Before creating a connection, data transfer daemon first check
the residual connection count to find if there is a pair of idle units
which can be used for the new data transfer connection. If there
is, it will find an idle network transit unit by the network transit
head list pointer, modify the idle marker of the unit to make it
turn into a non-idle unit, and then insert it into the end of the list.
After that the data transfer daemon traverses the local doubly
linked circular list to find the idle local transit unit whose number
is same to the network transit unit’s, and then modify the it’s idle
marker and insert it into the end of the list. All the messages of
the connection are transferred via these two transit units.