RE: iSCSI: DLB [T.31] ErrorRecoveryLevel 0.5

[Steve Reames <reames@diskdrive.com>]

Marjorie,
         Let me explain my thoughts, and maybe you or the list can tell me 
if I'm way off base.
         Suppose I'm an initiator, and both the target and I have 
negotiated ErrorRecoveryLevel 0. If I lose an incoming PDU, in theory I 
should just log out. But maybe, for some simple cases, I've added code to 
attempt to recover. I take a guess that maybe the target also can recover 
from simple errors. I throw a SNACK at the target and attempt to recover 
the lost PDU.
         There are two possible results: either it works, or I get a Reject 
PDU back from the target essentially saying it doesn't support SNACK. In 
the case of a Reject PDU, I log out, which is what I would have done 
anyway, so I haven't lost anything. If I recover, then hooray!
         What I'm hoping is that ErrrorRecoveryLevel 0 is not supposed to 
be interpreted as "not allowed to attempt recovery." My understanding 
(which may be wrong) is that when the target declares ErrorRecoveryLevel 0 
it may or may not support error recovery for some simple types of errors, 
but if it declares ErrorRecoveryLevel 1, then it has committed to me (the 
initiator) that it will recover.
         My original observation on David's comment [T.31] was that I 
didn't want language that *forbids* the initiator from at least making an 
effort at error recovery, even at ErrorRecoverLevel 0.

Thanks,
-Steve Reames
---------------------------------------------------------------------------
At 05:13 PM 7/10/2002 -0700, KRUEGER,MARJORIE (HP-Roseville,ex1) wrote:
>David,
>I don't understand what you are aquiescing to?  It seems to me that what
>Steve is proposing is a violation of the protocol - there is no way to
>negotiate "level 0 + SNACK", therefore as Mallikarjun pointed out, it
>wouldn't work.  If you've negotiated error level 0, the remote end of this
>session won't support SNACK (or shouldn't).  This sort of interpretation
>would create an interoperability (not to mention testing) nightmare - that's
>why the error recover levels were defined in the first place.  I think your
>original comment is valid.
>
>Marjorie Krueger
>Networked Storage Architecture
>Networked Storage Solutions Org.
>Hewlett-Packard
>
> > -----Original Message-----
> > From: Black_David@emc.com [mailto:Black_David@emc.com]
> > Sent: Wednesday, July 10, 2002 12:57 AM
> > To: reames@diskdrive.com; ips@ece.cmu.edu
> > Subject: RE: iSCSI: DLB [T.31]
> >
> >
> > Steve,
> >
> >> From DLB's comments:
> >>
> >>>[T.31] 9.16.1  Type
> >>>
> >>>   An iSCSI target that does not support recovery within connection MAY
> >>>   reject the status SNACK with a Reject PDU. If the target supports
> >>>   recovery within connection, it MAY reject the SNACK after which it
> >>>   MUST issue an Asynchronous Message PDU with an iSCSI event that indi-
> >>>   cates "Request Logout".
> >>>
> >>> This should be conditioned on the operational ErrorRecoveryLevel of the
> >>> session, not whether the target supports recovery within connection.
> >>
> >> I would prefer that this not be conditioned on the ErrorRecoveryLevel. If
> >> I am writing code, I may choose to support recovery-within-connection,
>but
> >> not all the features that would be required to move me up to
> >> ErrorRecoveryLevel 1. I would like SNACK and Reject PDUs to work properly
>
> >> for my code, even though it is technically only "ErrorRecoveryLevel 0.5".
> >> As I read it, changing the wording would allow the target to ignore
> >> my improved error recovery efforts unless I have a full
>ErrorRecoveryLevel 1
> >> implementation. David, I doubt that is what you intended, so maybe you
> >> want to word it a little differently.
> >
> > Actually, it was what I intended when I made that comment,
> > BUT, I had not considered the scenario you describe above ...
> > and so, I now agree with
> > you.  Therefore I'll withdraw my [T.31] comment provided that the
> > possibility of multiple "ErrorRecoveryLevel 0.5" levels of support is
> > described in the overview section to be added on error recovery.
> >
> > Thanks,
> > --David
> > ---------------------------------------------------
> > David L. Black, Senior Technologist
> > EMC Corporation, 42 South St., Hopkinton, MA  01748
> > +1 (508) 249-6449            FAX: +1 (508) 497-8018
> > black_david@emc.com       Mobile: +1 (978) 394-7754
> > ---------------------------------------------------
> >