Hyphen Restrictions

Vint Cerf vint at google.com
Wed Jan 5 13:49:25 CET 2011


My understanding is that this restriction is to reserve all subsets of octet
sequences of the form "ab--" for arbitrary octets "a", "b" for possible use
as prefixes.

You bring up an interesting point since the xn-- sequence appears only AFTER
conversion of a UNICODE string into ACE. Moreover, the reserved subset uses
only ASCII, in fact, because it is affixed to the ACE form of the UNICODE
string. So in some ways, the restriction is broader than is absolutely
necessary. On the other hand, it is far easier to state and to test for than
something more precise.

vint




On Wed, Jan 5, 2011 at 1:18 AM, Yoshiro YONEYA <yoshiro.yoneya at jprs.co.jp>wrote:

> Hi, all,
>
> I need clarification of RFC5891 section 4.2.3.1, which says:
>
> 4.2.3.1.  Hyphen Restrictions
>
>   The Unicode string MUST NOT contain "--" (two consecutive hyphens) in
>   the third and fourth character positions and MUST NOT start or end
>   with a "-" (hyphen).
>
> My question is that what "the third and fourth character positions" means.
> Does it mean third and fourth octet from the beginning of the string?
> For example:
>  beginning of the string
>    |
>    v 1   2   3   4   5 <-- position of octet
>    +---+---+---+---+---+
>    | a | b | - | - | c |
>    +---+---+---+---+---+
>              ^   ^
>              |   |
>      two consecutive hyphens
>
> Or does it mean third and fourth character from the beginning of the
> string?
> For example:
>  beginning of the string
>    |
>    v 1   2   3   4   5 <-- position of character
>    +---+---+---+---+---+
>    |<A>|<B>| - | - |<C>| here <A>, <B> and <C> stands for non-ASCII (multi-
>    +---+---+---+---+---+ octets) character
>              ^   ^
>              |   |
>      two consecutive hyphens
>
> My understanding for this restrictions is to preserve future ACE prefix,
> so I expect the answer for my question is former one.  Is that right?
>
> Regards,
>
> --
> Yoshiro YONEYA <yoshiro.yoneya at jprs.co.jp>
>
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> Idna-update at alvestrand.no
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