An SCU uses N-ACTION to ask the SCP to change the state of a UPS Instance as shown in Figure CC.1.1-1. Since all UPSs are created as instances of the UPS Push SOP Class, the Requested SOP Class UID (0000,0003) in the N-ACTION request shall be the UID of the UPS Push SOP Class. See CC.3.1 for further details.
To take control of a SCHEDULED UPS, an SCU shall generate a Transaction UID and submit a state change to IN PROGRESS including the Transaction UID in the submission. The SCU shall record and use the Transaction UID in future N-ACTION and N-SET requests for that UPS instance.
Notes: 1. The performing SCU may wish to record the Transaction UID in non-volatile storage. This would allow the SCU to retain control over the UPS after recovering from a crash.
2. If two SCUs try to take control of a UPS, the second SCU will get an error since the first SCU established the correct Transaction UID, so the Transaction UID provided by the second SCU is incorrect.
Upon completion of an IN PROGRESS UPS it controls, an SCU shall submit a state change to COMPLETED and include the Transaction UID for the UPS instance.
To cancel an IN PROGRESS UPS for which it has the Transaction UID, an SCU shall submit a state change to CANCELED and include the Transaction UID for the UPS instance.
Notes: 1. Prior to submitting the state change to CANCELED, the performing SCU can N-SET the values of Reason For Cancellation, Procedure Step Discontinuation Reason Code Sequence, Contact Display Name or Contact URI to provide information to observing SCUs about the context of the cancellation.
2. To request cancellation of an IN PROGRESS UPS for which it does not have the Transaction UID, an SCU uses the Request UPS Cancel action as described in CC.2.2, rather than a Change UPS State action.
Prior to submitting a state change to COMPLETED or CANCELED for a UPS instance it controls, the SCU shall perform any N-SETs necessary for the UPS to meet Final State requirements as described in section CC.18.104.22.168.
At any time after receipt of the N-ACTION-Response, the SCU may release the association on which it sent the N-ACTION-Request.