Checkpoint-recovery based virtual machine (VM) replication is an attractive technique for accommodating VM installations with high-availability. It provides seamless failover for the entire software stack executed in the VM regardless the application or the underlying operating system (OS), it runs on commodity hardware, and it is inherently capable of dealing with shared memory nondeterminism of symmetric multiprocessing (SMP) configurations. There have been several studies aiming at alleviating the overhead of replication, however, due to consistency requirements, network performance of the basic replication mechanism remains extremely poor. In this paper we revisit the replication protocol and extend it with speculative communication. Speculative communication silently acknowledges TCP packets of the VM, enabling the guest’s TCP stack to progress with transmission without exposing the messages to the clients before the corresponding execution state is checkpointed to the backup ...