A string contains the following:
A hex string containing the value for the
protection facility. This will be converted
from hex to the octets actually in the
packet by the agent. The agent will supply
the length field and the length octet is not
contained in this string.
An string containing the 3 characters DEF
for a circuit means use the PLE default (the
entry for the PLE default parameters may not
use the value DEF).
A zero length string mean no facility
requested.