• So that the R/3 System can find the destination system, these TCP/IP prerequisites must be fulfilled, in particular the IP addresses in the respective file hosts must be known.
• The name of the gateway and the dispatcher must be entered in the file services, for example, sapgw00 and sapdp00.
• In the R/3 System, Idocs are transmitted from the actual posting (update). Therefore, the TCP/IP link must also be created for the posting system.
• The SAP Gateway must have the right to start the external program (RFC server) via Remote Shell.
  As of release 3.0C, you can work in register mode. In this way, the connection between the external system program and Gateway remains open (see Registering Server Programs with the SAP Gateway in The RFC API).

For details on the TCP/IP settings, refer to the documentation BC - SAP Communication: Configuration.