Outbound IDoc interface - Status control

Hi,

I'm analyzing a requirement where I have to interface ECC with an external partner which reads information from an FTP.

Scenario would be ECC(IDoc)->PI 7.31 AEX->FTP.

As soon as the IDoc is triggered and successfully delivered to PI, I believe ECC manages its status and puts it into 03 (Data passed to port OK). In our scenario, it means that the file was posted into the FTP.

Once the partner picks up the XML file from FTP, the IDoc must be updated to status 06 (Translation OK) and, once it deliveries to final destination (another application), IDoc status must be updated in ECC to 12 (Dispatch OK).

So, my question is how to deal with this status management with PI, acknowledging to ECC the status 06 and 12 (or, eventually a 05 or 17 for error statuses) which are provided by the external partner?

Thanks!