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Computer Science

Agent Communications:Static Protocols

CPSC 601.68/CPSC 599.68Rob Kremer

Department of Computer ScienceUniversity of Calgary

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Static (Ad-hoc) Protocols

Merely “scripts” that the agent follows

• Typical notation is state diagrams• Petri nets are a generalization of state diagrams

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Ad-hoc Protocols: Petri NetsBuyer: Request to Buy

Seller: Request PaymentSeller: Accepting to Sell

Buyer: Accepting to Pay

Buyer: Paying for item

Seller: Accepting Payment

Seller: Handing out item

Buyer: Accepting item

Success

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Simple Petri Nets

“Place”

“Transition”

“Arc”

• Directed arcs only

• Strictly bipartite

• “Tokens” occupy places (a place can contain any number of tokens)

• A transition can fire only when there is sufficient tokens at all of the incoming places.

• The transition firing consumes tokens at the incoming side and emits tokens to the places on the outgoing side.

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Transitions

• If ALL of a transition’s incoming places have tokens, it MAY fire

• So if there’s a single token to available for more than one transition, only one of them MAY fire

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Transitions

• A transition will place tokens on all of it’s outgoing places IF it fires

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Transitions

• EVERY incoming place must have sufficient* tokens available in order for it to fire

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Transitions

• Places can contain any number of tokens

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• Lots of other kinds of Petri Nets– Weighted Petri nets– Coloured Petri nets– Hierarchy– Vector addition system with states– Prioritised Petri nets– Timed Petri nets– Stochastic Petri nets– etc.

• Could do the whole course on Petri Nets!• A decent quick description is on Wikipedia at

http://en.wikipedia.org/wiki/Petri_net

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Example: “call for proposals” A MIP-net (Multi-agent Interaction Protocol net) – combining two A-nets (Agent nets) and one IP-net (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/10.1145/870000/860791/p1052-ling.pdf?key1=860791&key2=2595358611&coll=&dl=ACM&CFID=15151515&CFTOKEN=6184618 Ling & Loke 2003, p.1053

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Example: A produce Protocol

This is an extension of colored Petri nets called “reference nets”

Reference:Lawrence Cabac, Daniel Moldt & Heiko Rolke. A Proposal for Structuring Petri Net-Based Agent Interaction Protocols. Lecture Notes in Computer Science: 24th International Conference on Application and Theory of Petri Nets, Eindhoven, Netherlands, June 2003. Springer, 2003. Also available: http://www.informatik.uni-hamburg.de/TGI/mitarbeiter/wimis/cabac/netcomponents.pdf.gz

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Issues

• Rigid and Brittle

• Hard to account for all eventualities

• See assignment 1 to convinceyourself of this