[e2e] Re: [Tsvwg] Really End-to-end or CRC vs everything else?
jonathan at DSG.Stanford.EDU
Mon Jun 11 16:06:49 PDT 2001
In message <18.104.22.168.2.20010611181116.046205a0 at mail.reed.com>,
"David P. Reed" writes:
>Not to belabor this, but just passing the check-function test is not
>sufficient. It must also pass a variety of other tests to be accepted in
>the context of a protocol (correct sequence number, correct address, ...).
>So the simple combinatoric argument you sketch below doesn't quite
>work. You can't synthesize just any datagram that passes the checksum. It
>also has to meet the other constraints.
Oh, sure Thanks for pointing it out; I never meant it to be taken
that literally. (In fact the next chapter of my dissertation talks
about this; I went through it in gory detail in in the ATM simulations
I did with Michael Greenwald, Jim Hughes, and Craig Partridge.)
You can interpret the result as applying to a check over just the
payload. Or you an apply the result to all the bits at a give layer,
by sitting down and counting how many bits of information are check by
the syntactic check at a given layer. Provided the ``check'' is a
function, not a relation, the proof still holds. (And yes, I did the
work to show how to extend it to a relation, though I didn't address
the specific example of TCP's window.)
The insight that we really cannot rank error-check functions _in
vacuo_, without reference to some error model[*]
(in the sense I use the phrase) is still valid.
[*] Or with reference to some input distribution, if you
buy the information-maximization story.
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