path: root/doc/rfc/rfc6120.txt

Internet Engineering Task Force (IETF)                    P. Saint-Andre
Request for Comments: 6120                                         Cisco
Obsoletes: 3920                                               March 2011
Category: Standards Track
ISSN: 2070-1721


        Extensible Messaging and Presence Protocol (XMPP): Core

Abstract

   The Extensible Messaging and Presence Protocol (XMPP) is an
   application profile of the Extensible Markup Language (XML) that
   enables the near-real-time exchange of structured yet extensible data
   between any two or more network entities.  This document defines
   XMPP's core protocol methods: setup and teardown of XML streams,
   channel encryption, authentication, error handling, and communication
   primitives for messaging, network availability ("presence"), and
   request-response interactions.  This document obsoletes RFC 3920.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6120.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   8
     1.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .   8
     1.2.   History  . . . . . . . . . . . . . . . . . . . . . . . .   8
     1.3.   Functional Summary . . . . . . . . . . . . . . . . . . .   9
     1.4.   Terminology  . . . . . . . . . . . . . . . . . . . . . .  11
   2.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .  13
     2.1.   Global Addresses . . . . . . . . . . . . . . . . . . . .  13
     2.2.   Presence . . . . . . . . . . . . . . . . . . . . . . . .  14
     2.3.   Persistent Streams . . . . . . . . . . . . . . . . . . .  14
     2.4.   Structured Data  . . . . . . . . . . . . . . . . . . . .  14
     2.5.   Distributed Network of Clients and Servers . . . . . . .  14
   3.  TCP Binding . . . . . . . . . . . . . . . . . . . . . . . . .  16
     3.1.   Scope  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     3.2.   Resolution of Fully Qualified Domain Names . . . . . . .  17
       3.2.1.   Preferred Process: SRV Lookup  . . . . . . . . . . .  17
       3.2.2.   Fallback Processes . . . . . . . . . . . . . . . . .  18
       3.2.3.   When Not to Use SRV  . . . . . . . . . . . . . . . .  18
       3.2.4.   Use of SRV Records with Add-On Services  . . . . . .  19
     3.3.   Reconnection . . . . . . . . . . . . . . . . . . . . . .  19
     3.4.   Reliability  . . . . . . . . . . . . . . . . . . . . . .  20
   4.  XML Streams . . . . . . . . . . . . . . . . . . . . . . . . .  20
     4.1.   Stream Fundamentals  . . . . . . . . . . . . . . . . . .  20
     4.2.   Opening a Stream . . . . . . . . . . . . . . . . . . . .  23
     4.3.   Stream Negotiation . . . . . . . . . . . . . . . . . . .  24
       4.3.1.   Basic Concepts . . . . . . . . . . . . . . . . . . .  24
       4.3.2.   Stream Features Format . . . . . . . . . . . . . . .  25
       4.3.3.   Restarts . . . . . . . . . . . . . . . . . . . . . .  27
       4.3.4.   Resending Features . . . . . . . . . . . . . . . . .  27
       4.3.5.   Completion of Stream Negotiation . . . . . . . . . .  27
       4.3.6.   Determination of Addresses . . . . . . . . . . . . .  28
       4.3.7.   Flow Chart . . . . . . . . . . . . . . . . . . . . .  29
     4.4.   Closing a Stream . . . . . . . . . . . . . . . . . . . .  31
     4.5.   Directionality . . . . . . . . . . . . . . . . . . . . .  32
     4.6.   Handling of Silent Peers . . . . . . . . . . . . . . . .  33
       4.6.1.   Dead Connection  . . . . . . . . . . . . . . . . . .  34
       4.6.2.   Broken Stream  . . . . . . . . . . . . . . . . . . .  34
       4.6.3.   Idle Peer  . . . . . . . . . . . . . . . . . . . . .  34
       4.6.4.   Use of Checking Methods  . . . . . . . . . . . . . .  35
     4.7.   Stream Attributes  . . . . . . . . . . . . . . . . . . .  35
       4.7.1.   from . . . . . . . . . . . . . . . . . . . . . . . .  35
       4.7.2.   to . . . . . . . . . . . . . . . . . . . . . . . . .  37
       4.7.3.   id . . . . . . . . . . . . . . . . . . . . . . . . .  38
       4.7.4.   xml:lang . . . . . . . . . . . . . . . . . . . . . .  39
       4.7.5.   version  . . . . . . . . . . . . . . . . . . . . . .  41
       4.7.6.   Summary of Stream Attributes . . . . . . . . . . . .  43
     4.8.   XML Namespaces . . . . . . . . . . . . . . . . . . . . .  43



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       4.8.1.   Stream Namespace . . . . . . . . . . . . . . . . . .  43
       4.8.2.   Content Namespace  . . . . . . . . . . . . . . . . .  43
       4.8.3.   XMPP Content Namespaces  . . . . . . . . . . . . . .  44
       4.8.4.   Other Namespaces . . . . . . . . . . . . . . . . . .  46
       4.8.5.   Namespace Declarations and Prefixes  . . . . . . . .  47
     4.9.   Stream Errors  . . . . . . . . . . . . . . . . . . . . .  48
       4.9.1.   Rules  . . . . . . . . . . . . . . . . . . . . . . .  48
         4.9.1.1.  Stream Errors Are Unrecoverable . . . . . . . . .  48
         4.9.1.2.  Stream Errors Can Occur During Setup  . . . . . .  49
         4.9.1.3.  Stream Errors When the Host Is Unspecified or
                   Unknown . . . . . . . . . . . . . . . . . . . . .  50
         4.9.1.4.  Where Stream Errors Are Sent  . . . . . . . . . .  50
       4.9.2.   Syntax . . . . . . . . . . . . . . . . . . . . . . .  51
       4.9.3.   Defined Stream Error Conditions  . . . . . . . . . .  52
         4.9.3.1.  bad-format  . . . . . . . . . . . . . . . . . . .  52
         4.9.3.2.  bad-namespace-prefix  . . . . . . . . . . . . . .  52
         4.9.3.3.  conflict  . . . . . . . . . . . . . . . . . . . .  53
         4.9.3.4.  connection-timeout  . . . . . . . . . . . . . . .  54
         4.9.3.5.  host-gone . . . . . . . . . . . . . . . . . . . .  54
         4.9.3.6.  host-unknown  . . . . . . . . . . . . . . . . . .  55
         4.9.3.7.  improper-addressing . . . . . . . . . . . . . . .  56
         4.9.3.8.  internal-server-error . . . . . . . . . . . . . .  56
         4.9.3.9.  invalid-from  . . . . . . . . . . . . . . . . . .  56
         4.9.3.10. invalid-namespace . . . . . . . . . . . . . . . .  57
         4.9.3.11. invalid-xml . . . . . . . . . . . . . . . . . . .  57
         4.9.3.12. not-authorized  . . . . . . . . . . . . . . . . .  58
         4.9.3.13. not-well-formed . . . . . . . . . . . . . . . . .  59
         4.9.3.14. policy-violation  . . . . . . . . . . . . . . . .  59
         4.9.3.15. remote-connection-failed  . . . . . . . . . . . .  60
         4.9.3.16. reset . . . . . . . . . . . . . . . . . . . . . .  60
         4.9.3.17. resource-constraint . . . . . . . . . . . . . . .  61
         4.9.3.18. restricted-xml  . . . . . . . . . . . . . . . . .  61
         4.9.3.19. see-other-host  . . . . . . . . . . . . . . . . .  62
         4.9.3.20. system-shutdown . . . . . . . . . . . . . . . . .  64
         4.9.3.21. undefined-condition . . . . . . . . . . . . . . .  64
         4.9.3.22. unsupported-encoding  . . . . . . . . . . . . . .  64
         4.9.3.23. unsupported-feature . . . . . . . . . . . . . . .  65
         4.9.3.24. unsupported-stanza-type . . . . . . . . . . . . .  65
         4.9.3.25. unsupported-version . . . . . . . . . . . . . . .  66
       4.9.4.   Application-Specific Conditions  . . . . . . . . . .  67
     4.10.  Simplified Stream Examples . . . . . . . . . . . . . . .  68
   5.  STARTTLS Negotiation  . . . . . . . . . . . . . . . . . . . .  69
     5.1.   Fundamentals . . . . . . . . . . . . . . . . . . . . . .  69
     5.2.   Support  . . . . . . . . . . . . . . . . . . . . . . . .  70
     5.3.   Stream Negotiation Rules . . . . . . . . . . . . . . . .  70
       5.3.1.   Mandatory-to-Negotiate . . . . . . . . . . . . . . .  70
       5.3.2.   Restart  . . . . . . . . . . . . . . . . . . . . . .  70
       5.3.3.   Data Formatting  . . . . . . . . . . . . . . . . . .  70



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       5.3.4.   Order of TLS and SASL Negotiations . . . . . . . . .  71
       5.3.5.   TLS Renegotiation  . . . . . . . . . . . . . . . . .  71
       5.3.6.   TLS Extensions . . . . . . . . . . . . . . . . . . .  72
     5.4.   Process  . . . . . . . . . . . . . . . . . . . . . . . .  72
       5.4.1.   Exchange of Stream Headers and Stream Features . . .  72
       5.4.2.   Initiation of STARTTLS Negotiation . . . . . . . . .  73
         5.4.2.1.  STARTTLS Command  . . . . . . . . . . . . . . . .  73
         5.4.2.2.  Failure Case  . . . . . . . . . . . . . . . . . .  73
         5.4.2.3.  Proceed Case  . . . . . . . . . . . . . . . . . .  74
       5.4.3.   TLS Negotiation  . . . . . . . . . . . . . . . . . .  74
         5.4.3.1.  Rules . . . . . . . . . . . . . . . . . . . . . .  74
         5.4.3.2.  TLS Failure . . . . . . . . . . . . . . . . . . .  75
         5.4.3.3.  TLS Success . . . . . . . . . . . . . . . . . . .  76
   6.  SASL Negotiation  . . . . . . . . . . . . . . . . . . . . . .  77
     6.1.   Fundamentals . . . . . . . . . . . . . . . . . . . . . .  77
     6.2.   Support  . . . . . . . . . . . . . . . . . . . . . . . .  77
     6.3.   Stream Negotiation Rules . . . . . . . . . . . . . . . .  77
       6.3.1.   Mandatory-to-Negotiate . . . . . . . . . . . . . . .  77
       6.3.2.   Restart  . . . . . . . . . . . . . . . . . . . . . .  78
       6.3.3.   Mechanism Preferences  . . . . . . . . . . . . . . .  78
       6.3.4.   Mechanism Offers . . . . . . . . . . . . . . . . . .  78
       6.3.5.   Data Formatting  . . . . . . . . . . . . . . . . . .  79
       6.3.6.   Security Layers  . . . . . . . . . . . . . . . . . .  80
       6.3.7.   Simple User Name . . . . . . . . . . . . . . . . . .  80
       6.3.8.   Authorization Identity . . . . . . . . . . . . . . .  80
       6.3.9.   Realms . . . . . . . . . . . . . . . . . . . . . . .  81
       6.3.10.  Round Trips  . . . . . . . . . . . . . . . . . . . .  81
     6.4.   Process  . . . . . . . . . . . . . . . . . . . . . . . .  82
       6.4.1.   Exchange of Stream Headers and Stream Features . . .  82
       6.4.2.   Initiation . . . . . . . . . . . . . . . . . . . . .  83
       6.4.3.   Challenge-Response Sequence  . . . . . . . . . . . .  84
       6.4.4.   Abort  . . . . . . . . . . . . . . . . . . . . . . .  84
       6.4.5.   SASL Failure . . . . . . . . . . . . . . . . . . . .  85
       6.4.6.   SASL Success . . . . . . . . . . . . . . . . . . . .  86
     6.5.   SASL Errors  . . . . . . . . . . . . . . . . . . . . . .  87
       6.5.1.   aborted  . . . . . . . . . . . . . . . . . . . . . .  88
       6.5.2.   account-disabled . . . . . . . . . . . . . . . . . .  88
       6.5.3.   credentials-expired  . . . . . . . . . . . . . . . .  88
       6.5.4.   encryption-required  . . . . . . . . . . . . . . . .  89
       6.5.5.   incorrect-encoding . . . . . . . . . . . . . . . . .  89
       6.5.6.   invalid-authzid  . . . . . . . . . . . . . . . . . .  89
       6.5.7.   invalid-mechanism  . . . . . . . . . . . . . . . . .  90
       6.5.8.   malformed-request  . . . . . . . . . . . . . . . . .  90
       6.5.9.   mechanism-too-weak . . . . . . . . . . . . . . . . .  90
       6.5.10.  not-authorized . . . . . . . . . . . . . . . . . . .  91
       6.5.11.  temporary-auth-failure . . . . . . . . . . . . . . .  91
     6.6.   SASL Definition  . . . . . . . . . . . . . . . . . . . .  91
   7.  Resource Binding  . . . . . . . . . . . . . . . . . . . . . .  92



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     7.1.   Fundamentals . . . . . . . . . . . . . . . . . . . . . .  92
     7.2.   Support  . . . . . . . . . . . . . . . . . . . . . . . .  93
     7.3.   Stream Negotiation Rules . . . . . . . . . . . . . . . .  93
       7.3.1.   Mandatory-to-Negotiate . . . . . . . . . . . . . . .  93
       7.3.2.   Restart  . . . . . . . . . . . . . . . . . . . . . .  93
     7.4.   Advertising Support  . . . . . . . . . . . . . . . . . .  93
     7.5.   Generation of Resource Identifiers . . . . . . . . . . .  94
     7.6.   Server-Generated Resource Identifier . . . . . . . . . .  94
       7.6.1.   Success Case . . . . . . . . . . . . . . . . . . . .  94
       7.6.2.   Error Cases  . . . . . . . . . . . . . . . . . . . .  95
         7.6.2.1.  Resource Constraint . . . . . . . . . . . . . . .  95
         7.6.2.2.  Not Allowed . . . . . . . . . . . . . . . . . . .  96
     7.7.   Client-Submitted Resource Identifier . . . . . . . . . .  96
       7.7.1.   Success Case . . . . . . . . . . . . . . . . . . . .  96
       7.7.2.   Error Cases  . . . . . . . . . . . . . . . . . . . .  97
         7.7.2.1.  Bad Request . . . . . . . . . . . . . . . . . . .  97
         7.7.2.2.  Conflict  . . . . . . . . . . . . . . . . . . . .  97
       7.7.3.   Retries  . . . . . . . . . . . . . . . . . . . . . .  99
   8.  XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . .  99
     8.1.   Common Attributes  . . . . . . . . . . . . . . . . . . . 100
       8.1.1.   to . . . . . . . . . . . . . . . . . . . . . . . . . 100
         8.1.1.1.  Client-to-Server Streams  . . . . . . . . . . . . 100
         8.1.1.2.  Server-to-Server Streams  . . . . . . . . . . . . 101
       8.1.2.   from . . . . . . . . . . . . . . . . . . . . . . . . 101
         8.1.2.1.  Client-to-Server Streams  . . . . . . . . . . . . 101
         8.1.2.2.  Server-to-Server Streams  . . . . . . . . . . . . 102
       8.1.3.   id . . . . . . . . . . . . . . . . . . . . . . . . . 103
       8.1.4.   type . . . . . . . . . . . . . . . . . . . . . . . . 103
       8.1.5.   xml:lang . . . . . . . . . . . . . . . . . . . . . . 103
     8.2.   Basic Semantics  . . . . . . . . . . . . . . . . . . . . 105
       8.2.1.   Message Semantics  . . . . . . . . . . . . . . . . . 105
       8.2.2.   Presence Semantics . . . . . . . . . . . . . . . . . 105
       8.2.3.   IQ Semantics . . . . . . . . . . . . . . . . . . . . 105
     8.3.   Stanza Errors  . . . . . . . . . . . . . . . . . . . . . 107
       8.3.1.   Rules  . . . . . . . . . . . . . . . . . . . . . . . 108
       8.3.2.   Syntax . . . . . . . . . . . . . . . . . . . . . . . 109
       8.3.3.   Defined Conditions . . . . . . . . . . . . . . . . . 110
         8.3.3.1.  bad-request . . . . . . . . . . . . . . . . . . . 110
         8.3.3.2.  conflict  . . . . . . . . . . . . . . . . . . . . 111
         8.3.3.3.  feature-not-implemented . . . . . . . . . . . . . 111
         8.3.3.4.  forbidden . . . . . . . . . . . . . . . . . . . . 112
         8.3.3.5.  gone  . . . . . . . . . . . . . . . . . . . . . . 113
         8.3.3.6.  internal-server-error . . . . . . . . . . . . . . 113
         8.3.3.7.  item-not-found  . . . . . . . . . . . . . . . . . 114
         8.3.3.8.  jid-malformed . . . . . . . . . . . . . . . . . . 114
         8.3.3.9.  not-acceptable  . . . . . . . . . . . . . . . . . 115
         8.3.3.10. not-allowed . . . . . . . . . . . . . . . . . . . 116
         8.3.3.11. not-authorized  . . . . . . . . . . . . . . . . . 116



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         8.3.3.12. policy-violation  . . . . . . . . . . . . . . . . 117
         8.3.3.13. recipient-unavailable . . . . . . . . . . . . . . 117
         8.3.3.14. redirect  . . . . . . . . . . . . . . . . . . . . 118
         8.3.3.15. registration-required . . . . . . . . . . . . . . 119
         8.3.3.16. remote-server-not-found . . . . . . . . . . . . . 119
         8.3.3.17. remote-server-timeout . . . . . . . . . . . . . . 120
         8.3.3.18. resource-constraint . . . . . . . . . . . . . . . 121
         8.3.3.19. service-unavailable . . . . . . . . . . . . . . . 121
         8.3.3.20. subscription-required . . . . . . . . . . . . . . 122
         8.3.3.21. undefined-condition . . . . . . . . . . . . . . . 123
         8.3.3.22. unexpected-request  . . . . . . . . . . . . . . . 123
       8.3.4.   Application-Specific Conditions  . . . . . . . . . . 124
     8.4.   Extended Content . . . . . . . . . . . . . . . . . . . . 125
   9.  Detailed Examples . . . . . . . . . . . . . . . . . . . . . . 128
     9.1.   Client-to-Server Examples  . . . . . . . . . . . . . . . 128
       9.1.1.   TLS  . . . . . . . . . . . . . . . . . . . . . . . . 128
       9.1.2.   SASL . . . . . . . . . . . . . . . . . . . . . . . . 130
       9.1.3.   Resource Binding . . . . . . . . . . . . . . . . . . 132
       9.1.4.   Stanza Exchange  . . . . . . . . . . . . . . . . . . 133
       9.1.5.   Close  . . . . . . . . . . . . . . . . . . . . . . . 134
     9.2.   Server-to-Server Examples  . . . . . . . . . . . . . . . 134
       9.2.1.   TLS  . . . . . . . . . . . . . . . . . . . . . . . . 134
       9.2.2.   SASL . . . . . . . . . . . . . . . . . . . . . . . . 136
       9.2.3.   Stanza Exchange  . . . . . . . . . . . . . . . . . . 137
       9.2.4.   Close  . . . . . . . . . . . . . . . . . . . . . . . 137
   10. Server Rules for Processing XML Stanzas . . . . . . . . . . . 138
     10.1.  In-Order Processing  . . . . . . . . . . . . . . . . . . 138
     10.2.  General Considerations . . . . . . . . . . . . . . . . . 140
     10.3.  No 'to' Address  . . . . . . . . . . . . . . . . . . . . 141
       10.3.1.  Message  . . . . . . . . . . . . . . . . . . . . . . 141
       10.3.2.  Presence . . . . . . . . . . . . . . . . . . . . . . 141
       10.3.3.  IQ . . . . . . . . . . . . . . . . . . . . . . . . . 141
     10.4.  Remote Domain  . . . . . . . . . . . . . . . . . . . . . 142
       10.4.1.  Existing Stream  . . . . . . . . . . . . . . . . . . 142
       10.4.2.  No Existing Stream . . . . . . . . . . . . . . . . . 142
       10.4.3.  Error Handling . . . . . . . . . . . . . . . . . . . 143
     10.5.  Local Domain . . . . . . . . . . . . . . . . . . . . . . 143
       10.5.1.  domainpart . . . . . . . . . . . . . . . . . . . . . 143
       10.5.2.  domainpart/resourcepart  . . . . . . . . . . . . . . 143
       10.5.3.  localpart@domainpart . . . . . . . . . . . . . . . . 143
         10.5.3.1. No Such User  . . . . . . . . . . . . . . . . . . 144
         10.5.3.2. User Exists . . . . . . . . . . . . . . . . . . . 144
       10.5.4.  localpart@domainpart/resourcepart  . . . . . . . . . 144
   11. XML Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 145
     11.1.  XML Restrictions . . . . . . . . . . . . . . . . . . . . 145
     11.2.  XML Namespace Names and Prefixes . . . . . . . . . . . . 146
     11.3.  Well-Formedness  . . . . . . . . . . . . . . . . . . . . 146
     11.4.  Validation . . . . . . . . . . . . . . . . . . . . . . . 147



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     11.5.  Inclusion of XML Declaration . . . . . . . . . . . . . . 147
     11.6.  Character Encoding . . . . . . . . . . . . . . . . . . . 147
     11.7.  Whitespace . . . . . . . . . . . . . . . . . . . . . . . 148
     11.8.  XML Versions . . . . . . . . . . . . . . . . . . . . . . 148
   12. Internationalization Considerations . . . . . . . . . . . . . 148
   13. Security Considerations . . . . . . . . . . . . . . . . . . . 148
     13.1.  Fundamentals . . . . . . . . . . . . . . . . . . . . . . 148
     13.2.  Threat Model . . . . . . . . . . . . . . . . . . . . . . 149
     13.3.  Order of Layers  . . . . . . . . . . . . . . . . . . . . 150
     13.4.  Confidentiality and Integrity  . . . . . . . . . . . . . 150
     13.5.  Peer Entity Authentication . . . . . . . . . . . . . . . 151
     13.6.  Strong Security  . . . . . . . . . . . . . . . . . . . . 151
     13.7.  Certificates . . . . . . . . . . . . . . . . . . . . . . 152
       13.7.1.  Certificate Generation . . . . . . . . . . . . . . . 152
         13.7.1.1. General Considerations  . . . . . . . . . . . . . 152
         13.7.1.2. Server Certificates . . . . . . . . . . . . . . . 153
         13.7.1.3. Client Certificates . . . . . . . . . . . . . . . 156
         13.7.1.4. XmppAddr Identifier Type  . . . . . . . . . . . . 156
       13.7.2.  Certificate Validation . . . . . . . . . . . . . . . 157
         13.7.2.1. Server Certificates . . . . . . . . . . . . . . . 158
         13.7.2.2. Client Certificates . . . . . . . . . . . . . . . 158
         13.7.2.3. Checking of Certificates in Long-Lived Streams  . 160
         13.7.2.4. Use of Certificates in XMPP Extensions  . . . . . 160
     13.8.  Mandatory-to-Implement TLS and SASL Technologies . . . . 160
       13.8.1.  For Authentication Only  . . . . . . . . . . . . . . 161
       13.8.2.  For Confidentiality Only . . . . . . . . . . . . . . 161
       13.8.3.  For Confidentiality and Authentication with
                Passwords  . . . . . . . . . . . . . . . . . . . . . 162
       13.8.4.  For Confidentiality and Authentication without
                Passwords  . . . . . . . . . . . . . . . . . . . . . 163
     13.9.  Technology Reuse . . . . . . . . . . . . . . . . . . . . 163
       13.9.1.  Use of Base 64 in SASL . . . . . . . . . . . . . . . 163
       13.9.2.  Use of DNS . . . . . . . . . . . . . . . . . . . . . 163
       13.9.3.  Use of Hash Functions  . . . . . . . . . . . . . . . 164
       13.9.4.  Use of SASL  . . . . . . . . . . . . . . . . . . . . 164
       13.9.5.  Use of TLS . . . . . . . . . . . . . . . . . . . . . 165
       13.9.6.  Use of UTF-8 . . . . . . . . . . . . . . . . . . . . 165
       13.9.7.  Use of XML . . . . . . . . . . . . . . . . . . . . . 166
     13.10. Information Leaks  . . . . . . . . . . . . . . . . . . . 166
       13.10.1. IP Addresses . . . . . . . . . . . . . . . . . . . . 166
       13.10.2. Presence Information . . . . . . . . . . . . . . . . 166
     13.11. Directory Harvesting . . . . . . . . . . . . . . . . . . 166
     13.12. Denial of Service  . . . . . . . . . . . . . . . . . . . 167
     13.13. Firewalls  . . . . . . . . . . . . . . . . . . . . . . . 169
     13.14. Interdomain Federation . . . . . . . . . . . . . . . . . 169
     13.15. Non-Repudiation  . . . . . . . . . . . . . . . . . . . . 169
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 170
     14.1.  XML Namespace Name for TLS Data  . . . . . . . . . . . . 170



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     14.2.  XML Namespace Name for SASL Data . . . . . . . . . . . . 170
     14.3.  XML Namespace Name for Stream Errors . . . . . . . . . . 170
     14.4.  XML Namespace Name for Resource Binding  . . . . . . . . 171
     14.5.  XML Namespace Name for Stanza Errors . . . . . . . . . . 171
     14.6.  GSSAPI Service Name  . . . . . . . . . . . . . . . . . . 171
     14.7.  Port Numbers and Service Names . . . . . . . . . . . . . 171
   15. Conformance Requirements  . . . . . . . . . . . . . . . . . . 172
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . . 181
     16.1.  Normative References . . . . . . . . . . . . . . . . . . 181
     16.2.  Informative References . . . . . . . . . . . . . . . . . 184
   Appendix A.  XML Schemas  . . . . . . . . . . . . . . . . . . . . 190
     A.1.   Stream Namespace . . . . . . . . . . . . . . . . . . . . 190
     A.2.   Stream Error Namespace . . . . . . . . . . . . . . . . . 192
     A.3.   STARTTLS Namespace . . . . . . . . . . . . . . . . . . . 193
     A.4.   SASL Namespace . . . . . . . . . . . . . . . . . . . . . 194
     A.5.   Client Namespace . . . . . . . . . . . . . . . . . . . . 196
     A.6.   Server Namespace . . . . . . . . . . . . . . . . . . . . 201
     A.7.   Resource Binding Namespace . . . . . . . . . . . . . . . 206
     A.8.   Stanza Error Namespace . . . . . . . . . . . . . . . . . 206
   Appendix B.  Contact Addresses  . . . . . . . . . . . . . . . . . 208
   Appendix C.  Account Provisioning . . . . . . . . . . . . . . . . 208
   Appendix D.  Differences from RFC 3920  . . . . . . . . . . . . . 208
   Appendix E.  Acknowledgements . . . . . . . . . . . . . . . . . . 210

1.  Introduction

1.1.  Overview

   The Extensible Messaging and Presence Protocol (XMPP) is an
   application profile of the Extensible Markup Language [XML] that
   enables the near-real-time exchange of structured yet extensible data
   between any two or more network entities.  This document defines
   XMPP's core protocol methods: setup and teardown of XML streams,
   channel encryption, authentication, error handling, and communication
   primitives for messaging, network availability ("presence"), and
   request-response interactions.

1.2.  History

   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 base Jabber protocol that would be suitable as an
   IETF instant messaging (IM) and presence technology in accordance
   with [IMP-REQS].  In October 2004, [RFC3920] and [RFC3921] were
   published, representing the most complete definition of XMPP at that
   time.




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   Since 2004 the Internet community has gained extensive implementation
   and deployment experience with XMPP, including formal
   interoperability testing carried out under the auspices of the XMPP
   Standards Foundation (XSF).  This document incorporates comprehensive
   feedback from software developers and XMPP service providers,
   including a number of backward-compatible modifications summarized
   under Appendix D.  As a result, this document reflects the rough
   consensus of the Internet community regarding the core features of
   XMPP 1.0, thus obsoleting RFC 3920.

1.3.  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 typically implemented
   using a distributed client-server architecture, wherein a client
   needs to connect to a server in order to gain access to the network
   and thus be allowed to exchange XML stanzas with other entities
   (which can be associated with other servers).  The process whereby a
   client connects to a server, exchanges XML stanzas, and ends the
   connection is:

   1.  Determine the IP address and port at which to connect, typically
       based on resolution of a fully qualified domain name
       (Section 3.2)

   2.  Open a Transmission Control Protocol [TCP] connection

   3.  Open an XML stream over TCP (Section 4.2)

   4.  Preferably negotiate Transport Layer Security [TLS] for channel
       encryption (Section 5)

   5.  Authenticate using a Simple Authentication and Security Layer
       [SASL] mechanism (Section 6)

   6.  Bind a resource to the stream (Section 7)

   7.  Exchange an unbounded number of XML stanzas with other entities
       on the network (Section 8)

   8.  Close the XML stream (Section 4.4)

   9.  Close the TCP connection



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   Within XMPP, one server can optionally connect to another server to
   enable inter-domain or inter-server communication.  For this to
   happen, the two servers need to negotiate a connection between
   themselves and then exchange XML stanzas; the process for doing so
   is:

   1.  Determine the IP address and port at which to connect, typically
       based on resolution of a fully qualified domain name
       (Section 3.2)

   2.  Open a TCP connection

   3.  Open an XML stream (Section 4.2)

   4.  Preferably negotiate TLS for channel encryption (Section 5)

   5.  Authenticate using a Simple Authentication and Security Layer
       [SASL] mechanism (Section 6) *

   6.  Exchange an unbounded number of XML stanzas both directly for the
       servers and indirectly on behalf of entities associated with each
       server, such as connected clients (Section 8)

   7.  Close the XML stream (Section 4.4)

   8.  Close the TCP connection

      * Interoperability Note: At the time of writing, most deployed
      servers still use the Server Dialback protocol [XEP-0220] to
      provide weak identity verification instead of using SASL with PKIX
      certificates to provide strong authentication, especially in cases
      where SASL negotiation would not result in strong authentication
      anyway (e.g., because TLS negotiation was not mandated by the peer
      server, or because the PKIX certificate presented by the peer
      server during TLS negotiation is self-signed and has not been
      previously accepted); for details, see [XEP-0220].  The solutions
      specified in this document offer a significantly stronger level of
      security (see also Section 13.6).

   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, those payloads are
   defined by various XMPP extensions.  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 applications.




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1.4.  Terminology

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

   Certain security-related terms are to be understood in the sense
   defined in [SEC-TERMS]; such terms include, but are not limited to,
   "assurance", "attack", "authentication", "authorization",
   "certificate", "certification authority", "certification path",
   "confidentiality", "credential", "downgrade", "encryption", "hash
   value", "identity", "integrity", "signature", "self-signed
   certificate", "sign", "spoof", "tamper", "trust", "trust anchor",
   "validate", and "verify".

   Certain terms related to certificates, domains, and application
   service identity are to be understood in the sense defined in
   [TLS-CERTS]; these include, but are not limited to, "PKIX
   certificate", "source domain", "derived domain", and the identifier
   types "CN-ID", "DNS-ID", and "SRV-ID".

   Other security-related terms are to be understood in the sense
   defined in the referenced specifications (for example, "denial of
   service" as described in [DOS] or "end entity certificate" as
   described in [PKIX]).

   The term "whitespace" is used to refer to any character or characters
   matching the "S" production from [XML], i.e., one or more instances
   of the SP, HTAB, CR, or LF rules defined in [ABNF].

   The terms "localpart", "domainpart", and "resourcepart" are defined
   in [XMPP-ADDR].

   The term "bare JID" refers to an XMPP address of the form
   <localpart@domainpart> (for an account at a server) or of the form
   <domainpart> (for a server).

   The term "full JID" refers to an XMPP address of the form
   <localpart@domainpart/resourcepart> (for a particular authorized
   client or device associated with an account) or of the form
   <domainpart/resourcepart> (for a particular resource or script
   associated with a server).

   The term "XML stream" (also "stream") is defined under Section 4.1.






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   The term "XML stanza" (also "stanza") is defined under Section 4.1.
   There are three kinds of stanzas: message, presence, and IQ (short
   for "Info/Query").  These communication primitives are defined under
   Sections 8.2.1, 8.2.2, and 8.2.3, respectively.

   The term "originating entity" refers to the entity that first
   generates a stanza that is sent over an XMPP network (e.g., a
   connected client, an add-on service, or a server).  The term
   "generated stanza" refers to the stanza so generated.

   The term "input stream" designates an XML stream over which a server
   receives data from a connected client or remote server, and the term
   "output stream" designates an XML stream over which a server sends
   data to a connected client or remote server.  The following terms
   designate some of the actions that a server can perform when
   processing data received over an input stream:

      route:  pass the data to a remote server for direct processing by
         the remote server or eventual delivery to a client associated
         with the remote server

      deliver:  pass the data to a connected client

      ignore:  discard the data without acting upon it or returning an
         error to the sender

   When the term "ignore" is used with regard to client processing of
   data it receives, the phrase "without acting upon it" explicitly
   includes not presenting the data to a human user.

   Following the "XML Notation" used in [IRI] to represent characters
   that cannot be rendered in ASCII-only documents, some examples in
   this document use the form "&#x...." as a notational device to
   represent [UNICODE] characters (e.g., the string "&#x0159;" stands
   for the Unicode character LATIN SMALL LETTER R WITH CARON); this form
   is definitely not to be sent over the wire in XMPP systems.

   Consistent with the convention used in [URI] to represent Uniform
   Resource Identifiers, XMPP addresses in running text are enclosed
   between '<' and '>' (although natively they are not URIs).

   In examples, lines have been wrapped for improved readability,
   "[...]" means elision, and the following prepended strings are used
   (these prepended strings are not to be sent over the wire):

   o  C: = a client

   o  E: = any XMPP entity



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   o  I: = an initiating entity

   o  P: = a peer server

   o  R: = a receiving entity

   o  S: = a server

   o  S1: = server1

   o  S2: = server2

   Readers need to be aware that the examples are not exhaustive and
   that, in examples for some protocol flows, the alternate steps shown
   would not necessarily be triggered by the exact data sent in the
   previous step; in all cases the protocol definitions specified in
   this document or in normatively referenced documents rule over any
   examples provided here.  All examples are fictional and the
   information exchanged (e.g., usernames and passwords) does not
   represent any existing users or servers.

2.  Architecture

   XMPP provides a technology for the asynchronous, end-to-end exchange
   of structured data by means of direct, persistent XML streams among a
   distributed network of globally addressable, presence-aware clients
   and servers.  Because this architectural style involves ubiquitous
   knowledge of network availability and a conceptually unlimited number
   of concurrent information transactions in the context of a given
   client-to-server or server-to-server session, we label it
   "Availability for Concurrent Transactions" (ACT) to distinguish it
   from the "Representational State Transfer" [REST] architectural style
   familiar from the World Wide Web.  Although the architecture of XMPP
   is similar in important ways to that of email (see [EMAIL-ARCH]), it
   introduces several modifications to facilitate communication in close
   to real time.  The salient features of this ACTive architectural
   style are as follows.

2.1.  Global Addresses

   As with email, XMPP uses globally unique addresses (based on the
   Domain Name System) in order to route and deliver messages over the
   network.  All XMPP entities are addressable on the network, most
   particularly clients and servers but also various additional services
   that can be accessed by clients and servers.  In general, server
   addresses are of the form <domainpart> (e.g., <im.example.com>),
   accounts hosted at a server are of the form <localpart@domainpart>
   (e.g., <juliet@im.example.com>, called a "bare JID"), and a



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   particular connected device or resource that is currently authorized
   for interaction on behalf of an account is of the form
   <localpart@domainpart/resourcepart> (e.g.,
   <juliet@im.example.com/balcony>, called a "full JID").  For
   historical reasons, XMPP addresses are often called Jabber IDs or
   JIDs.  Because the formal specification of the XMPP address format
   depends on internationalization technologies that are in flux at the
   time of writing, the format is defined in [XMPP-ADDR] instead of this
   document.  The terms "localpart", "domainpart", and "resourcepart"
   are defined more formally in [XMPP-ADDR].

2.2.  Presence

   XMPP includes the ability for an entity to advertise its network
   availability or "presence" to other entities.  In XMPP, this
   availability for communication is signaled end-to-end by means of a
   dedicated communication primitive: the <presence/> stanza.  Although
   knowledge of network availability is not strictly necessary for the
   exchange of XMPP messages, it facilitates real-time interaction
   because the originator of a message can know before initiating
   communication that the intended recipient is online and available.
   End-to-end presence is defined in [XMPP-IM].

2.3.  Persistent Streams

   Availability for communication is also built into each point-to-point
   "hop" through the use of persistent XML streams over long-lived TCP
   connections.  These "always-on" client-to-server and server-to-server
   streams enable each party to push data to the other party at any time
   for immediate routing or delivery.  XML streams are defined under
   Section 4.

2.4.  Structured Data

   The basic protocol data unit in XMPP is not an XML stream (which
   simply provides the transport for point-to-point communication) but
   an XML "stanza", which is essentially a fragment of XML that is sent
   over a stream.  The root element of a stanza includes routing
   attributes (such as "from" and "to" addresses), and the child
   elements of the stanza contain a payload for delivery to the intended
   recipient.  XML stanzas are defined under Section 8.

2.5.  Distributed Network of Clients and Servers

   In practice, XMPP consists of a network of clients and servers that
   inter-communicate (however, communication between any two given
   deployed servers is strictly discretionary and a matter of local
   service policy).  Thus, for example, the user <juliet@im.example.com>



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   associated with the server <im.example.com> might be able to exchange
   messages, presence, and other structured data with the user
   <romeo@example.net> associated with the server <example.net>.  This
   pattern is familiar from messaging protocols that make use of global
   addresses, such as the email network (see [SMTP] and [EMAIL-ARCH]).
   As a result, end-to-end communication in XMPP is logically peer-to-
   peer but physically client-to-server-to-server-to-client, as
   illustrated in the following diagram.

     example.net <--------------> im.example.com
        ^                                ^
        |                                |
        v                                v
   romeo@example.net           juliet@im.example.com

             Figure 1: Distributed Client-Server Architecture

      Informational Note: Architectures that employ XML streams
      (Section 4) and XML stanzas (Section 8) but that establish peer-
      to-peer connections directly between clients using technologies
      based on [LINKLOCAL] have been deployed, but such architectures
      are not defined in this specification and are best described as
      "XMPP-like"; for details, see [XEP-0174].  In addition, XML
      streams can be established end-to-end over any reliable transport,
      including extensions to XMPP itself; however, such methods are out
      of scope for this specification.

   The following paragraphs describe the responsibilities of clients and
   servers on the network.

   A client is an entity that establishes an XML stream with a server by
   authenticating using the credentials of a registered account (via
   SASL negotiation (Section 6)) and that then completes resource
   binding (Section 7) in order to enable delivery of XML stanzas
   between the server and the client over the negotiated stream.  The
   client then uses XMPP to communicate with its server, other clients,
   and any other entities on the network, where the server is
   responsible for delivering stanzas to other connected clients at the
   same server or routing them to remote servers.  Multiple clients can
   connect simultaneously to a server on behalf of the same registered
   account, where each client is differentiated by the resourcepart of
   an XMPP address (e.g., <juliet@im.example.com/balcony> vs.
   <juliet@im.example.com/chamber>), as defined under [XMPP-ADDR] and
   Section 7.







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   A server is an entity whose primary responsibilities are to:

   o  Manage XML streams (Section 4) with connected clients and deliver
      XML stanzas (Section 8) to those clients over the negotiated
      streams; this includes responsibility for ensuring that a client
      authenticates with the server before being granted access to the
      XMPP network.

   o  Subject to local service policies on server-to-server
      communication, manage XML streams (Section 4) with remote servers
      and route XML stanzas (Section 8) to those servers over the
      negotiated streams.

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

   o  Storing data that is used by clients (e.g., contact lists for
      users of XMPP-based instant messaging and presence applications as
      defined in [XMPP-IM]); 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 remote server or delivered to a connected
      client.

   o  Hosting add-on 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].

3.  TCP Binding

3.1.  Scope

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

      Informational Note: There is no necessary coupling of XML streams
      to TCP, and other transports are possible.  For example, two
      entities could connect to each other by means of [HTTP] as
      specified in [XEP-0124] and [XEP-0206].  However, this
      specification defines only a binding of XMPP to TCP.






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3.2.  Resolution of Fully Qualified Domain Names

   Because XML streams are sent over TCP, the initiating entity needs to
   determine the IPv4 or IPv6 address (and port) of the receiving entity
   before it can attempt to open an XML stream.  Typically this is done
   by resolving the receiving entity's fully qualified domain name or
   FQDN (see [DNS-CONCEPTS]).

3.2.1.  Preferred Process: SRV Lookup

   The preferred process for FQDN resolution is to use [DNS-SRV] records
   as follows:

   1.  The initiating entity constructs a DNS SRV query whose inputs
       are:

       *  a Service of "xmpp-client" (for client-to-server connections)
          or "xmpp-server" (for server-to-server connections)

       *  a Proto of "tcp"

       *  a Name corresponding to the "origin domain" [TLS-CERTS] of the
          XMPP service to which the initiating entity wishes to connect
          (e.g., "example.net" or "im.example.com")

   2.  The result is a query such as "_xmpp-client._tcp.example.net." or
       "_xmpp-server._tcp.im.example.com.".

   3.  If a response is received, it will contain one or more
       combinations of a port and FDQN, each of which is weighted and
       prioritized as described in [DNS-SRV].  (However, if the result
       of the SRV lookup is a single resource record with a Target of
       ".", i.e., the root domain, then the initiating entity MUST abort
       SRV processing at this point because according to [DNS-SRV] such
       a Target "means that the service is decidedly not available at
       this domain".)

   4.  The initiating entity chooses at least one of the returned FQDNs
       to resolve (following the rules in [DNS-SRV]), which it does by
       performing DNS "A" or "AAAA" lookups on the FDQN; this will
       result in an IPv4 or IPv6 address.

   5.  The initiating entity uses the IP address(es) from the
       successfully resolved FDQN (with the corresponding port number
       returned by the SRV lookup) as the connection address for the
       receiving entity.





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   6.  If the initiating entity fails to connect using that IP address
       but the "A" or "AAAA" lookups returned more than one IP address,
       then the initiating entity uses the next resolved IP address for
       that FDQN as the connection address.

   7.  If the initiating entity fails to connect using all resolved IP
       addresses for a given FDQN, then it repeats the process of
       resolution and connection for the next FQDN returned by the SRV
       lookup based on the priority and weight as defined in [DNS-SRV].

   8.  If the initiating entity receives a response to its SRV query but
       it is not able to establish an XMPP connection using the data
       received in the response, it SHOULD NOT attempt the fallback
       process described in the next section (this helps to prevent a
       state mismatch between inbound and outbound connections).

   9.  If the initiating entity does not receive a response to its SRV
       query, it SHOULD attempt the fallback process described in the
       next section.

3.2.2.  Fallback Processes

   The fallback process SHOULD be a normal "A" or "AAAA" address record
   resolution to determine the IPv4 or IPv6 address of the origin
   domain, where the port used is the "xmpp-client" port of 5222 for
   client-to-server connections or the "xmpp-server" port of 5269 for
   server-to-server connections (these are the default ports as
   registered with the IANA as described under Section 14.7).

   If connections via TCP are unsuccessful, the initiating entity might
   attempt to find and use alternative connection methods such as the
   HTTP binding (see [XEP-0124] and [XEP-0206]), which might be
   discovered using [DNS-TXT] records as described in [XEP-0156].

3.2.3.  When Not to Use SRV

   If the initiating entity has been explicitly configured to associate
   a particular FQDN (and potentially port) with the origin domain of
   the receiving entity (say, to "hardcode" an association from an
   origin domain of example.net to a configured FQDN of
   apps.example.com), the initiating entity is encouraged to use the
   configured name instead of performing the preferred SRV resolution
   process on the origin domain.








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3.2.4.  Use of SRV Records with Add-On Services

   Many XMPP servers are implemented in such a way that they can host
   add-on services (beyond those defined in this specification and
   [XMPP-IM]) at DNS domain names that typically are "subdomains" of the
   main XMPP service (e.g., conference.example.net for a [XEP-0045]
   service associated with the example.net XMPP service) or "subdomains"
   of the first-level domain of the underlying service (e.g.,
   muc.example.com for a [XEP-0045] service associated with the
   im.example.com XMPP service).  If an entity associated with a remote
   XMPP server wishes to communicate with such an add-on service, it
   would generate an appropriate XML stanza and the remote server would
   attempt to resolve the add-on service's DNS domain name via an SRV
   lookup on resource records such as "_xmpp-
   server._tcp.conference.example.net." or "_xmpp-
   server._tcp.muc.example.com.".  Therefore, if the administrators of
   an XMPP service wish to enable entities associated with remote
   servers to access such add-on services, they need to advertise the
   appropriate "_xmpp-server" SRV records in addition to the "_xmpp-
   server" record for their main XMPP service.  In case SRV records are
   not available, the fallback methods described under Section 3.2.2 can
   be used to resolve the DNS domain names of add-on services.

3.3.  Reconnection

   It can happen that an XMPP server goes offline unexpectedly while
   servicing TCP connections from connected clients and remote 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 performance and network
   congestion.  If an entity chooses to reconnect, it:

   o  SHOULD set the number of seconds that expire before reconnecting
      to an unpredictable number between 0 and 60 (this helps to ensure
      that not all entities attempt to reconnect at exactly the same
      number of seconds after being disconnected).

   o  SHOULD back off increasingly on the time between subsequent
      reconnection attempts (e.g., in accordance with "truncated binary
      exponential backoff" as described in [ETHERNET]) if the first
      reconnection attempt does not succeed.

   It is RECOMMENDED to make use of TLS session resumption [TLS-RESUME]
   when reconnecting.  A future version of this document, or a separate
   specification, might provide more detailed guidelines regarding
   methods for speeding the reconnection process.





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3.4.  Reliability

   The use of long-lived TCP connections in XMPP implies that the
   sending of XML stanzas over XML streams can be unreliable, since the
   parties to a long-lived TCP connection might not discover a
   connectivity disruption in a timely manner.  At the XMPP application
   layer, long connectivity disruptions can result in undelivered
   stanzas.  Although the core XMPP technology defined in this
   specification does not contain features to overcome this lack of
   reliability, there exist XMPP extensions for doing so (e.g.,
   [XEP-0198]).

4.  XML Streams

4.1.  Stream Fundamentals

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

   Definition of XML Stream:  An XML stream is a container for the
      exchange of XML elements between any two entities over a network.
      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 5) or SASL negotiation (Section 6)) 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 to" or "session with" the
      receiving entity.  The initial stream enables unidirectional
      communication from the initiating entity to the receiving entity;
      in order to enable exchange of stanzas 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 the basic unit of meaning
      in XMPP.  A stanza is a first-level element (at depth=1 of the
      stream) whose element name is "message", "presence", or "iq" and
      whose qualifying namespace is 'jabber:client' or 'jabber:server'.
      By contrast, a first-level element qualified by any other
      namespace is not an XML stanza (stream errors, stream features,
      TLS-related elements, SASL-related elements, etc.), nor is a



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      <message/>, <presence/>, or <iq/> element that is qualified by the
      'jabber:client' or 'jabber:server' namespace but that occurs at a
      depth other than one (e.g., a <message/> element contained within
      an extension element (Section 8.4) for reporting purposes), nor is
      a <message/>, <presence/>, or <iq/> element that is qualified by a
      namespace other than 'jabber:client' or 'jabber:server'.  An XML
      stanza typically contains one or more 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 8.4 in this specification).

   There are three kinds of stanzas: message, presence, and IQ (short
   for "Info/Query").  These stanza types provide three different
   communication primitives: a "push" mechanism for generalized
   messaging, a specialized "publish-subscribe" mechanism for
   broadcasting information about network availability, and a "request-
   response" mechanism for more structured exchanges of data (similar to
   [HTTP]).  Further explanations are provided under Section 8.2.1,
   Section 8.2.2, and Section 8.2.3, respectively.

   Consider the example of a client's connection to a server.  The
   client initiates an XML stream by sending a stream header to the
   server, preferably preceded by an XML declaration specifying the XML
   version and the character encoding supported (see Section 11.5 and
   Section 11.6).  Subject to local policies and service provisioning,
   the server then replies with a second XML stream back to the client,
   again preferably preceded by an XML declaration.  Once the client has
   completed SASL negotiation (Section 6) and resource binding
   (Section 7), the client can 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 as further
   described under Section 4.4.

   In essence, then, one XML stream functions as an envelope for the XML
   stanzas sent during a session and another XML stream functions as an
   envelope for the XML stanzas received during a session.  We can
   represent this in a simplistic fashion as follows.













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   +--------------------+--------------------+
   | INITIAL STREAM     |  RESPONSE STREAM   |
   +--------------------+--------------------+
   | <stream>           |                    |
   |--------------------|--------------------|
   |                    | <stream>           |
   |--------------------|--------------------|
   | <presence>         |                    |
   |   <show/>          |                    |
   | </presence>        |                    |
   |--------------------|--------------------|
   | <message to='foo'> |                    |
   |   <body/>          |                    |
   | </message>         |                    |
   |--------------------|--------------------|
   | <iq to='bar'       |                    |
   |     type='get'>    |                    |
   |   <query/>         |                    |
   | </iq>              |                    |
   |--------------------|--------------------|
   |                    | <iq from='bar'     |
   |                    |     type='result'> |
   |                    |   <query/>         |
   |                    | </iq>              |
   |--------------------|--------------------|
   | [ ... ]            |                    |
   |--------------------|--------------------|
   |                    | [ ... ]            |
   |--------------------|--------------------|
   | </stream>          |                    |
   |--------------------|--------------------|
   |                    | </stream>          |
   +--------------------+--------------------+

                Figure 2: A Simplistic View of Two Streams

   Those who are accustomed to thinking of XML in a document-centric
   manner might find the following analogies useful:

   o  The two XML streams are like two "documents" (matching the
      "document" production from [XML]) that are built up through the
      accumulation of XML stanzas.

   o  The root <stream/> element is like the "document entity" for each
      "document" (as described in Section 4.8 of [XML]).

   o  The XML stanzas sent over the streams are like "fragments" of the
      "documents" (as described in [XML-FRAG]).



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   However, these descriptions are merely analogies, because XMPP does
   not deal in documents and fragments but in streams and stanzas.

   The remainder of this section defines the following aspects of XML
   streams (along with related topics):

   o  How to open a stream (Section 4.2)

   o  The stream negotiation process (Section 4.3)

   o  How to close a stream (Section 4.4)

   o  The directionality of XML streams (Section 4.5)

   o  How to handle peers that are silent (Section 4.6)

   o  The XML attributes of a stream (Section 4.7)

   o  The XML namespaces of a stream (Section 4.8)

   o  Error handling related to XML streams (Section 4.9)

4.2.  Opening a Stream

   After connecting to the appropriate IP address and port of the
   receiving entity, the initiating entity opens a stream by sending a
   stream header (the "initial stream header") to the receiving entity.

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

   The receiving entity then replies by sending a stream header of its
   own (the "response stream header") to the initiating entity.












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

   The entities can then proceed with the remainder of the stream
   negotiation process.

4.3.  Stream Negotiation

4.3.1.  Basic Concepts

   Because the receiving entity for a stream acts as a gatekeeper to the
   domains it services, it imposes certain conditions for connecting as
   a client or as a peer server.  At a minimum, the initiating entity
   needs to authenticate with the receiving entity before it is allowed
   to send stanzas to the receiving entity (for client-to-server streams
   this means using SASL as described under Section 6).  However, the
   receiving entity can consider conditions other than authentication to
   be mandatory-to-negotiate, such as encryption using TLS as described
   under Section 5.  The receiving entity informs the initiating entity
   about such conditions by communicating "stream features": the set of
   particular protocol interactions that the initiating entity needs to
   complete before the receiving entity will accept XML stanzas from the
   initiating entity, as well as any protocol interactions that are
   voluntary-to-negotiate but that might improve the handling of an XML
   stream (e.g., establishment of application-layer compression as
   described in [XEP-0138]).

   The existence of conditions for connecting implies that streams need
   to be negotiated.  The order of layers (TCP, then TLS, then SASL,
   then XMPP as described under Section 13.3) implies that stream
   negotiation is a multi-stage process.  Further structure is imposed
   by two factors: (1) a given stream feature might be offered only to
   certain entities or only after certain other features have been
   negotiated (e.g., resource binding is offered only after SASL
   authentication), and (2) stream features can be either mandatory-to-
   negotiate or voluntary-to-negotiate.  Finally, for security reasons
   the parties to a stream need to discard knowledge that they gained
   during the negotiation process after successfully completing the
   protocol interactions defined for certain features (e.g., TLS in all
   cases and SASL in the case when a security layer might be




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   established, as defined in the specification for the relevant SASL
   mechanism).  This is done by flushing the old stream context and
   exchanging new stream headers over the existing TCP connection.

4.3.2.  Stream Features Format

   If the initiating entity includes in the initial stream header the
   'version' attribute set to a value of at least "1.0" (see
   Section 4.7.5), after sending the response stream header the
   receiving entity MUST send a <features/> child element (typically
   prefixed by the stream namespace prefix as described under
   Section 4.8.5) to the initiating entity in order to announce any
   conditions for continuation of the stream negotiation process.  Each
   condition takes the form of a child element of the <features/>
   element, qualified by a namespace that is different from the stream
   namespace and the content namespace.  The <features/> element can
   contain one child, contain multiple children, or be empty.

      Implementation Note: The order of child elements contained in any
      given <features/> element is not significant.

   If a particular stream feature is or can be mandatory-to-negotiate,
   the definition of that feature needs to do one of the following:

   1.  Declare that the feature is always mandatory-to-negotiate (e.g.,
       this is true of resource binding for XMPP clients); or

   2.  Specify a way for the receiving entity to flag the feature as
       mandatory-to-negotiate for this interaction (e.g., for STARTTLS,
       this is done by including an empty <required/> element in the
       advertisement for that stream feature, but that is not a generic
       format for all stream features); it is RECOMMENDED that stream
       feature definitions for new mandatory-to-negotiate features do so
       by including an empty <required/> element as is done for
       STARTTLS.

      Informational Note: Because there is no generic format for
      indicating that a feature is mandatory-to-negotiate, it is
      possible that a feature that is not understood by the initiating
      entity might be considered mandatory-to-negotiate by the receiving
      entity, resulting in failure of the stream negotiation process.
      Although such an outcome would be undesirable, the working group
      deemed it rare enough that a generic format was not needed.

   For security reasons, certain stream features necessitate the
   initiating entity to send a new initial stream header upon successful
   negotiation of the feature (e.g., TLS in all cases and SASL in the
   case when a security layer might be established).  If this is true of



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   a given stream feature, the definition of that feature needs to
   specify that a stream restart is expected after negotiation of the
   feature.

   A <features/> element that contains at least one mandatory-to-
   negotiate feature indicates that the stream negotiation is not
   complete and that the initiating entity MUST negotiate further
   features.

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

   A <features/> element MAY contain more than one mandatory-to-
   negotiate feature.  This means that the initiating entity can choose
   among the mandatory-to-negotiate features at this stage of the stream
   negotiation process.  As an example, perhaps a future technology will
   perform roughly the same function as TLS, so the receiving entity
   might advertise support for both TLS and the future technology at the
   same stage of the stream negotiation process.  However, this applies
   only at a given stage of the stream negotiation process and does not
   apply to features that are mandatory-to-negotiate at different stages
   (e.g., the receiving entity would not advertise both STARTTLS and
   SASL as mandatory-to-negotiate, or both SASL and resource binding as
   mandatory-to-negotiate, because TLS would need to be negotiated
   before SASL and because SASL would need to be negotiated before
   resource binding).

   A <features/> element that contains both mandatory-to-negotiate and
   voluntary-to-negotiate features indicates that the negotiation is not
   complete but that the initiating entity MAY complete the voluntary-
   to-negotiate feature(s) before it attempts to negotiate the
   mandatory-to-negotiate feature(s).

   R: <stream:features>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
        <compression xmlns='http://jabber.org/features/compress'>
          <method>zlib</method>
          <method>lzw</method>
        </compression>
      </stream:features>

   A <features/> element that contains only voluntary-to-negotiate
   features indicates that the stream negotiation is complete and that
   the initiating entity is cleared to send XML stanzas, but that the
   initiating entity MAY negotiate further features if desired.



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   R: <stream:features>
        <compression xmlns='http://jabber.org/features/compress'>
          <method>zlib</method>
          <method>lzw</method>
        </compression>
      </stream:features>

   An empty <features/> element indicates that the stream negotiation is
   complete and that the initiating entity is cleared to send XML
   stanzas.

   R: <stream:features/>

4.3.3.  Restarts

   On successful negotiation of a feature that necessitates a stream
   restart, both parties MUST consider the previous stream to be
   replaced but MUST NOT send a closing </stream> tag and MUST NOT
   terminate the underlying TCP connection; instead, the parties MUST
   reuse the existing connection, which might be in a new state (e.g.,
   encrypted as a result of TLS negotiation).  The initiating entity
   then MUST send a new initial stream header, which SHOULD be preceded
   by an XML declaration as described under Section 11.5.  When the
   receiving entity receives the new initial stream header, it MUST
   generate a new stream ID (instead of reusing the old stream ID)
   before sending a new response stream header (which SHOULD be preceded
   by an XML declaration as described under Section 11.5).

4.3.4.  Resending Features

   The receiving entity MUST send an updated list of stream features to
   the initiating entity after a stream restart.  The list of updated
   features MAY be empty if there are no further features to be
   advertised or MAY include any combination of features.

4.3.5.  Completion of Stream Negotiation

   The receiving entity indicates completion of the stream negotiation
   process by sending to the initiating entity either an empty
   <features/> element or a <features/> element that contains only
   voluntary-to-negotiate features.  After doing so, the receiving
   entity MAY send an empty <features/> element (e.g., after negotiation
   of such voluntary-to-negotiate features) but MUST NOT send additional
   stream features to the initiating entity (if the receiving entity has
   new features to offer, preferably limited to mandatory-to-negotiate
   or security-critical features, it can simply close the stream with a
   <reset/> stream error (Section 4.9.3.16) and then advertise the new
   features when the initiating entity reconnects, preferably closing



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   existing streams in a staggered way so that not all of the initiating
   entities reconnect at once).  Once stream negotiation is complete,
   the initiating entity is cleared to send XML stanzas over the stream
   for as long as the stream is maintained by both parties.

      Informational Note: Resource binding as specified under Section 7
      is an historical exception to the foregoing rule, since it is
      mandatory-to-negotiate for clients but uses XML stanzas for
      negotiation purposes.

   The initiating entity MUST NOT attempt to send XML stanzas
   (Section 8) to entities other than itself (i.e., the client's
   connected resource or any other authenticated resource of the
   client's account) or the server to which it is connected until stream
   negotiation has been completed.  Even if the initiating entity does
   attempt to do so, the receiving entity MUST NOT accept such stanzas
   and MUST close the stream with a <not-authorized/> stream error
   (Section 4.9.3.12).  This rule applies to XML stanzas only (i.e.,
   <message/>, <presence/>, and <iq/> elements qualified by the content
   namespace) and not to XML elements used for stream negotiation (e.g.,
   elements used to complete TLS negotiation (Section 5) or SASL
   negotiation (Section 6)).

4.3.6.  Determination of Addresses

   After the parties to an XML stream have completed the appropriate
   aspects of stream negotiation, the receiving entity for a stream MUST
   determine the initiating entity's JID.

   For client-to-server communication, both SASL negotiation (Section 6)
   and resource binding (Section 7) MUST be completed before the server
   can determine the client's address.  The client's bare JID
   (<localpart@domainpart>) MUST be the authorization identity (as
   defined by [SASL]), either (1) as directly communicated by the client
   during SASL negotiation (Section 6) or (2) as derived by the server
   from the authentication identity if no authorization identity was
   specified during SASL negotiation.  The resourcepart of the full JID
   (<localpart@domainpart/resourcepart>) MUST be the resource negotiated
   by the client and server during resource binding (Section 7).  A
   client MUST NOT attempt to guess at its JID but instead MUST consider
   its JID to be whatever the server returns to it during resource
   binding.  The server MUST ensure that the resulting JID (including
   localpart, domainpart, resourcepart, and separator characters)
   conforms to the canonical format for XMPP addresses defined in
   [XMPP-ADDR]; to meet this restriction, the server MAY replace the JID
   sent by the client with the canonicalized JID as determined by the
   server and communicate that JID to the client during resource
   binding.



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   For server-to-server communication, the initiating server's bare JID
   (<domainpart>) MUST be the authorization identity (as defined by
   [SASL]), either (1) as directly communicated by the initiating server
   during SASL negotiation (Section 6) or (2) as derived by the
   receiving server from the authentication identity if no authorization
   identity was specified during SASL negotiation.  In the absence of
   SASL negotiation, the receiving server MAY consider the authorization
   identity to be an identity negotiated within the relevant
   verification protocol (e.g., the 'from' attribute of the <result/>
   element in Server Dialback [XEP-0220]).

      Security Warning: Because it is possible for a third party to
      tamper with information that is sent over the stream before a
      security layer such as TLS is successfully negotiated, it is
      advisable for the receiving server to treat any such unprotected
      information with caution; this applies especially to the 'from'
      and 'to' addresses on the first initial stream header sent by the
      initiating entity.

4.3.7.  Flow Chart

   We summarize the foregoing rules in the following non-normative flow
   chart for the stream negotiation process, presented from the
   perspective of the initiating entity.



























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                   +---------------------+
                   | open TCP connection |
                   +---------------------+
                              |
                              v
                       +---------------+
                       | send initial  |<-------------------------+
                       | stream header |                          ^
                       +---------------+                          |
                              |                                   |
                              v                                   |
                      +------------------+                        |
                      | receive response |                        |
                      | stream header    |                        |
                      +------------------+                        |
                              |                                   |
                              v                                   |
                       +----------------+                         |
                       | receive stream |                         |
   +------------------>| features       |                         |
   ^   {OPTIONAL}      +----------------+                         |
   |                          |                                   |
   |                          v                                   |
   |       +<-----------------+                                   |
   |       |                                                      |
   |    {empty?} ----> {all voluntary?} ----> {some mandatory?}   |
   |       |      no          |          no         |             |
   |       | yes              | yes                 | yes         |
   |       |                  v                     v             |
   |       |           +---------------+    +----------------+    |
   |       |           | MAY negotiate |    | MUST negotiate |    |
   |       |           | any or none   |    | one feature    |    |
   |       |           +---------------+    +----------------+    |
   |       v                  |                     |             |
   |   +---------+            v                     |             |
   |   |  DONE   |<----- {negotiate?}               |             |
   |   +---------+   no       |                     |             |
   |                     yes  |                     |             |
   |                          v                     v             |
   |                          +--------->+<---------+             |
   |                                     |                        |
   |                                     v                        |
   +<-------------------------- {restart mandatory?} ------------>+
                  no                                     yes

                  Figure 3: Stream Negotiation Flow Chart





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4.4.  Closing a Stream

   An XML stream from one entity to another can be closed at any time,
   either because a specific stream error (Section 4.9) has occurred or
   in the absence of an error (e.g., when a client simply ends its
   session).

   A stream is closed by sending a closing </stream> tag.

   E: </stream:stream>

   If the parties are using either two streams over a single TCP
   connection or two streams over two TCP connections, the entity that
   sends the closing stream tag MUST behave as follows:

   1.  Wait for the other party to also close its outbound stream before
       terminating the underlying TCP connection(s); this gives the
       other party an opportunity to finish transmitting any outbound
       data to the closing entity before the termination of the TCP
       connection(s).

   2.  Refrain from sending any further data over its outbound stream to
       the other entity, but continue to process data received from the
       other entity (and, if necessary, process such data).

   3.  Consider both streams to be void if the other party does not send
       its closing stream tag within a reasonable amount of time (where
       the definition of "reasonable" is a matter of implementation or
       deployment).

   4.  After receiving a reciprocal closing stream tag from the other
       party or waiting a reasonable amount of time with no response,
       terminate the underlying TCP connection(s).

      Security Warning: In accordance with Section 7.2.1 of [TLS], to
      help prevent a truncation attack the party that is closing the
      stream MUST send a TLS close_notify alert and MUST receive a
      responding close_notify alert from the other party before
      terminating the underlying TCP connection(s).

   If the parties are using multiple streams over multiple TCP
   connections, there is no defined pairing of streams and therefore the
   behavior is a matter for implementation.








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4.5.  Directionality

   An XML stream is always unidirectional, by which is meant that XML
   stanzas can be sent in only one direction over the stream (either
   from the initiating entity to the receiving entity or from the
   receiving entity to the initiating entity).

   Depending on the type of session that has been negotiated and the
   nature of the entities involved, the entities might use:

   o  Two streams over a single TCP connection, where the security
      context negotiated for the first stream is applied to the second
      stream.  This is typical for client-to-server sessions, and a
      server MUST allow a client to use the same TCP connection for both
      streams.

   o  Two streams over two TCP connections, where each stream is
      separately secured.  In this approach, one TCP connection is used
      for the stream in which stanzas are sent from the initiating
      entity to the receiving entity, and the other TCP connection is
      used for the stream in which stanzas are sent from the receiving
      entity to the initiating entity.  This is typical for server-to-
      server sessions.

   o  Multiple streams over two or more TCP connections, where each
      stream is separately secured.  This approach is sometimes used for
      server-to-server communication between two large XMPP service
      providers; however, this can make it difficult to maintain
      coherence of data received over multiple streams in situations
      described under Section 10.1, which is why a server MAY close the
      stream with a <conflict/> stream error (Section 4.9.3.3) if a
      remote server attempts to negotiate more than one stream (as
      described under Section 4.9.3.3).

   This concept of directionality applies only to stanzas and explicitly
   does not apply to first-level children of the stream root that are
   used to bootstrap or manage the stream (e.g., first-level elements
   used for TLS negotiation, SASL negotiation, Server Dialback
   [XEP-0220], and Stream Management [XEP-0198]).

   The foregoing considerations imply that while completing STARTTLS
   negotiation (Section 5) and SASL negotiation (Section 6) two servers
   would use one TCP connection, but after the stream negotiation
   process is done that original 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



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   initiating server over a separate TCP connection.  This separate TCP
   connection is then secured using a new round of TLS and/or SASL
   negotiation.

      Implementation Note: For historical reasons, a server-to-server
      session always uses two TCP connections.  While that approach
      remains the standard behavior described in this document,
      extensions such as [XEP-0288] enable servers to negotiate the use
      of a single TCP connection for bidirectional stanza exchange.

      Informational Note: Although XMPP developers sometimes apply the
      terms "unidirectional" and "bidirectional" to the underlying TCP
      connection (e.g., calling the TCP connection for a client-to-
      server session "bidirectional" and the TCP connection for a
      server-to-server session "unidirectional"), strictly speaking a
      stream is always unidirectional (because the initiating entity and
      receiving entity always have a minimum of two streams, one in each
      direction) and a TCP connection is always bidirectional (because
      TCP traffic can be sent in both directions).  Directionality
      applies to the application-layer traffic sent over the TCP
      connection, not to the transport-layer traffic sent over the TCP
      connection itself.

4.6.  Handling of Silent Peers

   When an entity that is a party to a stream has not received any XMPP
   traffic from its stream peer for some period of time, the peer might
   appear to be silent.  There are several reasons why this might
   happen:

   1.  The underlying TCP connection is dead.

   2.  The XML stream is broken despite the fact that the underlying TCP
       connection is alive.

   3.  The peer is idle and simply has not sent any XMPP traffic over
       its XML stream to the entity.

   These three conditions are best handled separately, as described in
   the following sections.

      Implementation Note: For the purpose of handling silent peers, we
      treat a two unidirectional TCP connections as conceptually
      equivalent to a single bidirectional TCP connection (see
      Section 4.5); however, implementers need to be aware that, in the
      case of two unidirectional TCP connections, responses to traffic
      at the XMPP application layer will come back from the peer on the
      second TCP connection.  In addition, the use of multiple streams



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      in each direction (which is a somewhat frequent deployment choice
      for server-to-server connectivity among large XMPP service
      providers) further complicates application-level checking of XMPP
      streams and their underlying TCP connections, because there is no
      necessary correlation between any given initial stream and any
      given response stream.

4.6.1.  Dead Connection

   If the underlying TCP connection is dead, stream-level checks (e.g.,
   [XEP-0199] and [XEP-0198]) are ineffective.  Therefore, it is
   unnecessary to close the stream with or without an error, and it is
   appropriate instead to simply terminate the TCP connection.

   One common method for checking the TCP connection is to send a space
   character (U+0020) between XML stanzas, which is allowed for XML
   streams as described under Section 11.7; the sending of such a space
   character is properly called a "whitespace keepalive" (the term
   "whitespace ping" is often used, despite the fact that it is not a
   ping since no "pong" is possible).  However, this is not allowed
   during TLS negotiation or SASL negotiation, as described under
   Section 5.3.3 and Section 6.3.5.

4.6.2.  Broken Stream

   Even if the underlying TCP connection is alive, the peer might never
   respond to XMPP traffic that the entity sends, whether normal stanzas
   or specialized stream-checking traffic such as the application-level
   pings defined in [XEP-0199] or the more comprehensive Stream
   Management protocol defined in [XEP-0198].  In this case, it is
   appropriate for the entity to close a broken stream with a
   <connection-timeout/> stream error (Section 4.9.3.4).

4.6.3.  Idle Peer

   Even if the underlying TCP connection is alive and the stream is not
   broken, the peer might have sent no stanzas for a certain period of
   time.  In this case, the peer itself MAY close the stream (as
   described under Section 4.4) rather than leaving an unused stream
   open.  If the idle peer does not close the stream, the other party
   MAY either close the stream using the handshake described under
   Section 4.4 or close the stream with a stream error (e.g., <resource-
   constraint/> (Section 4.9.3.17) if the entity has reached a limit on
   the number of open TCP connections or <policy-violation/>
   (Section 4.9.3.14) if the connection has exceeded a local timeout
   policy).  However, consistent with the order of layers (specified
   under Section 13.3), the other party is advised to verify that the
   underlying TCP connection is alive and the stream is unbroken (as



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   described above) before concluding that the peer is idle.
   Furthermore, it is preferable to be liberal in accepting idle peers,
   since experience has shown that doing so improves the reliability of
   communication over XMPP networks and that it is typically more
   efficient to maintain a stream between two servers than to
   aggressively time out such a stream.

4.6.4.  Use of Checking Methods

   Implementers are advised to support whichever stream-checking and
   connection-checking methods they deem appropriate, but to carefully
   weigh the network impact of such methods against the benefits of
   discovering broken streams and dead TCP connections in a timely
   manner.  The length of time between the use of any particular check
   is very much a matter of local service policy and depends strongly on
   the network environment and usage scenarios of a given deployment and
   connection type.  At the time of writing, it is RECOMMENDED that any
   such check be performed not more than once every 5 minutes and that,
   ideally, such checks will be initiated by clients rather than
   servers.  Those who implement XMPP software and deploy XMPP services
   are encouraged to seek additional advice regarding appropriate timing
   of stream-checking and connection-checking methods, particularly when
   power-constrained devices are being used (e.g., in mobile
   environments).

4.7.  Stream Attributes

   The attributes of the root <stream/> element are defined in the
   following sections.

      Security Warning: Until and unless the confidentiality and
      integrity of the stream are protected via TLS as described under
      Section 5 or an equivalent security layer (such as the SASL GSSAPI
      mechanism), the attributes provided in a stream header could be
      tampered with by an attacker.

      Implementation Note: The attributes of the root <stream/> element
      are not prepended by a namespace prefix because, as explained in
      [XML-NAMES], "[d]efault namespace declarations do not apply
      directly to attribute names; the interpretation of unprefixed
      attributes is determined by the element on which they appear."

4.7.1.  from

   The 'from' attribute specifies an XMPP identity of the entity sending
   the stream element.





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   For initial stream headers in client-to-server communication, the
   'from' attribute is the XMPP identity of the principal controlling
   the client, i.e., a JID of the form <localpart@domainpart>.  The
   client might not know the XMPP identity, e.g., because the XMPP
   identity is assigned at a level other than the XMPP application layer
   (as in the Generic Security Service Application Program Interface
   [GSS-API]) or is derived by the server from information provided by
   the client (as in some deployments of end-user certificates with the
   SASL EXTERNAL mechanism).  Furthermore, if the client considers the
   XMPP identity to be private information then it is advised not to
   include a 'from' attribute before the confidentiality and integrity
   of the stream are protected via TLS or an equivalent security layer.
   However, if the client knows the XMPP identity then it SHOULD include
   the 'from' attribute after the confidentiality and integrity of the
   stream are protected via TLS or an equivalent security layer.

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

   For initial stream headers in server-to-server communication, the
   'from' attribute is one of the configured FQDNs of the server, i.e.,
   a JID of the form <domainpart>.  The initiating server might have
   more than one XMPP identity, e.g., in the case of a server that
   provides virtual hosting, so it will need to choose an identity that
   is associated with this output stream (e.g., based on the 'to'
   attribute of the stanza that triggered the stream negotiation
   attempt).  Because a server is a "public entity" on the XMPP network,
   it MUST include the 'from' attribute after the confidentiality and
   integrity of the stream are protected via TLS or an equivalent
   security layer.

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






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   For response stream headers in both client-to-server and server-to-
   server communication, the receiving entity MUST include the 'from'
   attribute and MUST set its value to one of the receiving entity's
   FQDNs (which MAY be an FQDN other than that specified in the 'to'
   attribute of the initial stream header, as described under
   Section 4.9.1.3 and Section 4.9.3.6).

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

   Whether or not the 'from' attribute is included, each entity MUST
   verify the identity of the other entity before exchanging XML stanzas
   with it, as described under Section 13.5.

      Interoperability Note: It is possible that implementations based
      on [RFC3920] will not include the 'from' address on any stream
      headers (even ones whose confidentiality and integrity are
      protected); an entity SHOULD be liberal in accepting such stream
      headers.

4.7.2.  to

   For initial stream headers in both client-to-server and server-to-
   server communication, the initiating entity MUST include the 'to'
   attribute and MUST set its value to a domainpart that the initiating
   entity knows or expects the receiving entity to service.  (The same
   information can be provided in other ways, such as a Server Name
   Indication during TLS negotiation as described in [TLS-EXT].)

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

   For response stream headers in client-to-server communication, if the
   client included a 'from' attribute in the initial stream header then
   the server MUST include a 'to' attribute in the response stream



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   header and MUST set its value to the bare JID specified in the 'from'
   attribute of the initial stream header.  If the client did not
   include a 'from' attribute in the initial stream header then the
   server MUST NOT include a 'to' attribute in the response stream
   header.

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

   For response stream headers in server-to-server communication, the
   receiving entity MUST include a 'to' attribute in the response stream
   header and MUST set its value to the domainpart specified in the
   'from' attribute of the initial stream header.

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

   Whether or not the 'to' attribute is included, each entity MUST
   verify the identity of the other entity before exchanging XML stanzas
   with it, as described under Section 13.5.

      Interoperability Note: It is possible that implementations based
      on [RFC3920] will not include the 'to' address on stream headers;
      an entity SHOULD be liberal in accepting such stream headers.

4.7.3.  id

   The 'id' attribute specifies a unique identifier for the stream,
   called a "stream ID".  The stream ID MUST be generated by the
   receiving entity when it sends a response stream header and MUST BE
   unique within the receiving application (normally a server).






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      Security Warning: The stream ID MUST be both unpredictable and
      non-repeating because it can be security-critical when reused by
      an authentication mechanisms, as is the case for Server Dialback
      [XEP-0220] and the "XMPP 0.9" authentication mechanism used before
      RFC 3920 defined the use of SASL in XMPP; for recommendations
      regarding randomness for security purposes, see [RANDOM].

   For initial stream headers, the initiating entity MUST NOT include
   the 'id' attribute; however, if the 'id' attribute is included, the
   receiving entity MUST ignore it.

   For response stream headers, the receiving entity MUST include the
   'id' attribute.

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

      Interoperability Note: In RFC 3920, the text regarding inclusion
      of the 'id' attribute was ambiguous, leading some implementations
      to leave the attribute off the response stream header.

4.7.4.  xml:lang

   The 'xml:lang' attribute specifies an entity's preferred or default
   language for any human-readable XML character data to be sent over
   the stream (an XML stanza can also possess an 'xml:lang' attribute,
   as discussed under Section 8.1.5).  The syntax of this attribute is
   defined in Section 2.12 of [XML]; in particular, 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 language identifier
   format defined in [LANGTAGS].

   For initial stream headers, the initiating entity SHOULD include the
   'xml:lang' attribute.










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

   For response stream headers, the receiving entity MUST include the
   'xml:lang' attribute.  The following rules apply:

   o  If the initiating entity included an 'xml:lang' attribute in its
      initial stream header and the receiving entity supports that
      language in the human-readable XML character data that it
      generates and sends to the initiating entity (e.g., in the <text/>
      element for stream and stanza errors), the value of the 'xml:lang'
      attribute MUST be the identifier for the initiating entity's
      preferred language (e.g., "de-CH").

   o  If the receiving entity supports a language that matches the
      initiating entity's preferred language according to the "lookup
      scheme" specified in Section 3.4 of [LANGMATCH] (e.g., "de"
      instead of "de-CH"), then the value of the 'xml:lang' attribute
      SHOULD be the identifier for the matching language.

   o  If the receiving entity does not support the initiating entity's
      preferred language or a matching language according to the lookup
      scheme (or if the initiating entity did not include the 'xml:lang'
      attribute in its initial stream header), then the value of the
      'xml:lang' attribute MUST be the identifier for the default
      language of the receiving entity (e.g., "en").


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

   If the initiating entity included the 'xml:lang' attribute in its
   initial stream header, the receiving entity SHOULD remember that
   value as the default xml:lang for all stanzas sent by the initiating
   entity over the current stream.  As described under Section 8.1.5,



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   the initiating entity MAY include the 'xml:lang' attribute in any XML
   stanzas it sends over the stream.  If the initiating entity does not
   include the 'xml:lang' attribute in any such stanza, the receiving
   entity SHOULD add the 'xml:lang' attribute to the stanza when routing
   it to a remote server or delivering it to a connected client, where
   the value of the attribute MUST be the identifier for the language
   preferred by the initiating entity (even if the receiving entity does
   not support that language for human-readable XML character data it
   generates and sends to the initiating entity, such as in stream or
   stanza errors).  If the initiating entity includes the 'xml:lang'
   attribute in any such stanza, the receiving entity MUST NOT modify or
   delete it when routing it to a remote server or delivering it to a
   connected client.

4.7.5.  version

   The inclusion of the version attribute set to a value of at least
   "1.0" signals support for the stream-related protocols defined in
   this specification, including TLS negotiation (Section 5), SASL
   negotiation (Section 6), stream features (Section 4.3.2), and stream
   errors (Section 4.9).

   The version of XMPP specified in this specification is "1.0"; in
   particular, XMPP 1.0 encapsulates the stream-related protocols as
   well as the basic semantics of the three defined XML stanza types
   (<message/>, <presence/>, and <iq/> as described under Sections
   8.2.1, 8.2.2, and 8.2.3, respectively).

   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 will be incremented only if the stream and
   stanza formats or obligatory 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 defined in the older
   specification.

   The minor version number will 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.,



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   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:

   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 close the stream with an <unsupported-
       version/> stream error (Section 4.9.3.25).

   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.



















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4.7.6.  Summary of Stream Attributes

   The following table summarizes the attributes of the root <stream/>
   element.

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

                        Figure 4: Stream Attributes

4.8.  XML Namespaces

   Readers are referred to the specification of XML namespaces
   [XML-NAMES] for a full understanding of the concepts used in this
   section, especially the concept of a "default namespace" as provided
   in Section 3 and Section 6.2 of that specification.

4.8.1.  Stream Namespace

   The root <stream/> element ("stream header") MUST be qualified by the
   namespace 'http://etherx.jabber.org/streams' (the "stream
   namespace").  If this rule is violated, the entity that receives the
   offending stream header MUST close the stream with a stream error,
   which SHOULD be <invalid-namespace/> (Section 4.9.3.10), although
   some existing implementations send <bad-format/> (Section 4.9.3.1)
   instead.

4.8.2.  Content Namespace

   An entity MAY declare a "content namespace" as the default namespace
   for data sent over the stream (i.e., data other than elements
   qualified by the stream namespace).  If so, (1) the content namespace
   MUST be other than the stream namespace, and (2) the content
   namespace MUST be the same for the initial stream and the response
   stream so that both streams are qualified consistently.  The content
   namespace applies to all first-level child elements sent over the
   stream unless explicitly qualified by another namespace (i.e., the
   content namespace is the default namespace).






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   Alternatively (i.e., instead of declaring the content namespace as
   the default namespace), an entity MAY explicitly qualify the
   namespace for each first-level child element of the stream, using so-
   called "prefix-free canonicalization".  These two styles are shown in
   the following examples.

   When a content namespace is declared as the default namespace, in
   rough outline a stream will look something like the following.

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

   When a content namespace is not declared as the default namespace and
   so-called "prefix-free canonicalization" is used instead, in rough
   outline a stream will look something like the following.

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

   Traditionally, most XMPP implementations have used the content-
   namespace-as-default-namespace style rather than the prefix-free
   canonicalization style for stream headers; however, both styles are
   acceptable since they are semantically equivalent.

4.8.3.  XMPP Content Namespaces

   XMPP as defined in this specification uses two content namespaces:
   'jabber:client' and 'jabber:server'.  These 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



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   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.  Support for these content namespaces implies
   support for the common attributes (Section 8.1) and basic semantics
   (Section 8.2) of all three core stanza types (message, presence, and
   IQ).

   An implementation MAY support content namespaces 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.

   An implementation MAY refuse to support any other content namespaces
   as default namespaces.  If an entity receives a first-level child
   element qualified by a content namespace it does not support, it MUST
   close the stream with an <invalid-namespace/> stream error
   (Section 4.9.3.10).

   Client implementations MUST support the 'jabber:client' content
   namespace as a default namespace.  The 'jabber:server' content
   namespace is out of scope for an XMPP client, and a client MUST NOT
   send stanzas qualified by the 'jabber:server' namespace.

   Server implementations MUST support as default content namespaces
   both the 'jabber:client' namespace (when the stream is used for
   communication between a client and a server) and the 'jabber:server'
   namespace (when the stream is used for communication between two
   servers).  When communicating with a connected client, a server MUST
   NOT send stanzas qualified by the 'jabber:server' namespace; when
   communicating with a peer server, a server MUST NOT send stanzas
   qualified by the 'jabber:client' namespace.

      Implementation Note: Because a client sends stanzas over a stream
      whose content namespace is 'jabber:client', if a server routes to
      a peer server a stanza it has received from a connected client
      then it needs to "re-scope" the stanza so that its content
      namespace is 'jabber:server'.  Similarly, if a server delivers to
      a connected client a stanza it has received from a peer server
      then it needs to "re-scope" the stanza so that its content
      namespace is 'jabber:client'.  This rule applies to XML stanzas as
      defined under Section 4.1 (i.e., a first-level <message/>,
      <presence/>, or <iq/> element qualified by the 'jabber:client' or
      'jabber:server' namespace), and by namespace inheritance to all
      child elements of a stanza.  However, the rule does not apply to
      elements qualified by namespaces other than 'jabber:client' and
      'jabber:server' nor to any children of such elements (e.g., a
      <message/> element contained within an extension element



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      (Section 8.4) for reporting purposes).  Although it is not
      forbidden for an entity to generate stanzas in which an extension
      element contains a child element qualified by the 'jabber:client'
      or 'jabber:server' namespace, existing implementations handle such
      stanzas inconsistently; therefore, implementers are advised to
      weigh the likely lack of interoperability against the possible
      utility of such stanzas.  Finally, servers are advised to apply
      stanza re-scoping to other stream connection methods and
      alternative XMPP connection methods, such as those specified in
      [XEP-0124], [XEP-0206], [XEP-0114], and [XEP-0225].

4.8.4.  Other Namespaces

   Either party to a stream MAY send data qualified by namespaces other
   than the content namespace and the stream namespace.  For example,
   this is how data related to TLS negotiation and SASL negotiation are
   exchanged, as well as XMPP extensions such as Stream Management
   [XEP-0198] and Server Dialback [XEP-0220].

      Interoperability Note: For historical reasons, some server
      implementations expect a declaration of the 'jabber:server:
      dialback' namespace on server-to-server streams, as explained in
      [XEP-0220].

   However, an XMPP server MUST NOT route or deliver data received over
   an input stream if that data is (a) qualified by another namespace
   and (b) addressed to an entity other than the server, unless the
   other party to the output stream over which the server would send the
   data has explicitly negotiated or advertised support for receiving
   arbitrary data from the server.  This rule is included because XMPP
   is designed for the exchange of XML stanzas (not arbitrary XML data),
   and because allowing an entity to send arbitrary data to other
   entities could significantly increase the potential for exchanging
   malicious information.  As an example of this rule, the server
   hosting the example.net domain would not route the following first-
   level XML element from <romeo@example.net> to <juliet@example.com>:

     <ns1:foo xmlns:ns1='http://example.org/ns1'
              from='romeo@example.net/resource1'
              to='juliet@example.com'>
       <ns1:bar/>
     </ns1:foo>

   This rule also applies to first-level elements that look like stanzas
   but that are improperly namespaced and therefore really are not
   stanzas at all (see also Section 4.8.5), for example:





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     <ns2:message xmlns:ns2='http://example.org/ns2'
                  from='romeo@example.net/resource1'
                  to='juliet@example.com'>
       <body>hi</body>
     </ns2:message>

   Upon receiving arbitrary first-level XML elements over an input
   stream, a server MUST either ignore the data or close the stream with
   a stream error, which SHOULD be <unsupported-stanza-type/>
   (Section 4.9.3.24).

4.8.5.  Namespace Declarations and Prefixes

   Because the content namespace is other than the stream namespace, if
   a content namespace is declared as the default namespace then the
   following statements are true:

   1.  The stream header needs to contain a namespace declaration for
       both the content namespace and the stream namespace.

   2.  The stream namespace declaration needs to include a namespace
       prefix for the stream namespace.

      Interoperability Note: For historical reasons, an implementation
      MAY accept only the prefix 'stream' for the stream namespace
      (resulting in prefixed names such as <stream:stream> and <stream:
      features>); this specification retains that allowance from
      [RFC3920] for the purpose of backward compatibility.
      Implementations are advised that using a prefix other than
      'stream' for the stream namespace might result in interoperability
      problems.  If an entity receives a stream header with a stream
      namespace prefix it does not accept, it MUST close the stream with
      a stream error, which SHOULD be <bad-namespace-prefix/>
      (Section 4.9.3.2), although some existing implementations send
      <bad-format/> (Section 4.9.3.1) instead.

   An implementation MUST NOT generate namespace prefixes for elements
   qualified by the content namespace (i.e., the default namespace for
   data sent over the stream) if the content namespace is 'jabber:
   client' or 'jabber:server'.  For example, the following is illegal:

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



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     <foo:message xmlns:foo='jabber:client'>
       <foo:body>foo</foo:body>
     </foo:message>

   An XMPP entity SHOULD NOT accept data that violates this rule (in
   particular, an XMPP server MUST NOT route or deliver such data to
   another entity without first correcting the error); instead it SHOULD
   either ignore the data or close the stream with a stream error, which
   SHOULD be <bad-namespace-prefix/> (Section 4.9.3.2).

   Namespaces declared in a stream header MUST apply only to that stream
   (e.g., the 'jabber:server:dialback' namespace used in Server Dialback
   [XEP-0220]).  In particular, because XML stanzas intended for routing
   or delivery over streams with other entities will lose the namespace
   context declared in the header of the stream in which those stanzas
   originated, namespaces for extended content within such stanzas MUST
   NOT be declared in that stream header (see also Section 8.4).  If
   either party to a stream declares such namespaces, the other party to
   the stream SHOULD close the stream with an <invalid-namespace/>
   stream error (Section 4.9.3.10).  In any case, an entity MUST ensure
   that such namespaces are properly declared (according to this
   section) when routing or delivering stanzas from an input stream to
   an output stream.

4.9.  Stream Errors

   The root stream element MAY contain an <error/> child element that is
   qualified by the stream namespace.  The error child SHALL be sent by
   a compliant entity if it perceives that a stream-level error has
   occurred.

4.9.1.  Rules

   The following rules apply to stream-level errors.

4.9.1.1.  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 an <error/> element with an appropriate child element specifying
   the error condition and then immediately close the stream as
   described under Section 4.4.









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

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

   The entity that receives the stream error then SHALL close the stream
   as explained under Section 4.4.

   C: </stream:stream>

4.9.1.2.  Stream Errors Can Occur During Setup

   If the error is triggered by the initial stream header, the receiving
   entity MUST still send the opening <stream> tag, include the <error/>
   element as a child of the stream element, and send the closing
   </stream> tag (preferably in the same TCP packet).

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.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='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.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>








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4.9.1.3.  Stream Errors When the Host Is Unspecified or Unknown

   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 value of the 'from' attribute returned by the receiving
   entity in the stream header sent before closing the stream MUST be
   either an authoritative FQDN for the receiving entity or the empty
   string.

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

4.9.1.4.  Where Stream Errors Are Sent

   When two TCP connections are used between the initiating entity and
   the receiving entity (one in each direction) rather than using a
   single bidirectional connection, the following rules apply:

   o  Stream-level errors related to the initial stream are returned by
      the receiving entity on the response stream via the same TCP
      connection.

   o  Stanza errors triggered by outbound stanzas sent from the
      initiating entity over the initial stream via the same TCP
      connection are returned by the receiving entity on the response
      stream via the other ("return") TCP connection, since they are
      inbound stanzas from the perspective of the initiating entity.



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4.9.2.  Syntax

   The syntax for stream errors is as follows, where XML data shown
   within the square brackets '[' and ']' is OPTIONAL.

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

   The "defined-condition" MUST correspond to one of the stream error
   conditions defined under Section 4.9.3.  However, because additional
   error conditions might be defined in the future, if an entity
   receives a stream error condition that it does not understand then it
   MUST treat the unknown condition as equivalent to <undefined-
   condition/> (Section 4.9.3.21).  If the designers of an XMPP protocol
   extension or the developers of an XMPP implementation need to
   communicate a stream error condition that is not defined in this
   specification, they can do so by defining an application-specific
   error condition element qualified by an application-specific
   namespace.

   The <error/> element:

   o  MUST contain a child element corresponding to one of the defined
      stream error conditions (Section 4.9.3); 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.

   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 4.9.4).

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



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   be shown in addition to the error message associated with the defined
   condition element (and, optionally, the application-specific
   condition element).

4.9.3.  Defined Stream Error Conditions

   The following stream-level error conditions are defined.

4.9.3.1.  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, which alternatively might trigger a <not-well-
   formed/> stream error (Section 4.9.3.13).)

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

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

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

4.9.3.2.  bad-namespace-prefix

   The entity has sent a namespace prefix that is unsupported, or has
   sent no namespace prefix on an element that needs such a prefix (see
   Section 11.2).

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

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




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   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.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>

4.9.3.3.  conflict

   The server either (1) is 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 or allows only one server-to-
   server stream for a given domain pair as a way of helping to ensure
   in-order processing as described under Section 10.1).

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

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.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>



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   If a client receives a <conflict/> stream error (Section 4.9.3.3),
   during the resource binding aspect of its reconnection attempt it
   MUST NOT blindly request the resourcepart it used during the former
   session but instead MUST choose a different resourcepart; details are
   provided under Section 7.

4.9.3.4.  connection-timeout

   One party is closing the stream because it has reason to believe that
   the other party has permanently lost the ability to communicate over
   the stream.  The lack of ability to communicate can be discovered
   using various methods, such as whitespace keepalives as specified
   under Section 4.4, XMPP-level pings as defined in [XEP-0199], and
   XMPP Stream Management as defined in [XEP-0198].

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

      Interoperability Note: RFC 3920 specified that the <connection-
      timeout/> stream error (Section 4.9.3.4) is to be used if the peer
      has not generated any traffic over the stream for some period of
      time.  That behavior is no longer recommended; instead, the error
      SHOULD be used only if the connected client or peer server has not
      responded to data sent over the stream.

4.9.3.5.  host-gone

   The value of the 'to' attribute provided in the initial stream header
   corresponds to an FQDN that is no longer serviced by the receiving
   entity.

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

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






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   S: <?xml version='1.0'?>
      <stream:stream
          from='im.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>

4.9.3.6.  host-unknown

   The value of the 'to' attribute provided in the initial stream header
   does not correspond to an FQDN that is serviced by the receiving
   entity.

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

   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='im.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>




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4.9.3.7.  improper-addressing

   A stanza sent between two servers lacks a 'to' or 'from' attribute,
   the 'from' or 'to' attribute has no value, or the value violates the
   rules for XMPP addresses [XMPP-ADDR].

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

   P: <message from='juliet@im.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>

4.9.3.8.  internal-server-error

   The server has experienced a misconfiguration or other internal error
   that prevents it from servicing the stream.

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

4.9.3.9.  invalid-from

   The data provided in a 'from' attribute does not match an authorized
   JID or validated domain as negotiated (1) between two servers using
   SASL or Server Dialback, or (2) between a client and a server via
   SASL 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@im.example.com'>
        <body>Neither, fair saint, if either thee dislike.</body>
      </message>







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

4.9.3.10.  invalid-namespace

   The stream namespace name is something other than
   "http://etherx.jabber.org/streams" (see Section 11.2) or the content
   namespace declared as the default namespace is not supported (e.g.,
   something other than "jabber:client" or "jabber:server").

   (In the following example, the client specifies a namespace of
   'http://wrong.namespace.example.org/' for the stream.)

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

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.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>

4.9.3.11.  invalid-xml

   The entity has sent invalid XML over the stream to a server that
   performs validation (see Section 11.4).

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




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   P: <iq from='example.net'
          id='l3b1vs75'
          to='im.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>

4.9.3.12.  not-authorized

   The entity has attempted to send XML stanzas or other outbound data
   before the stream has been authenticated, or otherwise is not
   authorized to perform an action related to stream negotiation; the
   receiving entity MUST NOT process the offending data before sending
   the stream error.

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

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

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.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>






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

4.9.3.13.  not-well-formed

   The initiating entity has sent XML that violates the well-formedness
   rules of [XML] or [XML-NAMES].

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

   C: <message>
        <foo:body>What is this foo?</foo:body>
      </message>

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

      Interoperability Note: In RFC 3920, the name of this error
      condition was "xml-not-well-formed" instead of "not-well-formed".
      The name was changed because the element name <xml-not-well-
      formed/> violates the constraint from Section 3 of [XML] that
      "names beginning with a match to (('X'|'x')('M'|'m')('L'|'l')) are
      reserved for standardization in this or future versions of this
      specification".

4.9.3.14.  policy-violation

   The entity has violated some local service policy (e.g., a stanza
   exceeds a configured size limit); the server MAY choose to specify
   the policy in the <text/> element or in 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@im.example.com' id='foo'>
        <body>[ ... the-emacs-manual ... ]</body>
      </message>






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

   S: </stream:stream>

4.9.3.15.  remote-connection-failed

   The server is unable to properly connect to a remote entity that is
   needed for authentication or authorization (e.g., in certain
   scenarios related to Server Dialback [XEP-0220]); this condition is
   not to be used when the cause of the error is within the
   administrative domain of the XMPP service provider, in which case the
   <internal-server-error/> condition is more appropriate.

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

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.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>

4.9.3.16.  reset

   The server is closing the stream because it has new (typically
   security-critical) features to offer, because the keys or
   certificates used to establish a secure context for the stream have
   expired or have been revoked during the life of the stream
   (Section 13.7.2.3), because the TLS sequence number has wrapped
   (Section 5.3.5), etc.  The reset applies to the stream and to any



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   security context established for that stream (e.g., via TLS and
   SASL), which means that encryption and authentication need to be
   negotiated again for the new stream (e.g., TLS session resumption
   cannot be used).

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

4.9.3.17.  resource-constraint

   The server lacks the system resources necessary to service the
   stream.

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

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.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>

4.9.3.18.  restricted-xml

   The entity has attempted to send restricted XML features such as a
   comment, processing instruction, DTD subset, or XML entity reference
   (see Section 11.1).

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




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   C: <message to='juliet@im.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>

4.9.3.19.  see-other-host

   The server will not provide service to the initiating entity but is
   redirecting traffic to another host under the administrative control
   of the same service provider.  The XML character data of the <see-
   other-host/> element returned by the server MUST specify the
   alternate FQDN or IP address at which to connect, which MUST be a
   valid domainpart or a domainpart plus port number (separated by the
   ':' character in the form "domainpart:port").  If the domainpart is
   the same as the source domain, derived domain, or resolved IPv4 or
   IPv6 address to which the initiating entity originally connected
   (differing only by the port number), then the initiating entity
   SHOULD simply attempt to reconnect at that address.  (The format of
   an IPv6 address MUST follow [IPv6-ADDR], which includes the enclosing
   the IPv6 address in square brackets '[' and ']' as originally defined
   by [URI].)  Otherwise, the initiating entity MUST resolve the FQDN
   specified in the <see-other-host/> element as described under
   Section 3.2.

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














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   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.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'>
          [2001:41D0:1:A49b::1]:9222
        </see-other-host>
      </stream:error>
      </stream:stream>

   When negotiating a stream with the host to which it has been
   redirected, the initiating entity MUST apply the same policies it
   would have applied to the original connection attempt (e.g., a policy
   requiring TLS), MUST specify the same 'to' address on the initial
   stream header, and MUST verify the identity of the new host using the
   same reference identifier(s) it would have used for the original
   connection attempt (in accordance with [TLS-CERTS]).  Even if the
   receiving entity returns a <see-other-host/> error before the
   confidentiality and integrity of the stream have been established
   (thus introducing the possibility of a denial-of-service attack), the
   fact that the initiating entity needs to verify the identity of the
   XMPP service based on the same reference identifiers implies that the
   initiating entity will not connect to a malicious entity.  To reduce
   the possibility of a denial-of-service attack, (a) the receiving
   entity SHOULD NOT close the stream with a <see-other-host/> stream
   error until after the confidentiality and integrity of the stream
   have been protected via TLS or an equivalent security layer (such as
   the SASL GSSAPI mechanism), and (b) the receiving entity MAY have a
   policy of following redirects only if it has authenticated the
   receiving entity.  In addition, the initiating entity SHOULD abort
   the connection attempt after a certain number of successive redirects
   (e.g., at least 2 but no more than 5).












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4.9.3.20.  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>

4.9.3.21.  undefined-condition

   The error condition is not one of those defined by the other
   conditions in this list; this error condition SHOULD NOT be used
   except in conjunction with an application-specific condition.

   S: <stream:error>
        <undefined-condition
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
        <app-error xmlns='http://example.org/ns'/>
      </stream:error>
      </stream:stream>

4.9.3.22.  unsupported-encoding

   The initiating entity has encoded the stream in an encoding that is
   not supported by the server (see Section 11.6) or has otherwise
   improperly encoded the stream (e.g., by violating the rules of the
   [UTF-8] encoding).

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










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   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.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>

4.9.3.23.  unsupported-feature

   The receiving entity has advertised a mandatory-to-negotiate stream
   feature that the initiating entity does not support, and has offered
   no other mandatory-to-negotiate feature alongside the unsupported
   feature.

   (In the following example, the receiving entity requires negotiation
   of an example feature, but the initiating entity does not support the
   feature.)

   R: <stream:features>
        <example xmlns='urn:xmpp:example'>
          <required/>
        </example>
      </stream:features>

   I: <stream:error>
        <unsupported-feature
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

4.9.3.24.  unsupported-stanza-type

   The initiating entity has sent a first-level child of the stream that
   is not supported by the server, either because the receiving entity
   does not understand the namespace or because the receiving entity
   does not understand the element name for the applicable namespace
   (which might be the content namespace declared as the default
   namespace).





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   (In the following example, the client attempts to send a first-level
   child element of <pubsub/> qualified by the 'jabber:client'
   namespace, but the schema for that namespace defines no such
   element.)

   C: <pubsub xmlns='jabber:client'>
        <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.example/2003/12/13/atom03'/>
              <id>tag:denmark.example,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>

4.9.3.25.  unsupported-version

   The 'version' attribute provided by the initiating entity in the
   stream header specifies a version of XMPP that is not supported by
   the server.

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





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

4.9.4.  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>
        <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='http://example.org/ns'/>
      </stream:error>
      </stream:stream>









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4.10.  Simplified Stream Examples

   This section contains two highly simplified examples of a stream-
   based connection between a client and a server; these examples are
   included for the purpose of illustrating the concepts introduced thus
   far, but the reader needs to be aware that these examples elide many
   details (see Section 9 for more complete examples).

   A basic connection:

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

   [ ... stream negotiation ... ]

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

   [ ... stream negotiation ... ]

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

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


   More detailed examples are provided under Section 9.

5.  STARTTLS Negotiation

5.1.  Fundamentals

   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 modeled after similar extensions for the [IMAP],





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   [POP3], and [ACAP] protocols as described in [USINGTLS].  The XML
   namespace name for the STARTTLS extension is
   'urn:ietf:params:xml:ns:xmpp-tls'.

5.2.  Support

   Support for STARTTLS is REQUIRED in XMPP client and server
   implementations.  An administrator of a given deployment MAY specify
   that TLS is mandatory-to-negotiate for client-to-server
   communication, server-to-server communication, or both.  An
   initiating entity SHOULD use TLS to secure its stream with the
   receiving entity before proceeding with SASL authentication.

5.3.  Stream Negotiation Rules

5.3.1.  Mandatory-to-Negotiate

   If the receiving entity advertises only the STARTTLS feature or if
   the receiving entity includes the <required/> child element as
   explained under Section 5.4.1, the parties MUST consider TLS as
   mandatory-to-negotiate.  If TLS is mandatory-to-negotiate, the
   receiving entity SHOULD NOT advertise support for any stream feature
   except STARTTLS during the initial stage of the stream negotiation
   process, because further stream features might depend on prior
   negotiation of TLS given the order of layers in XMPP (e.g., the
   particular SASL mechanisms offered by the receiving entity will
   likely depend on whether TLS has been negotiated).

5.3.2.  Restart

   After TLS negotiation, the parties MUST restart the stream.

5.3.3.  Data Formatting

   During STARTTLS negotiation, the entities MUST NOT send any
   whitespace as separators between XML elements (i.e., from the last
   character of the first-level <starttls/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-tls' namespace as sent by the initiating
   entity, until the last character of the first-level <proceed/>
   element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace
   as sent by the receiving entity).  This prohibition helps to ensure
   proper security layer byte precision.  Any such whitespace shown in
   the STARTTLS examples provided in this document is included only for
   the sake of readability.







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5.3.4.  Order of TLS and SASL Negotiations

   If the initiating entity chooses to use TLS, STARTTLS negotiation
   MUST be completed before proceeding to SASL negotiation (Section 6);
   this order of negotiation is necessary to help safeguard
   authentication information sent during SASL negotiation, as well as
   to make it possible to base the use of the SASL EXTERNAL mechanism on
   a certificate (or other credentials) provided during prior TLS
   negotiation.

5.3.5.  TLS Renegotiation

   The TLS protocol allows either party in a TLS-protected channel to
   initiate a new handshake that establishes new cryptographic
   parameters (see [TLS-NEG]).  The cases most commonly mentioned are:

   1.  Refreshing encryption keys.

   2.  Wrapping the TLS sequence number as explained in Section 6.1 of
       [TLS].

   3.  Protecting client credentials by completing server authentication
       first and then completing client authentication over the
       protected channel.

   Because it is relatively inexpensive to establish streams in XMPP,
   for the first two cases it is preferable to use an XMPP stream reset
   (as described under Section 4.9.3.16) instead of performing TLS
   renegotiation.

   The third case has improved security characteristics when the TLS
   client (which might be an XMPP server) presents credentials to the
   TLS server.  If communicating such credentials to an unauthenticated
   TLS server might leak private information, it can be appropriate to
   complete TLS negotiation for the purpose of authenticating the TLS
   server to the TLS client and then attempt TLS renegotiation for the
   purpose of authenticating the TLS client to the TLS server.  However,
   this case is extremely rare because the credentials presented by an
   XMPP server or XMPP client acting as a TLS client are almost always
   public (i.e., a PKIX certificate), and therefore providing those
   credentials before authenticating the XMPP server acting as a TLS
   server would not in general leak private information.

   As a result, implementers are encouraged to carefully weigh the costs
   and benefits of TLS renegotiation before supporting it in their
   software, and XMPP entities that act as TLS clients are discouraged





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   from attempting TLS renegotiation unless the certificate (or other
   credential information) sent during TLS negotiation is known to be
   private.

   Support for TLS renegotiation is strictly OPTIONAL.  However,
   implementations that support TLS renegotiation MUST implement and use
   the TLS Renegotiation Extension [TLS-NEG].

   If an entity that does not support TLS renegotiation detects a
   renegotiation attempt, then it MUST immediately close the underlying
   TCP connection without returning a stream error (since the violation
   has occurred at the TLS layer, not the XMPP layer, as described under
   Section 13.3).

   If an entity that supports TLS renegotiation detects a TLS
   renegotiation attempt that does not use the TLS Renegotiation
   Extension [TLS-NEG], then it MUST immediately close the underlying
   TCP connection without returning a stream error (since the violation
   has occurred at the TLS layer, not the XMPP layer as described under
   Section 13.3).

5.3.6.  TLS Extensions

   Either party to a stream MAY include any TLS extension during the TLS
   negotiation itself.  This is a matter for the TLS layer, not the XMPP
   layer.

5.4.  Process

5.4.1.  Exchange of Stream Headers and Stream Features

   The initiating entity resolves the FQDN of the receiving entity as
   specified under Section 3, opens a TCP connection to the advertised
   port at the resolved IP address, and sends an initial stream header
   to the receiving entity.

   I: <stream:stream
        from='juliet@im.example.com'
        to='im.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.




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   R: <stream:stream
        from='im.example.com'
        id='t7AMCin9zjMNwQKDnplntZPIDEI='
        to='juliet@im.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.  If the receiving entity supports TLS, the stream features
   MUST include an advertisement for support of STARTTLS negotiation,
   i.e., a <starttls/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-tls' namespace.

   If the receiving entity considers STARTTLS negotiation to be
   mandatory-to-negotiate, the <starttls/> element MUST contain an empty
   <required/> child element.

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

5.4.2.  Initiation of STARTTLS Negotiation

5.4.2.1.  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.

5.4.2.2.  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, close the XML stream, and terminate the underlying TCP
   connection.




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

   R: </stream:stream>

   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 did not offer the STARTTLS feature in its
       stream features.

   3.  The receiving entity cannot complete STARTTLS negotiation because
       of an internal error.

      Informational Note: STARTTLS failure is not triggered by TLS
      errors such as bad_certificate or handshake_failure, which are
      generated and handled during the TLS negotiation itself as
      described in [TLS].

   If the failure case occurs, the initiating entity MAY attempt to
   reconnect as explained under Section 3.3.

5.4.2.3.  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
   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.

5.4.3.  TLS Negotiation

5.4.3.1.  Rules

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





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   The following rules apply:

   1.  The entities MUST NOT send any further XML data until the TLS
       negotiation is complete.

   2.  When using any of the mandatory-to-implement (MTI) ciphersuites
       specified under Section 13.8, the receiving entity MUST present a
       certificate.

   3.  So that mutual certificate authentication will be possible, the
       receiving entity SHOULD send a certificate request to the
       initiating entity, and the initiating entity SHOULD send a
       certificate to the receiving entity (but for privacy reasons
       might opt not to send a certificate until after the receiving
       entity has authenticated to the initiating entity).

   4.  The receiving entity SHOULD choose which certificate to present
       based on the domainpart contained in the 'to' attribute of the
       initial stream header (in essence, this domainpart is
       functionally equivalent to the Server Name Indication defined for
       TLS in [TLS-EXT]).

   5.  To determine if the TLS negotiation will succeed, the initiating
       entity MUST attempt to validate the receiving entity's
       certificate in accordance with the certificate validation
       procedures specified under Section 13.7.2.

   6.  If the initiating entity presents a certificate, the receiving
       entity too MUST attempt to validate the initiating entity's
       certificate in accordance with the certificate validation
       procedures specified under Section 13.7.2.

   7.  Following successful TLS negotiation, all further data
       transmitted by either party MUST be protected with the negotiated
       algorithms, keys, and secrets (i.e., encrypted, integrity-
       protected, or both depending on the ciphersuite used).

      Security Warning: See Section 13.8 regarding ciphersuites that
      MUST be supported for TLS; naturally, other ciphersuites MAY be
      supported as well.

5.4.3.2.  TLS Failure

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






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   The receiving entity MUST NOT send a closing </stream> tag before
   terminating the TCP connection (since the failure has occurred at the
   TLS layer, not the XMPP layer as described under Section 13.3).

   The initiating entity MAY attempt to reconnect as explained under
   Section 3.3, with or without attempting TLS negotiation (in
   accordance with local service policy, user-configured preferences,
   etc.).

5.4.3.3.  TLS Success

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

   1.  The initiating entity MUST discard any information transmitted in
       layers above TCP that it obtained from the receiving entity in an
       insecure manner before TLS took effect (e.g., the receiving
       entity's 'from' address or the stream ID and stream features
       received from the receiving entity).

   2.  The receiving entity MUST discard any information transmitted in
       layers above TCP that it obtained from the initiating entity in
       an insecure manner before TLS took effect (e.g., the initiating
       entity's 'from' address).

   3.  The initiating entity MUST send a new initial stream header to
       the receiving entity over the encrypted connection (as specified
       under Section 4.3.3, the initiating entity MUST NOT send a
       closing </stream> tag before sending the new initial stream
       header, since the receiving entity and initiating entity MUST
       consider the original stream to be replaced upon success of the
       TLS negotiation).

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

   4.  The receiving entity MUST respond with a new response stream
       header over the encrypted connection (for which it MUST generate
       a new stream ID instead of reusing the old stream ID).







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   R: <stream:stream
        from='im.example.com'
        id='vgKi/bkYME8OAj4rlXMkpucAqe4='
        to='juliet@im.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 6 (see especially Section 6.4.1 regarding the few
       reasons why the SASL stream feature would not be offered here).

   R: <stream:features>
        <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
          <mechanism>EXTERNAL</mechanism>
          <mechanism>SCRAM-SHA-1-PLUS</mechanism>
          <mechanism>SCRAM-SHA-1</mechanism>
          <mechanism>PLAIN</mechanism>
        </mechanisms>
      </stream:features>

6.  SASL Negotiation

6.1.  Fundamentals

   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].  The XML namespace name for the SASL
   extension is 'urn:ietf:params:xml:ns:xmpp-sasl'.

6.2.  Support

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

6.3.  Stream Negotiation Rules

6.3.1.  Mandatory-to-Negotiate

   The parties to a stream MUST consider SASL as mandatory-to-negotiate.





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6.3.2.  Restart

   After SASL negotiation, the parties MUST restart the stream.

6.3.3.  Mechanism Preferences

   Any entity that will act as a SASL client or a SASL server MUST
   maintain an ordered list of its preferred SASL mechanisms according
   to the client or server, where the list is ordered according to local
   policy or user configuration (which SHOULD be in order of perceived
   strength to enable the strongest authentication possible).  The
   initiating entity MUST maintain its own preference order independent
   of the preference order of the receiving entity.  A client MUST try
   SASL mechanisms in its preference order.  For example, if the server
   offers the ordered list "PLAIN SCRAM-SHA-1 GSSAPI" or "SCRAM-SHA-1
   GSSAPI PLAIN" but the client's ordered list is "GSSAPI SCRAM-SHA-1",
   the client MUST try GSSAPI first and then SCRAM-SHA-1 but MUST NOT
   try PLAIN (since PLAIN is not on its list).

6.3.4.  Mechanism Offers

   If the receiving entity considers TLS negotiation (Section 5) to be
   mandatory-to-negotiate before it will accept authentication with a
   particular SASL mechanism, it MUST NOT advertise that mechanism in
   its list of available SASL mechanisms before TLS negotiation has been
   completed.

   The receiving entity SHOULD offer the SASL EXTERNAL mechanism if both
   of the following conditions hold:

   1.  During TLS negotiation the initiating entity presented a
       certificate that is acceptable to the receiving entity for
       purposes of strong identity verification in accordance with local
       service policies (e.g., because said certificate is unexpired, is
       unrevoked, and is anchored to a root trusted by the receiving
       entity).

   2.  The receiving entity expects that the initiating entity will be
       able to authenticate and authorize as the identity provided in
       the certificate; in the case of a server-to-server stream, the
       receiving entity might have such an expectation because a DNS
       domain name presented in the initiating entity's certificate
       matches the domain referenced in the 'from' attribute of the
       initial stream header, where the matching rules of [TLS-CERTS]
       apply; in the case of a client-to-server stream, the receiving
       entity might have such an expectation because the bare JID
       presented in the initiating entity's certificate matches a user
       account that is registered with the server or because other



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       information contained in the initiating entity's certificate
       matches that of an entity that has permission to use the server
       for access to an XMPP network.

   However, the receiving entity MAY offer the SASL EXTERNAL mechanism
   under other circumstances, as well.

   When the receiving entity offers the SASL EXTERNAL mechanism, the
   receiving entity SHOULD list the EXTERNAL mechanism first among its
   offered SASL mechanisms and the initiating entity SHOULD attempt SASL
   negotiation using the EXTERNAL mechanism first (this preference will
   tend to increase the likelihood that the parties can negotiate mutual
   certificate authentication).

   Section 13.8 specifies SASL mechanisms that MUST be supported;
   naturally, other SASL mechanisms MAY be supported as well.

      Informational Note: Best practices for the use of SASL in the
      context of XMPP are described in [XEP-0175] for the ANONYMOUS
      mechanism and in [XEP-0178] for the EXTERNAL mechanism.

6.3.5.  Data Formatting

   The following data formatting rules apply to the SASL negotiation:

   1.  During SASL negotiation, the entities MUST NOT send any
       whitespace as separators between XML elements (i.e., from the
       last character of the first-level <auth/> element qualified by
       the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace as sent by the
       initiating entity, until the last character of the first-level
       <success/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace as sent by the
       receiving entity).  This prohibition helps to ensure proper
       security layer byte precision.  Any such whitespace shown in the
       SASL examples provided in this document is included only for the
       sake of readability.

   2.  Any XML character data contained within the XML elements MUST be
       encoded using base 64, where the encoding adheres to the
       definition in Section 4 of [BASE64] and where the padding bits
       are set to zero.

   3.  As formally specified in the XML schema for the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace under Appendix A.4,
       the receiving entity MAY include one or more application-specific
       child elements inside the <mechanisms/> element to provide
       information that might be needed by the initiating entity in
       order to complete successful SASL negotiation using one or more



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       of the offered mechanisms; however, the syntax and semantics of
       all such elements are out of scope for this specification (see
       [XEP-0233] for one example).

6.3.6.  Security Layers

   Upon successful SASL negotiation that involves negotiation of a
   security layer, both the initiating entity and the receiving entity
   MUST discard any application-layer state (i.e, state from the XMPP
   layer, excluding state from the TLS negotiation or SASL negotiation).

6.3.7.  Simple User Name

   Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify
   that the authentication identity used in the context of such
   mechanisms is a "simple user name" (see Section 2 of [SASL] as well
   as [SASLPREP]).  The exact form of the simple user name in any
   particular mechanism or deployment thereof is a local matter, and a
   simple user name does not necessarily map to an application
   identifier such as a JID or JID component (e.g., a localpart).
   However, in the absence of local information provided by the server,
   an XMPP client SHOULD assume that the authentication identity for
   such a SASL mechanism is a simple user name equal to the localpart of
   the user's JID.

6.3.8.  Authorization Identity

   An authorization identity is an OPTIONAL identity included by the
   initiating entity to specify an identity to act as (see Section 2 of
   [SASL]).  In client-to-server streams, it would most likely be used
   by an administrator to perform some management task on behalf of
   another user, whereas in server-to-server streams it would most
   likely be used to specify a particular add-on service at an XMPP
   service (e.g., a multi-user chat server at conference.example.com
   that is hosted by the example.com XMPP service).  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.

   In the case of client-to-server communication, the value of an
   authorization identity MUST be a bare JID (<localpart@domainpart>)
   rather than a full JID (<localpart@domainpart/resourcepart>).

   In the case of server-to-server communication, the value of an
   authorization identity MUST be a domainpart only (<domainpart>).



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   If the initiating entity provides an authorization identity during
   SASL negotiation, the receiving entity is responsible for verifying
   that the initiating entity is in fact allowed to assume the specified
   authorization identity; if not, the receiving entity MUST return an
   <invalid-authzid/> SASL error as described under Section 6.5.6.

6.3.9.  Realms

   The receiving entity MAY include a realm when negotiating certain
   SASL mechanisms (e.g., both the GSSAPI and DIGEST-MD5 mechanisms
   allow the authentication exchange to include a realm, though in
   different ways, whereas the EXTERNAL, SCRAM, and PLAIN mechanisms do
   not).  If the receiving entity does not communicate a realm, the
   initiating entity MUST NOT assume that any realm exists.  The realm
   MUST be used only for the purpose of authentication; in particular,
   an initiating entity MUST NOT attempt to derive an XMPP domainpart
   from the realm information provided by the receiving entity.

6.3.10.  Round Trips

   [SASL] specifies that a using protocol such as XMPP can define two
   methods by which the protocol can save round trips where allowed for
   the SASL mechanism:

   1.  When the SASL client (the XMPP "initiating entity") requests an
       authentication exchange, it can include "initial response" data
       with its request if appropriate for the SASL mechanism in use.
       In XMPP, this is done by including the initial response as the
       XML character data of the <auth/> element.

   2.  At the end of the authentication exchange, the SASL server (the
       XMPP "receiving entity") can include "additional data with
       success" if appropriate for the SASL mechanism in use.  In XMPP,
       this is done by including the additional data as the XML
       character data of the <success/> element.

   For the sake of protocol efficiency, it is REQUIRED for clients and
   servers to support these methods and RECOMMENDED to use them;
   however, clients and servers MUST support the less efficient modes as
   well.











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6.4.  Process

   The process for SASL negotiation is as follows.

6.4.1.  Exchange of Stream Headers and Stream Features

   If SASL negotiation follows successful STARTTLS negotiation
   (Section 5), then the SASL negotiation occurs over the protected
   stream that has already been negotiated.  If not, the initiating
   entity resolves the FQDN of the receiving entity as specified under
   Section 3, opens a TCP connection to the advertised port at the
   resolved IP address, and sends an initial stream header to the
   receiving entity.  In either case, the receiving entity will receive
   an initial stream from the initiating entity.

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

   When the receiving entity processes an initial stream header from the
   initiating entity, it MUST send a response stream header to the
   initiating entity (for which it MUST generate a unique stream ID.  If
   TLS negotiation has already succeeded, then this stream ID MUST be
   different from the stream ID sent before TLS negotiation succeeded).

   R: <stream:stream
        from='im.example.com'
        id='vgKi/bkYME8OAj4rlXMkpucAqe4='
        to='juliet@im.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.  The stream features SHOULD include an advertisement for
   support of SASL negotiation, i.e., a <mechanisms/> element qualified
   by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace.  Typically there
   are only three cases in which support for SASL negotiation would not
   be advertised here:







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   o  TLS negotiation needs to happen before SASL can be offered (i.e.,
      TLS is required and the receiving entity is responding to the very
      first initial stream header it has received for this connection
      attempt).

   o  SASL negotiation is impossible for a server-to-server connection
      (i.e., the initiating server has not provided a certificate that
      would enable strong authentication and therefore the receiving
      server is falling back to weak identity verification using the
      Server Dialback protocol [XEP-0220]).

   o  SASL has already been negotiated (i.e., the receiving entity is
      responding to an initial stream header sent as a stream restart
      after successful SASL negotiation).

   The <mechanisms/> element MUST contain one <mechanism/> child element
   for each authentication mechanism the receiving entity offers to the
   initiating entity.  As noted, the order of <mechanism/> elements in
   the XML indicates the preference order of the SASL mechanisms
   according to the receiving entity (which is not necessarily the
   preference order according to the initiating entity).

   R: <stream:features>
        <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
          <mechanism>EXTERNAL</mechanism>
          <mechanism>SCRAM-SHA-1-PLUS</mechanism>
          <mechanism>SCRAM-SHA-1</mechanism>
          <mechanism>PLAIN</mechanism>
        </mechanisms>
      </stream:features>

6.4.2.  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, thus starting the
   handshake for that particular authentication mechanism.  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='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>





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   If the initiating entity subsequently sends another <auth/> element
   and the ongoing authentication handshake has not yet completed, the
   receiving entity MUST discard the ongoing handshake and MUST process
   a new handshake for the subsequently requested SASL mechanism.

6.4.3.  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).

   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).

   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 for this authentication
      mechanism.

   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.

6.4.4.  Abort

   The initiating entity aborts the handshake for this authentication
   mechanism 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
   a <failure/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace and containing an
   <aborted/> child element.



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

6.4.5.  SASL Failure

   The receiving entity reports failure of the handshake for this
   authentication mechanism by sending a <failure/> element qualified by
   the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace (the particular
   cause of failure MUST be communicated in an appropriate child element
   of the <failure/> element as defined under Section 6.5).

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

   Where appropriate for the chosen SASL mechanism, 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 (which it would if the receiving entity immediately
   returned a SASL failure and closed the stream).

   If the initiating entity attempts a reasonable number of retries with
   the same SASL mechanism and all attempts fail, it MAY fall back to
   the next mechanism in its ordered list by sending a new <auth/>
   request to the receiving entity, thus starting a new handshake for
   that authentication mechanism.  If all handshakes fail and there are
   no remaining mechanisms in the initiating entity's list of supported
   and acceptable mechanisms, the initiating entity SHOULD simply close
   the stream as described under Section 4.4 (instead of waiting for the
   stream to time out).

   If the initiating entity exceeds the number of retries, the receiving
   entity MUST close the stream with a stream error, which SHOULD be
   <policy-violation/> (Section 4.9.3.14), although some existing
   implementations send <not-authorized/> (Section 4.9.3.12) instead.

      Implementation Note: For server-to-server streams, if the
      receiving entity cannot offer the SASL EXTERNAL mechanism or any
      other SASL mechanism based on the security context established
      during TLS negotiation, the receiving entity MAY attempt to
      complete weak identity verification using the Server Dialback
      protocol [XEP-0220]; however, if according to local service
      policies weak identity verification is insufficient then the





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      receiving entity SHOULD instead close the stream with a <policy-
      violation/> stream error (Section 4.9.3.14) instead of waiting for
      the stream to time out.

6.4.6.  SASL Success

   Before considering the SASL handshake to be a success, if the
   initiating entity provided a 'from' attribute on an initial stream
   header whose confidentiality and integrity were protected via TLS or
   an equivalent security layer (such as the SASL GSSAPI mechanism) then
   the receiving entity SHOULD correlate the authentication identity
   resulting from the SASL negotiation with that 'from' address; if the
   two identities do not match then the receiving entity SHOULD
   terminate the connection attempt (however, the receiving entity might
   have legitimate reasons not to terminate the connection attempt, for
   example, because it has overridden a connecting client's address to
   correct the JID format or assign a JID based on information presented
   in an end-user certificate).

   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 the chosen SASL mechanism supports or requires it.
   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'/>

      Informational Note: For client-to-server streams, the
      authorization identity communicated during SASL negotiation is
      used to determine the canonical address for the initiating client
      according to the receiving server, as described under
      Section 4.3.6.

   Upon receiving the <success/> element, the initiating entity MUST
   initiate a new stream over the existing TCP connection by sending a
   new initial stream header to the receiving entity (as specified under
   Section 4.3.3, the initiating entity MUST NOT send a closing
   </stream> tag before sending the new initial stream header, since the
   receiving entity and initiating entity MUST consider the original
   stream to be replaced upon success of the SASL negotiation).









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

   Upon receiving the new initial stream header from the initiating
   entity, the receiving entity MUST respond by sending a new response
   stream header to the initiating entity (for which it MUST generate a
   new stream ID instead of reusing the old stream ID).

   R: <stream:stream
        from='im.example.com'
        id='gPybzaOzBmaADgxKXu9UClbprp0='
        to='juliet@im.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).

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

6.5.  SASL Errors

   The syntax of SASL errors is as follows, where the XML data shown
   within the square brackets '[' and ']' is OPTIONAL.

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <defined-condition/>
     [<text xml:lang='langcode'>
         OPTIONAL descriptive text
     </text>]
   </failure>

   The "defined-condition" MUST be one of the SASL-related error
   conditions defined in the following sections.  However, because
   additional error conditions might be defined in the future, if an
   entity receives a SASL error condition that it does not understand
   then it MUST treat the unknown condition as a generic authentication
   failure, i.e., as equivalent to <not-authorized/> (Section 6.5.10).



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   Inclusion of the <text/> element is OPTIONAL, and can be used to
   provide application-specific information about the error condition,
   which information MAY be displayed to a human but only as a
   supplement to the defined condition.

   Because XMPP itself defines an application profile of SASL and there
   is no expectation that more specialized XMPP applications will be
   built on top of SASL, the SASL error format does not provide
   extensibility for application-specific error conditions as is done
   for XML streams (Section 4.9.4) and XML stanzas (Section 8.3.4).

6.5.1.  aborted

   The receiving entity acknowledges that the authentication handshake
   has been aborted 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>

6.5.2.  account-disabled

   The account of the initiating entity has been temporarily disabled;
   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'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <account-disabled/>
        <text xml:lang='en'>Call 212-555-1212 for assistance.</text>
      </failure>

6.5.3.  credentials-expired

   The authentication failed because the initiating entity provided
   credentials that have expired; 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>





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

6.5.4.  encryption-required

   The mechanism requested by the initiating entity cannot be used
   unless the confidentiality and integrity of the underlying stream are
   protected (typically via TLS); sent in reply to an <auth/> element
   (with or without initial response data).

   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

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

6.5.5.  incorrect-encoding

   The data provided by the initiating entity could not be processed
   because the base 64 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.

   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>

6.5.6.  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>




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6.5.7.  invalid-mechanism

   The initiating entity did not specify 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>

6.5.8.  malformed-request

   The request is malformed (e.g., the <auth/> element includes initial
   response data but the mechanism does not allow that, or the data sent
   violates the syntax for the specified SASL mechanism); sent in reply
   to an <abort/>, <auth/>, <challenge/>, or <response/> element.

   (In the following example, the XML character data of the <auth/>
   element contains more than 255 UTF-8-encoded Unicode characters and
   therefore violates the "token" production for the SASL ANONYMOUS
   mechanism as specified in [ANONYMOUS].)

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

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

6.5.9.  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).

   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

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







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6.5.10.  not-authorized

   The authentication failed because the initiating entity did not
   provide proper credentials, or because some generic authentication
   failure has occurred but the receiving entity does not wish to
   disclose specific information about the cause of the failure; 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>

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

6.5.11.  temporary-auth-failure

   The authentication failed because of a temporary error condition
   within the receiving entity, and it is advisable for the initiating
   entity to 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>

6.6.  SASL Definition

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

   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



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      methods.  The initiating entity chooses one method from the list
      and sends it to the receiving entity as the value of the
      'mechanism' attribute possessed by an <auth/> element, optionally
      including an initial response to avoid a round trip.

   exchange sequence:  Challenges and responses are carried through the
      exchange of <challenge/> elements from receiving entity to
      initiating entity and <response/> elements from initiating entity
      to receiving entity.  The receiving entity reports failure by
      sending a <failure/> element and success by sending a <success/>
      element; the initiating entity aborts the exchange by sending an
      <abort/> element.  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 can be
      used in XMPP to denote the non-default <localpart@domainpart> of a
      client; an empty string is equivalent to an absent authorization
      identity.

7.  Resource Binding

7.1.  Fundamentals

   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.  That is, there MUST be an XMPP resource associated with the
   bare JID (<localpart@domainpart>) of the client, so that the address
   for use over that stream is a full JID of the form
   <localpart@domainpart/resource> (including the resourcepart).  This
   ensures that the server can deliver XML stanzas to and receive XML
   stanzas from the client in relation to entities other than the server
   itself or the client's account, as explained under Section 10.

      Informational Note: The client could exchange stanzas with the
      server itself or the client's account before binding a resource
      since the full JID is needed only for addressing outside the
      context of the stream negotiated between the client and the
      server, but this is not commonly done.





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   After a client has bound a resource to the stream, it is referred to
   as a "connected resource".  A server SHOULD allow an entity to
   maintain multiple connected resources simultaneously, where each
   connected resource is associated with a distinct XML stream and is
   differentiated from the other connected resources by a distinct
   resourcepart.

      Security Warning: A server SHOULD enable the administrator of an
      XMPP service to limit the number of connected resources in order
      to prevent certain denial-of-service attacks as described under
      Section 13.12.

   If, before completing the resource binding step, the client attempts
   to send an XML stanza to an entity other than the server itself or
   the client's account, the server MUST NOT process the stanza and MUST
   close the stream with a <not-authorized/> stream error
   (Section 4.9.3.12).

   The XML namespace name for the resource binding extension is
   'urn:ietf:params:xml:ns:xmpp-bind'.

7.2.  Support

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

7.3.  Stream Negotiation Rules

7.3.1.  Mandatory-to-Negotiate

   The parties to a stream MUST consider resource binding as mandatory-
   to-negotiate.

7.3.2.  Restart

   After resource binding, the parties MUST NOT restart the stream.

7.4.  Advertising Support

   Upon sending a new 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.

   The server MUST NOT include the resource binding stream feature until
   after the client has authenticated, typically by means of successful
   SASL negotiation.




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   S: <stream:stream
          from='im.example.com'
          id='gPybzaOzBmaADgxKXu9UClbprp0='
          to='juliet@im.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'/>
      </stream:features>

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

7.5.  Generation of Resource Identifiers

   A resourcepart MUST at a minimum be unique among the connected
   resources for that <localpart@domainpart>.  Enforcement of this
   policy is the responsibility of the server.

      Security Warning: A resourcepart can be security-critical.  For
      example, if a malicious entity can guess a client's resourcepart
      then it might be able to determine if the client (and therefore
      the controlling principal) is online or offline, thus resulting in
      a presence leak as described under Section 13.10.2.  To prevent
      that possibility, a client can either (1) generate a random
      resourcepart on its own or (2) ask the server to generate a
      resourcepart on its behalf.  One method for ensuring that the
      resourcepart is random is to generate a Universally Unique
      Identifier (UUID) as specified in [UUID].

7.6.  Server-Generated Resource Identifier

   A server MUST be able to generate an XMPP resourcepart on behalf of a
   client.  The resourcepart generated by the server MUST be random (see
   [RANDOM]).

7.6.1.  Success Case

   A client requests a server-generated resourcepart by sending an IQ
   stanza of type "set" (see Section 8.2.3) containing an empty <bind/>
   element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind'
   namespace.





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   C: <iq id='tn281v37' type='set'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
      </iq>

   Once the server has generated an XMPP resourcepart 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='tn281v37' type='result'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
         <jid>
           juliet@im.example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb
         </jid>
       </bind>
      </iq>

7.6.2.  Error Cases

   When a client asks the server to generate a resourcepart during
   resource binding, the following stanza error conditions are defined:

   o  The account has reached a limit on the number of simultaneous
      connected resources allowed.

   o  The client is otherwise not allowed to bind a resource to the
      stream.

   Naturally, it is possible that error conditions not specified here
   might occur, as described under Section 8.3.

7.6.2.1.  Resource Constraint

   If the account has reached a limit on the number of simultaneous
   connected resources allowed, the server MUST return a <resource-
   constraint/> stanza error (Section 8.3.3.18).

   S: <iq id='tn281v37' type='error'>
        <error type='wait'>
          <resource-constraint
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>








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7.6.2.2.  Not Allowed

   If the client is otherwise not allowed to bind a resource to the
   stream, the server MUST return a <not-allowed/> stanza error
   (Section 8.3.3.10).

   S: <iq id='tn281v37' type='error'>
        <error type='cancel'>
          <not-allowed
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

7.7.  Client-Submitted Resource Identifier

   Instead of asking the server to generate a resourcepart on its
   behalf, a client MAY attempt to submit a resourcepart that it has
   generated or that the controlling user has provided.

7.7.1.  Success Case

   A client asks its server to accept a client-submitted resourcepart 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='wy2xa82b4' type='set'>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <resource>balcony</resource>
        </bind>
      </iq>

   The server SHOULD accept the client-submitted resourcepart.  It does
   so by returning an IQ stanza of type "result" to the client,
   including a <jid/> child element that specifies the full JID for the
   connected resource and contains without modification the client-
   submitted text.

   S: <iq id='wy2xa82b4' type='result'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
         <jid>juliet@im.example.com/balcony</jid>
       </bind>
      </iq>

   Alternatively, in accordance with local service policies the server
   MAY refuse the client-submitted resourcepart and override it with a
   resourcepart that the server generates.




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   S: <iq id='wy2xa82b4' type='result'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
         <jid>
      juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
         </jid>
       </bind>
      </iq>

7.7.2.  Error Cases

   When a client attempts to submit its own XMPP resourcepart during
   resource binding, the following stanza error conditions are defined
   in addition to those described under Section 7.6.2:

   o  The provided resourcepart cannot be processed by the server.

   o  The provided resourcepart is already in use.

   Naturally, it is possible that error conditions not specified here
   might occur, as described under Section 8.3.

7.7.2.1.  Bad Request

   If the provided resourcepart cannot be processed by the server (e.g.,
   because it is of zero length or because it otherwise violates the
   rules for resourceparts specified in [XMPP-ADDR]), the server can
   return a <bad-request/> stanza error (Section 8.3.3.1) but SHOULD
   instead process the resourcepart so that it is in conformance.

   S: <iq id='wy2xa82b4' type='error'>
        <error type='modify'>
          <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

7.7.2.2.  Conflict

   If there is a currently connected client whose session has the
   resourcepart being requested by the newly connecting client, the
   server MUST do one of the following (which of these the server does
   is a matter for implementation or local service policy, although
   suggestions are provided below).

   1.  Override the resourcepart provided by the newly connecting client
       with a server-generated resourcepart.  This behavior is
       encouraged, because it simplifies the resource binding process
       for client implementations.




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   2.  Disallow the resource binding attempt of the newly connecting
       client and maintain the session of the currently connected
       client.  This behavior is neither encouraged nor discouraged,
       despite the fact that it was implicitly encouraged in RFC 3920;
       however, note that handling of the <conflict/> error is unevenly
       supported among existing client implementations, which often
       treat it as an authentication error and have been observed to
       discard cached credentials when receiving it.

   3.  Terminate the session of the currently connected client and allow
       the resource binding attempt of the newly connecting client.
       Although this was the traditional behavior of early XMPP server
       implementations, it is now discouraged because it can lead to a
       never-ending cycle of two clients effectively disconnecting each
       other; however, note that this behavior can be appropriate in
       some deployment scenarios or if the server knows that the
       currently connected client has a dead connection or broken stream
       as described under Section 4.6.

   If the server follows behavior #1, it returns an <iq/> stanza of type
   "result" to the newly connecting client, where the <jid/> child of
   the <bind/> element contains XML character data that indicates the
   full JID of the client, including the resourcepart that was generated
   by the server.

   S: <iq id='wy2xa82b4' type='result'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
         <jid>
      juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
         </jid>
       </bind>
      </iq>

   If the server follows behavior #2, it sends a <conflict/> stanza
   error (Section 8.3.3.2) in response to the resource binding attempt
   of the newly connecting client but maintains the XML stream so that
   the newly connecting client has an opportunity to negotiate a non-
   conflicting resourcepart (i.e., the newly connecting client needs to
   choose a different resourcepart before making another attempt to bind
   a resource).

   S: <iq id='wy2xa82b4' type='error'>
        <error type='modify'>
          <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>





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   If the server follows behavior #3, it returns a <conflict/> stream
   error (Section 4.9.3.3) to the currently connected client (as
   described under Section 4.9.3.3) and returns an IQ stanza of type
   "result" (indicating success) in response to the resource binding
   attempt of the newly connecting client.

   S: <iq id='wy2xa82b4' type='result'>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <jid>
            juliet@im.example.com/balcony
          </jid>
        </bind>
      </iq>

7.7.3.  Retries

   If an error occurs when a client submits a resourcepart, the server
   SHOULD allow a configurable but reasonable number of retries (at
   least 5 and no more than 10); this enables the client to tolerate
   incorrectly provided resourceparts (e.g., bad data formats or
   duplicate text strings) without being forced to reconnect.

   After the client has reached the retry limit, the server MUST close
   the stream with a <policy-violation/> stream error
   (Section 4.9.3.14).

8.  XML Stanzas

   After a client and a server (or two servers) have completed stream
   negotiation, either party can send XML stanzas.  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 in this specification; 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.

   Support for the XML stanza syntax and semantics defined in this
   specification is REQUIRED in XMPP client and server implementations.

      Security Warning: 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 closing tag).






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8.1.  Common Attributes

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

8.1.1.  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 'to' address, refer to Section 10.

8.1.1.1.  Client-to-Server Streams

   The following rules apply to inclusion of the 'to' attribute in
   stanzas sent from a connected client to its server over an XML stream
   qualified by the 'jabber:client' namespace.

   1.  A stanza with a specific intended recipient (e.g., a conversation
       partner, a remote service, the server itself, even another
       resource associated with the user's bare JID) MUST possess a 'to'
       attribute whose value is an XMPP address.

   2.  A stanza sent from a client to a server for direct processing by
       the server (e.g., roster processing as described in [XMPP-IM] or
       presence sent to the server for broadcasting to other entities)
       MUST NOT possess a 'to' attribute.

   The following rules apply to inclusion of the 'to' attribute in
   stanzas sent from a server to a connected client over an XML stream
   qualified by the 'jabber:client' namespace.

   1.  If the server has received the stanza from another connected
       client or from a peer server, the server MUST NOT modify the 'to'
       address before delivering the stanza to the client.

   2.  If the server has itself generated the stanza (e.g., a response
       to an IQ stanza of type "get" or "set", even if the stanza did
       not include a 'to' address), the stanza MAY include a 'to'
       address, which MUST be the full JID of the client; however, if
       the stanza does not include a 'to' address then the client MUST
       treat it as if the 'to' address were included with a value of the
       client's full JID.



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      Implementation Note: It is the server's responsibility to deliver
      only stanzas that are addressed to the client's full JID or the
      user's bare JID; thus, there is no need for the client to check
      the 'to' address of incoming stanzas.  However, if the client does
      check the 'to' address then it is suggested to check at most the
      bare JID portion (not the full JID), since the 'to' address might
      be the user's bare JID, the client's current full JID, or even a
      full JID with a different resourcepart (e.g., in the case of so-
      called "offline messages" as described in [XEP-0160]).

8.1.1.2.  Server-to-Server Streams

   The following rules apply to inclusion of the 'to' 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 'to' attribute whose value is an XMPP
       address; if a server receives a stanza that does not meet this
       restriction, it MUST close the stream with an <improper-
       addressing/> stream error (Section 4.9.3.7).

   2.  The domainpart of the JID contained in the stanza's 'to'
       attribute MUST match the FQDN of the receiving server (or any
       validated domain thereof) as communicated via SASL negotiation
       (see Section 6), Server Dialback (see [XEP-0220]), or similar
       means; if a server receives a stanza that does not meet this
       restriction, it MUST close the stream with a <host-unknown/>
       stream error (Section 4.9.3.6) or a <host-gone/> stream error
       (Section 4.9.3.5).

8.1.2.  from

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

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

8.1.2.1.  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 a server receives an XML stanza from a connected client, the
       server MUST add a 'from' attribute to the stanza or override the
       'from' attribute specified by the client, where the value of the



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       'from' attribute MUST be the full JID
       (<localpart@domainpart/resource>) determined by the server for
       the connected resource that generated the stanza (see
       Section 4.3.6), or the bare JID (<localpart@domainpart>) in the
       case of subscription-related presence stanzas (see [XMPP-IM]).

   2.  When the server generates a stanza on its own behalf for delivery
       to the client from the server itself, the stanza MUST include a
       'from' attribute whose value is the bare JID (i.e., <domainpart>)
       of the server as agreed upon during stream negotiation (e.g.,
       based on the 'to' attribute of the initial stream header).

   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 (<localpart@domainpart>).

   4.  A server MUST NOT send to the client a stanza without a 'from'
       attribute if the stanza was not generated by the server on its
       own behalf (e.g., if it was generated by another client or a peer
       server and the server is merely delivering it to the client on
       behalf of some other entity); therefore, when a client receives a
       stanza that does not include a 'from' attribute, it MUST assume
       that the stanza is from the user's account on the server.

8.1.2.2.  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 whose value is an XMPP
       address; if a server receives a stanza that does not meet this
       restriction, it MUST close the stream with an <improper-
       addressing/> stream error (Section 4.9.3.7).

   2.  The domainpart of the JID contained in the stanza's 'from'
       attribute MUST match the FQDN of the sending server (or any
       validated domain thereof) as communicated via SASL negotiation
       (see Section 6), Server Dialback (see [XEP-0220]), or similar
       means; if a server receives a stanza that does not meet this
       restriction, it MUST close the stream with an <invalid-from/>
       stream error (Section 4.9.3.9).

   Enforcement of these rules helps to prevent certain denial-of-service
   attacks as described under Section 13.12.



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8.1.3.  id

   The 'id' attribute is used by the originating entity to track any
   response or error stanza that it might receive in relation to the
   generated stanza from another entity (such as an intermediate server
   or the intended recipient).

   It is up to the originating entity whether the value of the 'id'
   attribute is unique only within its current stream or unique
   globally.

   For <message/> and <presence/> stanzas, it is RECOMMENDED for the
   originating entity to include an 'id' attribute; for <iq/> stanzas,
   it is REQUIRED.

   If the generated stanza includes an 'id' attribute then it is
   REQUIRED for the response or error stanza to also include an 'id'
   attribute, where the value of the 'id' attribute MUST match that of
   the generated stanza.

   The semantics of IQ stanzas impose additional restrictions as
   described under Section 8.2.3.

8.1.4.  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 defined in [XMPP-IM], whereas the values for IQ
   stanzas specify the part of the semantics for all structured request-
   response exchanges (no matter what the payload) and therefore are
   specified under Section 8.2.3.  The only 'type' value common to all
   three kinds of stanzas is "error" as described under Section 8.3.

8.1.5.  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
   [CHARSETS], "internationalization is for humans").  The value of the
   'xml:lang' attribute specifies the default language of any such
   human-readable XML character data.







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   <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 SHOULD add an 'xml:lang'
   attribute whose value is that specified for the client's output
   stream as defined under Section 4.7.4.

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

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

   If an inbound stanza 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 entity's input stream as
   defined under Section 4.7.4.

   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].

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









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8.2.  Basic Semantics

8.2.1.  Message Semantics

   The <message/> stanza is 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 will possess a
   'to' attribute that specifies the intended recipient of the message
   (see Section 8.1.1 and Section 10.3), unless the message is being
   sent to the bare JID of a connected client's account.  Upon receiving
   a message stanza with a 'to' address, a server SHOULD attempt to
   route or deliver it to the intended recipient (see Section 10 for
   general routing and delivery rules related to XML stanzas).

8.2.2.  Presence Semantics

   The <presence/> stanza is a specialized "broadcast" or "publish-
   subscribe" mechanism, whereby multiple entities receive information
   (in this case, network availability information) about an entity to
   which they have subscribed.  In general, a publishing client SHOULD
   send a presence stanza with no 'to' attribute, in which case the
   server to which the client is connected will broadcast that stanza to
   all subscribed entities.  However, a publishing client MAY also send
   a presence stanza with a 'to' attribute, in which case the server
   will route or deliver that stanza to the intended recipient.
   Although the <presence/> stanza is most often used by XMPP clients,
   it can also be used by servers, add-on services, and any other kind
   of XMPP entity.  See Section 10 for general routing and delivery
   rules related to XML stanzas, and [XMPP-IM] for rules specific to
   presence applications.

8.2.3.  IQ Semantics

   Info/Query, or IQ, is a "request-response" mechanism, similar in some
   ways to the Hypertext Transfer Protocol [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 8.4), 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 can 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>                      |
       | <------------------------- |
       |                            |

                     Figure 5: Semantics of IQ Stanzas

   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; if not, the recipient or an intermediate
       router MUST return a <bad-request/> stanza error
       (Section 8.3.3.1).

       *  get -- The stanza requests information, inquires about what
          data is needed in order to complete further operations, etc.

       *  set -- The stanza provides data that is needed for an
          operation to be completed, sets new values, replaces existing
          values, etc.

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






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       *  error -- The stanza reports an error that has occurred
          regarding processing or delivery of a get or set request (see
          Section 8.3).

   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 (or be
       empty if the generated stanza did not include an 'id' attribute).

   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" to provide obligatory
       information discovered through a get/result pair).

   5.  An IQ stanza of type "get" or "set" MUST contain exactly 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 8.3.

8.3.  Stanza Errors

   Stanza-related errors are handled in a manner similar to stream
   errors (Section 4.9).  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, which is a
   stanza that:

   o  is of the same kind (message, presence, or IQ) as the generated
      stanza that triggered the error

   o  has a 'type' attribute set to a value of "error"

   o  typically swaps the 'from' and 'to' addresses of the generated
      stanza

   o  mirrors the 'id' attribute (if any) of the generated stanza that
      triggered the error






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   o  contains an <error/> child element that specifies the error
      condition and therefore provides a hint regarding actions that the
      sender might be able to take in an effort to remedy the error
      (however, it is not always possible to remedy the error)

8.3.1.  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 error stanza SHOULD simply swap the 'from' and 'to' addresses
       from the generated stanza, unless doing so would (1) result in an
       information leak (see under Section 13.10) or other breach of
       security, or (2) force the sender of the error stanza to include
       a malformed JID in the 'from' or 'to' address of the error
       stanza.

   3.  If the generated stanza was <message/> or <presence/> and
       included an 'id' attribute then it is REQUIRED for the error
       stanza to also include an 'id' attribute.  If the generated
       stanza was <iq/> then the error stanza MUST include an 'id'
       attribute.  In all cases, the value of the 'id' attribute MUST
       match that of the generated stanza (or be empty if the generated
       stanza did not include an 'id' attribute).

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

   5.  The entity that returns an error stanza MAY pass along its JID to
       the sender of the generated stanza (e.g., for diagnostic or
       tracking purposes) through the addition of a 'by' attribute to
       the <error/> child element.

   6.  The entity that returns 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 (however, this is a
       courtesy only and the originating entity MUST NOT depend on
       receiving the original payload).  Naturally, the entity MUST NOT
       include the original data if it not well-formed XML, violates the
       XML restrictions of XMPP (see under Section 11.1), or is
       otherwise harmful (e.g., exceeds a size limit).

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




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

8.3.2.  Syntax

   The syntax for stanza-related errors is as follows, where XML data
   shown within the square brackets '[' and ']' is OPTIONAL, 'intended-
   recipient' is the JID of the entity to which the original stanza was
   addressed, 'sender' is the JID of the originating entity, and 'error-
   generator' is the entity that detects the fact that an error has
   occurred and thus returns an error stanza.

   <stanza-kind from='intended-recipient' to='sender' type='error'>
     [OPTIONAL to include sender XML here]
     <error [by='error-generator']
            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  auth -- retry after providing credentials

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

   The "defined-condition" MUST correspond to one of the stanza error
   conditions defined under Section 8.3.3.  However, because additional
   error conditions might be defined in the future, if an entity
   receives a stanza error condition that it does not understand then it
   MUST treat the unknown condition as equivalent to <undefined-
   condition/> (Section 8.3.3.21).  If the designers of an XMPP protocol
   extension or the developers of an XMPP implementation need to
   communicate a stanza error condition that is not defined in this



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   specification, they can do so by defining an application-specific
   error condition element qualified by an application-specific
   namespace.

   The <error/> element:

   o  MUST contain a defined condition element.

   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.

   The <text/> element is OPTIONAL.  If included, it is to be used only
   to provide descriptive or diagnostic information that supplements the
   meaning of a defined condition or application-specific condition.  It
   MUST 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
   defined condition element (and, optionally, the application-specific
   condition element).

      Interoperability Note: The syntax defined in [RFC3920] included a
      legacy 'code' attribute, whose semantics have been replaced by the
      defined condition elements; information about mapping defined
      condition elements to values of the legacy 'code' attribute can be
      found in [XEP-0086].

8.3.3.  Defined Conditions

   The following conditions are defined for use in stanza errors.

   The error-type value that is RECOMMENDED for each defined condition
   is the usual expected type; however, in some circumstances a
   different type might be more appropriate.

8.3.3.1.  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,




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   or an element that is qualified by a recognized namespace but that
   violates the defined syntax for the element); the associated error
   type SHOULD be "modify".

   C: <iq from='juliet@im.example.com/balcony'
          id='zj3v142b'
          to='im.example.com'
          type='subscribe'>
        <ping xmlns='urn:xmpp:ping'/>
      </iq>

   S: <iq from='im.example.com'
          id='zj3v142b'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='modify'>
          <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

8.3.3.2.  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='wy2xa82b4' type='set'>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <resource>balcony</resource>
        </bind>
      </iq>

   S: <iq id='wy2xa82b4' type='error'>
        <error type='cancel'>
          <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

8.3.3.3.  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 (e.g., the entity understands the namespace but
   does not recognize the element name); the associated error type
   SHOULD be "cancel" or "modify".







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   C: <iq from='juliet@im.example.com/balcony'
          id='9u2bax16'
          to='pubsub.example.com'
          type='get'>
        <pubsub xmlns='http://jabber.org/protocol/pubsub'>
          <subscriptions/>
        </pubsub>
      </iq>

   E: <iq from='pubsub.example.com'
          id='9u2bax16'
          to='juliet@im.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>

8.3.3.4.  forbidden

   The requesting entity does not possess the necessary permissions to
   perform an action that only certain authorized roles or individuals
   are allowed to complete (i.e., it typically relates to authorization
   rather than authentication); the associated error type SHOULD be
   "auth".

   C: <presence
          from='juliet@im.example.com/balcony'
          id='y2bs71v4'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          id='y2bs71v4'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='auth'>
          <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>





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8.3.3.5.  gone

   The recipient or server can no longer be contacted at this address,
   typically on a permanent basis (as opposed to the <redirect/> error
   condition, which is used for temporary addressing failures); the
   associated error type SHOULD be "cancel" and the error stanza SHOULD
   include a new address (if available) as the XML character data of the
   <gone/> element (which MUST be a Uniform Resource Identifier [URI] or
   Internationalized Resource Identifier [IRI] at which the entity can
   be contacted, typically an XMPP IRI as specified in [XMPP-URI]).

   C: <message
          from='juliet@im.example.com/churchyard'
          id='sj2b371v'
          to='romeo@example.net'
          type='chat'>
        <body>Thy lips are warm.</body>
      </message>

   S: <message
          from='romeo@example.net'
          id='sj2b371v'
          to='juliet@im.example.com/churchyard'
          type='error'>
        <error by='example.net'
               type='cancel'>
          <gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
            xmpp:romeo@afterlife.example.net
          </gone>
        </error>
      </message>

8.3.3.6.  internal-server-error

   The server has experienced a misconfiguration or other internal error
   that prevents it from processing the stanza; the associated error
   type SHOULD be "cancel".

   C: <presence
          from='juliet@im.example.com/balcony'
          id='y2bs71v4'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>







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   E: <presence
          from='characters@muc.example.com/JulieC'
          id='y2bs71v4'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='cancel'>
          <internal-server-error
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

8.3.3.7.  item-not-found

   The addressed JID or item requested cannot be found; the associated
   error type SHOULD be "cancel".

   C: <presence from='userfoo@example.com/bar'
                id='pwb2n78i'
                to='nosuchroom@conference.example.org/foo'/>

   S: <presence from='nosuchroom@conference.example.org/foo'
                id='pwb2n78i'
                to='userfoo@example.com/bar'
                type='error'>
        <error type='cancel'>
          <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

      Security Warning: 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 (for a more detailed discussion of presence
      authorization, refer to the discussion of presence subscriptions
      in [XMPP-IM]); instead it MUST return a <service-unavailable/>
      stanza error (Section 8.3.3.19).

8.3.3.8.  jid-malformed

   The sending entity has provided (e.g., during resource binding) or
   communicated (e.g., in the 'to' address of a stanza) an XMPP address
   or aspect thereof that violates the rules defined in [XMPP-ADDR]; the
   associated error type SHOULD be "modify".








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   C: <presence
          from='juliet@im.example.com/balcony'
          id='y2bs71v4'
          to='ch@r@cters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='ch@r@cters@muc.example.com/JulieC'
          id='y2bs71v4'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error by='muc.example.com'
               type='modify'>
          <jid-malformed
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

      Implementation Note: Enforcement of the format for XMPP localparts
      is primarily the responsibility of the service at which the
      associated account or entity is located (e.g., the example.com
      service is responsible for returning <jid-malformed/> errors
      related to all JIDs of the form <localpart@example.com>), whereas
      enforcement of the format for XMPP domainparts is primarily the
      responsibility of the service that seeks to route a stanza to the
      service identified by that domainpart (e.g., the example.org
      service is responsible for returning <jid-malformed/> errors
      related to stanzas that users of that service have to tried send
      to JIDs of the form <localpart@example.com>).  However, any entity
      that detects a malformed JID MAY return this error.

8.3.3.9.  not-acceptable

   The recipient or server understands the request but cannot process it
   because the request does not meet criteria defined by the recipient
   or server (e.g., a request to subscribe to information that does not
   simultaneously include configuration parameters needed by the
   recipient); the associated error type SHOULD be "modify".

   C: <message to='juliet@im.example.com' id='yt2vs71m'>
        <body>[ ... the-emacs-manual ... ]</body>
      </message>








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   S: <message from='juliet@im.example.com' id='yt2vs71m'>
        <error type='modify'>
          <not-acceptable
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

8.3.3.10.  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@im.example.com/balcony'
          id='y2bs71v4'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          id='y2bs71v4'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='cancel'>
          <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

8.3.3.11.  not-authorized

   The sender needs to provide credentials before being allowed to
   perform the action, or has provided improper credentials (the name
   "not-authorized", which was borrowed from the "401 Unauthorized"
   error of [HTTP], might lead the reader to think that this condition
   relates to authorization, but instead it is typically used in
   relation to authentication); the associated error type SHOULD be
   "auth".

   C: <presence
          from='juliet@im.example.com/balcony'
          id='y2bs71v4'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>





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   E: <presence
          from='characters@muc.example.com/JulieC'
          id='y2bs71v4'
          to='juliet@im.example.com/balcony'>
        <error type='auth'>
          <not-authorized xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

8.3.3.12.  policy-violation

   The entity has violated some local service policy (e.g., a message
   contains words that are prohibited by the service) and the server MAY
   choose to specify the policy in the <text/> element or in an
   application-specific condition element; the associated error type
   SHOULD be "modify" or "wait" depending on the policy being violated.

   (In the following example, the client sends an XMPP message
   containing words that are forbidden according to the server's local
   service policy.)

   C: <message from='romeo@example.net/foo'
               to='bill@im.example.com'
               id='vq71f4nb'>
        <body>%#&@^!!!</body>
      </message>

   S: <message from='bill@im.example.com'
               id='vq71f4nb'
               to='romeo@example.net/foo'>
        <error by='example.net' type='modify'>
          <policy-violation
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

8.3.3.13.  recipient-unavailable

   The intended recipient is temporarily unavailable, undergoing
   maintenance, etc.; the associated error type SHOULD be "wait".

   C: <presence
          from='juliet@im.example.com/balcony'
          id='y2bs71v4'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>




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   E: <presence
          from='characters@muc.example.com/JulieC'
          id='y2bs71v4'
          to='juliet@im.example.com/balcony'>
        <error type='wait'>
          <recipient-unavailable
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

      Security Warning: 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 (for a more detailed discussion of presence
      authorization, refer to the discussion of presence subscriptions
      in [XMPP-IM]); instead it MUST return a <service-unavailable/>
      stanza error (Section 8.3.3.19).

8.3.3.14.  redirect

   The recipient or server is redirecting requests for this information
   to another entity, typically in a temporary fashion (as opposed to
   the <gone/> error condition, which is used for permanent addressing
   failures); the associated error type SHOULD be "modify" and the error
   stanza SHOULD contain the alternate address in the XML character data
   of the <redirect/> element (which MUST be a URI or IRI with which the
   sender can communicate, typically an XMPP IRI as specified in
   [XMPP-URI]).

   C: <presence
          from='juliet@im.example.com/balcony'
          id='y2bs71v4'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          id='y2bs71v4'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='modify'>
          <redirect xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
            xmpp:characters@conference.example.org
          </redirect>
        </error>
      </presence>




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      Security Warning: An application receiving a stanza-level redirect
      SHOULD warn a human user of the redirection attempt and request
      approval before proceeding to communicate with the entity whose
      address is contained in the XML character data of the <redirect/>
      element, because that entity might have a different identity or
      might enforce different security policies.  The end-to-end
      authentication or signing of XMPP stanzas could help to mitigate
      this risk, since it would enable the sender to determine if the
      entity to which it has been redirected has the same identity as
      the entity it originally attempted to contact.  An application MAY
      have a policy of following redirects only if it has authenticated
      the receiving entity.  In addition, an application SHOULD abort
      the communication attempt after a certain number of successive
      redirects (e.g., at least 2 but no more than 5).

8.3.3.15.  registration-required

   The requesting entity is not authorized to access the requested
   service because prior registration is necessary (examples of prior
   registration include members-only rooms in XMPP multi-user chat
   [XEP-0045] and gateways to non-XMPP instant messaging services, which
   traditionally required registration in order to use the gateway
   [XEP-0100]); the associated error type SHOULD be "auth".

   C: <presence
          from='juliet@im.example.com/balcony'
          id='y2bs71v4'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          id='y2bs71v4'
          to='juliet@im.example.com/balcony'>
        <error type='auth'>
          <registration-required
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

8.3.3.16.  remote-server-not-found

   A remote server or service specified as part or all of the JID of the
   intended recipient does not exist or cannot be resolved (e.g., there
   is no _xmpp-server._tcp DNS SRV record, the A or AAAA fallback





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   resolution fails, or A/AAAA lookups succeed but there is no response
   on the IANA-registered port 5269); the associated error type SHOULD
   be "cancel".

   C: <message
          from='romeo@example.net/home'
          id='ud7n1f4h'
          to='bar@example.org'
          type='chat'>
       <body>yt?</body>
      </message>

   E: <message
          from='bar@example.org'
          id='ud7n1f4h'
          to='romeo@example.net/home'
          type='error'>
        <error type='cancel'>
          <remote-server-not-found
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

8.3.3.17.  remote-server-timeout

   A remote server or service specified as part or all of the JID of the
   intended recipient (or needed to fulfill a request) was resolved but
   communications could not be established within a reasonable amount of
   time (e.g., an XML stream cannot be established at the resolved IP
   address and port, or an XML stream can be established but stream
   negotiation fails because of problems with TLS, SASL, Server
   Dialback, etc.); the associated error type SHOULD be "wait" (unless
   the error is of a more permanent nature, e.g., the remote server is
   found but it cannot be authenticated or it violates security
   policies).

   C: <message
          from='romeo@example.net/home'
          id='ud7n1f4h'
          to='bar@example.org'
          type='chat'>
       <body>yt?</body>
      </message>








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   E: <message
          from='bar@example.org'
          id='ud7n1f4h'
          to='romeo@example.net/home'
          type='error'>
        <error type='wait'>
          <remote-server-timeout
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

8.3.3.18.  resource-constraint

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

   C: <iq from='romeo@example.net/foo'
          id='kj4vz31m'
          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='kj4vz31m'
          to='romeo@example.net/foo'
          type='error'>
        <error type='wait'>
          <resource-constraint
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

8.3.3.19.  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@im.example.com'>
        <body>Hello?</body>
      </message>






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

      Security Warning: An application MUST return a <service-
      unavailable/> stanza error (Section 8.3.3.19) instead of <item-
      not-found/> (Section 8.3.3.7) or <recipient-unavailable/>
      (Section 8.3.3.13) 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 (for a more detailed discussion of presence
      authorization, refer to [XMPP-IM]).

8.3.3.20.  subscription-required

   The requesting entity is not authorized to access the requested
   service because a prior subscription is necessary (examples of prior
   subscription include authorization to receive presence information as
   defined in [XMPP-IM] and opt-in data feeds for XMPP publish-subscribe
   as defined in [XEP-0060]); the associated error type SHOULD be
   "auth".

   C: <message
          from='romeo@example.net/orchard'
          id='pa73b4n7'
          to='playwright@shakespeare.example.com'
          type='chat'>
        <subject>ACT II, SCENE II</subject>
        <body>help, I forgot my lines!</body>
      </message>

   E: <message
          from='playwright@shakespeare.example.com'
          id='pa73b4n7'
          to='romeo@example.net/orchard'
          type='error'>
        <error type='auth'>
          <subscription-required
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>






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8.3.3.21.  undefined-condition

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

   C: <message
          from='northumberland@shakespeare.example'
          id='richard2-4.1.247'
          to='kingrichard@royalty.england.example'>
        <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>
      </message>

   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>

8.3.3.22.  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='o6hsv25z'
          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='o6hsv25z'
          to='romeo@example.net/foo'
          type='error'>
        <error type='modify'>
          <unexpected-request
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
          <not-subscribed
              xmlns='http://jabber.org/protocol/pubsub#errors'/>
        </error>
      </iq>

8.3.4.  Application-Specific Conditions

   As noted, an application MAY provide application-specific stanza
   error information by including a properly namespaced child within the
   error element.  Typically, the application-specific element
   supplements or further qualifies a defined element.  Thus, the
   <error/> element will contain two or three child elements.

   <iq id='ixc3v1b9' type='error'>
     <error type='modify'>
       <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       <too-many-parameters xmlns='http://example.org/ns'/>
     </error>
   </iq>














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   <message type='error' id='7h3baci9'>
     <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>
       <too-many-parameters xmlns='http://example.org/ns'/>
     </error>
   </message>

   An entity that receives an application-specific error condition it
   does not understand MUST ignore that condition but appropriately
   process the rest of the error stanza.

8.4.  Extended Content

   Although 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.

   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.  Such a 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 the data
   contained within the child element.  Such a child element is called
   an "extension element".  An extension element can be included either
   at the direct child level of the stanza or in any mix of levels.

   Similarly, "extension attributes" are allowed.  That is: a stanza
   itself (i.e., an <iq/>, <message/>, or <presence/> element qualified
   by the "jabber:client" or "jabber:server" content namespace) or any
   child element of such a stanza (whether an extension element or a
   child element qualified by the content namespace) MAY also include
   one or more attributes qualified by XML namespaces other than the
   content namespace or the reserved
   "http://www.w3.org/XML/1998/namespace" namespace (including the so-
   called "empty namespace" if the attribute is not prefixed as
   described under [XML-NAMES]).






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      Interoperability Note: For the sake of backward compatibility and
      maximum interoperability, an entity that generates a stanza SHOULD
      NOT include such attributes in the stanza itself or in child
      elements of the stanza that are qualified by the content
      namespaces "jabber:client" or "jabber:server" (e.g., the <body/>
      child of the <message/> stanza).

   An extension element or extension attribute is said to be "extended
   content" and the qualifying namespace for such an element or
   attribute is said to be an "extended namespace".

      Informational Note: Although extended namespaces for XMPP are
      commonly defined by the XMPP Standards Foundation (XSF) and by the
      IETF, no specification or IETF standards action is necessary to
      define extended namespaces, and any individual or organization is
      free to define XMPP extensions.

   To illustrate these concepts, several examples follow.

   The following stanza contains one direct child element whose extended
   namespace is 'jabber:iq:roster':

   <iq from='juliet@capulet.com/balcony'
       id='h83vxa4c'
       type='get'>
    <query xmlns='jabber:iq:roster'/>
   </iq>

   The following stanza contains two direct child elements with two
   different extended namespaces.

   <presence from='juliet@capulet.com/balcony'>
     <c xmlns='http://jabber.org/protocol/caps'
        hash='sha-1'
        node='http://code.google.com/p/exodus'
        ver='QgayPKawpkPSDYmwT/WM94uAlu0='/>
     <x xmlns='vcard-temp:x:update'>
       <photo>sha1-hash-of-image</photo>
     </x>
   </presence>

   The following stanza contains two child elements, one of which is
   qualified by the "jabber:client" or "jabber:server" content namespace
   and one of which is qualified by an extended namespace; the extension
   element in turn contains a child element that is qualified by a
   different extended namespace.





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   <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?</p>
       </body>
     </html>
   </message>

   It is conventional in the XMPP community for implementations to not
   generate namespace prefixes for elements that are qualified by
   extended namespaces (in the XML community, this convention is
   sometimes called "prefix-free canonicalization").  However, if an
   implementation generates such namespace prefixes then it MUST include
   the namespace declaration in the stanza itself or a child element of
   the stanza, not in the stream header (see Section 4.8.4).

   Routing entities (typically servers) SHOULD try to maintain prefixes
   when serializing XML stanzas for processing, but receiving entities
   MUST NOT depend on the prefix strings to have any particular value
   (the allowance for the 'stream' prefix, described under
   Section 4.8.5, is an exception to this rule, albeit for streams
   rather than stanzas).

   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) a server that is routing or
   delivering the stanza to the recipient.

   If a recipient receives a stanza that contains an element or
   attribute it does not understand, it MUST NOT attempt to process that
   XML data and instead MUST proceed as follows.

   o  If an intended recipient receives a message stanza whose only
      child element is qualified by a namespace it does not understand,
      then depending on the XMPP application it MUST either ignore the
      entire stanza or return a stanza error, which SHOULD be <service-
      unavailable/> (Section 8.3.3.19).

   o  If an intended recipient receives a presence stanza whose only
      child element is qualified by a namespace it does not understand,
      then it MUST ignore the child element by treating the presence
      stanza as if it contained no child element.







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   o  If an intended recipient receives a message or presence stanza
      that contains XML data qualified by a namespace it does not
      understand, then it MUST ignore the portion of the stanza
      qualified by the unknown namespace.

   o  If an intended recipient receives an IQ stanza of type "get" or
      "set" containing a child element qualified by a namespace it does
      not understand, then the entity MUST return an IQ stanza of type
      "error" with an error condition of <service-unavailable/>.

   If a server handles a stanza that is intended for delivery to another
   entity and that contains a child element it does not understand, it
   MUST route the stanza unmodified to a remote server or deliver the
   stanza unmodified to a connected client associated with a local
   account.

9.  Detailed Examples

   The detailed examples in this section further illustrate the
   protocols defined in this specification.

9.1.  Client-to-Server Examples

   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 "im.example.com", the
   server requires use of TLS, the client authenticates via the SASL
   SCRAM-SHA-1 mechanism as <juliet@im.example.com> with a password of
   "r0m30myr0m30", and the client binds a client-submitted 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.im.example.com and has opened a TCP connection to
   the advertised port at the resolved IP address.

9.1.1.  TLS

   Step 1: Client initiates stream to server:

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






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

   S: <stream:stream
        from='im.example.com'
        id='t7AMCin9zjMNwQKDnplntZPIDEI='
        to='juliet@im.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 (only the STARTTLS
   extension at this point, which is mandatory-to-negotiate):

   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 STARTTLS negotiation has
   failed, closes the XML stream, and terminates the TCP connection
   (thus, the stream negotiation process ends unsuccessfully and the
   parties do not move on to the next step):

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

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












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   Step 7: If TLS negotiation is successful, client initiates a new
   stream to server over the TLS-protected TCP connection:

   C: <stream:stream
        from='juliet@im.example.com'
        to='im.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 (thus, the stream negotiation process ends unsuccessfully
   and the parties do not move on to the next step):

9.1.2.  SASL

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

   S: <stream:stream
        from='im.example.com'
        id='vgKi/bkYME8OAj4rlXMkpucAqe4='
        to='juliet@im.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>SCRAM-SHA-1-PLUS</mechanism>
          <mechanism>SCRAM-SHA-1</mechanism>
          <mechanism>PLAIN</mechanism>
        </mechanisms>
      </stream:features>

   Step 9: Client selects an authentication mechanism (in this case,
   SCRAM-SHA-1), including initial response data:

   C: <auth xmlns="urn:ietf:params:xml:ns:xmpp-sasl"
            mechanism="SCRAM-SHA-1">
        biwsbj1qdWxpZXQscj1vTXNUQUF3QUFBQU1BQUFBTlAwVEFBQUFBQUJQVTBBQQ==
      </auth>

   The decoded base 64 data is
   "n,,n=juliet,r=oMsTAAwAAAAMAAAANP0TAAAAAABPU0AA".




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   Step 10: Server sends a challenge:

   S: <challenge xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
        cj1vTXNUQUF3QUFBQU1BQUFBTlAwVEFBQUFBQUJQVTBBQWUxMjQ2OTViLTY5Y
        TktNGRlNi05YzMwLWI1MWIzODA4YzU5ZSxzPU5qaGtZVE0wTURndE5HWTBaaT
        AwTmpkbUxUa3hNbVV0TkRsbU5UTm1ORE5rTURNeixpPTQwOTY=
      </challenge>

   The decoded base 64 data is "r=oMsTAAwAAAAMAAAANP0TAAAAAABPU0AAe12469
   5b-69a9-4de6-9c30-
   b51b3808c59e,s=NjhkYTM0MDgtNGY0Zi00NjdmLTkxMmUtNDlmNTNmNDNkMDMz,i=409
   6" (line breaks not included in actual data).

   Step 11: Client sends a response:

   C: <response xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
        Yz1iaXdzLHI9b01zVEFBd0FBQUFNQUFBQU5QMFRBQUFBQUFCUFUwQUFlMTI0N
        jk1Yi02OWE5LTRkZTYtOWMzMC1iNTFiMzgwOGM1OWUscD1VQTU3dE0vU3ZwQV
        RCa0gyRlhzMFdEWHZKWXc9
      </response>

   The decoded base 64 data is "c=biws,r=oMsTAAwAAAAMAAAANP0TAAAAAABPU0
   AAe124695b-69a9-4de6-9c30-b51b3808c59e,p=UA57tM/
   SvpATBkH2FXs0WDXvJYw=" (line breaks not included in actual data).

   Step 12: Server informs client of success, including additional data
   with success:

   S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        dj1wTk5ERlZFUXh1WHhDb1NFaVc4R0VaKzFSU289
      </success>

   The decoded base 64 data is "v=pNNDFVEQxuXxCoSEiW8GEZ+1RSo=".

   Step 12 (alt): Server returns a SASL error to client (thus, the
   stream negotiation process ends unsuccessfully and the parties do not
   move on to the next step):

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









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   Step 13: Client initiates a new stream to server:

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

9.1.3.  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='im.example.com'
        id='gPybzaOzBmaADgxKXu9UClbprp0='
        to='juliet@im.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'/>
      </stream:features>

   Upon being informed that resource binding is mandatory-to-negotiate,
   the client needs to bind a resource to the stream; here we assume
   that the client submits a human-readable text string.

   Step 15: Client binds a resource:

   C: <iq id='yhc13a95' type='set'>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <resource>balcony</resource>
        </bind>
      </iq>












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   Step 16: Server accepts submitted resourcepart and informs client of
   successful resource binding:

   S: <iq id='yhc13a95' type='result'>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <jid>
            juliet@im.example.com/balcony
          </jid>
        </bind>
      </iq>

   Step 16 (alt): Server returns error to client (thus, the stream
   negotiation process ends unsuccessfully and the parties do not move
   on to the next step):

   S: <iq id='yhc13a95' type='error'>
        <error type='cancel'>
          <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

9.1.4.  Stanza Exchange

   Now the client is allowed to send XML stanzas over the negotiated
   stream.

   C: <message from='juliet@im.example.com/balcony'
               id='ju2ba41c'
               to='romeo@example.net'
               type='chat'
               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 9.2).

   The intended recipient replies, and the message is delivered to the
   client.

   E: <message from='romeo@example.net/orchard'
               id='ju2ba41c'
               to='juliet@im.example.com/balcony'
               type='chat'
               xml:lang='en'>
        <body>Neither, fair saint, if either thee dislike.</body>
      </message>




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   The client can subsequently send and receive an unbounded number of
   subsequent XML stanzas over the stream.

9.1.5.  Close

   Desiring to send no further messages, the client closes its stream to
   the server but waits for incoming data from the server.

   C: </stream:stream>

   Consistent with Section 4.4, the server might send additional data to
   the client and then closes its stream to the client.

   S: </stream:stream>

   The client now sends a TLS close_notify alert, receives a responding
   close_notify alert from the server, and then terminates the
   underlying TCP connection.

9.2.  Server-to-Server Examples

   The following examples show the data flow for a server negotiating an
   XML stream with a peer server, exchanging XML stanzas, and closing
   the negotiated stream.  The initiating server ("Server1") is
   im.example.com; the receiving server ("Server2") is example.net and
   it requires use of TLS; im.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 how Server1 declares the content namespace "jabber:
   server" as the default namespace and uses prefixes for stream-related
   elements, whereas Server2 uses prefix-free canonicalization.

9.2.1.  TLS

   Step 1: Server1 initiates stream to Server2:

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







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   Step 2: Server2 responds by sending a response stream header to
   Server1:

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

   Step 3: Server2 sends stream features to Server1 (only the STARTTLS
   extension at this point, which is mandatory-to-negotiate):

   S2: <features xmlns='http://etherx.jabber.org/streams'>
         <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
           <required/>
         </starttls>
       </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 STARTTLS negotiation has
   failed, closes the stream, and terminates the TCP connection (thus,
   the stream negotiation process ends unsuccessfully and the parties do
   not move on to the next step):

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

   Step 6: Server1 and Server2 attempt to complete TLS negotiation via
   TCP (see [TLS] for details).

   Step 7: If TLS negotiation is successful, Server1 initiates a new
   stream to Server2 over the TLS-protected TCP connection:

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




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   Step 7 (alt): If TLS negotiation is unsuccessful, Server2 closes the
   TCP connection (thus, the stream negotiation process ends
   unsuccessfully and the parties do not move on to the next step).

9.2.2.  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
         from='example.net'
         id='RChdjlgj/TIBcbT9Keu31zDihH4='
         to='im.example.com'
         version='1.0'
         xmlns='http://etherx.jabber.org/streams'>

   S2: <features xmlns='http://etherx.jabber.org/streams'>
         <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
           <mechanism>EXTERNAL</mechanism>
         </mechanisms>
       </features>

   Step 9: Server1 selects the EXTERNAL mechanism (including an empty
   response of "="):

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

   Step 10: Server2 returns success:

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

   Step 10 (alt): Server2 informs Server1 of failed authentication
   (thus, the stream negotiation process ends unsuccessfully and the
   parties do not move on to the next step):

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










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   Step 11: Server1 initiates a new stream to Server2:

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

   Step 12: Server2 responds by sending a stream header to Server1 along
   with any additional features (or, in this case, an empty features
   element):

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

   S2: <features xmlns='http://etherx.jabber.org/streams'/>

9.2.3.  Stanza Exchange

   Now Server1 is allowed to send XML stanzas to Server2 over the
   negotiated stream from im.example.com to example.net; here we assume
   that the transferred stanzas are those shown earlier for client-to-
   server communication, albeit over a server-to-server stream qualified
   by the 'jabber:server' namespace.

   Server1 sends XML stanza to Server2:

   S1: <message from='juliet@im.example.com/balcony'
                id='ju2ba41c'
                to='romeo@example.net'
                type='chat'
                xml:lang='en'>
       <body>Art thou not Romeo, and a Montague?</body>
      </message>

9.2.4.  Close

   Desiring to send no further messages, Server1 closes its stream to
   Server2 but waits for incoming data from Server2.  (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.)




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   S1: </stream:stream>

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

   S2: </stream>

   Server1 now sends a TLS close_notify alert, receives a responding
   close_notify alert from Server2, and then terminates the underlying
   TCP connection.

10.  Server Rules for Processing XML Stanzas

   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.  This
   section provides general rules for processing XML stanzas.  However,
   particular XMPP applications MAY specify delivery rules that modify
   or supplement the following rules (e.g., a set of delivery rules for
   instant messaging and presence applications is defined in [XMPP-IM]).

10.1.  In-Order Processing

   An XMPP server MUST ensure in-order processing of the stanzas and
   other XML elements it receives over a given input stream from a
   connected client or remote server.

   In-order processing applies (a) to any XML elements used to negotiate
   and manage XML streams, and (b) to all uses of XML stanzas, including
   but not limited to the following:

   1.  Stanzas sent by a client to its server or to its own bare JID for
       direct processing by the server (e.g., in-order processing of a
       roster get and initial presence as described in [XMPP-IM]).

   2.  Stanzas sent by a connected client and intended for delivery to
       another entity associated with the server (e.g., stanzas
       addressed from <juliet@im.example.com> to
       <nurse@im.example.com>).  The server MUST ensure that it delivers
       stanzas addressed to the intended recipient in the order it
       receives them over the input stream from the sending client,
       treating stanzas addressed to the bare JID and the full JID of
       the intended recipient as equivalent for delivery purposes.






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   3.  Stanzas sent by a connected client and intended for delivery to
       an entity located at a remote domain (e.g., stanzas addressed
       from <juliet@im.example.com> to <romeo@example.net>).  The
       routing server MUST ensure that it routes stanzas addressed to
       the intended recipient in the order it receives them over the
       input stream from the sending client, treating stanzas addressed
       to the bare JID and the full JID of the intended recipient as
       equivalent for routing purposes.  To help ensure in-order
       processing, the routing server MUST route such stanzas over a
       single output stream to the remote domain, rather than sending
       some stanzas over one server-to-server stream and other stanzas
       over another server-to-server stream.

   4.  Stanzas routed from one server to another server for delivery to
       an entity associated with the remote domain (e.g., stanzas
       addressed from <juliet@im.example.com> to <romeo@example.net> and
       routed by <im.example.com> over a server-to-server stream to
       <example.net>).  The delivering server MUST ensure that it
       delivers stanzas to the intended recipient in the order it
       receives them over the input stream from the routing server,
       treating stanzas addressed to the bare JID and the full JID of
       the intended recipient as equivalent for delivery purposes.

   5.  Stanzas sent by one server to another server for direct
       processing by the server that is hosting the remote domain (e.g.,
       stanzas addressed from <im.example.com> to <example.net>).

   If the server's processing of a particular request could have an
   effect on its processing of subsequent data it might receive over
   that input stream (e.g., enforcement of communication policies), it
   MUST suspend processing of subsequent data until it has processed the
   request.

   In-order processing applies only to a single input stream.
   Therefore, a server is not responsible for ensuring the coherence of
   data it receives across multiple input streams associated with the
   same local account (e.g., stanzas received over two different input
   streams from <juliet@im.example.com/balcony> and
   <juliet@im.example.com/chamber>) or the same remote domain (e.g., two
   different input streams negotiated by a remote domain; however, a
   server MAY close the stream with a <conflict/> stream error
   (Section 4.9.3.3) if a remote server attempts to negotiate more than
   one stream, as described under Section 4.9.3.3).








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10.2.  General Considerations

   At high level, there are three primary considerations at play in
   server processing of XML stanzas, which sometimes are at odds but
   need to be managed in a consistent way:

   1.  It is good to deliver a stanza to the intended recipient if
       possible.

   2.  If a stanza cannot be delivered, it is helpful to inform the
       sender.

   3.  It is bad to facilitate directory harvesting attacks
       (Section 13.11) and presence leaks (Section 13.10.2).

   With regard to possible delivery-related attacks, the following
   points need to be kept in mind:

   1.  From the perspective of an attacker, there is little if any
       effective difference between the server's (i) delivering the
       stanza or storing it offline for later delivery (see [XMPP-IM])
       and (ii) silently ignoring it (because an error is not returned
       immediately in any of those cases); therefore, in scenarios where
       a server delivers a stanza or places the stanza into offline
       storage for later delivery, it needs to silently ignore the
       stanza if that account does not exist.

   2.  How a server processes stanzas sent to the bare JID
       <localpart@domainpart> has implications for directory harvesting,
       because an attacker could determine whether an account exists if
       the server responds differently depending on whether there is an
       account for a given bare JID.

   3.  How a server processes stanzas sent to a full JID has
       implications for presence leaks, because an attacker could send
       requests to multiple full JIDs and receive different replies
       depending on whether the user has a device currently online at
       that full JID.  The use of randomized resourceparts (whether
       generated by the client or the server as described under
       Section 7) significantly helps to mitigate this attack, so it is
       of somewhat lesser concern than the directory harvesting attack.

   Naturally, presence is not leaked if the entity to which a user's
   server returns an error already knows the user's presence or is
   authorized to do so (e.g., by means of a presence subscription or
   directed presence), and a server does not enable a directory





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   harvesting attack if it returns an error to an entity that already
   knows if a user exists (e.g., because the entity is in the user's
   contact list); these matters are discussed more fully in [XMPP-IM].

10.3.  No 'to' Address

   If the stanza possesses no 'to' attribute, the server MUST handle it
   directly on behalf of the entity that sent it, where the meaning of
   "handle it directly" depends on whether the stanza is message,
   presence, or IQ.  Because all stanzas received from other servers
   MUST possess a 'to' attribute, this rule applies only to stanzas
   received from a local entity (typically a client) that is connected
   to the server.

10.3.1.  Message

   If the server receives a message stanza with no 'to' attribute, it
   MUST treat the message as if the 'to' address were the bare JID
   <localpart@domainpart> of the sending entity.

10.3.2.  Presence

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

10.3.3.  IQ

   If the server receives an IQ stanza with no 'to' attribute, it MUST
   process the stanza on behalf of the account from which received the
   stanza, as follows:

   1.  If the IQ stanza is of type "get" or "set" and the server
       understands the namespace that qualifies the payload, the server
       MUST handle the stanza on behalf of the sending entity or return
       an appropriate error to the sending entity.  Although the meaning
       of "handle" is determined by the semantics of the qualifying
       namespace, in general the server will respond to the IQ stanza of
       type "get" or "set" by returning an appropriate IQ stanza of type
       "result" or "error", responding as if the server were the bare
       JID of the sending entity.  As an example, if the sending entity
       sends an IQ stanza of type "get" where the payload is qualified
       by the 'jabber:iq:roster' namespace (as described in [XMPP-IM]),
       then the server will return the roster associated with the
       sending entity's bare JID to the particular resource of the
       sending entity that requested the roster.




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   2.  If the IQ stanza is of type "get" or "set" and the server does
       not understand the namespace that qualifies the payload, the
       server MUST return an error to the sending entity, which MUST be
       <service-unavailable/>.

   3.  If the IQ stanza is of type "error" or "result", the server MUST
       handle the error or result in accordance with the payload of the
       associated IQ stanza or type "get" or "set" (if there is no such
       associated stanza, the server MUST ignore the error or result
       stanza).

10.4.  Remote Domain

   If the domainpart of the JID contained in the 'to' attribute does not
   match one of the configured FQDNs of the server, the server SHOULD
   attempt to route the stanza to the remote domain (subject to local
   service provisioning and security policies regarding inter-domain
   communication, since such communication is OPTIONAL for any given
   deployment).  As described in the following sections, there are two
   possible cases.

      Security Warning: These rules apply only client-to-server streams.
      As described under Section 8.1.1.2, a server MUST NOT accept a
      stanza over a server-to-server stream if the domainpart of the JID
      in the 'to' attribute does not match an FQDN serviced by the
      receiving server.

10.4.1.  Existing Stream

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

10.4.2.  No Existing Stream

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

   1.  Resolve the FQDN of the remote domain (as described under
       Section 13.9.2).

   2.  Negotiate a server-to-server stream between the two domains (as
       defined under Section 5 and Section 6).

   3.  Route the stanza to the authoritative server for the remote
       domain over the newly established stream.





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10.4.3.  Error Handling

   If routing of a stanza to the intended recipient's server is
   unsuccessful, the sender's server MUST return an error to the sender.
   If resolution of the remote domain is unsuccessful, the stanza error
   MUST be <remote-server-not-found/> (Section 8.3.3.16).  If resolution
   succeeds but streams cannot be negotiated, the stanza error MUST be
   <remote-server-timeout/> (Section 8.3.3.17).

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

10.5.  Local Domain

   If the domainpart of the JID contained in the 'to' attribute matches
   one of the configured FQDNs of the server, the server MUST first
   determine if the FQDN is serviced by the server itself 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.  However, the server MUST NOT route or "forward" the stanza
   to another domain, because it is the server's responsibility to
   process all stanzas for which the domainpart of the 'to' address
   matches one of the configured FQDNs of the server (among other
   things, this helps to prevent looping).

10.5.1.  domainpart

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

10.5.2.  domainpart/resourcepart

   If the JID contained in the 'to' attribute is of the form
   <domainpart/resourcepart>, then the server MUST either (a) handle the
   stanza as appropriate for the stanza kind or (b) return an error
   stanza to the sender.

10.5.3.  localpart@domainpart

   An address of this type is normally associated with an account on the
   server.  The following rules provide some general guidelines; more
   detailed rules in the context of instant messaging and presence
   applications are provided in [XMPP-IM].




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10.5.3.1.  No Such User

   If there is no local account associated with the
   <localpart@domainpart>, how the stanza is processed depends on the
   stanza type.

   o  For a message stanza, the server MUST either (a) silently ignore
      the stanza or (b) return a <service-unavailable/> stanza error
      (Section 8.3.3.19) to the sender.

   o  For a presence stanza, the server SHOULD ignore the stanza (or
      behave as described in [XMPP-IM]).

   o  For an IQ stanza, the server MUST return a <service-unavailable/>
      stanza error (Section 8.3.3.19) to the sender.

10.5.3.2.  User Exists

   If the JID contained in the 'to' attribute is of the form
   <localpart@domainpart>, how the stanza is processed depends on the
   stanza type.

   o  For a message stanza, if there exists at least one connected
      resource for the account then the server SHOULD deliver it to at
      least one of the connected resources.  If there exists no
      connected resource then the server MUST either (a) store the
      message offline for delivery when the account next has a connected
      resource or (b) return a <service-unavailable/> stanza error
      (Section 8.3.3.19).

   o  For a presence stanza, if there exists at least one connected
      resource that has sent initial presence (i.e., has a "presence
      session" as defined in [XMPP-IM]) then the server SHOULD deliver
      it to such resources.  If there exists no connected resource then
      the server SHOULD ignore the stanza (or behave as described in
      [XMPP-IM]).

   o  For an IQ stanza, the server MUST handle it directly on behalf of
      the intended recipient.

10.5.4.  localpart@domainpart/resourcepart

   If the JID contained in the 'to' attribute is of the form
   <localpart@domainpart/resourcepart> and the user exists but there is
   no connected resource that exactly matches the full JID, the stanza
   SHOULD be processed as if the JID were of the form
   <localpart@domainpart> as described under Section 10.5.3.2.




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   If the JID contained in the 'to' attribute is of the form
   <localpart@domainpart/resourcepart>, the user exists, and there is a
   connected resource that exactly matches the full JID, the server MUST
   deliver the stanza to that connected resource.

11.  XML Usage

11.1.  XML Restrictions

   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 or restricted form 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].  Furthermore, XMPP uses XML to define
   protocol data structures and extensions for the purpose of structured
   interactions between network entities and therefore adheres to the
   recommendations provided in [XML-GUIDE] regarding restrictions on the
   use of XML in IETF protocols.  As a result, 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 the predefined entities (Section 4.6 therein)

   An XMPP implementation MUST behave as follows with regard to these
   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 close the stream with a
       stream error, which SHOULD be <restricted-xml/>
       (Section 4.9.3.18), although some existing implementations send
       <bad-format/> (Section 4.9.3.1) instead.






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11.2.  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 under Section 4.8 for XML
   streams and Section 8.4 for XML stanzas.

11.3.  Well-Formedness

   In XML, there are two varieties of well-formedness:

   o  "XML-well-formedness" in accordance with the definition of "well-
      formed" from Section 2.1 of [XML].

   o  "Namespace-well-formedness" in accordance with the definition of
      "namespace-well-formed" from Section 7 of [XML-NAMES].

   The following rules apply:

   1.  An XMPP entity MUST NOT generate data that is not XML-well-
       formed.

   2.  An XMPP entity MUST NOT accept data that is not XML-well-formed;
       instead it MUST close the stream over which the data was received
       with a <not-well-formed/> stream error (Section 4.9.3.13).

   3.  An XMPP entity MUST NOT generate data that is not namespace-well-
       formed.

   4.  An XMPP entity MUST NOT accept data that is not namespace-well-
       formed (in particular, an XMPP server MUST NOT route or deliver
       data that is not namespace-well-formed); instead it MUST return
       either a <not-acceptable/> stanza error (Section 8.3.3.9) or
       close the stream with a <not-well-formed/> stream error
       (Section 4.9.3.13), where it is preferable to close the stream
       with a stream error because accepting such data can open an
       entity to certain denial-of-service attacks.

      Interoperability Note: Because these restrictions were
      underspecified in [RFC3920], it is possible that implementations
      based on that specification will send data that does not comply
      with these restrictions.





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11.4.  Validation

   A server is not responsible for ensuring that XML data delivered to a
   connected client or routed to a peer server is valid, in accordance
   with the definition of "valid" provided in Section 2.8 of [XML].  An
   implementation MAY choose to accept or send only data that has been
   explicitly validated against the schemas provided in this document,
   but such behavior is OPTIONAL.  Clients are advised not to 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.

      Informational Note: The terms "valid" and "well-formed" are
      distinct in XML.

11.5.  Inclusion of XML Declaration

   Before sending a stream header, an implementation SHOULD send an XML
   declaration (matching the "XMLDecl" production from [XML]).
   Applications MUST follow the rules provided in [XML] regarding the
   format of the XML declaration and the circumstances under which the
   XML declaration is included.

   Because external markup declarations are prohibited in XMPP (as
   described under Section 11.1), the standalone document declaration
   (matching the "SDDecl" production from [XML]) would have no meaning
   and therefore MUST NOT be included in an XML declaration sent over an
   XML stream.  If an XMPP entity receives an XML declaration containing
   a standalone document declaration set to a value of "no", the entity
   MUST either ignore the standalone document declaration or close the
   stream with a stream error, which SHOULD be <restricted-xml/>
   (Section 4.9.3.18).

11.6.  Character Encoding

   Implementations MUST support the UTF-8 transformation of Universal
   Character Set [UCS2] characters, as needed for conformance with
   [CHARSETS] and as 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 close the stream with a stream
   error, which SHOULD be <unsupported-encoding/> (Section 4.9.3.22),
   although some existing implementations send <bad-format/>
   (Section 4.9.3.1) instead.

   Because it is mandatory for an XMPP implementation to support all and
   only the UTF-8 encoding and because UTF-8 always has the same byte
   order, an implementation MUST NOT send a byte order mark ("BOM") at



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   the beginning of the data stream.  If an entity receives the
   [UNICODE] character U+FEFF anywhere in an XML stream (including as
   the first character of the stream), it MUST interpret that character
   as a zero width no-break space, not as a byte order mark.

11.7.  Whitespace

   Except where explicitly disallowed (e.g., during TLS negotiation
   (Section 5) and SASL negotiation (Section 6)), either entity MAY send
   whitespace as separators between XML stanzas or between any other
   first-level elements sent over the stream.  One common use for
   sending such whitespace is explained under Section 4.4.

11.8.  XML Versions

   XMPP is an application profile of XML 1.0.  A future version of XMPP
   might be defined in terms of higher versions of XML, but this
   specification defines XMPP only in terms of XML 1.0.

12.  Internationalization Considerations

   As specified under Section 11.6, XML streams MUST be encoded in
   UTF-8.

   As specified under Section 4.7, 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 8.1.5, 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.

   Internationalization of XMPP addresses is specified in [XMPP-ADDR].

13.  Security Considerations

13.1.  Fundamentals

   XMPP technologies are typically deployed using a decentralized
   client-server architecture.  As a result, several paths are possible
   when two XMPP entities need to communicate:

   1.  Both entities are servers.  In this case, the entities can
       establish a direct server-to-server stream between themselves.





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   2.  One entity is a server and the other entity is a client whose
       account is hosted on that server.  In this case, the entities can
       establish a direct client-to-server stream between themselves.

   3.  Both entities are clients whose accounts are hosted on the same
       server.  In this case, the entities cannot establish a direct
       stream between themselves, but there is only one intermediate
       entity between them, whose policies they might understand and in
       which they might have some level of trust (e.g., the server might
       require the use of Transport Layer Security for all client
       connections).

   4.  Both entities are clients but their accounts are hosted on
       different servers.  In this case, the entities cannot establish a
       direct stream between themselves and there are two intermediate
       entities between them; each client might have some trust in the
       server that hosts its account but might know nothing about the
       policies of the server to which the other client connects.

   This specification covers only the security of a direct XML stream
   between two servers or between a client and a server (cases #1 and
   #2), where each stream can be considered a single "hop" along a
   communication path.  The goal of security for a multi-hop path (cases
   #3 and #4), although very desirable, is out of scope for this
   specification.

   In accordance with [SEC-GUIDE], this specification covers
   communication security (confidentiality, data integrity, and peer
   entity authentication), non-repudiation, and systems security
   (unauthorized usage, inappropriate usage, and denial of service).  We
   also discuss common security issues such as information leaks,
   firewalls, and directory harvesting, as well as best practices
   related to the reuse of technologies such as base 64, DNS,
   cryptographic hash functions, SASL, TLS, UTF-8, and XML.

13.2.  Threat Model

   The threat model for XMPP is in essence the standard "Internet Threat
   Model" described in [SEC-GUIDE].  Attackers are assumed to be
   interested in and capable of launching the following attacks against
   unprotected XMPP systems:

   o  Eavesdropping

   o  Sniffing passwords

   o  Breaking passwords through dictionary attacks




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   o  Discovering usernames through directory harvesting attacks

   o  Replaying, inserting, deleting, or modifying stanzas

   o  Spoofing users

   o  Gaining unauthorized entry to a server or account

   o  Using a server or account inappropriately

   o  Denying service to other entities

   o  Subverting communication streams through man-in-the-middle attacks

   o  Gaining control over on-path servers

   Where appropriate, the following sections describe methods for
   protecting against these threats.

13.3.  Order of Layers

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

   1.  TCP

   2.  TLS

   3.  SASL

   4.  XMPP

   This order has important security implications, as described
   throughout these security considerations.

   Within XMPP, XML stanzas are further ordered on top of XML streams,
   as described under Section 4.

13.4.  Confidentiality and Integrity

   The use of Transport Layer Security (TLS) with appropriate
   ciphersuites provides a reliable mechanism to ensure the
   confidentiality and integrity of data exchanged between a client and
   a server or between two servers.  Therefore, TLS can help to protect
   against eavesdropping, password sniffing, man-in-the-middle attacks,
   and stanza replays, insertion, deletion, and modification over an XML
   stream.  XMPP clients and servers MUST support TLS as defined under
   Section 5.




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      Informational Note: The confidentiality and integrity of a stream
      can be protected by methods other than TLS, e.g., by means of a
      SASL mechanism that involves negotiation of a security layer.

      Security Warning: The use of TLS in XMPP applies to a single
      stream.  Because XMPP is typically deployed using a distributed
      client-server architecture (as explained under Section 2.5), a
      stanza might traverse multiple streams, and not all of those
      streams might be TLS-protected.  For example, a stanza sent from a
      client with a session at one server (e.g.,
      <romeo@example.net/orchard>) and intended for delivery to a client
      with a session at another server (e.g.,
      <juliet@example.com/balcony>) will traverse three streams: (1) the
      stream from the sender's client to its server, (2) the stream from
      the sender's server to the recipient's server, and (3) the stream
      from the recipient's server to the recipient's client.
      Furthermore, the stanza will be processed as cleartext within the
      sender's server and the recipient's server.  Therefore, even if
      the stream from the sender's client to its server is protected,
      the confidentiality and integrity of a stanza sent over that
      protected stream cannot be guaranteed when the stanza is processed
      by the sender's server, sent from the sender's server to the
      recipient's server, processed by the recipient's server, or sent
      from the recipient's server to the recipient's client.  Only a
      robust technology for end-to-end encryption could ensure the
      confidentiality and integrity of a stanza as it traverses all of
      the "hops" along a communication path (e.g., a technology that
      meets the requirements defined in [E2E-REQS]).  Unfortunately, the
      XMPP community has so far failed to produce an end-to-end
      encryption technology that might be suitable for widespread
      implementation and deployment, and definition of such a technology
      is out of scope for this document.

13.5.  Peer Entity Authentication

   The use of the Simple Authentication and Security Layer (SASL) for
   authentication provides a reliable mechanism for peer entity
   authentication.  Therefore, SASL helps to protect against user
   spoofing, unauthorized usage, and man-in-the middle attacks.  XMPP
   clients and servers MUST support SASL as defined under Section 6.

13.6.  Strong Security

   [STRONGSEC] defines "strong security" and its importance to
   communication over the Internet.  For the purpose of XMPP
   communication over client-to-server and server-to-server streams, the
   term "strong security" refers to the use of security technologies




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   that provide both mutual authentication and integrity checking (e.g.,
   a combination of TLS encryption and SASL authentication using
   appropriate SASL mechanisms).

   Implementations MUST support strong security.  Service provisioning
   SHOULD use strong security.

   An implementation SHOULD make it possible for an end user or service
   administrator to provision a deployment with specific trust anchors
   for the certificate presented by a connecting entity (either client
   or server); when an application is thus provisioned, it MUST NOT use
   a generic PKI trust store to authenticate the connecting entity.
   More detailed rules and guidelines regarding certificate validation
   are provided in the next section.

   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.

13.7.  Certificates

   Channel encryption of an XML stream using Transport Layer Security as
   described under Section 5, and in some cases also authentication as
   described under Section 6, is commonly based on a PKIX certificate
   presented by the receiving entity (or, in the case of mutual
   certificate authentication, both the receiving entity and the
   initiating entity).  This section describes best practices regarding
   the generation of PKIX certificates to be presented by XMPP entities
   and the verification of PKIX certificates presented by XMPP entities.

   In general, the following sections rely on and extend the rules and
   guidelines provided in the [PKIX] profile of [X509], and in
   [TLS-CERTS].  The reader is referred to those specifications for a
   detailed understanding of PKIX certificates and their use in TLS.

13.7.1.  Certificate Generation

13.7.1.1.  General Considerations

   The following rules apply to end entity public key certificates that
   are issued to XMPP servers or clients:

   1.  The certificate MUST conform to [PKIX].

   2.  The certificate MUST NOT contain a basicConstraints extension
       with the cA boolean set to TRUE.

   3.  The subject field MUST NOT be null.



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   4.  The signatureAlgorithm SHOULD be the PKCS #1 version 1.5
       signature algorithm with SHA-256 as defined by [PKIX-ALGO], or a
       stronger algorithm if available.

   5.  The certificate SHOULD include an Authority Information Access
       (AIA) extension that specifies the address of an Online
       Certificate Status Protocol [OCSP] responder (if not, a relying
       party would need to fall back on the use of Certificate
       Revocation Lists (CRLs) as described in [PKIX]).

   The following rules apply to certification authority (CA)
   certificates that are used by issuers of XMPP end entity
   certificates:

   1.  The certificate MUST conform to [PKIX].

   2.  The certificate MUST contain a keyUsage extension with the
       digitalSignature bit set.

   3.  The subject field MUST NOT be null.

   4.  The signatureAlgorithm SHOULD be the PKCS #1 version 1.5
       signature algorithm with SHA-256 as defined by [PKIX-ALGO], or a
       stronger algorithm if available.

   5.  For issuers of public key certificates, the issuer's certificate
       MUST contain a basicConstraints extension with the cA boolean set
       to TRUE.

13.7.1.2.  Server Certificates

13.7.1.2.1.  Rules

   In a PKIX certificate to be presented by an XMPP server (i.e., a
   "server certificate"), the certificate SHOULD include one or more
   XMPP addresses (i.e., domainparts) associated with XMPP services
   hosted at the server.  The rules and guidelines defined in
   [TLS-CERTS] apply to XMPP server certificates, with the following
   XMPP-specific considerations:

   o  Support for the DNS-ID identifier type [PKIX] is REQUIRED in XMPP
      client and server software implementations.  Certification
      authorities that issue XMPP-specific certificates MUST support the
      DNS-ID identifier type.  XMPP service providers SHOULD include the
      DNS-ID identifier type in certificate requests.






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   o  Support for the SRV-ID identifier type [PKIX-SRV] is REQUIRED for
      XMPP client and server software implementations (for verification
      purposes XMPP client implementations need to support only the
      "_xmpp-client" service type, whereas XMPP server implementations
      need to support both the "_xmpp-client" and "_xmpp-server" service
      types).  Certification authorities that issue XMPP-specific
      certificates SHOULD support the SRV-ID identifier type.  XMPP
      service providers SHOULD include the SRV-ID identifier type in
      certificate requests.

   o  Support for the XmppAddr identifier type (specified under
      Section 13.7.1.4) is encouraged in XMPP client and server software
      implementations for the sake of backward-compatibility, but is no
      longer encouraged in certificates issued by certification
      authorities or requested by XMPP service providers.

   o  DNS domain names in server certificates MAY contain the wildcard
      character '*' as the complete left-most label within the
      identifier.

13.7.1.2.2.  Examples

   For our first (relatively simple) example, consider a company called
   "Example Products, Inc."  It hosts an XMPP service at
   "im.example.com" (i.e., user addresses at the service are of the form
   "user@im.example.com"), and SRV lookups for the xmpp-client and xmpp-
   server services at "im.example.com" yield one machine, called
   "x.example.com", as follows:

   _xmpp-client._tcp.im.example.com. 400 IN SRV 20 0 5222 x.example.com
   _xmpp-server._tcp.im.example.com. 400 IN SRV 20 0 5269 x.example.com

   The certificate presented by x.example.com contains the following
   representations:

   o  An otherName type of SRVName (id-on-dnsSRV) containing an
      IA5String (ASCII) string of "_xmpp-client.im.example.com"

   o  An otherName type of SRVName (id-on-dnsSRV) containing an
      IA5String (ASCII) string of "_xmpp-server.im.example.com"

   o  A dNSName containing an ASCII string of "im.example.com"

   o  An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
      string of "im.example.com"

   o  A CN containing an ASCII string of "Example Products, Inc."




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   For our second (more complex) example, consider an ISP called
   "Example Internet Services".  It hosts an XMPP service at
   "example.net" (i.e., user addresses at the service are of the form
   "user@example.net"), but SRV lookups for the xmpp-client and xmpp-
   server services at "example.net" yield two machines ("x1.example.net"
   and "x2.example.net"), as follows:

   _xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x1.example.net.
   _xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x2.example.net.
   _xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x1.example.net.
   _xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x2.example.net.

   Example Internet Services also hosts chatrooms at chat.example.net,
   and provides an xmpp-server SRV record for that service as well (thus
   enabling entities from remote domains to access that service).  It
   also might provide other such services in the future, so it wishes to
   represent a wildcard in its certificate to handle such growth.

   The certificate presented by either x1.example.net or x2.example.net
   contains the following representations:

   o  An otherName type of SRVName (id-on-dnsSRV) containing an
      IA5String (ASCII) string of "_xmpp-client.example.net"

   o  An otherName type of SRVName (id-on-dnsSRV) containing an
      IA5String (ASCII) string of "_xmpp-server.example.net"

   o  An otherName type of SRVName (id-on-dnsSRV) containing an
      IA5String (ASCII) string of "_xmpp-server.chat.example.net"

   o  A dNSName containing an ASCII string of "example.net"

   o  A dNSName containing an ASCII string of "*.example.net"

   o  An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
      string of "example.net"

   o  An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
      string of "chat.example.net"

   o  A CN containing an ASCII string of "Example Internet Services"










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13.7.1.3.  Client Certificates

   In a PKIX certificate to be presented by an XMPP client controlled by
   a human user (i.e., a "client certificate"), 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 an XmppAddr as
   specified under Section 13.7.1.4.

13.7.1.4.  XmppAddr Identifier Type

   The XmppAddr identifier type is a UTF8String within an otherName
   entity inside the subjectAltName, using the [ASN.1] Object Identifier
   "id-on-xmppAddr" specified below.

   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
   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@im.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@im.example.com

   dotted display format:  subjectAltName=otherName:
      1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com

   URN notation:  subjectAltName=otherName:urn:oid:
      1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com

   Use of the "id-on-xmppAddr" format is RECOMMENDED in the generation
   of certificates, but all three formats MUST be supported for the
   purpose of certificate validation.

   The "id-on-xmppAddr" object identifier MAY be used in conjunction
   with the extended key usage extension specified in Section 4.2.1.12
   of [PKIX] in order to explicitly define and limit the intended use of
   a certificate to the XMPP network.



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13.7.2.  Certificate Validation

   When an XMPP entity is presented with a server certificate or client
   certificate by a peer for the purpose of encryption or authentication
   of XML streams as described under Section 5 and Section 6, the entity
   MUST attempt to validate the certificate to determine if the
   certificate will be considered a "trusted certificate", i.e., a
   certificate that is acceptable for encryption and/or authentication
   in accordance with the XMPP entity's local service policies or
   configured settings.

   For both server certificates and client certificates, the validating
   entity MUST do the following:

   1.  Attempt to verify the integrity of the certificate.

   2.  Attempt to verify that the certificate has been properly signed
       by the issuing Certificate Authority.

   3.  Attempt to validate the full certification path.

   4.  Check the rules for end entity public key certificates and
       certification authority certificates specified under
       Section 13.7.1.1 for the general case and under either
       Section 13.7.1.2 or Section 13.7.1.3 for XMPP server or client
       certificates, respectively.

   5.  Check certificate revocation messages via Certificate Revocation
       Lists (CRLs), the Online Certificate Status Protocol [OCSP], or
       both.

   If any of those validation attempts fail, the validating entity MUST
   unilaterally terminate the session.

   The following sections describe the additional identity verification
   rules that apply to server-to-server and client-to-server streams.

   Once the identity of the stream peer has been validated, the
   validating entity SHOULD also correlate the validated identity with
   the 'from' address (if any) of the stream header it received from the
   peer.  If the two identities do not match, the validating entity
   SHOULD terminate the connection attempt (however, there might be good
   reasons why the identities do not match, as described under
   Section 4.7.1).







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13.7.2.1.  Server Certificates

   For server certificates, the rules and guidelines defined in
   [TLS-CERTS] apply, with the proviso that the XmppAddr identifier
   specified under Section 13.7.1.4 is allowed as a reference
   identifier.

   The identities to be checked are set as follows:

   o  The initiating entity sets the source domain of its reference
      identifiers to the 'to' address it communicates in the initial
      stream header; i.e., this is the identity it expects the receiving
      entity to provide in a PKIX certificate.

   o  The receiving entity sets the source domain of its reference
      identifiers to the 'from' address communicated by the initiating
      entity in the initial stream header; i.e., this is the identity
      that the initiating entity is trying to assert.

   In the case of server-to-server communication, the matching procedure
   described in [TLS-CERTS] can be performed by an application server
   (receiving entity) when verifying an incoming server-to-server
   connection from a peer server (initiating entity).  In this case, the
   receiving entity verifies the identity of the initiating entity and
   uses as the source domain of its reference identifiers the DNS domain
   name asserted by the initiating entity in the 'from' attribute of the
   initial stream header.  However, the matching procedure described in
   [TLS-CERTS] remains unchanged and is applied in the same way.

13.7.2.2.  Client Certificates

   When an XMPP server validates a certificate presented by a client,
   there are three possible cases, as discussed in the following
   sections.

   The identities to be checked are set as follows:

   o  The client sets the source domain of its reference identifiers to
      the 'to' address it communicates in the initial stream header;
      i.e., this is the identity it expects the server to provide in a
      PKIX certificate.

   o  The server sets the bare JID of its reference identifiers to the
      'from' address communicated by the initiating entity in the
      initial stream header; i.e., this is the identity that the client
      is trying to assert.





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13.7.2.2.1.  Case #1

   If the client certificate appears to be certified by a certification
   path terminating in a trust anchor (as described in Section 6.1 of
   [PKIX]), the server MUST check the certificate for any instances of
   the XmppAddr as described under Section 13.7.1.4.  There are three
   possible sub-cases:

   Sub-Case #1:  The server finds one XmppAddr for which the domainpart
      of the represented JID matches one of the configured FQDNs of the
      server; the server SHOULD use this represented JID as the
      validated identity of the client.

   Sub-Case #2:  The server finds more than one XmppAddr for which the
      domainpart of the represented JID matches one of the configured
      FQDNs of the server; the server SHOULD use one of these
      represented JIDs as the validated identity of the client, choosing
      among them based on the bare JID contained in the 'from' address
      of the initial stream header (if any), based on the domainpart
      contained in the 'to' address of the initial stream header, or in
      accordance with local service policies (such as a lookup in a user
      database based on other information contained in the client
      certificate).

   Sub-Case #3:  The server finds no XmppAddrs, or finds at least one
      XmppAddr but the domainpart of the represented JID does not match
      one of the configured FQDNs of the server; the server MUST NOT use
      the represented JID (if any) as the validated identity of the
      client but instead MUST validate the identity of the client using
      other means in accordance with local service policies (such as a
      lookup in a user database based on other information contained in
      the client certificate).  If the identity cannot be so validated,
      the server MAY abort the validation process and terminate the TLS
      negotiation.

13.7.2.2.2.  Case #2

   If the client certificate is certified by a Certificate Authority not
   known to the server, the server MUST proceed as under Case #1, Sub-
   Case #3.

13.7.2.2.3.  Case #3

   If the client certificate is self-signed, the server MUST proceed as
   under Case #1, Sub-Case #3.






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13.7.2.3.  Checking of Certificates in Long-Lived Streams

   Because XMPP uses long-lived XML streams, it is possible that a
   certificate presented during stream negotiation might expire or be
   revoked while the stream is still live (this is especially relevant
   in the context of server-to-server streams).  Therefore, each party
   to a long-lived stream SHOULD:

   1.  Cache the expiration date of the certificate presented by the
       other party and any certificates on which that certificate
       depends (such as a root or intermediate certificate for a
       certification authority), and close the stream when any such
       certificate expires, with a stream error of <reset/>
       (Section 4.9.3.16).

   2.  Periodically query the Online Certificate Status Protocol [OCSP]
       responder listed in the Authority Information Access (AIA)
       extension of the certificate presented by the other party and any
       certificates on which that certificate depends (such as a root or
       intermediate certificate for a certification authority), and
       close the stream if any such certificate has been revoked, with a
       stream error of <reset/> (Section 4.9.3.16).  It is RECOMMENDED
       to query the OSCP responder at or near the time communicated via
       the nextUpdate field received in the OCSP response or, if the
       nextUpdate field is not set, to query every 24 hours.

   After the stream is closed, the initiating entity from the closed
   stream will need to reconnect and the receiving entity will need to
   authenticate the initiating entity based on whatever certificate it
   presents during negotiation of the new stream.

13.7.2.4.  Use of Certificates in XMPP Extensions

   Certificates MAY be used in extensions to XMPP for the purpose of
   application-layer encryption or authentication above the level of XML
   streams (e.g., for end-to-end encryption).  Such extensions will
   define their own certificate handling rules.  At a minimum, such
   extensions are encouraged to remain consistent with the rules defined
   in this specification, specifying additional rules only when
   necessary.

13.8.  Mandatory-to-Implement TLS and SASL Technologies

   The following TLS ciphersuites and SASL mechanisms are mandatory-to-
   implement (naturally, implementations MAY support other ciphersuites
   and mechanisms as well).  For security considerations related to TLS
   ciphersuites, see Section 13.9.4 and [TLS].  For security




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   considerations related to SASL mechanisms, see Section 13.9.4,
   [SASL], and specifications for particular SASL mechanisms such as
   [SCRAM], [DIGEST-MD5], and [PLAIN].

13.8.1.  For Authentication Only

   For authentication only, servers and clients MUST support the SASL
   Salted Challenge Response Authentication Mechanism [SCRAM] -- in
   particular, the SCRAM-SHA-1 and SCRAM-SHA-1-PLUS variants.

      Security Warning: Even though it is possible to complete
      authentication only without confidentiality, it is RECOMMENDED for
      servers and clients to protect the stream with TLS before
      attempting authentication with SASL, both to help protect the
      information exchanged during SASL negotiation and to help prevent
      certain downgrade attacks as described under Section 13.9.4 and
      Section 13.9.5.  Even if TLS is used, implementations SHOULD also
      enforce channel binding as described under Section 13.9.4.

      Interoperability Note: The SCRAM-SHA-1 or SASL-SCRAM-SHA-1-PLUS
      variants of the SCRAM mechanism replace the SASL DIGEST-MD5
      mechanism as XMPP's mandatory-to-implement password-based method
      for authentication only.  For backward-compatibility with existing
      deployed infrastructure, implementations are encouraged to
      continue supporting the DIGEST-MD5 mechanism as specified in
      [DIGEST-MD5]; however, there are known interoperability issues
      with DIGEST-MD5 that make it impractical in the long term.

13.8.2.  For Confidentiality Only

   For confidentiality only, servers MUST support TLS with the
   TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite.

      Security Warning: Because a connection with confidentiality only
      has weaker security properties than a connection with both
      confidentiality and authentication, it is RECOMMENDED for servers
      and clients to prefer connections with both qualities (e.g., by
      protecting the stream with TLS before attempting authentication
      with SASL).  In practice, confidentiality only is employed merely
      for server-to-server connections when the peer server does not
      present a trusted certificate and the servers use Server Dialback
      [XEP-0220] for weak identity verification, but TLS with
      confidentiality only is still desirable to protect the connection
      against casual eavesdropping.







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13.8.3.  For Confidentiality and Authentication with Passwords

   For both confidentiality and authentication with passwords, servers
   and clients MUST implement TLS with the TLS_RSA_WITH_AES_128_CBC_SHA
   ciphersuite plus SASL SCRAM, in particular the SCRAM-SHA-1 and
   SCRAM-SHA-1-PLUS variants (with SCRAM-SHA1-PLUS being preferred, as
   described under Section 13.9.4).

   As further explained in the following Security Warning, in certain
   circumstances a server MAY offer TLS with the
   TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite plus SASL PLAIN when it is
   not possible to offer more secure alternatives; in addition, clients
   SHOULD implement PLAIN over TLS in order to maximize interoperability
   with servers that are not able to deploy more secure alternatives.

      Security Warning: In practice, many servers offer, and many
      clients use, TLS plus SASL PLAIN.  The SCRAM-SHA-1 and especially
      SCRAM-SHA-1-PLUS variants of the SCRAM mechanism are strongly
      preferred over the PLAIN mechanism because of their superior
      security properties (including for SCRAM-SHA-1-PLUS the ability to
      enforce channel binding as described under Section 13.9.4).  A
      client SHOULD treat TLS plus SASL PLAIN as a technology of last
      resort to be used only when interacting with a server that does
      not offer SCRAM (or other alternatives that are more secure than
      TLS plus SASL PLAIN), MUST prefer more secure mechanisms (e.g.,
      EXTERNAL, SCRAM-SHA-1-PLUS, SCRAM-SHA-1, or the older DIGEST-MD5
      mechanism) to the PLAIN mechanism, and MUST NOT use the PLAIN
      mechanism if the stream does not at a minimum have confidentiality
      and integrity protection via TLS with full certificate validation
      as described under Section 13.7.2.1.  A server MUST NOT offer SASL
      PLAIN if the confidentiality and integrity of the stream are not
      protected via TLS or an equivalent security layer.  A server
      SHOULD NOT offer TLS plus SASL PLAIN unless it is unable to offer
      some variant of SASL SCRAM (or other alternatives that are more
      secure than TLS plus SASL PLAIN), e.g., because the XMPP service
      depends for authentication purposes on a database or directory
      that is not under the control of the XMPP administrators, such as
      Pluggable Authentication Modules (PAM), an Lightweight Directory
      Access Protocol (LDAP) directory [LDAP], or an Authentication,
      Authorization, and Accounting (AAA) key management protocol (for
      guidance, refer to [AAA]).  However, offering TLS plus SASL PLAIN
      even when the server supports more secure alternatives might be
      appropriate if the server needs to enable interoperability with an
      installed base of clients that do not yet support SCRAM or other
      alternatives that are more secure than TLS plus SASL PLAIN.






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13.8.4.  For Confidentiality and Authentication without Passwords

   For both confidentiality and authentication without passwords,
   servers MUST and clients SHOULD implement TLS with the
   TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite plus the SASL EXTERNAL
   mechanism (see Appendix A of [SASL]) with PKIX certificates.

13.9.  Technology Reuse

13.9.1.  Use of Base 64 in SASL

   Both the client and the server MUST verify any base 64 data received
   during SASL negotiation (Section 6).  An implementation MUST reject
   (not ignore) any characters that are not explicitly allowed by the
   base 64 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 base 64 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.

13.9.2.  Use of DNS

   XMPP typically relies on the Domain Name System (specifically
   [DNS-SRV] records) to resolve a fully qualified domain name to an IP
   address before a client connects to a server or before a peer server
   connects to another server.  Before attempting to negotiate an XML
   stream, the initiating entity MUST NOT proceed until it has resolved
   the DNS domain name of the receiving entity as specified under
   Section 3 (although it is not necessary to resolve the DNS domain
   name before each connection attempt, because DNS resolution results
   can be temporarily cached in accordance with time-to-live values).
   However, in the absence of a secure DNS option (e.g., as provided by
   [DNSSEC]), a malicious attacker with access to the DNS server data,
   or able to cause spoofed answers to be cached in a recursive
   resolver, can potentially cause the initiating entity to connect to
   any XMPP server chosen by the attacker.  Deployment and validation of
   server certificates help to prevent such attacks.




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13.9.3.  Use of Hash Functions

   XMPP itself does not directly mandate the use of any particular
   cryptographic hash function.  However, technologies on which XMPP
   depends (e.g., TLS and particular SASL mechanisms), as well as
   various XMPP extensions, might make use of cryptographic hash
   functions.  Those who implement XMPP technologies or who develop XMPP
   extensions are advised to closely monitor the state of the art
   regarding attacks against cryptographic hash functions in Internet
   protocols as they relate to XMPP.  For helpful guidance, refer to
   [HASHES].

13.9.4.  Use of SASL

   Because the initiating entity chooses an acceptable SASL mechanism
   from the list presented by the receiving entity, the initiating
   entity depends on the receiving entity's list for authentication.
   This dependency introduces the possibility of a downgrade attack if
   an attacker can gain control of the channel and therefore present a
   weak list of mechanisms.  To mitigate this attack, the parties SHOULD
   protect the channel using TLS before attempting SASL negotiation and
   either perform full certificate validation as described under
   Section 13.7.2.1 or use a SASL mechanism that provides channel
   bindings, such as SCRAM-SHA-1-PLUS.  (Protecting the channel via TLS
   with full certificate validation can help to ensure the
   confidentiality and integrity of the information exchanged during
   SASL negotiation.)

   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 (e.g.,
   TLS).  Such a binding is known as a "channel binding" (see
   [CHANNEL]).  Some SASL mechanisms provide channel bindings, which in
   the case of XMPP would typically be a binding to TLS (see
   [CHANNEL-TLS]).  If a SASL mechanism provides a channel binding
   (e.g., this is true of [SCRAM]), then XMPP entities using that
   mechanism SHOULD prefer the channel binding variant (e.g., preferring
   "SCRAM-SHA-1-PLUS" over "SCRAM-SHA-1").  If a SASL mechanism does not
   provide a channel 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.





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   Many deployed XMPP services authenticate client connections by means
   of passwords.  It is well known that most human users choose
   relatively weak passwords.  Although service provisioning is out of
   scope for this document, XMPP servers that allow password-based
   authentication SHOULD enforce minimal criteria for password strength
   to help prevent dictionary attacks.  Because all password-based
   authentication mechanisms are susceptible to password guessing
   attacks, XMPP servers MUST limit the number of retries allowed during
   SASL authentication, as described under Section 6.4.5.

   Some SASL mechanisms (e.g., [ANONYMOUS]) do not provide strong peer
   entity authentication of the client to the server.  Service
   administrators are advised to enable such mechanisms with caution.
   Best practices for the use of the SASL ANONYMOUS mechanism in XMPP
   are described in [XEP-0175].

13.9.5.  Use of TLS

   Implementations of TLS typically support multiple versions of the
   Transport Layer Security protocol as well as the older Secure Sockets
   Layer (SSL) protocol.  Because of known security vulnerabilities,
   XMPP servers and clients MUST NOT request, offer, or use SSL 2.0.
   For further details, see Appendix E.2 of [TLS] along with [TLS-SSL2].

   To prevent man-in-the-middle attacks, the TLS client (which might be
   an XMPP client or an XMPP server) MUST verify the certificate of the
   TLS server and MUST check its understanding of the server FQDN
   against the server's identity as presented in the TLS Certificate
   message as described under Section 13.7.2.1 (for further details, see
   [TLS-CERTS].

   Support for TLS renegotiation is strictly OPTIONAL.  However,
   implementations that support TLS renegotiation MUST implement and use
   the TLS Renegotiation Extension [TLS-NEG].  Further details are
   provided under Section 5.3.5.

13.9.6.  Use of UTF-8

   The use of UTF-8 makes it possible to transport non-ASCII characters,
   and thus enables character "spoofing" scenarios, in which a displayed
   value appears to be something other than it is.  Furthermore, there
   are known attack scenarios related to the decoding of UTF-8 data.  On
   both of these points, refer to [UTF-8] for more information.








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13.9.7.  Use of XML

   Because XMPP is an application profile of the Extensible Markup
   Language [XML], many of the security considerations described in
   [XML-MEDIA] and [XML-GUIDE] also apply to XMPP.  Several aspects of
   XMPP mitigate the risks described there, such as the prohibitions
   specified under Section 11.1 and the lack of external references to
   style sheets or transformations, but these mitigating factors are by
   no means comprehensive.

13.10.  Information Leaks

13.10.1.  IP Addresses

   A client's IP address and method of access MUST NOT be made public by
   a server (e.g., as typically occurs in [IRC]).

13.10.2.  Presence Information

   One of the core aspects of XMPP is presence: information about the
   network availability of an XMPP entity (i.e., whether the entity is
   currently online or offline).  A "presence leak" occurs when an
   entity's network availability is inadvertently and involuntarily
   revealed to a second entity that is not authorized to know the first
   entity's network availability.

   Although presence is discussed more fully in [XMPP-IM], it is
   important to note that an XMPP server MUST NOT leak presence.  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 depending on whether a
   potential attacker sends XML stanzas to the entity's bare JID
   (<localpart@domainpart>) or full JID
   (<localpart@domainpart/resourcepart>).

13.11.  Directory Harvesting

   If a server generates an error stanza in response to receiving a
   stanza for a user account that does not exist, using the <service-
   unavailable/> stanza error condition (Section 8.3.3.19) can help
   protect against directory harvesting attacks, since this is the same
   error condition that is returned if, for instance, the namespace of
   an IQ child element is not understood, or if "offline message
   storage" ([XEP-0160]) or message forwarding is not enabled for a
   domain.  However, subtle differences in the exact XML of error
   stanzas, as well as in the timing with which such errors are



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   returned, can enable an attacker to determine the network presence of
   a user when more advanced blocking technologies are not used (see for
   instance [XEP-0016] and [XEP-0191]).  Therefore, a server that
   exercises a higher level of caution might not return any error at all
   in response to certain kinds of received stanzas, so that a non-
   existent user appears to behave like a user that has no interest in
   conversing with the sender.

13.12.  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.

   Some considerations discussed in this document help to prevent
   denial-of-service attacks (e.g., the mandate that 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 or weakly verified via Server Dialback).

   In addition, [XEP-0205] provides a detailed discussion of potential
   denial-of-service attacks against XMPP systems along with 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.  If an entity
       attempts to connect but the maximum number of connection attempts
       has been reached, the receiving server MUST NOT allow the new
       connection to proceed.

   3.  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



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       but it has already reached the configured number of allowable
       resources, the receiving server MUST return a <resource-
       constraint/> stanza error (Section 8.3.3.18).

   4.  A server implementation SHOULD enable a server administrator to
       limit the size of stanzas it will accept from a connected client
       or peer server (where "size" is inclusive of all XML markup as
       defined in Section 2.4 of [XML], from the opening "<" character
       of the stanza to the closing ">" character).  A deployed server's
       maximum stanza size MUST NOT be smaller than 10000 bytes, which
       reflects a reasonable compromise between the benefits of
       expressiveness for originating entities and the costs of stanza
       processing for servers.  A server implementation SHOULD NOT
       blindly set 10000 bytes as the value for all deployments but
       instead SHOULD enable server administrators to set their own
       limits.  If a connected resource or peer server sends a stanza
       that violates the upper limit, the receiving server MUST either
       return a <policy-violation/> stanza error (Section 8.3.3.12),
       thus allowing the sender to recover, or close the stream with a
       <policy-violation/> stream error (Section 4.9.3.14).

   5.  A server implementation SHOULD enable a server administrator to
       limit the number of XML stanzas that a connected client is
       allowed to 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 a <policy-
       violation/> stanza error (Section 8.3.3.12).

   6.  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.

   7.  A server implementation MAY 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.

   8.  A server implementation MAY refuse to route or deliver any stanza
       that it considers to be abusive, with or without returning an
       error to the sender.

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






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13.13.  Firewalls

   Although DNS SRV records can instruct connecting entities to use TCP
   ports other than 5222 (client-to-server) and 5269 (server-to-server),
   communication using XMPP typically occurs over those ports, which are
   registered with the IANA (see Section 14).  Use of these well-known
   ports allows administrators to easily enable or disable XMPP activity
   through existing and commonly deployed firewalls.

13.14.  Interdomain Federation

   The term "federation" is commonly used to describe communication
   between two servers.

   Because service provisioning is a matter of policy, it is OPTIONAL
   for any given server to support federation.  If a particular server
   enables federation, it SHOULD enable strong security as previously
   described to ensure both authentication and confidentiality;
   compliant implementations SHOULD support TLS and SASL for this
   purpose.

   Before RFC 3920 defined TLS plus SASL EXTERNAL with certificates for
   encryption and authentication of server-to-server streams, the only
   method for weak identity verification of a peer server was Server
   Dialback as defined in [XEP-0220].  Even when [DNSSEC] is used,
   Server Dialback provides only weak identity verification and provides
   no confidentiality or integrity.  At the time of writing, Server
   Dialback is still the most widely used technique for some level of
   assurance over server-to-server streams.  This reality introduces the
   possibility of a downgrade attack from TLS + SASL EXTERNAL to Server
   Dialback if an attacker can gain control of the channel and therefore
   convince the initiating server that the receiving server does not
   support TLS or does not have an appropriate certificate.  To help
   prevent this attack, the parties SHOULD protect the channel using TLS
   before proceeding, even if the presented certificates are self-signed
   or otherwise untrusted.

13.15.  Non-Repudiation

   Systems that provide both peer entity authentication and data
   integrity have the potential to enable an entity to prove to a third
   party that another entity intended to send particular data.  Although
   XMPP systems can provide both peer entity authentication and data
   integrity, XMPP was never designed to provide non-repudiation.







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14.  IANA Considerations

   The following subsections update the registrations provided in
   [RFC3920].  This section is to be interpreted according to
   [IANA-GUIDE].

14.1.  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:  RFC 6120
   Description:  This is the XML namespace name for STARTTLS negotiation
      data in the Extensible Messaging and Presence Protocol (XMPP) as
      defined by RFC 6120.
   Registrant Contact:  IESG <iesg@ietf.org>

14.2.  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:  RFC 6120
   Description:  This is the XML namespace name for SASL negotiation
      data in the Extensible Messaging and Presence Protocol (XMPP) as
      defined by RFC 6120.
   Registrant Contact:  IESG <iesg@ietf.org>

14.3.  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:  RFC 6120
   Description:  This is the XML namespace name for stream error data in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by RFC 6120.
   Registrant Contact:  IESG <iesg@ietf.org>







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14.4.  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].)

   URI:  urn:ietf:params:xml:ns:xmpp-bind
   Specification:  RFC 6120
   Description:  This is the XML namespace name for resource binding in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by RFC 6120.
   Registrant Contact:  IESG <iesg@ietf.org>

14.5.  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:  RFC 6120
   Description:  This is the XML namespace name for stanza error data in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by RFC 6120.
   Registrant Contact:  IESG <iesg@ietf.org>

14.6.  GSSAPI Service Name

   The IANA has registered "xmpp" as a [GSS-API] service name, as
   defined under Section 6.6.

14.7.  Port Numbers and Service Names

   The IANA has registered "xmpp-client" and "xmpp-server" as keywords
   for [TCP] ports 5222 and 5269, respectively.  In accordance with
   [IANA-PORTS], this document updates the existing registration, as
   follows.

   Service Name:  xmpp-client
   Transport Protocol:  TCP
   Description:  A service offering support for connections by XMPP
      client applications
   Registrant:  IETF XMPP Working Group
   Contact:  IESG <iesg@ietf.org>
   Reference:  RFC 6120
   Port Number:  5222





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   Service Name:  xmpp-server
   Transport Protocol:  TCP
   Description:  A service offering support for connections by XMPP
      server applications
   Registrant:  IETF XMPP Working Group
   Contact:  IESG <iesg@ietf.org>
   Reference:  RFC 6120
   Port Number:  5269

15.  Conformance Requirements

   This section describes a protocol feature set that summarizes the
   conformance requirements of this specification.  This feature set is
   appropriate for use in software certification, interoperability
   testing, and implementation reports.  For each feature, this section
   provides the following information:

   o  A human-readable name

   o  An informational description

   o  A reference to the particular section of this document that
      normatively defines the feature

   o  Whether the feature applies to the Client role, the Server role,
      or both (where "N/A" signifies that the feature is not applicable
      to the specified role)

   o  Whether the feature MUST or SHOULD be implemented, where the
      capitalized terms are to be understood as described in [KEYWORDS]

   The feature set specified here attempts to adhere to the concepts and
   formats proposed by Larry Masinter within the IETF's NEWTRK Working
   Group in 2005, as captured in [INTEROP].  Although this feature set
   is more detailed than called for by [REPORTS], it provides a suitable
   basis for the generation of implementation reports to be submitted in
   support of advancing this specification from Proposed Standard to
   Draft Standard in accordance with [PROCESS].

   Feature:  bind-gen
   Description:  Generate a random resource on demand.
   Section:  Section 7.6
   Roles:  Client N/A, Server MUST.

   Feature:  bind-mtn
   Description:  Consider resource binding as mandatory-to-negotiate.
   Section:  Section 7.3.1
   Roles:  Client MUST, Server MUST.



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   Feature:  bind-restart
   Description:  Do not restart the stream after negotiation of resource
      binding.
   Section:  Section 7.3.2
   Roles:  Client MUST, Server MUST.

   Feature:  bind-support
   Description:  Support binding of client resources to an authenticated
      stream.
   Section:  Section 7
   Roles:  Client MUST, Server MUST.

   Feature:  sasl-correlate
   Description:  When authenticating a stream peer using SASL, correlate
      the authentication identifier resulting from SASL negotiation with
      the 'from' address (if any) of the stream header it received from
      the peer.
   Section:  Section 6.4.6
   Roles:  Client SHOULD, Server SHOULD.

   Feature:  sasl-errors
   Description:  Support SASL errors during the negotiation process.
   Section:  Section 6.5
   Roles:  Client MUST, Server MUST.

   Feature:  sasl-mtn
   Description:  Consider SASL as mandatory-to-negotiate.
   Section:  Section 6.3.1
   Roles:  Client MUST, Server MUST.

   Feature:  sasl-restart
   Description:  Initiate or handle a stream restart after SASL
      negotiation.
   Section:  Section 6.3.2
   Roles:  Client MUST, Server MUST.

   Feature:  sasl-support
   Description:  Support the Simple Authentication and Security Layer
      for stream authentication.
   Section:  Section 6
   Roles:  Client MUST, Server MUST.

   Feature:  security-mti-auth-scram
   Description:  Support the SASL SCRAM mechanism for authentication
      only (this implies support for both the SCRAM-SHA-1 and
      SCRAM-SHA-1-PLUS variants).
   Section:  Section 13.8
   Roles:  Client MUST, Server MUST.



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   Feature:  security-mti-both-external
   Description:  Support TLS with SASL EXTERNAL for confidentiality and
      authentication.
   Section:  Section 13.8
   Roles:  Client SHOULD, Server MUST.

   Feature:  security-mti-both-plain
   Description:  Support TLS using the TLS_RSA_WITH_AES_128_CBC_SHA
      ciphersuite plus the SASL PLAIN mechanism for confidentiality and
      authentication.
   Section:  Section 13.8
   Roles:  Client SHOULD, Server MAY.

   Feature:  security-mti-both-scram
   Description:  Support TLS using the TLS_RSA_WITH_AES_128_CBC_SHA
      ciphersuite plus the SCRAM-SHA-1 and SCRAM-SHA-1-PLUS variants of
      the SASL SCRAM mechanism for confidentiality and authentication.
   Section:  Section 13.8
   Roles:  Client MUST, Server MUST.

   Feature:  security-mti-confidentiality
   Description:  Support TLS using the TLS_RSA_WITH_AES_128_CBC_SHA
      ciphersuite for confidentiality only.
   Section:  Section 13.8
   Roles:  Client N/A, Server SHOULD.

   Feature:  stanza-attribute-from
   Description:  Support the common 'from' attribute for all stanza
      kinds.
   Section:  Section 8.1.2
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-attribute-from-stamp
   Description:  Stamp or rewrite the 'from' address of all stanzas
      received from connected clients.
   Section:  Section 8.1.2.1
   Roles:  Client N/A, Server MUST.

   Feature:  stanza-attribute-from-validate
   Description:  Validate the 'from' address of all stanzas received
      from peer servers.
   Section:  Section 8.1.2.2
   Roles:  Client N/A, Server MUST.

   Feature:  stanza-attribute-id
   Description:  Support the common 'id' attribute for all stanza kinds.
   Section:  Section 8.1.3
   Roles:  Client MUST, Server MUST.



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   Feature:  stanza-attribute-to
   Description:  Support the common 'to' attribute for all stanza kinds.
   Section:  Section 8.1.1
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-attribute-to-validate
   Description:  Ensure that all stanzas received from peer servers
      include a 'to' address.
   Section:  Section 8.1.1
   Roles:  Client N/A, Server MUST.

   Feature:  stanza-attribute-type
   Description:  Support the common 'type' attribute for all stanza
      kinds.
   Section:  Section 8.1.4
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-attribute-xmllang
   Description:  Support the common 'xml:lang' attribute for all stanza
      kinds.
   Section:  Section 8.1.5
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-error
   Description:  Generate and handle stanzas of type "error" for all
      stanza kinds.
   Section:  Section 8.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-error-child
   Description:  Ensure that stanzas of type "error" include an <error/>
      child element.
   Section:  Section 8.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-error-id
   Description:  Ensure that stanzas of type "error" preserve the 'id'
      provided in the triggering stanza.
   Section:  Section 8.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-error-reply
   Description:  Do not reply to a stanza of type "error" with another
      stanza of type "error".
   Section:  Section 8.3
   Roles:  Client MUST, Server MUST.





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   Feature:  stanza-extension
   Description:  Correctly process XML data qualified by an unsupported
      XML namespace, where "correctly process" means to ignore that
      portion of the stanza in the case of a message or presence stanza
      and return an error in the case of an IQ stanza (for the intended
      recipient), and to route or deliver the stanza (for a routing
      entity such as a server).
   Section:  Section 8.4
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-iq-child
   Description:  Include exactly one child element in an <iq/> stanza of
      type "get" or "set", zero or one child elements in an <iq/> stanza
      of type "result", and one or two child elements in an <iq/> stanza
      of type "error".
   Section:  Section 8.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-iq-id
   Description:  Ensure that all <iq/> stanzas include an 'id'
      attribute.
   Section:  Section 8.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-iq-reply
   Description:  Reply to an <iq/> stanza of type "get" or "set" with an
      <iq/> stanza of type "result" or "error".
   Section:  Section 8.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-iq-type
   Description:  Ensure that all <iq/> stanzas include a 'type'
      attribute whose value is "get", "set", "result", or "error".
   Section:  Section 8.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-kind-iq
   Description:  Support the <iq/> stanza.
   Section:  Section 8.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-kind-message
   Description:  Support the <message/> stanza.
   Section:  Section 8.2.1
   Roles:  Client MUST, Server MUST.






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   Feature:  stanza-kind-presence
   Description:  Support the <presence/> stanza.
   Section:  Section 8.2.2
   Roles:  Client MUST, Server MUST.

   Feature:  stream-attribute-initial-from
   Description:  Include a 'from' attribute in the initial stream
      header.
   Section:  Section 4.7.1
   Roles:  Client SHOULD, Server MUST.

   Feature:  stream-attribute-initial-lang
   Description:  Include an 'xml:lang' attribute in the initial stream
      header.
   Section:  Section 4.7.4
   Roles:  Client SHOULD, Server SHOULD.

   Feature:  stream-attribute-initial-to
   Description:  Include a 'to' attribute in the initial stream header.
   Section:  Section 4.7.2
   Roles:  Client MUST, Server MUST.

   Feature:  stream-attribute-response-from
   Description:  Include a 'from' attribute in the response stream
      header.
   Section:  Section 4.7.1
   Roles:  Client N/A, Server MUST.

   Feature:  stream-attribute-response-id
   Description:  Include an 'id' attribute in the response stream
      header.
   Section:  Section 4.7.3
   Roles:  Client N/A, Server MUST.

   Feature:  stream-attribute-response-id-unique
   Description:  Ensure that the 'id' attribute in the response stream
      header is unique within the context of the receiving entity.
   Section:  Section 4.7.3
   Roles:  Client N/A, Server MUST.

   Feature:  stream-attribute-response-to
   Description:  Include a 'to' attribute in the response stream header.
   Section:  Section 4.7.2
   Roles:  Client N/A, Server SHOULD.







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   Feature:  stream-error-generate
   Description:  Generate a stream error (followed by a closing stream
      tag and termination of the TCP connection) upon detecting a
      stream-related error condition.
   Section:  Section 4.9
   Roles:  Client MUST, Server MUST.

   Feature:  stream-fqdn-resolution
   Description:  Resolve FQDNs before opening a TCP connection to the
      receiving entity.
   Section:  Section 3.2
   Roles:  Client MUST, Server MUST.

   Feature:  stream-negotiation-complete
   Description:  Do not consider the stream negotiation process to be
      complete until the receiving entity sends a stream features
      advertisement that is empty or that contains only voluntary-to-
      negotiate features.
   Section:  Section 4.3.5
   Roles:  Client MUST, Server MUST.

   Feature:  stream-negotiation-features
   Description:  Send stream features after sending a response stream
      header.
   Section:  Section 4.3.2
   Roles:  Client N/A, Server MUST.

   Feature:  stream-negotiation-restart
   Description:  Consider the previous stream to be replaced upon
      negotiation of a stream feature that necessitates a stream
      restart, and send or receive a new initial stream header after
      negotiation of such a stream feature.
   Section:  Section 4.3.3
   Roles:  Client MUST, Server MUST.

   Feature:  stream-reconnect
   Description:  Reconnect with exponential backoff if a TCP connection
      is terminated unexpectedly.
   Section:  Section 3.3
   Roles:  Client MUST, Server MUST.

   Feature:  stream-tcp-binding
   Description:  Bind an XML stream to a TCP connection.
   Section:  Section 3
   Roles:  Client MUST, Server MUST.






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   Feature:  tls-certs
   Description:  Check the identity specified in a certificate that is
      presented during TLS negotiation.
   Section:  Section 13.7.2
   Roles:  Client MUST, Server MUST.

   Feature:  tls-mtn
   Description:  Consider TLS as mandatory-to-negotiate if STARTTLS is
      the only feature advertised or if the STARTTLS feature
      advertisement includes an empty <required/> element.
   Section:  Section 5.3.1
   Roles:  Client MUST, Server MUST.

   Feature:  tls-restart
   Description:  Initiate or handle a stream restart after TLS
      negotiation.
   Section:  Section 5.3.2
   Roles:  Client MUST, Server MUST.

   Feature:  tls-support
   Description:  Support Transport Layer Security for stream encryption.
   Section:  Section 5
   Roles:  Client MUST, Server MUST.

   Feature:  tls-correlate
   Description:  When validating a certificate presented by a stream
      peer during TLS negotiation, correlate the validated identity with
      the 'from' address (if any) of the stream header it received from
      the peer.
   Section:  Section 13.7.2
   Roles:  Client SHOULD, Server SHOULD.

   Feature:  xml-namespace-content-client
   Description:  Support 'jabber:client' as a content namespace.
   Section:  Section 4.8.2
   Roles:  Client MUST, Server MUST.

   Feature:  xml-namespace-content-server
   Description:  Support 'jabber:server' as a content namespace.
   Section:  Section 4.8.2
   Roles:  Client N/A, Server MUST.

   Feature:  xml-namespace-streams-declaration
   Description:  Ensure that there is a namespace declaration for the
      'http://etherx.jabber.org/streams' namespace.
   Section:  Section 4.8.1
   Roles:  Client MUST, Server MUST.




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   Feature:  xml-namespace-streams-prefix
   Description:  Ensure that all elements qualified by the
      'http://etherx.jabber.org/streams' namespace are prefixed by the
      prefix (if any) defined in the namespace declaration.
   Section:  Section 4.8.1
   Roles:  Client MUST, Server MUST.

   Feature:  xml-restriction-comment
   Description:  Do not generate or accept XML comments.
   Section:  Section 11.1
   Roles:  Client MUST, Server MUST.

   Feature:  xml-restriction-dtd
   Description:  Do not generate or accept internal or external DTD
      subsets.
   Section:  Section 11.1
   Roles:  Client MUST, Server MUST.

   Feature:  xml-restriction-pi
   Description:  Do not generate or accept XML processing instructions.
   Section:  Section 11.1
   Roles:  Client MUST, Server MUST.

   Feature:  xml-restriction-ref
   Description:  Do not generate or accept internal or external entity
      references with the exception of the predefined entities.
   Section:  Section 11.1
   Roles:  Client MUST, Server MUST.

   Feature:  xml-wellformed-xml
   Description:  Do not generate or accept data that is not XML-well-
      formed.
   Section:  Section 11.3
   Roles:  Client MUST, Server MUST.

   Feature:  xml-wellformed-ns
   Description:  Do not generate or accept data that is not namespace-
      well-formed.
   Section:  Section 11.3
   Roles:  Client MUST, Server MUST.











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16.  References

16.1.  Normative References

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

   [CHANNEL]       Williams, N., "On the Use of Channel Bindings to
                   Secure Channels", RFC 5056, November 2007.

   [CHANNEL-TLS]   Altman, J., Williams, N., and L. Zhu, "Channel
                   Bindings for TLS", RFC 5929, July 2010.

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

   [DNS-CONCEPTS]  Mockapetris, P., "Domain names - concepts and
                   facilities", STD 13, RFC 1034, November 1987.

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

   [IPv6-ADDR]     Kawamura, S. and M. Kawashima, "A Recommendation for
                   IPv6 Address Text Representation", RFC 5952,
                   August 2010.

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

   [LANGMATCH]     Phillips, A. and M. Davis, "Matching of Language
                   Tags", BCP 47, RFC 4647, September 2006.

   [LANGTAGS]      Phillips, A. and M. Davis, "Tags for Identifying
                   Languages", BCP 47, RFC 5646, September 2009.

   [OCSP]          Myers, M., Ankney, R., Malpani, A., Galperin, S., and
                   C. Adams, "X.509 Internet Public Key Infrastructure
                   Online Certificate Status Protocol - OCSP", RFC 2560,
                   June 1999.

   [PKIX]          Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
                   Housley, R., and W. Polk, "Internet X.509 Public Key
                   Infrastructure Certificate and Certificate Revocation
                   List (CRL) Profile", RFC 5280, May 2008.






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   [PKIX-ALGO]     Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                   Standards (PKCS) #1: RSA Cryptography Specifications
                   Version 2.1", RFC 3447, February 2003.

   [PKIX-SRV]      Santesson, S., "Internet X.509 Public Key
                   Infrastructure Subject Alternative Name for
                   Expression of Service Name", RFC 4985, August 2007.

   [PLAIN]         Zeilenga, K., "The PLAIN Simple Authentication and
                   Security Layer (SASL) Mechanism", RFC 4616,
                   August 2006.

   [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.

   [SCRAM]         Newman, C., Menon-Sen, A., Melnikov, A., and N.
                   Williams, "Salted Challenge Response Authentication
                   Mechanism (SCRAM) SASL and GSS-API Mechanisms",
                   RFC 5802, July 2010.

   [STRONGSEC]     Schiller, J., "Strong Security Requirements for
                   Internet Engineering Task Force Standard Protocols",
                   BCP 61, RFC 3365, August 2002.

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

   [TLS]           Dierks, T. and E. Rescorla, "The Transport Layer
                   Security (TLS) Protocol Version 1.2", RFC 5246,
                   August 2008.

   [TLS-CERTS]     Saint-Andre, P. and J. Hodges, "Representation and
                   Verification of Domain-Based Application Service
                   Identity within Internet Public Key Infrastructure
                   Using X.509 (PKIX) Certificates in the Context of
                   Transport Layer Security (TLS)", RFC 6125,
                   March 2011.

   [TLS-NEG]       Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
                   "Transport Layer Security (TLS) Renegotiation
                   Indication Extension", RFC 5746, February 2010.

   [TLS-SSL2]      Turner, S. and T. Polk, "Prohibiting Secure Sockets
                   Layer (SSL) Version 2.0", RFC 6176, March 2011.



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   [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.

   [UNICODE]       The Unicode Consortium, "The Unicode Standard,
                   Version 6.0", 2010,
                   <http://www.unicode.org/versions/Unicode6.0.0/>.

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

   [URI]           Berners-Lee, T., Fielding, R., and L. Masinter,
                   "Uniform Resource Identifier (URI): Generic Syntax",
                   STD 66, RFC 3986, January 2005.

   [X509]          International Telecommunications Union, "Information
                   technology - Open Systems Interconnection - The
                   Directory: Public-key and attribute certificate
                   frameworks", ITU-T Recommendation X.509, ISO Standard
                   9594-8, March 2000.

   [XML]           Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli,
                   J., and T. Bray, "Extensible Markup Language (XML)
                   1.0 (Fifth Edition)", World Wide Web Consortium
                   Recommendation REC-xml-20081126, November 2008,
                   <http://www.w3.org/TR/2008/REC-xml-20081126>.

   [XML-GUIDE]     Hollenbeck, S., Rose, M., and L. Masinter,
                   "Guidelines for the Use of Extensible Markup Language
                   (XML) within IETF Protocols", BCP 70, RFC 3470,
                   January 2003.

   [XML-MEDIA]     Murata, M., St. Laurent, S., and D. Kohn, "XML Media
                   Types", RFC 3023, January 2001.

   [XML-NAMES]     Thompson, H., Hollander, D., Layman, A., Bray, T.,
                   and R. Tobin, "Namespaces in XML 1.0 (Third
                   Edition)", World Wide Web Consortium
                   Recommendation REC-xml-names-20091208, December 2009,
                   <http://www.w3.org/TR/2009/REC-xml-names-20091208>.

   [XMPP-ADDR]     Saint-Andre, P., "Extensible Messaging and Presence
                   Protocol (XMPP): Address Format", RFC 6122,
                   March 2011.





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   [XMPP-IM]       Saint-Andre, P., "Extensible Messaging and Presence
                   Protocol (XMPP): Instant Messaging and Presence",
                   RFC 6121, March 2011.

16.2.  Informative References

   [AAA]           Housley, R. and B. Aboba, "Guidance for
                   Authentication, Authorization, and Accounting (AAA)
                   Key Management", BCP 132, RFC 4962, July 2007.

   [ABNF]          Crocker, D. and P. Overell, "Augmented BNF for Syntax
                   Specifications: ABNF", STD 68, RFC 5234,
                   January 2008.

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

   [ANONYMOUS]     Zeilenga, K., "Anonymous Simple Authentication and
                   Security Layer (SASL) Mechanism", RFC 4505,
                   June 2006.

   [ASN.1]         CCITT, "Recommendation X.208: Specification of
                   Abstract Syntax Notation One (ASN.1)", 1988.

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

   [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.

   [E2E-REQS]      Saint-Andre, P., "Requirements for End-to-End
                   Encryption in the Extensible Messaging and Presence
                   Protocol (XMPP)", Work in Progress, March 2010.

   [EMAIL-ARCH]    Crocker, D., "Internet Mail Architecture", RFC 5598,
                   July 2009.







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   [ETHERNET]      "Information technology - Telecommunications and
                   information exchange between systems - Local and
                   metropolitan area networks - Specific requirements -
                   Part 3: Carrier sense multiple access with collision
                   detection (CSMA/CD) access method and physical layer
                   specifications", IEEE Standard 802.3, September 1998.

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

   [HASHES]        Hoffman, P. and B. Schneier, "Attacks on
                   Cryptographic Hashes in Internet Protocols",
                   RFC 4270, November 2005.

   [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.

   [IANA-GUIDE]    Narten, T. and H. Alvestrand, "Guidelines for Writing
                   an IANA Considerations Section in RFCs", BCP 26,
                   RFC 5226, May 2008.

   [IANA-PORTS]    Cotton, M., Eggert, L., Touch, J., Westerlund, M.,
                   and S. Cheshire, "Internet Assigned Numbers Authority
                   (IANA) Procedures for the Management of the Transport
                   Protocol Port Number and Service Name Registry", Work
                   in Progress, February 2011.

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

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

   [INTEROP]       Masinter, L., "Formalizing IETF Interoperability
                   Reporting", Work in Progress, October 2005.

   [IRC]           Kalt, C., "Internet Relay Chat: Architecture",
                   RFC 2810, April 2000.

   [IRI]           Duerst, M. and M. Suignard, "Internationalized
                   Resource Identifiers (IRIs)", RFC 3987, January 2005.






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   [LDAP]          Zeilenga, K., "Lightweight Directory Access Protocol
                   (LDAP): Technical Specification Road Map", RFC 4510,
                   June 2006.

   [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.

   [PROCESS]       Bradner, S., "The Internet Standards Process --
                   Revision 3", BCP 9, RFC 2026, October 1996.

   [REPORTS]       Dusseault, L. and R. Sparks, "Guidance on
                   Interoperation and Implementation Reports for
                   Advancement to Draft Standard", BCP 9, RFC 5657,
                   September 2009.

   [REST]          Fielding, R., "Architectural Styles and the Design of
                   Network-based Software Architectures",  2000.

   [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.

   [SASLPREP]      Zeilenga, K., "SASLprep: Stringprep Profile for User
                   Names and Passwords", RFC 4013, February 2005.

   [SEC-TERMS]     Shirey, R., "Internet Security Glossary, Version 2",
                   RFC 4949, August 2007.

   [SMTP]          Klensin, J., "Simple Mail Transfer Protocol",
                   RFC 5321, October 2008.

   [SEC-GUIDE]     Rescorla, E. and B. Korver, "Guidelines for Writing
                   RFC Text on Security Considerations", BCP 72,
                   RFC 3552, July 2003.






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   [TLS-EXT]       Eastlake 3rd, D., "Transport Layer Security (TLS)
                   Extensions: Extension Definitions", RFC 6066,
                   January 2011.

   [TLS-RESUME]    Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
                   "Transport Layer Security (TLS) Session Resumption
                   without Server-Side State", RFC 5077, January 2008.

   [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.

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

   [XEP-0001]      Saint-Andre, P., "XMPP Extension Protocols", XSF
                   XEP 0001, March 2010.

   [XEP-0016]      Millard, P. and P. Saint-Andre, "Privacy Lists", XSF
                   XEP 0016, February 2007.

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

   [XEP-0060]      Millard, P., Saint-Andre, P., and R. Meijer,
                   "Publish-Subscribe", XSF XEP 0060, July 2010.

   [XEP-0071]      Saint-Andre, P., "XHTML-IM", XSF XEP 0071,
                   September 2008.

   [XEP-0077]      Saint-Andre, P., "In-Band Registration", XSF
                   XEP 0077, September 2009.

   [XEP-0086]      Norris, R. and P. Saint-Andre, "Error Condition
                   Mappings", XSF XEP 0086, February 2004.

   [XEP-0100]      Saint-Andre, P. and D. Smith, "Gateway Interaction",
                   XSF XEP 0100, October 2005.

   [XEP-0114]      Saint-Andre, P., "Jabber Component Protocol", XSF
                   XEP 0114, March 2005.

   [XEP-0124]      Paterson, I., Smith, D., and P. Saint-Andre,
                   "Bidirectional-streams Over Synchronous HTTP (BOSH)",
                   XSF XEP 0124, July 2010.



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   [XEP-0138]      Hildebrand, J. and P. Saint-Andre, "Stream
                   Compression", XSF XEP 0138, May 2009.

   [XEP-0156]      Hildebrand, J. and P. Saint-Andre, "Discovering
                   Alternative XMPP Connection Methods", XSF XEP 0156,
                   June 2007.

   [XEP-0160]      Saint-Andre, P., "Best Practices for Handling Offline
                   Messages", XSF XEP 0160, January 2006.

   [XEP-0174]      Saint-Andre, P., "Link-Local Messaging", XSF
                   XEP 0174, November 2008.

   [XEP-0175]      Saint-Andre, P., "Best Practices for Use of SASL
                   ANONYMOUS", XSF XEP 0175, September 2009.

   [XEP-0178]      Saint-Andre, P. and P. Millard, "Best Practices for
                   Use of SASL EXTERNAL with Certificates", XSF
                   XEP 0178, February 2007.

   [XEP-0191]      Saint-Andre, P., "Simple Communications Blocking",
                   XSF XEP 0191, February 2007.

   [XEP-0198]      Karneges, J., Hildebrand, J., Saint-Andre, P., Forno,
                   F., Cridland, D., and M. Wild, "Stream Management",
                   XSF XEP 0198, February 2011.

   [XEP-0199]      Saint-Andre, P., "XMPP Ping", XSF XEP 0199,
                   June 2009.

   [XEP-0205]      Saint-Andre, P., "Best Practices to Discourage Denial
                   of Service Attacks", XSF XEP 0205, January 2009.

   [XEP-0206]      Paterson, I. and P. Saint-Andre, "XMPP Over BOSH",
                   XSF XEP 0206, July 2010.

   [XEP-0220]      Miller, J., Saint-Andre, P., and P. Hancke, "Server
                   Dialback", XSF XEP 0220, March 2010.

   [XEP-0225]      Saint-Andre, P., "Component Connections", XSF
                   XEP 0225, October 2008.

   [XEP-0233]      Miller, M., Saint-Andre, P., and J. Hildebrand,
                   "Domain-Based Service Names in XMPP SASL
                   Negotiation", XSF XEP 0233, June 2010.

   [XEP-0288]      Hancke, P. and D. Cridland, "Bidirectional Server-to-
                   Server Connections", XSF XEP 0288, October 2010.



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   [XML-FRAG]      Grosso, P. and D. Veillard, "XML Fragment
                   Interchange", World Wide Web Consortium CR CR-xml-
                   fragment-20010212, February 2001,
                   <http://www.w3.org/TR/2001/CR-xml-fragment-20010212>.

   [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-URI]      Saint-Andre, P., "Internationalized Resource
                   Identifiers (IRIs) and Uniform Resource Identifiers
                   (URIs) for the Extensible Messaging and Presence
                   Protocol (XMPP)", RFC 5122, February 2008.
































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Appendix A.  XML Schemas

   The following schemas formally define various namespaces used in this
   document, in conformance with [XML-SCHEMA].  Because validation of
   XML streams and stanzas is optional, these schemas are not normative
   and are provided for descriptive purposes only.

A.1.  Stream Namespace

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

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

     <xs:import namespace='jabber:client'/>
     <xs:import namespace='jabber:server'/>
     <xs:import namespace='urn:ietf:params:xml:ns:xmpp-sasl'/>
     <xs:import namespace='urn:ietf:params:xml:ns:xmpp-streams'/>
     <xs:import namespace='urn:ietf:params:xml:ns:xmpp-tls'/>

     <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='1'/>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-sasl'
                   minOccurs='0'
                   maxOccurs='1'/>
           <xs:any namespace='##other'
                   minOccurs='0'
                   maxOccurs='unbounded'
                   processContents='lax'/>
           <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>





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             <xs:choice minOccurs='0' maxOccurs='unbounded'>
               <xs:element ref='server:message'/>
               <xs:element ref='server:presence'/>
               <xs:element ref='server:iq'/>
             </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'/>
         <xs:attribute ref='xml:lang' use='optional'/>
         <xs:anyAttribute namespace='##other' processContents='lax'/>
       </xs:complexType>
     </xs:element>

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

   </xs:schema>







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A.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'/>
     <xs:element name='not-authorized' type='empty'/>
     <xs:element name='not-well-formed' type='empty'/>
     <xs:element name='policy-violation' type='empty'/>
     <xs:element name='remote-connection-failed' type='empty'/>
     <xs:element name='reset' 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: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'/>



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         <xs:element ref='invalid-namespace'/>
         <xs:element ref='invalid-xml'/>
         <xs:element ref='not-authorized'/>
         <xs:element ref='not-well-formed'/>
         <xs:element ref='policy-violation'/>
         <xs:element ref='remote-connection-failed'/>
         <xs:element ref='reset'/>
         <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: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>

A.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'>






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     <xs:element name='starttls'>
       <xs:complexType>
         <xs:choice minOccurs='0' maxOccurs='1'>
           <xs:element name='required' type='empty'/>
         </xs:choice>
       </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>

A.4.  SASL Namespace

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

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

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

     <xs:element name='abort' type='empty'/>





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

     <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:sequence>
           <xs:choice minOccurs='0'>
             <xs:element name='aborted' type='empty'/>
             <xs:element name='account-disabled' type='empty'/>
             <xs:element name='credentials-expired' type='empty'/>
             <xs:element name='encryption-required' 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'/>
             <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:element ref='text' minOccurs='0' maxOccurs='1'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <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>



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

   </xs:schema>

A.5.  Client Namespace

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

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

     <xs:import
         namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>

     <xs:element name='message'>
        <xs:complexType>
           <xs:sequence>
             <xs:choice minOccurs='0' maxOccurs='unbounded'>
               <xs:element ref='subject'/>
               <xs:element ref='body'/>
               <xs:element ref='thread'/>
             </xs:choice>
             <xs:any     namespace='##other'
                         minOccurs='0'
                         maxOccurs='unbounded'
                         processContents='lax'/>
             <xs:element ref='error'
                         minOccurs='0'/>
           </xs:sequence>
           <xs:attribute name='from'
                         type='xs:string'
                         use='optional'/>
           <xs:attribute name='id'
                         type='xs:NMTOKEN'
                         use='optional'/>
           <xs:attribute name='to'
                         type='xs:string'
                         use='optional'/>
           <xs:attribute name='type'
                         use='optional'
                         default='normal'>



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

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

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

     <xs:element name='thread'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='parent'
                           type='xs:NMTOKEN'
                           use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>






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     <xs:element name='presence'>
       <xs:complexType>
         <xs:sequence>
           <xs:choice minOccurs='0' maxOccurs='unbounded'>
             <xs:element ref='show'/>
             <xs:element ref='status'/>
             <xs:element ref='priority'/>
           </xs:choice>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='unbounded'
                       processContents='lax'/>
           <xs:element ref='error'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='optional'/>
         <xs:attribute name='to'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='type' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NMTOKEN'>
               <xs:enumeration value='error'/>
               <xs:enumeration value='probe'/>
               <xs:enumeration value='subscribe'/>
               <xs:enumeration value='subscribed'/>
               <xs:enumeration value='unavailable'/>
               <xs:enumeration value='unsubscribe'/>
               <xs:enumeration value='unsubscribed'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>











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

     <xs:element name='status'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='string1024'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:simpleType name='string1024'>
       <xs:restriction base='xs:string'>
         <xs:minLength value='1'/>
         <xs:maxLength value='1024'/>
       </xs:restriction>
     </xs:simpleType>

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

     <xs:element name='iq'>
       <xs:complexType>
         <xs:sequence>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='1'
                       processContents='lax'/>
           <xs:element ref='error'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='required'/>





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         <xs:attribute name='to'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NMTOKEN'>
               <xs:enumeration value='error'/>
               <xs:enumeration value='get'/>
               <xs:enumeration value='result'/>
               <xs:enumeration value='set'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:sequence xmlns:err='urn:ietf:params:xml:ns:xmpp-stanzas'>
           <xs:group ref='err:stanzaErrorGroup'/>
           <xs:element ref='err:text'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='by'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NMTOKEN'>
               <xs:enumeration value='auth'/>
               <xs:enumeration value='cancel'/>
               <xs:enumeration value='continue'/>
               <xs:enumeration value='modify'/>
               <xs:enumeration value='wait'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>









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A.6.  Server Namespace

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

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

     <xs:import
         namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>

     <xs:element name='message'>
        <xs:complexType>
           <xs:sequence>
             <xs:choice minOccurs='0' maxOccurs='unbounded'>
               <xs:element ref='subject'/>
               <xs:element ref='body'/>
               <xs:element ref='thread'/>
             </xs:choice>
             <xs:any namespace='##other'
                     minOccurs='0'
                     maxOccurs='unbounded'
                     processContents='lax'/>
             <xs:element ref='error'
                         minOccurs='0'/>
           </xs:sequence>
           <xs:attribute name='from'
                         type='xs:string'
                         use='required'/>
           <xs:attribute name='id'
                         type='xs:NMTOKEN'
                         use='optional'/>
           <xs:attribute name='to'
                         type='xs:string'
                         use='required'/>
           <xs:attribute name='type'
                         use='optional'
                         default='normal'>
             <xs:simpleType>
               <xs:restriction base='xs:NMTOKEN'>
                 <xs:enumeration value='chat'/>
                 <xs:enumeration value='error'/>
                 <xs:enumeration value='groupchat'/>
                 <xs:enumeration value='headline'/>
                 <xs:enumeration value='normal'/>
               </xs:restriction>



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

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

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

     <xs:element name='thread'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:NMTOKEN'>
             <xs:attribute name='parent'
                           type='xs:NMTOKEN'
                           use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>














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

     <xs:element name='presence'>
       <xs:complexType>
         <xs:sequence>
           <xs:choice minOccurs='0' maxOccurs='unbounded'>
             <xs:element ref='show'/>
             <xs:element ref='status'/>
             <xs:element ref='priority'/>
           </xs:choice>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='unbounded'
                       processContents='lax'/>
           <xs:element ref='error'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='optional'/>
         <xs:attribute name='to'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='type' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NMTOKEN'>
               <xs:enumeration value='error'/>
               <xs:enumeration value='probe'/>
               <xs:enumeration value='subscribe'/>
               <xs:enumeration value='subscribed'/>
               <xs:enumeration value='unavailable'/>
               <xs:enumeration value='unsubscribe'/>
               <xs:enumeration value='unsubscribed'/>
             </xs:restriction>
           </xs:simpleType>



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

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

     <xs:element name='status'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='string1024'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:simpleType name='string1024'>
       <xs:restriction base='xs:string'>
         <xs:minLength value='1'/>
         <xs:maxLength value='1024'/>
       </xs:restriction>
     </xs:simpleType>

     <xs:element name='priority' type='xs:byte' default='0'/>

     <xs:element name='iq'>
       <xs:complexType>
         <xs:sequence>
           <xs:any namespace='##other'
                   minOccurs='0'
                   maxOccurs='1'
                   processContents='lax'/>
           <xs:element ref='error'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='from'
                       type='xs:string'
                       use='required'/>



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         <xs:attribute name='id'
                       type='xs:NMTOKEN'
                       use='required'/>
         <xs:attribute name='to'
                       type='xs:string'
                       use='required'/>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NMTOKEN'>
               <xs:enumeration value='error'/>
               <xs:enumeration value='get'/>
               <xs:enumeration value='result'/>
               <xs:enumeration value='set'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:sequence xmlns:err='urn:ietf:params:xml:ns:xmpp-stanzas'>
           <xs:group ref='err:stanzaErrorGroup'/>
           <xs:element ref='err:text'
                       minOccurs='0'/>
         </xs:sequence>
         <xs:attribute name='by'
                       type='xs:string'
                       use='optional'/>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NMTOKEN'>
               <xs:enumeration value='auth'/>
               <xs:enumeration value='cancel'/>
               <xs:enumeration value='continue'/>
               <xs:enumeration value='modify'/>
               <xs:enumeration value='wait'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>






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A.7.  Resource Binding Namespace

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

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

     <xs:element name='bind'>
       <xs:complexType>
         <xs:choice>
           <xs:element name='resource' type='resourceType'/>
           <xs:element name='jid' type='fullJIDType'/>
         </xs:choice>
       </xs:complexType>
     </xs:element>

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

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

   </xs:schema>

A.8.  Stanza Error Namespace

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

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

     <xs:element name='bad-request' type='empty'/>
     <xs:element name='conflict' type='empty'/>
     <xs:element name='feature-not-implemented' type='empty'/>



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     <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-authorized' type='empty'/>
     <xs:element name='policy-violation' 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: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='policy-violation'/>
         <xs:element ref='recipient-unavailable'/>
         <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:choice>
     </xs:group>





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     <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 B.  Contact Addresses

   Consistent with [MAILBOXES], organization that offer XMPP services
   are encouraged to provide an Internet mailbox of "XMPP" for inquiries
   related to that service, where the host portion of the resulting
   mailto URI is the organization's domain, not the domain of the XMPP
   service itself (e.g., the XMPP service might be offered at
   im.example.com but the Internet mailbox would be <xmpp@example.com>).

Appendix C.  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].  An XMPP
   server implementation or administrative function MUST ensure that any
   JID assigned during account provisioning (including localpart,
   domainpart, resourcepart, and separator characters) conforms to the
   canonical format for XMPP addresses defined in [XMPP-ADDR].

Appendix D.  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 (in addition to
   numerous changes of an editorial nature).

   o  Moved specification of the XMPP address format to a separate
      document.




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   o  Recommended or mandated use of the 'from' and 'to' attributes on
      stream headers.

   o  More fully specified the stream closing handshake.

   o  Specified the recommended stream reconnection algorithm.

   o  Changed the name of the <xml-not-well-formed/> stream error
      condition to <not-well-formed/> for compliance with the XML
      specification.

   o  Removed the unnecessary and unused <invalid-id/> stream error (see
      RFC 3920 for historical documentation).

   o  Specified return of the <restricted-xml/> stream error in response
      to receipt of prohibited XML features.

   o  More completely specified the format and handling of the <see-
      other-host/> stream error, including consistency with RFC 3986 and
      RFC 5952 with regard to IPv6 addresses (e.g., enclosing the IPv6
      address in square brackets '[' and ']').

   o  Specified that the SASL SCRAM mechanism is a mandatory-to-
      implement technology for client-to-server streams.

   o  Specified that TLS plus the SASL PLAIN mechanism is a mandatory-
      to-implement technology for client-to-server streams.

   o  Specified that support for the SASL EXTERNAL mechanism is required
      for servers but only recommended for clients (since end-user X.509
      certificates are difficult to obtain and not yet widely deployed).

   o  Removed the hard two-connection rule for server-to-server streams.

   o  More clearly specified the certificate profile for both public key
      certificates and issuer certificates.

   o  Added the <reset/> stream error (Section 4.9.3.16) condition to
      handle expired/revoked certificates or the addition of security-
      critical features to an existing stream.

   o  Added the <account-disabled/>, <credentials-expired/>,
      <encryption-required/>, and <malformed-request/> SASL error
      conditions to handle error flows mistakenly left out of RFC 3920
      or discussed in RFC 4422 but not in RFC 2222.

   o  Removed the unused <payment-required/> stanza error.




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   o  Removed the unnecessary requirement for escaping of characters
      that map to certain predefined entities, since they do not need to
      be escaped in XML.

   o  Clarified the process of DNS SRV lookups and fallbacks.

   o  Clarified the handling of SASL security layers.

   o  Clarified that a SASL simple user name is the localpart, not the
      bare JID.

   o  Clarified the stream negotiation process and associated flow
      chart.

   o  Clarified the handling of stream features.

   o  Added a 'by' attribute to the <error/> element for stanza errors
      so that the entity that has detected the error can include its JID
      for diagnostic or tracking purposes.

   o  Clarified the handling of data that violates the well-formedness
      definitions for XML 1.0 and XML namespaces.

   o  Specified the security considerations in more detail, especially
      with regard to presence leaks and denial-of-service attacks.

   o  Moved documentation of the Server Dialback protocol from this
      specification to a separate specification maintained by the XMPP
      Standards Foundation.

Appendix E.  Acknowledgements

   This document is an update to, and derived from, RFC 3920.  This
   document would have been impossible without the work of the
   contributors and commenters acknowledged there.

   Hundreds of people have provided implementation feedback, bug
   reports, requests for clarification, and suggestions for improvement
   since publication of RFC 3920.  Although the document editor has
   endeavored to address all such feedback, he is solely responsible for
   any remaining errors and ambiguities.

   Special thanks are due to Kevin Smith, Matthew Wild, Dave Cridland,
   Philipp Hancke, Waqas Hussain, Florian Zeitz, Ben Campbell, Jehan
   Pages, Paul Aurich, Justin Karneges, Kurt Zeilenga, Simon Josefsson,
   Ralph Meijer, Curtis King, and others for their comments during
   Working Group Last Call.




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   Thanks also to Yaron Sheffer and Elwyn Davies for their reviews on
   behalf of the Security Directorate and the General Area Review Team,
   respectively.

   The Working Group chairs were Ben Campbell and Joe Hildebrand.  The
   responsible Area Director was Gonzalo Camarillo.

Author's Address

   Peter Saint-Andre
   Cisco
   1899 Wyknoop Street, Suite 600
   Denver, CO  80202
   USA

   Phone: +1-303-308-3282
   EMail: psaintan@cisco.com


































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