Network Working Group P. Saint-Andre
Internet-Draft Cisco
Obsoletes: 3920 (if approved) August 14, 2009
Intended status: Standards Track
Expires: February 15, 2010
Extensible Messaging and Presence Protocol (XMPP): Core
draft-ietf-xmpp-3920bis-01
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Abstract
This document defines the core features of the Extensible Messaging
and Presence Protocol (XMPP), a technology for streaming Extensible
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Markup Language (XML) elements for the purpose of exchanging
structured information in close to real time between any two or more
network-aware entities. XMPP provides a generalized, extensible
framework for incrementally exchanging XML data, upon which a variety
of applications can be built. The framework includes methods for
stream setup and teardown, channel encryption, authentication of a
client to a server and of one server to another server, and
primitives for push-style messages, publication of network
availability information ("presence"), and request-response
interactions. This document also specifies the format for XMPP
addresses, which are fully internationalizable.
This document obsoletes RFC 3920.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2. Functional Summary . . . . . . . . . . . . . . . . . . . 10
1.3. Conventions . . . . . . . . . . . . . . . . . . . . . . 11
1.4. Acknowledgements . . . . . . . . . . . . . . . . . . . . 12
1.5. Discussion Venue . . . . . . . . . . . . . . . . . . . . 12
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3. Client . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4. Network . . . . . . . . . . . . . . . . . . . . . . . . 14
3. Addresses . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2. Domain Identifier . . . . . . . . . . . . . . . . . . . 15
3.3. Localpart . . . . . . . . . . . . . . . . . . . . . . . 17
3.4. Resource Identifier . . . . . . . . . . . . . . . . . . 17
3.5. Determination of Addresses . . . . . . . . . . . . . . . 18
4. TCP Binding . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2. Hostname Resolution . . . . . . . . . . . . . . . . . . 19
4.3. Client-to-Server Communication . . . . . . . . . . . . . 20
4.4. Server-to-Server Communication . . . . . . . . . . . . . 21
4.5. Reconnection . . . . . . . . . . . . . . . . . . . . . . 21
4.6. Other Bindings . . . . . . . . . . . . . . . . . . . . . 21
5. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 22
5.2. Stream Security . . . . . . . . . . . . . . . . . . . . 24
5.3. Stream Attributes . . . . . . . . . . . . . . . . . . . 24
5.3.1. from . . . . . . . . . . . . . . . . . . . . . . . . 25
5.3.2. to . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.3.3. id . . . . . . . . . . . . . . . . . . . . . . . . . 27
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5.3.4. xml:lang . . . . . . . . . . . . . . . . . . . . . . 28
5.3.5. version . . . . . . . . . . . . . . . . . . . . . . 29
5.3.6. Summary of Stream Attributes . . . . . . . . . . . . 31
5.4. Namespace Declarations . . . . . . . . . . . . . . . . . 31
5.5. Stream Features . . . . . . . . . . . . . . . . . . . . 31
5.6. Restarts During Stream Negotiation . . . . . . . . . . . 33
5.7. Closing a Stream . . . . . . . . . . . . . . . . . . . . 34
5.7.1. With Stream Error . . . . . . . . . . . . . . . . . 34
5.7.2. Without Stream Error . . . . . . . . . . . . . . . . 34
5.7.3. Handling of Idle Streams . . . . . . . . . . . . . . 34
5.8. Stream Errors . . . . . . . . . . . . . . . . . . . . . 35
5.8.1. Rules . . . . . . . . . . . . . . . . . . . . . . . 35
5.8.1.1. Stream Errors Are Unrecoverable . . . . . . . . . 36
5.8.1.2. Stream Errors Can Occur During Setup . . . . . . 36
5.8.1.3. Stream Errors When the Host is Unspecified or
Unknown . . . . . . . . . . . . . . . . . . . . . 37
5.8.1.4. Where Stream Errors Are Sent . . . . . . . . . . 38
5.8.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 38
5.8.3. Defined Stream Error Conditions . . . . . . . . . . 39
5.8.3.1. bad-format . . . . . . . . . . . . . . . . . . . 39
5.8.3.2. bad-namespace-prefix . . . . . . . . . . . . . . 40
5.8.3.3. conflict . . . . . . . . . . . . . . . . . . . . 41
5.8.3.4. connection-timeout . . . . . . . . . . . . . . . 41
5.8.3.5. host-gone . . . . . . . . . . . . . . . . . . . . 42
5.8.3.6. host-unknown . . . . . . . . . . . . . . . . . . 42
5.8.3.7. improper-addressing . . . . . . . . . . . . . . . 43
5.8.3.8. internal-server-error . . . . . . . . . . . . . . 43
5.8.3.9. invalid-from . . . . . . . . . . . . . . . . . . 44
5.8.3.10. invalid-id . . . . . . . . . . . . . . . . . . . 44
5.8.3.11. invalid-namespace . . . . . . . . . . . . . . . . 45
5.8.3.12. invalid-xml . . . . . . . . . . . . . . . . . . . 45
5.8.3.13. not-authorized . . . . . . . . . . . . . . . . . 46
5.8.3.14. policy-violation . . . . . . . . . . . . . . . . 47
5.8.3.15. remote-connection-failed . . . . . . . . . . . . 48
5.8.3.16. resource-constraint . . . . . . . . . . . . . . . 48
5.8.3.17. restricted-xml . . . . . . . . . . . . . . . . . 49
5.8.3.18. see-other-host . . . . . . . . . . . . . . . . . 49
5.8.3.19. system-shutdown . . . . . . . . . . . . . . . . . 50
5.8.3.20. undefined-condition . . . . . . . . . . . . . . . 51
5.8.3.21. unsupported-encoding . . . . . . . . . . . . . . 51
5.8.3.22. unsupported-stanza-type . . . . . . . . . . . . . 52
5.8.3.23. unsupported-version . . . . . . . . . . . . . . . 52
5.8.3.24. xml-not-well-formed . . . . . . . . . . . . . . . 53
5.8.4. Application-Specific Conditions . . . . . . . . . . 54
5.9. Simplified Stream Examples . . . . . . . . . . . . . . . 54
6. STARTTLS Negotiation . . . . . . . . . . . . . . . . . . . . 56
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 57
6.2. Rules . . . . . . . . . . . . . . . . . . . . . . . . . 57
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6.2.1. Data Formatting . . . . . . . . . . . . . . . . . . 57
6.2.2. Order of Negotiation . . . . . . . . . . . . . . . . 57
6.3. Process . . . . . . . . . . . . . . . . . . . . . . . . 58
6.3.1. Exchange of Stream Headers and Stream Features . . . 58
6.3.2. Initiation of STARTTLS Negotiation . . . . . . . . . 59
6.3.2.1. STARTTLS Command . . . . . . . . . . . . . . . . 59
6.3.2.2. Failure Case . . . . . . . . . . . . . . . . . . 59
6.3.2.3. Proceed Case . . . . . . . . . . . . . . . . . . 60
6.3.3. TLS Negotiation . . . . . . . . . . . . . . . . . . 60
6.3.3.1. Rules . . . . . . . . . . . . . . . . . . . . . . 60
6.3.3.2. TLS Failure . . . . . . . . . . . . . . . . . . . 61
6.3.3.3. TLS Success . . . . . . . . . . . . . . . . . . . 61
7. SASL Negotiation . . . . . . . . . . . . . . . . . . . . . . 62
7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 62
7.2. Rules . . . . . . . . . . . . . . . . . . . . . . . . . 62
7.2.1. Mechanism Preferences . . . . . . . . . . . . . . . 62
7.2.2. Mechanism Offers . . . . . . . . . . . . . . . . . . 63
7.2.3. Data Formatting . . . . . . . . . . . . . . . . . . 63
7.2.4. Security Layers . . . . . . . . . . . . . . . . . . 64
7.2.5. Simple Usernames . . . . . . . . . . . . . . . . . . 64
7.2.6. Authorization Identities . . . . . . . . . . . . . . 64
7.2.7. Realms . . . . . . . . . . . . . . . . . . . . . . . 65
7.2.8. Round Trips . . . . . . . . . . . . . . . . . . . . 65
7.3. Process . . . . . . . . . . . . . . . . . . . . . . . . 65
7.3.1. Exchange of Stream Headers and Stream Features . . . 65
7.3.2. Initiation . . . . . . . . . . . . . . . . . . . . . 67
7.3.3. Challenge-Response Sequence . . . . . . . . . . . . 67
7.3.4. Abort . . . . . . . . . . . . . . . . . . . . . . . 68
7.3.5. Failure . . . . . . . . . . . . . . . . . . . . . . 68
7.3.6. Success . . . . . . . . . . . . . . . . . . . . . . 69
7.4. SASL Errors . . . . . . . . . . . . . . . . . . . . . . 70
7.4.1. aborted . . . . . . . . . . . . . . . . . . . . . . 71
7.4.2. account-disabled . . . . . . . . . . . . . . . . . . 71
7.4.3. credentials-expired . . . . . . . . . . . . . . . . 71
7.4.4. encryption-required . . . . . . . . . . . . . . . . 71
7.4.5. incorrect-encoding . . . . . . . . . . . . . . . . . 72
7.4.6. invalid-authzid . . . . . . . . . . . . . . . . . . 72
7.4.7. invalid-mechanism . . . . . . . . . . . . . . . . . 72
7.4.8. malformed-request . . . . . . . . . . . . . . . . . 73
7.4.9. mechanism-too-weak . . . . . . . . . . . . . . . . . 73
7.4.10. not-authorized . . . . . . . . . . . . . . . . . . . 73
7.4.11. temporary-auth-failure . . . . . . . . . . . . . . . 74
7.4.12. transition-needed . . . . . . . . . . . . . . . . . 74
7.5. SASL Definition . . . . . . . . . . . . . . . . . . . . 74
8. Resource Binding . . . . . . . . . . . . . . . . . . . . . . 75
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 75
8.2. Advertising Support . . . . . . . . . . . . . . . . . . 76
8.3. Generation of Resource Identifiers . . . . . . . . . . . 77
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8.4. Server-Generated Resource Identifier . . . . . . . . . . 77
8.4.1. Success Case . . . . . . . . . . . . . . . . . . . . 77
8.4.2. Error Cases . . . . . . . . . . . . . . . . . . . . 78
8.4.2.1. Resource Constraint . . . . . . . . . . . . . . . 78
8.4.2.2. Not Allowed . . . . . . . . . . . . . . . . . . . 78
8.5. Client-Submitted Resource Identifier . . . . . . . . . . 78
8.5.1. Success Case . . . . . . . . . . . . . . . . . . . . 78
8.5.2. Error Cases . . . . . . . . . . . . . . . . . . . . 79
8.5.2.1. Bad Request . . . . . . . . . . . . . . . . . . . 79
8.5.2.2. Conflict . . . . . . . . . . . . . . . . . . . . 79
8.5.3. Retries . . . . . . . . . . . . . . . . . . . . . . 80
9. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 81
9.1. Common Attributes . . . . . . . . . . . . . . . . . . . 81
9.1.1. to . . . . . . . . . . . . . . . . . . . . . . . . . 81
9.1.1.1. Client-to-Server Streams . . . . . . . . . . . . 81
9.1.1.2. Server-to-Server Streams . . . . . . . . . . . . 82
9.1.2. from . . . . . . . . . . . . . . . . . . . . . . . . 82
9.1.2.1. Client-to-Server Streams . . . . . . . . . . . . 82
9.1.2.2. Server-to-Server Streams . . . . . . . . . . . . 83
9.1.3. id . . . . . . . . . . . . . . . . . . . . . . . . . 83
9.1.4. type . . . . . . . . . . . . . . . . . . . . . . . . 84
9.1.5. xml:lang . . . . . . . . . . . . . . . . . . . . . . 84
9.2. Basic Semantics . . . . . . . . . . . . . . . . . . . . 85
9.2.1. Message Semantics . . . . . . . . . . . . . . . . . 85
9.2.2. Presence Semantics . . . . . . . . . . . . . . . . . 85
9.2.3. IQ Semantics . . . . . . . . . . . . . . . . . . . . 86
9.3. Stanza Errors . . . . . . . . . . . . . . . . . . . . . 87
9.3.1. Rules . . . . . . . . . . . . . . . . . . . . . . . 88
9.3.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 88
9.3.3. Defined Conditions . . . . . . . . . . . . . . . . . 89
9.3.3.1. bad-request . . . . . . . . . . . . . . . . . . . 89
9.3.3.2. conflict . . . . . . . . . . . . . . . . . . . . 90
9.3.3.3. feature-not-implemented . . . . . . . . . . . . . 90
9.3.3.4. forbidden . . . . . . . . . . . . . . . . . . . . 91
9.3.3.5. gone . . . . . . . . . . . . . . . . . . . . . . 91
9.3.3.6. internal-server-error . . . . . . . . . . . . . . 92
9.3.3.7. item-not-found . . . . . . . . . . . . . . . . . 92
9.3.3.8. jid-malformed . . . . . . . . . . . . . . . . . . 93
9.3.3.9. not-acceptable . . . . . . . . . . . . . . . . . 93
9.3.3.10. not-allowed . . . . . . . . . . . . . . . . . . . 94
9.3.3.11. not-authorized . . . . . . . . . . . . . . . . . 94
9.3.3.12. not-modified . . . . . . . . . . . . . . . . . . 95
9.3.3.13. payment-required . . . . . . . . . . . . . . . . 96
9.3.3.14. policy-violation . . . . . . . . . . . . . . . . 96
9.3.3.15. recipient-unavailable . . . . . . . . . . . . . . 97
9.3.3.16. redirect . . . . . . . . . . . . . . . . . . . . 97
9.3.3.17. registration-required . . . . . . . . . . . . . . 98
9.3.3.18. remote-server-not-found . . . . . . . . . . . . . 98
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9.3.3.19. remote-server-timeout . . . . . . . . . . . . . . 99
9.3.3.20. resource-constraint . . . . . . . . . . . . . . . 99
9.3.3.21. service-unavailable . . . . . . . . . . . . . . . 100
9.3.3.22. subscription-required . . . . . . . . . . . . . . 100
9.3.3.23. undefined-condition . . . . . . . . . . . . . . . 101
9.3.3.24. unexpected-request . . . . . . . . . . . . . . . 102
9.3.4. Application-Specific Conditions . . . . . . . . . . 103
9.4. Extended Content . . . . . . . . . . . . . . . . . . . . 104
9.5. Stanza Size . . . . . . . . . . . . . . . . . . . . . . 105
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 106
10.1. Client-to-Server . . . . . . . . . . . . . . . . . . . . 106
10.1.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . 106
10.1.2. SASL . . . . . . . . . . . . . . . . . . . . . . . . 108
10.1.3. Resource Binding . . . . . . . . . . . . . . . . . . 109
10.1.4. Stanza Exchange . . . . . . . . . . . . . . . . . . 109
10.1.5. Close . . . . . . . . . . . . . . . . . . . . . . . 110
10.2. Server-to-Server Examples . . . . . . . . . . . . . . . 110
10.2.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . 111
10.2.2. SASL . . . . . . . . . . . . . . . . . . . . . . . . 112
10.2.3. Stanza Exchange . . . . . . . . . . . . . . . . . . 113
10.2.4. Close . . . . . . . . . . . . . . . . . . . . . . . 114
11. Server Rules for Processing XML Stanzas . . . . . . . . . . . 114
11.1. No 'to' Address . . . . . . . . . . . . . . . . . . . . 115
11.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . 115
11.1.2. Message . . . . . . . . . . . . . . . . . . . . . . 115
11.1.3. Presence . . . . . . . . . . . . . . . . . . . . . . 115
11.1.4. IQ . . . . . . . . . . . . . . . . . . . . . . . . . 115
11.2. Local Domain . . . . . . . . . . . . . . . . . . . . . . 116
11.2.1. Mere Domain . . . . . . . . . . . . . . . . . . . . 116
11.2.2. Domain with Resource . . . . . . . . . . . . . . . . 116
11.2.3. Localpart at Domain . . . . . . . . . . . . . . . . 116
11.2.3.1. No Such User . . . . . . . . . . . . . . . . . . 116
11.2.3.2. Bare JID . . . . . . . . . . . . . . . . . . . . 117
11.2.3.3. Full JID . . . . . . . . . . . . . . . . . . . . 117
11.3. Foreign Domain . . . . . . . . . . . . . . . . . . . . . 117
11.3.1. Existing Stream . . . . . . . . . . . . . . . . . . 117
11.3.2. No Existing Stream . . . . . . . . . . . . . . . . . 118
11.3.3. Error Handling . . . . . . . . . . . . . . . . . . . 118
12. XML Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 118
12.1. Restrictions . . . . . . . . . . . . . . . . . . . . . . 118
12.2. XML Namespace Names and Prefixes . . . . . . . . . . . . 119
12.2.1. Streams Namespace . . . . . . . . . . . . . . . . . 119
12.2.2. Default Namespace . . . . . . . . . . . . . . . . . 120
12.2.3. Extended Namespaces . . . . . . . . . . . . . . . . 121
12.3. Well-Formedness . . . . . . . . . . . . . . . . . . . . 122
12.4. Validation . . . . . . . . . . . . . . . . . . . . . . . 122
12.5. Inclusion of Text Declaration . . . . . . . . . . . . . 123
12.6. Character Encoding . . . . . . . . . . . . . . . . . . . 123
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12.7. Whitespace . . . . . . . . . . . . . . . . . . . . . . . 123
12.8. XML Versions . . . . . . . . . . . . . . . . . . . . . . 123
13. Compliance Requirements . . . . . . . . . . . . . . . . . . . 123
13.1. Servers . . . . . . . . . . . . . . . . . . . . . . . . 124
13.2. Clients . . . . . . . . . . . . . . . . . . . . . . . . 124
14. Internationalization Considerations . . . . . . . . . . . . . 125
15. Security Considerations . . . . . . . . . . . . . . . . . . . 125
15.1. High Security . . . . . . . . . . . . . . . . . . . . . 125
15.2. Certificates . . . . . . . . . . . . . . . . . . . . . . 126
15.2.1. Certificate Generation . . . . . . . . . . . . . . . 126
15.2.1.1. Server Certificates . . . . . . . . . . . . . . . 126
15.2.1.2. Client Certificates . . . . . . . . . . . . . . . 128
15.2.1.3. ASN.1 Object Identifier . . . . . . . . . . . . . 128
15.2.2. Certificate Validation . . . . . . . . . . . . . . . 129
15.2.2.1. Server Certificates . . . . . . . . . . . . . . . 129
15.2.2.2. Client Certificates . . . . . . . . . . . . . . . 131
15.2.2.3. Use of Certificates in XMPP Extensions . . . . . 132
15.3. Client-to-Server Communication . . . . . . . . . . . . . 132
15.4. Server-to-Server Communication . . . . . . . . . . . . . 133
15.5. Order of Layers . . . . . . . . . . . . . . . . . . . . 133
15.6. Mandatory-to-Implement Technologies . . . . . . . . . . 133
15.7. Hash Function Agility . . . . . . . . . . . . . . . . . 134
15.8. SASL Downgrade Attacks . . . . . . . . . . . . . . . . . 134
15.9. Lack of SASL Channel Binding to TLS . . . . . . . . . . 134
15.10. Use of base64 in SASL . . . . . . . . . . . . . . . . . 135
15.11. Stringprep Profiles . . . . . . . . . . . . . . . . . . 135
15.12. Address Spoofing . . . . . . . . . . . . . . . . . . . . 136
15.12.1. Address Forging . . . . . . . . . . . . . . . . . . 136
15.12.2. Address Mimicking . . . . . . . . . . . . . . . . . 137
15.13. Firewalls . . . . . . . . . . . . . . . . . . . . . . . 138
15.14. Denial of Service . . . . . . . . . . . . . . . . . . . 138
15.15. Presence Leaks . . . . . . . . . . . . . . . . . . . . . 139
15.16. Directory Harvesting . . . . . . . . . . . . . . . . . . 140
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 140
16.1. XML Namespace Name for TLS Data . . . . . . . . . . . . 140
16.2. XML Namespace Name for SASL Data . . . . . . . . . . . . 140
16.3. XML Namespace Name for Stream Errors . . . . . . . . . . 141
16.4. XML Namespace Name for Resource Binding . . . . . . . . 141
16.5. XML Namespace Name for Stanza Errors . . . . . . . . . . 141
16.6. Nodeprep Profile of Stringprep . . . . . . . . . . . . . 142
16.7. Resourceprep Profile of Stringprep . . . . . . . . . . . 142
16.8. GSSAPI Service Name . . . . . . . . . . . . . . . . . . 142
16.9. Port Numbers . . . . . . . . . . . . . . . . . . . . . . 142
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 143
17.1. Normative References . . . . . . . . . . . . . . . . . . 143
17.2. Informative References . . . . . . . . . . . . . . . . . 145
Appendix A. Nodeprep . . . . . . . . . . . . . . . . . . . . . . 149
A.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 149
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A.2. Character Repertoire . . . . . . . . . . . . . . . . . . 149
A.3. Mapping . . . . . . . . . . . . . . . . . . . . . . . . 150
A.4. Normalization . . . . . . . . . . . . . . . . . . . . . 150
A.5. Prohibited Output . . . . . . . . . . . . . . . . . . . 150
A.6. Bidirectional Characters . . . . . . . . . . . . . . . . 150
A.7. Notes . . . . . . . . . . . . . . . . . . . . . . . . . 151
Appendix B. Resourceprep . . . . . . . . . . . . . . . . . . . . 151
B.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 151
B.2. Character Repertoire . . . . . . . . . . . . . . . . . . 152
B.3. Mapping . . . . . . . . . . . . . . . . . . . . . . . . 152
B.4. Normalization . . . . . . . . . . . . . . . . . . . . . 152
B.5. Prohibited Output . . . . . . . . . . . . . . . . . . . 152
B.6. Bidirectional Characters . . . . . . . . . . . . . . . . 152
Appendix C. XML Schemas . . . . . . . . . . . . . . . . . . . . 153
C.1. Streams Namespace . . . . . . . . . . . . . . . . . . . 153
C.2. Stream Error Namespace . . . . . . . . . . . . . . . . . 154
C.3. STARTTLS Namespace . . . . . . . . . . . . . . . . . . . 157
C.4. SASL Namespace . . . . . . . . . . . . . . . . . . . . . 157
C.5. Resource Binding Namespace . . . . . . . . . . . . . . . 160
C.6. Stanza Error Namespace . . . . . . . . . . . . . . . . . 160
Appendix D. Contact Addresses . . . . . . . . . . . . . . . . . 162
Appendix E. Account Provisioning . . . . . . . . . . . . . . . . 162
Appendix F. Differences From RFC 3920 . . . . . . . . . . . . . 163
Appendix G. Copying Conditions . . . . . . . . . . . . . . . . . 164
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 165
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1. Introduction
1.1. Overview
The Extensible Messaging and Presence Protocol (XMPP) is an
application profile of the Extensible Markup Language [XML] for
streaming XML data in close to real time between any two (or more)
network-aware entities. XMPP is typically used to exchange messages,
share presence information, and engage in structured request-response
interactions. The basic syntax and semantics of XMPP were developed
originally within the Jabber open-source community, mainly in 1999.
In late 2002, the XMPP Working Group was chartered with developing an
adaptation of the core Jabber protocol that would be suitable as an
IETF instant messaging (IM) and presence technology. As a result of
work by the XMPP WG, [RFC3920] and [RFC3921] were published in
October 2004, representing the most complete definition of XMPP at
that time.
As a result of extensive implementation and deployment experience
with XMPP since 2004, as well as more formal interoperability testing
carried out under the auspices of the XMPP Standards Foundation
(XSF), this document reflects consensus from the XMPP developer
community regarding XMPP's core XML streaming technology. In
particular, this document incorporates the following backward-
compatible changes from RFC 3920:
o Incorporated corrections and errata
o Added examples throughout
o Clarified and more completely specified matters that were
underspecified
o Modified text to reflect updated technologies for which XMPP is a
using protocol, e.g., Transport Layer Security (TLS) and the
Simple Authentication and Security Layer (SASL)
o Defined several additional stream, stanza, and SASL error
conditions
o Removed the deprecated DIGEST-MD5 SASL mechanism [DIGEST-MD5] as a
mandatory-to-implement technology
o Added the TLS plus the SASL PLAIN mechanism [PLAIN] as a
mandatory-to-implement technology
o Defined of optional support for multiple resources over the same
connection
o Transferred historical documentation for the server dialback
protocol from this specification to a separate specification
Therefore, this document defines the core features of XMPP 1.0, thus
obsoleting RFC 3920.
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Note: [xmpp-im] defines the XMPP features needed to provide the
basic instant messaging and presence functionality that is
described in [IMP-REQS].
1.2. Functional Summary
This non-normative section provides a developer-friendly, functional
summary of XMPP; refer to the sections that follow for a normative
definition of XMPP.
The purpose of XMPP is to enable the exchange of relatively small
pieces of structured data (called "XML stanzas") over a network
between any two (or more) entities. XMPP is implemented using a
client-server architecture, wherein a client needs to connect to a
server in order to gain access to the network and thus be allowed to
exchange XML stanzas with other entities (which can be associated
with other servers). The process whereby a client connects to a
server, exchanges XML stanzas, and ends the connection is:
1. Determine the hostname and port at which to connect
2. Open a TCP connection
3. Open an XML stream
4. Complete TLS negotiation for channel encryption (recommended)
5. Complete SASL negotiation for authentication
6. Bind a resource to the stream
7. Exchange an unbounded number of XML stanzas with other entities
on the network
8. Close the XML stream
9. Close the TCP connection
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. Complete TLS negotiation for channel encryption (recommended)
5. Complete SASL negotiation for authentication *
6. Exchange an unbounded number of XML stanzas both directly for the
servers and indirectly on behalf of entities associated with each
server (e.g., connected clients)
7. Close the XML stream
8. Close the TCP connection
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* 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].
In the sections following discussion of XMPP architecture and XMPP
addresses, this document specifies how clients connect to servers and
specifies the basic semantics of XML stanzas. However, this document
does not define the "payloads" of the XML stanzas that might be
exchanged once a connection is successfully established; instead,
those payloads are defined by various XMPP extensions. For example,
[xmpp-im] defines extensions for basic instant messaging and presence
functionality. In addition, various specifications produced in the
XSF's XEP series [XEP-0001] define extensions for a wide range of
more advanced functionality.
1.3. Conventions
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 [SECTERMS]; 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,
and LF.
Following the "XML Notation" used in [IRI] to represent characters
that cannot be rendered in ASCII-only documents, some examples in
this document use the form "&#x...." as a notational device to
represent Unicode characters (e.g., the string "ř" stands for
the Unicode character LATIN SMALL LETTER R WITH CARON).
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In examples, lines have been wrapped for improved readability,
"[...]" means elision, and the following prepended strings are used
(these prepended strings are not to be sent over the wire):
o C: = a client
o E: = any XMPP entity
o I: = an initiating entity
o P: = a peer server
o R: = a receiving entity
o S: = a server
o S1: = server1
o S2: = server2
1.4. Acknowledgements
The editor of this document finds it impossible to appropriately
acknowledge the many individuals who have provided comments regarding
the protocols defined herein. 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.
1.5. Discussion Venue
The document editor and the broader XMPP developer community welcome
discussion and comments related to the topics presented in this
document. The preferred forum is the <standards@xmpp.org> mailing
list, for which archives and subscription information are available
at <http://mail.jabber.org/mailman/listinfo/standards>.
2. Architecture
2.1. Overview
XMPP assumes a client-server architecture, wherein a client utilizing
XMPP accesses a server (normally over a [TCP] connection) and servers
can also communicate with each other over TCP connections.
A simplified architectural diagram for a typical deployment is shown
here, where the entities have the following significance:
o romeo@example.net -- an XMPP user.
o example.net -- an XMPP server.
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o im.example.com -- an XMPP server.
o juliet@im.example.com -- an XMPP user.
example.net ---------------- im.example.com
| |
| |
romeo@example.net juliet@im.example.com
Note: Architectures that employ XML streams (Section 5) and XML
stanzas (Section 9) but that establish peer-to-peer connections
directly between clients using technologies based on [LINKLOCAL]
have been deployed, but such architectures are not XMPP and are
best described as "XMPP-like"; for details, see [XEP-0174]. 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.
2.2. Server
A SERVER is an entity whose primary responsibilities are to:
o Manage XML streams (Section 5) with local clients and deliver XML
stanzas (Section 9) to those clients over the negotiated XML
streams.
o Subject to local service policies on server-to-server
communication, manage XML streams (Section 5) with foreign servers
and route XML stanzas (Section 9) to those servers over the
negotiated XML streams.
Depending on the application, the secondary responsibilities of an
XMPP server 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 foreign server or delivered to a local entity.
o Hosting local services that also use XMPP as the basis for
communication but that provide additional functionality beyond
that defined in this document or in [xmpp-im]; examples include
multi-user conferencing services as specified in [XEP-0045] and
publish-subscribe services as specified in [XEP-0060].
2.3. Client
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
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completes resource binding (Section 8) in order to enable delivery of
XML stanzas via the server to the client. A client then uses XMPP to
communicate with its server, other clients, and any other accessible
entities on a network. Multiple clients can connect simultaneously
to a server on behalf of a local account, where each client is
differentiated by the resource identifier portion of an XMPP address
(e.g., <localpart@domain/home> vs. <localpart@domain/work>), as
defined under Section 3 and Section 8. The RECOMMENDED port for TCP
connections between a client and a server is 5222, as registered with
the IANA (see Section 16.9).
2.4. Network
Because each server is identified by a network address and because
server-to-server communication is a straightforward extension of the
client-to-server protocol, in practice the system consists of a
network of servers that inter-communicate. Thus, for example,
<juliet@im.example.com> is able to exchange messages, presence, and
other information with <romeo@example.net>. This pattern is familiar
from messaging protocols (such as [SMTP]) that make use of network
addressing standards. Communication between any two servers is
OPTIONAL. The RECOMMENDED port for TCP connections between servers
is 5269, as registered with the IANA (see Section 16.9).
3. Addresses
3.1. Overview
An ENTITY is anything that is network-addressable and that can
communicate using XMPP. For historical reasons, the native address
of an XMPP entity is called a JABBER IDENTIFIER or JID. A valid JID
contains a set of ordered elements formed of an XMPP localpart,
domain identifier, and resource identifier.
The syntax for a JID is defined as follows using the Augmented
Backus-Naur Form as specified in [ABNF].
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jid = [ localpart "@" ] domain [ "/" resource ]
localpart = 1*(nodepoint)
; a "nodepoint" is a UTF-8 encoded Unicode code
; point that satisfies the Nodeprep profile of
; stringprep
domain = fqdn / address-literal
fqdn = *(ldhlabel ".") toplabel
ldhlabel = letdig [*61(ldh) letdig]
toplabel = ALPHA *61(ldh) letdig
letdig = ALPHA / DIGIT
ldh = ALPHA / DIGIT / "-"
address-literal = IPv4address / IPv6address
; the "IPv4address" and "IPv6address" rules are
; defined in RFC 3986
resource = 1*(resourcepoint)
; a "resourcepoint" is a UTF-8 encoded Unicode
; code point that satisfies the Resourceprep
; profile of stringprep
All JIDs are based on the foregoing structure. One common use of
this structure is to identify a messaging and presence account, the
server that hosts the account, and a connected resource (e.g., a
specific device) in the form of <localpart@domain/resource>.
However, localparts other than clients are possible; for example, a
specific chat room offered by a multi-user conference service (see
[XEP-0045]) could be addressed as <room@service> (where "room" is the
name of the chat room and "service" is the hostname of the multi-user
conference service) and a specific occupant of such a room could be
addressed as <room@service/nick> (where "nick" is the occupant's room
nickname). Many other JID types are possible (e.g., <domain/
resource> could be a server-side script or service).
Each allowable portion of a JID (localpart, domain identifier, and
resource identifier) MUST NOT be more than 1023 bytes in length,
resulting in a maximum total size (including the '@' and '/'
separators) of 3071 bytes.
Note: While the format of a JID is consistent with [URI], an
entity's address on an XMPP network MUST be represented as a JID
(without a URI scheme) and not a [URI] or [IRI] as specified in
[XMPP-URI]; the latter specification is provided only for
identification and interaction outside the context of the XMPP
wire protocol itself.
3.2. Domain Identifier
The DOMAIN IDENTIFIER portion of a JID is that portion after the '@'
character (if any) and before the '/' character (if any); it is the
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primary identifier and is the only REQUIRED element of a JID (a mere
domain identifier is a valid JID). Typically a domain identifier
identifies the "home" server to which clients connect for XML routing
and data management functionality. However, it is not necessary for
an XMPP domain identifier to identify an entity that provides core
XMPP server functionality (e.g., a domain identifier can identity an
entity such as a multi-user conference service, a publish-subscribe
service, or a user directory).
Note: A single server can service multiple domain identifiers,
i.e., multiple local domains; this is typically referred to as
virtual hosting.
The domain identifier for every server or service that will
communicate over a network SHOULD be a fully qualified domain name
(see [DNS]); while the domain identifier MAY be either an Internet
Protocol (IPv4 or IPv6) address or a text label that is resolvable on
a local network (commonly called an "unqualified hostname"), it is
possible that domain identifiers that are IP addresses will not be
acceptable to other services for the sake of interdomain
communication. Furthermore, domain identifiers that are unqualified
hostnames MUST NOT be used on public networks but MAY be used on
private networks.
Note: If the domain identifier includes a final character
considered to be a label separator (dot) by [IDNA] or [DNS], this
character MUST be stripped from the domain identifier before the
JID of which it is a part is used for the purpose of routing an
XML stanza, comparing against another JID, or constructing an
[XMPP-URI]; in particular, the character MUST be stripped before
any other canonicalization steps are taken, such as application of
the [NAMEPREP] profile of [STRINGPREP] or completion of the
ToASCII operation as described in [IDNA].
A domain identifier MUST be an "internationalized domain name" as
defined in [IDNA], that is, "a domain name in which every label is an
internationalized label". When preparing a text label (consisting of
a sequence of Unicode code points) for representation as an
internationalized label in the process of constructing an XMPP domain
identifier or comparing two XMPP domain identifiers, an application
MUST ensure that for each text label it is possible to apply without
failing the ToASCII operation specified in [IDNA] with the
UseSTD3ASCIIRules flag set (thus forbidding ASCII code points other
than letters, digits, and hyphens). If the ToASCII operation can be
applied without failing, then the label is an internationalized
label. An internationalized domain name (and therefore an XMPP
domain identifier) is constructed from its constituent
internationalized labels by following the rules specified in [IDNA].
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Note: The ToASCII operation includes application of the [NAMEPREP]
profile of [STRINGPREP] and encoding using the algorithm specified
in [PUNYCODE]; for details, see [IDNA]. Although the output of
the ToASCII operation is not used in XMPP, it MUST be possible to
apply that operation without failing.
3.3. Localpart
The LOCALPART of a JID is an optional identifier placed before the
domain identifier and separated from the latter by the '@' character.
Typically a localpart uniquely identifies the entity requesting and
using network access provided by a server (i.e., a local account),
although it can also represent other kinds of entities (e.g., a chat
room associated with a multi-user conference service). The entity
represented by an XMPP localpart is addressed within the context of a
specific domain.
A localpart MUST NOT be zero bytes in length and, as for all portions
of a JID, MUST NOT be more than 1023 bytes in length.
A localpart MUST be formatted such that the Nodeprep profile of
[STRINGPREP] can be applied without failing (see Appendix A). Before
comparing two localparts, an application MUST first ensure that the
Nodeprep profile has been applied to each identifier (the profile
need not be applied each time a comparison is made, as long as it has
been applied before comparison).
3.4. Resource Identifier
The RESOURCE IDENTIFIER portion of a JID is an optional identifier
placed after the domain identifier and separated from the latter by
the '/' character. A resource identifier can modify either a
<localpart@domain> address or a mere <domain> address. Typically a
resource identifier uniquely identifies a specific connection (e.g.,
a device or location) or object (e.g., a participant in a multi-user
conference room) belonging to the entity associated with an XMPP
localpart at a local domain.
When an XMPP address does not include a resource identifier (i.e.,
when it is of the form <domain> or <localpart@domain>), it is
referred to as a BARE JID. When an XMPP address includes a resource
identifier (i.e., when it is of the form <domain/resource> or
<localpart@domain/resource>), is referred to as a FULL JID.
A resource identifier MUST NOT be zero bytes in length and, as for
all portions of a JID, MUST NOT be more than 1023 bytes in length.
A resource identifier MUST be formatted such that the Resourceprep
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profile of [STRINGPREP] can be applied without failing (see
Appendix B). Before comparing two resource identifiers, an
application MUST first ensure that the Resourceprep profile has been
applied to each identifier (the profile need not be applied each time
a comparison is made, as long as it has been applied before
comparison).
Note: For historical reasons, the term "resource identifier" is
used in XMPP to refer to the optional portion of an XMPP address
that follows the domain identifier and the "/" separator
character; this use of the term "resource identifier" is not to be
confused with the meanings of "resource" and "identifier" provided
in Section 1.1 of [URI].
XMPP entities SHOULD consider resource identifiers to be opaque
strings and SHOULD NOT impute meaning to any given resource
identifier. In paticular, the use of the '/' character as a
separator between the domain identifier and the resource identifier
does not imply that resource identifiers are hierarchical in the way
that, say, HTTP addresses are hierarchical; thus for example an XMPP
address of the form <localpart@domain/foo/bar> does not identify a
resource "bar" that exists below a resource "foo" in a hierarchy of
resources associated with the entity "localpart@domain".
3.5. Determination of Addresses
After the parties to an XML stream have completed the appropriate
aspects of stream negotiation (typically SASL negotiation (Section 7)
and, if appropriate, resource binding (Section 8)) the receiving
entity for a stream MUST determine the initiating entity's JID.
For server-to-server communication, the initiating server's JID MUST
be the authorization identity (as defined by [SASL]), either (1) as
directly communicated by the initiating server during SASL
negotiation (Section 7) or (2) as derived by the receiving server
from the authentication identity if no authorization identity was
specified during SASL negotiation (Section 7). (For information
about the determination of addresses in the absence of SASL
negotiation when the older server dialback protocol is used, see
[XEP-0220].)
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 7) or (2) as derived by the server from the
authentication identity if no authorization identity was specified
during SASL negotiation (Section 7). The resource identifier portion
of the full JID (<localpart@domain/resource>) MUST be the resource
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identifier negotiated by the client and server during resource
binding (Section 8).
The receiving entity MUST ensure that the resulting JID (including
localpart, domain identifier, resource identifier, and separator
characters) conforms to the rules and formats defined earlier in this
section; to meet this restriction, the receiving entity MAY replace
the JID sent by the initiating entity with the canonicalized JID as
determined by the receiving entity.
4. TCP Binding
4.1. Scope
As XMPP is defined in this specification, an initiating entity
(client or server) MUST open a Transmission Control Protocol [TCP]
connection at the receiving entity (server) before it negotiates XML
streams with the receiving entity. The rules specified in the
following sections apply to the TCP binding.
4.2. Hostname Resolution
Before opening the TCP connection, the initiating entity first MUST
resolve the Domain Name System (DNS) hostname associated with the
receiving entity and determine the appropriate TCP port for
communication with the receiving entity. The process is:
1. Attempt to resolve the hostname using (a) a [DNS-SRV] Service of
"xmpp-client" (for client-to-server connections) or "xmpp-server"
(for server-to-server connections) and (b) a Proto of "tcp",
resulting in resource records such as "_xmpp-
client._tcp.example.net." or "_xmpp-server._tcp.im.example.com.".
The result of the SRV lookup will be one or more combinations of
a port and hostname, which hostnames the initiating entity MUST
resolve according to returned SRV record weight (if the result of
the SRV lookup is a single RR with a Target of ".", i.e. the root
domain, the initiating entity MUST abort SRV processing but
SHOULD attempt a fallback resolution as described below). The
initiating entity uses the IP address(es) from the first
successfully resolved hostname (with the corresponding port
number returned by the SRV lookup) in order to connect to the
receiving entity. If the initiating entity fails to connect
using one of the IP addresses, the initiating entity uses the
next resolved IP address to connect. If the initiating entity
fails to connect using all resolved IP addresses, then the
initiating entity repeats the process of resolution and
connection for the next hostname returned by the SRV lookup.
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2. If the SRV lookup aborts or fails, the fallback SHOULD be a
normal IPv4 or IPv6 address record resolution to determine the IP
address, where the port used is the "xmpp-client" port of 5222
for client-to-server connections or the "xmpp-server" port 5269
for server-to-server connections.
3. For client-to-server connections, the fallback MAY be a [DNS-TXT]
lookup for alternative connection methods, for example as
described in [XEP-0156].
Note: If the initiating entity has been explicitly configured to
associate a particular hostname (and potentially port) with the
original hostname of the receiving entity (say, to "hardcode" an
association between an original hostname of example.net and a
configured hostname and of webcm.example.com:80), the initiating
entity SHALL use the configured name instead of performing the
foregoing resolution process on the original name.
Note: Many XMPP servers are implemented in such a way that they
can host additional services (beyond those defined in this
specification and [xmpp-im]) at hostnames that 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
additional 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 additional
services, it needs to advertise the appropriate "_xmpp-server" SRV
records in addition to the "_xmpp-server" record for its main XMPP
service.
4.3. Client-to-Server Communication
Because a client is subordinate to a server and therefore a client
authenticates to the server but the server does not necessarily
authenticate to the client, it is necessary to have only one TCP
connection between client and server. Thus the server MUST allow the
client to share a single TCP connection for XML stanzas sent from
client to server and from server to client (i.e., the inital stream
and response stream as specified under Section 5).
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4.4. Server-to-Server Communication
Because two servers are peers and therefore each peer MUST
authenticate with the other, the servers MUST use two TCP
connections: one for XML stanzas sent from the first server to the
second server and another (initiated by the second server) for XML
stanzas from the second server to the first server.
This rule applies only to XML stanzas (Section 9). Therefore during
STARTTLS negotiation (Section 6) and SASL negotiation (Section 7) the
servers would use one TCP connection, but after stream setup that TCP
connection would be used only for the initiating server to send XML
stanzas to the receiving server. In order for the receiving server
to send XML stanzas to the initiating server, the receiving server
would need to reverse the roles and negotiate an XML stream from the
receiving server to the initiating server.
4.5. 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.
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 require a new stream for
immediate communication (e.g., because the stream was idle and
therefore timed out), it SHOULD NOT assume that is needs to
reconnect immediately.
4.6. Other Bindings
There is no necessary coupling of an XML stream to a TCP connection.
For example, two entities could connect to each other via another
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transport, such as [HTTP] as specified in [XEP-0124] and [XEP-0206].
Although this specification neither encourages nor discourages other
bindings, it defines only a binding of XMPP to TCP.
5. XML Streams
5.1. Overview
Two fundamental concepts make possible the rapid, asynchronous
exchange of relatively small payloads of structured information
between presence-aware entities: XML streams and XML stanzas. These
terms are defined as follows.
Definition of XML Stream: An XML STREAM is a container for the
exchange of XML elements between any two entities over a network.
The start of an XML stream is denoted unambiguously by an opening
STREAM HEADER (i.e., an XML <stream> tag with appropriate
attributes and namespace declarations), while the end of the XML
stream is denoted unambiguously by a closing XML </stream> tag.
During the life of the stream, the entity that initiated it can
send an unbounded number of XML elements over the stream, either
elements used to negotiate the stream (e.g., to complete TLS
negotiation (Section 6) or SASL negotiation (Section 7)) or XML
stanzas. The INITIAL STREAM is negotiated from the initiating
entity (typically a client or server) to the receiving entity
(typically a server), and can be seen as corresponding to the
initiating entity's "connection" or "session" with the receiving
entity. The initial stream enables unidirectional communication
from the initiating entity to the receiving entity; in order to
enable information exchange from the receiving entity to the
initiating entity, the receiving entity MUST negotiate a stream in
the opposite direction (the RESPONSE STREAM).
Definition of XML Stanza: An XML STANZA is a discrete semantic unit
of structured information that is sent from one entity to another
over an XML stream, 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 herein are the
<message/>, <presence/>, and <iq/> elements qualified by the
default namespace for the stream, as described under Section 9;
for example, an XML element sent for the purpose of TLS
negotiation (Section 6) or SASL negotiation (Section 7) is not
considered to be an XML stanza, nor is a stream error or a stream
feature. An XML stanza MAY contain child elements (with
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accompanying attributes, elements, and XML character data) as
necessary in order to convey the desired information, which MAY be
qualified by any XML namespace (see [XML-NAMES] as well as
Section 9.4 herein).
Consider the example of a client's connection to a server. In order
to connect to a server, a client 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 12.5 and Section 12.6). 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 7) and resource binding (Section 8), 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 5.7.
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> |
+--------------------+
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Note: Those who are accustomed to thinking of XML in a document-
centric manner might view a client's connection to a server as
consisting of two open-ended XML documents: one from the client to
the server and one from the server to the client. On this
analogy, the two XML streams can be considered equivalent to two
"documents" (matching production [1] content of [XML]) that are
built up through the accumulation of XML stanzas, the root
<stream/> element can be considered equivalent to the "document
entity" for each "document" (as described in Section 4.8 of
[XML]), and the XML stanzas sent over the streams can be
considered equivalent to "fragments" of the "documents" as
described in [XML-FRAG]. However, this perspective is merely an
analogy; XMPP does not deal in documents and fragments but in
streams and stanzas.
5.2. Stream Security
For the purpose of stream security, both Transport Layer Security
(see Section 6) and the Simple Authentication and Security Layer (see
Section 7) are mandatory to implement. Use of these technologies
results in high security as described under Section 15.1.
The initial stream and the response stream MUST be secured
separately, although security in both directions MAY be established
via mechanisms that provide mutual authentication.
The initiating entity MUST NOT attempt to send XML stanzas
(Section 9) over the stream before the stream has been authenticated.
However, if it does attempt to do so, the receiving entity MUST NOT
accept such stanzas and MUST return a <not-authorized/> stream error.
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 6) or SASL negotiation (Section 7)).
5.3. Stream Attributes
The attributes of the root <stream/> element are defined in the
following sections.
Note: The attributes of the root <stream/> element are not
prepended by a 'stream:' 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."
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5.3.1. from
The 'from' attribute communicates an XMPP identity of the entity
sending the stream element.
Note: It is possible for an entity to have more than one XMPP
identity (e.g., in the case of a server that provides virtual
hosting). It is also possible that an entity does 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]).
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; if not, then it MUST NOT include the 'from' attribute.
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.
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).
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R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
Whether or not the 'from' attribute is included, each entity MUST
verify the identity of the other entity before exchanging XML stanzas
with it (see Section 15.3 and Section 15.4).
Note: It is possible that implementations based on an earlier
revision of this specification will not include the 'from' address
on stream headers; an entity SHOULD be liberal in accepting such
stream headers.
5.3.2. 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.
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.
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R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
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 (see Section 15.3 and Section 15.4).
Note: It is possible that implementations based on an earlier
revision of this specification will not include the 'to' address
on stream headers; an entity SHOULD be liberal in accepting such
stream headers.
5.3.3. id
The 'id' attribute communicates a unique identifier for the stream.
This identifier is called a STREAM ID. The stream ID MUST be
generated by the receiving entity when it sends a response stream
header, MUST BE unique within the receiving application (normally a
server), and 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.
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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'>
5.3.4. 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.
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. 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 an identifier for the initiating entity's preferred
language; 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 but MAY be the
identifier for the default language of the receiving entity; if the
receiving entity does not support the initiating entity's preferred
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language or a closely matching language (or 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.
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 9.1.5, the initiating entity MAY
include the 'xml:lang' attribute in any XML stanzas it sends over the
stream. If the initiating entity does not include the 'xml:lang'
attribute in any such stanza, the receiving entity SHOULD add the
'xml:lang' attribute to the stanza, whose value 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.
5.3.5. 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 6), SASL
negotiation (Section 7), Section 5.5, and stream errors
(Section 5.8).
The version of XMPP specified herein 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
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ignored by recipients and MUST NOT be sent.
The major version number will be incremented only if the stream and
stanza formats or required actions have changed so dramatically that
an older version entity would not be able to interoperate with a
newer version entity if it simply ignored the elements and attributes
it did not understand and took the actions specified in the older
specification.
The minor version number will be incremented only if significant new
capabilities have been added to the core protocol (e.g., a newly
defined value of the 'type' attribute for message, presence, or IQ
stanzas). The minor version number MUST be ignored by an entity with
a smaller minor version number, but MAY be used for informational
purposes by the entity with the larger minor version number (e.g.,
the entity with the larger minor version number would simply note
that its correspondent would not be able to understand that value of
the 'type' attribute and therefore would not send it).
The following rules apply to the generation and handling of the
'version' attribute within stream headers:
1. The initiating entity MUST set the value of the 'version'
attribute in the initial stream header to the highest version
number it supports (e.g., if the highest version number it
supports is that defined in this specification, it MUST set the
value to "1.0").
2. The receiving entity MUST set the value of the 'version'
attribute in the response stream header to either the value
supplied by the initiating entity or the highest version number
supported by the receiving entity, whichever is lower. The
receiving entity MUST perform a numeric comparison on the major
and minor version numbers, not a string match on
"<major>.<minor>".
3. If the version number included in the response stream header is
at least one major version lower than the version number included
in the initial stream header and newer version entities cannot
interoperate with older version entities as described, the
initiating entity SHOULD 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.
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5.3.6. 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 |
+----------+--------------------------+-------------------------+
5.4. Namespace Declarations
The stream element MUST possess both a streams namespace declaration
and a default namespace declaration (as "namespace declaration" is
defined in [XML-NAMES]). For detailed information regarding the
streams namespace and default namespace, see Section 12.2.
5.5. Stream Features
If the initiating entity includes the 'version' attribute set to a
value of at least "1.0" in the initial stream header, after sending
the response stream header the receiving entity MUST send a
<features/> child element (prefixed by the streams namespace prefix)
to the initiating entity in order to announce any stream-level
features that can be negotiated or capabilities that otherwise need
to be advertised.
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'>
R: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
Stream features are used mainly to advertise TLS negotiation
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(Section 6), SASL negotiation (Section 7), and resource binding
(Section 8); however, stream features also can be used to advertise
features associated with various XMPP extensions.
If it is mandatory for a feature to be successfully negotiated before
the initiating entity will be allowed to proceed with the sending of
XML stanzas or with further steps of the stream negotiation, the
advertisement of that feature SHOULD include an empty <required/>
child element but MAY include neither a <required/> element not an
<optional/> element (i.e., features default to required).
R: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<required/>
</bind>
</stream:features>
If successful negotiation of a feature is discretionary, the
advertisement of that feature MUST include an empty <optional/> child
element.
R: <stream:features>
<session xmlns='urn:ietf:params:xml:ns:xmpp-session'>
<optional/>
</session>
</stream:features>
If an entity does not understand or support a feature that has been
advertised, it MUST still inspect the feature advertisement to
determine if negotiation of the feature is mandatory. If negotiation
of an unsupported feature is mandatory (as determined by inclusion of
the <required/> child element or the absence of an <optional/> child
element), then the entity MUST abort the stream negotiation process.
If negotiation of an unsupported feature is discretionary (as
determined by inclusion of the <optional/> child element or the
absence of a child element), the entity MUST silently ignore the
associated feature advertisement and proceed with the stream
negotiation process.
Note: Implementations based on an earlier revision of this
specification do not include the <optional/> child element and
they include the <required/> child element only in the case of the
STARTTLS feature. Entities MUST accept stream feature
advertisements without the child elements, and SHOULD consider
consider negotiation of such features to be discretionary.
If it is necessary for a feature to be successfully negotiated before
the initiating entity is allowed to proceed with the sending a non-
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security-related feature or with further steps of the stream
negotiation, the receiving entity SHOULD NOT advertise any other
stream features until the mandatory feature has been successfully
negotiated; however, if the mandatory feature is security-critical
(e.g., STARTTLS or SASL) then the receiving entity MUST NOT advertise
any other stream features until the security-critical feature has
been successfully negotiated.
The order of child elements contained in any given <features/>
element is not significant.
After completing negotiation of any stream feature (even stream
features that do not require a stream restart), the receiving entity
MUST send an updated list of stream features to the initiating
entity. However, if there are no features to be advertised then the
receiving entity MUST send an empty <features/> element.
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'>
R: <stream:features/>
At any time after stream establishment, the receiving entity MAY send
additional or modified stream feature advertisements (e.g., because a
new feature has been enabled).
5.6. Restarts During Stream Negotiation
Certain stream features require the initiating entity to send a new
initial stream header on successful negotiation of the feature (e.g.,
after successful negotiation of TLS or SASL). Both parties MUST
consider the previous stream to be replaced on successful feature
negotiation 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).
When the receiving entity receives the new initial stream header, it
MUST generate a new stream ID (instead of re-using the old stream ID)
before sending a new response stream header.
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5.7. Closing a Stream
An XML stream between two entities can be closed because a 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 stream
error has occurred.
A stream is closed by sending a closing </stream> tag over the TCP
connection.
S: </stream:stream>
After an entity sends a closing stream tag, it MUST NOT send further
data over that stream.
5.7.1. With Stream Error
If a stream error has occurred, the entity that detects the error
MUST close the stream as described under Section 5.8.1.
5.7.2. 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.
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).
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.
5.7.3. 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
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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
communications 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 12.7; the sending of such a space character is properly
called a WHITESPACE KEEPALIVE (although the term "whitespace ping" is
often used, in fact 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 5.7.2.
5.8. Stream Errors
The root stream element MAY contain an <error/> child element that is
prefixed by the streams namespace prefix. The error child shall be
sent by a compliant entity if it perceives that a stream-level error
has occurred.
5.8.1. Rules
The following rules apply to stream-level errors.
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5.8.1.1. Stream Errors Are Unrecoverable
Stream-level errors are unrecoverable. Therefore, if an error occurs
at the level of the stream, the entity that detects the error MUST
send a <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></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 it MAY wait until
after it receives a closing </stream> tag from the entity to which it
sent the stream error.
C: </stream:stream>
5.8.1.2. 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>
5.8.1.3. 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>
5.8.1.4. Where Stream Errors Are Sent
When two XML streams are used between the initiating entity and the
receiving entity (one in each direction) rather than using a single
bidirectional stream, stanza errors triggered by stanzas sent over
the outbound stream are returned on the inbound stream (since they
are inbound stanzas from the perspective of the entity that sent the
triggering stanza), whereas stream errors related to the outbound
stream are returned on the outbound stream (since they are not
inbound stanzas from the perspective of the entity that sent the
triggering stanza but strictly related to the outbound stream
itself); the same is true, naturally, of any stream errors that are
related to the outbound stream but not triggered by an outbound
stanza.
5.8.2. Syntax
The syntax for stream errors is as follows, where "defined-condition"
is a placeholder for one of the conditions defined under
Section 5.8.3.
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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 5.8.3); this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace.
o MAY contain a <text/> child element containing XML character data
that describes the error in more detail; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace
and SHOULD possess an 'xml:lang' attribute specifying the natural
language of the XML character data.
o MAY contain a child element for an application-specific error
condition; this element MUST be qualified by an application-
defined namespace, and its structure is defined by that namespace
(see Section 5.8.4).
The <text/> element is OPTIONAL. If included, it 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).
5.8.3. Defined Stream Error Conditions
The following stream-level error conditions are defined.
5.8.3.1. bad-format
The entity has sent XML that cannot be processed.
(In the following example, the client sends an XMPP message that is
not well-formed XML.)
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C: <message>
<body>No closing body tag!
</message>
S: <stream:error>
<bad-format
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
This error MAY be used instead of the more specific XML-related
errors, such as <bad-namespace-prefix/>, <invalid-xml/>, <restricted-
xml/>, <unsupported-encoding/>, and <xml-not-well-formed/>. However,
the more specific errors are RECOMMENDED.
5.8.3.2. bad-namespace-prefix
The entity has sent a namespace prefix that is unsupported, or has
sent no namespace prefix on an element that requires such a prefix
(see Section 12.2).
(In the following example, the client specifies a namespace prefix of
"foobar" for the XML streams namespace.)
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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5.8.3.3. conflict
The server is either (1) closing the existing stream for this entity
because a new stream has been initiated that conflicts with the
existing stream, or (2) is refusing a new stream for this entity
because allowing the new stream would conflict with an existing
stream (e.g., because the server allows only a certain number of
connections from the same IP address).
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>
5.8.3.4. connection-timeout
The entity has not generated any traffic over the stream for some
period of time (configurable according to a local service policy) and
therefore the connection is being dropped.
P: <stream:error>
<connection-timeout
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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5.8.3.5. host-gone
The value of the 'to' attribute provided in the initial stream header
corresponds to a hostname that is no longer 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>
5.8.3.6. 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>
5.8.3.7. 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.)
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>
5.8.3.8. internal-server-error
The server has experienced a misconfiguration or an otherwise-
undefined internal error that prevents it from servicing the stream.
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S: <stream:error>
<internal-server-error
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.8.3.9. invalid-from
The JID or hostname provided in a 'from' address 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>
5.8.3.10. invalid-id
The stream ID or server dialback ID is invalid or does not match an
ID previously provided.
(In the following example, the server dialback ID is invalid; see
[XEP-0220].)
P: <db:verify
from='example.net'
to='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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5.8.3.11. invalid-namespace
The streams namespace name is something other than
"http://etherx.jabber.org/streams" (see Section 12.2) 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 streams namespace
of 'http://wrong.namespace.example.org/'.)
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>
5.8.3.12. invalid-xml
The entity has sent invalid XML over the stream to a server that
performs validation (see Section 12.4).
(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='some-id'
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>
5.8.3.13. not-authorized
The entity has attempted to send XML stanzas before the stream has
been authenticated, or otherwise is not authorized to perform an
action related to stream negotiation; the receiving entity MUST NOT
process the offending stanza before sending the stream error.
(In the following example, the client attempts to send XML stanzas
before authenticating with the server.)
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C: <?xml version='1.0'?>
<stream:stream
from='juliet@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>
5.8.3.14. policy-violation
The entity has violated some local service policy (e.g., the stanza
exceeds a configured size limit); the server MAY choose to specify
the policy in the <text/> element or 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'/>
</stream:error>
S: </stream:stream>
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5.8.3.15. remote-connection-failed
The server is unable to properly connect to a remote entity that is
required for authentication or authorization, 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>
5.8.3.16. resource-constraint
The server lacks the system resources necessary to service the
stream.
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C: <?xml version='1.0'?>
<stream:stream
from='juliet@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>
5.8.3.17. 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 12.1).
(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>
5.8.3.18. see-other-host
The server will not provide service to the initiating entity but is
redirecting traffic to another host; the XML character data of the
<see-other-host/> element returned by the server SHOULD specify the
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alternate hostname or IP address at which to connect, which SHOULD be
a valid domain identifier but MAY also include a port number. 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 domain identifier but MAY assume that it can connect to
that domain identifier at the standard XMPP ports (i.e., 5222 for
client-to-server connections and 5269 for server-to-server
connections).
C: <?xml version='1.0'?>
<stream:stream
from='juliet@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>
5.8.3.19. system-shutdown
The server is being shut down and all active streams are being
closed.
S: <stream:error>
<system-shutdown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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5.8.3.20. undefined-condition
The error condition is not one of those defined by the other
conditions in this list; this error condition SHOULD be used only in
conjunction with an application-specific condition.
S: <stream:error>
<undefined-condition
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<app-error xmlns='http://example.com/ns'/>
</stream:error>
</stream:stream>
5.8.3.21. unsupported-encoding
The initiating entity has encoded the stream in an encoding that is
not supported by the server (see Section 12.6) or has otherwise
improperly encoded the stream (e.g., by violating the rules of the
[UTF-8] encoding).
(In the following example, the client attempts to encode data using
UTF-16 instead of UTF-8.)
C: <?xml version='1.0' encoding='UTF-16'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
<stream:error>
<unsupported-encoding
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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5.8.3.22. unsupported-stanza-type
The initiating entity has sent a first-level child of the stream that
is not supported by the server or consistent with the default
namespace.
(In the following example, the client attempts to send an XML stanza
of <pubsub/> when the default namespace is "jabber:client".)
C: <pubsub>
<publish node='princely_musings'>
<item id='ae890ac52d0df67ed7cfdf51b644e901'>
<entry xmlns='http://www.w3.org/2005/Atom'>
<title>Soliloquy</title>
<summary>
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?
</summary>
<link rel='alternate' type='text/html'
href='http://denmark.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>
5.8.3.23. unsupported-version
The value of the 'version' attribute provided by the initiating
entity in the stream header specifies a version of XMPP that is not
supported by the server; the server MAY specify the version(s) it
supports in the <text/> element.
(In the following example, the client specifies an XMPP version of
"11.0" but the server supports only version "1.0" and "1.1".)
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C: <?xml version='1.0'?>
<stream:stream
from='juliet@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>
5.8.3.24. 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 well-formed XML.)
C: <message>
<body>No closing body tag!
</message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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5.8.4. Application-Specific Conditions
As noted, an application MAY provide application-specific stream
error information by including a properly-namespaced child in the
error element. The application-specific element SHOULD supplement or
further qualify a defined element. Thus the <error/> element will
contain two or three child elements.
C: <message>
<body>
My keyboard layout is:
QWERTYUIOP{}|
ASDFGHJKL:"
ZXCVBNM<>?
</body>
</message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<text xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
Some special application diagnostic information!
</text>
<escape-your-data xmlns='http://example.com/ns'/>
</stream:error>
</stream:stream>
5.9. 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 body tag!
</message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
More detailed examples are provided under Section 10.
6. STARTTLS Negotiation
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6.1. Overview
XMPP includes a method for securing the stream from tampering and
eavesdropping. This channel encryption method makes use of the
Transport Layer Security [TLS] protocol, specifically a "STARTTLS"
extension that is modelled after similar extensions for the [IMAP],
[POP3], and [ACAP] protocols as described in [USINGTLS]. The XML
namespace name for the STARTTLS extension is
'urn:ietf:params:xml:ns:xmpp-tls'.
Support for STARTTLS is REQUIRED in XMPP client and server
implementations. An administrator of a given deployment MAY require
the use of TLS for client-to-server communication, server-to-server
communication, or both. A deployed client SHOULD use TLS to secure
its stream with a server prior to attempting the completion of SASL
negotiation (Section 7), and deployed servers SHOULD use TLS between
two domains for the purpose of securing server-to-server
communication.
6.2. Rules
6.2.1. Data Formatting
During STARTTLS negotiation, the entities MUST NOT send any
whitespace within the root stream element 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 precision. Any such whitespace shown in
the STARTTLS examples provided in this document is included only for
the sake of readability.
6.2.2. Order of Negotiation
If the initiating entity chooses to use TLS, STARTTLS negotiation
MUST be completed before proceeding to SASL negotiation (Section 7);
this order of negotiation is required to help safeguard
authentication information sent during SASL negotiation, as well as
to make it possible to base the use of the SASL EXTERNAL mechanism on
a certificate (or other credentials) provided during prior TLS
negotiation.
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6.3. Process
6.3.1. Exchange of Stream Headers and Stream Features
The initiating entity resolves the hostname of the receiving entity
as specified under Section 4, opens a TCP connection to the
advertised port at the resolved IP address, and sends an initial
stream header to the receiving entity; if the initiating entity is
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 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
discretionary, the <starttls/> element MUST contain an empty
<optional/> child element.
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R: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<optional/>
</starttls>
</stream:features>
If the receiving entity considers STARTTLS negotiation to be
mandatory, the <starttls/> element MUST contain an empty <required/>
child element.
R: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
6.3.2. Initiation of STARTTLS Negotiation
6.3.2.1. STARTTLS Command
In order to begin the STARTTLS negotiation, the initiating entity
issues the STARTTLS command (i.e., a <starttls/> element qualified by
the 'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct the
receiving entity that it wishes to begin a STARTTLS negotiation to
secure the stream.
I: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
The receiving entity MUST reply with either a <proceed/> element
(proceed case) or a <failure/> element (failure case) qualified by
the 'urn:ietf:params:xml:ns:xmpp-tls' namespace.
6.3.2.2. Failure Case
If the failure case occurs, the receiving entity MUST return a
<failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
namespace, terminate the XML stream, and terminate the underlying TCP
connection.
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.
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2. The receiving entity does not offer STARTTLS negotiation either
temporarily or permanently.
3. The receiving entity cannot complete STARTTLS negotiation because
of an internal error.
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 4.5.
6.3.2.3. Proceed Case
If the proceed case occurs, the receiving entity MUST return a
<proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
namespace.
R: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
The receiving entity MUST consider the TLS negotiation to have begun
immediately after sending the closing '>' character of the <proceed/>
element to the initiating entity. The initiating entity MUST
consider the TLS negotiation to have begun immediately after
receiving the closing '>' character of the <proceed/> element from
the receiving entity.
The entities now proceed to TLS negotiation as explained in the next
section.
6.3.3. TLS Negotiation
6.3.3.1. Rules
In order to complete TLS negotiation over the TCP connection, the
entities MUST follow the process defined in [TLS].
The following rules apply:
1. The entities MUST NOT send any further XML data until the TLS
negotiation has either failed or succeeded.
2. 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 shall succeed; see Section 15.2.2
regarding certificate validation procedures.
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5. The receiving entity SHOULD choose which certificate to present
based on the 'to' attribute of the initial stream header.
Note: See Section 15.6 regarding ciphers that MUST be supported
for TLS; naturally, other ciphers MAY be supported as well.
6.3.3.2. TLS Failure
If the TLS negotiation results in failure, the receiving entity MUST
terminate the TCP connection.
The receiving entity MUST NOT send a closing </stream> tag before
terminating the TCP connection, since the receiving entity and
initiating entity MUST consider the original stream to be replaced
upon failure of the TLS negotiation.
6.3.3.3. TLS Success
If the TLS negotiation is successful, then the entities MUST proceed
as follows.
1. The receiving entity MUST discard any knowledge obtained in an
insecure manner from the initiating entity before TLS took
effect.
2. The initiating entity MUST discard any knowledge obtained in an
insecure manner from the receiving entity before TLS took effect.
3. The initiating entity MUST send a new initial stream header to
the receiving entity over the 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'>
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.
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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 7.
R: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>PLAIN</mechanism>
<required/>
</mechanisms>
</stream:features>
7. SASL Negotiation
7.1. Overview
XMPP includes a method for authenticating a stream by means of an
XMPP-specific profile of the Simple Authentication and Security Layer
protocol (see [SASL]). SASL provides a generalized method for adding
authentication support to connection-based protocols, and XMPP uses
an XML namespace profile of SASL that conforms to the profiling
requirements of [SASL].
Support for SASL negotiation is REQUIRED in XMPP client and server
implementations.
7.2. Rules
7.2.1. 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
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"GSSAPI DIGEST-MD5", the client shall try GSSAPI first and then
DIGEST-MD5 but shall never try PLAIN (since PLAIN is not on its
list).
7.2.2. Mechanism Offers
If the receiving entity considers TLS negotiation (Section 6) 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. However, the EXTERNAL mechanism
MAY be offered under other circumstances as well.
See Section 15.6 regarding mechanisms that MUST be supported;
naturally, other SASL mechanisms MAY be supported as well. Best
practices for the use of several SASL mechanisms in the context of
XMPP are described in [XEP-0175] and [XEP-0178].
7.2.3. Data Formatting
The following data formattting rules apply to the SASL negotiation:
1. During SASL negotiation, the entities MUST NOT send any
whitespace within the root stream element 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 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 C.4,
the receiving entity MAY include one or more application-specific
child elements inside the <mechanisms/> element to provide
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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.
7.2.4. 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).
7.2.5. Simple Usernames
It is possible that provision of a "simple username" is supported by
the selected SASL mechanism (e.g., this is supported by the DIGEST-
MD5 and CRAM-MD5 mechanisms but not by the EXTERNAL and GSSAPI
mechanisms). The simple username provided during authentication MUST
be as follows:
Client-to-server communication: The initiating entity's registered
account name, i.e., a user name as described under Section 3.3
(this is not a bare JID of the form <localpart@domain> but only
the localpart of the JID). The simple username MUST adhere to the
Nodeprep (Appendix A) profile of [STRINGPREP].
Server-to-server communication: The initiating entity's sending
domain, i.e., IP address or fully qualified domain name as
contained in an XMPP domain identifier. The simple username MUST
adhere to the [NAMEPREP] profile of [STRINGPREP].
7.2.6. Authorization Identities
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. If provided, the
value of the authorization identity MUST be a bare JID of the form
<domain> (i.e., an XMPP domain identifier only) for servers and a
bare JID of the form <localpart@domain> (i.e., localpart and domain
identifier) for clients.
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Note: The authorization identity communited 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 3.5.
7.2.7. 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.
7.2.8. 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
clients and servers to use these methods, however they MUST support
the less efficient modes as well.
7.3. Process
The process for SASL negotiation is as follows.
7.3.1. Exchange of Stream Headers and Stream Features
If SASL negotiation follows successful STARTTLS negotation
(Section 6), then the SASL negotiation occurs over the encrypted
stream that has already been negotiated. If not, the initiating
entity resolves the hostname of the receiving entity as specified
under Section 4, opens a TCP connection to the advertised port at the
resolved IP address, and sends an initial stream header to the
receiving entity; if the initiating entity is capable of STARTTLS
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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; 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.
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>
<required/>
</mechanisms>
</stream:features>
If the receiving entity considers SASL negotiation to be
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discretionary, the <mechanisms/> element MUST contain an empty
<optional/> child element.
R: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>PLAIN</mechanism>
<optional/>
</mechanisms>
</stream:features>
If the receiving entity considers SASL negotiation to be mandatory,
the <mechanisms/> element MUST contain an empty <required/> child
element.
R: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>PLAIN</mechanism>
<required/>
</mechanisms>
</stream:features>
7.3.2. Initiation
In order to begin the SASL negotiation, the initiating entity sends
an <auth/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace and includes an
appropriate value for the 'mechanism' attribute. This element MAY
contain XML character data (in SASL terminology, the "initial
response") if the mechanism supports or requires it; if the
initiating entity needs to send a zero-length initial response, it
MUST transmit the response as a single equals sign character ("="),
which indicates that the response is present but contains no data.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>UjBtMzBSMGNrcw==</auth>
7.3.3. Challenge-Response Sequence
If necessary, the receiving entity challenges the initiating entity
by sending a <challenge/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (which MUST be generated in accordance
with the definition of the SASL mechanism chosen by the initiating
entity).
The initiating entity responds to the challenge by sending a
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<response/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (which MUST be generated in accordance
with the definition of the SASL mechanism chosen by the initiating
entity).
If necessary, the receiving entity sends more challenges and the
initiating entity sends more responses.
This series of challenge/response pairs continues until one of three
things happens:
o The initiating entity aborts the handshake.
o The receiving entity reports failure of the handshake.
o The receiving entity reports success of the handshake.
These scenarios are described in the following sections.
7.3.4. Abort
The initiating entity aborts the handshake by sending an <abort/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
namespace.
I: <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Upon receiving an <abort/> element, the receiving entity MUST return
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>
7.3.5. Failure
The receiving entity reports failure of the handshake by sending a
<failure/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace (the particular cause of
failure MUST be communicated in an appropriate child element of the
<failure/> element as defined under Section 7.4).
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>
Where appropriate for the chosen SASL mechanism, the receiving entity
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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/>
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/>).
7.3.6. Success
The receiving entity reports success of the handshake by sending a
<success/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (in SASL terminology, "additional data
with success") if 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'/>
Note: The authorization identity communited 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 3.5.
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'
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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
XML stream header to the initiating entity.
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'>
<required/>
</bind>
</stream:features>
7.4. SASL Errors
The syntax of SASL errors is as follows:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<defined-condition/>
[<text xml:lang='langcode'>
OPTIONAL descriptive text
</text>]
</failure>
Where "defined-condition" is one of the SASL-related error conditions
defined in the following sections.
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.
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7.4.1. 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>
7.4.2. 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'>UjBtMzBSMGNrcw==</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>
7.4.3. 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>
7.4.4. 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).
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I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>UjBtMzBSMGNrcw==</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<encryption-required/>
</failure>
7.4.5. 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.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'>[ ... ]</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<incorrect-encoding/>
</failure>
7.4.6. 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>
7.4.7. 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/>
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</failure>
7.4.8. 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].)
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>
7.4.9. 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'>UjBtMzBSMGNrcw==</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism-too-weak/>
</failure>
7.4.10. 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/>
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</failure>
Note: This error condition includes but is not limited to the case
of incorrect credentials or an unknown username. In order to
discourage directory harvest attacks, no differentiation is made
between incorrect credentials and an unknown username.
7.4.11. 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.
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>
7.4.12. 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>
Note: An XMPP client MUST treat a <transition-needed/> 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.
7.5. SASL Definition
The profiling requirements of [SASL] require that the following
information be supplied by the definition of a using protocol.
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service name: "xmpp"
initiation sequence: After the initiating entity provides an opening
XML stream header and the receiving entity replies in kind, the
receiving entity provides a list of acceptable authentication
methods. The initiating entity chooses one method from the list
and sends it to the receiving entity as the value of the
'mechanism' attribute possessed by an <auth/> element, optionally
including an initial response to avoid a round trip.
exchange sequence: Challenges and responses are carried through the
exchange of <challenge/> elements from receiving entity to
initiating entity and <response/> elements from initiating entity
to receiving entity. The receiving entity reports failure by
sending a <failure/> element and success by sending a <success/>
element; the initiating entity aborts the exchange by sending an
<abort/> element. Upon successful negotiation, both sides
consider the original XML stream to be closed and new stream
headers are sent by both entities.
security layer negotiation: The security layer takes effect
immediately after sending the closing '>' character of the
<success/> element for the receiving entity, and immediately after
receiving the closing '>' character of the <success/> element for
the initiating entity. The order of layers is first [TCP], then
[TLS], then [SASL], then XMPP.
use of the authorization identity: The authorization identity can be
used in XMPP to denote the non-default <localpart@domain> of a
client or the sending <domain> of a server; an empty string is
equivalent to an absent authorization identity.
8. Resource Binding
8.1. Overview
After a client authenticates with a server, it MUST bind a specific
resource to the stream so that the server can properly address the
client (see Section 3). That is, there MUST be an XMPP resource
identifier associated with the bare JID (<localpart@domain>) of the
client, so that the address for use over that stream is a full JID of
the form <localpart@domain/resource>. 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, as explained under
Section 11 (the client could exchange stanzas with the server itself
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
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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 resource identifier;
however, 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 15.14.
If, before completing the resource binding step, the client attempts
to send an outbound XML stanza (i.e., a stanza not directed to the
server itself or to the client's own account), the server MUST NOT
process the stanza and MUST either 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.
8.2. 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; this <bind/>
element MUST include an empty <required/> element to explicitly
indicate that it is mandatory for the client to complete resource
binding at this stage of the stream negotiation process.
Note: 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'>
<required/>
</bind>
</stream:features>
Upon being so informed that resource binding is required, the client
MUST bind a resource to the stream as described in the following
sections.
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8.3. Generation of Resource Identifiers
A resource identifier MUST at a minimum be unique among the connected
resources for that <localpart@domain>. Enforcement of this policy is
the responsibility of the server.
A resource identifier can be security-critical. For example, if a
malicious entity can guess a client's resource identifier 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 15.15. To prevent that
possibility, a client can either (1) generate a random resource
identifier on its own or (2) ask the server to generate a resource
identifier on its behalf, which MUST be random (see [RANDOM]). When
generating a random resource identifier, it is RECOMMENDED that the
resource identifier be a Universally Unique Identifier (UUID), for
which the format specified in [UUID] is RECOMMENDED.
8.4. Server-Generated Resource Identifier
A server that supports resource binding MUST be able to generate an
XMPP resource identifier on behalf of a client.
8.4.1. Success Case
A client requests a server-generated resource identifier by sending
an IQ stanza of type "set" (see Section 9.2.3) containing an empty
<bind/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind'
namespace.
C: <iq id='bind_1' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</iq>
Once the server has generated an XMPP resource identifier for the
client, it MUST return an IQ stanza of type "result" to the client,
which MUST include a <jid/> child element that specifies the full JID
for the connected resource as determined by the server.
S: <iq id='bind_1' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb
</jid>
</bind>
</iq>
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8.4.2. Error Cases
When a client asks the server to generate a resource identifer during
resource binding, the following stanza error conditions are possible:
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.
8.4.2.1. Resource Constraint
If the account has reached a limit on the number of simultaneous
connected resources allowed, the server MUST return a <resource-
constraint/> error.
S: <iq id='bind_2' type='error'>
<error type='wait'>
<resource-constraint
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.4.2.2. Not Allowed
If the client is otherwise not allowed to bind a resource to the
stream, the server MUST return a <not-allowed/> error.
S: <iq id='bind_2' type='error'>
<error type='cancel'>
<not-allowed
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.5. Client-Submitted Resource Identifier
Instead of asking the server to generate a resource identifier on its
behalf, a client MAY attempt to submit a resource identifier that it
has generated or that the controlling user has provided.
8.5.1. Success Case
A client asks its server to accept a client-submitted resource
identifier by sending an IQ stanza of type "set" containing a <bind/>
element with a child <resource/> element containing non-zero-length
XML character data.
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C: <iq id='bind_2' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>balcony</resource>
</bind>
</iq>
The server SHOULD accept the client-submitted resource identifier.
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='bind_2' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>juliet@im.example.com/balcony</jid>
</bind>
</iq>
8.5.2. Error Cases
When a client attempts to submit its own XMPP resource identifier
during resource binding, the following stanza error conditions are
possible in addition to those described under Section 8.4.2:
o The provided resource identifier cannot be processed by the
server, e.g. because it is not in accordance with the Resourceprep
(Appendix B) profile of [STRINGPREP]).
o The provided resource identifier is already in use.
8.5.2.1. Bad Request
If the provided resource identifier cannot be processed by the
server, the server MAY return a <bad-request/> error (but SHOULD
instead apply the Resourceprep (Appendix B) profile of [STRINGPREP]
or otherwise process the resource identifier so that it is in
conformance).
S: <iq id='bind_2' type='error'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.5.2.2. Conflict
If there is already a connected resource of the same name, the server
MUST do one of the following:
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1. Not accept the resource identifier provided by the client but
instead override it with an XMPP resource identifier that the
server generates.
2. Terminate the current resource and allow the newly-requested
resource.
3. Disallow the newly-requested resource and maintain the current
resource.
Which of these the server does is up to the implementation, although
it is RECOMMENDED to implement case #1.
S: <iq id='bind_2' 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/> stream error to the
current resource and return an IQ stanza of type "result" (indicating
success) to the newly-requested resource.
S: <iq id='bind_2' type='result'/>
In case #3, the server MUST send a <conflict/> stanza error to the
newly-requested resource but maintain the XML stream for that
connection so that the newly-requested resource has an opportunity to
negotiate a non-conflicting resource identifier before sending
another request for resource binding.
S: <iq id='bind_2' type='error'>
<error type='modify'>
<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.5.3. Retries
If an error occurs when a client submits a resource identifier, 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 resource identifiers (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.
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9. XML Stanzas
After a client has connected to a server or two servers have
connected to each other, either party can send XML stanzas over the
negotiated stream. Three kinds of XML stanza are defined for the
'jabber:client' and 'jabber:server' namespaces: <message/>,
<presence/>, and <iq/>. In addition, there are five common
attributes for these stanza types. These common attributes, as well
as the basic semantics of the three stanza types, are defined herein;
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.
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).
Support for the XML stanza syntax and semantics defined herein is
REQUIRED in XMPP client and server implementations.
9.1. Common Attributes
The following five attributes are common to message, presence, and IQ
stanzas.
9.1.1. to
The 'to' attribute specifies the JID of the intended recipient for
the stanza.
<message to='romeo@example.net'>
<body>Art thou not Romeo, and a Montague?</body>
</message>
For information about server processing of inbound and outbound XML
stanzas based on the nature of the 'to' address, refer to Section 11.
9.1.1.1. Client-to-Server Streams
The following rules apply to 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.
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2. A stanza sent from a client to a server for direct processing by
the server on behalf of the client (e.g., presence sent to the
server for broadcasting to other entities) MUST NOT possess a
'to' attribute.
9.1.1.2. 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 domain identifier portion 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.
9.1.2. 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>
9.1.2.1. Client-to-Server Streams
The following rules apply to the 'from' attribute in the context of
XML streams qualified by the 'jabber:client' namespace (i.e., client-
to-server streams).
1. When the server receives an XML stanza from a client and the
stanza does not include a 'from' attribute, the server MUST add a
'from' attribute to the stanza, where the value of the 'from'
attribute is the full JID (<localpart@domain/resource>)
determined by the server for the connected resource that
generated the stanza (see Section 3.5), 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'
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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.
9.1.2.2. Server-to-Server Streams
The following rules apply to the 'from' attribute in the context of
XML streams qualified by the 'jabber:server' namespace (i.e., server-
to-server streams).
1. A stanza MUST possess a 'from' attribute 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 domain identifier portion of the JID contained in the 'from'
attribute MUST match the hostname of the sending server (or any
validated domain thereof) as communicated in the SASL negotiation
(see Section 7), server dialback (see [XEP-0220], or similar
means; if a server receives a stanza that does not meet this
restriction, it MUST generate an <invalid-from/> stream error.
Enforcement of these rules helps to prevent certain denial of service
attacks as described under Section 15.14.
9.1.3. 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 (session) or
unique globally.
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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.
Note: The semantics of IQ stanzas impose additional restrictions; see
Section 9.2.3.
9.1.4. type
The 'type' attribute specifies the purpose or context of the message,
presence, or IQ stanza. The particular allowable values for the
'type' attribute vary depending on whether the stanza is a message,
presence, or IQ stanza. The defined values for message and presence
stanzas are specific to instant messaging and presence applications
and therefore are specified in [xmpp-im], whereas the values for IQ
stanzas specify the role of an IQ stanza in a structured request-
response exchange and therefore are specified under Section 9.2.3.
The only 'type' value common to all three stanzas is "error"; see
Section 9.3.
9.1.5. xml:lang
A stanza SHOULD possess an 'xml:lang' attribute (as defined in
Section 2.12 of [XML]) if the stanza contains XML character data that
is intended to be presented to a human user (as explained in
[CHARSET], "internationalization is for humans"). The value of the
'xml:lang' attribute specifies the default language of any such
human-readable XML character data.
<presence from='romeo@example.net/orchard' xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
</presence>
The value of the 'xml:lang' attribute MAY be overridden by the 'xml:
lang' attribute of a specific child element.
<presence from='romeo@example.net/orchard' xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
<status xml:lang='cs'>Dvořím se Julii</status>
</presence
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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 5.3.4.
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 received by a client or server does not
possess an 'xml:lang' attribute, an implementation MUST assume that
the default language is that specified for the stream as defined
under Section 5.3.4.
The value of the 'xml:lang' attribute MUST conform to the NMTOKEN
datatype (as defined in Section 2.3 of [XML]) and MUST conform to the
format defined in [LANGTAGS].
A server MUST NOT modify or delete 'xml:lang' attributes on stanzas
it receives from other entities.
9.2. Basic Semantics
9.2.1. Message Semantics
The <message/> stanza can be seen as a "push" mechanism whereby one
entity pushes information to another entity, similar to the
communications that occur in a system such as email. All message
stanzas SHOULD possess a 'to' attribute that specifies the intended
recipient of the message; upon receiving such a stanza, a server
SHOULD route or deliver it to the intended recipient (see Section 11
for general routing and delivery rules related to XML stanzas).
9.2.2. Presence Semantics
The <presence/> stanza can be seen as a specialized broadcast or
"publish-subscribe" mechanism, whereby multiple entities receive
information (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,
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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 11 for general routing
and delivery rules related to XML stanzas, and [xmpp-im] for rules
specific to presence applications.
9.2.3. IQ Semantics
Info/Query, or IQ, is a request-response mechanism, similar in some
ways to 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 9.4), and the interaction is tracked by the
requesting entity through use of the 'id' attribute. Thus, IQ
interactions follow a common pattern of structured data exchange such
as get/result or set/result (although an error can be returned in
reply to a request if appropriate):
Requesting Responding
Entity Entity
---------- ----------
| |
| <iq id='1' type='get'> |
| [ ... payload ... ] |
| </iq> |
| -------------------------> |
| |
| <iq id='1' type='result'> |
| [ ... payload ... ] |
| </iq> |
| <------------------------- |
| |
| <iq id='2' type='set'> |
| [ ... payload ... ] |
| </iq> |
| -------------------------> |
| |
| <iq id='2' type='error'> |
| [ ... condition ... ] |
| </iq> |
| <------------------------- |
| |
To enforce these semantics, the following rules apply:
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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 9.3).
3. An entity that receives an IQ request of type "get" or "set" MUST
reply with an IQ response of type "result" or "error". The
response MUST preserve the 'id' attribute of the request.
4. An entity that receives a stanza of type "result" or "error" MUST
NOT respond to the stanza by sending a further IQ response of
type "result" or "error"; however, the requesting entity MAY send
another request (e.g., an IQ of type "set" to provide required
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 9.3.
9.3. Stanza Errors
Stanza-related errors are handled in a manner similar to stream
errors (Section 5.8). Unlike stream errors, stanza errors are
recoverable; therefore they do not result in termination of the XML
stream and underlying TCP connection. Instead, the entity that
discovers the error condition returns an ERROR STANZA to the sender,
i.e., a stanza of the same kind (message, presence, or IQ) whose
'type' attribute is set to a value of "error" and which contains an
<error/> child element that specifies the error condition. The
specified error condition provides a hint regarding actions that the
sender can take to remedy the error if possible.
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9.3.1. Rules
The following rules apply to stanza errors:
1. The receiving or processing entity that detects an error
condition in relation to a stanza SHOULD return an error stanza
(and MUST do so for IQ stanzas).
2. The entity that generates an error stanza MAY include the
original XML sent so that the sender can inspect and, if
necessary, correct the XML before attempting to resend.
3. An error stanza MUST contain an <error/> child element.
4. An <error/> child MUST NOT be included if the 'type' attribute
has a value other than "error" (or if there is no 'type'
attribute).
5. An entity that receives an error stanza MUST NOT respond to the
stanza with a further error stanza; this helps to prevent
looping.
9.3.2. Syntax
The syntax for stanza-related errors is:
<stanza-kind from='intended-recipient' to='sender' type='error'>
[OPTIONAL to include sender XML here]
<error type='error-type'>
<defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
[<text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
xml:lang='langcode'>
OPTIONAL descriptive text
</text>]
[OPTIONAL application-specific condition element]
</error>
</stanza-kind>
The "stanza-kind" MUST be one of message, presence, or iq.
The "error-type MUST be one of the following:
o 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 <error/> element:
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o MUST contain a child element corresponding to one of the stanza
error conditions defined under Section 9.3.3; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace.
o MAY contain a <text/> child element containing XML character data
that describes the error in more detail; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace
and SHOULD possess an 'xml:lang' attribute specifying the natural
language of the XML character data.
o MAY contain a child element for an application-specific error
condition; this element MUST be qualified by an application-
specific namespace that defines the syntax and semantics of the
element.
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).
9.3.3. Defined Conditions
The following conditions are defined for use in stanza errors.
9.3.3.1. bad-request
The sender has sent a stanza containing XML that does not conform to
the appropriate schema or that cannot be processed (e.g., an IQ
stanza that includes an unrecognized value of the 'type' attribute,
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='some-id'
to='im.example.com'
type='subscribe'>
<ping xmlns='urn:xmpp:ping'/>
</iq>
S: <iq from='im.example.com'
id='some-id'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
9.3.3.2. conflict
Access cannot be granted because an existing resource exists with the
same name or address; the associated error type SHOULD be "cancel".
C: <iq id='bind_2' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>balcony</resource>
</bind>
</iq>
S: <iq id='bind_2' type='error'>
<error type='cancel'>
<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
9.3.3.3. feature-not-implemented
The feature represented in the XML stanza is not implemented by the
intended recipient or an intermediate server and therefore the stanza
cannot be processed (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='subscriptions1'
to='pubsub.example.com'
type='get'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<subscriptions/>
</pubsub>
</iq>
E: <iq from='pubsub.example.com
id='subscriptions1'
to='juliet@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>
9.3.3.4. forbidden
The requesting entity does not possess the required permissions to
perform the action; the associated error type SHOULD be "auth".
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'
type='error'>
<error type='auth'>
<forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.5. gone
The recipient or server can no longer be contacted at this address,
typically on a permanent basis; the associated error type SHOULD be
"cancel" or "modify" and the error stanza SHOULD include a new
address as the XML character data of the <gone/> element (which MUST
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be a URI or IRI at which the entity can be contacted, typically an
XMPP IRI as specified in [XMPP-URI]).
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
xmpp:conference.example.com
</gone>
</error>
</presence>
9.3.3.6. internal-server-error
The server could not process the stanza because of a misconfiguration
or an otherwise-undefined internal server error; the associated error
type SHOULD be "wait" or "cancel".
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'
type='error'>
<error type='wait'>
<internal-server-error
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.7. item-not-found
The addressed JID or item requested cannot be found; the associated
error type SHOULD be "cancel" or "modify".
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C: <presence from='userfoo@example.com/bar'
to='nosuchroom@conference.example.org/foo'/>
S: <presence from='nosuchroom@conference.example.org/foo'
to='userfoo@example.com/bar'
type='error'>
<error type='cancel'>
<item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
Note: An application MUST NOT return this error if doing so would
provide information about the intended recipient's network
availability to an entity that is not authorized to know such
information; instead it MUST return a <service-unavailable/>
error.
9.3.3.8. jid-malformed
The sending entity has provided or communicated an XMPP address
(e.g., a value of the 'to' attribute) or aspect thereof (e.g., an
XMPP resource identifier) that does not adhere to the syntax defined
under Section 3; the associated error type SHOULD be "modify".
C: <presence
from='juliet@im.example.com/balcony'
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'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<jid-malformed
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.9. not-acceptable
The recipient or server understands the request but is refusing to
process it because it does not meet criteria defined by the recipient
or server (e.g., a local policy regarding stanza size limits or
acceptable words in messages); the associated error type SHOULD be
"modify".
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C: <message to='juliet@im.example.com' id='foo'>
<body>[ ... the-emacs-manual ... ]</body>
</message>
S: <message from='juliet@im.example.com' id='foo'>
<error type='modify'>
<not-acceptable
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
9.3.3.10. not-allowed
The recipient or server does not allow any entity to perform the
action (e.g., sending to entities at a blacklisted domain); the
associated error type SHOULD be "cancel".
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'
type='error'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.11. not-authorized
The sender needs to provide proper credentials before being allowed
to perform the action, or has provided improper credentials; the
associated error type SHOULD be "auth".
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C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'>
<error type='auth'>
<not-authorized xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.12. not-modified
The item requested has not changed since it was last requested; the
associated error type SHOULD be "continue".
C: <iq from='juliet@capulet.com/balcony'
id='roster2'
type='get'>
<query xmlns='jabber:iq:roster'>
<headers xmlns='http://jabber.org/protocol/shim'>
<header name='If-None-Match'>
some-long-opaque-string
</header>
</headers>
</query>
</iq>
S: <iq type='error'
to='juliet@capulet.com/balcony'
id='roster2'>
<query xmlns='jabber:iq:roster'>
<headers xmlns='http://jabber.org/protocol/shim'>
<header name='If-None-Match'>
some-long-opaque-string
</header>
</headers>
</query>
<error type='modify'>
<not-modified xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
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9.3.3.13. payment-required
The requesting entity is not authorized to access the requested
service because payment is required; the associated error type SHOULD
be "auth".
C: <iq from='romeo@example.net/foo'
id='items1'
to='pubsub.example.com'
type='get'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<items node='my_musings'/>
</pubsub>
</iq>
E: <iq from='pubsub.example.com'
id='items1'
to='romeo@example.net/foo'
type='error'>
<error type='auth'>
<payment-required
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
9.3.3.14. 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; 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='juliet@im.example.com' id='foo'>
<body>[ ... the-emacs-manual ... ]</body>
</message>
S: <message from='juliet@im.example.com' id='foo'>
<error type='cancel'>
<policy-violation
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
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9.3.3.15. recipient-unavailable
The intended recipient is temporarily unavailable; the associated
error type SHOULD be "wait".
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'>
<error type='wait'>
<recipient-unavailable
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
Note: An application MUST NOT return this error if doing so would
provide information about the intended recipient's network
availability to an entity that is not authorized to know such
information; instead it MUST return a <service-unavailable/>
error.
9.3.3.16. redirect
The recipient or server is redirecting requests for this information
to another entity, typically in a temporary fashion (the <gone/>
condition 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'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
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>
9.3.3.17. registration-required
The requesting entity is not authorized to access the requested
service because prior registration is required; the associated error
type SHOULD be "auth".
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'>
<error type='auth'>
<registration-required
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.18. remote-server-not-found
A remote server or service specified as part or all of the JID of the
intended recipient does not exist; the associated error type SHOULD
be "cancel".
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C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'>
<error type='cancel'>
<remote-server-not-found
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.19. remote-server-timeout
A remote server or service specified as part or all of the JID of the
intended recipient (or required to fulfill a request) could not be
contacted within a reasonable amount of time; the associated error
type SHOULD be "wait".
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'>
<error type='wait'>
<remote-server-timeout
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.20. resource-constraint
The server or recipient lacks the system resources necessary to
service the request; the associated error type SHOULD be "wait" or
"modify".
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C: <iq from='romeo@example.net/foo'
id='items1'
to='pubsub.example.com'
type='get'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<items node='my_musings'/>
</pubsub>
</iq>
E: <iq from='pubsub.example.com'
id='items1'
to='romeo@example.net/foo'
type='error'>
<error type='wait'>
<resource-constraint
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
9.3.3.21. 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>
An application MUST return a <service-unavailable/> error instead of
<item-not-found/> or <recipient-unavailable/> if sending one of the
latter errors would provide information about the intended
recipient's network availability to an entity that is not authorized
to know such information.
9.3.3.22. subscription-required
The requesting entity is not authorized to access the requested
service because a prior subscription is required; the associated
error type SHOULD be "auth".
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C: <message
from='romeo@example.net/orchard'
to='playbot@shakespeare.example.com'
<body>help</body>
</message>
E: <message
from='playbot@shakespeare.example.com'
to='romeo@example.net/orchard'
type='error'>
<error type='auth'>
<subscription-required
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
9.3.3.23. 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>
9.3.3.24. unexpected-request
The recipient or server understood the request but was not expecting
it at this time (e.g., the request was out of order); the associated
error type SHOULD be "wait" or "modify".
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C: <iq from='romeo@example.net/foo'
id='unsub1'
to='pubsub.example.com'
type='set'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<unsubscribe
node='my_musings'
jid='romeo@example.net'/>
</pubsub>
</iq>
E: <iq from='pubsub.example.com'
id='unsub1'
to='romeo@example.net/foo'
type='error'>
<error type='cancel'>
<unexpected-request
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<not-subscribed
xmlns='http://jabber.org/protocol/pubsub#errors'/>
</error>
</iq>
9.3.4. Application-Specific Conditions
As noted, an application MAY provide application-specific stanza
error information by including a properly-namespaced child in the
error element. The application-specific element SHOULD supplement or
further qualify a defined element. Thus, the <error/> element will
contain two or three child elements.
<iq id='some-id' type='error'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<too-many-parameters xmlns='http://example.com/ns'/>
</error>
</iq>
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<message type='error' id='another-id'>
<error type='modify'>
<undefined-condition
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<text xml:lang='en'
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
[ ... application-specific information ... ]
</text>
<too-many-parameters xmlns='http://example.com/ns'/>
</error>
</message>
An entity that receives an application-specific error condition it
does not understand MUST ignore the condition.
9.4. Extended Content
While the message, presence, and IQ stanzas provide basic semantics
for messaging, availability, and request-response interactions, XMPP
uses XML namespaces (see [XML-NAMES] to extend the basic stanza
syntax for the purpose of providing additional functionality.
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 all data
contained within the child element. Such a child element is called
an "extension element".
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 other than the
default namespace or the reserved "xml" prefix (including the "empty
namespace" if the attribute is not prefixed). 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 (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
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said to be an EXTENDED NAMESPACE.
Because an XML stanza is the primary unit of meaning in XMPP, any
prefixed element or attribute included in a stanza MUST use a prefix
that is declared in the stanza itself or a child element of the
stanza, not outside the context of the stanza (e.g., not on the
stream header).
Routing entities (typically servers) SHOULD try to maintain prefixes
when serializing XML stanzas for processing, but receiving entities
MUST NOT rely on the prefix strings having 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) an entity that is routing the
stanza to the recipient.
Recipient: If a recipient receives a stanza that contains a child
element it does not understand, it MUST silently ignore that
particular XML data, i.e., it MUST NOT process it or present it to
a user or associated application (if any). In particular:
* If an entity receives a message or presence stanza that
contains XML data qualified by a namespace it does not
understand, the portion of the stanza that qualified by the
unknown namespace MUST be ignored.
* If an entity receives a message stanza whose only child element
is qualified by a namespace it does not understand, it MUST
ignore the entire stanza.
* If an entity receives an IQ stanza of type "get" or "set"
containing a child element qualified by a namespace it does not
understand, the entity MUST return an IQ stanza of type "error"
with an error condition of <service-unavailable/>.
Router: If a routing entity (typically a server) handles a stanza
that contains a child element it does not understand, it MUST
ignore the associated XML data by routing or delivering it
untouched to the recipient.
9.5. 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 or a <policy-
violation/> stream error.
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10. Examples
10.1. Client-to-Server
The following examples show the XMPP data flow for a client
negotiating an XML stream with a server, exchanging XML stanzas, and
closing the negotiated stream. The server is "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.
Note: The alternate steps shown are provided only to illustrate
the protocol for failure cases; they are not exhaustive and would
not necessarily be triggered by the data sent in the examples.
10.1.1. 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 (STARTTLS extension
only at this point):
S: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
Step 4: Client sends STARTTLS command to server:
C: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5: Server informs client that it is allowed to proceed:
S: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5 (alt): Server informs client that 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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10.1.2. SASL
Step 8: Server responds by sending a stream header to client along
with any available stream features:
S: <stream:stream
from='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>
<required/>
</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'>UjBtMzBSMGNrcw==</auth>
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>
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'
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10.1.3. 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'>
<required/>
</bind>
</stream:features>
Upon being so 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='bind_1' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
balcony
</bind>
</iq>
Step 14: Server accepts submitted resource identifier and informs
client of successful resource binding:
S: <iq id='bind_1' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/balcony
</jid>
</bind>
</iq>
10.1.4. Stanza Exchange
Now the client is allowed to send XML stanzas over the negotiated
stream.
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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>
If necessary, sender's server negotiates XML streams with intended
recipient's server (see Section 10.2).
The intended recipient replies and the message is delivered to the
client.
E: <message from='romeo@example.net/orchard'
to='juliet@im.example.com/balcony'
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.
10.1.5. Close
Desiring to send no further messages, the client closes the stream.
C: </stream:stream>
Consistent with the recommended stream closing handshake, the server
closes the stream as well:
S: </stream:stream>
Client now terminates the underlying TCP connection.
10.2. Server-to-Server Examples
The following examples show the data flow for a server negotiating an
XML stream with another server, exchanging XML stanzas, and closing
the negotiated stream. The initiating server ("Server1") is
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.
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Note: The alternate steps shown are provided only to illustrate
the protocol for failure cases; they are not exhaustive and would
not necessarily be triggered by the data sent in the examples.
10.2.1. TLS
Step 1: Server1 initiates stream to Server2:
S1: <stream:stream
from='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'>
Step 3: Server2 sends stream features to Server1:
S2: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
Step 4: Server1 sends the STARTTLS command to Server2:
S1: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5: Server2 informs Server1 that it is allowed to proceed:
S2: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5 (alt): Server2 informs Server1 that STARTTLS negotiation has
failed and closes stream:
S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
S2: </stream:stream>
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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.
10.2.2. SASL
Step 8: Server2 sends a response stream header to Server1 along with
available stream features (including a preference for the SASL
EXTERNAL mechanism):
S2: <stream:stream
from='example.net'
id='RChdjlgj/TIBcbT9Keu31zDihH4='
to='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>
<required/>
</mechanisms>
</stream:features>
Step 9: Server1 selects the EXTERNAL mechanism, in this case with an
authorization identity encoded according to [BASE64]:
S1: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='EXTERNAL'/>eG1wcC5leGFtcGxlLmNvbQ</auth>
The decoded authorization identity is "im.example.com".
Step 10: Server2 determines that the authorization identity provided
by Server1 matches the information in the presented certificate and
therefore returns success:
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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'>
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/>
10.2.3. Stanza Exchange
Now Server1 is allowed to send XML stanzas to Server2 over the
negotiated stream; here we assume that the transferred stanzas are
those shown earlier for client-to-server communication, 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'
to='romeo@example.net'
xml:lang='en'>
<body>Art thou not Romeo, and a Montague?</body>
</message>
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The intended recipient replies and the message is delivered from
Server2 to Server1.
Server2 sends XML stanza to Server1:
S2: <message from='romeo@example.net/orchard'
to='juliet@im.example.com/balcony'
xml:lang='en'>
<body>Neither, fair saint, if either thee dislike.</body>
</message>
10.2.4. Close
Desiring to send no further messages, Server1 closes the stream. (In
practice, the stream would most likely remain open for some time,
since Server1 and Server2 do not immediately know if the stream will
be needed for further communication.)
S1: </stream:stream>
Consistent with the recommended stream closing handshake, Server2
closes the stream as well:
S2: </stream:stream>
Server1 now terminates the underlying TCP connection.
11. Server Rules for Processing XML Stanzas
An XMPP server MUST ensure in-order processing of XML stanzas between
any two entities. This includes stanzas sent by a client to its
server for direct processing by the server (e.g., in-order processing
of a roster get and initial presence as described in [xmpp-im]).
Beyond the requirement for in-order processing, each server
implementation will contain its own logic for processing stanzas it
receives. Such logic determines whether the server needs to ROUTE a
given stanza to another domain, DELIVER it to a local entity
(typically a connected client associated with a local account), or
HANDLE it directly within the server itself. The following rules
apply.
Note: Particular XMPP applications MAY specify delivery rules that
modify or supplement the following rules; for example, a set of
delivery rules for instant messaging and presence applications is
defined in [xmpp-im].
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11.1. No 'to' Address
11.1.1. 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.
11.1.2. 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.
11.1.3. 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).
11.1.4. 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. While the meaning of
"handle" is determined by the semantics of the qualifying
namespace, in general the server shall 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 shall 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
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<service-unavailable/>.
3. If the IQ stanza is of type "error" or "result", the server MUST
handle the error or result as appropriate for the request-
response interaction, responding as if the server were the bare
JID of the sending entity.
11.2. Local Domain
If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute matches one of the configured hostnames of the
server itself, the server MUST first determine if the hostname is
serviced by the server or by a specialized local service. If the
latter, the server MUST route the stanza to that service. If the
former, the server MUST proceed as follows.
11.2.1. Mere Domain
If the JID contained in the 'to' attribute is of the form <domain>,
then the server MUST either handle the stanza as appropriate for the
stanza kind or return an error stanza to the sender.
11.2.2. 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.
11.2.3. Localpart at Domain
Note: For addresses of this type, more detailed rules in the
context of instant messaging and presence applications are
provided in [xmpp-im].
11.2.3.1. No Such User
If there is no local account associated with the <localpart@domain>,
how the stanza shall be 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.
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11.2.3.2. Bare JID
If the JID contained in the 'to' attribute is of the form
<localpart@domain>, how the stanza shall be 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
resource, the server MUST either return an 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 MUST silently discard the stanza.
o For an IQ stanza, the server MUST handle it directly on behalf of
the intended recipient.
11.2.3.3. 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 shall be 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.
11.3. Foreign Domain
If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute does not match one of the configured hostnames
of the server itself, the server SHOULD attempt to route the stanza
to the foreign domain (subject to local service provisioning and
security policies regarding inter-domain communication, since such
communication is optional for any given deployment). There are two
possible cases.
11.3.1. Existing Stream
If a server-to-server stream already exists between the two domains,
the sender's server shall attempt to route the stanza to the
authoritative server for the foreign domain over the existing stream.
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11.3.2. No Existing Stream
If there exists no server-to-server stream between the two domains,
the sender's server shall proceed as follows:
1. Resolve the hostname of the foreign domain (as defined under
Section 15.4).
2. Negotiate a server-to-server stream between the two domains (as
defined under Section 6 and Section 7).
3. Route the stanza to the authoritative server for the foreign
domain over the newly-established stream.
11.3.3. Error Handling
If routing 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 foreign 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 foreign server cannot deliver the stanza to the
recipient, the foreign server shall return an appropriate error to
the sender by way of the sender's server.
12. XML Usage
12.1. Restrictions
The Extensible Messaging and Presence Protocol (XMPP) defines a class
of data objects called XML streams as well as the behavior of
computer programs that process XML streams. XMPP is an application
profile 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)
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o internal or external DTD subsets (Section 2.8 therein)
o internal or external entity references (Section 4.2 therein) with
the exception of predefined entities (Section 4.6 therein)
An XMPP implementation MUST behave as follows with regard to these
features:
1. An XMPP implementation MUST NOT inject characters matching such
features into an XML stream.
2. If an XMPP implementation receives characters matching such
features over an XML stream, it MUST return a stream error, which
SHOULD be <restricted-xml/> but MAY be <bad-format/>.
12.2. XML Namespace Names and Prefixes
XML namespaces (see [XML-NAMES]) are used within XMPP streams to
create strict boundaries of data ownership. The basic function of
namespaces is to separate different vocabularies of XML elements that
are structurally mixed together. Ensuring that XMPP streams are
namespace-aware enables any allowable XML to be structurally mixed
with any data element within XMPP. XMPP-specific rules for XML
namespace names and prefixes are defined in the following
subsections.
12.2.1. Streams Namespace
A streams namespace declaration is REQUIRED in all XML stream headers
and the name of the streams namespace MUST be
'http://etherx.jabber.org/streams'. If this rule is violated, the
entity that receives the offending stream header MUST return a stream
error to the sending entity, which SHOULD be <invalid-namespace/> but
MAY be <bad-format/>.
The element names of the <stream/> element and its <features/> and
<error/> children MUST be qualified by the streams namespace prefix
in all instances. If this rule is violated, the entity that receives
the offending element MUST return a stream error to the sending
entity, which SHOULD be <bad-format/>.
An implementation SHOULD generate only the 'stream:' prefix for these
elements, and for historical reasons MAY accept only the 'stream:'
prefix. If an entity receives a stream header with a streams
namespace prefix it does not accept, it MUST return a stream error to
the sending entity, which SHOULD be <bad-namespace-prefix/> but MAY
be <bad-format/>.
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12.2.2. Default Namespace
A default namespace declaration is REQUIRED and defines the allowable
first-level children of the root stream element. This namespace
declaration MUST be the same for the initial stream and the response
stream so that both streams are qualified consistently. The default
namespace declaration applies to the stream and all first-level child
element sent within a stream unless explicitly qualified by the
streams namespace or another namespace.
A server implementation MUST support the following two default
namespaces:
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.
If an implementation accepts a stream that is qualified by the
'jabber:client' or 'jabber:server' namespace, it MUST support the
common attributes (Section 9.1) and basic semantics (Section 9.2) of
all three core stanza types (message, presence, and IQ).
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.
An implementation MUST NOT generate namespace prefixes for elements
qualified by the default namespace if the default namespace is
'jabber:client' or 'jabber:server'.
Note: The 'jabber:client' and 'jabber:server' namespaces are
nearly identical but are used in different contexts (client-to-
server communication for 'jabber:client' and server-to-server
communication for 'jabber:server'). The only difference between
the two is that the 'to' and 'from' attributes are OPTIONAL on
stanzas sent over XML streams qualified by the 'jabber:client'
namespace, whereas they are REQUIRED on stanzas sent over XML
streams qualified by the 'jabber:server' namespace.
An implementation MAY support a default namespace other than "jabber:
client" or "jabber:server". However, because such namespaces would
define applications other than XMPP, they are to be defined in
separate specifications.
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12.2.3. Extended Namespaces
An EXTENDED NAMESPACE is an XML namespace that qualifies extended
content as defined under Section 9.4. For example, in the following
stanza, the extended namespace is 'jabber:iq:roster':
<iq from='juliet@capulet.com/balcony'
id='roster1'
type='get'>
<query xmlns='jabber:iq:roster'/>
</iq>
An XML stanza MAY contain XML data qualified by more than one
extended namespace, either at the direct child level of the stanza
(for presence and message stanzas) or in any mix of levels (for all
stanzas).
<presence from='juliet@capulet.com/balcony'>
<c xmlns='http://jabber.org/protocol/caps'
node='http://exodus.jabberstudio.org/caps'
ver='0.9'/>
<x xmlns='vcard-temp:x:update'>
<photo>sha1-hash-of-image</photo>
</x>
</presence>
<message to='juliet@capulet.com'>
<body>Hello?</body>
<html xmlns='http://jabber.org/protocol/xhtml-im'>
<body xmlns='http://www.w3.org/1999/xhtml'>
<p style='font-weight:bold'>Hello?</t>
</body>
</html>
</message>
<iq from='juliet@capulet.com/balcony'
id='roster2'
type='get'>
<query xmlns='jabber:iq:roster'>
<headers xmlns='http://jabber.org/protocol/shim'>
<header name='If-None-Match'>some-long-opaque-string</header>
</headers>
</query>
</iq>
An implementation SHOULD NOT generate namespace prefixes for elements
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qualified by content (as opposed to stream) namespaces other than the
default namespace. However, if included, the namespace declarations
for those prefixes MUST be included on the stanza root or a child
thereof, not at the level of the stream element (this helps to ensure
that any such namespace declaration is routed and delivered with the
stanza, instead of assumed from the stream).
12.3. Well-Formedness
There are two varieties of well-formedness:
o "XML-well-formedness" in accordance with the definition of "well-
formed" in Section 2.1 of [XML].
o "Namespace-well-formedness" in accordance with the definition of
"namespace-well-formed" in 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 SHOULD NOT route or deliver data that is not
namespace-well-formed, and SHOULD return a stanza error of <not-
acceptable/> or a stream error of <xml-not-well-formed/> in response
to the receipt of such data.
Note: Because these restrictions were underspecified in an earlier
revision of this specification, it is possible that
implementations based on that revision will send data that does
not comply with the restrictions; an entity SHOULD be liberal in
accepting such data.
12.4. 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
ignore any non-conformant elements or attributes on the incoming XML
stream.
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Note: The terms "valid" and "well-formed" are distinct in XML.
12.5. Inclusion of Text Declaration
Implementations SHOULD send a text declaration before sending a
stream header. Applications MUST follow the rules provided in [XML]
regarding the circumstances under which a text declaration is
included.
12.6. Character Encoding
Implementations MUST support the UTF-8 transformation of Universal
Character Set [UCS2] characters, as required by [CHARSET] and defined
in [UTF-8]. Implementations MUST NOT attempt to use any other
encoding. If one party to an XML stream detects that the other party
has attempted to send XML data with an encoding other than UTF-8, it
MUST return a stream error, which SHOULD be <unsupported-encoding/>
but MAY be <bad-format/>.
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.
12.7. Whitespace
Except where explicitly disallowed (e.g., during TLS negotiation
(Section 6) and SASL negotiation (Section 7)), either entity MAY send
whitespace within the root stream element as separators between XML
stanzas or between any other first-level elements sent over the
stream. One common use for sending such whitespace is explained
under Section 5.7.3.
12.8. 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 addresses XML 1.0 only.
13. Compliance Requirements
This section summarizes the specific aspects of the Extensible
Messaging and Presence Protocol that MUST be supported by servers and
clients in order to be considered compliant implementations, as well
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as additional protocol aspects that SHOULD be supported. For
compliance purposes, we draw a distinction between core protocols
(which MUST be supported by any server or client, regardless of the
specific application) and instant messaging and presence protocols
(which MUST be supported only by instant messaging and presence
applications built on top of the core protocols). Compliance
requirements that apply to all servers and clients are specified in
this section; compliance requirements for instant messaging and
presence applications are specified in the corresponding section of
[xmpp-im].
13.1. Servers
A server MUST support the following core protocols in order to be
considered compliant:
o Conformance with [IDNA] for domain identifiers, the Nodeprep
(Appendix A) profile of [STRINGPREP] for localparts, and the
Resourceprep (Appendix B) profile of [STRINGPREP] for resource
identifiers, as well as enforcement thereof for clients that
authenticate with the server
o XML streams (Section 5), including TLS negotiation (Section 6),
SASL negotiation (Section 7), stream features (Section 5.5), and
Resource Binding (Section 8)
o The basic semantics of the three defined stanza types (i.e.,
<message/>, <presence/>, and <iq/>)
o Generation (and, where appropriate, handling) of error syntax and
semantics related to streams, TLS, SASL, and XML stanzas
For backward compatibility with the large deployed base of XMPP
servers, server developers are advised to implement the server
dialback protocol first specified in [RFC3920] and now documented in
[XEP-0220], since that protocol is widely used for weak identity
verification of peer servers in the absence of domain certificates.
13.2. Clients
A client MUST support the following core protocols in order to be
considered compliant:
o XML streams (Section 5), including TLS negotiation (Section 6),
SASL negotiation (Section 7), stream features (Section 5.5), and
Resource Binding (Section 8)
o The basic semantics of the three defined stanza types (i.e.,
<message/>, <presence/>, and <iq/>)
o Handling (and, where appropriate, generation) of error syntax and
semantics related to streams, TLS, SASL, and XML stanzas
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In addition, a client SHOULD support the following core protocols:
o Conformance with [IDNA] for domain identifiers, the Nodeprep
(Appendix A) profile of [STRINGPREP] for localparts, and the
Resourceprep (Appendix B) profile of [STRINGPREP] for resource
identifiers.
14. Internationalization Considerations
As specified under Section 12.6, XML streams MUST be encoded in
UTF-8.
As specified under Section 5.3, an XML stream SHOULD include an 'xml:
lang' attribute specifying the default language for any XML character
data that is intended to be presented to a human user. As specified
under Section 9.1.5, an XML stanza SHOULD include an 'xml:lang'
attribute if the stanza contains XML character data that is intended
to be presented to a human user. A server SHOULD apply the default
'xml:lang' attribute to stanzas it routes or delivers on behalf of
connected entities, and MUST NOT modify or delete 'xml:lang'
attributes on stanzas it receives from other entities.
As specified under Section 3, a server MUST support and enforce
[IDNA] for domain identifiers, the Nodeprep (Appendix A) profile of
[STRINGPREP] for localparts, and the Resourceprep (Appendix B)
profile of [STRINGPREP] for resource identifiers; this enables XMPP
addresses to include a wide variety of Unicode characters outside the
US-ASCII range.
15. Security Considerations
15.1. High Security
For the purposes of XMPP communication (client-to-server and server-
to-server), the term "high security" refers to the use of security
technologies that provide both mutual authentication and integrity
checking; in particular, when using certificate-based authentication
to provide high security, a 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 Section 15.2 regarding certificate validation procedures.
Implementations MUST support high security. Service provisioning
SHOULD use high security, subject to local security policies.
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15.2. Certificates
Channel encryption of an XML stream using Transport Layer Security as
described under Section 6, and in some cases also authentication as
described under Section 7, 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.
For both client and server certificates, a certificate MUST conform
to the format defined in [X509]. Considerations specific to client
certificates or server certificates are described in the following
sections.
15.2.1. Certificate Generation
15.2.1.1. 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., domain identifiers) associated with domains
serviced at the server. The representations described in the
following sections are RECOMMENDED. These representations are
provided in preference order.
15.2.1.1.1. SRVName
A server's domain identifier SHOULD be represented as an SRVName,
i.e., as an otherName field of type "id-on-dnsSRV" as specified in
[X509-SRV].
15.2.1.1.2. dNSName
A server's domain identifier SHOULD be represented as a dNSName,
i.e., as a subjectAltName extension of type dNSName.
The dNSName MAY contain the wildcard character '*'. The wildcard
character applies only to the left-most domain name component or
component fragment and matches any single component or component
fragment. For instance, a dNSName of *.example.com matches
foo.example.com but not bar.foo.example.com or example.com itself;
similarly, a dNSName of im*.example.net matches im1.example.net and
im2.example.net but not chat.example.net or example.net itself.
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15.2.1.1.3. XmppAddr
A server's domain identifier 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 15.2.1.3. In server certificates, this representation is
included only for the sake of backward-compatibility.
15.2.1.1.4. Common Name
A server's domain identifier SHOULD NOT be represented as a Common
Name; instead, the Common Name field SHOULD be reserved for
representation of a human-friendly name.
15.2.1.1.5. 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.
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_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x2.example.net.
Example Internet Services also hosts chatrooms at chat.example.net,
and provides an xmpp-server SRV record for that service as well (thus
enabling entity from foreign 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"
15.2.1.2. 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 15.2.1.3.
15.2.1.3. ASN.1 Object Identifier
The [ASN.1] Object Identifier "id-on-xmppAddr" (also called an
XmppAddr) is defined as follows.
id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
id-on OBJECT IDENTIFIER ::= { id-pkix 8 } -- other name forms
id-on-xmppAddr OBJECT IDENTIFIER ::= { id-on 5 }
XmppAddr ::= UTF8String
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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.
15.2.2. 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 6 and Section 7, the entity
MUST validate the certificate to determine if the certificate shall
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
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.
15.2.2.1. Server Certificates
When an entity (client or server) validates a certificate presented
by an XMPP server, there are three possible cases, as discussed in
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the following sections.
15.2.2.1.1. 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 15.2.1.1.1,
Section 15.2.1.1.2, and Section 15.2.1.1.3. There are three possible
sub-cases:
Sub-Case #1: The entity finds at least one SRVName, dNSName, or
XmppAddr that matches the hostname to which it attempted to
connect; the entity MUST use this represented domain identifier as
the validated identity of the XMPP server. The server certificate
MUST be checked against the hostname as provided by the entity
(client or server), not the hostname as resolved via the Domain
Name System; e.g., if a user specifies a hostname of "example.net"
but a [DNS-SRV] lookup returns "x1.example.net", the certificate
MUST be checked as "example.net". A user-oriented client MAY
provide a configuration setting that enables a human user to
explicitly specify a hostname to be checked for connection
purposes.
Sub-Case #2: The entity finds no SRVName, dNSName, or XmppAddr that
matches the hostname to which it attempted to connect and a human
user has not permanently accepted the certificate during a
previous connection attempt; the entity MUST NOT use the
represented domain identifier (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 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 provide a setting that enables the check.
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Sub-Case #3: The entity finds no SRVName, dNSName, or XmppAddr that
matches the hostname to which it 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.
15.2.2.1.2. 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.
15.2.2.1.3. 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.
15.2.2.2. Client Certificates
When an XMPP server validates a certificate presented by a client,
there are three possible cases, as discussed in the following
sections.
15.2.2.2.1. 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 15.2.1.3. There
are three possible sub-cases:
Sub-Case #1: The server finds one XmppAddr for which the domain
identifier portion 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.
Sub-Case #2: The server finds more than one XmppAddr for which the
domain identifier portion 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 domain identifier portion 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.
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15.2.2.2.2. 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.
15.2.2.2.3. Case #3
If the client certificate is self-signed, the server MUST proceed as
under Case #1, Sub-Case #3.
15.2.2.3. 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 shall
define their own certificate handling rules, which at a minimum
SHOULD be consistent with the rules specified herein but MAY specify
additional rules.
15.3. Client-to-Server Communication
A compliant client implementation MUST support both TLS and SASL for
connections to a server.
The TLS protocol for encrypting XML streams (defined under Section 6)
provides a reliable mechanism for helping to ensure the
confidentiality and integrity of data exchanged between two entities.
The SASL protocol for authenticating XML streams (defined under
Section 7) provides a reliable mechanism for validating that a client
connecting to a server is who it claims to be.
Client-to-server communication MUST NOT proceed until the DNS
hostname asserted by the server has been resolved as specified under
Section 4. If there is a mismatch between the hostname to which a
client attempted to connect (e.g., "example.net") and the hostname to
which the client actually connects (e.g., "x1.example.net"), the
client MUST warn a human user about the mismatch and the human user
MUST approve the connection before the client proceeds; however, the
client MAY also allow the user to add the presented hostname to a
configured set of accepted hostnames to expedite future connections.
A client's IP address and method of access MUST NOT be made public by
a server, nor are any connections other than the original server
connection required. This helps to protect the client's server from
direct attack or identification by third parties.
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15.4. Server-to-Server Communication
A compliant server implementation MUST support both TLS and SASL for
inter-domain communication.
Because service provisioning is a matter of policy, it is optional
for any given domain to communicate with other domains, and server-
to-server communication can be disabled by the administrator of any
given deployment. If a particular domain enables inter-domain
communication, it SHOULD enable high security.
Administrators might want to require use of SASL for server-to-server
communication to ensure both authentication and confidentiality
(e.g., on an organization's private network). Compliant
implementations SHOULD support SASL for this purpose.
Server-to-server communication MUST NOT proceed until the DNS
hostnames asserted by both servers have been resolved as specified
under Section 4.
15.5. Order of Layers
The order of layers in which protocols MUST be stacked is:
1. TCP
2. TLS
3. SASL
4. XMPP
The rationale for this order is that [TCP] is the base connection
layer used by all of the protocols stacked on top of TCP, [TLS] is
often provided at the operating system layer, [SASL] is often
provided at the application layer, and XMPP is the application
itself.
15.6. Mandatory-to-Implement Technologies
At a minimum, all implementations MUST support the following
mechanisms:
for confidentiality only: TLS (using the
TLS_RSA_WITH_AES_128_CBC_SHA cipher)
for both confidentiality and authentication: TLS plus the SASL PLAIN
mechanism (See [PLAIN]) for password-based authentication and TLS
plus the SASL EXTERNAL mechanism (see Appendix A of [SASL]) for
non-password-based authentication (using the
TLS_RSA_WITH_AES_128_CBC_SHA cipher supporting peer certificates)
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Naturally, implementations MAY support other ciphers with TLS and MAY
support other SASL mechanisms.
Note: The use of TLS plus SASL PLAIN replaces the SASL DIGEST-MD5
mechanism as XMPP's mandatory-to-implement password-based method
for authentication. For backward-compatibility, implementations
are encouraged to continue supporting the SASL DIGEST-MD5
mechanism as specified in [DIGEST-MD5]. Refer to [PLAIN] for
important security considerations related to the SASL PLAIN
mechanism.
15.7. Hash Function Agility
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].
15.8. SASL Downgrade Attacks
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 prevent this attack, the parties SHOULD
protect the channel using TLS before attempting SASL negotiation.
15.9. Lack of SASL Channel Binding to TLS
The SASL framework itself does not provide a method for binding SASL
authentication to a security layer providing confidentiality and
integrity protection that was negotiated at a lower layer. 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.
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15.10. Use of base64 in SASL
Both the client and the server MUST verify any base64 data received
during SASL negotiation (Section 7). An implementation MUST reject
(not ignore) any characters that are not explicitly allowed by the
base64 alphabet; this helps to guard against creation of a covert
channel that could be used to "leak" information.
An implementation MUST NOT break on invalid input and MUST reject any
sequence of base64 characters containing the pad ('=') character if
that character is included as something other than the last character
of the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against
buffer overflow attacks and other attacks on the implementation.
While base 64 encoding visually hides otherwise easily recognized
information (such as passwords), it does not provide any
computational confidentiality.
All uses of base 64 encoding MUST follow the definition in Section 4
of [BASE64] and padding bits MUST be set to zero.
15.11. Stringprep Profiles
XMPP makes use of the [NAMEPREP] profile of [STRINGPREP] for
processing of domain identifiers; for security considerations related
to Nameprep, refer to the appropriate section of [NAMEPREP].
In addition, XMPP defines two profiles of [STRINGPREP]: Nodeprep
(Appendix A) for localparts and Resourceprep (Appendix B) for
resource identifiers.
The Unicode and ISO/IEC 10646 repertoires have many characters that
look similar. In many cases, users of security protocols might
perform visual matching, such as when comparing the names of trusted
third parties. Because it is impossible to map similar-looking
characters without a great deal of context (such as knowing the fonts
used), stringprep does nothing to map similar-looking characters
together, nor to prohibit some characters because they look like
others.
A localpart can be employed as one part of an entity's address in
XMPP. One common usage is as the username of an instant messaging
user; another is as the name of a multi-user conference room; and
many other kinds of entities could use localparts as part of their
addresses. The security of such services could be compromised based
on different interpretations of the internationalized localpart; for
example, a user entering a single internationalized localpart could
access another user's account information, or a user could gain
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access to a hidden or otherwise restricted chat room or service.
A resource identifier can be employed as one part of an entity's
address in XMPP. One common usage is as the name for an instant
messaging user's connected resource; another is as the nickname of a
user in a multi-user conference room; and many other kinds of
entities could use resource identifiers as part of their addresses.
The security of such services could be compromised based on different
interpretations of the internationalized resource identifier; for
example, a user could attempt to initiate multiple connections with
the same name, or a user could send a message to someone other than
the intended recipient in a multi-user conference room.
15.12. Address Spoofing
As discussed in [XEP-0165], there are two forms of address spoofing:
forging and mimicking.
15.12.1. Address Forging
In the context of XMPP technologies, address forging occurs when an
entity is able to generate an XML stanza whose 'from' address does
not correspond to the account credentials with which the entity
authenticated onto the network (or an authorization identity provided
during SASL negotiation (Section 7)). For example, address forging
occurs if an entity that authenticated as "juliet@im.example.com" is
able to send XML stanzas from "nurse@im.example.com" or
"romeo@example.net".
Address forging is difficult in XMPP systems, given the requirement
for sending servers to stamp 'from' addresses and for receiving
servers to verify sending domains via server-to-server
authentication. However, address forging is not impossible, since a
rogue server could forge JIDs at the sending domain by ignoring the
stamping requirement. A rogue server could even forge JIDs at other
domains by means of a DNS poisoning attack if [DNSSEC] is not used.
This specification does not define methods for discovering or
counteracting such rogue servers.
Note: An entity outside the security perimeter of a particular server
cannot reliably distinguish between bare JIDs of the form
<localpart@domain> at that server, since the server could forge any
such JID; therefore only the domain identifier can be authenticated
or authorized with any level of assurance.
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15.12.2. Address Mimicking
Address mimicking occus when an entity provides legitimate
authentication credentials for and sends XML stanzas from an account
whose JID appears to a human user to be the same as another JID. For
example, in some XMPP clients the address "paypa1@example.org"
(spelled with the number one as the final character of the localpart)
might appear to be the same as "paypal@example.org (spelled with the
lower-case version of the letter "L"), especially on casual visual
inspection; this phenomenon is sometimes called "typejacking". A
more sophisticated example of address mimicking might involve the use
of characters from outside the US-ASCII range, such as the Cherokee
characters U+13DA U+13A2 U+13B5 U+13AC U+13A2 U+13AC U+13D2 instead
of the US-ASCII characters "STPETER".
In some examples of address mimicking, it is unlikely that the
average user could tell the difference between the real JID and the
fake JID. (Naturally, there is no way to distinguish with full
certainty which is the fake JID and which is the real JID; in some
communication contexts, the JID with Cherokee characters might be the
real JID and the JID with US-ASCII characters might thus appear to be
the fake JID.) Because JIDs can contain almost any Unicode
character, it can be relatively easy to mimic some JIDs in XMPP
systems. The possibility of address mimicking introduces security
vulnerabilities of the kind that have also plagued the World Wide
Web, specifically the phenomenon known as phishing.
Mimicked addresses that involve characters from only one character
set or from the character set typically employed by a particular user
are not easy to combat (e.g., the simple typejacking attack
previously described, which relies on a surface similarity between
the characters "1" and "l" in some presentations). However, mimicked
addresses that involve characters from more than one character set,
or from a character set not typically employed by a particular user,
can be mitigated somewhat through intelligent presentation. In
particular, every human user of an XMPP technology presumably has a
preferred language (or, in some cases, a small set of preferred
languages), which an XMPP application SHOULD gather either explicitly
from the user or implicitly via the operating system of the user's
device. Furthermore, every language has a range (or a small set of
ranges) of characters normally used to represent that language in
textual form. Therefore, an XMPP application SHOULD warn the user
when presenting a JID that uses characters outside the normal range
of the user's preferred language(s). This recommendation is not
intended to discourage communication across language communities;
instead, it recognizes the existence of such language communities and
encourages due caution when presenting unfamiliar character sets to
human users.
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For more detailed recommendations regarding prevention of address
mimicking in XMPP systems, refer to [XEP-0165].
15.13. 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 16). Use of these well-known
ports allows administrators to easily enable or disable XMPP activity
through existing and commonly-deployed firewalls.
15.14. Denial of Service
[DOS] defines denial of service as follows:
A Denial-of-Service (DoS) attack is an attack in which one or more
machines target a victim and attempt to prevent the victim from
doing useful work. The victim can be a network server, client or
router, a network link or an entire network, an individual
Internet user or a company doing business using the Internet, an
Internet Service Provider (ISP), country, or any combination of or
variant on these.
[XEP-0205] provides a detailed discussion of potential denial of
service attacks against XMPP systems and best practices for
preventing such attacks. The recommendations include:
1. A server implementation SHOULD enable a server administrator to
limit the number of TCP connections that it will accept from a
given IP address at any one time. If an entity attempts to
connect but the maximum number of TCP connections has been
reached, the receiving server MUST NOT allow the new connection
to proceed.
2. A server implementation SHOULD enable a server administrator to
limit the number of TCP connection attempts that it will accept
from a given IP address in a given time period. (While it is
possible to limit the number of connections at the TCP layer
rather than at the XMPP application layer, this is not advisable
because limits at the TCP layer might result in an inability to
access non-XMPP services.) If an entity attempts to connect but
the maximum number of connections has been reached, the receiving
server MUST NOT allow the new connection to proceed.
3. A server MUST NOT process XML stanzas from clients that have not
yet provided appropriate authentication credentials and MUST NOT
process XML stanzas from peer servers whose identity it has not
either authenticated via SASL or weakly verified via server
dialback (see [XEP-0220]).
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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].
15.15. Presence Leaks
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
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availability is associated with a specific connected resource;
therefore, any disclosure of a connected resource's full JID
comprises a presence leak. To help prevent such a presence leak, a
server MUST NOT return different stanza errors if a potential
attacker sends XML stanzas to the entity's bare JID
(<localpart@domain>) or full JID (<localpart@domain/resource>).
15.16. 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]).
16. IANA Considerations
The following sections update the registrations provided in
[RFC3920].
16.1. XML Namespace Name for TLS Data
A URN sub-namespace for STARTTLS negotiation data in the Extensible
Messaging and Presence Protocol (XMPP) is defined as follows. (This
namespace name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-tls
Specification: XXXX
Description: This is the XML namespace name for STARTTLS negotiation
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@xmpp.org>
16.2. XML Namespace Name for SASL Data
A URN sub-namespace for SASL negotiation data in the Extensible
Messaging and Presence Protocol (XMPP) is defined as follows. (This
namespace name adheres to the format defined in [XML-REG].)
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URI: urn:ietf:params:xml:ns:xmpp-sasl
Specification: XXXX
Description: This is the XML namespace name for SASL negotiation
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@xmpp.org>
16.3. XML Namespace Name for Stream Errors
A URN sub-namespace for stream error data in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-streams
Specification: XXXX
Description: This is the XML namespace name for stream error data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@xmpp.org>
16.4. XML Namespace Name for Resource Binding
A URN sub-namespace for resource binding in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-bind
Specification: XXXX
Description: This is the XML namespace name for resource binding in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@xmpp.org>
16.5. XML Namespace Name for Stanza Errors
A URN sub-namespace for stanza error data in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-stanzas
Specification: XXXX
Description: This is the XML namespace name for stanza error data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
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16.6. Nodeprep Profile of Stringprep
The Nodeprep profile of stringprep is defined under Nodeprep
(Appendix A). The IANA has registered Nodeprep in the stringprep
profile registry.
Name of this profile:
Nodeprep
RFC in which the profile is defined:
XXXX
Indicator whether or not this is the newest version of the profile:
This is the first version of Nodeprep
16.7. Resourceprep Profile of Stringprep
The Resourceprep profile of stringprep is defined under Resourceprep
(Appendix B). The IANA has registered Resourceprep in the stringprep
profile registry.
Name of this profile:
Resourceprep
RFC in which the profile is defined:
XXXX
Indicator whether or not this is the newest version of the profile:
This is the first version of Resourceprep
16.8. GSSAPI Service Name
The IANA has registered "xmpp" as a GSSAPI [GSS-API] service name, as
defined under Section 7.5.
16.9. Port Numbers
The IANA has registered "xmpp-client" and "xmpp-server" as keywords
for [TCP] ports 5222 and 5269 respectively.
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These ports SHOULD be used for client-to-server and server-to-server
communications respectively, but other ports MAY be used.
17. References
17.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[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] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[IDNA] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[LANGTAGS]
Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 4646, September 2006.
[NAMEPREP]
Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[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.
[STRINGPREP]
Hoffman, P. and M. Blanchet, "Preparation of
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Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[TCP] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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-SRV]
Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
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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-
xml-20060816, August 2006,
<http://www.w3.org/TR/2006/REC-xml-20060816>.
[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>.
17.2. 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.
[DOS] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006.
[GSS-API] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[HASHES] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270, November 2005.
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[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.
[IRI] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[LINKLOCAL]
Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
May 2005.
[MAILBOXES]
Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
FUNCTIONS", RFC 2142, May 1997.
[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[PUNYCODE]
Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[RFC3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.
[RFC3921] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
RFC 3921, October 2004.
[SECTERMS]
Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[URN-OID] Mealling, M., "A URN Namespace of Object Identifiers",
RFC 3061, February 2001.
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[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.
[XEP-0030]
Hildebrand, J., Millard, P., Eatmon, R., and P. Saint-
Andre, "Service Discovery", XSF XEP 0030, June 2008.
[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-0165]
Saint-Andre, P., "Best Practices to Prevent JID
Mimicking", XSF XEP 0165, December 2007.
[XEP-0174]
Saint-Andre, P., "Link-Local Messaging", XSF XEP 0174,
November 2008.
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[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-0205]
Saint-Andre, P., "Best Practices to Discourage Denial of
Service Attacks", XSF XEP 0205, January 2009.
[XEP-0199]
Saint-Andre, P., "XMPP Ping", XSF XEP 0199, June 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.
[XEP-0271]
Saint-Andre, P., "XMPP Nodes", XSF XEP 0271, June 2009.
[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,
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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-01 (work in progress),
August 2009.
[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. Nodeprep
A.1. Introduction
This appendix defines the "Nodeprep" profile of stringprep. As such,
it specifies processing rules that will enable users to enter
internationalized localparts in the Extensible Messaging and Presence
Protocol (XMPP) and have the highest chance of getting the content of
the strings correct. (An XMPP localpart is the optional portion of
an XMPP address that precedes an XMPP domain identifier and the '@'
separator; it is often but not exclusively associated with an instant
messaging username.) These processing rules are intended only for
XMPP localparts and are not intended for arbitrary text or any other
aspect of an XMPP address.
This profile defines the following, as required by [STRINGPREP]:
o The intended applicability of the profile: internationalized
localparts within XMPP
o The character repertoire that is the input and output to
stringprep: Unicode 3.2, specified in Section 2 of this Appendix
o The mappings used: specified in Section 3
o The Unicode normalization used: specified in Section 4
o The characters that are prohibited as output: specified in Section
5
o Bidirectional character handling: specified in Section 6
A.2. Character Repertoire
This profile uses Unicode 3.2 with the list of unassigned code points
being Table A.1, both defined in Appendix A of [STRINGPREP].
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A.3. Mapping
This profile specifies mapping using the following tables from
[STRINGPREP]:
Table B.1
Table B.2
A.4. Normalization
This profile specifies the use of Unicode normalization form KC, as
described in [STRINGPREP].
A.5. Prohibited Output
This profile specifies the prohibition of using the following tables
from [STRINGPREP].
Table C.1.1
Table C.1.2
Table C.2.1
Table C.2.2
Table C.3
Table C.4
Table C.5
Table C.6
Table C.7
Table C.8
Table C.9
In addition, the following additional Unicode characters are also
prohibited:
U+0022 (QUOTATION MARK), i.e., "
U+0026 (AMPERSAND), i.e., &
U+0027 (APOSTROPHE), i.e., '
U+002F (SOLIDUS), i.e., /
U+003A (COLON), i.e., :
U+003C (LESS-THAN SIGN), i.e., <
U+003E (GREATER-THAN SIGN), i.e., >
U+0040 (COMMERCIAL AT), i.e., @
A.6. Bidirectional Characters
This profile specifies checking bidirectional strings, as described
in Section 6 of [STRINGPREP].
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A.7. Notes
Because the additional characters prohibited by Nodeprep are
prohibited after normalization, an implementation MUST NOT enable a
human user to input any Unicode code point whose decomposition
includes those characters; such code points include but are not
necessarily limited to the following (refer to [UNICODE] for complete
information).
o U+2100 (ACCOUNT OF)
o U+2101 (ADDRESSED TO THE SUBJECT)
o U+2105 (CARE OF)
o U+2106 (CADA UNA)
o U+226E (NOT LESS-THAN)
o U+226F (NOT GREATER-THAN)
o U+2A74 (DOUBLE COLON EQUAL)
o U+FE13 (SMALL COLON)
o U+FE60 (SMALL AMPERSAND)
o U+FE64 (SMALL LESS-THAN SIGN)
o U+FE65 (SMALL GREATER-THAN SIGN)
o U+FE6B (SMALL COMMERCIAL AT)
o U+FF02 (FULLWIDTH QUOTATION MARK)
o U+FF06 (FULLWIDTH AMPERSAND)
o U+FF07 (FULLWIDTH APOSTROPHE)
o U+FF0F (FULLWIDTH SOLIDUS)
o U+FF1A (FULLWIDTH COLON)
o U+FF1C (FULLWIDTH LESS-THAN SIGN)
o U+FF1E (FULLWIDTH GREATER-THAN SIGN)
o U+FF20 (FULLWIDTH COMMERCIAL AT)
Appendix B. Resourceprep
B.1. Introduction
This appendix defines the "Resourceprep" profile of stringprep. As
such, it specifies processing rules that will enable users to enter
internationalized resource identifiers in the Extensible Messaging
and Presence Protocol (XMPP) and have the highest chance of getting
the content of the strings correct. (An XMPP resource identifier is
the optional portion of an XMPP address that follows an XMPP domain
identifier and the '/' separator.) These processing rules are
intended only for XMPP resource identifiers and are not intended for
arbitrary text or any other aspect of an XMPP address.
This profile defines the following, as required by [STRINGPREP]:
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o The intended applicability of the profile: internationalized
resource identifiers within XMPP
o The character repertoire that is the input and output to
stringprep: Unicode 3.2, specified in Section 2 of this Appendix
o The mappings used: specified in Section 3
o The Unicode normalization used: specified in Section 4
o The characters that are prohibited as output: specified in Section
5
o Bidirectional character handling: specified in Section 6
B.2. Character Repertoire
This profile uses Unicode 3.2 with the list of unassigned code points
being Table A.1, both defined in Appendix A of [STRINGPREP].
B.3. Mapping
This profile specifies mapping using the following tables from
[STRINGPREP]:
Table B.1
B.4. Normalization
This profile specifies the use of Unicode normalization form KC, as
described in [STRINGPREP].
B.5. Prohibited Output
This profile specifies the prohibition of using the following tables
from [STRINGPREP].
Table C.1.2
Table C.2.1
Table C.2.2
Table C.3
Table C.4
Table C.5
Table C.6
Table C.7
Table C.8
Table C.9
B.6. Bidirectional Characters
This profile specifies checking bidirectional strings, as described
in Section 6 of [STRINGPREP].
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Appendix C. XML Schemas
Because validation of XML streams and stanzas is optional, the
following XML schemas are provided for descriptive purposes only.
These schemas are not normative.
The following schemas formally define various XML namespaces used in
the core XMPP protocols, in conformance with [XML-SCHEMA]. For
schemas defining the 'jabber:client' and 'jabber:server' namespaces,
refer to [xmpp-im].
C.1. Streams Namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='http://etherx.jabber.org/streams'
xmlns='http://etherx.jabber.org/streams'
elementFormDefault='unqualified'>
<xs:import namespace='jabber:client'/>
<xs:import namespace='jabber:server'/>
<xs: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'/>
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<xs:element ref='db:result'/>
<xs:element ref='db:verify'/>
</xs:choice>
</xs:choice>
<xs:element ref='error' minOccurs='0' maxOccurs='1'/>
</xs:sequence>
<xs:attribute name='from' type='xs:string' use='optional'/>
<xs:attribute name='id' type='xs:string' use='optional'/>
<xs:attribute name='to' type='xs:string' use='optional'/>
<xs:attribute name='version' type='xs:decimal' use='optional'/>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='features'>
<xs:complexType>
<xs:any namespace='##other'/>
</xs:complexType>
</xs:element>
<xs:element name='error'>
<xs:complexType>
<xs:sequence xmlns:err='urn:ietf:params:xml:ns:xmpp-streams'>
<xs:group ref='err:streamErrorGroup'/>
<xs:element ref='err:text'
minOccurs='0'
maxOccurs='1'/>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
C.2. Stream Error Namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-streams'
xmlns='urn:ietf:params:xml:ns:xmpp-streams'
elementFormDefault='qualified'>
<xs:element name='bad-format' type='empty'/>
<xs:element name='bad-namespace-prefix' type='empty'/>
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<xs:element name='conflict' type='empty'/>
<xs:element name='connection-timeout' type='empty'/>
<xs:element name='host-gone' type='empty'/>
<xs:element name='host-unknown' type='empty'/>
<xs:element name='improper-addressing' type='empty'/>
<xs:element name='internal-server-error' type='empty'/>
<xs:element name='invalid-from' type='empty'/>
<xs:element name='invalid-id' type='empty'/>
<xs:element name='invalid-namespace' type='empty'/>
<xs:element name='invalid-xml' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='policy-violation' type='empty'/>
<xs:element name='remote-connection-failed' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='restricted-xml' type='empty'/>
<xs:element name='see-other-host' type='xs:string'/>
<xs:element name='system-shutdown' type='empty'/>
<xs:element name='undefined-condition' type='empty'/>
<xs:element name='unsupported-encoding' type='empty'/>
<xs:element name='unsupported-stanza-type' type='empty'/>
<xs:element name='unsupported-version' type='empty'/>
<xs:element name='xml-not-well-formed' type='empty'/>
<xs:group name='streamErrorGroup'>
<xs:choice>
<xs:element ref='bad-format'/>
<xs:element ref='bad-namespace-prefix'/>
<xs:element ref='conflict'/>
<xs:element ref='connection-timeout'/>
<xs:element ref='host-gone'/>
<xs:element ref='host-unknown'/>
<xs:element ref='improper-addressing'/>
<xs:element ref='internal-server-error'/>
<xs:element ref='invalid-from'/>
<xs:element ref='invalid-id'/>
<xs:element ref='invalid-namespace'/>
<xs:element ref='invalid-xml'/>
<xs:element ref='not-authorized'/>
<xs:element ref='policy-violation'/>
<xs:element ref='remote-connection-failed'/>
<xs:element ref='resource-constraint'/>
<xs:element ref='restricted-xml'/>
<xs:element ref='see-other-host'/>
<xs:element ref='system-shutdown'/>
<xs:element ref='undefined-condition'/>
<xs:element ref='unsupported-encoding'/>
<xs:element ref='unsupported-stanza-type'/>
<xs:element ref='unsupported-version'/>
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<xs:element ref='xml-not-well-formed'/>
</xs:choice>
</xs:group>
<xs:element name='text'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</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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C.3. STARTTLS Namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-tls'
xmlns='urn:ietf:params:xml:ns:xmpp-tls'
elementFormDefault='qualified'>
<xs:element name='starttls'>
<xs:complexType>
<xs:choice minOccurs='0' maxOccurs='1'>
<xs:element name='optional' type='empty'/>
<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>
C.4. SASL Namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-sasl'
xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
elementFormDefault='qualified'>
<xs:element name='mechanisms'>
<xs:complexType>
<xs:sequence>
<xs:element name='mechanism'
minOccurs='1'
maxOccurs='unbounded'
type='xs:NMTOKEN'/>
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<xs:choice minOccurs='0' maxOccurs='1'>
<xs:element name='optional' type='empty'/>
<xs:element name='required' type='empty'/>
</xs:choice>
<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'/>
<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>
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<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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C.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:choice>
<xs:element name='resource' type='resourceType'/>
<xs:element name='jid' type='fullJIDType'/>
</xs:choice>
<xs:choice>
<xs:element name='optional' type='empty'/>
<xs:element name='required' type='empty'/>
</xs:choice>
</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>
C.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'
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xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
elementFormDefault='qualified'>
<xs:element name='bad-request' type='empty'/>
<xs:element name='conflict' type='empty'/>
<xs:element name='feature-not-implemented' type='empty'/>
<xs:element name='forbidden' type='empty'/>
<xs:element name='gone' type='xs:string'/>
<xs:element name='internal-server-error' type='empty'/>
<xs:element name='item-not-found' type='empty'/>
<xs:element name='jid-malformed' type='empty'/>
<xs:element name='not-acceptable' type='empty'/>
<xs:element name='not-allowed' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='not-modified' type='empty'/>
<xs:element name='payment-required' type='empty'/>
<xs:element name='recipient-unavailable' type='empty'/>
<xs:element name='redirect' type='xs:string'/>
<xs:element name='registration-required' type='empty'/>
<xs:element name='remote-server-not-found' type='empty'/>
<xs:element name='remote-server-timeout' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='service-unavailable' type='empty'/>
<xs:element name='subscription-required' type='empty'/>
<xs:element name='undefined-condition' type='empty'/>
<xs:element name='unexpected-request' type='empty'/>
<xs:element name='unknown-sender' type='empty'/>
<xs:group name='stanzaErrorGroup'>
<xs:choice>
<xs:element ref='bad-request'/>
<xs:element ref='conflict'/>
<xs:element ref='feature-not-implemented'/>
<xs:element ref='forbidden'/>
<xs:element ref='gone'/>
<xs:element ref='internal-server-error'/>
<xs:element ref='item-not-found'/>
<xs:element ref='jid-malformed'/>
<xs:element ref='not-acceptable'/>
<xs:element ref='not-authorized'/>
<xs:element ref='not-allowed'/>
<xs:element ref='not-modified'/>
<xs:element ref='payment-required'/>
<xs:element ref='recipient-unavailable'/>
<xs:element ref='redirect'/>
<xs:element ref='registration-required'/>
<xs:element ref='remote-server-not-found'/>
<xs:element ref='remote-server-timeout'/>
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<xs:element ref='resource-constraint'/>
<xs:element ref='service-unavailable'/>
<xs:element ref='subscription-required'/>
<xs:element ref='undefined-condition'/>
<xs:element ref='unexpected-request'/>
<xs:element ref='unknown-sender'/>
</xs:choice>
</xs:group>
<xs:element name='text'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
Appendix D. Contact Addresses
Consistent with [MAILBOXES], an organization that offers an XMPP
service SHOULD provide an Internet mailbox of "XMPP" for inquiries
related to that service, where the host portion of the resulting
mailto URI 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 E. Account Provisioning
Account provisioning is out of scope for this specification.
Possible methods for account provisioning include account creation by
a server administrator and in-band account registration using the
'jabber:iq:register' namespace as documented in [XEP-0077].
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Appendix F. Differences From RFC 3920
Based on consensus derived from implementation and deployment
experience as well as formal interoperability testing, the following
substantive modifications were made from RFC 3920.
o Corrected the ABNF syntax for JIDs to prevent zero-length
localparts, domain identifiers, and resource identifiers.
o To avoid confusion with the term "node" as used in [XEP-0030] and
[XEP-0060] (see also [XEP-0271]), changed the term "node
identifier" to "localpart" (but retained the name "Nodeprep" for
backward compatibility).
o Corrected the nameprep processing rules to require use of the
UseSTD3ASCIIRules flag.
o Recommended or mandated use of the 'from' and 'to' attributes on
stream headers.
o More fully specified stream closing handshake.
o Specified recommended stream reconnection algorithm.
o Specified return of <restricted-xml/> stream error in response to
receipt of prohibited XML features.
o Specified that TLS plus SASL PLAIN is a mandatory-to-implement
technology for client-to-server connections, since implementation
of SASL EXTERNAL is uncommon in XMPP clients, in part because
underlying security features such as end-user X.509 certificates
are not yet widely deployed.
o Added the <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 Added the <not-modified/> stanza error condition to enable
potential ETags usage.
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 process of DNS SRV lookups and fallbacks.
o Clarified handling of SASL security layers.
o Clarified handling of stream features, regularized use of the
<required/> child element, and defined use of the <optional/>
child element.
o Clarified handling of data that violates the well-formedness
definitions for XML 1.0 and XML namespaces.
o Specified 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
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order to more fully specify and clearly explain XMPP.
Appendix G. 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
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
B
Bare JID 17
C
Connected Resource 75
D
Domain Identifier 15
E
Entity 14
Error Stanza 87
Extended Content 104
F
Full JID 17
I
Initial Stream 22
IQ Stanza 86
J
Jabber Identifier 14
L
Localpart 17
M
Message Stanza 85
P
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Presence Stanza 85
R
Resource Identifier 17
Response Stream 22
S
Stream ID 27
W
Whitespace Keepalive 35
X
XML Stanza 22
XML Stream 22
Author's Address
Peter Saint-Andre
Cisco
Email: psaintan@cisco.com
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