In reply to uvm_user123:
to be honest, I did not really get your point. But I believe you are thinking too much in terms of clock or bus cycles. Your refernece model should be on the transaction level, i.e. it does not know anything about timing. It considers only the order of the data it gets. All the timing synchronisation should be done by your DUT. In the reference model you have to ensure 2 things. You should never loose any transaction and you should never change the order. Guaranteeing this all things are fine.