There are two kinds of transit units in a doubly linked circular
list of transit units (both the network doubly linked circular list
and the local doubly linked circular list): idle transit unit and non-
idle transit unit. In the beginning, every transit unit in the list is
the idle transit unit. When an idle transit unit is needed, it will be
obtained from the head of the list, become a non-idle transit unit,
and then be inserted into the tail of the list. Therefore, the idle
transit units are in the front of the list while the non-idle transit
units are in the end of the list. The linked list head pointer (both
the network list head pointer and the local list head pointer) in the
transit state structure points to the first idle transit head of the list.
The doubly linked circular list of transit units