Messages are often thought of as completely independent, such that any sender can send a message on any channel whenever it likes. However, messages are often associated. With Request-Reply (154) pairs, two messages appear independent, but the reply message has a one-to-one correspondence with the request message that caused it. Thus, the replier that processes the request message cannot simply send the reply message on any channel it wants; it must send it on the channel on which the requestor expects the reply.
