AH and ESP are commonly called protocols, though this is another case where the
use of this term is debatable. They are not really distinct protocols but are implemented
as headers that are inserted into IP datagrams, as you will see. They thus do
the “grunt work” of IPsec, and can be used together to provide both authentication
and privacy. However, they cannot operate on their own. To function properly,
they need the support of several other protocols and services (see Figure 29-1). The
most important of these include the following