The length (i.e. number of octets) of a Group ID. If no
length is given (i.e. it has a value of 0), the default
length of its cgmGdoiGroupIdType should be used as long as it
is not reprsented by an ASCII string. If the value has a
type that is represented by an ASCII string, a length MUST
be included. If the length given is not 0, it should match
the 'Payload Length' (subtracting the generic header length)
of the Identification Payload for a GDOI GROUPKEY-PULL
exchange.