You don't push from LDMS to LDSD but instead you pull via setting up a data connection and subsequently a data import mapping which you can schedule (in ServiceDesk).
The import is done on the schedule by the "Data Import" windows service usually running (as Best Practice) on an Application (APP) Server.
This service interacts with the ServiceDesk Framework also running on that Server. The framework talks to the SQL Server via the default SQL port (TCP 1433) so that's what you need to allow through your firewall so that ServiceDesk can PULL data from the LDMS Core Server Database.
Hope this helps.
Thanks Julian. We've been discussing this with the LANDESK guys in the UK and they've confirmed this approach.