Model Driven Design of Multiagent Systems - Agents and Artificial Intelligence - page 17

Fig. 9. The Contract Net Protocol
The core part of the contract the agents use for interaction is given with a
protocol. Figure 9 shows the diagram for the Contract Net Protocol 7 [5]. In this
protocol a manager agent tries to find an agent in a group of bidders that is
selected to provide a specific service for the manager. We present some more
details on the usefulness of this protocol in Section 5. The building blocks for
protocols are (i) actors, which might be separated into sub-actors, (ii) message
flows, and (ii) and messages. Actors refer to the participants of the protocols.
Role bindings are used to define the requirements to agent types that actually
could take the part of a specific actor in a protocol. Message flows mark specific
states in the protocol execution. If a message flow has more then one exiting arc
exactly one of these arcs can be chosen to continue protocol execution, which
means that the outgoing arcs have an xor semantics. An additional assumption
is that each message flow that receives a message spawns of an achieve goal
event, where the abstract goal's name is derived from the name of the incoming
message which is by definition unique, i.e. a message flow can receive at most
one message. To achieve such an abstract goal might turn out to be a complex
process within the agent and might very well involve the interaction with other
agents which are then, behind the scenes, again organized by using contracts.
Messages are defined by message types.
With these conventions the behavior that an agent needs to comply to when it
engages in a specific interaction is defined in the protocol description. However,
the protocol specifies the communication behavior only. All capabilities that are
available from the agent's body are addressed by spawning off achieve goal events
and the only direct body capability the protocol itself relies on is that the agent
is able to send the messages according to the specified message types. In this
7 Please do not confuse the name of the Contract Net Protocol with what we call
contract based communciation. The dual use of the term contract is basically pure
chance.