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OpenVPN: Building and Integrating Virtual Private Networks by Markus Feilner

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VPN Concepts—Overview

During the last ten years, many different VPN concepts have evolved. You may have noticed that I always added "network frames" in brackets when I spoke of tunneling IP packets. This became 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. 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 become the basis for other protocols.

The concept of GRE is pretty simple. A ...

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