Re: [idn] length restrictions on IDN label

[Erik Nordmark <Erik.Nordmark@sun.com>]

> >Please bring it to the DNSEXT working  group.
> >
> As a novice IETF pariticpant, I have little experience with DNSEXT WG.
> Would  Area Directors make comment on this issue, Erik ?

When and if there is a need to carry IDNs in the DNS protocol using octet
values above 127 the DNSEXT WG would be the place to discuss this.
However, I still haven't seen a fully worked out approach for doing this
that takes DNSSEC into account. (draft-hall-dm-idns is the clostest I've seen,
but it doesn't get into DNSSEC issues).

When and if this is approach, there are presumably multiple encodings
that can be used. One is UTF-8; another possibility would be to
define a bootstring algorithm from Unicode to octets in the range 0-255.
The latter would carry the bootstring compression features forward into
these encodings.

Taking length limits into account would be one part of that work.

> UTF8-form of a label may be used as protocol elements , in addition to
> presentation forms.
> In the latter case, there will be no problem as you said above: just
> display it.
> 
> But, in the former case of being used as protocols elements, utf8-form
> label length limit is
> of our concern. IDNA drafts does not rule out that utf-8 form of labels
> may be used as protocol
> elements. This will clairfy my point.

See above.

But perhaps IDNA should be more clear that there is a length issue for
such future work. For instance, would adding a sentence at the end of
this paragraph be helpful?
	If a signaling system which makes negotiation possible between old and
	new DNS clients and servers is standardized in the future, the encoding
	of the query in the DNS protocol itself can be changed from ACE to
	something else, such as UTF-8. The question whether or not this should
	be used is, however, a separate problem and is not discussed in this
	memo.
E.g. "Such work would have to take into account the length limit for IDNs
as specified in this document".

It might also be useful to make the IDNA specification more clear about
the length limit and its implications somewhere in the document.
Strawman:
	The length limit for internationalized labels is the limit
	of at most 63 octets of output from ToASCII. The design
	of the Punycode encoding is such that some sequences of up to 63 Unicode
	code points result in ToASCII output of 63 or less octets, but
	all possible Unicode strings of 64 code points result in ToASCII
	output of 64 or more octets.

	Implementation Note:
		Depending on the local encoding the software uses for
		Unicode this can correspond to a lot more than 63 octets.
		Similarely, the total length of a IDN expressed in the local
		encoding can significantly exceed 255 octets.

  Erik