Message passing refers to services performing a simple, one-way transfer operation between two programs. Like the other one-way messaging models, message passing generally leaves the sending program unblocked. Also, as with all forms of messaging, message passing is usually connectionless, which means that the sending application does not have to explicitly establish and manage a connection with the message’s intended recipient. Although message passing is not inherently a two-way communication model, two-way communication can be accomplished by sending the response in a separate message.