Message exchange patterns (MEP) are a type of design pattern that describe distributed communications. The patterns describe the interactions between the different participants on the network (multiple clients, multiple servers).

Common MEP’s include:
One-Way (I send you a message and don’t expect a response)
And…
Request-Response (I send you a message and expect a response)

These obviously are the simplest cases and more complicate patterns are formed from these building blocks.