Criteria for selecting logon descriptors

The logon descriptor contains data that is related to the node and the VTAM® CINIT. This data allows IMS to create a control block structure that supports a session.

IMS uses the following criteria (in sequence) to select a logon descriptor:

1. IMS uses existing control blocks for terminals. IMS does not look for a descriptor if it finds existing control blocks.
2. IMS determines whether the Logon exit routine is used to define logon descriptor names.

   For VTAM terminals, the exit routine extracts the name from the VTAM CINIT user data, or another appropriate algorithm. The LUTYPE and TS= (transmission service level) fields in the VTAM CINIT data must agree with the selected descriptor; otherwise, the logon is rejected. IMS rejects invalid logon descriptor names.

3. In response to a request to establish a session (logon), IMS examines the LOGOND parameter. The LOGOND parameter can indicate the logon descriptor for IMS to use.

   LOGOND is a parameter in the VTAM CINIT user data. LOGOND is also a keyword on the IMS /OPNDST command.

   Restriction: LOGOND is not valid for ISC parallel sessions that use VTAM.
4. If IMS does not find a logon descriptor name, it looks for a logon descriptor with the same name as the VTAM CINIT LUNAME.
   Recommendation: Use the node name on the logon descriptor if you expect any of the following situations:

   • You do not expect to add more of the same terminal type.
   • You do not have many of the terminal type.
   • You want to simplify the logic in the Logon exit routine.
   • For ISC, you want to specify parameters (such as OUTBUF=) other than the default ISC logon descriptor, and you have not coded an exit routine.

If these criteria do not yield a valid logon descriptor name, IMS selects a descriptor by using the default criteria.