The message itself can be structured to a different degree. On one end of the spectrum is an unstructured message that looks like an array of bytes to the middleware. An example of such an interface can be the UNIX IPC API. On the other end of the spectrum is a structured message where the middleware requires that every item of the message is both syntactically and semantically typed. An example of such an interface can be the MACH IPC API or MPI or MQSeries.