Internet Engineering Task Force (IETF)                    P. Saint-Andre
Request for Comments: 6121                                         Cisco
Obsoletes: 3921                                               March 2011
Category: Standards Track
ISSN: 2070-1721

Extensible Messaging and Presence Protocol (XMPP):
Instant Messaging and Presence

Abstract

   This document defines extensions to core features of the Extensible
   Messaging and Presence Protocol (XMPP) that provide basic instant
   messaging (IM) and presence functionality in conformance with the
   requirements in RFC 2779.  This document obsoletes RFC 3921.

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/rfc6121.

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
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   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.

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.2.  History . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Functional Summary  . . . . . . . . . . . . . . . . . . .   7
     1.5.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   8
   2.  Managing the Roster . . . . . . . . . . . . . . . . . . . . .   9
     2.1.  Syntax and Semantics  . . . . . . . . . . . . . . . . . .   9
       2.1.1.  Ver Attribute . . . . . . . . . . . . . . . . . . . .  10
       2.1.2.  Roster Items  . . . . . . . . . . . . . . . . . . . .  10
         2.1.2.1.  Approved Attribute  . . . . . . . . . . . . . . .  10
         2.1.2.2.  Ask Attribute . . . . . . . . . . . . . . . . . .  10
         2.1.2.3.  JID Attribute . . . . . . . . . . . . . . . . . .  11
         2.1.2.4.  Name Attribute  . . . . . . . . . . . . . . . . .  11
         2.1.2.5.  Subscription Attribute  . . . . . . . . . . . . .  11
         2.1.2.6.  Group Element . . . . . . . . . . . . . . . . . .  12
       2.1.3.  Roster Get  . . . . . . . . . . . . . . . . . . . . .  12
       2.1.4.  Roster Result . . . . . . . . . . . . . . . . . . . .  13
       2.1.5.  Roster Set  . . . . . . . . . . . . . . . . . . . . .  14
       2.1.6.  Roster Push . . . . . . . . . . . . . . . . . . . . .  14
     2.2.  Retrieving the Roster on Login  . . . . . . . . . . . . .  16
     2.3.  Adding a Roster Item  . . . . . . . . . . . . . . . . . .  17
       2.3.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  17
       2.3.2.  Success Case  . . . . . . . . . . . . . . . . . . . .  17
       2.3.3.  Error Cases . . . . . . . . . . . . . . . . . . . . .  18
     2.4.  Updating a Roster Item  . . . . . . . . . . . . . . . . .  22
       2.4.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  22
       2.4.2.  Success Case  . . . . . . . . . . . . . . . . . . . .  24
       2.4.3.  Error Cases . . . . . . . . . . . . . . . . . . . . .  24
     2.5.  Deleting a Roster Item  . . . . . . . . . . . . . . . . .  24
       2.5.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  24
       2.5.2.  Success Case  . . . . . . . . . . . . . . . . . . . .  25
       2.5.3.  Error Cases . . . . . . . . . . . . . . . . . . . . .  26
     2.6.  Roster Versioning . . . . . . . . . . . . . . . . . . . .  26
       2.6.1.  Stream Feature  . . . . . . . . . . . . . . . . . . .  26
       2.6.2.  Request . . . . . . . . . . . . . . . . . . . . . . .  26
       2.6.3.  Success Case  . . . . . . . . . . . . . . . . . . . .  27
   3.  Managing Presence Subscriptions . . . . . . . . . . . . . . .  30
     3.1.  Requesting a Subscription . . . . . . . . . . . . . . . .  30
       3.1.1.  Client Generation of Outbound Subscription Request  .  31
       3.1.2.  Server Processing of Outbound Subscription Request  .  32
       3.1.3.  Server Processing of Inbound Subscription Request . .  34
       3.1.4.  Client Processing of Inbound Subscription Request . .  35
       3.1.5.  Server Processing of Outbound Subscription Approval .  36
       3.1.6.  Server Processing of Inbound Subscription Approval  .  38

     3.2.  Canceling a Subscription  . . . . . . . . . . . . . . . .  40
       3.2.1.  Client Generation of Subscription Cancellation  . . .  40
       3.2.2.  Server Processing of Outbound Subscription
               Cancellation  . . . . . . . . . . . . . . . . . . . .  40
       3.2.3.  Server Processing of Inbound Subscription
               Cancellation  . . . . . . . . . . . . . . . . . . . .  41
     3.3.  Unsubscribing . . . . . . . . . . . . . . . . . . . . . .  43
       3.3.1.  Client Generation of Unsubscribe  . . . . . . . . . .  43
       3.3.2.  Server Processing of Outbound Unsubscribe . . . . . .  43
       3.3.3.  Server Processing of Inbound Unsubscribe  . . . . . .  44
     3.4.  Pre-Approving a Subscription Request  . . . . . . . . . .  46
       3.4.1.  Client Generation of Subscription Pre-Approval  . . .  46
       3.4.2.  Server Processing of Subscription Pre-Approval  . . .  47
   4.  Exchanging Presence Information . . . . . . . . . . . . . . .  48
     4.1.  Presence Fundamentals . . . . . . . . . . . . . . . . . .  48
     4.2.  Initial Presence  . . . . . . . . . . . . . . . . . . . .  49
       4.2.1.  Client Generation of Initial Presence . . . . . . . .  49
       4.2.2.  Server Processing of Outbound Initial Presence  . . .  50
       4.2.3.  Server Processing of Inbound Initial Presence . . . .  50
       4.2.4.  Client Processing of Initial Presence . . . . . . . .  51
     4.3.  Presence Probes . . . . . . . . . . . . . . . . . . . . .  51
       4.3.1.  Server Generation of Outbound Presence Probe  . . . .  52
       4.3.2.  Server Processing of Inbound Presence Probe . . . . .  53
         4.3.2.1.  Handling of the 'id' Attribute  . . . . . . . . .  55
     4.4.  Subsequent Presence Broadcast . . . . . . . . . . . . . .  57
       4.4.1.  Client Generation of Subsequent Presence Broadcast  .  57
       4.4.2.  Server Processing of Subsequent Outbound Presence . .  57
       4.4.3.  Server Processing of Subsequent Inbound Presence  . .  58
       4.4.4.  Client Processing of Subsequent Presence  . . . . . .  59
     4.5.  Unavailable Presence  . . . . . . . . . . . . . . . . . .  59
       4.5.1.  Client Generation of Unavailable Presence . . . . . .  59
       4.5.2.  Server Processing of Outbound Unavailable Presence  .  59
       4.5.3.  Server Processing of Inbound Unavailable Presence . .  61
       4.5.4.  Client Processing of Unavailable Presence . . . . . .  62
     4.6.  Directed Presence . . . . . . . . . . . . . . . . . . . .  62
       4.6.1.  General Considerations  . . . . . . . . . . . . . . .  62
       4.6.2.  Client Generation of Directed Presence  . . . . . . .  63
       4.6.3.  Server Processing of Outbound Directed Presence . . .  63
       4.6.4.  Server Processing of Inbound Directed Presence  . . .  64
       4.6.5.  Client Processing of Inbound Directed Presence  . . .  64
       4.6.6.  Server Processing of Presence Probes  . . . . . . . .  64
     4.7.  Presence Syntax . . . . . . . . . . . . . . . . . . . . .  65
       4.7.1.  Type Attribute  . . . . . . . . . . . . . . . . . . .  65
       4.7.2.  Child Elements  . . . . . . . . . . . . . . . . . . .  66
         4.7.2.1.  Show Element  . . . . . . . . . . . . . . . . . .  66
         4.7.2.2.  Status Element  . . . . . . . . . . . . . . . . .  67
         4.7.2.3.  Priority Element  . . . . . . . . . . . . . . . .  68
       4.7.3.  Extended Content  . . . . . . . . . . . . . . . . . .  69

   5.  Exchanging Messages . . . . . . . . . . . . . . . . . . . . .  69
     5.1.  One-to-One Chat Sessions  . . . . . . . . . . . . . . . .  69
     5.2.  Message Syntax  . . . . . . . . . . . . . . . . . . . . .  70
       5.2.1.  To Attribute  . . . . . . . . . . . . . . . . . . . .  70
       5.2.2.  Type Attribute  . . . . . . . . . . . . . . . . . . .  71
       5.2.3.  Body Element  . . . . . . . . . . . . . . . . . . . .  73
       5.2.4.  Subject Element . . . . . . . . . . . . . . . . . . .  74
       5.2.5.  Thread Element  . . . . . . . . . . . . . . . . . . .  75
     5.3.  Extended Content  . . . . . . . . . . . . . . . . . . . .  77
   6.  Exchanging IQ Stanzas . . . . . . . . . . . . . . . . . . . .  77
   7.  A Sample Session  . . . . . . . . . . . . . . . . . . . . . .  78
   8.  Server Rules for Processing XML Stanzas . . . . . . . . . . .  84
     8.1.  General Considerations  . . . . . . . . . . . . . . . . .  85
     8.2.  No 'to' Address . . . . . . . . . . . . . . . . . . . . .  85
     8.3.  Remote Domain . . . . . . . . . . . . . . . . . . . . . .  85
     8.4.  Local Domain  . . . . . . . . . . . . . . . . . . . . . .  86
     8.5.  Local User  . . . . . . . . . . . . . . . . . . . . . . .  86
       8.5.1.  No Such User  . . . . . . . . . . . . . . . . . . . .  86
       8.5.2.  localpart@domainpart  . . . . . . . . . . . . . . . .  86
         8.5.2.1.  Available or Connected Resources  . . . . . . . .  87
         8.5.2.2.  No Available or Connected Resources . . . . . . .  89
       8.5.3.  localpart@domainpart/resourcepart . . . . . . . . . .  90
         8.5.3.1.  Resource Matches  . . . . . . . . . . . . . . . .  90
         8.5.3.2.  No Resource Matches . . . . . . . . . . . . . . .  90
       8.5.4.  Summary of Message Delivery Rules . . . . . . . . . .  92
   9.  Handling of URIs  . . . . . . . . . . . . . . . . . . . . . .  93
   10. Internationalization Considerations . . . . . . . . . . . . .  94
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  94
   12. Conformance Requirements  . . . . . . . . . . . . . . . . . .  95
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  99
     13.1. Normative References  . . . . . . . . . . . . . . . . . .  99
     13.2. Informative References  . . . . . . . . . . . . . . . . .  99
   Appendix A.  Subscription States  . . . . . . . . . . . . . . . . 103
     A.1.  Defined States  . . . . . . . . . . . . . . . . . . . . . 103
     A.2.  Server Processing of Outbound Presence Subscription
           Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 104
       A.2.1.  Subscribe . . . . . . . . . . . . . . . . . . . . . . 105
       A.2.2.  Unsubscribe . . . . . . . . . . . . . . . . . . . . . 105
       A.2.3.  Subscribed  . . . . . . . . . . . . . . . . . . . . . 106
       A.2.4.  Unsubscribed  . . . . . . . . . . . . . . . . . . . . 106
     A.3.  Server Processing of Inbound Presence Subscription
           Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 106
       A.3.1.  Subscribe . . . . . . . . . . . . . . . . . . . . . . 107
       A.3.2.  Unsubscribe . . . . . . . . . . . . . . . . . . . . . 107
       A.3.3.  Subscribed  . . . . . . . . . . . . . . . . . . . . . 108
       A.3.4.  Unsubscribed  . . . . . . . . . . . . . . . . . . . . 109
   Appendix B.  Blocking Communication . . . . . . . . . . . . . . . 110
   Appendix C.  vCards . . . . . . . . . . . . . . . . . . . . . . . 110

   Appendix D.  XML Schema for jabber:iq:roster  . . . . . . . . . . 110
   Appendix E.  Differences From RFC 3921  . . . . . . . . . . . . . 112
   Appendix F.  Acknowledgements . . . . . . . . . . . . . . . . . . 113

1. Introduction

1.1. Overview

   The Extensible Messaging and Presence Protocol (XMPP) is an
   application profile of the Extensible Markup Language [XML] that
   enables the near-real-time exchange of structured yet extensible data
   between any two or more network entities.  The core features of XMPP
   defined in [XMPP-CORE] provide the building blocks for many types of
   near-real-time applications, which can be layered on top of the core
   by sending application-specific data qualified by particular XML
   namespaces (refer to [XML-NAMES]).  This document defines XMPP
   extensions that provide the basic functionality expected of an
   instant messaging (IM) and presence application as described in
   [IMP-REQS].

1.2. History

   The basic syntax and semantics of XMPP were developed originally
   within the Jabber open-source community, mainly in 1999.  In late
   2002, the XMPP Working Group was chartered with developing an
   adaptation of the core Jabber protocol that would be suitable as an
   IETF IM and presence technology in accordance with [IMP-REQS].  In
   October 2004, [RFC3920] and [RFC3921] were published, representing
   the most complete definition of XMPP at that time.

   Since 2004 the Internet community has gained extensive implementation
   and deployment experience with XMPP, including formal
   interoperability testing carried out under the auspices of the XMPP
   Standards Foundation (XSF).  This document incorporates comprehensive
   feedback from software developers and service providers, including a
   number of backward-compatible modifications summarized under
   Appendix E.  As a result, this document reflects the rough consensus
   of the Internet community regarding the IM and presence features of
   XMPP 1.0, thus obsoleting RFC 3921.

1.3. Requirements

   Traditionally, IM applications have combined the following factors:

   1.  The central point of focus is a list of one's contacts or
       "buddies" (in XMPP this list is called a "roster").

   2.  The purpose of using such an application is to exchange
       relatively brief text messages with particular contacts in close
       to real time -- often relatively large numbers of such messages
       in rapid succession, in the form of a one-to-one "chat session"
       as described under Section 5.1.

   3.  The catalyst for exchanging messages is "presence" -- i.e.,
       information about the network availability of particular contacts
       (thus knowing who is online and available for a one-to-one chat
       session).

   4.  Presence information is provided only to contacts that one has
       authorized by means of an explicit agreement called a "presence
       subscription".

   Thus at a high level this document assumes that a user needs to be
   able to complete the following use cases:

   o  Manage items in one's contact list

   o  Exchange messages with one's contacts

   o  Exchange presence information with one's contacts

   o  Manage presence subscriptions to and from one's contacts

   Detailed definitions of these functionality areas are contained in
   RFC 2779 [IMP-REQS], and the interested reader is referred to that
   document regarding in-depth requirements.  Although the XMPP IM and
   presence extensions specified herein meet the requirements of RFC
   2779, they were not designed explicitly with that specification in
   mind, since the base protocol evolved through an open development
   process within the Jabber open-source community before RFC 2779 was
   written.  Although XMPP protocol extensions addressing many other
   functionality areas have been defined in the XMPP Standards
   Foundation's XEP series (e.g., multi-user text chat as specified in
   [XEP-0045]), such extensions are not specified in this document
   because they are not mandated by RFC 2779.

      Implementation Note: RFC 2779 stipulates that presence services
      must be separable from IM services and vice-versa; i.e., it must
      be possible to use the protocol to provide a presence service, a
      messaging service, or both.  Although the text of this document
      assumes that implementations and deployments will want to offer a
      unified IM and presence service, it is not mandatory for an XMPP
      service to offer both a presence service and a messaging service,
      and the protocol makes it possible to offer separate and distinct

services for presence and for messaging. (For example, a
 presence-only service could return a <service-unavailable/> stanza
 error if a client attempts to send a <message/> stanza.)

1.4. Functional Summary

   This non-normative section provides a developer-friendly, functional
   summary of XMPP-based IM and presence features; consult the sections
   that follow for a normative definition of these features.

   [XMPP-CORE] specifies how an XMPP client connects to an XMPP server.
   In particular, it specifies the preconditions that need to be
   fulfilled before a client is allowed to send XML stanzas (the basic
   unit of meaning in XMPP) to other entities on an XMPP network.  These
   preconditions comprise negotiation of the XML stream and include
   exchange of XML stream headers, optional channel encryption via
   Transport Layer Security [TLS], mandatory authentication via Simple
   Authentication and Security Layer [SASL], and binding of a resource
   to the stream for client addressing.  The reader is referred to
   [XMPP-CORE] for details regarding these preconditions, and knowledge
   of [XMPP-CORE] is assumed herein.

      Interoperability Note: [RFC3921] specified one additional
      precondition: formal establishment of an instant messaging and
      presence session.  Implementation and deployment experience has
      shown that this additional step is unnecessary.  However, for
      backward compatibility an implementation MAY still offer that
      feature.  This enables older software to connect while letting
      newer software save a round trip.

   Upon fulfillment of the preconditions specified in [XMPP-CORE], an
   XMPP client has a long-lived XML stream with an XMPP server, which
   enables the user controlling that client to send and receive a
   potentially unlimited number of XML stanzas over the stream.  Such a
   stream can be used to exchange messages, share presence information,
   and engage in structured request-response interactions in close to
   real time.  After negotiation of the XML stream, the typical flow for
   an instant messaging and presence session is as follows:

   1.  Retrieve one's roster.  (See Section 2.2.)

   2.  Send initial presence to the server for broadcast to all
       subscribed contacts, thus "going online" from the perspective of
       XMPP communication.  (See Section 4.2.)

   3.  Exchange messages, manage presence subscriptions, perform roster
       updates, and in general process and generate other XML stanzas
       with particular semantics throughout the life of the session.
       (See Sections 5, 3, 2, and 6.)

   4.  Terminate the session when desired by sending unavailable
       presence and closing the underlying XML stream.  (See
       Section 4.5.)

1.5. Terminology

   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].

   This document inherits the terminology defined in [XMPP-CORE].

   The terms "automated client" and "interactive client" are to be
   understood in the sense defined in [TLS-CERTS].

   For convenience, this document employs the term "user" to refer to
   the owner of an XMPP account; however, account owners need not be
   humans and can be bots, devices, or other automated applications.

   Several other terms, such as "interested resource", are defined
   within the body of this document.

   Following the "XML Notation" used in [IRI] to represent characters
   that cannot be rendered in ASCII-only documents, some examples in
   this document use the form "&#x...." as a notational device to
   represent [UNICODE] characters (e.g., the string "ř" stands
   for the Unicode character LATIN SMALL LETTER R WITH CARON); this form
   is definitely not to be sent over the wire in XMPP systems.

   In examples, lines have been wrapped for improved readability,
   "[...]" means elision, and the following prepended strings are used
   (these prepended strings are not to be sent over the wire):

   o  C: = client

   o  CC: = contact's client

   o  CS: = contact's server

   o  S: = server

   o  UC: = user's client

   o  US: = user's server

   Readers need to be aware that the examples are not exhaustive and
   that, in examples for some protocol flows, the alternate steps shown
   would not necessarily be triggered by the exact data sent in the
   previous step; in all cases, the protocol definitions specified in
   this document or in normatively referenced documents rule over any
   examples provided here.  All examples are fictional and the
   information exchanged (e.g., usernames and passwords) does not
   represent any existing users or servers.

2. Managing the Roster

   In XMPP, a user's roster contains any number of specific contacts.  A
   user's roster is stored by the user's server on the user's behalf so
   that the user can access roster information from any device.  When
   the user adds items to the roster or modifies existing items, if an
   error does not occur then the server SHOULD store that data
   unmodified if at all possible and MUST return the data it has stored
   when an authorized client requests the roster.

      Security Warning: Because the user's roster can contain
      confidential data, the server MUST restrict access to this data so
      that only authorized entities (typically limited to the account
      owner) are able to retrieve, modify, or delete it.

   RFC 3921 assumed that the only place where a user stores their roster
   is the server where the user's account is registered and at which the
   user authenticates for access to the XMPP network.  This
   specification removes that strict coupling of roster storage to
   account registration and network authentication, with the result that
   a user could store their roster at another location, or could have
   multiple rosters that are stored in multiple locations.  However, in
   the absence of implementation and deployment experience with a more
   flexible roster storage model, this specification retains the
   terminology of RFC 3921 by using the terms "client" and "server" (and
   "the roster" instead of "a roster"), rather than coining a new term
   for "a place where a user stores a roster".  Future documents might
   provide normative rules for non-server roster storage or for the
   management of multiple rosters, but such rules are out of scope for
   this document.

2.1. Syntax and Semantics

Rosters are managed using <iq/> stanzas (see Section 8.2.3 of
 [XMPP-CORE]), specifically by means of a <query/> child element
 qualified by the 'jabber:iq:roster' namespace. The detailed syntax
 and semantics are defined in the following sections.

2.1.1. Ver Attribute

The 'ver' attribute is a string that identifies a particular version
 of the roster information. The value MUST be generated only by the
 server and MUST be treated by the client as opaque. The server can
 use any appropriate method for generating the version ID, such as a
 hash of the roster data or a strictly increasing sequence number.

 Inclusion of the 'ver' attribute is RECOMMENDED.

 Use of the 'ver' attribute is described more fully under Section 2.6.

 Interoperability Note: The 'ver' attribute of the <query/> element
 was not defined in RFC 3921 and is newly defined in this
 specification.

2.1.2. Roster Items

The <query/> element inside a roster set (Section 2.1.5) contains one
 <item/> child, and a roster result (Section 2.1.4) typically contains
 multiple <item/> children. Each <item/> element describes a unique
 "roster item" (sometimes also called a "contact").

 The syntax of the <item/> element is described in the following
 sections.

2.1.2.1. Approved Attribute

The boolean 'approved' attribute with a value of "true" is used to
 signal subscription pre-approval as described under Section 3.4 (the
 default is "false", in accordance with [XML-DATATYPES]).

 A server SHOULD include the 'approved' attribute to inform the client
 of subscription pre-approvals. A client MUST NOT include the
 'approved' attribute in the roster sets it sends to the server, but
 instead MUST use presence stanzas of type "subscribed" and
 "unsubscribed" to manage pre-approvals as described under
 Section 3.4.

 Interoperability Note: The 'approved' attribute of the <item/>
 element was not defined in RFC 3921 and is newly defined in this
 specification.

2.1.2.2. Ask Attribute

The 'ask' attribute of the <item/> element with a value of
 "subscribe" is used to signal various subscription sub-states that
 include a "Pending Out" aspect as described under Section 3.1.2.

   A server SHOULD include the 'ask' attribute to inform the client of
   "Pending Out" sub-states.  A client MUST NOT include the 'ask'
   attribute in the roster sets it sends to the server, but instead MUST
   use presence stanzas of type "subscribe" and "unsubscribe" to manage
   such sub-states as described under Section 3.1.2.

2.1.2.3. JID Attribute

The 'jid' attribute of the <item/> element specifies the Jabber
 Identifier (JID) that uniquely identifies the roster item.

 The 'jid' attribute is REQUIRED whenever a client or server adds,
 updates, deletes, or returns a roster item.

2.1.2.4. Name Attribute

The 'name' attribute of the <item/> element specifies the "handle" to
 be associated with the JID, as determined by the user (not the
 contact). Although the value of the 'name' attribute MAY have
 meaning to a human user, it is opaque to the server. However, the
 'name' attribute MAY be used by the server for matching purposes
 within the context of various XMPP extensions (one possible
 comparison method is that described for XMPP resourceparts in
 [XMPP-ADDR]).

 It is OPTIONAL for a client to include the 'name' attribute when
 adding or updating a roster item.

2.1.2.5. Subscription Attribute

The state of the presence subscription is captured in the
 'subscription' attribute of the <item/> element. The defined
 subscription-related values are:

 none: the user does not have a subscription to the contact's
 presence, and the contact does not have a subscription to the
 user's presence; this is the default value, so if the subscription
 attribute is not included then the state is to be understood as
 "none"

 to: the user has a subscription to the contact's presence, but the
 contact does not have a subscription to the user's presence

 from: the contact has a subscription to the user's presence, but the
 user does not have a subscription to the contact's presence

 both: the user and the contact have subscriptions to each other's
 presence (also called a "mutual subscription")

   In a roster result (Section 2.1.4), the client MUST ignore values of
   the 'subscription' attribute other than "none", "to", "from", or
   "both".

   In a roster push (Section 2.1.6), the client MUST ignore values of
   the 'subscription' attribute other than "none", "to", "from", "both",
   or "remove".

   In a roster set (Section 2.1.5), the 'subscription' attribute MAY be
   included with a value of "remove", which indicates that the item is
   to be removed from the roster; in a roster set the server MUST ignore
   all values of the 'subscription' attribute other than "remove".

   Inclusion of the 'subscription' attribute is OPTIONAL.

2.1.2.6. Group Element

The <group/> child element specifies a category or "bucket" into
 which the roster item is to be grouped by a client. An <item/>
 element MAY contain more than one <group/> element, which means that
 roster groups are not exclusive. Although the XML character data of
 the <group/> element MAY have meaning to a human user, it is opaque
 to the server. However, the <group/> element MAY be used by the
 server for matching purposes within the context of various XMPP
 extensions (one possible comparison method is that described for XMPP
 resourceparts in [XMPP-ADDR]).

 It is OPTIONAL for a client to include the <group/> element when
 adding or updating a roster item. If a roster set (Section 2.1.5)
 includes no <group/> element, then the item is to be interpreted as
 being affiliated with no group.

2.1.3. Roster Get

A "roster get" is a client's request for the server to return the
 roster; syntactically it is an IQ stanza of type "get" sent from
 client to server and containing a <query/> element qualified by the
 'jabber:iq:roster' namespace, where the <query/> element MUST NOT
 contain any <item/> child elements.

C: <iq from='juliet@example.com/balcony'
 id='bv1bs71f'
 type='get'>
 <query xmlns='jabber:iq:roster'/>
 </iq>

The expected outcome of sending a roster get is for the server to
return a roster result.

2.1.4. Roster Result

A "roster result" is the server's response to a roster get;
 syntactically it is an IQ stanza of type "result" sent from server to
 client and containing a <query/> element qualified by the 'jabber:iq:
 roster' namespace.

 The <query/> element in a roster result contains one <item/> element
 for each contact and therefore can contain more than one <item/>
 element.

 S: <iq id='bv1bs71f'
 to='juliet@example.com/chamber'
 type='result'>
 <query xmlns='jabber:iq:roster' ver='ver7'>
 <item jid='nurse@example.com'/>
 <item jid='romeo@example.net'/>
 </query>
 </iq>

 If the roster exists but there are no contacts in the roster, then
 the server MUST return an IQ-result containing a child <query/>
 element that in turn contains no <item/> children (i.e., the server
 MUST NOT return an empty <iq/> stanza of type "error").

 S: <iq id='bv1bs71f'
 to='juliet@example.com/chamber'
 type='result'>
 <query xmlns='jabber:iq:roster' ver='ver9'/>
 </iq>

 If the roster does not exist, then the server MUST return a stanza
 error with a condition of <item-not-found/>.

 S: <iq id='bv1bs71f'
 to='juliet@example.com/chamber'
 type='error'>
 <error type='cancel'>
 <item-not-found
 xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
 </error>
 </iq>

2.1.5. Roster Set

A "roster set" is a client's request for the server to modify (i.e.,
 create, update, or delete) a roster item; syntactically it is an IQ
 stanza of type "set" sent from client to server and containing a
 <query/> element qualified by the 'jabber:iq:roster' namespace.

 The following rules apply to roster sets:

 1. The <query/> element MUST contain one and only one <item/>
 element.

 2. The server MUST ignore any value of the 'subscription' attribute
 other than "remove" (see Section 2.1.2.5).

 Security Warning: Traditionally, the IQ stanza of the roster set
 included no 'to' address, with the result that all roster sets
 were sent from an authenticated resource (full JID) of the account
 whose roster was being updated. Furthermore, RFC 3921 required a
 server to perform special-case checking of roster sets to ignore
 the 'to' address; however, this specification has removed that
 special-casing, which means that a roster set might include a 'to'
 address other than that of the sender. Therefore, the entity that
 processes a roster set MUST verify that the sender of the roster
 set is authorized to update the roster, and if not return a
 <forbidden/> error.

 C: <iq from='juliet@example.com/balcony'
 id='rs1'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='nurse@example.com'/>
 </query>
 </iq>

2.1.6. Roster Push

A "roster push" is a newly created, updated, or deleted roster item
 that is sent from the server to the client; syntactically it is an IQ
 stanza of type "set" sent from server to client and containing a
 <query/> element qualified by the 'jabber:iq:roster' namespace.

 The following rules apply to roster pushes:

 1. The <query/> element in a roster push MUST contain one and only
 one <item/> element.

2. A receiving client MUST ignore the stanza unless it has no 'from'
 attribute (i.e., implicitly from the bare JID of the user's
 account) or it has a 'from' attribute whose value matches the
 user's bare JID <user@domainpart>.

 S: <iq id='a78b4q6ha463'
 to='juliet@example.com/chamber'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='nurse@example.com'/>
 </query>
 </iq>

 As mandated by the semantics of the IQ stanza as defined in
 [XMPP-CORE], each resource that receives a roster push from the
 server is supposed to reply with an IQ stanza of type "result" or
 "error" (however, it is known that many existing clients do not reply
 to roster pushes).

 C: <iq from='juliet@example.com/balcony'
 id='a78b4q6ha463'
 type='result'/>

 C: <iq from='juliet@example.com/chamber'
 id='a78b4q6ha463'
 type='result'/>

 Security Warning: Traditionally, a roster push included no 'from'
 address, with the result that all roster pushes were sent
 implicitly from the bare JID of the account itself. However, this
 specification allows entities other than the user's server to
 maintain roster information, which means that a roster push might
 include a 'from' address other than the bare JID of the user's
 account. Therefore, the client MUST check the 'from' address to
 verify that the sender of the roster push is authorized to update
 the roster. If the client receives a roster push from an
 unauthorized entity, it MUST NOT process the pushed data; in
 addition, the client can either return a stanza error of <service-
 unavailable/> error or refuse to return a stanza error at all (the
 latter behavior overrides a MUST-level requirement from
 [XMPP-CORE] for the purpose of preventing a presence leak).

 Implementation Note: There is no error case for client processing
 of roster pushes; if the server receives an IQ of type "error" in
 response to a roster push then it SHOULD ignore the error.

2.2. Retrieving the Roster on Login

Upon authenticating with a server and binding a resource (thus
 becoming a connected resource as defined in [XMPP-CORE]), a client
 SHOULD request the roster before sending initial presence (however,
 because receiving the roster is not necessarily desirable for all
 resources, e.g., a connection with limited bandwidth, the client's
 request for the roster is not mandatory). After a connected resource
 sends initial presence (see Section 4.2), it is referred to as an
 "available resource". If a connected resource or available resource
 requests the roster, it is referred to as an "interested resource".
 The server MUST send roster pushes to all interested resources.

 Implementation Note: Presence subscription requests are sent to
 available resources, whereas the roster pushes associated with
 subscription state changes are sent to interested resources.
 Therefore, if a resource wishes to receive both subscription
 requests and roster pushes, it MUST both send initial presence and
 request the roster.

 A client requests the roster by sending a roster get over its stream
 with the server.

 C: <iq from='juliet@example.com/balcony'
 id='hu2bac18'
 type='get'>
 <query xmlns='jabber:iq:roster'/>
 </iq>

 S: <iq id='hu2bac18'
 to='juliet@example.com/balcony'
 type='result'>
 <query xmlns='jabber:iq:roster' ver='ver11'>
 <item jid='romeo@example.net'
 name='Romeo'
 subscription='both'>
 <group>Friends</group>
 </item>
 <item jid='mercutio@example.com'
 name='Mercutio'
 subscription='from'/>
 <item jid='benvolio@example.net'
 name='Benvolio'
 subscription='both'/>
 </query>
 </iq>

If the server cannot process the roster get, it MUST return an
 appropriate stanza error as described in [XMPP-CORE] (such as
 <service-unavailable/> if the roster namespace is not supported or
 <internal-server-error/> if the server experiences trouble processing
 or returning the roster).

2.3. Adding a Roster Item

2.3.1. Request

At any time, a client can add an item to the roster. This is done by
 sending a roster set containing a new item.

 C: <iq from='juliet@example.com/balcony'
 id='ph1xaz53'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='nurse@example.com'
 name='Nurse'>
 <group>Servants</group>
 </item>
 </query>
 </iq>

2.3.2. Success Case

   If the server can successfully process the roster set for the new
   item (i.e., if no error occurs), it MUST create the item in the
   user's roster and proceed as follows.

   The server MUST return an IQ stanza of type "result" to the connected
   resource that sent the roster set.

S: <iq id='ph1xaz53'
 to='juliet@example.com/balcony'
 type='result'/>

   The server MUST also send a roster push containing the new roster
   item to all of the user's interested resources, including the
   resource that generated the roster set.

S: <iq to='juliet@example.com/balcony'
 id='a78b4q6ha463'
 type='set'>
 <query xmlns='jabber:iq:roster' ver='ver13'>
 <item jid='nurse@example.com'
 name='Nurse'
 subscription='none'>
 <group>Servants</group>
 </item>
 </query>
 </iq>

 S: <iq to='juliet@example.com/chamber'
 id='x81g3bdy4n19'
 type='set'>
 <query xmlns='jabber:iq:roster' ver='ver13'>
 <item jid='nurse@example.com'
 name='Nurse'
 subscription='none'>
 <group>Servants</group>
 </item>
 </query>
 </iq>

 As mandated by the semantics of the IQ stanza as defined in
 [XMPP-CORE], each resource that receives a roster push from the
 server is supposed to reply with an IQ stanza of type "result" or
 "error" (however, it is known that many existing clients do not reply
 to roster pushes).

 C: <iq from='juliet@example.com/balcony'
 id='a78b4q6ha463'
 type='result'/>

 C: <iq from='juliet@example.com/chamber'
 id='x81g3bdy4n19'
 type='result'/>

2.3.3. Error Cases

If the server cannot successfully process the roster set, it MUST
 return a stanza error. The following error cases are defined.
 Naturally, other stanza errors can occur, such as <internal-server-
 error/> if the server experiences an internal problem with processing
 the roster get, or even <not-allowed/> if the server only allows
 roster modifications by means of a non-XMPP method such as a web
 interface.

The server MUST return a <forbidden/> stanza error to the client if
 the sender of the roster set is not authorized to update the roster
 (where typically only an authenticated resource of the account itself
 is authorized).

 The server MUST return a <bad-request/> stanza error to the client if
 the roster set contains any of the following violations:

 1. The <query/> element contains more than one <item/> child
 element.

 2. The <item/> element contains more than one <group/> element, but
 there are duplicate groups (one possible comparison method for
 determining duplicates is that described for XMPP resourceparts
 in [XMPP-ADDR]).

 The server MUST return a <not-acceptable/> stanza error to the client
 if the roster set contains any of the following violations:

 1. The length of the 'name' attribute is greater than a server-
 configured limit.

 2. The XML character data of the <group/> element is of zero length
 (to remove an item from all groups, the client instead needs to
 exclude any <group/> element from the roster set).

3. The XML character data of the <group/> element is larger than a
server-configured limit.

Error: Roster set initiated by unauthorized entity

 C: <iq from='juliet@example.com/balcony'
 id='ix7s53v2'
 to='romeo@example.net'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='nurse@example.com'/>
 </query>
 </iq>

 S: <iq id='ix7s53v2'
 to='juliet@example.com/balcony'
 type='error'>
 <error type='auth'>
 <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
 </error>
 </iq>

Error: Roster set contains more than one item

 C: <iq from='juliet@example.com/balcony'
 id='nw83vcj4'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='nurse@example.com'
 name='Nurse'>
 <group>Servants</group>
 </item>
 <item jid='mother@example.com'
 name='Mom'>
 <group>Family</group>
 </item>
 </query>
 </iq>

 S: <iq id='nw83vcj4'
 to='juliet@example.com/balcony'
 type='error'>
 <error type='modify'>
 <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
 </error>
 </iq>

 Error: Roster set contains item with oversized handle

 C: <iq from='juliet@example.com/balcony'
 id='yl491b3d'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='nurse@example.com'
 name='[ ... some-very-long-handle ... ]'>
 <group>Servants</group>
 </item>
 </query>
 </iq>

 S: <iq id='yl491b3d'
 to='juliet@example.com/balcony'
 type='error'>
 <error type='modify'>
 <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
 </error>
 </iq>

 Error: Roster set contains duplicate groups

C: <iq from='juliet@example.com/balcony'
 id='tk3va749'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='nurse@example.com'
 name='Nurse'>
 <group>Servants</group>
 <group>Servants</group>
 </item>
 </query>
 </iq>

 S: <iq id='tk3va749'
 to='juliet@example.com/balcony'
 type='error'>
 <error type='modify'>
 <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
 </error>
 </iq>

 Error: Roster set contains empty group

 C: <iq from='juliet@example.com/balcony'
 id='fl3b486u'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='nurse@example.com'
 name='Nurse'>
 <group></group>
 </item>
 </query>
 </iq>

 S: <iq id='fl3b486u'
 to='juliet@example.com/balcony'
 type='error'>
 <error type='modify'>
 <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
 </error>
 </iq>

 Error: Roster set contains oversized group name

C: <iq from='juliet@example.com/balcony'
 id='qh3b4v19'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='nurse@example.com'
 name='Nurse'>
 <group>[ ... some-very-long-group-name ... ]</group>
 </item>
 </query>
 </iq>

 S: <iq id='qh3b4v19'
 to='juliet@example.com/balcony'
 type='error'>
 <error type='modify'>
 <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
 </error>
 </iq>

 Interoperability Note: Some servers return a <not-allowed/> stanza
 error to the client if the value of the <item/> element's 'jid'
 attribute matches the bare JID <localpart@domainpart> of the
 user's account.

2.4. Updating a Roster Item

2.4.1. Request

   Updating an existing roster item is done in the same way as adding a
   new roster item, i.e., by sending a roster set to the server.
   Because a roster item is atomic, the item MUST be updated exactly as
   provided in the roster set.

   There are several reasons why a client might update a roster item:

   1.  Adding a group

   2.  Deleting a group

   3.  Changing the handle

   4.  Deleting the handle

Consider a roster item that is defined as follows:

 <item jid='romeo@example.net'
 name='Romeo'>
 <group>Friends</group>
 </item>

 The user who has this item in her roster might want to add the item
 to another group.

 C: <iq from='juliet@example.com/balcony'
 id='di43b2x9'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'
 name='Romeo'>
 <group>Friends</group>
 <group>Lovers</group>
 </item>
 </query>
 </iq>

 Sometime later, the user might want to remove the item from the
 original group.

 C: <iq from='juliet@example.com/balcony'
 id='lf72v157'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'
 name='Romeo'>
 <group>Lovers</group>
 </item>
 </query>
 </iq>

 The user might want to remove the item from all groups.

 C: <iq from='juliet@example.com/balcony'
 id='ju4b62a5'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'/>
 </query>
 </iq>

 The user might also want to change the handle for the item.

C: <iq from='juliet@example.com/balcony'
 id='gb3sv487'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'
 name='MyRomeo'/>
 </query>
 </iq>

 The user might then want to remove the handle altogether.

 C: <iq from='juliet@example.com/balcony'
 id='o3bx66s5'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'
 name=''/>
 </query>
 </iq>

 Implementation Note: Including an empty 'name' attribute is
 equivalent to including no 'name' attribute; both actions set the
 name to the empty string.

2.4.2. Success Case

   As with adding a roster item, if the roster item can be successfully
   processed then the server MUST update the item in the user's roster,
   send a roster push to all of the user's interested resources, and
   send an IQ result to the initiating resource; details are provided
   under Section 2.3.

2.4.3. Error Cases

The error cases described under Section 2.3.3 also apply to updating
a roster item.

2.5. Deleting a Roster Item

2.5.1. Request

   At any time, a client can delete an item from his or her roster by
   sending a roster set and specifying a value of "remove" for the
   'subscription' attribute.

C: <iq from='juliet@example.com/balcony'
 id='hm4hs97y'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='nurse@example.com'
 subscription='remove'/>
 </query>
 </iq>

2.5.2. Success Case

As with adding a roster item, if the server can successfully process
 the roster set then it MUST update the item in the user's roster,
 send a roster push to all of the user's interested resources (with
 the 'subscription' attribute set to a value of "remove"), and send an
 IQ result to the initiating resource; details are provided under
 Section 2.3.

 In addition, the user's server might need to generate one or more
 subscription-related presence stanzas, as follows:

 1. If the user has a presence subscription to the contact, then the
 user's server MUST send a presence stanza of type "unsubscribe"
 to the contact (in order to unsubscribe from the contact's
 presence).

 2. If the contact has a presence subscription to the user, then the
 user's server MUST send a presence stanza of type "unsubscribed"
 to the contact (in order to cancel the contact's subscription to
 the user).

 3. If the presence subscription is mutual, then the user's server
 MUST send both a presence stanza of type "unsubscribe" and a
 presence stanza of type "unsubscribed" to the contact.

 S: <presence from='juliet@example.com'
 id='lm3ba81g'
 to='nurse@example.com'
 type='unsubscribe'/>

 S: <presence from='juliet@example.com'
 id='xb2c1v4k'
 to='nurse@example.com'
 type='unsubscribed'/>

2.5.3. Error Cases

If the value of the 'jid' attribute specifies an item that is not in
 the roster, then the server MUST return an <item-not-found/> stanza
 error.

 Error: Roster item not found

 C: <iq from='juliet@example.com/balcony'
 id='uj4b1ca8'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='[ ... non-existent-jid ... ]'
 subscription='remove'/>
 </query>
 </iq>

 S: <iq id='uj4b1ca8'
 to='juliet@example.com/balcony'
 type='error'>
 <error type='modify'>
 <item-not-found
 xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
 </error>
 </iq>

2.6. Roster Versioning

2.6.1. Stream Feature

If a server supports roster versioning, then it MUST advertise the
 following stream feature during stream negotiation.

 <ver xmlns='urn:xmpp:features:rosterver'/>

 The roster versioning stream feature is merely informative and
 therefore is never mandatory-to-negotiate.

2.6.2. Request

   If a client supports roster versioning and the server to which it has
   connected advertises support for roster versioning as described in
   the foregoing section, then the client SHOULD include the 'ver'
   element in its request for the roster.  If the server does not
   advertise support for roster versioning, the client MUST NOT include
   the 'ver' attribute.  If the client includes the 'ver' attribute in
   its roster get, it sets the attribute's value to the version ID
   associated with its last cache of the roster.

C: <iq from='romeo@example.net/home'
 id='r1h3vzp7'
 to='romeo@example.net'
 type='get'>
 <query xmlns='jabber:iq:roster' ver='ver14'/>
 </iq>

 If the client has not yet cached the roster or the cache is lost or
 corrupted, but the client wishes to bootstrap the use of roster
 versioning, it MUST set the 'ver' attribute to the empty string
 (i.e., ver="").

 Naturally, if the client does not support roster versioning or does
 not wish to bootstrap the use of roster versioning, it will not
 include the 'ver' attribute.

2.6.3. Success Case

Whether or not the roster has been modified since the version ID
 enumerated by the client, the server MUST either return the complete
 roster as described under Section 2.1.4 (including a 'ver' attribute
 that signals the latest version) or return an empty IQ-result (thus
 indicating that any roster modifications will be sent via roster
 pushes, as described below). In general, unless returning the
 complete roster would (1) use less bandwidth than sending individual
 roster pushes to the client (e.g., if the roster contains only a few
 items) or (2) the server cannot associate the version ID with any
 previous version it has on file, the server SHOULD send an empty IQ-
 result and then send the modifications (if any) via roster pushes.

 S: <iq from='romeo@example.net'
 id='r1h3vzp7'
 to='romeo@example.net/home'
 type='result'/>

 Implementation Note: This empty IQ-result is different from an
 empty <query/> element, thus disambiguating this usage from an
 empty roster.

 If roster versioning is enabled and the roster has not been modified
 since the version ID enumerated by the client, the server will simply
 not send any roster pushes to the client (until and unless some
 relevant event triggers a roster push during the lifetime of the
 client's session).

If the roster has been modified since the version ID enumerated by
 the client, the server MUST then send one roster push to the client
 for each roster item that has been modified since the version ID
 enumerated by the client. (We call a roster push that is sent for
 purposes of roster version synchronization an "interim roster push".)

 Definition: A "roster modification" is any change to the roster
 data that would result in a roster push to a connected client.
 Therefore, internal states related to roster processing within the
 server that would not result in a roster push to a connected
 client do not necessitate a change to the version.

 S: <iq from='romeo@example.net'
 id='ah382g67'
 to='romeo@example.net/home'
 type='set'>
 <query xmlns='jabber:iq:roster' ver='ver34'>
 <item jid='tybalt@example.org' subscription='remove'/>
 </query>
 </iq>

 S: <iq from='romeo@example.net'
 id='b2gs90j5'
 to='romeo@example.net/home'
 type='set'>
 <query xmlns='jabber:iq:roster' ver='ver42'>
 <item jid='bill@example.org' subscription='both'/>
 </query>
 </iq>

 S: <iq from='romeo@example.net'
 id='c73gs419'
 to='romeo@example.net/home'
 type='set'>
 <query xmlns='jabber:iq:roster' ver='ver72'>
 <item jid='nurse@example.org'
 name='Nurse'
 subscription='to'>
 <group>Servants</group>
 </item>
 </query>
 </iq>

S: <iq from='romeo@example.net'
 id='dh361f35'
 to='romeo@example.net/home'
 type='set'>
 <query xmlns='jabber:iq:roster' ver='ver96'>
 <item jid='juliet@example.org'
 name='Juliet'
 subscription='both'>
 <group>VIPs</group>
 </item>
 </query>
 </iq>

 These "interim roster pushes" can be understood as follows:

 1. Imagine that the client had an active presence session for the
 entire time between its cached roster version (say, "ver14") and
 the new roster version (say, "ver96").

 2. During that time, the client might have received roster pushes
 related to various roster versions (which might have been, say,
 "ver51" and "ver79"). However, some of those roster pushes might
 have contained intermediate updates to the same roster item
 (e.g., modifications to the subscription state for
 bill@example.org from "none" to "to" and from "to" to "both").

 3. The interim roster pushes would not include all of the
 intermediate steps, only the final result of all modifications
 applied to each item while the client was in fact offline (which
 might have been, say, "ver34", "ver42", "ver72", and "ver96").

 The client MUST handle an "interim roster push" in the same way it
 handles any roster push (indeed, from the client's perspective it
 cannot tell the difference between an "interim" roster push and a
 "live" roster push and therefore it has no way of knowing when it has
 received all of the interim roster pushes). When requesting the
 roster after reconnection, the client SHOULD request the version
 associated with the last roster push it received during its previous
 session, not the version associated with the roster result it
 received at the start of its previous session.

 When roster versioning is enabled, the server MUST include the
 updated roster version with each roster push. Roster pushes MUST
 occur in order of modification and the version contained in a roster
 push MUST be unique. Even if the client has not included the 'ver'
 attribute in its roster gets or sets, the server SHOULD include the
 'ver' attribute on all roster pushes and results that it sends to the
 client.

Implementation Note: Guidelines and more detailed examples for
 roster versioning are provided in [XEP-0237].

3. Managing Presence Subscriptions

 In order to protect the privacy of XMPP users, presence information
 is disclosed only to other entities that a user has approved. When a
 user has agreed that another entity is allowed to view its presence,
 the entity is said to have a "subscription" to the user's presence.
 An entity that has a subscription to a user's presence or to which a
 user has a presence subscription is called a "contact" (in this
 document the term "contact" is also used in a less strict sense to
 refer to a potential contact or any item in a user's roster).

 In XMPP, a subscription lasts across presence sessions; indeed, it
 lasts until the contact unsubscribes or the user cancels the
 previously granted subscription. (This model is different from that
 used for presence subscriptions in the Session Initiation Protocol
 (SIP), as defined in [SIP-PRES].)

 Subscriptions are managed within XMPP by sending presence stanzas
 containing specially defined attributes ("subscribe", "unsubscribe",
 "subscribed", and "unsubscribed").

 Implementation Note: When a server processes or generates an
 outbound presence stanza of type "subscribe", "subscribed",
 "unsubscribe", or "unsubscribed", the server MUST stamp the
 outgoing presence stanza with the bare JID <localpart@domainpart>
 of the sending entity, not the full JID
 <localpart@domainpart/resourcepart>. Enforcement of this rule
 simplifies the presence subscription model and helps to prevent
 presence leaks; for information about presence leaks, refer to the
 security considerations of [XMPP-CORE].

 Subscription states are reflected in the rosters of both the user and
 the contact. This section does not cover every possible case related
 to presence subscriptions, and mainly narrates the protocol flows for
 bootstrapping a mutual subscription between a user and a contact.
 Complete details regarding subscription states can be found under
 Appendix A.

3.1. Requesting a Subscription

   A "subscription request" is a request from a user for authorization
   to permanently subscribe to a contact's presence information;
   syntactically it is a presence stanza whose 'type' attribute has a
   value of "subscribe".  A subscription request is generated by a

   user's client, processed by the (potential) contact's server, and
   acted on by the contact via the contact's client.  The workflow is
   described in the following sections.

      Implementation Note: Presence subscription requests are sent to
      available resources, whereas the roster pushes associated with
      subscription state changes are sent to interested resources.
      Therefore, if a resource wishes to receive both subscription
      requests and roster pushes, it MUST both send initial presence and
      request the roster.

3.1.1. Client Generation of Outbound Subscription Request

A user's client generates a subscription request by sending a
 presence stanza of type "subscribe" and specifying a 'to' address of
 the potential contact's bare JID <contact@domainpart>.

 UC: <presence id='xk3h1v69'
 to='juliet@example.com'
 type='subscribe'/>

 When a user sends a presence subscription request to a potential
 instant messaging and presence contact, the value of the 'to'
 attribute MUST be a bare JID <contact@domainpart> rather than a full
 JID <contact@domainpart/resourcepart>, since the desired result is
 for the user to receive presence from all of the contact's resources,
 not merely the particular resource specified in the 'to' attribute.
 Use of bare JIDs also simplifies subscription processing, presence
 probes, and presence notifications by the user's server and the
 contact's server.

 For tracking purposes, a client SHOULD include an 'id' attribute in a
 presence subscription request.

 Implementation Note: Many XMPP clients prompt the user for
 information about the potential contact (e.g., "handle" and
 desired roster group) when generating an outbound presence
 subscription request and therefore send a roster set before
 sending the outbound presence subscription request. This behavior
 is OPTIONAL, because a client MAY instead wait until receiving the
 initial roster push from the server before uploading user-provided
 information about the contact. A server MUST process a roster set
 and outbound presence subscription request in either order (i.e.,
 in whatever order generated by the client).

3.1.2. Server Processing of Outbound Subscription Request

Upon receiving the outbound presence subscription request, the user's
 server MUST proceed as follows.

 1. Before processing the request, the user's server MUST check the
 syntax of the JID contained in the 'to' attribute (however, it is
 known that some existing implementations do not perform this
 check). If the JID is of the form
 <contact@domainpart/resourcepart> instead of
 <contact@domainpart>, the user's server SHOULD treat it as if the
 request had been directed to the contact's bare JID and modify
 the 'to' address accordingly. The server MAY also verify that
 the JID adheres to the format defined in [XMPP-ADDR] and possibly
 return a <jid-malformed/> stanza error.

 2. If the potential contact is hosted on the same server as the
 user, then the server MUST adhere to the rules specified under
 Section 3.1.3 when processing the subscription request and
 delivering it to the (local) contact.

 3. If the potential contact is hosted on a remote server, subject to
 local service policies the user's server MUST then route the
 stanza to that remote domain in accordance with core XMPP stanza
 processing rules. (This can result in returning an appropriate
 stanza error to the user, such as <remote-server-timeout/>.)

 As mentioned, before locally delivering or remotely routing the
 presence subscription request, the user's server MUST stamp the
 outbound subscription request with the bare JID <user@domainpart> of
 the user.

 US: <presence from='romeo@example.net'
 id='xk3h1v69'
 to='juliet@example.com'
 type='subscribe'/>

 If the presence subscription request cannot be locally delivered or
 remotely routed (e.g., because the request is malformed, the local
 contact does not exist, the remote server does not exist, an attempt
 to contact the remote server times out, or any other error is
 determined or experienced by the user's server), then the user's
 server MUST return an appropriate error stanza to the user. An
 example follows.

US: <presence from='juliet@example.com'
 id='xk3h1v69'
 to='romeo@example.net'
 type='error'>
 <error type='modify'>
 <remote-server-not-found
 xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
 </error>
 </presence>

 After locally delivering or remotely routing the presence
 subscription request, the user's server MUST then send a roster push
 to all of the user's interested resources, containing the potential
 contact with a subscription state of "none" and with notation that
 the subscription is pending (via an 'ask' attribute whose value is
 "subscribe").

 US: <iq id='b89c5r7ib574'
 to='romeo@example.net/foo'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item ask='subscribe'
 jid='juliet@example.com'
 subscription='none'/>
 </query>
 </iq>

 US: <iq id='b89c5r7ib575'
 to='romeo@example.net/bar'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item ask='subscribe'
 jid='juliet@example.com'
 subscription='none'/>
 </query>
 </iq>

 If a remote contact does not approve or deny the subscription request
 within some configurable amount of time, the user's server SHOULD
 resend the subscription request to the contact based on an
 implementation-specific algorithm (e.g., whenever a new resource
 becomes available for the user, or after a certain amount of time has
 elapsed); this helps to recover from transient, silent errors that
 might have occurred when the original subscription request was routed
 to the remote domain. When doing so, it is RECOMMENDED for the
 server to include an 'id' attribute so that it can track responses to
 the resent subscription request.

3.1.3. Server Processing of Inbound Subscription Request

Before processing the inbound presence subscription request, the
 contact's server SHOULD check the syntax of the JID contained in the
 'to' attribute. If the JID is of the form
 <contact@domainpart/resourcepart> instead of <contact@domainpart>,
 the contact's server SHOULD treat it as if the request had been
 directed to the contact's bare JID and modify the 'to' address
 accordingly. The server MAY also verify that the JID adheres to the
 format defined in [XMPP-ADDR] and possibly return a <jid-malformed/>
 stanza error.

 When processing the inbound presence subscription request, the
 contact's server MUST adhere to the following rules:

 1. Above all, the contact's server MUST NOT automatically approve
 subscription requests on the contact's behalf -- unless the
 contact has (a) pre-approved subscription requests from the user
 as described under Section 3.4, (b) configured its account to
 automatically approve subscription requests, or (c) accepted an
 agreement with its service provider that allows automatic
 approval (for instance, via an employment agreement within an
 enterprise deployment). Instead, if a subscription request
 requires approval then the contact's server MUST deliver that
 request to the contact's available resource(s) for approval or
 denial by the contact.

 2. If the contact exists and the user already has a subscription to
 the contact's presence, then the contact's server MUST auto-reply
 on behalf of the contact by sending a presence stanza of type
 "subscribed" from the contact's bare JID to the user's bare JID.
 Likewise, if the contact previously sent a presence stanza of
 type "subscribed" and the contact's server treated that as
 indicating "pre-approval" for the user's presence subscription
 (see Section 3.4), then the contact's server SHOULD also auto-
 reply on behalf of the contact.

 CS: <presence from='juliet@example.com'
 id='xk3h1v69'
 to='romeo@example.net'
 type='subscribed'/>

 3. Otherwise, if there is at least one available resource associated
 with the contact when the subscription request is received by the
 contact's server, then the contact's server MUST send that
 subscription request to all available resources in accordance
 with Section 8. As a way of acknowledging receipt of the
 presence subscription request, the contact's server MAY send a

       presence stanza of type "unavailable" from the bare JID of the
       contact to the bare JID of the user (the user's client MUST NOT
       assume that this acknowledgement provides presence information
       about the contact, since it comes from the contact's bare JID and
       is received before the subscription request has been approved).

   4.  Otherwise, if the contact has no available resources when the
       subscription request is received by the contact's server, then
       the contact's server MUST keep a record of the complete presence
       stanza comprising the subscription request, including any
       extended content contained therein (see Section 8.4 of
       [XMPP-CORE]), and then deliver the request when the contact next
       has an available resource.  The contact's server MUST continue to
       deliver the subscription request whenever the contact creates an
       available resource, until the contact either approves or denies
       the request.  (The contact's server MUST NOT deliver more than
       one subscription request from any given user when the contact
       next has an available resource; e.g., if the user sends multiple
       subscription requests to the contact while the contact is
       offline, the contact's server SHOULD store only one of those
       requests, such as the first request or last request, and MUST
       deliver only one of the requests when the contact next has an
       available resource; this helps to prevent "subscription request
       spam".)

      Security Warning: Until and unless the contact approves the
      subscription request as described under Section 3.1.4, the
      contact's server MUST NOT add an item for the user to the
      contact's roster.

      Security Warning: The mandate for the contact's server to store
      the complete stanza of the presence subscription request
      introduces the possibility of an application resource exhaustion
      attack (see Section 2.1.2 of [DOS]), for example, by a rogue
      server or a coordinated group of users (e.g., a botnet) against
      the contact's server or particular contact.  Server implementers
      are advised to consider the possibility of such attacks and
      provide tools for counteracting it, such as enabling service
      administrators to set limits on the number or size of inbound
      presence subscription requests that the server will store in
      aggregate or for any given contact.

3.1.4. Client Processing of Inbound Subscription Request

   When an interactive client receives a subscription request, it MUST
   present the request to the natural person controlling the client
   (i.e., the "contact") for approval, unless the contact has explicitly
   configured the client to automatically approve or deny some or all

subscription requests as described above. An automated client that
 is not controlled by a natural person will have its own application-
 specific rules for approving or denying subscription requests.

 A client approves a subscription request by sending a presence stanza
 of type "subscribed", which is processed as described under
 Section 3.1.5 for the contact's server and Section 3.1.6 for the
 user's server.

 CC: <presence id='h4v1c4kj'
 to='romeo@example.net'
 type='subscribed'/>

 A client denies a subscription request by sending a presence stanza
 of type "unsubscribed", which is processed as described under
 Section 3.2 for both the contact's server and the user's server.

 CC: <presence id='tb2m1b59'
 to='romeo@example.net'
 type='unsubscribed'/>

 For tracking purposes, a client SHOULD include an 'id' attribute in a
 subscription approval or subscription denial; this 'id' attribute
 MUST NOT mirror the 'id' attribute of the subscription request.

3.1.5. Server Processing of Outbound Subscription Approval

When the contact's client sends the subscription approval, the
 contact's server MUST stamp the outbound stanza with the bare JID
 <contact@domainpart> of the contact and locally deliver or remotely
 route the stanza to the user.

 CS: <presence from='juliet@example.com'
 id='h4v1c4kj'
 to='romeo@example.net'
 type='subscribed'/>

 The contact's server then MUST send an updated roster push to all of
 the contact's interested resources, with the 'subscription' attribute
 set to a value of "from". (Here we assume that the contact does not
 already have a subscription to the user; if that were the case, the
 'subscription' attribute would be set to a value of "both", as
 explained under Appendix A.)

CS: <iq id='a78b4q6ha463'
 to='juliet@example.com/balcony'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'
 subscription='from'/>
 </query>
 </iq>

 CS: <iq id='x81g3bdy4n19'
 to='juliet@example.com/chamber'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'
 subscription='from'/>
 </query>
 </iq>

 From the perspective of the contact, there now exists a subscription
 from the user, which is why the 'subscription' attribute is set to a
 value of "from". (Here we assume that the contact does not already
 have a subscription to the user; if that were the case, the
 'subscription' attribute would be set to a value of "both", as
 explained under Appendix A.)

 The contact's server MUST then also send current presence to the user
 from each of the contact's available resources.

 CS: <presence from='juliet@example.com/balcony'
 id='pw72bc5j'
 to='romeo@example.net'/>

 CS: <presence from='juliet@example.com/chamber'
 id='ux31da4q'
 to='romeo@example.net'/>

 In order to subscribe to the user's presence, the contact would then
 need to send a subscription request to the user. (XMPP clients will
 often automatically send the subscription request instead of
 requiring the contact to initiate the subscription request, since it
 is assumed that the desired end state is a mutual subscription.)
 Naturally, when the contact sends a subscription request to the user,
 the subscription states will be different from those shown in the
 foregoing examples (see Appendix A) and the roles will be reversed.

3.1.6. Server Processing of Inbound Subscription Approval

When the user's server receives a subscription approval, it MUST
 first check if the contact is in the user's roster with
 subscription='none' or subscription='from' and the 'ask' flag set to
 "subscribe" (i.e., a subscription state of "None + Pending Out",
 "None + Pending Out+In", or "From + Pending Out"; see Appendix A).
 If this check is successful, then the user's server MUST:

 1. Deliver the inbound subscription approval to all of the user's
 interested resources (this helps to give the user's client(s)
 proper context regarding the subscription approval so that they
 can differentiate between a roster push originated by another of
 the user's resources and a subscription approval received from
 the contact). This MUST occur before sending the roster push
 described in the next step.

 US: <presence from='juliet@example.com'
 id='h4v1c4kj'
 to='romeo@example.net'
 type='subscribed'/>

 2. Initiate a roster push to all of the user's interested resources,
 containing an updated roster item for the contact with the
 'subscription' attribute set to a value of "to" (if the
 subscription state was "None + Pending Out" or "None + Pending
 Out+In") or "both" (if the subscription state was "From + Pending
 Out").

 US: <iq id='b89c5r7ib576'
 to='romeo@example.net/foo'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='juliet@example.com'
 subscription='to'/>
 </query>
 </iq>

 US: <iq id='b89c5r7ib577'
 to='romeo@example.net/bar'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='juliet@example.com'
 subscription='to'/>
 </query>
 </iq>

3. The user's server MUST also deliver the available presence stanza
 received from each of the contact's available resources to each
 of the user's available resources.

 [ ... to resource1 ... ]

 US: <presence from='juliet@example.com/balcony'
 id='pw72bc5j'
 to='romeo@example.net'/>

 [ ... to resource2 ... ]

 US: <presence from='juliet@example.com/balcony'
 id='pw72bc5j'
 to='romeo@example.net'/>

 [ ... to resource1 ... ]

 US: <presence from='juliet@example.com/chamber'
 id='ux31da4q'
 to='romeo@example.net'/>

 [ ... to resource2 ... ]

 US: <presence from='juliet@example.com/chamber'
 id='ux31da4q'
 to='romeo@example.net'/>

 Implementation Note: If the user's account has no available
 resources when the inbound subscription approval notification is
 received, the user's server MAY keep a record of the notification
 (ideally the complete presence stanza) and then deliver the
 notification when the account next has an available resource.
 This behavior provides more complete signaling to the user
 regarding the reasons for the roster change that occurred while
 the user was offline.

 Otherwise -- that is, if the user does not exist, if the contact is
 not in the user's roster, or if the contact is in the user's roster
 with a subscription state other than those described in the foregoing
 check -- then the user's server MUST silently ignore the subscription
 approval notification by not delivering it to the user, not modifying
 the user's roster, and not generating a roster push to the user's
 interested resources.

 From the perspective of the user, there now exists a subscription to
 the contact's presence (which is why the 'subscription' attribute is
 set to a value of "to").

3.2. Canceling a Subscription

3.2.1. Client Generation of Subscription Cancellation

If a contact would like to cancel a subscription that it has
 previously granted to a user, to cancel a subscription pre-approval
 (Section 3.4), or to deny a subscription request, it sends a presence
 stanza of type "unsubscribed".

 CC: <presence id='ij5b1v7g'
 to='romeo@example.net'
 type='unsubscribed'/>

3.2.2. Server Processing of Outbound Subscription Cancellation

   Upon receiving the outbound subscription cancellation, the contact's
   server MUST proceed as follows.

   1.  If the user's bare JID is not yet in the contact's roster or is
       in the contact's roster with a state of "None", "None + Pending
       Out", or "To", the contact's server SHOULD NOT route or deliver
       the presence stanza of type "unsubscribed" to the user and MUST
       NOT send presence notifications of type "unavailable" to the user
       as described below.

   2.  If the user's bare JID is in the contact's roster with a state of
       "None", "None + Pending Out", or "To" and the 'approved' flag is
       set to "true" (thus signaling a subscription pre-approval as
       described under Section 3.4), the contact's server MUST remove
       the pre-approval and MUST NOT route or deliver the presence
       stanza of type "unsubscribed" to the user.

   3.  Otherwise, as shown in the following examples, the contact's
       server MUST route or deliver both presence notifications of type
       "unavailable" and presence stanzas of type "unsubscribed" to the
       user and MUST send a roster push to the contact.

   While the user is still subscribed to the contact's presence (i.e.,
   before the contact's server routes or delivers the presence stanza of
   type "unsubscribed" to the user), the contact's server MUST send a
   presence stanza of type "unavailable" from all of the contact's
   online resources to the user.

CS: <presence from='juliet@example.com/balcony'
 id='i8bsg3h3'
 type='unavailable'/>

 CS: <presence from='juliet@example.com/chamber'
 id='bvx2c9mk'
 type='unavailable'/>

 Then the contact's server MUST route or deliver the presence stanza
 of type "unsubscribed" to the user, making sure to stamp the outbound
 subscription cancellation with the bare JID <contact@domainpart> of
 the contact.

 CS: <presence from='juliet@example.com'
 id='ij5b1v7g'
 to='romeo@example.net'
 type='unsubscribed'/>

 The contact's server then MUST send a roster push with the updated
 roster item to all of the contact's interested resources, where the
 subscription state is now either "none" or "to" (see Appendix A).

 CS: <iq id='pw3f2v175b34'
 to='juliet@example.com/balcony'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'
 subscription='none'/>
 </query>
 </iq>

 CS: <iq id='zu2y3f571v35'
 to='juliet@example.com/chamber'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'
 subscription='none'/>
 </query>
 </iq>

3.2.3. Server Processing of Inbound Subscription Cancellation

   When the user's server receives the inbound subscription
   cancellation, it MUST first check if the contact is in the user's
   roster with subscription='to' or subscription='both' (see
   Appendix A).  If this check is successful, then the user's server
   MUST:

1. Deliver the inbound subscription cancellation to all of the
 user's interested resources (this helps to give the user's
 client(s) proper context regarding the subscription cancellation
 so that they can differentiate between a roster push originated
 by another of the user's resources and a subscription
 cancellation received from the contact). This MUST occur before
 sending the roster push described in the next step.

 US: <presence from='juliet@example.com'
 id='ij5b1v7g'
 to='romeo@example.net'
 type='unsubscribed'/>

 2. Initiate a roster push to all of the user's interested resources,
 containing an updated roster item for the contact with the
 'subscription' attribute set to a value of "none" (if the
 subscription state was "To" or "To + Pending In") or "from" (if
 the subscription state was "Both").

 US: <iq id='h37h3u1bv400'
 to='romeo@example.net/foo'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='juliet@example.com'
 subscription='none'/>
 </query>
 </iq>

 US: <iq id='h37h3u1bv401'
 to='romeo@example.net/bar'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='juliet@example.com'
 subscription='none'/>
 </query>
 </iq>

 The user's server MUST also deliver the inbound presence stanzas of
 type "unavailable".

 Implementation Note: If the user's account has no available
 resources when the inbound unsubscribed notification is received,
 the user's server MAY keep a record of the notification (ideally
 the complete presence stanza) and then deliver the notification
 when the account next has an available resource. This behavior
 provides more complete signaling to the user regarding the reasons
 for the roster change that occurred while the user was offline.

   Otherwise -- that is, if the user does not exist, if the contact is
   not in the user's roster, or if the contact is in the user's roster
   with a subscription state other than those described in the foregoing
   check -- then the user's server MUST silently ignore the unsubscribed
   notification by not delivering it to the user, not modifying the
   user's roster, and not generating a roster push to the user's
   interested resources.

3.3. Unsubscribing

3.3.1. Client Generation of Unsubscribe

If a user would like to unsubscribe from a contact's presence, it
 sends a presence stanza of type "unsubscribe".

 UC: <presence id='ul4bs71n'
 to='juliet@example.com'
 type='unsubscribe'/>

3.3.2. Server Processing of Outbound Unsubscribe

Upon receiving the outbound unsubscribe, the user's server MUST
 proceed as follows.

 1. If the contact is hosted on the same server as the user, then the
 server MUST adhere to the rules specified under Section 3.3.3
 when processing the subscription request.

 2. If the contact is hosted on a remote server, subject to local
 service policies the user's server MUST then route the stanza to
 that remote domain in accordance with core XMPP stanza processing
 rules. (This can result in returning an appropriate stanza error
 to the user, such as <remote-server-timeout/>.)

 As mentioned, before locally delivering or remotely routing the
 unsubscribe, the user's server MUST stamp the stanza with the bare
 JID <user@domainpart> of the user.

 US: <presence from='romeo@example.net'
 id='ul4bs71n'
 to='juliet@example.com'
 type='unsubscribe'/>

 The user's server then MUST send a roster push with the updated
 roster item to all of the user's interested resources, where the
 subscription state is now either "none" or "from" (see Appendix A).

US: <iq id='h37h3u1bv402'
 to='romeo@example.net/foo'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='juliet@example.com'
 subscription='none'/>
 </query>
 </iq>

 US: <iq to='romeo@example.net/bar'
 type='set'
 id='h37h3u1bv403'>
 <query xmlns='jabber:iq:roster'>
 <item jid='juliet@example.com'
 subscription='none'/>
 </query>
 </iq>

3.3.3. Server Processing of Inbound Unsubscribe

When the contact's server receives the unsubscribe notification, it
 MUST first check if the user's bare JID is in the contact's roster
 with subscription='from' or subscription='both' (i.e., a subscription
 state of "From", "From + Pending Out", or "Both"; see Appendix A).
 If this check is successful, then the contact's server MUST:

 1. Deliver the inbound unsubscribe to all of the contact's
 interested resources (this helps to give the contact's client(s)
 proper context regarding the unsubscribe so that they can
 differentiate between a roster push originated by another of the
 contact's resources and an unsubscribe received from the user).
 This MUST occur before sending the roster push described in the
 next step.

 CS: <presence from='romeo@example.net'
 id='ul4bs71n'
 to='juliet@example.com'
 type='unsubscribe'/>

 2. Initiate a roster push to all of the contact's interested
 resources, containing an updated roster item for the user with
 the 'subscription' attribute set to a value of "none" (if the
 subscription state was "From" or "From + Pending Out") or "to"
 (if the subscription state was "Both").

CS: <iq id='tn2b5893g1s4'
 to='juliet@example.com/balcony'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'
 subscription='none'/>
 </query>
 </iq>

 CS: <iq id='sp3b56n27hrp'
 to='juliet@example.com/chamber'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item jid='romeo@example.net'
 subscription='none'/>
 </query>
 </iq>

 3. Generate an outbound presence stanza of type "unavailable" from
 each of the contact's available resources to the user.

 CS: <presence from='juliet@example.com/balcony'
 id='o5v91w49'
 to='romeo@example.net'
 type='unavailable'/>

 CS: <presence from='juliet@example.com/chamber'
 id='n6b1c37k'
 to='romeo@example.net'
 type='unavailable'/>

 Implementation Note: If the contact's account has no available
 resources when the inbound unsubscribe notification is received,
 the contact's server MAY keep a record of the notification
 (ideally the complete presence stanza) and then deliver the
 notification when the account next has an available resource.
 This behavior provides more complete signaling to the user
 regarding the reasons for the roster change that occurred while
 the user was offline.

 Otherwise -- that is, if the contact does not exist, if the user is
 not in the contact's roster, or if the user's bare JID is in the
 contact's roster with a subscription state other than those described
 in the foregoing check -- then the contact's server MUST silently
 ignore the unsubscribe stanza by not delivering it to the contact,
 not modifying the contact's roster, and not generating a roster push
 to the contact's interested resources. However, if the contact's
 server is keeping track of an inbound presence subscription request

   from the user to the contact but the user is not yet in the contact's
   roster (functionally equivalent to a subscription state of "None +
   Pending In" where the contact never added the user to the contact's
   roster), then the contact's server MUST simply remove any record of
   the inbound presence subscription request (it cannot remove the user
   from the contact's roster because the user was never added to the
   contact's roster).

      Implementation Note: The user's client MUST NOT depend on
      receiving the unavailable presence notification from the contact,
      since it MUST consider its presence subscription to the contact,
      and its presence information about the contact, to be null and
      void when it sends the presence stanza of type "unsubscribe" or
      when it receives the roster push triggered by the unsubscribe
      request.

3.4. Pre-Approving a Subscription Request

If a user has not received a subscription request from a contact, the
 user can "pre-approve" such a request so that it will be
 automatically approved by the user's server.

 Support for subscription pre-approvals is OPTIONAL on the part of
 clients and servers. If a server supports subscription pre-
 approvals, then it MUST advertise the following stream feature during
 stream negotiation.

 

 The subscription pre-approval stream feature is merely informative
 and therefore is never mandatory-to-negotiate.

3.4.1. Client Generation of Subscription Pre-Approval

If the server to which a client connects has advertised support for
 subscription pre-approvals, the client MAY generate a subscription
 pre-approval by sending a presence stanza of type "subscribed" to the
 contact.

 UC: <presence id='pg81vx64'
 to='juliet@example.com'
 type='subscribed'/>

 If the server does not advertise support for subscription pre-
 approvals, the client MUST NOT attempt to pre-approve subscription
 requests from potential or actual contacts.

3.4.2. Server Processing of Subscription Pre-Approval

Upon receiving the presence stanza of type "subscribed", the user's
 server MUST proceed as follows if it supports subscription pre-
 approvals.

 1. If the contact is in the user's roster with a state of "Both",
 "From", or "From + Pending Out", the user's server MUST silently
 ignore the stanza.

 2. If the contact is in the user's roster with a state of "To +
 Pending In", "None + Pending In", or "None + Pending Out+In", the
 user's server MUST handle the stanza as a normal subscription
 approval (see under Section 3.1.5) by updating the existing
 roster item to a state of "Both", "From", or "From + Pending Out"
 (respectively), pushing the modified roster item to all of the
 user's interested resources, and routing the presence stanza of
 type "subscribed" to the contact.

 3. If the contact is in the user's roster with a state of "To",
 "None", or "None + Pending Out", the user's server MUST note the
 subscription pre-approval by setting the 'approved' flag to a
 value of "true", then push the modified roster item to all of the
 user's interested resources. However, the user's server MUST NOT
 route the presence stanza of type "subscribed" to the contact.

 4. If the contact is not yet in the user's roster, the user's server
 MUST create a roster item for the contact with a state of "None"
 and set the 'approved' flag to a value of "true", then push the
 roster item to all of the user's interested resources. However,
 the user's server MUST NOT route the presence stanza of type
 "subscribed" to the contact.

 An example of the roster push follows.

 US: <iq id='h3bs81vs763f'
 to='romeo@example.net/bar'
 type='set'>
 <query xmlns='jabber:iq:roster'>
 <item approved='true'
 jid='juliet@example.com'
 subscription='none'/>
 </query>
 </iq>

 When the 'approved' flag is set to "true", the user's server MUST NOT
 deliver a presence stanza of type "subscribe" from the contact to the
 user, but instead MUST automatically respond to such a stanza on

   behalf of the user by returning a presence stanza of type
   "subscribed" from the bare JID of the user to the bare JID of the
   contact.

      Implementation Note: It is a matter of implementation or local
      service policy whether the server maintains a record of the
      subscription approval after it has received a presence
      subscription request from the contact.  If the server does not
      maintain such a record, upon receiving the subscription request it
      will not include the 'approved' attribute in the roster item for
      the contact (i.e., in subsequent roster pushes and roster
      results).  If the server maintains such a record, it will always
      include the 'approved' attribute (set to "true") in the roster
      item for the contact, until and unless the user sends a presence
      stanza of type "unsubscribed" to the contact (or removes the
      contact from the roster entirely).

      Implementation Note: A client can cancel a pre-approval by sending
      a presence stanza of type "unsubscribed", as described more fully
      under Section 3.2.  In this case, the user's server would send a
      roster push to all of the user's interested resources with the
      'approved' attribute removed.  (Alternatively, the client can
      simply remove the roster item entirely.)

4.  Exchanging Presence Information

4.1. Presence Fundamentals

The concept of presence refers to an entity's availability for
 communication over a network. At the most basic level, presence is a
 boolean "on/off" variable that signals whether an entity is available
 or unavailable for communication (the terms "online" and "offline"
 are also used). In XMPP, an entity's availability is signaled when
 its client generates a <presence/> stanza with no 'type' attribute,
 and an entity's lack of availability is signaled when its client
 generates a <presence/> stanza whose 'type' attribute has a value of
 "unavailable".

 XMPP presence typically follows a "publish-subscribe" or "observer"
 pattern, wherein an entity sends presence to its server, and its
 server then broadcasts that information to all of the entity's
 contacts who have a subscription to the entity's presence (in the
 terminology of [IMP-MODEL], an entity that generates presence is a
 "presentity" and the entities that receive presence are
 "subscribers"). A client generates presence for broadcast to all
 subscribed entities by sending a presence stanza to its server with
 no 'to' address, where the presence stanza has either no 'type'
 attribute or a 'type' attribute whose value is "unavailable". This

kind of presence is called "broadcast presence". (A client can also
 send "directed presence", i.e., a presence stanza with a 'to'
 address; this is less common but is sometimes used to send presence
 to entities that are not subscribed to the user's presence; see
 Section 4.6.)

 After a client completes the preconditions specified in [XMPP-CORE],
 it can establish a "presence session" at its server by sending
 initial presence (Section 4.2), where the presence session is
 terminated by sending unavailable presence (Section 4.5). For the
 duration of its presence session, a connected resource (in the
 terminology of [XMPP-CORE]) is said to be an "available resource".

 In XMPP, applications that combine messaging and presence
 functionality, the default type of communication for which presence
 signals availability is messaging; however, it is not necessary for
 XMPP applications to combine messaging and presence functionality,
 and they can provide standalone presence features without messaging
 (in addition, XMPP servers do not require information about network
 availability in order to successfully route message and IQ stanzas).

 Informational Note: In the examples that follow, the user is
 <juliet@example.com>, she has two available resources ("balcony"
 and "chamber"), and she has three contacts in her roster with a
 subscription state of "from" or "both": <romeo@example.net>,
 <mercutio@example.com>, and <benvolio@example.net>.

4.2. Initial Presence

4.2.1. Client Generation of Initial Presence

After completing the preconditions described in [XMPP-CORE]
 (REQUIRED) and requesting the roster (RECOMMENDED), a client signals
 its availability for communication by sending "initial presence" to
 its server, i.e., a presence stanza with no 'to' address (indicating
 that it is meant to be broadcast by the server on behalf of the
 client) and no 'type' attribute (indicating the user's availability).

 UC: <presence/>

 The initial presence stanza MAY contain the <priority/> element, the
 <show/> element, and one or more instances of the <status/> element,
 as well as extended content; details are provided under Section 4.7.

4.2.2. Server Processing of Outbound Initial Presence

Upon receiving initial presence from a client, the user's server MUST
 send the initial presence stanza from the full JID
 <user@domainpart/resourcepart> of the user to all contacts that are
 subscribed to the user's presence; such contacts are those for which
 a JID is present in the user's roster with the 'subscription'
 attribute set to a value of "from" or "both".

 US: <presence from='juliet@example.com/balcony'
 to='romeo@example.net'/>

 US: <presence from='juliet@example.com/balcony'
 to='mercutio@example.com'/>

 US: <presence from='juliet@example.com/balcony'
 to='benvolio@example.net'/>

 The user's server MUST also broadcast initial presence from the
 user's newly available resource to all of the user's available
 resources, including the resource that generated the presence
 notification in the first place (i.e., an entity is implicitly
 subscribed to its own presence).

 [... to the "balcony" resource ...]

 US: <presence from='juliet@example.com/balcony'
 to='juliet@example.com'/>

 [... to the "chamber" resource ...]

 US: <presence from='juliet@example.com/balcony'
 to='juliet@example.com'/>

 In the absence of presence information about the user's contacts, the
 user's server MUST also send presence probes to the user's contacts
 on behalf of the user as specified under Section 4.3.

4.2.3. Server Processing of Inbound Initial Presence

Upon receiving presence from the user, the contact's server MUST
 deliver the user's presence stanza to all of the contact's available
 resources.

 [ ... to resource1 ... ]

 CS: <presence from='juliet@example.com/balcony'
 to='romeo@example.net'/>

[ ... to resource2 ... ]

 CS: <presence from='juliet@example.com/balcony'
 to='romeo@example.net'/>

4.2.4. Client Processing of Initial Presence

   When the contact's client receives presence from the user, the
   following behavior is suggested for interactive clients:

   1.  If the user's bare JID is in the contact's roster, display the
       presence information in an appropriate roster interface.

   2.  If the user is not in the contact's roster but the contact and
       the user are actively exchanging message or IQ stanzas, display
       the presence information in the user interface for that
       communication session (see also Section 4.6 and Section 5.1).

   3.  Otherwise, ignore the presence information and do not display it
       to the contact.

4.3. Presence Probes

A "presence probe" is a request for a contact's current presence
 information, sent on behalf of a user by the user's server;
 syntactically it is a presence stanza whose 'type' attribute has a
 value of "probe". In the context of presence subscriptions, the
 value of the 'from' address MUST be the bare JID of the subscribed
 user and the value of the 'to' address MUST be the bare JID of the
 contact to which the user is subscribed, since presence subscriptions
 are based on the bare JID.

 US: <presence from='juliet@example.com'
 id='ign291v5'
 to='romeo@example.net'
 type='probe'/>

 Interoperability Note: RFC 3921 specified that probes are sent
 from the full JID, not the bare JID (a rule that was changed
 because subscriptions are based on the bare JID). Some existing
 implementations send from the full JID instead of the bare JID.

 Probes can also be sent by an entity that has received presence
 outside the context of a presence subscription, typically when the
 contact has sent directed presence as described under Section 4.6; in
 this case the value of the 'from' or 'to' address can be a full JID
 instead of a bare JID. See Section 4.6 for a complete discussion.

   Presence probes SHOULD NOT be sent by a client, because in general a
   client will not need to send them since the task of gathering
   presence from a user's contacts is managed by the user's server.
   However, if a user's client generates an outbound presence probe then
   the user's server SHOULD route the probe (if the contact is at
   another server) or process the probe (if the contact is at the same
   server) and MUST NOT use its receipt of the presence probe from a
   connected client as the sole cause for returning a stanza or stream
   error to the client.

4.3.1. Server Generation of Outbound Presence Probe

When a server needs to discover the availability of a user's contact,
 it sends a presence probe from the bare JID <user@domainpart> of the
 user to the bare JID <contact@domainpart> of the contact.

 Implementation Note: Although presence probes are intended for
 sending to contacts (i.e., entities to which a user is
 subscribed), a server MAY send a presence probe to the full JID of
 an entity from which the user has received presence information
 during the current session.

 The user's server SHOULD send a presence probe whenever the user
 starts a new presence session by sending initial presence; however,
 the server MAY choose not to send the probe at that point if it has
 what it deems to be reliable and up-to-date presence information
 about the user's contacts (e.g., because the user has another
 available resource or because the user briefly logged off and on
 before the new presence session began). In addition, a server MAY
 periodically send a presence probe to a contact if it has not
 received presence information or other traffic from the contact in
 some configurable amount of time; this can help to prevent "ghost"
 contacts who appear to be online but in fact are not.

 US: <presence from='juliet@example.com'
 id='ign291v5'
 to='romeo@example.net'
 type='probe'/>

 US: <presence from='juliet@example.com'
 id='xv291f38'
 to='mercutio@example.com'
 type='probe'/>

 Naturally, the user's server does not need to send a presence probe
 to a contact if the contact's account resides on the same server as
 the user, since the server possesses the contact's information
 locally.

4.3.2. Server Processing of Inbound Presence Probe

Upon receiving a presence probe to the contact's bare JID from the
 user's server on behalf of the user, the contact's server MUST reply
 as follows:

 1. If the contact account does not exist or the user's bare JID is
 in the contact's roster with a subscription state other than
 "From", "From + Pending Out", or "Both" (as explained under
 Appendix A), then the contact's server SHOULD return a presence
 stanza of type "unsubscribed" in response to the presence probe
 (this will trigger a protocol flow for canceling the user's
 subscription to the contact as described under Section 3.2;
 however, this MUST NOT result in cancellation of a subscription
 pre-approval as described under Section 3.4). Here the 'from'
 address MUST be the bare JID of the contact, since specifying a
 full JID would constitute a presence leak as described in
 [XMPP-CORE].

 CS: <presence from='mercutio@example.com'
 id='xv291f38'
 to='juliet@example.com'
 type='unsubscribed'/>

 However, if a server receives a presence probe from a configured
 domain of the server itself or another such trusted service, it MAY
 provide presence information about the user to that entity.

 2. Else, if the contact has moved temporarily or permanently to
 another address, then the server SHOULD return a presence stanza
 of type "error" with a stanza error condition of <redirect/>
 (temporary) or <gone/> (permanent) that includes the new address
 of the contact.

 CS: <presence from='mercutio@example.com'
 id='xv291f38'
 to='juliet@example.com'
 type='error'>
 <error type='modify'>
 <gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
 xmpp:la-mer@example.com
 </gone>
 </error>
 </presence>

 3. Else, if the contact has no available resources, then the server
 SHOULD reply to the presence probe by sending to the user a
 presence stanza of type "unavailable" (although sending

unavailable presence here is preferable because it results in a
 deterministic answer to the probe, it is not mandatory because it
 can greatly increase the number of presence notifications
 generated by the contact's server). Here the 'from' address is
 the bare JID because there is no available resource associated
 with the contact. If appropriate in accordance with local
 security policies this presence notification MAY include the full
 XML of the last unavailable presence stanza that the server
 received from the contact (including the 'id' of the original
 stanza), but if not then the presence notification SHOULD simply
 indicate that the contact is unavailable without any of the
 details originally provided. In any case, the presence
 notification returned to the probing entity SHOULD include
 information about the time when the last unavailable presence
 stanza was generated (formatted using the XMPP delayed delivery
 extension [DELAY]).

 CS: <presence from='mercutio@example.com'
 id='xv291f38'
 to='juliet@example.com'
 type='unavailable'>
 <delay xmlns='urn:xmpp:delay'
 stamp='2002-09-10T23:41:07Z'/>
 </presence>

 4. Else, if the contact has at least one available resource, then
 the server MUST reply to the presence probe by sending to the
 user the full XML of the last presence stanza with no 'to'
 attribute received by the server from each of the contact's
 available resources. Here the 'from' addresses are the full JIDs
 of each available resource.

 CS: <presence from='romeo@example.net/foo'
 id='hzf1v27k'
 to='juliet@example.com'/>

 CS: <presence from='romeo@example.net/bar'
 id='ps6t1fu3'
 to='juliet@example.com'>
 <show>away</show>
 </presence>

 Implementation Note: By "full XML" is meant the complete stanza
 from the opening <presence> tag to the closing </presence> tag,
 including all elements and attributes whether qualified by the
 content namespace or extended namespaces; however, in accordance

with [XMPP-CORE], the contact's server will need to transform the
 content namespace from 'jabber:client' to 'jabber:server' if it
 sends the complete stanza over a server-to-server stream.

 If the contact's server receives a presence probe addressed to a full
 JID of the contact, the server MUST NOT return presence information
 about any resource except the resource specified by the 'to' address
 of the probe. Rules #1 and #2 for a bare JID probe apply equally to
 the case of a full JID probe. If there is a resource matching the
 full JID and the probing entity has authorization via a presence
 subscription to see the contact's presence, then the server MUST
 return an available presence notification, which SHOULD communicate
 only the fact that the resource is available (not detailed
 information such as the <show/>, <status/>, <priority/>, or presence
 extensions).

 CS: <presence from='romeo@example.net/bar'
 to='lobby@chat.example.com'/>

 Implementation Note: See Section 4.6 regarding rules that
 supplement the foregoing for handling of directed presence.

4.3.2.1. Handling of the 'id' Attribute

The handling of the 'id' attribute in relation to presence probes was
 unspecified in RFC 3921. Although the pattern of "send a probe and
 receive a reply" might seem like a request-response protocol similar
 to the XMPP <iq/> stanza, in fact it is not because the response to a
 probe might consist of multiple presence stanzas (one for each
 available resource currently active for the contact). For this
 reason, if the contact currently has available resources then the
 contact's server SHOULD preserve the 'id' attribute of the contact's
 original presence stanza (if any) when sending those presence
 notifications to the probing entity. By contrast, if the contact
 currently has no available resources, the probing entity is not
 authorized (via presence subscription) to see the contact's presence,
 or an error occurs in relation to the probe, then the contact's
 server SHOULD mirror the 'id' of the user's presence probe when
 replying to the probing entity.

 The following examples illustrate the difference.

 In the first scenario, Juliet sends presence from her "chamber"
 resource.

CC: <presence from='juliet@example.com/chamber' id='pres1'>
 <show>dnd</show>
 <status>busy!</status>
 </presence>

 She also sends presence from her "balcony" resource.

 CC: <presence from='juliet@example.com/balcony' id='pres2'>
 <show>away</show>
 <status>stepped away</status>
 </presence>

 Romeo's server then sends a probe to Juliet.

 US: <presence from='romeo@example.net' id='probe1' type='probe'/>

 Juliet's server then sends both of her presence notifications to
 Romeo, preserving the 'id' attributes included in the stanzas that
 her client has sent.

 CS: <presence from='juliet@example.com/chamber' id='pres1'>
 <show>dnd</show>
 <status>busy!</status>
 </presence>

 CS: <presence from='juliet@example.com/balcony' id='pres2'>
 <show>away</show>
 <status>stepped away</status>
 </presence>

 In the second scenario, Juliet is offline when Romeo's server sends a
 probe.

 US: <presence from='romeo@example.net'
 id='probe2'
 type='probe'/>

 Juliet's server replies with an unavailable notification, mirroring
 the 'id' of Rome's presence probe because there is no 'id' to
 preserve from an available notification that her client has sent.

 CS: <presence from='juliet@example.com'
 id='probe2'
 type='unavailable'/>

4.4. Subsequent Presence Broadcast

4.4.1. Client Generation of Subsequent Presence Broadcast

After sending initial presence, at any time during its session the
 user's client can update its availability for broadcast by sending a
 presence stanza with no 'to' address and no 'type' attribute.

 UC: <presence>
 <show>away</show>
 </presence>

 The presence broadcast MAY contain the <priority/> element, the
 <show/> element, and one or more instances of the <status/> element,
 as well as extended content; details are provided under Section 4.7.

 However, a user SHOULD send a presence update only to broadcast
 information that is relevant to the user's availability for
 communication or the communication capabilities of the resource.
 Information that is not relevant in this way might be of interest to
 the user's contacts but SHOULD be sent via other means, such as the
 "publish-subscribe" method described in [XEP-0163].

4.4.2. Server Processing of Subsequent Outbound Presence

   Upon receiving a presence stanza expressing updated availability, the
   user's server MUST broadcast the full XML of that presence stanza to
   the contacts who are in the user's roster with a subscription type of
   "from" or "both".

      Interoperability Note: RFC 3921 specified that the user's server
      would check to make sure that it had not received a presence error
      from the contact before sending subsequent presence notifications.
      That rule has been removed because this specification uses
      presence stanzas of type "unsubscribe" (not "error") to solve
      subscription synchronization problems, in part because such
      stanzas change the contact's subscription state in the user's
      roster to either "none" or "to" (see Section 3.3 and Appendix A),
      thus obviating the need for the error check.

      Interoperability Note: If the subscription type is "both", some
      existing server implementations send subsequent presence
      notifications to a contact only if the contact is online according
      to the user's server (that is, if the user's server never received
      a positive indication that the contact is online in response to
      the presence probe it sent to the contact, the user's server does
      not send subsequent presence notifications from the user to the

contact). This behavior is perceived to save bandwidth, since
 most presence subscriptions are bidirectional and many contacts
 will not be online at any given time.

 US: <presence from='juliet@example.com/balcony'
 to='romeo@example.net'>
 <show>away</show>
 </presence>

 US: <presence from='juliet@example.com/balcony'
 to='benvolio@example.net'>
 <show>away</show>
 </presence>

 US: <presence from='juliet@example.com/balcony'
 to='mercutio@example.com'>
 <show>away</show>
 </presence>

 Implementation Note: See Section 4.6 regarding rules that
 supplement the foregoing for handling of directed presence.

 The user's server MUST also send the presence stanza to all of the
 user's available resources (including the resource that generated the
 presence notification in the first place).

 US: <presence from='juliet@example.com/balcony'
 to='juliet@example.com/chamber'>
 <show>away</show>
 </presence>

 US: <presence from='juliet@example.com/balcony'
 to='juliet@example.com/balcony'>
 <show>away</show>
 </presence>

4.4.3. Server Processing of Subsequent Inbound Presence

Upon receiving presence from the user, the contact's server MUST
 deliver the user's presence stanza to all of the contact's available
 resources.

 [ ... to resource1 ... ]

 CS: <presence from='juliet@example.com/balcony'
 to='romeo@example.net'>
 <show>away</show>
 </presence>

[ ... to resource2 ... ]

 CS: <presence from='juliet@example.com/balcony'
 to='romeo@example.net'>
 <show>away</show>
 </presence>

4.4.4. Client Processing of Subsequent Presence

   From the perspective of the contact's client, there is no significant
   difference between initial presence broadcast and subsequent
   presence, so the contact's client follows the rules for processing of
   inbound presence defined under Section 4.4.3.

4.5. Unavailable Presence

4.5.1. Client Generation of Unavailable Presence

Before ending its presence session with a server, the user's client
 SHOULD gracefully become unavailable by sending "unavailable
 presence", i.e., a presence stanza that possesses no 'to' attribute
 and that possesses a 'type' attribute whose value is "unavailable".

 UC: <presence type='unavailable'/>

 Optionally, the unavailable presence stanza MAY contain one or more
 <status/> elements specifying the reason why the user is no longer
 available.

 UC: <presence type='unavailable'>
 <status>going on vacation</status>
 </presence>

 However, the unavailable presence stanza MUST NOT contain the
 <priority/> element or the <show/> element, since these elements
 apply only to available resources.

4.5.2. Server Processing of Outbound Unavailable Presence

   The user's server MUST NOT depend on receiving unavailable presence
   from an available resource, since the resource might become
   unavailable ungracefully (e.g., the resource's XML stream might be
   closed with or without a stream error for any of the reasons
   described in [XMPP-CORE]).

If an available resource becomes unavailable for any reason (either
 gracefully or ungracefully), the user's server MUST broadcast
 unavailable presence to all contacts that are in the user's roster
 with a subscription type of "from" or "both".

 Interoperability Note: RFC 3921 specified that the user's server
 would check to make sure that it had not received a presence error
 from the contact before sending unavailable presence
 notifications. That rule has been removed because this
 specification uses presence stanzas of type "unsubscribe" (not
 "error") to solve subscription synchronization problems, in part
 because such stanzas change the contact's subscription state in
 the user's roster to either "none" or "to" (see Section 3.3 and
 Appendix A), thus obviating the need for the error check.

 Implementation Note: Even if the user's server does not broadcast
 the user's subsequent presence notifications to contacts who are
 offline (as described under Section 4.4.2), it MUST broadcast the
 user's unavailable presence notification; if it did not do so, the
 last presence received by the contact's server would be the user's
 initial presence for the presence session, with the result that
 the contact would consider the user to be online.

 Implementation Note: See Section 4.6 regarding rules that
 supplement the foregoing for handling of directed presence.

 If the unavailable notification was gracefully received from the
 client, then the server MUST broadcast the full XML of the presence
 stanza.

 US: <presence from='juliet@example.com/balcony'
 to='romeo@example.net'
 type='unavailable'>
 <status>going on vacation</status>
 </presence>

 US: <presence from='juliet@example.com/balcony'
 to='benvolio@example.net'
 type='unavailable'>
 <status>going on vacation</status>
 </presence>

 US: <presence from='juliet@example.com/balcony'
 to='mercutio@example.com'
 type='unavailable'>
 <status>going on vacation</status>
 </presence>

The user's server MUST also send the unavailable notification to all
 of the user's available resources (as well as to the resource that
 generated the unavailable presence in the first place).

 US: <presence from='juliet@example.com/balcony'
 to='juliet@example.com/chamber'
 type='unavailable'>
 <status>going on vacation</status>
 </presence>

 If the server detects that the user has gone offline ungracefully,
 then the server MUST generate the unavailable presence broadcast on
 the user's behalf.

 Implementation Note: Any presence stanza with no 'type' attribute
 and no 'to' attribute that the client sends after the server
 broadcasts or generates an unavailable presence notification MUST
 be routed or delivered by the user's server to all subscribers
 (i.e., MUST be treated as equivalent to initial presence for a new
 presence session).

4.5.3. Server Processing of Inbound Unavailable Presence

Upon receiving an unavailable notification from the user, the
 contact's server MUST deliver the user's presence stanza to all of
 the contact's available resources.

 [ ... to resource1 ... ]

 CS: <presence from='juliet@example.com/balcony'
 to='romeo@example.net'
 type='unavailable'>
 <status>going on vacation</status>
 </presence>

 [ ... to resource2 ... ]

 CS: <presence from='juliet@example.com/balcony'
 to='romeo@example.net'
 type='unavailable'>
 <status>going on vacation</status>
 </presence>

 Implementation Note: If the contact's server does not broadcast
 subsequent presence notifications to users who are offline (as
 described under Section 4.4.2), it MUST also update its internal
 representation of which entities are online by noting that the
 user is unavailable.

4.5.4. Client Processing of Unavailable Presence

   From the perspective of the contact's client, there is no significant
   difference between available presence broadcast and unavailable
   presence broadcast, so in general the contact's client follows the
   rules for processing of inbound presence defined under Section 4.4.3.

   However, if the contact receives an unavailable notification from the
   bare JID of the user (rather than the full JID of a particular
   available resource), the contact's client SHOULD treat the
   unavailable notification as applying to all resources.

4.6. Directed Presence

   This section supplements the rules for client and server processing
   of presence notifications and presence probes, but only for the
   special case of directed presence.

4.6.1. General Considerations

   In general, a client sends directed presence when it wishes to share
   availability information with an entity that is not subscribed to its
   presence, typically on a temporary basis.  Common uses of directed
   presence include casual one-to-one chat sessions as described under
   Section 5.1 and multi-user chat rooms as described in [XEP-0045].

   The temporary relationship established by sharing directed presence
   with another entity is secondary to the permanent relationship
   established through a presence subscription.  Therefore, the acts of
   creating, modifying, or canceling a presence subscription MUST take
   precedence over the rules specified in the following subsections.
   For example, if a user shares directed presence with a contact but
   then adds the contact to the user's roster by completing the presence
   subscription "handshake", the user's server MUST treat the contact
   just as it would any normal subscriber as described under Section 3,
   for example, by sending subsequent presence broadcasts to the
   contact.  As another example, if the user then cancels the contact's
   subscription to the user's presence, the user's server MUST handle
   the cancellation just as it normally would as described under
   Section 3.2, which includes sending unavailable presence to the
   contact even if the user has sent directed presence to the contact.

   XMPP servers typically implement directed presence by keeping a list
   of the entities (bare JIDs or full JIDs) to which a user has sent
   directed presence during the user's current session for a given
   resource (full JID), then clearing the list when the user goes
   offline (e.g., by sending a broadcast presence stanza of type
   "unavailable").  The server MUST remove from the directed presence

   list (or its functional equivalent) any entity to which the user
   sends directed unavailable presence and SHOULD remove any entity that
   sends unavailable presence to the user.

4.6.2. Client Generation of Directed Presence

   As noted, directed presence is a client-generated presence stanza
   with a 'to' attribute whose value is the bare JID or full JID of the
   other entity and with either no 'type' attribute (indicating
   availability) or a 'type' attribute whose value is "unavailable".

4.6.3. Server Processing of Outbound Directed Presence

   When the user's server receives a directed presence stanza, it SHOULD
   process it according to the following rules.

   1.  If the user sends directed available or unavailable presence to a
       contact that is in the user's roster with a subscription type of
       "from" or "both" after having sent initial presence and before
       sending unavailable presence broadcast (i.e., during the user's
       presence session), the user's server MUST locally deliver or
       remotely route the full XML of that presence stanza but SHOULD
       NOT otherwise modify the contact's status regarding presence
       broadcast (i.e., it SHOULD include the contact's JID in any
       subsequent presence broadcasts initiated by the user).

   2.  If the user sends directed presence to an entity that is not in
       the user's roster with a subscription type of "from" or "both"
       after having sent initial presence and before sending unavailable
       presence broadcast (i.e., during the user's presence session),
       the user's server MUST locally deliver or remotely route the full
       XML of that presence stanza to the entity but MUST NOT modify the
       contact's status regarding available presence broadcast (i.e., it
       MUST NOT include the entity's JID in any subsequent broadcasts of
       available presence initiated by the user); however, if the
       available resource from which the user sent the directed presence
       becomes unavailable, the user's server MUST route that
       unavailable presence to the entity (if the user has not yet sent
       directed unavailable presence to that entity).

   3.  If the user sends directed presence without first sending initial
       presence or after having sent unavailable presence broadcast
       (i.e., the resource is connected but not available), the user's
       server MUST treat the entity to which the user sends directed
       presence as in case #2 above.

4.6.4. Server Processing of Inbound Directed Presence

   From the perspective of the contact's server, there is no significant
   difference between presence broadcast and directed presence, so the
   contact's server follows the rules for processing of inbound presence
   defined under Sections 4.3.2, 4.4.3, and 4.5.3.

4.6.5. Client Processing of Inbound Directed Presence

   From the perspective of the contact's client, there is no significant
   difference between presence broadcast and directed presence, so the
   contact's client follows the rules for processing of inbound presence
   defined under Section 4.4.3.

4.6.6. Server Processing of Presence Probes

   If a user's client has sent directed presence to another entity
   (e.g., a one-to-one chat partner or a multi-user chat room), after
   some time the entity or its server might want to know if the client
   is still online.  This scenario is especially common in the case of
   multi-user chat rooms, in which the user might be a participant for a
   long period of time.  If the user's client goes offline without the
   chat room being informed (either by the client or the client's
   server), the user's representation in the room might become a "ghost"
   that appears to be participating but that in fact is no longer
   present in the room.  To detect such "ghosts", some multi-user chat
   room implementations send presence probes to users that have joined
   the room.

   In the case of directed presence, the probing entity SHOULD send the
   probe from the JID that received directed presence (whether a full
   JID or a bare JID).  The probe SHOULD be sent to the user's full JID,
   not the user's bare JID without a resourcepart, because the temporary
   "authorization" involved with directed presence is based on the full
   JID from which the user sent directed presence to the probing entity.
   When the user's server receives a probe, it MUST first apply any
   logic associated with presence subscriptions as described under
   Section 4.3.2.  If the probing entity does not have a subscription to
   the user's presence, then the server MUST check if the user has sent
   directed presence to the entity during its current session; if so,
   the server SHOULD answer the probe with only mere presence of type
   "available" or "unavailable" (i.e., not including child elements) and
   only for that full JID (i.e., not for any other resources that might
   be currently associated with the user's bare JID).

4.7. Presence Syntax

4.7.1. Type Attribute

The absence of a 'type' attribute signals that the relevant entity is
 available for communication (see Section 4.2 and Section 4.4).

 A 'type' attribute with a value of "unavailable" signals that the
 relevant entity is not available for communication (see Section 4.5).

 The XMPP presence stanza is also used to negotiate and manage
 subscriptions to the presence of other entities. These tasks are
 completed via presence stanzas of type "subscribe", "unsubscribe",
 "subscribed", and "unsubscribed" as described under Section 3.

 If a user and contact are associated with different XMPP servers,
 those servers also use a special presence stanza of type "probe" in
 order to determine the availability of the entity on the peer server;
 details are provided under Section 4.3. Clients SHOULD NOT send
 presence stanzas of type "probe".

 The values of the 'type' attribute can be summarized as follows:

 o error -- An error has occurred regarding processing of a
 previously sent presence stanza; if the presence stanza is of type
 "error", it MUST include an <error/> child element (refer to
 [XMPP-CORE]).

 o probe -- A request for an entity's current presence; SHOULD be
 generated only by a server on behalf of a user.

 o subscribe -- The sender wishes to subscribe to the recipient's
 presence.

 o subscribed -- The sender has allowed the recipient to receive
 their presence.

 o unavailable -- The sender is no longer available for
 communication.

 o unsubscribe -- The sender is unsubscribing from the receiver's
 presence.

 o unsubscribed -- The subscription request has been denied or a
 previously granted subscription has been canceled.

If the value of the 'type' attribute is not one of the foregoing
 values, the recipient or an intermediate router SHOULD return a
 stanza error of <bad-request/>.

 Implementation Note: There is no default value for the 'type'
 attribute of the <presence/> element.

 Implementation Note: There is no value of "available" for the
 'type' attribute of the <presence/> element.

4.7.2. Child Elements

In accordance with the default namespace declaration, a presence
 stanza is qualified by the 'jabber:client' or 'jabber:server'
 namespace, which defines certain child elements of presence stanzas,
 in particular the <show/>, <status/>, and <priority/> elements.
 These child elements are used to provide more detailed information
 about an entity's availability. Typically these child elements are
 included only if the presence stanza possesses no 'type' attribute,
 although exceptions are noted in the text that follows.

4.7.2.1. Show Element

The OPTIONAL <show/> element specifies the particular availability
 sub-state of an entity or a specific resource thereof. A presence
 stanza MUST NOT contain more than one <show/> element. There are no
 attributes defined for the <show/> element. The <show/> element MUST
 NOT contain mixed content (as defined in Section 3.2.2 of [XML]).
 The XML character data of the <show/> element is not meant for
 presentation to a human user. The XML character data MUST be one of
 the following (additional availability states could be defined
 through extended content elements):

 o away -- The entity or resource is temporarily away.

 o chat -- The entity or resource is actively interested in chatting.

 o dnd -- The entity or resource is busy (dnd = "Do Not Disturb").

 o xa -- The entity or resource is away for an extended period (xa =
 "eXtended Away").

 If no <show/> element is provided, the entity is assumed to be online
 and available.

 Any specialized processing of availability states by recipients and
 intermediate routers is up to the implementation (e.g., incorporation
 of availability states into stanza routing and delivery logic).

4.7.2.2. Status Element

The OPTIONAL <status/> element contains human-readable XML character
 data specifying a natural-language description of an entity's
 availability. It is normally used in conjunction with the show
 element to provide a detailed description of an availability state
 (e.g., "In a meeting") when the presence stanza has no 'type'
 attribute.

 <presence from='romeo@example.net/orchard'
 xml:lang='en'>
 <show>dnd</show>
 <status>Wooing Juliet</status>
 </presence>

 There are no attributes defined for the <status/> element, with the
 exception of the 'xml:lang' attribute inherited from [XML]. The
 <status/> element MUST NOT contain mixed content (as defined in
 Section 3.2.2 of [XML]). Multiple instances of the <status/> element
 MAY be included, but only if each instance possesses an 'xml:lang'
 attribute with a distinct language value (either explicitly or by
 inheritance from the 'xml:lang' value of an element farther up in the
 XML hierarchy, which from the sender's perspective can include the
 XML stream header as described in [XMPP-CORE]).

 <presence from='romeo@example.net/orchard'
 id='jx62vs97'
 xml:lang='en'>
 <show>dnd</show>
 <status>Wooing Juliet</status>
 <status xml:lang='cs'>Dvořím se Julii</status>
 </presence>

 A presence stanza of type "unavailable" MAY also include a <status/>
 element to provide detailed information about why the entity is going
 offline.

 <presence from='romeo@example.net/orchard'
 id='oy6sb241'
 type='unavailable'
 xml:lang='en'>
 <status>Busy IRL</status>
 </presence>

The <status/> child MAY also be sent in a subscription-related
 presence stanza (i.e., type "subscribe", "subscribed", "unsubscribe",
 or "unsubscribed") to provide a description of the action. An
 interactive client MAY present this <status/> information to a human
 user (see Section 11).

 <presence from='romeo@example.net'
 id='uc51xs63'
 to='nurse@example.com'
 type='subscribe'>
 <status>Hi, Juliet told me to add you to my buddy list.</status>
 </presence>

4.7.2.3. Priority Element

The OPTIONAL <priority/> element contains non-human-readable XML
 character data that specifies the priority level of the resource.
 The value MUST be an integer between -128 and +127. A presence
 stanza MUST NOT contain more than one <priority/> element. There are
 no attributes defined for the <priority/> element. The <priority/>
 element MUST NOT contain mixed content (as defined in Section 3.2.2
 of [XML]).

 <presence xml:lang='en'>
 <show>dnd</show>
 <status>Wooing Juliet</status>
 <status xml:lang='cs'>Dvořím se Julii</status>
 <priority>1</priority>
 </presence>

 If no priority is provided, the processing server or client MUST
 consider the priority to be zero ("0").

 The client's server MAY override the priority value provided by the
 client (e.g., in order to impose a message handling rule of
 delivering a message intended for the account's bare JID to all of
 the account's available resources). If the server does so, it MUST
 communicate the modified priority value when it echoes the client's
 presence back to itself and sends the presence notification to the
 user's contacts (because this modified priority value is typically
 the default value of zero, communicating the modified priority value
 can be done by not including the <priority/> child element).

 For information regarding the semantics of priority values in stanza
 processing within instant messaging and presence applications, refer
 to Section 8.

4.7.3. Extended Content

As described in [XMPP-CORE], an XML stanza MAY contain any child
 element that is qualified by a namespace other than the default
 namespace; this applies to the presence stanza as well.

 (In the following example, the presence stanza includes entity
 capabilities information as defined in [XEP-0115].)

 <presence from='romeo@example.net'>
 <c xmlns='http://jabber.org/protocol/caps'
 hash='sha-1'
 node='http://psi-im.org'
 ver='q07IKJEyjvHSyhy//CH0CxmKi8w='/>
 </presence>

 Any extended content included in a presence stanza SHOULD represent
 aspects of an entity's availability for communication or provide
 information about communication-related capabilities.

5. Exchanging Messages

Once a client has authenticated with a server and bound a resource to
 an XML stream as described in [XMPP-CORE], an XMPP server will route
 XML stanzas to and from that client. One kind of stanza that can be
 exchanged is <message/> (if, that is, messaging functionality is
 enabled on the server). Exchanging messages is a basic use of XMPP
 and occurs when a user generates a message stanza that is addressed
 to another entity. As defined under Section 8, the sender's server
 is responsible for delivering the message to the intended recipient
 (if the recipient is on the same local server) or for routing the
 message to the recipient's server (if the recipient is on a remote
 server). Thus a message stanza is used to "push" information to
 another entity.

5.1. One-to-One Chat Sessions

   In practice, instant messaging activity between human users tends to
   occur in the form of a conversational burst that we call a "chat
   session": the exchange of multiple messages between two parties in
   relatively rapid succession within a relatively brief period of time.

   When a human user intends to engage in such a chat session with a
   contact (rather than sending a single message to which no reply is
   expected), the message type generated by the user's client SHOULD be
   "chat" and the contact's client SHOULD preserve that message type in
   subsequent replies.  The user's client also SHOULD include a

<thread/> element with its initial message, which the contact's
 client SHOULD also preserve during the life of the chat session (see
 Section 5.2.5).

 The user's client SHOULD address the initial message in a chat
 session to the bare JID <contact@domainpart> of the contact (rather
 than attempting to guess an appropriate full JID
 <contact@domainpart/resourcepart> based on the <show/>, <status/>, or
 <priority/> value of any presence notifications it might have
 received from the contact). Until and unless the user's client
 receives a reply from the contact, it SHOULD send any further
 messages to the contact's bare JID. The contact's client SHOULD
 address its replies to the user's full JID
 <user@domainpart/resourcepart> as provided in the 'from' address of
 the initial message. Once the user's client receives a reply from
 the contact's full JID, it SHOULD address its subsequent messages to
 the contact's full JID as provided in the 'from' address of the
 contact's replies, thus "locking in" on that full JID. A client
 SHOULD "unlock" after having received a <message/> or <presence/>
 stanza from any other resource controlled by the peer (or a presence
 stanza from the locked resource); as a result, it SHOULD address its
 next message(s) in the chat session to the bare JID of the peer (thus
 "unlocking" the previous "lock") until it receives a message from one
 of the peer's full JIDs.

 When two parties engage in a chat session but do not share presence
 with each other based on a presence subscription, they SHOULD send
 directed presence to each other so that either party can easily
 discover if the peer goes offline during the course of the chat
 session. However, a client MUST provide a way for a user to disable
 such presence sharing globally or to enable it only with particular
 entities. Furthermore, a party SHOULD send directed unavailable
 presence to the peer when it has reason to believe that the chat
 session is over (e.g., if, after some reasonable amount of time, no
 subsequent messages have been exchanged between the parties).

 An example of a chat session is provided under Section 7.

5.2. Message Syntax

The following sections describe the syntax of the <message/> stanza.

5.2.1. To Attribute

An instant messaging client specifies an intended recipient for a
 message by providing the JID of the intended recipient in the 'to'
 attribute of the <message/> stanza.

If the message is being sent outside the context of any existing chat
 session or received message, the value of the 'to' address SHOULD be
 of the form <localpart@domainpart> rather than of the form
 <localpart@domainpart/resourcepart> (see Section 5.1).

 <message
 from='juliet@example.com/balcony'
 id='ktx72v49'
 to='romeo@example.net'
 type='chat'
 xml:lang='en'>
 <body>Art thou not Romeo, and a Montague?</body>
 </message>

 If the message is being sent in reply to a message previously
 received from an address of the form
 <localpart@domainpart/resourcepart> (e.g., within the context of a
 one-to-one chat session as described under Section 5.1), the value of
 the 'to' address SHOULD be of the form
 <localpart@domainpart/resourcepart> rather than of the form
 <localpart@domainpart> unless the sender has knowledge (e.g., via
 presence) that the intended recipient's resource is no longer
 available.

 <message
 from='romeo@example.net/orchard'
 id='sl3nx51f'
 to='juliet@example.com/balcony'
 type='chat'
 xml:lang='en'>
 <body>Neither, fair saint, if either thee dislike.</body>
 </message>

5.2.2. Type Attribute

   Common uses of the message stanza in instant messaging applications
   include: single messages; messages sent in the context of a one-to-
   one chat session; messages sent in the context of a multi-user chat
   room; alerts, notifications, or other information to which no reply
   is expected; and errors.  These uses are differentiated via the
   'type' attribute.  Inclusion of the 'type' attribute is RECOMMENDED.
   If included, the 'type' attribute MUST have one of the following
   values:

   o  chat -- The message is sent in the context of a one-to-one chat
      session.  Typically an interactive client will present a message
      of type "chat" in an interface that enables one-to-one chat
      between the two parties, including an appropriate conversation
      history.  Detailed recommendations regarding one-to-one chat
      sessions are provided under Section 5.1.

   o  error -- The message is generated by an entity that experiences an
      error when processing a message received from another entity (for
      details regarding stanza error syntax, refer to [XMPP-CORE]).  A
      client that receives a message of type "error" SHOULD present an
      appropriate interface informing the original sender regarding the
      nature of the error.

   o  groupchat -- The message is sent in the context of a multi-user
      chat environment (similar to that of [IRC]).  Typically a
      receiving client will present a message of type "groupchat" in an
      interface that enables many-to-many chat between the parties,
      including a roster of parties in the chatroom and an appropriate
      conversation history.  For detailed information about XMPP-based
      groupchat, refer to [XEP-0045].

   o  headline -- The message provides an alert, a notification, or
      other transient information to which no reply is expected (e.g.,
      news headlines, sports updates, near-real-time market data, or
      syndicated content).  Because no reply to the message is expected,
      typically a receiving client will present a message of type
      "headline" in an interface that appropriately differentiates the
      message from standalone messages, chat messages, and groupchat
      messages (e.g., by not providing the recipient with the ability to
      reply).  If the 'to' address is the bare JID, the receiving server
      SHOULD deliver the message to all of the recipient's available
      resources with non-negative presence priority and MUST deliver the
      message to at least one of those resources; if the 'to' address is
      a full JID and there is a matching resource, the server MUST
      deliver the message to that resource; otherwise the server MUST
      either silently ignore the message or return an error (see
      Section 8).

   o  normal -- The message is a standalone message that is sent outside
      the context of a one-to-one conversation or groupchat, and to
      which it is expected that the recipient will reply.  Typically a
      receiving client will present a message of type "normal" in an
      interface that enables the recipient to reply, but without a
      conversation history.  The default value of the 'type' attribute
      is "normal".

   An IM application SHOULD support all of the foregoing message types.
   If an application receives a message with no 'type' attribute or the
   application does not understand the value of the 'type' attribute
   provided, it MUST consider the message to be of type "normal" (i.e.,
   "normal" is the default).

   Guidelines for server handling of different message types is provided
   under Section 8.

   Although the 'type' attribute is OPTIONAL, it is considered polite to
   mirror the type in any replies to a message; furthermore, some
   specialized applications (e.g., a multi-user chat service) MAY at
   their discretion enforce the use of a particular message type (e.g.,
   type='groupchat').

5.2.3. Body Element

The <body/> element contains human-readable XML character data that
 specifies the textual contents of the message; this child element is
 normally included but is OPTIONAL.

 <message
 from='juliet@example.com/balcony'
 id='b4vs9km4'
 to='romeo@example.net'
 type='chat'
 xml:lang='en'>
 <body>Wherefore art thou, Romeo?</body>
 </message>

 There are no attributes defined for the <body/> element, with the
 exception of the 'xml:lang' attribute. Multiple instances of the
 <body/> element MAY be included in a message stanza for the purpose
 of providing alternate versions of the same body, but only if each
 instance possesses an 'xml:lang' attribute with a distinct language
 value (either explicitly or by inheritance from the 'xml:lang' value
 of an element farther up in the XML hierarchy, which from the
 sender's perspective can include the XML stream header as described
 in [XMPP-CORE]).

<message
 from='juliet@example.com/balcony'
 id='z94nb37h'
 to='romeo@example.net'
 type='chat'
 xml:lang='en'>
 <body>Wherefore art thou, Romeo?</body>
 <body xml:lang='cs'>
 PročeŽ jsi ty, Romeo?
 </body>
 </message>

 The <body/> element MUST NOT contain mixed content (as defined in
 Section 3.2.2 of [XML]).

5.2.4. Subject Element

The <subject/> element contains human-readable XML character data
 that specifies the topic of the message.

 <message
 from='juliet@example.com/balcony'
 id='c8xg3nf8'
 to='romeo@example.net'
 type='chat'
 xml:lang='en'>
 <subject>I implore you!</subject>
 <body>Wherefore art thou, Romeo?</body>
 </message>

 There are no attributes defined for the <subject/> element, with the
 exception of the 'xml:lang' attribute inherited from [XML]. Multiple
 instances of the <subject/> element MAY be included for the purpose
 of providing alternate versions of the same subject, but only if each
 instance possesses an 'xml:lang' attribute with a distinct language
 value (either explicitly or by inheritance from the 'xml:lang' value
 of an element farther up in the XML hierarchy, which from the
 sender's perspective can include the XML stream header as described
 in [XMPP-CORE]).

<message
 from='juliet@example.com/balcony'
 id='jk3v47gw'
 to='romeo@example.net'
 type='chat'
 xml:lang='en'>
 <subject>I implore you!</subject>
 <subject xml:lang='cs'>
 Úpěnlivě prosím!
 </subject>
 <body>Wherefore art thou, Romeo?</body>
 <body xml:lang='cs'>
 Pročež jsi ty, Romeo?
 </body>
 </message>

 The <subject/> element MUST NOT contain mixed content (as defined in
 Section 3.2.2 of [XML]).

5.2.5. Thread Element

The primary use of the XMPP <thread/> element is to uniquely identify
 a conversation thread or "chat session" between two entities
 instantiated by <message/> stanzas of type 'chat'. However, the XMPP
 <thread/> element MAY also be used to uniquely identify an analogous
 thread between two entities instantiated by <message/> stanzas of
 type 'headline' or 'normal', or among multiple entities in the
 context of a multi-user chat room instantiated by <message/> stanzas
 of type 'groupchat'. It MAY also be used for <message/> stanzas not
 related to a human conversation, such as a game session or an
 interaction between plugins. The <thread/> element is not used to
 identify individual messages, only conversations or messaging
 sessions.

 The inclusion of the <thread/> element is OPTIONAL. Because the
 <thread/> element identifies the particular conversation thread to
 which a message belongs, a message stanza MUST NOT contain more than
 one <thread/> element.

 The <thread/> element MAY possess a 'parent' attribute that
 identifies another thread of which the current thread is an offshoot
 or child. The 'parent' attribute MUST conform to the syntax of the
 <thread/> element itself and its value MUST be different from the XML
 character data of the <thread/> element on which the 'parent'
 attribute is included.

Implementation Note: The ability to specify both a parent thread
 and a child thread introduces the possibility of conflicts between
 thread identifiers for overlapping threads. For example, one
 <thread/> element might contain XML character data of "foo" and a
 'parent' attribute whose value is "bar", a second <thread/>
 element might contain XML character data of "bar" and a 'parent'
 attribute whose value is "baz", and a third <thread/> element
 might contain XML character data of "baz" and a 'parent' attribute
 whose value is once again "foo". It is up to the implementation
 how it will treat conflicts between such overlapping thread
 identifiers (e.g., whether it will "chain together" thread
 identifiers by showing "foo" as both a parent and grandchild of
 "baz" in a multi-level user interface, or whether it will show
 only one level of dependency at a time).

 The value of the <thread/> element is not human-readable and MUST be
 treated as opaque by entities; no semantic meaning can be derived
 from it, and only exact comparisons can be made against it. The
 value of the <thread/> element MUST uniquely identify the
 conversation thread either between the conversation partners or more
 generally (one way to ensure uniqueness is by generating a
 universally unique identifier (UUID) as described in [UUID]).

 Security Warning: An application that generates a ThreadID MUST
 ensure that it does not reveal identifying information about the
 entity (e.g., the MAC address of the device on which the XMPP
 application is running).

 The <thread/> element MUST NOT contain mixed content (as defined in
 Section 3.2.2 of [XML]).

 <message
 from='juliet@example.com/balcony'
 to='romeo@example.net'
 type='chat'
 xml:lang='en'>
 <subject>I implore you!</subject>
 <subject xml:lang='cs'>
 Úpěnlivě prosím!
 </subject>
 <body>Wherefore art thou, Romeo?</body>
 <body xml:lang='cs'>
 Pročež jsi ty, Romeo?
 </body>
 <thread parent='e0ffe42b28561960c6b12b944a092794b9683a38'>
 0e3141cd80894871a68e6fe6b1ec56fa
 </thread>
 </message>

For detailed recommendations regarding use of the <thread/> element,
refer to [XEP-0201].

5.3. Extended Content

As described in [XMPP-CORE], an XML stanza MAY contain any child
 element that is qualified by a namespace other than the default
 namespace; this applies to the message stanza as well. Guidelines
 for handling extended content on the part of both routing servers and
 end recipients are provided in Section 8.4 of [XMPP-CORE].

 (In the following example, the message stanza includes an XHTML-
 formatted version of the message as defined in [XEP-0071]).)

 <message
 from='juliet@example.com/balcony'
 to='romeo@example.net'
 type='chat'
 xml:lang='en'>
 <body>Wherefore art thou, Romeo?</body>
 <html xmlns='http://jabber.org/protocol/xhtml-im'>
 <body xmlns='http://www.w3.org/1999/xhtml'>
 Wherefore art
 thou, Romeo?
 </body>
 </html>
 </message>

6. Exchanging IQ Stanzas

As described in [XMPP-CORE], IQ stanzas provide a structured request-
 response mechanism. The basic semantics of that mechanism (e.g.,
 that the 'id' attribute is mandatory) are defined in [XMPP-CORE],
 whereas the specific semantics needed to complete particular use
 cases are defined in all instances by the extended namespace that
 qualifies the direct child element of an IQ stanza of type "get" or
 "set". The 'jabber:client' and 'jabber:server' namespaces do not
 define any children of IQ stanzas other than the <error/> element
 common to all stanza types. This document defines one such extended
 namespace, for Managing the Roster (Section 2). However, an IQ
 stanza MAY contain structured information qualified by any extended
 namespace.

7. A Sample Session

The examples in this section illustrate a possible instant messaging
 and presence session. The user is <romeo@example.net>, he has an
 available resource whose resourcepart is "orchard", and he has the
 following individuals in his roster:

 o <juliet@example.com> (subscription="both" and she has two
 available resources, "chamber" and "balcony")

 o <benvolio@example.net> (subscription="to")

 o <mercutio@example.org> (subscription="from")

 First, the user completes the preconditions (stream establishment,
 TLS and SASL negotiation, and resource binding) described in
 [XMPP-CORE]; those protocol flows are not reproduced here.

 Next, the user requests his roster.

 Example 1: User requests current roster from server

 UC: <iq from='romeo@example.net/orchard'
 id='hf61v3n7'
 type='get'>
 <query xmlns='jabber:iq:roster'/>
 </iq>

 Example 2: User receives roster from server

 US: <iq id='hf61v3n7'
 to='romeo@example.net/orchard'
 type='result'>
 <query xmlns='jabber:iq:roster'>
 <item jid='juliet@example.com'
 name='Juliet'
 subscription='both'>
 <group>Friends</group>
 </item>
 <item jid='benvolio@example.org'
 name='Benvolio'
 subscription='to'/>
 <item jid='mercutio@example.org'
 name='Mercutio'
 subscription='from'/>
 </query>
 </iq>

Now the user begins a presence session.

 Example 3: User sends initial presence

 UC: <presence/>

 Example 4: User's server sends presence probes to contacts with
 subscription="to" and subscription="both" on behalf of the user

 US: <presence
 from='romeo@example.net'
 to='juliet@example.com'
 type='probe'/>

 US: <presence
 from='romeo@example.net'
 to='benvolio@example.org'
 type='probe'/>

 Example 5: User's server sends initial presence to contacts with
 subscription="from" and subscription="both" on behalf of the user's
 available resource, as well as to user

 US: <presence
 from='romeo@example.net/orchard'
 to='juliet@example.com'/>

 US: <presence
 from='romeo@example.net/orchard'
 to='mercutio@example.org'/>

 US: <presence
 from='romeo@example.net/orchard'
 to='romeo@example.net'/>

 Example 6: Contacts' servers reply to presence probe on behalf of all
 available resources

 CS: <presence
 from='juliet@example.com/balcony'
 to='romeo@example.net'
 xml:lang='en'>
 <show>away</show>
 <status>be right back</status>
 <priority>0</priority>
 </presence>

CS: <presence
 from='juliet@example.com/chamber'
 to='romeo@example.net'>
 <priority>1</priority>
 </presence>

 CS: <presence
 from='benvolio@example.org/pda'
 to='romeo@example.net'
 xml:lang='en'>
 <show>dnd</show>
 <status>gallivanting</status>
 </presence>

 Example 7: Contacts' servers deliver user's initial presence to all
 available resources

 CS: <presence
 from='romeo@example.net/orchard'
 to='juliet@example.com'/>

 CS: <presence
 from='romeo@example.net/orchard'
 to='juliet@example.com'/>

 CS: <presence
 from='romeo@example.net/orchard'
 to='mercutio@example.org'/>

 Example 8: User sends directed presence to another user not in his
 roster

 UC: <presence
 from='romeo@example.net/orchard'
 to='nurse@example.com'
 xml:lang='en'>
 <show>dnd</show>
 <status>courting Juliet</status>
 <priority>0</priority>
 </presence>

 Now the user engages in a chat session with one of his contacts.

Example 9: A threaded conversation

 CC: <message
 from='juliet@example.com/balcony'
 to='romeo@example.net'
 type='chat'
 xml:lang='en'>
 <body>My ears have not yet drunk a hundred words</body>
 <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
 </message>

 CC: <message
 from='juliet@example.com/balcony'
 to='romeo@example.net'
 type='chat'
 xml:lang='en'>
 <body>Of that tongue's utterance, yet I know the sound:</body>
 <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
 </message>

 CC: <message
 from='juliet@example.com/balcony'
 to='romeo@example.net'
 type='chat'
 xml:lang='en'>
 <body>Art thou not Romeo, and a Montague?</body>
 <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
 </message>

 UC: <message
 from='romeo@example.net/orchard'
 to='juliet@example.com/balcony'
 type='chat'
 xml:lang='en'>
 <body>Neither, fair saint, if either thee dislike.</body>
 <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
 </message>

 CC: <message
 from='juliet@example.com/balcony'
 to='romeo@example.net/orchard'
 type='chat'
 xml:lang='en'>
 <body>How cam'st thou hither, tell me, and wherefore?</body>
 <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
 </message>

 And so on.

The user can also send subsequent presence broadcast.

 Example 10: User sends updated available presence for broadcast

 UC: <presence xml:lang='en'>
 <show>away</show>
 <status>I shall return!</status>
 <priority>1</priority>
 </presence>

 Example 11: User's server broadcasts updated presence to the contacts
 who have a subscription of type "both" or "from" (but not to the
 entity to which the user sent directed presence)

 US: <presence
 from='romeo@example.net/orchard'
 to='juliet@example.com'
 xml:lang='en'>
 <show>away</show>
 <status>I shall return!</status>
 <priority>1</priority>
 </presence>

 US: <presence
 from='romeo@example.net/orchard'
 to='mercutio@example.org'
 xml:lang='en'>
 <show>away</show>
 <status>I shall return!</status>
 <priority>1</priority>
 </presence>

 Example 12: Contacts' servers deliver updated presence

 CS: <presence
 from='romeo@example.net/orchard'
 to='juliet@example.com'
 xml:lang='en'>
 <show>away</show>
 <status>I shall return!</status>
 <priority>1</priority>
 </presence>

CS: <presence
 from='romeo@example.net/orchard'
 to='juliet@example.com'
 xml:lang='en'>
 <show>away</show>
 <status>I shall return!</status>
 <priority>1</priority>
 </presence>

 CS: <presence
 from='romeo@example.net/orchard'
 to='mercutio@example.org'
 xml:lang='en'>
 <show>away</show>
 <status>I shall return!</status>
 <priority>1</priority>
 </presence>

 Example 13: One of the contact's resources broadcasts unavailable
 notification

 CC: <presence from='juliet@example.com/chamber' type='unavailable'/>

 Example 14: Contact's server sends unavailable notification to user

 CS: <presence
 from='juliet@example.com/chamber'
 to='romeo@example.net'
 type='unavailable'/>

 Now the user ends his presence session.

 Example 15: User sends unavailable notification

 UC: <presence type='unavailable' xml:lang='en'>
 <status>gone home</status>
 </presence>

 Example 16: User's server broadcasts unavailable notification to
 contacts as well as to the entity to whom the user sent directed
 presence

US: <presence
 from='romeo@example.net/orchard'
 to='juliet@example.com'
 type='unavailable'
 xml:lang='en'>
 <status>gone home</status>
 </presence>

 US: <presence
 from='romeo@example.net/orchard'
 to='mercutio@example.org'
 type='unavailable'
 xml:lang='en'>
 <status>gone home</status>
 </presence>

 US: <presence
 from='romeo@example.net/orchard'
 to='nurse@example.com'
 type='unavailable'
 xml:lang='en'>
 <status>gone home</status>
 </presence>

 Finally the user closes his stream and the server responds in kind.

 Example 17: User closes stream

 UC: </stream:stream>

 Example 18: User's server closes stream

 US: </stream:stream>

 THE END

8. Server Rules for Processing XML Stanzas

   Basic server rules for processing XML stanzas are defined in
   [XMPP-CORE], and the reader is referred to that specification for
   underlying rules and security implications.  This section defines
   supplementary rules for XMPP instant messaging and presence servers.

   Some delivery rules defined in this section specify the use of
   "offline storage", i.e., the server's act of storing a message stanza
   on behalf of the user and then delivering it when the user next
   becomes available.  For recommendations regarding offline message
   storage, refer to [XEP-0160].

8.1. General Considerations

   [XMPP-CORE] discusses general considerations for stanza delivery, in
   particular the tradeoffs between (i) providing an acceptable level of
   service regarding stanza delivery and (ii) preventing directory
   harvesting attacks and presence leaks.  However, the concept of a
   directory harvesting attack does not apply if a contact is known to
   and trusted by a user (because the contact is in the user's roster as
   described under Section 2).  Similarly, the concept of a presence
   leak does not apply if a contact is authorized to know a user's
   presence (by means of a presence subscription as described under
   Section 3) or if the user has voluntarily sent presence to an entity
   (by means of directed presence as described under Section 4.6).
   Therefore, in cases where the following sections guard against
   directory harvesting attacks and presence leaks by providing an
   alternative of (a) silently ignoring a stanza or (b) returning an
   error, a server SHOULD return an error if the originating entity is
   in the user's roster (when the error would reveal whether the user's
   account exists) or is authorized to receive presence from the user or
   has received directed presence from the user (when the error would
   reveal the presence of a user's resource).

      Security Warning: All of the stanza processing rules described
      below are defined with the understanding that they will be applied
      subject to enforcement of relevant privacy and security policies,
      such as those deployed by means of [XEP-0016] or [XEP-0191].  The
      conformance language (MUST, SHOULD, etc.) in the following
      sections is not meant to override any such local service policies.

8.2. No 'to' Address

If the stanza possesses no 'to' attribute, the rules defined in
[XMPP-CORE] apply.

8.3. Remote Domain

   If the domainpart of the address contained in the 'to' attribute of
   an outbound stanza does not match a configured domain of the server
   itself, then the rules provided in Section 10.4 of [XMPP-CORE] apply.

      Interoperability Note: RFC 3921 specified how to use the _im._xmpp
      and _pres._xmpp SRV records [IMP-SRV] as a fallback method for
      discovering whether a remote instant messaging and presence
      service communicates via XMPP.  Because those SRV records have not
      been widely deployed, this document no longer specifies their use,
      and new implementations are not encouraged.

8.4. Local Domain

If the domainpart of the JID contained in the 'to' attribute matches
 one of the configured domains of the server, the domain is serviced
 by the server itself (not by a specialized local service), and the
 JID is of the form <domainpart> or <domainpart/resourcepart>, the
 rules defined in [XMPP-CORE] apply.

8.5. Local User

If the 'to' address specifies a bare JID <localpart@domainpart> or
 full JID <localpart@domainpart/resourcepart> where the domainpart of
 the JID matches a configured domain that is serviced by the server
 itself, the server MUST proceed as follows.

8.5.1. No Such User

If the user account identified by the 'to' attribute does not exist,
 how the stanza is processed depends on the stanza type.

 o For an IQ stanza, the server MUST return a <service-unavailable/>
 stanza error to the sender.

 o For a message stanza, the server MUST either (a) silently ignore
 the message or (b) return a <service-unavailable/> stanza error to
 the sender.

 o For a presence stanza with no 'type' attribute or a 'type'
 attribute of "unavailable", the server MUST silently ignore the
 stanza.

 o For a presence stanza of type "subscribe", "subscribed",
 "unsubscribe", or "unsubscribed", the server MUST silently ignore
 the stanza.

 o For a presence stanza of type "probe", the server MUST either (a)
 silently ignore the stanza or (b) return a presence stanza of type
 "unsubscribed".

8.5.2. localpart@domainpart

If the JID contained in the 'to' attribute is of the form
 <localpart@domainpart>, then the server MUST adhere to the following
 rules.

8.5.2.1. Available or Connected Resources

If there is at least one available resource or connected resource,
how the stanza is processed depends on the stanza type.

8.5.2.1.1. Message

For a message stanza of type "normal":

 o If all of the available resources have a negative presence
 priority then the server SHOULD either (a) store the message
 offline for later delivery or (b) return a stanza error to the
 sender, which SHOULD be <service-unavailable/>.

 o If there is one available resource with a non-negative presence
 priority then the server MUST deliver the message to that
 resource.

 o If there is more than one resource with a non-negative presence
 priority then the server MUST either (a) deliver the message to
 the "most available" resource or resources (according to the
 server's implementation-specific algorithm, e.g., treating the
 resource or resources with the highest presence priority as "most
 available") or (b) deliver the message to all of the non-negative
 resources.

 For a message stanza of type "chat":

 o If the only available resource has a negative presence priority
 then the server SHOULD either (a) store the message offline for
 later delivery or (b) return a stanza error to the sender, which
 SHOULD be <service-unavailable/>.

 o If the only available resource has a non-negative presence
 priority then the server MUST deliver the message to that
 resource.

 o If there is more than one resource with a non-negative presence
 priority then the server MUST either (a) deliver the message to
 the "most available" resource or resources (according to the
 server's implementation-specific algorithm, e.g., treating the
 resource or resources with the highest presence priority as "most
 available") or (b) deliver the message to all of the non-negative
 resources that have opted in to receive chat messages.

 For a message stanza of type "groupchat", the server MUST NOT deliver
 the stanza to any of the available resources but instead MUST return
 a stanza error to the sender, which SHOULD be <service-unavailable/>.

For a message stanza of type "headline":

 o If the only available resource has a negative presence priority
 then the server MUST silently ignore the stanza.

 o If the only available resource has a non-negative presence
 priority then the server MUST deliver the message to that
 resource.

 o If there is more than one resource with a non-negative presence
 priority then the server MUST deliver the message to all of the
 non-negative resources.

 For a message stanza of type "error", the server MUST silently ignore
 the message.

 However, for any message type the server MUST NOT deliver the stanza
 to any available resource with a negative priority; if the only
 available resource has a negative priority, the server SHOULD handle
 the message as if there were no available resources or connected
 resources as described under Section 8.5.2.2.

 In all cases, the server MUST NOT rewrite the 'to' attribute (i.e.,
 it MUST leave it as <localpart@domainpart> rather than change it to
 <localpart@domainpart/resourcepart>).

8.5.2.1.2. Presence

For a presence stanza with no type or of type "unavailable", the
 server MUST deliver it to all available resources.

 For a presence stanza of type "subscribe", "subscribed",
 "unsubscribe", or "unsubscribed", the server MUST adhere to the rules
 defined under Section 3 and summarized under Appendix A.

 For a presence stanza of type "probe", the server MUST handle it
 directly as described under Section 4.3.

 In all cases, the server MUST NOT rewrite the 'to' attribute (i.e.,
 it MUST leave it as <localpart@domainpart> rather than change it to
 <localpart@domainpart/resourcepart>).

8.5.2.1.3. IQ

   For an IQ stanza, the server itself MUST reply on behalf of the user
   with either an IQ result or an IQ error, and MUST NOT deliver the IQ
   stanza to any of the user's available resources.  Specifically, if
   the semantics of the qualifying namespace define a reply that the

server can provide on behalf of the user, then the server MUST reply
 to the stanza on behalf of the user by returning either an IQ stanza
 of type "result" or an IQ stanza of type "error" that is appropriate
 to the original payload; if not, then the server MUST reply with a
 <service-unavailable/> stanza error.

8.5.2.2. No Available or Connected Resources

   If there are no available resources or connected resources associated
   with the user, how the stanza is processed depends on the stanza
   type.

8.5.2.2.1. Message

For a message stanza of type "normal" or "chat", the server SHOULD
 either (a) add the message to offline storage or (b) return a stanza
 error to the sender, which SHOULD be <service-unavailable/>.

 For a message stanza of type "groupchat", the server MUST return an
 error to the sender, which SHOULD be <service-unavailable/>.

 For a message stanza of type "headline" or "error", the server MUST
 silently ignore the message.

8.5.2.2.2. Presence

   For a presence stanza with no type or of type "unavailable", the
   server SHOULD silently ignore the stanza by not storing it for later
   delivery and not replying to it on behalf of the user.

   For a presence stanza of type "subscribe", "subscribed",
   "unsubscribe", or "unsubscribed", the server MUST adhere to the rules
   defined under Section 3 and summarized under Appendix A.

   For a presence stanza of type "probe", the server MUST handle it
   directly as described under Section 4.3.

8.5.2.2.3. IQ

For an IQ stanza, the server itself MUST reply on behalf of the user
 with either an IQ result or an IQ error. Specifically, if the
 semantics of the qualifying namespace define a reply that the server
 can provide on behalf of the user, then the server MUST reply to the
 stanza on behalf of the user by returning either an IQ stanza of type
 "result" or an IQ stanza of type "error" that is appropriate to the
 original payload; if not, then the server MUST reply with a <service-
 unavailable/> stanza error.

8.5.3. localpart@domainpart/resourcepart

If the domainpart of the JID contained in the 'to' attribute of an
 inbound stanza matches one of the configured domains of the server
 itself and the JID contained in the 'to' attribute is of the form
 <localpart@domainpart/resourcepart>, then the server MUST adhere to
 the following rules.

8.5.3.1. Resource Matches

If an available resource or connected resource exactly matches the
 full JID, how the stanza is processed depends on the stanza type.

 o For an IQ stanza of type "get" or "set", if the intended recipient
 does not share presence with the requesting entity either by means
 of a presence subscription of type "both" or "from" or by means of
 directed presence, then the server SHOULD NOT deliver the IQ
 stanza but instead SHOULD return a <service-unavailable/> stanza
 error to the requesting entity. This policy helps to prevent
 presence leaks (see Section 11).

 o For an IQ stanza of type "result" or "error", the server MUST
 deliver the stanza to the resource.

 o For a message stanza, the server MUST deliver the stanza to the
 resource.

 o For a presence stanza with no 'type' attribute or a 'type'
 attribute of "unavailable", the server MUST deliver the stanza to
 the resource.

 o For a presence stanza of type "subscribe", "subscribed",
 "unsubscribe", or "unsubscribed", the server MUST follow the
 guidelines provided under Section 3.

 o For a presence stanza of type "probe", the server MUST follow the
 guidelines provided under Section 4.3.

8.5.3.2. No Resource Matches

If no available resource or connected resource exactly matches the
full JID, how the stanza is processed depends on the stanza type.

8.5.3.2.1. Message

For a message stanza of type "normal", "groupchat", or "headline",
 the server MUST either (a) silently ignore the stanza or (b) return
 an error stanza to the sender, which SHOULD be <service-
 unavailable/>.

 For a message stanza of type "chat":

 o If there is no available or connected resource, the server MUST
 either (a) store the message offline for later delivery or (b)
 return an error stanza to the sender, which SHOULD be <service-
 unavailable/>.

 o If all of the available resources have a negative presence
 priority then the server SHOULD (a) store the message offline for
 later delivery or (b) return a stanza error to the sender, which
 SHOULD be <service-unavailable/>.

 o If there is one available resource with a non-negative presence
 priority then the server MUST deliver the message to that
 resource.

 o If there is more than one resource with a non-negative presence
 priority then the server MUST either (a) deliver the message to
 the "most available" resource or resources (according to the
 server's implementation-specific algorithm, e.g., treating the
 resource or resources with the highest presence priority as "most
 available") or (b) deliver the message to all of the non-negative
 resources that have opted in to receive chat messages.

 For a message stanza of type "error", the server MUST silently ignore
 the stanza.

8.5.3.2.2. Presence

   For a presence stanza with no 'type' attribute or a 'type' attribute
   of "unavailable", the server MUST silently ignore the stanza.

   For a presence stanza of type "subscribe", the server MUST follow the
   guidelines provided under Section 3.1.3.

   For a presence stanza of type "subscribed", "unsubscribe", or
   "unsubscribed", the server MUST ignore the stanza.

   For a presence stanza of type "probe", the server MUST follow the
   guidelines provided under Section 4.3.

8.5.3.2.3. IQ

For an IQ stanza, the server MUST return a <service-unavailable/>
stanza error to the sender.

8.5.4. Summary of Message Delivery Rules

   The following table summarizes the message (not stanza) delivery
   rules described earlier in this section.  The left column shows
   various combinations of conditions (non-existent account, no active
   resources, only one resource and it has a negative presence priority,
   only one resource and it has a non-negative presence priority, or
   more than one resource and each one has a non-negative presence
   priority) and 'to' addresses (bare JID, full JID matching an
   available resource, or full JID matching no available resource).  The
   subsequent columns list the four primary message types (normal, chat,
   groupchat, or headline) along with six possible delivery options:
   storing the message offline (O), bouncing the message with a stanza
   error (E), silently ignoring the message (S), delivering the message
   to the resource specified in the 'to' address (D), delivering the
   message to the "most available" resource or resources according to
   the server's implementation-specific algorithm, e.g., treating the
   resource or resources with the highest presence priority as "most
   available" (M), or delivering the message to all resources with non-
   negative presence priority (A -- where for chat messages "all
   resources" can mean the set of resources that have explicitly opted
   in to receiving every chat message).  The '/' character stands for
   "exclusive or".  The server SHOULD observe the rules given in section
   8.1 when choosing which action to take for a particular message.

   Table 1: Message Delivery Rules

   +----------------------------------------------------------+
   | Condition        | Normal | Chat  | Groupchat | Headline |
   +----------------------------------------------------------+
   | ACCOUNT DOES NOT EXIST                                   |
   |  bare            |  S/E   |  S/E  |     E     |    S     |
   |  full            |  S/E   |  S/E  |    S/E    |   S/E    |
   +----------------------------------------------------------+
   | ACCOUNT EXISTS, BUT NO ACTIVE RESOURCES                  |
   |  bare            |  O/E   |  O/E  |     E     |    S     |
   |  full (no match) |  S/E   |  O/E  |    S/E    |   S/E    |
   +----------------------------------------------------------+
   | 1+ NEGATIVE RESOURCES BUT ZERO NON-NEGATIVE RESOURCES    |
   |  bare            |  O/E   |  O/E  |     E     |    S     |
   |  full match      |   D    |   D   |     D     |    D     |
   |  full no match   |  S/E   |  O/E  |    S/E    |   S/E    |
   +----------------------------------------------------------+
   | 1 NON-NEGATIVE RESOURCE                                  |
   |  bare            |   D    |   D   |     E     |    D     |
   |  full match      |   D    |   D   |     D     |    D     |
   |  full no match   |  S/E   |   D   |    S/E    |   S/E    |
   +----------------------------------------------------------+
   | 1+ NON-NEGATIVE RESOURCES                                |
   |  bare            |  M/A   |  M/A* |     E     |    A     |
   |  full match      |   D    |  D/A* |     D     |    D     |
   |  full no match   |  S/E   |  M/A* |    S/E    |   S/E    |
   +----------------------------------------------------------+

      * For messages of type "chat", a server SHOULD NOT act in
      accordance with option (A) unless clients can explicitly opt in to
      receiving all chat messages; however, methods for opting in are
      outside the scope of this specification.

9. Handling of URIs

   The addresses of XMPP entities as used in communication over an XMPP
   network (e.g., in the 'from' and 'to' addresses of an XML stanza)
   MUST NOT be prepended with a Uniform Resource Identifier [URI]
   scheme.

   However, an application that is external to XMPP itself (e.g., a page
   on the World Wide Web) might need to identify an XMPP entity either
   as a URI or as an Internationalized Resource Identifier [IRI], and an
   XMPP client might need to interact with such an external application
   (for example, an XMPP client might be invoked by clicking a link
   provided on a web page).  In the context of such interactions, XMPP
   clients are encouraged to handle addresses that are encoded as

   "xmpp:" URIs and IRIs as specified in [XMPP-URI] and further
   described in [XEP-0147].  Although XMPP clients are also encouraged
   to handle addresses that are encoded as "im:" URIs as specified in
   [CPIM] and "pres:" URIs as specified in [CPP], they can do so by
   removing the "im:" or "pres:" scheme and entrusting address
   resolution to the server as specified under Section 8.3.

10. Internationalization Considerations

For internationalization considerations, refer to the relevant
section of [XMPP-CORE].

11. Security Considerations

   Core security considerations for XMPP are provided in Section 13 of
   [XMPP-CORE], including discussion of channel encryption,
   authentication, information leaks, denial-of-service attacks, and
   interdomain federation.

   Section 13.1 of [XMPP-CORE] outlines the architectural roles of
   clients and servers in typical deployments of XMPP, and discusses the
   security properties associated with those roles.  These roles have an
   impact on the security of instant messages, presence subscriptions,
   and presence notifications as described in this document.  In
   essence, an XMPP user registers (or has provisioned) an account on an
   XMPP server and therefore places some level of trust in the server to
   complete various tasks on the user's behalf, enforce security
   policies, etc.  Thus it is the server's responsibility to:

   1.  Preferably mandate the use of channel encryption for
       communication with local clients and remote servers.

   2.  Authenticate any client that wishes to access the user's account.

   3.  Process XML stanzas to and from clients that have authenticated
       as the user (specifically with regard to instant messaging and
       presence functionality, store the user's roster, process inbound
       and outbound subscription requests and responses, generate and
       handle presence probes, broadcast outbound presence
       notifications, route outbound messages, and deliver inbound
       messages and presence notifications).

   As discussed in Sections 13.1 and 13.4 of [XMPP-CORE], even if the
   server fulfills the foregoing responsibilities, the client does not
   have any assurance that stanzas it might exchange with other clients
   (whether on the same server or a remote server) are protected for all
   hops along the XMPP communication path, or within the server itself.
   It is the responsibility of the client to use an appropriate

technology for encryption and signing of XML stanzas if it wishes to
 ensure end-to-end confidentiality and integrity of its
 communications.

 Additional considerations that apply only to instant messaging and
 presence applications of XMPP are defined in several places within
 this document; specifically:

 o When a server processes an inbound presence stanza of type "probe"
 whose intended recipient is a user associated with one of the
 server's configured domains, the server MUST NOT reveal the user's
 presence if the sender is an entity that is not authorized to
 receive that information as determined by presence subscriptions
 (see Section 4).

 o A user's server MUST NOT leak the user's network availability to
 entities who are not authorized to know the user's presence. In
 XMPP itself, authorization takes the form of an explicit
 subscription from a contact to the user (as described under
 Section 3). However, some XMPP deployments might consider an
 entity to be authorized if there is an existing trust relationship
 between the entity and the user who is generating presence
 information (as an example, a corporate deployment of XMPP might
 automatically add the user's presence information to a private
 directory of employees if the organization mandates the sharing of
 presence information as part of an employment agreement).

 o When a server processes an outbound presence stanza with no type
 or of type "unavailable", it MUST follow the rules defined under
 Section 4 in order to ensure that such presence information is not
 sent to entities that are not authorized to know such information.

 o A client MAY ignore the <status/> element when contained in a
 presence stanza of type "subscribe", "unsubscribe", "subscribed",
 or "unsubscribed"; this can help prevent "presence subscription
 spam".

12. 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].

 Feature: message-body
 Description: Support the <body/> child element of the <message/>
 stanza.
 Section: Section 5.2.3
 Roles: Client MUST, Server N/A.

 Feature: message-subject
 Description: Support the <subject/> child element of the <message/>
 stanza.
 Section: Section 5.2.4
 Roles: Client SHOULD, Server N/A.

 Feature: message-thread
 Description: Support the <thread/> child element of the <message/>
 stanza.
 Section: Section 5.2.5
 Roles: Client SHOULD, Server N/A.

 Feature: message-type-support
 Description: Support reception of messages of type "normal", "chat",
 "groupchat", "headline", and "error".
 Section: Section 5.2.2
 Roles: Client SHOULD, Server N/A.

 Feature: message-type-deliver
 Description: Appropriately deliver messages of type "normal",
 "chat", "groupchat", "headline", and "error".
 Section: Section 8
 Roles: Client N/A, Server SHOULD.

Feature: presence-notype
 Description: Treat a presence stanza with no 'type' attribute as
 indicating availability.
 Section: Section 4.7.1
 Roles: Client MUST, Server MUST.

 Feature: presence-probe
 Description: Send and receive presence stanzas with a 'type'
 attribute of "probe" for the discovery of presence information.
 Section: Section 4.7.1
 Roles: Client N/A, Server MUST.

 Feature: presence-sub-approval
 Description: Treat an outbound presence stanza of type "subscribed"
 as the act of approving a presence subscription request previously
 received from another entity, and treat an inbound presence stanza
 of type "subscribed" as a subscription approval from another
 entity.
 Section: Section 3.1
 Roles: Client MUST, Server MUST.

 Feature: presence-sub-cancel
 Description: Treat an outbound presence stanza of type
 "unsubscribed" as the act of denying a subscription request
 received from another entity or canceling a subscription approval
 previously granted to another entity, and treat an inbound
 presence stanza of type "unsubscribed" as an subscription denial
 or cancellation from another entity.
 Section: Section 3.2
 Roles: Client MUST, Server MUST.

 Feature: presence-sub-preapproval
 Description: Treat an outbound presence stanza of type "subscribed"
 in certain circumstances as the act of pre-approving a
 subscription request received from another entity; this includes
 support for the 'approved' attribute of the <item/> element within
 the 'jabber:iq:roster' namespace.
 Section: Section 3.4
 Roles: Client MAY, Server MAY.

 Feature: presence-sub-request
 Description: Treat an outbound presence stanza of type "subscribe"
 as the act of requesting a subscription to the presence
 information of another entity, and treat an inbound presence
 stanza of type "subscribe" as a presence subscription request from
 another entity.
 Section: Section 3.1
 Roles: Client MUST, Server MUST.

Feature: presence-sub-unsubscribe
 Description: Treat an outbound presence stanza of type "unsubscribe"
 as the act of unsubscribing from another entity, and treat an
 inbound presence stanza of type "unsubscribe" as an unsubscribe
 notification from another entity.
 Section: Section 3.3
 Roles: Client MUST, Server MUST.

 Feature: presence-unavailable
 Description: Treat a presence stanza with a 'type' attribute of
 "unavailable" as indicating lack of availability.
 Section: Section 4.7.1
 Roles: Client MUST, Server MUST.

 Feature: roster-get
 Description: Treat an IQ stanza of type "get" containing an empty
 <query/> element qualified by the 'jabber:iq:roster' namespace as
 a request to retrieve the roster information associated with an
 account on a server.
 Section: Section 2.1.3
 Roles: Client MUST, Server MUST.

 Feature: roster-set
 Description: Treat an IQ stanza of type "set" containing a <query/>
 element qualified by the 'jabber:iq:roster' namespace as a request
 to add or update the item contained in the <query/> element.
 Section: Section 2.1.5
 Roles: Client MUST, Server MUST.

 Feature: roster-push
 Description: Send a roster push to each interested resource whenever
 the server-side representation of the roster information
 materially changes, or handle such a push when received from the
 server.
 Section: Section 2.1.6
 Roles: Client MUST, Server MUST.

 Feature: roster-version
 Description: Treat the 'ver' attribute of the <query/> element
 qualified by the 'jabber:iq:roster' namespace as an identifier of
 the particular version of roster information being sent or
 received.
 Section: Section 2.1.1
 Roles: Client SHOULD, Server MUST.

13. References

13.1. Normative References

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

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

 [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>.

 [XML-NAMES]
 Bray, T., Hollander, D., and A. Layman, "Namespaces in
 XML", W3C REC-xml-names, January 1999,
 <http://www.w3.org/TR/REC-xml-names>.

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

13.2. Informative References

   [CPIM]     Peterson, J., "Common Profile for Instant Messaging
              (CPIM)", RFC 3860, August 2004.

   [CPP]      Peterson, J., "Common Profile for Presence (CPP)",
              RFC 3859, August 2004.

   [DOS]      Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, December 2006.

   [IMP-MODEL]
              Day, M., Rosenberg, J., and H. Sugano, "A Model for
              Presence and Instant Messaging", RFC 2778, February 2000.

   [IMP-REQS]
              Day, M., Aggarwal, S., and J. Vincent, "Instant Messaging
              / Presence Protocol Requirements", RFC 2779,
              February 2000.

   [IMP-SRV]  Peterson, J., "Address Resolution for Instant Messaging
              and Presence", RFC 3861, August 2004.

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

 [IRC] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
 April 2000.

 [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.

 [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.

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

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

 [SIP-PRES]
 Rosenberg, J., "A Presence Event Package for the Session
 Initiation Protocol (SIP)", RFC 3856, August 2004.

 [TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
 (TLS) Protocol Version 1.2", RFC 5246, August 2008.

 [TLS-CERTS]
 Saint-Andre, P. and J. Hodges, "Representation and
 Verification of Domain-Based Application Service Identity
 within Internet Public Key Infrastructure Using X.509
 (PKIX) Certificates in the Context of Transport Layer
 Security (TLS)", RFC 6125, March 2011.

 [UNICODE] The Unicode Consortium, "The Unicode Standard, Version
 6.0", 2010,
 <http://www.unicode.org/versions/Unicode6.0.0/>.

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

   [UUID]     Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              July 2005.

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

   [XEP-0045]
              Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
              July 2008.

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

   [XEP-0071]
              Saint-Andre, P., "XHTML-IM", XSF XEP 0071, September 2008.

   [XEP-0115]
              Hildebrand, J., Saint-Andre, P., and R. Troncon, "Entity
              Capabilities", XSF XEP 0115, February 2008.

   [XEP-0147]
              Saint-Andre, P., "XMPP URI Scheme Query Components", XSF
              XEP 0147, September 2006.

   [XEP-0160]
              Saint-Andre, P., "Best Practices for Handling Offline
              Messages", XSF XEP 0160, January 2006.

   [XEP-0163]
              Saint-Andre, P. and K. Smith, "Personal Eventing
              Protocol", XSF XEP 0163, July 2010.

   [XEP-0191]
              Saint-Andre, P., "Simple Communications Blocking", XSF
              XEP 0191, February 2007.

   [XEP-0201]
              Saint-Andre, P., Paterson, I., and K. Smith, "Best
              Practices for Message Threads", XSF XEP 0201,
              November 2010.

   [XEP-0237]
              Saint-Andre, P. and D. Cridland, "Roster Versioning", XSF
              XEP 0237, March 2010.

[XML-DATATYPES]
 Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes
 Second Edition", W3C REC-xmlschema-2, October 2004,
 <http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/>.

 [XML-SCHEMA]
 Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech,
 "XML Schema Part 1: Structures Second Edition", World Wide
 Web Consortium Recommendation REC-xmlschema-1-20041028,
 October 2004,
 <http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>.

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

 [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.

 [VCARD] Dawson, F. and T. Howes, "vCard MIME Directory Profile",
 RFC 2426, September 1998.

Appendix A. Subscription States

   This section provides detailed information about subscription states
   and server processing of subscription-related presence stanzas (i.e.,
   presence stanzas of type "subscribe", "subscribed", "unsubscribe",
   and "unsubscribed").

A.1. Defined States

   There are four primary subscription states (these states are
   described from the perspective of the user, not the contact):

   None:  The user does not have a subscription to the contact's
      presence, and the contact does not have a subscription to the
      user's presence.

   To:  The user has a subscription to the contact's presence, but the
      contact does not have a subscription to the user's presence.

   From:  The contact has a subscription to the user's presence, but the
      user does not have a subscription to the contact's presence.

   Both:  Both the user and the contact have subscriptions to each
      other's presence (i.e., the union of 'from' and 'to').

      Implementation Note: For the purpose of processing subscription-
      related presence stanzas as described in the following sections, a
      subscription state of "None" includes the case of the contact not
      being in the user's roster at all, i.e., an unknown entity from
      the perspective of the user's roster.

   The foregoing states are supplemented by various sub-states related
   to pending inbound and outbound subscriptions, thus yielding nine
   possible subscription states:

   1.  "None" = Contact and user are not subscribed to each other, and
       neither has requested a subscription from the other; this is
       reflected in the user's roster by subscription='none'.

   2.  "None + Pending Out" = Contact and user are not subscribed to
       each other, and user has sent contact a subscription request but
       contact has not replied yet; this is reflected in the user's
       roster by subscription='none' and ask='subscribe'.

   3.  "None + Pending In" = Contact and user are not subscribed to each
       other, and contact has sent user a subscription request but user
       has not replied yet.  This state might or might not be reflected
       in the user's roster, as follows: if the user has created a

       roster item for the contact then the server MUST maintain that
       roster item and also note the existence of the inbound presence
       subscription request, whereas if the user has not created a
       roster item for the contact then the user's server MUST note the
       existence of the inbound presence subscription request but MUST
       NOT create a roster item for the contact (instead, the server
       MUST wait until the user has approved the subscription request
       before adding the contact to the user's roster).

   4.  "None + Pending Out+In" = Contact and user are not subscribed to
       each other, contact has sent user a subscription request but user
       has not replied yet, and user has sent contact a subscription
       request but contact has not replied yet; this is reflected in the
       user's roster by subscription='none' and ask='subscribe'.

   5.  "To" = User is subscribed to contact (one-way); this is reflected
       in the user's roster by subscription='to'.

   6.  "To + Pending In" = User is subscribed to contact, and contact
       has sent user a subscription request but user has not replied
       yet; this is reflected in the user's roster by subscription='to'.

   7.  "From" = Contact is subscribed to user (one-way); this is
       reflected in the user's roster by subscription='from'.

   8.  "From + Pending Out" = Contact is subscribed to user, and user
       has sent contact a subscription request but contact has not
       replied yet; this is reflected in the user's roster by
       subscription='from' and ask='subscribe'.

   9.  "Both" = User and contact are subscribed to each other (two-way);
       this is reflected in the user's roster by subscription='both'.

A.2. Server Processing of Outbound Presence Subscription Stanzas

   Outbound presence subscription stanzas enable the user to manage his
   or her subscription to the contact's presence (via the "subscribe"
   and "unsubscribe" types), and to manage the contact's access to the
   user's presence (via the "subscribed" and "unsubscribed" types).

   The following rules apply to outbound routing of the stanza as well
   as changes to the user's roster.  (These rules are described from the
   perspective of the user, not the contact.  In addition, "S.N." stands
   for SHOULD NOT and "M.N." stands for MUST NOT.)

A.2.1. Subscribe

A.2.2. Unsubscribe

A.2.3. Subscribed

A.2.4. Unsubscribed

A.3. Server Processing of Inbound Presence Subscription Stanzas

   Inbound presence subscription stanzas request a subscription-related
   action from the user (via the "subscribe" type), inform the user of
   subscription-related actions taken by the contact (via the

   "unsubscribe" type), or enable the user to manage the contact's
   access to the user's presence information (via the "subscribed" and
   "unsubscribed" types).

   The following rules apply to delivery of the inbound stanza as well
   as changes to the user's roster.  (These rules for server processing
   of inbound presence subscription stanzas are described from the
   perspective of the user, not the contact.  In addition, "S.N." stands
   for SHOULD NOT.)

A.3.1. Subscribe

A.3.2. Unsubscribe

   When the user's server receives a presence stanza of type
   "unsubscribe" for the user from the contact, if the stanza results in
   a subscription state change from the user's perspective then the
   user's server MUST change the state, MUST deliver the presence stanza
   from the contact to the user, and SHOULD auto-reply by sending a
   presence stanza of type "unsubscribed" to the contact on behalf of
   the user.  Otherwise the user's server MUST NOT change the state and
   (because there is no state change) SHOULD NOT deliver the stanza.
   These rules are summarized in the following table.

A.3.3. Subscribed

   When the user's server receives a presence stanza of type
   "subscribed" for the user from the contact, if there is no pending
   outbound request for access to the contact's presence information,
   then it MUST NOT change the subscription state and (because there is
   no state change) SHOULD NOT deliver the stanza to the user.  If there
   is a pending outbound request for access to the contact's presence
   information and the inbound presence stanza of type "subscribed"
   results in a subscription state change, then the user's server MUST
   change the subscription state and MUST deliver the stanza to the
   user.  If the user already is subscribed to the contact's presence
   information, the inbound presence stanza of type "subscribed" does
   not result in a subscription state change; therefore the user's
   server MUST NOT change the subscription state and (because there is
   no state change) SHOULD NOT deliver the stanza to the user.  These
   rules are summarized in the following table.

A.3.4. Unsubscribed

Appendix B. Blocking Communication

   Sections 2.3.5 and 5.4.10 of [IMP-REQS] require that a compliant
   instant messaging and presence technology needs to enable a user to
   block communications from selected users.  Protocols for doing so are
   specified in [XEP-0016] and [XEP-0191].

Appendix C.  vCards

   Sections 3.1.3 and 4.1.4 of [IMP-REQS] require that it be possible to
   retrieve out-of-band contact information for other users (e.g.,
   telephone number or email address).  An XML representation of the
   vCard specification defined in RFC 2426 [VCARD] is in common use
   within the XMPP community to provide such information but is out of
   scope for this specification (documentation of this protocol is
   contained in [XEP-0054]).

Appendix D. XML Schema for jabber:iq:roster

The following schema formally defines the 'jabber:iq:roster'
 namespace used in this document, in conformance with [XML-SCHEMA].
 Because validation of XML streams and stanzas is optional, this
 schema is not normative and is provided for descriptive purposes
 only. For schemas defining core XMPP namespaces, refer to
 [XMPP-CORE].

 <?xml version='1.0' encoding='UTF-8'?>

 <xs:schema
 xmlns:xs='http://www.w3.org/2001/XMLSchema'
 targetNamespace='jabber:iq:roster'
 xmlns='jabber:iq:roster'
 elementFormDefault='qualified'>

 <xs:element name='query'>
 <xs:complexType>
 <xs:sequence>
 <xs:element ref='item'
 minOccurs='0'
 maxOccurs='unbounded'/>
 </xs:sequence>
 <xs:attribute name='ver'
 type='xs:string'
 use='optional'/>
 </xs:complexType>
 </xs:element>

<xs:element name='item'>
 <xs:complexType>
 <xs:sequence>
 <xs:element ref='group'
 minOccurs='0'
 maxOccurs='unbounded'/>
 </xs:sequence>
 <xs:attribute name='approved'
 type='xs:boolean'
 use='optional'/>
 <xs:attribute name='ask'
 use='optional'>
 <xs:simpleType>
 <xs:restriction base='xs:NMTOKEN'>
 <xs:enumeration value='subscribe'/>
 </xs:restriction>
 </xs:simpleType>
 </xs:attribute>
 <xs:attribute name='jid'
 type='xs:string'
 use='required'/>
 <xs:attribute name='name'
 type='xs:string'
 use='optional'/>
 <xs:attribute name='subscription'
 use='optional'
 default='none'>
 <xs:simpleType>
 <xs:restriction base='xs:NMTOKEN'>
 <xs:enumeration value='both'/>
 <xs:enumeration value='from'/>
 <xs:enumeration value='none'/>
 <xs:enumeration value='remove'/>
 <xs:enumeration value='to'/>
 </xs:restriction>
 </xs:simpleType>
 </xs:attribute>
 </xs:complexType>
 </xs:element>

 <xs:element name='group' type='xs:string'/>

 </xs:schema>

Appendix E. Differences From RFC 3921

Based on consensus derived from implementation and deployment
 experience as well as formal interoperability testing, the following
 substantive modifications were made from [RFC3921] (in addition to
 numerous changes of an editorial nature).

 o The protocol for session establishment was determined to be
 unnecessary and therefore the content previously defined in
 Section 3 of RFC 3921 was removed. However, for the sake of
 backward-compatibility server implementations are encouraged to
 advertise support for the feature, even though session
 establishment is a "no-op".

 o In order to more seamlessly repair lack of synchronization in
 subscription states between rosters located at different servers,
 clarified and modified error handling related to presence
 subscription requests, presence probes and presence notifications.

 o Changed the 'from' address for presence probes so that it is the
 bare JID, not the full JID.

 o Adjusted and clarified stanza delivery rules based on
 implementation and deployment experience.

 o Explicitly specified that a server is allowed to deliver a message
 stanza of type "normal" or "chat" to all resources if it has a
 method for allowing resources to opt in to such behavior.

 o Allowed a server to use its own algorithm for determining the
 "most available" resource for the purpose of message delivery, but
 mentioned the recommended algorithm from RFC 3921 (based on
 presence priority) as one possible algorithm.

 o Added optional versioning of roster information to save bandwidth
 in cases where the roster has not changed (or has changed very
 little) between sessions; the relevant protocol interactions were
 originally described in [XEP-0237].

 o Added optional server support for pre-approved presence
 subscriptions via presence stanzas of type "subscribed", including
 a new 'approved' attribute that can be set to "true" (for a pre-
 approved subscription) or "false" (the default).

 o Added optional 'parent' attribute to <thread/> element.

o Moved the protocol for communications blocking (specified in
 Section 10 of RFC 3921) back to [XEP-0016], from which it was
 originally taken.

 o Recommended returning presence unavailable in response to probes.

 o Clarified handling of presence probes sent to full JIDs.

 o Explicitly specified that the default value for the presence
 <priority/> element is zero.

 o Removed recommendation to support the "_im" and "_pres" SRV
 records.

Appendix F. Acknowledgements

   This document is an update to, and derived from, RFC 3921.  This
   document would have been impossible without the work of the
   contributors and commenters acknowledged there.

   Hundreds of people have provided implementation feedback, bug
   reports, requests for clarification, and suggestions for improvement
   since publication of RFC 3921.  Although the document editor has
   endeavored to address all such feedback, he is solely responsible for
   any remaining errors and ambiguities.

   Some of the text about roster versioning was borrowed from
   [XEP-0237], and some of the text about message threads was borrowed
   from [XEP-0201].

   Special thanks are due to Kevin Smith, Matthew Wild, Dave Cridland,
   Waqas Hussain, Philipp Hancke, Florian Zeitz, Jonas Lindberg, Jehan
   Pages, Tory Patnoe, and others for their comments during Working
   Group Last Call.

   Thanks also to Richard Barnes for his review on behalf of the
   Security Directorate.

   The Working Group chairs were Ben Campbell and Joe Hildebrand.  The
   responsible Area Director was Gonzalo Camarillo.

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