1. A provider network, comprising; a network substrate; and a plurality of host devices coupled to the network substrate, wherein each host device includes a plurality of virtual machines (VMs) and a virtual machine monitor (VMM) that monitors the plurality of VMs on the respective host device, wherein each VMM is configured to: obtain one or more Internet Protocol (IP) packets generated by a source VM on the respective host device, wherein the one or more IP packets are not checksummed by the source VM; encapsulate the one or more IP packets according to an encapsulation protocol to generate a network substrate packet that includes the one or more IP packets as a payload, wherein the network substrate packet includes network substrate packet metadata; generate a checksum of at least the payload of the network substrate packet according to a checksum algorithm; write the generated checksum to a checksum field of the network substrate packet metadata; and send the network substrate packet to a destination on the provider network over the network substrate.