IDNs- 11/18 IDNA- Issues and Rationale

[bwhhisc]

Written November 18 by John Klensin IDNA

http://www.ietf.org/internet-drafts/draft-klensin-idnabis-issues-05.txt

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4
1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 4
1.3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Applicability and Function of IDNA . . . . . . . . . . . . 5
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.5.1. Documents and Standards . . . . . . . . . . . . . . . 6
1.5.2. Terminology about Characters and Character Sets . . . 6
1.5.3. DNS-related Terminology . . . . . . . . . . . . . . . 7
1.5.4. Terminology Specific to IDNA . . . . . . . . . . . . . 7
1.5.5. Punycode is an Algorithm, not a Name . . . . . . . . . 10
1.5.6. Other Terminology Issues . . . . . . . . . . . . . . . 10
2. The Original (2003) IDNA Model . . . . . . . . . . . . . . . . 11
2.1. Proposed label . . . . . . . . . . . . . . . . . . . . . . 12
2.2. Permitted Character Identification . . . . . . . . . . . . 12
2.3. Character Mappings . . . . . . . . . . . . . . . . . . . . 12
2.4. Registry Restrictions . . . . . . . . . . . . . . . . . . 12
2.5. Punycode Conversion . . . . . . . . . . . . . . . . . . . 13
2.6. Lookup or Insertion in the Zone . . . . . . . . . . . . . 13
3. A Revised IDNA Model . . . . . . . . . . . . . . . . . . . . . 13
3.1. Localization: The Role of the Local System and User
Interface . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2. IDN Processing in the IDNA200x Model . . . . . . . . . . . 14
3.2.1. Summary of Effects . . . . . . . . . . . . . . . . . . 14
4. IDNA200x Document List . . . . . . . . . . . . . . . . . . . . 15
5. Permitted Characters: An Inclusion List . . . . . . . . . . . 15
5.1. A Tiered Model of Permitted Characters and Labels . . . . 15
5.1.1. ALWAYS . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1.2. MAYBE . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1.3. CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 18
5.1.4. NEVER . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2. Layered Restrictions: Tables, Context, Registration,
Applications . . . . . . . . . . . . . . . . . . . . . . . 19
5.3. A New Character List -- History . . . . . . . . . . . . . 19
5.4. Understanding New Issues and Constraints . . . . . . . . . 20
5.5. ALWAYS, MAYBE, and Contextual Rules . . . . . . . . . . . 20
6. Issues that Any Solution Must Address . . . . . . . . . . . . 21
6.1. Display and Network Order . . . . . . . . . . . . . . . . 21
6.2. Entry and Display in Applications . . . . . . . . . . . . 22
6.3. The Ligature and Digraph Problem . . . . . . . . . . . . . 23
6.4. Right-to-left Text . . . . . . . . . . . . . . . . . . . . 25
7. IDNs and the Robustness Principle . . . . . . . . . . . . . . 25
8. Migration and Version Synchronization . . . . . . . . . . . . 26
8.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 26
8.2. More Flexibility in User Agents . . . . . . . . . . . . . 29
8.3. The Question of Prefix Changes . . . . . . . . . . . . . . 31