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    Re: TCP limitations (was Re: ISCSI: Urgent Flag requirement violates TCP.)



    > > But the Urgent pointer only helps when you happen to have a sequence
    > > hole.  So how in practice can it be worth the effort?
    > 
    > It will permit a high speed endpoint to deliver full pipe aggregate
    > throughput across multiple streams.
    
    Then I still have another question/concern.  What happens with out-of-sequence
    data that arrives that doesn't have an Urgent pointer within it, either due
    to coalescence or because there just doesn't happen to be an ISCSI header
    in the packet?
    
    I would hope it's buffered even though the receiver doesn't know where it
    should ultimately wind up.  Indeed, if it isn't buffered, then the TCP
    exhibits the "Failure to retain above-sequence data" problem documented in
    RFC 2525, and is not unconditionally complaint with the TCP standard.
    
    But if there's sufficient buffer for this case, then there pretty much
    needs to be sufficient buffer for the general case (all data in flight,
    whether or not it contains an ISCSI header), and also a mechanism to copy
    the data from its initial buffer to its final location.  In which case,
    what the Urgent framing gains is some performance (avoiding a buffer copy),
    but not a savings in memory.  However, you only get this performance gain
    for a stream that is likely slowing down, so it still seems to be a corner
    case and not a significant win.
    
    		Vern
    


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Last updated: Tue Sep 04 01:06:18 2001
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