Good catch. The ACE prefix indicates to a parser that the following string is the Punycoded transformation of an IDNA-compatible Unicode string, but is not actually a part of that transformation.
From: ua-eai-bounces@icann.org [mailto:ua-eai-bounces@icann.org]
On Behalf Of Jiankang Yao
Sent: Sunday, January 22, 2017 6:27 PM
To: Don Hollander <don.hollander@icann.org>; ua-eai@icann.org
Cc: ua-coordination@icann.org
Subject: Re: [UA-EAI] [UA-Coordination] Revised UASG013 - Quick Guide to EAI
Hello,
The definition
"Punycode is an algorithm that converts an IDN into an ASCII string that starts with xn--. The Punycode
algorithm is bi-directional. " needs some adjustment.
xn--abcdeddaa
where "xn--" is a prefix and "abcdeddaa" is output of the Punycode algorithm. So Punycode output does not include "xn--".
======================================following are texts from RFC5890===============================
" the string following "xn--" must be the valid output of the Punycode algorithm
and must be convertible into valid U-label form."
"An "A-label" is the ASCII-Compatible Encoding (ACE, see
Section 2.3.2.5) form of an IDNA-valid string. It must be a
complete label: IDNA is defined for labels, not for parts of them
and not for complete domain names. This means, by definition,
that every A-label will begin with the IDNA ACE prefix, "xn--"
(see Section 2.3.2.5), followed by a string that is a valid output
of the Punycode algorithm [RFC3492] and hence a maximum of 59
ASCII characters in length. "
Jiankang Yao
From: Don Hollander
Date: 2017-01-22 17:33
To: ua-eai@icann.org
Subject: [UA-Coordination] Revised UASG013 - Quick Guide
to EAI
Last month I sent out a formatted Quick Guide to EAI.
This resulted in a number of comments from Ajay, Stuart, Mark, Lars & Dennis.
These enhancements were mostly focused on the Email Service Provider.
I have revised the source document which is attached.
And for reference, I’m also including the formatted earlier draft.
Could I please get any final comments by the end of this week (25th)?
Thanks.
Don
Don Hollander
Universal Acceptance Steering Group
Skype: don_hollander