LU 6.2 sync point services is a session-level protocol. The maximum service level parameters are exchanged at session activation. However, when the conversation is allocated, the service level can be lower than the parameters exchanged at session activation. This can be caused by the inability of the other logical unit to support a higher service level. Synchronization of protected resources is the application program's responsibility. It is a means of recovery from errors or failures that allows the data network to divide its transmissions into logical units of work and commit or back out the information in these divisions of work.

The sync point function is divided between VTAM® and a VTAM application program. LU 6.2 provides the facilities to allow the application program to implement a sync point facility by supporting the flow of information between application programs and the ability to control a session by rejecting, holding, or releasing a session based on a session identifier.

Persistent LU-LU session support does not allow LU 6.2 sync point sessions to be retained. Instead, they end when an application fails and they cannot be recovered.

The SYNCLVL operand on the APPL definition statement indicates the level of synchronization for this application program. If you do not code SYNCLVL, confirmation requests and sync point processing are not supported.

On the APPL definition statement, you can also specify (in conjunction with sync point support) when the ATTN exit for an LU 6.2 application program is to be scheduled. This exit routine can be invoked when the last session (ATNLOSS=LAST) is deactivated between an LU and MODE (LU-mode) pair. The ATTN exit can also be invoked for all session deactivations by specifying a value of ALL on the ATNLOSS operand. The LU 6.2 application program can use this operand without implementing sync point support.

For more details on sync point support, see z/OS Communications Server: SNA Programmer's LU 6.2 Guide.