The following issues are decided by the system in the process of message passing:
1) Whether the communication link is established or not
2) Whether the receiver is ready to receive the message
3) One or more messages can be sent to the same destination node
The message passing can be of two types: Synchronous and Asynchronous
In Synchronous, the message passing is executed on synchronous communication network. In this case, the sender and receiver processes must be synchronized in time and space. Time synchronization means both processes must be ready before message transmission takes place. The space synchronization needs the availability of interconnection link.
In Asynchronous message passing, message-sending and receiving are not synchronized in time and space. Now, the store and forward technology may be used. The blocked messages are buffered for later transmission. Because of finite size buffer the system may be blocked even in buffered Asynchronous non-blocking system.