GRE: carry any protocol through any network
GRE (Generic Routing Encapsulation) wraps any network-layer packet inside an IP packet. This creates a virtual tunnel — private IP packets (10.x.x.x) travel across the public internet by riding inside public IP packets.
GRE is the basis for many overlay network technologies: site-to-site VPNs, DMVPN, and SD-WAN overlays all use GRE or GRE variants.