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
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.
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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 [RFC2811] and multi-party chat technologies based on the Extensible Messaging and Presence Protocol (XMPP) [RFC6120] [XEP-0045], the Message Session Relay Protocol (MSRP) [RFC4975] [RFC7701], and Centralized Conferencing (XCON) [RFC5239] [XCON-SYSTEM]. Recent chat room technologies also allow internationalized nicknames because they support characters from outside the ASCII range [RFC20], 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).
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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 [RFC5322] 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.
Many important terms used in this document are defined in [RFC7564], [RFC6365], 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 [RFC2119].
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 [RFC5893]) 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.
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 [RFC7564]. In addition, the entity MUST ensure that the string is encoded as UTF-8 [RFC3629].
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).
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:
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).
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).
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 [RFC7564] 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.
[RFC7564] 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].