent communication is essential for facilitating cooperation,
coordination, and other aspects of agent interaction
 Two major approaches to agent communication
 Direct communication, and
 Indirect communication
 Early work on agent communication focused on direct
communication with the use of different protocols
 Protocols are efficient means for information exchange and
coordination; however, they restrict agents’ autonomy; agents
have to strictly follow the admissible sequence of messages
imposed by the protocol.

Introduction
 Agent communication is based on human conversations.
 Message contains a verb and a content.
 For instance: inform (it rains) inform is the verb and it rains
is the content. F(P): F- force; P- message content
 Several types of speech acts exist in human conversations:
 Representatives, which give information
 Directives, which require something from the recipient
 Commissives, which commit the sender to do something in the
future
 Expressives, which describe the mental states of the sender
 Declarations, which process an act just by saying it
 Agents do not use all the types of verbs present in a natural
language but a smaller subset.

Direct Communication
 Communication between cognitive agents (agents that have a
representation of themselves, of other agents, and of the
world, and reason about their courses of action) where the
recipient is formally defined
 Several elements require attention when considering direct
communication:
 an agent communication language
 ontologies
 communication support

Indirect Communication
 Message is released in the environment. Any agent sufficiently
close to it can perceive it.
 Indirect communication is used by reactive agents (agents that
do not have a representation of themselves, of other agents,
and of the world.
 Can only be used for limited coordination, since it is not
possible to represent a wide range of messages.
 Examples of indirect communication include pheromones
(example the MANTA project ) and forces (AI, robotics, and
multi-agent systems).

Agent Communication Versus
Multi-agent Communication
 Agent communication (as opposed to multi-agent
communication) is a one-to-one communication in which a
speaker utters speech acts to a single addressee.
 Several features characterize one-to-one communication:
 Communication is between two agents: a speaker and an
addressee.
 Communication is privileged between two agents, that is, it is
not possible for other agents to hear the communication.
 Entering and leaving communications are subject to agreement
from other
 Terms of termination of communication are necessarily known
before beginning the communication.

Agent Communication Versus
Multi-agent Communication
 Multi-agent communication allow for indirect communication.
That is, it should allow agents to listen to messages transmitted
in their vicinity (either physical or logical) but not too far-away
communication.
 It is then important to add the notion of earshot. In human
communication, earshot can range from whispering (private
communications) to shouting where even further agents can
hear.
 Multi-agent systems are open systems; as a consequence,
agents can enter and leave at any moment during the
conversation.
 Termination of communications is not necessarily decided in
advance. Termination can happen if no more communication is
needed.

Direct Communication:
Agent Communication Languages
 Communicating requires the use of common language. For
example, we communicate using English, to understand each
other.
 Agent communication languages do not offer a rich set of
communicative acts as natural languages.
 Currently, there exist two main agent communication
languages: KQML (Knowledge Query and Manipulation
Language) and FIPA ACL.
 KQML allow data exchange between distributed knowledge
bases. KQML is composed of
 7 performatives for conversation management (sorry, error, etc.),
 17 performatives for the conversation (ask-if, tell, etc.), and
 11 performatives for conversation sending (forward, broker-all,
etc.).

KQML Performatives (abstracted from EIWG, 1994)

 KQML is the older communication languages and is gradually
replaced by FIPA ACL for several reasons:
 There was a need for clear semantics for the language, (make a
clear different between KQML and FIPA ACL)
 It was recommended to have a smaller set of communicative acts,
while keeping the autonomy of agents,
 FIPA ACL has implementation of various protocols, and
 FIPA ACL supports communication at various levels.

The KQML Language
 Knowledge Query and Manipulation Language (KQML) is a
language that is designed to support interaction among
intelligent software agents.
 It was developed by the ARPA-supported Knowledge Sharing
Effort and independently implemented by several research
groups.
 It has been successfully used to implement a variety of
information systems using different software architectures.
 The KQML language simplifies its implementation by allowing
KQML messages to carry arbitrary useful information, such as
the names and addresses of the sending and receiving agents,
a unique message identifier, and notations by any intervening
agents.

KQML: the 3 layers
The KQML language can be viewed as consisting of three layers: the content,
message and communication layers, as shown in Figure.
• The content layer specifies the actual content of the message, and the KQML
standard itself has nothing to say about this.
• The set of performatives provided by the language, and shown in Table,
constitute the message layer, which in turn forms the core of the language.
• This layer of abstraction provides the performative and specifies the protocol for
delivering the message that subsumes the content.
• KQML specifies several protocols, including synchronous (where a blocking
query waits for an expected reply) and asynchronous (which involves non-
blocking messages).
• The communication layer encodes low level communication parameters, such
as the identities of the sender and the recipient, and unique identifiers for the
particular speech act.

Example of an instantiated performative
A KQML message from agent joe representing a query about the
price of a share of IBM stock might be encoded as:
In this message, the KQML performative is ask-one, the
content is (price ibm ?price), the ontology assumed
by the query is identified by the token nyse-ticks, the
receiver of the message is to be a server identified as
stock-server and the query is written in a language called
LPROLOG. The value of the :content keyword is the content
level, the values of the :reply-with, :sender, :receiver
keywords form the communication layer
and the performative name, with the :language and
:ontology form the message layer.

Applications of KQML Language
 KQML has been used as the communication language in several
technology integration experiments in the ARPA Rome Lab Planning
Initiative.
 One of these experiments supported an integrated planning and
scheduling system for military transportation logistics linking a
planning agent, with a scheduler (in Common Lisp), a knowledge
base, and a case based reasoning tool (in Common Lisp).
 All of the components integrated were pre-existing systems which
were not designed to work in a cooperative distributed environment.

FIPA ACL
 The Foundation for Intelligent Physical Agents (FIPA) was formed in
1996 to produce software standards for heterogeneous and
interacting agents and agent-based systems.
 FIPA’s agent communication language, like KQML, is based on speech
act theory: messages are actions or communicative acts, as they are
intended to perform some action by virtue of being sent.
 FIPA ACL is superficially similar to KQML. Its syntax is identical to
KQML’s except for different names for some reserved primitives.
 Thus, it maintains the KQML approach of separating the outer
language from the inner language. The outer language defines the
intended meaning of the message; the inner, or content, language
denotes the variety of interesting agent architectures.
 Thus, KQML introduces a small number of KQML performatives that
agents use to describe the metadata specifying the information
requirements and capabilities; it also introduces a special class of
agents called communication facilitators.
 A facilitator is an agent that performs various useful communication
services, such as maintaining a registry of service names, forwarding
messages to named services, routing messages based on content,
matchmaking between information providers and clients, and
providing mediation and translation services.

Agent Communication
 This chapter will describe FIPA ACL.
 F(P); where F is force P is message content
 For example: inform (it rains); Force (F) is the performative verb
 Performatives in FIPA ACL are arranged into five categories:
 Passing information,
 Requesting information,
 Negotiation,
 Performing actions, and
 Error handling.
 However, we note that agent-based applications can be (and
have been) developed using traditional third generation
languages like Lisp, C, or Prolog, and object-oriented (OO)
languages such as C++ or Smalltalk.

Primitive
performatives

FIPA ACL
 Inform: an agent informs recipients about something it
believes and it believes recipients do not yet believe it.
 Request: request that recipient perform actions.
 KQML is lack of semantics associated with performative. FIPA
ACL addresses this issue via two parts:
 Feasible pre condition: pre condition that have to be evaluated to
true to execute the performative.
 Rational effect: expresses what the sender hopes to bring about.
 Semantics are expresses via FIPA SL (refer table 6.2)

Ontologies
 It is possible that two agents speak about the same concept
but not with the same term: using the terms “painter” and
“artist,” for instance.
 In order to avoid these differences, it is important to use a
common vocabulary also called an ontology
 An ontology defines the basic terms and relations comprising
the vocabulary of a topic area as well as the rules for
combining terms and relations to define extensions to the
vocabulary
 Example of ontology for computer science department is
shown next:

Ontologies

Communication Support
 Communicating requires a communication support, which is a
mechanism that stores, retrieves, and directs messages to the
agents.
 Communication support is present in agent platforms such as
JADE and MadKit , where it is possible to send messages to
 Agents,
 The roles they play in a group, or
 A group.

Structuring Communication
Three main different approaches exist to structure
direct communication:
1. Protocols: Main approach to represent communication in MAS
systems. The way to structure communication in a rigid manner.
A finite set of performatives is allowed for a particular state of
the interaction.
2. Dialogue games: offer more flexibility in communication in
comparison with protocols. The communicative acts are paired,
for instance, a question is paired with answer. It is possible to
embed dialogue games into other dialogue games.
3. Argumentation systems: agents challenge the content of each
message and as a consequence, agents have to justify their
message content via arguments. Argumentation is frequently
used in negotiation-based systems.

Protocol
 Research on protocols in multi-agent systems share work
with communication protocol engineering and interaction
protocol engineering.
 Interaction protocol engineering composed of several
phases from analysis to execution

Interaction Protocol Engineering
1. Analysis: This phase describes informally the protocol. The
exact sequence of messages, the content, and the different
requirements the protocol should follow.
2. Formal Description : To avoid misunderstandings and to
ease validation. Several formal description techniques are
available for interaction protocol designers:
 Finite State Machines : Based on graph theory, where each state
corresponds to an interaction state and a transition sends or
receives a message. Example include AgenTalk , and COOL
 Petri nets: Allow multiple flows and synchronization on the
same net.
 UML-based notations: Example FIPA UAML and Agent UML
 Others: temporal logic and Z language

3. Validation: The validation can be structural (checking for
deadlock, termination, etc.) or behavioral (checking whether
the protocol achieves its goals).
Two techniques exist for the validation of interaction
protocols:
 Reachability analysis : A graph of all the reachable states of the
protocol is built-validation is based on graph properties.
 Model checking: A graph of reachable states is built, but this
time the property is expressed as a temporal logic formula.
 Another kind of validation is the agent communication
language compliance verification, which checks whether an
agent correctly implements the protocol and agent
communication language.

4. Protocol Synthesis: The actual implementation. First the
skeleton of the protocol is generated. Then the semantics
associated with each transition is added. The first phase is
automatic and the second one is manual. System unable to
generate the semantics. The advantage is that it reduces the
effort required for generating a protocol since it s not necessary
to rewrite each time the semantics of the protocol modified.
5. Conformance Testing: Checks whether the implemented
protocol contains the requirements defined in the analysis
phase.

Multi-Agent Interaction Protocol
1. The Contract Net Protocol:
 The most renowned and widely used protocol in multi-agent
systems.
 Describes a protocol involving two kinds of agents:
 (1) a task manager that has to perform a task and does not have the
skills for that, and
 (2) bidders that will perform the task.
 The Contract Net protocol is composed of four phases:
 Task manager Advertises the task to potential bidders (the message also
contains the constraints associated with the task- deadline)
 bidder can bid for the task
 task manager selects one bidder based on its bid and awards it, and
 performing the task.

 The Contract Net Protocol:
When task execution completes, the awarded bidder
informs the manager that the task is realized (the
inform-done message), or sends the result (the
inform-result message), or sends the failure
message if it fails performing the task.
The first message sent is the cfp (call for proposal)
message, where the task is advertised with its
constraints.
The bidders can answer either by propose
denoting that they agree to realize the task subject
to the constraints associated with it (as expressed
in the message content), or by refuse if the bidder
refuses to execute the task—the reason refusal is
provided by the bidder in the message content
field.
After the deadline, the manager agent selects a bid
based on the constraints.
It then sends an accept proposal to the awarded
bidder and a reject-proposal to all other bidders.

2. Auction Protocols:
 Many auction protocols are implemented in multi-agent
systems.
 Example: English auction and the Dutch auction
 English auction is an ascending open cry auction where the
price of the item for sale increases as long as at least one
auction participant is willing to pay. It stops when no
participants is willing to raise the price beyond the current
bid.
 Dutch auction protocol is a descending open cry auction.
Unlike the English auction, the price decreases as long as a
participant does not inform it accepts to pay the price.

The first message inform(start-auction)
informs auction participants that the auction
has begun.
The second message is sent by the auctioneer
(the cfp message).
It informs the auction participants of the starting
bid price.
The participants can respond by a propose
message including a bid equal to or greater
than the current bid.
This can continue iteratively until a deadline is
reached. After a deadline, the auctioneer selects
a bid and informs the winner with the accept-
proposal message.
The other participants who placed bids receive
the reject-proposal message. The auction
continues as long as there are at least two
offers.
The auctioneer may close the auction (the
inform(end of auction)) if there are less than
two offers.
The protocol allows the auctioneer to check whether the hammer price is greater than the
reservation price, and to allocate the sold good to the winner conditioned on this.

3. Sian’s Learning Protocol:
 A simple protocol where an agent proposes a hypothesis to other
agents. Those agents provide their opinion on whether the
hypothesis is correct or incorrect, and whether they want to modify
it.
 Each agent votes on the correctness of the hypothesis. The agent
that proposed the hypothesis gathers the votes and, using some
heuristics, decides whether the hypothesis is accepted or not. If the
hypothesis is accepted, it informs other agents of the acceptance.

The first message is a query-if message containing
the hypothesis to validate.
The other agents can answer either by a confirm message
denoting that the hypothesis is correct, by a disagree
message denoting that the hypothesis is wrong, by a
noopinion message denoting that they have no idea about
this hypothesis, or by a modify message requesting to
modify the hypothesis.
Based on heuristics and answers from other agents, the initiator
decides whether the hypothesis is true.
If it decides that the hypothesis is true, it informs the other agents via
an assert message that the hypothesis is considered true.

Dialogue Games
 Conceived with the idea of restoring agents’ autonomy.
 Only pairs of communicative acts are defined, e.g., a request
with an answer.
 Dialogue games are interactions between two or more players,
where each utters according to a defined set of rules.
 Example of dialogues:
 Information-seeking dialogues are those where one
participant seeks the answer to some question(s) from another
participant
 Inquiry dialogues are those where the participants collaborate
to answer some question or questions where answers are not
known to any single participant.
 Persuasion dialogues involve one participant seeking to
persuade another to accept a proposition he or she does not
currently endorse

Dialogue Games
 Example of dialogues:
 Negotiation dialogues are those where the participants
bargain over the division of some scarce resource.
 Deliberation dialogues are those where the participants
collaborate to decide what action or course of action should be
adopted in some situation.
 Eristic dialogues are those where participants quarrel
verbally as a substitute for physical fighting, aiming to vent
perceived grievances.

Dialogue Games: Elements
 Commencement rules: Rules that define the
circumstances under which the dialogue commences.
 Locutions rules: Rules that indicate what utterances
are permitted, and permit those asserting propositions
that are subsequently questioned or contested to justify
their assertions.
 Combination rules: Rules that define the dialogical
contexts under which particular locutions are permitted
or not, or obligatory or not.
 Commitments rules: Rules that define the
circumstances under which participants express
commitment to a proposition.
 Termination rules: Rules that define the circumstances
under which the dialogue ends.

Argumentation Systems
 The previous two Communication assumes what the
agents say to be true.
 Argumentation Systems allow to challenge agents on
their beliefs.
 Agent can use it to, for example, to deal with
inconsistent information.

Multiparty Communication
Proposal
 Communication between agents is private to these agents.
It is then not possible to send the same message to all the
agents and to see the answers without using a multicast or
a broadcast mechanism.
 An agent cannot overhear the communication and intervene
if needed.
 In Internet applications, there are three types of modes
 Public,
 Private and
 Secret

Multiparty Communication Proposal
 Public:
 Resembles the Internet multi-party communication, where potentially all
agents are able to “hear” a message.
 The agent communication not only contains the sender and the addressees
but auditors and over hearers as well.
 Private:
 Private communication is certainly the less frequent mode of communication
in multi-agent systems.
 The aim of this mode is to offer some privacy to agents during the
conversation.
 Agents outside the communication group may be aware of this
communication but cannot perceive the content.
 The aim of this mode of communication is to let agents outside the
communicating group learn that some agents are acquainted.
 Secret:
 Secret communications is similar to current one-to-one communication.
 Communication to be aware of this communication. The approach is the
same as private communications

A purchase negotiation dialogue
game
 The example deals with buying a new car under some
specific constraints such as price, quality, number of
seats, etc.
 The different communicative acts (also called locutions)
in this dialogue game are:

Chapter 6 agent communications--agent communications

More Related Content

What's hot (20)

Viewers also liked (14)

Similar to Chapter 6 agent communications--agent communications (20)

Recently uploaded (20)

Chapter 6 agent communications--agent communications