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RFC 6121

 
 
 

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

Part 3 of 5, p. 48 to 77
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4.  Exchanging Presence Information

4.1.  Presence Fundamentals

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

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

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

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

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

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

4.2.  Initial Presence

4.2.1.  Client Generation of Initial Presence

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

   UC: <presence/>

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

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4.2.2.  Server Processing of Outbound Initial Presence

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

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

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

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

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

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

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

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

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

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

4.2.3.  Server Processing of Inbound Initial Presence

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

   [ ... to resource1 ... ]

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

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   [ ... to resource2 ... ]

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

4.2.4.  Client Processing of Initial Presence

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

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

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

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

4.3.  Presence Probes

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

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

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

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

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

4.3.1.  Server Generation of Outbound Presence Probe

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

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

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

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

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

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

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4.3.2.  Server Processing of Inbound Presence Probe

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

4.3.2.1.  Handling of the 'id' Attribute

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

   The following examples illustrate the difference.

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

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

   She also sends presence from her "balcony" resource.

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

   Romeo's server then sends a probe to Juliet.

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

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

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

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

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

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

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

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

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4.4.  Subsequent Presence Broadcast

4.4.1.  Client Generation of Subsequent Presence Broadcast

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

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

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

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

4.4.2.  Server Processing of Subsequent Outbound Presence

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

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

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

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

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

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

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

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

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

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

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

4.4.3.  Server Processing of Subsequent Inbound Presence

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

   [ ... to resource1 ... ]

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

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   [ ... to resource2 ... ]

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

4.4.4.  Client Processing of Subsequent Presence

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

4.5.  Unavailable Presence

4.5.1.  Client Generation of Unavailable Presence

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

   UC: <presence type='unavailable'/>

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

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

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

4.5.2.  Server Processing of Outbound Unavailable Presence

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

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

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

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

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

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

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

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

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

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

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

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

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

4.5.3.  Server Processing of Inbound Unavailable Presence

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

   [ ... to resource1 ... ]

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

   [ ... to resource2 ... ]

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

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

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4.5.4.  Client Processing of Unavailable Presence

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

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

4.6.  Directed Presence

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

4.6.1.  General Considerations

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

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

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

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

4.6.2.  Client Generation of Directed Presence

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

4.6.3.  Server Processing of Outbound Directed Presence

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

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

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

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

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4.6.4.  Server Processing of Inbound Directed Presence

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

4.6.5.  Client Processing of Inbound Directed Presence

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

4.6.6.  Server Processing of Presence Probes

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

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

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4.7.  Presence Syntax

4.7.1.  Type Attribute

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

4.7.2.  Child Elements

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

4.7.2.1.  Show Element

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

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

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

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

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

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

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

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4.7.2.2.  Status Element

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

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

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

   <presence from='romeo@example.net/orchard'
             id='jx62vs97'
             xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
     <status xml:lang='cs'>Dvo&#x0159;&#x00ED;m se Julii</status>
   </presence>

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

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

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

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

4.7.2.3.  Priority Element

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

   <presence xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
     <status xml:lang='cs'>Dvo&#x0159;&#x00ED;m se Julii</status>
     <priority>1</priority>
   </presence>

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

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

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

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4.7.3.  Extended Content

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

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

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

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

5.  Exchanging Messages

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

5.1.  One-to-One Chat Sessions

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

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

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

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

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

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

5.2.  Message Syntax

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

5.2.1.  To Attribute

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

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

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

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

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

5.2.2.  Type Attribute

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

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

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

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

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

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

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

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

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

5.2.3.  Body Element

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

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

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

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   <message
       from='juliet@example.com/balcony'
       id='z94nb37h'
       to='romeo@example.net'
       type='chat'
       xml:lang='en'>
     <body>Wherefore art thou, Romeo?</body>
     <body xml:lang='cs'>
        Pro&#x010D;e&#x017D; jsi ty, Romeo?
      </body>
   </message>

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

5.2.4.  Subject Element

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

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

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

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   <message
       from='juliet@example.com/balcony'
       id='jk3v47gw'
       to='romeo@example.net'
       type='chat'
       xml:lang='en'>
     <subject>I implore you!</subject>
     <subject xml:lang='cs'>
       &#x00DA;p&#x011B;nliv&#x011B; pros&#x00ED;m!
     </subject>
     <body>Wherefore art thou, Romeo?</body>
     <body xml:lang='cs'>
        Pro&#x010D;e&#x017E; jsi ty, Romeo?
      </body>
   </message>

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

5.2.5.  Thread Element

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

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

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

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

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

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

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

   <message
       from='juliet@example.com/balcony'
       to='romeo@example.net'
       type='chat'
       xml:lang='en'>
     <subject>I implore you!</subject>
     <subject xml:lang='cs'>
       &#x00DA;p&#x011B;nliv&#x011B; pros&#x00ED;m!
     </subject>
     <body>Wherefore art thou, Romeo?</body>
     <body xml:lang='cs'>
        Pro&#x010D;e&#x017E; jsi ty, Romeo?
     </body>
     <thread parent='e0ffe42b28561960c6b12b944a092794b9683a38'>
       0e3141cd80894871a68e6fe6b1ec56fa
     </thread>
   </message>

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

5.3.  Extended Content

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

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

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

6.  Exchanging IQ Stanzas

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


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