Re: iSCSI: NOP-Out closing the command window

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

Sandeep,

You are right the sequences are not documented now and that is why I put
them out here - to hear comments.

A PDU with P=0 from the target (no ITT) could be hadndled as a mistake.

The 3-way sequence is probably the simplest for the code to handle (no
special path for a request that has no answer).

Julo


Sandeep Joshi <sandeepj@research.bell-labs.com> on 27-07-2001 20:34:32

Please respond to Sandeep Joshi <sandeepj@research.bell-labs.com>

To:   Julian Satran/Haifa/IBM@IBMIL
cc:   ips@ece.cmu.edu
Subject:  Re: iSCSI: NOP-Out closing the command window




Julian Satran wrote:
>
> Sandeep,
>
> They are 3 distinguishing elements P and the ITT+TTT.
>
> Julo
>

I believe you are commenting on my question about the validity
about the 2nd sequence.. correct ?  But this 2nd sequence isn't
documented in Sec 2.12 or 2.13.

Incidentally, there is a similar ping type mentioned in Sec 2.13.1
and its used to test the connection from the target end.  Could
you please say what the response would be :
  T->I  NOP +P=0 T=ff I=ff
  I->T  < response or it none ? >

And I still prefer that the ping response from the initiator *not*
have a valid ITT/cmdSN.  A 3-way ping looks forbidding and may open
up new problems.

-Sandeep

> Sandeep Joshi <sandeepj@research.bell-labs.com> on 27-07-2001 18:53:20
>
> Please respond to Sandeep Joshi <sandeepj@research.bell-labs.com>
>
> To:   ips@ece.cmu.edu
> cc:
> Subject:  Re: iSCSI: NOP-Out closing the command window
>
> If the NOP-Out does not have the ping-bit set, then it is a
> ping response and *not* a new command being issued by the
> initiator.
>
> Hence, it seems that the NOP-OUT with P=0 need not carry a
> new cmdSN (Mark's 2nd option).
>
> Btw, I did not know the 2nd sequence (P=0 from initiator?)
> below was valid
>   > I->T NOP +P=0 +I=x+ Data
>   > T->I NOP +P=0 +I=x
>
> -Sandeep
>
> Julian Satran wrote:
> >
> > Mark,
> >
> > Definitely a problem .  How about stating (the obvious) that NOP as any
> > thing carying an ITT expectes an answer
> > wheter it carries an echo (P=1) or not (P=0).
> >
> > If it does not carry an ITT it does not.
> >
> > We can have the following sequences all valid:
> >
> > I->T NOP +P=1 +I=x+ Data
> > T->I NOP +P=0 + Data
> >
> > I->T NOP +P=0 +I=x+ Data
> > T->I NOP +P=0 +I=x
> >
> > T->I NOP +P=1 +TTT
> > I->T NOP +P=0 I=1 + TTT (no ITT)
> >
> > All would be permitted today if we remove the tie between ITT and P say
> > that NOP must have an ITT if issued at initiators initiative.
> >
> > We might add as valid (today it is not, it is explicitly forbidden):
> >
> > T->I NOP +P=1 +TTT
> > I->T NOP +P=1+ I= 1 TTT + ITT + Data
> > T->I NOP +P=0  +ITT + Data
> >
> > The last requires us to "tweak" the termination rule (a target is
> forbiden
> > to answer a P=1 with a P=1
> >
> > comments?
> > Julo
> >
> > Mark Bakke <mbakke@cisco.com> on 27-07-2001 16:25:20
> >
> > Please respond to Mark Bakke <mbakke@cisco.com>
> >
> > To:   IPS <ips@ece.cmu.edu>
> > cc:
> > Subject:  iSCSI: NOP-Out closing the command window
> >
> > When sending a NOP-Out without the P bit set, there's
> > no response to update ExpCmdSN to keep the window open.
> >
> > On an otherwise idle session, sending a long enough
> > sequence of these NOP-Outs can close the command window
> > permanently.
> >
> > In case of a stuck command window, please break glass...
> >
> > The easy solution is to turn on the P bit, and get the
> > responses to update the window, but that defaults the
> > purpose of allowing the P bit to not be set in the first
> > place.
> >
> > Another easy solution (but I almost hate to mention it)
> > is not to have NOP-Out update the CmdSN.  This seems to
> > have the possibility of breaking other things.
> >
> > I suppose we could come up with a more complicated rule,
> > like "if the NOP-Out's CmdSN would be the last (or perhaps
> > penultimate) CmdSN allowed by the current window, it MUST
> > set the P bit."  Or something like that.
> >
> > Anyway, I see three possible solutions.  Any thoughts?
> >
> > --
> > Mark A. Bakke
> > Cisco Systems
> > mbakke@cisco.com
> > 763.398.1054