# Guaranteed

The following figure shows the message flow for the guaranteed communication service level:

For the execution of IMS transactions in transactional mode (commit-then-send, known as Commit Mode 0), the following rules apply:

- You must set the Substation ES service level for a particular recipe to guaranteed.
- The synchronization level is set for the Tpipe. IMS maintains sequence numbers for recoverable input and output for the Tpipe.
- Acknowledgment is always requested by both IMS and Substation ES IMS Interface.
- TIBCO messaging applications must publish the messages with the TIBCO application transactional protocols.

If Substation ES and Substation ES IMS Interface must initiate protected conversations, Substation ES acquires and owns the unit of work (UOW) context identification. The UOW context is passed to Substation ES IMS Interface and provides the context ID in the State Data segment of the OTMA message prefix.

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