True synchronous replication requires that every host write I/O operation be committed simultaneously to both local storage and remote secondary storage before an acknowledgment is returned to the production application. This constraint imposes latency penalties and distance limitations typically restricted to metro campus topologies.

In contrast, Zerto uses an asynchronous Continuous Data Protection (CDP) model. By operating at the hypervisor layer via an I/O filter/Virtual Replication Appliance (VRA), it intercepts host write requests, immediately returns local write completion to avoid application latency, and asynchronously streams the delta changes across the WAN to the target site. These changes are tracked in a target journal with continuous checkpoints, delivering RPOs of seconds without the architectural constraints of synchronous replication.