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Internet Engineering Task Force (IETF) P. Saint-Andre Request for Comments: 6122 Cisco Updates: 3920 March 2011 Category: Standards Track ISSN: 2070-1721

 Extensible Messaging and Presence Protocol (XMPP): Address Format

Abstract

 This document defines the format for addresses used in the Extensible
 Messaging and Presence Protocol (XMPP), including support for
 non-ASCII characters.  This document updates RFC 3920.

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/rfc6122.

Copyright Notice

 Copyright (c) 2011 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 6122 XMPP Address Format March 2011

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
 2.  Addresses  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.1.  Fundamentals . . . . . . . . . . . . . . . . . . . . . . .  4
   2.2.  Domainpart . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.3.  Localpart  . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.4.  Resourcepart . . . . . . . . . . . . . . . . . . . . . . .  8
 3.  Internationalization Considerations  . . . . . . . . . . . . .  9
 4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   4.1.  Reuse of Stringprep  . . . . . . . . . . . . . . . . . . .  9
   4.2.  Reuse of Unicode . . . . . . . . . . . . . . . . . . . . .  9
   4.3.  Address Spoofing . . . . . . . . . . . . . . . . . . . . .  9
     4.3.1.  Address Forging  . . . . . . . . . . . . . . . . . . . 10
     4.3.2.  Address Mimicking  . . . . . . . . . . . . . . . . . . 10
 5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   5.1.  Nodeprep Profile of Stringprep . . . . . . . . . . . . . . 13
   5.2.  Resourceprep Profile of Stringprep . . . . . . . . . . . . 14
 6.  Conformance Requirements . . . . . . . . . . . . . . . . . . . 14
 7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   7.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
   7.2.  Informative References . . . . . . . . . . . . . . . . . . 17
 Appendix A.  Nodeprep  . . . . . . . . . . . . . . . . . . . . . . 19
   A.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 19
   A.2.  Character Repertoire . . . . . . . . . . . . . . . . . . . 19
   A.3.  Mapping  . . . . . . . . . . . . . . . . . . . . . . . . . 19
   A.4.  Normalization  . . . . . . . . . . . . . . . . . . . . . . 19
   A.5.  Prohibited Output  . . . . . . . . . . . . . . . . . . . . 20
   A.6.  Bidirectional Characters . . . . . . . . . . . . . . . . . 20
   A.7.  Notes  . . . . . . . . . . . . . . . . . . . . . . . . . . 20
 Appendix B.  Resourceprep  . . . . . . . . . . . . . . . . . . . . 21
   B.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 21
   B.2.  Character Repertoire . . . . . . . . . . . . . . . . . . . 22
   B.3.  Mapping  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   B.4.  Normalization  . . . . . . . . . . . . . . . . . . . . . . 22
   B.5.  Prohibited Output  . . . . . . . . . . . . . . . . . . . . 22
   B.6.  Bidirectional Characters . . . . . . . . . . . . . . . . . 22
 Appendix C.  Differences from RFC 3920 . . . . . . . . . . . . . . 22
 Appendix D.  Acknowledgements  . . . . . . . . . . . . . . . . . . 23

Saint-Andre Standards Track [Page 2] RFC 6122 XMPP Address Format March 2011

1. Introduction

1.1. Overview

 The Extensible Messaging and Presence Protocol (XMPP) is an
 application profile of the Extensible Markup Language [XML] 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] in 2002, and defined canonically by
 [RFC3920] in 2004.
 As specified in RFC 3920, the XMPP address format reuses the
 "stringprep" technology for preparation of non-ASCII characters
 [STRINGPREP], including the Nameprep profile for internationalized
 domain names as specified in [NAMEPREP] and [IDNA2003] 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] 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 (mostly
 within the PRECIS Working Group) in order to find a replacement for
 the Nodeprep and Resourceprep profiles of stringprep defined in RFC
 3920.  Because all other aspects of revised documentation for XMPP
 have been incorporated into [XMPP], the XMPP Working Group decided to
 temporarily split the XMPP address format into a separate document so
 as not to significantly delay publication of improved documentation
 for XMPP.  It is expected that this document will be obsoleted as
 soon as work on a new approach to preparation and comparison of
 internationalized addresses has been completed.
 Therefore, this specification provides corrected documentation of the
 XMPP address format using the internationalization technologies
 available in 2004 (when RFC 3920 was published).  Although this
 document normatively references [IDNA2003] and [NAMEPREP], XMPP
 software implementations are encouraged to begin migrating to
 IDNA2008 (see [IDNA-PROTO] and related documents) because the
 specification that obsoletes this one will use IDNA2008 rather than
 IDNA2003.
 This document updates RFC 3920.

Saint-Andre Standards Track [Page 3] RFC 6122 XMPP Address Format March 2011

1.2. Terminology

 Many important terms used in this document are defined in [IDNA2003],
 [STRINGPREP], [UNICODE], and [XMPP].
 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 [KEYWORDS].

2. Addresses

2.1. 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] code points, encoded using [UTF-8], 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].

Saint-Andre Standards Track [Page 4] RFC 6122 XMPP Address Format March 2011

    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
                    ;
                    ; note well that reuse of the IP-literal rule
                    ; from RFC 3986 implies that IPv6 addresses are
                    ; enclosed in square brackets (i.e., beginning
                    ; with '[' and ending with ']'), which was not
                    ; the case in RFC 3920
                    ;
    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.
 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.
 For the purpose of communication over an XMPP network (e.g., in the
 'to' or 'from' address of an XMPP stanza), an entity's address MUST
 be represented as a JID, not as a Uniform Resource Identifier [URI]
 or Internationalized Resource Identifier [IRI].  An XMPP IRI
 [XMPP-URI] is in essence a JID prepended with 'xmpp:'; however, the
 native addressing format used in XMPP is that of a mere JID without a
 URI scheme.  [XMPP-URI] is provided only for identification and
 interaction outside the context of XMPP itself, for example when

Saint-Andre Standards Track [Page 5] RFC 6122 XMPP Address Format March 2011

 linking to a JID from a web page.  See [XMPP-URI] for a description
 of the process for securely extracting a JID from an XMPP URI or IRI.
    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).

2.2. 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 XMPP service MUST be a fully qualified
 domain name (FQDN; see [DNS]), IPv4 address, IPv6 address, or
 unqualified hostname (i.e., a text label that is resolvable on a
 local network).
    Interoperability Note: Domainparts that are IP addresses might not
    be accepted by other services for the sake of server-to-server
    communication, and domainparts that are unqualified hostnames
    cannot be used on public networks because they are resolvable only
    on a local network.
 If the domainpart includes a final character considered to be a label
 separator (dot) by [IDNA2003] or [DNS], 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].  In particular, the
 character MUST be stripped before any other canonicalization steps
 are taken, such as application of the [NAMEPREP] profile of
 [STRINGPREP] or completion of the ToASCII operation as described in
 [IDNA2003].
 A domainpart consisting of a fully qualified domain name MUST be an
 "internationalized domain name" as defined in [IDNA2003]; that is, it
 MUST be "a domain name in which every label is an internationalized
 label" and MUST follow the rules for construction of

Saint-Andre Standards Track [Page 6] RFC 6122 XMPP Address Format March 2011

 internationalized domain names specified in [IDNA2003].  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] 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] profile of
 [STRINGPREP] and encoding using the algorithm specified in
 [PUNYCODE]; for details, see [IDNA2003].)  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]) 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).
 A domainpart MUST NOT be zero bytes in length and MUST NOT be more
 than 1023 bytes in length.  This rule is to be enforced after any
 mapping or normalization resulting from application of the Nameprep
 profile of stringprep (e.g., in Nameprep some characters can be
 mapped to nothing, which might result in a string of zero length).
 Naturally, the length limits of [DNS] apply, and nothing in this
 document is to be interpreted as overriding those more fundamental
 limits.
 In the terms of IDNA2008 [IDNA-DEFS], the domainpart of a JID is a
 "domain name slot".

2.3. 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 (i.e., <localpart@domainpart>).

Saint-Andre Standards Track [Page 7] RFC 6122 XMPP Address Format March 2011

 A localpart MUST be formatted such that the Nodeprep profile of
 [STRINGPREP] can be applied without failing (see Appendix A).  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).
 A localpart MUST NOT be zero bytes in length and MUST NOT be more
 than 1023 bytes in length.  This rule is to be enforced after any
 mapping or normalization resulting from application of the Nodeprep
 profile of stringprep (e.g., in Nodeprep some characters can be
 mapped to nothing, which might result in a string of zero length).

2.4. 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 domain (i.e.,
 <localpart@domainpart/resourcepart>).
 A resourcepart MUST be formatted such that the Resourceprep profile
 of [STRINGPREP] can be applied without failing (see Appendix B).
 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).
 A resourcepart MUST NOT be zero bytes in length and MUST NOT be more
 than 1023 bytes in length.  This rule is to be enforced after any
 mapping or normalization resulting from application of the
 Resourceprep profile of stringprep (e.g., in Resourceprep some
 characters can be mapped to nothing, which might result in a string
 of zero length).
    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], this specification
    uses the term "resourcepart" instead of "resource identifier" (as
    in RFC 3920).

Saint-Andre Standards Track [Page 8] RFC 6122 XMPP Address Format March 2011

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

3. Internationalization Considerations

 XMPP servers MUST, and XMPP clients SHOULD, support [IDNA2003] for
 domainparts (including the [NAMEPREP] profile of [STRINGPREP]), the
 Nodeprep (Appendix A) profile of [STRINGPREP] for localparts, and the
 Resourceprep (Appendix B) profile of [STRINGPREP] 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].

4. Security Considerations

4.1. Reuse of Stringprep

 The security considerations described in [STRINGPREP] apply to the
 Nodeprep (Appendix A) and Resourceprep (Appendix B) profiles defined
 in this document for XMPP localparts and resourceparts.  The security
 considerations described in [STRINGPREP] and [NAMEPREP] apply to the
 Nameprep profile that is reused here for XMPP domainparts.

4.2. Reuse of Unicode

 The security considerations described in [UNICODE-SEC] apply to the
 use of Unicode characters in XMPP addresses.

4.3. Address Spoofing

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

Saint-Andre Standards Track [Page 9] RFC 6122 XMPP Address Format March 2011

4.3.1. 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 negotiation of SASL authentication [SASL] as described in
 [XMPP]).  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]).  However, address forging is possible if:
 o  A poorly implemented server ignores the requirement for stamping
    the 'from' address.  This would enable any entity that
    authenticated with the server to send stanzas from any
    localpart@domainpart as long as the domainpart matches the sending
    domain of the server.
 o  An actively malicious server generates stanzas on behalf of any
    registered account.
 Therefore, an entity outside the security perimeter of a particular
 server cannot reliably distinguish between 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 poorly implemented or rogue servers.  However, the
 end-to-end authentication or signing of XMPP stanzas could help to
 mitigate this risk, since it would require the rogue server to
 generate false credentials in addition to modifying 'from' addresses.
 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] are not used.

4.3.2. 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

Saint-Andre Standards Track [Page 10] RFC 6122 XMPP Address Format March 2011

 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:
 o  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.
 o  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

Saint-Andre Standards Track [Page 11] RFC 6122 XMPP Address Format March 2011

    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, two or more
    confusable resources could be bound at the same time to the same
    account (resulting in inconsistent authorization decisions in an
    XMPP application that uses full JIDs), 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], it is also true (as noted in
 [IDNA-DEFS]) 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).  Such policies are also
     appropriate for XMPP services that allow temporary or permanent
     registration of XMPP resourceparts, e.g., during resource binding
     [XMPP] or upon joining an XMPP-based chat room [XEP-0045].  For
     related considerations in the context of domain name
     registration, refer to Section 4.3 of [IDNA-PROTO] and Section
     3.2 of [IDNA-RATIONALE].  Note well that methods for enforcing
     such restrictions are out of scope for this document.

Saint-Andre Standards Track [Page 12] RFC 6122 XMPP Address Format March 2011

 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.

5. IANA Considerations

 The following sections update the registrations provided in
 [RFC3920].

5.1. Nodeprep Profile of Stringprep

 The Nodeprep profile of stringprep is defined under Nodeprep
 (Appendix A).  The IANA has registered Nodeprep in the "Stringprep
 Profiles" registry.
 Name of this profile:
    Nodeprep
 RFC in which the profile is defined:
    RFC 6122
 Indicator whether or not this is the newest version of the profile:
    This is the first version of Nodeprep

Saint-Andre Standards Track [Page 13] RFC 6122 XMPP Address Format March 2011

5.2. Resourceprep Profile of Stringprep

 The Resourceprep profile of stringprep is defined under Resourceprep
 (Appendix B).  The IANA has registered Resourceprep in the
 "Stringprep Profiles" registry.
 Name of this profile:
    Resourceprep
 RFC in which the profile is defined:
    RFC 6122
 Indicator whether or not this is the newest version of the profile:
    This is the first version of Resourceprep

6. 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:
 o  A human-readable name
 o  An informational description
 o  A reference to the particular section of this document that
    normatively defines the feature
 o  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)
 o  Whether the feature MUST or SHOULD be implemented, where the
    capitalized terms are to be understood as described in [KEYWORDS]
 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].  Although this feature set
 is more detailed than called for by [REPORTS], 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].

Saint-Andre Standards Track [Page 14] RFC 6122 XMPP Address Format March 2011

 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, and
    conforms to the underlying length limits of the DNS.
 Section:  Section 2.2
 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 2.2
 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 2.3
 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 2.3
 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 2.4
 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 2.4
 Roles:  Client SHOULD, Server MUST.

Saint-Andre Standards Track [Page 15] RFC 6122 XMPP Address Format March 2011

7. References

7.1. Normative References

 [ABNF]            Crocker, D., Ed. and P. Overell, "Augmented BNF for
                   Syntax Specifications: ABNF", STD 68, RFC 5234,
                   January 2008.
 [DNS]             Mockapetris, P., "Domain names - implementation and
                   specification", STD 13, RFC 1035, November 1987.
 [IDNA2003]        Faltstrom, P., Hoffman, P., and A. Costello,
                   "Internationalizing Domain Names in Applications
                   (IDNA)", RFC 3490, March 2003.
                   See Section 1 for an explanation of why the
                   normative reference to an obsoleted specification
                   is needed.
 [KEYWORDS]        Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.
 [NAMEPREP]        Hoffman, P. and M. Blanchet, "Nameprep: A
                   Stringprep Profile for Internationalized Domain
                   Names (IDN)", RFC 3491, March 2003.
                   See Section 1 for an explanation of why the
                   normative reference to an obsoleted specification
                   is needed.
 [STRINGPREP]      Hoffman, P. and M. Blanchet, "Preparation of
                   Internationalized Strings ("stringprep")",
                   RFC 3454, December 2002.
 [UNICODE]         The Unicode Consortium, "The Unicode Standard,
                   Version 3.2.0", 2000.  The Unicode Standard,
                   Version 3.2.0 is defined by The Unicode Standard,
                   Version 3.0 (Reading, MA, Addison-Wesley, 2000.
                   ISBN 0-201-61633-5), as amended by the Unicode
                   Standard Annex #27: Unicode 3.1
                   (http://www.unicode.org/reports/tr27/) and by the
                   Unicode Standard Annex #28: Unicode 3.2
                   (http://www.unicode.org/reports/tr28/).
 [UNICODE-SEC]     The Unicode Consortium, "Unicode Technical Report
                   #36: Unicode Security Considerations", 2008,
                   <http://www.unicode.org/reports/tr36/>.

Saint-Andre Standards Track [Page 16] RFC 6122 XMPP Address Format March 2011

 [UTF-8]           Yergeau, F., "UTF-8, a transformation format of ISO
                   10646", STD 63, RFC 3629, November 2003.
 [XMPP]            Saint-Andre, P., "Extensible Messaging and Presence
                   Protocol (XMPP): Core", RFC 6120, March 2011.

7.2. Informative References

 [DNSSEC]          Arends, R., Austein, R., Larson, M., Massey, D.,
                   and S. Rose, "DNS Security Introduction and
                   Requirements", RFC 4033, March 2005.
 [IDNA-DEFS]       Klensin, J., "Internationalized Domain Names for
                   Applications (IDNA): Definitions and Document
                   Framework", RFC 5890, August 2010.
 [IDNA-PROTO]      Klensin, J., "Internationalized Domain Names in
                   Applications (IDNA): Protocol", RFC 5891,
                   August 2010.
 [IDNA-RATIONALE]  Klensin, J., "Internationalized Domain Names for
                   Applications (IDNA): Background, Explanation, and
                   Rationale", RFC 5894, August 2010.
 [INTEROP]         Masinter, L., "Formalizing IETF Interoperability
                   Reporting", Work in Progress, October 2005.
 [IRI]             Duerst, M. and M. Suignard, "Internationalized
                   Resource Identifiers (IRIs)", RFC 3987,
                   January 2005.
 [PROCESS]         Bradner, S., "The Internet Standards Process --
                   Revision 3", BCP 9, RFC 2026, October 1996.
 [PUNYCODE]        Costello, A., "Punycode: A Bootstring encoding of
                   Unicode for Internationalized Domain Names in
                   Applications (IDNA)", RFC 3492, March 2003.
 [REPORTS]         Dusseault, L. and R. Sparks, "Guidance on
                   Interoperation and Implementation Reports for
                   Advancement to Draft Standard", BCP 9, RFC 5657,
                   September 2009.
 [RFC3920]         Saint-Andre, P., Ed., "Extensible Messaging and
                   Presence Protocol (XMPP): Core", RFC 3920,
                   October 2004.

Saint-Andre Standards Track [Page 17] RFC 6122 XMPP Address Format March 2011

 [RFC5952]         Kawamura, S. and M. Kawashima, "A Recommendation
                   for IPv6 Address Text Representation", RFC 5952,
                   August 2010.
 [SASL]            Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
                   Authentication and Security Layer (SASL)",
                   RFC 4422, June 2006.
 [URI]             Berners-Lee, T., Fielding, R., and L. Masinter,
                   "Uniform Resource Identifier (URI): Generic
                   Syntax", STD 66, RFC 3986, January 2005.
 [XEP-0029]        Kaes, C., "Definition of Jabber Identifiers
                   (JIDs)", XSF XEP 0029, October 2003.
 [XEP-0030]        Hildebrand, J., Millard, P., Eatmon, R., and P.
                   Saint-Andre, "Service Discovery", XSF XEP 0030,
                   June 2008.
 [XEP-0045]        Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
                   July 2008.
 [XEP-0060]        Millard, P., Saint-Andre, P., and R. Meijer,
                   "Publish-Subscribe", XSF XEP 0060, July 2010.
 [XEP-0165]        Saint-Andre, P., "Best Practices to Discourage JID
                   Mimicking", XSF XEP 0045, December 2007.
 [XML]             Paoli, J., Maler, E., Sperberg-McQueen, C.,
                   Yergeau, F., and T. Bray, "Extensible Markup
                   Language (XML) 1.0 (Fourth Edition)", World Wide
                   Web Consortium Recommendation REC-xml-20060816,
                   August 2006,
                   <http://www.w3.org/TR/2006/REC-xml-20060816>.
 [XMPP-URI]        Saint-Andre, P., "Internationalized Resource
                   Identifiers (IRIs) and Uniform Resource Identifiers
                   (URIs) for the Extensible Messaging and Presence
                   Protocol (XMPP)", RFC 5122, February 2008.

Saint-Andre Standards Track [Page 18] RFC 6122 XMPP Address Format March 2011

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

A.2. Character Repertoire

 This profile uses Unicode 3.2 with the list of unassigned code points
 in Table A.1, both as defined in Appendix A of [STRINGPREP].

A.3. Mapping

 This profile specifies mapping using the following tables from
 [STRINGPREP]:
    Table B.1
    Table B.2

A.4. Normalization

 This profile specifies the use of Unicode Normalization Form KC, as
 described in [STRINGPREP].

Saint-Andre Standards Track [Page 19] RFC 6122 XMPP Address Format March 2011

A.5. Prohibited Output

 This profile specifies the prohibition of using the following tables
 from [STRINGPREP].
    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].

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] for complete
 information):

Saint-Andre Standards Track [Page 20] RFC 6122 XMPP Address Format March 2011

 o  U+2100 (ACCOUNT OF)
 o  U+2101 (ADDRESSED TO THE SUBJECT)
 o  U+2105 (CARE OF)
 o  U+2106 (CADA UNA)
 o  U+226E (NOT LESS-THAN)
 o  U+226F (NOT GREATER-THAN)
 o  U+2A74 (DOUBLE COLON EQUAL)
 o  U+FE13 (PRESENTATION FORM FOR VERTICAL COLON)
 o  U+FE60 (SMALL AMPERSAND)
 o  U+FE64 (SMALL LESS-THAN SIGN)
 o  U+FE65 (SMALL GREATER-THAN SIGN)
 o  U+FE6B (SMALL COMMERCIAL AT)
 o  U+FF02 (FULLWIDTH QUOTATION MARK)
 o  U+FF06 (FULLWIDTH AMPERSAND)
 o  U+FF07 (FULLWIDTH APOSTROPHE)
 o  U+FF0F (FULLWIDTH SOLIDUS)
 o  U+FF1A (FULLWIDTH COLON)
 o  U+FF1C (FULLWIDTH LESS-THAN SIGN)
 o  U+FF1E (FULLWIDTH GREATER-THAN SIGN)
 o  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]:
 o  The intended applicability of the profile: internationalized
    resourceparts within XMPP
 o  The character repertoire that is the input and output to
    stringprep: Unicode 3.2, specified in B.2
 o  The mappings used: specified in B.3
 o  The Unicode normalization used: specified in B.4
 o  The characters that are prohibited as output: specified in B.5

Saint-Andre Standards Track [Page 21] RFC 6122 XMPP Address Format March 2011

 o  Bidirectional character handling: specified in B.6

B.2. Character Repertoire

 This profile uses Unicode 3.2 with the list of unassigned code points
 in Table A.1, both as defined in Appendix A of [STRINGPREP].

B.3. Mapping

 This profile specifies mapping using the following tables from
 [STRINGPREP]:
    Table B.1

B.4. Normalization

 This profile specifies the use of Unicode Normalization Form KC, as
 described in [STRINGPREP].

B.5. Prohibited Output

 This profile specifies the prohibition of using the following tables
 from [STRINGPREP].
    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].

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.

Saint-Andre Standards Track [Page 22] RFC 6122 XMPP Address Format March 2011

 o  Corrected the ABNF syntax to ensure consistency with [URI] and
    [IRI], including consistency with RFC 3986 and [RFC5952] with
    regard to IPv6 addresses (e.g., enclosing the IPv6 address in
    square brackets '[' and ']' -- see also Section 4.9.3.19 of
    [XMPP]).
 o  Corrected the ABNF syntax to prevent zero-length localparts,
    domainparts, and resourceparts (and also noted that the underlying
    length limits from the DNS apply to domainparts).
 o  To avoid confusion with the term "node" as used in [XEP-0030] and
    [XEP-0060], changed the term "node identifier" to "localpart" (but
    retained the name "Nodeprep" for backward compatibility).
 o  To avoid confusion with the terms "resource" and "identifier" as
    used in [URI], changed the term "resource identifier" to
    "resourcepart".
 o  Corrected the Nameprep processing rules to require use of the
    UseSTD3ASCIIRules flag.

Appendix D. Acknowledgements

 Thanks to Ben Campbell, Waqas Hussain, Jehan Pages, and Florian Zeitz
 for their feedback.  Thanks also to Richard Barnes and Elwyn Davies
 for their reviews on behalf of the Security Directorate and the
 General Area Review Team, respectively.
 The Working Group chairs were Ben Campbell and Joe Hildebrand.  The
 responsible Area Director was Gonzalo Camarillo.
 Some text in this document was borrowed or adapted from [IDNA-DEFS],
 [IDNA-PROTO], [IDNA-RATIONALE], and [XEP-0165].

Author's Address

 Peter Saint-Andre
 Cisco
 1899 Wyknoop Street, Suite 600
 Denver, CO  80202
 USA
 Phone: +1-303-308-3282
 EMail: psaintan@cisco.com

Saint-Andre Standards Track [Page 23]

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