The Gateway carries out RFC services within the SAP world, which are based on TCP/IP. These services enable SAP Systems and external programs to communicate with one another.
RFC services can be used either in the ABAP program or for the external programs using the interfaces. RFC can be used between processes of an instance or a system, or between systems.
Since the RFC protocol is based on the CPI-C protocol, each RFC connection is also a CPI-C connection. Native CPI-C calls are not supported anymore, although they are still mentioned at some positions and in some error codes.
The following figure shows three different SAP systems which use gateways to use RFC connections.
Here you can see the differences between the installation types ABAP -only, Java-only, and dual-stack:
For systems which contain AS ABAP (ABAP-only and dual-stack), each instance contains a gateway that is started and monitored by the ABAP Dispatcher.
As long as the system is installed with ASCS instance (ABAP Central Services, made up of the message server and standalone enqueue server), this does not contain a gateway.
In a Java-only system (in the figure below on the right), one gateway is enough for the whole system since the instances communicate with each other and not using the RFC. The gateway is used for RFC/JCo connections to other systems.
This documentation is intended for system and network administrators, and describes the functions, configuration, and monitoring of the Gateway.
In the SAP System, an Gateway is started for each application server.
For certain constellations, an Gateway is used as a separately installed SAP instance or used outside an SAP System:
Starting external partner programs on remote systems which do not recognize any remote shells (Windows)
Application-specific decoupling of communication (in certain cases)
You should install the Gateway in the same way as you install all other SAP programs, that is in the executable files directory in the corresponding directory structure of the system.
More information: Starting and Stopping the Stand-Alone Gateway