path: root/doc/rfc/rfc3921.txt

Network Working Group                                P. Saint-Andre, Ed.
Request for Comments: 3921                    Jabber Software Foundation
Category: Standards Track                                   October 2004


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

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

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

























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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.   Syntax of XML Stanzas  . . . . . . . . . . . . . . . . . . .   4
   3.   Session Establishment  . . . . . . . . . . . . . . . . . . .  10
   4.   Exchanging Messages  . . . . . . . . . . . . . . . . . . . .  13
   5.   Exchanging Presence Information  . . . . . . . . . . . . . .  16
   6.   Managing Subscriptions . . . . . . . . . . . . . . . . . . .  26
   7.   Roster Management  . . . . . . . . . . . . . . . . . . . . .  27
   8.   Integration of Roster Items and Presence Subscriptions . . .  32
   9.   Subscription States  . . . . . . . . . . . . . . . . . . . .  56
   10.  Blocking Communication . . . . . . . . . . . . . . . . . . .  62
   11.  Server Rules for Handling XML Stanzas  . . . . . . . . . . .  85
   12.  IM and Presence Compliance Requirements  . . . . . . . . . .  88
   13.  Internationalization Considerations  . . . . . . . . . . . .  89
   14.  Security Considerations  . . . . . . . . . . . . . . . . . .  89
   15.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  90
   16.  References . . . . . . . . . . . . . . . . . . . . . . . . .  91
   A.   vCards . . . . . . . . . . . . . . . . . . . . . . . . . . .  93
   B.   XML Schemas. . . . . . . . . . . . . . . . . . . . . . . . .  93
   C.   Differences Between Jabber IM/Presence Protocols and XMPP. . 105
   Contributors . . . . . . . . . . . . . . . . . . . . . . . . . .  106
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . .  106
   Author's Address. . . . . . . . . . . . . . . . . . . . . . . . . 106
   Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 107

1.  Introduction

1.1.  Overview

   The Extensible Messaging and Presence Protocol (XMPP) is a protocol
   for streaming XML [XML] elements in order to exchange messages and
   presence information in close to real time.  The core features of
   XMPP are defined in Extensible Messaging and Presence Protocol
   (XMPP): Core [XMPP-CORE].  These features -- mainly XML streams, use
   of TLS and SASL, and the <message/>, <presence/>, and <iq/> children
   of the stream root -- 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 [XML-NAMES].  This memo describes extensions to and
   applications of the core features of XMPP that provide the basic
   functionality expected of an instant messaging (IM) and presence
   application as defined in RFC 2779 [IMP-REQS].








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1.2.  Requirements

   For the purposes of this memo, the requirements of a basic instant
   messaging and presence application are defined by [IMP-REQS], which
   at a high level stipulates that a user must be able to complete the
   following use cases:

   o  Exchange messages with other users
   o  Exchange presence information with other users
   o  Manage subscriptions to and from other users
   o  Manage items in a contact list (in XMPP this is called a "roster")
   o  Block communications to or from specific other users

   Detailed definitions of these functionality areas are contained in
   [IMP-REQS], and the interested reader is directed to that document
   regarding the requirements addressed herein.

   [IMP-REQS] also stipulates that presence services must be separable
   from instant messaging services; 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 memo 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.

   Note: While XMPP-based instant messaging and presence meets the
   requirements of [IMP-REQS], it was 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.  Note also that although protocols
   addressing many other functionality areas have been defined in the
   Jabber community, such protocols are not included in this memo
   because they are not required by [IMP-REQS].

1.3.  Terminology

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

   The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14, RFC 2119 [TERMS].







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2.  Syntax of XML Stanzas

   The basic semantics and common attributes of XML stanzas qualified by
   the 'jabber:client' and 'jabber:server' namespaces are defined in
   [XMPP-CORE].  However, these namespaces also define various child
   elements, as well as values for the common 'type' attribute, that are
   specific to instant messaging and presence applications.  Thus,
   before addressing particular "use cases" for such applications, we
   here further describe the syntax of XML stanzas, thereby
   supplementing the discussion in [XMPP-CORE].

2.1.  Message Syntax

   Message stanzas qualified by the 'jabber:client' or 'jabber:server'
   namespace are used to "push" information to another entity.  Common
   uses 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.

2.1.1.  Types of Message

   The 'type' attribute of a message stanza is RECOMMENDED; if included,
   it specifies the conversational context of the message, thus
   providing a hint regarding presentation (e.g., in a GUI).  If
   included, the 'type' attribute MUST have one of the following values:

   o  chat -- The message is sent in the context of a one-to-one chat
      conversation.  A compliant client SHOULD present the message in an
      interface enabling one-to-one chat between the two parties,
      including an appropriate conversation history.

   o  error -- An error has occurred related to a previous message sent
      by the sender (for details regarding stanza error syntax, refer to
      [XMPP-CORE]).  A compliant client 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 compliant 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 memo.

   o  headline -- The message is probably generated by an automated
      service that delivers or broadcasts content (news, sports, market
      information, RSS feeds, etc.).  No reply to the message is
      expected, and a compliant client SHOULD present the message in an



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      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).

   o  normal -- The message is a single 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 compliant client
      SHOULD present the message in an interface enabling the recipient
      to reply, but without a conversation history.

   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).  The "error" type MUST be generated only in
   response to an error related to a message received from another
   entity.

   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').

2.1.2.  Child Elements

   As described under extended namespaces (Section 2.4), a message
   stanza MAY contain any properly-namespaced child element.

   In accordance with the default namespace declaration, by default a
   message stanza is qualified by the 'jabber:client' or 'jabber:server'
   namespace, which defines certain allowable children of message
   stanzas.  If the message stanza is of type "error", it MUST include
   an <error/> child; for details, see [XMPP-CORE].  Otherwise, the
   message stanza MAY contain any of the following child elements
   without an explicit namespace declaration:

   1.  <subject/>
   2.  <body/>
   3.  <thread/>

2.1.2.1.  Subject

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



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   subject, but only if each instance possesses an 'xml:lang' attribute
   with a distinct language value.  The <subject/> element MUST NOT
   contain mixed content (as defined in Section 3.2.2 of [XML]).

2.1.2.2.  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.  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
   but only if each instance possesses an 'xml:lang' attribute with a
   distinct language value.  The <body/> element MUST NOT contain mixed
   content (as defined in Section 3.2.2 of [XML]).

2.1.2.3.  Thread

   The <thread/> element contains non-human-readable XML character data
   specifying an identifier that is used for tracking a conversation
   thread (sometimes referred to as an "instant messaging session")
   between two entities.  The value of the <thread/> element is
   generated by the sender and SHOULD be copied back in any replies.  If
   used, it MUST be unique to that conversation thread within the stream
   and MUST be consistent throughout that conversation (a client that
   receives a message from the same full JID but with a different thread
   ID MUST assume that the message in question exists outside the
   context of the existing conversation thread).  The use of the
   <thread/> element is OPTIONAL and 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 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]).

2.2.  Presence Syntax

   Presence stanzas are used qualified by the 'jabber:client' or
   'jabber:server' namespace to express an entity's current network
   availability (offline or online, along with various sub-states of the
   latter and optional user-defined descriptive text), and to notify
   other entities of that availability.  Presence stanzas are also used
   to negotiate and manage subscriptions to the presence of other
   entities.






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2.2.1.  Types of Presence

   The 'type' attribute of a presence stanza is OPTIONAL.  A presence
   stanza that does not possess a 'type' attribute is used to signal to
   the server that the sender is online and available for communication.
   If included, the 'type' attribute specifies a lack of availability, a
   request to manage a subscription to another entity's presence, a
   request for another entity's current presence, or an error related to
   a previously-sent presence stanza.  If included, the 'type' attribute
   MUST have one of the following values:

   o  unavailable -- Signals that the entity is no longer available for
      communication.

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

   o  error -- An error has occurred regarding processing or delivery of
      a previously-sent presence stanza.

   For detailed information regarding presence semantics and the
   subscription model used in the context of XMPP-based instant
   messaging and presence applications, refer to Exchanging Presence
   Information (Section 5) and Managing Subscriptions (Section 6).

2.2.2.  Child Elements

   As described under extended namespaces (Section 2.4), a presence
   stanza MAY contain any properly-namespaced child element.

   In accordance with the default namespace declaration, by default a
   presence stanza is qualified by the 'jabber:client' or
   'jabber:server' namespace, which defines certain allowable children
   of presence stanzas.  If the presence stanza is of type "error", it
   MUST include an <error/> child; for details, see [XMPP-CORE].  If the
   presence stanza possesses no 'type' attribute, it MAY contain any of



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   the following child elements (note that the <status/> child MAY be
   sent in a presence stanza of type "unavailable" or, for historical
   reasons, "subscribe"):

   1.  <show/>
   2.  <status/>
   3.  <priority/>

2.2.2.1.  Show

   The OPTIONAL <show/> element contains non-human-readable XML
   character data that specifies the particular availability status of
   an entity or specific resource.  A presence stanza MUST NOT contain
   more than one <show/> element.  The <show/> element MUST NOT possess
   any attributes.  If provided, the XML character data value MUST be
   one of the following (additional availability types 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.

2.2.2.2.  Status

   The OPTIONAL <status/> element contains XML character data specifying
   a natural-language description of availability status.  It is
   normally used in conjunction with the show element to provide a
   detailed description of an availability state (e.g., "In a meeting").
   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.

2.2.2.3.  Priority

   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.  If no priority is provided,



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   a server SHOULD consider the priority to be zero.  For information
   regarding the semantics of priority values in stanza routing within
   instant messaging and presence applications, refer to Server Rules
   for Handling XML Stanzas (Section 11).

2.3.  IQ Syntax

   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], whereas the specific semantics
   required to complete particular use cases are defined in all cases by
   an extended namespace (Section 2.4) (note that the 'jabber:client'
   and 'jabber:server' namespaces do not define any children of IQ
   stanzas other than the common <error/>).  This memo defines two such
   extended namespaces, one for Roster Management (Section 7) and the
   other for Blocking Communication (Section 10); however, an IQ stanza
   MAY contain structured information qualified by any extended
   namespace.

2.4.  Extended Namespaces

   While the three XML stanza kinds defined in the "jabber:client" or
   "jabber:server" namespace (along with their attributes and child
   elements) provide a basic level of functionality for messaging and
   presence, XMPP uses XML namespaces to extend the stanzas for the
   purpose of providing additional functionality.  Thus a message or
   presence stanza MAY contain one or more optional child elements
   specifying content that extends the meaning of the message (e.g., an
   XHTML-formatted version of the message body), and an IQ stanza MAY
   contain one such child element.  This child element MAY have any name
   and MUST possess an 'xmlns' namespace declaration (other than
   "jabber:client", "jabber:server", or
   "http://etherx.jabber.org/streams") that defines all data contained
   within the child element.

   Support for any given extended namespace is OPTIONAL on the part of
   any implementation (aside from the extended namespaces defined
   herein).  If an entity does not understand such a namespace, the
   entity's expected behavior depends on whether the entity is (1) the
   recipient or (2) an entity that is routing the stanza to the
   recipient:

   Recipient: If a recipient receives a stanza that contains a child
      element it does not understand, it SHOULD ignore that specific XML
      data, i.e., it SHOULD not process it or present it to a user or
      associated application (if any).  In particular:





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      *  If an entity receives a message or presence stanza that
         contains XML data qualified by a namespace it does not
         understand, the portion of the stanza that is in the unknown
         namespace SHOULD be ignored.

      *  If an entity receives a message stanza whose only child element
         is qualified by a namespace it does not understand, it MUST
         ignore the entire stanza.

      *  If an entity receives an IQ stanza of type "get" or "set"
         containing a child element qualified by a namespace it does not
         understand, the entity SHOULD return an IQ stanza of type
         "error" with an error condition of <service-unavailable/>.

   Router: If a routing entity (usually a server) handles a stanza that
      contains a child element it does not understand, it SHOULD ignore
      the associated XML data by passing it on untouched to the
      recipient.

3.  Session Establishment

   Most instant messaging and presence applications based on XMPP are
   implemented via a client-server architecture that requires a client
   to establish a session on a server in order to engage in the expected
   instant messaging and presence activities.  However, there are
   several pre-conditions that MUST be met before a client can establish
   an instant messaging and presence session.  These are:

   1.  Stream Authentication -- a client MUST complete stream
       authentication as documented in [XMPP-CORE] before attempting to
       establish a session or send any XML stanzas.
   2.  Resource Binding -- after completing stream authentication, a
       client MUST bind a resource to the stream so that the client's
       address is of the form <user@domain/resource>, after which the
       entity is now said to be a "connected resource" in the
       terminology of [XMPP-CORE].

   If a server supports sessions, it MUST include a <session/> element
   qualified by the 'urn:ietf:params:xml:ns:xmpp-session' namespace in
   the stream features it advertises to a client after the completion of
   stream authentication as defined in [XMPP-CORE]:










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   Server advertises session establishment feature to client:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='c2s_345'
       from='example.com'
       version='1.0'>
   <stream:features>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
   </stream:features>

   Upon being so informed that session establishment is required (and
   after completing resource binding), the client MUST establish a
   session if it desires to engage in instant messaging and presence
   functionality; it completes this step by sending to the server an IQ
   stanza of type "set" containing an empty <session/> child element
   qualified by the 'urn:ietf:params:xml:ns:xmpp-session' namespace:

   Step 1: Client requests session with server:

   <iq to='example.com'
       type='set'
       id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
   </iq>

   Step 2: Server informs client that session has been created:

   <iq from='example.com'
       type='result'
       id='sess_1'/>

   Upon establishing a session, a connected resource (in the terminology
   of [XMPP-CORE]) is said to be an "active resource".

   Several error conditions are possible.  For example, the server may
   encounter an internal condition that prevents it from creating the
   session, the username or authorization identity may lack permissions
   to create a session, or there may already be an active resource
   associated with a resource identifier of the same name.

   If the server encounters an internal condition that prevents it from
   creating the session, it MUST return an error.






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   Step 2 (alt): Server responds with error (internal server error):

   <iq from='example.com' type='error' id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
     <error type='wait'>
       <internal-server-error
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   If the username or resource is not allowed to create a session, the
   server MUST return an error (e.g., forbidden).

   Step 2 (alt): Server responds with error (username or resource not
   allowed to create session):

   <iq from='example.com' type='error' id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
     <error type='auth'>
       <forbidden
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   If there is already an active resource of the same name, the server
   MUST either (1) terminate the active resource and allow the
   newly-requested session, or (2) disallow the newly-requested session
   and maintain the active resource.  Which of these the server does is
   up to the implementation, although it is RECOMMENDED to implement
   case #1.  In case #1, the server SHOULD send a <conflict/> stream
   error to the active resource, terminate the XML stream and underlying
   TCP connection for the active resource, and return a IQ stanza of
   type "result" (indicating success) to the newly-requested session. In
   case #2, the server SHOULD send a <conflict/> stanza error to the
   newly-requested session but maintain the XML stream for that
   connection so that the newly-requested session has an opportunity to
   negotiate a non-conflicting resource identifier before sending
   another request for session establishment.

   Step 2 (alt): Server informs existing active resource of resource
   conflict (case #1):

   <stream:error>
     <conflict xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>





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   Step 2 (alt): Server informs newly-requested session of resource
   conflict (case #2):

   <iq from='example.com' type='error' id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
     <error type='cancel'>
       <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   After establishing a session, a client SHOULD send initial presence
   and request its roster as described below, although these actions are
   OPTIONAL.

   Note: Before allowing the creation of instant messaging and presence
   sessions, a server MAY require prior account provisioning.  Possible
   methods for account provisioning include account creation by a server
   administrator as well as in-band account registration using the
   'jabber:iq:register' namespace; the latter method is out of scope for
   this memo, but is documented in [JEP-0077], published by the Jabber
   Software Foundation [JSF].

4.  Exchanging Messages

   Exchanging messages is a basic use of XMPP and is brought about when
   a user generates a message stanza that is addressed to another
   entity.  As defined under Server Rules for Handling XML Stanzas
   (Section 11), 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).

   For information regarding the syntax of message stanzas as well as
   their defined attributes and child elements, refer to Message Syntax
   (Section 2.1).

4.1.  Specifying an Intended Recipient

   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 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.  If the message is being sent




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   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>.

4.2.  Specifying a Message Type

   As noted, it is RECOMMENDED for a message stanza to possess a 'type'
   attribute whose value captures the conversational context (if any) of
   the message (see Type (Section 2.1.1)).

   The following example shows a valid value of the 'type' attribute:

   Example: A message of a defined type:

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

4.3.  Specifying a Message Body

   A message stanza MAY (and often will) contain a child <body/> element
   whose XML character data specifies the primary meaning of the message
   (see Body (Section 2.1.2.2)).

   Example: A message with a body:

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

4.4.  Specifying a Message Subject

   A message stanza MAY contain one or more child <subject/> elements
   specifying the topic of the message (see Subject (Section 2.1.2.1)).








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   Example: A message with a subject:

   <message
       to='romeo@example.net'
       from='juliet@example.com/balcony'
       type='chat'
       xml:lang='en'>
     <subject>I implore you!</subject>
     <subject
         xml:lang='cz'>&#x00DA;p&#x011B;nliv&#x011B; prosim!</subject>
     <body>Wherefore art thou, Romeo?</body>
     <body xml:lang='cz'>Pro&#x010D;e&#x017D; jsi ty, Romeo?</body>
   </message>

4.5.  Specifying a Conversation Thread

   A message stanza MAY contain a child <thread/> element specifying the
   conversation thread in which the message is situated, for the purpose
   of tracking the conversation (see Thread (Section 2.1.2.3)).

   Example: A threaded conversation:

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

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

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



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5.  Exchanging Presence Information

   Exchanging presence information is made relatively straightforward
   within XMPP by using presence stanzas.  However, we see here a
   contrast to the handling of messages: although a client MAY send
   directed presence information to another entity by including a 'to'
   address, normally presence notifications (i.e., presence stanzas with
   no 'type' or of type "unavailable" and with no 'to' address) are sent
   from a client to its server and then broadcasted by the server to any
   entities that are subscribed to the presence of the sending entity
   (in the terminology of RFC 2778 [IMP-MODEL], these entities are
   subscribers).  This broadcast model does not apply to
   subscription-related presence stanzas or presence stanzas of type
   "error", but to presence notifications only as defined above.  (Note:
   While presence information MAY be provided on a user's behalf by an
   automated service, normally it is provided by the user's client.)

   For information regarding the syntax of presence stanzas as well as
   their defined attributes and child elements, refer to [XMPP-CORE].

5.1.  Client and Server Presence Responsibilities

5.1.1.  Initial Presence

   After establishing a session, a client SHOULD send initial presence
   to the server in order to signal its availability for communications.
   As defined herein, the initial presence stanza (1) MUST possess no
   'to' address (signalling that it is meant to be broadcasted by the
   server on behalf of the client) and (2) MUST possess no 'type'
   attribute (signalling the user's availability).  After sending
   initial presence, an active resource is said to be an "available
   resource".

   Upon receiving initial presence from a client, the user's server MUST
   do the following if there is not already one or more available
   resources for the user (if there is already one or more available
   resources for the user, the server obviously does not need to send
   the presence probes, since it already possesses the requisite
   information):

   1.  Send presence probes (i.e., presence stanzas whose 'type'
       attribute is set to a value of "probe") from the full JID (e.g.,
       <user@example.com/resource>) of the user to all contacts to which
       the user is subscribed in order to determine if they are
       available; such contacts are those for which a JID is present in
       the user's roster with the 'subscription' attribute set to a
       value of "to" or "both".  (Note: The user's server MUST NOT send
       presence probes to contacts from which the user is blocking



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       inbound presence notifications, as described under Blocking
       Inbound Presence Notifications (Section 10.10).)

   2.  Broadcast initial presence from the full JID (e.g.,
       <user@example.com/resource>) of the user to all contacts that are
       subscribed to the user's presence information; 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: The user's server MUST NOT broadcast initial presence to
       contacts to which the user is blocking outbound presence
       notifications, as described under Blocking Outbound Presence
       Notifications (Section 10.11).)

   In addition, the user's server MUST broadcast initial presence from
   the user's new available resource to any of the user's existing
   available resources (if any).

   Upon receiving initial presence from the user, the contact's server
   MUST deliver the user's presence stanza to the full JIDs
   (<contact@example.org/resource>) associated with all of the contact's
   available resources, but only if the user is in the contact's roster
   with a subscription state of "to" or "both" and the contact has not
   blocked inbound presence notifications from the user's bare or full
   JID (as defined under Blocking Inbound Presence Notifications
   (Section 10.10)).

   If the user's server receives a presence stanza of type "error" in
   response to the initial presence that it sent to a contact on behalf
   of the user, it SHOULD NOT send further presence updates to that
   contact (until and unless it receives a presence stanza from the
   contact).

5.1.2.  Presence Broadcast

   After sending initial presence, the user MAY update its presence
   information for broadcasting at any time during its session by
   sending a presence stanza with no 'to' address and either no 'type'
   attribute or a 'type' attribute with a value of "unavailable". (Note:
   A user's client SHOULD NOT send a presence update to broadcast
   information that changes independently of the user's presence and
   availability.)

   If the presence stanza lacks a 'type' attribute (i.e., expresses
   availability), the user's server MUST broadcast the full XML of that
   presence stanza to all contacts (1) that are in the user's roster
   with a subscription type of "from" or "both", (2) to whom the user





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   has not blocked outbound presence notifications, and (3) from whom
   the server has not received a presence error during the user's
   session (as well as to any of the user's other available resources).

   If the presence stanza has a 'type' attribute set to a value of
   "unavailable", the user's server MUST broadcast the full XML of that
   presence stanza to all entities that fit the above description, as
   well as to any entities to which the user has sent directed available
   presence during the user's session (if the user has not yet sent
   directed unavailable presence to that entity).

5.1.3.  Presence Probes

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

   1.  If the user is not in the contact's roster with a subscription
       state of "From", "From + Pending Out", or "Both" (as defined
       under Subscription States (Section 9)), the contact's server MUST
       return a presence stanza of type "error" in response to the
       presence probe (however, if a server receives a presence probe
       from a subdomain of the server's hostname or another such trusted
       service, it MAY provide presence information about the user to
       that entity).  Specifically:

       *  if the user is in the contact's roster with a subscription
          state of "None", "None + Pending Out", or "To" (or is not in
          the contact's roster at all), the contact's server MUST return
          a <forbidden/> stanza error in response to the presence probe.

       *  if the user is in the contact's roster with a subscription
          state of "None + Pending In", "None + Pending Out/In", or "To
          + Pending In", the contact's server MUST return a
          <not-authorized/> stanza error in response to the presence
          probe.

   2.  Else, if the contact is blocking presence notifications to the
       user's bare JID or full JID (using either a default list or
       active list as defined under Blocking Outbound Presence
       Notifications (Section 10.11)), the server MUST NOT reply to the
       presence probe.

   3.  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.





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   4.  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 (again, subject to privacy lists in force for each
       session).

5.1.4.  Directed Presence

   A user MAY send directed presence to another entity (i.e., a presence
   stanza with a 'to' attribute whose value is the JID of the other
   entity and with either no 'type' attribute or a 'type' attribute
   whose value is "unavailable").  There are three possible cases:

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

   2.  If the user sends directed presence to an entity that is not in
       the user's roster with a subscription type of "from" or "both"
       after having sent initial presence and before sending unavailable
       presence broadcast, 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 broadcast 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 active 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.








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5.1.5.  Unavailable Presence

   Before ending its session with a server, a client SHOULD gracefully
   become unavailable by sending a final presence stanza that possesses
   no 'to' attribute and that possesses a 'type' attribute whose value
   is "unavailable" (optionally, the final presence stanza MAY contain
   one or more <status/> elements specifying the reason why the user is
   no longer available).  However, the user's server MUST NOT depend on
   receiving final presence from an available resource, since the
   resource may become unavailable unexpectedly or may be timed out by
   the server.  If one of the user's resources becomes unavailable for
   any reason (either gracefully or ungracefully), the user's server
   MUST broadcast unavailable presence to all contacts (1) that are in
   the user's roster with a subscription type of "from" or "both", (2)
   to whom the user has not blocked outbound presence, and (3) from whom
   the server has not received a presence error during the user's
   session; the user's server MUST also send that unavailable presence
   stanza to any of the user's other available resources, as well as to
   any entities to which the user has sent directed presence during the
   user's session for that resource (if the user has not yet sent
   directed unavailable presence to that entity).  Any presence stanza
   with no 'type' attribute and no 'to' attribute that is sent after
   sending directed unavailable presence or broadcasted unavailable
   presence MUST be broadcasted by the server to all subscribers.

5.1.6.  Presence Subscriptions

   A subscription request is a presence stanza whose 'type' attribute
   has a value of "subscribe".  If the subscription request is being
   sent to an instant messaging contact, the JID supplied in the 'to'
   attribute SHOULD be of the form <contact@example.org> rather than
   <contact@example.org/resource>, since the desired result is normally
   for the user to receive presence from all of the contact's resources,
   not merely the particular resource specified in the 'to' attribute.

   A user's server MUST NOT automatically approve subscription requests
   on the user's behalf.  All subscription requests MUST be directed to
   the user's client, specifically to one or more available resources
   associated with the user.  If there is no available resource
   associated with the user when the subscription request is received by
   the user's server, the user's server MUST keep a record of the
   subscription request and deliver the request when the user next
   creates an available resource, until the user either approves or
   denies the request.  If there is more than one available resource
   associated with the user when the subscription request is received by
   the user's server, the user's server MUST broadcast that subscription
   request to all available resources in accordance with Server Rules
   for Handling XML Stanzas (Section 11).  (Note: If an active resource



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   has not provided initial presence, the server MUST NOT consider it to
   be available and therefore MUST NOT send subscription requests to
   it.)   However, if the user receives a presence stanza of type
   "subscribe" from a contact to whom the user has already granted
   permission to see the user's presence information (e.g., in cases
   when the contact is seeking to resynchronize subscription states),
   the user's server SHOULD auto-reply on behalf of the user.  In
   addition, the user's server MAY choose to re-send an unapproved
   pending 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.

5.2.  Specifying Availability Status

   A client MAY provide further information about its availability
   status by using the <show/> element (see Show (Section 2.2.2.1)).

   Example: Availability status:

   <presence>
     <show>dnd</show>
   </presence>

5.3.  Specifying Detailed Status Information

   In conjunction with the <show/> element, a client MAY provide
   detailed status information by using the <status/> element (see
   Status (Section 2.2.2.2)).

   Example: Detailed status information:

   <presence xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
     <status xml:lang='cz'>Ja dvo&#x0159;&#x00ED;m Juliet</status>
   </presence>













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5.4.  Specifying Presence Priority

   A client MAY provide a priority for its resource by using the
   <priority/> element (see Priority (Section 2.2.2.3)).

   Example: Presence priority:

   <presence xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
     <status xml:lang='cz'>Ja dvo&#x0159;&#x00ED;m Juliet</status>
     <priority>1</priority>
   </presence>

5.5.  Presence Examples

   The examples in this section illustrate the presence-related
   protocols described above.  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 is "chamber" and another whose
      resource is "balcony")

   o  benvolio@example.org (subscription="to")

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

   Example 1: User sends initial presence:

   <presence/>

   Example 2: User's server sends presence probes to contacts with
   subscription="to" and subscription="both" on behalf of the user's
   available resource:

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

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





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   Example 3: User's server sends initial presence to contacts with
   subscription="from" and subscription="both" on behalf of the user's
   available resource:

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

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

   Example 4: Contacts' servers reply to presence probe on behalf of all
   available resources:

   <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>

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

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

   Example 5: Contacts' servers deliver user's initial presence to all
   available resources or return error to user:

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







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

   <presence
       type='error'
       from='mercutio@example.org'
       to='romeo@example.net/orchard'>
     <error type='cancel'>
       <gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </presence>

   Example 6: User sends directed presence to another user not in his
   roster:

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

   Example 7: User sends updated available presence information for
   broadcasting:

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

   Example 8: User's server broadcasts updated presence information only
   to one contact (not those from whom an error was received or to whom
   the user sent directed presence):

   <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>





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   Example 9: Contact's server delivers updated presence information to
   all of the contact's available resources:

   [to "balcony" resource...]
   <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>

   [to "chamber" resource...]
   <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 10: One of the contact's resources broadcasts final presence:

   <presence from='juliet@example.com/balcony' type='unavailable'/>

   Example 11: Contact's server sends unavailable presence information
   to user:

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

   Example 12: User sends final presence:

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









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   Example 13: User's server broadcasts unavailable presence information
   to contact as well as to the person to whom the user sent directed
   presence:

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

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

6.  Managing Subscriptions

   In order to protect the privacy of instant messaging users and any
   other entities, presence and availability 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 information.  A
   subscription lasts across sessions; indeed, it lasts until the
   subscriber unsubscribes or the subscribee cancels the
   previously-granted subscription.  Subscriptions are managed within
   XMPP by sending presence stanzas containing specially-defined
   attributes.

   Note: There are important interactions between subscriptions and
   rosters; these are defined under Integration of Roster Items and
   Presence Subscriptions (Section 8), and the reader must refer to that
   section for a complete understanding of presence subscriptions.

6.1.  Requesting a Subscription

   A request to subscribe to another entity's presence is made by
   sending a presence stanza of type "subscribe".

   Example: Sending a subscription request:

   <presence to='juliet@example.com' type='subscribe'/>






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   For client and server responsibilities regarding presence
   subscription requests, refer to Presence Subscriptions (Section
   5.1.6).

6.2.  Handling a Subscription Request

   When a client receives a subscription request from another entity, it
   MUST either approve the request by sending a presence stanza of type
   "subscribed" or refuse the request by sending a presence stanza of
   type "unsubscribed".

   Example: Approving a subscription request:

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

   Example: Refusing a presence subscription request:

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

6.3.  Cancelling a Subscription from Another Entity

   If a user would like to cancel a previously-granted subscription
   request, it sends a presence stanza of type "unsubscribed".

   Example: Cancelling a previously granted subscription request:

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

6.4.  Unsubscribing from Another Entity's Presence

   If a user would like to unsubscribe from the presence of another
   entity, it sends a presence stanza of type "unsubscribe".

   Example: Unsubscribing from an entity's presence:

   <presence to='juliet@example.com' type='unsubscribe'/>

7.  Roster Management

   In XMPP, one's contact list is called a roster, which consists of any
   number of specific roster items, each roster item being identified by
   a unique JID (usually of the form <contact@domain>).  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.







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   Note: There are important interactions between rosters and
   subscriptions; these are defined under Integration of Roster Items
   and Presence Subscriptions (Section 8), and the reader must refer to
   that section for a complete understanding of roster management.

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

   The "key" or unique identifier for each roster item is a JID,
   encapsulated in the 'jid' attribute of the <item/> element (which is
   REQUIRED).  The value of the 'jid' attribute SHOULD be of the form
   <user@domain> if the item is associated with another (human) instant
   messaging user.

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

   o  "none" -- the user does not have a subscription to the contact's
      presence information, and the contact does not have a subscription
      to the user's presence information

   o  "to" -- the user has a subscription to the contact's presence
      information, but the contact does not have a subscription to the
      user's presence information

   o  "from" -- the contact has a subscription to the user's presence
      information, but the user does not have a subscription to the
      contact's presence information

   o  "both" -- both the user and the contact have subscriptions to each
      other's presence information

   Each <item/> element MAY contain a 'name' attribute, which sets the
   "nickname" to be associated with the JID, as determined by the user
   (not the contact).  The value of the 'name' attribute is opaque.

   Each <item/> element MAY contain one or more <group/> child elements,
   for use in collecting roster items into various categories.  The XML
   character data of the <group/> element is opaque.







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7.2.  Business Rules

   A server MUST ignore any 'to' address on a roster "set", and MUST
   treat any roster "set" as applying to the sender.  For added safety,
   a client SHOULD check the "from" address of a "roster push" (incoming
   IQ of type "set" containing a roster item) to ensure that it is from
   a trusted source; specifically, the stanza MUST either have no 'from'
   attribute (i.e., implicitly from the server) or have a 'from'
   attribute whose value matches the user's bare JID (of the form
   <user@domain>) or full JID (of the form <user@domain/resource>);
   otherwise, the client SHOULD ignore the "roster push".

7.3.  Retrieving One's Roster on Login

   Upon connecting to the server and becoming an active 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 OPTIONAL).  If an available resource does
   not request the roster during a session, the server MUST NOT send it
   presence subscriptions and associated roster updates.

   Example: Client requests current roster from server:

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

   Example: Client receives roster from server:

   <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'>
         <group>Friends</group>
       </item>
       <item jid='benvolio@example.org'
             name='Benvolio'
             subscription='both'>
         <group>Friends</group>
       </item>
     </query>



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   </iq>

7.4.  Adding a Roster Item

   At any time, a user MAY add an item to his or her roster.

   Example: Client adds a new item:

   <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>

   The server MUST update the roster information in persistent storage,
   and also push the change out to all of the user's available resources
   that have requested the roster.  This "roster push" consists of an IQ
   stanza of type "set" from the server to the client and enables all
   available resources to remain in sync with the server-based roster
   information.

   Example: Server (1) pushes the updated roster information to all
   available resources that have requested the roster and (2) replies
   with an IQ result to the sending resource:

   <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>

   <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>



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

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

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

   Example: Resources reply with an IQ result to the server:

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

7.5.  Updating a Roster Item

   Updating an existing roster item (e.g., changing the group) is done
   in the same way as adding a new roster item, i.e., by sending the
   roster item in an IQ set to the server.

   Example: User updates roster item (added group):

   <iq from='juliet@example.com/chamber' type='set' id='roster_3'>
     <query xmlns='jabber:iq:roster'>
       <item jid='romeo@example.net'
             name='Romeo'
             subscription='both'>
         <group>Friends</group>
         <group>Lovers</group>
       </item>
     </query>
   </iq>

   As with adding a roster item, when updating a roster item the server
   MUST update the roster information in persistent storage, and also
   initiate a roster push to all of the user's available resources that
   have requested the roster.







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7.6.  Deleting a Roster Item

   At any time, a user MAY delete an item from his or her roster by
   sending an IQ set to the server and making sure that the value of the
   'subscription' attribute is "remove" (a compliant server MUST ignore
   any other values of the 'subscription' attribute when received from a
   client).

   Example: Client removes an item:

   <iq from='juliet@example.com/balcony' type='set' id='roster_4'>
     <query xmlns='jabber:iq:roster'>
       <item jid='nurse@example.com' subscription='remove'/>
     </query>
   </iq>

   As with adding a roster item, when deleting a roster item the server
   MUST update the roster information in persistent storage, initiate a
   roster push to all of the user's available resources that have
   requested the roster (with the 'subscription' attribute set to a
   value of "remove"), and send an IQ result to the initiating resource.

   For further information about the implications of this command, see
   Removing a Roster Item and Cancelling All Subscriptions (Section
   8.6).

8.  Integration of Roster Items and Presence Subscriptions

8.1.  Overview

   Some level of integration between roster items and presence
   subscriptions is normally expected by an instant messaging user
   regarding the user's subscriptions to and from other contacts.  This
   section describes the level of integration that MUST be supported
   within XMPP instant messaging applications.

   There are four primary subscription states:

   o  None -- the user does not have a subscription to the contact's
      presence information, and the contact does not have a subscription
      to the user's presence information










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   o  To -- the user has a subscription to the contact's presence
      information, but the contact does not have a subscription to the
      user's presence information

   o  From -- the contact has a subscription to the user's presence
      information, but the user does not have a subscription to the
      contact's presence information

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

   Each of these states is reflected in the roster of both the user and
   the contact, thus resulting in durable subscription states.

   Narrative explanations of how these subscription states interact with
   roster items in order to complete certain defined use cases are
   provided in the following sub-sections.  Full details regarding
   server and client handling of all subscription states (including
   pending states between the primary states listed above) is provided
   in Subscription States (Section 9).

   The server MUST NOT send presence subscription requests or roster
   pushes to unavailable resources, nor to available resources that have
   not requested the roster.

   The 'from' and 'to' addresses are OPTIONAL in roster pushes; if
   included, their values SHOULD be the full JID of the resource for
   that session.  A client MUST acknowledge each roster push with an IQ
   stanza of type "result" (for the sake of brevity, these stanzas are
   not shown in the following examples but are required by the IQ
   semantics defined in [XMPP-CORE]).

8.2.  User Subscribes to Contact

   The process by which a user subscribes to a contact, including the
   interaction between roster items and subscription states, is
   described below.

   1.  In preparation for being able to render the contact in the user's
       client interface and for the server to keep track of the
       subscription, the user's client SHOULD perform a "roster set" for
       the new roster item.  This request consists of sending an IQ
       stanza of type='set' containing a <query/> element qualified by
       the 'jabber:iq:roster' namespace, which in turn contains an
       <item/> element that defines the new roster item; the <item/>
       element MUST possess a 'jid' attribute, MAY possess a 'name'
       attribute, MUST NOT possess a 'subscription' attribute, and MAY
       contain one or more <group/> child elements:



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   <iq type='set' id='set1'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   2.  As a result, the user's server (1) MUST initiate a roster push
       for the new roster item to all available resources associated
       with this user that have requested the roster, setting the
       'subscription' attribute to a value of "none"; and (2) MUST reply
       to the sending resource with an IQ result indicating the success
       of the roster set:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='none'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <iq type='result' id='set1'/>

   3.  If the user wants to request a subscription to the contact's
       presence information, the user's client MUST send a presence
       stanza of type='subscribe' to the contact:

   <presence to='contact@example.org' type='subscribe'/>

   4.  As a result, the user's server MUST initiate a second roster push
       to all of the user's available resources that have requested the
       roster, setting the contact to the pending sub-state of the
       'none' subscription state; this pending sub-state is denoted by
       the inclusion of the ask='subscribe' attribute in the roster
       item:









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   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='none'
           ask='subscribe'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   Note: If the user did not create a roster item before sending the
   subscription request, the server MUST now create one on behalf of the
   user, then send a roster push to all of the user's available
   resources that have requested the roster, absent the 'name' attribute
   and the <group/> child shown above.

   5.  The user's server MUST also stamp the presence stanza of type
       "subscribe" with the user's bare JID (i.e., <user@example.com>)
       as the 'from' address (if the user provided a 'from' address set
       to the user's full JID, the server SHOULD remove the resource
       identifier).  If the contact is served by a different host than
       the user, the user's server MUST route the presence stanza to the
       contact's server for delivery to the contact (this case is
       assumed throughout; however, if the contact is served by the same
       host, then the server can simply deliver the presence stanza
       directly):

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='subscribe'/>

   Note: If the user's server receives a presence stanza of type "error"
   from the contact's server, it MUST deliver the error stanza to the
   user, whose client MAY determine that the error is in response to the
   outgoing presence stanza of type "subscribe" it sent previously
   (e.g., by tracking an 'id' attribute) and then choose to resend the
   "subscribe" request or revert the roster to its previous state by
   sending a presence stanza of type "unsubscribe" to the contact.

   6.  Upon receiving the presence stanza of type "subscribe" addressed
       to the contact, the contact's server MUST determine if there is
       at least one available resource from which the contact has
       requested the roster.  If so, it MUST deliver the subscription
       request to the contact (if not, the contact's server MUST store
       the subscription request offline for delivery when this condition



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       is next met; normally this is done by adding a roster item for
       the contact to the user's roster, with a state of "None + Pending
       In" as defined under Subscription States (Section 9), however a
       server SHOULD NOT push or deliver roster items in that state to
       the contact).  No matter when the subscription request is
       delivered, the contact must decide whether or not to approve it
       (subject to the contact's configured preferences, the contact's
       client MAY approve or refuse the subscription request without
       presenting it to the contact).  Here we assume the "happy path"
       that the contact approves the subscription request (the alternate
       flow of declining the subscription request is defined in Section
       8.2.1).  In this case, the contact's client (1) SHOULD perform a
       roster set specifying the desired nickname and group for the user
       (if any); and (2) MUST send a presence stanza of type
       "subscribed" to the user in order to approve the subscription
       request.

   <iq type='set' id='set2'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence to='user@example.com' type='subscribed'/>

   7.  As a result, the contact's server (1) MUST initiate a roster push
       to all available resources associated with the contact that have
       requested the roster, containing a roster item for the user with
       the subscription state set to 'from' (the server MUST send this
       even if the contact did not perform a roster set); (2) MUST
       return an IQ result to the sending resource indicating the
       success of the roster set; (3) MUST route the presence stanza of
       type "subscribed" to the user, first stamping the 'from' address
       as the bare JID (<contact@example.org>) of the contact; and (4)
       MUST send available presence from all of the contact's available
       resources to the user:











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   <iq type='set' to='contact@example.org/resource'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='from'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <iq type='result' to='contact@example.org/resource' id='set2'/>

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='subscribed'/>

   <presence
       from='contact@example.org/resource'
       to='user@example.com'/>

   Note: If the contact's server receives a presence stanza of type
   "error" from the user's server, it MUST deliver the error stanza to
   the contact, whose client MAY determine that the error is in response
   to the outgoing presence stanza of type "subscribed" it sent
   previously (e.g., by tracking an 'id' attribute) and then choose to
   resend the "subscribed" notification or revert the roster to its
   previous state by sending a presence stanza of type "unsubscribed" to
   the user.

   8.  Upon receiving the presence stanza of type "subscribed" addressed
       to the user, the user's server MUST first verify that the contact
       is in the user's roster with either of the following states: (a)
       subscription='none' and ask='subscribe' or (b)
       subscription='from' and ask='subscribe'.  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 MUST NOT route it to the user, modify the
       user's roster, or generate a roster push to the user's available
       resources).  If the contact is in the user's roster with either
       of those states, the user's server (1) MUST deliver the presence
       stanza of type "subscribed" from the contact to the user; (2)
       MUST initiate a roster push to all of the user's available
       resources that have requested the roster, containing an updated
       roster item for the contact with the 'subscription' attribute set





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       to a value of "to"; and (3) MUST deliver the available presence
       stanza received from each of the contact's available resources to
       each of the user's available resources:

   <presence
       to='user@example.com'
       from='contact@example.org'
       type='subscribed'/>

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='to'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <presence
       from='contact@example.org/resource'
       to='user@example.com/resource'/>

   9.  Upon receiving the presence stanza of type "subscribed", the user
       SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "subscribe" to the contact or "denying" it by
       sending a presence stanza of type "unsubscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).

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

8.2.1.  Alternate Flow: Contact Declines Subscription Request

   The above activity flow represents the "happy path" regarding the
   user's subscription request to the contact.  The main alternate flow
   occurs if the contact refuses the user's subscription request, as
   described below.







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   1.  If the contact wants to refuse the request, the contact's client
       MUST send a presence stanza of type "unsubscribed" to the user
       (instead of the presence stanza of type "subscribed" sent in Step
       6 of Section 8.2):

   <presence to='user@example.com' type='unsubscribed'/>

   2.  As a result, the contact's server MUST route the presence stanza
       of type "unsubscribed" to the user, first stamping the 'from'
       address as the bare JID (<contact@example.org>) of the contact:

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unsubscribed'/>

   Note: If the contact's server previously added the user to the
   contact's roster for tracking purposes, it MUST remove the relevant
   item at this time.

   3.  Upon receiving the presence stanza of type "unsubscribed"
       addressed to the user, the user's server (1) MUST deliver that
       presence stanza to the user and (2) MUST initiate a roster push
       to all of the user's available resources that have requested the
       roster, containing an updated roster item for the contact with
       the 'subscription' attribute set to a value of "none" and with no
       'ask' attribute:

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unsubscribed'/>

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='none'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   4.  Upon receiving the presence stanza of type "unsubscribed", the
       user SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the contact or "denying" it by



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       sending a presence stanza of type "subscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).

   As a result of this activity, the contact is now in the user's roster
   with a subscription state of "none", whereas the user is not in the
   contact's roster at all.

8.3.  Creating a Mutual Subscription

   The user and contact can build on the "happy path" described above to
   create a mutual subscription (i.e., a subscription of type "both").
   The process is described below.

   1.  If the contact wants to create a mutual subscription, the contact
       MUST send a subscription request to the user (subject to the
       contact's configured preferences, the contact's client MAY send
       this automatically):

   <presence to='user@example.com' type='subscribe'/>

   2.  As a result, the contact's server (1) MUST initiate a roster push
       to all available resources associated with the contact that have
       requested the roster, with the user still in the 'from'
       subscription state but with a pending 'to' subscription denoted
       by the inclusion of the ask='subscribe' attribute in the roster
       item; and (2) MUST route the presence stanza of type "subscribe"
       to the user, first stamping the 'from' address as the bare JID
       (<contact@example.org>) of the contact:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='from'
           ask='subscribe'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>








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   <presence
       from='contact@example.org'
       to='user@example.com'
       type='subscribe'/>

   Note: If the contact's server receives a presence stanza of type
   "error" from the user's server, it MUST deliver the error stanza to
   the contact, whose client MAY determine that the error is in response
   to the outgoing presence stanza of type "subscribe" it sent
   previously (e.g., by tracking an 'id' attribute) and then choose to
   resend the "subscribe" request or revert the roster to its previous
   state by sending a presence stanza of type "unsubscribe" to the user.

   3.  Upon receiving the presence stanza of type "subscribe" addressed
       to the user, the user's server must determine if there is at
       least one available resource for which the user has requested the
       roster.  If so, the user's server MUST deliver the subscription
       request to the user (if not, it MUST store the subscription
       request offline for delivery when this condition is next met). No
       matter when the subscription request is delivered, the user must
       then decide whether or not to approve it (subject to the user's
       configured preferences, the user's client MAY approve or refuse
       the subscription request without presenting it to the user).
       Here we assume the "happy path" that the user approves the
       subscription request (the alternate flow of declining the
       subscription request is defined in Section 8.3.1).  In this case,
       the user's client MUST send a presence stanza of type
       "subscribed" to the contact in order to approve the subscription
       request.

   <presence to='contact@example.org' type='subscribed'/>

   4.  As a result, the user's server (1) MUST initiate a roster push to
       all of the user's available resources that have requested the
       roster, containing a roster item for the contact with the
       'subscription' attribute set to a value of "both"; (2) MUST route
       the presence stanza of type "subscribed" to the contact, first
       stamping the 'from' address as the bare JID (<user@example.com>)
       of the user; and (3) MUST send to the contact the full XML of the
       last presence stanza with no 'to' attribute received by the
       server from each of the user's available resources (subject to
       privacy lists in force for each session):









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   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='both'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='subscribed'/>

   <presence
       from='user@example.com/resource'
       to='contact@example.org'/>

   Note: If the user's server receives a presence stanza of type "error"
   from the contact's server, it MUST deliver the error stanza to the
   user, whose client MAY determine that the error is in response to the
   outgoing presence stanza of type "subscribed" it sent previously
   (e.g., by tracking an 'id' attribute) and then choose to resend the
   subscription request or revert the roster to its previous state by
   sending a presence stanza of type "unsubscribed" to the contact.

   5.  Upon receiving the presence stanza of type "subscribed" addressed
       to the contact, the contact's server MUST first verify that the
       user is in the contact's roster with either of the following
       states: (a) subscription='none' and ask='subscribe' or (b)
       subscription='from' and ask='subscribe'.  If the user is not in
       the contact's roster with either of those states, the contact's
       server MUST silently ignore the presence stanza of type
       "subscribed" (i.e., it MUST NOT route it to the contact, modify
       the contact's roster, or generate a roster push to the contact's
       available resources).  If the user is in the contact's roster
       with either of those states, the contact's server (1) MUST
       deliver the presence stanza of type "subscribed" from the user to
       the contact; (2) MUST initiate a roster push to all available
       resources associated with the contact that have requested the
       roster, containing an updated roster item for the user with the
       'subscription' attribute set to a value of "both"; and (3) MUST
       deliver the available presence stanza received from each of the
       user's available resources to each of the contact's available
       resources:




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   <presence
       from='user@example.com'
       to='contact@example.org'
       type='subscribed'/>

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='both'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com/resource'
       to='contact@example.org/resource'/>

   6.  Upon receiving the presence stanza of type "subscribed", the
       contact SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "subscribe" to the user or "denying" it by sending
       a presence stanza of type "unsubscribe" to the user; this step
       does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the contact's server know that it MUST no longer send
       notification of the subscription state change to the contact (see
       Section 9.4).

   The user and the contact now have a mutual subscription to each
   other's presence -- i.e., the subscription is of type "both".

8.3.1.  Alternate Flow: User Declines Subscription Request

   The above activity flow represents the "happy path" regarding the
   contact's subscription request to the user.  The main alternate flow
   occurs if the user refuses the contact's subscription request, as
   described below.

   1.  If the user wants to refuse the request, the user's client MUST
       send a presence stanza of type "unsubscribed" to the contact
       (instead of the presence stanza of type "subscribed" sent in Step
       3 of Section 8.3):

   <presence to='contact@example.org' type='unsubscribed'/>




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   2.  As a result, the user's server MUST route the presence stanza of
       type "unsubscribed" to the contact, first stamping the 'from'
       address as the bare JID (<user@example.com>) of the user:

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribed'/>

   3.  Upon receiving the presence stanza of type "unsubscribed"
       addressed to the contact, the contact's server (1) MUST deliver
       that presence stanza to the contact; and (2) MUST initiate a
       roster push to all available resources associated with the
       contact that have requested the roster, containing an updated
       roster item for the user with the 'subscription' attribute set to
       a value of "from" and with no 'ask' attribute:

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribed'/>

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='from'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   4.  Upon receiving the presence stanza of type "unsubscribed", the
       contact SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the user or "denying" it by
       sending a presence stanza of type "subscribe" to the user; this
       step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the contact's server know that it MUST no longer send
       notification of the subscription state change to the contact (see
       Section 9.4).

   As a result of this activity, there has been no change in the
   subscription state; i.e., the contact is in the user's roster with a
   subscription state of "to" and the user is in the contact's roster
   with a subscription state of "from".



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8.4.  Unsubscribing

   At any time after subscribing to a contact's presence information, a
   user MAY unsubscribe.  While the XML that the user sends to make this
   happen is the same in all instances, the subsequent subscription
   state is different depending on the subscription state obtaining when
   the unsubscribe "command" is sent.  Both possible scenarios are
   described below.

8.4.1.  Case #1: Unsubscribing When Subscription is Not Mutual

   In the first case, the user has a subscription to the contact's
   presence information but the contact does not have a subscription to
   the user's presence information (i.e., the subscription is not yet
   mutual).

   1.  If the user wants to unsubscribe from the contact's presence
       information, the user MUST send a presence stanza of type
       "unsubscribe" to the contact:

   <presence to='contact@example.org' type='unsubscribe'/>

   2.  As a result, the user's server (1) MUST send a roster push to all
       of the user's available resources that have requested the roster,
       containing an updated roster item for the contact with the
       'subscription' attribute set to a value of "none"; and (2) MUST
       route the presence stanza of type "unsubscribe" to the contact,
       first stamping the 'from' address as the bare JID
       (<user@example.com>) of the user:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='none'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>






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   3.  Upon receiving the presence stanza of type "unsubscribe"
       addressed to the contact, the contact's server (1) MUST initiate
       a roster push to all available resources associated with the
       contact that have requested the roster, containing an updated
       roster item for the user with the 'subscription' attribute set to
       a value of "none" (if the contact is unavailable or has not
       requested the roster, the contact's server MUST modify the roster
       item and send that modified item the next time the contact
       requests the roster); and (2) MUST deliver the "unsubscribe"
       state change notification to the contact:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='none'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>

   4.  Upon receiving the presence stanza of type "unsubscribe", the
       contact SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribed" to the user or "denying" it by
       sending a presence stanza of type "subscribed" to the user; this
       step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the contact's server know that it MUST no longer send
       notification of the subscription state change to the contact (see
       Section 9.4).

   5.  The contact's server then (1) MUST send a presence stanza of type
       "unsubscribed" to the user; and (2) SHOULD send unavailable
       presence from all of the contact's available resources to the
       user:

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unsubscribed'/>




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   <presence
       from='contact@example.org/resource'
       to='user@example.com'
       type='unavailable'/>

   6.  When the user's server receives the presence stanzas of type
       "unsubscribed" and "unavailable", it MUST deliver them to the
       user:

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unsubscribed'/>

   <presence
       from='contact@example.org/resource'
       to='user@example.com'
       type='unavailable'/>

   7.  Upon receiving the presence stanza of type "unsubscribed", the
       user SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the contact or "denying" it by
       sending a presence stanza of type "subscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).

8.4.2.  Case #2: Unsubscribing When Subscription is Mutual

   In the second case, the user has a subscription to the contact's
   presence information and the contact also has a subscription to the
   user's presence information (i.e., the subscription is mutual).

   1.  If the user wants to unsubscribe from the contact's presence
       information, the user MUST send a presence stanza of type
       "unsubscribe" to the contact:

   <presence to='contact@example.org' type='unsubscribe'/>

   2.  As a result, the user's server (1) MUST send a roster push to all
       of the user's available resources that have requested the roster,
       containing an updated roster item for the contact with the
       'subscription' attribute set to a value of "from"; and (2) MUST
       route the presence stanza of type "unsubscribe" to the contact,
       first stamping the 'from' address as the bare JID
       (<user@example.com>) of the user:



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   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='from'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>

   3.  Upon receiving the presence stanza of type "unsubscribe"
       addressed to the contact, the contact's server (1) MUST initiate
       a roster push to all available resources associated with the
       contact that have requested the roster, containing an updated
       roster item for the user with the 'subscription' attribute set to
       a value of "to" (if the contact is unavailable or has not
       requested the roster, the contact's server MUST modify the roster
       item and send that modified item the next time the contact
       requests the roster); and (2) MUST deliver the "unsubscribe"
       state change notification to the contact:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='to'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>

   4.  Upon receiving the presence stanza of type "unsubscribe", the
       contact SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribed" to the user or "denying" it by
       sending a presence stanza of type "subscribed" to the user; this



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       step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the contact's server know that it MUST no longer send
       notification of the subscription state change to the contact (see
       Section 9.4).

   5.  The contact's server then (1) MUST send a presence stanza of type
       "unsubscribed" to the user; and (2) SHOULD send unavailable
       presence from all of the contact's available resources to the
       user:

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unsubscribed'/>

   <presence
       from='contact@example.org/resource'
       to='user@example.com'
       type='unavailable'/>

   6.  When the user's server receives the presence stanzas of type
       "unsubscribed" and "unavailable", it MUST deliver them to the
       user:

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unsubscribed'/>

   <presence
       from='contact@example.org/resource'
       to='user@example.com'
       type='unavailable'/>

   7.  Upon receiving the presence stanza of type "unsubscribed", the
       user SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the contact or "denying" it by
       sending a presence stanza of type "subscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).

   Note: Obviously this does not result in removal of the roster item
   from the user's roster, and the contact still has a subscription to
   the user's presence information.  In order to both completely cancel



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   a mutual subscription and fully remove the roster item from the
   user's roster, the user SHOULD update the roster item with
   subscription='remove' as defined under Removing a Roster Item and
   Cancelling All Subscriptions (Section 8.6).

8.5.  Cancelling a Subscription

   At any time after approving a subscription request from a user, a
   contact MAY cancel that subscription.  While the XML that the contact
   sends to make this happen is the same in all instances, the
   subsequent subscription state is different depending on the
   subscription state obtaining when the cancellation was sent.  Both
   possible scenarios are described below.

8.5.1.  Case #1: Cancelling When Subscription is Not Mutual

   In the first case, the user has a subscription to the contact's
   presence information but the contact does not have a subscription to
   the user's presence information (i.e., the subscription is not yet
   mutual).

   1.  If the contact wants to cancel the user's subscription, the
       contact MUST send a presence stanza of type "unsubscribed" to the
       user:

   <presence to='user@example.com' type='unsubscribed'/>

   2.  As a result, the contact's server (1) MUST send a roster push to
       all of the contact's available resources that have requested the
       roster, containing an updated roster item for the user with the
       'subscription' attribute set to a value of "none"; (2) MUST route
       the presence stanza of type "unsubscribed" to the user, first
       stamping the 'from' address as the bare JID
       (<contact@example.org>) of the contact; and (3) SHOULD send
       unavailable presence from all of the contact's available
       resources to the user:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='none'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>




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   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unsubscribed'/>

   <presence
       from='contact@example.org/resource'
       to='user@example.com'
       type='unavailable'/>

   3.  Upon receiving the presence stanza of type "unsubscribed"
       addressed to the user, the user's server (1) MUST initiate a
       roster push to all of the user's available resources that have
       requested the roster, containing an updated roster item for the
       contact with the 'subscription' attribute set to a value of
       "none" (if the user is unavailable or has not requested the
       roster, the user's server MUST modify the roster item and send
       that modified item the next time the user requests the roster);
       (2) MUST deliver the "unsubscribed" state change notification to
       all of the user's available resources; and (3) MUST deliver the
       unavailable presence to all of the user's available resources:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='none'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unsubscribed'/>

   <presence
       from='contact@example.org/resource'
       to='user@example.com'
       type='unavailable'/>

   4.  Upon receiving the presence stanza of type "unsubscribed", the
       user SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the contact or "denying" it by
       sending a presence stanza of type "subscribe" to the contact;



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       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).

8.5.2.  Case #2: Cancelling When Subscription is Mutual

   In the second case, the user has a subscription to the contact's
   presence information and the contact also has a subscription to the
   user's presence information (i.e., the subscription is mutual).

   1.  If the contact wants to cancel the user's subscription, the
       contact MUST send a presence stanza of type "unsubscribed" to the
       user:

   <presence to='user@example.com' type='unsubscribed'/>

   2.  As a result, the contact's server (1) MUST send a roster push to
       all of the contact's available resources that have requested the
       roster, containing an updated roster item for the user with the
       'subscription' attribute set to a value of "to"; (2) MUST route
       the presence stanza of type "unsubscribed" to the user, first
       stamping the 'from' address as the bare JID
       (<contact@example.org>) of the contact; and (3) SHOULD send
       unavailable presence from all of the contact's available
       resources to all of the user's available resources:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='to'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unsubscribed'/>

   <presence
       from='contact@example.org/resource'
       to='user@example.com'
       type='unavailable'/>




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   3.  Upon receiving the presence stanza of type "unsubscribed"
       addressed to the user, the user's server (1) MUST initiate a
       roster push to all of the user's available resources that have
       requested the roster, containing an updated roster item for the
       contact with the 'subscription' attribute set to a value of
       "from" (if the user is unavailable or has not requested the
       roster, the user's server MUST modify the roster item and send
       that modified item the next time the user requests the roster);
       and (2) MUST deliver the "unsubscribed" state change notification
       to all of the user's available resources; and (3) MUST deliver
       the unavailable presence to all of the user's available
       resources:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='from'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unsubscribed'/>

   <presence
       from='contact@example.org/resource'
       to='user@example.com'
       type='unavailable'/>

   4.  Upon receiving the presence stanza of type "unsubscribed", the
       user SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the contact or "denying" it by
       sending a presence stanza of type "subscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).

   Note: Obviously this does not result in removal of the roster item
   from the contact's roster, and the contact still has a subscription
   to the user's presence information.  In order to both completely
   cancel a mutual subscription and fully remove the roster item from



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   the contact's roster, the contact should update the roster item with
   subscription='remove' as defined under Removing a Roster Item and
   Cancelling All Subscriptions (Section 8.6).

8.6.  Removing a Roster Item and Cancelling All Subscriptions

   Because there may be many steps involved in completely removing a
   roster item and cancelling subscriptions in both directions, the
   roster management protocol includes a "shortcut" method for doing so.
   The process may be initiated no matter what the current subscription
   state is by sending a roster set containing an item for the contact
   with the 'subscription' attribute set to a value of "remove":

   <iq type='set' id='remove1'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='remove'/>
     </query>
   </iq>

   When the user removes a contact from his or her roster by setting the
   'subscription' attribute to a value of "remove", the user's server
   (1) MUST automatically cancel any existing presence subscription
   between the user and the contact (both 'to' and 'from' as
   appropriate); (2) MUST remove the roster item from the user's roster
   and inform all of the user's available resources that have requested
   the roster of the roster item removal; (3) MUST inform the resource
   that initiated the removal of success; and (4) SHOULD send
   unavailable presence from all of the user's available resources to
   the contact:

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribed'/>










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   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='remove'/>
     </query>
   </iq>

   <iq type='result' id='remove1'/>

   <presence
       from='user@example.com/resource'
       to='contact@example.org'
       type='unavailable'/>

   Upon receiving the presence stanza of type "unsubscribe", the
   contact's server (1) MUST initiate a roster push to all available
   resources associated with the contact that have requested the roster,
   containing an updated roster item for the user with the
   'subscription' attribute set to a value of "to" (if the contact is
   unavailable or has not requested the roster, the contact's server
   MUST modify the roster item and send that modified item the next time
   the contact requests the roster); and (2) MUST also deliver the
   "unsubscribe" state change notification to all of the contact's
   available resources:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='to'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>

   Upon receiving the presence stanza of type "unsubscribed", the
   contact's server (1) MUST initiate a roster push to all available
   resources associated with the contact that have requested the roster,
   containing an updated roster item for the user with the
   'subscription' attribute set to a value of "none" (if the contact is
   unavailable or has not requested the roster, the contact's server



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   MUST modify the roster item and send that modified item the next time
   the contact requests the roster); and (2) MUST also deliver the
   "unsubscribe" state change notification to all of the contact's
   available resources:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='none'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribed'/>

   Upon receiving the presence stanza of type "unavailable" addressed to
   the contact, the contact's server MUST deliver the unavailable
   presence to all of the user's available resources:

   <presence
       from='user@example.com/resource'
       to='contact@example.org'
       type='unavailable'/>

   Note: When the user removes the contact from the user's roster, the
   end state of the contact's roster is that the user is still in the
   contact's roster with a subscription state of "none"; in order to
   completely remove the roster item for the user, the contact needs to
   also send a roster removal request.

9.  Subscription States

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

9.1.  Defined States

   There are nine possible subscription states, which are described here
   from the user's (not contact's) perspective:




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   1.  "None" = contact and user are not subscribed to each other, and
       neither has requested a subscription from the other

   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

   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
       contact has approved subscription request from user)

   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

   5.  "To" = user is subscribed to contact (one-way)

   6.  "To + Pending In" = user is subscribed to contact, and contact
       has sent user a subscription request but user has not replied yet

   7.  "From" = contact is subscribed to user (one-way)

   8.  "From + Pending Out" = contact is subscribed to user, and user
       has sent contact a subscription request but contact has not
       replied yet

   9.  "Both" = user and contact are subscribed to each other (two-way)

9.2.  Server Handling of Outbound Presence Subscription Stanzas

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

   Because it is possible for the user's server and the contact's server
   to lose synchronization regarding subscription states, the user's
   server MUST without exception route all outbound presence stanzas of
   type "subscribe" or "unsubscribe" to the contact so that the user is
   able to resynchronize his or her subscription to the contact's
   presence information if needed.






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   The user's server SHOULD NOT route a presence stanza of type
   "subscribed" or "unsubscribed" to the contact if the stanza does not
   result in a subscription state change from the user's perspective,
   and MUST NOT make a state change.  If the stanza results in a
   subscription state change, the user's server MUST route the stanza to
   the contact and MUST make the appropriate state change.  These rules
   are summarized in the following tables.

   Table 1: Recommended handling of outbound "subscribed" stanzas

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

   Table 2: Recommended handling of outbound "unsubscribed" stanzas

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

9.3.  Server Handling 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 contact to manage the user's
   access to the contact's presence information (via the "subscribed"
   and "unsubscribed" types).



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   When the user's server receives a subscription request for the user
   from the contact (i.e., a presence stanza of type "subscribe"), it
   MUST deliver that request to the user for approval if the user has
   not already granted the contact access to the user's presence
   information and if there is no pending inbound subscription request;
   however, the user's server SHOULD NOT deliver the new request if
   there is a pending inbound subscription request, since the previous
   subscription request will have been recorded.  If the user has
   already granted the contact access to the user's presence
   information, the user's server SHOULD auto-reply to an inbound
   presence stanza of type "subscribe" from the contact by sending a
   presence stanza of type "subscribed" to the contact on behalf of the
   user; this rule enables the contact to resynchronize the subscription
   state if needed.  These rules are summarized in the following table.

   Table 3: Recommended handling of inbound "subscribe" stanzas

   +------------------------------------------------------------------+
   |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
   +------------------------------------------------------------------+
   |  "None"                  |  yes       |  "None + Pending In"     |
   |  "None + Pending Out"    |  yes       |  "None + Pending Out/In" |
   |  "None + Pending In"     |  no        |  no state change         |
   |  "None + Pending Out/In" |  no        |  no state change         |
   |  "To"                    |  yes       |  "To + Pending In"       |
   |  "To + Pending In"       |  no        |  no state change         |
   |  "From"                  |  no *      |  no state change         |
   |  "From + Pending Out"    |  no *      |  no state change         |
   |  "Both"                  |  no *      |  no state change         |
   +------------------------------------------------------------------+

   * Server SHOULD auto-reply with "subscribed" stanza

   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 SHOULD auto-reply by sending a presence stanza of type
   "unsubscribed" to the contact on behalf of the user, MUST deliver the
   "unsubscribe" stanza to the user, and MUST change the state.  If no
   subscription state change results, the user's server SHOULD NOT
   deliver the stanza and MUST NOT change the state.  These rules are
   summarized in the following table.









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   Table 4: Recommended handling of inbound "unsubscribe" stanzas

   +------------------------------------------------------------------+
   |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
   +------------------------------------------------------------------+
   |  "None"                  |  no        |  no state change         |
   |  "None + Pending Out"    |  no        |  no state change         |
   |  "None + Pending In"     |  yes *     |  "None"                  |
   |  "None + Pending Out/In" |  yes *     |  "None + Pending Out"    |
   |  "To"                    |  no        |  no state change         |
   |  "To + Pending In"       |  yes *     |  "To"                    |
   |  "From"                  |  yes *     |  "None"                  |
   |  "From + Pending Out"    |  yes *     |  "None + Pending Out     |
   |  "Both"                  |  yes *     |  "To"                    |
   +------------------------------------------------------------------+

   * Server SHOULD auto-reply with "unsubscribed" stanza

   When the user's server receives a presence stanza of type
   "subscribed" for the user from the contact, it MUST NOT deliver the
   stanza to the user and MUST NOT change the subscription state if
   there is no pending outbound request for access to the contact's
   presence information.  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,
   the user's server MUST deliver the stanza to the user and MUST change
   the subscription state.  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 SHOULD NOT deliver the stanza to the user
   and MUST NOT change the subscription state.  These rules are
   summarized in the following table.

   Table 5: Recommended handling of inbound "subscribed" stanzas

   +------------------------------------------------------------------+
   |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
   +------------------------------------------------------------------+
   |  "None"                  |  no        |  no state change         |
   |  "None + Pending Out"    |  yes       |  "To"                    |
   |  "None + Pending In"     |  no        |  no state change         |
   |  "None + Pending Out/In" |  yes       |  "To + Pending In"       |
   |  "To"                    |  no        |  no state change         |
   |  "To + Pending In"       |  no        |  no state change         |
   |  "From"                  |  no        |  no state change         |
   |  "From + Pending Out"    |  yes       |  "Both"                  |
   |  "Both"                  |  no        |  no state change         |
   +------------------------------------------------------------------+



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   When the user's server receives a presence stanza of type
   "unsubscribed" for the user from the contact, it MUST deliver the
   stanza to the user and MUST change the subscription state 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.  Otherwise, the user's server SHOULD NOT
   deliver the stanza and MUST NOT change the subscription state.  These
   rules are summarized in the following table.

   Table 6: Recommended handling of inbound "unsubscribed" stanzas

   +------------------------------------------------------------------+
   |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
   +------------------------------------------------------------------+
   |  "None"                  |  no        |  no state change         |
   |  "None + Pending Out"    |  yes       |  "None"                  |
   |  "None + Pending In"     |  no        |  no state change         |
   |  "None + Pending Out/In" |  yes       |  "None + Pending In"     |
   |  "To"                    |  yes       |  "None"                  |
   |  "To + Pending In"       |  yes       |  "None + Pending In"     |
   |  "From"                  |  no        |  no state change         |
   |  "From + Pending Out"    |  yes       |  "From"                  |
   |  "Both"                  |  yes       |  "From"                  |
   +------------------------------------------------------------------+

9.4.  Server Delivery and Client Acknowledgement of Subscription
      Requests and State Change Notifications

   When a server receives an inbound presence stanza of type "subscribe"
   (i.e., a subscription request) or of type "subscribed",
   "unsubscribe", or "unsubscribed" (i.e., a subscription state change
   notification), in addition to sending the appropriate roster push (or
   updated roster when the roster is next requested by an available
   resource), it MUST deliver the request or notification to the
   intended recipient at least once.  A server MAY require the recipient
   to acknowledge receipt of all state change notifications (and MUST
   require acknowledgement in the case of subscription requests, i.e.,
   presence stanzas of type "subscribe").  In order to require
   acknowledgement, a server SHOULD send the request or notification to
   the recipient each time the recipient logs in, until the recipient
   acknowledges receipt of the notification by "affirming" or "denying"
   the notification, as shown in the following table:









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   Table 7: Acknowledgement of subscription state change notifications

   +--------------------------------------------------+
   |  STANZA TYPE   |  ACCEPT        |  DENY          |
   +--------------------------------------------------+
   |  subscribe     |  subscribed    |  unsubscribed  |
   |  subscribed    |  subscribe     |  unsubscribe   |
   |  unsubscribe   |  unsubscribed  |  subscribed    |
   |  unsubscribed  |  unsubscribe   |  subscribe     |
   +--------------------------------------------------+

   Obviously, given the foregoing subscription state charts, some of the
   acknowledgement stanzas will be routed to the contact and result in
   subscription state changes, while others will not.  However, any such
   stanzas MUST result in the server's no longer sending the
   subscription state notification to the user.

   Because a user's server MUST automatically generate outbound presence
   stanzas of type "unsubscribe" and "unsubscribed" upon receiving a
   roster set with the 'subscription' attribute set to a value of
   "remove" (see Removing a Roster Item and Cancelling All Subscriptions
   (Section 8.6)), the server MUST treat a roster remove request as
   equivalent to sending both of those presence stanzas for purposes of
   determining whether to continue sending subscription state change
   notifications of type "subscribe" or "subscribed" to the user.

10.  Blocking Communication

   Most instant messaging systems have found it necessary to implement
   some method for users to block communications from particular other
   users (this is also required by sections 5.1.5, 5.1.15, 5.3.2, and
   5.4.10 of [IMP-REQS]).  In XMPP this is done by managing one's
   privacy lists using the 'jabber:iq:privacy' namespace.

   Server-side privacy lists enable successful completion of the
   following use cases:

   o  Retrieving one's privacy lists.

   o  Adding, removing, and editing one's privacy lists.

   o  Setting, changing, or declining active lists.

   o  Setting, changing, or declining the default list (i.e., the list
      that is active by default).

   o  Allowing or blocking messages based on JID, group, or subscription
      type (or globally).



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   o  Allowing or blocking inbound presence notifications based on JID,
      group, or subscription type (or globally).

   o  Allowing or blocking outbound presence notifications based on JID,
      group, or subscription type (or globally).

   o  Allowing or blocking IQ stanzas based on JID, group, or
      subscription type (or globally).

   o  Allowing or blocking all communications based on JID, group, or
      subscription type (or globally).

   Note: Presence notifications do not include presence subscriptions,
   only presence information that is broadcasted to entities that are
   subscribed to a user's presence information.  Thus this includes
   presence stanzas with no 'type' attribute or of type='unavailable'
   only.

10.1.  Syntax and Semantics

   A user MAY define one or more privacy lists, which are stored by the
   user's server.  Each <list/> element contains one or more rules in
   the form of <item/> elements, and each <item/> element uses
   attributes to define a privacy rule type, a specific value to which
   the rule applies, the relevant action, and the place of the item in
   the processing order.

   The syntax is as follows:

   <iq>
     <query xmlns='jabber:iq:privacy'>
       <list name='foo'>
         <item
             type='[jid|group|subscription]'
             value='bar'
             action='[allow|deny]'
             order='unsignedInt'>
           [<message/>]
           [<presence-in/>]
           [<presence-out/>]
           [<iq/>]
         </item>
       </list>
     </query>
   </iq>






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   If the type is "jid", then the 'value' attribute MUST contain a valid
   Jabber ID.  JIDs SHOULD be matched in the following order:

   1.  <user@domain/resource> (only that resource matches)

   2.  <user@domain> (any resource matches)

   3.  <domain/resource> (only that resource matches)

   4.  <domain> (the domain itself matches, as does any user@domain,
       domain/resource, or address containing a subdomain)

   If the type is "group", then the 'value' attribute SHOULD contain the
   name of a group in the user's roster.  (If a client attempts to
   update, create, or delete a list item with a group that is not in the
   user's roster, the server SHOULD return to the client an
   <item-not-found/> stanza error.)

   If the type is "subscription", then the 'value' attribute MUST be one
   of "both", "to", "from", or "none" as defined under Roster Syntax and
   Semantics (Section 7.1), where "none" includes entities that are
   totally unknown to the user and therefore not in the user's roster at
   all.

   If no 'type' attribute is included, the rule provides the
   "fall-through" case.

   The 'action' attribute MUST be included and its value MUST be either
   "allow" or "deny".

   The 'order' attribute MUST be included and its value MUST be a
   non-negative integer that is unique among all items in the list.  (If
   a client attempts to create or update a list with non-unique order
   values, the server MUST return to the client a <bad-request/> stanza
   error.)

   The <item/> element MAY contain one or more child elements that
   enable an entity to specify more granular control over which kinds of
   stanzas are to be blocked (i.e., rather than blocking all stanzas).
   The allowable child elements are:

   o  <message/> -- blocks incoming message stanzas
   o  <iq/> -- blocks incoming IQ stanzas
   o  <presence-in/> -- blocks incoming presence notifications
   o  <presence-out/> -- blocks outgoing presence notifications






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   Within the 'jabber:iq:privacy' namespace, the <query/> child of an IQ
   stanza of type "set" MUST NOT include more than one child element
   (i.e., the stanza MUST contain only one <active/> element, one
   <default/> element, or one <list/> element); if a sending entity
   violates this rule, the receiving entity MUST return a <bad-request/>
   stanza error.

   When a client adds or updates a privacy list, the <list/> element
   SHOULD contain at least one <item/> child element; when a client
   removes a privacy list, the <list/> element MUST NOT contain any
   <item/> child elements.

   When a client updates a privacy list, it must include all of the
   desired items (i.e., not a "delta").

10.2.  Business Rules

   1.  If there is an active list set for a session, it affects only the
       session(s) for which it is activated, and only for the duration
       of the session(s); the server MUST apply the active list only and
       MUST NOT apply the default list (i.e., there is no "layering" of
       lists).

   2.  The default list applies to the user as a whole, and is processed
       if there is no active list set for the target session/resource to
       which a stanza is addressed, or if there are no current sessions
       for the user.

   3.  If there is no active list set for a session (or there are no
       current sessions for the user), and there is no default list,
       then all stanzas SHOULD BE accepted or appropriately processed by
       the server on behalf of the user in accordance with the Server
       Rules for Handling XML Stanzas (Section 11).

   4.  Privacy lists MUST be the first delivery rule applied by a
       server, superseding (1) the routing and delivery rules specified
       in Server Rules for Handling XML Stanzas (Section 11), and (2)
       the handling of subscription-related presence stanzas (and
       corresponding generation of roster pushes) specified in
       Integration of Roster Items and Presence Subscriptions (Section
       8).

   5.  The order in which privacy list items are processed by the server
       is important.  List items MUST be processed in ascending order
       determined by the integer values of the 'order' attribute for
       each <item/>.





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   6.  As soon as a stanza is matched against a privacy list rule, the
       server MUST appropriately handle the stanza in accordance with
       the rule and cease processing.

   7.  If no fall-through item is provided in a list, the fall-through
       action is assumed to be "allow".

   8.  If a user updates the definition for an active list, subsequent
       processing based on that active list MUST use the updated
       definition (for all resources to which that active list currently
       applies).

   9.  If a change to the subscription state or roster group of a roster
       item defined in an active or default list occurs during a user's
       session, subsequent processing based on that list MUST take into
       account the changed state or group (for all resources to which
       that list currently applies).

   10. When the definition for a rule is modified, the server MUST send
       an IQ stanza of type "set" to all connected resources, containing
       a <query/> element with only one <list/> child element, where the
       'name' attribute is set to the name of the modified privacy list.
       These "privacy list pushes" adhere to the same semantics as the
       "roster pushes" used in roster management, except that only the
       list name itself (not the full list definition or the "delta") is
       pushed to the connected resources.  It is up to the receiving
       resource to determine whether to retrieve the modified list
       definition, although a connected resource SHOULD do so if the
       list currently applies to it.

   11. When a resource attempts to remove a list or specify a new
       default list while that list applies to a connected resource
       other than the sending resource, the server MUST return a
       <conflict/> error to the sending resource and MUST NOT make the
       requested change.

10.3.  Retrieving One's Privacy Lists

   Example: Client requests names of privacy lists from server:

   <iq from='romeo@example.net/orchard' type='get' id='getlist1'>
     <query xmlns='jabber:iq:privacy'/>
   </iq>








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   Example: Server sends names of privacy lists to client, preceded by
   active list and default list:

   <iq type='result' id='getlist1' to='romeo@example.net/orchard'>
     <query xmlns='jabber:iq:privacy'>
       <active name='private'/>
       <default name='public'/>
       <list name='public'/>
       <list name='private'/>
       <list name='special'/>
     </query>
   </iq>

   Example: Client requests a privacy list from server:

   <iq from='romeo@example.net/orchard' type='get' id='getlist2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='public'/>
     </query>
   </iq>

   Example: Server sends a privacy list to client:

   <iq type='result' id='getlist2' to='romeo@example.net/orchard'>
     <query xmlns='jabber:iq:privacy'>
       <list name='public'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='1'/>
         <item action='allow' order='2'/>
       </list>
     </query>
   </iq>

   Example: Client requests another privacy list from server:

   <iq from='romeo@example.net/orchard' type='get' id='getlist3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='private'/>
     </query>
   </iq>









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   Example: Server sends another privacy list to client:

   <iq type='result' id='getlist3' to='romeo@example.net/orchard'>
     <query xmlns='jabber:iq:privacy'>
       <list name='private'>
         <item type='subscription'
               value='both'
               action='allow'
               order='10'/>
         <item action='deny' order='15'/>
       </list>
     </query>
   </iq>

   Example: Client requests yet another privacy list from server:

   <iq from='romeo@example.net/orchard' type='get' id='getlist4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='special'/>
     </query>
   </iq>

   Example: Server sends yet another privacy list to client:

   <iq type='result' id='getlist4' to='romeo@example.net/orchard'>
     <query xmlns='jabber:iq:privacy'>
       <list name='special'>
         <item type='jid'
               value='juliet@example.com'
               action='allow'
               order='6'/>
         <item type='jid'
               value='benvolio@example.org'
               action='allow'
               order='7'/>
         <item type='jid'
               value='mercutio@example.org'
               action='allow'
               order='42'/>
         <item action='deny' order='666'/>
       </list>
     </query>
   </iq>

   In this example, the user has three lists: (1) 'public', which allows
   communications from everyone except one specific entity (this is the
   default list); (2) 'private', which allows communications only with




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   contacts who have a bidirectional subscription with the user (this is
   the active list); and (3) 'special', which allows communications only
   with three specific entities.

   If the user attempts to retrieve a list but a list by that name does
   not exist, the server MUST return an <item-not-found/> stanza error
   to the user:

   Example: Client attempts to retrieve non-existent list:

   <iq to='romeo@example.net/orchard' type='error' id='getlist5'>
     <query xmlns='jabber:iq:privacy'>
       <list name='The Empty Set'/>
     </query>
     <error type='cancel'>
       <item-not-found
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   The user is allowed to retrieve only one list at a time.  If the user
   attempts to retrieve more than one list in the same request, the
   server MUST return a <bad request/> stanza error to the user:

   Example: Client attempts to retrieve more than one list:

   <iq to='romeo@example.net/orchard' type='error' id='getlist6'>
     <query xmlns='jabber:iq:privacy'>
       <list name='public'/>
       <list name='private'/>
       <list name='special'/>
     </query>
     <error type='modify'>
       <bad-request
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

10.4.  Managing Active Lists

   In order to set or change the active list currently being applied by
   the server, the user MUST send an IQ stanza of type "set" with a
   <query/> element qualified by the 'jabber:iq:privacy' namespace that
   contains an empty <active/> child element possessing a 'name'
   attribute whose value is set to the desired list name.






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   Example: Client requests change of active list:

   <iq from='romeo@example.net/orchard' type='set' id='active1'>
     <query xmlns='jabber:iq:privacy'>
       <active name='special'/>
     </query>
   </iq>

   The server MUST activate and apply the requested list before sending
   the result back to the client.

   Example: Server acknowledges success of active list change:

   <iq type='result' id='active1' to='romeo@example.net/orchard'/>

   If the user attempts to set an active list but a list by that name
   does not exist, the server MUST return an <item-not-found/> stanza
   error to the user:

   Example: Client attempts to set a non-existent list as active:

   <iq to='romeo@example.net/orchard' type='error' id='active2'>
     <query xmlns='jabber:iq:privacy'>
       <active name='The Empty Set'/>
     </query>
     <error type='cancel'>
       <item-not-found
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   In order to decline the use of any active list, the connected
   resource MUST send an empty <active/> element with no 'name'
   attribute.

   Example: Client declines the use of active lists:

   <iq from='romeo@example.net/orchard' type='set' id='active3'>
     <query xmlns='jabber:iq:privacy'>
       <active/>
     </query>
   </iq>

   Example: Server acknowledges success of declining any active list:

   <iq type='result' id='active3' to='romeo@example.net/orchard'/>





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10.5.  Managing the Default List

   In order to change its default list (which applies to the user as a
   whole, not only the sending resource), the user MUST send an IQ
   stanza of type "set" with a <query/> element qualified by the
   'jabber:iq:privacy' namespace that contains an empty <default/> child
   element possessing a 'name' attribute whose value is set to the
   desired list name.

   Example: User requests change of default list:

   <iq from='romeo@example.net/orchard' type='set' id='default1'>
     <query xmlns='jabber:iq:privacy'>
       <default name='special'/>
     </query>
   </iq>

   Example: Server acknowledges success of default list change:

   <iq type='result' id='default1' to='romeo@example.net/orchard'/>

   If the user attempts to change which list is the default list but the
   default list is in use by at least one connected resource other than
   the sending resource, the server MUST return a <conflict/> stanza
   error to the sending resource:

   Example: Client attempts to change the default list but that list is
   in use by another resource:

   <iq to='romeo@example.net/orchard' type='error' id='default1'>
     <query xmlns='jabber:iq:privacy'>
       <default name='special'/>
     </query>
     <error type='cancel'>
       <conflict
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   If the user attempts to set a default list but a list by that name
   does not exist, the server MUST return an <item-not-found/> stanza
   error to the user:









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   Example: Client attempts to set a non-existent list as default:

   <iq to='romeo@example.net/orchard' type='error' id='default1'>
     <query xmlns='jabber:iq:privacy'>
       <default name='The Empty Set'/>
     </query>
     <error type='cancel'>
       <item-not-found
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   In order to decline the use of a default list (i.e., to use the
   domain's stanza routing rules at all times), the user MUST send an
   empty <default/> element with no 'name' attribute.

   Example: Client declines the use of the default list:

   <iq from='romeo@example.net/orchard' type='set' id='default2'>
     <query xmlns='jabber:iq:privacy'>
       <default/>
     </query>
   </iq>

   Example: Server acknowledges success of declining any default list:

   <iq type='result' id='default2' to='romeo@example.net/orchard'/>

   If one connected resource attempts to decline the use of a default
   list for the user as a whole but the default list currently applies
   to at least one other connected resource, the server MUST return a
   <conflict/> error to the sending resource:

   Example: Client attempts to decline a default list but that list is
   in use by another resource:

   <iq to='romeo@example.net/orchard' type='error' id='default3'>
     <query xmlns='jabber:iq:privacy'>
       <default/>
     </query>
     <error type='cancel'>
       <conflict
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>






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10.6.  Editing a Privacy List

   In order to edit a privacy list, the user MUST send an IQ stanza of
   type "set" with a <query/> element qualified by the
   'jabber:iq:privacy' namespace that contains one <list/> child element
   possessing a 'name' attribute whose value is set to the list name the
   user would like to edit.  The <list/> element MUST contain one or
   more <item/> elements, which specify the user's desired changes to
   the list by including all elements in the list (not the "delta").

   Example: Client edits a privacy list:

   <iq from='romeo@example.net/orchard' type='set' id='edit1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='public'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='3'/>
         <item type='jid'
               value='paris@example.org'
               action='deny'
               order='5'/>
         <item action='allow' order='68'/>
       </list>
     </query>
   </iq>

   Example: Server acknowledges success of list edit:

   <iq type='result' id='edit1' to='romeo@example.net/orchard'/>

   Note: The value of the 'order' attribute for any given item is not
   fixed.  Thus in the foregoing example if the user would like to add 4
   items between the "tybalt@example.com" item and the
   "paris@example.org" item, the user's client MUST renumber the
   relevant items before submitting the list to the server.

   The server MUST now send a "privacy list push" to all connected
   resources:

   Example: Privacy list push on list edit:

   <iq to='romeo@example.net/orchard' type='set' id='push1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='public'/>
     </query>
   </iq>



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   <iq to='romeo@example.net/home' type='set' id='push2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='public'/>
     </query>
   </iq>

   In accordance with the semantics of IQ stanzas defined in
   [XMPP-CORE], each connected resource MUST return an IQ result to the
   server as well:

   Example: Acknowledging receipt of privacy list pushes:

   <iq from='romeo@example.net/orchard'
       type='result'
       id='push1'/>

   <iq from='romeo@example.net/home'
       type='result'
       id='push2'/>

10.7.  Adding a New Privacy List

   The same protocol used to edit an existing list is used to create a
   new list.  If the list name matches that of an existing list, the
   request to add a new list will overwrite the old one.  As with list
   edits, the server MUST also send a "privacy list push" to all
   connected resources.

10.8.  Removing a Privacy List

   In order to remove a privacy list, the user MUST send an IQ stanza of
   type "set" with a <query/> element qualified by the
   'jabber:iq:privacy' namespace that contains one empty <list/> child
   element possessing a 'name' attribute whose value is set to the list
   name the user would like to remove.

   Example: Client removes a privacy list:

   <iq from='romeo@example.net/orchard' type='set' id='remove1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='private'/>
     </query>
   </iq>

   Example: Server acknowledges success of list removal:

   <iq type='result' id='remove1' to='romeo@example.net/orchard'/>




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   If a user attempts to remove a list that is currently being applied
   to at least one resource other than the sending resource, the server
   MUST return a <conflict/> stanza error to the user; i.e., the user
   MUST first set another list to active or default before attempting to
   remove it.  If the user attempts to remove a list but a list by that
   name does not exist, the server MUST return an <item-not-found/>
   stanza error to the user.  If the user attempts to remove more than
   one list in the same request, the server MUST return a <bad request/>
   stanza error to the user.

10.9.  Blocking Messages

   Server-side privacy lists enable a user to block incoming messages
   from other entities based on the entity's JID, roster group, or
   subscription status (or globally).  The following examples illustrate
   the protocol.  (Note: For the sake of brevity, IQ stanzas of type
   "result" are not shown in the following examples, nor are "privacy
   list pushes".)

   Example: User blocks based on JID:

   <iq from='romeo@example.net/orchard' type='set' id='msg1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='message-jid-example'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='3'>
           <message/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive messages from the entity with the specified JID.

   Example: User blocks based on roster group:

   <iq from='romeo@example.net/orchard' type='set' id='msg2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='message-group-example'>
         <item type='group'
               value='Enemies'
               action='deny'
               order='4'>
           <message/>
         </item>



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       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive messages from any entities in the specified roster
   group.

   Example: User blocks based on subscription type:

   <iq from='romeo@example.net/orchard' type='set' id='msg3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='message-sub-example'>
         <item type='subscription'
               value='none'
               action='deny'
               order='5'>
           <message/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive messages from any entities with the specified
   subscription type.

   Example: User blocks globally:

   <iq from='romeo@example.net/orchard' type='set' id='msg4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='message-global-example'>
         <item action='deny' order='6'>
           <message/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive messages from any other users.

10.10.  Blocking Inbound Presence Notifications

   Server-side privacy lists enable a user to block incoming presence
   notifications from other entities based on the entity's JID, roster
   group, or subscription status (or globally).  The following examples
   illustrate the protocol.



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   Note: Presence notifications do not include presence subscriptions,
   only presence information that is broadcasted to the user because the
   user is currently subscribed to a contact's presence information.
   Thus this includes presence stanzas with no 'type' attribute or of
   type='unavailable' only.

   Example: User blocks based on JID:

   <iq from='romeo@example.net/orchard' type='set' id='presin1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presin-jid-example'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='7'>
           <presence-in/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive presence notifications from the entity with the
   specified JID.

   Example: User blocks based on roster group:

   <iq from='romeo@example.net/orchard' type='set' id='presin2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presin-group-example'>
         <item type='group'
               value='Enemies'
               action='deny'
               order='8'>
           <presence-in/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive presence notifications from any entities in the
   specified roster group.








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   Example: User blocks based on subscription type:

   <iq from='romeo@example.net/orchard' type='set' id='presin3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presin-sub-example'>
         <item type='subscription'
               value='to'
               action='deny'
               order='9'>
           <presence-in/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive presence notifications from any entities with the
   specified subscription type.

   Example: User blocks globally:

   <iq from='romeo@example.net/orchard' type='set' id='presin4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presin-global-example'>
         <item action='deny' order='11'>
           <presence-in/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive presence notifications from any other users.

10.11.  Blocking Outbound Presence Notifications

   Server-side privacy lists enable a user to block outgoing presence
   notifications to other entities based on the entity's JID, roster
   group, or subscription status (or globally).  The following examples
   illustrate the protocol.

   Note: Presence notifications do not include presence subscriptions,
   only presence information that is broadcasted to contacts because
   those contacts are currently subscribed to the user's presence
   information.  Thus this includes presence stanzas with no 'type'
   attribute or of type='unavailable' only.





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   Example: User blocks based on JID:

   <iq from='romeo@example.net/orchard' type='set' id='presout1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presout-jid-example'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='13'>
           <presence-out/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not send presence notifications to the entity with the specified
   JID.

   Example: User blocks based on roster group:

   <iq from='romeo@example.net/orchard' type='set' id='presout2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presout-group-example'>
         <item type='group'
               value='Enemies'
               action='deny'
               order='15'>
           <presence-out/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not send presence notifications to any entities in the specified
   roster group.

   Example: User blocks based on subscription type:

   <iq from='romeo@example.net/orchard' type='set' id='presout3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presout-sub-example'>
         <item type='subscription'
               value='from'
               action='deny'
               order='17'>
           <presence-out/>



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         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not send presence notifications to any entities with the
   specified subscription type.

   Example: User blocks globally:

   <iq from='romeo@example.net/orchard' type='set' id='presout4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presout-global-example'>
         <item action='deny' order='23'>
           <presence-out/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not send presence notifications to any other users.

10.12.  Blocking IQ Stanzas

   Server-side privacy lists enable a user to block incoming IQ stanzas
   from other entities based on the entity's JID, roster group, or
   subscription status (or globally).  The following examples illustrate
   the protocol.

   Example: User blocks based on JID:

   <iq from='romeo@example.net/orchard' type='set' id='iq1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='iq-jid-example'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='29'>
           <iq/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive IQ stanzas from the entity with the specified JID.



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   Example: User blocks based on roster group:

   <iq from='romeo@example.net/orchard' type='set' id='iq2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='iq-group-example'>
         <item type='group'
               value='Enemies'
               action='deny'
               order='31'>
           <iq/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive IQ stanzas from any entities in the specified roster
   group.

   Example: User blocks based on subscription type:

   <iq from='romeo@example.net/orchard' type='set' id='iq3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='iq-sub-example'>
         <item type='subscription'
               value='none'
               action='deny'
               order='17'>
           <iq/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive IQ stanzas from any entities with the specified
   subscription type.














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   Example: User blocks globally:

   <iq from='romeo@example.net/orchard' type='set' id='iq4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='iq-global-example'>
         <item action='deny' order='1'>
           <iq/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive IQ stanzas from any other users.

10.13.  Blocking All Communication

   Server-side privacy lists enable a user to block all stanzas from and
   to other entities based on the entity's JID, roster group, or
   subscription status (or globally).  Note that this includes
   subscription-related presence stanzas, which are excluded by Blocking
   Inbound Presence Notifications (Section 10.10).  The following
   examples illustrate the protocol.

   Example: User blocks based on JID:

   <iq from='romeo@example.net/orchard' type='set' id='all1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='all-jid-example'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='23'/>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive any communications from, nor send any stanzas to,
   the entity with the specified JID.

   Example: User blocks based on roster group:

   <iq from='romeo@example.net/orchard' type='set' id='all2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='all-group-example'>
         <item type='group'
               value='Enemies'



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               action='deny'
               order='13'/>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive any communications from, nor send any stanzas to,
   any entities in the specified roster group.

   Example: User blocks based on subscription type:

   <iq from='romeo@example.net/orchard' type='set' id='all3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='all-sub-example'>
         <item type='subscription'
               value='none'
               action='deny'
               order='11'/>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive any communications from, nor send any stanzas to,
   any entities with the specified subscription type.

   Example: User blocks globally:

   <iq from='romeo@example.net/orchard' type='set' id='all4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='all-global-example'>
         <item action='deny' order='7'/>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive any communications from, nor send any stanzas to,
   any other users.

10.14.  Blocked Entity Attempts to Communicate with User

   If a blocked entity attempts to send message or presence stanzas to
   the user, the user's server SHOULD silently drop the stanza and MUST
   NOT return an error to the sending entity.





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   If a blocked entity attempts to send an IQ stanza of type "get" or
   "set" to the user, the user's server MUST return to the sending
   entity a <service-unavailable/> stanza error, since this is the
   standard error code sent from a client that does not understand the
   namespace of an IQ get or set.  IQ stanzas of other types SHOULD be
   silently dropped by the server.

   Example: Blocked entity attempts to send IQ get:

   <iq type='get'
       to='romeo@example.net'
       from='tybalt@example.com/pda'
       id='probing1'>
     <query xmlns='jabber:iq:version'/>
   </iq>

   Example: Server returns error to blocked entity:

   <iq type='error'
       from='romeo@example.net'
       to='tybalt@example.com/pda'
       id='probing1'>
     <query xmlns='jabber:iq:version'/>
     <error type='cancel'>
       <service-unavailable
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

10.15.  Higher-Level Heuristics

   When building a representation of a higher-level privacy heuristic, a
   client SHOULD use the simplest possible representation.

   For example, the heuristic "block all communications with any user
   not in my roster" could be constructed in any of the following ways:

   o  allow communications from all JIDs in my roster (i.e., listing
      each JID as a separate list item), but block communications with
      everyone else

   o  allow communications from any user who is in one of the groups
      that make up my roster (i.e., listing each group as a separate
      list item), but block communications from everyone else

   o  allow communications from any user with whom I have a subscription
      of 'both' or 'to' or 'from' (i.e., listing each subscription value
      separately), but block communications from everyone else



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   o  block communications from anyone whose subscription state is
      'none'

   The final representation is the simplest and SHOULD be used; here is
   the XML that would be sent in this case:

   <iq type='set' id='heuristic1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='heuristic-example'>
         <item type='subscription'
               value='none'
               action='deny'
               order='437'/>
       </list>
     </query>
   </iq>

11.  Server Rules for Handling XML Stanzas

   Basic routing and delivery rules for servers are defined in
   [XMPP-CORE].  This section defines additional rules for
   XMPP-compliant instant messaging and presence servers.

11.1.  Inbound Stanzas

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute of an inbound stanza matches a hostname of the
   server itself and the JID contained in the 'to' attribute is of the
   form <user@example.com> or <user@example.com/resource>, the server
   MUST first apply any privacy lists (Section 10) that are in force,
   then follow the rules defined below:

   1.  If the JID is of the form <user@domain/resource> and an available
       resource matches the full JID, the recipient's server MUST
       deliver the stanza to that resource.

   2.  Else if the JID is of the form <user@domain> or <user@domain/
       resource> and the associated user account does not exist, the
       recipient's server (a) SHOULD silently ignore the stanza (i.e.,
       neither deliver it nor return an error) if it is a presence
       stanza, (b) MUST return a <service-unavailable/> stanza error to
       the sender if it is an IQ stanza, and (c) SHOULD return a
       <service-unavailable/> stanza error to the sender if it is a
       message stanza.

   3.  Else if the JID is of the form <user@domain/resource> and no
       available resource matches the full JID, the recipient's server
       (a) SHOULD silently ignore the stanza (i.e., neither deliver it



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       nor return an error) if it is a presence stanza, (b) MUST return
       a <service-unavailable/> stanza error to the sender if it is an
       IQ stanza, and (c) SHOULD treat the stanza as if it were
       addressed to <user@domain> if it is a message stanza.

   4.  Else if the JID is of the form <user@domain> and there is at
       least one available resource available for the user, the
       recipient's server MUST follow these rules:

       1.  For message stanzas, the server SHOULD deliver the stanza to
           the highest-priority available resource (if the resource did
           not provide a value for the <priority/> element, the server
           SHOULD consider it to have provided a value of zero).  If two
           or more available resources have the same priority, the
           server MAY use some other rule (e.g., most recent connect
           time, most recent activity time, or highest availability as
           determined by some hierarchy of <show/> values) to choose
           between them or MAY deliver the message to all such
           resources.  However, the server MUST NOT deliver the stanza
           to an 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 (defined below).  In addition, 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>).

       2.  For presence stanzas other than those of type "probe", the
           server MUST deliver the stanza to all available resources;
           for presence probes, the server SHOULD reply based on the
           rules defined in Presence Probes (Section 5.1.3).  In
           addition, 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>).

       3.  For IQ stanzas, 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 available resources.
           Specifically, if the semantics of the qualifying namespace
           define a reply that the server can provide, the server MUST
           reply to the stanza on behalf of the user; if not, the server
           MUST reply with a <service-unavailable/> stanza error.

   5.  Else if the JID is of the form <user@domain> and there are no
       available resources associated with the user, how the stanza is
       handled depends on the stanza type:





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       1.  For presence stanzas of type "subscribe", "subscribed",
           "unsubscribe", and "unsubscribed", the server MUST maintain a
           record of the stanza and deliver the stanza at least once
           (i.e., when the user next creates an available resource); in
           addition, the server MUST continue to deliver presence
           stanzas of type "subscribe" until the user either approves or
           denies the subscription request (see also Presence
           Subscriptions (Section 5.1.6)).

       2.  For all other presence stanzas, the server SHOULD silently
           ignore the stanza by not storing it for later delivery or
           replying to it on behalf of the user.

       3.  For message stanzas, 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 some other means (e.g., to the user's email account).
           However, if offline message storage or message forwarding is
           not enabled, the server MUST return to the sender a
           <service-unavailable/> stanza error.  (Note: Offline message
           storage and message forwarding are not defined in XMPP, since
           they are strictly a matter of implementation and service
           provisioning.)

       4.  For IQ stanzas, 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, the server MUST reply to the
           stanza on behalf of the user; if not, the server MUST reply
           with a <service-unavailable/> stanza error.

11.2.  Outbound Stanzas

   If the hostname of the domain identifier portion of the address
   contained in the 'to' attribute of an outbound stanza matches a
   hostname of the server itself, the server MUST deliver the stanza to
   a local entity according the rules for Inbound Stanzas (Section
   11.1).

   If the hostname of the domain identifier portion of the address
   contained in the 'to' attribute of an outbound stanza does not match
   a hostname of the server itself, the server MUST attempt to route the
   stanza to the foreign domain.  The recommended order of actions is as
   follows:







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   1.  First attempt to resolve the foreign hostname using an [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].

   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.

12.  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; note well that the requirements
   described here supplement but do not supersede the core requirements.
   Note also that 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.

12.1.  Servers

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





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   o  All server-related instant messaging and presence syntax and
      semantics defined in this document, including presence broadcast
      on behalf of clients, presence subscriptions, roster storage and
      manipulation, privacy lists, and IM-specific routing and delivery
      rules

12.2.  Clients

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

   o  Generation and handling 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

   o  All client-related instant messaging syntax and semantics defined
      in this document, including presence subscriptions, roster
      management, and privacy lists

   o  End-to-end object encryption as defined in End-to-End Object
      Encryption in the Extensible Messaging and Presence Protocol
      (XMPP) [XMPP-E2E]

   A client MUST also handle addresses that are encoded as "im:" URIs as
   specified in [CPIM], and MAY do so by removing the "im:" scheme and
   entrusting address resolution to the server as specified under
   Outbound Stanzas (Section 11.2).

13.  Internationalization Considerations

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

14.  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 memo; specifically:








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   o  When a server processes an inbound stanza of any kind whose
      intended recipient is a user associated with one of the server's
      hostnames, the server MUST first apply any privacy lists (Section
      10) that are in force (see Server Rules for Handling XML Stanzas
      (Section 11)).

   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
      information if the sender is an entity that is not authorized to
      receive that information as determined by presence subscriptions
      (see Client and Server Presence Responsibilities (Section 5.1)).

   o  When a server processes an outbound presence stanza with no type
      or of type "unavailable", it MUST follow the rules defined under
      Client and Server Presence Responsibilities (Section 5.1) 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.

15.  IANA Considerations

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

15.1.  XML Namespace Name for Session Data

   A URN sub-namespace for session-related data in the Extensible
   Messaging and Presence Protocol (XMPP) is defined as follows.  (This
   namespace name adheres to the format defined in The IETF XML Registry
   [XML-REG].)

   URI: urn:ietf:params:xml:ns:xmpp-session
   Specification: RFC 3921
   Description: This is the XML namespace name for session-related data
      in the Extensible Messaging and Presence Protocol (XMPP) as
      defined by RFC 3921.
   Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>






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15.2.  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: RFC 3921
   Description: Instant messaging protocol label for the Extensible
      Messaging and Presence Protocol (XMPP) as defined by RFC 3921.
   Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>

15.3.  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: RFC 3921
   Description: Presence protocol label for the Extensible Messaging and
      Presence Protocol (XMPP) as defined by RFC 3921.
   Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>

16.  References

16.1.  Normative References

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

   [IMP-REQS]  Day, M., Aggarwal, S., Mohr, G., 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.

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



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   [XML]       Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler,
               "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C
               REC-xml, October 2000, <http://www.w3.org/TR/REC-xml>.

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

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

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

16.2.  Informative References

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

   [IRC]       Oikarinen, J. and D. Reed, "Internet Relay Chat
               Protocol", RFC 1459, May 1993.

   [JEP-0054]  Saint-Andre, P., "vcard-temp", JSF JEP 0054, March 2003.

   [JEP-0077]  Saint-Andre, P., "In-Band Registration", JSF JEP 0077,
               August 2004.

   [JEP-0078]  Saint-Andre, P., "Non-SASL Authentication", JSF JEP 0078,
               July 2004.

   [JSF]       Jabber Software Foundation, "Jabber Software Foundation",
               <http://www.jabber.org/>.

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

   [XML-REG]   Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
               January 2004.












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Appendix A.  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
   [JEP-0054], published by the Jabber Software Foundation [JSF]).

Appendix B.  XML Schemas

   The following XML schemas are descriptive, not normative.  For
   schemas defining the core features of XMPP, refer to [XMPP-CORE].

B.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'/>
               <xs:element ref='body'/>
               <xs:element ref='thread'/>
             </xs:choice>
             <xs:any     namespace='##other'
                         minOccurs='0'
                         maxOccurs='unbounded'/>
             <xs:element ref='error'
                         minOccurs='0'/>











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



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       <xs:complexType>
         <xs:sequence>
           <xs:choice minOccurs='0' maxOccurs='unbounded'>
             <xs:element ref='show'/>
             <xs:element ref='status'/>
             <xs:element ref='priority'/>
           </xs:choice>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='error'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='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>

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



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     </xs:element>

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

     <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'>
           <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'>



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           <xs:group   ref='err:stanzaErrorGroup'/>
           <xs:element ref='err:text'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='code' type='xs:byte' use='optional'/>
         <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>

B.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'/>

     <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>




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           <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>
     </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>



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

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



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

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



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                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='code' type='xs:byte' use='optional'/>
         <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>

B.3  session

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

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-session'
       xmlns='urn:ietf:params:xml:ns:xmpp-session'
       elementFormDefault='qualified'>

     <xs:element name='session' type='empty'/>

     <xs:simpleType name='empty'>
       <xs:restriction base='xs:string'>
         <xs:enumeration value=''/>
       </xs:restriction>
     </xs:simpleType>

   </xs:schema>

B.4  jabber:iq:privacy

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

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='jabber:iq:privacy'





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       xmlns='jabber:iq:privacy'
       elementFormDefault='qualified'>

     <xs:element name='query'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='active'
                       minOccurs='0'/>
           <xs:element ref='default'
                       minOccurs='0'/>
           <xs:element ref='list'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

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

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

     <xs:element name='list'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='item'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
         </xs:sequence>



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RFC 3921                        XMPP IM                     October 2004


         <xs:attribute name='name'
                       type='xs:string'
                       use='required'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='item'>
       <xs:complexType>
         <xs:sequence>
           <xs:element name='iq'
                       minOccurs='0'
                       type='empty'/>
           <xs:element name='message'
                       minOccurs='0'
                       type='empty'/>
           <xs:element name='presence-in'
                       minOccurs='0'
                       type='empty'/>
           <xs:element name='presence-out'
                       minOccurs='0'
                       type='empty'/>
         </xs:sequence>
         <xs:attribute name='action' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='allow'/>
               <xs:enumeration value='deny'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute name='order'
                       type='xs:unsignedInt'
                       use='required'/>
         <xs:attribute name='type' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='group'/>
               <xs:enumeration value='jid'/>
               <xs:enumeration value='subscription'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute name='value'
                       type='xs:string'
                       use='optional'/>
       </xs:complexType>
     </xs:element>




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     <xs:simpleType name='empty'>
       <xs:restriction base='xs:string'>
         <xs:enumeration value=''/>
       </xs:restriction>
     </xs:simpleType>

   </xs:schema>

B.5  jabber:iq:roster

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

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

     <xs:element name='query'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='item'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <xs:element name='item'>
       <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: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'>




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               <xs:enumeration value='both'/>
               <xs:enumeration value='from'/>
               <xs:enumeration value='none'/>
               <xs:enumeration value='remove'/>
               <xs:enumeration value='to'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

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

   </xs:schema>

Appendix C.  Differences Between Jabber IM/Presence Protocols and XMPP

   This section is non-normative.

   XMPP has been adapted from the protocols originally developed in the
   Jabber open-source community, which can be thought of as "XMPP 0.9".
   Because there exists a large installed base of Jabber implementations
   and deployments, it may be helpful to specify the key differences
   between the relevant Jabber protocols and XMPP in order to expedite
   and encourage upgrades of those implementations and deployments to
   XMPP.  This section summarizes the differences that relate
   specifically to instant messaging and presence applications, while
   the corresponding section of [XMPP-CORE] summarizes the differences
   that relate to all XMPP applications.

C.1  Session Establishment

   The client-to-server authentication protocol developed in the Jabber
   community assumed that every client is an IM client and therefore
   initiated an IM session upon successful authentication and resource
   binding, which are performed simultaneously (documentation of this
   protocol is contained in [JEP-0078], published by the Jabber Software
   Foundation [JSF]).  XMPP maintains a stricter separation between core
   functionality and IM functionality; therefore, an IM session is not
   created until the client specifically requests one using the protocol
   defined under Session Establishment (Section 3).










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C.2  Privacy Lists

   The Jabber community began to define a protocol for communications
   blocking (privacy lists) in late 2001, but that effort was deprecated
   once the XMPP Working Group was formed.  Therefore the protocol
   defined under Blocking Communication (Section 10) is the only such
   protocol defined for use in the Jabber community.

Contributors

   Most of the core aspects of the Extensible Messaging and Presence
   Protocol were developed originally within the Jabber open-source
   community in 1999.  This community was founded by Jeremie Miller, who
   released source code for the initial version of the jabberd server in
   January 1999.  Major early contributors to the base protocol also
   included Ryan Eatmon, Peter Millard, Thomas Muldowney, and Dave
   Smith.  Work specific to instant messaging and presence by the XMPP
   Working Group has concentrated especially on IM session establishment
   and communication blocking (privacy lists); the session establishment
   protocol was mainly developed by Rob Norris and Joe Hildebrand, and
   the privacy lists protocol was originally contributed by Peter
   Millard.

Acknowledgements

   Thanks are due to a number of individuals in addition to the
   contributors listed.  Although it is difficult to provide a complete
   list, the following individuals were particularly helpful in defining
   the protocols or in commenting on the specifications in this memo:
   Thomas Charron, Richard Dobson, Schuyler Heath, Jonathan Hogg, Craig
   Kaes, Jacek Konieczny, Lisa Dusseault, Alexey Melnikov, Keith
   Minkler, Julian Missig, Pete Resnick, Marshall Rose, Jean-Louis
   Seguineau, Alexey Shchepin, Iain Shigeoka, and David Waite.  Thanks
   also to members of the XMPP Working Group and the IETF community for
   comments and feedback provided throughout the life of this memo.

Author's Address

   Peter Saint-Andre (editor)
   Jabber Software Foundation

   EMail: stpeter@jabber.org









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RFC 3921                        XMPP IM                     October 2004


Full Copyright Statement

   Copyright (C) The Internet Society (2004).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
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   The IETF invites any interested party to bring to its attention any
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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