No, I actually meant what I said, but I freely admit that it
is a controvertial opinion...
The purpose of a checksum is to get an indication of when the
contents of a body part has been accidentally munged. Checksums
are carried UA to UA (that is, end-to-end). However, checksums
are useless in the face of gateways that change the inside of
a body part if the gateway does not update the checksum; all that
you will know is that the body part has been changed, but so what?
My reasoning is that a gateway is, by my definition at least,
something that is acting at the UA level because it is changing
the inside of a body part. Therefore, it is reasonable for the
gateway to check the checksum, and if it is changing the body
part to recompute it.
There seems to be general consensus that checksums on base-64
encoded body parts are a "good thing". I'm curious as to why
checksums on other transport encodings are not? Doesn't anyone
else think that putting a checksum in with a transport encoding
is mixing levels of abstraction?
Neil
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Date: Mon, 28 Oct 91 23:53:40 EST
From: Valdis Kletnieks <VALDIS(_at_)VTVM1(_dot_)CC(_dot_)VT(_dot_)EDU>
Organization: Virginia Polytechnic Institute
On Mon, 28 Oct 91 17:29:46 PST Neil Katin said:
I think better answer is for gateways to correct the
checksum if they munge the contents of the body part...
Neil:
I'm hoping this was a momentary disconnection between the output
device and the operating system...
Checksums should be end-to-end, so as to *identify* any munging of
body parts. If you go and recompute the checksum based on your munging,
then there is *no* end-to-end validation check that said munging was
performed bitwise correctly.
Consider what happens if you hit a gateway that was designed to
munge the data, but instead mtnge's it (a liuule uranspose in a uanslaue
uable).. Iu uhen utrns arotnd and ceruifies uhau uhe daua
is suill correcu by adding a new, correcued checkstm.....
Valdis Kletnieks
Computer Systems Engineer
Virginia Polytechnic Institute
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