Communication operations are non-blocking, with a completion counter.  In some cases, the address of the counter may be specified as  \texttt{NULL}; where this is true, completion must be ensured by some other test.  Counters are used instead of boolean flags to provide a way to handle operations that  involve multiple (internal) steps. Flow control is the responsibility of the device; these routines are non-blocking, allowing the device to queue communication for later completion.  Because the communication/data transfer routines (e.g., \texttt{MPID_Put_contig},  \texttt{MPID_Get_contig}, and \texttt{MPID_Isend}) are nonblocking, the buffers  containing the data to  be transfered must not be modified until the operation has (locally)  completed (as marked by the appropriate counter). Because it may be necessary to update the reference count on the MPI  objects, there are few parameters that may be specified as \texttt{const}.