Backoff and Reverse

Elliott at Elliott at
Mon Mar 1 10:37:41 PST 2004

* From the T10 Reflector (t10 at, posted by:
* Elliott, Robert (Server Storage) 
Backoff Reverse Path is an internal signal in the expander.  It is used
when the expander forwards an OPEN that crosses on a wire with an
incoming OPEN and determines that the incoming OPEN must win (due to AWT
and source SAS address comparison of the two OPENs).  

For example, the expander sends an OPEN (Y to Z) out phy B.  On link B,
it crosses on the wire with another OPEN (Y to Z).

phy Y]---link A----[phy A --- ECM/ECR --- phy B]---link B----[phy Z

This means the request which caused the expander's outgoing OPEN needs
to be backed off.  There are two options:
a) Backoff Retry - the source phy has to rearbitrate in the expander for
the destination phy.  In the example this is not chosen because phy A
can service the request - b) is a better option.

b) Backoff Reverse Path - the source phy is the destination of the
incoming OPEN, so the request is going to lose on that phy's physical
link too.  The expander puts out the request after AIP on that link, so
it treated as a "must win."  In the example, phy B sends
BackoffReversePath since phy A can service its request.  phy A puts open
OPEN (Z to Y).  Since this is after the AIP had been sent for the OPEN
(Y to Z), it must win.

Rob Elliott, elliott at
Hewlett-Packard Industry Standard Server Storage Advanced Technology 

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