Data can be transferred between two SAP systems reliably and safely via transactional RFC (tRFC).
This RFC was renamed from asynchronous to transactional RFC, because asynchronous RFC has another meaning in SAP systems.
The remote system need not be available at the time when the RFC client program is executing a tRFC. In SAP systems, the tRFC component stores the called RFC function together with the corresponding data in the database, including a unique transaction identifier (TID). This ensures that the called function module executed exactly once in the RFC server system.
For more information on tRFC in ABAP, see Transactional Remote Function Calls.
For external systems, the RFC library provides some special RFC calls. However, the transactional RFC cannot be fully implemented in the RFC library.