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    Re: Symmetric vs Asymmetric


    • To: ips@ece.cmu.edu
    • Subject: Re: Symmetric vs Asymmetric
    • From: Pierre Labat <pierre_labat@hp.com>
    • Date: Mon, 11 Sep 2000 16:08:29 -0700
    • Content-Transfer-Encoding: 7bit
    • Content-Type: text/plain; charset=us-ascii
    • Organization: Hewlett Packard ATM-SISL
    • Sender: owner-ips@ece.cmu.edu

    1) I don't like the asymmetric model because when solving the
    synchronization
    of the commands by sending the commands on a same TCP connexion,
    it just moves the synchronization problem in an other place :
    how to synchronize the commands and their data (pb described but Matt
    and Somesh).
    
    2)The advantage of the asymmetric model (when two connections at least):
    
    "avoid the problem of commands flow controlled by large transfer of
    data"
    is not an advantage compared to the symmetric model. Because to do so
    the asymmetric model needs at least two connections. In this case,
    the symmetric model could avoid this problem too :
       - opening two TCP connections one for the normal command
    (read/write,...)
            and a second one for the urgent commands (task management
    etc...)
    
    3) As stated it seems that we
      needs a flow control of the commands because if we use the SCSI
      one (TASK SET FULL) the traffic we will instable and composed of
      a sequence of bursts with regularly a full
     RoundTripTime*Throughput/size of a command
      number of commands to retry.
      The command window in the iSCSI draft seems not to be a good
      mechanism to flow control the commands, because if there is a mix
      of slow and fast commands, the result will be a task set half full
      but a window closed, hence the traffic blocked.
       Why not replace in the draft the  "MAXCmdRN" by a credit that is a
    number
       of commands in flight allowed (not yet completed) independent of the
    "CmdRN"?
       Doing that, the  flow will not be blocked by a slow command but will
       blocked only upon the filling of the task set.
    
    
    Pierre
    
    


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