WCF to acknowledge a received message

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C#

WCF

Hi
Consider this scenario; we have a WCF Service A and a WCF Service B.

1. Service A will send a request message to Service B
2. Service B will receive the request message and sends an acknowledgement to Service A to say that the request message has been received
3. Service B will process the message and send a response message back to Service A, to say your request has been completed
4. Service A will receive the response message and sends an acknowledgement to Service B, to say that the response message has been received

The question is, in step 2, how and what is the best method to use to send the acknowledgement from Service B to Service A, before Service B start processing the message.

Any examples would be great, many thanks!