When VTAM® opens an ACB that is capable of persistence, regardless of whether persistence is enabled, VTAM begins tracking data about the RUs that flow across each session. This data tracking permits the recovering application to resynchronize the restored sessions.

The recovering application's ability to restore the sessions pending recovery and resynchronize RU flow depends on complete data tracking information. By providing information about the most recent RUs that are processed, VTAM enables the application to determine which RUs were not processed and to resynchronize the sessions appropriately. VTAM tracks data for all applications that are capable of persistence. VTAM tracks data through the life of the session, including the time when the session is pending recovery. In a sysplex that supports multinode persistent sessions, this data is maintained in the multinode persistent sessions coupling facility structure for access by other VTAM end nodes in the sysplex.

When an application begins restoring the sessions, VTAM reconnects the sessions and sends information to the recovering application about the last RUs that were received on the sessions. The information is returned in the OPNDST OPTCD=RESTORE command and is in the format of the session state control vector (control vector hex 29). A copy of the BIND (as mapped by ISTDBIND) is also provided. See Table 1 for more information pertaining to control vector hex 29 and Table 1 for more information pertaining to the BIND. A 4-byte pointer in the NIB (NIBRPARM) indicates where the recovery information is stored.

VTAM tracks additional information for recovery of LU 6.1 and LU 6.2 sessions supported through the record application program interface (RAPI) as shown in Table 1.