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Versions: (draft-miller-xmpp-core) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 RFC 3920

Network Working Group                                     P. Saint-Andre
Internet-Draft                                                 J. Miller
Expires: November 2, 2003                     Jabber Software Foundation
                                                            May 04, 2003


                               XMPP Core
                        draft-ietf-xmpp-core-12

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 November 2, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   This document describes the core features of the Extensible Messaging
   and Presence Protocol (XMPP), a protocol for streaming XML elements
   in order to exchange messages and presence information in close to
   real time. XMPP is used mainly for the purpose of building instant
   messaging (IM) and presence applications, such as the servers and
   clients that comprise the Jabber network.









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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.2   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.3   Discussion Venue . . . . . . . . . . . . . . . . . . . . . .  5
   1.4   Intellectual Property Notice . . . . . . . . . . . . . . . .  5
   2.    Generalized Architecture . . . . . . . . . . . . . . . . . .  6
   2.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.2   Server . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.3   Client . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.4   Gateway  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.5   Network  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.    Addressing Scheme  . . . . . . . . . . . . . . . . . . . . .  8
   3.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.2   Domain Identifier  . . . . . . . . . . . . . . . . . . . . .  8
   3.3   Node Identifier  . . . . . . . . . . . . . . . . . . . . . .  8
   3.4   Resource Identifier  . . . . . . . . . . . . . . . . . . . .  9
   4.    XML Streams  . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.2   Stream Attributes  . . . . . . . . . . . . . . . . . . . . . 11
   4.3   Namespace Declarations . . . . . . . . . . . . . . . . . . . 12
   4.3.1 Stream Namespace Declaration . . . . . . . . . . . . . . . . 12
   4.3.2 Default Namespace Declaration  . . . . . . . . . . . . . . . 13
   4.4   Stream Features  . . . . . . . . . . . . . . . . . . . . . . 13
   4.5   Stream Errors  . . . . . . . . . . . . . . . . . . . . . . . 14
   4.5.1 Rules  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.5.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.5.3 Conditions . . . . . . . . . . . . . . . . . . . . . . . . . 15
   4.5.4 Extensibility  . . . . . . . . . . . . . . . . . . . . . . . 16
   4.6   Simple Streams Example . . . . . . . . . . . . . . . . . . . 16
   5.    Stream Encryption  . . . . . . . . . . . . . . . . . . . . . 19
   5.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   5.2   Narrative  . . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.3   Client-to-Server Example . . . . . . . . . . . . . . . . . . 21
   5.4   Server-to-Server Example . . . . . . . . . . . . . . . . . . 23
   6.    Stream Authentication  . . . . . . . . . . . . . . . . . . . 26
   6.1   SASL Authentication  . . . . . . . . . . . . . . . . . . . . 26
   6.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   6.1.2 Narrative  . . . . . . . . . . . . . . . . . . . . . . . . . 27
   6.1.3 SASL Definition  . . . . . . . . . . . . . . . . . . . . . . 29
   6.1.4 Client-to-Server Example . . . . . . . . . . . . . . . . . . 30
   6.1.5 Server-to-Server Example . . . . . . . . . . . . . . . . . . 32
   6.2   Dialback Authentication  . . . . . . . . . . . . . . . . . . 35
   6.2.1 Dialback Protocol  . . . . . . . . . . . . . . . . . . . . . 37
   7.    XML Stanzas  . . . . . . . . . . . . . . . . . . . . . . . . 42
   7.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 42
   7.2   Common Attributes  . . . . . . . . . . . . . . . . . . . . . 42



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   7.2.1 to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
   7.2.2 from . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
   7.2.3 id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
   7.2.4 type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
   7.2.5 xml:lang . . . . . . . . . . . . . . . . . . . . . . . . . . 43
   7.3   Message Stanzas  . . . . . . . . . . . . . . . . . . . . . . 44
   7.3.1 Types of Message . . . . . . . . . . . . . . . . . . . . . . 44
   7.3.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 44
   7.4   Presence Stanzas . . . . . . . . . . . . . . . . . . . . . . 46
   7.4.1 Types of Presence  . . . . . . . . . . . . . . . . . . . . . 46
   7.4.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 46
   7.5   IQ Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 48
   7.5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 48
   7.5.2 Types of IQ  . . . . . . . . . . . . . . . . . . . . . . . . 48
   7.5.3 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 49
   7.6   Extended Namespaces  . . . . . . . . . . . . . . . . . . . . 49
   7.7   Stanza Errors  . . . . . . . . . . . . . . . . . . . . . . . 50
   7.7.1 Rules  . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
   7.7.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
   7.7.3 Conditions . . . . . . . . . . . . . . . . . . . . . . . . . 51
   7.7.4 Extensibility  . . . . . . . . . . . . . . . . . . . . . . . 52
   8.    XML Usage within XMPP  . . . . . . . . . . . . . . . . . . . 54
   8.1   Restrictions . . . . . . . . . . . . . . . . . . . . . . . . 54
   8.2   Namespaces . . . . . . . . . . . . . . . . . . . . . . . . . 54
   8.3   Validation . . . . . . . . . . . . . . . . . . . . . . . . . 54
   8.4   Character Encodings  . . . . . . . . . . . . . . . . . . . . 55
   8.5   Inclusion of Text Declaration  . . . . . . . . . . . . . . . 55
   9.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 56
   9.1   XML Namespace Name for TLS Data  . . . . . . . . . . . . . . 56
   9.2   XML Namespace Name for SASL Data . . . . . . . . . . . . . . 56
   9.3   XML Namespace Name for Stream Errors . . . . . . . . . . . . 56
   9.4   XML Namespace Name for Stanza Errors . . . . . . . . . . . . 57
   9.5   Existing Registrations . . . . . . . . . . . . . . . . . . . 57
   10.   Internationalization Considerations  . . . . . . . . . . . . 58
   11.   Security Considerations  . . . . . . . . . . . . . . . . . . 59
   11.1  High Security  . . . . . . . . . . . . . . . . . . . . . . . 59
   11.2  Client-to-Server Communications  . . . . . . . . . . . . . . 59
   11.3  Server-to-Server Communications  . . . . . . . . . . . . . . 60
   11.4  Firewalls  . . . . . . . . . . . . . . . . . . . . . . . . . 60
   11.5  Mandatory to Implement Technologies  . . . . . . . . . . . . 60
         Normative References . . . . . . . . . . . . . . . . . . . . 61
         Informative References . . . . . . . . . . . . . . . . . . . 63
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 63
   A.    XML Schemas  . . . . . . . . . . . . . . . . . . . . . . . . 65
   A.1   Streams namespace  . . . . . . . . . . . . . . . . . . . . . 65
   A.2   TLS namespace  . . . . . . . . . . . . . . . . . . . . . . . 66
   A.3   SASL namespace . . . . . . . . . . . . . . . . . . . . . . . 66
   A.4   Dialback namespace . . . . . . . . . . . . . . . . . . . . . 67



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   A.5   Client namespace . . . . . . . . . . . . . . . . . . . . . . 68
   A.6   Server namespace . . . . . . . . . . . . . . . . . . . . . . 72
   A.7   Stream error namespace . . . . . . . . . . . . . . . . . . . 76
   A.8   Stanza error namespace . . . . . . . . . . . . . . . . . . . 77
   B.    Revision History . . . . . . . . . . . . . . . . . . . . . . 79
   B.1   Changes from draft-ietf-xmpp-core-09 . . . . . . . . . . . . 79
   B.2   Changes from draft-ietf-xmpp-core-09 . . . . . . . . . . . . 79
   B.3   Changes from draft-ietf-xmpp-core-08 . . . . . . . . . . . . 79
   B.4   Changes from draft-ietf-xmpp-core-07 . . . . . . . . . . . . 80
   B.5   Changes from draft-ietf-xmpp-core-06 . . . . . . . . . . . . 80
   B.6   Changes from draft-ietf-xmpp-core-05 . . . . . . . . . . . . 80
   B.7   Changes from draft-ietf-xmpp-core-04 . . . . . . . . . . . . 80
   B.8   Changes from draft-ietf-xmpp-core-03 . . . . . . . . . . . . 80
   B.9   Changes from draft-ietf-xmpp-core-02 . . . . . . . . . . . . 81
   B.10  Changes from draft-ietf-xmpp-core-01 . . . . . . . . . . . . 81
   B.11  Changes from draft-ietf-xmpp-core-00 . . . . . . . . . . . . 81
   B.12  Changes from draft-miller-xmpp-core-02 . . . . . . . . . . . 81
         Intellectual Property and Copyright Statements . . . . . . . 83

































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1. Introduction

1.1 Overview

   The Extensible Messaging and Presence Protocol (XMPP) is an open XML
   [1] 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 and presence technology. As a result of work by the
   XMPP WG, the current document defines the core features of XMPP; XMPP
   IM [23] defines the extensions required to provide the instant
   messaging (IM) and presence functionality defined in RFC 2779 [2].

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 [3].

1.3 Discussion Venue

   The authors welcome discussion and comments related to the topics
   presented in this document. The preferred forum is the
   <xmppwg@jabber.org> mailing list, for which archives and subscription
   information are available at <http://www.jabber.org/cgi-bin/mailman/
   listinfo/xmppwg/>.

1.4 Intellectual Property Notice

   This document is in full compliance with all provisions of Section 10
   of RFC 2026. Parts of this specification use the term "jabber" for
   identifying namespaces and other protocol syntax. Jabber[tm] is a
   registered trademark of Jabber, Inc.  Jabber, Inc. grants permission
   to the IETF for use of the Jabber trademark in association with this
   specification and its successors, if any.













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2. Generalized Architecture

2.1 Overview

   Although XMPP is not wedded to any specific network architecture, to
   this point it usually has been implemented via a typical
   client-server architecture, wherein a client utilizing XMPP accesses
   a server over a TCP [4] 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 to manage
   connections from or sessions for other entities (in the form of XML
   streams to and from authorized clients, servers, and other entities)
   and to route appropriately-addressed XML data "stanzas" 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 users of XMPP-based IM 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. Compliant server
   implementations MUST ensure in-order processing of XML stanzas
   between any two entities.

2.3 Client




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   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 polling or some other mechanism), this specification defines a
   binding for 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 connecting over a discrete TCP
   socket and differentiated by the resource identifier of a Jabber
   Identifier (e.g., user@domain/home vs. user@domain/work) as defined
   under Section 3. The port registered with the IANA [5] for
   connections between a Jabber client and a Jabber server is 5222.

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), 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. There
   are two methods for negotiating a connection between any two servers:
   primarily SASL authentication (Section 6.1) and secondarily server
   dialback (Section 6.2).














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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 [24]. In particular, a valid Jabber Identifier (JID)
   contains a set of ordered elements formed of a domain identifier,
   node identifier, and resource identifier in the following format:
   [node@]domain[/resource].

   All JIDs are based on the foregoing structure. The most common use of
   this structure is to identify an IM user, the server to which the
   user connects, and the user's active session or connection (e.g., a
   specific client) in the form of <user@domain/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).

3.2 Domain Identifier

   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 (a multi-user chat service, a
   user directory, a gateway to a foreign messaging system, etc.).

   The domain identifier for every server or service that will
   communicate over a network SHOULD resolve to a Fully Qualified Domain
   Name. A domain identifier MUST conform to RFC 952 [6] and RFC 1123
   [7]. A domain identifier MUST be no more than 1023 bytes in length
   and MUST conform to the nameprep [8] profile of stringprep [9].

3.3 Node Identifier

   The node identifier is an optional secondary identifier. It usually
   represents the entity requesting and using network access provided by
   the server or gateway (i.e., a client), although it can also



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   represent other kinds of entities (e.g., a multi-user 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 IM applications of XMPP this address is called a "bare
   JID" and is of the form <user@domain>.

   A node identifier MUST be no more than 1023 bytes in length and MUST
   conform to the nodeprep [10] profile of stringprep [9].

3.4 Resource Identifier

   The resource identifier is an optional tertiary identifier, which may
   modify either a <user@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 as the authzid value
   provided during stream authentication. An entity may maintain
   multiple resources simultaneously.

   A resource identifier MUST be no more than 1023 bytes in length and
   MUST conform to the resourceprep [11] profile of stringprep [9].




























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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 may be defined as follows:

   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, and 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,
      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 the XML specification [1]).
      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.

   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 supported and the character
   encoding (see also Section 8.4). 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 authenticated with the server
   (see Section 6), the client MAY send an unlimited 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



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   underlying TCP connection as well.

   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 graphically 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 the XMPP address of 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.



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   o  from -- The 'from' attribute SHOULD be used only in the XML stream
      header from the receiving entity to the initiating entity, and
      MUST be set to the XMPP address of the receiving entity 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. 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  version -- If the initiating entity complies with the protocol
      defined herein, it MUST include a 'version' attribute in the XML
      stream header it sends to the receiving entity, and it MUST set
      the value of the 'version' attribute to "1.0". If the initiating
      entity includes the version attribute and the receiving entity
      supports XMPP 1.0, the receiving entity MUST reciprocate by
      including the attribute in its response.

   We can summarize these values as follows:

           |  initiating to receiving  |  receiving to initiating
   ------------------------------------------------------------
   to      |  hostname of receiver     |  silently ignored
   from    |  silently ignored         |  hostname of receiver
   id      |  silently ignored         |  session key
   version |  signals XMPP 1.0 support |  signals XMPP 1.0 support


4.3 Namespace Declarations

   The stream element MUST possess both a stream namespace declaration
   and a default namespace declaration (as "namespace declaration" is
   defined in the XML namespaces specification [12]).

4.3.1 Stream Namespace Declaration

   A stream namespace declaration (e.g., 'xmlns:stream') is REQUIRED in
   both XML streams. A compliant entity SHOULD accept any namespace
   prefix on the <stream/> element; however, for historical reasons some
   entities MAY accept only a 'stream' prefix, resulting in the use of a
   <stream:stream/> element as the stream root. The name of the stream



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   namespace MUST be "http://etherx.jabber.org/streams".

4.3.2 Default Namespace Declaration

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

   Since XML streams function as containers for any XML stanzas sent
   asynchronously between network endpoints, it should be possible to
   scope an XML stream with any default namespace declaration. At a
   minimum, a compliant implementation MUST support the following two
   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

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

   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 scoped by the 'jabber:client' or 'jabber:server'
   namespace, it MUST support all three core stanza types (message,
   presence, and IQ) as described herein and defined in the schema.

4.4 Stream Features

   If the initiating entity sends a "version='1.0'" flag in its
   initiating stream element, the receiving entity MUST send a features
   child element 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 for SASL and
   TLS negotiation only, but it could be used for other negotiable
   features in the future (usage is defined under Stream Encryption
   (Section 5) and Stream Authentication (Section 6) below). If an
   entity does not understand or support some features, it SHOULD
   silently ignore them.



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4.5 Stream Errors

   The root stream element MAY contain an error child element (e.g.,
   <stream:error/> if the stream namespace prefix is 'stream'). 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.

4.5.1 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 close the
      underlying TCP connection.

   o  If the error occurs while the stream is being set up, the
      receiving entity MUST still send the opening and closing stream
      tags and include the error element as a child of the stream
      element. 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.


4.5.2 Syntax

   The syntax for stream errors is as follows:

   <stream:error class='error-class'>
     <condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
       <descriptive-element-name/>
     </condition>
   </stream:error>

   The value of the 'class' attribute must be one of the following:

   o  address -- the condition relates to the JID or domain to which the
      stream was addressed

   o  format -- the condition relates to XML format or structure

   o  redirect -- the condition relates to a host redirection

   o  server -- the condition relates to the internal state of the



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      server

   The <condition/> element MUST contain a child element that specifies
   a particular stream-level error condition, as defined in the next
   section. (Note: the XML namespace name
   'urn:ietf:params:xml:ns:xmpp-streams' that scopes the <condition/>
   element adheres to the format defined in The IETF XML Registry [25].)

4.5.3 Conditions

   The following stream-level error conditions are defined:

   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; the associated class is
      "address".

   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; the associated class is
      "address".

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

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

   o  <invalid-namespace/> -- the stream namespace name is something
      other than "http://etherx.jabber.org/streams" or the dialback
      namespace name is something other than "jabber:server:dialback";
      the associated class is "format".

   o  <nonmatching-hosts/> -- the hostname provided in a 'from' address
      does not match the hostname (or any validated domain) negotiated
      via SASL or dialback; the associated class is "address".

   o  <not-authorized/> -- the entity does not possess sufficient
      privileges to perform the desired action; the associated class is
      "access".

   o  <remote-connection-failed/> -- the server is unable to properly
      connect to a remote resource that is required for authentication
      or authorization; the associated class is "server".




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   o  <resource-constraint/> -- the server is resource-contrained and is
      unable to service the stream; the associated class is "server".

   o  <see-other-host/> -- the server will not provide service to the
      initiating entity but is redirecting traffic to another host; this
      element SHOULD contain CDATA specifying the alternate hostname or
      IP address to which the initiating entity MAY attempt to connect;
      the associated class is "redirect".

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

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

   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; this element
      MAY contain CDATA specifying the XMPP version(s) supported by the
      server; the associated class is "format".

   o  <xml-not-well-formed/> -- the initiating entity has sent XML that
      is not well-formed as defined by the XML specification [1]; the
      associated class is "format".


4.5.4 Extensibility

   If desired, an XMPP application MAY provide custom error information;
   this MUST be contained in a properly-namespaced child of the
   <condition/> element (i.e., the namespace name MUST NOT be one of the
   namespace names defined herein).

4.6 Simple Streams Example

   The following is 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):












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   A basic session:

   C: <?xml version='1.0'?>
      <stream:stream
          to='shakespeare.lit'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   S: <?xml version='1.0'?>
      <stream:stream
          from='shakespeare.lit'
          id='id_123456789'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   ... authentication ...
   C:   <message from='juliet@shakespeare.lit'
                 to='romeo@shakespeare.lit'>
   C:     <body>Art thou not Romeo, and a Montague?</body>
   C:   </message>
   S:   <message from='romeo@shakespeare.lit'
                 to='juliet@shakespeare.lit'>
   S:     <body>Neither, fair saint, if either thee dislike.</body>
   S:   </message>
   C: </stream:stream>
   S: </stream:stream>

























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   A session gone bad:

   C: <?xml version='1.0'?>
      <stream:stream
          to='shakespeare.lit'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   S: <?xml version='1.0'?>
      <stream:stream
          from='shakespeare.lit'
          id='id_123456789'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   ... authentication ...
   C: <message><body>Bad XML, no closing body tag!</message>
   S: <stream:error class='format'>
        <condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
          <xml-not-well-formed/>
        </condition>
      </stream:error>
   S: </stream:stream>




























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5. Stream Encryption

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) [13] protocol, along with a "STARTTLS"
   extension that is modelled after similar extensions for the IMAP
   [26], POP3 [27], and ACAP [28] protocols as described in RFC 2595
   [29]. The namespace name for the STARTTLS extension is
   'urn:ietf:params:xml:ns:xmpp-tls', which adheres to the format
   defined in The IETF XML Registry [25].)

   TLS SHOULD be used between any initiating entity and any receiving
   entity (e.g., a stream from a client to a server or from one server
   to another). An administrator of a given domain MAY require use of
   TLS for either or both client-to-server communications and
   server-to-server communications. Servers SHOULD use TLS betweeen two
   domains for the purpose of securing server-to-server communications.
   When the remote domain is already known, the server can verify the
   credentials of the known domain by comparing known keys or
   certificates. When the remote domain is not recognized, it may still
   be possible to verify a certificate if it is signed by a common
   trusted authority. Even if there is no way to verify certificates
   (e.g., an unknown domain with a self-signed certificate, or a
   certificate signed by an unrecognized authority), if the servers
   choose to communicate despite the lack of verified credentials, TLS
   still SHOULD be used to provide channel encryption.

   The following business rules apply:

   1.  An initiating entity that complies with this specification MUST
       include the "version='1.0'" flag in the initiating stream header.

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

   3.  If the initiating entity chooses to use TLS for stream
       encryption, TLS negotiation MUST be completed before proceeding
       to SASL negotiation.

   4.  The initiating entity MUST validate the certificate presented by
       the receiving entity:




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       Case 1 -- The initiating entity has been configured with a set of
       trusted root certificates: Normal certificate validation
          processing is appropriate, and SHOULD follow the rules defined
          for HTTP over TLS [14]. The trusted roots may be either a
          well-known public set or a manually configured Root CA (e.g.,
          an organization's own Certificate Authority or a self-signed
          Root CA for the service as described under High Security
          (Section 11.1)). This case is RECOMMENDED.

       Case 2 -- The initiating entity has been configured with the
       receiving entity's self-signed service certificate: Simple
          comparison of public keys is appropriate. This case is NOT
          RECOMMENDED (see High Security (Section 11.1) for details).

       If the above methods fail, the certificate SHOULD be presented to
       a user for approval; if presented, the receiver MUST deliver the
       entire certificate chain to the user, who SHOULD be given the
       option to store the Root CA certificate (not the service or End
       Entity certificate) and to not be queried again regarding
       acceptance of the certificate for some reasonable period of time.

   5.  If the TLS negotiation is successful, the receiving entity MUST
       discard any knowledge obtained from the initiating entity before
       TLS takes effect.

   6.  If the TLS negotiation is successful, the initiating entity MUST
       discard any knowledge obtained from the receiving entity before
       TLS takes effect.

   7.  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.

   8.  If the TLS negotiation results in success, the initiating entity
       SHOULD continue with SASL negotiation.

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


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



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       stream by sending the opening XML stream header to the receiving
       entity, including the "version='1.0'" flag.

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

   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 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 scoped by the
       '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 receiving entity MUST ignore any further XML data sent over
       the XML stream but keep the underlying TCP connection open for
       the purpose of completing the TLS negotiation.

   6.  The initiating entity and receiving entity attempt to complete a
       TLS negotiation in accordance with RFC 2246 [13].

   7.  If the TLS negotiation is successful, the initiating entity
       SHOULD initiate a new stream by sending an opening XML stream
       header to the receiving entity. If the TLS negotiation is
       unsuccessful, the receiving entity MUST terminate the TCP
       connection.

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


5.3 Client-to-Server Example

   The following example shows the data flow for a client securing a
   stream using STARTTLS (the IANA registers port 5222 for
   client-to-server communications using XMPP/Jabber, but another port
   MAY be used).



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   Step 1: Client initiates stream to server:

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

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

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       version='1.0'>

   Step 3: Server sends the STARTTLS extension to the 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 the 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.






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

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

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

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

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       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 SHOULD continue with stream authentication (Section
   6).

5.4 Server-to-Server Example

   The following example shows the data flow for two servers securing a
   stream using STARTTLS (the IANA registers port 5269 for
   server-to-server communications using XMPP/Jabber, but another port
   MAY be used).

   Step 1: Server1 initiates stream to Server2:

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







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

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       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'>
       <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'
       version='1.0'>

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



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

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       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>
     </mechanisms>
   </stream:features>

   Step 9: Server1 SHOULD continue with stream authentication (Section
   6).

































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6. Stream Authentication

   XMPP includes two methods for enforcing authentication at the level
   of XML streams. The secure and preferred method for authenticating
   streams between two entities uses an XMPP adaptation of the Simple
   Authentication and Security Layer (SASL) [15]. If SASL negotiation is
   not possible, some level of trust MAY be established between servers
   based on existing trust in DNS; the authentication method used in
   this case is the server dialback protocol that is native to XMPP (no
   such ad-hoc method is defined between a client and a server). If SASL
   is used for server-to-server authentication, the servers MUST NOT use
   dialback. For further information about the relative merits of these
   two methods, consult Security Considerations (Section 11).

   Stream authentication is REQUIRED for all direct communications
   between two entities; if an entity sends a stanza to an
   unauthenticated stream, the receiving entity SHOULD silently drop the
   stanza, MUST NOT process it, and MAY terminate both the XML stream
   and the underlying TCP connection.

6.1 SASL Authentication

6.1.1 Overview

   The Simple Authentication and Security Layer (SASL) provides a
   generalized method for adding authentication support to
   connection-based protocols. XMPP uses a generic XML namespace profile
   for SASL that conforms to section 4 ("Profiling Requirements") of RFC
   2222 [15] (the XMPP-specific namespace name is
   'urn:ietf:params:xml:ns:xmpp-sasl'), which adheres to the format
   defined in The IETF XML Registry [25].)

   The following business rules apply:

   1.  If TLS is used for stream encryption, SASL SHOULD NOT be used for
       anything except stream authentication (i.e., a security layer
       SHOULD NOT be negotiated using SASL). Conversely, if a security
       layer is to be negotiated via SASL, TLS SHOULD NOT be used.

   2.  If the initiating entity is capable of authenticating via SASL,
       it MUST include the "version='1.0'" flag in the initiating stream
       header.

   3.  If the receiving entity is capable of accepting authentications
       via SASL, it MUST send one or more authentication mechanisms
       within a <mechanisms/> element scoped by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace in response to the
       opening stream tag received from the initiating entity.



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   4.  If the SASL negotiation 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.

   5.  If the SASL negotiation 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.

   The following syntax rules apply:

   1.  The initial challenge MUST include a realm, nonce, qop, charset,
       and algorithm.

   2.  The inital response for client-to-server negotiation MUST include
       all and only a username, realm, nonce, cnonce, nc, qop,
       digest-uri, response, charset, and authzid.

   3.  The inital response for server-to-server negotiation MUST include
       all and only a realm, nonce, cnonce, nc, qop, digest-uri,
       response, and charset.

   4.  The realm-value MUST be no more than 1023 bytes in length and
       MUST conform to the nameprep [8] profile of stringprep [9].

   5.  The username-value MUST be no more than 1023 bytes in length and
       MUST conform to the nodeprep [10] profile of stringprep [9].

   6.  The response-value MUST be computed in accordance with the
       relevant SASL mechanism as defined by the appropriate RFC (e.g.,
       RFC 2831 [16] for digest authentication).

   7.  The authzid-value MUST be a Jabber Identifier of the form
       <user@domain/resource>, i.e., it MUST include a node identifier,
       domain identifier, and resource identifier.


6.1.2 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 a
       '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 response, the receiving



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       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 scoped by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace, which in turn is a
       child of a <features/> element in the streams namespace. If
       channel encryption must 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 methods prior to channel encryption. If the
       initiating entity presents a valid initiating entity certificate
       during prior TLS negotiation, the receiving entity MAY offer the
       SASL EXTERNAL mechanism to the initiating entity during stream
       authentication (see RFC 2222 [15]).

   3.  The initiating entity selects a mechanism by sending an <auth/>
       element scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl'
       namespace to the receiving entity and including the appropriate
       value for the 'mechanism' attribute; this element MAY optionally
       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, the authentication credentials shall be taken
       from the certificate presented during prior TLS negotiation.

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

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

   2.  The receiving entity reports failure of the handshake by sending



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       a <failure/> element scoped 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. The
       following conditions are defined:

       *  <authentication-mechanism-too-weak/>

       *  <invalid-realm/>

       *  <not-authorized/>

       *  <password-transition-required/>

       *  <temporary-authentication-failure/>

   3.  The receiving entity reports success of the handshake by sending
       a <success/> element scoped by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
       entity; this element MAY optionally contain character data (in
       SASL terminology "additional data with success").

   Any character data contained within these elements MUST be encoded
   using base64.

6.1.3 SASL Definition

   Section 4 of the SASL specification [15] requires 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
      '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. (All of these elements are scoped by the
      'urn:ietf:params:xml:ns:xmpp-sasl' namespace.)



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   security layer negotiation: If a security layer is negotiated, both
      sides consider the original stream closed and new <stream> headers
      are sent by both entities. The security layer takes effect
      immediately following the ">" character of the <response/> element
      for the client and immediately following the closing ">" character
      of the <succeed/> element for the server. (Both of these elements
      are scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace.)

   use of the authorization identity: The authorization identity is used
      by xmpp only in negotiation between a client and a server, and
      denotes the "full JID" (user@domain/resource) requested by the
      user or application associated with the client.


6.1.4 Client-to-Server Example

   The following example shows the data flow for a client authenticating
   with a server using SASL (the IANA registers port 5222 for
   client-to-server communications using XMPP/Jabber, but another port
   MAY be used).

   Step 1: Client initiates stream to server:

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

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

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       from='domain'
       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>
       <mechanism>PLAIN</mechanism>
     </mechanisms>
   </stream:features>





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   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 the client:

   <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi
       xxb3A9ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz
   </challenge>

   The decoded challenge is:

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

   Step 6: Client responds to the challenge:

   <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       dXNlcm5hbWU9InJvYiIscmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik
       9BNk1HOXRFUUdtMmhoIixjbm9uY2U9Ik9BNk1IWGg2VnFUclJrIixuYz0w
       MDAwMDAwMSxxb3A9YXV0aCxkaWdlc3QtdXJpPSJ4bXBwL2NhdGFjbHlzbS
       5jeCIscmVzcG9uc2U9ZDM4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxNDNh
       ZjcsY2hhcnNldD11dGYtOCxhdXRoemlkPSJyb2JAY2F0YWNseXNtLmN4L2
       15UmVzb3VyY2Ui
   </response>

   The decoded response is:

   username="rob",realm="cataclysm.cx",\
   nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\
   nc=00000001,qop=auth,digest-uri="xmpp/cataclysm.cx",\
   response=d388dad90d4bbd760a152321f2143af7,charset=utf-8,\
   authzid="rob@cataclysm.cx/myResource"

   Step 7: Server sends another challenge to the client:

   <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
   </challenge>

   The decoded challenge is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd





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   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'>
     <mechanism-too-weak/>
   </failure>

   Step 10: Client initiates a new stream to the server:

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

   Step 11: Server responds by sending a stream header to the client,
   with the stream already authenticated (not followed by further stream
   features):

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       from='domain'
       version='1.0'>


6.1.5 Server-to-Server Example

   The following example shows the data flow for a server authenticating
   with another server using SASL (the IANA registers port 5269 for
   server-to-server communications using XMPP/Jabber, but another port
   MAY be used).

   Step 1: Server1 initiates stream to Server2:

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




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

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       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'>
       cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi
       xxb3A9ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz
   </challenge>

   The decoded challenge is:

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

   Step 6: Server1 responds to the challenge:

   <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi
       xjbm9uY2U9Ik9BNk1IWGg2VnFUclJrIixuYz0wMDAwMDAwMSxxb3A9YXV0
       aCxkaWdlc3QtdXJpPSJ4bXBwL2NhdGFjbHlzbS5jeCIscmVzcG9uc2U9ZD
       M4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcsY2hhcnNldD11dGYt
       OAo=
   </response>

   The decoded response is:




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

   Step 7: Server2 sends another challenge to Server1:

   <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
   </challenge>

   The decoded challenge is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd

   Step 8: Server1 responds to the challenge:

   <response 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'>
     <temporary-auth-failure/>
   </failure>

   Step 10: Server1 initiates a new stream to Server2:

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

   Step 11: Server2 responds by sending a stream header to Server1, with
   the stream already authenticated (not followed by further stream
   features):

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       version='1.0'>







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6.2 Dialback Authentication

   XMPP includes a protocol-level method for verifying that a connection
   between two servers can be trusted as much as the DNS can be trusted.
   The method is called dialback and is used only within XML streams
   that are declared under the "jabber:server" namespace.

   The purpose of the dialback protocol is to make server spoofing more
   difficult, and thus to make it more difficult to forge XML stanzas.
   Dialback is decidedly not intended as a mechanism for securing or
   encrypting the streams between servers as is done via SASL and TLS,
   only for helping to prevent the spoofing of a server and the sending
   of false data from it. In particular, dialback authentication is
   susceptible to DNS poisoning attacks unless DNSSec [30] is used.
   Furthermore, even if the DNS information is accurate, dialback
   authentication cannot protect from attacks where the attacker is
   capable of hijacking the IP address of the remote domain. Domains
   requiring more robust security SHOULD use TLS and SASL as defined
   above.

   Server dialback is made possible by the existence of DNS, since one
   server can verify that another server which is connecting to it is
   authorized to represent a given hostname. All DNS hostname
   resolutions MUST first resolve the hostname using an SRV [18] record
   of _jabber._tcp.server. If the SRV lookup fails, the fallback is a
   normal A lookup to determine the IP address, using the jabber-server
   port of 5269 assigned by the Internet Assigned Numbers Authority [5].

   The method for generating and verifying the keys used in the dialback
   protocol MUST take into account the hostnames being used, the random
   ID generated for the stream, and a secret known by the authoritative
   server's network. Generating unique but verifiable keys is important
   to prevent common man-in-the-middle attacks and server spoofing.

   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



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      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.

   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.

   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    |
       | ----------------------> |
       |                         |
       |  establish connection   |
       | <---------------------- |
       |                         |
       |   send stream header    |
       | <---------------------- |
       |                         |                   Authoritative
       |   send dialback key     |                       Server
       | ----------------------> |                   -------------
       |                         |                         |
                                 |  establish connection   |
                                 | ----------------------> |
                                 |                         |
                                 |   send stream header    |
                                 | ----------------------> |



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                                 |                         |
                                 |  establish connection   |
                                 | <---------------------- |
                                 |                         |
                                 |   send stream header    |
                                 | <---------------------- |
                                 |                         |
                                 |   send dialback key     |
                                 | ----------------------> |
                                 |                         |
                                 |  validate dialback key  |
                                 | <---------------------- |
                                 |
       |  report dialback result |
       | <---------------------- |
       |                         |


6.2.1 Dialback Protocol

   The 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:









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   <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.

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

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

        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
        SHOULD generate a <not-authorized/> stream error condition and
        terminate both the XML stream and the underlying TCP connection.

   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:






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   <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
        terminate both the XML stream and the underlying TCP connection
        between it and Authoritative Server. 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
        between it and Authoritative Server. If either of the foregoing
        stream errors 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,



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        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 a <nonmatching-hosts/> 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'
       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 a <nonmatching-hosts/> stream
        error condition and terminate both the XML stream and the
        underlying TCP connection.

   10.  Receiving Server informs Originating Server of the result:







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   <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
   <invalid-xml/> 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 a <nonmatching-hosts/> 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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7. XML Stanzas

7.1 Overview

   Once the XML streams in each direction have been authenticated and
   (if desired) encrypted, XML stanzas can be sent over the streams.
   Three XML stanza types are defined for the 'jabber:client' and
   'jabber:server' namespaces: <message/>, <presence/>, and <iq/>.

   In essence, the <message/> stanza type 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.
   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. The <iq/> element can be seen as a
   "request-response" mechanism similar to HTTP, whereby two entities
   can engage in a structured conversation using 'get' or 'set' requests
   and 'result' or 'error' responses.

7.2 Common Attributes

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

7.2.1 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) MAY legitimately lack 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 <invalid-xml/> stream error
   condition and terminate both the XML stream and the underlying TCP
   connection.

   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.

7.2.2 from




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

   In the 'jabber:client' namespace, a client MUST NOT include a 'from'
   attribute on the stanzas it sends to a server; if a server receives a
   stanza from a client and the stanza possesses a 'from' attribute, it
   MUST ignore the value of the 'from' attribute and MAY return an error
   to the sender. In addition, a server MUST stamp stanzas received from
   a client with the user@domain/resource (full JID) of the connected
   resource that generated the stanza as defined by the authzid provided
   in the SASL negotiation.

   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 <invalid-xml/> stream error
   condition. Furthermore, the domain identifier portion of the JID
   contained in the 'from' attribute MUST match the hostname of the
   sending server (or any validated domain) as communicated in the SASL
   negotiation or dialback negotiation; if a server receives a stanza
   that does not meet this restriction, it MUST generate a
   <nonmatching-hosts/> stream error condition. Both of these conditions
   MUST result in closing of the stream and termination of the
   underlying TCP connection.

7.2.3 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 use of
   IQ stanzas). If the stanza sent by the sending entity is an IQ stanza
   of type "get" or "set", the receiving entity MUST include an 'id'
   attribute with the same value in any replies of type "result" or
   "error". The value of the 'id' attribute is NOT REQUIRED to be unique
   either globally, within a domain, or within a stream.

7.2.4 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, and thus are defined in the
   following sections.

7.2.5 xml:lang

   A stanza SHOULD possess an 'xml:lang' attribute (as defined in
   Section 2.12 of the XML specification [1]) specifying the default
   language of any XML character data contained in the stanza or its
   child elements. The value of the 'xml:lang' attribute MUST be an



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   NMTOKEN and MUST conform to the format defined in RFC 3066 [17].

7.3 Message Stanzas

   Message stanzas in the 'jabber:client' or 'jabber:server' namespace
   are used to "push" information to another entity. Common uses in the
   context of instant messaging 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. These messages
   types are identified more fully below.

7.3.1 Types of Message

   The 'type' attribute of a message stanza is OPTIONAL; if included, it
   specifies the conversational context of the message. The sending of a
   message stanza without a 'type' attribute signals that the message
   stanza is a single message. However, the 'type' attribute MAY also
   have one of the following values:

   o  chat

   o  error

   o  groupchat

   o  headline

   For information about the meaning of these message types, refer to
   XMPP IM [23].

7.3.2 Children

   As described under extended namespaces (Section 7.6), 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 Section 7.7. If the message stanza
   has no 'type' attribute or has a 'type' attribute with a value of
   "chat", "groupchat", or "headline", it MAY contain any of the
   following child elements without an explicit namespace declaration:

7.3.2.1 Subject

   The <subject/> element specifies the topic of the message. The
   <subject/> element SHOULD NOT possess any attributes, with the



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   exception of the 'xml:lang' attribute. Multiple instances of the
   <subject/> element MAY be included for the purpose of providing
   alternate versions of the same subject, but only if each instance
   possesses an 'xml:lang' attribute with a distinct language value. The
   <subject> element MUST NOT contain mixed content.

7.3.2.2 Body

   The <body/> element contains the textual contents of the message;
   this child element is normally included but NOT REQUIRED. The <body/>
   element SHOULD 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.

7.3.2.3 Thread

   The <thread/> element contains a random string that is generated by
   the sender and that SHOULD be copied back in replies; it is used for
   tracking a conversation thread (sometimes referred to as an "IM
   session") between two entities. If used, it MUST be unique to that
   conversation thread within the stream and MUST be consistent
   throughout that conversation. The use of the <thread/> element is
   optional and is not used to identify individual messages, only
   conversations. Only one <thread/> element MAY be included in a
   message stanza, and it MUST NOT possess any attributes. The <thread/>
   element MUST be treated as an opaque string by entities; no semantic
   meaning may be derived from it, and only exact, case-insensitve
   comparisons may be made against it. The <thread> element MUST NOT
   contain mixed content.

   The method for generating thread IDs SHOULD be as follows:

   1.  concatenate the sender's full JID (user@domain/resource) with the
       recipient's full JID

   2.  concatenate these JID strings with a full ISO-8601 timestamp
       including year, month, day, hours, minutes, seconds, and UTC
       offset in the following format: YYYY-MM-DD-Thh:mm:ssTZD (where
       TZD is either "Z" for UTC or "[+|-]hh:mm" for an offset from UTC)

   3.  hash the resulting string according to the SHA1 algorithm

   4.  convert the hexidecimal SHA1 output to all lowercase






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7.4 Presence Stanzas

   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.

7.4.1 Types of Presence

   The 'type' attribute of a presence stanza is optional. A presence
   stanza that does not possess a 'type' attribute is used to signal to
   the server that the sender is online and available for communication.
   If included, the 'type' attribute specifies a lack of availability, a
   request to manage a subscription to another entity's presence, a
   request for another entity's current presence, or an error related to
   a previously-sent presence stanza. The 'type' attribute MAY 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. In general
      SHOULD NOT be sent by a client.

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

   Detailed information about presence semantics and about the
   subscription model used within XMPP can be found in XMPP IM [23].

7.4.2 Children

   As described under extended namespaces (Section 7.6), a presence
   stanza MAY contain any properly-namespaced child element.



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   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 Section 7.7. 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"):

7.4.2.1 Show

   The optional <show/> element specifies a particular availability
   status of an entity or specific resource (if a <show/> element is not
   provided, default availability is assumed). Only one <show/> element
   MAY be included in a presence stanza, and it SHOULD NOT possess any
   attributes. The CDATA value SHOULD be one of the following (values
   other than these four SHOULD be ignored; additional availability
   types could be defined through a properly-namespaced child element of
   the presence stanza):

   o  away

   o  chat

   o  xa

   o  dnd

   For information about the meaning of these values, refer to XMPP IM
   [23].

7.4.2.2 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 SHOULD 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.

7.4.2.3 Priority

   The optional <priority/> element specifies the priority level of the
   connected resource. The value may be any integer between -128 and
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   stanza, and it MUST NOT possess any attributes. For information
   regarding the semantics of priority values in stanza routing within
   IM applications, see XMPP IM [23].

7.5 IQ Stanzas

7.5.1 Overview

   Info/Query, or IQ, is a request-response mechanism, similar in some
   ways to HTTP [31]. IQ stanzas in the 'jabber:client' or
   'jabber:server' namespace 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.

   Most IQ interactions follow a common pattern of structured data
   exchange such as get/result or set/result (although an error may be
   returned in response 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'>  |
       | <------------------------ |
       |                           |

   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). 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).

7.5.2 Types of IQ




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   The 'type' attribute of an IQ stanza is REQUIRED. The 'type'
   attribute specifies a distinct step within a request-response
   interaction. The value SHOULD be one of the following (all other
   values SHOULD be ignored):

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

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

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

   o  error -- An error has occurred regarding processing or delivery of
      a previously-sent get or set.


7.5.3 Children

   As described under extended namespaces (Section 7.6), an IQ stanza
   MAY contain any properly-namespaced child element. Note that an IQ
   stanza contains no children in the 'jabber:client' or 'jabber:server'
   namespace since it is a vessel for XML in another namespace.

   An IQ stanza of type "get" or "set" MUST include one and only one
   child element. An IQ stanza of type "error" SHOULD include the child
   element contained in the associated "set" or "get" and MUST include
   an <error/> child; for details, see Section 7.7.

7.6 Extended Namespaces

   While the core data elements 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 core data elements
   for the purpose of providing additional functionality. Thus a
   message, presence, or IQ stanza MAY house one or more optional child
   elements containing content that extends the meaning of the message
   (e.g., an encrypted form of the message body). 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. 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



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   stanza to the recipient. In particular:

   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 in an extended namespace it does not
         understand, the portion of the stanza that is in the unknown
         namespace SHOULD be ignored.

      *  If an entity receives a message stanza without a <body/>
         element but containing only a child element bound by a
         namespace it does not understand, it MUST ignore the entire
         stanza.

      *  If an entity receives an IQ stanza in 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.


7.7 Stanza Errors

   As defined herein, stanza-related errors are handled in a manner
   similar to stream errors (Section 4.5).

7.7.1 Rules

   The following rules apply to stanza-related errors:

   o  A stanza whose 'type' attribute has a value of "error" MUST
      contain an <error/> child element.

   o  The receiving or processing entity that returns an error to the
      sending entity SHOULD include the original XML sent along with the
      <error/> element and its children so that the sender can inspect
      and if necessary correct the XML before re-sending.

   o  An entity that receives a stanza whose 'type' attribute has a
      value of "error" MUST NOT respond to the stanza with a further
      stanza of type "error"; this helps to prevent looping.




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   o  An <error/> child MUST NOT be included if the 'type' attribute has
      a value other than "error".


7.7.2 Syntax

   The syntax for stanza-related errors is as follows:

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

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

   The value of the 'class' attribute MUST be one of the following:

   o  access -- the condition relates to access rights, permissions, or
      authorization

   o  address -- the condition relates to the JID or domain to which the
      stanza was addressed

   o  format -- the condition relates to XML format or structure

   o  recipient -- the condition relates to the state or capabilities of
      the recipient (which may be the server)

   o  server -- the condition relates to the internal state of the
      server

   The <condition/> element MUST contain a child element that specifies
   a particular stanza-related error condition, and its namespace name
   MUST be 'urn:ietf:params:xml:ns:xmpp-stanzas', which adheres to the
   format defined in The IETF XML Registry [25].

7.7.3 Conditions

   The following stanza-related error conditions are defined:

   o  <bad-request/> -- the sender has sent XML that is malformed or
      cannot be processed (e.g., a client-generated stanza includes a
      'from' address, or an IQ stanza includes an unrecognized value of
      the 'type' attribute); the associated class is "format".



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

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

   o  <forbidden/> -- the stanza is understood but the action is
      forbidden; the associated class is "access".

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

   o  <item-not-found/> -- the addressed JID or item requested cannot be
      found; the associated class is "address".

   o  <jid-malformed/> -- the value of the 'to' attribute in the
      sender's stanza does not adhere to the syntax defined in
      Addressing (Section 3); the associated class is "address".

   o  <not-allowed/> -- the action is not permitted when attempted by
      the sender; the associated class is "access".

   o  <recipient-unavailable/> -- the specific recipient requested is
      currently unavailable; the associated class is "recipient".

   o  <registration-required/> -- the user is not authorized to access
      the requested service because registration is required; the
      associated class is "access".

   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 class is "address".

   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 class
      is "server".

   o  <service-unavailable/> -- the service requested is currently
      unavailable on the server; the associated class is "server".


7.7.4 Extensibility

   If desired, an XMPP application MAY provide custom error information



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   by including a properly-namespaced child of the appropriate
   descriptive element name, for example:

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







































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8. XML Usage within XMPP

8.1 Restrictions

   XMPP is a simplified and specialized protocol for streaming XML
   elements in order to exchange messages and presence 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 must support the full XML specification [1]. 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 the XML specification [1])

   o  processing instructions (Section 2.6)

   o  internal or external DTD subsets (Section 2.8)

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

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

8.2 Namespaces

   XML Namespaces [12] 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 XML to be structurally mixed with any
   data element within XMPP.

   Additionally, an XMPP implementation MAY be more strict about
   namespace prefixes than the XML namespace specification requires.

8.3 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. 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,



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   and schemas are included herein for descriptive purposes only.

8.4 Character Encodings

   Software implementing XML streams MUST support the UTF-8 (RFC 2279
   [19]) and UTF-16 (RFC 2781 [20]) transformations of Universal
   Character Set (ISO/IEC 10646-1 [21]) characters. Software MUST NOT
   attempt to use any other encoding for transmitted data. The encodings
   of the transmitted and received streams are independent. Software MAY
   select either UTF-8 or UTF-16 for the transmitted stream, and SHOULD
   deduce the encoding of the received stream as described in the XML
   specification [1]. For historical reasons, existing implementations
   MAY support UTF-8 only.

8.5 Inclusion of Text Declaration

   An application MAY send a text declaration. Applications MUST follow
   the rules in the XML specification [1] regarding the circumstances
   under which a text declaration is included.
































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9. IANA Considerations

9.1 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.

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

   Specification: [RFCXXXX]

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

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


9.2 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.

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

   Specification: [RFCXXXX]

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

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


9.3 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.

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

   Specification: [RFCXXXX]

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





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


9.4 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.

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

   Specification: [RFCXXXX]

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

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


9.5 Existing Registrations

   The IANA registers "xmpp" as a GSSAPI [22] service name, as specified
   in Section 6.1.3.

   Additionally, the IANA registers "jabber-client" and "jabber-server"
   as keywords for TCP ports 5222 and 5269 respectively.

























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10. Internationalization Considerations

   Each XML stanza SHOULD include the 'xml:lang' attribute as described
   above. Servers MUST NOT modify or delete 'xml:lang' attributes from
   stanzas they receive from other entities.














































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11. Security Considerations

11.1 High Security

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

   Standalone, self-signed service certificates SHOULD NOT be used;
   rather, an entity that wishes to generate a self-signed service
   certificate SHOULD first generate a self-signed Root CA certificate
   and then generate a signed service certificate. Entities that
   communicate with the service SHOULD be configured with the Root CA
   certificate rather than the service certificate; this avoids problems
   associated with simple comparison of service certificates. If a
   self-signed service certificate is used, an entity SHOULD NOT trust
   it if it is changed to another self-signed certificate or a
   certificate signed by an unrecognized authority.

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

11.2 Client-to-Server Communications

   The TLS protocol for encrypting XML streams (defined under 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
   Section 6.1) provides a reliable mechanism for validating that a
   client connecting to a server is who it claims to be.

   The IP address and method of access of clients MUST NOT be made
   available 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.

   End-to-end encryption of message bodies and presence status
   information MAY be effected through use of the methods defined in
   End-to-End Object Encryption in XMPP [32].




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11.3 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 the lower-security Dialback
   Protocol (Section 6.2) as a fallback mechanism that helps to prevent
   the spoofing of domains.

   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. In the absence of
   high security, a domain MAY use server dialback for inter-domain
   communications.

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

11.4 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 [5]. Use of these well-known ports allows
   administrators to easily enable or disable XMPP activity through
   existing and commonly-deployed firewalls.

11.5 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 (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher
      supporting client-side certificates)










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Normative References

   [1]   World Wide Web Consortium, "Extensible Markup Language (XML)
         1.0 (Second Edition)", W3C xml, October 2000, <http://
         www.w3.org/TR/2000/REC-xml-20001006>.

   [2]   Day, M., Aggarwal, S., Mohr, G. and J. Vincent, "A Model for
         Presence and Instant Messaging", RFC 2779, February 2000,
         <http://www.ietf.org/rfc/rfc2779.txt>.

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

   [4]   University of Southern California, "Transmission Control
         Protocol", RFC 793, September 1981, <http://www.ietf.org/rfc/
         rfc0793.txt>.

   [5]   Internet Assigned Numbers Authority, "Internet Assigned Numbers
         Authority", January 1998, <http://www.iana.org/>.

   [6]   Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet host
         table specification", RFC 952, October 1985.

   [7]   Braden, R., "Requirements for Internet Hosts - Application and
         Support", STD 3, RFC 1123, October 1989.

   [8]   Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile
         for Internationalized Domain Names (draft-ietf-idn-nameprep-11,
         work in progress)", June 2002.

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

   [10]  Saint-Andre, P. and J. Hildebrand, "Nodeprep: A Stringprep
         Profile for Node Identifiers in XMPP
         (draft-ietf-xmpp-nodeprep-02, work in progress)", April 2003.

   [11]  Saint-Andre, P. and J. Hildebrand, "Resourceprep: A Stringprep
         Profile for Resource Identifiers in XMPP
         (draft-ietf-xmpp-resourceprep-02, work in progress)", April
         2003.

   [12]  World Wide Web Consortium, "Namespaces in XML", W3C xml-names,
         January 1999, <http://www.w3.org/TR/1999/
         REC-xml-names-19990114/>.

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



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         1999.

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

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

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

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

   [18]  Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
         location of services (DNS SRV)", RFC 2052, October 1996.

   [19]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
         2279, January 1998.

   [20]  Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO 10646",
         RFC 2781, February 2000.

   [21]  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.

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






















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Informative References

   [23]  Saint-Andre, P. and J. Miller, "XMPP Instant Messaging
         (draft-ietf-xmpp-im-11, work in progress)", May 2003.

   [24]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 2396, August
         1998, <http://www.ietf.org/rfc/rfc2396.txt>.

   [25]  Mealling, M., "The IANA XML Registry",
         draft-mealling-iana-xmlns-registry-04 (work in progress), June
         2002.

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

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

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

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

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

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

   [32]  Saint-Andre, P. and J. Hildebrand, "End-to-End Object
         Encryption in XMPP (draft-ietf-xmpp-e2e-02, work in progress)",
         April 2003.


Authors' Addresses

   Peter Saint-Andre
   Jabber Software Foundation

   EMail: stpeter@jabber.org
   URI:   http://www.jabber.org/people/stpeter.php







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   Jeremie Miller
   Jabber Software Foundation

   EMail: jeremie@jabber.org
   URI:   http://www.jabber.org/people/jer.php














































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Appendix A. XML Schemas

   The following XML schemas are descriptive, not normative.

A.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='qualified'>

     <xs:element name='stream'>
       <xs:complexType>
         <xs:element ref='features' minOccurs='0' maxOccurs='1'/>
         <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
         <xs:choice maxOccurs='1'>
           <xs:any namespace='jabber:client'/>
           <xs:any namespace='jabber:server'/>
         </xs:choice>
         <xs:attribute name='to' type='xs:string' use='optional'/>
         <xs:attribute name='from' type='xs:string' use='optional'/>
         <xs:attribute name='id' type='xs:ID' use='optional'/>
         <xs:attribute name='version' type='xs:decimal' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='features'>
       <xs:complexType>
         <xs:any
             namespace='##other'
             minOccurs='0'
             maxOccurs='unbounded'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:any maxOccurs='1'
                 namespace='urn:ietf:params:xml:ns:xmpp-streams'/>
         <xs:attribute name='class' use='required'/>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='address'/>
               <xs:enumeration value='format'/>
               <xs:enumeration value='redirect'/>



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               <xs:enumeration value='server'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>


A.2 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:element
             ref='required'
             minOccurs='0'
             maxOccurs='1'>
       </xs:complexType>
     </xs:element>

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

   </xs:schema>


A.3 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>



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

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

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

     <xs:element name='challenge' type='xs:string'/>
     <xs:element name='response' type='xs:string'/>
     <xs:element name='abort'/>
     <xs:element name='success'/>
     <xs:element name='failure'>
       <xs:complexType>
         <xs:choice maxOccurs='1'>
           <xs:element ref='invalid-realm'/>
           <xs:element ref='mechanism-too-weak'/>
           <xs:element ref='not-authorized'/>
           <xs:element ref='password-transition-required'/>
           <xs:element ref='temporary-auth-failure'/>
         </xs:choice>
       </xs:complexType>
     </xs:element>

     <xs:element name='invalid-realm'/>
     <xs:element name='mechanism-too-weak'/>
     <xs:element name='not-authorized'/>
     <xs:element name='password-transition-required'/>
     <xs:element name='temporary-auth-failure'/>

   </xs:schema>


A.4 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'>




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


A.5 Client namespace

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

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'



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       targetNamespace='jabber:client'
       xmlns='jabber:client'
       elementFormDefault='qualified'>

     <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='to' type='xs:string' use='optional'/>
           <xs:attribute name='from' type='xs:string' use='optional'/>
           <xs:attribute name='id' type='xs:ID' use='optional'/>
           <xs:attribute name='xml:lang'
                         type='xs:NMTOKEN'
                         use='optional'/>
           <xs:attribute name='type' use='optional'>
             <xs:simpleType>
               <xs:restriction base='xs:NCName'>
                 <xs:enumeration value='chat'/>
                 <xs:enumeration value='groupchat'/>
                 <xs:enumeration value='headline'/>
                 <xs:enumeration value='error'/>
               </xs:restriction>
             </xs:simpleType>
           </xs:attribute>
        </xs:complexType>
     </xs:element>

     <xs:element name='body' type='xs:string'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='subject' type='xs:string'>



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

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

     <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='to' type='xs:string' use='optional'/>
         <xs:attribute name='from' type='xs:string' use='optional'/>
         <xs:attribute name='id' type='xs:ID' use='optional'/>
         <xs:attribute name='xml:lang'
                       type='xs:NMTOKEN'
                       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='error'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

     <xs:element name='show'>
       <xs:simpleType>
         <xs:restriction base='xs:NCName'>



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           <xs:enumeration value='away'/>
           <xs:enumeration value='chat'/>
           <xs:enumeration value='xa'/>
           <xs:enumeration value='dnd'/>
         </xs:restriction>
       </xs:simpleType>
     </xs:element>

     <xs:element name='status' type='xs:string'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='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='to' type='xs:string' use='optional'/>
         <xs:attribute name='from' type='xs:string' use='optional'/>
         <xs:attribute name='id' type='xs:ID' 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 name='xml:lang'
                       type='xs:NMTOKEN'
                       use='optional'/>
       </xs:complexType>
     </xs:element>




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     <xs:element name='error'>
       <xs:complexType>
         <xs:any maxOccurs='1'
                 namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>
         <xs:attribute name='class' use='required'/>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='access'/>
               <xs:enumeration value='address'/>
               <xs:enumeration value='format'/>
               <xs:enumeration value='recipient'/>
               <xs:enumeration value='server'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>


A.6 Server namespace

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

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

     <xs: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='to' type='xs:string' use='required'/>
           <xs:attribute name='from' type='xs:string' use='required'/>
           <xs:attribute name='id' type='xs:ID' use='optional'/>



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           <xs:attribute name='xml:lang'
                         type='xs:NMTOKEN'
                         use='optional'/>
           <xs:attribute name='type' use='optional'>
             <xs:simpleType>
               <xs:restriction base='xs:NCName'>
                 <xs:enumeration value='chat'/>
                 <xs:enumeration value='groupchat'/>
                 <xs:enumeration value='headline'/>
                 <xs:enumeration value='error'/>
               </xs:restriction>
             </xs:simpleType>
           </xs:attribute>
        </xs:complexType>
     </xs:element>

     <xs:element name='body' type='xs:string'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

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

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

     <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'



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               maxOccurs='unbounded'/>
           <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
         </xs:sequence>
         <xs:attribute name='to' type='xs:string' use='required'/>
         <xs:attribute name='from' type='xs:string' use='required'/>
         <xs:attribute name='id' type='xs:ID' use='optional'/>
           <xs:attribute name='xml:lang'
                         type='xs:NMTOKEN'
                         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='error'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </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='xa'/>
           <xs:enumeration value='dnd'/>
         </xs:restriction>
       </xs:simpleType>
     </xs:element>

     <xs:element name='status' type='xs:string'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='priority' type='xs:byte'/>

     <xs:element name='iq'>



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       <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='to' type='xs:string' use='required'/>
         <xs:attribute name='from' type='xs:string' use='required'/>
         <xs:attribute name='id' type='xs:ID' 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 name='xml:lang'
                       type='xs:NMTOKEN'
                       use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:any maxOccurs='1'
                 namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>
         <xs:attribute name='class' use='required'/>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='access'/>
               <xs:enumeration value='address'/>
               <xs:enumeration value='format'/>
               <xs:enumeration value='recipient'/>
               <xs:enumeration value='server'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>





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A.7 Stream error 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-streams'
       xmlns='urn:ietf:params:xml:ns:xmpp-streams'
       elementFormDefault='qualified'>

     <xs:element name='condition'>
       <xs:complexType>
         <xs:any
             namespace='##other'
             minOccurs='0'
             maxOccurs='1'/>
         <xs:choice maxOccurs='1'>
           <xs:element ref='host-gone'/>
           <xs:element ref='host-unknown'/>
           <xs:element ref='internal-server-error'/>
           <xs:element ref='invalid-id'/>
           <xs:element ref='invalid-namespace'/>
           <xs:element ref='nonmatching-hosts'/>
           <xs:element ref='not-authorized'/>
           <xs:element ref='remote-connection-failed'/>
           <xs:element ref='resource-constraint'/>
           <xs:element ref='see-other-host'/>
           <xs:element ref='system-shutdown'/>
           <xs:element ref='unsupported-stanza-type'/>
           <xs:element ref='unsupported-version'/>
           <xs:element ref='xml-not-well-formed'/>
         </xs:choice>
       </xs:complexType>

     <xs:element name='host-gone'/>
     <xs:element name='host-unknown'/>
     <xs:element name='internal-server-error'/>
     <xs:element name='invalid-id'/>
     <xs:element name='invalid-namespace'/>
     <xs:element name='nonmatching-hosts'/>
     <xs:element name='not-authorized'/>
     <xs:element name='remote-connection-failed'/>
     <xs:element name='resource-constraint'/>
     <xs:element name='see-other-host' type='xs:string'/>
     <xs:element name='system-shutdown'/>
     <xs:element name='unsupported-stanza-type'/>
     <xs:element name='unsupported-version' type='xs:string'/>
     <xs:element name='xml-not-well-formed'/>



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   </xs:schema>


A.8 Stanza error 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-stanzas'
       xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
       elementFormDefault='qualified'>

     <xs:element name='condition'>
       <xs:complexType>
         <xs:choice>
           <xs:element ref='bad-request'/>
           <xs:element ref='feature-not-implemented'/>
           <xs:element ref='forbidden'/>
           <xs:element ref='internal-server-error'/>
           <xs:element ref='item-not-found'/>
           <xs:element ref='jid-malformed'/>
           <xs:element ref='not-allowed'/>
           <xs:element ref='recipient-unavailable'/>
           <xs:element ref='registration-required'/>
           <xs:element ref='remote-server-not-found'/>
           <xs:element ref='remove-server-timeout'/>
           <xs:element ref='service-unavailable'/>
         </xs:choice>
       </xs:complexType>

     <xs:element name='bad-request' type='conditionType'/>
     <xs:element name='feature-not-implemented' type='conditionType'/>
     <xs:element name='forbidden' type='conditionType'/>
     <xs:element name='internal-server-error' type='conditionType'/>
     <xs:element name='item-not-found' type='conditionType'/>
     <xs:element name='jid-malformed' type='conditionType'/>
     <xs:element name='not-allowed' type='conditionType'/>
     <xs:element name='recipient-unavailable' type='conditionType'/>
     <xs:element name='registration-required' type='conditionType'/>
     <xs:element name='remote-server-not-found' type='conditionType'/>
     <xs:element name='remote-server-timeout' type='conditionType'/>
     <xs:element name='service-unavailable' type='conditionType'/>

     <xs:complexType name='conditionType'>
       <xs:any namespace='##other' minOccurs='0' maxOccurs='1'/>
     </xs:complexType>




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   </xs:schema>


















































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Appendix B. Revision History

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

B.1 Changes from draft-ietf-xmpp-core-09

   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.


B.2 Changes from draft-ietf-xmpp-core-09

   o  Fixed several dialback error conditions.

   o  Cleaned up business 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.


B.3 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.




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B.4 Changes from draft-ietf-xmpp-core-07

   o  Made several small editorial changes.


B.5 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 [25].

   o  Removed note to RFC Editor regarding provisional namespace names.


B.6 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.


B.7 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.


B.8 Changes from draft-ietf-xmpp-core-03

   o  Clarified rules and procedures for TLS and SASL.



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   o  Amplified stream error code syntax per list discussion.

   o  Made numerous small editorial changes.


B.9 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".


B.10 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.

   o  Corrected error in Stream Authentication regarding "version='1.0'"
      flag.

   o  Made numerous small editorial changes.


B.11 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.


B.12 Changes from draft-miller-xmpp-core-02

   o  Brought Streams Authentication section into line with discussion
      on list and at IETF 55 meeting.




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   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.

   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 types 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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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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