Conditions one should ensure that no license cost is incured for communication with NON SAP apps

HI ,

We have customers who still run the ECC version of SAP . They have plans to upgrade but have not migrated till date.

what are our options to pull data from these systems and push data back to these system without impacting the license cost .

If some one can explain in detail what all needs to be ensured while doing such activities, it would be a great help to me.