Terminating an ISC VTAM session

Terminating an ISC VTAM® session releases a logical unit from its current connection to the VTAM application program, making the LU available for sessions with other VTAM applications, or terminating communications altogether.

Definitions: The two types of session termination are normal and abnormal. Normal termination allows both half sessions to complete normal processing before the session is terminated. Abnormal termination forces the session to terminate unconditionally.

Session termination can be invoked by the IMS master terminal operator, the VTAM network operator, or either half session.

Because of this variety of session termination methods, each IMS network installation must determine specific procedures for session termination. When developing these procedures, consider the requirements for session-termination processing.

Normal termination

Normal termination of an ISC session occurs with the flow of the data flow control indicators stop bracket initiation (SBI) and bracket initiation stopped (BIS). Normal termination can be initiated by either half session.

Normal session termination also occurs when the MTO invokes an orderly termination of the IMS network by the IMS /CHECKPOINT command with the FREEZE, PURGE, or DUMPQ parameter and the QUIESCE parameter. The QUIESCE parameter ensures that message queues are emptied before the session is terminated.

When all terminals have indicated that shutdown is complete, IMS:
  • Performs checkpoint processing and then issues the VTAM CLSDST macro instruction, when acting as primary half session.
  • Awaits UNBIND, if acting as secondary half session.

CLSDST causes VTAM to send the UNBIND command. CLSDST or UNBIND releases the logical units from session with IMS. Any further data transmission to the logical units is prohibited.

During the processing of an orderly session termination, the IMS master terminal operator can terminate the network unconditionally by using an IMS /CLSDST, /STOP, or /CHECKPOINT command.

Abnormal termination

Abnormal session termination can occur as a result of transmission or protocol errors, or errors in data that make that data unacceptable to the receiving message processing program. Because an ISC session involves two peer-level systems, error recovery processing and abnormal session termination processes can differ. IMS-detected error conditions requiring abnormal session termination result in IMS issuing a VTAM CLSDST macro when IMS is the primary half session, or TERMSESS with OPTCD=UNCOND when IMS is the secondary half session.