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XMPP


Network Working Group                                P. Saint-Andre, Ed.
Internet-Draft                                                       XSF                                 XMPP Standards Foundation
Obsoletes: 3920 (if approved)                              July 17, 2007
Intended status: Standards Track                          April 17, 2007
Expires: October 19, 2007 January 18, 2008


        Extensible Messaging and Presence Protocol (XMPP): Core
                     draft-saintandre-rfc3920bis-02
                     draft-saintandre-rfc3920bis-03

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   This Internet-Draft will expire on October 19, 2007. January 18, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This memo document defines the core features of the Extensible Messaging
   and Presence Protocol (XMPP), a technology for streaming Extensible
   Markup Language (XML) elements in order to exchange structured
   information in close to real time between any two network-aware
   entities.  XMPP provides a generalized, extensible framework for
   incrementally exchanging XML data, upon which a variety of
   applications can be built.  The framework includes methods for stream



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   setup and teardown, channel encryption, authentication of a client to



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   a server and of one server to another server, and primitives for
   push-style messages, publication of presence and network availability
   information, information
   ("presence"), and request-response interactions between any two XMPP
   entities.  This document also specifies the format for XMPP
   addresses, which are fully internationalizable.

   This document obsoletes RFC 3920.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.2.   Functional Summary . . . . . . . . . . . . . . . . . . .   5
     1.3.   Conventions  . . . . . . . . . . . . . . . . . . . . . .   6   7
   2.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   6   7
     2.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .   6   7
     2.2.   Server . . . . . . . . . . . . . . . . . . . . . . . . .   7   8
     2.3.   Client . . . . . . . . . . . . . . . . . . . . . . . . .   7   8
     2.4.   Network  . . . . . . . . . . . . . . . . . . . . . . . .   7   9
   3.  Addresses . . . . . . . . . . . . . . . . . . . . . . . . . .   8   9
     3.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .   8   9
     3.2.   Domain Identifier  . . . . . . . . . . . . . . . . . . .   9  10
     3.3.   Node Identifier  . . . . . . . . . . . . . . . . . . . .  10  11
     3.4.   Resource Identifier  . . . . . . . . . . . . . . . . . .  10  11
     3.5.   Determination of Addresses . . . . . . . . . . . . . . .  11  12
   4.  TCP Binding . . . . . . . . . . . . . . . . . . . . . . . . .  11  12
   5.  XML Streams . . . . . . . . . . . . . . . . . . . . . . . . .  12  13
     5.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  12  13
     5.2.   Stream Security  . . . . . . . . . . . . . . . . . . . .  14  15
     5.3.   Stream Attributes  . . . . . . . . . . . . . . . . . . .  15  16
     5.4.   Namespace Declarations . . . . . . . . . . . . . . . . .  18  22
     5.5.   Stream Features  . . . . . . . . . . . . . . . . . . . .  18  22
     5.6.   Closing Streams  . . . . . . . . . . . . . . . . . . . .  18  23
     5.7.   Reconnection . . . . . . . . . . . . . . . . . . . . . .  19  24
     5.8.   Stream Errors  . . . . . . . . . . . . . . . . . . . . .  19  24
     5.9.   Simplified Stream Examples . . . . . . . . . . . . . . .  23  41
   6.  TLS  STARTTLS Negotiation  . . . . . . . . . . . . . . . . . . . . . . .  25  43
     6.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  25  43
     6.2.   Narrative  . . .   Rules  . . . . . . . . . . . . . . . . . . . .  28
   7.  SASL Negotiation . . . . .  44
     6.3.   Process  . . . . . . . . . . . . . . . . .  29
     7.1.   Overview . . . . . . .  44
     6.4.   Representation of JIDs in Certificates . . . . . . . . .  48
   7.  SASL Negotiation  . . . . . . . .  29
     7.2.   Narrative . . . . . . . . . . . . . .  49
     7.1.   Overview . . . . . . . . .  31
     7.3.   SASL Definition . . . . . . . . . . . . . . .  49
     7.2.   Rules  . . . . .  33
     7.4.   SASL Errors . . . . . . . . . . . . . . . . . . . .  49
     7.3.   Process  . .  34
   8.  Resource Binding . . . . . . . . . . . . . . . . . . . . . .  34
     8.1.   Binding Multiple Resources  51
     7.4.   SASL Definition  . . . . . . . . . . . . . . .  38
   9.  XML Stanzas . . . . .  56



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     7.5.   SASL Errors  . . . . . . . . . . . . . . . . . . . .  39



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     9.1.   Common Attributes . .  57
   8.  Resource Binding  . . . . . . . . . . . . . . . . .  40
     9.2.   Basic Semantics . . . . .  60
     8.1.   Overview . . . . . . . . . . . . . . .  42
     9.3.   Stanza Errors . . . . . . . . .  60
     8.2.   Advertising Support  . . . . . . . . . . . .  44
     9.4.   Extended Namespaces . . . . . .  60
     8.3.   Server-Generated Resource Identifier . . . . . . . . . .  61
     8.4.   Client-Generated Resource Identifier . .  48
   10. Examples . . . . . . . .  62
     8.5.   Binding Multiple Resources . . . . . . . . . . . . . . .  64
   9.  XML Stanzas . . .  49
     10.1.  Client-to-Server . . . . . . . . . . . . . . . . . . . .  49
     10.2.  Server-to-Server Examples . .  67
     9.1.   Common Attributes  . . . . . . . . . . . . .  55
   11. Server Rules for Handling XML Stanzas . . . . . .  67
     9.2.   Basic Semantics  . . . . . .  58
     11.1.  No 'to' Address . . . . . . . . . . . . . .  71
     9.3.   Stanza Errors  . . . . . .  59
     11.2.  Foreign Domain . . . . . . . . . . . . . . .  73
     9.4.   Extended Content . . . . . .  59
     11.3.  Local Domain . . . . . . . . . . . . . .  88
   10. Examples  . . . . . . . .  60
     11.4.  Mere Domain or Specific Resource . . . . . . . . . . . .  60
     11.5.  Node in Same Domain . . . . . .  89
     10.1.  Client-to-Server . . . . . . . . . . . .  60
   12. XML Usage . . . . . . . .  89
     10.2.  Server-to-Server Examples  . . . . . . . . . . . . . . .  95
   11. Server Rules for Processing XML Stanzas . . .  61
     12.1.  Restrictions . . . . . . . .  99
     11.1.  No 'to' Address  . . . . . . . . . . . . . .  61
     12.2.  XML Namespace Names and Prefixes . . . . . .  99
     11.2.  Local Domain . . . . . .  61
     12.3.  Validation . . . . . . . . . . . . . . . . 100
     11.3.  Resource at Local Domain . . . . . . .  63
     12.4.  Inclusion of Text Declaration . . . . . . . . . 100
     11.4.  Node at Local Domain . . . .  63
     12.5.  Character Encoding . . . . . . . . . . . . . . 101
     11.5.  Foreign Domain . . . . .  63
     12.6.  White Space . . . . . . . . . . . . . . . . 101
   12. XML Usage . . . . . .  63
   13. Compliance Requirements . . . . . . . . . . . . . . . . . . .  63
     13.1.  Servers . 102
     12.1.  Restrictions . . . . . . . . . . . . . . . . . . . . . . 102
     12.2.  XML Namespace Names and Prefixes .  64
     13.2.  Clients . . . . . . . . . . . 103
     12.3.  Validation . . . . . . . . . . . . .  64
   14. Internationalization Considerations . . . . . . . . . . 105
     12.4.  Inclusion of Text Declaration  . . .  65
   15. Security Considerations . . . . . . . . . . 106
     12.5.  Character Encoding . . . . . . . . .  65
     15.1.  High Security . . . . . . . . . . 106
     12.6.  White Space  . . . . . . . . . . .  65
     15.2.  Certificate Validation . . . . . . . . . . . 106
   13. Compliance Requirements . . . . . .  65
     15.3.  Client-to-Server Communications . . . . . . . . . . . .  66
     15.4.  Server-to-Server Communications . 106
     13.1.  Servers  . . . . . . . . . . .  67
     15.5.  Order of Layers . . . . . . . . . . . . . 107
     13.2.  Clients  . . . . . . .  67
     15.6.  Lack of SASL Channel Binding to TLS . . . . . . . . . .  68
     15.7.  Mandatory-to-Implement Technologies . . . . . . . 107
   14. Internationalization Considerations . . .  68
     15.8.  Firewalls . . . . . . . . . . 107
   15. Security Considerations . . . . . . . . . . . . .  68
     15.9.  Use of base64 in SASL . . . . . . 108
     15.1.  High Security  . . . . . . . . . . .  69
     15.10. Stringprep Profiles . . . . . . . . . . 108
     15.2.  Certificate Validation . . . . . . . .  69
   16. IANA Considerations . . . . . . . . . 108
     15.3.  Client-to-Server Communication . . . . . . . . . . . .  70
     16.1.  XML Namespace Name for TLS Data . 109
     15.4.  Server-to-Server Communication . . . . . . . . . . .  70
     16.2.  XML Namespace Name for SASL Data . . 110
     15.5.  Order of Layers  . . . . . . . . . .  70
     16.3.  XML Namespace Name for Stream Errors . . . . . . . . . .  70
     16.4.  XML Namespace Name for Resource 110
     15.6.  Lack of SASL Channel Binding to TLS  . . . . . . . .  71
     16.5.  XML Namespace Name for Stanza Errors . . 110
     15.7.  Mandatory-to-Implement Technologies  . . . . . . . .  71
     16.6.  Nodeprep Profile of Stringprep . . 111
     15.8.  Firewalls  . . . . . . . . . . .  71
     16.7.  Resourceprep Profile of Stringprep . . . . . . . . . . .  72
     16.8.  GSSAPI Service Name . 111
     15.9.  Use of base64 in SASL  . . . . . . . . . . . . . . . . .  72
     16.9.  Port Numbers 111
     15.10. Stringprep Profiles  . . . . . . . . . . . . . . . . . . 111
     15.11. Address Spoofing . . . .  72
   17. References . . . . . . . . . . . . . . . . 112
     15.12. Denial of Service  . . . . . . . . .  72
     17.1.  Normative References . . . . . . . . . . 114
     15.13. Presence Leaks . . . . . . . .  72
     17.2.  Informative References . . . . . . . . . . . . . 115
     15.14. Directory Harvesting . . . .  75



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   Appendix A.  Nodeprep . . . . . . . . . . . . . . 116
   16. IANA Considerations . . . . . . . .  77
     A.1.   Introduction . . . . . . . . . . . . . 116



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     16.1.  XML Namespace Name for TLS Data  . . . . . . . . .  77
     A.2.   Character Repertoire . . . 116
     16.2.  XML Namespace Name for SASL Data . . . . . . . . . . . . 116
     16.3.  XML Namespace Name for Stream Errors . . .  78
     A.3.   Mapping . . . . . . . 116
     16.4.  XML Namespace Name for Resource Binding  . . . . . . . . 117
     16.5.  XML Namespace Name for Stanza Errors . . . . . . . . .  78
     A.4.   Normalization . 117
     16.6.  Nodeprep Profile of Stringprep . . . . . . . . . . . . . 117
     16.7.  Resourceprep Profile of Stringprep . . . . . . .  78
     A.5.   Prohibited Output . . . . 118
     16.8.  GSSAPI Service Name  . . . . . . . . . . . . . . .  78
     A.6.   Bidirectional Characters . . . 118
     16.9.  Port Numbers . . . . . . . . . . . . .  79
   Appendix B.  Resourceprep . . . . . . . . . 118
   17. References  . . . . . . . . . . .  79
     B.1.   Introduction . . . . . . . . . . . . . . 118
     17.1.  Normative References . . . . . . . .  79
     B.2.   Character Repertoire . . . . . . . . . . 118
     17.2.  Informative References . . . . . . . .  79
     B.3.   Mapping . . . . . . . . . 120
   Appendix A.  Nodeprep . . . . . . . . . . . . . . .  80
     B.4.   Normalization . . . . . . . 124
     A.1.   Introduction . . . . . . . . . . . . . .  80
     B.5.   Prohibited Output . . . . . . . . 124
     A.2.   Character Repertoire . . . . . . . . . . .  80
     B.6.   Bidirectional Characters . . . . . . . 124
     A.3.   Mapping  . . . . . . . . .  80
   Appendix C.  Server Dialback . . . . . . . . . . . . . . . 124
     A.4.   Normalization  . . .  80
     C.1.   Overview . . . . . . . . . . . . . . . . . . 124
     A.5.   Prohibited Output  . . . . . .  80
     C.2.   Order of Events . . . . . . . . . . . . . 125
     A.6.   Bidirectional Characters . . . . . . .  82
     C.3.   Protocol . . . . . . . . . 125
   Appendix B.  Resourceprep . . . . . . . . . . . . . . .  82
     C.4.   Reuse of Negotiated Connections . . . . . 125
     B.1.   Introduction . . . . . . .  88
     C.5.   Dialback Key Generation . . . . . . . . . . . . . . . 125
     B.2.   Character Repertoire .  89
     C.6.   Advertisement . . . . . . . . . . . . . . . . . 126
     B.3.   Mapping  . . . .  90
   Appendix D.  XML Schemas . . . . . . . . . . . . . . . . . . . .  92
     D.1.   Streams namespace 126
     B.4.   Normalization  . . . . . . . . . . . . . . . . . . .  92
     D.2.   Stream error namespace . . 126
     B.5.   Prohibited Output  . . . . . . . . . . . . . . .  93
     D.3.   TLS namespace . . . . 126
     B.6.   Bidirectional Characters . . . . . . . . . . . . . . . . 127
   Appendix C.  XML Schemas  .  95
     D.4.   SASL namespace . . . . . . . . . . . . . . . . . . . 127
     C.1.   Streams namespace  . .  95
     D.5.   Resource binding namespace . . . . . . . . . . . . . . .  97
     D.6.   Dialback namespace . . 127
     C.2.   Stream error namespace . . . . . . . . . . . . . . . . .  99
     D.7.   Server dialback stream feature 128
     C.3.   TLS namespace  . . . . . . . . 101
     D.8.   Stanza error namespace . . . . . . . . . . . . . 131
     C.4.   SASL namespace . . . . 101
   Appendix E.  Contact Addresses . . . . . . . . . . . . . . . . . 103
   Appendix F.  Differences From RFC 3920 131
     C.5.   Resource binding namespace . . . . . . . . . . . . . 103
   Author's Address . . 133
     C.6.   Stanza error namespace . . . . . . . . . . . . . . . . . 135
   Appendix D.  Contact Addresses  . . . . . 104
   Intellectual Property and Copyright Statements . . . . . . . . . 105 . . . 136
   Appendix E.  Account Provisioning . . . . . . . . . . . . . . . . 137
   Appendix F.  Differences From RFC 3920  . . . . . . . . . . . . . 137
   Appendix G.  Copying Conditions . . . . . . . . . . . . . . . . . 138
   Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 139
   Intellectual Property and Copyright Statements  . . . . . . . . . 140











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1.  Introduction

1.1.  Overview

   The Extensible Messaging and Presence Protocol (XMPP) is an
   Extensible Markup Language XML [XML] technology 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 late 2002, the XMPP WG Working Group was
   chartered with developing an adaptation of the core Jabber protocol
   that would be suitable as an IETF instant messaging (IM) and presence
   technology.  As a result of work by the XMPP WG WG, [RFC3920] was
   published in October 2004.

   As a result of extensive implementation and deployment experience
   with XMPP since that time, as well as implementation
   experience and more formal interoperability testing completed since the
   publication of RFC 3920,
   testing, this document defines reflects consensus from the core features of XMPP 1.0; the extensions required to provide the instant messaging
   and presence functionality defined in [IMP-REQS] are specified in
   [XMPP-IM].

   This developer
   community regarding XMPP's core XML streaming technology.  In
   particular, this document obsoletes incorporates the following backward-
   compatible changes from RFC 3920.

1.2.  Functional Summary

   The purpose 3920:

   o  Corrections and errata
   o  Additional examples throughout
   o  Clarifications and more complete specification of matters that
      were underspecified
   o  Modifications to reflect updated technologies for which XMPP is to enable a
      using protocol (e.g., Transport Layer Security and the exchange Simple
      Authentication and Security Layer)
   o  Definition of relatively small
   pieces several additional error conditions
   o  Addition of structured data TLS plus SASL PLAIN as a mandatory-to-implement
      technology
   o  Definition of optional support for multiple resources over the
      same connection
   o  Removal of historical documentation for the server dialback
      protocol from this specification to a separate specification

   Therefore, this document defines the core features of XMPP 1.0 and
   obsoletes RFC 3920.

   Note: The XMPP extensions required to provide the basic instant
   messaging and presence functionality defined in [IMP-REQS] are
   specified in [XMPP-IM].

1.2.  Functional Summary

   This non-normative section provides a developer-friendly, functional
   summary of XMPP; refer to the sections that follow for a normative
   definition of XMPP.



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   The purpose of XMPP is to enable the exchange of relatively small
   pieces of structured data (called "XML stanzas") over a network
   between any two (or more) entities.  XMPP is implemented using a
   client-server architecture, wherein a client must connect to a server
   in order to gain access to the network and thus be allowed to
   exchange XML stanzas with other entities.  The process whereby a
   client connects to a server, exchanges XML stanzas, and ends the
   connection is as follows: is:

   1.  Determine the hostname and port at which to connect
   2.  Open a TCP connection
   3.  Open an XML stream
   4.  Complete TLS negotiation for channel encryption (RECOMMENDED) (recommended)
   5.  Complete SASL negotiation for authentication
   6.  Bind a resource to the stream
   7.  Exchange an unbounded number of XML stanzas with other entities
       on the network
   8.  Close the XML stream when further communications are not needed or
       desired
   9.  Close the TCP connection. connection

   In the sections following discussion of XMPP architecture and XMPP
   addresses, this document specifies how clients connect to servers and
   specifies the basic semantics of XML stanzas, but stanzas.  However, this document
   does not define the "payloads" of the XML stanzas that might be
   exchanged once a connection is successfully established; instead,
   definition of such semantics is provided by various XMPP extensions (e.g.,
   [XMPP-IM] defines extensions for



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   functionality, and various specifications produced in the XMPP
   Standards Foundation's XEP series define extensions for a wide range
   of more advanced functionality).

   Within the client-server architecture used by XMPP, one server may
   optionally connect to another server to enable inter-domain or inter-
   server communication.  For this to happen, the two servers must
   negotiate a connection between themselves and then exchange XML
   stanzas; the process for doing so is as follows: is:

   1.  Determine the hostname and port at which to connect
   2.  Open a TCP connection
   3.  Open an XML stream
   4.  Complete TLS negotiation for channel encryption (RECOMMENDED) (recommended)
   5.  Complete SASL negotiation for authentication
   6.  Exchange an unbounded number of XML stanzas both directly for the
       servers and indirectly on behalf of entities associated with each
       server (e.g., connected clients)
   7.  Close the XML stream when further communications are not needed or
       desired





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   8.  Close the TCP connection. connection

   Note: Depending on local service policies, a service may wish to use
   the older server dialback protocol to provide weak identity
   verification in cases where SASL negotiation would not result in
   strong authentication (e.g., because the certificate presented by the
   peer service during TLS negotiation is self-signed and thus provides
   only weak identity); for details, see
   Appendix C. [XEP-0220].

1.3.  Conventions

   The following keywords are to be interpreted as described in [TERMS]:
   "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD",
   "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

   In examples, lines have been wrapped for improved readability. readability,
   "[...]" means elision, and the following prepended strings are used:

   o  C: = client
   o  E: = any XMPP entity
   o  I: = initiating entity
   o  P: = peer server
   o  R: = receiving entity
   o  S: = server
   o  S1: = server1
   o  S2: = server2


2.  Architecture

2.1.  Overview

   XMPP assumes a client-server architecture, wherein a client utilizing
   XMPP accesses a server (normally over a [TCP] connection) and servers
   can also communicate with each other over TCP connections.
   Architectures that use the syntax of XML stanzas (Section 9) but that
   establish peer-to-peer connections directly between clients using
   technologies based on [LINKLOCAL] have been deployed, but such
   architectures are not XMPP and are best described as "XMPP-like"; for
   details, see [XEP-0174].



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   An

   A simplified architectural diagram for a typical deployment is shown
   here, where the entities have the following significance:

   o  romeo@example.net -- an XMPP user.
   o  example.net -- an XMPP server.
   o  juliet@example.com  example.com -- an XMPP user. server.
   o  example.com  juliet@example.com -- an XMPP server. user.

     example.net -------------------- example.com
        |                                |
        |                                |
   romeo@example.net               juliet@example.com



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   Note: Architectures that employ the syntax of XML stanzas (Section 9)
   but that establish peer-to-peer connections directly between clients
   using technologies based on [LINKLOCAL] have been deployed, but such
   architectures are not XMPP and are best described as "XMPP-like"; for
   details, see [XEP-0174].

2.2.  Server

   A server acts as SERVER is an intelligent abstraction layer for XMPP
   communications.  Its entity whose primary responsibilities are: are to:

   o  to manage connections from other entities, in the form of  Manage XML streams (Section 5) to and from authorized clients, servers, with local clients and
      other entities deliver XML
      stanzas (Section 9) to those clients over the negotiated XML
      streams.
   o  Subject to local service policies on server-to-server
      communication, manage XML streams (Section 5) with foreign servers
      and route appropriately-addressed XML stanzas (Section 9) among
      such entities to those servers over the
      negotiated XML streams

   Most XMPP-compliant servers also assume responsibility for streams.

   Depending on the
   storage application, the secondary responsibilities of an
   XMPP server may include:

   o  Storing XML data that is used by clients (e.g., contact lists for
      users of XMPP-based instant messaging and presence applications);
      in this case, the relevant XML data stanza is processed handled directly by the
      server itself on behalf of the client and is not routed to another entity.

2.3.  Client

   Most clients connect directly to a
      foreign server over or delivered to a [TCP] connection and local entity.
   o  Hosting local services that also use XMPP to take full advantage of as the basis for
      communication but that provide additional functionality provided by a
   server and any associated services.  Multiple resources (e.g.,
   devices beyond
      that defined in this document or locations) MAY connect simultaneously to a server on in [XMPP-IM]; examples include
      multi-user conferencing services as specified in [XEP-0045] and
      publish-subscribe services as specified in [XEP-0060].

2.3.  Client

   A CLIENT is an entity that establiishes an XML stream with a server
   by authenticating using the credentials of a local account and that
   then completes resource binding (Section 8) in order to enable
   delivery of XML stanzas via the server to the client.  A client then
   uses XMPP to communicate with its server, other clients, and any
   other accessible entities on a network.  Multiple clients may connect
   simultaneously to a server on behalf of each authorized client, a local account, with each resource
   client differentiated by the resource identifier portion of an XMPP
   address (e.g., <node@domain/home> vs. <node@domain/work>) <node@domain/work>), as defined
   under Addresses
   (Section 3) Section 3 and Resource Binding (Section 8). Section 8.  The RECOMMENDED port for TCP
   connections between a client and a server is 5222, as registered with
   the IANA (see Port Numbers (Section 16.9)). Section 16.9).





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2.4.  Network

   Because each server is identified by a network address and because
   server-to-server communications are communication is a straightforward extension of



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   client-to-server protocol, in practice, practice the system consists of a
   network of servers that inter-communicate.  Thus, for example,
   <juliet@example.com> is able to exchange messages, presence, and
   other information with <romeo@example.net>.  This pattern is familiar
   from messaging protocols (such as [SMTP]) that make use of network
   addressing standards.  Communications  Communication between any two servers are is
   OPTIONAL.  If enabled, such communications communication SHOULD occur over XML
   streams that are bound to [TCP] connections.  The RECOMMENDED port
   for tCP connections between servers is 5269, as registered with the
   IANA (see Port Numbers (Section 16.9)). Section 16.9.


3.  Addresses

3.1.  Overview

   An entity ENTITY is anything that can be considered a network endpoint
   (i.e., an ID on the network) is network-addressable and that can
   communicate using XMPP.
   All such entities are uniquely addressable on the network.  For historical reasons, the native address
   of an XMPP entity is called a Jabber
   Identifier JABBER IDENTIFIER or JID.  A valid JID
   contains a set of ordered elements formed of a an XMPP domain
   identifier, node identifier, and resource identifier.

   The syntax for a JID is defined as follows using the Augmented
   Backus-Naur Form as defined specified in [ABNF].

      jid             = [ node "@" ] domain [ "/" resource ]
      node            = 1*(nodepoint)
                        ; a "nodepoint" is a UTF-8 encoded Unicode code
                        ; point that satisfies the Nodeprep profile of
                        ; stringprep
      domain          = fqdn / address-literal / idnlabel
      fqdn            = (idnlabel 1*("." idnlabel))
                        ; an "idnlabel" is an internationalized label
                        ; as described in RFC 3490
      address-literal = IPv4address / IPv6address
                        ; the "IPv4address" and "IPv6address" rules are
                        ; defined in Appendix B of RFC 3513
      resource        = 1*(resourcepoint)
                        ; a "resourcepoint" is a UTF-8 encoded Unicode
                        ; code point that satisfies the Resourceprep
                        ; profile of stringprep

   All JIDs are based on the foregoing structure.  One common use of
   this structure is to identify a messaging and presence account, the



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   server that hosts the account, and a connected resource (e.g., a
   specific device) in the form of <node@domain/resource>.  However,



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   node types other than clients are possible; for example, a specific
   chat room offered by a multi-user chat conference service (see [XEP-0045])
   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 conference
   service) and a specific occupant of such a room could be addressed as
   <room@service/nick> (where "nick" is the occupant's room nickname).
   Many other JID types are possible (e.g., <domain/resource> could be a
   server-side script or service).

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

   Note: While the format of a JID is consistent with [URI], an entity's
   address on an XMPP network MUST be a JID (without a URI scheme) and
   not a [URI] or [IRI] as specified in [XMPP-URI]; the latter
   specification is provided only for use by non-XMPP applications.

3.2.  Domain Identifier

   The DOMAIN IDENTIFIER portion of a JID is that portion after the '@'
   character (if any) and before the '/' character (if any); it is the
   primary identifier and is the only REQUIRED element of a JID (a mere
   domain identifier is a valid JID).
   It usually represents  Typically a domain identifier
   identifies the network or "home" server to which other
   entities clients connect for XML routing
   and data management capabilities.
   Note that a functionality.  (Note: A single server may host
   multiple domain identifiers (local
   domains), and that identifiers, i.e., multiple local domains.)  However,
   it is not necessary for an XMPP domain identifiers identifier to
   reference entities identify an
   entity that provide traditional provides core XMPP server functionality (e.g., a multi-user chat service or a domain
   identifier may identity an entity such as a multi-user conference
   service, a publish-subscribe service, or a user directory).

   The domain identifier for every server or service that will
   communicate over a network SHOULD be a fully qualified domain name
   (see [DNS]) but MAY be either an IPv4 or IPv6 address or a text label
   (commonly called an "unqualified hostname") that is resolvable on a
   local network.  If the domain identifier includes a final character
   considered to be a label separator (dot) by [IDNA] or [STD13], this
   character MUST be stripped from the domain identifier before the JID
   of which it is a part is used for the purpose of routing an XML
   stanza, comparing against another JID, or constructing an [XMPP-URI];
   in particular, the character should be stripped before any other
   canonicalization steps are taken (such as application of the
   [NAMEPREP] profile of [STRINGPREP] or completion of the ToASCII
   operation as described in [IDNA]).



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   A domain identifier MUST be an "internationalized domain name" as
   defined in [IDNA], that is, "a domain name in which every label is an
   internationalized label".  When preparing a text label (consisting of
   a sequence of Unicode code points) for representation as an



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   internationalized label in the process of constructing an XMPP domain
   identifier or comparing two XMPP domain identifiers, an application
   MUST ensure that for each text label it is possible to apply without
   failing the ToASCII operation specified in [IDNA] with the
   UseSTD3ASCIIRules flag set (thus forbidding ASCII code points other
   than letters, digits, and hyphens).  If the ToASCII operation can be
   applied without failing, then the label is an internationalized
   label.  An internationalized domain name (and therefore an XMPP
   domain identifier) is constructed from its constituent
   internationalized labels by following the rules specified in [IDNA].
   Note:
   (Note: The ToASCII operation includes application of the [NAMEPREP]
   profile of [STRINGPREP] and encoding using the algorithm specified in
   [PUNYCODE]; for details, see [IDNA]. [IDNA].)

3.3.  Node Identifier

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

   A node identifier MUST be formatted such that the Nodeprep profile of
   [STRINGPREP] can be applied without failing (see Appendix A).  Before
   comparing two node identifiers, an application MUST first apply the
   Nodeprep profile to each identifier.

3.4.  Resource Identifier

   The RESOURCE IDENTIFIER portion of a JID is an optional tertiary
   identifier placed after the domain identifier and separated from the
   latter by the '/' character.  A resource identifier may modify either
   a <node@domain> address or a mere <domain> address.  It usually represents  Typically a
   resource identifier uniquely identifies a specific connection (e.g.,
   a device or location) or object (e.g., a participant in a multi-user chat
   conference room) belonging to the entity associated with a an XMPP node identifier.
   identifier at a local domain.  A resource identifier has no semantic
   meaning and is opaque to both servers and other clients, and clients.  A resource



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   identifier is typically defined by negotiated between a client implementation when it provides the information necessary to
   complete Resource Binding (Section 8) (although it may be generated
   by and a server on behalf of a client), during
   resource binding (Section 8), after which the entity is referred to
   as a "connected resource" CONNECTED RESOURCE and its address (of the form
   <node@domain/resource>) is referrred referred to as a "full JID" (<node@domain/resource>). FULL JID.  An entity MAY
   maintain multiple connected resources simultaneously, with each
   connected resource differentiated by a distinct resource identifier.



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   A resource identifier MUST be formatted such that the Resourceprep
   profile of [STRINGPREP] can be applied without failing (see
   Appendix B).  Before comparing two resource identifiers, an
   application MUST first apply the Resourceprep profile to each
   identifier.

3.5.  Determination of Addresses

   After SASL negotiation (Section 7) and, if appropriate, Resource
   Binding resource
   binding (Section 8), the receiving entity for a stream MUST determine
   the initiating entity's JID.

   For server-to-server communications, communication, the initiating entity's JID
   SHOULD be the authorization identity, derived from the authentication
   identity, as
   identity (as defined by [SASL], [SASL]), if no authorization identity was
   specified during SASL negotiation (Section 7).

   For client-to-server communications, communication, the "bare JID" bare JID (<node@domain>)
   SHOULD be the authorization identity, derived from the authentication
   identity, as
   identity (as defined in [SASL], by [SASL]), if no authorization identity was
   specified during SASL negotiation (Section 7); the resource
   identifier portion of the "full JID" full JID (<node@domain/resource>) SHOULD be
   the resource identifier negotiated by the client and server during
   Resource Binding
   resource binding (Section 8).

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


4.  TCP Binding

   Although there

   As XMPP is no necessary coupling of defined herein, an XML stream to initiating entity (client or server)
   MUST open a [TCP]
   connection (e.g., two entities could connect to each other via
   another transport, e.g.  [HTTP] as specified in [XEP-0124] and
   [XEP-0206]), this specification defines a binding of XMPP to TCP
   only.

   Therefore, as XMPP is defined herein, an initiating entity (client or
   server) MUST open a TCP TCP connection at the receiving entity (server) before it
   negotiates XML streams with the receiving entity.  However, prior to
   opening the TCP connection connection, the initiating entity first MUST resolve
   the Domain Name System (DNS) hostname associated with the receiving
   entity and determine the appropriate TCP port for
   communications communication with the receiving entity.  The process is as follows:



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   the receiving entity.  The process is:

   1.  Attempt to resolve the hostname using a [DNS-SRV] Service of
       "xmpp-client" (for client-to-server connections) or "xmpp-server"
       (for server-to-server connections) and Proto of "tcp", resulting
       in resource records such as "_xmpp-client._tcp.example.com." or
       "_xmpp-server._tcp.example.com."; the IP address and port at
       which the initiating entity attempts to connect to the receiving
       entity shall be those specified in the SRV lookup result.
   2.  If the SRV lookup fails, the fallback SHOULD be a normal IPv4 or
       [IPv6] address record resolution to determine the IP address,
       where the port used is the "xmpp-client" port of 5222 for client-
       to-server connectionsn connections or the "xmpp-server" port 5269 for client- server-
       to-server connections.
   3.  However,  For client-to-server connections, the fallback MAY be a DNS TXT [DNS-TXT]
       lookup (see [DNS-TXT]) for alternative connection methods, for example as
       described in [XEP-0156].

   TCP connections are handled differently in client-to-server
   communications
   communication and server-to-server communications. ommunication.  Specifically:

   o  Because a client is subordinate to a server and therefore a client
      authenticates to the server but the server does not authenticate
      to the client, it is necessary to have only one TCP connection
      between client and server.  Thus the server MUST allow the client
      to share a single TCP connection for XML stanzas sent from client
      to server and from server to client (i.e., the inital stream and
      response stream as specified under XML Streams (Section 5)). Section 5).
   o  Because two servers are peers and therefore each peers peer must
      authenticate with the other, the servers MUST use two TCP
      connections: one for XML stanzas sent from the first server to the
      second server and another (initiated by the second server) for
      stanzas from the second server to the first server.

   Note: There is no necessary coupling of an XML stream to a [TCP]
   connection.  For example, two entities could connect to each other
   via another transport, such as [HTTP] as specified in [XEP-0124] and
   [XEP-0206].  However, this specification defines a binding of XMPP to
   TCP only.


5.  XML Streams

5.1.  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



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   terms are defined as follows: follows.

   Definition of XML Stream:  An XML STREAM is a container for the
      exchange of XML elements between any two entities over a network.
      The start of an XML stream is denoted unambiguously by an opening
      XML <stream> tag (with appropriate attributes and namespace
      declarations), while the end of the XML stream is denoted
      unambiguously by a closing XML </stream> tag.  During the life of



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      the stream, the entity that initiated it can send an unbounded
      number of XML elements over the stream, either elements used to
      negotiate the stream (e.g., to complete TLS negotiation
      (Section 6) or SASL negotiation (Section 7)) or XML stanzas.  The
      "initial stream"
      INITIAL STREAM is negotiated from the initiating entity (usually (typically
      a client or server) to the receiving entity (usually (typically a server),
      and can be seen as corresponding to the initiating entity's
      "connection" or "session" with the receiving entity.  The initial
      stream enables unidirectional communication from the initiating
      entity to the receiving entity; in order to enable information
      exchange from the receiving entity to the initiating entity, the
      receiving entity MUST negotiate a stream in the opposite direction
      (the
      "response stream"). RESPONSE STREAM).
   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, and is the basic unit of meaning in XMPP.  An
      XML stanza exists at the direct child level of the root <stream/>
      element and is said to be well-balanced if it matches the
      production [43] content of [XML].  The start of any XML stanza is
      denoted unambiguously by the element start tag at depth=1 of the
      XML stream (e.g., <presence>), and the end of any XML stanza is
      denoted unambiguously by the corresponding close tag at depth=1
      (e.g., </presence>); a server MUST NOT process, deliver, or route process a partial stanza
      and MUST NOT attach meaning to the transmission timing of any part
      of a stanza (before receipt of the close tag).  The only XML
      stanzas defined herein are the <message/>, <presence/>, and <iq/>
      elements qualified by the default namespace for the stream, as
      described under XML Stanzas (Section 9); Section 9; an XML element sent for the purpose of
      TLS negotiation (Section 6), 6) or SASL negotiation (Section 7), or server dialback (Appendix C) 7) is not
      considered to be an XML stanza.  An XML stanza MAY contain child
      elements (with accompanying attributes, elements, and XML
      character data) as necessary in order to convey the desired
      information, which MAY be qualified by any XML namespace (see
      [XML-NAMES]).
      [XML-NAMES] as well as Section 9.4 herein).

   Consider the example of a client's connection to a server.  In order
   to connect to a server, a client MUST initiate an XML stream by
   sending an opening <stream> tag to the server, optionally preceded by
   a text declaration specifying the XML version and the character
   encoding supported (see Inclusion of Text Declaration (Section 12.4) Section 12.4 and Character Encoding (Section 12.5)). Section 12.5).  Subject to



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   local policies and service provisioning, the server SHOULD then reply
   with a second XML stream back to the client, again optionally
   preceded by a text declaration.  Once the client has completed SASL
   negotiation (Section 7), 7) and resource binding (Section 8), the client
   MAY send an unbounded number of XML stanzas over the stream to any recipient on the network. any.
   When the client desires to close the stream, it simply sends a
   closing </stream> tag



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   Those who are accustomed to thinking of 5.6).

   In essence, then, an XML in a document-centric
   manner may wish to view a client's connection to a server stream acts as
   consisting of two open-ended XML documents: one from an envelope for all the XML
   stanzas sent during a connection.  We can represent this in a
   simplistic fashion as follows.

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

   Note: Those who are accustomed to thinking of XML in a document-
   centric manner may wish to view a client's connection to 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; 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 connection.  We can represent this in a
   simplistic fashion as follows:

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

5.2.  Stream Security

   For the purpose of stream security, both Transport Layer Security
   (see Section 6) and the Simple Authentication and Security Layer (see
   Section 7) are mandatory to implement.




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   When negotiating XML streams in XMPP 1.0, TLS SHOULD be used as
   defined under TLS negotiation (Section 6) Section 6 and SASL MUST be used as defined under SASL negotiation (Section 7).
   Section 7.  The initial stream and the response stream MUST be
   secured separately, although security in both directions MAY be
   established via mechanisms that provide mutual authentication.  An

   The initiating entity SHOULD NOT attempt to send XML Stanzas stanzas
   (Section 9) over the stream before the stream has been authenticated,
   but authenticated.
   However, if it does, then does attempt to do so, the other receiving entity MUST NOT
   accept such stanzas and SHOULD MUST return a <not-authorized/> stream error
   and then terminate both the XML stream and the underlying TCP connection; note well that



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   this
   connection.  Note: This applies to XML stanzas only (i.e.,
   <message/>, <presence/>, and <iq/> elements qualified by the default
   namespace) and not to XML elements used for stream negotiation (e.g.,
   elements used to complete TLS negotiation (Section 6) or SASL
   negotiation (Section 7)).

5.3.  Stream Attributes

   The attributes of the stream root <stream/> element are as follows:

   o follows.

5.3.1.  from --

   In client-to-server communications, communication, the 'from' attribute SHOULD be
   included in the XML stream header sent from the initiating entity to
   the receiving entity and (if included) MUST be set to the account
   name (i.e., "bare JID" bare JID = <node@domain>) of the entity controlling the
   client.

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   In server-to-server
      communications, communication, the 'from' attribute SHOULD be
   included in the XML stream header sent from the initiating entity to
   the receiving entity and (if included) MUST be set to a hostname
   serviced by the initiating entity.









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   P: <?xml version='1.0'?>
      <stream:stream
          from='example.net'
          to='example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>

   In both client-to-server and server-to-server communications, the
   'from' attribute MUST be included in the XML stream header by which
   the receiving entity responds to the initiating entity and MUST be
   set to a hostname serviced by the receiving entity that is granting
   access to the initiating entity.

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   Note: Each entity MUST verify the identity of the other entity before
   exchanging XML stanzas with it (see the Client-to-Server
      Communications (Section 15.3) Section 15.3 and Server-to-Server Communications
      (Section 15.4) sections of this document for details).
   o Section 15.4).

5.3.2.  to --

   In both client-to-server and server-to-server communications, the
   'to' attribute SHOULD be included in the XML stream header sent from
   the initiating entity to the receiving entity and (if included) MUST
   be set to a hostname serviced by the receiving entity.

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   In client-to-server communications, communication, if the client included a 'from'
   address in the initial stream header then the server SHOULD include a
   'to' attribute in the XML stream header by which it replies to the
   client and (if included) MUST set the 'to' attribute to the bare JID



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   specified in the 'from' attribute of the XML stream header sent from
   the initiating entity to the receiving entity.

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   In server-to-server communications, communication, if the initiating entity included
   a 'from' address in the initial stream header then the receiving
   entity SHOULD include a 'to' attribute in the XML stream header by
   which it replies to the initiating entity and (if included) MUST set
   the 'to' attribute to the hostname specified in the 'from' attribute
   of the XML stream header sent from the initiating entity to the
   receiving entity.

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
          to='example.net'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>

   Note: Each entity MUST verify the identity of the other entity before
   exchanging XML stanzas with it (see the Client-to-Server
      Communications (Section 15.3) Section 15.3 and Server-to-Server Communications
      (Section 15.4) sections of this document for details). Section 15.4).

5.3.3.  id

   There SHOULD NOT be an '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.

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'



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   o  id --


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

   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
   identifier for the initiating entity's two streams with the receiving
   entity, and MUST be unique within the receiving application (normally
   a server).  Note well that the

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   Note: The stream ID may be security-critical and therefore MUST be
   both unpredictable and nonrepeating (see [RANDOM] for recommendations
   regarding randomness for security purposes).  There SHOULD NOT be an '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

5.3.4.  xml:lang --

   An 'xml:lang' attribute (as defined in Section 2.12 of [XML]) SHOULD
   be included by the initiating entity on the header for the initial
   stream to specify the default language of any human-readable XML
   character data it sends over that stream.

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   If the attribute is included, the receiving entity SHOULD remember
   that value as the default for both the initial stream and the
   response stream; if the attribute is not included, the receiving
   entity SHOULD use a configurable default value for both streams,
   which it MUST communicate in the header for the response stream.







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   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   For all stanzas sent over the initial stream, if the initiating
   entity does not include an 'xml:lang' attribute, the receiving entity
   SHOULD apply the default value; if the initiating entity does include
   an 'xml:lang' attribute, the receiving entity MUST NOT modify or
   delete it (see also xml:lang (Section 9.1.5)). Section 9.1.5).  The value of the 'xml:lang'
   attribute MUST be an conform to the NMTOKEN datatype (as defined in Section
   2.3 of [XML]) and MUST conform to the format defined in [LANGTAGS].
   o

5.3.5.  version --

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

   We can summarize as follows:

   +----------+--------------------------+-------------------------+
   |          | initiating to receiving  | receiving to initiating |
   +----------+--------------------------+-------------------------+
   | to       | JID of receiver          | JID of initiator        |
   | from     | JID of initiator         | JID of receiver         |
   | id       | silently ignored         | stream identifier       |
   | xml:lang | default language         | default language        |
   | version  | XMPP 1.0 supported       | XMPP 1.0 supported      |
   +----------+--------------------------+-------------------------+

   Note: The attributes of the root <stream/> element are not prepended



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   by a 'stream:' prefix because, in accordance with Section 5.3 of XML
   namespaces specification [XML-NAMES], the default namespace does not
   apply to attribute names.

5.3.1.  Version Support

   The version of XMPP specified herein is "1.0"; in particular, this
   encapsulates the stream-related protocols (TLS negotiation
   (Section 6), SASL negotiation (Section 7), and Stream Errors stream errors
   (Section 5.8)), as well as the semantics of the three defined XML
   stanza types (<message/>, <presence/>, and <iq/>).

   The numbering scheme for XMPP versions is "<major>.<minor>".  The
   major and minor numbers MUST be treated as separate integers and each
   number MAY be incremented higher than a single digit.  Thus, "XMPP
   2.4" would be a lower version than "XMPP 2.13", which in turn would
   be lower than "XMPP 12.3".  Leading zeros (e.g., "XMPP 6.01") MUST be
   ignored by recipients and MUST NOT be sent.

   The major version number should be incremented only if the stream and
   stanza formats or required actions have changed so dramatically that
   an older version entity would not be able to interoperate with a
   newer version entity if it simply ignored the elements and attributes
   it did not understand and took the actions specified in the older
   specification.

   The minor version number indicates should be incremented only if significant
   new capabilities,
   and capabilities have been added to the core protocol (e.g., a newly
   defined value of the 'type' attribute for message, presence, or IQ
   stanzas).  The minor version number MUST be ignored by an entity with



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   a smaller minor version number, but used for informational purposes
   by the entity with the larger minor version number.  For example, a minor version number might
   indicate the ability to process a newly defined value of the 'type'
   attribute for message, presence, or IQ stanzas; (e.g., the entity
   with the larger minor version number would simply note that its
   correspondent would not be able to understand that value of the
   'type' attribute and therefore would not send it. it).

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

   1.  The initiating entity MUST set the value of the 'version'
       attribute on the initial stream header to the highest version
       number it supports (e.g., if the highest version number it
       supports is that defined in this specification, it MUST set the
       value to "1.0").
   2.  The receiving entity MUST set the value of the 'version'
       attribute on the response stream header to either the value
       supplied by the initiating entity or the highest version number
       supported by the receiving entity, whichever is lower.  The
       receiving entity MUST perform a numeric comparison on the major
       and minor version numbers, not a string match on
       "<major>.<minor>".



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   3.  If the version number included in the response stream header is
       at least one major version lower than the version number included
       in the initial stream header and newer version entities cannot
       interoperate with older version entities as described above, described, the
       initiating entity SHOULD generate an <unsupported-version/>
       stream error and terminate the XML stream and underlying TCP
       connection.
   4.  If either entity receives a stream header with no 'version'
       attribute, the entity MUST consider the version supported by the
       other entity to be "0.9" and SHOULD NOT include a 'version'
       attribute in the stream header it sends in reply.

5.4.  Namespace Declarations

   The stream

5.3.6.  Summary

   We can summarize the attributes of the root <stream/> element MUST possess both a streams namespace declaration
   and a default namespace declaration as
   follows.

   +----------+--------------------------+-------------------------+
   |          | initiating to receiving  | receiving to initiating |
   +----------+--------------------------+-------------------------+
   | to       | JID of receiver          | JID of initiator        |
   | from     | JID of initiator         | JID of receiver         |
   | id       | silently ignored         | stream identifier       |
   | xml:lang | default language         | default language        |
   | version  | XMPP 1.0 supported       | XMPP 1.0 supported      |
   +----------+--------------------------+-------------------------+




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   Note: The attributes of the root <stream/> element are not prepended
   by a 'stream:' prefix because, in accordance with Section 5.3 of
   [XML-NAMES], the default namespace does not apply to attribute names.

5.4.  Namespace Declarations

   The stream element MUST possess both a streams namespace declaration
   and a default namespace declaration (as "namespace declaration" is
   defined in the [XML-NAMES]).  For detailed information regarding the
   streams namespace and default namespace, see Namespace Names and
   Prefixes (Section 12.2). Section 12.2.

5.5.  Stream Features

   If the initiating entity includes the 'version' attribute set to a
   value of at least "1.0" in the initial stream header, after sending
   the header for the response stream the receiving entity MUST send a
   <features/> child element (prefixed by the streams namespace prefix)
   to the initiating entity in order to announce any stream-level
   features that can be negotiated (or capabilities that otherwise need
   to be advertised).  Currently, this is

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
   S: <stream:features>
        <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
          <required/>
        </starttls>
      </stream:features>

   Stream features are used only mainly to advertise TLS negotiation
   (Section 6), SASL negotiation (Section 7), and resource binding
   (Section 8), and server dialback (Appendix C) as
   defined herein; 8); however, the stream features functionality also can be used to advertise other negotiable
   features as well. associated with various XMPP extensions.  If an entity does
   not understand or support some features, a feature, it SHOULD silently ignore them. that
   feature.

   If one or more security features (e.g., TLS and SASL) need to be
   successfully negotiated before a non-security-related feature (e.g., Resource Binding)
   resource binding) can be offered, the non-security-
   related non-security-related feature
   SHOULD NOT be included in the stream features that are advertised
   before the relevant security features have been negotiated.



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   If a feature must be negotiated before the initiating entity may
   proceed, that feature SHOULD include a <required/> child element.

5.6.  Closing Streams

   At any time after XML streams have been negotiated between two
   entities, either entity MAY close its stream

   If there are no features to the other entity
   (even be advertised (e.g., in the absence of a stream error) by sending reset
   initiated after successful SASL negotiation for a closing stream
   tag:



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   connection, or after resource binding for a client-to-server stream)
   then the receiving entity MUST include an empty <stream:features/>
   element after sending a stream header to the initiating entity.

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
   S: <stream:features/>

5.6.  Closing Streams

   At any time after XML streams have been negotiated between two
   entities, either entity MAY close its stream to the other entity
   (even in the absence of a stream error) by sending a closing stream
   tag:

   C: </stream:stream>

   The entity that sends the closing stream tag SHOULD wait for the
   other entity to also close its stream:

   S: </stream:stream>

   However, the entity that sends the first closing stream tag MAY
   consider both streams to be void if the other entity does not send
   its closing stream tag within a reasonable amount of time (where the
   definition of "reasonable" is left up to the implementation or
   deployment).

   After an entity sends a closing stream tag, it MUST NOT send further
   data over that stream.

   After the entity that sent the first closing stream tag receives a
   reciprocal closing stream tag from the other entity, it MUST
   terminate the underlying TCP connection.

   Note: There is one TCP connection for client-to-server streams, but



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   there are two TCP connections for server-to-server streams.

5.7.  Reconnection

   It can happen that an XMPP server goes offline while servicing
   connections from clients and from other servers.  Because the number
   of such connections can be quite large, the reconnection algorithm
   employed by entities that seek to reconnect can have a significant
   impact on software and network performance.  The following guidelines
   are RECOMMENDED:

   o  The time to live (TTL) specified in Domain Name System records
      SHOULD be honored, even if DNS results are cached; if the TTL has
      not expired, an entity that seeks to reconnect SHOULD NOT re-
      resolve DNS the server hostname before reconnecting.
   o  The time that expires before an entity first seeks to reconnect
      SHOULD be randomized (e.g., so that all clients do not attempt to
      reconnect 30 seconds after being disconnected).
   o  If the first reconnection attempt does not succeed, an entity
      SHOULD back off exponentially on the time between subsequent
      reconnection attempts.

5.8.  Stream Errors

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





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5.8.1.  Rules

   The following rules apply to stream-level errors:

   o  It is assumed that all stream-level errors.

5.8.1.1.  Stream Errors Are Unrecoverable

   Stream-level errors are unrecoverable;
      therefore, 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

   C: <message><body></message>

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




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5.8.1.2.  Stream Errors Can Occur During Setup

   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

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://wrong.ns.example.org/'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
      <stream:error>
        <invalid-namespace
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.1.3.  Stream Errors When the Host is Unspecified

   If the initiating entity provides no 'to' attribute or provides an
   unknown host in the 'to' attribute (or provides no 'to' attribute
      at all), and the server error occurs during stream
   setup, the receiving entity SHOULD provide the server's its authoritative hostname
   in the 'from' attribute of the stream header sent before termination.














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   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://wrong.ns.example.org/'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <invalid-namespace
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.2.  Syntax

   The syntax for stream errors is as follows:

   <stream:error>
     <defined-condition follows, where "defined-condition"
   is a placeholder for one of the conditions defined under
   Section 5.8.3.

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

   The <error/> element:

   o  MUST contain a child element corresponding to one of the defined
      stanza
      stream error conditions defined in the text that follows; (Section 5.8.3); this element MUST be
      qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace namespace.
   o  MAY contain a <text/> child element containing XML character data
      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 specifying the natural
      language of the XML character data data.



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   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
      (see Section 5.8.4).

   The <text/> element is OPTIONAL.  If included, it SHOULD be used only



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

5.8.3.  Defined Stream Error Conditions

   The following stream-level error conditions are defined:

   o  <bad-format/> -- the defined.

5.8.3.1.  bad-format

   The entity has sent XML that cannot be processed;
      this processed.

   C: <message><body></message>

   S: <stream:error>
        <bad-format
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

   This error MAY be used instead of the more specific XML-related
   errors, such as <bad-namespace-prefix/>, <invalid-xml/>,
      <restricted-xml/>, <restricted-
   xml/>, <unsupported-encoding/>, and <xml-not-well-
      formed/>, although <xml-not-well-formed/>.  However,
   the more specific errors are preferred.
   o  <bad-namespace-prefix/> -- the

5.8.3.2.  bad-namespace-prefix

   The entity has sent a namespace prefix that is unsupported, or has
   sent no namespace prefix on an element that requires such a prefix
   (see XML Namespace Names and Prefixes
      (Section 12.2)).
   o  <conflict/> -- the Section 12.2).












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   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:foobar='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <bad-namespace-prefix
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.3.  conflict

   The server is either (1) closing the active existing stream for this entity
   because a new stream has been initiated that conflicts with the
   existing stream.
   o  <connection-timeout/> -- stream (2) is refusing a new stream for this entity because
   allowing the new stream would conflict with an existing stream (e.g.,
   because the server allows only a certain number of connections for
   the same IP address).




















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   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <conflict
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.4.  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/> -- policy) and
   therefore the connection is being dropped.

   S: <stream:error>
        <connection-timeout
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.5.  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











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   P: <?xml version='1.0'?>
      <stream:stream
          from='example.net'
          to='foo.example.org'
          version='1.0'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
          to='example.net'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <host-gone
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.6.  host-unknown

   The value of the 'to' attribute provided by the initiating entity in
   the stream header does not correspond to a hostname that is hosted by
   the server.
   o  <improper-addressing/> -- a stanza sent between two servers lacks
      a 'to' or 'from' attribute (or the























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   P: <?xml version='1.0'?>
      <stream:stream
          from='example.net'
          to='example.org'
          version='1.0'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
          to='example.net'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <host-unknown
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.7.  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

   P: <message from='juliet@example.com'>
        <body>Wherefore art thou?</body>
      </message>

   S: <stream:error>
        <improper-addressing
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.8.  internal-server-error

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

   S: <stream:error>
        <internal-server-error
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>



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5.8.3.9.  invalid-from

   The JID or hostname provided in a 'from' address does not match an
   authorized JID or validated domain negotiated between servers via SASL or dialback,
   SASL, or between a client and a server via authentication and
   resource binding.
   o  <invalid-id/> -- the

   P: <message from='romeo@example.org' to='juliet@example.com'>
        <body>Neither, fair saint, if either thee dislike.</body>
      </message>

   S: <stream:error>
        <improper-addressing
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.10.  invalid-id

   The stream ID or server dialback ID is invalid or does not match an
   ID previously provided.
   o  <invalid-namespace/> -- the provided (the following example is from server
   dialback; see [XEP-0220]).

   P: <db:verify
          from='example.net'
          to='example.com'
          id='unknown-id-here'
          type='invalid'/>

   S: <stream:error>
        <invalid-id
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.11.  invalid-namespace

   The streams namespace name is something other than
   "http://etherx.jabber.org/streams" or the dialback
      namespace name is something other than "jabber:server:dialback" (see XML Namespace Names and Prefixes (Section 12.2)). Section 12.2).












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   o  <invalid-xml/> -- the


   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://wrong.ns.example.org/'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <invalid-namespace
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.12.  invalid-xml

   The entity has sent invalid XML over the stream to a server that
   performs validation (see Validation
      (Section 12.3)).
   o  <not-authorized/> -- Section 12.3).  (In the following example, a
   peer attempts to send an IQ stanza of type "subscribe" but there is
   no such value for the 'type' attribute.)

   P: <iq from='example.net'
          id='some-id'
          to='example.com'
          type='subscribe'>
        <ping xmlns='urn:xmpp:ping'/>
      </iq>

   S: <stream:error>
        <invalid-xml
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.13.  not-authorized

   The entity has attempted to send XML stanzas before the stream has
   been authenticated, or otherwise is not authorized to perform an
   action related to stream negotiation; the receiving entity MUST NOT



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   process the offending stanza before sending the stream error.
   o  <policy-violation/> -- the entity has violated some local service
      policy (e.g.,  (In
   the entity is on following example, a provisioned blacklist); the
      server MAY choose client attempts to specify the policy in send XML stanzas before
   authenticating with the <text/> element or
      an server.)

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'

   C: <message to='romeo@example.net'>
        <body>Wherefore art thou?</body>
      </message>

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

5.8.3.14.  policy-violation

   The entity has violated some local service policy (e.g., the stanza
   exceeds a configured size limit); the server MAY choose to specify
   the policy in the <text/> element or an application-specific
   condition element.
   o  <remote-connection-failed/> -- the

   C: <message to='juliet@example.com' id='foo'>
        <body>[ ... the-emacs-manual ... ]</body>
      </message>

   S: <stream:error>
        <policy-violation
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>




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   S: </stream:stream>

5.8.3.15.  remote-connection-failed

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

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <remote-connection-failed
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.16.  resource-constraint

   The server lacks the system resources necessary to service the
   stream.
   o  <restricted-xml/> -- the

















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   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <resource-constraint
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.17.  restricted-xml

   The entity has attempted to send restricted XML features such as a
   comment, processing instruction, DTD, entity reference, or unescaped
   character (see Restrictions
      (Section 12.1)).
   o  <see-other-host/> -- the Section 12.1).

   C: <message to='juliet@example.com'>
        <body><!--just a comment!--></body>
      </message>

   S: <stream:error>
        <restricted-xml
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.18.  see-other-host

   The server will not provide service to the initiating entity but is
   redirecting traffic to another host; the XML character data of the
   <see-other-host/> element returned by the server SHOULD specify the
   alternate hostname or IP address at which to connect, which SHOULD be
   a valid domain identifier but may also include a port number; if no
   port is specified, the initiating entity SHOULD perform a [DNS-SRV]
   lookup on the provided domain identifier but MAY assume that it can



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   connect to that domain identifier at the standard XMPP ports (5222
   for client-to-server connections and 5269 for server-to-server
   connections).
   o  <system-shutdown/> -- the

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <see-other-host
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
          xmpp.example.com:9090
        </see-other-host>
      </stream:error>
      </stream:stream>

5.8.3.19.  system-shutdown

   The server is being shut down and all active streams are being
   closed.
   o  <undefined-condition/> -- the

   S: <stream:error>
        <system-shutdown
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.20.  undefined-condition

   The error condition is not one of those defined by the other
   conditions in this list; this error condition SHOULD be used only in
   conjunction with an application-specific condition.
   o  <unsupported-encoding/> -- the






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   S: <stream:error>
        <undefined-condition
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
        <app-error xmlns='some-application-ns'/>
      </stream:error>
      </stream:stream>

5.8.3.21.  unsupported-encoding

   The initiating entity has encoded the stream in an encoding that is
   not supported by the server (see
      Character Encoding (Section 12.5)).
   o  <unsupported-stanza-type/> -- the Section 12.5).

   C: <?xml version='1.0' encoding='UTF-16'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
      <stream:error>
        <unsupported-encoding
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.22.  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/> --












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   C: <pubsub>
        <publish node='princely_musings'>
          <item id='ae890ac52d0df67ed7cfdf51b644e901'>
            <entry xmlns='http://www.w3.org/2005/Atom'>
              <title>Soliloquy</title>
              <summary>
   To be, or not to be: that is the question:
   Whether 'tis nobler in the mind to suffer
   The slings and arrows of outrageous fortune,
   Or to take arms against a sea of troubles,
   And by opposing end them?
              </summary>
              <link rel='alternate' type='text/html'
                    href='http://denmark.lit/2003/12/13/atom03'/>
              <id>tag:denmark.lit,2003:entry-32397</id>
              <published>2003-12-13T18:30:02Z</published>
              <updated>2003-12-13T18:30:02Z</updated>
            </entry>
          </item>
        </publish>
      </pubsub>

   S: <stream:error>
        <unsupported-stanza-type
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.23.  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.

















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   o


   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='11.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
      <stream:error>
        <unsupported-version
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.8.3.24.  xml-not-well-formed

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

   C: <message><body></message>

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

5.8.4.  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:








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   C: <message>
        <body>
          My keyboard layout is:

          QWERTYUIOP{}|
          ASDFGHJKL:"
          ZXCVBNM<>?
        </body>
      </message>

   S: <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>

5.9.  Simplified Stream Examples

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
























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   A basic connection:

   C: <?xml version='1.0'?>
      <stream:stream
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          xml:lang='en'
          from='juliet@example.com'
          to='example.com'
          version='1.0'>
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'

   S: <?xml version='1.0'?>
      <stream:stream
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          xml:lang='en'
          from='example.com'
          id='someid'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'>
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   [ ...  encryption, authentication, and resource binding ... ]

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

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

   C: </stream:stream>

   S: </stream:stream>













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   A connection gone bad:

   C: <?xml version='1.0'?>
      <stream:stream
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          xml:lang='en'
          from='juliet@example.com'
          to='example.com'
          version='1.0'>
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          xml:lang='en'
          from='example.com'
          id='someid'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@example.com'
          version='1.0'>
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   [ ...  encryption, authentication, and resource binding ... ]

   C:   <message from='juliet@example.com/balcony'
                 to='romeo@example.net'
                 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>
   S:
      </stream:stream>


   More detailed examples are provided under Section 10.


6.  TLS  STARTTLS Negotiation

6.1.  Overview

   XMPP includes a method for securing the stream from tampering and
   eavesdropping.  This channel encryption method makes use of the
   Transport Layer Security (TLS) protocol [TLS], along with (see [TLS]), specifically a
   "STARTTLS" extension that is modelled after similar extensions for
   the [IMAP], [POP3], and [ACAP] protocols as described in [USINGTLS].



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   The XML namespace name for the STARTTLS extension is
   'urn:ietf:params:xml:ns:xmpp-tls'.

   Support for STARTTLS is REQUIRED in XMPP client and server
   implementations.  An administrator of a given domain MAY deployment may require
   the use of TLS for client-to-server communications, communication, server-to-server communications,
   communication, or both.  Clients SHOULD  A deployed client should use TLS to secure the streams
   its stream with a server prior to attempting the completion of SASL
   negotiation (Section 7), and



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   two domains for the purpose of securing server-to-server communications.
   communication.

6.2.  Rules

6.2.1.  Data Formatting

   The following rules apply:

   1.   An initiating entity that complies with this specification entities MUST
        include the 'version' attribute set to a value NOT send any white space characters (matching
   production [3] content of "1.0" in [XML]) within the
        initial root stream header.
   2.   If the TLS negotiation occurs element as
   separators between two servers,
        communications MUST NOT proceed until the Domain Name System
        (DNS) hostnames asserted by elements (any white space characters shown in the servers have been resolved (see
        Server-to-Server Communications (Section 15.4)).
   3.   When a receiving entity that complies with
   STARTTLS examples provided in this specification
        receives an initial stream header that includes document are included for the 'version'
        attribute set to a value sake
   of at least "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 readability only); this prohibition helps to ensure proper
   security layer byte precision.

6.2.2.  Order of other stream features it supports.
   4. Negotiation

   If the initiating entity chooses to use TLS, TLS STARTTLS negotiation
   MUST be completed before proceeding to SASL negotiation; negotiation (Section 7);
   this order of negotiation is required to help safeguard
   authentication information sent during SASL negotiation, as well as
   to make it possible to base the use of the SASL EXTERNAL mechanism on
   a certificate (or other credentials) provided during prior TLS
   negotiation.
   5.   During TLS negotiation, an

6.3.  Process

6.3.1.  Exchange of Stream Headers and Stream Features

   The initiating entity MUST NOT send any white space
        characters (matching production [3] content resolves the hostname of [XML]) within the
        root stream element receiving entity
   as separators between elements (any white
        space characters shown in specified under Section 4, opens a TCP connection to the TLS examples that follow are
        included for
   advertised port at the sake of readability only); this prohibition
        helps resolved IP address, and sends an initial
   stream header to ensure proper security layer byte precision.
   6.   The the receiving entity; if the initiating entity is
   capable of STARTTLS negotiation, it MUST consider include the TLS negotiation 'version'
   attribute set to have
        begun immediately after sending the closing ">" character a value of at least "1.0" in the
        <proceed/> element to the initiating entity. initial stream
   header.







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   I: <stream:stream
        from='juliet@example.com'
        to='example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   The initiating receiving entity MUST consider send a response stream header to the TLS negotiation to have begun
        immediately after receiving
   initiating entity over the closing ">" character of TCP connection opened by the
        <proceed/> element from initiating
   entity (for client-to-server streams) or over a new TCP connection
   (for server-to-server streams); if the receiving entity.
   7.   The initiating entity is capable of
   STARTTLS negotiation, it MUST validate include the certificate presented by 'version' attribute set to
   a value of at least "1.0" in the response stream header.

   R: <stream:stream
        from='example.com'
        id='t7AMCin9zjMNwQKDnplntZPIDEI='
        to='juliet@example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   The receiving entity; see Certificate Validation (Section 15.2)
        regarding certificate validation procedures.
   8.   Certificates entity then MUST be checked against the hostname as provided by send stream features to the initiating entity (e.g.,
   entity, which SHOULD include a user), not <starttls/> element (qualified by the hostname as
        resolved via
   'urn:ietf:params:xml:ns:xmpp-tls' namespace) to indicate that the Domain Name System; e.g., if
   receiving entity supports STARTTLS negotiation.

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

   If the user specifies
        a hostname of "example.net" but a [DNS-SRV] lookup returned
        "im.example.net", receiving entity requires the certificate MUST be checked use of STARTTLS, it SHOULD
   include an empty <required/> element as
        "example.net".  If a JID for an XMPP client (e.g., an end user
        account) is represented in child of the <starttls/>
   element.

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

6.3.2.  Initiation of STARTTLS Negotiation

   In order to begin the STARTTLS negotiation, the initiating entity
   issues the STARTTLS command (i.e., a certificate, it MUST be represented <starttls/> element qualified by
   the 'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct the



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        as a UTF8String within an otherName


   receiving entity inside the
        subjectAltName, using the [ASN.1] Object Identifier "id-on-
        xmppAddr" specified in Section 6.1.1 of this document.  If that it wishes to begin a JID
        for an XMPP server is represented in STARTTLS negotiation to
   secure the stream.

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

   The receiving entity MUST reply with either a certificate, it SHOULD be
        represented as <proceed/> element
   (proceed case) or a UTF8String within an otherName entity inside <failure/> element (failure case) qualified by
   the subjectAltName, using 'urn:ietf:params:xml:ns:xmpp-tls' namespace.

   If the [ASN.1] Object Identifier "id-on-
        xmppAddr" specified in Section 6.1.1 of this document; however, failure case occurs, the JID for an XMPP server MAY also or instead be represented as receiving entity MUST return a subjectAltName extension of type dNSName, where the dNSName
        may contain
   <failure/> element qualified by the wildcard character '*', which applies only to 'urn:ietf:params:xml:ns:xmpp-tls'
   namespace, terminate the left-most domain name component or component fragment XML stream, and is
        considered to match any single component or component fragment
        (e.g., *.example.com matches foo.example.com terminate the underlying TCP
   connection.  Causes for the failure case include but are not
        bar.foo.example.com, and im*.example.net matches im1.example.net
        and im2.example.net but limited
   to:

   1.  The initiating entity has sent a malformed STARTTLS command.
   2.  The receiving entity does not chat.example.net).
   9. offer STARTTLS negotiation either
       temporarily or permanently.
   3.  The receiving entity cannot complete STARTTLS negotiation because
       of an internal error.

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

   R: </stream:stream>

   If the TLS negotiation is successful, proceed case occurs, the initiating receiving entity MUST
        send return a new stream header to
   <proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
   namespace.

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

   Note: The receiving entity.
   10.  If entity MUST consider the TLS negotiation is successful, to have
   begun immediately after sending the receiving entity MUST
        discard any knowledge obtained in an insecure manner from closing '>' character of the
   <proceed/> element to the initiating entity.  The initiating entity before TLS takes effect.
   11.  If
   MUST consider the TLS negotiation is successful, to have begun immediately after
   receiving the initiating entity MUST
        discard any knowledge obtained in an insecure manner closing '>' character of the <proceed/> element from
   the receiving entity before entity.

6.3.3.  TLS takes effect.
   12.  If the Negotiation

   The entities MUST now attempt to complete TLS negotiation is successful, over the receiving entity
   TCP connection by following the process defined in [TLS].

   Note: The entities MUST NOT offer send any further XML data until the STARTTLS extension to TLS
   negotiation has either failed or succeeded.

   If the initiating receiving entity along
        with presents a certificate during TLS
   negotiation, the other stream features that are offered after initiating entity MUST validate the new
        stream header is received and responded to.
   13.  If certificate in



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   order to determine if the TLS negotiation is successful, shall succeed (see
   Section 15.2 regarding certificate validation procedures).
   Specifically, the certificate MUST be checked against the hostname as
   provided by the initiating entity (e.g., a user), not the hostname as
   resolved via the Domain Name System; e.g., if the user specifies a
   hostname of "example.net" but a [DNS-SRV] lookup returns
   "xmpp.example.net", the certificate MUST
        continue with SASL negotiation.
   14. be checked as "example.net".
   See Section 6.4 for information about the representation of XMPP
   addresses in certificates.

   Note: See Section 15.7 regarding ciphers that MUST be supported for
   TLS; naturally, other ciphers MAY be supported as well.

6.3.4.  Failure

   If the TLS negotiation results in failure, the receiving entity MUST
   terminate both the XML stream and the underlying TCP connection.
   15.  See Mandatory-to-Implement Technologies (Section 15.7) regarding
        mechanisms that

   Note: It is not necessary to send a closing </stream> tag before
   terminating the TCP connection, since the receiving entity and
   initiating entity MUST consider the original stream to be supported.

6.1.1.  ASN.1 Object Identifier for XMPP Address

   The [ASN.1] Object Identifier "id-on-xmppAddr" described above is
   defined as follows:

   id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
           dod(6) internet(1) security(5) mechanisms(5) pkix(7) }

   id-on  OBJECT IDENTIFIER ::= { id-pkix 8 }  -- other name forms

   id-on-xmppAddr  OBJECT IDENTIFIER ::= { id-on 5 }

   XmppAddr ::= UTF8String



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   This Object Identifier MAY also be represented in dotted display
   format (i.e., "1.3.6.1.5.5.7.8.5") or in closed upon
   failure of the Uniform Resource Name
   notation specified in [URN-OID] (i.e., "urn:oid:1.3.6.1.5.5.7.8.5").

   Thus for example TLS negotiation.

6.3.5.  Success

   If the JID "example.com" TLS negotiation is successful, then the entities MUST proceed
   as included follows.

   The receiving entity MUST discard any knowledge obtained in a certificate
   might be formatted as "subjectAltName=otherName:
   1.3.6.1.5.5.7.8.5;UTF8:example.com".

6.2.  Narrative

   When an
   insecure manner from the initiating entity secures a stream with a receiving before TLS took effect.

   The initiating entity
   using TLS, MUST discard any knowledge obtained in an
   insecure manner from the steps involved are as follows:

   1. receiving entity before TLS took effect.

   The initiating entity opens MUST send a TCP connection and initiates the
       stream by sending the opening XML new stream header to the receiving
       entity, including
   entity over the 'version' attribute set secured TCP connection.

   I: <stream:stream
        from='juliet@example.com'
        to='example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   Note: It is not necessary to send a value of at
       least "1.0".
   2.  The receiving entity responds by opening a TCP connection and closing </stream> tag before
   sending an XML the initial stream header to header, since the receiving entity and
   initiating entity, including entity MUST consider the 'version' attribute set original stream to a value be closed upon



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   success of at least "1.0".
   3. the TLS negotiation.

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

   R: <stream:stream
        from='example.com'
        id='vgKi/bkYME8OAj4rlXMkpucAqe4='
        to='juliet@example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   The receiving entity SHOULD also
       include a list of supported SASL mechanisms in the MUST send stream
       features.
   4.  The features to the initiating entity issues
   entity, which MUST NOT include the STARTTLS command (i.e., a
       <starttls/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct feature but which SHOULD
   include the
       receiving entity that it wishes to begin a SASL stream feature as described under Section 7.

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

6.4.  Representation of JIDs in Certificates

   TLS negotiation to
       secure the stream.
   5.  The receiving entity MUST reply with either a <proceed/> element
       or is commonly based on a <failure/> element qualified digital certificate presented
   by the
       'urn:ietf:params:xml:ns:xmpp-tls' namespace.  If receiving entity (or in the failure case
       occurs, of mutual authentication both
   the receiving entity MUST terminate both the XML stream and the underlying TCP connection (failure cases include when the initiating entity sends entity).

   If a malformed STARTTLS command, when the
       receiving entity does not offer TLS negotiation either
       temporarily or permanently, and when the receiving entity cannot
       complete TLS negotiation because of JID for an internal error).  If the
       proceed case occurs, the entities XMPP user account is represented in a certificate, it
   MUST attempt to complete the
       TLS negotiation over be represented as a UTF8String within an otherName entity inside
   the TCP connection and MUST NOT send any
       further XML data until subjectAltName, using the TLS negotiation is complete.
   6.  The initiating entity and receiving entity attempt to complete a
       TLS negotiation [ASN.1] Object Identifier "id-on-
   xmppAddr" specified in accordance with [TLS].




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   7.  If the TLS negotiation is unsuccessful, the receiving entity MUST
       terminate the TCP connection. this section.

   If the TLS negotiation is
       successful, the initiating entity MUST initiate a new stream by
       sending JID for an opening XML stream header to the receiving entity (it XMPP server is not necessary to send represented in a closing </stream> tag first, since the
       receiving entity and initiating entity MUST consider the original
       stream to certificate, it
   SHOULD be closed upon successful TLS negotiation).
   8.  Upon receiving the new stream header from the initiating entity,
       the receiving entity MUST respond by sending represented as a new XML stream
       header to the initiating UTF8String within an otherName entity along with the available features
       (but not including
   inside the STARTTLS feature) and SHOULD include an
       updated list of SASL mechanisms so that subjectAltName, using the initiating entity can
       detect any changes [ASN.1] Object Identifier "id-
   on-xmppAddr" specified in this section.  However, in addition to the list or
   instead of SASL mechanisms supported by the receiving entity.

   Examples of TLS negotiation are provided under Section 10.


7.  SASL Negotiation

7.1.  Overview

   XMPP includes a method for authenticating "id-on-xmppAddr" representation, it MAY be represented
   as a stream by means of an
   XMPP-specific profile subjectAltName extension of type dNSName; this dNSName MAY
   contain the Simple Authentication and Security Layer
   protocol (see [SASL]).  SASL provides a generalized method for adding
   authentication support wildcard character '*', which applies only to connection-based protocols, the left-
   most domain name component or component fragment and XMPP uses is considered to
   match any single component or component fragment (e.g., *.example.com
   matches foo.example.com but not bar.foo.example.com, and
   im*.example.net matches im1.example.net and im2.example.net but not



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   chat.example.net).

   The [ASN.1] Object Identifier "id-on-xmppAddr" is defined as follows.

   id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
           dod(6) internet(1) security(5) mechanisms(5) pkix(7) }

   id-on  OBJECT IDENTIFIER ::= { id-pkix 8 }  -- other name forms

   id-on-xmppAddr  OBJECT IDENTIFIER ::= { id-on 5 }

   XmppAddr ::= UTF8String

   This Object Identifier MAY be represented in dotted display format
   (i.e., "1.3.6.1.5.5.7.8.5") or in the Uniform Resource Name notation
   specified in [URN-OID] (i.e., "urn:oid:1.3.6.1.5.5.7.8.5").

   Thus for example the JID "juliet@example.com" as included in a
   certificate could be formatted in any of the following three ways:

   o  subjectAltName=otherName:id-on-xmppAddr;UTF8:juliet@example.com
   o  subjectAltName=otherName:1.3.6.1.5.5.7.8.5;UTF8:juliet@example.com
   o  subjectAltName=otherName:urn:oid:1.3.6.1.5.5.7.8.5;UTF8:juliet@
      example.com


7.  SASL Negotiation

7.1.  Overview

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

   Support for SASL is REQUIRED in XMPP client and server
   implementations.

7.2.  Rules

7.2.1.  Data Formatting

   The following formatting rules apply:

   1.   If apply to the data sent during SASL negotiation occurs between two servers,
        communications
   negotiation:





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   1.  The entities MUST NOT proceed until send any white space characters (matching
       production [3] content of [XML]) within the Domain Name System
        (DNS) hostnames asserted by the servers have been resolved (see
        Server-to-Server Communications (Section 15.4)).
   2.   If the initiating entity is capable of SASL negotiation, it MUST
        include the 'version' attribute set to a value of at least "1.0"
        in the initial stream header.
   3.   If the receiving entity is capable of SASL negotiation, it MUST
        advertise one or more authentication mechanisms within a
        <mechanisms/> element qualified by the
        'urn:ietf:params:xml:ns:xmpp-sasl' namespace in reply to the
        opening stream tag received from the initiating entity (if the
        opening stream tag included the 'version' attribute set to a
        value of at least "1.0").
   4.   During SASL negotiation, an entity MUST NOT send any white space
        characters (matching production [3] content of [XML]) within the
        root stream element as separators between elements (any white



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        space characters shown in root stream element
       as separators between elements (any white space characters shown
       in the SASL examples that follow provided in this document are included for
       the sake of readability only); this prohibition helps to ensure
       proper security layer byte precision.
   5.
   2.  Any XML character data contained within the XML elements used
        during SASL negotiation MUST be
       encoded using base64, where the encoding adheres to the
       definition in Section 4 of RFC 3548
        [BASE64].
   6.   If [BASE64] and where the receiving entity does not include padding bits
       are set to zero.

7.2.2.  Security Layers

   Upon successful SASL negotiation that involves negotiation of a 'realm' value,
   security layer, the initiating entity must default it to the domain identifier
        portion of MUST discard any knowledge
   obtained from the receiving entity's JID.
   7.   If provision of a "simple username" is supported by entity which was not obtained from the selected
   SASL mechanism (e.g., this is supported by the DIGEST-MD5 and
        CRAM-MD5 mechanisms but not by negotiation itself.

   Upon successful SASL negotiation that involves negotiation of a
   security layer, the EXTERNAL and GSSAPI
        mechanisms), during authentication receiving entity MUST discard any knowledge
   obtained from the initiating entity SHOULD
        provide as the simple username its which was not obtained from the
   SASL negotiation itself.  The receiving entity SHOULD also include an
   updated list of SASL mechanisms with the stream features so that the
   initiating entity is able to detect any changes to the list of
   mechanisms supported by the receiving entity.

7.2.3.  Simple Usernames

   If provision of a "simple username" is supported by the selected SASL
   mechanism (e.g., this is supported by the DIGEST-MD5 and CRAM-MD5
   mechanisms but not by the EXTERNAL and GSSAPI mechanisms), during
   authentication the initiating entity SHOULD provide as the simple
   username its sending domain (IP address or fully qualified domain
   name as contained in a an XMPP domain identifier) in the case of
   server-to-server communications communication or its registered account name (user
   or node name as contained in an XMPP node identifier) in the case of
   client-to-server communications. communication.  In either case, the initiating
   entity MUST ensure that the username adheres to the [NAMEPREP] or
   Nodeprep (Appendix A) profile of [STRINGPREP] (as appropriate) before
   sending it to the receiving entity.  (Note: Account provisioning is out of scope for this
        specification; possible methods for account provisioning include
        account creation by a server administrator and in-band account
        registration using the 'jabber:iq:register' namespace as
        documented in [XEP-0077].)
   8.

7.2.4.  Authorization Identities

   If the initiating entity wishes to act on behalf of another entity
   and the selected SASL mechanism supports transmission of an
   authorization identity, the initiating entity MUST provide an
   authorization identity during SASL negotiation.  If the initiating
   entity does not wish to act on behalf of another entity, it MUST NOT



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   provide an authorization identity.  As specified in [SASL], the
   initiating entity MUST NOT provide an authorization identity unless
   the authorization identity is different from the default
   authorization identity derived from the authentication identity.  If
   provided, the value of the authorization identity MUST be of the form
   <domain> (i.e., a an XMPP domain identifier only) for servers and of
   the form <node@domain> (i.e., node identifier and domain identifier)
   for clients.
   9.

7.3.  Process

   The process for SASL negotiation is as follows.

7.3.1.  Exchange of Stream Headers and Stream Features

   If SASL negotiation follows successful STARTTLS negotation
   (Section 6), then the SASL negotiation is successful, occurs over the existing
   stream.  If not, the initiating entity
        MUST send resolves the hostname of the
   receiving entity as specified under Section 4, opens a new TCP connection
   to the advertised port at the resolved IP address, and sends an
   initial stream header to the receiving entity.
   10.  Upon successful SASL negotiation that involves negotiation entity; if the initiating
   entity is capable of STARTTLS negotiation, it MUST include the
   'version' attribute set to a
        security layer, value of at least "1.0" in the initial
   stream header.

   I: <stream:stream
        from='juliet@example.com'
        to='example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   The receiving entity MUST discard any knowledge
        obtained from the initiating entity which was not obtained from send a response stream header to the
   initiating entity; if the receiving entity is capable of SASL negotiation itself;
   negotiation, it MUST include the 'version' attribute set to a value
   of at least "1.0" in the response stream header.

   R: <stream:stream
        from='example.com'
        id='vgKi/bkYME8OAj4rlXMkpucAqe4='
        to='juliet@example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   The receiving entity SHOULD also MUST send new stream features (including an updated list of SASL to the initiating



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        mechanisms) so


   entity, which SHOULD include the SASL stream feature, i.e., a
   <mechanisms/> element (qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace) that contains one
   <mechanism/> child element for each authentication mechanism the initiating
   receiving entity can detect any changes offers to the list initiating entity.  The order of
   <mechanism/> elements in the XML indicates the preference order of
   the SASL mechanisms supported by according to the receiving entity.
   11.  Upon successful SASL negotiation that involves negotiation of a
        security layer,

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

   Note: If the initiating entity MUST discard any knowledge
        obtained from presents a valid certificate during
   prior TLS negotiation, the receiving entity which was not obtained from SHOULD offer the SASL
   EXTERNAL mechanism to the initiating entity during SASL negotiation itself.
   12.  See Mandatory-to-Implement Technologies
   (refer to [SASL]), although the EXTERNAL mechanism MAY be offered
   under other circumstances as well.

   Note: If TLS negotiation (Section 15.7) 6) needs to be completed before a
   particular authentication mechanism may be used, the receiving entity
   MUST NOT provide that mechanism in the list of available SASL
   authentication mechanisms prior to TLS negotiation.

   Note: See Section 15.7 regarding mechanisms that MUST be supported;
   naturally, other SASL mechanisms MAY be supported as well (best
   practices for the use of several SASL mechanisms in the context of
   XMPP are described in [XEP-0175] and [XEP-0178]).

7.2.  Narrative

   When an initiating entity authenticates

   If successful SASL negotiation is required for interaction with a receiving entity using
   SASL, the steps involved are as follows:

   1.  The initiating entity requests SASL authentication by including
       the 'version' attribute in the opening XML stream header sent to the
   receiving entity, with the value set to "1.0".
   2.  After sending an XML stream header in reply, the receiving entity
       advertises a list of available SASL authentication mechanisms as
       stream features; each of these is it SHOULD signal that fact by including a <mechanism/>
   <required/> element included as a child within a of the <mechanisms/> container element.

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

   Note: The receiving entity MAY include an application-specific child
   element qualified by inside the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace, which in turn
       is a child of a <features/> <mechanisms/> element in the streams namespace.  If
       TLS negotiation (Section 6) needs order to be completed before a
       particular authentication mechanism may be used, the receiving
       entity MUST NOT provide
   information that mechanism in the list of available
       SASL authentication mechanisms prior to TLS negotiation.  If may be needed by the initiating entity presents a valid certificate during prior TLS
       negotiation, the receiving entity SHOULD offer the SASL EXTERNAL
       mechanism in order to the initiating entity during complete



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   successful SASL negotiation (refer
       to [SASL]), although using one or more of the EXTERNAL mechanism MAY be offered under
       other circumstances as well.  If successful SASL negotiation is
       required for interaction with
   mechanisms; however, the receiving entity, it SHOULD
       signal that fact by including a <required/> syntax and semantics of any such element as a child are
   out of scope for this specification.

7.3.2.  Initiation

   In order to begin the SASL negotiation, the <mechanisms/> element.
   3.  The initiating entity selects a mechanism by sending sends
   an <auth/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the receiving entity and including includes an
   appropriate value for the 'mechanism' attribute.  This element MAY
   contain XML character data (in SASL terminology, the "initial
   response") if the mechanism supports or requires it; if the
   initiating entity needs to send a zero-length initial response, it
   MUST transmit the response as a single equals sign ("="), which
   indicates that the response is present but contains no data.




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   4.

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

7.3.3.  Challenge-Response Sequence

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

   R: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
      cmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLHFvcD0i
      YXV0aCIsY2hhcnNldD11dGYtOCxhbGdvcml0aG09bWQ1LXNlc3MK
      </challenge>

   The decoded challenge is:

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

   Note: If the receiving entity does not specify a 'realm' value, the
   initiating entity MUST default it to the domain identifier portion of
   the receiving entity's JID.

   The initiating entity responds to the challenge by sending to the
       receiving entity a
   <response/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
   contain XML character data (which MUST be computed generated in accordance
   with the definition of the SASL mechanism chosen by the initiating
   entity).
   6.



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   I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
      dXNlcm5hbWU9Imp1bGlldCIscmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2
      TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAwMDAwMDAx
      LHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20iLHJlc3BvbnNl
      PWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNoYXJzZXQ9dXRmLTgK
      </response>

   The decoded response is:

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

   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.

   o  The initiating entity aborts the handshake.
   o  The receiving entity reports failure of the handshake.
   o  The receiving entity reports success of the handshake.

   These scenarios are described in the following sections.

7.3.4.  Abort

   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.
   namespace.

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

   Upon receiving an <abort/> element, the receiving entity MUST return
   an <aborted/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace.

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

   The receiving entity SHOULD allow a configurable but reasonable
   number of retries (at least 2 and no more than 5),
       after which it MUST terminate the TCP connection; 5); this enables the
   initiating entity (e.g., an end-user client) to tolerate
       incorrectly-provided incorrectly-
   provided credentials (e.g., a mistyped password) without being forced
   to reconnect.
   2.

   If the initiating entity exceeds the number of retries, the receiving
   entity MUST terminate the TCP connection.



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7.3.5.  Failure

   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 7.4)). Section 7.5).

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <not-authorized/>
      </failure>

   If the failure case occurs, the receiving entity SHOULD allow a
   configurable but reasonable number of retries (at least 2), after which it MUST terminate the
       TCP connection; 2 and no more
   than 5); this enables the initiating entity (e.g., an end-
       user end-user
   client) to tolerate incorrectly-provided credentials (e.g., a
   mistyped password) without being forced to reconnect.
   3.

   If the initiating entity exceeds the number of retries, the receiving
   entity MUST terminate the TCP connection.

7.3.6.  Success

   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; namespace; this element MAY
   contain XML character data (in SASL terminology, "additional data
   with success") if required by the chosen SASL mechanism; if the
   receiving entity needs to send additional data of zero length, it
   MUST transmit the data as a single equals sign ("=").

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

   The decoded value for subsequent authentication is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd

   Upon receiving the <success/> element, the initiating entity MUST
   initiate a new stream over the existing TCP connection by sending an
       opening XML
   initial stream header to the receiving entity (it is not entity.









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   I: <stream:stream
        from='juliet@example.com'
        to='example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   Note: It is not necessary to send a closing </stream> tag first, before
   sending the initial stream header, since the receiving entity and
   initiating entity MUST consider the original stream to be closed upon
   sending or receiving the <success/>
       element). element.

   Upon receiving the new initial stream header from the initiating entity,
   the receiving entity MUST respond by sending a new XML stream header
   to the initiating entity, along with entity.

   R: <stream:stream
        from='example.com'
        id='gPybzaOzBmaADgxKXu9UClbprp0='
        to='juliet@example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   The receiving entity MUST also send stream features, containing any
   further available features or containing no features (via an empty
   <features/> element (to signify
       that no additional features are available); element); any such additional features not defined herein
   MUST be defined by the relevant extension to XMPP.  As noted, if SASL negotiation involved
       establishment of a security layer, the receiving entity SHOULD
       send an updated list of SASL mechanisms so that the initiating
       entity can detect any changes to the list of mechanisms supported
       by the receiving entity.

7.3.

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

7.4.  SASL Definition

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

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



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      'mechanism' attribute possessed by an <auth/> element, optionally
      including an initial response to avoid a round trip.
   exchange sequence:  Challenges and responses are carried through the
      exchange of <challenge/> elements from receiving entity to
      initiating entity and <response/> elements from initiating entity
      to receiving entity.  The receiving entity reports failure by
      sending a <failure/> element and success by sending a <success/>
      element; the initiating entity aborts the exchange by sending an
      <abort/> element.  Upon successful negotiation, both sides
      consider the original XML stream to be closed and new stream
      headers are sent by both entities.
   security layer negotiation:  The security layer takes effect
      immediately after sending the closing ">" '>' character of the
      <success/> element for the receiving entity, and immediately after
      receiving the closing ">" '>' character of the <success/> element for
      the initiating entity.  The order of layers is first [TCP], then
      [TLS], then [SASL], then XMPP.
   use of the authorization identity:  The authorization identity may be
      used by xmpp XMPP to denote the non-default <node@domain> of a client
      or the sending <domain> of a server; an empty string is equivalent
      to an absent authorization identity.




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7.4.

7.5.  SASL Errors

   The following SASL-related error conditions are defined:

   o  <aborted/> -- defined.

7.5.1.  aborted

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

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

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

7.5.2.  incorrect-encoding

   The data provided by the initiating entity could not be processed
   because the [BASE64] encoding is incorrect (e.g., because the
   encoding does not adhere to the definition in Section 4 of [BASE64]);
   sent in reply to a <response/> element or an <auth/> element with
   initial response data.
   o  <invalid-authzid/> --







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

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

7.5.3.  invalid-authzid

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

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

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

7.5.4.  invalid-mechanism

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

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

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

7.5.5.  malformed-request

   The challenge or response request is malformed (e.g., the <auth/> element includes an
   initial response but the mechanism does not allow that); sent in
   reply to an <abort/>, <auth/>, <challenge/>, or <response/> element.
   o  <mechanism-too-weak/> --

   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='ANONYMOUS'>[ ... ]</auth>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <malformed-request/>
      </failure>




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7.5.6.  mechanism-too-weak

   The mechanism requested by the initiating entity is weaker than
   server policy permits for that initiating entity; sent in reply to a <response/> element or an
   <auth/> element with (with or without initial response data.
   o  <not-authorized/> -- data) or a
   <response/> element.

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

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

7.5.7.  not-authorized

   The authentication failed because the initiating entity did not
   provide proper credentials (this
      includes but is not limited to the case of an unknown username,
      and no differentiation is made between an unknown username and
      incorrect credentials); credentials; sent in reply to a <response/> element or
   an <auth/> element with initial response data.
   o  <temporary-auth-failure/> -- The authentication failed because of

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

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <not-authorized/>
      </failure>

   Note: This error condition includes but is not limited to the case of
   incorrect credentials or an unknown username.  In order to discourage
   directory harvest attacks, no differentiation is made between
   incorrect credentials and an unknown username.

7.5.8.  temporary-auth-failure

   The authentication failed because of a temporary error condition
   within the receiving entity, and the initiating entity should try
   again later; sent in reply to an <auth/> element or <response/>
   element.

   Examples of SASL negotiation are provided under Section 10.

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

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




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8.  Resource Binding

8.1.  Overview

   After a client authenticates with a server, it MUST bind a specific
   resource to the stream so that the server can properly address the
   client (see addresses (Section 3)) and route XML stanzas to and from



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   the client (see stanza delivery rules (Section 11)).  That is, Section 3), i.e., there MUST be a an XMPP resource
   identifier associated with the "bare JID" bare JID (<node@domain>) of the client; this ensures client
   with the result that the address for use over that stream is a "full JID" full
   JID of the form <node@domain/resource>.  This ensures that the server
   can deliver XML stanzas to and receive XML stanzas from the client
   (see Section 11).  After binding a resource to the stream, the client
   is referred to as a CONNECTED RESOURCE. connected resource.

   If, before completing the resource binding step, the client attempts
   to send an outbound XML stanza (i.e., a stanza not directed to the
   server itself or to the client's own account), the server MUST NOT
   process the stanza and SHOULD return a <not-authorized/> stream error
   to the client.

   Support for resource binding is REQUIRED in XMPP client and server
   implementations.

8.2.  Advertising Support

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

   Server advertises to explicitly indicate that resource
   binding feature to client:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       xml:lang='en' must be completed at this stage of the stream negotiation
   process.  (Note: The server SHOULD NOT include the <bind/> stream
   feature until after successful SASL negotiation.)















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   S: <stream:stream
          from='example.com'
       id='c2s_345'
          id='gPybzaOzBmaADgxKXu9UClbprp0='
          to='juliet@example.com'
       version='1.0'>
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

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

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

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








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   Client asks server to bind a resource:

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

8.3.  Server-Generated Resource Identifier

   A server that supports resource binding MUST be able to generate a an
   XMPP resource identifier on behalf of a client.  A  The resource
   identifier generated by the server MUST at a minimum be currently unique among
   the connected resources for that
   <node@domain>.

   If <node@domain> and SHOULD be random
   since the resource identifier may be security-critical.  It is
   RECOMMENDED that the server-generated resource identifier be a
   Universally Unique Identifier (UUID), for which the format specified
   in [UUID] is RECOMMENDED.

   It is RECOMMENDED for the client wishes to specify ask its server to generate an
   appropriate resource identifier on its behalf, rather than generating
   a resource on its own and requesting that the server accept the
   client-generated resource identifier, it MUST send identifer.

8.3.1.  Success Case

   A client requests a server-generated resource identifier by sending
   an IQ stanza of type "set" that contains the desired resource
   identifier as the non-zero-length XML character data of a <resource/> (see Section 9.2.3) containing an empty
   <bind/> element that is a child of qualified by the <bind/> element.

   Client binds a resource: 'urn:ietf:params:xml:ns:xmpp-bind'
   namespace.

   C: <iq type='set' id='bind_2'> id='bind_1' type='set'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>balcony</resource>
     </bind> xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
      </iq>

   Once the server has generated a an XMPP resource identifier for the client or
   accepted the resource identifier provided by the
   client, it MUST return an IQ stanza of type "result" to the client,



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   which MUST include a <jid/> child element that specifies the full JID
   for the connected resource as determined by the server.

   Server informs client of successful resource binding:

   S: <iq type='result' id='bind_2'> id='bind_1' type='result'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <jid>juliet@example.com/balcony</jid>
         <jid>
           juliet@example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb
         </jid>
       </bind>
      </iq>

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

   When a client supplies a resource identifier, the following stanza
   error conditions are

8.3.2.  Error Case

   It is possible (see Stanza Errors (Section 9.3)):




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   o  The provided resource identifier cannot be processed by that the
      server, e.g. because it is not in accordance with Resourceprep
      (Appendix B).
   o  The client is not allowed to bind a resource to
   the stream (e.g., because the node or user has reached a limit on the
   number of connected resources allowed).
   o  The provided resource identifier is already in use but  In this case, the server
      does not allow binding of multiple connected resources with the
      same identifier.

   The protocol for these
   MUST return a <not-allowed/> stanza error conditions is as follows.

   Resource identifier cannot be processed:

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

   Client is not allowed to bind a resource: the client.

   S: <iq type='error' id='bind_2'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>someresource</resource>
     </bind> id='bind_1' type='error'>
        <error type='cancel'>
          <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

   If there is already a connected resource of the same name, the server
   MUST do one of the following:

   1.  Not accept

8.4.  Client-Generated Resource Identifier

   A client MAY attempt to specify the resource identifier provided by on its own
   rather than asking the client but
       instead override it with server to generate a resource identifier that the server
       generates.
   2.  Terminate the current resource and allow the newly-requested
       resource.
   3.  Disallow the newly-requested on
   its behalf.

8.4.1.  Success Case

   A client asks its server to accept a client-generated resource and maintain the current
       resource.

   Which
   identifier by sending an IQ stanza of these the type "set" containing a <bind/>
   element with a child <resource/> element containing non-zero-length
   XML character data.

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

   The server does is up to MAY accept the implementation, although resource identifier provided by the client,
   in which case it is RECOMMENDED returns an IQ stanza of type "result" to implement case #1.  In case #2, the server MUST
   send client,
   including a <conflict/> stream error to <jid/> child element that specifies the current resource, terminate full JID for the
   connected resource.




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

   However, the XML stream and underlying TCP connection for server MAY instead override the current
   resource, client-generated
   resource identifier and return generate a IQ stanza resource identifier on behalf of type "result" (indicating
   success) to
   the newly-requested resource.  In case #3, client, as shown in the server
   MUST either (a) return previous section.

8.4.2.  Error Cases

   When a server-generated client attempts to set its own XMPP resource name or (b) send a
   <conflict/> identifier during
   resource binding, the following stanza error conditions are possible:

   o  The client is not allowed to the newly-requested bind a resource but maintain to the XML stream for that connection so that (e.g.,
      because the newly-requested
   resource node or user has an opportunity to negotiate reached a non-conflicting resource
   identifier before sending another request for limit on the number of
      connected resources allowed).
   o  The provided resource binding.

   Resource identifier cannot be processed by the
      server, e.g. because it is not in use:

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

   If, before completing accordance with the Resourceprep
      (Appendix B) profile of [STRINGPREP]).
   o  The provided resource identifier is already in use but the server
      does not allow binding step, of multiple connected resources with the
      same identifier.

8.4.2.1.  Not Allowed

   If the client attempts
   to send an outbound XML stanza (i.e., a stanza is not directed allowed to the
   server itself or bind a resource to the client's own account), stream, the
   server MUST NOT
   process return a <not-allowed/> error.

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

8.4.2.2.  Bad Request

   If the stanza and provided resource identifier cannot be processed by the
   server, the server SHOULD return a <not-authorized/> stanza <bad-request/> error
   to the client.

8.1.  Binding Multiple Resources

   A server (but MAY support binding
   instead apply the Resourceprep (Appendix B) profile of multiple resources to [STRINGPREP]
   or otherwise process the same
   stream.  This functionality resource identifier so that it is desirable in certain environments
   (e.g., for devices that are unable to open more than one TCP
   connection or when a machine runs an
   conformance).

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



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      </iq>

8.4.2.3.  Conflict

   If there is already a server supports binding connected resource of multiple
   resources to a stream, it the same name, the server
   MUST enable a client to unbind resources.
   This shall be completed by sending an IQ-set with a child element do one of
   <unbind/> qualified by the 'urn:ietf:params:xml:ns:xmpp-bind'
   namespace, which in turn has a child element of <resource/> whose XML
   character data specifies following:

   1.  Not accept the resource to be unbound:

   <iq type='set' id='unbind_1'>
     <unbind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>someresource</resource>
     </unbind>
   </iq>

   If the server does not understand identifier provided by the <unbind/> element, client but
       instead override it MUST
   return with an error of <bad-request/>.  Otherwise, if there is no such XMPP resource for identifier that stream, the
       server MUST return an error of <item-



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   not-found/>.  When the client unbinds generates.
   2.  Terminate the only current resource associated
   with the stream, and allow the server SHOULD close newly-requested
       resource.
   3.  Disallow the stream newly-requested resource and terminate maintain the
   TCP connection.

   A server SHOULD advertise its support for current
       resource.

   Which of these the
   'urn:ietf:params:xml:ns:xmpp-bind' namespace by returning an
   appropriate stream feature as follows:

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

   When a client binds multiple resources server does is up to the same stream, proper
   management of 'from' addresses implementation, although
   it is imperative. RECOMMENDED to implement case #1.

   In particular, a
   client case #2, the server MUST specify a 'from' address on every stanza it sends over send a <conflict/> stream error to which it has bound multiple resources, where the 'from'
   address is the full JID (<node@domain.tld/resource>) associated with
   current resource, terminate the relevant resource.  If a client does not specify a 'from' address
   on a stanza it sends over a XML stream to which it has bound multiple
   resources (or if it specifies as and underlying TCP
   connection for the 'from' address a full JID other
   than one current resource, and return an IQ stanza of type
   "result" (indicating success) to the bound resources), newly-requested resource.

   In case #3, the server MUST return the send a <conflict/> stanza error to the client with an <unknown-sender/> stanza error.

   Naturally,
   newly-requested resource but maintain the rules regarding validation of asserted 'from'
   addresses still apply (see Section 11).


9. XML Stanzas

   After a client stream for that
   connection so that the newly-requested resource has connected an opportunity to
   negotiate a non-conflicting resource identifier before sending
   another request for resource binding.

8.5.  Binding Multiple Resources

   A server or two servers have
   connected MAY support binding of multiple resources to each other, either party can send XML stanzas over the
   negotiated same
   stream.  Three kinds of XML stanza are defined for the
   'jabber:client' and 'jabber:server' namespaces: <message/>,
   <presence/>, and <iq/>.  In addition, there are five common
   attributes for these kinds of stanza.  These common attributes, as
   well as the basic semantics of the three stanza kinds, are defined
   herein; more detailed information regarding the syntax of XML stanzas
   for instant messaging and presence applications  This functionality is provided desirable in
   [XMPP-IM], and certain environments
   (e.g., for other applications in the relevant devices that are unable to open more than one TCP
   connection or when a machine runs a local XMPP extension
   specifications.

   An XML stanza client daemon that is the basic unit
   used by multiple applications).

8.5.1.  Support

   If a server supports binding of meaning in XMPP. multiple resources to a stream, it
   MUST enable a client to unbind resources.  A server that supports
   unbinding MUST
   NOT process, deliver, or route also support binding of multiple resources.  Thus a partial stanza and
   client can discover whether a server MUST NOT
   attach meaning to the transmission timing supports binding of any child element within multiple
   resources by determining if the server advertises a stanza. stream feature of
   <unbind/>, as follows.






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9.1.  Common Attributes

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

9.1.1.  to

   The 'to' attribute specifies the JID of


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

8.5.2.  Binding an Additional Resource

   A connected client binds an additional resource by following the intended recipient
   protocol for binding of the stanza.

   In the 'jabber:client' namespace, a stanza with a specific intended
   recipient MUST possess a 'to' attribute, whereas a original resource, i.e., by sending an IQ
   stanza sent from a
   client to of type "set" containing a server for direct processing <bind/> element qualified by that server (e.g.,
   presence sent to the server for broadcasting
   'urn:ietf:params:xml:ns:xmpp-bind' namespace (either empty to other entities)
   SHOULD NOT possess a 'to' attribute.

   In the 'jabber:server' namespace, a stanza MUST possess a 'to'
   attribute; if a request
   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 generation of the 'to' attribute is invalid or cannot be contacted,
   the entity discovering that fact (usually the sender's resource identifier or recipient's
   server) MUST return an appropriate error to the sender, setting the
   'from' attribute of the error stanza containing a
   <resource/> element with XML character data to the value provided in the
   'to' attribute of the offending stanza.

9.1.2.  from

   The 'from' attribute specifies the JID request client
   generation of the sender.

   When resource identifier).

8.5.3.  Unbinding a server receives Resource

8.5.3.1.  Success Case

   A client unbinds a resource by sending an XML IQ stanza within the context of type "set"
   containing an
   authenticated stream <unbind/> element qualified by the 'jabber:client'
   'urn:ietf:params:xml:ns:xmpp-bind' namespace, it
   MUST do one which in turn contains
   a child element of <resource/> whose XML character data specifies the following:
   1.  validate that the value of
   resource to be unbound:

   C: <iq id='unbind_1' type='set'>
        <unbind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <resource>someresource</resource>
        </unbind>
      </iq>

   If no error occurs, the 'from' attribute provided by server MUST unbind the
       client is that of a connected resource for the associated entity
   2.  add a and no longer
   accept stanzas whose 'from' address to the stanza whose value is specifies the full JID
       (<node@domain/resource>) determined by the server for the
       connected resource associated
   with that generated the stanza (see Determination
       of Addresses (Section 3.5)), or the bare JID (<node@domain>) in
       the case of subscription-related presence stanzas (see [XMPP-IM]
       for details)

   If resource.

   S: <iq id='unbind_1' type='result'/>

   When a client attempts to send an XML stanza for which the value of unbinds the
   'from' attribute does not exactly match one of only resource associated with the connected
   resources for that entity, stream,
   the server SHOULD return an <invalid-
   from/> close the stream error to and terminate the client.  If a client attempts to send an TCP connection.

   S: <iq id='unbind_1' type='result'/>

   S: <stream:stream>






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   XML stanza over


8.5.3.2.  Error Cases

8.5.3.2.1.  Unbind Not Supported

   If the server does not understand the <unbind/> element, it MUST
   return a stream that stanza error, which SHOULD be <bad-request/>.

   S: <iq id='unbind_1' type='error'>
        <error type='modify'>
          <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

8.5.3.2.2.  No Such Resource

   If there is not yet authenticated, no such resource for that stream, the server
   SHOULD MUST return a <not-authorized/> stream
   an error of <item-not-found/>.

   S: <iq id='unbind_1' type='error'>
        <error type='cancel'>
          <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

8.5.4.  From Addresses

   When a client binds multiple resources to the client.  If
   generated, both same stream, proper
   management of these conditions 'from' addresses is imperative.  In particular, a
   client MUST result in closure of the
   stream and termination of the underlying TCP connection; this helps
   to prevent specify a denial of service attack launched from a rogue client.

   When a server generates a stanza from the server itself for delivery
   to a connected client (e.g., in the context of data storage services
   provided by the server 'from' address on behalf of the client), the every stanza MUST
   either (1) not include a 'from' attribute or (2) include it sends over a
   stream to which it has bound multiple resources, where the 'from'
   attribute whose value
   address is the account's bare JID (<node@domain>) or
   connected resource's full JID (<node@domain/resource>).  A server
   MUST NOT send to the client a stanza without a 'from' attribute if
   the stanza was not generated by (<node@domain.tld/resource>) associated with
   the server itself.  When relevant resource.  If a client
   receives a stanza that does not include specify a 'from' attribute, address
   on a stanza it sends over a stream to which it has bound multiple
   resources, the server MUST
   assume that return the stanza is from the server to which the client is
   connected.

   In the 'jabber:server' namespace, a stanza MUST possess a 'from'
   attribute; if a server receives a stanza that does not meet this
   restriction, it MUST generate with an <improper-addressing/> stream error
   condition.  Furthermore,
   <unknown-sender/> stanza error.

   C: <message to='romeo@example.net'>
        <body>Wherefore art thou?</body>
      </message>

   S: <message from='romeo@example.net'
               type='error'>
        <body>Wherefore art thou?</body>
        <error type='modify'>
          <unknown-sender xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>




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   Naturally, the domain identifier portion rules regarding validation of the JID
   contained in the asserted 'from' attribute MUST match
   addresses still apply (see Section 11).


9.  XML Stanzas

   After a client has connected to a server or two servers have
   connected to each other, either party can send XML stanzas over the hostname
   negotiated stream.  Three kinds of XML stanza are defined for the
   sending server (or any validated domain thereof, such
   'jabber:client' and 'jabber:server' namespaces: <message/>,
   <presence/>, and <iq/>.  In addition, there are five common
   attributes for these stanza types.  These common attributes, as a validated
   local domain hosted by the sending server) well
   as communicated in the
   SASL negotiation, dialback negotiation or other means; if a server
   receives a stanza that does not meet this restriction, it MUST
   generate an <invalid-from/> stream error condition.  Both of these
   conditions MUST result in closure basic semantics of the stream and termination of three stanza types, are defined herein;
   more detailed information regarding the underlying TCP connection; this helps to prevent a denial of
   service attack launched from a rogue server.

9.1.3.  id

   The optional 'id' attribute MAY be used by a sending entity for
   internal tracking syntax of XML stanzas that it sends for
   instant messaging and presence applications is provided in [XMPP-IM],
   and receives (especially for tracking the request-response interaction inherent other applications in the
   semantics of IQ stanzas).  It relevant XMPP extension
   specifications.

   An XML stanza is OPTIONAL for the value basic unit of the 'id'
   attribute to be unique globally, within meaning in XMPP.  A server MUST
   NOT process a domain, or partial stanza and a server MUST NOT attach meaning to
   the transmission timing of any child element within a stream.
   The stanza.

   Support for the XML stanza syntax and semantics of IQ stanzas impose additional restrictions; see defined herein is
   REQUIRED in XMPP client and server implementations.

9.1.  Common Attributes

   The following five attributes are common to message, presence, and IQ
   Semantics (Section 9.2.3).

9.1.4.  type
   stanzas.

9.1.1.  to

   The 'type' 'to' attribute specifies detailed information about the purpose
   or context JID of the message, presence, or IQ intended recipient for
   the stanza.

   <message to='romeo@example.net'>
     <body>Art thou not Romeo, and a Montague?</body>
   </message>

   For information about server processing of inbound and outbound XML
   stanzas based on the nature of the 'to' address, refer to Section 11.

9.1.1.1.  Client-to-Server Streams

   The particular
   allowable values for following rules apply to the 'type' 'to' attribute vary depending on whether in the stanza is a message, presence, or IQ; context of XML
   streams qualified by the values for message and 'jabber:client' namespace (i.e., client-to-
   server streams).




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   presence stanzas are


   1.  A stanza with a specific intended recipient MUST possess a 'to'
       attribute.
   2.  A stanza sent from a client to instant messaging and a server for direct processing by
       the server (e.g., presence
   applications and therefore are defined in [XMPP-IM], whereas sent to the
   values server for IQ stanzas specify the role of an IQ stanza in a
   structured request-response "conversation" and thus are defined under
   IQ Semantics (Section 9.2.3) below.  The only 'type' value common broadcasting to
   all three stanzas is "error"; see Stanza Errors (Section 9.3).

9.1.5.  xml:lang

   A stanza
       other entities) SHOULD NOT possess an 'xml:lang' attribute (as defined in
   Section 2.12 of [XML]) if the stanza contains XML character data that
   is intended to be presented to a human user (as explained in
   [CHARSET], "internationalization is for humans"). 'to' attribute.

9.1.1.2.  Server-to-Server Streams

   The value of following rules apply to the
   'xml:lang' 'to' attribute specifies in the default language context of any such
   human-readable XML character data, which MAY be overridden
   streams qualified by the
   'xml:lang' attribute of 'jabber:server' namespace (i.e., server-to-
   server streams).

   1.  A stanza MUST possess a specific child element.  If 'to' attribute; if a server receives a
       stanza that does not possess an 'xml:lang' attribute, an implementation meet this restriction, it MUST assume
   that the default language is that specified for generate an
       <improper-addressing/> stream error and terminate both the XML
       stream as defined
   under Stream Attributes (Section 5.3) above. and the underlying TCP connection with the offending
       server.

9.1.2.  from

   The value 'from' attribute specifies the JID of the 'xml:
   lang' attribute MUST be an NMTOKEN sender.

   <message from='juliet@example.com/balcony'
            to='romeo@example.net'>
     <body>Art thou not Romeo, and MUST conform a Montague?</body>
   </message>

9.1.2.1.  Client-to-Server Streams

   The following rules apply to the format
   defined 'from' attribute in [LANGTAGS].

9.2.  Basic Semantics

9.2.1.  Message Semantics

   The <message/> the context of
   XML streams qualified by the 'jabber:client' namespace (i.e., client-
   to-server streams).

   1.  When the server receives an XML stanza kind can be seen as from a "push" mechanism whereby
   one entity pushes information to another entity, similar to client and the
   communications that occur in
       stanza does not include a system such as email.  All message
   stanzas SHOULD possess 'from' attribute, the server MUST add a 'to'
       'from' attribute that specifies to the intended
   recipient stanza, where the value of the message; upon receiving such a stanza, a 'from'
       attribute is the full JID (<node@domain/resource>) determined by
       the server
   SHOULD route for the connected resource that generated the stanza
       (see Section 3.5), or deliver it to the intended recipient bare JID (<node@domain>) in the case of
       subscription-related presence stanzas (see Server
   Rules for Handling [XMPP-IM]).
   2.  When the server receives an XML Stanzas (Section 11) for general routing stanza from a client and
   delivery rules related to XML stanzas).

9.2.2.  Presence Semantics

   The <presence/> element can be seen as the
       stanza includes a specialized broadcast or
   "publish-subscribe" mechanism, whereby multiple entities receive
   information about an entity to which they have subscribed (in this
   case, network availability information).  In general, a publishing
   entity SHOULD send a presence stanza with no 'to' 'from' attribute, in which
   case the server to which MUST either (a)
       validate that the entity value of the 'from' attribute provided by the
       client is connected SHOULD broadcast or
   multiplex that stanza to all subscribing entities.  However, a
   publishing entity MAY also send a presence stanza with of a 'to'
   attribute, in which case connected resource for the server SHOULD route associated entity
       or deliver that
   stanza to (b) override the intended recipient.  See Server Rules for Handling XML
   Stanzas (Section 11) for general routing and delivery rules related provided 'from' attribute by adding a 'from'
       attribute as specified under Rule #1.





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   to XML stanzas, and [XMPP-IM] for rules specific to presence
   applications.

9.2.3.  IQ Semantics

   Info/Query, or IQ, is a request-response mechanism, similar in some
   ways to [HTTP].  The semantics of IQ enable an entity to make a
   request of, and receive a response from, another entity.  The data
   content of


   3.  When the request and response is defined by server generates a stanza from the schema or other
   structural definition associated with server for delivery
       to the XML namespace that
   qualifies client on behalf of the direct child element account of the IQ element (see extended
   namespaces (Section 9.4)), and connected client
       (e.g., in the interaction is tracked context of data storage services provided by the
   requesting entity through use
       server on behalf of the 'id' attribute.  Thus, IQ
   interactions follow client), the stanza MUST either (a) not
       include a common pattern of structured data exchange such
   as get/result 'from' attribute or set/result (although an error may be returned in
   reply to (b) include a request if appropriate):

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

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

   1.  The 'id' attribute is REQUIRED for IQ stanzas.
   2.  The 'type' 'from' attribute is REQUIRED for IQ stanzas.  The
       whose value MUST
       be one of the following:
       *  get -- The stanza is the account's bare JID (<node@domain>).
   4.  When the server generates a request for information or
          requirements.
       *  set -- The stanza provides required data, sets new values, or
          replaces existing values.
       *  result -- The stanza is a response to a successful get or set
          request.
       *  error -- An error has occurred regarding processing or from the server itself for
       delivery of a previously-sent get or set (see Stanza Errors
          (Section 9.3)).



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   3.  An entity that receives an IQ request of type "get" or "set" MUST
       reply with an IQ response of type "result" or "error" (the
       response MUST preserve to the 'id' attribute of client, the request).
   4.  An entity that receives a stanza MUST include a 'from'
       attribute whose value is the mere domain (<domain>) of type "result" or "error" the
       server.
   5.  A server MUST NOT respond send to the client a stanza by sending without 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).
   5.  An IQ stanza of type "get" or "set" MUST contain one and only one
       child element that specifies the semantics of 'from'
       attribute if the particular
       request or response.
   6.  An IQ stanza of type "result" MUST include zero or one child
       elements.
   7.  An IQ stanza of type "error" SHOULD include the child element
       contained in was not generated by the associated "get" server (e.g., if
       it was generated by another client or "set" and MUST include an
       <error/> child; for details, see Stanza Errors (Section 9.3).

9.3.  Stanza Errors

   Stanza-related errors are handled in another server); therefore,
       when a client receives a manner similar to stream
   errors (Section 5.8).  However, unlike stream errors, stanza errors
   are recoverable; therefore error stanzas that does not include hints regarding
   actions a 'from'
       attribute, it MUST assume that the original sender can take in order stanza is from the server to remedy
       which the
   error.

9.3.1.  Rules client is connected.

9.1.2.2.  Server-to-Server Streams

   The following rules apply to stanza-related errors:

   o  The receiving or processing entity that detects an error condition the 'from' attribute in relation to a the context of
   XML streams qualified by the 'jabber:server' namespace (i.e., server-
   to-server streams).

   1.  A stanza SHOULD return an "error stanza" (and MUST
      do so for IQ stanzas), where such an "error stanza" is possess a stanza of
      the same kind (message, presence, or IQ) whose 'type' attribute is
      set to 'from' attribute; if a value of "error".
   o  The entity server receives a
       stanza that generates does not meet this restriction, it MUST generate an
       <improper-addressing/> stream error stanza SHOULD include and terminate the
      original XML sent so that underlying
       TCP connection.
   2.  The domain identifier portion of the sender can inspect and, if
      necessary, correct JID contained in the XML before attempting to resend.
   o  An error stanza MUST contain an <error/> child element.
   o  An <error/> child 'from'
       attribute MUST NOT be included if match the 'type' attribute has
      a value other than "error" hostname of the sending server (or any
       validated domain thereof, such as a validated local domain hosted
       by the sending server) as communicated in the SASL negotiation
       (see Section 7), server dialback (see [XEP-0220], or similar
       means; if there is no 'type' attribute).
   o  An entity that a server receives an error a stanza that does not meet this
       restriction, it MUST NOT respond to the
      stanza with a further generate an <invalid-from/> stream error stanza; this and
       terminate the underlying TCP connection.

   Enforcement of these rules helps to prevent looping.

9.3.2.  Syntax a denial of service
   attack launched from a rogue server.

9.1.3.  id

   The syntax 'id' attribute MAY be used by a sending entity for stanza-related errors internal
   tracking of stanzas that it sends and receives (especially for
   tracking the request-response interaction inherent in the semantics
   of IQ stanzas).  It is as follows: OPTIONAL for the value of the 'id' attribute
   to be unique globally, within a domain, or within a stream.  The
   semantics of IQ stanzas impose additional restrictions; see
   Section 9.2.3.



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   <stanza-kind 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'
             xml:lang='langcode'>
         OPTIONAL descriptive text
       </text>]
       [OPTIONAL application-specific condition element]
     </error>
   </stanza-kind>


9.1.4.  type

   The "stanza-kind" is one 'type' attribute specifies detailed information about the purpose
   or context of the message, presence, or iq. IQ stanza.  The value of particular
   allowable values for the <error/> element's 'type' attribute MUST be one of vary depending on whether
   the following:

   o  cancel -- do not retry (the error stanza is unrecoverable)
   o  continue -- proceed (the condition was only a warning)
   o  modify -- retry after changing the data sent
   o  auth -- retry after providing credentials
   o  wait -- retry after waiting (the error is temporary) message, presence, or IQ stanza.  The <error/> element:

   o  MUST contain a child element corresponding to one of the defined
      stanza error conditions values
   for message and presence stanzas are specific to instant messaging
   and presence applications and therefore are specified below; this element MUST be
      qualified by in [XMPP-IM],
   whereas the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace.
   o  MAY contain a <text/> child containing XML character data that
      describes values for IQ stanzas specify the error role of an IQ stanza in more detail; this element MUST be qualified
      by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace
   a structured request-response exchange and thus are specified under
   Section 9.2.3.  The only 'type' value common to all three stanzas is
   "error"; see Section 9.3.

9.1.5.  xml:lang

   A stanza 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 attribute (as defined by in
   Section 2.12 of [XML]) if the stanza contains XML character data that namespace.

   The <text/> element
   is OPTIONAL.  If included, it SHOULD intended to be used only presented to provide descriptive or diagnostic information that supplements the
   meaning of a defined condition or application-specific condition.  It
   SHOULD NOT be interpreted programmatically by an application.  It
   SHOULD NOT be used as the error message presented to a user, but MAY
   be shown human user (as explained in addition to
   [CHARSET], "internationalization is for humans").  The value of the error message associated with
   'xml:lang' attribute specifies the
   included condition element (or elements).

   Finally, to maintain backward compatibility, default language of any such
   human-readable XML character data.

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

   The value of the schema (specified in
   [XMPP-IM]) allows 'xml:lang' attribute MAY be overridden by the optional inclusion 'xml:
   lang' attribute of a 'code' attribute on specific child element.

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

   If an outbound stanza generated by a client does not possess an 'xml:
   lang' attribute, the
   <error/> element; client's server MAY add an 'xml:lang' attribute
   whose value is that specified for details, see [XEP-0086]. the stream as defined under
   Section 5.3.











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9.3.3.  Defined Conditions

   The following conditions are defined for use in


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

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

   If an inbound stanza errors.

   o  <bad-request/> -- the sender has sent XML received does not possess an 'xml:lang'
   attribute, an implementation MUST assume that the default language is malformed or
      that cannot be processed (e.g., an IQ stanza
   that includes an
      unrecognized specified for the stream as defined under Section 5.3.

   The value of the 'type' attribute); the associated error
      type SHOULD be "modify".
   o  <conflict/> -- access cannot be granted because an existing
      resource exists with 'xml:lang' attribute MUST conform to the same name or address; NMTOKEN
   datatype (as defined in Section 2.3 of [XML]) and MUST conform to the associated
      error type SHOULD be "cancel".
   o  <feature-not-implemented/> -- the feature requested is not
      implemented by the recipient or
   format defined in [LANGTAGS].

   A server and therefore cannot MUST NOT modify or delete 'xml:lang' attributes on stanzas
   it receives from other entities.

9.2.  Basic Semantics

9.2.1.  Message Semantics

   The <message/> stanza can be
      processed; seen as a "push" mechanism whereby one
   entity pushes information to another entity, similar to the associated error type
   communications that occur in a system such as email.  All message
   stanzas SHOULD be "cancel" or
      "modify".
   o  <forbidden/> -- the requesting entity does not possess a 'to' attribute that specifies the
      required permissions to perform the action; intended
   recipient of the associated error
      type message; upon receiving such a stanza, a server
   SHOULD be "auth".
   o  <gone/> -- route or deliver it to the intended recipient or server (see Section 11
   for general routing and delivery rules related to XML stanzas).

9.2.2.  Presence Semantics

   The <presence/> stanza can no longer be contacted at seen as a specialized broadcast or
   "publish-subscribe" mechanism, whereby multiple entities receive
   information about an entity to which they have subscribed (in this address (the error stanza MAY contain
   case, network availability information).  In general, a new address publishing
   entity (client) SHOULD send a presence stanza with no 'to' attribute,
   in which case the
      XML character data of the <gone/> element); server to which the associated error
      type entity is connected SHOULD be "cancel"
   broadcast or "modify".
   o  <internal-server-error/> -- multiplex that stanza to all subscribing entities.
   However, a publishing entity MAY also send a presence stanza with a
   'to' attribute, in which case the server could not process the SHOULD route or deliver that
   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/> -- to the sending entity has provided or
      communicated an intended recipient.  See Section 11 for general routing
   and delivery rules related to XML stanzas, and [XMPP-IM] for rules



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   specific to presence applications.

9.2.3.  IQ Semantics

   Info/Query, or aspect thereof (e.g., IQ, is a resource identifier) that does not
      adhere request-response mechanism, similar in some
   ways to [HTTP].  The semantics of IQ enable an entity to make a
   request of, and receive a response from, another entity.  The data
   content of the syntax request and response is defined under Addresses (Section 3); by the schema or other
   structural definition associated error type SHOULD be "modify".
   o  <not-acceptable/> -- with the recipient or server understands XML namespace that
   qualifies the
      request but direct child element of the IQ element (see
   Section 9.4), and the interaction is refusing to process it because it does not meet
      criteria defined tracked by the recipient or server (e.g., requesting entity
   through use of the 'id' attribute.  Thus, IQ interactions follow a local policy
      regarding stanza size limits
   common pattern of structured data exchange such as get/result or acceptable words in messages); the
      associated set/
   result (although an error type SHOULD may be "modify".
   o  <not-allowed/> -- the recipient or server does not allow any
      entity to perform the action (e.g., sending returned in reply to entities at a
      blacklisted domain); the associated error type SHOULD be "cancel".
   o  <not-authorized/> -- the sender must provide proper credentials
      before being allowed request if
   appropriate):

   Requesting                  Responding
     Entity                      Entity
   ----------                  ----------
       |                            |
       | <iq id='1' type='get'>     |
       |   [ ... payload ... ]      |
       | </iq>                      |
       | -------------------------> |
       |                            |
       | <iq id='1' type='result'>  |
       |   [ ... payload ... ]      |
       | </iq>                      |
       | <------------------------- |
       |                            |
       | <iq id='2' type='set'>     |
       |   [ ... payload ... ]      |
       | </iq>                      |
       | -------------------------> |
       |                            |
       | <iq id='2' type='error'>   |
       |   [ ... condition ... ]    |
       | </iq>                      |
       | <------------------------- |
       |                            |

   In order to perform the action, or has provided
      improper credentials; the associated error type SHOULD be "auth".
   o  <not-modified/> -- the item requested has not changed since it was
      last requested; the associated error type SHOULD be "continue".
   o  <payment-required/> -- enforce these semantics, the requesting entity following rules apply:

   1.  The 'id' attribute is not authorized to
      access the requested service because payment REQUIRED for IQ stanzas.
   2.  The 'type' attribute is required; the
      associated error type SHOULD REQUIRED for IQ stanzas.  The value MUST
       be "auth". one of the following:





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   o  <recipient-unavailable/>


       *  get -- the intended recipient The stanza is temporarily
      unavailable; the associated error type SHOULD be "wait" (note: an
      application MUST NOT return this error if doing so would provide a request for information about the intended recipient's network availability to
      an entity that is not authorized to know such information).
   o  <redirect/> or
          requirements.
       *  set -- the recipient The stanza provides required data, sets new values, or server
          replaces existing values.
       *  result -- The stanza is redirecting requests for
      this information a response to another entity, usually temporarily (the error
      stanza SHOULD contain the alternate address, which MUST be a valid
      JID, in the XML character data of the <redirect/> element); the
      associated successful get or set
          request.
       *  error type SHOULD be "modify".
   o  <registration-required/> -- the requesting entity is not
      authorized to access the requested service because prior
      registration is required; the associated An error type SHOULD be
      "auth".
   o  <remote-server-not-found/> -- a remote server or service specified
      as part has occurred regarding processing or all of the JID
          delivery of the intended recipient does not
      exist; the associated error type SHOULD be "cancel".
   o  <remote-server-timeout/> -- a remote server previously-sent get or service specified
      as part set (see Section 9.3).
   3.  An entity that receives an IQ request of type "get" or all "set" MUST
       reply with an IQ response of type "result" or "error".  The
       response MUST preserve the JID 'id' attribute of the intended recipient (or required
      to fulfill a request) could not be contacted within request.
   4.  An entity that receives a reasonable
      amount stanza of time; the associated error type SHOULD be "wait".
   o  <resource-constraint/> -- the server "result" or recipient lacks the system
      resources necessary "error" MUST
       NOT respond to service the request; the associated error stanza by sending a further IQ response of
       type SHOULD be "wait".
   o  <service-unavailable/> -- the server "result" or recipient does not
      currently provide the requested service; the associated error type
      SHOULD be "cancel".
   o  <subscription-required/> -- "error"; however, the requesting entity is not
      authorized MAY send
       another request (e.g., an IQ of type "set" in order to access the requested service because provide
       required information discovered through a subscription
      is required; the associated error type SHOULD be "auth".
   o  <undefined-condition/> -- the error condition is not one get/result pair).
   5.  An IQ stanza of those
      defined by the other conditions in this list; any error type may
      be associated with this condition, "get" or "set" MUST contain one and it SHOULD be used only in
      conjunction with an application-specific condition.
   o  <unexpected-request/> -- the recipient or server understood the
      request but was not expecting it at this time (e.g., one
       child element that specifies the request
      was out semantics of order); the associated error particular
       request.
   6.  An IQ stanza of type SHOULD be "wait" "result" MUST include zero or
      "modify".
   o  <unknown-sender/> -- the one child
       elements.
   7.  An IQ stanza 'from' address specified by a
      remote server or connected client is not known to of type "error" MAY include the receiving
      server child element
       contained in the associated "get" or is not valid "set" and MUST include an
       <error/> child; for details, see Section 9.3.

9.3.  Stanza Errors

   Stanza-related errors are handled in a manner similar to stream
   errors (Section 5.8).  Unlike stream errors, stanza errors are
   recoverable; therefore they do not result in termination of the stream; XML
   stream and underlying TCP connection.  Instead, the entity that
   discovers the associated error type
      SHOULD be "modify".

9.3.4.  Application-Specific Conditions

   As noted, condition returns an application MAY provide application-specific ERROR STANZA to the sender,
   i.e., a stanza
   error information by including of the same kind (message, presence, or IQ) whose
   'type' attribute is set to a properly-namespaced value of "error" and which contains an
   <error/> child in element that specifies the error element. condition.  The application-specific element SHOULD supplement or



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   further qualify
   specified error condition provides 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>

9.4.  Extended Namespaces

   While hint regarding actions that the message, presence, and IQ stanza kinds provide basic
   semantics for messaging, availability, and request-response
   interactions, XMPP uses XML namespaces
   sender can take to extend the stanzas for remedy the
   purpose of providing additional functionality.  Thus a message or
   presence error.

9.3.1.  Rules

   The following rules apply to stanza MAY contain one errors:

   1.  The receiving or more optional child elements
   specifying content processing entity that extends the meaning of the message (e.g., detects an
   XHTML-formatted version of the message body as described error
       condition in
   [XEP-0071]), and relation to a stanza SHOULD return an error stanza
       (and MUST do so for IQ stanzas).
   2.  The entity that generates an error stanza MAY include the
       original XML sent so that the sender can inspect and, if
       necessary, correct the XML before attempting to resend.



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   3.  An error stanza MUST contain one such an <error/> child element.
   This
   4.  An <error/> 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 NOT be included if the child element.  Such 'type' attribute
       has a child element is said to
   be defined by an EXTENDED NAMESPACE.

   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 value other than "error" (or if there is (1) the recipient or (2) an no 'type'
       attribute).
   5.  An entity that is routing the receives an error stanza MUST NOT respond to the recipient:







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   Recipient:  If a recipient receives a
       stanza that contains with a child
      element it does not understand, it SHOULD ignore that specific XML
      data, i.e., it SHOULD not process it or present it further error stanza; this helps to a user or
      associated application (if any).  In particular:
      *  If an entity receives a message or presence stanza that
         contains prevent
       looping.

9.3.2.  Syntax

   The syntax for stanza-related errors is:

   <stanza-kind from='intended-recipient' to='sender' type='error'>
     [OPTIONAL to include sender XML data qualified by a namespace it does not
         understand, the portion here]
     <error type='error-type'>
       <defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       [<text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
             xml:lang='langcode'>
         OPTIONAL descriptive text
       </text>]
       [OPTIONAL application-specific condition element]
     </error>
   </stanza-kind>

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

   The value of the stanza that is in <error/> element's 'type' attribute MUST be one of
   the unknown
         namespace SHOULD following:

   o  cancel -- do not retry (the error cannot be ignored.
      *  If an entity receives a message stanza whose remedied)
   o  continue -- proceed (the condition was only child element
         is qualified by a namespace it does not understand, it MUST
         ignore warning)
   o  modify -- retry after changing the entire stanza.
      *  If an entity receives an IQ stanza of type "get" or "set"
         containing data sent
   o  auth -- retry after providing credentials
   o  wait -- retry after waiting (the error is temporary)

   The <error/> element:

   o  MUST contain a child element qualified by a namespace it does not
         understand, corresponding to one of the entity SHOULD return an IQ defined
      stanza of type
         "error" with an error condition of <service-unavailable/>.
   Router:  If a routing entity (usually a server) handles a stanza that
      contains conditions specified; this element MUST be qualified
      by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace.
   o  MAY contain a <text/> child element it does not understand, it SHOULD ignore
      the associated containing XML character data by passing it on untouched to
      that describes the
      recipient.


10.  Examples

10.1.  Client-to-Server

   The following examples show error in more detail; this element MUST be
      qualified by the data flow for a client negotiating an
   XML stream with a server, exchanging XML stanzas, 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace
      and closing the
   negotiated stream.  The server is "example.com", SHOULD possess an 'xml:lang' attribute specifying the server requires
   use natural
      language of TLS, the client authenticates via the SASL DIGEST-MD5
   mechanism as "juliet@example.com", and the client binds the resource
   "balcony" to the stream.  (Note: The alternate steps shown below are
   provided to illustrate the protocol XML character data.
   o  MAY contain a child element for failure cases; they are not
   exhaustive and would not necessarily an application-specific error
      condition; this element MUST be triggered qualified by the data sent in
   the examples.)

   Step 1: Client initiates stream to server:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       xml:lang='en'
       from='juliet@example.com'
       to='example.com'
       version='1.0'> an application-
      defined namespace, and its structure is defined by that namespace.



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

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       xml:lang='en'
       from='example.com'
       id='c2s_123'
       to='juliet@example.com'
       version='1.0'>

   Step 3: Server sends stream features to client (STARTTLS extension
   and authentication mechanisms):

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

   Step 4: Client sends STARTTLS command to server:

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

   Step 5: Server informs client that it


   The <text/> element is allowed OPTIONAL.  If included, it SHOULD be used only
   to proceed:

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

   Step 5 (alt): Server informs client provide descriptive or diagnostic information that TLS negotiation has failed
   and closes both XML stream and TCP connection:

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

   Step 6: Client and server attempt to complete TLS negotiation over supplements the existing TCP connection (see [TLS] for details).

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

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       xml:lang='en'
       from='juliet@example.com'
       to='example.com'
       version='1.0'>




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

   Step 8: Server responds defined condition or application-specific condition.  It
   SHOULD NOT be interpreted programmatically by sending an application.  It
   SHOULD NOT be used as the error message presented to a stream header user, but MAY
   be shown in addition to client along the error message associated with any available stream features (notice that the server now shows
   a different set of SASL mechanisms; here
   included condition element (or elements).

   Finally, to maintain backward compatibility, the server accepts schema for the SASL
   PLAIN mechanism once
   default namespace (specified in [XMPP-IM]) allows the stream has been secured via TLS):

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

   Step 9: Client selects an authentication mechanism, in this case
   [DIGEST-MD5] with an empty authorization identity ("="):

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

   Step 10: Server sends optional
   inclusion of a [BASE64] encoded challenge to client:

   <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   cmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLHFvcD0i
   YXV0aCIsY2hhcnNldD11dGYtOCxhbGdvcml0aG09bWQ1LXNlc3MK
   </challenge>

   The decoded challenge is:

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

   Note: When 'code' attribute on the server sends <error/> element; for details,
   see [XEP-0086].

9.3.3.  Defined Conditions

   The following conditions are defined for use in stanza errors.

9.3.3.1.  bad-request

   The sender has sent a DIGEST-MD5 challenge stanza containing XML that does not conform to
   the client, the
   qop list must appropriate schema or that cannot be quoted since it is a list rather than a single item
   (even if there is only one item in processed (e.g., an IQ
   stanza that includes an unrecognized value of the list); however, when 'type' attribute);
   the
   client sends its response to associated error type SHOULD be "modify".

   C: <iq from='juliet@example.com/balcony'
          id='some-id'
          to='example.com'
          type='subscribe'>
        <ping xmlns='urn:xmpp:ping'/>
      </iq>

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

9.3.3.2.  conflict

   Access cannot be granted because an existing resource exists with the server (see below),
   same name or address; the qop must not associated error type SHOULD be quoted since it is a single item rather than a list. "cancel".







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   Step 10 (alt): Server returns error to client:

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

   Step 11: Client sends a [BASE64] encoded response to the challenge:

   <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   dXNlcm5hbWU9Imp1bGlldCIscmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2
   TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAwMDAwMDAx
   LHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20iLHJlc3BvbnNl
   PWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNoYXJzZXQ9dXRmLTgK
   </response>


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

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

9.3.3.3.  feature-not-implemented

   The decoded response is:

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

   Step 12: Server informs client of success and includes [BASE64]
   encoded value for subsequent authentication:

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

   The decoded value for subsequent authentication is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd

   Step 12 (alt): Server returns error to client:

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











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   Step 13: Client initiates a new stream to server:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       xml:lang='en'
       from='juliet@example.com'
       to='example.com'
       version='1.0'>

   Step 14: Server responds by sending a stream header to client along
   with supported features (in this case resource binding):

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

   Upon being so informed that resource binding feature represented in the XML stanza is required, not implemented by the client
   MUST bind a resource to
   recipient or server and therefore the stream; here we assume that stanza cannot be processed; the client
   binds a resource called "balcony".

   Step 15: Client binds a resource:
   associated error type SHOULD be "cancel" or "modify".

   C: <iq type='set' id='bind_1'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>balcony</resource>
     </bind> from='juliet@example.com/balcony'
          id='subscriptions1'
          to='pubsub.example.com'
          type='get'>
        <pubsub xmlns='http://jabber.org/protocol/pubsub'>
          <subscriptions/>
        </pubsub>
      </iq>

   E: <iq from='pubsub.example.com
          id='subscriptions1'
          to='juliet@example.com/balcony'
          type='error'>
        <error type='cancel'>
          <feature-not-implemented
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
          <unsupported
              xmlns='http://jabber.org/protocol/pubsub#errors'
              feature='retrieve-subscriptions'/>
        </error>
      </iq>

9.3.3.4.  forbidden

   The requesting entity does not possess the required permissions to
   perform the action; the associated error type SHOULD be "auth".







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   Step 16: Server informs client of successful resource binding:

   <iq type='result'


   C: <presence
          from='juliet@example.com/balcony'
          to='characters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'
       id='bind_1'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <jid>juliet@example.com/balcony</jid>
     </bind>
   </iq>

   Now
          type='error'>
        <error type='auth'>
          <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.5.  gone

   The recipient or server can no longer be contacted at this address
   (the error stanza MAY contain a new address in the client is allowed to send XML stanzas over character data
   of the negotiated
   stream.

   Client sends XML stanza to other entity:

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

   If necessary, sender's server negotiates XML streams with intended
   recipient's server (see Server-to-Server Examples (Section 10.2)).

   The intended recipient replies and the message is delivered to <gone/> element); the
   client.

   Client receives XML stanza from other entity:

   <message from='romeo@example.net/orchard' associated error type SHOULD be "cancel"
   or "modify".

   C: <presence
          from='juliet@example.com/balcony'
          to='characters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'
            xml:lang='en'>
     <body>Neither, fair saint, if either thee dislike.</body>
   </message>

   Desiring to send no further messages, the client closes the stream.

   Client closes
          type='error'>
        <error type='modify'>
          <gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
            conference.example.com
          </gone>
        </error>
      </presence>

9.3.3.6.  internal-server-error

   <internal-server-error/> -- the stream:

   </stream:stream>

   Consistent with server could not process the recommended stream closing handshake, stanza
   because of a misconfiguration or an otherwise-undefined internal
   server
   closes stream as well:

   Server closes error; the stream:

   </stream:stream> associated error type SHOULD be "wait" or "cancel".







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   Client now terminates the underlying TCP connection.

10.2.  Server-to-Server Examples


   C: <presence
          from='juliet@example.com/balcony'
          to='characters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'
          type='error'>
        <error type='wait'>
          <internal-server-error
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.7.  item-not-found

   The following examples show the data flow for a server negotiating an
   XML stream with another server, exchanging XML stanzas, and closing addressed JID or item requested cannot be found; the negotiated stream.  The initiating server ("Server1") is
   "example.com"; associated
   error type SHOULD be "cancel" or "modify".

   C: <iq id='unbind_1' type='set'>
        <unbind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <resource>someresource</resource>
        </unbind>
      </iq>

   S: <iq id='unbind_1' type='error'>
        <error type='cancel'>
          <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

   An application MUST NOT return this error if doing so would provide
   information about the receiving server ("Server2") intended recipient's network availability to an
   entity that is example.net and not authorized to know such information; instead it
   requires use of TLS; example.com presents
   SHOULD return a certificate and
   authenticates via the SASL EXTERNAL mechanism.  (Note: <service-unavailable/> error.

9.3.3.8.  jid-malformed

   The alternate
   steps shown below are sending entity has provided to illustrate or communicated an XMPP address
   (e.g., a value of the protocol for failure
   cases; they are not exhaustive and would 'to' attribute) or aspect thereof (e.g., an
   XMPP resource identifier) that does not necessarily adhere to the syntax defined
   under Section 3; the associated error type SHOULD be triggered "modify".







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   C: <presence
          from='juliet@example.com/balcony'
          to='ch@r@cters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='ch@r@cters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'
          type='error'>
        <error type='modify'>
          <jid-malformed
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.9.  not-acceptable

   The recipient or server understands the request but is refusing to
   process it because it does not meet criteria defined by the data sent recipient
   or server (e.g., a local policy regarding stanza size limits or
   acceptable words in messages); the examples.)

   Step 1: Server1 initiates stream associated error type SHOULD be
   "modify".

   C: <message to='juliet@example.com' id='foo'>
        <body>[ ... the-emacs-manual ... ]</body>
      </message>

   S: <message from='juliet@example.com' id='foo'>
        <error type='modify'>
          <not-acceptable
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

9.3.3.10.  not-allowed

   The recipient or server does not allow any entity to Server2:

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

   Step 2: Server2 responds by perform the
   action (e.g., sending a stream tag to Server1:

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

   Step 3: Server2 sends stream features to Server1 (STARTTLS extension
   and authentication mechanisms):

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

   Step 4: Server1 sends entities at a blacklisted domain); the STARTTLS command to Server2:

   <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
   associated error type SHOULD be "cancel".











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   Step 5: Server2 informs Server1 that it is


   C: <presence
          from='juliet@example.com/balcony'
          to='characters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'>
        <error type='cancel'>
          <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.11.  not-authorized

   The sender must provide proper credentials before being allowed to proceed:

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

   Step 5 (alt): Server2 informs Server1 that TLS negotiation
   perform the action, or 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 provided improper credentials; the
   associated error type SHOULD be "auth".

   C: <presence
          from='juliet@example.com/balcony'
          to='characters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'>
        <error type='auth'>
          <not-authorized xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.12.  not-modified

   The item requested has not changed since it was last requested; the
   associated error type SHOULD be "continue".













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  C: <iq from='juliet@capulet.com/balcony'
         id='roster2'
         type='get'>
      <query xmlns='jabber:iq:roster'>
         <headers xmlns='http://jabber.org/protocol/shim'>
           <header name='If-None-Match'>some-long-opaque-string</header>
         </headers>
      </query>
     </iq>

  S: <iq type='error'
         to='juliet@capulet.com/balcony'
         id='roster2'>
       <query xmlns='jabber:iq:roster'>
         <headers xmlns='http://jabber.org/protocol/shim'>
           <header name='If-None-Match'>some-long-opaque-string</header>
         </headers>
       </query>
       <error type='modify'>
         <not-modified xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       </error>
     </iq>

9.3.3.13.  payment-required

   The requesting entity is successful, Server1 initiates a new
   stream not authorized to Server2:

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

   Step 7 (alt): If TLS negotiation access the requested
   service because payment is unsuccessful, Server2 closes TCP
   connection.

   Step 8: Server2 responds by sending a stream header to Server1 along
   with available stream features (notice that Server2 now prefers required; the
   SASL EXTERNAL mechanism):

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='example.net'
       id='s2s_234'
       to='example.com'
       version='1.0'>
   <stream:features>
     <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       <mechanism>EXTERNAL</mechanism>
       <mechanism>DIGEST-MD5</mechanism>
       <required/>
     </mechanisms>
   </stream:features> associated error type SHOULD
   be "auth".

   C: <iq from='romeo@example.net/foo'
          id='items1'
          to='pubsub.example.com'
          type='get'>
        <pubsub xmlns='http://jabber.org/protocol/pubsub'>
          <items node='my_musings'/>
        </pubsub>
      </iq>

   E: <iq from='pubsub.example.com'
          id='items1'
          to='romeo@example.net/foo'
          type='error'>
        <error type='auth'>
          <payment-required
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>




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   Step 9: Server1 selects


9.3.3.14.  recipient-unavailable

   The intended recipient is temporarily unavailable; the EXTERNAL mechanism, in associated
   error type SHOULD be "wait".

   C: <presence
          from='juliet@example.com/balcony'
          to='characters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'>
        <error type='wait'>
          <recipient-unavailable
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

   An application MUST NOT return this case with error if doing so would provide
   information about the intended recipient's network availability to an
   authorization identity encoded according
   entity that is not authorized to [BASE64]:

   <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
         mechanism='EXTERNAL'/>ZXhhbXBsZS5jb20K</auth> know such information; instead it
   SHOULD return a <service-unavailable/> error.

9.3.3.15.  redirect

   The decoded authorization identity recipient or server is "example.com".

   Step 10: Server2 determines that the authorization identity provided
   by Server1 matches the valid id-xmppAddr-on or Common Name redirecting requests for this information
   to another entity, typically in the
   presented certificate and therefore returns success:

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

   Step 11 (alt): Server2 informs Server1 of failed authentication:

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <not-authorized/>
   </failure>
   </stream:stream>

   Step 12: Server1 initiates a new stream to Server2:

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

   Step 13: Server2 responds by sending temporary fashion (the error stanza
   SHOULD contain the alternate address, which SHOULD be a stream header to Server1 along
   with any additional features (or, valid JID, in this case, an empty features
   element):

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

   Now Server1 is allowed to send
   the XML stanzas to Server2 over character data of the
   negotiated stream; here we assume that <redirect/> element); the transferred stanzas are
   those shown earlier for client-to-server communications. associated
   error type SHOULD be "modify".

   C: <presence
          from='juliet@example.com/balcony'
          to='characters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'
          type='error'>
        <error type='modify'>
          <redirect xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
            characters@conference.example.org
          </redirect>
        </error>



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   Server1 sends XML stanza to Server2:

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


      </presence>

9.3.3.16.  registration-required

   The intended recipient replies and the message requesting entity is delivered from
   Server2 to Server1.

   Server2 sends XML stanza to Server1:

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

   Desiring not authorized to send no further messages, Server1 closes the stream.  (In
   practice, access the stream would most likely remain open for some time,
   since Server1 and Server2 do not immediately know if requested
   service because prior registration is required; the stream will associated error
   type SHOULD be needed for further communications.)

   Server1 closes the stream:

   </stream:stream>

   Consistent with the recommended stream closing handshake, Server2
   closes stream "auth".

   C: <presence
          from='juliet@example.com/balcony'
          to='characters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'>
        <error type='auth'>
          <registration-required
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.17.  remote-server-not-found

   A remote server or service specified as well:

   Server2 closes part or all of the stream:

   </stream:stream>

   Server1 now terminates JID of the underlying TCP connection.


11.  Server Rules for Handling XML Stanzas

   Compliant
   intended recipient does not exist; the associated error type SHOULD
   be "cancel".

   C: <presence
          from='juliet@example.com/balcony'
          to='characters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'>
        <error type='wait'>
          <remote-server-not-found
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>








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9.3.3.18.  remote-server-timeout

   A remote server implementations MUST ensure in-order processing or service specified as part or all of
   XML stanzas between any two entities.  This includes stanzas sent by
   a client to its server for direct processing by the server.

   Beyond JID of the requirement for in-order processing, each server
   implementation will contain its own "delivery tree" for handling
   stanzas it receives.  Such
   intended recipient (or required to fulfill a tree determines whether request) could not be
   contacted within a stanza needs
   to reasonable amount of time; the associated error
   type SHOULD be routed to another domain, processed direct, "wait".

   C: <presence
          from='juliet@example.com/balcony'
          to='characters@chat.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@chat.example.com/JulieC'
          to='juliet@example.com/balcony'>
        <error type='wait'>
          <remote-server-timeout
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.19.  resource-constraint

   The server or delivered recipient lacks the system resources necessary to a
   service the request; the associated error type SHOULD be "wait".

   C: <iq from='romeo@example.net/foo'
          id='items1'
          to='pubsub.example.com'
          type='get'>
        <pubsub xmlns='http://jabber.org/protocol/pubsub'>
          <items node='my_musings'/>
        </pubsub>
      </iq>

   E: <iq from='pubsub.example.com'
          id='items1'
          to='romeo@example.net/foo'
          type='error'>
        <error type='wait'>
          <resource-constraint
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>






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   resource associated with a connected node.


9.3.3.20.  service-unavailable

   The following rules
   apply.

11.1.  No 'to' Address

   If server or recipient does not currently provide the stanza possesses no 'to' attribute, requested
   service; the server associated error type SHOULD process
   it directly on behalf of the entity that sent it.  Because all
   stanzas received from other servers MUST possess be "cancel".

   C: <message from='romeo@example.net/foo'
               to='juliet@example.com'>
        <body>Hello?</body>
      </message>

   S: <message from='juliet@example.com/foo'
               to='romeo@example.net'>
        <error type='cancel'>
          <service-unavailable
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

   An application SHOULD return a 'to' attribute,
   this rule applies only <service-unavailable/> error instead
   of <item-not-found/> or <recipient-unavailable/> if sending one of
   the latter errors would provide information about the intended
   recipient's network availability to stanzas received from a registered an entity
   (such as a client) that is connected not authorized
   to know such information.

9.3.3.21.  subscription-required

   The requesting entity is not authorized to access the server.  If the server
   receives requested
   service because a presence stanza with no 'to' attribute, subscription is required; the server associated error type
   SHOULD
   broadcast it to the entities that are subscribed to the sending
   entity's presence, if applicable (the semantics be "auth".

   C: <message
          from='romeo@example.net/orchard'
          to='playbot@shakespeare.example.com'
        <body>help</body>
      </message>

   E: <message
         from='playbot@shakespeare.example.com'
         to='romeo@example.net/orchard'
         type='error'>
        <error type='auth'>
          <subscription-required
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>






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9.3.3.22.  undefined-condition

   The error condition is not one of presence broadcast
   for presence applications are those defined in [XMPP-IM]).  If by the server
   receives an IQ stanza of other
   conditions in this list; any error type "get" or "set" may be associated with no 'to' attribute this
   condition, and it understands the namespace that qualifies the content of SHOULD be used only in conjunction with an
   application-specific condition.

   C: <message
          from='northumberland@shakespeare.lit'
          id='richard2-4.1.247'
          to='kingrichard@royalty.england.lit'>
        <body>My lord, dispatch; read o'er these articles.</body>
        <amp xmlns='http://jabber.org/protocol/amp'>
          <rule action='notify'
                condition='deliver'
                value='stored'/>
        </amp>

   S: <message from='example.org'
               id='amp1'
               to='northumberland@example.net/field'
               type='error'>
        <amp xmlns='http://jabber.org/protocol/amp'
             from='kingrichard@example.org'
             status='error'
             to='northumberland@example.net/field'>
          <rule action='error'
                condition='deliver'
                value='stored'/>
        </amp>
        <error type='modify'>
          <undefined-condition
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
          <failed-rules xmlns='http://jabber.org/protocol/amp#errors'>
            <rule action='error'
                  condition='deliver'
                  value='stored'/>
          </failed-rules>
        </error>
      </message>

9.3.3.23.  unexpected-request

   The recipient or server understood the
   stanza, request but was not expecting
   it MUST either process the stanza directly on behalf of
   sending entity (where the meaning of "process" is determined by at this time (e.g., the
   semantics request was out of order); the qualifying namespace) or return an associated
   error to the
   sending entity.

11.2.  Foreign Domain

   If the hostname of the domain identifier portion of type SHOULD be "wait" or "modify".





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   C: <iq from='romeo@example.net/foo'
          id='unsub1'
          to='pubsub.example.com'
          type='set'>
        <pubsub xmlns='http://jabber.org/protocol/pubsub'>
           <unsubscribe
               node='my_musings'
               jid='romeo@example.net'/>
        </pubsub>
      </iq>

   E: <iq from='pubsub.example.com'
          id='unsub1'
          to='romeo@example.net/foo'
          type='error'>
        <error type='cancel'>
          <unexpected-request
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
          <not-subscribed
              xmlns='http://jabber.org/protocol/pubsub#errors'/>
        </error>
      </iq>

9.3.3.24.  unknown-sender

   The stanza 'from' address specified by a connected client is not
   valid for the JID contained
   in stream (e.g., the 'to' attribute stanza does not match one of the configured hostnames
   of the server itself or include a configured local domain thereof, 'from'
   address when multiple resources are bound to the server
   SHOULD route stream); the
   associated error type SHOULD be "modify".

   C: <message to='romeo@example.net'>
        <body>Wherefore art thou?</body>
      </message>

   S: <message from='romeo@example.net'
               type='error'>
        <body>Wherefore art thou?</body>
        <error type='modify'>
          <unknown-sender xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

9.3.4.  Application-Specific Conditions

   As noted, an application MAY provide application-specific stanza to
   error information by including a properly-namespaced child in the foreign domain (subject to local
   service provisioning and security policies regarding inter-domain
   communication, since such communication is OPTIONAL).  There are two
   possible cases:

   A server-to-server stream already exists between
   error element.  The application-specific element SHOULD supplement or
   further qualify a defined element.  Thus, the <error/> element will



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   contain two domains:
      The sender's server routes or three child elements:

   <iq id='some-id' type='error'>
     <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'>
         [ ... application-specific information ... ]
       </text>
       <special-application-condition xmlns='application-ns'/>
     </error>
   </message>

9.4.  Extended Content

   While the stanza message, presence, and IQ stanzas provide basic semantics
   for messaging, availability, and request-response interactions, XMPP
   uses XML namespaces (see [XML-NAMES] to extend the authoritative server basic stanza
   syntax for the foreign domain over the existing stream
   There exists no server-to-server stream between the two domains:  The
      sender's server (1) resolves purpose of providing additional functionality.  Thus a
   message or presence stanza MAY contain one or more optional child
   elements specifying content that extends the hostname meaning of the foreign domain
      (as defined under Server-to-Server Communications (Section 15.4)),
      (2) negotiates a server-to-server stream between message
   (e.g., an XHTML-formatted version of the two domains
      (as defined under TLS negotiation (Section 6) message body as described in
   [XEP-0071]), and SASL negotiation
      (Section 7)), an IQ stanza of type "get" or "set" MUST contain one
   such child element.  This child element MAY have any name and (3) routes 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 stanza child element.  Such a child element is
   said to the authoritative
      server for the foreign domain over the newly-established stream

   If routing be EXTENDED CONTENT and its namespace name is said to the recipient's server be an
   EXTENDED NAMESPACE.

   Support for any given extended namespace is unsuccessful, OPTIONAL on the sender's
   server MUST return part of
   any implementation.  If an error to the sender; if entity does not understand such a
   namespace, the recipient's server
   can be contacted but delivery by entity's expected behavior depends on whether the recipient's server to
   entity is (1) the recipient or (2) an entity that is unsuccessful, routing the recipient's server MUST return an
   error
   stanza to the sender by way of the sender's server. recipient:







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11.3.  Local Domain


   Recipient:  If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute matches one of the configured hostnames of the
   server, or one of the configured local domains hosted by the server,
   the server MUST either process the stanza itself or route the stanza
   to a specialized service recipient receives a stanza that is responsible for contains a child
      element it does not understand, it SHOULD silently ignore that local domain,
      particular XML data, i.e., it SHOULD not process it or return an error present it
      to the sender (if the service providing the local
   domain is not available).

11.4.  Mere Domain a user or Specific Resource 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 hostname of the domain identifier portion of the JID contained
   in stanza that qualified by the 'to' attribute matches
         unknown namespace SHOULD be ignored.
      *  If an entity receives a configured hostname of the server
   itself and the JID contained in the 'to' attribute message stanza whose only child element
         is of the form
   <domain> or <domain/resource>, the server (or qualified by a defined resource
   thereof) namespace it does not understand, it MUST either process the stanza as appropriate for
         ignore the entire stanza.
      *  If an entity receives an IQ stanza
   kind of type "get" or "set"
         containing a child element qualified by a namespace it does not
         understand, the entity SHOULD return an error IQ stanza to the sender.

11.5.  Node in Same Domain

   If the hostname of the domain identifier portion type
         "error" with an error condition of the JID contained
   in the 'to' attribute matches <service-unavailable/>.
   Router:  If a routing entity (typically a server) handles a configured hostname of the server
   itself and the JID contained in the 'to' attribute is of the form
   <node@domain> or <node@domain/resource>, the server SHOULD deliver
   the stanza to the intended recipient of
      that contains a child element it does not understand, it SHOULD
      ignore the stanza as represented associated XML data by routing or delivering it
      untouched to the JID contained in the 'to' attribute. recipient.


10.  Examples

10.1.  Client-to-Server

   The following rules apply:

   1.  If examples show the JID contains XMPP data flow for a resource identifier (i.e., is of the form
       <node@domain/resource>) and there exists client
   negotiating an XML stream with a connected resource
       that exactly matches the full JID, server, exchanging XML stanzas, and
   closing the recipient's negotiated stream.  The server SHOULD
       deliver the stanza to is "example.com", the stream or connection that exactly
       matches
   server requires use of TLS, the resource identifier.
   2.  If client authenticates via the JID contains a resource identifier SASL
   DIGEST-MD5 mechanism as "juliet@example.com", and there exists no
       connected resource that exactly matches the full JID, the
       recipient's server SHOULD return client binds a <service-unavailable/> stanza
       error
   server-generated resource to the sender.
   3.  If the JID stream.  It is of the form <node@domain> and there exists at least
       one connected resource for the node, assumed that before
   sending the recipient's server
       SHOULD deliver initial stream header, the stanza client has already resolved an
   SRV record of _xmpp-client._tcp.example.com and has opened a TCP
   connection to the advertised port at least one of the connected
       resources, according resolved IP address.

   Note: The alternate steps shown are provided only to application-specific rules.

   Particular XMPP applications MAY specify delivery rules that modify
   or supplement illustrate the foregoing rules; for example, a set of delivery
   rules
   protocol for instant messaging failure cases; they are not exhaustive and presence applications is defined would not
   necessarily be triggered by the data sent in
   [XMPP-IM]. the examples.













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12.  XML Usage

12.1.  Restrictions

   XMPP is


10.1.1.  TLS

   Step 1: Client initiates stream to server:

   C: <stream:stream
        from='juliet@example.com'
        to='example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   Step 2: Server responds by sending a simplified and specialized protocol for streaming XML
   elements in order stream header to exchange structured information in close client:

   S: <stream:stream
        from='example.com'
        id='t7AMCin9zjMNwQKDnplntZPIDEI='
        to='juliet@example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   Step 3: Server sends stream features to real
   time.  Because XMPP does not require the parsing of arbitrary and
   complete XML documents, there client (STARTTLS extension
   only at this point):

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

   Step 4: Client sends STARTTLS command to server:

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

   Step 5: Server informs client that it is no requirement allowed to proceed:

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

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

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

   S: </stream:stream>

   Step 6: Client and server attempt to
   support the full feature set of [XML].  In particular, complete TLS negotiation over



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   the following
   restrictions apply.

   With regard existing TCP connection (see [TLS] for details).

   Step 7: If TLS negotiation is successful, client initiates a new
   stream to XML generation, an XMPP implementation MUST NOT inject
   into an XML server:

   C: <stream:stream
        from='juliet@example.com'
        to='example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

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

10.1.2.  SASL

   Step 8: Server responds by sending a stream header to client along
   with any of the following:

   o  comments (as defined available stream features:

   S: <stream:stream
        from='example.com'
        id='vgKi/bkYME8OAj4rlXMkpucAqe4='
        to='juliet@example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

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

   Step 9: Client selects an authentication mechanism, in Section 2.5 of [XML])
   o  processing instructions (Section 2.6 therein)
   o  internal or external DTD subsets (Section 2.8 therein)
   o  internal or external entity references (Section 4.2 therein) this case
   [DIGEST-MD5] with
      the exception of predefined entities (Section 4.6 therein)
   o  character data or attribute values containing unescaped characters
      that map to the predefined entities (Section 4.6 therein); such
      characters MUST be escaped

   With regard to XML processing, if an empty authorization identity ("="):

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








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   such restricted XML data, it MUST return Core                      July 2007


   Step 10: Server sends a <restricted-xml/> stream
   error.

12.2.  XML Namespace Names and Prefixes

   XML namespaces (see [XML-NAMES]) are used within all XMPP-compliant
   XML [BASE64] encoded challenge to create strict boundaries of data ownership. client:

   S: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
      cmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLHFvcD0i
      YXV0aCIsY2hhcnNldD11dGYtOCxhbGdvcml0aG09bWQ1LXNlc3MK
      </challenge>

   The basic
   function of namespaces is to separate different vocabularies of XML
   elements that are structurally mixed together.  Ensuring that XMPP-
   compliant XML is namespace-aware enables any allowable XML decoded challenge is:

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

   Note: When the server sends a DIGEST-MD5 challenge to be
   structurally mixed with any data element within XMPP.  Rules for XML
   namespace names and prefixes are defined in the following
   subsections.

12.2.1.  Streams Namespace

   A streams namespace declaration is REQUIRED in all XML stream
   headers.  The name of client, the streams namespace MUST
   qop list must be
   'http://etherx.jabber.org/streams'.  The element names of quoted since it is a list rather than a single item
   (even if there is only one item in the
   <stream/> element and list); however, when the
   client sends its <features/> and <error/> children MUST be
   qualified by response to the streams namespace prefix in all instances.  An
   implementation SHOULD generate only server, the 'stream:' prefix for these
   elements, qop must not be quoted
   since it is a single item rather than a list.

   Step 10 (alt): Server returns error to client:

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

   S: </stream:stream>

   Step 11: Client sends a [BASE64] encoded response to the challenge:

   C: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
      dXNlcm5hbWU9Imp1bGlldCIscmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2
      TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAwMDAwMDAx
      LHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20iLHJlc3BvbnNl
      PWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNoYXJzZXQ9dXRmLTgK
      </response>

   The decoded response is:

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

   Step 12: Server informs client of success and includes [BASE64]
   encoded value for historical reasons MAY accept only the 'stream:'
   prefix. subsequent authentication:

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



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12.2.2.  Default Namespace

   A default namespace declaration is REQUIRED and is used in all XML
   streams in order to define the allowable first-level children of the
   root stream element.  This namespace declaration MUST be the same for
   the initial stream and the response stream so that both streams are
   qualified consistently.


   The default namespace declaration applies decoded value for subsequent authentication is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd

   Step 12 (alt): Server returns error to
   the stream and all stanzas sent within client:

   S: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <not-authorized/>
      </failure>

   Step 13: Client initiates a new stream (unless explicitly
   qualified by another namespace, or to server:

   C: <stream:stream
        from='juliet@example.com'
        to='example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

10.1.3.  Resource Binding

   Step 14: Server responds by the prefix of the streams
   namespace or the dialback namespace).

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

   o  jabber:client -- this default namespace is declared when the
      stream is used for communications between sending a stream header to client and a server
   o  jabber:server -- along
   with supported features (in this default namespace case resource binding):

   S: <stream:stream
        from='example.com'
        id='gPybzaOzBmaADgxKXu9UClbprp0='
        to='juliet@example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

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

   Upon being so informed that resource binding is declared when required, the
      stream is used for communications between two servers

   A client implementation
   MUST support bind a resource to the 'jabber:client' default
   namespace, and for historical reasons MAY support only stream; here we assume that default
   namespace.

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

   Note: The 'jabber:client' and 'jabber:server' namespaces are nearly
   identical but are used in different contexts (client-to-server
   communications for 'jabber:client' generate a resource identifier on its behalf.







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   Step 15: Client binds a resource:

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

   Step 16: Server generates resource identifier and server-to-server
   communications for 'jabber:server').  The only difference between informs client of
   successful resource binding:

   S: <iq id='bind_1' type='result'>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <jid>
            juliet@example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb
          </jid>
        </bind>
      </iq>

10.1.4.  Stanza Exchange

   Now the
   two client is that allowed to send XML stanzas over the 'to' and 'from' attributes are OPTIONAL on stanzas
   sent within 'jabber:client', whereas they are REQUIRED on stanzas
   sent within 'jabber:server'.  If a compliant implementation accepts a
   stream that is qualified by the 'jabber:client' or 'jabber:server'
   namespace, it MUST support the common attributes (Section 9.1) negotiated
   stream.

   C: <message
        from='juliet@example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb'
        to='romeo@example.net'
        xml:lang='en'>
        <body>Art thou not Romeo, and
   basic semantics (Section 9.2) of all three core stanza kinds
   (message, presence, a Montague?</body>
      </message>

   If necessary, sender's server negotiates XML streams with intended
   recipient's server (see Section 10.2).

   The intended recipient replies and IQ).

12.2.3.  Dialback Namespace

   A dialback namespace declaration the message is REQUIRED for all elements used in
   server dialback (Appendix C). delivered to the
   client.

   E: <message
        from='romeo@example.net/63a6be96-1ea4-11dc-a38b-000bcd821cgc'
        to='juliet@example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb'
        xml:lang='en'>
        <body>Neither, fair saint, if either thee dislike.</body>
      </message>

   The name client may send and receive an unbounded number of subsequent XML
   stanzas over the dialback namespace
   MUST be 'jabber:server:dialback'.  All elements qualified by this stream.







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   namespace MUST be prefixed.  An implementation SHOULD generate only


10.1.5.  Close

   Desiring to send no further messages, the 'db:' prefix for such elements and MAY accept only client closes the 'db:'
   prefix.

12.3.  Validation

   A stream.

   C: </stream:stream>

   Consistent with the recommended stream closing handshake, server is not responsible for validating
   closes stream as well:

   S: </stream:stream>

   Client now terminates the underlying TCP connection.

10.2.  Server-to-Server Examples

   The following examples show the XML elements forwarded
   to a client or another server; an implementation MAY choose to
   provide only validated data elements but this is OPTIONAL (although flow for a server negotiating an implementation MUST NOT accept
   XML that is not well-formed).
   Clients SHOULD NOT rely on stream with another server, exchanging XML stanzas, and closing
   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 negotiated stream.  Validation of XML streams
   and stanzas  The initiating server ("Server1") is OPTIONAL, and schemas are included herein for
   descriptive purposes only.

12.4.  Inclusion of Text Declaration

   Implementations SHOULD send a text declaration before sending a
   stream header.  Applications MUST follow the rules in [XML] regarding
   example.com; the circumstances under which a text declaration receiving server ("Server2") is included.

12.5.  Character Encoding

   Implementations MUST support the [UTF-8] transformation of Universal
   Character Set ([UCS2]) characters, as required by [CHARSET].
   Implementations MUST NOT attempt to use any other encoding.

12.6.  White Space

   Except where explicitly disallowed (i.e., during TLS negotiation
   (Section 6) example.net and SASL negotiation [SASL]), either entity MAY send
   white space characters (matching production [3] content it
   requires use of [XML])
   within the root stream element as separators between XML stanzas or
   between any other first-level elements sent over TLS; example.com presents a certificate and
   authenticates via the stream; one
   common use for sending such white space characters SASL EXTERNAL mechanism.  It is to check assumed that
   before sending the
   viability initial stream header, Server1 has already
   resolved an SRV record of the underlying _xmpp-client._tcp.example.net and has
   opened a TCP connection after a period of
   inactivity.


13.  Compliance Requirements

   This section summarizes to the specific aspects of advertised port at the Extensible
   Messaging resolved IP
   address.

   Note: The alternate steps shown are provided only to illustrate the
   protocol for failure cases; they are not exhaustive and Presence Protocol that MUST would not
   necessarily be supported triggered by servers and
   clients the data sent 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 examples.

10.2.1.  TLS

   Step 1: Server1 initiates stream to Server2:

   S1: <stream:stream
         from='example.com'
         to='example.net'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>











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   specific application) and instant messaging and presence protocols
   (which MUST be supported only


   Step 2: Server2 responds by instant messaging and presence
   applications built on top of sending a stream tag to Server1:

   S2: <stream:stream
         from='example.net'
         id='hTiXkW+ih9k2SqdGkk/AZi0OJ/Q='
         to='example.com'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>

   Step 3: Server2 sends stream features to Server1:

   S2: <stream:features>
         <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
           <required/>
         </starttls>
       </stream:features>

   Step 4: Server1 sends the core protocols).  Compliance
   requirements STARTTLS command to Server2:

   S1: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   Step 5: Server2 informs Server1 that apply it is allowed to all servers proceed:

   S2: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   Step 5 (alt): Server2 informs Server1 that TLS negotiation has failed
   and clients are specified in
   this section; compliance requirements for instant messaging closes stream:

   S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   S2: </stream:stream>

   Step 6: Server1 and
   presence applications are specified in the corresponding section of
   [XMPP-IM].

13.1.  Servers

   In addition Server2 attempt to all defined requirements with regard complete TLS negotiation via
   TCP.

   Step 7: If TLS negotiation is successful, Server1 initiates a new
   stream to security, XML
   usage, and internationalization, Server2:

   S1: <stream:stream
         from='example.com'
         to='example.net'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>

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



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10.2.2.  SASL

   Step 8: Server2 responds by sending a server MUST support the following
   core protocols in order stream header to be considered compliant:

   o  Conformance Server1 along
   with [IDNA] for domain identifiers, the Nodeprep
      (Appendix A) profile of [STRINGPREP] for node identifiers, and the
      Resourceprep (Appendix B) profile of [STRINGPREP] for resource
      identifiers, and enforcement thereof available stream features (including a preference for clients that authenticate
      with the server.
   o  XML streams (Section 5), including TLS negotiation (Section 6), SASL negotiation (Section 7), and Resource Binding (Section 8)
   o  The basic semantics of
   EXTERNAL mechanism):

   S2: <stream:stream
         from='example.net'
         id='RChdjlgj/TIBcbT9Keu31zDihH4='
         to='example.com'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>

   S2: <stream:features>
         <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
           <mechanism>EXTERNAL</mechanism>
           <mechanism>DIGEST-MD5</mechanism>
           <required/>
         </mechanisms>
       </stream:features>

   Step 9: Server1 selects the three defined stanza kinds (i.e.,
      <message/>, <presence/>, and <iq/>) as specified EXTERNAL mechanism, in stanza
      semantics (Section 9.2)
   o  Generation (and, where appropriate, handling) of error syntax and
      semantics related this case with an
   authorization identity encoded according to streams, TLS, SASL, and XML stanzas

   In addition, for historical reasons a server SHOULD support [BASE64]:

   S1: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
             mechanism='EXTERNAL'/>ZXhhbXBsZS5jb20K</auth>

   The decoded authorization identity is "example.com".

   Step 10: Server2 determines that the
   following core protocol:

   o  Server dialback (Appendix C)

13.2.  Clients

   A client MUST support authorization identity provided
   by Server1 matches the following core protocols information in order to be
   considered compliant:

   o  XML streams (Section 5), including TLS negotiation (Section 6),
      SASL negotiation (Section 7), and Resource Binding (Section 8)
   o  The basic semantics of the three defined stanza kinds (i.e.,
      <message/>, <presence/>, presented certificate and <iq/>) as specified in stanza
      semantics (Section 9.2)
   o  Handling (and, where appropriate, generation)
   therefore returns success:

   S2: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

   Step 11 (alt): Server2 informs Server1 of error syntax and
      semantics related to streams, TLS, SASL, and XML stanzas

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

   S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
         <not-authorized/>
       </failure>

   S2: </stream:stream>








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   o  Conformance


   Step 12: Server1 initiates a new stream to Server2:

   S1: <stream:stream
         from='example.com'
         to='example.net'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>

   Step 13: Server2 responds by sending a stream header to Server1 along
   with [IDNA] for domain identifiers, the Nodeprep
      (Appendix A) profile of [STRINGPREP] for node identifiers, and the
      Resourceprep (Appendix B) profile of [STRINGPREP] for resource
      identifiers.


14.  Internationalization Considerations

   XML streams MUST be encoded any additional features (or, in UTF-8 as specified under Character
   Encoding (Section 12.5).  As specified under Stream Attributes
   (Section 5.3), this case, an empty features
   element):

   S2: <stream:stream
         from='example.net'
         id='MbbV2FeojySpUIP6J91qaa+TWHM='
         to='example.com'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>

   S2: <stream:features/>

10.2.3.  Stanza Exchange

   Now Server1 is allowed to send XML stream SHOULD include an 'xml:lang' attribute
   specifying stanzas to Server2 over the default language
   negotiated stream; here we assume that the transferred stanzas are
   those shown earlier for any client-to-server communication.

   Server1 sends XML character data sent over stanza to Server2:

   S1: <message from='juliet@example.com/balcony'
                to='romeo@example.net'
                xml:lang='en'>
       <body>Art thou not Romeo, and a Montague?</body>
      </message>

   The intended recipient replies and the stream that message is intended to be presented delivered from
   Server2 to a human user.  As
   specified under xml:lang (Section 9.1.5), an Server1.

   Server2 sends XML stanza SHOULD
   include an 'xml:lang' attribute if the stanza contains XML character
   data that is intended to be presented Server1:

   S2: <message from='romeo@example.net/orchard'
                to='juliet@example.com/balcony'
                xml:lang='en'>
         <body>Neither, fair saint, if either thee dislike.</body>
       </message>




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10.2.4.  Close

   Desiring to a human user.  A server
   SHOULD apply send no further messages, Server1 closes the default 'xml:lang' attribute to stanzas it routes or
   delivers on behalf of connected entities, stream.  (In
   practice, the stream would most likely remain open for some time,
   since Server1 and Server2 do not immediately know if the stream will
   be needed for further communication.)

   S1: </stream:stream>

   Consistent with the recommended stream closing handshake, Server2
   closes stream as well:

   S2: </stream:stream>

   Server1 now terminates the underlying TCP connection.


11.  Server Rules for Processing XML Stanzas

   An XMPP server MUST NOT modify or
   delete 'xml:lang' attributes ensure in-order processing of XML stanzas it receives from other between
   any two entities.


15.  Security Considerations

15.1.  High Security

   For the purposes of XMPP communications (client-to-server and server-
   to-server), the term "high security" refers  This includes stanzas sent by a client to its
   server for direct processing by the use server (e.g., in-order processing
   of security
   technologies that provide both mutual authentication a roster get and integrity-
   checking; initial presence as decribed in particular, when using certificate-based authentication [XMPP-IM]).

   Beyond the requirement for in-order processing, each server
   implementation will contain its own logic for processing stanzas it
   receives.  Such logic determines whether the server needs to provide high security, a chain-of-trust SHOULD be established out-
   of-band, although a shared certificate authority signing certificates
   could allow ROUTE a previously unknown certificate
   given stanza to establish trust in-
   band.  See Section 15.2 below regarding certificate validation
   procedures.

   Implementations MUST support high security.  Service provisioning
   SHOULD use high security, subject another domain, DELIVER it to a local security policies.

15.2.  Certificate Validation

   When an XMPP peer communicates entity
   (typically a connected client associated with another peer securely, a local account), or
   HANDLE it MUST
   validate directly within the peer's certificate.  There are three possible cases:

   Case #1: server itself.  The peer contains an End Entity certificate which appears
      to be certified by following rules
   apply.

   Note: Particular XMPP applications MAY specify delivery rules that
   modify or supplement the following rules; for example, a chain set of certificates terminating in a trust
      anchor (as described
   delivery rules for instant messaging and presence applications is
   defined in Section 6.1 [XMPP-IM].

11.1.  No 'to' Address

11.1.1.  Overview

   If the stanza possesses no 'to' attribute, the server SHOULD handle
   it directly on behalf of [X509]). the entity that sent it.  Because all
   stanzas received from other servers MUST possess a 'to' attribute,
   this rule applies only to stanzas received from a local entity (such
   as a client) that is connected to the server.





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   Case #2:  The peer certificate is certified by


11.1.2.  Message

   If the server receives a Certificate
      Authority not known message stanza with no 'to' attribute, it
   SHOULD handle it directly, which may include returning an error to
   the validating peer.
   Case #3:  The peer certificate is self-signed.

   In Case #1, sending entity.

11.1.3.  Presence

   If the validating peer MUST do one of two things:
   1.  Verify server receives a presence stanza with no 'to' attribute, it
   SHOULD broadcast it to the peer certificate according entities that are subscribed to the rules
   sending entity's presence, if applicable (the semantics of [X509].
       The certificate SHOULD then be checked against presence
   broadcast for presence applications are defined in [XMPP-IM]).

11.1.4.  IQ

   If the expected
       identity server receives an IQ stanza of type "get" or "set" with no
   'to' attribute, it MUST do the peer following following:

   1.  If it understands the rules described in [HTTP-TLS],
       except namespace that if present an [ASN.1] Object Identifier qualifies the content of "id-on-
       xmppAddr" (represented as a UTF8String in an otherName entity
       inside the subjectAltName)
       stanza, it MUST be used as either handle the identity.  If one stanza directly on behalf of these checks fails, user-oriented clients MUST either notify
       sending entity (where the user (clients MAY give meaning of "handle" is determined by
       the user semantics of the opportunity qualifying namespace) or return an
       appropriate error to continue
       with the connection in any case) or terminate sending entity.
   2.  If it does not understand the connection with
       a bad certificate error.  Automated clients SHOULD terminate namespace that qualifies the
       connection (with a bad certificate error) and log
       content of the stanza, it MUST return an error to an
       appropriate audit log.  Automated clients MAY provide a
       configuration setting that disables this check, but MUST provide
       a setting that enables it.
   2.  The peer the sending
       entity, which SHOULD show be <service-unavailable/>.

11.2.  Local Domain

   If the certificate to a user for approval,
       including hostname of the entire certificate chain.  The peer MUST cache domain identifier portion of the
       certificate (or some non-forgeable representation such as a
       hash).  In future connections, JID contained
   in the peer MUST verify that 'to' attribute matches one of the same
       certificate was presented and MUST notify configured hostnames of the user if it has
       changed.

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

15.3.  Client-to-Server Communications

   A compliant client implementation
   server itself, the server MUST support both TLS and SASL for
   connections do one of the following:

   1.  Handle the stanza itself.
   2.  Route the stanza to a server.

   The TLS protocol specialized service that is responsible for encrypting XML streams (defined under TLS
   negotiation (Section 6)) 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 SASL
   negotiation (Section 7)) provides a reliable mechanism for validating
       that a client connecting to a server is who it claims local domain.
   3.  Return an error to be.

   Client-to-server communications MUST NOT proceed until the DNS
   hostname asserted by sender if the server has been resolved as specified under
   TCP Binding (Section 4).  If there service providing the local
       domain is a mismatch between not available.

11.3.  Resource at Local Domain

   If the hostname
   to which of the domain identifier portion of the JID contained
   in the 'to' attribute matches a client attempted to connect (e.g., "example.net") configured hostname of the server
   itself and the
   hostname to which JID contained in the client actually connects (e.g.,
   "im.example.net"), 'to' attribute is of the client MUST warn form
   <domain/resource>, the server (or a human user about defined resource thereof) MUST
   either handle the stanza as appropriate for the stanza kind or return
   an error stanza to the sender.




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   mismatch and the human user MUST approve the connection before


11.4.  Node at Local Domain

   If the
   client proceeds; however, hostname of the client MAY allow domain identifier portion of the user to add JID contained
   in the
   presented hostname to 'to' attribute matches a configured set of accepted hostnames in order
   to expedite future connections.

   The IP address and method of access hostname of clients MUST NOT be made
   public by a server, nor are any connections other than the original server connection required.  This helps to protect
   itself and the client's
   server from direct attack JID contained in the 'to' attribute is a bare JID
   (<node@domain>) or identification by third parties.

15.4.  Server-to-Server Communications

   A compliant full JID (<node@domain/resource>), the server implementation MUST support both TLS and SASL for
   inter-domain communications.  For historical reasons, a compliant
   implementation
   SHOULD also support Server Dialback (Appendix C).

   Because service provisioning is a matter of policy, it is OPTIONAL
   for any given domain deliver the stanza to communicate with other domains, and server-
   to-server communications MAY be disabled by the administrator intended recipient of any
   given deployment. the stanza as
   represented by the JID contained in the 'to' attribute.  The
   following rules apply:

   1.  If the JID contains an XMPP resource identifier (i.e., is of the
       form <node@domain/resource>) and there exists a particular domain enables inter-domain
   communications, it connected
       resource that exactly matches the full JID, the recipient's
       server SHOULD enable high security.

   Administrators may want deliver the stanza to require use that connection.
   2.  If the JID contains an XMPP resource identifier and there exists
       no connected resource that exactly matches the full JID, the
       recipient's server SHOULD return a <service-unavailable/> stanza
       error to the sender.
   3.  If the JID is of SASL the form <node@domain> and there exists at least
       one connected resource for server-to-server
   communications in order the node, the recipient's server
       SHOULD deliver the stanza to ensure both authentication at least one of the connected
       resources if the stanza is a message or presence stanza and
   confidentiality (e.g.,
       SHOULD handle it directly on behalf of the node if the stanza is
       an organization's private network).
   Compliant implementations SHOULD support SASL for this purpose.

   Server-to-server communications MUST NOT proceed until IQ stanza.

11.5.  Foreign Domain

11.5.1.  Overview

   If the DNS hostname of the domain identifier portion of the JID contained
   in the 'to' attribute does not match one of the configured hostnames asserted by both servers have been resolved as specified
   under TCP Binding (Section 4).

   Server dialback helps protect against
   of the server itself, the server SHOULD attempt to route the stanza
   to the foreign domain spoofing, thus making it
   more difficult (subject to spoof XML stanzas.  It local service provisioning and
   security policies regarding inter-domain communication, since such
   communication is not a mechanism optional for
   authenticating, securing, or encrypting streams any given deployment).  There are two
   possible cases.

11.5.2.  Existing Stream

   If a server-to-server stream already exists between servers as is
   done via SASL and TLS, and results in weak verification of the two domains,
   the sender's server
   identities only.  Furthermore, it is susceptible shall attempt to DNS poisoning
   attacks unless [DNSSEC] is used, and even if route the DNS information is
   accurate, dialback cannot protect from attacks where stanza to the attacker is
   capable of hijacking
   authoritative server for the IP address of foreign domain over the remote domain.  Domains
   requiring robust security SHOULD use TLS and SASL. existing stream.

11.5.3.  No Existing Stream

   If SASL is used
   for there exists no server-to-server authentication, dialback SHOULD NOT be used
   since it is unnecessary.

15.5.  Order of Layers

   The order of layers in which protocols MUST be stacked is stream between the two domains,
   the sender's server shall proceed as follows:




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   1.  TCP
   2.  TLS
   3.  SASL
   4.  XMPP

   The rationale for this order is that [TCP] is  Resolve the base connection
   layer used by all hostname of the protocols stacked on top of TCP, [TLS] is
   often provided at the operating system layer, [SASL] is often
   provided at foreign domain (as defined under
       Section 15.4).
   2.  Negotiate a server-to-server stream between the application layer, two domains (as
       defined under Section 6 and XMPP is Section 7).
   3.  Route the application
   itself.

15.6.  Lack of SASL Channel Binding to TLS

   The SASL framework does not provide a mechanism to bind SASL
   authentication to a security layer providing confidentiality and
   integrity protection that was negotiated at a lower layer.  This lack
   of a "channel binding" prevents SASL from being able stanza to verify that the source and destination end points authoritative server for the foreign
       domain over the newly-established stream.

11.5.4.  Error Handling

   If routing to which the lower layer's
   security intended recipient's server is bound are equivalent unsuccessful, the
   sender's server MUST return an error to the end points that SASL sender, which SHOULD be
   <remote-server-not-found/> if resolution of the foreign domain is
   authenticating.
   unsuccessful and <remote-server-timeout/> if resolution succeeds but
   streams cannot be negotiated.

   If stream negotiation with the end points are not identical, intended recipient's server is
   successful but the lower
   layer's security foreign server cannot be trusted deliver the stanza to protect data transmitted
   between the SASL authenticated entities.  In such a situation, a SASL
   security layer should be negotiated that effectively ignores
   recipient, the
   presence foreign server shall return an error to the sender by
   way of the lower layer security.

15.7.  Mandatory-to-Implement Technologies

   At sender's server.


12.  XML Usage

12.1.  Restrictions

   XMPP is 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) simplified and specialized protocol for both:  TLS plus SASL PLAIN for client-to-server connections and
      TLS plus SASL EXTERNAL for server-to-server connections (using the
      TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher supporting peer certificates)

   Naturally, implementations MAY support other ciphers with TLS and MAY
   support other SASL mechanisms.

15.8.  Firewalls

   Communications using streaming XML
   elements in order to exchange structured information in close to real
   time.  Because XMPP normally occur over [TCP] connections on
   port 5222 (client-to-server) or port 5269 (server-to-server), as
   registered with does not require the IANA (see IANA Considerations (Section 16)).  Use parsing of these well-known ports allows administrators to easily enable or
   disable XMPP activity through existing arbitrary and commonly-deployed
   firewalls.



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15.9.  Use needs to
   support the full feature set of base64 in SASL

   Both [XML].  In particular, the client and following
   features of XML are prohibited in XMPP:

   o  comments (as defined in Section 2.5 of [XML])
   o  processing instructions (Section 2.6 therein)
   o  internal or external DTD subsets (Section 2.8 therein)
   o  internal or external entity references (Section 4.2 therein) with
      the server MUST verify any [BASE64] data received
   during SASL negotiation exception of predefined entities (Section 7).  An implementation MUST reject
   (not ignore) any 4.6 therein)
   o  character data or attribute values containing unescaped characters
      that are not explicitly allowed by the
   base64 alphabet; this helps map to guard against creation of a covert
   channel that could the predefined entities (Section 4.6 therein); such
      characters MUST be used escaped

   An XMPP implementation MUST behave as follow with regard to "leak" information. these
   features:

   1.  An XMPP implementation MUST NOT break on invalid input and MUST reject any sequence of
   base64 inject characters containing the pad ('=') character if matching such
       features into an XML stream.





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   2.  If an XMPP implementation receives characters matching such
       features over an XML stream, it MUST return a stream error, which
       SHOULD be <restricted-xml/> but MAY be <bad-format/>.

12.2.  XML Namespace Names and Prefixes

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

12.2.1.  Streams Namespace

   A streams namespace declaration is REQUIRED in all XML stream headers
   and the name of the data (e.g., "=AAA" or "BBBB=CCC"); streams namespace MUST be
   'http://etherx.jabber.org/streams'.  If this helps rule is violated, the
   entity that receives the offending stream header MUST return a stream
   error to guard against
   buffer overflow attacks and other attacks on the implementation.
   Base 64 encoding visually hides otherwise easily recognized
   information, such as passwords, sending entity, which SHOULD be <invalid-namespace/> but does not provide any
   computational confidentiality.  Base 64 encoding
   MAY be <bad-format/>.

   The element names of the <stream/> element and its <features/> and
   <error/> children MUST follow be qualified by the
   definition streams namespace prefix
   in Section 4 of [BASE64].

15.10.  Stringprep Profiles

   XMPP makes use of all instances.  If this rule is violated, the [NAMEPREP] profile of [STRINGPREP] for
   processing of domain identifiers; for security considerations related
   to Nameprep, refer entity that receives
   the offending element MUST return a stream error to the appropriate section of [NAMEPREP].

   In addition, XMPP defines two profiles of [STRINGPREP]: Nodeprep
   (Appendix A) sending
   entity, which SHOULD be <bad-format/>.

   An implementation SHOULD generate only the 'stream:' prefix for node identifiers these
   elements, and Resourceprep (Appendix B) for
   resource identifiers.

   The Unicode and ISO/IEC 10646 repertoires have many characters that
   look similar.  In many cases, users of security protocols might do
   visual matching, such as when comparing historical reasons MAY accept only the names of trusted third
   parties.  Because it is impossible to map similar-looking characters
   without 'stream:'
   prefix.  If an entity receives a great deal of context, such as knowing the fonts used,
   stringprep stream header with a streams
   namespace prefix it does nothing to map similar-looking characters together,
   nor not accept, it MUST return a stream error to prohibit some characters because they look like others.

   A node identifier can
   the sending entity, which SHOULD be employed as one part of an entity's address
   in XMPP.  One common usage <bad-namespace-prefix/> but MAY
   be <bad-format/>.

12.2.2.  Default Namespace

   A default namespace declaration is as the username of an instant messaging
   user; another REQUIRED and is as used in all XML
   streams in order to define the name of a multi-user chat room; many other
   kinds of entities could use node identifiers as part of their
   addresses.  The security allowable first-level children of such services could the
   root stream element.  This namespace declaration MUST be compromised based
   on different interpretations of the internationalized node
   identifier; same for example, a user entering a single internationalized
   node identifier could access another user's account information, or a
   user could gain access
   the initial stream and the response stream so that both streams are
   qualified consistently.  The default namespace declaration applies to an otherwise restricted chat room
   the stream and all stanzas sent within a stream (unless explicitly
   qualified by another namespace or
   service.

   A resource identifier can be employed as one part by the prefix of an entity's
   address in XMPP.  One common usage is as the name for an instant streams
   namespace).




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   messaging user's connected resource; another is as


   A server implementation MUST support the nickname of a
   user in a multi-user chat room; many other kinds of entities could
   use resource identifiers as part of their addresses.  The security of
   such services could be compromised based on different interpretations
   of following two default
   namespaces (for historical reasons, an implementation MAY support
   only these two default namespaces):

   o  jabber:client -- this default namespace is declared when the internationalized resource identifier;
      stream is used for example, a user
   could attempt to initiate multiple connections with the same name, or communication between a user could send client and a message to someone other than server
   o  jabber:server -- this default namespace is declared when the intended
   recipient in a multi-user chat room.


16.  IANA Considerations

16.1.  XML Namespace Name
      stream is used for TLS Data communication between two servers

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

   URI:  urn:ietf:params:xml:ns:xmpp-tls
   Specification:  RFC 3920
   Description:  This for historical reasons MAY support only that default
   namespace.

   If an implementation accepts a stream that is qualified by the XML
   'jabber:client' or 'jabber:server' namespace, it MUST support the
   common attributes (Section 9.1) and basic semantics (Section 9.2) of
   all three core stanza types (message, presence, and IQ).

   An implementation MUST NOT generate namespace name prefixes for TLS-related data in elements
   qualified by the Extensible Messaging default namespace if the default namespace is
   'jabber:client' or 'jabber:server'.

   Note: The 'jabber:client' and Presence Protocol (XMPP) as defined
      by RFC 3920.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

16.2.  XML Namespace Name 'jabber:server' namespaces are nearly
   identical but are used in different contexts (client-to-server
   communication for SASL Data

   A URN sub-namespace 'jabber:client' and server-to-server communication
   for SASL-related data in 'jabber:server').  The only difference between the Extensible Messaging
   and Presence Protocol (XMPP) two is defined as follows.  (This namespace
   name adheres to that
   the format defined in [XML-REG].)

   URI:  urn:ietf:params:xml:ns:xmpp-sasl
   Specification:  RFC 3920
   Description:  This is 'to' and 'from' attributes are OPTIONAL on stanzas sent over XML
   streams qualified by the 'jabber:client' namespace, whereas they are
   REQUIRED on stanzas sent over XML streams qualified by the 'jabber:
   server' namespace.

12.2.3.  Extended Namespaces

   An EXTENDED NAMESPACE is an XML namespace name for SASL-related data in (other than the Extensible Messaging and Presence Protocol (XMPP) as defined
      by RFC 3920.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

16.3. default
   namespace) that is used to qualify XML Namespace Name for Stream Errors

   A URN sub-namespace for stream-related error data contained within an XML
   stanza.  For example, in the Extensible
   Messaging and Presence Protocol (XMPP) is defined as follows.  (This following stanza, the extended namespace name adheres to
   is 'jabber:iq:roster':

   <iq from='juliet@capulet.com/balcony'
       id='roster1'
       type='get'>
    <query xmlns='jabber:iq:roster'/>
   </iq>

   An XML stanza MAY contain XML data qualified by more than one
   extended namespace, either at the format defined direct child level of the stanza
   (for presence and message stanzas) or in [XML-REG].) any mix of levels (for all
   stanzas).



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   URI:  urn:ietf:params:xml:ns:xmpp-streams
   Specification:  RFC 3920
   Description:  This is


   <presence from='juliet@capulet.com/balcony'>
     <c xmlns='http://jabber.org/protocol/caps'
        node='http://exodus.jabberstudio.org/caps'
        ver='0.9'/>
     <x xmlns='vcard-temp:x:update'>
       <photo>sha1-hash-of-image</photo>
     </x>
   </presence>


   <message to='juliet@capulet.com'>
     <body>Hello?</body>
     <html xmlns='http://jabber.org/protocol/xhtml-im'>
       <body xmlns='http://www.w3.org/1999/xhtml'>
         <p style='font-weight:bold'>Hello?</t>
       </body>
     </html>
   </message>


   <iq from='juliet@capulet.com/balcony'
       id='roster2'
       type='get'>
    <query xmlns='jabber:iq:roster'>
       <headers xmlns='http://jabber.org/protocol/shim'>
         <header name='If-None-Match'>some-long-opaque-string</header>
       </headers>
    </query>
   </iq>

   An implementation SHOULD NOT generate namespace prefixes for elements
   qualified by content (as opposed to stream) namespaces other than
   'jabber:client' and 'jabber:server'.  However, if included, the XML
   namespace name declarations for stream-related error
      data in those prefixes MUST be included on the Extensible Messaging
   stanza root or a child thereof, not at the level of the stream
   element (this helps to ensure that any such namespace declaration is
   routed and Presence Protocol (XMPP) as
      defined by RFC 3920.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

16.4.  XML Namespace Name for Resource Binding delivered with the stanza, instead of assumed from the
   stream).

12.3.  Validation

   A URN sub-namespace server is not responsible for resource binding in the Extensible Messaging
   and Presence Protocol (XMPP) ensuring that XML data delivered to a
   client or routed to another server is defined valid as follows.  (This namespace
   name adheres to the format term "valid" is
   defined in [XML-REG].)

   URI:  urn:ietf:params:xml:ns:xmpp-bind
   Specification:  RFC 3920
   Description:  This Section 2.8 of [XML].  An implementation MAY choose to
   provide only validated data, but such behavior is OPTIONAL.  A client
   SHOULD NOT rely on the XML namespace name for resource binding in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by RFC 3920.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

16.5.  XML Namespace Name for Stanza Errors

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

   URI:  urn:ietf:params:xml:ns:xmpp-stanzas
   Specification:  RFC 3920
   Description:  This is the XML namespace name for stanza-related error send data in that does not conform to
   the Extensible Messaging schemas, and Presence Protocol (XMPP) as
      defined by RFC 3920.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

16.6.  Nodeprep Profile of Stringprep

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

   Name of this profile:

      Nodeprep

   RFC in which SHOULD ignore any non-conformant elements or
   attributes on the profile is defined: incoming XML stream.



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      RFC 3920

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

      This is the first version of Nodeprep

16.7.  Resourceprep Profile of Stringprep


   Note: The Resourceprep profile of stringprep terms "valid" and "well-formed" are distinct in XML.  All
   XMPP data MUST be well-formed as the term "well-formed" is defined under Resourceprep
   (Appendix B).  The IANA has registered Resourceprep in the stringprep
   profile registry.

   Name
   Section 2.1 of this profile:

      Resourceprep

   RFC [XML].

12.4.  Inclusion of Text Declaration

   Implementations SHOULD send a text declaration before sending a
   stream header.  Applications MUST follow the rules in which [XML] regarding
   the profile is defined:

      RFC 3920

   Indicator whether or not this circumstances under which a text declaration is included.

12.5.  Character Encoding

   Implementations MUST support the newest version of the profile:

      This is the first version [UTF-8] transformation of Resourceprep

16.8.  GSSAPI Service Name

   The IANA has registered "xmpp" Universal
   Character Set ([UCS2]) characters, as required by [CHARSET].
   Implementations MUST NOT attempt to use any other encoding.  If an
   implementation detects that an entity has attempted to send XML data
   with an encoding other than UTF-8, it MUST return a GSSAPI [GSS-API] service name, as
   defined under SASL Definition stream error,
   which SHOULD be <unsupported-encoding/>.

12.6.  White Space

   Except where explicitly disallowed (i.e., during TLS negotiation
   (Section 7.3).

16.9.  Port Numbers

   The IANA has registered "xmpp-client" 6) and "xmpp-server" SASL negotiation (Section 7)), either entity MAY send
   white space characters (matching production [3] content of [XML])
   within the root stream element as keywords separators between XML stanzas or
   between any other first-level elements sent over the stream; one
   common use for [TCP] ports 5222 sending such white space characters is to check the
   viability of the underlying TCP connection after a period of
   inactivity.


13.  Compliance Requirements

   This section summarizes the specific aspects of the Extensible
   Messaging and 5269 respectively.

   These ports 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 used for client-to-server 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 server-to-server
   communications respectively, but their use is OPTIONAL.


17.  References

17.1.  Normative References

   [ABNF]     Crocker, D. instant messaging and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

   [BASE64]   Josefsson, S., "The Base16, Base32, presence protocols
   (which MUST be supported only by instant messaging and Base64 Data
              Encodings", RFC 4648, October 2006. presence
   applications built on top of the core protocols).  Compliance
   requirements that apply to all servers and clients are specified in
   this section; compliance requirements for instant messaging and
   presence applications are specified in the corresponding section of
   [XMPP-IM].





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

   [DIGEST-MD5]
              Leach, P.


13.1.  Servers

   In addition to all defined requirements with regard to security, XML
   usage, and C. Newman, "Using Digest Authentication as internationalization, a
              SASL Mechanism", RFC 2831, May 2000.

   [DNS-SRV]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR server MUST support the following
   core protocols in order to be considered compliant:

   o  Conformance with [IDNA] for
              specifying domain identifiers, the location Nodeprep
      (Appendix A) profile of services (DNS SRV)", RFC 2782,
              February 2000.

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

   [GSS-API]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [HMAC]     National Institute the
      Resourceprep (Appendix B) profile of Standards and Technology, "The
              Keyed-Hash Message Authentication Code (HMAC)", FIPS PUB
              198, March 2002, <http://csrc.nist.gov/publications/fips/
              fips198/fips-198a.pdf>.

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

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

   [IPv6]     Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [LANGTAGS]
              Phillips, A. and M. Davis, "Tags [STRINGPREP] for Identifying
              Languages", BCP 47, RFC 4646, September 2006.

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

   [PUNYCODE]
              Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

   [RANDOM]   Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements resource
      identifiers, as well as enforcement thereof for Security", BCP 106, RFC 4086, June 2005.



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   [SASL]     Melnikov, A. clients that
      authenticate with the server.
   o  XML streams (Section 5), including TLS negotiation (Section 6),
      SASL negotiation (Section 7), and K. Zeilenga, "Simple Authentication Resource Binding (Section 8)
   o  The basic semantics of the three defined stanza types (i.e.,
      <message/>, <presence/>, and
              Security Layer (SASL)", RFC 4422, June 2006.

   [SHA]      National Institute <iq/>) as specified in stanza
      semantics (Section 9.2)
   o  Generation (and, where appropriate, handling) of Standards error syntax and Technology, "Secure
              Hash Standard", FIPS PUB 180-2, August 2002, <http://
              csrc.nist.gov/publications/fips/fips180-2/
              fips180-2withchangenotice.pdf>.

   [STRINGPREP]
              Hoffman, P.
      semantics related to streams, TLS, SASL, and M. Blanchet, "Preparation XML stanzas

   For backward compatibility with the large deployed base of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

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

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

   [TLS]      Dierks, T. implement the server
   dialback protocol first specified in [RFC3920] and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [UCS2]     International Organization now documented in
   [XEP-0220], since that protocol is widely used for Standardization,
              "Information Technology - Universal Multiple-octet coded
              Character Set (UCS) - Amendment 2: UCS Transformation
              Format 8 (UTF-8)", ISO Standard 10646-1 Addendum 2,
              October 1996.

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

   [X509]     Housley, R., Polk, W., Ford, W., peer servers in the absence of domain certificates.

13.2.  Clients

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

   o  XML streams (Section 5), including TLS negotiation (Section 6),
      SASL negotiation (Section 7), and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate Resource Binding (Section 8)
   o  The basic semantics of the three defined stanza types (i.e.,
      <message/>, <presence/>, and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C., <iq/>) as specified in Section 9.2
   o  Handling (and, where appropriate, generation) of error syntax and E. Maler,
              "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C REC-
              xml, October 2000, <http://www.w3.org/TR/REC-xml>.

   [XML-NAMES]
              Bray, T., Hollander, D.,
      semantics related to streams, TLS, SASL, and A. Layman, "Namespaces XML stanzas

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

   o  Conformance with [IDNA] for domain identifiers, the Nodeprep
      (Appendix A) profile of [STRINGPREP] for node identifiers, and the
      Resourceprep (Appendix B) profile of [STRINGPREP] for resource
      identifiers.


14.  Internationalization Considerations

   XML streams MUST be encoded in
              XML", W3C REC-xml-names, January 1999,
              <http://www.w3.org/TR/REC-xml-names>. UTF-8 as specified under Section 12.5.



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17.2.  Informative References

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

   [ASN.1]    CCITT, "Recommendation X.208: Specification of Abstract
              Syntax Notation One (ASN.1)", 1988.

   [DNSSEC]   Arends, R., Austein, R., Larson, M., Massey, D.,


   As specified under Section 5.3, an XML stream SHOULD include an 'xml:
   lang' attribute specifying the default language for any XML character
   data sent over the stream that is intended to be presented to a human
   user.  As specified under Section 9.1.5, an XML stanza SHOULD include
   an 'xml:lang' attribute if the stanza contains XML character data
   that is intended to be presented to a human user.  A server SHOULD
   apply the default 'xml:lang' attribute to stanzas it routes or
   delivers on behalf of connected entities, and S.
              Rose, "DNS MUST NOT modify or
   delete 'xml:lang' attributes on stanzas it receives from other
   entities.


15.  Security Introduction and Requirements",
              RFC 4033, March 2005.

   [DNS-TXT]  Rosenbaum, R., "Using Considerations

15.1.  High Security

   For the Domain Name System To Store
              Arbitrary String Attributes", RFC 1464, May 1993.

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

   [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

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

   [IRI]      Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [LINKLOCAL]
              Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration purposes of IPv4 Link-Local Addresses", RFC 3927,
              May 2005.

   [MAILBOXES]
              Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
              FUNCTIONS", RFC 2142, May 1997.

   [POP3]     Myers, J. XMPP communication (client-to-server and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [STD13]    Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.




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   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [URN-OID]  Mealling, M., "A URN Namespace server-
   to-server), the term "high security" refers to the use of Object Identifiers",
              RFC 3061, February 2001.

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

   [XEP-0045]
              Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
              April 2007.

   [XEP-0071]
              Saint-Andre, P., "XHTML-IM", XSF XEP 0071, March 2007.

   [XEP-0077]
              Saint-Andre, P., "In-Band Registration", XSF XEP 0077,
              January 2006.

   [XEP-0086]
              Norris, R. and P. Saint-Andre, "Error Condition Mappings",
              XSF XEP 0086, February 2004.

   [XEP-0124]
              Paterson, I., Smith, D., and P. Saint-Andre,
              "Bidirectional-streams Over Synchronous HTTP (BOSH)", XSF
              XEP 0124, February 2007.

   [XEP-0156]
              Hildebrand, J. and P. Saint-Andre, "Discovering
              Alternative XMPP Connection Methods", XSF XEP 0156,
              January 2007.

   [XEP-0157]
              Saint-Andre, P. security
   technologies that provide both mutual authentication and J. Konieczny, "Contact Addresses for integrity-
   checking; in particular, when using certificate-based authentication
   to provide high security, a chain-of-trust SHOULD be established out-
   of-band, although a shared certificate authority signing certificates
   could allow a previously unknown certificate to establish trust in-
   band.  See Section 15.2 regarding certificate validation procedures.

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

15.2.  Certificate Validation

   When an XMPP Services", XSF XEP 0157, January 2007.

   [XEP-0174]
              Saint-Andre, P., "Link-Local Messaging", XSF XEP 0174,
              March 2007.

   [XEP-0175]
              Saint-Andre, P., "Best Practices for Use peer communicates with another peer securely, it MUST
   validate the peer's certificate.  There are three possible cases:

   Case #1:  The peer contains an End Entity certificate that appears to
      be certified by a chain of SASL
              ANONYMOUS", XSF XEP 0175, September 2006. certificates terminating in a trust
      anchor (as described in Section 6.1 of [X509]).
   Case #2:  The peer certificate is certified by a Certificate
      Authority not known to the validating peer.
   Case #3:  The peer certificate is self-signed.

   In Case #1, the validating peer MUST do one of two things:
   1.  Verify the peer certificate according to the rules of [X509].
       The certificate SHOULD then be checked against the expected
       identity of the peer following the rules described in [HTTP-TLS],
       except that if present an [ASN.1] Object Identifier of "id-on-
       xmppAddr" (represented as a UTF8String in an otherName entity
       inside the subjectAltName) MUST be used as the identity.  If one
       of these checks fails, user-oriented clients MUST either notify



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   [XEP-0178]
              Saint-Andre, P. and P. Millard, "Best Practices for Use of
              SASL EXTERNAL


       the user (clients MAY give the user the opportunity to continue
       with Certificates", XSF XEP 0178,
              February 2007.

   [XEP-0206]
              Paterson, I., "XMPP Over BOSH", XSF XEP 0206,
              February 2007.

   [XML-REG]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [XMPP-IM]  Saint-Andre, P., "Extensible Messaging the connection anyway) or terminate the connection with a
       bad certificate error.  Automated clients SHOULD terminate the
       connection (with a bad certificate error) and log the error to an
       appropriate audit log.  Automated clients MAY provide a
       configuration setting that disables this check, but MUST provide
       a setting that enables it.
   2.  The peer SHOULD show the certificate to a user for approval,
       including the entire certificate chain.  The peer MUST cache the
       certificate (or some non-forgeable representation such as a
       hash).  In future connections, the peer MUST verify that the same
       certificate was presented and Presence
              Protocol (XMPP): Instant Messaging MUST notify the user if it has
       changed.

   In Case #2 and  Presence",
              draft-saintandre-rfc3921bis-02 (work Case #3, implementations SHOULD act as in progress),
              April 2007.

   [XMPP-URI]
              Saint-Andre, P., "Internationalized Resource Identifiers
              (IRIs) Rule #2 for
   Case #1.

15.3.  Client-to-Server Communication

   A compliant client implementation MUST support both TLS and Uniform Resource  Identifiers (URIs) SASL for
   connections to a server.

   The TLS protocol for encrypting XML streams (defined under Section 6)
   provides a reliable mechanism for helping to ensure the
              Extensible Messaging
   confidentiality and Presence Protocol (XMPP)",
              draft-saintandre-rfc4622bis-00 (work in progress),
              April 2007.


Appendix A.  Nodeprep

A.1.  Introduction

   This appendix defines the "Nodeprep" profile data integrity of [STRINGPREP].  As
   such, it specifies processing rules data exchanged between two
   entities.

   The SASL protocol for authenticating XML streams (defined under
   Section 7) provides a reliable mechanism for validating that will enable users a client
   connecting to a server is who it claims to be.

   Client-to-server communication MUST NOT proceed until the DNS
   hostname asserted by the server has been resolved as specified under
   Section 4.  If there is a mismatch between the hostname to which a
   client attempted to connect (e.g., "example.net") and the hostname to
   which the client actually connects (e.g., "xmpp.example.net"), the
   client MUST warn a human user about the mismatch and the human user
   MUST approve the connection before the client proceeds; however, the
   client MAY also allow the user to enter
   internationalized node identifiers in add the Extensible Messaging and
   Presence Protocol (XMPP) presented hostname to a
   configured set of accepted hostnames in order to expedite future
   connections.

   The IP address and have the highest chance method of getting the
   content access of clients MUST NOT be made
   public by a server, nor are any connections other than the strings correct.  (An XMPP node identifier is original
   server connection required.  This helps to protect the
   optional portion of an client's
   server from direct attack or identification by third parties.





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15.4.  Server-to-Server Communication

   A compliant server implementation MUST support both TLS and SASL for
   inter-domain communication.

   Because service provisioning is a matter of policy, it is optional
   for any given domain identifier to communicate with other domains, and server-
   to-server communication may be disabled by the '@' separator; administrator of any
   given deployment.  If a particular domain enables inter-domain
   communication, it is often but not exclusively associated
   with an instant messaging username.)  These processing rules are
   intended only should enable high security.

   Administrators may want to require use of SASL for XMPP node identifiers server-to-server
   communication in order to ensure both authentication and are not intended
   confidentiality (e.g., on an organization's private network).
   Compliant implementations SHOULD support SASL for
   arbitrary text or any other aspect this purpose.

   Server-to-server communication MUST NOT proceed until the DNS
   hostnames asserted by both servers have been resolved as specified
   under Section 4.

15.5.  Order of an Layers

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

   1.  TCP
   2.  TLS
   3.  SASL
   4.  XMPP address.

   This profile defines

   The rationale for this order is that [TCP] is the following, as required base connection
   layer used by [STRINGPREP]:

   o  The intended applicability all of the profile: internationalized node
      identifiers within XMPP
   o  The character repertoire that protocols stacked on top of TCP, [TLS] is
   often provided at the operating system layer, [SASL] is often
   provided at the input application layer, and output to
      stringprep: Unicode 3.2, specified in Section 2 of this Appendix





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   o  The mappings used: specified in Section 3
   o  The Unicode normalization used: specified in Section 4
   o is the application
   itself.

15.6.  Lack of SASL Channel Binding to TLS

   The characters SASL framework does not provide a mechanism to bind SASL
   authentication to a security layer providing confidentiality and
   integrity protection that are prohibited as output: specified in Section
      5
   o  Bidirectional character handling: specified in Section 6

A.2.  Character Repertoire was negotiated at a lower layer.  This profile uses Unicode 3.2 with the list lack
   of unassigned code points a "channel binding" prevents SASL from being Table A.1, both defined in Appendix A of [STRINGPREP].

A.3.  Mapping

   This profile specifies mapping using able to verify that
   the following tables from
   [STRINGPREP]:

      Table B.1
      Table B.2

A.4.  Normalization

   This profile specifies source and destination end points to which the use of Unicode normalization form KC, as
   described in [STRINGPREP].

A.5.  Prohibited Output

   This profile specifies lower layer's
   security is bound are equivalent to the prohibition of using end points that SASL is
   authenticating.  If the end points are not identical, the following tables
   from [STRINGPREP].

      Table C.1.1
      Table C.1.2
      Table C.2.1
      Table C.2.2
      Table C.3
      Table C.4
      Table C.5
      Table C.6
      Table C.7
      Table C.8
      Table C.9 lower
   layer's security cannot be trusted to protect data transmitted
   between the SASL authenticated entities.  In addition, such a situation, a SASL
   security layer SHOULD be negotiated that effectively ignores the following Unicode characters are also prohibited:

      #x22 (")
   presence of the lower layer security.



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      #x26 (&)
      #x27 (')
      #x2F (/)
      #x3A (:)
      #x3C (<)
      #x3E (>)
      #x40 (@)

A.6.  Bidirectional Characters

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


Appendix B.  Resourceprep

B.1.  Introduction

   This appendix defines the "Resourceprep" profile of [STRINGPREP].  As
   such, it specifies processing rules that will enable users to enter
   internationalized resource identifiers in


15.7.  Mandatory-to-Implement Technologies

   At a minimum, all implementations MUST support the Extensible Messaging
   and Presence Protocol (XMPP) and have following
   mechanisms:

   for authentication:  the highest chance of getting SASL [DIGEST-MD5] mechanism
   for confidentiality:  TLS (using the content of TLS_RSA_WITH_3DES_EDE_CBC_SHA
      cipher)
   for both:  TLS plus SASL PLAIN for client-to-server connections and
      TLS plus SASL EXTERNAL for server-to-server connections (using the strings correct.  (An
      TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher supporting peer certificates)

   Naturally, implementations MAY support other ciphers with TLS and MAY
   support other SASL mechanisms.

15.8.  Firewalls

   Communication using XMPP resource identifier is normally occurs over [TCP] connections on
   port 5222 (client-to-server) or port 5269 (server-to-server), as
   registered with the optional portion IANA (see Section 16).  Use of an these well-known
   ports allows administrators to easily enable or disable XMPP address that follows a domain
   identifier activity
   through existing and commonly-deployed firewalls.

15.9.  Use of base64 in SASL

   Both the '/' separator.)  These processing rules are
   intended only for XMPP resource identifiers client and the server MUST verify any [BASE64] data received
   during SASL negotiation (Section 7).  An implementation MUST reject
   (not ignore) any characters that are not intended for
   arbitrary text or any other aspect of an XMPP address.

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

   o  The intended applicability of the profile: internationalized
      resource identifiers within XMPP
   o  The character repertoire
   base64 alphabet; this helps to guard against creation of a covert
   channel that is the could be used to "leak" information.  An implementation
   MUST NOT break on invalid input and output to
      stringprep: Unicode 3.2, specified in Section 2 MUST reject any sequence of this Appendix
   o  The mappings used: specified in Section 3
   o  The Unicode normalization used: specified in Section 4
   o  The
   base64 characters containing the pad ('=') character if that are prohibited
   character is included as output: specified in Section
      5
   o  Bidirectional something other than the last character handling: specified of
   the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against
   buffer overflow attacks and other attacks on the implementation.
   Base 64 encoding visually hides otherwise easily recognized
   information, such as passwords, but does not provide any
   computational confidentiality.  Base 64 encoding MUST follow the
   definition in Section 6

B.2.  Character Repertoire

   This 4 of [BASE64] and padding bits MUST be set to
   zero.

15.10.  Stringprep Profiles

   XMPP makes use of the [NAMEPREP] profile uses Unicode 3.2 with of [STRINGPREP] for
   processing of domain identifiers; for security considerations related
   to Nameprep, refer to the list appropriate section of unassigned code points
   being Table A.1, both defined in Appendix A [NAMEPREP].

   In addition, XMPP defines two profiles of [STRINGPREP]. [STRINGPREP]: Nodeprep



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B.3.  Mapping

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

      Table B.1

B.4.  Normalization

   This profile specifies the use of


   (Appendix A) for node identifiers and Resourceprep (Appendix B) for
   resource identifiers.

   The Unicode normalization form KC, as
   described in [STRINGPREP].

B.5.  Prohibited Output

   This profile specifies the prohibition and ISO/IEC 10646 repertoires have many characters that
   look similar.  In many cases, users of using the following tables
   from [STRINGPREP].

      Table C.1.2
      Table C.2.1
      Table C.2.2
      Table C.3
      Table C.4
      Table C.5
      Table C.6
      Table C.7
      Table C.8
      Table C.9

B.6.  Bidirectional Characters

   This profile specifies checking bidirectional strings, security protocols might do
   visual matching, such as described
   in Section 6 when comparing the names of [STRINGPREP].


Appendix C.  Server Dialback

C.1.  Overview

   Server dialback is a reverse DNS lookup method whose results are
   communicated over XML streams, thus making trusted third
   parties.  Because it more difficult to spoof
   XMPP server domains and XML stanzas sent over XML streams between
   servers.  Server dialback is not impossible to map similar-looking characters
   without a security mechanism, and results
   only in weak verification great deal of server identities (see Server-to-Server
   Communications (Section 15.4) regarding this method's security
   characteristics).  Domains requiring robust security SHOULD use TLS
   and SASL; see Server-to-Server Communications (Section 15.4) for
   details.  If SASL is used for server-to-server authentication,
   dialback SHOULD NOT context, such as knowing the fonts used,
   stringprep does nothing to map similar-looking characters together,
   nor to prohibit some characters because they look like others.

   A node identifier can be used since it employed as one part of an entity's address
   in XMPP.  One common usage is unnecessary.  Documentation



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   user; another is included mainly for as the sake name of backward-compatibility
   with existing implementations and deployments.  However, depending on
   local policies, a service may wish to multi-user conference room; many
   other kinds of entities could use dialback to provide weak
   identity verification in cases where SASL negotiation would not
   result in strong authentication (e.g., because the certificate
   presented by the peer service during TLS negotiation is self-signed
   and thus provides even weaker identity verification than DNS). node identifiers as part of their
   addresses.  The server dialback method is made possible by the existence security of the
   Domain Name System (DNS), since one server can (normally) discover
   the authoritative server for a given domain.  Because dialback
   depends such services could be compromised based
   on DNS, inter-domain communications MUST NOT proceed until
   the Domain Name System (DNS) hostnames asserted by different interpretations of the servers have
   been resolved (see Server-to-Server Communications (Section 15.4)).

   Server dialback is uni-directional, and results in weak identity
   verification for one stream in one direction.  Because server
   dialback is not an authentication mechanism, mutual authentication is
   not possible via dialback.  Therefore, server dialback MUST be
   completed in each direction in order to enable bi-directional
   communications between two domains.

   The method internationalized node
   identifier; for generating and verifying the keys used in server
   dialback MUST take into account the hostnames being used, the stream
   ID generated by the receiving server, and example, a user entering a single internationalized
   node identifier could access another user's account information, or a secret known by
   user could gain access to a hidden or otherwise restricted chat room
   or service.

   A resource identifier can be employed as one part of an entity's
   address in XMPP.  One common usage is as the
   authoritative server's network; see Appendix C.5 name for an instant
   messaging user's connected resource; another is as the recommended
   algorithm.

   Any error that occurs during dialback negotiation MUST be considered nickname of a stream error, resulting
   user in termination a multi-user conference room; many other kinds of the stream and entities
   could use resource identifiers as part of the
   underlying TCP connection. their addresses.  The possible error conditions are
   specified in
   security of such services could be compromised based on different
   interpretations of the protocol description below.

   The following terminology applies:

   o  ORIGINATING SERVER -- internationalized resource identifier; for
   example, a user could attempt to initiate multiple connections with
   the server that is attempting same name, or a user could send a message to establish someone other than
   the intended recipient in a
      connection between multi-user conference room.

15.11.  Address Spoofing

   As discussed in [XEP-0165], there are two domains.
   o  RECEIVING SERVER -- forms of address spoofing:
   forging and mimicking.

15.11.1.  Address Forging

   In the server that context of XMPP technologies, address forging occurs when an
   entity is trying to authenticate that
      the Originating Server represents the domain which it claims able to
      be.
   o  AUTHORITATIVE SERVER -- the server that answers generate an XML stanza whose 'from' address does
   not correspond to the DNS
      hostname asserted by the Originating Server; for basic
      environments this will be account credentials with which the Originating Server, but it could be
      a separate machine in entity
   authenticated onto the Originating Server's network. network (or an authorization identity provided
   during SASL negotiation (Section 7).  For example, address forging
   occurs if an entity that authenticated as "juliet@example.com" is
   able to send XML stanzas from "nurse@example.com" or
   "romeo@example.net".



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C.2.  Order of Events

   The following


   Address forging is a brief summary of the order of events difficult in dialback:

   1.  The Originating Server establishes an XML stream with XMPP systems, given the
       Receiving Server.
   2.  The Originating Server sends requirement
   for sending servers to stamp 'from' addresses and for receiving
   servers to verify sending domains via server-to-server
   authentication.  However, address forging is not impossible, since a 'key' value over
   rogue server could forge JIDs at the connection to sending domain by ignoring the Receiving Server.
   3.  The Receiving Server establishes
   stamping requirement.  A rogue server could even forge JIDs at other
   domains by means of a DNS poisoning attack if [DNSSEC] is not used.
   This specification does not define methods for discovering or
   counteracting such rogue servers.

15.11.2.  Address Mimicking

   Address mimicking occus when an XML stream with the
       Authoritative Server.
   4.  The Receiving Server entity provides legitimate
   authentication credentials for and sends XML stanzas from an account
   whose JID appears to a human user to be the same 'key' value to as another JID.  For
   example, in some XMPP clients the
       Authoritative Server for verification.
   5.  The Authoritative Server replies that key is valid or invalid.
   6.  The Receiving Server informs address "paypa1@example.org"
   (spelled with the Originating Server whether it is
       authenticated or not.

   We can represent this flow of events graphically number one as follows:

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

C.3.  Protocol

   This section describes the detailed protocol interaction between final character of the node
   identifier) may appear to be the same as "paypal@example.org (spelled
   with the
   Originating Server, lower-case version of the Receiving Server, and letter "L"), especially on casual
   visual inspection; this phenomenon is sometimes called "typejacking".
   A more sophisticated example of address mimicking might involve the Authoritative
   Server.

   This section uses
   use of characters from outside the following domain names, IP addresses, stream



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   IDs, and shared secret in US-ASCII range, such as the examples:

   o  The Originating Server is "example.org" (there is no IP
   Cherokee characters U+13DA U+13A2 U+13B5 U+13AC U+13A2 U+13AC U+13D2
   instead of the US-ASCII charac