The send, receive, and reply operations may be synchronous or asynchronous. A synchronous operation blocks a process till the operation completes. An asynchronous operation is non-blocking and only initiates the operation. The caller could discover completion by some other mechanism discussed later. The notion of synchronous operations requires an understanding of what it means for an operation to complete. In the case of remote assignment, both the send and receive complete when the message has been delivered to the receiver. In the case of remote procedure call, the send, receive, and reply complete when the result has been delivered to the sender, assuming there is a return value. Otherwise, the send and receive complete when the procedure finishes execution. During the time the procedure is executing, the sender and receiver are in a rendezvous, as mentioned before. Note that synchronous/asynchronous implies blocking/not blocking but not vice versa, that is, not every blocking operation is synchronous and not every non blocking operation is asynchronous. For instance, a send that blocks till the receiver machine has received the message is blocking but not synchronous since the receiver process may not have received it. Similarly, we will see later a Xinu receive that is non-blocking but is not asynchronous. These definitions of synchronous/asynchronous operations are similar but not identical to the ones given in your text books, which tend to equate synchronous with blocking.