They were able to make IPC fast by using synchronous IPC, fixed size messages, and optimizing for the above things. In either case, the client uses Send to send a message to server. Server may be local or remote. In the message case, the server calls Receive to receive the message. In the RPC case, the server may not actually call Recieve .
Client is blocked waiting for a reply in either case.
Performance of IPC Round-trip time of small messages was 2.7ms. Local IPC was around 480 usec. Process Groups and Multicasting A process can belong to one or more process groups. A process joins a process group by knowing the name of the group. Multicasting allows a process to send a message to all members of a process group.