13.5 CONCLUSIONS

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In this chapter we elaborated on a few network access layer security or layer 2 forwarding/tunneling protocols, including, for example, the L2F protocol, PPTP, and L2TP. The protocols provide some means for virtual private networking. It is, however, important to note that if a protocol or protocol implementation's cryptography is weak or inherently flawed (e.g., MS-PPTP), the resulting security is no better than the security of a protocol that does not use cryptography at all. In fact, the resulting security may even be worse (because the claimed use of crytography may seduce people into transmitting sensitive data they would not transmit under normal circumstances). Note, however, that the fact that weak or inherently flawed cryptography can make the overall security worse is true for any protocol, not just the layer 2 forwarding/tunneling protocols addressed so far. This argument should always be kept in mind and considered with care when cryptography is used.

Because of the weak cryptography that is typically built into layer 2 tunneling protocols, the use of L2TP and IPsec SAs between the LACs and the LNSs provides an interesting possibility to establish VPN channels or tunnels. In Chapter 14, we focus entirely on layer 3 tunneling in general and the IPsec protocols in particular.

A final word is due to the term VPN. We have said that layer 2 tunneling and layer 3 tunneling protocols can be used for virtual private networking. According to RFC 2828, a VPN is "a restricted-use, logical computer network that is constructed from the system resources of a relatively public, physical network (such as the Internet), often by using encryption, and often by tunneling links of the virtual network across the real network" [20]. According to this definition, the use of encryption is not mandatory for VPNs. Consequently, there are some alternative technologies and notions of virtual private networking in use today. These technologies use controlled route leaking (i.e., route filtering) or label switching instead of cryptography to provide VPNs. For example, multi-protocol label switching (MPLS) is a heavily promoted and widely deployed technology to provide a feature similar to a closed user group in a TCP/IP-based network [21, 22]. MPLS implements label switching to make sure that IP packets can only reach legitimate members of a particular host group. There is no cryptographic protection in use. Consequently, any subscriber of an MPLS network has to trust the provider not to eavesdrop on its communications and not to manipulate the IP traffic. Sometimes this level of trust is justified. Sometimes, however, this level of trust is not justified and the subscriber is then well advised to look into and consider the use of VPN technologies that employ cryptography in one way or another.

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