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 in Figure 6.22. The original IP packet is encapsulated into a GRE packet. The GRE packet itself is encapsulated into an IP packet. This means an IP packet is transported inside an IP packet. The outer IP packet is forwarded like an ordinary IP packet within an IP network. Understanding Changing Telecommunications - Building a Successful Telecom Business Original packet UPD/TCP Payload GRE Tunnelling UPD/TCP Payload Unprotected data GRE Checksum coverage protection Figure 6.22 The GRE tunnelling principle 6.11.3 SECURITY IN REMOTE ACCESS Secure shell (SSH) is a protocol that permits secure remote access over a network between two computers. The protocol negotiates and establishes an encrypted connection between what is called an SSH client and an SSH server. The SSH protocol consists of three major components: Transport layer protocol. This protocol provides server authentication, confidentiality and integrity. User authentication protocol. This protocol authenticates the client to the server. Connection control. This protocol multiplexes the encrypted tunnel onto several logical channels. SSH can handle seve