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    RE: iSCSI spec qs.



    Hello Julian,
    
    I am still not sure what value add is provided by the Synch and Steering
    model.
    
    1. Does it avoid allocating any memory for assembly at the TCP layer ? If
    so, how would TCP recover lost packets ?
    
    2. Does this require modifications to the "legacy TCP implementations" ? If
    so, what are the modifications required ?
    
    3. How does the synch and steering layer help according to the proposal ?
    You would still need to allocate the buffers at the Synch and steering layer
    to provide redundant functionality that is already provided by TCP.
    
    The way I think of TCP is a dedicated, reliable connection. What you send at
    one end is what you get at the other end with all packet recovery taken care
    of at the TCP layer. So, in that case, iSCSI message framing is not an
    issue.
    
    Thanks for your input..
    
    Regards,
    
    Nimesh
    
    
    
    -----Original Message-----
    From: julian_satran@il.ibm.com [mailto:julian_satran@il.ibm.com]
    Sent: Friday, March 16, 2001 2:29 AM
    To: Nimesh Ray
    Subject: Re: iSCSI spec qs.
    
    
    
    
    There issue is for both TCP and iSCSI. If you are building adapter that
    have to place
    the data somewhere and data arrives out of order you have to place it
    somewhere (even if you don't deliver it). That may be the final destination
    or some temporary memory.
    
    In the later case you will have:
    
    1- to provide large amounts of temporary memory (possibly on the card)
    2-copy data later
    
    Both are expensive and should be avoided.
    
    Regards,
    Julo
    
    "Nimesh Ray" <nimesh@confluencenetworks.com> on 15/03/2001 22:47:57
    
    Please respond to "Nimesh Ray" <nimesh@confluencenetworks.com>
    
    To:   Julian Satran/Haifa/IBM@IBMIL
    cc:
    Subject:  iSCSI spec qs.
    
    
    
    
    Hello Julian,
    
    This is in reference to the iSCSI spec section 1.2.8 Message
    Synchronization
    and Steering. The following statement in section 1.2.8.2 page 21.
    
    "On the incoming path, the Synch and Steering layer extracts the Synch &
    Steering information from the TCP stream. Then it helps deliver (steer) the
    data stream to its final location and/or recover iSCSI PDU boundaries when
    some TCP packets are lost or received out of order"
    
    Seems like the assumption above is that you can receive packets out of
    order
    or lose packets if you are a client of TCP. Now as far as my understanding
    goes, TCP guarantees in-order and in-sequence guaranteed delivery of
    packets
    to the clients of TCP, in this case iSCSI. So, the receiver of the message
    should see exactly what the sender sent, with all the error recovery of
    lost
    packets done by the TCP layer. I believe that is exactly the rationale that
    went into selecting iSCSI over TCP.
    
    So, unless there is something else I do not understand, the "Synch and
    Steering" layer above TCP is not needed.
    
    Could you please clarify on this ?
    
    Thanks,
    
    Nimesh
    
    
    


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