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Versions: (draft-saintandre-xmpp-6122bis) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 RFC 7622

XMPP                                                      P. Saint-Andre
Internet-Draft                                       Cisco Systems, Inc.
Obsoletes: 6122 (if approved)                             March 27, 2013
Intended status: Standards Track
Expires: September 28, 2013


   Extensible Messaging and Presence Protocol (XMPP): Address Format
                       draft-ietf-xmpp-6122bis-06

Abstract

   This document defines the address format for the Extensible Messaging
   and Presence Protocol (XMPP), including support for code points
   outside the ASCII range.  This document obsoletes RFC 6122.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 28, 2013.

Copyright Notice

   Copyright (c) 2013 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.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Addresses . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Fundamentals  . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Domainpart  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Localpart . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  Resourcepart  . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Enforcement in JIDs and JID Parts . . . . . . . . . . . . . .   9
   4.  Internationalization Considerations . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     5.1.  Reuse of PRECIS . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  Reuse of Unicode  . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Address Spoofing  . . . . . . . . . . . . . . . . . . . .  12
       5.3.1.  Address Forging . . . . . . . . . . . . . . . . . . .  12
       5.3.2.  Address Mimicking . . . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Subclasses  . . . . . . . . . . . . . . . . . . . . . . .  14
     6.2.  Usage of IdentifierClass  . . . . . . . . . . . . . . . .  15
     6.3.  Usage of FreeformClass  . . . . . . . . . . . . . . . . .  15
   7.  Conformance Requirements  . . . . . . . . . . . . . . . . . .  16
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Differences from RFC 6122  . . . . . . . . . . . . .  22
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  22
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

1.1.  Overview

   The Extensible Messaging and Presence Protocol (XMPP) [RFC6120] 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 then defined canonically
   by [RFC3920] in 2004 and [RFC6122] in 2011.










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   As specified in RFC 3920 and RFC 6122, the XMPP address format used
   the "stringprep" technology for preparation of non-ASCII characters
   [RFC3454].  Following the migration of internationalized domain names
   away from stringprep, this document defines the XMPP address format
   in a way that no longer depends on stringprep.  Instead, this
   document builds upon the internationalization framework defined by
   the IETF's PRECIS Working Group [FRAMEWORK].

   This document obsoletes RFC 6122.

1.2.  Terminology

   Many important terms used in this document are defined in
   [FRAMEWORK], [RFC5890], [RFC6120], [RFC6365], and [UNICODE].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

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 ("JID").  A valid JID
   is a string of [UNICODE] code points, encoded using UTF-8 [RFC3629],
   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 (ABNF) as specified in [RFC5234].

   jid           = [ localpart "@" ] domainpart [ "/" resourcepart ]
   localpart     = 1*1023(localpoint)
                   ;
                   ; a "localpoint" is a UTF-8 encoded
                   ; Unicode code point that conforms to
                   ; the "LocalpartIdentifierClass" subclass
                   ; of the PRECIS IdentifierClass
                   ;
   domainpart    = IP-literal / IPv4address / ifqdn
                   ;
                   ; the "IPv4address" and "IP-literal"
                   ; rules are defined in RFC 3986, and



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                   ; 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 ']')
                   ;
   ifqdn         = 1*1023(domainpoint)
                   ;
                   ; a "domainpoint" is a UTF-8 encoded
                   ; Unicode code point that conforms to
                   ; RFC 5890
                   ;
   resourcepart  = 1*1023(resourcepoint)
                   ;
                   ; a "resourcepoint" is a UTF-8 encoded
                   ; Unicode code point that conforms to
                   ; the "ResourcepartFreeformClass" subclass
                   ; of the PRECIS FreeformClass
                   ;


   All JIDs are based on the foregoing structure.  However, note that
   the foregoing structure does not capture all of the rules and
   restrictions that apply to JIDs, which are described below.

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

      Implementation Note: The length limits on JIDs and JID parts are
      based on octets (bytes), not characters.  UTF-8 encoding can
      result in more than one octet per character.

      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., under Unicode Normalization Form KC U+FE6B SMALL COMMERCIAL
      AT decomposes to U+0040 COMMERCIAL AT, although this is not true
      under Unicode Normalization C, which is used in this
      specification).





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   This document defines the native format for JIDs; see [RFC5122] for
   information about the representation of a JID as a Uniform Resource
   Identifier (URI) [RFC3986] or Internationalized Resource Identifier
   (IRI) [RFC3987] and the extraction of a JID from an XMPP URI or IRI.

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 [XEP-0045], a publish-subscribe service
   [XEP-0060], or a user directory).

   The domainpart for every XMPP service MUST be a fully-qualified
   domain name (FQDN), an IPv4 address, an IPv6 address, or an
   unqualified hostname (i.e., a text label that is resolvable on a
   local network).

      Informational Note: The term "fully-qualified domain name" is not
      well defined.  In [RFC1034] it also called an absolute domain
      name, and the two terms are associated in [RFC1535].  The earliest
      use of the term can be found in [RFC1123].  References to those
      older specifications ought not to be construed as limiting the
      characters of a fully-qualified domain name to the ASCII range;
      for example, [RFC5890] mentions that a fully-qualified domain name
      can contain one or more U-labels.

      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 [RFC1034], 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 or IRI [RFC5122].  In particular, the
   character MUST be stripped before any other canonicalization steps
   are taken.

   In general, the content of a domainpart is an Internationalized
   Domain Name ("IDN") as described in the specifications for



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   Internationalized Domain Names in Applications (commonly called
   "IDNA2008") [RFC5890], and a domainpart is an "IDNA-aware domain name
   slot".  The following rules apply to a domainpart that consists of a
   fully-qualified domain name:

   o  The domainpart MUST contain only NR-LDH labels and U-labels as
      defined in [RFC5890] and MUST consist only of Unicode code points
      that conform to the rules specified in [RFC5892].


   o  The domainpart MUST NOT include A-labels as defined in [RFC5890];
      each A-label MUST be converted to a U-label during preparation of
      a domainpart, and comparison MUST be performed using U-labels, not
      A-labels.


   o  After conversion of A-labels to U-labels if necessary, all
      uppercase and titlecase code points within the domainpart MUST be
      mapped to their lowercase equivalents.


   o  After (and in addition to) case mapping and width mapping, other
      mappings MAY be applied to the domainpart, such as those defined
      in [I-D.yoneya-precis-mappings] or [RFC5895].

   After any and all normalization, conversion, and mapping of code
   points, a domainpart MUST NOT be zero octets in length and MUST NOT
   be more than 1023 octets in length.  (Naturally, the length limits of
   [RFC1034] apply, and nothing in this document is to be interpreted as
   overriding those more fundamental limits.)

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 [XEP-0045]).  The
   entity represented by an XMPP localpart is addressed within the
   context of a specific domain (i.e., <localpart@domainpart>).

   A localpart MUST NOT be zero octets in length and MUST NOT be more
   than 1023 octets in length.  This rule is to be enforced after any
   normalization and mapping of code points.

   A localpart MUST consist only of Unicode code points that conform to
   the "LocalpartIdentifierClass" subclass of the "IdentifierClass" base



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   string class defined in [FRAMEWORK].  The LocalpartIdentifierClass
   subclass includes all code points allowed by the IdentifierClass base
   class, with the exception of the following characters that are
   explicitly disallowed in XMPP localparts:

      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., @

   The normalization and mapping rules for the LocalpartIdentifierClass
   are as follows, where the operations specified MUST be completed in
   the order shown:

   1.  All characters MUST be mapped using Unicode Normalization Form C
       (NFC).


   2.  Uppercase and titlecase characters MUST be mapped to their
       lowercase equivalents.


   3.  Fullwidth and halfwidth characters MUST be mapped to their
       decomposition equivalents (see [I-D.yoneya-precis-mappings]).


   4.  Additional mappings MAY be applied, such as those defined in
       [I-D.yoneya-precis-mappings].


   With regard to directionality, applications MUST apply the "Bidi
   Rule" defined in [RFC5893] (i.e., each of the six conditions of the
   Bidi Rule must be satisfied).

2.4.  Resourcepart





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   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 [XEP-0045])
   belonging to the entity associated with an XMPP localpart at a domain
   (i.e., <localpart@domainpart/resourcepart>).

   A resourcepart MUST NOT be zero octets in length and MUST NOT be more
   than 1023 octets in length.  This rule is to be enforced after any
   normalization and mapping of code points.

   A resourcepart MUST consist only of Unicode code points that conform
   to the "FreeformClass" base string class defined in [FRAMEWORK].
   (Note that there is no XMPP-specific subclass for resourceparts.)

   The normalization and mapping rules for the resourcepart of a JID are
   as follows, where the operations specified MUST be completed in the
   order shown:

   1.  All characters MUST be mapped using Unicode Normalization Form C
       (NFC).


   2.  Uppercase and titlecase characters MAY be mapped to their
       lowercase equivalents.


   3.  Fullwidth and halfwidth characters MAY be mapped to their
       decomposition equivalents (see [I-D.yoneya-precis-mappings]).


   4.  Additional mappings MAY be applied, such as those defined in
       [I-D.yoneya-precis-mappings].


   With regard to directionality, applications MUST apply the "Bidi
   Rule" defined in [RFC5893] (i.e., each of the six conditions of the
   Bidi Rule must be satisfied).

   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



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      <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@domainpart".


   o  The '@' character is allowed in the resourcepart and is often used
      in the "nick" shown in XMPP chatrooms [XEP-0045].  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.

   In some contexts, it might be appropriate to apply more restrictive
   rules to the preparation and comparison of XMPP resourceparts.  For
   example, in the context of XMPP Multi-User Chat [XEP-0045], it might
   be appropriate to apply the rules specified in
   [I-D.ietf-precis-nickname].  However, the application of such more
   restrictive rules is out of scope for resourceparts in general and is
   properly defined in specifications for the relevant XMPP extensions.

3.  Enforcement in JIDs and JID Parts

   Enforcement of the XMPP address format rules is the responsibility of
   XMPP servers.  Although XMPP clients SHOULD prepare complete JIDs and
   parts of JIDs in accordance with the rules before including them in
   protocol slots within XML streams (such that JIDs and parts of JIDs
   are in conformance), XMPP servers MUST enforce the rules wherever
   possible and reject stanzas and other XML elements that violate the
   rules (for stanzas, by returning a <jid-malformed/> error to the
   sender as described in Section 8.3.3.8 of [RFC6120]).

   Enforcement applies to complete JIDs and to parts of JIDs.  To
   facilitate implementation, this document defines the concepts of "JID
   slot", "localpart slot", and "resourcepart slot" (similar to the
   concept of a "domain name slot" for IDNA2008 defined in
   Section 2.3.2.6 of [RFC5890]):

   JID Slot:  An XML element or attribute explicitly designated in XMPP
      or in XMPP extensions for carrying a complete JID.


   Localpart Slot:  An XML element or attribute explicitly designated in
      XMPP or in XMPP extensions for carrying the localpart of a JID.







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   Resourcepart Slot:  An XML element or attribute explicitly designated
      in XMPP or in XMPP extensions for carrying the resourcepart of a
      JID.

   A server is responsible for enforcing the address format rules when
   receiving protocol elements from clients where the server is expected
   to handle such elements directly or to use them for purposes of
   routing a stanza to another domain or delivering a stanza to a local
   entity; two examples from [RFC6120] are the 'to' attribute on XML
   stanzas (which is a JID slot used by XMPP servers for routing of
   outbound stanzas) and the <resource/> child of the <bind/> element
   (which is a resourcepart slot used by XMPP servers for binding of a
   resource to an account for routing of stanzas between the server and
   a particular client).

   A server is not responsible for enforcing the rules when the protocol
   elements are intended for communication among other entities,
   typically within the payload of a stanza that the server is merely
   routing to another domain or delivering to a local entity, such as a
   connected client or an add-on service.  Two examples are the
   'initiator' attribute in the Jingle extension [XEP-0166] (which is a
   JID slot used for client-to-client coordination of multimedia
   sessions) and the 'nick' attribute in the Multi-User Chat extension
   [XEP-0045] (which is a resourcepart slot used for administrative
   purposes in the context of XMPP chatrooms).  In such cases, clients
   SHOULD enforce the rules themselves and not depend on the server to
   do so, and client implementers need to understand that not enforcing
   the rules can lead to a degraded user experience or to security
   vulnerabilities.  However, when an add-on service (e.g., a multi-user
   chat service) handles a stanza directly, it ought to enforce the
   rules as well, as defined by the relevant specification for that type
   of service.

   This document does not provide an exhaustive list of JID slots,
   localpart slots, or resourcepart slots.  However, implementers of
   core XMPP servers are advised to consider as JID slots at least the
   following elements and attributes when they are handled directly or
   used for purposes of routing to another domain or delivering to a
   local entity:

   o  The 'from' and 'to' stream attributes and the 'from' and 'to'
      stanza attributes [RFC6120].

   o  The 'jid' attribute of the roster <item/> element for contact list
      management [RFC6121].






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   o  The 'value' attribute of the <item/> element for Privacy Lists
      [RFC3921] [XEP-0016] when the value of the 'type' attribute is
      "jid".

   o  The 'jid' attribute of the <item/> element for Service Discovery
      defined in [XEP-0030].

   o  The <value/> element for Data Forms [XEP-0004], when the 'type'
      attribute is "jid-single" or "jid-multi".

   o  The 'jid' attribute of the <conference/> element for Bookmark
      Storage [XEP-0048].

   o  The <JABBERID/> of the <vCard/> element for vCard 3.0 [XEP-0054]
      and the <uri/> child of the <impp/> element for vCard 4.0
      [XEP-0292] when the XML character data identifies an XMPP URI
      [RFC5122].

   o  The 'from' attribute of the <delay/> element for Delayed Delivery
      [XEP-0203].

   o  The 'jid' attribute of the <item/> element for the Blocking
      Command [XEP-0191].

   o  The 'from' and 'to' attributes of the <result/> and <verify/>
      elements for Server Dialback [RFC3921], [XEP-0220].

   o  The 'from' and 'to' attributes of the <amp/> element for Advanced
      Message Processing [XEP-0079].

   o  The 'from' and 'to' attributes of the <iq/>, <message/>, and
      <presence/> elements for the Jabber Component Protocol [XEP-0114].

   Developers of XMPP clients and specialized XMPP add-on services are
   advised to check the appropriate specifications for JID slots,
   localpart slots, and resourcepart slots in XMPP protocol extensions
   such as Service Discovery [XEP-0030], Multi-User Chat [XEP-0045],
   Publish-Subscribe [XEP-0060], SOCKS5 Bytestreams [XEP-0065], In-Band
   Registration [XEP-0077], Roster Item Exchange [XEP-0144], and Jingle
   [XEP-0166].

4.  Internationalization Considerations

   XMPP applications MUST support IDNA2008 for domainparts as described
   under Section 2.2, the "LocalpartIdentifierClass" subclass for
   localparts as described under Section 2.3, and the "FreeformClass"
   base string class for resourceparts as described under Section 2.4.
   This enables XMPP addresses to include a wide variety of characters



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   outside the ASCII range.  Rules for enforcement of the XMPP address
   format are provided in [RFC6120] and specifications for various XMPP
   extensions.

      Implementation Note: For backward compatibility, many XMPP
      applications support IDNA2003 [RFC3490] for domainparts, and the
      stringprep [RFC3454] profiles Nodeprep and Resourceprep [RFC3920]
      for localparts and resourceparts.

5.  Security Considerations

5.1.  Reuse of PRECIS

   The security considerations described in [FRAMEWORK] apply to the
   "IdentifierClass" and "FreeformClass" base string classes used in
   this document for XMPP localparts and resourceparts.  The security
   considerations described in [RFC5890] apply to internationalized
   domain names, which are used here for XMPP domainparts.

5.2.  Reuse of Unicode

   The security considerations described in [UTR39] apply to the use of
   Unicode characters in XMPP addresses.

5.3.  Address Spoofing

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

5.3.1.  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 [RFC4422] as described in
   [RFC6120]).  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 [RFC6120]).  However, address forging is possible if:








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   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 at the domain or domains hosted at that server.

   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 the kind of poorly implemented or rogue servers just
   described.  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 for signing or encryption
   of each stanza, in addition to modifying 'from' addresses.

5.3.2.  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.
   Because many characters are visually similar, it is relatively easy
   to mimic JIDs in XMPP systems.  As one simple example, the localpart
   "ju1iet" (using the Arabic numeral one as the third character) might
   appear the same as the localpart "juliet" (using lowercase "L" as the
   third character).

   As explained in [RFC5890], [FRAMEWORK], [UTR36], and [UTR39], there
   is no straightforward solution to the problem of visually similar
   characters.  Furthermore, IDNA and PRECIS technologies do not attempt
   to define such a solution.  As a result, XMPP domainparts,
   localparts, and resourceparts could contain such characters, leading
   to security vulnerabilities such as the following:

   o  A domainpart is always employed as one part of an entity's address
      in XMPP.  One common usage is as the address of a server or
      server-side service, such as a multi-user chat service [XEP-0045].
      The security of such services could be compromised based on
      different interpretations of the internationalized domainpart; for
      example, a user might authorize a malicious entity at a fake
      server to view the user's presence information, or a user could
      join chatrooms at a fake multi-user chat service.




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   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
      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
      private message to someone other than the intended recipient in a
      multi-user chat room.

   XMPP services and clients are strongly encouraged to define and
   implement consistent policies regarding the registration, storage,
   and presentation of visually similar characters in XMPP systems.  In
   particular, service providers and software implementers are strongly
   encouraged to use the policies recommended in [FRAMEWORK].

6.  IANA Considerations

6.1.  Subclasses

   The IANA shall add the following entry to the PRECIS Subclass
   Registry:

   Subclass:  LocalpartIdentifierClass.

   Base Class:  IdentifierClass.

   Exclusions:  Eight legacy characters in the ASCII range.

   Specification:  RFC XXXX.  [Note to RFC Editor: please change XXXX to
      the number issued for this specification.]





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6.2.  Usage of IdentifierClass

   The IANA shall add the following entry to the PRECIS Usage Registry:

   Applicability:  Localparts of XMPP addresses.

   Base Class:  IdentifierClass.

   Subclass:  Yes, LocalpartIdentifierClass.

   Normalization:  NFC.

   Case Mapping:  Map uppercase and titlecase characters to lowercase.

   Width Mapping:  Map fullwidth and halfwidth characters to their
      decomposition equivalents.

   Additional Mappings:  None required or recommended.

   Directionality:  The "Bidi Rule" defined in RFC 5893 applies.

   Specification:  RFC XXXX.  [Note to RFC Editor: please change XXXX to
      the number issued for this specification.]

6.3.  Usage of FreeformClass

   The IANA shall add the following entry to the PRECIS Usage Registry:

   Applicability:  Resourceparts of XMPP addresses.

   Base Class:  FreeformClass

   Subclass:  No.

   Replaces:  The Resourceprep profile of Stringprep.

   Normalization:  NFC.

   Case Mapping:  Optional.

   Width Mapping:  Optional.

   Additional Mappings:  None required or recommended.

   Directionality:  The "Bidi Rule" defined in RFC 5893 applies.

   Specification:  RFC XXXX.  [Note to RFC Editor: please change XXXX to
      the number issued for this specification.]



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7.  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 [RFC2119]

   The feature set specified here provides a basis for interoperability
   testing and follows the spirit of a proposal made by Larry Masinter
   within the IETF's NEWTRK Working Group in 2005 [INTEROP].

   Feature:  address-domain-length

   Description:  Ensure that the domainpart of an XMPP address is at
      least one octet in length and at most 1023 octets in length, and
      that it conforms to the underlying length limits of the DNS.

   Section:  Section 2.2

   Roles:  Server MUST, client SHOULD.

   Feature:  address-domain-prep

   Description:  Ensure that the domainpart of an XMPP address conforms
      to IDNA2008, that it contains only NR-LDH labels and U-labels (not
      A-labels), and that all uppercase and titlecase code points are
      mapped to their lowercase equivalents.

   Section:  Section 2.2

   Roles:  Server MUST, client SHOULD.

   Feature:  address-localpart-length



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   Description:  Ensure that the localpart of an XMPP address is at
      least one octet in length and at most 1023 octets in length.

   Section:  Section 2.3

   Roles:  Server MUST, client SHOULD.

   Feature:  address-localpart-prep

   Description:  Ensure that the localpart of an XMPP address conforms
      to the "LocalpartIdentifierClass" subclass, that all code points
      are normalized using NFC, and that all uppercase and titlecase
      code points are mapped to their lowercase equivalents.

   Section:  Section 2.3

   Roles:  Server MUST, client SHOULD.

   Feature:  address-resource-length

   Description:  Ensure that the resourcepart of an XMPP address is at
      least one octet in length and at most 1023 octets in length.

   Section:  Section 2.4

   Roles:  Server MUST, client SHOULD.

   Feature:  address-resource-prep

   Description:  Ensure that the resourcepart of an XMPP address
      conforms to the "FreeformClass" base string class from the PRECIS
      framework, with all code points normalized using NFC.

   Section:  Section 2.4

   Roles:  Server MUST, client SHOULD.

8.  References

8.1.  Normative References

   [FRAMEWORK]
              Saint-Andre, P. and M. Blanchet, "Precis Framework:
              Handling Internationalized Strings in Protocols", draft-
              ietf-precis-framework-06 (work in progress), September
              2012.





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   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

   [RFC5892]  Faltstrom, P., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, August 2010.

   [RFC5893]  Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5893, August 2010.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [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/).

   [UTR36]    The Unicode Consortium, "Unicode Technical Report #36:
              Unicode Security Considerations", 2008,
              <http://www.unicode.org/reports/tr36/>.

8.2.  Informative References





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   [INTEROP]  Masinter, L., "Formalizing IETF Interoperability
              Reporting", Work in Progress, October 2005.

   [I-D.yoneya-precis-mappings]
              YONEYA, Y. and T. NEMOTO, "Mapping characters for PRECIS
              classes", draft-yoneya-precis-mappings-03 (work in
              progress), October 2012.

   [I-D.ietf-precis-nickname]
              Saint-Andre, P., "Preparation and Comparison of
              Nicknames", draft-ietf-precis-nickname-05 (work in
              progress), November 2012.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC1535]  Gavron, E., "A Security Problem and Proposed Correction
              With Widely Deployed DNS Software", RFC 1535, October
              1993.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [RFC3490]  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.

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.

   [RFC3921]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence", RFC
              3921, October 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.




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   [RFC5122]  Saint-Andre, P., "Internationalized Resource Identifiers
              (IRIs) and Uniform Resource Identifiers (URIs) for the
              Extensible Messaging and Presence Protocol (XMPP)", RFC
              5122, February 2008.

   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, August 2010.

   [RFC5895]  Resnick, P. and P. Hoffman, "Mapping Characters for
              Internationalized Domain Names in Applications (IDNA)
              2008", RFC 5895, September 2010.

   [RFC6121]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence", RFC
              6121, March 2011.

   [RFC6122]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Address Format", RFC 6122, March 2011.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              September 2011.

   [UTR39]    The Unicode Consortium, "Unicode Technical Report #39:
              Unicode Security Mechanisms", August 2010,
              <http://unicode.org/reports/tr39/>.

   [XEP-0004]
              Eatmon, R., Hildebrand, J., Miller, J., Muldowney, T., and
              P. Saint-Andre, "Data Forms", XSF XEP 0004, August 2007.

   [XEP-0016]
              Millard, P. and P. Saint-Andre, "Privacy Lists", XSF XEP
              0016, February 2007.

   [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, February
              2012.




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   [XEP-0048]
              Blackman, R., Millard, P., and P. Saint-Andre,
              "Bookmarks", XSF XEP 0048, November 2007.

   [XEP-0054]
              Saint-Andre, P., "vcard-temp", XSF XEP 0054, July 2008.

   [XEP-0060]
              Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
              Subscribe", XSF XEP 0060, July 2010.

   [XEP-0065]
              Smith, D., Miller, M., Saint-Andre, P., and J. Karneges,
              "SOCKS5 Bytestreams", XSF XEP 0065, April 2011.

   [XEP-0077]
              Saint-Andre, P., "In-Band Registration", XSF XEP 0077,
              January 2012.

   [XEP-0079]
              Miller, M. and P. Saint-Andre, "Advanced Message
              Processing", XSF XEP 0079, November 2005.

   [XEP-0114]
              Saint-Andre, P., "Jabber Component Protocol", XSF XEP
              0114, March 2005.

   [XEP-0144]
              Saint-Andre, P., "Roster Item Exchange", XSF XEP 0144,
              August 2005.

   [XEP-0165]
              Saint-Andre, P., "Best Practices to Discourage JID
              Mimicking", XSF XEP 0165, December 2007.

   [XEP-0166]
              Ludwig, S., Beda, J., Saint-Andre, P., McQueen, R., Egan,
              S., and J. Hildebrand, "Jingle", XSF XEP 0166, December
              2009.

   [XEP-0191]
              Saint-Andre, P., "Blocking Command", XSF XEP 0191, July
              2012.

   [XEP-0203]
              Saint-Andre, P., "Delayed Delivery", XSF XEP 0203,
              September 2009.




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   [XEP-0220]
              Miller, J., Saint-Andre, P., and P. Hancke, "Server
              Dialback", XSF XEP 0220, August 2012.

   [XEP-0292]
              Saint-Andre, P. and S. Mizzi, "vCard4 Over XMPP", XSF XEP
              0292, October 2011.

   [XML]      Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J.,
              and T. Bray, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, November 2008,
              <http://www.w3.org/TR/2008/REC-xml-20081126>.

Appendix A.  Differences from RFC 6122

   Based on consensus derived from working group discussion,
   implementation and deployment experience, and formal interoperability
   testing, the following substantive modifications were made from RFC
   6122.

   o  Changed domainpart preparation to use IDNA2008 (instead of
      IDNA2003).

   o  Changed localpart preparation to use the LocalpartIdentifierClass
      subclass of the PRECIS IdentifierClass (instead of the Nodeprep
      profile of Stringprep).

   o  Changed resourcepart preparation to use the PRECIS FreeformClass
      (instead of the Resourceprep profile of Stringprep).

   o  Specified that internationalized labels within domainparts must be
      U-labels (instead of should be U-labels).

   o  Specified the use of Unicode normalization form C (instead of KC
      as specified in the Nodeprep and Resourceprep profiles of
      Stringprep).

   o  Specified that fullwidth and halfwidth should be mapped to their
      decomposition equivalents (previously handled through the use of
      NFKC).

   o  Specified that servers must enforce the address formatting rules.

Appendix B.  Acknowledgements

   Thanks to Miguel Garcia, Joe Hildebrand, and Florian Zeitz for their
   feedback.



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   Some text in this document was borrowed or adapted from [RFC5890],
   [RFC5891], [RFC5894], and [XEP-0165].

Author's Address

   Peter Saint-Andre
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO  80202
   USA

   Phone: +1-303-308-3282
   Email: psaintan@cisco.com





































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