Effectiveness of Coloured Petri nets for Modelling and Analysing the Contract Net Protocol

Abstract. The Contract Net Protocol was developed to facilitate contract negotiation in MultiAgent Systems, between an auctioneer and many bidders. It is therefore important to analyse the protocol to ensure that it terminates correctly and satisfies other important properties. There have been few attempts to model and analyse this protocol in the literature. The main paper on its verification, published in 2004, suggests that Coloured Petri nets are inadequate for this task. This seems to be due to a misunderstanding of what Coloured Petri Nets are. The main aim of this paper is therefore to show how Coloured Petri nets can be used effectively to model and analyse this protocol. We present a model of the protocol implemented in CPN Tools. The level of abstraction excludes details of the messages communicated between the agents and processing that does not affect the operation of the protocol. We analyse the protocol and show that it terminates correctly (there are no deadlocks or livelocks, and the terminal states show that the auctioneer and bidders have consistent states)and that there is no dead code (all procedures are executable). We also demonstrate that the channel bounds and number of terminal states depend linearly on the number of bidder agents participating in the negotiations for up to 6 bidders and conjecture that this is true in general.