GRE (Generic Routing Encapsulation)
Updated
GRE wraps the packets of one network inside IP packets of another, creating a tunnel. It adds no encryption, so it is often combined with IPsec when the tunnel crosses the internet. It is defined in RFC 2784.
In practice
GRE inside IPsec is a common way to run a routing protocol such as BGP between a mobile operator and a customer datacenter over the internet.