The DDCMP module is a data link control procedure that ensures a reliable
   data communication path between communication devices connected by data
   links. DDCMP has been designed to operate over full-duplex and half-duplex
   synchronous and asynchronous channels in both point-to-point and multipoint
   modes.  It can be used in a variety of applications such as distributed
   computer networking, host/front end processing, remote terminal
   concentration, and remote job entry-exit system operation.

   The hierarchical relationship of the entities that constitute the DDCMP
   module:

                   DDCMP
                      Port
                      Link
                        Logical station

   The DDCMP entity is the top-level entity in the hierarchy of entities
   belonging to the DDCMP module.