Identifying LUs and sessions

After a session has been established, the application program has both the communication identifier (CID) that identifies the session and the symbolic name of the LU (the session partner). For details, refer to the sections on the LOGON and SCIP exit routines and on the OPNDST and OPNSEC macroinstructions in Establishing and terminating sessions with logical units.

The CID must be specified in the RPL for all communications directed to a particular session.

When a RECEIVE macroinstruction issued in the any-mode (described in the following) is completed, VTAM® provides the CID of the session on which the request or response is received. If the application program requires the LU's symbolic name, the application program has three ways to relate the CID to the LU's symbolic (user-supplied) name:

The application program can use an INQUIRE OPTCD=CIDXLATE macroinstruction to translate the CID into a symbolic name.
The application program can maintain a table of CIDs and their symbolic equivalents. Applications that are enabled for persistence should be capable of rebuilding this table during recovery. This is because the CID of a particular session is different after VTAM recovers.
When the application program establishes a session with the LU, the application program can initially assign a 4-byte value to the session (by putting the value in the USERFLD field of the NIB), and VTAM returns the value each time that session's data satisfies RECEIVE. The 4-byte value can be anything the application program chooses to associate with the session. For example, it can be used to identify the session (and thus the LU), or it can contain the address of a subroutine that is to handle that session's data.