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    Re: iSCSI : On the subject of R2T and Task Tags.



    
    The draft must provide for seperate fields to track the task and the R2T
    along the lines of RX_ID and SEQ_ID in Fibre Channel. Implementations are
    free to use them as they please and must use 0xFFFFFFFF as their un-used
    values in these fields. 
    
    The advantage of defining the fields explicitly in the header as opposed
    to allowing targets to vertically encode a portion of the tag field for
    tracking the R2T is that these standard fields are interpret-able 
    by iSCSI Analyzers and will make debugging and tracking the 
    sequence of the I/O easier than implementation specific 
    vertical encoding into a single field. 
    
    Merging the 2 into a single field is akin to attempting to merge the 
    OX_ID, SEQ_ID and SEQ_CNT fields of FC into a single field and 
    allowing implementation specific vertical encoding. Standard defined
    fields make for easier introp verification and debugging.
    
    - Santosh
    
    
    > 
    > Doug,
    > 
    > The draft make should not make any statements about how a target chooses to
    > assign Target Task Tags. Some may choose to assign it per R2T, others may
    > choose to assign it per task, based on how they choose to implement the
    > target. Since the initiator is required to reflect it, I do not believe
    > there are any interoperability issues. If an implementation wishes to do a
    > Task-SubTask breakdown, they can choose to partition the 32 bits into
    > portions that can be used for identifying resources within the target.
    > 
    > Venkat Rangan
    > Rhapsody Networks Inc.
    > http://www.rhapsodynetworks.com
    > 
    > 
    > -----Original Message-----
    > From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
    > Douglas Otis
    > Sent: Wednesday, January 10, 2001 5:27 PM
    > To: Matt Wakeley; IPS Reflector
    > Subject: RE: iSCSI : On the subject of R2T and Task Tags.
    > 
    > 
    > Matt,
    > 
    > With independent error checking requiring an additional level of error
    > handling requesting repeated reads, there could be old responses combined
    > with the retry from failed data digests. (If requesting overlapped reads are
    > prohibited, retry is an exception.) Without some means of identifying which
    > retry phase data is associated, a complete set would be difficult to
    > confirm.  As order of each data portion is not mandated and these transfers
    > themselves may overlap, byte counts can not reveal completion.  Should does
    > not mean 'must.'
    > 
    > Doug
    > 
    > > Pierre Labat wrote:
    > >
    > > > I agree that the draft must not impose to the target to have a
    > > different tag
    > > > for each R2T (it is up to the target implementation to decide).
    > >  I disagree
    > > > to impose a unique target tag for the whole IO.  It imposes the
    > > target to do
    > > > score boarding
    > > > to know when all the data have been received. Letting the
    > > possiblility to the
    > > > target
    > > > to have one tag per R2T (or add a subtag identifying the R2T as
    > > proposed Santosh)
    > > >
    > > > allows the target to dispense with the score boarding. The
    > > target knows that all
    > > > the
    > > > data has been received when a data PDU with the "F" bit set has
    > > been received
    > > > for each R2T.
    > >
    > > There is no need to do scoreboarding.  Just count the number of bytes
    > > received.  If they all add up, then you've received all the data.
    > >  I for one
    > > would not rely on a PDU with the "F" bit set.  I would want to
    > > cross check it
    > > with the correct number of bytes.
    > >
    > >
    > > >
    > > > Regards,
    > > >
    > > > Pierre
    > > >
    > > > >
    > > > >
    > > -Matt Wakeley
    > > Agilent Technologies
    > >
    > 
    > 
    > 
    
    
    -- 
    #################################
    Santosh Rao
    Software Design Engineer,
    HP, Cupertino.
    email : santoshr@cup.hp.com
    Phone : 408-447-3751
    #################################
    


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Last updated: Tue Sep 04 01:05:54 2001
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