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    RE: iSCSI: Out of order commands



    Paul,
    
    I should have seen this comming. 
    iSCSI assumes that TCP is there to take care of the transport window and 
    the transport window is as
    large as it should be. However we are talking about the application 
    window.  The application has advertize it's own window in addition to the 
    transport windows. With transport having things in order we can avoid 
    having to have a complete or even partial duplicate of the windows.  If we 
    allow transport to deliver not in order we have to have them duplicated 
    (or at least the control part - and whatever DMA schemes we put in place 
    won't help).
    
    Julo
    
    
    
    
    Paul Koning <ni1d@arrl.net>
    09-11-01 04:25
    Please respond to Paul Koning
    
     
            To:     Julian Satran/Haifa/IBM@IBMIL
            cc:     ips@ece.cmu.edu
            Subject:        RE: iSCSI: Out of order commands
    
     
    
    Excerpt of message (sent 8 November 2001) by Julian Satran:
    > Ron,
    >
    > Targets will most likely advertise a total (command and data) window
    > larger than they can accommodate on any long haul link.  With the 
    current
    > ordering rules nothing bad will happen.
    
    This is a very bad idea.
    
    The whole notion of a window is that you advertise what you can
    handle.  You DO NOT, EVER, advertise more than you can handle in the
    hope that you won't be caught.
    
    Long haul links are no excuse.  The way you deal with those is by
    having more buffer capacity, and issuing window increases early enough
    to keep data flowing.
    
    In a window flow control scheme, a target that advertises resources it
    does not have is defective.
    
    This is all VERY old hat.  TCP has understood how to do this for
    ages.  In particular, it has always been well understood that you HAVE
    to have more buffers in order to run at high speed over long delay
    links.
    
    In essence, what you're saying is that we have an application protocol
    here that is so unusual that it should throw out the lessons of the
    past 40 years, which have taught us how you construct correct
    protocols.  This makes no sense whatsoever to me.
    
    I think the notion of out of order commands on a single connection is
    somewhat strange, but your reason for opposing it is several order of
    magnitude stranger.
    
                        paul
    
    
    
    
    


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Last updated: Fri Nov 09 12:17:39 2001
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