Network Working Group                                P. Saint-Andre, Ed.
Internet-Draft                                 XMPP Standards Foundation
Obsoletes: 3921 (if approved)                           October 29, 2007
Intended status: Standards Track
Expires: May 1, 2008


Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and
                                Presence
                     draft-saintandre-rfc3921bis-04

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

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

   This document obsoletes RFC 3921.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Functional Summary . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  7
     1.5.  Discussion Venue . . . . . . . . . . . . . . . . . . . . .  7
   2.  Managing the Roster  . . . . . . . . . . . . . . . . . . . . .  8
     2.1.  Syntax and Semantics . . . . . . . . . . . . . . . . . . .  8
       2.1.1.  Items  . . . . . . . . . . . . . . . . . . . . . . . .  8
       2.1.2.  Roster Get . . . . . . . . . . . . . . . . . . . . . .  8
       2.1.3.  Roster Set . . . . . . . . . . . . . . . . . . . . . .  9
       2.1.4.  Roster Push  . . . . . . . . . . . . . . . . . . . . .  9
       2.1.5.  Roster Result  . . . . . . . . . . . . . . . . . . . . 10
       2.1.6.  Subscription Attribute . . . . . . . . . . . . . . . . 10
     2.2.  Retrieving the Roster on Login . . . . . . . . . . . . . . 11
     2.3.  Adding a Roster Item . . . . . . . . . . . . . . . . . . . 12
       2.3.1.  Success Case . . . . . . . . . . . . . . . . . . . . . 12
       2.3.2.  Error Cases  . . . . . . . . . . . . . . . . . . . . . 14
     2.4.  Updating a Roster Item . . . . . . . . . . . . . . . . . . 17
       2.4.1.  Success Case . . . . . . . . . . . . . . . . . . . . . 17
       2.4.2.  Error Cases  . . . . . . . . . . . . . . . . . . . . . 19
     2.5.  Deleting a Roster Item . . . . . . . . . . . . . . . . . . 19
       2.5.1.  Success Case . . . . . . . . . . . . . . . . . . . . . 19
       2.5.2.  Error Cases  . . . . . . . . . . . . . . . . . . . . . 20
   3.  Managing Presence Subscriptions  . . . . . . . . . . . . . . . 20
     3.1.  Requesting a Subscription  . . . . . . . . . . . . . . . . 21
       3.1.1.  Client Generation of Outbound Subscription Request . . 21
       3.1.2.  Server Processing of Outbound Subscription Request . . 21
       3.1.3.  Server Processing of Inbound Subscription Request  . . 23
       3.1.4.  Client Processing of Inbound Subscription Request  . . 24
       3.1.5.  Server Processing of Outbound Subscription Approval  . 24
       3.1.6.  Server Processing of Inbound Subscription Approval . . 25
     3.2.  Cancelling a Subscription  . . . . . . . . . . . . . . . . 27
       3.2.1.  Client Generation of Subscription Cancellation . . . . 27
       3.2.2.  Server Processing of Outbound Subscription
               Cancellation . . . . . . . . . . . . . . . . . . . . . 27
       3.2.3.  Server Processing of Inbound Subscription
               Cancellation . . . . . . . . . . . . . . . . . . . . . 28
     3.3.  Unsubscribing  . . . . . . . . . . . . . . . . . . . . . . 29
       3.3.1.  Client Generation of Unsubscribe . . . . . . . . . . . 29
       3.3.2.  Server Processing of Outbound Unsubscribe  . . . . . . 29
       3.3.3.  Server Processing of Inbound Unsubscribe . . . . . . . 30
   4.  Exchanging Presence Information  . . . . . . . . . . . . . . . 30
     4.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 30
     4.2.  Initial Presence . . . . . . . . . . . . . . . . . . . . . 31
       4.2.1.  Client Generation of Initial Presence  . . . . . . . . 31



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       4.2.2.  Server Processing of Outbound Presence . . . . . . . . 31
       4.2.3.  Server Processing of Inbound Presence  . . . . . . . . 32
       4.2.4.  Client Processing of Inbound Presence  . . . . . . . . 32
     4.3.  Presence Probes  . . . . . . . . . . . . . . . . . . . . . 33
       4.3.1.  Server Generation of Outbound Presence Probe . . . . . 33
       4.3.2.  Server Processing of Inbound Presence Probe  . . . . . 34
     4.4.  Subsequent Presence Broadcast  . . . . . . . . . . . . . . 35
       4.4.1.  Client Generation of Presence Broadcast  . . . . . . . 35
       4.4.2.  Server Processing of Outbound Presence . . . . . . . . 35
       4.4.3.  Server Processing of Inbound Presence  . . . . . . . . 36
       4.4.4.  Client Processing of Inbound Presence  . . . . . . . . 36
     4.5.  Unavailable Presence . . . . . . . . . . . . . . . . . . . 37
       4.5.1.  Client Generation of Unavailable Presence  . . . . . . 37
       4.5.2.  Server Processing of Outbound Unavailable Presence . . 37
       4.5.3.  Server Processing of Inbound Unavailable Presence  . . 38
       4.5.4.  Client Processing of Inbound Unavailable Presence  . . 39
     4.6.  Directed Presence  . . . . . . . . . . . . . . . . . . . . 39
       4.6.1.  Client Generation of Directed Presence . . . . . . . . 39
       4.6.2.  Server Processing of Outbound Directed Presence  . . . 39
       4.6.3.  Server Processing of Inbound Directed Presence . . . . 40
       4.6.4.  Client Processing of Inbound Directed Presence . . . . 40
     4.7.  Presence Syntax  . . . . . . . . . . . . . . . . . . . . . 40
       4.7.1.  Type Attribute . . . . . . . . . . . . . . . . . . . . 40
       4.7.2.  Child Elements . . . . . . . . . . . . . . . . . . . . 41
       4.7.3.  Show Element . . . . . . . . . . . . . . . . . . . . . 42
       4.7.4.  Status Element . . . . . . . . . . . . . . . . . . . . 42
       4.7.5.  Priority Element . . . . . . . . . . . . . . . . . . . 43
       4.7.6.  Extended Content . . . . . . . . . . . . . . . . . . . 44
   5.  Exchanging Messages  . . . . . . . . . . . . . . . . . . . . . 44
     5.1.  Attributes . . . . . . . . . . . . . . . . . . . . . . . . 45
       5.1.1.  To Attribute . . . . . . . . . . . . . . . . . . . . . 45
       5.1.2.  Type Attribute . . . . . . . . . . . . . . . . . . . . 45
     5.2.  Child Elements . . . . . . . . . . . . . . . . . . . . . . 47
       5.2.1.  Body . . . . . . . . . . . . . . . . . . . . . . . . . 47
       5.2.2.  Subject  . . . . . . . . . . . . . . . . . . . . . . . 48
       5.2.3.  Thread . . . . . . . . . . . . . . . . . . . . . . . . 48
     5.3.  Extended Content . . . . . . . . . . . . . . . . . . . . . 49
   6.  Exchanging IQ Stanzas  . . . . . . . . . . . . . . . . . . . . 49
   7.  A Sample Session . . . . . . . . . . . . . . . . . . . . . . . 50
   8.  Server Rules for Processing XML Stanzas  . . . . . . . . . . . 56
     8.1.  No Such User . . . . . . . . . . . . . . . . . . . . . . . 56
     8.2.  Full JID at Local Domain . . . . . . . . . . . . . . . . . 56
       8.2.1.  Available Resource Matches . . . . . . . . . . . . . . 56
       8.2.2.  No Available Resource Matches  . . . . . . . . . . . . 57
     8.3.  Bare JID at Local Domain . . . . . . . . . . . . . . . . . 57
       8.3.1.  Available Resources  . . . . . . . . . . . . . . . . . 57
         8.3.1.1.  Message  . . . . . . . . . . . . . . . . . . . . . 57
         8.3.1.2.  Presence . . . . . . . . . . . . . . . . . . . . . 58



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         8.3.1.3.  IQ . . . . . . . . . . . . . . . . . . . . . . . . 58
       8.3.2.  No Available Resources . . . . . . . . . . . . . . . . 58
         8.3.2.1.  Message  . . . . . . . . . . . . . . . . . . . . . 59
         8.3.2.2.  Presence . . . . . . . . . . . . . . . . . . . . . 59
         8.3.2.3.  IQ . . . . . . . . . . . . . . . . . . . . . . . . 59
     8.4.  Foreign Domain . . . . . . . . . . . . . . . . . . . . . . 59
   9.  IM and Presence Compliance Requirements  . . . . . . . . . . . 61
     9.1.  Servers  . . . . . . . . . . . . . . . . . . . . . . . . . 61
     9.2.  Clients  . . . . . . . . . . . . . . . . . . . . . . . . . 61
   10. Internationalization Considerations  . . . . . . . . . . . . . 62
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 62
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 62
     12.1. Instant Messaging SRV Protocol Label Registration  . . . . 63
     12.2. Presence SRV Protocol Label Registration . . . . . . . . . 63
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 63
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 63
     13.2. Informative References . . . . . . . . . . . . . . . . . . 64
   Appendix A.  Subscription States . . . . . . . . . . . . . . . . . 65
     A.1.  Defined States . . . . . . . . . . . . . . . . . . . . . . 66
     A.2.  Server Processing of Outbound Presence Subscription
           Stanzas  . . . . . . . . . . . . . . . . . . . . . . . . . 67
       A.2.1.  Subscribe  . . . . . . . . . . . . . . . . . . . . . . 67
       A.2.2.  Unsubscribe  . . . . . . . . . . . . . . . . . . . . . 68
       A.2.3.  Subscribed . . . . . . . . . . . . . . . . . . . . . . 68
       A.2.4.  Unsubscribed . . . . . . . . . . . . . . . . . . . . . 69
     A.3.  Server Processing of Inbound Presence Subscription
           Stanzas  . . . . . . . . . . . . . . . . . . . . . . . . . 69
       A.3.1.  Subscribe  . . . . . . . . . . . . . . . . . . . . . . 70
       A.3.2.  Unsubscribe  . . . . . . . . . . . . . . . . . . . . . 70
       A.3.3.  Subscribed . . . . . . . . . . . . . . . . . . . . . . 71
       A.3.4.  Unsubscribed . . . . . . . . . . . . . . . . . . . . . 72
   Appendix B.  Blocking Communication  . . . . . . . . . . . . . . . 73
   Appendix C.  vCards  . . . . . . . . . . . . . . . . . . . . . . . 73
   Appendix D.  XML Schemas . . . . . . . . . . . . . . . . . . . . . 73
     D.1.  jabber:client  . . . . . . . . . . . . . . . . . . . . . . 73
     D.2.  jabber:server  . . . . . . . . . . . . . . . . . . . . . . 77
     D.3.  jabber:iq:roster . . . . . . . . . . . . . . . . . . . . . 81
   Appendix E.  Differences From RFC 3921 . . . . . . . . . . . . . . 83
   Appendix F.  Copying Conditions  . . . . . . . . . . . . . . . . . 83
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 84
   Intellectual Property and Copyright Statements . . . . . . . . . . 86









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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 [XMPP-CORE]
   provide the building blocks for many types of near-real-time
   applications, which may be layered on top of the core by sending
   application-specific data qualified by particular XML namespaces
   (refer to [XML-NAMES]).  This document describes XMPP extensions that
   provide the basic functionality expected of an instant messaging (IM)
   and presence application as defined in [IMP-REQS].

   This document obsoletes RFC 3921.

1.2.  Requirements

   Traditionally, instant messaging applications have combined the
   following factors:

   1.  The central point of focus is a list of one's contacts or
       "buddies" (in XMPP this list is called a ROSTER).
   2.  The purpose of using such an application is to exchange
       relatively brief text messages with each of one's contacts in
       close to real time -- often relatively large numbers of such
       messages in rapid succession, in the form of one-to-one "chat
       sessions".
   3.  The catalyst for exchanging messages is PRESENCE -- i.e.,
       information about the network availability of each of one's
       contacts (thus knowing who is online and available for a chat
       session).
   4.  Presence information is provided only to contacts that a user has
       authorized via 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



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   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 required 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/> error if a client attempt to send a <message/>
   stanzas.)

1.3.  Functional Summary

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

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

   Upon fulfillment of the preconditions specified in [XMPP-CORE], an
   XMPP client has a long-lived XML stream with an XMPP server, which
   enables the user 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



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   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 communications.  (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.)
   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 [XMPP-CORE].

   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 may be bots, devices, or other non-human
   applications.

   In examples, lines have been wrapped for improved readability,
   "[...]" means elision, and the following prepended strings are used:

   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.  Discussion Venue

   The editor welcomes 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>>.





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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 may 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.  Items

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

   The syntax of the <item/> is as follows:

   1.  The 'jid' attribute is REQUIRED; the value of this attribute is
       Jabber Identifier (JID) that uniquely identifies the roster item.
   2.  The 'name' attribute is OPTIONAL; the value of this attribute
       specifies the "handle" to be associated with the JID, as
       determined by the user (not the contact).  The value of the
       'name' attribute is typicall opaque to the server but may have
       meaning to a human user.
   3.  The 'subscription' attribute is OPTIONAL; see Section 2.1.6.
   4.  The 'ask' attribute is OPTIONAL and is used to specify certain
       subscription sub-states; for details, see Section 3.1.2.
   5.  The <group/> element is OPTIONAL; the XML character data of this
       element specifies a category into which the roster item should be
       grouped by a client.  The <item/> MAY more than one <group/>
       element.  The XML character data of the <group/> element is
       typicall opaque to the server but may have meaning to a human
       user.

2.1.2.  Roster Get

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

   The <query/> element in a roster get MUST NOT contain any <item/>
   child elements.






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

2.1.3.  Roster Set

   A ROSTER SET 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.  A receiving server MUST ignore any value of the 'subscription'
       attribute other than "remove" (see Section 2.1.6).
   3.  A receiving 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'
          type='set'
          id='roster_set'>
       <query xmlns='jabber:iq:roster'>
         <item jid='nurse@example.com'/>
       </query>
     </iq>

2.1.4.  Roster Push

   A ROSTER PUSH 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 get 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 server) or it has a 'from'
       attribute whose value matches the user's bare JID <user@domain>.








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

2.1.5.  Roster Result

   A ROSTER RESULT 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 MAY contain more than one <item/>
   element.

   S: <iq to='juliet@example.com/chamber'
          type='result'
          id='roster_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 <query/> element MUST be
   empty.

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

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




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

   In a roster push, a receiving 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
   "remove", which indicates that the item is to be removed from the
   roster; a receiving 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 may not be desirable for all resources, e.g., a connection
   with limited bandwidth, the client's request for the roster is
   recommended and not required).  After a connected resource sends
   initial presence (see Section 4.2), it is referred to as an available
   resource.  If a connected resource requests the roster but does not
   send initial presence, the server MUST NOT send it presence
   subscription requests and SHOULD NOT send it associated roster
   pushes.  If an available resource does not request the roster during
   a session, the server SHOULD NOT send it presence subscription
   requests and MUST NOT send it associated roster pushes.  Therefore,
   if a client wishes to engage in the full range of interactions
   related to contact lists (rosters) and presence subscriptions, it
   SHOULD both request the roster and send initial presence.  For the
   sake of brevity, the term INTERESTED RESOURCE is used herein to refer
   to the concept of "an available resource that has requested the
   roster".

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

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





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   S: <iq to='juliet@example.com/balcony' type='result' id='roster_1'>
       <query xmlns='jabber:iq:roster'>
         <item jid='romeo@example.net'
               name='Romeo'
               subscription='both'>
           <group>Friends</group>
         </item>
         <item jid='mercutio@example.org'
               name='Mercutio'
               subscription='from'/>
         <item jid='nurse@example.com'
               name='Nurse'
               subscription='to'/>
         <item jid='benvolio@example.org'
               name='Benvolio'
               subscription='both'/>
       </query>
     </iq>

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

2.3.  Adding a Roster Item

2.3.1.  Success Case

   At any time, a client may add an item to the roster by sending a
   roster set to the server.

   Note: When the item added represents another IM user, the value of
   the 'jid' 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.

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

   If the server can successfully process the roster set (i.e., if none



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   of the error cases occurs), it MUST create the roster item in
   persistent storage and send a roster push containing the new roster
   item to all of the user's interested resources.

   S: <iq to='juliet@example.com/balcony'
          type='set'
          id='a78b4q6ha463'>
        <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'
          type='set'
          id='a78b4q6ha464'>
        <query xmlns='jabber:iq:roster'>
          <item jid='nurse@example.com'
                name='Nurse'
                subscription='none'>
            <group>Servants</group>
          </item>
        </query>
      </iq>

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

   S: <iq to='juliet@example.com/balcony' type='result' id='roster_2'/>

   As required by the semantics of the IQ stanza as defined in
   [XMPP-CORE], each resource that received the roster push MUST reply
   with an IQ stanza of type "result" (or "error").

   C: <iq from='juliet@example.com/balcony'
         type='result'
         id='a78b4q6ha463'/>
   C: <iq from='juliet@example.com/chamber'
         type='result'
         id='a78b4q6ha464'/>

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




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2.3.2.  Error Cases

   If the server cannot successfully process the roster set, it MUST
   return an error.  The following error cases are defined (naturally,
   other stanza errors may occur, e.g., <internal-server-error/>).

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

   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
       [XMPP-CORE]).

   The server SHOULD return a <not-acceptable/> 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).




















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   Error: Roster set contains more than one item

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

   Error: Roster set contains item with oversized handle

   C: <iq from='juliet@example.com/balcony' type='set' id='roster_4'>
        <query xmlns='jabber:iq:roster'>
          <item jid='nurse@example.com'
                name='[ ... some-very-long-handle ... ]'>
            <group>Servants</group>
          </item>
        </query>
      </iq>

   S: <iq to='juliet@example.com/balcony' type='error' id='roster_4'>
        <error type='modify'>
          <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>














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   Error: Roster set contains duplicate groups

   C: <iq from='juliet@example.com/balcony' type='set' id='roster_5'>
        <query xmlns='jabber:iq:roster'>
          <item jid='nurse@example.com'
                name='Nurse'>
            <group>Servants</group>
            <group>Servants</group>
          </item>
        </query>
      </iq>

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

   Error: Roster set contains empty group

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

   S: <iq to='juliet@example.com/balcony' type='error' id='roster_6'>
        <error type='modify'>
          <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

















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   Error: Roster set contains oversized group

   C: <iq from='juliet@example.com/balcony' type='set' id='roster_7'>
        <query xmlns='jabber:iq:roster'>
          <item jid='nurse@example.com'
                name='Nurse'>
            <group>[ ... some-very-long-group-name ... ]</group>
          </item>
        </query>
      </iq>

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

   The server MUST return a <not-allowed/> 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).

   Error: Roster set contains sender's JID

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

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

2.4.  Updating a Roster Item

2.4.1.  Success Case

   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 shall be updated exactly as
   provided in the roster set.

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





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   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 may want to add the item to
   another group.

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

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

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

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

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

   The user may then want to remove the handle altogether (note:



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   including an empty 'name' attribute is equivalent to including no
   'name' attribute).

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

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

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

   As with adding a roster item, when updating a roster item 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.

2.4.2.  Error Cases

   The error cases described under Section 2.3.2 also apply to updating
   a roster item.

2.5.  Deleting a Roster Item

2.5.1.  Success Case

   At any time, a client may 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' type='set' id='delete_1'>
        <query xmlns='jabber:iq:roster'>
          <item jid='nurse@example.com' subscription='remove'/>
        </query>
      </iq>

   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



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   value of "remove"), and send an IQ result to the initiating resource;
   for details, see Section 2.3.

   The server MUST also generate a presence stanza of type "unsubscribe"
   and a presence stanza of type "unsubscribed" from the user to the
   contact.

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

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

2.5.2.  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/> error.

   Error: Roster item not found

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

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


3.  Managing Presence Subscriptions

   In order to protect the privacy of instant messaging users, presence
   information is disclosed only to other entities that the user has
   approved.  When a user has agreed that another entity may 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 less strict sense to
   refer to a potential contact or an item in a user's roster).



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   In XMPP, a subscription lasts across 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>.  This rule helps to prevent presence
   leaks; for details, see the security considerations of [XMPP-CORE].

3.1.  Requesting a Subscription

   A SUBSCRIPTION REQUEST 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.

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'/>

   Typically the user's client prompts the user for information about
   the potential contact ("handle" and desired roster group) and
   generates a roster set with that information before sending the
   subscription request, but that behavior is recommended rather than
   required.

3.1.2.  Server Processing of Outbound Subscription Request

   As mentioned, 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'/>

   If the potential contact is hosted on the same server, the server
   MUST adhere to the rules specified in the next section in processing



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   the subscription request and delivering it to the (local) contact.

   If the potential contact is hosted on a different server, the user's
   server then routes the stanza to that foreign domain in accordance
   with core XMPP stanza processing rules.

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

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

   Note: Because the server must send this roster push, a client MAY
   simply wait for the roster push rather than proactively adding the
   contact to the user's roster before sending the subscription request.

   Note: If the contact does not approve or deny the subscription
   request within some configurable amount of time, the user's server
   MAY choose to 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 elapsed); this helps to recover from transient, silent
   errors that may have occurred in relation to the original
   subscription request.







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3.1.3.  Server Processing of Inbound Subscription Request

   The contact's server MUST adhere to the following rules when
   processing the inbound subscription request:

   1.  Above all, the contact's server MUST NOT automatically approve
       subscription requests on the contact's behalf; instead, if a
       subscription request requires approval then the contact's server
       MUST deliver that request to the contact's interested resource(s)
       for approval or denial by the contact.
   2.  If the contact does not exist, 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 user's presence, then the contact's server SHOULD 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 MAY 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
       interested resource associated with the contact when the
       subscription request is received by the contact's server, the
       contact's server MUST broadcast that subscription request to all
       interested resources in accordance with Server Rules for
       Processing XML Stanzas (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
       interested resources when the subscription request is received by
       the contact's server, 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 interested resource, until
       the contact either approves or denies the request.  (Note: The
       contact's server MUST NOT deliver more than one subscription
       request from any given user when the contact next has an
       interested 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



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       deliver only one of the requests when the contact next has an
       interested 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'/>

   Note: Before approving a presence subscription, the contact MAY
   generate a roster set that includes a handle for the user and that
   places the user in one or more roster groups; see Section 2.3.

   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'/>

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 route or deliver 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.







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

   CS: <iq to='juliet@example.com/chamber'
           type='set'
           id='a78b4q6ha464'>
         <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
   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 a
   subscription='none' or subscription='from' and the 'ask' flag set to
   "subscribe" (i.e., a subscription states of "None + Pending Out" or
   "From + Pending Out"; see Appendix A).  If the contact is not in the
   user's roster with either of those states, the user's server MUST
   silently ignore the presence stanza of type "subscribed" (i.e., it



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   MUST NOT route it to the user, modify the user's roster, or generate
   a roster push to the user's interested resources).

   If the foregoing 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".

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

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

   From the perspective of the user, there now exists a subscription to
   the contact's presence.

   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.


















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   [ ... 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

   As mentioned, 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'/>

   If the user is hosted on the same server, the server MUST adhere to
   the rules specified in the next section when processing the
   subscription cancellation.

   If the user is hosted on a different server, the contact's server
   then routes the stanza to that foreign domain in accordance with core
   XMPP stanza processing rules.




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   If the user is in the contact's roster, 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 to='juliet@example.com/balcony'
           type='set'
           id='a78b4q6ha465'>
         <query xmlns='jabber:iq:roster'>
           <item jid='romeo@example.net'
                 subscription='none'/>
         </query>
       </iq>

   CS: <iq to='juliet@example.com/chamber'
           type='set'
           id='a78b4q6ha466'>
         <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 modify the subscription state and send a roster
   push to the user's interested resources, where the subscription state
   is now either "none" or "from" (see Appendix A).

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

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



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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

   As mentioned, the user's server MUST stamp the outbound unsubscribe
   with the bare JID <user@domain> of the user.

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

   If the contact is hosted on the same server, the server MUST adhere
   to the rules specified in the next section when processing the
   unsubscribe.

   If the contact is hosted on a different server, the user's server
   then routes the stanza to that foreign domain in accordance with core
   XMPP stanza processing rules.

   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 to='romeo@example.net/foo'
           type='set'
           id='h37h3u1bv402'>
         <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>



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3.3.3.  Server Processing of Inbound Unsubscribe

   When the contact's server receives the inbound unsubscribe, it MUST
   modify the subscription state and send a roster push to the contact's
   interested resources, where the subscription state is now either
   "none" or "to" (see Appendix A).

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

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


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 principal's availability is signalled
   when a client controlled by the principal 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".  In XMPP-based
   applications that combine messaging and presence functionality, the
   default type of communication for which presence signals availability
   is messaging; however, XMPP-based applications are not required 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).

   XMPP presence typically follows a "publish-subscribe" or "observer"



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   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
   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 BROADCASTED PRESENCE.  (A client MAY 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 principal's presence; see
   Section 4.6.)

   After a client completes the preconditions specified in [XMPP-CORE],
   it can establish a PRESENCE SESSION at its server by sending initial
   presence (Section 4.2).  Such a presence session is terminated by
   sending unavailable presence (Section 4.5).

4.2.  Initial Presence

4.2.1.  Client Generation of Initial Presence

   After completing the preconditions described in [XMPP-CORE]
   (REQUIRED) and requesting the roster (RECOMMENDED), a client SHOULD
   signal 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 broadcasted by the server on
   behalf of the client) and no 'type' attribute (indicating the user's
   availability).  After sending initial presence, a connected resource
   (in the terminology of [XMPP-CORE]) is said to be an AVAILABLE
   RESOURCE.

   UC: <presence/>

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".

   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



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   benvolio@example.com.

   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.com'/>

   The user's server MUST also broadcast initial presence from the
   user's newly available resource to all of the user's available
   resources.

   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 SHOULD 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'/>

4.2.4.  Client Processing of Inbound Presence

   When the contact's client receives presence from the user and the
   user is in the contact's roster, it SHOULD display the presence
   information in an appropriate roster interface.

   If the user is not in the contact's roster but the contact and the



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   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).

   Otherwise, the client SHOULD ignore the presence information and not
   display it to the contact.

4.3.  Presence Probes

   A PRESENCE PROBE is a presence stanza whose 'type' attribute has a
   value of "probe".  The value of the 'from' address SHOULD be the full
   JID <user@domain/resource> of the probing user and the value of the
   'to' address SHOULD be the bare JID <contact@domain> of the contact
   whose availability the user wants to discover.

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

   A presence probe SHOULD NOT be sent by a client.  Instead, it is
   designed to be sent by a user's server on the user's behalf in order
   to discover the availability of the user's contacts.

4.3.1.  Server Generation of Outbound Presence Probe

   When a server needs to discover the availability of a user's contact,
   it SHOULD send a presence probe from the full JID
   <user@domain/resource> of the user to the bare JID <contact@domain>
   of the contact.  The server SHOULD send a probe to a contact only if
   the contact is in the user's roster with the 'subscription' attribute
   set to a value of "to" or "both" (i.e., if the user is subscribed to
   the contact's presence).

   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).











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

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

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

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).

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

   2.  Else, if the contact has no available resources, the server MUST
       either (1) 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, or (2) 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>




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4.4.  Subsequent Presence Broadcast

4.4.1.  Client Generation of Presence Broadcast

   After sending initial presence, the user's client may 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>

   Note: A user SHOULD NOT send a presence update to broadcast
   information that changes independently of the user's availability for
   communication.

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".

   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.com'>
         <show>away</show>
       </presence>

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

   Note: As an optimization, if the subscription state is "both" then
   the user's server MAY choose to broadcast subsequent presence only if
   the server has received available presence from the contact at some
   point during the user's session; i.e., if the server never received
   available presence from the contact and the user has a mutual



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   presence subscription with the contact, it MAY decline to send
   subsequent presence to the contact.

   The user's server MUST also send the presence stanza to all of the
   user's available resources.

   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 and the
   user is in the contact's roster, it SHOULD display the presence
   information in an appropriate roster interface.

   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).

   Otherwise, the client SHOULD ignore the presence information and not



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   display it to the contact.

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

   US: <presence type='unavailable'>
         <status>going on vacation</status>
       </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 may become unavailable
   ungracefully (e.g., the resource may 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 contact has at least one resource that is online according to
       the user's server.
   3.  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.

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






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

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

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

   The user's server MUST also send the unavailable presence stanza to
   all of the user's remaining available resources.

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

   Note: Any presence stanza with no 'type' attribute and no 'to'
   attribute that is sent after sending broadcasted unavailable presence
   MUST be broadcasted by the 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
   contact's available resources.

   [ ... to resource1 ... ]

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

   [ ... to resource2 ... ]

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

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




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4.5.4.  Client Processing of Inbound Unavailable Presence

   When the contact's client receives unavailable presence from the user
   and the user is in the contact's roster, it SHOULD display the
   unavailable presence information in an appropriate roster interface.

   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.

   Otherwise, the client SHOULD 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
   defined above, but only for the special case of directed presence.

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 or a 'type' attribute whose value
   is "unavailable".

   If a user exchanges messages or 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, subject to user approval
   (either explicitly for this entity or implicitly via a configuration
   setting).  If a client supports this feature, it MUST allow the user
   to disable the feature in order to prevent presence sharing with
   unknown entities.

4.6.2.  Server Processing of Outbound Directed Presence

   When the user's server receives the 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 broadcasted unavailable presence (i.e., during the user's
       presence session), the user's server MUST route or deliver the
       full XML of that presence stanza but SHOULD NOT otherwise modify
       the contact's status regarding broadcasted presence (i.e., it
       SHOULD include the contact's JID in any subsequent presence



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       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 broadcasted
       unavailable presence (i.e., during the user's presence session),
       the user's server MUST route or deliver 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
       become 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 entities to which the user sends directed
       presence in the same way that it treats the entities listed 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 broadcasted presence and directed presence, so the contact's
   server follows the existing rules for processing of inbound presence.

4.6.4.  Client Processing of Inbound Directed Presence

   When the contact's client receives presence from the user and the
   user is in the contact's roster, it SHOULD display the presence
   information in an appropriate roster interface.

   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 an appropriate user
   interface.

   Otherwise, the client SHOULD 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).




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   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
      [XMPP-CORE]).
   o  probe -- A request for an entity's current presence; SHOULD be
      generated only by a server on behalf of a user.
   o  subscribe -- The sender wishes to subscribe to the recipient's
      presence.
   o  subscribed -- The sender has allowed the recipient to receive
      their presence.
   o  unsubscribe -- The sender is unsubscribing from another entity's
      presence.
   o  unsubscribed -- The subscription request has been denied or a
      previously-granted subscription has been cancelled.
   o  unavailable -- Signals that the entity is no longer available for
      communication.

   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.  Therefore if the intended recipient or
   intermediate router receives a <presence/> element whose 'type' value
   is undefined, it SHOULD return a stanza error of <bad-request/>.

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/>



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   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 properly-namespaced child element of the presence
   stanza):

   o  away -- The entity or resource is temporarily away.
   o  chat -- The entity or resource is actively interested in chatting.
   o  dnd -- The entity or resource is busy (dnd = "Do Not Disturb").
   o  xa -- The entity or resource is away for an extended period (xa =
      "eXtended Away").

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

   Any specialized processing of availability states by recipients and
   intermediate routers is up to the implementation (e.g., incorporation
   of availability states into server 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



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   (either explicitly or by inheritance from the 'xml:lang' value of an
   element farther up in the XML hierarchy, which may include the XML
   stream header as described in [XMPP-CORE]).

   <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'>
     <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/> to a human user (see
   Section 11).

   <presence from='romeo@example.net'
             to='nurse@example.com'
             type='subscribe'>
     <status>Hi, Juliet said I should 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 SHOULD



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   consider the priority to be zero.

   For information regarding the semantics of priority values in stanza
   processing within instant messaging and presence applications, refer
   to Server Rules for Processing XML Stanzas (Section 8).

4.7.6.  Extended Content

   As described in [XMPP-CORE], an XML stanza MAY contain any properly-
   namespaced child element; 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'
        node='http://psi-im.org/caps'
        ver='0.11'/>
   </presence>

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


5.  Exchanging Messages

   Once a client has authenticated with a server and bound a resource to
   an XML stream as described in [XMPP-CORE], an XMPP server will route
   XML stanzas to and from that client (if, that is, messaging
   functionality is enabled and the server is not a presence-only
   service).  One kind of stanza that may be exchanged is <message/>.
   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 Server Rules for Processing XML Stanzas (Section 8),
   the sender's server is responsible for delivering the message to the
   intended recipient (if the recipient is on the same server) or for
   routing the message to the recipient's server (if the recipient is on
   a different server).  Thus a message stanza is used to "push"
   information to another entity.

   As noted under Section 4.6, if a user exchanges messages 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.

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



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5.1.  Attributes

5.1.1.  To Attribute

   An instant messaging client SHOULD specify 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 chat session), 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>
   </message>

5.1.2.  Type Attribute

   Common uses of the message stanza in instant messaging applications
   include single messages, messages sent in the context of a chat
   conversation, messages sent in the context of a multi-user chat room,
   headlines and other alerts, 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:






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   o  chat -- The message is sent in the context of a one-to-one chat
      conversation.  A receiving client SHOULD present the message in an
      interface enabling one-to-one chat between the two parties,
      including an appropriate conversation history.  (For detailed
      recommendations regarding chat sessions, refer to [XEP-0201].)
   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 [XMPP-CORE]).  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]).  A receiving client
      SHOULD present the message in an interface enabling many-to-many
      chat between the parties, including a roster of parties in the
      chatroom and an appropriate conversation history.  Full definition
      of XMPP-based groupchat protocols is out of scope for this
      document (for details, refer to [XEP-0045]).
   o  headline -- The message is probably generated by an automated
      service that delivers or broadcasts content (news, sports, market
      information, syndicated content, alerts, etc.), or the sender
      wishes the message to be delivered as if it were so generated.  No
      reply to the message is expected, and a receiving client SHOULD
      present the message in an interface that appropriately
      differentiates the message from standalone messages, chat
      sessions, or groupchat sessions (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.  A receiving
      client SHOULD present the message in an interface enabling the
      recipient to reply, but without a conversation history.  This is
      the default value of the 'type' attribute.

   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').





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5.2.  Child Elements

   An XMPP message stanza MAY contain any allowable child elements
   qualified by the 'jabber:client' (or 'jabber:server') namespace, as
   well as any other properly-namespaced child element that consists of
   extended content.  The defined payloads are described in the
   following sections and extended content payloads are described in the
   appropriate XMPP extension specifications (not herein).

5.2.1.  Body

   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
       to='romeo@example.net'
       from='juliet@example.com/balcony'
       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 may include the
   XML stream header as described in [XMPP-CORE]).

   <message
       to='romeo@example.net'
       from='juliet@example.com/balcony'
       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
   Section 3.2.2 of [XML]).







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5.2.2.  Subject

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

   <message
       to='romeo@example.net'
       from='juliet@example.com/balcony'
       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 may include the XML
   stream header as described in [XMPP-CORE]).

   <message
       to='romeo@example.net'
       from='juliet@example.com/balcony'
       type='chat'
       xml:lang='en'>
     <subject>I implore you!</subject>
     <subject xml:lang='cs'>
       &#x00DA;p&#x011B;nliv&#x011B; pros&#x00EDm!
     </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]).

5.2.3.  Thread

   The primary use of the XMPP <thread/> element is to uniquely identify
   a conversation thread or "chat session" between two entities
   instantiated by <message/> stanzas of type 'chat'.  However, the XMPP
   <thread/> element may also be used to uniquely identify an analogous
   thread between two entities instantiated by <message/> stanzas of



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   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 conversation, such as a game session or between plugins.

   The value of the <thread/> element MUST be a universally unique
   identifier (UUID) as described in [UUID].

   The use of the <thread/> element is OPTIONAL.  The <thread/> element
   is not used to identify individual messages, only conversations.  A
   message stanza MUST NOT contain more than one <thread/> element.  The
   <thread/> element MUST NOT possess any attributes.  The value of the
   <thread/> element is not human-readable and MUST be treated as opaque
   by entities; no semantic meaning may be derived from it, and only
   exact comparisons may be made against it.  The <thread/> element MUST
   NOT contain mixed content (as defined in Section 3.2.2 of [XML]).

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

5.3.  Extended Content

   As described in [XMPP-CORE], an XML stanza MAY contain any properly-
   namespaced child element; 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
       to='romeo@example.net'
       from='juliet@example.com/balcony'
       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 [XMPP-CORE], IQ stanzas provide a structured request-
   response mechanism.  The basic semantics of that mechanism (e.g.,
   that the 'id' attribute is required) are defined in [XMPP-CORE],



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   whereas the specific semantics required to complete particular use
   cases are defined in all cases 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/> 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.


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

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

   Next, the user requests his roster.

   Example 1: User requests current roster from server:

   UC: <iq from='romeo@example.net/balcony' type='get' id='ex1'>
         <query xmlns='jabber:iq:roster'/>
       </iq>












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   Example 2: User receives roster from server:

   US: <iq to='romeo@example.net/balcony' type='result' id='ex1'>
         <query xmlns='jabber:iq:roster'>
           <item jid='romeo@example.com'
                 name='Juliet'
                 subscription='both'>
             <group>Friends</group>
           </item>
           <item jid='benvolio@example.org'
                 name='Benvolio'
                 subscription='to'/>
           <item jid='mercutio@example.org'
                 name='Mercutio'
                 subscription='from'/>
         </query>
       </iq>

   Now the user begins a presence session.

   Example 3: User sends initial presence:

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

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

   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'



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           to='mercutio@example.org'/>

   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 or return unsubscribed to user:

   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'
           type='unsubscribed'/>








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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 has a threaded conversation (chat session) with one of
   his contacts.

   Example 9: A threaded conversation

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

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

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

   And so on.

   The user can also send subsequent broadcasted presence.




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

   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 contact
   as well as to the person to whom the user sent directed presence:

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

   US: <presence
           type='unavailable'
           from='romeo@example.net/orchard'
           to='nurse@example.com'
           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
   [XMPP-CORE].  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 [XMPP-CORE] shall apply.

8.1.  No Such User

   If the user account does not exist, how the stanza shall be processed
   depends on the stanza type.

   o  For a message stanza, the server SHOULD 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 SHOULD silently ignore the
      stanza (i.e., not return an error or a presence stanza of type
      "unsubscribed").
   o  For a presence stanza of type "subscribe", "subscribed",
      "unsubscribe", or "unsubscribed", the server MUST follow the
      guidelines provided under Section 3.
   o  For an IQ stanza, the server MUST return a <service-unavailable/>
      stanza error to the sender.

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.

8.2.1.  Available Resource Matches

   If an available resource exactly matches the full JID, the
   recipient's server MUST deliver the stanza to that resource.

   However, for 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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8.2.2.  No Available Resource Matches

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

   o  For a message, the server SHOULD treat the stanza as if it were
      addressed to <user@domain> as described in the next section.
   o  For a presence stanza with no 'type' attribute or a 'type'
      attribute of "unavailable", the server SHOULD silently ignore the
      stanza (i.e., not return an error or a presence stanza of type
      "unsubscribed").
   o  For a presence stanza of type "subscribe", "subscribed",
      "unsubscribe", or "unsubscribed", the server MUST follow the
      guidelines provided under Section 3.
   o  For an IQ stanza, the server MUST return a <service-unavailable/>
      stanza error to the sender.

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 Resources

   If there is at least one available resource, how the stanza shall be
   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", "error", "groupchat", or
   "normal", the server SHOULD 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



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   resources from the tie, and MUST deliver the message to at least one
   of the highest-priority resources.

   However, for any message type the server MUST NOT deliver the stanza
   to any available resource with a negative priority; if the only
   available resource has a negative priority, the server SHOULD handle
   the message as if there were no available resources 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
   <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 Resources

   If there are no available resources associated with the user, how the
   stanza shall be processed depends on the stanza type.






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8.3.2.1.  Message

   For a message stanza of type "headline", the server SHOULD NOT store
   the stanza on behalf of the user and deliver it when the user next
   becomes available, but instead SHOULD either forward the stanza to
   the user via a non-XMPP messaging system (e.g., to the user's email
   account) or return a <service-unavailable/> stanza error to the
   sender.

   For a message stanza of type "chat", "error", "groupchat", or
   "normal", the server MAY choose to store the stanza on behalf of the
   user and deliver it when the user next becomes available, 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, the server MUST return a <service-
   unavailable/> stanza error to the sender.

   Note: Offline message storage and message forwarding are not defined
   in XMPP, since they are strictly a matter of implementation and
   service provisioning.  However, for recommendations regarding offline
   message storage refer to [XEP-0160].

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.  Foreign 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



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   to route the stanza to the foreign domain.  If there exists an active
   stream between the two peers, the server shall 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 foreign domain (or use a cached
   resolution of the foreign domain to an IP address).  The recommended
   order of attempted resolutions is as follows:

   1.  Attempt to resolve the foreign hostname using a DNS service
       location record [SRV] Service of "xmpp-server" and Proto of
       "tcp", resulting in resource records such as "_xmpp-
       server._tcp.example.com.", as specified in [XMPP-CORE].
   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
       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 [XMPP-CORE].

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

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

   Third, route the stanza to the foreign 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.






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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 [XMPP-CORE].  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, and is not required to support both if only a presence
   service or an instant messaging service is desired.

9.1.  Servers

   In addition to 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:

   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 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)
   o  End-to-end object signing and encryption as defined in [XMPP-E2E]

   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.



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10.  Internationalization Considerations

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


11.  Security Considerations

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

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

   o  When a server processes an inbound presence stanza of type "probe"
      whose intended recipient is a user associated with one of the
      server's 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
      Exchanging Presence Information (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
      Exchanging Presence Information (Section 4) in order to ensure
      that such presence information is not broadcasted 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 who does not exist, the use of the <service-
      unavailable/> error condition helps protect against well-known
      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.
   o  A client MAY ignore the <status/> element when contained in a
      presence stanza of type "subscribe", "unsubscribe", "subscribed",
      or "unsubscribed" in order to help prevent "presence subscription
      spam".


12.  IANA Considerations

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



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

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:

   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.

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




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   [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.

   [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-CORE]
              Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", draft-saintandre-rfc3920bis-04
              (work in progress), October 2007.

   [XMPP-E2E]
              Saint-Andre, P., "End-to-End Signing and Object Encryption
              for the Extensible Messaging and Presence Protocol
              (XMPP)", RFC 3923, October 2004.

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.

   [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



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              Security Layer (SASL)", RFC 4422, June 2006.

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

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

   [XEP-0045]
              Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
              April 2007.

   [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, August 2007.

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

   [XEP-0201]
              Saint-Andre, P., Paterson, I., and K. Smith, "Best
              Practices for Message Threads", XSF XEP 0201, August 2007.

   [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='none' 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.

   Note: In the following tables, "S.N." stand for SHOULD NOT.

A.2.1.  Subscribe

   Table 1: Processing of outbound "subscribe" stanzas

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

   [1] 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.

   Note: In the following tables, "S.N." stand 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.  A protocol for doing so is
   specified in [XEP-0016].


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 XMPP (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 [XMPP-CORE].

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' type='xs:NMTOKEN'/>

     <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'/>
               <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>



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     <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'/>
         </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'>



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           <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

   <?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'/>



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     <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'/>
        </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>



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     </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' type='xs:NMTOKEN'/>

     <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='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>



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           </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'/>
         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='id'



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                       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>
         <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'



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       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'>
       <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>



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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, server
      implementations may still want to advertise support for the
      feature in order to ensure backward-compatibility, even though
      session establishment is a "no-op".
   o  The protocol for communications blocking specified in Section 10
      of RFC 3921 has been moved to [XEP-0016].
   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.

   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

   The Contributor grants third parties the irrevocable right to copy,
   use and distribute the Contribution, with or without modification, in
   any medium, without royalty, provided that, unless separate
   permission is granted, redistributed modified works:

   1.  do not contain misleading author, version, name of work, or
       endorsement information, and
   2.  do not claim endorsement of the modified work by the Contributor,
       or any organization the Contributor belongs to, the Internet
       Engineering Task Force (IETF), Internet Research Task Force
       (IRTF), Internet Engineering Steering Group (IESG), Internet
       Architecture Board (IAB), Internet Assigned Numbers Authority
       (IANA), Internet Society (ISOC), Request For Comments (RFC)
       Editor, or any combination or variation of such terms (including
       without limitation the IETF "4 diamonds" logo), or any terms that
       are confusingly similar thereto, and
   3.  remove any claims of status as an Internet Standard, including
       without limitation removing the RFC boilerplate.




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   The IETF suggests that any citation or excerpt of unmodified text
   reference the RFC or other document from which the text is derived.


Index

   A
      Available Resource  31

   B
      Broadcasted Presence  31

   C
      Contact  20

   D
      Directed Presence  31

   I
      Initial Presence  31
      Interested Resource  11

   P
      Presence  5
      Presence Probe  33
      Presence Session  31
      Presence Subscription  20

   R
      Roster  5
      Roster Get  8
      Roster Push  9
      Roster Result  10
      Roster Set  9

   S
      Subscription Request  21

   U
      Unavailable Presence  37











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Author's Address

   Peter Saint-Andre (editor)
   XMPP Standards Foundation

   Email: stpeter@jabber.org
   URI:   https://stpeter.im/












































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Full Copyright Statement

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