Date post: | 15-Jan-2016 |
Category: |
Documents |
Upload: | brent-coulbourne |
View: | 220 times |
Download: | 0 times |
Negotiation
Martin Beer, School of Computing & Management
Sciences, Sheffield Hallam University, Sheffield,
United Kingdom
Introduction
• Principles of Negotiation– Game Theoretic Approaches
• Evaluation Criteria• Voting• Auctions• General Equilibrium Markets• Contract Nets
– Heuristic-based Negotiation– Argumentation-based Negotiation
Principles of Negotiation
• Negotiation = Interaction amongst Agents based on communication for the purpose of coming to an agreement
• Distributed conflict resolution• Decision Making• Proposal accepted, refined,
criticized or refuted
Principles of Negotiation
Collectively Motivated AgentsCommon Goals
Coordination to achieve Common goal
Self-interested AgentsOwn Goals
Coordination for Coherent Behaviour
Coordination
Distributed Search through a space of possible solutions
Negotiation Includes
• A communication language
• A negotiation Protocol
• A decision Process by which an agent decides upon its– Position– Concessions– Criteria for agreement etc.
Negotiation Strategies
• Negotiation can take a number of different forms– Single or multi-party negotiation– May include a single shot message by
each party or a conversation with several messages going back and forth
Negotiation Techniques
• Game Theoretic Negotiation– Evaluation Criteria– Voting
Auctions– General Equilibrium Market Mechanisms– Contract Nets
• Heuristic-based Negotiation• Argument-based negotiation
Game Theoretic Negotiation• Evaluation Criteria
– Criteria to evaluate negotiation protocols among self-interested agents
– Agents are supposed to behave rationally– Rational behavior = an agent prefers a greater utility
(payoff) than a smaller one– Payoff maximization
• Individual payoffs• Group payoffs• Social welfare
– Social Welfare• The sum of agents’ utilities (payoffs) in a given solution• Measures the global good of the agents• Problem: How do we compare utilities?
Utility• Utility is the means by which we
optimise the results of the negotiation• Utility often equates to price, this may
not always be the case. It may be– Speed of response– “closeness”– Some combination of functions etc.
• Any function that can be readily computed can be used.
Pareto Efficiency• A solution x
– i.e. a payoff vector p(x1, … xn) is Pareto efficient (i.e. Pareto optimal) if there is no other solution x’ such that at least one agent is better off in x’ than in x and no agent is worse off in x’ than in x.
• Measures global good– Does not require utility comparison
• Social Welfare Pereto Efficiency
Individual Rationality (IR)
• IR of an agent participation = The agent’s payoff in the negotiated solution is no less than the payoff that the agent would get by not participating in the negotiation
• A mechanism is IR if the participation is IR for all agents
Stability• A protocol is stable if once the agents arrived at a
solution they do not deviate from itDominant Strategy
– The agent is best off using a specific strategy no matter what strategies the other agents use
Nash Equilibrium– The strategy profile S*
A = < S*1 …. S*
n >– For each I, S*
I is the agent’s best strategy given that the other agents use strategies
< S*1 …. S*
i-1 ,S*i+1 …. S*
n >P
– Problems• No Nash Equilibrium• Multiple Nash Equilibria• Guarantees stability only in the beginning of the game
Prisoners Dilemma
• Two prisoners are collectively charged with a crime and held in separate cells, with no way of meeting or communicating.
• They are told that:– If one confesses and the other does not, the
confessor will be freed, and the other will be jailed for three years
– If both confess, then each will be jailed for two years
• Both prisoners know that if neither confesses, then they will each be jailed for one year.
Prisoner’s Dilemma – Possible Outcomes
Note – 4 is immediate release
Player j
Cooperate Defect
Player i Cooperate 4,4 1,4
Defect 4,1 3,3
Prisoner’s Dilemma – Strategic Considerations
• The individual rational action is to defect• This guarantees a payoff of at least 2 whereas
cooperating guarantees a payoff of at most 1• Logic Says that this is not the best alternative – if
both cooperate the payoff is 3• This is the fundamental problem with multi-agent
systems• It appears to imply that cooperation will not occur with
self-interested agents• Can we get over this?
Yes – by repeating the problem many times
Computational Efficiency
• To achieve perfect rationality– The number of options to consider is too big– Sometimes no algorithm finds the optimal solution
• Bounded Rationality– Limits the time/computation for options
consideration– Prunes the search space– Imposes restrictions on the types of spaces
Voting• Truthful Voters
– Rank feasible social outcomes based on agents’ individual ranking of those outcomes
– A – set of n agents– O – set of m feasible outcomes– Each agent has a preference relation
<I : O x O, asymmetric and transitive
• Social Choice Rule– Input: the agent preference relations (<1, …. <n)– Output: elements of O sorted according to the
input – gives the social preferences relation <*
Properties of the Social Choice Rule
• A social preference ordering <* should exist for all possible inputs (individual preferences)
• <* should be defined for every pair (o,o’) O• <* should be asymmetric & transitive over O• The outcomes should be Pareto efficient
If I A, o<I o’ then o <* o’
• The scheme is independent of irrelevant alternativesIf I A, < and <‘ satisfy o <I o’ and o <’I o’ then the social
ranking of o and o’ is the same in these two situations
• No agent should be a dictator in the sense thato <I o’ implies o<* o’ for all preferences of the other agents
Arrow’s Impossibility Theorem• No Social choice rule satisfies all of the six conditions
listed on the last slide• Alternatives
– Binary Protocol• Alternatives are voted pair-wise• The chosen alternative depends on the agenda• That is the order of the pairings• This is the method used in parliamentary debates
– Borda Protocol• Assigns an alternative |O| points for the highest preference, |O|
-1 points for the second and so on …..• The counts are summed across the voters and the alternative
with the highest count becomes the social choice• Winner turns loser and loser turns winner if the lowest ranked
alternative is removed
Auctions - Theory• The auctioneer wants to sell an item at the highest possible
payment and the bidders want to acquire the item at the lowest possible price
• A centralised protocol includes– One auctioneer– Several bidders
• The auctioneer introduces the item for sale, which can be a combination of other items, or have multiple attributes
• Bidders make offers. This may be repeated for several times, depending on the auction type
• The auctioneer determines the winner
• Auction Characteristics– Simple protocols– Centralised– Allows collusion “behind the scenes”– May favour auctioneer
Auction Settings• Private Value Auctions
– The value to a bidder agent depends on its private preferences
– Assumed to be known exactly
• Common Value Auctions– The good’s value depends entirely on other
agents’ valuation
• Correlated Value Auctions– The good’s value depends on internal and external
valuations
English (First-price Open Cry) Auction
– Each bidder announces openly its bid– When no bidder is willing to raise anyone, the auction ends– The highest bidder wins the item at the price of the bid
• Strategy– In private value auctions the dominant strategy is to always
bid a small amount more than the current highest bid and stop when the private value is reached
– In correlated value auctions the bidder increases the price at a constant rate or at a rate it thinks appropriate
First-Price Sealed-Bid Auction
• Each bidder submits one bid without knowing the other’s bids
• The highest Bidder wins the item and pays the amount of their bid
• Strategy– No dominant strategy– Bid less than its true valuation but it is dependant
on the other agents bids which are not known
Dutch (descending) Auction
• The auctioneer continuously lowers the price until one of the bidders takes the item at the current price
• Strategy– Strategically equivalent to the first-price
sealed-bid auction– Efficient for real time
Vickery (Second-Price Sealed Bid) Auctions
• Each bidder submits one bid without knowing the others’ bids
• The highest bidder wins but pays the price of the second bid
• Strategy– The bidder dominant strategy is to bid its
true valuation
All-Pay Auctions
• Each participating bidder has to pay the amount of their bid (or some other amount) to the auctioneer
Problems with Auction Protocols
• They are not collusion proof• Lying Auctioneer
– Problem with Vickery Auction– Problem with English Auction – use skills that bid in the
auction to increase bidders’ valuation of the item– The auctioneer bids the highest second price to obtain its
reservation price – may lead to the auctioneer keeping the item
– Common value auctions suffer from the winner’s curse: agents should bid less than their valuation prices (as winning the auction means its valuation is too high)
– Interrelated auctions – the bidder may lie about the value of an item to get a combination of items at its valuation price
General Equilibrium Markets• General Equilibrium Theory =A macroeconomic
theory• A set of goods available at different prices• Two types of agents – Consumers & Producers• The producer’s profits are profits are divided among
the consumers according to predetermined proportions that need not be equal
• The producers’ profits are divided among consumers according the shares they ‘own’
• Prices may change and the agents may change their consumption and production plans but– Actual production and consumption only occur when the
market has reached a general equilibrium
Contract Nets• General Equilibrium Market Mechanisms use
– Global prices– A centralised mediator
• Drawbacks– Not all prices are global– Bottleneck of the mediator– Mediator – point of failure– Agents have no direct control over the agents to which they
send information• Need a more distributed solution• Task allocation via negotiation – Contract Net
– A kind of bridge between game theoretic negotiation and heuristic-based one
– Formal model for making bids and awarding decisions
Contract Net• Protocol
– The agents suggest contracts to each other and make their accepting/rejecting decisions based on marginal cost calculations
– The agent can take the roles of both Contractor and Contractee
– It can also contract out tasks that it received earlier via another contract
– The agents do not know the tasks and cost functions of other agents
– Task allocation improves with each step - hill climbing in the space of task allocations where the high-metric of the hill is social welfare
– It is an anytime algorithm• Contracting can be terminated anytime• The worth of each agent’s solution increases monotonically
Social Function increases monotonically
Contract Net• Problem
– Task allocation stuck in a local equilibrium = no contract net is individually rational and the task allocation is not globally optimal
• Possible solution– Different contract types
• O – one task• C – cluster contracts• S – swap contracts• M – multi-agent contracts
– For each of the four contract types there exists task allocations \for which there is an IR contract under one type but no IR contracts under the other three types
– Under all four contract types there are initial task allocations for which no IR sequence of contracts will lead to the optimal solution (social welfare)
Contract Net• Main differences as compared to game theoretic
negotiation– An agent may reject an IR contract– An agent may accept a non-IR contract– The order of accepting IR contracts may lead to different pay
offs– Each contract is made by evaluating just a single contract
instead of doing look ahead in the future• Untruthful Agents
– An agent may lie about its marginal costs– An agent may lie about what tasks it ahs
• Hide tasks• Phantom tasks• Decoy tasks
– Sometimes lying may be beneficial
Heuristic-based Negotiation• Produce good rather than optimal solution• Heuristic-based negotiation
– Computational approximations of game theoretic techniques
– Informal negotiation models• No central mediator• Utterances are private between negotiating
agents• The protocol does not prescribe an optimal
course of action• Central concern: the agent’s decision making
heuristically during the course of negotiation
Argumentation-based Negotiation
• Arguments used to persuade the party to accept a negotiation proposal
• Different types of agents• Each Argument type defines preconditions for
its usage. If the preconditions are met, then the agent may use the argument
• The agent needs a strategy to decide which argument to use
• Most of the times assumes the BDI model
Argumentation-based Negotiation
• Strategies– Appeal to past promise– Promise a future reward– Appeal to self interest– Threat