RFC3921 - 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.

Abstract

This memo describes extensions to and applications of the core
features of the Extensible Messaging and Presence Protocol (XMPP)
that provide the basic instant messaging (IM) and presence
functionality defined in RFC 2779.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Syntax of XML Stanzas . . . . . . . . . . . . . . . . . . . 4
3. Session Establishment . . . . . . . . . . . . . . . . . . . 10
4. Exchanging Messages . . . . . . . . . . . . . . . . . . . . 13
5. Exchanging Presence Information . . . . . . . . . . . . . . 16
6. Managing Subscriptions . . . . . . . . . . . . . . . . . . . 26
7. Roster Management . . . . . . . . . . . . . . . . . . . . . 27
8. Integration of Roster Items and Presence Subscriptions . . . 32
9. Subscription States . . . . . . . . . . . . . . . . . . . . 56
10. Blocking Communication . . . . . . . . . . . . . . . . . . . 62
11. Server Rules for Handling XML Stanzas . . . . . . . . . . . 85
12. IM and Presence Compliance Requirements . . . . . . . . . . 88
13. Internationalization Considerations . . . . . . . . . . . . 89
14. Security Considerations . . . . . . . . . . . . . . . . . . 89
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 90
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 91
A. vCards . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
B. XML Schemas. . . . . . . . . . . . . . . . . . . . . . . . . 93
C. Differences Between Jabber IM/Presence Protocols and XMPP. . 105
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 106
Author's Address. . . . . . . . . . . . . . . . . . . . . . . . . 106
Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 107

1. Introduction

1.1. Overview

The Extensible Messaging and Presence Protocol (XMPP) is a protocol
for streaming XML [XML] elements in order to exchange messages and
presence information in close to real time. The core features of
XMPP are defined in Extensible Messaging and Presence Protocol
(XMPP): Core [XMPP-CORE]. These features -- mainly XML streams, use
of TLS and SASL, and the , , 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].

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1.2. Requirements

For the purposes of this memo, the requirements of a basic instant
messaging and presence application are defined by [IMP-REQS], which
at a high level stipulates that a user must be able to complete the
following use cases:

o Exchange messages with other users
o Exchange presence information with other users
o Manage subscriptions to and from other users
o Manage items in a contact list (in XMPP this is called a "roster")
o Block communications to or from specific other users

Detailed definitions of these functionality areas are contained in
[IMP-REQS], and the interested reader is directed to that document
regarding the requirements addressed herein.

[IMP-REQS] also stipulates that presence services must be separable
from instant messaging services; i.e., it must be possible to use the
protocol to provide a presence service, an instant messaging service,
or both. Although the text of this memo assumes that implementations
and deployments will want to offer a unified instant messaging and
presence service, there is no requirement that a service must offer
both a presence service and an instant messaging service, and the
protocol makes it possible to offer separate and distinct services
for presence and for instant messaging.

Note: While XMPP-based instant messaging and presence meets the
requirements of [IMP-REQS], it was not designed explicitly with that
specification in mind, since the base protocol evolved through an
open development process within the Jabber open-source community
before RFC 2779 was written. Note also that although protocols
addressing many other functionality areas have been defined in the
Jabber community, such protocols are not included in this memo
because they are not required by [IMP-REQS].

1.3. Terminology

This memo inherits the terminology defined in [XMPP-CORE].

The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14, RFC 2119 [TERMS].

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2. Syntax of XML Stanzas

The basic semantics and common attributes of XML stanzas qualified by
the 'jabber:client' and 'jabber:server' namespaces are defined in
[XMPP-CORE]. However, these namespaces also define various child
elements, as well as values for the common 'type' attribute, that are
specific to instant messaging and presence applications. Thus,
before addressing particular "use cases" for such applications, we
here further describe the syntax of XML stanzas, thereby
supplementing the discussion in [XMPP-CORE].

2.1. Message Syntax

Message stanzas qualified by the 'jabber:client' or 'jabber:server'
namespace are used to "push" information to another entity. Common
uses in instant messaging applications include single messages,
messages sent in the context of a chat conversation, messages sent in
the context of a multi-user chat room, headlines and other alerts,
and errors.

2.1.1. Types of Message

The 'type' attribute of a message stanza is RECOMMENDED; if included,
it specifies the conversational context of the message, thus
providing a hint regarding presentation (e.g., in a GUI). If
included, the 'type' attribute MUST have one of the following values:

o chat -- The message is sent in the context of a one-to-one chat
conversation. A compliant client SHOULD present the message in an
interface enabling one-to-one chat between the two parties,
including an appropriate conversation history.

o error -- An error has occurred related to a previous message sent
by the sender (for details regarding stanza error syntax, refer to
[XMPP-CORE]). A compliant client SHOULD present an appropriate
interface informing the sender of the nature of the error.

o groupchat -- The message is sent in the context of a multi-user
chat environment (similar to that of [IRC]). A compliant client
SHOULD present the message in an interface enabling many-to-many
chat between the parties, including a roster of parties in the
chatroom and an appropriate conversation history. Full definition
of XMPP-based groupchat protocols is out of scope for this memo.

o headline -- The message is probably generated by an automated
service that delivers or broadcasts content (news, sports, market
information, RSS feeds, etc.). No reply to the message is
expected, and a compliant client SHOULD present the message in an

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interface that appropriately differentiates the message from
standalone messages, chat sessions, or groupchat sessions (e.g.,
by not providing the recipient with the ability to reply).

o normal -- The message is a single message that is sent outside the
context of a one-to-one conversation or groupchat, and to which it
is expected that the recipient will reply. A compliant client
SHOULD present the message in an interface enabling the recipient
to reply, but without a conversation history.

An IM application SHOULD support all of the foregoing message types;
if an application receives a message with no 'type' attribute or the
application does not understand the value of the 'type' attribute
provided, it MUST consider the message to be of type "normal" (i.e.,
"normal" is the default). The "error" type MUST be generated only in
response to an error related to a message received from another
entity.

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

2.1.2. Child Elements

As described under extended namespaces (Section 2.4), a message
stanza MAY contain any properly-namespaced child element.

In accordance with the default namespace declaration, by default a
message stanza is qualified by the 'jabber:client' or 'jabber:server'
namespace, which defines certain allowable children of message
stanzas. If the message stanza is of type "error", it MUST include
an 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

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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.

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2.2.1. Types of Presence

The 'type' attribute of a presence stanza is OPTIONAL. A presence
stanza that does not possess a 'type' attribute is used to signal to
the server that the sender is online and available for communication.
If included, the 'type' attribute specifies a lack of availability, a
request to manage a subscription to another entity's presence, a
request for another entity's current presence, or an error related to
a previously-sent presence stanza. If included, the 'type' attribute
MUST have one of the following values:

o unavailable -- Signals that the entity is no longer available for
communication.

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

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

o unsubscribe -- The sender is unsubscribing from another entity's
presence.

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

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

o error -- An error has occurred regarding processing or delivery of
a previously-sent presence stanza.

For detailed information regarding presence semantics and the
subscription model used in the context of XMPP-based instant
messaging and presence applications, refer to Exchanging Presence
Information (Section 5) and Managing Subscriptions (Section 6).

2.2.2. Child Elements

As described under extended namespaces (Section 2.4), a presence
stanza MAY contain any properly-namespaced child element.

In accordance with the default namespace declaration, by default a
presence stanza is qualified by the 'jabber:client' or
'jabber:server' namespace, which defines certain allowable children
of presence stanzas. If the presence stanza is of type "error", it
MUST include an child; for details, see [XMPP-CORE]. If the
presence stanza possesses no 'type' attribute, it MAY contain any of

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the following child elements (note that the 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,

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a server SHOULD consider the priority to be zero. For information
regarding the semantics of priority values in stanza routing within
instant messaging and presence applications, refer to Server Rules
for Handling XML Stanzas (Section 11).

2.3. IQ Syntax

IQ stanzas provide a structured request-response mechanism. The
basic semantics of that mechanism (e.g., that the 'id' attribute is
REQUIRED) are defined in [XMPP-CORE], whereas the specific semantics
required to complete particular use cases are defined in all cases by
an extended namespace (Section 2.4) (note that the 'jabber:client'
and 'jabber:server' namespaces do not define any children of IQ
stanzas other than the common ). This memo defines two such
extended namespaces, one for Roster Management (Section 7) and the
other for Blocking Communication (Section 10); however, an IQ stanza
MAY contain structured information qualified by any extended
namespace.

2.4. Extended Namespaces

While the three XML stanza kinds defined in the "jabber:client" or
"jabber:server" namespace (along with their attributes and child
elements) provide a basic level of functionality for messaging and
presence, XMPP uses XML namespaces to extend the stanzas for the
purpose of providing additional functionality. Thus a message or
presence stanza MAY contain one or more optional child elements
specifying content that extends the meaning of the message (e.g., an
XHTML-formatted version of the message body), and an IQ stanza MAY
contain one such child element. This child element MAY have any name
and MUST possess an 'xmlns' namespace declaration (other than
"jabber:client", "jabber:server", or
"http://etherx.jabber.org/streams") that defines all data contained
within the child element.

Support for any given extended namespace is OPTIONAL on the part of
any implementation (aside from the extended namespaces defined
herein). If an entity does not understand such a namespace, the
entity's expected behavior depends on whether the entity is (1) the
recipient or (2) an entity that is routing the stanza to the
recipient:

Recipient: If a recipient receives a stanza that contains a child
element it does not understand, it SHOULD ignore that specific XML
data, i.e., it SHOULD not process it or present it to a user or
associated application (if any). In particular:

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* If an entity receives a message or presence stanza that
contains XML data qualified by a namespace it does not
understand, the portion of the stanza that is in the unknown
namespace SHOULD be ignored.

* If an entity receives a message stanza whose only child element
is qualified by a namespace it does not understand, it MUST
ignore the entire stanza.

* If an entity receives an IQ stanza of type "get" or "set"
containing a child element qualified by a namespace it does not
understand, the entity SHOULD return an IQ stanza of type
"error" with an error condition of .

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]:

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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.

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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):

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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

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outside the context of any existing chat session or received message,
the value of the 'to' address SHOULD be of the form
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)).

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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

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5. Exchanging Presence Information

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

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

5.1. Client and Server Presence Responsibilities

5.1.1. Initial Presence

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

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

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

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

2. Broadcast initial presence from the full JID (e.g.,
) 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

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

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

5.1.3. Presence Probes

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

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

* if the user is in the contact's roster with a subscription
state of "None", "None + Pending Out", or "To" (or is not in
the contact's roster at all), the contact's server MUST return
a 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.

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

5.1.4. Directed Presence

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

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

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

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

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5.1.5. Unavailable Presence

Before ending its session with a server, a client SHOULD gracefully
become unavailable by sending a final presence stanza that possesses
no 'to' attribute and that possesses a 'type' attribute whose value
is "unavailable" (optionally, the final presence stanza MAY contain
one or more 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

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

5.2. Specifying Availability Status

A client MAY provide further information about its availability
status by using the 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

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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:

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

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

away
be right back
0

dnd
gallivanting

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

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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

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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

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

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:

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

6.2. Handling a Subscription Request

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

Example: Approving a subscription request:

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.

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

7.1. Syntax and Semantics

Rosters are managed using IQ stanzas, specifically by means of a
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.

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7.2. Business Rules

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

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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

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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.

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7.6. Deleting a Roster Item

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

Example: Client removes an item:

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

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

8. Integration of Roster Items and Presence Subscriptions

8.1. Overview

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

There are four primary subscription states:

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

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

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

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

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

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

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

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

8.2. User Subscribes to Contact

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

1. In preparation for being able to render the contact in the user's
client interface and for the server to keep track of the
subscription, the user's client SHOULD perform a "roster set" for
the new roster item. This request consists of sending an IQ
stanza of type='set' containing a 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:

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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:

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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

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

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:

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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

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

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.

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

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

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

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

8.3. Creating a Mutual Subscription

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

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

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

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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):

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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:

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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):

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

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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

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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:

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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:

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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

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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:

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

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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

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

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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

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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

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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

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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:

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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

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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:

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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.

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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

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).

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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

* 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.

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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 |
+------------------------------------------------------------------+

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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

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:

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Table 7: Acknowledgement of subscription state change notifications

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).

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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:

[]
[]
[]
[]

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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

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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 .

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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:

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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:

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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

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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.

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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:

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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:

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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:

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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:

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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:

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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:

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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.

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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.

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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.

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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:

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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.

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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.

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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.

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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

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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 instant messaging and presence servers.

11.1. Inbound Stanzas

If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute of an inbound stanza matches a hostname of the
server itself and the JID contained in the 'to' attribute is of the
form or , the server
MUST first apply any privacy lists (Section 10) that are in force,
then follow the rules defined below:

1. If the JID is of the form and an available
resource matches the full JID, the recipient's server MUST
deliver the stanza to that resource.

2. Else if the JID is of the form or and the associated user account does not exist, the
recipient's server (a) SHOULD silently ignore the stanza (i.e.,
neither deliver it nor return an error) if it is a presence
stanza, (b) MUST return a stanza error to
the sender if it is an IQ stanza, and (c) SHOULD return a
stanza error to the sender if it is a
message stanza.

3. Else if the JID is of the form and no
available resource matches the full JID, the recipient's server
(a) SHOULD silently ignore the stanza (i.e., neither deliver it

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nor return an error) if it is a presence stanza, (b) MUST return
a stanza error to the sender if it is an
IQ stanza, and (c) SHOULD treat the stanza as if it were
addressed to if it is a message stanza.

4. Else if the JID is of the form and there is at
least one available resource available for the user, the
recipient's server MUST follow these rules:

1. For message stanzas, the server SHOULD deliver the stanza to
the highest-priority available resource (if the resource did
not provide a value for the element, the server
SHOULD consider it to have provided a value of zero). If two
or more available resources have the same priority, the
server MAY use some other rule (e.g., most recent connect
time, most recent activity time, or highest availability as
determined by some hierarchy of values) to choose
between them or MAY deliver the message to all such
resources. However, the server MUST NOT deliver the stanza
to an available resource with a negative priority; if the
only available resource has a negative priority, the server
SHOULD handle the message as if there were no available
resources (defined below). In addition, the server MUST NOT
rewrite the 'to' attribute (i.e., it MUST leave it as
rather than change it to ).

2. For presence stanzas other than those of type "probe", the
server MUST deliver the stanza to all available resources;
for presence probes, the server SHOULD reply based on the
rules defined in Presence Probes (Section 5.1.3). In
addition, the server MUST NOT rewrite the 'to' attribute
(i.e., it MUST leave it as rather than change
it to ).

3. For IQ stanzas, the server itself MUST reply on behalf of the
user with either an IQ result or an IQ error, and MUST NOT
deliver the IQ stanza to any of the available resources.
Specifically, if the semantics of the qualifying namespace
define a reply that the server can provide, the server MUST
reply to the stanza on behalf of the user; if not, the server
MUST reply with a stanza error.

5. Else if the JID is of the form and there are no
available resources associated with the user, how the stanza is
handled depends on the stanza type:

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1. For presence stanzas of type "subscribe", "subscribed",
"unsubscribe", and "unsubscribed", the server MUST maintain a
record of the stanza and deliver the stanza at least once
(i.e., when the user next creates an available resource); in
addition, the server MUST continue to deliver presence
stanzas of type "subscribe" until the user either approves or
denies the subscription request (see also Presence
Subscriptions (Section 5.1.6)).

2. For all other presence stanzas, the server SHOULD silently
ignore the stanza by not storing it for later delivery or
replying to it on behalf of the user.

3. For message stanzas, the server MAY choose to store the
stanza on behalf of the user and deliver it when the user
next becomes available, or forward the message to the user
via some other means (e.g., to the user's email account).
However, if offline message storage or message forwarding is
not enabled, the server MUST return to the sender a
stanza error. (Note: Offline message
storage and message forwarding are not defined in XMPP, since
they are strictly a matter of implementation and service
provisioning.)

4. For IQ stanzas, the server itself MUST reply on behalf of the
user with either an IQ result or an IQ error. Specifically,
if the semantics of the qualifying namespace define a reply
that the server can provide, the server MUST reply to the
stanza on behalf of the user; if not, the server MUST reply
with a stanza error.

11.2. Outbound Stanzas

If the hostname of the domain identifier portion of the address
contained in the 'to' attribute of an outbound stanza matches a
hostname of the server itself, the server MUST deliver the stanza to
a local entity according the rules for Inbound Stanzas (Section
11.1).

If the hostname of the domain identifier portion of the address
contained in the 'to' attribute of an outbound stanza does not match
a hostname of the server itself, the server MUST attempt to route the
stanza to the foreign domain. The recommended order of actions is as
follows:

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1. First attempt to resolve the foreign hostname using an [SRV]
Service of "xmpp-server" and Proto of "tcp", resulting in
resource records such as "_xmpp-server._tcp.example.com.", as
specified in [XMPP-CORE].

2. If the "xmpp-server" address record resolution fails, attempt to
resolve the "_im" or "_pres" [SRV] Service as specified in
[IMP-SRV], using the "_im" Service for stanzas and the
"_pres" Service for stanzas (it is up to the
implementation how to handle stanzas). This will result in
one or more resolutions of the form "_im..example.com." or
"_pres..example.com.", where "" would be a label
registered in the Instant Messaging SRV Protocol Label registry
or the Presence SRV Protocol Label registry: either "_xmpp" for
an XMPP-aware domain or some other IANA-registered label (e.g.,
"_simple") for a non-XMPP-aware domain.

3. If both SRV address record resolutions fail, attempt to perform a
normal IPv4/IPv6 address record resolution to determine the IP
address using the "xmpp-server" port of 5269 registered with the
IANA, as specified in [XMPP-CORE].

Administrators of server deployments are strongly encouraged to keep
the _im._xmpp, _pres._xmpp, and _xmpp._tcp SRV records properly
synchronized, since different implementations might perform the "_im"
and "_pres" lookups before the "xmpp-server" lookup.

12. IM and Presence Compliance Requirements

This section summarizes the specific aspects of the Extensible
Messaging and Presence Protocol that MUST be supported by instant
messaging and presence servers and clients in order to be considered
compliant implementations. All such applications MUST comply with
the requirements specified in [XMPP-CORE]. The text in this section
specifies additional compliance requirements for instant messaging
and presence servers and clients; note well that the requirements
described here supplement but do not supersede the core requirements.
Note also that a server or client MAY support only presence or
instant messaging, and is not required to support both if only a
presence service or an instant messaging service is desired.

12.1. Servers

In addition to core server compliance requirements, an instant
messaging and presence server MUST additionally support the following
protocols:

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o All server-related instant messaging and presence syntax and
semantics defined in this document, including presence broadcast
on behalf of clients, presence subscriptions, roster storage and
manipulation, privacy lists, and IM-specific routing and delivery
rules

12.2. Clients

In addition to core client compliance requirements, an instant
messaging and presence client MUST additionally support the following
protocols:

o Generation and handling of the IM-specific semantics of XML
stanzas as defined by the XML schemas, including the 'type'
attribute of message and presence stanzas as well as their child
elements

o All client-related instant messaging syntax and semantics defined
in this document, including presence subscriptions, roster
management, and privacy lists

o End-to-end object encryption as defined in End-to-End Object
Encryption in the Extensible Messaging and Presence Protocol
(XMPP) [XMPP-E2E]

A client MUST also handle addresses that are encoded as "im:" URIs as
specified in [CPIM], and MAY do so by removing the "im:" scheme and
entrusting address resolution to the server as specified under
Outbound Stanzas (Section 11.2).

13. Internationalization Considerations

For internationalization considerations, refer to the relevant
section of [XMPP-CORE].

14. Security Considerations

Core security considerations for XMPP are defined in the relevant
section of [XMPP-CORE].

Additional considerations that apply only to instant messaging and
presence applications of XMPP are defined in several places within
this memo; specifically:

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o When a server processes an inbound stanza of any kind whose
intended recipient is a user associated with one of the server's
hostnames, the server MUST first apply any privacy lists (Section
10) that are in force (see Server Rules for Handling XML Stanzas
(Section 11)).

o When a server processes an inbound presence stanza of type "probe"
whose intended recipient is a user associated with one of the
server's hostnames, the server MUST NOT reveal the user's presence
information if the sender is an entity that is not authorized to
receive that information as determined by presence subscriptions
(see Client and Server Presence Responsibilities (Section 5.1)).

o When a server processes an outbound presence stanza with no type
or of type "unavailable", it MUST follow the rules defined under
Client and Server Presence Responsibilities (Section 5.1) in order
to ensure that such presence information is not broadcasted to
entities that are not authorized to know such information.

o When a server generates an error stanza in response to receiving a
stanza for a user who does not exist, the use of the
error condition helps protect against
well-known dictionary attacks, since this is the same error
condition that is returned if, for instance, the namespace of an
IQ child element is not understood, or if offline message storage
or message forwarding is not enabled for a domain.

15. IANA Considerations

For a number of related IANA considerations, refer to the relevant
section of [XMPP-CORE].

15.1. XML Namespace Name for Session Data

A URN sub-namespace for session-related data in the Extensible
Messaging and Presence Protocol (XMPP) is defined as follows. (This
namespace name adheres to the format defined in The IETF XML Registry
[XML-REG].)

URI: urn:ietf:params:xml:ns:xmpp-session
Specification: RFC 3921
Description: This is the XML namespace name for session-related data
in the Extensible Messaging and Presence Protocol (XMPP) as
defined by RFC 3921.
Registrant Contact: IETF, XMPP Working Group,

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15.2. Instant Messaging SRV Protocol Label Registration

Address Resolution for Instant Messaging and Presence [IMP-SRV]
defines an Instant Messaging SRV Protocol Label registry for
protocols that can provide services that conform to the "_im" SRV
Service label. Because XMPP is one such protocol, the IANA registers
the "_xmpp" protocol label in the appropriate registry, as follows:

Protocol label: _xmpp
Specification: RFC 3921
Description: Instant messaging protocol label for the Extensible
Messaging and Presence Protocol (XMPP) as defined by RFC 3921.
Registrant Contact: IETF, XMPP Working Group,

15.3. Presence SRV Protocol Label Registration

Address Resolution for Instant Messaging and Presence [IMP-SRV]
defines a Presence SRV Protocol Label registry for protocols that can
provide services that conform to the "_pres" SRV Service label.
Because XMPP is one such protocol, the IANA registers the "_xmpp"
protocol label in the appropriate registry, as follows:

Protocol label: _xmpp
Specification: RFC 3921
Description: Presence protocol label for the Extensible Messaging and
Presence Protocol (XMPP) as defined by RFC 3921.
Registrant Contact: IETF, XMPP Working Group,

16. References

16.1. Normative References

[CPIM] Peterson, J., "Common Profile for Instant Messaging
(CPIM)", RFC 3860, August 2004.

[IMP-REQS] Day, M., Aggarwal, S., Mohr, G., and J. Vincent, "Instant
Messaging/Presence Protocol Requirements", RFC 2779,
February 2000.

[IMP-SRV] Peterson, J., "Address Resolution for Instant Messaging
and Presence", RFC 3861, August 2004.

[SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.

[TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

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RFC 3921 XMPP IM October 2004

[XML] Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler,
"Extensible Markup Language (XML) 1.0 (2nd ed)", W3C
REC-xml, October 2000, .

[XML-NAMES] Bray, T., Hollander, D., and A. Layman, "Namespaces in
XML", W3C REC-xml-names, January 1999,
.

[XMPP-CORE] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.

[XMPP-E2E] Saint-Andre, P., "End-to-End Object Encryption in the
Extensible Messaging and Presence Protocol (XMPP)", RFC
3923, October 2004.

16.2. Informative References

[IMP-MODEL] Day, M., Rosenberg, J., and H. Sugano, "A Model for
Presence and Instant Messaging", RFC 2778, February 2000.

[IRC] Oikarinen, J. and D. Reed, "Internet Relay Chat
Protocol", RFC 1459, May 1993.

[JEP-0054] Saint-Andre, P., "vcard-temp", JSF JEP 0054, March 2003.

[JEP-0077] Saint-Andre, P., "In-Band Registration", JSF JEP 0077,
August 2004.

[JEP-0078] Saint-Andre, P., "Non-SASL Authentication", JSF JEP 0078,
July 2004.

[JSF] Jabber Software Foundation, "Jabber Software Foundation",
.

[VCARD] Dawson, F. and T. Howes, "vCard MIME Directory Profile",
RFC 2426, September 1998.

[XML-REG] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.

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Appendix A. vCards

Sections 3.1.3 and 4.1.4 of [IMP-REQS] require that it be possible to
retrieve out-of-band contact information for other users (e.g.,
telephone number or email address). An XML representation of the
vCard specification defined in RFC 2426 [VCARD] is in common use
within the Jabber community to provide such information but is out of
scope for XMPP (documentation of this protocol is contained in
[JEP-0054], published by the Jabber Software Foundation [JSF]).

Appendix B. XML Schemas

The following XML schemas are descriptive, not normative. For
schemas defining the core features of XMPP, refer to [XMPP-CORE].

B.1 jabber:client

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B.2 jabber:server

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B.3 session

B.4 jabber:iq:privacy

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B.5 jabber:iq:roster

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Appendix C. Differences Between Jabber IM/Presence Protocols and XMPP

This section is non-normative.

XMPP has been adapted from the protocols originally developed in the
Jabber open-source community, which can be thought of as "XMPP 0.9".
Because there exists a large installed base of Jabber implementations
and deployments, it may be helpful to specify the key differences
between the relevant Jabber protocols and XMPP in order to expedite
and encourage upgrades of those implementations and deployments to
XMPP. This section summarizes the differences that relate
specifically to instant messaging and presence applications, while
the corresponding section of [XMPP-CORE] summarizes the differences
that relate to all XMPP applications.

C.1 Session Establishment

The client-to-server authentication protocol developed in the Jabber
community assumed that every client is an IM client and therefore
initiated an IM session upon successful authentication and resource
binding, which are performed simultaneously (documentation of this
protocol is contained in [JEP-0078], published by the Jabber Software
Foundation [JSF]). XMPP maintains a stricter separation between core
functionality and IM functionality; therefore, an IM session is not
created until the client specifically requests one using the protocol
defined under Session Establishment (Section 3).

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C.2 Privacy Lists

The Jabber community began to define a protocol for communications
blocking (privacy lists) in late 2001, but that effort was deprecated
once the XMPP Working Group was formed. Therefore the protocol
defined under Blocking Communication (Section 10) is the only such
protocol defined for use in the Jabber community.

Contributors

Most of the core aspects of the Extensible Messaging and Presence
Protocol were developed originally within the Jabber open-source
community in 1999. This community was founded by Jeremie Miller, who
released source code for the initial version of the jabberd server in
January 1999. Major early contributors to the base protocol also
included Ryan Eatmon, Peter Millard, Thomas Muldowney, and Dave
Smith. Work specific to instant messaging and presence by the XMPP
Working Group has concentrated especially on IM session establishment
and communication blocking (privacy lists); the session establishment
protocol was mainly developed by Rob Norris and Joe Hildebrand, and
the privacy lists protocol was originally contributed by Peter
Millard.

Acknowledgements

Thanks are due to a number of individuals in addition to the
contributors listed. Although it is difficult to provide a complete
list, the following individuals were particularly helpful in defining
the protocols or in commenting on the specifications in this memo:
Thomas Charron, Richard Dobson, Schuyler Heath, Jonathan Hogg, Craig
Kaes, Jacek Konieczny, Lisa Dusseault, Alexey Melnikov, Keith
Minkler, Julian Missig, Pete Resnick, Marshall Rose, Jean-Louis
Seguineau, Alexey Shchepin, Iain Shigeoka, and David Waite. Thanks
also to members of the XMPP Working Group and the IETF community for
comments and feedback provided throughout the life of this memo.

Author's Address

Peter Saint-Andre (editor)
Jabber Software Foundation

EMail: stpeter@jabber.org

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