The three business problems a typical organization is trying to solve with a Virtual Private Network are:
Intra-organizational communication (intranet).
Communication with other organizations (extranet).
Access of mobile users, home workers, remote office, and so on, through inexpensive dial-up media (Virtual Private Dial-up Network)
The three types of VPN solutions usually span most of the topologies and technologies offered by VPN service providers, but differ greatly in the level of security required in their implementation.
Intra-organizational communications usually are not protected well by the end hosts or the firewalls. The VPN service used to implement intra-organizational communication therefore must offer high levels of isolation and security. Intra-organizational communications also require guaranteed quality of service for mission-critical processes.
These are the two major reasons why we don't see many organizations using Internet, which cannot offer end-to-end quality of service, isolation, or security, as the infrastructure for their intra-organizational communications. Intranet VPNs were thus usually implemented with traditional technologies like X.25, Frame Relay, or ATM.
Inter-organizational communications frequently take place between central sites of the organizations?usually using dedicated security devices, such as firewalls or encryption gear similar to the setup demonstrated in Figure 7-3. These communications also might have less stringent quality of service requirements. This set of requirements makes the Internet more and more suitable for inter-organizational communications; therefore, it's no surprise that more and more business-to-business traffic takes place over the Internet.
Remote user access into a corporate network, typically from changing or unknown locations, is always riddled with security issues, which have to be resolved on an end-to-end basis using such technologies as encryption or one-time passwords. Thus, the security requirements for VPDN services were never as high as the requirements for Intranet communications. It's no surprise that most of the VPDN services today are implemented on top of Internet Protocol (IP), either over the Internet or using the private backbone of a service provider, as illustrated in Figure 7-4. The protocols used to implement VPDN service over IP include Layer 2 Forwarding (L2F) or Layer 2 Transport Protocol (L2TP).
The VPDN technology uses a number of special terms that are unique to the VPDN world:
Network Access Server (NAS)? The Remote Access Server (RAS) managed by the service provider that accepts the customer call, performs the initial authentication, and forwards the call (via L2F or L2TP) to the customer's gateway.
Home Gateway? A customer-managed router that accepts the call forwarded by the NAS, performs additional authentication and authorization, and terminates the PPP session from the dial-up user. The PPP session parameters (including network addresses, such as an IP address) are negotiated between the dial-up user and the home gateway; NAS only forwards frames of Point-to-Point Protocol (PPP) between the two.
The details of VPDN, L2F, and L2TP are beyond the scope of this book. Please refer to RFC 2341 Cisco Layer Two Forwarding (Protocol) "L2F" and RFC 2661 Layer Two Tunneling Protocol "L2TP" for additional information on these topics.