Re: Urgent as Framing Hint?

["David P. Reed" <dpreed@reed.com>]

Hey, if you "integrate" TCP with iSCSI on both ends, you can use the 
following kludge to frame segments.

Use a variant TCP checksum on each datagram that starts a segment.  The 
variant checksum is created by adding in a suitable constant magic 
value.  This has a negligible effect on end-to-end error detection, and 
allows distinguishing buffer boundaries.

Since TCP's checksum is invisible to lower layers, it is "private" to the 
particular connection.

The receiver simply implements its checksum processing to accept two 
possible outcomes as "correct", and flags one of the outcomes as a segment 
hint.

I personally like this idea a lot, because any "middleboxes" that screw 
with checksums (even to check them) or translate streams will be discovered 
and shamed publicly for their egregious violation of layering.

Yes, it's a kludge, but it's one with a small, very localized effect that 
need only be supported by the iSCSI folks.

My $.02.
At 05:47 AM 12/1/00 -0600, Randall R. Stewart wrote:
>Matt Wakeley wrote:
> > I am glad I put the urgent pointer proposal out there, because others have
> > pointed out how there may be problems with using it.  I still believe that
> > *if* TCP implementations were implemented correctly, it would work to a
> > degree.  You however, have insisted that it was a "modification to 
> TCP", when
> > in fact, it was never intended to be.
> >
>Matt:
>
>I am not sure what you mena by "work to a degree". I am quite sure
>after looking at TCP and hearing feedback from David Reed, that in
>ALL TCP implementations your idea will work a lot of the times. But
>I am also just as sure that your idea will NOT work when faced with
>a more than one packet loss..
>
>If working with only single packet losses is what you had in mind
>then I am sure it will "work to a degree" right now. The real
>question is do you want a solution that will break under heavy
>load with multiple packet losses?
>
>I currently prefer the "magic sequence" proposal where you have
>a special escape sequence you can look for inside the data stream.
>I am not sure that this is managable for 10Gb data streams since
>it will involve a lot of horse power to do it.. but so far it
>is the only solution I can see that works reliably (if you have
>enough CPU)...
>
>
>R
>--
>Randall R. Stewart
>randall@stewart.chicago.il.us or rrs@cisco.com
>815-342-5222 (cell) 815-477-2127 (work)

- David
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