In his Morals by Agreement, David Gauthier intends to develop a theory of practical rationality related to each individual s pursuit of his or her own interest or ends but from which moral principles can be derived.

The paper makes a comparison of alternatives between the construction of enterprise VPN and the tunnel protocol to realise VPN from the angle of engineering and constructs a flawless security.

This paper introduces VPN and its key techniques that include the tunnelling technique and tunnel protocol,and based on this,probes into the application of VPN in the practice by taking the requirements and methods of configuring VPN to realize the remote accessing to campus-wide network.

First, it introduces the producing background, the functions that can be realized and the key technology of VPN; then, analyzes tunnel technology and tunnel protocols to VPN, and emphasizes work principle of the second tunnel protocol and the third tunnel protocol IPsec; last, shows the advantages and applications of VPN.

L2TP is a Internet tunneling protocol for industry standard.

In this paper, with the VPN implementation requirements in mind, we perform a comparative research on the existing tunneling protocols including GRE, L2TP, IPSec and IP/IP.

Common tunneling protocols are GRE, L2TP and IPSec.