MultiAgent System Communication Paradigms Abstract MultiAgent Systems seem

Multi-Agent System Communication Paradigms Abstract: Multi-Agent Systems seem like a good idea -- solve problems by using several different specialized agents who cooperate to reach a solution. Aside from obvious problems (such as disagreements, inconsistencies, and composition), communication is a significant impediment. Agents may be written by different people in different styles, with different objectives. Simple communication protocols tend to be brittle and fail because not all agents my behave identically, and my respond in unanticipated ways. Attempts to formalize and standardize communication, such as the one the Foundation for Intelligent Physical Agents (FIPA) developed using BDI (Belief, Desire, Intension) semantics, encountered difficulties that made implementation and conformance difficult. Some of these problems stem from the FIPA standard's requirement that all agents model the beliefs, desires, and intentions of all other agents, which makes for a rather unnaturally omniscient society of agents. One way to address this issue is to neglect the modeling of the "minds" of other agents, and model only the social commitments agents undertake (or fulfill) in the course of their conversations. To achieve this, conversations are modeled as sequences of speech acts, where rules or policies (which may be considered social norms) translate from the speech acts to the instantiation or deletion of social commitments among the participant agents. 2005/07/27 Commitment-based Conversations Rob Kremer University of Calgary Department of Computer Science Calgary, CANADA kremer@cpsc. ucalgary. ca 1

Simple Conversation: naive What time is it? 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications It’s 2: 00 2

Simple Conversation: more complex What time is it? <nod> Just a sec, I’ll check <nod> It’s 2: 00 <nod> Thanks <nod> 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 3

Simple Conversation: failure What time is it? I don’t know, my watch stopped Go ask someone else. <ignore> (didn’t hear) 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 4

State Machine Eg: “call for proposals” A MIP-net (Multi-agent Interaction Protocol net) – combining two A-nets (Agent nets) and one IPnet (Interaction Protocol net). Reference: Sea Ling & Seng Wai Loke. A Formal Compositional Model of Multiagent Interaction. AAMAS’ 03 – International Conference on Autonomous Agents and Multi. Agent System, July 14 -18, 2003, Melbourne, Australia. ACM, 2003. Also available: http: //delivery. acm. org/10. 1145/870 000/860791/p 1052 ling. pdf? key 1=860791&key 2=2595 358611&coll=&dl=ACM&CFID=151 51515&CFTOKEN=6184618 2020 -11 -03 Ling & Loke 2003, p. 1053 CPSC 609. 68/599. 68: Agent Communications 5

BDI • Belief – Environment • Desire – Goals • Intention – The current desire(s) chosen by the selection function with the beliefs and desires as input 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 6

Motivation • Agents aren’t objects – therefore they must communicate in order to influence other agents (as opposed to invoking methods on other agents) • We treat such communications just like other actions, and call it “speech act theory” 1, 2 1 John Austin. How to Do Things With Words. Oxford University Press: Oxford, England, 1962 2 John Searle, Speech acts: An Essay in the Philosophy of Language. Cambridge University Press: Cambridge, England, 1969. 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 7

Speech Acts • Performative verbs – Eg: request, inform, promise • Aspects: – Locutionary act: making an utterance – Illocutionary act: action performed in saying something – Perlocution: the effect of the act • Felicit conditions: – There exist an accepted procedure, and circumstances and persons must be specified – Must be executed correctly and completely – Must be sincere, and must have uptake 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 9

Example Request • Normal I/O conditions: – Hearer not deaf – Not uttered in a play or film • Preparatory conditions: – – Speaker must correctly choose the act Hearer must be able Speaker must believe the hearer is able Hearer wouldn’t have done the act anyway • Sincerity conditions: – Not sarcasm, etc. 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 10

Types of Speech Acts • Representatives: commits the speaker to the truth of a proposition – Eg: Informing • Directives: attempt to get the hearer to do something – Eg: Requesting • Commissives: commit to a course of action – Eg: Promising • Expressives: express some psychological state – Eg: Thanking • Declarations: effects some change in an institutional state of affairs – Eg: Declaring war 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 11

Rational Action • Speech act theory can be considered as a specialization of a more general theory of rational action 1 • Eg: “A request is an attempt on the part of spkr, by doing , to bring about a state where, ideally, (i) addr intends (relative to the spkr still having that goal, and addr still being helpfully inclined to spkr), and (ii) addr actually eventually does , or at least brings about a state where addr believes it is mutually believed that it wants the ideal situation” 1, p. 241 1 P. R. Cohen and H. J. Levesque. Rational interaction as the basis for communication. In P. R. Cohen, J. Morgan, and M. E. Polleck (eds), Intentions in Communications, pp 221 -56. The MIT Press: Cambridge, MA, 1990. 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 12

FIPA Messages performative the type of the communicative act of the message sender the sender of the message receiver the intended recipients of the message reply-to subsequent messages in this conversation thread are to be directed to the agent named content the content of the message; equivalently denotes the object of the action language the language in which the content parameter is expressed encoding the specific encoding of the content field ontology the ontology(s) used to give a meaning to the symbols in the content expression protocol the interaction protocol that the sending agent is employing with this ACL message conversation-id an expression used to identify the sequence of communicative acts that form a conversation reply-with an expression that will be used by the responding agent to identify this message in-reply-to an expression that references an earlier action to which this message is a reply-by a time and/or date expression indicating the latest time the sending agent wants a reply 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications See FIPA standard SC 00061 G 13

FIPA Performatives (informally) accept-proposal agree cancel call-for-proposal confirm disconfirm failure Inform inform-if inform-ref not-understood propagate propose proxy query-if query-ref refuse reject-proposal request-whenever subscribe 2020 -11 -03 accepting a previous proposal agreeing to perform some action inform another agent that the agent no longer need perform some action calling for proposals to perform an action informs a given proposition is true informs a given proposition is false an action was attempted but failed a given proposition is true inform whether or not a proposition is true inform the object which corresponds to a descriptor did not understand what the receiver just did pass a message on submitting a proposal to perform an action pass on an embedded message asking whether or not a proposition is true asking for the object referred to refusing to perform an action rejecting a proposal during negotiation request to perform some action when some proposition becomes true request to perform some action each time the proposition becomes true requesting to notify of the value of a reference whenever the object changes CPSC 609. 68/599. 68: Agent Communications See FIPA standard SC 00037 J 14

FIPA Semantics Semantic content Agent Proposition <i, inform (j, φ )> FP: Biφ ￢ Bi(Bifjφ Uifjφ) Feasibility Precondition RE: Bjφ Rational Effect 2020 -11 -03 “j believes φ” Bjφ Bj￢φ CPSC 609. 68/599. 68: Agent Communications Ujφ Uj ￢φ (where Uj means “j is uncertain about φ”) 15

FIPA inform semantics • Formally: <i, inform (j, φ )> FP: Biφ ￢ Bi(Bifjφ Uifjφ) RE: Bjφ • Example: (inform : sender (agent-identifier : name i) : receiver (set (agent-identifier : name j)) : content "weather (today, raining)“ : language Prolog) 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 16

FIPA request semantics • Formally: <i, request (j, a )> FP: FP(a) [ij] Bi Agent (j, a) ￢Bi Ij Done (a) RE: Done (a) FP(a) [ij] denotes the part of the FPs of a which are mental attitudes of i. • Example: (request : sender (agent-identifier : name i) : receiver (set (agent-identifier : name j)) : content "open "db. txt" for input“ : language vb) 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 17

Plan Preconditions: P Predicate Postconditions: P Predicate Actions: Seq Action hd: tail: Plan 2020 -11 -03 Action // returns head(Plan. Action) Plan // returns {Pre, Post, tail(Actions)} CPSC 609. 68/599. 68: Agent Communications 18

BDI Agent BDIagent B: P Bel D: P Des I: P Int get. Next. Percept: Percept brf: P Bel Percept P Bel options: P Bel P Int P Des filter: P Bel P Des P Int plan: P Bel P Int Plan execute: 2020 -11 -03 Plan CPSC 609. 68/599. 68: Agent Communications 19

Basics • First order model logic with identity • Three primitive attitudes: B ip U ip C ip i (implicityly) believes p i is uncertain about p but thinks that p is more likely than ¬p (choice) i desires that p currently holds • Other attitudes: PGip I ip 2020 -11 -03 i has p as a persistent goal (desire? ) i has the intention to bring about p CPSC 609. 68/599. 68: Agent Communications 20

Actions a 1 ; a 2 a 1 | a 2 sequence in which a 2 follows a 1. nondeterministic choice in which either a 1 happens or a 2 happens, but not both. Feasible(a, p) a can take place and, if it does, p will be true just after that. (Feasible(a) Feasible(a, True) Possible(p) a. Feasible(a, p). Done(a, p) a has just taken place and p was true just before that. (Done(a) Done(a, True) Agent(i, a) i is the only agent that ever performs (in the past, present or future) the actions a. Single(a) a is not a sequence. ¬Single(a 1; a 2), but Single(a 1|a 2) iff Single(a 1)/Single(a 2) 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 21

Abbreviations Bifip Bip / Bi¬p Brefi lx (x) y. Bi(lx (x) = y). Agent i believes that it knows the (x which is) . Uifip Uip / Ui¬p Urefi lx (x) y. Ui(lx (x) = y). ABn, i, jp 2020 -11 -03 Bi. Bj. Bi …p. n is the number of B operators alternating between i and j. CPSC 609. 68/599. 68: Agent Communications 22

Property: intending to achieve a RE ( x. Bi ak=x) / RE(ak)=p / ¬Ci¬Possible(Done(ak)) (Iip Ii. Done(a 1| … |an) // there exists an action // who’s RE is p // that i thinks should be done // then if i intends p, then i intents one of the act that can achieve it Where a 1, . . . , an are all the acts of type ak “If I intent to break the vase, then I intent to either drop it, or smash it with a hammer” 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 23

Property: satisfiability of intent = Ii. Done(a) Bi. Feasible(a) / Ii. Bi. Feasible(a) If agent i intends a, then it needs to believe a is feasible or at least have the intent to discover if a is feasible “If I intend to build a perpetual motion machine, then I have to believe it’s possible or to at least discover if it’s possible. ” 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 24

Property: intent of act implies intent of RE = Ii. Done(a) Ii. RE(a) If agent i intends a, then it also intents the RE of a “If I intend to drop the vase, then I also intend to break the vase” 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 25

Property: observing an act = Bi(Done(a) / Agent(j, a) Ij. RE(a)) If agent i observes j doing a, then i will come to believe the j intends the RE of a. “If I see Jane hammering the vase, then I believe that Jane intends to break the vase” 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 26

BDI Issues • Sometimes end up with expressions like “I believe that you believe that…” • Calls on agents to have a omniscient view of all the other agents • FIPA is based on BDI semantics 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 27

Social Commitments Many utterances imply some sort of “conversational” social commitment • Eg: a request commits the receiver to reply Other social commitments are negotiated • Eg: “wash my car” Basic Agent body: • When an agent observes (or sends or receives) a message, it uses policies (rules) as social norms that generate (or delete) social commitments • Agents spend their free time trying to fulfill the social commitments for which it is a debtor 2020 -11 -03 CPSC 609. 68/599. 68: Agent Communications 28

FIPA Performatives inform proxy propagate request cancel confirm disconfirm inform-ref query-if request-when propose query-ref call-for-participation request-whenever subscribe 2005/07/27 agree accept-proposal failure not-understood reject-proposal refuse Commitment-based Conversations 29

FIPA Performatives performative inform Added catagories proxy propagate ack reply request cancel reply-propose-discharge affirmative-reply confirm disconfirm inform-ref query-if request-when propose query-ref call-for-participation request-whenever subscribe 2005/07/27 agree accept-proposal negative-reply failure not-understood reject-proposal refuse Commitment-based Conversations 30

FIPA Performatives performative inform Added catagories Arranged in a lattice proxy propagate ack reply request cancel reply-propose-discharge affirmative-reply confirm disconfirm inform-ref query-if request-when propose query-ref call-for-participation request-whenever subscribe 2005/07/27 agree accept-proposal negative-reply failure not-understood reject-proposal refuse Commitment-based Conversations 31

FIPA Performatives performative inform proxy propagate ack Added catagories Arranged in a lattice Extended reply request cancel reply-propose-discharge affirmative-reply propose-discharge nack confirm disconfirm done inform-ref query-if request-when propose query-ref call-for-participation request-whenever subscribe 2005/07/27 agree notify accept-proposal negative-reply timeout failure not-understood reject-proposal refuse Commitment-based Conversations 32

Policies P-inform commits the addressee to acknowledge P-ack releases informed agents of the commitment to acknowledge P-request commits the proposed agents to reply P-counteroffer commits addressees to reply P-reply releases proposed agents of the commitment to reply and releases counteroffered agents of the commitment to reply P-agree an acceptance realizes the shared uptake of proposed/counteroffered commitments P-done releases accepted agents of the commitment earlier agreed to 2005/07/27 Commitment-based Conversations 33

Policies Commitment Operators Performatives Policies Performative P-inform P-ack Inform Ack P-request Reply P-reply Agree P-agree Confirm P-confirm 2005/07/27 Commitment Operators Commitment-operator Operator: add Commitment: (receiver, sender, ack) Commitment-operator Operator: delete Commitment: (sender, receiver, ack) Social Commitments action communication-act Commitment-operator Operator: add Commitment: (reciever, sender, reply) ack Commitment-operator Operator: delete Commitment: (receiver, sender, reply) reply Commitment-operator Operator: add Commitment: (receiver, sender, content) Commitment-operator Operator: delete Commitment: (receiver, sender, content) Commitment-based Conversations Unspecified action 34

Example: Informally Alice Can you attend this meeting? (nod) Bob (performative: request, content: attend(Bob, x)) (performative: agree, content: request|attend(Bob, x)) Sure. . . (performative: ack, content: agree|request|attend(Bob, x)) I’m here (nod) (performative: propose, content: discharge|attend(Bob, x)) (performative: ack, content: discharge|attend(Bob, x)) Thanks for coming. (performative: accept-proposal, content: discharge|attend(Bob, x)) (nod) (performative: ack, content: accept-proposal|discharge|attend(Bob, x)) 2005/07/27 Commitment-based Conversations 35

Example: Perf. Lattice and Commitments Bob Alice ack(Alice, Bob, x) inform ack(Bob, Alice, x) request ack reply(Bob, Alice, x) inform ack reply agree ack(Alice, Bob, x) act(Bob, Alice, x) inform ack reply-propose-discharge(Alice, Bob, x) propose/discharge propose-discharge(Bob, Alice, x) ack inform ack(Bob, Alice, x) reply agree accept-proposal ack 2005/07/27 Commitment-based Conversations 36

Example: Implementation Details Bob Alice inform request/* ack consider(Alice, Bob, x) inform ack(Alice, Bob, x) reply(Bob, Alice, x) ack reply agree/request|* ack evaluate(Alice, Bob, x) ack(Alice, Bob, x) reply-propose-discharge(Alice, Bob, x) decide(Bob, Alice, x)` ack(Bob, Alice, x) inform act(Bob, Alice, x) ack reply propose/discharge|* propose-discharge(Bob, Alice, x) ack inform reply accept(Bob, Alice, x) ack(Bob, Alice, x) agree accept-proposal/propose/discharge|* ack 2005/07/27 Commitment-based Conversations 37

Conclusions • Arranging performatives in a lattice simplifies interpretation • Messages (performatives) → policies → commitment operators → shared social commitments • Easily observable by 3 rd parties • Agents do not have the be implemented in the SC style (eg. could be BDI internally) • Turn taking arrises naturally 2005/07/27 Commitment-based Conversations 38