MPI defines four communication modes which are selected via the specific SEND routine used. The communication mode instructs MPI on the algorithm that should be used for sending the message: synchronous, buffered, ready, or standard. The choice of mode is separate from the selection of blocking/nonblocking; in other words, each communication mode can be applied to either a blocking or a nonblocking send. It should be noted that the RECV routine does not specify the communication mode — it is simply blocking or nonblocking. The table below provides the names of the MPI routines corresponding to each communication mode.

Over the next few pages, we'll discuss each communication mode separately in the context of making blocking calls. Nonblocking calls are covered in Nonblocking Communication.