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    RE: Framing Formats



    In my previous posting I was referring to the TCP-unaware variety. At
    the heart of the matter is whether we trust the underlying transport
    mechanism or not. If we don't, then there has to be some sort of
    error-recovery procedure for when information/data is lost in transit.
    If we do trust it, then the iSCSI specification is simplified and we
    assume that the protocol is riding on an error free link. 
    
    But to go back to the point of this thread....
    
    Byte and word stuffing mechanisms have been proven to work for many
    years. Picking a 32 bit number reduces the chances of it showing up in
    the data stream. Even if it does the stuffing mechanism would mask it. 
    
    Mike
    
     
    -----Original Message-----
    From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu] On Behalf Of
    Black_David@emc.com
    Sent: Tuesday, July 10, 2001 7:07 AM
    To: Julian_Satran@il.ibm.com
    Cc: ips@ece.cmu.edu
    Subject: Framing Formats
    
    NOTE: iSCSI tag removed because there's FCIP-related
    material in this message.
    
    > We don't want this thread to be started again in this forum.
    > For anything related to framing go to the TSWG or to the RDMA mailing
    list.
    
    I think Julian may have jumped for that conclusion a little too quickly.
    Certainly, there are topics that belong on those mailing lists:
    
    - TCP-aware framing mechanisms are in the domain of TSVWG.  Use
    	of URG and PSH for framing is not going to happen.
    - RDMA mechanism discussions belong on the RDMA list - this
    	is not an official IETF list.
    
    Beyond these, there are a couple of topics that are fair game
    for the IPS list:
    
    - TCP-unaware framing mechanisms intended for specification as
    	part of an IPS protocol; this includes markers, as well as
    	the synchronization recovery mechanism proposed for FCIP.
    - Discussion of appropriate requirements (e.g., should markers
    	be specified as "MUST implement" with some additional
    	rules about which end of a connection controls their
    	usage on what traffic?).
    
    The requirements issue is definitely an open issue at this point
    in time.  As for alternatives to markers, a fairly high level of rigor
    will be expected of approaches that scan data looking for some
    distinctive pattern - it basically needs to be the case that this
    sort of scan and detect algorithm cannot get confused by any
    pattern appearing in a data payload (read or write data for
    iSCSI).  A motivating example to think about is where trace
    data that includes the distinctive pattern (e.g., from a sniffer)
    is being written to or read from a file.
    Byte and word stuffing mechanisms to ensure that
    the pattern the algorithm is looking for can't appear in data
    are one way to accomplish this, but there are others.
    
    While I'm on this topic, let me note that the algorithm in
    Annex A of the -03 FCIP draft does not meet this "cannot
    get confused" criterion.  For FCIP, it is possible to meet
    this criterion without resorting to stuffing techniques - the
    basic idea is to scan enough incoming data so that there
    have to be valid headers in there, note every possible
    header found, and their relationships based on length
    fields.  If the resulting situation is ambiguous/confusing,
    restart and/or abandon the attempt to re-establish
    synchronization.  A considerably improved algorithm
    should appear in the -04 version of the FCIP draft based
    on some offline discussion with some of the draft authors.
    
    Thanks,
    --David
    
    ---------------------------------------------------
    David L. Black, Senior Technologist
    EMC Corporation, 42 South St., Hopkinton, MA  01748
    +1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
    black_david@emc.com       Mobile: +1 (978) 394-7754
    ---------------------------------------------------
    
    
    


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Last updated: Tue Sep 04 01:04:20 2001
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