Before an LPI connection can be used to transfer data, it needs to be opened. This is performed by sending Open messages, starting with an Open request. The format of the Open messages is defined by the interface being used. For example, the 3270 emulator uses the Function Management Interface (FMI) to communicate with the local 2.1 node.
The interface also defines the initiator of the Open request. In this case, the 3270 emulator sends the Open(SSCP) Request, and the local 2.1 node sends the Open(PLU) Request.
On the Open(SSCP) Request, the 3270 emulator sets all the source and destination LPIs to zero, except for the source index, which can be used by the 3270 emulator for internal routing (for example, to distinguish between two sessions).
The DL-BASE and DMOD ensure that Open messages are routed to a suitable destination. If a routing procedure is used, it should always first call sbpurcvx to process Open responses; when sbpurcvx indicates that it has not processed a received message, and the received message is an Open OK response, the application is informed that the connection was established successfully.