VPNs Illustrated: Tunnels, VPNs, and IPsec

12. ESP

12.1 Introduction

The Encapsulating Security Payload (ESP) protocol provides the same authentication, data integrity, and antireplay protection that AH provides but adds the IPsec confidentiality function. In tunnel mode, ESP also provides limited protection from traffic analysis. The ESP specification is RFC 2406 [Kent and Atkinson 1998b].

Except for the data authenticated and the placement of the authentication data in the packet, the ESP authentication function is identical to that in AH. Given this, we might wonder why ESP has its own authentication function or even why, given that the data is encrypted, we need authentication at all. It happens that unauthenticated ESP is vulnerable to certain remarkably simple cut-and-paste ...