ACRE is designed so that agents will use pre-designed interaction protocols for their communication. A protocol is the description of the messages that agents are allowed to send and in what order. Whenever two agents decide to make use of a protocol, the messages they send are considered to be part of a conversation. Each conversation should follow the rules set out by a defined protocol.
Protocols are defined as states and transitions. Each transition contains some rules that a message must match against so as to be a legal message. The state of a conversation will change every time a legal message is sent by one agent to another. Visually, each state is shown as a circle with a name in it. Transitions are shown as arrows with the message rules attached. This can be seen below.
Every protocol has one start state associated with it. This is the state a conversation is in at the very beginning, before any message have been sent. Visually, a start state is shown by an arrow pointing to it that doesn't come from another state. An end state shows that the conversation has come to an end and that no more messages should be sent. This is shown by a double-line circle around the state. A protocol can have any number of end states.
The method by which a conversation follows a protocol is illustrated in the following examples.