Completing session initiation

After the half sessions are in sync, either half session can decide to accept or reject the session.

• If the primary half session does not want to continue the session, it sends UNBIND.
• If the primary half session wants to continue the session, it sends start data traffic (SDT).
  • If the secondary half session responds positively, the session is formally bound and traffic on the session can begin.
  • If the secondary half session wants to discontinue the session, it rejects the primary half session's SDT, whereupon the primary must respond with an UNBIND and the session terminates.

Session initiation, including allocation of the LTERM subpool, is completed when a start data traffic (SDT) response has been received if IMS is the primary half session or when an SDT response is sent if IMS is the secondary half session. The LTERM subpool remains allocated to the other half-session name, even across session or subsystem failures, until the session is terminated by mutual consent of both half sessions (symmetrical shutdown). This requires that, after they have been allocated, all subsequent session binds for the same half session must specify the same bind session qualifiers that were active at abnormal session termination until the subpool is released through normal session termination.

If the session is terminated prior to the completion of SDT, any newly attempted subpool allocation caused by a session cold start is backed out, and the subpool is returned to an available-for-allocation status.

In IMS, the master terminal operator must always be notified of any session that is rejected prior to a start data traffic (SDT) completion. In an ISC session, notification to the IMS master terminal operator is optional for normal initiation and termination sequences. Operator notification is specified by the OPTIONS keyword parameters MTOMSG and NOMTOMSG on the TYPE and TERMINAL macros or by an ETO logon descriptor.