During the last ten years, many different VPN concepts have evolved. You may have noticed that I added 'network frames' in parenthesis when I spoke of tunneling IP packets. This was necessary because, in principle, tunneling can be done on almost all layers of the OSI model.
A proposed standard for tunneling
The General Routing Encapsulation (GRE) provides a standard for tunneling data, which was defined in 1994 in Request for Comments (RFCs) 1701 and 1702, and later in RFCs 2784 and 2890. Perhaps because this definition is not a protocol definition, but more or less a standard proposal on how to tunnel data, this implementation has found its way into many devices and has become the basis for other protocols.
The concept of ...