Transcript Multiagent Systems

DAI: Agent interaction in
MAS
Material collected, assembled and
extended by
S. Costantini,
Computer Sc. Dept. Univ. of L’Aquila
Many thanks to all colleagues that
share teaching material on the web.
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Distributed Artificial
Intelligence – Definition
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A multiagent system (MAS) is a system in which
several interacting, intelligent agents pursue some
set of goals or tasks that are beyond their individual
capabilities.
Distributed problem solving considers how the task of
solving a particular problem can be divided among a
number of agents that cooperate in dividing and
sharing knowledge about the problem and about its
evolving solutions
DAI is the study, construction and application of MAS.
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Agent communication and
interaction
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Communication protocols enable agents to
exchange and understand messages
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Propose, accept, reject, retract, disagree or
counterpropse a course of action
Interaction protocols enable agents to have
conversations
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Agent1 proposes an action to Agent2
Agent2 evaluates the proposal and sends to
Agent1: acceptance, counterproposal,
disagreement or rejection
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Agent communications coordination
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Agents communicate in
order to achieve better the
goals of themselves or of
the society in which they
exist.
MAS to maintain global
coherence (behaving as a
unit) without explicit
global control.
Agents determine common
goals and common tasks,
avoid conflicts and pool
knowledge and evidence.
Coordination
Cooperation
Planning
Competition
Negotiation
Centralized
Distributed
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Communications - meaning
Three aspects to the formal study of
communication:
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1.
2.
3.
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Syntax: how the symbols of
communication are structured
Semantics: what the symbols denote
Pragmatics: how the symbols are
interpreted
Meaning is a combination of semantics
and pragmatics.
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Message types
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Communication could be active, passive or
both (agent is master, slave or peer)
Two message types: assertions and queries.
All agents accept information by means of
assertions.
Passive agent: accepts queries, sends replies
Active agent: issue queries, make assertions
Peer agent: all of the above.
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Communication levels
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Communication protocols are typically
specified at several levels:
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Method of interconnection
Syntax
Meaning (semantics)
Binary, multicast, broadcast.
Data structure of a protocol:
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Sender, receiver(s), language, encoding and
decoding functions, actions to be taken by the
receiver
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Speech acts
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Speech act theory used for analyzing human
communication
Aspects of speech act theory:
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Locution: physical utterance by the speaker
Illocution: intended meaning
Perlocution: action that result from the loction
Message contained within the protocol maybe
ambiguous or require decomposition,
however, the communication protocol itself
should clearly identify the type of message.
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Knowledge Query and
Manipulation Language (KQML)
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KQML is a protocol for
exchanging information and
knowledge.
The semantics of the
communications protocol must
be domain independent, while
the semantics of the enclosed
message may depend on the
domain.
KQML performatives: evaluate,
ask-one, ask-all, reply, sorry,
cancel, ready, advertise,
broadcast, etc.
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Example:
(tell
:sender Agent1
:receiver Agent2
:language KIF
:ontology Blocks-World
:content (AND (BLOCK A) (BLOCK
B) (On A B))
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Languages: KIF (Knowledge
Interchange Format), Prolog,
Lisp, etc.
An ontology is a specification of
objects, concepts and
relationships in an area of
interest.
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Interaction protocols coordination
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DAI involves distributed control and distributed data.
Agents have a degree of autonomy in generating new
actions and deciding which goal to pursue next.
Knowledge of the system's overall state is dispersed
throughout the system.
Coordination activities on a search problem may be
represented as:
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defining goal graph,
assigning regions of the graph to appropriate agents
decisions about which areas of the graph to explore
traversing the graph
ensuring that successful traversal is reported.
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Coordination
• Coordination is a mechanism for ensuring that agents’
activities retain some desired relationship/s (sequence,
complementarity, etc.).
• Control is the extent to which coordination information
must be elaborated by each agent.
• The range of control is from none to total. Control is
inversely related to autonomy: no control corresponds to
total autonomy, and being totally controlled corresponds
to zero autonomy.
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Cooperation
Agents which have some degree of
autonomy can cooperate with other such
agents on tasks and activities, for their
own or mutual benefit.
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Advantages of Cooperation
Complete tasks quicker through shared
effort
•
• By sharing resources, achieve tasks
otherwise not possible
• Make use of complementary
capabilities
•
Avoid harmful interactions
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Modes of Cooperation
•
accidental: not intended
• unilaterally intended: one agent intentionally
helps another
•
mutual cooperation: two or more agents
intentionally collaborate
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Degrees of Cooperation
• Fully cooperative (benevolent): Agents always
attempt to assist other agents that request or need their
help
• Antagonistic: Agents do not cooperate with others
and may even try to block their goals
• Partly cooperative: Agents sometimes or to some
extent will assist other agents
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Cooperation protocols
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Basic strategy is to decompose and then distribute
tasks
Decomposition done by system designer or by agents
Distribution mechanisms:
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Market mechanism: generalized agreements or mutual
selection
Contract net: announce, bid and award cycles
Multiagent planning: planning agents perform task
assignment
Organizational structure: agents have fixed responsibilities
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Blackboard systems
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Offer a support for cooperation
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blackboard structure for exchanging
proposals/requests
control component (optional) to match
proposals and requests
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based on Knowledge bases (KS’s) that are
application-dependent
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Blackboard systems
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Independence of
expertise
(knowledge sources)
Common interaction
language
Event-based
activation
Need for control
…..
…..
Blackboard
…..
…..
Executing
Activated
KS
Library
Of
KSs
Events
Control
components
Pending
KS
Activations
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Negotiation (Def 1)
The process of improving agreement (reducing
inconsistency and uncertainty) on common
viewpoints or plans through the structured
exchange of relevant knowledge
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Negotiation (Def 2)
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Negotiation is a process involving at least
two parties aimed at reaching an
agreement that is acceptable by the
parties involved
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Negotiation (Def 3)
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Occurs when agents with different goals
want to reach a joint decision
Negotiation mechanism ideally:
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Efficiency
Stability
Simplicity
Distribution
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Prior to negotiation: an agent has
identified a resource or service it
requires but cannot supply: then, it needs
to identify an agent/s which it believes
potentially could.
•
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Negotiation
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Negotiation protocol is a set of rules which
govern the interactions of participants
Negotiation Strategy is a decision making
model, which participants employ in order to
achieve their goal in line with a negotiation
protocol
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Elements of Negotiation
• bargaining (promising something in
exchange for something else),
• bidding (offering a service or capability at
a specific ‘price’),
• contracting (committing to provide a
service or capability at a specific ‘price’)
may be part of the negotiation process.
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Negotiation Process
• Agent contacts other agent(s) to offer or
request resource or service (often under
specified conditions)
• other agent/s identify what can be
supplied under what conditions
• initiating agent/s accepts conditions
• supplying agent/s commit to provide
resource or service.
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Forms of Negotiation
• In single-stage negotiation, initiating agent makes a
request and the respondent accepts or rejects this.
• In multi-stage negotiation, agents iterate through
more than one stage of offer/counter-offer.
• Negotiation protocols: a structured procedure for
one or more stages of the negotiation process.
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Basic Negotiation Techniques
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Game theoretic
 Negotiation process seen as a game where
agents are players and make moves
Heuristic
 Agents make proposals and modify their
proposals if not accepted according to heuristic
strategies
Argumentation based
 Agents try to convince others to accept their
proposals by logical arguments
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Classification of negotiation protocols
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Number of participants - 1-1, 1-N
(Auction), N-M (Market)
Existence of third party - direct, mediated
negotiation
Degree of automation - semi, fully
automated
…
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Contract Net Protocol
Agents coordinate their activities through contracts
to accomplish specific goals.
1. An agent acting as a manager decomposes its
contract (the task or problem it was assigned
with) into subcontracts to be accomplished by
other potential contractor agents.
2. For each subcontract, the manager announces a
task to the network of agents.
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Contract Net Protocol (cont’d)
3. Agents receive and evaluate the
announcement.
4. Agents with appropriate resources, expertise,
and information reply to the manager with bids
that indicate their ability to achieve the
announced task.
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Contract Net Protocol (cont’d)
5. The manager evaluates the bids received and
awards the task to the most suitable agent, called
the contractor.
6. Finally, manager and contractor exchange
information together during the accomplishment of
the task
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Contract Net Protocol
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Advantages:
 dynamic task allocation via self-bidding
 agents can be introduced and removed
dynamically
 it provides natural load-balancing (as busy
agents need not bid)
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Contract Net Protocol
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Disadvantages:
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contract net protocol does not detect or resolve
conflicts
the agents in the contract net are considered
helpful and non-antagonistic (which in real-world
scenarios is not realistic)
it is rather communication-intensive, the costs of
which can be quite high
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Interacting tasks
Basic contract net model does not consider
interacting tasks
• e.g., two agents each need all of an available
resource;
• e.g., several agents all need a resource or another
agent’s help at or about the same time
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Possible Approaches
to Interacting tasks
1. Agents negotiate directly on resolving problem.
2. A mediator coordinates the interaction to reduce
or eliminate the conflict, or negotiates with
agents on resolving it.
3.
The agents not only plan how to accomplish their
tasks/goals but communicate these to others and
negotiate potential conflicts, modifying plans
accordingly.
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Multi Agent Planning
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Multi-agent planning emphasizes certain
avoidance of inconsistent and conflicting
situations, which is critical in applications such as
air-traffic control
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By forming a multi-agent plan, the nodes
determine all of their actions and interactions
beforehand, leaving nothing to chance
Two types of multi-agent planning
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centralized
distributed
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Centralized Multi Agent
Planning (1)
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Separate agents form their individual plans and
then they are sent to a central coordinator, that
analyses them and finds potential plan conflicts
The central coordinator can
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identify critical regions of plans around which agents should
synchronize
insert plan steps for sending and waiting for synchronization
messages to ensure proper synchronization
the individual partial plans can then be merged into a multi-agent
plans with conflicting interactions eliminated
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Centralized Multi Agent
Planning (2)
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Disadvantages of centralized planning:
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It is static – e.g. the agents first form their
plans and then coordinate them, hence can
miss opportunities for cooperation that would
have been possible had the agents built their
individual plans concurrently with reasoning
about what other nodes are doing
It is difficult to create a central coordinator
AOSE
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Distributed Multi Agent
Planning
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It allows agents to model each other's plans
Agents communicate in order to build and update
their individual plans and their models of others
until all conflicts are resolved
Multi Agent Planning requires that nodes share
and process substantial amounts of information
and hence it generally involves more computation
and communication than other approaches
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