Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence
draft-saintandre-rfc3921bis-01

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Copyright © The IETF Trust (2007).

Abstract

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

This document obseletes RFC 3921.

Table of Contents

1.  Introduction
    1.1.  Overview
    1.2.  Requirements
    1.3.  Typical Session Flow
    1.4.  Terminology
2.  Managing the Roster
    2.1.  Syntax and Semantics
    2.2.  Business Rules
    2.3.  Retrieving One's Roster on Login
    2.4.  Adding a Roster Item
    2.5.  Updating a Roster Item
    2.6.  Deleting a Roster Item
3.  Managing Presence Subscriptions
    3.1.  Requesting a Subscription
    3.2.  Handling a Subscription Request
    3.3.  Cancelling a Subscription from Another Entity
    3.4.  Unsubscribing from Another Entity's Presence
4.  Exchanging Presence Information
    4.1.  Overview
    4.2.  Initial Presence
    4.3.  Presence Probes
    4.4.  Subsequent Presence Broadcast
    4.5.  Unavailable Presence
    4.6.  Directed Presence
    4.7.  Presence Syntax
5.  Exchanging Messages
    5.1.  Overview
    5.2.  Specifying a Message Body
    5.3.  Specifying a Message Subject
    5.4.  Specifying a Conversation Thread
    5.5.  Message Errors
    5.6.  Extended Namespaces
6.  Exchanging IQ Stanzas
7.  Examples
8.  Server Rules for Handling XML Stanzas
    8.1.  Inbound Stanzas
    8.2.  Outbound Stanzas
9.  IM and Presence Compliance Requirements
    9.1.  Servers
    9.2.  Clients
10.  Internationalization Considerations
11.  Security Considerations
12.  IANA Considerations
    12.1.  Instant Messaging SRV Protocol Label Registration
    12.2.  Presence SRV Protocol Label Registration
13.  References
    13.1.  Normative References
    13.2.  Informative References
Appendix A.  Integration of Roster Management and Presence Subscriptions
    A.1.  Overview
    A.2.  User Subscribes to Contact
    A.3.  Creating a Mutual Subscription
    A.4.  Unsubscribing
    A.5.  Cancelling a Subscription
    A.6.  Removing a Roster Item and Cancelling All Subscriptions
Appendix B.  Subscription States
    B.1.  Defined States
    B.2.  Server Handling of Outbound Presence Subscription Stanzas
    B.3.  Server Handling of Inbound Presence Subscription Stanzas
    B.4.  Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications
Appendix C.  Blocking Communication
Appendix D.  vCards
Appendix E.  XML Schemas
    E.1.  jabber:client
    E.2.  jabber:server
    E.3.  jabber:iq:roster
Appendix F.  Differences From RFC 3921
§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Introduction

1.1.  Overview

The Extensible Messaging and Presence Protocol (XMPP) is a protocol for streaming Extensible Markup Language (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.) [XML] elements in order to exchange messages, presence (availability) information, and other structured data in close to real time. The core features of XMPP are defined in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.). 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 (see [XML‑NAMES] (Bray, T., Hollander, D., and A. Layman, “Namespaces in XML,” January 1999.)). This document describes extensions to the core features of XMPP that provide the basic functionality expected of an instant messaging (IM) and presence application as defined in [IMP‑REQS] (Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” February 2000.).

This document obsoletes RFC 3921.

1.2.  Requirements

Traditionally, instant messaging applications have combined the following factors:

1. The central point of focus is a list of one's contacts or "buddies" (in XMPP this list is called a ROSTER).
2. The purpose of using such an application is to exchange relatively brief text messages with each of one's contacts in close to real time -- often relatively large numbers of such messages in rapid succession, in the form of one-to-one "chat sessions".
3. The catalyst for exchanging messages is PRESENCE -- i.e., knowledge about the network availability of each of one's contacts (thus knowing who is online and available for a chat session).
4. Presence information is provided only to contacts that a user has authorized via a presence subscription.

Thus at a high level this document assumes that a user must be able to complete the following use cases:

• Manage items in a contact list
• Exchange messages with one's contacts
• Exchange presence information with one's contacts
• Manage presence subscriptions to and from one's contacts

Detailed definitions of these functionality areas are contained in [IMP‑REQS] (Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” February 2000.), and the interested reader should refer to that document regarding the requirements addressed herein. While the XMPP instant messaging and presence extensions specified herein meet the requirements of [IMP‑REQS] (Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” February 2000.), they were 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 XMPP protocol extensions addressing many other functionality areas have been defined in the Jabber Software Foundation's XEP series, such extensions are not included in this document because they are not required by [IMP‑REQS] (Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” February 2000.).

Note: [IMP‑REQS] (Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” February 2000.) stipulates that presence services must be separable from instant messaging services and vice-versa; 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 document 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. (For example, a presence-only service could return <service-unavailable/> errors in response to attempts to route <message/> stanzas.)

1.3.  Typical Session Flow

[XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.) specifies how an XMPP client connects to an XMPP server. In particular, it specifies the preconditions (including XML stream establishment, authentication, and binding of a resource to the stream) that must be fulfilled before a client is allowed to send XML stanzas (the basic unit of meaning in XMPP) to other entities on an XMPP network. The reader is referred to [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.) for details, and knowledge of [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.) is assumed herein.

Upon fulfillment of the preconditions specified in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.), an XMPP client has a session with an XMPP server and may send and receive a potentially unlimited number of XML stanzas over the underlying XML stream. The typical flow for an instant messaging and presence session is as follows:

1. Retrieve one's roster. (See Section 2.3 (Retrieving One's Roster on Login).)
2. Send initial presence to the server for broadcasting to all subscribed contacts, thus "going online" from the perspective of XMPP communications. (See Section 4.2 (Initial Presence).)
3. Exchange messages, manage presence subscriptions, perform roster updates, and in general process and generate other XML stanzas with particular semantics throughout the life of the session.
4. Terminate the session when desired by sending unavailable presence and closing the underlying XML stream. (See Section 4.5 (Unavailable Presence).)

1.4.  Terminology

This document inherits the terminology defined in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.).

The following keywords are to be interpreted as described in [TERMS] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.): "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

2.  Managing the Roster

In XMPP, one's roster contains any number of specific contacts. 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.

Note: There are important interactions between rosters and subscriptions; these are defined under Integration of Managing the Roster and Presence Subscriptions (Integration of Roster Management and Presence Subscriptions), and the reader must refer to that section for a complete understanding of roster management (however, such an understanding is necessary only for developers of XMPP servers, since most of the complexity is shielded from clients).

2.1.  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 Jabber Identifier or JID, encapsulated in the 'jid' attribute of the <item/> element (which is REQUIRED).

Note: When the item added represents another IM user, the value of the 'jid' attribute MUST be of the form <contact@domain> rather <contact@domain/resource>.

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:

• "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
• "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
• "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
• "both" -- both the user and the contact have subscriptions to each other's presence information (also called a "mutual subscription")

Each <item/> element MAY possess a 'name' attribute, which sets the "handle" 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.

2.2.  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 (i.e., an 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".

2.3.  Retrieving One's Roster on Login

Upon authenticating with a server and binding a resource (thus becoming a CONNECTED 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 recommended and not required). If an available resource does not request the roster during a session, the server MUST NOT send it presence subscriptions and associated roster updates. For the sake of brevity, the term INTERESTED RESOURCE is used herein to refer to the concept of "an available resource that has requested the roster".

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'/>
    <item jid='benvolio@example.org'
          name='Benvolio'
          subscription='both'/>
  </query>
</iq>

2.4.  Adding a Roster Item

At any time, a user MAY add an item to his or her roster by sending an IQ stanza of type "set" containing a <query/> element that in turn contains one <item/> element.

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>

Note: The <query/> element MUST NOT contain more than one <item/> child element when the client sends an IQ set to the server. In addition, the <item/> element MAY contain more than one <group/> element but the XML character data of each <group/> element MUST specify distinct groups (where duplicates are to be determined using the Resourceprep profile of stringprep as defined in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.)). If either of these rules is violated, the server MUST return a <bad-format/> error to the client.

Note: A client MUST NOT add itself to its own roster; i.e., the value of the <item/> element's 'jid' attribute MUST NOT match the bare JID (node@domain) portion of the <iq/> element's 'from' attribute. If this rule is violated, the server MUST return a <not-allowed/> error to the client.

If the server can successfully process the roster addition, it MUST update the roster information in persistent storage and push the change out to all of the user's interested resources. This "roster push" consists of an IQ stanza of type "set" from the server to the client and enables all interested resources to remain in sync with the server-based roster information.

Example: Server (1) pushes the updated roster information to all interested resources 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>
    </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] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.), 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'/>

2.5.  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 (add group):

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

Example: User updates roster item (remove group):

<iq from='juliet@example.com/chamber' type='set' id='roster_4'>
  <query xmlns='jabber:iq:roster'>
    <item jid='romeo@example.net'
          name='Romeo'>
      <group>Lovers</group>
    </item>
  </query>
</iq>

Example: User updates roster item (change handle):

<iq from='juliet@example.com/chamber' type='set' id='roster_5'>
  <query xmlns='jabber:iq:roster'>
    <item jid='romeo@example.net'
          name='MyRomeo'>
      <group>Lovers</group>
    </item>
  </query>
</iq>

Example: User updates roster item (remove handle):

<iq from='juliet@example.com/chamber' type='set' id='roster_6'>
  <query xmlns='jabber:iq:roster'>
    <item jid='romeo@example.net'
          name=''>
      <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, initiate a roster push to all of the user's interested resources, and send an IQ result to the initiating resource.

2.6.  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 setting the value of the 'subscription' attribute to "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_7'>
  <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 interested resources (with the 'subscription' attribute set to a value of "remove"), and send an IQ result to the initiating resource.

This command will result in cancellation of existing presence subscriptions; for details, see Removing a Roster Item and Cancelling All Subscriptions (Removing a Roster Item and Cancelling All Subscriptions).

3.  Managing Presence 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. In particular, 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@domain> rather than <contact@domain/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.

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

3.1.  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 from='romeo@example.net'
          to='juliet@example.com'
          type='subscribe'/>

A user's server MUST NOT automatically approve subscription requests on the user's behalf. All subscription requests MUST be delivered to the user's client, specifically to one or more of the user's interested resources. If the user has no interested resources when the subscription request is received by the user's server, the user's server MUST keep a record of the complete subscription request (including any extended namespacescontained therein) and deliver the request when the user next has an interested resource, until the user either approves or denies the request. If there is more than one interested 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 interested resources in accordance with Server Rules for Handling XML Stanzas (Server Rules for Handling XML Stanzas). 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.

Note: When a contact generates a subscription request to a user, the contact's server MUST stamp the outgoing presence stanza with the bare JID (<contact@domain>) of the contact, not the full JID (<contact@domain/resource>). The same is true for presence stanzas of type "subscribed", "unsubscribe", and "unsubscribed".

Note: If a user sends a presence subscription request to a contact but has not already added the contact to the user's roster, the user's server MUST send a roster push to all of the user's interested resources. Thus a client MAY simply wait for the roster push rather than proactively adding to the contact to the user's roster.

3.2.  Handling a Subscription Request

When a client receives a subscription request from another entity, it SHOULD 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 from='juliet@example.com'
          to='romeo@example.net'
          type='subscribed'/>

Example: Refusing a presence subscription request:

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

3.3.  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 from='juliet@example.com'
          to='romeo@example.net'
          type='unsubscribed'/>

3.4.  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 from='romeo@example.net'
          to='juliet@example.com'
          type='unsubscribe'/>

4.  Exchanging Presence Information

4.1.  Overview

The concept of presence refers to an entity's availability for communication over a network. At the most basic level, presence is a boolean "on/off" variable that signals whether an entity is available or unavailable for communication; the terms "online" and "offline" are also used. In XMPP, a principal's availability is signalled when a client controlled by the principal generates a <presence/> stanza with no 'type' attribute, and an entity's lack of availability is signalled when a client or entity generates a <presence/> stanza whose 'type' attribute has a value of "unavailable". In XMPP-based applications that combine messaging and presence functionality, the default type of communication for which presence signals availability is messaging; however, XMPP-based applications are not required to combine messaging and presence functionality, and can provide standalone presence features without messaging (in addition, XMPP servers do not require presence information in order to successfully route message and IQ stanzas).

XMPP presence typically follows a "publish-subscribe" or "observer" pattern, wherein an entity sends presence information to its server, and its server then broadcasts or multiplexes that information to all of the entity's contacts who have a subscription to the entity's presence (in the terminology of [IMP‑MODEL] (Day, M., Rosenberg, J., and H. Sugano, “A Model for Presence and Instant Messaging,” February 2000.), an entity that generates presence information is a "presentity" and the entities that receive presence information are "subscribers"). A client generates presence for broadcasting to all subscribed entities by sending a presence stanza to its server with no 'to' address, where the presence stanza has either no 'type' attribute or a 'type' attribute whose value is "unavailable". A user's server MUST NOT leak the user's network availability to entities who are not authorized to know the user's presence, either via an explicit subscription as described herein or via an existing trust relationship (such as presence-enabled user directories within organizations). However, a client MAY also send directed presence (Directed Presence) to entities that are not subscribed to the principal's presence (this does not constitute a presence leak, since it is initiated by the client); this is done by specifying a 'to' address on the relevant presence stanza. (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.)

After a client completes the preconditions specified in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.), it can establish a "presence session" at its server by sending "initial presence" as described under Section 4.2 (Initial Presence), that is by sending a presence stanza with to 'type' or 'to' attribute.

The XMPP presence stanza is also used to negotiate and manage subscriptions to the presence of other entities. These tasks are completed via presence stanzas of type "subscribe", "unsubscribe", "subscribed", and "unsubscribed" as described under Section 3 (Managing Presence Subscriptions).

If a user and contact are associated with different XMPP servers, those servers also use a special presence stanza of type "probe" in order to determine the availability of the entity on the peer server. Clients SHOULD NOT send presence stanzas of type "probe".

Naturally, a presence stanza may also be of type "error".

The values of the 'type' attribute are summarized in the following list (the reader is reminded that a presence stanza with no 'type' attribute signals that the relevant entity is available for communication):

• unavailable -- Signals that the entity is no longer available for communication.
• subscribe -- The sender wishes to subscribe to the recipient's presence.
• subscribed -- The sender has allowed the recipient to receive their presence.
• unsubscribe -- The sender is unsubscribing from another entity's presence.
• unsubscribed -- The subscription request has been denied or a previously-granted subscription has been cancelled.
• probe -- A request for an entity's current presence; SHOULD be generated only by a server on behalf of a user.
• error -- An error has occurred regarding processing or delivery of a previously-sent presence stanza.

4.2.  Initial Presence

After completing the preconditions described in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.) (REQUIRED) and requesting the roster (RECOMMENDED), a client SHOULD send INITIAL PRESENCE to its 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, a connected resource (in the terminology of [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.)) 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 a PRESENCE PROBE (i.e., presence stanza whose 'type' attribute is set to a value of "probe") from the full JID (e.g., <juliet@example.com/balcony>) of the user to each contact 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".
2. Broadcast initial presence from the full JID (e.g., <juliet@example.com/balcony>) 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".

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

Upon receiving initial presence from the user, if the contact address does not exist or the user is not in the contact's roster with a subscription state of "to" or "both", then the contact's server MUST return a presence stanza of type "unsubscribe" to the user. Otherwise, if the user is in the contact's roster with a subscription state of "to" or "both", then the contact's server MUST deliver the user's presence stanza to the full JIDs (e.g., <romeo@example.net/orchard>) associated with all of the contact's available resources.

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).

4.3.  Presence Probes

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

1. If the contact account does not exist or the user is in the contact's roster with a subscription state other than "From", "From + Pending Out", or "Both" (as defined under Subscription States (Subscription States)), the contact's server MUST return a presence stanza of type "unsubscribed" 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).
2. 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.
3. 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.

4.4.  Subsequent Presence Broadcast

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

Upon receiving such a presence stanza expressing updated 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" and (2) from whom the server has not received unavailable presence, presence of type "unsubscribe", or a presence error during the user's session, as well as to the user's other available resources. (Note: Regarding rule 2 above, if the subscription state is "both" then the server MAY broadcast subsequent presence only if the server has received available presence from the contact at some point during the user's session; i.e., if the server never received available presence from the contact and the user has a mutual presence subscription with the contact, it MAY decline to send subsequent presence to the contact.)

Upon receiving subsequent presence from the user, if the contact address does not exist or the user is not in the contact's roster with a subscription state of "to" or "both", then the contact's server MUST return a presence stanza of type "unsubscribe" to the user. Otherwise, if the user is in the contact's roster with a subscription state of "to" or "both", then the contact's server MUST deliver the user's presence stanza to the full JIDs (e.g., <romeo@example.net/orchard>) associated with all of the contact's available resources.

4.5.  Unavailable Presence

Before ending its session with a server, a client SHOULD gracefully become unavailable by sending UNAVAILABLE PRESENCE, i.e., a presence stanza that possesses no 'to' attribute and that possesses a 'type' attribute whose value is "unavailable" (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" and (2) from whom the server has not received unavailable presence, presence of type "unsubscribe", or a presence error during the user's session; the user's server MUST also send that unavailable presence stanza to the user's other available resources. (Note: Regarding rule 2 above, if the subscription state is "both" then the server MAY broadcast unavailable presence only if the server has received available presence from the contact at some point during the user's session; i.e., if the server never received available presence from the contact and the user has a mutual presence subscription with the contact, it MAY decline to send unavailable presence to the contact). If the unavailable presence stanza was received from the client, the server MUST broadcast the full XML of that presence stanza to all entities that fit the above description.

Any presence stanza with no 'type' attribute and no 'to' attribute that is sent after sending broadcasted unavailable presence MUST be broadcasted by the server to all subscribers (i.e., MUST be treated as equivalent to "initial presence" for a new presence session).

4.6.  Directed Presence

This section supplements and in some respects modifies the rules defined above, but only for the special case of "directed presence".

BROADCASTED PRESENCE is generated when a client sends a presence stanza with no type (or with type "unavailable") and no 'to' address over the XML to its server. However, a user MAY also 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 available or unavailable presence to a contact that is in the user's roster with a subscription type of "from" or "both" after having sent initial presence and before sending broadcasted unavailable presence, the user's server MUST route or deliver the full XML of that presence stanza but SHOULD NOT otherwise modify the contact's status regarding broadcasted presence (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 broadcasted unavailable presence, 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 route that unavailable presence to the entity (if the user has not yet sent directed unavailable presence to that entity).
3. If the user sends directed presence without first sending initial presence or after having sent unavailable presence broadcast (i.e., the resource is connected but not available), the user's server MUST treat the entities to which the user sends directed presence in the same way that it treats the entities listed in case #2 above.

A server SHOULD respond to presence probes from entities to which a user has sent directed presence (before sending directed or broadcasted unavailable presence), even if such entities are not in the user's roster with a subscription type of "from" or "both". For instance, a user may join a multi-user chat room (see [XEP‑0045] (Saint-Andre, P., “Multi-User Chat,” September 2006.)) by sending directed presence to the room:

<presence from='romeo@example.net/orchard'
          to='shakespeare@chat.example.com/RM'/>

In order to discover if the user remains online, the chat service SHOULD send a presence probe to the user on behalf of the room:

<presence from='shakespeare@chat.example.com'
          to='romeo@example.net'/>

If the user is still online, the user's server SHOULD send an empty available presence to the requesting entity:

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

If the user is now offline, the user's server SHOULD send an empty unavailable presence to the requesting entity:

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

4.7.  Presence Syntax

In accordance with the default namespace declaration, a presence stanza is qualified by the 'jabber:client' or 'jabber:server' namespace, which defines certain allowable children of presence stanzas, in particular the <show/>, <status/>, and <priority/> elements. These child elements are used to provide more detailed information about an entity's availability. Typically these child elements are provided only if the presence stanza possesses no 'type' attribute, although exceptions are noted in the text that follows.

4.7.1.  Availability States

The OPTIONAL <show/> element contains non-human-readable XML character data that specifies the particular availability sub-state of an entity or a specific resource thereof. A presence stanza MUST NOT contain more than one <show/> element. 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):

• away -- The entity or resource is temporarily away.
• chat -- The entity or resource is actively interested in chatting.
• dnd -- The entity or resource is busy (dnd = "Do Not Disturb").
• 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.

Example: Availability status:

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

4.7.2.  Availability Description

The OPTIONAL <status/> element contains XML character data specifying a natural-language description of an entity's availability. It is normally used in conjunction with the show element to provide a detailed description of an availability state (e.g., "In a meeting") when the presence stanza has no 'type' attribute. 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 (either explicitly or by inheritance of the 'xml:lang' value of an element farther up in the XML hierarchy, which may include the XML stream header as described in Section 4.4 of [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.)).

Example: Availability description:

<presence from='romeo@example.net/orchard'
          xml:lang='en'>
  <show>dnd</show>
  <status>Wooing Juliet</status>
  <status xml:lang='cz'>Ja dvo&#x0159;&#x00ED;m Juliet</status>
</presence>

A presence stanza of type "unavailable" MAY also include a <status/> element to provide detailed information about why the entity is going offline.

Example: Unavailability description:

<presence from='romeo@example.net/orchard'
          type='unavailable'>
  <status>Busy IRL</status>
</presence>

The <status/> child MAY also be sent in a subscription-related presence stanza (i.e., type "subscribe", "subscribed", "unsubscribe", or "unsubscribed") to provide a description of the action:

Example: Description of subscription request:

<presence from='romeo@example.net'
          to='nurse@example.com'
          type='subscribe'>
  <status>Hi, Juliet said I should add you to my buddy list.</status>
</presence>

4.7.3.  Resource 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, a server SHOULD consider the priority to be zero.

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>

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 (Server Rules for Handling XML Stanzas).

4.7.4.  Presence Errors

If the presence stanza is of type "error", it MUST include an <error/> child element; for details, see [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.).

4.7.5.  Extended Namespaces

As described in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.), an XML stanza MAY contain any properly-namespaced child element; this applies to the presence stanza as well.

5.  Exchanging Messages

5.1.  Overview

Once a client has authenticated with a server and bound a resource to an XML stream as described in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.), an XMPP server will route XML stanzas to and from that client. One type of stanza that may be exchanged is <message/>. 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 (Server Rules for Handling XML Stanzas), 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). Thus the message stanza is used to "push" information to another entity.

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 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>.

Common uses of the message stanza 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. These uses are differentiated via 'type' attribute. Inclusion of the 'type' attribute is RECOMMENDED. If included, the 'type' attribute MUST have one of the following values:

• 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.
• error -- An error has occurred related to a previous message sent by the sender (for details regarding stanza error syntax, refer to [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.)). A compliant client SHOULD present an appropriate interface informing the sender of the nature of the error.
• groupchat -- The message is sent in the context of a multi-user chat environment (similar to that of [IRC] (Kalt, C., “Internet Relay Chat: Architecture,” April 2000.)). 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 document (for details see [XEP‑0045] (Saint-Andre, P., “Multi-User Chat,” September 2006.)).
• headline -- The message is probably generated by an automated service that delivers or broadcasts content (news, sports, market information, syndicated content, etc.). No reply to the message is expected, and a compliant client SHOULD present the message in an 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).
• 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. This is the default value of the 'type' attribute.

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').

In addition to the 'type' attribute (which differentiates the conversational context of the message), an XMPP message stanza MAY contain any allowable child elements qualified by the 'jabber:client' (or 'jabber:server') namespace, as well as any other properly-namespaced child element. These payloads are described in the text that follows.

5.2.  Specifying a Message 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 (either explicitly or by inheritance of the 'xml:lang' value of an element farther up in the XML hierarchy, which may include the XML stream header as described in Section 4.4 of [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.)). The <body/> element MUST NOT contain mixed content (as defined in Section 3.2.2 of [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.)).

A message stanza often will contain a child <body/> element whose XML character data specifies the primary meaning of the message.

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>

5.3.  Specifying a Message 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 subject, but only if each instance possesses an 'xml:lang' attribute with a distinct language value (either explicitly or by inheritance of the 'xml:lang' value of an element farther up in the XML hierarchy, which may include the XML stream header as described in Section 4.4 of [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.)). The <subject/> element MUST NOT contain mixed content (as defined in Section 3.2.2 of [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.)).

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>

5.4.  Specifying a Conversation Thread

The primary use of the XMPP <thread/> element is to uniquely identify a conversation thread or "chat session" between two entities instantiated by <message/> stanzas of type 'chat'. However, the XMPP <thread/> element may also be used to uniquely identify an analogous thread between two entities instantiated by <message/> stanzas of type 'headline' or 'normal', or among multiple entities in the context of a multi-user chat room instantiated by <message/> stanzas of type 'groupchat'. It may also be used for <message/> stanzas not related to a conversation, such as a game session or between plugins.

The value of the <thread/> element MUST be a universally unique identifier (UUID) as described in [UUID] (Leach, P., Mealling, M., and R. Salz, “A Universally Unique IDentifier (UUID) URN Namespace,” July 2005.).

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] (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.)).

5.5.  Message Errors

If the message stanza is of type "error", it MUST include an <error/> child; for details, see [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.).

5.6.  Extended Namespaces

As described in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.), an XML stanza MAY contain any properly-namespaced child element; this applies to the message stanza as well.

6.  Exchanging IQ Stanzas

As described in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.), 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] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.), whereas the specific semantics required to complete particular use cases are defined in all cases by an extended namespace. Note that the 'jabber:client' and 'jabber:server' namespaces do not define any children of IQ stanzas other than the common <error/>. This document defines such an extended namespace, for Managing the Roster (Managing the Roster). However, an IQ stanza MAY contain structured information qualified by any extended namespace.

7.  Examples

The examples in this section illustrate a possible instant messaging and presence session. 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:

• juliet@example.com (subscription="both" and she has two available resources, one whose resource is "chamber" and another whose resource is "balcony")
• benvolio@example.org (subscription="to")
• mercutio@example.org (subscription="from")

First, the user completes the preconditions (stream establishment, TLS and SASL negotiation, and resource binding) described in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.); those packet flows are not reproduced here.

Next, the user requests his roster:

Example 1: Client requests current roster from server:

<iq from='romeo@example.net/balcony' type='get' id='ex1'>
  <query xmlns='jabber:iq:roster'/>
</iq>

Example 2: Client receives roster from server:

<iq to='romeo@example.com/balcony' type='result' id='ex1'>
  <query xmlns='jabber:iq:roster'>
    <item jid='romeo@example.net'
          name='Juliet'
          subscription='both'>
      <group>Friends</group>
    </item>
    <item jid='benvolio@example.org'
          name='Benvolio'
          subscription='to'/>
    <item jid='mercutio@example.org'
          name='Mercutio'
          subscription='from'/>
  </query>
</iq>

Now the user sends initial presence.

Example 3: User sends initial presence:

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

Example 4: 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'/>

Example 5: 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 6: 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 7: 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'/>

<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 8: 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>

Now the user has a threaded conversation (chat session) with one of his contacts.

Example 9: 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>

And so on.

The user can also send subsequent presence broadcast.

Example 10: 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 11: 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>

Example 12: 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 13: One of the contact's resources broadcasts final presence:

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

Example 14: Contact's server sends unavailable presence information to user:

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

Example 15: User sends unavailable presence:

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

Example 16: 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
    type='unavailable'
    from='romeo@example.net/orchard'
    to='nurse@example.com'
    xml:lang='en'>
  <status>gone home</status>
</presence>

Now the user closes his stream:

</stream:stream>

And the user's server closes its stream as well:

</stream:stream>

THE END

8.  Server Rules for Handling XML Stanzas

Basic routing and delivery rules for servers are defined in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.). This section defines additional rules for XMPP-compliant instant messaging and presence servers.

8.1.  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 follow the rules defined in the following list:

1. If the JID is of the form <user@domain/resource> and an available resource exactly 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 connected or available resource exactly matches the full JID, 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 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 in the text that follows). 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 (Presence Probes). 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 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 shall be handled depends on the stanza type:
   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 and requests the roster); 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 Managing Presence Subscriptions (Managing Presence Subscriptions)).
   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.

8.2.  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 (Inbound Stanzas).

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:

1. First attempt to resolve the foreign hostname using an [SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) Service of "xmpp-server" and Proto of "tcp", resulting in resource records such as "_xmpp-server._tcp.example.com.", as specified in [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.).
2. If the "xmpp-server" address record resolution fails, attempt to resolve the "_im" or "_pres" [SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) Service as specified in [IMP‑SRV] (Peterson, J., “Address Resolution for Instant Messaging and Presence,” August 2004.), 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] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.).

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.

9.  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] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.). 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.

9.1.  Servers

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

• 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, and IM-specific routing and delivery rules

9.2.  Clients

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

• 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
• All client-related instant messaging syntax and semantics defined in this document, including presence subscriptions, and roster management
• End-to-end object encryption as defined in End-to-End Object Encryption in the Extensible Messaging and Presence Protocol (XMPP) (Saint-Andre, P., “End-to-End Signing and Object Encryption for the Extensible Messaging and Presence Protocol (XMPP),” October 2004.) [XMPP‑E2E]

A client MUST also handle addresses that are encoded as "im:" URIs as specified in [CPIM] (Peterson, J., “Common Profile for Instant Messaging (CPIM),” August 2004.), and MAY do so by removing the "im:" scheme and entrusting address resolution to the server as specified under Outbound Stanzas (Outbound Stanzas).

10.  Internationalization Considerations

For internationalization considerations, refer to the relevant section of [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.).

11.  Security Considerations

Core security considerations for XMPP are defined in the relevant section of [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.).

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

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

12.  IANA Considerations

For a number of related IANA considerations, refer to the relevant section of [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.).

12.1.  Instant Messaging SRV Protocol Label Registration

Address Resolution for Instant Messaging and Presence (Peterson, J., “Address Resolution for Instant Messaging and Presence,” August 2004.) [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>

12.2.  Presence SRV Protocol Label Registration

Address Resolution for Instant Messaging and Presence (Peterson, J., “Address Resolution for Instant Messaging and Presence,” August 2004.) [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>

13.  References

13.1. Normative References

13.2. Informative References

Appendix A.  Integration of Roster Management and Presence Subscriptions

This Appendix provides a detailed description of how roster management and presence subscriptions are integrated in XMPP-based instant messaging and presence applications, mainly for the benefit of server developers (most of the complexity specified here can be safely ignored by client developers).

A.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:

• 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
• 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
• 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
• 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 (Subscription States).

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] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.)).

A.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 as follows.

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:

   <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 interested resources, 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:

   <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 interested resources, 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 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 (Subscription States), 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 Appendix A.2.1 (Alternate Flow: Contact Declines Subscription Request)). In this case, the contact's client (1) SHOULD perform a roster set specifying the desired handle 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:

   <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 interested resources, containing an updated roster item for the contact with the 'subscription' attribute set 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 (Subscription States) 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 Appendix B.4 (Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications)).

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.

A.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 follows:

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 Appendix A.2 (User Subscribes to Contact)):

   <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 interested resources, 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 sending a presence stanza of type "subscribe" to the contact; this step does not necessarily affect the subscription state (see Subscription States (Subscription States) 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 Appendix B.4 (Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications)).

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.

A.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 as follows:

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>
   
   <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 Appendix A.3.1 (Alternate Flow: User Declines Subscription Request)). 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 interested resources, 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:

   <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:

   <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 (Subscription States) 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 Appendix B.4 (Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications)).

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

A.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 follows:

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 Appendix A.3 (Creating a Mutual Subscription)):

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

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 (Subscription States) 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 Appendix B.4 (Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications)).

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".

A.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 in the text that follows.

A.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 interested resources, 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'/>
   

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 (Subscription States) 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 Appendix B.4 (Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications)).
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 (Subscription States) 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 Appendix B.4 (Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications)).

A.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 interested resources, 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:

   <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 step does not necessarily affect the subscription state (see Subscription States (Subscription States) 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 Appendix B.4 (Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications)).
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 (Subscription States) 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 Appendix B.4 (Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications)).

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 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 (Removing a Roster Item and Cancelling All Subscriptions).

A.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 in the text that follows.

A.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 interested resources, 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>
   
   <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 interested resources, 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; this step does not necessarily affect the subscription state (see Subscription States (Subscription States) 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 Appendix B.4 (Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications)).

A.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 interested resources, 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'/>
   

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 interested resources, 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 (Subscription States) 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 Appendix B.4 (Server Delivery and Client Acknowledgement of Subscription Requests and State Change Notifications)).

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 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 (Removing a Roster Item and Cancelling All Subscriptions).

A.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 interested resources 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'/>

<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 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.

Appendix B.  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").

B.1.  Defined States

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

1. "None" = contact and user are not subscribed to each other, and neither has requested a subscription from the other; this is reflected in the roster by subscription='none'
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; this is reflected in the roster by subscription='none' and ask='subscribe'
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 user has approved subscription request from contact); this is reflected in the roster by subscription='none'
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; this is reflected in the roster by subscription='none' and ask='subscribe'
5. "To" = user is subscribed to contact (one-way); this is reflected in the roster by subscription='to'
6. "To + Pending In" = user is subscribed to contact, and contact has sent user a subscription request but user has not replied yet; this is reflected in the roster by subscription='to'
7. "From" = contact is subscribed to user (one-way); this is reflected in the roster by subscription='from'
8. "From + Pending Out" = contact is subscribed to user, and user has sent contact a subscription request but contact has not replied yet; this is reflected in the roster by subscription='none' and ask='subscribe'
9. "Both" = user and contact are subscribed to each other (two-way); this is reflected in the roster by subscription='both'

B.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).

The following rules apply:

1. Regarding outbound presence stanzas of type "subscribe", the user's server MUST without exception route the stanza to the contact.
2. Regarding outbound presence stanzas of type "unsubscribe", the user's server MUST without exception route the stanza to the contact.
3. Regarding outbound presence stanzas of type "subscribed", if the stanza does not result in a subscription state change from the user's perspective then the user's server SHOULD NOT route the stanza to the contact; however, if the stanza results in a subscription state change, the user's server MUST route the stanza to the contact.
4. Regarding outbound presence stanzas of type "unsubscribed", if the stanza does not result in a subscription state change from the user's perspective then the user's server SHOULD NOT route the stanza to the contact; however, if the stanza results in a subscription state change, the user's server MUST route the stanza to the contact.

The tables shown that follow summarize the state transitions associated with the handling of outbound presence subscription stanzas.

Table 1: Recommended handling of outbound "subscribe" stanzas

+----------------------------------------------------------------+
|  EXISTING STATE          |  ROUTE?  |  NEW STATE               |
+----------------------------------------------------------------+
|  "None"                  |  yes     |  "None + Pending Out"    |
|  "None + Pending Out"    |  yes     |  no state change         |
|  "None + Pending In"     |  yes     |  "None + Pending Out+In" |
|  "None + Pending Out+In" |  yes     |  no state change         |
|  "To"                    |  yes     |  no state change         |
|  "To + Pending In"       |  yes     |  no state change         |
|  "From"                  |  yes     |  "From + Pending Out"    |
|  "From + Pending Out"    |  yes     |  no state change         |
|  "Both"                  |  yes     |  no state change         |
+----------------------------------------------------------------+

Table 2: Recommended handling of outbound "unsubscribe" stanzas

+----------------------------------------------------------------+
|  EXISTING STATE          |  ROUTE?  |  NEW STATE               |
+----------------------------------------------------------------+
|  "None"                  |  yes     |  no state change         |
|  "None + Pending Out"    |  yes     |  "None"                  |
|  "None + Pending In"     |  yes     |  no state change         |
|  "None + Pending Out+In" |  yes     |  "None + Pending In"     |
|  "To"                    |  yes     |  "None"                  |
|  "To + Pending In"       |  yes     |  "Pending In"            |
|  "From"                  |  yes     |  no state change         |
|  "From + Pending Out"    |  yes     |  "From"                  |
|  "Both"                  |  yes     |  "From"                  |
+----------------------------------------------------------------+

Table 3: 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 4: 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"                    |
+----------------------------------------------------------------+

B.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).

The following rules apply:

1. Regarding inbound presence stanzas of type "subscribe", 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 then the user's server MUST route the stanza to the user; however, if there is a pending inbound subscription request then the user's server SHOULD NOT deliver the new request. If the user has already granted the contact access to the user's presence information, the user's server SHOULD auto-reply 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.
2. Regarding inbound presence stanzas of type "unsubscribe", if the stanza does not result in a subscription state change from the user's perspective then the user's server SHOULD NOT route the stanza to the contact; however, if the stanza results in a subscription state change, the user's server MUST route the stanza to the contact.
3. Regarding inbound presence stanzas of type "subscribed", if the stanza does not result in a subscription state change from the user's perspective then the user's server SHOULD NOT route the stanza to the contact; however, if the stanza results in a subscription state change, the user's server MUST route the stanza to the contact.
4. Regarding inbound presence stanzas of type "unsubscribed", if the stanza does not result in a subscription state change from the user's perspective then the user's server SHOULD NOT route the stanza to the contact; however, if the stanza results in a subscription state change, the user's server MUST route the stanza to the contact.

The tables shown that follow summarize the state transitions associated with the handling of inbound presence subscription stanzas.

Table 5: 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.

Table 6: 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 7: 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         |
+------------------------------------------------------------------+

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 8: 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"                  |
+------------------------------------------------------------------+

B.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:

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 (Removing a Roster Item and Cancelling All Subscriptions)), 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.

Appendix C.  Blocking Communication

Sections 2.3.5 and 5.4.10 of [IMP‑REQS] (Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” February 2000.) requires that a compliant instant messaging and presence technology must enable a user to block communications from selected users. A protocol for doing so is specified in [XEP‑0016] (Millard, P. and P. Saint-Andre, “Privacy Lists,” November 2006.) and a simplified "front-end" to that protocol is specified in [XEP‑0191] (Saint-Andre, P., “Simple Communications Blocking,” November 2006.).

Appendix D.  vCards

Sections 3.1.3 and 4.1.4 of [IMP‑REQS] (Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” February 2000.) 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 (Dawson, F. and T. Howes, “vCard MIME Directory Profile,” September 1998.) [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 [XEP‑0054] (Saint-Andre, P., “vcard-temp,” March 2003.)).

Appendix E.  XML Schemas

The following XML schemas are descriptive, not normative. For schemas defining stream-related features of XMPP, refer to [XMPP‑CORE] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.).

E.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'/>
        </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'>
    <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>
  </xs:element>

  <xs:element name='status'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='string1024'>
          <xs:attribute ref='xml:lang' use='optional'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <xs:simpleType name='string1024'>
    <xs:restriction base='xs:string'>
      <xs:minLength value='1'/>
      <xs:maxLength value='1024'/>
    </xs:restriction>
  </xs:simpleType>

  <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'>
        <xs:group   ref='err:stanzaErrorGroup'/>
        <xs:element ref='err:text'
                    minOccurs='0'/>
      </xs:sequence>
      <xs:attribute name='code' type='xs:unsignedShort' 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>

E.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>
        <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>
      <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>

  <xs:element name='status'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='string1024'>
          <xs:attribute ref='xml:lang' use='optional'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <xs:simpleType name='string1024'>
    <xs:restriction base='xs:string'>
      <xs:minLength value='1'/>
      <xs:maxLength value='1024'/>
    </xs:restriction>
  </xs:simpleType>

  <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'
                    minOccurs='0'/>
      </xs:sequence>
      <xs:attribute name='code' type='xs:unsignedShort' 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>

E.3.  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'>
            <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 F.  Differences From RFC 3921

This section is informative.

Based on consensus derived from interoperability testing and implementation experience, the following modifications were made from RFC 3921. In addition, several smaller changes were made to more clearly specify and explain the protocols.

• The protocol for session establishment was determined to be unnecessary and therefore the content previously defined in Section 3 of RFC 3921 was removed. However, server implementations may still want to advertise support for the feature in order to ensure backwards-compatibility, even though it is a "no-op".
• The protocol for communications blocking specified in Section 10 of RFC 3921 has been moved to [XEP‑0016] (Millard, P. and P. Saint-Andre, “Privacy Lists,” November 2006.) and a simplified "front-end" to that functionality has been defined in [XEP‑0191] (Saint-Andre, P., “Simple Communications Blocking,” November 2006.) to ease the task of implementing communications blocking in servers and clients.
• In order to more seamlessly repair lack of synchronization in subscription states between servers, error handling related to presence probes and presence notifications was modified to return presence stanzas of type "unbsubscribe" or "unsubscribed" rather than error stanzas.

Author's Address

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