iSCSI - Change Proposal X bit

["Julian Satran" <Julian_Satran@il.ibm.com>]

Dear colleagues,

A while ago Santosh Rao suggested on this list to remove the X bit as the 
reassignment function is now handled by a Task Management and "delayed" 
PDUs can be caught be the command window check.

The clear advantage of this change is that the target does not have to 
check for the X bit and may drop the command if it is out of the window or 
exists already.

However in order to drop "old" commands that might in the pipe on a 
sluggish connection - removing the X bit will require the initiator to 
issue an immediate NOP requiring a NOP response on every open connection 
whenever CmdSN wraps around (becomes equal to InitCmdSN).

We may want the new BHS to look like

Byte /    0       |       1       |       2       |       3       |
   /              |               |               |               |
  |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|  
  +---------------+---------------+---------------+---------------+
 0|I| Opcode      | Opcode-specific fields                        |
  +---------------+---------------+---------------+---------------+
 4|TotalAHSLength | DataSegmentLength                             |
  +---------------+---------------+---------------+---------------+
 8| LUN or Opcode-specific fields                                 |
  +                                                               +
12|                                                               |
  +---------------+---------------+---------------+---------------+
16| Initiator Task Tag or Opcode-specific fields                  |
  +---------------+---------------+---------------+---------------+
20/ Opcode-specific fields                                        /
 +/                                                               /
  +---------------+---------------+---------------+---------------+
48

and move the direction bit in the opcode 1 bit up

Alternatively we may make the X bit reserved.

Comments?

Julo