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    Re: iSCSI: NOP-Out closing the command window



    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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