There are two levels of communication protocol.
The underlying 'transport layer' is DDE (Dynamic Data Exchange). This provides a simple and well-known method of communicating between two Windows processes.
The higher-level protocol is KQML, a KIF-based language which uses 'performatives' (in plain English, commands) to instruct an agent. KQML is sent as a string via DDE, and parsed at the other end. Actual message 'content' must be in KIF, whose semantics is defined by part of the Enterprise Ontology. Each agent must interpret the KIF and translate to its internal format. For example, a database agent might translate the KIF to SQL or ODBC commands. The response must also be in KIF. For pointers to further information on this subject, see Further information.
There are some behaviours and KQML performatives that must be implemented in every agent. For example, when each agent starts, it should register itself with the Enterprise Task Manager. When the agent quits, it should unregister itself. Also, the agent should respond to requests for information exchange, such as start-task.
Some default behaviour, such as (un)registration, is provided in default Agent Toolkit methods, allowing the agent programmer to quickly get ahead with programming agent-specific functionality.