To secure network traffic between sites and users, organizations use virtual private networks (VPNs) to create end-to-end private network connections. A VPN is virtual in that it carries information within a private network, but that information is actually transported over a public network. A VPN is private in that the traffic is encrypted to keep the data confidential while it is transported across the public network.

The figure shows a collection of various types of VPNs managed by an enterprise’s main site. The tunnel enables remote sites and users to access the main site’s network resources securely.

Modern VPNs now support encryption features, such as Internet Protocol Security (IPsec) and Secure Sockets Layer (SSL) to secure network traffic between sites.

Confidentiality is achieved by encrypting the data, as shown in the figure. The degree of confidentiality depends on the encryption algorithm and the length of the key used in the encryption algorithm. If someone tries to hack the key through a brute-force attack, the number of possibilities to try is a function of the length of the key. The time to process all the possibilities is a function of the computer power of the attacking device. The shorter the key, the easier it is to break. A 64-bit key can take approximately one year to break with a relatively sophisticated computer. A 128-bit key with the same machine can take roughly 1019 or 10 quintillion years to decrypt.

When conducting business long distance, you must know who is at the other end of the phone, email, or fax. The same is true of VPN networks. The device on the other end of the VPN tunnel must be authenticated before the communication path is considered secure. The figure highlights the two peer authentication methods.

The two main IPsec protocols are Authentication Header (AH) and Encapsulation Security Protocol (ESP). The IPsec protocol is the first building block of the framework. The choice of AH or ESP establishes which other building blocks are available.

AH uses IP protocol 51 and is appropriate only when confidentiality is not required or permitted. It provides data authentication and integrity, but it does not provide data confidentiality (encryption). All text is transported unencrypted.

ESP uses IP protocol 50 and provides both confidentiality and authentication. It provides confidentiality by performing encryption on the IP packet. ESP provides authentication for the inner IP packet and ESP header. Authentication provides data origin authentication and data integrity. Although both encryption and authentication are optional in ESP, at a minimum, one of them must be selected.

If ESP is selected as the IPsec protocol, an encryption algorithm must also be selected. Cisco products support 3DES, AES, and SEAL. However, 3DES should be avoided. If 3DES must be implemented, then configure short key lifetimes.

ESP can also provide integrity and authentication. First, the payload is encrypted. Next, the encrypted payload is sent through a hash algorithm, such as SHA-256 or higher. The hash provides authentication and data integrity for the data payload. Note that MD5 and SHA-1 should be avoided.

Optionally, ESP can also enforce anti-replay protection. Anti-replay protection verifies that each packet is unique and is not duplicated. This protection ensures that a hacker cannot intercept packets and insert changed packets into the data stream. Anti-replay works by keeping track of packet sequence numbers and using a sliding window on the destination end.

When a connection is established between a source and destination, their counters are initialized at zero. Each time a packet is sent, a sequence number is appended to the packet by the source. The destination uses the sliding window to determine which sequence numbers are expected. The destination verifies that the sequence number of the packet is not duplicated and is received in the correct order.

ESP and AH can be applied to IP packets in two different modes, transport mode and tunnel mode, as shown in the figure below.