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    Re: DataSegmentLength of 0



    
    
    Barry,
    
    OK - it reads:
    
    1.1.1     DataSegmentLength
    
       This is the data payload length of a SCSI Data-In or SCSI Data-Out PDU;
       sending of 0 length data segments should be avoided, but initiators and
       targets must be able to properly receive 0 length data segments.
    
    
       Regards,
       Julo
    
    "Barry Reinhold" <bbrtrebia@mediaone.net> on 07-06-2001 17:06:19
    
    Please respond to "Barry Reinhold" <bbrtrebia@mediaone.net>
    
    To:   Julian Satran/Haifa/IBM@IBMIL
    cc:   "Jon Sreekanth" <jon.sreekanth@trebia.com>, "James Smart"
          <james.smart@trebia.com>
    Subject:  DataSegmentLength of 0
    
    
    
    
    Julian,
    Clause 2.7.6, DataSegmentLength,
    
    States the following:
    
       "This is the data payload length of a SCSI Data-In or SCSI Data-Out
       PDU; sending of 0 length data segments should be avoided."
    
    For devices that are going to act as protocol translators between Fibre
    Channel and iSCSI it is going to be just about impossible not to send a
    data
    segment length of 0 if we get an underrun condition. (This happens on
    inquery responses. From the Fibre Channel side there is no way to tell that
    the data has been sent until you get a FCP_RSP. At this point the iSCSI
    data
    frames have already been sent with all the data, but the F bit still needs
    to be set. The translating device therefore needs to send a frame with a 0
    length data segment and the F bit set.)  Hence, I suspect almost all
    devices
    that are not acting as end points to IO operations will have this problem
    and you will see iSCSI data frames with the F bit set and the 0 length data
    segments. I agree that we don't want to send 0 length segments, but I do
    think we need to be able to handle them if they are generated.
    
    In order to make people aware of the need to handle 0 length frames I was
    looking for wording changes to the spec along these lines:
    
         "This is the data payload length of a SCSI Data-In or SCSI Data_Out
    PDU;
    sending of 0 length data segments should be avoided, but devices must be
    able to properly receive 0 length data segments."
    
    In this way it makes it clear that reception of 0 length data segments is
    required (which I think was the intent) and devices will be more likely to
    interoperate.
    Barry Reinhold
    Principal Architect
    Trebia Networks
    barry.reinhold@trebia.com
    603-868-5144/603-659-0885/978-929-0830 x138
    
    
    
    
    


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Last updated: Tue Sep 04 01:04:26 2001
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