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    Re: iSCSI: remove CDB from iSCSI header



    
    I agree with Matt.  Basically, then, the iSCSI header digest should cover
    everything in the first 48 bytes.  All of the messages, with the exception
    of the SCSI Command Request, could easily be done this way, and I think
    that we should modify that to fit Matt's idea of treating the CDB (and
    sense data) the same as SCSI data for digest purposes.  
    
    Here's what they would all look like:
    
    01 - SCSI Command Request
    
    The iSCSI header digest currently covers the 48-byte iSCSI header,
    which includes the CDB.
    
    The data digest covers everything else, which includes extended CDB,
    bidi-read transfer length, and immediate data.
    
    Since the command request is the only operation that really breaks
    the notion of Matt's model, I would suggest that we start the CDB
    after the 48-byte iSCSI header, and that it not be covered by the
    header digest.  We can move the bidi-read length inside the iSCSI
    header, and end up with:
    
      +----------------------------------------------------
     0| Normal iSCSI header stuff for command request
      +----------------------------------------------------
    32| Bidi-read length
      +--------------------------------------------------
    36/ Reserved
     +/
      +---------------------------------------------------
      | (optional header digest goes here)
      +-------------------------------------------------
    48| CDB
      +--------------------------------------------------
      | Additional CDB
      +---------------------------------------------------
      | Immediate Data
      +----------------------------------------------------
      | (optional data digest goes here)
      +-----------------------------------------------------
    
    The header digest would cover the first 48 bytes, and would exactly
    match the rest of the iSCSI operations.  The data digest would include
    the CDB (now contiguous with the additional CDB, as an added bonus)
    and the immediate data.  Anyone concerned with having a separate
    data integrity check for just the data when an additional CDB is
    being used should refrain from using the immediate data feature.
    
    While we are on the topic, is anyone really planning to make use of bidi-read
    and immediate data?
    
    
    Here is my understanding of what the digests cover with the remainder
    of the iSCSI operations:
    
    81 - SCSI Response
    91 - Asynchronous Event
    
    iSCSI header digest covers 48-byte iSCSI header
    iSCSI data digest covers the sense data, if present
    
    02 - Task management command
    82 - Task management response
    06 - Logout command
    86 - Logout response
    90 - R2T
    
    iSCSI header digest covers 48-byte header
    no data, so no data digest present
    
    05 - SCSI Data write
    85 - SCSI Data read
    00 - NOP-Out
    80 - NOP-In
    
    iSCSI header digest covers 48-byte header
    iSCSI data digest covers the data
    
    04 - Text command
    84 - Text response
    03 - Login command
    83 - Login response
    
    iSCSI header digest covers 48-byte iSCSI header
    iSCSI data digest covers the text parameters
    
    ??? Should the text be padded to a 4-byte boundary before adding the data CRC?
    I think that it already says so somewhere in the spec.
    
    --
    Mark
    
    Matt Wakeley wrote:
    > 
    > I don't think I like this.  Header followed by digest followed by more header.
    > 
    > I think this is a good opportunity to think about removing the CDB from the
    > header.  This will reduce the amount of dead space used by headers that do not
    > contain a CDB.  This will be beneficial when sending multiple smaller PDUs (in
    > order to keep the CRC coverage high) by reducing the iSCSI header overhead.
    > 
    > Change the iSCSI command so that there is an iSCSI header (verified by the
    > header digest) followed by CDB "payload" (verified by the data digest).  No
    > options for immediate data.  Keep it simple, like it is in Fibre Channel.
    > 
    > -Matt Wakeley
    > Agilent Technologies
    > 
    > julian_satran@il.ibm.com wrote:
    > >
    > > yes,
    > >
    > > Julo
    > >
    > > "Barry Reinhold" <bbrtrebia@mediaone.net> on 22/01/2001 17:49:29
    > >
    > > Please respond to "Barry Reinhold" <bbrtrebia@mediaone.net>
    > >
    > > To:   mbakke@cisco.com, Julian Satran/Haifa/IBM@IBMIL
    > > cc:   ips@ece.cmu.edu
    > > Subject:  RE: Coverage of Data Digest when using Header Digests
    > >
    > > Ok,
    > >      So if the bidi-read length and ADDCDB fields are not in the data
    > > digest
    > > then I assume we have to figure them into the header digest even though
    > > they
    > > are located past the header digets. Is that the expected behavior?
    > >
    > > -----Original Message-----
    > > From: mbakke@cisco.com [mailto:mbakke@cisco.com]
    > > Sent: Monday, January 22, 2001 10:22 AM
    > > To: julian_satran@il.ibm.com
    > > Cc: ips@ece.cmu.edu; Barry Reinhold
    > > Subject: Re: Coverage of Data Digest when using Header Digests
    > >
    > > Barry-
    > >
    > > In particular, the data digest covers only the SCSI Data part of an
    > > iSCSI message; the header digest covers everything else.  This means
    > > that in an 8k write, the data digest will cover only the 8k, and the
    > > header digest will cover everything else.
    > >
    > > Hope this helps,
    > >
    > > Mark
    > >
    > > julian_satran@il.ibm.com wrote:
    > > >
    > > > Barry,
    > > >
    > > > Considering that one of the reasons to have a separate header and data
    > > > digest was to enable data
    > > > to carried through proxies, virtualizers etc. the current thinking is
    > > that
    > > > the data digest will cover only the data and the header (including
    > > > extensions) will be covered by the header digest.
    > > >
    > > > Julo
    > > >
    > > > "Barry Reinhold" <bbrtrebia@mediaone.net> on 21/01/2001 21:50:04
    > > >
    > > > Please respond to "Barry Reinhold" <bbrtrebia@mediaone.net>
    > > >
    > > > To:   Julian Satran/Haifa/IBM@IBMIL
    > > > cc:
    > > > Subject:  Coverage of Data Digest when using Header Digests
    > > >
    > > > Julian,
    > > >      Is the Data Digest intended to cover whatever follows the 48 byte
    > > > iSCSI
    > > > header? In particular in a command frame which has a CDB > 16 bytes, uses
    > > > bidi, has immediate data, and is using both header and data digests -
    > > what
    > > > would the data digest cover?
    > > >                                                              - barry
    > > > reinhold
    > >
    > > --
    > > Mark A. Bakke
    > > Cisco Systems
    > > mbakke@cisco.com
    > > 763.398.1054
    
    -- 
    Mark A. Bakke
    Cisco Systems
    mbakke@cisco.com
    763.398.1054
    


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Last updated: Tue Sep 04 01:05:45 2001
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