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Extensible Messaging and Presence Protocol (XMPP): Core :: RFC6120

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.

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

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Table of Contents

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

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

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

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

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

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

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

1. Introduction

1.1. Overview

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

1.2. History

The basic syntax and semantics of XMPP were developed originally
within the Jabber open-source community, mainly in 1999. In late
2002, the XMPP Working Group was chartered with developing an
adaptation of the base Jabber protocol that would be suitable as an
IETF instant messaging (IM) and presence technology in accordance
with [IMP-REQS]. In October 2004, [RFC3920] and [RFC3921] were
published, representing the most complete definition of XMPP at that
time.

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

1.3. Functional Summary

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

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

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

2. Open a Transmission Control Protocol [TCP] connection

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

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

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

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

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

8. Close the XML stream (Section 4.4)

9. Close the TCP connection

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

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

2. Open a TCP connection

3. Open an XML stream (Section 4.2)

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

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

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

7. Close the XML stream (Section 4.4)

8. Close the TCP connection

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

This document specifies how clients connect to servers and specifies
the basic semantics of XML stanzas. However, this document does not
define the "payloads" of the XML stanzas that might be exchanged once
a connection is successfully established; instead, those payloads are
defined by various XMPP extensions. For example, [XMPP-IM] defines
extensions for basic instant messaging and presence functionality.
In addition, various specifications produced in the XSF's XEP series
[XEP-0001] define extensions for a wide range of applications.

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1.4. Terminology

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

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

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

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

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

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

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

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

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

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

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

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

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

deliver: pass the data to a connected client

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

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

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

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

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

o C: = a client

o E: = any XMPP entity

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o I: = an initiating entity

o P: = a peer server

o R: = a receiving entity

o S: = a server

o S1: = server1

o S2: = server2

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

2. Architecture

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

2.1. Global Addresses

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

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particular connected device or resource that is currently authorized
for interaction on behalf of an account is of the form
(e.g.,
, 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 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

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associated with the server might be able to exchange
messages, presence, and other structured data with the user
associated with the server . 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., vs.
), as defined under [XMPP-ADDR] and
Section 7.

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A server is an entity whose primary responsibilities are to:

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

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

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

o Storing data that is used by clients (e.g., contact lists for
users of XMPP-based instant messaging and presence applications as
defined in [XMPP-IM]); in this case, the relevant XML stanza is
handled directly by the server itself on behalf of the client and
is not routed to a remote server or delivered to a connected
client.

o Hosting add-on services that also use XMPP as the basis for
communication but that provide additional functionality beyond
that defined in this document or in [XMPP-IM]; examples include
multi-user conferencing services as specified in [XEP-0045] and
publish-subscribe services as specified in [XEP-0060].

3. TCP Binding

3.1. Scope

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

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

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3.2. Resolution of Fully Qualified Domain Names

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

3.2.1. Preferred Process: SRV Lookup

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

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

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

* a Proto of "tcp"

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

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

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

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

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

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

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

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

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

3.2.2. Fallback Processes

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

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

3.2.3. When Not to Use SRV

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

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3.2.4. Use of SRV Records with Add-On Services

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

3.3. Reconnection

It can happen that an XMPP server goes offline unexpectedly while
servicing TCP connections from connected clients and remote servers.
Because the number of such connections can be quite large, the
reconnection algorithm employed by entities that seek to reconnect
can have a significant impact on software performance and network
congestion. If an entity chooses to reconnect, it:

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

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

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

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3.4. Reliability

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

4. XML Streams

4.1. Stream Fundamentals

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

Definition of XML Stream: An XML stream is a container for the
exchange of XML elements between any two entities over a network.
The start of an XML stream is denoted unambiguously by an opening
"stream header" (i.e., an XML tag with appropriate
attributes and namespace declarations), while the end of the XML
stream is denoted unambiguously by a closing XML tag.
During the life of the stream, the entity that initiated it can
send an unbounded number of XML elements over the stream, either
elements used to negotiate the stream (e.g., to complete TLS
negotiation (Section 5) or SASL negotiation (Section 6)) or XML
stanzas. The "initial stream" is negotiated from the initiating
entity (typically a client or server) to the receiving entity
(typically a server), and can be seen as corresponding to the
initiating entity's "connection to" or "session with" the
receiving entity. The initial stream enables unidirectional
communication from the initiating entity to the receiving entity;
in order to enable exchange of stanzas from the receiving entity
to the initiating entity, the receiving entity MUST negotiate a
stream in the opposite direction (the "response stream").

Definition of XML Stanza: An XML stanza is the basic unit of meaning
in XMPP. A stanza is a first-level element (at depth=1 of the
stream) whose element name is "message", "presence", or "iq" and
whose qualifying namespace is 'jabber:client' or 'jabber:server'.
By contrast, a first-level element qualified by any other
namespace is not an XML stanza (stream errors, stream features,
TLS-related elements, SASL-related elements, etc.), nor is a

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, , or element that is qualified by the
'jabber:client' or 'jabber:server' namespace but that occurs at a
depth other than one (e.g., a element contained within
an extension element (Section 8.4) for reporting purposes), nor is
a , , or 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 tag to the server as further
described under Section 4.4.

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

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+--------------------+--------------------+
| INITIAL STREAM | RESPONSE STREAM |
+--------------------+--------------------+
| | |
|--------------------|--------------------|
| | |
|--------------------|--------------------|
| | |
| | |
| | |
|--------------------|--------------------|
| | |
| | |
| | |
|--------------------|--------------------|
| | |
| | |
| | |
|--------------------|--------------------|
| | |
| | |
| | |
|--------------------|--------------------|
| [ ... ] | |
|--------------------|--------------------|
| | [ ... ] |
|--------------------|--------------------|
| | |
|--------------------|--------------------|
| | |
+--------------------+--------------------+

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 element is like the "document entity" for each
"document" (as described in Section 4.8 of [XML]).

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

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

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

o How to open a stream (Section 4.2)

o The stream negotiation process (Section 4.3)

o How to close a stream (Section 4.4)

o The directionality of XML streams (Section 4.5)

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

o The XML attributes of a stream (Section 4.7)

o The XML namespaces of a stream (Section 4.8)

o Error handling related to XML streams (Section 4.9)

4.2. Opening a Stream

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

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

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

4.3. Stream Negotiation

4.3.1. Basic Concepts

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

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

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

4.3.2. Stream Features Format

If the initiating entity includes in the initial stream header the
'version' attribute set to a value of at least "1.0" (see
Section 4.7.5), after sending the response stream header the
receiving entity MUST send a 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
element, qualified by a namespace that is different from the stream
namespace and the content namespace. The element can
contain one child, contain multiple children, or be empty.

Implementation Note: The order of child elements contained in any
given 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 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 element as is done for
STARTTLS.

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

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

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

A 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.

A 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 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).

zlib
lzw

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

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zlib
lzw

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

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 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
element or a element that contains only
voluntary-to-negotiate features. After doing so, the receiving
entity MAY send an empty 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
stream error (Section 4.9.3.16) and then advertise the new
features when the initiating entity reconnects, preferably closing

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

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

The initiating entity MUST NOT attempt to send XML stanzas
(Section 8) to entities other than itself (i.e., the client's
connected resource or any other authenticated resource of the
client's account) or the server to which it is connected until stream
negotiation has been completed. Even if the initiating entity does
attempt to do so, the receiving entity MUST NOT accept such stanzas
and MUST close the stream with a stream error
(Section 4.9.3.12). This rule applies to XML stanzas only (i.e.,
, , and 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
() 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
() MUST be the resource negotiated
by the client and server during resource binding (Section 7). A
client MUST NOT attempt to guess at its JID but instead MUST consider
its JID to be whatever the server returns to it during resource
binding. The server MUST ensure that the resulting JID (including
localpart, domainpart, resourcepart, and separator characters)
conforms to the canonical format for XMPP addresses defined in
[XMPP-ADDR]; to meet this restriction, the server MAY replace the JID
sent by the client with the canonicalized JID as determined by the
server and communicate that JID to the client during resource
binding.

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For server-to-server communication, the initiating server's bare JID
() 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
element in Server Dialback [XEP-0220]).

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

4.3.7. Flow Chart

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

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

Figure 3: Stream Negotiation Flow Chart

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4.4. Closing a Stream

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

A stream is closed by sending a closing tag.

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

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

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

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

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

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

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

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4.5. Directionality

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

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

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

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

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

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

The foregoing considerations imply that while completing STARTTLS
negotiation (Section 5) and SASL negotiation (Section 6) two servers
would use one TCP connection, but after the stream negotiation
process is done that original TCP connection would be used only for
the initiating server to send XML stanzas to the receiving server.
In order for the receiving server to send XML stanzas to the
initiating server, the receiving server would need to reverse the
roles and negotiate an XML stream from the receiving server to the

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

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

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

4.6. Handling of Silent Peers

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

1. The underlying TCP connection is dead.

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

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

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

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

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

4.6.1. Dead Connection

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

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

4.6.2. Broken Stream

Even if the underlying TCP connection is alive, the peer might never
respond to XMPP traffic that the entity sends, whether normal stanzas
or specialized stream-checking traffic such as the application-level
pings defined in [XEP-0199] or the more comprehensive Stream
Management protocol defined in [XEP-0198]. In this case, it is
appropriate for the entity to close a broken stream with a
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., (Section 4.9.3.17) if the entity has reached a limit on
the number of open TCP connections or
(Section 4.9.3.14) if the connection has exceeded a local timeout
policy). However, consistent with the order of layers (specified
under Section 13.3), the other party is advised to verify that the
underlying TCP connection is alive and the stream is unbroken (as

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

4.6.4. Use of Checking Methods

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

4.7. Stream Attributes

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

4.7.1. from

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

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For initial stream headers in client-to-server communication, the
'from' attribute is the XMPP identity of the principal controlling
the client, i.e., a JID of the form . 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.

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 . 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.

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

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].)

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

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

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.

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

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

4.7.3. id

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

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

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

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

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

4.7.4. xml:lang

The 'xml:lang' attribute specifies an entity's preferred or default
language for any human-readable XML character data to be sent over
the stream (an XML stanza can also possess an 'xml:lang' attribute,
as discussed under Section 8.1.5). The syntax of this attribute is
defined in Section 2.12 of [XML]; in particular, the value of the
'xml:lang' attribute MUST conform to the NMTOKEN datatype (as defined
in Section 2.3 of [XML]) and MUST conform to the language identifier
format defined in [LANGTAGS].

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

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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
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").

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

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

4.7.5. version

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

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

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

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

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

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the entity with the larger minor version number would simply note
that its correspondent would not be able to understand that value of
the 'type' attribute and therefore would not send it).

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

1. The initiating entity MUST set the value of the 'version'
attribute in the initial stream header to the highest version
number it supports (e.g., if the highest version number it
supports is that defined in this specification, it MUST set the
value to "1.0").

2. The receiving entity MUST set the value of the 'version'
attribute in the response stream header to either the value
supplied by the initiating entity or the highest version number
supported by the receiving entity, whichever is lower. The
receiving entity MUST perform a numeric comparison on the major
and minor version numbers, not a string match on
".".

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 stream error (Section 4.9.3.25).

4. If either entity receives a stream header with no 'version'
attribute, the entity MUST consider the version supported by the
other entity to be "0.9" and SHOULD NOT include a 'version'
attribute in the response stream header.

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4.7.6. Summary of Stream Attributes

The following table summarizes the attributes of the root
element.

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 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 (Section 4.9.3.10), although
some existing implementations send (Section 4.9.3.1)
instead.

4.8.2. Content Namespace

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

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

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

foo

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.

foo

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

4.8.3. XMPP Content Namespaces

XMPP as defined in this specification uses two content namespaces:
'jabber:client' and 'jabber:server'. These namespaces are nearly
identical but are used in different contexts (client-to-server
communication for 'jabber:client' and server-to-server communication
for 'jabber:server'). The only difference between the two is that

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the 'to' and 'from' attributes are OPTIONAL on stanzas sent over XML
streams qualified by the 'jabber:client' namespace, whereas they are
REQUIRED on stanzas sent over XML streams qualified by the 'jabber:
server' namespace. Support for these content namespaces implies
support for the common attributes (Section 8.1) and basic semantics
(Section 8.2) of all three core stanza types (message, presence, and
IQ).

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

An implementation MAY refuse to support any other content namespaces
as default namespaces. If an entity receives a first-level child
element qualified by a content namespace it does not support, it MUST
close the stream with an 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 ,
, or 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
element contained within an extension element

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

4.8.4. Other Namespaces

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

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

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

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

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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
(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 and ); 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
(Section 4.9.3.2), although some existing implementations send
(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:

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foo

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

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C: No closing tag!

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

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 tag, include the
element as a child of the stream element, and send the closing
tag (preferably in the same TCP packet).

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4.9.1.3. Stream Errors When the Host Is Unspecified or Unknown

If the initiating entity provides no 'to' attribute or provides an
unknown host in the 'to' attribute and the error occurs during stream
setup, the value of the 'from' attribute returned by the receiving
entity in the stream header sent before closing the stream MUST be
either an authoritative FQDN for the receiving entity or the empty
string.

4.9.1.4. Where Stream Errors Are Sent

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

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

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

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4.9.2. Syntax

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

[
OPTIONAL descriptive text
]
[OPTIONAL application-specific condition element]

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

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

4.9.3. Defined Stream Error Conditions

The following stream-level error conditions are defined.

4.9.3.1. bad-format

The entity has sent XML that cannot be processed.

(In the following example, the client sends an XMPP message that is
not well-formed XML, which alternatively might trigger a stream error (Section 4.9.3.13).)

C:
No closing tag!

This error can be used instead of the more specific XML-related
errors, such as , , , , and . 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.)

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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).

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If a client receives a 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].

Interoperability Note: RFC 3920 specified that the 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.)

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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.)

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4.9.3.7. improper-addressing

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

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

P:
Wherefore art thou?

4.9.3.8. internal-server-error

The server has experienced a misconfiguration or other internal error
that prevents it from servicing the 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:
Neither, fair saint, if either thee dislike.

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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.)

4.9.3.11. invalid-xml

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

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

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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:
Wherefore art thou?

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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:
What is this foo?

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 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 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:
[ ... the-emacs-manual ... ]

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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
condition is more appropriate.

4.9.3.16. reset

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

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

4.9.3.17. resource-constraint

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

4.9.3.18. restricted-xml

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

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

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This message has no subject.

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 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 element as described under
Section 3.2.

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[2001:41D0:1:A49b::1]:9222

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 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 stream
error until after the confidentiality and integrity of the stream
have been protected via TLS or an equivalent security layer (such as
the SASL GSSAPI mechanism), and (b) the receiving entity MAY have a
policy of following redirects only if it has authenticated the
receiving entity. In addition, the initiating entity SHOULD abort
the connection attempt after a certain number of successive redirects
(e.g., at least 2 but no more than 5).

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4.9.3.20. system-shutdown

The server is being shut down and all active streams are being
closed.

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.

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.)

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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.)

4.9.3.24. unsupported-stanza-type

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

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

Soliloquy

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?

tag:denmark.example,2003:entry-32397
2003-12-13T18:30:02Z
2003-12-13T18:30:02Z

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.

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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 element will
contain two or three child elements.

My keyboard layout is:

QWERTYUIOP{}|
ASDFGHJKL:"
ZXCVBNM<>?

Some special application diagnostic information!

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4.10. Simplified Stream Examples

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

A basic connection:

[ ... stream negotiation ... ]

C:
Art thou not Romeo, and a Montague?

S:
Neither, fair saint, if either thee dislike.

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A connection gone bad:

[ ... stream negotiation ... ]

C:
No closing tag!

More detailed examples are provided under Section 9.

5. STARTTLS Negotiation

5.1. Fundamentals

XMPP includes a method for securing the stream from tampering and
eavesdropping. This channel encryption method makes use of the
Transport Layer Security [TLS] protocol, specifically a "STARTTLS"
extension that is modeled after similar extensions for the [IMAP],

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

5.2. Support

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

5.3. Stream Negotiation Rules

5.3.1. Mandatory-to-Negotiate

If the receiving entity advertises only the STARTTLS feature or if
the receiving entity includes the 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 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
element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace
as sent by the receiving entity). This prohibition helps to ensure
proper security layer byte precision. Any such whitespace shown in
the STARTTLS examples provided in this document is included only for
the sake of readability.

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5.3.4. Order of TLS and SASL Negotiations

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

5.3.5. TLS Renegotiation

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

1. Refreshing encryption keys.

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

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

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

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

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

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

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

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

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

5.3.6. TLS Extensions

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

5.4. Process

5.4.1. Exchange of Stream Headers and Stream Features

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

The receiving entity MUST send a response stream header to the
initiating entity over the TCP connection opened by the initiating
entity.

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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 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 element MUST contain an empty
child element.

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 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.

The receiving entity MUST reply with either a element
(proceed case) or a 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
element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
namespace, close the XML stream, and terminate the underlying TCP
connection.

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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
element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
namespace.

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

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

5.4.3. TLS Negotiation

5.4.3.1. Rules

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

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The following rules apply:

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

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

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

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

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

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

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

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

5.4.3.2. TLS Failure

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

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The receiving entity MUST NOT send a closing 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 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).

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

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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).

EXTERNAL
SCRAM-SHA-1-PLUS
SCRAM-SHA-1
PLAIN

6. SASL Negotiation

6.1. Fundamentals

XMPP includes a method for authenticating a stream by means of an
XMPP-specific profile of the Simple Authentication and Security Layer
protocol (see [SASL]). SASL provides a generalized method for adding
authentication support to connection-based protocols, and XMPP uses
an XML namespace profile of SASL that conforms to the profiling
requirements of [SASL]. The XML namespace name for the SASL
extension is 'urn:ietf:params:xml:ns:xmpp-sasl'.

6.2. Support

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

6.3. Stream Negotiation Rules

6.3.1. Mandatory-to-Negotiate

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

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6.3.2. Restart

After SASL negotiation, the parties MUST restart the stream.

6.3.3. Mechanism Preferences

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

6.3.4. Mechanism Offers

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

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

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

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

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

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

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

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

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

6.3.5. Data Formatting

The following data formatting rules apply to the SASL negotiation:

1. During SASL negotiation, the entities MUST NOT send any
whitespace as separators between XML elements (i.e., from the
last character of the first-level 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
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 element to provide
information that might be needed by the initiating entity in
order to complete successful SASL negotiation using one or more

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

6.3.6. Security Layers

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

6.3.7. Simple User Name

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

6.3.8. Authorization Identity

An authorization identity is an OPTIONAL identity included by the
initiating entity to specify an identity to act as (see Section 2 of
[SASL]). In client-to-server streams, it would most likely be used
by an administrator to perform some management task on behalf of
another user, whereas in server-to-server streams it would most
likely be used to specify a particular add-on service at an XMPP
service (e.g., a multi-user chat server at conference.example.com
that is hosted by the example.com XMPP service). If the initiating
entity wishes to act on behalf of another entity and the selected
SASL mechanism supports transmission of an authorization identity,
the initiating entity MUST provide an authorization identity during
SASL negotiation. If the initiating entity does not wish to act on
behalf of another entity, it MUST NOT provide an authorization
identity.

In the case of client-to-server communication, the value of an
authorization identity MUST be a bare JID ()
rather than a full JID ().

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

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If the initiating entity provides an authorization identity during
SASL negotiation, the receiving entity is responsible for verifying
that the initiating entity is in fact allowed to assume the specified
authorization identity; if not, the receiving entity MUST return an
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 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 element.

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

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6.4. Process

The process for SASL negotiation is as follows.

6.4.1. Exchange of Stream Headers and Stream Features

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

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).

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 element qualified
by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace. Typically there
are only three cases in which support for SASL negotiation would not
be advertised here:

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

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

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

The element MUST contain one child element
for each authentication mechanism the receiving entity offers to the
initiating entity. As noted, the order of 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).

EXTERNAL
SCRAM-SHA-1-PLUS
SCRAM-SHA-1
PLAIN

6.4.2. Initiation

In order to begin the SASL negotiation, the initiating entity sends
an 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: AGp1bGlldAByMG0zMG15cjBtMzA=

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If the initiating entity subsequently sends another 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 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
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 element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace.

Upon receiving an element, the receiving entity MUST return
a element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace and containing an
child element.

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6.4.5. SASL Failure

The receiving entity reports failure of the handshake for this
authentication mechanism by sending a 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 element as defined under Section 6.5).

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
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
(Section 4.9.3.14), although some existing
implementations send (Section 4.9.3.12) instead.

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

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receiving entity SHOULD instead close the stream with a 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
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 ("=").

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 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
tag before sending the new initial stream header, since the
receiving entity and initiating entity MUST consider the original
stream to be replaced upon success of the SASL negotiation).

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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).

The receiving entity MUST also send stream features, containing any
further available features or containing no features (via an empty
element).

6.5. SASL Errors

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

[
OPTIONAL descriptive text
]

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 (Section 6.5.10).

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Inclusion of the 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
element.

6.5.2. account-disabled

The account of the initiating entity has been temporarily disabled;
sent in reply to an element (with or without initial response
data) or a element.

I: AGp1bGlldAByMG0zMG15cjBtMzA=

Call 212-555-1212 for assistance.

6.5.3. credentials-expired

The authentication failed because the initiating entity provided
credentials that have expired; sent in reply to a element
or an element with initial response data.

I:
[ ... ]

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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 element
(with or without initial response data).

I: AGp1bGlldAByMG0zMG15cjBtMzA=

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 element or an element with initial
response data.

I: [ ... ]

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
element or an element with initial response data.

I:
[ ... ]

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6.5.7. invalid-mechanism

The initiating entity did not specify a mechanism, or requested a
mechanism that is not supported by the receiving entity; sent in
reply to an element.

6.5.8. malformed-request

The request is malformed (e.g., the 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 , , , or element.

(In the following example, the XML character data of the
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: [ ... some-long-token ... ]

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
element (with or without initial response data).

I: AGp1bGlldAByMG0zMG15cjBtMzA=

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6.5.10. not-authorized

The authentication failed because the initiating entity did not
provide proper credentials, or because some generic authentication
failure has occurred but the receiving entity does not wish to
disclose specific information about the cause of the failure; sent in
reply to a element or an element with initial
response data.

I:
[ ... ]

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 element or a
element.

I:
[ ... ]

6.6. SASL Definition

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

service name: "xmpp"

initiation sequence: After the initiating entity provides an opening
XML stream header and the receiving entity replies in kind, the
receiving entity provides a list of acceptable authentication

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

exchange sequence: Challenges and responses are carried through the
exchange of elements from receiving entity to
initiating entity and elements from initiating entity
to receiving entity. The receiving entity reports failure by
sending a element and success by sending a
element; the initiating entity aborts the exchange by sending an
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
element for the receiving entity, and immediately after
receiving the closing '>' character of the 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 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 () of the client, so that the address
for use over that stream is a full JID of the form
(including the resourcepart). This
ensures that the server can deliver XML stanzas to and receive XML
stanzas from the client in relation to entities other than the server
itself or the client's account, as explained under Section 10.

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

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

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

If, before completing the resource binding step, the client attempts
to send an XML stanza to an entity other than the server itself or
the client's account, the server MUST NOT process the stanza and MUST
close the stream with a 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
element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace
in the stream features it presents to the client.

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

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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 . 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
element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind'
namespace.

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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 child element that specifies the full JID for the
connected resource as determined by the server.

juliet@im.example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb

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 stanza error (Section 8.3.3.18).

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7.6.2.2. Not Allowed

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

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 element with
a child element containing non-zero-length XML character
data.

balcony

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

juliet@im.example.com/balcony

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

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juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb

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 stanza error (Section 8.3.3.1) but SHOULD
instead process the resourcepart so that it is in conformance.

7.7.2.2. Conflict

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

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

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2. Disallow the resource binding attempt of the newly connecting
client and maintain the session of the currently connected
client. This behavior is neither encouraged nor discouraged,
despite the fact that it was implicitly encouraged in RFC 3920;
however, note that handling of the 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 stanza of type
"result" to the newly connecting client, where the child of
the element contains XML character data that indicates the
full JID of the client, including the resourcepart that was generated
by the server.

juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb

If the server follows behavior #2, it sends a 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).

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If the server follows behavior #3, it returns a 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.

juliet@im.example.com/balcony

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 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: , , and . In addition, there
are five common attributes for these stanza types. These common
attributes, as well as the basic semantics of the three stanza types,
are defined in this specification; more detailed information
regarding the syntax of XML stanzas for instant messaging and
presence applications is provided in [XMPP-IM], and for other
applications in the relevant XMPP extension specifications.

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

Security Warning: A server MUST NOT process a partial stanza and
MUST NOT attach meaning to the transmission timing of any part of
a stanza (before receipt of the closing tag).

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8.1. Common Attributes

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

8.1.1. to

The 'to' attribute specifies the JID of the intended recipient for
the stanza.

Art thou not Romeo, and a Montague?

For information about server processing of inbound and outbound XML
stanzas based on the 'to' address, refer to Section 10.

8.1.1.1. Client-to-Server Streams

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

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

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

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

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

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

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

8.1.1.2. Server-to-Server Streams

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

1. A stanza MUST possess a 'to' attribute whose value is an XMPP
address; if a server receives a stanza that does not meet this
restriction, it MUST close the stream with an 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
stream error (Section 4.9.3.6) or a stream error
(Section 4.9.3.5).

8.1.2. from

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

Art thou not Romeo, and a Montague?

8.1.2.1. Client-to-Server Streams

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

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

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'from' attribute MUST be the full JID
() determined by the server for
the connected resource that generated the stanza (see
Section 4.3.6), or the bare JID () 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., )
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 ().

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 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
stream error (Section 4.9.3.9).

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

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8.1.3. id

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

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

For and stanzas, it is RECOMMENDED for the
originating entity to include an 'id' attribute; for stanzas,
it is REQUIRED.

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

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

8.1.4. type

The 'type' attribute specifies the purpose or context of the message,
presence, or IQ stanza. The particular allowable values for the
'type' attribute vary depending on whether the stanza is a message,
presence, or IQ stanza. The defined values for message and presence
stanzas are specific to instant messaging and presence applications
and therefore are defined in [XMPP-IM], whereas the values for IQ
stanzas specify the part of the semantics for all structured request-
response exchanges (no matter what the payload) and therefore are
specified under Section 8.2.3. The only 'type' value common to all
three kinds of stanzas is "error" as described under Section 8.3.

8.1.5. xml:lang

A stanza SHOULD possess an 'xml:lang' attribute (as defined in
Section 2.12 of [XML]) if the stanza contains XML character data that
is intended to be presented to a human user (as explained in
[CHARSETS], "internationalization is for humans"). The value of the
'xml:lang' attribute specifies the default language of any such
human-readable XML character data.

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dnd
Wooing Juliet

The value of the 'xml:lang' attribute MAY be overridden by the 'xml:
lang' attribute of a specific child element.

dnd
Wooing Juliet
Dvořím se Julii

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:
dnd
Wooing Juliet

S:
dnd
Wooing Juliet

If an inbound stanza received by a client or server does not possess
an 'xml:lang' attribute, an implementation MUST assume that the
default language is that specified for the entity's input stream as
defined under Section 4.7.4.

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

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

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8.2. Basic Semantics

8.2.1. Message Semantics

The 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 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 stanza is most often used by XMPP clients,
it can also be used by servers, add-on services, and any other kind
of XMPP entity. See Section 10 for general routing and delivery
rules related to XML stanzas, and [XMPP-IM] for rules specific to
presence applications.

8.2.3. IQ Semantics

Info/Query, or IQ, is a "request-response" mechanism, similar in some
ways to the Hypertext Transfer Protocol [HTTP]. The semantics of IQ
enable an entity to make a request of, and receive a response from,
another entity. The data content of the request and response is
defined by the schema or other structural definition associated with
the XML namespace that qualifies the direct child element of the IQ
element (see Section 8.4), and the interaction is tracked by the
requesting entity through use of the 'id' attribute. Thus, IQ
interactions follow a common pattern of structured data exchange such
as get/result or set/result (although an error can be returned in
reply to a request if appropriate):

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Requesting Responding
Entity Entity
---------- ----------
| |
| |
| [ ... payload ... ] |
| |
| -------------------------> |
| |
| |
| [ ... payload ... ] |
| |
| <------------------------- |
| |
| |
| [ ... payload ... ] |
| |
| -------------------------> |
| |
| |
| [ ... condition ... ] |
| |
| <------------------------- |
| |

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 stanza error
(Section 8.3.3.1).

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

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

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

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

3. An entity that receives an IQ request of type "get" or "set" MUST
reply with an IQ response of type "result" or "error". The
response MUST preserve the 'id' attribute of the request (or be
empty if the generated stanza did not include an 'id' attribute).

4. An entity that receives a stanza of type "result" or "error" MUST
NOT respond to the stanza by sending a further IQ response of
type "result" or "error"; however, the requesting entity MAY send
another request (e.g., an IQ of type "set" to provide obligatory
information discovered through a get/result pair).

5. An IQ stanza of type "get" or "set" MUST contain exactly one
child element, which specifies the semantics of the particular
request.

6. An IQ stanza of type "result" MUST include zero or one child
elements.

7. An IQ stanza of type "error" MAY include the child element
contained in the associated "get" or "set" and MUST include an
child; for details, see Section 8.3.

8.3. Stanza Errors

Stanza-related errors are handled in a manner similar to stream
errors (Section 4.9). Unlike stream errors, stanza errors are
recoverable; therefore, they do not result in termination of the XML
stream and underlying TCP connection. Instead, the entity that
discovers the error condition returns an error stanza, which is a
stanza that:

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

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

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

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

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o contains an 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 or and
included an 'id' attribute then it is REQUIRED for the error
stanza to also include an 'id' attribute. If the generated
stanza was 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 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 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 child MUST NOT be included if the 'type' attribute
has a value other than "error" (or if there is no 'type'
attribute).

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

8.3.2. Syntax

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

[OPTIONAL to include sender XML here]

[
OPTIONAL descriptive text
]
[OPTIONAL application-specific condition element]

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 (Section 8.3.3.21). If the designers of an XMPP protocol
extension or the developers of an XMPP implementation need to
communicate a stanza error condition that is not defined in this

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

The element:

o MUST contain a defined condition element.

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

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

8.3.3. Defined Conditions

The following conditions are defined for use in stanza errors.

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

8.3.3.1. bad-request

The sender has sent a stanza containing XML that does not conform to
the appropriate schema or that cannot be processed (e.g., an IQ
stanza that includes an unrecognized value of the 'type' attribute,

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

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".

balcony

8.3.3.3. feature-not-implemented

The feature represented in the XML stanza is not implemented by the
intended recipient or an intermediate server and therefore the stanza
cannot be processed (e.g., the entity understands the namespace but
does not recognize the element name); the associated error type
SHOULD be "cancel" or "modify".

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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".

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8.3.3.5. gone

The recipient or server can no longer be contacted at this address,
typically on a permanent basis (as opposed to the 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
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:
Thy lips are warm.

xmpp:romeo@afterlife.example.net

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".

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8.3.3.7. item-not-found

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

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
stanza error (Section 8.3.3.19).

8.3.3.8. jid-malformed

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

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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 errors
related to all JIDs of the form ), 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 errors
related to stanzas that users of that service have to tried send
to JIDs of the form ). 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:
[ ... the-emacs-manual ... ]

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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".

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".

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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 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:
%#&@^!!!

8.3.3.13. recipient-unavailable

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

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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 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 element (which MUST be a URI or IRI with which the
sender can communicate, typically an XMPP IRI as specified in
[XMPP-URI]).

xmpp:characters@conference.example.org

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Security Warning: An application receiving a stanza-level redirect
SHOULD warn a human user of the redirection attempt and request
approval before proceeding to communicate with the entity whose
address is contained in the XML character data of the
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".

8.3.3.16. remote-server-not-found

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

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

C:
yt?

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:
yt?

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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".

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:
Hello?

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Security Warning: An application MUST return a stanza error (Section 8.3.3.19) instead of (Section 8.3.3.7) or
(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:
ACT II, SCENE II
help, I forgot my lines!

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8.3.3.21. undefined-condition

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

C:
My lord, dispatch; read o'er these articles.

8.3.3.22. unexpected-request

The recipient or server understood the request but was not expecting
it at this time (e.g., the request was out of order); the associated
error type SHOULD be "wait" or "modify".

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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
element will contain two or three child elements.

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[ ... application-specific information ... ]

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

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

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

sha1-hash-of-image

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

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Hello?

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 (Section 8.3.3.19).

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

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

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

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 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:

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

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

Step 4: Client sends STARTTLS command to server:

Step 5: Server informs client that it is allowed to proceed:

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):

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

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

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:

SCRAM-SHA-1-PLUS
SCRAM-SHA-1
PLAIN

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

C:
biwsbj1qdWxpZXQscj1vTXNUQUF3QUFBQU1BQUFBTlAwVEFBQUFBQUJQVTBBQQ==

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

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Step 10: Server sends a challenge:

S:
cj1vTXNUQUF3QUFBQU1BQUFBTlAwVEFBQUFBQUJQVTBBQWUxMjQ2OTViLTY5Y
TktNGRlNi05YzMwLWI1MWIzODA4YzU5ZSxzPU5qaGtZVE0wTURndE5HWTBaaT
AwTmpkbUxUa3hNbVV0TkRsbU5UTm1ORE5rTURNeixpPTQwOTY=

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:
Yz1iaXdzLHI9b01zVEFBd0FBQUFNQUFBQU5QMFRBQUFBQUFCUFUwQUFlMTI0N
jk1Yi02OWE5LTRkZTYtOWMzMC1iNTFiMzgwOGM1OWUscD1VQTU3dE0vU3ZwQV
RCa0gyRlhzMFdEWHZKWXc9

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:
dj1wTk5ERlZFUXh1WHhDb1NFaVc4R0VaKzFSU289

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):

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Step 13: Client initiates a new stream to server:

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):

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:

balcony

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

juliet@im.example.com/balcony

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):

9.1.4. Stanza Exchange

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

C:
Art thou not Romeo, and a Montague?

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:
Neither, fair saint, if either thee dislike.

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

9.1.5. Close

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

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

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:

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

S2:

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

S2:

Step 4: Server1 sends the STARTTLS command to Server2:

S1:

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

S2:

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:

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:

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

9.2.2. SASL

Step 8: Server2 sends a response stream header to Server1 along with
available stream features (including a preference for the SASL
EXTERNAL mechanism):

S2:

EXTERNAL

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

S1: =

Step 10: Server2 returns success:

S2:

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:

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Step 11: Server1 initiates a new stream to Server2:

S1:

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:

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:
Art thou not Romeo, and a Montague?

9.2.4. Close

Desiring to send no further messages, Server1 closes its stream to
Server2 but waits for incoming data from Server2. (In practice, the
stream would most likely remain open for some time, since Server1 and
Server2 do not immediately know if the stream will be needed for
further communication.)

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S1:

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

S2:

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 to
). The server MUST ensure that it delivers
stanzas addressed to the intended recipient in the order it
receives them over the input stream from the sending client,
treating stanzas addressed to the bare JID and the full JID of
the intended recipient as equivalent for delivery purposes.

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3. Stanzas sent by a connected client and intended for delivery to
an entity located at a remote domain (e.g., stanzas addressed
from to ). 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 to and
routed by over a server-to-server stream to
). 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 to ).

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 and
) 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 stream error
(Section 4.9.3.3) if a remote server attempts to negotiate more than
one stream, as described under Section 4.9.3.3).

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10.2. General Considerations

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

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

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

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

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

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

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

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

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

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

10.3. No 'to' Address

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

10.3.1. Message

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

10.3.2. Presence

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

10.3.3. IQ

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

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

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

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

10.4. Remote Domain

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

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

10.4.1. Existing Stream

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

10.4.2. No Existing Stream

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

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

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

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

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10.4.3. Error Handling

If routing of a stanza to the intended recipient's server is
unsuccessful, the sender's server MUST return an error to the sender.
If resolution of the remote domain is unsuccessful, the stanza error
MUST be (Section 8.3.3.16). If resolution
succeeds but streams cannot be negotiated, the stanza error MUST be
(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
, 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
, then the server MUST either (a) handle the
stanza as appropriate for the stanza kind or (b) return an error
stanza to the sender.

10.5.3. localpart@domainpart

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

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10.5.3.1. No Such User

If there is no local account associated with the
, 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 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
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
, 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 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
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
as described under Section 10.5.3.2.

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If the JID contained in the 'to' attribute is of the form
, 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
(Section 4.9.3.18), although some existing implementations send
(Section 4.9.3.1) instead.

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11.2. XML Namespace Names and Prefixes

XML namespaces (see [XML-NAMES]) are used within XMPP streams to
create strict boundaries of data ownership. The basic function of
namespaces is to separate different vocabularies of XML elements that
are structurally mixed together. Ensuring that XMPP streams are
namespace-aware enables any allowable XML to be structurally mixed
with any data element within XMPP. XMPP-specific rules for XML
namespace names and prefixes are defined under Section 4.8 for XML
streams and Section 8.4 for XML stanzas.

11.3. Well-Formedness

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

o "XML-well-formedness" in accordance with the definition of "well-
formed" from Section 2.1 of [XML].

o "Namespace-well-formedness" in accordance with the definition of
"namespace-well-formed" from Section 7 of [XML-NAMES].

The following rules apply:

1. An XMPP entity MUST NOT generate data that is not XML-well-
formed.

2. An XMPP entity MUST NOT accept data that is not XML-well-formed;
instead it MUST close the stream over which the data was received
with a 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 stanza error (Section 8.3.3.9) or
close the stream with a stream error
(Section 4.9.3.13), where it is preferable to close the stream
with a stream error because accepting such data can open an
entity to certain denial-of-service attacks.

Interoperability Note: Because these restrictions were
underspecified in [RFC3920], it is possible that implementations
based on that specification will send data that does not comply
with these restrictions.

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11.4. Validation

A server is not responsible for ensuring that XML data delivered to a
connected client or routed to a peer server is valid, in accordance
with the definition of "valid" provided in Section 2.8 of [XML]. An
implementation MAY choose to accept or send only data that has been
explicitly validated against the schemas provided in this document,
but such behavior is OPTIONAL. Clients are advised not to rely on
the ability to send data that does not conform to the schemas, and
SHOULD ignore any non-conformant elements or attributes on the
incoming XML stream.

Informational Note: The terms "valid" and "well-formed" are
distinct in XML.

11.5. Inclusion of XML Declaration

Before sending a stream header, an implementation SHOULD send an XML
declaration (matching the "XMLDecl" production from [XML]).
Applications MUST follow the rules provided in [XML] regarding the
format of the XML declaration and the circumstances under which the
XML declaration is included.

Because external markup declarations are prohibited in XMPP (as
described under Section 11.1), the standalone document declaration
(matching the "SDDecl" production from [XML]) would have no meaning
and therefore MUST NOT be included in an XML declaration sent over an
XML stream. If an XMPP entity receives an XML declaration containing
a standalone document declaration set to a value of "no", the entity
MUST either ignore the standalone document declaration or close the
stream with a stream error, which SHOULD be
(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 (Section 4.9.3.22),
although some existing implementations send
(Section 4.9.3.1) instead.

Because it is mandatory for an XMPP implementation to support all and
only the UTF-8 encoding and because UTF-8 always has the same byte
order, an implementation MUST NOT send a byte order mark ("BOM") at

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the beginning of the data stream. If an entity receives the
[UNICODE] character U+FEFF anywhere in an XML stream (including as
the first character of the stream), it MUST interpret that character
as a zero width no-break space, not as a byte order mark.

11.7. Whitespace

Except where explicitly disallowed (e.g., during TLS negotiation
(Section 5) and SASL negotiation (Section 6)), either entity MAY send
whitespace as separators between XML stanzas or between any other
first-level elements sent over the stream. One common use for
sending such whitespace is explained under Section 4.4.

11.8. XML Versions

XMPP is an application profile of XML 1.0. A future version of XMPP
might be defined in terms of higher versions of XML, but this
specification defines XMPP only in terms of XML 1.0.

12. Internationalization Considerations

As specified under Section 11.6, XML streams MUST be encoded in
UTF-8.

As specified under Section 4.7, an XML stream SHOULD include an 'xml:
lang' attribute specifying the default language for any XML character
data that is intended to be presented to a human user. As specified
under Section 8.1.5, an XML stanza SHOULD include an 'xml:lang'
attribute if the stanza contains XML character data that is intended
to be presented to a human user. A server SHOULD apply the default
'xml:lang' attribute to stanzas it routes or delivers on behalf of
connected entities, and MUST NOT modify or delete 'xml:lang'
attributes on stanzas it receives from other entities.

Internationalization of XMPP addresses is specified in [XMPP-ADDR].

13. Security Considerations

13.1. Fundamentals

XMPP technologies are typically deployed using a decentralized
client-server architecture. As a result, several paths are possible
when two XMPP entities need to communicate:

1. Both entities are servers. In this case, the entities can
establish a direct server-to-server stream between themselves.

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2. One entity is a server and the other entity is a client whose
account is hosted on that server. In this case, the entities can
establish a direct client-to-server stream between themselves.

3. Both entities are clients whose accounts are hosted on the same
server. In this case, the entities cannot establish a direct
stream between themselves, but there is only one intermediate
entity between them, whose policies they might understand and in
which they might have some level of trust (e.g., the server might
require the use of Transport Layer Security for all client
connections).

4. Both entities are clients but their accounts are hosted on
different servers. In this case, the entities cannot establish a
direct stream between themselves and there are two intermediate
entities between them; each client might have some trust in the
server that hosts its account but might know nothing about the
policies of the server to which the other client connects.

This specification covers only the security of a direct XML stream
between two servers or between a client and a server (cases #1 and
#2), where each stream can be considered a single "hop" along a
communication path. The goal of security for a multi-hop path (cases
#3 and #4), although very desirable, is out of scope for this
specification.

In accordance with [SEC-GUIDE], this specification covers
communication security (confidentiality, data integrity, and peer
entity authentication), non-repudiation, and systems security
(unauthorized usage, inappropriate usage, and denial of service). We
also discuss common security issues such as information leaks,
firewalls, and directory harvesting, as well as best practices
related to the reuse of technologies such as base 64, DNS,
cryptographic hash functions, SASL, TLS, UTF-8, and XML.

13.2. Threat Model

The threat model for XMPP is in essence the standard "Internet Threat
Model" described in [SEC-GUIDE]. Attackers are assumed to be
interested in and capable of launching the following attacks against
unprotected XMPP systems:

o Eavesdropping

o Sniffing passwords

o Breaking passwords through dictionary attacks

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o Discovering usernames through directory harvesting attacks

o Replaying, inserting, deleting, or modifying stanzas

o Spoofing users

o Gaining unauthorized entry to a server or account

o Using a server or account inappropriately

o Denying service to other entities

o Subverting communication streams through man-in-the-middle attacks

o Gaining control over on-path servers

Where appropriate, the following sections describe methods for
protecting against these threats.

13.3. Order of Layers

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

1. TCP

2. TLS

3. SASL

4. XMPP

This order has important security implications, as described
throughout these security considerations.

Within XMPP, XML stanzas are further ordered on top of XML streams,
as described under Section 4.

13.4. Confidentiality and Integrity

The use of Transport Layer Security (TLS) with appropriate
ciphersuites provides a reliable mechanism to ensure the
confidentiality and integrity of data exchanged between a client and
a server or between two servers. Therefore, TLS can help to protect
against eavesdropping, password sniffing, man-in-the-middle attacks,
and stanza replays, insertion, deletion, and modification over an XML
stream. XMPP clients and servers MUST support TLS as defined under
Section 5.

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Informational Note: The confidentiality and integrity of a stream
can be protected by methods other than TLS, e.g., by means of a
SASL mechanism that involves negotiation of a security layer.

Security Warning: The use of TLS in XMPP applies to a single
stream. Because XMPP is typically deployed using a distributed
client-server architecture (as explained under Section 2.5), a
stanza might traverse multiple streams, and not all of those
streams might be TLS-protected. For example, a stanza sent from a
client with a session at one server (e.g.,
) and intended for delivery to a client
with a session at another server (e.g.,
) will traverse three streams: (1) the
stream from the sender's client to its server, (2) the stream from
the sender's server to the recipient's server, and (3) the stream
from the recipient's server to the recipient's client.
Furthermore, the stanza will be processed as cleartext within the
sender's server and the recipient's server. Therefore, even if
the stream from the sender's client to its server is protected,
the confidentiality and integrity of a stanza sent over that
protected stream cannot be guaranteed when the stanza is processed
by the sender's server, sent from the sender's server to the
recipient's server, processed by the recipient's server, or sent
from the recipient's server to the recipient's client. Only a
robust technology for end-to-end encryption could ensure the
confidentiality and integrity of a stanza as it traverses all of
the "hops" along a communication path (e.g., a technology that
meets the requirements defined in [E2E-REQS]). Unfortunately, the
XMPP community has so far failed to produce an end-to-end
encryption technology that might be suitable for widespread
implementation and deployment, and definition of such a technology
is out of scope for this document.

13.5. Peer Entity Authentication

The use of the Simple Authentication and Security Layer (SASL) for
authentication provides a reliable mechanism for peer entity
authentication. Therefore, SASL helps to protect against user
spoofing, unauthorized usage, and man-in-the middle attacks. XMPP
clients and servers MUST support SASL as defined under Section 6.

13.6. Strong Security

[STRONGSEC] defines "strong security" and its importance to
communication over the Internet. For the purpose of XMPP
communication over client-to-server and server-to-server streams, the
term "strong security" refers to the use of security technologies

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that provide both mutual authentication and integrity checking (e.g.,
a combination of TLS encryption and SASL authentication using
appropriate SASL mechanisms).

Implementations MUST support strong security. Service provisioning
SHOULD use strong security.

An implementation SHOULD make it possible for an end user or service
administrator to provision a deployment with specific trust anchors
for the certificate presented by a connecting entity (either client
or server); when an application is thus provisioned, it MUST NOT use
a generic PKI trust store to authenticate the connecting entity.
More detailed rules and guidelines regarding certificate validation
are provided in the next section.

The initial stream and the response stream MUST be secured
separately, although security in both directions MAY be established
via mechanisms that provide mutual authentication.

13.7. Certificates

Channel encryption of an XML stream using Transport Layer Security as
described under Section 5, and in some cases also authentication as
described under Section 6, is commonly based on a PKIX certificate
presented by the receiving entity (or, in the case of mutual
certificate authentication, both the receiving entity and the
initiating entity). This section describes best practices regarding
the generation of PKIX certificates to be presented by XMPP entities
and the verification of PKIX certificates presented by XMPP entities.

In general, the following sections rely on and extend the rules and
guidelines provided in the [PKIX] profile of [X509], and in
[TLS-CERTS]. The reader is referred to those specifications for a
detailed understanding of PKIX certificates and their use in TLS.

13.7.1. Certificate Generation

13.7.1.1. General Considerations

The following rules apply to end entity public key certificates that
are issued to XMPP servers or clients:

1. The certificate MUST conform to [PKIX].

2. The certificate MUST NOT contain a basicConstraints extension
with the cA boolean set to TRUE.

3. The subject field MUST NOT be null.

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4. The signatureAlgorithm SHOULD be the PKCS #1 version 1.5
signature algorithm with SHA-256 as defined by [PKIX-ALGO], or a
stronger algorithm if available.

5. The certificate SHOULD include an Authority Information Access
(AIA) extension that specifies the address of an Online
Certificate Status Protocol [OCSP] responder (if not, a relying
party would need to fall back on the use of Certificate
Revocation Lists (CRLs) as described in [PKIX]).

The following rules apply to certification authority (CA)
certificates that are used by issuers of XMPP end entity
certificates:

1. The certificate MUST conform to [PKIX].

2. The certificate MUST contain a keyUsage extension with the
digitalSignature bit set.

3. The subject field MUST NOT be null.

4. The signatureAlgorithm SHOULD be the PKCS #1 version 1.5
signature algorithm with SHA-256 as defined by [PKIX-ALGO], or a
stronger algorithm if available.

5. For issuers of public key certificates, the issuer's certificate
MUST contain a basicConstraints extension with the cA boolean set
to TRUE.

13.7.1.2. Server Certificates

13.7.1.2.1. Rules

In a PKIX certificate to be presented by an XMPP server (i.e., a
"server certificate"), the certificate SHOULD include one or more
XMPP addresses (i.e., domainparts) associated with XMPP services
hosted at the server. The rules and guidelines defined in
[TLS-CERTS] apply to XMPP server certificates, with the following
XMPP-specific considerations:

o Support for the DNS-ID identifier type [PKIX] is REQUIRED in XMPP
client and server software implementations. Certification
authorities that issue XMPP-specific certificates MUST support the
DNS-ID identifier type. XMPP service providers SHOULD include the
DNS-ID identifier type in certificate requests.

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o Support for the SRV-ID identifier type [PKIX-SRV] is REQUIRED for
XMPP client and server software implementations (for verification
purposes XMPP client implementations need to support only the
"_xmpp-client" service type, whereas XMPP server implementations
need to support both the "_xmpp-client" and "_xmpp-server" service
types). Certification authorities that issue XMPP-specific
certificates SHOULD support the SRV-ID identifier type. XMPP
service providers SHOULD include the SRV-ID identifier type in
certificate requests.

o Support for the XmppAddr identifier type (specified under
Section 13.7.1.4) is encouraged in XMPP client and server software
implementations for the sake of backward-compatibility, but is no
longer encouraged in certificates issued by certification
authorities or requested by XMPP service providers.

o DNS domain names in server certificates MAY contain the wildcard
character '*' as the complete left-most label within the
identifier.

13.7.1.2.2. Examples

For our first (relatively simple) example, consider a company called
"Example Products, Inc." It hosts an XMPP service at
"im.example.com" (i.e., user addresses at the service are of the form
"user@im.example.com"), and SRV lookups for the xmpp-client and xmpp-
server services at "im.example.com" yield one machine, called
"x.example.com", as follows:

_xmpp-client._tcp.im.example.com. 400 IN SRV 20 0 5222 x.example.com
_xmpp-server._tcp.im.example.com. 400 IN SRV 20 0 5269 x.example.com

The certificate presented by x.example.com contains the following
representations:

o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of "_xmpp-client.im.example.com"

o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of "_xmpp-server.im.example.com"

o A dNSName containing an ASCII string of "im.example.com"

o An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
string of "im.example.com"

o A CN containing an ASCII string of "Example Products, Inc."

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For our second (more complex) example, consider an ISP called
"Example Internet Services". It hosts an XMPP service at
"example.net" (i.e., user addresses at the service are of the form
"user@example.net"), but SRV lookups for the xmpp-client and xmpp-
server services at "example.net" yield two machines ("x1.example.net"
and "x2.example.net"), as follows:

_xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x1.example.net.
_xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x2.example.net.
_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x1.example.net.
_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x2.example.net.

Example Internet Services also hosts chatrooms at chat.example.net,
and provides an xmpp-server SRV record for that service as well (thus
enabling entities from remote domains to access that service). It
also might provide other such services in the future, so it wishes to
represent a wildcard in its certificate to handle such growth.

The certificate presented by either x1.example.net or x2.example.net
contains the following representations:

o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of "_xmpp-client.example.net"

o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of "_xmpp-server.example.net"

o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of "_xmpp-server.chat.example.net"

o A dNSName containing an ASCII string of "example.net"

o A dNSName containing an ASCII string of "*.example.net"

o An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
string of "example.net"

o An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
string of "chat.example.net"

o A CN containing an ASCII string of "Example Internet Services"

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13.7.1.3. Client Certificates

In a PKIX certificate to be presented by an XMPP client controlled by
a human user (i.e., a "client certificate"), it is RECOMMENDED for
the certificate to include one or more JIDs associated with an XMPP
user. If included, a JID MUST be represented as an XmppAddr as
specified under Section 13.7.1.4.

13.7.1.4. XmppAddr Identifier Type

The XmppAddr identifier type is a UTF8String within an otherName
entity inside the subjectAltName, using the [ASN.1] Object Identifier
"id-on-xmppAddr" specified below.

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

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

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

XmppAddr ::= UTF8String

As an alternative to the "id-on-xmppAddr" notation, this Object
Identifier MAY be represented in dotted display format (i.e.,
"1.3.6.1.5.5.7.8.5") or in the Uniform Resource Name notation
specified in [URN-OID] (i.e., "urn:oid:1.3.6.1.5.5.7.8.5").

Thus for example the JID as included in a
certificate could be formatted in any of the following three ways:

id-on-xmppAddr:
subjectAltName=otherName:id-on-xmppAddr;UTF8:juliet@im.example.com

dotted display format: subjectAltName=otherName:
1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com

URN notation: subjectAltName=otherName:urn:oid:
1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com

Use of the "id-on-xmppAddr" format is RECOMMENDED in the generation
of certificates, but all three formats MUST be supported for the
purpose of certificate validation.

The "id-on-xmppAddr" object identifier MAY be used in conjunction
with the extended key usage extension specified in Section 4.2.1.12
of [PKIX] in order to explicitly define and limit the intended use of
a certificate to the XMPP network.

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13.7.2. Certificate Validation

When an XMPP entity is presented with a server certificate or client
certificate by a peer for the purpose of encryption or authentication
of XML streams as described under Section 5 and Section 6, the entity
MUST attempt to validate the certificate to determine if the
certificate will be considered a "trusted certificate", i.e., a
certificate that is acceptable for encryption and/or authentication
in accordance with the XMPP entity's local service policies or
configured settings.

For both server certificates and client certificates, the validating
entity MUST do the following:

1. Attempt to verify the integrity of the certificate.

2. Attempt to verify that the certificate has been properly signed
by the issuing Certificate Authority.

3. Attempt to validate the full certification path.

4. Check the rules for end entity public key certificates and
certification authority certificates specified under
Section 13.7.1.1 for the general case and under either
Section 13.7.1.2 or Section 13.7.1.3 for XMPP server or client
certificates, respectively.

5. Check certificate revocation messages via Certificate Revocation
Lists (CRLs), the Online Certificate Status Protocol [OCSP], or
both.

If any of those validation attempts fail, the validating entity MUST
unilaterally terminate the session.

The following sections describe the additional identity verification
rules that apply to server-to-server and client-to-server streams.

Once the identity of the stream peer has been validated, the
validating entity SHOULD also correlate the validated identity with
the 'from' address (if any) of the stream header it received from the
peer. If the two identities do not match, the validating entity
SHOULD terminate the connection attempt (however, there might be good
reasons why the identities do not match, as described under
Section 4.7.1).

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13.7.2.1. Server Certificates

For server certificates, the rules and guidelines defined in
[TLS-CERTS] apply, with the proviso that the XmppAddr identifier
specified under Section 13.7.1.4 is allowed as a reference
identifier.

The identities to be checked are set as follows:

o The initiating entity sets the source domain of its reference
identifiers to the 'to' address it communicates in the initial
stream header; i.e., this is the identity it expects the receiving
entity to provide in a PKIX certificate.

o The receiving entity sets the source domain of its reference
identifiers to the 'from' address communicated by the initiating
entity in the initial stream header; i.e., this is the identity
that the initiating entity is trying to assert.

In the case of server-to-server communication, the matching procedure
described in [TLS-CERTS] can be performed by an application server
(receiving entity) when verifying an incoming server-to-server
connection from a peer server (initiating entity). In this case, the
receiving entity verifies the identity of the initiating entity and
uses as the source domain of its reference identifiers the DNS domain
name asserted by the initiating entity in the 'from' attribute of the
initial stream header. However, the matching procedure described in
[TLS-CERTS] remains unchanged and is applied in the same way.

13.7.2.2. Client Certificates

When an XMPP server validates a certificate presented by a client,
there are three possible cases, as discussed in the following
sections.

The identities to be checked are set as follows:

o The client sets the source domain of its reference identifiers to
the 'to' address it communicates in the initial stream header;
i.e., this is the identity it expects the server to provide in a
PKIX certificate.

o The server sets the bare JID of its reference identifiers to the
'from' address communicated by the initiating entity in the
initial stream header; i.e., this is the identity that the client
is trying to assert.

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13.7.2.2.1. Case #1

If the client certificate appears to be certified by a certification
path terminating in a trust anchor (as described in Section 6.1 of
[PKIX]), the server MUST check the certificate for any instances of
the XmppAddr as described under Section 13.7.1.4. There are three
possible sub-cases:

Sub-Case #1: The server finds one XmppAddr for which the domainpart
of the represented JID matches one of the configured FQDNs of the
server; the server SHOULD use this represented JID as the
validated identity of the client.

Sub-Case #2: The server finds more than one XmppAddr for which the
domainpart of the represented JID matches one of the configured
FQDNs of the server; the server SHOULD use one of these
represented JIDs as the validated identity of the client, choosing
among them based on the bare JID contained in the 'from' address
of the initial stream header (if any), based on the domainpart
contained in the 'to' address of the initial stream header, or in
accordance with local service policies (such as a lookup in a user
database based on other information contained in the client
certificate).

Sub-Case #3: The server finds no XmppAddrs, or finds at least one
XmppAddr but the domainpart of the represented JID does not match
one of the configured FQDNs of the server; the server MUST NOT use
the represented JID (if any) as the validated identity of the
client but instead MUST validate the identity of the client using
other means in accordance with local service policies (such as a
lookup in a user database based on other information contained in
the client certificate). If the identity cannot be so validated,
the server MAY abort the validation process and terminate the TLS
negotiation.

13.7.2.2.2. Case #2

If the client certificate is certified by a Certificate Authority not
known to the server, the server MUST proceed as under Case #1, Sub-
Case #3.

13.7.2.2.3. Case #3

If the client certificate is self-signed, the server MUST proceed as
under Case #1, Sub-Case #3.

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13.7.2.3. Checking of Certificates in Long-Lived Streams

Because XMPP uses long-lived XML streams, it is possible that a
certificate presented during stream negotiation might expire or be
revoked while the stream is still live (this is especially relevant
in the context of server-to-server streams). Therefore, each party
to a long-lived stream SHOULD:

1. Cache the expiration date of the certificate presented by the
other party and any certificates on which that certificate
depends (such as a root or intermediate certificate for a
certification authority), and close the stream when any such
certificate expires, with a stream error of
(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 (Section 4.9.3.16). It is RECOMMENDED
to query the OSCP responder at or near the time communicated via
the nextUpdate field received in the OCSP response or, if the
nextUpdate field is not set, to query every 24 hours.

After the stream is closed, the initiating entity from the closed
stream will need to reconnect and the receiving entity will need to
authenticate the initiating entity based on whatever certificate it
presents during negotiation of the new stream.

13.7.2.4. Use of Certificates in XMPP Extensions

Certificates MAY be used in extensions to XMPP for the purpose of
application-layer encryption or authentication above the level of XML
streams (e.g., for end-to-end encryption). Such extensions will
define their own certificate handling rules. At a minimum, such
extensions are encouraged to remain consistent with the rules defined
in this specification, specifying additional rules only when
necessary.

13.8. Mandatory-to-Implement TLS and SASL Technologies

The following TLS ciphersuites and SASL mechanisms are mandatory-to-
implement (naturally, implementations MAY support other ciphersuites
and mechanisms as well). For security considerations related to TLS
ciphersuites, see Section 13.9.4 and [TLS]. For security

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considerations related to SASL mechanisms, see Section 13.9.4,
[SASL], and specifications for particular SASL mechanisms such as
[SCRAM], [DIGEST-MD5], and [PLAIN].

13.8.1. For Authentication Only

For authentication only, servers and clients MUST support the SASL
Salted Challenge Response Authentication Mechanism [SCRAM] -- in
particular, the SCRAM-SHA-1 and SCRAM-SHA-1-PLUS variants.

Security Warning: Even though it is possible to complete
authentication only without confidentiality, it is RECOMMENDED for
servers and clients to protect the stream with TLS before
attempting authentication with SASL, both to help protect the
information exchanged during SASL negotiation and to help prevent
certain downgrade attacks as described under Section 13.9.4 and
Section 13.9.5. Even if TLS is used, implementations SHOULD also
enforce channel binding as described under Section 13.9.4.

Interoperability Note: The SCRAM-SHA-1 or SASL-SCRAM-SHA-1-PLUS
variants of the SCRAM mechanism replace the SASL DIGEST-MD5
mechanism as XMPP's mandatory-to-implement password-based method
for authentication only. For backward-compatibility with existing
deployed infrastructure, implementations are encouraged to
continue supporting the DIGEST-MD5 mechanism as specified in
[DIGEST-MD5]; however, there are known interoperability issues
with DIGEST-MD5 that make it impractical in the long term.

13.8.2. For Confidentiality Only

For confidentiality only, servers MUST support TLS with the
TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite.

Security Warning: Because a connection with confidentiality only
has weaker security properties than a connection with both
confidentiality and authentication, it is RECOMMENDED for servers
and clients to prefer connections with both qualities (e.g., by
protecting the stream with TLS before attempting authentication
with SASL). In practice, confidentiality only is employed merely
for server-to-server connections when the peer server does not
present a trusted certificate and the servers use Server Dialback
[XEP-0220] for weak identity verification, but TLS with
confidentiality only is still desirable to protect the connection
against casual eavesdropping.

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13.8.3. For Confidentiality and Authentication with Passwords

For both confidentiality and authentication with passwords, servers
and clients MUST implement TLS with the TLS_RSA_WITH_AES_128_CBC_SHA
ciphersuite plus SASL SCRAM, in particular the SCRAM-SHA-1 and
SCRAM-SHA-1-PLUS variants (with SCRAM-SHA1-PLUS being preferred, as
described under Section 13.9.4).

As further explained in the following Security Warning, in certain
circumstances a server MAY offer TLS with the
TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite plus SASL PLAIN when it is
not possible to offer more secure alternatives; in addition, clients
SHOULD implement PLAIN over TLS in order to maximize interoperability
with servers that are not able to deploy more secure alternatives.

Security Warning: In practice, many servers offer, and many
clients use, TLS plus SASL PLAIN. The SCRAM-SHA-1 and especially
SCRAM-SHA-1-PLUS variants of the SCRAM mechanism are strongly
preferred over the PLAIN mechanism because of their superior
security properties (including for SCRAM-SHA-1-PLUS the ability to
enforce channel binding as described under Section 13.9.4). A
client SHOULD treat TLS plus SASL PLAIN as a technology of last
resort to be used only when interacting with a server that does
not offer SCRAM (or other alternatives that are more secure than
TLS plus SASL PLAIN), MUST prefer more secure mechanisms (e.g.,
EXTERNAL, SCRAM-SHA-1-PLUS, SCRAM-SHA-1, or the older DIGEST-MD5
mechanism) to the PLAIN mechanism, and MUST NOT use the PLAIN
mechanism if the stream does not at a minimum have confidentiality
and integrity protection via TLS with full certificate validation
as described under Section 13.7.2.1. A server MUST NOT offer SASL
PLAIN if the confidentiality and integrity of the stream are not
protected via TLS or an equivalent security layer. A server
SHOULD NOT offer TLS plus SASL PLAIN unless it is unable to offer
some variant of SASL SCRAM (or other alternatives that are more
secure than TLS plus SASL PLAIN), e.g., because the XMPP service
depends for authentication purposes on a database or directory
that is not under the control of the XMPP administrators, such as
Pluggable Authentication Modules (PAM), an Lightweight Directory
Access Protocol (LDAP) directory [LDAP], or an Authentication,
Authorization, and Accounting (AAA) key management protocol (for
guidance, refer to [AAA]). However, offering TLS plus SASL PLAIN
even when the server supports more secure alternatives might be
appropriate if the server needs to enable interoperability with an
installed base of clients that do not yet support SCRAM or other
alternatives that are more secure than TLS plus SASL PLAIN.

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13.8.4. For Confidentiality and Authentication without Passwords

For both confidentiality and authentication without passwords,
servers MUST and clients SHOULD implement TLS with the
TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite plus the SASL EXTERNAL
mechanism (see Appendix A of [SASL]) with PKIX certificates.

13.9. Technology Reuse

13.9.1. Use of Base 64 in SASL

Both the client and the server MUST verify any base 64 data received
during SASL negotiation (Section 6). An implementation MUST reject
(not ignore) any characters that are not explicitly allowed by the
base 64 alphabet; this helps to guard against creation of a covert
channel that could be used to "leak" information.

An implementation MUST NOT break on invalid input and MUST reject any
sequence of base 64 characters containing the pad ('=') character if
that character is included as something other than the last character
of the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against
buffer overflow attacks and other attacks on the implementation.

While base 64 encoding visually hides otherwise easily recognized
information (such as passwords), it does not provide any
computational confidentiality.

All uses of base 64 encoding MUST follow the definition in Section 4
of [BASE64] and padding bits MUST be set to zero.

13.9.2. Use of DNS

XMPP typically relies on the Domain Name System (specifically
[DNS-SRV] records) to resolve a fully qualified domain name to an IP
address before a client connects to a server or before a peer server
connects to another server. Before attempting to negotiate an XML
stream, the initiating entity MUST NOT proceed until it has resolved
the DNS domain name of the receiving entity as specified under
Section 3 (although it is not necessary to resolve the DNS domain
name before each connection attempt, because DNS resolution results
can be temporarily cached in accordance with time-to-live values).
However, in the absence of a secure DNS option (e.g., as provided by
[DNSSEC]), a malicious attacker with access to the DNS server data,
or able to cause spoofed answers to be cached in a recursive
resolver, can potentially cause the initiating entity to connect to
any XMPP server chosen by the attacker. Deployment and validation of
server certificates help to prevent such attacks.

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13.9.3. Use of Hash Functions

XMPP itself does not directly mandate the use of any particular
cryptographic hash function. However, technologies on which XMPP
depends (e.g., TLS and particular SASL mechanisms), as well as
various XMPP extensions, might make use of cryptographic hash
functions. Those who implement XMPP technologies or who develop XMPP
extensions are advised to closely monitor the state of the art
regarding attacks against cryptographic hash functions in Internet
protocols as they relate to XMPP. For helpful guidance, refer to
[HASHES].

13.9.4. Use of SASL

Because the initiating entity chooses an acceptable SASL mechanism
from the list presented by the receiving entity, the initiating
entity depends on the receiving entity's list for authentication.
This dependency introduces the possibility of a downgrade attack if
an attacker can gain control of the channel and therefore present a
weak list of mechanisms. To mitigate this attack, the parties SHOULD
protect the channel using TLS before attempting SASL negotiation and
either perform full certificate validation as described under
Section 13.7.2.1 or use a SASL mechanism that provides channel
bindings, such as SCRAM-SHA-1-PLUS. (Protecting the channel via TLS
with full certificate validation can help to ensure the
confidentiality and integrity of the information exchanged during
SASL negotiation.)

The SASL framework itself does not provide a method for binding SASL
authentication to a security layer providing confidentiality and
integrity protection that was negotiated at a lower layer (e.g.,
TLS). Such a binding is known as a "channel binding" (see
[CHANNEL]). Some SASL mechanisms provide channel bindings, which in
the case of XMPP would typically be a binding to TLS (see
[CHANNEL-TLS]). If a SASL mechanism provides a channel binding
(e.g., this is true of [SCRAM]), then XMPP entities using that
mechanism SHOULD prefer the channel binding variant (e.g., preferring
"SCRAM-SHA-1-PLUS" over "SCRAM-SHA-1"). If a SASL mechanism does not
provide a channel binding, then the mechanism cannot provide a way to
verify that the source and destination end points to which the lower
layer's security is bound are equivalent to the end points that SASL
is authenticating; furthermore, if the end points are not identical,
then the lower layer's security cannot be trusted to protect data
transmitted between the SASL-authenticated entities. In such a
situation, a SASL security layer SHOULD be negotiated that
effectively ignores the presence of the lower-layer security.

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Many deployed XMPP services authenticate client connections by means
of passwords. It is well known that most human users choose
relatively weak passwords. Although service provisioning is out of
scope for this document, XMPP servers that allow password-based
authentication SHOULD enforce minimal criteria for password strength
to help prevent dictionary attacks. Because all password-based
authentication mechanisms are susceptible to password guessing
attacks, XMPP servers MUST limit the number of retries allowed during
SASL authentication, as described under Section 6.4.5.

Some SASL mechanisms (e.g., [ANONYMOUS]) do not provide strong peer
entity authentication of the client to the server. Service
administrators are advised to enable such mechanisms with caution.
Best practices for the use of the SASL ANONYMOUS mechanism in XMPP
are described in [XEP-0175].

13.9.5. Use of TLS

Implementations of TLS typically support multiple versions of the
Transport Layer Security protocol as well as the older Secure Sockets
Layer (SSL) protocol. Because of known security vulnerabilities,
XMPP servers and clients MUST NOT request, offer, or use SSL 2.0.
For further details, see Appendix E.2 of [TLS] along with [TLS-SSL2].

To prevent man-in-the-middle attacks, the TLS client (which might be
an XMPP client or an XMPP server) MUST verify the certificate of the
TLS server and MUST check its understanding of the server FQDN
against the server's identity as presented in the TLS Certificate
message as described under Section 13.7.2.1 (for further details, see
[TLS-CERTS].

Support for TLS renegotiation is strictly OPTIONAL. However,
implementations that support TLS renegotiation MUST implement and use
the TLS Renegotiation Extension [TLS-NEG]. Further details are
provided under Section 5.3.5.

13.9.6. Use of UTF-8

The use of UTF-8 makes it possible to transport non-ASCII characters,
and thus enables character "spoofing" scenarios, in which a displayed
value appears to be something other than it is. Furthermore, there
are known attack scenarios related to the decoding of UTF-8 data. On
both of these points, refer to [UTF-8] for more information.

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13.9.7. Use of XML

Because XMPP is an application profile of the Extensible Markup
Language [XML], many of the security considerations described in
[XML-MEDIA] and [XML-GUIDE] also apply to XMPP. Several aspects of
XMPP mitigate the risks described there, such as the prohibitions
specified under Section 11.1 and the lack of external references to
style sheets or transformations, but these mitigating factors are by
no means comprehensive.

13.10. Information Leaks

13.10.1. IP Addresses

A client's IP address and method of access MUST NOT be made public by
a server (e.g., as typically occurs in [IRC]).

13.10.2. Presence Information

One of the core aspects of XMPP is presence: information about the
network availability of an XMPP entity (i.e., whether the entity is
currently online or offline). A "presence leak" occurs when an
entity's network availability is inadvertently and involuntarily
revealed to a second entity that is not authorized to know the first
entity's network availability.

Although presence is discussed more fully in [XMPP-IM], it is
important to note that an XMPP server MUST NOT leak presence. In
particular at the core XMPP level, real-time addressing and network
availability is associated with a specific connected resource;
therefore, any disclosure of a connected resource's full JID
comprises a presence leak. To help prevent such a presence leak, a
server MUST NOT return different stanza errors depending on whether a
potential attacker sends XML stanzas to the entity's bare JID
() or full JID
().

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 stanza error condition (Section 8.3.3.19) can help
protect against directory harvesting attacks, since this is the same
error condition that is returned if, for instance, the namespace of
an IQ child element is not understood, or if "offline message
storage" ([XEP-0160]) or message forwarding is not enabled for a
domain. However, subtle differences in the exact XML of error
stanzas, as well as in the timing with which such errors are

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returned, can enable an attacker to determine the network presence of
a user when more advanced blocking technologies are not used (see for
instance [XEP-0016] and [XEP-0191]). Therefore, a server that
exercises a higher level of caution might not return any error at all
in response to certain kinds of received stanzas, so that a non-
existent user appears to behave like a user that has no interest in
conversing with the sender.

13.12. Denial of Service

[DOS] defines denial of service as follows:

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

Some considerations discussed in this document help to prevent
denial-of-service attacks (e.g., the mandate that a server MUST NOT
process XML stanzas from clients that have not yet provided
appropriate authentication credentials and MUST NOT process XML
stanzas from peer servers whose identity it has not either
authenticated via SASL or weakly verified via Server Dialback).

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

1. A server implementation SHOULD enable a server administrator to
limit the number of TCP connections that it will accept from a
given IP address at any one time. If an entity attempts to
connect but the maximum number of TCP connections has been
reached, the receiving server MUST NOT allow the new connection
to proceed.

2. A server implementation SHOULD enable a server administrator to
limit the number of TCP connection attempts that it will accept
from a given IP address in a given time period. If an entity
attempts to connect but the maximum number of connection attempts
has been reached, the receiving server MUST NOT allow the new
connection to proceed.

3. A server implementation SHOULD enable a server administrator to
limit the number of connected resources it will allow an account
to bind at any one time. If a client attempts to bind a resource

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but it has already reached the configured number of allowable
resources, the receiving server MUST return a 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 stanza error (Section 8.3.3.12),
thus allowing the sender to recover, or close the stream with a
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 stanza error (Section 8.3.3.12).

6. A server implementation SHOULD enable a server administrator to
limit the amount of bandwidth it will allow a connected client or
peer server to use in a given time period.

7. A server implementation MAY enable a server administrator to
limit the types of stanzas (based on the extended content
"payload") that it will allow a connected resource or peer server
send over an active connection. Such limits and restrictions are
a matter of deployment policy.

8. A server implementation MAY refuse to route or deliver any stanza
that it considers to be abusive, with or without returning an
error to the sender.

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

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13.13. Firewalls

Although DNS SRV records can instruct connecting entities to use TCP
ports other than 5222 (client-to-server) and 5269 (server-to-server),
communication using XMPP typically occurs over those ports, which are
registered with the IANA (see Section 14). Use of these well-known
ports allows administrators to easily enable or disable XMPP activity
through existing and commonly deployed firewalls.

13.14. Interdomain Federation

The term "federation" is commonly used to describe communication
between two servers.

Because service provisioning is a matter of policy, it is OPTIONAL
for any given server to support federation. If a particular server
enables federation, it SHOULD enable strong security as previously
described to ensure both authentication and confidentiality;
compliant implementations SHOULD support TLS and SASL for this
purpose.

Before RFC 3920 defined TLS plus SASL EXTERNAL with certificates for
encryption and authentication of server-to-server streams, the only
method for weak identity verification of a peer server was Server
Dialback as defined in [XEP-0220]. Even when [DNSSEC] is used,
Server Dialback provides only weak identity verification and provides
no confidentiality or integrity. At the time of writing, Server
Dialback is still the most widely used technique for some level of
assurance over server-to-server streams. This reality introduces the
possibility of a downgrade attack from TLS + SASL EXTERNAL to Server
Dialback if an attacker can gain control of the channel and therefore
convince the initiating server that the receiving server does not
support TLS or does not have an appropriate certificate. To help
prevent this attack, the parties SHOULD protect the channel using TLS
before proceeding, even if the presented certificates are self-signed
or otherwise untrusted.

13.15. Non-Repudiation

Systems that provide both peer entity authentication and data
integrity have the potential to enable an entity to prove to a third
party that another entity intended to send particular data. Although
XMPP systems can provide both peer entity authentication and data
integrity, XMPP was never designed to provide non-repudiation.

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14. IANA Considerations

The following subsections update the registrations provided in
[RFC3920]. This section is to be interpreted according to
[IANA-GUIDE].

14.1. XML Namespace Name for TLS Data

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

URI: urn:ietf:params:xml:ns:xmpp-tls
Specification: RFC 6120
Description: This is the XML namespace name for STARTTLS negotiation
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by RFC 6120.
Registrant Contact: IESG

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

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

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14.4. XML Namespace Name for Resource Binding

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

URI: urn:ietf:params:xml:ns:xmpp-bind
Specification: RFC 6120
Description: This is the XML namespace name for resource binding in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by RFC 6120.
Registrant Contact: IESG

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

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
Reference: RFC 6120
Port Number: 5222

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Service Name: xmpp-server
Transport Protocol: TCP
Description: A service offering support for connections by XMPP
server applications
Registrant: IETF XMPP Working Group
Contact: IESG
Reference: RFC 6120
Port Number: 5269

15. Conformance Requirements

This section describes a protocol feature set that summarizes the
conformance requirements of this specification. This feature set is
appropriate for use in software certification, interoperability
testing, and implementation reports. For each feature, this section
provides the following information:

o A human-readable name

o An informational description

o A reference to the particular section of this document that
normatively defines the feature

o Whether the feature applies to the Client role, the Server role,
or both (where "N/A" signifies that the feature is not applicable
to the specified role)

o Whether the feature MUST or SHOULD be implemented, where the
capitalized terms are to be understood as described in [KEYWORDS]

The feature set specified here attempts to adhere to the concepts and
formats proposed by Larry Masinter within the IETF's NEWTRK Working
Group in 2005, as captured in [INTEROP]. Although this feature set
is more detailed than called for by [REPORTS], it provides a suitable
basis for the generation of implementation reports to be submitted in
support of advancing this specification from Proposed Standard to
Draft Standard in accordance with [PROCESS].

Feature: bind-gen
Description: Generate a random resource on demand.
Section: Section 7.6
Roles: Client N/A, Server MUST.

Feature: bind-mtn
Description: Consider resource binding as mandatory-to-negotiate.
Section: Section 7.3.1
Roles: Client MUST, Server MUST.

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Feature: bind-restart
Description: Do not restart the stream after negotiation of resource
binding.
Section: Section 7.3.2
Roles: Client MUST, Server MUST.

Feature: bind-support
Description: Support binding of client resources to an authenticated
stream.
Section: Section 7
Roles: Client MUST, Server MUST.

Feature: sasl-correlate
Description: When authenticating a stream peer using SASL, correlate
the authentication identifier resulting from SASL negotiation with
the 'from' address (if any) of the stream header it received from
the peer.
Section: Section 6.4.6
Roles: Client SHOULD, Server SHOULD.

Feature: sasl-errors
Description: Support SASL errors during the negotiation process.
Section: Section 6.5
Roles: Client MUST, Server MUST.

Feature: sasl-mtn
Description: Consider SASL as mandatory-to-negotiate.
Section: Section 6.3.1
Roles: Client MUST, Server MUST.

Feature: sasl-restart
Description: Initiate or handle a stream restart after SASL
negotiation.
Section: Section 6.3.2
Roles: Client MUST, Server MUST.

Feature: sasl-support
Description: Support the Simple Authentication and Security Layer
for stream authentication.
Section: Section 6
Roles: Client MUST, Server MUST.

Feature: security-mti-auth-scram
Description: Support the SASL SCRAM mechanism for authentication
only (this implies support for both the SCRAM-SHA-1 and
SCRAM-SHA-1-PLUS variants).
Section: Section 13.8
Roles: Client MUST, Server MUST.

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Feature: security-mti-both-external
Description: Support TLS with SASL EXTERNAL for confidentiality and
authentication.
Section: Section 13.8
Roles: Client SHOULD, Server MUST.

Feature: security-mti-both-plain
Description: Support TLS using the TLS_RSA_WITH_AES_128_CBC_SHA
ciphersuite plus the SASL PLAIN mechanism for confidentiality and
authentication.
Section: Section 13.8
Roles: Client SHOULD, Server MAY.

Feature: security-mti-both-scram
Description: Support TLS using the TLS_RSA_WITH_AES_128_CBC_SHA
ciphersuite plus the SCRAM-SHA-1 and SCRAM-SHA-1-PLUS variants of
the SASL SCRAM mechanism for confidentiality and authentication.
Section: Section 13.8
Roles: Client MUST, Server MUST.

Feature: security-mti-confidentiality
Description: Support TLS using the TLS_RSA_WITH_AES_128_CBC_SHA
ciphersuite for confidentiality only.
Section: Section 13.8
Roles: Client N/A, Server SHOULD.

Feature: stanza-attribute-from
Description: Support the common 'from' attribute for all stanza
kinds.
Section: Section 8.1.2
Roles: Client MUST, Server MUST.

Feature: stanza-attribute-from-stamp
Description: Stamp or rewrite the 'from' address of all stanzas
received from connected clients.
Section: Section 8.1.2.1
Roles: Client N/A, Server MUST.

Feature: stanza-attribute-from-validate
Description: Validate the 'from' address of all stanzas received
from peer servers.
Section: Section 8.1.2.2
Roles: Client N/A, Server MUST.

Feature: stanza-attribute-id
Description: Support the common 'id' attribute for all stanza kinds.
Section: Section 8.1.3
Roles: Client MUST, Server MUST.

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Feature: stanza-attribute-to
Description: Support the common 'to' attribute for all stanza kinds.
Section: Section 8.1.1
Roles: Client MUST, Server MUST.

Feature: stanza-attribute-to-validate
Description: Ensure that all stanzas received from peer servers
include a 'to' address.
Section: Section 8.1.1
Roles: Client N/A, Server MUST.

Feature: stanza-attribute-type
Description: Support the common 'type' attribute for all stanza
kinds.
Section: Section 8.1.4
Roles: Client MUST, Server MUST.

Feature: stanza-attribute-xmllang
Description: Support the common 'xml:lang' attribute for all stanza
kinds.
Section: Section 8.1.5
Roles: Client MUST, Server MUST.

Feature: stanza-error
Description: Generate and handle stanzas of type "error" for all
stanza kinds.
Section: Section 8.3
Roles: Client MUST, Server MUST.

Feature: stanza-error-child
Description: Ensure that stanzas of type "error" include an
child element.
Section: Section 8.3
Roles: Client MUST, Server MUST.

Feature: stanza-error-id
Description: Ensure that stanzas of type "error" preserve the 'id'
provided in the triggering stanza.
Section: Section 8.3
Roles: Client MUST, Server MUST.

Feature: stanza-error-reply
Description: Do not reply to a stanza of type "error" with another
stanza of type "error".
Section: Section 8.3
Roles: Client MUST, Server MUST.

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Feature: stanza-extension
Description: Correctly process XML data qualified by an unsupported
XML namespace, where "correctly process" means to ignore that
portion of the stanza in the case of a message or presence stanza
and return an error in the case of an IQ stanza (for the intended
recipient), and to route or deliver the stanza (for a routing
entity such as a server).
Section: Section 8.4
Roles: Client MUST, Server MUST.

Feature: stanza-iq-child
Description: Include exactly one child element in an stanza of
type "get" or "set", zero or one child elements in an stanza
of type "result", and one or two child elements in an stanza
of type "error".
Section: Section 8.2.3
Roles: Client MUST, Server MUST.

Feature: stanza-iq-id
Description: Ensure that all stanzas include an 'id'
attribute.
Section: Section 8.2.3
Roles: Client MUST, Server MUST.

Feature: stanza-iq-reply
Description: Reply to an stanza of type "get" or "set" with an
stanza of type "result" or "error".
Section: Section 8.2.3
Roles: Client MUST, Server MUST.

Feature: stanza-iq-type
Description: Ensure that all 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 stanza.
Section: Section 8.2.3
Roles: Client MUST, Server MUST.

Feature: stanza-kind-message
Description: Support the stanza.
Section: Section 8.2.1
Roles: Client MUST, Server MUST.

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Feature: stanza-kind-presence
Description: Support the stanza.
Section: Section 8.2.2
Roles: Client MUST, Server MUST.

Feature: stream-attribute-initial-from
Description: Include a 'from' attribute in the initial stream
header.
Section: Section 4.7.1
Roles: Client SHOULD, Server MUST.

Feature: stream-attribute-initial-lang
Description: Include an 'xml:lang' attribute in the initial stream
header.
Section: Section 4.7.4
Roles: Client SHOULD, Server SHOULD.

Feature: stream-attribute-initial-to
Description: Include a 'to' attribute in the initial stream header.
Section: Section 4.7.2
Roles: Client MUST, Server MUST.

Feature: stream-attribute-response-from
Description: Include a 'from' attribute in the response stream
header.
Section: Section 4.7.1
Roles: Client N/A, Server MUST.

Feature: stream-attribute-response-id
Description: Include an 'id' attribute in the response stream
header.
Section: Section 4.7.3
Roles: Client N/A, Server MUST.

Feature: stream-attribute-response-id-unique
Description: Ensure that the 'id' attribute in the response stream
header is unique within the context of the receiving entity.
Section: Section 4.7.3
Roles: Client N/A, Server MUST.

Feature: stream-attribute-response-to
Description: Include a 'to' attribute in the response stream header.
Section: Section 4.7.2
Roles: Client N/A, Server SHOULD.

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Feature: stream-error-generate
Description: Generate a stream error (followed by a closing stream
tag and termination of the TCP connection) upon detecting a
stream-related error condition.
Section: Section 4.9
Roles: Client MUST, Server MUST.

Feature: stream-fqdn-resolution
Description: Resolve FQDNs before opening a TCP connection to the
receiving entity.
Section: Section 3.2
Roles: Client MUST, Server MUST.

Feature: stream-negotiation-complete
Description: Do not consider the stream negotiation process to be
complete until the receiving entity sends a stream features
advertisement that is empty or that contains only voluntary-to-
negotiate features.
Section: Section 4.3.5
Roles: Client MUST, Server MUST.

Feature: stream-negotiation-features
Description: Send stream features after sending a response stream
header.
Section: Section 4.3.2
Roles: Client N/A, Server MUST.

Feature: stream-negotiation-restart
Description: Consider the previous stream to be replaced upon
negotiation of a stream feature that necessitates a stream
restart, and send or receive a new initial stream header after
negotiation of such a stream feature.
Section: Section 4.3.3
Roles: Client MUST, Server MUST.

Feature: stream-reconnect
Description: Reconnect with exponential backoff if a TCP connection
is terminated unexpectedly.
Section: Section 3.3
Roles: Client MUST, Server MUST.

Feature: stream-tcp-binding
Description: Bind an XML stream to a TCP connection.
Section: Section 3
Roles: Client MUST, Server MUST.

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Feature: tls-certs
Description: Check the identity specified in a certificate that is
presented during TLS negotiation.
Section: Section 13.7.2
Roles: Client MUST, Server MUST.

Feature: tls-mtn
Description: Consider TLS as mandatory-to-negotiate if STARTTLS is
the only feature advertised or if the STARTTLS feature
advertisement includes an empty element.
Section: Section 5.3.1
Roles: Client MUST, Server MUST.

Feature: tls-restart
Description: Initiate or handle a stream restart after TLS
negotiation.
Section: Section 5.3.2
Roles: Client MUST, Server MUST.

Feature: tls-support
Description: Support Transport Layer Security for stream encryption.
Section: Section 5
Roles: Client MUST, Server MUST.

Feature: tls-correlate
Description: When validating a certificate presented by a stream
peer during TLS negotiation, correlate the validated identity with
the 'from' address (if any) of the stream header it received from
the peer.
Section: Section 13.7.2
Roles: Client SHOULD, Server SHOULD.

Feature: xml-namespace-content-client
Description: Support 'jabber:client' as a content namespace.
Section: Section 4.8.2
Roles: Client MUST, Server MUST.

Feature: xml-namespace-content-server
Description: Support 'jabber:server' as a content namespace.
Section: Section 4.8.2
Roles: Client N/A, Server MUST.

Feature: xml-namespace-streams-declaration
Description: Ensure that there is a namespace declaration for the
'http://etherx.jabber.org/streams' namespace.
Section: Section 4.8.1
Roles: Client MUST, Server MUST.

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Feature: xml-namespace-streams-prefix
Description: Ensure that all elements qualified by the
'http://etherx.jabber.org/streams' namespace are prefixed by the
prefix (if any) defined in the namespace declaration.
Section: Section 4.8.1
Roles: Client MUST, Server MUST.

Feature: xml-restriction-comment
Description: Do not generate or accept XML comments.
Section: Section 11.1
Roles: Client MUST, Server MUST.

Feature: xml-restriction-dtd
Description: Do not generate or accept internal or external DTD
subsets.
Section: Section 11.1
Roles: Client MUST, Server MUST.

Feature: xml-restriction-pi
Description: Do not generate or accept XML processing instructions.
Section: Section 11.1
Roles: Client MUST, Server MUST.

Feature: xml-restriction-ref
Description: Do not generate or accept internal or external entity
references with the exception of the predefined entities.
Section: Section 11.1
Roles: Client MUST, Server MUST.

Feature: xml-wellformed-xml
Description: Do not generate or accept data that is not XML-well-
formed.
Section: Section 11.3
Roles: Client MUST, Server MUST.

Feature: xml-wellformed-ns
Description: Do not generate or accept data that is not namespace-
well-formed.
Section: Section 11.3
Roles: Client MUST, Server MUST.

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16. References

16.1. Normative References

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

[CHANNEL] Williams, N., "On the Use of Channel Bindings to
Secure Channels", RFC 5056, November 2007.

[CHANNEL-TLS] Altman, J., Williams, N., and L. Zhu, "Channel
Bindings for TLS", RFC 5929, July 2010.

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

[DNS-CONCEPTS] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.

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

[IPv6-ADDR] Kawamura, S. and M. Kawashima, "A Recommendation for
IPv6 Address Text Representation", RFC 5952,
August 2010.

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

[LANGMATCH] Phillips, A. and M. Davis, "Matching of Language
Tags", BCP 47, RFC 4647, September 2006.

[LANGTAGS] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.

[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and
C. Adams, "X.509 Internet Public Key Infrastructure
Online Certificate Status Protocol - OCSP", RFC 2560,
June 1999.

[PKIX] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.

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[PKIX-ALGO] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.

[PKIX-SRV] Santesson, S., "Internet X.509 Public Key
Infrastructure Subject Alternative Name for
Expression of Service Name", RFC 4985, August 2007.

[PLAIN] Zeilenga, K., "The PLAIN Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4616,
August 2006.

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

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

[SCRAM] Newman, C., Menon-Sen, A., Melnikov, A., and N.
Williams, "Salted Challenge Response Authentication
Mechanism (SCRAM) SASL and GSS-API Mechanisms",
RFC 5802, July 2010.

[STRONGSEC] Schiller, J., "Strong Security Requirements for
Internet Engineering Task Force Standard Protocols",
BCP 61, RFC 3365, August 2002.

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

[TLS] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.

[TLS-CERTS] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service
Identity within Internet Public Key Infrastructure
Using X.509 (PKIX) Certificates in the Context of
Transport Layer Security (TLS)", RFC 6125,
March 2011.

[TLS-NEG] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation
Indication Extension", RFC 5746, February 2010.

[TLS-SSL2] Turner, S. and T. Polk, "Prohibiting Secure Sockets
Layer (SSL) Version 2.0", RFC 6176, March 2011.

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

[UNICODE] The Unicode Consortium, "The Unicode Standard,
Version 6.0", 2010,
.

[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,
.

[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,
.

[XMPP-ADDR] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Address Format", RFC 6122,
March 2011.

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[XMPP-IM] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
RFC 6121, March 2011.

16.2. Informative References

[AAA] Housley, R. and B. Aboba, "Guidance for
Authentication, Authorization, and Accounting (AAA)
Key Management", BCP 132, RFC 4962, July 2007.

[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
January 2008.

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

[ANONYMOUS] Zeilenga, K., "Anonymous Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4505,
June 2006.

[ASN.1] CCITT, "Recommendation X.208: Specification of
Abstract Syntax Notation One (ASN.1)", 1988.

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

[DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and
S. Rose, "DNS Security Introduction and
Requirements", RFC 4033, March 2005.

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

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

[E2E-REQS] Saint-Andre, P., "Requirements for End-to-End
Encryption in the Extensible Messaging and Presence
Protocol (XMPP)", Work in Progress, March 2010.

[EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.

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[ETHERNET] "Information technology - Telecommunications and
information exchange between systems - Local and
metropolitan area networks - Specific requirements -
Part 3: Carrier sense multiple access with collision
detection (CSMA/CD) access method and physical layer
specifications", IEEE Standard 802.3, September 1998.

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

[HASHES] Hoffman, P. and B. Schneier, "Attacks on
Cryptographic Hashes in Internet Protocols",
RFC 4270, November 2005.

[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
June 1999.

[IANA-GUIDE] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26,
RFC 5226, May 2008.

[IANA-PORTS] Cotton, M., Eggert, L., Touch, J., Westerlund, M.,
and S. Cheshire, "Internet Assigned Numbers Authority
(IANA) Procedures for the Management of the Transport
Protocol Port Number and Service Name Registry", Work
in Progress, February 2011.

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

[IMP-REQS] Day, M., Aggarwal, S., and J. Vincent, "Instant
Messaging / Presence Protocol Requirements",
RFC 2779, February 2000.

[INTEROP] Masinter, L., "Formalizing IETF Interoperability
Reporting", Work in Progress, October 2005.

[IRC] Kalt, C., "Internet Relay Chat: Architecture",
RFC 2810, April 2000.

[IRI] Duerst, M. and M. Suignard, "Internationalized
Resource Identifiers (IRIs)", RFC 3987, January 2005.

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[LDAP] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map", RFC 4510,
June 2006.

[LINKLOCAL] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses",
RFC 3927, May 2005.

[MAILBOXES] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES,
ROLES AND FUNCTIONS", RFC 2142, May 1997.

[POP3] Myers, J. and M. Rose, "Post Office Protocol -
Version 3", STD 53, RFC 1939, May 1996.

[PROCESS] Bradner, S., "The Internet Standards Process --
Revision 3", BCP 9, RFC 2026, October 1996.

[REPORTS] Dusseault, L. and R. Sparks, "Guidance on
Interoperation and Implementation Reports for
Advancement to Draft Standard", BCP 9, RFC 5657,
September 2009.

[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", 2000.

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

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

[SASLPREP] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.

[SEC-TERMS] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.

[SMTP] Klensin, J., "Simple Mail Transfer Protocol",
RFC 5321, October 2008.

[SEC-GUIDE] Rescorla, E. and B. Korver, "Guidelines for Writing
RFC Text on Security Considerations", BCP 72,
RFC 3552, July 2003.

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[TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
January 2011.

[TLS-RESUME] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption
without Server-Side State", RFC 5077, January 2008.

[URN-OID] Mealling, M., "A URN Namespace of Object
Identifiers", RFC 3061, February 2001.

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

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

[XEP-0001] Saint-Andre, P., "XMPP Extension Protocols", XSF
XEP 0001, March 2010.

[XEP-0016] Millard, P. and P. Saint-Andre, "Privacy Lists", XSF
XEP 0016, February 2007.

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

[XEP-0060] Millard, P., Saint-Andre, P., and R. Meijer,
"Publish-Subscribe", XSF XEP 0060, July 2010.

[XEP-0071] Saint-Andre, P., "XHTML-IM", XSF XEP 0071,
September 2008.

[XEP-0077] Saint-Andre, P., "In-Band Registration", XSF
XEP 0077, September 2009.

[XEP-0086] Norris, R. and P. Saint-Andre, "Error Condition
Mappings", XSF XEP 0086, February 2004.

[XEP-0100] Saint-Andre, P. and D. Smith, "Gateway Interaction",
XSF XEP 0100, October 2005.

[XEP-0114] Saint-Andre, P., "Jabber Component Protocol", XSF
XEP 0114, March 2005.

[XEP-0124] Paterson, I., Smith, D., and P. Saint-Andre,
"Bidirectional-streams Over Synchronous HTTP (BOSH)",
XSF XEP 0124, July 2010.

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[XEP-0138] Hildebrand, J. and P. Saint-Andre, "Stream
Compression", XSF XEP 0138, May 2009.

[XEP-0156] Hildebrand, J. and P. Saint-Andre, "Discovering
Alternative XMPP Connection Methods", XSF XEP 0156,
June 2007.

[XEP-0160] Saint-Andre, P., "Best Practices for Handling Offline
Messages", XSF XEP 0160, January 2006.

[XEP-0174] Saint-Andre, P., "Link-Local Messaging", XSF
XEP 0174, November 2008.

[XEP-0175] Saint-Andre, P., "Best Practices for Use of SASL
ANONYMOUS", XSF XEP 0175, September 2009.

[XEP-0178] Saint-Andre, P. and P. Millard, "Best Practices for
Use of SASL EXTERNAL with Certificates", XSF
XEP 0178, February 2007.

[XEP-0191] Saint-Andre, P., "Simple Communications Blocking",
XSF XEP 0191, February 2007.

[XEP-0198] Karneges, J., Hildebrand, J., Saint-Andre, P., Forno,
F., Cridland, D., and M. Wild, "Stream Management",
XSF XEP 0198, February 2011.

[XEP-0199] Saint-Andre, P., "XMPP Ping", XSF XEP 0199,
June 2009.

[XEP-0205] Saint-Andre, P., "Best Practices to Discourage Denial
of Service Attacks", XSF XEP 0205, January 2009.

[XEP-0206] Paterson, I. and P. Saint-Andre, "XMPP Over BOSH",
XSF XEP 0206, July 2010.

[XEP-0220] Miller, J., Saint-Andre, P., and P. Hancke, "Server
Dialback", XSF XEP 0220, March 2010.

[XEP-0225] Saint-Andre, P., "Component Connections", XSF
XEP 0225, October 2008.

[XEP-0233] Miller, M., Saint-Andre, P., and J. Hildebrand,
"Domain-Based Service Names in XMPP SASL
Negotiation", XSF XEP 0233, June 2010.

[XEP-0288] Hancke, P. and D. Cridland, "Bidirectional Server-to-
Server Connections", XSF XEP 0288, October 2010.

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[XML-FRAG] Grosso, P. and D. Veillard, "XML Fragment
Interchange", World Wide Web Consortium CR CR-xml-
fragment-20010212, February 2001,
.

[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,
.

[XMPP-URI] Saint-Andre, P., "Internationalized Resource
Identifiers (IRIs) and Uniform Resource Identifiers
(URIs) for the Extensible Messaging and Presence
Protocol (XMPP)", RFC 5122, February 2008.

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Appendix A. XML Schemas

The following schemas formally define various namespaces used in this
document, in conformance with [XML-SCHEMA]. Because validation of
XML streams and stanzas is optional, these schemas are not normative
and are provided for descriptive purposes only.

A.1. Stream Namespace

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A.2. Stream Error Namespace

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A.3. STARTTLS Namespace

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A.4. SASL Namespace

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A.5. Client Namespace

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A.6. Server Namespace

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A.7. Resource Binding Namespace

A.8. Stanza Error Namespace

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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 ).

Appendix C. Account Provisioning

Account provisioning is out of scope for this specification.
Possible methods for account provisioning include account creation by
a server administrator and in-band account registration using the
'jabber:iq:register' namespace as documented in [XEP-0077]. An XMPP
server implementation or administrative function MUST ensure that any
JID assigned during account provisioning (including localpart,
domainpart, resourcepart, and separator characters) conforms to the
canonical format for XMPP addresses defined in [XMPP-ADDR].

Appendix D. Differences from RFC 3920

Based on consensus derived from implementation and deployment
experience as well as formal interoperability testing, the following
substantive modifications were made from RFC 3920 (in addition to
numerous changes of an editorial nature).

o Moved specification of the XMPP address format to a separate
document.

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RFC 6120 XMPP Core March 2011

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 stream error
condition to for compliance with the XML
specification.

o Removed the unnecessary and unused stream error (see
RFC 3920 for historical documentation).

o Specified return of the stream error in response
to receipt of prohibited XML features.

o More completely specified the format and handling of the 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 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 , ,
, and 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 stanza error.

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RFC 6120 XMPP Core March 2011

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 element for stanza errors
so that the entity that has detected the error can include its JID
for diagnostic or tracking purposes.

o Clarified the handling of data that violates the well-formedness
definitions for XML 1.0 and XML namespaces.

o Specified the security considerations in more detail, especially
with regard to presence leaks and denial-of-service attacks.

o Moved documentation of the Server Dialback protocol from this
specification to a separate specification maintained by the XMPP
Standards Foundation.

Appendix E. Acknowledgements

This document is an update to, and derived from, RFC 3920. This
document would have been impossible without the work of the
contributors and commenters acknowledged there.

Hundreds of people have provided implementation feedback, bug
reports, requests for clarification, and suggestions for improvement
since publication of RFC 3920. Although the document editor has
endeavored to address all such feedback, he is solely responsible for
any remaining errors and ambiguities.

Special thanks are due to Kevin Smith, Matthew Wild, Dave Cridland,
Philipp Hancke, Waqas Hussain, Florian Zeitz, Ben Campbell, Jehan
Pages, Paul Aurich, Justin Karneges, Kurt Zeilenga, Simon Josefsson,
Ralph Meijer, Curtis King, and others for their comments during
Working Group Last Call.

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Thanks also to Yaron Sheffer and Elwyn Davies for their reviews on
behalf of the Security Directorate and the General Area Review Team,
respectively.

The Working Group chairs were Ben Campbell and Joe Hildebrand. The
responsible Area Director was Gonzalo Camarillo.

Author's Address

Peter Saint-Andre
Cisco
1899 Wyknoop Street, Suite 600
Denver, CO 80202
USA

Phone: +1-303-308-3282
EMail: psaintan@cisco.com

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RFC, FYI, BCP