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