The transit cache state block contains a transit cache state
structure which mainly has three fields: network linked list head
pointer, local linked list head pointer and residual connection
count. The network linked list head pointer points to the first idle
transit unit of the network doubly linked circular list. It is used
when network data transfer daemon receives message or local
data transfer daemon sends message. The local linked list head
pointer points to the first idle transit unit of the local doubly
linked circular list. It is used when local data transfer daemon
receives message or network data transfer daemon send message.
The residual connection count indicates that how many data
transfer connections the transit cache can still support to create. It
is used for the data transfer daemon to create a data transfer
connection.