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Internet Application Protocol Collation Registry
RFC 4790

Document type: RFC - Proposed Standard (March 2007; Errata)
Was draft-newman-i18n-comparator (individual in app area)
Document stream: IETF
Last updated: 2013-03-02
Other versions: plain text, pdf, html

IETF State: (None)
Consensus: Unknown
Document shepherd: No shepherd assigned

IESG State: RFC 4790 (Proposed Standard)
Responsible AD: Lisa Dusseault
Send notices to: chris.newman@sun.com,arnt@oryx.com

Network Working Group                                          C. Newman
Request for Comments: 4790                              Sun Microsystems
Category: Standards Track                                      M. Duerst
                                                Aoyama Gakuin University
                                                          A. Gulbrandsen
                                                                    Oryx
                                                              March 2007

            Internet Application Protocol Collation Registry

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Many Internet application protocols include string-based lookup,
   searching, or sorting operations.  However, the problem space for
   searching and sorting international strings is large, not fully
   explored, and is outside the area of expertise for the Internet
   Engineering Task Force (IETF).  Rather than attempt to solve such a
   large problem, this specification creates an abstraction framework so
   that application protocols can precisely identify a comparison
   function, and the repertoire of comparison functions can be extended
   in the future.

Newman, et al.              Standards Track                     [Page 1]
RFC 4790                   Collation Registry                 March 2007

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  4
   2.  Collation Definition and Purpose . . . . . . . . . . . . . . .  4
     2.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  Some Other Terms Used in this Document . . . . . . . . . .  5
     2.4.  Sort Keys  . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Collation Identifier Syntax  . . . . . . . . . . . . . . . . .  6
     3.1.  Basic Syntax . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Wildcards  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Ordering Direction . . . . . . . . . . . . . . . . . . . .  7
     3.4.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.5.  Naming Guidelines  . . . . . . . . . . . . . . . . . . . .  7
   4.  Collation Specification Requirements . . . . . . . . . . . . .  8
     4.1.  Collation/Server Interface . . . . . . . . . . . . . . . .  8
     4.2.  Operations Supported . . . . . . . . . . . . . . . . . . .  8
       4.2.1.  Validity . . . . . . . . . . . . . . . . . . . . . . .  9
       4.2.2.  Equality . . . . . . . . . . . . . . . . . . . . . . .  9
       4.2.3.  Substring  . . . . . . . . . . . . . . . . . . . . . .  9
       4.2.4.  Ordering . . . . . . . . . . . . . . . . . . . . . . . 10
     4.3.  Sort Keys  . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.4.  Use of Lookup Tables . . . . . . . . . . . . . . . . . . . 11
   5.  Application Protocol Requirements  . . . . . . . . . . . . . . 11
     5.1.  Character Encoding . . . . . . . . . . . . . . . . . . . . 11
     5.2.  Operations . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.3.  Wildcards  . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.4.  String Comparison  . . . . . . . . . . . . . . . . . . . . 12
     5.5.  Disconnected Clients . . . . . . . . . . . . . . . . . . . 12
     5.6.  Error Codes  . . . . . . . . . . . . . . . . . . . . . . . 13
     5.7.  Octet Collation  . . . . . . . . . . . . . . . . . . . . . 13
   6.  Use by Existing Protocols  . . . . . . . . . . . . . . . . . . 13
   7.  Collation Registration . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Collation Registration Procedure . . . . . . . . . . . . . 14
     7.2.  Collation Registration Format  . . . . . . . . . . . . . . 15
       7.2.1.  Registration Template  . . . . . . . . . . . . . . . . 15
       7.2.2.  The Collation Element  . . . . . . . . . . . . . . . . 15
       7.2.3.  The Identifier Element . . . . . . . . . . . . . . . . 16
       7.2.4.  The Title Element  . . . . . . . . . . . . . . . . . . 16
       7.2.5.  The Operations Element . . . . . . . . . . . . . . . . 16
       7.2.6.  The Specification Element  . . . . . . . . . . . . . . 16
       7.2.7.  The Submitter Element  . . . . . . . . . . . . . . . . 16
       7.2.8.  The Owner Element  . . . . . . . . . . . . . . . . . . 16
       7.2.9.  The Version Element  . . . . . . . . . . . . . . . . . 17
       7.2.10. The Variable Element . . . . . . . . . . . . . . . . . 17
     7.3.  Structure of Collation Registry  . . . . . . . . . . . . . 17
     7.4.  Example Initial Registry Summary . . . . . . . . . . . . . 18

Newman, et al.              Standards Track                     [Page 2]
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   8.  Guidelines for Expert Reviewer . . . . . . . . . . . . . . . . 18
   9.  Initial Collations . . . . . . . . . . . . . . . . . . . . . . 19
     9.1.  ASCII Numeric Collation  . . . . . . . . . . . . . . . . . 20
       9.1.1.  ASCII Numeric Collation Description  . . . . . . . . . 20
       9.1.2.  ASCII Numeric Collation Registration . . . . . . . . . 20
     9.2.  ASCII Casemap Collation  . . . . . . . . . . . . . . . . . 21
       9.2.1.  ASCII Casemap Collation Description  . . . . . . . . . 21
       9.2.2.  ASCII Casemap Collation Registration . . . . . . . . . 22
     9.3.  Octet Collation  . . . . . . . . . . . . . . . . . . . . . 22
       9.3.1.  Octet Collation Description  . . . . . . . . . . . . . 22
       9.3.2.  Octet Collation Registration . . . . . . . . . . . . . 23
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 24
     13.2. Informative References . . . . . . . . . . . . . . . . . . 24

Newman, et al.              Standards Track                     [Page 3]
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1.  Introduction

   The Application Configuration Access Protocol ACAP [11] specification
   introduced the concept of a comparator (which we call collation in
   this document), but failed to create an IANA registry.  With the
   introduction of stringprep [6] and the Unicode Collation Algorithm
   [7], it is now time to create that registry and populate it with some
   initial values appropriate for an international community.  This
   specification replaces and generalizes the definition of a comparator
   in ACAP, and creates a collation registry.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as defined in "Key words for
   use in RFCs to Indicate Requirement Levels" [1].

   The attribute syntax specifications use the Augmented Backus-Naur
   Form (ABNF) [2] notation, including the core rules defined in
   Appendix A.  The ABNF production "Language-tag" is imported from
   Language Tags [5] and "reg-name" from URI: Generic Syntax [4].

2.  Collation Definition and Purpose

2.1.  Definition

   A collation is a named function which takes two arbitrary length
   strings as input and can be used to perform one or more of three
   basic comparison operations: equality test, substring match, and
   ordering test.

2.2.  Purpose

   Collations are an abstraction for comparison functions so that these
   comparison functions can be used in multiple protocols.  The details
   of a particular comparison operation can be specified by someone with
   appropriate expertise, independent of the application protocols that
   use that collation.  This is similar to the way a charset [13]
   separates the details of octet to character mapping from a protocol
   specification, such as MIME [9], or the way SASL [10] separates the
   details of an authentication mechanism from a protocol specification,
   such as ACAP [11].

Newman, et al.              Standards Track                     [Page 4]
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   Here is a small diagram to help illustrate the value of this
   abstraction:

   +-------------------+                         +-----------------+
   | IMAP i18n SEARCH  |--+                      | Basic           |
   +-------------------+  |                   +--| Collation Spec  |
                          |                   |  +-----------------+
   +-------------------+  |  +-------------+  |  +-----------------+
   | ACAP i18n SEARCH  |--+--| Collation   |--+--| A stringprep    |
   +-------------------+  |  | Registry    |  |  | Collation Spec  |
                          |  +-------------+  |  +-----------------+
   +-------------------+  |                   |  +-----------------+
   | ...other protocol |--+                   |  | locale-specific |
   +-------------------+                      +--| Collation Spec  |
                                                 +-----------------+

   Thus IMAP, ACAP, and future application protocols with international
   search capability simply specify how to interface to the collation
   registry instead of each protocol specification having to specify all
   the collations it supports.

2.3.  Some Other Terms Used in this Document

   The terms client, server, and protocol are used in somewhat unusual
   senses.

   Client means a user, or a program acting directly on behalf of a
   user.  This may be a mail reader acting as an IMAP client, or it may
   be an interactive shell, where the user can type protocol commands/
   requests directly, or it may be a script or program written by the
   user.

   Server means a program that performs services requested by the
   client.  This may be a traditional server such as an HTTP server, or
   it may be a Sieve [14] interpreter running a Sieve script written by
   a user.  A server needs to use the operations provided by collations
   in order to fulfill the client's requests.

   The protocol describes how the client tells the server what it wants
   done, and (if applicable) how the server tells the client about the
   results.  IMAP is a protocol by this definition, and so is the Sieve
   language.

2.4.  Sort Keys

   One component of a collation is a transformation, which turns a
   string into a sort key, which is then used while sorting.

Newman, et al.              Standards Track                     [Page 5]
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   The transformation can range from an identity mapping (e.g., the
   i;octet collation Section 9.3) to a mapping that makes the string
   unreadable to a human.

   This is an implementation detail of collations or servers.  A
   protocol SHOULD NOT expose it to clients, since some collations leave
   the sort key's format up to the implementation, and current
   conformant implementations are known to use different formats.

3.  Collation Identifier Syntax

3.1.  Basic Syntax

   The collation identifier itself is a single US-ASCII string.  The
   identifier MUST NOT be longer than 254 characters, and obeys the
   following grammar:

     collation-char  = ALPHA / DIGIT / "-" / ";" / "=" / "."

     collation-id    = collation-prefix ";" collation-core-name
                       *collation-arg

     collation-scope = Language-tag / "vnd-" reg-name

     collation-core-name = ALPHA *( ALPHA / DIGIT / "-" )

     collation-arg   = ";" ALPHA *( ALPHA / DIGIT ) "="
                       1*( ALPHA / DIGIT / "." )

   Note: the ABNF production "Language-tag" is imported from Language
   Tags [5] and "reg-name" from URI: Generic Syntax [4].

   There is a special identifier called "default".  For protocols that
   have a default collation, "default" refers to that collation.  For
   other protocols, the identifier "default" MUST match no collations,
   and servers SHOULD treat it in the same way as they treat nonexistent
   collations.

3.2.  Wildcards

   The string a client uses to select a collation MAY contain one or
   more wildcard ("*") characters that match zero or more collation-
   chars.  Wildcard characters MUST NOT be adjacent.  If the wildcard
   string matches multiple collations, the server SHOULD attempt to
   select a widely useful collation in preference to a narrowly useful
   one.

Newman, et al.              Standards Track                     [Page 6]
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     collation-wild  =  ("*" / (ALPHA ["*"])) *(collation-char ["*"])
                         ; MUST NOT exceed 254 characters total

3.3.  Ordering Direction

   When used as a protocol element for ordering, the collation
   identifier MAY be prefixed by either "+" or "-" to explicitly specify
   an ordering direction. "+" has no effect on the ordering operation,
   while "-" inverts the result of the ordering operation.  In general,
   collation-order is used when a client requests a collation, and
   collation-selected is used when the server informs the client of the
   selected collation.

     collation-selected =  ["+" / "-"] collation-id

     collation-order =  ["+" / "-"] collation-wild

3.4.  URIs

   Some protocols are designed to use URIs [4] to refer to collations
   rather than simple tokens.  A special section of the IANA URL space
   is reserved for such usage.  The "collation-uri" form is used to
   refer to a specific named collation (the collation registration may
   not actually be present).  The "collation-auri" form is an abstract
   name for an ordering, a collation pattern or a vendor private
   collator.

     collation-uri   =  "http://www.iana.org/assignments/collation/"
                        collation-id ".xml"

     collation-auri  =  ( "http://www.iana.org/assignments/collation/"
                        collation-order ".xml" ) / other-uri

     other-uri       =  <absoluteURI>
                     ;  excluding the IANA collation namespace.

3.5.  Naming Guidelines

   While this specification makes no absolute requirements on the
   structure of collation identifiers, naming consistency is important,
   so the following initial guidelines are provided.

   Collation identifiers with an international audience typically begin
   with "i;".  Collation identifiers intended for a particular language
   or locale typically begin with a language tag [5] followed by a ";".
   After the first ";" is normally the name of the general collation
   algorithm, followed by a series of algorithm modifications separated
   by the ";" delimiter.  Parameterized modifications will use "=" to

Newman, et al.              Standards Track                     [Page 7]
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   delimit the parameter from the value.  The version numbers of any
   lookup tables used by the algorithm SHOULD be present as
   parameterized modifications.

   Collation identifiers of the form *;vnd-hostname;* are reserved for
   vendor-specific collations created by the owner of the hostname
   following the "vnd-" prefix (e.g., vnd-example.com for the vendor
   example.com).  Registration of such collations (or the name space as
   a whole), with intended use of the "Vendor", is encouraged when a
   public specification or open-source implementation is available, but
   is not required.

4.  Collation Specification Requirements

4.1.  Collation/Server Interface

   The collation itself defines what it operates on.  Most collations
   are expected to operate on character strings.  The i;octet
   (Section 9.3) collation operates on octet strings.  The i;ascii-
   numeric (Section 9.1) operation operates on numbers.

   This specification defines the collation interface in terms of octet
   strings.  However, implementations may choose to use character
   strings instead.  Such implementations may not be able to implement
   e.g., i;octet.  Since i;octet is not currently mandatory to implement
   for any protocol, this should not be a problem.

4.2.  Operations Supported

   A collation specification MUST state which of the three basic
   operations are supported (equality, substring, ordering) and how to
   perform each of the supported operations on any two input character
   strings, including empty strings.  Collations must be deterministic,
   i.e., given a collation with a specific identifier, and any two fixed
   input strings, the result MUST be the same for the same operation.

   In general, collation operations should behave as their names
   suggest.  While a collation may be new, the operations are not, so
   the new collation's operations should be similar to those of older
   collations.  For example, a date/time collation should not provide a
   "substring" operation that would morph IMAP substring SEARCH into
   e.g., a date-range search.

   A non-obvious consequence of the rules for each collation operation
   is that, for any single collation, either none or all of the
   operations can return "undefined".  For example, it is not possible
   to have an equality operation that never returns "undefined", and a
   substring operation that occasionally does.

Newman, et al.              Standards Track                     [Page 8]
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4.2.1.  Validity

   The validity test takes one string as argument.  It returns valid if
   its input string is a valid input to the collation's other
   operations, and invalid if not.  (In other words, a string is valid
   if it is equal to itself according to the collation's equality
   operation.)

   The validity test is provided by all collations.  It MUST NOT be
   listed separately in the collation registration.

4.2.2.  Equality

   The equality test always returns "match" or "no-match" when it is
   supplied valid input, and MAY return "undefined" if one or both input
   strings are not valid.

   The equality test MUST be reflexive and symmetric.  For valid input,
   it MUST be transitive.

   If a collation provides either a substring or an ordering test, it
   MUST also provide an equality test.  The substring and/or ordering
   tests MUST be consistent with the equality test.

   The return values of the equality test are called "match", "no-match"
   and "undefined" in this document.

4.2.3.  Substring

   The substring matching operation determines if the first string is a
   substring of the second string, i.e., if one or more substrings of
   the second string is equal to the first, as defined by the
   collation's equality operation.

   A collation that supports substring matching will automatically
   support two special cases of substring matching: prefix and suffix
   matching, if those special cases are supported by the application
   protocol.  It returns "match" or "no-match" when it is supplied valid
   input and returns "undefined" when supplied invalid input.

   Application protocols MAY return position information for substring
   matches.  If this is done, the position information SHOULD include
   both the starting offset and the ending offset for each match.  This
   is important because more sophisticated collations can match strings
   of unequal length (for example, a pre-composed accented character can
   match a decomposed accented character).  In general, overlapping
   matches SHOULD be reported (as when "ana" occurs twice within
   "banana"), although there are cases where a collation may decide not

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   to.  For example, in a collation which treats all whitespace
   sequences as identical, the substring operation could be defined such
   that " 1 " (SP "1" SP) is reported just once within "  1  " (SP SP
   "1" SP SP), not four times (SP SP "1" SP, SP "1" SP, SP "1" SP SP and
   SP SP "1" SP SP), since the four matches are, in a sense, the same
   match.

   A string is a substring of itself.  The empty string is a substring
   of all strings.

   Note that the substring operation of some collations can match
   strings of unequal length.  For example, a pre-composed accented
   character can match a decomposed accented character.  The Unicode
   Collation Algorithm [7] discusses this in more detail.

   The return values of the substring operation are called "match", "no-
   match", and "undefined" in this document.

4.2.4.  Ordering

   The ordering operation determines how two strings are ordered.  It
   MUST be reflexive.  For valid input, it MUST be transitive and
   trichotomous.

   Ordering returns "less" if the first string is listed before the
   second string, according to the collation; "greater", if the second
   string is listed before the first string; and "equal", if the two
   strings are equal, as defined by the collation's equality operation.
   If one or both strings are invalid, the result of ordering is
   "undefined".

   When the collation is used with a "+" prefix, the behavior is the
   same as when used with no prefix.  When the collation is used with a
   "-" prefix, the result of the ordering operation of the collation
   MUST be reversed.

   The return values of the ordering operation are called "less",
   "equal", "greater", and "undefined" in this document.

4.3.  Sort Keys

   A collation specification SHOULD describe the internal transformation
   algorithm to generate sort keys.  This algorithm can be applied to
   individual strings, and the result can be stored to potentially
   optimize future comparison operations.  A collation MAY specify that
   the sort key is generated by the identity function.  The sort key may
   have no meaning to a human.  The sort key may not be valid input to
   the collation.

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4.4.  Use of Lookup Tables

   Some collations use customizable lookup tables, e.g., because the
   tables depend on locale, and may be modified after shipping the
   software.  Collations that use more than one customizable lookup
   table in a documented format MUST assign numbers to the tables they
   use.  This permits an application protocol command to access the
   tables used by a server collation, so that clients and servers use
   the same tables.

5.  Application Protocol Requirements

   This section describes the requirements and issues that an
   application protocol needs to consider if it offers searching,
   substring matching and/or sorting, and permits the use of characters
   outside the US-ASCII charset.

5.1.  Character Encoding

   The protocol specification has to make sure that it is clear on which
   characters (rather than just octets) the collations are used.  This
   can be done by specifying the protocol itself in terms of characters
   (e.g., in the case of a query language), by specifying a single
   character encoding for the protocol (e.g., UTF-8 [3]), or by
   carefully describing the relevant issues of character encoding
   labeling and conversion.  In the later case, details to consider
   include how to handle unknown charsets, any charsets that are
   mandatory-to-implement, any issues with byte-order that might apply,
   and any transfer encodings that need to be supported.

5.2.  Operations

   The protocol must specify which of the operations defined in this
   specification (equality matching, substring matching, and ordering)
   can be invoked in the protocol, and how they are invoked.  There may
   be more than one way to invoke an operation.

   The protocol MUST provide a mechanism for the client to select the
   collation to use with equality matching, substring matching, and
   ordering.

   If a protocol needs a total ordering and the collation chosen does
   not provide it because the ordering operation returns "undefined" at
   least once, the recommended fallback is to sort all invalid strings
   after the valid ones, and use i;octet to order the invalid strings.

   Although the collation's substring function provides a list of
   matches, a protocol need not provide all that to the client.  It may

Newman, et al.              Standards Track                    [Page 11]
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   provide only the first matching substring, or even just the
   information that the substring search matched.  In this way,
   collations can be used with protocols that are defined such that "x
   is a substring of y" returns true-false.

   If the protocol provides positional information for the results of a
   substring match, that positional information SHOULD fully specify the
   substring(s) in the result that matches, independent of the length of
   the search string.  For example, returning both the starting and
   ending offset of the match would suffice, as would the starting
   offset and a length.  Returning just the starting offset is not
   acceptable.  This rule is necessary because advanced collations can
   treat strings of different lengths as equal (for example, pre-
   composed and decomposed accented characters).

5.3.  Wildcards

   The protocol MUST specify whether it allows the use of wildcards in
   collation identifiers.  If the protocol allows wildcards, then:
      The protocol MUST specify how comparisons behave in the absence of
      explicit collation negotiation, or when a collation of "default"
      is requested.  The protocol MAY specify that the default collation
      used in such circumstances is sensitive to server configuration.

      The protocol SHOULD provide a way to list available collations
      matching a given wildcard pattern, or patterns.

5.4.  String Comparison

   If a protocol compares strings in any nontrivial way, using a
   collation may be appropriate.  As an example, many protocols use
   case-independent strings.  In many cases, a simple ASCII mapping to
   upper/lower case works well.  In other cases, it may be better to use
   a specifiable collation; for example, so that a server can treat "i"
   and "I" as equivalent in Italy, and different in Turkey (Turkish also
   has a dotted upper-case" I" and a dotless lower-case "i").

   Protocol designers should consider, in each case, whether to use a
   specifiable collation.  Keywords often have other needs than user
   variables, and search arguments may be different again.

5.5.  Disconnected Clients

   If the protocol supports disconnected clients, and a collation is
   used that can use configurable tables (e.g., to support
   locale-specific extensions), then the client may not be able to
   reproduce the server's collation operations while offline.

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   A mechanism to download such tables has been discussed.  Such a
   mechanism is not included in the present specification, since the
   problem is not yet well understood.

5.6.  Error Codes

   The protocol specification should consider assigning protocol error
   codes for the following circumstances:

   o  The client requests the use of a collation by identifier or
      pattern, but no implemented collation matches that pattern.

   o  The client attempts to use a collation for an operation that is
      not supported by that collation -- for example, attempting to use
      the "i;ascii-numeric" collation for substring matching.

   o  The client uses an equality or substring matching collation, and
      the result is an error.  It may be appropriate to distinguish
      between the two input strings, particularly when one is supplied
      by the client and the other is stored by the server.  It might
      also be appropriate to distinguish the specific case of an invalid
      UTF-8 string.

5.7.  Octet Collation

   The i;octet (Section 9.3) collation is only usable with protocols
   based on octet-strings.  Clients and servers MUST NOT use i;octet
   with other protocols.

   If the protocol permits the use of collations with data structures
   other than strings, the protocol MUST describe the default behavior
   for a collation with those data structures.

6.  Use by Existing Protocols

   This section is informative.

   Both ACAP [11] and Sieve [14] are standards track specifications that
   used collations prior to the creation of this specification and
   registry.  Those standards do not meet all the application protocol
   requirements described in Section 5.

   These protocols allow the use of the i;octet (Section 9.3) collation
   working directly on UTF-8 data, as used in these protocols.

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   In Sieve, all matches are either true or false.  Accordingly, Sieve
   servers must treat "undefined" and "no-match" results of the equality
   and substring operations as false, and only "match" as true.

   In ACAP and Sieve, there are no invalid strings.  In this document's
   terms, invalid strings sort after valid strings.

   IMAP [15] also collates, although that is explicit only when the
   COMPARATOR [17] extension is used.  The built-in IMAP substring
   operation and the ordering provided by the SORT [16] extension may
   not meet the requirements made in this document.

   Other protocols may be in a similar position.

   In IMAP, the default collation is i;ascii-casemap, because its
   operations are understood to match IMAP's built-in operations.

7.  Collation Registration

7.1.  Collation Registration Procedure

   The IETF will create a mailing list, collation@ietf.org, which can be
   used for public discussion of collation proposals prior to
   registration.  Use of the mailing list is strongly encouraged.  The
   IESG will appoint a designated expert who will monitor the
   collation@ietf.org mailing list and review registrations.

   The registration procedure begins when a completed registration
   template is sent to iana@iana.org and collation@ietf.org.  The
   designated expert is expected to tell IANA and the submitter of the
   registration within two weeks whether the registration is approved,
   approved with minor changes, or rejected with cause.  When a
   registration is rejected with cause, it can be re-submitted if the
   concerns listed in the cause are addressed.  Decisions made by the
   designated expert can be appealed to the IESG Applications Area
   Director, then to the IESG.  They follow the normal appeals procedure
   for IESG decisions.

   Collation registrations in a standards track, BCP, or IESG-approved
   experimental RFC are owned by the IETF, and changes to the
   registration follow normal procedures for updating such documents.
   Collation registrations in other RFCs are owned by the RFC author(s).
   Other collation registrations are owned by the individual(s) listed
   in the contact field of the registration, and IANA will preserve this
   information.

   If the registration is a change of an existing collation, it MUST be
   approved by the owner.  In the event the owner cannot be contacted

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   for a period of one month, and the designated expert deems the change
   necessary, the IESG MAY re-assign ownership to an appropriate party.

7.2.  Collation Registration Format

   Registration of a collation is done by sending a well-formed XML
   document to collation@ietf.org and iana@iana.org.

7.2.1.  Registration Template

   Here is a template for the registration:

   <?xml version='1.0'?>
   <!DOCTYPE collation SYSTEM 'collationreg.dtd'>
   <collation rfc="YYYY" scope="global" intendedUse="common">
     <identifier>collation identifier</identifier>
     <title>technical title for collation</title>
     <operations>equality order substring</operations>
     <specification>specification reference</specification>
     <owner>email address of owner or IETF</owner>
     <submitter>email address of submitter</submitter>
     <version>1</version>
   </collation>

7.2.2.  The Collation Element

   The root of the registration document MUST be a <collation> element.
   The collation element contains the other elements in the
   registration, which are described in the following sub-subsections,
   in the order given here.

   The <collation> element MAY include an "rfc=" attribute if the
   specification is in an RFC.  The "rfc=" attribute gives only the
   number of the RFC, without any prefix, such as "RFC", or suffix, such
   as ".txt".

   The <collation> element MUST include a "scope=" attribute, which MUST
   have one of the values "global", "local", or "other".

   The <collation> element MUST include an "intendedUse=" attribute,
   which must have one of the values "common", "limited", "vendor", or
   "deprecated".  Collation specifications intended for "common" use are
   expected to reference standards from standards bodies with
   significant experience dealing with the details of international
   character sets.

   Be aware that future revisions of this specification may add
   additional function types, as well as additional XML attributes,

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   values, and elements.  Any system that automatically parses these XML
   documents MUST take this into account to preserve future
   compatibility.

7.2.3.  The Identifier Element

   The <identifier> element gives the precise identifier of the
   collation, e.g., i;ascii-casemap.  The <identifier> element is
   mandatory.

7.2.4.  The Title Element

   The <title> element gives the title of the collation.  The <title>
   element is mandatory.

7.2.5.  The Operations Element

   The <operations> element lists which of the three operations
   ("equality", "order" or "substring") the collation provides,
   separated by single spaces.  The <operations> element is mandatory.

7.2.6.  The Specification Element

   The <specification> element describes where to find the
   specification.  The <specification> element is mandatory.  It MAY
   have a URI attribute.  There may be more than one <specification>
   element, in which case, they together form the specification.

   If it is discovered that parts of a collation specification conflict,
   a new revision of the collation is necessary, and the
   collation@ietf.org mailing list should be notified.

7.2.7.  The Submitter Element

   The <submitter> element provides an RFC 2822 [12] email address for
   the person who submitted the registration.  It is optional if the
   <owner> element contains an email address.

   There may be more than one <submitter> element.

7.2.8.  The Owner Element

   The <owner> element contains either the four letters "IETF" or an
   email address of the owner of the registration.  The <owner> element
   is mandatory.  There may be more than one <owner> element.  If so,
   all owners are equal.  Each owner can speak for all.

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7.2.9.  The Version Element

   The <version> element MUST be included when the registration is
   likely to be revised, or has been revised in such a way that the
   results change for one or more input strings.  The <version> element
   is optional.

7.2.10.  The Variable Element

   The <variable> element specifies an optional variable to control the
   collation's behaviour, for example whether it is case sensitive.  The
   <variable> element is optional.  When <variable> is used, it must
   contain <name> and <default> elements, and it may contain one or more
   <value> elements.

7.2.10.1.  The Name Element

   The <name> element specifies the name value of a variable.  The
   <name> element is mandatory.

7.2.10.2.  The Default Element

   The <default> element specifies the default value of a variable.  The
   <default> element is mandatory.

7.2.10.3.  The Value Element

   The <value> element specifies a legal value of a variable.  The
   <value> element is optional.  If one or more <value> elements are
   present, only those values are legal.  If none are, then the
   variable's legal values do not form an enumerated set, and the rules
   MUST be specified in an RFC accompanying the registration.

7.3.  Structure of Collation Registry

   Once the registration is approved, IANA will store each XML
   registration document in a URL of the form
   http://www.iana.org/assignments/collation/collation-id.xml, where
   collation-id is the content of the identifier element in the
   registration.  Both the submitter and the designated expert are
   responsible for verifying that the XML is well-formed.  The
   registration document should avoid using new elements.  If any are
   necessary, it is important to be consistent with other registrations.

   IANA will also maintain a text summary of the registry under the name
   http://www.iana.org/assignments/collation/collation-index.html.  This
   summary is divided into four sections.  The first section is for
   collations intended for common use.  This section is intended for

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   collation registrations published in IESG-approved RFCs, or for
   locally scoped collations from the primary standards body for that
   locale.  The designated expert is encouraged to reject collation
   registrations with an intended use of "common" if the expert believes
   it should be "limited", as it is desirable to keep the number of
   "common" registrations small and of high quality.  The second section
   is reserved for limited-use collations.  The third section is
   reserved for registered vendor-specific collations.  The final
   section is reserved for deprecated collations.

7.4.  Example Initial Registry Summary

   The following is an example of how IANA might structure the initial
   registry summary.html file:

     Collation                              Functions Scope Reference
     ---------                              --------- ----- ---------
   Common Use Collations:
     i;ascii-casemap                        e, o, s   Local [RFC 4790]

   Limited Use Collations:
     i;octet                                e, o, s   Other [RFC 4790]
     i;ascii-numeric                        e, o      Other [RFC 4790]

   Vendor Collations:

   Deprecated Collations:

   References
   ----------
   [RFC 4790]  Newman, C., Duerst, M., Gulbrandsen, A., "Internet
               Application Protocol Collation Registry", RFC 4790,
               Sun Microsystems, March 2007.

8.  Guidelines for Expert Reviewer

   The expert reviewer appointed by the IESG has fairly broad latitude
   for this registry.  While a number of collations are expected
   (particularly customizations of the UCA for localized use), an
   explosion of collations (particularly common-use collations) is not
   desirable for widespread interoperability.  However, it is important
   for the expert reviewer to provide cause when rejecting a
   registration, and, when possible, to describe corrective action to

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   permit the registration to proceed.  The following table includes
   some example reasons to reject a registration with cause:

   o  The registration is not a well-formed XML document.

   o  The registration has an intended use of "common", but there is no
      evidence the collation will be widely deployed, so it should be
      listed as "limited".

   o  The registration has an intended use of "common", but it is
      redundant with the functionality of a previously registered
      "common" collation.

   o  The registration has an intended use of "common", but the
      specification is not detailed enough to allow interoperable
      implementations by others.

   o  The collation identifier fails to precisely identify the version
      numbers of relevant tables to use.

   o  The registration fails to meet one of the "MUST" requirements in
      Section 4.

   o  The collation identifier fails to meet the syntax in Section 3.

   o  The collation specification referenced in the registration is
      vague or has optional features without a clear behavior specified.

   o  The referenced specification does not adequately address security
      considerations specific to that collation.

   o  The registration's operations are needlessly different from those
      of traditional operations.

   o  The registration's XML is needlessly different from that of
      already registered collations.

9.  Initial Collations

   This section registers the three collations that were originally
   defined in [11], and are implemented in most [14] engines.  Some of
   the behavior of these collations is perhaps not ideal, such as
   i;ascii-casemap accepting non-ASCII input.  Compatibility with widely
   deployed code was judged more important than fixing the collations.
   Some of the aspects of these collations are necessary to maintain
   compatibility with widely deployed code.

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9.1.  ASCII Numeric Collation

9.1.1.  ASCII Numeric Collation Description

   The "i;ascii-numeric" collation is a simple collation intended for
   use with arbitrarily-sized, unsigned decimal integer numbers stored
   as octet strings.  US-ASCII digits (0x30 to 0x39) represent digits of
   the numbers.  Before converting from string to integer, the input
   string is truncated at the first non-digit character.  All input is
   valid; strings that do not start with a digit represent positive
   infinity.

   The collation supports equality and ordering, but does not support
   the substring operation.

   The equality operation returns "match" if the two strings represent
   the same number (i.e., leading zeroes and trailing non-digits are
   disregarded), and "no-match" if the two strings represent different
   numbers.

   The ordering operation returns "less" if the first string represents
   a smaller number than the second, "equal" if they represent the same
   number, and "greater" if the first string represents a larger number
   than the second.

   Some examples: "0" is less than "1", and "1" is less than
   "4294967298". "4294967298", "04294967298", and "4294967298b" are all
   equal. "04294967298" is less than "". "", "x", and "y" are equal.

9.1.2.  ASCII Numeric Collation Registration

   <?xml version='1.0'?>
   <!DOCTYPE collation SYSTEM 'collationreg.dtd'>
   <collation rfc="4790" scope="other" intendedUse="limited">
     <identifier>i;ascii-numeric</identifier>
     <title>ASCII Numeric</title>
     <operations>equality order</operations>
     <specification>RFC 4790</specification>
     <owner>IETF</owner>
     <submitter>chris.newman@sun.com</submitter>
   </collation>

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9.2.  ASCII Casemap Collation

9.2.1.  ASCII Casemap Collation Description

   The "i;ascii-casemap" collation is a simple collation that operates
   on octet strings and treats US-ASCII letters case-insensitively.  It
   provides equality, substring, and ordering operations.  All input is
   valid.  Note that letters outside ASCII are not treated case-
   insensitively.

   Its equality, ordering, and substring operations are as for i;octet,
   except that at first, the lower-case letters (octet values 97-122) in
   each input string are changed to upper case (octet values 65-90).

   Care should be taken when using OS-supplied functions to implement
   this collation, as it is not locale sensitive.  Functions, such as
   strcasecmp and toupper, are sometimes locale sensitive, and may
   inappropriately map lower-case letters other than a-z to upper case.

   The i;ascii-casemap collation is well-suited for use with many
   Internet protocols and computer languages.  Use with natural language
   is often inappropriate; even though the collation apparently supports
   languages such as Swahili and English, in real-world use, it tends to
   mis-sort a number of types of string:

   o  people and place names containing non-ASCII,

   o  words such as "naive" (if spelled with an accent, the accented
      character could push the word to the wrong spot in a sorted list),

   o  names such as "Lloyd" (which, in Welsh, sorts after "Lyon", unlike
      in English),

   o  strings containing euro and pound sterling symbols, quotation
      marks other than '"', dashes/hyphens, etc.

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9.2.2.  ASCII Casemap Collation Registration

   <?xml version='1.0'?>
   <!DOCTYPE collation SYSTEM 'collationreg.dtd'>
   <collation rfc="4790" scope="local" intendedUse="common">
     <identifier>i;ascii-casemap</identifier>
     <title>ASCII Casemap</title>
     <operations>equality order substring</operations>
     <specification>RFC 4790</specification>
     <owner>IETF</owner>
     <submitter>chris.newman@sun.com</submitter>
   </collation>

9.3.  Octet Collation

9.3.1.  Octet Collation Description

   The "i;octet" collation is a simple and fast collation intended for
   use on binary octet strings rather than on character data.  Protocols
   that want to make this collation available have to do so by
   explicitly allowing it.  If not explicitly allowed, it MUST NOT be
   used.  It never returns an "undefined" result.  It provides equality,
   substring, and ordering operations.

   The ordering algorithm is as follows:

   1.  If both strings are the empty string, return the result "equal".

   2.  If the first string is empty and the second is not, return the
       result "less".

   3.  If the second string is empty and the first is not, return the
       result "greater".

   4.  If both strings begin with the same octet value, remove the first
       octet from both strings and repeat this algorithm from step 1.

   5.  If the unsigned value (0 to 255) of the first octet of the first
       string is less than the unsigned value of the first octet of the
       second string, then return "less".

   6.  If this step is reached, return "greater".

   This algorithm is roughly equivalent to the C library function
   memcmp, with appropriate length checks added.

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   The matching operation returns "match" if the sorting algorithm would
   return "equal".  Otherwise, the matching operation returns "no-
   match".

   The substring operation returns "match" if the first string is the
   empty string, or if there exists a substring of the second string of
   length equal to the length of the first string, which would result in
   a "match" result from the equality function.  Otherwise, the
   substring operation returns "no-match".

9.3.2.  Octet Collation Registration

   This collation is defined with intendedUse="limited" because it can
   only be used by protocols that explicitly allow it.

   <?xml version='1.0'?>
   <!DOCTYPE collation SYSTEM 'collationreg.dtd'>
   <collation rfc="4790" scope="global" intendedUse="limited">
     <identifier>i;octet</identifier>
     <title>Octet</title>
     <operations>equality order substring</operations>
     <specification>RFC 4790</specification>
     <owner>IETF</owner>
     <submitter>chris.newman@sun.com</submitter>
   </collation>

10.  IANA Considerations

   Section 7 defines how to register collations with IANA.  Section 9
   defines a list of predefined collations that have been registered
   with IANA.

11.  Security Considerations

   Collations will normally be used with UTF-8 strings.  Thus, the
   security considerations for UTF-8 [3], stringprep [6], and Unicode
   TR-36 [8] also apply, and are normative to this specification.

12.  Acknowledgements

   The authors want to thank all who have contributed to this document,
   including Brian Carpenter, John Cowan, Dave Cridland, Mark Davis,
   Spencer Dawkins, Lisa Dusseault, Lars Eggert, Frank Ellermann, Philip
   Guenther, Tony Hansen, Ted Hardie, Sam Hartman, Kjetil Torgrim Homme,
   Michael Kay, John Klensin, Alexey Melnikov, Jim Melton, and Abhijit
   Menon-Sen.

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13.  References

13.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [2]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 4234, October 2005.

   [3]   Yergeau, F., "UTF-8, a transformation format of ISO 10646",
         STD 63, RFC 3629, November 2003.

   [4]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifier (URI): Generic Syntax", RFC 3986,
         January 2005.

   [5]   Phillips, A. and M. Davis, "Tags for Identifying Languages",
         BCP 47, RFC 4646, September 2006.

   [6]   Hoffman, P. and M. Blanchet, "Preparation of Internationalized
         Strings ("stringprep")", RFC 3454, December 2002.

   [7]   Davis, M. and K. Whistler, "Unicode Collation Algorithm version
         14", May 2005,
         <http://www.unicode.org/reports/tr10/tr10-14.html>.

   [8]   Davis, M. and M. Suignard, "Unicode Security Considerations",
         February 2006, <http://www.unicode.org/reports/tr36/>.

13.2.  Informative References

   [9]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part One: Format of Internet Message Bodies",
         RFC 2045, November 1996.

   [10]  Melnikov, A., "Simple Authentication and Security Layer
         (SASL)", RFC 4422, June 2006.

   [11]  Newman, C. and J. Myers, "ACAP -- Application Configuration
         Access Protocol", RFC 2244, November 1997.

   [12]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.

   [13]  Freed, N. and J. Postel, "IANA Charset Registration
         Procedures", BCP 19, RFC 2978, October 2000.

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   [14]  Showalter, T., "Sieve: A Mail Filtering Language", RFC 3028,
         January 2001.

   [15]  Crispin, M., "Internet Message Access Protocol - Version
         4rev1", RFC 3501, March 2003.

   [16]  Crispin, M. and K. Murchison, "Internet Message Access Protocol
         - Sort and Thread Extensions", Work in Progress, May 2004.

   [17]  Newman, C. and A. Gulbrandsen, "Internet Message Access
         Protocol Internationalization", Work in Progress, January 2006.

Authors' Addresses

   Chris Newman
   Sun Microsystems
   1050 Lakes Drive
   West Covina, CA  91790
   USA

   EMail: chris.newman@sun.com

   Martin Duerst
   Aoyama Gakuin University
   5-10-1 Fuchinobe
   Sagamihara, Kanagawa  229-8558
   Japan

   Phone: +81 42 759 6329
   Fax:   +81 42 759 6495
   EMail: duerst@it.aoyama.ac.jp
   URI:   http://www.sw.it.aoyama.ac.jp/D%C3%BCrst/

   Note: Please write "Duerst" with u-umlaut wherever possible, for
   example as "D&#252;rst" in XML and HTML.

   Arnt Gulbrandsen
   Oryx Mail Systems GmbH
   Schweppermannstr. 8
   81671 Munich
   Germany

   Fax:   +49 89 4502 9758
   EMail: arnt@oryx.com
   URI:   http://www.oryx.com/arnt/

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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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