First, replicas need to check if they are receiving a valid sequence of operations from each client. Most checks are simple, such as verifying if a BEGIN is always followed by a COMMIT/ROLLBACK and if the unique identifiers that are sent are valid. There is one additional aspect that could be exploited by a Byzantine client: the client first executes operations in the coordinator and later propagates the complete se- quence of operations (and results) to all replicas. At this moment, the coordinator does not execute the op- erations, as it has already executed them. A Byzantine client could exploit this behavior by sending a sequence of operations during the COMMIT PBFT.
