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rfc:rfc3921

Network Working Group P. Saint-Andre, Ed. Request for Comments: 3921 Jabber Software Foundation Category: Standards Track October 2004

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

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

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

Saint-Andre Standards Track [Page 1] RFC 3921 XMPP IM October 2004

Table of Contents

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

1. Introduction

1.1. Overview

 The Extensible Messaging and Presence Protocol (XMPP) is a protocol
 for streaming XML [XML] elements in order to exchange messages and
 presence information in close to real time.  The core features of
 XMPP are defined in Extensible Messaging and Presence Protocol
 (XMPP): Core [XMPP-CORE].  These features -- mainly XML streams, use
 of TLS and SASL, and the <message/>, <presence/>, and <iq/> children
 of the stream root -- provide the building blocks for many types of
 near-real-time applications, which may be layered on top of the core
 by sending application-specific data qualified by particular XML
 namespaces [XML-NAMES].  This memo describes extensions to and
 applications of the core features of XMPP that provide the basic
 functionality expected of an instant messaging (IM) and presence
 application as defined in RFC 2779 [IMP-REQS].

Saint-Andre Standards Track [Page 2] RFC 3921 XMPP IM October 2004

1.2. Requirements

 For the purposes of this memo, the requirements of a basic instant
 messaging and presence application are defined by [IMP-REQS], which
 at a high level stipulates that a user must be able to complete the
 following use cases:
 o  Exchange messages with other users
 o  Exchange presence information with other users
 o  Manage subscriptions to and from other users
 o  Manage items in a contact list (in XMPP this is called a "roster")
 o  Block communications to or from specific other users
 Detailed definitions of these functionality areas are contained in
 [IMP-REQS], and the interested reader is directed to that document
 regarding the requirements addressed herein.
 [IMP-REQS] also stipulates that presence services must be separable
 from instant messaging services; i.e., it must be possible to use the
 protocol to provide a presence service, an instant messaging service,
 or both.  Although the text of this memo assumes that implementations
 and deployments will want to offer a unified instant messaging and
 presence service, there is no requirement that a service must offer
 both a presence service and an instant messaging service, and the
 protocol makes it possible to offer separate and distinct services
 for presence and for instant messaging.
 Note: While XMPP-based instant messaging and presence meets the
 requirements of [IMP-REQS], it was not designed explicitly with that
 specification in mind, since the base protocol evolved through an
 open development process within the Jabber open-source community
 before RFC 2779 was written.  Note also that although protocols
 addressing many other functionality areas have been defined in the
 Jabber community, such protocols are not included in this memo
 because they are not required by [IMP-REQS].

1.3. Terminology

 This memo inherits the terminology defined in [XMPP-CORE].
 The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in BCP
 14, RFC 2119 [TERMS].

Saint-Andre Standards Track [Page 3] RFC 3921 XMPP IM October 2004

2. Syntax of XML Stanzas

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

2.1. Message Syntax

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

2.1.1. Types of Message

 The 'type' attribute of a message stanza is RECOMMENDED; if included,
 it specifies the conversational context of the message, thus
 providing a hint regarding presentation (e.g., in a GUI).  If
 included, the 'type' attribute MUST have one of the following values:
 o  chat -- The message is sent in the context of a one-to-one chat
    conversation.  A compliant client SHOULD present the message in an
    interface enabling one-to-one chat between the two parties,
    including an appropriate conversation history.
 o  error -- An error has occurred related to a previous message sent
    by the sender (for details regarding stanza error syntax, refer to
    [XMPP-CORE]).  A compliant client SHOULD present an appropriate
    interface informing the sender of the nature of the error.
 o  groupchat -- The message is sent in the context of a multi-user
    chat environment (similar to that of [IRC]).  A compliant client
    SHOULD present the message in an interface enabling many-to-many
    chat between the parties, including a roster of parties in the
    chatroom and an appropriate conversation history.  Full definition
    of XMPP-based groupchat protocols is out of scope for this memo.
 o  headline -- The message is probably generated by an automated
    service that delivers or broadcasts content (news, sports, market
    information, RSS feeds, etc.).  No reply to the message is
    expected, and a compliant client SHOULD present the message in an

Saint-Andre Standards Track [Page 4] RFC 3921 XMPP IM October 2004

    interface that appropriately differentiates the message from
    standalone messages, chat sessions, or groupchat sessions (e.g.,
    by not providing the recipient with the ability to reply).
 o  normal -- The message is a single message that is sent outside the
    context of a one-to-one conversation or groupchat, and to which it
    is expected that the recipient will reply.  A compliant client
    SHOULD present the message in an interface enabling the recipient
    to reply, but without a conversation history.
 An IM application SHOULD support all of the foregoing message types;
 if an application receives a message with no 'type' attribute or the
 application does not understand the value of the 'type' attribute
 provided, it MUST consider the message to be of type "normal" (i.e.,
 "normal" is the default).  The "error" type MUST be generated only in
 response to an error related to a message received from another
 entity.
 Although the 'type' attribute is OPTIONAL, it is considered polite to
 mirror the type in any replies to a message; furthermore, some
 specialized applications (e.g., a multi-user chat service) MAY at
 their discretion enforce the use of a particular message type (e.g.,
 type='groupchat').

2.1.2. Child Elements

 As described under extended namespaces (Section 2.4), a message
 stanza MAY contain any properly-namespaced child element.
 In accordance with the default namespace declaration, by default a
 message stanza is qualified by the 'jabber:client' or 'jabber:server'
 namespace, which defines certain allowable children of message
 stanzas.  If the message stanza is of type "error", it MUST include
 an <error/> child; for details, see [XMPP-CORE].  Otherwise, the
 message stanza MAY contain any of the following child elements
 without an explicit namespace declaration:
 1.  <subject/>
 2.  <body/>
 3.  <thread/>

2.1.2.1. Subject

 The <subject/> element contains human-readable XML character data
 that specifies the topic of the message.  The <subject/> element MUST
 NOT possess any attributes, with the exception of the 'xml:lang'
 attribute.  Multiple instances of the <subject/> element MAY be
 included for the purpose of providing alternate versions of the same

Saint-Andre Standards Track [Page 5] RFC 3921 XMPP IM October 2004

 subject, but only if each instance possesses an 'xml:lang' attribute
 with a distinct language value.  The <subject/> element MUST NOT
 contain mixed content (as defined in Section 3.2.2 of [XML]).

2.1.2.2. Body

 The <body/> element contains human-readable XML character data that
 specifies the textual contents of the message; this child element is
 normally included but is OPTIONAL.  The <body/> element MUST NOT
 possess any attributes, with the exception of the 'xml:lang'
 attribute.  Multiple instances of the <body/> element MAY be included
 but only if each instance possesses an 'xml:lang' attribute with a
 distinct language value.  The <body/> element MUST NOT contain mixed
 content (as defined in Section 3.2.2 of [XML]).

2.1.2.3. Thread

 The <thread/> 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 <thread/> 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
 <thread/> element is OPTIONAL and is not used to identify individual
 messages, only conversations.  A message stanza MUST NOT contain more
 than one <thread/> element.  The <thread/> element MUST NOT possess
 any attributes.  The value of the <thread/> 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 <thread/>
 element MUST NOT contain mixed content (as defined in Section 3.2.2
 of [XML]).

2.2. Presence Syntax

 Presence stanzas are used qualified by the 'jabber:client' or
 'jabber:server' namespace to express an entity's current network
 availability (offline or online, along with various sub-states of the
 latter and optional user-defined descriptive text), and to notify
 other entities of that availability.  Presence stanzas are also used
 to negotiate and manage subscriptions to the presence of other
 entities.

Saint-Andre Standards Track [Page 6] RFC 3921 XMPP IM October 2004

2.2.1. Types of Presence

 The 'type' attribute of a presence stanza is OPTIONAL.  A presence
 stanza that does not possess a 'type' attribute is used to signal to
 the server that the sender is online and available for communication.
 If included, the 'type' attribute specifies a lack of availability, a
 request to manage a subscription to another entity's presence, a
 request for another entity's current presence, or an error related to
 a previously-sent presence stanza.  If included, the 'type' attribute
 MUST have one of the following values:
 o  unavailable -- Signals that the entity is no longer available for
    communication.
 o  subscribe -- The sender wishes to subscribe to the recipient's
    presence.
 o  subscribed -- The sender has allowed the recipient to receive
    their presence.
 o  unsubscribe -- The sender is unsubscribing from another entity's
    presence.
 o  unsubscribed -- The subscription request has been denied or a
    previously-granted subscription has been cancelled.
 o  probe -- A request for an entity's current presence; SHOULD be
    generated only by a server on behalf of a user.
 o  error -- An error has occurred regarding processing or delivery of
    a previously-sent presence stanza.
 For detailed information regarding presence semantics and the
 subscription model used in the context of XMPP-based instant
 messaging and presence applications, refer to Exchanging Presence
 Information (Section 5) and Managing Subscriptions (Section 6).

2.2.2. Child Elements

 As described under extended namespaces (Section 2.4), a presence
 stanza MAY contain any properly-namespaced child element.
 In accordance with the default namespace declaration, by default a
 presence stanza is qualified by the 'jabber:client' or
 'jabber:server' namespace, which defines certain allowable children
 of presence stanzas.  If the presence stanza is of type "error", it
 MUST include an <error/> child; for details, see [XMPP-CORE].  If the
 presence stanza possesses no 'type' attribute, it MAY contain any of

Saint-Andre Standards Track [Page 7] RFC 3921 XMPP IM October 2004

 the following child elements (note that the <status/> child MAY be
 sent in a presence stanza of type "unavailable" or, for historical
 reasons, "subscribe"):
 1.  <show/>
 2.  <status/>
 3.  <priority/>

2.2.2.1. Show

 The OPTIONAL <show/> 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 <show/> element.  The <show/> 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 <show/> element is provided, the entity is assumed to be online
 and available.

2.2.2.2. Status

 The OPTIONAL <status/> 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 <status/> element MUST NOT possess any attributes, with the
 exception of the 'xml:lang' attribute.  Multiple instances of the
 <status/> 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 <priority/> element contains non-human-readable XML
 character data that specifies the priority level of the resource. The
 value MUST be an integer between -128 and +127.  A presence stanza
 MUST NOT contain more than one <priority/> element.  The <priority/>
 element MUST NOT possess any attributes.  If no priority is provided,

Saint-Andre Standards Track [Page 8] RFC 3921 XMPP IM October 2004

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

2.3. IQ Syntax

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

2.4. Extended Namespaces

 While the three XML stanza kinds defined in the "jabber:client" or
 "jabber:server" namespace (along with their attributes and child
 elements) provide a basic level of functionality for messaging and
 presence, XMPP uses XML namespaces to extend the stanzas for the
 purpose of providing additional functionality.  Thus a message or
 presence stanza MAY contain one or more optional child elements
 specifying content that extends the meaning of the message (e.g., an
 XHTML-formatted version of the message body), and an IQ stanza MAY
 contain one such child element.  This child element MAY have any name
 and MUST possess an 'xmlns' namespace declaration (other than
 "jabber:client", "jabber:server", or
 "http://etherx.jabber.org/streams") that defines all data contained
 within the child element.
 Support for any given extended namespace is OPTIONAL on the part of
 any implementation (aside from the extended namespaces defined
 herein).  If an entity does not understand such a namespace, the
 entity's expected behavior depends on whether the entity is (1) the
 recipient or (2) an entity that is routing the stanza to the
 recipient:
 Recipient: If a recipient receives a stanza that contains a child
    element it does not understand, it SHOULD ignore that specific XML
    data, i.e., it SHOULD not process it or present it to a user or
    associated application (if any).  In particular:

Saint-Andre Standards Track [Page 9] RFC 3921 XMPP IM October 2004

  • If an entity receives a message or presence stanza that

contains XML data qualified by a namespace it does not

       understand, the portion of the stanza that is in the unknown
       namespace SHOULD be ignored.
  • If an entity receives a message stanza whose only child element

is qualified by a namespace it does not understand, it MUST

       ignore the entire stanza.
  • If an entity receives an IQ stanza of type "get" or "set"

containing a child element qualified by a namespace it does not

       understand, the entity SHOULD return an IQ stanza of type
       "error" with an error condition of <service-unavailable/>.
 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 <user@domain/resource>, 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 <session/> element
 qualified by the 'urn:ietf:params:xml:ns:xmpp-session' namespace in
 the stream features it advertises to a client after the completion of
 stream authentication as defined in [XMPP-CORE]:

Saint-Andre Standards Track [Page 10] RFC 3921 XMPP IM October 2004

 Server advertises session establishment feature to client:
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     id='c2s_345'
     from='example.com'
     version='1.0'>
 <stream:features>
   <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
   <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
 </stream:features>
 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 <session/> child element
 qualified by the 'urn:ietf:params:xml:ns:xmpp-session' namespace:
 Step 1: Client requests session with server:
 <iq to='example.com'
     type='set'
     id='sess_1'>
   <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
 </iq>
 Step 2: Server informs client that session has been created:
 <iq from='example.com'
     type='result'
     id='sess_1'/>
 Upon establishing a session, a connected resource (in the terminology
 of [XMPP-CORE]) is said to be an "active resource".
 Several error conditions are possible.  For example, the server may
 encounter an internal condition that prevents it from creating the
 session, the username or authorization identity may lack permissions
 to create a session, or there may already be an active resource
 associated with a resource identifier of the same name.
 If the server encounters an internal condition that prevents it from
 creating the session, it MUST return an error.

Saint-Andre Standards Track [Page 11] RFC 3921 XMPP IM October 2004

 Step 2 (alt): Server responds with error (internal server error):
 <iq from='example.com' type='error' id='sess_1'>
   <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
   <error type='wait'>
     <internal-server-error
         xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>
 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):
 <iq from='example.com' type='error' id='sess_1'>
   <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
   <error type='auth'>
     <forbidden
         xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>
 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 <conflict/> 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 <conflict/> 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):
 <stream:error>
   <conflict xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
 </stream:error>
 </stream:stream>

Saint-Andre Standards Track [Page 12] RFC 3921 XMPP IM October 2004

 Step 2 (alt): Server informs newly-requested session of resource
 conflict (case #2):
 <iq from='example.com' type='error' id='sess_1'>
   <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
   <error type='cancel'>
     <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>
 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 <message/> stanza.  If the message is being
 sent in reply to a message previously received from an address of the
 form <user@domain/resource> (e.g., within the context of a chat
 session), the value of the 'to' address SHOULD be of the form
 <user@domain/resource> rather than of the form <user@domain> unless
 the sender has knowledge (via presence) that the intended recipient's
 resource is no longer available.  If the message is being sent

Saint-Andre Standards Track [Page 13] RFC 3921 XMPP IM October 2004

 outside the context of any existing chat session or received message,
 the value of the 'to' address SHOULD be of the form <user@domain>
 rather than of the form <user@domain/resource>.

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

4.3. Specifying a Message Body

 A message stanza MAY (and often will) contain a child <body/> 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:
 <message
     to='romeo@example.net'
     from='juliet@example.com/balcony'
     type='chat'
     xml:lang='en'>
   <body>Wherefore art thou, Romeo?</body>
   <body xml:lang='cz'>Pro&#x010D;e&#x017D; jsi ty, Romeo?</body>
 </message>

4.4. Specifying a Message Subject

 A message stanza MAY contain one or more child <subject/> elements
 specifying the topic of the message (see Subject (Section 2.1.2.1)).

Saint-Andre Standards Track [Page 14] RFC 3921 XMPP IM October 2004

 Example: A message with a subject:
 <message
     to='romeo@example.net'
     from='juliet@example.com/balcony'
     type='chat'
     xml:lang='en'>
   <subject>I implore you!</subject>
   <subject
       xml:lang='cz'>&#x00DA;p&#x011B;nliv&#x011B; prosim!</subject>
   <body>Wherefore art thou, Romeo?</body>
   <body xml:lang='cz'>Pro&#x010D;e&#x017D; jsi ty, Romeo?</body>
 </message>

4.5. Specifying a Conversation Thread

 A message stanza MAY contain a child <thread/> 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:
 <message
     to='romeo@example.net/orchard'
     from='juliet@example.com/balcony'
     type='chat'
     xml:lang='en'>
   <body>Art thou not Romeo, and a Montague?</body>
   <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
 </message>
 <message
     to='juliet@example.com/balcony'
     from='romeo@example.net/orchard'
     type='chat'
     xml:lang='en'>
   <body>Neither, fair saint, if either thee dislike.</body>
   <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
 </message>
 <message
     to='romeo@example.net/orchard'
     from='juliet@example.com/balcony'
     type='chat'
     xml:lang='en'>
   <body>How cam'st thou hither, tell me, and wherefore?</body>
   <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
 </message>

Saint-Andre Standards Track [Page 15] RFC 3921 XMPP IM October 2004

5. Exchanging Presence Information

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

5.1. Client and Server Presence Responsibilities

5.1.1. Initial Presence

 After establishing a session, a client SHOULD send initial presence
 to the server in order to signal its availability for communications.
 As defined herein, the initial presence stanza (1) MUST possess no
 'to' address (signalling that it is meant to be broadcasted by the
 server on behalf of the client) and (2) MUST possess no 'type'
 attribute (signalling the user's availability).  After sending
 initial presence, an active resource is said to be an "available
 resource".
 Upon receiving initial presence from a client, the user's server MUST
 do the following if there is not already one or more available
 resources for the user (if there is already one or more available
 resources for the user, the server obviously does not need to send
 the presence probes, since it already possesses the requisite
 information):
 1.  Send presence probes (i.e., presence stanzas whose 'type'
     attribute is set to a value of "probe") from the full JID (e.g.,
     <user@example.com/resource>) of the user to all contacts to which
     the user is subscribed in order to determine if they are
     available; such contacts are those for which a JID is present in
     the user's roster with the 'subscription' attribute set to a
     value of "to" or "both".  (Note: The user's server MUST NOT send
     presence probes to contacts from which the user is blocking

Saint-Andre Standards Track [Page 16] RFC 3921 XMPP IM October 2004

     inbound presence notifications, as described under Blocking
     Inbound Presence Notifications (Section 10.10).)
 2.  Broadcast initial presence from the full JID (e.g.,
     <user@example.com/resource>) of the user to all contacts that are
     subscribed to the user's presence information; such contacts are
     those for which a JID is present in the user's roster with the
     'subscription' attribute set to a value of "from" or "both".
     (Note: The user's server MUST NOT broadcast initial presence to
     contacts to which the user is blocking outbound presence
     notifications, as described under Blocking Outbound Presence
     Notifications (Section 10.11).)
 In addition, the user's server MUST broadcast initial presence from
 the user's new available resource to any of the user's existing
 available resources (if any).
 Upon receiving initial presence from the user, the contact's server
 MUST deliver the user's presence stanza to the full JIDs
 (<contact@example.org/resource>) associated with all of the contact's
 available resources, but only if the user is in the contact's roster
 with a subscription state of "to" or "both" and the contact has not
 blocked inbound presence notifications from the user's bare or full
 JID (as defined under Blocking Inbound Presence Notifications
 (Section 10.10)).
 If the user's server receives a presence stanza of type "error" in
 response to the initial presence that it sent to a contact on behalf
 of the user, it SHOULD NOT send further presence updates to that
 contact (until and unless it receives a presence stanza from the
 contact).

5.1.2. Presence Broadcast

 After sending initial presence, the user MAY update its presence
 information for broadcasting at any time during its session by
 sending a presence stanza with no 'to' address and either no 'type'
 attribute or a 'type' attribute with a value of "unavailable". (Note:
 A user's client SHOULD NOT send a presence update to broadcast
 information that changes independently of the user's presence and
 availability.)
 If the presence stanza lacks a 'type' attribute (i.e., expresses
 availability), the user's server MUST broadcast the full XML of that
 presence stanza to all contacts (1) that are in the user's roster
 with a subscription type of "from" or "both", (2) to whom the user

Saint-Andre Standards Track [Page 17] RFC 3921 XMPP IM October 2004

 has not blocked outbound presence notifications, and (3) from whom
 the server has not received a presence error during the user's
 session (as well as to any of the user's other available resources).
 If the presence stanza has a 'type' attribute set to a value of
 "unavailable", the user's server MUST broadcast the full XML of that
 presence stanza to all entities that fit the above description, as
 well as to any entities to which the user has sent directed available
 presence during the user's session (if the user has not yet sent
 directed unavailable presence to that entity).

5.1.3. Presence Probes

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

state of "None", "None + Pending Out", or "To" (or is not in

        the contact's roster at all), the contact's server MUST return
        a <forbidden/> stanza error in response to the presence probe.
  • if the user is in the contact's roster with a subscription

state of "None + Pending In", "None + Pending Out/In", or "To

        + Pending In", the contact's server MUST return a
        <not-authorized/> stanza error in response to the presence
        probe.
 2.  Else, if the contact is blocking presence notifications to the
     user's bare JID or full JID (using either a default list or
     active list as defined under Blocking Outbound Presence
     Notifications (Section 10.11)), the server MUST NOT reply to the
     presence probe.
 3.  Else, if the contact has no available resources, the server MUST
     either (1) reply to the presence probe by sending to the user the
     full XML of the last presence stanza of type "unavailable"
     received by the server from the contact, or (2) not reply at all.

Saint-Andre Standards Track [Page 18] RFC 3921 XMPP IM October 2004

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

5.1.4. Directed Presence

 A user MAY send directed presence to another entity (i.e., a presence
 stanza with a 'to' attribute whose value is the JID of the other
 entity and with either no 'type' attribute or a 'type' attribute
 whose value is "unavailable").  There are three possible cases:
 1.  If the user sends directed presence to a contact that is in the
     user's roster with a subscription type of "from" or "both" after
     having sent initial presence and before sending unavailable
     presence broadcast, the user's server MUST route or deliver the
     full XML of that presence stanza (subject to privacy lists) but
     SHOULD NOT otherwise modify the contact's status regarding
     presence broadcast (i.e., it SHOULD include the contact's JID in
     any subsequent presence broadcasts initiated by the user).
 2.  If the user sends directed presence to an entity that is not in
     the user's roster with a subscription type of "from" or "both"
     after having sent initial presence and before sending unavailable
     presence broadcast, the user's server MUST route or deliver the
     full XML of that presence stanza to the entity but MUST NOT
     modify the contact's status regarding available presence
     broadcast (i.e., it MUST NOT include the entity's JID in any
     subsequent broadcasts of available presence initiated by the
     user); however, if the available resource from which the user
     sent the directed presence become unavailable, the user's server
     MUST broadcast that unavailable presence to the entity (if the
     user has not yet sent directed unavailable presence to that
     entity).
 3.  If the user sends directed presence without first sending initial
     presence or after having sent unavailable presence broadcast
     (i.e., the resource is active but not available), the user's
     server MUST treat the entities to which the user sends directed
     presence in the same way that it treats the entities listed in
     case #2 above.

Saint-Andre Standards Track [Page 19] RFC 3921 XMPP IM October 2004

5.1.5. Unavailable Presence

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

5.1.6. Presence Subscriptions

 A subscription request is a presence stanza whose 'type' attribute
 has a value of "subscribe".  If the subscription request is being
 sent to an instant messaging contact, the JID supplied in the 'to'
 attribute SHOULD be of the form <contact@example.org> rather than
 <contact@example.org/resource>, since the desired result is normally
 for the user to receive presence from all of the contact's resources,
 not merely the particular resource specified in the 'to' attribute.
 A user's server MUST NOT automatically approve subscription requests
 on the user's behalf.  All subscription requests MUST be directed to
 the user's client, specifically to one or more available resources
 associated with the user.  If there is no available resource
 associated with the user when the subscription request is received by
 the user's server, the user's server MUST keep a record of the
 subscription request and deliver the request when the user next
 creates an available resource, until the user either approves or
 denies the request.  If there is more than one available resource
 associated with the user when the subscription request is received by
 the user's server, the user's server MUST broadcast that subscription
 request to all available resources in accordance with Server Rules
 for Handling XML Stanzas (Section 11).  (Note: If an active resource

Saint-Andre Standards Track [Page 20] RFC 3921 XMPP IM October 2004

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

5.2. Specifying Availability Status

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

5.3. Specifying Detailed Status Information

 In conjunction with the <show/> element, a client MAY provide
 detailed status information by using the <status/> element (see
 Status (Section 2.2.2.2)).
 Example: Detailed status information:
 <presence xml:lang='en'>
   <show>dnd</show>
   <status>Wooing Juliet</status>
   <status xml:lang='cz'>Ja dvo&#x0159;&#x00ED;m Juliet</status>
 </presence>

Saint-Andre Standards Track [Page 21] RFC 3921 XMPP IM October 2004

5.4. Specifying Presence Priority

 A client MAY provide a priority for its resource by using the
 <priority/> element (see Priority (Section 2.2.2.3)).
 Example: Presence priority:
 <presence xml:lang='en'>
   <show>dnd</show>
   <status>Wooing Juliet</status>
   <status xml:lang='cz'>Ja dvo&#x0159;&#x00ED;m Juliet</status>
   <priority>1</priority>
 </presence>

5.5. Presence Examples

 The examples in this section illustrate the presence-related
 protocols described above.  The user is romeo@example.net, he has an
 available resource whose resource identifier is "orchard", and he has
 the following individuals in his roster:
 o  juliet@example.com (subscription="both" and she has two available
    resources, one whose resource is "chamber" and another whose
    resource is "balcony")
 o  benvolio@example.org (subscription="to")
 o  mercutio@example.org (subscription="from")
 Example 1: User sends initial presence:
 <presence/>
 Example 2: User's server sends presence probes to contacts with
 subscription="to" and subscription="both" on behalf of the user's
 available resource:
 <presence
     type='probe'
     from='romeo@example.net/orchard'
     to='juliet@example.com'/>
 <presence
     type='probe'
     from='romeo@example.net/orchard'
     to='benvolio@example.org'/>

Saint-Andre Standards Track [Page 22] RFC 3921 XMPP IM October 2004

 Example 3: User's server sends initial presence to contacts with
 subscription="from" and subscription="both" on behalf of the user's
 available resource:
 <presence
     from='romeo@example.net/orchard'
     to='juliet@example.com'/>
 <presence
     from='romeo@example.net/orchard'
     to='mercutio@example.org'/>
 Example 4: Contacts' servers reply to presence probe on behalf of all
 available resources:
 <presence
     from='juliet@example.com/balcony'
     to='romeo@example.net/orchard'
     xml:lang='en'>
   <show>away</show>
   <status>be right back</status>
   <priority>0</priority>
 </presence>
 <presence
     from='juliet@example.com/chamber'
     to='romeo@example.net/orchard'>
   <priority>1</priority>
 </presence>
 <presence
     from='benvolio@example.org/pda'
     to='romeo@example.net/orchard'
     xml:lang='en'>
   <show>dnd</show>
   <status>gallivanting</status>
 </presence>
 Example 5: Contacts' servers deliver user's initial presence to all
 available resources or return error to user:
 <presence
     from='romeo@example.net/orchard'
     to='juliet@example.com/chamber'/>

Saint-Andre Standards Track [Page 23] RFC 3921 XMPP IM October 2004

 <presence
     from='romeo@example.net/orchard'
     to='juliet@example.com/balcony'/>
 <presence
     type='error'
     from='mercutio@example.org'
     to='romeo@example.net/orchard'>
   <error type='cancel'>
     <gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </presence>
 Example 6: User sends directed presence to another user not in his
 roster:
 <presence
     from='romeo@example.net/orchard'
     to='nurse@example.com'
     xml:lang='en'>
   <show>dnd</show>
   <status>courting Juliet</status>
   <priority>0</priority>
 </presence>
 Example 7: User sends updated available presence information for
 broadcasting:
 <presence xml:lang='en'>
   <show>away</show>
   <status>I shall return!</status>
   <priority>1</priority>
 </presence>
 Example 8: User's server broadcasts updated presence information only
 to one contact (not those from whom an error was received or to whom
 the user sent directed presence):
 <presence
     from='romeo@example.net/orchard'
     to='juliet@example.com'
     xml:lang='en'>
   <show>away</show>
   <status>I shall return!</status>
   <priority>1</priority>
 </presence>

Saint-Andre Standards Track [Page 24] RFC 3921 XMPP IM October 2004

 Example 9: Contact's server delivers updated presence information to
 all of the contact's available resources:
 [to "balcony" resource...]
 <presence
     from='romeo@example.net/orchard'
     to='juliet@example.com'
     xml:lang='en'>
   <show>away</show>
   <status>I shall return!</status>
   <priority>1</priority>
 </presence>
 [to "chamber" resource...]
 <presence
     from='romeo@example.net/orchard'
     to='juliet@example.com'
     xml:lang='en'>
   <show>away</show>
   <status>I shall return!</status>
   <priority>1</priority>
 </presence>
 Example 10: One of the contact's resources broadcasts final presence:
 <presence from='juliet@example.com/balcony' type='unavailable'/>
 Example 11: Contact's server sends unavailable presence information
 to user:
 <presence
     type='unavailable'
     from='juliet@example.com/balcony'
     to='romeo@example.net/orchard'/>
 Example 12: User sends final presence:
 <presence from='romeo@example.net/orchard'
           type='unavailable'
           xml:lang='en'>
   <status>gone home</status>
 </presence>

Saint-Andre Standards Track [Page 25] RFC 3921 XMPP IM October 2004

 Example 13: User's server broadcasts unavailable presence information
 to contact as well as to the person to whom the user sent directed
 presence:
 <presence
     type='unavailable'
     from='romeo@example.net/orchard'
     to='juliet@example.com'
     xml:lang='en'>
   <status>gone home</status>
 </presence>
 <presence
     from='romeo@example.net/orchard'
     to='nurse@example.com'
     xml:lang='en'>
   <status>gone home</status>
 </presence>

6. Managing Subscriptions

 In order to protect the privacy of instant messaging users and any
 other entities, presence and availability information is disclosed
 only to other entities that the user has approved.  When a user has
 agreed that another entity may view its presence, the entity is said
 to have a subscription to the user's presence information.  A
 subscription lasts across sessions; indeed, it lasts until the
 subscriber unsubscribes or the subscribee cancels the
 previously-granted subscription.  Subscriptions are managed within
 XMPP by sending presence stanzas containing specially-defined
 attributes.
 Note: There are important interactions between subscriptions and
 rosters; these are defined under Integration of Roster Items and
 Presence Subscriptions (Section 8), and the reader must refer to that
 section for a complete understanding of presence subscriptions.

6.1. Requesting a Subscription

 A request to subscribe to another entity's presence is made by
 sending a presence stanza of type "subscribe".
 Example: Sending a subscription request:
 <presence to='juliet@example.com' type='subscribe'/>

Saint-Andre Standards Track [Page 26] RFC 3921 XMPP IM October 2004

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

6.2. Handling a Subscription Request

 When a client receives a subscription request from another entity, it
 MUST either approve the request by sending a presence stanza of type
 "subscribed" or refuse the request by sending a presence stanza of
 type "unsubscribed".
 Example: Approving a subscription request:
 <presence to='romeo@example.net' type='subscribed'/>
 Example: Refusing a presence subscription request:
 <presence to='romeo@example.net' type='unsubscribed'/>

6.3. Cancelling a Subscription from Another Entity

 If a user would like to cancel a previously-granted subscription
 request, it sends a presence stanza of type "unsubscribed".
 Example: Cancelling a previously granted subscription request:
 <presence to='romeo@example.net' type='unsubscribed'/>

6.4. Unsubscribing from Another Entity's Presence

 If a user would like to unsubscribe from the presence of another
 entity, it sends a presence stanza of type "unsubscribe".
 Example: Unsubscribing from an entity's presence:
 <presence to='juliet@example.com' type='unsubscribe'/>

7. Roster Management

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

Saint-Andre Standards Track [Page 27] RFC 3921 XMPP IM October 2004

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

7.1. Syntax and Semantics

 Rosters are managed using IQ stanzas, specifically by means of a
 <query/> child element qualified by the 'jabber:iq:roster' namespace.
 The <query/> element MAY contain one or more <item/> children, each
 describing a unique roster item or "contact".
 The "key" or unique identifier for each roster item is a JID,
 encapsulated in the 'jid' attribute of the <item/> element (which is
 REQUIRED).  The value of the 'jid' attribute SHOULD be of the form
 <user@domain> if the item is associated with another (human) instant
 messaging user.
 The state of the presence subscription in relation to a roster item
 is captured in the 'subscription' attribute of the <item/> element.
 Allowable values for this attribute are:
 o  "none" -- the user does not have a subscription to the contact's
    presence information, and the contact does not have a subscription
    to the user's presence information
 o  "to" -- the user has a subscription to the contact's presence
    information, but the contact does not have a subscription to the
    user's presence information
 o  "from" -- the contact has a subscription to the user's presence
    information, but the user does not have a subscription to the
    contact's presence information
 o  "both" -- both the user and the contact have subscriptions to each
    other's presence information
 Each <item/> element MAY contain a 'name' attribute, which sets the
 "nickname" to be associated with the JID, as determined by the user
 (not the contact).  The value of the 'name' attribute is opaque.
 Each <item/> element MAY contain one or more <group/> child elements,
 for use in collecting roster items into various categories.  The XML
 character data of the <group/> element is opaque.

Saint-Andre Standards Track [Page 28] RFC 3921 XMPP IM October 2004

7.2. Business Rules

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

7.3. Retrieving One's Roster on Login

 Upon connecting to the server and becoming an active resource, a
 client SHOULD request the roster before sending initial presence
 (however, because receiving the roster may not be desirable for all
 resources, e.g., a connection with limited bandwidth, the client's
 request for the roster is OPTIONAL).  If an available resource does
 not request the roster during a session, the server MUST NOT send it
 presence subscriptions and associated roster updates.
 Example: Client requests current roster from server:
 <iq from='juliet@example.com/balcony' type='get' id='roster_1'>
   <query xmlns='jabber:iq:roster'/>
 </iq>
 Example: Client receives roster from server:
 <iq to='juliet@example.com/balcony' type='result' id='roster_1'>
   <query xmlns='jabber:iq:roster'>
     <item jid='romeo@example.net'
           name='Romeo'
           subscription='both'>
       <group>Friends</group>
     </item>
     <item jid='mercutio@example.org'
           name='Mercutio'
           subscription='from'>
       <group>Friends</group>
     </item>
     <item jid='benvolio@example.org'
           name='Benvolio'
           subscription='both'>
       <group>Friends</group>
     </item>
   </query>

Saint-Andre Standards Track [Page 29] RFC 3921 XMPP IM October 2004

 </iq>

7.4. Adding a Roster Item

 At any time, a user MAY add an item to his or her roster.
 Example: Client adds a new item:
 <iq from='juliet@example.com/balcony' type='set' id='roster_2'>
   <query xmlns='jabber:iq:roster'>
     <item jid='nurse@example.com'
           name='Nurse'>
       <group>Servants</group>
     </item>
   </query>
 </iq>
 The server MUST update the roster information in persistent storage,
 and also push the change out to all of the user's available resources
 that have requested the roster.  This "roster push" consists of an IQ
 stanza of type "set" from the server to the client and enables all
 available resources to remain in sync with the server-based roster
 information.
 Example: Server (1) pushes the updated roster information to all
 available resources that have requested the roster and (2) replies
 with an IQ result to the sending resource:
 <iq to='juliet@example.com/balcony'
     type='set'
     id='a78b4q6ha463'>
   <query xmlns='jabber:iq:roster'>
     <item jid='nurse@example.com'
           name='Nurse'
           subscription='none'>
       <group>Servants</group>
     </item>
   </query>
 </iq>
 <iq to='juliet@example.com/chamber'
     type='set'
     id='a78b4q6ha464'>
   <query xmlns='jabber:iq:roster'>
     <item jid='nurse@example.com'
           name='Nurse'
           subscription='none'>
       <group>Servants</group>

Saint-Andre Standards Track [Page 30] RFC 3921 XMPP IM October 2004

     </item>
   </query>
 </iq>
 <iq to='juliet@example.com/balcony' type='result' id='roster_2'/>
 As required by the semantics of the IQ stanza kind as defined in
 [XMPP-CORE], each resource that received the roster push MUST reply
 with an IQ stanza of type "result" (or "error").
 Example: Resources reply with an IQ result to the server:
 <iq from='juliet@example.com/balcony'
     to='example.com'
     type='result'
     id='a78b4q6ha463'/>
 <iq from='juliet@example.com/chamber'
     to='example.com'
     type='result'
     id='a78b4q6ha464'/>

7.5. Updating a Roster Item

 Updating an existing roster item (e.g., changing the group) is done
 in the same way as adding a new roster item, i.e., by sending the
 roster item in an IQ set to the server.
 Example: User updates roster item (added group):
 <iq from='juliet@example.com/chamber' type='set' id='roster_3'>
   <query xmlns='jabber:iq:roster'>
     <item jid='romeo@example.net'
           name='Romeo'
           subscription='both'>
       <group>Friends</group>
       <group>Lovers</group>
     </item>
   </query>
 </iq>
 As with adding a roster item, when updating a roster item the server
 MUST update the roster information in persistent storage, and also
 initiate a roster push to all of the user's available resources that
 have requested the roster.

Saint-Andre Standards Track [Page 31] RFC 3921 XMPP IM October 2004

7.6. Deleting a Roster Item

 At any time, a user MAY delete an item from his or her roster by
 sending an IQ set to the server and making sure that the value of the
 'subscription' attribute is "remove" (a compliant server MUST ignore
 any other values of the 'subscription' attribute when received from a
 client).
 Example: Client removes an item:
 <iq from='juliet@example.com/balcony' type='set' id='roster_4'>
   <query xmlns='jabber:iq:roster'>
     <item jid='nurse@example.com' subscription='remove'/>
   </query>
 </iq>
 As with adding a roster item, when deleting a roster item the server
 MUST update the roster information in persistent storage, initiate a
 roster push to all of the user's available resources that have
 requested the roster (with the 'subscription' attribute set to a
 value of "remove"), and send an IQ result to the initiating resource.
 For further information about the implications of this command, see
 Removing a Roster Item and Cancelling All Subscriptions (Section
 8.6).

8. Integration of Roster Items and Presence Subscriptions

8.1. Overview

 Some level of integration between roster items and presence
 subscriptions is normally expected by an instant messaging user
 regarding the user's subscriptions to and from other contacts.  This
 section describes the level of integration that MUST be supported
 within XMPP instant messaging applications.
 There are four primary subscription states:
 o  None -- the user does not have a subscription to the contact's
    presence information, and the contact does not have a subscription
    to the user's presence information

Saint-Andre Standards Track [Page 32] RFC 3921 XMPP IM October 2004

 o  To -- the user has a subscription to the contact's presence
    information, but the contact does not have a subscription to the
    user's presence information
 o  From -- the contact has a subscription to the user's presence
    information, but the user does not have a subscription to the
    contact's presence information
 o  Both -- both the user and the contact have subscriptions to each
    other's presence information (i.e., the union of 'from' and 'to')
 Each of these states is reflected in the roster of both the user and
 the contact, thus resulting in durable subscription states.
 Narrative explanations of how these subscription states interact with
 roster items in order to complete certain defined use cases are
 provided in the following sub-sections.  Full details regarding
 server and client handling of all subscription states (including
 pending states between the primary states listed above) is provided
 in Subscription States (Section 9).
 The server MUST NOT send presence subscription requests or roster
 pushes to unavailable resources, nor to available resources that have
 not requested the roster.
 The 'from' and 'to' addresses are OPTIONAL in roster pushes; if
 included, their values SHOULD be the full JID of the resource for
 that session.  A client MUST acknowledge each roster push with an IQ
 stanza of type "result" (for the sake of brevity, these stanzas are
 not shown in the following examples but are required by the IQ
 semantics defined in [XMPP-CORE]).

8.2. User Subscribes to Contact

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

Saint-Andre Standards Track [Page 33] RFC 3921 XMPP IM October 2004

 <iq type='set' id='set1'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         name='MyContact'>
       <group>MyBuddies</group>
     </item>
   </query>
 </iq>
 2.  As a result, the user's server (1) MUST initiate a roster push
     for the new roster item to all available resources associated
     with this user that have requested the roster, setting the
     'subscription' attribute to a value of "none"; and (2) MUST reply
     to the sending resource with an IQ result indicating the success
     of the roster set:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='none'
         name='MyContact'>
       <group>MyBuddies</group>
     </item>
   </query>
 </iq>
 <iq type='result' id='set1'/>
 3.  If the user wants to request a subscription to the contact's
     presence information, the user's client MUST send a presence
     stanza of type='subscribe' to the contact:
 <presence to='contact@example.org' type='subscribe'/>
 4.  As a result, the user's server MUST initiate a second roster push
     to all of the user's available resources that have requested the
     roster, setting the contact to the pending sub-state of the
     'none' subscription state; this pending sub-state is denoted by
     the inclusion of the ask='subscribe' attribute in the roster
     item:

Saint-Andre Standards Track [Page 34] RFC 3921 XMPP IM October 2004

 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='none'
         ask='subscribe'
         name='MyContact'>
       <group>MyBuddies</group>
     </item>
   </query>
 </iq>
 Note: If the user did not create a roster item before sending the
 subscription request, the server MUST now create one on behalf of the
 user, then send a roster push to all of the user's available
 resources that have requested the roster, absent the 'name' attribute
 and the <group/> child shown above.
 5.  The user's server MUST also stamp the presence stanza of type
     "subscribe" with the user's bare JID (i.e., <user@example.com>)
     as the 'from' address (if the user provided a 'from' address set
     to the user's full JID, the server SHOULD remove the resource
     identifier).  If the contact is served by a different host than
     the user, the user's server MUST route the presence stanza to the
     contact's server for delivery to the contact (this case is
     assumed throughout; however, if the contact is served by the same
     host, then the server can simply deliver the presence stanza
     directly):
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='subscribe'/>
 Note: If the user's server receives a presence stanza of type "error"
 from the contact's server, it MUST deliver the error stanza to the
 user, whose client MAY determine that the error is in response to the
 outgoing presence stanza of type "subscribe" it sent previously
 (e.g., by tracking an 'id' attribute) and then choose to resend the
 "subscribe" request or revert the roster to its previous state by
 sending a presence stanza of type "unsubscribe" to the contact.
 6.  Upon receiving the presence stanza of type "subscribe" addressed
     to the contact, the contact's server MUST determine if there is
     at least one available resource from which the contact has
     requested the roster.  If so, it MUST deliver the subscription
     request to the contact (if not, the contact's server MUST store
     the subscription request offline for delivery when this condition

Saint-Andre Standards Track [Page 35] RFC 3921 XMPP IM October 2004

     is next met; normally this is done by adding a roster item for
     the contact to the user's roster, with a state of "None + Pending
     In" as defined under Subscription States (Section 9), however a
     server SHOULD NOT push or deliver roster items in that state to
     the contact).  No matter when the subscription request is
     delivered, the contact must decide whether or not to approve it
     (subject to the contact's configured preferences, the contact's
     client MAY approve or refuse the subscription request without
     presenting it to the contact).  Here we assume the "happy path"
     that the contact approves the subscription request (the alternate
     flow of declining the subscription request is defined in Section
     8.2.1).  In this case, the contact's client (1) SHOULD perform a
     roster set specifying the desired nickname and group for the user
     (if any); and (2) MUST send a presence stanza of type
     "subscribed" to the user in order to approve the subscription
     request.
 <iq type='set' id='set2'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>
 <presence to='user@example.com' type='subscribed'/>
 7.  As a result, the contact's server (1) MUST initiate a roster push
     to all available resources associated with the contact that have
     requested the roster, containing a roster item for the user with
     the subscription state set to 'from' (the server MUST send this
     even if the contact did not perform a roster set); (2) MUST
     return an IQ result to the sending resource indicating the
     success of the roster set; (3) MUST route the presence stanza of
     type "subscribed" to the user, first stamping the 'from' address
     as the bare JID (<contact@example.org>) of the contact; and (4)
     MUST send available presence from all of the contact's available
     resources to the user:

Saint-Andre Standards Track [Page 36] RFC 3921 XMPP IM October 2004

 <iq type='set' to='contact@example.org/resource'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         subscription='from'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>
 <iq type='result' to='contact@example.org/resource' id='set2'/>
 <presence
     from='contact@example.org'
     to='user@example.com'
     type='subscribed'/>
 <presence
     from='contact@example.org/resource'
     to='user@example.com'/>
 Note: If the contact's server receives a presence stanza of type
 "error" from the user's server, it MUST deliver the error stanza to
 the contact, whose client MAY determine that the error is in response
 to the outgoing presence stanza of type "subscribed" it sent
 previously (e.g., by tracking an 'id' attribute) and then choose to
 resend the "subscribed" notification or revert the roster to its
 previous state by sending a presence stanza of type "unsubscribed" to
 the user.
 8.  Upon receiving the presence stanza of type "subscribed" addressed
     to the user, the user's server MUST first verify that the contact
     is in the user's roster with either of the following states: (a)
     subscription='none' and ask='subscribe' or (b)
     subscription='from' and ask='subscribe'.  If the contact is not
     in the user's roster with either of those states, the user's
     server MUST silently ignore the presence stanza of type
     "subscribed" (i.e., it MUST NOT route it to the user, modify the
     user's roster, or generate a roster push to the user's available
     resources).  If the contact is in the user's roster with either
     of those states, the user's server (1) MUST deliver the presence
     stanza of type "subscribed" from the contact to the user; (2)
     MUST initiate a roster push to all of the user's available
     resources that have requested the roster, containing an updated
     roster item for the contact with the 'subscription' attribute set

Saint-Andre Standards Track [Page 37] RFC 3921 XMPP IM October 2004

     to a value of "to"; and (3) MUST deliver the available presence
     stanza received from each of the contact's available resources to
     each of the user's available resources:
 <presence
     to='user@example.com'
     from='contact@example.org'
     type='subscribed'/>
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='to'
         name='MyContact'>
       <group>MyBuddies</group>
     </item>
   </query>
 </iq>
 <presence
     from='contact@example.org/resource'
     to='user@example.com/resource'/>
 9.  Upon receiving the presence stanza of type "subscribed", the user
     SHOULD acknowledge receipt of that subscription state
     notification through either "affirming" it by sending a presence
     stanza of type "subscribe" to the contact or "denying" it by
     sending a presence stanza of type "unsubscribe" to the contact;
     this step does not necessarily affect the subscription state (see
     Subscription States (Section 9) for details), but instead lets
     the user's server know that it MUST no longer send notification
     of the subscription state change to the user (see Section 9.4).
 From the perspective of the user, there now exists a subscription to
 the contact's presence information; from the perspective of the
 contact, there now exists a subscription from the user.

8.2.1. Alternate Flow: Contact Declines Subscription Request

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

Saint-Andre Standards Track [Page 38] RFC 3921 XMPP IM October 2004

 1.  If the contact wants to refuse the request, the contact's client
     MUST send a presence stanza of type "unsubscribed" to the user
     (instead of the presence stanza of type "subscribed" sent in Step
     6 of Section 8.2):
 <presence to='user@example.com' type='unsubscribed'/>
 2.  As a result, the contact's server MUST route the presence stanza
     of type "unsubscribed" to the user, first stamping the 'from'
     address as the bare JID (<contact@example.org>) of the contact:
 <presence
     from='contact@example.org'
     to='user@example.com'
     type='unsubscribed'/>
 Note: If the contact's server previously added the user to the
 contact's roster for tracking purposes, it MUST remove the relevant
 item at this time.
 3.  Upon receiving the presence stanza of type "unsubscribed"
     addressed to the user, the user's server (1) MUST deliver that
     presence stanza to the user and (2) MUST initiate a roster push
     to all of the user's available resources that have requested the
     roster, containing an updated roster item for the contact with
     the 'subscription' attribute set to a value of "none" and with no
     'ask' attribute:
 <presence
     from='contact@example.org'
     to='user@example.com'
     type='unsubscribed'/>
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='none'
         name='MyContact'>
       <group>MyBuddies</group>
     </item>
   </query>
 </iq>
 4.  Upon receiving the presence stanza of type "unsubscribed", the
     user SHOULD acknowledge receipt of that subscription state
     notification through either "affirming" it by sending a presence
     stanza of type "unsubscribe" to the contact or "denying" it by

Saint-Andre Standards Track [Page 39] RFC 3921 XMPP IM October 2004

     sending a presence stanza of type "subscribe" to the contact;
     this step does not necessarily affect the subscription state (see
     Subscription States (Section 9) for details), but instead lets
     the user's server know that it MUST no longer send notification
     of the subscription state change to the user (see Section 9.4).
 As a result of this activity, the contact is now in the user's roster
 with a subscription state of "none", whereas the user is not in the
 contact's roster at all.

8.3. Creating a Mutual Subscription

 The user and contact can build on the "happy path" described above to
 create a mutual subscription (i.e., a subscription of type "both").
 The process is described below.
 1.  If the contact wants to create a mutual subscription, the contact
     MUST send a subscription request to the user (subject to the
     contact's configured preferences, the contact's client MAY send
     this automatically):
 <presence to='user@example.com' type='subscribe'/>
 2.  As a result, the contact's server (1) MUST initiate a roster push
     to all available resources associated with the contact that have
     requested the roster, with the user still in the 'from'
     subscription state but with a pending 'to' subscription denoted
     by the inclusion of the ask='subscribe' attribute in the roster
     item; and (2) MUST route the presence stanza of type "subscribe"
     to the user, first stamping the 'from' address as the bare JID
     (<contact@example.org>) of the contact:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         subscription='from'
         ask='subscribe'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>

Saint-Andre Standards Track [Page 40] RFC 3921 XMPP IM October 2004

 <presence
     from='contact@example.org'
     to='user@example.com'
     type='subscribe'/>
 Note: If the contact's server receives a presence stanza of type
 "error" from the user's server, it MUST deliver the error stanza to
 the contact, whose client MAY determine that the error is in response
 to the outgoing presence stanza of type "subscribe" it sent
 previously (e.g., by tracking an 'id' attribute) and then choose to
 resend the "subscribe" request or revert the roster to its previous
 state by sending a presence stanza of type "unsubscribe" to the user.
 3.  Upon receiving the presence stanza of type "subscribe" addressed
     to the user, the user's server must determine if there is at
     least one available resource for which the user has requested the
     roster.  If so, the user's server MUST deliver the subscription
     request to the user (if not, it MUST store the subscription
     request offline for delivery when this condition is next met). No
     matter when the subscription request is delivered, the user must
     then decide whether or not to approve it (subject to the user's
     configured preferences, the user's client MAY approve or refuse
     the subscription request without presenting it to the user).
     Here we assume the "happy path" that the user approves the
     subscription request (the alternate flow of declining the
     subscription request is defined in Section 8.3.1).  In this case,
     the user's client MUST send a presence stanza of type
     "subscribed" to the contact in order to approve the subscription
     request.
 <presence to='contact@example.org' type='subscribed'/>
 4.  As a result, the user's server (1) MUST initiate a roster push to
     all of the user's available resources that have requested the
     roster, containing a roster item for the contact with the
     'subscription' attribute set to a value of "both"; (2) MUST route
     the presence stanza of type "subscribed" to the contact, first
     stamping the 'from' address as the bare JID (<user@example.com>)
     of the user; and (3) MUST send to the contact the full XML of the
     last presence stanza with no 'to' attribute received by the
     server from each of the user's available resources (subject to
     privacy lists in force for each session):

Saint-Andre Standards Track [Page 41] RFC 3921 XMPP IM October 2004

 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='both'
         name='MyContact'>
       <group>MyBuddies</group>
     </item>
   </query>
 </iq>
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='subscribed'/>
 <presence
     from='user@example.com/resource'
     to='contact@example.org'/>
 Note: If the user's server receives a presence stanza of type "error"
 from the contact's server, it MUST deliver the error stanza to the
 user, whose client MAY determine that the error is in response to the
 outgoing presence stanza of type "subscribed" it sent previously
 (e.g., by tracking an 'id' attribute) and then choose to resend the
 subscription request or revert the roster to its previous state by
 sending a presence stanza of type "unsubscribed" to the contact.
 5.  Upon receiving the presence stanza of type "subscribed" addressed
     to the contact, the contact's server MUST first verify that the
     user is in the contact's roster with either of the following
     states: (a) subscription='none' and ask='subscribe' or (b)
     subscription='from' and ask='subscribe'.  If the user is not in
     the contact's roster with either of those states, the contact's
     server MUST silently ignore the presence stanza of type
     "subscribed" (i.e., it MUST NOT route it to the contact, modify
     the contact's roster, or generate a roster push to the contact's
     available resources).  If the user is in the contact's roster
     with either of those states, the contact's server (1) MUST
     deliver the presence stanza of type "subscribed" from the user to
     the contact; (2) MUST initiate a roster push to all available
     resources associated with the contact that have requested the
     roster, containing an updated roster item for the user with the
     'subscription' attribute set to a value of "both"; and (3) MUST
     deliver the available presence stanza received from each of the
     user's available resources to each of the contact's available
     resources:

Saint-Andre Standards Track [Page 42] RFC 3921 XMPP IM October 2004

 <presence
     from='user@example.com'
     to='contact@example.org'
     type='subscribed'/>
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         subscription='both'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>
 <presence
     from='user@example.com/resource'
     to='contact@example.org/resource'/>
 6.  Upon receiving the presence stanza of type "subscribed", the
     contact SHOULD acknowledge receipt of that subscription state
     notification through either "affirming" it by sending a presence
     stanza of type "subscribe" to the user or "denying" it by sending
     a presence stanza of type "unsubscribe" to the user; this step
     does not necessarily affect the subscription state (see
     Subscription States (Section 9) for details), but instead lets
     the contact's server know that it MUST no longer send
     notification of the subscription state change to the contact (see
     Section 9.4).
 The user and the contact now have a mutual subscription to each
 other's presence -- i.e., the subscription is of type "both".

8.3.1. Alternate Flow: User Declines Subscription Request

 The above activity flow represents the "happy path" regarding the
 contact's subscription request to the user.  The main alternate flow
 occurs if the user refuses the contact's subscription request, as
 described below.
 1.  If the user wants to refuse the request, the user's client MUST
     send a presence stanza of type "unsubscribed" to the contact
     (instead of the presence stanza of type "subscribed" sent in Step
     3 of Section 8.3):
 <presence to='contact@example.org' type='unsubscribed'/>

Saint-Andre Standards Track [Page 43] RFC 3921 XMPP IM October 2004

 2.  As a result, the user's server MUST route the presence stanza of
     type "unsubscribed" to the contact, first stamping the 'from'
     address as the bare JID (<user@example.com>) of the user:
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='unsubscribed'/>
 3.  Upon receiving the presence stanza of type "unsubscribed"
     addressed to the contact, the contact's server (1) MUST deliver
     that presence stanza to the contact; and (2) MUST initiate a
     roster push to all available resources associated with the
     contact that have requested the roster, containing an updated
     roster item for the user with the 'subscription' attribute set to
     a value of "from" and with no 'ask' attribute:
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='unsubscribed'/>
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         subscription='from'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>
 4.  Upon receiving the presence stanza of type "unsubscribed", the
     contact SHOULD acknowledge receipt of that subscription state
     notification through either "affirming" it by sending a presence
     stanza of type "unsubscribe" to the user or "denying" it by
     sending a presence stanza of type "subscribe" to the user; this
     step does not necessarily affect the subscription state (see
     Subscription States (Section 9) for details), but instead lets
     the contact's server know that it MUST no longer send
     notification of the subscription state change to the contact (see
     Section 9.4).
 As a result of this activity, there has been no change in the
 subscription state; i.e., the contact is in the user's roster with a
 subscription state of "to" and the user is in the contact's roster
 with a subscription state of "from".

Saint-Andre Standards Track [Page 44] RFC 3921 XMPP IM October 2004

8.4. Unsubscribing

 At any time after subscribing to a contact's presence information, a
 user MAY unsubscribe.  While the XML that the user sends to make this
 happen is the same in all instances, the subsequent subscription
 state is different depending on the subscription state obtaining when
 the unsubscribe "command" is sent.  Both possible scenarios are
 described below.

8.4.1. Case #1: Unsubscribing When Subscription is Not Mutual

 In the first case, the user has a subscription to the contact's
 presence information but the contact does not have a subscription to
 the user's presence information (i.e., the subscription is not yet
 mutual).
 1.  If the user wants to unsubscribe from the contact's presence
     information, the user MUST send a presence stanza of type
     "unsubscribe" to the contact:
 <presence to='contact@example.org' type='unsubscribe'/>
 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
     (<user@example.com>) of the user:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='none'
         name='MyContact'>
       <group>MyBuddies</group>
     </item>
   </query>
 </iq>
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='unsubscribe'/>

Saint-Andre Standards Track [Page 45] RFC 3921 XMPP IM October 2004

 3.  Upon receiving the presence stanza of type "unsubscribe"
     addressed to the contact, the contact's server (1) MUST initiate
     a roster push to all available resources associated with the
     contact that have requested the roster, containing an updated
     roster item for the user with the 'subscription' attribute set to
     a value of "none" (if the contact is unavailable or has not
     requested the roster, the contact's server MUST modify the roster
     item and send that modified item the next time the contact
     requests the roster); and (2) MUST deliver the "unsubscribe"
     state change notification to the contact:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         subscription='none'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='unsubscribe'/>
 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:
 <presence
     from='contact@example.org'
     to='user@example.com'
     type='unsubscribed'/>

Saint-Andre Standards Track [Page 46] RFC 3921 XMPP IM October 2004

 <presence
     from='contact@example.org/resource'
     to='user@example.com'
     type='unavailable'/>
 6.  When the user's server receives the presence stanzas of type
     "unsubscribed" and "unavailable", it MUST deliver them to the
     user:
 <presence
     from='contact@example.org'
     to='user@example.com'
     type='unsubscribed'/>
 <presence
     from='contact@example.org/resource'
     to='user@example.com'
     type='unavailable'/>
 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:
 <presence to='contact@example.org' type='unsubscribe'/>
 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
     (<user@example.com>) of the user:

Saint-Andre Standards Track [Page 47] RFC 3921 XMPP IM October 2004

 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='from'
         name='MyContact'>
       <group>MyBuddies</group>
     </item>
   </query>
 </iq>
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='unsubscribe'/>
 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:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         subscription='to'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='unsubscribe'/>
 4.  Upon receiving the presence stanza of type "unsubscribe", the
     contact SHOULD acknowledge receipt of that subscription state
     notification through either "affirming" it by sending a presence
     stanza of type "unsubscribed" to the user or "denying" it by
     sending a presence stanza of type "subscribed" to the user; this

Saint-Andre Standards Track [Page 48] RFC 3921 XMPP IM October 2004

     step does not necessarily affect the subscription state (see
     Subscription States (Section 9) for details), but instead lets
     the contact's server know that it MUST no longer send
     notification of the subscription state change to the contact (see
     Section 9.4).
 5.  The contact's server then (1) MUST send a presence stanza of type
     "unsubscribed" to the user; and (2) SHOULD send unavailable
     presence from all of the contact's available resources to the
     user:
 <presence
     from='contact@example.org'
     to='user@example.com'
     type='unsubscribed'/>
 <presence
     from='contact@example.org/resource'
     to='user@example.com'
     type='unavailable'/>
 6.  When the user's server receives the presence stanzas of type
     "unsubscribed" and "unavailable", it MUST deliver them to the
     user:
 <presence
     from='contact@example.org'
     to='user@example.com'
     type='unsubscribed'/>
 <presence
     from='contact@example.org/resource'
     to='user@example.com'
     type='unavailable'/>
 7.  Upon receiving the presence stanza of type "unsubscribed", the
     user SHOULD acknowledge receipt of that subscription state
     notification through either "affirming" it by sending a presence
     stanza of type "unsubscribe" to the contact or "denying" it by
     sending a presence stanza of type "subscribe" to the contact;
     this step does not necessarily affect the subscription state (see
     Subscription States (Section 9) for details), but instead lets
     the user's server know that it MUST no longer send notification
     of the subscription state change to the user (see Section 9.4).
 Note: Obviously this does not result in removal of the roster item
 from the user's roster, and the contact still has a subscription to
 the user's presence information.  In order to both completely cancel

Saint-Andre Standards Track [Page 49] RFC 3921 XMPP IM October 2004

 a mutual subscription and fully remove the roster item from the
 user's roster, the user SHOULD update the roster item with
 subscription='remove' as defined under Removing a Roster Item and
 Cancelling All Subscriptions (Section 8.6).

8.5. Cancelling a Subscription

 At any time after approving a subscription request from a user, a
 contact MAY cancel that subscription.  While the XML that the contact
 sends to make this happen is the same in all instances, the
 subsequent subscription state is different depending on the
 subscription state obtaining when the cancellation was sent.  Both
 possible scenarios are described below.

8.5.1. Case #1: Cancelling When Subscription is Not Mutual

 In the first case, the user has a subscription to the contact's
 presence information but the contact does not have a subscription to
 the user's presence information (i.e., the subscription is not yet
 mutual).
 1.  If the contact wants to cancel the user's subscription, the
     contact MUST send a presence stanza of type "unsubscribed" to the
     user:
 <presence to='user@example.com' type='unsubscribed'/>
 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
     (<contact@example.org>) of the contact; and (3) SHOULD send
     unavailable presence from all of the contact's available
     resources to the user:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         subscription='none'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>

Saint-Andre Standards Track [Page 50] RFC 3921 XMPP IM October 2004

 <presence
     from='contact@example.org'
     to='user@example.com'
     type='unsubscribed'/>
 <presence
     from='contact@example.org/resource'
     to='user@example.com'
     type='unavailable'/>
 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:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='none'
         name='MyContact'>
       <group>MyBuddies</group>
     </item>
   </query>
 </iq>
 <presence
     from='contact@example.org'
     to='user@example.com'
     type='unsubscribed'/>
 <presence
     from='contact@example.org/resource'
     to='user@example.com'
     type='unavailable'/>
 4.  Upon receiving the presence stanza of type "unsubscribed", the
     user SHOULD acknowledge receipt of that subscription state
     notification through either "affirming" it by sending a presence
     stanza of type "unsubscribe" to the contact or "denying" it by
     sending a presence stanza of type "subscribe" to the contact;

Saint-Andre Standards Track [Page 51] RFC 3921 XMPP IM October 2004

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

8.5.2. Case #2: Cancelling When Subscription is Mutual

 In the second case, the user has a subscription to the contact's
 presence information and the contact also has a subscription to the
 user's presence information (i.e., the subscription is mutual).
 1.  If the contact wants to cancel the user's subscription, the
     contact MUST send a presence stanza of type "unsubscribed" to the
     user:
 <presence to='user@example.com' type='unsubscribed'/>
 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
     (<contact@example.org>) 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:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         subscription='to'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>
 <presence
     from='contact@example.org'
     to='user@example.com'
     type='unsubscribed'/>
 <presence
     from='contact@example.org/resource'
     to='user@example.com'
     type='unavailable'/>

Saint-Andre Standards Track [Page 52] RFC 3921 XMPP IM October 2004

 3.  Upon receiving the presence stanza of type "unsubscribed"
     addressed to the user, the user's server (1) MUST initiate a
     roster push to all of the user's available resources that have
     requested the roster, containing an updated roster item for the
     contact with the 'subscription' attribute set to a value of
     "from" (if the user is unavailable or has not requested the
     roster, the user's server MUST modify the roster item and send
     that modified item the next time the user requests the roster);
     and (2) MUST deliver the "unsubscribed" state change notification
     to all of the user's available resources; and (3) MUST deliver
     the unavailable presence to all of the user's available
     resources:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='from'
         name='MyContact'>
       <group>MyBuddies</group>
     </item>
   </query>
 </iq>
 <presence
     from='contact@example.org'
     to='user@example.com'
     type='unsubscribed'/>
 <presence
     from='contact@example.org/resource'
     to='user@example.com'
     type='unavailable'/>
 4.  Upon receiving the presence stanza of type "unsubscribed", the
     user SHOULD acknowledge receipt of that subscription state
     notification through either "affirming" it by sending a presence
     stanza of type "unsubscribe" to the contact or "denying" it by
     sending a presence stanza of type "subscribe" to the contact;
     this step does not necessarily affect the subscription state (see
     Subscription States (Section 9) for details), but instead lets
     the user's server know that it MUST no longer send notification
     of the subscription state change to the user (see Section 9.4).
 Note: Obviously this does not result in removal of the roster item
 from the contact's roster, and the contact still has a subscription
 to the user's presence information.  In order to both completely
 cancel a mutual subscription and fully remove the roster item from

Saint-Andre Standards Track [Page 53] RFC 3921 XMPP IM October 2004

 the contact's roster, the contact should update the roster item with
 subscription='remove' as defined under Removing a Roster Item and
 Cancelling All Subscriptions (Section 8.6).

8.6. Removing a Roster Item and Cancelling All Subscriptions

 Because there may be many steps involved in completely removing a
 roster item and cancelling subscriptions in both directions, the
 roster management protocol includes a "shortcut" method for doing so.
 The process may be initiated no matter what the current subscription
 state is by sending a roster set containing an item for the contact
 with the 'subscription' attribute set to a value of "remove":
 <iq type='set' id='remove1'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='remove'/>
   </query>
 </iq>
 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:
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='unsubscribe'/>
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='unsubscribed'/>

Saint-Andre Standards Track [Page 54] RFC 3921 XMPP IM October 2004

 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='contact@example.org'
         subscription='remove'/>
   </query>
 </iq>
 <iq type='result' id='remove1'/>
 <presence
     from='user@example.com/resource'
     to='contact@example.org'
     type='unavailable'/>
 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:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         subscription='to'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='unsubscribe'/>
 Upon receiving the presence stanza of type "unsubscribed", the
 contact's server (1) MUST initiate a roster push to all available
 resources associated with the contact that have requested the roster,
 containing an updated roster item for the user with the
 'subscription' attribute set to a value of "none" (if the contact is
 unavailable or has not requested the roster, the contact's server

Saint-Andre Standards Track [Page 55] RFC 3921 XMPP IM October 2004

 MUST modify the roster item and send that modified item the next time
 the contact requests the roster); and (2) MUST also deliver the
 "unsubscribe" state change notification to all of the contact's
 available resources:
 <iq type='set'>
   <query xmlns='jabber:iq:roster'>
     <item
         jid='user@example.com'
         subscription='none'
         name='SomeUser'>
       <group>SomeGroup</group>
     </item>
   </query>
 </iq>
 <presence
     from='user@example.com'
     to='contact@example.org'
     type='unsubscribed'/>
 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:
 <presence
     from='user@example.com/resource'
     to='contact@example.org'
     type='unavailable'/>
 Note: When the user removes the contact from the user's roster, the
 end state of the contact's roster is that the user is still in the
 contact's roster with a subscription state of "none"; in order to
 completely remove the roster item for the user, the contact needs to
 also send a roster removal request.

9. Subscription States

 This section provides detailed information about subscription states
 and server handling of subscription-related presence stanzas (i.e.,
 presence stanzas of type "subscribe", "subscribed", "unsubscribe",
 and "unsubscribed").

9.1. Defined States

 There are nine possible subscription states, which are described here
 from the user's (not contact's) perspective:

Saint-Andre Standards Track [Page 56] RFC 3921 XMPP IM October 2004

 1.  "None" = contact and user are not subscribed to each other, and
     neither has requested a subscription from the other
 2.  "None + Pending Out" = contact and user are not subscribed to
     each other, and user has sent contact a subscription request but
     contact has not replied yet
 3.  "None + Pending In" = contact and user are not subscribed to each
     other, and contact has sent user a subscription request but user
     has not replied yet (note: contact's server SHOULD NOT push or
     deliver roster items in this state, but instead SHOULD wait until
     contact has approved subscription request from user)
 4.  "None + Pending Out/In" = contact and user are not subscribed to
     each other, contact has sent user a subscription request but user
     has not replied yet, and user has sent contact a subscription
     request but contact has not replied yet
 5.  "To" = user is subscribed to contact (one-way)
 6.  "To + Pending In" = user is subscribed to contact, and contact
     has sent user a subscription request but user has not replied yet
 7.  "From" = contact is subscribed to user (one-way)
 8.  "From + Pending Out" = contact is subscribed to user, and user
     has sent contact a subscription request but contact has not
     replied yet
 9.  "Both" = user and contact are subscribed to each other (two-way)

9.2. Server Handling of Outbound Presence Subscription Stanzas

 Outbound presence subscription stanzas enable the user to manage his
 or her subscription to the contact's presence information (via the
 "subscribe" and "unsubscribe" types), and to manage the contact's
 access to the user's presence information (via the "subscribed" and
 "unsubscribed" types).
 Because it is possible for the user's server and the contact's server
 to lose synchronization regarding subscription states, the user's
 server MUST without exception route all outbound presence stanzas of
 type "subscribe" or "unsubscribe" to the contact so that the user is
 able to resynchronize his or her subscription to the contact's
 presence information if needed.

Saint-Andre Standards Track [Page 57] RFC 3921 XMPP IM October 2004

 The user's server SHOULD NOT route a presence stanza of type
 "subscribed" or "unsubscribed" to the contact if the stanza does not
 result in a subscription state change from the user's perspective,
 and MUST NOT make a state change.  If the stanza results in a
 subscription state change, the user's server MUST route the stanza to
 the contact and MUST make the appropriate state change.  These rules
 are summarized in the following tables.
 Table 1: Recommended handling of outbound "subscribed" stanzas
 +----------------------------------------------------------------+
 |  EXISTING STATE          |  ROUTE?  |  NEW STATE               |
 +----------------------------------------------------------------+
 |  "None"                  |  no      |  no state change         |
 |  "None + Pending Out"    |  no      |  no state change         |
 |  "None + Pending In"     |  yes     |  "From"                  |
 |  "None + Pending Out/In" |  yes     |  "From + Pending Out"    |
 |  "To"                    |  no      |  no state change         |
 |  "To + Pending In"       |  yes     |  "Both"                  |
 |  "From"                  |  no      |  no state change         |
 |  "From + Pending Out"    |  no      |  no state change         |
 |  "Both"                  |  no      |  no state change         |
 +----------------------------------------------------------------+
 Table 2: Recommended handling of outbound "unsubscribed" stanzas
 +----------------------------------------------------------------+
 |  EXISTING STATE          |  ROUTE?  |  NEW STATE               |
 +----------------------------------------------------------------+
 |  "None"                  |  no      |  no state change         |
 |  "None + Pending Out"    |  no      |  no state change         |
 |  "None + Pending In"     |  yes     |  "None"                  |
 |  "None + Pending Out/In" |  yes     |  "None + Pending Out"    |
 |  "To"                    |  no      |  no state change         |
 |  "To + Pending In"       |  yes     |  "To"                    |
 |  "From"                  |  yes     |  "None"                  |
 |  "From + Pending Out"    |  yes     |  "None + Pending Out"    |
 |  "Both"                  |  yes     |  "To"                    |
 +----------------------------------------------------------------+

9.3. Server Handling of Inbound Presence Subscription Stanzas

 Inbound presence subscription stanzas request a subscription-related
 action from the user (via the "subscribe" type), inform the user of
 subscription-related actions taken by the contact (via the
 "unsubscribe" type), or enable the contact to manage the user's
 access to the contact's presence information (via the "subscribed"
 and "unsubscribed" types).

Saint-Andre Standards Track [Page 58] RFC 3921 XMPP IM October 2004

 When the user's server receives a subscription request for the user
 from the contact (i.e., a presence stanza of type "subscribe"), it
 MUST deliver that request to the user for approval if the user has
 not already granted the contact access to the user's presence
 information and if there is no pending inbound subscription request;
 however, the user's server SHOULD NOT deliver the new request if
 there is a pending inbound subscription request, since the previous
 subscription request will have been recorded.  If the user has
 already granted the contact access to the user's presence
 information, the user's server SHOULD auto-reply to an inbound
 presence stanza of type "subscribe" from the contact by sending a
 presence stanza of type "subscribed" to the contact on behalf of the
 user; this rule enables the contact to resynchronize the subscription
 state if needed.  These rules are summarized in the following table.
 Table 3: Recommended handling of inbound "subscribe" stanzas
 +------------------------------------------------------------------+
 |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
 +------------------------------------------------------------------+
 |  "None"                  |  yes       |  "None + Pending In"     |
 |  "None + Pending Out"    |  yes       |  "None + Pending Out/In" |
 |  "None + Pending In"     |  no        |  no state change         |
 |  "None + Pending Out/In" |  no        |  no state change         |
 |  "To"                    |  yes       |  "To + Pending In"       |
 |  "To + Pending In"       |  no        |  no state change         |
 |  "From"                  |  no *      |  no state change         |
 |  "From + Pending Out"    |  no *      |  no state change         |
 |  "Both"                  |  no *      |  no state change         |
 +------------------------------------------------------------------+
  • Server SHOULD auto-reply with "subscribed" stanza
 When the user's server receives a presence stanza of type
 "unsubscribe" for the user from the contact, if the stanza results in
 a subscription state change from the user's perspective then the
 user's server SHOULD auto-reply by sending a presence stanza of type
 "unsubscribed" to the contact on behalf of the user, MUST deliver the
 "unsubscribe" stanza to the user, and MUST change the state.  If no
 subscription state change results, the user's server SHOULD NOT
 deliver the stanza and MUST NOT change the state.  These rules are
 summarized in the following table.

Saint-Andre Standards Track [Page 59] RFC 3921 XMPP IM October 2004

 Table 4: Recommended handling of inbound "unsubscribe" stanzas
 +------------------------------------------------------------------+
 |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
 +------------------------------------------------------------------+
 |  "None"                  |  no        |  no state change         |
 |  "None + Pending Out"    |  no        |  no state change         |
 |  "None + Pending In"     |  yes *     |  "None"                  |
 |  "None + Pending Out/In" |  yes *     |  "None + Pending Out"    |
 |  "To"                    |  no        |  no state change         |
 |  "To + Pending In"       |  yes *     |  "To"                    |
 |  "From"                  |  yes *     |  "None"                  |
 |  "From + Pending Out"    |  yes *     |  "None + Pending Out     |
 |  "Both"                  |  yes *     |  "To"                    |
 +------------------------------------------------------------------+
  • Server SHOULD auto-reply with "unsubscribed" stanza
 When the user's server receives a presence stanza of type
 "subscribed" for the user from the contact, it MUST NOT deliver the
 stanza to the user and MUST NOT change the subscription state if
 there is no pending outbound request for access to the contact's
 presence information.  If there is a pending outbound request for
 access to the contact's presence information and the inbound presence
 stanza of type "subscribed" results in a subscription state change,
 the user's server MUST deliver the stanza to the user and MUST change
 the subscription state.  If the user already has access to the
 contact's presence information, the inbound presence stanza of type
 "subscribed" does not result in a subscription state change;
 therefore the user's server SHOULD NOT deliver the stanza to the user
 and MUST NOT change the subscription state.  These rules are
 summarized in the following table.
 Table 5: Recommended handling of inbound "subscribed" stanzas
 +------------------------------------------------------------------+
 |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
 +------------------------------------------------------------------+
 |  "None"                  |  no        |  no state change         |
 |  "None + Pending Out"    |  yes       |  "To"                    |
 |  "None + Pending In"     |  no        |  no state change         |
 |  "None + Pending Out/In" |  yes       |  "To + Pending In"       |
 |  "To"                    |  no        |  no state change         |
 |  "To + Pending In"       |  no        |  no state change         |
 |  "From"                  |  no        |  no state change         |
 |  "From + Pending Out"    |  yes       |  "Both"                  |
 |  "Both"                  |  no        |  no state change         |
 +------------------------------------------------------------------+

Saint-Andre Standards Track [Page 60] RFC 3921 XMPP IM October 2004

 When the user's server receives a presence stanza of type
 "unsubscribed" for the user from the contact, it MUST deliver the
 stanza to the user and MUST change the subscription state if there is
 a pending outbound request for access to the contact's presence
 information or if the user currently has access to the contact's
 presence information.  Otherwise, the user's server SHOULD NOT
 deliver the stanza and MUST NOT change the subscription state.  These
 rules are summarized in the following table.
 Table 6: Recommended handling of inbound "unsubscribed" stanzas
 +------------------------------------------------------------------+
 |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
 +------------------------------------------------------------------+
 |  "None"                  |  no        |  no state change         |
 |  "None + Pending Out"    |  yes       |  "None"                  |
 |  "None + Pending In"     |  no        |  no state change         |
 |  "None + Pending Out/In" |  yes       |  "None + Pending In"     |
 |  "To"                    |  yes       |  "None"                  |
 |  "To + Pending In"       |  yes       |  "None + Pending In"     |
 |  "From"                  |  no        |  no state change         |
 |  "From + Pending Out"    |  yes       |  "From"                  |
 |  "Both"                  |  yes       |  "From"                  |
 +------------------------------------------------------------------+

9.4. Server Delivery and Client Acknowledgement of Subscription

    Requests and State Change Notifications
 When a server receives an inbound presence stanza of type "subscribe"
 (i.e., a subscription request) or of type "subscribed",
 "unsubscribe", or "unsubscribed" (i.e., a subscription state change
 notification), in addition to sending the appropriate roster push (or
 updated roster when the roster is next requested by an available
 resource), it MUST deliver the request or notification to the
 intended recipient at least once.  A server MAY require the recipient
 to acknowledge receipt of all state change notifications (and MUST
 require acknowledgement in the case of subscription requests, i.e.,
 presence stanzas of type "subscribe").  In order to require
 acknowledgement, a server SHOULD send the request or notification to
 the recipient each time the recipient logs in, until the recipient
 acknowledges receipt of the notification by "affirming" or "denying"
 the notification, as shown in the following table:

Saint-Andre Standards Track [Page 61] RFC 3921 XMPP IM October 2004

 Table 7: Acknowledgement of subscription state change notifications
 +--------------------------------------------------+
 |  STANZA TYPE   |  ACCEPT        |  DENY          |
 +--------------------------------------------------+
 |  subscribe     |  subscribed    |  unsubscribed  |
 |  subscribed    |  subscribe     |  unsubscribe   |
 |  unsubscribe   |  unsubscribed  |  subscribed    |
 |  unsubscribed  |  unsubscribe   |  subscribe     |
 +--------------------------------------------------+
 Obviously, given the foregoing subscription state charts, some of the
 acknowledgement stanzas will be routed to the contact and result in
 subscription state changes, while others will not.  However, any such
 stanzas MUST result in the server's no longer sending the
 subscription state notification to the user.
 Because a user's server MUST automatically generate outbound presence
 stanzas of type "unsubscribe" and "unsubscribed" upon receiving a
 roster set with the 'subscription' attribute set to a value of
 "remove" (see Removing a Roster Item and Cancelling All Subscriptions
 (Section 8.6)), the server MUST treat a roster remove request as
 equivalent to sending both of those presence stanzas for purposes of
 determining whether to continue sending subscription state change
 notifications of type "subscribe" or "subscribed" to the user.

10. Blocking Communication

 Most instant messaging systems have found it necessary to implement
 some method for users to block communications from particular other
 users (this is also required by sections 5.1.5, 5.1.15, 5.3.2, and
 5.4.10 of [IMP-REQS]).  In XMPP this is done by managing one's
 privacy lists using the 'jabber:iq:privacy' namespace.
 Server-side privacy lists enable successful completion of the
 following use cases:
 o  Retrieving one's privacy lists.
 o  Adding, removing, and editing one's privacy lists.
 o  Setting, changing, or declining active lists.
 o  Setting, changing, or declining the default list (i.e., the list
    that is active by default).
 o  Allowing or blocking messages based on JID, group, or subscription
    type (or globally).

Saint-Andre Standards Track [Page 62] RFC 3921 XMPP IM October 2004

 o  Allowing or blocking inbound presence notifications based on JID,
    group, or subscription type (or globally).
 o  Allowing or blocking outbound presence notifications based on JID,
    group, or subscription type (or globally).
 o  Allowing or blocking IQ stanzas based on JID, group, or
    subscription type (or globally).
 o  Allowing or blocking all communications based on JID, group, or
    subscription type (or globally).
 Note: Presence notifications do not include presence subscriptions,
 only presence information that is broadcasted to entities that are
 subscribed to a user's presence information.  Thus this includes
 presence stanzas with no 'type' attribute or of type='unavailable'
 only.

10.1. Syntax and Semantics

 A user MAY define one or more privacy lists, which are stored by the
 user's server.  Each <list/> element contains one or more rules in
 the form of <item/> elements, and each <item/> 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:
 <iq>
   <query xmlns='jabber:iq:privacy'>
     <list name='foo'>
       <item
           type='[jid|group|subscription]'
           value='bar'
           action='[allow|deny]'
           order='unsignedInt'>
         [<message/>]
         [<presence-in/>]
         [<presence-out/>]
         [<iq/>]
       </item>
     </list>
   </query>
 </iq>

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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.  <user@domain/resource> (only that resource matches)
 2.  <user@domain> (any resource matches)
 3.  <domain/resource> (only that resource matches)
 4.  <domain> (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
 <item-not-found/> 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 <bad-request/> stanza
 error.)
 The <item/> 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  <message/> -- blocks incoming message stanzas
 o  <iq/> -- blocks incoming IQ stanzas
 o  <presence-in/> -- blocks incoming presence notifications
 o  <presence-out/> -- blocks outgoing presence notifications

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 Within the 'jabber:iq:privacy' namespace, the <query/> child of an IQ
 stanza of type "set" MUST NOT include more than one child element
 (i.e., the stanza MUST contain only one <active/> element, one
 <default/> element, or one <list/> element); if a sending entity
 violates this rule, the receiving entity MUST return a <bad-request/>
 stanza error.
 When a client adds or updates a privacy list, the <list/> element
 SHOULD contain at least one <item/> child element; when a client
 removes a privacy list, the <list/> element MUST NOT contain any
 <item/> 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 <item/>.

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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 <query/> element with only one <list/> 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
     <conflict/> 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:
 <iq from='romeo@example.net/orchard' type='get' id='getlist1'>
   <query xmlns='jabber:iq:privacy'/>
 </iq>

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 Example: Server sends names of privacy lists to client, preceded by
 active list and default list:
 <iq type='result' id='getlist1' to='romeo@example.net/orchard'>
   <query xmlns='jabber:iq:privacy'>
     <active name='private'/>
     <default name='public'/>
     <list name='public'/>
     <list name='private'/>
     <list name='special'/>
   </query>
 </iq>
 Example: Client requests a privacy list from server:
 <iq from='romeo@example.net/orchard' type='get' id='getlist2'>
   <query xmlns='jabber:iq:privacy'>
     <list name='public'/>
   </query>
 </iq>
 Example: Server sends a privacy list to client:
 <iq type='result' id='getlist2' to='romeo@example.net/orchard'>
   <query xmlns='jabber:iq:privacy'>
     <list name='public'>
       <item type='jid'
             value='tybalt@example.com'
             action='deny'
             order='1'/>
       <item action='allow' order='2'/>
     </list>
   </query>
 </iq>
 Example: Client requests another privacy list from server:
 <iq from='romeo@example.net/orchard' type='get' id='getlist3'>
   <query xmlns='jabber:iq:privacy'>
     <list name='private'/>
   </query>
 </iq>

Saint-Andre Standards Track [Page 67] RFC 3921 XMPP IM October 2004

 Example: Server sends another privacy list to client:
 <iq type='result' id='getlist3' to='romeo@example.net/orchard'>
   <query xmlns='jabber:iq:privacy'>
     <list name='private'>
       <item type='subscription'
             value='both'
             action='allow'
             order='10'/>
       <item action='deny' order='15'/>
     </list>
   </query>
 </iq>
 Example: Client requests yet another privacy list from server:
 <iq from='romeo@example.net/orchard' type='get' id='getlist4'>
   <query xmlns='jabber:iq:privacy'>
     <list name='special'/>
   </query>
 </iq>
 Example: Server sends yet another privacy list to client:
 <iq type='result' id='getlist4' to='romeo@example.net/orchard'>
   <query xmlns='jabber:iq:privacy'>
     <list name='special'>
       <item type='jid'
             value='juliet@example.com'
             action='allow'
             order='6'/>
       <item type='jid'
             value='benvolio@example.org'
             action='allow'
             order='7'/>
       <item type='jid'
             value='mercutio@example.org'
             action='allow'
             order='42'/>
       <item action='deny' order='666'/>
     </list>
   </query>
 </iq>
 In this example, the user has three lists: (1) 'public', which allows
 communications from everyone except one specific entity (this is the
 default list); (2) 'private', which allows communications only with

Saint-Andre Standards Track [Page 68] RFC 3921 XMPP IM October 2004

 contacts who have a bidirectional subscription with the user (this is
 the active list); and (3) 'special', which allows communications only
 with three specific entities.
 If the user attempts to retrieve a list but a list by that name does
 not exist, the server MUST return an <item-not-found/> stanza error
 to the user:
 Example: Client attempts to retrieve non-existent list:
 <iq to='romeo@example.net/orchard' type='error' id='getlist5'>
   <query xmlns='jabber:iq:privacy'>
     <list name='The Empty Set'/>
   </query>
   <error type='cancel'>
     <item-not-found
         xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>
 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 <bad request/> stanza error to the user:
 Example: Client attempts to retrieve more than one list:
 <iq to='romeo@example.net/orchard' type='error' id='getlist6'>
   <query xmlns='jabber:iq:privacy'>
     <list name='public'/>
     <list name='private'/>
     <list name='special'/>
   </query>
   <error type='modify'>
     <bad-request
         xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>

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
 <query/> element qualified by the 'jabber:iq:privacy' namespace that
 contains an empty <active/> 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:
 <iq from='romeo@example.net/orchard' type='set' id='active1'>
   <query xmlns='jabber:iq:privacy'>
     <active name='special'/>
   </query>
 </iq>
 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:
 <iq type='result' id='active1' to='romeo@example.net/orchard'/>
 If the user attempts to set an active list but a list by that name
 does not exist, the server MUST return an <item-not-found/> stanza
 error to the user:
 Example: Client attempts to set a non-existent list as active:
 <iq to='romeo@example.net/orchard' type='error' id='active2'>
   <query xmlns='jabber:iq:privacy'>
     <active name='The Empty Set'/>
   </query>
   <error type='cancel'>
     <item-not-found
         xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>
 In order to decline the use of any active list, the connected
 resource MUST send an empty <active/> element with no 'name'
 attribute.
 Example: Client declines the use of active lists:
 <iq from='romeo@example.net/orchard' type='set' id='active3'>
   <query xmlns='jabber:iq:privacy'>
     <active/>
   </query>
 </iq>
 Example: Server acknowledges success of declining any active list:
 <iq type='result' id='active3' to='romeo@example.net/orchard'/>

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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 <query/> element qualified by the
 'jabber:iq:privacy' namespace that contains an empty <default/> child
 element possessing a 'name' attribute whose value is set to the
 desired list name.
 Example: User requests change of default list:
 <iq from='romeo@example.net/orchard' type='set' id='default1'>
   <query xmlns='jabber:iq:privacy'>
     <default name='special'/>
   </query>
 </iq>
 Example: Server acknowledges success of default list change:
 <iq type='result' id='default1' to='romeo@example.net/orchard'/>
 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 <conflict/> stanza
 error to the sending resource:
 Example: Client attempts to change the default list but that list is
 in use by another resource:
 <iq to='romeo@example.net/orchard' type='error' id='default1'>
   <query xmlns='jabber:iq:privacy'>
     <default name='special'/>
   </query>
   <error type='cancel'>
     <conflict
         xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>
 If the user attempts to set a default list but a list by that name
 does not exist, the server MUST return an <item-not-found/> stanza
 error to the user:

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 Example: Client attempts to set a non-existent list as default:
 <iq to='romeo@example.net/orchard' type='error' id='default1'>
   <query xmlns='jabber:iq:privacy'>
     <default name='The Empty Set'/>
   </query>
   <error type='cancel'>
     <item-not-found
         xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>
 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 <default/> element with no 'name' attribute.
 Example: Client declines the use of the default list:
 <iq from='romeo@example.net/orchard' type='set' id='default2'>
   <query xmlns='jabber:iq:privacy'>
     <default/>
   </query>
 </iq>
 Example: Server acknowledges success of declining any default list:
 <iq type='result' id='default2' to='romeo@example.net/orchard'/>
 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
 <conflict/> error to the sending resource:
 Example: Client attempts to decline a default list but that list is
 in use by another resource:
 <iq to='romeo@example.net/orchard' type='error' id='default3'>
   <query xmlns='jabber:iq:privacy'>
     <default/>
   </query>
   <error type='cancel'>
     <conflict
         xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>

Saint-Andre Standards Track [Page 72] RFC 3921 XMPP IM October 2004

10.6. Editing a Privacy List

 In order to edit a privacy list, the user MUST send an IQ stanza of
 type "set" with a <query/> element qualified by the
 'jabber:iq:privacy' namespace that contains one <list/> child element
 possessing a 'name' attribute whose value is set to the list name the
 user would like to edit.  The <list/> element MUST contain one or
 more <item/> 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:
 <iq from='romeo@example.net/orchard' type='set' id='edit1'>
   <query xmlns='jabber:iq:privacy'>
     <list name='public'>
       <item type='jid'
             value='tybalt@example.com'
             action='deny'
             order='3'/>
       <item type='jid'
             value='paris@example.org'
             action='deny'
             order='5'/>
       <item action='allow' order='68'/>
     </list>
   </query>
 </iq>
 Example: Server acknowledges success of list edit:
 <iq type='result' id='edit1' to='romeo@example.net/orchard'/>
 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:
 <iq to='romeo@example.net/orchard' type='set' id='push1'>
   <query xmlns='jabber:iq:privacy'>
     <list name='public'/>
   </query>
 </iq>

Saint-Andre Standards Track [Page 73] RFC 3921 XMPP IM October 2004

 <iq to='romeo@example.net/home' type='set' id='push2'>
   <query xmlns='jabber:iq:privacy'>
     <list name='public'/>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard'
     type='result'
     id='push1'/>
 <iq from='romeo@example.net/home'
     type='result'
     id='push2'/>

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 <query/> element qualified by the
 'jabber:iq:privacy' namespace that contains one empty <list/> 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:
 <iq from='romeo@example.net/orchard' type='set' id='remove1'>
   <query xmlns='jabber:iq:privacy'>
     <list name='private'/>
   </query>
 </iq>
 Example: Server acknowledges success of list removal:
 <iq type='result' id='remove1' to='romeo@example.net/orchard'/>

Saint-Andre Standards Track [Page 74] RFC 3921 XMPP IM October 2004

 If a user attempts to remove a list that is currently being applied
 to at least one resource other than the sending resource, the server
 MUST return a <conflict/> 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 <item-not-found/>
 stanza error to the user.  If the user attempts to remove more than
 one list in the same request, the server MUST return a <bad request/>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='msg1'>
   <query xmlns='jabber:iq:privacy'>
     <list name='message-jid-example'>
       <item type='jid'
             value='tybalt@example.com'
             action='deny'
             order='3'>
         <message/>
       </item>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='msg2'>
   <query xmlns='jabber:iq:privacy'>
     <list name='message-group-example'>
       <item type='group'
             value='Enemies'
             action='deny'
             order='4'>
         <message/>
       </item>

Saint-Andre Standards Track [Page 75] RFC 3921 XMPP IM October 2004

     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='msg3'>
   <query xmlns='jabber:iq:privacy'>
     <list name='message-sub-example'>
       <item type='subscription'
             value='none'
             action='deny'
             order='5'>
         <message/>
       </item>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='msg4'>
   <query xmlns='jabber:iq:privacy'>
     <list name='message-global-example'>
       <item action='deny' order='6'>
         <message/>
       </item>
     </list>
   </query>
 </iq>
 As a result of creating and applying the foregoing list, the user
 will not receive messages from any other users.

10.10. Blocking Inbound Presence Notifications

 Server-side privacy lists enable a user to block incoming presence
 notifications from other entities based on the entity's JID, roster
 group, or subscription status (or globally).  The following examples
 illustrate the protocol.

Saint-Andre Standards Track [Page 76] RFC 3921 XMPP IM October 2004

 Note: Presence notifications do not include presence subscriptions,
 only presence information that is broadcasted to the user because the
 user is currently subscribed to a contact's presence information.
 Thus this includes presence stanzas with no 'type' attribute or of
 type='unavailable' only.
 Example: User blocks based on JID:
 <iq from='romeo@example.net/orchard' type='set' id='presin1'>
   <query xmlns='jabber:iq:privacy'>
     <list name='presin-jid-example'>
       <item type='jid'
             value='tybalt@example.com'
             action='deny'
             order='7'>
         <presence-in/>
       </item>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='presin2'>
   <query xmlns='jabber:iq:privacy'>
     <list name='presin-group-example'>
       <item type='group'
             value='Enemies'
             action='deny'
             order='8'>
         <presence-in/>
       </item>
     </list>
   </query>
 </iq>
 As a result of creating and applying the foregoing list, the user
 will not receive presence notifications from any entities in the
 specified roster group.

Saint-Andre Standards Track [Page 77] RFC 3921 XMPP IM October 2004

 Example: User blocks based on subscription type:
 <iq from='romeo@example.net/orchard' type='set' id='presin3'>
   <query xmlns='jabber:iq:privacy'>
     <list name='presin-sub-example'>
       <item type='subscription'
             value='to'
             action='deny'
             order='9'>
         <presence-in/>
       </item>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='presin4'>
   <query xmlns='jabber:iq:privacy'>
     <list name='presin-global-example'>
       <item action='deny' order='11'>
         <presence-in/>
       </item>
     </list>
   </query>
 </iq>
 As a result of creating and applying the foregoing list, the user
 will not receive presence notifications from any other users.

10.11. Blocking Outbound Presence Notifications

 Server-side privacy lists enable a user to block outgoing presence
 notifications to other entities based on the entity's JID, roster
 group, or subscription status (or globally).  The following examples
 illustrate the protocol.
 Note: Presence notifications do not include presence subscriptions,
 only presence information that is broadcasted to contacts because
 those contacts are currently subscribed to the user's presence
 information.  Thus this includes presence stanzas with no 'type'
 attribute or of type='unavailable' only.

Saint-Andre Standards Track [Page 78] RFC 3921 XMPP IM October 2004

 Example: User blocks based on JID:
 <iq from='romeo@example.net/orchard' type='set' id='presout1'>
   <query xmlns='jabber:iq:privacy'>
     <list name='presout-jid-example'>
       <item type='jid'
             value='tybalt@example.com'
             action='deny'
             order='13'>
         <presence-out/>
       </item>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='presout2'>
   <query xmlns='jabber:iq:privacy'>
     <list name='presout-group-example'>
       <item type='group'
             value='Enemies'
             action='deny'
             order='15'>
         <presence-out/>
       </item>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='presout3'>
   <query xmlns='jabber:iq:privacy'>
     <list name='presout-sub-example'>
       <item type='subscription'
             value='from'
             action='deny'
             order='17'>
         <presence-out/>

Saint-Andre Standards Track [Page 79] RFC 3921 XMPP IM October 2004

       </item>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='presout4'>
   <query xmlns='jabber:iq:privacy'>
     <list name='presout-global-example'>
       <item action='deny' order='23'>
         <presence-out/>
       </item>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='iq1'>
   <query xmlns='jabber:iq:privacy'>
     <list name='iq-jid-example'>
       <item type='jid'
             value='tybalt@example.com'
             action='deny'
             order='29'>
         <iq/>
       </item>
     </list>
   </query>
 </iq>
 As a result of creating and applying the foregoing list, the user
 will not receive IQ stanzas from the entity with the specified JID.

Saint-Andre Standards Track [Page 80] RFC 3921 XMPP IM October 2004

 Example: User blocks based on roster group:
 <iq from='romeo@example.net/orchard' type='set' id='iq2'>
   <query xmlns='jabber:iq:privacy'>
     <list name='iq-group-example'>
       <item type='group'
             value='Enemies'
             action='deny'
             order='31'>
         <iq/>
       </item>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='iq3'>
   <query xmlns='jabber:iq:privacy'>
     <list name='iq-sub-example'>
       <item type='subscription'
             value='none'
             action='deny'
             order='17'>
         <iq/>
       </item>
     </list>
   </query>
 </iq>
 As a result of creating and applying the foregoing list, the user
 will not receive IQ stanzas from any entities with the specified
 subscription type.

Saint-Andre Standards Track [Page 81] RFC 3921 XMPP IM October 2004

 Example: User blocks globally:
 <iq from='romeo@example.net/orchard' type='set' id='iq4'>
   <query xmlns='jabber:iq:privacy'>
     <list name='iq-global-example'>
       <item action='deny' order='1'>
         <iq/>
       </item>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='all1'>
   <query xmlns='jabber:iq:privacy'>
     <list name='all-jid-example'>
       <item type='jid'
             value='tybalt@example.com'
             action='deny'
             order='23'/>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='all2'>
   <query xmlns='jabber:iq:privacy'>
     <list name='all-group-example'>
       <item type='group'
             value='Enemies'

Saint-Andre Standards Track [Page 82] RFC 3921 XMPP IM October 2004

             action='deny'
             order='13'/>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='all3'>
   <query xmlns='jabber:iq:privacy'>
     <list name='all-sub-example'>
       <item type='subscription'
             value='none'
             action='deny'
             order='11'/>
     </list>
   </query>
 </iq>
 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:
 <iq from='romeo@example.net/orchard' type='set' id='all4'>
   <query xmlns='jabber:iq:privacy'>
     <list name='all-global-example'>
       <item action='deny' order='7'/>
     </list>
   </query>
 </iq>
 As a result of creating and applying the foregoing list, the user
 will not receive any communications from, nor send any stanzas to,
 any other users.

10.14. Blocked Entity Attempts to Communicate with User

 If a blocked entity attempts to send message or presence stanzas to
 the user, the user's server SHOULD silently drop the stanza and MUST
 NOT return an error to the sending entity.

Saint-Andre Standards Track [Page 83] RFC 3921 XMPP IM October 2004

 If a blocked entity attempts to send an IQ stanza of type "get" or
 "set" to the user, the user's server MUST return to the sending
 entity a <service-unavailable/> 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:
 <iq type='get'
     to='romeo@example.net'
     from='tybalt@example.com/pda'
     id='probing1'>
   <query xmlns='jabber:iq:version'/>
 </iq>
 Example: Server returns error to blocked entity:
 <iq type='error'
     from='romeo@example.net'
     to='tybalt@example.com/pda'
     id='probing1'>
   <query xmlns='jabber:iq:version'/>
   <error type='cancel'>
     <service-unavailable
         xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>

10.15. Higher-Level Heuristics

 When building a representation of a higher-level privacy heuristic, a
 client SHOULD use the simplest possible representation.
 For example, the heuristic "block all communications with any user
 not in my roster" could be constructed in any of the following ways:
 o  allow communications from all JIDs in my roster (i.e., listing
    each JID as a separate list item), but block communications with
    everyone else
 o  allow communications from any user who is in one of the groups
    that make up my roster (i.e., listing each group as a separate
    list item), but block communications from everyone else
 o  allow communications from any user with whom I have a subscription
    of 'both' or 'to' or 'from' (i.e., listing each subscription value
    separately), but block communications from everyone else

Saint-Andre Standards Track [Page 84] RFC 3921 XMPP IM October 2004

 o  block communications from anyone whose subscription state is
    'none'
 The final representation is the simplest and SHOULD be used; here is
 the XML that would be sent in this case:
 <iq type='set' id='heuristic1'>
   <query xmlns='jabber:iq:privacy'>
     <list name='heuristic-example'>
       <item type='subscription'
             value='none'
             action='deny'
             order='437'/>
     </list>
   </query>
 </iq>

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 <user@example.com> or <user@example.com/resource>, 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 <user@domain/resource> 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 <user@domain> or <user@domain/
     resource> 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 <service-unavailable/> stanza error to
     the sender if it is an IQ stanza, and (c) SHOULD return a
     <service-unavailable/> stanza error to the sender if it is a
     message stanza.
 3.  Else if the JID is of the form <user@domain/resource> and no
     available resource matches the full JID, the recipient's server
     (a) SHOULD silently ignore the stanza (i.e., neither deliver it

Saint-Andre Standards Track [Page 85] RFC 3921 XMPP IM October 2004

     nor return an error) if it is a presence stanza, (b) MUST return
     a <service-unavailable/> stanza error to the sender if it is an
     IQ stanza, and (c) SHOULD treat the stanza as if it were
     addressed to <user@domain> if it is a message stanza.
 4.  Else if the JID is of the form <user@domain> 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 <priority/> 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 <show/> 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
         <user@domain> rather than change it to <user@domain/
         resource>).
     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 <user@domain> rather than change
         it to <user@domain/resource>).
     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 <service-unavailable/> stanza error.
 5.  Else if the JID is of the form <user@domain> and there are no
     available resources associated with the user, how the stanza is
     handled depends on the stanza type:

Saint-Andre Standards Track [Page 86] RFC 3921 XMPP IM October 2004

     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
         <service-unavailable/> 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 <service-unavailable/> 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:

Saint-Andre Standards Track [Page 87] RFC 3921 XMPP IM October 2004

 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 <message/> stanzas and the
     "_pres" Service for <presence/> stanzas (it is up to the
     implementation how to handle <iq/> stanzas).  This will result in
     one or more resolutions of the form "_im.<proto>.example.com." or
     "_pres.<proto>.example.com.", where "<proto>" 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:

Saint-Andre Standards Track [Page 88] RFC 3921 XMPP IM October 2004

 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:

Saint-Andre Standards Track [Page 89] RFC 3921 XMPP IM October 2004

 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
    <service-unavailable/> 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, <xmppwg@jabber.org>

Saint-Andre Standards Track [Page 90] RFC 3921 XMPP IM October 2004

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, <xmppwg@jabber.org>

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, <xmppwg@jabber.org>

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.

Saint-Andre Standards Track [Page 91] 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, <http://www.w3.org/TR/REC-xml>.
 [XML-NAMES] Bray, T., Hollander, D., and A. Layman, "Namespaces in
             XML", W3C REC-xml-names, January 1999,
             <http://www.w3.org/TR/REC-xml-names>.
 [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",
             <http://www.jabber.org/>.
 [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.

Saint-Andre Standards Track [Page 92] RFC 3921 XMPP IM October 2004

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

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='jabber:client'
     xmlns='jabber:client'
     elementFormDefault='qualified'>
   <xs:import namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   <xs:element name='message'>
      <xs:complexType>
         <xs:sequence>
           <xs:choice minOccurs='0' maxOccurs='unbounded'>
             <xs:element ref='subject'/>
             <xs:element ref='body'/>
             <xs:element ref='thread'/>
           </xs:choice>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='error'
                       minOccurs='0'/>

Saint-Andre Standards Track [Page 93] RFC 3921 XMPP IM October 2004

         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='optional'/>
         <xs:attribute name='to'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='type' use='optional' default='normal'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='chat'/>
               <xs:enumeration value='error'/>
               <xs:enumeration value='groupchat'/>
               <xs:enumeration value='headline'/>
               <xs:enumeration value='normal'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute ref='xml:lang' use='optional'/>
      </xs:complexType>
   </xs:element>
   <xs:element name='body'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:string'>
           <xs:attribute ref='xml:lang' use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:element name='subject'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:string'>
           <xs:attribute ref='xml:lang' use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:element name='thread' type='xs:NMTOKEN'/>
   <xs:element name='presence'>

Saint-Andre Standards Track [Page 94] RFC 3921 XMPP IM October 2004

     <xs:complexType>
       <xs:sequence>
         <xs:choice minOccurs='0' maxOccurs='unbounded'>
           <xs:element ref='show'/>
           <xs:element ref='status'/>
           <xs:element ref='priority'/>
         </xs:choice>
         <xs:any     namespace='##other'
                     minOccurs='0'
                     maxOccurs='unbounded'/>
         <xs:element ref='error'
                     minOccurs='0'/>
       </xs:sequence>
       <xs:attribute name='from'
                     type='xs:string'
                     use='optional'/>
       <xs:attribute name='id'
                     type='xs:NMTOKEN'
                     use='optional'/>
       <xs:attribute name='to'
                     type='xs:string'
                     use='optional'/>
       <xs:attribute name='type' use='optional'>
         <xs:simpleType>
           <xs:restriction base='xs:NCName'>
             <xs:enumeration value='error'/>
             <xs:enumeration value='probe'/>
             <xs:enumeration value='subscribe'/>
             <xs:enumeration value='subscribed'/>
             <xs:enumeration value='unavailable'/>
             <xs:enumeration value='unsubscribe'/>
             <xs:enumeration value='unsubscribed'/>
           </xs:restriction>
         </xs:simpleType>
       </xs:attribute>
       <xs:attribute ref='xml:lang' use='optional'/>
     </xs:complexType>
   </xs:element>
   <xs:element name='show'>
     <xs:simpleType>
       <xs:restriction base='xs:NCName'>
         <xs:enumeration value='away'/>
         <xs:enumeration value='chat'/>
         <xs:enumeration value='dnd'/>
         <xs:enumeration value='xa'/>
       </xs:restriction>
     </xs:simpleType>

Saint-Andre Standards Track [Page 95] RFC 3921 XMPP IM October 2004

   </xs:element>
   <xs:element name='status'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:string'>
           <xs:attribute ref='xml:lang' use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:element name='priority' type='xs:byte'/>
   <xs:element name='iq'>
     <xs:complexType>
       <xs:sequence>
         <xs:any     namespace='##other'
                     minOccurs='0'/>
         <xs:element ref='error'
                     minOccurs='0'/>
       </xs:sequence>
       <xs:attribute name='from'
                     type='xs:string'
                     use='optional'/>
       <xs:attribute name='id'
                     type='xs:NMTOKEN'
                     use='required'/>
       <xs:attribute name='to'
                     type='xs:string'
                     use='optional'/>
       <xs:attribute name='type' use='required'>
         <xs:simpleType>
           <xs:restriction base='xs:NCName'>
             <xs:enumeration value='error'/>
             <xs:enumeration value='get'/>
             <xs:enumeration value='result'/>
             <xs:enumeration value='set'/>
           </xs:restriction>
         </xs:simpleType>
       </xs:attribute>
       <xs:attribute ref='xml:lang' use='optional'/>
     </xs:complexType>
   </xs:element>
   <xs:element name='error'>
     <xs:complexType>
       <xs:sequence  xmlns:err='urn:ietf:params:xml:ns:xmpp-stanzas'>

Saint-Andre Standards Track [Page 96] RFC 3921 XMPP IM October 2004

         <xs:group   ref='err:stanzaErrorGroup'/>
         <xs:element ref='err:text'
                     minOccurs='0'/>
       </xs:sequence>
       <xs:attribute name='code' type='xs:byte' use='optional'/>
       <xs:attribute name='type' use='required'>
         <xs:simpleType>
           <xs:restriction base='xs:NCName'>
             <xs:enumeration value='auth'/>
             <xs:enumeration value='cancel'/>
             <xs:enumeration value='continue'/>
             <xs:enumeration value='modify'/>
             <xs:enumeration value='wait'/>
           </xs:restriction>
         </xs:simpleType>
       </xs:attribute>
     </xs:complexType>
   </xs:element>
 </xs:schema>

B.2 jabber:server

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='jabber:server'
     xmlns='jabber:server'
     elementFormDefault='qualified'>
   <xs:import namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   <xs:element name='message'>
      <xs:complexType>
         <xs:sequence>
           <xs:choice minOccurs='0' maxOccurs='unbounded'>
             <xs:element ref='subject'/>
             <xs:element ref='body'/>
             <xs:element ref='thread'/>
           </xs:choice>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='error'
                       minOccurs='0'/>
         </xs:sequence>

Saint-Andre Standards Track [Page 97] RFC 3921 XMPP IM October 2004

         <xs:attribute name='from'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='optional'/>
         <xs:attribute name='to'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='type' use='optional' default='normal'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='chat'/>
               <xs:enumeration value='error'/>
               <xs:enumeration value='groupchat'/>
               <xs:enumeration value='headline'/>
               <xs:enumeration value='normal'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute ref='xml:lang' use='optional'/>
      </xs:complexType>
   </xs:element>
   <xs:element name='body'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:string'>
           <xs:attribute ref='xml:lang' use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:element name='subject'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:string'>
           <xs:attribute ref='xml:lang' use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:element name='thread' type='xs:NMTOKEN'/>
   <xs:element name='presence'>
     <xs:complexType>

Saint-Andre Standards Track [Page 98] RFC 3921 XMPP IM October 2004

       <xs:sequence>
         <xs:choice minOccurs='0' maxOccurs='unbounded'>
           <xs:element ref='show'/>
           <xs:element ref='status'/>
           <xs:element ref='priority'/>
         </xs:choice>
         <xs:any     namespace='##other'
                     minOccurs='0'
                     maxOccurs='unbounded'/>
         <xs:element ref='error'
                     minOccurs='0'/>
       </xs:sequence>
       <xs:attribute name='from'
                     type='xs:string'
                     use='required'/>
       <xs:attribute name='id'
                     type='xs:NMTOKEN'
                     use='optional'/>
       <xs:attribute name='to'
                     type='xs:string'
                     use='required'/>
       <xs:attribute name='type' use='optional'>
         <xs:simpleType>
           <xs:restriction base='xs:NCName'>
             <xs:enumeration value='error'/>
             <xs:enumeration value='probe'/>
             <xs:enumeration value='subscribe'/>
             <xs:enumeration value='subscribed'/>
             <xs:enumeration value='unavailable'/>
             <xs:enumeration value='unsubscribe'/>
             <xs:enumeration value='unsubscribed'/>
           </xs:restriction>
         </xs:simpleType>
       </xs:attribute>
       <xs:attribute ref='xml:lang' use='optional'/>
     </xs:complexType>
   </xs:element>
   <xs:element name='show'>
     <xs:simpleType>
       <xs:restriction base='xs:NCName'>
         <xs:enumeration value='away'/>
         <xs:enumeration value='chat'/>
         <xs:enumeration value='dnd'/>
         <xs:enumeration value='xa'/>
       </xs:restriction>
     </xs:simpleType>
   </xs:element>

Saint-Andre Standards Track [Page 99] RFC 3921 XMPP IM October 2004

   <xs:element name='status'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:string'>
           <xs:attribute ref='xml:lang' use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:element name='priority' type='xs:byte'/>
   <xs:element name='iq'>
     <xs:complexType>
       <xs:sequence>
         <xs:any     namespace='##other'
                     minOccurs='0'/>
         <xs:element ref='error'
                     minOccurs='0'/>
       </xs:sequence>
       <xs:attribute name='from'
                     type='xs:string'
                     use='required'/>
       <xs:attribute name='id'
                     type='xs:NMTOKEN'
                     use='required'/>
       <xs:attribute name='to'
                     type='xs:string'
                     use='required'/>
       <xs:attribute name='type' use='required'>
         <xs:simpleType>
           <xs:restriction base='xs:NCName'>
             <xs:enumeration value='error'/>
             <xs:enumeration value='get'/>
             <xs:enumeration value='result'/>
             <xs:enumeration value='set'/>
           </xs:restriction>
         </xs:simpleType>
       </xs:attribute>
       <xs:attribute ref='xml:lang' use='optional'/>
     </xs:complexType>
   </xs:element>
   <xs:element name='error'>
     <xs:complexType>
       <xs:sequence  xmlns:err='urn:ietf:params:xml:ns:xmpp-stanzas'>
         <xs:group   ref='err:stanzaErrorGroup'/>
         <xs:element ref='err:text'

Saint-Andre Standards Track [Page 100] RFC 3921 XMPP IM October 2004

                     minOccurs='0'/>
       </xs:sequence>
       <xs:attribute name='code' type='xs:byte' use='optional'/>
       <xs:attribute name='type' use='required'>
         <xs:simpleType>
           <xs:restriction base='xs:NCName'>
             <xs:enumeration value='auth'/>
             <xs:enumeration value='cancel'/>
             <xs:enumeration value='continue'/>
             <xs:enumeration value='modify'/>
             <xs:enumeration value='wait'/>
           </xs:restriction>
         </xs:simpleType>
       </xs:attribute>
     </xs:complexType>
   </xs:element>
 </xs:schema>

B.3 session

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='urn:ietf:params:xml:ns:xmpp-session'
     xmlns='urn:ietf:params:xml:ns:xmpp-session'
     elementFormDefault='qualified'>
   <xs:element name='session' type='empty'/>
   <xs:simpleType name='empty'>
     <xs:restriction base='xs:string'>
       <xs:enumeration value=''/>
     </xs:restriction>
   </xs:simpleType>
 </xs:schema>

B.4 jabber:iq:privacy

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='jabber:iq:privacy'

Saint-Andre Standards Track [Page 101] RFC 3921 XMPP IM October 2004

     xmlns='jabber:iq:privacy'
     elementFormDefault='qualified'>
   <xs:element name='query'>
     <xs:complexType>
       <xs:sequence>
         <xs:element ref='active'
                     minOccurs='0'/>
         <xs:element ref='default'
                     minOccurs='0'/>
         <xs:element ref='list'
                     minOccurs='0'
                     maxOccurs='unbounded'/>
       </xs:sequence>
     </xs:complexType>
   </xs:element>
   <xs:element name='active'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:NMTOKEN'>
           <xs:attribute name='name'
                         type='xs:string'
                         use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:element name='default'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:NMTOKEN'>
           <xs:attribute name='name'
                         type='xs:string'
                         use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:element name='list'>
     <xs:complexType>
       <xs:sequence>
         <xs:element ref='item'
                     minOccurs='0'
                     maxOccurs='unbounded'/>
       </xs:sequence>

Saint-Andre Standards Track [Page 102] RFC 3921 XMPP IM October 2004

       <xs:attribute name='name'
                     type='xs:string'
                     use='required'/>
     </xs:complexType>
   </xs:element>
   <xs:element name='item'>
     <xs:complexType>
       <xs:sequence>
         <xs:element name='iq'
                     minOccurs='0'
                     type='empty'/>
         <xs:element name='message'
                     minOccurs='0'
                     type='empty'/>
         <xs:element name='presence-in'
                     minOccurs='0'
                     type='empty'/>
         <xs:element name='presence-out'
                     minOccurs='0'
                     type='empty'/>
       </xs:sequence>
       <xs:attribute name='action' use='required'>
         <xs:simpleType>
           <xs:restriction base='xs:NCName'>
             <xs:enumeration value='allow'/>
             <xs:enumeration value='deny'/>
           </xs:restriction>
         </xs:simpleType>
       </xs:attribute>
       <xs:attribute name='order'
                     type='xs:unsignedInt'
                     use='required'/>
       <xs:attribute name='type' use='optional'>
         <xs:simpleType>
           <xs:restriction base='xs:NCName'>
             <xs:enumeration value='group'/>
             <xs:enumeration value='jid'/>
             <xs:enumeration value='subscription'/>
           </xs:restriction>
         </xs:simpleType>
       </xs:attribute>
       <xs:attribute name='value'
                     type='xs:string'
                     use='optional'/>
     </xs:complexType>
   </xs:element>

Saint-Andre Standards Track [Page 103] RFC 3921 XMPP IM October 2004

   <xs:simpleType name='empty'>
     <xs:restriction base='xs:string'>
       <xs:enumeration value=''/>
     </xs:restriction>
   </xs:simpleType>
 </xs:schema>

B.5 jabber:iq:roster

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='jabber:iq:roster'
     xmlns='jabber:iq:roster'
     elementFormDefault='qualified'>
   <xs:element name='query'>
     <xs:complexType>
       <xs:sequence>
         <xs:element ref='item'
                     minOccurs='0'
                     maxOccurs='unbounded'/>
       </xs:sequence>
     </xs:complexType>
   </xs:element>
   <xs:element name='item'>
     <xs:complexType>
       <xs:sequence>
         <xs:element ref='group'
                     minOccurs='0'
                     maxOccurs='unbounded'/>
       </xs:sequence>
       <xs:attribute name='ask' use='optional'>
         <xs:simpleType>
           <xs:restriction base='xs:NCName'>
             <xs:enumeration value='subscribe'/>
           </xs:restriction>
         </xs:simpleType>
       </xs:attribute>
       <xs:attribute name='jid' type='xs:string' use='required'/>
       <xs:attribute name='name' type='xs:string' use='optional'/>
       <xs:attribute name='subscription' use='optional'>
         <xs:simpleType>
           <xs:restriction base='xs:NCName'>

Saint-Andre Standards Track [Page 104] RFC 3921 XMPP IM October 2004

             <xs:enumeration value='both'/>
             <xs:enumeration value='from'/>
             <xs:enumeration value='none'/>
             <xs:enumeration value='remove'/>
             <xs:enumeration value='to'/>
           </xs:restriction>
         </xs:simpleType>
       </xs:attribute>
     </xs:complexType>
   </xs:element>
   <xs:element name='group' type='xs:string'/>
 </xs:schema>

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).

Saint-Andre Standards Track [Page 105] RFC 3921 XMPP IM October 2004

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

Saint-Andre Standards Track [Page 106] RFC 3921 XMPP IM October 2004

Full Copyright Statement

 Copyright (C) The Internet Society (2004).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/S HE
 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
 INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the IETF's procedures with respect to rights in IETF Documents can
 be found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Saint-Andre Standards Track [Page 107]

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