Automatic and explicit buffering -Automatic buffering provides
a queue with indefinite length; thus ensuring the sender will never
have to block while waiting to copy a message. There are no specifications
how automatic buffering will be provided; one scheme may reserve sufficiently large memory where much of the memory is wasted. Explicit buffering specifies how large the buffer is. In this situation, the sender may be blocked while waiting for available space in the queue. However, it is less likely memory will be wasted with explicit buffering.