secure communication path from the employee to the intranet, a secure tunnel is created. 6.11.1 L2TP One of the most frequently used protocols today is the layer 2 tunnelling protocol (L2TP). Another tunnelling protocol is MIP (mobile IP). Security Transport "IPsec" Transport Payload Added headers Transport "IPsec" Tunnel Tunnel end point Figure 6.21 Layer 3 tunnelling L2TP is a secure protocol used for connecting VPNs over public lines such as the Internet. It is essentially a combination of two other secure communications protocols: PPTP and Cisco Systems' L2F. L2TP is a popular mechanism for accessing corporate networks. It provides a tunnelling facility that can be used both in a voluntary and in mandatory manner. In the voluntary case the user's PC establishes an L2TP connection to the corporate tunnel server over any ISP or Internet access method. In the mandatory case the ISP's remote access concentrator creates a tunnel from a specific user and to a specific destination automatically as the user logs in. This destination could be a particular kind of ISP or service, a server in a corporation that the ISP has an agreement with, or ISP equipment in a location that is closer to the served corporate than the access concentrator. Both models are useful in their own situations. The main advantage of the voluntary tunnelling model is that there is no need for agreements with the ISP. The method is mainly used for road-warrior access from around the world. The main advantages of the mandatory tunnelling method are that there is no need for extra configuration of software on the PCs, and the omission of the tunnelling and security headers from the packets on the typical network bottleneck, the access link. The latter is particularly useful in the case of mobile communication. 6.11.2 GENERIC ROUTE ENCAPSULATION (GRE) PROTOCOL The GRE protocol is used in conjunction with the point-to-point tunnelling protocol (PPTP) to create VPNs between clients or between clients and servers. GRE encapsulation is achieved as shown