Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence
draft-ietf-xmpp-3921bis-02

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Abstract

This document defines extensions to 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 obsoletes RFC 3921.

Table of Contents

1.  Introduction
    1.1.  Overview
    1.2.  Requirements
    1.3.  Functional Summary
    1.4.  Conventions
    1.5.  Acknowledgements
    1.6.  Discussion Venue
2.  Managing the Roster
    2.1.  Syntax and Semantics
        2.1.1.  Ver Attribute
        2.1.2.  Roster Items
            2.1.2.1.  Ask Attribute
            2.1.2.2.  Jid Attribute
            2.1.2.3.  Name Attribute
            2.1.2.4.  Subscription Attribute
            2.1.2.5.  Group Element
        2.1.3.  Roster Get
        2.1.4.  Roster Set
        2.1.5.  Roster Push
        2.1.6.  Roster Result
        2.1.7.  Subscription Attribute
    2.2.  Retrieving the Roster on Login
    2.3.  Adding a Roster Item
        2.3.1.  Request
        2.3.2.  Success Case
        2.3.3.  Error Cases
    2.4.  Updating a Roster Item
        2.4.1.  Request
        2.4.2.  Success Case
        2.4.3.  Error Cases
    2.5.  Deleting a Roster Item
        2.5.1.  Request
        2.5.2.  Success Case
        2.5.3.  Error Cases
    2.6.  Roster Versioning
        2.6.1.  Request
        2.6.2.  Success Case
3.  Managing Presence Subscriptions
    3.1.  Requesting a Subscription
        3.1.1.  Client Generation of Outbound Subscription Request
        3.1.2.  Server Processing of Outbound Subscription Request
        3.1.3.  Server Processing of Inbound Subscription Request
        3.1.4.  Client Processing of Inbound Subscription Request
        3.1.5.  Server Processing of Outbound Subscription Approval
        3.1.6.  Server Processing of Inbound Subscription Approval
    3.2.  Cancelling a Subscription
        3.2.1.  Client Generation of Subscription Cancellation
        3.2.2.  Server Processing of Outbound Subscription Cancellation
        3.2.3.  Server Processing of Inbound Subscription Cancellation
    3.3.  Unsubscribing
        3.3.1.  Client Generation of Unsubscribe
        3.3.2.  Server Processing of Outbound Unsubscribe
        3.3.3.  Server Processing of Inbound Unsubscribe
4.  Exchanging Presence Information
    4.1.  Overview
    4.2.  Initial Presence
        4.2.1.  Client Generation of Initial Presence
        4.2.2.  Server Processing of Outbound Presence
        4.2.3.  Server Processing of Inbound Presence
        4.2.4.  Client Processing of Inbound Presence
    4.3.  Presence Probes
        4.3.1.  Server Generation of Outbound Presence Probe
        4.3.2.  Server Processing of Inbound Presence Probe
    4.4.  Subsequent Presence Broadcast
        4.4.1.  Client Generation of Presence Broadcast
        4.4.2.  Server Processing of Outbound Presence
        4.4.3.  Server Processing of Inbound Presence
        4.4.4.  Client Processing of Inbound Presence
    4.5.  Unavailable Presence
        4.5.1.  Client Generation of Unavailable Presence
        4.5.2.  Server Processing of Outbound Unavailable Presence
        4.5.3.  Server Processing of Inbound Unavailable Presence
        4.5.4.  Client Processing of Inbound Unavailable Presence
    4.6.  Directed Presence
        4.6.1.  Client Generation of Directed Presence
        4.6.2.  Server Processing of Outbound Directed Presence
        4.6.3.  Server Processing of Inbound Directed Presence
        4.6.4.  Client Processing of Inbound Directed Presence
    4.7.  Presence Syntax
        4.7.1.  Type Attribute
        4.7.2.  Child Elements
        4.7.3.  Show Element
        4.7.4.  Status Element
        4.7.5.  Priority Element
        4.7.6.  Extended Content
5.  Exchanging Messages
    5.1.  One-to-One Chat Sessions
    5.2.  Message Syntax
        5.2.1.  To Attribute
        5.2.2.  Type Attribute
        5.2.3.  Body Element
        5.2.4.  Subject Element
        5.2.5.  Thread Element
    5.3.  Extended Content
6.  Exchanging IQ Stanzas
7.  A Sample Session
8.  Server Rules for Processing XML Stanzas
    8.1.  No Such User
    8.2.  Full JID at Local Domain
        8.2.1.  Resource Matches
        8.2.2.  No Resource Matches
    8.3.  Bare JID at Local Domain
        8.3.1.  Available or Connected Resources
            8.3.1.1.  Message
            8.3.1.2.  Presence
            8.3.1.3.  IQ
        8.3.2.  No Available or Connected Resources
            8.3.2.1.  Message
            8.3.2.2.  Presence
            8.3.2.3.  IQ
    8.4.  Remote Domain
9.  Handling of URIs
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.  Conformance Requirements
14.  References
    14.1.  Normative References
    14.2.  Informative References
Appendix A.  Subscription States
    A.1.  Defined States
    A.2.  Server Processing of Outbound Presence Subscription Stanzas
        A.2.1.  Subscribe
        A.2.2.  Unsubscribe
        A.2.3.  Subscribed
        A.2.4.  Unsubscribed
    A.3.  Server Processing of Inbound Presence Subscription Stanzas
        A.3.1.  Subscribe
        A.3.2.  Unsubscribe
        A.3.3.  Subscribed
        A.3.4.  Unsubscribed
Appendix B.  Blocking Communication
Appendix C.  vCards
Appendix D.  XML Schemas
    D.1.  jabber:client
    D.2.  jabber:server
    D.3.  jabber:iq:roster
Appendix E.  Differences From RFC 3921
Appendix F.  Copying Conditions
§  Index
§  Author's Address

1.  Introduction

1.1.  Overview

The Extensible Messaging and Presence Protocol (XMPP) is an application profile of the Extensible Markup Language [XML] (Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fourth Edition),” August 2006.) for streaming XML data in close to real time between any two (or more) network-aware entities. XMPP is typically used to exchange messages, share presence information, and engage in structured request-response interactions. The core features of XMPP defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.) provide the building blocks for many types of near-real-time applications, which can be layered on top of the core by sending application-specific data qualified by particular XML namespaces (refer to [XML‑NAMES] (Bray, T., Hollander, D., and A. Layman, “Namespaces in XML,” January 1999.)). This document defines XMPP extensions 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.).

As a result of extensive implementation and deployment experience with XMPP since 2004, as well as more formal interoperability testing carried out under the auspices of the XMPP Standards Foundation (XSF), this document reflects consensus from the XMPP developer community regarding XMPP's basic instant messaging and presence features. In particular, this document incorporates the following backward-compatible changes from RFC 3921:

• Incorporated corrections and errata
• Added examples throughout
• Clarified and more completely specified matters that were underspecified
• Removed the protocol for session establishment, which was deemed unnecessary
• Modified error handling related to presence stanzas to more seamlessly repair lack of synchronization in subscription states between rosters located at different servers
• Added optional server support for pre-approved presence subscriptions
• Added optional 'parent' attribute to <thread/> element
• Transferred documentation for the communications blocking protocol from this specification to a separate specification

Therefore, this document defines the basic instant messaging and presence features of XMPP 1.0, thus obsoleting 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 particular contacts in close to real time -- often relatively large numbers of such messages in rapid succession, in the form of a one-to-one CHAT SESSION as described under Section 5.1 (One-to-One Chat Sessions).
3. The catalyst for exchanging messages is PRESENCE -- i.e., information about the network availability of particular contacts (thus knowing who is online and available for a one-to-one chat session).
4. Presence information is provided only to contacts that one has authorized by means of an explicit agreement called 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 one's 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 RFC 2779 [IMP‑REQS] (Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” February 2000.), and the interested reader is referred to that document regarding the requirements addressed herein. While the XMPP instant messaging and presence extensions specified herein meet the requirements of RFC 2779, 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. Although XMPP protocol extensions addressing many other functionality areas have been defined in the XMPP Standards Foundation's XEP series (e.g., multi-user text chat as specified in [XEP‑0045] (Saint-Andre, P., “Multi-User Chat,” July 2008.)), such extensions are not specified in this document because they are not mandated by RFC 2779.

Note: RFC 2779 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 a <service-unavailable/> stanza error if a client attempt to send a <message/> stanza.)

1.3.  Functional Summary

This non-normative section provides a developer-friendly, functional summary of XMPP-based instant messaging and presence features; consult the sections that follow for a normative definition of these features.

[xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.) specifies how an XMPP client connects to an XMPP server. In particular, it specifies the preconditions 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. These preconditions comprise negotiation of the XML stream and include XML stream establishment, optional channel encryption via Transport Layer Security [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.), mandatory authentication via Simple Authentication and Security Layer [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.), and binding of a resource to the stream for client addressing. The reader is referred to [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.) for details regarding these preconditions, and knowledge of [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.) is assumed herein.

Note: [RFC3921] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.) specified one additional precondition: formal establishment of an instant messaging and presence session. Implementation and deployment experience has shown that this additional step is unnecessary. However, for backward compatibility an implementation SHOULD still offer that feature and note in the stream feature that negotiation of the feature is discretionary (via the <optional/> child element). This enables older software to connect while saving newer software to skip a round trip.

Upon fulfillment of the preconditions specified in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), an XMPP client has a long-lived XML stream with an XMPP server, which enables the user controlling that client to send and receive a potentially unlimited number of XML stanzas over the stream. Such a stream can be used to exchange messages, share presence information, and engage in structured request-response interactions in close to real time. After negotiation of the XML stream, the typical flow for an instant messaging and presence session is as follows:

1. Retrieve one's roster. (See Section 2.2 (Retrieving the Roster on Login).)
2. Send initial presence to the server for broadcasting to all subscribed contacts, thus "going online" from the perspective of XMPP communication. (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. (See Section 5 (Exchanging Messages), Section 3 (Managing Presence Subscriptions), Section 2 (Managing the Roster), and Section 6 (Exchanging IQ Stanzas).)
4. Terminate the session when desired by sending unavailable presence and closing the underlying XML stream. (See Section 4.5 (Unavailable Presence).)

1.4.  Conventions

This document inherits the terminology defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.).

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".

For convenience, this document employs the term "user" to refer to the owner of an XMPP account; however, account owners need not be human persons and can be bots, devices, or other non-human applications.

Following the "XML Notation" used in [IRI] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) to represent characters that cannot be rendered in ASCII-only documents, some examples in this document use the form "&#x...." as a notational device to represent Unicode characters (e.g., the string "&#x0159;" stands for the Unicode character LATIN SMALL LETTER R WITH CARON).

In examples, lines have been wrapped for improved readability, "[...]" means elision, and the following prepended strings are used (these prepended strings are not to be sent over the wire):

• C: = client
• CC: = contact's client
• CS: = contact's server
• S: = server
• UC: = user's client
• US: = user's server

1.5.  Acknowledgements

The editor of this document finds it impossible to appropriately acknowledge the many individuals who have provided comments regarding the protocols defined herein. However, thanks are due to those who have who have provided implementation feedback, bug reports, requests for clarification, and suggestions for improvement since the publication of the RFC this document supersedes. The editor has endeavored to address all such feedback, but is solely responsible for any remaining errors and ambiguities.

Portions of the text about roster versioning has been borrowed from [XEP‑0237] (Saint-Andre, P. and D. Cridland, “Roster Versioning,” May 2009.).

1.6.  Discussion Venue

[[ RFC Editor: please remove this section. ]]

The document editor and the broader XMPP developer community welcome discussion and comments related to the topics presented in this document. The primary and preferred venue is the <xmpp@ietf.org> mailing list, for which archives and subscription information are available at https://www.ietf.org/mailman/listinfo/xmpp. Related discussions often occur on the <standards@xmpp.org> mailing list, for which archives and subscription information are available at http://mail.jabber.org/mailman/listinfo/standards.

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 can access roster information from any resource. Because the user's roster can contain confidential data, the server MUST restrict access to this data so that only authorized entities (typically limited to the account owner) are able to retrieve, modify, or delete it.

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 detailed syntax and semantics are defined in the following sections.

2.1.1.  Ver Attribute

The 'ver' attribute is a string that identifies a particular version of the roster information. The value MUST be generated only by the server and MUST be treated by the client as opaque. The server can use any appropriate method for generating the version ID, such as a hash of the roster data or a strictly-increasing sequence number.

Inclusion of the 'ver' attribute is RECOMMENDED.

Use of the 'ver' attribute is described more fully under Section 2.6 (Roster Versioning).

2.1.2.  Roster Items

The <query/> element MAY contain one or more <item/> children, each describing a unique ROSTER ITEM or "contact".

The syntax of the <item/> element is described in the following sections.

2.1.2.1.  Ask Attribute

The 'ask' attribute is used to specify certain subscription sub-states; for details, see Section 3.1.2 (Server Processing of Outbound Subscription Request).

Inclusion of the 'ask' attribute is OPTIONAL.

2.1.2.2.  Jid Attribute

The 'jid' attribute specifies the Jabber Identifier (JID) that uniquely identifies the roster item.

Inclusion of the 'jid' attribute is REQUIRED.

2.1.2.3.  Name Attribute

The 'name' attribute specifies the "handle" to be associated with the JID, as determined by the user (not the contact). Although the value of the 'name' attribute MAY have meaning to a human user, it is opaque to the server. However, the 'name' attribute MAY be used by the server for matching purposes within the context of various XMPP extensions, in which case the values MUST be compared only after application of the Resourceprep profile of stringprep as defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.).

Inclusion of the 'name' attribute is OPTIONAL.

2.1.2.4.  Subscription Attribute

The 'subscription' attribute is OPTIONAL; see Section 2.1.7 (Subscription Attribute).

Inclusion of the 'subscription' attribute is OPTIONAL.

2.1.2.5.  Group Element

The <group/> child element specifies a category or "bucket" into which the roster item is to be grouped by a client. An <item/> element MAY contain more than one <group/> element, so that roster groups are not exclusive. Although the XML character data of the <group/> element MAY have meaning to a human user, it is opaque to the server. However, the <group/> element MAY be used by the server for matching purposes within the context of various XMPP extensions, in which case the data MUST be compared only after application of the Resourceprep profile of stringprep as defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.).

Inclusion of the <group/> child element is OPTIONAL.

2.1.3.  Roster Get

A ROSTER GET is a client's request for the server to send the roster; syntactically it is an IQ stanza of type "get" sent from client to server and containing a <query/> element qualified by the 'jabber:iq:roster' namespace, where the <query/> element MUST NOT contain any <item/> child elements.

C: <iq from='juliet@example.com/balcony'
       id='rg1'
       type='get'>
    <query xmlns='jabber:iq:roster'/>
  </iq>

The expected outcome of sending a roster get is for the server to return a roster result.

2.1.4.  Roster Set

A ROSTER SET is a client's request for the server to modify (i.e., create, update, or delete) a roster item; syntactically it is an IQ stanza of type "set" sent from client to server and containing a <query/> element qualified by the 'jabber:iq:roster' namespace.

The following rules apply to roster sets:

1. The <query/> element MUST contain one and only one <item/> element.
2. The server MUST ignore any value of the 'subscription' attribute other than "remove" (see Section 2.1.7 (Subscription Attribute)).
3. The server MUST ignore any 'to' address specified on the IQ stanza and MUST handle the IQ stanza as if it included no 'to' attribute.

C: <iq from='juliet@example.com/balcony'
       id='rs1'
       type='set'>
    <query xmlns='jabber:iq:roster'>
      <item jid='nurse@example.com'/>
    </query>
  </iq>

2.1.5.  Roster Push

A ROSTER PUSH is a newly created, updated, or deleted roster item that is sent from the server to the client; syntactically it is an IQ stanza of type "set" sent from server to client and containing a <query/> element qualified by the 'jabber:iq:roster' namespace.

The following rules apply to roster pushes:

1. The <query/> element in a roster push MUST contain one and only one <item/> element.
2. A receiving client MUST ignore the stanza unless it has no 'from' attribute (i.e., implicitly from the user's bare JID) or it has a 'from' attribute whose value matches the user's bare JID <user@domain>.

S: <iq id='a78b4q6ha463'
       to='juliet@example.com/chamber'
       type='set'>
    <query xmlns='jabber:iq:roster'>
      <item jid='nurse@example.com'/>
    </query>
  </iq>

As mandated by the semantics of the IQ stanza as defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), each resource that receives a roster push MUST reply with an IQ stanza of type "result" (or "error").

C: <iq from='juliet@example.com/balcony'
       id='a78b4q6ha463'
       type='result'/>

C: <iq from='juliet@example.com/chamber'
       id='a78b4q6ha463'
       type='result'/>

Note: There is no error case for client processing of roster pushes; if the server receives an IQ of type "error" in response to a roster push it SHOULD ignore the error.

2.1.6.  Roster Result

A ROSTER RESULT is the server's response to a roster get; syntactically it is an IQ stanza of type "result" sent from server to client and containing a <query/> element qualified by the 'jabber:iq:roster' namespace.

The <query/> element in a roster result contains one <item/> element for each contact and therefore can contain more than one <item/> element.

S: <iq id='rg1'
       to='juliet@example.com/chamber'
       type='result'>
    <query xmlns='jabber:iq:roster' ver='ver7'>
      <item jid='nurse@example.com'/>
      <item jid='romeo@example.net'/>
    </query>
  </iq>

If there are no contacts in the roster, then the server MUST return an IQ-result containing a child <query/> element that in turn contains no <item/> children (e.g., the server MUST NOT return an empty <iq/> stanza element).

S: <iq to='juliet@example.com/chamber'
       id='roster_result'
       type='result'>
    <query xmlns='jabber:iq:roster' ver='ver9'/>
  </iq>

2.1.7.  Subscription Attribute

The state of the presence subscription in relation to a roster item is captured in the 'subscription' attribute of the <item/> element. Allowable subscription-related values for this attribute are:

• "none" -- the user does not have a subscription to the contact's presence, and the contact does not have a subscription to the user's presence; this is the default value, so if the subscription attribute is not included then the state is to be understood as "none"
• "to" -- the user has a subscription to the contact's presence, but the contact does not have a subscription to the user's presence
• "from" -- the contact has a subscription to the user's presence, but the user does not have a subscription to the contact's presence
• "both" -- both the user and the contact have subscriptions to each other's presence (also called a "mutual subscription")

In a roster result, the client MUST ignore values of the 'subscription' attribute other than "none", "to", "from", or "both".

In a roster push, the client MUST ignore values of the 'subscription' attribute other than "none", "to", "from", "both", or "remove".

In a roster set, the value of the 'subscription' attribute MAY be included with a value of "remove", which indicates that the item is to be removed from the roster; the server MUST ignore all values of the 'subscription' attribute other than "remove".

2.2.  Retrieving the 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 is not necessarily desirable for all resources, e.g., a connection with limited bandwidth, the client's request for the roster is not mandatory). After a connected resource sends initial presence (see Section 4.2 (Initial Presence)), it is referred to as an available resource. If a connected resource or available resource requests the roster, it is referred to as an INTERESTED RESOURCE. The server MUST send roster pushes to all interested resources.

Note: Presence subscription requests are sent to available resources, whereas the roster pushes associated with subscription state changes are sent to interested resources. Therefore if a resource wishes to receive both subscription requests and roster pushes, it MUST both send initial presence and request the roster.

A client requests the roster by sending a roster get over its stream to the server.

C: <iq from='juliet@example.com/balcony'
       id='roster_1'
       type='get'>
     <query xmlns='jabber:iq:roster'/>
   </iq>

S: <iq id='roster_1'
       to='juliet@example.com/balcony'
       type='result'>
     <query xmlns='jabber:iq:roster' ver='ver11'>
       <item jid='romeo@example.net'
             name='Romeo'
             subscription='both'>
         <group>Friends</group>
       </item>
       <item jid='mercutio@example.com'
             name='Mercutio'
             subscription='from'/>
       <item jid='benvolio@example.net'
             name='Benvolio'
             subscription='both'/>
     </query>
   </iq>

If the server cannot process the roster get, it MUST return an appropriate stanza error as described in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.) (such as <service-unavailable/> if the roster namespace is not supported or <internal-server-error/> if the server experiences trouble processing or returning the roster).

2.3.  Adding a Roster Item

2.3.1.  Request

At any time, a client can add an item to the roster. This is done by sending a roster set containing a new item.

C: <iq from='juliet@example.com/balcony'
       id='roster_2'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='nurse@example.com'
             name='Nurse'>
         <group>Servants</group>
       </item>
     </query>
   </iq>

Note: When a user adds a contact for the purpose of tracking the user's presence subscription to a contact, the user's client SHOULD send a presence subscription request to the contact before generating any roster set related to the contact. This enables the user's server to enforce any policies relevant to presence subscriptions (e.g., a prohibition on presence subscriptions to full JIDs). For details, see Section 3 (Managing Presence Subscriptions).

2.3.2.  Success Case

If the server can successfully process the roster set (i.e., if none of the error cases occurs), it MUST create the roster item in persistent storage.

The server MUST then return an IQ stanza of type "result" to the connected resource that sent the roster set.

S: <iq id='roster_2'
       to='juliet@example.com/balcony'
       type='result'/>

The server MUST also send a roster push containing the new roster item to all of the user's interested resources, including the resource that generated the roster set.

S: <iq to='juliet@example.com/balcony'
       id='a78b4q6ha463'
       type='set'>
     <query xmlns='jabber:iq:roster' ver='ver13'>
       <item jid='nurse@example.com'
             name='Nurse'
             subscription='none'>
         <group>Servants</group>
       </item>
     </query>
   </iq>

S: <iq to='juliet@example.com/chamber'
       id='a78b4q6ha464'
       type='set'>
     <query xmlns='jabber:iq:roster' ver='ver13'>
       <item jid='nurse@example.com'
             name='Nurse'
             subscription='none'>
         <group>Servants</group>
       </item>
     </query>
   </iq>

As mandated by the semantics of the IQ stanza as defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), each resource that receives a roster push MUST reply with an IQ stanza of type "result" (or "error").

C: <iq from='juliet@example.com/balcony'
       id='a78b4q6ha463'
       type='result'/>

C: <iq from='juliet@example.com/chamber'
       id='a78b4q6ha464'
       type='result'/>

2.3.3.  Error Cases

If the server cannot successfully process the roster set, it MUST return a stanza error. The following error cases are defined (naturally, other stanza errors can occur, such as <internal-server-error/>).

The server SHOULD return a <bad-request/> stanza error to the client if the roster set violates any of the following conditions:

1. The <query/> element contains more than one <item/> child element.
2. The <item/> element contains more than one <group/> element, but there are duplicate groups (where duplicates are determined using the Resourceprep profile of stringprep as defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.)).

The server SHOULD return a <not-acceptable/> stanza error to the client if the roster set violates any of the following conditions:

1. The value of the 'name' attribute is greater than a server-configured limit.
2. The XML character data of the <group/> element is of zero length.
3. The XML character data of the <group/> element is greater than a server-configured limit.

Alternatively, the server MAY ignore the foregoing violations and process the roster set as best as possible (e.g., process only the first <item/> element, ignore duplicate <group/> elements, place the roster item in no group or a default group if the <group/> element is empty, and truncate 'name' attributes and <group/> elements that are too long).

Error: Roster set contains more than one item

C: <iq from='juliet@example.com/balcony'
       id='roster_3'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='nurse@example.com'
             name='Nurse'>
         <group>Servants</group>
       </item>
       <item jid='mother@example.com'
             name='Mom'>
         <group>Family</group>
       </item>
     </query>
   </iq>

S: <iq id='roster_3'
       to='juliet@example.com/balcony'
       type='error'>
    <error type='modify'>
      <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    </error>
  </iq>

Error: Roster set contains item with oversized handle

C: <iq from='juliet@example.com/balcony'
       id='roster_4'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='nurse@example.com'
             name='[ ... some-very-long-handle ... ]'>
         <group>Servants</group>
       </item>
     </query>
   </iq>

S: <iq id='roster_4'
       to='juliet@example.com/balcony'
       type='error'>
     <error type='modify'>
       <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

Error: Roster set contains duplicate groups

C: <iq from='juliet@example.com/balcony'
       id='roster_5'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='nurse@example.com'
             name='Nurse'>
         <group>Servants</group>
         <group>Servants</group>
       </item>
     </query>
   </iq>

S: <iq id='roster_5'
       to='juliet@example.com/balcony'
       type='error'>
     <error type='modify'>
       <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

Error: Roster set contains empty group

C: <iq from='juliet@example.com/balcony'
       id='roster_6'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='nurse@example.com'
             name='Nurse'>
         <group></group>
       </item>
     </query>
   </iq>

S: <iq id='roster_6'
       to='juliet@example.com/balcony'
       type='error'>
     <error type='modify'>
       <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

Error: Roster set contains oversized group

C: <iq from='juliet@example.com/balcony'
       id='roster_7'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='nurse@example.com'
             name='Nurse'>
         <group>[ ... some-very-long-group-name ... ]</group>
       </item>
     </query>
   </iq>

S: <iq id='roster_7'
       to='juliet@example.com/balcony'
       type='error'>
     <error type='modify'>
       <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

The server MUST return a <not-allowed/> stanza error to the client if the value of the <item/> element's 'jid' attribute matches the bare JID <node@domain> portion of the <iq/> element's 'from' attribute (i.e., a JID MUST NOT be allowed to add itself to its own roster).

Error: Roster set contains sender's JID

C: <iq from='juliet@example.com/balcony'
       id='roster_8'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='juliet@example.com'/>
      </query>
    </iq>

S: <iq id='roster_8'
       to='juliet@example.com/balcony'
       type='error'>
      <error type='cancel'>
        <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
      </error>
    </iq>

2.4.  Updating a Roster Item

2.4.1.  Request

Updating an existing roster item is done in the same way as adding a new roster item, i.e., by sending a roster set to the server. Because a roster item is atomic, the item MUST be updated exactly as provided in the roster set.

There are several reasons why a client might update a roster item:

1. Adding a group
2. Deleting a group
3. Changing the handle
4. Deleting the handle

Consider a roster item that is defined as follows:

    <item jid='romeo@example.net'
          name='Romeo'>
      <group>Friends</group>
    </item>

The user who has this item in her roster might want to add the item to another group.

C: <iq from='juliet@example.com/balcony'
       id='update_1'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='romeo@example.net'
             name='Romeo'>
         <group>Friends</group>
         <group>Lovers</group>
       </item>
     </query>
   </iq>

The user might then want to remove the item from the original group.

C: <iq from='juliet@example.com/balcony'
       id='update_2'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='romeo@example.net'
             name='Romeo'>
         <group>Lovers</group>
       </item>
     </query>
   </iq>

The user might then want to change the handle for the item.

C: <iq from='juliet@example.com/balcony'
       id='update_3'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='romeo@example.net'
             name='MyRomeo'>
         <group>Lovers</group>
       </item>
     </query>
   </iq>

The user might then want to remove the handle altogether (note: including an empty 'name' attribute is equivalent to including no 'name' attribute).

C: <iq from='juliet@example.com/balcony'
       id='update_4'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='romeo@example.net'
             name=''>
         <group>Lovers</group>
       </item>
     </query>
   </iq>

The user might then want to remove the item from all groups.

C: <iq from='juliet@example.com/balcony'
       id='update_5'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='romeo@example.net'/>
     </query>
   </iq>

2.4.2.  Success Case

As with adding a roster item, if the roster item can be successfully processed then the server MUST update the roster information in persistent storage, send a roster push to all of the user's interested resources, and send an IQ result to the initiating resource; for details, see Section 2.3 (Adding a Roster Item).

2.4.3.  Error Cases

The error cases described under Section 2.3.3 (Error Cases) also apply to updating a roster item.

2.5.  Deleting a Roster Item

2.5.1.  Request

At any time, a client can delete an item from his or her roster by sending a roster set and specifying the value of the 'subscription' attribute to be "remove".

C: <iq from='juliet@example.com/balcony'
       id='delete_1'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='nurse@example.com' subscription='remove'/>
     </query>
   </iq>

2.5.2.  Success Case

As with adding a roster item, if the server can successfully process the roster set then it MUST update the roster information in persistent storage, send 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; for details, see Section 2.3 (Adding a Roster Item).

If the user has a presence subscription to the contact or the contact has a presence subscription to the user, the user's server MUST also generate a presence stanza of type "unsubscribe" (to unsubscribe from the contact's presence) or a presence stanza of type "unsubscribed" (to cancel the contact's subscription to the user), or both.

S: <presence from='juliet@example.com'
             to='nurse@example.com'
             type='unsubscribe'/>

S: <presence from='juliet@example.com'
             to='nurse@example.com'
             type='unsubscribed'/>

2.5.3.  Error Cases

If the value of the 'jid' attribute specifies an item that is not in the roster, then the server MUST return an <item-not-found/> stanza error.

Error: Roster item not found

C: <iq from='juliet@example.com/balcony'
       id='delete_2'
       type='set'>
     <query xmlns='jabber:iq:roster'>
       <item jid='[ ... non-existent-jid ... ]'
             subscription='remove'/>
     </query>
   </iq>

S: <iq id='delete_2'
       to='juliet@example.com/balcony'
       type='error'>
     <error type='modify'>
       <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

2.6.  Roster Versioning

2.6.1.  Request

If a client supports roster versioning, it MUST include the 'ver' element in its request for the roster, where the 'ver' attribute is set to the version ID associated with its last cache of the roster.

C: <iq from='romeo@montague.lit/home'
       id='r1h3vzp7'
       to='romeo@montague.lit'
       type='get'>
     <query xmlns='jabber:iq:roster' ver='ver14'/>
   </iq>

If the client has not yet cached the roster or the cache is lost or corrupted, but the client wishes to bootstrap the use of roster versioning, it MUST set the 'ver' attribute to the empty string (i.e., ver="").

Naturally, if the client does not support roster versioning or does not wish to bootstrap the use of roster versioning, it will behave like an RFC-3921-compliant client by not including the 'ver' attribute.

2.6.2.  Success Case

Whether or not the roster has been modified since the version ID enumerated by the client, the server MUST either return the complete roster as described in RFC 3921 (including a 'ver' attribute that signals the latest version) or return an empty IQ-result (thus indicating that any roster modifications will be sent via roster pushes, as described below). In general, unless returning the complete roster would (1) use less bandwidth than sending individual roster pushes to the client (e.g., if the roster contains only a few items) or (2) the server cannot associate the version ID with any previous version it has on file, the server SHOULD send an empty IQ-result and then send the modifications (if any) via roster pushes.

S: <iq from='romeo@montague.lit'
       id='r1h3vzp7'
       to='romeo@montague.lit/home'
       type='result'/>

Note: This empty IQ-result is different from an empty <query/> element, thus disambiguating this usage from an empty roster.

If the roster has not been modified since the version ID enumerated by the client, the server will simply not send any roster pushes to the client (until and unless some relevant event triggers a roster push during the lifetime of the client's session).

If the roster has been modified since the version ID enumerated by the client, the server MUST then send one roster push to the client for each roster item that has been modified since the version ID enumerated by the client. (We call a roster push that is sent for purposes of roster version synchronization an "interim roster push".)

Definition: A ROSTER MODIFICATION is any modification to the roster data that would result in a roster push to a connected client. Therefore internal states related to roster processing within the server that would not result in a roster push to a connected client do not necessitate a change to the version.

S: <iq from='romeo@montague.lit'
       id='ah382g67'
       to='romeo@montague.lit/home'
       type='set'>
     <query xmlns='jabber:iq:roster' ver='ver34'>
       <item jid='tybalt@shakespeare.lit' subscription='remove'/>
     </query>
   </iq>

S: <iq from='romeo@montague.lit'
       id='b2gs90j5'
       to='romeo@montague.lit/home'
       type='set'>
     <query xmlns='jabber:iq:roster' ver='ver42'>
       <item jid='bill@shakespeare.lit' subscription='both'/>
     </query>
   </iq>

S: <iq from='romeo@montague.lit'
       id='c73gs419'
       to='romeo@montague.lit/home'
       type='set'>
     <query xmlns='jabber:iq:roster' ver='ver72'>
       <item jid='nurse@shakespeare.lit'
             name='Nurse'
             subscription='to'>
         <group>Servants</group>
       </item>
     </query>
   </iq>

S: <iq from='romeo@montague.lit'
       id='dh361f35'
       to='romeo@montague.lit/home'
       type='set'>
     <query xmlns='jabber:iq:roster' ver='ver96'>
       <item jid='juliet@shakespeare.lit'
             name='Juliet'
             subscription='both'>
         <group>VIPs</group>
       </item>
     </query>
   </iq>

These "interim roster pushes" can be understood as follows:

1. Imagine that the client had an active presence session for the entire time between its cached roster version (say, "ver14") and the new roster version (say, "ver96").
2. During that time, the client might have received roster pushes related to various roster versions. However, some of those roster pushes might have contained intermediate updates to the same roster item (e.g., modifications to the subscription state for bill@shakespeare.lit from "none" to "to" and from "to" to "both").
3. The interim roster pushes would not include all of the intermediate steps, only the final result of all modifications applied to each item while the client was in fact offline (say, "ver34", "ver42", "ver72", and "ver96").

The client MUST handle an "interim roster push" in the same way it handles any roster push (indeed, from the client's perspective it cannot tell the difference between an "interim" roster push and a "live" roster push). If the client's session ends before it receives all of the interim roster pushes, when requesting the roster after reconnection it SHOULD request the version associated with the last roster push it received during the session that was disconnected, not the version associated with the roster result it received at the start of the session that was disconnected.

When roster versioning is enabled, the server MUST include the updated roster version with each roster push. Roster pushes MUST occur in order of modification and the version contained in a roster push MUST be unique. Even if the client has not included the 'ver' attribute in its roster gets or sets, the server SHOULD include the 'ver' attribute on all roster pushes and results that it sends to the client.

Note: Implementation guidelines and more detailed examples for roster versioning are provided in [XEP‑0237] (Saint-Andre, P. and D. Cridland, “Roster Versioning,” May 2009.).

3.  Managing Presence Subscriptions

In order to protect the privacy of instant messaging users, presence information is disclosed only to other entities that a user has approved. When a user has agreed that another entity is allowed to view its presence, the entity is said to have a SUBSCRIPTION to the user's presence. An entity that has a subscription to a user's presence or to which a user has a presence subscription is called a CONTACT (in this document the term "contact" is also used in a less strict sense to refer to a potential contact or an item in a user's roster).

In XMPP, a subscription lasts across presence sessions; indeed, it lasts until the contact unsubscribes or the user cancels the previously-granted subscription.

Subscriptions are managed within XMPP by sending presence stanzas containing specially-defined attributes ("subscribe", "unsubscribe", "subscribed", and "unsubscribed").

Note: When a server processes or generates an outbound presence stanza of type "subscribe", "subscribed", "unsubscribe", or "unsubscribed", the server MUST stamp the outgoing presence stanza with the bare JID <node@domain> of the sending entity, not the full JID <node@domain/resource>. Enforcement of this rule simplifies the presence subscription model and helps to prevent presence leaks; for information about presence leaks, refer to the security considerations of [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.).

Subscription states are reflected in the rosters of both the user and the contact. Complete details regarding these subscription states can be found Appendix A (Subscription States); those details are not provided in this section, which simply narrates the protocol flows for common use cases related to presence subscriptions.

3.1.  Requesting a Subscription

A SUBSCRIPTION REQUEST is a request from a user for authorization to permanently subscribe to a contact's presence information; syntactically it is a presence stanza whose 'type' attribute has a value of "subscribe". A subscription request is generated by a user's client, processed by the (potential) contact's server, and acted on by the contact via the contact's client. The workflow is described in the following sections.

Note: Presence subscription requests are sent to available resources, whereas the roster pushes associated with subscription state changes are sent to interested resources. Therefore if a resource wishes to receive both subscription requests and roster pushes, it MUST both send initial presence and request the roster.

3.1.1.  Client Generation of Outbound Subscription Request

A user's client generates a subscription request by sending a presence stanza of type "subscribe" and specifying a 'to' address of the potential contact's bare JID <contact@domain>.

UC: <presence id='xk3h1v69'
              to='juliet@example.com'
              type='subscribe'/>

When a user sends a presence subscription request to a potential instant messaging and presence contact, the value of the 'to' attribute MUST be a bare JID <contact@domain> rather a full JID <contact@domain/resource>, since the desired result is for the user to receive presence from all of the contact's resources, not merely the particular resource specified in the 'to' attribute. Use of bare JIDs also simplifies subscription processing, presence probes, and presence notifications by the user's server and the contact's server.

For tracking purposes, a client SHOULD include an 'id' attribute in a presence subscription request.

Although many XMPP clients prompt the user for information about the potential contact (e.g., "handle" and desired roster group) when generating an outbound presence subscription request, the client MUST NOT send a roster set before sending the presence subscription request, but instead MUST wait until receiving the initial roster push from the server. This enables the user's server to enforce any policies relevant to presence subscriptions (e.g., a prohibition on presence subscriptions to full JIDs).

3.1.2.  Server Processing of Outbound Subscription Request

Upon receiving the outbound presence subscription request, the user's server MUST proceed as follows.

1. Before processing the request, the user's server SHOULD check the syntax of the JID contained in the 'to' attribute. If the JID is of the form <contact@domain/resource> instead of <contact@domain>, the user's server SHOULD treat it as if the request had been directed to the contact's bare JID and modify the 'to' address accordingly. The server MAY also verify that the JID adheres to the format defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), including checking against the relevant stringprep profiles.
2. If the potential contact is hosted on the same server as the user, then the server MUST adhere to the rules specified in the next section in processing the subscription request and delivering it to the (local) contact.
3. If the potential contact is hosted on a remote server, subject to local service policies the user's server MUST then route the stanza to that remote domain in accordance with core XMPP stanza processing rules. (This can result in returning an appropriate stanza error to the user, such as <remote-server-timeout/>.)

As mentioned, before locally delivering or remotely routing the presence subscription request, the user's server MUST stamp the outbound subscription request with the bare JID <user@domain> of the user.

US: <presence from='romeo@example.net'
              id='xk3h1v69'
              to='juliet@example.com'
              type='subscribe'/>

If the presence subscription request cannot be locally delivered or remotely routed (e.g., because the request is malformed, the local contact does not exist, the remote server does not exist, an attempt to contact the remote server times out, or any other error determined or experienced by the user's server), then the user's server MUST return an appropriate error stanza to the user. An example follows.

US: <presence from='juliet@example.com'
              id='xk3h1v69'
              to='romeo@example.net'
              type='error'>
      <error type='modify'>
        <remote-server-not-found
            xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
      </error>
    </presence>

After locally delivering or remotely routing the presence subscription request, the user's server MUST then send a roster push to all of the user's interested resources, containing the potential contact with a subscription state of "none" and with notation that the subscription is pending (via an 'ask' attribute whose value is "subscribe").

US: <iq id='b89c5r7ib574'
        to='romeo@example.net/foo'
        type='set'>
      <query xmlns='jabber:iq:roster'>
        <item ask='subscribe'
              jid='juliet@example.com'
              subscription='none'/>
      </query>
    </iq>

US: <iq id='b89c5r7ib575'
        to='romeo@example.net/bar'
        type='set'>
      <query xmlns='jabber:iq:roster'>
        <item ask='subscribe'
              jid='juliet@example.com'
              subscription='none'/>
        </item>
      </query>
    </iq>

If the contact does not approve or deny the subscription request within some configurable amount of time, the user's server SHOULD resend the 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 might have occurred in relation to the original subscription request.

3.1.3.  Server Processing of Inbound Subscription Request

Before processing the inbound presence subscription request, the contact's server SHOULD check the syntax of the JID contained in the 'to' attribute. If the JID is of the form <contact@domain/resource> instead of <contact@domain>, the contact's server SHOULD treat it as if the request had been directed to the contact's bare JID and modify the 'to' address accordingly. The server MAY also verify that the JID adheres to the format defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), including checking against the relevant stringprep profiles.

When processing the inbound presence subscription request, the contact's server MUST adhere to the following rules:

1. Above all, the contact's server MUST NOT automatically approve subscription requests on the contact's behalf (unless the contact has configured its account to automatically approve subscription requests or has accepted an agreement with its service provider that allows such behavior, for instance via an employment agreement within an enterprise deployment). Instead, if a subscription request requires approval then the contact's server MUST deliver that request to the contact's available resource(s) for approval or denial by the contact.
2. If the contact does not exist, then the contact's server MUST automatically return a presence stanza of type "unsubscribed" to the user.

   CS: <presence from='juliet@example.com'
                 id='xk3h1v69'
                 to='romeo@example.net'
                 type='unsubscribed'/>
   

3. If the contact exists and the user already has a subscription to the contact's presence, then the contact's server MUST auto-reply on behalf of the contact by sending a presence stanza of type "subscribed" from the contact's bare JID to the user's bare JID. If the contact previously sent a presence stanza of type "subscribed" and the contact's server treated that as indicating "pre-approval" for the user's presence subscription (see Appendix A (Subscription States)), then the contact's server SHOULD also auto-reply on behalf of the contact.

   CS: <presence from='juliet@example.com'
                 id='xk3h1v69'
                 to='romeo@example.net'
                 type='subscribed'/>
   

4. If the contact exists, the user does not already have a subscription to the contact's presence, and there is at least one available resource associated with the contact when the subscription request is received by the contact's server, then the contact's server MUST broadcast that subscription request to all available resources in accordance with Section 8 (Server Rules for Processing XML Stanzas). As a way of acknowledging receipt of the presence subscription request, the contact's server MAY send a presence stanza of type "unavailable" from the bare JID of the contact to the bare JID of the user.
5. If the contact exists, the user does not already have a subscription to the contact's presence, and the contact has no available resources when the subscription request is received by the contact's server, then the contact's server MUST keep a record of the complete presence stanza comprising the subscription request, including any extended content contained therein, and deliver the request when the contact next has an available resource. The contact's server MUST continue to deliver the subscription request whenever the contact creates an available resource, until the contact either approves or denies the request. (The contact's server MUST NOT deliver more than one subscription request from any given user when the contact next has an available resource; e.g., if the user sends multiple subscription requests to the contact while the contact is offline, the contact's server SHOULD store only one of those requests, such as the first request or last request, and MUST deliver only one of the requests when the contact next has an available resource; this helps to prevent "subscription request spam".) As a way of acknowledging receipt of the presence subscription request, the contact's server MAY send a presence stanza of type "unavailable" from the bare JID of the contact to the bare JID of the user.

Note: Until and unless the contact approves the subscription request as described under Section 3.1.4 (Client Processing of Inbound Subscription Request), the contact's server MUST NOT add an item for the user to the contact's roster.

3.1.4.  Client Processing of Inbound Subscription Request

When the contact's client receives a subscription request from the user, it MUST present the request to the contact for approval (unless the contact has explicitly configured the client to automatically approve or deny some or all subscription requests).

A subscription request is approved by sending a presence stanza of type "subscribed", which is processed as described in the following sections for both the contact's server and the user's server.

CC: <presence to='romeo@example.net' type='subscribed'/>

A subscription request is denied by sending a presence stanza of type "unsubscribed", which is processed as described under Section 3.2 (Cancelling a Subscription) for both the contact's server and the user's server.

CC: <presence to='romeo@example.net' type='unsubscribed'/>

3.1.5.  Server Processing of Outbound Subscription Approval

When the contact's client sends the subscription approval, the contact's server MUST stamp the outbound stanza with the bare JID <contact@domain> of the contact and locally deliver or remotely route the stanza to the user.

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

The contact's server then MUST send a roster push to all of the contact's interested resources.

CS: <iq id='a78b4q6ha463'
        to='juliet@example.com/balcony'
        type='set'>
      <query xmlns='jabber:iq:roster'>
        <item jid='romeo@example.net'
              subscription='from'/>
      </query>
    </iq>

CS: <iq id='a78b4q6ha464'
        to='juliet@example.com/chamber'
        type='set'>
      <query xmlns='jabber:iq:roster'>
        <item jid='romeo@example.net'
              subscription='from'/>
      </query>
    </iq>

The contact's server MUST then also send current presence to the user from each of the contact's available resources.

CS: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'/>

CS: <presence from='juliet@example.com/chamber'
              to='romeo@example.net'/>

From the perspective of the contact, there now exists a subscription from the user.

In order to subscribe to the user's presence, the contact would then send a subscription request to the user. (XMPP clients will often automatically send the subscription request instead of requiring the contact to initiate the subscription request, since it is assumed that the desired end state is a mutual subscription.) Naturally, when the contact sends a subscription request to the user, the subscription states will be different from those shown in the foregoing examples (see Appendix A (Subscription States)) and the roles will be reversed.

3.1.6.  Server Processing of Inbound Subscription Approval

When the user's server receives the subscription approval, it MUST first check if the contact is in the user's roster with subscription='none' or subscription='from' and the 'ask' flag set to "subscribe" (i.e., a subscription state of "None + Pending Out", "None + Pending Out+In", or "From + Pending Out"; see Appendix A (Subscription States)). If this check is successful, the user's server MUST:

1. 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" (if the subscription state was "None + Pending Out" or "None + Pending Out+In") or "both" (if the subscription state was "From + Pending Out").

   US: <iq id='b89c5r7ib576'
           to='romeo@example.net/foo'
           type='set'>
         <query xmlns='jabber:iq:roster'>
           <item jid='juliet@example.com'
                 subscription='to'/>
         </query>
       </iq>
   
   US: <iq id='b89c5r7ib577'
           to='romeo@example.net/bar'
           type='set'>
         <query xmlns='jabber:iq:roster'>
           <item jid='juliet@example.com'
                 subscription='to'/>
           </item>
         </query>
       </iq>
   

2. Deliver the inbound subscription approval to all of the user's interested resources (this helps to give the user's client(s) proper context regarding the subscription approval so that they can differentiate between a roster push originated by another of the user's resources and a subscription approval received from the contact).

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

3. The user's server MUST also deliver the available presence stanza received from each of the contact's available resources to each of the user's available resources.

   [ ... to resource1 ... ]
   
   US: <presence from='juliet@example.com/balcony'
                 to='romeo@example.net'/>
   
   [ ... to resource2 ... ]
   
   US: <presence from='juliet@example.com/balcony'
                 to='romeo@example.net'/>
   
   [ ... to resource1 ... ]
   
   US: <presence from='juliet@example.com/chamber'
                 to='romeo@example.net'/>
   
   [ ... to resource2 ... ]
   
   US: <presence from='juliet@example.com/chamber'
                 to='romeo@example.net'/>
   

Otherwise -- that is, if the user does not exist, if the contact is not in the user's roster, or if the contact is in the user's roster with a subscription state other than those described in the foregoing check -- then the user's server MUST silently ignore the stanza by not delivering it to the user, not modifying the user's roster, and not generating a roster push to the user's interested resources.

From the perspective of the user, there now exists a subscription to the contact's presence.

3.2.  Cancelling a Subscription

3.2.1.  Client Generation of Subscription Cancellation

If a contact would like to cancel a subscription that it has previously granted to a user (or deny a subscription request), it sends a presence stanza of type "unsubscribed".

CC: <presence to='romeo@example.net' type='unsubscribed'/>

3.2.2.  Server Processing of Outbound Subscription Cancellation

Upon receiving the outound subscription cancellation, the contact's server MUST proceed as follows.

1. If the user is hosted on the same server as the contact, then the server MUST adhere to the rules specified in the next section in processing the subscription cancellation.
2. If the user is hosted on a remote server, subject to local service policies the contact's server MUST then route the stanza to that remote domain in accordance with core XMPP stanza processing rules. (This can result in returning an appropriate stanza error to the contact, such as <remote-server-timeout/>.)

As mentioned, before locally delivering or remotely routing the stanza, the contact's server MUST stamp the outbound subscription cancellation with the bare JID <contact@domain> of the contact.

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

The contact's server then MUST send a roster push with the updated roster item to all of the contact's interested resources, where the subscription state is now either "none" or "to" (see Appendix A (Subscription States)).

CS: <iq id='a78b4q6ha465'
        to='juliet@example.com/balcony'
        type='set'>
      <query xmlns='jabber:iq:roster'>
        <item jid='romeo@example.net'
              subscription='none'/>
      </query>
    </iq>

CS: <iq id='a78b4q6ha466'
        to='juliet@example.com/chamber'
        type='set'>
      <query xmlns='jabber:iq:roster'>
        <item jid='romeo@example.net'
              subscription='none'/>
      </query>
    </iq>

3.2.3.  Server Processing of Inbound Subscription Cancellation

When the user's server receives the inbound subscription cancellation, it MUST first check if the contact is in the user's roster with subscription='to' or subscription='both' (see Appendix A (Subscription States)). If this check is successful, the user's server MUST:

1. 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 subscription state was "To" or "To + Pending In") or "from" (if the subscription state was "Both").

   US: <iq id='h37h3u1bv400'
           to='romeo@example.net/foo'
           type='set'>
         <query xmlns='jabber:iq:roster'>
           <item jid='juliet@example.com'
                 subscription='none'/>
         </query>
       </iq>
   
   US: <iq id='h37h3u1bv401'
           to='romeo@example.net/bar'
           type='set'>
         <query xmlns='jabber:iq:roster'>
           <item jid='juliet@example.com'
                 subscription='none'/>
           </item>
         </query>
       </iq>
   

2. Deliver the inbound subscription cancellation to all of the user's interested resources (this helps to give the user's client(s) proper context regarding the subscription cancellation so that they can differentiate between a roster push originated by another of the user's resources and a subscription cancellation received from the contact).

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

Otherwise -- that is, if the user does not exist, if the contact is not in the user's roster, or if the contact is in the user's roster with a subscription state other than those described in the foregoing check -- then the user's server MUST silently ignore the stanza by not delivering it to the user, not modifying the user's roster, and not generating a roster push to the user's interested resources.

3.3.  Unsubscribing

3.3.1.  Client Generation of Unsubscribe

If a user would like to unsubscribe from a contact's presence, it sends a presence stanza of type "unsubscribe".

UC: <presence to='juliet@example.com' type='unsubscribe'/>

3.3.2.  Server Processing of Outbound Unsubscribe

Upon receiving the outbound unsubscribe, the user's server MUST proceed as follows.

1. If the contact is hosted on the same server as the user, then the server MUST adhere to the rules specified in the next section in processing the subscription request.
2. If the contact is hosted on a remote server, subject to local service policies the user's server MUST then route the stanza to that remote domain in accordance with core XMPP stanza processing rules. (This can result in returning an appropriate stanza error to the user, such as <remote-server-timeout/>.)

As mentioned, before locally delivering or remotely routing the unsubscrbe, the user's server MUST stamp the stanza with the bare JID <user@domain> of the user.

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

The user's server then MUST send a roster push with the updated roster item to all of the user's interested resources, where the subscription state is now either "none" or "from" (see Appendix A (Subscription States)).

US: <iq id='h37h3u1bv402'
        to='romeo@example.net/foo'
        type='set'>
      <query xmlns='jabber:iq:roster'>
        <item jid='juliet@example.com'
              subscription='none'/>
      </query>
    </iq>

US: <iq to='romeo@example.net/bar'
        type='set'
        id='h37h3u1bv403'>
      <query xmlns='jabber:iq:roster'>
        <item jid='juliet@example.com'
              subscription='none'/>
        </item>
      </query>
    </iq>

3.3.3.  Server Processing of Inbound Unsubscribe

When the contact's server receives the unsubscribe notification, it MUST first check if the user is in the contact's roster with subscription='from' or subscription='both' (i.e., a subscription state of "From", "From + Pending Out", or "Both"; see Appendix A (Subscription States)). If this check is successful, the contact's server MUST:

1. Initiate a roster push to all of the contact's interested resources, containing an updated roster item for the contact with the 'subscription' attribute set to a value of "none" (if the subscription state was "From" or "From + Pending Out") or "to" (if the subscription state was "Both").

   CS: <iq id='a78b4q6ha467'
           to='juliet@example.com/balcony'
           type='set'>
         <query xmlns='jabber:iq:roster'>
           <item jid='romeo@example.net'
                 subscription='none'/>
         </query>
       </iq>
   
   CS: <iq id='a78b4q6ha468'
           to='juliet@example.com/chamber'
           type='set'>
         <query xmlns='jabber:iq:roster'>
           <item jid='romeo@example.net'
                 subscription='none'/>
         </query>
       </iq>
   

2. Deliver the inbound unsubscribe to all of the contact's interested resources (this helps to give the contact's client(s) proper context regarding the unsubscribe so that they can differentiate between a roster push originated by another of the contact's resources and an unsubscribe received from the user).

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

3. Deliver the inbound unavailable notification to all of the contact's interested resources.

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

Note: The user's client MUST NOT depend on receiving the unsubscribe and the unavailable presence notification, since it MUST consider its presence subscription to the contact, and its presence information about the contact, to be null and void when it sends the presence stanza of type "unusbscribe" or when receives the roster push triggered by the unsubscribe request.

Otherwise -- that is, if the contact does not exist, if the user is not in the contact's roster, or if the user is in the contact's roster with a subscription state other than those described in the foregoing check -- then the contact's server MUST silently ignore the stanza by not delivering it to the contact, not modifying the contact's roster, and not generating a roster push to the contact's interested resources. However, if the contact's server is keeping track of an inbound presence subscription request from the user to the contact but the user is not in the contact's roster (functionally equivalent to a subscription state of "None + Pending In" where the contact never added the user to the contact's roster), then the contact's server MUST simply remove any record of the inbound presence subscription request (it cannot remove the user from the contact's roster because the user was never added to the contact's roster).

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 user's availability is signalled when a client controlled by the user generates a <presence/> stanza with no 'type' attribute, and an entity's lack of availability is signalled when a client generates a <presence/> stanza whose 'type' attribute has a value of "unavailable".

XMPP presence typically follows a "publish-subscribe" or "observer" pattern, wherein an entity sends presence to its server, and its server then broadcasts that information to all of the entity's contacts who have a subscription to the entity's presence (in the terminology of [IMP‑MODEL] (Day, M., Rosenberg, J., and H. Sugano, “A Model for Presence and Instant Messaging,” February 2000.), an entity that generates presence is a "presentity" and the entities that receive presence 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". This kind of presence is called BROADCAST PRESENCE. (A client can also send DIRECTED PRESENCE, i.e., a presence stanza with a 'to' address; this is less common but is sometimes used to send presence to entities that are not subscribed to the user's presence; see Section 4.6 (Directed Presence).)

After a client completes the preconditions specified in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), it can establish a PRESENCE SESSION at its server by sending initial presence (Initial Presence), where the presence session is terminated by sending unavailable presence (Unavailable Presence). For the duration of its presence session, a connected resource (in the terminology of [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.)) is said to be an AVAILABLE RESOURCE.

In XMPP-based applications that combine messaging and presence functionality, the default type of communication for which presence signals availability is messaging; however, it is not necessary for XMPP-based applications to combine messaging and presence functionality, and can provide standalone presence features without messaging (in addition, XMPP servers do not require information about network availability in order to successfully route message and IQ stanzas).

Note: In the following examples, the "user" is juliet@example.com and the user has three contacts in her roster with a subscription state of "from" or "both": romeo@example.net, mercutio@example.com, and benvolio@example.net.

4.2.  Initial Presence

4.2.1.  Client Generation of Initial Presence

After completing the preconditions described in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.) (REQUIRED) and requesting the roster (RECOMMENDED), a client signals its availability for communication by sending INITIAL PRESENCE to its server, i.e., a presence stanza with no 'to' address (indicating that it is meant to be broadcast by the server on behalf of the client) and no 'type' attribute (indicating the user's availability).

UC: <presence/>

The initial presence stanza MAY contain the <priority/> element, the <show/> element, and one or more instances of the <status/> element, as well as extended content.

4.2.2.  Server Processing of Outbound Presence

Upon receiving initial presence from a client, the user's server MUST send the initial presence stanza from the full JID <user@domain/resource> of the user to all contacts that are subscribed to the user's presence; such contacts are those for which a JID is present in the user's roster with the 'subscription' attribute set to a value of "from" or "both".

US: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'/>

US: <presence from='juliet@example.com/balcony'
              to='mercutio@example.com'/>

US: <presence from='juliet@example.com/balcony'
              to='benvolio@example.net'/>

The user's server MUST also broadcast initial presence from the user's newly available resource to all of the user's available resources (including the resource that generated the presence notification in the first place).

US: <presence from='juliet@example.com/balcony'
              to='juliet@example.com/balcony'/>

US: <presence from='juliet@example.com/balcony'
              to='juliet@example.com/chamber'/>

In the absence of presence information about the user's contacts, the user's server MUST also send presence probes to the user's contacts on behalf of the user as specified under Section 4.3 (Presence Probes).

4.2.3.  Server Processing of Inbound Presence

Upon receiving presence from the user, the contact's server MUST deliver the user's presence stanza to all of the contact's available resources.

[ ... to resource1 ... ]

CS: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'/>

[ ... to resource2 ... ]

CS: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'/>

If there is no such contact, the contact's server MUST silently ignore the presence stanza.

4.2.4.  Client Processing of Inbound Presence

When the contact's client receives presence from the user, it SHOULD proceed as follows:

1. If the user is in the contact's roster, the client MUST display the presence information in an appropriate roster interface.
2. If the user is not in the contact's roster but the contact and the user are actively exchanging message or IQ stanzas, the contact's client SHOULD display the presence information in the user interface for that chat session (see also Section 4.6 (Directed Presence) and Section 5.1 (One-to-One Chat Sessions)).
3. Otherwise, the client MUST ignore the presence information and not display it to the contact.

4.3.  Presence Probes

A PRESENCE PROBE is a request for a contact's current presence information, sent on behalf of a user by the user's server; syntactically it is a presence stanza whose 'type' attribute has a value of "probe". The value of the 'from' address MUST be the full JID <user@domain/resource> of the user and the value of the 'to' address MUST be the bare JID <contact@domain> of the contact to which the user is subscribed.

US: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'
              type='probe'/>

Note: Although presence probes MAY be sent by a client, in general a client will not need to send them since the task of gathering presence from a user's contacts is managed by the user's server. However, if a client generates an outbound presence probe then the user's server SHOULD route the probe (if the contact is at another server) or process the probe (if the contact is at the same server) and MUST NOT return a stanza or stream error to the client.

If a server receives a presence probe intended for a full JID <contact@domain/resource>, it SHOULD handle it on behalf of the connected resource by returning only the presence information for that particular resource (and in any case MUST NOT deliver it to the resource).

4.3.1.  Server Generation of Outbound Presence Probe

When a server needs to discover the availability of a user's contact, it sends a presence probe from the full JID <user@domain/resource> of the user to the bare JID <contact@domain> of the contact. The server MUST NOT send a probe to a contact if the user is not subscribed to the contact's presence (i.e., if the contact is not in the user's roster with the 'subscription' attribute set to a value of "to" or "both".

The user's server SHOULD send a presence probe whenever the user starts a new presence session by sending initial presence; however, the server MAY choose not to send the probe at that point if it has what it deems to be reliable and up-to-date presence information about the user's contacts (e.g., because the user has another available resource or because the user briefly logged off and on before the new presence session began). In addition, a server MAY periodically send a presence probe to a contact if it has not received presence information or other traffic from the contact in some configurable amount of time; this can help to prevent "ghost" contacts who appear to be online but in fact are not.

US: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'
              type='probe'/>

US: <presence from='juliet@example.com/balcony'
              to='benvolio@example.net'
              type='probe'/>

Naturally, the user's server does not need to send a presence probe to a contact if the contact's account resides on the same server as the user, since the server possesses contact's information locally.

4.3.2.  Server Processing of Inbound Presence Probe

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 Appendix A (Subscription States)) and the contact has not sent directed presence to the user (as defined under Section 4.6 (Directed Presence)), then 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 configured hostname of the server itself or another such trusted service, it MAY provide presence information about the user to that entity).

   CS: <presence from='mercutio@example.com'
                 to='juliet@example.com'
                 type='unsubscribed'/>
   

2. Else, if the contact has no available resources, then the server SHOULD 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 (however, the server MAY opt to not reply at all).
3. Else, if the contact has at least one available resource, then the server MUST reply to the presence probe by sending to the user the full XML of the last presence stanza with no 'to' attribute received by the server from each of the contact's available resources.

   CS: <presence from='romeo@example.net/foo'
                 to='juliet@example.com'/>
   
   CS: <presence from='romeo@example.net/bar'
                 to='juliet@example.com'>
         <show>away</show>
       </presence>
   

4.4.  Subsequent Presence Broadcast

4.4.1.  Client Generation of Presence Broadcast

After sending initial presence, the user's client can update its availability for broadcasting at any time during its session by sending a presence stanza with no 'to' address and no 'type' attribute.

UC: <presence>
      <show>away</show>
    </presence>

The presence broadcast MAY contain the <priority/> element, the <show/> element, and one or more instances of the <status/> element, as well as extended content.

However, a user SHOULD send a presence update only to broadcast information that is relevant to the user's availability for communication or the communication capabilities of the connected resource. Information that is not relevant in this way can be of interest to the user's contacts but SHOULD be sent via other means, such as the XMPP message stanza.

4.4.2.  Server Processing of Outbound Presence

Upon receiving a presence stanza expressing updated availability, the user's server MUST broadcast the full XML of that presence stanza to the contacts who meet all of the following criteria:

1. The contact is in the user's roster with a subscription type of "from" or "both".
2. The last presence stanza received from the contact during the user's presence session was not of type "error" or "unsubscribe".

As an optimization, if the subscription type is "both", then the server SHOULD send subsequent presence notifications to a contact only if the contact is online according to the user's server. That is, if the user's server never received a positive indication that the contact is online in response to the presence probe it sent to the contact or if the last presence stanza it received from the contact during the user's presence session was of type "unavailable", the user's server SHOULD NOT send subsequent presence notifications from the user to the contact. This optimization helps to save bandwidth, since most presence subscriptions are bidirectional and many contacts will not be online at any given time.

US: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'>
      <show>away</show>
    </presence>

US: <presence from='juliet@example.com/balcony'
              to='benvolio@example.net'>
      <show>away</show>
    </presence>

US: <presence from='juliet@example.com/balcony'
              to='mercutio@example.com'>
      <show>away</show>
    </presence>

See Section 4.6 (Directed Presence) regarding rules that supplement the foregoing for handling of directed presence.

The user's server MUST also send the presence stanza to all of the user's available resources (including the resource that generated the presence notification in the first place).

US: <presence from='juliet@example.com/balcony'
              to='juliet@example.com/chamber'>
      <show>away</show>
    </presence>

US: <presence from='juliet@example.com/balcony'
              to='juliet@example.com/balcony'>
      <show>away</show>
    </presence>

4.4.3.  Server Processing of Inbound Presence

Upon receiving presence from the user, the contact's server MUST deliver the user's presence stanza to all of the contact's available resources.

[ ... to resource1 ... ]

CS: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'>
      <show>away</show>
    </presence>

[ ... to resource2 ... ]

CS: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'>
      <show>away</show>
    </presence>

4.4.4.  Client Processing of Inbound Presence

When the contact's client receives presence from the user, it SHOULD proceed as follows:

1. If the user is in the contact's roster, the client MUST display the presence information in an appropriate roster interface.
2. If the user is not in the contact's roster but the contact and the user are actively exchanging message or IQ stanzas, the contact's client SHOULD display the presence information in the user interface for that chat session (see also Section 4.6 (Directed Presence) and Section 5.1 (One-to-One Chat Sessions)).
3. Otherwise, the client MUST ignore the presence information and not display it to the contact.

4.5.  Unavailable Presence

4.5.1.  Client Generation of Unavailable Presence

Before ending its presence session with a server, the user's client SHOULD gracefully become unavailable by sending UNAVAILABLE PRESENCE, i.e., a presence stanza that possesses no 'to' attribute and that possesses a 'type' attribute whose value is "unavailable".

UC: <presence type='unavailable'/>

Optionally, the unavailable presence stanza MAY contain one or more <status/> elements specifying the reason why the user is no longer available.

UC: <presence type='unavailable'>
      <status>going on vacation</status>
    </presence>

However, the unavailable presence stanza MUST NOT contain the <priority/> element or the <show/> element, since these elements apply only to available presence.

4.5.2.  Server Processing of Outbound Unavailable Presence

The user's server MUST NOT depend on receiving unavailable presence from an available resource, since the resource can become unavailable ungracefully (e.g., the resource can be timed out by the server because of inactivity).

If an available resource becomes unavailable for any reason (either gracefully or ungracefully), the user's server MUST broadcast unavailable presence to all contacts that meet all of the following criteria:

1. The contact is in the user's roster with a subscription type of "from" or "both".
2. The last presence stanza received from the contact during the user's presence session was not of type "error" or "unsubscribe".

See Section 4.6 (Directed Presence) regarding rules that supplement the foregoing for handling of directed presence.

The optimization employed for subsequent presence broadcast during a user's presence session MUST NOT be employed for unavailable presence broadcast; if it were, the last presence received by the contact's server would be the user's initial presence for the presence session, with the result that the contact would consider the user to be online.

If the unavailable notification was gracefully received from the client, then the server MUST broadcast the full XML of the presence stanza.

US: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'
              type='unavailable'/>
      <status>going on vacation</status>
    </presence>

US: <presence from='juliet@example.com/balcony'
              to='benvolio@example.net'
              type='unavailable'>
      <status>going on vacation</status>
    </presence>

US: <presence from='juliet@example.com/balcony'
              to='mercutio@example.com'
              type='unavailable'>
      <status>going on vacation</status>
    </presence>

The user's server MUST also send the unavailable notification to all of the user's available resources (including the resource that generated the presence notification in the first place).

US: <presence from='juliet@example.com/balcony'
              to='juliet@example.com/chamber'
              type='unavailable'>
      <status>going on vacation</status>
    </presence>

If the server detects that the user has gone offline ungracefully, then the server MUST generate the unavailable presence broadcast on the user's behalf.

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

4.5.3.  Server Processing of Inbound Unavailable Presence

Upon receiving an unavailable notification from the user, the contact's server MUST deliver the user's presence stanza to all of the contact's available resources.

[ ... to resource1 ... ]

CS: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'
              type='unavailable'>
      <status>going on vacation</status>
    </presence>

[ ... to resource2 ... ]

CS: <presence from='juliet@example.com/balcony'
              to='romeo@example.net'
              type='unavailable'>
      <status>going on vacation</status>
    </presence>

If the contact's server is optimizing subsequent presence delivery as described under Section 4.4 (Subsequent Presence Broadcast), it MUST also note that the user is unavailable and appropriately update its internal representation of which entities are online.

4.5.4.  Client Processing of Inbound Unavailable Presence

When the contact's client receives an unavailable notification from the user, it SHOULD proceed as follows:

1. If the user is in the contact's roster, the client MUST display the unavailable notification in an appropriate roster interface.
2. If the user is not in the contact's roster but the contact and the user are actively exchanging message or IQ stanzas, the contact's client SHOULD display the unavailable notification in the user interface for that chat session (see also Section 4.6 (Directed Presence) and Section 5.1 (One-to-One Chat Sessions)).
3. Otherwise, the client MUST ignore the unavailable notification and not display it to the contact.

Typically, presence is presence about a particular connected resource. However, it is possible for the contact to receive an unavailable notification from the bare JID of the user. In this case, the presence notificatio SHOULD be treated as related to a resource identifier of zero length, which does not supersede or overshadow other resources associated with the same bare JID.

4.6.  Directed Presence

This section supplements and in some respects modifies the rules for client and server processing of presence notifications, but only for the special case of directed presence.

4.6.1.  Client Generation of Directed Presence

As noted, directed presence is a presence stanza with a 'to' attribute whose value is the bare JID or full JID of the other entity and with either no 'type' attribute (indicating availability) or a 'type' attribute whose value is "unavailable".

Information about the use of directed presence in the context of a one-to-one chat session is provided under Section 5.1 (One-to-One Chat Sessions).

4.6.2.  Server Processing of Outbound Directed Presence

When the user's server receives a directed presence stanza, it SHOULD process it according to the following rules.

1. If the user sends directed available or unavailable presence to a contact that is in the user's roster with a subscription type of "from" or "both" after having sent initial presence and before sending unavailable presence broadcast (i.e., during the user's presence session), the user's server MUST locally deliver or remotely route the full XML of that presence stanza but SHOULD NOT otherwise modify the contact's status regarding presence broadcast (i.e., it SHOULD include the contact's JID in any subsequent presence broadcasts initiated by the user).
2. If the user sends directed presence to an entity that is not in the user's roster with a subscription type of "from" or "both" after having sent initial presence and before sending unavailable presence broadcast (i.e., during the user's presence session), the user's server MUST locally deliver or remotely route the full XML of that presence stanza to the entity but MUST NOT modify the contact's status regarding available presence broadcast (i.e., it MUST NOT include the entity's JID in any subsequent broadcasts of available presence initiated by the user); however, if the available resource from which the user sent the directed presence becomes unavailable, the user's server MUST route that unavailable presence to the entity (if the user has not yet sent directed unavailable presence to that entity).
3. If the user sends directed presence without first sending initial presence or after having sent unavailable presence broadcast (i.e., the resource is connected but not available), the user's server MUST treat the entity to which the user sends directed presence as in case #2 above.

4.6.3.  Server Processing of Inbound Directed Presence

From the perspective of the contact's server, there is no difference between presence broadcast and directed presence, so the contact's server follows the existing rules for processing of inbound presence.

4.6.4.  Client Processing of Inbound Directed Presence

When the contact's client receives directed presence from the user, it SHOULD proceed as follows:

1. If the user is in the contact's roster, the client MUST display the presence information in an appropriate roster interface.
2. If the user is not in the contact's roster but the contact and the user are actively exchanging message or IQ stanzas, the contact's client SHOULD display the presence information in the user interface for that chat session (see also Section 4.6 (Directed Presence) and Section 5.1 (One-to-One Chat Sessions)).
3. Otherwise, the client MUST ignore the presence information and not display it to the contact.

4.7.  Presence Syntax

4.7.1.  Type Attribute

The absence of a 'type' attribute signals that the relevant entity is available for communication (see Section 4.2 (Initial Presence) and Section 4.4 (Subsequent Presence Broadcast)).

A 'type' attribute with a value of "unavailable" signals that the relevant entity is not available for communication (see Section 4.5 (Unavailable Presence)).

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; for details, see Section 4.3 (Presence Probes). Clients SHOULD NOT send presence stanzas of type "probe".

The values of the 'type' attribute can be summarized as follows:

• error -- An error has occurred regarding processing of a previously-sent presence stanza; if the presence stanza is of type "error", it MUST include an <error/> child element (refer to [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.)).
• probe -- A request for an entity's current presence; SHOULD be generated only by a server on behalf of a user.
• subscribe -- The sender wishes to subscribe to the recipient's presence.
• subscribed -- The sender has allowed the recipient to receive their presence.
• unavailable -- Signals that the entity is no longer available for communication.
• unsubscribe -- The sender is unsubscribing from the receiver's presence.
• unsubscribed -- The subscription request has been denied or a previously-granted subscription has been cancelled.

If the value of the 'type' attribute is not one of the foregoing values, the recipient or an intermediate router SHOULD return a stanza error of <bad-request/>.

Note: There is no default value for the 'type' attribute of the <presence/> element; in particular, there is no value of "available".

4.7.2.  Child Elements

In accordance with the default namespace declaration, a presence stanza is qualified by the 'jabber:client' or 'jabber:server' namespace, which defines certain 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.

4.7.3.  Show Element

The OPTIONAL <show/> element specifies the particular availability sub-state of an entity or a specific resource thereof. A presence stanza MUST NOT contain more than one <show/> element. There are no attributes defined for the <show/> element. The XML character data of the <show/> element is not human-readable. The XML character data MUST be one of the following (additional availability states could be defined through a child element of the presence stanza that is qualified by a namespace other than the default namespace):

• 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.

Any specialized processing of availability states by recipients and intermediate routers is up to the implementation (e.g., incorporation of availability states into stanza routing and delivery logic).

4.7.4.  Status Element

The OPTIONAL <status/> element contains human-readable XML character data specifying a natural-language description of an entity's availability. It is normally used in conjunction with the show element to provide a detailed description of an availability state (e.g., "In a meeting") when the presence stanza has no 'type' attribute.

<presence from='romeo@example.net/orchard'
          xml:lang='en'>
  <show>dnd</show>
  <status>Wooing Juliet</status>
</presence>

There are no attributes defined for the <status/> element, with the exception of the 'xml:lang' attribute inherited from XML. Multiple instances of the <status/> element MAY be included, but only if each instance possesses an 'xml:lang' attribute with a distinct language value (either explicitly or by inheritance from the 'xml:lang' value of an element farther up in the XML hierarchy, which can include the XML stream header as described in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.)).

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

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

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

The <status/> child MAY also be sent in a subscription-related presence stanza (i.e., type "subscribe", "subscribed", "unsubscribe", or "unsubscribed") to provide a description of the action. The receiving client MAY present this <status/> information to a human user (see Section 11 (Security Considerations)).

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

4.7.5.  Priority Element

The OPTIONAL <priority/> element contains non-human-readable XML character data that specifies the priority level of the resource. The value MUST be an integer between -128 and +127. A presence stanza MUST NOT contain more than one <priority/> element. There are no attributes defined for the <priority/> element.

<presence xml:lang='en'>
  <show>dnd</show>
  <status>Wooing Juliet</status>
  <status xml:lang='cs'>Dvo&#x0159;&#x00ED;m se Julii</status>
  <priority>1</priority>
</presence>

If no priority is provided, the processing server or client MUST consider the priority to be zero ("0").

For information regarding the semantics of priority values in stanza processing within instant messaging and presence applications, refer to Section 8 (Server Rules for Processing XML Stanzas).

4.7.6.  Extended Content

As described in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), an XML stanza MAY contain any child element that is qualified by a namespace other than the default namespace; this applies to the presence stanza as well.

(In the following example, the presence stanza includes entity capabilities information as defined in [XEP‑0115] (Hildebrand, J., Saint-Andre, P., and R. Tronçon, “Entity Capabilities,” February 2008.)).)

<presence from='romeo@example.net'>
  <c xmlns='http://jabber.org/protocol/caps'
     hash='sha-1'
     node='http://psi-im.org'
     ver='q07IKJEyjvHSyhy//CH0CxmKi8w='/>
</presence>

Any extended content included in a presence stanza SHOULD represent aspects of an entity's availability for communication or provide information about communication-related capabilities.

5.  Exchanging Messages

Once a client has authenticated with a server and bound a resource to an XML stream as described in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), an XMPP server will route XML stanzas to and from that client. One kind of stanza that can be exchanged is <message/> (if, that is, messaging functionality is enabled and the server is not a presence-only service). Exchanging messages is a basic use of XMPP and occurs when a user generates a message stanza that is addressed to another entity. As defined under Section 8 (Server Rules for Processing XML Stanzas), the sender's server is responsible for delivering the message to the intended recipient (if the recipient is on the same local server) or for routing the message to the recipient's server (if the recipient is on a remote server). Thus a message stanza is used to "push" information to another entity.

5.1.  One-to-One Chat Sessions

In practice, instant messaging activity between human users tends to occur in form of a conversational burst that we call a CHAT SESSION: the exchange of at least several messages between two parties in relatively rapid succession within a relatively brief period of time.

When a human user intends to engage in such a chat session with a contact (rather than sending a single message to which no reply is expected), the user's client SHOULD send a message of type "chat" and the contact's client SHOULD preserve that message type in subsequent replies. The user's client also SHOULD include a <thread/> element with its initial message, which the contact's client SHOULD also preserve during the life of the chat session.

The user's client MUST address the initial message in a chat session to the bare JID <contact@domain> of the contact (rather than attempting to guess an appropriate full JID <contact@domain/resource> based on the <show/>, <status/>, or <priority/> value of any presence notifications it has received from the contact). Until and unless the user's client receives a reply from the contact, it MUST continue sending any further messages to the contact's bare JID. The contact's client SHOULD address its subsequent replies to the user's full JID <user@domain/resource> as provided in the 'from' address of the initial message. Once the user's client receives a reply from the contact's full JID, it SHOULD address its subsequent messages to the contact's full JID as provided in the 'from' address of the contact's replies, thus "locking in" on that full JID.

When two parties engage in a chat session but do not share presence with each other based on a presence subscription, they SHOULD send directed presence to each other so that either party can easily discover if the peer changes its availability or goes offline during the course of the chat session. However, a client MUST provide a way for a user to disable such presence sharing globally or to enable it only with particular entities. Furthermore, a party SHOULD send directed unavailable to the peer when it has reason to believe that the chat session is over (e.g., if, after some reasonable amount of time, no subsequent messages have been exchanged between the parties).

If a party receives a presence change from the peer during a one-to-one chat session (e.g., a new resource comes online or the existing resource sends modified presence), then it SHOULD address its next message(s) in the chat session to the bare JID of the peer (thus "unlocking" the previous "lock") until it receives a message from one of the peer's full JIDs.

An example of a chat session is provided under Section 7 (A Sample Session).

5.2.  Message Syntax

The following sections describe the syntax of the <message/> stanza.

5.2.1.  To Attribute

An instant messaging client specifies an intended recipient for a message by providing the JID of an entity other than the sender in the 'to' attribute of the <message/> stanza.

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

<message
    from='juliet@example.com/balcony'
    to='romeo@example.net'
    type='chat'
    xml:lang='en'>
  <body>Art thou not Romeo, and a Montague?</body>
</message>

If the message is being sent in reply to a message previously received from an address of the form <user@domain/resource> (e.g., within the context of a one-to-one chat session as described under Section 5.1 (One-to-One Chat Sessions)), 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.

<message
    from='romeo@example.net/orchard'
    to='juliet@example.com/balcony'
    type='chat'
    xml:lang='en'>
  <body>Neither, fair saint, if either thee dislike.</body>
</message>

5.2.2.  Type Attribute

Common uses of the message stanza in instant messaging applications include: single messages; messages sent in the context of a one-to-one chat session; messages sent in the context of a multi-user chat room; alerts, notifications, or other information to which no reply is expected; and errors. These uses are differentiated via the 'type' attribute. Inclusion of the 'type' attribute is RECOMMENDED. If included, the 'type' attribute MUST have one of the following values:

• chat -- The message is sent in the context of a one-to-one chat session. Typically a receiving client will present message of type "chat" in an interface that enables one-to-one chat between the two parties, including an appropriate conversation history. Detailed recommendations regarding one-to-one chat sessions are provided under Section 5.1 (One-to-One Chat Sessions).
• error -- The message is generated by an entity that experiences an error in processing a message received from another entity (for details regarding stanza error syntax, refer to [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.)). A client that receives a message of type "error" 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.)). Typically a receiving client will present a message of type "groupchat" in an interface that enables many-to-many chat between the parties, including a roster of parties in the chatroom and an appropriate conversation history. For detailed information about XMPP-based groupchat, refer to [XEP‑0045] (Saint-Andre, P., “Multi-User Chat,” July 2008.).
• headline -- The message provides an alert, a notification, or other information to which no reply is expected (e.g., news headlines, sports updates, near-real-time market data, and syndicated content). Because no reply to the message is expected, typically a receiving client will present a message of type "headline" in an interface that appropriately differentiates the message from standalone messages, chat messages, or groupchat messages (e.g., by not providing the recipient with the ability to reply). The receiving server SHOULD deliver the message to all of the recipient's available resources.
• normal -- The message is a standalone message that is sent outside the context of a one-to-one conversation or groupchat, and to which it is expected that the recipient will reply. Typically a receiving client will present a message of type "normal" in an interface that enables the recipient to reply, but without a conversation history. The default value of the 'type' attribute is "normal".

An IM application SHOULD support all of the foregoing message types. If an application receives a message with no 'type' attribute or the application does not understand the value of the 'type' attribute provided, it MUST consider the message to be of type "normal" (i.e., "normal" is the default).

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

5.2.3.  Body Element

The <body/> element contains human-readable XML character data that specifies the textual contents of the message; this child element is normally included but is OPTIONAL.

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

There are no attributes defined for the <body/> element, with the exception of the 'xml:lang' attribute. Multiple instances of the <body/> element MAY be included in a message stanza, but only if each instance possesses an 'xml:lang' attribute with a distinct language value (either explicitly or by inheritance from the 'xml:lang' value of an element farther up in the XML hierarchy, which can include the XML stream header as described in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.)).

<message
    from='juliet@example.com/balcony'
    to='romeo@example.net'
    type='chat'
    xml:lang='en'>
  <body>Wherefore art thou, Romeo?</body>
  <body xml:lang='cs'>
     Pro&#x010D;e&#x017D; jsi ty, Romeo?
   </body>
</message>

The <body/> element MUST NOT contain mixed content (as defined in Section 3.2.2 of [XML] (Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fourth Edition),” August 2006.)).

5.2.4.  Subject Element

The <subject/> element contains human-readable XML character data that specifies the topic of the message.

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

There are no attributes defined for the <subject/> element, with the exception of the 'xml:lang' attribute inherited from XML. Multiple instances of the <subject/> element MAY be included for the purpose of providing alternate versions of the same subject, but only if each instance possesses an 'xml:lang' attribute with a distinct language value (either explicitly or by inheritance from the 'xml:lang' value of an element farther up in the XML hierarchy, which can include the XML stream header as described in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.)).

<message
    from='juliet@example.com/balcony'
    to='romeo@example.net'
    type='chat'
    xml:lang='en'>
  <subject>I implore you!</subject>
  <subject xml:lang='cs'>
    &#x00DA;p&#x011B;nliv&#x011B; pros&#x00ED;m!
  </subject>
  <body>Wherefore art thou, Romeo?</body>
  <body xml:lang='cs'>
     Pro&#x010D;e&#x017E; jsi ty, Romeo?
   </body>
</message>

The <subject/> element MUST NOT contain mixed content (as defined in Section 3.2.2 of [XML] (Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fourth Edition),” August 2006.)).

5.2.5.  Thread Element

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

The inclusion of the <thread/> element is OPTIONAL. Because the <thread/> element uniquely identifies the particular conversation thread to which a message belongs, a message stanza MUST NOT contain more than one <thread/> element.

The value of the <thread/> element is not human-readable and MUST be treated as opaque by entities; no semantic meaning can be derived from it, and only exact comparisons can be made against it. The value of the <thread/> element MUST 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 <thread/> element MAY possess a 'parent' attribute that identifies another thread of which the current thread is an offshoot or child; the value of the 'parent' MUST conform to the syntax of the <thread/> element itself.

The <thread/> element MUST NOT contain mixed content (as defined in Section 3.2.2 of [XML] (Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fourth Edition),” August 2006.)).

<message
    from='juliet@example.com/balcony'
    to='romeo@example.net'
    type='chat'
    xml:lang='en'>
  <subject>I implore you!</subject>
  <subject xml:lang='cs'>
    &#x00DA;p&#x011B;nliv&#x011B; pros&#x00ED;m!
  </subject>
  <body>Wherefore art thou, Romeo?</body>
  <body xml:lang='cs'>
     Pro&#x010D;e&#x017E; jsi ty, Romeo?
  </body>
  <thread parent='e0ffe42b28561960c6b12b944a092794b9683a38'>
    0e3141cd80894871a68e6fe6b1ec56fa
  </thread>
</message>

For detailed recommendations regarding use of the <thread/> element, refer to [XEP‑0201] (Saint-Andre, P., Paterson, I., and K. Smith, “Best Practices for Message Threads,” February 2008.).

5.3.  Extended Content

As described in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), an XML stanza MAY contain any child element that is qualified by a namespace other than the default namespace; this applies to the message stanza as well.

(In the following example, the message stanza includes an XHTML-formatted version of the message as defined in [XEP‑0071] (Saint-Andre, P., “XHTML-IM,” September 2008.)).)

<message
    from='juliet@example.com/balcony'
    to='romeo@example.net'
    type='chat'
    xml:lang='en'>
  <body>Wherefore art thou, Romeo?</body>
  <html xmlns='http://jabber.org/protocol/xhtml-im'>
    <body xmlns='http://www.w3.org/1999/xhtml'>
      Wherefore <span style='font-style: italic'>art</span>
      thou, <span style='color:red'>Romeo</span>?
    </body>
  </html>
</message>

6.  Exchanging IQ Stanzas

As described in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), IQ stanzas provide a structured request-response mechanism. The basic semantics of that mechanism (e.g., that the 'id' attribute is mandatory) are defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.), whereas the specific semantics needed to complete particular use cases are defined in all instances by the extended namespace that qualifies the direct child element of an IQ stanza of type "get" or "set". The 'jabber:client' and 'jabber:server' namespaces do not define any children of IQ stanzas other than the <error/> element common to all stanza types. This document defines one such extended namespace, for Managing the Roster (Managing the Roster). However, an IQ stanza MAY contain structured information qualified by any extended namespace.

As noted under Section 4.6 (Directed Presence), if a user exchanges IQ stanzas with another entity but does not share presence with the entity based on a presence subscription, it is RECOMMENDED for the user's client to send directed presence to the other entity.

7.  A Sample Session

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 identifier is "chamber" and another whose resource identifier is "balcony")
• benvolio@example.net (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,” September 2009.); those protocol flows are not reproduced here.

Next, the user requests his roster.

Example 1: User requests current roster from server:

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

Example 2: User receives roster from server:

US: <iq to='romeo@example.net/balcony'
        id='ex1'
        type='result'>
      <query xmlns='jabber:iq:roster'>
        <item jid='juliet@example.com'
              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 begins a presence session.

Example 3: User sends initial presence:

UC: <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:

US: <presence
        from='romeo@example.net/orchard'
        to='juliet@example.com'
        type='probe'/>

US: <presence
        from='romeo@example.net/orchard'
        to='benvolio@example.org'
        type='probe'/>

Example 5: User's server sends initial presence to contacts with subscription="from" and subscription="both" on behalf of the user's available resource:

US: <presence
        from='romeo@example.net/orchard'
        to='juliet@example.com'/>

US: <presence
        from='romeo@example.net/orchard'
        to='mercutio@example.org'/>

Example 6: Contacts' servers reply to presence probe on behalf of all available resources:

CS: <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>

CS: <presence
        from='juliet@example.com/chamber'
        to='romeo@example.net/orchard'>
      <priority>1</priority>
    </presence>

CS: <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:

CS: <presence
        from='romeo@example.net/orchard'
        to='juliet@example.com'/>

CS: <presence
        from='romeo@example.net/orchard'
        to='juliet@example.com'/>

CS: <presence
        from='mercutio@example.org'
        to='romeo@example.net'/>

Example 8: User sends directed presence to another user not in his roster:

UC: <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 engages in a chat session with one of his contacts.

Example 9: A threaded conversation

CC: <message
        from='juliet@example.com/balcony'
        to='romeo@example.net'
        type='chat'
        xml:lang='en'>
      <body>My ears have not yet drunk a hundred words</body>
      <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
    </message>

CC: <message
        from='juliet@example.com/balcony'
        to='romeo@example.net'
        type='chat'
        xml:lang='en'>
      <body>Of that tongue's utterance, yet I know the sound:</body>
      <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
    </message>

CC: <message
        from='juliet@example.com/balcony'
        to='romeo@example.net'
        type='chat'
        xml:lang='en'>
      <body>Art thou not Romeo, and a Montague?</body>
      <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
    </message>

UC: <message
        from='romeo@example.net/orchard'
        to='juliet@example.com/balcony'
        type='chat'
        xml:lang='en'>
      <body>Neither, fair saint, if either thee dislike.</body>
      <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
    </message>

CC: <message
        from='juliet@example.com/balcony'
        to='romeo@example.net/orchard'
        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 for broadcasting:

UC: <presence xml:lang='en'>
      <show>away</show>
      <status>I shall return!</status>
      <priority>1</priority>
    </presence>

Example 11: User's server broadcasts updated presence only to one contact:

US: <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 to all of the contact's available resources ("balcony" and "chamber"):

CS: <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>

CS: <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 unavailable notification:

CC: <presence from='juliet@example.com/chamber' type='unavailable'/>

Example 14: Contact's server sends unavailable notification to user:

CS: <presence
        from='juliet@example.com/chamber'
        to='romeo@example.net'
        type='unavailable'/>

Now the user ends his presence session.

Example 15: User sends unavailable notification:

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

Example 16: User's server broadcasts unavailable notification to contacts as well as to the person to whom the user sent directed presence:

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

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

Finally the user closes his stream and the server responds in kind.

Example 17: User closes stream:

UC: </stream:stream>

Example 18: User's server closes stream:

US: </stream:stream>

THE END

8.  Server Rules for Processing XML Stanzas

Basic server rules for processing XML stanzas are defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.). This section defines supplementary rules for XMPP instant messaging and presence servers; in the absence of a supplementary rule defined below (e.g., for stanzas without a 'to' address), the rule defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.) applies.

8.1.  No Such User

If the user account identified by the 'to' attribute does not exist, how the stanza is processed depends on the stanza type.

• For an IQ stanza, the server MUST return a <service-unavailable/> stanza error to the sender.
• For a message stanza, the server MUST return a <service-unavailable/> stanza error to the sender.
• For a presence stanza with no 'type' attribute or a 'type' attribute of "unavailable", the server MUST silently ignore the stanza.
• For a presence stanza of type "subscribe", the server MUST return a presence stanza of type "unsubscribed".
• For a presence stanza of type "subscribed", "unsubscribe", or "unsubscribed", the server MUST silently ignore the stanza.

8.2.  Full JID at Local Domain

If the hostname of the domain identifier portion of the JID contained in the 'to' attribute of an inbound stanza matches one of the configured hostnames of the server itself and the JID contained in the 'to' attribute is of the form <user@domain/resource>, then the server MUST adhere to the following rules (subject to enforcement of relevant privacy and security policies, such as those deployed by means of [XEP‑0016] (Millard, P. and P. Saint-Andre, “Privacy Lists,” February 2007.) or [XEP‑0191] (Saint-Andre, P., “Simple Communications Blocking,” February 2007.)).

8.2.1.  Resource Matches

If an available or connected resource exactly matches the full JID, how the stanza is processed depends on the stanza type.

• For an IQ stanzas of type "get" or "set", if the intended recipient does not share presence with the requesting entity either by means of a presence subscription of type "both" or "from" or by means of directed presence, then the server SHOULD NOT deliver the IQ stanza but instead SHOULD return a <service-unavailable/> stanza error to the requesting entity. This policy helps to prevent presence leaks (see Section 11 (Security Considerations)).
• For a message stanza, the server MUST deliver the stanza to the resource.
• For a presence stanza with no 'type' attribute or a 'type' attribute of "unavailable", the server MUST deliver the stanza to the resource.
• For a presence stanza of type "subscribe", the server MUST follow the guidelines provided under Section 3.1.3 (Server Processing of Inbound Subscription Request).
• For a presence stanza of type "subscribe", "subscribed", "unsubscribe", or "unsubscribed", the server MUST follow the guidelines provided under Section 3 (Managing Presence Subscriptions).

8.2.2.  No Resource Matches

If no connected or available resource exactly matches the full JID, how the stanza is processed depends on the stanza type.

• For an IQ stanza, the server MUST return a <service-unavailable/> stanza error to the sender.
• For a message stanza of type "chat", "headline", or "normal", the server SHOULD treat the stanza as if it were addressed to <user@domain> as described in the next section (but without modifying the value of the 'to' attribute).
• For a message stanza of type "error", the server MUST silently ignore the stanza.
• For a message stanza of type "groupchat", the server MUST return a stanza error to the sender, which SHOULD be <service-unavailable/>.
• For a presence stanza with no 'type' attribute or a 'type' attribute of "unavailable", the server MUST silently ignore the stanza.
• For a presence stanza of type "subscribe", the server MUST follow the guidelines provided under Section 3.1.3 (Server Processing of Inbound Subscription Request).
• For a presence stanza of type "subscribed", "unsubscribe", or "unsubscribed", the server MUST ignore the stanza.

8.3.  Bare JID at Local Domain

If the hostname of the domain identifier portion of the JID contained in the 'to' attribute of an inbound stanza matches one of the configured hostnames of the server itself and the JID contained in the 'to' attribute is of the form <user@domain>, then the server MUST adhere to the following rules.

8.3.1.  Available or Connected Resources

If there is at least one available or connected resource, how the stanza is processed depends on the stanza type.

8.3.1.1.  Message

For a message stanza of type "headline", the server SHOULD deliver the stanza to all available resources.

For a message stanza of type "chat" or "normal", the server SHOULD deliver the stanza to the highest-priority available resource. If there is not one highest-priority available resource but instead the highest priority is asserted by two or more available resources, these resources are said to form a "delivery tie". In the case of a delivery tie, a server SHOULD deliver the message to all of the tied resources. However, before delivering the message, a server MAY remove one or more resources from the tie. Methods for doing so are outside the scope of this specification, but could include factors such as the resource's time of connection, time of last network or application activity, availability as determined by some hierarchy of <show/> values, or user-configured rules. Nevertheless, a server MUST NOT remove all resources from the tie, and MUST deliver the message to at least one of the highest-priority resources (subject to appropriate security policies as described under Section 11 (Security Considerations) and in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.)).

For a message stanza of type "groupchat", the server MUST NOT deliver the stanza to any of the available resources but instead MUST return a stanza error to the sender, which SHOULD be <service-unavailable/>.

For a message stanza of type "error", the server MUST silently discard the message (i.e., neither deliver it to the intended recipient nor return a stanza error to the sender).

However, for any message type the server MUST NOT deliver the stanza to any 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 or connected resources as described under Section 8.3.2 (No Available or Connected Resources).

In all cases, 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>).

8.3.1.2.  Presence

For a presence stanza of type "probe", the server MUST handle it directly as described under Section 4.3 (Presence Probes).

For a presence stanza with no type or of type "unavailable", the server MUST deliver the stanza to all available resources.

For a presence stanza of type "subscribe", "subscribed", "unsubscribe", or "unsubscribed", the server MUST adhere to the rules defined under Section 3 (Managing Presence Subscriptions) and summarized under Appendix A (Subscription States).

In all cases, 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>).

8.3.1.3.  IQ

For an IQ stanza, the server itself MUST reply on behalf of the user with either an IQ result or an IQ error, and MUST NOT deliver the IQ stanza to any of the user's available resources. Specifically, if the semantics of the qualifying namespace define a reply that the server can provide on behalf of the user, then the server MUST reply to the stanza on behalf of the user by returning either an IQ stanza of type "result" or an IQ stanza of type "error" that is appropriate to the original payload; if not, then the server MUST reply with a <service-unavailable/> stanza error.

8.3.2.  No Available or Connected Resources

If there are no available or connected resources associated with the user, how the stanza is processed depends on the stanza type.

8.3.2.1.  Message

In order to properly handle message stanzas, it is strongly RECOMMENDED for an implementation to support OFFLINE STORAGE, i.e., the server SHOULD store the message stanza on behalf of the user and deliver it when the user next becomes available. For recommendations regarding offline message storage refer to [XEP‑0160] (Saint-Andre, P., “Best Practices for Handling Offline Messages,” January 2006.).

For a message stanza of type "chat" or "normal", the server SHOULD add the message to offline storage or forward the message to the user via a non-XMPP messaging system (e.g., to the user's email account). However, if offline message storage or message forwarding is not enabled or available (e.g., because a size limit has been reached on offline messages), then the server MUST return a <service-unavailable/> stanza error to the sender.

For a message stanza of type "headline", according to local service policies the server MUST either (1) add the message to offline storage or (2) silently discard the message (i.e., neither deliver it to the intended recipient nor return an error to the sender).

For a message stanza of type "groupchat", the server SHOULD NOT add the message to offline storage but instead SHOULD return an error to the sender.

For a message stanza of type "error", the server MUST NOT add the message to offline storage but instead SHOULD silently discard the message (i.e., neither deliver it to the intended recipient nor return an error to the sender).

8.3.2.2.  Presence

For a presence stanza with no type or of type "unavailable" or "probe", the server SHOULD silently ignore the stanza by not storing it for later delivery and not replying to it on behalf of the user.

For a presence stanza of type "subscribe", "subscribed", "unsubscribe", or "unsubscribed", the server MUST adhere to the rules defined under Section 3 (Managing Presence Subscriptions) and summarized under Appendix A (Subscription States).

8.3.2.3.  IQ

For an IQ stanza, 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 on behalf of the user, then the server MUST reply to the stanza on behalf of the user by returning either an IQ stanza of type "result" or an IQ stanza of type "error" that is appropriate to the original payload; if not, then the server MUST reply with a <service-unavailable/> stanza error.

8.4.  Remote Domain

If the hostname of the domain identifier portion of the address contained in the 'to' attribute of an outbound stanza does not match a configured hostname of the server itself, then the server MUST attempt to route the stanza to the remote domain. If there exists an active stream between the two peers, then the server MUST route the stanza over that stream for processing by the peer server. If not, then the server MUST do the following.

First, resolve the hostname of the remote domain (or use a cached resolution of the remote domain to an IP address). The RECOMMENDED order of attempted resolutions is as follows:

1. Attempt to resolve the remote hostname using a DNS service location record [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 a 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,” September 2009.).
2. If the "xmpp-server" address record resolution fails, attempt to resolve the "_im" or "_pres" SRV 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,” September 2009.).

If the server cannot resolve the remote domain, it MUST return a <remote-server-not-found/> stanza error.

Second, negotiate XML streams with the remote domain by following the process defined in [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.). If the server can resolve the remote domain but cannot establish streams with the XMPP service at that domain, it MUST return a <remote-server-timeout/> stanza error.

Third, route the stanza to the remote domain for processing by the peer server.

Note: 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.  Handling of URIs

The addresses of XMPP entities as used in communication over an XMPP network (e.g., in the 'from' and 'to' addresses of an XML stanza) MUST NOT be prepended with a Uniform Resource Identifier [URI] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) scheme. However, an application that is external to XMPP itself (e.g., a page on the World Wide Web) might need to identify an XMPP entity either as a URI or as an Internationalized Resource Identifier [IRI] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.), and an XMPP client might need to interact with such an external application (for example, an XMPP client might be invoked by clicking a link provided on a web page).

In the context of such interactions, an XMPP client SHOULD handle addresses that are encoded as "xmpp:" URIs and IRIs as specified in [XMPP‑URI] (Saint-Andre, P., “Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP),” February 2008.) and further described in [XEP‑0147] (Saint-Andre, P., “XMPP URI Scheme Query Components,” September 2006.). A client SHOULD also handle addresses that are encoded as "im:" URIs as specified in [CPIM] (Peterson, J., “Common Profile for Instant Messaging (CPIM),” August 2004.) and "pres:" URIs as specified in [CPP] (Peterson, J., “Common Profile for Presence (CPP),” August 2004.), although it MAY do so by removing the "im:" or "pres:" scheme and entrusting address resolution to the server as specified under Section 8.4 (Remote Domain).

10.  Internationalization Considerations

For internationalization considerations, refer to the relevant section of [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.).

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,” September 2009.).

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 if the sender is an entity that is not authorized to receive that information as determined by presence subscriptions (see Section 4 (Exchanging Presence Information)).
• 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).
• When a server processes an outbound presence stanza with no type or of type "unavailable", it MUST follow the rules defined under Section 4 (Exchanging Presence Information) in order to ensure that such presence information is not sent to entities that are not authorized to know such information.
• A client MAY ignore the <status/> element when contained in a presence stanza of type "subscribe", "unsubscribe", "subscribed", or "unsubscribed"; this can help prevent "presence subscription spam".

12.  IANA Considerations

The following sections update the registrations provided in [RFC3921] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.).

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,” September 2009.).

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:

XXXX

Description:

Instant messaging protocol label for the Extensible Messaging and Presence Protocol (XMPP) as defined by XXXX.

Registrant Contact:

IETF, XMPP Working Group, <xmppwg@xmpp.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:

XXXX

Description:

Presence protocol label for the Extensible Messaging and Presence Protocol (XMPP) as defined by XXXX.

Registrant Contact:

IETF, XMPP Working Group, <xmppwg@xmpp.org>

13.  Conformance Requirements

This section describes a protocol feature set that summarizes the conformance requirements of this specification. This feature set is appropriate for use in software certification, interoperability testing, and implementation reports. For each feature, this section provides the following information:

• A human-readable name
• An informational description
• A reference to the particular section of this document that normatively defines the feature
• Whether the feature applies to the Client role, the Server role, or both (where "N/A" signifies that the feature is not applicable to the specified role)
• Whether the feature MUST or SHOULD be implemented, where the capitalized terms are to be understood as described in [TERMS] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.)

Note: The feature set specified here attempts to adhere to the concepts and formats proposed by Larry Masinter within the IETF's NEWTRK Working Group in 2005, as captured in [INTEROP] (Masinter, L., “Formalizing IETF Interoperability Reporting,” October 2005.). Although this feature set is more detailed than called for by [REPORTS] (Dusseault, L. and R. Sparks, “Guidance on Interoperation and Implementation Reports for Advancement to Draft Standard,” September 2009.), it provides a suitable basis for the generation of implementation reports to be submitted in support of advancing this specification from Proposed Standard to Draft Standard in accordance with [PROCESS] (Bradner, S., “The Internet Standards Process -- Revision 3,” October 1996.).

Feature:

message-body

Description:

Support the <body/> child element of the <message/> stanza.

Section:

Section 5.2.3 (Body Element)

Roles:

Client MUST, Server N/A.

Feature:

message-subject

Description:

Support the <subject/> child element of the <message/> stanza.

Section:

Section 5.2.4 (Subject Element)

Roles:

Client SHOULD, Server N/A.

Feature:

message-thread

Description:

Support the <thread/> child element of the <message/> stanza.

Section:

Section 5.2.5 (Thread Element)

Roles:

Client SHOULD, Server N/A.

Feature:

message-type

Description:

Differentiate between messages of type "normal", "chat", "groupchat", "headline", and "error".

Section:

Section 5.2.2 (Type Attribute)

Roles:

Client MUST, Server N/A.

Feature:

presence-notype

Description:

Treat a presence stanza with no 'type' attribute as indicating availability.

Section:

Section 4.7.1 (Type Attribute)

Roles:

Client MUST, Server MUST.

Feature:

presence-probe

Description:

Send and receive presence stanzas with a 'type' attribute of "probe" for the discovery of presence information.

Section:

Section 4.7.1 (Type Attribute)

Roles:

Client N/A, Server MUST.

Feature:

presence-sub-approval

Description:

Treat an outbound presence stanza of type "subscribed" as the act of approving a presence subscription request previously received from another entity, and treat an inbound presence stanza of type "subscribed" as a subscription approval from another entity.

Section:

Section 3.1 (Requesting a Subscription)

Roles:

Client MUST, Server MUST.

Feature:

presence-sub-cancel

Description:

Treat an outbound presence stanza of type "unsubscribed" as the act of denying a subscription request received from another entity or cancelling a subscription approval previously granted to another entity, and treat an inbound presence stanza of type "unsubscribed" as an subscription denial or cancellation from another entity.

Section:

Section 3.2 (Cancelling a Subscription)

Roles:

Client MUST, Server MUST.

Feature:

presence-sub-request

Description:

Treat an outbound presence stanza of type "subscribe" as the act of requesting a subscription to the presence information of another entity, and treat an inbound presence stanza of type "subscribe" as a presence subscription request from another entity.

Section:

Section 3.1 (Requesting a Subscription)

Roles:

Client MUST, Server MUST.

Feature:

presence-sub-unsubscribe

Description:

Treat an outbound presence stanza of type "unsubscribe" as the act of unsubscribing from another entity, and treat an inbound presence stanza of type "unsubscribe" as an unsubscribe notification from another entity.

Section:

Section 3.3 (Unsubscribing)

Roles:

Client MUST, Server MUST.

Feature:

presence-unavailable

Description:

Treat a presence stanza with a 'type' attribute of "unavailable" as indicating lack of availability.

Section:

Section 4.7.1 (Type Attribute)

Roles:

Client MUST, Server MUST.

Feature:

roster-get

Description:

Treat an IQ stanza of type "get" containing an empty <query/> element qualified by the 'jabber:iq:roster' namespace as a request to retrieve the roster information associated with an account on a server.

Section:

Section 2.1.3 (Roster Get)

Roles:

Client MUST, Server MUST.

Feature:

roster-set

Description:

Treat an IQ stanza of type "set" containing a <query/> element qualified by the 'jabber:iq:roster' namespace as a request to add or update the item contained in the <query/> element.

Section:

Section 2.1.4 (Roster Set)

Roles:

Client MUST, Server MUST.

Feature:

roster-push

Description:

Send a roster push to each interested resource whenever the server-side representation of the roster information materially changes, or handle such a push when received from the server.

Section:

Section 2.1.5 (Roster Push)

Roles:

Client MUST, Server MUST.

Feature:

roster-version

Description:

Treat the 'ver' attribute of the <query/> element qualified by the 'jabber:iq:roster' namespace as an identifier of the particular version of roster information being sent or received.

Section:

Section 2.1.1 (Ver Attribute)

Roles:

Client MUST, Server MUST.

14.  References

14.1. Normative References

14.2. Informative References

Appendix A.  Subscription States

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

A.1.  Defined States

There are four primary subscription states (note: these states are described from the perspective of the user, not the contact):

• None -- the user does not have a subscription to the contact's presence, and the contact does not have a subscription to the user's presence
• To -- the user has a subscription to the contact's presence, but the contact does not have a subscription to the user's presence
• From -- the contact has a subscription to the user's presence, but the user does not have a subscription to the contact's presence
• Both -- both the user and the contact have subscriptions to each other's presence (i.e., the union of 'from' and 'to')

These states are supplemented by various pending sub-states to yield nine possible subscription states:

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 user's 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 user's 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; this state might or might not be reflected in the user's roster, as follows: if the user has created a roster item for the contact then the server MUST maintain that roster item and also note the existence of the inbound presence subscription request, whereas if the user has not created a roster item for the contact then the user's server MUST note the existence of the inbound presence subscription request but MUST NOT create a roster item for the contact (instead, the server MUST wait until the user has approved the subscription request before adding the contact to the user's roster).
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 user's roster by subscription='none' and ask='subscribe'
5. "To" = user is subscribed to contact (one-way); this is reflected in the user's 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 user's roster by subscription='to'
7. "From" = contact is subscribed to user (one-way); this is reflected in the user's 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 user's roster by subscription='from' and ask='subscribe'
9. "Both" = user and contact are subscribed to each other (two-way); this is reflected in the user's roster by subscription='both'

A.2.  Server Processing of Outbound Presence Subscription Stanzas

Outbound presence subscription stanzas enable the user to manage his or her subscription to the contact's presence (via the "subscribe" and "unsubscribe" types), and to manage the contact's access to the user's presence (via the "subscribed" and "unsubscribed" types).

The following rules apply to outbound routing of the stanza as well as changes to the user's roster.

Note: The rules for server processing of outbound presence subscription stanzas are described from the perspective of the user, not the contact. In addition, "S.N." stands for SHOULD NOT.

A.2.1.  Subscribe

Table 1: Processing of outbound "subscribe" stanzas

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

Note: A state change to "pending out" includes setting the 'ask' flag to a value of "subscribe" in the user's roster.

A.2.2.  Unsubscribe

Table 2: Processing of outbound "unsubscribe" stanzas

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

A.2.3.  Subscribed

Table 3: Processing of outbound "subscribed" stanzas

+-----------------------------------------------------------------+
|  EXISTING STATE          |  ROUTE?  |  NEW STATE                |
+-----------------------------------------------------------------+
|  "None"                  |  S.N.    |  no state change [1]      |
|  "None + Pending Out"    |  S.N.    |  no state change          |
|  "None + Pending In"     |  MUST    |  "From"                   |
|  "None + Pending Out+In" |  MUST    |  "From + Pending Out"     |
|  "To"                    |  S.N.    |  no state change          |
|  "To + Pending In"       |  MUST    |  "Both"                   |
|  "From"                  |  S.N.    |  no state change          |
|  "From + Pending Out"    |  S.N.    |  no state change          |
|  "Both"                  |  S.N.    |  no state change          |
+-----------------------------------------------------------------+

[1] A server MAY note the fact that the user wishes to allow the contact to be subscribed to the user's presence and automatically approve any subscription request received from the contact; if it does so, upon the receiving presence stanza of type "subscribed" from the user's client it MUST add a roster item for the contact to the user's roster and set the 'ask' flag to a value of "subscribed". However, the user's server still SHOULD NOT route the presence stanza of type "subscribed" to the contact. This optional functionality applies only if the contact is not already in the user's roster or if the contact is in the user's roster with a state of "None" (not including a state of "None + Pending Out").

A.2.4.  Unsubscribed

Table 4: Processing of outbound "unsubscribed" stanzas

+-----------------------------------------------------------------+
|  EXISTING STATE          |  ROUTE?  |  NEW STATE                |
+-----------------------------------------------------------------+
|  "None"                  |  S.N.    |  no state change          |
|  "None + Pending Out"    |  S.N.    |  no state change          |
|  "None + Pending In"     |  MUST    |  "None"                   |
|  "None + Pending Out+In" |  MUST    |  "None + Pending Out"     |
|  "To"                    |  S.N.    |  no state change          |
|  "To + Pending In"       |  MUST    |  "To"                     |
|  "From"                  |  MUST    |  "None"                   |
|  "From + Pending Out"    |  MUST    |  "None + Pending Out"     |
|  "Both"                  |  MUST    |  "To"                     |
+-----------------------------------------------------------------+

A.3.  Server Processing 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 user to manage the contact's access to the user's presence information (via the "subscribed" and "unsubscribed" types).

The following rules apply to delivery of the inbound stanza as well as changes to the user's roster.

Note: The rules for server processing of inbound presence subscription stanzas are described from the perspective of the user, not the contact. In addition, "S.N." stands for SHOULD NOT.

A.3.1.  Subscribe

Table 5: Processing of inbound "subscribe" stanzas

+------------------------------------------------------------------+
|  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
+------------------------------------------------------------------+
|  "None"                  |  MUST [1]  |  "None + Pending In"     |
|  "None + Pending Out"    |  MUST      |  "None + Pending Out+In" |
|  "None + Pending In"     |  S.N.      |  no state change         |
|  "None + Pending Out+In" |  S.N.      |  no state change         |
|  "To"                    |  MUST      |  "To + Pending In"       |
|  "To + Pending In"       |  S.N.      |  no state change         |
|  "From"                  |  S.N. [2]  |  no state change         |
|  "From + Pending Out"    |  S.N. [2]  |  no state change         |
|  "Both"                  |  S.N. [2]  |  no state change         |
+------------------------------------------------------------------+

[1] If the user previously sent presence of type "subscribed" as described under Appendix A.2.3 (Subscribed), then the server MAY auto-reply with "subscribed" and change the state to "From" rather than "None + Pending In".

[2] Server SHOULD auto-reply with "subscribed".

A.3.2.  Unsubscribe

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 MUST change the state and SHOULD auto-reply by sending a presence stanza of type "unsubscribed" to the contact on behalf of the user. Otherwise the user's server MUST NOT change the state and SHOULD NOT deliver the stanza. These rules are summarized in the following table.

Table 6: Processing of inbound "unsubscribe" stanzas

+------------------------------------------------------------------+
|  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
+------------------------------------------------------------------+
|  "None"                  |  S.N.      |  no state change         |
|  "None + Pending Out"    |  S.N.      |  no state change         |
|  "None + Pending In"     |  S.N. [1]  |  "None"                  |
|  "None + Pending Out+In" |  S.N. [1]  |  "None + Pending Out"    |
|  "To"                    |  S.N.      |  no state change         |
|  "To + Pending In"       |  S.N. [1]  |  "To"                    |
|  "From"                  |  S.N. [1]  |  "None"                  |
|  "From + Pending Out"    |  S.N. [1]  |  "None + Pending Out     |
|  "Both"                  |  S.N. [1]  |  "To"                    |
+------------------------------------------------------------------+

[1] Server SHOULD auto-reply with "unsubscribed".

A.3.3.  Subscribed

When the user's server receives a presence stanza of type "subscribed" for the user from the contact, if there is no pending outbound request for access to the contact's presence information, then it MUST NOT change the subscription state and SHOULD NOT deliver the stanza to the user. 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, then the user's server MUST change the subscription state but SHOULD NOT deliver the stanza to the user. 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 MUST NOT change the subscription state and SHOULD NOT deliver the stanza to the user. These rules are summarized in the following table.

Table 7: Processing of inbound "subscribed" stanzas

+------------------------------------------------------------------+
|  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
+------------------------------------------------------------------+
|  "None"                  |  S.N.      |  no state change         |
|  "None + Pending Out"    |  S.N.      |  "To"                    |
|  "None + Pending In"     |  S.N.      |  no state change         |
|  "None + Pending Out+In" |  S.N.      |  "To + Pending In"       |
|  "To"                    |  S.N.      |  no state change         |
|  "To + Pending In"       |  S.N.      |  no state change         |
|  "From"                  |  S.N.      |  no state change         |
|  "From + Pending Out"    |  S.N.      |  "Both"                  |
|  "Both"                  |  S.N.      |  no state change         |
+------------------------------------------------------------------+

A.3.4.  Unsubscribed

When the user's server receives a presence stanza of type "unsubscribed" for the user from the contact, 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, then the user's server MUST change the subscription state but SHOULD NOT deliver the stanza to the user. Otherwise, the user's server MUST NOT change the subscription state and SHOULD NOT deliver the stanza. These rules are summarized in the following table.

Table 8: Processing of inbound "unsubscribed" stanzas

+------------------------------------------------------------------+
|  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
+------------------------------------------------------------------+
|  "None"                  |  S.N.      |  no state change         |
|  "None + Pending Out"    |  S.N.      |  "None"                  |
|  "None + Pending In"     |  S.N.      |  no state change         |
|  "None + Pending Out+In" |  S.N.      |  "None + Pending In"     |
|  "To"                    |  S.N.      |  "None"                  |
|  "To + Pending In"       |  S.N.      |  "None + Pending In"     |
|  "From"                  |  S.N.      |  no state change         |
|  "From + Pending Out"    |  S.N.      |  "From"                  |
|  "Both"                  |  S.N.      |  "From"                  |
+------------------------------------------------------------------+

Appendix B.  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.) require that a compliant instant messaging and presence technology must enable a user to block communications from selected users. Protocols for doing so are specified in [XEP‑0016] (Millard, P. and P. Saint-Andre, “Privacy Lists,” February 2007.) and [XEP‑0191] (Saint-Andre, P., “Simple Communications Blocking,” February 2007.).

Appendix C.  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 this specification (documentation of this protocol is contained in [XEP‑0054] (Saint-Andre, P., “vcard-temp,” July 2008.)).

Appendix D.  XML Schemas

Because validation of XML streams and stanzas is optional, the following XML schemas are provided for descriptive purposes only. These schemas are not normative.

The following schemas formally define various XML namespaces used in the core XMPP protocols, in conformance with [XML‑SCHEMA] (Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech, “XML Schema Part 1: Structures Second Edition,” October 2004.). For schemas defining namespaces for XML streams and other core aspects of XMPP, refer to [xmpp‑core] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” September 2009.).

D.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'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:NMTOKEN'>
          <xs:attribute name='parent'
                        type='xs:NMTOKEN'
                        use='optional'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <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='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>

D.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'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:NMTOKEN'>
          <xs:attribute name='parent'
                        type='xs:NMTOKEN'
                        use='optional'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <xs:element name='subject'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:NMTOKEN'>
          <xs:attribute name='parent'
                        type='xs:NMTOKEN'
                        use='optional'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <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='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>

D.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:attribute name='ver'
                    type='xs:string'
                    use='optional'/>
    </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:enumeration value='subscribed'/>
          </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'
                    default='none'>
        <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 E.  Differences From RFC 3921

Based on consensus derived from implementation and deployment experience as well as formal interoperability testing, the following substantive modifications were made from RFC 3921.

• 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, for the sake of backward-compatibility server implementations are encouraged to advertise support for the feature, even though session establishment is a "no-op".
• In order to more seamlessly repair lack of synchronization in subscription states between rosters located at different servers, error handling related to presence probes and presence notifications was modified to return presence stanzas of type "unsubscribe" or "unsubscribed" rather than error stanzas.
• Added optional versioning of roster information to save bandwidth in cases where the roster has not changed (or has changed very little) between sessions; the relevant protocol interactions were originally described in [XEP‑0237] (Saint-Andre, P. and D. Cridland, “Roster Versioning,” May 2009.).
• Added optional server support for pre-approved presence subscriptions via presence stanzas of type "subscribed" and the optional "subscribed" value for the 'ask' flag.
• Added optional 'parent' attribute to <thread/> element
• Moved the protocol for communications blocking (specified in Section 10 of RFC 3921) back to [XEP‑0016] (Millard, P. and P. Saint-Andre, “Privacy Lists,” February 2007.), from which it was originally taken.

In addition, numerous changes of an editorial nature were made in order to more fully specify and clearly explain the protocols.

Appendix F.  Copying Conditions

Regarding this entire document or any portion of it, the author makes no guarantees and is not responsible for any damage resulting from its use. The author grants irrevocable permission to anyone to use, modify, and distribute it in any way that does not diminish the rights of anyone else to use, modify, and distribute it, provided that redistributed derivative works do not contain misleading author or version information. Derivative works need not be licensed under similar terms.

Index

Author's Address