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IETF RFC 7700
Last modified on Tuesday, December 15th, 2015
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Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 7700 &yet
Category: Standards Track December 2015
ISSN: 2070-1721
Preparation, Enforcement, and Comparison of
Internationalized Strings Representing Nicknames
 Abstract
This document describes methods for handling Unicode strings
representing memorable, human-friendly names (called "nicknames",
"display names", or "petnames") for people, devices, accounts,
websites, and other entities.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7700.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Saint-Andre Standards Track PAGE 1 
RFC 7700 PRECIS: Nickname December 2015
Table of Contents
1. Introduction ....................................................2
1.1. Overview ...................................................2
1.2. Terminology ................................................3
2. Nickname Profile ................................................3
2.1. Rules ......................................................3
2.2. Preparation ................................................5
2.3. Enforcement ................................................5
2.4. Comparison .................................................5
3. Examples ........................................................5
4. Use in Application Protocols ....................................6
5. IANA Considerations .............................................7
6. Security Considerations .........................................8
6.1. Reuse of PRECIS ............................................8
6.2. Reuse of Unicode ...........................................8
6.3. Visually Similar Characters ................................8
7. References ......................................................8
7.1. Normative References .......................................8
7.2. Informative References .....................................9
Acknowledgements ..................................................11
Author's Address ..................................................11
1. Introduction
1.1. Overview
A number of technologies and applications provide the ability for a
person to choose a memorable, human-friendly name in a communications
context, or to set such a name for another entity such as a device,
account, contact, or website. Such names are variously called
"nicknames" (e.g., in chat room applications), "display names" (e.g.,
in Internet mail), or "petnames" (see [PETNAME-SYSTEMS]); for
consistency, these are all called "nicknames" in this document.
Nicknames are commonly supported in technologies for textual chat
rooms, e.g., Internet Relay Chat [RFC 2811] and multi-party chat
technologies based on the Extensible Messaging and Presence Protocol
(XMPP) [RFC 6120] [XEP-0045], the Message Session Relay Protocol
(MSRP) [RFC 4975] [RFC 7701], and Centralized Conferencing (XCON)
[RFC 5239] [XCON-SYSTEM]. Recent chat room technologies also allow
internationalized nicknames because they support characters from
outside the ASCII range [RFC 20], typically by means of the Unicode
character set [Unicode]. Although such nicknames tend to be used
primarily for display purposes, they are sometimes used for
programmatic purposes as well (e.g., kicking users or avoiding
nickname conflicts).
Saint-Andre Standards Track PAGE 2
RFC 7700 PRECIS: Nickname December 2015
A similar usage enables a person to set their own preferred display
name or to set a preferred display name for another user (e.g., the
"display-name" construct in the Internet message format [RFC 5322] and
[XEP-0172] in XMPP).
Memorable, human-friendly names are also used in contexts other than
personal messaging, such as names for devices (e.g., in a network
visualization application), websites (e.g., for bookmarks in a web
browser), accounts (e.g., in a web interface for a list of payees in
a bank account), people (e.g., in a contact list application), and
the like.
The rules specified in this document can be applied in all of the
foregoing contexts.
To increase the likelihood that memorable, human-friendly names will
work in ways that make sense for typical users throughout the world,
this document defines rules for preparing, enforcing, and comparing
internationalized nicknames.
1.2. Terminology
Many important terms used in this document are defined in [RFC 7564],
[RFC 6365], and [Unicode].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC 2119].
2. Nickname Profile
2.1. Rules
The following rules apply within the Nickname profile of the PRECIS
FreeformClass.
1. Width Mapping Rule: There is no width-mapping rule (such a rule
is not necessary because width mapping is performed as part of
normalization using Normalization Form KC (NFKC) as specified
below).
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RFC 7700 PRECIS: Nickname December 2015
2. Additional Mapping Rule: The additional mapping rule consists of
the following sub-rules.
1. Any instances of non-ASCII space MUST be mapped to ASCII
space (U+0020); a non-ASCII space is any Unicode code point
having a general category of "Zs", naturally with the
exception of U+0020.
2. Any instances of the ASCII space character at the beginning
or end of a nickname MUST be removed (e.g., "stpeter " is
mapped to "stpeter").
3. Interior sequences of more than one ASCII space character
MUST be mapped to a single ASCII space character (e.g.,
"St Peter" is mapped to "St Peter").
3. Case Mapping Rule: Unicode Default Case Folding MUST be applied,
as defined in the Unicode Standard [Unicode] (at the time of this
writing, the algorithm is specified in Chapter 3 of
[Unicode7.0]). In applications that prohibit conflicting
nicknames, this rule helps to reduce the possibility of confusion
by ensuring that nicknames differing only by case (e.g.,
"stpeter" vs. "StPeter") would not be presented to a human user
at the same time.
4. Normalization Rule: The string MUST be normalized using Unicode
NFKC. Because NFKC is more "aggressive" in finding matches than
other normalization forms (in the terminology of Unicode, it
performs both canonical and compatibility decomposition before
recomposing code points), this rule helps to reduce the
possibility of confusion by increasing the number of characters
that would match (e.g., U+2163 ROMAN NUMERAL FOUR would match the
combination of U+0049 LATIN CAPITAL LETTER I and U+0056 LATIN
CAPITAL LETTER V).
5. Directionality Rule: There is no directionality rule. The "Bidi
Rule" (defined in [RFC 5893]) and similar rules are unnecessary
and inapplicable to nicknames, because it is perfectly acceptable
for a given nickname to be presented differently in different
layout systems (e.g., a user interface that is configured to
handle primarily a right-to-left script versus an interface that
is configured to handle primarily a left-to-right script), as
long as the presentation is consistent in any given layout
system.
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RFC 7700 PRECIS: Nickname December 2015
2.2. Preparation
An entity that prepares a string for subsequent enforcement according
to this profile MUST ensure that the string consists only of Unicode
code points that conform to the FreeformClass string class defined in
[RFC 7564]. In addition, the entity MUST ensure that the string is
encoded as UTF-8 [RFC 3629].
2.3. Enforcement
An entity that performs enforcement according to this profile MUST
prepare a string as described in Section 2.2 and MUST also apply the
following rules specified in Section 2.1 in the order shown:
1. Additional Mapping Rule
2. Normalization Rule
3. Directionality Rule
After all of the foregoing rules have been enforced, the entity MUST
ensure that the nickname is not zero bytes in length (this is done
after enforcing the rules to prevent applications from mistakenly
omitting a nickname entirely, because when internationalized
characters are accepted, a non-empty sequence of characters can
result in a zero-length nickname after canonicalization).
2.4. Comparison
An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in Section 2.2 and MUST
apply the following rules specified in Section 2.1 in the order
shown:
1. Additional Mapping Rule
2. Case Mapping Rule
3. Normalization Rule
4. Directionality Rule
The two strings are to be considered equivalent if they are an exact
octet-for-octet match (sometimes called "bit-string identity").
3. Examples
The following examples illustrate a small number of nicknames that
are consistent with the format defined above, along with the output
string resulting from application of the PRECIS rules (note that the
characters < and > are used to delineate the actual nickname and are
not part of the nickname strings).
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RFC 7700 PRECIS: Nickname December 2015
Table 1: A Sample of Legal Nicknames
+---------------------------+-----------------------------------+
| # | Nickname | Output for Comparison |
+---------------------------+-----------------------------------+
| 1 | <Foo> | <foo> |
+---------------------------+-----------------------------------+
| 2 | <foo> | <foo> |
+---------------------------+-----------------------------------+
| 3 | <Foo Bar> | <foo bar> |
+---------------------------+-----------------------------------+
| 4 | <foo bar> | <foo bar> |
+---------------------------+-----------------------------------+
| 5 | <Σ> | GREEK SMALL LETTER SIGMA (U+03C3) |
+---------------------------+-----------------------------------+
| 6 | <σ> | GREEK SMALL LETTER SIGMA (U+03C3) |
+---------------------------+-----------------------------------+
| 7 | <ς> | GREEK SMALL LETTER SIGMA (U+03C3) |
+---------------------------+-----------------------------------+
| 8 | <♚> | BLACK CHESS KING (U+265A) |
+---------------------------+-----------------------------------+
| 9 | <Richard Ⅳ> | <richard iv> |
+---------------------------+-----------------------------------+
Regarding examples 5, 6, and 7: applying Unicode Default Case Folding
to GREEK CAPITAL LETTER SIGMA (U+03A3) results in GREEK SMALL LETTER
SIGMA (U+03C3), and the same is true of GREEK SMALL LETTER FINAL
SIGMA (U+03C2); therefore, the comparison operation defined in
Section 2.4 would result in matching of the nicknames in examples 5,
6, and 7. Regarding example 8: symbol characters such as BLACK CHESS
KING (U+265A) are allowed by the PRECIS FreeformClass and thus can be
used in nicknames. Regarding example 9: applying Unicode Default
Case Folding to ROMAN NUMERAL FOUR (U+2163) results in SMALL ROMAN
NUMERAL FOUR (U+2173), and applying NFKC to SMALL ROMAN NUMERAL FOUR
(U+2173) results in LATIN SMALL LETTER I (U+0069) LATIN SMALL LETTER
V (U+0086).
4. Use in Application Protocols
This specification defines only the PRECIS-based rules for handling
of nickname strings. It is the responsibility of an application
protocol (e.g., MSRP, XCON, or XMPP) or application definition to
specify the protocol slots in which nickname strings can appear, the
entities that are expected to enforce the rules governing nickname
strings, and when in protocol processing or interface handling the
rules need to be enforced. See Section 6 of [RFC 7564] for guidelines
about using PRECIS profiles in applications.
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RFC 7700 PRECIS: Nickname December 2015
Above and beyond the PRECIS-based rules specified here, application
protocols can also define application-specific rules governing
nickname strings (rules regarding the minimum or maximum length of
nicknames, further restrictions on allowable characters or character
ranges, safeguards to mitigate the effects of visually similar
characters, etc.).
Naturally, application protocols can also specify rules governing the
actual use of nicknames in applications (reserved nicknames,
authorization requirements for using nicknames, whether certain
nicknames can be prohibited, handling of duplicates, the relationship
between nicknames and underlying identifiers such as SIP URIs or
Jabber IDs, etc.).
Entities that enforce the rules specified in this document are
encouraged to be liberal in what they accept by following this
procedure:
1. Where possible, map characters (e.g, through width mapping,
additional mapping, case mapping, or normalization) and accept
the mapped string.
2. If mapping is not possible (e.g., because a character is
disallowed in the FreeformClass), reject the string.
5. IANA Considerations
The IANA shall add the following entry to the PRECIS Profiles
Registry:
Name: Nickname
Base Class: FreeformClass
Applicability: Nicknames in messaging and text conferencing
technologies; petnames for devices, accounts, and people; and
other uses of nicknames or petnames.
Replaces: None
Width Mapping Rule: None (handled via NFKC)
Additional Mapping Rule: Map non-ASCII space characters to ASCII
space, strip leading and trailing space characters, map interior
sequences of multiple space characters to a single ASCII space.
Case Mapping Rule: Map uppercase and titlecase characters to
lowercase using Unicode Default Case Folding.
Saint-Andre Standards Track PAGE 7
RFC 7700 PRECIS: Nickname December 2015
Normalization Rule: NFKC
Directionality Rule: None
Enforcement: To be specified by applications.
Specification: RFC 7700 (this document)
6. Security Considerations
6.1. Reuse of PRECIS
The security considerations described in [RFC 7564] apply to the
FreeformClass string class used in this document for nicknames.
6.2. Reuse of Unicode
The security considerations described in [UTS39] apply to the use of
Unicode characters in nicknames.
6.3. Visually Similar Characters
[RFC 7564] describes some of the security considerations related to
visually similar characters, also called "confusable characters" or
"confusables".
Although the mapping rules defined in Section 2 of this document are
designed, in part, to reduce the possibility of confusion about
nicknames, this document does not provide more-detailed
recommendations regarding the handling of visually similar
characters, such as those provided in [UTS39].
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC 3629, November
2003, <http://www.rfc-editor.org/info/RFC 3629>.
Saint-Andre Standards Track PAGE 8
RFC 7700 PRECIS: Nickname December 2015
[RFC 5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
for Internationalized Domain Names for Applications
(IDNA)", RFC 5893, DOI 10.17487/RFC 5893, August 2010,
<http://www.rfc-editor.org/info/RFC 5893>.
[RFC 6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
DOI 10.17487/RFC 6365, September 2011,
<http://www.rfc-editor.org/info/RFC 6365>.
[RFC 7564] Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
Preparation, Enforcement, and Comparison of
Internationalized Strings in Application Protocols",
RFC 7564, DOI 10.17487/RFC 7564, May 2015,
<http://www.rfc-editor.org/info/RFC 7564>.
[Unicode] The Unicode Consortium, "The Unicode Standard",
<http://www.unicode.org/versions/latest/>.
[Unicode7.0]
The Unicode Consortium, "The Unicode Standard, Version
7.0.0", 2014,
<http://www.unicode.org/versions/Unicode7.0.0/>.
[UTS39] The Unicode Consortium, "Unicode Technical Standard #39:
Unicode Security Mechanisms", November 2013,
<http://unicode.org/reports/tr39/>.
7.2. Informative References
[PETNAME-SYSTEMS]
Stiegler, M., "An Introduction to Petname Systems",
updated June 2012, February 2005,
<http://www.skyhunter.com/marcs/petnames/
IntroPetNames.html>.
[RFC 20] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC 20, October 1969,
<http://www.rfc-editor.org/info/RFC 20>.
[RFC 2811] Kalt, C., "Internet Relay Chat: Channel Management",
RFC 2811, DOI 10.17487/RFC 2811, April 2000,
<http://www.rfc-editor.org/info/RFC 2811>.
[RFC 4975] Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
"The Message Session Relay Protocol (MSRP)", RFC 4975,
DOI 10.17487/RFC 4975, September 2007,
<http://www.rfc-editor.org/info/RFC 4975>.
Saint-Andre Standards Track PAGE 9
RFC 7700 PRECIS: Nickname December 2015
[RFC 5239] Barnes, M., Boulton, C., and O. Levin, "A Framework for
Centralized Conferencing", RFC 5239, DOI 10.17487/RFC 5239,
June 2008, <http://www.rfc-editor.org/info/RFC 5239>.
[RFC 5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC 5322, October 2008,
<http://www.rfc-editor.org/info/RFC 5322>.
[RFC 6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC 6120,
March 2011, <http://www.rfc-editor.org/info/RFC 6120>.
[RFC 7701] Niemi, A., Garcia-Martin, M., and G. Sandbakken, "Multi-
party Chat Using the Message Session Relay Protocol
(MSRP)", RFC 7701, DOI 10.17487/RFC 7701, December 2015,
<http://www.rfc-editor.org/info/RFC 7701>.
[XCON-SYSTEM]
Barnes, M., Boulton, C., and S. Loreto, "Chatrooms within
a Centralized Conferencing (XCON) System", Work in
Progress, draft-boulton-xcon-session-chat-08, July 2012.
[XEP-0045]
Saint-Andre, P., "Multi-User Chat", XSF XEP 0045, February
2012, <http://xmpp.org/extensions/xep-0045.html>.
[XEP-0172]
Saint-Andre, P. and V. Mercier, "User Nickname", XSF
XEP 0172, March 2012,
<http://xmpp.org/extensions/xep-0172.html>.
Saint-Andre Standards Track PAGE 10
RFC 7700 PRECIS: Nickname December 2015
Acknowledgements
Thanks to Kim Alvefur, Mary Barnes, Ben Campbell, Dave Cridland,
Miguel Garcia, Salvatore Loreto, Enrico Marocco, Matt Miller, and
Yoshiro YONEYA for their reviews and comments.
Paul Kyzivat and Melinda Shore reviewed the document for the General
Area Review Team and Operations Directorate, respectively.
During IESG review, Ben Campbell and Kathleen Moriarty provided
comments that led to further improvements.
Thanks to Matt Miller as Document Shepherd, Pete Resnick and Andrew
Sullivan as IANA Designated Experts, Marc Blanchet and Alexey
Melnikov as working group Chairs, and Barry Leiba as Area Director.
The author wishes to acknowledge Cisco Systems, Inc., for employing
him during his work on earlier draft versions of this document.
Author's Address
Peter Saint-Andre
&yet
Email: peter@andyet.com
URI: https://andyet.com/
Saint-Andre Standards Track PAGE 11
RFC TOTAL SIZE: 22898 bytes
PUBLICATION DATE: Tuesday, December 15th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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