Design considerations for secondary logical units

In an IMS ISC session, the bidder is always the primary logical unit (the bind sender), and the first speaker is always the secondary logical unit (the bind receiver). When providing a system design for a secondary logical unit that is to communicate with IMS within an ISC network, consider the following points:

• The first speaker (secondary logical unit) must resolve bracket contention and be prepared to receive interrupts (for example an VTAM® command or bracket indicator) at all times. If the first speaker is not prepared to do so, a deadlock can result (for example, if both the primary logical unit and the secondary logical unit use the VTAM SEND macro, and issue POST=RESPONSE rather than POST=SCHED).
• When terminating a session using the VTAM TERMSESS macro, the system designer should be aware of the differences involved in issuing CONDITIONAL versus UNCONDITIONAL. Sending CONDITIONAL permits the primary logical unit to clean up and control session termination. Sending UNCONDITIONAL causes immediate termination of the session by VTAM prior to notification of the primary logical unit. If the primary logical unit is in a suspended state as, for example, when waiting for a response, the system designer should be aware that UNCONDITIONAL session termination can have unanticipated results.