How does a requestor that has received a reply know which request this is the reply for?

My application is using Messaging to perform a Request-Reply and has received a reply message.
How does a requestor that has received a reply know which request this is the reply for?
Each reply message should contain a Correlation Identifier, a unique identifier that indicates which request message this reply is for.
There are six parts to Correlation Identifier:


Enterprise Integration PatternsFind the full description of this pattern in:
Enterprise Integration Patterns
Gregor Hohpe and Bobby Woolf
ISBN 0321200683
650 pages
Addison-Wesley

http://www.eaipatterns.com/CorrelationIdentifier.html