Another SAS revision 4 errata

Hoglund, Tim thoglund at lsil.com
Fri Jun 27 09:14:41 PDT 2003


* From the T10 Reflector (t10 at t10.org), posted by:
* "Hoglund, Tim" <thoglund at lsil.com>
*
Rob-

Page 211, 7.15.9.1 State description (for XL6:Open_Response_Wait state),
response generation is flawed as follows:

To determine whether a Backoff Retry response or a Backoff Reverse Path
response should be sent, items c), d), e), f) each compare the source SAS
address and connection rate of the received OPEN address frame to the
destination SAS address and connection rate of the transmitted OPEN address
frame.

While this works for a topology containing a single expander, it will fail
(cause bad routing of OPEN address frames) for topologies containing
multiple expanders as follows:

Consider expanders X1, X2 and end devices A, B, C below:

  A---X1---X2---B
           |
           C

A attempts to open B (source = A, dest = B)
C attempts to open A (source = C, dest = A)

Assume arbitration priority of Open(A to B) is higher than Open(C to A).

If the opens cross on the wire between X1,X2 (both of which will be in
XL6:Open_Response_Wait state) the following will occur:

  X1 received OPEN frame source = C
  X1 transmitted OPEN frame dest = B

  X2 received OPEN frame source = A
  X2 transmitted OPEN frame dest = A

  X1 will continue to wait due to the lower priority of the received OPEN
address frame.
  X2 will generate Backoff Reverse Path and forward the Open(A to B) to C in
error.

  (If arbitration priority was reversed, X2 would wait and X1 would generate
Backoff Retry in error.)

What should be compared in each of these cases is the source SAS address and
connection rate of the transmitted OPEN address frame to the destination SAS
address and connection rate of the received OPEN address frame.

The comparisons for this example then would be:

  X1 received OPEN frame dest = A
  X1 transmitted OPEN frame source = A

  X2 received OPEN frame dest = B
  X2 transmitted OPEN frame source = C

  X1 will continue to wait due to the lower priority of the received OPEN
address frame.
  X2 will generate Backoff Retry and arbitrate for a new path to device B
correctly.

  (If arbitration priority was reversed, X2 would wait and X1 would generate
Backoff Reverse Path correctly.)


TimH

~~~~~~~~~~~~~~~~~~~~~~~
Tim.Hoglund at lsil.com
LSI Logic Corporation
Storage Std Products
719/533-7450 (Voice)
719/533-7480 (Fax)
~~~~~~~~~~~~~~~~~~~~~~~


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