The information exchanged between an external program and the transaction program (R/2: X1SA, R/3: Dispatcher service) for the logon is determined in the SAP system.

The SAP logon protocol consists of the protocol header (12 bytes) and the actual logon to the SAP System.

The protocol header, the first 12 bytes of the logon sequence, has the following structure:

For a correct logon, a CONNECT message must be sent to the transaction program in the desired SAP target system. This logon/start request must always be sent in EBCDIC code.

• If you are using an external security system (RACF, ACF2, TOP SECRET..), transfer of user and client ID via SAP SignOn Exit (SONEX) for CPI-C connections is not supported. A valid user/password combination must therefore be available
  in the SAP system.
• From R/2 Release 5.0 and in R/3, only specific user IDs
  are permitted (user master record type CPIC).

After a CONNECT request is sent from an external system, the SAP system sends a response. The type of message depends on whether the logon was correct or not.

After a connection request (CONN...) is sent with valid logon data, the SAP system returns the request to start CPI-C data transfer. This acknowledgement is always sent in EBCDIC format.

Within the SAP system, the specified ABAP FORM routine has already been activated and performed up to the first RECEIVE statement. This routine then waits for data from the partner.

Positive response: APPCCPIC1  

After a connection request (CONN...) is sent with invalid logon data, the SAP system termintes the conversation and sends an error message if the request had a valid format.

The negative response from the SAP system consists of the general protocol header beginning with FREE..., an error number and an error text (see the second example in the above illustration).