The perfectly synchronized round model provides the powerful abstraction of crash-stop failures with atomic message delivery. This abstraction makes distributed programming very easy. We present an implementation of this abstraction in a distributed system with general message omissions. Protocols devised using our abstraction (i.e., in the perfectly synchronized round model) are automatically transformed into protocols for the omission model. The transformation is achieved using a round shifting technique with a constant time complexity overhead. This transformation is in a precise sense optimal. Furthermore, and rather surprisingly, no automatic transformation from a weaker model, say the traditional crash-stop model (with no atomic message delivery), onto an even stronger model than the general-omission one, say the send-omission model, can provide better time complexity performance.