| 6.1 |  | Virtual Private Networks |
| 6.1.3 |  | VPN technology options |

| Figure shows the methods of protection implemented on different layers. With implementation of encryption on one layer, this layer and all layers above it are automatically protected. Network layer protection offers one of the most flexible solutions. It is media independent as well as application independent.

Providing privacy and other cryptographic services at the application layer was also very popular in the past. In some situations, it is still heavily used today. However, application layer security is application specific and protection methods need be reimplemented in every application.

Some standardization has been successful at Layer 4 of the OSI model with protocols such as Secure Socket Layer (SSL) providing privacy, authenticity, and integrity to TCP-based applications. SSL is used heavily in modern e-commerce sites. However, SSL fails to address the issues of flexibility, ease of implementation, and application independence. One of the latest technologies available, Transport Layer Security (TLS), will address many of the limitations of SSL.

Protection at lower levels of the OSI stack, especially the Data Link layer, was also used in communication systems of the past. This provided protocol independent protection on specific untrusted links. However, Data Link layer protection is expensive to deploy on a large scale because there is a need to protect every single link separately. Data Link layer protection allows for man-in-the-middle attacks on intermediate stations, or routers, and is usually proprietary.

Because of the above limitations, Layer 3 has become the most popular level to apply cryptographic protection to network traffic.