Direct communication
With direct communication each process that requires to communicate must explicitly name the recipient or sender of the communication. The send and receive primitives are
o Send (P,message) - Send a message to process P.
o Receive (Q,message) - Receive a message from process Q.
Indirect communication
With indirect communication, the messages are sent to and received from mailboxes or ports. A mailbox can be viewed abstractly as an object into which messages can be placed by processes and from which messages can be removed. Every mailbox has a unique identification. Two processes can communicate only if they share a mailbox. The primitives are
o Send (A, message) - Send a message to mailbox A.
o Receive (A, message) - Receive a message from mailbox A.