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Versions: 00 01 02 03 04 05 06 07 08 09                                 
Network Working Group                                P. Saint-Andre, Ed.
Internet-Draft                                 XMPP Standards Foundation
Obsoletes: 3920 (if approved)                            October 5, 2007
Intended status: Standards Track
Expires: April 7, 2008


        Extensible Messaging and Presence Protocol (XMPP): Core
                     draft-saintandre-rfc3920bis-04

Status of this Memo

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   This Internet-Draft will expire on April 7, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This 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 or more 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
   a server and of one server to another server, and primitives for
   push-style messages, publication of network availability 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  . . . . . . . . . . . . . . . . . . . . . . . .   8
     1.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .   8
     1.2.   Functional Summary . . . . . . . . . . . . . . . . . . .   9
     1.3.   Conventions  . . . . . . . . . . . . . . . . . . . . . .  10
     1.4.   Discussion Venue . . . . . . . . . . . . . . . . . . . .  10
   2.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .  10
     2.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  10
     2.2.   Server . . . . . . . . . . . . . . . . . . . . . . . . .  11
     2.3.   Client . . . . . . . . . . . . . . . . . . . . . . . . .  12
     2.4.   Network  . . . . . . . . . . . . . . . . . . . . . . . .  12
   3.  Addresses . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.2.   Domain Identifier  . . . . . . . . . . . . . . . . . . .  13
     3.3.   Node Identifier  . . . . . . . . . . . . . . . . . . . .  14
     3.4.   Resource Identifier  . . . . . . . . . . . . . . . . . .  15
     3.5.   Determination of Addresses . . . . . . . . . . . . . . .  15
   4.  TCP Binding . . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.1.   Scope  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.2.   Hostname Resolution  . . . . . . . . . . . . . . . . . .  16
     4.3.   Client-to-Server Communications  . . . . . . . . . . . .  17
     4.4.   Server-to-Server Communications  . . . . . . . . . . . .  17
     4.5.   Other Bindings . . . . . . . . . . . . . . . . . . . . .  17
   5.  XML Streams . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.2.   Stream Security  . . . . . . . . . . . . . . . . . . . .  19
     5.3.   Stream Attributes  . . . . . . . . . . . . . . . . . . .  20
       5.3.1.   from . . . . . . . . . . . . . . . . . . . . . . . .  20
       5.3.2.   to . . . . . . . . . . . . . . . . . . . . . . . . .  21
       5.3.3.   id . . . . . . . . . . . . . . . . . . . . . . . . .  22
       5.3.4.   xml:lang . . . . . . . . . . . . . . . . . . . . . .  23
       5.3.5.   version  . . . . . . . . . . . . . . . . . . . . . .  24
       5.3.6.   Summary  . . . . . . . . . . . . . . . . . . . . . .  25
     5.4.   Namespace Declarations . . . . . . . . . . . . . . . . .  25
     5.5.   Stream Features  . . . . . . . . . . . . . . . . . . . .  26
     5.6.   Closing Streams  . . . . . . . . . . . . . . . . . . . .  27
     5.7.   Reconnection . . . . . . . . . . . . . . . . . . . . . .  27



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     5.8.   Stream Errors  . . . . . . . . . . . . . . . . . . . . .  28
       5.8.1.   Rules  . . . . . . . . . . . . . . . . . . . . . . .  28
         5.8.1.1.  Stream Errors Are Unrecoverable . . . . . . . . .  28
         5.8.1.2.  Stream Errors Can Occur During Setup  . . . . . .  28
         5.8.1.3.  Stream Errors When the Host is Unspecified  . . .  29
       5.8.2.   Syntax . . . . . . . . . . . . . . . . . . . . . . .  30
       5.8.3.   Defined Stream Error Conditions  . . . . . . . . . .  31
         5.8.3.1.  bad-format  . . . . . . . . . . . . . . . . . . .  31
         5.8.3.2.  bad-namespace-prefix  . . . . . . . . . . . . . .  31
         5.8.3.3.  conflict  . . . . . . . . . . . . . . . . . . . .  32
         5.8.3.4.  connection-timeout  . . . . . . . . . . . . . . .  33
         5.8.3.5.  host-gone . . . . . . . . . . . . . . . . . . . .  33
         5.8.3.6.  host-unknown  . . . . . . . . . . . . . . . . . .  34
         5.8.3.7.  improper-addressing . . . . . . . . . . . . . . .  35
         5.8.3.8.  internal-server-error . . . . . . . . . . . . . .  35
         5.8.3.9.  invalid-from  . . . . . . . . . . . . . . . . . .  36
         5.8.3.10. invalid-id  . . . . . . . . . . . . . . . . . . .  36
         5.8.3.11. invalid-namespace . . . . . . . . . . . . . . . .  37
         5.8.3.12. invalid-xml . . . . . . . . . . . . . . . . . . .  37
         5.8.3.13. not-authorized  . . . . . . . . . . . . . . . . .  38
         5.8.3.14. policy-violation  . . . . . . . . . . . . . . . .  39
         5.8.3.15. remote-connection-failed  . . . . . . . . . . . .  40
         5.8.3.16. resource-constraint . . . . . . . . . . . . . . .  40
         5.8.3.17. restricted-xml  . . . . . . . . . . . . . . . . .  41
         5.8.3.18. see-other-host  . . . . . . . . . . . . . . . . .  41
         5.8.3.19. system-shutdown . . . . . . . . . . . . . . . . .  42
         5.8.3.20. undefined-condition . . . . . . . . . . . . . . .  43
         5.8.3.21. unsupported-encoding  . . . . . . . . . . . . . .  43
         5.8.3.22. unsupported-stanza-type . . . . . . . . . . . . .  44
         5.8.3.23. unsupported-version . . . . . . . . . . . . . . .  44
         5.8.3.24. xml-not-well-formed . . . . . . . . . . . . . . .  45
       5.8.4.   Application-Specific Conditions  . . . . . . . . . .  46
     5.9.   Simplified Stream Examples . . . . . . . . . . . . . . .  46
   6.  STARTTLS Negotiation  . . . . . . . . . . . . . . . . . . . .  48
     6.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  49
     6.2.   Rules  . . . . . . . . . . . . . . . . . . . . . . . . .  49
       6.2.1.   Data Formatting  . . . . . . . . . . . . . . . . . .  49
       6.2.2.   Order of Negotiation . . . . . . . . . . . . . . . .  49
     6.3.   Process  . . . . . . . . . . . . . . . . . . . . . . . .  49
       6.3.1.   Exchange of Stream Headers and Stream Features . . .  49
       6.3.2.   Initiation of STARTTLS Negotiation . . . . . . . . .  51
         6.3.2.1.  STARTTLS Command  . . . . . . . . . . . . . . . .  51
         6.3.2.2.  Failure Case  . . . . . . . . . . . . . . . . . .  51
         6.3.2.3.  Proceed Case  . . . . . . . . . . . . . . . . . .  51
       6.3.3.   TLS Negotiation  . . . . . . . . . . . . . . . . . .  52
         6.3.3.1.  Rules . . . . . . . . . . . . . . . . . . . . . .  52
         6.3.3.2.  TLS Failure . . . . . . . . . . . . . . . . . . .  52
         6.3.3.3.  TLS Success . . . . . . . . . . . . . . . . . . .  52



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     6.4.   Representation of JIDs in Certificates . . . . . . . . .  54
       6.4.1.   Client Certificates  . . . . . . . . . . . . . . . .  54
       6.4.2.   Server Certificates  . . . . . . . . . . . . . . . .  54
       6.4.3.   ASN.1 Object Identifier  . . . . . . . . . . . . . .  54
   7.  SASL Negotiation  . . . . . . . . . . . . . . . . . . . . . .  55
     7.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  55
     7.2.   Rules  . . . . . . . . . . . . . . . . . . . . . . . . .  55
       7.2.1.   Data Formatting  . . . . . . . . . . . . . . . . . .  55
       7.2.2.   Security Layers  . . . . . . . . . . . . . . . . . .  56
       7.2.3.   Simple Usernames . . . . . . . . . . . . . . . . . .  56
       7.2.4.   Authorization Identities . . . . . . . . . . . . . .  56
     7.3.   Process  . . . . . . . . . . . . . . . . . . . . . . . .  57
       7.3.1.   Exchange of Stream Headers and Stream Features . . .  57
       7.3.2.   Initiation . . . . . . . . . . . . . . . . . . . . .  59
       7.3.3.   Challenge-Response Sequence  . . . . . . . . . . . .  59
       7.3.4.   Abort  . . . . . . . . . . . . . . . . . . . . . . .  60
       7.3.5.   Failure  . . . . . . . . . . . . . . . . . . . . . .  61
       7.3.6.   Success  . . . . . . . . . . . . . . . . . . . . . .  61
     7.4.   SASL Definition  . . . . . . . . . . . . . . . . . . . .  62
     7.5.   SASL Errors  . . . . . . . . . . . . . . . . . . . . . .  63
       7.5.1.   aborted  . . . . . . . . . . . . . . . . . . . . . .  63
       7.5.2.   incorrect-encoding . . . . . . . . . . . . . . . . .  63
       7.5.3.   invalid-authzid  . . . . . . . . . . . . . . . . . .  64
       7.5.4.   invalid-mechanism  . . . . . . . . . . . . . . . . .  64
       7.5.5.   malformed-request  . . . . . . . . . . . . . . . . .  64
       7.5.6.   mechanism-too-weak . . . . . . . . . . . . . . . . .  65
       7.5.7.   not-authorized . . . . . . . . . . . . . . . . . . .  65
       7.5.8.   temporary-auth-failure . . . . . . . . . . . . . . .  65
   8.  Resource Binding  . . . . . . . . . . . . . . . . . . . . . .  66
     8.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  66
     8.2.   Advertising Support  . . . . . . . . . . . . . . . . . .  66
     8.3.   Server-Generated Resource Identifier . . . . . . . . . .  67
       8.3.1.   Success Case . . . . . . . . . . . . . . . . . . . .  67
       8.3.2.   Error Case . . . . . . . . . . . . . . . . . . . . .  68
     8.4.   Client-Generated Resource Identifier . . . . . . . . . .  68
       8.4.1.   Success Case . . . . . . . . . . . . . . . . . . . .  68
       8.4.2.   Error Cases  . . . . . . . . . . . . . . . . . . . .  69
         8.4.2.1.  Not Allowed . . . . . . . . . . . . . . . . . . .  69
         8.4.2.2.  Bad Request . . . . . . . . . . . . . . . . . . .  69
         8.4.2.3.  Conflict  . . . . . . . . . . . . . . . . . . . .  69
     8.5.   Binding Multiple Resources . . . . . . . . . . . . . . .  70
       8.5.1.   Support  . . . . . . . . . . . . . . . . . . . . . .  70
       8.5.2.   Binding an Additional Resource . . . . . . . . . . .  70
       8.5.3.   Unbinding a Resource . . . . . . . . . . . . . . . .  71
         8.5.3.1.  Success Case  . . . . . . . . . . . . . . . . . .  71
         8.5.3.2.  Error Cases . . . . . . . . . . . . . . . . . . .  71
       8.5.4.   From Addresses . . . . . . . . . . . . . . . . . . .  72
   9.  XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . .  72



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     9.1.   Common Attributes  . . . . . . . . . . . . . . . . . . .  73
       9.1.1.   to . . . . . . . . . . . . . . . . . . . . . . . . .  73
         9.1.1.1.  Client-to-Server Streams  . . . . . . . . . . . .  73
         9.1.1.2.  Server-to-Server Streams  . . . . . . . . . . . .  73
       9.1.2.   from . . . . . . . . . . . . . . . . . . . . . . . .  74
         9.1.2.1.  Client-to-Server Streams  . . . . . . . . . . . .  74
         9.1.2.2.  Server-to-Server Streams  . . . . . . . . . . . .  75
       9.1.3.   id . . . . . . . . . . . . . . . . . . . . . . . . .  75
       9.1.4.   type . . . . . . . . . . . . . . . . . . . . . . . .  75
       9.1.5.   xml:lang . . . . . . . . . . . . . . . . . . . . . .  76
     9.2.   Basic Semantics  . . . . . . . . . . . . . . . . . . . .  77
       9.2.1.   Message Semantics  . . . . . . . . . . . . . . . . .  77
       9.2.2.   Presence Semantics . . . . . . . . . . . . . . . . .  77
       9.2.3.   IQ Semantics . . . . . . . . . . . . . . . . . . . .  77
     9.3.   Stanza Errors  . . . . . . . . . . . . . . . . . . . . .  79
       9.3.1.   Rules  . . . . . . . . . . . . . . . . . . . . . . .  79
       9.3.2.   Syntax . . . . . . . . . . . . . . . . . . . . . . .  79
       9.3.3.   Defined Conditions . . . . . . . . . . . . . . . . .  81
         9.3.3.1.  bad-request . . . . . . . . . . . . . . . . . . .  81
         9.3.3.2.  conflict  . . . . . . . . . . . . . . . . . . . .  81
         9.3.3.3.  feature-not-implemented . . . . . . . . . . . . .  82
         9.3.3.4.  forbidden . . . . . . . . . . . . . . . . . . . .  82
         9.3.3.5.  gone  . . . . . . . . . . . . . . . . . . . . . .  83
         9.3.3.6.  internal-server-error . . . . . . . . . . . . . .  83
         9.3.3.7.  item-not-found  . . . . . . . . . . . . . . . . .  84
         9.3.3.8.  jid-malformed . . . . . . . . . . . . . . . . . .  84
         9.3.3.9.  not-acceptable  . . . . . . . . . . . . . . . . .  85
         9.3.3.10. not-allowed . . . . . . . . . . . . . . . . . . .  85
         9.3.3.11. not-authorized  . . . . . . . . . . . . . . . . .  85
         9.3.3.12. not-modified  . . . . . . . . . . . . . . . . . .  86
         9.3.3.13. payment-required  . . . . . . . . . . . . . . . .  87
         9.3.3.14. recipient-unavailable . . . . . . . . . . . . . .  87
         9.3.3.15. redirect  . . . . . . . . . . . . . . . . . . . .  88
         9.3.3.16. registration-required . . . . . . . . . . . . . .  88
         9.3.3.17. remote-server-not-found . . . . . . . . . . . . .  89
         9.3.3.18. remote-server-timeout . . . . . . . . . . . . . .  89
         9.3.3.19. resource-constraint . . . . . . . . . . . . . . .  89
         9.3.3.20. service-unavailable . . . . . . . . . . . . . . .  90
         9.3.3.21. subscription-required . . . . . . . . . . . . . .  90
         9.3.3.22. undefined-condition . . . . . . . . . . . . . . .  91
         9.3.3.23. unexpected-request  . . . . . . . . . . . . . . .  92
         9.3.3.24. unknown-sender  . . . . . . . . . . . . . . . . .  93
       9.3.4.   Application-Specific Conditions  . . . . . . . . . .  93
     9.4.   Extended Content . . . . . . . . . . . . . . . . . . . .  94
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  95
     10.1.  Client-to-Server . . . . . . . . . . . . . . . . . . . .  95
       10.1.1.  TLS  . . . . . . . . . . . . . . . . . . . . . . . .  96
       10.1.2.  SASL . . . . . . . . . . . . . . . . . . . . . . . .  97



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       10.1.3.  Resource Binding . . . . . . . . . . . . . . . . . .  99
       10.1.4.  Stanza Exchange  . . . . . . . . . . . . . . . . . . 100
       10.1.5.  Close  . . . . . . . . . . . . . . . . . . . . . . . 100
     10.2.  Server-to-Server Examples  . . . . . . . . . . . . . . . 101
       10.2.1.  TLS  . . . . . . . . . . . . . . . . . . . . . . . . 101
       10.2.2.  SASL . . . . . . . . . . . . . . . . . . . . . . . . 103
       10.2.3.  Stanza Exchange  . . . . . . . . . . . . . . . . . . 104
       10.2.4.  Close  . . . . . . . . . . . . . . . . . . . . . . . 105
   11. Server Rules for Processing XML Stanzas . . . . . . . . . . . 105
     11.1.  No 'to' Address  . . . . . . . . . . . . . . . . . . . . 105
       11.1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . 105
       11.1.2.  Message  . . . . . . . . . . . . . . . . . . . . . . 106
       11.1.3.  Presence . . . . . . . . . . . . . . . . . . . . . . 106
       11.1.4.  IQ . . . . . . . . . . . . . . . . . . . . . . . . . 106
     11.2.  Local Domain . . . . . . . . . . . . . . . . . . . . . . 106
       11.2.1.  Mere Domain  . . . . . . . . . . . . . . . . . . . . 106
       11.2.2.  Resource at Domain . . . . . . . . . . . . . . . . . 106
       11.2.3.  Node at Local Domain . . . . . . . . . . . . . . . . 107
     11.3.  Foreign Domain . . . . . . . . . . . . . . . . . . . . . 107
       11.3.1.  Existing Stream  . . . . . . . . . . . . . . . . . . 107
       11.3.2.  No Existing Stream . . . . . . . . . . . . . . . . . 107
       11.3.3.  Error Handling . . . . . . . . . . . . . . . . . . . 108
   12. XML Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 108
     12.1.  Restrictions . . . . . . . . . . . . . . . . . . . . . . 108
     12.2.  XML Namespace Names and Prefixes . . . . . . . . . . . . 109
       12.2.1.  Streams Namespace  . . . . . . . . . . . . . . . . . 109
       12.2.2.  Default Namespace  . . . . . . . . . . . . . . . . . 109
       12.2.3.  Extended Namespaces  . . . . . . . . . . . . . . . . 110
     12.3.  Validation . . . . . . . . . . . . . . . . . . . . . . . 112
     12.4.  Inclusion of Text Declaration  . . . . . . . . . . . . . 112
     12.5.  Character Encoding . . . . . . . . . . . . . . . . . . . 112
     12.6.  White Space  . . . . . . . . . . . . . . . . . . . . . . 112
   13. Compliance Requirements . . . . . . . . . . . . . . . . . . . 112
     13.1.  Servers  . . . . . . . . . . . . . . . . . . . . . . . . 113
     13.2.  Clients  . . . . . . . . . . . . . . . . . . . . . . . . 113
   14. Internationalization Considerations . . . . . . . . . . . . . 114
   15. Security Considerations . . . . . . . . . . . . . . . . . . . 114
     15.1.  High Security  . . . . . . . . . . . . . . . . . . . . . 114
     15.2.  Certificate Validation . . . . . . . . . . . . . . . . . 115
     15.3.  Client-to-Server Communication . . . . . . . . . . . . . 115
     15.4.  Server-to-Server Communication . . . . . . . . . . . . . 116
     15.5.  Order of Layers  . . . . . . . . . . . . . . . . . . . . 116
     15.6.  Lack of SASL Channel Binding to TLS  . . . . . . . . . . 117
     15.7.  Mandatory-to-Implement Technologies  . . . . . . . . . . 117
     15.8.  Firewalls  . . . . . . . . . . . . . . . . . . . . . . . 118
     15.9.  Use of base64 in SASL  . . . . . . . . . . . . . . . . . 118
     15.10. Stringprep Profiles  . . . . . . . . . . . . . . . . . . 118
     15.11. Address Spoofing . . . . . . . . . . . . . . . . . . . . 119



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       15.11.1. Address Forging  . . . . . . . . . . . . . . . . . . 119
       15.11.2. Address Mimicking  . . . . . . . . . . . . . . . . . 119
     15.12. Denial of Service  . . . . . . . . . . . . . . . . . . . 121
     15.13. Presence Leaks . . . . . . . . . . . . . . . . . . . . . 122
     15.14. Directory Harvesting . . . . . . . . . . . . . . . . . . 122
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 122
     16.1.  XML Namespace Name for TLS Data  . . . . . . . . . . . . 123
     16.2.  XML Namespace Name for SASL Data . . . . . . . . . . . . 123
     16.3.  XML Namespace Name for Stream Errors . . . . . . . . . . 123
     16.4.  XML Namespace Name for Resource Binding  . . . . . . . . 123
     16.5.  XML Namespace Name for Stanza Errors . . . . . . . . . . 124
     16.6.  Nodeprep Profile of Stringprep . . . . . . . . . . . . . 124
     16.7.  Resourceprep Profile of Stringprep . . . . . . . . . . . 124
     16.8.  GSSAPI Service Name  . . . . . . . . . . . . . . . . . . 125
     16.9.  Port Numbers . . . . . . . . . . . . . . . . . . . . . . 125
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . . 125
     17.1.  Normative References . . . . . . . . . . . . . . . . . . 125
     17.2.  Informative References . . . . . . . . . . . . . . . . . 127
   Appendix A.  Nodeprep . . . . . . . . . . . . . . . . . . . . . . 130
     A.1.   Introduction . . . . . . . . . . . . . . . . . . . . . . 130
     A.2.   Character Repertoire . . . . . . . . . . . . . . . . . . 131
     A.3.   Mapping  . . . . . . . . . . . . . . . . . . . . . . . . 131
     A.4.   Normalization  . . . . . . . . . . . . . . . . . . . . . 131
     A.5.   Prohibited Output  . . . . . . . . . . . . . . . . . . . 131
     A.6.   Bidirectional Characters . . . . . . . . . . . . . . . . 132
   Appendix B.  Resourceprep . . . . . . . . . . . . . . . . . . . . 132
     B.1.   Introduction . . . . . . . . . . . . . . . . . . . . . . 132
     B.2.   Character Repertoire . . . . . . . . . . . . . . . . . . 133
     B.3.   Mapping  . . . . . . . . . . . . . . . . . . . . . . . . 133
     B.4.   Normalization  . . . . . . . . . . . . . . . . . . . . . 133
     B.5.   Prohibited Output  . . . . . . . . . . . . . . . . . . . 133
     B.6.   Bidirectional Characters . . . . . . . . . . . . . . . . 133
   Appendix C.  XML Schemas  . . . . . . . . . . . . . . . . . . . . 133
     C.1.   Streams namespace  . . . . . . . . . . . . . . . . . . . 134
     C.2.   Stream error namespace . . . . . . . . . . . . . . . . . 135
     C.3.   STARTTLS namespace . . . . . . . . . . . . . . . . . . . 137
     C.4.   SASL namespace . . . . . . . . . . . . . . . . . . . . . 137
     C.5.   Resource binding namespace . . . . . . . . . . . . . . . 139
     C.6.   Stanza error namespace . . . . . . . . . . . . . . . . . 141
   Appendix D.  Contact Addresses  . . . . . . . . . . . . . . . . . 142
   Appendix E.  Account Provisioning . . . . . . . . . . . . . . . . 143
   Appendix F.  Differences From RFC 3920  . . . . . . . . . . . . . 143
   Appendix G.  Copying Conditions . . . . . . . . . . . . . . . . . 144
   Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 145
   Intellectual Property and Copyright Statements  . . . . . . . . . 146





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1.  Introduction

1.1.  Overview

   The Extensible Messaging and Presence Protocol (XMPP) is an
   application profile of the Extensible Markup Language [XML] for
   streaming XML data in close to real time between any two (or more)
   network-aware entities.  XMPP is typically used to exchange messages,
   share presence information, and engage in structured request-response
   interactions.  The basic syntax and semantics of XMPP were developed
   originally within the Jabber open-source community, mainly in 1999.
   In late 2002, the XMPP 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, [RFC3920] and [RFC3921] were published in
   October 2004, representing the most complete definition of XMPP at
   that time.

   As a result of extensive implementation and deployment experience
   with XMPP since 2004, as well as more formal interoperability testing
   carried out under the auspices of the XMPP Standards Foundation
   (XSF), this document reflects consensus from the XMPP developer
   community regarding XMPP's core XML streaming technology.  In
   particular, this document incorporates the following backward-
   compatible changes from RFC 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 a
      using protocol, e.g., Transport Layer Security (TLS) and the
      Simple Authentication and Security Layer (SASL)
   o  Definition of several additional error conditions
   o  Addition of 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].




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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.

   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:

   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)
   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
   9.  Close the TCP 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.  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 XMPP extensionsl.  For
   example, [XMPP-IM] defines extensions for basic instant messaging and
   presence functionality.  In addition, various specifications produced
   in the XSF's XEP series [XEP-0001] 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:

   1.  Determine the hostname and port at which to connect
   2.  Open a TCP connection
   3.  Open an XML stream





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   4.  Complete TLS negotiation for channel encryption (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
   8.  Close the TCP 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 [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,
   "[...]" 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

1.4.  Discussion Venue

   The editor welcomes discussion and comments related to the topics
   presented in this document.  The preferred forum is the
   <standards@xmpp.org> mailing list, for which archives and
   subscription information are available at
   <<http://mail.jabber.org/mailman/listinfo/standards>>.


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



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   can also communicate with each other over TCP connections.

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

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


   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 is an entity whose primary responsibilities are to:

   o  Manage XML streams (Section 5) with local clients and 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 XML stanzas (Section 9) to those servers over the
      negotiated XML streams.

   Depending on the 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 stanza is handled directly by the
      server itself on behalf of the client and is not routed to a
      foreign server or delivered to a local entity.
   o  Hosting local services that also use XMPP as the basis for
      communication but that provide additional functionality beyond
      that defined in this document or in [XMPP-IM]; examples include
      multi-user conferencing services as specified in [XEP-0045] and
      publish-subscribe services as specified in [XEP-0060].




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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 a local account, where each
   client is differentiated by the resource identifier portion of an
   XMPP address (e.g., <node@domain/home> vs. <node@domain/work>), as
   defined under Section 3 and Section 8.  The RECOMMENDED port for TCP
   connections between a client and a server is 5222, as registered with
   the IANA (see Section 16.9).

2.4.  Network

   Because each server is identified by a network address and because
   server-to-server communication is a straightforward extension of the
   client-to-server protocol, in 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.  Communication between any two servers is
   OPTIONAL.  If enabled, such 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 Section 16.9).


3.  Addresses

3.1.  Overview

   An ENTITY is anything that is network-addressable and that can
   communicate using XMPP.  For historical reasons, the native address
   of an XMPP entity is called a JABBER IDENTIFIER or JID.  A valid JID
   contains a set of ordered elements formed of 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 specified in [ABNF].








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      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
   server that hosts the account, and a connected resource (e.g., a
   specific device) in the form of <node@domain/resource>.  However,
   node types other than clients are possible; for example, a specific
   chat room offered by a multi-user 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 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).  Typically a domain identifier
   identifies the "home" server to which clients connect for XML routing
   and data management functionality.  (Note: A single server may



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   service multiple domain identifiers, i.e., multiple local domains.)
   However, it is not necessary for an XMPP domain identifier to
   identify an entity that provides core XMPP server functionality
   (e.g., 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]).

   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
   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: The ToASCII operation includes application of the [NAMEPREP]
   profile of [STRINGPREP] and encoding using the algorithm specified in
   [PUNYCODE]; for details, see [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.  Typically a node identifier uniquely
   identifies the entity requesting and using network access provided by
   a server (i.e., a local account), although it can also represent
   other kinds of entities (e.g., a chat room associated with a multi-
   user conference service).  The entity represented by an XMPP node



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   identifier is addressed within the context of a specific domain.
   When the domain is an XMPP server and the entity is a local account
   on the server, the resulting address (of the form <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.  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
   conference room) belonging to the entity associated with an XMPP node
   identifier at a local domain.  XMPP entities SHOULD consider resource
   identifiers to be opaque strings and SHOULD NOT impute meaning to any
   given resource identifier.  A resource identifier is negotiated
   between a client and a server during resource binding (Section 8),
   after which the entity is referred to as a CONNECTED RESOURCE and its
   address (of the form <node@domain/resource>) is referred to as a FULL
   JID.  An entity MAY maintain multiple connected resources
   simultaneously, with each connected resource differentiated by a
   distinct resource identifier.

   A resource identifier MUST be formatted such that the Resourceprep
   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 (Section 8), the receiving entity for a stream MUST determine
   the initiating entity's JID.

   For server-to-server communication, the initiating entity's JID
   SHOULD be the authorization identity (as defined by [SASL]), either
   (1) as directly communicated by the initiating entity during SASL
   negotiation (Section 7) or (2) as derived from the authentication
   identity if no authorization identity was specified during SASL
   negotiation (Section 7).




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   For client-to-server communication, the client's bare JID
   (<node@domain>) SHOULD be the authorization identity (as defined by
   [SASL]), either (1) as directly communicated by the initiating entity
   during SASL negotiation (Section 7) or (2) as derived from the
   authentication identity if no authorization identity was specified
   during SASL negotiation (Section 7).  The resource identifier portion
   of the full JID (<node@domain/resource>) SHOULD be the resource
   identifier negotiated by the client and server during resource
   binding (Section 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

4.1.  Scope

   As XMPP is defined in this specification, an initiating entity
   (client or server) MUST open a Transmission Control Protocol [TCP]
   connection at the receiving entity (server) before it negotiates XML
   streams with the receiving entity.  The rules specified in the
   following sections apply to the TCP binding.

4.2.  Hostname Resolution

   Before opening the TCP 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
   communication with 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 result of the SRV lookup
       will be one or more combinations of a port and hostname; the
       initiating entity MUST resolve one of the hostnames in order to
       determine an IP address at which to connect.
   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 connections or the "xmpp-server" port 5269 for server-
       to-server connections.



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   3.  For client-to-server connections, the fallback MAY be a [DNS-TXT]
       lookup for alternative connection methods, for example as
       described in [XEP-0156].

4.3.  Client-to-Server Communications

   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 Section 5).

4.4.  Server-to-Server Communications

   Because two servers are peers and therefore each 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 XML
   stanzas from the second server to the first server.

   This rule applies only to XML stanzas (Section 9).  Therefore during
   STARTTLS negotiation (Section 6) and SASL negotiation (Section 7) the
   servers would use one TCP connection, but after stream setup that TCP
   connection would be used only for the initiating server to send XML
   stanzas to the receiving server.  In order for the receiving server
   to send XML stanzas to the initiating server, the receiving server
   would need to reverse the roles and negotiate an XML stream from the
   receiving server to the initiating server.

4.5.  Other Bindings

   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
   terms are defined as follows.




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   Definition of XML Stream:  An XML STREAM is a container for the
      exchange of XML elements between any two entities over a network.
      The start of an XML stream is denoted unambiguously by an opening
      STREAM HEADER (i.e., an 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 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 is negotiated from the initiating
      entity (typically a client or server) to the receiving entity
      (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).
   Definition of XML Stanza:  An XML STANZA is a discrete semantic unit
      of structured information that is sent from one entity to another
      over an XML stream.  An XML stanza 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 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 Section 9; an XML element sent for the purpose of
      TLS negotiation (Section 6) or SASL negotiation (Section 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] 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 a stream header to the server, optionally preceded by a text
   declaration specifying the XML version and the character encoding
   supported (see Section 12.4 and Section 12.5).  Subject to local
   policies and service provisioning, the server SHOULD then reply with
   a second XML stream back to the client, again optionally preceded by



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   a text declaration.  Once the client has completed SASL negotiation
   (Section 7) and resource binding (Section 8), the client MAY send an
   unbounded number of XML stanzas over the stream.  When the client
   desires to close the stream, it simply sends a closing </stream> tag
   to the server (see Section 5.6).

   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>              |
   |--------------------|
   | <iq from='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.

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



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   Section 7) are mandatory to implement.

   When negotiating XML streams in XMPP 1.0, TLS SHOULD be used as
   defined under Section 6 and SASL MUST be used as defined under
   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.

   The initiating entity SHOULD NOT attempt to send XML stanzas
   (Section 9) over the stream before the stream has been authenticated.
   However, if it does attempt to do so, the receiving entity MUST NOT
   accept such stanzas and MUST return a <not-authorized/> stream error
   and then terminate both the XML stream and the underlying TCP
   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 root <stream/> element are as follows.

5.3.1.  from

   In client-to-server communication, the 'from' attribute SHOULD be
   included in the initial stream header and (if included) MUST be set
   to the account name (i.e., 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 communication, the 'from' attribute SHOULD be
   included in the initial stream header 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 response stream header 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 Section 15.3 and 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 initial stream header 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 communication, if the client included a 'from'
   address in the initial stream header then the server SHOULD include a
   'to' attribute in the response stream header and (if included) MUST
   set the 'to' attribute to the bare JID specified in the 'from'
   attribute of the initial stream header.



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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'>

   In server-to-server 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 response stream header
   and (if included) MUST set the 'to' attribute to the hostname
   specified in the 'from' attribute of the initial stream header.

   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 Section 15.3 and Section 15.4).

5.3.3.  id

   There SHOULD NOT be an 'id' attribute in the initial stream header;
   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'
          xmlns:stream='http://etherx.jabber.org/streams'>

   The 'id' attribute MUST be included in the response XML stream
   header.  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



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   within the receiving application (normally a server).

   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).

5.3.4.  xml:lang

   An 'xml:lang' attribute (as defined in Section 2.12 of [XML]) SHOULD
   be included in the initial stream header 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 response stream header.

   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'>




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   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 Section 9.1.5).  The value of the 'xml:lang'
   attribute MUST conform to the NMTOKEN datatype (as defined in Section
   2.3 of [XML]) and MUST conform to the format defined in [LANGTAGS].

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.

   The version of XMPP specified herein is "1.0"; in particular, XMPP
   1.0 encapsulates the stream-related protocols (TLS negotiation
   (Section 6), SASL negotiation (Section 7), and stream errors
   (Section 5.8)), as well as the basic 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 should be incremented only if significant
   new 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
   a smaller minor version number, but MAY be used for informational
   purposes by the entity with the larger minor version number (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).

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





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   1.  The initiating entity MUST set the value of the 'version'
       attribute in 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 in 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>".
   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, 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 response stream header.

5.3.6.  Summary

   We can summarize the attributes of the root <stream/> element 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
   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 [XML-NAMES]).  For detailed information regarding the



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   streams namespace and default namespace, see 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 response stream header the receiving entity MUST send a
   <features/> child element (prefixed by the streams namespace prefix)
   to the initiating entity in order to announce any stream-level
   features that can be negotiated (or capabilities that otherwise need
   to be advertised).

   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 mainly to advertise TLS negotiation
   (Section 6), SASL negotiation (Section 7), and resource binding
   (Section 8); however, stream features also can be used to advertise
   features associated with various XMPP extensions.  If an entity does
   not understand or support a feature, it SHOULD silently ignore the
   associated 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) can be offered, the non-security-related feature
   SHOULD NOT be included in the stream features that are advertised
   before the relevant security features have been negotiated.

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

   If there are no features to be advertised (e.g., in the stream reset
   initiated after successful SASL negotiation for a server-to-server
   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 response stream header.



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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'>
   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 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 or connections.

   Note: Closing of XML streams is handled differently in the case of a
   stream error; see Section 5.8.1.1.

5.7.  Reconnection

   It can happen that an XMPP server goes offline while servicing
   connections from local 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:



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   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 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 shall be
   sent by a compliant entity if it perceives that a stream-level error
   has occurred.

5.8.1.  Rules

   The following rules apply to stream-level errors.

5.8.1.1.  Stream Errors Are Unrecoverable

   Stream-level errors are unrecoverable.  Therefore, if an error occurs
   at the level of the stream, the entity that detects the error MUST
   send a stream error to the other entity, send a closing </stream>
   tag, and immediately terminate the underlying TCP connection.

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

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

5.8.1.2.  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 immediately terminate the underlying TCP connection.








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   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.namespace.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 and the error occurs during stream
   setup, the receiving entity SHOULD provide 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.namespace.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, 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'>
       [ ... descriptive text ... ]
     </text>]
     [application-specific condition element]
   </stream:error>

   The <error/> element:

   o  MUST contain a child element corresponding to one of the defined
      stream error conditions (Section 5.8.3); this element MUST be
      qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' 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.



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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
   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.

5.8.3.1.  bad-format

   The entity has sent XML that cannot be processed.

   (In the following example, the client sends an XMPP message that is
   not well-formed XML.)

   C: <message>
        <body>No closing body tag!
      </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/>, <unsupported-encoding/>, and <xml-not-well-formed/>.  However,
   the more specific errors are preferred.

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 Section 12.2).

   (In the following example, the client specifies a namespace prefix of
   "foobar" for the XML streams namespace.)




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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 existing stream for this entity
   because a new stream has been initiated that conflicts with the
   existing stream, or (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 from 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) and
   therefore the connection is being dropped.

   P: <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 in the initial stream header
   corresponds to a hostname that is no longer hosted by the receiving
   entity.

   (In the following example, the peer specifies a 'to' address of
   "foo.example.com" when connecting to the "example.com" server, but
   the server no longer hosts a service at that address.)







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   P: <?xml version='1.0'?>
      <stream:stream
          from='example.net'
          to='foo.example.com'
          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 in the initial stream
   header does not correspond to a hostname that is hosted by the
   receiving entity.

   (In the following example, the peer specifies a 'to' address of
   "example.org" when connecting to the "example.com" server, but the
   server knows nothing of that address.)



















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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).

   (In the following example, the peer sends a stanza without a 'to'
   address.)

   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 internal error that prevents it from servicing the stream.






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

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 between a client and a server via authentication and
   resource binding.

   (In the following example, a peer that has authenticated only as
   "example.net" attempts to send a stanza from an address at
   "example.org".)

   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.

   (In the following example, the server dialback ID is invalid; 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>





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5.8.3.11.  invalid-namespace

   The streams namespace name is something other than
   "http://etherx.jabber.org/streams" (see Section 12.2).

   (In the following example, the client specifies a streams namespace
   of 'http://wrong.namespace.example.org/' instead of the correct
   namespace of "http://etherx.jabber.org/streams".)

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://wrong.namespace.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 Section 12.3).

   (In the following example, the peer attempts to send an IQ stanza of
   type "subscribe" but there is no such value for the 'type'
   attribute.)











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   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
   process the offending stanza before sending the stream error.

   (In the following example, the client attempts to send XML stanzas
   before authenticating with the server.)





























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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'

   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.

   (In the following example, the client sends an XMPP message that is
   too large according to the server's local service policy.)

   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>

   S: </stream:stream>



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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.

   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.



















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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 Section 12.1).

   (In the following example, the client sends an XMPP message
   containing an XML comment.)

   C: <message to='juliet@example.com'>
        <!--<subject/>-->
        <body>This message has no subject.</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



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   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
   connect to that domain identifier at the standard XMPP ports (i.e.,
   5222 for client-to-server connections and 5269 for server-to-server
   connections).

   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.

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








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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.

   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 Section 12.5).

   (In the following example, the client attempts to encode data using
   UTF-16 instead of UTF-8.)

   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>








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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 or consistent with the default
   namespace.

   (In the following example, the client attempts to send an XML stanza
   of <pubsub/> when the default namespace is "jabber:client".)

   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.

   (In the following example, the client specifies an XMPP version of
   "11.0" but the server supports only version "1.0" and "1.1".)





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   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'/>
        <text xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
          1.0, 1.1
        </text>
      </stream:error>
      </stream:stream>

5.8.3.24.  xml-not-well-formed

   The initiating entity has sent XML that is not well-formed as defined
   by [XML].

   (In the following example, the client sends an XMPP message that is
   not well-formed XML.)

   C: <message>
        <body>No closing body tag!
      </message>

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








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5.8.4.  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:

   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; 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
          from='juliet@example.com'
          to='example.com'
          version='1.0'
          xml:lang='en'
          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'>

   [ ... channel encryption ... ]

   [ ... authentication ... ]

   [ ... resource binding ... ]

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

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

   C: </stream:stream>

   S: </stream:stream>









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   A connection gone bad:

   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'>

   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'>

   [ ... channel encryption ... ]

   [ ... authentication ... ]

   [ ... resource binding ... ]


   C:   <message from='juliet@example.com/balcony'
                 to='romeo@example.net'
                 xml:lang='en'>
          <body>No closing body tag!
        </message>

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


   More detailed examples are provided under Section 10.


6.  STARTTLS Negotiation






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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, specifically a "STARTTLS"
   extension that is modelled after similar extensions for the [IMAP],
   [POP3], and [ACAP] protocols as described in [USINGTLS].  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 deployment may require
   the use of TLS for client-to-server communication, server-to-server
   communication, or both.  A deployed client should use TLS to secure
   its stream with a server prior to attempting the completion of SASL
   negotiation (Section 7), and deployed servers should use TLS between
   two domains for the purpose of securing server-to-server
   communication.

6.2.  Rules

6.2.1.  Data Formatting

   The entities MUST NOT send any white space characters (matching
   production [3] content of [XML]) within the root stream element as
   separators between elements (any white space characters shown in the
   STARTTLS examples provided in this document are included only for the
   sake of readability); this prohibition helps to ensure proper
   security layer byte precision.

6.2.2.  Order of Negotiation

   If the initiating entity chooses to use TLS, STARTTLS negotiation
   MUST be completed before proceeding to SASL 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.

6.3.  Process

6.3.1.  Exchange of Stream Headers and Stream Features

   The initiating entity resolves the hostname of the receiving entity
   as specified under Section 4, opens a TCP connection to the
   advertised port at the resolved IP address, and sends an initial
   stream header to the receiving entity; if the initiating entity is



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   capable of STARTTLS negotiation, it MUST include the 'version'
   attribute set to a 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 send a response stream header to the
   initiating entity over the TCP connection opened by the initiating
   entity; if the receiving entity is capable of STARTTLS 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='t7AMCin9zjMNwQKDnplntZPIDEI='
        to='juliet@example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   The receiving entity then MUST send stream features to the initiating
   entity, which SHOULD include a <starttls/> element (qualified by the
   'urn:ietf:params:xml:ns:xmpp-tls' namespace) to indicate that the
   receiving entity supports STARTTLS negotiation.

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

   If the receiving entity requires the use of STARTTLS, it SHOULD
   include an empty <required/> element as a child of the <starttls/>
   element.

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






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6.3.2.  Initiation of STARTTLS Negotiation

6.3.2.1.  STARTTLS Command

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

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

   The receiving entity MUST reply with either a <proceed/> element
   (proceed case) or a <failure/> element (failure case) qualified by
   the 'urn:ietf:params:xml:ns:xmpp-tls' namespace.

6.3.2.2.  Failure Case

   If the failure case occurs, the receiving entity MUST return a
   <failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
   namespace, terminate the XML stream, and terminate the underlying TCP
   connection.  Causes for the failure case include but are not limited
   to:

   1.  The initiating entity has sent a malformed STARTTLS command.
   2.  The receiving entity does not 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 failure case occurs, the initiating entity MAY attempt to
   reconnect as explained under Section 5.7.

6.3.2.3.  Proceed Case

   If the proceed case occurs, the receiving entity MUST return a
   <proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
   namespace.

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

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



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   consider the TLS negotiation to have begun immediately after
   receiving the closing '>' character of the <proceed/> element from
   the receiving entity.

   The entities now proceed to TLS negotiation as explained in the next
   section.

6.3.3.  TLS Negotiation

6.3.3.1.  Rules

   In order to complete TLS negotiation over the TCP connection, the
   entities MUST follow the process defined in [TLS].

   The following rules apply:

   1.  The entities MUST NOT send any further XML data until the TLS
       negotiation has either failed or succeeded.
   2.  If the receiving entity presents a certificate during TLS
       negotiation, the initiating entity MUST validate the certificate
       in order to determine if the TLS negotiation 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 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.

6.3.3.2.  TLS Failure

   If the TLS negotiation results in failure, the receiving entity MUST
   terminate the TCP connection.

   The receiving entity MUST NOT send a closing </stream> tag before
   terminating the TCP connection, since the receiving entity and
   initiating entity MUST consider the original stream to be closed upon
   failure of the TLS negotiation.

6.3.3.3.  TLS Success

   If the TLS negotiation is successful, then the entities MUST proceed
   as follows.




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   1.  The receiving entity MUST discard any knowledge obtained in an
       insecure manner from the initiating entity before TLS took
       effect.
   2.  The initiating entity MUST discard any knowledge obtained in an
       insecure manner from the receiving entity before TLS took effect.
   3.  The initiating entity MUST send a new initial stream header to
       the receiving entity over the 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: The initiating entity MUST NOT send a closing </stream> tag
       before sending the initial stream header, since the receiving
       entity and initiating entity MUST consider the original stream to
       be closed upon success of the TLS negotiation.
   4.  The receiving entity MUST respond with a 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'

   5.  The receiving entity also MUST send stream features to the
       initiating entity, which MUST NOT include the STARTTLS feature
       but which SHOULD include the 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>







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6.4.  Representation of JIDs in Certificates

   TLS negotiation is commonly based on a digital certificate presented
   by the receiving entity (or, in the case of mutual authentication,
   both the receiving entity and the initiating entity).

6.4.1.  Client Certificates

   In a certificate to be presented by an XMPP client, it is RECOMMENDED
   for the certificate to include one or more JIDs associated with an
   XMPP user.  If included, a JID MUST be represented as a UTF8String
   within an otherName entity inside the subjectAltName, using the
   [ASN.1] Object Identifier "id-on-xmppAddr" specified under
   Section 6.4.3.

6.4.2.  Server Certificates

   In a certificate to be presented by an XMPP server, it is RECOMMENDED
   for the certificate to include one or more JIDs associated with
   domains serviced at the server.  If included, the following
   representation is RECOMMENDED:

   1.  A JID MUST be represented as a subjectAltName extension of type
       dNSName.  This dNSName MAY contain the wildcard character '*',
       which applies only to the left-most domain name component or
       component fragment and 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
       chat.example.net).
   2.  A JID SHOULD be represented as a UTF8String within an otherName
       entity inside the subjectAltName, using the [ASN.1] Object
       Identifier "id-on-xmppAddr" specified under Section 6.4.3.

6.4.3.  ASN.1 Object Identifier

   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

   As an alternative to the "id-on-xmppAddr" notation, this Object



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   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:

   id-on-xmppAddr:
      subjectAltName=otherName:id-on-xmppAddr;UTF8:juliet@example.com
   dotted display format:
      subjectAltName=otherName:1.3.6.1.5.5.7.8.5;UTF8:juliet@example.com
   URN notation:  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 generalized method for adding
   authentication support to connection-based protocols, and XMPP uses
   an XML namespace profile of SASL that conforms to the profiling
   requirements of [SASL].

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

7.2.  Rules

7.2.1.  Data Formatting

   The following formatting rules apply to the data sent during SASL
   negotiation:

   1.  The entities MUST NOT send any white space characters (matching
       production [3] content of [XML]) within the root stream element
       as separators between elements (any white space characters shown
       in the SASL examples provided in this document are included for
       the sake of readability only); this prohibition helps to ensure
       proper security layer byte precision.
   2.  Any XML character data contained within the XML elements MUST be
       encoded using base64, where the encoding adheres to the
       definition in Section 4 of [BASE64] and where the padding bits
       are set to zero.





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7.2.2.  Security Layers

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

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

   Provision of a "simple username" may be 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).  The
   simple username provided during authentication SHOULD be as follows:

   Client-to-server communication:  The initiating entity's registered
      account name, i.e., user name or node name as contained in an XMPP
      node identifier.  The simple username MUST adhere to the Nodeprep
      (Appendix A) profile of [STRINGPREP].
   Server-to-server communication:  The initiating entity's sending
      domain, i.e., IP address or fully qualified domain name as
      contained in an XMPP domain identifier.  The simple username MUST
      adhere to the [NAMEPREP] profile of [STRINGPREP].

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
   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., an XMPP domain identifier only) for servers and of
   the form <node@domain> (i.e., node identifier and domain identifier)
   for clients.





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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 occurs over the existing
   stream.  If not, the initiating entity resolves the hostname of the
   receiving entity as specified under Section 4, opens a TCP connection
   to the advertised port at the resolved IP address, and sends an
   initial stream header to the receiving entity; if the initiating
   entity is capable of STARTTLS negotiation, it MUST include the
   'version' attribute set to a 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 send a response stream header to the
   initiating entity; if the receiving entity is capable of SASL
   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 also MUST send stream features to the initiating
   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
   receiving entity offers to the initiating entity.  The order of
   <mechanism/> elements in the XML indicates the preference order of
   the SASL mechanisms according to the receiving entity.





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   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 presents a valid certificate during
   prior TLS negotiation, the receiving entity SHOULD offer the SASL
   EXTERNAL mechanism to the initiating entity during SASL negotiation
   (refer to [SASL]) and SHOULD prefer that mechanism.  However, the
   EXTERNAL mechanism MAY be offered under other circumstances as well.

   Note: If TLS negotiation (Section 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]).

   If successful SASL negotiation is required for interaction with the
   receiving entity, the receiving entity SHOULD signal that fact by
   including a <required/> element as a child of the <mechanisms/>
   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: As formally specified in the XML schema for the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace, the receiving entity
   MAY include an application-specific child element inside the
   <mechanisms/> element in order to provide information that may be
   needed by the initiating in order to complete successful SASL
   negotiation using one or more of the offered mechanisms; however, the
   syntax and semantics of any such element are out of scope for this
   specification.





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7.3.2.  Initiation

   In order to begin the SASL negotiation, the initiating entity sends
   an <auth/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace and 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 character ("="),
   which indicates that the response is present but contains no data.

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

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







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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:

   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.

   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); this enables the
   initiating entity (e.g., an end-user client) to tolerate incorrectly-
   provided credentials (e.g., a mistyped password) without being forced
   to reconnect.

   If the initiating entity exceeds the number of retries, the receiving
   entity MUST return a stream error (which SHOULD be <policy-



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   violation/>) and terminate the TCP connection.

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 (the particular cause of
   failure SHOULD be communicated in an appropriate child element of the
   <failure/> element as defined under 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 and no more
   than 5); this enables the initiating entity (e.g., an end-user
   client) to tolerate incorrectly-provided credentials (e.g., a
   mistyped password) without being forced to reconnect.

   If the initiating entity exceeds the number of retries, the receiving
   entity MUST return a stream error (which SHOULD be <policy-
   violation/>) and 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; 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 character ("=").

   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
   initial stream header to the receiving 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 initiating entity MUST NOT send a closing </stream> tag
   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.

   Upon receiving the initial stream header from the initiating entity,
   the receiving entity MUST respond by sending a response XML stream
   header to the initiating 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); any such additional features not defined herein
   MUST be defined by the relevant extension to XMPP.

   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 in 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.

7.5.  SASL Errors

   The following SASL-related error conditions are defined.

7.5.1.  aborted

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

   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.







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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.

   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.

   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 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.

   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 an
   <auth/> element (with or without initial response 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; sent in reply to a <response/> element or
   an <auth/> element with initial response data.

   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 a <response/>
   element.

   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 Section 3).  That is, there MUST be an XMPP resource
   identifier associated with the bare JID (<node@domain>) of the
   client, with the result that the address for use over that stream is
   a 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.

   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 sending a response stream header to the client after successful
   SASL negotiation, the server MUST include a <bind/> element qualified
   by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace in the stream
   features it presents to the client; this <bind/> element SHOULD
   include an empty <required/> element to explicitly indicate that
   resource binding must be completed at this stage of the stream
   negotiation process.  (Note: The server SHOULD NOT include the
   resource binding stream feature until after successful SASL
   negotiation.)

   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>



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      </stream:features>

   Upon being so informed that resource binding is required, the client
   MUST bind a resource to the stream as described in the following
   sections.

8.3.  Server-Generated Resource Identifier

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

8.3.1.  Success Case

   A client requests a server-generated resource identifier by sending
   an IQ stanza of type "set" (see Section 9.2.3) containing an empty
   <bind/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind'
   namespace.

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

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

   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>






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8.3.2.  Error Case

   It is possible that 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).  In this case, the server
   MUST return a <not-allowed/> stanza error to the client.

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

8.4.  Client-Generated Resource Identifier

   A client MAY attempt to specify the resource identifier on its own
   rather than asking the server to generate a resource identifier on
   its behalf.

8.4.1.  Success Case

   A client asks its server to accept a client-generated resource
   identifier by sending an IQ stanza of 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 MAY accept the resource identifier provided by the client,
   in which case it returns an IQ stanza of type "result" to the client,
   including a <jid/> child element that specifies the full JID for the
   connected resource.

   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 server MAY instead override the client-generated
   resource identifier and generate a resource identifier on behalf of
   the client, as shown in the previous section.





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8.4.2.  Error Cases

   When a client attempts to set its own XMPP resource identifier during
   resource binding, the following stanza error conditions are possible:

   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 cannot be processed by the
      server, e.g. because it is not in 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 of multiple connected resources with the
      same identifier.

8.4.2.1.  Not Allowed

   If the client is not allowed to bind a resource to the stream, the
   server MUST 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 provided resource identifier cannot be processed by the
   server, the server MAY return a <bad-request/> error (but SHOULD
   instead apply the Resourceprep (Appendix B) profile of [STRINGPREP]
   or otherwise process the resource identifier so that it is in
   conformance).

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

8.4.2.3.  Conflict

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

   1.  Not accept the resource identifier provided by the client but
       instead override it with an XMPP resource identifier that the
       server generates.



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   2.  Terminate the current resource and allow the newly-requested
       resource.
   3.  Disallow the newly-requested resource and maintain the current
       resource.

   Which of these the server does is up to the implementation, although
   it is RECOMMENDED to implement case #1.

   In case #2, the server MUST send a <conflict/> stream error to the
   current resource, terminate the XML stream and underlying TCP
   connection for the current resource, and return an IQ stanza of type
   "result" (indicating success) to the newly-requested resource.

   In case #3, the server MUST send a <conflict/> stanza error to the
   newly-requested resource but maintain the XML stream for that
   connection so that the newly-requested resource has an opportunity to
   negotiate a non-conflicting resource identifier before sending
   another request for resource binding.

8.5.  Binding Multiple Resources

   A server MAY support binding of multiple resources to the same
   stream.  This functionality is desirable in certain environments
   (e.g., for devices that are unable to open more than one TCP
   connection or when a machine runs a local XMPP client daemon that is
   used by multiple applications).

8.5.1.  Support

   If a server supports binding of multiple resources to a stream, it
   MUST enable a client to unbind resources.  A server that supports
   unbinding MUST also support binding of multiple resources.  Thus a
   client can discover whether a server supports binding of multiple
   resources by determining if the server advertises a stream feature of
   <unbind/>, as follows.

   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
   protocol for binding of the original resource, i.e., by sending an IQ
   stanza of type "set" containing a <bind/> element qualified by the



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   'urn:ietf:params:xml:ns:xmpp-bind' namespace (either empty to request
   server generation of the resource identifier or containing a
   <resource/> element with XML character data to request client
   generation of the resource identifier).

8.5.3.  Unbinding a Resource

8.5.3.1.  Success Case

   A client unbinds a resource by sending an IQ stanza of type "set"
   containing an <unbind/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-bind' namespace, which in turn contains
   a child element of <resource/> whose XML character data specifies the
   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 server MUST unbind the resource and no longer
   accept stanzas whose 'from' address specifies the full JID associated
   with that resource.

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

   When a client unbinds the only resource associated with the stream,
   the server SHOULD close the stream and terminate the TCP connection.

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

   S: </stream:stream>

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 stanza error, which SHOULD be <bad-request/> or <feature-
   not-implemented/>.

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




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8.5.3.2.2.  No Such Resource

   If there is no such resource for that stream, the server MUST return
   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 same stream, proper
   management of 'from' addresses is imperative.  In particular, a
   client MUST specify a 'from' address on every stanza it sends over a
   stream to which it has bound multiple resources, where the 'from'
   address is the full JID (<node@domain.tld/resource>) associated with
   the relevant resource.  If a client does not specify a 'from' address
   on a stanza it sends over a stream to which it has bound multiple
   resources, the server MUST return the stanza to the client with an
   <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>

   Naturally, the rules regarding validation of asserted 'from'
   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
   negotiated 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 stanza types.  These common attributes, as well
   as the basic semantics of the three stanza types, are defined herein;



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   more detailed information regarding the syntax of XML stanzas for
   instant messaging and presence applications is provided in [XMPP-IM],
   and for other applications in the relevant XMPP extension
   specifications.

   An XML stanza is the basic unit of meaning in XMPP.  A server MUST
   NOT process a partial stanza and a server MUST NOT attach meaning to
   the transmission timing of any child element within a stanza.

   Support for the XML stanza syntax and semantics 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
   stanzas.

9.1.1.  to

   The 'to' attribute specifies the JID of the 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 following rules apply to the 'to' attribute in the context of XML
   streams qualified by the 'jabber:client' namespace (i.e., client-to-
   server streams).

   1.  A stanza with a specific intended recipient MUST possess a 'to'
       attribute.
   2.  A stanza sent from a client to a server for direct processing by
       the server (e.g., presence sent to the server for broadcasting to
       other entities) SHOULD NOT possess a 'to' attribute.

9.1.1.2.  Server-to-Server Streams

   The following rules apply to the 'to' attribute in the context of XML
   streams qualified by the 'jabber:server' namespace (i.e., server-to-
   server streams).





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

9.1.2.  from

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

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

9.1.2.1.  Client-to-Server Streams

   The following rules apply to the 'from' attribute in 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 from a client and the
       stanza does not include a 'from' attribute, the server MUST add a
       'from' attribute to the stanza, where the value of the 'from'
       attribute is the full JID (<node@domain/resource>) determined by
       the server for the connected resource that generated the stanza
       (see Section 3.5), or the bare JID (<node@domain>) in the case of
       subscription-related presence stanzas (see [XMPP-IM]); the only
       exception to this rule occurs when multiple resources are bound
       to the same stream as described under Section 8.5.
   2.  When the server receives an XML stanza from a client and the
       stanza includes a 'from' attribute, the server MUST either (a)
       validate that the value of the 'from' attribute provided by the
       client is that of a connected resource for the associated entity
       or (b) override the provided 'from' attribute by adding a 'from'
       attribute as specified under Rule #1.
   3.  When the server generates a stanza from the server for delivery
       to the client on behalf of the account of the connected client
       (e.g., in the context of data storage services provided by the
       server on behalf of the client), the stanza MUST either (a) not
       include a 'from' attribute or (b) include a 'from' attribute
       whose value is the account's bare JID (<node@domain>).
   4.  When the server generates a stanza from the server itself for
       delivery to the client, the stanza MUST include a 'from'
       attribute whose value is the mere domain (<domain>) of the
       server.





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   5.  A server MUST NOT send to the client a stanza without a 'from'
       attribute if the stanza was not generated by the server (e.g., if
       it was generated by another client or another server); therefore,
       when a client receives a stanza that does not include a 'from'
       attribute, it MUST assume that the stanza is from the server to
       which the client is connected.

9.1.2.2.  Server-to-Server Streams

   The following rules apply to the 'from' attribute in the context of
   XML streams qualified by the 'jabber:server' namespace (i.e., server-
   to-server streams).

   1.  A stanza MUST possess a 'from' attribute; if a server receives a
       stanza that does not meet this restriction, it MUST generate an
       <improper-addressing/> stream error and terminate the underlying
       TCP connection.
   2.  The domain identifier portion of the JID contained in the 'from'
       attribute MUST match the hostname of the sending server (or any
       validated domain thereof) as communicated in the SASL negotiation
       (see Section 7), server dialback (see [XEP-0220], or similar
       means; if a server receives a stanza that does not meet this
       restriction, it MUST generate an <invalid-from/> stream error and
       terminate the underlying TCP connection.

   Enforcement of these rules helps to prevent a denial of service
   attack launched from a rogue server.

9.1.3.  id

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

9.1.4.  type

   The 'type' attribute specifies the purpose or context of the message,
   presence, or IQ stanza.  The particular allowable values for the
   'type' attribute vary depending on whether the stanza is a message,
   presence, or IQ stanza.  The defined values for message and presence
   stanzas are specific to instant messaging and presence applications
   and therefore are specified in [XMPP-IM], whereas the values for IQ
   stanzas specify the role of an IQ stanza in 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



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   Section 9.3.

9.1.5.  xml:lang

   A stanza SHOULD possess an 'xml:lang' attribute (as defined in
   Section 2.12 of [XML]) if the stanza contains XML character data that
   is intended to be presented to a human user (as explained in
   [CHARSET], "internationalization is for humans").  The value of the
   'xml:lang' attribute specifies the 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 'xml:lang' attribute MAY be overridden by the 'xml:
   lang' attribute of a 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 client's server MAY add an 'xml:lang' attribute
   whose value is that specified for the stream as defined under
   Section 5.3.

   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'
                xml:lang='en'>
        <show>dnd</show>
        <status>Wooing Juliet</status>
      </presence>

   If an inbound stanza received received by a client or server does not
   possess an 'xml:lang' attribute, an implementation MUST assume that
   the default language is that specified for the stream as defined
   under Section 5.3.

   The value of the 'xml:lang' attribute MUST conform to the NMTOKEN



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   datatype (as defined in Section 2.3 of [XML]) and MUST conform to the
   format defined in [LANGTAGS].

   A server 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 seen as a "push" mechanism whereby one
   entity pushes information to another entity, similar to the
   communications that occur in a system such as email.  All message
   stanzas SHOULD possess a 'to' attribute that specifies the intended
   recipient of the message; upon receiving such a stanza, a server
   SHOULD route or deliver it to the intended recipient (see Section 11
   for general routing and delivery rules related to XML stanzas).

9.2.2.  Presence Semantics

   The <presence/> stanza can be seen as 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 (client) SHOULD send a presence stanza with no 'to' attribute,
   in which case the server to which the entity is connected SHOULD
   broadcast or multiplex that stanza to all subscribing entities.
   However, a publishing entity MAY also send a presence stanza with a
   'to' attribute, in which case the server SHOULD route or deliver that
   stanza to the intended recipient.  See Section 11 for general routing
   and delivery rules related 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 the request and response is defined by the schema or other
   structural definition associated with the XML namespace that
   qualifies the direct child element of the IQ element (see
   Section 9.4), and the interaction is tracked by the requesting entity
   through use of the 'id' attribute.  Thus, IQ interactions follow a
   common pattern of structured data exchange such as get/result or set/
   result (although an error may be returned in reply to a request if
   appropriate):





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   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 enforce these semantics, the following rules apply:

   1.  The 'id' attribute is REQUIRED for IQ stanzas.
   2.  The 'type' attribute is REQUIRED for IQ stanzas.  The value MUST
       be one of the following:
       *  get -- The stanza is a request for information or
          requirements.
       *  set -- The stanza provides required data, sets new values, or
          replaces existing values.
       *  result -- The stanza is a response to a successful get or set
          request.
       *  error -- An error has occurred regarding processing or
          delivery of a previously-sent get or set request (see
          Section 9.3).
   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 the 'id' attribute of the request.
   4.  An entity that receives a stanza of type "result" or "error" MUST
       NOT respond to the stanza by sending a further IQ response of
       type "result" or "error"; however, 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).




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   5.  An IQ stanza of type "get" or "set" MUST contain one and only one
       child element, which specifies the semantics of the particular
       request.
   6.  An IQ stanza of type "result" MUST include zero or one child
       elements.
   7.  An IQ stanza of type "error" MAY include the child element
       contained in the associated "get" or "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 XML
   stream and underlying TCP connection.  Instead, the entity that
   discovers the error condition returns an ERROR STANZA to the sender,
   i.e., a stanza of the same kind (message, presence, or IQ) whose
   'type' attribute is set to a value of "error" and which contains an
   <error/> child element that specifies the error condition.  The
   specified error condition provides a hint regarding actions that the
   sender can take to remedy the error.

9.3.1.  Rules

   The following rules apply to stanza errors:

   1.  The receiving or processing entity that detects an error
       condition in 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.
   3.  An error stanza MUST contain an <error/> child element.
   4.  An <error/> child MUST NOT be included if the 'type' attribute
       has a value other than "error" (or if there is no 'type'
       attribute).
   5.  An entity that receives an error stanza MUST NOT respond to the
       stanza with a further error stanza; this helps to prevent
       looping.

9.3.2.  Syntax

   The syntax for stanza-related errors is:








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   <stanza-kind from='intended-recipient' to='sender' type='error'>
     [OPTIONAL 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>

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

   The "error-type MUST be one of the following:

   o  cancel -- do not retry (the error cannot be remedied)
   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)

   The <error/> element:

   o  MUST contain a child element corresponding to one of the stanza
      error conditions defined under Section 9.3.3; this element MUST be
      qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' 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-stanzas' namespace
      and SHOULD possess an 'xml:lang' attribute specifying the natural
      language of the XML character data.
   o  MAY contain a child element for an application-specific error
      condition; this element MUST be qualified by an application-
      specific namespace that defines the syntax and semantics of the
      element.

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







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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 stanza containing XML that does not conform to
   the appropriate schema or that cannot be processed (e.g., an IQ
   stanza that includes an unrecognized value of the 'type' attribute);
   the associated error type SHOULD be "modify".

   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
   same name or address; the associated error type SHOULD be "cancel".

   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>








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9.3.3.3.  feature-not-implemented

   The feature represented in the XML stanza is not implemented by the
   intended recipient or an intermediate server and therefore the stanza
   cannot be processed; the associated error type SHOULD be "cancel" or
   "modify".

   C: <iq 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".

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



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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 XML character data
   of the <gone/> element); the 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'
          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

   The server could not process the stanza because of a misconfiguration
   or an otherwise-undefined internal server error; the associated error
   type SHOULD be "wait" or "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'
          type='error'>
        <error type='wait'>
          <internal-server-error
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>






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9.3.3.7.  item-not-found

   The addressed JID or item requested cannot be found; the 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>

   Note: An application MUST NOT return this error if doing so would
   provide information about the intended recipient's network
   availability to an entity that is not authorized to know such
   information; instead it SHOULD return a <service-unavailable/> error.

9.3.3.8.  jid-malformed

   The sending entity has provided or communicated an XMPP address
   (e.g., a value of the 'to' attribute) or aspect thereof (e.g., an
   XMPP resource identifier) that does not adhere to the syntax defined
   under Section 3; the associated error type SHOULD be "modify".

   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>







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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 recipient
   or server (e.g., a local policy regarding stanza size limits or
   acceptable words in messages); the 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 perform the
   action (e.g., sending to entities at a blacklisted domain); 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='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
   perform the action, or has provided improper credentials; the
   associated error type SHOULD be "auth".







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   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".

   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>






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9.3.3.13.  payment-required

   The requesting entity is not authorized to access the requested
   service because payment is required; the 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>

9.3.3.14.  recipient-unavailable

   The intended recipient is temporarily unavailable; the 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>

   Note: An application MUST NOT return this error if doing so would
   provide information about the intended recipient's network
   availability to an entity that is not authorized to know such



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   information; instead it SHOULD return a <service-unavailable/> error.

9.3.3.15.  redirect

   The recipient or server is redirecting requests for this information
   to another entity, typically in a temporary fashion; the associated
   error type SHOULD be "modify" and the error stanza SHOULD contain the
   alternate address (which SHOULD be a valid JID) in the XML character
   data of the <redirect/> element.

   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>
      </presence>

9.3.3.16.  registration-required

   The requesting entity is not authorized to access the requested
   service because prior registration is required; 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'>
          <registration-required
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>




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9.3.3.17.  remote-server-not-found

   A remote server or service specified as part or all of the JID of the
   intended recipient does not exist; the associated error type SHOULD
   be "cancel".

   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>

9.3.3.18.  remote-server-timeout

   A remote server or service specified as part or all of the JID of the
   intended recipient (or required to fulfill a request) could not be
   contacted within a reasonable amount of time; the 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'>
          <remote-server-timeout
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.19.  resource-constraint

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




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   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>

9.3.3.20.  service-unavailable

   The server or recipient does not currently provide the requested
   service; the associated error type SHOULD 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 <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 an entity that is not authorized
   to know such information.

9.3.3.21.  subscription-required

   The requesting entity is not authorized to access the requested
   service because a prior subscription is required; the associated
   error type SHOULD be "auth".



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   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>

9.3.3.22.  undefined-condition

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





























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   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 request but was not expecting
   it at this time (e.g., the request was out of order); the associated
   error 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 stream (e.g., the stanza does not include a 'from'
   address when multiple resources are bound to the stream as described
   under Section 8.5.4); 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
   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



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   contain two 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 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 basic stanza
   syntax for the purpose of providing additional functionality.  Thus a
   message or presence stanza MAY contain one or more optional child
   elements specifying content that extends the meaning of the message
   (e.g., an XHTML-formatted version of the message body as described in
   [XEP-0071]), and an IQ stanza of type "get" or "set" MUST contain one
   such child element.  This child element MAY have any name and MUST
   possess a namespace declaration (other than "jabber:client", "jabber:
   server", or "http://etherx.jabber.org/streams") that defines all data
   contained within the child element.  Such a child element is said to
   be EXTENDED CONTENT and its namespace name is said to be 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 is (1) the recipient or (2) an entity that is routing the
   stanza to the recipient.







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   Recipient:  If a recipient receives a stanza that contains a child
      element it does not understand, it SHOULD silently ignore that
      particular XML data, i.e., it SHOULD not process it or present it
      to a user or associated application (if any).  In particular:
      *  If an entity receives a message or presence stanza that
         contains XML data qualified by a namespace it does not
         understand, the portion of the stanza that qualified by the
         unknown namespace SHOULD be ignored.
      *  If an entity receives a message stanza whose only child element
         is qualified by a namespace it does not understand, it MUST
         ignore the entire stanza.
      *  If an entity receives an IQ stanza of type "get" or "set"
         containing a child element qualified by a namespace it does not
         understand, the entity SHOULD return an IQ stanza of type
         "error" with an error condition of <service-unavailable/>.
   Router:  If a routing entity (typically a server) handles a stanza
      that contains a child element it does not understand, it SHOULD
      ignore the associated XML data by routing or delivering it
      untouched to the recipient.


10.  Examples

10.1.  Client-to-Server

   The following examples show the XMPP data flow for a client
   negotiating an XML stream with a server, exchanging XML stanzas, and
   closing the negotiated stream.  The server is "example.com", the
   server requires use of TLS, the client authenticates via the SASL
   DIGEST-MD5 mechanism as "juliet@example.com", and the client binds a
   server-generated resource to the stream.  It is assumed that before
   sending the initial stream header, the client has already resolved an
   SRV record of _xmpp-client._tcp.example.com and has opened a TCP
   connection to the advertised port at the resolved IP address.

   Note: The alternate steps shown are provided only to illustrate the
   protocol for failure cases; they are not exhaustive and would not
   necessarily be triggered by the data sent in the examples.













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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 response stream header to
   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 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 allowed to proceed:

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

   Step 5 (alt): Server informs client that 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>




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

   Step 7: If TLS negotiation is successful, client initiates a new
   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 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 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 this case
   [DIGEST-MD5] with an empty authorization identity ("="):

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







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

   S: <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

   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 subsequent authentication:

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

   The decoded value for subsequent authentication is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd





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

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

   Step 13: Client initiates a new 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'

10.1.3.  Resource Binding

   Step 14: Server responds by sending a stream header to client along
   with supported features (in this 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 required, the client
   MUST bind a resource to the stream; here we assume that the client
   asks the server to generate a resource identifier on its behalf.

   Step 15: Client binds a resource:

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





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   Step 16: Server generates resource identifier and 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 client is allowed to send XML stanzas over the 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 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 the message is 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 client may send and receive an unbounded number of subsequent XML
   stanzas over the stream.

10.1.5.  Close

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

   C: </stream:stream>

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




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   S: </stream:stream>

   Client now terminates the underlying TCP connection.

10.2.  Server-to-Server Examples

   The following examples show the data flow for a server negotiating an
   XML stream with another server, exchanging XML stanzas, and closing
   the negotiated stream.  The initiating server ("Server1") is
   example.com; the receiving server ("Server2") is example.net and it
   requires use of TLS; example.com presents a certificate and
   authenticates via the SASL EXTERNAL mechanism.  It is assumed that
   before sending the initial stream header, Server1 has already
   resolved an SRV record of _xmpp-server._tcp.example.net and has
   opened a TCP connection to the advertised port at the resolved IP
   address.

   Note: The alternate steps shown are provided only to illustrate the
   protocol for failure cases; they are not exhaustive and would not
   necessarily be triggered by the data sent in 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'>

   Step 2: Server2 responds by sending a response stream header 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'>









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   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 STARTTLS command to Server2:

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

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

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

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

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

   S2: </stream:stream>

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

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

   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 sends a response stream header to Server1 along with
   available stream features (including a preference for the SASL
   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 EXTERNAL mechanism, in this case with an
   authorization identity encoded according to [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 authorization identity provided
   by Server1 matches the information in the presented certificate and
   therefore returns success:

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

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

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

   S2: </stream:stream>








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   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 any additional features (or, in 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 stanzas to Server2 over the
   negotiated stream; here we assume that the transferred stanzas are
   those shown earlier for client-to-server communication.

   Server1 sends XML 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 message is delivered from
   Server2 to Server1.

   Server2 sends XML stanza to 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 send no further messages, Server1 closes the 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 ensure in-order processing of XML stanzas between
   any two entities.  This includes stanzas sent by a client to its
   server for direct processing by the server (e.g., in-order processing
   of a roster get and initial presence as described in [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 ROUTE a
   given stanza to another domain, DELIVER it to a local entity
   (typically a connected client associated with a local account), or
   HANDLE it directly within the server itself.  The following rules
   apply.

   Note: Particular XMPP applications MAY specify delivery rules that
   modify or supplement the following rules; for example, a set of
   delivery rules for instant messaging and presence applications is
   defined in [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 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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11.1.2.  Message

   If the server receives a message stanza with no 'to' attribute, it
   SHOULD handle it directly, which may include returning an error to
   the sending entity.

11.1.3.  Presence

   If the server receives a presence stanza with no 'to' attribute, it
   SHOULD broadcast it to the entities that are subscribed to the
   sending entity's presence, if applicable (the semantics of presence
   broadcast for presence applications are defined in [XMPP-IM]).

11.1.4.  IQ

   If the server receives an IQ stanza of type "get" or "set" with no
   'to' attribute, it MUST do the following:

   1.  If it understands the namespace that qualifies the content of the
       stanza, it MUST either handle the stanza directly on behalf of
       sending entity (where the meaning of "handle" is determined by
       the semantics of the qualifying namespace) or return an
       appropriate error to the sending entity.
   2.  If it does not understand the namespace that qualifies the
       content of the stanza, it MUST return an error to the sending
       entity, which SHOULD be <service-unavailable/>.

11.2.  Local Domain

   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 itself, the server MUST first determine if the hostname is
   serviced by the server or by a specialized local service.  If the
   latter, the server MUST route the stanza to that service.  If the
   former, the server MUST proceed as follows.

11.2.1.  Mere Domain

   If the JID contained in the 'to' attribute is of the form <domain>,
   then the server MUST either handle the stanza as appropriate for the
   stanza kind or return an error stanza to the sender.

11.2.2.  Resource at Domain

   If the JID contained in the 'to' attribute is of the form <domain/
   resource>, then the server MUST either handle the stanza as
   appropriate for the stanza kind or return an error stanza to the
   sender.



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11.2.3.  Node at Local Domain

   If the JID contained in the 'to' attribute is of the form
   <node@domain> (bare JID) or <node@domain/resource> (full JID), then
   the server SHOULD deliver the stanza to the intended recipient.  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 connected
       resource that exactly matches the full JID, the recipient's
       server SHOULD deliver the stanza to 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 the form <node@domain> and there exists at least
       one connected resource for the node, the recipient's server
       SHOULD deliver the stanza to at least one of the connected
       resources if the stanza is a message or presence stanza and
       SHOULD handle it directly on behalf of the node if the stanza is
       an IQ stanza.

   Note: More detailed rules in the context of instant messaging and
   presence applications are provided in [XMPP-IM].

11.3.  Foreign Domain

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute does not match one of the configured hostnames
   of the server itself, the server SHOULD attempt to route the stanza
   to the foreign domain (subject to local service provisioning and
   security policies regarding inter-domain communication, since such
   communication is optional for any given deployment).  There are two
   possible cases.

11.3.1.  Existing Stream

   If a server-to-server stream already exists between the two domains,
   the sender's server shall attempt to route the stanza to the
   authoritative server for the foreign domain over the existing stream.

11.3.2.  No Existing Stream

   If there exists no server-to-server stream between the two domains,
   the sender's server shall proceed as follows:






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   1.  Resolve the hostname of the foreign domain (as defined under
       Section 15.4).
   2.  Negotiate a server-to-server stream between the two domains (as
       defined under Section 6 and Section 7).
   3.  Route the stanza to the authoritative server for the foreign
       domain over the newly-established stream.

11.3.3.  Error Handling

   If routing to the intended recipient's server is unsuccessful, the
   sender's server MUST return an error to the sender, which SHOULD be
   <remote-server-not-found/> if resolution of the foreign domain is
   unsuccessful and <remote-server-timeout/> if resolution succeeds but
   streams cannot be negotiated.

   If stream negotiation with the intended recipient's server is
   successful but the foreign server cannot deliver the stanza to the
   recipient, the foreign server shall return an error to the sender by
   way of the sender's server.


12.  XML Usage

12.1.  Restrictions

   The Extensible Messaging and Presence Protocol (XMPP) defines a class
   of data objects called XML streams as well as the behavior of
   computer programs that process XML streams.  XMPP is an application
   profile of the Extensible Markup Language [XML], and a complete XML
   stream (including start and end stream tags) is a conforming XML
   document.

   However, XMPP does not deal with XML documents but with XML streams.
   Because XMPP does not require the parsing of arbitrary and complete
   XML documents, there is no requirement that XMPP needs to support the
   full feature set of [XML].  In particular, the following 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 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

   An XMPP implementation MUST behave as follow with regard to these



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   features:

   1.  An XMPP implementation MUST NOT inject characters matching such
       features into an XML stream.
   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 XMPP streams to
   create strict boundaries of data ownership.  The basic function of
   namespaces is to separate different vocabularies of XML elements that
   are structurally mixed together.  Ensuring that XMPP streams are
   namespace-aware enables any allowable XML to be structurally mixed
   with any data element within XMPP.  XMPP-specific 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
   and the name of the streams namespace MUST be
   'http://etherx.jabber.org/streams'.  If this rule is violated, the
   entity that receives the offending stream header MUST return a stream
   error to the sending entity, which SHOULD be <invalid-namespace/> but
   MAY be <bad-format/>.

   The element names of the <stream/> element and its <features/> and
   <error/> children MUST be qualified by the streams namespace prefix
   in all instances.  If this rule is violated, the entity that receives
   the offending element MUST return a stream error to the sending
   entity, which SHOULD be <bad-format/>.

   An implementation SHOULD generate only the 'stream:' prefix for these
   elements, and for historical reasons MAY accept only the 'stream:'
   prefix.  If an entity receives a stream header with a streams
   namespace prefix it does not accept, it MUST return a stream error to
   the sending entity, which SHOULD be <bad-namespace-prefix/> but MAY
   be <bad-format/>.

12.2.2.  Default Namespace

   A default namespace declaration is REQUIRED and defines the allowable
   first-level children of the root stream element.  This namespace
   declaration MUST be the same for the initial stream and the response
   stream so that both streams are qualified consistently.  The default
   namespace declaration applies to the stream and all first-level child



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   element sent within a stream unless explicitly qualified by the
   streams namespace or another namespace).

   A server implementation MUST support the 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
      stream is used for communication between a client and a server
   o  jabber:server -- this default namespace is declared when the
      stream is used for communication between two servers

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

   If an implementation accepts a stream that is qualified by the
   'jabber:client' or 'jabber:server' namespace, it MUST support the
   common attributes (Section 9.1) and basic semantics (Section 9.2) of
   all three core stanza types (message, presence, and IQ).

   An implementation MUST NOT generate namespace prefixes for elements
   qualified by the default namespace if the default namespace is
   'jabber:client' or 'jabber:server'.

   Note: The 'jabber:client' and 'jabber:server' namespaces are nearly
   identical but are used in different contexts (client-to-server
   communication for 'jabber:client' and server-to-server communication
   for 'jabber:server').  The only difference between the two is that
   the '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.

   An implementation MAY support a default namespace other than "jabber:
   client" or "jabber:server".  However, because such namespaces would
   define applications other than XMPP, they are to be defined in
   separate specifications.

12.2.3.  Extended Namespaces

   An EXTENDED NAMESPACE is an XML namespace that qualifies extended
   content as defined under Section 9.4.  For example, in the following
   stanza, the extended namespace is 'jabber:iq:roster':







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   <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 direct child level of the stanza
   (for presence and message stanzas) or in any mix of levels (for all
   stanzas).

   <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 the
   default namespace.  However, if included, the namespace declarations
   for those prefixes MUST be included on the 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 delivered with the
   stanza, instead of assumed from the stream).



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12.3.  Validation

   A server is not responsible for ensuring that XML data delivered to a
   client or routed to another server is valid, in accorfdance with the
   definition of "valid" provided in 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 ability to
   send data that does not conform to the schemas, and SHOULD ignore any
   non-conformant elements or attributes on the incoming XML stream.

   Note: The terms "valid" and "well-formed" are distinct in XML.  All
   XMPP data MUST be well-formed, in accordance with the definition of
   "well-formed" provided in Section 2.1 of [XML].

12.4.  Inclusion of Text Declaration

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

12.5.  Character Encoding

   Implementations MUST support the UTF-8 transformation of Universal
   Character Set [UCS2] characters, as required by [CHARSET] and defined
   in [UTF-8].  Implementations MUST NOT attempt to use any other
   encoding.  If one party to an XML stream detects that the other party
   has attempted to send XML data with an encoding other than UTF-8, it
   MUST return a stream error, which SHOULD be <unsupported-encoding/>.

12.6.  White Space

   Except where explicitly disallowed (e.g., during TLS negotiation
   (Section 6) and SASL negotiation (Section 7)), either entity MAY send
   white space characters (matching production [3] content of [XML])
   within the root stream element as separators between XML stanzas or
   between any other first-level elements sent over the stream; one
   common use for 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 Presence Protocol that MUST be supported by servers and
   clients in order to be considered compliant implementations, as well
   as additional protocol aspects that SHOULD be supported.  For



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   compliance purposes, we draw a distinction between core protocols
   (which MUST be supported by any server or client, regardless of the
   specific application) and instant messaging and presence protocols
   (which MUST be supported only by instant messaging and presence
   applications built on top of the core protocols).  Compliance
   requirements that apply to all servers and clients are specified in
   this section; compliance requirements for instant messaging and
   presence applications are specified in the corresponding section of
   [XMPP-IM].

13.1.  Servers

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

   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, as well as enforcement thereof 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 the three defined stanza types (i.e.,
      <message/>, <presence/>, and <iq/>)
   o  Generation (and, where appropriate, handling) of error syntax and
      semantics related to streams, TLS, SASL, and XML stanzas

   For backward compatibility with the large deployed base of XMPP
   servers, server developers are advised to implement the server
   dialback protocol first specified in [RFC3920] and now documented in
   [XEP-0220], since that protocol is widely used for weak identity
   verification of 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 Resource Binding (Section 8)
   o  The basic semantics of the three defined stanza types (i.e.,
      <message/>, <presence/>, and <iq/>)
   o  Handling (and, where appropriate, generation) of error syntax and
      semantics related to streams, TLS, SASL, and XML stanzas

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





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   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

   As specified under Section 12.5, XML streams MUST be encoded in
   UTF-8.

   As specified under Section 5.3, an XML stream SHOULD include an 'xml:
   lang' attribute specifying the default language for any XML character
   data 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 MUST NOT modify or delete 'xml:lang'
   attributes on stanzas it receives from other entities.

   As specified under Section 3, a server MUST support and enforce
   [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; this enables XMPP
   addresses to include a wide variety of Unicode characters outside the
   US-ASCII range.


15.  Security Considerations

15.1.  High Security

   For the purposes of XMPP communication (client-to-server and server-
   to-server), the term "high security" refers to the use of security
   technologies that provide both mutual authentication and integrity
   checking; in particular, when using certificate-based authentication
   to provide high security, a chain-of-trust SHOULD be established out-
   of-band, although a shared certification 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.






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15.2.  Certificate Validation

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

   Case #1:  The peer contains an End Entity certificate that appears to
      be certified by a chain of certificates terminating in a trust
      anchor (as described in Section 6.1 of [X509]).
   Case #2:  The peer certificate is certified by a Certificate
      Authority not known to the validating peer.
   Case #3:  The peer certificate is self-signed.

   In Case #1, the validating peer MUST do one of two things:
   1.  Verify the peer certificate according to the rules of [X509].
       The certificate SHOULD then be checked against the expected
       identity of the peer following the rules described in [HTTP-TLS],
       except that 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
       the user (clients MAY give the user the opportunity to continue
       with 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 MUST notify the user if it has
       changed.

   In Case #2 and Case #3, implementations SHOULD act as in Rule #2 for
   Case #1.

15.3.  Client-to-Server Communication

   A compliant client implementation MUST support both TLS and 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
   confidentiality and data integrity of data exchanged between two
   entities.

   The SASL protocol for authenticating XML streams (defined under



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   Section 7) provides a reliable mechanism for validating that 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 add the presented hostname to a
   configured set of accepted hostnames in order to expedite future
   connections.

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

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 to communicate with other domains, and server-
   to-server communication may be disabled by the administrator of any
   given deployment.  If a particular domain enables inter-domain
   communication, it should enable high security.

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

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

   1.  TCP
   2.  TLS
   3.  SASL





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   4.  XMPP

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

15.6.  Lack of SASL Channel Binding to TLS

   The SASL framework itself does not provide a method for binding SASL
   authentication to a security layer providing confidentiality and
   integrity protection that was negotiated at a lower layer.  Some SASL
   mechanisms provide such a binding.  However, if a SASL mechanism does
   not provide such a binding, then the mechanism cannot provide a way
   to verify that the source and destination end points to which the
   lower layer's security is bound are equivalent to the end points that
   SASL is authenticating; furthermore, if the end points are not
   identical, then the lower layer's security cannot be trusted to
   protect data transmitted between the SASL-authenticated entities.  In
   such a situation, a SASL security layer SHOULD be negotiated that
   effectively ignores the presence of the lower-layer security.

15.7.  Mandatory-to-Implement Technologies

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

   for authentication only:  the SASL [DIGEST-MD5] mechanism
   for confidentiality only:  TLS (using the
      TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher)
   for both authentication and confidentiality:  TLS plus SASL PLAIN for
      password-based authentication or TLS plus SASL EXTERNAL for non-
      password-based authentication (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.

   Note: The use of TLS plus SASL plain for replaces the SASL DIGEST-MD5
   mechanism as XMPP's mandatory-to-implement password-based
   authentication mechanism.  Implementations are encouraged to continue
   supporting the SASL DIGEST-MD5 mechanism as specified in
   [DIGEST-MD5].







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15.8.  Firewalls

   Communication using XMPP normally occurs over TCP connections on port
   5222 (client-to-server) or port 5269 (server-to-server), as
   registered with the IANA (see Section 16).  Use of these well-known
   ports allows administrators to easily enable or disable XMPP activity
   through existing and commonly-deployed firewalls.

15.9.  Use of base64 in SASL

   Both the 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 explicitly allowed by the
   base64 alphabet; this helps to guard against creation of a covert
   channel that could be used to "leak" information.  An implementation
   MUST NOT break on invalid input and MUST reject any sequence of
   base64 characters containing the pad ('=') character if that
   character is included as something other than the last character of
   the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against
   buffer overflow attacks and other attacks on the implementation.
   While base 64 encoding visually hides otherwise easily recognized
   information (such as passwords), it does not provide any
   computational confidentiality.  All uses of base 64 encoding MUST
   follow the definition in Section 4 of [BASE64] and padding bits MUST
   be set to zero.

15.10.  Stringprep Profiles

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

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

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

   A node identifier can be employed as one part of an entity's address
   in XMPP.  One common usage is as the username of an instant messaging
   user; another is as the name of a multi-user conference room; many
   other kinds of entities could use node identifiers as part of their



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   addresses.  The security of such services could be compromised based
   on different interpretations of the internationalized node
   identifier; for example, a user entering a single internationalized
   node identifier could access another user's account information, or a
   user could gain access to 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 name for an instant
   messaging user's connected resource; another is as the nickname of a
   user in a multi-user conference 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 the internationalized resource identifier; for
   example, a user could attempt to initiate multiple connections with
   the same name, or a user could send a message to someone other than
   the intended recipient in a multi-user conference room.

15.11.  Address Spoofing

   As discussed in [XEP-0165], there are two forms of address spoofing:
   forging and mimicking.

15.11.1.  Address Forging

   In the context of XMPP technologies, address forging occurs when an
   entity is able to generate an XML stanza whose 'from' address does
   not correspond to the account credentials with which the entity
   authenticated onto the 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".

   Address forging is difficult in XMPP systems, given the 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
   rogue server could forge JIDs at the sending domain by ignoring the
   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 entity provides legitimate
   authentication credentials for and sends XML stanzas from an account



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   whose JID appears to a human user to be the same as another JID.  For
   example, in some XMPP clients the address "paypa1@example.org"
   (spelled with the number one as the final character of the node
   identifier) may appear to be the same as "paypal@example.org (spelled
   with the lower-case version of the letter "L"), especially on casual
   visual inspection; this phenomenon is sometimes called "typejacking".
   A more sophisticated example of address mimicking might involve the
   use of characters from outside the US-ASCII range, such as the
   Cherokee characters U+13DA U+13A2 U+13B5 U+13AC U+13A2 U+13AC U+13D2
   instead of the US-ASCII characters "STPETER".

   In some examples of address mimicking, it is unlikely that the
   average user could tell the difference between the real JID and the
   fake JID.  (Naturally, there is no way to distinguish with full
   certainty which is the fake JID and which is the real JID; in some
   communication contexts, the JID with Cherokee characters may be the
   real JID and the JID with US-ASCII characters may thus appear to be
   the fake JID.)  Because JIDs can contain almost any Unicode
   character, it may be relatively easy to mimic some JIDs in XMPP
   systems.  The possibility of address mimicking introduces security
   vulnerabilities of the kind that have also plagued the World Wide
   Web, specifically the phenomenon known as phishing.

   Mimicked addresses that involve characters from only one character
   set or from the character set typically employed by a particular user
   are not easy to combat (e.g., the simple typejacking attack
   previously described, which relies on a surface similarity between
   the characters "1" and "l" in some presentations).  However, mimicked
   addresses that involve characters from more than one character set,
   or from a character set not typically employed by a particular user,
   can be mitigated somewhat through intelligent presentation.  In
   particular, every human user of an XMPP technology presumably has a
   preferred language (or, in some cases, a small set of preferred
   languages), which an XMPP application SHOULD gather either explicitly
   from the user or implicitly via the operating system of the user's
   device.  Furthermore, every language has a range (or a small set of
   ranges) of characters normally used to represent that language in
   textual form.  Therefore, an XMPP application SHOULD warn the user
   when presenting a JID that uses characters outside the normal range
   of the user's preferred language(s).  This recommendation is not
   intended to discourage communication across language communities;
   instead, it recognizes the existence of such language communities and
   encourages due caution when presenting unfamiliar character sets to
   human users.

   For more detailed recommendations regarding prevention of address
   mimicking in XMPP systems, refer to [XEP-0165].




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15.12.  Denial of Service

   [DOS] defines denial of service as follows:

      A Denial-of-Service (DoS) attack is an attack in which one or more
      machines target a victim and attempt to prevent the victim from
      doing useful work.  The victim can be a network server, client or
      router, a network link or an entire network, an individual
      Internet user or a company doing business using the Internet, an
      Internet Service Provider (ISP), country, or any combination of or
      variant on these.

   [XEP-0205] provides a detailed discussion of potential denial of
   service attacks against XMPP systems and best practices for
   preventing such attacks.  The recommendations include:

   1.  A server implementation SHOULD enable a server administrator to
       limit the number of TCP connections that it will accept from a
       given IP address at any one time.  If an entity attempts to
       connect but the maximum number of TCP connections has been
       reached, the receiving server MUST NOT allow the new connection
       to proceed.
   2.  A server implementation SHOULD enable a server administrator to
       limit the number of TCP connection attempts that it will accept
       from a given IP address in a given time period.  (While it is
       possible to limit the number of connections at the TCP layer
       rather than at the XMPP application layer, care must be taken in
       doing so since limits at the TCP layer might result in an
       inability to access non-XMPP services.)  If an entity attempts to
       connect but the maximum number of connections has been reached,
       the receiving server MUST NOT allow the new connection to
       proceed.
   3.  A server MUST NOT process XML stanzas from clients that have not
       yet provided appropriate authentication credentials and MUST NOT
       process XML stanzas from peer servers whose identity it has not
       either authenticated via SASL.
   4.  A server implementation SHOULD enable a server administrator to
       limit the number of connected resources it will allow an account
       to bind at any one time.  If a client attempts to bind a resource
       but it has already reached the configured number of allowable
       resources, the receiving server MUST return a <not-allowed/>
       stanza error.
   5.  A server implementation SHOULD enable a server administrator to
       limit the size of stanzas it will accept from a connected client
       or peer server.  If a connected resource or peer server sends a
       stanza that violates the upper limit, the receiving server SHOULD
       NOT process the stanza and instead SHOULD return a <not-allowed/>
       stanza error.  Alternatively (e.g., if the sender has sent an



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       egregiously large stanza), the server MAY instead return a
       <policy-violation/> stream error.
   6.  A server implementation SHOULD enable a server administrator to
       limit the number of XML stanzas that a connected client may send
       to distinct recipients within a given time period.  If a
       connected client sends too many stanzas to distinct recipients in
       a given time period, the receiving server SHOULD NOT process the
       stanza and instead SHOULD return an <unexpected-request/> stanza
       error.
   7.  A server implementation SHOULD enable a server administrator to
       limit the amount of bandwidth it will allow a connected client or
       peer server to use in a given time period.
   8.  A server implementation SHOULD enable a server administrator to
       limit the types of stanzas (based on the extended content
       "payload") that it will allow a connected resource or peer server
       send over an active connection.  Such limits and restrictions are
       a matter of deployment policy.

   For more detailed recommendations regarding denial of service attacks
   in XMPP systems, refer to [XEP-0205].

15.13.  Presence Leaks

   One of the core aspects of XMPP is presence, i.e., widespread
   information about the network availability of XMPP entities.
   Although presence is discussed more fully in [XMPP-IM], it is
   important to note that an XMPP server MUST NOT disclose an entity's
   presence to entities that are not authorized to know that information
   (such a disclosure is called a "presence leak").  In particular at
   the core XMPP level, real-time addressing and network availability is
   associated with a specific connected resource; therefore, any
   disclosure of a connected resource's full JID comprises a presence
   leak.  To help prevent such a presence leak, a server MUST NOT return
   different stanza errors if a potential attacker sends XML stanzas to
   the entity's bare JID (<node@domain>) or full JID
   (<node@domain/resource>).

15.14.  Directory Harvesting

   To help prevent directory harvesting attacks, a server MUST NOT
   return different stanza errors if a potential attacker sends XML
   stanzas to an existing entity or a nonexistent entity.  The stanza
   error returned in both cases SHOULD be <service-unavailable/>.


16.  IANA Considerations

   The following sections update the registrations provided in



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   [RFC3920].

16.1.  XML Namespace Name for TLS Data

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

   URI:  urn:ietf:params:xml:ns:xmpp-tls
   Specification:  XXXX
   Description:  This is the XML namespace name for STARTTLS negotiation
      data in the Extensible Messaging and Presence Protocol (XMPP) as
      defined by XXXX.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

16.2.  XML Namespace Name for SASL Data

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

   URI:  urn:ietf:params:xml:ns:xmpp-sasl
   Specification:  XXXX
   Description:  This is the XML namespace name for SASL negotiation
      data in the Extensible Messaging and Presence Protocol (XMPP) as
      defined by XXXX.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

16.3.  XML Namespace Name for Stream Errors

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

   URI:  urn:ietf:params:xml:ns:xmpp-streams
   Specification:  XXXX
   Description:  This is the XML namespace name for stream error data in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by XXXX.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

16.4.  XML Namespace Name for Resource Binding

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





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   URI:  urn:ietf:params:xml:ns:xmpp-bind
   Specification:  XXXX
   Description:  This is the XML namespace name for resource binding in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by XXXX.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

16.5.  XML Namespace Name for Stanza Errors

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

   URI:  urn:ietf:params:xml:ns:xmpp-stanzas
   Specification:  XXXX
   Description:  This is the XML namespace name for stanza error data in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by XXXX.
   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 the profile is defined:

      XXXX

   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

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

   Name of this profile:






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      Resourceprep

   RFC in which the profile is defined:

      XXXX

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

      This is the first version of Resourceprep

16.8.  GSSAPI Service Name

   The IANA has registered "xmpp" as a GSSAPI [GSS-API] service name, as
   defined under Section 7.4.

16.9.  Port Numbers

   The IANA has registered "xmpp-client" and "xmpp-server" as keywords
   for [TCP] ports 5222 and 5269 respectively.

   These ports SHOULD be used for client-to-server and server-to-server
   communications respectively, but other ports MAY be used.


17.  References

17.1.  Normative References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

   [BASE64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

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

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

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

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




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   [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 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.

   [RANDOM]   Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [SASL]     Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

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

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

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

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

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

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

   [UUID]     Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              July 2005.



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

   [XML]      Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F.,
              and T. Bray, "Extensible Markup Language (XML) 1.0 (Fourth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20060816, August 2006,
              <http://www.w3.org/TR/2006/REC-xml-20060816>.

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

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., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [DNS-TXT]  Rosenbaum, R., "Using the Domain Name System To Store
              Arbitrary String Attributes", RFC 1464, May 1993.

   [DOS]      Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, December 2006.

   [GSS-API]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [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.

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

   [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [IMP-REQS]



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              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 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. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [PUNYCODE]
              Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.

   [RFC3921]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence",
              RFC 3921, October 2004.

   [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.

   [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 of Object Identifiers",
              RFC 3061, February 2001.

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




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   [XEP-0001]
              Saint-Andre, P., "XMPP Extension Protocols", XSF XEP 0001,
              December 2006.

   [XEP-0045]
              Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
              April 2007.

   [XEP-0060]
              Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
              Subscribe", XSF XEP 0060, September 2007.

   [XEP-0071]
              Saint-Andre, P., "XHTML-IM", XSF XEP 0071, August 2007.

   [XEP-0077]
              Saint-Andre, P., "In-Band Registration", XSF XEP 0077,
              January 2006.

   [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,
              June 2007.

   [XEP-0157]
              Saint-Andre, P. and J. Konieczny, "Contact Addresses for
              XMPP Services", XSF XEP 0157, January 2007.

   [XEP-0165]
              Saint-Andre, P., "Best Practices to Prevent JID
              Mimicking", XSF XEP 0165, July 2007.

   [XEP-0174]
              Saint-Andre, P., "Link-Local Messaging", XSF XEP 0174,
              June 2007.

   [XEP-0175]
              Saint-Andre, P., "Best Practices for Use of SASL
              ANONYMOUS", XSF XEP 0175, September 2006.

   [XEP-0178]
              Saint-Andre, P. and P. Millard, "Best Practices for Use of
              SASL EXTERNAL with Certificates", XSF XEP 0178,



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              February 2007.

   [XEP-0205]
              Saint-Andre, P., "Best Practices to Discourage Denial of
              Service Attacks", XSF XEP 0205, July 2007.

   [XEP-0206]
              Paterson, I., "XMPP Over BOSH", XSF XEP 0206, June 2007.

   [XEP-0220]
              Saint-Andre, P. and J. Miller, "Server Dialback", XSF
              XEP 0220, July 2007.

   [XML-REG]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [XML-SCHEMA]
              Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech,
              "XML Schema Part 1: Structures Second Edition", World Wide
              Web Consortium Recommendation REC-xmlschema-1-20041028,
              October 2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>.

   [XMPP-IM]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and  Presence",
              draft-saintandre-rfc3921bis-03 (work in progress),
              July 2007.

   [XMPP-URI]
              Saint-Andre, P., "Internationalized Resource Identifiers
              (IRIs) and Uniform Resource Identifiers (URIs) for the
              Extensible Messaging and Presence Protocol (XMPP)",
              draft-saintandre-rfc4622bis-01 (work in progress),
              June 2007.


Appendix A.  Nodeprep

A.1.  Introduction

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



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

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

   o  The intended applicability of the profile: internationalized node
      identifiers within XMPP
   o  The character repertoire that is the input and output to
      stringprep: Unicode 3.2, specified in Section 2 of this Appendix
   o  The mappings used: specified in Section 3
   o  The Unicode normalization used: specified in Section 4
   o  The characters that are prohibited as output: specified in Section
      5
   o  Bidirectional character handling: specified in Section 6

A.2.  Character Repertoire

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

A.3.  Mapping

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

      Table B.1
      Table B.2

A.4.  Normalization

   This profile specifies the use of Unicode normalization form KC, as
   described in [STRINGPREP].

A.5.  Prohibited Output

   This profile specifies the prohibition of using 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







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      Table C.5
      Table C.6
      Table C.7
      Table C.8
      Table C.9

   In addition, the following Unicode characters are also prohibited:

      #x22 (")
      #x26 (&)
      #x27 (')
      #x2F (/)
      #x3A (:)
      #x3C (<)
      #x3E (>)
      #x40 (@)

A.6.  Bidirectional Characters

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


Appendix B.  Resourceprep

B.1.  Introduction

   This appendix defines the "Resourceprep" profile of stringprep.  As
   such, it specifies processing rules that will enable users to enter
   internationalized resource identifiers in the Extensible Messaging
   and Presence Protocol (XMPP) and have the highest chance of getting
   the content of the strings correct.  (An XMPP resource identifier is
   the optional portion of an XMPP address that follows an XMPP domain
   identifier and the '/' separator.)  These processing rules are
   intended only for XMPP resource identifiers and are not intended for
   arbitrary text or any other aspect of an XMPP address.

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

   o  The intended applicability of the profile: internationalized
      resource identifiers within XMPP
   o  The character repertoire that is the input and output to
      stringprep: Unicode 3.2, specified in Section 2 of this Appendix
   o  The mappings used: specified in Section 3
   o  The Unicode normalization used: specified in Section 4
   o  The characters that are prohibited as output: specified in Section
      5




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   o  Bidirectional character handling: specified in Section 6

B.2.  Character Repertoire

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

B.3.  Mapping

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

      Table B.1

B.4.  Normalization

   This profile specifies the use of Unicode normalization form KC, as
   described in [STRINGPREP].

B.5.  Prohibited Output

   This profile specifies the prohibition 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, as described
   in Section 6 of [STRINGPREP].


Appendix C.  XML Schemas

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

   The following schemas formally define various XML namespaces used in



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   the core XMPP protocols, in conformance with [XML-SCHEMA].  For
   schemas defining the 'jabber:client' and 'jabber:server' namespaces,
   refer to [XMPP-IM].

C.1.  Streams namespace

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

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

     <xs:import namespace='jabber:client'/>
     <xs:import namespace='jabber:server'/>

     <xs:element name='stream'>
       <xs:complexType>
         <xs:sequence xmlns:client='jabber:client'
                      xmlns:server='jabber:server'>
           <xs:element ref='features' minOccurs='0' maxOccurs='1'/>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-tls'
                   minOccurs='0'
                   maxOccurs='unbounded'/>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-sasl'
                   minOccurs='0'
                   maxOccurs='unbounded'/>
           <xs:choice minOccurs='0' maxOccurs='1'>
             <xs:choice minOccurs='0' maxOccurs='unbounded'>
               <xs:element ref='client:message'/>
               <xs:element ref='client:presence'/>
               <xs:element ref='client:iq'/>
             </xs:choice>
             <xs:choice minOccurs='0' maxOccurs='unbounded'>
               <xs:element ref='server:message'/>
               <xs:element ref='server:presence'/>
               <xs:element ref='server:iq'/>
               <xs:element ref='db:result'/>
               <xs:element ref='db:verify'/>
             </xs:choice>
           </xs:choice>
           <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
         </xs:sequence>
         <xs:attribute name='from' type='xs:string' use='optional'/>
         <xs:attribute name='id' type='xs:string' use='optional'/>
         <xs:attribute name='to' type='xs:string' use='optional'/>
         <xs:attribute name='version' type='xs:decimal' use='optional'/>



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         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='features'>
       <xs:complexType>
         <xs:any namespace='##other'/>
       </xs:complexType>
     </xs:element>

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

   </xs:schema>

C.2.  Stream error namespace

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

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

     <xs:element name='bad-format' type='empty'/>
     <xs:element name='bad-namespace-prefix' type='empty'/>
     <xs:element name='conflict' type='empty'/>
     <xs:element name='connection-timeout' type='empty'/>
     <xs:element name='host-gone' type='empty'/>
     <xs:element name='host-unknown' type='empty'/>
     <xs:element name='improper-addressing' type='empty'/>
     <xs:element name='internal-server-error' type='empty'/>
     <xs:element name='invalid-from' type='empty'/>
     <xs:element name='invalid-id' type='empty'/>
     <xs:element name='invalid-namespace' type='empty'/>
     <xs:element name='invalid-xml' type='empty'/>



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     <xs:element name='not-authorized' type='empty'/>
     <xs:element name='policy-violation' type='empty'/>
     <xs:element name='remote-connection-failed' type='empty'/>
     <xs:element name='resource-constraint' type='empty'/>
     <xs:element name='restricted-xml' type='empty'/>
     <xs:element name='see-other-host' type='xs:string'/>
     <xs:element name='system-shutdown' type='empty'/>
     <xs:element name='undefined-condition' type='empty'/>
     <xs:element name='unsupported-encoding' type='empty'/>
     <xs:element name='unsupported-stanza-type' type='empty'/>
     <xs:element name='unsupported-version' type='empty'/>
     <xs:element name='xml-not-well-formed' type='empty'/>

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

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



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         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

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

   </xs:schema>

C.3.  STARTTLS namespace

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

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

     <xs:element name='starttls'>
       <xs:complexType>
         <xs:sequence>
           <xs:element name='required'
                       minOccurs='0'
                       type='empty'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

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

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

   </xs:schema>

C.4.  SASL namespace

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

   <xs:schema



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

     <xs:element name='mechanisms'>
       <xs:complexType>
         <xs:sequence>
           <xs:element name='mechanism'
                       minOccurs='0'
                       maxOccurs='unbounded'
                       type='xs:NMTOKEN'/>
           <xs:element name='required'
                       minOccurs='0'
                       maxOccurs='1'
                       type='empty'/>
           <xs:any namespace='##other'
                   minOccurs='0'
                   minOccurs='unbounded'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

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

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

     <xs:element name='failure'>
       <xs:complexType>
         <xs:choice minOccurs='0'>
           <xs:element name='aborted' type='empty'/>
           <xs:element name='incorrect-encoding' type='empty'/>
           <xs:element name='invalid-authzid' type='empty'/>
           <xs:element name='invalid-mechanism' type='empty'/>
           <xs:element name='malformed-request' type='empty'/>



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           <xs:element name='mechanism-too-weak' type='empty'/>
           <xs:element name='not-authorized' type='empty'/>
           <xs:element name='temporary-auth-failure' type='empty'/>
         </xs:choice>
       </xs:complexType>
     </xs:element>

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

   </xs:schema>

C.5.  Resource binding namespace



































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

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

     <xs:element name='bind'>
       <xs:complexType>
         <xs:sequence>
           <xs:choice minOccurs='0' maxOccurs='1'>
             <xs:element name='resource' type='resourceType'/>
             <xs:element name='jid' type='fullJIDType'/>
           </xs:choice>
           <xs:element name='required'
                       minOccurs='0'
                       maxOccurs='1'
                       type='empty'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <xs:element name='unbind'>
       <xs:complexType>
         <xs:sequence minOccurs='0'>
           <xs:element name='resource' type='resourceType'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <xs:simpleType name='resourceType'>
       <xs:restriction base='xs:string'>
         <xs:minLength value='1'/>
         <xs:maxLength value='1023'/>
       </xs:restriction>
     </xs:simpleType>

     <xs:simpleType name='fullJIDType'>
       <xs:restriction base='xs:string'>
         <xs:minLength value='8'/>
         <xs:maxLength value='3071'/>
       </xs:restriction>
     </xs:simpleType>

   </xs:schema>

C.6.  Stanza error namespace



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

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

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

     <xs:group name='stanzaErrorGroup'>
       <xs:choice>
         <xs:element ref='bad-request'/>
         <xs:element ref='conflict'/>
         <xs:element ref='feature-not-implemented'/>
         <xs:element ref='forbidden'/>
         <xs:element ref='gone'/>
         <xs:element ref='internal-server-error'/>
         <xs:element ref='item-not-found'/>
         <xs:element ref='jid-malformed'/>
         <xs:element ref='not-acceptable'/>
         <xs:element ref='not-authorized'/>
         <xs:element ref='not-allowed'/>
         <xs:element ref='not-modified'/>
         <xs:element ref='payment-required'/>
         <xs:element ref='recipient-unavailable'/>



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         <xs:element ref='redirect'/>
         <xs:element ref='registration-required'/>
         <xs:element ref='remote-server-not-found'/>
         <xs:element ref='remote-server-timeout'/>
         <xs:element ref='resource-constraint'/>
         <xs:element ref='service-unavailable'/>
         <xs:element ref='subscription-required'/>
         <xs:element ref='undefined-condition'/>
         <xs:element ref='unexpected-request'/>
         <xs:element ref='unknown-sender'/>
       </xs:choice>
     </xs:group>

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

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

   </xs:schema>


Appendix D.  Contact Addresses

   Consistent with [MAILBOXES], an organization that offers an XMPP
   service should provide an Internet mailbox of "XMPP" for inquiries
   related to that service, where the host portion of the resulting
   mailto URI should be the organization's domain, not necessarily the
   domain of the XMPP service itself (e.g., the XMPP service might be
   offered at xmpp.example.net but the Internet mailbox should be
   <xmpp@example.net>).

   In addition, the XMPP service should provide a way to discover the
   XMPP contact address(es) of the service administrator(s), as
   specified in [XEP-0157].






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Appendix E.  Account Provisioning

   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].


Appendix F.  Differences From RFC 3920

   Based on consensus derived from implementation and deployment
   experience as well as formal interoperability testing, the following
   substantive modifications were made from RFC 3920.

   o  Corrected the ABNF syntax for JIDs to prevent zero-length node
      identifiers, domain identifiers, and resource identifiers.
   o  Corrected the nameprep processing rules to require use of the
      UseSTD3ASCIIRules flag.
   o  Encouraged use of the 'from' and 'to' attributes on stream
      headers.
   o  More fully specified stream closing handshake.
   o  Specified recommended stream reconnection algorithm.
   o  Specified return of <restricted-xml/> stream error in response to
      receipt of prohibited XML features.
   o  Specified that SASL mechanisms must be sent both before and after
      negotiation of SASL security layers.
   o  Specified that TLS plus SASL PLAIN is a mandatory-to-implement
      technology for client-to-server connections, since implementation
      of SASL EXTERNAL is uncommon in XMPP clients, in part because
      underlying security features such as end-user X.509 certificates
      are not yet widely deployed.
   o  Added the <malformed-request/> SASL error condition to handle an
      error case discussed in RFC 4422 but not in RFC 2222.
   o  More fully specified binding of multiple resources to the same
      stream.
   o  Added the <unknown-sender/> stanza error condition to provide
      appropriate handling of stanzas when multiple resources are bound
      to the same stream.
   o  Added the <not-modified/> stanza error condition to enable
      potential ETags usage.
   o  Moved historical documentation of the server dialback protocol
      from this specification to a separate specification maintained by
      the XMPP Standards Foundation.

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





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Appendix G.  Copying Conditions

   The Contributor grants third parties the irrevocable right to copy,
   use and distribute the Contribution, with or without modification, in
   any medium, without royalty, provided that, unless separate
   permission is granted, redistributed modified works:

   1.  do not contain misleading author, version, name of work, or
       endorsement information, and
   2.  do not claim endorsement of the modified work by the Contributor,
       or any organization the Contributor belongs to, the Internet
       Engineering Task Force (IETF), Internet Research Task Force
       (IRTF), Internet Engineering Steering Group (IESG), Internet
       Architecture Board (IAB), Internet Assigned Numbers Authority
       (IANA), Internet Society (ISOC), Request For Comments (RFC)
       Editor, or any combination or variation of such terms (including
       without limitation the IETF "4 diamonds" logo), or any terms that
       are confusingly similar thereto, and
   3.  remove any claims of status as an Internet Standard, including
       without limitation removing the RFC boilerplate.

   The IETF suggests that any citation or excerpt of unmodified text
   reference the RFC or other document from which the text is derived.


Index

   B
      Bare JID  15

   C
      Connected Resource  15

   D
      Domain Identifier  13

   E
      Entity  12
      Error Stanza  79
      Extended Content  94

   F
      Full JID  15

   I
      Initial Stream  18
      IQ Stanza  77




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   J
      Jabber Identifier  12

   M
      Message Stanza  77

   N
      Node Identifier  14

   P
      Presence Stanza  77

   R
      Resource Identifier  15
      Response Stream  18

   X
      XML Stanza  18
      XML Stream  18


Author's Address

   Peter Saint-Andre (editor)
   XMPP Standards Foundation

   Email: stpeter@jabber.org
   URI:   https://stpeter.im/























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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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