Message passing system:-
The function of message system is to permit processes to communicate with one another without the requirement to resort (option) to shared data. Communication between the user processes is accomplished by the passing of messages. An IPC facility gives at least two operations: send and receive. If processes P and Q need to communicate, they must send messages to and receive messages from each other; a communication link must exist among them.
Methods for logically implementing links
o Direct or indirect communication.
o Symmetric or asymmetric communication.