01-131r4 SPI-4 negotiation message rewrite posted

Elliott, Robert Robert.Elliott at COMPAQ.com
Sun Sep 23 17:44:19 PDT 2001


* From the T10 Reflector (t10 at t10.org), posted by:
* "Elliott, Robert" <Robert.Elliott at compaq.com>
*
This document been posted to the T10 web site:
    01-131r4 SPI-4 negotiation message rewrite

While resolving the pSCSI WG request on 01-131r3 to clarify
the results of changing IU_REQ, this scenario was identified
as a problem:

  WDTR IN -> WDTR OUT -> BUS FREE

If both initiator and target agree that IU_REQ was turned off, 
the initiator will start selecting with attention and sending
IDENTIFY messages to start each connection.

However, the target might have gone BUS FREE because of
parity errors on the WDTR OUT, not because it negotiation
succeeded and it recognized that IU_REQ changed.  This leaves
the initiator thinking IU_REQ is 0 while the target thinks
it is 1.  The initiator will select with attention and
send IDENTIFY, which will result in a MESSAGE REJECT from
the target, which only expects PPR OUT or ABORT TASK at
that time.

The solution incorporated in r4 is to require the target
to always run SDTR after the WDTR:

  WDTR IN -> WDTR OUT -> SDTR IN -> SDTR OUT -> BUS FREE

Even if the SDTR OUT has parity errors and the synchronous
rate is unknown, both sides know that the WDTR succeeded and
can assume that IU_REQ was changed to 0.  This way, the
initiator can safely send an IDENTIFY that the target won't
reject, after which either side can renegotiate if desired.

If you have any issues with this, please submit a SPI-4 
letter ballot comment.

---
Rob Elliott, Compaq Server Storage
Robert.Elliott at compaq.com
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