If the data transfer daemon needs to send a message in a
connection, it should traverse the doubly linked circular list of
target subsystem from the end to find the transit unit that belongs
to this connection, and then put the message into the unit. When
the data transfer daemon wants to receive message in a
connection, it will traverse the doubly linked circular list of itself
subsystem from the end to find the transit unit which belongs to
the connection, and then get the message from the unit.