FW: Backoff and Reverse

John Lohmeyer john.lohmeyer at lsil.com
Mon Mar 1 09:28:21 PST 2004


* From the T10 Reflector (t10 at t10.org), posted by:
* John Lohmeyer <john.lohmeyer at lsil.com>
*

>----- Original Message ----- 
>From: "Hugh Curley" <hcurley at indra.com>
>To: <t10 at t10.org>
>Sent: Friday, February 27, 2004 7:21 AM
>Subject: Backoff and Reverse
>
>
>>
>>
>>
>> > Is there any legitimate reason for the Backoff and Reverse Expander
>> > operation as shown in figures H7 and H1 of SAS 1 Rev3?
>> >
>> > My thinking is:
>> > If a SSP connection is open, regardless of who opened it, both
>devices
>may
>> > send any SSP frames they may have.  Therefore, if Device A opened
>the
>> > connection, and Device B has frames, it would respond with Open
>Accept.
>> >
>> > Device B would not refuse A's offer of connection because of
>connection
>> > rate.  If A's rate is too fast for B, the connection will be refused
>by
>> phy
>> > Y (Open Reject(Connection Rate not Supported)).  If A's rate is
>slower
>> than
>> > B wants, B will just accept what is offered because B is a target.
>> >
>> > I was told that the only reason for Backoff and Reverse is if the
>"to"
>> > device (B in this case) desired a different protocol.  But here, B
>is a
>> > single protocol device, it cannot talk any other protocol.
>> >
>> > I could understand the operation if there was another expander in
>the
>path
>> > and both A and B made open requests for the other at approximately
>the
>> same
>> > time.  The connection request gets through A's expander (the one
>shown)
>> and
>> > now in B's expander both requests are vying for the same resources.
>If
>> B's
>> > request wins, then that will be forwarded to A's expander and the
>Backoff
>> > and Reverse operation will be as shown.
>> >
>> > My reason for this question is:
>> > On previous occassions when I have noted this type of inconsistency
>in
>the
>> > standard, further understanding showed that the standard was correct
>and
>I
>> > was wrong.  Now, I am concerned that there is something that I do
>not
>> > understand.  Is it possible to have a Backoff and Reverse with a SAS
>> service
>> > delivery subsystem and the "to" device is an SSP only target?
>> >
>> > Thank you,
>> >
>> >
>> > Hugh Curley
>> > hcurley at Indra.com
>> >
>> >
>>

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