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Versions: (RFC 3921) 00 01 02 03 04 05 06 07 08

Network Working Group                                     P. Saint-Andre
Internet-Draft                                                     Cisco
Obsoletes: 3921 (if approved)                              March 8, 2009
Intended status: Standards Track
Expires: September 9, 2009


Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and
                                Presence
                     draft-saintandre-rfc3921bis-08

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   This Internet-Draft will expire on September 9, 2009.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Abstract

   This document defines extensions to core features of the Extensible



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   Messaging and Presence Protocol (XMPP) that provide basic instant
   messaging (IM) and presence functionality in conformance with RFC
   2779.

   This document obsoletes RFC 3921.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
     1.3.  Functional Summary . . . . . . . . . . . . . . . . . . . .  8
     1.4.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  9
     1.5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . .  9
     1.6.  Discussion Venue . . . . . . . . . . . . . . . . . . . . . 10
   2.  Managing the Roster  . . . . . . . . . . . . . . . . . . . . . 10
     2.1.  Syntax and Semantics . . . . . . . . . . . . . . . . . . . 10
       2.1.1.  Roster Items . . . . . . . . . . . . . . . . . . . . . 10
         2.1.1.1.  Ask Attribute  . . . . . . . . . . . . . . . . . . 10
         2.1.1.2.  Jid Attribute  . . . . . . . . . . . . . . . . . . 10
         2.1.1.3.  Name Attribute . . . . . . . . . . . . . . . . . . 10
         2.1.1.4.  Subscription Attribute . . . . . . . . . . . . . . 11
         2.1.1.5.  Group Element  . . . . . . . . . . . . . . . . . . 11
       2.1.2.  Roster Get . . . . . . . . . . . . . . . . . . . . . . 11
       2.1.3.  Roster Set . . . . . . . . . . . . . . . . . . . . . . 12
       2.1.4.  Roster Push  . . . . . . . . . . . . . . . . . . . . . 12
       2.1.5.  Roster Result  . . . . . . . . . . . . . . . . . . . . 13
       2.1.6.  Subscription Attribute . . . . . . . . . . . . . . . . 14
     2.2.  Retrieving the Roster on Login . . . . . . . . . . . . . . 14
     2.3.  Adding a Roster Item . . . . . . . . . . . . . . . . . . . 15
       2.3.1.  Request  . . . . . . . . . . . . . . . . . . . . . . . 15
       2.3.2.  Success Case . . . . . . . . . . . . . . . . . . . . . 16
       2.3.3.  Error Cases  . . . . . . . . . . . . . . . . . . . . . 17
     2.4.  Updating a Roster Item . . . . . . . . . . . . . . . . . . 21
       2.4.1.  Request  . . . . . . . . . . . . . . . . . . . . . . . 21
       2.4.2.  Success Case . . . . . . . . . . . . . . . . . . . . . 23
       2.4.3.  Error Cases  . . . . . . . . . . . . . . . . . . . . . 23
     2.5.  Deleting a Roster Item . . . . . . . . . . . . . . . . . . 23
       2.5.1.  Request  . . . . . . . . . . . . . . . . . . . . . . . 23
       2.5.2.  Success Case . . . . . . . . . . . . . . . . . . . . . 24
       2.5.3.  Error Cases  . . . . . . . . . . . . . . . . . . . . . 24
   3.  Managing Presence Subscriptions  . . . . . . . . . . . . . . . 25
     3.1.  Requesting a Subscription  . . . . . . . . . . . . . . . . 25
       3.1.1.  Client Generation of Outbound Subscription Request . . 26
       3.1.2.  Server Processing of Outbound Subscription Request . . 26
       3.1.3.  Server Processing of Inbound Subscription Request  . . 28
       3.1.4.  Client Processing of Inbound Subscription Request  . . 29



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       3.1.5.  Server Processing of Outbound Subscription Approval  . 30
       3.1.6.  Server Processing of Inbound Subscription Approval . . 31
     3.2.  Cancelling a Subscription  . . . . . . . . . . . . . . . . 32
       3.2.1.  Client Generation of Subscription Cancellation . . . . 32
       3.2.2.  Server Processing of Outbound Subscription
               Cancellation . . . . . . . . . . . . . . . . . . . . . 32
       3.2.3.  Server Processing of Inbound Subscription
               Cancellation . . . . . . . . . . . . . . . . . . . . . 33
     3.3.  Unsubscribing  . . . . . . . . . . . . . . . . . . . . . . 34
       3.3.1.  Client Generation of Unsubscribe . . . . . . . . . . . 34
       3.3.2.  Server Processing of Outbound Unsubscribe  . . . . . . 34
       3.3.3.  Server Processing of Inbound Unsubscribe . . . . . . . 35
   4.  Exchanging Presence Information  . . . . . . . . . . . . . . . 36
     4.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 36
     4.2.  Initial Presence . . . . . . . . . . . . . . . . . . . . . 37
       4.2.1.  Client Generation of Initial Presence  . . . . . . . . 37
       4.2.2.  Server Processing of Outbound Presence . . . . . . . . 38
       4.2.3.  Server Processing of Inbound Presence  . . . . . . . . 38
       4.2.4.  Client Processing of Inbound Presence  . . . . . . . . 39
     4.3.  Presence Probes  . . . . . . . . . . . . . . . . . . . . . 39
       4.3.1.  Server Generation of Outbound Presence Probe . . . . . 40
       4.3.2.  Server Processing of Inbound Presence Probe  . . . . . 40
     4.4.  Subsequent Presence Broadcast  . . . . . . . . . . . . . . 41
       4.4.1.  Client Generation of Presence Broadcast  . . . . . . . 41
       4.4.2.  Server Processing of Outbound Presence . . . . . . . . 42
       4.4.3.  Server Processing of Inbound Presence  . . . . . . . . 43
       4.4.4.  Client Processing of Inbound Presence  . . . . . . . . 43
     4.5.  Unavailable Presence . . . . . . . . . . . . . . . . . . . 44
       4.5.1.  Client Generation of Unavailable Presence  . . . . . . 44
       4.5.2.  Server Processing of Outbound Unavailable Presence . . 44
       4.5.3.  Server Processing of Inbound Unavailable Presence  . . 45
       4.5.4.  Client Processing of Inbound Unavailable Presence  . . 46
     4.6.  Directed Presence  . . . . . . . . . . . . . . . . . . . . 46
       4.6.1.  Client Generation of Directed Presence . . . . . . . . 47
       4.6.2.  Server Processing of Outbound Directed Presence  . . . 47
       4.6.3.  Server Processing of Inbound Directed Presence . . . . 47
       4.6.4.  Client Processing of Inbound Directed Presence . . . . 48
     4.7.  Presence Syntax  . . . . . . . . . . . . . . . . . . . . . 48
       4.7.1.  Type Attribute . . . . . . . . . . . . . . . . . . . . 48
       4.7.2.  Child Elements . . . . . . . . . . . . . . . . . . . . 49
       4.7.3.  Show Element . . . . . . . . . . . . . . . . . . . . . 49
       4.7.4.  Status Element . . . . . . . . . . . . . . . . . . . . 50
       4.7.5.  Priority Element . . . . . . . . . . . . . . . . . . . 51
       4.7.6.  Extended Content . . . . . . . . . . . . . . . . . . . 51
   5.  Exchanging Messages  . . . . . . . . . . . . . . . . . . . . . 52
     5.1.  One-to-One Chat Sessions . . . . . . . . . . . . . . . . . 52
     5.2.  Message Syntax . . . . . . . . . . . . . . . . . . . . . . 53
       5.2.1.  To Attribute . . . . . . . . . . . . . . . . . . . . . 53



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       5.2.2.  Type Attribute . . . . . . . . . . . . . . . . . . . . 54
       5.2.3.  Body Element . . . . . . . . . . . . . . . . . . . . . 55
       5.2.4.  Subject Element  . . . . . . . . . . . . . . . . . . . 56
       5.2.5.  Thread Element . . . . . . . . . . . . . . . . . . . . 56
     5.3.  Extended Content . . . . . . . . . . . . . . . . . . . . . 58
   6.  Exchanging IQ Stanzas  . . . . . . . . . . . . . . . . . . . . 58
   7.  A Sample Session . . . . . . . . . . . . . . . . . . . . . . . 59
   8.  Server Rules for Processing XML Stanzas  . . . . . . . . . . . 66
     8.1.  No Such User . . . . . . . . . . . . . . . . . . . . . . . 66
     8.2.  Full JID at Local Domain . . . . . . . . . . . . . . . . . 66
       8.2.1.  Resource Matches . . . . . . . . . . . . . . . . . . . 66
       8.2.2.  No Resource Matches  . . . . . . . . . . . . . . . . . 67
     8.3.  Bare JID at Local Domain . . . . . . . . . . . . . . . . . 67
       8.3.1.  Available or Connected Resources . . . . . . . . . . . 67
         8.3.1.1.  Message  . . . . . . . . . . . . . . . . . . . . . 67
         8.3.1.2.  Presence . . . . . . . . . . . . . . . . . . . . . 68
         8.3.1.3.  IQ . . . . . . . . . . . . . . . . . . . . . . . . 69
       8.3.2.  No Available or Connected Resources  . . . . . . . . . 69
         8.3.2.1.  Message  . . . . . . . . . . . . . . . . . . . . . 69
         8.3.2.2.  Presence . . . . . . . . . . . . . . . . . . . . . 70
         8.3.2.3.  IQ . . . . . . . . . . . . . . . . . . . . . . . . 70
     8.4.  Remote Domain  . . . . . . . . . . . . . . . . . . . . . . 70
   9.  IM and Presence Compliance Requirements  . . . . . . . . . . . 71
     9.1.  Servers  . . . . . . . . . . . . . . . . . . . . . . . . . 71
     9.2.  Clients  . . . . . . . . . . . . . . . . . . . . . . . . . 72
   10. Internationalization Considerations  . . . . . . . . . . . . . 72
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 72
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 73
     12.1. Instant Messaging SRV Protocol Label Registration  . . . . 73
     12.2. Presence SRV Protocol Label Registration . . . . . . . . . 74
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 74
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 74
     13.2. Informative References . . . . . . . . . . . . . . . . . . 75
   Appendix A.  Subscription States . . . . . . . . . . . . . . . . . 76
     A.1.  Defined States . . . . . . . . . . . . . . . . . . . . . . 77
     A.2.  Server Processing of Outbound Presence Subscription
           Stanzas  . . . . . . . . . . . . . . . . . . . . . . . . . 78
       A.2.1.  Subscribe  . . . . . . . . . . . . . . . . . . . . . . 78
       A.2.2.  Unsubscribe  . . . . . . . . . . . . . . . . . . . . . 79
       A.2.3.  Subscribed . . . . . . . . . . . . . . . . . . . . . . 79
       A.2.4.  Unsubscribed . . . . . . . . . . . . . . . . . . . . . 80
     A.3.  Server Processing of Inbound Presence Subscription
           Stanzas  . . . . . . . . . . . . . . . . . . . . . . . . . 80
       A.3.1.  Subscribe  . . . . . . . . . . . . . . . . . . . . . . 81
       A.3.2.  Unsubscribe  . . . . . . . . . . . . . . . . . . . . . 81
       A.3.3.  Subscribed . . . . . . . . . . . . . . . . . . . . . . 82
       A.3.4.  Unsubscribed . . . . . . . . . . . . . . . . . . . . . 83
   Appendix B.  Blocking Communication  . . . . . . . . . . . . . . . 84



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   Appendix C.  vCards  . . . . . . . . . . . . . . . . . . . . . . . 84
   Appendix D.  XML Schemas . . . . . . . . . . . . . . . . . . . . . 84
     D.1.  jabber:client  . . . . . . . . . . . . . . . . . . . . . . 84
     D.2.  jabber:server  . . . . . . . . . . . . . . . . . . . . . . 89
     D.3.  jabber:iq:roster . . . . . . . . . . . . . . . . . . . . . 93
   Appendix E.  Differences From RFC 3921 . . . . . . . . . . . . . . 94
   Appendix F.  Copying Conditions  . . . . . . . . . . . . . . . . . 95
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 96










































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1.  Introduction

1.1.  Overview

   The Extensible Messaging and Presence Protocol (XMPP) is an
   application profile of the Extensible Markup Language [XML] for
   streaming XML data in close to real time between any two (or more)
   network-aware entities.  XMPP is typically used to exchange messages,
   share presence information, and engage in structured request-response
   interactions.  The core features of XMPP defined in [rfc3920bis]
   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 defined in [IMP-REQS].

   As a result of extensive implementation and deployment experience
   with XMPP since 2004, as well as more formal interoperability testing
   carried out under the auspices of the XMPP Standards Foundation
   (XSF), this document reflects consensus from the XMPP developer
   community regarding XMPP's basic instant messaging and presence
   features.  In particular, this document incorporates the following
   backward-compatible changes from RFC 3921:

   o  Incorporated corrections and errata
   o  Added examples throughout
   o  Clarified and more completely specified matters that were
      underspecified
   o  Removed the protocol for session establishment, which was deemed
      unnecessary
   o  Modified error handling related to presence stanzas to more
      seamlessly repair lack of synchronization in subscription states
      between rosters located at different servers
   o  Added optional server support for pre-approved presence
      subscriptions
   o  Added optional 'parent' attribute to <thread/> element
   o  Transferred documentation for the communications blocking protocol
      from this specification to a separate specification

   Therefore, this document defines the basic instant messaging and
   presence features of XMPP 1.0, thus obsoleting RFC 3921.

1.2.  Requirements

   Traditionally, instant messaging applications have combined the
   following factors:




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   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 must 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 the requirements addressed herein.  While the XMPP
   instant messaging 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.

      Note: RFC 2779 stipulates that presence services must be separable
      from instant messaging services and vice-versa; i.e., it must be
      possible to use the protocol to provide a presence service, an
      instant messaging service, or both.  Although the text of this
      document assumes that implementations and deployments will want to
      offer a unified instant messaging and presence service, there is
      no requirement that a service must offer both a presence service
      and an instant messaging service, and the protocol makes it
      possible to offer separate and distinct services for presence and
      for instant messaging.  (For example, a presence-only service
      could return a <service-unavailable/> stanza error if a client
      attempt to send a <message/> stanza.)



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1.3.  Functional Summary

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

   [rfc3920bis] specifies how an XMPP client connects to an XMPP server.
   In particular, it specifies the preconditions that must 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 XML
   stream establishment, 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 [rfc3920bis] for
   details regarding these preconditions, and knowledge of [rfc3920bis]
   is assumed herein.

      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 SHOULD still offer that feature
      and note in the stream feature that negotiation of the feature is
      discretionary (via the <optional/> child element).  This enables
      older software to connect while saving newer software to skip a
      round trip.

   Upon fulfillment of the preconditions specified in [rfc3920bis], 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 broadcasting 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 Section 5, Section 3, Section 2, and Section 6.)





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

1.4.  Conventions

   This document inherits the terminology defined in [rfc3920bis].

   The following keywords are to be interpreted as described in [TERMS]:
   "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD",
   "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

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

   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 "&#x0159;" stands for
   the Unicode character LATIN SMALL LETTER R WITH CARON).

   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

1.5.  Acknowledgements

   The editor of this document finds it impossible to appropriately
   acknowledge the many individuals who have provided comments regarding
   the protocols defined herein.  However, thanks are due to those who
   have who have provided implementation feedback, bug reports, requests
   for clarification, and suggestions for improvement since the
   publication of the RFC this document supersedes.  The editor has
   endeavored to address all such feedback, but is solely responsible
   for any remaining errors and ambiguities.







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1.6.  Discussion Venue

   The document editor and the broader XMPP developer community welcome
   discussion and comments related to the topics presented in this
   document.  The preferred forum is the <standards@xmpp.org> mailing
   list, for which archives and subscription information are available
   at <http://mail.jabber.org/mailman/listinfo/standards>.


2.  Managing the Roster

   In XMPP, one'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 resource.

2.1.  Syntax and Semantics

   Rosters are managed using IQ stanzas, 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.  Roster Items

   The <query/> element MAY contain one or more <item/> children, each
   describing a unique ROSTER ITEM or "contact".

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

2.1.1.1.  Ask Attribute

   The 'ask' attribute is used to specify certain subscription sub-
   states; for details, see Section 3.1.2.

   Inclusion of the 'ask' attribute is OPTIONAL.

2.1.1.2.  Jid Attribute

   The 'jid' attribute specifies the Jabber Identifier (JID) that
   uniquely identifies the roster item.

   Inclusion of the 'jid' attribute is REQUIRED.

2.1.1.3.  Name Attribute

   The 'name' attribute specifies the "handle" to be associated with the
   JID, as determined by the user (not the contact).  Although the value



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   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, in which case the values MUST be compared only after
   application of the Resourceprep profile of stringprep as defined in
   [rfc3920bis].

   Inclusion of the 'name' attribute is OPTIONAL.

2.1.1.4.  Subscription Attribute

   The 'subscription' attribute is OPTIONAL; see Section 2.1.6.

   Inclusion of the 'subscription' attribute is OPTIONAL.

2.1.1.5.  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, so 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,
   in which case the data MUST be compared only after application of the
   Resourceprep profile of stringprep as defined in [rfc3920bis].

   Inclusion of the <group/> child element is OPTIONAL.

2.1.2.  Roster Get

   A ROSTER GET is a client's request for the server to send 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='rg1'
          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.






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2.1.3.  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 2.1.6).
   3.  The server MUST ignore any 'to' address specified on the IQ
       stanza and MUST handle the IQ stanza as if it included no 'to'
       attribute.

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

2.1.4.  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 user's bare JID) or it has a
       'from' attribute whose value matches the user's bare JID
       <user@domain>.

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



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   As mandated by the semantics of the IQ stanza as defined in
   [rfc3920bis], each resource that receives a roster push MUST reply
   with an IQ stanza of type "result" (or "error").

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

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

      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 it SHOULD ignore the error.

2.1.5.  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='rg1'
          to='juliet@example.com/chamber'
          type='result'>
       <query xmlns='jabber:iq:roster'>
         <item jid='nurse@example.com'/>
         <item jid='romeo@example.net'/>
       </query>
     </iq>

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

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





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2.1.6.  Subscription Attribute

   The state of the presence subscription in relation to a roster item
   is captured in the 'subscription' attribute of the <item/> element.
   Allowable subscription-related values for this attribute are:

   o  "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
   o  "to" -- the user has a subscription to the contact's presence, but
      the contact does not have a subscription to the user's presence
   o  "from" -- the contact has a subscription to the user's presence,
      but the user does not have a subscription to the contact's
      presence
   o  "both" -- both the user and the contact have subscriptions to each
      other's presence (also called a "mutual subscription")

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

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

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

2.2.  Retrieving the Roster on Login

   Upon authenticating with a server and binding a resource (thus
   becoming a connected resource), 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.

      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



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   to the server.

   C: <iq from='juliet@example.com/balcony'
          id='roster_1'
          type='get'>
        <query xmlns='jabber:iq:roster'/>
      </iq>


   S: <iq id='roster_1'
          to='juliet@example.com/balcony'
          type='result'>
        <query xmlns='jabber:iq:roster'>
          <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 [rfc3920bis] (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.












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   C: <iq from='juliet@example.com/balcony'
          id='roster_2'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <item jid='nurse@example.com'
                name='Nurse'>
            <group>Servants</group>
          </item>
        </query>
      </iq>

      Note: When a user adds a contact for the purpose of tracking the
      user's presence subscription to a contact, the user's client MUST
      send a presence subscription request to the contact before
      generating any roster set related to the contact.  This enables
      the user's server to enforce any policies relevant to presence
      subscriptions (e.g., a prohibition on presence subscriptions to
      full JIDs).  For details, see Section 3.

2.3.2.  Success Case

   If the server can successfully process the roster set (i.e., if none
   of the error cases occurs), it MUST create the roster item in
   persistent storage.

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

   S: <iq id='roster_2'
          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.
















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

   S: <iq to='juliet@example.com/chamber'
          id='a78b4q6ha464'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <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
   [rfc3920bis], each resource that receives a roster push MUST reply
   with an IQ stanza of type "result" (or "error").

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

   C: <iq from='juliet@example.com/chamber'
          id='a78b4q6ha464'
          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/>).

   The server SHOULD return a <bad-request/> stanza error to the client
   if the roster set violates any of the following conditions:






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   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 (where duplicates are determined using
       the Resourceprep profile of stringprep as defined in
       [rfc3920bis]).

   The server SHOULD return a <not-acceptable/> stanza error to the
   client if the roster set violates any of the following conditions:

   1.  The value of the 'name' attribute is greater than a server-
       configured limit.
   2.  The XML character data of the <group/> element is of zero length.
   3.  The XML character data of the <group/> element is greater than a
       server-configured limit.

   Alternatively, the server MAY ignore the foregoing violations and
   process the roster set as best as possible (e.g., process only the
   first <item/> element, ignore duplicate <group/> elements, place the
   roster item in no group or a default group if the <group/> element is
   empty, and truncate 'name' attributes and <group/> elements that are
   too long).

   Error: Roster set contains more than one item

   C: <iq from='juliet@example.com/balcony'
          id='roster_3'
          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='roster_3'
          to='juliet@example.com/balcony'
          type='error'>
       <error type='modify'>
         <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       </error>
     </iq>




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   Error: Roster set contains item with oversized handle

   C: <iq from='juliet@example.com/balcony'
          id='roster_4'
          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='roster_4'
          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='roster_5'
          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='roster_5'
          to='juliet@example.com/balcony'
          type='error'>
        <error type='modify'>
          <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>









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   Error: Roster set contains empty group

   C: <iq from='juliet@example.com/balcony'
          id='roster_6'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <item jid='nurse@example.com'
                name='Nurse'>
            <group></group>
          </item>
        </query>
      </iq>

   S: <iq id='roster_6'
          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

   C: <iq from='juliet@example.com/balcony'
          id='roster_7'
          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='roster_7'
          to='juliet@example.com/balcony'
          type='error'>
        <error type='modify'>
          <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

   The server MUST return a <not-allowed/> stanza error to the client if
   the value of the <item/> element's 'jid' attribute matches the bare
   JID <node@domain> portion of the <iq/> element's 'from' attribute
   (i.e., a JID MUST NOT be allowed to add itself to its own roster).





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   Error: Roster set contains sender's JID

   C: <iq from='juliet@example.com/balcony'
          id='roster_8'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <item jid='juliet@example.com'/>
         </query>
       </iq>

   S: <iq id='roster_8'
          to='juliet@example.com/balcony'
          type='error'>
         <error type='cancel'>
           <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
         </error>
       </iq>

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.








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   C: <iq from='juliet@example.com/balcony'
          id='update_1'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <item jid='romeo@example.net'
                name='Romeo'>
            <group>Friends</group>
            <group>Lovers</group>
          </item>
        </query>
      </iq>

   The user might then want to remove the item from the original group.

   C: <iq from='juliet@example.com/balcony'
          id='update_2'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <item jid='romeo@example.net'
                name='Romeo'>
            <group>Lovers</group>
          </item>
        </query>
      </iq>

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

   C: <iq from='juliet@example.com/balcony'
          id='update_3'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <item jid='romeo@example.net'
                name='MyRomeo'>
            <group>Lovers</group>
          </item>
        </query>
      </iq>

   The user might then want to remove the handle altogether (note:
   including an empty 'name' attribute is equivalent to including no
   'name' attribute).










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   C: <iq from='juliet@example.com/balcony'
          id='update_4'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <item jid='romeo@example.net'
                name=''>
            <group>Lovers</group>
          </item>
        </query>
      </iq>

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

   C: <iq from='juliet@example.com/balcony'
          id='update_5'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <item jid='romeo@example.net'/>
        </query>
      </iq>

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 roster information in
   persistent storage, send a roster push to all of the user's
   interested resources, and send an IQ result to the initiating
   resource; for details, see 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 the value of the 'subscription'
   attribute to be "remove".

   C: <iq from='juliet@example.com/balcony'
          id='delete_1'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <item jid='nurse@example.com' subscription='remove'/>
        </query>



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      </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 roster information in
   persistent storage, 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;
   for details, see Section 2.3.

   If the user has a presence subscription to the contact or the contact
   has a presence subscription to the user, the user's server MUST also
   generate a presence stanza of type "unsubscribe" (to unsubscribe from
   the contact's presence) or a presence stanza of type "unsubscribed"
   (to cancel the contact's subscription to the user), or both.

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

   S: <presence from='juliet@example.com'
                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, the server MUST return an <item-not-found/> stanza error.

   Error: Roster item not found

   C: <iq from='juliet@example.com/balcony'
          id='delete_2'
          type='set'>
        <query xmlns='jabber:iq:roster'>
          <item jid='[ ... non-existent-jid ... ]'
                subscription='remove'/>
        </query>
      </iq>

   S: <iq id='delete_2'
          to='juliet@example.com/balcony'
          type='error'>
        <error type='modify'>
          <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>



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3.  Managing Presence Subscriptions

   In order to protect the privacy of instant messaging 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 an 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.

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

      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 <node@domain> of the sending entity, not the
      full JID <node@domain/resource>.  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 [rfc3920bis].

   Subscription states are reflected in the rosters of both the user and
   the contact.  Complete details regarding these subscription states
   can be found Appendix A; those details are not provided in this
   section, which simply narrates the protocol flows for common use
   cases related to presence subscriptions.

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.

      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



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      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@domain>.

   UC: <presence 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@domain> rather a full JID
   <contact@domain/resource>, 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.

      Although 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, the
      client MUST NOT send a roster set before sending the presence
      subscription request, but instead MUST wait until receiving the
      initial roster push from the server.  This enables the user's
      server to enforce any policies relevant to presence subscriptions
      (e.g., a prohibition on presence subscriptions to full JIDs).

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 SHOULD check the
       syntax of the JID contained in the 'to' attribute.  If the JID is
       of the form <contact@domain/resource> instead of
       <contact@domain>, 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 [rfc3920bis], including
       checking against the relevant stringprep profiles.
   2.  If the potential contact is hosted on the same server as the
       user, the server MUST adhere to the rules specified in the next
       section in processing the subscription request and delivering it
       to the (local) contact.





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   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@domain> of the
   user.

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

   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'/>
           </item>
         </query>
       </iq>

   If the contact does not approve or deny the subscription request
   within some configurable amount of time, the user's server SHOULD re-
   send 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



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   elapsed); this helps to recover from transient, silent errors that
   might have occurred in relation to the original 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@domain/resource>
   instead of <contact@domain>, 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 [rfc3920bis], including checking
   against the relevant stringprep profiles.

   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 configured its account to automatically approve subscription
       requests or has accepted an agreement with its service provider
       that allows such behavior, 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 does not exist, then the contact's server MUST
       automatically return a presence stanza of type "unsubscribed" to
       the user.

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

   3.  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.
       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
       Appendix A), then the contact's server SHOULD also auto-reply on
       behalf of the contact.
   4.  If the contact exists, the user does not already have a
       subscription to the contact's presence, and 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 broadcast that subscription request to



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       all available resources in accordance with Section 8.
   5.  If the contact exists, the user does not already have a
       subscription to the contact's presence, and 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, and 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".)

      Note: 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.

3.1.4.  Client Processing of Inbound Subscription Request

   When the contact's client receives a subscription request from the
   user, it MUST present the request to the contact for approval (unless
   the contact has explicitly configured the client to automatically
   approve or deny some or all subscription requests).

   A subscription request is approved by sending a presence stanza of
   type "subscribed", which is processed as described in the following
   sections for both the contact's server and the user's server.

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

   A subscription request is denied 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 to='romeo@example.net' type='unsubscribed'/>








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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@domain> of the contact and locally deliver or remotely route
   the stanza to the user.

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

   The contact's server then MUST send a roster push to all of the
   contact's interested resources.

   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='a78b4q6ha464'
           to='juliet@example.com/chamber'
           type='set'>
         <query xmlns='jabber:iq:roster'>
           <item jid='romeo@example.net'
                 subscription='from'/>
         </query>
       </iq>

   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'
                 to='romeo@example.net'/>

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

   From the perspective of the contact, there now exists a subscription
   from the user.

   In order to subscribe to the user's presence, the contact would then
   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



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   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 the 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, the user's server MUST 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'/>
           </item>
         </query>
       </iq>

   (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 stanza 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.




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   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'
                 to='romeo@example.net'/>

   [ ... to resource2 ... ]

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

   [ ... to resource1 ... ]

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

   [ ... to resource2 ... ]

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

3.2.  Cancelling 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 (or deny a subscription request), it
   sends a presence stanza of type "unsubscribed".

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

3.2.2.  Server Processing of Outbound Subscription Cancellation

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

   1.  If the user is hosted on the same server as the contact, the
       server MUST adhere to the rules specified in the next section in
       processing the subscription cancellation.
   2.  If the user is hosted on a remote server, subject to local
       service policies the contact'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 contact, such as <remote-server-timeout/>.)




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   As mentioned, before locally delivering or remotely routing the
   stanza, the contact's server MUST stamp the outbound subscription
   cancellation with the bare JID <contact@domain> of the contact.

   CS: <presence from='juliet@example.com'
                 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='a78b4q6ha465'
           to='juliet@example.com/balcony'
           type='set'>
         <query xmlns='jabber:iq:roster'>
           <item jid='romeo@example.net'
                 subscription='none'/>
         </query>
       </iq>

   CS: <iq id='a78b4q6ha466'
           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, the user's server MUST
   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").










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   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'/>
           </item>
         </query>
       </iq>

   (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 stanza 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 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, the
       server MUST adhere to the rules specified in the next section in
       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/>.)



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

   US: <presence from='romeo@example.net'
                 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'/>
           </item>
         </query>
       </iq>

3.3.3.  Server Processing of Inbound Unsubscribe

   When the contact's server receives the subscription approval, it MUST
   first check if the user 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, the contact's server MUST initiate a
   roster push to all of the contact'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
   "From" or "From + Pending Out") or "to" (if the subscription state
   was "Both").








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   CS: <iq id='a78b4q6ha467'
           to='juliet@example.com/balcony'
           type='set'>
         <query xmlns='jabber:iq:roster'>
           <item jid='romeo@example.net'
                 subscription='none'/>
         </query>
       </iq>

   CS: <iq id='a78b4q6ha468'
           to='juliet@example.com/chamber'
           type='set'>
         <query xmlns='jabber:iq:roster'>
           <item jid='romeo@example.net'
                 subscription='none'/>
         </query>
       </iq>

   (Otherwise -- that is, if the contact does not exist, if the user is
   not in the contact's roster, or if the user 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
   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.)


4.  Exchanging Presence Information

4.1.  Overview

   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, a user's availability is signalled when a
   client controlled by the user generates a <presence/> stanza with no
   'type' attribute, and an entity's lack of availability is signalled
   when a 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 broadcasting to all



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   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 [rfc3920bis],
   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
   [rfc3920bis]) is said to be an AVAILABLE RESOURCE.

   In XMPP-based 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-based applications to combine messaging and presence
   functionality, and 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).

      Note: In the following examples, the "user" is juliet@example.com
      and the user 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 [rfc3920bis]
   (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.





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4.2.2.  Server Processing of Outbound Presence

   Upon receiving initial presence from a client, the user's server MUST
   send the initial presence stanza from the full JID
   <user@domain/resource> 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).

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

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

   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 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'/>



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   If there is no such contact, the contact's server MUST silently
   ignore the presence stanza.

4.2.4.  Client Processing of Inbound Presence

   When the contact's client receives presence from the user, it SHOULD
   proceed as follows:

   1.  If the user is in the contact's roster, the client MUST 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, the
       contact's client SHOULD display the presence information in the
       user interface for that chat session (see also Section 4.6 and
       Section 5.1).
   3.  Otherwise, the client MUST ignore the presence information and
       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".  The value of the 'from' address MUST be the full
   JID <user@domain/resource> of the user and the value of the 'to'
   address MUST be the bare JID <contact@domain> of the contact to which
   the user is subscribed.

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

      Note: Although presence probes MAY be sent by a client, 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 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 return a stanza or stream error to the
      client.

   If a server receives a presence probe intended for a full JID
   <contact@domain/resource>, it SHOULD treat the probe as if the 'to'
   address was a bare JID, but MAY instead handle it on behalf of the
   connected resource by returning only the presence information for
   that particular resources (and in any case MUST NOT deliver it to the
   resource).




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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 full JID <user@domain/resource> of
   the user to the bare JID <contact@domain> of the contact.  The server
   MUST NOT send a probe to a contact if the user is not subscribed to
   the contact's presence (i.e., if the contact is not in the user's
   roster with the 'subscription' attribute set to a value of "to" or
   "both".

   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/balcony'
                 to='romeo@example.net'
                 type='probe'/>

   US: <presence from='juliet@example.com/balcony'
                 to='benvolio@example.net'
                 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 contact's information locally.

4.3.2.  Server Processing of Inbound Presence Probe

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

   1.  If the contact account does not exist or the user is in the
       contact's roster with a subscription state other than "From",
       "From + Pending Out", or "Both" (as defined under Appendix A),
       the contact's server MUST return a presence stanza of type
       "unsubscribed" in response to the presence probe (however, if a
       server receives a presence probe from a configured hostname of
       the server itself or another such trusted service, it MAY provide
       presence information about the user to that entity).




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

   2.  Else, if the contact has no available resources, the server
       SHOULD reply to the presence probe by sending to the user the
       full XML of the last presence stanza of type "unavailable"
       received by the server from the contact (however, the server MAY
       opt to not reply at all).
   3.  Else, if the contact has at least one available resource, 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.

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

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

4.4.  Subsequent Presence Broadcast

4.4.1.  Client Generation of Presence Broadcast

   After sending initial presence, the user's client can update its
   availability for broadcasting at any time during its session 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.

   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 connected
   resource.  Information that is not relevant in this way can be of
   interest to the user's contacts but SHOULD be sent via other means,
   such as the XMPP message stanza.





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4.4.2.  Server Processing of 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 meet all of the following criteria:

   1.  The contact is in the user's roster with a subscription type of
       "from" or "both".
   2.  The last presence stanza received from the contact during the
       user's presence session was not of type "error" or "unsubscribe".

   As an optimization, if the subscription type is "both", the server
   SHOULD 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 or if the last presence stanza it received from the contact
   during the user's presence session was of type "unavailable", the
   user's server SHOULD NOT send subsequent presence notifications from
   the user to the contact.  This optimization helps 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>

      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).







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   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 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 Inbound Presence

   When the contact's client receives presence from the user, it SHOULD
   proceed as follows:

   1.  If the user is in the contact's roster, the client MUST 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, the
       contact's client SHOULD display the presence information in the
       user interface for that chat session (see also Section 4.6 and
       Section 5.1).
   3.  Otherwise, the client MUST ignore the presence information and
       not display it to the contact.







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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 presence.

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 can become unavailable
   ungracefully (e.g., the resource can be timed out by the server
   because of inactivity).

   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 meet all of the following
   criteria:

   1.  The contact is in the user's roster with a subscription type of
       "from" or "both".
   2.  The last presence stanza received from the contact during the
       user's presence session was not of type "error" or "unsubscribe".

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

      The optimization employed for subsequent presence broadcast during
      a user's presence session MUST NOT be employed for unavailable
      presence broadcast; if it were, 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



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      the user to be online.

   If the unavailable presence stanza was gracefully received from the
   client, 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 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'
                 type='unavailable'>
         <status>going on vacation</status>
       </presence>

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

      Note: Any presence stanza with no 'type' attribute and no 'to'
      attribute that is sent after sending unavailable presence
      broadcast MUST be sent 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 unavailable presence from the user, the contact's
   server MUST deliver the user's presence stanza to all of the



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   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>

   If the contact's server is optimizing subsequent presence delivery as
   described under Section 4.4, it MUST also note that the user is
   unavailable and appropriately update its internal representation of
   which entities are online.

4.5.4.  Client Processing of Inbound Unavailable Presence

   When the contact's client receives unavailable presence from the
   user, it SHOULD proceed as follows:

   1.  If the user is in the contact's roster, the client MUST display
       the unavailable 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, the
       contact's client SHOULD display the unavailable presence
       information in the user interface for that chat session (see also
       Section 4.6 and Section 5.1).
   3.  Otherwise, the client MUST ignore the unavailable presence
       information and not display it to the contact.

4.6.  Directed Presence

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







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4.6.1.  Client Generation of Directed Presence

   As noted, directed presence is a 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".

   Information about the use of directed presence in the context of a
   one-to-one chat session is provided under Section 5.1.

4.6.2.  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.

4.6.3.  Server Processing of Inbound Directed Presence

   From the perspective of the contact's server, there is no difference
   between presence broadcast and directed presence, so the contact's
   server follows the existing rules for processing of inbound presence.



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4.6.4.  Client Processing of Inbound Directed Presence

   When the contact's client receives directed presence from the user,
   it SHOULD proceed as follows:

   1.  If the user is in the contact's roster, the client MUST 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, the
       contact's client SHOULD display the presence information in the
       user interface for that chat session (see also Section 4.6 and
       Section 5.1).
   3.  Otherwise, the client MUST ignore the presence information and
       not display it to the contact.

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;
   for details, see 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
      [rfc3920bis]).
   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.





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   o  subscribed -- The sender has allowed the recipient to receive
      their presence.
   o  unavailable -- Signals that the entity 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 cancelled.

   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/>.

      Note: There is no default value for the 'type' attribute of the
      <presence/> element; in particular, there is no value of
      "available".

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 allowable children 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 provided only if the presence stanza possesses no 'type'
   attribute, although exceptions are noted in the text that follows.

4.7.3.  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.  The <show/>
   element MUST NOT possess any attributes.  The XML character data of
   the <show/> element is not human-readable.  The XML character data
   MUST be one of the following (additional availability states could be
   defined through a child element of the presence stanza that is
   qualified by a namespace other than the default namespace):

   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.




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   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).

4.7.4.  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>

   The <status/> element MUST NOT possess any attributes, with the
   exception of the 'xml:lang' attribute.  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 can include the XML
   stream header as described in [rfc3920bis]).

   <presence from='romeo@example.net/orchard'
             xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
     <status xml:lang='cs'>Dvo&#x0159;&#x00ED;m 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'
             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.  The
   receiving client MAY present this <status/> information to a human



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   user (see Section 11).

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

4.7.5.  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.  The
   <priority/> element MUST NOT possess any attributes.

   <presence xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
     <status xml:lang='cs'>Dvo&#x0159;&#x00ED;m 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").

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

4.7.6.  Extended Content

   As described in [rfc3920bis], 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



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   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 [rfc3920bis], 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 and the server is not a presence-only service).  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 form of a conversational burst that we call a CHAT SESSION:
   the exchange of at least several 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 user's client SHOULD send a message of type "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.

   The user's client MUST address the initial message in a chat session
   to the bare JID <contact@domain> (rather than attempting to guess an
   appropriate full JID <contact@domain/resource>).  Until and unless
   the user's client receives a reply from the contact, it MUST continue
   sending any further messages to the contact's bare JID.  The
   contact's client SHOULD address its subsequent replies to the user's
   full JID <user@domain/resource> 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.

   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



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   discover if the other party changes its availability or 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 to the other party 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 an entity other than the sender 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 <user@domain> rather than of the form
   <user@domain/resource>.

   <message
       from='juliet@example.com/balcony'
       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 <user@domain/resource> (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
   <user@domain/resource> rather than of the form <user@domain> unless
   the sender has knowledge (via presence) that the intended recipient's
   resource is no longer available.

   <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>



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   </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 a receiving client will present 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 in processing a message received from another entity (for
      details regarding stanza error syntax, refer to [rfc3920bis]).  A
      client that receives a message of type "error" SHOULD present an
      appropriate interface informing the sender of 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 information to which no reply is expected (e.g., news
      headlines, sports updates, near-real-time market data, and
      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, or groupchat
      messages (e.g., by not providing the recipient with the ability to
      reply).  The receiving server SHOULD deliver the message to all of
      the recipient's available resources.
   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



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      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).

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

   The <body/> element MUST NOT possess any attributes, with the
   exception of the 'xml:lang' attribute.  Multiple instances of the
   <body/> element MAY be included in a message stanza, 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 can include the
   XML stream header as described in [rfc3920bis]).

   <message
       from='juliet@example.com/balcony'
       to='romeo@example.net'
       type='chat'
       xml:lang='en'>
     <body>Wherefore art thou, Romeo?</body>
     <body xml:lang='cs'>
        Pro&#x010D;e&#x017D; jsi ty, Romeo?
      </body>
   </message>

   The <body/> element MUST NOT contain mixed content (as defined in



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

   The <subject/> element MUST NOT possess any attributes, with the
   exception of the 'xml:lang' attribute.  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 can include the XML
   stream header as described in [rfc3920bis]).

   <message
       from='juliet@example.com/balcony'
       to='romeo@example.net'
       type='chat'
       xml:lang='en'>
     <subject>I implore you!</subject>
     <subject xml:lang='cs'>
       &#x00DA;p&#x011B;nliv&#x011B; pros&#x00ED;m!
     </subject>
     <body>Wherefore art thou, Romeo?</body>
     <body xml:lang='cs'>
        Pro&#x010D;e&#x017E; 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



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   <thread/> element can 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 messagingg
   sessions.

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

   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 be a universally unique
   identifier (UUID) as described in [UUID].

   The <thread/> element MAY possess a 'parent' attribute that
   identifies another thread of which the current thread is an offshoot
   or child; the value of the 'parent' MUST conform to the syntax of the
   <thread/> element itself.

   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'>
       &#x00DA;p&#x011B;nliv&#x011B; pros&#x00ED;m!
     </subject>
     <body>Wherefore art thou, Romeo?</body>
     <body xml:lang='cs'>
        Pro&#x010D;e&#x017E; jsi ty, Romeo?
     </body>
     <thread parent='e0ffe42b28561960c6b12b944a092794b9683a38'>
       0e3141cd80894871a68e6fe6b1ec56fa
     </thread>
   </message>

   For detailed recommendations regarding use of the <thread/> element,



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   refer to [XEP-0201].

5.3.  Extended Content

   As described in [rfc3920bis], 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.

   (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 <span style='font-style: italic'>art</span>
         thou, <span style='color:red'>Romeo</span>?
       </body>
     </html>
   </message>


6.  Exchanging IQ Stanzas

   As described in [rfc3920bis], 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
   [rfc3920bis], 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.

   As noted under Section 4.6, if a user exchanges IQ stanzas with
   another entity but does not share presence with the entity based on a
   presence subscription, it is RECOMMENDED for the user's client to
   send directed presence to the other entity.






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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 resource identifier is "orchard", and he has
   the following individuals in his roster:

   o  juliet@example.com (subscription="both" and she has two available
      resources, one whose resource identifier is "chamber" and another
      whose resource identifier is "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
   [rfc3920bis]; 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/balcony'
           id='ex1'
           type='get'>
         <query xmlns='jabber:iq:roster'/>
       </iq>

   Example 2: User receives roster from server:

   US: <iq to='romeo@example.net/balcony'
           id='ex1'
           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.



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   Example 3: User sends initial presence:

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

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

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

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

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

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






















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   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/orchard'
           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/orchard'>
         <priority>1</priority>
       </presence>

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











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   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.






































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   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.



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   The user can also send subsequent presence broadcast.

   Example 10: User sends updated available presence for broadcasting:

   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 only to one
   contact:

   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>

   Example 12: Contact's server delivers updated presence to all of the
   contact's available resources ("balcony" and "chamber"):

   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>

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

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




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   Example 14: Contact's server sends unavailable presence 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 presence:

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

   Example 16: User's server broadcasts unavailable presence to contacts
   as well as to the person 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='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




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8.  Server Rules for Processing XML Stanzas

   Basic server rules for processing XML stanzas are defined in
   [rfc3920bis].  This section defines supplementary rules for XMPP
   instant messaging and presence servers; in the absence of a
   supplementary rule defined below (e.g., for stanzas without a 'to'
   address), the rule defined in [rfc3920bis] applies.

8.1.  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 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", the server MUST return
      a presence stanza of type "unsubscribed".
   o  For a presence stanza of type "subscribed", "unsubscribe", or
      "unsubscribed", the server MUST silently ignroe the stanza.

8.2.  Full JID at Local Domain

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute of an inbound stanza matches one of the
   configured hostnames of the server itself and the JID contained in
   the 'to' attribute is of the form <user@domain/resource>, the server
   MUST adhere to the following rules (subject to enforcement of
   relevant privacy and security policies, such as those deployed by
   means of [XEP-0016] or [XEP-0191]).

8.2.1.  Resource Matches

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

   o  For an IQ stanzas 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).




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   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", the server MUST follow
      the guidelines provided under Section 3.1.3.
   o  For a presence stanza of type "subscribe", "subscribed",
      "unsubscribe", or "unsubscribed", the server MUST follow the
      guidelines provided under Section 3.

8.2.2.  No Resource Matches

   If no connected or available resource exactly matches the full JID,
   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 SHOULD treat the stanza as if it
      were addressed to <user@domain> as described in the next section
      (but without modifying the value of the 'to' attribute).
   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", the server MUST follow
      the guidelines provided under Section 3.1.3.
   o  For a presence stanza of type "subscribed", "unsubscribe", or
      "unsubscribed", the server MUST ignore the stanza.

8.3.  Bare JID at Local Domain

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute of an inbound stanza matches one of the
   configured hostnames of the server itself and the JID contained in
   the 'to' attribute is of the form <user@domain>, the server MUST
   adhere to the following rules.

8.3.1.  Available or Connected Resources

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

8.3.1.1.  Message

   For a message stanza of type "headline", the server SHOULD deliver
   the stanza to all available resources.

   For a message stanza of type "chat" or "normal", the server SHOULD



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   deliver the stanza to the highest-priority available resource.  If
   there is not one highest-priority available resource but instead the
   highest priority is asserted by two or more available resources,
   these resources are said to form a "delivery tie".  In the case of a
   delivery tie, a server SHOULD deliver the message to all of the tied
   resources.  However, before delivering the message, a server MAY
   remove one or more resources from the tie.  Methods for doing so are
   outside the scope of this specification, but could include factors
   such as the resource's time of connection, time of last network or
   application activity, availability as determined by some hierarchy of
   <show/> values, or user-configured rules.  Nevertheless, a server
   MUST NOT remove all resources from the tie, and MUST deliver the
   message to at least one of the highest-priority resources (subject to
   appropriate security policies as described under Section 11 and in
   [rfc3920bis]).

   For a message stanza of type "groupchat", the server SHOULD NOT
   deliver the stanza to any of the available resources but instead
   SHOULD return an error to the sender.

   For a message stanza of type "error", the server SHOULD silently
   discard the message (i.e., neither deliver it to the intended
   recipient nor return an error to the sender).

   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 or connected resources as
   described under Section 8.3.2.

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

8.3.1.2.  Presence

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

   For a presence stanza with no type or of type "unavailable", the
   server MUST deliver the stanza 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.

   In all cases, the server MUST NOT rewrite the 'to' attribute (i.e.,
   it MUST leave it as <user@domain> rather than change it to



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   <user@domain/resource>).

8.3.1.3.  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, 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, the server MUST reply with a <service-
   unavailable/> stanza error.

8.3.2.  No Available or Connected Resources

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

8.3.2.1.  Message

   In order to properly handle message stanzas, it is strongly
   RECOMMENDED for an implementation to support OFFLINE STORAGE, i.e.,
   the server SHOULD store the message stanza on behalf of the user and
   deliver it when the user next becomes available.  For recommendations
   regarding offline message storage refer to [XEP-0160].

   For a message stanza of type "chat" or "normal", the server SHOULD
   add the message to offline storage or forward the message to the user
   via a non-XMPP messaging system (e.g., to the user's email account).
   However, if offline message storage or message forwarding is not
   enabled or available (e.g., because a size limit has been reached on
   offline messages), the server MUST return a <service-unavailable/>
   stanza error to the sender.

   For a message stanza of type "headline", according to local service
   policies the server MUST either (1) add the message to offline
   storage or (2) silently discard the message (i.e., neither deliver it
   to the intended recipient nor return an error to the sender).

   For a message stanza of type "groupchat", the server SHOULD NOT add
   the message to offline storage but instead SHOULD return an error to
   the sender.

   For a message stanza of type "error", the server MUST NOT add the
   message to offline storage but instead SHOULD silently discard the
   message (i.e., neither deliver it to the intended recipient nor
   return an error to the sender).



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8.3.2.2.  Presence

   For a presence stanza with no type or of type "unavailable" or
   "probe", 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.

8.3.2.3.  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, 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, the server MUST reply with a <service-
   unavailable/> stanza error.

8.4.  Remote Domain

   If the hostname of the domain identifier portion of the address
   contained in the 'to' attribute of an outbound stanza does not match
   a configured hostname of the server itself, the server MUST attempt
   to route the stanza to the remote domain.  If there exists an active
   stream between the two peers, the server MUST route the stanza over
   that stream for processing by the peer server.  If not, the server
   MUST do the following.

   First, resolve the hostname of the remote domain (or use a cached
   resolution of the remote domain to an IP address).  The RECOMMENDED
   order of attempted resolutions is as follows:

   1.  Attempt to resolve the remote hostname using a DNS service
       location record [SRV] Service of "xmpp-server" and a Proto of
       "tcp", resulting in resource records such as "_xmpp-
       server._tcp.example.com.", as specified in [rfc3920bis].
   2.  If the "xmpp-server" address record resolution fails, attempt to
       resolve the "_im" or "_pres" SRV Service as specified in
       [IMP-SRV], using the "_im" Service for <message/> stanzas and the
       "_pres" Service for <presence/> stanzas (it is up to the
       implementation how to handle <iq/> stanzas).  This will result in
       one or more resolutions of the form "_im.<proto>.example.com." or
       "_pres.<proto>.example.com.", where "<proto>" would be a label
       registered in the Instant Messaging SRV Protocol Label registry
       or the Presence SRV Protocol Label registry: either "_xmpp" for



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       an XMPP-aware domain or some other IANA-registered label (e.g.,
       "_simple") for a non-XMPP-aware domain.
   3.  If both SRV address record resolutions fail, attempt to perform a
       normal IPv4/IPv6 address record resolution to determine the IP
       address using the "xmpp-server" port of 5269 registered with the
       IANA, as specified in [rfc3920bis].

   If the server cannot resolve the remote domain, it MUST return a
   <remote-server-not-found/> stanza error.

   Second, negotiate XML streams with the remote domain by following the
   process defined in [rfc3920bis].  If the server can resolve the
   remote domain but cannot establish streams with the XMPP service at
   that domain, it MUST return a <remote-server-timeout/> stanza error.

   Third, route the stanza to the remote domain for processing by the
   peer server.

      Note: Administrators of server deployments are strongly encouraged
      to keep the _im._xmpp, _pres._xmpp, and _xmpp._tcp SRV records
      properly synchronized, since different implementations might
      perform the "_im" and "_pres" lookups before the "xmpp-server"
      lookup.


9.  IM and Presence Compliance Requirements

   This section summarizes the specific aspects of the Extensible
   Messaging and Presence Protocol that MUST be supported by instant
   messaging and presence servers and clients in order to be considered
   compliant implementations.  All such applications MUST comply with
   the requirements specified in [rfc3920bis].  The text in this section
   specifies additional compliance requirements for instant messaging
   and presence servers and clients (the requirements described here
   supplement but do not supersede the core requirements).

      Note: A server or client MAY support only presence or instant
      messaging; therefore is not necessary to support both if only a
      presence service or an instant messaging service is desired.

9.1.  Servers

   In addition to the core server compliance requirements, an instant
   messaging and presence server MUST additionally support all server-
   related instant messaging and presence syntax and semantics defined
   in this document, including:





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   o  Presence broadcast on behalf of clients as specified under
      Section 4
   o  Presence subscriptions as specified under Section 3
   o  Roster storage and management as specified under Section 2
   o  IM-specific routing and delivery rules as specified under
      Section 8

9.2.  Clients

   In addition to the core client compliance requirements, an instant
   messaging and presence client MUST additionally support the following
   protocols:

   o  Generation and processing of the IM-specific semantics of XML
      stanzas as defined by the XML schemas, including the 'type'
      attribute of message and presence stanzas as well as their child
      elements (see Section 5 and Section 4)
   o  All client-related instant messaging syntax and semantics defined
      in this document, including presence subscriptions and roster
      management (see Section 3 and Section 2)

   A client MUST also handle addresses that are encoded as "im:" URIs as
   specified in [CPIM] and "pres:" URIs as specified in [CPP], although
   it MAY do so by removing the "im:" or "pres:" scheme and entrusting
   address resolution to the server as specified under Section 8.4.  A
   client SHOULD also handle addresses that are encoded as "xmpp:" URIs
   and IRIs as specified in [XMPP-URI], although here again it MAY do so
   by removing the scheme and entrusting address resolution to the
   server.


10.  Internationalization Considerations

   For internationalization considerations, refer to the relevant
   section of [rfc3920bis].


11.  Security Considerations

   Core security considerations for XMPP are defined in the relevant
   section of [rfc3920bis].

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






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   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 hostnames, 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,
      either via an explicit subscription as described herein or via an
      existing trust relationship (such as presence-enabled user
      directories within organizations).
   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  When a server generates an error stanza in response to receiving a
      stanza for a user account that does not exist, the use of the
      <service-unavailable/> stanza error condition can help protect
      against dictionary attacks, since this is the same error condition
      that is returned if, for instance, the namespace of an IQ child
      element is not understood, or if offline message storage or
      message forwarding is not enabled for a domain.  However, subtle
      differences in the exact XML of error stanzas, as well as in the
      timing with which such errors are returned, can enable an attacker
      to determine the network presence of a user when more advanced
      blocking technologies are not used (see for instance [XEP-0016]
      and [XEP-0191]).
   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.  IANA Considerations

   The following sections update the registrations provided in
   [RFC3921].

   For a number of related IANA considerations, refer to the relevant
   section of [rfc3920bis].

12.1.  Instant Messaging SRV Protocol Label Registration

   Address Resolution for Instant Messaging and Presence [IMP-SRV]
   defines an Instant Messaging SRV Protocol Label registry for
   protocols that can provide services that conform to the "_im" SRV
   Service label.  Because XMPP is one such protocol, the IANA registers
   the "_xmpp" protocol label in the appropriate registry, as follows:



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   Protocol label:  _xmpp
   Specification:  XXXX
   Description:  Instant messaging protocol label for the Extensible
      Messaging and Presence Protocol (XMPP) as defined by XXXX.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

12.2.  Presence SRV Protocol Label Registration

   Address Resolution for Instant Messaging and Presence [IMP-SRV]
   defines a Presence SRV Protocol Label registry for protocols that can
   provide services that conform to the "_pres" SRV Service label.
   Because XMPP is one such protocol, the IANA registers the "_xmpp"
   protocol label in the appropriate registry, as follows:

   Protocol label:  _xmpp
   Specification:  XXXX
   Description:  Presence protocol label for the Extensible Messaging
      and Presence Protocol (XMPP) as defined by XXXX.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>


13.  References

13.1.  Normative References

   [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.

   [rfc3920bis]
              Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", draft-saintandre-rfc3920bis-09
              (work in progress), March 2009.

   [SRV]      Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

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

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



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   [XML]      Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F.,
              and T. Bray, "Extensible Markup Language (XML) 1.0 (Fourth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20060816, August 2006,
              <http://www.w3.org/TR/2006/REC-xml-20060816>.

   [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-URI]
              Saint-Andre, P., "Internationalized Resource Identifiers
              (IRIs) and Uniform Resource Identifiers (URIs) for the
              Extensible Messaging and Presence Protocol (XMPP)",
              RFC 4622, July 2006.

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.

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

   [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.

   [RFC3921]  Saint-Andre, P., "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.

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

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



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

   [XEP-0054]
              Saint-Andre, P., "vcard-temp", XSF XEP 0054, March 2003.

   [XEP-0071]
              Saint-Andre, P., "XHTML-IM", XSF XEP 0071, August 2007.

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

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

   [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,
              February 2008.

   [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>.

   [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").







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A.1.  Defined States

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

   o  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
   o  To -- the user has a subscription to the contact's presence, but
      the contact does not have a subscription to the user's presence
   o  From -- the contact has a subscription to the user's presence, but
      the user does not have a subscription to the contact's presence
   o  Both -- both the user and the contact have subscriptions to each
      other's presence (i.e., the union of 'from' and 'to')

   These states are supplemented by various pending sub-states to yield
   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 (note: contact's server SHOULD NOT push or
       deliver roster items in this state, but instead SHOULD wait until
       user has approved subscription request from contact); this is
       reflected in the user's roster by subscription='none'
   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'




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   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.

      Note: The rules for server processing of outbound presence
      subscription stanzas are described from the perspective of the
      user, not the contact.  In addition, "S.N." stands for SHOULD NOT.

A.2.1.  Subscribe

   Table 1: Processing of outbound "subscribe" stanzas

   +-----------------------------------------------------------------+
   |  EXISTING STATE          |  ROUTE?  |  NEW STATE                |
   +-----------------------------------------------------------------+
   |  "None"                  |  MUST    |  "None + Pending Out"     |
   |  "None + Pending Out"    |  MUST    |  no state change          |
   |  "None + Pending In"     |  MUST    |  "None + Pending Out+In"  |
   |  "None + Pending Out+In" |  MUST    |  no state change          |
   |  "To"                    |  MUST    |  no state change          |
   |  "To + Pending In"       |  MUST    |  no state change          |
   |  "From"                  |  MUST    |  "From + Pending Out"     |
   |  "From + Pending Out"    |  MUST    |  no state change          |
   |  "Both"                  |  MUST    |  no state change          |
   +-----------------------------------------------------------------+

      Note: A state change to "pending out" includes setting the 'ask'
      flag to a value of "subscribe" in the user's roster.














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A.2.2.  Unsubscribe

   Table 2: Processing of outbound "unsubscribe" stanzas

   +-----------------------------------------------------------------+
   |  EXISTING STATE          |  ROUTE?  |  NEW STATE                |
   +-----------------------------------------------------------------+
   |  "None"                  |  MUST    |  no state change          |
   |  "None + Pending Out"    |  MUST    |  "None"                   |
   |  "None + Pending In"     |  MUST    |  no state change          |
   |  "None + Pending Out+In" |  MUST    |  "None + Pending In"      |
   |  "To"                    |  MUST    |  "None"                   |
   |  "To + Pending In"       |  MUST    |  "Pending In"             |
   |  "From"                  |  MUST    |  no state change          |
   |  "From + Pending Out"    |  MUST    |  "From"                   |
   |  "Both"                  |  MUST    |  "From"                   |
   +-----------------------------------------------------------------+

A.2.3.  Subscribed

   Table 3: Processing of outbound "subscribed" stanzas

   +-----------------------------------------------------------------+
   |  EXISTING STATE          |  ROUTE?  |  NEW STATE                |
   +-----------------------------------------------------------------+
   |  "None"                  |  S.N.    |  no state change [1]      |
   |  "None + Pending Out"    |  S.N.    |  no state change          |
   |  "None + Pending In"     |  MUST    |  "From"                   |
   |  "None + Pending Out+In" |  MUST    |  "From + Pending Out"     |
   |  "To"                    |  S.N.    |  no state change          |
   |  "To + Pending In"       |  MUST    |  "Both"                   |
   |  "From"                  |  S.N.    |  no state change          |
   |  "From + Pending Out"    |  S.N.    |  no state change          |
   |  "Both"                  |  S.N.    |  no state change          |
   +-----------------------------------------------------------------+

   [1] A server MAY note the fact that the user wishes to allow the
   contact to be subscribed to the user's presence and automatically
   approve any subscription request received from the contact; if it
   does so, upon the receiving presence stanza of type "subscribed" from
   the user's client it MUST add a roster item for the contact to the
   user's roster and set the 'ask' flag to a value of "subscribed".
   However, the user's server still SHOULD NOT route the presence stanza
   of type "subscribed" to the contact.  This optional functionality
   applies only if the contact is not already in the user's roster or if
   the contact is in the user's roster with a state of "None" (not
   including a state of "None + Pending Out").




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A.2.4.  Unsubscribed

   Table 4: Processing of outbound "unsubscribed" stanzas

   +-----------------------------------------------------------------+
   |  EXISTING STATE          |  ROUTE?  |  NEW STATE                |
   +-----------------------------------------------------------------+
   |  "None"                  |  S.N.    |  no state change          |
   |  "None + Pending Out"    |  S.N.    |  no state change          |
   |  "None + Pending In"     |  MUST    |  "None"                   |
   |  "None + Pending Out+In" |  MUST    |  "None + Pending Out"     |
   |  "To"                    |  S.N.    |  no state change          |
   |  "To + Pending In"       |  MUST    |  "To"                     |
   |  "From"                  |  MUST    |  "None"                   |
   |  "From + Pending Out"    |  MUST    |  "None + Pending Out"     |
   |  "Both"                  |  MUST    |  "To"                     |
   +-----------------------------------------------------------------+

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.

      Note: The 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.


















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A.3.1.  Subscribe

   Table 5: Processing of inbound "subscribe" stanzas

   +------------------------------------------------------------------+
   |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
   +------------------------------------------------------------------+
   |  "None"                  |  MUST [1]  |  "None + Pending In"     |
   |  "None + Pending Out"    |  MUST      |  "None + Pending Out+In" |
   |  "None + Pending In"     |  S.N.      |  no state change         |
   |  "None + Pending Out+In" |  S.N.      |  no state change         |
   |  "To"                    |  MUST      |  "To + Pending In"       |
   |  "To + Pending In"       |  S.N.      |  no state change         |
   |  "From"                  |  S.N. [2]  |  no state change         |
   |  "From + Pending Out"    |  S.N. [2]  |  no state change         |
   |  "Both"                  |  S.N. [2]  |  no state change         |
   +------------------------------------------------------------------+

   [1] If the user previously sent presence of type "subscribed" as
   described under Appendix A.2.3, then the server MAY auto-reply with
   "subscribed" and change the state to "From" rather than "None +
   Pending In".

   [2] Server SHOULD auto-reply with "subscribed".

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 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
   SHOULD NOT deliver the stanza.  These rules are summarized in the
   following table.
















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   Table 6: Processing of inbound "unsubscribe" stanzas

   +------------------------------------------------------------------+
   |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
   +------------------------------------------------------------------+
   |  "None"                  |  S.N.      |  no state change         |
   |  "None + Pending Out"    |  S.N.      |  no state change         |
   |  "None + Pending In"     |  S.N. [1]  |  "None"                  |
   |  "None + Pending Out+In" |  S.N. [1]  |  "None + Pending Out"    |
   |  "To"                    |  S.N.      |  no state change         |
   |  "To + Pending In"       |  S.N. [1]  |  "To"                    |
   |  "From"                  |  S.N. [1]  |  "None"                  |
   |  "From + Pending Out"    |  S.N. [1]  |  "None + Pending Out     |
   |  "Both"                  |  S.N. [1]  |  "To"                    |
   +------------------------------------------------------------------+

   [1] Server SHOULD auto-reply with "unsubscribed".

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 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 but SHOULD
   NOT deliver the stanza to the user.  If the user already has access
   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 SHOULD NOT deliver the stanza to the user.  These rules are
   summarized in the following table.

















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   Table 7: Processing of inbound "subscribed" stanzas

   +------------------------------------------------------------------+
   |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
   +------------------------------------------------------------------+
   |  "None"                  |  S.N.      |  no state change         |
   |  "None + Pending Out"    |  S.N.      |  "To"                    |
   |  "None + Pending In"     |  S.N.      |  no state change         |
   |  "None + Pending Out+In" |  S.N.      |  "To + Pending In"       |
   |  "To"                    |  S.N.      |  no state change         |
   |  "To + Pending In"       |  S.N.      |  no state change         |
   |  "From"                  |  S.N.      |  no state change         |
   |  "From + Pending Out"    |  S.N.      |  "Both"                  |
   |  "Both"                  |  S.N.      |  no state change         |
   +------------------------------------------------------------------+

A.3.4.  Unsubscribed

   When the user's server receives a presence stanza of type
   "unsubscribed" for the user from the contact, if there is a pending
   outbound request for access to the contact's presence information or
   if the user currently has access to the contact's presence
   information, then the user's server MUST change the subscription
   state but SHOULD NOT deliver the stanza to the user.  Otherwise, the
   user's server MUST NOT change the subscription state and SHOULD NOT
   deliver the stanza.  These rules are summarized in the following
   table.

   Table 8: Processing of inbound "unsubscribed" stanzas

   +------------------------------------------------------------------+
   |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
   +------------------------------------------------------------------+
   |  "None"                  |  S.N.      |  no state change         |
   |  "None + Pending Out"    |  S.N.      |  "None"                  |
   |  "None + Pending In"     |  S.N.      |  no state change         |
   |  "None + Pending Out+In" |  S.N.      |  "None + Pending In"     |
   |  "To"                    |  S.N.      |  "None"                  |
   |  "To + Pending In"       |  S.N.      |  "None + Pending In"     |
   |  "From"                  |  S.N.      |  no state change         |
   |  "From + Pending Out"    |  S.N.      |  "From"                  |
   |  "Both"                  |  S.N.      |  "From"                  |
   +------------------------------------------------------------------+








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Appendix B.  Blocking Communication

   Sections 2.3.5 and 5.4.10 of [IMP-REQS] require that a compliant
   instant messaging and presence technology must 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 Jabber 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 Schemas

   Because validation of XML streams and stanzas is optional, the
   following XML schemas are provided for descriptive purposes only.
   These schemas are not normative.

   The following schemas formally define various XML namespaces used in
   the core XMPP protocols, in conformance with [XML-SCHEMA].  For
   schemas defining namespaces for XML streams and other core aspects of
   XMPP, refer to [rfc3920bis].

D.1.  jabber:client

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

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

     <xs:import
         namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>

     <xs:element name='message'>
        <xs:complexType>
           <xs:sequence>
             <xs:choice minOccurs='0' maxOccurs='unbounded'>
               <xs:element ref='subject'/>



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               <xs:element ref='body'/>
               <xs:element ref='thread'/>
             </xs:choice>
             <xs:any     namespace='##other'
                         minOccurs='0'
                         maxOccurs='unbounded'/>
             <xs:element ref='error'
                         minOccurs='0'/>
           </xs:sequence>
           <xs:attribute name='from'
                         type='xs:string'
                         use='optional'/>
           <xs:attribute name='id'
                         type='xs:NMTOKEN'
                         use='optional'/>
           <xs:attribute name='to'
                         type='xs:string'
                         use='optional'/>
           <xs:attribute name='type'
                         use='optional'
                         default='normal'>
             <xs:simpleType>
               <xs:restriction base='xs:NCName'>
                 <xs:enumeration value='chat'/>
                 <xs:enumeration value='error'/>
                 <xs:enumeration value='groupchat'/>
                 <xs:enumeration value='headline'/>
                 <xs:enumeration value='normal'/>
               </xs:restriction>
             </xs:simpleType>
           </xs:attribute>
           <xs:attribute ref='xml:lang' use='optional'/>
        </xs:complexType>
     </xs:element>

     <xs:element name='body'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:string'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='subject'>
       <xs:complexType>
         <xs:simpleContent>



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           <xs:extension base='xs:string'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='thread'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='parent'
                           type='xs:NMTOKEN'
                           use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='presence'>
       <xs:complexType>
         <xs:sequence>
           <xs:choice minOccurs='0' maxOccurs='unbounded'>
             <xs:element ref='show'/>
             <xs:element ref='status'/>
             <xs:element ref='priority'/>
           </xs:choice>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='error'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='optional'/>
         <xs:attribute name='to'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='type' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='error'/>
               <xs:enumeration value='probe'/>
               <xs:enumeration value='subscribe'/>



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               <xs:enumeration value='subscribed'/>
               <xs:enumeration value='unavailable'/>
               <xs:enumeration value='unsubscribe'/>
               <xs:enumeration value='unsubscribed'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='show'>
       <xs:simpleType>
         <xs:restriction base='xs:NCName'>
           <xs:enumeration value='away'/>
           <xs:enumeration value='chat'/>
           <xs:enumeration value='dnd'/>
           <xs:enumeration value='xa'/>
         </xs:restriction>
       </xs:simpleType>
     </xs:element>

     <xs:element name='status'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='string1024'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:simpleType name='string1024'>
       <xs:restriction base='xs:string'>
         <xs:minLength value='1'/>
         <xs:maxLength value='1024'/>
       </xs:restriction>
     </xs:simpleType>

     <xs:element name='priority' type='xs:byte'/>

     <xs:element name='iq'>
       <xs:complexType>
         <xs:sequence>
           <xs:any     namespace='##other'
                       minOccurs='0'/>
           <xs:element ref='error'
                       minOccurs='0'/>



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         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='required'/>
         <xs:attribute name='to'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='error'/>
               <xs:enumeration value='get'/>
               <xs:enumeration value='result'/>
               <xs:enumeration value='set'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:sequence xmlns:err='urn:ietf:params:xml:ns:xmpp-stanzas'>
           <xs:group ref='err:stanzaErrorGroup'/>
           <xs:element ref='err:text'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='auth'/>
               <xs:enumeration value='cancel'/>
               <xs:enumeration value='continue'/>
               <xs:enumeration value='modify'/>
               <xs:enumeration value='wait'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>

D.2.  jabber:server



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

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

     <xs:import
         namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>

     <xs:element name='message'>
        <xs:complexType>
           <xs:sequence>
             <xs:choice minOccurs='0' maxOccurs='unbounded'>
               <xs:element ref='subject'/>
               <xs:element ref='body'/>
               <xs:element ref='thread'/>
             </xs:choice>
             <xs:any namespace='##other'
                     minOccurs='0'
                     maxOccurs='unbounded'/>
             <xs:element ref='error'
                         minOccurs='0'/>
           </xs:sequence>
           <xs:attribute name='from'
                         type='xs:string'
                         use='required'/>
           <xs:attribute name='id'
                         type='xs:NMTOKEN'
                         use='optional'/>
           <xs:attribute name='to'
                         type='xs:string'
                         use='required'/>
           <xs:attribute name='type'
                         use='optional'
                         default='normal'>
             <xs:simpleType>
               <xs:restriction base='xs:NCName'>
                 <xs:enumeration value='chat'/>
                 <xs:enumeration value='error'/>
                 <xs:enumeration value='groupchat'/>
                 <xs:enumeration value='headline'/>
                 <xs:enumeration value='normal'/>
               </xs:restriction>
             </xs:simpleType>
           </xs:attribute>
           <xs:attribute ref='xml:lang' use='optional'/>



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        </xs:complexType>
     </xs:element>

     <xs:element name='body'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:string'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='subject'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:string'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='thread'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='parent'
                           type='xs:NMTOKEN'
                           use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='subject'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='parent'
                           type='xs:NMTOKEN'
                           use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='presence'>



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       <xs:complexType>
         <xs:sequence>
           <xs:choice minOccurs='0' maxOccurs='unbounded'>
             <xs:element ref='show'/>
             <xs:element ref='status'/>
             <xs:element ref='priority'/>
           </xs:choice>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='error'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='optional'/>
         <xs:attribute name='to'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='type' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='error'/>
               <xs:enumeration value='probe'/>
               <xs:enumeration value='subscribe'/>
               <xs:enumeration value='subscribed'/>
               <xs:enumeration value='unavailable'/>
               <xs:enumeration value='unsubscribe'/>
               <xs:enumeration value='unsubscribed'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='show'>
       <xs:simpleType>
         <xs:restriction base='xs:NCName'>
           <xs:enumeration value='away'/>
           <xs:enumeration value='chat'/>
           <xs:enumeration value='dnd'/>
           <xs:enumeration value='xa'/>
         </xs:restriction>
       </xs:simpleType>



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     </xs:element>

     <xs:element name='status'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='string1024'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:simpleType name='string1024'>
       <xs:restriction base='xs:string'>
         <xs:minLength value='1'/>
         <xs:maxLength value='1024'/>
       </xs:restriction>
     </xs:simpleType>

     <xs:element name='priority' type='xs:byte'/>

     <xs:element name='iq'>
       <xs:complexType>
         <xs:sequence>
           <xs:any namespace='##other'
                   minOccurs='0'/>
           <xs:element ref='error'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='required'/>
         <xs:attribute name='to'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='error'/>
               <xs:enumeration value='get'/>
               <xs:enumeration value='result'/>
               <xs:enumeration value='set'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>



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         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:sequence xmlns:err='urn:ietf:params:xml:ns:xmpp-stanzas'>
           <xs:group ref='err:stanzaErrorGroup'/>
           <xs:element ref='err:text'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='auth'/>
               <xs:enumeration value='cancel'/>
               <xs:enumeration value='continue'/>
               <xs:enumeration value='modify'/>
               <xs:enumeration value='wait'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>

D.3.  jabber:iq:roster

   <?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:complexType>
     </xs:element>

     <xs:element name='item'>



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       <xs:complexType>
         <xs:sequence>
           <xs:element ref='group'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
         </xs:sequence>
         <xs:attribute name='ask' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='subscribe'/>
               <xs:enumeration value='subscribed'/>
             </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'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <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 RFC 3921.

   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".




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   o  In order to more seamlessly repair lack of synchronization in
      subscription states between rosters located at different servers,
      error handling related to presence probes and presence
      notifications was modified to return presence stanzas of type
      "unsubscribe" or "unsubscribed" rather than error stanzas.
   o  Added optional server support for pre-approved presence
      subscriptions via presence stanzas of type "subscribed" and the
      optional "subscribed" value for the 'ask' flag.
   o  Added optional 'parent' attribute to <thread/> element
   o  The protocol for communications blocking specified in Section 10
      of RFC 3921 has been moved to [XEP-0016].

   In addition, numerous changes of an editorial nature were made in
   order to more fully specify and clearly explain the protocols.


Appendix F.  Copying Conditions

   Regarding this entire document or any portion of it, the author makes
   no guarantees and is not responsible for any damage resulting from
   its use.  The author grants irrevocable permission to anyone to use,
   modify, and distribute it in any way that does not diminish the
   rights of anyone else to use, modify, and distribute it, provided
   that redistributed derivative works do not contain misleading author
   or version information.  Derivative works need not be licensed under
   similar terms.


Index

   A
      Available Resource  37

   C
      Chat Session  52
      Contact  25

   D
      Directed Presence  37

   I
      Initial Presence  37

   O
      Offline Message Storage  69

   P
      Presence  7



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      Presence Broadcast  37
      Presence Probe  39
      Presence Session  37
      Presence Subscription  25

   R
      Roster  7
      Roster Get  11
      Roster Push  12
      Roster Result  13
      Roster Set  12

   S
      Subscription Request  25

   U
      Unavailable Presence  44


Author's Address

   Peter Saint-Andre
   Cisco

   Email: psaintan@cisco.com


























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