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.
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
 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
 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
 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
 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
 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
 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.
 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
 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.
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.
 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
 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
 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>
 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.
 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.
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.
 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.
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.
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
 <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.
 +--------------------+--------------------+
 | 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]).
 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.
 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
 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
 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.
 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
 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.
 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.
 +---------------------+
 | 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
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.
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
 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
 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
 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.
 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'>
 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
 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).
 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.
 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,
 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.,
 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.
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).
 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
 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
 (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:
 <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'>
 <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.
 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>
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.
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
 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'>
 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>
 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'>
 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>
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>
 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.)
 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>
 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>
 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
 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.)
 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'>
 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).
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'>
 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).
 (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年12月13日T18:30:02Z</published>
 <updated>2003年12月13日T18:30:02Z</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'>
 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>
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>
 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],
 [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.
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
 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.
 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.
 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].
 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.
 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).
 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.
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
 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
 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>).
 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.
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:
 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>
 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.
 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
 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).
 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).
 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>
 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>
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>
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
 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.
 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.
 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.
 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>
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.
 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.
 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>
 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).
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.
 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
 '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.
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.
 <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.
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):
 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.
 * 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
 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).
 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
 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,
 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".
 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>
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>
 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".
 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>
 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>
 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>
 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>
 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
 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>
 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>
 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>
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".
 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>
 <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]).
 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.
 <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.
 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'>
 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).
 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".
 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>
 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>
 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>
 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'>
 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'>
 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>
 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.)
 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.
 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).
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
 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.
 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.
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].
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.
 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.
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.
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
 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.
 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
 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.
 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
 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.
 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.
 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."
 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"
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.
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).
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.
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.
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
 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.
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.
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.
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.
 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.
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
 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
 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].
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.
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>
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
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
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.
 [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.
 [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.
 [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.
 [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.
 [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.
 [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.
 [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.
 [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.
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>
 <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>
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'/>
 <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'>
 <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'/>
 <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>
 <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'>
 <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>
 <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>
 <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'/>
 <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>
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>
 </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>
 <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>
 </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'/>
 <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>
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'/>
 <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>
 <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.
 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.
 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.
 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