Coordinating the restart process

Half sessions use rules to coordinate restart after a session failure.

The rules used by the half sessions include:
  • The restart always occurs from the most currently completed sync point.
  • The session bind establishes the half-duplex state at the current sync point.
  • When the primary half session wants to restart:
    • It sends a BIND request to select the proper half-duplex state.
    • The secondary half session cannot change this state in the BIND response, unless the secondary half session does not want to restart.
    • When the secondary half session does not want to restart, it sends a BIND response to preclude restart by setting the bracket state to between-brackets.
  • When the primary half session does not want to restart:
    • The primary half session sends a BIND request setting the bracket state to between-brackets.
    • If the secondary half session wants to restart, it sends a BIND response that establishes the proper half-duplex state and sets the bracket state to in-brackets.
    • If the secondary half session does not want to restart, the secondary half session' response to the BIND does not change (it matches the state set by the primary half session).
  • STSN is used to resynchronize any pending requests for sync-point responses.
    • If no mismatch occurred, agreement exists on the current sync point. The restart is attempted after sending start data traffic (SDT).
    • It might be necessary for the half session in send state to send LUSTATUS - NO-OP, CD to adjust the half-duplex state to the current sync point based on the STSN.
    • If a mismatch occurred and the session continues, the half session in send state sends LUSTATUS - NO-OP, EB to place the session into contention.
  • The session in send state at the final restart point reestablishes the session by sending an explicit ATTACH.
Half sessions assuming a secondary role must reject a session bind in these situations:
  • Session bind parameters indicate in-brackets, other half session speaks first, and response mode or conversational output (or equivalent) are immediately available at data traffic active state. This is a session restart logic error.
  • On restart following an abnormal failure or shutdown sequence, session bind parameters indicate half-session names different from those in effect during the previous session between IMS and another logical unit.

In an ISC session, either subsystem can assume a primary or a secondary half-session role, and either subsystem can request message resynchronization; therefore, a set of rules must be established by which these half-session pairs can maintain message or sync point integrity.

These topics describe the format of the STSN and how the STSN is used to complete resynchronization and recovery.