Network Working Group C. Newman
Request for Comments: 4790 Sun Microsystems
Category: Standards Track M. Duerst
Aoyama Gakuin University
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 (C) The IETF Trust (2007).
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 2007Table 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]RFC 4790 Collation Registry March 2007
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]RFC 4790 Collation Registry March 20071. Introduction
The Application Configuration Access Protocol ACAP  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  and the Unicode Collation Algorithm
, 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" .
The attribute syntax specifications use the Augmented Backus-Naur
Form (ABNF)  notation, including the core rules defined in
Appendix A. The ABNF production "Language-tag" is imported from
Language Tags  and "reg-name" from URI: Generic Syntax .
2. Collation Definition and Purpose2.1. 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
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 
separates the details of octet to character mapping from a protocol
specification, such as MIME , or the way SASL  separates the
details of an authentication mechanism from a protocol specification,
such as ACAP .
Newman, et al. Standards Track [Page 4]RFC 4790 Collation Registry March 2007
Here is a small diagram to help illustrate the value of this
| 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
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
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  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
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]RFC 4790 Collation Registry March 2007
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 Syntax3.1. Basic Syntax
The collation identifier itself is a single US-ASCII string. The
identifier MUST NOT be longer than 254 characters, and obeys the
collation-char = ALPHA / DIGIT / "-" / ";" / "=" / "."
collation-id = collation-prefix ";" collation-core-name
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  and "reg-name" from URI: Generic Syntax .
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
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
Newman, et al. Standards Track [Page 6]RFC 4790 Collation Registry March 2007
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
collation-selected = ["+" / "-"] collation-id
collation-order = ["+" / "-"] collation-wild
Some protocols are designed to use URIs  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
collation-uri = "http://www.iana.org/assignments/collation/"
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  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]RFC 4790 Collation Registry March 2007
delimit the parameter from the value. The version numbers of any
lookup tables used by the algorithm SHOULD be present as
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 Requirements4.1. 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]RFC 4790 Collation Registry March 20074.2.1. 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
The validity test is provided by all collations. It MUST NOT be
listed separately in the collation registration.
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.
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
Newman, et al. Standards Track [Page 9]RFC 4790 Collation Registry March 2007
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
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  discusses this in more detail.
The return values of the substring operation are called "match", "no-
match", and "undefined" in this document.
The ordering operation determines how two strings are ordered. It
MUST be reflexive. For valid input, it MUST be transitive and
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
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
Newman, et al. Standards Track [Page 10]RFC 4790 Collation Registry March 20074.4. 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 ), 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.
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
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]RFC 4790 Collation Registry March 2007
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).
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.
Newman, et al. Standards Track [Page 12]RFC 4790 Collation Registry March 2007
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
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  and Sieve  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.
Newman, et al. Standards Track [Page 13]RFC 4790 Collation Registry March 2007
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  also collates, although that is explicit only when the
COMPARATOR  extension is used. The built-in IMAP substring
operation and the ordering provided by the SORT  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 Registration7.1. Collation Registration Procedure
The IETF will create a mailing list, email@example.com, 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
firstname.lastname@example.org mailing list and review registrations.
The registration procedure begins when a completed registration
template is sent to email@example.com and firstname.lastname@example.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
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
Newman, et al. Standards Track [Page 14]RFC 4790 Collation Registry March 2007
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 email@example.com and firstname.lastname@example.org.
7.2.1. Registration Template
Here is a template for the registration:
<!DOCTYPE collation SYSTEM 'collationreg.dtd'>
<collation rfc="YYYY" scope="global" intendedUse="common">
<title>technical title for collation</title>
<operations>equality order substring</operations>
<owner>email address of owner or IETF</owner>
<submitter>email address of submitter</submitter>
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
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
Be aware that future revisions of this specification may add
additional function types, as well as additional XML attributes,
Newman, et al. Standards Track [Page 15]RFC 4790 Collation Registry March 2007
values, and elements. Any system that automatically parses these XML
documents MUST take this into account to preserve future
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
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
email@example.com mailing list should be notified.
7.2.7. The Submitter Element
The <submitter> element provides an RFC 2822  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.
Newman, et al. Standards Track [Page 16]RFC 4790 Collation Registry March 20077.2.9. 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
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
18.104.22.168. The Name Element
The <name> element specifies the name value of a variable. The
<name> element is mandatory.
22.214.171.124. The Default Element
The <default> element specifies the default value of a variable. The
<default> element is mandatory.
126.96.36.199. 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
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
summary is divided into four sections. The first section is for
collations intended for common use. This section is intended for
Newman, et al. Standards Track [Page 17]RFC 4790 Collation Registry March 2007
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]
[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
Newman, et al. Standards Track [Page 18]RFC 4790 Collation Registry March 2007
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
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
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 , and are implemented in most  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.
Newman, et al. Standards Track [Page 19]RFC 4790 Collation Registry March 20079.1. ASCII Numeric Collation9.1.1. 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
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
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
<!DOCTYPE collation SYSTEM 'collationreg.dtd'>
<collation rfc="4790" scope="other" intendedUse="limited">
Newman, et al. Standards Track [Page 20]RFC 4790 Collation Registry March 20079.2. ASCII Casemap Collation9.2.1. 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-
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
o strings containing euro and pound sterling symbols, quotation
marks other than '"', dashes/hyphens, etc.
Newman, et al. Standards Track [Page 21]RFC 4790 Collation Registry March 20079.2.2. ASCII Casemap Collation Registration
<!DOCTYPE collation SYSTEM 'collationreg.dtd'>
<collation rfc="4790" scope="local" intendedUse="common">
<operations>equality order substring</operations>
9.3. Octet Collation9.3.1. 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
3. If the second string is empty and the first is not, return the
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.
Newman, et al. Standards Track [Page 22]RFC 4790 Collation Registry March 2007
The matching operation returns "match" if the sorting algorithm would
return "equal". Otherwise, the matching operation returns "no-
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.
<!DOCTYPE collation SYSTEM 'collationreg.dtd'>
<collation rfc="4790" scope="global" intendedUse="limited">
<operations>equality order substring</operations>
10. IANA Considerations
Section 7 defines how to register collations with IANA. Section 9
defines a list of predefined collations that have been registered
11. Security Considerations
Collations will normally be used with UTF-8 strings. Thus, the
security considerations for UTF-8 , stringprep , and Unicode
TR-36  also apply, and are normative to this specification.
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
Newman, et al. Standards Track [Page 23]RFC 4790 Collation Registry March 200713. References13.1. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
 Yergeau, F., "UTF-8, a transformation format of ISO 10646",
STD 63, RFC 3629, November 2003.
 Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", RFC 3986,
 Phillips, A. and M. Davis, "Tags for Identifying Languages",
BCP 47, RFC 4646, September 2006.
 Hoffman, P. and M. Blanchet, "Preparation of Internationalized
Strings ("stringprep")", RFC 3454, December 2002.
 Davis, M. and K. Whistler, "Unicode Collation Algorithm version
14", May 2005,
 Davis, M. and M. Suignard, "Unicode Security Considerations",
February 2006, <http://www.unicode.org/reports/tr36/>.
13.2. Informative References
 Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
 Melnikov, A., "Simple Authentication and Security Layer
(SASL)", RFC 4422, June 2006.
 Newman, C. and J. Myers, "ACAP -- Application Configuration
Access Protocol", RFC 2244, November 1997.
 Resnick, P., "Internet Message Format", RFC 2822, April 2001.
 Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
Newman, et al. Standards Track [Page 24]RFC 4790 Collation Registry March 2007
 Showalter, T., "Sieve: A Mail Filtering Language", RFC 3028,
 Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
 Crispin, M. and K. Murchison, "Internet Message Access Protocol
- Sort and Thread Extensions", Work in Progress, May 2004.
 Newman, C. and A. Gulbrandsen, "Internet Message Access
Protocol Internationalization", Work in Progress, January 2006.
1050 Lakes Drive
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Phone: +81 42 759 6329
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Note: Please write "Duerst" with u-umlaut wherever possible, for
example as "Dürst" in XML and HTML.
Oryx Mail Systems GmbH
Fax: +49 89 4502 9758
Newman, et al. Standards Track [Page 25]RFC 4790 Collation Registry March 2007Full Copyright Statement
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Newman, et al. Standards Track [Page 26]