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Multiagent Systems & Societies of Agents (II). Authors : Michael N. Huns & Larry M. Stephens Speaker : Shabbir Ali Syed CSCE 976, April 8 th 2002. Agent Interaction Protocols. Govern the exchange of a series of messages among agents Case 1: Agents have conflicting goals - PowerPoint PPT Presentation
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Multiagent Systems &Societies of Agents (II)
Authors: Michael N. Huns & Larry M. Stephens
Speaker: Shabbir Ali Syed
CSCE 976, April 8th 2002
Agent Interaction Protocols
Govern the exchange of a series of messages among agents
Case 1: Agents have conflicting goals
Case 2: Agents have similar goals
Agents with conflicting goals
Conflicting goals or simply self-interested
• Maximize payoff (utility functions)
Agents with similar goals
Objective: maintain globally coherent performance without violating autonomous behavior of agents– Determine shared goals– Determine common tasks– Avoid unnecessary conflicts– Pool knowledge and evidence
Some Interaction Protocols
1. Coordination Protocols
2. Cooperation Protocols
3. Contract Net
4. Blackboard Systems
5. Negotiation
6. Multi-Agent Belief Maintenance
7. Market Mechanisms
1. Coordination Protocols
Done between multiple agents to satisfy individual or group goals
Why coordination is needed:– Maintain dependencies between actions
– Meet global constraints
– When no one agent has sufficient competence, resources, or information to achieve system goals
Distributed AI (DAI)
Distributing data and controlAgents have autonomy to generate new actions and to
decide which goals to pursue next
Disadvantages:– KB is distributed, so each agent has only a partial and
imprecise perspective of KB
– Degree of uncertainty in actions
– Difficult to attain coherent global behavior
Goal GraphIt is a AND/OR graph with the leaves representing
the goalsActivities are:
– Defining Goal Graph, including identification and classification of dependencies.
– Assigning particular regions of the graph to appropriate agents
– Controlling decisions about which areas of the graph to explore
– Traversing the graph– Ensuring that successful traversal is reported
Agent Structures
Commitment– Pledges to undertake a specified course of action– As situation changes agents must evaluate whether existing
commitments are still valid– Internal and Belief consistent
Convention– Provide a means to manage commitments in changing
circumstances– Provides degree of predictability: Agents can take into
consideration future conflicts, dependencies, and activities of other agents
Limited B/W Social Convention
INVOKE WHENLocal commitment droppedLocal commitment satisfied
ACTIONSRule1: IF Local commitment satisfied
THEN inform all related commitmentsRule2: IF Local commitments dropped because unattainable or motivation not
presentTHEN inform all strongly related commitments.
Rule3: IF Local commitments dropped because unattainable or motivation not present
AND communication resources not overburdenedTHEN inform all weakly related commitments.
Basic Joint Action Convention
• INVOKE WHENStatus of commitment to joint action changes
Status of commitment to attaining joint action in present tem context changes
Status of joint commitment of a team member changes.
ACTIONS
Rule1: IF status of commitment to joint action changes
OR
IF status of commitment to present team context changes
THEN inform all other team members of these changes
Rule 2: IF status of joint commitment of a team member changes
THEN determine whether joint commitment still viable.
Cooperating Agents
Each agent should share status of its commitment to:– the shared objective– the given team framework
If belief changes, should inform all agents
2. Cooperation Protocols
Divide and Conquer Approach• Smaller sub-tasks require less capable agents• Fewer resources
Distributing Criteria:• Avoid overloading critical resources• Assign tasks to agents with matching capabilities• Make an agent with wide view assign tasks to other agents• Assign overlapping responsibilities to agents to achieve coherence• Assign highly independent tasks to agents in spatial or semantic
proximity-minimizes communication and synchronization costs• Reassign tasks if necessary for completing urgent tasks
Methods for task distribution
3. Contract NetBest know and widely applied to distribute tasks.Connection problem: finding an appropriate agent to work on a given task
Manager:• Announces a task that needs to be performed• Receives and evaluates bids from potential contractors.• Award a contract to a suitable contractor.• Receive and synthesize results.Contactor:• Receive task announcement• Evaluate my capability to respond• Respond (decline, bid)• Perform the task if my bid is accepted• Report my results.
Contract Net
Task Announcement
• Addressee: Contractor
• Eligibility Specification: Contractors should meet certain criteria to make bids.
• Task Abstraction: A brief description of the task, is used by contractors to rank tasks from several task announcements.
• Bid Specification: Tells contractors , what info. must be provide with the bids.Manager compares different contractors on basis of bids.
• Expiration Time: Deadline for receiving bids.
Limitations
• Task must be awarded anyway, even if a better contractor is busy.
• Manager is under no obligation to inform other customers that an award has already been made.
• All potential contractors can be busy and so not send bids to the manager
• A potential contractor ranks the proposed task below other tasks, and so may not send bids.
• No contractor even if idle is able to handle the task.
Proposed solution
• Manager: Requests immediate response bids.
Contractors: eligible but busy.
ineligible
uninterested.• Manager: directed contracts
Contractors:acceptance
refusal
4. Blackboard Systems
Characteristics of BB systems1. Independence of expertise
2. Diversity of problem-solving techniques
3. Flexible representation of blackboard information
4. Common interaction language
5. Event based activation
6. Need for control
7. Incremental solution generation
Knowledge source: KS
BB Systems: characteristicsIndependence of expertise:
A specialist (KS) can act independently of the other
Diversity of problem-solving techniques: Internal representation of each KS is hidden from others
Flexible representation of blackboard information:
No restriction as to what can be placed on the blackboard
Common interaction language:KS’s should be able to correctly interpret information posted by
other KS’S
BB Systems: characteristicsEvent based activation:
KS’s give their preferences and blackboard triggers them whenever it occurs
Need for control: Triggered KS, evaluates quality of its contributioninforms Control Component about the costestimates benefits and decides how to trigger for better problem solving
Incremental solution generation:KS contributes as needed (refining, contradicting, initiating)
Diagram for blackboard
5. Negotiation
Joint decision reached by two or more agents, each trying to reach an individual goal
Features:– Language used by participating agents
– Protocols followed by agents as they negotiate
– Decision process used for concession, criteria for agreement and to determine position
Attributes of negotiation
• Efficiency: agents should not waste resources in coming to an agreement
• Stability: no agent should have an incentive to deviate from agreed upon strategies
• Simplicity: the negotiation mechanism should impose low computational and bandwidth demands on the agents
• Distribution: no central decision maker
• Symmetry: should not be biased against any agent for arbitrary reasons
Systems for Negotiation
Two types– Environment centered
– Agent centered
Environment Centered
Rules by which agents can interact productively and fairly irrespective of their capabilities or intentions
– Task-oriented domain
– State-oriented domain
– Worth-oriented domain
Agent Centered
Best strategy for an agent to follow in a given environment
– Task-oriented domain
Task-Oriented Domain
• Agents have set of tasks
• Resources needed are available
• Agents can achieve tasks without help nor interference
• Agents can benefit by sharing some tasks Example: Internet downloading
Example: Internet downloading Constraints
• Each agent declares documents it wants
• Common documents are assigned by the “toss of coin”
• Agents pay for the documents they download
• Agents are granted access to all documents in common set
Mechanism is
simple, accurate, systematic, and distributed
(no document downloaded twice)
Agent Centered: Approaches
1. Speech act classifiers together with a possible world semantics
used to formalize negotiation protocols and their components
2. Unified Negotiation Protocol Assumption: agents are economically rational• Set of agents must be small• Must have a common language• Must have a common problem abstraction• Must reach a common solution
Speech Act
An agent forms and maintains its commitments to achieve a task individually iff:– It has not pre-committed itself to another agent
to adopt and achieve a task– It has a goal to achieve the task individually– It is willing to achieve the task individually
Unified Negotiation Protocol
1. Deal: joint plan between agents that would satisfy all their goals
2. Utility: amount agent is willing to pay minus cost of deal (to be maximized)
3. Negotiation set: set of all deals that have a positive utility for all agents
1. Conflict: negotiation set is empty
2. Compromise: agents agree to negotiate
3. Co-operative: all deals in negotiation set are preferred by both agents over achieving their goals
Human(x)=>Mortal(x) Human(socrate)
Mortal(socrate)
Justification node
fact fact
Derived fact
+
IN IN
IN
+
IN IN IN OUT
IN
+ ++ -
Drank-Fountain-of-youth(socrate) Human(x)=>Mortal(x) Human(socrate)
MORTAL(SOCRATE)
ININ
- ++
IN
OUT
6. Multiagent Belief Maintenance
High level interaction among agents
Relies on Truth Maintenance Systems (TMS): Data structure (AI) that keeps track of the truth of
a fact in a KB given the truth of the facts it is derived from (which constitute its support or justification)
TMS (or Reason Maintenance System, RMS)
– Ensure integrity of agents knowledge
– Ensure that its stable• Datum that has a valid justification is believed
• Datum that lacks a valid justification is disbelieved
– Well founded• Permits no set of its beliefs to be mutually dependent
– Logically consistent• Stable at the time consistency is determined and has no logical
contradiction
• No datum is both believed and disbelieved at same time
Justification based TMS (JTMS)
Datum:• Set of justification.• Associated status.
– INTERNAL:Believed because of a valid local justification.
– EXTERNAL:Believed because another agent asserted it.
– OUT: Disbelieved.
A communicated Datum must be:– INTERNAL to at least one of the agents that believes it.– Either INTERNAL or EXTERNAL to the rest.
TMS before Justification
Resultant Network
Multi-agent TMS
Invoked by addition or removal of justifications:– Belief changes should be resolved with as few agents as
possible
– Changing as few beliefs as possible
When invoked:– Unlabels data
– Chooses labeling for unlabelled shared data
– Initiates labeling
7. Market Mechanisms
For large or unknown # of agents– The goods being traded– Consumer agents that are trading the goods– Producer agents, with their technology for
transforming some goods into others– Bidding & trading behaviors of agents
Competitive Equilibrium
• Consumers bid to maximize their utility, subject to budget constraints
• Producers bid to maximize their profits, subject to technological capabilities
• Net demand is zero for all goods
Rational action: maximizes preferences for an agent(including past commitments)
Societies of agents
• Intelligent agents work well in groups (societies) not in isolation
• Distributed system is a better solution• Peer to peer better than client server
Social commitments: commitments of an agent to another
Social dependence
Social dependence (x y a p):
Agent x depends on agent y with regard to act a for realizing state p, when p is a goal of x and x is unable to realize p while y is able to do so.
Types of dependencies
• Voluntary: agents adopt roles that bind them to certain commitments
• Compound: mutual dependence occurs when x and y depend on each other for realizing a common goal p
• Reciprocal: x and y depend on each other for realizing different goals
Co-operation
Form of mutual dependence
Agents form a co-operative team when:
• All agents share a common goal
• Each agent is required to do its share to achieve the common goal by the group itself or a subgroup
• Each agent adopts a request to do its share
Conclusion
• Characteristics of multi-agent systems.
• Mechanisms for agents communication.
• High level agent interaction protocols.
• Societies of agents
Future work
To develop protocols or societies in which
the effects of deception and misinformation
can be constrained
QUESTIONS & DISCUSSION
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