# Reliable

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

For the execution of IMS transactions in reliable mode (send-then-commit, known as Commit Mode 1), the following rules apply:

- IBM Resource Recovery Service (RRS) is not required to complete a request with Substation ES.
- Transaction pipes need not be synchronized.
- The synchronization level is specified as None in the OTMA message prefix.
- Message receipt from IMS by Substation ES IMS Interface is in random order because XCF does not guarantee ordering.

Note: Currently, send-then-commit with confirmation is not supported.
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