Combining mark vs combining character?

[Vint Cerf]

Simon,

I am pretty sure that the terms "combining mark" and "combining character"
as used in IDNA2008 mean the same thing.

neither are permitted as the initial character of a Unicode domain label

vint




On Wed, Jan 5, 2011 at 5:06 AM, Simon Josefsson <simon at josefsson.org> wrote:

> Hi,
>
> I need a clarification regarding this paragraph in section 4.2.3.2 of
> RFC 5891:
>
>   The Unicode string MUST NOT begin with a combining mark or combining
>   character (see The Unicode Standard, Section 2.11 [Unicode] for an
>   exact definition).
>
> And this in section 5.4:
>
>   Putative U-labels with any of the following characteristics MUST be
>   rejected prior to DNS lookup:
> ...
>   o  Labels whose first character is a combining mark (see The Unicode
>      Standard, Section 2.11 [Unicode]).
>
> The reference to [Unicode] is not normative, which would be a problem
> for any implementer.
>
> Reading section 2.11 of Unicode 5.0 discuss "combining character" but
> not "combining mark".
>
> There is a section 7.9 in Unicode 5.0 called "Combining Marks".
>
> A section that discuss both Combining Marks and Combining Characters in
> the same section is section 3.11 on "Canonical Ordering Behaviour".
>
> There is one section 3.6 on "Combination" that gives the precice
> definition of a "Combining character":
>
>   Combining character: A character with the General Category of
>   Combining Mark (M).
>
> Is this the intended definition of Combining character by RFC 5891?
>
> Questions:
>
> 1) Does RFC 5891 refer to "combining mark" and "combining character" as
> the same thing?
>
> 2) Is there a significant difference between the requirement in 4.2.3.2
> and 5.4?  The latter section only mentions "combining mark" and not
> "combining character".
>
> 3) What is the precice definition of a "combining mark"?
>
> /Simon
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