HP-UX IPSec version A.02.01 Administrator's Guide

HP-UX IPSec Overview
IPsec Protocol Suite
Chapter 140
On the remote system (System B), the recipient ESP module processes
the inbound ESP packet as follows:
1. The recipient ESP module calculates its own authentication value for
the encrypted payload using its copy of the authentication key
(KeyA).
2. The recipient ESP compares its authentication value with the
transmitted authentication value (the HMAC). If the values match,
the recipient then uses its copy of the encryption key (KeyE) to
decrypt the encrypted portion of the packet and extract the original
payload.
Figure 1-3 ESP Processing
Transport and Tunnel Modes
ESP can be used in transport mode or tunnel mode.