Failover and Fallback procedures we know very well.
Suppose initial message was supposed to send to primary peer, but peer went down and message has been forwarded to Secondary peer i.e. Failover. Now suppose if Primary Peer has come up and according to DIAMETER theory the message should go to primary peer. So the next message will be Update message. Which will have the same session id which was used with the initial message.
Now my doubt is how the primary peer will recognize the session-id of update message. Because it has no info for that session. The session was created with the secondary peer.