Stupid U-label question

[John C Klensin]

--On Tuesday, 19 August, 2008 01:12 +0200 Frank Ellermann
<hmdmhdfmhdjmzdtjmzdtzktdkztdjz at gmail.com> wrote:

> Hi, triggered by an error - I tried a punycode decode
> instead of an IDNA2003 decode - I tested the effect
> of xn--xn--4ca-cxa with two tools, both told me "this
> is no A-label".
> 
> In other words xn--4caä (ending with an umlauted a)
> is no U-label.  I wasn't aware that U-labels must not
> start with "xn--", is that mentioned in the drafts ?

I don't remember what IDNA2003 says, if anything, but the
IDNA2008 drafts very clearly (even more clearly in  the versions
I'm trying to wrap up and post, I think) say that, if an
application is IDNA conforming, and a label appears in a domain
name context, then anything starting with "xn--" is required to
be an A-label.

I think not having such a rule leads to bad places with no real
benefits, unless one believes that a someone might actually need
a U-label of xn--4caä for a legitimate, non-demonstration,
purpose.

   john