Network Working Group P. Saint-Andre
Internet-Draft Cisco
Obsoletes: 3920 (if approved) May 7, 2010
Intended status: Standards Track
Expires: November 8, 2010
Extensible Messaging and Presence Protocol (XMPP): Core
draft-ietf-xmpp-3920bis-08
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.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 8, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2. History . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3. Functional Summary . . . . . . . . . . . . . . . . . . . 9
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . 11
1.5. Acknowledgements . . . . . . . . . . . . . . . . . . . . 12
1.6. Discussion Venue . . . . . . . . . . . . . . . . . . . . 12
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1. Global Addresses . . . . . . . . . . . . . . . . . . . . 13
2.2. Presence . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3. Persistent Streams . . . . . . . . . . . . . . . . . . . 13
2.4. Structured Data . . . . . . . . . . . . . . . . . . . . 13
2.5. Distributed Network of Clients and Servers . . . . . . . 14
3. TCP Binding . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2. Hostname Resolution . . . . . . . . . . . . . . . . . . 16
3.2.1. Preferred Process: SRV Lookup . . . . . . . . . . . 16
3.2.2. Fallback Processes . . . . . . . . . . . . . . . . . 17
3.2.3. When Not to Use SRV . . . . . . . . . . . . . . . . 17
3.2.4. SRV Security Checks . . . . . . . . . . . . . . . . 17
3.2.5. Use of SRV Records with Add-On Services . . . . . . 17
3.3. Directionality . . . . . . . . . . . . . . . . . . . . . 18
3.4. Reconnection . . . . . . . . . . . . . . . . . . . . . . 19
3.5. Reliability . . . . . . . . . . . . . . . . . . . . . . 19
4. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2. Stream Negotiation . . . . . . . . . . . . . . . . . . . 22
4.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . 22
4.2.2. Stream Features Format . . . . . . . . . . . . . . . 23
4.2.3. Restarts . . . . . . . . . . . . . . . . . . . . . . 24
4.2.4. Resending Features . . . . . . . . . . . . . . . . . 25
4.2.5. Completion of Stream Negotiation . . . . . . . . . . 25
4.2.6. Determination of Addresses . . . . . . . . . . . . . 25
4.2.7. State Chart . . . . . . . . . . . . . . . . . . . . 26
4.3. Closing a Stream . . . . . . . . . . . . . . . . . . . . 28
4.3.1. With Stream Error . . . . . . . . . . . . . . . . . 28
4.3.2. Without Stream Error . . . . . . . . . . . . . . . . 28
4.3.3. Handling of Idle Streams . . . . . . . . . . . . . . 29
4.4. Stream Attributes . . . . . . . . . . . . . . . . . . . 30
4.4.1. from . . . . . . . . . . . . . . . . . . . . . . . . 30
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4.4.2. to . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.4.3. id . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.4.4. xml:lang . . . . . . . . . . . . . . . . . . . . . . 33
4.4.5. version . . . . . . . . . . . . . . . . . . . . . . 35
4.4.6. Summary of Stream Attributes . . . . . . . . . . . . 36
4.5. Namespace Declarations . . . . . . . . . . . . . . . . . 36
4.5.1. Declaration of Streams Namespace . . . . . . . . . . 36
4.5.2. Declaration of Default Namespace . . . . . . . . . . 37
4.5.3. Declaration of Other Namespaces . . . . . . . . . . 38
4.6. Stream Errors . . . . . . . . . . . . . . . . . . . . . 38
4.6.1. Rules . . . . . . . . . . . . . . . . . . . . . . . 39
4.6.1.1. Stream Errors Are Unrecoverable . . . . . . . . . 39
4.6.1.2. Stream Errors Can Occur During Setup . . . . . . 39
4.6.1.3. Stream Errors When the Host is Unspecified or
Unknown . . . . . . . . . . . . . . . . . . . . . 40
4.6.1.4. Where Stream Errors Are Sent . . . . . . . . . . 41
4.6.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 41
4.6.3. Defined Stream Error Conditions . . . . . . . . . . 42
4.6.3.1. bad-format . . . . . . . . . . . . . . . . . . . 42
4.6.3.2. bad-namespace-prefix . . . . . . . . . . . . . . 43
4.6.3.3. conflict . . . . . . . . . . . . . . . . . . . . 44
4.6.3.4. connection-timeout . . . . . . . . . . . . . . . 44
4.6.3.5. host-gone . . . . . . . . . . . . . . . . . . . . 45
4.6.3.6. host-unknown . . . . . . . . . . . . . . . . . . 45
4.6.3.7. improper-addressing . . . . . . . . . . . . . . . 46
4.6.3.8. internal-server-error . . . . . . . . . . . . . . 46
4.6.3.9. invalid-from . . . . . . . . . . . . . . . . . . 47
4.6.3.10. invalid-id . . . . . . . . . . . . . . . . . . . 47
4.6.3.11. invalid-namespace . . . . . . . . . . . . . . . . 48
4.6.3.12. invalid-xml . . . . . . . . . . . . . . . . . . . 48
4.6.3.13. not-authorized . . . . . . . . . . . . . . . . . 49
4.6.3.14. policy-violation . . . . . . . . . . . . . . . . 50
4.6.3.15. remote-connection-failed . . . . . . . . . . . . 51
4.6.3.16. reset . . . . . . . . . . . . . . . . . . . . . . 51
4.6.3.17. resource-constraint . . . . . . . . . . . . . . . 52
4.6.3.18. restricted-xml . . . . . . . . . . . . . . . . . 52
4.6.3.19. see-other-host . . . . . . . . . . . . . . . . . 53
4.6.3.20. system-shutdown . . . . . . . . . . . . . . . . . 53
4.6.3.21. undefined-condition . . . . . . . . . . . . . . . 54
4.6.3.22. unsupported-encoding . . . . . . . . . . . . . . 54
4.6.3.23. unsupported-feature . . . . . . . . . . . . . . . 55
4.6.3.24. unsupported-stanza-type . . . . . . . . . . . . . 55
4.6.3.25. unsupported-version . . . . . . . . . . . . . . . 56
4.6.3.26. xml-not-well-formed . . . . . . . . . . . . . . . 57
4.6.4. Application-Specific Conditions . . . . . . . . . . 58
4.7. Simplified Stream Examples . . . . . . . . . . . . . . . 58
5. STARTTLS Negotiation . . . . . . . . . . . . . . . . . . . . 60
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 61
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5.2. Stream Negotiation Rules . . . . . . . . . . . . . . . . 61
5.2.1. Mandatory-to-Negotiate . . . . . . . . . . . . . . . 61
5.2.2. Restart . . . . . . . . . . . . . . . . . . . . . . 61
5.2.3. Data Formatting . . . . . . . . . . . . . . . . . . 61
5.2.4. Order of Negotiation . . . . . . . . . . . . . . . . 62
5.2.5. Renegotiation . . . . . . . . . . . . . . . . . . . 62
5.2.6. TLS Extensions . . . . . . . . . . . . . . . . . . . 62
5.3. Process . . . . . . . . . . . . . . . . . . . . . . . . 62
5.3.1. Exchange of Stream Headers and Stream Features . . . 62
5.3.2. Initiation of STARTTLS Negotiation . . . . . . . . . 63
5.3.2.1. STARTTLS Command . . . . . . . . . . . . . . . . 63
5.3.2.2. Failure Case . . . . . . . . . . . . . . . . . . 64
5.3.2.3. Proceed Case . . . . . . . . . . . . . . . . . . 64
5.3.3. TLS Negotiation . . . . . . . . . . . . . . . . . . 65
5.3.3.1. Rules . . . . . . . . . . . . . . . . . . . . . . 65
5.3.3.2. TLS Failure . . . . . . . . . . . . . . . . . . . 65
5.3.3.3. TLS Success . . . . . . . . . . . . . . . . . . . 66
6. SASL Negotiation . . . . . . . . . . . . . . . . . . . . . . 67
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 67
6.2. Stream Negotiation Rules . . . . . . . . . . . . . . . . 67
6.2.1. Mandatory-to-Negotiate . . . . . . . . . . . . . . . 67
6.2.2. Restart . . . . . . . . . . . . . . . . . . . . . . 67
6.2.3. Mechanism Preferences . . . . . . . . . . . . . . . 68
6.2.4. Mechanism Offers . . . . . . . . . . . . . . . . . . 68
6.2.5. Data Formatting . . . . . . . . . . . . . . . . . . 68
6.2.6. Security Layers . . . . . . . . . . . . . . . . . . 69
6.2.7. Simple Username . . . . . . . . . . . . . . . . . . 69
6.2.8. Authorization Identity . . . . . . . . . . . . . . . 69
6.2.9. Realms . . . . . . . . . . . . . . . . . . . . . . . 70
6.2.10. Round Trips . . . . . . . . . . . . . . . . . . . . 70
6.3. Process . . . . . . . . . . . . . . . . . . . . . . . . 71
6.3.1. Exchange of Stream Headers and Stream Features . . . 71
6.3.2. Initiation . . . . . . . . . . . . . . . . . . . . . 72
6.3.3. Challenge-Response Sequence . . . . . . . . . . . . 72
6.3.4. Abort . . . . . . . . . . . . . . . . . . . . . . . 73
6.3.5. Failure . . . . . . . . . . . . . . . . . . . . . . 73
6.3.6. Success . . . . . . . . . . . . . . . . . . . . . . 74
6.4. SASL Errors . . . . . . . . . . . . . . . . . . . . . . 75
6.4.1. aborted . . . . . . . . . . . . . . . . . . . . . . 75
6.4.2. account-disabled . . . . . . . . . . . . . . . . . . 76
6.4.3. credentials-expired . . . . . . . . . . . . . . . . 76
6.4.4. encryption-required . . . . . . . . . . . . . . . . 76
6.4.5. incorrect-encoding . . . . . . . . . . . . . . . . . 76
6.4.6. invalid-authzid . . . . . . . . . . . . . . . . . . 77
6.4.7. invalid-mechanism . . . . . . . . . . . . . . . . . 77
6.4.8. malformed-request . . . . . . . . . . . . . . . . . 77
6.4.9. mechanism-too-weak . . . . . . . . . . . . . . . . . 78
6.4.10. not-authorized . . . . . . . . . . . . . . . . . . . 78
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6.4.11. temporary-auth-failure . . . . . . . . . . . . . . . 78
6.4.12. transition-needed . . . . . . . . . . . . . . . . . 79
6.5. SASL Definition . . . . . . . . . . . . . . . . . . . . 79
7. Resource Binding . . . . . . . . . . . . . . . . . . . . . . 80
7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 80
7.2. Stream Negotiation Rules . . . . . . . . . . . . . . . . 81
7.2.1. Mandatory-to-Negotiate . . . . . . . . . . . . . . . 81
7.2.2. Restart . . . . . . . . . . . . . . . . . . . . . . 81
7.3. Advertising Support . . . . . . . . . . . . . . . . . . 81
7.4. Generation of Resource Identifiers . . . . . . . . . . . 82
7.5. Server-Generated Resource Identifier . . . . . . . . . . 82
7.5.1. Success Case . . . . . . . . . . . . . . . . . . . . 82
7.5.2. Error Cases . . . . . . . . . . . . . . . . . . . . 83
7.5.2.1. Resource Constraint . . . . . . . . . . . . . . . 83
7.5.2.2. Not Allowed . . . . . . . . . . . . . . . . . . . 83
7.6. Client-Submitted Resource Identifier . . . . . . . . . . 83
7.6.1. Success Case . . . . . . . . . . . . . . . . . . . . 83
7.6.2. Error Cases . . . . . . . . . . . . . . . . . . . . 84
7.6.2.1. Bad Request . . . . . . . . . . . . . . . . . . . 84
7.6.2.2. Conflict . . . . . . . . . . . . . . . . . . . . 85
7.6.3. Retries . . . . . . . . . . . . . . . . . . . . . . 86
8. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 86
8.1. Common Attributes . . . . . . . . . . . . . . . . . . . 86
8.1.1. to . . . . . . . . . . . . . . . . . . . . . . . . . 86
8.1.1.1. Client-to-Server Streams . . . . . . . . . . . . 87
8.1.1.2. Server-to-Server Streams . . . . . . . . . . . . 87
8.1.2. from . . . . . . . . . . . . . . . . . . . . . . . . 87
8.1.2.1. Client-to-Server Streams . . . . . . . . . . . . 88
8.1.2.2. Server-to-Server Streams . . . . . . . . . . . . 88
8.1.3. id . . . . . . . . . . . . . . . . . . . . . . . . . 89
8.1.4. type . . . . . . . . . . . . . . . . . . . . . . . . 89
8.1.5. xml:lang . . . . . . . . . . . . . . . . . . . . . . 90
8.2. Basic Semantics . . . . . . . . . . . . . . . . . . . . 91
8.2.1. Message Semantics . . . . . . . . . . . . . . . . . 91
8.2.2. Presence Semantics . . . . . . . . . . . . . . . . . 91
8.2.3. IQ Semantics . . . . . . . . . . . . . . . . . . . . 91
8.3. Stanza Errors . . . . . . . . . . . . . . . . . . . . . 93
8.3.1. Rules . . . . . . . . . . . . . . . . . . . . . . . 93
8.3.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 94
8.3.3. Defined Conditions . . . . . . . . . . . . . . . . . 95
8.3.3.1. bad-request . . . . . . . . . . . . . . . . . . . 95
8.3.3.2. conflict . . . . . . . . . . . . . . . . . . . . 96
8.3.3.3. feature-not-implemented . . . . . . . . . . . . . 96
8.3.3.4. forbidden . . . . . . . . . . . . . . . . . . . . 97
8.3.3.5. gone . . . . . . . . . . . . . . . . . . . . . . 98
8.3.3.6. internal-server-error . . . . . . . . . . . . . . 98
8.3.3.7. item-not-found . . . . . . . . . . . . . . . . . 99
8.3.3.8. jid-malformed . . . . . . . . . . . . . . . . . . 99
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8.3.3.9. not-acceptable . . . . . . . . . . . . . . . . . 100
8.3.3.10. not-allowed . . . . . . . . . . . . . . . . . . . 100
8.3.3.11. not-authorized . . . . . . . . . . . . . . . . . 101
8.3.3.12. payment-required . . . . . . . . . . . . . . . . 101
8.3.3.13. policy-violation . . . . . . . . . . . . . . . . 102
8.3.3.14. recipient-unavailable . . . . . . . . . . . . . . 102
8.3.3.15. redirect . . . . . . . . . . . . . . . . . . . . 103
8.3.3.16. registration-required . . . . . . . . . . . . . . 104
8.3.3.17. remote-server-not-found . . . . . . . . . . . . . 104
8.3.3.18. remote-server-timeout . . . . . . . . . . . . . . 105
8.3.3.19. resource-constraint . . . . . . . . . . . . . . . 106
8.3.3.20. service-unavailable . . . . . . . . . . . . . . . 106
8.3.3.21. subscription-required . . . . . . . . . . . . . . 107
8.3.3.22. undefined-condition . . . . . . . . . . . . . . . 107
8.3.3.23. unexpected-request . . . . . . . . . . . . . . . 108
8.3.4. Application-Specific Conditions . . . . . . . . . . 109
8.4. Extended Content . . . . . . . . . . . . . . . . . . . . 110
8.5. Stanza Size . . . . . . . . . . . . . . . . . . . . . . 112
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 113
9.1. Client-to-Server Examples . . . . . . . . . . . . . . . 113
9.1.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . 113
9.1.2. SASL . . . . . . . . . . . . . . . . . . . . . . . . 115
9.1.3. Resource Binding . . . . . . . . . . . . . . . . . . 116
9.1.4. Stanza Exchange . . . . . . . . . . . . . . . . . . 117
9.1.5. Close . . . . . . . . . . . . . . . . . . . . . . . 117
9.2. Server-to-Server Examples . . . . . . . . . . . . . . . 118
9.2.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . 118
9.2.2. SASL . . . . . . . . . . . . . . . . . . . . . . . . 120
9.2.3. Stanza Exchange . . . . . . . . . . . . . . . . . . 121
9.2.4. Close . . . . . . . . . . . . . . . . . . . . . . . 122
10. Server Rules for Processing XML Stanzas . . . . . . . . . . . 122
10.1. No 'to' Address . . . . . . . . . . . . . . . . . . . . 122
10.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . 122
10.1.2. Message . . . . . . . . . . . . . . . . . . . . . . 123
10.1.3. Presence . . . . . . . . . . . . . . . . . . . . . . 123
10.1.4. IQ . . . . . . . . . . . . . . . . . . . . . . . . . 123
10.2. Local Domain . . . . . . . . . . . . . . . . . . . . . . 123
10.2.1. Mere Domain . . . . . . . . . . . . . . . . . . . . 124
10.2.2. Domain with Resource . . . . . . . . . . . . . . . . 124
10.2.3. Localpart at Domain . . . . . . . . . . . . . . . . 124
10.2.3.1. No Such User . . . . . . . . . . . . . . . . . . 124
10.2.3.2. Bare JID . . . . . . . . . . . . . . . . . . . . 124
10.2.3.3. Full JID . . . . . . . . . . . . . . . . . . . . 125
10.3. Remote Domain . . . . . . . . . . . . . . . . . . . . . 125
10.3.1. Existing Stream . . . . . . . . . . . . . . . . . . 125
10.3.2. No Existing Stream . . . . . . . . . . . . . . . . . 125
10.3.3. Error Handling . . . . . . . . . . . . . . . . . . . 126
11. XML Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 126
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11.1. Restrictions . . . . . . . . . . . . . . . . . . . . . . 126
11.2. XML Namespace Names and Prefixes . . . . . . . . . . . . 127
11.3. Well-Formedness . . . . . . . . . . . . . . . . . . . . 127
11.4. Validation . . . . . . . . . . . . . . . . . . . . . . . 128
11.5. Inclusion of XML Declaration . . . . . . . . . . . . . . 128
11.6. Character Encoding . . . . . . . . . . . . . . . . . . . 128
11.7. Whitespace . . . . . . . . . . . . . . . . . . . . . . . 128
11.8. XML Versions . . . . . . . . . . . . . . . . . . . . . . 129
12. Internationalization Considerations . . . . . . . . . . . . . 129
13. Security Considerations . . . . . . . . . . . . . . . . . . . 129
13.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 129
13.2. Threat Model . . . . . . . . . . . . . . . . . . . . . . 130
13.3. Order of Layers . . . . . . . . . . . . . . . . . . . . 131
13.4. Confidentiality and Integrity . . . . . . . . . . . . . 131
13.5. Peer Entity Authentication . . . . . . . . . . . . . . . 131
13.6. Strong Security . . . . . . . . . . . . . . . . . . . . 131
13.7. Certificates . . . . . . . . . . . . . . . . . . . . . . 132
13.7.1. Certificate Generation . . . . . . . . . . . . . . . 132
13.7.1.1. General Considerations . . . . . . . . . . . . . 132
13.7.1.2. Server Certificates . . . . . . . . . . . . . . . 133
13.7.1.3. Client Certificates . . . . . . . . . . . . . . . 135
13.7.1.4. ASN.1 Object Identifier . . . . . . . . . . . . . 135
13.7.2. Certificate Validation . . . . . . . . . . . . . . . 136
13.7.2.1. Server Certificates . . . . . . . . . . . . . . . 137
13.7.2.2. Client Certificates . . . . . . . . . . . . . . . 138
13.7.2.3. Checking of Certificates in Long-Lived Streams . 139
13.7.2.4. Use of Certificates in XMPP Extensions . . . . . 140
13.8. Mandatory-to-Implement Technologies . . . . . . . . . . 140
13.9. Technology Reuse . . . . . . . . . . . . . . . . . . . . 141
13.9.1. Use of base64 in SASL . . . . . . . . . . . . . . . 141
13.9.2. Use of DNS . . . . . . . . . . . . . . . . . . . . . 141
13.9.3. Use of Hash Functions . . . . . . . . . . . . . . . 142
13.9.4. Use of SASL . . . . . . . . . . . . . . . . . . . . 142
13.9.5. Use of TLS . . . . . . . . . . . . . . . . . . . . . 143
13.9.6. Use of UTF-8 . . . . . . . . . . . . . . . . . . . . 143
13.9.7. Use of XML . . . . . . . . . . . . . . . . . . . . . 143
13.10. Information Leaks . . . . . . . . . . . . . . . . . . . 143
13.10.1. IP Addresses . . . . . . . . . . . . . . . . . . . . 143
13.10.2. Presence Information . . . . . . . . . . . . . . . . 143
13.11. Directory Harvesting . . . . . . . . . . . . . . . . . . 144
13.12. Denial of Service . . . . . . . . . . . . . . . . . . . 144
13.13. Firewalls . . . . . . . . . . . . . . . . . . . . . . . 146
13.14. Interdomain Federation . . . . . . . . . . . . . . . . . 146
13.15. Non-Repudiation . . . . . . . . . . . . . . . . . . . . 146
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 146
14.1. XML Namespace Name for TLS Data . . . . . . . . . . . . 147
14.2. XML Namespace Name for SASL Data . . . . . . . . . . . . 147
14.3. XML Namespace Name for Stream Errors . . . . . . . . . . 147
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14.4. XML Namespace Name for Resource Binding . . . . . . . . 147
14.5. XML Namespace Name for Stanza Errors . . . . . . . . . . 148
14.6. GSSAPI Service Name . . . . . . . . . . . . . . . . . . 148
14.7. Port Numbers . . . . . . . . . . . . . . . . . . . . . . 148
15. Conformance Requirements . . . . . . . . . . . . . . . . . . 148
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 158
16.1. Normative References . . . . . . . . . . . . . . . . . . 158
16.2. Informative References . . . . . . . . . . . . . . . . . 160
Appendix A. XML Schemas . . . . . . . . . . . . . . . . . . . . 164
A.1. Streams Namespace . . . . . . . . . . . . . . . . . . . 165
A.2. Stream Error Namespace . . . . . . . . . . . . . . . . . 166
A.3. STARTTLS Namespace . . . . . . . . . . . . . . . . . . . 168
A.4. SASL Namespace . . . . . . . . . . . . . . . . . . . . . 168
A.5. Resource Binding Namespace . . . . . . . . . . . . . . . 171
A.6. Stanza Error Namespace . . . . . . . . . . . . . . . . . 171
Appendix B. Contact Addresses . . . . . . . . . . . . . . . . . 173
Appendix C. Account Provisioning . . . . . . . . . . . . . . . . 173
Appendix D. Differences from RFC 3920 . . . . . . . . . . . . . 173
Appendix E. Copying Conditions . . . . . . . . . . . . . . . . . 174
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 175
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1. Introduction
1.1. Overview
The Extensible Messaging and Presence Protocol (XMPP) is an
application profile of the Extensible Markup Language [XML] 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 core Jabber protocol that would be suitable as an
IETF instant messaging (IM) and presence technology in accordance
with [IMP-REQS]. In October 2004, [RFC3920] and [RFC3921] were
published, representing the most complete definition of XMPP at that
time.
Since 2004 the Internet community has gained extensive implementation
and deployment experience with XMPP, including formal
interoperability testing carried out under the auspices of the XMPP
Standards Foundation (XSF). This document incorporates comprehensive
feedback from software developers and 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 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
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is:
1. Determine the hostname and port at which to connect
2. Open a Transmission Control Protocol [TCP] connection
3. Open an XML stream over TCP
4. Negotiate Transport Layer Security [TLS] for channel encryption
(recommended)
5. Negotiate a Simple Authentication and Security Layer [SASL]
mechanism for authentication
6. Bind a resource to the stream
7. Exchange an unbounded number of XML stanzas with other entities
on the network
8. Close the XML stream
9. Close the TCP connection
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 hostname and port at which to connect
2. Open a TCP connection
3. Open an XML stream
4. Negotiate TLS for channel encryption (recommended)
5. Negotiate a SASL mechanism for authentication *
6. Exchange an unbounded number of XML stanzas both directly for the
servers and indirectly on behalf of entities associated with each
server (e.g., connected clients)
7. Close the XML stream
8. Close the TCP connection
* Implementation Note: Depending on local service policies, it is
possible that a deployed server will use the older server dialback
protocol to provide weak identity verification in cases where SASL
negotiation would not result in strong authentication (e.g.,
because TLS negotiation was not mandated by the peer server, or
because the certificate presented by the peer server during TLS
negotiation is self-signed and thus provides only weak identity);
for details, see [XEP-0220].
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
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[XEP-0001] define extensions for a wide range of applications.
1.4. Terminology
The following capitalized keywords are to be interpreted as described
in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT";
"SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY",
"OPTIONAL".
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", "confidentiality",
"credential", "downgrade", "encryption", "fingerprint", "hash value",
"identity", "integrity", "signature", "security perimeter", "self-
signed certificate", "sign", "spoof", "tamper", "trust", "trust
anchor", "trust chain", "validate", "verify". Other security-related
terms (for example, "denial of service") are to be understood in the
sense defined in the referenced specifications.
The term "whitespace" is used to refer to any character that matches
production [3] content of [XML], i.e., any instance of SP, HT, CR, or
LF.
With regard to XML stanzas or parts thereof, the term "silently
ignore" means (1) for a routing entity (typically a server) to route
the data unmodified to another server or to deliver the data
unmodified to a client, or (2) for a client to discard the data
without presenting it a human user or associated application.
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] code points (e.g., the string "ř" stands
for the Unicode character LATIN SMALL LETTER R WITH CARON).
Following the convention used in [URI] to represent Uniform Resource
Indentifiers, XMPP addresses in running text are enclosed between '<'
and '>'.
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
1.5. Acknowledgements
The editor of this document finds it impossible to appropriately
acknowledge the many individuals who have provided comments regarding
the protocols defined in this specification. However, thanks are due
to those who have who have provided implementation feedback, bug
reports, requests for clarification, and suggestions for improvement
since the publication of the RFC this document supersedes. The
editor has endeavored to address all such feedback, but is solely
responsible for any remaining errors and ambiguities.
1.6. Discussion Venue
The document editor and the broader XMPP developer community welcome
discussion and comments related to the topics presented in this
document. The primary and preferred venue is the <xmpp@ietf.org>
mailing list, for which archives and subscription information are
available at <https://www.ietf.org/mailman/listinfo/xmpp>. Related
discussions often occur on the <standards@xmpp.org> mailing list, for
which archives and subscription information are available at
<http://mail.jabber.org/mailman/listinfo/standards>.
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.
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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 "domain.tld" (e.g., "im.example.com"),
accounts hosted at a server are of the form "localpart@domain" (e.g.,
"juliet@im.example.com"), and a particular connected device or
resource that is currently authorized for interaction on behalf of an
account is of the form "localpart@domain/resource" (e.g.,
"juliet@im.example.com/balcony"). 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.
2.2. Presence
XMPP includes the ability for an entity to advertise its network
availability or "presence" to other entities. Such availability for
communication is signalled end-to-end via dedicated communication
primitives in XMPP (the <presence/> stanza). Although knowledge of
network availability is not strictly necessary for the exchange of
XMPP messages, it facilitates real-time interaction because the
originator of a message can know before initiating communication that
the intended recipient is online and available. End-to-end presence
is defined in [XMPP-IM].
2.3. Persistent Streams
Availability for communication is also built into a point-to-point
"hop" through the use of persistent XML streams over long-lived TCP
connections. These "always-on" client-to-server or 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
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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 OPTIONAL). Thus, for example, the user
<juliet@im.example.com> associated with the server <im.example.com>
might be able to exchange messages, presence, and other structured
data with the user <romeo@example.net> associated with the server
<example.net>. This pattern is familiar from messaging protocols
that make use of global addresses, such as the email network (see
[SMTP] and [EMAIL-ARCH]). As a result, end-to-end communication in
XMPP is logically peer-to-peer but physically client-to-server-to-
server-to-client, as illustrated in the following diagram.
example.net ---------------- im.example.com
| |
| |
romeo@example.net juliet@im.example.com
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 local account 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 local entities or
routing them to remote entities. Multiple clients can connect
simultaneously to a server on behalf of the same local account, where
each client is differentiated by the resourcepart of an XMPP address
(e.g., <juliet@im.example.com/balcony> vs.
<juliet@im.example.com/chamber>), as defined under [XMPP-ADDR] and
Section 7.
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A SERVER is an entity whose primary responsibilities are to:
o Manage XML streams (Section 4) with local 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 XML 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 local entity.
o Hosting local services that also use XMPP as the basis for
communication but that provide additional functionality beyond
that defined in this document or in [XMPP-IM]; examples include
multi-user conferencing services as specified in [XEP-0045] and
publish-subscribe services as specified in [XEP-0060].
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 at 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. Hostname Resolution
Because XML streams are sent over TCP, the initiating entity needs to
determine the IPv4 or IPv6 address (and port) of the receiving
entity's "origin domain" before it can attempt to connect to the XMPP
network.
3.2.1. Preferred Process: SRV Lookup
The preferred process for hostname 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" of the XMPP
service to which the initiating entity wishes to connect
(e.g., "example.net" or "im.example.com")
2. The resulting 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 hostname, each of which is weighted 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 (but SHOULD attempt the fallback process described in
the next section).
4. The initiating entity resolves at least one of the returned
hostnames, in order according to the returned SRV record weight.
It resolves the hostname using a DNS "A" or "AAAA" lookup on the
hostname, which will result in an IPv4 or IPv6 address.
5. The initiating entity uses the IP address(es) from the first
successfully resolved hostname (with the corresponding port
number returned by the SRV lookup) as the connection address for
the receiving entity.
6. If the initiating entity fails to connect using that IP address
but the "A" or "AAAA" lookup returned more than one IP address,
then the initiating entity uses the next resolved IP address for
that hostname as the connection address.
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7. If the initiating entity fails to connect using all resolved IP
addresses for a given hostname, then it repeats the process of
resolution and connection for the next hostname returned by the
SRV lookup.
8. If the initiating entity fails to connect using any hostname
returned by the SRV lookup, then it can either abort the
connection attempt or use 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 5269 for
server-to-server connections.
For client-to-server connections, the fallback MAY be a [DNS-TXT]
lookup for alternative connection methods, for example 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 hostname (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 hostname of
webcm.example.com:80), the initiating entity SHALL use the configured
name instead of performing the preferred SRV resolution process on
the origin name.
3.2.4. SRV Security Checks
In the absence of a secure DNS option (e.g., as provided by
[DNSSEC]), the initiating entity SHOULD check that the target domain
name returned in the SRV record matches the origin domain that was
queried. If the target domain name is not within the trust
boundaries of the origin domain, the connecting application SHOULD
verify with the human user (if any) that the SRV target domain name
is suitable for use before executing any connections to the IP
address of the XMPP service.
3.2.5. 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
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[XMPP-IM]) at hostnames that typically are subdomains of the hostname
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 host (e.g.,
muc.example.com for a [XEP-0045] service associated with the
im.example.com XMPP service). If an entity from a remote domain
wishes to use such add-on services, it would generate an appropriate
XML stanza and the remote domain itself would attempt to resolve the
service's hostname via an SRV lookup on resource records such as
"_xmpp-server._tcp.conference.example.net." or "_xmpp-
server._tcp.muc.example.com.". Therefore if a service wishes to
enable entities from remote domains to access these add-on services,
it needs to advertise the appropriate "_xmpp-server" SRV records in
addition to the "_xmpp-server" record for its main XMPP service.
3.3. Directionality
A client is subordinate to a server. As a result, a client
authenticates to the server but the server does not necessarily
authenticate to the client. Therefore the server MUST allow the
client to share a single, bidirectional TCP connection for XML
stanzas sent from client to server and from server to client (i.e.,
the inital stream and response stream as specified under Section 4).
By contrast, any two given servers are peers. As a result, each peer
MUST authenticate to the other. This can be accomplished in two
ways:
o Through one-way authentication on two, unidirectional TCP
connections: one for XML stanzas sent from the first server to the
second server and another, initiated by the second server, for XML
stanzas from the second server to the first server (this is the
"legacy" approach in the absence of mutual authentication).
o Through mutual authentication on a single, bidirectional TCP
connection (this is possible using TLS and SASL with certificates,
however methods for explicitly signalling the use of a single,
bidirectional TCP connection are out of scope for this
specification).
The directionality of a stream applies only to XML stanzas
(Section 8). Therefore during STARTTLS negotiation (Section 5) and
SASL negotiation (Section 6) two servers would use one TCP
connection, but after stream setup that TCP connection would be used
only for the initiating server to send XML stanzas to the receiving
server. In order for the receiving server to send XML stanzas to the
initiating server, the receiving server would need to reverse the
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roles and negotiate an XML stream from the receiving server to the
initiating server.
3.4. Reconnection
It can happen that an XMPP server goes offline while servicing TCP
connections from local clients and from other servers. Because the
number of such connections can be quite large, the reconnection
algorithm employed by entities that seek to reconnect can have a
significant impact on software and network performance. The
following guidelines are RECOMMENDED:
o The time to live (TTL) specified in Domain Name System records
MUST be honored, even if DNS results are cached; if the TTL has
not expired, an entity that seeks to reconnect MUST NOT re-resolve
the server hostname before reconnecting.
o The time that expires before an entity first seeks to reconnect
MUST be randomized (e.g., so that all clients do not attempt to
reconnect exactly 30 seconds after being disconnected).
o If the first reconnection attempt does not succeed, an entity MUST
back off increasingly on the time between subsequent reconnection
attempts, e.g. in accordance with the exponential backoff
principle.
Implementation Note: Because it is possible that a disconnected
entity cannot determine the cause of disconnection (e.g., because
there was no explicit stream error) or does not need a new stream
for immediate communication (e.g., because the stream was idle and
therefore timed out), it SHOULD NOT assume that it needs to
reconnect immediately.
3.5. 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]).
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4. XML Streams
4.1. Overview
Two fundamental concepts make possible the rapid, asynchronous
exchange of relatively small payloads of structured information
between presence-aware entities: XML streams and XML stanzas. These
terms are defined as follows.
Definition of XML Stream: An XML STREAM is a container for the
exchange of XML elements between any two entities over a network.
The start of an XML stream is denoted unambiguously by an opening
STREAM HEADER (i.e., an XML <stream> tag with appropriate
attributes and namespace declarations), while the end of the XML
stream is denoted unambiguously by a closing XML </stream> tag.
During the life of the stream, the entity that initiated it can
send an unbounded number of XML elements over the stream, either
elements used to negotiate the stream (e.g., to complete TLS
negotiation (Section 5) or SASL negotiation (Section 6)) or XML
stanzas. The INITIAL STREAM is negotiated from the initiating
entity (typically a client or server) to the receiving entity
(typically a server), and can be seen as corresponding to the
initiating entity's "connection to" or "session with" the
receiving entity. The initial stream enables unidirectional
communication from the initiating entity to the receiving entity;
in order to enable exchange of stanzas from the receiving entity
to the initiating entity, the receiving entity MUST negotiate a
stream in the opposite direction (the RESPONSE STREAM).
Definition of XML Stanza: An XML STANZA is a discrete semantic unit
of structured information that is sent from one entity to another
over an XML stream, and is the basic unit of meaning in XMPP. An
XML stanza exists at the direct child level of the root <stream/>
element; the start of any XML stanza is denoted unambiguously by
the element start tag at depth=1 of the XML stream (e.g.,
<presence>), and the end of any XML stanza is denoted
unambiguously by the corresponding close tag at depth=1 (e.g.,
</presence>). The only XML stanzas defined in this specification
are the <message/>, <presence/>, and <iq/> elements qualified by
the default namespace for the stream, as described under
Section 8; by contrast, an XML element sent for the purpose of TLS
negotiation (Section 5) or SASL negotiation (Section 6) is not
considered to be an XML stanza, nor is a stream error or a stream
feature. 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).
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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, optionally preceded by a text 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 optionally preceded by a text declaration. Once the client has
completed SASL negotiation (Section 6) and resource binding
(Section 7), the client can send an unbounded number of XML stanzas
over the stream. When the client desires to close the stream, it
simply sends a closing </stream> tag to the server as further
described under Section 4.3.
In essence, then, an XML stream acts as an envelope for all the XML
stanzas sent during a connection. We can represent this in a
simplistic fashion as follows.
+--------------------+
| <stream> |
|--------------------|
| <presence> |
| <show/> |
| </presence> |
|--------------------|
| <message to='foo'> |
| <body/> |
| </message> |
|--------------------|
| <iq to='bar'> |
| <query/> |
| </iq> |
|--------------------|
| <iq from='bar'> |
| <query/> |
| </iq> |
|--------------------|
| [ ... ] |
|--------------------|
| </stream> |
+--------------------+
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 production
[1] content of [XML]) that are built up through the accumulation
of XML stanzas.
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o The root <stream/> element is like the "document entity" for each
"document" (as described in Section 4.8 of [XML]).
o The XML stanzas sent over the streams are like "fragments" of the
"documents" (as described in [XML-FRAG]).
However, these analogies are merely that, 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:
o The stream negotation process
o How to close a stream
o The XML attributes of a stream
o The XML namespaces of a stream
4.2. Stream Negotiation
4.2.1. Overview
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, typically using SASL as
described under Section 6. However, the receiving entity can
consider conditions other than authentication to be mandatory, 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 are mandatory for the initiating entity to complete
before the receiving entity will accept XML stanzas from the
initiating entity (e.g., authentication), as well as any protocol
interactions that are voluntary but that might improve the handling
of an XML stream (e.g., establishment of application-layer
compression).
The existence of conditions for connecting implies that streams need
to be negotiated. The order of layers (TCP, then TLS, then SASL,
then XMPP; see 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
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defined for certain features (e.g., TLS in all cases and SASL in the
case when a security layer might be established); this is done by
flushing the old stream context and exchanging new stream headers
over the existing TCP connection.
4.2.2. Stream Features Format
If the initiating entity includes the 'version' attribute set to a
value of at least "1.0" in the initial stream header, after sending
the response stream header the receiving entity MUST send a
<features/> child element (prefixed by the streams namespace prefix)
to the initiating entity in order to announce any conditions for
continuation of the stream negotiation process. Each condition takes
the form of a child element of the <features/> element, qualified by
a namespace that is different from the streams namespace and the
default namespace. The <features/> element can contain one child,
contain multiple children, or be empty.
Implementation Note: The order of child elements contained in any
given <features/> element is not significant.
If a particular stream feature is or can be mandatory-to-negotiate,
the definition of that feature needs to either declare that the
feature is always mandatory-to-negotiate (e.g., this is true of
resource binding for XMPP clients) or specify a way for the receiving
entity to flag the feature as mandatory-to-negotiate for this
interaction (e.g., this is done for TLS by including an empty
<required/> element in the advertisement for that stream feature).
For security reasons, certain stream features necessitate the
initiating entity to send a new initial stream header upon successful
negotiation of the feature (e.g., TLS in all cases and SASL in the
case when a security layer might be established). If this is true of
a given stream feature, the definition of that feature needs to
declare that a stream restart is expected after negotiation of the
feature.
A <features/> element that contains at least one mandatory feature
indicates that the stream negotiation is not complete and that the
initiating entity MUST negotiate further features.
R: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
A <features/> element MAY contain more than one mandatory feature.
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This means that the initiating entity can choose among the mandatory
features. For 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.
A <features/> element that contains both mandatory and voluntary
features indicates that the negotiation is not complete but that the
initiating entity MAY complete the voluntary feature(s) before it
attempts to negotiate the mandatory feature(s).
R: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
<compression xmlns='http://jabber.org/features/compress'>
<method>zlib</method>
<method>lzw</method>
</compression>
</stream:features>
A <features/> element that contains only voluntary features indicates
that the stream negotiation is complete and that the initiating
entity is cleared to send XML stanzas, but that the initiating entity
MAY negotiate further features if desired.
R: <stream:features>
<session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
<compression xmlns='http://jabber.org/features/compress'>
<method>zlib</method>
<method>lzw</method>
</compression>
</stream:features>
An empty <features/> element indicates that the stream negotiation is
complete and that the initiating entity is cleared to send XML
stanzas.
R: <stream:features/>
4.2.3. 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 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 re-using
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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).
For the sake of backward compatibility, the receiving entity MUST
accept stream restarts at any stage in the stream negotiation process
even if the receiving entity has not indicated that a stream restart
is mandatory at that stage.
4.2.4. Resending Features
After completing negotiation of any stream feature (even stream
features that do not necessitate a stream restart), the receiving
entity MUST send an updated list of stream features to the initiating
entity, where the list MAY be empty if there are no further features
to be advertised or MAY include any combination of features.
4.2.5. Completion of Stream Negotiation
The receiving entity indicates completion of the stream negotiation
process by sending to the initiating entity either an empty
<features/> element or a <features/> element that contains only
voluntary features. After doing so, the receiving entity MUST NOT
send additional stream features to the initiating entity (if the
receiving entity has new features to offer, it can simply close the
stream using a <reset/> stream error and then advertise the new
features when the initiating entity reconnects, preferably closing
existing streams in a staggered way so that not all of the initiating
entities reconnect at once). Once stream negotiation is complete,
the initiating entity is cleared to send XML stanzas over the stream
for as long as the stream is maintained by both parties.
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. However, if it does attempt to do
so, the receiving entity MUST NOT accept such stanzas and MUST return
a <not-authorized/> stream error. This rule applies to XML stanzas
only (i.e., <message/>, <presence/>, and <iq/> elements qualified by
the default namespace) and not to XML elements used for stream
negotiation (e.g., elements used to complete TLS negotiation
(Section 5) or SASL negotiation (Section 6)).
4.2.6. Determination of Addresses
After the parties to an XML stream have completed the appropriate
aspects of stream negotiation (typically SASL negotiation (Section 6)
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and, for client-to-server streams, resource binding (Section 7)) the
receiving entity for a stream MUST determine the initiating entity's
JID.
For client-to-server communication, the client's bare JID
(<localpart@domain>) 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 (Section 6). The resourcepart of the full
JID (<localpart@domain/resource>) 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 SASL negotiation
(bare JID) or resource binding (full JID).
For server-to-server communication, the initiating server's JID MUST
be the authentication identity communicated by the initiating entity
during SASL negotiation (Section 6); in the absence of SASL
negotiation (e.g., when the older server dialback protocol is used as
specified in [XEP-0220]), the receiving server MAY consider the
authentication identity to be the 'from' address on the initial
stream header sent after TLS negotiation (information contained in
stream headers sent before TLS negotation can be spoofed). Note that
authorization identities are not allowed during SASL negotiation for
server-to-server streams; for details, see under Section 6.2.8.
The receiving entity 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 receiving entity MAY
replace the JID sent by the initiating entity with the canonicalized
JID as determined by the receiving entity.
4.2.7. State Chart
We summarize the foregoing rules in the following non-normative state
chart for the stream negotiation process, presented from the
perspective of the initiating entity.
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+------------+
| open TCP |
| connection |
+------------+
|
+---------------+
| send initial |<-------------------------+
| stream header | ^
+---------------+ |
| |
+------------------+ |
| receive response | |
| stream header | |
+------------------+ |
| |
+----------------+ |
| receive stream | |
+------------------>| features | |
^ +----------------+ |
| | |
| +<-----------------+ |
| | |
| {empty?} ----> {all voluntary?} ----> {some mandatory?} |
| | no | no | |
| | yes | yes | yes |
| | | | |
| | +---------------+ +----------------+ |
| | | MAY negotiate | | MUST negotiate | |
| | | any or none | | one feature | |
| | +---------------+ +----------------+ |
| | | | |
| +----------+ +-----------+ | |
| | process |<-----| negotiate | | |
| | complete | no | a feature | | |
| +----------+ +-----------+ | |
| | | |
| yes | | |
| | | |
| +--------->+<---------+ |
| | |
| | |
+<-------------------------- {restart mandatory?} ------------>+
no yes
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4.3. Closing a Stream
An XML stream between two entities can be closed because a specific
stream error has occurred or in some cases in the absence of an
error. Where feasible, it is preferable to close a stream only if a
specific stream error has occurred.
A stream is closed by sending a closing </stream> tag over the TCP
connection.
S: </stream:stream>
The following sections describe how to handle closing of a stream in
the error case and the non-error case.
Implementation Note: As described under Section 4.3.3, an idle
stream SHOULD NOT be considered a trigger for a stream error.
After an entity sends a closing stream tag, it MUST NOT initiate the
sending of further data over that stream (but MAY respond to data
sent by the other entity).
4.3.1. With Stream Error
If a specific stream error has occurred, the entity that detects the
error MUST close the stream as described under Section 4.6.1.
4.3.2. Without Stream Error
At any time after XML streams have been negotiated between two
entities, either entity MAY close its stream to the other party in
the absence of a stream error by sending a closing stream tag.
P: </stream:stream>
The entity that sends the closing stream tag SHOULD wait for the
other party to also close its stream before terminated the underlying
TCP connection. This gives the other party an opportunity to finish
transmitting any data in the opposite direction before the TCP
connection is terminated.
S: </stream:stream>
However, the entity that sends the first closing stream tag MAY
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).
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After the entity that sent the first closing stream tag receives a
reciprocal closing stream tag from the other party (or if it
considers the stream to be void after a reasonable amount of time),
it MUST terminate the underlying TCP connection or connections.
4.3.3. Handling of Idle Streams
An XML stream can become idle, i.e., neither entity has sent XMPP
traffic over the stream for some period of time. The idle timeout
period is a matter of implementation and local service policy;
however, it is RECOMMENDED to be liberal in accepting idle streams,
since experience has shown that doing so improves the reliability of
communication over XMPP networks. In particular, it is typically
more efficient to maintain a stream between two servers than it is to
aggressively timeout such a stream, especially with regard to
synchronization of presence information as described in [XMPP-IM];
therefore it is RECOMMENDED to maintain such a stream since
experience has shown that server-to-server streams are cyclical and
typically need to be re-established every 24 to 48 hours if they are
timed out.
An XML stream can appear idle at the XMPP level because the
underlying TCP connection has become idle (e.g., a client's network
connection has been lost). One common method for preventing a TCP
connection from going idle or for detecting an idle TCP connection is
to send a space character (U+0020) over the TCP connection 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). Other connection-testing methods include the application-
level pings described in [XEP-0199] and the more comprehensive stream
management protocol described in [XEP-0198]. Implementers are
advised to support whichever connection-testing methods they deem
appropriate, but to carefully weigh the network impact of such
methods against the benefits of discovering idle streams in a timely
manner. The length of time between the use of any particular
connection test is a matter of implementation and local service
policy; however, it is RECOMMENDED that any such test be performed
not more than once every 60 seconds.
To close an idle stream with a local client or remote server, a
server MUST close the stream without error as explained under
Section 4.3.2.
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4.4. Stream Attributes
The attributes of the root <stream/> element are defined in the
following sections.
Security Note: Until and unless the confidentiality and integrity
of a stream header is ensured via Transport Layer Security as
described under Section 5, an entity SHOULD NOT trust the
attributes provided in a stream header.
Implementation Note: The attributes of the root <stream/> element
are not prepended by a namespace prefix because, as explained in
[XML-NAMES], "[d]efault namespace declarations do not apply
directly to attribute names; the interpretation of unprefixed
attributes is determined by the element on which they appear."
4.4.1. from
The 'from' attribute communicates an XMPP identity of the entity
sending the stream element.
For initial stream headers in client-to-server communication, if the
client knows the XMPP identity of the principal controlling the
client (typically an account name of the form <localpart@domain>),
then it MAY include the 'from' attribute and set its value to that
identity. However, because the client might not know the XMPP
identity of the principal controlling the entity (e.g., because the
XMPP identity is assigned at a level other than the XMPP application
layer, as in the General Security Service Application Program
Interface [GSS-API]), inclusion of the 'from' address is OPTIONAL.
Security Note: Including the XMPP identity before the stream is
protected via TLS can expose that identity to eavesdroppers.
I: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For initial stream headers in server-to-server communication, a
server MUST include the 'from' attribute and MUST set its value to a
hostname serviced by the initiating entity (because the initiating
entity might have more than one XMPP identity, e.g., in the case of a
server that provides virtual hosting, it will need to choose an
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identity that is associated with this stream).
I: <?xml version='1.0'?>
<stream:stream
from='example.net'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers in both client-to-server and server-to-
server communication, the receiving entity MUST include the 'from'
attribute and MUST set its value to a hostname serviced by the
receiving entity (which MAY be a hostname other than that specified
in the 'to' attribute of the initial stream header).
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
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 (see Section 13.5.
Interoperability Note: It is possible that implementations based
on the predecessor to this specification will not include the
'from' address on stream headers; an entity SHOULD be liberal in
accepting such stream headers.
4.4.2. 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 hostname 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].)
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I: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers in client-to-server communication, if the
client included a 'from' attribute in the initial stream header then
the server MUST include a 'to' attribute in the response stream
header and MUST set its value to the bare JID specified in the 'from'
attribute of the initial stream header. If the client did not
include a 'from' attribute in the initial stream header then the
server MUST NOT include a 'to' attribute in the response stream
header.
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers in server-to-server communication, the
receiving entity MUST include a 'to' attribute in the response stream
header and MUST set its value to the hostname specified in the 'from'
attribute of the initial stream header.
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
to='example.net'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
Whether or not the 'to' attribute is included, each entity MUST
verify the identity of the other entity before exchanging XML stanzas
with it, as described under Section 13.5.
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Interoperability Note: It is possible that implementations based
on the predecessor to this specification will not include the 'to'
address on stream headers; an entity SHOULD be liberal in
accepting such stream headers.
4.4.3. id
The 'id' attribute communicates a unique identifier for the stream,
called a STREAM ID. The stream ID MUST be generated by the receiving
entity when it sends a response stream header and MUST BE unique
within the receiving application (normally a server).
Security Note: The stream ID MUST be both unpredictable and
nonrepeating because it can be security-critical (see [RANDOM] for
recommendations regarding randomness for security purposes).
For initial stream headers, the initiating entity MUST NOT include
the 'id' attribute; however, if the 'id' attribute is included, the
receiving entity MUST silently ignore it.
For response stream headers, the receiving entity MUST include the
'id' attribute.
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
4.4.4. xml:lang
The 'xml:lang' attribute communicates an entity's preferred or
default language for any human-readable XML character data to be sent
over the stream. The syntax of this attribute is defined in Section
2.12 of [XML]; in particular, the value of the 'xml:lang' attribute
MUST conform to the NMTOKEN datatype (as defined in Section 2.3 of
[XML]) and MUST conform to the language identifier format defined in
[LANGTAGS].
For initial stream headers, the initiating entity SHOULD include the
'xml:lang' attribute.
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I: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers, the receiving entity MUST include the
'xml:lang' attribute. The following rules apply:
o If the initiating entity included an 'xml:lang' attribute in its
initial stream header and the receiving entity supports that
language in the human-readable XML character data that it
generates and sends to the initiating entity (e.g., in the <text/>
element for stream and stanza errors), the value of the 'xml:lang'
attribute MUST be the identifier for the initiating entity's
preferred language (e.g., "de-CH").
o If the receiving entity supports a language that closely matches
the initiating entity's preferred language (e.g., "de" instead of
"de-CH"), then the value of the 'xml:lang' attribute SHOULD be the
identifier for the matching language (e.g., "de") but MAY be the
identifier for the default language of the receiving entity (e.g.,
"en").
o If the receiving entity does not support the initiating entity's
preferred language or a closely matching language (or if the
initiating entity did not include the 'xml:lang' attribute in its
initial stream header), then the value of the 'xml:lang' attribute
MUST be the identifier for the default language of the receiving
entity (e.g., "en").
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
If the initiating entity included the 'xml:lang' attribute in its
initial stream header, the receiving entity SHOULD remember that
value as the default xml:lang for all stanzas sent by the initiating
entity. As described under Section 8.1.5, the initiating entity MAY
include the 'xml:lang' attribute in any XML stanzas it sends over the
stream. If the initiating entity does not include the 'xml:lang'
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attribute in any such stanza, the receiving entity SHOULD add the
'xml:lang' attribute to the stanza, 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.
4.4.5. 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 4.2.2), and stream
errors (Section 4.6).
The version of XMPP specified in this specification is "1.0"; in
particular, XMPP 1.0 encapsulates the stream-related protocols as
well as the basic semantics of the three defined XML stanza types
(<message/>, <presence/>, and <iq/>).
The numbering scheme for XMPP versions is "<major>.<minor>". The
major and minor numbers MUST be treated as separate integers and each
number MAY be incremented higher than a single digit. Thus, "XMPP
2.4" would be a lower version than "XMPP 2.13", which in turn would
be lower than "XMPP 12.3". Leading zeros (e.g., "XMPP 6.01") MUST be
ignored by recipients and MUST NOT be sent.
The major version number will be incremented only if the stream and
stanza formats or obligatory actions have changed so dramatically
that an older version entity would not be able to interoperate with a
newer version entity if it simply ignored the elements and attributes
it did not understand and took the actions specified in the older
specification.
The minor version number will be incremented only if significant new
capabilities have been added to the core protocol (e.g., a newly
defined value of the 'type' attribute for message, presence, or IQ
stanzas). The minor version number MUST be ignored by an entity with
a smaller minor version number, but MAY be used for informational
purposes by the entity with the larger minor version number (e.g.,
the entity with the larger minor version number would simply note
that its correspondent would not be able to understand that value of
the 'type' attribute and therefore would not send it).
The following rules apply to the generation and handling of the
'version' attribute within stream headers:
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1. The initiating entity MUST set the value of the 'version'
attribute in the initial stream header to the highest version
number it supports (e.g., if the highest version number it
supports is that defined in this specification, it MUST set the
value to "1.0").
2. The receiving entity MUST set the value of the 'version'
attribute in the response stream header to either the value
supplied by the initiating entity or the highest version number
supported by the receiving entity, whichever is lower. The
receiving entity MUST perform a numeric comparison on the major
and minor version numbers, not a string match on
"<major>.<minor>".
3. If the version number included in the response stream header is
at least one major version lower than the version number included
in the initial stream header and newer version entities cannot
interoperate with older version entities as described, the
initiating entity SHOULD generate an <unsupported-version/>
stream error.
4. If either entity receives a stream header with no 'version'
attribute, the entity MUST consider the version supported by the
other entity to be "0.9" and SHOULD NOT include a 'version'
attribute in the response stream header.
4.4.6. Summary of Stream Attributes
The following table summarizes the attributes of the root <stream/>
element.
+----------+--------------------------+-------------------------+
| | initiating to receiving | receiving to initiating |
+----------+--------------------------+-------------------------+
| to | JID of receiver | JID of initiator |
| from | JID of initiator | JID of receiver |
| id | silently ignored | stream identifier |
| xml:lang | default language | default language |
| version | XMPP 1.0+ supported | XMPP 1.0+ supported |
+----------+--------------------------+-------------------------+
4.5. Namespace Declarations
4.5.1. Declaration of Streams Namespace
A streams namespace declaration is REQUIRED in all XML stream
headers, and the name of the streams namespace MUST be
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'http://etherx.jabber.org/streams'. If this rule is violated, the
entity that receives the offending stream header MUST return a stream
error to the sending entity, which SHOULD be <invalid-namespace/> but
MAY be <bad-format/>.
The element names of the <stream/> element and its <features/> and
<error/> children MUST be qualified by the streams namespace prefix
in all instances. If this rule is violated, the entity that receives
the offending element MUST return a stream error to the sending
entity, which SHOULD be <bad-format/>.
Interoperability Note: For historical reasons, an implementation
MAY accept only the 'stream:' prefix for these elements. If an
entity receives a stream header with a streams namespace prefix it
does not accept, it MUST return a stream error to the sending
entity, which SHOULD be <bad-namespace-prefix/> but MAY be <bad-
format/>.
4.5.2. Declaration of Default Namespace
A default namespace declaration is REQUIRED and defines the allowable
first-level children of the root stream element. This namespace
declaration MUST be the same for the initial stream and the response
stream so that both streams are qualified consistently. The default
namespace declaration applies to the stream and all first-level child
elements sent over a stream unless explicitly qualified by the
streams namespace or another namespace.
A server implementation MUST support the following two default
namespaces:
o jabber:client -- this default namespace is declared when the
stream is used for communication between a client and a server
o jabber:server -- this default namespace is declared when the
stream is used for communication between two servers
A client implementation MUST support the 'jabber:client' default
namespace.
Informational Note: The 'jabber:client' and 'jabber:server'
namespaces are nearly identical but are used in different contexts
(client-to-server communication for 'jabber:client' and server-to-
server communication for 'jabber:server'). The only difference
between the two is that the 'to' and 'from' attributes are
OPTIONAL on stanzas sent over XML streams qualified by the
'jabber:client' namespace, whereas they are REQUIRED on stanzas
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sent over XML streams qualified by the 'jabber:server' namespace.
If an implementation accepts a stream that is qualified by the
'jabber:client' or 'jabber:server' namespace, it MUST support the
common attributes (Section 8.1) and basic semantics (Section 8.2) of
all three core stanza types (message, presence, and IQ).
An implementation MUST NOT generate namespace prefixes for elements
qualified by the default namespace if the default namespace is
'jabber:client' or 'jabber:server'.
Implementation Note: Because a client sends stanzas over a stream
whose default namespace is 'jabber:client', if the server to which
the client is connected needs to route a client-generated stanza
to another server then it MUST "re-scope" the stanza so that its
default namespace is 'jabber:server' (i.e., it MUST NOT send a
stanza qualified by the 'jabber:client' namespace over a stream
whose default namespace is 'jabber:server'). Similarly, a routing
server MUST "re-scope" a stanza received over a server-to-server
stream (whose default namespace is 'jabber:server') so that the
stanza is qualified by the 'jabber:client' namespace before
sending it over a client-to-server stream (whose default namespace
is 'jabber:client').
An implementation MAY support a default namespace other than 'jabber:
client' or 'jabber:server'. However, because such namespaces would
define applications other than XMPP, they are to be defined in
separate specifications.
Interoperability Note: For historical reasons, an implementation
MAY refuse to support any other default namespaces. If an entity
receives a stream header with a default namespace it does not
support, it MUST return an <invalid-namespace/> stream error.
4.5.3. Declaration of Other Namespaces
Because an XML stanza is the primary unit of meaning in XMPP and
because an XML stanza can be routed outside the context of the stream
in which it originated, a stream header MUST NOT include namespace
declarations for namespaces that are different from the streams
namespace and the default namespace.
4.6. Stream Errors
The root stream element MAY contain an <error/> child element that is
prefixed by the streams namespace prefix. The error child SHALL be
sent by a compliant entity if it perceives that a stream-level error
has occurred.
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4.6.1. Rules
The following rules apply to stream-level errors.
4.6.1.1. Stream Errors Are Unrecoverable
Stream-level errors are unrecoverable. Therefore, if an error occurs
at the level of the stream, the entity that detects the error MUST
send an <error/> element with an appropriate child element that
specifies the error condition and at the same time send a closing
</stream> tag.
C: <message><body>No closing tag!</message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
The entity that generates the stream error then SHOULD immediately
terminate the underlying TCP connection, although as explained under
Section 4.3 it MAY wait until after it receives a closing </stream>
tag from the entity to which it sent the stream error.
C: </stream:stream>
4.6.1.2. Stream Errors Can Occur During Setup
If the error is triggered by the initial stream header, the receiving
entity MUST still send the opening <stream> tag, include the <error/>
element as a child of the stream element, and send the closing
</stream> tag (preferably all at the same time).
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C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://wrong.namespace.example.org/'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<invalid-namespace
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.1.3. 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 receiving entity SHOULD provide an authoritative hostname
in the 'from' attribute of the stream header sent before termination,
but absent such an authoritative hostname MAY instead simply populate
the response stream's 'from' attribute with the value of the initial
stream header's 'to' attribute (where the value of the 'from'
attribute MAY be empty if the initiating entity provided no 'to'
attribute).
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C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='unknown.host.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<host-unknown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.1.4. 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 Stanza errors triggered by outbound stanzas sent from the
initiating entity over the initial stream via the first TCP
connection are returned by the receiving entity on the response
stream via the second TCP connection (since they are inbound
stanzas from the perspective of the initiating entity).
o By contrast, stream-level errors related to the initial stream are
returned by the receiving entity on the response stream via the
first TCP connection.
4.6.2. Syntax
The syntax for stream errors is as follows, where "defined-condition"
is a placeholder for one of the conditions defined under
Section 4.6.3 and XML data shown within the square brackets '[' and
']' is OPTIONAL.
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<stream:error>
<defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
[<text xmlns='urn:ietf:params:xml:ns:xmpp-streams'
xml:lang='langcode'>
[ ... descriptive text ... ]
</text>]
[application-specific condition element]
</stream:error>
The <error/> element:
o MUST contain a child element corresponding to one of the defined
stream error conditions (Section 4.6.3); this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace.
o MAY contain a <text/> child element containing XML character data
that describes the error in more detail; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace
and SHOULD possess an 'xml:lang' attribute specifying the natural
language of the XML character data.
o MAY contain a child element for an application-specific error
condition; this element MUST be qualified by an application-
defined namespace, and its structure is defined by that namespace
(see Section 4.6.4).
The <text/> element is OPTIONAL. If included, it MUST be used only
to provide descriptive or diagnostic information that supplements the
meaning of a defined condition or application-specific condition. It
MUST NOT be interpreted programmatically by an application. It MUST
NOT be used as the error message presented to a human user, but MAY
be shown in addition to the error message associated with the defined
condition element (and, optionally, the application-specific
condition element).
4.6.3. Defined Stream Error Conditions
The following stream-level error conditions are defined.
4.6.3.1. 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 an <xml-not-
well-formed/> stream error.)
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C: <message>
<body>No closing tag!
</message>
S: <stream:error>
<bad-format
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
This error MAY be used instead of the more specific XML-related
errors, such as <bad-namespace-prefix/>, <invalid-xml/>, <restricted-
xml/>, <unsupported-encoding/>, and <xml-not-well-formed/>. However,
the more specific errors are RECOMMENDED.
4.6.3.2. 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 streams namespace.)
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:foobar='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<bad-namespace-prefix
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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4.6.3.3. 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).
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<conflict
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.4. connection-timeout
The entity has not generated any traffic over the stream for some
period of time (configurable according to a local service policy) and
therefore the connection is being dropped.
P: <stream:error>
<connection-timeout
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
As explained under Section 4.3.3, it is RECOMMENDED to not return an
error when closing an idle stream.
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4.6.3.5. host-gone
The value of the 'to' attribute provided in the initial stream header
corresponds to a hostname that is no longer serviced by the receiving
entity.
(In the following example, the peer specifies a 'to' address of
"foo.im.example.com" when connecting to the "im.example.com" server,
but the server no longer hosts a service at that address.)
P: <?xml version='1.0'?>
<stream:stream
from='example.net'
to='foo.im.example.com'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
to='example.net'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<host-gone
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.6. host-unknown
The value of the 'to' attribute provided in the initial stream header
does not correspond to a hostname 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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P: <?xml version='1.0'?>
<stream:stream
from='example.net'
to='example.org'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
to='example.net'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<host-unknown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.7. 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 is not a
valid XMPP address.
(In the following example, the peer sends a stanza without a 'to'
address over a server-to-server stream.)
P: <message from='juliet@im.example.com'>
<body>Wherefore art thou?</body>
</message>
S: <stream:error>
<improper-addressing
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.8. internal-server-error
The server has experienced a misconfiguration or an otherwise-
undefined internal error that prevents it from servicing the stream.
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S: <stream:error>
<internal-server-error
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.9. invalid-from
The JID or hostname provided in a 'from' address is not a valid JID
or does not match an authorized JID or validated domain as negotiated
between servers via SASL or server dialback, or as negotiated between
a client and a server via authentication and resource binding.
(In the following example, a peer that has authenticated only as
"example.net" attempts to send a stanza from an address at
"example.org".)
P: <message from='romeo@example.org' to='juliet@im.example.com'>
<body>Neither, fair saint, if either thee dislike.</body>
</message>
S: <stream:error>
<invalid-from
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.10. invalid-id
The stream ID or server dialback ID is invalid or does not match an
ID previously provided.
(In the following example, the server dialback ID is invalid; see
[XEP-0220].)
P: <db:verify
from='example.net'
to='im.example.com'
id='unknown-id'
type='invalid'/>
S: <stream:error>
<invalid-id
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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4.6.3.11. invalid-namespace
The streams namespace name is something other than
"http://etherx.jabber.org/streams" (see Section 11.2) or the default
namespace is not supported (e.g., something other than "jabber:
client" or "jabber:server").
(In the following example, the client specifies a namespace of
'http://wrong.namespace.example.org/' for the stream.)
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://wrong.namespace.example.org/'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<invalid-namespace
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.12. invalid-xml
The entity has sent invalid XML over the stream to a server that
performs validation (see Section 11.4).
(In the following example, the peer attempts to send an IQ stanza of
type "subscribe" but the XML schema defines no such value for the
'type' attribute.)
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P: <iq from='example.net'
id='l3b1vs75'
to='im.example.com'
type='subscribe'>
<ping xmlns='urn:xmpp:ping'/>
</iq>
S: <stream:error>
<invalid-xml
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.13. not-authorized
The entity has attempted to send XML stanzas before the stream has
been authenticated, or otherwise is not authorized to perform an
action related to stream negotiation; the receiving entity MUST NOT
process the offending stanza before sending the stream error.
(In the following example, the client attempts to send XML stanzas
before authenticating with the server.)
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C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
C: <message to='romeo@example.net'>
<body>Wherefore art thou?</body>
</message>
S: <stream:error>
<not-authorized
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.14. policy-violation
The entity has violated some local service policy (e.g., the stanza
exceeds a configured size limit); the server MAY choose to specify
the policy in the <text/> element or in an application-specific
condition element.
(In the following example, the client sends an XMPP message that is
too large according to the server's local service policy.)
C: <message to='juliet@im.example.com' id='foo'>
<body>[ ... the-emacs-manual ... ]</body>
</message>
S: <stream:error>
<policy-violation
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<stanza-too-big xmlns='urn:xmpp:errors'/>
</stream:error>
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S: </stream:stream>
4.6.3.15. remote-connection-failed
The server is unable to properly connect to a remote entity that is
needed for authentication or authorization, such as a remote
authentication database or (in server dialback) the authoritative
server.
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<remote-connection-failed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.16. reset
The server is closing the stream because it has new (typically
security-critical) features to offer or because it needs to reset the
stream for some other reason (e.g., because the certificates used to
establish a secure context for the stream have expired or have been
revoked during the life of the stream).
S: <stream:error>
<reset
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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4.6.3.17. resource-constraint
The server lacks the system resources necessary to service the
stream.
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<resource-constraint
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.18. restricted-xml
The entity has attempted to send restricted XML features such as a
comment, processing instruction, DTD subset, or XML entity reference
(see Section 11.1).
(In the following example, the client sends an XMPP message
containing an XML comment.)
C: <message to='juliet@im.example.com'>
<!--<subject/>-->
<body>This message has no subject.</body>
</message>
S: <stream:error>
<restricted-xml
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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4.6.3.19. see-other-host
The server will not provide service to the initiating entity but is
redirecting traffic to another host; the XML character data of the
<see-other-host/> element returned by the server SHOULD specify the
alternate hostname or IP address at which to connect, which SHOULD be
a valid domainpart but MAY also include a port number. When it
receives a see-other-host stream error, the initiating entity SHOULD
cleanly handle the disconnection and then reconnect to the host
specified in the <see-other-host/> element; if no port is specified,
the initiating entity SHOULD perform a [DNS-SRV] lookup on the
provided domainpart but MAY assume that it can connect to that
domainpart at the standard XMPP ports (i.e., 5222 for client-to-
server connections and 5269 for server-to-server connections).
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<see-other-host
xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
[2001:41D0:1:A49b::1]:9222
</see-other-host>
</stream:error>
</stream:stream>
4.6.3.20. system-shutdown
The server is being shut down and all active streams are being
closed.
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S: <stream:error>
<system-shutdown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.21. undefined-condition
The error condition is not one of those defined by the other
conditions in this list; this error condition SHOULD be used only in
conjunction with an application-specific condition.
S: <stream:error>
<undefined-condition
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<app-error xmlns='http://example.com/ns'/>
</stream:error>
</stream:stream>
4.6.3.22. 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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C: <?xml version='1.0' encoding='UTF-16'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
<stream:error>
<unsupported-encoding
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.23. unsupported-feature
The receiving entity has advertised a mandatory stream feature that
the initiating entity does not support, and has offered no other
mandatory feature alongside the unsupported feature.
(In the following example, the receiving entity requires negotiation
of an example feature but the initiating entity does not support the
feature.)
R: <stream:features>
<example xmlns='urn:xmpp:example'/>
</stream:features>
I: <stream:error>
<unsupported-feature
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.24. unsupported-stanza-type
The initiating entity has sent a first-level child of the stream that
is not supported by the server or consistent with the default
namespace.
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(In the following example, the client attempts to send an XML stanza
of <pubsub/> when the default namespace is "jabber:client".)
C: <pubsub>
<publish node='princely_musings'>
<item id='ae890ac52d0df67ed7cfdf51b644e901'>
<entry xmlns='http://www.w3.org/2005/Atom'>
<title>Soliloquy</title>
<summary>
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?
</summary>
<link rel='alternate' type='text/html'
href='http://denmark.example/2003/12/13/atom03'/>
<id>tag:denmark.example,2003:entry-32397</id>
<published>2003-12-13T18:30:02Z</published>
<updated>2003-12-13T18:30:02Z</updated>
</entry>
</item>
</publish>
</pubsub>
S: <stream:error>
<unsupported-stanza-type
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
4.6.3.25. unsupported-version
The value of the 'version' attribute provided by the initiating
entity in the stream header specifies a version of XMPP that is not
supported by the server; the server MAY specify the version(s) it
supports in the <text/> element.
(In the following example, the client specifies an XMPP version of
"11.0" but the server supports only version "1.0" and "1.1".)
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C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='11.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
<stream:error>
<unsupported-version
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<text xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
1.0, 1.1
</text>
</stream:error>
</stream:stream>
4.6.3.26. xml-not-well-formed
The initiating entity has sent XML that violates the well-formedness
rules of [XML] or [XML-NAMES].
(In the following example, the client sends an XMPP message that is
not namespace-well-formed.)
C: <message>
<foo:body>What is this foo?</foo:body>
</message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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4.6.4. Application-Specific Conditions
As noted, an application MAY provide application-specific stream
error information by including a properly-namespaced child in the
error element. The application-specific element SHOULD supplement or
further qualify a defined element. Thus the <error/> element will
contain two or three child elements.
C: <message>
<body>
My keyboard layout is:
QWERTYUIOP{}|
ASDFGHJKL:"
ZXCVBNM<>?
</body>
</message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<text xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
Some special application diagnostic information!
</text>
<escape-your-data xmlns='http://example.com/ns'/>
</stream:error>
</stream:stream>
4.7. Simplified Stream Examples
This section contains two 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.
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A basic connection:
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
[ ... channel encryption ... ]
[ ... authentication ... ]
[ ... resource binding ... ]
C: <message from='juliet@im.example.com/balcony'
to='romeo@example.net'
xml:lang='en'>
<body>Art thou not Romeo, and a Montague?</body>
</message>
S: <message from='romeo@example.net/orchard'
to='juliet@im.example.com/balcony'
xml:lang='en'>
<body>Neither, fair saint, if either thee dislike.</body>
</message>
C: </stream:stream>
S: </stream:stream>
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A connection gone bad:
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
[ ... channel encryption ... ]
[ ... authentication ... ]
[ ... resource binding ... ]
C: <message from='juliet@im.example.com/balcony'
to='romeo@example.net'
xml:lang='en'>
<body>No closing tag!
</message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
More detailed examples are provided under Section 9.
5. STARTTLS Negotiation
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5.1. Overview
XMPP includes a method for securing the stream from tampering and
eavesdropping. This channel encryption method makes use of the
Transport Layer Security [TLS] protocol, specifically a "STARTTLS"
extension that is modelled after similar extensions for the [IMAP],
[POP3], and [ACAP] protocols as described in [USINGTLS]. The XML
namespace name for the STARTTLS extension is
'urn:ietf:params:xml:ns:xmpp-tls'.
Support for STARTTLS is REQUIRED in XMPP client and server
implementations. An administrator of a given deployment MAY
necessitate the use of TLS for client-to-server communication,
server-to-server communication, or both. A deployed client SHOULD
use TLS to secure its stream with a server prior to attempting the
completion of SASL negotiation (Section 6), and deployed servers
SHOULD use TLS between two domains for the purpose of securing
server-to-server communication.
5.2. Stream Negotiation Rules
5.2.1. Mandatory-to-Negotiate
If the receiving entity advertises only the STARTTLS feature or if
the receiving entity includes the <required/> child element as
explained under Section 5.3.1, 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).
5.2.2. Restart
After TLS negotiation, the parties MUST restart the stream.
5.2.3. 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 <starttls/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-tls' namespace at depth=1 of the stream
as sent by the initiating entity, until the last character of the
<proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
namespace at depth=1 of the stream as sent by the receiving entity).
This prohibition helps to ensure proper security layer byte
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precision. Any such whitespace shown in the STARTTLS examples
provided in this document is included only for the sake of
readability.
5.2.4. Order of Negotiation
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.
5.2.5. Renegotiation
XMPP entities MUST NOT attempt TLS renegotiation, and if either party
to a TLS-protected stream detects a TLS renegotiation attempt 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; see Section 13.3).
5.2.6. 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.
Because XMPP services are often provided in so-called virtual hosting
deployments, it is RECOMMENDED for the initiating entity to include
an extension of type "server_name" as defined in [TLS-EXT]. If an
XMPP service hosts multiple domains on the same IP address, it MUST
enable support for the "server_name" extension.
5.3. Process
5.3.1. Exchange of Stream Headers and Stream Features
The initiating entity resolves the hostname 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; if the initiating entity is
capable of STARTTLS negotiation, it MUST include the 'version'
attribute set to a value of at least "1.0" in the initial stream
header.
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I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity MUST send a response stream header to the
initiating entity over the TCP connection opened by the initiating
entity; if the receiving entity is capable of STARTTLS negotiation,
it MUST include the 'version' attribute set to a value of at least
"1.0" in the response stream header.
R: <stream:stream
from='im.example.com'
id='t7AMCin9zjMNwQKDnplntZPIDEI='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
The receiving entity then MUST send stream features to the initiating
entity. If the receiving entity supports TLS, the stream features
MUST include an advertisement for support of STARTTLS negotiation,
i.e., a <starttls/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-tls' namespace.
If the receiving entity considers STARTTLS negotiation to be
mandatory, the <starttls/> element SHOULD contain an empty
<required/> child element.
R: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
5.3.2. Initiation of STARTTLS Negotiation
5.3.2.1. STARTTLS Command
In order to begin the STARTTLS negotiation, the initiating entity
issues the STARTTLS command (i.e., a <starttls/> element qualified by
the 'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct the
receiving entity that it wishes to begin a STARTTLS negotiation to
secure the stream.
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I: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
The receiving entity MUST reply with either a <proceed/> element
(proceed case) or a <failure/> element (failure case) qualified by
the 'urn:ietf:params:xml:ns:xmpp-tls' namespace.
5.3.2.2. Failure Case
If the failure case occurs, the receiving entity MUST return a
<failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
namespace, terminate the XML stream, and terminate the underlying TCP
connection.
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
R: </stream:stream>
Causes for the failure case include but are not limited to:
1. The initiating entity has sent a malformed STARTTLS command.
2. The receiving entity does not offer STARTTLS negotiation either
temporarily (e.g., at this stage in the stream negotiation
process) or permanently (i.e., the feature is not supported).
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 unknown certificate authority;
those errors 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 3.4.
5.3.2.3. Proceed Case
If the proceed case occurs, the receiving entity MUST return a
<proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
namespace.
R: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
The receiving entity MUST consider the TLS negotiation to have begun
immediately after sending the closing '>' character of the <proceed/>
element to the initiating entity. The initiating entity MUST
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consider the TLS negotiation to have begun immediately after
receiving the closing '>' character of the <proceed/> element from
the receiving entity.
The entities now proceed to TLS negotiation as explained in the next
section.
5.3.3. TLS Negotiation
5.3.3.1. Rules
In order to complete TLS negotiation over the TCP connection, the
entities MUST follow the process defined in [TLS].
The following rules apply:
1. The entities MUST NOT send any further XML data until the TLS
negotiation has either failed or succeeded.
2. The receiving entity MUST present a certificate.
3. The receiving entity SHOULD send a certificate request to the
initiating entity so that mutual authentication will be possible.
4. The initiating entity MUST validate the certificate to determine
if the TLS negotiation will succeed; see Section 13.7.2 regarding
certificate validation procedures.
5. The receiving entity SHOULD choose which certificate to present
based on the 'to' attribute of the initial stream header or the
TLS "server_name" extension.
Security Note: See Section 13.8 regarding ciphers that MUST be
supported for TLS; naturally, other ciphers MAY be supported as
well.
5.3.3.2. TLS Failure
If the TLS negotiation results in failure, the receiving entity MUST
terminate the TCP connection.
The receiving entity MUST NOT send a closing </stream> tag before
terminating the TCP connection, since the receiving entity and
initiating entity MUST consider the original stream to be replaced
upon failure of the TLS negotiation.
If the failure case occurs and TLS negotation was voluntary
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(advertised by means of the <optional/> child element) instead of
mandatory (advertised by means of the <required/> child element), the
initiating entity SHOULD attempt to reconnect as explained under
Section 3.4 but without attempting TLS negotiation.
5.3.3.3. 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.
I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
Implementation Note: The initiating entity MUST NOT send a
closing </stream> tag before sending the new initial stream
header, since the receiving entity and initiating entity MUST
consider the original stream to be replaced upon success of the
TLS negotiation.
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 re-using the old stream ID).
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R: <stream:stream
from='im.example.com'
id='vgKi/bkYME8OAj4rlXMkpucAqe4='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
5. The receiving entity also MUST send stream features to the
initiating entity, which MUST NOT include the STARTTLS feature
but which SHOULD include the SASL stream feature as described
under Section 6.
R: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>
6. SASL Negotiation
6.1. Overview
XMPP includes a method for authenticating a stream by means of an
XMPP-specific profile of the Simple Authentication and Security Layer
protocol (see [SASL]). SASL provides a generalized method for adding
authentication support to connection-based protocols, and XMPP uses
an XML namespace profile of SASL that conforms to the profiling
requirements of [SASL].
Support for SASL negotiation is REQUIRED in XMPP client and server
implementations.
6.2. Stream Negotiation Rules
6.2.1. Mandatory-to-Negotiate
The parties to a stream MUST consider SASL as mandatory-to-negotiate.
6.2.2. Restart
After SASL negotiation, the parties MUST restart the stream.
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6.2.3. 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 by the perceived
strength of the mechanisms. A server MUST offer and a client MUST
try SASL mechanisms in the order of their perceived strength. For
example, if the server offers the ordered list "PLAIN DIGEST-MD5
GSSAPI" or "DIGEST-MD5 GSSAPI PLAIN" but the client's ordered list is
"GSSAPI DIGEST-MD5", the client MUST try GSSAPI first and then
DIGEST-MD5 but MUST never try PLAIN (since PLAIN is not on its list).
6.2.4. Mechanism Offers
If the receiving entity considers TLS negotiation (Section 5) to be
mandatory before use of a particular SASL authentication mechanism
will be acceptable, the receiving entity MUST NOT advertise that
mechanism in its list of available SASL authentication mechanisms
prior to successful TLS negotiation.
If during prior 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, the receiving entity MUST offer the SASL EXTERNAL mechanism
to the initiating entity during SASL negotiation (refer to [SASL])
and SHOULD prefer that mechanism to enable mutual authentication.
However, the EXTERNAL mechanism MAY be offered under other
circumstances as well.
See Section 13.8 regarding mechanisms that MUST be supported;
naturally, other SASL mechanisms MAY be supported as well. 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.
6.2.5. Data Formatting
The following data formattting 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 <auth/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace at depth=1 of the
stream as sent by the initiating entity, until the last character
of the <success/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace at depth=1 of the
stream as sent by the receiving entity). This prohibition helps
to ensure proper security layer byte precision. Any such
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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 base64, where the encoding adheres to the
definition in Section 4 of [BASE64] and where the padding bits
are set to zero.
3. As formally specified in the XML schema for the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace under Appendix A.4,
the receiving entity MAY include one or more application-specific
child elements inside the <mechanisms/> element to provide
information that might be needed by the initiating entity in
order to complete successful SASL negotiation using one or more
of the offered mechanisms; however, the syntax and semantics of
all such elements are out of scope for this specification.
6.2.6. Security Layers
Upon successful SASL negotiation that involves negotiation of a
security layer, both the initiating entity and the receiving MUST
discard any application-layer state (i.e, state from the XMPP layer,
excluding state from the TLS negotiation or SASL negotiation).
6.2.7. Simple Username
Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) have the
concept of a "simple username". The exact form of the simple
username in any particular mechanism is a local matter, and a simple
username does not necessarily map to an application identifier such
as a JID or a JID component.
6.2.8. Authorization Identity
An authorization identity is an optional identity specified by the
initiating entity, which is typically used by an administrator to
perform some management task on behalf of another user. If the
initiating entity wishes to act on behalf of another entity and the
selected SASL mechanism supports transmission of an authorization
identity, the initiating entity MUST provide an authorization
identity during SASL negotiation. If the initiating entity does not
wish to act on behalf of another entity, it MUST NOT provide an
authorization identity. As specified in [SASL], the initiating
entity MUST NOT provide an authorization identity unless the
authorization identity is different from the default authorization
identity derived from the authentication identity.
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Authorization identities are allowed only for client-to-server
streams. Because a server-to-server stream is valid only for the
particular domain pair authenticated during stream negotiation,
authorization of any additional domains for the stream needs to be
completed by means of protocol extensions above the core XMPP stream
layer. Therefore in the case of server-to-server communication the
initiating server MUST NOT attempt to negotiate an authorization
identity during SASL negotiation and the receiving server MUST NOT
accept an authorization identity from the initiating server.
In the case of client-to-server communication, the value of an
authorization identity MUST be a bare JID of the form
<localpart@domain> (i.e., localpart and domainpart).
Informational Note: The authorization identity communicated during
SASL negotiation is used to determine the canonical address for
the initiating client or server according to the receiving server,
as described under Section 4.2.6.
6.2.9. Realms
The receiving entity MAY include a realm when negotiating certain
SASL mechanisms. 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
hostname from the realm information provided by the receiving entity.
6.2.10. Round Trips
[SASL] specifies that a using protocol such as XMPP can define two
methods by which the protocol can save round trips where allowed for
the SASL mechanism:
1. When the SASL client (the XMPP "initiating entity") requests an
authentication exchange, it can include "initial response" data
with its request if appropriate for the SASL mechanism in use.
In XMPP this is done by including the initial response as the XML
character data of the <auth/> element.
2. At the end of the authentication exchange, the SASL server (the
XMPP "receiving entity") can include "additional data with
success" if appropriate for the SASL mechanism in use. In XMPP
this is done by including the additional data as the XML
character data of the <success/> element.
For the sake of protocol efficiency, it is RECOMMENDED for XMPP
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clients and servers to use these methods, however they MUST support
the less efficient modes as well.
6.3. Process
The process for SASL negotiation is as follows.
6.3.1. Exchange of Stream Headers and Stream Features
If SASL negotiation follows successful STARTTLS negotation
(Section 5), then the SASL negotiation occurs over the encrypted
stream that has already been negotiated. If not, the initiating
entity resolves the hostname 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; if the initiating entity is capable of STARTTLS
negotiation, it MUST include the 'version' attribute set to a value
of at least "1.0" in the initial stream header.
I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity MUST send a response stream header to the
initiating entity (for which it MUST generate a new stream ID instead
of re-using the old stream ID); if the receiving entity is capable of
SASL negotiation, it MUST include the 'version' attribute set to a
value of at least "1.0" in the response stream header.
R: <stream:stream
from='im.example.com'
id='vgKi/bkYME8OAj4rlXMkpucAqe4='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
The receiving entity also MUST send stream features to the initiating
entity. If the receiving entity supports SASL, the stream features
MUST include an advertisement for support of SASL negotiation, i.e.,
a <mechanisms/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace.
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The <mechanisms/> element MUST contain one <mechanism/> child element
for each authentication mechanism the receiving entity offers to the
initiating entity. The order of <mechanism/> elements in the XML
indicates the preference order of the SASL mechanisms according to
the receiving entity; however the initiating entity MUST maintain its
own preference order independent of the preference order of the
receiving entity.
R: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>
6.3.2. Initiation
In order to begin the SASL negotiation, the initiating entity sends
an <auth/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace and includes an
appropriate value for the 'mechanism' attribute. This element MAY
contain XML character data (in SASL terminology, the "initial
response") if the mechanism supports or requires it; if the
initiating entity needs to send a zero-length initial response, it
MUST transmit the response as a single equals sign character ("="),
which indicates that the response is present but contains no data.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
6.3.3. Challenge-Response Sequence
If necessary, the receiving entity challenges the initiating entity
by sending a <challenge/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (which MUST be generated in accordance
with the definition of the SASL mechanism chosen by the initiating
entity).
The initiating entity responds to the challenge by sending a
<response/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (which MUST be generated in accordance
with the definition of the SASL mechanism chosen by the initiating
entity).
If necessary, the receiving entity sends more challenges and the
initiating entity sends more responses.
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This series of challenge/response pairs continues until one of three
things happens:
o The initiating entity aborts the handshake.
o The receiving entity reports failure of the handshake.
o The receiving entity reports success of the handshake.
These scenarios are described in the following sections.
6.3.4. Abort
The initiating entity aborts the handshake by sending an <abort/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
namespace.
I: <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Upon receiving an <abort/> element, the receiving entity MUST return
a <failure/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace and containing an
<aborted/> child element.
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
<aborted/>
</failure>
6.3.5. Failure
The receiving entity reports failure of the handshake by sending a
<failure/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace (the particular cause of
failure MUST be communicated in an appropriate child element of the
<failure/> element as defined under Section 6.4).
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>
Where appropriate for the chosen SASL mechanism, the receiving entity
SHOULD allow a configurable but reasonable number of retries (at
least 2 and no more than 5); this enables the initiating entity
(e.g., an end-user client) to tolerate incorrectly-provided
credentials (e.g., a mistyped password) without being forced to
reconnect.
If the initiating entity attempts a reasonable number of retries with
the same SASL mechanism and all attempts fail, it MAY fall back to
the next mechanism in its ordered list by sending a new <auth/>
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request to the receiving entity. If there are no remaining
mechanisms in its list, the initiating entity SHOULD instead send an
<abort/> element to the receiving entity.
If the initiating entity exceeds the number of retries, the receiving
entity MUST return a stream error (which SHOULD be <policy-
violation/> but MAY be <not-authorized/>).
6.3.6. Success
The receiving entity reports success of the handshake by sending a
<success/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (in SASL terminology, "additional data
with success") if the chosen SASL mechanism supports or requires it;
if the receiving entity needs to send additional data of zero length,
it MUST transmit the data as a single equals sign character ("=").
R: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Informational Note: The authorization identity communicated during
SASL negotiation is used to determine the canonical address for
the initiating client or server according to the receiving server,
as described under Section 4.2.6.
Upon receiving the <success/> element, the initiating entity MUST
initiate a new stream over the existing TCP connection by sending a
new initial stream header to the receiving entity.
I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
Implementation Note: The initiating entity MUST NOT send a closing
</stream> tag before sending the new initial stream header, since
the receiving entity and initiating entity MUST consider the
original stream to be replaced upon sending or receiving the
<success/> element.
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 re-using the old stream ID).
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R: <stream:stream
from='im.example.com'
id='gPybzaOzBmaADgxKXu9UClbprp0='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity MUST also send stream features, containing any
further available features or containing no features (via an empty
<features/> element).
R: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>
6.4. SASL Errors
The syntax of SASL errors is as follows, where "defined-condition" is
one of the SASL-related error conditions defined in the following
sections and XML data shown within the square brackets '[' and ']' is
OPTIONAL.
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<defined-condition/>
[<text xml:lang='langcode'>
OPTIONAL descriptive text
</text>]
</failure>
Inclusion of a defined condition is REQUIRED.
Inclusion of the <text/> element is OPTIONAL, and can be used to
provide application-specific information about the error condition,
which information MAY be displayed to a human but only as a
supplement to the defined condition.
6.4.1. aborted
The receiving entity acknowledges an <abort/> element sent by the
initiating entity; sent in reply to the <abort/> element.
I: <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<aborted/>
</failure>
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6.4.2. account-disabled
The account of the initiating entity has been temporarily disabled;
sent in reply to an <auth/> element (with or without initial response
data) or a <response/> element.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<account-disabled/>
<text xml:lang='en'>Call 212-555-1212 for assistance.</text>
</failure>
6.4.3. credentials-expired
The authentication failed because the initiating entity provided
credentials that have expired; sent in reply to a <response/> element
or an <auth/> element with initial response data.
I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<credentials-expired/>
</failure>
6.4.4. encryption-required
The mechanism requested by the initiating entity cannot be used
unless the underlying stream is encrypted; sent in reply to an
<auth/> element (with or without initial response data).
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<encryption-required/>
</failure>
6.4.5. incorrect-encoding
The data provided by the initiating entity could not be processed
because the [BASE64] encoding is incorrect (e.g., because the
encoding does not adhere to the definition in Section 4 of [BASE64]);
sent in reply to a <response/> element or an <auth/> element with
initial response data.
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I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'>[ ... ]</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<incorrect-encoding/>
</failure>
6.4.6. invalid-authzid
The authzid provided by the initiating entity is invalid, either
because it is incorrectly formatted or because the initiating entity
does not have permissions to authorize that ID; sent in reply to a
<response/> element or an <auth/> element with initial response data.
I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<invalid-authzid/>
</failure>
6.4.7. invalid-mechanism
The initiating entity did not provide a mechanism or requested a
mechanism that is not supported by the receiving entity; sent in
reply to an <auth/> element.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='CRAM-MD5'/>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<invalid-mechanism/>
</failure>
6.4.8. malformed-request
The request is malformed (e.g., the <auth/> element includes initial
response data but the mechanism does not allow that, or the data sent
violates the syntax for the specified SASL mechanism); sent in reply
to an <abort/>, <auth/>, <challenge/>, or <response/> element.
(In the following example, the XML character data of the <auth/>
element contains more than 255 UTF-8-encoded Unicode characters and
therefore violates the "token" production for the SASL ANONYMOUS
mechanism as specified in [ANONYMOUS].)
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I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='ANONYMOUS'>[ ... some-long-token ... ]</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<malformed-request/>
</failure>
6.4.9. mechanism-too-weak
The mechanism requested by the initiating entity is weaker than
server policy permits for that initiating entity; sent in reply to an
<auth/> element (with or without initial response data).
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism-too-weak/>
</failure>
6.4.10. not-authorized
The authentication failed because the initiating entity did not
provide proper credentials or the receiving entity has detected an
attack but wishes to disclose as little information as possible to
the attacker; sent in reply to a <response/> element or an <auth/>
element with initial response data.
I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>
Security Note: 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.
6.4.11. temporary-auth-failure
The authentication failed because of a temporary error condition
within the receiving entity, and it is advisable for the initiating
entity to try again later; sent in reply to an <auth/> element or a
<response/> element.
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I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
6.4.12. transition-needed
The authentication failed because the mechanism cannot be used until
the initiating entity provides (for one time only) a plaintext
password so that the receiving entity can build a hashed password for
use in future authentication attempts; sent in reply to an <auth/>
element with or without initial response data.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='CRAM-MD5'>[ ... ]</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<transition-needed/>
</failure>
Security Note: An XMPP client MUST treat a <transition-needed/>
SASL error with extreme caution, SHOULD NOT provide a plaintext
password over an XML stream that is not encrypted via Transport
Layer Security, and MUST warn a human user before allowing the
user to provide a plaintext password over an unencrypted
connection.
6.5. SASL Definition
The profiling requirements of [SASL] require that the following
information be supplied by the definition of a using protocol.
service name: "xmpp"
initiation sequence: After the initiating entity provides an opening
XML stream header and the receiving entity replies in kind, the
receiving entity provides a list of acceptable authentication
methods. The initiating entity chooses one method from the list
and sends it to the receiving entity as the value of the
'mechanism' attribute possessed by an <auth/> element, optionally
including an initial response to avoid a round trip.
exchange sequence: Challenges and responses are carried through the
exchange of <challenge/> elements from receiving entity to
initiating entity and <response/> elements from initiating entity
to receiving entity. The receiving entity reports failure by
sending a <failure/> element and success by sending a <success/>
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element; the initiating entity aborts the exchange by sending an
<abort/> element. Upon successful negotiation, both sides
consider the original XML stream to be closed and new stream
headers are sent by both entities.
security layer negotiation: The security layer takes effect
immediately after sending the closing '>' character of the
<success/> element for the receiving entity, and immediately after
receiving the closing '>' character of the <success/> element for
the initiating entity. The order of layers is first [TCP], then
[TLS], then [SASL], then XMPP.
use of the authorization identity: The authorization identity can be
used in XMPP to denote the non-default <localpart@domain> of a
client; an empty string is equivalent to an absent authorization
identity.
7. Resource Binding
7.1. Overview
After a client authenticates with a server, it MUST bind a specific
resource to the stream so that the server can properly address the
client. That is, there MUST be an XMPP resource associated with the
bare JID (<localpart@domain>) of the client, so that the address for
use over that stream is a full JID of the form
<localpart@domain/resource> (including the resourcepart). This
ensures that the server can deliver XML stanzas to and receive XML
stanzas from the client in relation to entities other than the server
itself or the client's account, as explained under Section 10 (the
client could exchange stanzas with the server itself or the client's
account before binding a resource since the full JID is needed only
for addressing outside the context of the stream negotiated between
the client and the server, but this is not commonly done).
After a client has bound a resource to the stream, it is referred to
as a CONNECTED RESOURCE. A server SHOULD allow an entity to maintain
multiple connected resources simultaneously, where each connected
resource is associated with a distinct XML stream and differentiated
from the other connected resources by a distinct resourcepart.
Security Note: A server MUST 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
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either silently ignore the stanza or return a <not-authorized/>
stream error to the client.
Support for resource binding is REQUIRED in XMPP client and server
implementations.
7.2. Stream Negotiation Rules
7.2.1. Mandatory-to-Negotiate
The parties to a stream MUST consider resource binding as mandatory-
to-negotiate.
7.2.2. Restart
After resource binding, the parties MUST NOT restart the stream.
7.3. Advertising Support
Upon sending a new response stream header to the client after
successful SASL negotiation, the server MUST include a <bind/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace
in the stream features it presents to the client.
The server MUST NOT include the resource binding stream feature until
after the client has authenticated, typically by means of successful
SASL negotiation.
S: <stream:stream
from='im.example.com'
id='gPybzaOzBmaADgxKXu9UClbprp0='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>
Upon being informed that resource binding is mandatory, the client
MUST bind a resource to the stream as described in the following
sections.
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7.4. Generation of Resource Identifiers
A resourcepart MUST at a minimum be unique among the connected
resources for that <localpart@domain>. Enforcement of this policy is
the responsibility of the server.
Security Note: 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, which MUST be random (see [RANDOM]).
When generating a random resourcepart, it is RECOMMENDED that the
resourcepart be a Universally Unique Identifier (UUID), for which
the format specified in [UUID] is RECOMMENDED.
7.5. Server-Generated Resource Identifier
A server that supports resource binding MUST be able to generate an
XMPP resourcepart on behalf of a client.
7.5.1. Success Case
A client requests a server-generated resourcepart by sending an IQ
stanza of type "set" (see Section 8.2.3) containing an empty <bind/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind'
namespace.
C: <iq id='tn281v37' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</iq>
Once the server has generated an XMPP resourcepart for the client, it
MUST return an IQ stanza of type "result" to the client, which MUST
include a <jid/> child element that specifies the full JID for the
connected resource as determined by the server.
S: <iq id='tn281v37' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb
</jid>
</bind>
</iq>
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7.5.2. Error Cases
When a client asks the server to generate a resourcepart during
resource binding, the following stanza error conditions are defined
(and others not specified here are possible; see under Section 8.3):
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.
7.5.2.1. Resource Constraint
If the account has reached a limit on the number of simultaneous
connected resources allowed, the server MUST return a <resource-
constraint/> stanza error.
S: <iq id='wy2xa82b4' type='error'>
<error type='wait'>
<resource-constraint
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
7.5.2.2. Not Allowed
If the client is otherwise not allowed to bind a resource to the
stream, the server MUST return a <not-allowed/> stanza error.
S: <iq id='wy2xa82b4' type='error'>
<error type='cancel'>
<not-allowed
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
7.6. 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.
7.6.1. Success Case
A client asks its server to accept a client-submitted resourcepart by
sending an IQ stanza of type "set" containing a <bind/> element with
a child <resource/> element containing non-zero-length XML character
data.
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C: <iq id='wy2xa82b4' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>balcony</resource>
</bind>
</iq>
The server SHOULD accept the client-submitted resourcepart. It does
so by returning an IQ stanza of type "result" to the client,
including a <jid/> child element that specifies the full JID for the
connected resource and contains without modification the client-
submitted text.
S: <iq id='wy2xa82b4' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>juliet@im.example.com/balcony</jid>
</bind>
</iq>
Alternatively, in accordance with local service policies the server
MAY refuse the client-submitted resourcepart and override it with a
resourcepart that the server generates.
S: <iq id='wy2xa82b4' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
</jid>
</bind>
</iq>
7.6.2. 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 7.5.2 (and others not
specified here are possible; see under Section 8.3):
o The provided resourcepart cannot be processed by the server.
o The provided resourcepart is already in use.
7.6.2.1. Bad Request
If the provided resourcepart cannot be processed by the server (e.g.
because it is of zero length or because it is not in accordance with
the Resourceprep profile of stringprep specified in [XMPP-ADDR]), the
server MAY return a <bad-request/> stanza error (but SHOULD instead
apply the Resourceprep profile or otherwise process the resourcepart
so that it is in conformance).
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S: <iq id='wy2xa82b4' type='error'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
7.6.2.2. Conflict
If there is already a connected resource of the same name, the server
MUST do one of the following:
1. Not accept the resourcepart provided by the client but instead
override it with a resourcepart that the server generates.
2. Disallow the newly-requested resource and maintain the current
resource.
3. Terminate the current resource and allow the newly-requested
resource.
Which of these the server does is up to the implementation, although
it is RECOMMENDED to implement case #1.
S: <iq id='wy2xa82b4' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
</jid>
</bind>
</iq>
In case #2, the server MUST send a <conflict/> stanza error to the
newly-requested resource but maintain the XML stream for that
connection so that the newly-requested resource has an opportunity to
negotiate a non-conflicting resourcepart before sending another
request for resource binding.
S: <iq id='wy2xa82b4' type='error'>
<error type='modify'>
<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
In case #3, the server MUST send a <conflict/> stream error to the
current resource and return an IQ stanza of type "result" (indicating
success) to the newly-requested resource.
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S: <iq id='wy2xa82b4' type='result'/>
7.6.3. Retries
If an error occurs when a client submits a resourcepart, the server
SHOULD allow a configurable but reasonable number of retries (at
least 2 and no more than 5); 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 return
a <policy-violation/> stream error to the client.
8. XML Stanzas
After a client and a server (or two servers) have completed stream
negotiation, either party can send XML stanzas. Three kinds of XML
stanza are defined for the 'jabber:client' and 'jabber:server'
namespaces: <message/>, <presence/>, and <iq/>. In addition, there
are five common attributes for these stanza types. These common
attributes, as well as the basic semantics of the three stanza types,
are defined in this specification; more detailed information
regarding the syntax of XML stanzas for instant messaging and
presence applications is provided in [XMPP-IM], and for other
applications in the relevant XMPP extension specifications.
Support for the XML stanza syntax and semantics defined in this
specification is REQUIRED in XMPP client and server implementations.
Security Note: A server MUST NOT process a partial stanza and MUST
NOT attach meaning to the transmission timing of any part of a
stanza (before receipt of the close tag).
8.1. Common Attributes
The following five attributes are common to message, presence, and IQ
stanzas.
8.1.1. to
The 'to' attribute specifies the JID of the intended recipient for
the stanza.
<message to='romeo@example.net'>
<body>Art thou not Romeo, and a Montague?</body>
</message>
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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 the
context of XML streams qualified by the 'jabber:client' namespace
(i.e., client-to-server streams).
1. A stanza with a specific intended recipient MUST possess a 'to'
attribute whose value is an XMPP address.
2. A stanza sent from a client to a server for direct processing by
the server as described in [XMPP-IM] for rosters (e.g., presence
sent to the server for broadcasting to other entities) MUST NOT
possess a 'to' attribute.
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 generate an <improper-addressing/> stream
error.
2. The domainpart of the JID in the 'to' atttribute MUST match a
hostname serviced by the receiving server; if a server receives a
stanza that does not meet this restriction, it MUST generate a
<host-unknown/> or <host-gone/> stream error.
8.1.2. from
The 'from' attribute specifies the JID of the sender.
<message from='juliet@im.example.com/balcony'
to='romeo@example.net'>
<body>Art thou not Romeo, and a Montague?</body>
</message>
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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 the server receives an XML stanza from a client and the
stanza does not include a 'from' attribute, the server MUST add a
'from' attribute to the stanza, where the value of the 'from'
attribute is the full JID (<localpart@domain/resource>)
determined by the server for the connected resource that
generated the stanza (see Section 4.2.6), or the bare JID
(<localpart@domain>) in the case of subscription-related presence
stanzas (see [XMPP-IM]).
2. When the server receives an XML stanza from a client and the
stanza includes a 'from' attribute, the server MUST either (a)
validate that the value of the 'from' attribute provided by the
client is that of a connected resource for the associated entity
or (b) override the provided 'from' attribute by adding a 'from'
attribute as specified under Rule #1.
3. When the server generates a stanza from the server for delivery
to the client on behalf of the account of the connected client
(e.g., in the context of data storage services provided by the
server on behalf of the client), the stanza MUST either (a) not
include a 'from' attribute or (b) include a 'from' attribute
whose value is the account's bare JID (<localpart@domain>).
4. When the server generates a stanza from the server itself for
delivery to the client, the stanza MUST include a 'from'
attribute whose value is the bare JID (i.e., <domain>) of the
server.
5. A server MUST NOT send to the client a stanza without a 'from'
attribute if the stanza was not generated by the server (e.g., if
it was generated by another client or another server); therefore,
when a client receives a stanza that does not include a 'from'
attribute, it MUST assume that the stanza is from the server to
which the client is connected.
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).
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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 generate an <improper-addressing/> stream
error.
2. The domainpart of the JID contained in the 'from' attribute MUST
match the hostname 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
generate an <invalid-from/> stream error.
Enforcement of these rules helps to prevent certain denial of service
attacks as described under Section 13.12.
8.1.3. id
The 'id' attribute is used by the entity that generates a stanza
("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 will be unique only within its current stream or unique
globally.
For <message/> and <presence/> stanzas, it is RECOMMENDED for the
originating entity to include an 'id' attribute; for <iq/> stanzas,
it is REQUIRED.
If the generated stanza includes an 'id' attribute then it is
REQUIRED for the response or error stanza to also include an 'id'
attribute, where the value of the 'id' attribute MUST match that of
the generated stanza.
The semantics of IQ stanzas impose additional restrictions; see
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 role of an IQ stanza in a structured request-
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response exchange and therefore are specified under Section 8.2.3.
The only 'type' value common to all three stanzas is "error"; see
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
[CHARSET], "internationalization is for humans"). The value of the
'xml:lang' attribute specifies the default language of any such
human-readable XML character data.
<presence from='romeo@example.net/orchard' xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
</presence>
The value of the 'xml:lang' attribute MAY be overridden by the 'xml:
lang' attribute of a specific child element.
<presence from='romeo@example.net/orchard' xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
<status xml:lang='cs'>Dvořím se Julii</status>
</presence
If an outbound stanza generated by a client does not possess an 'xml:
lang' attribute, the client's server SHOULD add an 'xml:lang'
attribute whose value is that specified for the stream as defined
under Section 4.4.4.
C: <presence from='romeo@example.net/orchard'>
<show>dnd</show>
<status>Wooing Juliet</status>
</presence>
S: <presence from='romeo@example.net/orchard'
to='juliet@im.example.com'
xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
</presence>
If an inbound stanza received by a client or server does not possess
an 'xml:lang' attribute, an implementation MUST assume that the
default language is that specified for the stream as defined under
Section 4.4.4.
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The value of the 'xml:lang' attribute MUST conform to the NMTOKEN
datatype (as defined in Section 2.3 of [XML]) and MUST conform to the
format defined in [LANGTAGS].
A server MUST NOT modify or delete 'xml:lang' attributes on stanzas
it receives from other entities.
8.2. Basic Semantics
8.2.1. Message Semantics
The <message/> stanza can be seen as a "push" mechanism whereby one
entity pushes information to another entity, similar to the
communications that occur in a system such as email. All message
stanzas SHOULD possess a 'to' attribute that specifies the intended
recipient of the message; upon receiving such a stanza, a server
SHOULD route or deliver it to the intended recipient (see Section 10
for general routing and delivery rules related to XML stanzas).
8.2.2. Presence Semantics
The <presence/> stanza can be seen as 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
entity (client) SHOULD send a presence stanza with no 'to' attribute,
in which case the server to which the entity is connected SHOULD
broadcast that stanza to all subscribed entities. However, a
publishing entity MAY also send a presence stanza with a 'to'
attribute, in which case the server SHOULD route or deliver that
stanza to the intended recipient. See Section 10 for general routing
and delivery rules related to XML stanzas, and [XMPP-IM] for rules
specific to presence applications.
8.2.3. IQ Semantics
Info/Query, or IQ, is a request-response mechanism, similar in some
ways to the Hypertext Transfer Protocol [HTTP]. The semantics of IQ
enable an entity to make a request of, and receive a response from,
another entity. The data content of the request and response is
defined by the schema or other structural definition associated with
the XML namespace that qualifies the direct child element of the IQ
element (see Section 8.4), and the interaction is tracked by the
requesting entity through use of the 'id' attribute. Thus, IQ
interactions follow a common pattern of structured data exchange such
as get/result or set/result (although an error can be returned in
reply to a request if appropriate):
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Requesting Responding
Entity Entity
---------- ----------
| |
| <iq id='1' type='get'> |
| [ ... payload ... ] |
| </iq> |
| -------------------------> |
| |
| <iq id='1' type='result'> |
| [ ... payload ... ] |
| </iq> |
| <------------------------- |
| |
| <iq id='2' type='set'> |
| [ ... payload ... ] |
| </iq> |
| -------------------------> |
| |
| <iq id='2' type='error'> |
| [ ... condition ... ] |
| </iq> |
| <------------------------- |
| |
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 the value is other than one of the
following strings, the recipient or an intermediate router MUST
return a stanza error of <bad-request/>):
* get -- The stanza requests information, inquires about what
data is needed in order to complete further operations, etc.
* set -- The stanza provides data that is needed for an
operation to be completed, sets new values, replaces existing
values, etc.
* result -- The stanza is a response to a successful get or set
request.
* error -- The stanza reports an error that has occurred
regarding processing or delivery of a previously-sent get or
set request (see Section 8.3).
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3. An entity that receives an IQ request of type "get" or "set" MUST
reply with an IQ response of type "result" or "error". The
response MUST preserve the 'id' attribute of the request.
4. An entity that receives a stanza of type "result" or "error" MUST
NOT respond to the stanza by sending a further IQ response of
type "result" or "error"; however, the requesting entity MAY send
another request (e.g., an IQ of type "set" to provide obligatory
information discovered through a get/result pair).
5. An IQ stanza of type "get" or "set" MUST contain exactly one
child element, which specifies the semantics of the particular
request.
6. An IQ stanza of type "result" MUST include zero or one child
elements.
7. An IQ stanza of type "error" MAY include the child element
contained in the associated "get" or "set" and MUST include an
<error/> child; for details, see Section 8.3.
8.3. Stanza Errors
Stanza-related errors are handled in a manner similar to stream
errors (Section 4.6). 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 mirrors the 'id' attribute (if any) of the generated stanza that
triggered the error
o contains an <error/> child element that specifies the error
condition and therefore provides a hint regarding actions that the
sender can take to remedy the error (if possible)
8.3.1. Rules
The following rules apply to stanza errors:
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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. If the generated stanza included an 'id' attribute then it is
REQUIRED for the error stanza to also include an 'id' attribute,
where the value of the 'id' attribute MUST match that of the
generated stanza.
3. An error stanza MUST contain an <error/> child element.
4. 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).
5. An <error/> child MUST NOT be included if the 'type' attribute
has a value other than "error" (or if there is no 'type'
attribute).
6. 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.
<stanza-kind from='intended-recipient' to='sender' type='error'>
[OPTIONAL to include sender XML here]
<error type='error-type'>
<defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
[<text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
xml:lang='langcode'>
OPTIONAL descriptive text
</text>]
[OPTIONAL application-specific condition element]
</error>
</stanza-kind>
The "stanza-kind" MUST be one of message, presence, or iq.
The "error-type" MUST be one of the following:
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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.
The <error/> element:
o MUST contain a defined condition element.
o MAY contain a <text/> child element containing XML character data
that describes the error in more detail; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace
and SHOULD possess an 'xml:lang' attribute specifying the natural
language of the XML character data.
o MAY contain a child element for an application-specific error
condition; this element MUST be qualified by an application-
specific namespace that defines the syntax and semantics of the
element.
The <text/> element is OPTIONAL. If included, it MUST be used only
to provide descriptive or diagnostic information that supplements the
meaning of a defined condition or application-specific condition. It
MUST NOT be interpreted programmatically by an application. It MUST
NOT be used as the error message presented to a human user, but MAY
be shown in addition to the error message associated with the defined
condition element (and, optionally, the application-specific
condition element).
8.3.3. Defined Conditions
The following conditions are defined for use in stanza errors.
8.3.3.1. bad-request
The sender has sent a stanza containing XML that does not conform to
the appropriate schema or that cannot be processed (e.g., an IQ
stanza that includes an unrecognized value of the 'type' attribute,
or an element that is qualified by a recognized namespace but that
violates the defined syntax for the element); the associated error
type SHOULD be "modify".
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C: <iq from='juliet@im.example.com/balcony'
id='zj3v142b'
to='im.example.com'
type='subscribe'>
<ping xmlns='urn:xmpp:ping'/>
</iq>
S: <iq from='im.example.com'
id='zj3v142b'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.3.3.2. conflict
Access cannot be granted because an existing resource exists with the
same name or address; the associated error type SHOULD be "cancel".
C: <iq id='wy2xa82b4' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>balcony</resource>
</bind>
</iq>
S: <iq id='wy2xa82b4' type='error'>
<error type='cancel'>
<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.3.3.3. feature-not-implemented
The feature represented in the XML stanza is not implemented by the
intended recipient or an intermediate server and therefore the stanza
cannot be processed (e.g., the entity understands the namespace but
does not recognize the element name); the associated error type
SHOULD be "cancel" or "modify".
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C: <iq from='juliet@im.example.com/balcony'
id='9u2bax16'
to='pubsub.example.com'
type='get'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<subscriptions/>
</pubsub>
</iq>
E: <iq from='pubsub.example.com
id='9u2bax16'
to='juliet@im.example.com/balcony'
type='error'>
<error type='cancel'>
<feature-not-implemented
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<unsupported
xmlns='http://jabber.org/protocol/pubsub#errors'
feature='retrieve-subscriptions'/>
</error>
</iq>
8.3.3.4. forbidden
The requesting entity does not possess the necessary permissions to
perform the action; the associated error type SHOULD be "auth".
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
<error type='auth'>
<forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
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8.3.3.5. gone
The recipient or server can no longer be contacted at this address,
typically on a permanent basis (as opposed to the <redirect/> error
condition, which is used for temporary addressing failures); the
associated error type SHOULD be "cancel" and the error stanza SHOULD
include a new address (if available) as the XML character data of the
<gone/> element (which MUST be a Uniform Resource Identifier [URI] or
Internationalized Resource Identifier [IRI] at which the entity can
be contacted, typically an XMPP IRI as specified in [XMPP-URI]).
C: <message
from='juliet@im.example.com/churchyard'
id='sj2b371v'
to='romeo@example.net'
type='chat'>
<body>Thy lips are warm.</body>
</message>
S: <message
from='romeo@example.net'
id='sj2b371v'
to='juliet@im.example.com/churchyard'
type='error'>
<error type='cancel'>
<gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
8.3.3.6. internal-server-error
The server could not process the stanza because of a misconfiguration
or an otherwise-undefined internal server error; the associated error
type SHOULD be "cancel".
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C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
<error type='wait'>
<internal-server-error
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
8.3.3.7. item-not-found
The addressed JID or item requested cannot be found; the associated
error type SHOULD be "cancel".
C: <presence from='userfoo@example.com/bar'
id='pwb2n78i'
to='nosuchroom@conference.example.org/foo'/>
S: <presence from='nosuchroom@conference.example.org/foo'
id='pwb2n78i'
to='userfoo@example.com/bar'
type='error'>
<error type='cancel'>
<item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
Security Note: An application MUST NOT return this error if doing
so would provide information about the intended recipient's
network availability to an entity that is not authorized to know
such information; instead it MUST return a <service-unavailable/>
stanza error.
8.3.3.8. jid-malformed
The sending entity has provided or communicated an XMPP address
(e.g., a value of the 'to' attribute) or aspect thereof (e.g., an
XMPP resourcepart) that does not adhere to the syntax defined in
[XMPP-ADDR]; the associated error type SHOULD be "modify".
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C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='ch@r@cters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='ch@r@cters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<jid-malformed
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
8.3.3.9. not-acceptable
The recipient or server understands the request but cannot process it
because the request does not meet criteria defined by the recipient
or server (e.g., a request to subscribe to information that does not
simultaneously include configuration parameters needed by the
recipient); the associated error type SHOULD be "modify".
C: <message to='juliet@im.example.com' id='yt2vs71m'>
<body>[ ... the-emacs-manual ... ]</body>
</message>
S: <message from='juliet@im.example.com' id='yt2vs71m'>
<error type='modify'>
<not-acceptable
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
8.3.3.10. not-allowed
The recipient or server does not allow any entity to perform the
action (e.g., sending to entities at a blacklisted domain); the
associated error type SHOULD be "cancel".
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C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
8.3.3.11. not-authorized
The sender needs to provide credentials before being allowed to
perform the action, or has provided improper credentials; the
associated error type SHOULD be "auth".
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'>
<error type='auth'>
<not-authorized xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
8.3.3.12. payment-required
The requesting entity is not authorized to access the requested
service because payment is necessary; the associated error type
SHOULD be "auth".
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C: <iq from='romeo@example.net/foo'
id='7isf2v4'
to='pubsub.example.com'
type='get'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<items node='my_musings'/>
</pubsub>
</iq>
E: <iq from='pubsub.example.com'
id='7isf2v4'
to='romeo@example.net/foo'
type='error'>
<error type='auth'>
<payment-required
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.3.3.13. policy-violation
The entity has violated some local service policy (e.g., a message
contains words that are prohibited by the servive); the server MAY
choose to specify the policy in the <text/> element or in an
application-specific condition element; the associated error type
SHOULD be "modify" or "wait" depending on the policy being violated.
(In the following example, the client sends an XMPP message that is
too large according to the server's local service policy.)
C: <message to='bill@im.example.com' id='vq71f4nb'>
<body>%#&@^!!!</body>
</message>
S: <message from='bill@im.example.com' id='vq71f4nb'>
<error type='cancel'>
<policy-violation
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
8.3.3.14. recipient-unavailable
The intended recipient is temporarily unavailable, undergoing
maintenance, etc.; the associated error type SHOULD be "wait".
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C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'>
<error type='wait'>
<recipient-unavailable
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
Security Note: An application MUST NOT return this error if doing
so would provide information about the intended recipient's
network availability to an entity that is not authorized to know
such information; instead it MUST return a <service-unavailable/>
stanza error.
8.3.3.15. redirect
The recipient or server is redirecting requests for this information
to another entity, typically in a temporary fashion (as opposed to
the <gone/> error condition, which is used for permanent addressing
failures); the associated error type SHOULD be "modify" and the error
stanza SHOULD contain the alternate address in the XML character data
of the <redirect/> element (which MUST be a URI or IRI at which the
entity can be contacted, typically an XMPP IRI as specified in
[XMPP-URI]).
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C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<redirect xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
xmpp:characters@conference.example.org
</redirect>
</error>
</presence>
8.3.3.16. registration-required
The requesting entity is not authorized to access the requested
service because prior registration is necessary; the associated error
type SHOULD be "auth".
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'>
<error type='auth'>
<registration-required
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
8.3.3.17. remote-server-not-found
A remote server or service specified as part or all of the JID of the
intended recipient does not exist or cannot be resolved; the
associated error type SHOULD be "cancel".
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C: <message
from='romeo@example.net/home'
id='ud7n1f4h'
to='bar@example.org'
type='chat'>
<body>yt?</body>
</message>
E: <message
from='bar@example.org'
id='ud7n1f4h'
to='romeo@example.net/home'
type='error'>
<error type='cancel'>
<remote-server-not-found
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
8.3.3.18. 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; the associated error type SHOULD be "wait".
C: <message
from='romeo@example.net/home'
id='ud7n1f4h'
to='bar@example.org'
type='chat'>
<body>yt?</body>
</message>
E: <message
from='bar@example.org'
id='ud7n1f4h'
to='romeo@example.net/home'
type='error'>
<error type='wait'>
<remote-server-timeout
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
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8.3.3.19. resource-constraint
The server or recipient is busy or lacks the system resources
necessary to service the request; the associated error type SHOULD be
"wait".
C: <iq from='romeo@example.net/foo'
id='kj4vz31m'
to='pubsub.example.com'
type='get'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<items node='my_musings'/>
</pubsub>
</iq>
E: <iq from='pubsub.example.com'
id='kj4vz31m'
to='romeo@example.net/foo'
type='error'>
<error type='wait'>
<resource-constraint
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.3.3.20. service-unavailable
The server or recipient does not currently provide the requested
service; the associated error type SHOULD be "cancel".
C: <message from='romeo@example.net/foo'
to='juliet@im.example.com'>
<body>Hello?</body>
</message>
S: <message from='juliet@im.example.com/foo'
to='romeo@example.net'>
<error type='cancel'>
<service-unavailable
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
Security Note: An application MUST return a <service-unavailable/>
stanza error instead of <item-not-found/> or <recipient-
unavailable/> if sending one of the latter errors would provide
information about the intended recipient's network availability to
an entity that is not authorized to know such information.
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8.3.3.21. subscription-required
The requesting entity is not authorized to access the requested
service because a prior subscription is necessary; the associated
error type SHOULD be "auth".
C: <message
from='romeo@example.net/orchard'
id='pa73b4n7'
to='playwright@shakespeare.example.com'
type='chat'>
<subject>ACT II, SCENE II</subject>
<body>help, I forgot my lines!</body>
</message>
E: <message
from='playwright@shakespeare.example.com'
id='pa73b4n7'
to='romeo@example.net/orchard'
type='error'>
<error type='auth'>
<subscription-required
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
8.3.3.22. 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 be used only in conjunction with an
application-specific condition.
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C: <message
from='northumberland@shakespeare.example'
id='richard2-4.1.247'
to='kingrichard@royalty.england.example'>
<body>My lord, dispatch; read o'er these articles.</body>
<amp xmlns='http://jabber.org/protocol/amp'>
<rule action='notify'
condition='deliver'
value='stored'/>
</amp>
S: <message from='example.org'
id='amp1'
to='northumberland@example.net/field'
type='error'>
<amp xmlns='http://jabber.org/protocol/amp'
from='kingrichard@example.org'
status='error'
to='northumberland@example.net/field'>
<rule action='error'
condition='deliver'
value='stored'/>
</amp>
<error type='modify'>
<undefined-condition
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<failed-rules xmlns='http://jabber.org/protocol/amp#errors'>
<rule action='error'
condition='deliver'
value='stored'/>
</failed-rules>
</error>
</message>
8.3.3.23. unexpected-request
The recipient or server understood the request but was not expecting
it at this time (e.g., the request was out of order); the associated
error type SHOULD be "wait" or "modify".
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C: <iq from='romeo@example.net/foo'
id='o6hsv25z'
to='pubsub.example.com'
type='set'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<unsubscribe
node='my_musings'
jid='romeo@example.net'/>
</pubsub>
</iq>
E: <iq from='pubsub.example.com'
id='o6hsv25z'
to='romeo@example.net/foo'
type='error'>
<error type='cancel'>
<unexpected-request
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<not-subscribed
xmlns='http://jabber.org/protocol/pubsub#errors'/>
</error>
</iq>
8.3.4. Application-Specific Conditions
As noted, an application MAY provide application-specific stanza
error information by including a properly-namespaced child within the
error element. Typically, the application-specific element
supplements or further qualifies a defined element. Thus, the
<error/> element will contain two or three child elements.
<iq id='ixc3v1b9' type='error'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<too-many-parameters xmlns='http://example.com/ns'/>
</error>
</iq>
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<message type='error' id='7h3baci9'>
<error type='modify'>
<undefined-condition
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<text xml:lang='en'
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
[ ... application-specific information ... ]
</text>
<too-many-parameters xmlns='http://example.com/ns'/>
</error>
</message>
An entity that receives an application-specific error condition it
does not understand MUST silently ignore the condition.
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., the <iq/>, <message/>, and <presence/> elements
qualified by the "jabber:client" or "jabber:server" namespace
declared as the default namespace for the stream) and any child
element of such a stanza (whether a child element qualifed by the
default namespace or an extension element) MAY also include one or
more attributes that are qualified by XML namespaces that are
different from the default namespace or the reserved "xml" prefix
(including the "empty namespace" if the attribute is not prefixed).
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 default namespace
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(e.g., the message <body/> element).
An extension element or extension attribute is said to be EXTENDED
CONTENT and the namespace name for such an element or attribute is
said to be an EXTENDED NAMESPACE.
To illustrate these concepts, several examples follow.
The following stanza contains one direct child element whose extended
namespace is 'jabber:iq:roster':
<iq from='juliet@capulet.com/balcony'
id='h83vxa4c'
type='get'>
<query xmlns='jabber:iq:roster'/>
</iq>
The following stanza contains two direct child elements with two
different extended namespaces.
<presence from='juliet@capulet.com/balcony'>
<c xmlns='http://jabber.org/protocol/caps'
hash='sha-1'
node='http://code.google.com/p/exodus'
ver='QgayPKawpkPSDYmwT/WM94uAlu0='/>
<x xmlns='vcard-temp:x:update'>
<photo>sha1-hash-of-image</photo>
</x>
</presence>
The following stanza contains two child elements, one of which is
qualified by the default namespace and one of which is qualified by
an extended namespace; the extension element in turn contains a child
element that is qualified by a different extended namespace.
<message to='juliet@capulet.com'>
<body>Hello?</body>
<html xmlns='http://jabber.org/protocol/xhtml-im'>
<body xmlns='http://www.w3.org/1999/xhtml'>
<p style='font-weight:bold'>Hello?</t>
</body>
</html>
</message>
It is conventional in the XMPP community for implementations to not
generate namespace prefixes for elements that are qualified by
extended namespaces (outside the XMPP community, this convention is
sometimes called "prefix-free canonicalization"). However, if an
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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 4.5.3).
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.
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.
Recipient: If a recipient receives a stanza that contains an element
or attribute it does not understand, it MUST silently ignore that
XML data. In particular:
* If an entity receives a message or presence stanza that
contains XML data qualified by a namespace it does not
understand, the portion of the stanza qualified by the unknown
namespace MUST be silently ignored.
* If an entity receives a message stanza whose only child element
is qualified by a namespace it does not understand, it MUST
silently ignore the entire stanza.
* If an entity receives an IQ stanza of type "get" or "set"
containing a child element qualified by a namespace it does not
understand, the entity MUST return an IQ stanza of type "error"
with an error condition of <service-unavailable/>.
Server: If a server handles a stanza that contains a child element
it does not understand, it MUST silently ignore the associated XML
data by routing it unmodified to another server or delivering it
unmodified to the recipient.
8.5. Stanza Size
XMPP is optimized for the exchange of relatively large numbers of
relatively small stanzas. A client or server MAY enforce a maximum
stanza size. The maximum stanza size MUST NOT be smaller than 10000
bytes, from the opening "<" character to the closing ">" character.
If an entity receives a stanza that exceeds its maximum stanza size,
it MUST return a <not-acceptable/> stanza error.
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9. Examples
The examples in this section further illustrate the protocols defined
in this specification. The examples are not exhaustive and not
normative. The alternate steps shown illustrate the handling of
failure cases but would not necessarily be triggered by the data sent
in the examples.
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
PLAIN mechanism as "juliet@im.example.com", and the client binds a
client-submitted resource to the stream. It is assumed that before
sending the initial stream header, the client has already resolved an
SRV record of _xmpp-client._tcp.im.example.com and has opened a TCP
connection to the advertised port at the resolved IP address.
9.1.1. TLS
Step 1: Client initiates stream to server:
C: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
Step 2: Server responds by sending a response stream header to
client:
S: <stream:stream
from='im.example.com'
id='t7AMCin9zjMNwQKDnplntZPIDEI='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
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Step 3: Server sends stream features to client (only the STARTTLS
extension at this point):
S: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
Step 4: Client sends STARTTLS command to server:
C: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5: Server informs client that it is allowed to proceed:
S: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5 (alt): Server informs client that STARTTLS negotiation has
failed and closes both XML stream and TCP connection:
S: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
S: </stream:stream>
Step 6: Client and server attempt to complete TLS negotiation over
the existing TCP connection (see [TLS] for details).
Step 7: If TLS negotiation is successful, client initiates a new
stream to server:
C: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
Step 7 (alt): If TLS negotiation is unsuccessful, server closes TCP
connection.
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9.1.2. SASL
Step 8: Server responds by sending a stream header to client along
with any available stream features:
S: <stream:stream
from='im.example.com'
id='vgKi/bkYME8OAj4rlXMkpucAqe4='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
S: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>
Step 9: Client selects an authentication mechanism, in this case
[PLAIN]:
C: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
The decoded base64 data is "U+0000julietU+0000r0m30myr0m30", i.e., a
username of "juliet" and a password of "r0m30myr0m30" with the actual
ASCII 0 (or NUL) character as separator.
Step 10: Server informs client of success:
S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 10 (alt): Server returns error to client:
S: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>
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Step 11: Client initiates a new stream to server:
C: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
9.1.3. Resource Binding
Step 12: Server responds by sending a stream header to client along
with supported features (in this case resource binding):
S: <stream:stream
from='im.example.com'
id='gPybzaOzBmaADgxKXu9UClbprp0='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>
Upon being informed that resource binding is mandatory, the client
needs to bind a resource to the stream; here we assume that the
client submits a human-readable text string.
Step 13: Client binds a resource:
C: <iq id='yhc13a95' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
balcony
</bind>
</iq>
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Step 14: Server accepts submitted resourcepart and informs client of
successful resource binding:
S: <iq id='yhc13a95' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/balcony
</jid>
</bind>
</iq>
9.1.4. Stanza Exchange
Now the client is allowed to send XML stanzas over the negotiated
stream.
C: <message from='juliet@im.example.com/balcony'
id='ju2ba41c'
to='romeo@example.net'
type='chat'
xml:lang='en'>
<body>Art thou not Romeo, and a Montague?</body>
</message>
If necessary, sender's server negotiates XML streams with intended
recipient's server (see Section 9.2).
The intended recipient replies and the message is delivered to the
client.
E: <message from='romeo@example.net/orchard'
id='ju2ba41c'
to='juliet@im.example.com/balcony'
type='chat'
xml:lang='en'>
<body>Neither, fair saint, if either thee dislike.</body>
</message>
The client can subsequently send and receive an unbounded number of
subsequent XML stanzas over the stream.
9.1.5. Close
Desiring to send no further messages, the client closes the stream.
C: </stream:stream>
Consistent with the recommended stream closing handshake, the server
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closes the stream as well:
S: </stream:stream>
Client now 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 another 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.
9.2.1. TLS
Step 1: Server1 initiates stream to Server2:
S1: <stream:stream
from='im.example.com'
to='example.net'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
Step 2: Server2 responds by sending a response stream header to
Server1:
S2: <stream:stream
from='example.net'
id='hTiXkW+ih9k2SqdGkk/AZi0OJ/Q='
to='im.example.com'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
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Step 3: Server2 sends stream features to Server1 (only the STARTTLS
extension at this point):
S2: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
Step 4: Server1 sends the STARTTLS command to Server2:
S1: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5: Server2 informs Server1 that it is allowed to proceed:
S2: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5 (alt): Server2 informs Server1 that STARTTLS negotiation has
failed and closes stream:
S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
S2: </stream:stream>
Step 6: Server1 and Server2 attempt to complete TLS negotiation via
TCP (see [TLS] for details).
Step 7: If TLS negotiation is successful, Server1 initiates a new
stream to Server2:
S1: <stream:stream
from='im.example.com'
to='example.net'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
Step 7 (alt): If TLS negotiation is unsuccessful, Server2 closes TCP
connection.
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9.2.2. SASL
Step 8: Server2 sends a response stream header to Server1 along with
available stream features (including a preference for the SASL
EXTERNAL mechanism):
S2: <stream:stream
from='example.net'
id='RChdjlgj/TIBcbT9Keu31zDihH4='
to='im.example.com'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
S2: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
</mechanisms>
</stream:features>
Step 9: Server1 selects the EXTERNAL mechanism (including an empty
response of "="):
S1: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='EXTERNAL'/>=</auth>
Step 10: Server2 returns success:
S2: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 10 (alt): Server2 informs Server1 of failed authentication:
S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>
S2: </stream:stream>
Step 11: Server1 initiates a new stream to Server2:
S1: <stream:stream
from='im.example.com'
to='example.net'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
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Step 12: Server2 responds by sending a stream header to Server1 along
with any additional features (or, in this case, an empty features
element):
S2: <stream:stream
from='example.net'
id='MbbV2FeojySpUIP6J91qaa+TWHM='
to='im.example.com'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
S2: <stream:features/>
9.2.3. Stanza Exchange
Now Server1 is allowed to send XML stanzas to Server2 over the
negotiated stream; here we assume that the transferred stanzas are
those shown earlier for client-to-server communication, albeit over a
server-to-server stream qualified by the 'jabber:server' namespace.
Server1 sends XML stanza to Server2:
S1: <message from='juliet@im.example.com/balcony'
id='ju2ba41c'
to='romeo@example.net'
type='chat'
xml:lang='en'>
<body>Art thou not Romeo, and a Montague?</body>
</message>
The intended recipient replies and the message is delivered from
Server2 to Server1.
Server2 sends XML stanza to Server1:
S2: <message from='romeo@example.net/orchard'
id='ju2ba41c'
to='juliet@im.example.com/balcony'
type='chat'
xml:lang='en'>
<body>Neither, fair saint, if either thee dislike.</body>
</message>
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9.2.4. Close
Desiring to send no further messages, Server1 closes the stream. (In
practice, the stream would most likely remain open for some time,
since Server1 and Server2 do not immediately know if the stream will
be needed for further communication.)
S1: </stream:stream>
Consistent with the recommended stream closing handshake, Server2
closes the stream as well:
S2: </stream:stream>
Server1 now terminates the underlying TCP connection.
10. Server Rules for Processing XML Stanzas
An XMPP server MUST ensure in-order processing of XML stanzas between
any two entities. This includes stanzas sent by a client to its
server for direct processing by the server (e.g., in-order processing
of a roster get and initial presence as described in [XMPP-IM]).
Beyond the requirement for in-order processing, each server
implementation will contain its own logic for processing stanzas it
receives. Such logic determines whether the server needs to ROUTE a
given stanza to another domain, DELIVER it to a local entity
(typically a connected client associated with a local account), or
HANDLE it directly within the server itself. The following rules
apply.
Implementation Note: Particular XMPP applications MAY specify
delivery rules that modify or supplement the following rules; for
example, a set of delivery rules for instant messaging and
presence applications is defined in [XMPP-IM].
10.1. No 'to' Address
10.1.1. Overview
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 (such as a client) that is connected to
the server.
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10.1.2. Message
If the server receives a message stanza with no 'to' attribute, it
MUST treat the message as if the 'to' address were the bare JID
<localpart@domain> of the sending entity.
10.1.3. 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).
10.1.4. IQ
If the server receives an IQ stanza with no 'to' attribute, it MUST
process the stanza on behalf of the account from which received the
stanza, as follows:
1. If the IQ stanza is of type "get" or "set" and the server
understands the namespace that qualifies the payload, the server
MUST handle the stanza on behalf of the sending entity or return
an appropriate error to the sending entity. Although the meaning
of "handle" is determined by the semantics of the qualifying
namespace, in general the server will respond to the IQ stanza of
type "get" or "set" by returning an appropriate IQ stanza of type
"result" or "error", responding as if the server were the bare
JID of the sending entity. As an example, if the sending entity
sends an IQ stanza of type "get" where the payload is qualified
by the 'jabber:iq:roster' namespace (as described in [XMPP-IM]),
then the server will return the roster associated with the
sending entity's bare JID to the particular resource of the
sending entity that requested the roster.
2. If the IQ stanza is of type "get" or "set" and the server does
not understand the namespace that qualifies the payload, the
server MUST return an error to the sending entity, which MUST be
<service-unavailable/>.
3. If the IQ stanza is of type "error" or "result", the server MUST
handle the error or result in accordance with the payload of the
associated IQ stanza or type "get" of "set".
10.2. Local Domain
If the hostname of the domainpart of the JID contained in the 'to'
attribute matches one of the configured hostnames of the server
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itself, the server MUST first determine if the hostname is serviced
by the server or by a specialized local service. If the latter, the
server MUST route the stanza to that service. If the former, the
server MUST proceed as follows.
10.2.1. Mere Domain
If the JID contained in the 'to' attribute is of the form <domain>,
then the server MUST either handle the stanza as appropriate for the
stanza kind or return an error stanza to the sender.
10.2.2. Domain with Resource
If the JID contained in the 'to' attribute is of the form <domain/
resource>, then the server MUST either handle the stanza as
appropriate for the stanza kind or return an error stanza to the
sender.
10.2.3. Localpart at Domain
For addresses of this type, more detailed rules in the context of
instant messaging and presence applications are provided in
[XMPP-IM].
10.2.3.1. No Such User
If there is no local account associated with the <localpart@domain>,
how the stanza is processed depends on the stanza type.
o For a message stanza, the server MUST return a <service-
unavailable/> stanza error to the sender.
o For a presence stanza, the server SHOULD silently discard the
stanza.
o For an IQ stanza, the server MUST return a <service-unavailable/>
stanza error to the sender.
10.2.3.2. Bare JID
If the JID contained in the 'to' attribute is of the form
<localpart@domain>, 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 the server SHOULD deliver it to at least
one of the connected resources. If there exists no connected
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resource, the server MUST either return a <service-unavailable/>
stanza error or store the message offline for delivery when the
account next has a connected resource.
o For a presence stanza, if there exists at least one connected
resource for the account the server SHOULD deliver it to at least
one of the connected resources. If there exists no connected
resource, the server SHOULD silently discard the stanza.
o For an IQ stanza, the server MUST handle it directly on behalf of
the intended recipient.
10.2.3.3. Full JID
If the JID contained in the 'to' attribute is of the form
<localpart@domain/resource> and there is no connected resource that
exactly matches the full JID, the stanza is processed as if the JID
were of the form <localpart@domain>.
If the JID contained in the 'to' attribute is of the form
<localpart@domain/resource> and there is a connected resource that
exactly matches the full JID, the server SHOULD deliver the stanza to
that connected resource.
10.3. Remote Domain
If the domainpart of the JID contained in the 'to' attribute does not
match one of the configured hostnames of the server itself, the
server SHOULD attempt to route the stanza to the remote domain
(subject to local service provisioning and security policies
regarding inter-domain communication, since such communication is
optional for any given deployment). There are two possible cases.
10.3.1. Existing Stream
If a server-to-server stream already exists between the two domains,
the sender's server will attempt to route the stanza to the
authoritative server for the remote domain over the existing stream.
10.3.2. 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 hostname of the remote domain, as described under
Section 13.9.2).
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2. Negotiate a server-to-server stream between the two domains (as
defined under Section 5 and Section 6).
3. Route the stanza to the authoritative server for the remote
domain over the newly-established stream.
10.3.3. Error Handling
If routing of a stanza to the intended recipient's server is
unsuccessful, the sender's server MUST return an error to the sender.
If resolution of the remote domain is unsuccessful, the stanza error
MUST be <remote-server-not-found/>. If resolution succeeds but
streams cannot be negotiated, the stanza error MUST be <remote-
server-timeout/>.
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.
11. XML Usage
11.1. 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]. In particular, the following features of
XML are prohibited in XMPP:
o comments (as defined in Section 2.5 of [XML])
o processing instructions (Section 2.6 therein)
o internal or external DTD subsets (Section 2.8 therein)
o internal or external entity references (Section 4.2 therein) with
the exception of the predefined entities (Section 4.6 therein)
An XMPP implementation MUST behave as follows with regard to these
features:
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1. An XMPP implementation MUST NOT inject characters matching such
features into an XML stream.
2. If an XMPP implementation receives characters matching such
features over an XML stream, it MUST return a stream error, which
SHOULD be <restricted-xml/> but MAY be <bad-format/>.
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 4.5 for XML
streams and Section 8.4 for XML stanzas.
11.3. Well-Formedness
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.
An XMPP entity MUST NOT generate data that is not XML-well-formed.
An XMPP entity MUST NOT accept data that is not XML-well-formed;
instead it MUST return an <xml-not-well-formed/> stream error and
close the stream over which the data was received.
An XMPP entity MUST NOT generate data that is not namespace-well-
formed. An XMPP server MUST NOT route or deliver data that is not
namespace-well-formed, and MUST return a stanza error of <not-
acceptable/> or a stream error of <xml-not-well-formed/> in response
to the receipt of such data.
Interoperability Note: Because these restrictions were
underspecified in the predecessor to this specification, it is
possible that implementations based on that predecessor will send
data that does not comply with the restrictions.
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11.4. Validation
A server is not responsible for ensuring that XML data delivered to a
client or routed to another server is valid, in accordance with the
definition of "valid" provided in Section 2.8 of [XML]. An
implementation MAY choose to accept or provide only validated data,
but such behavior is OPTIONAL. A client SHOULD NOT rely on the
ability to send data that does not conform to the schemas, and SHOULD
silently 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 production [23] content of [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.
11.6. Character Encoding
Implementations MUST support the UTF-8 transformation of Universal
Character Set [UCS2] characters, as needed for conformance with
[CHARSET] 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 return a stream error, which
SHOULD be <unsupported-encoding/> but MAY be <bad-format/>.
Implementation Note: Because it is mandatory for an XMPP
implementation to support all and only the UTF-8 encoding and
because UTF-8 always has the same byte order, an implementation
MUST NOT send a byte order mark ("BOM") at the beginning of the
data stream. If an entity receives the Unicode character U+FEFF
anywhere in an XML stream (including as the first character of the
stream), it MUST interpret that character as a zero width no-break
space, not as a byte order mark.
11.7. Whitespace
Except where explicitly disallowed (e.g., during TLS negotiation
(Section 5) and SASL negotiation (Section 6)), either entity MAY send
whitespace as separators between XML stanzas or between any other
first-level elements sent over the stream. One common use for
sending such whitespace is explained under Section 4.3.3.
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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 4.4, 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. Overview
XMPP technologies are typically deployed using a decentralized
client-server architecture. As a result, several paths are possible
when two XMPP entities need to communicate:
1. Both entities are servers. In this case, the entities can
establish a direct server-to-server stream between themselves.
2. One entity is a server and the other entity is a client whose
account is hosted on that server. In this case, the entities can
establish a direct client-to-server stream between themselves.
3. Both entities are clients whose accounts are hosted on the same
server. In this case, the entities cannot establish a direct
stream between themselves, but there is only one intermediate
entity between them, whose policies they might understand and in
which they might have some level of trust (e.g., the server might
require the use of Transport Layer Security for all client
connections).
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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 re-use of technologies such as base64, DNS,
cryptographic hash functions, SASL, TLS, UTF-8, and XML.
13.2. Threat Model
The threat model for XMPP is in essence the standard "Internet Threat
Model" described in [SEC-GUIDE]. Attackers are assumed to be
interested in and capable of launching the following attacks against
unprotected XMPP systems:
o Eavesdropping
o Sniffing passwords
o Breaking passwords through dictionary attacks
o Discovering usernames through directory harvesting attacks
o Replaying, inserting, deleting, or modifying stanzas
o Spoofing users
o Gaining unauthorized entry to a server or account
o Using a server or account inappropriately
o Denying service to other entities
o Subverting communication streams through man-in-the-middle attacks
o Gaining control over on-path servers
Where appropriate, the following sections describe methods for
protecting against these threats.
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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 characteristics, as described
throughout these security considerations.
13.4. Confidentiality and Integrity
The use of Transport Layer Security (TLS) for encrypting XML streams
provides a reliable mechanism for the ensuring the confidentiality
and integrity of data exchanged between a client and a server or
between two servers. Therefore TLS helps 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.
13.5. Peer Entity Authentication
The use of the Simple Authentication and Security Layer (SASL) for
authentication provides a reliable mechanism for peer entity
authentication. Therefore SASL helps to protect against user
spoofing, unauthorized usage, and man-in-the middle attacks. XMPP
clients and servers MUST support SASL as defined under Section 6.
13.6. Strong Security
[STRONGSEC] defines "strong security" and its importance to
communication over the Internet. For the purpose of XMPP
communication over client-to-server and server-to-server streams, the
term "strong security" refers to the use of security technologies
that provide both mutual authentication and integrity checking (e.g.,
a combination of TLS encryption and SASL authentication using
appropriate SASL mechanisms). In particular, when using certificate-
based authentication to provide strong security, a trust chain SHOULD
be established out-of-band, although a shared certification authority
signing certificates could allow a previously unknown certificate to
establish trust in-band. See the next section regarding certificate
validation procedures.
Implementations MUST support strong security. Service provisioning
SHOULD use strong security, subject to local security policies.
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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 digital certificate
presented by the receiving entity (or, in the case of mutual
authentication, both the receiving entity and the initiating entity).
This section describes best practices regarding the generation of
digital certificates to be presented by XMPP entities and the
verification of digital certificates presented by XMPP entities.
Considerations specific to certificate geneneration and validation
with regard to client certificates or server certificates are
described in the following sections.
13.7.1. Certificate Generation
13.7.1.1. General Considerations
The following rules apply to public key certificates that are issued
to XMPP entities:
1. The certificate MUST conform to [X509].
2. The certificate MUST NOT contain a basicConstraints extension
with the cA boolean set to TRUE.
3. The subject field MUST NOT be null.
4. The hash algorithm for the signature SHOULD be SHA-256 as defined
by [X509-ALGO].
5. The certificate SHOULD include an Authority Information Access
(AIA) extension that specifies the address of an Online
Certificate Status Protocol [OCSP] responder.
The following rules apply to issuers of XMPP certificates:
1. The certificate MUST conform to [X509].
2. The certificate MUST contain a keyUsage extension with the
digitalSignature bit set.
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3. The subject field MUST NOT be null.
4. The hash algorithm for the signature SHOULD be SHA-256 as defined
by [X509-ALGO].
5. For issuers of public key certificates, the issuer's certificate
MUST contain a basicConstraints extension with the cA boolean set
to TRUE.
6. For issuers of access certificates, the issuer's certificate MUST
NOT contain a basicConstraints extension with the cA boolean set
to TRUE.
13.7.1.2. Server Certificates
In a digital certificate to be presented by an XMPP server (i.e., a
SERVER CERTIFICATE), it is RECOMMENDED for the certificate to include
one or more JIDs (i.e., domainparts) associated with domains serviced
at the server. The representations described in the following
sections are RECOMMENDED. These representations are provided in
preference order.
13.7.1.2.1. SRVName
A server's domainpart SHOULD be represented as an SRVName, i.e., as
an otherName field of type "id-on-dnsSRV" as specified in [X509-SRV].
13.7.1.2.2. dNSName
A server's domainpart SHOULD be represented as a dNSName, i.e., as a
subjectAltName extension of type dNSName.
The dNSName MAY contain one instance of the wildcard character '*'.
The wildcard character applies only to the left-most domain name
component and matches any single component (thus a dNSName of
*.example.com matches foo.example.com but not bar.foo.example.com or
example.com itself). The wildcard character is not allowed in
component fragments (thus a dNSName of im*.example.net is not allowed
and SHALL NOT be taken to match im1.example.net and im2.example.net).
13.7.1.2.3. XmppAddr
A server's domainpart MAY be represented as an XmppAddr, i.e., as a
UTF8String within an otherName entity inside the subjectAltName,
using the [ASN.1] Object Identifier "id-on-xmppAddr" specified under
Section 13.7.1.4. In server certificates, this representation is
included only for the sake of backward-compatibility.
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13.7.1.2.4. Common Name
A server's domainpart SHOULD NOT be represented as a Common Name;
instead, the Common Name field SHOULD be reserved for representation
of a human-friendly name.
13.7.1.2.5. Examples
For our first (relatively simple) example, consider a company called
"Example Products, Inc." It hosts an XMPP service at
"im.example.com" (i.e., user addresses at the service are of the form
"user@im.example.com"), and SRV lookups for the xmpp-client and xmpp-
server services at "im.example.com" yield one machine, called
"x.example.com", as follows:
_xmpp-client._tcp.im.example.com. 400 IN SRV 20 0 5222 x.example.com
_xmpp-server._tcp.im.example.com. 400 IN SRV 20 0 5269 x.example.com
The certificate presented by x.example.com contains the following
representations:
o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of: "_xmpp-client.im.example.com"
o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of: "_xmpp-server.im.example.com"
o A dNSName containing an ASCII string of "im.example.com"
o An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
string of: "im.example.com"
o A CN containing an ASCII string of "Example Products, Inc."
For our second (more complex) example, consider an ISP called
"Example Internet Services". It hosts an XMPP service at
"example.net" (i.e., user addresses at the service are of the form
"user@example.net"), but SRV lookups for the xmpp-client and xmpp-
server services at "example.net" yield two machines ("x1.example.net"
and "x2.example.net"), as follows:
_xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x1.example.net.
_xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x2.example.net.
_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x1.example.net.
_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x2.example.net.
Example Internet Services also hosts chatrooms at chat.example.net,
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and provides an xmpp-server SRV record for that service as well (thus
enabling entity from remote domains to access that service). It also
might provide other such services in the future, so it wishes to
represent a wildcard in its certificate to handle such growth.
The certificate presented by either x1.example.net or x2.example.net
contains the following representations:
o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of: "_xmpp-client.example.net"
o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of: "_xmpp-server.example.net"
o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of: "_xmpp-server.chat.example.net"
o A dNSName containing an ASCII string of "example.net"
o A dNSName containing an ASCII string of "*.example.net"
o An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
string of: "example.net"
o An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
string of: "chat.example.net"
o A CN containing an ASCII string of "Example Internet Services"
13.7.1.3. Client Certificates
In a digital 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, i.e.,
as a UTF8String within an otherName entity inside the subjectAltName,
using the [ASN.1] Object Identifier "id-on-xmppAddr" specified under
Section 13.7.1.4.
13.7.1.4. ASN.1 Object Identifier
The [ASN.1] Object Identifier "id-on-xmppAddr" (also called an
XmppAddr) is defined as follows.
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id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
id-on OBJECT IDENTIFIER ::= { id-pkix 8 } -- other name forms
id-on-xmppAddr OBJECT IDENTIFIER ::= { id-on 5 }
XmppAddr ::= UTF8String
As an alternative to the "id-on-xmppAddr" notation, this Object
Identifier MAY be represented in dotted display format (i.e.,
"1.3.6.1.5.5.7.8.5") or in the Uniform Resource Name notation
specified in [URN-OID] (i.e., "urn:oid:1.3.6.1.5.5.7.8.5").
Thus for example the JID "juliet@im.example.com" as included in a
certificate could be formatted in any of the following three ways:
id-on-xmppAddr:
subjectAltName=otherName:id-on-xmppAddr;UTF8:juliet@im.example.com
dotted display format: subjectAltName=otherName:
1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com
URN notation: subjectAltName=otherName:urn:oid:
1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com
Use of the "id-on-xmppAddr" format is RECOMMENDED in the generation
of certificates, but all three formats MUST be supported for the
purpose of certificate validation.
The "id-on-xmppAddr" object identifier MAY be used on conjuction with
the extended key usage extension specified in Section 4.2.1.12 of
[X509] in order to explicitly define and limit the intended use of a
certificate to the XMPP network.
13.7.2. Certificate Validation
When an XMPP entity is presented with a server certificate or client
certificate by a peer for the purpose of encryption or authentication
of XML streams as described under Section 5 and Section 6, the entity
MUST 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 verify the integrity of the certificate, MUST verify that
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the certificate has been properly signed by the issuing Certificate
Authority, and MUST support certificate revocation messages. An
implementation MUST enable a human user to view information about the
full chain of certificates.
The following sections describe certificate validation rules for
server-to-server and client-to-server streams.
13.7.2.1. Server Certificates
When an entity (client or server) validates a certificate presented
by an XMPP server, there are three possible cases, as discussed in
the following sections.
13.7.2.1.1. Case #1
If the server certificate appears to be certified by a chain of
certificates terminating in a trust anchor (as described in Section
6.1 of [X509]), the entity MUST check the certificate for any
instances of the SRVName, dNSName, and XmppAddr (in that order of
preference) as described under Section 13.7.1.2.1,
Section 13.7.1.2.2, and Section 13.7.1.2.3. There are three possible
sub-cases:
Sub-Case #1: The server's certificate includes at least one
presented identity that matches the reference identity to which
the entity attempted to connect, in accordance with the matching
rules specified in [TLS-CERTS]. In this case, the entity MUST use
the matched domainpart as the validated identity of the XMPP
server.
Sub-Case #2: The server's certificate includes no presented identity
that matches the reference identity to which the entity attempted
to connect and a human user has not permanently accepted the
certificate during a previous connection attempt. In this case,
the entity MUST NOT use the presented domainpart (if any) as the
validated identity of the XMPP server. Instead, if the connecting
entity is a user-oriented client then it MUST either (1)
automatically terminate the connection with a bad certificate
error or (2) show the certificate (including the entire
certificate chain) to the user and give the user the choice of
terminating the connecting or accepting the certificate
temporarily (i.e., for this connection attempt only) or
permanently (i.e., for all future connection attempts) and then
continuing with the connection; if a user permanently accepts a
certificate in this way, the client MUST cache the certificate (or
some non-forgeable representation such as a hash value) and in
future connection attempts behave as in Sub-Case #3. (It is the
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resposibility of the human user to verify the hash value or
fingerprint of the certificate with the peer over a trusted
communication layer.) If the connecting entity is an XMPP server
or an automated client, the application SHOULD terminate the
connection (with a bad certificate error) and log the error to an
appropriate audit log; an XMPP server or automated client MAY
provide a configuration setting that disables this check, but MUST
enable the check by default.
Sub-Case #3: The server's certificate includes no presented identity
that matches the reference identity to which the entity attempted
to connect but a human user has permanently accepted the
certificate during a previous connection attempt; the entity MUST
verify that the cached certificate was presented and MUST notify
the user if the certificate has changed.
13.7.2.1.2. Case #2
If the server certificate is certified by a Certificate Authority not
known to the entity, the entity MUST proceed as under Case #1, Sub-
Case #2 or Case #1, Sub-Case #3 as appropriate.
13.7.2.1.3. Case #3
If the server certificate is self-signed, the entity MUST proceed as
under Case #1, Sub-Case #2 or Case #1, Sub-Case #3 as appropriate.
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.
13.7.2.2.1. Case #1
If the client certificate appears to be certified by a chain of
certificates terminating in a trust anchor (as described in Section
6.1 of [X509]), 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 hostnames of
the server itself; the server SHOULD use this represented JID as
the validated identity of the client.
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Sub-Case #2: The server finds more than one XmppAddr for which the
domainpart of the represented JID matches one of the configured
hostnames of the server itself; the server SHOULD use one of these
represented JIDs as the validated identity of the client, choosing
among them according to local service policies or based on the
'to' address of the initial stream header.
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 hostnames of the server itself; the server
MUST NOT use the represented JID (if any) as the validated
identity of the client but instead MUST either validate the
identity of the client using other means.
13.7.2.2.2. Case #2
If the client certificate is certified by a Certificate Authority not
known to the server, the server MUST proceed as under Case #1, Sub-
Case #3.
13.7.2.2.3. Case #3
If the client certificate is self-signed, the server MUST proceed as
under Case #1, Sub-Case #3.
13.7.2.3. Checking of Certificates in Long-Lived Streams
Because XMPP uses long-lived XML streams, it is possible that a
certificate presented during stream negotiation might expire or be
revoked while the stream is still live (this is especially relevant
in the context of server-to-server streams). Therefore, each party
to a long-lived stream SHOULD:
1. Cache the expiration date of the certificate presented by the
other party and any certificates on which that certificate
depends (such as a root or intermediate certificate for a
certification authority), and terminate the stream when any such
certificate expires.
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
terminate the stream if any such certificate has been revoked.
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If the identity presented by the initiating entity changes between
the old stream and the new stream, then the receiving entity MUST
perform a full re-authentication on 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, which at a minimum
SHOULD be consistent with the rules defined in this specification but
MAY specify additional rules.
13.8. Mandatory-to-Implement Technologies
At a minimum, all implementations MUST support the following
mechanisms unless otherwise specified:
for authentication only: the SASL Salted Challenge Response
mechanism [SCRAM] (preferred) and the SASL PLAIN mechanism [PLAIN]
(not preferred)
for confidentiality only: TLS (using the
TLS_RSA_WITH_AES_128_CBC_SHA cipher)
for both confidentiality and authentication with passwords: TLS plus
the SASL Salted Challenge Response mechanism (see [SCRAM]) and TLS
plus the SASL PLAIN mechanism (see [PLAIN])
for both confidentiality and authentication without passwords: TLS
plus the SASL EXTERNAL mechanism (see Appendix A of [SASL]) using
the TLS_RSA_WITH_AES_128_CBC_SHA cipher supporting peer
certificates (clients SHOULD support this, and servers MUST)
Naturally, implementations MAY support other ciphers with TLS and MAY
support other SASL mechanisms.
Interoperability Note: The use of TLS plus SASL SCRAM or SASL
PLAIN replaces the SASL DIGEST-MD5 mechanism as XMPP's mandatory-
to-implement password-based method for authentication. For
backward-compatibility with existing deployed infrastructure,
implementations are encouraged to continue supporting the SASL
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. The use of the SASL SCRAM mechanism
is strongly preferred over the SASL PLAIN mechanism because of its
superior security properties, and PLAIN is intended to be a
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fallback only for implementations that do not yet support SCRAM.
Refer to [SCRAM] and [PLAIN] for important security considerations
related to these SASL mechanisms.
13.9. Technology Reuse
13.9.1. Use of base64 in SASL
Both the client and the server MUST verify any base64 data received
during SASL negotiation (Section 6). An implementation MUST reject
(not silently ignore) any characters that are not explicitly allowed
by the base64 alphabet; this helps to guard against creation of a
covert channel that could be used to "leak" information.
An implementation MUST NOT break on invalid input and MUST reject any
sequence of base64 characters containing the pad ('=') character if
that character is included as something other than the last character
of the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against
buffer overflow attacks and other attacks on the implementation.
While base 64 encoding visually hides otherwise easily recognized
information (such as passwords), it does not provide any
computational confidentiality.
All uses of base 64 encoding MUST follow the definition in Section 4
of [BASE64] and padding bits MUST be set to zero.
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 the DNS hostname
asserted by the receiving entity has been resolved as specified under
Section 3.
In the absence of a secure DNS option (e.g., as provided by
[DNSSEC]), the initiating entity MUST check that the target domain
name returned in a DNS SRV record matches the original service domain
that was queried. If the target domain name (e.g.,
"connect.example.net") is not in the queried domain (e.g.,
"example.com"), the connecting application MUST warn a human user (if
any) about the mismatch and require the user to approve the
connection before the client proceeds; however, the client MAY also
allow the user to add the presented hostname to a configured set of
accepted hostnames to expedite future connections.
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13.9.3. Use of Hash Functions
XMPP itself does not directly mandate the use of any particular 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 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
hashes 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 help prevent this attack, the parties
SHOULD protect the channel using TLS before attempting 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. Such a
binding is known as a "channel binding" (see [CHANNEL]). Some SASL
mechanisms provide channel bindings. However, 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.
Most XMPP servers authenticate account 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.
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].
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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.
See Appendix E.2 of [TLS] for further details.
13.9.6. Use of UTF-8
The use of UTF-8 makes it possible to transport non-ASCII characters,
and thus enables character "spoofing" scenarios, in which a displayed
value appears to be something other than it is. Furthermore, there
are known attack scenarios related to the decoding of UTF-8 data. On
both of these points, refer to [UTF-8] for more information.
13.9.7. Use of XML
Because XMPP is an application profile of the Extensible Markup
Language [XML], many of the security considerations described in
[XML-MEDIA] and [XML-GUIDE] also apply to XMPP. Several aspects of
XMPP mitigate the risks described there, such as the prohibitions
specified under Section 11.1 and the lack of external references to
style sheets or transformations, but these mitigating factors are by
no means comprehensive.
13.10. Information Leaks
13.10.1. IP Addresses
A client's IP address and method of access MUST NOT be made public by
a server.
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
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server MUST NOT return different stanza errors if a potential
attacker sends XML stanzas to the entity's bare JID
(<localpart@domain>) or full JID (<localpart@domain/resource>).
13.11. Directory Harvesting
When a server generates an error stanza in response to receiving a
stanza for a user account that does not exist, the use of the
<service-unavailable/> stanza error condition can help protect
against dictionary 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 or message
forwarding is not enabled for a domain. However, subtle differences
in the exact XML of error stanzas, as well as in the timing with
which such errors are returned, can enable an attacker to determine
the network presence of a user when more advanced blocking
technologies are not used (see for instance [XEP-0016] and
[XEP-0191]).
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.
[XEP-0205] provides a detailed discussion of potential denial of
service attacks against XMPP systems and best practices for
preventing such attacks. The recommendations include:
1. A server implementation SHOULD enable a server administrator to
limit the number of TCP connections that it will accept from a
given IP address at any one time. If an entity attempts to
connect but the maximum number of TCP connections has been
reached, the receiving server MUST NOT allow the new connection
to proceed.
2. A server implementation SHOULD enable a server administrator to
limit the number of TCP connection attempts that it will accept
from a given IP address in a given time period. 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.
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3. A server MUST NOT process XML stanzas from clients that have not
yet provided appropriate authentication credentials and MUST NOT
process XML stanzas from peer servers whose identity it has not
either authenticated via SASL or weakly verified via server
dialback (see [XEP-0220]).
4. A server implementation SHOULD enable a server administrator to
limit the number of connected resources it will allow an account
to bind at any one time. If a client attempts to bind a resource
but it has already reached the configured number of allowable
resources, the receiving server MUST return a <resource-
constraint/> stanza error.
5. A server implementation SHOULD enable a server administrator to
limit the size of stanzas it will accept from a connected client
or peer server. If a connected resource or peer server sends a
stanza that violates the upper limit, the receiving server SHOULD
NOT process the stanza and instead SHOULD return a <not-allowed/>
stanza error. Alternatively (e.g., if the sender has sent an
egregiously large stanza), the server MAY instead return a
<policy-violation/> stream error.
6. A server implementation SHOULD enable a server administrator to
limit the number of XML stanzas that a connected client 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 an <unexpected-
request/> stanza error.
7. A server implementation SHOULD enable a server administrator to
limit the amount of bandwidth it will allow a connected client or
peer server to use in a given time period.
8. A server implementation 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.
9. 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
Communication using XMPP normally occurs over TCP connections on port
5222 (client-to-server) or port 5269 (server-to-server), as
registered with the IANA (see Section 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,
dialback provides only weak identity verification and provides no
confidentiality or integrity. At the time of this 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
dialback if an attacker can gain control of the channel and therefore
convince the initiating server that the receiving server does not
support TLS or does not have an appropriate certificate. To help
prevent this attack, the parties SHOULD protect the channel using TLS
before proceeding, even if the presented certificates are self-signed
or otherwise untrusted.
13.15. Non-Repudiation
Systems that provide both peer entity authentication and data
integrity have the potential to enable an entity to prove to a third
party that another entity intended to send particular data. Although
XMPP systems can provide both peer entity authentication and data
integrity, XMPP was never designed to provide non-repudiation.
14. IANA Considerations
The following sections update the registrations provided in
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[RFC3920].
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: XXXX
Description: This is the XML namespace name for STARTTLS negotiation
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmpp@ietf.org>
14.2. XML Namespace Name for SASL Data
A URN sub-namespace for SASL negotiation data in the Extensible
Messaging and Presence Protocol (XMPP) is defined as follows. (This
namespace name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-sasl
Specification: XXXX
Description: This is the XML namespace name for SASL negotiation
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmpp@ietf.org>
14.3. XML Namespace Name for Stream Errors
A URN sub-namespace for stream error data in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-streams
Specification: XXXX
Description: This is the XML namespace name for stream error data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmpp@ietf.org>
14.4. XML Namespace Name for Resource Binding
A URN sub-namespace for resource binding in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
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URI: urn:ietf:params:xml:ns:xmpp-bind
Specification: XXXX
Description: This is the XML namespace name for resource binding in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmpp@ietf.org>
14.5. XML Namespace Name for Stanza Errors
A URN sub-namespace for stanza error data in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-stanzas
Specification: XXXX
Description: This is the XML namespace name for stanza error data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmpp@ietf.org>
14.6. GSSAPI Service Name
The IANA has registered "xmpp" as a [GSS-API] service name, as
defined under Section 6.5.
14.7. Port Numbers
The IANA has registered "xmpp-client" and "xmpp-server" as keywords
for [TCP] ports 5222 and 5269 respectively.
These ports SHOULD be used for client-to-server and server-to-server
communications respectively, but other ports MAY be used.
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
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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 [TERMS]
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 7.5
Roles: Client N/A, Server MUST.
Feature: bind-mtn
Description: Consider resource binding as mandatory-to-negotiate.
Section: Section 7.2.1
Roles: Client MUST, Server MUST.
Feature: bind-restart
Description: Do not restart the stream after negotiation of resource
binding.
Section: Section 7.2.2
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-errors
Description: Support SASL errors during the negotiation process.
Section: Section 6.4
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Roles: Client MUST, Server MUST.
Feature: sasl-mtn
Description: Consider SASL as mandatory-to-negotiate.
Section: Section 6.2.1
Roles: Client MUST, Server MUST.
Feature: sasl-restart
Description: Initiate or handle a stream restart after SASL
negotiation.
Section: Section 6.2.2
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: sasl-whitespace
Description: Ensure that no whitespace is sent between XML elements
during SASL negotiation.
Section: Section 6.2.5
Roles: Client MUST, Server MUST.
Feature: security-mti-auth-plain
Description: Support the SASL PLAIN mechanism for authentication
only.
Section: Section 13.8
Roles: Client MUST, Server MUST.
Feature: security-mti-auth-scram
Description: Support the SASL Salted Challenge Response mechanism
for authentication only.
Section: Section 13.8
Roles: Client MUST, Server MUST.
Feature: security-mti-both-external
Description: Support TLS with SASL EXTERNAL for confidentiality and
authentication.
Section: Section 13.8
Roles: Client SHOULD, Server MUST.
Feature: security-mti-both-plain
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Description: Support TLS with SASL PLAIN for confidentiality and
authentication.
Section: Section 13.8
Roles: Client MUST, Server MUST.
Feature: security-mti-both-scram
Description: Support TLS with SASL SCRAM 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
cipher for confidentiality only.
Section: Section 13.8
Roles: Client MUST, Server MUST.
Feature: stanza-attribute-from
Description: Support the common 'from' attribute for all stanza
kinds.
Section: Section 8.1.1
Roles: Client MUST, Server MUST.
Feature: stanza-attribute-from-validate
Description: Validate the 'from' address of all stanzas received
from connected clients or peer servers.
Section: Section 8.1.2
Roles: Client N/A, Server MUST.
Feature: stanza-attribute-id
Description: Support the common 'id' attribute for all stanza kinds.
Section: Section 8.1.3
Roles: Client MUST, Server MUST.
Feature: stanza-attribute-to
Description: Support the common 'to' attribute for all stanza kinds.
Section: Section 8.1.1
Roles: Client MUST, Server MUST.
Feature: stanza-attribute-to-validate
Description: Ensure that all stanzas received from peer servers
include a 'to' address.
Section: Section 8.1.1
Roles: Client N/A, Server MUST.
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Feature: stanza-attribute-type
Description: Support the common 'type' attribute for all stanza
kinds.
Section: Section 8.1.4
Roles: Client MUST, Server MUST.
Feature: stanza-attribute-xmllang
Description: Support the common 'xml:lang' attribute for all stanza
kinds.
Section: Section 8.1.5
Roles: Client MUST, Server MUST.
Feature: stanza-error
Description: Generate and handle stanzas of type "error" for all
stanza kinds.
Section: Section 8.3
Roles: Client MUST, Server MUST.
Feature: stanza-error-child
Description: Ensure that stanzas of type "error" include an <error/>
child element.
Section: Section 8.3
Roles: Client MUST, Server MUST.
Feature: stanza-error-id
Description: Ensure that stanzas of type "error" preserve the 'id'
provided in the triggering stanza.
Section: Section 8.3
Roles: Client MUST, Server MUST.
Feature: stanza-error-reply
Description: Do not reply to a stanza of type "error" with another
stanza of type "error".
Section: Section 8.3
Roles: Client MUST, Server MUST.
Feature: stanza-extension
Description: Correctly process XML data qualified by an unsupported
XML namespace, where "correctly process" means to silently 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
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Roles: Client MUST, Server MUST.
Feature: stanza-iq-child
Description: Include exactly one child element in an <iq/> stanza of
type "get" or "set", zero or one child elements in an <iq/> stanza
of type "result", and one or two child elements in an <iq/> stanza
of type "error".
Section: Section 8.2.3
Roles: Client MUST, Server MUST.
Feature: stanza-iq-id
Description: Ensure that all <iq/> stanzas include an 'id'
attribute.
Section: Section 8.2.3
Roles: Client MUST, Server MUST.
Feature: stanza-iq-reply
Description: Reply to an <iq/> stanza of type "get" or "set" with an
<iq/> stanza of type "result" or "error".
Section: Section 8.2.3
Roles: Client MUST, Server MUST.
Feature: stanza-iq-type
Description: Ensure that all <iq/> stanzas include a 'type'
attribute whose value is "get", "set", "result", or "error".
Section: Section 8.2.3
Roles: Client MUST, Server MUST.
Feature: stanza-kind-iq
Description: Support the <iq/> stanza.
Section: Section 8.2.3
Roles: Client MUST, Server MUST.
Feature: stanza-kind-message
Description: Support the <message/> stanza.
Section: Section 8.2.1
Roles: Client MUST, Server MUST.
Feature: stanza-kind-presence
Description: Support the <presence/> stanza.
Section: Section 8.2.2
Roles: Client MUST, Server MUST.
Feature: stream-attribute-initial-from
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Description: Include a 'from' attribute in the initial stream
header.
Section: Section 4.4.1
Roles: Client SHOULD, Server SHOULD.
Feature: stream-attribute-initial-lang
Description: Include an 'xml:lang' attribute in the initial stream
header.
Section: Section 4.4.4
Roles: Client SHOULD, Server SHOULD.
Feature: stream-attribute-initial-to
Description: Include a 'to' attribute in the initial stream header.
Section: Section 4.4.2
Roles: Client SHOULD, Server SHOULD.
Feature: stream-attribute-response-from
Description: Include a 'from' attribute in the response stream
header.
Section: Section 4.4.1
Roles: Client N/A, Server MUST.
Feature: stream-attribute-response-id
Description: Include an 'id' attribute in the response stream
header.
Section: Section 4.4.3
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 4.4.3
Roles: Client N/A, Server MUST.
Feature: stream-attribute-response-to
Description: Include a 'to' attribute in the response stream header.
Section: Section 4.4.2
Roles: Client N/A, Server SHOULD.
Feature: stream-error-generate
Description: Generate a stream error (followed by a closing stream
tag and termination of the TCP connection) upon detecting a
stream-related error condition.
Section: Section 4.6
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Roles: Client MUST, Server MUST.
Feature: stream-hostname-resolution
Description: Resolve hostnames before opening a TCP connection.
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 4.2.5
Roles: Client MUST, Server MUST.
Feature: stream-negotiation-features
Description: Send stream features after sending a response stream
header.
Section: Section 4.2.2
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 4.2.3
Roles: Client MUST, Server MUST.
Feature: stream-reconnect
Description: Reconnect with exponential backoff if a TCP connection
is terminated unexpectedly.
Section: Section 3.4
Roles: Client MUST, Server MUST.
Feature: stream-tcp-binding
Description: Bind an XML stream to a TCP connection.
Section: Section 3
Roles: Client MUST, Server MUST.
Feature: tls-certs
Description: Check the identity specified in a certificate that is
presented during TLS negotiation.
Section: Section 13.7.2
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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 includes an
empty <required/> element.
Section: Section 5.2.1
Roles: Client MUST, Server MUST.
Feature: tls-restart
Description: Initiate or handle a stream restart after TLS
negotiation.
Section: Section 5.2.2
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-whitespace
Description: Ensure that no whitespace is sent between XML elements
during TLS negotiation.
Section: Section 5.2.3
Roles: Client MUST, Server MUST.
Feature: xml-namespace-default
Description: Ensure that there is a default namespace for the stream
(other than the streams namespace).
Section: Section 4.5.2
Roles: Client MUST, Server MUST.
Feature: xml-namespace-default-client
Description: Support 'jabber:client' as a default namespace.
Section: Section 4.5.2
Roles: Client MUST, Server MUST.
Feature: xml-namespace-default-server
Description: Support 'jabber:server' as a default namespace.
Section: Section 4.5.2
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.
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Section: Section 4.5.1
Roles: Client MUST, Server MUST.
Feature: xml-namespace-streams-prefix
Description: Ensure that all elements qualified by the
'http://etherx.jabber.org/streams' namespace are prefixed by the
prefix defined in the namespace declaration.
Section: Section 4.5.1
Roles: Client MUST, Server MUST.
Feature: xml-restriction-comment
Description: Do not generate or accept XML comments.
Section: Section 11.1
Roles: Client MUST, Server MUST.
Feature: xml-restriction-dtd
Description: Do not generate or accept internal or external DTD
subsets.
Section: Section 11.1
Roles: Client MUST, Server MUST.
Feature: xml-restriction-pi
Description: Do not generate or accept XML processing instructions.
Section: Section 11.1
Roles: Client MUST, Server MUST.
Feature: xml-restriction-ref
Description: Do not generate or accept internal or external entity
references with the exception of the predefined entities.
Section: Section 11.1
Roles: Client MUST, Server MUST.
Feature: xml-wellformed-xml
Description: Do not generate or accept data that is not XML-well-
formed.
Section: Section 11.3
Roles: Client MUST, Server MUST.
Feature: xml-wellformed-ns
Description: Do not generate or accept data that is not namespace-
well-formed.
Section: Section 11.3
Roles: Client MUST, Server MUST.
16. References
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16.1. Normative References
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[CHARSET] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[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.
[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 (SCRAM) SASL and GSS-API
Mechanism", draft-ietf-sasl-scram-11 (work in progress),
February 2010.
[TCP] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[TLS-CERTS]
Saint-Andre, P. and J. Hodges, "Representation and
Verification of Application Server Identity in
Certificates Used with Transport Layer Security (TLS)",
draft-saintandre-tls-server-id-check-04 (work in
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progress), April 2010.
[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
3.2.0", 2000.
The Unicode Standard, Version 3.2.0 is defined by The
Unicode Standard, Version 3.0 (Reading, MA, Addison-
Wesley, 2000. ISBN 0-201-61633-5), as amended by the
Unicode Standard Annex #27: Unicode 3.1
(http://www.unicode.org/reports/tr27/) and by the Unicode
Standard Annex #28: Unicode 3.2
(http://www.unicode.org/reports/tr28/).
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[UUID] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
July 2005.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[X509] 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.
[X509-ALGO]
Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[X509-SRV]
Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, August 2007.
[XML] Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F.,
and T. Bray, "Extensible Markup Language (XML) 1.0 (Fourth
Edition)", World Wide Web Consortium Recommendation REC-
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xml-20060816, August 2006,
<http://www.w3.org/TR/2006/REC-xml-20060816>.
[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]
Layman, A., Hollander, D., Tobin, R., and T. Bray,
"Namespaces in XML 1.1 (Second Edition)", World Wide Web
Consortium Recommendation REC-xml-names11-20060816,
August 2006, <http://www.w3.org/TR/REC-xml-names>.
[XMPP-ADDR]
Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Address Format",
draft-ietf-xmpp-address-00 (work in progress), April 2010.
16.2. Informative References
[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.
[CHANNEL] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[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.
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[DOS] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006.
[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
[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.
[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", draft-ietf-newtrk-interop-reports-00 (work in
progress), October 2005.
[IRI] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[LINKLOCAL]
Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
May 2005.
[MAILBOXES]
Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
FUNCTIONS", RFC 2142, May 1997.
[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[PROCESS] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[REPORTS] Dusseault, L. and R. Sparks, "Guidance on Interoperation
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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.
[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.
[SEC-TERMS]
Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[STRONGSEC]
Schiller, J., "Strong Security Requirements for Internet
Engineering Task Force Standard Protocols", BCP 61,
RFC 3365, August 2002.
[TLS-EXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[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.
[XEP-0001]
Saint-Andre, P., "XMPP Extension Protocols", XSF XEP 0001,
January 2008.
[XEP-0016]
Millard, P. and P. Saint-Andre, "Privacy Lists", XSF
XEP 0016, February 2007.
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[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, September 2008.
[XEP-0071]
Saint-Andre, P., "XHTML-IM", XSF XEP 0071, September 2008.
[XEP-0077]
Saint-Andre, P., "In-Band Registration", XSF XEP 0077,
January 2006.
[XEP-0124]
Paterson, I., Smith, D., and P. Saint-Andre,
"Bidirectional-streams Over Synchronous HTTP (BOSH)", XSF
XEP 0124, April 2009.
[XEP-0156]
Hildebrand, J. and P. Saint-Andre, "Discovering
Alternative XMPP Connection Methods", XSF XEP 0156,
June 2007.
[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, November 2007.
[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., and F.
Forno, "Stream Management", XSF XEP 0198, June 2009.
[XEP-0199]
Saint-Andre, P., "XMPP Ping", XSF XEP 0199, June 2009.
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[XEP-0205]
Saint-Andre, P., "Best Practices to Discourage Denial of
Service Attacks", XSF XEP 0205, January 2009.
[XEP-0206]
Paterson, I., "XMPP Over BOSH", XSF XEP 0206,
October 2008.
[XEP-0220]
Saint-Andre, P. and J. Miller, "Server Dialback", XSF
XEP 0220, October 2008.
[XML-FRAG]
Grosso, P. and D. Veillard, "XML Fragment Interchange",
World Wide Web Consortium CR CR-xml-fragment-20010212,
February 2001,
<http://www.w3.org/TR/2001/CR-xml-fragment-20010212>.
[XML-REG] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[XML-SCHEMA]
Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech,
"XML Schema Part 1: Structures Second Edition", World Wide
Web Consortium Recommendation REC-xmlschema-1-20041028,
October 2004,
<http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>.
[XMPP-IM] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
draft-ietf-xmpp-3921bis-06 (work in progress), March 2010.
[XMPP-URI]
Saint-Andre, P., "Internationalized Resource Identifiers
(IRIs) and Uniform Resource Identifiers (URIs) for the
Extensible Messaging and Presence Protocol (XMPP)",
RFC 5122, February 2008.
Appendix A. XML Schemas
Because validation of XML streams and stanzas is optional, the
following XML schemas are provided for descriptive purposes only.
These schemas are not normative.
The following schemas formally define various XML namespaces used in
the core XMPP protocols, in conformance with [XML-SCHEMA]. For
schemas defining the 'jabber:client' and 'jabber:server' namespaces,
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refer to [XMPP-IM].
A.1. Streams Namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='http://etherx.jabber.org/streams'
xmlns='http://etherx.jabber.org/streams'
elementFormDefault='unqualified'>
<xs:import namespace='jabber:client'/>
<xs:import namespace='jabber:server'/>
<xs:import namespace='urn:ietf:params:xml:ns:xmpp-sasl'/>
<xs:import namespace='urn:ietf:params:xml:ns:xmpp-streams'/>
<xs:import namespace='urn:ietf:params:xml:ns:xmpp-tls'/>
<xs:element name='stream'>
<xs:complexType>
<xs:sequence xmlns:client='jabber:client'
xmlns:server='jabber:server'>
<xs:element ref='features' minOccurs='0' maxOccurs='1'/>
<xs:any namespace='urn:ietf:params:xml:ns:xmpp-tls'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:any namespace='urn:ietf:params:xml:ns:xmpp-sasl'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:choice minOccurs='0' maxOccurs='1'>
<xs:choice minOccurs='0' maxOccurs='unbounded'>
<xs:element ref='client:message'/>
<xs:element ref='client:presence'/>
<xs:element ref='client:iq'/>
</xs:choice>
<xs:choice minOccurs='0' maxOccurs='unbounded'>
<xs:element ref='server:message'/>
<xs:element ref='server:presence'/>
<xs:element ref='server:iq'/>
</xs:choice>
</xs:choice>
<xs:element ref='error' minOccurs='0' maxOccurs='1'/>
</xs:sequence>
<xs:attribute name='from' type='xs:string' use='optional'/>
<xs:attribute name='id' type='xs:string' use='optional'/>
<xs:attribute name='to' type='xs:string' use='optional'/>
<xs:attribute name='version' type='xs:decimal' use='optional'/>
<xs:attribute ref='xml:lang' use='optional'/>
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</xs:complexType>
</xs:element>
<xs:element name='features'>
<xs:complexType>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'/>
</xs:complexType>
</xs:element>
<xs:element name='error'>
<xs:complexType>
<xs:sequence xmlns:err='urn:ietf:params:xml:ns:xmpp-streams'>
<xs:group ref='err:streamErrorGroup'/>
<xs:element ref='err:text'
minOccurs='0'
maxOccurs='1'/>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
A.2. Stream Error Namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-streams'
xmlns='urn:ietf:params:xml:ns:xmpp-streams'
elementFormDefault='qualified'>
<xs:element name='bad-format' type='empty'/>
<xs:element name='bad-namespace-prefix' type='empty'/>
<xs:element name='conflict' type='empty'/>
<xs:element name='connection-timeout' type='empty'/>
<xs:element name='host-gone' type='empty'/>
<xs:element name='host-unknown' type='empty'/>
<xs:element name='improper-addressing' type='empty'/>
<xs:element name='internal-server-error' type='empty'/>
<xs:element name='invalid-from' type='empty'/>
<xs:element name='invalid-id' type='empty'/>
<xs:element name='invalid-namespace' type='empty'/>
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<xs:element name='invalid-xml' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='policy-violation' type='empty'/>
<xs:element name='remote-connection-failed' type='empty'/>
<xs:element name='reset' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='restricted-xml' type='empty'/>
<xs:element name='see-other-host' type='xs:string'/>
<xs:element name='system-shutdown' type='empty'/>
<xs:element name='undefined-condition' type='empty'/>
<xs:element name='unsupported-encoding' type='empty'/>
<xs:element name='unsupported-stanza-type' type='empty'/>
<xs:element name='unsupported-version' type='empty'/>
<xs:element name='xml-not-well-formed' type='empty'/>
<xs:group name='streamErrorGroup'>
<xs:choice>
<xs:element ref='bad-format'/>
<xs:element ref='bad-namespace-prefix'/>
<xs:element ref='conflict'/>
<xs:element ref='connection-timeout'/>
<xs:element ref='host-gone'/>
<xs:element ref='host-unknown'/>
<xs:element ref='improper-addressing'/>
<xs:element ref='internal-server-error'/>
<xs:element ref='invalid-from'/>
<xs:element ref='invalid-id'/>
<xs:element ref='invalid-namespace'/>
<xs:element ref='invalid-xml'/>
<xs:element ref='not-authorized'/>
<xs:element ref='policy-violation'/>
<xs:element ref='remote-connection-failed'/>
<xs:element ref='reset'/>
<xs:element ref='resource-constraint'/>
<xs:element ref='restricted-xml'/>
<xs:element ref='see-other-host'/>
<xs:element ref='system-shutdown'/>
<xs:element ref='undefined-condition'/>
<xs:element ref='unsupported-encoding'/>
<xs:element ref='unsupported-stanza-type'/>
<xs:element ref='unsupported-version'/>
<xs:element ref='xml-not-well-formed'/>
</xs:choice>
</xs:group>
<xs:element name='text'>
<xs:complexType>
<xs:simpleContent>
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<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
A.3. STARTTLS Namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-tls'
xmlns='urn:ietf:params:xml:ns:xmpp-tls'
elementFormDefault='qualified'>
<xs:element name='starttls'>
<xs:complexType>
<xs:choice minOccurs='0' maxOccurs='1'>
<xs:element name='required' type='empty'/>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:element name='proceed' type='empty'/>
<xs:element name='failure' type='empty'/>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
A.4. SASL Namespace
<?xml version='1.0' encoding='UTF-8'?>
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<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-sasl'
xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
elementFormDefault='qualified'>
<xs:element name='mechanisms'>
<xs:complexType>
<xs:sequence>
<xs:element name='mechanism'
minOccurs='1'
maxOccurs='unbounded'
type='xs:NMTOKEN'/>
<xs:any namespace='##other'
minOccurs='0'
minOccurs='unbounded'/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='abort' type='empty'/>
<xs:element name='auth'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute name='mechanism'
type='xs:NMTOKEN'
use='required'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='challenge' type='xs:string'/>
<xs:element name='response' type='xs:string'/>
<xs:element name='success' type='xs:string'/>
<xs:element name='failure'>
<xs:complexType>
<xs:sequence>
<xs:choice minOccurs='0'>
<xs:element name='aborted' type='empty'/>
<xs:element name='account-disabled' type='empty'/>
<xs:element name='credentials-expired' type='empty'/>
<xs:element name='encryption-required' type='empty'/>
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<xs:element name='incorrect-encoding' type='empty'/>
<xs:element name='invalid-authzid' type='empty'/>
<xs:element name='invalid-mechanism' type='empty'/>
<xs:element name='malformed-request' type='empty'/>
<xs:element name='mechanism-too-weak' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='temporary-auth-failure' type='empty'/>
<xs:element name='transition-needed' type='empty'/>
</xs:choice>
<xs:element ref='text' minOccurs='0' maxOccurs='1'/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='text'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
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A.5. Resource Binding Namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-bind'
xmlns='urn:ietf:params:xml:ns:xmpp-bind'
elementFormDefault='qualified'>
<xs:element name='bind'>
<xs:complexType>
<xs:choice>
<xs:element name='resource' type='resourceType'/>
<xs:element name='jid' type='fullJIDType'/>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:simpleType name='fullJIDType'>
<xs:restriction base='xs:string'>
<xs:minLength value='8'/>
<xs:maxLength value='3071'/>
</xs:restriction>
</xs:simpleType>
<xs:simpleType name='resourceType'>
<xs:restriction base='xs:string'>
<xs:minLength value='1'/>
<xs:maxLength value='1023'/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
A.6. Stanza Error Namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-stanzas'
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
elementFormDefault='qualified'>
<xs:element name='bad-request' type='empty'/>
<xs:element name='conflict' type='empty'/>
<xs:element name='feature-not-implemented' type='empty'/>
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<xs:element name='forbidden' type='empty'/>
<xs:element name='gone' type='xs:string'/>
<xs:element name='internal-server-error' type='empty'/>
<xs:element name='item-not-found' type='empty'/>
<xs:element name='jid-malformed' type='empty'/>
<xs:element name='not-acceptable' type='empty'/>
<xs:element name='not-allowed' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='payment-required' type='empty'/>
<xs:element name='policy-violation' type='empty'/>
<xs:element name='recipient-unavailable' type='empty'/>
<xs:element name='redirect' type='xs:string'/>
<xs:element name='registration-required' type='empty'/>
<xs:element name='remote-server-not-found' type='empty'/>
<xs:element name='remote-server-timeout' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='service-unavailable' type='empty'/>
<xs:element name='subscription-required' type='empty'/>
<xs:element name='undefined-condition' type='empty'/>
<xs:element name='unexpected-request' type='empty'/>
<xs:group name='stanzaErrorGroup'>
<xs:choice>
<xs:element ref='bad-request'/>
<xs:element ref='conflict'/>
<xs:element ref='feature-not-implemented'/>
<xs:element ref='forbidden'/>
<xs:element ref='gone'/>
<xs:element ref='internal-server-error'/>
<xs:element ref='item-not-found'/>
<xs:element ref='jid-malformed'/>
<xs:element ref='not-acceptable'/>
<xs:element ref='not-authorized'/>
<xs:element ref='not-allowed'/>
<xs:element ref='payment-required'/>
<xs:element ref='policy-violation'/>
<xs:element ref='recipient-unavailable'/>
<xs:element ref='redirect'/>
<xs:element ref='registration-required'/>
<xs:element ref='remote-server-not-found'/>
<xs:element ref='remote-server-timeout'/>
<xs:element ref='resource-constraint'/>
<xs:element ref='service-unavailable'/>
<xs:element ref='subscription-required'/>
<xs:element ref='undefined-condition'/>
<xs:element ref='unexpected-request'/>
</xs:choice>
</xs:group>
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<xs:element name='text'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
Appendix B. Contact Addresses
Consistent with [MAILBOXES], an organization that offers an XMPP
service SHOULD provide an Internet mailbox of "XMPP" for inquiries
related to that service, where the host portion of the resulting
mailto URI MUST be the organization's domain, not the domain of the
XMPP service itself (e.g., the XMPP service might be offered at
im.example.com but the Internet mailbox would be <xmpp@example.com>).
Appendix C. Account Provisioning
Account provisioning is out of scope for this specification.
Possible methods for account provisioning include account creation by
a server administrator and in-band account registration using the
'jabber:iq:register' namespace as documented in [XEP-0077].
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.
o Moved specification of the XMPP address format to a separate
document.
o Recommended or mandated use of the 'from' and 'to' attributes on
stream headers.
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o More fully specified the stream closing handshake.
o Specified the recommended stream reconnection algorithm.
o Specified return of the <restricted-xml/> stream error in response
to receipt of prohibited XML features.
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
if mutual authentication is successful.
o More clearly specified the certificate profile for both public key
certificates and issuer certificates.
o Added the <reset/> streams error condition to handle expired/
revoked certificates or the addition of security-critical features
to an existing stream.
o Added the <account-disabled/>, <credentials-expired/>,
<encryption-required/>, <malformed-request/>, and <transition-
needed/> 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 unnecessary requirement for escaping of characters that
map to certain predefined entities, which 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 the stream negotiation process and associated state
chart.
o Clarified the handling of stream features.
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 historical documentation of the server dialback protocol
from this specification to a separate specification maintained by
the XMPP Standards Foundation.
In addition, numerous changes of an editorial nature were made in
order to more fully specify and clearly explain XMPP.
Appendix E. Copying Conditions
Regarding this entire document or any portion of it, the author makes
no guarantees and is not responsible for any damage resulting from
its use. The author grants irrevocable permission to anyone to use,
modify, and distribute it in any way that does not diminish the
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rights of anyone else to use, modify, and distribute it, provided
that redistributed derivative works do not contain misleading author
or version information. Derivative works need not be licensed under
similar terms.
Index
C
Connected Resource 80
E
Error Stanza 93
Extended Content 110
I
Initial Stream 20
IQ Stanza 91
M
Message Stanza 91
P
Presence Stanza 91
R
Response Stream 20
S
Stream ID 33
W
Whitespace Keepalive 29
X
XML Stanza 20
XML Stream 20
Author's Address
Peter Saint-Andre
Cisco
Email: psaintan@cisco.com
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