Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence
Network Working Group P. Saint-Andre, Ed.
Request for Comments: 3921 Jabber Software Foundation
Category: Standards Track October 2004
Extensible Messaging and Presence Protocol (XMPP):
Instant Messaging and Presence
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This memo describes extensions to and applications of the core
features of the Extensible Messaging and Presence Protocol (XMPP)
that provide the basic instant messaging (IM) and presence
functionality defined in RFC 2779.
Saint-Andre Standards Track [Page 1]
RFC 3921 XMPP IM October 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Syntax of XML Stanzas . . . . . . . . . . . . . . . . . . . 4
3. Session Establishment . . . . . . . . . . . . . . . . . . . 10
4. Exchanging Messages . . . . . . . . . . . . . . . . . . . . 13
5. Exchanging Presence Information . . . . . . . . . . . . . . 16
6. Managing Subscriptions . . . . . . . . . . . . . . . . . . . 26
7. Roster Management . . . . . . . . . . . . . . . . . . . . . 27
8. Integration of Roster Items and Presence Subscriptions . . . 32
9. Subscription States . . . . . . . . . . . . . . . . . . . . 56
10. Blocking Communication . . . . . . . . . . . . . . . . . . . 62
11. Server Rules for Handling XML Stanzas . . . . . . . . . . . 85
12. IM and Presence Compliance Requirements . . . . . . . . . . 88
13. Internationalization Considerations . . . . . . . . . . . . 89
14. Security Considerations . . . . . . . . . . . . . . . . . . 89
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 90
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 91
A. vCards . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
B. XML Schemas. . . . . . . . . . . . . . . . . . . . . . . . . 93
C. Differences Between Jabber IM/Presence Protocols and XMPP. . 105
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 106
Author's Address. . . . . . . . . . . . . . . . . . . . . . . . . 106
Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 107
1. Introduction
1.1. Overview
The Extensible Messaging and Presence Protocol (XMPP) is a protocol
for streaming XML [XML] elements in order to exchange messages and
presence information in close to real time. The core features of
XMPP are defined in Extensible Messaging and Presence Protocol
(XMPP): Core [XMPP-CORE]. These features -- mainly XML streams, use
of TLS and SASL, and the , , and children
of the stream root -- provide the building blocks for many types of
near-real-time applications, which may be layered on top of the core
by sending application-specific data qualified by particular XML
namespaces [XML-NAMES]. This memo describes extensions to and
applications of the core features of XMPP that provide the basic
functionality expected of an instant messaging (IM) and presence
application as defined in RFC 2779 [IMP-REQS].
Saint-Andre Standards Track [Page 2]
RFC 3921 XMPP IM October 2004
1.2. Requirements
For the purposes of this memo, the requirements of a basic instant
messaging and presence application are defined by [IMP-REQS], which
at a high level stipulates that a user must be able to complete the
following use cases:
o Exchange messages with other users
o Exchange presence information with other users
o Manage subscriptions to and from other users
o Manage items in a contact list (in XMPP this is called a "roster")
o Block communications to or from specific other users
Detailed definitions of these functionality areas are contained in
[IMP-REQS], and the interested reader is directed to that document
regarding the requirements addressed herein.
[IMP-REQS] also stipulates that presence services must be separable
from instant messaging services; i.e., it must be possible to use the
protocol to provide a presence service, an instant messaging service,
or both. Although the text of this memo assumes that implementations
and deployments will want to offer a unified instant messaging and
presence service, there is no requirement that a service must offer
both a presence service and an instant messaging service, and the
protocol makes it possible to offer separate and distinct services
for presence and for instant messaging.
Note: While XMPP-based instant messaging and presence meets the
requirements of [IMP-REQS], it was not designed explicitly with that
specification in mind, since the base protocol evolved through an
open development process within the Jabber open-source community
before RFC 2779 was written. Note also that although protocols
addressing many other functionality areas have been defined in the
Jabber community, such protocols are not included in this memo
because they are not required by [IMP-REQS].
1.3. Terminology
This memo inherits the terminology defined in [XMPP-CORE].
The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14, RFC 2119 [TERMS].
Saint-Andre Standards Track [Page 3]
RFC 3921 XMPP IM October 2004
2. Syntax of XML Stanzas
The basic semantics and common attributes of XML stanzas qualified by
the 'jabber:client' and 'jabber:server' namespaces are defined in
[XMPP-CORE]. However, these namespaces also define various child
elements, as well as values for the common 'type' attribute, that are
specific to instant messaging and presence applications. Thus,
before addressing particular "use cases" for such applications, we
here further describe the syntax of XML stanzas, thereby
supplementing the discussion in [XMPP-CORE].
2.1. Message Syntax
Message stanzas qualified by the 'jabber:client' or 'jabber:server'
namespace are used to "push" information to another entity. Common
uses in instant messaging applications include single messages,
messages sent in the context of a chat conversation, messages sent in
the context of a multi-user chat room, headlines and other alerts,
and errors.
2.1.1. Types of Message
The 'type' attribute of a message stanza is RECOMMENDED; if included,
it specifies the conversational context of the message, thus
providing a hint regarding presentation (e.g., in a GUI). If
included, the 'type' attribute MUST have one of the following values:
o chat -- The message is sent in the context of a one-to-one chat
conversation. A compliant client SHOULD present the message in an
interface enabling one-to-one chat between the two parties,
including an appropriate conversation history.
o error -- An error has occurred related to a previous message sent
by the sender (for details regarding stanza error syntax, refer to
[XMPP-CORE]). A compliant client SHOULD present an appropriate
interface informing the sender of the nature of the error.
o groupchat -- The message is sent in the context of a multi-user
chat environment (similar to that of [IRC]). A compliant client
SHOULD present the message in an interface enabling many-to-many
chat between the parties, including a roster of parties in the
chatroom and an appropriate conversation history. Full definition
of XMPP-based groupchat protocols is out of scope for this memo.
o headline -- The message is probably generated by an automated
service that delivers or broadcasts content (news, sports, market
information, RSS feeds, etc.). No reply to the message is
expected, and a compliant client SHOULD present the message in an
Saint-Andre Standards Track [Page 4]
RFC 3921 XMPP IM October 2004
interface that appropriately differentiates the message from
standalone messages, chat sessions, or groupchat sessions (e.g.,
by not providing the recipient with the ability to reply).
o normal -- The message is a single message that is sent outside the
context of a one-to-one conversation or groupchat, and to which it
is expected that the recipient will reply. A compliant client
SHOULD present the message in an interface enabling the recipient
to reply, but without a conversation history.
An IM application SHOULD support all of the foregoing message types;
if an application receives a message with no 'type' attribute or the
application does not understand the value of the 'type' attribute
provided, it MUST consider the message to be of type "normal" (i.e.,
"normal" is the default). The "error" type MUST be generated only in
response to an error related to a message received from another
entity.
Although the 'type' attribute is OPTIONAL, it is considered polite to
mirror the type in any replies to a message; furthermore, some
specialized applications (e.g., a multi-user chat service) MAY at
their discretion enforce the use of a particular message type (e.g.,
type='groupchat').
2.1.2. Child Elements
As described under extended namespaces (Section 2.4), a message
stanza MAY contain any properly-namespaced child element.
In accordance with the default namespace declaration, by default a
message stanza is qualified by the 'jabber:client' or 'jabber:server'
namespace, which defines certain allowable children of message
stanzas. If the message stanza is of type "error", it MUST include
an child; for details, see [XMPP-CORE]. Otherwise, the
message stanza MAY contain any of the following child elements
without an explicit namespace declaration:
1.
2.
3.
2.1.2.1. Subject
The element contains human-readable XML character data
that specifies the topic of the message. The element MUST
NOT possess any attributes, with the exception of the 'xml:lang'
attribute. Multiple instances of the element MAY be
included for the purpose of providing alternate versions of the same
Saint-Andre Standards Track [Page 5]
RFC 3921 XMPP IM October 2004
subject, but only if each instance possesses an 'xml:lang' attribute
with a distinct language value. The element MUST NOT
contain mixed content (as defined in Section 3.2.2 of [XML]).
2.1.2.2. Body
The
element contains human-readable XML character data that
specifies the textual contents of the message; this child element is
normally included but is OPTIONAL. The
element MUST NOT
possess any attributes, with the exception of the 'xml:lang'
attribute. Multiple instances of the
element MAY be included
but only if each instance possesses an 'xml:lang' attribute with a
distinct language value. The
element MUST NOT contain mixed
content (as defined in Section 3.2.2 of [XML]).
2.1.2.3. Thread
The element contains non-human-readable XML character data
specifying an identifier that is used for tracking a conversation
thread (sometimes referred to as an "instant messaging session")
between two entities. The value of the element is
generated by the sender and SHOULD be copied back in any replies. If
used, it MUST be unique to that conversation thread within the stream
and MUST be consistent throughout that conversation (a client that
receives a message from the same full JID but with a different thread
ID MUST assume that the message in question exists outside the
context of the existing conversation thread). The use of the
element is OPTIONAL and is not used to identify individual
messages, only conversations. A message stanza MUST NOT contain more
than one element. The element MUST NOT possess
any attributes. The value of the element MUST be treated
as opaque by entities; no semantic meaning may be derived from it,
and only exact comparisons may be made against it. The
element MUST NOT contain mixed content (as defined in Section 3.2.2
of [XML]).
2.2. Presence Syntax
Presence stanzas are used qualified by the 'jabber:client' or
'jabber:server' namespace to express an entity's current network
availability (offline or online, along with various sub-states of the
latter and optional user-defined descriptive text), and to notify
other entities of that availability. Presence stanzas are also used
to negotiate and manage subscriptions to the presence of other
entities.
Saint-Andre Standards Track [Page 6]
RFC 3921 XMPP IM October 2004
2.2.1. Types of Presence
The 'type' attribute of a presence stanza is OPTIONAL. A presence
stanza that does not possess a 'type' attribute is used to signal to
the server that the sender is online and available for communication.
If included, the 'type' attribute specifies a lack of availability, a
request to manage a subscription to another entity's presence, a
request for another entity's current presence, or an error related to
a previously-sent presence stanza. If included, the 'type' attribute
MUST have one of the following values:
o unavailable -- Signals that the entity is no longer available for
communication.
o subscribe -- The sender wishes to subscribe to the recipient's
presence.
o subscribed -- The sender has allowed the recipient to receive
their presence.
o unsubscribe -- The sender is unsubscribing from another entity's
presence.
o unsubscribed -- The subscription request has been denied or a
previously-granted subscription has been cancelled.
o probe -- A request for an entity's current presence; SHOULD be
generated only by a server on behalf of a user.
o error -- An error has occurred regarding processing or delivery of
a previously-sent presence stanza.
For detailed information regarding presence semantics and the
subscription model used in the context of XMPP-based instant
messaging and presence applications, refer to Exchanging Presence
Information (Section 5) and Managing Subscriptions (Section 6).
2.2.2. Child Elements
As described under extended namespaces (Section 2.4), a presence
stanza MAY contain any properly-namespaced child element.
In accordance with the default namespace declaration, by default a
presence stanza is qualified by the 'jabber:client' or
'jabber:server' namespace, which defines certain allowable children
of presence stanzas. If the presence stanza is of type "error", it
MUST include an child; for details, see [XMPP-CORE]. If the
presence stanza possesses no 'type' attribute, it MAY contain any of
Saint-Andre Standards Track [Page 7]
RFC 3921 XMPP IM October 2004
the following child elements (note that the child MAY be
sent in a presence stanza of type "unavailable" or, for historical
reasons, "subscribe"):
1.
2.
3.
2.2.2.1. Show
The OPTIONAL element contains non-human-readable XML
character data that specifies the particular availability status of
an entity or specific resource. A presence stanza MUST NOT contain
more than one element. The element MUST NOT possess
any attributes. If provided, the XML character data value MUST be
one of the following (additional availability types could be defined
through a properly-namespaced child element of the presence stanza):
o away -- The entity or resource is temporarily away.
o chat -- The entity or resource is actively interested in chatting.
o dnd -- The entity or resource is busy (dnd = "Do Not Disturb").
o xa -- The entity or resource is away for an extended period (xa =
"eXtended Away").
If no element is provided, the entity is assumed to be online
and available.
2.2.2.2. Status
The OPTIONAL element contains XML character data specifying
a natural-language description of availability status. It is
normally used in conjunction with the show element to provide a
detailed description of an availability state (e.g., "In a meeting").
The element MUST NOT possess any attributes, with the
exception of the 'xml:lang' attribute. Multiple instances of the
element MAY be included but only if each instance possesses
an 'xml:lang' attribute with a distinct language value.
2.2.2.3. Priority
The OPTIONAL 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 element. The
element MUST NOT possess any attributes. If no priority is provided,
Saint-Andre Standards Track [Page 8]
RFC 3921 XMPP IM October 2004
a server SHOULD consider the priority to be zero. For information
regarding the semantics of priority values in stanza routing within
instant messaging and presence applications, refer to Server Rules
for Handling XML Stanzas (Section 11).
2.3. IQ Syntax
IQ stanzas provide a structured request-response mechanism. The
basic semantics of that mechanism (e.g., that the 'id' attribute is
REQUIRED) are defined in [XMPP-CORE], whereas the specific semantics
required to complete particular use cases are defined in all cases by
an extended namespace (Section 2.4) (note that the 'jabber:client'
and 'jabber:server' namespaces do not define any children of IQ
stanzas other than the common ). This memo defines two such
extended namespaces, one for Roster Management (Section 7) and the
other for Blocking Communication (Section 10); however, an IQ stanza
MAY contain structured information qualified by any extended
namespace.
2.4. Extended Namespaces
While the three XML stanza kinds defined in the "jabber:client" or
"jabber:server" namespace (along with their attributes and child
elements) provide a basic level of functionality for messaging and
presence, XMPP uses XML namespaces to extend the stanzas for the
purpose of providing additional functionality. Thus a message or
presence stanza MAY contain one or more optional child elements
specifying content that extends the meaning of the message (e.g., an
XHTML-formatted version of the message body), and an IQ stanza MAY
contain one such child element. This child element MAY have any name
and MUST possess an 'xmlns' namespace declaration (other than
"jabber:client", "jabber:server", or
"http://etherx.jabber.org/streams") that defines all data contained
within the child element.
Support for any given extended namespace is OPTIONAL on the part of
any implementation (aside from the extended namespaces defined
herein). If an entity does not understand such a namespace, the
entity's expected behavior depends on whether the entity is (1) the
recipient or (2) an entity that is routing the stanza to the
recipient:
Recipient: If a recipient receives a stanza that contains a child
element it does not understand, it SHOULD ignore that specific XML
data, i.e., it SHOULD not process it or present it to a user or
associated application (if any). In particular:
Saint-Andre Standards Track [Page 9]
RFC 3921 XMPP IM October 2004
* If an entity receives a message or presence stanza that
contains XML data qualified by a namespace it does not
understand, the portion of the stanza that is in the unknown
namespace SHOULD be ignored.
* If an entity receives a message stanza whose only child element
is qualified by a namespace it does not understand, it MUST
ignore the entire stanza.
* If an entity receives an IQ stanza of type "get" or "set"
containing a child element qualified by a namespace it does not
understand, the entity SHOULD return an IQ stanza of type
"error" with an error condition of .
Router: If a routing entity (usually a server) handles a stanza that
contains a child element it does not understand, it SHOULD ignore
the associated XML data by passing it on untouched to the
recipient.
3. Session Establishment
Most instant messaging and presence applications based on XMPP are
implemented via a client-server architecture that requires a client
to establish a session on a server in order to engage in the expected
instant messaging and presence activities. However, there are
several pre-conditions that MUST be met before a client can establish
an instant messaging and presence session. These are:
1. Stream Authentication -- a client MUST complete stream
authentication as documented in [XMPP-CORE] before attempting to
establish a session or send any XML stanzas.
2. Resource Binding -- after completing stream authentication, a
client MUST bind a resource to the stream so that the client's
address is of the form , after which the
entity is now said to be a "connected resource" in the
terminology of [XMPP-CORE].
If a server supports sessions, it MUST include a element
qualified by the 'urn:ietf:params:xml:ns:xmpp-session' namespace in
the stream features it advertises to a client after the completion of
stream authentication as defined in [XMPP-CORE]:
Saint-Andre Standards Track [Page 10]
RFC 3921 XMPP IM October 2004
Server advertises session establishment feature to client:
Upon being so informed that session establishment is required (and
after completing resource binding), the client MUST establish a
session if it desires to engage in instant messaging and presence
functionality; it completes this step by sending to the server an IQ
stanza of type "set" containing an empty child element
qualified by the 'urn:ietf:params:xml:ns:xmpp-session' namespace:
Step 1: Client requests session with server:
Step 2: Server informs client that session has been created:
Upon establishing a session, a connected resource (in the terminology
of [XMPP-CORE]) is said to be an "active resource".
Several error conditions are possible. For example, the server may
encounter an internal condition that prevents it from creating the
session, the username or authorization identity may lack permissions
to create a session, or there may already be an active resource
associated with a resource identifier of the same name.
If the server encounters an internal condition that prevents it from
creating the session, it MUST return an error.
Saint-Andre Standards Track [Page 11]
RFC 3921 XMPP IM October 2004
Step 2 (alt): Server responds with error (internal server error):
If the username or resource is not allowed to create a session, the
server MUST return an error (e.g., forbidden).
Step 2 (alt): Server responds with error (username or resource not
allowed to create session):
If there is already an active resource of the same name, the server
MUST either (1) terminate the active resource and allow the
newly-requested session, or (2) disallow the newly-requested session
and maintain the active resource. Which of these the server does is
up to the implementation, although it is RECOMMENDED to implement
case #1. In case #1, the server SHOULD send a stream
error to the active resource, terminate the XML stream and underlying
TCP connection for the active resource, and return a IQ stanza of
type "result" (indicating success) to the newly-requested session. In
case #2, the server SHOULD send a stanza error to the
newly-requested session but maintain the XML stream for that
connection so that the newly-requested session has an opportunity to
negotiate a non-conflicting resource identifier before sending
another request for session establishment.
Step 2 (alt): Server informs existing active resource of resource
conflict (case #1):
Saint-Andre Standards Track [Page 12]
RFC 3921 XMPP IM October 2004
Step 2 (alt): Server informs newly-requested session of resource
conflict (case #2):
After establishing a session, a client SHOULD send initial presence
and request its roster as described below, although these actions are
OPTIONAL.
Note: Before allowing the creation of instant messaging and presence
sessions, a server MAY require prior account provisioning. Possible
methods for account provisioning include account creation by a server
administrator as well as in-band account registration using the
'jabber:iq:register' namespace; the latter method is out of scope for
this memo, but is documented in [JEP-0077], published by the Jabber
Software Foundation [JSF].
4. Exchanging Messages
Exchanging messages is a basic use of XMPP and is brought about when
a user generates a message stanza that is addressed to another
entity. As defined under Server Rules for Handling XML Stanzas
(Section 11), the sender's server is responsible for delivering the
message to the intended recipient (if the recipient is on the same
server) or for routing the message to the recipient's server (if the
recipient is on a different server).
For information regarding the syntax of message stanzas as well as
their defined attributes and child elements, refer to Message Syntax
(Section 2.1).
4.1. Specifying an Intended Recipient
An instant messaging client SHOULD specify an intended recipient for
a message by providing the JID of an entity other than the sender in
the 'to' attribute of the stanza. If the message is being
sent in reply to a message previously received from an address of the
form (e.g., within the context of a chat
session), the value of the 'to' address SHOULD be of the form
rather than of the form unless
the sender has knowledge (via presence) that the intended recipient's
resource is no longer available. If the message is being sent
Saint-Andre Standards Track [Page 13]
RFC 3921 XMPP IM October 2004
outside the context of any existing chat session or received message,
the value of the 'to' address SHOULD be of the form
rather than of the form .
4.2. Specifying a Message Type
As noted, it is RECOMMENDED for a message stanza to possess a 'type'
attribute whose value captures the conversational context (if any) of
the message (see Type (Section 2.1.1)).
The following example shows a valid value of the 'type' attribute:
Example: A message of a defined type:
Wherefore art thou, Romeo?4.3. Specifying a Message Body A message
stanza MAY (and often will) contain a child element whose XML
character data specifies the primary meaning of the message (see
Body (Section 2.1.2.2)). Example: A message with a body: Wherefore
art thou, Romeo?PročeŽ jsi ty, Romeo?4.4. Specifying a
Message Subject A message stanza MAY contain one or more child
elements specifying the topic of the message (see Subject (Section
2.1.2.1)). Saint-Andre Standards Track [Page 14]
RFC 3921 XMPP IM
October 2004 Example: A message with a subject: I implore you!
Úpěnlivě prosim! Wherefore art thou,
Romeo?PročeŽ jsi ty, Romeo?4.5. Specifying a Conversation
Thread A message stanza MAY contain a child element specifying the
conversation thread in which the message is situated, for the
purpose of tracking the conversation (see Thread (Section
2.1.2.3)). Example: A threaded conversation: Art thou not Romeo,
and a Montague?e0ffe42b28561960c6b12b944a092794b9683a38 Neither,
fair saint, if either thee
dislike.e0ffe42b28561960c6b12b944a092794b9683a38 How cam'st thou
hither, tell me, and
wherefore?e0ffe42b28561960c6b12b944a092794b9683a38 Saint-Andre
Standards Track [Page 15]
RFC 3921 XMPP IM October 2004 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., ) 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 Saint-Andre Standards Track [Page
16]
RFC 3921 XMPP IM October 2004 inbound presence notifications,
as described under Blocking Inbound Presence Notifications (Section
10.10).) 2. Broadcast initial presence from the full JID (e.g., )
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 () 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 Saint-Andre Standards Track
[Page 17]
RFC 3921 XMPP IM October 2004 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 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 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. Saint-Andre
Standards Track [Page 18]
RFC 3921 XMPP IM October 2004 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. Saint-Andre
Standards Track [Page 19]
RFC 3921 XMPP IM October 2004 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 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 rather
than , 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 Saint-Andre Standards Track [Page 20]
RFC 3921 XMPP
IM October 2004 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 element (see Show (Section 2.2.2.1)). Example:
Availability status: dnd 5.3. Specifying Detailed Status
Information In conjunction with the element, a client MAY provide
detailed status information by using the element (see Status
(Section 2.2.2.2)). Example: Detailed status information: dnd
Wooing Juliet Ja dvořím Juliet Saint-Andre Standards Track
[Page 21]
RFC 3921 XMPP IM October 2004 5.4. Specifying Presence
Priority A client MAY provide a priority for its resource by using
the element (see Priority (Section 2.2.2.3)). Example: Presence
priority: dnd Wooing Juliet Ja dvořím Juliet 1 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: Example 2: User's server
sends presence probes to contacts with subscription="to" and
subscription="both" on behalf of the user's available resource:
Saint-Andre Standards Track [Page 22]
RFC 3921 XMPP IM October 2004
Example 3: User's server sends initial presence to contacts with
subscription="from" and subscription="both" on behalf of the user's
available resource: Example 4: Contacts' servers reply to presence
probe on behalf of all available resources: away be right back 0 1
dnd gallivanting Example 5: Contacts' servers deliver user's
initial presence to all available resources or return error to
user: Saint-Andre Standards Track [Page 23]
RFC 3921 XMPP IM
October 2004 Example 6: User sends directed presence to another
user not in his roster: dnd courting Juliet 0 Example 7: User sends
updated available presence information for broadcasting: away I
shall return! 1 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):
away I shall return! 1 Saint-Andre Standards Track [Page 24]
RFC 3921 XMPP IM October 2004 Example 9: Contact's server delivers
updated presence information to all of the contact's available
resources: [to "balcony" resource...] away I shall return! 1 [to
"chamber" resource...] away I shall return! 1 Example 10: One of
the contact's resources broadcasts final presence: Example 11:
Contact's server sends unavailable presence information to user:
Example 12: User sends final presence: gone home Saint-Andre
Standards Track [Page 25]
RFC 3921 XMPP IM October 2004 Example 13:
User's server broadcasts unavailable presence information to
contact as well as to the person to whom the user sent directed
presence: gone home gone home 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: Saint-Andre Standards Track [Page 26]
RFC 3921 XMPP IM October 2004 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: Example: Refusing a presence subscription request: 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: 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: 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 ). 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.
Saint-Andre Standards Track [Page 27]
RFC 3921 XMPP IM October 2004
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 child element qualified by the
'jabber:iq:roster' namespace. The element MAY contain one or more
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 element (which is
REQUIRED). The value of the 'jid' attribute SHOULD be of the form
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
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 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 element
MAY contain one or more child elements, for use in collecting
roster items into various categories. The XML character data of the
element is opaque. Saint-Andre Standards Track [Page 28]
RFC 3921
XMPP IM October 2004 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 ) or
full JID (of the form ); 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: Example: Client
receives roster from server: Friends Friends Friends Saint-Andre
Standards Track [Page 29]
RFC 3921 XMPP IM October 2004 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: Servants 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:
Servants Servants Saint-Andre Standards Track [Page 30]
RFC 3921
XMPP IM October 2004 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:
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): Friends Lovers 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. Saint-Andre Standards Track [Page
31]
RFC 3921 XMPP IM October 2004 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: 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 Saint-Andre Standards Track
[Page 32]
RFC 3921 XMPP IM October 2004 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 element qualified
by the 'jabber:iq:roster' namespace, which in turn contains an
element that defines the new roster item; the element MUST possess
a 'jid' attribute, MAY possess a 'name' attribute, MUST NOT possess
a 'subscription' attribute, and MAY contain one or more child
elements: Saint-Andre Standards Track [Page 33]
RFC 3921 XMPP IM
October 2004 MyBuddies 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: MyBuddies 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: 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: Saint-Andre Standards Track [Page 34]
RFC 3921 XMPP IM
October 2004 MyBuddies 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 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., ) 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): 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 Saint-Andre
Standards Track [Page 35]
RFC 3921 XMPP IM October 2004 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. SomeGroup 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 () of the contact; and
(4) MUST send available presence from all of the contact's
available resources to the user: Saint-Andre Standards Track [Page
36]
RFC 3921 XMPP IM October 2004 SomeGroup 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 Saint-Andre Standards
Track [Page 37]
RFC 3921 XMPP IM October 2004 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: MyBuddies 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. Saint-Andre Standards
Track [Page 38]
RFC 3921 XMPP IM October 2004 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): 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 () of the contact: 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: MyBuddies 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 Saint-Andre Standards Track [Page 39]
RFC 3921 XMPP
IM October 2004 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): 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 () of the contact: SomeGroup Saint-Andre Standards Track
[Page 40]
RFC 3921 XMPP IM October 2004 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. 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 () 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): Saint-Andre
Standards Track [Page 41]
RFC 3921 XMPP IM October 2004 MyBuddies
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: Saint-Andre Standards Track [Page 42]
RFC 3921 XMPP IM
October 2004 SomeGroup 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):
Saint-Andre Standards Track [Page 43]
RFC 3921 XMPP IM October 2004
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 () of the user: 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: SomeGroup 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".
Saint-Andre Standards Track [Page 44]
RFC 3921 XMPP IM October 2004
8.4. Unsubscribing At any time after subscribing to a contact's
presence information, a user MAY unsubscribe. While the XML that
the user sends to make this happen is the same in all instances,
the subsequent subscription state is different depending on the
subscription state obtaining when the unsubscribe "command" is
sent. Both possible scenarios are described below. 8.4.1. Case #1:
Unsubscribing When Subscription is Not Mutual In the first case,
the user has a subscription to the contact's presence information
but the contact does not have a subscription to the user's presence
information (i.e., the subscription is not yet mutual). 1. If the
user wants to unsubscribe from the contact's presence information,
the user MUST send a presence stanza of type "unsubscribe" to the
contact: 2. As a result, the user's server (1) MUST send a roster
push to all of the user's available resources that have requested
the roster, containing an updated roster item for the contact with
the 'subscription' attribute set to a value of "none"; and (2) MUST
route the presence stanza of type "unsubscribe" to the contact,
first stamping the 'from' address as the bare JID () of the user:
MyBuddies Saint-Andre Standards Track [Page 45]
RFC 3921 XMPP IM
October 2004 3. Upon receiving the presence stanza of type
"unsubscribe" addressed to the contact, the contact's server (1)
MUST initiate a roster push to all available resources associated
with the contact that have requested the roster, containing an
updated roster item for the user with the 'subscription' attribute
set to a value of "none" (if the contact is unavailable or has not
requested the roster, the contact's server MUST modify the roster
item and send that modified item the next time the contact requests
the roster); and (2) MUST deliver the "unsubscribe" state change
notification to the contact: SomeGroup 4. Upon receiving the
presence stanza of type "unsubscribe", the contact SHOULD
acknowledge receipt of that subscription state notification through
either "affirming" it by sending a presence stanza of type
"unsubscribed" to the user or "denying" it by sending a presence
stanza of type "subscribed" to the user; this step does not
necessarily affect the subscription state (see Subscription States
(Section 9) for details), but instead lets the contact's server
know that it MUST no longer send notification of the subscription
state change to the contact (see Section 9.4). 5. The contact's
server then (1) MUST send a presence stanza of type "unsubscribed"
to the user; and (2) SHOULD send unavailable presence from all of
the contact's available resources to the user: Saint-Andre
Standards Track [Page 46]
RFC 3921 XMPP IM October 2004 6. When the
user's server receives the presence stanzas of type "unsubscribed"
and "unavailable", it MUST deliver them to the user: 7. Upon
receiving the presence stanza of type "unsubscribed", the user
SHOULD acknowledge receipt of that subscription state notification
through either "affirming" it by sending a presence stanza of type
"unsubscribe" to the contact or "denying" it by sending a presence
stanza of type "subscribe" to the contact; this step does not
necessarily affect the subscription state (see Subscription States
(Section 9) for details), but instead lets the user's server know
that it MUST no longer send notification of the subscription state
change to the user (see Section 9.4). 8.4.2. Case #2: Unsubscribing
When Subscription is Mutual In the second case, the user has a
subscription to the contact's presence information and the contact
also has a subscription to the user's presence information (i.e.,
the subscription is mutual). 1. If the user wants to unsubscribe
from the contact's presence information, the user MUST send a
presence stanza of type "unsubscribe" to the contact: 2. As a
result, the user's server (1) MUST send a roster push to all of the
user's available resources that have requested the roster,
containing an updated roster item for the contact with the
'subscription' attribute set to a value of "from"; and (2) MUST
route the presence stanza of type "unsubscribe" to the contact,
first stamping the 'from' address as the bare JID () of the user:
Saint-Andre Standards Track [Page 47]
RFC 3921 XMPP IM October 2004
MyBuddies 3. Upon receiving the presence stanza of type
"unsubscribe" addressed to the contact, the contact's server (1)
MUST initiate a roster push to all available resources associated
with the contact that have requested the roster, containing an
updated roster item for the user with the 'subscription' attribute
set to a value of "to" (if the contact is unavailable or has not
requested the roster, the contact's server MUST modify the roster
item and send that modified item the next time the contact requests
the roster); and (2) MUST deliver the "unsubscribe" state change
notification to the contact: SomeGroup 4. Upon receiving the
presence stanza of type "unsubscribe", the contact SHOULD
acknowledge receipt of that subscription state notification through
either "affirming" it by sending a presence stanza of type
"unsubscribed" to the user or "denying" it by sending a presence
stanza of type "subscribed" to the user; this Saint-Andre Standards
Track [Page 48]
RFC 3921 XMPP IM October 2004 step does not
necessarily affect the subscription state (see Subscription States
(Section 9) for details), but instead lets the contact's server
know that it MUST no longer send notification of the subscription
state change to the contact (see Section 9.4). 5. The contact's
server then (1) MUST send a presence stanza of type "unsubscribed"
to the user; and (2) SHOULD send unavailable presence from all of
the contact's available resources to the user: 6. When the user's
server receives the presence stanzas of type "unsubscribed" and
"unavailable", it MUST deliver them to the user: 7. Upon receiving
the presence stanza of type "unsubscribed", the user SHOULD
acknowledge receipt of that subscription state notification through
either "affirming" it by sending a presence stanza of type
"unsubscribe" to the contact or "denying" it by sending a presence
stanza of type "subscribe" to the contact; this step does not
necessarily affect the subscription state (see Subscription States
(Section 9) for details), but instead lets the user's server know
that it MUST no longer send notification of the subscription state
change to the user (see Section 9.4). Note: Obviously this does not
result in removal of the roster item from the user's roster, and
the contact still has a subscription to the user's presence
information. In order to both completely cancel Saint-Andre
Standards Track [Page 49]
RFC 3921 XMPP IM October 2004 a mutual
subscription and fully remove the roster item from the user's
roster, the user SHOULD update the roster item with
subscription='remove' as defined under Removing a Roster Item and
Cancelling All Subscriptions (Section 8.6). 8.5. Cancelling a
Subscription At any time after approving a subscription request
from a user, a contact MAY cancel that subscription. While the XML
that the contact sends to make this happen is the same in all
instances, the subsequent subscription state is different depending
on the subscription state obtaining when the cancellation was sent.
Both possible scenarios are described below. 8.5.1. Case #1:
Cancelling When Subscription is Not Mutual In the first case, the
user has a subscription to the contact's presence information but
the contact does not have a subscription to the user's presence
information (i.e., the subscription is not yet mutual). 1. If the
contact wants to cancel the user's subscription, the contact MUST
send a presence stanza of type "unsubscribed" to the user: 2. As a
result, the contact's server (1) MUST send a roster push to all of
the contact's available resources that have requested the roster,
containing an updated roster item for the user with the
'subscription' attribute set to a value of "none"; (2) MUST route
the presence stanza of type "unsubscribed" to the user, first
stamping the 'from' address as the bare JID () of the contact; and
(3) SHOULD send unavailable presence from all of the contact's
available resources to the user: SomeGroup Saint-Andre Standards
Track [Page 50]
RFC 3921 XMPP IM October 2004 3. Upon receiving the
presence stanza of type "unsubscribed" addressed to the user, the
user's server (1) MUST initiate a roster push to all of the user's
available resources that have requested the roster, containing an
updated roster item for the contact with the 'subscription'
attribute set to a value of "none" (if the user is unavailable or
has not requested the roster, the user's server MUST modify the
roster item and send that modified item the next time the user
requests the roster); (2) MUST deliver the "unsubscribed" state
change notification to all of the user's available resources; and
(3) MUST deliver the unavailable presence to all of the user's
available resources: MyBuddies 4. Upon receiving the presence
stanza of type "unsubscribed", the user SHOULD acknowledge receipt
of that subscription state notification through either "affirming"
it by sending a presence stanza of type "unsubscribe" to the
contact or "denying" it by sending a presence stanza of type
"subscribe" to the contact; Saint-Andre Standards Track [Page 51]
RFC 3921 XMPP IM October 2004 this step does not necessarily affect
the subscription state (see Subscription States (Section 9) for
details), but instead lets the user's server know that it MUST no
longer send notification of the subscription state change to the
user (see Section 9.4). 8.5.2. Case #2: Cancelling When
Subscription is Mutual In the second case, the user has a
subscription to the contact's presence information and the contact
also has a subscription to the user's presence information (i.e.,
the subscription is mutual). 1. If the contact wants to cancel the
user's subscription, the contact MUST send a presence stanza of
type "unsubscribed" to the user: 2. As a result, the contact's
server (1) MUST send a roster push to all of the contact's
available resources that have requested the roster, containing an
updated roster item for the user with the 'subscription' attribute
set to a value of "to"; (2) MUST route the presence stanza of type
"unsubscribed" to the user, first stamping the 'from' address as
the bare JID () of the contact; and (3) SHOULD send unavailable
presence from all of the contact's available resources to all of
the user's available resources: SomeGroup Saint-Andre Standards
Track [Page 52]
RFC 3921 XMPP IM October 2004 3. Upon receiving the
presence stanza of type "unsubscribed" addressed to the user, the
user's server (1) MUST initiate a roster push to all of the user's
available resources that have requested the roster, containing an
updated roster item for the contact with the 'subscription'
attribute set to a value of "from" (if the user is unavailable or
has not requested the roster, the user's server MUST modify the
roster item and send that modified item the next time the user
requests the roster); and (2) MUST deliver the "unsubscribed" state
change notification to all of the user's available resources; and
(3) MUST deliver the unavailable presence to all of the user's
available resources: MyBuddies 4. Upon receiving the presence
stanza of type "unsubscribed", the user SHOULD acknowledge receipt
of that subscription state notification through either "affirming"
it by sending a presence stanza of type "unsubscribe" to the
contact or "denying" it by sending a presence stanza of type
"subscribe" to the contact; this step does not necessarily affect
the subscription state (see Subscription States (Section 9) for
details), but instead lets the user's server know that it MUST no
longer send notification of the subscription state change to the
user (see Section 9.4). Note: Obviously this does not result in
removal of the roster item from the contact's roster, and the
contact still has a subscription to the user's presence
information. In order to both completely cancel a mutual
subscription and fully remove the roster item from Saint-Andre
Standards Track [Page 53]
RFC 3921 XMPP IM October 2004 the
contact's roster, the contact should update the roster item with
subscription='remove' as defined under Removing a Roster Item and
Cancelling All Subscriptions (Section 8.6). 8.6. Removing a Roster
Item and Cancelling All Subscriptions Because there may be many
steps involved in completely removing a roster item and cancelling
subscriptions in both directions, the roster management protocol
includes a "shortcut" method for doing so. The process may be
initiated no matter what the current subscription state is by
sending a roster set containing an item for the contact with the
'subscription' attribute set to a value of "remove": When the user
removes a contact from his or her roster by setting the
'subscription' attribute to a value of "remove", the user's server
(1) MUST automatically cancel any existing presence subscription
between the user and the contact (both 'to' and 'from' as
appropriate); (2) MUST remove the roster item from the user's
roster and inform all of the user's available resources that have
requested the roster of the roster item removal; (3) MUST inform
the resource that initiated the removal of success; and (4) SHOULD
send unavailable presence from all of the user's available
resources to the contact: Saint-Andre Standards Track [Page 54]
RFC 3921 XMPP IM October 2004 Upon receiving the presence stanza of
type "unsubscribe", the contact's server (1) MUST initiate a roster
push to all available resources associated with the contact that
have requested the roster, containing an updated roster item for
the user with the 'subscription' attribute set to a value of "to"
(if the contact is unavailable or has not requested the roster, the
contact's server MUST modify the roster item and send that modified
item the next time the contact requests the roster); and (2) MUST
also deliver the "unsubscribe" state change notification to all of
the contact's available resources: SomeGroup Upon receiving the
presence stanza of type "unsubscribed", the contact's server (1)
MUST initiate a roster push to all available resources associated
with the contact that have requested the roster, containing an
updated roster item for the user with the 'subscription' attribute
set to a value of "none" (if the contact is unavailable or has not
requested the roster, the contact's server Saint-Andre Standards
Track [Page 55]
RFC 3921 XMPP IM October 2004 MUST modify the
roster item and send that modified item the next time the contact
requests the roster); and (2) MUST also deliver the "unsubscribe"
state change notification to all of the contact's available
resources: SomeGroup Upon receiving the presence stanza of type
"unavailable" addressed to the contact, the contact's server MUST
deliver the unavailable presence to all of the user's available
resources: Note: When the user removes the contact from the user's
roster, the end state of the contact's roster is that the user is
still in the contact's roster with a subscription state of "none";
in order to completely remove the roster item for the user, the
contact needs to also send a roster removal request. 9.
Subscription States This section provides detailed information
about subscription states and server handling of
subscription-related presence stanzas (i.e., presence stanzas of
type "subscribe", "subscribed", "unsubscribe", and "unsubscribed").
9.1. Defined States There are nine possible subscription states,
which are described here from the user's (not contact's)
perspective: Saint-Andre Standards Track [Page 56]
RFC 3921 XMPP IM
October 2004 1. "None" = contact and user are not subscribed to
each other, and neither has requested a subscription from the other
2. "None + Pending Out" = contact and user are not subscribed to
each other, and user has sent contact a subscription request but
contact has not replied yet 3. "None + Pending In" = contact and
user are not subscribed to each other, and contact has sent user a
subscription request but user has not replied yet (note: contact's
server SHOULD NOT push or deliver roster items in this state, but
instead SHOULD wait until contact has approved subscription request
from user) 4. "None + Pending Out/In" = contact and user are not
subscribed to each other, contact has sent user a subscription
request but user has not replied yet, and user has sent contact a
subscription request but contact has not replied yet 5. "To" = user
is subscribed to contact (one-way) 6. "To + Pending In" = user is
subscribed to contact, and contact has sent user a subscription
request but user has not replied yet 7. "From" = contact is
subscribed to user (one-way) 8. "From + Pending Out" = contact is
subscribed to user, and user has sent contact a subscription
request but contact has not replied yet 9. "Both" = user and
contact are subscribed to each other (two-way) 9.2. Server Handling
of Outbound Presence Subscription Stanzas Outbound presence
subscription stanzas enable the user to manage his or her
subscription to the contact's presence information (via the
"subscribe" and "unsubscribe" types), and to manage the contact's
access to the user's presence information (via the "subscribed" and
"unsubscribed" types). Because it is possible for the user's server
and the contact's server to lose synchronization regarding
subscription states, the user's server MUST without exception route
all outbound presence stanzas of type "subscribe" or "unsubscribe"
to the contact so that the user is able to resynchronize his or her
subscription to the contact's presence information if needed.
Saint-Andre Standards Track [Page 57]
RFC 3921 XMPP IM October 2004
The user's server SHOULD NOT route a presence stanza of type
"subscribed" or "unsubscribed" to the contact if the stanza does
not result in a subscription state change from the user's
perspective, and MUST NOT make a state change. If the stanza
results in a subscription state change, the user's server MUST
route the stanza to the contact and MUST make the appropriate state
change. These rules are summarized in the following tables. Table
1: Recommended handling of outbound "subscribed" stanzas
+----------------------------------------------------------------+
| EXISTING STATE | ROUTE? | NEW STATE |
+----------------------------------------------------------------+
| "None" | no | no state change | | "None + Pending Out" | no | no
state change | | "None + Pending In" | yes | "From" | | "None +
Pending Out/In" | yes | "From + Pending Out" | | "To" | no | no
state change | | "To + Pending In" | yes | "Both" | | "From" | no |
no state change | | "From + Pending Out" | no | no state change | |
"Both" | no | no state change |
+----------------------------------------------------------------+
Table 2: Recommended handling of outbound "unsubscribed" stanzas
+----------------------------------------------------------------+
| EXISTING STATE | ROUTE? | NEW STATE |
+----------------------------------------------------------------+
| "None" | no | no state change | | "None + Pending Out" | no | no
state change | | "None + Pending In" | yes | "None" | | "None +
Pending Out/In" | yes | "None + Pending Out" | | "To" | no | no
state change | | "To + Pending In" | yes | "To" | | "From" | yes |
"None" | | "From + Pending Out" | yes | "None + Pending Out" | |
"Both" | yes | "To" |
+----------------------------------------------------------------+
9.3. Server Handling of Inbound Presence Subscription Stanzas
Inbound presence subscription stanzas request a
subscription-related action from the user (via the "subscribe"
type), inform the user of subscription-related actions taken by the
contact (via the "unsubscribe" type), or enable the contact to
manage the user's access to the contact's presence information (via
the "subscribed" and "unsubscribed" types). Saint-Andre Standards
Track [Page 58]
RFC 3921 XMPP IM October 2004 When the user's
server receives a subscription request for the user from the
contact (i.e., a presence stanza of type "subscribe"), it MUST
deliver that request to the user for approval if the user has not
already granted the contact access to the user's presence
information and if there is no pending inbound subscription
request; however, the user's server SHOULD NOT deliver the new
request if there is a pending inbound subscription request, since
the previous subscription request will have been recorded. If the
user has already granted the contact access to the user's presence
information, the user's server SHOULD auto-reply to an inbound
presence stanza of type "subscribe" from the contact by sending a
presence stanza of type "subscribed" to the contact on behalf of
the user; this rule enables the contact to resynchronize the
subscription state if needed. These rules are summarized in the
following table. Table 3: Recommended handling of inbound
"subscribe" stanzas
+------------------------------------------------------------------+
| EXISTING STATE | DELIVER? | NEW STATE |
+------------------------------------------------------------------+
| "None" | yes | "None + Pending In" | | "None + Pending Out" | yes
| "None + Pending Out/In" | | "None + Pending In" | no | no state
change | | "None + Pending Out/In" | no | no state change | | "To"
| yes | "To + Pending In" | | "To + Pending In" | no | no state
change | | "From" | no * | no state change | | "From + Pending Out"
| no * | no state change | | "Both" | no * | no state change |
+------------------------------------------------------------------+
* Server SHOULD auto-reply with "subscribed" stanza When the user's
server receives a presence stanza of type "unsubscribe" for the
user from the contact, if the stanza results in a subscription
state change from the user's perspective then the user's server
SHOULD auto-reply by sending a presence stanza of type
"unsubscribed" to the contact on behalf of the user, MUST deliver
the "unsubscribe" stanza to the user, and MUST change the state. If
no subscription state change results, the user's server SHOULD NOT
deliver the stanza and MUST NOT change the state. These rules are
summarized in the following table. Saint-Andre Standards Track
[Page 59]
RFC 3921 XMPP IM October 2004 Table 4: Recommended
handling of inbound "unsubscribe" stanzas
+------------------------------------------------------------------+
| EXISTING STATE | DELIVER? | NEW STATE |
+------------------------------------------------------------------+
| "None" | no | no state change | | "None + Pending Out" | no | no
state change | | "None + Pending In" | yes * | "None" | | "None +
Pending Out/In" | yes * | "None + Pending Out" | | "To" | no | no
state change | | "To + Pending In" | yes * | "To" | | "From" | yes
* | "None" | | "From + Pending Out" | yes * | "None + Pending Out |
| "Both" | yes * | "To" |
+------------------------------------------------------------------+
* Server SHOULD auto-reply with "unsubscribed" stanza When the
user's server receives a presence stanza of type "subscribed" for
the user from the contact, it MUST NOT deliver the stanza to the
user and MUST NOT change the subscription state if there is no
pending outbound request for access to the contact's presence
information. If there is a pending outbound request for access to
the contact's presence information and the inbound presence stanza
of type "subscribed" results in a subscription state change, the
user's server MUST deliver the stanza to the user and MUST change
the subscription state. If the user already has access to the
contact's presence information, the inbound presence stanza of type
"subscribed" does not result in a subscription state change;
therefore the user's server SHOULD NOT deliver the stanza to the
user and MUST NOT change the subscription state. These rules are
summarized in the following table. Table 5: Recommended handling of
inbound "subscribed" stanzas
+------------------------------------------------------------------+
| EXISTING STATE | DELIVER? | NEW STATE |
+------------------------------------------------------------------+
| "None" | no | no state change | | "None + Pending Out" | yes |
"To" | | "None + Pending In" | no | no state change | | "None +
Pending Out/In" | yes | "To + Pending In" | | "To" | no | no state
change | | "To + Pending In" | no | no state change | | "From" | no
| no state change | | "From + Pending Out" | yes | "Both" | |
"Both" | no | no state change |
+------------------------------------------------------------------+
Saint-Andre Standards Track [Page 60]
RFC 3921 XMPP IM October 2004
When the user's server receives a presence stanza of type
"unsubscribed" for the user from the contact, it MUST deliver the
stanza to the user and MUST change the subscription state if there
is a pending outbound request for access to the contact's presence
information or if the user currently has access to the contact's
presence information. Otherwise, the user's server SHOULD NOT
deliver the stanza and MUST NOT change the subscription state.
These rules are summarized in the following table. Table 6:
Recommended handling of inbound "unsubscribed" stanzas
+------------------------------------------------------------------+
| EXISTING STATE | DELIVER? | NEW STATE |
+------------------------------------------------------------------+
| "None" | no | no state change | | "None + Pending Out" | yes |
"None" | | "None + Pending In" | no | no state change | | "None +
Pending Out/In" | yes | "None + Pending In" | | "To" | yes | "None"
| | "To + Pending In" | yes | "None + Pending In" | | "From" | no |
no state change | | "From + Pending Out" | yes | "From" | | "Both"
| yes | "From" |
+------------------------------------------------------------------+
9.4. Server Delivery and Client Acknowledgement of Subscription
Requests and State Change Notifications When a server receives an
inbound presence stanza of type "subscribe" (i.e., a subscription
request) or of type "subscribed", "unsubscribe", or "unsubscribed"
(i.e., a subscription state change notification), in addition to
sending the appropriate roster push (or updated roster when the
roster is next requested by an available resource), it MUST deliver
the request or notification to the intended recipient at least
once. A server MAY require the recipient to acknowledge receipt of
all state change notifications (and MUST require acknowledgement in
the case of subscription requests, i.e., presence stanzas of type
"subscribe"). In order to require acknowledgement, a server SHOULD
send the request or notification to the recipient each time the
recipient logs in, until the recipient acknowledges receipt of the
notification by "affirming" or "denying" the notification, as shown
in the following table: Saint-Andre Standards Track [Page 61]
RFC 3921 XMPP IM October 2004 Table 7: Acknowledgement of subscription
state change notifications
+--------------------------------------------------+ | STANZA TYPE
| ACCEPT | DENY |
+--------------------------------------------------+ | subscribe |
subscribed | unsubscribed | | subscribed | subscribe | unsubscribe
| | unsubscribe | unsubscribed | subscribed | | unsubscribed |
unsubscribe | subscribe |
+--------------------------------------------------+ Obviously,
given the foregoing subscription state charts, some of the
acknowledgement stanzas will be routed to the contact and result in
subscription state changes, while others will not. However, any
such stanzas MUST result in the server's no longer sending the
subscription state notification to the user. Because a user's
server MUST automatically generate outbound presence stanzas of
type "unsubscribe" and "unsubscribed" upon receiving a roster set
with the 'subscription' attribute set to a value of "remove" (see
Removing a Roster Item and Cancelling All Subscriptions (Section
8.6)), the server MUST treat a roster remove request as equivalent
to sending both of those presence stanzas for purposes of
determining whether to continue sending subscription state change
notifications of type "subscribe" or "subscribed" to the user. 10.
Blocking Communication Most instant messaging systems have found it
necessary to implement some method for users to block
communications from particular other users (this is also required
by sections 5.1.5, 5.1.15, 5.3.2, and 5.4.10 of [IMP-REQS]). In
XMPP this is done by managing one's privacy lists using the
'jabber:iq:privacy' namespace. Server-side privacy lists enable
successful completion of the following use cases: o Retrieving
one's privacy lists. o Adding, removing, and editing one's privacy
lists. o Setting, changing, or declining active lists. o Setting,
changing, or declining the default list (i.e., the list that is
active by default). o Allowing or blocking messages based on JID,
group, or subscription type (or globally). Saint-Andre Standards
Track [Page 62]
RFC 3921 XMPP IM October 2004 o Allowing or
blocking inbound presence notifications based on JID, group, or
subscription type (or globally). o Allowing or blocking outbound
presence notifications based on JID, group, or subscription type
(or globally). o Allowing or blocking IQ stanzas based on JID,
group, or subscription type (or globally). o Allowing or blocking
all communications based on JID, group, or subscription type (or
globally). Note: Presence notifications do not include presence
subscriptions, only presence information that is broadcasted to
entities that are subscribed to a user's presence information. Thus
this includes presence stanzas with no 'type' attribute or of
type='unavailable' only. 10.1. Syntax and Semantics A user MAY
define one or more privacy lists, which are stored by the user's
server. Each element contains one or more rules in the form of
elements, and each element uses attributes to define a privacy rule
type, a specific value to which the rule applies, the relevant
action, and the place of the item in the processing order. The
syntax is as follows: [] [] [] [] Saint-Andre Standards Track [Page
63]
RFC 3921 XMPP IM October 2004 If the type is "jid", then the
'value' attribute MUST contain a valid Jabber ID. JIDs SHOULD be
matched in the following order: 1. (only that resource matches) 2.
(any resource matches) 3. (only that resource matches) 4. (the
domain itself matches, as does any user@domain, domain/resource, or
address containing a subdomain) If the type is "group", then the
'value' attribute SHOULD contain the name of a group in the user's
roster. (If a client attempts to update, create, or delete a list
item with a group that is not in the user's roster, the server
SHOULD return to the client an stanza error.) If the type is
"subscription", then the 'value' attribute MUST be one of "both",
"to", "from", or "none" as defined under Roster Syntax and
Semantics (Section 7.1), where "none" includes entities that are
totally unknown to the user and therefore not in the user's roster
at all. If no 'type' attribute is included, the rule provides the
"fall-through" case. The 'action' attribute MUST be included and
its value MUST be either "allow" or "deny". The 'order' attribute
MUST be included and its value MUST be a non-negative integer that
is unique among all items in the list. (If a client attempts to
create or update a list with non-unique order values, the server
MUST return to the client a stanza error.) The element MAY contain
one or more child elements that enable an entity to specify more
granular control over which kinds of stanzas are to be blocked
(i.e., rather than blocking all stanzas). The allowable child
elements are: o -- blocks incoming message stanzas o -- blocks
incoming IQ stanzas o -- blocks incoming presence notifications o
-- blocks outgoing presence notifications Saint-Andre Standards
Track [Page 64]
RFC 3921 XMPP IM October 2004 Within the
'jabber:iq:privacy' namespace, the child of an IQ stanza of type
"set" MUST NOT include more than one child element (i.e., the
stanza MUST contain only one element, one element, or one element);
if a sending entity violates this rule, the receiving entity MUST
return a stanza error. When a client adds or updates a privacy
list, the element SHOULD contain at least one child element; when a
client removes a privacy list, the element MUST NOT contain any
child elements. When a client updates a privacy list, it must
include all of the desired items (i.e., not a "delta"). 10.2.
Business Rules 1. If there is an active list set for a session, it
affects only the session(s) for which it is activated, and only for
the duration of the session(s); the server MUST apply the active
list only and MUST NOT apply the default list (i.e., there is no
"layering" of lists). 2. The default list applies to the user as a
whole, and is processed if there is no active list set for the
target session/resource to which a stanza is addressed, or if there
are no current sessions for the user. 3. If there is no active list
set for a session (or there are no current sessions for the user),
and there is no default list, then all stanzas SHOULD BE accepted
or appropriately processed by the server on behalf of the user in
accordance with the Server Rules for Handling XML Stanzas (Section
11). 4. Privacy lists MUST be the first delivery rule applied by a
server, superseding (1) the routing and delivery rules specified in
Server Rules for Handling XML Stanzas (Section 11), and (2) the
handling of subscription-related presence stanzas (and
corresponding generation of roster pushes) specified in Integration
of Roster Items and Presence Subscriptions (Section 8). 5. The
order in which privacy list items are processed by the server is
important. List items MUST be processed in ascending order
determined by the integer values of the 'order' attribute for each
. Saint-Andre Standards Track [Page 65]
RFC 3921 XMPP IM October
2004 6. As soon as a stanza is matched against a privacy list rule,
the server MUST appropriately handle the stanza in accordance with
the rule and cease processing. 7. If no fall-through item is
provided in a list, the fall-through action is assumed to be
"allow". 8. If a user updates the definition for an active list,
subsequent processing based on that active list MUST use the
updated definition (for all resources to which that active list
currently applies). 9. If a change to the subscription state or
roster group of a roster item defined in an active or default list
occurs during a user's session, subsequent processing based on that
list MUST take into account the changed state or group (for all
resources to which that list currently applies). 10. When the
definition for a rule is modified, the server MUST send an IQ
stanza of type "set" to all connected resources, containing a
element with only one child element, where the 'name' attribute is
set to the name of the modified privacy list. These "privacy list
pushes" adhere to the same semantics as the "roster pushes" used in
roster management, except that only the list name itself (not the
full list definition or the "delta") is pushed to the connected
resources. It is up to the receiving resource to determine whether
to retrieve the modified list definition, although a connected
resource SHOULD do so if the list currently applies to it. 11. When
a resource attempts to remove a list or specify a new default list
while that list applies to a connected resource other than the
sending resource, the server MUST return a error to the sending
resource and MUST NOT make the requested change. 10.3. Retrieving
One's Privacy Lists Example: Client requests names of privacy lists
from server: Saint-Andre Standards Track [Page 66]
RFC 3921 XMPP IM
October 2004 Example: Server sends names of privacy lists to
client, preceded by active list and default list: Example: Client
requests a privacy list from server: Example: Server sends a
privacy list to client: Example: Client requests another privacy
list from server: Saint-Andre Standards Track [Page 67]
RFC 3921
XMPP IM October 2004 Example: Server sends another privacy list to
client: Example: Client requests yet another privacy list from
server: Example: Server sends yet another privacy list to client:
In this example, the user has three lists: (1) 'public', which
allows communications from everyone except one specific entity
(this is the default list); (2) 'private', which allows
communications only with Saint-Andre Standards Track [Page 68]
RFC 3921 XMPP IM October 2004 contacts who have a bidirectional
subscription with the user (this is the active list); and (3)
'special', which allows communications only with three specific
entities. If the user attempts to retrieve a list but a list by
that name does not exist, the server MUST return an stanza error to
the user: Example: Client attempts to retrieve non-existent list:
The user is allowed to retrieve only one list at a time. If the
user attempts to retrieve more than one list in the same request,
the server MUST return a stanza error to the user: Example: Client
attempts to retrieve more than one list: 10.4. Managing Active
Lists In order to set or change the active list currently being
applied by the server, the user MUST send an IQ stanza of type
"set" with a element qualified by the 'jabber:iq:privacy' namespace
that contains an empty child element possessing a 'name' attribute
whose value is set to the desired list name. Saint-Andre Standards
Track [Page 69]
RFC 3921 XMPP IM October 2004 Example: Client
requests change of active list: The server MUST activate and apply
the requested list before sending the result back to the client.
Example: Server acknowledges success of active list change: If the
user attempts to set an active list but a list by that name does
not exist, the server MUST return an stanza error to the user:
Example: Client attempts to set a non-existent list as active: In
order to decline the use of any active list, the connected resource
MUST send an empty element with no 'name' attribute. Example:
Client declines the use of active lists: Example: Server
acknowledges success of declining any active list: Saint-Andre
Standards Track [Page 70]
RFC 3921 XMPP IM October 2004 10.5.
Managing the Default List In order to change its default list
(which applies to the user as a whole, not only the sending
resource), the user MUST send an IQ stanza of type "set" with a
element qualified by the 'jabber:iq:privacy' namespace that
contains an empty child element possessing a 'name' attribute whose
value is set to the desired list name. Example: User requests
change of default list: Example: Server acknowledges success of
default list change: If the user attempts to change which list is
the default list but the default list is in use by at least one
connected resource other than the sending resource, the server MUST
return a stanza error to the sending resource: Example: Client
attempts to change the default list but that list is in use by
another resource: If the user attempts to set a default list but a
list by that name does not exist, the server MUST return an stanza
error to the user: Saint-Andre Standards Track [Page 71]
RFC 3921
XMPP IM October 2004 Example: Client attempts to set a non-existent
list as default: In order to decline the use of a default list
(i.e., to use the domain's stanza routing rules at all times), the
user MUST send an empty element with no 'name' attribute. Example:
Client declines the use of the default list: Example: Server
acknowledges success of declining any default list: If one
connected resource attempts to decline the use of a default list
for the user as a whole but the default list currently applies to
at least one other connected resource, the server MUST return a
error to the sending resource: Example: Client attempts to decline
a default list but that list is in use by another resource:
Saint-Andre Standards Track [Page 72]
RFC 3921 XMPP IM October 2004
10.6. Editing a Privacy List In order to edit a privacy list, the
user MUST send an IQ stanza of type "set" with a element qualified
by the 'jabber:iq:privacy' namespace that contains one child
element possessing a 'name' attribute whose value is set to the
list name the user would like to edit. The element MUST contain one
or more elements, which specify the user's desired changes to the
list by including all elements in the list (not the "delta").
Example: Client edits a privacy list: Example: Server acknowledges
success of list edit: Note: The value of the 'order' attribute for
any given item is not fixed. Thus in the foregoing example if the
user would like to add 4 items between the "tybalt@example.com"
item and the "paris@example.org" item, the user's client MUST
renumber the relevant items before submitting the list to the
server. The server MUST now send a "privacy list push" to all
connected resources: Example: Privacy list push on list edit:
Saint-Andre Standards Track [Page 73]
RFC 3921 XMPP IM October 2004
In accordance with the semantics of IQ stanzas defined in
[XMPP-CORE], each connected resource MUST return an IQ result to
the server as well: Example: Acknowledging receipt of privacy list
pushes: 10.7. Adding a New Privacy List The same protocol used to
edit an existing list is used to create a new list. If the list
name matches that of an existing list, the request to add a new
list will overwrite the old one. As with list edits, the server
MUST also send a "privacy list push" to all connected resources.
10.8. Removing a Privacy List In order to remove a privacy list,
the user MUST send an IQ stanza of type "set" with a element
qualified by the 'jabber:iq:privacy' namespace that contains one
empty child element possessing a 'name' attribute whose value is
set to the list name the user would like to remove. Example: Client
removes a privacy list: Example: Server acknowledges success of
list removal: Saint-Andre Standards Track [Page 74]
RFC 3921 XMPP
IM October 2004 If a user attempts to remove a list that is
currently being applied to at least one resource other than the
sending resource, the server MUST return a stanza error to the
user; i.e., the user MUST first set another list to active or
default before attempting to remove it. If the user attempts to
remove a list but a list by that name does not exist, the server
MUST return an stanza error to the user. If the user attempts to
remove more than one list in the same request, the server MUST
return a stanza error to the user. 10.9. Blocking Messages
Server-side privacy lists enable a user to block incoming messages
from other entities based on the entity's JID, roster group, or
subscription status (or globally). The following examples
illustrate the protocol. (Note: For the sake of brevity, IQ stanzas
of type "result" are not shown in the following examples, nor are
"privacy list pushes".) Example: User blocks based on JID: As a
result of creating and applying the foregoing list, the user will
not receive messages from the entity with the specified JID.
Example: User blocks based on roster group: Saint-Andre Standards
Track [Page 75]
RFC 3921 XMPP IM October 2004 As a result of
creating and applying the foregoing list, the user will not receive
messages from any entities in the specified roster group. Example:
User blocks based on subscription type: As a result of creating and
applying the foregoing list, the user will not receive messages
from any entities with the specified subscription type. Example:
User blocks globally: As a result of creating and applying the
foregoing list, the user will not receive messages from any other
users. 10.10. Blocking Inbound Presence Notifications Server-side
privacy lists enable a user to block incoming presence
notifications from other entities based on the entity's JID, roster
group, or subscription status (or globally). The following examples
illustrate the protocol. Saint-Andre Standards Track [Page 76]
RFC 3921 XMPP IM October 2004 Note: Presence notifications do not
include presence subscriptions, only presence information that is
broadcasted to the user because the user is currently subscribed to
a contact's presence information. Thus this includes presence
stanzas with no 'type' attribute or of type='unavailable' only.
Example: User blocks based on JID: As a result of creating and
applying the foregoing list, the user will not receive presence
notifications from the entity with the specified JID. Example: User
blocks based on roster group: As a result of creating and applying
the foregoing list, the user will not receive presence
notifications from any entities in the specified roster group.
Saint-Andre Standards Track [Page 77]
RFC 3921 XMPP IM October 2004
Example: User blocks based on subscription type: As a result of
creating and applying the foregoing list, the user will not receive
presence notifications from any entities with the specified
subscription type. Example: User blocks globally: As a result of
creating and applying the foregoing list, the user will not receive
presence notifications from any other users. 10.11. Blocking
Outbound Presence Notifications Server-side privacy lists enable a
user to block outgoing presence notifications to other entities
based on the entity's JID, roster group, or subscription status (or
globally). The following examples illustrate the protocol. Note:
Presence notifications do not include presence subscriptions, only
presence information that is broadcasted to contacts because those
contacts are currently subscribed to the user's presence
information. Thus this includes presence stanzas with no 'type'
attribute or of type='unavailable' only. Saint-Andre Standards
Track [Page 78]
RFC 3921 XMPP IM October 2004 Example: User blocks
based on JID: As a result of creating and applying the foregoing
list, the user will not send presence notifications to the entity
with the specified JID. Example: User blocks based on roster group:
As a result of creating and applying the foregoing list, the user
will not send presence notifications to any entities in the
specified roster group. Example: User blocks based on subscription
type: Saint-Andre Standards Track [Page 79]
RFC 3921 XMPP IM
October 2004 As a result of creating and applying the foregoing
list, the user will not send presence notifications to any entities
with the specified subscription type. Example: User blocks
globally: As a result of creating and applying the foregoing list,
the user will not send presence notifications to any other users.
10.12. Blocking IQ Stanzas Server-side privacy lists enable a user
to block incoming IQ stanzas from other entities based on the
entity's JID, roster group, or subscription status (or globally).
The following examples illustrate the protocol. Example: User
blocks based on JID: As a result of creating and applying the
foregoing list, the user will not receive IQ stanzas from the
entity with the specified JID. Saint-Andre Standards Track [Page
80]
RFC 3921 XMPP IM October 2004 Example: User blocks based on
roster group: As a result of creating and applying the foregoing
list, the user will not receive IQ stanzas from any entities in the
specified roster group. Example: User blocks based on subscription
type: As a result of creating and applying the foregoing list, the
user will not receive IQ stanzas from any entities with the
specified subscription type. Saint-Andre Standards Track [Page 81]
RFC 3921 XMPP IM October 2004 Example: User blocks globally: As a
result of creating and applying the foregoing list, the user will
not receive IQ stanzas from any other users. 10.13. Blocking All
Communication Server-side privacy lists enable a user to block all
stanzas from and to other entities based on the entity's JID,
roster group, or subscription status (or globally). Note that this
includes subscription-related presence stanzas, which are excluded
by Blocking Inbound Presence Notifications (Section 10.10). The
following examples illustrate the protocol. Example: User blocks
based on JID: As a result of creating and applying the foregoing
list, the user will not receive any communications from, nor send
any stanzas to, the entity with the specified JID. Example: User
blocks based on roster group: As a result of creating and applying
the foregoing list, the user will not receive any communications
from, nor send any stanzas to, any entities in the specified roster
group. Example: User blocks based on subscription type: As a result
of creating and applying the foregoing list, the user will not
receive any communications from, nor send any stanzas to, any
entities with the specified subscription type. Example: User blocks
globally: As a result of creating and applying the foregoing list,
the user will not receive any communications from, nor send any
stanzas to, any other users. 10.14. Blocked Entity Attempts to
Communicate with User If a blocked entity attempts to send message
or presence stanzas to the user, the user's server SHOULD silently
drop the stanza and MUST NOT return an error to the sending entity.
Saint-Andre Standards Track [Page 83]
RFC 3921 XMPP IM October 2004
If a blocked entity attempts to send an IQ stanza of type "get" or
"set" to the user, the user's server MUST return to the sending
entity a stanza error, since this is the standard error code sent
from a client that does not understand the namespace of an IQ get
or set. IQ stanzas of other types SHOULD be silently dropped by the
server. Example: Blocked entity attempts to send IQ get: Example:
Server returns error to blocked entity: 10.15. Higher-Level
Heuristics When building a representation of a higher-level privacy
heuristic, a client SHOULD use the simplest possible
representation. For example, the heuristic "block all
communications with any user not in my roster" could be constructed
in any of the following ways: o allow communications from all JIDs
in my roster (i.e., listing each JID as a separate list item), but
block communications with everyone else o allow communications from
any user who is in one of the groups that make up my roster (i.e.,
listing each group as a separate list item), but block
communications from everyone else o allow communications from any
user with whom I have a subscription of 'both' or 'to' or 'from'
(i.e., listing each subscription value separately), but block
communications from everyone else Saint-Andre Standards Track [Page
84]
RFC 3921 XMPP IM October 2004 o block communications from
anyone whose subscription state is 'none' The final representation
is the simplest and SHOULD be used; here is the XML that would be
sent in this case: 11. Server Rules for Handling XML Stanzas Basic
routing and delivery rules for servers are defined in [XMPP-CORE].
This section defines additional rules for XMPP-compliant