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XMPP Working Group                                  P. Saint-Andre (ed.)
Internet-Draft                                Jabber Software Foundation
Expires: July 6, 2004                                    January 6, 2004


        Extensible Messaging and Presence Protocol (XMPP): Core
                        draft-ietf-xmpp-core-21

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   This Internet-Draft will expire on July 6, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

   This memo defines the core features of the Extensible Messaging and
   Presence Protocol (XMPP), a protocol for streaming Extensible Markup
   Language (XML) elements in order to exchange structured information
   in close to real time between any two network endpoints.  While XMPP
   provides a generalized, extensible framework for exchanging XML data,
   it is used mainly for the purpose of building instant messaging and
   presence applications that meet the requirements of RFC 2779.









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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Generalized Architecture . . . . . . . . . . . . . . . . . . .  4
   3.  Addressing Scheme  . . . . . . . . . . . . . . . . . . . . . .  6
   4.  XML Streams  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Use of TLS . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   6.  Use of SASL  . . . . . . . . . . . . . . . . . . . . . . . . . 26
   7.  Resource Binding . . . . . . . . . . . . . . . . . . . . . . . 36
   8.  Server Dialback  . . . . . . . . . . . . . . . . . . . . . . . 39
   9.  XML Stanzas  . . . . . . . . . . . . . . . . . . . . . . . . . 46
   10. Server Rules for Handling XML Stanzas  . . . . . . . . . . . . 55
   11. XML Usage within XMPP  . . . . . . . . . . . . . . . . . . . . 57
   12. Core Compliance Requirements . . . . . . . . . . . . . . . . . 60
   13. Internationalization Considerations  . . . . . . . . . . . . . 61
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 61
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 66
       Normative References . . . . . . . . . . . . . . . . . . . . . 69
       Informative References . . . . . . . . . . . . . . . . . . . . 71
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 72
   A.  Nodeprep . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
   B.  Resourceprep . . . . . . . . . . . . . . . . . . . . . . . . . 74
   C.  XML Schemas  . . . . . . . . . . . . . . . . . . . . . . . . . 76
   D.  Differences Between Core Jabber Protocol and XMPP  . . . . . . 83
   E.  Revision History . . . . . . . . . . . . . . . . . . . . . . . 85
       Intellectual Property and Copyright Statements . . . . . . . . 94

























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1. Introduction

1.1 Overview

   The Extensible Messaging and Presence Protocol (XMPP) is an open XML
   [XML] protocol for near-real-time messaging, presence, and
   request-response services.  The basic syntax and semantics were
   developed originally within the Jabber open-source community, mainly
   in 1999.  In 2002, the XMPP WG was chartered with developing an
   adaptation of the Jabber protocol that would be suitable as an IETF
   instant messaging (IM) and presence technology.  As a result of work
   by the XMPP WG, the current memo defines the core features of XMPP;
   Extensible Messaging and Presence Protocol (XMPP): Instant Messaging
   and Presence [XMPP-IM] defines the extensions required to provide the
   instant messaging and presence functionality defined in RFC 2779
   [IMP-REQS].

1.2 Terminology

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

1.3 Contributors

   Most of the core aspects of the Extensible Messaging and Presence
   Protocol were developed originally within the Jabber open-source
   community in 1999.  This community was founded by Jeremie Miller, who
   released source code for the initial version of the jabberd server in
   January 1999.  Major early contributors to the base protocol also
   included Ryan Eatmon, Peter Millard, Thomas Muldowney, and Dave
   Smith.  Work by the XMPP Working Group has concentrated especially on
   security and internationalization; in these areas, protocols for the
   use of TLS and SASL were originally contributed by Rob Norris, and
   stringprep profiles were originally contributed by Joe Hildebrand.
   The error code syntax was suggested by Lisa Dusseault.

1.4 Acknowledgements

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



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   members of the XMPP Working Group and the IETF community for comments
   and feedback provided throughout the life of this memo.

2. Generalized Architecture

2.1 Overview

   Although XMPP is not wedded to any specific network architecture, to
   date it usually has been implemented via a typical client-server
   architecture, wherein a client utilizing XMPP accesses a server over
   a TCP [TCP] socket.

   The following diagram provides a high-level overview of this
   architecture (where "-" represents communications that use XMPP and
   "=" represents communications that use any other protocol).

   C1 -  S1 - S2 - C3
        /  \
   C2 -     G1 = FN1 = FC1

   The symbols are as follows:

   o  C1, C2, C3 -- XMPP clients

   o  S1, S2 -- XMPP servers

   o  G1 -- A gateway that translates between XMPP and the protocol(s)
      used on a foreign (non-XMPP) messaging network

   o  FN1 -- A foreign messaging network

   o  FC1 -- A client on a foreign messaging network


2.2 Server

   A server acts as an intelligent abstraction layer for XMPP
   communications.  Its primary responsibilities are:

   o  to manage connections from or sessions for other entities, in the
      form of XML streams (Section 4) to and from authorized clients,
      servers, and other entities

   o  to route appropriately-addressed XML stanzas (Section 9) among
      such entities over XML streams

   Most XMPP-compliant servers also assume responsibility for the
   storage of data that is used by clients (e.g., contact lists for



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   users of XMPP-based instant messaging and presence applications); in
   this case, the XML data is processed directly by the server itself on
   behalf of the client and is not routed to another entity.

2.3 Client

   Most clients connect directly to a server over a [TCP] socket and use
   XMPP to take full advantage of the functionality provided by a server
   and any associated services.  Although there is no necessary coupling
   of an XML stream to a TCP socket (e.g., a client could connect via
   HTTP [HTTP] polling or some other mechanism), this specification
   defines a binding of XMPP to TCP only.  Multiple resources (e.g.,
   devices or locations) MAY connect simultaneously to a server on
   behalf of each authorized client, with each resource differentiated
   by the resource identifier of an XMPP address (e.g., <node@domain/
   home> vs. <node@domain/work>) as defined under Addressing Scheme
   (Section 3).  The RECOMMENDED port for connections between a client
   and a server is 5222, as registered with the IANA (see Port Numbers
   (Section 15.9)).

2.4 Gateway

   A gateway is a special-purpose server-side service whose primary
   function is to translate XMPP into the protocol used by a foreign
   (non-XMPP) messaging system, as well as to translate the return data
   back into XMPP.  Examples are gateways to Internet Relay Chat (IRC),
   Short Message Service (SMS), SIMPLE, SMTP, and legacy instant
   messaging networks such as AIM, ICQ, MSN Messenger, and Yahoo!
   Instant Messenger.  Communications between gateways and servers, and
   between gateways and the foreign messaging system, are not defined in
   this document.

2.5 Network

   Because each server is identified by a network address and because
   server-to-server communications are a straightforward extension of
   the client-to-server protocol, in practice the system consists of a
   network of servers that inter-communicate.  Thus user-a@domain1 is
   able to exchange messages, presence, and other information with
   user-b@domain2.  This pattern is familiar from messaging protocols
   (such as SMTP) that make use of network addressing standards.
   Communications between any two servers are OPTIONAL.  If enabled,
   such communications SHOULD occur over XML streams that are bound to
   [TCP] sockets.  The RECOMMENDED port for connections between servers
   is 5269, as registered with the IANA (see Port Numbers (Section
   15.9)).





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3. Addressing Scheme

3.1 Overview

   An entity is anything that can be considered a network endpoint
   (i.e., an ID on the network) and that can communicate using XMPP.
   All such entities are uniquely addressable in a form that is
   consistent with RFC 2396 [URI].  For historical reasons, the address
   of an XMPP entity is called a Jabber Identifier or JID.  A valid JID
   contains a set of ordered elements formed of a domain identifier,
   node identifier, and resource identifier.

   The syntax for a JID is defined below using Augmented Backus-Naur
   Form as defined in [ABNF].  The IPv4address and IPv6address rules are
   defined in Appendix B of [IPv6]; the allowable character sequences
   that conform to the node rule are defined by the Nodeprep (Appendix
   A) profile of [STRINGPREP] as documented in this memo; the allowable
   character sequences that conform to the resource rule are defined by
   the Resourceprep (Appendix B) profile of [STRINGPREP] as documented
   in this memo; and the sub-domain rule makes reference to the concept
   of a domain label as described in [IDNA].

      jid             = [ node "@" ] domain [ "/" resource ]
      domain          = fqdn / address-literal
      fqdn            = (sub-domain 1*("." sub-domain))
      sub-domain      = ([IDNA] conformant domain label)
      address-literal = IPv4address / IPv6address

   All JIDs are based on the foregoing structure.  The most common use
   of this structure is to identify an instant messaging user, the
   server to which the user connects, and the user's connected resource
   (e.g., a specific client) in the form of <user@host/resource>.
   However, node types other than clients are possible; for example, a
   specific chat room offered by a multi-user chat service could be
   addressed as <room@service> (where "room" is the name of the chat
   room and "service" is the hostname of the multi-user chat service)
   and a specific occupant of such a room could be addressed as
   <room@service/nick> (where "nick" is the occupant's room nickname).
   Many other JID types are possible (e.g., <domain/resource> could be a
   server-side script or service).

   Each allowable portion of a JID (node identifier, domain identifier,
   and resource identifier) MUST NOT be more than 1023 bytes in length,
   resulting in a maximum total size (including the '@' and '/'
   separators) of 3071 bytes.

3.2 Domain Identifier




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   The domain identifier is the primary identifier and is the only
   REQUIRED element of a JID (a mere domain identifier is a valid JID).
   It usually represents the network gateway or "primary" server to
   which other entities connect for XML routing and data management
   capabilities.  However, the entity referenced by a domain identifier
   is not always a server, and may be a service that is addressed as a
   subdomain of a server and that provides functionality above and
   beyond the capabilities of a server (e.g., a multi-user chat service,
   a user directory, or a gateway to a foreign messaging system).

   The domain identifier for every server or service that will
   communicate over a network MAY be an IP address but SHOULD be a fully
   qualified domain name (see [DNS]).  A domain identifier MUST be an
   "internationalized domain name" as defined in [IDNA], to which the
   Nameprep [NAMEPREP] profile of stringprep [STRINGPREP] can be applied
   without failing.  Before comparing two domain identifiers, a server
   MUST (and a client SHOULD) first apply the Nameprep profile to the
   labels (as defined in [IDNA]) that make up each identifier.

3.3 Node Identifier

   The node identifier is an optional secondary identifier placed before
   the domain identifier and separated from the latter by the '@'
   character.  It usually represents the entity requesting and using
   network access provided by the server or gateway (i.e., a client),
   although it can also represent other kinds of entities (e.g., a chat
   room associated with a multi-user chat service).  The entity
   represented by a node identifier is addressed within the context of a
   specific domain; within instant messaging and presence applications
   of XMPP this address is called a "bare JID" and is of the form
   <node@domain>.

   A node identifier MUST be formatted such that the Nodeprep (Appendix
   A) profile of [STRINGPREP] can be applied to it without failing.
   Before comparing two node identifiers, a server MUST (and a client
   SHOULD) first apply the Nodeprep profile to each identifier.

3.4 Resource Identifier

   The resource identifier is an optional tertiary identifier placed
   after the domain identifier and separated from the latter by the '/'
   character.  A resource identifier may modify either a <node@domain>
   or mere <domain> address.  It usually represents a specific session,
   connection (e.g., a device or location), or object (e.g., a
   participant in a multi-user chat room) belonging to the entity
   associated with a node identifier.  A resource identifier is opaque
   to both servers and other clients, and is typically defined by a
   client implementation when it provides the information necessary to



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   complete Resource Binding (Section 7) (although it may be generated
   by a server on behalf of a client), after which it is referred to as
   a "connected resource".  An entity MAY maintain multiple connected
   resources simultaneously, with each connected resource differentiated
   by a distinct resource identifier.

   A resource identifier MUST be formatted such that the Resourceprep
   (Appendix B) profile of [STRINGPREP] can be applied to it without
   failing.  Before comparing two resource identifiers, a server MUST
   (and a client SHOULD) first apply the Resourceprep profile to each
   identifier.

3.5 Determination of Addresses

   After SASL negotiation (Section 6) and, if appropriate, Resource
   Binding (Section 7), the receiving entity for a stream MUST determine
   the initiating entity's JID.

   For server-to-server communications, the initiating entity's JID
   SHOULD be the authorization identity, derived from the authentication
   identity as defined by the Simple Authentication and Security Layer
   (SASL) specification [SASL] if no authorization identity was
   specified during SASL negotiation (Section 6).

   For client-to-server communications, the "bare JID" (<node@domain>)
   SHOULD be the authorization identity, derived from the authentication
   identity as defined in [SASL] if no authorization identity was
   specified during SASL negotiation (Section 6); the resource
   identifier portion of the "full JID" (<node@domain/resource>) SHOULD
   be the resource identifier negotiated by the client and server during
   Resource Binding (Section 7).

   The receiving entity MUST ensure that the resulting JID (including
   node identifier, domain identifier, resource identifier, and
   separator characters) conforms to the rules and formats defined
   earlier in this section; to meet this restriction, the receiving
   entity may need to replace the JID sent by the initiating entity with
   the canonicalized JID as determined by the receiving entity.

4. XML Streams

4.1 Overview

   Two fundamental concepts make possible the rapid, asynchronous
   exchange of relatively small payloads of structured information
   between presence-aware entities: XML streams and XML stanzas.  These
   terms are defined as follows:




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   Definition of XML Stream: An XML stream is a container for the
      exchange of XML elements between any two entities over a network.
      An XML stream is negotiated from an initiating entity (usually a
      client or server) to a receiving entity (usually a server),
      normally over a [TCP] socket, and corresponds to the initiating
      entity's "session" with the receiving entity.  The start of the
      XML stream is denoted unambiguously by an opening XML <stream> tag
      (with appropriate attributes and namespace declarations), while
      the end of the XML stream is denoted unambiguously by a closing
      XML </stream> tag.  An XML stream is unidirectional; in order to
      enable bidirectional information exchange, the initiating entity
      and receiving entity MUST negotiate one stream in each direction
      (the "initial stream" and the "response stream"), normally over
      the same TCP connection.

   Definition of XML Stanza: An XML stanza is a discrete semantic unit
      of structured information that is sent from one entity to another
      over an XML stream.  An XML stanza exists at the direct child
      level of the root <stream/> element and is said to be
      well-balanced if it matches production [43] content of [XML].  The
      start of any XML stanza is denoted unambiguously by the element
      start tag at depth=1 of the XML stream (e.g., <presence>), and the
      end of any XML stanza is denoted unambiguously by the
      corresponding close tag at depth=1 (e.g., </presence>).  An XML
      stanza MAY contain child elements (with accompanying attributes,
      elements, and CDATA) as necessary in order to convey the desired
      information.  The only defined XML stanzas are <message/>,
      <presence/>, and <iq/> as defined under XML Stanzas (Section 9);
      an XML element sent for the purpose of Transport Layer Security
      (TLS) negotiation (Section 5), Simple Authentication and Security
      Layer (SASL) negotiation (Section 6), or server dialback (Section
      8) is not considered to be an XML stanza.

   Consider the example of a client's session with a server.  In order
   to connect to a server, a client MUST initiate an XML stream by
   sending an opening <stream> tag to the server, optionally preceded by
   a text declaration specifying the XML version and the character
   encoding supported (see Inclusion of Text Declaration (Section 11.4);
   see also Character Encoding (Section 11.5)).  Subject to local
   policies and service provisioning, the server SHOULD then reply with
   a second XML stream back to the client, again optionally preceded by
   a text declaration.  Once the client has completed SASL negotiation
   (Section 6), the client MAY send an unbounded number of XML stanzas
   over the stream to any recipient on the network.  When the client
   desires to close the stream, it simply sends a closing </stream> tag
   to the server (alternatively, the stream may be closed by the
   server), after which both the client and server SHOULD close the
   underlying TCP connection as well.



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   Those who are accustomed to thinking of XML in a document-centric
   manner may wish to view a client's session with a server as
   consisting of two open-ended XML documents: one from the client to
   the server and one from the server to the client.  From this
   perspective, the root <stream/> element can be considered the
   document entity for each "document", and the two "documents" are
   built up through the accumulation of XML stanzas sent over the two
   XML streams.  However, this perspective is a convenience only, and
   XMPP does not deal in documents but in XML streams and XML stanzas.

   In essence, then, an XML stream acts as an envelope for all the XML
   stanzas sent during a session.  We can represent this in a simplistic
   fashion as follows:

   |--------------------|
   | <stream>           |
   |--------------------|
   | <presence>         |
   |   <show/>          |
   | </presence>        |
   |--------------------|
   | <message to='foo'> |
   |   <body/>          |
   | </message>         |
   |--------------------|
   | <iq to='bar'>      |
   |   <query/>         |
   | </iq>              |
   |--------------------|
   | ...                |
   |--------------------|
   | </stream>          |
   |--------------------|


4.2 Stream Attributes

   The attributes of the stream element are as follows:

   o  to -- The 'to' attribute SHOULD be used only in the XML stream
      header from the initiating entity to the receiving entity, and
      MUST be set to a hostname serviced by the receiving entity.  There
      SHOULD be no 'to' attribute set in the XML stream header by which
      the receiving entity replies to the initiating entity; however, if
      a 'to' attribute is included, it SHOULD be silently ignored by the
      initiating entity.

   o  from -- The 'from' attribute SHOULD be used only in the XML stream



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      header from the receiving entity to the initiating entity, and
      MUST be set to a hostname serviced by the receiving entity that is
      granting access to the initiating entity.  There SHOULD be no
      'from' attribute on the XML stream header sent from the initiating
      entity to the receiving entity; however, if a 'from' attribute is
      included, it SHOULD be silently ignored by the receiving entity.

   o  id -- The 'id' attribute SHOULD be used only in the XML stream
      header from the receiving entity to the initiating entity.  This
      attribute is a unique identifier created by the receiving entity
      to function as a session key for the initiating entity's streams
      with the receiving entity, and MUST be unique within the receiving
      application (normally a server).  Note well that the stream ID may
      be security-critical and therefore MUST be both unpredictable and
      non-repeating.  There SHOULD be no 'id' attribute on the XML
      stream header sent from the initiating entity to the receiving
      entity; however, if an 'id' attribute is included, it SHOULD be
      silently ignored by the receiving entity.

   o  xml:lang -- An 'xml:lang' attribute (as defined in Section 2.12 of
      [XML]) SHOULD be included by the initiating entity on the header
      for the initial stream to specify the default language of any
      human-readable XML character data it sends over that stream.  If
      the attribute is included, the receiving entity SHOULD remember
      that value as the default for both the initial stream and the
      response stream; if the attribute is not included, the receiving
      entity SHOULD use a configurable default value for both streams,
      which it MUST communicate in the header for the response stream.
      For all stanzas sent over the initial stream, if the initiating
      entity does not include an 'xml:lang' attribute, the receiving
      entity SHOULD apply the default value; if the initiating entity
      does include an 'xml:lang' attribute, the receiving entity MUST
      NOT modify or delete it (see also xml:lang (Section 9.1.5)).  The
      value of the 'xml:lang' attribute MUST be an NMTOKEN (as defined
      in Section 2.3 of [XML]) and MUST conform to the format defined in
      RFC 3066 [LANGTAGS].

   o  version -- The presence of the version attribute set to a value of
      "1.0" signals support for the stream-related protocols (including
      stream features) defined in this specification.  Detailed rules
      regarding generation and handling of this attribute are defined
      below.

   We can summarize as follows:

            |  initiating to receiving  |  receiving to initiating
   ---------+---------------------------+-----------------------
   to       |  hostname of receiver     |  silently ignored



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   from     |  silently ignored         |  hostname of receiver
   id       |  silently ignored         |  session key
   xml:lang |  default language         |  default language
   version  |  signals XMPP 1.0 support |  signals XMPP 1.0 support


4.2.1 Version Support

   The version of XMPP specified herein is "1.0"; in particular, this
   encapsulates the stream-related protocols (Use of TLS (Section 5),
   Use of SASL (Section 6), and Stream Errors (Section 4.6)) as well as
   the semantics of the three defined XML stanza types (<message/>,
   <presence/>, and <iq/>).  The major version number should be
   incremented only if the stream and stanza formats or required actions
   have changed so dramatically that an older version entity would not
   be able to interoperate with a newer version entity if it simply
   ignored the elements and attributes it did not understand and took
   the actions specified in the older specification.  The minor version
   number indicates new capabilities, and MUST be ignored by an entity
   with a smaller minor version number, but used for informational
   purposes by the node with the larger minor version number.  For
   example, a minor version number might indicate the ability to process
   a newly defined value of the 'type' attribute for message, presence,
   or IQ stanzas; the node with the larger minor version number would
   simply note that its correspondent would not be able to understand
   that value of the 'type' attribute and therefore would not send it.

   The following rules apply to the generation and handling by
   implementations of the 'version' attribute within stream headers:

   1.  The initiating entity MUST set the value of the 'version'
       attribute on the initial stream header to the highest version
       number it supports (e.g., if the highest version number it
       supports is that defined in this specification, it MUST set the
       value to "1.0").

   2.  The receiving entity MUST set the value of the 'version'
       attribute on the response stream header to either the value
       supplied by the initiating entity or highest version number
       supported by the receiving entity, whichever is lower. The
       receiving entity MUST perform a numeric comparison, not a string
       match.

   3.  If the version number included in the response stream header is
       at least one major version lower than the version number included
       in the initial stream header and newer version entities cannot
       interoperate with older version entities as described above, the
       initiating entity SHOULD generate an <unsupported-version/>



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       stream error and terminate the XML stream and underlying TCP
       connection.

   4.  If either entity receives a stream header with no 'version'
       attribute, the entity MUST consider the version supported by the
       other entity to be "0.0" and SHOULD NOT include a 'version'
       attribute in the stream header it sends in reply.


4.3 Namespace Declarations

   The stream element MUST possess both a streams namespace declaration
   and a default namespace declaration (as "namespace declaration" is
   defined in the XML namespaces specification [XML-NAMES]).  For
   detailed information regarding the streams namespace and default
   namespace, see Namespace Names and Prefixes (Section 11.2).

4.4 Stream Features

   If the initiating entity includes the 'version' attribute set to a
   value of "1.0" in the initial stream header, the receiving entity
   MUST send a <features/> child element (prefixed by the streams
   namespace prefix) to the initiating entity in order to announce any
   stream-level features that can be negotiated (or capabilities that
   otherwise need to be advertised).  Currently this is used only to
   advertise Use of TLS (Section 5), Use of SASL (Section 6), and
   Resource Binding (Section 7) as defined herein, and for Session
   Establishment as defined in [XMPP-IM]; however, the stream features
   functionality could be used to advertise other negotiable features in
   the future.  If an entity does not understand or support some
   features, it SHOULD silently ignore them.

4.5 Stream Security

   When negotiating XML streams in XMPP 1.0, TLS SHOULD be used as
   defined under Use of TLS (Section 5) and SASL MUST be used as defined
   under Use of SASL (Section 6).  If the initiating entity attempts to
   send an XML Stanza (Section 9) before the stream has been
   authenticated, the receiving entity SHOULD return a <not-authorized/>
   stream error to the initiating entity and then terminate both the XML
   stream and the underlying TCP connection.

4.6 Stream Errors

   The root stream element MAY contain an <error/> child element that is
   prefixed by the streams namespace prefix.  The error child MUST be
   sent by a compliant entity (usually a server rather than a client) if
   it perceives that a stream-level error has occurred.



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4.6.1 Rules

   The following rules apply to stream-level errors:

   o  It is assumed that all stream-level errors are unrecoverable;
      therefore, if an error occurs at the level of the stream, the
      entity that detects the error MUST send a stream error to the
      other entity, send a closing </stream> tag, and terminate the
      underlying TCP connection.

   o  If the error occurs while the stream is being set up, the
      receiving entity MUST still send the opening <stream> tag, include
      the <error/> element as a child of the stream element, send the
      closing </stream> tag, and terminate the underlying TCP
      connection.  In this case, if the initiating entity provides an
      unknown host in the 'to' attribute (or provides no 'to' attribute
      at all), the server SHOULD provide the server's authoritative
      hostname in the 'from' attribute of the stream header sent before
      termination.


4.6.2 Syntax

   The syntax for stream errors is as follows:

   <stream:error>
     <defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
     <text xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
       OPTIONAL descriptive text
     </text>
     [OPTIONAL application-specific condition element]
   </stream:error>

   The <error/> element:

   o  MUST contain a child element corresponding to one of the defined
      stanza error conditions defined below; this element MUST be
      qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace

   o  MAY contain a <text/> child containing CDATA that describes the
      error in more detail; this element MUST be qualified by the
      'urn:ietf:params:xml:ns:xmpp-streams' namespace and SHOULD possess
      an 'xml:lang' attribute

   o  MAY contain a child element for an application-specific error
      condition; this element MUST be qualified by an
      application-defined namespace, and its structure is defined by
      that namespace



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   The <text/> element is OPTIONAL.  If included, it SHOULD be used only
   to provide descriptive or diagnostic information that supplements the
   meaning of a defined condition or application-specific condition.  It
   SHOULD NOT be interpreted programmatically by an application.  It
   SHOULD NOT be used as the error message presented to a user, but MAY
   be shown in addition to the error message associated with the
   included condition element (or elements).

4.6.3 Defined Conditions

   The following stream-level error conditions are defined:

   o  <bad-format/> -- the entity has sent XML that cannot be processed;
      this error MAY be used rather than more specific XML-related
      errors such as <bad-namespace-prefix/>, <invalid-xml/>,
      <restricted-xml/>, <unsupported-encoding/>, and
      <xml-not-well-formed/>, although the more specific errors are
      preferred.

   o  <bad-namespace-prefix/> -- the entity has sent a namespace prefix
      that is unsupported, or has sent no namespace prefix on an element
      that requires such a prefix (see XML Namespace Names and Prefixes
      (Section 11.2)).

   o  <conflict/> -- the server is closing the active stream for this
      entity because a new stream has been initiated that conflicts with
      the existing stream.

   o  <connection-timeout/> -- the entity has not generated any traffic
      over the stream for some period of time (configurable according to
      a local service policy).

   o  <host-gone/> -- the value of the 'to' attribute provided by the
      initiating entity in the stream header corresponds to a hostname
      that is no longer hosted by the server.

   o  <host-unknown/> -- the value of the 'to' attribute provided by the
      initiating entity in the stream header does not correspond to a
      hostname that is hosted by the server.

   o  <improper-addressing/> -- a stanza sent between two servers lacks
      a 'to' or 'from' attribute (or the attribute has no value).

   o  <internal-server-error/> -- the server has experienced a
      misconfiguration or an otherwise-undefined internal error that
      prevents it from servicing the stream.

   o  <invalid-from/> -- the JID or hostname provided in a 'from'



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      address does not match an authorized JID or validated domain
      negotiated between servers via SASL or dialback, or between a
      client and a server via authentication and resource binding.

   o  <invalid-id/> -- the stream ID or dialback ID is invalid or does
      not match an ID previously provided.

   o  <invalid-namespace/> -- the streams namespace name is something
      other than "http://etherx.jabber.org/streams" or the dialback
      namespace name is something other than "jabber:server:dialback"
      (see XML Namespace Names and Prefixes (Section 11.2)).

   o  <invalid-xml/> -- the entity has sent invalid XML over the stream
      to a server that performs validation (see Validation (Section
      11.3)).

   o  <not-authorized/> -- the entity has attempted to send data before
      the stream has been authenticated, or otherwise is not authorized
      to perform an action related to stream negotiation; the receiving
      entity MUST NOT process the offending stanza before sending the
      stream error.

   o  <policy-violation/> -- the entity has violated some local service
      policy; the server MAY choose to specify the policy in the <text/>
      element.

   o  <remote-connection-failed/> -- the server is unable to properly
      connect to a remote entity that is required for authentication or
      authorization.

   o  <resource-constraint/> -- the server lacks the system resources
      necessary to service the stream.

   o  <restricted-xml/> -- the entity has attempted to send restricted
      XML features such as a comment, processing instruction, DTD,
      entity reference, or unescaped character (see Restrictions
      (Section 11.1)).

   o  <see-other-host/> -- the server will not provide service to the
      initiating entity but is redirecting traffic to another host; the
      server SHOULD specify the alternate hostname or IP address (which
      MUST be a valid domain identifier) in the CDATA of the
      <see-other-host/> element.

   o  <system-shutdown/> -- the server is being shut down and all active
      streams are being closed.

   o  <undefined-condition/> -- the error condition is not one of those



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      defined by the other conditions in this list; this error condition
      SHOULD be used only in conjunction with an application-specific
      condition.

   o  <unsupported-encoding/> -- the initiating entity has encoded the
      stream in an encoding that is not supported by the server (see
      Character Encoding (Section 11.5)).

   o  <unsupported-stanza-type/> -- the initiating entity has sent a
      first-level child of the stream that is not supported by the
      server.

   o  <unsupported-version/> -- the value of the 'version' attribute
      provided by the initiating entity in the stream header specifies a
      version of XMPP that is not supported by the server; the server
      MAY specify the version(s) it supports in the <text/> element.

   o  <xml-not-well-formed/> -- the initiating entity has sent XML that
      is not well-formed as defined by [XML].


4.6.4 Application-Specific Conditions

   As noted, an application MAY provide application-specific stream
   error information by including a properly-namespaced child in the
   error element.  The application-specific element SHOULD supplement or
   further qualify a defined element.  Thus the <error/> element will
   contain two or three child elements:

   <stream:error>
     <xml-not-well-formed
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
       <text xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
         Some special application diagnostic information!
       </text>
     <escape-your-data xmlns='application-ns'/>
   </stream:error>
   </stream:stream>


4.7 Simplified Stream Examples

   This section contains two simplified examples of a stream-based
   "session" of a client on a server (where the "C" lines are sent from
   the client to the server, and the "S" lines are sent from the server
   to the client); these examples are included for the purpose of
   illustrating the concepts introduced thus far.




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   A basic "session":

   C: <?xml version='1.0'?>
      <stream:stream
          to='example.com'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='someid'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   ...  encryption, authentication, and resource binding ...
   C:   <message from='juliet@example.com'
                 to='romeo@example.net'
                 xml:lang='en'>
   C:     <body>Art thou not Romeo, and a Montague?</body>
   C:   </message>
   S:   <message from='romeo@example.net'
                 to='juliet@example.com'
                 xml:lang='en'>
   S:     <body>Neither, fair saint, if either thee dislike.</body>
   S:   </message>
   C: </stream:stream>
   S: </stream:stream>

   A "session" gone bad:

   C: <?xml version='1.0'?>
      <stream:stream
          to='example.com'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='someid'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   ...  encryption, authentication, and resource binding ...
   C: <message xml:lang='en'>
        <body>Bad XML, no closing body tag!
      </message>



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   S: <stream:error>
       <xml-not-well-formed
           xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
   S: </stream:stream>


5. Use of TLS

5.1 Overview

   XMPP includes a method for securing the stream from tampering and
   eavesdropping.  This channel encryption method makes use of the
   Transport Layer Security (TLS) protocol [TLS], along with a
   "STARTTLS" extension that is modelled after similar extensions for
   the IMAP [IMAP], POP3 [POP3], and ACAP [ACAP] protocols as described
   in RFC 2595 [USINGTLS].  The namespace name for the STARTTLS
   extension is 'urn:ietf:params:xml:ns:xmpp-tls'.

   An administrator of a given domain MAY require the use of TLS for
   client-to-server communications, server-to-server communications, or
   both.  Clients SHOULD use TLS to secure the streams prior to
   attempting to complete SASL negotiation (Section 6), and servers
   SHOULD use TLS between two domains for the purpose of securing
   server-to-server communications.

   The following rules apply:

   1.   An initiating entity that complies with this specification MUST
        include the 'version' attribute set to a value of "1.0" in the
        initial stream header.

   2.   If the TLS negotiation occurs between two servers,
        communications MUST NOT proceed until the Domain Name System
        (DNS) hostnames asserted by the servers have been resolved (see
        Server-to-Server Communications (Section 14.4)).

   3.   When a receiving entity that complies with this specification
        receives an initial stream header that includes the 'version'
        attribute set to a value of "1.0", after sending a stream header
        in reply (including the version flag) it MUST include a
        <starttls/> element (qualified by the
        'urn:ietf:params:xml:ns:xmpp-tls' namespace) along with the list
        of other stream features it supports.

   4.   If the initiating entity chooses to use TLS, TLS negotiation
        MUST be completed before proceeding to SASL negotiation; this
        order of negotiation is required in order to help safeguard



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        authentication information sent during SASL negotiation, as well
        as to make it possible to base the use of the SASL EXTERNAL
        mechanism on a certificate provided during prior TLS
        negotiation.

   5.   During TLS negotiation, an entity MUST NOT send any white space
        characters (matching production [3] content of [XML]) within the
        root stream element as separators between elements (any white
        space characters shown in the TLS examples below are included
        for the sake of readability only); this prohibition helps to
        ensure proper security layer byte precision.

   6.   The receiving entity MUST consider the TLS negotiation to have
        begun immediately after sending the closing ">" character of the
        <proceed/> element.  The initiating entity MUST consider the TLS
        negotiation to have begun immediately after receiving the
        closing ">" character of the <proceed/> element from the
        receiving entity.

   7.   The initiating entity MUST validate the certificate presented by
        the receiving entity; see Certificate Validation (Section 14.2)
        regarding certificate validation procedures.

   8.   Certificates MUST be checked against the hostname as provided by
        the initiating entity (e.g., a user), not the hostname as
        resolved via the Domain Name System; e.g., if the user or
        certificate specifies a hostname of "example.com" but a DNS SRV
        [SRV] lookup returned "im.example.com", the certificate MUST be
        checked as "example.com". If a JID is represented in a
        certificate, it SHOULD be encoded using the otherName choice of
        the subjectAltName type along with a type-id of "xmpp" (as these
        terms are profiled in [X509]).

   9.   Certificates MUST be checked against the hostname as provided by
        the initiating entity (e.g., a user), not the hostname as
        resolved via the Domain Name System; e.g., if the user or
        certificate specifies a hostname of "example.com" but a DNS SRV
        [SRV] lookup returned "im.example.com", the certificate MUST be
        checked as "example.com". If a JID is represented in a
        certificate, it SHOULD be encoded using the otherName choice of
        the subjectAltName type along with a type-id of "xmpp" (as these
        terms are profiled in [X509]).

   10.  If the TLS negotiation is successful, the receiving entity MUST
        discard any knowledge obtained in an insecure manner from the
        initiating entity before TLS takes effect.

   11.  If the TLS negotiation is successful, the initiating entity MUST



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        discard any knowledge obtained in an insecure manner from the
        receiving entity before TLS takes effect.

   12.  If the TLS negotiation is successful, the receiving entity MUST
        NOT offer the STARTTLS extension to the initiating entity along
        with the other stream features that are offered when the stream
        is restarted.

   13.  If the TLS negotiation is successful, the initiating entity MUST
        continue with SASL negotiation.

   14.  If the TLS negotiation results in failure, the receiving entity
        MUST terminate both the XML stream and the underlying TCP
        connection.

   15.  See Mandatory-to-Implement Technologies (Section 14.7) regarding
        mechanisms that MUST be supported.


5.2 Narrative

   When an initiating entity secures a stream with a receiving entity,
   the steps involved are as follows:

   1.  The initiating entity opens a TCP connection and initiates the
       stream by sending the opening XML stream header to the receiving
       entity, including the 'version' attribute set to a value of
       "1.0".

   2.  The receiving entity responds by opening a TCP connection and
       sending an XML stream header to the initiating entity, including
       the 'version' attribute set to a value of "1.0".

   3.  The receiving entity offers the STARTTLS extension to the
       initiating entity by including it with the list of other
       supported stream features (if TLS is required for interaction
       with the receiving entity, it SHOULD signal that fact by
       including a <required/> element as a child of the <starttls/>
       element).

   4.  The initiating entity issues the STARTTLS command (i.e., a
       <starttls/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct the
       receiving entity that it wishes to begin a TLS negotiation to
       secure the stream.

   5.  The receiving entity MUST reply with either a <proceed/> element
       or a <failure/> element qualified by the



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       'urn:ietf:params:xml:ns:xmpp-tls' namespace.  If the failure case
       occurs, the receiving entity MUST terminate both the XML stream
       and the underlying TCP connection.  If the proceed case occurs,
       the entities MUST attempt to complete the TLS negotiation over
       the TCP connection and MUST NOT send any further XML data until
       the TLS negotiation is complete.

   6.  The initiating entity and receiving entity attempt to complete a
       TLS negotiation in accordance with [TLS].

   7.  If the TLS negotiation is unsuccessful, the receiving entity MUST
       terminate the TCP connection (it is not necessary to send a
       closing </stream> tag first, since the receiving entity and
       initiating entity MUST consider the original stream to be closed
       upon sending or receiving the <success/> element).  If the TLS
       negotiation is successful, the initiating entity MUST initiate a
       new stream by sending an opening XML stream header to the
       receiving entity.

   8.  Upon receiving the new stream header from the initiating entity,
       the receiving entity MUST respond by sending a new XML stream
       header to the initiating entity along with the available features
       (but NOT including the STARTTLS feature).


5.3 Client-to-Server Example

   The following example shows the data flow for a client securing a
   stream using STARTTLS (note: the alternate steps shown below are
   provided to illustrate the protocol for failure cases; they are not
   exhaustive and would not necessarily be triggered by the data sent in
   the example).

   Step 1: Client initiates stream to server:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       to='example.com'
       version='1.0'>

   Step 2: Server responds by sending a stream tag to client:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='c2s_123'
       from='example.com'



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       version='1.0'>

   Step 3: Server sends the STARTTLS extension to client along with
   authentication mechanisms and any other stream features:

   <stream:features>
     <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
       <required/>
     </starttls>
     <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       <mechanism>DIGEST-MD5</mechanism>
       <mechanism>PLAIN</mechanism>
     </mechanisms>
   </stream:features>

   Step 4: Client sends the STARTTLS command to server:

   <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   Step 5: Server informs client to proceed:

   <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   Step 5 (alt): Server informs client that TLS negotiation has failed
   and closes both stream and TCP connection:

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
   </stream:stream>

   Step 6: Client and server attempt to complete TLS negotiation over
   the existing TCP connection.

   Step 7: If TLS negotiation is successful, client initiates a new
   stream to server:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       to='example.com'
       version='1.0'>

   Step 7 (alt): If TLS negotiation is unsuccessful, Server2 closes TCP
   connection.

   Step 8: Server responds by sending a stream header to client along
   with any available stream features:

   <stream:stream



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       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='example.com'
       id='c2s_234'
       version='1.0'>
   <stream:features>
     <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       <mechanism>DIGEST-MD5</mechanism>
       <mechanism>PLAIN</mechanism>
       <mechanism>EXTERNAL</mechanism>
     </mechanisms>
   </stream:features>

   Step 9: Client continues with SASL negotiation (Section 6).

5.4 Server-to-Server Example

   The following example shows the data flow for two servers securing a
   stream using STARTTLS (note: the alternate steps shown below are
   provided to illustrate the protocol for failure cases; they are not
   exhaustive and would not necessarily be triggered by the data sent in
   the example).

   Step 1: Server1 initiates stream to Server2:

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       to='example.com'
       version='1.0'>

   Step 2: Server2 responds by sending a stream tag to Server1:

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='example.com'
       id='s2s_123'
       version='1.0'>

   Step 3: Server2 sends the STARTTLS extension to Server1 along with
   authentication mechanisms and any other stream features:

   <stream:features>
     <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
       <required/>
     </starttls>
     <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>



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       <mechanism>DIGEST-MD5</mechanism>
       <mechanism>KERBEROS_V4</mechanism>
     </mechanisms>
   </stream:features>

   Step 4: Server1 sends the STARTTLS command to Server2:

   <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   Step 5: Server2 informs Server1 to proceed:

   <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   Step 5 (alt): Server2 informs Server1 that TLS negotiation has failed
   and closes stream:

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
   </stream:stream>

   Step 6: Server1 and Server2 attempt to complete TLS negotiation via
   TCP.

   Step 7: If TLS negotiation is successful, Server1 initiates a new
   stream to Server2:

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       to='example.com'
       version='1.0'>

   Step 7 (alt): If TLS negotiation is unsuccessful, server closes TCP
   connection.

   Step 8: Server2 responds by sending a stream header to Server1 along
   with any available stream features:

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='example.com'
       id='s2s_234'
       version='1.0'>
   <stream:features>
     <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       <mechanism>DIGEST-MD5</mechanism>
       <mechanism>KERBEROS_V4</mechanism>
       <mechanism>EXTERNAL</mechanism>



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     </mechanisms>
   </stream:features>

   Step 9: Server1 continues with SASL negotiation (Section 6).

6. Use of SASL

6.1 Overview

   XMPP includes a method for authenticating a stream by means of an
   XMPP-specific profile of the Simple Authentication and Security Layer
   (SASL) protocol [SASL].  SASL provides a generalized method for
   adding authentication support to connection-based protocols, and XMPP
   uses a generic XML namespace profile for SASL that conforms to the
   profiling requirements of [SASL].

   The following rules apply:

   1.   If the SASL negotiation occurs between two servers,
        communications MUST NOT proceed until the Domain Name System
        (DNS) hostnames asserted by the servers have been resolved (see
        Server-to-Server Communications (Section 14.4)).

   2.   If the initiating entity is capable of SASL negotiation, it MUST
        include the 'version' attribute set to a value of "1.0" in the
        initial stream header.

   3.   If the receiving entity is capable of SASL negotiation, it MUST
        send one or more authentication mechanisms within a <mechanisms/
        > element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
        namespace in reply to the opening stream tag received from the
        initiating entity (if the opening stream tag included the
        'version' attribute set to a value of "1.0").

   4.   During SASL negotiation, an entity MUST NOT send any white space
        characters (matching production [3] content of [XML]) within the
        root stream element as separators between elements (any white
        space characters shown in the SASL examples below are included
        for the sake of readability only); this prohibition helps to
        ensure proper security layer byte precision.

   5.   Any character data contained within the XML elements used during
        SASL negotiation MUST be encoded using base64, where the
        encoding adheres to the definition in Section 3 of RFC 3548
        [BASE64].

   6.   If provision of a "simple username" is supported by the selected
        SASL mechanism (e.g., this is supported by the DIGEST-MD5 and



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        CRAM-MD5 mechanisms but not by the EXTERNAL and GSSAPI
        mechanisms), during authentication the initiating entity SHOULD
        provide its sending domain (in the case of server-to-server
        communications) or registered account name (in the case of
        client-to-server communications) as the simple username.

   7.   If the initiating entity wishes to act on behalf of another
        entity and the selected SASL mechanism supports transmission of
        an authorization identity, the initiating entity MUST provide an
        authorization identity during SASL negotiation.  If the
        initiating entity does not wish to act on behalf of another
        entity, it MUST NOT provide an authorization identity.  As
        specified in [SASL], the initiating entity MUST NOT provide an
        authorization identity unless the authorization identity is
        different from the default authorization identity derived from
        the authentication identity as described in [SASL].  If
        provided, the value of the authorization identity MUST be of the
        form <domain> (i.e., a domain identifier only) for servers and
        of the form <node@domain> (i.e., node identifier and domain
        identifier) for clients.

   8.   Upon successful SASL negotiation that involves negotiation of a
        security layer, the receiving entity MUST discard any knowledge
        obtained from the initiating entity which was not obtained from
        the SASL negotiation itself.

   9.   Upon successful SASL negotiation that involves negotiation of a
        security layer, the initiating entity MUST discard any knowledge
        obtained from the receiving entity which was not obtained from
        the SASL negotiation itself.

   10.  See Mandatory-to-Implement Technologies (Section 14.7) regarding
        mechanisms that MUST be supported.


6.2 Narrative

   When an initiating entity authenticates with a receiving entity, the
   steps involved are as follows:

   1.  The initiating entity requests SASL authentication by including
       the 'version' attribute in the opening XML stream header sent to
       the receiving entity, with the value set to "1.0".

   2.  After sending an XML stream header in reply, the receiving entity
       sends a list of available SASL authentication mechanisms; each of
       these is a <mechanism/> element included as a child within a
       <mechanisms/> container element qualified by the



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       'urn:ietf:params:xml:ns:xmpp-sasl' namespace, which in turn is a
       child of a <features/> element in the streams namespace.  If Use
       of TLS (Section 5) needs to be established before a particular
       authentication mechanism may be used, the receiving entity MUST
       NOT provide that mechanism in the list of available SASL
       authentication mechanisms prior to TLS negotiation.  If the
       initiating entity presents a valid certificate during prior TLS
       negotiation, the receiving entity SHOULD offer the SASL EXTERNAL
       mechanism to the initiating entity during SASL negotiation (refer
       to [SASL]), although the EXTERNAL mechanism MAY be offered under
       other circumstances as well.

   3.  The initiating entity selects a mechanism by sending an <auth/>
       element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
       namespace to the receiving entity and including an appropriate
       value for the 'mechanism' attribute; this element MAY contain
       character data (in SASL terminology, the "initial response") if
       the mechanism supports or requires it.  If the initiating entity
       selects the EXTERNAL mechanism for authentication and presented a
       certificate during prior TLS negotiation, the authentication
       credentials SHOULD be taken from that certificate.

   4.  If necessary, the receiving entity challenges the initiating
       entity by sending a <challenge/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
       entity; this element MAY contain character data (which MUST be
       computed in accordance with the definition of the SASL mechanism
       chosen by the initiating entity).

   5.  The initiating entity responds to the challenge by sending a
       <response/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the receiving
       entity; this element MAY contain character data (which MUST be
       computed in accordance with the definition of the SASL mechanism
       chosen by the initiating entity).

   6.  If necessary, the receiving entity sends more challenges and the
       initiating entity sends more responses.

   This series of challenge/response pairs continues until one of three
   things happens:

   1.  The initiating entity aborts the handshake by sending an <abort/>
       element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
       namespace to the receiving entity.  Upon receiving an <abort/>
       element, the receiving entity SHOULD allow a configurable but
       reasonable number of retries (at least 2), after which it MUST
       terminate the TCP connection; this allows the initiating entity



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       (e.g., an end-user client) to tolerate incorrectly-provided
       credentials (e.g., a mistyped password) without being forced to
       reconnect.

   2.  The receiving entity reports failure of the handshake by sending
       a <failure/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
       entity (the particular cause of failure SHOULD be communicated in
       an appropriate child element of the <failure/> element as defined
       under SASL Errors (Section 6.4)).  If the failure case occurs,
       the receiving entity SHOULD allow a configurable but reasonable
       number of retries (at least 2), after which it MUST terminate the
       TCP connection; this allows the initiating entity (e.g., an
       end-user client) to tolerate incorrectly-provided credentials
       (e.g., a mistyped password) without being forced to reconnect.

   3.  The receiving entity reports success of the handshake by sending
       a <success/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
       entity; this element MAY contain character data (in SASL
       terminology, "additional data with success") if required by the
       chosen SASL mechanism.  Upon receiving the <success/> element,
       the initiating entity MUST initiate a new stream by sending an
       opening XML stream header to the receiving entity (it is not
       necessary to send a closing </stream> tag first, since the
       receiving entity and initiating entity MUST consider the original
       stream to be closed upon sending or receiving the <success/>
       element).  Upon receiving the new stream header from the
       initiating entity, the receiving entity MUST respond by sending a
       new XML stream header to the initiating entity, along with any
       available features (but NOT including the STARTTLS feature) or an
       empty <features/> element (to signify that no additional features
       are available); any such additional features not defined herein
       MUST be defined by the relevant extension to XMPP.


6.3 SASL Definition

   The profiling requirements of [SASL] require that the following
   information be supplied by a protocol definition:

   service name: "xmpp"

   initiation sequence: After the initiating entity provides an opening
      XML stream header and the receiving entity replies in kind, the
      receiving entity provides a list of acceptable authentication
      methods.  The initiating entity chooses one method from the list
      and sends it to the receiving entity as the value of the



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      'mechanism' attribute possessed by an <auth/> element, optionally
      including an initial response to avoid a round trip.

   exchange sequence: Challenges and responses are carried through the
      exchange of <challenge/> elements from receiving entity to
      initiating entity and <response/> elements from initiating entity
      to receiving entity.  The receiving entity reports failure by
      sending a <failure/> element and success by sending a <success/>
      element; the initiating entity aborts the exchange by sending an
      <abort/> element.  Upon successful negotiation, both sides
      consider the original XML stream to be closed and new stream
      headers are sent by both entities.

   security layer negotiation: The security layer takes effect
      immediately after sending the closing ">" character of the
      <success/> element for the receiving entity, and immediately after
      receiving the closing ">" character of the <success/> element for
      the initiating entity.  The order of layers is first [TCP], then
      [TLS], then [SASL], then XMPP.

   use of the authorization identity: The authorization identity may be
      used by xmpp to denote the <node@domain> of a client or the
      sending <domain> of a server.


6.4 SASL Errors

   The following SASL-related error conditions are defined:

   o  <aborted/> -- The receiving entity acknowledges an <abort/>
      element sent by the initiating entity; sent in reply to the
      <abort/> element.

   o  <incorrect-encoding/> -- The data provided by the initiating
      entity could not be processed because the [BASE64] encoding is
      incorrect (e.g., because the encoding does not adhere to the the
      definition in Section 3 of [BASE64]); sent in reply to a
      <response/> element or an <auth/> element with initial challenge
      data.

   o  <invalid-authzid/> -- The authzid provided by the initiating
      entity is invalid, either because it is incorrectly formatted or
      because the initiating entity does not have permissions to
      authorize that ID; sent in reply to a <response/> element or an
      <auth/> element with initial challenge data.

   o  <invalid-mechanism/> -- The initiating entity did not provide a
      mechanism or requested a mechanism that is not supported by the



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      receiving entity; sent in reply to an <auth/> element.

   o  <mechanism-too-weak/> -- The mechanism requested by the initiating
      entity is weaker than server policy permits for that initiating
      entity; sent in reply to a <response/> element or an <auth/>
      element with initial challenge data.

   o  <not-authorized/> -- The authentication failed because the
      initiating entity did not provide valid credentials (this includes
      but is not limited to the case of an unknown username); sent in
      reply to a <response/> element or an <auth/> element with initial
      challenge data.

   o  <temporary-auth-failure/> -- The authentication failed because of
      a temporary error condition within the receiving entity; sent in
      reply to an <auth/> element or <response/> element.


6.5 Client-to-Server Example

   The following example shows the data flow for a client authenticating
   with a server using SASL, normally after successful TLS negotiation
   (note: the alternate steps shown below are provided to illustrate the
   protocol for failure cases; they are not exhaustive and would not
   necessarily be triggered by the data sent in the example).

   Step 1: Client initiates stream to server:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       to='example.com'
       version='1.0'>

   Step 2: Server responds with a stream tag sent to client:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='c2s_234'
       from='example.com'
       version='1.0'>

   Step 3: Server informs client of available authentication mechanisms:

   <stream:features>
     <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       <mechanism>DIGEST-MD5</mechanism>



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       <mechanism>PLAIN</mechanism>
     </mechanisms>
   </stream:features>

   Step 4: Client selects an authentication mechanism:

   <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
         mechanism='DIGEST-MD5'/>

   Step 5: Server sends a [BASE64] encoded challenge to client:

   <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9ImF1dGgi
   LGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
   </challenge>

   The decoded challenge is:

   realm="somerealm",nonce="OA6MG9tEQGm2hh",\
   qop="auth",charset=utf-8,algorithm=md5-sess

   Step 5 (alt): Server returns error to client:

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <incorrect-encoding/>
   </failure>
   </stream:stream>

   Step 6: Client sends a [BASE64] encoded response to the challenge:

   <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
   T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
   MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
   LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
   YXJzZXQ9dXRmLTgK
   </response>

   The decoded response is:

   username="somenode",realm="somerealm",\
   nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\
   nc=00000001,qop=auth,digest-uri="xmpp/example.com",\
   response=d388dad90d4bbd760a152321f2143af7,charset=utf-8

   Step 7: Server sends another [BASE64] encoded challenge to client:

   <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>



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   cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
   </challenge>

   The decoded challenge is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd

   Step 7 (alt): Server returns error to client:

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <temporary-auth-failure/>
   </failure>
   </stream:stream>

   Step 8: Client responds to the challenge:

   <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

   Step 9: Server informs client of successful authentication:

   <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

   Step 9 (alt): Server informs client of failed authentication:

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <temporary-auth-failure/>
   </failure>
   </stream:stream>

   Step 10: Client initiates a new stream to server:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       to='example.com'
       version='1.0'>

   Step 11: Server responds by sending a stream header to client along
   with any additional features (or an empty features element):

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='c2s_345'
       from='example.com'
       version='1.0'>
   <stream:features>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>



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     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
   </stream:features>


6.6 Server-to-Server Example

   The following example shows the data flow for a server authenticating
   with another server using SASL, normally after successful TLS
   negotiation (note: the alternate steps shown below are provided to
   illustrate the protocol for failure cases; they are not exhaustive
   and would not necessarily be triggered by the data sent in the
   example).

   Step 1: Server1 initiates stream to Server2:

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       to='example.com'
       version='1.0'>

   Step 2: Server2 responds with a stream tag sent to Server1:

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='example.com'
       id='s2s_234'
       version='1.0'>

   Step 3: Server2 informs Server1 of available authentication
   mechanisms:

   <stream:features>
     <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       <mechanism>DIGEST-MD5</mechanism>
       <mechanism>KERBEROS_V4</mechanism>
     </mechanisms>
   </stream:features>

   Step 4: Server1 selects an authentication mechanism:

   <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
         mechanism='DIGEST-MD5'/>

   Step 5: Server2 sends a [BASE64] encoded challenge to Server1:

   <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>



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   dXNlcm5hbWU9ImV4YW1wbGUuY29tIixyZWFsbT0ic29tZXJlYWxtIixub25j
   ZT0iT0E2TUc5dEVRR20yaGgiLHFvcD0iYXV0aCIsY2hhcnNldD11dGYtOCxh
   bGdvcml0aG09bWQ1LXNlc3MK
   </challenge>

   The decoded challenge is:

   username="example.com",realm="somerealm",\
   nonce="OA6MG9tEQGm2hh",qop="auth",\
   charset=utf-8,algorithm=md5-sess

   Step 5 (alt): Server2 returns error to Server1:

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <incorrect-encoding/>
   </failure>
   </stream:stream>

   Step 6: Server1 sends a [BASE64] encoded response to the challenge:

   <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   dXNlcm5hbWU9ImV4YW1wbGUuY29tIixyZWFsbT0ic29tZXJlYWxtIixub25j
   ZT0iT0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5j
   PTAwMDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5j
   b20iLHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3
   LGNoYXJzZXQ9dXRmLTgK
   </response>

   The decoded response is:

   username="example.com",realm="somerealm",\
   nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\
   nc=00000001,qop=auth,digest-uri="xmpp/example.com",\
   response=d388dad90d4bbd760a152321f2143af7,charset=utf-8

   Step 7: Server2 sends another [BASE64] encoded challenge to Server1:

   <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
   </challenge>

   The decoded challenge is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd

   Step 7 (alt): Server2 returns error to Server1:

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>



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     <invalid-authzid/>
   </failure>
   </stream:stream>

   Step 8: Server1 responds to the challenge:

   <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

   Step 8 (alt): Server1 aborts negotiation:

   <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

   Step 9: Server2 informs Server1 of successful authentication:

   <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

   Step 9 (alt): Server2 informs Server1 of failed authentication:

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <aborted/>
   </failure>
   </stream:stream>

   Step 10: Server1 initiates a new stream to Server2:

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       to='example.com'
       version='1.0'>

   Step 11: Server2 responds by sending a stream header to Server1 along
   with any additional features (or an empty features element):

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='example.com'
       id='s2s_345'
       version='1.0'>
   <stream:features/>


7. Resource Binding

   After SASL negotiation (Section 6) with the receiving entity, the
   initiating entity MAY want or need to bind a specific resource to
   that stream.  In general this applies only to clients: in order to



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   conform to the addressing format (Section 3) and stanza delivery
   rules (Section 10) specified herein, there MUST be a resource
   identifier associated with the <node@domain> of the client (which is
   either generated by the server or provided by the client
   application); this ensures that the address for use over that stream
   is a "full JID" of the form <node@domain/resource>.

   Upon receiving a success indication within the SASL negotiation, the
   client MUST send a new stream header to the server, to which the
   server MUST respond with a stream header as well as a list of
   available stream features.  Specifically, if the server requires the
   client to bind a resource to the stream after successful SASL
   negotiation, it MUST include an empty <bind/> element qualified by
   the 'urn:ietf:params:xml:ns:xmpp-bind' namespace in the stream
   features list it presents to the client upon sending the header for
   the response stream sent after successful SASL negotiation (but not
   before):

   Server advertises resource binding feature to client:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='c2s_345'
       from='example.com'
       version='1.0'>
   <stream:features>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
   </stream:features>

   Upon being so informed that resource binding is required, the client
   MUST bind a resource to the stream by sending to the server an IQ
   stanza of type "set" (see IQ Semantics (Section 9.2.3)) containing
   data qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace.

   If the client wishes to allow the server to generate the resource
   identifier on its behalf, it sends an IQ stanza of type "set" that
   contains an empty <bind/> element:

   Client asks server to bind a resource:

   <iq type='set' id='bind_1'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
   </iq>

   A server that supports resource binding MUST be able to generate a
   resource identifier on behalf of a client.  A resource identifier
   generated by the server MUST be unique for that <node@domain>.



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   If the client wishes to specify the resource identifier, it sends an
   IQ stanza of type "set" that contains the desired resource identifier
   as the CDATA of a <resource/> element that is a child of the <bind/>
   element:

   Client binds a resource:

   <iq type='set' id='bind_2'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>someresource</resource>
     </bind>
   </iq>

   Once the server has generated a resource identifier for the client or
   accepted the resource identifier provided by the client, it MUST
   return an IQ stanza of type "result" to the client, which MUST
   include a <jid/> child element that specifies the full JID for the
   connected resource as determined by the server:

   Server informs client of successful resource binding:

   <iq type='result' id='bind_2'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <jid>somenode@example.com/someresource</jid>
     </bind>
   </iq>

   A server is NOT REQUIRED to accept the resource identifier provided
   by the client, and MAY override it with a resource identifier that
   the server generates; in this case, the server SHOULD NOT return a
   stanza error (e.g., <forbidden/>) to the client but instead SHOULD
   communicate the generated resource identifier to the client in the IQ
   result as shown above.

   When a client supplies a resource identifier, the following stanza
   error conditions are possible (see Stanza Errors (Section 9.3)):

   o  The provided resource identifier cannot be processed by the server
      in accordance with Resourceprep (Appendix B).

   o  The client is not allowed to bind a resource to the stream (e.g.,
      because the client has reached a limit on the number of connected
      resources allowed).

   o  The provided resource identifier is already in use but the server
      does not allow binding of multiple connected resources with the
      same identifier.




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   The protocol for these error conditions is shown below.

   Resource identifier cannot be processed:

   <iq type='error' id='bind_2'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>someresource</resource>
     </bind>
     <error type='modify'>
       <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   Client is not allowed to bind a resource:

   <iq type='error' id='bind_2'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>someresource</resource>
     </bind>
     <error type='cancel'>
       <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   Resource identifier is in use:

   <iq type='error' id='bind_2'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>someresource</resource>
     </bind>
     <error type='cancel'>
       <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>


8. Server Dialback

8.1 Overview

   The Jabber protocols from which XMPP was adapted include a "server
   dialback" method for protecting against domain spoofing, thus making
   it more difficult to spoof XML stanzas (see Server-to-Server
   Communications (Section 14.4) regarding this method's security
   characteristics).  Server dialback also makes it easier to deploy
   systems in which outbound messages and inbound messages are handled
   by different machines for the same domain.  Server dialback is not a
   security mechanism, and domains requiring robust security SHOULD use



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   TLS and SASL; see Server-to-Server Communications (Section 14.4) for
   details.

   The server dialback method is made possible by the existence of the
   Domain Name System (DNS), since one server can (normally) discover
   the authoritative server for a given domain. Because dialback depends
   on DNS, inter-domain communications MUST NOT proceed until the Domain
   Name System (DNS) hostnames asserted by the servers have been
   resolved (see Server-to-Server Communications (Section 14.4)).

   The method for generating and verifying the keys used in server
   dialback MUST take into account the hostnames being used, the stream
   ID generated by the receiving server, and a secret known by the
   authoritative server's network. The stream ID is security-critical in
   server dialback and therefore MUST be both unpredictable and
   non-repeating.

   Any error that occurs during dialback negotiation MUST be considered
   a stream error, resulting in termination of the stream and of the
   underlying TCP connection.  The possible error conditions are
   specified in the protocol description below.

   The following terminology applies:

   o  Originating Server -- the server that is attempting to establish a
      connection between two domains.

   o  Receiving Server -- the server that is trying to authenticate that
      Originating Server represents the domain which it claims to be.

   o  Authoritative Server -- the server that answers to the DNS
      hostname asserted by Originating Server; for basic environments
      this will be Originating Server, but it could be a separate
      machine in Originating Server's network.


8.2 Order of Events

   The following is a brief summary of the order of events in dialback:

   1.  Originating Server establishes a connection to Receiving Server.

   2.  Originating Server sends a 'key' value over the connection to
       Receiving Server.

   3.  Receiving Server establishes a connection to Authoritative
       Server.




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   4.  Receiving Server sends the same 'key' value to Authoritative
       Server.

   5.  Authoritative Server replies that key is valid or invalid.

   6.  Receiving Server informs Originating Server whether it is
       authenticated or not.

   We can represent this flow of events graphically as follows:

   Originating               Receiving
      Server                     Server
   -----------               ---------
       |                         |
       |   establish connection  |
       | ----------------------> |
       |                         |
       |   send stream header    |
       | ----------------------> |
       |                         |
       |   send stream header    |
       | <---------------------- |
       |                         |                   Authoritative
       |   send dialback key     |                       Server
       | ----------------------> |                   -------------
       |                         |                         |
                                 |   establish connection  |
                                 | ----------------------> |
                                 |                         |
                                 |   send stream header    |
                                 | ----------------------> |
                                 |                         |
                                 |   send stream header    |
                                 | <---------------------- |
                                 |                         |
                                 |   send verify request   |
                                 | ----------------------> |
                                 |                         |
                                 |   send verify response  |
                                 | <---------------------- |
                                 |
       |  report dialback result |
       | <---------------------- |
       |                         |







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8.3 Protocol

   The detailed protocol interaction between the servers is as follows:

   1.   Originating Server establishes TCP connection to Receiving
        Server.

   2.   Originating Server sends a stream header to Receiving Server:

   <stream:stream
       xmlns:stream='http://etherx.jabber.org/streams'
       xmlns='jabber:server'
       xmlns:db='jabber:server:dialback'>

        Note: The 'to' and 'from' attributes are NOT REQUIRED on the
        root stream element.  The inclusion of the xmlns:db namespace
        declaration with the name shown indicates to Receiving Server
        that Originating Server supports dialback.  If the namespace
        name is incorrect, then Receiving Server MUST generate an
        <invalid-namespace/> stream error condition and terminate both
        the XML stream and the underlying TCP connection.

   3.   Receiving Server SHOULD send a stream header back to Originating
        Server, including a unique ID for this interaction:

   <stream:stream
       xmlns:stream='http://etherx.jabber.org/streams'
       xmlns='jabber:server'
       xmlns:db='jabber:server:dialback'
       id='457F9224A0...'>

        Note: The 'to' and 'from' attributes are NOT REQUIRED on the
        root stream element.  If the namespace name is incorrect, then
        Originating Server MUST generate an <invalid-namespace/> stream
        error condition and terminate both the XML stream and the
        underlying TCP connection.  Note well that Receiving Server is
        NOT REQUIRED to reply and MAY silently terminate the XML stream
        and underlying TCP connection depending on security policies in
        place; however, if Receiving Server desires to proceed, it MUST
        sent a stream header back to Originating Server.

   4.   Originating Server sends a dialback key to Receiving Server:

   <db:result
       to='Receiving Server'
       from='Originating Server'>
     98AF014EDC0...
   </db:result>



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        Note: This key is not examined by Receiving Server, since
        Receiving Server does not keep information about Originating
        Server between sessions.  The key generated by Originating
        Server MUST be based in part on the value of the ID provided by
        Receiving Server in the previous step, and in part on a secret
        shared by Originating Server and Authoritative Server.  If the
        value of the 'to' address does not match a hostname recognized
        by Receiving Server, then Receiving Server MUST generate a
        <host-unknown/> stream error condition and terminate both the
        XML stream and the underlying TCP connection.  If the value of
        the 'from' address matches a domain with which Receiving Server
        already has an established connection, then Receiving Server
        MUST maintain the existing connection until it validates whether
        the new connection is legitimate; additionally, Receiving Server
        MAY choose to generate a <not-authorized/> stream error
        condition for the new connection and then terminate both the XML
        stream and the underlying TCP connection related to the new
        request.

   5.   Receiving Server establishes a TCP connection back to the domain
        name asserted by Originating Server, as a result of which it
        connects to Authoritative Server.  (Note: As an optimization, an
        implementation MAY reuse an existing trusted connection here
        rather than opening a new TCP connection.)

   6.   Receiving Server sends Authoritative Server a stream header:

   <stream:stream
       xmlns:stream='http://etherx.jabber.org/streams'
       xmlns='jabber:server'
       xmlns:db='jabber:server:dialback'>

        Note: The 'to' and 'from' attributes are NOT REQUIRED on the
        root stream element.  If the namespace name is incorrect, then
        Authoritative Server MUST generate an <invalid-namespace/>
        stream error condition and terminate both the XML stream and the
        underlying TCP connection.

   7.   Authoritative Server sends Receiving Server a stream header:

   <stream:stream
       xmlns:stream='http://etherx.jabber.org/streams'
       xmlns='jabber:server'
       xmlns:db='jabber:server:dialback'
       id='1251A342B...'>

        Note: If the namespace name is incorrect, then Receiving Server
        MUST generate an <invalid-namespace/> stream error condition and



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        terminate both the XML stream and the underlying TCP connection
        between it and Authoritative Server.  If a stream error occurs
        between Receiving Server and Authoritative Server, then
        Receiving Server MUST generate a <remote-connection-failed/>
        stream error condition and terminate both the XML stream and the
        underlying TCP connection between it and Originating Server.

   8.   Receiving Server sends Authoritative Server a stanza requesting
        that Authoritative Server verify a key:

   <db:verify
       from='Receiving Server'
       to='Originating Server'
       id='457F9224A0...'>
     98AF014EDC0...
   </db:verify>

        Note: Passed here are the hostnames, the original identifier
        from Receiving Server's stream header to Originating Server in
        Step 3, and the key that Originating Server sent to Receiving
        Server in Step 4.  Based on this information as well as shared
        secret information within the Authoritative Server's network,
        the key is verified.  Any verifiable method MAY be used to
        generate the key.  If the value of the 'to' address does not
        match a hostname recognized by Authoritative Server, then
        Authoritative Server MUST generate a <host-unknown/> stream
        error condition and terminate both the XML stream and the
        underlying TCP connection.  If the value of the 'from' address
        does not match the hostname represented by Receiving Server when
        opening the TCP connection (or any validated domain), then
        Authoritative Server MUST generate an <invalid-from/> stream
        error condition and terminate both the XML stream and the
        underlying TCP connection.

   9.   Authoritative Server sends a stanza back to Receiving Server
        verifying whether the key was valid or invalid:

   <db:verify
       from='Originating Server'
       to='Receiving Server'
       type='valid'
       id='457F9224A0...'/>

         or

   <db:verify
       from='Originating Server'
       to='Receiving Server'



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       type='invalid'
       id='457F9224A0...'/>

        Note: If the ID does not match that provided by Receiving Server
        in Step 3, then Receiving Server MUST generate an <invalid-id/>
        stream error condition and terminate both the XML stream and the
        underlying TCP connection.  If the value of the 'to' address
        does not match a hostname recognized by Receiving Server, then
        Receiving Server MUST generate a <host-unknown/> stream error
        condition and terminate both the XML stream and the underlying
        TCP connection.  If the value of the 'from' address does not
        match the hostname represented by Originating Server when
        opening the TCP connection (or any validated domain), then
        Receiving Server MUST generate an <invalid-from/> stream error
        condition and terminate both the XML stream and the underlying
        TCP connection.  After returning the verification to Receiving
        Server, Authoritative Server SHOULD terminate the stream between
        them.

   10.  Receiving Server informs Originating Server of the result:

   <db:result
       from='Receiving Server'
       to='Originating Server'
       type='valid'/>

        Note: At this point the connection has either been validated via
        a type='valid', or reported as invalid.  If the connection is
        invalid, then Receiving Server MUST terminate both the XML
        stream and the underlying TCP connection.  If the connection is
        validated, data can be sent by Originating Server and read by
        Receiving Server; before that, all data stanzas sent to
        Receiving Server SHOULD be silently dropped.

   Even if dialback negotiation is successful, a server MUST verify that
   all XML stanzas received from the other server include a 'from'
   attribute and a 'to' attribute; if a stanza does not meet this
   restriction, the server that receives the stanza MUST generate an
   <improper-addressing/> stream error condition and terminate both the
   XML stream and the underlying TCP connection.  Furthermore, a server
   MUST verify that the 'from' attribute of stanzas received from the
   other server includes a validated domain for the stream; if a stanza
   does not meet this restriction, the server that receives the stanza
   MUST generate an <invalid-from/> stream error condition and terminate
   both the XML stream and the underlying TCP connection.  Both of these
   checks help to prevent spoofing related to particular stanzas.





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9. XML Stanzas

   After TLS negotiation (Section 5) if desired, SASL negotiation
   (Section 6), and Resource Binding (Section 7) if necessary, XML
   stanzas can be sent over the streams.  Three kinds of XML stanza are
   defined for the 'jabber:client' and 'jabber:server' namespaces:
   <message/>, <presence/>, and <iq/>.  In addition, there are five
   common attributes for these kinds of stanza.  These common
   attributes, as well as the basic semantics of the three stanza kinds,
   are defined herein; more detailed information regarding the syntax of
   XML stanzas in relation to instant messaging and presence
   applications is provided in [XMPP-IM].

9.1 Common Attributes

   The following five attributes are common to message, presence, and IQ
   stanzas:

9.1.1 to

   The 'to' attribute specifies the JID of the intended recipient for
   the stanza.

   In the 'jabber:client' namespace, a stanza SHOULD possess a 'to'
   attribute, although a stanza sent from a client to a server for
   handling by that server (e.g., presence sent to the server for
   broadcasting to other entities) SHOULD NOT possess a 'to' attribute.

   In the 'jabber:server' namespace, a stanza MUST possess a 'to'
   attribute; if a server receives a stanza that does not meet this
   restriction, it MUST generate an <improper-addressing/> stream error
   condition and terminate both the XML stream and the underlying TCP
   connection with the offending server.

   If the value of the 'to' attribute is invalid or cannot be contacted,
   the entity discovering that fact (usually the sender's or recipient's
   server) MUST return an appropriate error to the sender, setting the
   'from' attribute of the error stanza to the value provided in the
   'to' attribute of the offending stanza.

9.1.2 from

   The 'from' attribute specifies the JID of the sender.

   When a server receives an XML stanza within the context of an
   authenticated stream qualified by the 'jabber:client' namespace, it
   MUST do one of the following:




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   1.  validate that the value of the 'from' attribute provided by the
       client is that of a connected resource for the associated entity

   2.  add a 'from' address to the stanza whose value is the full JID
       (<node@domain/resource>) determined by the server for the
       connected resource that generated the stanza (see Determination
       of Addresses (Section 3.5))

   If a client attempts to send an XML stanza for which the value of the
   'from' attribute does not match one of the connected resources for
   that entity, the server SHOULD return an <invalid-from/> stream error
   to the client.  If a client attempts to send an XML stanza over a
   stream that is not yet authenticated, the server SHOULD return a
   <not-authorized/> stream error to the client.  If generated, both of
   these conditions MUST result in closing of the stream and termination
   of the underlying TCP connection; this helps to prevent a denial of
   service attack launched from a rogue client.

   When a server generates a stanza from the server itself for delivery
   to a connected client (e.g., in the context of data storage services
   provided by the server on behalf of the client), the stanza MUST
   either (1) not include a 'from' attribute or (2) include a 'from'
   attribute whose value is the account's bare JID (<node@domain>) or
   client's full JID (<node@domain/resource>).  A server MUST NOT send
   to the client a stanza without a 'from' attribute if the stanza was
   not generated by the server itself.  When a client receives a stanza
   that does not include a 'from' attribute, it MUST assume that the
   stanza is from the server to which the client is connected.

   In the 'jabber:server' namespace, a stanza MUST possess a 'from'
   attribute; if a server receives a stanza that does not meet this
   restriction, it MUST generate an <improper-addressing/> stream error
   condition.  Furthermore, the domain identifier portion of the JID
   contained in the 'from' attribute MUST match the hostname (or any
   validated domain) of the sending server as communicated in the SASL
   negotiation or dialback negotiation; if a server receives a stanza
   that does not meet this restriction, it MUST generate an
   <invalid-from/> stream error condition.  Both of these conditions
   MUST result in closing of the stream and termination of the
   underlying TCP connection; this helps to prevent a denial of service
   attack launched from a rogue server.

9.1.3 id

   The optional 'id' attribute MAY be used by a sending entity for
   internal tracking of stanzas that it sends and receives (especially
   for tracking the request-response interaction inherent in the
   semantics of IQ stanzas).  The value of the 'id' attribute is NOT



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   REQUIRED to be unique either globally, within a domain, or within a
   stream.  The semantics of IQ stanzas impose additional restrictions;
   see IQ Semantics (Section 9.2.3).

9.1.4 type

   The 'type' attribute specifies detailed information about the purpose
   or context of the message, presence, or IQ stanza.  The particular
   allowable values for the 'type' attribute vary depending on whether
   the stanza is a message, presence, or IQ; the values for message and
   presence stanzas are specific to instant messaging and presence
   applications and therefore are defined in [XMPP-IM], whereas the
   values for IQ stanzas specify the role of an IQ stanza in a
   structured request-response "conversation" and thus are defined under
   IQ Semantics (Section 9.2.3) below.  The only 'type' value common to
   all three stanzas is "error", for which see Stanza Errors (Section
   9.3).

9.1.5 xml:lang

   A stanza SHOULD possess an 'xml:lang' attribute (as defined in
   Section 2.12 of [XML]) if the stanza contains XML character data that
   is intended to be presented to a human user (as explained in RFC 2277
   [CHARSET], "internationalization is for humans").  The value of the
   'xml:lang' attribute specifies the default language of any such
   human-readable XML character data, which MAY be overridden by the
   'xml:lang' attribute of a specific child element.  If a stanza does
   not possess an 'xml:lang' attribute, an implementation MUST assume
   that the default language is that specified for the stream as defined
   under Stream Attributes (Section 4.2) above.  The value of the
   'xml:lang' attribute MUST be an NMTOKEN and MUST conform to the
   format defined in RFC 3066 [LANGTAGS].

9.2 Basic Semantics

9.2.1 Message Semantics

   The <message/> stanza kind can be seen as a "push" mechanism whereby
   one entity pushes information to another entity, similar to the
   communications that occur in a system such as email.  All message
   stanzas SHOULD possess a 'to' attribute that specifies the intended
   recipient of the message; upon receiving such a stanza, a server
   SHOULD route or deliver it to the intended recipient (see Server
   Rules for Handling XML Stanzas (Section 10) for general routing and
   delivery rules related to XML stanzas).

9.2.2 Presence Semantics




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   The <presence/> element can be seen as a basic broadcast or
   "publish-subscribe" mechanism, whereby multiple entities receive
   information (in this case, presence information) about an entity to
   which they have subscribed.  In general, a publishing entity SHOULD
   send a presence stanza with no 'to' attribute, in which case the
   server to which the entity is connected SHOULD broadcast or multiplex
   that stanza to all subscribing entities.  However, a publishing
   entity MAY also send a presence stanza with a 'to' attribute, in
   which case the server SHOULD route or deliver that stanza to the
   intended recipient.  See Server Rules for Handling XML Stanzas
   (Section 10) for general routing and delivery rules related to XML
   stanzas, and [XMPP-IM] for presence-specific rules in the context of
   an instant messaging and presence application.

9.2.3 IQ Semantics

   Info/Query, or IQ, is a request-response mechanism, similar in some
   ways to [HTTP].  The semantics of IQ enable an entity to make a
   request of, and receive a response from, another entity.  The data
   content of the request and response is defined by the namespace
   declaration of a direct child element of the IQ element, and the
   interaction is tracked by the requesting entity through use of the
   'id' attribute.  Thus IQ interactions follow a common pattern of
   structured data exchange such as get/result or set/result (although
   an error may be returned in reply to a request if appropriate):

   Requesting                 Responding
     Entity                     Entity
   ----------                 ----------
       |                           |
       | <iq type='get' id='1'>    |
       | ------------------------> |
       |                           |
       | <iq type='result' id='1'> |
       | <------------------------ |
       |                           |
       | <iq type='set' id='2'>    |
       | ------------------------> |
       |                           |
       | <iq type='error' id='2'>  |
       | <------------------------ |
       |                           |

   In order to enforce these semantics, the following rules apply:

   1.  The 'id' attribute is REQUIRED for IQ stanzas.

   2.  The 'type' attribute is REQUIRED for IQ stanzas.  The value MUST



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       be one of the following:

   3.

       *  get -- The stanza is a request for information or
          requirements.

       *  set -- The stanza provides required data, sets new values, or
          replaces existing values.

       *  result -- The stanza is a response to a successful get or set
          request.

       *  error -- An error has occurred regarding processing or
          delivery of a previously-sent get or set (see Stanza Errors
          (Section 9.3)).

   4.  An entity that receives an IQ request of type "get" or "set" MUST
       reply with an IQ response of type "result" or "error" (which
       response MUST preserve the 'id' attribute of the request).

   5.  An entity that receives a stanza of type "result" or "error" MUST
       NOT respond to the stanza by sending a further IQ response of
       type "result" or "error"; however, as shown above, the requesting
       entity MAY send another request (e.g., an IQ of type "set" in
       order to provide required information discovered through a get/
       result pair).

   6.  An IQ stanza of type "get" or "set" MUST contain one and only one
       child element (properly-namespaced as defined in [XMPP-IM]) that
       specifies the semantics of the particular request or response.

   7.  An IQ stanza of type "result" MUST include zero or one child
       elements.

   8.  An IQ stanza of type "error" SHOULD include the child element
       contained in the associated "get" or "set" and MUST include an
       <error/> child; for details, see Stanza Errors (Section 9.3).


9.3 Stanza Errors

   Stanza-related errors are handled in a manner similar to stream
   errors (Section 4.6).  However, unlike stream errors, stanza errors
   are recoverable; therefore error stanzas include hints regarding
   actions that the original sender can take in order to remedy the
   error.




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9.3.1 Rules

   The following rules apply to stanza-related errors:

   o  The receiving or processing entity that detects an error condition
      in relation to a stanza MUST return to the sending entity a stanza
      of the same kind (message, presence, or IQ) whose 'type' attribute
      is set to a value of "error" (such a stanza is called an "error
      stanza" herein).

   o  The entity that generates an error stanza SHOULD (but is NOT
      REQUIRED to) include the original XML sent so that the sender can
      inspect and if necessary correct the XML before attempting to
      resend.

   o  An error stanza MUST contain an <error/> child element.

   o  An <error/> child MUST NOT be included if the 'type' attribute has
      a value other than "error" (or if there is no 'type' attribute).

   o  An entity that receives an error stanza MUST NOT respond to the
      stanza with a further error stanza; this helps to prevent looping.


9.3.2 Syntax

   The syntax for stanza-related errors is as follows:

   <stanza-name to='sender' type='error'>
     [RECOMMENDED to include sender XML here]
     <error type='error-type'>
       <defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       <text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
         OPTIONAL descriptive text
       </text>
       [OPTIONAL application-specific condition element]
     </error>
   </stanza-name>

   The stanza-name is one of message, presence, or iq.

   The value of the <error/> element's 'type' attribute MUST be one of
   the following:

   o  cancel -- do not retry (the error is unrecoverable)

   o  continue -- proceed (the condition was only a warning)




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   o  modify -- retry after changing the data sent

   o  auth -- retry after providing credentials

   o  wait -- retry after waiting (the error is temporary)

   The <error/> element:

   o  MUST contain a child element corresponding to one of the defined
      stanza error conditions specified below; this element MUST be
      qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace.

   o  MAY contain a <text/> child containing CDATA that describes the
      error in more detail; this element MUST be qualified by the
      'urn:ietf:params:xml:ns:xmpp-stanzas' namespace and SHOULD possess
      an 'xml:lang' attribute.

   o  MAY contain a child element for an application-specific error
      condition; this element MUST be qualified by an
      application-defined namespace, and its structure is defined by
      that namespace.

   The <text/> element is OPTIONAL.  If included, it SHOULD be used only
   to provide descriptive or diagnostic information that supplements the
   meaning of a defined condition or application-specific condition.  It
   SHOULD NOT be interpreted programmatically by an application.  It
   SHOULD NOT be used as the error message presented to a user, but MAY
   be shown in addition to the error message associated with the
   included condition element (or elements).

   Finally, to maintain backward compatibility, the schema (specified in
   [XMPP-IM]) allows the optional inclusion of a 'code' attribute on the
   <error/> element.

9.3.3 Defined Conditions

   The following stanza-related error conditions are defined for use in
   stanza errors.

   o  <bad-request/> -- the sender has sent XML that is malformed or
      that cannot be processed (e.g., an IQ stanza that includes an
      unrecognized value of the 'type' attribute); the associated error
      type SHOULD be "modify".

   o  <conflict/> -- access cannot be granted because an existing
      resource or session exists with the same name or address; the
      associated error type SHOULD be "cancel".




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   o  <feature-not-implemented/> -- the feature requested is not
      implemented by the recipient or server and therefore cannot be
      processed; the associated error type SHOULD be "cancel".

   o  <forbidden/> -- the requesting entity does not possess the
      required permissions to perform the action; the associated error
      type SHOULD be "auth".

   o  <gone/> -- the recipient or server can no longer be contacted at
      this address (the error stanza MAY contain a new address in the
      CDATA of the <gone/> element); the associated error type SHOULD be
      "modify".

   o  <internal-server-error/> -- the server could not process the
      stanza because of a misconfiguration or an otherwise-undefined
      internal server error; the associated error type SHOULD be "wait".

   o  <item-not-found/> -- the addressed JID or item requested cannot be
      found; the associated error type SHOULD be "cancel".

   o  <jid-malformed/> -- the value of the 'to' attribute in the
      sender's stanza does not adhere to the syntax defined in
      Addressing Scheme (Section 3); the associated error type SHOULD be
      "modify".

   o  <not-acceptable/> -- the recipient or server understands the
      request but is refusing to process it because it does not meet
      criteria defined by the recipient or server (e.g., a local policy
      regarding acceptable words in messages); the associated error type
      SHOULD be "modify".

   o  <not-allowed/> -- the recipient or server does not allow any
      entity to perform the action; the associated error type SHOULD be
      "cancel".

   o  <not-authorized/> -- the sender must provide proper credentials
      before being allowed to perform the action, or has provided
      improper credentials; the associated error type SHOULD be "auth".

   o  <payment-required/> -- the requesting entity is not authorized to
      access the requested service because payment is required; the
      associated error type SHOULD be "auth".

   o  <recipient-unavailable/> -- the intended recipient is temporarily
      unavailable; the associated error type SHOULD be "wait" (note: an
      application MUST NOT return this error if doing so would provide
      information about the intended recipient's network availability to
      an entity that is not authorized to know such information).



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   o  <redirect/> -- the recipient or server is redirecting requests for
      this information to another entity, usually temporarily (the error
      stanza SHOULD contain the alternate address, which MUST be a valid
      JID, in the CDATA of the <redirect/> element); the associated
      error type SHOULD be "modify".

   o  <registration-required/> -- the requesting entity is not
      authorized to access the requested service because registration is
      required; the associated error type SHOULD be "auth".

   o  <remote-server-not-found/> -- a remote server or service specified
      as part or all of the JID of the intended recipient does not
      exist; the associated error type SHOULD be "cancel".

   o  <remote-server-timeout/> -- a remote server or service specified
      as part or all of the JID of the intended recipient could not be
      contacted within a reasonable amount of time; the associated error
      type SHOULD be "wait".

   o  <resource-constraint/> -- the server or recipient lacks the system
      resources necessary to service the request; the associated error
      type SHOULD be "wait".

   o  <service-unavailable/> -- the server or recipient does not
      currently provide the requested service; the associated error type
      SHOULD be "cancel".

   o  <subscription-required/> -- the requesting entity is not
      authorized to access the requested service because a subscription
      is required; the associated error type SHOULD be "auth".

   o  <undefined-condition/> -- the error condition is not one of those
      defined by the other conditions in this list; any error type may
      be associated with this condition, and it SHOULD be used only in
      conjunction with an application-specific condition.

   o  <unexpected-request/> -- the recipient or server understood the
      request but was not expecting it at this time (e.g., the request
      was out of order); the associated error type SHOULD be "wait".


9.3.4 Application-Specific Conditions

   As noted, an application MAY provide application-specific stanza
   error information by including a properly-namespaced child in the
   error element.  The application-specific element SHOULD supplement or
   further qualify a defined element.  Thus the <error/> element will
   contain two or three child elements:



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   <iq type='error' id='some-id'>
     <error type='modify'>
       <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       <too-many-parameters xmlns='application-ns'/>
     </error>
   </iq>


   <message type='error' id='another-id'>
     <error type='modify'>
       <undefined-condition
             xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       <text xml:lang='en'
             xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
         Some special application diagnostic information...
       </text>
       <special-application-condition xmlns='application-ns'/>
     </error>
   </message>


10. Server Rules for Handling XML Stanzas

   Compliant server implementations MUST ensure in-order processing of
   XML stanzas between any two entities.

   Beyond the requirement for in-order processing, each server
   implementation will contain its own "delivery tree" for handling
   stanzas it receives.  Such a tree determines whether a stanza needs
   to be routed to another domain, processed internally, or delivered to
   a resource associated with a connected node.  The following rules
   apply:

10.1 No 'to' Address

   If the stanza possesses no 'to' attribute, the server SHOULD process
   it on behalf of the entity that sent it.  Because all stanzas
   received from other servers MUST possess a 'to' attribute, this rule
   applies only to stanzas received from a registered entity (such as a
   client) that is connected to the server.  If the server receives a
   presence stanza with no 'to' attribute, the server SHOULD broadcast
   it to the entities that are subscribed to the sending entity's
   presence, if applicable (the semantics of presence broadcast for
   instant messaging and presence applications are defined in
   [XMPP-IM]).  If the server receives an IQ stanza of type "get" or
   "set" with no 'to' attribute and it understands the namespace that
   qualifies the content of the stanza, it MUST either process the
   stanza on behalf of sending entity (where the meaning of "process" is



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   determined by the semantics of the qualifying namespace) or return an
   error to the sending entity.

10.2 Foreign Domain

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute does not match one of the configured hostnames
   of the server itself or a subdomain thereof, the server SHOULD route
   the stanza to the foreign domain (subject to local service
   provisioning and security policies regarding inter-domain
   communication).  There are two possible cases:

   A server-to-server stream already exists between the two domains: The
      sender's server routes the stanza to the authoritative server for
      the foreign domain over the existing stream

   There exists no server-to-server stream between the two domains: The
      sender's server (1) resolves the hostname of the foreign domain
      (as defined under Server-to-Server Communications (Section 14.4)),
      (2) negotiates a server-to-server stream between the two domains
      (as defined under Use of TLS (Section 5) and Use of SASL (Section
      6)), and (3) routes the stanza to the authoritative server for the
      foreign domain over the newly-established stream

   If routing to the recipient's server is unsuccessful, the sender's
   server MUST return an error to the sender; if the recipient's server
   can be contacted but delivery by the recipient's server to the
   recipient is unsuccessful, the recipient's server MUST return an
   error to the sender by way of the sender's server.

10.3 Subdomain

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute matches a subdomain of one of the configured
   hostnames of the server itself, the server MUST either process the
   stanza itself or route the stanza to a specialized service that is
   responsible for that subdomain (if the subdomain is configured), or
   return an error to the sender (if the subdomain is not configured).

10.4 Mere Domain or Specific Resource

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute matches a configured hostname of the server
   itself and the JID contained in the 'to' attribute is of the form
   <domain> or <domain/resource>, the server (or a defined resource
   thereof) MUST either process the stanza as appropriate for the stanza
   kind or return an error stanza to the sender.




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10.5 Node in Same Domain

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute matches a configured hostname of the server
   itself and the JID contained in the 'to' attribute is of the form
   <node@domain> or <node@domain/resource>, the server SHOULD deliver
   the stanza to the intended recipient of the stanza as represented by
   the JID contained in the 'to' attribute.  The following rules apply:

   1.  If the JID contains a resource identifier (i.e., is of the form
       <node@domain/resource>) and there exists a connected resource
       that matches the full JID, the recipient's server SHOULD deliver
       the stanza to the stream or session that exactly matches the
       resource identifier.

   2.  If the JID contains a resource identifier and there exists no
       connected resource that matches the full JID, the recipient's
       server SHOULD return a <service-unavailable/> stanza error to the
       sender.

   3.  If the JID is of the form <node@domain> and there exists at least
       one connected resource for the node, the recipient's server MUST
       deliver the stanza to at least one of the connected resources,
       according to application-specific rules (a set of delivery rules
       for instant messaging and presence applications is defined in
       [XMPP-IM]).


11. XML Usage within XMPP

11.1 Restrictions

   XMPP is a simplified and specialized protocol for streaming XML
   elements in order to exchange structured information in close to real
   time.  Because XMPP does not require the parsing of arbitrary and
   complete XML documents, there is no requirement that XMPP needs to
   support the full feature set of [XML].  In particular, the following
   restrictions apply.

   With regard to XML generation, an XMPP implementation MUST NOT inject
   into an XML stream any of the following:

   o  comments (as defined in Section 2.5 of [XML])

   o  processing instructions (Section 2.6 therein)

   o  internal or external DTD subsets (Section 2.8 therein)




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   o  internal or external entity references (Section 4.2 therein) with
      the exception of predefined entities (Section 4.6 therein)

   o  character data or attribute values containing unescaped characters
      that map to the predefined entities (Section 4.6 therein); such
      characters MUST be escaped

   With regard to XML processing, if an XMPP implementation receives
   such restricted XML data, it MUST ignore the data.

11.2 XML Namespace Names and Prefixes

   XML Namespaces [XML-NAMES] are used within all XMPP-compliant XML to
   create strict boundaries of data ownership.  The basic function of
   namespaces is to separate different vocabularies of XML elements that
   are structurally mixed together.  Ensuring that XMPP-compliant XML is
   namespace-aware enables any allowable XML to be structurally mixed
   with any data element within XMPP.  Rules for XML namespace names and
   prefixes are defined in the following subsections.

11.2.1 Streams Namespace

   A streams namespace declaration is REQUIRED in all XML stream
   headers.  The name of the streams namespace MUST be 'http://
   etherx.jabber.org/streams'.  The element names of the <stream/>
   element and its <features/> and <error/> children MUST be qualified
   by the streams namespace prefix in all instances.  An implementation
   SHOULD generate only the 'stream:' prefix for these elements, and for
   historical reasons MAY accept only the 'stream:' prefix.

11.2.2 Default Namespace

   A default namespace declaration is REQUIRED and is used in all XML
   streams in order to define the allowable first-level children of the
   root stream element.  This namespace declaration MUST be the same for
   the initial stream and the response stream so that both streams are
   qualified consistently.  The default namespace declaration applies to
   the stream and all stanzas sent within a stream (unless explicitly
   qualified by another namespace, or by the prefix of the streams
   namespace or the dialback namespace).

   A server implementation MUST support the following two default
   namespaces (for historical reasons, some implementations MAY support
   only these two default namespaces):

   o  jabber:client -- this default namespace is declared when the
      stream is used for communications between a client and a server




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   o  jabber:server -- this default namespace is declared when the
      stream is used for communications between two servers

   A client implementation MUST support the 'jabber:client' default
   namespace, and for historical reasons MAY support only that default
   namespace.

   An implementation MUST NOT generate namespace prefixes for elements
   in the default namespace if the default namespace is 'jabber:client'
   or 'jabber:server'.  An implementation SHOULD NOT generate namespace
   prefixes for elements qualified by content (as opposed to stream)
   namespaces other than 'jabber:client' and 'jabber:server'.

   Note: The 'jabber:client' and 'jabber:server' namespaces are nearly
   identical but are used in different contexts (client-to-server
   communications for 'jabber:client' and server-to-server
   communications for 'jabber:server').  The only difference between the
   two is that the 'to' and 'from' attributes are OPTIONAL on stanzas
   sent within 'jabber:client', whereas they are REQUIRED on stanzas
   sent within 'jabber:server'.  If a compliant implementation accepts a
   stream that is qualified by the 'jabber:client' or 'jabber:server'
   namespace, it MUST support the common attributes (Section 9.1) and
   basic semantics (Section 9.2) of all three core stanza kinds
   (message, presence, and IQ).

11.2.3 Dialback Namespace

   A dialback namespace declaration is REQUIRED for all elements used in
   server dialback (Section 8).  The name of the dialback namespace MUST
   be 'jabber:server:dialback'.  All elements qualified by this
   namespace MUST be prefixed.  An implementation SHOULD generate only
   the 'db:' prefix for such elements and MAY accept only the 'db:'
   prefix.

11.3 Validation

   Except as noted with regard to 'to' and 'from' addresses for stanzas
   within the 'jabber:server' namespace, a server is not responsible for
   validating the XML elements forwarded to a client or another server;
   an implementation MAY choose to provide only validated data elements
   but is NOT REQUIRED to do so (although an implementation MUST NOT
   accept XML that is not well-formed).  Clients SHOULD NOT rely on the
   ability to send data which does not conform to the schemas, and
   SHOULD ignore any non-conformant elements or attributes on the
   incoming XML stream.  Validation of XML streams and stanzas is NOT
   REQUIRED or recommended, and schemas are included herein for
   descriptive purposes only.




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11.4 Inclusion of Text Declaration

   Implementations SHOULD send a text declaration before sending a
   stream header.  Applications MUST follow the rules in [XML] regarding
   the circumstances under which a text declaration is included.

11.5 Character Encoding

   Implementations MUST support the UTF-8 (RFC 3269 [UTF-8])
   transformation of Universal Character Set (ISO/IEC 10646-1 [UCS2])
   characters, as required by RFC 2277 [CHARSET].  Implementations MUST
   NOT attempt to use any other encoding.

12. Core Compliance Requirements

   This section summarizes the specific aspects of the Extensible
   Messaging and Presence Protocol that MUST be supported by servers and
   clients in order to be considered compliant implementations, as well
   as additional protocol aspects that SHOULD be supported.  For
   compliance purposes, we draw a distinction between core protocols
   (which MUST be supported by any server or client, regardless of the
   specific application) and instant messaging protocols (which MUST be
   supported only by instant messaging and presence applications built
   on top of the core protocols).  Compliance requirements that apply to
   all servers and clients are specified in this section; compliance
   requirements for instant messaging servers and clients are specified
   in the corresponding section of [XMPP-IM].

12.1 Servers

   In addition to all defined requirements with regard to security, XML
   usage, and internationalization, a server MUST support the following
   core protocols in order to be considered compliant:

   o  Application of the [NAMEPREP], Nodeprep (Appendix A), and
      Resourceprep (Appendix B) profiles of [STRINGPREP] to addresses
      (including ensuring that domain identifiers are internationalized
      domain names as defined in [IDNA])

   o  XML streams (Section 4), including Use of TLS (Section 5), Use of
      SASL (Section 6), and Resource Binding (Section 7)

   o  The basic semantics of the three defined stanza kinds (i.e.,
      <message/>, <presence/>, and <iq/>) as specified in stanza
      semantics (Section 9.2)

   o  Generation (and, where appropriate, handling) of error syntax and
      semantics related to streams, TLS, SASL, and XML stanzas



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   In addition, a server SHOULD support the following core protocol:

   o  Server dialback (Section 8)


12.2 Clients

   A client MUST support the following core protocols in order to be
   considered compliant:

   o  XML streams (Section 4), including Use of TLS (Section 5), Use of
      SASL (Section 6), and Resource Binding (Section 7)

   o  The basic semantics of the three defined stanza kinds (i.e.,
      <message/>, <presence/>, and <iq/>) as specified in stanza
      semantics (Section 9.2)

   o  Handling (and, where appropriate, generation) of error syntax and
      semantics related to streams, TLS, SASL, and XML stanzas

   In addition, a client SHOULD support the following core protocols:

   o  Generation of addresses to which the [NAMEPREP], Nodeprep
      (Appendix A), and Resourceprep (Appendix B) profiles of
      [STRINGPREP] can be applied without failing


13. Internationalization Considerations

   XML streams MUST be encoded in UTF-8 as specified under Character
   Encoding (Section 11.5).  As specified under Stream Attributes
   (Section 4.2), an XML stream SHOULD include an 'xml:lang' attribute
   that is treated as the default language for any XML character data
   sent over the stream that is intended to be presented to a human
   user.  As specified under xml:lang (Section 9.1.5), an XML stanza
   SHOULD include an 'xml:lang' attribute if the stanza contains XML
   character data that is intended to be presented to a human user.  A
   server SHOULD apply the default 'xml:lang' attribute to stanzas it
   routes or delivers on behalf of connected entities, and MUST NOT
   modify or delete 'xml:lang' attributes from stanzas it receives from
   other entities.

14. Security Considerations

14.1 High Security

   For the purposes of XMPP communications (client-to-server and
   server-to-server), the term "high security" refers to the use of



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   security technologies that provide both mutual authentication and
   integrity-checking; in particular, when using certificate-based
   authentication to provide high security, a chain-of-trust SHOULD be
   established out-of-band, although a shared certificate authority
   signing certificates could allow a previously unknown certificate to
   establish trust in-band. See Section 14.2 below regarding certificate
   validation procedures.

   Implementations MUST support high security.  Service provisioning
   SHOULD use high security, subject to local security policies.

14.2 Certificate Validation

   When an XMPP peer communicates with another peer securely, it MUST
   validate the peer's certificate.  There are three possible cases:

   Case #1: The peer contains an End Entity certificate which appears to
      be certified by a chain of certificates terminating in a trust
      anchor (as described in Section 6.1 of [X509]).

   Case #2: The peer certificate is certified by a Certificate Authority
      not known to the validating peer.

   Case #3: The peer certificate is self-signed.

   In Case #1, the validating peer MUST do one of two things:

   1.  Verify the peer certificate according to the rules of [X509].
       The certificate SHOULD then be checked against the expected
       identity of the peer following the rules described in [HTTP-TLS],
       except that a subjectAltName extension of type "xmpp" MUST be
       used as the identity if present.  If one of these checks fails,
       user-oriented clients MUST either notify the user (clients MAY
       give the user the opportunity to continue with the connection in
       any case) or terminate the connection with a bad certificate
       error.  Automated clients SHOULD terminate the connection (with a
       bad certificate error) and log the error to an appropriate audit
       log.  Automated clients MAY provide a configuration setting that
       disables this check, but MUST provide a setting which enables it.

   2.  The peer SHOULD show the certificate to a user for approval,
       including the entire certificate chain.  The peer MUST Cache the
       certificate (or some non-forgeable representation such as a
       hash).  In future connections, the peer MUST verify that the same
       certificate was presented and MUST notify the user if it has
       changed.

   In Case #2 and Case #3, implementations SHOULD act as in (2) above.



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14.3 Client-to-Server Communications

   A compliant implementation MUST support both TLS and SASL for
   connections to a server.

   The TLS protocol for encrypting XML streams (defined under Use of TLS
   (Section 5)) provides a reliable mechanism for helping to ensure the
   confidentiality and data integrity of data exchanged between two
   entities.

   The SASL protocol for authenticating XML streams (defined under Use
   of SASL (Section 6)) provides a reliable mechanism for validating
   that a client connecting to a server is who it claims to be.

   Client-to-server communications MUST NOT proceed until the DNS
   hostname asserted by the server has been resolved.  Such resolutions
   SHOULD first attempt to resolve the hostname using an [SRV] Service
   of "xmpp-client" and Proto of "tcp", resulting in resource records
   such as "_xmpp-client._tcp.example.com." (the use of the string
   "xmpp-client" for the service identifier is consistent with the IANA
   registration).  If the SRV lookup fails, the fallback is a normal
   IPv4/IPv6 address record resolution to determine the IP address,
   using the "xmpp-client" port of 5222 registered with the IANA.

   The IP address and method of access of clients MUST NOT be made
   public by a server, nor are any connections other than the original
   server connection required.  This helps to protect the client's
   server from direct attack or identification by third parties.

14.4 Server-to-Server Communications

   A compliant implementation MUST support both TLS and SASL for
   inter-domain communications.  For historical reasons, a compliant
   implementation SHOULD also support Server Dialback (Section 8).

   Because service provisioning is a matter of policy, it is OPTIONAL
   for any given domain to communicate with other domains, and
   server-to-server communications MAY be disabled by the administrator
   of any given deployment.  If a particular domain enables inter-domain
   communications, it SHOULD enable high security.

   Administrators may want to require use of SASL for server-to-server
   communications in order to ensure both authentication and
   confidentiality (e.g., on an organization's private network).
   Compliant implementations SHOULD support SASL for this purpose.

   Inter-domain connections MUST NOT proceed until the DNS hostnames
   asserted by the servers have been resolved.  Such resolutions MUST



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   first attempt to resolve the hostname using an [SRV] Service of
   "xmpp-server" and Proto of "tcp", resulting in resource records such
   as "_xmpp-server._tcp.example.com." (the use of the string
   "xmpp-server" for the service identifier is consistent with the IANA
   registration; note well that the "xmpp-server" service identifier
   supersedes the earlier use of a "jabber" service identifier, since
   the earlier usage did not conform to [SRV]; implementations desiring
   to be backward compatible should continue to look for or answer to
   the "jabber" service identifier as well).  If the SRV lookup fails,
   the fallback is a normal IPv4/IPv6 address record resolution to
   determine the IP address, using the "xmpp-server" port of 5269
   registered with the IANA.

   Server dialback helps protect against domain spoofing, thus making it
   more difficult to spoof XML stanzas.  It is not a mechanism for
   authenticating, securing, or encrypting streams between servers as is
   done via SASL and TLS.  Furthermore, it is susceptible to DNS
   poisoning attacks unless DNSSec [DNSSEC] is used, and even if the DNS
   information is accurate, dialback cannot protect from attacks where
   the attacker is capable of hijacking the IP address of the remote
   domain.  Domains requiring robust security SHOULD use TLS and SASL.
   If SASL is used for server-to-server authentication, dialback SHOULD
   NOT be used since it is unnecessary.

14.5 Order of Layers

   The order of layers in which protocols MUST be stacked is as follows:

   1.  TCP

   2.  TLS

   3.  SASL

   4.  XMPP

   The rationale for this order is that [TCP] is the base connection
   layer used by all of the protocols stacked on top of TCP, [TLS] is
   often provided at the operating system layer, [SASL] is often
   provided at the application layer, and XMPP is the application
   itself.

14.6 Lack of SASL Channel Binding to TLS

   The SASL framework does not provide a mechanism to bind SASL
   authentication to a security layer providing confidentiality and
   integrity protection that was negotiated at a lower layer.  This lack
   of a "channel binding" prevents SASL from being able to verify that



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   the source and destination end points to which the lower layer's
   security is bound are equivalent to the end points that SASL is
   authenticating.  If the end points are not identical, the lower
   layer's security cannot be trusted to protect data transmitted
   between the SASL authenticated entities.  In such a situation, a SASL
   security layer should be negotiated which effectively ignores the
   presence of the lower layer security.

14.7 Mandatory-to-Implement Technologies

   At a minimum, all implementations MUST support the following
   mechanisms:

   for authentication: the SASL [DIGEST-MD5] mechanism

   for confidentiality: TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA
      cipher)

   for both: TLS plus SASL EXTERNAL(using the
      TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher supporting client-side
      certificates)


14.8 Firewalls

   Communications using XMPP normally occur over [TCP] sockets on port
   5222 (client-to-server) or port 5269 (server-to-server), as
   registered with the IANA (see IANA Considerations (Section 15)).  Use
   of these well-known ports allows administrators to easily enable or
   disable XMPP activity through existing and commonly-deployed
   firewalls.

14.9 Use of base64 in SASL

   Both the client and the server MUST verify any [BASE64] data received
   during SASL negotiation.  An implementation MUST reject (not ignore)
   any characters that are not explicitly allowed by the base64
   alphabet; this helps to guard against creation of a covert channel
   that could be used to "leak" information.  An implementation MUST NOT
   break on invalid input and MUST reject any sequence of base64
   characters containing the pad ('=') character if that character is
   included as something other than the last character of the data (e.g.
   "=AAA" or "BBBB=CCC"); this helps to guard against buffer overflow
   attacks and other attacks on the implementation.  Base encoding
   visually hides otherwise easily recognized information, such as
   passwords, but does not provide any computational confidentiality.
   Base 64 encoding MUST follow the definition in Section 3 of RFC 3548
   [BASE64].



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14.10 Stringprep Profiles

   XMPP makes use of the [NAMEPREP] profile of [STRINGPREP] for
   processing of domain identifiers; for security considerations related
   to Nameprep, refer to the appropriate section of [NAMEPREP].

   In addition, XMPP defines two profiles of [STRINGPREP]: Nodeprep
   (Appendix A) for node identifiers and Resourceprep (Appendix B) for
   resource identifiers.

   The Unicode and ISO/IEC 10646 repertoires have many characters that
   look similar.  In many cases, users of security protocols might do
   visual matching, such as when comparing the names of trusted third
   parties.  Because it is impossible to map similar-looking characters
   without a great deal of context such as knowing the fonts used,
   stringprep does nothing to map similar-looking characters together
   nor to prohibit some characters because they look like others.

   A node identifier can be employed as one part of an entity's address
   in XMPP.  One common usage is as the username of an instant messaging
   user; another is as the name of a multi-user chat room; and many
   other kinds of entities could use node identifiers as part of their
   addresses.  The security of such services could be compromised based
   on different interpretations of the internationalized node
   identifier; for example, a user entering a single internationalized
   node identifier could access another user's account information, or a
   user could gain access to an otherwise restricted chat room or
   service.

   A resource identifier can be employed as one part of an entity's
   address in XMPP.  One common usage is as the name for an instant
   messaging user's connected resource (active session); another is as
   the nickname of a user in a multi-user chat room; and many other
   kinds of entities could use resource identifiers as part of their
   addresses.  The security of such services could be compromised based
   on different interpretations of the internationalized resource
   identifier; for example, a user could attempt to initiate multiple
   sessions with the same name, or a user could send a message to
   someone other than the intended recipient in a multi-user chat room.

15. IANA Considerations

15.1 XML Namespace Name for TLS Data

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



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   URI: urn:ietf:params:xml:ns:xmpp-tls

   Specification: XXXX

   Description: This is the XML namespace name for TLS-related data in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by XXXX.

   Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>


15.2 XML Namespace Name for SASL Data

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

   URI: urn:ietf:params:xml:ns:xmpp-sasl

   Specification: XXXX

   Description: This is the XML namespace name for SASL-related data in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by XXXX.

   Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>


15.3 XML Namespace Name for Stream Errors

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

   URI: urn:ietf:params:xml:ns:xmpp-streams

   Specification: XXXX

   Description: This is the XML namespace name for stream-related error
      data in the Extensible Messaging and Presence Protocol (XMPP) as
      defined by XXXX.

   Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>


15.4 XML Namespace Name for Resource Binding

   A URN sub-namespace for resource binding in the Extensible Messaging



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   and Presence Protocol (XMPP) is defined as follows.  (This namespace
   name adheres to the format defined in [XML-REG].)

   URI: urn:ietf:params:xml:ns:xmpp-bind

   Specification: XXXX

   Description: This is the XML namespace name for resource binding in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by XXXX.

   Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>


15.5 XML Namespace Name for Stanza Errors

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

   URI: urn:ietf:params:xml:ns:xmpp-stanzas

   Specification: XXXX

   Description: This is the XML namespace name for stanza-related error
      data in the Extensible Messaging and Presence Protocol (XMPP) as
      defined by XXXX.

   Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>


15.6 Nodeprep Profile of Stringprep

   The Nodeprep profile of stringprep is defined under Nodeprep
   (Appendix A).  The IANA registers Nodeprep in the stringprep profile
   registry.

   Name of this profile:

      Nodeprep

   RFC in which the profile is defined:

      XXXX

   Indicator whether or not this is the newest version of the profile:





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      This is the first version of Nodeprep


15.7 Resourceprep Profile of Stringprep

   The Resourceprep profile of stringprep is defined under Resourceprep
   (Appendix B).  The IANA registers Resourceprep in the stringprep
   profile registry.

   Name of this profile:

      Resourceprep

   RFC in which the profile is defined:

      XXXX

   Indicator whether or not this is the newest version of the profile:

      This is the first version of Resourceprep


15.8 GSSAPI Service Name

   The IANA registers "xmpp" as a GSSAPI [GSS-API] service name, as
   defined under SASL Definition (Section 6.3).

15.9 Port Numbers

   The IANA currently registers "jabber-client" and "jabber-server" as
   keywords for [TCP] ports 5222 and 5269 respectively.  The IANA shall
   change these registrations to "xmpp-client" and "xmpp-server"
   respectively.

   These ports SHOULD be used for client-to-server and server-to-server
   communications respectively, but their use is NOT REQUIRED.

Normative References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [BASE64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, July 2003.

   [CHARSET]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.




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   [DIGEST-MD5]
              Leach, P. and C. Newman, "Using Digest Authentication as a
              SASL Mechanism", RFC 2831, May 2000.

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [GSS-API]  Linn, J., "Generic Security Service Application Program
              Interface, Version 2", RFC 2078, January 1997.

   [HTTP-TLS]
              Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

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

   [IPv6]     Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.

   [LANGTAGS]
              Alvestrand, H., "Tags for the Identification of
              Languages", BCP 47, RFC 3066, January 2001.

   [IDNA]     Faltstrom, P., Hoffman, P. and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [NAMEPREP]
              Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)", RFC
              3491, March 2003.

   [SASL]     Myers, J., "Simple Authentication and Security Layer
              (SASL)", RFC 2222, October 1997.

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

   [STRINGPREP]
              Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("STRINGPREP")", RFC 3454,
              December 2002.

   [TCP]      Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.




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

   [TLS]      Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A.
              and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246,
              January 1999.

   [UCS2]     International Organization for Standardization,
              "Information Technology - Universal Multiple-octet coded
              Character Set (UCS) - Amendment 2: UCS Transformation
              Format 8 (UTF-8)", ISO Standard 10646-1 Addendum 2,
              October 1996.

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [X509]     Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
              "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C
              REC-xml, October 2000, <http://www.w3.org/TR/REC-xml>.

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

Informative References

   [ACAP]     Newman, C. and J. Myers, "ACAP -- Application
              Configuration Access Protocol", RFC 2244, November 1997.

   [DNSSEC]   Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [HTTP]     Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [IMAP]     Crispin, M., "Internet Message Access Protocol - Version
              4rev1", RFC 2060, December 1996.

   [JSF]      Jabber Software Foundation, "Jabber Software Foundation",
              <http://www.jabber.org/>.




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   [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [URI]      Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [USINGTLS]
              Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
              2595, June 1999.

   [XML-REG]  Mealling, M., "The IETF XML Registry",
              draft-mealling-iana-xmlns-registry-05 (work in progress),
              June 2003.

   [XMPP-IM]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence",
              draft-ietf-xmpp-im-19 (work in progress), November 2003.


Author's Address

   Peter Saint-Andre
   Jabber Software Foundation

   EMail: stpeter@jabber.org

Appendix A. Nodeprep

A.1 Introduction

   This appendix defines the "Nodeprep" profile of [STRINGPREP].  As
   such, it specifies processing rules that will enable users to enter
   internationalized node identifiers in the Extensible Messaging and
   Presence Protocol (XMPP) and have the highest chance of getting the
   content of the strings correct.  (An XMPP node identifier is the
   optional portion of an XMPP address that precedes a domain identifier
   and the '@' separator; it is often but not exclusively associated
   with an instant messaging username.) These processing rules are
   intended only for XMPP node identifiers and are not intended for
   arbitrary text or any other aspect of an XMPP address.

   This profile defines the following, as required by [STRINGPREP]:

   o  The intended applicability of the profile: internationalized node
      identifiers within XMPP

   o  The character repertoire that is the input and output to



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      stringprep: Unicode 3.2, specified in Section 2 of this Appendix

   o  The mappings used: specified in Section 3

   o  The Unicode normalization used: specified in Section 4

   o  The characters that are prohibited as output: specified in Section
      5

   o  Bidirectional character handling: specified in Section 6


A.2 Character Repertoire

   This profile uses Unicode 3.2 with the list of unassigned code points
   being Table A.1, both defined in Appendix A of [STRINGPREP].

A.3 Mapping

   This profile specifies mapping using the following tables from
   [STRINGPREP]:

      Table B.1

      Table B.2


A.4 Normalization

   This profile specifies using Unicode normalization form KC, as
   described in [STRINGPREP].

A.5 Prohibited Output

   This profile specifies prohibiting use of the following tables from
   [STRINGPREP].

      Table C.1.1

      Table C.1.2

      Table C.2.1

      Table C.2.2

      Table C.3





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      Table C.4

      Table C.5

      Table C.6

      Table C.7

      Table C.8

      Table C.9

   In addition, the following Unicode characters are also prohibited:

      #x22 (")

      #x26 (&)

      #x27 (')

      #x2F (/)

      #x3A (:)

      #x3C (<)

      #x3E (>)

      #x40 (@)


A.6 Bidirectional Characters

   This profile specifies checking bidirectional strings as described in
   Section 6 of [STRINGPREP].

Appendix B. Resourceprep

B.1 Introduction

   This appendix defines the "Resourceprep" profile of [STRINGPREP].  As
   such, it specifies processing rules that will enable users to enter
   internationalized resource identifiers in the Extensible Messaging
   and Presence Protocol (XMPP) and have the highest chance of getting
   the content of the strings correct.  (An XMPP resource identifier is
   the optional portion of an XMPP address that follows a domain
   identifier and the '/' separator; it is often but not exclusively
   associated with an instant messaging session name.) These processing



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   rules are intended only for XMPP resource identifiers and are not
   intended for arbitrary text or any other aspect of an XMPP address.

   This profile defines the following, as required by [STRINGPREP]:

   o  The intended applicability of the profile: internationalized
      resource identifiers within XMPP

   o  The character repertoire that is the input and output to
      stringprep: Unicode 3.2, specified in Section 2 of this Appendix

   o  The mappings used: specified in Section 3

   o  The Unicode normalization used: specified in Section 4

   o  The characters that are prohibited as output: specified in Section
      5

   o  Bidirectional character handling: specified in Section 6


B.2 Character Repertoire

   This profile uses Unicode 3.2 with the list of unassigned code points
   being Table A.1, both defined in Appendix A of [STRINGPREP].

B.3 Mapping

   This profile specifies mapping using the following tables from
   [STRINGPREP]:

      Table B.1


B.4 Normalization

   This profile specifies using Unicode normalization form KC, as
   described in [STRINGPREP].

B.5 Prohibited Output

   This profile specifies prohibiting use of the following tables from
   [STRINGPREP].

      Table C.1.2






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      Table C.2.1

      Table C.2.2

      Table C.3

      Table C.4

      Table C.5

      Table C.6

      Table C.7

      Table C.8

      Table C.9


B.6 Bidirectional Characters

   This profile specifies checking bidirectional strings as described in
   Section 6 of [STRINGPREP].

Appendix C. XML Schemas

   The following XML schemas are descriptive, not normative.  For
   schemas defining the 'jabber:client' and 'jabber:server' namespaces,
   refer to [XMPP-IM].

C.1 Streams namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='http://etherx.jabber.org/streams'
       xmlns='http://etherx.jabber.org/streams'
       elementFormDefault='unqualified'>

     <xs:import namespace='http://www.w3.org/XML/1998/namespace'
                schemaLocation='http://www.w3.org/2001/xml.xsd'/>

     <xs:element name='stream'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='features' minOccurs='0' maxOccurs='1'/>
           <xs:choice minOccurs='0' maxOccurs='1'>



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             <xs:any namespace='jabber:client'
                     minOccurs='0'
                     maxOccurs='unbounded'/>
             <xs:any namespace='jabber:server'
                     minOccurs='0'
                     maxOccurs='unbounded'/>
           </xs:choice>
           <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
         </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='version' type='xs:decimal' use='optional'/>
         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='features'>
       <xs:complexType>
         <xs:choice minOccurs='0' maxOccurs='unbounded'>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-tls'/>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-sasl'/>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-bind'/>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-session'/>
           <xs:any namespace='##other'/>
         </xs:choice>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:sequence>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-streams'
                   maxOccurs='2'/>
           <xs:any
               namespace='##other'
               minOccurs='0'
               maxOccurs='1'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

   </xs:schema>


C.2 Stream error namespace

   <?xml version='1.0' encoding='UTF-8'?>



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   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       xmlns:xml='http://www.w3.org/XML/1998/namespace'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-streams'
       xmlns='urn:ietf:params:xml:ns:xmpp-streams'
       elementFormDefault='qualified'>

     <xs:import namespace='http://www.w3.org/XML/1998/namespace'
                schemaLocation='http://www.w3.org/2001/xml.xsd'/>

     <xs:element name='bad-format' type='empty'/>
     <xs:element name='bad-namespace-prefix' type='empty'/>
     <xs:element name='conflict' type='empty'/>
     <xs:element name='connection-timeout' type='empty'/>
     <xs:element name='host-gone' type='empty'/>
     <xs:element name='host-unknown' type='empty'/>
     <xs:element name='improper-addressing' type='empty'/>
     <xs:element name='internal-server-error' type='empty'/>
     <xs:element name='invalid-from' type='empty'/>
     <xs:element name='invalid-id' type='empty'/>
     <xs:element name='invalid-namespace' type='empty'/>
     <xs:element name='invalid-xml' type='empty'/>
     <xs:element name='not-authorized' type='empty'/>
     <xs:element name='policy-violation' type='empty'/>
     <xs:element name='remote-connection-failed' type='empty'/>
     <xs:element name='resource-constraint' type='empty'/>
     <xs:element name='restricted-xml' type='empty'/>
     <xs:element name='see-other-host' type='xs:string'/>
     <xs:element name='system-shutdown' type='empty'/>
     <xs:element name='undefined-condition' type='empty'/>
     <xs:element name='unsupported-encoding' type='empty'/>
     <xs:element name='unsupported-stanza-type' type='empty'/>
     <xs:element name='unsupported-version' type='empty'/>
     <xs:element name='xml-not-well-formed' type='empty'/>

     <xs:element name='text' type='xs:string'>
       <xs:complexType>
         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

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

   </xs:schema>



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C.3 TLS namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-tls'
       xmlns='urn:ietf:params:xml:ns:xmpp-tls'
       elementFormDefault='qualified'>

     <xs:element name='starttls'>
       <xs:complexType>
         <xs:sequence>
           <xs:element
               ref='required'
               minOccurs='0'
               maxOccurs='1'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <xs:element name='required' type='empty'/>
     <xs:element name='proceed' type='empty'/>
     <xs:element name='failure' type='empty'/>

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

   </xs:schema>


C.4 SASL namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-sasl'
       xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
       elementFormDefault='qualified'>

     <xs:element name='mechanisms'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='mechanism' maxOccurs='unbounded'/>



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         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <xs:element name='mechanism' type='xs:string'/>

     <xs:element name='auth'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:string'>
             <xs:attribute name='mechanism'
                           type='xs:NMTOKEN'
                           use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='challenge' type='xs:NMTOKEN'/>
     <xs:element name='response' type='xs:NMTOKEN'/>
     <xs:element name='abort' type='empty'/>
     <xs:element name='success' type='empty'/>

     <xs:element name='failure'>
       <xs:complexType>
         <xs:choice maxOccurs='1'>
           <xs:element ref='aborted'/>
           <xs:element ref='incorrect-encoding'/>
           <xs:element ref='invalid-authzid'/>
           <xs:element ref='invalid-mechanism'/>
           <xs:element ref='mechanism-too-weak'/>
           <xs:element ref='not-authorized'/>
           <xs:element ref='temporary-auth-failure'/>
         </xs:choice>
       </xs:complexType>
     </xs:element>

     <xs:element name='aborted' type='empty'/>
     <xs:element name='incorrect-encoding' type='empty'/>
     <xs:element name='invalid-authzid' type='empty'/>
     <xs:element name='invalid-mechanism' type='empty'/>
     <xs:element name='mechanism-too-weak' type='empty'/>
     <xs:element name='not-authorized' type='empty'/>
     <xs:element name='temporary-auth-failure' type='empty'/>

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



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       </xs:restriction>
     </xs:simpleType>

   </xs:schema>


C.5 Resource binding namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-bind'
       xmlns='urn:ietf:params:xml:ns:xmpp-bind'
       elementFormDefault='qualified'>

     <xs:element name='bind'>
       <xs:complexType>
         <xs:choice minOccurs='0' maxOccurs='1'>
           <xs:element ref='resource'/>
           <xs:element ref='jid'/>
         </xs:choice>
       </xs:complexType>
     </xs:element>

     <xs:element name='resource' type='xs:string'/>
     <xs:element name='jid' type='xs:string'/>

   </xs:schema>


C.6 Dialback namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='jabber:server:dialback'
       xmlns='jabber:server:dialback'
       elementFormDefault='qualified'>

     <xs:element name='result'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='from' type='xs:string' use='required'/>
             <xs:attribute name='to' type='xs:string' use='required'/>
             <xs:attribute name='type' use='optional'>



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               <xs:simpleType>
                 <xs:restriction base='xs:NCName'>
                   <xs:enumeration value='invalid'/>
                   <xs:enumeration value='valid'/>
                 </xs:restriction>
               </xs:simpleType>
             </xs:attribute>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='verify'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <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='optional'>
               <xs:simpleType>
                 <xs:restriction base='xs:NCName'>
                   <xs:enumeration value='invalid'/>
                   <xs:enumeration value='valid'/>
                 </xs:restriction>
               </xs:simpleType>
             </xs:attribute>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

   </xs:schema>


C.7 Stanza error namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       xmlns:xml='http://www.w3.org/XML/1998/namespace'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-stanzas'
       xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
       elementFormDefault='qualified'>

     <xs:import namespace='http://www.w3.org/XML/1998/namespace'
                schemaLocation='http://www.w3.org/2001/xml.xsd'/>



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     <xs:element name='bad-request' type='empty'/>
     <xs:element name='conflict' type='empty'/>
     <xs:element name='feature-not-implemented' type='empty'/>
     <xs:element name='forbidden' type='empty'/>
     <xs:element name='gone' type='xs:string'/>
     <xs:element name='internal-server-error' type='empty'/>
     <xs:element name='item-not-found' type='empty'/>
     <xs:element name='jid-malformed' type='empty'/>
     <xs:element name='not-acceptable' type='empty'/>
     <xs:element name='not-allowed' type='empty'/>
     <xs:element name='payment-required' type='empty'/>
     <xs:element name='recipient-unavailable' type='empty'/>
     <xs:element name='redirect' type='xs:string'/>
     <xs:element name='registration-required' type='empty'/>
     <xs:element name='remote-server-not-found' type='empty'/>
     <xs:element name='remote-server-timeout' type='empty'/>
     <xs:element name='resource-constraint' type='empty'/>
     <xs:element name='service-unavailable' type='empty'/>
     <xs:element name='subscription-required' type='empty'/>
     <xs:element name='undefined-condition' type='empty'/>
     <xs:element name='unexpected-request' type='empty'/>

     <xs:element name='text' type='xs:string'>
       <xs:complexType>
         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

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

   </xs:schema>


Appendix D. Differences Between Core Jabber Protocol and XMPP

   This section is non-normative.

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



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   [XMPP-IM] summarizes the differences that relate specifically to
   instant messaging and presence applications.

D.1 Channel Encryption

   It is common practice in the Jabber community to use SSL for channel
   encryption on ports other than 5222 and 5269 (the convention is to
   use ports 5223 and 5270).  XMPP uses TLS over the IANA-registered
   ports for channel encryption, as defined under Use of TLS (Section 5)
   herein.

D.2 Authentication

   The client-server authentication protocol developed in the Jabber
   community uses a basic IQ interaction qualified by the
   'jabber:iq:auth' namespace (documentation of this protocol is
   contained in "JEP-0078: Non-SASL Authentication", published by the
   Jabber Software Foundation [JSF]).  XMPP uses SASL for
   authentication, as defined under Use of SASL (Section 6) herein.

   The Jabber community does not currently possess an authentication
   protocol for server-to-server communications, only the Server
   Dialback (Section 8) protocol to prevent server spoofing.  XMPP
   augments Server Dialback with a true server-to-server authentication
   protocol, as defined under Use of SASL (Section 6) herein.

D.3 Resource Binding

   Resource binding in the Jabber community is handled via the
   'jabber:iq:auth' namespace that is also used for client
   authentication with a server.  XMPP defines a dedicated namespace for
   resource binding as well as the ability for a server to generate a
   resource identifier on behalf of a client, as defined under Resource
   Binding (Section 7).

D.4 JID Processing

   JID processing was somewhat loosely defined by the Jabber community
   (documentation of forbidden characters and case handling is contained
   in "JEP-0029: Definition of Jabber Identifiers", published by the
   Jabber Software Foundation [JSF]).  XMPP specifies the use of
   [NAMEPREP] for domain identifiers and supplements Nameprep with two
   additional [STRINGPREP] profiles for JID processing: Nodeprep
   (Appendix A) for node identifiers and Resourceprep (Appendix B) for
   resource identifiers .

D.5 Error Handling




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   Stream-related errors are handled in the Jabber community via simple
   CDATA text in a <stream:error/> element.  In XMPP, stream-related
   errors are handled via an extensible mechanism defined under Stream
   Errors (Section 4.6) herein.

   Stanza-related errors are handled in the Jabber community via
   HTTP-style error codes.  In XMPP, stanza-related errors are handled
   via an extensible mechanism defined under Stanza Errors (Section 9.3)
   herein.  (Documentation of a mapping between Jabber and XMPP error
   handling mechanisms is contained in "JEP-0086: Legacy Errors",
   published by the Jabber Software Foundation [JSF].)

D.6 Internationalization

   Although use of UTF-8 has always been standard practice within the
   Jabber community, the community did not define mechanisms for
   specifying the language of human-readable text provided in CDATA
   sections.  XMPP specifies the use of the 'xml:lang' attribute in such
   contexts, as defined under Stream Attributes (Section 4.2) and
   xml:lang (Section 9.1.5) herein.

D.7 Stream Version Attribute

   The Jabber community does not include a 'version' attribute in stream
   headers.  XMPP specifies inclusion of that attribute, with a value of
   '1.0', as a way to signal support for the stream features
   (authentication, encryption, etc.) defined under Version Support
   (Section 4.2.1) herein.

Appendix E. Revision History

   Note to RFC Editor: please remove this entire appendix, and the
   corresponding entries in the table of contents, prior to publication.

E.1 Changes from draft-ietf-xmpp-core-20

   o  Completed changes necessary to address IESG feedback.


E.2 Changes from draft-ietf-xmpp-core-19

   o  Fixed several typographical errors.

   o  Restricted values of 'type' attribute for IQ stanzas to those
      defined in the schema (i.e., changed SHOULD to MUST) to ensure
      consistency with text in XMPP IM.

   o  Added reference to RFC 3548.



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   o  Added the <not-authorized/> stanza error.

   o  Replaced RFC 2222 reference with reference to
      draft-ietf-sasl-rfc2222bis.

   o  Further clarified role and usage of user names in SASL mechanisms.

   o  Added mention of 'code' attribute on error element.

   o  Clarified several sentences in the dialback narrative.

   o  Clarified use of stringprep profiles and added reference to RFC
      3490.

   o  Added security consideration regarding lack of SASL channel
      binding to TLS per discussion at IETF 58 meeting.

   o  Adjusted formatting to conform to RFC Editor requirements.


E.3 Changes from draft-ietf-xmpp-core-18

   o  Added the 'xml:lang' attribute to the root <stream/> element per
      previous consensus and list discussion.

   o  Changed "jabber-server" and "jabber-client" service names to
      "xmpp-server" and "xmpp-client".

   o  Added the <gone/>, <not-acceptable/>, and <redirect/> stanza
      errors.

   o  Changed dataype of <see-other-host/> stream error and of <gone/>
      and <redirect/> stanza errors to xs:string so that these elements
      may contain programmatic information.

   o  Removed <invalid-realm/> and <bad-protocol/> SASL errors.

   o  Removed references to RFC 952 and RFC 1123 (domain name format is
      handled by reference to Nameprep).

   o  Changed address record resolution text so that it is not specific
      to IPv4.

   o  Clarified text in appendices regarding scope of Nodeprep and
      Resourceprep.

   o  Removed requirement that receiving entity terminate the TCP
      connection upon receiving an <abort/> element from or sending a



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      <failure/> element to the initiating entity during SASL
      negotiation.

   o  Removed recommendation that TLS and SASL security layer should not
      both be used simultaneously.

   o  Added subsection to Security Considerations regarding use of
      base64 in SASL.

   o  Specified rules regarding inclusion of username in SASL
      negotiation.

   o  Adjusted content related to SASL authorization identities, since
      the previous text did not track SASL.

   o  Added section on resource binding to compensate for changes to
      SASL authorization identity text.

   o  Specified ABNF for JIDs.

   o  Checked all references.

   o  Completed a thorough proofreading and consistency check of the
      entire text.


E.4 Changes from draft-ietf-xmpp-core-17

   o  Specified that UTF-8 is the only allowable encoding.

   o  Added stream errors for <bad-namespace-prefix/>, <invalid-xml/>,
      and <restricted-xml/>, as well as a <bad-format/> error for
      generic XML error conditions.

   o  Folded Nodeprep and Resourceprep profiles into this document.

   o  Moved most delivery handling rules from XMPP IM to XMPP Core.

   o  Moved detailed stanza syntax descriptions from XMPP Core to XMPP
      IM.

   o  Moved stanza schemas from XMPP Core to XMPP IM.


E.5 Changes from draft-ietf-xmpp-core-16

   o  Added <conflict/> and <unsupported-encoding/> stream errors.




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   o  Changed the datatype for the <see-other-host/> and
      <unsupported-version/> stream errors from 'xs:string' to 'empty'.

   o  Further clarified server handling of the basic stanza kinds.

   o  Further clarified character encoding rules per list discussion.

   o  Specified meaning of version='1.0' flag in stream headers.

   o  Added stream closure to SASL failure cases in order to mirror
      handling of TLS failures.

   o  Added section on compliance requirements for server and client
      implementations.

   o  Added non-normative section on differences between Jabber usage
      and XMPP specifications.


E.6 Changes from draft-ietf-xmpp-core-15

   o  Added <connection-timeout/> and <policy-violation/> stream errors.

   o  Added <aborted/> SASL error and clarified <bad-protocol/> error.

   o  Made 'id' required for IQ stanzas.


E.7 Changes from draft-ietf-xmpp-core-14

   o  Added SRV lookup for client-to-server communications.

   o  Changed server SRV record to conform to RFC 2782; specifically,
      the service identifier was changed from 'jabber' to
      'jabber-server'.


E.8 Changes from draft-ietf-xmpp-core-13

   o  Clarified stream restart after successful TLS and SASL
      negotiation.

   o  Clarified requirement for resolution of DNS hostnames.

   o  Clarified text regarding namespaces.

   o  Clarified examples regarding empty <stream:features/> element.




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   o  Added several more SASL error conditions.

   o  Changed <invalid-xml/> stream error to <improper-addressing/> and
      added to schema.

   o  Made small editorial changes and fixed several schema errors.


E.9 Changes from draft-ietf-xmpp-core-12

   o  Moved server dialback to a separate section; clarified its
      security characteristics and its role in the protocol.

   o  Adjusted error handling syntax and semantics per list discussion.

   o  Further clarified length of node identifiers and total length of
      JIDs.

   o  Documented message type='normal'.

   o  Corrected several small errors in the TLS and SASL sections.

   o  Corrected several errors in the schemas.


E.10 Changes from draft-ietf-xmpp-core-11

   o  Corrected several small errors in the TLS and SASL sections.

   o  Made small editorial changes and fixed several schema errors.


E.11 Changes from draft-ietf-xmpp-core-10

   o  Adjusted TLS content regarding certificate validation process.

   o  Specified that stanza error extensions for specific applications
      are to be properly namespaced children of the relevant descriptive
      element.

   o  Clarified rules for inclusion of the 'id' attribute.

   o  Specified that the 'xml:lang' attribute SHOULD be included (per
      list discussion).

   o  Made small editorial changes and fixed several schema errors.





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E.12 Changes from draft-ietf-xmpp-core-09

   o  Fixed several dialback error conditions.

   o  Cleaned up rules regarding TLS and certificate processing based on
      off-list feedback.

   o  Changed <stream-condition/> and <stanza-condition/> elements to
      <condition/>.

   o  Added or modified several stream and stanza error conditions.

   o  Specified only one child allowed for IQ, or two if type="error".

   o  Fixed several errors in the schemas.


E.13 Changes from draft-ietf-xmpp-core-08

   o  Incorporated list discussion regarding addressing, SASL, TLS, TCP,
      dialback, namespaces, extensibility, and the meaning of 'ignore'
      for routers and recipients.

   o  Specified dialback error conditions.

   o  Made small editorial changes to address RFC Editor requirements.


E.14 Changes from draft-ietf-xmpp-core-07

   o  Made several small editorial changes.


E.15 Changes from draft-ietf-xmpp-core-06

   o  Added text regarding certificate validation in TLS negotiation per
      list discussion.

   o  Clarified nature of XML restrictions per discussion with W3C, and
      moved XML Restrictions subsection under "XML Usage within XMPP".

   o  Further clarified that XML streams are unidirectional.

   o  Changed stream error and stanza error namespace names to conform
      to the format defined in The IETF XML Registry.

   o  Removed note to RFC Editor regarding provisional namespace names.




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E.16 Changes from draft-ietf-xmpp-core-05

   o  Added <invalid-namespace/> as a stream error condition.

   o  Adjusted security considerations per discussion at IETF 56 and on
      list.


E.17 Changes from draft-ietf-xmpp-core-04

   o  Added server-to-server examples for TLS and SASL.

   o  Changed error syntax, rules, and examples based on list
      discussion.

   o  Added schemas for the TLS, stream error, and stanza error
      namespaces.

   o  Added note to RFC Editor regarding provisional namespace names.

   o  Made numerous small editorial changes and clarified text
      throughout.


E.18 Changes from draft-ietf-xmpp-core-03

   o  Clarified rules and procedures for TLS and SASL.

   o  Amplified stream error code syntax per list discussion.

   o  Made numerous small editorial changes.


E.19 Changes from draft-ietf-xmpp-core-02

   o  Added dialback schema.

   o  Removed all DTDs since schemas provide more complete definitions.

   o  Added stream error codes.

   o  Clarified error code "philosophy".


E.20 Changes from draft-ietf-xmpp-core-01

   o  Updated the addressing restrictions per list discussion and added
      references to the new Nodeprep and Resourceprep profiles.



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   o  Corrected error in Use of SASL regarding 'version' attribute.

   o  Made numerous small editorial changes.


E.21 Changes from draft-ietf-xmpp-core-00

   o  Added information about TLS from list discussion.

   o  Clarified meaning of "ignore" based on list discussion.

   o  Clarified information about Universal Character Set data and
      character encodings.

   o  Provided base64-decoded information for examples.

   o  Fixed several errors in the schemas.

   o  Made numerous small editorial fixes.


E.22 Changes from draft-miller-xmpp-core-02

   o  Brought Use of SASL section into line with discussion on list and
      at IETF 55 meeting.

   o  Added information about the optional 'xml:lang' attribute per
      discussion on list and at IETF 55 meeting.

   o  Specified that validation is neither required nor recommended, and
      that the formal definitions (DTDs and schemas) are included for
      descriptive purposes only.

   o  Specified that the response to an IQ stanza of type "get" or "set"
      must be an IQ stanza of type "result" or "error".

   o  Specified that compliant server implementations must process
      stanzas in order.

   o  Specified that for historical reasons some server implementations
      may accept 'stream:' as the only valid namespace prefix on the
      root stream element.

   o  Clarified the difference between 'jabber:client' and
      'jabber:server' namespaces, namely, that 'to' and 'from'
      attributes are required on all stanzas in the latter but not the
      former.




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   o  Fixed typo in Step 9 of the dialback protocol (changed db:result
      to db:verify).

   o  Removed references to TLS pending list discussion.

   o  Removed the non-normative appendix on OpenPGP usage pending its
      inclusion in a separate I-D.

   o  Simplified the architecture diagram, removed most references to
      services, and removed references to the 'jabber:component:*'
      namespaces.

   o  Noted that XMPP activity respects firewall administration
      policies.

   o  Further specified the scope and uniqueness of the 'id' attribute
      in all stanza kinds and the <thread/> element in message stanzas.

   o  Nomenclature changes: (1) from "chunks" to "stanzas"; (2) from
      "host" to "server" and from "node" to "client" (except with regard
      to definition of the addressing scheme).






























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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
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Full Copyright Statement

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   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION



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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Acknowledgment

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











































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XMPP Working Group                                  P. Saint-Andre (ed.)
Internet-Draft                                Jabber Software Foundation
Expires: July 6, 2004                                    January 6, 2004


  Extensible Messaging and Presence Protocol (XMPP): Instant Messaging
                              and Presence
                         draft-ietf-xmpp-im-20

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on July 6, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

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











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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .    3
   2.  Syntax of XML Stanzas  . . . . . . . . . . . . . . . . . . .    4
   3.  Session Establishment  . . . . . . . . . . . . . . . . . . .   11
   4.  Exchanging Messages  . . . . . . . . . . . . . . . . . . . .   14
   5.  Exchanging Presence Information  . . . . . . . . . . . . . .   16
   6.  Managing Subscriptions . . . . . . . . . . . . . . . . . . .   25
   7.  Roster Management  . . . . . . . . . . . . . . . . . . . . .   26
   8.  Integration of Roster Items and Presence Subscriptions . . .   31
   9.  Subscription States  . . . . . . . . . . . . . . . . . . . .   54
   10. Blocking Communication . . . . . . . . . . . . . . . . . . .   59
   11. Server Rules for Handling XML Stanzas  . . . . . . . . . . .   79
   12. IM and Presence Compliance Requirements  . . . . . . . . . .   81
   13. Internationalization Considerations  . . . . . . . . . . . .   82
   14. Security Considerations  . . . . . . . . . . . . . . . . . .   82
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . .   83
       Normative References . . . . . . . . . . . . . . . . . . . .   84
       Informative References . . . . . . . . . . . . . . . . . . .   85
       Author's Address . . . . . . . . . . . . . . . . . . . . . .   85
   A.  vCards . . . . . . . . . . . . . . . . . . . . . . . . . . .   85
   B.  XML Schemas  . . . . . . . . . . . . . . . . . . . . . . . .   86
   C.  Differences Between Jabber IM/Presence and XMPP  . . . . . .   98
   D.  Revision History . . . . . . . . . . . . . . . . . . . . . .   99
       Intellectual Property and Copyright Statements . . . . . . .  106


























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1. Introduction

1.1 Overview

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

1.2 Requirements

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

   o  Exchange messages with other users

   o  Exchange presence information with other users

   o  Manage subscriptions to and from other users

   o  Manage items in a contact list (in XMPP this is called a "roster")

   o  Block communications to or from specific other users

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

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



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

1.3 Terminology

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

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

1.4 Contributors

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

1.5 Acknowledgements

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

2. Syntax of XML Stanzas




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

2.1 Message Syntax

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

2.1.1 Types of Message

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

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

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

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

   o  headline -- The message is probably generated by an automated
      service that delivers or broadcasts content (news, sports, market
      information, RSS feeds, etc.).  No reply to the message is
      expected, and a compliant client SHOULD present the message an
      interface that appropriately differentiates the message from
      standalone messages, chat sessions, or groupchat sessions (e.g.,
      by not providing the recipient with the ability to reply).



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   o  normal -- The message is a single message that is sent outside the
      context of a one-to-one conversation or groupchat, and to which it
      is expected that the recipient will reply.  A compliant client
      SHOULD present the message in an interface enabling the recipient
      to reply.

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

   Although the 'type' attribute is NOT REQUIRED, it is considered
   polite to mirror the type in any replies to a message; furthermore,
   some specialized applications (e.g., a multi-user chat service) MAY
   at their discretion enforce the use of a particular message type
   (e.g., type='groupchat').

2.1.2 Child Elements

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

   In accordance with the default namespace declaration, by default a
   message stanza is in the 'jabber:client' or 'jabber:server'
   namespace, which defines certain allowable children of message
   stanzas.  If the message stanza is of type "error", it MUST include
   an <error/> child; for details, see [XMPP-CORE].  Otherwise, the
   message stanza MAY contain any of the following child elements
   without an explicit namespace declaration:

   1.  <subject/>

   2.  <body/>

   3.  <thread/>


2.1.2.1 Subject

   The <subject/> element contains XML character data that specifies the
   topic of the message.  The <subject/> element MUST NOT possess any
   attributes, with the exception of the 'xml:lang' attribute.  Multiple
   instances of the <subject/> element MAY be included for the purpose
   of providing alternate versions of the same subject, but only if each
   instance possesses an 'xml:lang' attribute with a distinct language



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   value.  The <subject/> element MUST NOT contain mixed content (as
   defined in Section 3.2.2 of [XML]).

2.1.2.2 Body

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

2.1.2.3 Thread

   The <thread/> element contains XML character data that specifies an
   identifier that is used for tracking a conversation thread (sometimes
   referred to as an "instant messaging session") between two entities.
   The value of the <thread/> element is generated by the sender and
   that SHOULD be copied back in any replies.  If used, it MUST be
   unique to that conversation thread within the stream and MUST be
   consistent throughout that conversation (a client that receives a
   message from the same full JID but with a different thread ID MUST
   assume that the message in question exists outside the context of the
   existing conversation thread).  The use of the <thread/> element is
   OPTIONAL and is not used to identify individual messages, only
   conversations.  A message stanza MUST NOT contain more than one
   <thread/> element.  The <thread/> element MUST NOT possess any
   attributes.  The value of the <thread/> element MUST be treated as
   opaque by entities; no semantic meaning may be derived from it, and
   only exact comparisons may be made against it.  The <thread/> element
   MUST NOT contain mixed content (as defined in Section 3.2.2 of
   [XML]).

2.2 Presence Syntax

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

2.2.1 Types of Presence

   The 'type' attribute of a presence stanza is OPTIONAL.  A presence
   stanza that does not possess a 'type' attribute is used to signal to



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   the server that the sender is online and available for communication.
   If included, the 'type' attribute specifies a lack of availability, a
   request to manage a subscription to another entity's presence, a
   request for another entity's current presence, or an error related to
   a previously-sent presence stanza.  If included, the 'type' attribute
   MUST have one of the following values:

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

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

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

   o  unsubscribe -- A notification that an entity is unsubscribing from
      another entity's presence.

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

   o  probe -- A request for an entity's current presence; SHOULD be
      generated only by a server and SHOULD NOT be generated by a
      client.

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

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

2.2.2 Child Elements

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

   In accordance with the default namespace declaration, by default a
   presence stanza is in the 'jabber:client' or 'jabber:server'
   namespace, which defines certain allowable children of presence
   stanzas.  If the presence stanza is of type "error", it MUST include
   an <error/> child; for details, see [XMPP-CORE].  If the presence
   stanza possesses no 'type' attribute, it MAY contain any of the
   following child elements (note that the <status/> child MAY be sent
   in a presence stanza of type "unavailable" or, for historical
   reasons, "subscribe"):



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   1.  <show/>

   2.  <status/>

   3.  <priority/>


2.2.2.1 Show

   The OPTIONAL <show/> element contains XML character data that
   specifies the particular availability status of an entity or specific
   resource.  A presence stanza MUST NOT contain more than one <show/>
   element.  The <show/> element MUST NOT possess any attributes.  The
   XML character data contained in the <show/> element is not intended
   to be human-readable.  If provided, the CDATA value MUST be one of
   the following (additional availability types could be defined through
   a properly-namespaced child element of the presence stanza):

   o  away -- The entity or resource is temporarily away.

   o  chat -- The entity or resource is actively interested in chatting.

   o  xa -- The entity or resource is away for an extended period (xa =
      "eXtended Away").

   o  dnd -- The entity or resource is busy (dnd = "Do Not Disturb").

   If no <show/> element is provided, the entity is assumed to be online
   and available.

2.2.2.2 Status

   The OPTIONAL <status/> element contains a natural-language
   description of availability status.  It is normally used in
   conjunction with the show element to provide a detailed description
   of an availability state (e.g., "In a meeting").  The <status/>
   element MUST NOT possess any attributes, with the exception of the
   'xml:lang' attribute.  Multiple instances of the <status/> element
   MAY be included but only if each instance possesses an 'xml:lang'
   attribute with a distinct language value.

2.2.2.3 Priority

   The OPTIONAL <priority/> element contains XML character data that
   specifies the priority level of the resource.  The value may be any
   integer between -128 and +127.  A presence stanza MUST NOT contain
   more than one <priority/> element.  The <priority/> element MUST NOT
   possess any attributes.  If no priority is provided, a server SHOULD



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

2.3 IQ Syntax

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

2.4 Extended Namespaces

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

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

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

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



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

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

   Router: If a routing entity (usually a server) handles a stanza that
      contains a child element it does not understand, it SHOULD ignore
      the associated XML data by passing it on untouched to the
      recipient.


3. Session Establishment

   Most instant messaging and presence applications based on XMPP are
   implemented via a client-server architecture that requires a client
   to establish a session on a server in order to engage in the expected
   instant messaging and presence activities.  However, there are
   several pre-conditions that MUST be met before a client can establish
   an instant messaging and presence session.  These are:

   1.  Stream Authentication -- a client MUST complete stream
       authentication as documented in [XMPP-CORE] before attempting to
       establish a session or send any XML stanzas.

   2.  Resource Binding -- after completing stream authentication, a
       client MUST bind a resource to the stream so that the client's
       address is of the form <user@domain/resource>, after which the
       entity is said to be a "connected resource" in the terminology of
       [XMPP-CORE].

   If a server supports sessions, it MUST include a <session/> element
   qualified by the 'urn:ietf:params:xml:ns:xmpp-session' namespace in
   the stream features it advertises to a client after the completion of
   stream authentication as defined in [XMPP-CORE]:

   Server advertises session establishment feature to client:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='c2s_345'
       from='example.com'
       version='1.0'>
   <stream:features>



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     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'>
   </stream:features>

   Upon being so informed that session establishment is required (and
   after completing resource binding), the client MUST establish a
   session if it desires to engage in instant messaging and presence
   functionality; it completes this step by sending to the server an IQ
   stanza of type "set" containing an empty <session/> child element
   qualified by the 'urn:ietf:params:xml:ns:xmpp-session' namespace:

   Step 1: Client requests session with server:

   <iq from='someuser@example.com'
       to='example.com'
       type='set'
       id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
   </iq>

   Step 2: Server informs client that session has been created:

   <iq from='example.com'
       to='someuser@example.com'
       type='result'
       id='sess_1'/>

   Upon establishing a session, a connected resource (in the terminology
   of [XMPP-CORE]) is said to be an "active resource".

   Several error conditions are possible.  For example, the server may
   encounter an internal condition that prevents it from creating the
   session, the username or authorization identity may lack permissions
   to create a session, or there may already be an active resource
   associated with a resource identifier of the same name.

   If the server encounters an internal condition that prevents it from
   creating the session, it MUST return an error.

   Step 2 (alt): Server responds with error (internal server error):

   <iq type='error' id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
     <error type='wait'>
       <internal-server-error
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>



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   If the username or resource is not allowed to create a session, the
   server MUST return an error (e.g., forbidden).

   Step 2 (alt): Server responds with error (username or resource not
   allowed to create session):

   <iq type='error' id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
     <error type='auth'>
       <forbidden
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   If there is already an active resource of the same name, the server
   MUST either (1) terminate the active resource and allow the
   newly-requested session, or (2) disallow the newly-requested session
   and maintain the active resource.  Which of these the server does is
   up to the implementation, although it is RECOMMENDED to implement
   case #1.  In case #1, the server SHOULD send a <conflict/> stream
   error to the active resource, terminate the XML stream and underlying
   TCP connection for the active resource, and return a IQ stanza of
   type "result" (indicating success) to the newly-requested session. In
   case #2, the server SHOULD send a <conflict/> stanza error to the
   newly-requested session but maintain the XML stream for that
   connection so that the newly-requested session has an opportunity to
   negotiate a non-conflicting resource identifier before sending
   another request for session establishment.

   Step 2 (alt): Server informs active resource of resource conflict
   (case #1):

   <stream:error>
     <conflict xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>

   Step 2 (alt): Server informs newly-requested session of resource
   conflict (case #2):

   <iq type='error' id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
     <error type='cancel'>
       <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   After establishing a session, a client SHOULD send initial presence



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   and request its roster as described below, although these actions are
   NOT REQUIRED.

   Note: Before allowing the creation of instant messaging and presence
   sessions, a server MAY require prior account provisioning.  Possible
   methods for account provisioning include account creation by a server
   administrator as well as in-band account registration using the
   'jabber:iq:register' namespace; the latter method is documented by
   the Jabber Software Foundation [JSF] at <http://www.jabber.org/
   protocol/> but is out of scope for this memo.

4. Exchanging Messages

   Exchanging messages is a basic use of XMPP and is brought about when
   a user generates a message stanza that is addressed to another
   entity.  As defined under Server Rules for Handling XML Stanzas
   (Section 11), the sender's server is responsible for delivering the
   message to the intended recipient (if the recipient is on the same
   server) or for routing the message to the recipient's server (if the
   recipient is on a different server).

   For information regarding the syntax of message stanzas as well as
   their defined attributes and child elements, refer to Message Syntax
   (Section 2.1).

4.1 Specifying an Intended Recipient

   An instant messaging client SHOULD specify an intended recipient for
   a message by providing the JID of an entity other than the sender in
   the 'to' attribute of the <message/> stanza.  If the message is being
   sent in reply to a message previously received from an address of the
   form <user@domain/resource> (e.g., within the context of a chat
   session), the value of the 'to' address SHOULD be the full JID (of
   the form <user@domain/resource>) rather than merely of the form
   <user@domain> unless the sender has knowledge (via presence) that the
   intended recipient's resource is no longer available.  If the message
   is being sent outside the context of any existing chat session or
   received message, the value of the 'to' address SHOULD be of the form
   <user@domain> rather than of the form <user@domain/resource>.

4.2 Specifying a Message Type

   As noted, it is RECOMMENDED for a message stanza to possess a 'type'
   attribute whose value captures the conversational context (if any) of
   the message (see Type (Section 2.1.1)).

   The following example shows a valid value of the 'type' attribute:




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   Example: A message of a defined type:

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


4.3 Specifying a Message Body

   A message stanza MAY (and often will) contain a child <body/> element
   whose XML character data specifies the primary meaning of the message
   (see Body (Section 2.1.2.2)).

   Example: A message with a body:

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


4.4 Specifying a Message Subject

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

   Example: A message with a subject:

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




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4.5 Specifying a Conversation Thread

   A message stanza MAY contain a child <thread/> element specifying the
   conversation thread in which the message is situated, for the purpose
   of tracking the conversation (see Thread (Section 2.1.2.3)).

   Example: A threaded conversation:

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

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

   <message
       to='romeo@example.net/orchard'
       from='juliet@example.com/balcony'
       type='chat'>
     <body>How cam'st thou hither, tell me, and wherefore?</body>
     <thread>e0ffe42b28561960c6b12b944a092794b9683a38</thread>
   </message>


5. Exchanging Presence Information

   Exchanging presence information is made relatively straightforward
   within XMPP by using presence stanzas.  However, we see here a
   contrast to the handling of messages: although a client MAY send
   directed presence information to another entity, normally presence
   information is sent from a client to its server (with no 'to'
   address) and then broadcasted by the server to any entities that are
   subscribed to the presence of the sending entity (in the terminology
   of RFC 2778 [IMP-MODEL], we can say that the only watchers in XMPP
   are subscribers).  (Note: While presence information MAY be provided
   on a user's behalf by an automated service, normally it is provided
   by the user's client.)

   For information regarding the syntax of presence stanzas as well as



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   their defined attributes and child elements, refer to [XMPP-CORE].

5.1 Client and Server Presence Responsibilities

   After establishing a session, a client SHOULD (but is NOT REQUIRED
   to) send initial presence to the server in order to signal its
   availability for communications.  As defined herein, the initial
   presence stanza (1) MUST possess no 'to' address (signalling that it
   is meant to be handled by the server on behalf of the client) and (2)
   MUST possess no 'type' attribute (signalling the user's
   availability).  After sending initial presence, an active resource is
   said to be an "available resource".

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

   1.  Send presence probes (i.e., presence stanzas whose 'type'
       attribute is set to a value of "probe") from the full JID (e.g.,
       <user@example.com/resource>) of the user to the bare JID (e.g.,
       <contact@example.org>) of any contacts to which the user is
       subscribed in order to determine if they are available; such
       contacts are those which are present in the user's roster with
       the 'subscription' attribute set to a value of "to" or "both".

   2.  Broadcast initial presence from the full JID (e.g.,
       <user@example.com/resource>) of the user to the bare JID (e.g.,
       <contact@example.org>) of any contacts that are subscribed to the
       user's presence; such contacts are those which are present in the
       user's roster with the 'subscription' attribute set to a value of
       "from" or "both".

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

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

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



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       service, it MAY provide presence information about the user to
       that entity). Specifically:

       *  if the user is in the contact's roster with a subscription
          state of "None", "None + Pending Out", or "To", the contact's
          server MUST return a <forbidden/> stanza error in response to
          the presence probe.

       *  if the user is in the contact's roster with a subscription
          state of "None + Pending In", "None + Pending Out/In", or "To
          + Pending In", the contact's server MUST return a
          <not-authorized/> stanza error in response to the presence
          probe.

   2.  Else, if the contact is blocking presence notifications to the
       user's bare JID or full JID (using either a default list or
       active list as defined under Blocking Outbound Presence
       Notifications (Section 10.11)), the server MUST NOT reply to the
       presence probe.

   3.  Else, if the contact has no available resources, the server MUST
       either (1) reply to the presence probe by sending to the user the
       full XML of the last presence stanza of type "unavailable"
       received by the server from the contact, or (2) not reply at all.

   4.  Else, if the contact has at least one available resource, the
       server MUST reply to the presence probe by sending to the user
       the full XML of the last presence stanza received by the server
       from each of the contact's available resources (again, subject to
       privacy rules for each session).

   Upon receiving initial presence from the user, the contact's server
   MUST deliver the user's presence stanza to the full JIDs
   (<contact@example.org/resource>) associated with all of the contact's
   available resources, but only if the user is in the contact's roster
   with a subscription state of "to" or "both" and the contact has not
   blocked inbound presence notifications from the user's bare or full
   JID (as defined under Blocking Inbound Presence Notifications
   (Section 10.10)).

   If the user's server receives a presence stanza of type "error" in
   response to the initial presence that it sent to a contact on behalf
   of the user, it SHOULD NOT send further presence updates to that
   contact (until and unless it receives a presence stanza from the
   contact).

   After sending initial presence, the user MAY update its presence
   information for broadcasting at any time during its session by



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   sending a presence stanza with no 'to' address and either no 'type'
   attribute or a 'type' attribute with a value of "unavailable".
   (Note: A user's client SHOULD NOT send a presence update to broadcast
   information that changes independently of the user's presence and
   availability.) If the presence stanza lacks a 'type' attribute (i.e.,
   expresses availability), the user's server MUST broadcast the full
   XML of that presence stanza to all contacts (1) that are in the
   user's roster with a subscription type of "from" or "both", (2) to
   whom the user has not blocked outbound presence, and (3) from whom
   the server has not received a presence error during the user's
   session (as well as to any of the user's other available resources).
   If the presence stanza has a 'type' attribute set to a value of
   "unavailable", the user's server MUST broadcast the full XML of that
   presence stanza to all entities that fit the above description, as
   well as to any entities to which the user has sent directed available
   presence during the user's session (if the user has not yet sent
   directed unavailable presence to that entity).

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

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

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

   3.  If the user sends directed presence without first sending initial
       presence or after having sent unavailable presence broadcast



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       (i.e., the resource is active but not available), the user's
       server MUST treat the entities to which the user sends directed
       presence in the same way that it treats the entities listed in
       case 2 above.

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

5.2 Specifying Availability Status

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

   Example: Availability status:

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


5.3 Specifying Detailed Status Information

   In conjunction with the  <show/> element, a client MAY provide
   detailed status information by using the <status/> element (see
   Status (Section 2.2.2.2)).

   Example: Detailed status information:




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   <presence xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
     <status xml:lang='cz'>Ja dvo&#x0159;&#x00ED;m Juliet</status>
   </presence>


5.4 Specifying Presence Priority

   A client MAY provide a priority for its resource by using the
   <priority/> element (see Priority (Section 2.2.2.3)).

   Example: Presence priority:

   <presence xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
     <status xml:lang='cz'>Ja dvo&#x0159;&#x00ED;m Juliet</status>
     <priority>1</priority>
   </presence>


5.5 Presence Examples

   The examples in this section illustrate the presence-related
   protocols described above.  The user is romeo@example.net, he has an
   available resource whose resource identifier is "orchard", and he has
   the following individuals in his roster:

   o  juliet@example.com (subscription="both" and she has two available
      resources, one whose resource is "chamber" and another whose
      resource is "balcony")

   o  benvolio@example.org (subscription="to")

   o  mercutio@example.org (subscription="from")

   Example 1: User sends initial presence:

   <presence/>

   Example 2: User's server sends presence probes to contacts with
   subscription="to" and subscription="both" on behalf of the user's
   available resource:

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



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       to='juliet@example.com'/>

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

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

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

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

   Example 4: Contacts' server replies to presence probe on behalf of
   all of the contact's available resources:

   <presence
       from='juliet@example.com/balcony'
       to='romeo@example.net/orchard'
       xml:lang='en'>
     <show>away</show>
     <status>be right back</status>
     <priority>0</priority>
   </presence>

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

   <presence
       from='benvolio@example.org/pda'
       to='romeo@example.net/orchard'
       xml:lang='en'>
     <show>dnd</show>
     <status>gallivanting</status>
   </presence>

   Example 5: Contact's server delivers user's initial presence to all
   of the contact's available resources or returns error to user:




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

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

   <presence
       type='error'
       from='mercutio@example.org'
       to='romeo@example.net/orchard'>
     <error type='cancel'>
       <gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </presence>

   Example 6: User sends directed presence to another user not in his
   roster:

   <presence
       from='romeo@example.net/orchard'
       to='nurse@example.com'
       xml:lang='en'>
     <show>dnd</show>
     <status>courting Juliet</status>
     <priority>0</priority>
   </presence>

   Example 7: User sends updated available presence information for
   broadcasting:

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

   Example 8: Updated presence information is delivered only to one
   contact (not those from whom an error was received or to whom the
   user sent directed presence):

   <presence
       from='romeo@example.net/orchard'
       to='juliet@example.com/chamber'
       xml:lang='en'>
     <show>away</show>
     <status>I shall return!</status>



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     <priority>1</priority>
   </presence>

   <presence
       from='romeo@example.net/orchard'
       to='juliet@example.com/balcony'
       xml:lang='en'>
     <show>away</show>
     <status>I shall return!</status>
     <priority>1</priority>
   </presence>

   Example 9: One of the contact's resources sends final presence:

   <presence type='unavailable'/>

   Example 10: Contact's server sends unavailable presence information
   to user:

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

   Example 11: User sends final presence:

   <presence type='unavailable' xml:lang='en'>
     <status>gone home</status>
   </presence>

   Example 12: Unavailable presence information is delivered to
   contact's one remaining resource as well as to the person to whom the
   user sent directed presence:

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

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



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6. Managing Subscriptions

   In order to protect the privacy of instant messaging users and any
   other entities, presence and availability information is disclosed
   only to other entities that the user has approved.  When a user has
   agreed that another entity may view its presence, the entity is said
   to have a subscription to the user's presence information.  A
   subscription lasts across sessions; indeed, it lasts until the
   subscriber unsubscribes or the subscribee cancels the
   previously-granted subscription.  Subscriptions are managed within
   XMPP by sending presence stanzas containing specially-defined
   attributes.

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

6.1 Requesting a Subscription

   A request to subscribe to another entity's presence is made by
   sending a presence stanza of type "subscribe".

   Example: Sending a subscription request:

   <presence to='juliet@example.com' type='subscribe'/>

   If the subscription request is being sent to an instant messaging
   contact, the JID supplied in the 'to' attribute SHOULD be of the form
   <contact@example.org> rather than <contact@example.org/resource>,
   since the desired result is normally for the user to receive presence
   from all of the contact's resources, not merely the particular
   resource specified in the 'to' attribute.

   A user's server MUST NOT automatically approve subscription requests
   on the user's behalf.  All subscription requests MUST be directed to
   the user's client, specifically to one or more available resources
   associated with the user.  If there is no available resource
   associated with the user when the subscription request is received by
   the user's server, the user's server MUST keep a record of the
   subscription request and deliver the request when the user next
   creates an available resource, until the user either approves or
   denies the request.  If there is more than one available resource
   associated with the user when the subscription request is received by
   the user's server, the user's server MUST broadcast that subscription
   request to all available resources in accordance with Server Rules
   for Handling XML Stanzas (Section 11).  (Note: If an active resource
   has not provided initial presence, the server MUST NOT consider it to



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   be available and therefore MUST NOT send subscription requests to
   it.)  However, f the user receives a presence stanza of type
   "subscribe" from a contact to whom the user has already granted
   permission to see the user's presence (e.g., in cases when the
   contact is seeking to resynchronize subscription states), the user's
   server SHOULD auto-reply on behalf of the user.

6.2 Handling a Subscription Request

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

   Example: Approving a subscription request:

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

   Example: Refusing a presence subscription request:

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


6.3 Cancelling a Subscription from Another Entity

   If a user would like to cancel a previously-granted subscription
   request, it sends a presence stanza of type "unsubscribed".

   Example: Cancelling a previously granted subscription request:

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


6.4 Unsubscribing from Another Entity's Presence

   If a user would like to unsubscribe from the presence of another
   entity, it sends a presence stanza of type "unsubscribe".

   Example: Unsubscribing from an entity's presence:

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


7. Roster Management

   In XMPP, one's contact list is called a roster, which consists of any
   number of specific roster items, each roster item being identified by
   a unique JID (usually of the form <contact@example.org>).  A user's



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   roster is stored by the user's server on the user's behalf so that
   the user may access roster information from any resource.

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

7.1 Syntax and Semantics

   Rosters are managed using IQ stanzas, specifically by means of a
   <query/> child element qualified by the 'jabber:iq:roster' namespace.
   The <query/> element MAY contain one or more <item/> children, each
   describing a unique roster item or "contact".

   The "key" or unique identifier for each roster item is a JID,
   encapsulated in the required 'jid' attribute of the <item/> element.
   The value of the 'jid' attribute SHOULD be of the form <user@domain>,
   especially if the item is associated with another (human) instant
   messaging user.

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

   o  "none" -- the user does not have a subscription to the contact,
      and the contact does not have a subscription to the user

   o  "to" -- the user has a subscription to the contact, but the
      contact does not have a subscription to the user

   o  "from" -- the contact has a subscription to the user, but the user
      does not have a subscription to the contact

   o  "both" -- both the user and the contact have subscriptions to each
      other

   Each <item/> element MAY contain a 'name' attribute, which sets the
   "nickname" to be associated with the JID, as determined by the user
   (not the contact).  The value of the 'name' attribute is opaque.

   Each <item/> element MAY contain one or more <group/> child elements,
   for use in collecting roster items into various categories.  The
   CDATA text of the <group/> element is opaque.

7.2 Business Rules

   A server MUST ignore any 'to' address on a roster "set", and MUST



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

7.3 Retrieving One's Roster on Login

   Upon connecting to the server, a client SHOULD request the roster
   (however, because receiving the roster may not be desirable for all
   resources, e.g., a connection with limited bandwidth, the client's
   request for the roster is NOT REQUIRED).  If an available resource
   does not request the roster during a session, the server MUST NOT
   send it presence subscriptions and associated roster updates.

   Example: Client requests current roster from server:

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

   Example: Client receives roster from the server:

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







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7.4 Adding a Roster Item

   At any time, a user MAY add an item to his or her roster.

   Example: Client adds a new item:

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

   The server MUST update the roster information in persistent storage,
   and also push the change out to all of the user's available resources
   that have requested the roster.  This "roster push" consists of an IQ
   set from the server to the client and enables all available resources
   to remain in sync with the server-based roster information.

   Example: Server (1) pushes the updated roster information to all
   available resources that have requested the roster and (2) replies
   with an IQ result to the sending resource:

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

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

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





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   Example: Connected resources reply with an IQ result to the server:

   <iq from='juliet@example.com/balcony'
       to='example.com'
       type='result'/>
   <iq from='juliet@example.com/chamber'
       to='example.com'
       type='result'/>


7.5 Updating a Roster Item

   Updating an existing roster item (e.g., changing the group) is done
   in the same way as adding a new roster item, i.e., by sending the
   roster item in an IQ set to the server.

   Example: User updates roster item (added group):

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

   As with adding a roster item, when updating a roster item the server
   MUST update the roster information in persistent storage, and also
   initiate a roster push to all of the user's available resources that
   have requested the roster.

7.6 Deleting a Roster Item

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

   Example: Client removes an item:

   <iq from='juliet@example.com/balcony' type='set' id='roster_4'>
     <query xmlns='jabber:iq:roster'>
       <item jid='nurse@example.com' subscription='remove'/>
     </query>
   </iq>



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

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

8. Integration of Roster Items and Presence Subscriptions

8.1 Overview

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

   There are four primary subscription states:

   o  None -- Neither the user nor the contact is subscribed to the
      other's presence

   o  To -- The user is subscribed to the contact's presence but there
      is no subscription from the contact to the user

   o  From -- There is a subscription from the contact to the user, but
      the user has not subscribed to the contact's presence

   o  Both -- Both the user and the contact are subscribed to each
      other's presence (i.e., the union of 'from' and 'to')

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

   If an active resource does not both send initial presence and request
   the roster, the server MUST NOT send it presence subscription
   requests or roster pushes.

   The 'from' and 'to' addresses are OPTIONAL in roster pushes; if



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

8.2 User Subscribes to Contact

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

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

   <iq type='set' id='set1'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   2.  As a result, the user's server (1) MUST initiate a roster push
       for the new roster item to all available resources associated
       with this user that have requested the roster, setting the
       'subscription' attribute to a value of "none"; and (2) MUST reply
       with an IQ result related to the roster set:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='none'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>



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   <iq type='result' id='set1'/>

   3.  If the user wants to request a subscription to the contact's
       presence, the user's client MUST send a presence stanza of
       type='subscribe' to the contact:

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

   4.  As a result, the user's server MUST initiate a second roster push
       to all of the user's available resources that have requested the
       roster, setting the contact to the pending sub-state of the
       'none' subscription state; this pending sub-state is denoted by
       the inclusion of the ask='subscribe' attribute in the roster
       item:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='none'
           ask='subscribe'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

       Note: If the user did not create a roster item before sending the
       subscription request, the server MUST now create one on behalf of
       the user with the 'subscription' attribute set to a value of
       "none", then send a roster push to all of the user's available
       resources that have requested the roster, absent the 'name'
       attribute and the <group/> child.

   5.  The user's server MUST also stamp the presence stanza of type
       "subscribe" with the user's bare JID (i.e., <user@example.com>)
       as the 'from' address.  If the contact is served by a different
       host than the user, the user's server MUST route the presence
       stanza to the contact's server for delivery to the contact (this
       case is assumed throughout; however, if the contact is served by
       the same host, then the server can simply deliver the presence
       stanza directly):

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='subscribe'/>




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       Note: If the user's server receives a presence stanza of type
       "error" from the contact's server in response to the outgoing
       presence stanza of type "subscribe", it MUST revert the
       subscription state from the new state ("None + Pending Out") to
       the previous state ("None"), and MUST deliver the error stanza to
       the user.

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

   <iq type='set' id='set2'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence to='user@example.com' type='subscribed'/>

   7.  As a result, the contact's server (1) MUST initiate a roster push
       to all available resources associated with the contact that have
       requested the roster, containing a roster item for the user with
       the subscription state set to 'from' (the server MUST send this
       even if the contact did not perform a roster set); (2) MUST
       return an IQ result related to the roster set; (3) MUST route the
       presence stanza of type "subscribed" to the user; and (4) MUST



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       send available presence from all of the contact's available
       resources to the user:

   <iq type='set' to='contact@example.org/resource'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='from'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <iq type='result' to='contact@example.org/resource' id='set2'/>

   <presence
       from='contact@example.org/resource'
       to='user@example.com'
       type='subscribed'/>

   <presence
       from='contact@example.org/resource'
       to='user@example.com'/>

       Note: If the contact's server receives a presence stanza of type
       "error" from the user's server in response to the outgoing
       presence stanza of type "subscribed", it MUST revert the
       subscription state from the new state ("From") to the previous
       state ("None + Pending In"), and MUST deliver the error stanza to
       the contact.

   8.  Upon receiving the presence stanza of type "subscribed" addressed
       to the user, the user's server MUST first verify that the contact
       is in the user's roster with either of the following states: (a)
       subscription='none' and ask='subscribe' or (b)
       subscription='from' and ask='subscribe'.  If the contact is not
       in the user's roster with either of those states, the user's
       server MUST silently ignore the presence stanza of type
       "subscribed" (i.e., it MUST NOT route it to the user, modify the
       user's roster, or generate a roster push to the user's available
       resources).  If the contact is in the user's roster with either
       of those states, the user's server (1) MUST deliver the presence
       stanza of type "subscribed" from the contact to the user; (2)
       MUST initiate a roster push to all of the user's available
       resources that have requested the roster, containing an updated
       roster item for the contact with the 'subscription' attribute set
       to a value of "to"; and (3) MUST deliver the available presence



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       stanza received from each of the contact's available resources to
       each of the user's available resources:

   <presence
       to='user@example.com'
       from='contact@example.org'
       type='subscribed'/>

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='to'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <presence
       from='contact@example.org/resource'
       to='user@example.com/resource'/>

   9.  Upon receiving the presence stanza of type "subscribed", the user
       SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "subscribe" to the contact or "denying" it by
       sending a presence stanza of type "unsubscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).

   From the perspective of the user, there now exists a subscription to
   the contact; from the perspective of the contact, there now exists a
   subscription from the user.  (Note: If at this point the user sends
   another subscription request to the contact, the user's server MUST
   silently ignore that request.)

8.2.1 Alternate Flow: Contact Declines Subscription Request

   The above activity flow represents the "happy path" related to the
   user's subscription request to the contact.  The main alternate flow
   occurs if the contact refuses the user's subscription request.

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



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       6 of Section 8.2):

   <presence to='user@example.com' type='unsubscribed'/>

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

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

       Note: if the contact's server previously added the user to the
       contact's roster for tracking purposes, it MUST remove the
       relevant item at this time.

   3.  Upon receiving the presence stanza of type "unsubscribed"
       addressed to the user, the user's server (1) MUST deliver that
       presence stanza to the user and (2) MUST initiate a roster push
       to all of the user's available resources that have requested the
       roster, containing an updated roster item for the contact with
       the 'subscription' attribute set to a value of "none" and with no
       'ask' attribute:

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

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='none'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   4.  Upon receiving the presence stanza of type "unsubscribed", the
       user SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the contact or "denying" it by
       sending a presence stanza of type "subscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets



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       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).

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

8.3 Creating a Mutual Subscription

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

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

   <presence to='user@example.com' type='subscribe'/>

   2.  As a result, the contact's server (1) MUST initiate a roster push
       to all available resources associated with the contact that have
       requested the roster, with the user still in the 'from'
       subscription state but with a pending 'to' subscription denoted
       by the inclusion of the ask='subscribe' attribute in the roster
       item; and (2) MUST route the presence stanza of type "subscribe"
       to the user, first stamping the 'from' address as the bare JID
       (<contact@example.org>) of the contact:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='from'
           ask='subscribe'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='subscribe'/>

       Note: If the contact's server receives a presence stanza of type
       "error" from the user's server in response to the outgoing



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       presence stanza of type "subscribe", it MUST revert the
       subscription state from the new state ("From + Pending Out") to
       the previous state ("From"), and MUST deliver the error stanza to
       the contact.

   3.  Upon receiving the presence stanza of type "subscribe" addressed
       to the user, the user's server must determine if there is at
       least one available resource for which the user has requested the
       roster.  If so, the user's server MUST deliver the subscription
       request to the user (if not, it MUST store the subscription
       request offline for delivery when this condition is next met).
       No matter when the subscription request is delivered, the user
       must then decide whether or not to approve it (subject to
       configured preferences, the user's client MAY approve or refuse
       the subscription request without presenting it to the user).
       Here we assume the "happy path" that the user approves the
       subscription request (the alternate flow of declining the
       subscription request is defined in Section 8.3.1).  In this case,
       the user's client MUST send a presence stanza of type
       "subscribed" to the contact in order to approve the subscription
       request.

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

   4.  As a result, the user's server (1) MUST initiate a roster push to
       all of the user's available resources that have requested the
       roster, containing a roster item for the contact with the
       'subscription' attribute set to a value of "both"; (2) MUST route
       the presence stanza of type "subscribed" to the contact, first
       stamping the 'from' address as the bare JID (<user@example.com>)
       of the user; and (3) MUST send available presence from each of
       the user's available resources to the contact:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='both'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='subscribed'/>



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   <presence
       from='user@example.com/resource'
       to='contact@example.org'/>

       Note: If the user's server receives a presence stanza of type
       "error" from the contact's server in response to the outgoing
       presence stanza of type "subscribed", it MUST revert the
       subscription state from the new state ("Both") to the previous
       state ("To + Pending In"), and MUST deliver the error stanza to
       the user.

   5.  Upon receiving the presence stanza of type "subscribed" addressed
       to the contact, the contact's server MUST first verify that the
       user is in the contact's roster with either of the following
       states: (a) subscription='none' and ask='subscribe' or (b)
       subscription='from' and ask='subscribe'.  If the user is not in
       the contact's roster with either of those states, the contact's
       server MUST silently ignore the presence stanza of type
       "subscribed" (i.e., it MUST NOT route it to the contact, modify
       the contact's roster, or generate a roster push to the contact's
       available resources).  If the user is in the contact's roster
       with either of those states, the contact's server (1) MUST
       deliver the presence stanza of type "subscribed" from the user to
       the contact; (2) MUST initiate a roster push to all available
       resources associated with the contact that have requested the
       roster, containing an updated roster item for the user with the
       'subscription' attribute set to a value of "both"; and (3) MUST
       deliver the available presence stanza received from each of the
       user's available resources to each of the contact's available
       resources:

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='subscribed'/>

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='both'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence



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       from='user@example.com/resource'
       to='contact@example.org/resource'/>

   6.  Upon receiving the presence stanza of type "subscribed", the
       contact SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "subscribe" to the user or "denying" it by sending
       a presence stanza of type "unsubscribe" to the user; this step
       does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the contact's server know that it MUST no longer send
       notification of the subscription state change to the contact (see
       Section 9.4).

   The user and the contact now have a mutual subscription to each
   other's presence -- i.e., the subscription is of type "both".  The
   user's server MUST now send the user's current presence information
   to the contact.  (Note: If at this point the user sends a
   subscription request to the contact or the contact sends a
   subscription request to the user, the sending user's server MUST
   silently ignore that request and not route it to the intended
   recipient.)

8.3.1 Alternate Flow: User Declines Subscription Request

   The above activity flow represents the "happy path" related to the
   contact's subscription request to the user.  The main alternate flow
   occurs if the user refuses the contact's subscription request.

   1.  If the user wants to refuse the request, the user's client MUST
       send a presence stanza of type "unsubscribed" to the contact
       (instead of the presence stanza of type "subscribed" sent in Step
       3 of Section 8.3):

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

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

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

   3.  Upon receiving the presence stanza of type "unsubscribed"
       addressed to the contact, the contact's server (1) MUST deliver
       that presence stanza to the contact; and (2) MUST initiate a



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       roster push to all available resources associated with the
       contact that have requested the roster, containing an updated
       roster item for the user with the 'subscription' attribute set to
       a value of "from" and with no 'ask' attribute:

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

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='from'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   4.  Upon receiving the presence stanza of type "unsubscribed", the
       contact SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the user or "denying" it by
       sending a presence stanza of type "subscribe" to the user; this
       step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the contact's server know that it MUST no longer send
       notification of the subscription state change to the contact (see
       Section 9.4).

   As a result of this activity, there has been no change in the
   subscription state; i.e., the contact is in the user's roster with a
   subscription state of "to" and the user is in the contact's roster
   with a subscription state of "from".

8.4 Unsubscribing

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

8.4.1 Case #1: Unsubscribing When Subscription is Not Mutual




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   In the first case, the user has a subscription to the contact but the
   contact does not have a subscription to the user (i.e., the
   subscription is not yet mutual).

   1.  If the user wants to unsubscribe from the contact's presence, the
       user MUST send a presence stanza of type "unsubscribe" to the
       contact:

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

   2.  As a result, the user's server (1) MUST send a roster push to all
       of the user's available resources that have requested the roster,
       containing an updated roster item for the contact with the
       'subscription' attribute set to a value of "none"; and (2) MUST
       route the presence stanza of type "unsubscribe" to the contact,
       first stamping the 'from' address as the bare JID
       (<user@example.com>) of the user:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='none'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>

   3.  Upon receiving the presence stanza of type "unsubscribe"
       addressed to the contact, the contact's server (1) MUST initiate
       a roster push to all available resources associated with the
       contact that have requested the roster, containing an updated
       roster item for the user with the 'subscription' attribute set to
       a value of "none" (if the contact is offline, the contact's
       server MUST modify the roster item and send that modified item
       the next time the contact requests the roster); and (2) MUST
       deliver the "unsubscribe" state change notification to the
       contact:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item



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           jid='user@example.com'
           subscription='none'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>

   4.  Upon receiving the presence stanza of type "unsubscribe", the
       contact SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribed" to the user or "denying" it by
       sending a presence stanza of type "subscribed" to the user; this
       step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the contact's server know that it MUST no longer send
       notification of the subscription state change to the contact (see
       Section 9.4).

   5.  The contact's server then (1) MUST send a presence stanza of type
       "unsubscribed" to the user; and (2) SHOULD send unavailable
       presence from the contact to the user:

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

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unavailable'/>

   6.  When the user's server receives a presence stanza of type
       "unsubscribed" and/or unavailable presence, it MUST deliver them
       to the user:

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

   <presence



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       from='contact@example.org'
       to='user@example.com'
       type='unavailable'/>

   7.  Upon receiving the presence stanza of type "unsubscribed", the
       user SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the contact or "denying" it by
       sending a presence stanza of type "subscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).


8.4.2 Case #2: Unsubscribing When Subscription is Mutual

   In the second case, the user has a subscription to the contact and
   the contact also has a subscription to the user (i.e., the
   subscription is mutual).

   1.  If the user wants to unsubscribe from the contact's presence, the
       user MUST send a presence stanza of type "unsubscribe" to the
       contact:

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

   2.  As a result, the user's server (1) MUST send a roster push to all
       of the user's available resources that have requested the roster,
       containing an updated roster item for the contact with the
       'subscription' attribute set to a value of "from"; and (2) MUST
       route the presence stanza of type "unsubscribe" to the contact,
       first stamping the 'from' address as the bare JID
       (<user@example.com>) of the user:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='from'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'



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       to='contact@example.org'
       type='unsubscribe'/>

   3.  Upon receiving the presence stanza of type "unsubscribe"
       addressed to the contact, the contact's server (1) MUST initiate
       a roster push to all available resources associated with the
       contact that have requested the roster, containing an updated
       roster item for the user with the 'subscription' attribute set to
       a value of "to" (if the contact is offline, the contact's server
       MUST modify the roster item and send that modified item the next
       time the contact requests the roster); and (2) MUST deliver the
       "unsubscribe" state change notification to the contact:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='to'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>

   4.  Upon receiving the presence stanza of type "unsubscribe", the
       contact SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribed" to the user or "denying" it by
       sending a presence stanza of type "subscribed" to the user; this
       step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the contact's server know that it MUST no longer send
       notification of the subscription state change to the contact (see
       Section 9.4).

   5.  The contact's server then (1) MUST send a presence stanza of type
       "unsubscribed" to the user; and (2) SHOULD send unavailable
       presence from the contact to the user:

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



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   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unavailable'/>

   6.  When the user's server receives a presence stanza of type
       "unsubscribed" and/or unavailable presence, it MUST deliver them
       to the user:

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

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unavailable'/>

   7.  Upon receiving the presence stanza of type "unsubscribed", the
       user SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the contact or "denying" it by
       sending a presence stanza of type "subscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).

   Note: Obviously this does not result in removal of the roster item
   from the user's roster, and the contact still has a subscription to
   the user's presence.  In order to both completely cancel a mutual
   subscription and fully remove the roster item from the user's roster,
   the user SHOULD update the roster item with subscription='remove' as
   defined under Removing a Roster Item and Cancelling All Subscriptions
   (Section 8.6).

8.5 Cancelling a Subscription

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

8.5.1 Case #1: Cancelling When Subscription is Not Mutual




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   In the first case, the user has a subscription to the contact but the
   contact does not have a subscription to the user (i.e., the
   subscription is not yet mutual).

   1.  If the contact wants to cancel the user's subscription, the
       contact MUST send a presence stanza of type "unsubscribed" to the
       user:

   <presence to='user@example.com' type='unsubscribed'/>

   2.  As a result, the contact's server (1) MUST send a roster push to
       all of the contact's available resources that have requested the
       roster, containing an updated roster item for the user with the
       'subscription' attribute set to a value of "none"; (2) MUST route
       the presence stanza of type "unsubscribed" to the user, first
       stamping the 'from' address as the bare JID
       (<contact@example.org>) of the contact; and (3) SHOULD send
       unavailable presence from the contact to the user:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='none'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

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

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unavailable'/>

   3.  Upon receiving the presence stanza of type "unsubscribed"
       addressed to the user, the user's server (1) MUST initiate a
       roster push to all of the user's available resources that have
       requested the roster, containing an updated roster item for the
       contact with the 'subscription' attribute set to a value of
       "none" (if the user is offline, the user's server MUST modify the
       roster item and send that modified item the next time the user
       requests the roster); (2) MUST deliver the "unsubscribed" state



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       change notification to the user; and (3) MUST deliver the
       unavailable presence to the user:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='none'
           name='MyContact'>
         <group>MyBuddies</group>
       </item>
     </query>
   </iq>

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

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unavailable'/>

   4.  Upon receiving the presence stanza of type "unsubscribed", the
       user SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the contact or "denying" it by
       sending a presence stanza of type "subscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).


8.5.2 Case #2: Cancelling When Subscription is Mutual

   In the second case, the user has a subscription to the contact and
   the contact also has a subscription to the user (i.e., the
   subscription is mutual).

   1.  If the contact wants to cancel the user's subscription, the
       contact MUST send a presence stanza of type "unsubscribed" to the
       user:

   <presence to='user@example.com' type='unsubscribed'/>





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   2.  As a result, the contact's server (1) MUST send a roster push to
       all of the contact's available resources that have requested the
       roster, containing an updated roster item for the user with the
       'subscription' attribute set to a value of "to"; (2) MUST route
       the presence stanza of type "unsubscribed" to the user, first
       stamping the 'from' address as the bare JID
       (<contact@example.org>) of the contact; and (3) SHOULD send
       unavailable presence from the contact to the user:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='to'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

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

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unavailable'/>

   3.  Upon receiving the presence stanza of type "unsubscribed"
       addressed to the user, the user's server (1) MUST initiate a
       roster push to all of the user's available resources that have
       requested the roster, containing an updated roster item for the
       contact with the 'subscription' attribute set to a value of
       "from" (if the user is offline, the user's server MUST modify the
       roster item and send that modified item the next time the user
       requests the roster); and (2) MUST deliver the "unsubscribed"
       state change notification to the user; and (3) MUST deliver the
       unavailable presence to the user:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='from'
           name='MyContact'>
         <group>MyBuddies</group>



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       </item>
     </query>
   </iq>

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

   <presence
       from='contact@example.org'
       to='user@example.com'
       type='unavailable'/>

   4.  Upon receiving the presence stanza of type "unsubscribed", the
       user SHOULD acknowledge receipt of that subscription state
       notification through either "affirming" it by sending a presence
       stanza of type "unsubscribe" to the contact or "denying" it by
       sending a presence stanza of type "subscribe" to the contact;
       this step does not necessarily affect the subscription state (see
       Subscription States (Section 9) for details), but instead lets
       the user's server know that it MUST no longer send notification
       of the subscription state change to the user (see Section 9.4).

   Note: Obviously this does not result in removal of the roster item
   from the contact's roster, and the contact still has a subscription
   to the user's presence.  In order to both completely cancel a mutual
   subscription and fully remove the roster item from the contact's
   roster, the contact should update the roster item with
   subscription='remove' as defined under Removing a Roster Item and
   Cancelling All Subscriptions (Section 8.6).

8.6 Removing a Roster Item and Cancelling All Subscriptions

   Because there may be many steps involved in completely removing a
   roster item and cancelling subscriptions in both directions, the
   roster management protocol includes a "shortcut" method for doing so.
   The process may be initiated no matter what the current subscription
   state is by sending a roster set containing an item for the contact
   with the 'subscription' attribute set to a value of "remove":

   <iq type='set' id='remove1'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='remove'/>
     </query>
   </iq>



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   When the user removes a contact from his or her roster by setting the
   'subscription' attribute to a value of "remove", the user's server
   (1) MUST automatically cancel any existing presence subscription
   between the user and the contact (both 'to' and 'from' as
   appropriate); (2) MUST remove the roster item from the user's roster
   and inform all of the user's available resources that have requested
   the roster of the roster item removal; (3) MUST inform the resource
   that initiated the removal of success; and (4) SHOULD send
   unavailable presence to the contact:

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>

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

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='contact@example.org'
           subscription='remove'/>
     </query>
   </iq>

   <iq type='result' id='remove1'/>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unavailable'/>

   Upon receiving the presence stanza of type "unsubscribe", the
   contact's server (1) MUST initiate a roster push to all available
   resources associated with the contact that have requested the roster,
   containing an updated roster item for the user with the
   'subscription' attribute set to a value of "to" (if the contact is
   offline, the contact's server MUST modify the roster item and send
   that modified item the next time the contact requests the roster);
   and (2) MUST also deliver the "unsubscribe" state change notification
   to the contact:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item



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           jid='user@example.com'
           subscription='to'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unsubscribe'/>

   Upon receiving the presence stanza of type "unsubscribed", the
   contact's server (1) MUST initiate a roster push to all available
   resources associated with the contact that have requested the roster,
   containing an updated roster item for the user with the
   'subscription' attribute set to a value of "none" (if the contact is
   offline, the contact's server MUST modify the roster item and send
   that modified item the next time the contact requests the roster);
   and (2) MUST also deliver the "unsubscribe" state change notification
   to the contact:

   <iq type='set'>
     <query xmlns='jabber:iq:roster'>
       <item
           jid='user@example.com'
           subscription='none'
           name='SomeUser'>
         <group>SomeGroup</group>
       </item>
     </query>
   </iq>

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

   Upon receiving the presence stanza of type "unavailable" addressed to
   the contact, the contact's server MUST deliver the unavailable
   presence to the user:

   <presence
       from='user@example.com'
       to='contact@example.org'
       type='unavailable'/>




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   Note that when the user removes the contact from the user's roster,
   the end state of the contact's roster is that the user is still in
   the contact's roster with a subscription state of "none"; in order to
   completely remove the roster item for the user, the contact needs to
   also send a roster removal request.

9. Subscription States

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

9.1 Defined States

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

   1.  "None" = contact and user are not subscribed to each other, and
       neither has requested a subscription from the other

   2.  "None + Pending Out" = contact and user are not subscribed to
       each other, and user has sent contact a subscription request but
       contact has not replied yet

   3.  "None + Pending In" = contact and user are not subscribed to each
       other, and contact has sent user a subscription request but user
       has not replied yet (note: contact's server SHOULD NOT push or
       deliver roster items in this state, but instead SHOULD wait until
       contact has approved subscription request from user)

   4.  "None + Pending Out/In" = contact and user are not subscribed to
       each other, contact has sent user a subscription request but user
       has not replied yet, and user has sent contact a subscription
       request but contact has not replied yet

   5.  "To" = user is subscribed to contact (one-way)

   6.  "To + Pending In" = user is subscribed to contact, and contact
       has send user a subscription request but user has not replied yet

   7.  "From" = contact is subscribed to user (one-way)

   8.  "From + Pending Out" = contact is subscribed to user, and user
       has sent contact a subscription request but contact has not
       replied yet

   9.  "Both" = user and contact are subscribed to each other (two-way)



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9.2 Server Handling of Outbound Presence Subscription Stanzas

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

   Because it is possible for the user's server and the contact's server
   to lose synchronization regarding subscription states, the user's
   server MUST route all outbound presence stanzas of type "subscribe"
   or "unsubscribe" to the contact so that the user is able to
   resynchronize his or her subscription to the contact if needed.

   The user's server SHOULD NOT route a presence stanza of type
   "subscribed" or "unsubscribed" to the contact if the stanza does not
   result in a subscription state change from the user's perspective,
   and MUST NOT make a state change.  If the stanza results in a
   subscription state change, the user's server MUST route the stanza to
   the contact and MUST make the appropriate state change.  These rules
   are summarized in the following tables.

   Table 1: Recommended handling of outbound "subscribed" stanzas

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

   Table 2: Recommended handling of outbound "unsubscribed" stanzas

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



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   |  "To + Pending In"       |  yes     |  "To"                    |
   |  "From"                  |  yes     |  "None"                  |
   |  "From + Pending Out"    |  yes     |  "None + Pending Out"    |
   |  "Both"                  |  yes     |  "To"                    |
   +----------------------------------------------------------------+


9.3 Server Handling of Inbound Presence Subscription Stanzas

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

   When the user's server receives a subscription request for the user
   from the contact (i.e., a presence stanza of type "subscribe"), it
   MUST deliver that request to the user for approval if the user has
   not already granted the contact access to the user's presence
   information and if there is no pending inbound subscription request;
   however, the user's server SHOULD NOT deliver the new request if
   there is a pending inbound subscription request, since the previous
   subscription request will have been recorded.  If the user has
   already granted the contact access to the user's presence
   information, the user's server SHOULD auto-reply to an inbound
   presense stanza of type "subscribe" from the contact by sending a
   presence stanza of type "subscribed" to the contact on behalf of the
   user; this rule enables the contact to resynchronize the subscription
   state if needed.  These rules are summarized in the following table.

   Table 3: Recommended handling of inbound "subscribe" stanzas

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

   * Server SHOULD auto-reply with "subscribed" stanza



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   When the user's server receives a presence stanza of type
   "unsubscribe" for the user from the contact, if the stanza results in
   a subscription state change from the user's perspective then the
   user's server SHOULD auto-reply by sending a presence stanza of type
   "unsubscribed" to the contact on behalf of the user, MUST deliver the
   "unsubscribe" stanza to the user, and MUST change the state.  If no
   subscription state change results, the user's server SHOULD NOT
   deliver the stanza and MUST NOT change the state.  These rules are
   summarized in the following table.

   Table 4: Recommended handling of inbound "unsubscribe" stanzas

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

   * Server SHOULD auto-reply with "unsubscribed" stanza

   When the user's server receives a presence stanza of type
   "subscribed" for the user from the contact, it MUST NOT deliver the
   stanza to the user and MUST NOT change the subscription state if
   there is no pending outbound request for access to the contact's
   presence information.  If there is a pending outbound request for
   access to the contact's presence information and the inbound presence
   stanza of type "subscribed" results in a subscription state change,
   the user's server MUST deliver the stanza to the user and MUST change
   the subscription state.  If the user already has access to the
   contact's presence information, the inbound presence stanza of type
   "subscribed" does not result in a subscription state change;
   therefore the user's server SHOULD NOT deliver the stanza to the user
   and MUST NOT change the subscription state. These rules are
   summarized in the following table.

   Table 5: Recommended handling of inbound "subscribed" stanzas

   +------------------------------------------------------------------+
   |  EXISTING STATE          |  DELIVER?  |  NEW STATE               |
   +------------------------------------------------------------------+



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   |  "None"                  |  no        |  no state change         |
   |  "None + Pending Out"    |  yes       |  "To"                    |
   |  "None + Pending In"     |  no        |  no state change         |
   |  "None + Pending Out/In" |  yes       |  "To + Pending In"       |
   |  "To"                    |  no        |  no state change         |
   |  "To + Pending In"       |  no        |  no state change         |
   |  "From"                  |  no        |  no state change         |
   |  "From + Pending Out"    |  yes       |  "Both"                  |
   |  "Both"                  |  no        |  no state change         |
   +------------------------------------------------------------------+

   When the user's server receives a presence stanza of type
   "unsubscribed" for the user from the contact, it MUST deliver the
   stanza to the user and MUST change the subscription state if there is
   a pending outbound request for access to the contact's presence
   information or if the user currently has access to the contact's
   presence information.  Otherwise, the user's server SHOULD NOT
   deliver the stanza and MUST NOT change the subscription state.  These
   rules are summarized in the following table.

   Table 6: Recommended handling of inbound "unsubscribed" stanzas

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


9.4 Server Delivery and Client Acknowledgement of Subscription State
    Change Notifications

   When a server receives an inbound presence stanza of type
   "subscribe", "subscribed", "unsubscribe", or "unsubscribed" that
   consists of a subscription state change request or notification, in
   addition to sending the appropriate roster push (or updated roster
   when the roster is next requested), it MUST deliver the request or
   notification to the intended recipient at least once.  A server MAY
   require the recipient to acknowledge receipt of all state change
   notifications (and MUST require acknowledgement in the case of



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   subscription requests, i.e., presence stanzas of type "subscribe").
   In order to require acknowledgement, a server SHOULD send the
   notification to the recipient each time the recipient logs in, until
   the recipient acknowledges receipt of the notification by "affirming"
   or "denying" the notification, as shown in the following table:

   Table 7: Acknowledgement of subscription state change notifications

   +--------------------------------------------------+
   |  STANZA TYPE   |  ACCEPT        |  DENY          |
   +--------------------------------------------------+
   |  subscribe     |  subscribed    |  unsubscribed  |
   |  subscribed    |  subscribe     |  unsubscribe   |
   |  unsubscribe   |  unsubscribed  |  subscribed    |
   |  unsubscribed  |  unsubscribe   |  subscribe     |
   +--------------------------------------------------+

   Obviously, given the foregoing subscription state charts, some of the
   acknowledgement stanzas will be routed to the contact and result in
   subscription state changes, while others will not.  However, any such
   stanzas MUST result in the server's no longer sending the
   subscription state notification to the user.

   Because a user's server MUST automatically generate outbound presence
   stanzas of type "unsubscribe" and "unsubscribed" upon receiving a
   roster set with the 'subscription' attribute set to a value of
   "remove" (see Removing a Roster Item and Cancelling All Subscriptions
   (Section 8.6)), the server MUST treat a roster remove request as
   equivalent to sending both of those presence stanzas for purposes of
   determining whether to continue sending subscription state change
   notifications of type "subscribe" or "subscribed" to the user.

10. Blocking Communication

   Most instant messaging systems have found it necessary to implement
   some method for users to block communications from particular other
   users (this is also required by sections 5.1.5, 5.1.15, 5.3.2, and
   5.4.10 of [IMP-REQS]).  In XMPP this is done using the
   'jabber:iq:privacy' namespace by managing one's privacy lists.

   Server-side privacy lists enable successful completion of the
   following use cases:

   o  Retrieving one's privacy lists.

   o  Adding, removing, and editing one's privacy lists.

   o  Setting, changing, or declining active lists.



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   o  Setting, changing, or declining the default list (i.e., the list
      that is active by default).

   o  Allowing or blocking messages based on JID, group, or subscription
      type (or globally).

   o  Allowing or blocking inbound presence notifications based on JID,
      group, or subscription type (or globally).

   o  Allowing or blocking outbound presence notifications based on JID,
      group, or subscription type (or globally).

   o  Allowing or blocking IQs based on JID, group, or subscription type
      (or globally).

   o  Allowing or blocking all communications based on JID, group, or
      subscription type (or globally).

   Note: presence notifications do not include presence subscriptions,
   only presence information that is broadcasted to entities that are
   subscribed to a user's presence information.  Thus this includes
   presence stanzas with no 'type' attribute or of type='unavailable'
   only.

10.1 Syntax and Semantics

   A user MAY define one or more privacy lists, which are stored by the
   user's server.  Each <list/> element contains one or more rules in
   the form of <item/> elements, and each <item/> element uses
   attributes to define a privacy rule type, a specific value to which
   the rules applies, the relevant action, and the place of the item in
   the processing order.

   The syntax is as follows:

   <iq>
     <query xmlns='jabber:iq:privacy'>
       <list name='foo'>
         <item
             type='[jid|group|subscription]'
             value='bar'
             action='[allow|deny]'
             order='unsignedInt'>
           [<message/>]
           [<presence-in/>]
           [<presence-out/>]
           [<iq/>]
         </item>



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       </list>
     </query>
   </iq>

   If the type is "jid", then the 'value' attribute MUST contain a valid
   Jabber ID.  JIDs are matched in the following order: <user@domain/
   resource>, then <user@domain>, then <domain/resource>, then <domain>.
   If the value is <user@domain>, then any resource for that user@domain
   matches.  If the value is <domain/resource>, then only that resource
   matches.  If the value is <domain>, then any user@domain (or
   subdomain) matches.

   If the type is "group", then the 'value' attribute SHOULD contain the
   name of a group in the user's roster.  (If a client attempts to
   update, create, or delete a list item with a group that is not in the
   user's roster, the server SHOULD return to the client an
   <item-not-found/> stanza error.)

   If the type is "subscription", then the 'value' attribute MUST be one
   of "both", "to", "from", or "none" as defined under Roster Syntax and
   Semantics (Section 7.1).

   If no 'type' attribute is included, the rule provides the
   "fall-through" case.

   The 'action' attribute MUST be included and its value MUST be either
   "accept" or "deny".

   The 'order' attribute MUST be included and its value MUST be a
   non-negative integer that is unique among all items in the list.  (If
   a client attempts to create or update a list with non-unique order
   values, the server MUST return to the client a <bad-request/> stanza
   error.)

   The <item/> element MAY contain one or more child elements that
   enable an entity to specify more granular control over which kinds of
   stanzas are to be blocked (i.e., rather than blocking all stanzas).
   The allowable child elements are:

   o  <message/> -- blocks incoming message stanzas

   o  <iq/> -- blocks incoming IQ stanzas

   o  <presence-in/> -- blocks incoming presence notifications

   o  <presence-out/> -- blocks outgoing presence notifications

   Within the 'jabber:iq:privacy' namespace, the <query/> child of a



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   client-generated IQ stanza of type "set" MUST NOT include more than
   one child element (i.e., the stanza must contain only one <active/>
   element, one <default/> element, or one <list/> element); if a client
   violates this rule, the server MUST return to the client a
   <bad-request/> stanza error.

   When a client adds or updates a privacy list, the <list/> element
   SHOULD contain at least one <item/> child element; when a client
   removes a privacy list, the <list/> element SHOULD contain no <item/>
   child element.

   When a client updates a privacy list, it must include all of the
   desired items (i.e., not a "delta").

10.2 Business Rules

   1.   If there is an active list set for a session, it affects only
        the session for which it is activated, and only for the duration
        of the session.  The server MUST apply the active list only and
        MUST NOT apply the default list.

   2.   The default list applies to the user as a whole, and is
        processed if there is no active list set for the target session/
        resource to which a stanza is addressed, or if there are no
        current sessions for the user.

   3.   If there is no active list set for a session (or there are no
        current sessions for the user), and there is no default list,
        then all stanzas SHOULD BE accepted or appropriately processed
        by the server on behalf of the user.

   4.   Privacy lists MUST be the first rule applied by a server,
        superseding (1) the routing and delivery rules specified in
        Server Rules for Handling XML Stanzas (Section 11), and (2) the
        handling of subscription-related presence stanzas (and
        corresponding generation of roster pushes) specified in
        Integration of Roster Items and Presence Subscriptions (Section
        8).

   5.   The order in which privacy list items are processed by the
        server is important.  List items MUST be processed in ascending
        order determined by the integer values of the 'order' attribute
        for each <item/>.

   6.   As soon as a stanza is matched against a privacy list, the
        server SHOULD appropriately handle the stanza and cease
        processing.




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   7.   If no fall-through item is provided in a list, the fall-through
        action is assumed to be "accept".

   8.   If a user updates the definition for an active list, subsequent
        processing based on that active list MUST use the updated
        definition (for all resources to which that active list
        currently applies).

   9.   If a user modifies an active list by adding, modifying, or
        deleting an item with the result that subsequent outbound
        presence notifications will be blocked to certain subscribed
        contacts, the server MUST also send unavailable presence from
        the user to the affected contacts.

   10.  If a user modifies an active list by adding, modifying, or
        deleting an item with the result that subsequent outbound
        presence notifications will no longer be blocked to certain
        subscribed contacts, the server MUST also send last available
        presence from the user to the affected contacts.

   11.  If a change to the subscription state or roster group of a
        roster item defined in an active or default list occurs during a
        user's session, subsequent processing based on that list MUST
        take into account the changed state or group (for all resources
        to which that list currently applies).


10.3 Retrieving One's Privacy Lists

   Example: Client requests names of privacy lists from server:

   <iq from='romeo@example.org/orchard' type='get' id='getlist1'>
     <query xmlns='jabber:iq:privacy'/>
   </iq>

   Example: Server sends names of privacy lists to client, preceded by
   active list and default list:

   <iq type='result' id='getlist1' to='romeo@example.net/orchard'>
     <query xmlns='jabber:iq:privacy'>
       <active name='private'/>
       <default name='public'/>
       <list name='public'/>
       <list name='private'/>
       <list name='special'/>
     </query>
   </iq>




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   Example: Client requests a privacy list from server:

   <iq from='romeo@example.org/orchard' type='get' id='getlist2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='public'/>
     </query>
   </iq>

   Example: Server sends a privacy list to client:

   <iq type='result' id='getlist2' to='romeo@example.net/orchard'>
     <query xmlns='jabber:iq:privacy'>
       <list name='public'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='1'/>
         <item action='allow' order='2'/>
       </list>
     </query>
   </iq>

   Example: Client requests another privacy list from server:

   <iq from='romeo@example.org/orchard' type='get' id='getlist3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='private'/>
     </query>
   </iq>

   Example: Server sends another privacy list to client:

   <iq type='result' id='getlist3' to='romeo@example.net/orchard'>
     <query xmlns='jabber:iq:privacy'>
       <list name='private'>
         <item type='subscription'
               value='both'
               action='allow'
               order='10'/>
         <item action='deny' order='15'/>
       </list>
     </query>
   </iq>

   Example: Client requests yet another privacy list from server:

   <iq from='romeo@example.org/orchard' type='get' id='getlist4'>
     <query xmlns='jabber:iq:privacy'>



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       <list name='special'/>
     </query>
   </iq>

   Example: Server sends yet another privacy list to client:

   <iq type='result' id='getlist4' to='romeo@example.net/orchard'>
     <query xmlns='jabber:iq:privacy'>
       <list name='special'>
         <item type='jid'
               value='juliet@example.com'
               action='allow'
               order='6'/>
         <item type='jid'
               value='benvolio@example.org'
               action='allow'
               order='7'/>
         <item type='jid'
               value='mercutio@example.org'
               action='allow'
               order='42'/>
         <item action='deny' order='666'/>
       </list>
     </query>
   </iq>

   In this example, the user has three lists: (1) 'public', which allows
   communications from everyone except one specific entity (this is the
   default list); (2) 'private', which allows communications only with
   contacts who have a bidirectional subscription with the user (this is
   the active list); and (3) 'special', which allows communications only
   with three specific entities.

   If the user attempts to retrieve a list but a list by that name does
   not exist, the server MUST return an <item-not-found/> stanza error
   to the user:

   Example: Client attempts to retrieve non-existent list:

   <iq to='romeo@example.org/orchard' type='error' id='getlist5'>
     <query xmlns='jabber:iq:privacy'>
       <list name='The Empty Set'/>
     </query>
     <error type='cancel'>
       <item-not-found
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>



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   The user is allowed to retrieve only one list at a time.  If the user
   attempts to retrieve more than one list in the same request, the
   server MUST return a <bad request/> stanza error to the user:

   Example: Client attempts to retrieve more than one list:

   <iq to='romeo@example.org/orchard' type='error' id='getlist6'>
     <query xmlns='jabber:iq:privacy'>
       <list name='public'/>
       <list name='private'/>
       <list name='special'/>
     </query>
     <error type='modify'>
       <bad-request
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>


10.4 Managing Active Lists

   In order to set or change the active list currently being applied by
   the server, the user MUST send an IQ stanza of type "set" with a
   <query/> element qualified by the 'jabber:iq:privacy' namespace that
   contains an empty <active/> child element possessing a 'name'
   attribute whose value is set to the desired list name.

   Example: Client requests change of active list:

   <iq from='romeo@example.org/orchard' type='set' id='active1'>
     <query xmlns='jabber:iq:privacy'>
       <active name='special'/>
     </query>
   </iq>

   The server MUST activate and apply the requested list before sending
   the result back to the client.

   Example: Server acknowledges success of active list change:

   <iq type='result' id='active1' to='romeo@example.org/orchard'/>

   If the user attempts to set an active list but a list by that name
   does not exist, the server MUST return an <item-not-found/> stanza
   error to the user:

   Example: Client attempts to set a non-existent list as active:




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   <iq to='romeo@example.org/orchard' type='error' id='active2'>
     <query xmlns='jabber:iq:privacy'>
       <active name='The Empty Set'/>
     </query>
     <error type='cancel'>
       <item-not-found
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   In order to decline the use of any active list, the user MUST send an
   empty <active/> element with no name.

   Example: Client declines the use of active lists:

   <iq from='romeo@example.org/orchard' type='set' id='active2'>
     <query xmlns='jabber:iq:privacy'>
       <active/>
     </query>
   </iq>


10.5 Managing the Default List

   In order to change its default list, the user MUST send an IQ stanza
   of type "set" with a <query/> element qualified by the
   'jabber:iq:privacy' namespace that contains an empty <default/> child
   element possessing a 'name' attribute whose value is set to the
   desired list name.

   Example: Client requests change of default list:

   <iq from='romeo@example.org/orchard' type='set' id='default1'>
     <query xmlns='jabber:iq:privacy'>
       <default name='special'/>
     </query>
   </iq>

   Example: Server acknowledges success of default list change:

   <iq type='result' id='default1' to='romeo@example.org/orchard'/>

   If the user attempts to set a default list but a list by that name
   does not exist, the server MUST return an <item-not-found/> stanza
   error to the user:

   Example: Client attempts to set a non-existent list as default:




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   <iq to='romeo@example.org/orchard' type='error' id='default2'>
     <query xmlns='jabber:iq:privacy'>
       <default name='The Empty Set'/>
     </query>
     <error type='cancel'>
       <item-not-found
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   In order to decline the use of a default list (i.e., to use the
   domain's stanza routing rules at all times), the user MUST send an
   empty <default/> element with no name.

   Example: Client declines the use of the default list:

   <iq from='romeo@example.org/orchard' type='set' id='default2'>
     <query xmlns='jabber:iq:privacy'>
       <default/>
     </query>
   </iq>


10.6 Editing a Privacy List

   In order to edit a privacy list, the user MUST send an IQ stanza of
   type "set" with a <query/> element qualified by the
   'jabber:iq:privacy' namespace that contains one <list/> child element
   possessing a 'name' attribute whose value is set to the list name the
   user would like to edit.  The <list/> element MUST contain one or
   more <item/> elements, which specify the user's desired changes to
   the list by including all elements in the list (not the "delta").

   Example: Client edits a privacy list:

   <iq from='romeo@example.org/orchard' type='set' id='edit1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='public'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='3'/>
         <item type='jid'
               value='paris@example.org'
               action='deny'
               order='5'/>
         <item action='allow' order='68'/>
       </list>



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     </query>
   </iq>

   Example: Server acknowledges success of list edit:

   <iq type='result' id='edit1' to='juliet@example.com/balcony'/>

   Note: The value of the 'order' attribute for any given item is not
   fixed.  Thus in the foregoing example if the user would like to add 4
   items between the "tybalt@example.com" item and the
   "paris@example.org" item, the user's client MUST renumber the
   relevant items before submitting the list to the server.

10.7 Adding a New Privacy List

   The same protocol used to edit an existing list is used to create a
   new list.  If the list name matches that of an existing list, the
   request to add a new list will overwrite the old one.

10.8 Removing a Privacy List

   In order to remove a privacy list, the user MUST send an IQ stanza of
   type "set" with a <query/> element qualified by the
   'jabber:iq:privacy' namespace that contains one empty <list/> child
   element possessing a 'name' attribute whose value is set to the list
   name the user would like to remove.

   Example: Client removes a privacy list:

   <iq from='romeo@example.org/orchard' type='set' id='remove1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='private'/>
     </query>
   </iq>

   Example: Server acknowledges success of list removal:

   <iq type='result' id='remove1' to='juliet@example.com/balcony'/>

   If a user attempts to remove an active list or the default list, the
   server MUST return a <conflict/> stanza error to the user.  The user
   MUST first set another list to active or default before removing it.

   If the user attempts to remove a list but a list by that name does
   not exist, the server MUST return an <item-not-found/> stanza error
   to the user:

   If the user attempts to remove more than one list in the same



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   request, the server MUST return a <bad request/> stanza error to the
   user.

10.9 Blocking Messages

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

   Example: User blocks based on JID:

   <iq from='romeo@example.org/orchard' type='set' id='msg1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='message-jid-example'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='3'>
           <message/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive messages from the user with the specified JID.

   Example: User blocks based on roster group:

   <iq from='romeo@example.org/orchard' type='set' id='msg2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='message-group-example'>
         <item type='group'
               value='Enemies'
               action='deny'
               order='4'>
           <message/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive messages from any users in the specified roster
   group.

   Example: User blocks based on subscription type:



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   <iq from='romeo@example.org/orchard' type='set' id='msg3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='message-sub-example'>
         <item type='subscription'
               value='none'
               action='deny'
               order='5'>
           <message/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive messages from any users with the specified
   subscription type.

   Example: User blocks globally:

   <iq from='romeo@example.org/orchard' type='set' id='msg4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='message-global-example'>
         <item action='deny' order='6'>
           <message/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive messages from any other users.

10.10 Blocking Inbound Presence Notifications

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

   Note: presence notifications do not include presence subscriptions,
   only presence information that is broadcasted to the user because the
   user is currently subscribed to a contact's presence information.
   Thus this includes presence stanzas with no 'type' attribute or of
   type='unavailable' only.

   Example: User blocks based on JID:

   <iq from='romeo@example.org/orchard' type='set' id='presin1'>



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     <query xmlns='jabber:iq:privacy'>
       <list name='presin-jid-example'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='7'>
           <presence-in/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive presence notifications from the user with the
   specified JID.

   Example: User blocks based on roster group:

   <iq from='romeo@example.org/orchard' type='set' id='presin2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presin-group-example'>
         <item type='group'
               value='Enemies'
               action='deny'
               order='8'>
           <presence-in/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive presence notifications from any users in the
   specified roster group.

   Example: User blocks based on subscription type:

   <iq from='romeo@example.org/orchard' type='set' id='presin3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presin-sub-example'>
         <item type='subscription'
               value='to'
               action='deny'
               order='9'>
           <presence-in/>
         </item>
       </list>
     </query>



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   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive presence notifications from any users with the
   specified subscription type.

   Example: User blocks globally:

   <iq from='romeo@example.org/orchard' type='set' id='presin4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presin-global-example'>
         <item action='deny' order='11'>
           <presence-in/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive presence notifications from any other users.

10.11 Blocking Outbound Presence Notifications

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

   Note: presence notifications do not include presence subscriptions,
   only presence information that is broadcasted to contacts because
   those contacts are currently subscribed to the user's presence
   information.  Thus this includes presence stanzas with no 'type'
   attribute or of type='unavailable' only.

   Example: User blocks based on JID:

   <iq from='romeo@example.org/orchard' type='set' id='presout1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presout-jid-example'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='13'>
           <presence-out/>
         </item>
       </list>
     </query>
   </iq>



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   As a result of creating and applying the foregoing list, the user
   will not send presence notifications to the user with the specified
   JID.

   Example: User blocks based on roster group:

   <iq from='romeo@example.org/orchard' type='set' id='presout2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presout-group-example'>
         <item type='group'
               value='Enemies'
               action='deny'
               order='15'>
           <presence-out/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not send presence notifications to any users in the specified
   roster group.

   Example: User blocks based on subscription type:

   <iq from='romeo@example.org/orchard' type='set' id='presout3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presout-sub-example'>
         <item type='subscription'
               value='from'
               action='deny'
               order='17'>
           <presence-out/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not send presence notifications to any users with the specified
   subscription type.

   Example: User blocks globally:

   <iq from='romeo@example.org/orchard' type='set' id='presout4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='presout-global-example'>
         <item action='deny' order='23'>



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           <presence-out/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not send presence notifications to any other users.

10.12 Blocking IQs

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

   Example: User blocks based on JID:

   <iq from='romeo@example.org/orchard' type='set' id='iq1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='iq-jid-example'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='29'>
           <iq/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive IQ stanzas from the user with the specified JID.

   Example: User blocks based on roster group:

   <iq from='romeo@example.org/orchard' type='set' id='iq2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='iq-group-example'>
         <item type='group'
               value='Enemies'
               action='deny'
               order='31'>
           <iq/>
         </item>
       </list>
     </query>
   </iq>



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   As a result of creating and applying the foregoing list, the user
   will not receive IQ stanzas from any users in the specified roster
   group.

   Example: User blocks based on subscription type:

   <iq from='romeo@example.org/orchard' type='set' id='iq3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='iq-sub-example'>
         <item type='subscription'
               value='none'
               action='deny'
               order='17'>
           <iq/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive IQ stanzas from any users with the specified
   subscription type.

   Example: User blocks globally:

   <iq from='romeo@example.org/orchard' type='set' id='iq4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='iq-global-example'>
         <item action='deny' order='1'>
           <iq/>
         </item>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive IQ stanzas from any other users.

10.13 Blocking All Communication

   Server-side privacy lists enable a user to block all stanzas from and
   to other users based on the other user's JID, roster group, or
   subscription status (or globally).  Note that this includes
   subscription-related presence stanzas, which are excluded by Blocking
   Inbound Presence Notifications (Section 10.10).  The following
   examples illustrate the protocol.

   Example: User blocks based on JID:



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   <iq from='romeo@example.org/orchard' type='set' id='all1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='all-jid-example'>
         <item type='jid'
               value='tybalt@example.com'
               action='deny'
               order='23'/>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive any communications from, nor send any stanzas to,
   the user with the specified JID.

   Example: User blocks based on roster group:

   <iq from='romeo@example.org/orchard' type='set' id='all2'>
     <query xmlns='jabber:iq:privacy'>
       <list name='all-group-example'>
         <item type='group'
               value='Enemies'
               action='deny'
               order='13'/>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive any communications from, nor send any stanzas to,
   any users in the specified roster group.

   Example: User blocks based on subscription type:

   <iq from='romeo@example.org/orchard' type='set' id='all3'>
     <query xmlns='jabber:iq:privacy'>
       <list name='all-sub-example'>
         <item type='subscription'
               value='none'
               action='deny'
               order='11'/>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive any communications from, nor send any stanzas to,
   any users with the specified subscription type.



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   Example: User blocks globally:

   <iq from='romeo@example.org/orchard' type='set' id='all4'>
     <query xmlns='jabber:iq:privacy'>
       <list name='all-global-example'>
         <item action='deny' order='7'/>
       </list>
     </query>
   </iq>

   As a result of creating and applying the foregoing list, the user
   will not receive any communications from, nor send any stanzas to,
   any other users.

10.14 Blocked Entity Attempts to Communicate with User

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

   If a blocked entity attempts to send an IQ stanza of type "get" or
   "set" to the user, the user's server MUST return to the sending
   entity a <feature-not-implemented/> stanza error, since this is the
   standard error code sent from a client that does not understand the
   namespace of an IQ get or set.  IQ stanzas of other types SHOULD be
   silently dropped by the server.

   Example: Blocked entity attempts to send IQ get:

   <iq type='get'
       to='romeo@example.net'
       from='tybalt@example.com/pda'
       id='probing1'>
     <query xmlns='jabber:iq:version'/>
   </iq>

   Example: Server returns error to blocked entity:

   <iq type='error'
       from='romeo@example.net'
       to='tybalt@example.com/pda'
       id='probing1'>
     <query xmlns='jabber:iq:version'/>
     <error type='cancel'>
       <feature-not-implemented
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>



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10.15 Higher-Level Heuristics

   When building a representation of a higher-level privacy heuristic, a
   client SHOULD use the simplest possible representation.

   For example, the heuristic "block all communications with any user
   not in my roster" could be constructed in any of the following ways:

   o  allow communications from all JIDs in my roster (i.e., listing
      each JID as a separate list item), but block communications with
      everyone else

   o  allow communications from any user who is in one of the groups
      that make up my roster (i.e., listing each group as a separate
      list item), but block communications from everyone else

   o  allow communications from any user with whom I have a subscription
      of 'both' or 'to' or 'from' (i.e., listing each subscription value
      separately), but block communications from everyone else

   o  block communications from anyone whose subscription state is
      'none'

   The final representation is the simplest and SHOULD be used; here is
   the XML that would be sent in this case:

   <iq type='set' id='heuristic1'>
     <query xmlns='jabber:iq:privacy'>
       <list name='heuristic-example'>
         <item type='subscription'
               value='none'
               action='deny'
               order='437'/>
       </list>
     </query>
   </iq>


11. Server Rules for Handling XML Stanzas

   Basic routing and delivery rules for servers are defined in
   [XMPP-CORE].  This section defines additional rules for
   XMPP-compliant instant messaging and presence servers.

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute of a stanza matches the hostname of the server
   itself and the JID contained in the 'to' attribute is of the form
   <user@example.com> or <user@example.com/resource>, the server MUST



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   first apply any privacy rules (Section 10) that are in force.  If
   privacy rules allow the stanza, it SHOULD be routed or delivered to
   the intended recipient of the stanza as represented by the JID
   contained in the 'to' attribute.  The following additional rules
   apply to instant messaging and presence applications, over and above
   those defined in [XMPP-CORE]:

   1.  If the JID is of the form <user@domain/resource> and an active
       resource matches the full JID, the recipient's server MUST
       deliver the stanza to that resource.

   2.  If the JID is of the form <user@domain/resource> and no active
       resource matches the full JID, the recipient's server (a) SHOULD
       silently ignore the stanza (i.e., neither deliver it nor return
       an error) if it is a presence stanza, (b) MUST return a
       <service-unavailable/> stanza error to the sender if it is an IQ
       stanza, and (c) SHOULD treat the stanza as if it were addressed
       to <user@domain> if it is a message stanza.

   3.  If the JID is of the form <user@domain> and there is at least one
       available resource available for the user, the recipient's server
       MUST follow these rules:

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

       2.  For presence stanzas other than those of type "probe", the
           server MUST deliver the stanza to all available resources;
           for presence probes, the server SHOULD reply based on the
           rules defined in Client and Server Presence Responsibilities
           (Section 5.1).  In addition, the server MUST NOT rewrite the
           'to' attribute (i.e., it MUST leave it as <user@domain>
           rather than change it to <user@domain/resource>).

       3.  For IQ stanzas, the server itself MUST reply on behalf of the



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           user with either an IQ result or an IQ error, and MUST NOT
           deliver the IQ stanza to any of the available resources.
           Specifically, if the semantics of the qualifying namespace
           define a reply that the server can provide, the server MUST
           reply to the stanza on behalf of the user; if not, the server
           MUST reply with a <service-unavailable/> stanza error.

   4.  If the JID is of the form <user@domain> and there are no
       available resources associated with the user, how the stanza is
       handled depends on the stanza type:

       1.  For presence stanzas of type "subscribe", the server MUST
           maintain a record of the stanza and deliver the request when
           the user next creates an available resource, until the user
           either approves or denies it.

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

       3.  For message stanzas, the server MAY choose to store the
           stanza on behalf of the user and deliver it when the user
           next becomes available.  However, if offline message storage
           is not enabled, the server MUST return to the sender a
           <service-unavailable/> stanza error.  (Note: offline message
           storage is not defined in XMPP since it strictly is a matter
           of implementation and service provisioning.)

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


12. IM and Presence Compliance Requirements

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



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   presence service or an instant messaging service is desired.

12.1 Servers

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

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


12.2 Clients

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

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

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

   o  End-to-end object encryption as defined in XMPP e2e [XMPP-E2E]


13. Internationalization Considerations

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

14. Security Considerations

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

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





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

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

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


15. IANA Considerations

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

15.1 XML Namespace Name for Session Data

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

   URI: urn:ietf:params:xml:ns:xmpp-session

   Specification: XXXX

   Description: This is the XML namespace name for session-related data
      in the Extensible Messaging and Presence Protocol (XMPP) as
      defined by XXXX.

   Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>


15.2 Instant Messaging SRV Protocol Label Registration

   Address Resolution for Instant Messaging and Presence [IMP-SRV]
   defines an Instant Messaging SRV Protocol Label registry for
   protocols that can provide services that conform to the "_im" SRV



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   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@jabber.org>


15.3 Presence SRV Protocol Label Registration

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

   Protocol label: _xmpp

   Specification: XXXX

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

   Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>

Normative References

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

   [IMP-SRV]  Peterson, J., "Address Resolution for Instant Messaging
              and Presence", draft-ietf-impp-srv-04 (work in progress),
              October 2003.

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

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
              "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C
              REC-xml, October 2000, <http://www.w3.org/TR/REC-xml>.

   [XML-NAMES]



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              Bray, T., Hollander, D. and A. Layman, "Namespaces in
              XML", W3C REC-xml-names, January 1999, <http://www.w3.org/
              TR/REC-xml-names>.

   [XMPP-CORE]
              Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", draft-ietf-xmpp-core-20 (work in
              progress), November 2003.

   [XMPP-E2E]
              Saint-Andre, P., "End-to-End Object Encryption in the
              Extensible Messaging and Presence Protocol (XMPP)",
              draft-ietf-xmpp-e2e-05 (work in progress), August 2003.

Informative References

   [JSF]      Jabber Software Foundation, "Jabber Software Foundation",
              <http://www.jabber.org/>.

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

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

   [XML-REG]  Mealling, M., "The IETF XML Registry",
              draft-mealling-iana-xmlns-registry-05 (work in progress),
              June 2003.


Author's Address

   Peter Saint-Andre
   Jabber Software Foundation

   EMail: stpeter@jabber.org

Appendix A. vCards

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



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Appendix B. XML Schemas

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

B.1 jabber:client

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       xmlns:xml='http://www.w3.org/XML/1998/namespace'
       targetNamespace='jabber:client'
       xmlns='jabber:client'
       elementFormDefault='qualified'>

     <xs:import namespace='http://www.w3.org/XML/1998/namespace'
                schemaLocation='http://www.w3.org/2001/xml.xsd'/>

     <xs:element name='message'>
        <xs:complexType>
           <xs:sequence>
              <xs:element ref='subject'
                          minOccurs='0'
                          maxOccurs='unbounded'/>
              <xs:element ref='body'
                          minOccurs='0'
                          maxOccurs='unbounded'/>
              <xs:element ref='thread'
                          minOccurs='0'
                          maxOccurs='1'/>
              <xs:any     namespace='##other'
                          minOccurs='0'
                          maxOccurs='unbounded'/>
              <xs:element ref='error'
                          minOccurs='0'
                          maxOccurs='1'/>
           </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'>



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             <xs:simpleType>
               <xs:restriction base='xs:NCName'>
                 <xs:enumeration value='chat'/>
                 <xs:enumeration value='error'/>
                 <xs:enumeration value='groupchat'/>
                 <xs:enumeration value='headline'/>
                 <xs:enumeration value='normal'/>
               </xs:restriction>
             </xs:simpleType>
           </xs:attribute>
           <xs:attribute ref='xml:lang' use='optional'/>
        </xs:complexType>
     </xs:element>

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

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

     <xs:element name='thread' type='xs:NMTOKEN'/>

     <xs:element name='presence'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='show'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:element ref='status'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='priority'
                       minOccurs='0'
                       maxOccurs='1'/>



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           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='error'
                       minOccurs='0'
                       maxOccurs='1'/>
         </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='subscribe'/>
               <xs:enumeration value='subscribed'/>
               <xs:enumeration value='unsubscribe'/>
               <xs:enumeration value='unsubscribed'/>
               <xs:enumeration value='unavailable'/>
               <xs:enumeration value='probe'/>
               <xs:enumeration value='error'/>
             </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='xs:string'>



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

     <xs:element name='priority' type='xs:byte'/>

     <xs:element name='iq'>
       <xs:complexType>
         <xs:sequence>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:element ref='error'
                       minOccurs='0'
                       maxOccurs='1'/>
         </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='get'/>
               <xs:enumeration value='set'/>
               <xs:enumeration value='result'/>
               <xs:enumeration value='error'/>
             </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>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-stanzas'
                   maxOccurs='1'/>
           <text   namespace='urn:ietf:params:xml:ns:xmpp-stanzas'
                   minOccurs='0'



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                   maxOccurs='1'/>
           <xs:any
               namespace='##other'
               minOccurs='0'
               maxOccurs='1'/>
         </xs:sequence>
         <xs:attribute name='code' type='xs:byte' use='optional'/>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='cancel'/>
               <xs:enumeration value='continue'/>
               <xs:enumeration value='modify'/>
               <xs:enumeration value='auth'/>
               <xs:enumeration value='wait'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>


B.2 jabber:server

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       xmlns:xml='http://www.w3.org/XML/1998/namespace'
       targetNamespace='jabber:server'
       xmlns='jabber:server'
       elementFormDefault='qualified'>

     <xs:import namespace='http://www.w3.org/XML/1998/namespace'
                schemaLocation='http://www.w3.org/2001/xml.xsd'/>

     <xs:element name='message'>
        <xs:complexType>
           <xs:sequence>
              <xs:element ref='subject'
                          minOccurs='0'
                          maxOccurs='unbounded'/>
              <xs:element ref='body'
                          minOccurs='0'
                          maxOccurs='unbounded'/>
              <xs:element ref='thread'



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                          minOccurs='0'
                          maxOccurs='1'/>
              <xs:any     namespace='##other'
                          minOccurs='0'
                          maxOccurs='unbounded'/>
              <xs:element ref='error'
                          minOccurs='0'
                          maxOccurs='1'/>
           </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'/>



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           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='thread' type='xs:NMTOKEN'/>

     <xs:element name='presence'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='show'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:element ref='status'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='priority'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='error'
                       minOccurs='0'
                       maxOccurs='1'/>
         </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='subscribe'/>
               <xs:enumeration value='subscribed'/>
               <xs:enumeration value='unsubscribe'/>
               <xs:enumeration value='unsubscribed'/>
               <xs:enumeration value='unavailable'/>
               <xs:enumeration value='probe'/>
               <xs:enumeration value='error'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>



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         <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='xs:string'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='priority' type='xs:byte'/>

     <xs:element name='iq'>
       <xs:complexType>
         <xs:sequence>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:element ref='error'
                       minOccurs='0'
                       maxOccurs='1'/>
         </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>



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             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='get'/>
               <xs:enumeration value='set'/>
               <xs:enumeration value='result'/>
               <xs:enumeration value='error'/>
             </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>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-stanzas'
                   maxOccurs='1'/>
           <text   namespace='urn:ietf:params:xml:ns:xmpp-stanzas'
                   minOccurs='0'
                   maxOccurs='1'/>
           <xs:any
               namespace='##other'
               minOccurs='0'
               maxOccurs='1'/>
         </xs:sequence>
         <xs:attribute name='code' type='xs:byte' use='optional'/>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='cancel'/>
               <xs:enumeration value='continue'/>
               <xs:enumeration value='modify'/>
               <xs:enumeration value='auth'/>
               <xs:enumeration value='wait'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>


B.3 session

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema



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       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-session'
       xmlns='urn:ietf:params:xml:ns:xmpp-session'
       elementFormDefault='qualified'>

     <xs:element name='session' type='empty'/>

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

   </xs:schema>


B.4 jabber:iq:privacy

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='jabber:iq:privacy'
       xmlns='jabber:iq:privacy'
       elementFormDefault='qualified'>

     <xs:element name='query'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='active'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:element ref='default'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:element ref='list'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <xs:element name='active'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='name'
                           type='xs:string'



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                           use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

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

     <xs:element name='list'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='item'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
         </xs:sequence>
         <xs:attribute name='name'
                       type='xs:string'
                       use='required'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='item'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='iq'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:element ref='message'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:element ref='presence-in'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:element ref='presence-out'
                       minOccurs='0'
                       maxOccurs='1'/>
         </xs:sequence>
         <xs:attribute name='action' use='required'>



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           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='allow'/>
               <xs:enumeration value='deny'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute name='order'
                       type='xs:unsignedInt'
                       use='required'/>
         <xs:attribute name='type' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='group'/>
               <xs:enumeration value='jid'/>
               <xs:enumeration value='subscription'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute name='value'
                       type='xs:string'
                       use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='iq' type='empty'/>
     <xs:element name='message' type='empty'/>
     <xs:element name='presence-in' type='empty'/>
     <xs:element name='presence-out' type='empty'/>

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

   </xs:schema>


B.5 jabber:iq:roster

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='jabber:iq:roster'
       xmlns='jabber:iq:roster'
       elementFormDefault='qualified'>



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     <xs:element name='query'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='item'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <xs:element name='item'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='group'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
         </xs:sequence>
         <xs:attribute name='ask' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='subscribe'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute name='jid' type='xs:string' use='required'/>
         <xs:attribute name='name' type='xs:string' use='optional'/>
         <xs:attribute name='subscription' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='to'/>
               <xs:enumeration value='from'/>
               <xs:enumeration value='both'/>
               <xs:enumeration value='none'/>
               <xs:enumeration value='remove'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

     <xs:element name='group' type='xs:string'/>

   </xs:schema>


Appendix C. Differences Between Jabber IM/Presence and XMPP

   This section is non-normative.



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

C.1 Session Establishment

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

C.2 Privacy Rules

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

Appendix D. Revision History

   Note to RFC Editor: please remove this entire appendix, and the
   corresponding entries in the table of contents, prior to publication.

D.1 Changes from draft-ietf-xmpp-im-19

   o  Completed changes necessary to address IESG feedback.


D.2 Changes from draft-ietf-xmpp-im-18

   o  Added presence type of "probe" to XML schema.

   o  Added error 'code' attribute to XML schema.




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   o  Added protocol label registrations for "_im" and "_pres" service
      labels.

   o  Adjusted formatting to conform to RFC Editor requirements.

   o  Clarified meaning of "connected", "active", and "available"
      resources.


D.3 Changes from draft-ietf-xmpp-im-17

   o  Added missing server handling rules for <user@domain/resource>
      cases.

   o  Further clarified privacy rules syntax by describing child
      elements of <item/>.

   o  Clarified several points related to session establishment.

   o  Checked all references.

   o  Completed a thorough proofreading and consistency check of the
      entire text.


D.4 Changes from draft-ietf-xmpp-im-16

   o  Added sentence to make explicit that blocking all communication
      includes subscription-related presence stanzas.

   o  Added clause to make explicit that privacy rules must be applied
      before handling of subscription-related presence stanzas and
      corresponding generation of roster pushes.

   o  Added syntax and semantics section for the 'jabber:iq:roster'
      namespace.

   o  Removed content about 'jabber:iq:last' namespace.

   o  Added several internal references from the security considerations
      section to other sections of this document.

   o  Moved most delivery handling rules from XMPP IM to Extensible
      Messaging and Presence Protocol (XMPP): Core.

   o  Moved detailed stanza syntax descriptions from Extensible
      Messaging and Presence Protocol (XMPP): Core to XMPP IM.




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   o  Moved stanza schemas from Extensible Messaging and Presence
      Protocol (XMPP): Core to XMPP IM.


D.5 Changes from draft-ietf-xmpp-im-15

   o  Specified stream error to be sent to active resource if there is a
      conflict regarding session creation.

   o  Fixed several more typographical errors in the privacy rules
      examples.

   o  Corrected an error regarding server handling of IQ stanzas sent to
      bare JIDs.

   o  Added section on compliance requirements for instant messaging
      server and client implementations.

   o  Added non-normative section on differences between Jabber usage
      and XMPP specifications.


D.6 Changes from draft-ietf-xmpp-im-14

   o  Added subscription state charts.

   o  Fixed several typographical errors in the privacy rules examples.

   o  Changed datatype of 'order' attribute in privacy rules from
      nonNegativeInteger to unsignedInt.


D.7 Changes from draft-ietf-xmpp-im-13

   o  Made one small change to privacy list syntax rules.


D.8 Changes from draft-ietf-xmpp-im-12

   o  Clarified meaning of the default message type as well as handling
      of unknown or unsupported types.

   o  Made several small editorial changes.


D.9 Changes from draft-ietf-xmpp-im-11





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   o  Further clarified subscription syntax and semantics.

   o  Further clarified presence responsibilities for clients and
      servers.

   o  Added 'xml:lang' example to presence status.

   o  Added subsection on presence priority.

   o  Defined server handling of unsolicited presence stanzas of type
      "subscribed".

   o  Specified default resource priority if not provided.

   o  Corrected several errors in the schemas.

   o  Added privacy list business rule regarding roster changes.

   o  Removed the 'jabber:iq:privacy:error' namespace (not necessary).

   o  Documented message type='normal'.

   o  Made numerous small editorial changes throughout.


D.10 Changes from draft-ietf-xmpp-im-10

   o  Clarified presence responsibilities for servers and clients.

   o  Clarified the routing and delivery rules for servers.

   o  Made the 'xml:lang' examples more complete.

   o  Corrected several errors in the unsubscribe workflow.

   o  Made small editorial changes in several sections.


D.11 Changes from draft-ietf-xmpp-im-09

   o  Clarified rules regarding allowable JID types in rosters.

   o  Further clarified the semantics and routing implications of
      presence priorities.

   o  Removed several obsolete subsections.





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D.12 Changes from draft-ietf-xmpp-im-08

   o  Removed authorization content (now addressed in Extensible
      Messaging and Presence Protocol (XMPP): Core).

   o  Added protocol for initiating an IM session, including schema and
      IANA registration template.

   o  Corrected <*-condition/> elements to be <condition/>.

   o  Made small editorial changes to address RFC Editor requirements.


D.13 Changes from draft-ietf-xmpp-im-07

   o  Added several error cases for resource authorization and updated
      relevant schema.


D.14 Changes from draft-ietf-xmpp-im-06

   o  Specified that IQ result stanzas are required in response to
      roster pushes.

   o  Changed stanza error namespace names to conform to the format
      defined in "The IETF XML Registry" as specified in Extensible
      Messaging and Presence Protocol (XMPP): Core.

   o  Removed note to RFC Editor regarding provisional namespace names.


D.15 Changes from draft-ietf-xmpp-im-05

   o  Removed use of ask='unsubscribe' per list discussion.

   o  Clarified handling of resource conflict during authorization.

   o  Added schemas for jabber:iq:auth, jabber:iq:auth:error, and
      jabber:iq:privacy:error.

   o  Corrected several small protocol errors in the examples.

   o  Clarified semantics of message types.


D.16 Changes from draft-ietf-xmpp-im-04





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   o  Specified sending of unavailable presence after unsubscribe and
      subscription-cancellation actions.

   o  Further specified syntax and business rules for privacy lists.

   o  Brought error codes into line with definitions in
      draft-ietf-xmpp-core.

   o  Added note to RFC Editor regarding provisional namespace names.

   o  Removed vCard content and DTD, instead pointing to JSF
      documentation.


D.17 Changes from draft-ietf-xmpp-im-03

   o  Fixed order processing on privacy rules per list discussion.

   o  Made numerous small editorial changes.


D.18 Changes from draft-ietf-xmpp-im-02

   o  Added a great deal more detail to the narrative regarding
      server-side privacy rules as well as the interaction between
      rosters and subscriptions.

   o  Removed DTDs in favor of schemas (with the exception of vCard
      XML).

   o  Removed non-normative documentation of authentication using
      jabber:iq:auth and of in-band registration using
      jabber:iq:register, since these are maintained by the Jabber
      Software Foundation and are not part of the XMPP specification.


D.19 Changes from draft-ietf-xmpp-im-01

   o  Made numerous small editorial changes.


D.20 Changes from draft-ietf-xmpp-im-00

   o  Moved registration and authentication via jabber:iq:auth to
      non-normative appendices.

   o  Changed initial presence stanza from MUST be empty to SHOULD be
      empty.



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   o  Specified that user or clients should not send presence stanzas of
      type='probe'.

   o  Specified the algorithm for digest passwords.


D.21 Changes from draft-miller-xmpp-im-02

   o  Added information about the 'jabber:iq:last' protocol to meet the
      requirement defined in section 3.2.4 of RFC 2779.

   o  Added information about the 'jabber:iq:privacy' protocol to meet
      the requirement defined in section 2.3.5 of RFC 2779.

   o  Added information about the vCard XML protocol to meet the
      requirement defined in sections 3.1.3 and 4.1.4 of RFC 2779.

   o  Changed the material describing authentication (but not resource
      authorization) with 'jabber:iq:auth' to non-normative.

   o  Noted that the only watchers are subscribers.

   o  Nomenclature changes: (1) from "chunks" to "stanzas"; (2) from
      "host" to "server"; (3) from "node" to "client" or "user" (as
      appropriate).


























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Intellectual Property Statement

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   This document and translations of it may be copied and furnished to
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Acknowledgment

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