06/28 draft: MaxCmdRN scheme

[Costa Sapuntzakis <csapuntz@cisco.com>]

I propose to remove MaxCmdRN from the iSCSI PDUs in which it currently 
appears and placing it in a separate iSCSI PDU.

We need some way of ordering updates to MaxCmdRN. I propose the following
mechanism:

Update MaxCmdRN

   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| Opcode (0x8A) |      0        |             MaxCmdRN          |
      +---------------+---------------+---------------+---------------+
     4| Length                                                        |
      +---------------+---------------+---------------+---------------+
     8| UpdateRN                      |               0               |
      +---------------------------------------------------------------+

MaxCmdRN: the maximum CmdRN that can be accepted by the client
UpdateRN: the next CmdRN that is expect at the target

Algorithm at initiator to update MaxCmdRN:

The initiator keeps the following state for each session:
   session.maxCmdRN
   session.updateRN

if packet.updateRN > session.updateRN ||
   (packet.updateRN == session.updateRN &&
    packet.maxCmdRN > session.maxCmdRN)
   session.maxCmdRN = packet.maxCmdRN

(all arithmetic is done in sequence space)




Solicit MaxCmdRN

   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| Opcode (0x0f) |      0                                        |
      +---------------+---------------+---------------+---------------+
     4| Length                                                        |
      +---------------+---------------+---------------+---------------+

   Used to probe closed windows/get initial MaxCmdRN.


Also, something needs to be said about shrinking windows. Is it allowed?

-Costa