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RFC 3921


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

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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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