Hi, I have some problem in MPI Function MPI_Issend. That is non-blocking send in synchronous mode. That is, when start this function, the send process send the request-to-send to the destination process. Then the receiver send back a permission-to-send message . And then the sender now sends the message with non-blocking types : The control will return to the sender as soon as possible. My question is : Is it right - the process of MPI_Issend which I describe above ? And how can i know the size of that request for synchronous mode ? Thanks.