Lookup CONTEXTJ test

[Vint Cerf]

simon:

from RFC5894

3.1.2.2.  Rules and Their Application

   Rules have descriptions such as "Must follow a character from Script
   XYZ", "Must occur only if the entire label is in Script ABC", or
   "Must occur only if the previous and subsequent characters have the
   DFG property".  The actual rules may be DEFINED or NULL.  If present,
   they may have values of "True" (character may be used in any position
   in any label), "False" (character may not be used in any label), or
   may be a set of procedural rules that specify the context in which
   the character is permitted.

   Because it is easier to identify these characters than to know that
   they are actually needed in IDNs or how to establish exactly the
   right rules for each one, a rule may have a null value in a given
   version of the tables.  Characters associated with null rules are not
   permitted to appear in putative labels for either registration or
   lookup.  Of course, a later version of the tables might contain a
   non-null rule.

   The actual rules and their descriptions are in Sections 2 and 3 of
   the Tables document [RFC5892].  That document also specifies the
   creation of a registry for future rules.



On Sun, Jan 9, 2011 at 5:30 AM, Simon Josefsson <simon at josefsson.org> wrote:

> Vint Cerf <vint at google.com> writes:
>
> > simon:
> >
> > Suppose you are (well, your software is) examining a string to determine
> > whether to look it up in the DNS and, on examination, you discover a
> > character in an apparent U-label that is labeled "CONTEXTJ". CONTEXTJ is
> one
> > of the several special handling rules in IDNA2008. Upon examination, you
> > discover that the character does NOT conform to the rule associated with
> > CONTEXTJ. At this point you should cease further examination and reject
> the
> > string as not being acceptable even for lookup in the DNS.  If the
> character
> > satisfies the associated CONTEXTJ rule, you may continue to examine the
> > string prior to looking it up.
> >
> > Your interpretation (2) is the correct one. The idea is to allow the use
> of
> > "joiner" characters only under specific conditions.
>
> Thank you, this is clear to me now.
>
> > A "null" rule is a condition that has no specific actions associated with
> > it. It's like defining a class of characters (perhaps by their Unicode
> > properties) for purposes of singling them out for special treatment, and
> > then not saying what should be done about them. If there is no rule, and
> if
> > a character in a string under examination meets the condition, the string
> > must be rejected if the condition does not have a defined rule (action)
> > associated with it. The lack of a rule means there is no test to perform
> and
> > it is interpreted in IDNA2008 as having failed implicitly. "null rule"
> means
> > a rule that is "empty" "has no content" "missing" "awol"
>
> When would this situation occur?
>
> /Simon
>
> >
> > v
> >
> >
> >
> >
> > On Sat, Jan 8, 2011 at 5:07 AM, Simon Josefsson <simon at josefsson.org>
> wrote:
> >
> >> Hi,
> >>
> >> I need help with interpretation regarding section 5.4 which says:
> >>
> >>   Putative U-labels with any of the following characteristics MUST be
> >>   rejected prior to DNS lookup:
> >> ...
> >>   o  Labels containing code points that are identified in the Tables
> >>      document as "CONTEXTJ", i.e., requiring exceptional contextual
> >>      rule processing on lookup, but that do not conform to those rules.
> >>
> >> I have trouble understand the bullet text.  To me, it seems as if the
> >> first part of the sentence, namely:
> >>
> >>    Labels containing code points that are identified in the Tables
> >>    document as "CONTEXTJ"
> >>
> >> says one thing but the rest of the sentence, namely:
> >>
> >>      requiring exceptional contextual rule processing on lookup, but
> >>      that do not conform to those rules.
> >>
> >> says a different thing.
> >>
> >> What is not clear to me is whether the test on a particular label is
> >> intended to fail if and only if:
> >>
> >> 1) the label has any code point with the CONTEXTJ property.
> >>
> >> 2) the label has any code point with the CONTEXTJ property AND the rule
> >>   fails.
> >>
> >> Interpretation 2) makes the most sense to me, but the normative part of
> >> the sentence suggests otherwise so I am looking for clarification.
> >>
> >> The text goes on and says:
> >>
> >>      Note that this implies that a rule must be defined, not null: a
> >>      character that requires a contextual rule but for which the rule
> >>      is null is treated in this step as having failed to conform to the
> >>      rule.
> >>
> >> What is a "null rule"?  I cannot find any definition.
> >>
> >> /Simon
> >> _______________________________________________
> >> Idna-update mailing list
> >> Idna-update at alvestrand.no
> >> http://www.alvestrand.no/mailman/listinfo/idna-update
> >>
> > _______________________________________________
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> > Idna-update at alvestrand.no
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>
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