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RFC 3921

 
 
 

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

Part 2 of 4, p. 16 to 44
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5.  Exchanging Presence Information

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

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

5.1.  Client and Server Presence Responsibilities

5.1.1.  Initial Presence

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

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

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

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

   2.  Broadcast initial presence from the full JID (e.g.,
       <user@example.com/resource>) of the user to all contacts that are
       subscribed to the user's presence information; such contacts are
       those for which a JID is present in the user's roster with the
       'subscription' attribute set to a value of "from" or "both".
       (Note: The user's server MUST NOT broadcast initial presence to
       contacts to which the user is blocking outbound presence
       notifications, as described under Blocking Outbound Presence
       Notifications (Section 10.11).)

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

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

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

5.1.2.  Presence Broadcast

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

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

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

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

5.1.3.  Presence Probes

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

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

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

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

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

   3.  Else, if the contact has no available resources, the server MUST
       either (1) reply to the presence probe by sending to the user the
       full XML of the last presence stanza of type "unavailable"
       received by the server from the contact, or (2) not reply at all.

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   4.  Else, if the contact has at least one available resource, the
       server MUST reply to the presence probe by sending to the user
       the full XML of the last presence stanza with no 'to' attribute
       received by the server from each of the contact's available
       resources (again, subject to privacy lists in force for each
       session).

5.1.4.  Directed Presence

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

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

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

   3.  If the user sends directed presence without first sending initial
       presence or after having sent unavailable presence broadcast
       (i.e., the resource is active but not available), the user's
       server MUST treat the entities to which the user sends directed
       presence in the same way that it treats the entities listed in
       case #2 above.

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5.1.5.  Unavailable Presence

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

5.1.6.  Presence Subscriptions

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

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

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   has not provided initial presence, the server MUST NOT consider it to
   be available and therefore MUST NOT send subscription requests to
   it.)   However, if the user receives a presence stanza of type
   "subscribe" from a contact to whom the user has already granted
   permission to see the user's presence information (e.g., in cases
   when the contact is seeking to resynchronize subscription states),
   the user's server SHOULD auto-reply on behalf of the user.  In
   addition, the user's server MAY choose to re-send an unapproved
   pending subscription request to the contact based on an
   implementation-specific algorithm (e.g., whenever a new resource
   becomes available for the user, or after a certain amount of time has
   elapsed); this helps to recover from transient, silent errors that
   may have occurred in relation to the original subscription request.

5.2.  Specifying Availability Status

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

   Example: Availability status:

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

5.3.  Specifying Detailed Status Information

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

   Example: Detailed status information:

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

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5.4.  Specifying Presence Priority

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

   Example: Presence priority:

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

5.5.  Presence Examples

   The examples in this section illustrate the presence-related
   protocols described above.  The user is romeo@example.net, he has an
   available resource whose resource identifier is "orchard", and he has
   the following individuals in his roster:

   o  juliet@example.com (subscription="both" and she has two available
      resources, one whose resource is "chamber" and another whose
      resource is "balcony")

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

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

   Example 1: User sends initial presence:

   <presence/>

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

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

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

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

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

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

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

   <presence
       from='juliet@example.com/balcony'
       to='romeo@example.net/orchard'
       xml:lang='en'>
     <show>away</show>
     <status>be right back</status>
     <priority>0</priority>
   </presence>

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

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

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

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

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

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

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

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

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

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

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

   <presence
       from='romeo@example.net/orchard'
       to='juliet@example.com'
       xml:lang='en'>
     <show>away</show>
     <status>I shall return!</status>
     <priority>1</priority>
   </presence>

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

   [to "balcony" resource...]
   <presence
       from='romeo@example.net/orchard'
       to='juliet@example.com'
       xml:lang='en'>
     <show>away</show>
     <status>I shall return!</status>
     <priority>1</priority>
   </presence>

   [to "chamber" resource...]
   <presence
       from='romeo@example.net/orchard'
       to='juliet@example.com'
       xml:lang='en'>
     <show>away</show>
     <status>I shall return!</status>
     <priority>1</priority>
   </presence>

   Example 10: One of the contact's resources broadcasts final presence:

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

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

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

   Example 12: User sends final presence:

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

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

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

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

6.  Managing Subscriptions

   In order to protect the privacy of instant messaging users and any
   other entities, presence and availability information is disclosed
   only to other entities that the user has approved.  When a user has
   agreed that another entity may view its presence, the entity is said
   to have a subscription to the user's presence information.  A
   subscription lasts across sessions; indeed, it lasts until the
   subscriber unsubscribes or the subscribee cancels the
   previously-granted subscription.  Subscriptions are managed within
   XMPP by sending presence stanzas containing specially-defined
   attributes.

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

6.1.  Requesting a Subscription

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

   Example: Sending a subscription request:

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

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

6.2.  Handling a Subscription Request

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

   Example: Approving a subscription request:

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

   Example: Refusing a presence subscription request:

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

6.3.  Cancelling a Subscription from Another Entity

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

   Example: Cancelling a previously granted subscription request:

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

6.4.  Unsubscribing from Another Entity's Presence

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

   Example: Unsubscribing from an entity's presence:

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

7.  Roster Management

   In XMPP, one's contact list is called a roster, which consists of any
   number of specific roster items, each roster item being identified by
   a unique JID (usually of the form <contact@domain>).  A user's roster
   is stored by the user's server on the user's behalf so that the user
   may access roster information from any resource.

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

7.1.  Syntax and Semantics

   Rosters are managed using IQ stanzas, specifically by means of a
   <query/> child element qualified by the 'jabber:iq:roster' namespace.
   The <query/> element MAY contain one or more <item/> children, each
   describing a unique roster item or "contact".

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

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

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

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

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

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

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

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

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7.2.  Business Rules

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

7.3.  Retrieving One's Roster on Login

   Upon connecting to the server and becoming an active resource, a
   client SHOULD request the roster before sending initial presence
   (however, because receiving the roster may not be desirable for all
   resources, e.g., a connection with limited bandwidth, the client's
   request for the roster is OPTIONAL).  If an available resource does
   not request the roster during a session, the server MUST NOT send it
   presence subscriptions and associated roster updates.

   Example: Client requests current roster from server:

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

   Example: Client receives roster from server:

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

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   </iq>

7.4.  Adding a Roster Item

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

   Example: Client adds a new item:

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

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

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

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

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

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

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

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

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

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

7.5.  Updating a Roster Item

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

   Example: User updates roster item (added group):

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

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

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7.6.  Deleting a Roster Item

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

   Example: Client removes an item:

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

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

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

8.  Integration of Roster Items and Presence Subscriptions

8.1.  Overview

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

   There are four primary subscription states:

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

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

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

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

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

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

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

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

8.2.  User Subscribes to Contact

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

8.2.1.  Alternate Flow: Contact Declines Subscription Request

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

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

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

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

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

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

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

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

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

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

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

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

8.3.  Creating a Mutual Subscription

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

8.3.1.  Alternate Flow: User Declines Subscription Request

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

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

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

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

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

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

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

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

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

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


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