Guidance on Designing Label Generation Rulesets (LGRs) Supporting Variant Labels
RFC 8228

Document Type RFC - Informational (August 2017; No errata)
Was draft-freytag-lager-variant-rules (individual in art area)
Last updated 2017-08-22
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IESG IESG state RFC 8228 (Informational)
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Send notices to "Scott Hollenbeck" <shollenbeck@verisign.com>
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Internet Engineering Task Force (IETF)                        A. Freytag
Request for Comments: 8228                                   August 2017
Category: Informational
ISSN: 2070-1721

   Guidance on Designing Label Generation Rulesets (LGRs) Supporting
                             Variant Labels

Abstract

   Rules for validating identifier labels and alternate representations
   of those labels (variants) are known as Label Generation Rulesets
   (LGRs); they are used for the implementation of identifier systems
   such as Internationalized Domain Names (IDNs).  This document
   describes ways to design LGRs to support variant labels.  In
   designing LGRs, it is important to ensure that the label generation
   rules are consistent and well behaved in the presence of variants.
   The design decisions can then be expressed using the XML
   representation of LGRs that is defined in RFC 7940.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It has been approved for publication by the Internet
   Engineering Steering Group (IESG).  Not all documents approved by the
   IESG are a candidate for any level of Internet Standard; see
   Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8228.

Freytag                       Informational                     [Page 1]
RFC 8228                      Variant Rules                  August 2017

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Variant Relations . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Symmetry and Transitivity . . . . . . . . . . . . . . . . . .   5
   4.  A Word on Notation  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Variant Mappings  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Variant Labels  . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Variant Types and Label Dispositions  . . . . . . . . . . . .   7
   8.  Allocatable Variants  . . . . . . . . . . . . . . . . . . . .   8
   9.  Blocked Variants  . . . . . . . . . . . . . . . . . . . . . .   9
   10. Pure Variant Labels . . . . . . . . . . . . . . . . . . . . .  10
   11. Reflexive Variants  . . . . . . . . . . . . . . . . . . . . .  11
   12. Limiting Allocatable Variants by Subtyping  . . . . . . . . .  12
   13. Allowing Mixed Originals  . . . . . . . . . . . . . . . . . .  14
   14. Handling Out-of-Repertoire Variants . . . . . . . . . . . . .  15
   15. Conditional Variants  . . . . . . . . . . . . . . . . . . . .  16
   16. Making Conditional Variants Well Behaved  . . . . . . . . . .  18
   17. Variants for Sequences  . . . . . . . . . . . . . . . . . . .  19
   18. Corresponding XML Notation  . . . . . . . . . . . . . . . . .  21
   19. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   20. Security Considerations . . . . . . . . . . . . . . . . . . .  23
   21. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     21.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     21.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  24
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  24

Freytag                       Informational                     [Page 2]
RFC 8228                      Variant Rules                  August 2017

1.  Introduction

   Label Generation Rulesets (LGRs) that define the set of permissible
   labels may be applied to identifier systems that rely on labels, such
   as the Domain Name System (DNS) [RFC1034] [RFC1035].  To date, LGRs
   have mostly been used to define policies for implementing
   Internationalized Domain Names (IDNs) using IDNA2008 [RFC5890]
   [RFC5891] [RFC5892] [RFC5893] [RFC5894] in the DNS.  This document
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