# Request/Reply Message Flows in CICS

When SXXREQR receives control, it does a series of steps.

## Procedure

1. Get parameters from either CommArea or Container.
2. Analyze parameters for validity.
3. Check for active RED interface in CICS.
4. Get access to the user data based on input passed.
5. Two methods of SSES selection are available; Primary/Alternative and Round-Robin.
6. Send the RED message to Substation ES using one of three functions (Send, Send with Confirm, or Send with Reply) indicating the RED Service to use.
   - All Substation ES (could be one or many on same or different LPARs) monitoring SXXREQR requests validate Service availability.
   - SXXREQR transfers the message to Substation ES.
   - SXXREQR waits for response (if required) with a timeout. The timeout value is 20 seconds by default.
   - Substation ES handles the message and forwards it to its destination, and then gets a reply and returns it to the waiting SXXREQR program through a Recipe which uses program SXCREPLY.
7. SXXREQR returns the results of the call in the parameter control block and response message as indicated.

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