SPD contains entries, each of which defines a subset of IP traffic and
points to an SA for that traffic. In more complex environments, there
may be multiple entries that potentially relate to a single SA or multiple
SAs associated with a single SPD entry. Each SPD entry is defined by a
set of IP and upper-layer protocol field values, called selectors. In effect,
these selectors are used to filter outgoing traffic in order to map it into
a particular SA.