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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: February 20, 2004                    Jabber Software Foundation
                                                         August 22, 2003


                               XMPP Core
                        draft-ietf-xmpp-core-17

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   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 February 20, 2004.

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. While XMPP provides a generalized, extensible framework
   for transporting structured information, it is used mainly for the
   purpose of building instant messaging and presence applications that
   meet the requirements of RFC 2779.








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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  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  . . . . . . . . . . . . . . . . . . . . . .  9
   3.4   Resource Identifier  . . . . . . . . . . . . . . . . . . . .  9
   4.    XML Streams  . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.2   Stream Attributes  . . . . . . . . . . . . . . . . . . . . . 11
   4.2.1 Version Support  . . . . . . . . . . . . . . . . . . . . . . 12
   4.3   Namespace Declarations . . . . . . . . . . . . . . . . . . . 13
   4.4   Stream Features  . . . . . . . . . . . . . . . . . . . . . . 13
   4.5   Stream Encryption and Authentication . . . . . . . . . . . . 14
   4.6   Stream Errors  . . . . . . . . . . . . . . . . . . . . . . . 14
   4.6.1 Rules  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.6.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.6.3 Defined Conditions . . . . . . . . . . . . . . . . . . . . . 15
   4.6.4 Application-Specific Conditions  . . . . . . . . . . . . . . 17
   4.7   Simple Streams Example . . . . . . . . . . . . . . . . . . . 17
   5.    Stream Encryption  . . . . . . . . . . . . . . . . . . . . . 20
   5.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.2   Narrative  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   5.3   Client-to-Server Example . . . . . . . . . . . . . . . . . . 23
   5.4   Server-to-Server Example . . . . . . . . . . . . . . . . . . 24
   6.    Stream Authentication  . . . . . . . . . . . . . . . . . . . 27
   6.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 27
   6.2   Narrative  . . . . . . . . . . . . . . . . . . . . . . . . . 28
   6.3   SASL Errors  . . . . . . . . . . . . . . . . . . . . . . . . 30
   6.4   SASL Definition  . . . . . . . . . . . . . . . . . . . . . . 31
   6.5   Client-to-Server Example . . . . . . . . . . . . . . . . . . 31
   6.6   Server-to-Server Example . . . . . . . . . . . . . . . . . . 34
   7.    Server Dialback  . . . . . . . . . . . . . . . . . . . . . . 38
   7.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 38
   7.2   Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 40
   8.    XML Stanzas  . . . . . . . . . . . . . . . . . . . . . . . . 45
   8.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 45



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   8.2   Common Attributes  . . . . . . . . . . . . . . . . . . . . . 45
   8.2.1 to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
   8.2.2 from . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
   8.2.3 id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
   8.2.4 type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
   8.2.5 xml:lang . . . . . . . . . . . . . . . . . . . . . . . . . . 46
   8.3   Message Stanzas  . . . . . . . . . . . . . . . . . . . . . . 47
   8.3.1 Types of Message . . . . . . . . . . . . . . . . . . . . . . 47
   8.3.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 47
   8.4   Presence Stanzas . . . . . . . . . . . . . . . . . . . . . . 49
   8.4.1 Types of Presence  . . . . . . . . . . . . . . . . . . . . . 49
   8.4.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 50
   8.5   IQ Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 51
   8.5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 51
   8.5.2 Types of IQ  . . . . . . . . . . . . . . . . . . . . . . . . 52
   8.5.3 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 52
   8.6   Extended Namespaces  . . . . . . . . . . . . . . . . . . . . 52
   8.7   Stanza Errors  . . . . . . . . . . . . . . . . . . . . . . . 53
   8.7.1 Rules  . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
   8.7.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
   8.7.3 Defined Conditions . . . . . . . . . . . . . . . . . . . . . 55
   8.7.4 Application-Specific Conditions  . . . . . . . . . . . . . . 57
   9.    XML Usage within XMPP  . . . . . . . . . . . . . . . . . . . 58
   9.1   Restrictions . . . . . . . . . . . . . . . . . . . . . . . . 58
   9.2   XML Namespace Names and Prefixes . . . . . . . . . . . . . . 58
   9.2.1 Stream Namespace . . . . . . . . . . . . . . . . . . . . . . 58
   9.2.2 Default Namespace  . . . . . . . . . . . . . . . . . . . . . 59
   9.2.3 Dialback Namespace . . . . . . . . . . . . . . . . . . . . . 59
   9.3   Validation . . . . . . . . . . . . . . . . . . . . . . . . . 60
   9.4   Inclusion of Text Declaration  . . . . . . . . . . . . . . . 60
   9.5   Character Encodings  . . . . . . . . . . . . . . . . . . . . 60
   10.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 61
   10.1  XML Namespace Name for TLS Data  . . . . . . . . . . . . . . 61
   10.2  XML Namespace Name for SASL Data . . . . . . . . . . . . . . 61
   10.3  XML Namespace Name for Stream Errors . . . . . . . . . . . . 61
   10.4  XML Namespace Name for Stanza Errors . . . . . . . . . . . . 62
   10.5  Existing Registrations . . . . . . . . . . . . . . . . . . . 62
   11.   Internationalization Considerations  . . . . . . . . . . . . 63
   12.   Security Considerations  . . . . . . . . . . . . . . . . . . 64
   12.1  High Security  . . . . . . . . . . . . . . . . . . . . . . . 64
   12.2  Client-to-Server Communications  . . . . . . . . . . . . . . 64
   12.3  Server-to-Server Communications  . . . . . . . . . . . . . . 65
   12.4  Order of Layers  . . . . . . . . . . . . . . . . . . . . . . 66
   12.5  Firewalls  . . . . . . . . . . . . . . . . . . . . . . . . . 66
   12.6  Mandatory to Implement Technologies  . . . . . . . . . . . . 66
   13.   Compliance Requirements  . . . . . . . . . . . . . . . . . . 67
   13.1  Servers  . . . . . . . . . . . . . . . . . . . . . . . . . . 67
   13.2  Clients  . . . . . . . . . . . . . . . . . . . . . . . . . . 67



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   14.   Differences Between Jabber and XMPP  . . . . . . . . . . . . 69
   14.1  Authentication . . . . . . . . . . . . . . . . . . . . . . . 69
   14.2  Channel Encryption . . . . . . . . . . . . . . . . . . . . . 69
   14.3  JID Processing . . . . . . . . . . . . . . . . . . . . . . . 69
   14.4  Error Handling . . . . . . . . . . . . . . . . . . . . . . . 69
   14.5  Internationalization . . . . . . . . . . . . . . . . . . . . 70
   14.6  Stream Version Attribute . . . . . . . . . . . . . . . . . . 70
         Normative References . . . . . . . . . . . . . . . . . . . . 71
         Informative References . . . . . . . . . . . . . . . . . . . 73
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 73
   A.    XML Schemas  . . . . . . . . . . . . . . . . . . . . . . . . 74
   A.1   Stream namespace . . . . . . . . . . . . . . . . . . . . . . 74
   A.2   Stream error namespace . . . . . . . . . . . . . . . . . . . 75
   A.3   TLS namespace  . . . . . . . . . . . . . . . . . . . . . . . 76
   A.4   SASL namespace . . . . . . . . . . . . . . . . . . . . . . . 77
   A.5   Dialback namespace . . . . . . . . . . . . . . . . . . . . . 78
   A.6   Client namespace . . . . . . . . . . . . . . . . . . . . . . 79
   A.7   Server namespace . . . . . . . . . . . . . . . . . . . . . . 83
   A.8   Stanza error namespace . . . . . . . . . . . . . . . . . . . 87
   B.    Revision History . . . . . . . . . . . . . . . . . . . . . . 89
   B.1   Changes from draft-ietf-xmpp-core-16 . . . . . . . . . . . . 89
   B.2   Changes from draft-ietf-xmpp-core-15 . . . . . . . . . . . . 89
   B.3   Changes from draft-ietf-xmpp-core-14 . . . . . . . . . . . . 89
   B.4   Changes from draft-ietf-xmpp-core-13 . . . . . . . . . . . . 90
   B.5   Changes from draft-ietf-xmpp-core-12 . . . . . . . . . . . . 90
   B.6   Changes from draft-ietf-xmpp-core-11 . . . . . . . . . . . . 90
   B.7   Changes from draft-ietf-xmpp-core-10 . . . . . . . . . . . . 90
   B.8   Changes from draft-ietf-xmpp-core-09 . . . . . . . . . . . . 91
   B.9   Changes from draft-ietf-xmpp-core-08 . . . . . . . . . . . . 91
   B.10  Changes from draft-ietf-xmpp-core-07 . . . . . . . . . . . . 91
   B.11  Changes from draft-ietf-xmpp-core-06 . . . . . . . . . . . . 91
   B.12  Changes from draft-ietf-xmpp-core-05 . . . . . . . . . . . . 92
   B.13  Changes from draft-ietf-xmpp-core-04 . . . . . . . . . . . . 92
   B.14  Changes from draft-ietf-xmpp-core-03 . . . . . . . . . . . . 92
   B.15  Changes from draft-ietf-xmpp-core-02 . . . . . . . . . . . . 92
   B.16  Changes from draft-ietf-xmpp-core-01 . . . . . . . . . . . . 93
   B.17  Changes from draft-ietf-xmpp-core-00 . . . . . . . . . . . . 93
   B.18  Changes from draft-miller-xmpp-core-02 . . . . . . . . . . . 93
         Intellectual Property and Copyright Statements . . . . . . . 95












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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 (IM) and presence technology. As a result of work
   by the XMPP WG, the current document defines the core features of
   XMPP; XMPP IM [24] defines the extensions required to provide the
   instant messaging 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 instant messaging
   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 JID (e.g.,
   <user@domain/home> vs. <user@domain/work>) as defined under
   Addressing Scheme (Section 3). The port registered with the IANA for
   connections between a client and a server is 5222 (see IANA
   Considerations (Section 10)).

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.
   Communications between any two servers are OPTIONAL; if enabled, such
   communications occur over XML streams that are normally bound to TCP
   sockets, using port 5269 as registered with the IANA (see IANA
   Considerations (Section 10)).












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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 [25]. For historical reasons, the address of such an
   entity is called a Jabber Identifier or JID. A valid JID contains a
   set of ordered elements formed of a domain identifier, node
   identifier, and resource identifier in the following format:
   [node@]domain[/resource]. Each allowable portion of a JID (node
   identifier, domain identifier, and resource identifier) may be up to
   1023 bytes in length, resulting in a maximum total size (including
   the '@' and '/' separators) of 3071 bytes.

   All JIDs are based on the foregoing structure. The most common use of
   this structure is to identify an instant messaging user, the server
   to which the user connects, and the user's 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 (e.g., a multi-user chat service,
   a user directory, or a gateway to a foreign messaging system).

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




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3.3 Node Identifier

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

   Definition of XML Stanza: An XML stanza is a discrete semantic unit
      of structured information that is sent from one entity to another
      over an XML stream. An XML stanza exists at the direct child level
      of the root <stream/> element and is said to be well-balanced if
      it matches production [43] content of 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. An XML element sent for the purpose of stream
      encryption, stream authentication, or server dialback is not
      considered to be an XML stanza.

   Consider the example of a client's session with a server. In order to
   connect to a server, a client MUST initiate an XML stream by sending
   an opening <stream> tag to the server, optionally preceded by a text
   declaration specifying the XML version supported and the character
   encoding (see also Character Encodings (Section 9.5)). 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



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   the network. When the client desires to close the stream, it simply
   sends a closing </stream> tag to the server (alternatively, the
   stream may be closed by the server), after which both the client and
   server SHOULD close the underlying TCP connection as well.

   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 JID of the receiving entity. There SHOULD be no
      'to' attribute set in the XML stream header by which the receiving



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      entity replies to the initiating entity; however, if a 'to'
      attribute is included, it SHOULD be silently ignored by the
      initiating entity.

   o  from -- The 'from' attribute SHOULD be used only in the XML stream
      header from the receiving entity to the initiating entity, and
      MUST be set to the JID 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, and MUST be unique within the receiving
      application (normally a server). 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 -- The presence of the version attribute set to a value of
      "1.0" signals support for the stream features defined in this
      specification. Detailed rules regarding generation and handling of
      this attribute are defined below.

   We can summarize as follows:

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


4.2.1 Version Support

   The following rules apply to the generation and handling of the
   'version' attribute:

   1.  If the initiating entity complies with the XML streams protocol
       defined herein (including Stream Encryption (Section 5), Stream
       Authentication (Section 6), and Stream Errors (Section 4.6)), it
       MUST include the '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".



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   2.  If the initiating entity includes the 'version' attribute set to
       a value of "1.0" in its stream header and the receiving entity
       supports XMPP 1.0, the receiving entity MUST reciprocate by
       including the 'version' attribute set to a value of "1.0" in its
       stream header response.

   3.  If the initiating entity does not include the 'version' attribute
       in its stream header, the receiving entity still SHOULD include
       the 'version' attribute set to a value of "1.0" in its stream
       header response.

   4.  If the initiating entity includes the 'version' attribute set to
       a value other than "1.0", the receiving entity SHOULD include the
       'version' attribute set to a value of "1.0" in its stream header
       response, but MAY at its discretion generate an
       <unsupported-version/> stream error and terminate the XML stream
       and underlying TCP connection.

   5.  If the receiving entity includes the 'version' attribute set to a
       value other than "1.0" in its stream header response, the
       initiating entity SHOULD generate an <unsupported-version/>
       stream error and terminate the XML stream and underlying TCP
       connection.


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]). For detailed
   information regarding the stream namespace and default namespace, see
   Namespace Names and Prefixes (Section 9.2).

4.4 Stream Features

   If the initiating entity includes the 'version' attribute set to a
   value of "1.0" in its initiating stream element, the receiving entity
   MUST send a <features/> child element (prefixed by the stream
   namespace prefix) to the initiating entity in order to announce any
   stream-level features that can be negotiated (or capabilities that
   otherwise need to be advertised). Currently this is used for TLS and
   SASL 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 Encryption and Authentication

   XML streams in XMPP 1.0 SHOULD be encrypted as defined under Stream
   Encryption (Section 5) and MUST be authenticated as defined under
   Stream Authentication (Section 6). If the initiating entity attempts
   to send an XML stanza before the stream is authenticated, the
   receiving entity SHOULD return a <not-authorized/> stream error to
   the initiating entity and then terminate both the XML stream and the
   underlying TCP connection.

4.6 Stream Errors

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

4.6.1 Rules

   The following rules apply to stream-level errors:

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

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


4.6.2 Syntax

   The syntax for stream errors is as follows:

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



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   </stream:error>

   The <error/> element:

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

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

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

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

   Note: the XML namespace name 'urn:ietf:params:xml:ns:xmpp-streams'
   that qualifies the descriptive element adheres to the format defined
   in The IETF XML Registry [26].

4.6.3 Defined Conditions

   The following stream-level error conditions are defined:

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

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

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

   o  <host-unknown/> -- the value of the 'to' attribute provided by the



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      initiating entity in the stream header does not correspond to a
      hostname that is hosted by the server.

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

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

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

   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".

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

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

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

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

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

   o  <see-other-host/> -- the server will not provide service to the
      initiating entity but is redirecting traffic to another host; the
      server SHOULD specify the alternate hostname or IP address in the
      <text/> element.

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

   o  <undefined-condition/> -- the error condition is not one of those
      defined by the other conditions in this list; this error condition



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      SHOULD be used only in conjuction with an application-specific
      condition.

   o  <unsupported-encoding/> -- the initiating entity has encoded the
      stream in an encoding that is not supported by the server.

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

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

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


4.6.4 Application-Specific Conditions

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

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


4.7 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):

   A basic session:

   C: <?xml version='1.0'?>
      <stream:stream



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          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'
                 xml:lang='en'>
   C:     <body>Art thou not Romeo, and a Montague?</body>
   C:   </message>
   S:   <message from='romeo@shakespeare.lit'
                 to='juliet@shakespeare.lit'
                 xml:lang='en'>
   S:     <body>Neither, fair saint, if either thee dislike.</body>
   S:   </message>
   C: </stream:stream>
   S: </stream:stream>

   A stream 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 xml:lang='en'>
        <body>Bad XML, no closing body tag!
      </message>
   S: <stream:error>
       <xml-not-well-formed
           xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>



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   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
   [27], POP3 [28], and ACAP [29] protocols as described in RFC 2595
   [30]. 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 [26].

   An administrator of a given domain MAY require the use of TLS for
   client-to-server communications, server-to-server communications, or
   both. Servers SHOULD use TLS betweeen two domains for the purpose of
   securing server-to-server communications. See Mandatory to Implement
   Technologies (Section 12.6) regarding mechanisms that MUST be
   supported.

   The following rules apply:

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

   2.   If the TLS negotiation occurs between two servers,
        communications MUST NOT proceed until the DNS hostnames asserted
        by the servers have been resolved (see Server-to-Server
        Communications (Section 12.3)).

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

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

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



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   6.   The initiating entity MUST validate the certificate presented by
        the receiving entity; there are two cases:

        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 12.1)). 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 12.1) for details).

        If the above methods fail, the certificate SHOULD be presented
        to a human (e.g., an end user or server administrator) for
        approval; if presented, the receiver MUST deliver the entire
        certificate chain to the human, 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.

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

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

   9.   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.

   10.  If the TLS negotiation is successful, the initiating entity
        SHOULD continue with SASL negotiation.

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







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5.2 Narrative

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

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

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

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

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

   5.  The receiving entity MUST reply with either a <proceed/> element
       or a <failure/> element qualified by the
       '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 unsuccessful, the receiving entity MUST
       terminate the TCP connection. If the TLS negotiation is
       successful, the initiating entity MUST initiate a new stream by
       sending an opening XML stream header to the receiving entity.

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



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5.3 Client-to-Server Example

   The following example shows the data flow for a client securing a
   stream using STARTTLS (the IANA-registered port 5222 SHOULD be used;
   see IANA Considerations (Section 10)).

   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 client:

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

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

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

   Step 4: Client sends the STARTTLS command to server:

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

   Step 5: Server informs client to proceed:

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

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

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



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   </stream:stream>

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

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

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       to='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 client along
   with any remaining negotiable stream features:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='capulet.com'
       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-registered port 5269 SHOULD be used;
   see IANA Considerations (Section 10)).

   Step 1: Server1 initiates stream to Server2:

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



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       to='montague.net'
       version='1.0'>

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

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='montague.net'
       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'



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       to='montague.net'
       version='1.0'>

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

   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'
       from='montague.net'
       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

6.1 Overview

   XMPP includes a method for authenticating a stream using an XMPP
   adaptation of the Simple Authentication and Security Layer (SASL)
   [15]. SASL provides a generalized method for adding authentication
   support to connection-based protocols, and XMPP uses a generic XML
   namespace profile for SASL that conforms to 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 [26]). Finally, see Mandatory to
   Implement Technologies (Section 12.6) regarding mechanisms that MUST
   be supported.

   The following rules apply:

   1.  If the SASL negotiation occurs between two servers,
       communications MUST NOT proceed until the DNS hostnames asserted
       by the servers have been resolved (see Server-to-Server
       Communications (Section 12.3)).

   2.  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.

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

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

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

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



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   7.  The initiating entity MUST provide an authzid during SASL
       negotiation. The authzid-value MUST be a valid JID of the form
       <domain> (i.e., a domain identifier only) for servers and of the
       form <user@domain/resource> (i.e., node identifier, domain
       identifier, and resource identifier) for clients. The initiating
       entity MAY process the authzid-value in accordance with the rules
       defined in Addressing Scheme (Section 3) before providing it to
       the receiving entity, but is NOT REQUIRED to do so; however, the
       receiving entity MUST verify that the authzid-value provided by
       the initiating entity conforms to the rules defined therein.

   8.  Any character data contained within the XML elements used during
       SASL negotiation MUST be encoded using base64.


6.2 Narrative

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

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

   2.  After sending an XML stream header in response, the receiving
       entity sends a list of available SASL authentication mechanisms;
       each of these is a <mechanism/> element included as a child
       within a <mechanisms/> container element qualified by the
       '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 needs to be established before a particular
       authentication mechanism may be used, the receiving entity MUST
       NOT provide that mechanism in the list of available SASL
       authentication methods prior to channel encryption. If the
       initiating entity presents a valid certificate during prior TLS
       negotiation, the receiving entity MAY offer the SASL EXTERNAL
       mechanism to the initiating entity during stream authentication
       (refer to RFC 2222 [15]).

   3.  The initiating entity selects a mechanism by sending an <auth/>
       element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
       namespace to the receiving entity and including an appropriate
       value for the 'mechanism' attribute; this element MAY 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.



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   4.  If necessary, the receiving entity challenges the initiating
       entity by sending a <challenge/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
       entity; this element MAY 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 qualified 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 qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
       namespace to the receiving entity. Upon receiving the <abort/>
       element, the receiving entity MUST terminate the TCP connection.

   2.  The receiving entity reports failure of the handshake by sending
       a <failure/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
       entity (the particular cause of failure SHOULD be communicated in
       an appropriate child element of the <failure/> element as defined
       under SASL Errors (Section 6.3)). Immediately after sending the
       <failure/> element, the receiving entity MUST terminate both the
       XML stream and the underlying TCP connection.

   3.  The receiving entity reports success of the handshake by sending
       a <success/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
       entity; this element MAY optionally contain character data (in
       SASL terminology "additional data with success"). Upon receiving
       the <success/> element, the initiating entity MUST initiate a new
       stream by sending an opening XML stream header to the receiving
       entity (it is not necessary to send a closing </stream:stream>
       element first, since the receiving entity and initiating entity
       MUST consider the original stream to be closed upon sending or
       receiving the <success/> element). Upon receiving the new stream
       header from the initiating entity, the receiving entity MUST
       respond by sending a new XML stream header to the initiating
       entity, along with any remaining available features (but NOT



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       including the STARTTLS feature or any authentication mechanisms)
       or an empty features element (to signify that no additional
       features are available); note that any such additional features
       are not defined herein, and MUST be defined by the relevant
       extension to XMPP.


6.3 SASL Errors

   The following SASL-related error conditions are defined:

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

   o  <bad-protocol/> -- The data provided by the initiating entity
      could not be processed, e.g. because does not adhere to the
      protocol for the requested mechanism; sent in response to the
      <response/> element.

   o  <encryption-required/> -- The mechanism chosen by the initiating
      entity may be used only if the stream is already encrypted; sent
      in response to the <auth/> element.

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

   o  <invalid-mechanism/> -- The initiating entity did not provide a
      mechanism or requested a mechanism that is not supported by the
      receiving entity; sent in response to the <auth/> element.

   o  <invalid-realm/> -- The realm provided by the initiating entity
      (in mechanisms that support the concept of a realm) does not match
      one of the hostnames served by the receiving entity; sent in
      response to a <response/> element.

   o  <mechanism-too-weak/> -- The mechanism requested by the initiating
      entity is weaker than server policy permits for that initiating
      entity; sent in response to the <response/> element.

   o  <not-authorized/> -- The authentication failed because the
      initiating entity did not provide valid credentials (this includes
      the case of an unknown username); sent in response to a <response/
      > element.

   o  <temporary-auth-failure/> -- The authentication failed because of



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      a temporary error condition within the receiving entity; sent in
      response to an <auth/> element or <response/> element.


6.4 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 qualified by the
      'urn:ietf:params:xml:ns:xmpp-sasl' namespace.) Upon successful
      negotiation, both sides consider the original XML stream closed
      and new <stream> headers are sent by both entities.

   security layer negotiation: The security layer takes effect
      immediately after sending the closing ">" character of the
      <success/> element for the server, and immediately after receiving
      the closing ">" character of the <success/> element for the client
      (this element is qualified by the
      'urn:ietf:params:xml:ns:xmpp-sasl' namespace).

   use of the authorization identity: The authorization identity is used
      by xmpp to denote the "full JID" (<user@domain/resource>) of a
      client or the sending domain of a server.


6.5 Client-to-Server Example

   The following example shows the data flow for a client authenticating
   with a server using SASL (the IANA-registered port 5222 SHOULD be
   used; see IANA Considerations (Section 10)).




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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 with a stream tag sent to client:

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

   Step 3: Server informs client of available authentication mechanisms:

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

   Step 4: Client selects an authentication mechanism:

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

   Step 5: Server sends a base64-encoded challenge to client:

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

   The decoded challenge is:

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

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

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <mechanism-too-weak/>



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   </failure>
   </stream:stream>

   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 client:

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

   The decoded challenge is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd

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

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

   Step 8: Client responds to the challenge:

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

   Step 9: Server informs client of successful authentication:

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





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

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

   Step 10: Client initiates a new stream to server:

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

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

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


6.6 Server-to-Server Example

   The following example shows the data flow for a server authenticating
   with another server using SASL (the IANA-registered port 5269 SHOULD
   be used; see IANA Considerations (Section 10)).

   Step 1: Server1 initiates stream to Server2:

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

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

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='montague.net'
       id='12345678'



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       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 5 (alt): Server2 returns error to Server1:

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

   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 5 (alt): Server2 returns error to Server1:

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

   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>
   </stream:stream>

   Step 10: Server1 initiates a new stream to Server2:

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

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

   <stream:stream



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













































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7. Server Dialback

7.1 Overview

   The Jabber protocol from which XMPP was adapted includes a "server
   dialback" method for protecting against domain spoofing, thus making
   it more difficult to spoof XML stanzas (see Server-to-Server
   Communications (Section 12.3) regarding this method's security
   characteristics). Server dialback also makes it easier to deploy
   systems in which outbound messages and inbound messages are handled
   by different machines for the same domain. The server dialback method
   is made possible by the existence of DNS, since one server can
   (normally) discover the authoritative server for a given domain.

   Because dialback depends on the Domain Name System, inter-domain
   communications MUST NOT proceed until the DNS hostnames asserted by
   the servers have been resolved (see Server-to-Server Communications
   (Section 12.3)).

   The method for generating and verifying the keys used in server
   dialback 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.

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

   The following terminology applies:

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

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

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

   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.



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



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       | <---------------------- |
       |                         |


7.2 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:

   <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'



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       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
        MUST maintain the existing connection until it validates whether
        the new connection is legitimate; additionally, Receiving Server
        MAY choose to generate a <not-authorized/> stream error
        condition for the new connection and then terminate both the XML
        stream and the underlying TCP connection related to the new
        request.

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

   6.   Receiving Server sends Authoritative Server a stream header:

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

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

   7.   Authoritative Server sends Receiving Server a stream header:

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



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       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,
        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'



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       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:

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

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

   Even if dialback negotiation is successful, a server MUST verify that
   all XML stanzas received from the other server include a 'from'
   attribute and a 'to' attribute; if a stanza does not meet this
   restriction, the server that receives the stanza MUST generate an
   <improper-addressing/> stream error condition and terminate both the
   XML stream and the underlying TCP connection. Furthermore, a server
   MUST verify that the 'from' attribute of stanzas received from the



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   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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8. XML Stanzas

8.1 Overview

   Once XML streams in both directions 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.

8.2 Common Attributes

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

8.2.1 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) SHOULD NOT possess a 'to' attribute.

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

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

8.2.2 from

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



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   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
   an XML 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. When a client sends an XML stanza
   within the context of an authenticated stream, the server MUST stamp
   the stanza with the full JID (<user@domain/resource>) of the
   connected resource that generated the stanza as defined by the
   authzid provided in the SASL negotiation. If a client attempts to
   send an XML stanza before the stream is authenticated, the server
   SHOULD return a <not-authorized/> stream error to the client and then
   terminate both the XML stream and the underlying TCP connection.

   In the 'jabber:server' namespace, a stanza MUST possess a 'from'
   attribute; if a server receives a stanza that does not meet this
   restriction, it MUST generate an <improper-addressing/> stream error
   condition. Furthermore, the domain identifier portion of the JID
   contained in the 'from' attribute MUST match the hostname 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.

8.2.3 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). The 'id' attribute is required for 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.

8.2.4 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.

8.2.5 xml:lang

   A stanza SHOULD possess an 'xml:lang' attribute (as defined in



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   Section 2.12 of the XML specification [1]) if the stanza contains XML
   character data that is intended to be presented to a human user (as
   explained in RFC 2277 [20], "internationalization is for humans").
   The value of the 'xml:lang' attribute specifies the default language
   of any such XML character data, which MAY be overridden by the
   'xml:lang' attribute of a specific child element. The value of the
   attribute MUST be an NMTOKEN and MUST conform to the format defined
   in RFC 3066 [17].

8.3 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.

8.3.1 Types of Message

   The 'type' attribute of a message stanza is RECOMMENDED; if included,
   it specifies the conversational context of the message. The 'type'
   attribute SHOULD have one of the following values:

   o  chat

   o  error

   o  groupchat

   o  headline

   o  normal

   A message stanza with a different value, or without a 'type'
   attribute, SHOULD be handled as if the 'type' were "normal".

   For information regarding the meaning of these message types in the
   context of XMPP-based instant messaging and presence applications,
   refer to XMPP IM [24].

8.3.2 Children

   As described under extended namespaces (Section 8.6), 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



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   stanzas. If the message stanza is of type "error", it MUST include an
   <error/> child; for details, see Stanza Errors (Section 8.7).
   Otherwise, the message stanza MAY contain any of the following child
   elements without an explicit namespace declaration:

   1.  <subject/>

   2.  <body/>

   3.  <thread/>


8.3.2.1 Subject

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

8.3.2.2 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.

8.3.2.3 Thread

   The <thread/> element contains a 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 "instant
   messaging session") between two entities. If used, it MUST be unique
   to that conversation thread within the stream and MUST be consistent
   throughout that conversation (a client that receives a message from
   the same full JID but with a different thread ID MUST assume that the
   message in question exists outside the context of the existing
   conversation thread). The use of the <thread/> element is OPTIONAL
   and is not used to identify individual messages, only conversations.
   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 comparisons may be made against it.



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   The <thread/> element MUST NOT contain mixed content.

8.4 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.

8.4.1 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.

   For information regarding presence semantics and the subscription
   model used in the context of XMPP-based instant messaging and
   presence applications, refer to XMPP IM [24].





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8.4.2 Children

   As described under extended namespaces (Section 8.6), a presence
   stanza MAY contain any properly-namespaced child element.

   In accordance with the default namespace declaration, by default a
   presence stanza is in the 'jabber:client' or 'jabber:server'
   namespace, which defines certain allowable children of presence
   stanzas. If the presence stanza is of type "error", it MUST include
   an <error/> child; for details, see Stanza Errors (Section 8.7). 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"):

   1.  <show/>

   2.  <status/>

   3.  <priority/>


8.4.2.1 Show

   The OPTIONAL <show/> element specifies the 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  dnd

   o  xa

   For information regarding the meaning of these values in the context
   of XMPP-based instant messaging and presence applications, refer to
   XMPP IM [24].

8.4.2.2 Status

   The OPTIONAL <status/> element contains a natural-language



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   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.

8.4.2.3 Priority

   The OPTIONAL <priority/> element specifies the priority level of the
   connected resource. The value may be any integer between -128 and
   +127. Only one <priority/> element MAY be included in a presence
   stanza, and it MUST NOT possess any attributes. If no priority is
   provided, a server SHOULD consider the priority to be zero. For
   information regarding the semantics of priority values in stanza
   routing within instant messaging applications, refer to XMPP IM [24].

8.5 IQ Stanzas

8.5.1 Overview

   Info/Query, or IQ, is a request-response mechanism, similar in some
   ways to HTTP [32]. 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'>  |



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       | <------------------------ |
       |                           |

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

8.5.2 Types of IQ

   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.


8.5.3 Children

   As described under extended namespaces (Section 8.6), an IQ stanza
   MAY contain any properly-namespaced child element. Note that an IQ
   stanza of type "get", "set", or "result" 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 "result" MUST include zero or one
   child elements. An IQ stanza of type "error" SHOULD include the child
   element contained in the associated "get" or "set" and MUST include
   an <error/> child; for details, see Stanza Errors (Section 8.7).

8.6 Extended Namespaces




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   While the three XML stanza types defined in the "jabber:client" or
   "jabber:server" namespace (along with their attributes and child
   elements) provide a basic level of functionality for messaging and
   presence, XMPP uses XML namespaces to extend the stanzas for the
   purpose of providing additional functionality. Thus a message,
   presence, or IQ stanza MAY house one or more optional child elements
   containing content that extends the meaning of the message (e.g., an
   XHTML-formatted version of the message body). 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
   stanza to the recipient:

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

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

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

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

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


8.7 Stanza Errors

   Stanza-related errors are handled in a manner similar to stream
   errors (Section 4.6), except that hints are also provided to the



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   receiving application regarding actions to take in reponse to the
   error.

8.7.1 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 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.

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


8.7.2 Syntax

   The syntax for stanza-related errors is as follows:

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

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

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

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

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

   o  modify -- retry after changing the data sent



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   o  auth -- retry after providing credentials

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

   The <error/> element:

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

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

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

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

   Note: the XML namespace name 'urn:ietf:params:xml:ns:xmpp-stanzas'
   that qualifies the descriptive element adheres to the format defined
   in The IETF XML Registry [26].

8.7.3 Defined Conditions

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

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

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

   o  <feature-not-implemented/> -- the feature requested is not



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      implemented by the recipient or server and therefore cannot be
      processed; the associated error type SHOULD be "cancel".

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

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

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

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

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

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

   o  <recipient-unavailable/> -- the specific recipient requested is
      currently unavailable; the associated error type SHOULD be "wait".

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

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

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

   o  <resource-constraint/> -- the server is resource-contrained and is
      unable to service the request; the associated error type SHOULD be
      "wait".

   o  <service-unavailable/> -- the service requested is currently
      unavailable on the server; the associated error type SHOULD be



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      "cancel".

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

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

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


8.7.4 Application-Specific Conditions

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

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


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










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9. XML Usage within XMPP

9.1 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 needs to 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)

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

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

9.2 XML Namespace Names and Prefixes

   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. Rules for XML namespace names and prefixes
   are defined below.

9.2.1 Stream Namespace

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



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   historical reasons MAY accept only the 'stream:' prefix.

9.2.2 Default Namespace

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

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

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

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

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

   An implementation MUST NOT generate namespace prefixes for elements
   in the default namespace if the default namespace is 'jabber:client'
   or 'jabber:server'. An implementation SHOULD NOT generate namespace
   prefixes for elements qualified by "extended" namespaces as described
   under Extended Namespaces (Section 8.6).

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

9.2.3 Dialback Namespace

   A dialback namespace declaration is REQUIRED for all elements used in
   server dialback. The name of the dialback namespace MUST be



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   'jabber:server:dialback'. All elements qualified by this namespace
   MUST be prefixed. An implementation SHOULD generate only the 'db:'
   prefix for such elements and MAY accept only the 'db:' prefix.

9.3 Validation

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

9.4 Inclusion of Text Declaration

   Implementations SHOULD send a text declaration before sending a
   stream header. Applications MUST follow the rules in the XML
   specification [1] regarding the circumstances under which a text
   declaration is included.

9.5 Character Encodings

   Implementations MUST support UTF-8 (RFC 2279 [19]) as required by RFC
   2277 [20], and SHOULD support UTF-16 (RFC 2781 [21]) for the sake of
   efficiency in encoding certain writing systems. If the character
   encoding is not specified in a text declaration sent before the
   stream header, implementations SHOULD deduce the encoding as
   described in Appendix F of the XML specification [1]. If an
   initiating entity attempts to use an encoding that the receiving
   entity does not understand, the receiving entity MUST reply with an
   <unsupported-encoding/> stream error. Note well that the encodings of
   the initial stream and response stream are independent, and MAY be
   different.













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10. IANA Considerations

10.1 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>


10.2 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>


10.3 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>


10.4 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>


10.5 Existing Registrations

   The IANA registers "xmpp" as a GSSAPI [23] service name, as specified
   in SASL Definition (Section 6.4).

   Additionally, the IANA registers "jabber-client" and "jabber-server"
   as keywords for TCP ports 5222 and 5269 respectively. These ports
   SHOULD be used for client-to-server and server-to-server
   communications respectively, but their use is NOT REQUIRED. The use
   of the string "jabber" in these keywords is historical.






















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11. Internationalization Considerations

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














































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12. Security Considerations

12.1 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.

12.2 Client-to-Server Communications

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

   The TLS protocol for encrypting XML streams (defined under Stream
   Encryption (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
   Stream Authentication (Section 6)) provides a reliable mechanism for
   validating that a client connecting to a server is who it claims to
   be.

   Client-to-server communications MUST NOT proceed until the DNS
   hostname asserted by the server has been resolved. Such resolutions
   SHOULD first attempt to resolve the hostname using an SRV [18]
   Service of "jabber-client" and Proto of "tcp", resulting in resource
   records such as "_jabber-client._tcp.shakespeare.lit." (the use of
   the string "jabber-client" for the service identifier is consistent



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   with the existing IANA registration). If the SRV lookup fails, the
   fallback is a normal A lookup to determine the IP address, using the
   "jabber-client" port of 5222 assigned by the Internet Assigned
   Numbers Authority [5].

   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.

12.3 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 Server Dialback (Section 7).

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

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

   Inter-domain connections MUST NOT proceed until the DNS hostnames
   asserted by the servers have been resolved. Such resolutions MUST
   first attempt to resolve the hostname using an SRV [18] Service of
   "jabber-server" and Proto of "tcp", resulting in resource records
   such as "_jabber-server._tcp.shakespeare.lit." (the use of the string
   "jabber-server" for the service identifer is consistent with the
   existing IANA registration; note well that the "jabber-server"
   service identifier supersedes the earlier use of a "jabber" service
   identifier, since the earlier usage did not conform to RFC 2782
   [18]). 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].

   Server dialback helps protect against domain spoofing, thus making it
   more difficult to spoof XML stanzas. It is not a mechanism for
   authenticating, securing, or encrypting streams between servers as is
   done via SASL and TLS. Furthermore, it is susceptible to DNS
   poisoning attacks unless DNSSec [31] is used, and even if the DNS
   information is accurate, dialback cannot protect from attacks where
   the attacker is capable of hijacking the IP address of the remote



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   domain. Domains requiring robust security SHOULD use TLS and SASL. If
   SASL is used for server-to-server authentication, dialback SHOULD NOT
   be used since it is unnecessary.

12.4 Order of Layers

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

   1.  TCP

   2.  TLS

   3.  SASL

   4.  XMPP

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

12.5 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] (see IANA Considerations (Section 10)).
   Use of these well-known ports allows administrators to easily enable
   or disable XMPP activity through existing and commonly-deployed
   firewalls.

12.6 Mandatory to Implement Technologies

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

   for authentication: the SASL DIGEST-MD5 mechanism

   for confidentiality: TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA
      cipher)

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








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13. Compliance Requirements

   This section summarizes the specific aspects of the Extensible
   Messaging and Presence Protocol that MUST be supported by servers and
   clients in order to be considered compliant implementations, as well
   as additional protocol aspects that SHOULD be supported. For
   compliance purposes, we draw a distinction between core protocols
   (which MUST be supported by any server or client, regardless of the
   specific application) and instant messaging protocols (which MUST be
   supported only by instant messaging applications built on top of the
   core protocols). Compliance requirements that apply to all servers
   and clients are specified in this section; compliance requirements
   for instant messaging servers and clients are specified in the
   corresponding section of XMPP IM [24].

13.1 Servers

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

   o  Enforcement of the nodeprep [10] and resourceprep [11] profiles of
      stringprep

   o  XML streams (Section 4) as defined in this document, including
      stream encryption (Section 5) using TLS and stream authentication
      (Section 6) using SASL

   o  The basic semantics of the three defined XML stanzas (Section 8)
      (i.e., <message/>, <presence/>, and <iq/>)

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

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

   o  Server dialback (Section 7)


13.2 Clients

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

   o  XML streams (Section 4) as defined in this document, including
      stream encryption (Section 5) using TLS and stream authentication
      (Section 6) using SASL

   o  The basic semantics of the three defined XML stanzas (Section 8)



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      (i.e., <message/>, <presence/>, and <iq/>)

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

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

   o  Generation of addresses in accordance with the nodeprep [10] and
      resourceprep [11] profiles of stringprep










































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14. Differences Between Jabber and XMPP

   This section is non-normative.

   XMPP has been adapted from the protocols originally developed in the
   Jabber open-source community, which can be thought of as "XMPP 0.9".
   Because there exists a large installed base of Jabber implementations
   and deployments, it may be helpful to specify the key differences
   between Jabber and XMPP in order to expedite and encourage upgrades
   of those implementations and deployments to XMPP. This section
   summarizes the core differences, and the corresponding section of
   XMPP IM [24] summarizes the differences that relate specifically to
   instant messaging applications.

14.1 Authentication

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

   The Jabber community does not currently possess an authentication
   protocol for server-to-server communications, only the Server
   Dialback (Section 7) protocol to prevent server spoofing. XMPP
   augments Server Dialback with a true server-to-server authentication
   protocol, as defined in the Stream Authentication (Section 6) section
   of this document.

14.2 Channel Encryption

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

14.3 JID Processing

   JID processing was somewhat loosely defined by the Jabber community
   (documentation of forbidden characters and case handling is contained
   in "JEP-0029: Definition of Jabber Identifiers", published by the
   Jabber Software Foundation [33]). XMPP defines two stringprep [9]
   profiles for JID processing: nodeprep [10] and resourceprep [11].

14.4 Error Handling




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

   Stanza-related errors are handled in the Jabber community via
   HTTP-style error codes. In XMPP, stanza-related errors are handled
   via an extensible mechanism defined in the Stanza Errors (Section
   8.7) section of this document. (Documentation of a mapping between
   Jabber and XMPP error handling mechanisms is contained in "JEP-0086:
   Legacy Errors", published by the Jabber Software Foundation [33].)

14.5 Internationalization

   Although use of UTF-8 has always been standard practice within the
   Jabber community, the community did not define mechanisms for
   specifying the language of human-readable text provided in CDATA
   sections. XMPP specifies the use of the 'xml:lang' attribute in such
   contexts, as defined in the xml:lang (Section 8.2.5) section of this
   document.

14.6 Stream Version Attribute

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























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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. and J. Vincent, "Instant Messaging /
         Presence Protocol Requirements", RFC 2779, February 2000.

   [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-03 (work in progress), June 2003.

   [11]  Saint-Andre, P. and J. Hildebrand, "Resourceprep: A Stringprep
         Profile for Resource Identifiers in XMPP",
         draft-ietf-xmpp-resourceprep-03 (work in progress), June 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
         1999.




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

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

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

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

   [22]  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.

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




















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Informative References

   [24]  Saint-Andre, P. and J. Miller, "XMPP Instant Messaging",
         draft-ietf-xmpp-im-16 (work in progress), August 2003.

   [25]  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>.

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

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

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

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

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

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

   [32]  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.

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


Authors' Addresses

   Peter Saint-Andre
   Jabber Software Foundation

   EMail: stpeter@jabber.org


   Jeremie Miller
   Jabber Software Foundation

   EMail: jeremie@jabber.org



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Appendix A. XML Schemas

   The following XML schemas are descriptive, not normative.

A.1 Stream namespace

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

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

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

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

     <xs:element name='error'>
       <xs:complexType>



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

   </xs:schema>


A.2 Stream error namespace

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

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

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

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



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

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

   </xs:schema>


A.3 TLS namespace

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

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

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

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

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

   </xs:schema>




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A.4 SASL namespace

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

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

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

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

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

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

     <xs:element name='failure'>
       <xs:complexType>
         <xs:choice maxOccurs='1'>
           <xs:element ref='aborted'/>
           <xs:element ref='bad-protocol'/>
           <xs:element ref='encryption-required'/>
           <xs:element ref='invalid-authzid'/>
           <xs:element ref='invalid-mechanism'/>
           <xs:element ref='invalid-realm'/>
           <xs:element ref='mechanism-too-weak'/>
           <xs:element ref='not-authorized'/>
           <xs:element ref='temporary-auth-failure'/>
         </xs:choice>
       </xs:complexType>
     </xs:element>



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     <xs:element name='aborted' type='empty'/>
     <xs:element name='bad-protocol' type='empty'/>
     <xs:element name='encryption-required' type='empty'/>
     <xs:element name='invalid-authzid' type='empty'/>
     <xs:element name='invalid-mechanism' type='empty'/>
     <xs:element name='invalid-realm' type='empty'/>
     <xs:element name='mechanism-too-weak' type='empty'/>
     <xs:element name='not-authorized' type='empty'/>
     <xs:element name='temporary-auth-failure' type='empty'/>

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

   </xs:schema>


A.5 Dialback namespace

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

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

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




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     <xs:element name='verify'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='from' type='xs:string' use='required'/>
             <xs:attribute name='to' type='xs:string' use='required'/>
             <xs:attribute name='id' type='xs:NMTOKEN' 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.6 Client namespace

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

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       xmlns:xml='http://www.w3.org/XML/1998/namespace'
       targetNamespace='jabber:client'
       xmlns='jabber:client'
       elementFormDefault='qualified'>

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

     <xs:element name='message'>
        <xs:complexType>
           <xs:sequence>
              <xs:element ref='subject'
                          minOccurs='0'
                          maxOccurs='unbounded'/>
              <xs:element ref='body'
                          minOccurs='0'
                          maxOccurs='unbounded'/>
              <xs:element ref='thread'
                          minOccurs='0'



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

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

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

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

     <xs:element name='presence'>
       <xs:complexType>



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         <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:NMTOKEN'
                       use='optional'/>
         <xs:attribute ref='xml:lang' 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='dnd'/>



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

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

     <xs:element name='error'>
       <xs:complexType>



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         <xs:sequence>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-stanzas'
                   maxOccurs='1'/>
           <text   namespace='urn:ietf:params:xml:ns:xmpp-stanzas'
                   minOccurs='0'
                   maxOccurs='1'/>
           <xs:any
               namespace='##other'
               minOccurs='0'
               maxOccurs='1'/>
         </xs:sequence>
         <xs:attribute name='type' use='required'/>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='cancel'/>
               <xs:enumeration value='continue'/>
               <xs:enumeration value='modify'/>
               <xs:enumeration value='auth'/>
               <xs:enumeration value='wait'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>


A.7 Server namespace

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

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       xmlns:xml='http://www.w3.org/XML/1998/namespace'
       targetNamespace='jabber:server'
       xmlns='jabber:server'
       elementFormDefault='qualified'>

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

     <xs:element name='message'>
        <xs:complexType>
           <xs:sequence>
              <xs:element ref='subject'
                          minOccurs='0'
                          maxOccurs='unbounded'/>



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

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

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



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

     <xs:element name='presence'>
       <xs:complexType>
         <xs:sequence>
           <xs:element ref='show'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:element ref='status'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='priority'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='unbounded'/>
           <xs:element ref='error'
                       minOccurs='0'
                       maxOccurs='1'/>
         </xs:sequence>
         <xs:attribute name='to'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='from'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='optional'/>
         <xs:attribute ref='xml:lang' 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>



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

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



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     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:sequence>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-stanzas'
                   maxOccurs='1'/>
           <text   namespace='urn:ietf:params:xml:ns:xmpp-stanzas'
                   minOccurs='0'
                   maxOccurs='1'/>
           <xs:any
               namespace='##other'
               minOccurs='0'
               maxOccurs='1'/>
         </xs:sequence>
         <xs:attribute name='type' use='required'/>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='cancel'/>
               <xs:enumeration value='continue'/>
               <xs:enumeration value='modify'/>
               <xs:enumeration value='auth'/>
               <xs:enumeration value='wait'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>


A.8 Stanza error namespace

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

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

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

     <xs:element name='bad-request' type='empty'/>
     <xs:element name='conflict' type='empty'/>



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

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

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

   </xs:schema>





















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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-16

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

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

   o  Further clarified server handling of the basic stanza types.

   o  Further clarified character encoding rules per list discussion.

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

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

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

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


B.2 Changes from draft-ietf-xmpp-core-15

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

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

   o  Made 'id' required for IQ stanzas.


B.3 Changes from draft-ietf-xmpp-core-14

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

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







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B.4 Changes from draft-ietf-xmpp-core-13

   o  Clarified stream restart after successful TLS and SASL
      negotiation.

   o  Clarified requirement for resolution of DNS hostnames.

   o  Clarified text regarding namespaces.

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

   o  Added several more SASL error conditions.

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

   o  Made small editorial changes and fixed several schema errors.


B.5 Changes from draft-ietf-xmpp-core-12

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

   o  Adjusted error handling syntax and semantics per list discussion.

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

   o  Documented message type='normal'.

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

   o  Corrected several errors in the schemas.


B.6 Changes from draft-ietf-xmpp-core-11

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

   o  Made small editorial changes and fixed several schema errors.


B.7 Changes from draft-ietf-xmpp-core-10

   o  Adjusted TLS content regarding certificate validation process.

   o  Specified that stanza error extensions for specific applications



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      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.8 Changes from draft-ietf-xmpp-core-09

   o  Fixed several dialback error conditions.

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

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

   o  Added or modified several stream and stanza error conditions.

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

   o  Fixed several errors in the schemas.


B.9 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.


B.10 Changes from draft-ietf-xmpp-core-07

   o  Made several small editorial changes.


B.11 Changes from draft-ietf-xmpp-core-06

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



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   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 [26].

   o  Removed note to RFC Editor regarding provisional namespace names.


B.12 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.13 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.14 Changes from draft-ietf-xmpp-core-03

   o  Clarified rules and procedures for TLS and SASL.

   o  Amplified stream error code syntax per list discussion.

   o  Made numerous small editorial changes.


B.15 Changes from draft-ietf-xmpp-core-02

   o  Added dialback schema.




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   o  Removed all DTDs since schemas provide more complete definitions.

   o  Added stream error codes.

   o  Clarified error code "philosophy".


B.16 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'
      attribute.

   o  Made numerous small editorial changes.


B.17 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.18 Changes from draft-miller-xmpp-core-02

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

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

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

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



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   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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