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    Re: iSCSI: NOP-Out closing the command window



    Sandeep,
    
    They are 3 distinguishing elements P and the ITT+TTT.
    
    Julo
    
    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
    
    
    
    


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Last updated: Tue Sep 04 01:04:11 2001
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