Ned, my point isn't that cancellation should always be possible everywhere.
What I'm interested in is defining those situations where cancellation may
be possible or advisable, and what (or whether) guidelines and/or standards
can be devised to implement reasonable and useful systems.
A signature match is neither necessary nor sufficient.
That's false. There are situations where it is both. And, yes, there are
situations where it is neither; but they don't prove this negative.
Cancellation of messages in transit is absolutely unacceptable
False again. It's sometimes acceptable and sometimes not.
You are happy to point out situations and systems where cancellation would
be bad or difficult; great--important thing to do. Just don't deny that
there are other situations and systems where it would be good and easy.
MIME defined an 8bit CTE, even though there were TONS of systems where it
can't be used. Same thing here.
It may be that, after some discussion, we will agree that there are so many
inapplicable systems and situations that the whole idea is just not worth
the trouble; but one or two examples don't make me believe that.
Steve Dorner, Qualcomm Incorporated