Authentication Header (AH)
The Authentication Header (AH) Protocol is designed to authenticate the source host
and to ensure the integrity of the payload carried in the IP packet. The protocol uses a
hash function and a symmetric (secret) key to create a message digest; the digest is
inserted in the authentication header (see MAC in Chapter 29). The AH is then placed
in the appropriate location, based on the mode (transport or tunnel). Figure 30.6 shows
the fields and the position of the authentication header in transport mode.