Re: iSCSI: Question on StatRN usage

["Prasenjit Sarkar/Almaden/IBM" <psarkar@almaden.ibm.com>]

Now I'm confused - the Pittsburgh draft indicates that StatRN usage is
optional.
When did it become mandatory?

<!--StartFragment-->Responses in transit from the target to the initiator
are also num-
     bered.  The StatRN (Status Reference Number) is used for this pur-
     pose.  If the target uses data packet numbering and all the inbound
     data have been acknowledged, or the target is able to regenerate
     inbound data, then the target may free all the resources allocated
     for the task execution just after sending a response.  The same
     holds for targets not allowing full command recovery.  The result
     summary, just enough to rebuild the status PDU, will be kept by
     those iSCSI target implementations that support status recovery



Satran, Smith, Sapuntzakis, Meth                                [Page 5]

iSCSI                         June 2000


     after connection failure.  As the only cause for long delays in
     responses can be failed connections and received responses free-up
     resources, we felt that score boarding responses at the initiator
     could be accomplished by simple bitmaps and there is no need to
     flow-control responses. Status acknowledgment is done by the ini-
     tiator through ExpStatRN (Expected Status RN) and large difference
     between StatRN and ExpStatRN indicates a failed connection.

     iSCSI initiators are required to implement the numbering scheme if
     they support more than one connection.

     iSCSI targets are not required to use the numbering scheme for
     ordered delivery even when they support multiple connections. How-
     ever they are required to provide ExpCmdRN and MaxCmdRN values that
     will enable the initiator to make progress.
<!--EndFragment-->


   Prasenjit Sarkar
   Research Staff Member
   IBM Almaden Research
   San Jose


julian_satran@il.ibm.com@ece.cmu.edu on 10/18/2000 01:57:12 AM

Sent by:  owner-ips@ece.cmu.edu


To:   ips@ece.cmu.edu
cc:
Subject:  Re: iSCSI: Question on StatRN usage





Mallikarjun,

StatRN implementation is MUST. Its role is to count statuses (responses)
that where sent and allow
bulk acknowledgement. As such I did not feel any need to place restrictions
and 0 is a legal value.
As the standard allows 2**32 Initiator Tags you could have 2**32 in flight
status responses (! obviously I think it will never
happen).  Obviously if you know that the target has N outstanding commands
and finished commands and it gets
from an initiator an ExpStatRN more the N distant from where it is
something is wrong!

Julo




"Mallikarjun C." <cbm@rose.hp.com> on 18/10/2000 03:58:54

Please respond to cbm@rose.hp.com

To:   ips@ece.cmu.edu
cc:
Subject:  iSCSI: Question on StatRN usage




Julian,

Could you please comment on the following on StatRN usage?  Let me
know if I am misinterpreting the draft.

Current iSCSI draft seems to allow StatRN values from 0 through
2**32-1.  It does not qualify the value of 0 as it does for CmdRNs.
Assuming that 0 is legal, how would a receiving initiator distinguish
between the target implementation that numbers (assigns StatRNs)
read data PDUs and the one that doesn't?  A StatRN of 0 in a read data
packet could be miscontrued and scoreboarded at the initiator, when
all the target was saying is that it doesn't implement StatRN for
data (assuming that 0 happens to be a legal StatRN at the moment).

Options are:
- explicitly state that StatRN value of 0 is not legal.  This is
  simple, and my preference.  A statRN of 0 in data PDUs in this case
  indicates non-implementation.
- rely on a login dialogue to call out the capability.

Even if we choose the first option, I propose introducing a login/text
key request/response through which an initiator can ask a target not
to number iSCSI data PDUs since he cannot scoreboard all the (potentially
unlimited # per each read command) data PDUs.  Note that the converse
case for target is not critical since target controls the write data
transfers
and can hope to have scoreboarding room always available.

Thanks!
--
Mallikarjun
M/S 5601
Networked Storage Architecture
HP Storage Organization
Hewlett-Packard, Roseville.
cbm@rose.hp.com