path: root/doc/rfc/rfc6122.txt

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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L