This document describes the usage of Generic Routing Encapsulation (GRE) header for Network Virtualization, called NVGRE, in multi- tenant datacenters. Network Virtualization decouples virtual networks and addresses from physical network infrastructure, providing isolation and concurrency between multiple virtual networks on the same physical network infrastructure. This document also introduces a Network Virtualization framework to illustrate the use cases, but the focus is on specifying the data plane aspect of NVGRE.