RFC 6121
Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence
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
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.
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'/>
[ ... 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.
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.
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
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
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.
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'/>
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
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>
[ ... 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]).
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>
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.
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
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.
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).
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.
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).
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'>Dvořím 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>
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'>Dvořím 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.
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
<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.
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:
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".
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]).
<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'>
PročeŽ 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]).
<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'>
Úpěnlivě prosím!
</subject>
<body>Wherefore art thou, Romeo?</body>
<body xml:lang='cs'>
Pročež 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.
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'>
Úpěnlivě prosím!
</subject>
<body>Wherefore art thou, Romeo?</body>
<body xml:lang='cs'>
Pročež jsi ty, Romeo?
</body>
<thread parent='e0ffe42b28561960c6b12b944a092794b9683a38'>
0e3141cd80894871a68e6fe6b1ec56fa
</thread>
</message>
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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