Understanding ACA -- Reply to Kurt Chan's note

Charles Monia monia at am.shrmsg.shr.mts.dec.com
Thu Jan 5 08:27:59 PST 1995

My .02

Quoting from Kurt's note:

>  Is there a means for a Target to be required to retain sense data
>  until it receives acknowledgement from the Initiator that the sense
>  data has been delivered?  

In my opinion, this has to be addressed in the SCSI protocol definition. If
there is a significant possibility of unacceptable packet loss, some layer
within the transport system, possibly the SCSI protocol layer itself, must be
prepared to deal with the problem.

>  On a channel model, the REQ/ACK handshakes
>  on the sense data may be sufficient.  However, on a FC network using
>  FCP in class 3, the current definition of ACA is insufficient to
>  ensure sense data preservation.  After several readings, it appears
>  that there are still some channel-based assumptions in SAM and other
>  X3T10 documents.

The only assumption in SAM is that all recoverable communications errors are
handled within the transport system transparently to the application.
Therefore, any error of this type that is reported to an application is
considered fatal.


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