in direct communication link Between each pair of processes, there exists exactly one link

Messages are sent to and received from mailboxes (or ports)

is a process that can send messages to a mailbox

is a process that can receive messages from mailbox

send(A, message) & receive(A, message)

A link is established by creating and sharing a mailbox

Each pair of processes may have several links

in indirect communication link A link is associated with … processes

Blocking send, blocking receive are

non-Blocking send, non-blocking receive are

The sending process sends the message and resumes operation, immediately.

The receiver blocks until a message is available. It resumes operation only after receiving the message

The receiver retrieves either a valid message or a null and resumes operation, immediately.

The sending process is blocked until the message is received by the receiving process or by the mailbox

The sender must block until the recipient receives the message

The sender never blocks.

The link cannot have any messages waiting in it.

The queue has finite length n thus, at most n messages can reside in it

If the queue is full, the sender must block until there is an available space

The queue's length is infinite; thus, any number of messages can wait in it

A socket is an endpoint for communication