I agree with Keith about the inadvisability of rewriting the grammar.
Save it for SIP/3.0.
In any case, I think the issue isn't with ABNF per se, its with the way
ABNF was used for SIP. ABNF isn't actually able to represent everything
that was desired for sip, so some things were simply handled as
exceptions in the text.
Of course the "right" thing to do would have been to either give up on
anything that couldn't legitimately be represented, or else switch to
some other specification language that could express what was intended.
But there is a big tradeoff there. At least many people are capable of
reading ABNF, and a reasonable number of those can accurately understand
what it means. A somewhat smaller, but still significant, number can
write it correctly. Its important that the people involved in the
standardization process, and that implement the standard, be able to
understand it. A more powerful specification language, that fewer people
could understand, might not be such a great choice.
Thanks,
Paul
Keith Moore wrote:
eburger wrote:
Personally, I don't understand why we didn't just use YACC.
Exercise: write an RFC ?822 parse in yacc. It can be done, but not
easily, as the RFC ?822 grammar is not LALR(1).
I am really in favor of using a formal language for specifying grammar. As Munjo states, the ABNF is essentially documentation fluff - useless at best and wrong at worst.
As for this particular suggestion, I am ambivalent as to whether we hack ABNF, adopt YACC, or even adopt ASN.1.
One lesson from the DRUMS effort - rewrite an existing grammar at your
peril. Even if the old specification has errors, writing a new grammar
which verifiably recognizes exactly the same language as the old one can
be difficult. And even if you restrict the language that new
implementations can emit, you generally have to accept the old language
for compatibility's sake. So you end up with two grammars where one was
formerly (almost) good enough.
Keith
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_______________________________________________
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This list is for NEW development of the core SIP Protocol
Use [email protected] for questions on current sip
Use [email protected] for new developments on the application of sip
