Extensible Messaging and Presence Protocol (XMPP): Address Format
draft-ietf-xmpp-address-06

Abstract

This document defines the format for addresses used in the Extensible Messaging and Presence Protocol (XMPP), including support for non-ASCII characters.

Status of this Memo

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This Internet-Draft will expire on April 28, 2011.

Copyright Notice

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Table of Contents

1.  Introduction
2.  Terminology
3.  Acknowledgements
4.  Addresses
    4.1.  Fundamentals
    4.2.  Domainpart
    4.3.  Localpart
    4.4.  Resourcepart
5.  Internationalization Considerations
6.  Security Considerations
    6.1.  Reuse of Stringprep
    6.2.  Reuse of Unicode
    6.3.  Address Spoofing
        6.3.1.  Address Forging
        6.3.2.  Address Mimicking
7.  IANA Considerations
    7.1.  Nodeprep Profile of Stringprep
    7.2.  Resourceprep Profile of Stringprep
8.  Conformance Requirements
9.  References
    9.1.  Normative References
    9.2.  Informative References
Appendix A.  Nodeprep
    A.1.  Introduction
    A.2.  Character Repertoire
    A.3.  Mapping
    A.4.  Normalization
    A.5.  Prohibited Output
    A.6.  Bidirectional Characters
    A.7.  Notes
Appendix B.  Resourceprep
    B.1.  Introduction
    B.2.  Character Repertoire
    B.3.  Mapping
    B.4.  Normalization
    B.5.  Prohibited Output
    B.6.  Bidirectional Characters
Appendix C.  Differences From RFC 3920
§  Author's Address

1.  Introduction

The Extensible Messaging and Presence Protocol (XMPP) is an application profile of the Extensible Markup Language [XML] (Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fourth Edition),” August 2006.) for streaming XML data in close to real time between any two or more network-aware entities. The address format for XMPP entities was originally developed in the Jabber open-source community in 1999, first described by [XEP‑0029] (Kaes, C., “Definition of Jabber Identifiers (JIDs),” October 2003.) in 2002, and defined canonically by [RFC3920] (Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.) in 2004.

As specified in RFC 3920, the XMPP address format re-uses the "stringprep" technology for preparation of non-ASCII characters [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.), including the Nameprep profile for internationalized domain names as specified in [NAMEPREP] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.) and [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) along with two XMPP-specific profiles for the localpart and resourcepart.

Since the publication of RFC 3920, IDNA2003 has been superseded by IDNA2008 (see [IDNA‑PROTO] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” August 2010.) and related documents), which is not based on stringprep. Following the lead of the IDNA community, other technology communities that use stringprep have begun discussions about migrating away from stringprep toward more "modern" approaches. The XMPP community is participating in those discussions in order to find a replacement for the Nodeprep and Resourceprep profiles of stringprep defined in RFC 3920. However, work on updated handling of internationalized addresses is currently in progress within the PRECIS Working Group and at the time of this writing it seems that such work might take several years to complete. Because all other aspects of revised documentation for XMPP have been incorporated into [XMPP] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2010.), the XMPP Working Group decided to split the XMPP address format into a separate specification so as not to significantly delay publication of improved documentation for XMPP while awaiting the conclusion of work on updated handling of internationalized addresses.

Therefore, this specification provides corrected documentation of the XMPP address format using the internationalization technologies available in 2004 (when RFC 3920 was published), with the intent that this specification will be superseded as soon as work on a new approach to preparation and comparison of internationalized strings has been defined by the PRECIS Working Group and applied to the specific cases of XMPP localparts and resourceparts. In the meantime, this document normatively references [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) and [NAMEPREP] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.); XMPP software implementations are encouraged to begin migrating to IDNA2008 (see [IDNA‑PROTO] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” August 2010.) and related documents) because it is nearly certain that the specification superseding this one will re-use IDNA2008.

2.  Terminology

Many important terms used in this document are defined in [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.), [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.), [UNICODE] (The Unicode Consortium, “The Unicode Standard, Version 3.2.0,” 2000.), and [XMPP] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2010.).

The keywords "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 [KEYWORDS] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Acknowledgements

Some text in this document was borrowed or adapted from [IDNA‑DEFS] (Klensin, J., “Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework,” August 2010.), [IDNA‑PROTO] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” August 2010.), [IDNA‑RATIONALE] (Klensin, J., “Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale,” August 2010.), and [XEP‑0165] (Saint-Andre, P., “Best Practices to Discourage JID Mimicking,” December 2007.).

4.  Addresses

4.1.  Fundamentals

An XMPP entity is anything that is network-addressable and that can communicate using XMPP. For historical reasons, the native address of an XMPP entity is called a Jabber Identifier or JID. A valid JID is a string of [UNICODE] (The Unicode Consortium, “The Unicode Standard, Version 3.2.0,” 2000.) code points, encoded using [UTF‑8] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.), and structured as an ordered sequence of localpart, domainpart, and resourcepart (where the first two parts are demarcated by the '@' character used as a separator, and the last two parts are similarly demarcated by the '/' character).

The syntax for a JID is defined as follows using the Augmented Backus-Naur Form as specified in [ABNF] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.).

   jid           = [ localpart "@" ] domainpart [ "/" resourcepart ]
   localpart     = 1*(nodepoint)
                   ; a "nodepoint" is a UTF-8 encoded Unicode code
                   ; point that satisfies the Nodeprep profile of
                   ; stringprep
   domainpart    = IP-literal / IPv4address / ifqdn
                   ; the "IPv4address" and "IP-literal" rules are
                   ; defined in RFC 3986, and the first-match-wins
                   ; (a.k.a. "greedy") algorithm described in RFC
                   ; 3986 applies to the matching process
   ifqdn         = 1*(namepoint)
                   ; a "namepoint" is a UTF-8 encoded Unicode
                   ; code point that satisfies the Nameprep
                   ; profile of stringprep
   resourcepart  = 1*(resourcepoint)
                   ; a "resourcepoint" is a UTF-8 encoded Unicode
                   ; code point that satisfies the Resourceprep
                   ; profile of stringprep

All JIDs are based on the foregoing structure. One common use of this structure is to identify a messaging and presence account, the server that hosts the account, and a connected resource (e.g., a specific device) in the form of <localpart@domainpart/resourcepart>. However, localparts other than clients are possible; for example, a specific chat room offered by a multi-user chat service (see [XEP‑0045] (Saint-Andre, P., “Multi-User Chat,” July 2008.)) is addressed as <room@service> (where "room" is the name of the chat room and "service" is the hostname of the multi-user chat service) and a specific occupant of such a room could be addressed as <room@service/nick> (where "nick" is the occupant's room nickname). Many other JID types are possible (e.g., <domainpart/resourcepart> could be a server-side script or service).

Each allowable portion of a JID (localpart, domainpart, and resourcepart) MUST NOT be zero bytes in length and MUST NOT be more than 1023 bytes in length, resulting in a maximum total size (including the '@' and '/' separators) of 3071 bytes.

An entity's address on an XMPP network MUST be represented as a JID (without a URI scheme) and not a [URI] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) or [IRI] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) as specified in [XMPP‑URI] (Saint-Andre, P., “Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP),” February 2008.); the latter specification is provided only for identification and interaction outside the context of XMPP itself.

Implementation Note: When dividing a JID into its component parts, an implementation needs to match the separator characters '@' and '/' before applying any transformation algorithms, which might decompose certain Unicode code points to the separator characters (e.g., U+FE6B SMALL COMMERCIAL AT might decompose into U+0040 COMMERCIAL AT).

4.2.  Domainpart

The DOMAINPART of a JID is that portion after the '@' character (if any) and before the '/' character (if any); it is the primary identifier and is the only REQUIRED element of a JID (a mere domainpart is a valid JID). Typically a domainpart identifies the "home" server to which clients connect for XML routing and data management functionality. However, it is not necessary for an XMPP domainpart to identify an entity that provides core XMPP server functionality (e.g., a domainpart can identify an entity such as a multi-user chat service, a publish-subscribe service, or a user directory).

The domainpart for every server or service that will communicate over a network SHOULD be a fully qualified domain name or "FQDN" (see [DNS] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.)); although the domainpart is allowed to be either an Internet Protocol (IPv4 or IPv6) address or a text label that is resolvable on a local network (commonly called an "unqualified hostname"), it is possible that domainparts that are IP addresses will not be acceptable to other services for the sake of interdomain communication. Furthermore, domainparts that are unqualified hostnames MUST NOT be used on public networks but MAY be used on private networks.

Implementation Note: If the domainpart includes a final character considered to be a label separator (dot) by [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) or [DNS] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.), this character MUST be stripped from the domainpart before the JID of which it is a part is used for the purpose of routing an XML stanza, comparing against another JID, or constructing an [XMPP‑URI] (Saint-Andre, P., “Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP),” February 2008.); in particular, the character MUST be stripped before any other canonicalization steps are taken, such as application of the [NAMEPREP] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.) profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) or completion of the ToASCII operation as described in [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.).

A domainpart MUST NOT be zero bytes in length and MUST NOT be more than 1023 bytes in length.

A domainpart consisting of a fully qualified domain name MUST be an "internationalized domain name" as defined in [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.), that is, it MUST be "a domain name in which every label is an internationalized label" and MUST follow the rules for construction of internationalized domain names specified in [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.). When preparing a text label (consisting of a sequence of UTF-8 encoded Unicode code points) for representation as an internationalized label in the process of constructing an XMPP domainpart or comparing two XMPP domainparts, an application MUST ensure that for each text label it is possible to apply without failing the ToASCII operation specified in [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) with the UseSTD3ASCIIRules flag set (thus forbidding ASCII code points other than letters, digits, and hyphens). If the ToASCII operation can be applied without failing, then the label is an internationalized label. (Note: The ToASCII operation includes application of the [NAMEPREP] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.) profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) and encoding using the algorithm specified in [PUNYCODE] (Costello, A., “Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA),” March 2003.); for details, see [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.).) Although XMPP applications do not communicate the output of the ToASCII operation (called an "ACE label") over the wire, it MUST be possible to apply that operation without failing to each internationalized label. If an XMPP application receives as input an ACE label, it SHOULD convert that ACE label to an internationalized label using the ToUnicode operation (see [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.)) before including the label in an XMPP domainpart that will be communicated over the wire on an XMPP network (however, instead of converting the label, there are legitimate reasons why an application might instead refuse the input altogether and return an error to the entity that provided the offending data).

In the terms of IDNA2008 [IDNA‑DEFS] (Klensin, J., “Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework,” August 2010.), the domainpart of a JID is a "domain name slot".

4.3.  Localpart

The LOCALPART of a JID is an optional identifier placed before the domainpart and separated from the latter by the '@' character. Typically a localpart uniquely identifies the entity requesting and using network access provided by a server (i.e., a local account), although it can also represent other kinds of entities (e.g., a chat room associated with a multi-user chat service). The entity represented by an XMPP localpart is addressed within the context of a specific domain.

A localpart MUST NOT be zero bytes in length and MUST NOT be more than 1023 bytes in length.

A localpart MUST be formatted such that the Nodeprep profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) can be applied without failing (see Appendix A (Nodeprep)). Before comparing two localparts, an application MUST first ensure that the Nodeprep profile has been applied to each identifier (the profile need not be applied each time a comparison is made, as long as it has been applied before comparison).

4.4.  Resourcepart

The resourcepart of a JID is an optional identifier placed after the domainpart and separated from the latter by the '/' character. A resourcepart can modify either a <localpart@domainpart> address or a mere <domainpart> address. Typically a resourcepart uniquely identifies a specific connection (e.g., a device or location) or object (e.g., an occupant in a multi-user chat room) belonging to the entity associated with an XMPP localpart at a local domain.

When an XMPP address does not include a resourcepart (i.e., when it is of the form <domainpart> or <localpart@domainpart>), it is referred to as a BARE JID. When an XMPP address includes a resourcepart (i.e., when it is of the form <domainpart/resourcepart> or <localpart@domainpart/resourcepart>), is referred to as a FULL JID.

A resourcepart MUST NOT be zero bytes in length and MUST NOT be more than 1023 bytes in length.

A resourcepart MUST be formatted such that the Resourceprep profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) can be applied without failing (see Appendix B (Resourceprep)). Before comparing two resourceparts, an application MUST first ensure that the Resourceprep profile has been applied to each identifier (the profile need not be applied each time a comparison is made, as long as it has been applied before comparison).

Informational Note: For historical reasons, the term "resource identifier" is often used in XMPP to refer to the optional portion of an XMPP address that follows the domainpart and the "/" separator character; to help prevent confusion between an XMPP "resource identifier" and the meanings of "resource" and "identifier" provided in Section 1.1 of [URI] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), this specification uses the term "resourcepart" instead of "resource identifier" (as in RFC 3920).

XMPP entities SHOULD consider resourceparts to be opaque strings and SHOULD NOT impute meaning to any given resourcepart. In particular:

• Use of the '/' character as a separator between the domainpart and the resourcepart does not imply that XMPP addresses are hierarchical in the way that, say, HTTP addresses are hierarchical; thus for example an XMPP address of the form <localpart@domainpart/foo/bar> does not identify a resource "bar" that exists below a resource "foo" in a hierarchy of resources associated with the entity "localpart@domain".
  
  
• The '@' character is allowed in the resourcepart, and is often used in the "nick" shown in XMPP chatrooms. For example, the JID <room@chat.example.com/user@host> describes an entity who is an occupant of the room <room@chat.example.com> with an (asserted) nick of <user@host>. However, chatroom services do not necessarily check such an asserted nick against the occupant's real JID.

5.  Internationalization Considerations

XMPP servers MUST, and XMPP clients SHOULD, support [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) for domainparts (including the [NAMEPREP] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.) profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.)), the Nodeprep (Nodeprep) profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) for localparts, and the Resourceprep (Resourceprep) profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) for resourceparts; this enables XMPP addresses to include a wide variety of characters outside the US-ASCII range. Rules for enforcement of the XMPP address format are provided in [XMPP] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2010.).

6.  Security Considerations

6.1.  Reuse of Stringprep

The security considerations described in [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) apply to the Nodeprep (Nodeprep) and Resourceprep (Resourceprep) profiles defined in this document for XMPP localparts and resourceparts. The security considerations described in [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) and [NAMEPREP] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.) apply to the Nameprep profile that is re-used here for XMPP domainparts.

6.2.  Reuse of Unicode

The security considerations described in [UNICODE‑SEC] (The Unicode Consortium, “Unicode Technical Report #36: Unicode Security Considerations,” 2008.) apply to the use of Unicode characters in XMPP addresses.

6.3.  Address Spoofing

There are two forms of address spoofing: forging and mimicking.

6.3.1.  Address Forging

In the context of XMPP technologies, address forging occurs when an entity is able to generate an XML stanza whose 'from' address does not correspond to the account credentials with which the entity authenticated onto the network (or an authorization identity provided during SASL negotiation). For example, address forging occurs if an entity that authenticated as "juliet@im.example.com" is able to send XML stanzas from "nurse@im.example.com" or "romeo@example.net".

Address forging is difficult in XMPP systems, given the requirement for sending servers to stamp 'from' addresses and for receiving servers to verify sending domains via server-to-server authentication (see [XMPP] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2010.)). However, address forging is not impossible, since a rogue server could forge JIDs at the sending domain by ignoring the stamping requirement. Therefore, an entity outside the security perimeter of a particular server cannot reliably distinguish between bare JIDs of the form <localpart@domainpart> at that server and thus can authenticate only the domainpart of such JIDs with any level of assurance. This specification does not define methods for discovering or counteracting such rogue servers.

Furthermore, it is possible for an attacker to forge JIDs at other domains by means of a DNS poisoning attack if DNS security extensions [DNSSEC] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.) are not used.

6.3.2.  Address Mimicking

Address mimicking occurs when an entity provides legitimate authentication credentials for and sends XML stanzas from an account whose JID appears to a human user to be the same as another JID. For example, in some XMPP clients the address "ju1iet@example.org" (spelled with the number one as the third character of the localpart) might appear to be the same as "juliet@example.org (spelled with the lower-case version of the letter "L"), especially on casual visual inspection; this phenomenon is sometimes called "typejacking". A more sophisticated example of address mimicking might involve the use of characters from outside the familiar Latin extended-A block of Unicode code points, such as the characters U+13DA U+13A2 U+13B5 U+13AC U+13A2 U+13AC U+13D2 from the Cherokee block instead of the similar-looking US-ASCII characters "STPETER".

In some examples of address mimicking, it is unlikely that the average user could tell the difference between the real JID and the fake JID. (Indeed, there is no programmatic way to distinguish with full certainty which is the fake JID and which is the real JID; in some communication contexts, the JID formed of Cherokee characters might be the real JID and the JID formed of US-ASCII characters might thus appear to be the fake JID.) Because JIDs can contain almost any properly-encoded Unicode code point, it can be relatively easy to mimic some JIDs in XMPP systems. The possibility of address mimicking introduces security vulnerabilities of the kind that have also plagued the World Wide Web, specifically the phenomenon known as phishing.

These problems arise because Unicode and ISO/IEC 10646 repertoires have many characters that look similar (so-called "confusable characters" or "confusables"). In many cases, XMPP users might perform visual matching, such as when comparing the JIDs of communication partners. Because it is impossible to map similar-looking characters without a great deal of context (such as knowing the fonts used), stringprep and stringprep-based technologies such as Nameprep, Nodeprep, and Resourceprep do nothing to map similar-looking characters together, nor do they prohibit some characters because they look like others. As a result, XMPP localparts and resourceparts could contain confusable characters, producing JIDs that appear to mimic other JIDs and thus leading to security vulnerabilities such as the following:

• A localpart can be employed as one part of an entity's address in XMPP. One common usage is as the username of an instant messaging user; another is as the name of a multi-user chat room; and many other kinds of entities could use localparts as part of their addresses. The security of such services could be compromised based on different interpretations of the internationalized localpart; for example, a user entering a single internationalized localpart could access another user's account information, or a user could gain access to a hidden or otherwise restricted chat room or service.
  
  
• A resourcepart can be employed as one part of an entity's address in XMPP. One common usage is as the name for an instant messaging user's connected resource; another is as the nickname of a user in a multi-user chat room; and many other kinds of entities could use resourceparts as part of their addresses. The security of such services could be compromised based on different interpretations of the internationalized resourcepart; for example, a user could attempt to bind multiple resources with the same name, or a user could send a message to someone other than the intended recipient in a multi-user chat room.

Despite the fact that some specific suggestions about identification and handling of confusable characters appear in the Unicode Security Considerations [UNICODE‑SEC] (The Unicode Consortium, “Unicode Technical Report #36: Unicode Security Considerations,” 2008.), it is also true (as noted in [IDNA‑DEFS] (Klensin, J., “Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework,” August 2010.)) that "there are no comprehensive technical solutions to the problems of confusable characters". Mimicked JIDs that involve characters from only one script, or from the script typically employed by a particular user or community of language users, are not easy to combat (e.g., the simple typejacking attack previously described, which relies on a surface similarity between the characters "1" and "l" in some presentations). However, mimicked addresses that involve characters from more than one script, or from a script not typically employed by a particular user or community of language users, can be mitigated somewhat through the application of appropriate registration policies at XMPP services and presentation policies in XMPP client software. Therefore the following policies are encouraged:

1. Because an XMPP service that allows registration of XMPP user accounts (localparts) plays a role similar to that of a registry for DNS domain names, such a service SHOULD establish a policy about the scripts or blocks of characters it will allow in localparts at the service. Such a policy is likely to be informed by the languages and scripts that are used to write registered account names; in particular, to reduce confusion, the service MAY forbid registration of XMPP localparts that contain characters from more than one script and to restrict registrations to characters drawn from a very small number of scripts (e.g., scripts that are well-understood by the administrators of the service). For related considerations in the context of domain name registration, refer to Section 4.3 of [IDNA‑PROTO] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” August 2010.) and Section 3.2 of [IDNA‑RATIONALE] (Klensin, J., “Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale,” August 2010.). Note well that methods for enforcing such restrictions are out of scope for this document.
   
   
2. Because every human user of an XMPP client presumably has a preferred language (or, in some cases, a small set of preferred languages), an XMPP client SHOULD gather that information either explicitly from the user or implicitly via the operating system of the user's device. Furthermore, because most languages are typically represented by a single script (or a small set of scripts) and most scripts are typically contained in one or more blocks of characters, an XMPP client SHOULD warn the user when presenting a JID that mixes characters from more than one script or block, or that uses characters outside the normal range of the user's preferred language(s). This recommendation is not intended to discourage communication across different communities of language users; instead, it recognizes the existence of such communities and encourages due caution when presenting unfamiliar scripts or characters to human users.

7.  IANA Considerations

The following sections update the registrations provided in [RFC3920] (Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.).

7.1.  Nodeprep Profile of Stringprep

The Nodeprep profile of stringprep is defined under Nodeprep (Nodeprep). The IANA has registered Nodeprep in the stringprep profile registry.

Name of this profile:

Nodeprep

RFC in which the profile is defined:

XXXX

Indicator whether or not this is the newest version of the profile:

This is the first version of Nodeprep

7.2.  Resourceprep Profile of Stringprep

The Resourceprep profile of stringprep is defined under Resourceprep (Resourceprep). The IANA has registered Resourceprep in the stringprep profile registry.

Name of this profile:

Resourceprep

RFC in which the profile is defined:

XXXX

Indicator whether or not this is the newest version of the profile:

This is the first version of Resourceprep

8.  Conformance Requirements

This section describes a protocol feature set that summarizes the conformance requirements of this specification. This feature set is appropriate for use in software certification, interoperability testing, and implementation reports. For each feature, this section provides the following information:

• A human-readable name
  
  
• An informational description
  
  
• A reference to the particular section of this document that normatively defines the feature
  
  
• Whether the feature applies to the Client role, the Server role, or both (where "N/A" signifies that the feature is not applicable to the specified role)
  
  
• Whether the feature MUST or SHOULD be implemented, where the capitalized terms are to be understood as described in [KEYWORDS] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.)

The feature set specified here attempts to adhere to the concepts and formats proposed by Larry Masinter within the IETF's NEWTRK Working Group in 2005, as captured in [INTEROP] (Masinter, L., “Formalizing IETF Interoperability Reporting,” October 2005.). Although this feature set is more detailed than called for by [REPORTS] (Dusseault, L. and R. Sparks, “Guidance on Interoperation and Implementation Reports for Advancement to Draft Standard,” September 2009.), it provides a suitable basis for the generation of implementation reports to be submitted in support of advancing this specification from Proposed Standard to Draft Standard in accordance with [PROCESS] (Bradner, S., “The Internet Standards Process -- Revision 3,” October 1996.).

Feature:

address-domain-length

Description:

Ensure that the domainpart of an XMPP address is at least one byte in length and at most 1023 bytes in length.

Section:

Section 4.2 (Domainpart)

Roles:

Both MUST.

Feature:

address-domain-prep

Description:

Ensure that the domainpart of an XMPP address conforms to the Nameprep profile of Stringprep.

Section:

Section 4.2 (Domainpart)

Roles:

Client SHOULD, Server MUST.

Feature:

address-localpart-length

Description:

Ensure that the localpart of an XMPP address is at least one byte in length and at most 1023 bytes in length.

Section:

Section 4.3 (Localpart)

Roles:

Both MUST.

Feature:

address-localpart-prep

Description:

Ensure that the localpart of an XMPP address conforms to the Nodeprep profile of Stringprep.

Section:

Section 4.3 (Localpart)

Roles:

Client SHOULD, Server MUST.

Feature:

address-resource-length

Description:

Ensure that the resourcepart of an XMPP address is at least one byte in length and at most 1023 bytes in length.

Section:

Section 4.4 (Resourcepart)

Roles:

Both MUST.

Feature:

address-resource-prep

Description:

Ensure that the resourcepart of an XMPP address conforms to the Resourceprep profile of Stringprep.

Section:

Section 4.2 (Domainpart)

Roles:

Client SHOULD, Server MUST.

9.  References

9.1. Normative References

9.2. Informative References

Appendix A.  Nodeprep

A.1.  Introduction

This appendix defines the "Nodeprep" profile of stringprep. As such, it specifies processing rules that will enable users to enter internationalized localparts in the Extensible Messaging and Presence Protocol (XMPP) and have the highest chance of getting the content of the strings correct. (An XMPP localpart is the optional portion of an XMPP address that precedes an XMPP domainpart and the '@' separator; it is often but not exclusively associated with an instant messaging username.) These processing rules are intended only for XMPP localparts and are not intended for arbitrary text or any other aspect of an XMPP address.

This profile defines the following, as required by [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.):

• The intended applicability of the profile: internationalized localparts within XMPP
• The character repertoire that is the input and output to stringprep: Unicode 3.2, specified in Section 2 of this Appendix
• The mappings used: specified in Section 3
• The Unicode normalization used: specified in Section 4
• The characters that are prohibited as output: specified in Section 5
• Bidirectional character handling: specified in Section 6

A.2.  Character Repertoire

This profile uses Unicode 3.2 with the list of unassigned code points being Table A.1, both defined in Appendix A of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).

A.3.  Mapping

This profile specifies mapping using the following tables from [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.):

Table B.1

Table B.2

A.4.  Normalization

This profile specifies the use of Unicode normalization form KC, as described in [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).

A.5.  Prohibited Output

This profile specifies the prohibition of using the following tables from [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).

Table C.1.1

Table C.1.2

Table C.2.1

Table C.2.2

Table C.3

Table C.4

Table C.5

Table C.6

Table C.7

Table C.8

Table C.9

In addition, the following additional Unicode characters are also prohibited:

U+0022 (QUOTATION MARK), i.e., "

U+0026 (AMPERSAND), i.e., &

U+0027 (APOSTROPHE), i.e., '

U+002F (SOLIDUS), i.e., /

U+003A (COLON), i.e., :

U+003C (LESS-THAN SIGN), i.e., <

U+003E (GREATER-THAN SIGN), i.e., >

U+0040 (COMMERCIAL AT), i.e., @

A.6.  Bidirectional Characters

This profile specifies checking bidirectional strings, as described in Section 6 of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).

A.7.  Notes

Because the additional characters prohibited by Nodeprep are prohibited after normalization, an implementation MUST NOT enable a human user to input any Unicode code point whose decomposition includes those characters; such code points include but are not necessarily limited to the following (refer to [UNICODE] (The Unicode Consortium, “The Unicode Standard, Version 3.2.0,” 2000.) for complete information).

• U+2100 (ACCOUNT OF)
• U+2101 (ADDRESSED TO THE SUBJECT)
• U+2105 (CARE OF)
• U+2106 (CADA UNA)
• U+226E (NOT LESS-THAN)
• U+226F (NOT GREATER-THAN)
• U+2A74 (DOUBLE COLON EQUAL)
• U+FE13 (SMALL COLON)
• U+FE60 (SMALL AMPERSAND)
• U+FE64 (SMALL LESS-THAN SIGN)
• U+FE65 (SMALL GREATER-THAN SIGN)
• U+FE6B (SMALL COMMERCIAL AT)
• U+FF02 (FULLWIDTH QUOTATION MARK)
• U+FF06 (FULLWIDTH AMPERSAND)
• U+FF07 (FULLWIDTH APOSTROPHE)
• U+FF0F (FULLWIDTH SOLIDUS)
• U+FF1A (FULLWIDTH COLON)
• U+FF1C (FULLWIDTH LESS-THAN SIGN)
• U+FF1E (FULLWIDTH GREATER-THAN SIGN)
• U+FF20 (FULLWIDTH COMMERCIAL AT)

Appendix B.  Resourceprep

B.1.  Introduction

This appendix defines the "Resourceprep" profile of stringprep. As such, it specifies processing rules that will enable users to enter internationalized resourceparts in the Extensible Messaging and Presence Protocol (XMPP) and have the highest chance of getting the content of the strings correct. (An XMPP resourcepart is the optional portion of an XMPP address that follows an XMPP domainpart and the '/' separator.) These processing rules are intended only for XMPP resourceparts and are not intended for arbitrary text or any other aspect of an XMPP address.

This profile defines the following, as required by [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.):

• The intended applicability of the profile: internationalized resourceparts within XMPP
• The character repertoire that is the input and output to stringprep: Unicode 3.2, specified in Section 2 of this Appendix
• The mappings used: specified in Section 3
• The Unicode normalization used: specified in Section 4
• The characters that are prohibited as output: specified in Section 5
• Bidirectional character handling: specified in Section 6

B.2.  Character Repertoire

This profile uses Unicode 3.2 with the list of unassigned code points being Table A.1, both defined in Appendix A of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).

B.3.  Mapping

This profile specifies mapping using the following tables from [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.):

Table B.1

B.4.  Normalization

This profile specifies the use of Unicode normalization form KC, as described in [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).

B.5.  Prohibited Output

This profile specifies the prohibition of using the following tables from [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).

Table C.1.2

Table C.2.1

Table C.2.2

Table C.3

Table C.4

Table C.5

Table C.6

Table C.7

Table C.8

Table C.9

B.6.  Bidirectional Characters

This profile specifies checking bidirectional strings, as described in Section 6 of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).

Appendix C.  Differences From RFC 3920

Based on consensus derived from implementation and deployment experience as well as formal interoperability testing, the following substantive modifications were made from RFC 3920.

• Corrected the ABNF syntax to (1) ensure consistency with [URI] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) and [IRI] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.), and (2) prevent zero-length localparts, domainparts, and resourceparts.
• To avoid confusion with the term "node" as used in [XEP‑0030] (Hildebrand, J., Millard, P., Eatmon, R., and P. Saint-Andre, “Service Discovery,” June 2008.) and [XEP‑0060] (Millard, P., Saint-Andre, P., and R. Meijer, “Publish-Subscribe,” September 2008.), changed the term "node identifier" to "localpart" (but retained the name "Nodeprep" for backward compatibility).
• To avoid confusion with the terms "resource" and "identifier" as used in [URI] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), changed the term "resource identifier" to "resourcepart".
• Corrected the nameprep processing rules to require use of the UseSTD3ASCIIRules flag.

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