I also had trouble finding much discussion on it, but here's a draft standard (from 2001) that seems to cover some of the basis of selecting one: https://www.ietf.org/proceedings/50/I-D/idn-aceid-01.txt
More or less, it seems they were chosen arbitrarily, at least at that time..: Therefore, the current situation letting independent ACE proposal > authors arbitrarily select an ACE identifier, hence permitting domain > name registrants registrer such names, may hinder deployment of MDN > technology. Not seeing any further follow-up on the draft. Jeff On Wed, Jun 25, 2025 at 11:38 PM Gihan Dias <[email protected]> wrote: > On 26/06/2025 09:02, jeffb wrote: > > Possibly a good place to start: > https://datatracker.ietf.org/doc/html/rfc3490 > > Specifically Section 5.. though I have no idea how to register/who > controls ACE prefixes: > https://datatracker.ietf.org/doc/html/rfc3490#section-5 > > Jeff, > > Thanks. > > It doesn't discuss *why* xn-- was chosen, though it does say: > > The ACE prefix, used in the conversion operations (section 4 > <https://datatracker.ietf.org/doc/html/rfc3490#section-4>), is two > alphanumeric ASCII characters followed by two hyphen-minuses. It > cannot be any of the prefixes already used in earlier documents, > which includes the following: "bl--", "bq--", "dq--", "lq--", "mq--", > "ra--", "wq--" and "zq--". > > However, I couldn’t find a discussion of ACE prefixes, or how any of the > above were selected. > > Gihan > -- jeffb | [email protected]
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