Optimistic Approach
In Time Warp, the smallest timestamp among all unprocessed event messages is called Global Virtual Time (GVT). No event with timestamp smaller than GVT will ever be rolled back.
An event may do two things that have to be rolled back: it may modify the state of the logical process, and/or it may send event messages to other processes.
To be able to roll back, periodically the process's state (state vector) is saved, and old state is restored on rollback.