RFC 6120
Extensible Messaging and Presence Protocol (XMPP): Core
Part 1 of 40 🎀
Extensible Messaging and Presence Protocol (XMPP): Core ✨
Okay bestie, let's start with the basics 💅 This is RFC 6120, from the Internet Engineering Task Force (IETF), written by P. Saint-Andre at Cisco. It's a Standards Track doc, dated March 2011, with ISSN 2070-1721. It obsoletes RFC 3920 🫶
Abstract
okay so XMPP is an application profile of XML 🦋 It's built for near-real-time swapping of structured but still extensible data between any two or more network entities, which is giving main character energy 👑 This doc defines XMPP's core methods: setting up and tearing down XML streams, channel encryption, authentication, error handling, and the basic communication primitives for messaging, network availability (aka "presence"), and request-response interactions 💖 It also obsoletes RFC 3920, so that one is officially retired 🌸
Status of This Memo
this is an Internet Standards Track document 🔥
it's a product of the IETF, which means it represents the consensus of the IETF community. It went through public review and got approved for publication by the Internet Engineering Steering Group (IESG) ✨ If you want more info on Internet Standards, check out Section 2 of RFC 5741 🧚
for the current status, any errata, and how to give feedback, go to http://www.rfc-editor.org/info/rfc6120 🌈
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. 💜
this document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) as they stand on the date of publication 🎀 Please actually read these, bestie, because they spell out your rights and restrictions for this document 🙈
also, if you extract code components from this document, they must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they come without warranty as described in the Simplified BSD License 💅
Table of Contents 📚
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 8
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2. History . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3. Functional Summary . . . . . . . . . . . . . . . . . . . 9
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . 11
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1. Global Addresses . . . . . . . . . . . . . . . . . . . . 13
2.2. Presence . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3. Persistent Streams . . . . . . . . . . . . . . . . . . . 14
2.4. Structured Data . . . . . . . . . . . . . . . . . . . . 14
2.5. Distributed Network of Clients and Servers . . . . . . . 14
3. TCP Binding . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2. Resolution of Fully Qualified Domain Names . . . . . . . 17
3.2.1. Preferred Process: SRV Lookup . . . . . . . . . . . 17
3.2.2. Fallback Processes . . . . . . . . . . . . . . . . . 18
3.2.3. When Not to Use SRV . . . . . . . . . . . . . . . . 18
3.2.4. Use of SRV Records with Add-On Services . . . . . . 19
3.3. Reconnection . . . . . . . . . . . . . . . . . . . . . . 19
3.4. Reliability . . . . . . . . . . . . . . . . . . . . . . 20
4. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1. Stream Fundamentals . . . . . . . . . . . . . . . . . . 20
4.2. Opening a Stream . . . . . . . . . . . . . . . . . . . . 23
4.3. Stream Negotiation . . . . . . . . . . . . . . . . . . . 24
4.3.1. Basic Concepts . . . . . . . . . . . . . . . . . . . 24
4.3.2. Stream Features Format . . . . . . . . . . . . . . . 25
4.3.3. Restarts . . . . . . . . . . . . . . . . . . . . . . 27
4.3.4. Resending Features . . . . . . . . . . . . . . . . . 27
4.3.5. Completion of Stream Negotiation . . . . . . . . . . 27
4.3.6. Determination of Addresses . . . . . . . . . . . . . 28
4.3.7. Flow Chart . . . . . . . . . . . . . . . . . . . . . 29
4.4. Closing a Stream . . . . . . . . . . . . . . . . . . . . 31
4.5. Directionality . . . . . . . . . . . . . . . . . . . . . 32
4.6. Handling of Silent Peers . . . . . . . . . . . . . . . . 33
4.6.1. Dead Connection . . . . . . . . . . . . . . . . . . 34
4.6.2. Broken Stream . . . . . . . . . . . . . . . . . . . 34
4.6.3. Idle Peer . . . . . . . . . . . . . . . . . . . . . 34
4.6.4. Use of Checking Methods . . . . . . . . . . . . . . 35
4.7. Stream Attributes . . . . . . . . . . . . . . . . . . . 35
4.7.1. from . . . . . . . . . . . . . . . . . . . . . . . . 35
4.7.2. to . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.7.3. id . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.7.4. xml:lang . . . . . . . . . . . . . . . . . . . . . . 39
4.7.5. version . . . . . . . . . . . . . . . . . . . . . . 41
4.7.6. Summary of Stream Attributes . . . . . . . . . . . . 43
4.8. XML Namespaces . . . . . . . . . . . . . . . . . . . . . 43that's the whole map for part 1 🧚 the actual sections (starting with 1. Introduction) are coming in the next parts, so stay tuned 💖
4.8.1. Stream Namespace . . . . . . . . . . . . . . . . . . 43 4.8.2. Content Namespace . . . . . . . . . . . . . . . . . 43 4.8.3. XMPP Content Namespaces . . . . . . . . . . . . . . 44 4.8.4. Other Namespaces . . . . . . . . . . . . . . . . . . 46 4.8.5. Namespace Declarations and Prefixes . . . . . . . . 47 4.9. Stream Errors . . . . . . . . . . . . . . . . . . . . . 48 4.9.1. Rules . . . . . . . . . . . . . . . . . . . . . . . 48 4.9.1.1. Stream Errors Are Unrecoverable . . . . . . . . . 48 4.9.1.2. Stream Errors Can Occur During Setup . . . . . . 49 4.9.1.3. Stream Errors When the Host Is Unspecified or Unknown . . . . . . . . . . . . . . . . . . . . . 50 4.9.1.4. Where Stream Errors Are Sent . . . . . . . . . . 50 4.9.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 51 4.9.3. Defined Stream Error Conditions . . . . . . . . . . 52 4.9.3.1. bad-format . . . . . . . . . . . . . . . . . . . 52 4.9.3.2. bad-namespace-prefix . . . . . . . . . . . . . . 52 4.9.3.3. conflict . . . . . . . . . . . . . . . . . . . . 53 4.9.3.4. connection-timeout . . . . . . . . . . . . . . . 54 4.9.3.5. host-gone . . . . . . . . . . . . . . . . . . . . 54 4.9.3.6. host-unknown . . . . . . . . . . . . . . . . . . 55 4.9.3.7. improper-addressing . . . . . . . . . . . . . . . 56 4.9.3.8. internal-server-error . . . . . . . . . . . . . . 56 4.9.3.9. invalid-from . . . . . . . . . . . . . . . . . . 56 4.9.3.10. invalid-namespace . . . . . . . . . . . . . . . . 57 4.9.3.11. invalid-xml . . . . . . . . . . . . . . . . . . . 57 4.9.3.12. not-authorized . . . . . . . . . . . . . . . . . 58 4.9.3.13. not-well-formed . . . . . . . . . . . . . . . . . 59 4.9.3.14. policy-violation . . . . . . . . . . . . . . . . 59 4.9.3.15. remote-connection-failed . . . . . . . . . . . . 60 4.9.3.16. reset . . . . . . . . . . . . . . . . . . . . . . 60 4.9.3.17. resource-constraint . . . . . . . . . . . . . . . 61 4.9.3.18. restricted-xml . . . . . . . . . . . . . . . . . 61 4.9.3.19. see-other-host . . . . . . . . . . . . . . . . . 62 4.9.3.20. system-shutdown . . . . . . . . . . . . . . . . . 64 4.9.3.21. undefined-condition . . . . . . . . . . . . . . . 64 4.9.3.22. unsupported-encoding . . . . . . . . . . . . . . 64 4.9.3.23. unsupported-feature . . . . . . . . . . . . . . . 65 4.9.3.24. unsupported-stanza-type . . . . . . . . . . . . . 65 4.9.3.25. unsupported-version . . . . . . . . . . . . . . . 66 4.9.4. Application-Specific Conditions . . . . . . . . . . 67 4.10. Simplified Stream Examples . . . . . . . . . . . . . . . 68
- STARTTLS Negotiation . . . . . . . . . . . . . . . . . . . . 69 5.1. Fundamentals . . . . . . . . . . . . . . . . . . . . . . 69 5.2. Support . . . . . . . . . . . . . . . . . . . . . . . . 70 5.3. Stream Negotiation Rules . . . . . . . . . . . . . . . . 70 5.3.1. Mandatory-to-Negotiate . . . . . . . . . . . . . . . 70 5.3.2. Restart . . . . . . . . . . . . . . . . . . . . . . 70 5.3.3. Data Formatting . . . . . . . . . . . . . . . . . . 70 5.3.4. Order of TLS and SASL Negotiations . . . . . . . . . 71 5.3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . 71 5.3.6. TLS Extensions . . . . . . . . . . . . . . . . . . . 72 5.4. Process . . . . . . . . . . . . . . . . . . . . . . . . 72 5.4.1. Exchange of Stream Headers and Stream Features . . . 72 5.4.2. Initiation of STARTTLS Negotiation . . . . . . . . . 73 5.4.2.1. STARTTLS Command . . . . . . . . . . . . . . . . 73 5.4.2.2. Failure Case . . . . . . . . . . . . . . . . . . 73 5.4.2.3. Proceed Case . . . . . . . . . . . . . . . . . . 74 5.4.3. TLS Negotiation . . . . . . . . . . . . . . . . . . 74 5.4.3.1. Rules . . . . . . . . . . . . . . . . . . . . . . 74 5.4.3.2. TLS Failure . . . . . . . . . . . . . . . . . . . 75 5.4.3.3. TLS Success . . . . . . . . . . . . . . . . . . . 76
- SASL Negotiation . . . . . . . . . . . . . . . . . . . . . . 77 6.1. Fundamentals . . . . . . . . . . . . . . . . . . . . . . 77 6.2. Support . . . . . . . . . . . . . . . . . . . . . . . . 77 6.3. Stream Negotiation Rules . . . . . . . . . . . . . . . . 77 6.3.1. Mandatory-to-Negotiate . . . . . . . . . . . . . . . 77 6.3.2. Restart . . . . . . . . . . . . . . . . . . . . . . 78 6.3.3. Mechanism Preferences . . . . . . . . . . . . . . . 78 6.3.4. Mechanism Offers . . . . . . . . . . . . . . . . . . 78 6.3.5. Data Formatting . . . . . . . . . . . . . . . . . . 79 6.3.6. Security Layers . . . . . . . . . . . . . . . . . . 80 6.3.7. Simple User Name . . . . . . . . . . . . . . . . . . 80 6.3.8. Authorization Identity . . . . . . . . . . . . . . . 80 6.3.9. Realms . . . . . . . . . . . . . . . . . . . . . . . 81 6.3.10. Round Trips . . . . . . . . . . . . . . . . . . . . 81 6.4. Process . . . . . . . . . . . . . . . . . . . . . . . . 82 6.4.1. Exchange of Stream Headers and Stream Features . . . 82 6.4.2. Initiation . . . . . . . . . . . . . . . . . . . . . 83 6.4.3. Challenge-Response Sequence . . . . . . . . . . . . 84 6.4.4. Abort . . . . . . . . . . . . . . . . . . . . . . . 84 6.4.5. SASL Failure . . . . . . . . . . . . . . . . . . . . 85 6.4.6. SASL Success . . . . . . . . . . . . . . . . . . . . 86 6.5. SASL Errors . . . . . . . . . . . . . . . . . . . . . . 87 6.5.1. aborted . . . . . . . . . . . . . . . . . . . . . . 88 6.5.2. account-disabled . . . . . . . . . . . . . . . . . . 88 6.5.3. credentials-expired . . . . . . . . . . . . . . . . 88 6.5.4. encryption-required . . . . . . . . . . . . . . . . 89 6.5.5. incorrect-encoding . . . . . . . . . . . . . . . . . 89 6.5.6. invalid-authzid . . . . . . . . . . . . . . . . . . 89 6.5.7. invalid-mechanism . . . . . . . . . . . . . . . . . 90 6.5.8. malformed-request . . . . . . . . . . . . . . . . . 90 6.5.9. mechanism-too-weak . . . . . . . . . . . . . . . . . 90 6.5.10. not-authorized . . . . . . . . . . . . . . . . . . . 91 6.5.11. temporary-auth-failure . . . . . . . . . . . . . . . 91 6.6. SASL Definition . . . . . . . . . . . . . . . . . . . . 91
- Resource Binding . . . . . . . . . . . . . . . . . . . . . . 92
okay bestie, this is the map of the whole spec picking up at section 7.1 💖 every heading is listed with its page number, so you can jump straight to the good stuff 🗺️✨
7.1. Fundamentals . . . . . . . . . . . . . . . . . . . . . . 92
7.2. Support . . . . . . . . . . . . . . . . . . . . . . . . 93
7.3. Stream Negotiation Rules . . . . . . . . . . . . . . . . 93
7.3.1. Mandatory-to-Negotiate . . . . . . . . . . . . . . . 93
7.3.2. Restart . . . . . . . . . . . . . . . . . . . . . . 93
7.4. Advertising Support . . . . . . . . . . . . . . . . . . 93
7.5. Generation of Resource Identifiers . . . . . . . . . . . 94
7.6. Server-Generated Resource Identifier . . . . . . . . . . 94
7.6.1. Success Case . . . . . . . . . . . . . . . . . . . . 94
7.6.2. Error Cases . . . . . . . . . . . . . . . . . . . . 95
7.6.2.1. Resource Constraint . . . . . . . . . . . . . . . 95
7.6.2.2. Not Allowed . . . . . . . . . . . . . . . . . . . 96
7.7. Client-Submitted Resource Identifier . . . . . . . . . . 96
7.7.1. Success Case . . . . . . . . . . . . . . . . . . . . 96
7.7.2. Error Cases . . . . . . . . . . . . . . . . . . . . 97
7.7.2.1. Bad Request . . . . . . . . . . . . . . . . . . . 97
7.7.2.2. Conflict . . . . . . . . . . . . . . . . . . . . 97
7.7.3. Retries . . . . . . . . . . . . . . . . . . . . . . 99
8. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 99
8.1. Common Attributes . . . . . . . . . . . . . . . . . . . 100
8.1.1. to . . . . . . . . . . . . . . . . . . . . . . . . . 100
8.1.1.1. Client-to-Server Streams . . . . . . . . . . . . 100
8.1.1.2. Server-to-Server Streams . . . . . . . . . . . . 101
8.1.2. from . . . . . . . . . . . . . . . . . . . . . . . . 101
8.1.2.1. Client-to-Server Streams . . . . . . . . . . . . 101
8.1.2.2. Server-to-Server Streams . . . . . . . . . . . . 102
8.1.3. id . . . . . . . . . . . . . . . . . . . . . . . . . 103
8.1.4. type . . . . . . . . . . . . . . . . . . . . . . . . 103
8.1.5. xml:lang . . . . . . . . . . . . . . . . . . . . . . 103
8.2. Basic Semantics . . . . . . . . . . . . . . . . . . . . 105
8.2.1. Message Semantics . . . . . . . . . . . . . . . . . 105
8.2.2. Presence Semantics . . . . . . . . . . . . . . . . . 105
8.2.3. IQ Semantics . . . . . . . . . . . . . . . . . . . . 105
8.3. Stanza Errors . . . . . . . . . . . . . . . . . . . . . 107
8.3.1. Rules . . . . . . . . . . . . . . . . . . . . . . . 108
8.3.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 109
8.3.3. Defined Conditions . . . . . . . . . . . . . . . . . 110
8.3.3.1. bad-request . . . . . . . . . . . . . . . . . . . 110
8.3.3.2. conflict . . . . . . . . . . . . . . . . . . . . 111
8.3.3.3. feature-not-implemented . . . . . . . . . . . . . 111
8.3.3.4. forbidden . . . . . . . . . . . . . . . . . . . . 112
8.3.3.5. gone . . . . . . . . . . . . . . . . . . . . . . 113
8.3.3.6. internal-server-error . . . . . . . . . . . . . . 113
8.3.3.7. item-not-found . . . . . . . . . . . . . . . . . 114
8.3.3.8. jid-malformed . . . . . . . . . . . . . . . . . . 114
8.3.3.9. not-acceptable . . . . . . . . . . . . . . . . . 115
8.3.3.10. not-allowed . . . . . . . . . . . . . . . . . . . 116
8.3.3.11. not-authorized . . . . . . . . . . . . . . . . . 116
8.3.3.12. policy-violation . . . . . . . . . . . . . . . . 117
8.3.3.13. recipient-unavailable . . . . . . . . . . . . . . 117
8.3.3.14. redirect . . . . . . . . . . . . . . . . . . . . 118
8.3.3.15. registration-required . . . . . . . . . . . . . . 119
8.3.3.16. remote-server-not-found . . . . . . . . . . . . . 119
8.3.3.17. remote-server-timeout . . . . . . . . . . . . . . 120
8.3.3.18. resource-constraint . . . . . . . . . . . . . . . 121
8.3.3.19. service-unavailable . . . . . . . . . . . . . . . 121
8.3.3.20. subscription-required . . . . . . . . . . . . . . 122
8.3.3.21. undefined-condition . . . . . . . . . . . . . . . 123
8.3.3.22. unexpected-request . . . . . . . . . . . . . . . 123
8.3.4. Application-Specific Conditions . . . . . . . . . . 124
8.4. Extended Content . . . . . . . . . . . . . . . . . . . . 125
9. Detailed Examples . . . . . . . . . . . . . . . . . . . . . . 128
9.1. Client-to-Server Examples . . . . . . . . . . . . . . . 128
9.1.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . 128
9.1.2. SASL . . . . . . . . . . . . . . . . . . . . . . . . 130
9.1.3. Resource Binding . . . . . . . . . . . . . . . . . . 132
9.1.4. Stanza Exchange . . . . . . . . . . . . . . . . . . 133
9.1.5. Close . . . . . . . . . . . . . . . . . . . . . . . 134
9.2. Server-to-Server Examples . . . . . . . . . . . . . . . 134
9.2.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . 134
9.2.2. SASL . . . . . . . . . . . . . . . . . . . . . . . . 136
9.2.3. Stanza Exchange . . . . . . . . . . . . . . . . . . 137
9.2.4. Close . . . . . . . . . . . . . . . . . . . . . . . 137
10. Server Rules for Processing XML Stanzas . . . . . . . . . . . 138
10.1. In-Order Processing . . . . . . . . . . . . . . . . . . 138
10.2. General Considerations . . . . . . . . . . . . . . . . . 140
10.3. No 'to' Address . . . . . . . . . . . . . . . . . . . . 141
10.3.1. Message . . . . . . . . . . . . . . . . . . . . . . 141
10.3.2. Presence . . . . . . . . . . . . . . . . . . . . . . 141
10.3.3. IQ . . . . . . . . . . . . . . . . . . . . . . . . . 141
10.4. Remote Domain . . . . . . . . . . . . . . . . . . . . . 142
10.4.1. Existing Stream . . . . . . . . . . . . . . . . . . 142
10.4.2. No Existing Stream . . . . . . . . . . . . . . . . . 142
10.4.3. Error Handling . . . . . . . . . . . . . . . . . . . 143
10.5. Local Domain . . . . . . . . . . . . . . . . . . . . . . 143
10.5.1. domainpart . . . . . . . . . . . . . . . . . . . . . 143
10.5.2. domainpart/resourcepart . . . . . . . . . . . . . . 143
10.5.3. localpart@domainpart . . . . . . . . . . . . . . . . 143
10.5.3.1. No Such User . . . . . . . . . . . . . . . . . . 144
10.5.3.2. User Exists . . . . . . . . . . . . . . . . . . . 144
10.5.4. localpart@domainpart/resourcepart . . . . . . . . . 144
11. XML Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 145
11.1. XML Restrictions . . . . . . . . . . . . . . . . . . . . 145
11.2. XML Namespace Names and Prefixes . . . . . . . . . . . . 146
11.3. Well-Formedness . . . . . . . . . . . . . . . . . . . . 146
11.4. Validation . . . . . . . . . . . . . . . . . . . . . . . 147
11.5. Inclusion of XML Declaration . . . . . . . . . . . . . . 147
11.6. Character Encoding . . . . . . . . . . . . . . . . . . . 147
11.7. Whitespace . . . . . . . . . . . . . . . . . . . . . . . 148
11.8. XML Versions . . . . . . . . . . . . . . . . . . . . . . 148
12. Internationalization Considerations . . . . . . . . . . . . . 148
13. Security Considerations . . . . . . . . . . . . . . . . . . . 148
13.1. Fundamentals . . . . . . . . . . . . . . . . . . . . . . 148
13.2. Threat Model . . . . . . . . . . . . . . . . . . . . . . 149
13.3. Order of Layers . . . . . . . . . . . . . . . . . . . . 150
13.4. Confidentiality and Integrity . . . . . . . . . . . . . 150
13.5. Peer Entity Authentication . . . . . . . . . . . . . . . 151
13.6. Strong Security . . . . . . . . . . . . . . . . . . . . 151
13.7. Certificates . . . . . . . . . . . . . . . . . . . . . . 152
13.7.1. Certificate Generation . . . . . . . . . . . . . . . 152
13.7.1.1. General Considerations . . . . . . . . . . . . . 152
13.7.1.2. Server Certificates . . . . . . . . . . . . . . . 153
13.7.1.3. Client Certificates . . . . . . . . . . . . . . . 156
13.7.1.4. XmppAddr Identifier Type . . . . . . . . . . . . 156
13.7.2. Certificate Validation . . . . . . . . . . . . . . . 157
13.7.2.1. Server Certificates . . . . . . . . . . . . . . . 158
13.7.2.2. Client Certificates . . . . . . . . . . . . . . . 158
13.7.2.3. Checking of Certificates in Long-Lived Streams . 160
13.7.2.4. Use of Certificates in XMPP Extensions . . . . . 160
13.8. Mandatory-to-Implement TLS and SASL Technologies . . . . 160
13.8.1. For Authentication Only . . . . . . . . . . . . . . 161
13.8.2. For Confidentiality Only . . . . . . . . . . . . . . 161
13.8.3. For Confidentiality and Authentication with
Passwords . . . . . . . . . . . . . . . . . . . . . 162
13.8.4. For Confidentiality and Authentication without
Passwords . . . . . . . . . . . . . . . . . . . . . 163
13.9. Technology Reuse . . . . . . . . . . . . . . . . . . . . 163
13.9.1. Use of Base 64 in SASL . . . . . . . . . . . . . . . 163
13.9.2. Use of DNS . . . . . . . . . . . . . . . . . . . . . 163
13.9.3. Use of Hash Functions . . . . . . . . . . . . . . . 164
13.9.4. Use of SASL . . . . . . . . . . . . . . . . . . . . 164
13.9.5. Use of TLS . . . . . . . . . . . . . . . . . . . . . 165
13.9.6. Use of UTF-8 . . . . . . . . . . . . . . . . . . . . 165
13.9.7. Use of XML . . . . . . . . . . . . . . . . . . . . . 166
13.10. Information Leaks . . . . . . . . . . . . . . . . . . . 166
13.10.1. IP Addresses . . . . . . . . . . . . . . . . . . . . 166
13.10.2. Presence Information . . . . . . . . . . . . . . . . 166
13.11. Directory Harvesting . . . . . . . . . . . . . . . . . . 166
13.12. Denial of Service . . . . . . . . . . . . . . . . . . . 167
13.13. Firewalls . . . . . . . . . . . . . . . . . . . . . . . 169
13.14. Interdomain Federation . . . . . . . . . . . . . . . . . 169
13.15. Non-Repudiation . . . . . . . . . . . . . . . . . . . . 169
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 170
14.1. XML Namespace Name for TLS Data . . . . . . . . . . . . 170
14.2. XML Namespace Name for SASL Data . . . . . . . . . . . . 170
14.3. XML Namespace Name for Stream Errors . . . . . . . . . . 170
14.4. XML Namespace Name for Resource Binding . . . . . . . . 171
14.5. XML Namespace Name for Stanza Errors . . . . . . . . . . 171
14.6. GSSAPI Service Name . . . . . . . . . . . . . . . . . . 171
14.7. Port Numbers and Service Names . . . . . . . . . . . . . 171
15. Conformance Requirements . . . . . . . . . . . . . . . . . . 172
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 181
16.1. Normative References . . . . . . . . . . . . . . . . . . 181
16.2. Informative References . . . . . . . . . . . . . . . . . 184
Appendix A. XML Schemas . . . . . . . . . . . . . . . . . . . . 190
A.1. Stream Namespace . . . . . . . . . . . . . . . . . . . . 190
A.2. Stream Error Namespace . . . . . . . . . . . . . . . . . 192
A.3. STARTTLS Namespace . . . . . . . . . . . . . . . . . . . 193
A.4. SASL Namespace . . . . . . . . . . . . . . . . . . . . . 194
A.5. Client Namespace . . . . . . . . . . . . . . . . . . . . 196
A.6. Server Namespace . . . . . . . . . . . . . . . . . . . . 201
A.7. Resource Binding Namespace . . . . . . . . . . . . . . . 206
A.8. Stanza Error Namespace . . . . . . . . . . . . . . . . . 206
Appendix B. Contact Addresses . . . . . . . . . . . . . . . . . 208that's the whole rest of the map, bestie 🧚 every section from 7.1 through Appendix B, page numbers and all 💅 the real content starts rolling in the next part, so stay tuned 🦋
1. Introduction ✨
1.1. Overview 💖
okay so, XMPP (the Extensible Messaging and Presence Protocol) is an application profile of the Extensible Markup Language [XML] 🎀 and it's built for the near-real-time exchange of structured yet extensible data between any two or more network entities. no cap, it's giving chat app energy. 🦋
this doc defines XMPP's core protocol methods: setting up and tearing down XML streams 🌊, channel encryption 🔐, authentication 🪪, error handling 🚨, and the communication primitives for messaging 💌, network availability (aka "presence") 🟢, and request-response interactions 🔁.
1.2. History 📜
the basic syntax and semantics of XMPP were originally developed within the Jabber open-source community, mainly in 1999 🕰️. then in late 2002 the XMPP Working Group got chartered to adapt the base Jabber protocol into an IETF instant messaging (IM) and presence technology, following [IMP-REQS] 💅. in October 2004, [RFC3920] and [RFC3921] were published, and they were the most complete definition of XMPP at that point 👑.
since 2004 the Internet community has gotten loads of implementation and deployment experience with XMPP ✨, including formal interoperability testing run under the XMPP Standards Foundation (XSF) 🧪. this document takes in comprehensive feedback from software developers and XMPP service providers 🫶, including a bunch of backward-compatible changes summarized under Appendix D. so this doc reflects the rough consensus of the Internet community on the core features of XMPP 1.0, and that's why it obsoletes RFC 3920 🎉. iconic glow-up, periodt 💜.
1.3. Functional Summary 🧚
this non-normative section is a developer-friendly overview of XMPP 🌸. the normative definitions come in the sections that follow, so this is just the vibe check, bestie 🙈.
the whole purpose of XMPP is to let entities swap relatively small pieces of structured data (called "XML stanzas") over a network 📦, between any two (or more) entities. XMPP is usually built as a distributed client-server setup 🖥️, where a client has to connect to a server to get onto the network and then trade XML stanzas with other entities (who might be hanging out on other servers) 🌐. the full journey of a client connecting to a server, exchanging stanzas, and leaving looks like this:
- Determine the IP address and port at which to connect, typically based on resolution of a fully qualified domain name (Section 3.2) 🗺️
- Open a Transmission Control Protocol [TCP] connection 🔌
- Open an XML stream over TCP (Section 4.2) 🌊
- Preferably negotiate Transport Layer Security [TLS] for channel encryption (Section 5) 🔐
- Authenticate using a Simple Authentication and Security Layer [SASL] mechanism (Section 6) 🪪
- Bind a resource to the stream (Section 7) 🎀
- Exchange an unbounded number of XML stanzas with other entities on the network (Section 8) 💌
- Close the XML stream (Section 4.4) 👋
- Close the TCP connection 🥀
within XMPP, one server can optionally connect to another server to enable inter-domain or inter-server communication 🌉. for that to happen, the two servers need to negotiate a connection between themselves and then swap XML stanzas. the steps for doing so are:
- Determine the IP address and port at which to connect, typically based on resolution of a fully qualified domain name (Section 3.2) 🗺️
- Open a TCP connection 🔌
- Open an XML stream (Section 4.2) 🌊
- Preferably negotiate TLS for channel encryption (Section 5) 🔐
- Authenticate using a Simple Authentication and Security Layer [SASL] mechanism (Section 6) * 🪪
- Exchange an unbounded number of XML stanzas both directly for the servers and indirectly on behalf of entities associated with each server, such as connected clients (Section 8) 🫂
- Close the XML stream (Section 4.4) 👋
- Close the TCP connection 🥀
* Interoperability Note: at the time of writing, most deployed servers still use the Server Dialback protocol [XEP-0220] 🙃 for weak identity verification, instead of SASL with PKIX certificates for strong authentication. this is especially true when SASL negotiation wouldn't result in strong authentication anyway (for example, because the peer server didn't require TLS negotiation, or because the PKIX certificate the peer server showed during TLS negotiation is self-signed and hasn't been accepted before). for the details, see [XEP-0220]. the solutions in this document offer a significantly stronger level of security (see also Section 13.6) 💪🔥.
this document covers how clients connect to servers and the basic semantics of XML stanzas 🧠. it does not define the "payloads" of the XML stanzas that get exchanged once a connection is up 🙅♀️. those payloads are defined by various XMPP extensions instead. for example, [XMPP-IM] defines extensions for basic instant messaging and presence 💬🟢. plus, the XSF's XEP series [XEP-0001] has tons of specs that define extensions for a wide range of applications 📚🌈.
1.4. Terminology 📖
the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [KEYWORDS] 🎯. these are the rules of the game, so read them carefully 🫡.
certain security-related terms mean what they mean in [SEC-TERMS] 🔒. that list includes, but isn't limited to, "assurance", "attack", "authentication", "authorization", "certificate", "certification authority", "certification path", "confidentiality", "credential", "downgrade", "encryption", "hash value", "identity", "integrity", "signature", "self-signed certificate", "sign", "spoof", "tamper", "trust", "trust anchor", "validate", and "verify" 🕵️♀️.
certain terms about certificates, domains, and application service identity mean what they mean in [TLS-CERTS] 🪪. these include, but aren't limited to, "PKIX certificate", "source domain", "derived domain", and the identifier types "CN-ID", "DNS-ID", and "SRV-ID" 🆔.
other security-related terms are defined in the referenced specifications instead (for example, "denial of service" as described in [DOS], or "end entity certificate" as described in [PKIX]) 📎.
the term "whitespace" refers to any character or characters matching the "S" production from [XML], which means one or more instances of the SP, HTAB, CR, or LF rules defined in [ABNF] ⬜.
the terms "localpart", "domainpart", and "resourcepart" are defined in [XMPP-ADDR] 🏷️.
the term "bare JID" refers to an XMPP address of the form <localpart@domainpart> (for an account at a server) or of the form <domainpart> (for a server) 🧾.
the term "full JID" refers to an XMPP address of the form <localpart@domainpart/resourcepart> (for a particular authorized client or device associated with an account) or of the form <domainpart/resourcepart> (for a particular resource or script associated with a server) 📱.
the term "XML stream" (also "stream") is defined under Section 4.1 🌊, and so is the term "XML stanza" (also "stanza") 📦. there are three kinds of stanzas: message, presence, and IQ (short for "Info/Query") 💌🟢🔁. these communication primitives are defined under Sections 8.2.1, 8.2.2, and 8.2.3, respectively 🎀.
the term "originating entity" refers to the entity that first generates a stanza sent over an XMPP network (for example, a connected client, an add-on service, or a server) 🌟. the term "generated stanza" refers to the stanza that was generated that way ✨.
the term "input stream" means an XML stream over which a server receives data from a connected client or a remote server 📥. the term "output stream" means an XML stream over which a server sends data to a connected client or a remote server 📤. the following terms describe some of the actions a server can take when it's processing data received over an input stream 🧐:
- route: pass the data to a remote server, for direct processing by the remote server or eventual delivery to a client associated with the remote server 🛫
- deliver: pass the data to a connected client 📬
- ignore: discard the data without acting upon it or returning an error to the sender 🙈
when "ignore" is used for client processing of data it receives, the phrase "without acting upon it" explicitly includes not presenting the data to a human user 🫣.
following the "XML Notation" used in [IRI] to represent characters that can't be rendered in ASCII-only documents, some examples in this document use the form &#x.... as a notational device for [UNICODE] characters 🌍. for example, the string ř stands for the Unicode character LATIN SMALL LETTER R WITH CARON 🇨🇿. this form is definitely not to be sent over the wire in XMPP systems 🚫.
consistent with the convention in [URI] for representing Uniform Resource Identifiers, XMPP addresses in running text are enclosed between '<' and '>' (even though natively they aren't URIs) 🪄.
in the examples, lines have been wrapped for readability, [...] means elision, and the following prepended strings are used (these prepended strings are not to be sent over the wire) 🧁:
C:= a clientE:= any XMPP entityI:= an initiating entityP:= a peer serverR:= a receiving entityS:= a serverS1:= server1S2:= server2
heads up, though: the examples aren't exhaustive 👀. in some protocol flows, the alternate steps shown wouldn't necessarily be triggered by the exact data sent in the previous step. in every case, the protocol definitions in this document or in normatively referenced documents rule over any examples here 👑. all examples are fictional, and the info exchanged (like usernames and passwords) doesn't represent any real users or servers 🧸.
2. Architecture 🏗️
2.1. Overview 🌸
hold up, this part is the main character moment 💅. XMPP gives you a technology for the asynchronous, end-to-end exchange of structured data 📨. it does that with direct, persistent XML streams among a distributed network of globally addressable, presence-aware clients and servers 🌐🟢. because this architectural style involves ubiquitous knowledge of network availability and a conceptually unlimited number of concurrent information transactions within a given client-to-server or server-to-server session, we call it "Availability for Concurrent Transactions" (ACT) 🦋. that way it's not confused with the "Representational State Transfer" [REST] style everyone knows from the World Wide Web 🕸️. XMPP's architecture is similar in important ways to email (see [EMAIL-ARCH]) 📧, but it has several modifications that make communication happen in close to real time ⚡. the salient features of this ACTive architectural style are as follows 💖.
2.1. Global Addresses ✨
okay so, just like email, XMPP uses globally unique addresses (built on the Domain Name System) so messages can actually find their way across the network 🌐 every XMPP entity is addressable, most importantly clients and servers, but also all the extra services that clients and servers can reach 💖 a server address looks like <domainpart> (like <im.example.com>), an account hosted on a server looks like <localpart@domainpart> (like <juliet@im.example.com>, called a "bare JID" 🎀), and a specific connected device or resource that's currently allowed to act for an account looks like <localpart@domainpart/resourcepart> (like <juliet@im.example.com/balcony>, called a "full JID" 🦋). historically these addresses get called Jabber IDs or JIDs, which is such a throwback 🧚 the formal address format is defined in [XMPP-ADDR] and not in this document, because the internationalization tech it depends on was still in flux when this was written 🫶 the terms "localpart", "domainpart", and "resourcepart" are defined more formally in [XMPP-ADDR] too 💅
2.2. Presence 🌸
XMPP has a way for an entity to advertise its network availability, aka "presence" 👀 this availability signal is sent end-to-end using a dedicated primitive: the <presence/> stanza 💜 presence isn't strictly required for XMPP messages to get exchanged, but it makes real-time chatting so much smoother, because the sender can know before starting a conversation that the person they want is online and available ✨ end-to-end presence is defined in [XMPP-IM] 🔥
2.3. Persistent Streams 🦋
availability for communication is also built into each point-to-point "hop" 🛸 this happens through persistent XML streams running over long-lived TCP connections, so basically always-on 💖 these always-on client-to-server and server-to-server streams let each party push data to the other at any time, for immediate routing or delivery 🚀 XML streams are defined under Section 4 📚
2.4. Structured Data 🎀
the basic protocol data unit in XMPP isn't the XML stream (that's just the transport for point-to-point communication) 🙅♀️ it's an XML "stanza", which is basically a fragment of XML sent over a stream 🌷 the root element of a stanza carries routing attributes (like the "from" and "to" addresses), and its child elements hold the payload for the intended recipient 💌 XML stanzas are defined under Section 8 ✨
2.5. Distributed Network of Clients and Servers 🌍
in practice, XMPP is a network of clients and servers that talk to each other (but communication between any two deployed servers is strictly discretionary and up to local service policy, so no obligation there 🙃). so for example, the user <juliet@im.example.com>, on the server <im.example.com>, might be able to swap messages, presence, and other structured data with the user <romeo@example.net>, on the server <example.net> 💕 this pattern is super familiar from messaging protocols that use global addresses, like the email network (see [SMTP] and [EMAIL-ARCH]) 📧 so end-to-end communication in XMPP is logically peer-to-peer but physically client-to-server-to-server-to-client 🫶 here's the diagram:
example.net <--------------> im.example.com
^ ^
| |
v v
romeo@example.net juliet@im.example.com Figure 1: Distributed Client-Server ArchitectureInformational Note: Architectures that employ XML streams (Section 4) and XML stanzas (Section 8) but that establish peer-to-peer connections directly between clients using technologies based on [LINKLOCAL] have been deployed, but such architectures are not defined in this specification and are best described as "XMPP-like"; for details, see [XEP-0174]. In addition, XML streams can be established end-to-end over any reliable transport, including extensions to XMPP itself; however, such methods are out of scope for this specification. 🙈
the following paragraphs describe the responsibilities of clients and servers on the network 📋
a client is an entity that establishes an XML stream with a server by authenticating with the credentials of a registered account (via SASL negotiation (Section 6)) 🔐 then it completes resource binding (Section 7), which enables delivery of XML stanzas between the server and the client over the negotiated stream 🎯 the client then uses XMPP to talk to its server, other clients, and any other entities on the network 💬 the server is responsible for delivering stanzas to other connected clients at the same server, or routing them to remote servers 🚚 multiple clients can connect at the same time to a server on behalf of the same registered account, and each one is differentiated by the resourcepart of its XMPP address (like <juliet@im.example.com/balcony> vs. <juliet@im.example.com/chamber>), as defined under [XMPP-ADDR] and Section 7 👑
a server is an entity whose primary responsibilities are to:
- Manage XML streams (Section 4) with connected clients and deliver XML stanzas (Section 8) to those clients over the negotiated streams; this includes responsibility for ensuring that a client authenticates with the server before being granted access to the XMPP network. 🛡️
- Subject to local service policies on server-to-server communication, manage XML streams (Section 4) with remote servers and route XML stanzas (Section 8) to those servers over the negotiated streams. 🌐
depending on the application, the secondary responsibilities of an XMPP server can include:
- Storing data that is used by clients (e.g., contact lists for users of XMPP-based instant messaging and presence applications as defined in [XMPP-IM]); in this case, the relevant XML stanza is handled directly by the server itself on behalf of the client and is not routed to a remote server or delivered to a connected client. 📒
- Hosting add-on services that also use XMPP as the basis for communication but that provide additional functionality beyond that defined in this document or in [XMPP-IM]; examples include multi-user conferencing services as specified in [XEP-0045] and publish-subscribe services as specified in [XEP-0060]. 🎉
- TCP Binding 🔌
3.1. Scope 🎯
as XMPP is defined in this specification, an initiating entity (client or server) MUST open a Transmission Control Protocol [TCP] connection to the receiving entity (server) before it negotiates XML streams with the receiving entity 🙌 the parties then keep that TCP connection for as long as the XML streams are in use, and the rules in the following sections apply to the TCP binding 📜
Informational Note: There is no necessary coupling of XML streams to TCP, and other transports are possible. For example, two entities could connect to each other by means of [HTTP] as specified in [XEP-0124] and [XEP-0206]. However, this specification defines only a binding of XMPP to TCP. 🧃
3.2. Resolution of Fully Qualified Domain Names 🗺️
because XML streams are sent over TCP, the initiating entity needs to figure out the IPv4 or IPv6 address (and port) of the receiving entity before it can try to open an XML stream 🔍 this is usually done by resolving the receiving entity's fully qualified domain name, or FQDN (see [DNS-CONCEPTS]) 🌸
3.2.1. Preferred Process: SRV Lookup 💅
the preferred way to resolve an FQDN is to use [DNS-SRV] records, like this:
- The initiating entity constructs a DNS SRV query whose inputs are:
- a Service of "xmpp-client" (for client-to-server connections) or "xmpp-server" (for server-to-server connections)
- a Proto of "tcp"
- a Name corresponding to the "origin domain" [TLS-CERTS] of the XMPP service to which the initiating entity wishes to connect (e.g., "example.net" or "im.example.com")
- The result is a query such as "_xmpp-client._tcp.example.net." or "_xmpp-server._tcp.im.example.com.". ✨
- If a response is received, it will contain one or more combinations of a port and FDQN, each of which is weighted and prioritized as described in [DNS-SRV]. (However, if the result of the SRV lookup is a single resource record with a Target of ".", i.e., the root domain, then the initiating entity MUST abort SRV processing at this point because according to [DNS-SRV] such a Target "means that the service is decidedly not available at this domain".) 🚫
- The initiating entity chooses at least one of the returned FQDNs to resolve (following the rules in [DNS-SRV]), which it does by performing DNS "A" or "AAAA" lookups on the FDQN; this will result in an IPv4 or IPv6 address. 🔎
- The initiating entity uses the IP address(es) from the successfully resolved FDQN (with the corresponding port number returned by the SRV lookup) as the connection address for the receiving entity. 🎯
- If the initiating entity fails to connect using that IP address but the "A" or "AAAA" lookups returned more than one IP address, then the initiating entity uses the next resolved IP address for that FDQN as the connection address. 🔁
- If the initiating entity fails to connect using all resolved IP addresses for a given FDQN, then it repeats the process of resolution and connection for the next FQDN returned by the SRV lookup based on the priority and weight as defined in [DNS-SRV]. 🧭
- If the initiating entity receives a response to its SRV query but it is not able to establish an XMPP connection using the data received in the response, it SHOULD NOT attempt the fallback process described in the next section (this helps to prevent a state mismatch between inbound and outbound connections). 🙅♀️
- If the initiating entity does not receive a response to its SRV query, it SHOULD attempt the fallback process described in the next section. 🫶
3.2.2. Fallback Processes 🛟
the fallback process SHOULD be a normal "A" or "AAAA" address record resolution to determine the IPv4 or IPv6 address of the origin domain, where the port used is the "xmpp-client" port of 5222 for client-to-server connections or the "xmpp-server" port of 5269 for server-to-server connections (these are the default ports as registered with the IANA as described under Section 14.7) 📮
if connections via TCP are unsuccessful, the initiating entity might attempt to find and use alternative connection methods such as the HTTP binding (see [XEP-0124] and [XEP-0206]), which might be discovered using [DNS-TXT] records as described in [XEP-0156] 🌈
3.2.3. When Not to Use SRV 🙋♀️
if the initiating entity has been explicitly configured to associate a particular FQDN (and potentially port) with the origin domain of the receiving entity (say, to "hardcode" an association from an origin domain of example.net to a configured FQDN of apps.example.com), the initiating entity is encouraged to use the configured name instead of performing the preferred SRV resolution process on the origin domain 🎀
3.2.4. Use of SRV Records with Add-On Services ✨
okay so a lot of XMPP servers can host add-on services (beyond the ones defined in this spec and [XMPP-IM]) on DNS names that are usually "subdomains" of the main XMPP service, like conference.example.net for a [XEP-0045] service tied to example.net 🎀, or "subdomains" of the first-level domain of the underlying service, like muc.example.com for a [XEP-0045] service tied to im.example.com 🦋. If an entity linked to a remote XMPP server wants to talk to one of these add-on services, it generates an appropriate XML stanza, and the remote server tries to resolve the add-on service's DNS domain name with an SRV lookup on records like _xmpp-server._tcp.conference.example.net. or _xmpp-server._tcp.muc.example.com. 🧚
So bestie, if the admins of an XMPP service want entities on remote servers to reach those add-on services, they need to advertise the matching "_xmpp-server" SRV records in addition to the "_xmpp-server" record for their main XMPP service 💅. And if SRV records aren't available, the fallback methods under Section 3.2.2 can be used to resolve the DNS domain names of add-on services 🫶.
3.3. Reconnection 🔁
It can happen that an XMPP server goes offline unexpectedly while it's serving TCP connections from connected clients and remote servers 😭. Since there can be a lot of those connections, the reconnection algorithm that entities use can seriously affect software performance and network congestion 💀. So if an entity chooses to reconnect, it:
- SHOULD set the number of seconds that expire before reconnecting to an unpredictable number between 0 and 60 🎲 (this helps make sure not every entity tries to reconnect at the exact same number of seconds after being disconnected).
- SHOULD back off increasingly on the time between subsequent reconnection attempts 📈 (for example, in accordance with "truncated binary exponential backoff" as described in [ETHERNET]) if the first reconnection attempt doesn't succeed.
It is RECOMMENDED to use TLS session resumption [TLS-RESUME] when reconnecting 🔐. A future version of this document, or a separate specification, might give more detailed guidelines on methods for speeding up the reconnection process ✨.
3.4. Reliability 💌
The use of long-lived TCP connections in XMPP means that sending XML stanzas over XML streams can be unreliable 😬, since the parties to a long-lived TCP connection might not notice a connectivity disruption in a timely manner. At the XMPP application layer, long connectivity disruptions can result in undelivered stanzas 📭. The core XMPP technology in this spec doesn't have features to overcome this lack of reliability, but there are XMPP extensions that do (for example, [XEP-0198]) 🛟.
4. XML Streams 🌊
4.1. Stream Fundamentals 🌸
Two fundamental concepts make the rapid, asynchronous exchange of relatively small payloads of structured information between XMPP entities possible: XML streams and XML stanzas 🧚. Here's how each one is defined.
Definition of XML Stream: An XML stream is a container for exchanging XML elements between any two entities over a network 📦. The start of an XML stream is marked unambiguously by an opening "stream header" (basically an XML <stream> tag with the right attributes and namespace declarations), and the end is marked unambiguously by a closing XML </stream> tag 🚪. While the stream is alive, the entity that initiated it can send an unbounded number of XML elements over it, either elements used to negotiate the stream (for example, to complete TLS negotiation (Section 5) or SASL negotiation (Section 6)) or XML stanzas 💜. The "initial stream" goes from the initiating entity (typically a client or server) to the receiving entity (typically a server), and it's basically the initiating entity's "connection to" or "session with" the receiving entity 🦋. The initial stream only enables one-way communication from the initiating entity to the receiving entity, so in order to exchange stanzas back from the receiving entity to the initiating entity, the receiving entity MUST negotiate a stream in the opposite direction (the "response stream") 🔁.
Definition of XML Stanza: An XML stanza is the basic unit of meaning in XMPP 👑. A stanza is a first-level element (at depth=1 of the stream) whose element name is "message", "presence", or "iq" and whose qualifying namespace is 'jabber:client' or 'jabber:server' 🎀. A first-level element qualified by any other namespace is not an XML stanza (that includes stream errors, stream features, TLS-related elements, SASL-related elements, and so on) 🙈. A <message/>, <presence/>, or <iq/> element that's qualified by 'jabber:client' or 'jabber:server' but sits at any depth other than one is also not a stanza (for example, a <message/> element nested inside an extension element (Section 8.4) for reporting purposes) 🫣. And a <message/>, <presence/>, or <iq/> element qualified by a namespace other than 'jabber:client' or 'jabber:server' isn't one either 🚫. An XML stanza usually contains one or more child elements (with attributes, elements, and XML character data) as needed to convey the desired information, and those child elements MAY be qualified by any XML namespace (see [XML-NAMES] as well as Section 8.4 in this specification) 🌈.
There are three kinds of stanzas: message, presence, and IQ (short for "Info/Query") 🍓. These give you three different communication primitives: a "push" mechanism for generalized messaging, a specialized "publish-subscribe" mechanism for broadcasting info about network availability, and a "request-response" mechanism for more structured exchanges of data (similar to [HTTP]) 🧁. More on those under Section 8.2.1, Section 8.2.2, and Section 8.2.3, respectively.
Let's look at a client's connection to a server 💖. The client opens an XML stream by sending a stream header to the server, preferably preceded by an XML declaration that specifies the XML version and the character encoding it supports (see Section 11.5 and Section 11.6) 📝. Depending on local policies and service provisioning, the server then replies with a second XML stream back to the client, again preferably preceded by an XML declaration ✨. Once the client has finished SASL negotiation (Section 6) and resource binding (Section 7), it can send an unbounded number of XML stanzas over the stream 🚀. When the client wants to close the stream, it just sends a closing </stream> tag to the server, as described further under Section 4.4 🫶.
So basically, one XML stream works as an envelope for the XML stanzas sent during a session, and another XML stream works as an envelope for the XML stanzas received during that session 💌. Here's a simplified picture of that 👇.
+--------------------+--------------------+
| INITIAL STREAM | RESPONSE STREAM |
+--------------------+--------------------+
| <stream> | |
|--------------------|--------------------|
| | <stream> |
|--------------------|--------------------|
| <presence> | |
| <show/> | |
| </presence> | |
|--------------------|--------------------|
| <message to='foo'> | |
| <body/> | |
| </message> | |
|--------------------|--------------------|
| <iq to='bar' | |
| type='get'> | |
| <query/> | |
| </iq> | |
|--------------------|--------------------|
| | <iq from='bar' |
| | type='result'> |
| | <query/> |
| | </iq> |
|--------------------|--------------------|
| [ ... ] | |
|--------------------|--------------------|
| | [ ... ] |
|--------------------|--------------------|
| </stream> | |
|--------------------|--------------------|
| | </stream> |
+--------------------+--------------------+Figure 2: A Simplistic View of Two Streams 🖼️
If you're used to thinking of XML in a document-centric way, these analogies might help 📚:
- The two XML streams are like two "documents" (matching the "document" production from [XML]) that get built up as XML stanzas accumulate 🧱.
- The root <stream/> element is like the "document entity" for each "document" (as described in Section 4.8 of [XML]) 🌿.
- The XML stanzas sent over the streams are like "fragments" of the "documents" (as described in [XML-FRAG]) 🧩.
That said, these are only analogies 🙃. XMPP doesn't really deal in documents and fragments; it deals in streams and stanzas 💅.
The rest of this section covers the following aspects of XML streams (plus related topics) 🌷:
- How to open a stream (Section 4.2)
- The stream negotiation process (Section 4.3)
- How to close a stream (Section 4.4)
- The directionality of XML streams (Section 4.5)
- How to handle peers that are silent (Section 4.6)
- The XML attributes of a stream (Section 4.7)
- The XML namespaces of a stream (Section 4.8)
- Error handling related to XML streams (Section 4.9)
4.2. Opening a Stream 🚪
After connecting to the appropriate IP address and port of the receiving entity, the initiating entity opens a stream by sending a stream header (the "initial stream header") to the receiving entity 🔌.
I: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>Then the receiving entity replies with a stream header of its own (the "response stream header") to the initiating entity 🙌.
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>From there, the entities can carry on with the rest of the stream negotiation process 🤝.
4.3. Stream Negotiation 🤝
4.3.1. Basic Concepts ✨
okay so the receiving entity for a stream is basically the bouncer 💅 for every domain it looks after, so it has some rules before it lets you in as a client or a peer server. at the very least, the initiating entity has to authenticate with the receiving entity before it can send stanzas (for client-to-server streams that means using SASL, see Section 6). but the receiving entity can also make other things mandatory-to-negotiate, like encryption with TLS (see Section 5). it tells you about all of this by sending "stream features": the list of protocol stuff you need to finish before it'll accept XML stanzas from you 🔐, plus any optional extras that could make handling the XML stream better (like application-layer compression, see [XEP-0138]) 🎀.
conditions mean streams need to be negotiated, and the layer order (TCP, then TLS, then SASL, then XMPP, see Section 13.3) means it's a multi-stage process 🪜. it gets even more structured because (1) some features are only offered to certain entities or only after other features are done (like resource binding only showing up after SASL auth 🙈), and (2) features can be either mandatory-to-negotiate or voluntary-to-negotiate. and for security reasons, both parties have to throw away what they learned during negotiation after finishing certain features (TLS always, and SASL when a security layer might get set up, per that SASL mechanism's spec) 🧹. they do this by flushing the old stream context and swapping in new stream headers over the same TCP connection.
4.3.2. Stream Features Format 💖
if the initiating entity puts a version attribute in its initial stream header set to at least "1.0" (see Section 4.7.5), then after sending the response stream header, the receiving entity MUST send a <features/> child element (usually with the stream namespace prefix, see Section 4.8.5) to the initiating entity so it can announce whatever it needs before the negotiation can keep going 📣. each condition is a child element of <features/>, in a namespace that's different from both the stream namespace and the content namespace. <features/> can have one child, a bunch of children, or nothing at all 🧁.
Implementation Note: the order of child elements inside any given <features/> element doesn't matter 🙌.
if a stream feature is or could be mandatory-to-negotiate, its definition needs to do one of these things:
- Declare that the feature is always mandatory-to-negotiate (like resource binding for XMPP clients) 🪪; or
- Specify a way for the receiving entity to flag the feature as mandatory-to-negotiate for that interaction (like STARTTLS, where you include an empty
<required/>element in the advertisement, though that's not a generic format for every feature) 🚩. it is RECOMMENDED that new mandatory-to-negotiate features do this by including an empty<required/>element, just like STARTTLS does ✨.
Informational Note: there's no generic way to say a feature is mandatory-to-negotiate, so a feature the initiating entity doesn't understand could get treated as mandatory by the receiving entity, and the stream negotiation could fail. the working group said that would be undesirable, but they thought it was rare enough that they didn't need a generic format 😬.
for security reasons, some stream features make the initiating entity send a new initial stream header after the feature is successfully negotiated (TLS always, and SASL when a security layer might get set up). if that's true for a feature, its definition has to say a stream restart is expected after negotiating it 🔁.
a <features/> element with at least one mandatory-to-negotiate feature means the negotiation isn't finished yet, and the initiating entity MUST negotiate further features 🚧.
R: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>a <features/> element MAY contain more than one mandatory-to-negotiate feature 🎲. that means the initiating entity can pick between them at this stage. like, maybe a future tech does roughly what TLS does, so the receiving entity could advertise both TLS and the new tech at the same stage 🔮. but this only works within one stage, and it doesn't apply to features that are mandatory at different stages. the receiving entity would NOT advertise both STARTTLS and SASL as mandatory-to-negotiate, or both SASL and resource binding as mandatory-to-negotiate, because TLS has to come before SASL and SASL has to come before resource binding 🙅♀️.
a <features/> element with both mandatory-to-negotiate and voluntary-to-negotiate features means the negotiation isn't done, but the initiating entity MAY finish the voluntary features before it tries the mandatory ones 🍓.
R: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
<compression xmlns='http://jabber.org/features/compress'>
<method>zlib</method>
<method>lzw</method>
</compression>
</stream:features>a <features/> element with only voluntary-to-negotiate features means the stream negotiation is complete and the initiating entity is cleared to send XML stanzas 🎉. it MAY negotiate more features if it wants to, though 🧚.
R: <stream:features>
<compression xmlns='http://jabber.org/features/compress'>
<method>zlib</method>
<method>lzw</method>
</compression>
</stream:features>and an empty <features/> element means the negotiation is complete and the initiating entity can start sending XML stanzas 🦋.
R: <stream:features/>4.3.3. Restarts 🔄
once a feature that needs a stream restart is successfully negotiated, both parties MUST treat the previous stream as replaced. but they MUST NOT send a closing </stream> tag, and they MUST NOT kill the underlying TCP connection 🙅♀️. instead, they MUST reuse the existing connection, which might be in a totally new state (like encrypted after TLS negotiation 🔐). then the initiating entity MUST send a new initial stream header, which SHOULD come after an XML declaration (see Section 11.5). when the receiving entity gets that new initial stream header, it MUST generate a new stream ID (not reuse the old one) before sending a new response stream header, which SHOULD also come after an XML declaration (see Section 11.5) 💅.
4.3.4. Resending Features 📝
the receiving entity MUST send an updated list of stream features to the initiating entity after a stream restart 🔁. the updated list MAY be empty if there's nothing else to advertise, or it MAY include any mix of features 🌈.
4.3.5. Completion of Stream Negotiation 🏁
the receiving entity says the stream negotiation is done by sending either an empty <features/> element or a <features/> element that has only voluntary-to-negotiate features 🎀. after that, the receiving entity MAY send an empty <features/> element (like after the voluntary features get negotiated), but it MUST NOT send any more stream features to the initiating entity 🚫. if it has new features to offer, ideally just mandatory-to-negotiate or security-critical ones, it can close the stream with a <reset/> stream error (Section 4.9.3.16) and then advertise the new features when the initiating entity reconnects. ideally it closes existing streams in a staggered way, so not every initiating entity reconnects at once 🌊. once negotiation is done, the initiating entity is cleared to send XML stanzas over the stream for as long as both parties keep the stream alive 💜.
Informational Note: resource binding (Section 7) is a historical exception to that rule. it's mandatory-to-negotiate for clients but it uses XML stanzas for the negotiation itself 🙃.
the initiating entity MUST NOT try to send XML stanzas (Section 8) to anything other than itself (meaning the client's connected resource or any other authenticated resource on the client's account) or the server it's connected to until stream negotiation is complete 🛑. even if it tries anyway, the receiving entity MUST NOT accept those stanzas and MUST close the stream with a <not-authorized/> stream error (Section 4.9.3.12). this rule only covers XML stanzas, meaning <message/>, <presence/>, and <iq/> elements in the content namespace. it does NOT cover XML elements used for stream negotiation, like the ones used to finish TLS (Section 5) or SASL (Section 6) negotiation 🫶.
4.3.6. Determination of Addresses ✨
okay so once both sides finish the right stream negotiation stuff, the receiving entity for a stream MUST determine the initiating entity's JID 🆔
for client-to-server stuff, SASL negotiation (Section 6) AND resource binding (Section 7) MUST be completed before the server can figure out the client's address 🫶 the client's bare JID (<localpart@domainpart>) MUST be the authorization identity (as defined by [SASL]), either (1) as the client directly tells the server during SASL negotiation (Section 6) or (2) as the server derives it from the authentication identity if no authorization identity was specified during SASL negotiation. the resourcepart of the full JID (<localpart@domainpart/resourcepart>) MUST be the resource the client and server agreed on during resource binding (Section 7) 💅 a client MUST NOT attempt to guess at its JID, but instead MUST consider its JID to be whatever the server hands back during resource binding 🙈 the server MUST ensure that the resulting JID (including localpart, domainpart, resourcepart, and separator characters) conforms to the canonical format for XMPP addresses defined in [XMPP-ADDR]. to meet this restriction, the server MAY replace the JID sent by the client with the canonicalized JID as determined by the server, and communicate that JID to the client during resource binding 🎀
for server-to-server communication, the initiating server's bare JID (<domainpart>) MUST be the authorization identity (as defined by [SASL]), either (1) as the initiating server directly communicates it during SASL negotiation (Section 6) or (2) as the receiving server derives it from the authentication identity if no authorization identity was specified during SASL negotiation. without SASL negotiation, the receiving server MAY consider the authorization identity to be an identity negotiated within the relevant verification protocol (e.g., the from attribute of the <result/> element in Server Dialback [XEP-0220]) 🧚
Security Warning: Because it is possible for a third party to tamper with information that is sent over the stream before a security layer such as TLS is successfully negotiated, it is advisable for the receiving server to treat any such unprotected information with caution. this applies especially to thefromandtoaddresses on the first initial stream header sent by the initiating entity 😭
4.3.7. Flow Chart 🦋
we're wrapping the whole stream negotiation vibe into one flow chart. it's non-normative, so it's just a summary, presented from the initiating entity's point of view 🌸
+---------------------+
| open TCP connection |
+---------------------+
|
v
+---------------+
| send initial |<-------------------------+
| stream header | ^
+---------------+ |
| |
v |
+------------------+ |
| receive response | |
| stream header | |
+------------------+ |
| |
v |
+----------------+ |
| receive stream | |
+------------------>| features | |
^ {OPTIONAL} +----------------+ |
| | |
| v |
| +<-----------------+ |
| | |
| {empty?} ----> {all voluntary?} ----> {some mandatory?} |
| | no | no | |
| | yes | yes | yes |
| | v v |
| | +---------------+ +----------------+ |
| | | MAY negotiate | | MUST negotiate | |
| | | any or none | | one feature | |
| | +---------------+ +----------------+ |
| v | | |
| +---------+ v | |
| | DONE |<----- {negotiate?} | |
| +---------+ no | | |
| yes | | |
| v v |
| +--------->+<---------+ |
| | |
| v |
+<-------------------------- {restart mandatory?} ------------>+
no yes
Figure 3: Stream Negotiation Flow Chart4.4. Closing a Stream 🚪
an XML stream between two entities can be closed at any time 💀 that can happen because a specific stream error (Section 4.9) popped up, or with no error at all (like when a client just wants to end its session) 🌈
a stream gets closed by sending a closing </stream> tag 💜
E: </stream:stream>if the parties are using either two streams over a single TCP connection OR two streams over two TCP connections, the entity that sends the closing stream tag MUST behave as follows:
- wait for the other party to also close its outbound stream before terminating the underlying TCP connection(s). this gives the other party a chance to finish sending any outbound data to the closing entity before the TCP connection(s) get terminated 🍓
- refrain from sending any further data over its outbound stream to the other entity, but keep processing data received from the other entity (and, if necessary, process such data) 🫧
- consider both streams to be void if the other party doesn't send its closing stream tag within a reasonable amount of time (where the definition of "reasonable" is a matter of implementation or deployment) ⏳
- after receiving a reciprocal closing stream tag from the other party, or waiting a reasonable amount of time with no response, terminate the underlying TCP connection(s) 👑
Security Warning: In accordance with Section 7.2.1 of [TLS], to help prevent a truncation attack, the party that is closing the stream MUST send a TLS close_notify alert and MUST receive a responding close_notify alert from the other party before terminating the underlying TCP connection(s). this one is SERIOUS bestie, don't skip it 🔥
if the parties are using multiple streams over multiple TCP connections, there is no defined pairing of streams, so the behavior is up to implementation 🤷♀️
4.5. Directionality 🧭
an XML stream is always unidirectional, which just means XML stanzas can only go in one direction over the stream (either from the initiating entity to the receiving entity, or from the receiving entity to the initiating entity) 🦋
depending on the type of session that got negotiated and who the entities are, they might use:
- two streams over a single TCP connection, where the security context negotiated for the first stream is applied to the second stream. this is typical for client-to-server sessions, and a server MUST allow a client to use the same TCP connection for both streams 💖
- two streams over two TCP connections, where each stream is separately secured. in this approach, one TCP connection is used for the stream in which stanzas are sent from the initiating entity to the receiving entity, and the other TCP connection is used for the stream in which stanzas are sent from the receiving entity to the initiating entity. this is typical for server-to-server sessions 🌸
- multiple streams over two or more TCP connections, where each stream is separately secured. this approach is sometimes used for server-to-server communication between two large XMPP service providers. however, it can make it hard to keep the data received over multiple streams coherent in the situations described under Section 10.1, which is why a server MAY close the stream with a
<conflict/>stream error (Section 4.9.3.3) if a remote server tries to negotiate more than one stream (as described under Section 4.9.3.3) 🫠
this concept of directionality only applies to stanzas. it explicitly does NOT apply to first-level children of the stream root that are used to bootstrap or manage the stream (e.g., first-level elements used for TLS negotiation, SASL negotiation, Server Dialback [XEP-0220], and Stream Management [XEP-0198]) 🧚
the stuff above means that while completing STARTTLS negotiation (Section 5) and SASL negotiation (Section 6), two servers would use one TCP connection. after the stream negotiation process is done, though, that original TCP connection only gets used for the initiating server to send XML stanzas to the receiving server 🎀 for the receiving server to send XML stanzas back to the initiating server, the receiving server would need to flip the roles and negotiate an XML stream from the receiving server to the initiating server over a separate TCP connection. that separate TCP connection then gets secured with a brand-new round of TLS and/or SASL negotiation 💅
Implementation Note: For historical reasons, a server-to-server session always uses two TCP connections. that remains the standard behavior described in this document, but extensions such as [XEP-0288] let servers negotiate the use of a single TCP connection for bidirectional stanza exchange ✨
Informational Note: XMPP developers sometimes call the underlying TCP connection "bidirectional" or "unidirectional" (like calling the TCP connection for a client-to-server session "bidirectional" and the one for a server-to-server session "unidirectional"). strictly speaking though, a stream is always unidirectional (because the initiating entity and receiving entity always have a minimum of two streams, one in each direction), and a TCP connection is always bidirectional (because TCP traffic can be sent in both directions). directionality applies to the application-layer traffic sent over the TCP connection, not to the transport-layer traffic sent over the TCP connection itself 🤓
4.6. Handling of Silent Peers 🌸
okay so bestie, picture this: you've been chatting with your stream buddy and suddenly they've gone totally quiet for ages 🥺. Is the connection dead? Is the XML stream broken? Or are they just chilling and haven't had anything to say? There are three possible reasons, and they're all different:
- The underlying TCP connection is dead. 💀
- The XML stream is broken even though the underlying TCP connection is alive. 🧩
- The peer is idle and simply has not sent any XMPP traffic over its XML stream to the entity. 😴
These three conditions are best handled separately, as described in the following sections. 💅
Implementation Note: For the purpose of handling silent peers, we treat a two unidirectional TCP connections as conceptually equivalent to a single bidirectional TCP connection (see Section 4.5); however, implementers need to be aware that, in the case of two unidirectional TCP connections, responses to traffic at the XMPP application layer will come back from the peer on the second TCP connection. In addition, the use of multiple streams in each direction (which is a somewhat frequent deployment choice for server-to-server connectivity among large XMPP service providers) further complicates application-level checking of XMPP streams and their underlying TCP connections, because there is no necessary correlation between any given initial stream and any given response stream. 🦋
4.6.1. Dead Connection 💀
If the underlying TCP connection is dead, stream-level checks (e.g., [XEP-0199] and [XEP-0198]) are ineffective. 🙈 Therefore, it is unnecessary to close the stream with or without an error, and it is appropriate instead to simply terminate the TCP connection.
One common method for checking the TCP connection is to send a space character (U+0020) between XML stanzas, which is allowed for XML streams as described under Section 11.7. The sending of such a space character is properly called a "whitespace keepalive" (the term "whitespace ping" is often used, despite the fact that it is not a ping since no "pong" is possible 🏓). However, this is not allowed during TLS negotiation or SASL negotiation, as described under Section 5.3.3 and Section 6.3.5. 🚫
4.6.2. Broken Stream 🧩
Even if the underlying TCP connection is alive, the peer might never respond to XMPP traffic that the entity sends. That's true whether it's normal stanzas or specialized stream-checking traffic such as the application-level pings defined in [XEP-0199] or the more comprehensive Stream Management protocol defined in [XEP-0198]. ✨ In this case, it is appropriate for the entity to close a broken stream with a <connection-timeout/> stream error (Section 4.9.3.4). 👋
4.6.3. Idle Peer 😴
Even if the underlying TCP connection is alive and the stream is not broken, the peer might have sent no stanzas for a certain period of time. In this case, the peer itself MAY close the stream (as described under Section 4.4) rather than leaving an unused stream open. 🍃
If the idle peer does not close the stream, the other party MAY either close the stream using the handshake described under Section 4.4 or close the stream with a stream error. For the stream error, the examples given are <resource-constraint/> (Section 4.9.3.17) if the entity has reached a limit on the number of open TCP connections, or <policy-violation/> (Section 4.9.3.14) if the connection has exceeded a local timeout policy. 🚦
However, consistent with the order of layers (specified under Section 13.3), the other party is advised to verify that the underlying TCP connection is alive and the stream is unbroken (as described above) before concluding that the peer is idle. 🔍 Furthermore, it is preferable to be liberal in accepting idle peers, since experience has shown that doing so improves the reliability of communication over XMPP networks. It's also typically more efficient to maintain a stream between two servers than to aggressively time out such a stream. 🫶
4.6.4. Use of Checking Methods ⏰
Implementers are advised to support whichever stream-checking and connection-checking methods they deem appropriate, but to carefully weigh the network impact of such methods against the benefits of discovering broken streams and dead TCP connections in a timely manner. ⚖️ The length of time between the use of any particular check is very much a matter of local service policy. It depends strongly on the network environment and usage scenarios of a given deployment and connection type. 🌍
At the time of writing, it is RECOMMENDED that any such check be performed not more than once every 5 minutes and that, ideally, such checks will be initiated by clients rather than servers. 💜 Those who implement XMPP software and deploy XMPP services are encouraged to seek additional advice regarding appropriate timing of stream-checking and connection-checking methods. This is especially true when power-constrained devices are being used (e.g., in mobile environments). 📱🔋
4.7. Stream Attributes 🎀
The attributes of the root <stream/> element are defined in the following sections. 📖
Security Warning: Until and unless the confidentiality and integrity of the stream are protected via TLS as described under Section 5 or an equivalent security layer (such as the SASL GSSAPI mechanism), the attributes provided in a stream header could be tampered with by an attacker. 🚨 Please don't skip this one, bestie. Anyone in the middle could mess with your stream header, so lock things down first! 🔐
Implementation Note: The attributes of the root <stream/> element are not prepended by a namespace prefix because, as explained in [XML-NAMES], "[d]efault namespace declarations do not apply directly to attribute names; the interpretation of unprefixed attributes is determined by the element on which they appear." 🧁
4.7.1. from 💌
The 'from' attribute specifies an XMPP identity of the entity sending the stream element. 🪪
For initial stream headers in client-to-server communication, the 'from' attribute is the XMPP identity of the principal controlling the client, i.e., a JID of the form <localpart@domainpart>. The client might not know the XMPP identity, though. Maybe it's because the XMPP identity is assigned at a level other than the XMPP application layer (as in the Generic Security Service Application Program Interface [GSS-API]). Or maybe the server derives it from information the client provided (as in some deployments of end-user certificates with the SASL EXTERNAL mechanism). 🤷♀️
Furthermore, if the client considers the XMPP identity to be private information, then it is advised not to include a 'from' attribute before the confidentiality and integrity of the stream are protected via TLS or an equivalent security layer. 🙊 However, if the client knows the XMPP identity, then it SHOULD include the 'from' attribute after the confidentiality and integrity of the stream are protected via TLS or an equivalent security layer. 🔒✨
Here's what that looks like:
I: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>For initial stream headers in server-to-server communication, the 'from' attribute is one of the configured FQDNs of the server, i.e., a JID of the form <domainpart>. 🏠 The initiating server might have more than one XMPP identity, e.g., in the case of a server that provides virtual hosting. So it will need to choose an identity that is associated with this output stream. For example, it might base that choice on the 'to' attribute of the stanza that triggered the stream negotiation attempt. 🎯 Because a server is a "public entity" on the XMPP network, it MUST include the 'from' attribute after the confidentiality and integrity of the stream are protected via TLS or an equivalent security layer. 🛡️
I: <?xml version='1.0'?>
<stream:stream
from='example.net'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>For response stream headers in both client-to-server and server-to-server communication, the receiving entity MUST include the 'from' attribute and MUST set its value to one of the receiving entity's FQDNs. The value MAY be an FQDN other than the one specified in the 'to' attribute of the initial stream header, as described under Section 4.9.1.3 and Section 4.9.3.6. 🌈
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>Whether or not the 'from' attribute is included, each entity MUST verify the identity of the other entity before exchanging XML stanzas with it, as described under Section 13.5. 🪪✅
Interoperability Note: It is possible that implementations based on [RFC3920] will not include the 'from' address on any stream headers (even ones whose confidentiality and integrity are protected). An entity SHOULD be liberal in accepting such stream headers. 🥰
4.7.2. to 🎯
For initial stream headers in both client-to-server and server-to-server communication, the initiating entity MUST include the 'to' attribute and MUST set its value to a domainpart that the initiating entity knows or expects the receiving entity to service. (The same information can be provided in other ways, such as a Server Name Indication during TLS negotiation as described in [TLS-EXT].) 🤝
I: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>For response stream headers in client-to-server communication, things are a little bit specific. If the client included a 'from' attribute in the initial stream header, then the server MUST include a 'to' attribute in the response stream header and MUST set its value to the bare JID specified in the 'from' attribute of the initial stream header. 💖 If the client did not include a 'from' attribute in the initial stream header, then the server MUST NOT include a 'to' attribute in the response stream header. 🙅♀️
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>For response stream headers in server-to-server communication, the receiving entity MUST include a 'to' attribute in the response stream header and MUST set its value to the domainpart specified in the 'from' attribute of the initial stream header. 🌐
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
to='example.net'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>Whether or not the 'to' attribute is included, each entity MUST verify the identity of the other entity before exchanging XML stanzas with it, as described under Section 13.5. 🔐
Interoperability Note: It is possible that implementations based on [RFC3920] will not include the 'to' address on stream headers. An entity SHOULD be liberal in accepting such stream headers. 🫶
4.7.3. id 🆔
Okay so the id attribute is basically the stream's unique ID tag, and we call it a "stream ID" 🫶 The stream ID MUST be generated by the receiving entity when it sends a response stream header and MUST BE unique within the receiving application (normally a server) ✨
Security Warning: The stream ID MUST be both unpredictable and non-repeating because it can be security-critical when reused by an authentication mechanisms, as is the case for Server Dialback [XEP-0220] and the "XMPP 0.9" authentication mechanism used before RFC 3920 defined the use of SASL in XMPP; for recommendations regarding randomness for security purposes, see [RANDOM]. 😭🔐
For initial stream headers, the initiating entity MUST NOT include the id attribute 🙅♀️ Bestie, if it sneaks in anyway, the receiving entity MUST ignore it 💅
For response stream headers, the receiving entity MUST include the id attribute 💖
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>Interoperability Note: In RFC 3920, the text about including the id attribute was ambiguous, so some implementations left it off the response stream header 🙈
4.7.4. xml:lang 🌍
The xml:lang attribute tells the other side an entity's preferred or default language for any human-readable XML character data sent over the stream 🗣️ (a stanza can also have its own xml:lang, as we'll see under Section 8.1.5). The syntax comes from Section 2.12 of [XML], and the value of xml:lang 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 xml:lang ✨
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. Here's how it plays out 🎀
- If the initiating entity included
xml:langin its initial stream header and the receiving entity supports that language for the human-readable XML character data it generates and sends back (like in the<text/>element for stream and stanza errors), the value ofxml:langMUST be the identifier for the initiating entity's preferred language (e.g., "de-CH"). - If the receiving entity supports a language that matches the initiating entity's preferred language according to the "lookup scheme" specified in Section 3.4 of [LANGMATCH] (e.g., "de" instead of "de-CH"), then the value of
xml:langSHOULD be the identifier for the matching language 🌸 - If the receiving entity does not support the initiating entity's preferred language or a matching language according to the lookup scheme (or if the initiating entity did not include
xml:langin its initial stream header), then the value ofxml:langMUST be the identifier for the default language of the receiving entity (e.g., "en") 🇺🇸
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>If the initiating entity included xml:lang in its initial stream header, the receiving entity SHOULD remember that value as the default xml:lang for all stanzas sent by the initiating entity over the current stream 🧚 As described under Section 8.1.5, the initiating entity MAY include xml:lang in any XML stanzas it sends over the stream. If it doesn't include xml:lang in a stanza, the receiving entity SHOULD add xml:lang to that stanza when routing it to a remote server or delivering it to a connected client, and the value of the attribute MUST be the identifier for the language preferred by the initiating entity (even if the receiving entity doesn't support that language for the human-readable XML character data it sends back, like in stream or stanza errors) 💜 If the initiating entity DOES include xml:lang in such a stanza, the receiving entity MUST NOT modify or delete it when routing it to a remote server or delivering it to a connected client 🙌
4.7.5. version 🔢
Including the version attribute with a value of at least "1.0" signals support for the stream-related protocols defined in this specification, which covers TLS negotiation (Section 5), SASL negotiation (Section 6), stream features (Section 4.3.2), and stream errors (Section 4.9) 🔥
The version of XMPP specified here is "1.0" 💅 In particular, XMPP 1.0 wraps up the stream-related protocols plus the basic semantics of the three XML stanza types (<message/>, <presence/>, and <iq/>, as described under Sections 8.2.1, 8.2.2, and 8.2.3, respectively).
Version numbers look like <major>.<minor>. The major and minor numbers MUST be treated as separate integers, and each number MAY be incremented higher than a single digit 🌈 So "XMPP 2.4" is a lower version than "XMPP 2.13", which is lower than "XMPP 12.3" 📈 Leading zeros (e.g., "XMPP 6.01") MUST be ignored by recipients and MUST NOT be sent 🙈
The major version number only gets bumped if the stream and stanza formats or obligatory actions have changed so dramatically that an older version entity couldn't interoperate with a newer one by just ignoring the elements and attributes it doesn't understand and taking the actions defined in the older specification 💀
The minor version number only gets bumped if significant new capabilities have been added to the core protocol (for example, 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 one (e.g., that entity would just note that its correspondent can't understand that type value and therefore wouldn't send it) 🦋
Here are the rules for generating and handling the version attribute in stream headers 📝
- The initiating entity MUST set the value of the
versionattribute in the initial stream header to the highest version number it supports (e.g., if the highest version it supports is the one defined in this specification, it MUST set the value to "1.0"). - The receiving entity MUST set the value of the
versionattribute 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>". - If the version number in the response stream header is at least one major version lower than the version number in the initial stream header, and newer version entities can't interoperate with older ones as described, the initiating entity SHOULD close the stream with an
<unsupported-version/>stream error (Section 4.9.3.25) 🚪 - If either entity receives a stream header with no
versionattribute, the entity MUST consider the version supported by the other entity to be "0.9", and it SHOULD NOT include aversionattribute in the response stream header 🕰️
4.7.6. Summary of Stream Attributes 📋
Here's the cheat sheet for the attributes of the root <stream/> element 🧁
+----------+--------------------------+-------------------------+
| | initiating to receiving | receiving to initiating |
+----------+--------------------------+-------------------------+
| to | JID of receiver | JID of initiator |
| from | JID of initiator | JID of receiver |
| id | ignored | stream identifier |
| xml:lang | default language | default language |
| version | XMPP 1.0+ supported | XMPP 1.0+ supported |
+----------+--------------------------+-------------------------+
Figure 4: Stream Attributes4.8. XML Namespaces 🎀
Readers are referred to the specification of XML namespaces [XML-NAMES] for a full understanding of the concepts used in this section, especially the "default namespace" concept from Section 3 and Section 6.2 of that specification 🤓
4.8.1. Stream Namespace 👑
The root <stream/> element (the "stream header") MUST be qualified by the namespace http://etherx.jabber.org/streams (the "stream namespace") 🌸 If this rule is broken, the entity that receives the offending stream header MUST close the stream with a stream error, which SHOULD be <invalid-namespace/> (Section 4.9.3.10), although some existing implementations send <bad-format/> (Section 4.9.3.1) instead 🙃
4.8.2. Content Namespace 💅
okay so bestie, an entity MAY declare a "content namespace" as the default namespace for data sent over the stream 💖 (basically anything that isn't qualified by the stream namespace). If it does, (1) the content namespace MUST be other than the stream namespace, and (2) the content namespace MUST be the same for the initial stream and the response stream so both streams are qualified consistently 🎀. The content namespace applies to all first-level child elements sent over the stream unless they're explicitly qualified by another namespace (aka the content namespace is the default namespace) ✨.
Alternatively (instead of declaring the content namespace as the default namespace), an entity MAY explicitly qualify the namespace for each first-level child element of the stream, using so-called "prefix-free canonicalization" 🦋. These two styles are shown in the examples below 👀.
When a content namespace is declared as the default namespace, a stream will look something like this in rough outline 🌸:
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<message>
<body>foo</body>
</message>
</stream:stream>When a content namespace is NOT declared as the default namespace and so-called "prefix-free canonicalization" is used instead, a stream will look something like this in rough outline 🧚:
<stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='http://etherx.jabber.org/streams'>
<message xmlns='jabber:client'>
<body>foo</body>
</message>
</stream>Traditionally, most XMPP implementations have used the content-namespace-as-default-namespace style for stream headers rather than prefix-free canonicalization 💅. Both styles are acceptable though, since they are semantically equivalent 🫶.
4.8.3. XMPP Content Namespaces 🌈
XMPP as defined in this specification uses two content namespaces: 'jabber:client' and 'jabber:server' 💜. They're nearly identical but get used in different contexts: 'jabber:client' is for client-to-server communication and 'jabber:server' is for server-to-server communication 🔥. The only real difference is that the 'to' and 'from' attributes are OPTIONAL on stanzas sent over XML streams qualified by 'jabber:client', whereas they are REQUIRED on stanzas sent over XML streams qualified by 'jabber:server' 😭. Supporting these content namespaces implies supporting the common attributes (Section 8.1) and basic semantics (Section 8.2) of all three core stanza types (message, presence, and IQ) 👑.
An implementation MAY support content namespaces other than 'jabber:client' or 'jabber:server' ✨. However, because such namespaces would define applications other than XMPP, they are to be defined in separate specifications 📚.
An implementation MAY refuse to support any other content namespaces as default namespaces 🙈. If an entity receives a first-level child element qualified by a content namespace it does not support, it MUST close the stream with an <invalid-namespace/> stream error (Section 4.9.3.10) 💀.
Client implementations MUST support the 'jabber:client' content namespace as a default namespace 🍓. The 'jabber:server' content namespace is out of scope for an XMPP client, so a client MUST NOT send stanzas qualified by 'jabber:server' 🙅.
Server implementations MUST support both 'jabber:client' (when the stream is used for communication between a client and a server) and 'jabber:server' (when the stream is used for communication between two servers) as default content namespaces 🫶. When communicating with a connected client, a server MUST NOT send stanzas qualified by 'jabber:server'. When communicating with a peer server, a server MUST NOT send stanzas qualified by 'jabber:client' 🚫.
Implementation Note: Because a client sends stanzas over a stream whose content namespace is'jabber:client', if a server routes to a peer server a stanza it received from a connected client, then it needs to "re-scope" the stanza so its content namespace is'jabber:server'🔁. Similarly, if a server delivers to a connected client a stanza it received from a peer server, then it needs to "re-scope" the stanza so its content namespace is'jabber:client'🔁. This rule applies to XML stanzas as defined under Section 4.1 (i.e., a first-level<message/>,<presence/>, or<iq/>element qualified by'jabber:client'or'jabber:server'), and by namespace inheritance to all child elements of a stanza 🧬. However, the rule does NOT apply to elements qualified by namespaces other than'jabber:client'and'jabber:server', nor to any children of such elements (e.g., a<message/>element contained within an extension element (Section 8.4) for reporting purposes) 🙌. Although it is not forbidden for an entity to generate stanzas in which an extension element contains a child element qualified by'jabber:client'or'jabber:server', existing implementations handle such stanzas inconsistently. So implementers are advised to weigh the likely lack of interoperability against the possible utility of such stanzas 🤔. Finally, servers are advised to apply stanza re-scoping to other stream connection methods and alternative XMPP connection methods, such as those specified in [XEP-0124], [XEP-0206], [XEP-0114], and [XEP-0225] 🌟.
4.8.4. Other Namespaces 🧃
Either party to a stream MAY send data qualified by namespaces other than the content namespace and the stream namespace 🎈. For example, this is how data related to TLS negotiation and SASL negotiation gets exchanged, as well as XMPP extensions such as Stream Management [XEP-0198] and Server Dialback [XEP-0220] 🔐.
Interoperability Note: For historical reasons, some server implementations expect a declaration of the 'jabber:server:dialback' namespace on server-to-server streams, as explained in [XEP-0220] 🕰️.
However, an XMPP server MUST NOT route or deliver data received over an input stream if that data is (a) qualified by another namespace and (b) addressed to an entity other than the server, unless the other party to the output stream over which the server would send the data has explicitly negotiated or advertised support for receiving arbitrary data from the server 🛑. This rule exists because XMPP is designed for the exchange of XML stanzas (not arbitrary XML data), and because allowing an entity to send arbitrary data to other entities could significantly increase the potential for exchanging malicious information 🚨. As an example, the server hosting the example.net domain would not route the following first-level XML element from <romeo@example.net> to <juliet@example.com> 👀:
<ns1:foo xmlns:ns1='http://example.org/ns1'
from='romeo@example.net/resource1'
to='juliet@example.com'>
<ns1:bar/>
</ns1:foo>This rule also applies to first-level elements that look like stanzas but are improperly namespaced, and therefore really are not stanzas at all (see also Section 4.8.5) 🙃. For example:
<ns2:message xmlns:ns2='http://example.org/ns2'
from='romeo@example.net/resource1'
to='juliet@example.com'>
<body>hi</body>
</ns2:message>Upon receiving arbitrary first-level XML elements over an input stream, a server MUST either ignore the data or close the stream with a stream error, which SHOULD be <unsupported-stanza-type/> (Section 4.9.3.24) 🚫✨.
4.8.5. Namespace Declarations and Prefixes 🏷️
Because the content namespace is other than the stream namespace, if a content namespace is declared as the default namespace then the following statements are true 💁:
- The stream header needs to contain a namespace declaration for both the content namespace and the stream namespace.
- The stream namespace declaration needs to include a namespace prefix for the stream namespace.
Interoperability Note: For historical reasons, an implementation MAY accept only the prefix'stream'for the stream namespace (resulting in prefixed names such as<stream:stream>and<stream:features>). This specification retains that allowance from [RFC3920] for the purpose of backward compatibility 🕰️. Implementations are advised that using a prefix other than'stream'for the stream namespace might result in interoperability problems ⚠️. If an entity receives a stream header with a stream namespace prefix it does not accept, it MUST close the stream with a stream error, which SHOULD be<bad-namespace-prefix/>(Section 4.9.3.2), although some existing implementations send<bad-format/>(Section 4.9.3.1) instead 😅.
An implementation MUST NOT generate namespace prefixes for elements qualified by the content namespace (i.e., the default namespace for data sent over the stream) if the content namespace is 'jabber:client' or 'jabber:server' 🙅♀️. For example, the following is illegal:
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<foo:message xmlns:foo='jabber:client'>
<foo:body>foo</foo:body>
</foo:message>An XMPP entity SHOULD NOT accept data that violates this rule. In particular, an XMPP server MUST NOT route or deliver such data to another entity without first correcting the error. Instead, it SHOULD either ignore the data or close the stream with a stream error, which SHOULD be <bad-namespace-prefix/> (Section 4.9.3.2) 🧹.
Namespaces declared in a stream header MUST apply only to that stream (e.g., the 'jabber:server:dialback' namespace used in Server Dialback [XEP-0220]) 🔒. In particular, because XML stanzas intended for routing or delivery over streams with other entities will lose the namespace context declared in the header of the stream they originated on, namespaces for extended content within such stanzas MUST NOT be declared in that stream header (see also Section 8.4) 📦. If either party to a stream declares such namespaces, the other party to the stream SHOULD close the stream with an <invalid-namespace/> stream error (Section 4.9.3.10) 💅. In any case, an entity MUST ensure that such namespaces are properly declared (according to this section) when routing or delivering stanzas from an input stream to an output stream 🎀.
4.9. Stream Errors 🚨
The root stream element MAY contain an <error/> child element that is qualified by the stream namespace 🧯. The error child SHALL be sent by a compliant entity if it perceives that a stream-level error has occurred 🙏.
4.9.1. Rules 📜
The following rules apply to stream-level errors 💖.
4.9.1.1. Stream Errors Are Unrecoverable ✨
okay so stream-level errors are a no-take-backs situation 💀 If something goes wrong at the stream level, the entity that spots it MUST send an <error/> element with a proper child element naming the error condition, and then immediately close the stream as described under Section 4.4. no second chances 🫠
C: <message><body>No closing tag!</message>
S: <stream:error>
<not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>then the entity that gets the stream error SHALL close the stream as explained under Section 4.4 🔒
C: </stream:stream>4.9.1.2. Stream Errors Can Occur During Setup 🎀
if the error is triggered by the initial stream header, the receiving entity MUST still send the opening <stream> tag, put the <error/> element inside the stream element as a child, and send the closing </stream> tag (preferably in the same TCP packet) 📦 it's giving "I'm still polite even when I'm rejecting you" 💅
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://wrong.namespace.example.org/'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<invalid-namespace
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>4.9.1.3. Stream Errors When the Host Is Unspecified or Unknown 🕵️♀️
so if the initiating entity gives no 'to' attribute, or gives an unknown host in the 'to' attribute, and the error happens during stream setup, the 'from' attribute that the receiving entity returns in the stream header (sent before closing the stream) MUST be either an authoritative FQDN for the receiving entity or the empty string 🫶
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='unknown.host.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<host-unknown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>4.9.1.4. Where Stream Errors Are Sent 🗺️
when two TCP connections are used between the initiating entity and the receiving entity (one in each direction) instead of one bidirectional connection, these rules apply:
- stream-level errors related to the initial stream are returned by the receiving entity on the response stream, via the same TCP connection 🔁
- stanza errors triggered by outbound stanzas sent from the initiating entity over the initial stream (via the same TCP connection) are returned by the receiving entity on the response stream via the other ("return") TCP connection, since they are inbound stanzas from the perspective of the initiating entity 📬
4.9.2. Syntax 🧩
the syntax for stream errors looks like this, where XML data shown inside the square brackets '[' and ']' is OPTIONAL 👀
<stream:error>
<defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
[<text xmlns='urn:ietf:params:xml:ns:xmpp-streams'
xml:lang='langcode'>
OPTIONAL descriptive text
</text>]
[OPTIONAL application-specific condition element]
</stream:error>the "defined-condition" MUST correspond to one of the stream error conditions defined under Section 4.9.3 💖 but because more error conditions might be defined in the future, if an entity gets a stream error condition it doesn't understand, it MUST treat that unknown condition as equivalent to <undefined-condition/> (Section 4.9.3.21) 🤷♀️ and if the designers of an XMPP protocol extension, or the developers of an XMPP implementation, need to communicate a stream error condition that isn't defined in this specification, they can define an application-specific error condition element qualified by an application-specific namespace 🌈
the <error/> element:
- MUST contain a child element corresponding to one of the defined stream error conditions (Section 4.9.3); this element MUST be qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace 🎯
- MAY contain a
<text/>child element with 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 🗣️ - MAY contain a child element for an application-specific error condition; this element MUST be qualified by an application-defined namespace, and its structure is defined by that namespace (see Section 4.9.4) 🧁
the <text/> element is OPTIONAL. if it's included, it MUST be used only to provide descriptive or diagnostic information that supplements the meaning of a defined condition or application-specific condition 📝 it MUST NOT be interpreted programmatically by an application 🙅♀️ it MUST NOT be used as the error message presented to a human user, but MAY be shown in addition to the error message associated with the defined condition element (and, optionally, the application-specific condition element) ✨
4.9.3. Defined Stream Error Conditions 🎀
the following stream-level error conditions are defined 👑
4.9.3.1. bad-format 🍓
the entity has sent XML that cannot be processed 😵💫
(in the following example, the client sends an XMPP message that isn't well-formed XML, which could also trigger a <not-well-formed/> stream error (Section 4.9.3.13).)
C: <message>
<body>No closing tag!
</message>
S: <stream:error>
<bad-format
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>this error can be used instead of the more specific XML-related errors, such as <bad-namespace-prefix/>, <invalid-xml/>, <not-well-formed/>, <restricted-xml/>, and <unsupported-encoding/>. however, the more specific errors are RECOMMENDED 🧠
4.9.3.2. bad-namespace-prefix 🌸
the entity has sent a namespace prefix that is unsupported, or has sent no namespace prefix on an element that needs one (see Section 11.2) 🙈
(in the following example, the client specifies a namespace prefix of "foobar" for the XML stream namespace.)
C: <?xml version='1.0'?>
<foobar:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:foobar='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<bad-namespace-prefix
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>4.9.3.3. conflict 🥊
the server either (1) is closing the existing stream for this entity because a new stream has been initiated that conflicts with it, or (2) is refusing a new stream for this entity because allowing it would conflict with an existing stream 🚫 (for example, the server might allow only a certain number of connections from the same IP address, or only one server-to-server stream for a given domain pair, to help ensure in-order processing as described under Section 10.1) 📦
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<conflict
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>if a client receives a <conflict/> stream error (Section 4.9.3.3) during the resource binding part of its reconnection attempt, it MUST NOT blindly request the resourcepart it used during the former session 🙅♀️ instead, it MUST choose a different resourcepart; details are under Section 7 🫠
4.9.3.4. connection-timeout ⏰
one party closes the stream because it has reason to believe the other party has permanently lost the ability to communicate over it 📵 that lost ability can be found using different methods, like whitespace keepalives (Section 4.4), XMPP-level pings as defined in [XEP-0199], and XMPP Stream Management as defined in [XEP-0198] 🛰️
P: <stream:error>
<connection-timeout
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>Interoperability Note: RFC 3920 specified that the <connection-timeout/> stream error (Section 4.9.3.4) is to be used if the peer has not generated any traffic over the stream for some period of time. That behavior is no longer recommended; instead, the error SHOULD be used only if the connected client or peer server has not responded to data sent over the stream. 🧃
4.9.3.5. host-gone 🏠
the value of the 'to' attribute in the initial stream header corresponds to an FQDN that the receiving entity no longer services 👋
(in the following example, the peer specifies a 'to' address of "foo.im.example.com" when connecting to the "im.example.com" server, but the server no longer hosts a service at that address.)
P: <?xml version='1.0'?>
<stream:stream
from='example.net'
to='foo.im.example.com'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
to='example.net'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<host-gone
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>4.9.3.6. host-unknown ✨
Okay so, bestie, this one's about the to attribute in the initial stream header. If the value doesn't correspond to an FQDN that the receiving entity actually serves, that's a host-unknown moment 💀. The server basically goes "who?" 🙈
(In the example below, the peer says its to address is "example.org" while connecting to the "im.example.com" server, but the server has no clue who "example.org" is.)
P: <?xml version='1.0'?> <stream:stream from='example.net' to='example.org' version='1.0' xmlns='jabber:server' xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?> <stream:stream from='im.example.com' id='g4qSvGvBxJ+xeAd7QKezOQJFFlw=' to='example.net' version='1.0' xml:lang='en' xmlns='jabber:server' xmlns:stream='http://etherx.jabber.org/streams'> <stream:error> <host-unknown xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.7. improper-addressing 🎀
This one's giving "missing address" energy 😭. A stanza sent between two servers either lacks a to or from attribute, has one with no value, or has a value that breaks the rules for XMPP addresses [XMPP-ADDR]. Not it 💅.
(In the example below, the peer sends a stanza with no to address over a server-to-server stream.)
P: <message from='juliet@im.example.com'> <body>Wherefore art thou?</body> </message>
S: <stream:error> <improper-addressing xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.8. internal-server-error 🔥
Ngl this is the server having a moment 😩. It's been misconfigured or hit some other internal error that stops it from servicing the stream. Not the peer's fault, just the server being a little dramatic 💀.
S: <stream:error> <internal-server-error xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.9. invalid-from 🦋
Okay this one is so serious 💜. The data in a from attribute doesn't match an authorized JID or validated domain that was negotiated, either (1) between two servers using SASL or Server Dialback, or (2) between a client and a server via SASL authentication and resource binding. Basically, you're trying to speak as someone you're not, and the vibes are off 🙅♀️.
(In the example below, a peer that has only authenticated as "example.net" tries to send a stanza from an address at "example.org".)
P: <message from='romeo@example.org' to='juliet@im.example.com'> <body>Neither, fair saint, if either thee dislike.</body> </message>
S: <stream:error> <invalid-from xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.10. invalid-namespace 🌸
The stream namespace name has to be "http://etherx.jabber.org/streams" (see Section 11.2), and the content namespace declared as the default has to be supported. So if it's something other than "jabber:client" or "jabber:server", the server is like "nope, not in this lifetime" 💅.
(In the example below, the client picks a namespace of 'http://wrong.namespace.example.org/' for the stream.)
C: <?xml version='1.0'?> <stream:stream from='juliet@im.example.com' to='im.example.com' version='1.0' xmlns='jabber:client' xmlns:stream='http://wrong.namespace.example.org/'>
S: <?xml version='1.0'?> <stream:stream from='im.example.com' id='++TR84Sm6A3hnt3Q065SnAbbk3Y=' to='juliet@im.example.com' version='1.0' xml:lang='en' xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams'> <stream:error> <invalid-namespace xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.11. invalid-xml 🧚
This is when the entity sends invalid XML over the stream to a server that does validation (see Section 11.4). Rude 😤.
(In the example below, the peer tries to send an IQ stanza of type "subscribe", but the XML schema doesn't define that value for the 'type' attribute.)
P: <iq from='example.net' id='l3b1vs75' to='im.example.com' type='subscribe'> <ping xmlns='urn:xmpp:ping'/> </iq>
S: <stream:error> <invalid-xml xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.12. not-authorized 🔒
Okay, so this one's about trying to get in before you've got the vibe check passed 🙈. The entity has tried to send XML stanzas or other outbound data before the stream is authenticated, or it's otherwise not authorized to do something related to stream negotiation. The receiving entity MUST NOT process the offending data before sending the stream error. No peeking, bestie 🚫.
(In the example below, the client tries to send XML stanzas before authenticating with the server.)
C: <?xml version='1.0'?> <stream:stream from='juliet@im.example.com' to='im.example.com' version='1.0' xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?> <stream:stream from='im.example.com' id='++TR84Sm6A3hnt3Q065SnAbbk3Y=' to='juliet@im.example.com' version='1.0' xml:lang='en' xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams'>
C: <message to='romeo@example.net'> <body>Wherefore art thou?</body> </message>
S: <stream:error> <not-authorized xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.13. not-well-formed 💖
This is the XML being a little messy 🍓. The initiating entity has sent XML that breaks the well-formedness rules of [XML] or [XML-NAMES].
(In the example below, the client sends an XMPP message that isn't namespace-well-formed.)
C: <message> <foo:body>What is this foo?</foo:body> </message>
S: <stream:error> <not-well-formed xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
Interoperability Note: In RFC 3920, the name of this error condition was "xml-not-well-formed" instead of "not-well-formed". The name was changed because the element name <xml-not-well-formed/> violates the constraint from Section 3 of [XML] that "names beginning with a match to (('X'|'x')('M'|'m')('L'|'l')) are reserved for standardization in this or future versions of this specification". 🧐
4.9.3.14. policy-violation 📏
The entity has broken some local service policy, like sending a stanza that's bigger than the configured size limit 😳. The server MAY choose to specify the policy in the <text/> element or in an application-specific condition element. It's optional, so the server gets to decide how much it wants to explain 💅.
(In the example below, the client sends an XMPP message that's too large according to the server's local service policy.)
C: <message to='juliet@im.example.com' id='foo'> <body>[ ... the-emacs-manual ... ]</body> </message>
S: <stream:error> <policy-violation xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> <stanza-too-big xmlns='urn:xmpp:errors'/> </stream:error>
S: </stream:stream>
4.9.3.15. remote-connection-failed 🌈
Okay so this one is about the server being unable to connect to a remote entity it needs for authentication or authorization (for example, in certain Server Dialback [XEP-0220] scenarios). This condition is not to be used when the cause of the error is within the administrative domain of the XMPP service provider, in which case the <internal-server-error/> condition is more appropriate. Know your lane, bestie 🫶.
C: <?xml version='1.0'?> <stream:stream from='juliet@im.example.com' to='im.example.com' version='1.0' xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?> <stream:stream from='im.example.com' id='++TR84Sm6A3hnt3Q065SnAbbk3Y=' to='juliet@im.example.com' version='1.0' xml:lang='en' xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams'> <stream:error> <remote-connection-failed xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.16. reset 🔄
The server is closing the stream because it has new (typically security-critical) features to offer, because the keys or certificates used to establish a secure context for the stream have expired or been revoked during the life of the stream (Section 13.7.2.3), because the TLS sequence number has wrapped (Section 5.3.5), and so on. ✨ The reset applies to the stream and to any security context set up for that stream (for example, via TLS and SASL), which means encryption and authentication need to be negotiated again for the new stream. Even TLS session resumption can't be used here 😭.
S: <stream:error> <reset xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.17. resource-constraint 🦋
The server doesn't have the system resources it needs to service the stream. It's running on fumes 🪫.
C: <?xml version='1.0'?> <stream:stream from='juliet@im.example.com' to='im.example.com' version='1.0' xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?> <stream:stream from='im.example.com' id='++TR84Sm6A3hnt3Q065SnAbbk3Y=' to='juliet@im.example.com' version='1.0' xml:lang='en' xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams'> <stream:error> <resource-constraint xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.18. restricted-xml 🚫
This is when the entity tries to send restricted XML features like a comment, processing instruction, DTD subset, or XML entity reference (see Section 11.1). No fancy XML extras, babe 💅.
(In the example below, the client sends an XMPP message containing an XML comment.)
C: <message to='juliet@im.example.com'> <!--<subject/>--> <body>This message has no subject.</body> </message>
S: <stream:error> <restricted-xml xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </stream:error> </stream:stream>
4.9.3.19. see-other-host 🦋
okay so this one is giving redirect. 💅 The server won't serve the initiating entity, but it's sending them to another host under the same service provider's admin control. The XML character data of the <see-other-host/> element MUST specify the alternate FQDN or IP address to connect to. That value MUST be a valid domainpart, or a domainpart plus port number, separated by the ':' character in the form "domainpart:port" 🎀
If the domainpart is the same as the source domain, the derived domain, or the resolved IPv4 or IPv6 address the initiating entity originally connected to (differing only by the port number), then the initiating entity SHOULD simply attempt to reconnect at that address ✨ An IPv6 address MUST follow [IPv6-ADDR], which means wrapping it in square brackets '[' and ']' as originally defined by [URI] 🧚 Otherwise, the initiating entity MUST resolve the FQDN in the <see-other-host/> element as described under Section 3.2 🌸
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<see-other-host
xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
[2001:41D0:1:A49b::1]:9222
</see-other-host>
</stream:error>
</stream:stream>When negotiating a stream with the host it's been redirected to, the initiating entity MUST apply the same policies it would have applied to the original connection attempt (for example, a policy requiring TLS) 🔐 It MUST also specify the same 'to' address on the initial stream header, and it MUST verify the identity of the new host using the same reference identifier(s) it would have used for the original connection attempt (in accordance with [TLS-CERTS]) 💖
Okay, real talk, this part is lowkey intense 😭 Even if the receiving entity returns a <see-other-host/> error before the confidentiality and integrity of the stream are set up (which opens the door to a denial-of-service attack), the fact that the initiating entity has to verify the identity of the XMPP service using the same reference identifiers means it won't connect to a malicious entity. Iconic protection 👑
To reduce the chance of a denial-of-service attack, (a) the receiving entity SHOULD NOT close the stream with a <see-other-host/> stream error until after the confidentiality and integrity of the stream have been protected via TLS or an equivalent security layer (such as the SASL GSSAPI mechanism), and (b) the receiving entity MAY have a policy of following redirects only if it has authenticated the receiving entity 🙈 And on top of that, the initiating entity SHOULD abort the connection attempt after a certain number of successive redirects (for example, at least 2 but no more than 5) 🛑
4.9.3.20. system-shutdown 🌈
The server is being shut down, and all active streams are being closed. Bye bye, everybody 😭
S: <stream:error>
<system-shutdown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>4.9.3.21. undefined-condition 💀
This is the catch-all for an error condition that isn't one of the others in this list. It SHOULD NOT be used except in conjunction with an application-specific condition. Think of it as the "I don't know what happened" error, and honestly, that's a vibe we all relate to 🙃
S: <stream:error>
<undefined-condition
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<app-error xmlns='http://example.org/ns'/>
</stream:error>
</stream:stream>4.9.3.22. unsupported-encoding 🍓
The initiating entity has encoded the stream in an encoding the server doesn't support (see Section 11.6), or it has otherwise improperly encoded the stream, for example by violating the rules of the [UTF-8] encoding. Not it 🙅♀️
(In the following example, the client tries to encode data using UTF-16 instead of UTF-8.)
C: <?xml version='1.0' encoding='UTF-16'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<unsupported-encoding
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>4.9.3.23. unsupported-feature 🧚
The receiving entity has advertised a mandatory-to-negotiate stream feature that the initiating entity doesn't support, and it hasn't offered any other mandatory-to-negotiate feature alongside the unsupported one. Ghosted by a feature 👻
(In the following example, the receiving entity requires negotiation of an example feature, but the initiating entity doesn't support it.)
R: <stream:features>
<example xmlns='urn:xmpp:example'>
<required/>
</example>
</stream:features>
I: <stream:error>
<unsupported-feature
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>4.9.3.24. unsupported-stanza-type 💜
The initiating entity has sent a first-level child of the stream that the server doesn't support. This happens either because the receiving entity doesn't understand the namespace, or because it doesn't understand the element name for the applicable namespace (which might be the content namespace declared as the default namespace) 📦
(In the following example, the client tries to send a first-level child element of <pubsub/> qualified by the 'jabber:client' namespace, but the schema for that namespace defines no such element. Very Shakespearean of it, honestly 🎭)
C: <pubsub xmlns='jabber:client'>
<publish node='princely_musings'>
<item id='ae890ac52d0df67ed7cfdf51b644e901'>
<entry xmlns='http://www.w3.org/2005/Atom'>
<title>Soliloquy</title>
<summary>
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?
</summary>
<link rel='alternate' type='text/html'
href='http://denmark.example/2003/12/13/atom03'/>
<id>tag:denmark.example,2003:entry-32397</id>
<published>2003-12-13T18:30:02Z</published>
<updated>2003-12-13T18:30:02Z</updated>
</entry>
</item>
</publish>
</pubsub>
S: <stream:error>
<unsupported-stanza-type
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>4.9.3.25. unsupported-version 🎀
The 'version' attribute the initiating entity put in the stream header specifies a version of XMPP that the server doesn't support. Version 11.0 is giving "not yet, babe" 🙈
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='11.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<unsupported-version
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>4.9.4. Application-Specific Conditions ✨
As we've already covered, 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. So the <error/> element ends up with two or three child elements. Layers of drama 💅
C: <message>
<body>
My keyboard layout is:
QWERTYUIOP{}|
ASDFGHJKL:"
ZXCVBNM<>?
</body>
</message>
S: <stream:error>
<not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<text xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
Some special application diagnostic information!
</text>
<escape-your-data xmlns='http://example.org/ns'/>
</stream:error>
</stream:stream>4.10. Simplified Stream Examples 🌸
Okay so this section has two super simplified examples of a stream-based connection between a client and a server 🧁 They're here to illustrate the concepts we've covered so far, but you need to know they skip over a lot of details. For the more complete examples, check out Section 9 🔍
A basic connection:
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
[ ... stream negotiation ... ]
C: <message from='juliet@im.example.com/balcony'
to='romeo@example.net'
xml:lang='en'>
<body>Art thou not Romeo, and a Montague?</body>
</message>
S: <message from='romeo@example.net/orchard'
to='juliet@im.example.com/balcony'
xml:lang='en'>
<body>Neither, fair saint, if either thee dislike.</body>
</message>
C: </stream:stream>
S: </stream:stream>A connection gone bad:
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
[ ... stream negotiation ... ]
C: <message from='juliet@im.example.com/balcony'
to='romeo@example.net'
xml:lang='en'>
<body>No closing tag!
</message>
S: <stream:error>
<not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>More detailed examples are provided under Section 9. 🦋
5. STARTTLS Negotiation 🔐
5.1. Fundamentals ✨
okay so XMPP has a way to keep the stream safe from tampering and eavesdropping 🔒 this is the main character move for channel encryption, fr fr. It uses Transport Layer Security [TLS], specifically a "STARTTLS" extension modeled on similar ones for [IMAP], [POP3], and [ACAP], as described in [USINGTLS]. The XML namespace name for STARTTLS is 'urn:ietf:params:xml:ns:xmpp-tls' 🎀
5.2. Support 💅
Support for STARTTLS is REQUIRED in XMPP client and server implementations 🫶 An administrator of a given deployment MAY specify that TLS is mandatory-to-negotiate for client-to-server communication, server-to-server communication, or both. An initiating entity SHOULD use TLS to secure its stream with the receiving entity before it moves on to SASL authentication 🔐
5.3. Stream Negotiation Rules 🌸
5.3.1. Mandatory-to-Negotiate
If the receiving entity advertises only the STARTTLS feature, or if it includes the <required/> child element as explained under Section 5.4.1, the parties MUST consider TLS mandatory-to-negotiate. Once TLS is mandatory-to-negotiate, the receiving entity SHOULD NOT advertise support for any stream feature except STARTTLS during the initial stage of stream negotiation. The reason is that later stream features might depend on TLS already being negotiated, since of the order of layers in XMPP. For example, the SASL mechanisms the receiving entity offers will likely depend on whether TLS has been negotiated 🧚
5.3.2. Restart
After TLS negotiation, the parties MUST restart the stream 🔄
5.3.3. Data Formatting
During STARTTLS negotiation, the entities MUST NOT send any whitespace as separators between XML elements. That covers everything from the last character of the first-level <starttls/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace as sent by the initiating entity, until the last character of the first-level <proceed/> element qualified by the same namespace as sent by the receiving entity. This prohibition helps ensure proper security layer byte precision 🔢 Any whitespace shown in the STARTTLS examples in this document is only there for readability, so don't copy it in your code 🙈
5.3.4. Order of TLS and SASL Negotiations
If the initiating entity chooses to use TLS, STARTTLS negotiation MUST be completed before SASL negotiation (Section 6) begins. This order is necessary to help safeguard authentication information sent during SASL negotiation. It also makes it possible to base use of the SASL EXTERNAL mechanism on a certificate (or other credentials) provided during prior TLS negotiation 💜
5.3.5. TLS Renegotiation
TLS lets either party in a TLS-protected channel start a new handshake that establishes new cryptographic parameters (see [TLS-NEG]). The most common reasons are:
- Refreshing encryption keys 🔑
- Wrapping the TLS sequence number as explained in Section 6.1 of [TLS]
- Protecting client credentials by completing server authentication first, and then completing client authentication over the protected channel
Establishing streams in XMPP is pretty cheap, so for the first two cases it is preferable to use an XMPP stream reset (as described under Section 4.9.3.16) instead of doing TLS renegotiation 🦋
The third case has better security characteristics when the TLS client (which might be an XMPP server) presents credentials to the TLS server. If sending those credentials to an unauthenticated TLS server might leak private information, it can be appropriate to complete TLS negotiation to authenticate the TLS server to the TLS client, and then attempt TLS renegotiation to authenticate the TLS client to the TLS server. However, this case is extremely rare, because the credentials presented by an XMPP server or XMPP client acting as a TLS client are almost always public (i.e., a PKIX certificate). Providing those credentials before authenticating the XMPP server acting as a TLS server would not, in general, leak private information 🙌
So implementers are encouraged to weigh the costs and benefits of TLS renegotiation carefully before supporting it in their software. XMPP entities that act as TLS clients are discouraged from attempting TLS renegotiation unless the certificate (or other credential information) sent during TLS negotiation is known to be private 👀
Support for TLS renegotiation is strictly OPTIONAL. However, implementations that support TLS renegotiation MUST implement and use the TLS Renegotiation Extension [TLS-NEG] ✨
If an entity that does not support TLS renegotiation detects a renegotiation attempt, it MUST immediately close the underlying TCP connection without returning a stream error. The violation happened at the TLS layer, not the XMPP layer, as described under Section 13.3, so a stream error isn't the right move 💀
If an entity that supports TLS renegotiation detects a TLS renegotiation attempt that does not use the TLS Renegotiation Extension [TLS-NEG], it MUST immediately close the underlying TCP connection without returning a stream error. Again, the violation happened at the TLS layer, not the XMPP layer, as described under Section 13.3 🫠
5.3.6. TLS Extensions
Either party to a stream MAY include any TLS extension during the TLS negotiation itself. This is a matter for the TLS layer, not the XMPP layer 🧁
5.4. Process 🎀
5.4.1. Exchange of Stream Headers and Stream Features
The initiating entity resolves the FQDN of the receiving entity as specified under Section 3, opens a TCP connection to the advertised port at the resolved IP address, and sends an initial stream header to the receiving entity 💌
I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>The receiving entity MUST send a response stream header to the initiating entity over the TCP connection that the initiating entity opened 🩷
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'>After that, the receiving entity MUST send stream features to the initiating entity. If the receiving entity supports TLS, the stream features MUST include an advertisement for STARTTLS negotiation, meaning a <starttls/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace 🌈
If the receiving entity considers STARTTLS negotiation to be mandatory-to-negotiate, the <starttls/> element MUST contain an empty <required/> child element ⭐
R: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>5.4.2. Initiation of STARTTLS Negotiation
5.4.2.1. STARTTLS Command
To begin STARTTLS negotiation, the initiating entity issues the STARTTLS command (a <starttls/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace). This tells the receiving entity that it wants to start 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 (the proceed case) or a <failure/> element (the failure case), qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace 🦋
5.4.2.2. Failure Case
If the failure case happens, the receiving entity MUST return a <failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace, close the XML stream, and terminate the underlying TCP connection 😭
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>R: </stream:stream>Causes for the failure case include, but are not limited to:
- The initiating entity has sent a malformed STARTTLS command.
- The receiving entity did not offer the STARTTLS feature in its stream features.
- The receiving entity cannot complete STARTTLS negotiation because of an internal error.
Informational Note: STARTTLS failure is not triggered by TLS errors such as bad_certificate or handshake_failure. Those are generated and handled during the TLS negotiation itself, as described in [TLS] 🧠
If the failure case happens, the initiating entity MAY attempt to reconnect as explained under Section 3.3 🔁
5.4.2.3. Proceed Case
If the proceed case happens, 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 TLS negotiation to have begun immediately after sending the closing '>' character of the <proceed/> element. The initiating entity MUST consider TLS negotiation to have begun immediately after receiving the closing '>' character of the <proceed/> element from the receiving entity 🎬
The entities now move on to TLS negotiation, which is explained in the next section 🌟
5.4.3. TLS Negotiation
5.4.3.1. Rules
To complete TLS negotiation over the TCP connection, the entities MUST follow the process defined in [TLS] 📜
The following rules apply:
- The entities MUST NOT send any further XML data until TLS negotiation is complete 🤐
- When using any of the mandatory-to-implement (MTI) ciphersuites specified under Section 13.8, the receiving entity MUST present a certificate 🪪
- So that mutual certificate authentication will be possible, the receiving entity SHOULD send a certificate request to the initiating entity, and the initiating entity SHOULD send a certificate to the receiving entity. (For privacy reasons, the initiating entity might opt not to send a certificate until after the receiving entity has authenticated to the initiating entity.) 🫶
- The receiving entity SHOULD choose which certificate to present based on the domainpart contained in the
'to'attribute of the initial stream header. In essence, this domainpart is functionally equivalent to the Server Name Indication defined for TLS in [TLS-EXT] 🎯 - To determine whether TLS negotiation will succeed, the initiating entity MUST attempt to validate the receiving entity's certificate in accordance with the certificate validation procedures specified under Section 13.7.2 🔍
- If the initiating entity presents a certificate, the receiving entity too MUST attempt to validate the initiating entity's certificate in accordance with the certificate validation procedures specified under Section 13.7.2 🧐
- Following successful TLS negotiation, all further data transmitted by either party MUST be protected with the negotiated algorithms, keys, and secrets. That means encrypted, integrity-protected, or both, depending on the ciphersuite used 🔒
Security Warning: See Section 13.8 regarding ciphersuites that MUST be supported for TLS. Naturally, other ciphersuites MAY be supported as well 🚨
5.4.3.2. TLS Failure 💔
okay so if the TLS negotiation goes wrong, the receiving entity MUST terminate the TCP connection. no drama, just close it. 🔌
and bestie, the receiving entity MUST NOT send a closing </stream> tag before terminating the TCP connection. the failure happened at the TLS layer, not the XMPP layer (the closing-tag stuff lives under Section 13.3), so the stream tag isn't the right move here 🙅♀️
the initiating entity MAY try to reconnect as explained under Section 3.3, with or without attempting TLS negotiation again, depending on local service policy, user-configured preferences, etc. you get to choose your comeback era 💅
5.4.3.3. TLS Success 🎉
if the TLS negotiation works, then the entities MUST proceed as follows. this is the glow-up part ✨
- the initiating entity MUST discard any information it got from the receiving entity in an insecure manner before TLS took effect (for example, the receiving entity's
fromaddress, or the stream ID and stream features it received). anything that came in before the encryption was on? toss it 🗑️
the initiating entity's stream header looks like this:
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 discard any information it got from the initiating entity in an insecure manner before TLS took effect (for example, the initiating entity's
fromaddress). same energy, other side of the chat 🫶 - the initiating entity MUST send a new initial stream header to the receiving entity over the encrypted connection. as specified under Section 4.3.3, the initiating entity MUST NOT send a closing
</stream>tag before sending the new initial stream header, since the receiving entity and initiating entity MUST consider the original stream to be replaced upon success of the TLS negotiation. the old stream is literally retired once TLS works, it's giving fresh start 🌸
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 respond with a new response stream header over the encrypted connection, and for that it MUST generate a new stream ID instead of reusing the old stream ID. no recycling the old ID, periodt 🦋
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. those features MUST NOT include the STARTTLS feature, but they SHOULD include the SASL stream feature as described under Section 6. (see especially Section 6.4.1 for the few reasons the SASL stream feature wouldn't be offered here, because sometimes it just isn't 🤷♀️)
R: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>SCRAM-SHA-1-PLUS</mechanism>
<mechanism>SCRAM-SHA-1</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>6. SASL Negotiation 👑
6.1. Fundamentals
XMPP has a way to authenticate a stream using an XMPP-specific profile of the Simple Authentication and Security Layer protocol (see [SASL]). SASL is a generalized method for adding authentication support to connection-based protocols, and XMPP uses an XML namespace profile of SASL that conforms to the profiling requirements of [SASL]. the XML namespace name for the SASL extension is urn:ietf:params:xml:ns:xmpp-sasl. it's the SASL namespace, and it's giving organized 📚
6.2. Support
Support for SASL negotiation is REQUIRED in XMPP client and server implementations. no skipping this one, it's a must-have for the squad 💜
6.3. Stream Negotiation Rules
6.3.1. Mandatory-to-Negotiate
The parties to a stream MUST consider SASL as mandatory-to-negotiate. it's not optional-optional, it's the real deal 🔒
6.3.2. Restart
After SASL negotiation, the parties MUST restart the stream. fresh stream, fresh start ✨
6.3.3. Mechanism Preferences
Any entity that will act as a SASL client or a SASL server MUST maintain an ordered list of its preferred SASL mechanisms according to the client or server. the list is ordered according to local policy or user configuration, which SHOULD be in order of perceived strength so you can get the strongest authentication possible. the initiating entity MUST maintain its own preference order independent of the preference order of the receiving entity. and a client MUST try SASL mechanisms in its preference order. it's giving "my list, my rules" 🙌
for example, say the server offers PLAIN SCRAM-SHA-1 GSSAPI or SCRAM-SHA-1 GSSAPI PLAIN, but the client's ordered list is GSSAPI SCRAM-SHA-1. the client MUST try GSSAPI first and then SCRAM-SHA-1, but it MUST NOT try PLAIN (since PLAIN is not on its list). PLAIN got left on read 💀
6.3.4. Mechanism Offers
If the receiving entity considers TLS negotiation (Section 5) to be mandatory-to-negotiate before it will accept authentication with a particular SASL mechanism, it MUST NOT advertise that mechanism in its list of available SASL mechanisms before TLS negotiation has been completed. no sneak peeks, TLS first 🔐
the receiving entity SHOULD offer the SASL EXTERNAL mechanism if both of the following conditions hold:
- during TLS negotiation, the initiating entity presented a certificate that is acceptable to the receiving entity for purposes of strong identity verification in accordance with local service policies (for example, because said certificate is unexpired, is unrevoked, and is anchored to a root trusted by the receiving entity).
- the receiving entity expects that the initiating entity will be able to authenticate and authorize as the identity provided in the certificate. in the case of a server-to-server stream, the receiving entity might have such an expectation because a DNS domain name presented in the initiating entity's certificate matches the domain referenced in the
fromattribute of the initial stream header, where the matching rules of [TLS-CERTS] apply. in the case of a client-to-server stream, the receiving entity might have such an expectation because the bare JID presented in the initiating entity's certificate matches a user account that is registered with the server, or because other information contained in the initiating entity's certificate matches that of an entity that has permission to use the server for access to an XMPP network. the vibe check is strong here 🕵️♀️
however, the receiving entity MAY offer the SASL EXTERNAL mechanism under other circumstances, as well. so there's wiggle room, bestie 🌈
when the receiving entity offers the SASL EXTERNAL mechanism, it SHOULD list EXTERNAL first among its offered SASL mechanisms, and the initiating entity SHOULD attempt SASL negotiation using the EXTERNAL mechanism first. this preference tends to increase the likelihood that the parties can negotiate mutual certificate authentication, so it's giving main character energy 🎀
Section 13.8 specifies SASL mechanisms that MUST be supported. naturally, other SASL mechanisms MAY be supported as well. ✨
Informational Note: Best practices for the use of SASL in the context of XMPP are described in [XEP-0175] for the ANONYMOUS mechanism and in [XEP-0178] for the EXTERNAL mechanism.
6.3.5. Data Formatting
The following data formatting rules apply to the SASL negotiation:
- during SASL negotiation, the entities MUST NOT send any whitespace as separators between XML elements. that covers everything from the last character of the first-level
<auth/>element qualified by theurn:ietf:params:xml:ns:xmpp-saslnamespace as sent by the initiating entity, until the last character of the first-level<success/>element qualified by theurn:ietf:params:xml:ns:xmpp-saslnamespace as sent by the receiving entity. this prohibition helps to ensure proper security layer byte precision, so no sneaky spaces 🙅♀️ any whitespace shown in the SASL examples provided in this document is only there for readability. - any XML character data contained within the XML elements MUST be encoded using base 64, where the encoding adheres to the definition in Section 4 of [BASE64] and where the padding bits are set to zero. base64 with zeroed padding bits, bestie, very precise 🧮
- as formally specified in the XML schema for the
urn:ietf:params:xml:ns:xmpp-saslnamespace under Appendix A.4, the receiving entity MAY include one or more application-specific child elements inside the<mechanisms/>element. these give the initiating entity info it might need to finish SASL negotiation using one or more of the offered mechanisms. however, the syntax and semantics of all such elements are out of scope for this specification (see [XEP-0233] for one example). that's a whole other story, not ours 📖
6.3.6. Security Layers
Upon successful SASL negotiation that involves negotiation of a security layer, both the initiating entity and the receiving entity MUST discard any application-layer state (i.e, state from the XMPP layer, excluding state from the TLS negotiation or SASL negotiation). fresh slate on the XMPP side, but the TLS and SASL stuff stays 🧹
6.3.7. Simple User Name
Some SASL mechanisms (for example, CRAM-MD5, DIGEST-MD5, and SCRAM) specify that the authentication identity used in the context of such mechanisms is a "simple user name" (see Section 2 of [SASL] as well as [SASLPREP]). the exact form of the simple user name in any particular mechanism or deployment is a local matter. a simple user name does not necessarily map to an application identifier such as a JID or JID component (for example, a localpart). however, in the absence of local information provided by the server, an XMPP client SHOULD assume that the authentication identity for such a SASL mechanism is a simple user name equal to the localpart of the user's JID. so it's basically the part before the @, lowkey iconic 🦋
6.3.8. Authorization Identity
An authorization identity is an OPTIONAL identity included by the initiating entity to specify an identity to act as (see Section 2 of [SASL]). in client-to-server streams, it would most likely be used by an administrator to do some management task on behalf of another user. in server-to-server streams, it would most likely be used to specify a particular add-on service at an XMPP service. for example, a multi-user chat server at conference.example.com that is hosted by the example.com XMPP service. 🧚
if the initiating entity wants 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 want to act on behalf of another entity, it MUST NOT provide an authorization identity. pick your lane, bestie 🛣️
in the case of client-to-server communication, the value of an authorization identity MUST be a bare JID (<localpart@domainpart>) rather than a full JID (<localpart@domainpart/resourcepart>). no resource part allowed here 🙅♀️
in the case of server-to-server communication, the value of an authorization identity MUST be a domainpart only (<domainpart>). just the domain, that's it 🌸
if the initiating entity provides an authorization identity during SASL negotiation, the receiving entity is responsible for verifying that the initiating entity is actually allowed to assume the specified authorization identity. if not, the receiving entity MUST return an <invalid-authzid/> SASL error as described under Section 6.5.6. verification is the receiving entity's job, and if the vibes are off, it gets an error 💅
6.3.9. Realms ✨
okay so the receiving entity MAY include a realm when negotiating certain SASL mechanisms 🎀 (GSSAPI and DIGEST-MD5 both let the auth exchange carry a realm, just in different ways, whereas EXTERNAL, SCRAM, and PLAIN don't). 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. bestie, this part is key 🙈: an initiating entity MUST NOT attempt to derive an XMPP domainpart from the realm information provided by the receiving entity. hands off the domainpart 💅
6.3.10. Round Trips 🦋
[SASL] says a using protocol like XMPP can define two ways to save round trips where the SASL mechanism allows it 🧚:
- when the SASL client (the XMPP "initiating entity") requests an authentication exchange, it can include "initial response" data with its request, if that's appropriate for the SASL mechanism in use. in XMPP, that means putting the initial response in the XML character data of the
<auth/>element. - at the end of the authentication exchange, the SASL server (the XMPP "receiving entity") can include "additional data with success", if that's appropriate for the SASL mechanism in use. in XMPP, that means putting the additional data in the XML character data of the
<success/>element.
for the sake of protocol efficiency, it is REQUIRED for clients and servers to support these methods and RECOMMENDED to use them 🔥; however, clients and servers MUST support the less efficient modes as well. efficient is iconic, but the basic modes still have to work 💖
6.4. Process ✨
the process for SASL negotiation is as follows 👑
6.4.1. Exchange of Stream Headers and Stream Features 🧚
if SASL negotiation follows successful STARTTLS negotiation (Section 5), then the SASL negotiation happens over the protected stream that's already been negotiated 🔒. if not, the initiating entity resolves the FQDN of the receiving entity as specified under Section 3, opens a TCP connection to the advertised port at the resolved IP address, and sends an initial stream header to the receiving entity. either way, the receiving entity will receive an initial stream from the initiating entity.
I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>when the receiving entity processes an initial stream header from the initiating entity, it MUST send a response stream header to the initiating entity (for which it MUST generate a unique stream ID. if TLS negotiation has already succeeded, then this stream ID MUST be different from the stream ID sent before TLS negotiation succeeded) 🌈
R: <stream:stream
from='im.example.com'
id='vgKi/bkYME8OAj4rlXMkpucAqe4='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>the receiving entity also MUST send stream features to the initiating entity 🎀. the stream features SHOULD include an advertisement for support of SASL negotiation, i.e., a <mechanisms/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace. typically there are only three cases where support for SASL negotiation wouldn't be advertised here 🙃:
- TLS negotiation needs to happen before SASL can be offered (i.e., TLS is required and the receiving entity is responding to the very first initial stream header it has received for this connection attempt).
- SASL negotiation is impossible for a server-to-server connection (i.e., the initiating server has not provided a certificate that would enable strong authentication and therefore the receiving server is falling back to weak identity verification using the Server Dialback protocol [XEP-0220]).
- SASL has already been negotiated (i.e., the receiving entity is responding to an initial stream header sent as a stream restart after successful SASL negotiation).
the <mechanisms/> element MUST contain one <mechanism/> child element for each authentication mechanism the receiving entity offers to the initiating entity. as noted, the order of <mechanism/> elements in the XML shows the preference order of the SASL mechanisms according to the receiving entity 📋 (which is not necessarily the preference order according to the initiating entity). the receiving entity's order is not the same as the initiator's, so don't mix them up 😭
R: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>SCRAM-SHA-1-PLUS</mechanism>
<mechanism>SCRAM-SHA-1</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>6.4.2. Initiation 🌸
to begin 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 🚀. that starts the handshake for that particular authentication mechanism. this element MAY contain XML character data (in SASL terminology, the "initial response") if the mechanism supports or requires it. if the initiating entity needs to send a zero-length initial response, it MUST transmit the response as a single equals sign character ("="), which means the response is there but has no data 🫶.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>if the initiating entity sends another <auth/> element while the ongoing authentication handshake hasn't finished yet, the receiving entity MUST discard the ongoing handshake and MUST process a new handshake for the newly requested SASL mechanism. no double-booking the handshake, babe 💀
6.4.3. Challenge-Response Sequence 💜
if necessary, the receiving entity challenges the initiating entity by sending a <challenge/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace. this element MAY contain XML character data (which MUST be generated in accordance with the definition of the SASL mechanism the initiating entity chose) 🍓.
the initiating entity answers the challenge by sending a <response/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace. this element MAY contain XML character data (which MUST be generated in accordance with the definition of the SASL mechanism the initiating entity chose).
if necessary, the receiving entity sends more challenges and the initiating entity sends more responses.
this back-and-forth of challenge/response pairs keeps going until one of three things happens:
- the initiating entity aborts the handshake for this authentication mechanism.
- the receiving entity reports failure of the handshake.
- the receiving entity reports success of the handshake.
these scenarios are described in the following sections 📖
6.4.4. Abort 🛑
the initiating entity aborts the handshake for this authentication mechanism by sending an <abort/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace.
I: <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>when the receiving entity gets an <abort/> element, it MUST return a <failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace and containing an <aborted/> child element 🙅♀️.
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<aborted/>
</failure>6.4.5. SASL Failure 💀
the receiving entity reports failure of the handshake for this authentication mechanism by sending a <failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace (the particular cause of failure MUST be communicated in an appropriate child element of the <failure/> element, as defined under Section 6.5) ✨.
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>where it makes sense for the chosen SASL mechanism, the receiving entity SHOULD allow a configurable but reasonable number of retries (at least 2 and no more than 5) 🫠. this lets the initiating entity (e.g., an end-user client) survive wrong credentials, like a typo'd password, without being forced to reconnect. otherwise the receiving entity would immediately return a SASL failure and close the stream, and nobody wants that main-character-exit 🙄
if the initiating entity tries a reasonable number of retries with the same SASL mechanism and they all fail, it MAY fall back to the next mechanism in its ordered list by sending a new <auth/> request to the receiving entity, which starts a new handshake for that authentication mechanism 🔄. if every handshake fails and there are no mechanisms left in the initiating entity's list of supported and acceptable mechanisms, the initiating entity SHOULD simply close the stream as described under Section 4.4 (instead of waiting for the stream to time out) 🥀.
if the initiating entity exceeds the number of retries, the receiving entity MUST close the stream with a stream error, which SHOULD be <policy-violation/> (Section 4.9.3.14), although some existing implementations send <not-authorized/> (Section 4.9.3.12) instead 🚫.
Implementation Note: for server-to-server streams, if the receiving entity cannot offer the SASL EXTERNAL mechanism or any other SASL mechanism based on the security context established during TLS negotiation, the receiving entity MAY attempt to complete weak identity verification using the Server Dialback protocol [XEP-0220]. however, if according to local service policies weak identity verification is insufficient, then the receiving entity SHOULD instead close the stream with a <policy-violation/> stream error (Section 4.9.3.14) instead of waiting for the stream to time out 🧚✨
6.4.6. SASL Success ✨
Okay bestie, before the SASL handshake counts as a success 🎀, there's a check. If the initiating entity sent a 'from' attribute on an initial stream header, and that stream's confidentiality and integrity were protected by TLS or an equivalent security layer (like the SASL GSSAPI mechanism), then the receiving entity SHOULD match the authentication identity from the SASL negotiation against that 'from' address. If the two don't match, the receiving entity SHOULD terminate the connection attempt. BUT (and this is lowkey important 👀) it might have legit reasons not to terminate, like if it overrode a connecting client's address to fix the JID format, or assigned a JID based on info in an end-user certificate. 💅
The receiving entity reports that the handshake worked by sending a <success/> element in the urn:ietf:params:xml:ns:xmpp-sasl namespace. This element MAY contain XML character data (SASL calls it "additional data with success") if the chosen SASL mechanism supports or requires that. And if the receiving entity needs to send additional data that's zero length, it MUST send it as a single equals sign character ("=") 🧚.
R: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>Informational Note: For client-to-server streams, the authorization identity shared during SASL negotiation is used by the receiving server to decide the canonical address for the initiating client. See Section 4.3.6 for the details 🌸.
Once the initiating entity gets that <success/> element, it MUST start a brand new stream over the existing TCP connection by sending a new initial stream header to the receiving entity (see Section 4.3.3). It MUST NOT send a closing </stream> tag before that new header, because the receiving and initiating entities MUST both consider the original stream replaced once SASL succeeds. Iconic stream replacement 🦋.
I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>When the receiving entity gets that new initial stream header, it MUST answer with a new response stream header. It also MUST generate a brand new stream ID for that response, instead of reusing the old one 🫶.
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, either listing any further available features or sending none at all (an empty <features/> element) ✨.
R: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>6.5. SASL Errors 💅
Here's the syntax for SASL errors. The XML data inside the square brackets '[' and ']' is OPTIONAL 🎀.
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<defined-condition/>
[<text xml:lang='langcode'>
OPTIONAL descriptive text
</text>]
</failure>The "defined-condition" MUST be one of the SASL-related error conditions defined in the following sections. Since new error conditions might show up in the future, though, if an entity gets a SASL error condition it doesn't understand, it MUST treat that unknown condition as a generic authentication failure. In other words, it's equivalent to <not-authorized/> (Section 6.5.10) 💀.
Including the <text/> element is OPTIONAL. It can carry application-specific info about the error, which MAY be shown to a human, but only as a supplement to the defined condition 🌈.
Because XMPP defines its own application profile of SASL, and nobody's expected to build more specialized XMPP apps on top of SASL, the SASL error format doesn't offer extensibility for application-specific error conditions the way XML streams (Section 4.9.4) and XML stanzas (Section 8.3.4) do 🧚.
6.5.1. aborted
The receiving entity acknowledges that the initiating entity aborted the authentication handshake. This is the reply to an <abort/> element 🦋.
I: <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<aborted/>
</failure>6.5.2. account-disabled
The initiating entity's account has been temporarily disabled. This is sent in reply to an <auth/> element (with or without initial response data) or a <response/> element 🙈.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<account-disabled/>
<text xml:lang='en'>Call 212-555-1212 for assistance.</text>
</failure>6.5.3. credentials-expired
The auth failed because the initiating entity gave credentials that have expired. This is sent in reply to a <response/> element, or to an <auth/> element with initial response data ⏰.
I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<credentials-expired/>
</failure>6.5.4. encryption-required
The mechanism the initiating entity asked for can't be used unless the confidentiality and integrity of the underlying stream are protected (typically via TLS). This is sent in reply to an <auth/> element (with or without initial response data) 🔐.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<encryption-required/>
</failure>6.5.5. incorrect-encoding
The data from the initiating entity couldn't be processed because the base 64 encoding is wrong. For example, the encoding doesn't follow the definition in Section 4 of [BASE64]. This is sent in reply to a <response/> element or an <auth/> element with initial response data 🍓.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'>[ ... ]</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<incorrect-encoding/>
</failure>6.5.6. invalid-authzid
The authzid from the initiating entity is invalid. Either it's formatted wrong, or the initiating entity doesn't have permission to authorize that ID. This is sent in reply to a <response/> element or an <auth/> element with initial response data 🚫.
I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<invalid-authzid/>
</failure>6.5.7. invalid-mechanism
The initiating entity either didn't pick a mechanism, or asked for one the receiving entity doesn't support. This is sent in reply to an <auth/> element 🫠.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='CRAM-MD5'/>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<invalid-mechanism/>
</failure>6.5.8. malformed-request
The request is malformed. For example, the <auth/> element has initial response data but the mechanism doesn't allow that, or the data sent breaks the syntax for the specified SASL mechanism. This is 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, so it violates the "token" production for the SASL ANONYMOUS mechanism as specified in [ANONYMOUS].)
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='ANONYMOUS'>[ ... some-long-token ... ]</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<malformed-request/>
</failure>6.5.9. mechanism-too-weak
The mechanism the initiating entity asked for is weaker than server policy allows for that entity. This is sent in reply to an <auth/> element (with or without initial response data) 🛡️.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism-too-weak/>
</failure>6.5.10. not-authorized
The auth failed because the initiating entity didn't provide proper credentials. It also covers the case where some generic authentication failure happened, but the receiving entity doesn't want to share specifics about what went wrong. This is sent in reply to a <response/> element or an <auth/> element with initial response data 🙅.
I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>Security Warning: This error condition includes, but is not limited to, the case of incorrect credentials or a nonexistent username. To discourage directory harvest attacks, NO differentiation is made between incorrect credentials and a nonexistent username. Please don't leak extra info here, bestie, it's a whole vibe-killer for security 🚨.
6.5.11. temporary-auth-failure
The auth failed because of a temporary error condition within the receiving entity. It's advisable for the initiating entity to try again later. This is sent in reply to an <auth/> element or a <response/> element 🌙.
I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>6.6. SASL Definition ✨
okay so [SASL] is super picky and needs a bunch of info from whatever protocol is using it, and XMPP is spilling all of it right here 💅
- service name:
"xmpp"🎀 - initiation sequence: once the initiating entity sends its opening XML stream header and the receiving entity replies in kind, the receiving entity hands over a list of acceptable authentication methods 📋. The initiating entity picks one and sends it as the value of the
'mechanism'attribute on an<auth/>element. It can also optionally tuck in an initial response to skip a round trip ✨ - exchange sequence: challenges go from receiving entity to initiating entity via
<challenge/>, and responses go the other way via<response/>. The receiving entity reports failure with<failure/>and success with<success/>, and the initiating entity aborts with<abort/>🌸. Upon successful negotiation, both sides consider the original XML stream closed, and both send new stream headers 🦋 - security layer negotiation: the security layer kicks in immediately after the receiving entity sends the closing '>' of its
<success/>, and immediately after the initiating entity receives the closing '>' of that<success/>💖. The order of layers is first [TCP], then [TLS], then [SASL], then XMPP 🧚 - use of the authorization identity: XMPP can use this to denote the non-default
<localpart@domainpart>of a client. An empty string is equivalent to an absent authorization identity 🫶
7. Resource Binding 👑
7.1. Fundamentals
okay bestie, this is the big one 🙈. After a client authenticates with a server, it MUST bind a specific resource to the stream so the server can properly address the client. Basically there MUST be an XMPP resource attached to the client's bare JID (<localpart@domainpart>), so the address for that stream becomes a full JID of the form <localpart@domainpart/resource>, resourcepart included 💅. This is what lets the server deliver and receive XML stanzas in relation to entities other than the server itself or the client's account, as explained under Section 10 ✨
Informational Note: the client could swap stanzas with the server itself or with its own account before binding a resource, since the full JID is only needed for addressing outside the context of the stream negotiated between the client and the server. Not commonly done though 🌈
After a client has bound a resource to the stream, it's called a "connected resource" 🎀. A server SHOULD allow an entity to maintain multiple connected resources at the same time. Each connected resource is tied to a distinct XML stream and is told apart from the others by a distinct resourcepart 🦋
Security Warning: a server SHOULD enable the administrator of an XMPP service to limit the number of connected resources, to help prevent certain denial-of-service attacks described under Section 13.12 🚨
If, before completing the resource binding step, the client tries to send an XML stanza to an entity other than the server itself or the client's account, the server MUST NOT process the stanza. It MUST close the stream with a <not-authorized/> stream error (Section 4.9.3.12) 💀
The XML namespace name for the resource binding extension is urn:ietf:params:xml:ns:xmpp-bind 🍓
7.2. Support
Support for resource binding is REQUIRED in XMPP client and server implementations. no excuses, bestie 👑
7.3. Stream Negotiation Rules
7.3.1. Mandatory-to-Negotiate
The parties to a stream MUST consider resource binding as mandatory-to-negotiate ✨
7.3.2. Restart
After resource binding, the parties MUST NOT restart the stream 🙅♀️
7.4. Advertising Support
Upon sending a new response stream header to the client after successful SASL negotiation, the server MUST include a <bind/> element qualified by the urn:ietf:params:xml:ns:xmpp-bind namespace in the stream features it presents to the client 🎀.
The server MUST NOT include the resource binding stream feature until after the client has authenticated, typically through successful SASL negotiation. it's giving "wait your turn" energy 💅
S: <stream:stream
from='im.example.com'
id='gPybzaOzBmaADgxKXu9UClbprp0='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>Upon being informed that resource binding is mandatory-to-negotiate, the client MUST bind a resource to the stream as described in the following sections 🫶
7.5. Generation of Resource Identifiers
A resourcepart MUST at a minimum be unique among the connected resources for that <localpart@domainpart>. Enforcing this policy is the server's job 👑
Security Warning: a resourcepart can be security-critical 😭. For example, if a malicious entity can guess a client's resourcepart, it might be able to tell whether the client (and therefore the controlling principal) is online or offline. That's a presence leak, as described under Section 13.10.2. To prevent that, a client can either (1) generate a random resourcepart on its own or (2) ask the server to generate one on its behalf. One method for making sure the resourcepart is random is to generate a Universally Unique Identifier (UUID) as specified in [UUID] 🔥
7.6. Server-Generated Resource Identifier
A server MUST be able to generate an XMPP resourcepart on behalf of a client. The resourcepart the server generates MUST be random (see [RANDOM]) 🧚
7.6.1. Success Case
A client asks for a server-generated resourcepart by sending an IQ stanza of type "set" (see Section 8.2.3) containing an empty <bind/> element qualified by the urn:ietf:params:xml:ns:xmpp-bind namespace 💖
C: <iq id='tn281v37' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</iq>Once the server has generated an XMPP resourcepart for the client, it MUST return an IQ stanza of type "result" to the client. That stanza MUST include a <jid/> child element specifying the full JID for the connected resource, as determined by the server ✨
S: <iq id='tn281v37' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb
</jid>
</bind>
</iq>7.6.2. Error Cases
When a client asks the server to generate a resourcepart during resource binding, the following stanza error conditions are defined 🙈:
- The account has reached a limit on the number of simultaneous connected resources allowed.
- The client is otherwise not allowed to bind a resource to the stream.
Naturally, other error conditions not specified here might occur too, as described under Section 8.3 🌸
7.6.2.1. Resource Constraint
If the account has reached a limit on the number of simultaneous connected resources allowed, the server MUST return a <resource-constraint/> stanza error (Section 8.3.3.18) 🚫
S: <iq id='tn281v37' type='error'>
<error type='wait'>
<resource-constraint
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>7.6.2.2. Not Allowed
If the client is otherwise not allowed to bind a resource to the stream, the server MUST return a <not-allowed/> stanza error (Section 8.3.3.10) 🙅♀️
S: <iq id='tn281v37' type='error'>
<error type='cancel'>
<not-allowed
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>7.7. Client-Submitted Resource Identifier
Instead of asking the server to generate a resourcepart, a client MAY attempt to submit a resourcepart that it has generated or that the controlling user has provided 🎀
7.7.1. Success Case
A client asks its server to accept a client-submitted resourcepart by sending an IQ stanza of type "set" containing a <bind/> element with a child <resource/> element that has non-zero-length XML character data 💅
C: <iq id='wy2xa82b4' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>balcony</resource>
</bind>
</iq>The server SHOULD accept the client-submitted resourcepart. It does that 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 the client-submitted text without modification 🦋
S: <iq id='wy2xa82b4' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>juliet@im.example.com/balcony</jid>
</bind>
</iq>Alternatively, in accordance with local service policies, the server MAY refuse the client-submitted resourcepart and override it with a resourcepart that the server generates ✨
S: <iq id='wy2xa82b4' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
</jid>
</bind>
</iq>7.7.2. Error Cases
When a client attempts to submit its own XMPP resourcepart during resource binding, the following stanza error conditions are defined in addition to those described under Section 7.6.2 🌈:
- The provided resourcepart cannot be processed by the server.
- The provided resourcepart is already in use.
Naturally, other error conditions not specified here might occur too, as described under Section 8.3 💜
7.7.2.1. Bad Request
If the provided resourcepart cannot be processed by the server (for example, because it has zero length or because it otherwise violates the rules for resourceparts specified in [XMPP-ADDR]), the server can return a <bad-request/> stanza error (Section 8.3.3.1). It SHOULD instead process the resourcepart so that it conforms 🙈
S: <iq id='wy2xa82b4' type='error'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>7.7.2.2. Conflict 💥
okay so bestie, if there's already a connected client whose session has the same resourcepart the newly connecting client is asking for, the server MUST do one of the following. Which one it picks is up to the implementation or local service policy, though we've got suggestions below 💅
- Override the resourcepart from the newly connecting client with a server-generated resourcepart ✨ This one is encouraged, because it makes the resource binding process simpler for client implementations 🫶
- Disallow the resource binding attempt of the newly connecting client and keep the session of the currently connected client 🙅♀️ This is neither encouraged nor discouraged, even though it was implicitly encouraged in RFC 3920. Heads up though: handling of the
<conflict/>error is unevenly supported across client implementations. A lot of them treat it like an authentication error, and they've been seen throwing away cached credentials when they get it 😭 - Terminate the session of the currently connected client and allow the resource binding attempt of the newly connecting client ⚡ This was the traditional behavior of early XMPP server implementations, but it's now discouraged because it can lead to a never-ending cycle of two clients knocking each other offline 🔄 It can still be appropriate in some deployments, though, or if the server knows the currently connected client has a dead connection or broken stream, as described under Section 4.6.
If the server goes with behavior #1, it sends back an <iq/> stanza of type "result" to the newly connecting client. The <jid/> child of the <bind/> element holds the full JID, including the resourcepart the server generated 🎀
S: <iq id='wy2xa82b4' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
</jid>
</bind>
</iq>If the server goes with behavior #2, it sends a <conflict/> stanza error (Section 8.3.3.2) in response to the resource binding attempt of the newly connecting client. It keeps the XML stream open though, so the newly connecting client can negotiate a non-conflicting resourcepart. In other words, it has to pick a different resourcepart before trying to bind a resource again 🌸
S: <iq id='wy2xa82b4' type='error'>
<error type='modify'>
<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>If the server goes with behavior #3, it sends a <conflict/> stream error (Section 4.9.3.3) to the currently connected client, as described under Section 4.9.3.3. Then it returns an IQ stanza of type "result" (meaning success) to the newly connecting client's resource binding attempt 👑
S: <iq id='wy2xa82b4' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/balcony
</jid>
</bind>
</iq>7.7.3. Retries 🔁
If an error happens when a client submits a resourcepart, the server SHOULD allow a configurable but reasonable number of retries, at least 5 and no more than 10. This lets the client recover from a slightly wrong resourcepart, like bad data formats or duplicate text strings, without being forced to reconnect 🫶
Once the client has hit the retry limit, the server MUST close the stream with a <policy-violation/> stream error (Section 4.9.3.14) 🚫
8. XML Stanzas 🌈
After a client and a server (or two servers) finish stream negotiation, either party can send XML stanzas. Three kinds of XML stanza are defined for the 'jabber:client' and 'jabber:server' namespaces: <message/>, <presence/>, and <iq/>. There are also five common attributes for these stanza types. Those common attributes, plus the basic semantics of the three stanza types, are defined in this specification. For more detailed syntax for instant messaging and presence, check [XMPP-IM]. For other applications, check the relevant XMPP extension specifications 📚
Support for the XML stanza syntax and semantics defined in this specification is REQUIRED in XMPP client and server implementations ✨
Security Warning: A server MUST NOT process a partial stanza and MUST NOT attach meaning to the transmission timing of any part of a stanza (before receipt of the closing tag). 🚨
8.1. Common Attributes 💜
The following five attributes are common to message, presence, and IQ stanzas 🦋
8.1.1. to 🎯
The 'to' attribute specifies the JID of the intended recipient for the stanza 💌
<message to='romeo@example.net'>
<body>Art thou not Romeo, and a Montague?</body>
</message>For how servers process inbound and outbound XML stanzas based on the 'to' address, see Section 10 🧚
8.1.1.1. Client-to-Server Streams 🌷
These rules cover the 'to' attribute in stanzas a connected client sends to its server over an XML stream qualified by the 'jabber:client' namespace 💅
- A stanza with a specific intended recipient (like a conversation partner, a remote service, the server itself, or even another resource tied to the user's bare JID) MUST have a 'to' attribute whose value is an XMPP address 📮
- A stanza that a client sends to a server for direct processing by the server (like roster processing as described in [XMPP-IM], or presence sent to the server for broadcasting to other entities) MUST NOT have a 'to' attribute 🙅♀️
These rules cover the 'to' attribute in stanzas a server sends to a connected client over an XML stream qualified by the 'jabber:client' namespace 🎀
- If the server got the stanza from another connected client or from a peer server, the server MUST NOT modify the 'to' address before delivering the stanza to the client 🔒
- If the server generated the stanza itself (for example, a response to an IQ stanza of type "get" or "set", even if that stanza didn't include a 'to' address), the stanza MAY include a 'to' address, and that address MUST be the full JID of the client. But if the stanza doesn't include a 'to' address, the client MUST treat it as if the 'to' address were the client's full JID 💖
Implementation Note: It's the server's job to deliver only stanzas addressed to the client's full JID or the user's bare JID, so the client doesn't need to check the 'to' address of incoming stanzas. Still, if the client does check it, it's suggested to check at most the bare JID portion (not the full JID). The 'to' address might be the user's bare JID, the client's current full JID, or even a full JID with a different resourcepart (like in the case of so-called "offline messages" as described in [XEP-0160]) 🤓
8.1.1.2. Server-to-Server Streams 🛰️
These rules cover the 'to' attribute in the context of XML streams qualified by the 'jabber:server' namespace, which means server-to-server streams 🌐
- A stanza MUST have a 'to' attribute whose value is an XMPP address. If a server gets a stanza that doesn't meet this restriction, it MUST close the stream with an
<improper-addressing/>stream error (Section 4.9.3.7) 💀 - The domainpart of the JID in the stanza's 'to' attribute MUST match the FQDN of the receiving server (or any validated domain of it). That validation comes from SASL negotiation (see Section 6), Server Dialback (see [XEP-0220]), or similar means. If a server gets a stanza that doesn't meet this restriction, it MUST close the stream with a
<host-unknown/>stream error (Section 4.9.3.6) or a<host-gone/>stream error (Section 4.9.3.5) 🚪
8.1.2. from 🪞
The 'from' attribute specifies the JID of the sender 🫶
<message from='juliet@im.example.com/balcony'
to='romeo@example.net'>
<body>Art thou not Romeo, and a Montague?</body>
</message>8.1.2.1. Client-to-Server Streams 💫
These rules cover the 'from' attribute in the context of XML streams qualified by the 'jabber:client' namespace, meaning client-to-server streams 🦋
- When a server receives an XML stanza from a connected client, the server MUST add a 'from' attribute to the stanza or override the 'from' attribute the client specified. The value of that 'from' attribute MUST be the full JID (
<localpart@domainpart/resource>) the server determined for the connected resource that generated the stanza (see Section 4.3.6). The exception is subscription-related presence stanzas, which use the bare JID (<localpart@domainpart>) instead (see [XMPP-IM]) 📮 - When the server generates a stanza on its own behalf for delivery to the client, the stanza MUST include a 'from' attribute whose value is the bare JID (that is,
<domainpart>) of the server. That was agreed on during stream negotiation, for example based on the 'to' attribute of the initial stream header 🌸 - When the server generates a stanza for delivery to the client on behalf of the connected client's account (for example, in data storage services the server provides for the client), the stanza MUST either (a) not include a 'from' attribute or (b) include a 'from' attribute whose value is the account's bare JID (
<localpart@domainpart>) 🗂️ - A server MUST NOT send a stanza without a 'from' attribute to the client if the stanza wasn't generated by the server on its own behalf. For example, if another client or a peer server generated it and the server is just delivering it to the client for someone else. So when a client receives a stanza with no 'from' attribute, it MUST assume the stanza is from the user's account on the server 🧁
8.1.2.2. Server-to-Server Streams 🌍
These rules cover the 'from' attribute in the context of XML streams qualified by the 'jabber:server' namespace, meaning server-to-server streams 🛰️
- A stanza MUST have a 'from' attribute whose value is an XMPP address. If a server gets a stanza that doesn't meet this restriction, it MUST close the stream with an
<improper-addressing/>stream error (Section 4.9.3.7) 💀 - The domainpart of the JID in the stanza's 'from' attribute MUST match the FQDN of the sending server (or any validated domain of it). That validation comes from SASL negotiation (see Section 6), Server Dialback (see [XEP-0220]), or similar means. If a server gets a stanza that doesn't meet this restriction, it MUST close the stream with an
<invalid-from/>stream error (Section 4.9.3.9) 🚫
Enforcing these rules helps prevent certain denial-of-service attacks, as described under Section 13.12 🛡️✨
8.1.3. id ✨
Okay bestie, the id attribute is the originating entity's tracking tag 🏷️. It's how the sender keeps tabs on any response or error stanza that comes back about the stanza it generated, whether that reply comes from an intermediate server or from the intended recipient 💌.
Whether the id value only has to be unique within the current stream or unique globally is totally up to the originating entity 🤷♀️.
For <message/> and <presence/> stanzas, it is RECOMMENDED that the originating entity includes an id attribute 🎀. For <iq/> stanzas, it is REQUIRED 🚨.
If the generated stanza includes an id attribute, then the response or error stanza REQUIRED to also include an id attribute, and that id value MUST match the one on the generated stanza 🪞.
The semantics of IQ stanzas add extra restrictions on top of this, as described under Section 8.2.3 💅.
8.1.4. type 🎭
The type attribute tells you the purpose or context of a message, presence, or IQ stanza. The allowable values depend on which kind of stanza you're looking at. The defined values for message and presence stanzas are all about instant messaging and presence apps, so they live in [XMPP-IM] 💬. IQ values are different: they cover the semantics of every structured request-response exchange (no matter what the payload is), and they're specified under Section 8.2.3 🔁.
The only type value shared by all three kinds of stanzas is "error", as described under Section 8.3 ❌.
8.1.5. xml:lang 🌍
A stanza SHOULD have an xml:lang attribute (as defined in Section 2.12 of [XML]) if it contains XML character data that's meant to be shown to a human user (as explained in [CHARSETS], "internationalization is for humans") 🫶. The value of xml:lang sets the default language for any of that human-readable character data 🗣️.
<presence from='romeo@example.net/orchard' xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
</presence>The value of xml:lang MAY be overridden by the xml:lang attribute of a specific child element 🙌.
<presence from='romeo@example.net/orchard' xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
<status xml:lang='cs'>Dvořím se Julii</status>
</presence>If an outbound stanza generated by a client does not have an xml:lang attribute, the client's server SHOULD add one whose value is the one specified for the client's output stream, as defined under Section 4.7.4 💖.
C: <presence from='romeo@example.net/orchard'>
<show>dnd</show>
<status>Wooing Juliet</status>
</presence>
S: <presence from='romeo@example.net/orchard'
to='juliet@im.example.com'
xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
</presence>If an inbound stanza received by a client or server does not have an xml:lang attribute, an implementation MUST assume the default language is the one specified for the entity's input stream, as defined under Section 4.7.4 📥.
The value of xml:lang 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. Hands off, that's the sender's language choice 🔒.
8.2. Basic Semantics 🌟
8.2.1. Message Semantics 💌
The <message/> stanza is a "push" mechanism, where one entity pushes information to another, kind of like email 📨. All message stanzas will have a to attribute that names the intended recipient (see Section 8.1.1 and Section 10.3), unless the message is being sent to the bare JID of a connected client's account. When a server receives a message stanza with a to address, it SHOULD try to route or deliver it to the intended recipient (see Section 10 for the general routing and delivery rules for XML stanzas) 🚚.
8.2.2. Presence Semantics 📡
The <presence/> stanza is a specialized "broadcast" or "publish-subscribe" mechanism. Multiple entities get network availability information about an entity they've subscribed to, so it's giving group chat status update 🟢. In general, a publishing client SHOULD send a presence stanza with no to attribute. Then the server it's connected to broadcasts that stanza to all subscribed entities 📣. A publishing client MAY also send a presence stanza with a to attribute, in which case the server routes or delivers that stanza to the intended recipient 🎯.
Presence is mostly used by XMPP clients, but servers, add-on services, and any other kind of XMPP entity can use it too 🧚. See Section 10 for the general routing and delivery rules for XML stanzas, and [XMPP-IM] for the rules specific to presence applications 📚.
8.2.3. IQ Semantics 🔁
Info/Query, or IQ, is a "request-response" mechanism, a bit like the Hypertext Transfer Protocol [HTTP] 🌐. IQ lets one entity make a request of another and receive a response back. The data in the request and response is defined by the schema or other structural definition tied to the XML namespace that qualifies the direct child element of the IQ element (see Section 8.4). The requesting entity tracks the whole interaction using the id attribute 🕵️♀️. So IQ exchanges follow a common pattern of structured data exchange, like get/result or set/result. An error can also come back in reply to a request if that's appropriate 💀:
Requesting Responding
Entity Entity
---------- ----------
| |
| <iq id='1' type='get'> |
| [ ... payload ... ] |
| </iq> |
| -------------------------> |
| |
| <iq id='1' type='result'> |
| [ ... payload ... ] |
| </iq> |
| <------------------------- |
| |
| <iq id='2' type='set'> |
| [ ... payload ... ] |
| </iq> |
| -------------------------> |
| |
| <iq id='2' type='error'> |
| [ ... condition ... ] |
| </iq> |
| <------------------------- |
| |
Figure 5: Semantics of IQ StanzasTo enforce these semantics, the following rules apply 📜:
- The
idattribute is REQUIRED for IQ stanzas 🎀. - The
typeattribute is REQUIRED for IQ stanzas. Its value MUST be one of the following. If it isn't, the recipient or an intermediate router MUST return a<bad-request/>stanza error (Section 8.3.3.1) 🙅♀️.get-- The stanza requests information, asks what data is needed to complete further operations, etc.set-- The stanza provides data needed for an operation to be completed, sets new values, replaces existing values, etc.result-- The stanza is a response to a successful get or set request.error-- The stanza reports an error that has occurred regarding processing or delivery of a get or set request (see Section 8.3).
- 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
idattribute of the request (or be empty if the generated stanza did not include anidattribute) 🪞. - An entity that receives a stanza of type "result" or "error" MUST NOT respond to it by sending another IQ response of type "result" or "error". The requesting entity MAY, however, send another request. For example, it could send an IQ of type "set" to provide obligatory information it found through a get/result pair 🔄.
- An IQ stanza of type "get" or "set" MUST contain exactly one child element, which specifies the semantics of the particular request 🧁.
- An IQ stanza of type "result" MUST include zero or one child elements 🍓.
- An IQ stanza of type "error" MAY include the child element contained in the associated "get" or "set", and it MUST include an
<error/>child. For details, see Section 8.3 ❌.
8.3. Stanza Errors 🚨
Stanza errors get handled in a way that's similar to stream errors (Section 4.9). The difference is that stanza errors are recoverable 💪. So they do NOT end the XML stream or the underlying TCP connection. Instead, the entity that discovers the error condition returns an error stanza. An error stanza:
- is the same kind (message, presence, or IQ) as the generated stanza that triggered the error 🧩
- has a
typeattribute set to a value of "error" 🙈 - typically swaps the
fromandtoaddresses of the generated stanza 🔀 - mirrors the
idattribute (if any) of the generated stanza that triggered the error 🪞 - contains an
<error/>child element that specifies the error condition, and therefore gives a hint about actions the sender might be able to take to remedy the error (however, it is not always possible to remedy the error) 🩹
8.3.1. Rules ✨
okay bestie, here are the rules for stanza errors 💅
- The receiving or processing entity that detects an error condition in relation to a stanza SHOULD return an error stanza (and it MUST do so for IQ stanzas) 🫶
- The error stanza SHOULD simply swap the 'from' and 'to' addresses from the generated stanza 🔄, unless doing that would (1) result in an information leak (see under Section 13.10) or other breach of security, or (2) force the sender of the error stanza to include a malformed JID in the 'from' or 'to' address of the error stanza.
- If the generated stanza was
<message/>or<presence/>and included an 'id' attribute, then it is REQUIRED for the error stanza to also include an 'id' attribute. If the generated stanza was<iq/>, then the error stanza MUST include an 'id' attribute. In all cases, the value of the 'id' attribute MUST match that of the generated stanza (or be empty if the generated stanza did not include an 'id' attribute) 🆔 - An error stanza MUST contain an
<error/>child element 🎀 - The entity that returns an error stanza MAY pass along its JID to the sender of the generated stanza (for example, for diagnostic or tracking purposes) by adding a 'by' attribute to the
<error/>child element 🕵️♀️ - The entity that returns an error stanza MAY include the original XML so the sender can inspect and, if necessary, correct the XML before trying to resend 🧐 Just know this is a courtesy only, and the originating entity MUST NOT depend on receiving the original payload. Naturally, the entity MUST NOT include the original data if it is not well-formed XML, violates the XML restrictions of XMPP (see under Section 11.1), or is otherwise harmful (e.g., exceeds a size limit) 🚫
- An
<error/>child MUST NOT be included if the 'type' attribute has a value other than "error" (or if there is no 'type' attribute) 🙅♀️ - An entity that receives an error stanza MUST NOT respond to that stanza with a further error stanza. This helps prevent looping 🔁💀
8.3.2. Syntax 🧩
The syntax for stanza-related errors is below. XML data inside the square brackets '[' and ']' is OPTIONAL. 'intended-recipient' is the JID of the entity the original stanza was addressed to, 'sender' is the JID of the originating entity, and 'error-generator' is the entity that detects that an error has occurred and therefore returns an error stanza.
<stanza-kind from='intended-recipient' to='sender' type='error'>
[OPTIONAL to include sender XML here]
<error [by='error-generator']
type='error-type'>
<defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
[<text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
xml:lang='langcode'>
OPTIONAL descriptive text
</text>]
[OPTIONAL application-specific condition element]
</error>
</stanza-kind>The "stanza-kind" MUST be one of message, presence, or iq.
The "error-type" MUST be one of the following:
auth-- retry after providing credentials 🔑cancel-- do not retry (the error cannot be remedied) 🛑continue-- proceed (the condition was only a warning) ⚠️modify-- retry after changing the data sent ✏️wait-- retry after waiting (the error is temporary) ⏳
The "defined-condition" MUST correspond to one of the stanza error conditions defined under Section 8.3.3. However, because additional error conditions might be defined in the future, if an entity receives a stanza error condition that it does not understand, then it MUST treat the unknown condition as equivalent to <undefined-condition/> (Section 8.3.3.21). If the designers of an XMPP protocol extension or the developers of an XMPP implementation need to communicate a stanza error condition that is not defined in this specification, they can do so by defining an application-specific error condition element qualified by an application-specific namespace. 🌈
The <error/> element:
- MUST contain a defined condition element.
- 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. - MAY contain a child element for an application-specific error condition. This element MUST be qualified by an application-specific namespace that defines the syntax and semantics of the element.
The <text/> element is OPTIONAL. If included, it is to be used only to provide descriptive or diagnostic information that supplements the meaning of a defined condition or application-specific condition. It MUST NOT be interpreted programmatically by an application. It SHOULD NOT be used as the error message presented to a human user, but it MAY be shown in addition to the error message associated with the defined condition element (and, optionally, the application-specific condition element). 📝
Interoperability Note: The syntax defined in [RFC3920] included a legacy 'code' attribute, whose semantics have been replaced by the defined condition elements. Information about mapping defined condition elements to values of the legacy 'code' attribute can be found in [XEP-0086]. 🕰️
8.3.3. Defined Conditions 🌸
The following conditions are defined for use in stanza errors.
The error-type value that is RECOMMENDED for each defined condition is the usual expected type. However, in some circumstances a different type might be more appropriate. 💭
8.3.3.1. bad-request 🙈
The sender has sent a stanza containing XML that does not conform to the appropriate schema or that cannot be processed. For example, this happens with an IQ stanza that includes an unrecognized value of the 'type' attribute, or an element that is qualified by a recognized namespace but violates the defined syntax for that element. The associated error type SHOULD be "modify".
C: <iq from='juliet@im.example.com/balcony'
id='zj3v142b'
to='im.example.com'
type='subscribe'>
<ping xmlns='urn:xmpp:ping'/>
</iq>
S: <iq from='im.example.com'
id='zj3v142b'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>8.3.3.2. conflict ⚔️
Access cannot be granted because an existing resource exists with the same name or address. The associated error type SHOULD be "cancel".
C: <iq id='wy2xa82b4' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>balcony</resource>
</bind>
</iq>
S: <iq id='wy2xa82b4' type='error'>
<error type='cancel'>
<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>8.3.3.3. feature-not-implemented 🚧
The feature represented in the XML stanza is not implemented by the intended recipient or an intermediate server, so the stanza cannot be processed. For example, the entity understands the namespace but does not recognize the element name. The associated error type SHOULD be "cancel" or "modify".
C: <iq from='juliet@im.example.com/balcony'
id='9u2bax16'
to='pubsub.example.com'
type='get'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<subscriptions/>
</pubsub>
</iq>
E: <iq from='pubsub.example.com'
id='9u2bax16'
to='juliet@im.example.com/balcony'
type='error'>
<error type='cancel'>
<feature-not-implemented
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<unsupported
xmlns='http://jabber.org/protocol/pubsub#errors'
feature='retrieve-subscriptions'/>
</error>
</iq>8.3.3.4. forbidden 🚪
The requesting entity does not have the necessary permissions to perform an action that only certain authorized roles or individuals are allowed to complete. This typically relates to authorization rather than authentication. The associated error type SHOULD be "auth".
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
<error type='auth'>
<forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>8.3.3.5. gone 👻
The recipient or server can no longer be contacted at this address, typically on a permanent basis. (This is different from the <redirect/> error condition, which is used for temporary addressing failures.) The associated error type SHOULD be "cancel", and the error stanza SHOULD include a new address (if available) as the XML character data of the <gone/> element. That address MUST be a Uniform Resource Identifier [URI] or Internationalized Resource Identifier [IRI] at which the entity can be contacted, typically an XMPP IRI as specified in [XMPP-URI]. 🗺️
C: <message
from='juliet@im.example.com/churchyard'
id='sj2b371v'
to='romeo@example.net'
type='chat'>
<body>Thy lips are warm.</body>
</message>
S: <message
from='romeo@example.net'
id='sj2b371v'
to='juliet@im.example.com/churchyard'
type='error'>
<error by='example.net'
type='cancel'>
<gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
xmpp:romeo@afterlife.example.net
</gone>
</error>
</message>8.3.3.6. internal-server-error 🔧
The server has hit a misconfiguration or other internal error that prevents it from processing the stanza. The associated error type SHOULD be "cancel".
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
<error type='cancel'>
<internal-server-error
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>8.3.3.7. item-not-found ✨
okay so this one is for when the JID or item you asked for just... isn't there 😭🔍 the error type SHOULD be "cancel", bc you didn't do anything wrong, it's just gone missing 🫶
C: <presence from='userfoo@example.com/bar'
id='pwb2n78i'
to='nosuchroom@conference.example.org/foo'/>
S: <presence from='nosuchroom@conference.example.org/foo'
id='pwb2n78i'
to='userfoo@example.com/bar'
type='error'>
<error type='cancel'>
<item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>Security Warning: the app MUST NOT send this error if it would reveal whether the recipient is online to someone who isn't allowed to know that 🙈 (for the full tea on presence permissions, check the presence subscription bit in [XMPP-IM]). instead it MUST send a <service-unavailable/> stanza error (Section 8.3.3.19) 💜
8.3.3.8. jid-malformed
this is the "that address is busted" error 💅 it shows up when the sender gives an XMPP address (like during resource binding, or in a stanza's to address) that breaks the rules in [XMPP-ADDR]. the error type SHOULD be "modify", so it's like "fix it and try again, bestie" 🍓
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='ch@r@cters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='ch@r@cters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
<error by='muc.example.com'
type='modify'>
<jid-malformed
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>Implementation Note: okay the localpart rules are mostly the job of the service where the account lives 🏠 (so example.com handles<jid-malformed/>errors for everything shaped likelocalpart@example.com). the domainpart rules are mostly the job of whoever is trying to route the stanza to that service 📬 (so example.org is on the hook for<jid-malformed/>errors when its users try to send tolocalpart@example.com). but honestly? any entity that spots a malformed JID MAY return this error 🌸
8.3.3.9. not-acceptable
this one means "I see what you're asking for, but you didn't include the stuff I need to actually do it" 🧚 like asking to subscribe to something without the config details the recipient or server needs. the error type SHOULD be "modify" ✨
C: <message to='juliet@im.example.com' id='yt2vs71m'>
<body>[ ... the-emacs-manual ... ]</body>
</message>
S: <message from='juliet@im.example.com' id='yt2vs71m'>
<error type='modify'>
<not-acceptable
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>8.3.3.10. not-allowed
this is the "nobody gets to do that, period" error 🚫💅 like sending to a domain that's been blacklisted. the error type SHOULD be "cancel" 🥀
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>8.3.3.11. not-authorized
okay this name is literally so misleading 😭 it's borrowed from the "401 Unauthorized" error in [HTTP], so you'd think it's about authorization, but nope. it's about authentication, aka "show me your ID first" 🪪 you get this when you need to provide credentials before you can do the thing, or when the credentials you gave weren't it. the error type SHOULD be "auth" 💖
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'>
<error type='auth'>
<not-authorized xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>8.3.3.12. policy-violation
this one is for when you break some local service rule 🙊 like the server bans certain words and you typed them anyway. the server MAY put the details in the <text/> element or in an application-specific condition element, and the error type SHOULD be "modify" or "wait" depending on which policy got broken 🔥
(in the example below, the client sends an XMPP message with words the server's local policy doesn't allow 🤭)
C: <message from='romeo@example.net/foo'
to='bill@im.example.com'
id='vq71f4nb'>
<body>%#&@^!!!</body>
</message>
S: <message from='bill@im.example.com'
id='vq71f4nb'
to='romeo@example.net/foo'>
<error by='example.net' type='modify'>
<policy-violation
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>8.3.3.13. recipient-unavailable
the recipient is just temporarily out of it 💤 maybe they're in maintenance or whatever. the error type SHOULD be "wait", so basically "come back later, babe" 🌙
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'>
<error type='wait'>
<recipient-unavailable
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>Security Warning: the app MUST NOT send this error if it would reveal whether the recipient is online to someone who isn't allowed to know that 🙈 (again, the presence subscription stuff in [XMPP-IM] is the tea). instead it MUST send a <service-unavailable/> stanza error (Section 8.3.3.19) 💜
8.3.3.14. redirect
this is the "they moved, here's the new address" error 📦 it's temporary, unlike <gone/> which is for permanent stuff. the error type SHOULD be "modify", and the error stanza SHOULD carry the alternate address in the XML character data of the <redirect/> element. that address MUST be a URI or IRI the sender can actually talk to, usually an XMPP IRI like the ones in [XMPP-URI] 🔗
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<redirect xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
xmpp:characters@conference.example.org
</redirect>
</error>
</presence>Security Warning: an app that gets a stanza-level redirect SHOULD warn a real human and ask for approval before it talks to the address in the <redirect/> element 🚨 because that entity might be a totally different identity or have different security rules. end-to-end authentication or signing of XMPP stanzas could help here, since the sender could check whether the redirected entity is the same one it originally tried to reach 🔐. an app MAY have a policy of only following redirects to entities it has authenticated. also, an app SHOULD give up after a certain number of back-to-back redirects (e.g., at least 2 but no more than 5) so it doesn't spiral 🌀
8.3.3.15. registration-required
you have to sign up first, bestie 📝 examples are members-only rooms in XMPP multi-user chat [XEP-0045] and gateways to non-XMPP instant messaging services, which usually make you register before you can use them [XEP-0100]. the error type SHOULD be "auth" 🎀
C: <presence
from='juliet@im.example.com/balcony'
id='y2bs71v4'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
id='y2bs71v4'
to='juliet@im.example.com/balcony'>
<error type='auth'>
<registration-required
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>8.3.3.16. remote-server-not-found
this is for when the server part of the recipient's JID is ghosting you 👻 the remote server or service doesn't exist or can't be resolved. like there's no _xmpp-server._tcp DNS SRV record, the A or AAAA fallback lookup fails, or the A/AAAA lookups work but nothing answers on the IANA-registered port 5269. the error type SHOULD be "cancel" 🌈
C: <message
from='romeo@example.net/home'
id='ud7n1f4h'
to='bar@example.org'
type='chat'>
<body>yt?</body>
</message>
E: <message
from='bar@example.org'
id='ud7n1f4h'
to='romeo@example.net/home'
type='error'>
<error type='cancel'>
<remote-server-not-found
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>8.3.3.17. remote-server-timeout 🌸
okay so this one is for when a remote server or service is part of the recipient's JID (or is needed to finish a request) and it resolves 🗺️ but nobody can actually connect. ngl that's so frustrating, like you found the door but it won't open 😭. for example, an XML stream can't be set up at the resolved IP address and port, or the stream starts but negotiation fails because of TLS, SASL, Server Dialback, etc. the error type SHOULD be "wait", unless it's more permanent, like the remote server is found but can't be authenticated or it breaks security policies. then it's giving "cancel" energy 💅
C: <message from='romeo@example.net/home' id='ud7n1f4h' to='bar@example.org' type='chat'> <body>yt?</body> </message>
E: <message from='bar@example.org' id='ud7n1f4h' to='romeo@example.net/home' type='error'> <error type='wait'> <remote-server-timeout xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </message>
8.3.3.18. resource-constraint 🎀
this is the "I'm slammed rn" error 🥲. the server or recipient is busy or doesn't have the system resources it needs to handle the request. the error type SHOULD be "wait" 🫶
C: <iq from='romeo@example.net/foo' id='kj4vz31m' to='pubsub.example.com' type='get'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <items node='my_musings'/> </pubsub> </iq>
E: <iq from='pubsub.example.com' id='kj4vz31m' to='romeo@example.net/foo' type='error'> <error type='wait'> <resource-constraint xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </iq>
8.3.3.19. service-unavailable 🦋
the server or recipient just doesn't offer the service you asked for right now 🙈. the error type SHOULD be "cancel" 💀
C: <message from='romeo@example.net/foo' to='juliet@im.example.com'> <body>Hello?</body> </message>
S: <message from='juliet@im.example.com/foo' to='romeo@example.net'> <error type='cancel'> <service-unavailable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </message>
Security Warning: okay this one is serious, bestie 🚨 An application MUST return a<service-unavailable/>stanza error (Section 8.3.3.19) instead of<item-not-found/>(Section 8.3.3.7) or<recipient-unavailable/>(Section 8.3.3.13) if sending one of the latter errors would reveal information about the intended recipient's network availability to an entity that isn't authorized to know it. (for a deeper dive into presence authorization, check [XMPP-IM] 🔐)
8.3.3.20. subscription-required 🔑
the requesting entity isn't allowed to use the service because it needs a prior subscription first 🫠. examples are authorization to receive presence information as defined in [XMPP-IM], and opt-in data feeds for XMPP publish-subscribe as defined in [XEP-0060]. the error type SHOULD be "auth" 🧚
C: <message from='romeo@example.net/orchard' id='pa73b4n7' to='playwright@shakespeare.example.com' type='chat'> <subject>ACT II, SCENE II</subject> <body>help, I forgot my lines!</body> </message>
E: <message from='playwright@shakespeare.example.com' id='pa73b4n7' to='romeo@example.net/orchard' type='error'> <error type='auth'> <subscription-required xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </message>
8.3.3.21. undefined-condition 👻
this is the catch-all for when the error doesn't match any of the other conditions on this list 🤷♀️. any error type can go with it, but it SHOULD NOT be used unless it's paired with an application-specific condition. no lone wolves here 🐺
C: <message from='northumberland@shakespeare.example' id='richard2-4.1.247' to='kingrichard@royalty.england.example'> <body>My lord, dispatch; read o'er these articles.</body> <amp xmlns='http://jabber.org/protocol/amp'> <rule action='notify' condition='deliver' value='stored'/> </amp> </message>
S: <message from='example.org' id='amp1' to='northumberland@example.net/field' type='error'> <amp xmlns='http://jabber.org/protocol/amp' from='kingrichard@example.org' status='error' to='northumberland@example.net/field'> <rule action='error' condition='deliver' value='stored'/> </amp> <error type='modify'> <undefined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> <failed-rules xmlns='http://jabber.org/protocol/amp#errors'> <rule action='error' condition='deliver' value='stored'/> </failed-rules> </error> </message>
8.3.3.22. unexpected-request 🕰️
the recipient or server got the request and understood it, but wasn't expecting it at this moment. like, the steps happened out of order 🙃. the error type SHOULD be "wait" or "modify" 💅
C: <iq from='romeo@example.net/foo' id='o6hsv25z' to='pubsub.example.com' type='set'> <pubsub xmlns='http://jabber.org/protocol/pubsub'> <unsubscribe node='my_musings' jid='romeo@example.net'/> </pubsub> </iq>
E: <iq from='pubsub.example.com' id='o6hsv25z' to='romeo@example.net/foo' type='error'> <error type='modify'> <unexpected-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> <not-subscribed xmlns='http://jabber.org/protocol/pubsub#errors'/> </error> </iq>
8.3.4. Application-Specific Conditions ✨
as we said, an application MAY add its own stanza error info by putting a properly namespaced child inside the error element 🧁. usually that extra element supplements or further qualifies one of the defined elements, so the <error/> element ends up with two or three child elements. it's like adding a sparkly accessory to a look 💖
<iq id='ixc3v1b9' type='error'> <error type='modify'> <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> <too-many-parameters xmlns='http://example.org/ns'/> </error> </iq> <message type='error' id='7h3baci9'> <error type='modify'> <undefined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> <text xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'> [ ... application-specific information ... ] </text> <too-many-parameters xmlns='http://example.org/ns'/> </error> </message>
okay and the rule for anyone reading these: an entity that gets an application-specific error condition it doesn't understand MUST ignore that condition but still process the rest of the error stanza properly 🫶🌈
8.4. Extended Content ✨
okay so, the message, presence, and IQ stanzas already give us the basics for messaging, availability, and request-response stuff 💅. but XMPP wants more, so it uses XML namespaces (see [XML-NAMES]) to extend the basic stanza syntax and add extra functionality 🎀.
A message or presence stanza MAY contain one or more optional child elements that add meaning to the message, like an XHTML-formatted version of the message body described in [XEP-0071] 🌸. An IQ stanza of type "get" or "set" MUST contain one of these child elements 🫶. The child element MAY have any name, and it MUST have a namespace declaration (other than "jabber:client", "jabber:server", or "http://etherx.jabber.org/streams") that defines the data inside it ✨. That child is called an "extension element" 🦋. An extension element can sit right at the direct child level of the stanza, or it can show up in any mix of levels, bestie 💖.
Same energy for "extension attributes" 💜. A stanza itself (an <iq/>, <message/>, or <presence/> element qualified by "jabber:client" or "jabber:server") or any child element of that stanza (an extension element, or a child qualified by the content namespace) MAY also have one or more attributes qualified by XML namespaces other than the content namespace or the reserved "http://www.w3.org/XML/1998/namespace" namespace 🧚. That includes the so-called "empty namespace" if the attribute isn't prefixed, as described under [XML-NAMES] 🙈.
Interoperability Note: For backward compatibility and maximum interop 🫶, an entity that generates a stanza SHOULD NOT include these attributes in the stanza itself, or in child elements of the stanza that are qualified by the content namespaces "jabber:client" or "jabber:server" (like the<body/>child of the<message/>stanza) 🍓.
An extension element or extension attribute is called "extended content", and the namespace that qualifies it is called an "extended namespace" 🌈.
Informational Note: Extended namespaces for XMPP are commonly defined by the XMPP Standards Foundation (XSF) and by the IETF, but no spec or IETF standards action is needed to define one. Any individual or organization is free to define XMPP extensions 👑.
okay, now for the examples, because they make everything click ✨.
The following stanza has one direct child element whose extended namespace is 'jabber:iq:roster':
<iq from='juliet@capulet.com/balcony'
id='h83vxa4c'
type='get'>
<query xmlns='jabber:iq:roster'/>
</iq>The following stanza has two direct child elements, each with a different extended namespace 💅.
<presence from='juliet@capulet.com/balcony'>
<c xmlns='http://jabber.org/protocol/caps'
hash='sha-1'
node='http://code.google.com/p/exodus'
ver='QgayPKawpkPSDYmwT/WM94uAlu0='/>
<x xmlns='vcard-temp:x:update'>
<photo>sha1-hash-of-image</photo>
</x>
</presence>The following stanza has two child elements. One is qualified by the "jabber:client" or "jabber:server" content namespace, and the other is qualified by an extended namespace 🎀. That extension element then contains a child element that is qualified by a different extended namespace 😭.
<message to='juliet@capulet.com'>
<body>Hello?</body>
<html xmlns='http://jabber.org/protocol/xhtml-im'>
<body xmlns='http://www.w3.org/1999/xhtml'>
<p style='font-weight:bold'>Hello?</p>
</body>
</html>
</message>it's conventional in the XMPP community for implementations to NOT generate namespace prefixes for elements qualified by extended namespaces (in the XML world this is sometimes called "prefix-free canonicalization") 🦋. BUT if an implementation DOES generate those prefixes, then it MUST include the namespace declaration in the stanza itself or in a child element of the stanza, NOT in the stream header (see Section 4.8.4) 💀.
Routing entities (usually servers) SHOULD try to keep prefixes when serializing XML stanzas for processing 🌸. Receiving entities MUST NOT depend on prefix strings having any particular value. The allowance for the 'stream' prefix (described under Section 4.8.5) is the exception to this, though that one is about streams, not stanzas 🙈.
Support for any given extended namespace is OPTIONAL for any implementation ✨. If an entity doesn't understand a namespace, what it should do depends on whether it is (1) the recipient or (2) a server routing or delivering the stanza to the recipient 🫶.
If a recipient gets a stanza with an element or attribute it doesn't understand, it MUST NOT try to process that XML data. Instead, it MUST do the following 💅:
- If an intended recipient gets a message stanza whose only child element is qualified by a namespace it doesn't understand, then depending on the XMPP application it MUST either ignore the entire stanza or return a stanza error, which SHOULD be
<service-unavailable/>(Section 8.3.3.19) 🔥. - If an intended recipient gets a presence stanza whose only child element is qualified by a namespace it doesn't understand, then it MUST ignore that child element by treating the presence stanza as if it had no child element 🧚.
- If an intended recipient gets a message or presence stanza that contains XML data qualified by a namespace it doesn't understand, then it MUST ignore the portion of the stanza qualified by that unknown namespace 💜.
- If an intended recipient gets an IQ stanza of type "get" or "set" containing a child element qualified by a namespace it doesn't understand, then the entity MUST return an IQ stanza of type "error" with an error condition of
<service-unavailable/>😭.
If a server handles a stanza that's meant for delivery to another entity and that stanza contains a child element the server doesn't understand, it MUST route the stanza unmodified to a remote server, or deliver the stanza unmodified to a connected client associated with a local account 🌈.
9. Detailed Examples 🎀
The detailed examples in this section show the protocols from this spec in action, so you can see exactly how they play out 🧚.
9.1. Client-to-Server Examples
The following examples show the XMPP data flow for a client negotiating an XML stream with a server, exchanging XML stanzas, and closing the negotiated stream 💖. The server is "im.example.com". The server requires TLS, and the client authenticates with the SASL SCRAM-SHA-1 mechanism as <juliet@im.example.com> with a password of "r0m30myr0m30" 🔥. The client also binds a client-submitted resource to the stream 👑. It's assumed that before sending the initial stream header, the client has already resolved an SRV record of _xmpp-client._tcp.im.example.com and opened a TCP connection to the advertised port at the resolved IP address 🦋.
9.1.1. TLS
Step 1: Client initiates stream to server 🌸:
C: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>Step 2: Server responds by sending a response stream header to the client ✨:
S: <stream:stream
from='im.example.com'
id='t7AMCin9zjMNwQKDnplntZPIDEI='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>Step 3: Server sends stream features to the client. At this point it's only the STARTTLS extension, and it's mandatory-to-negotiate 🔒:
S: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>Step 4: Client sends the STARTTLS command to the server 🫶:
C: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>Step 5: Server tells the client it's allowed to proceed 🎀:
S: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>Step 5 (alt): Server tells the client that STARTTLS negotiation has failed, closes the XML stream, and terminates the TCP connection 💀. So the stream negotiation ends unsuccessfully, and the parties don't move on to the next step:
S: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
</stream:stream>Step 6: Client and server try to finish TLS negotiation over the existing TCP connection (see [TLS] for details) 🌈.
Step 7: If TLS negotiation succeeds, the client starts a new stream to the server over the TLS-protected TCP connection 🦋:
C: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>Step 7 (alt): If TLS negotiation fails, the server closes the TCP connection 😭. So the stream negotiation ends unsuccessfully, and the parties don't move on to the next step:
9.1.2. SASL 🎀
Step 8: okay so the server answers with its own stream header, plus whatever stream features it's got available. ✨
S: <stream:stream
from='im.example.com'
id='vgKi/bkYME8OAj4rlXMkpucAqe4='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>SCRAM-SHA-1-PLUS</mechanism>
<mechanism>SCRAM-SHA-1</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>Step 9: the client picks an authentication mechanism (here it's SCRAM-SHA-1) and sends along its initial response data. 💅
C: <auth xmlns="urn:ietf:params:xml:ns:xmpp-sasl"
mechanism="SCRAM-SHA-1">
biwsbj1qdWxpZXQscj1vTXNUQUF3QUFBQU1BQUFBTlAwVEFBQUFBQUJQVTBBQQ==
</auth>The decoded base64 data is n,,n=juliet,r=oMsTAAwAAAAMAAAANP0TAAAAAABPU0AA. 🦋
Step 10: the server sends a challenge. 🌸
S: <challenge xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
cj1vTXNUQUF3QUFBQU1BQUFBTlAwVEFBQUFBQUJQVTBBQWUxMjQ2OTViLTY5Y
TktNGRlNi05YzMwLWI1MWIzODA4YzU5ZSxzPU5qaGtZVE0wTURndE5HWTBaaT
AwTmpkbUxUa3hNbVV0TkRsbU5UTm1ORE5rTURNeixpPTQwOTY=
</challenge>The decoded base64 data is r=oMsTAAwAAAAMAAAANP0TAAAAAABPU0AAe124695b-69a9-4de6-9c30-b51b3808c59e,s=NjhkYTM0MDgtNGY0Zi00NjdmLTkxMmUtNDlmNTNmNDNkMDMz,i=4096 (the line breaks aren't part of the actual data). 🤓
Step 11: the client sends its response. 💖
C: <response xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
Yz1iaXdzLHI9b01zVEFBd0FBQUFNQUFBQU5QMFRBQUFBQUFCUFUwQUFlMTI0N
jk1Yi02OWE5LTRkZTYtOWMzMC1iNTFiMzgwOGM1OWUscD1VQTU3dE0vU3ZwQV
RCa0gyRlhzMFdEWHZKWXc9
</response>The decoded base64 data is c=biws,r=oMsTAAwAAAAMAAAANP0TAAAAAABPU0AAe124695b-69a9-4de6-9c30-b51b3808c59e,p=UA57tM/SvpATBkH2FXs0WDXvJYw= (again, no line breaks in the real data). 🫶
Step 12: the server says "you're in" 🎉 and includes some extra data with the success message. ✨
S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dj1wTk5ERlZFUXh1WHhDb1NFaVc4R0VaKzFSU289
</success>The decoded base64 data is v=pNNDFVEQxuXxCoSEiW8GEZ+1RSo=. 👑
Step 12 (alt): if it goes wrong, the server sends a SASL error instead. Then the stream negotiation ends unsuccessfully and nobody moves on to the next step. 💀
S: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>
</stream>Step 13: the client starts a brand new stream to the server. Fresh start, new era. 🧚
C: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>9.1.3. Resource Binding 🌈
Step 14: the server replies with a stream header and its supported features. In this case the only feature is resource binding. 🦋
S: <stream:stream
from='im.example.com'
id='gPybzaOzBmaADgxKXu9UClbprp0='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>Once the client hears that resource binding is mandatory-to-negotiate, it needs to bind a resource to the stream. Here we're pretending the client sends a human-readable text string. 🎀
Step 15: the client binds a resource. 💅
C: <iq id='yhc13a95' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>balcony</resource>
</bind>
</iq>Step 16: the server accepts the resourcepart and tells the client the binding worked. Bestie got her balcony. 🌸
S: <iq id='yhc13a95' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/balcony
</jid>
</bind>
</iq>Step 16 (alt): if the server says no, it sends an error. The stream negotiation ends unsuccessfully and the parties don't move on. Rough. 😭
S: <iq id='yhc13a95' type='error'>
<error type='cancel'>
<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>9.1.4. Stanza Exchange 💌
Now the client is finally allowed to send XML stanzas over the negotiated stream. The main character moment is happening. 🔥
C: <message from='juliet@im.example.com/balcony'
id='ju2ba41c'
to='romeo@example.net'
type='chat'
xml:lang='en'>
<body>Art thou not Romeo, and a Montague?</body>
</message>If it's needed, the sender's server negotiates XML streams with the intended recipient's server (see Section 9.2). 🧚
The intended recipient replies, and the message gets delivered to the client. 💜
E: <message from='romeo@example.net/orchard'
id='ju2ba41c'
to='juliet@im.example.com/balcony'
type='chat'
xml:lang='en'>
<body>Neither, fair saint, if either thee dislike.</body>
</message>After that, the client can send and receive an unbounded number of XML stanzas over the stream. Literally no limit, bestie. ✨
9.1.5. Close 🦋
Done texting for now? The client closes its stream to the server, but it keeps listening for incoming data from the server. 🫶
C: </stream:stream>Consistent with Section 4.4, the server might send some more data to the client, and then it closes its stream to the client. 🌙
S: </stream:stream>Last step: the client sends a TLS close_notify alert, gets a responding close_notify alert from the server, and then terminates the underlying TCP connection. Bye bye, bestie! 👋💖
9.2. Server-to-Server Examples 🌐
These examples show the data flow for a server negotiating an XML stream with a peer server, exchanging XML stanzas, and closing the negotiated stream. The initiating server ("Server1") is im.example.com. The receiving server ("Server2") is example.net, and it requires use of TLS. im.example.com presents a certificate and authenticates via the SASL EXTERNAL mechanism. We assume that, before sending the initial stream header, Server1 has already resolved an SRV record of _xmpp-server._tcp.example.net and opened a TCP connection to the advertised port at the resolved IP address. Notice how Server1 declares the content namespace jabber:server as the default namespace and uses prefixes for stream-related elements, whereas Server2 uses prefix-free canonicalization. 🤩
9.2.1. TLS 🔐
Step 1: Server1 initiates a 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 with a response stream header. 💅
S2: <stream
from='example.net'
id='hTiXkW+ih9k2SqdGkk/AZi0OJ/Q='
to='im.example.com'
version='1.0'
xmlns='http://etherx.jabber.org/streams'>Step 3: Server2 sends its stream features. At this point it's only the STARTTLS extension, and that one is mandatory-to-negotiate. 🔒
S2: <features xmlns='http://etherx.jabber.org/streams'>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</features>Step 4: Server1 sends the STARTTLS command to Server2. 🌸
S1: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>Step 5: Server2 tells Server1 it's allowed to proceed. Green light! 🚦
S2: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>Step 5 (alt): if STARTTLS negotiation fails, Server2 tells Server1, closes the stream, and terminates the TCP connection. The stream negotiation ends unsuccessfully and nobody moves on. 💀
S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
</stream>Step 6: Server1 and Server2 try to finish the TLS negotiation over TCP (see [TLS] for details). 🧁
Step 7: if TLS negotiation succeeds, Server1 starts a new stream to Server2 over the TLS-protected TCP connection. Protected and pretty. 🛡️
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 fails, Server2 closes the TCP connection. The stream negotiation ends unsuccessfully and the parties don't move on. 😭
9.2.2. SASL 🔑
Step 8: Server2 sends a response stream header to Server1, along with its available stream features. It's showing a preference for the SASL EXTERNAL mechanism. 💖
S2: <stream
from='example.net'
id='RChdjlgj/TIBcbT9Keu31zDihH4='
to='im.example.com'
version='1.0'
xmlns='http://etherx.jabber.org/streams'>
S2: <features xmlns='http://etherx.jabber.org/streams'>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
</mechanisms>
</features>Step 9: Server1 selects the EXTERNAL mechanism and includes an empty response of "=". 🎯
S1: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='EXTERNAL'>=</auth>Step 10: Server2 returns success. ✅
S2: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>Step 10 (alt): if authentication fails, Server2 tells Server1 so. The stream negotiation ends unsuccessfully and nobody moves on. 🙈
S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>
</stream>Step 11: Server1 starts a new stream to Server2. Round two! 🌈
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 replies with a stream header and any additional features. In this case it's just an empty features element, so nothing else to add. 🍓
S2: <stream
from='example.net'
id='MbbV2FeojySpUIP6J91qaa+TWHM='
to='im.example.com'
version='1.0'
xmlns='http://etherx.jabber.org/streams'>
S2: <features xmlns='http://etherx.jabber.org/streams'/>9.2.3. Stanza Exchange 💌
Now Server1 is allowed to send XML stanzas to Server2 over the negotiated stream, going from im.example.com to example.net. We're assuming the stanzas are the same ones we saw earlier for client-to-server communication, just sent over a server-to-server stream qualified by the jabber:server namespace. Iconic reuse. 🔁
Server1 sends an XML stanza to Server2: 🦋
S1: <message from='juliet@im.example.com/balcony'
id='ju2ba41c'
to='romeo@example.net'
type='chat'
xml:lang='en'>
<body>Art thou not Romeo, and a Montague?</body>
</message>9.2.4. Close 🌙
Done sending for now? Server1 closes its stream to Server2 but keeps waiting for incoming data from Server2. (In practice the stream would probably stay open for a while, since Server1 and Server2 don't immediately know whether they'll need it for more communication. Ngl, that's smart.) 🧠
S1: </stream:stream>Consistent with the recommended stream closing handshake, Server2 closes the stream too. 🤝
S2: </stream>Finally, Server1 sends a TLS close_notify alert, gets a responding close_notify alert from Server2, and then terminates the underlying TCP connection. Everybody's out. 👑
10. Server Rules for Processing XML Stanzas ✨
okay bestie, so every server has its own brain for deciding what to do with a stanza it gets 🧠💅 It figures out whether to route it to another domain, deliver it to someone local (usually a connected client on a local account), or handle it right there in the server itself. This section gives the general rules for that. BUT, and this is the tea, particular XMPP applications MAY specify delivery rules that modify or supplement these, like the delivery rules for instant messaging and presence in [XMPP-IM] 🌸
10.1. In-Order Processing
okay so the server MUST keep stanzas and other XML elements in order when they come in over a given input stream, whether that stream is from a connected client or a remote server 🎀 this is non-negotiable, it's giving reliability ✨
In-order processing covers two things: (a) the XML elements used to negotiate and manage XML streams, and (b) every kind of XML stanza, including (but not limited to) these 🦋
- Stanzas a client sends to its own server, or to its own bare JID, so the server processes them directly (for example, a roster get and initial presence, as described in [XMPP-IM]) 💜
- Stanzas a connected client sends to someone else on the same server (like from <juliet@im.example.com> to <nurse@im.example.com>). The server MUST deliver these to the recipient in the order it got them from the sending client's input stream 💖 It also treats the bare JID and the full JID of the recipient as the same thing for delivery 🫶
- Stanzas a connected client sends to someone at a remote domain (like from <juliet@im.example.com> to <romeo@example.net>). The routing server MUST route these in the order it got them from the client's input stream, and it treats the bare JID and full JID as equivalent for routing too 🧚 To help keep the order, the routing server MUST send them all over a single output stream to that remote domain. It is NOT allowed to split them across different server-to-server streams 🙅♀️
- Stanzas that one server routes to another server, for someone at the remote domain (like from <juliet@im.example.com> to <romeo@example.net>, routed by <im.example.com> over a server-to-server stream to <example.net>). The delivering server MUST deliver them in the order it got them from the routing server's input stream, and again the bare JID and full JID count as the same recipient 🌈
- Stanzas one server sends to another server so that the hosting server processes them directly (like from <im.example.com> to <example.net>) ⭐
If the server's processing of one request could change how it handles the data that comes after it on that input stream (like enforcing communication policies), it MUST pause processing the later data until it has finished the request 🛑 no skipping ahead, bestie 💅
In-order processing only applies to a single input stream. So a server is NOT on the hook for keeping data coherent across multiple input streams for the same local account (like stanzas coming in over two different streams from <juliet@im.example.com/balcony> and <juliet@im.example.com/chamber>), or across multiple streams from the same remote domain (like two different streams a remote domain negotiated). Still, a server MAY close the stream with a <conflict/> stream error (Section 4.9.3.3) if a remote server tries to negotiate more than one stream, as described under Section 4.9.3.3 🙈
10.2. General Considerations
okay so at a high level, there are three main things a server juggles when it processes XML stanzas 🎀 they sometimes clash, but they have to be balanced in a consistent way ✨
- It is good to deliver a stanza to the intended recipient when possible 💖
- If a stanza can't be delivered, it's helpful to tell the sender 💌
- It is bad to make it easy for someone to do directory harvesting attacks (Section 13.11) or presence leaks (Section 13.10.2) 😭🚫
With regard to possible delivery-related attacks, keep these in mind 👀
- From an attacker's point of view, there is basically no real difference between (i) the server delivering the stanza or storing it offline for later delivery (see [XMPP-IM]), and (ii) the server silently ignoring it, since no error comes back right away in any of those cases. So when a server delivers a stanza or puts it into offline storage, it needs to silently ignore the stanza if that account doesn't exist 🙈
- How a server handles stanzas sent to the bare JID <localpart@domainpart> has implications for directory harvesting. An attacker could figure out whether an account exists if the server responds differently depending on whether there is an account for that bare JID 🕵️♀️
- How a server handles stanzas sent to a full JID has implications for presence leaks. An attacker could send requests to multiple full JIDs and get different replies depending on whether the user has a device online at that full JID. Randomized resourceparts (whether the client or the server generates them, as described under Section 7) really help mitigate this, so it's a bit less of a worry than directory harvesting 🌸
Naturally, presence doesn't leak if the entity that gets the error from the user's server already knows the user's presence or is authorized to know it (for example, through a presence subscription or directed presence). And a server doesn't enable a directory harvesting attack if it returns an error to an entity that already knows whether the user exists (for example, because the entity is in the user's contact list). [XMPP-IM] talks about these matters more fully 📚✨
10.3. No 'to' Address
okay so if a stanza has no 'to' attribute, the server MUST handle it directly on behalf of the entity that sent it 🧚 what "handle it directly" means depends on whether the stanza is a message, a presence, or an IQ. Stanzas from other servers MUST always have a 'to' attribute, so this rule only applies to stanzas from a local entity (usually a client) connected to the server 💅
10.3.1. Message
if the server gets a message stanza with no 'to' attribute, it MUST treat the message as if the 'to' address were the bare JID <localpart@domainpart> of the sending entity 💌
10.3.2. Presence
if the server gets a presence stanza with no 'to' attribute, it MUST broadcast it to the entities subscribed to the sending entity's presence, if that applies ([XMPP-IM] defines what broadcasting means for presence apps) 🌈
10.3.3. IQ
if the server gets an IQ stanza with no 'to' attribute, it MUST process the stanza on behalf of the account it came from, like this 🎀
- If the IQ stanza is type
"get"or"set"AND the server understands the namespace qualifying the payload, the server MUST handle the stanza on behalf of the sending entity, or return an appropriate error to it. What "handle" means depends on the semantics of the qualifying namespace, but generally the server responds to a"get"or"set"IQ with an appropriate IQ of type"result"or"error", responding as if the server were the bare JID of the sending entity 🦋 For example, if the sending entity sends a"get"IQ whose payload is qualified by the'jabber:iq:roster'namespace (as described in [XMPP-IM]), the server returns the roster for the sending entity's bare JID to the exact resource that asked for it 🌸 - If the IQ stanza is type
"get"or"set"AND the server does NOT understand the namespace qualifying the payload, the server MUST return an error to the sending entity, and that error MUST be<service-unavailable/>🙅♀️ - If the IQ stanza is type
"error"or"result", the server MUST handle the error or result according to the payload of the associated IQ stanza of type"get"or"set". If there is no such associated stanza, the server MUST ignore the error or result stanza 🫶
10.4. Remote Domain
if the domainpart of the JID in the 'to' attribute does NOT match one of the server's configured FQDNs, the server SHOULD try to route the stanza to the remote domain 🌍 BUT this depends on local service provisioning and security policies about inter-domain communication, since that communication is OPTIONAL for any given deployment ✨ As the next sections explain, there are two possible cases 💖
Security Warning: These rules apply only to client-to-server streams. As described under Section 8.1.1.2, a server MUST NOT accept a stanza over a server-to-server stream if the domainpart of the JID in the 'to' attribute doesn't match an FQDN the receiving server serves. This one is SERIOUS, bestie 🚨😭
10.4.1. Existing Stream
if a server-to-server stream already exists between the two domains, the sender's server SHOULD try to route the stanza to the authoritative server for the remote domain over that existing stream 🦋
10.4.2. No Existing Stream
if there is NO server-to-server stream between the two domains yet, the sender's server will go through these steps 🧚
- Resolve the FQDN of the remote domain (as described under Section 13.9.2) 🔍
- Negotiate a server-to-server stream between the two domains (as defined under Section 5 and Section 6) 🤝
- Route the stanza to the authoritative server for the remote domain over the newly established stream 🚀
10.4.3. Error Handling
if routing a stanza to the intended recipient's server doesn't work, the sender's server MUST return an error to the sender 😬 If resolving the remote domain fails, the stanza error MUST be <remote-server-not-found/> (Section 8.3.3.16). If resolution works but the streams can't be negotiated, the stanza error MUST be <remote-server-timeout/> (Section 8.3.3.17) 💀
If stream negotiation with the intended recipient's server works, but the remote server can't deliver the stanza to the recipient, the remote server MUST return an appropriate error to the sender, routed through the sender's server 🫶
10.5. Local Domain 👑
okay so this is where the server becomes the main character 💅. If the domainpart of the JID in the 'to' attribute matches one of the configured FQDNs of the server, the server MUST first check whether that FQDN is served by the server itself or by a specialized local service 🧚. If it's the latter, the server MUST route the stanza to that service. If it's the former, the server MUST proceed as follows. The server MUST NOT route or "forward" the stanza to another domain, because processing every stanza for a configured FQDN is the server's job. It also helps prevent looping 🔁.
10.5.1. domainpart
If the JID in the 'to' attribute is of the form <domainpart>, then the server MUST either (a) handle the stanza as appropriate for the stanza kind or (b) return an error stanza to the sender 🎀.
10.5.2. domainpart/resourcepart
If the JID in the 'to' attribute is of the form <domainpart/resourcepart>, then the server MUST either (a) handle the stanza as appropriate for the stanza kind or (b) return an error stanza to the sender 🌸.
10.5.3. localpart@domainpart
an address like this is usually tied to an account on the server 🧸. The rules below give some general guidelines. More detailed rules for instant messaging and presence apps are in [XMPP-IM].
10.5.3.1. No Such User
if there's no local account for the <localpart@domainpart>, the way the stanza gets handled depends on what kind of stanza it is 🙈.
- For a message stanza, the server MUST either (a) silently ignore the stanza or (b) return a
<service-unavailable/>stanza error (Section 8.3.3.19) to the sender. - For a presence stanza, the server SHOULD ignore the stanza (or behave as described in [XMPP-IM]).
- For an IQ stanza, the server MUST return a
<service-unavailable/>stanza error (Section 8.3.3.19) to the sender.
10.5.3.2. User Exists 🫶
if the JID in the 'to' attribute is of the form <localpart@domainpart>, the stanza handling again depends on the stanza type 💖.
- For a message stanza, if there's at least one connected resource for the account, then the server SHOULD deliver it to at least one of the connected resources. If there's no connected resource, then the server MUST either (a) store the message offline for delivery when the account next has a connected resource or (b) return a
<service-unavailable/>stanza error (Section 8.3.3.19). - For a presence stanza, if there's at least one connected resource that has sent initial presence (i.e., has a "presence session" as defined in [XMPP-IM]), then the server SHOULD deliver it to such resources. If there's no connected resource, then the server SHOULD ignore the stanza (or behave as described in [XMPP-IM]).
- For an IQ stanza, the server MUST handle it directly on behalf of the intended recipient 🧚.
10.5.4. localpart@domainpart/resourcepart
if the JID in the 'to' attribute is of the form <localpart@domainpart/resourcepart> and the user exists but no connected resource exactly matches the full JID, the stanza SHOULD be processed as if the JID were <localpart@domainpart>, as described under Section 10.5.3.2 ✨.
if the JID in the 'to' attribute is of the form <localpart@domainpart/resourcepart>, the user exists, and a connected resource exactly matches the full JID, then the server MUST deliver the stanza to that connected resource. no notes, that's the exact-match moment 🎯.
11. XML Usage ✨
11.1. XML Restrictions
XMPP defines a class of data objects called XML streams, plus how programs are supposed to process them 🧠. XMPP is an application profile, or restricted form, of the Extensible Markup Language [XML]. A complete XML stream (including the start and end stream tags) is a conforming XML document 💜.
BUT (ok lowercase, it's just "but") XMPP doesn't deal with XML documents, it deals with XML streams. Since XMPP doesn't need to parse arbitrary, complete XML documents, it doesn't have to support the full feature set of [XML]. XMPP also uses XML to define protocol data structures and extensions for structured interactions between network entities. That means it follows the recommendations in [XML-GUIDE] about restricting XML in IETF protocols 📜. So the following XML features are prohibited in XMPP:
- comments (as defined in Section 2.5 of [XML])
- processing instructions (Section 2.6 therein)
- internal or external DTD subsets (Section 2.8 therein)
- internal or external entity references (Section 4.2 therein) with the exception of the predefined entities (Section 4.6 therein)
An XMPP implementation MUST behave as follows with regard to these features 🔒:
- An XMPP implementation MUST NOT inject characters matching such features into an XML stream.
- If an XMPP implementation receives characters matching such features over an XML stream, it MUST close the stream with a stream error, which SHOULD be
<restricted-xml/>(Section 4.9.3.18), although some existing implementations send<bad-format/>(Section 4.9.3.1) instead.
11.2. XML Namespace Names and Prefixes
XML namespaces (see [XML-NAMES]) keep data ownership strictly separated 🌈. Their basic job is to keep different vocabularies of XML elements apart when they get mixed together. When XMPP streams are namespace-aware, any allowable XML can be mixed structurally with any data element within XMPP 🦋. The XMPP-specific rules for XML namespace names and prefixes are in Section 4.8 for XML streams and Section 8.4 for XML stanzas.
11.3. Well-Formedness
there are two kinds of well-formedness in XML 🫧:
- "XML-well-formedness" in accordance with the definition of "well-formed" from Section 2.1 of [XML].
- "Namespace-well-formedness" in accordance with the definition of "namespace-well-formed" from Section 7 of [XML-NAMES].
these 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 close the stream over which the data was received with a
<not-well-formed/>stream error (Section 4.9.3.13). - An XMPP entity MUST NOT generate data that is not namespace-well-formed.
- An XMPP entity MUST NOT accept data that is not namespace-well-formed (in particular, an XMPP server MUST NOT route or deliver data that is not namespace-well-formed); instead it MUST return either a
<not-acceptable/>stanza error (Section 8.3.3.9) or close the stream with a<not-well-formed/>stream error (Section 4.9.3.13), where it is preferable to close the stream with a stream error because accepting such data can open an entity to certain denial-of-service attacks 🚨.
Interoperability Note: Because these restrictions were underspecified in [RFC3920], it is possible that implementations based on that specification will send data that does not comply with these restrictions.
11.4. Validation
a server isn't responsible for making sure the XML data it delivers to a connected client or routes to a peer server is valid, as defined in Section 2.8 of [XML] 🤷♀️. An implementation MAY choose to accept or send only data that has been explicitly validated against the schemas provided in this document, but such behavior is OPTIONAL. Clients are advised not to rely on the ability to send data that doesn't conform to the schemas. They SHOULD ignore any non-conformant elements or attributes on the incoming XML stream 🙃.
Informational Note: The terms "valid" and "well-formed" are distinct in XML.
11.5. Inclusion of XML Declaration
Before sending a stream header, an implementation SHOULD send an XML declaration (matching the "XMLDecl" production from [XML]). Applications MUST follow the rules in [XML] about the format of the XML declaration and when it gets included 🎀.
Because external markup declarations are prohibited in XMPP (as described under Section 11.1), the standalone document declaration (matching the "SDDecl" production from [XML]) would have no meaning. So it MUST NOT be included in an XML declaration sent over an XML stream. If an XMPP entity receives an XML declaration with a standalone document declaration set to a value of "no", the entity MUST either ignore the standalone document declaration or close the stream with a stream error, which SHOULD be <restricted-xml/> (Section 4.9.3.18) 🫠.
11.6. Character Encoding
Implementations MUST support the UTF-8 transformation of Universal Character Set [UCS2] characters, as needed for conformance with [CHARSETS] and as defined in [UTF-8]. Implementations MUST NOT attempt to use any other encoding 🔤. If one party to an XML stream detects that the other party tried to send XML data in an encoding other than UTF-8, it MUST close the stream with a stream error, which SHOULD be <unsupported-encoding/> (Section 4.9.3.22), although some existing implementations send <bad-format/> (Section 4.9.3.1) instead.
Because every XMPP implementation has to support all and only the UTF-8 encoding, and because UTF-8 always has the same byte order, an implementation MUST NOT send a byte order mark ("BOM") at the beginning of the data stream 🌊. If an entity receives the [UNICODE] character U+FEFF anywhere in an XML stream (including as the first character of the stream), it MUST interpret that character as a zero width no-break space, not as a byte order mark.
11.7. Whitespace
Except where explicitly disallowed (e.g., during TLS negotiation (Section 5) and SASL negotiation (Section 6)), either entity MAY send whitespace as separators between XML stanzas or between any other first-level elements sent over the stream ⭐. One common use for sending such whitespace is explained under Section 4.4.
11.8. XML Versions
XMPP is an application profile of XML 1.0 🧁. A future version of XMPP might be defined in terms of higher versions of XML, but this specification defines XMPP only in terms of XML 1.0.
12. Internationalization Considerations 🌍
As specified under Section 11.6, XML streams MUST be encoded in UTF-8 💖.
As specified under Section 4.7, an XML stream SHOULD include an 'xml:lang' attribute specifying the default language for any XML character data that is intended to be presented to a human user. As specified under Section 8.1.5, an XML stanza SHOULD include an 'xml:lang' attribute if the stanza contains XML character data intended to be presented to a human user. A server SHOULD apply the default 'xml:lang' attribute to stanzas it routes or delivers on behalf of connected entities, and MUST NOT modify or delete 'xml:lang' attributes on stanzas it receives from other entities 🫶.
Internationalization of XMPP addresses is specified in [XMPP-ADDR] 🦋.
13. Security Considerations 🔐
13.1. Fundamentals ✨
okay so XMPP is usually deployed as a decentralized client-server setup 🌸, which means there are a few different paths two XMPP entities can take when they wanna talk:
- Both entities are servers. In this case, the entities can establish a direct server-to-server stream between themselves.
- One entity is a server and the other entity is a client whose account is hosted on that server. In this case, the entities can establish a direct client-to-server stream between themselves.
- Both entities are clients whose accounts are hosted on the same server. In this case, the entities cannot establish a direct stream between themselves, but there is only one intermediate entity between them, whose policies they might understand and in which they might have some level of trust (e.g., the server might require the use of Transport Layer Security for all client connections).
- Both entities are clients but their accounts are hosted on different servers. In this case, the entities cannot establish a direct stream between themselves and there are two intermediate entities between them; each client might have some trust in the server that hosts its account but might know nothing about the policies of the server to which the other client connects.
this spec only covers the security of a direct XML stream between two servers or between a client and a server (cases #1 and #2) 🎀, where each stream can be treated as a single "hop" along a communication path. the goal of security for a multi-hop path (cases #3 and #4) is very desirable, but it's out of scope for this specification 😭.
In accordance with [SEC-GUIDE], this specification covers communication security (confidentiality, data integrity, and peer entity authentication), non-repudiation, and systems security (unauthorized usage, inappropriate usage, and denial of service). We also discuss common security issues such as information leaks, firewalls, and directory harvesting, as well as best practices related to the reuse of technologies such as base 64, DNS, cryptographic hash functions, SASL, TLS, UTF-8, and XML 💜.
13.2. Threat Model
the threat model for XMPP is basically the standard "Internet Threat Model" described in [SEC-GUIDE] 🦋. attackers are assumed to be interested in and capable of launching the following attacks against unprotected XMPP systems:
- Eavesdropping
- Sniffing passwords
- Breaking passwords through dictionary attacks
- Discovering usernames through directory harvesting attacks
- Replaying, inserting, deleting, or modifying stanzas
- Spoofing users
- Gaining unauthorized entry to a server or account
- Using a server or account inappropriately
- Denying service to other entities
- Subverting communication streams through man-in-the-middle attacks
- Gaining control over on-path servers
where it makes sense, the following sections describe methods for protecting against these threats 🛡️✨.
13.3. Order of Layers
the order of layers in which protocols MUST be stacked is as follows 🍓:
- TCP
- TLS
- SASL
- XMPP
this order has important security implications, as described throughout these security considerations 👑.
within XMPP, XML stanzas are further ordered on top of XML streams, as described under Section 4.
13.4. Confidentiality and Integrity
using Transport Layer Security (TLS) with appropriate ciphersuites gives you a reliable way to keep data exchanged between a client and a server, or between two servers, confidential and intact 🔐💅. so TLS can help protect against eavesdropping, password sniffing, man-in-the-middle attacks, and stanza replays, insertion, deletion, and modification over an XML stream. XMPP clients and servers MUST support TLS as defined under Section 5 🫶.
Informational Note: The confidentiality and integrity of a stream can be protected by methods other than TLS, e.g., by means of a SASL mechanism that involves negotiation of a security layer.
Security Warning: The use of TLS in XMPP applies to a single stream. Because XMPP is typically deployed using a distributed client-server architecture (as explained under Section 2.5), a stanza might traverse multiple streams, and not all of those streams might be TLS-protected. For example, a stanza sent from a client with a session at one server (e.g.,<romeo@example.net/orchard>) and intended for delivery to a client with a session at another server (e.g.,<juliet@example.com/balcony>) will traverse three streams: (1) the stream from the sender's client to its server, (2) the stream from the sender's server to the recipient's server, and (3) the stream from the recipient's server to the recipient's client. Furthermore, the stanza will be processed as cleartext within the sender's server and the recipient's server. Therefore, even if the stream from the sender's client to its server is protected, the confidentiality and integrity of a stanza sent over that protected stream cannot be guaranteed when the stanza is processed by the sender's server, sent from the sender's server to the recipient's server, processed by the recipient's server, or sent from the recipient's server to the recipient's client. Only a robust technology for end-to-end encryption could ensure the confidentiality and integrity of a stanza as it traverses all of the "hops" along a communication path (e.g., a technology that meets the requirements defined in [E2E-REQS]). Unfortunately, the XMPP community has so far failed to produce an end-to-end encryption technology that might be suitable for widespread implementation and deployment, and definition of such a technology is out of scope for this document 😭🙈.
13.5. Peer Entity Authentication
using the Simple Authentication and Security Layer (SASL) for authentication gives you a reliable mechanism for peer entity authentication 🪪. so SASL helps protect against user spoofing, unauthorized usage, and man-in-the-middle attacks. XMPP clients and servers MUST support SASL as defined under Section 6 💖.
13.6. Strong Security
[STRONGSEC] defines "strong security" and explains why it matters for communication over the Internet 🌈. for XMPP communication over client-to-server and server-to-server streams, the term "strong security" refers to security technologies that provide both mutual authentication and integrity checking (e.g., a combination of TLS encryption and SASL authentication using appropriate SASL mechanisms) ✨.
Implementations MUST support strong security. Service provisioning SHOULD use strong security 💅.
An implementation SHOULD make it possible for an end user or service administrator to provision a deployment with specific trust anchors for the certificate presented by a connecting entity (either client or server). When an application is provisioned that way, it MUST NOT use a generic PKI trust store to authenticate the connecting entity. More detailed rules and guidelines regarding certificate validation are provided in the next section 🧚.
The initial stream and the response stream MUST be secured separately, although security in both directions MAY be established via mechanisms that provide mutual authentication 🦋.
13.7. Certificates
channel encryption of an XML stream using Transport Layer Security as described under Section 5, and in some cases also authentication as described under Section 6, is commonly based on a PKIX certificate presented by the receiving entity (or, in the case of mutual certificate authentication, both the receiving entity and the initiating entity) 🎀. this section describes best practices for generating the PKIX certificates that XMPP entities present, and for verifying the PKIX certificates that XMPP entities present 📜.
in general, the following sections rely on and extend the rules and guidelines provided in the [PKIX] profile of [X509], and in [TLS-CERTS]. the reader is referred to those specifications for a detailed understanding of PKIX certificates and their use in TLS 🤓.
13.7.1. Certificate Generation
13.7.1.1. General Considerations
the following rules apply to end entity public key certificates that are issued to XMPP servers or clients 🍓:
- The certificate MUST conform to [PKIX].
- The certificate MUST NOT contain a
basicConstraintsextension with thecAboolean set toTRUE. - The subject field MUST NOT be null.
- The
signatureAlgorithmSHOULD be the PKCS #1 version 1.5 signature algorithm with SHA-256 as defined by [PKIX-ALGO], or a stronger algorithm if available. - The certificate SHOULD include an Authority Information Access (AIA) extension that specifies the address of an Online Certificate Status Protocol [OCSP] responder (if not, a relying party would need to fall back on the use of Certificate Revocation Lists (CRLs) as described in [PKIX]) 🙈.
the following rules apply to certification authority (CA) certificates that are used by issuers of XMPP end entity certificates 👑:
- The certificate MUST conform to [PKIX].
- The certificate MUST contain a
keyUsageextension with thedigitalSignaturebit set. - The subject field MUST NOT be null.
- The
signatureAlgorithmSHOULD be the PKCS #1 version 1.5 signature algorithm with SHA-256 as defined by [PKIX-ALGO], or a stronger algorithm if available. - For issuers of public key certificates, the issuer's certificate MUST contain a
basicConstraintsextension with thecAboolean set toTRUE💜.
13.7.1.2. Server Certificates ✨
13.7.1.2.1. Rules ✨
Okay bestie, so a PKIX certificate that an XMPP server shows off (aka a "server certificate" 🎀) SHOULD include one or more XMPP addresses (a.k.a. domainparts) for the XMPP services hosted at that server. The rules and guidelines in [TLS-CERTS] apply to these certs too, with some XMPP-specific extras 💅:
- Support for the DNS-ID identifier type [PKIX] is REQUIRED in XMPP client and server software implementations. Certification authorities that issue XMPP-specific certificates MUST support the DNS-ID identifier type. XMPP service providers SHOULD include the DNS-ID identifier type in certificate requests. 🌸
- Support for the SRV-ID identifier type [PKIX-SRV] is REQUIRED for XMPP client and server software implementations. For verification purposes, XMPP client implementations only need to support the
"_xmpp-client"service type, whereas XMPP server implementations need to support both the"_xmpp-client"and"_xmpp-server"service types. Certification authorities that issue XMPP-specific certificates SHOULD support the SRV-ID identifier type. XMPP service providers SHOULD include the SRV-ID identifier type in certificate requests. 🦋 - Support for the XmppAddr identifier type (specified under Section 13.7.1.4) is encouraged in XMPP client and server software implementations for the sake of backward-compatibility, but it is no longer encouraged in certificates issued by certification authorities or requested by XMPP service providers. 🙈
- DNS domain names in server certificates MAY contain the wildcard character
'*'as the complete left-most label within the identifier. ✨
13.7.1.2.2. Examples
Example number one, nice and simple 🧚. Consider a company called "Example Products, Inc." It hosts an XMPP service at "im.example.com" (so user addresses at the service look like "user@im.example.com"), and SRV lookups for the xmpp-client and xmpp-server services at "im.example.com" give back one machine, called "x.example.com", like this:
_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.comThe certificate presented by x.example.com contains these representations:
- An otherName type of SRVName (id-on-dnsSRV) containing an IA5String (ASCII) string of
"_xmpp-client.im.example.com" - An otherName type of SRVName (id-on-dnsSRV) containing an IA5String (ASCII) string of
"_xmpp-server.im.example.com" - A dNSName containing an ASCII string of
"im.example.com" - An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8 string of
"im.example.com" - A CN containing an ASCII string of
"Example Products, Inc."
Example number two, a bit more complex 👑. Consider an ISP called "Example Internet Services". It hosts an XMPP service at "example.net" (so user addresses look like "user@example.net"), but SRV lookups for the xmpp-client and xmpp-server services at "example.net" give back two machines ("x1.example.net" and "x2.example.net"), like this:
_xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x1.example.net.
_xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x2.example.net.
_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x1.example.net.
_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x2.example.net.Example Internet Services also hosts chatrooms at chat.example.net, and provides an xmpp-server SRV record for that service as well (which enables entities from remote domains to access that service) 🌈. It also might provide other such services in the future, so it wants to represent a wildcard in its certificate to handle that growth.
The certificate presented by either x1.example.net or x2.example.net contains these representations:
- An otherName type of SRVName (id-on-dnsSRV) containing an IA5String (ASCII) string of
"_xmpp-client.example.net" - An otherName type of SRVName (id-on-dnsSRV) containing an IA5String (ASCII) string of
"_xmpp-server.example.net" - An otherName type of SRVName (id-on-dnsSRV) containing an IA5String (ASCII) string of
"_xmpp-server.chat.example.net" - A dNSName containing an ASCII string of
"example.net" - A dNSName containing an ASCII string of
"*.example.net" - An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8 string of
"example.net" - An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8 string of
"chat.example.net" - A CN containing an ASCII string of
"Example Internet Services"
13.7.1.3. Client Certificates
For a PKIX certificate to be presented by an XMPP client controlled by a human user (a "client certificate" 🍓), it is RECOMMENDED for the certificate to include one or more JIDs associated with an XMPP user. If included, a JID MUST be represented as an XmppAddr, as specified under Section 13.7.1.4. 💜
13.7.1.4. XmppAddr Identifier Type
The XmppAddr identifier type is a UTF8String within an otherName entity inside the subjectAltName, using the [ASN.1] Object Identifier "id-on-xmppAddr" defined below. 🔥
id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
id-on OBJECT IDENTIFIER ::= { id-pkix 8 } -- other name forms
id-on-xmppAddr OBJECT IDENTIFIER ::= { id-on 5 }
XmppAddr ::= UTF8StringAs 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").
So for example, the JID <juliet@im.example.com> included in a certificate could be formatted in any of these 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.comUsing the "id-on-xmppAddr" format is RECOMMENDED when generating certificates, but all three formats MUST be supported for the purpose of certificate validation. ✨
The "id-on-xmppAddr" object identifier MAY be used together with the extended key usage extension specified in Section 4.2.1.12 of [PKIX] to explicitly define and limit the intended use of a certificate to the XMPP network. 🫶
13.7.2. Certificate Validation
When an XMPP entity is presented with a server certificate or client certificate by a peer, for the purpose of encryption or authentication of XML streams as described under Section 5 and Section 6, the entity MUST attempt to validate the certificate to determine whether it will be considered a "trusted certificate". That means a certificate that is acceptable for encryption and/or authentication according to the XMPP entity's local service policies or configured settings. 🔐
For both server certificates and client certificates, the validating entity MUST do the following:
- Attempt to verify the integrity of the certificate.
- Attempt to verify that the certificate has been properly signed by the issuing Certificate Authority.
- Attempt to validate the full certification path.
- Check the rules for end entity public key certificates and certification authority certificates specified under Section 13.7.1.1 for the general case, and under either Section 13.7.1.2 or Section 13.7.1.3 for XMPP server or client certificates, respectively.
- Check certificate revocation messages via Certificate Revocation Lists (CRLs), the Online Certificate Status Protocol [OCSP], or both.
If any of those validation attempts fail, the validating entity MUST unilaterally terminate the session. 🚨
The following sections describe the additional identity verification rules that apply to server-to-server and client-to-server streams. 🌸
Once the identity of the stream peer has been validated, the validating entity SHOULD also correlate the validated identity with the 'from' address (if any) of the stream header it received from the peer. If the two identities do not match, the validating entity SHOULD terminate the connection attempt. However, there might be good reasons why the identities do not match, as described under Section 4.7.1. 🙈
13.7.2.1. Server Certificates
For server certificates, the rules and guidelines defined in [TLS-CERTS] apply, with the proviso that the XmppAddr identifier specified under Section 13.7.1.4 is allowed as a reference identifier. 💖
The identities to be checked are set as follows:
- The initiating entity sets the source domain of its reference identifiers to the 'to' address it communicates in the initial stream header. In other words, this is the identity it expects the receiving entity to provide in a PKIX certificate. 🧚
- The receiving entity sets the source domain of its reference identifiers to the 'from' address communicated by the initiating entity in the initial stream header. In other words, this is the identity that the initiating entity is trying to assert. 👑
In the case of server-to-server communication, the matching procedure described in [TLS-CERTS] can be performed by an application server (receiving entity) when verifying an incoming server-to-server connection from a peer server (initiating entity). In this case, the receiving entity verifies the identity of the initiating entity and uses, as the source domain of its reference identifiers, the DNS domain name asserted by the initiating entity in the 'from' attribute of the initial stream header. However, the matching procedure described in [TLS-CERTS] remains unchanged and is applied in the same way. 🦋
13.7.2.2. Client Certificates
When an XMPP server validates a certificate presented by a client, there are three possible cases, as discussed in the following sections. 🌈
The identities to be checked are set as follows:
- The client sets the source domain of its reference identifiers to the 'to' address it communicates in the initial stream header. In other words, this is the identity it expects the server to provide in a PKIX certificate. 💅
- The server sets the bare JID of its reference identifiers to the 'from' address communicated by the initiating entity in the initial stream header. In other words, this is the identity that the client is trying to assert. 🎀
13.7.2.2.1. Case #1
okay so if the client cert looks like it chains up to a trust anchor (see Section 6.1 of [PKIX]) 💅, the server MUST check the cert for any XmppAddr like Section 13.7.1.4 describes. there are three sub-cases, and they're kinda giving drama 🎀
Sub-Case #1: the server finds ONE XmppAddr whose domainpart matches one of the server's configured FQDNs ✨. the server SHOULD use that represented JID as the validated identity of the client. clean and simple, iconic 👑
Sub-Case #2: the server finds MORE than one XmppAddr whose domainpart matches one of its configured FQDNs 🫶. the server SHOULD pick one of those represented JIDs as the validated identity, and it chooses based on the bare JID in the 'from' address of the initial stream header (if there is one), or on the domainpart in the 'to' address of that header, or on local service policies (like looking someone up in a user database using other info from the client cert). choices, choices 🍓
Sub-Case #3: the server finds NO XmppAddrs, OR it finds at least one but the domainpart doesn't match any configured FQDN 💀. the server MUST NOT use the represented JID (if there is one) as the validated identity, and instead it MUST validate the client some other way, following local service policies (like a user database lookup based on other info in the cert). if it still can't validate the identity, the server MAY abort the validation and end the TLS negotiation 🙈
13.7.2.2.2. Case #2
if the client cert is certified by a Certificate Authority the server doesn't know 🥺, the server MUST do what it does in Case #1, Sub-Case #3. not my favorite, but rules are rules 📜
13.7.2.2.3. Case #3
if the client cert is self-signed 🧚, the server MUST do what it does in Case #1, Sub-Case #3. same energy as Case #2, literally 💜
13.7.2.3. Checking of Certificates in Long-Lived Streams
okay so XMPP uses long-lived XML streams 🌊, which means a cert the other side showed during stream negotiation could expire or get revoked while the stream is still open. this is extra relevant for server-to-server streams, so pay attention 👀 Because of that, each party on a long-lived stream SHOULD do the following:
- Cache the expiration date of the cert the other party presented, plus any certs it depends on (like a root or intermediate cert for a certification authority). if ANY of those expire, close the stream with a stream error of
<reset/>(Section 4.9.3.16). bye bye 👋 - Periodically query the Online Certificate Status Protocol [OCSP] responder listed in the Authority Information Access (AIA) extension of the cert the other party presented, plus any certs it depends on (like a root or intermediate cert for a certification authority). if any of them has been revoked, close the stream with a stream error of
<reset/>(Section 4.9.3.16). It is RECOMMENDED to query the OSCP responder at or near the time given in the nextUpdate field of the OCSP response. if the nextUpdate field isn't set, query every 24 hours instead ⏰
after the stream is closed, the initiating entity has to reconnect 🔁, and the receiving entity has to authenticate it again based on whatever cert it presents during negotiation of the new stream. fresh start, no shortcuts 💖
13.7.2.4. Use of Certificates in XMPP Extensions
certs MAY be used in XMPP extensions for application-layer encryption or authentication above the XML stream level (e.g., for end-to-end encryption) 🔐. those extensions will define their own cert handling rules. at the very least, extensions are encouraged to stay consistent with the rules in this spec, and only add extra rules when they really need to 🎀
13.8. Mandatory-to-Implement TLS and SASL Technologies
okay so these are the TLS ciphersuites and SASL mechanisms that are mandatory-to-implement 🌸 (obviously, implementations MAY support other ciphersuites and mechanisms too). for security stuff about TLS ciphersuites, see Section 13.9.4 and [TLS]. for security stuff about SASL mechanisms, see Section 13.9.4, [SASL], and the specs for particular SASL mechanisms like [SCRAM], [DIGEST-MD5], and [PLAIN] 📚
13.8.1. For Authentication Only
for auth only, servers and clients MUST support the SASL Salted Challenge Response Authentication Mechanism [SCRAM] 🧂, specifically the SCRAM-SHA-1 and SCRAM-SHA-1-PLUS variants ✨
Security Warning: Even though you CAN do auth without confidentiality, it is RECOMMENDED for servers and clients to protect the stream with TLS before attempting SASL auth 🔒. that helps protect the info exchanged during SASL negotiation AND helps prevent certain downgrade attacks (see Section 13.9.4 and Section 13.9.5). even when TLS is used, implementations SHOULD also enforce channel binding as described under Section 13.9.4 💀
Interoperability Note: the SCRAM-SHA-1 or SCRAM-SHA-1-PLUS variants of SCRAM replace SASL DIGEST-MD5 as XMPP's mandatory-to-implement password-based method for auth only 🔄. for backward compatibility with existing deployed infrastructure, implementations are encouraged to keep supporting DIGEST-MD5 as specified in [DIGEST-MD5] 🫠. BUT there are known interoperability issues with DIGEST-MD5 that make it impractical in the long term, so... yeah 😭
13.8.2. For Confidentiality Only
for confidentiality only, servers MUST support TLS with the TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite 🔐
Security Warning: a connection with confidentiality only has weaker security properties than one with both confidentiality and authentication 🙃. so it is RECOMMENDED for servers and clients to prefer connections with both (e.g., by protecting the stream with TLS before attempting SASL auth). in practice, confidentiality only is only really used for server-to-server connections when the peer server doesn't present a trusted cert and the servers use Server Dialback [XEP-0220] for weak identity verification 🧐. TLS with confidentiality only is still desirable, though, to protect the connection against casual eavesdropping 👀
13.8.3. For Confidentiality and Authentication with Passwords
for both confidentiality AND auth with passwords, servers and clients MUST implement TLS with the TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite plus SASL SCRAM, specifically the SCRAM-SHA-1 and SCRAM-SHA-1-PLUS variants (with SCRAM-SHA1-PLUS being preferred, as described under Section 13.9.4) 🥇
as explained in the next Security Warning, a server MAY offer TLS with the TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite plus SASL PLAIN in certain circumstances when it can't offer more secure options 😬. also, clients SHOULD implement PLAIN over TLS to maximize interoperability with servers that can't deploy more secure alternatives 🤝
Security Warning: in practice, lots of servers offer, and lots of clients use, TLS plus SASL PLAIN 😩. SCRAM-SHA-1 and especially SCRAM-SHA-1-PLUS are strongly preferred over PLAIN because they have way superior security properties (including, for SCRAM-SHA-1-PLUS, the ability to enforce channel binding as described under Section 13.9.4) 💪. a client SHOULD treat TLS plus SASL PLAIN as a technology of last resort, only used when talking to a server that doesn't offer SCRAM (or other alternatives more secure than TLS plus SASL PLAIN) 🆘. a client MUST prefer more secure mechanisms (e.g., EXTERNAL, SCRAM-SHA-1-PLUS, SCRAM-SHA-1, or the older DIGEST-MD5 mechanism) over PLAIN, and it MUST NOT use PLAIN if the stream doesn't at minimum have confidentiality and integrity protection via TLS with full certificate validation as described under Section 13.7.2.1 🚫. a server MUST NOT offer SASL PLAIN if the confidentiality and integrity of the stream aren't protected via TLS or an equivalent security layer 🛑. a server SHOULD NOT offer TLS plus SASL PLAIN unless it's unable to offer some variant of SASL SCRAM (or other alternatives more secure than TLS plus SASL PLAIN). for example, that could happen because the XMPP service depends for auth on a database or directory that isn't under the control of the XMPP administrators, like Pluggable Authentication Modules (PAM), a Lightweight Directory Access Protocol (LDAP) directory [LDAP], or an Authentication, Authorization, and Accounting (AAA) key management protocol (for guidance, refer to [AAA]) 🗂️. however, offering TLS plus SASL PLAIN even when the server supports more secure alternatives might be appropriate if the server needs to enable interoperability with an installed base of clients that don't support SCRAM or other more secure alternatives yet 🫶
13.8.4. For Confidentiality and Authentication without Passwords
for both confidentiality AND auth without passwords, servers MUST and clients SHOULD implement TLS with the TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite plus the SASL EXTERNAL mechanism (see Appendix A of [SASL]) with PKIX certificates 🎟️
13.9. Technology Reuse
13.9.1. Use of Base 64 in SASL
both the client and the server MUST verify any base 64 data they receive during SASL negotiation (Section 6) 🔍. an implementation MUST reject (not ignore) any characters that aren't explicitly allowed by the base 64 alphabet 🚫. this helps guard against a covert channel that could be used to "leak" info 🕵️♀️
an implementation MUST NOT break on invalid input, and it MUST reject any sequence of base 64 characters that contains the pad ('=') character if that character is anywhere other than the last character of the data (e.g., "=AAA" or "BBBB=CCC") 🙅♀️. this helps guard against buffer overflow attacks and other attacks on the implementation 🛡️
ngl, base 64 encoding visually hides stuff that's otherwise easy to spot (like passwords) 👻, but it does NOT provide any computational confidentiality 💀
All uses of base 64 encoding MUST follow the definition in Section 4 of [BASE64], and padding bits MUST be set to zero 🌈
13.9.2. Use of DNS ✨
okay so XMPP basically leans on the Domain Name System (specifically [DNS-SRV] records) to turn a fully qualified domain name into an IP address 🌐 before a client connects to a server, or before a peer server connects to another server. bestie, before the initiating entity even starts negotiating an XML stream, it MUST NOT proceed until it has resolved the DNS domain name of the receiving entity, as specified under Section 3 💅. (It doesn't have to re-resolve before every single connection attempt, since DNS results can be cached for a while based on their time-to-live values.) BUT, and this is a big but 😭, if there's no secure DNS option (like the one from [DNSSEC]), a sneaky attacker who can touch the DNS server data, or who can get fake answers cached in a recursive resolver, could potentially trick the initiating entity into connecting to any XMPP server they pick 💀. The good news is that deploying and validating server certificates helps stop that kind of attack 🫶.
13.9.3. Use of Hash Functions 🔐
XMPP itself doesn't directly require any particular cryptographic hash function. However, the technologies XMPP depends on (like TLS and certain SASL mechanisms), plus a bunch of XMPP extensions, might use cryptographic hash functions. So if you're implementing XMPP technologies or developing XMPP extensions, you're advised to keep a close eye on the state of the art when it comes to attacks against cryptographic hash functions in Internet protocols as they relate to XMPP 👀. For helpful guidance, check out [HASHES] 🌸.
13.9.4. Use of SASL 🎀
Okay this one is a whole saga 😭. The initiating entity picks an acceptable SASL mechanism from the list the receiving entity presents, which means the initiating entity is relying on that list for authentication. That creates the possibility of a downgrade attack: if an attacker gains control of the channel, they can present a weak list of mechanisms 💀. To mitigate this, the parties SHOULD protect the channel using TLS before attempting SASL negotiation, and they SHOULD either perform full certificate validation as described under Section 13.7.2.1 or use a SASL mechanism that provides channel bindings, such as SCRAM-SHA-1-PLUS. (Protecting the channel via TLS with full certificate validation can help keep the information exchanged during SASL negotiation confidential and intact 🔒.)
The SASL framework itself doesn't give you a way to bind SASL authentication to a security layer that was negotiated at a lower layer (like TLS). That binding is called a "channel binding" (see [CHANNEL]) 🦋. Some SASL mechanisms do provide channel bindings, which in XMPP would usually mean binding to TLS (see [CHANNEL-TLS]). If a SASL mechanism does provide a channel binding (this is true of [SCRAM], for example), then XMPP entities using that mechanism SHOULD prefer the channel binding variant, like preferring SCRAM-SHA-1-PLUS over SCRAM-SHA-1 👑. If a SASL mechanism doesn't provide a channel binding, then there's no way for it to verify that the source and destination end points the lower layer's security is bound to are the same end points SASL is authenticating. And if those end points aren't identical, the lower layer's security can't be trusted to protect data sent between the SASL-authenticated entities 😭. In that situation, a SASL security layer SHOULD be negotiated that effectively ignores the presence of the lower-layer security 🙈.
Many deployed XMPP services authenticate client connections with passwords. And it's well known that most humans pick pretty weak passwords 💀. Service provisioning is out of scope for this document, but XMPP servers that allow password-based authentication SHOULD enforce minimal criteria for password strength to help prevent dictionary attacks 🔥. Because every password-based mechanism is vulnerable to password guessing, XMPP servers MUST limit the number of retries allowed during SASL authentication, as described under Section 6.4.5 🚫.
Some SASL mechanisms (like [ANONYMOUS]) don't provide strong peer entity authentication of the client to the server. So service admins are advised to turn those on with caution ⚠️. Best practices for using the SASL ANONYMOUS mechanism in XMPP are described in [XEP-0175] 🧚.
13.9.5. Use of TLS 🔒
Implementations of TLS usually support multiple versions of the Transport Layer Security protocol, plus the older Secure Sockets Layer (SSL) protocol 🕰️. Because of known security vulnerabilities, XMPP servers and clients MUST NOT request, offer, or use SSL 2.0 🚫. For more details, see Appendix E.2 of [TLS], along with [TLS-SSL2].
To prevent man-in-the-middle attacks, the TLS client (which might be an XMPP client or an XMPP server) MUST verify the certificate of the TLS server, and it MUST check its understanding of the server FQDN against the server's identity as presented in the TLS Certificate message, as described under Section 13.7.2.1 (for further details, see [TLS-CERTS]) 🕵️♀️.
Support for TLS renegotiation is strictly OPTIONAL 🎀. However, implementations that do support TLS renegotiation MUST implement and use the TLS Renegotiation Extension [TLS-NEG] ✨. Further details are provided under Section 5.3.5 💖.
13.9.6. Use of UTF-8 🌈
Using UTF-8 makes it possible to transport non-ASCII characters, and that enables "spoofing" scenarios, where a displayed value looks like something other than what it actually is 👀. There are also known attack scenarios related to decoding UTF-8 data 💀. On both of these points, check [UTF-8] for more info 🦋.
13.9.7. Use of XML 🧾
Since XMPP is an application profile of the Extensible Markup Language [XML], a lot of the security considerations described in [XML-MEDIA] and [XML-GUIDE] also apply to XMPP 💅. Several parts of XMPP help reduce those risks, like the prohibitions specified under Section 11.1 and the lack of external references to style sheets or transformations. BUT those mitigating factors are by no means comprehensive, so don't get too comfy 😬.
13.10. Information Leaks 🫣
13.10.1. IP Addresses 🌍
A client's IP address and method of access MUST NOT be made public by a server (this is typically what happens in [IRC], so please don't do that) 🙅♀️.
13.10.2. Presence Information 🟢
Presence is one of the core aspects of XMPP. It's information about the network availability of an XMPP entity, aka whether the entity is online or offline 💜. A "presence leak" happens when an entity's network availability gets revealed, accidentally and without consent, to a second entity that isn't authorized to know it 😱.
Presence is discussed more fully in [XMPP-IM], but the big takeaway is this: an XMPP server MUST NOT leak presence 🚨. Specifically, at the core XMPP level, real-time addressing and network availability are tied to a specific connected resource. So disclosing a connected resource's full JID counts as a presence leak 💀. To help prevent that, a server MUST NOT return different stanza errors depending on whether a potential attacker sends XML stanzas to the entity's bare JID (<localpart@domainpart>) or its full JID (<localpart@domainpart/resourcepart>) 🙈.
13.11. Directory Harvesting 🕵️
If a server generates an error stanza in response to a stanza for a user account that doesn't exist, using the <service-unavailable/> stanza error condition (Section 8.3.3.19) can help protect against directory harvesting attacks 🍓. That's because it's the same error condition returned when, say, the namespace of an IQ child element isn't understood, or when "offline message storage" ([XEP-0160]) or message forwarding isn't enabled for a domain 🧚. BUT, subtle differences in the exact XML of error stanzas, plus differences in the timing of when those errors come back, can let an attacker figure out a user's network presence when more advanced blocking technologies aren't in use (see for instance [XEP-0016] and [XEP-0191]) 😭. So a server that wants a higher level of caution might not return any error at all for certain kinds of received stanzas 🫶. That way, a nonexistent user looks like someone who just has no interest in chatting with the sender 🌸.
13.12. Denial of Service 💀
okay so [DOS] defines a denial-of-service attack as one where one or more machines target a victim and try to stop it from doing useful work 🙅♀️. The victim can be a server, a client, or a router, a link, a whole network, one individual internet user, a company doing business online, an ISP, a country, or any mix of those. Literally anything can get hit 😭
Some of the stuff we already talked about actually helps prevent DoS attacks 🛡️. For example, a server MUST NOT process XML stanzas from clients that haven't given it proper authentication credentials, and it MUST NOT process XML stanzas from peer servers whose identity it hasn't either authenticated via SASL or weakly verified via Server Dialback. Main character defense energy 👑
[XEP-0205] goes way deeper into possible DoS attacks against XMPP systems and gives best practices for stopping them 🔍. Here's what it recommends:
- A server implementation SHOULD let a server administrator limit how many TCP connections it accepts from a given IP address at any one time. If an entity tries to connect but the maximum number of TCP connections has already been reached, the receiving server MUST NOT let the new connection go through 🚫.
- A server implementation SHOULD let a server administrator limit how many TCP connection attempts it accepts from a given IP address in a given time period. If an entity tries to connect but the maximum number of connection attempts has already been reached, the receiving server MUST NOT let the new connection go through 🙅♀️.
- A server implementation SHOULD let a server administrator limit how many connected resources an account is allowed to bind at any one time. If a client tries to bind a resource but it has already hit the configured number of allowable resources, the receiving server MUST return a
<resource-constraint/>stanza error (Section 8.3.3.18) 💅. - A server implementation SHOULD let a server administrator limit the size of stanzas it accepts from a connected client or peer server. Here "size" counts all the XML markup as defined in Section 2.4 of [XML], from the opening "<" character of the stanza all the way to the closing ">" character 📏. A deployed server's maximum stanza size MUST NOT be smaller than 10000 bytes. That number is a reasonable compromise between letting originating entities be expressive and not making stanza processing too expensive for servers ⚖️. A server implementation SHOULD NOT blindly set 10000 bytes as the value for every deployment, and instead SHOULD let server administrators set their own limits 🙈. If a connected resource or peer server sends a stanza that breaks the upper limit, the receiving server MUST either return a
<policy-violation/>stanza error (Section 8.3.3.12), which lets the sender recover, or close the stream with a<policy-violation/>stream error (Section 4.9.3.14) 🧚. - A server implementation SHOULD let a server administrator limit how many XML stanzas 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 that time period, the receiving server SHOULD NOT process the stanza and SHOULD instead return a
<policy-violation/>stanza error (Section 8.3.3.12) 🚦. - A server implementation SHOULD let a server administrator limit how much bandwidth it allows a connected client or peer server to use in a given time period 📉.
- A server implementation MAY let a server administrator limit the types of stanzas (based on the extended content "payload") that a connected resource or peer server is allowed to send over an active connection. Those limits and restrictions are a matter of deployment policy 🎀.
- A server implementation MAY refuse to route or deliver any stanza it considers abusive, with or without sending an error back to the sender 🗑️.
For more detailed recommendations on DoS attacks in XMPP systems, check out [XEP-0205] 📚.
13.13. Firewalls 🧱
Okay this one is so practical 🥰. DNS SRV records can tell connecting entities to use TCP ports other than 5222 (client-to-server) and 5269 (server-to-server). But XMPP communication typically happens over those two ports anyway, and they're registered with the IANA (see Section 14) ✨. Because they're well-known, admins can easily turn XMPP activity on or off through the firewalls that basically everyone already runs 🔥.
13.14. Interdomain Federation 🌐
"Federation" is the word people use for communication between two servers 🫶.
Since service provisioning is a matter of policy, it is OPTIONAL for any given server to support federation 🤷♀️. If a server does enable federation, it SHOULD enable strong security as described earlier so it gets both authentication and confidentiality 🔐. Compliant implementations SHOULD support TLS and SASL for this purpose 💜.
Here's the tea 🍵. Before RFC 3920 defined TLS plus SASL EXTERNAL with certificates for encrypting and authenticating server-to-server streams, the only way to weakly verify a peer server's identity was Server Dialback, defined in [XEP-0220] 🕰️. Even with [DNSSEC] in the mix, Server Dialback only gives weak identity verification and provides no confidentiality or integrity 😬. At the time of writing, Server Dialback is still the most widely used technique for any level of assurance over server-to-server streams 📊.
This creates a possible downgrade attack from TLS + SASL EXTERNAL to Server Dialback 👀. If an attacker takes control of the channel, they can convince the initiating server that the receiving server doesn't support TLS or doesn't have an appropriate certificate. To help prevent this, the parties SHOULD protect the channel using TLS before proceeding, even if the presented certificates are self-signed or otherwise untrusted 🛡️. Don't let anyone talk you out of TLS, bestie 🙅♀️.
13.15. Non-Repudiation 🖋️
Systems that provide both peer entity authentication and data integrity can let one entity prove to a third party that another entity meant to send particular data 📜. XMPP systems can provide both peer entity authentication and data integrity, but XMPP was never designed to provide non-repudiation. So no, it's not giving legal receipt 🙈.
14. IANA Considerations 📋
The following subsections update the registrations provided in [RFC3920]. This section is to be interpreted according to [IANA-GUIDE] 📖.
14.1. XML Namespace Name for TLS Data 🔒
A URN sub-namespace for STARTTLS negotiation data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-tls
Specification: RFC 6120
Description: This is the XML namespace name for STARTTLS negotiation
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by RFC 6120.
Registrant Contact: IESG <iesg@ietf.org>14.2. XML Namespace Name for SASL Data 🔑
A URN sub-namespace for SASL negotiation data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-sasl
Specification: RFC 6120
Description: This is the XML namespace name for SASL negotiation
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by RFC 6120.
Registrant Contact: IESG <iesg@ietf.org>14.3. XML Namespace Name for Stream Errors 🚨
A URN sub-namespace for stream error data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-streams
Specification: RFC 6120
Description: This is the XML namespace name for stream error data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by RFC 6120.
Registrant Contact: IESG <iesg@ietf.org>14.4. XML Namespace Name for Resource Binding 🎀
A URN sub-namespace for resource binding in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-bind
Specification: RFC 6120
Description: This is the XML namespace name for resource binding in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by RFC 6120.
Registrant Contact: IESG <iesg@ietf.org>14.5. XML Namespace Name for Stanza Errors 🙅♀️
A URN sub-namespace for stanza error data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-stanzas
Specification: RFC 6120
Description: This is the XML namespace name for stanza error data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by RFC 6120.
Registrant Contact: IESG <iesg@ietf.org>14.6. GSSAPI Service Name 🧩
The IANA has registered "xmpp" as a [GSS-API] service name, as defined under Section 6.6 ✨.
14.7. Port Numbers and Service Names 🔌
The IANA has registered "xmpp-client" and "xmpp-server" as keywords for [TCP] ports 5222 and 5269, respectively. Per [IANA-PORTS], this document updates the existing registration, like so 🎉.
Service Name: xmpp-client
Transport Protocol: TCP
Description: A service offering support for connections by XMPP
client applications
Registrant: IETF XMPP Working Group
Contact: IESG <iesg@ietf.org>
Reference: RFC 6120
Port Number: 5222
Service Name: xmpp-server
Transport Protocol: TCP
Description: A service offering support for connections by XMPP
server applications
Registrant: IETF XMPP Working Group
Contact: IESG <iesg@ietf.org>
Reference: RFC 6120
Port Number: 526915. Conformance Requirements ✅
This section describes a protocol feature set that summarizes the conformance requirements of this specification 📝. The feature set is good for software certification, interoperability testing, and implementation reports 🧪. For each feature, this section gives:
- A human-readable name 🏷️
- An informational description 💬
- A reference to the particular section of this document that normatively defines the feature 🔗
- Whether the feature applies to the Client role, the Server role, or both (where "N/A" means the feature doesn't apply to the specified role) 🎭
- Whether the feature MUST or SHOULD be implemented, where the capitalized terms are to be understood as described in [KEYWORDS] 💖
The feature set here sticks to the ideas and formats Larry Masinter put forward in the IETF's NEWTRK Working Group in 2005, as captured in [INTEROP] 💅✨. It's more detailed than [REPORTS] asks for, but it's a solid base for the implementation reports you'll need to move this spec from Proposed Standard to Draft Standard under [PROCESS] 🎀📋.
bind-gen🌸 Generate a random resource on demand.Section: Section 7.6·Roles: Client N/A, Server MUST
bind-mtn🦋 Consider resource binding as mandatory-to-negotiate.Section: Section 7.3.1·Roles: Client MUST, Server MUST
bind-restart💖 Do not restart the stream after negotiation of resource binding.Section: Section 7.3.2·Roles: Client MUST, Server MUST
bind-support🧚 Support binding of client resources to an authenticated stream.Section: Section 7·Roles: Client MUST, Server MUST
sasl-correlate🔐 When authenticating a stream peer using SASL, correlate the authentication identifier from SASL negotiation with the 'from' address (if any) in the stream header that peer sent you 👀.Section: Section 6.4.6·Roles: Client SHOULD, Server SHOULD
sasl-errors🙈 Support SASL errors during the negotiation process.Section: Section 6.5·Roles: Client MUST, Server MUST
sasl-mtn👑 Consider SASL as mandatory-to-negotiate.Section: Section 6.3.1·Roles: Client MUST, Server MUST
sasl-restart🔄 Initiate or handle a stream restart after SASL negotiation.Section: Section 6.3.2·Roles: Client MUST, Server MUST
sasl-support✨ Support the Simple Authentication and Security Layer for stream authentication.Section: Section 6·Roles: Client MUST, Server MUST
security-mti-auth-scram🛡️ Support the SASL SCRAM mechanism for authentication only. This implies support for both the SCRAM-SHA-1 and SCRAM-SHA-1-PLUS variants 💜.Section: Section 13.8·Roles: Client MUST, Server MUST
security-mti-both-external🌈 Support TLS with SASL EXTERNAL for confidentiality and authentication.Section: Section 13.8·Roles: Client SHOULD, Server MUST
security-mti-both-plain🍓 Support TLS using theTLS_RSA_WITH_AES_128_CBC_SHAciphersuite plus the SASL PLAIN mechanism for confidentiality and authentication.Section: Section 13.8·Roles: Client SHOULD, Server MAY
security-mti-both-scram💅 Support TLS using theTLS_RSA_WITH_AES_128_CBC_SHAciphersuite plus the SCRAM-SHA-1 and SCRAM-SHA-1-PLUS variants of the SASL SCRAM mechanism for confidentiality and authentication.Section: Section 13.8·Roles: Client MUST, Server MUST
security-mti-confidentiality🫶 Support TLS using theTLS_RSA_WITH_AES_128_CBC_SHAciphersuite for confidentiality only.Section: Section 13.8·Roles: Client N/A, Server SHOULD
stanza-attribute-from💌 Support the common 'from' attribute for all stanza kinds.Section: Section 8.1.2·Roles: Client MUST, Server MUST
stanza-attribute-from-stamp🏷️ Stamp or rewrite the 'from' address of all stanzas received from connected clients.Section: Section 8.1.2.1·Roles: Client N/A, Server MUST
stanza-attribute-from-validate🔍 Validate the 'from' address of all stanzas received from peer servers.Section: Section 8.1.2.2·Roles: Client N/A, Server MUST
stanza-attribute-id🆔 Support the common 'id' attribute for all stanza kinds.Section: Section 8.1.3·Roles: Client MUST, Server MUST
stanza-attribute-to📬 Support the common 'to' attribute for all stanza kinds.Section: Section 8.1.1·Roles: Client MUST, Server MUST
stanza-attribute-to-validate📭 Ensure that all stanzas received from peer servers include a 'to' address.Section: Section 8.1.1·Roles: Client N/A, Server MUST
stanza-attribute-type🏷️ Support the common 'type' attribute for all stanza kinds.Section: Section 8.1.4·Roles: Client MUST, Server MUST
stanza-attribute-xmllang🌍 Support the common 'xml:lang' attribute for all stanza kinds.Section: Section 8.1.5·Roles: Client MUST, Server MUST
stanza-error💀 Generate and handle stanzas of type "error" for all stanza kinds.Section: Section 8.3·Roles: Client MUST, Server MUST
stanza-error-child🧾 Ensure that stanzas of type "error" include an<error/>child element.Section: Section 8.3·Roles: Client MUST, Server MUST
stanza-error-id🪪 Ensure that stanzas of type "error" preserve the 'id' provided in the triggering stanza.Section: Section 8.3·Roles: Client MUST, Server MUST
stanza-error-reply🙅 Do not reply to a stanza of type "error" with another stanza of type "error".Section: Section 8.3·Roles: Client MUST, Server MUST
stanza-extension🧩 Correctly process XML data qualified by an unsupported XML namespace. "Correctly process" means ignoring that portion of the stanza in the case of a message or presence stanza, and returning an error in the case of an IQ stanza (for the intended recipient). It also means routing or delivering the stanza (for a routing entity such as a server).Section: Section 8.4·Roles: Client MUST, Server MUST
stanza-iq-child🎯 Include exactly one child element in an<iq/>stanza of type "get" or "set", zero or one child elements in an<iq/>stanza of type "result", and one or two child elements in an<iq/>stanza of type "error".Section: Section 8.2.3·Roles: Client MUST, Server MUST
stanza-iq-id🆔 Ensure that all<iq/>stanzas include an 'id' attribute.Section: Section 8.2.3·Roles: Client MUST, Server MUST
stanza-iq-reply🔁 Reply to an<iq/>stanza of type "get" or "set" with an<iq/>stanza of type "result" or "error".Section: Section 8.2.3·Roles: Client MUST, Server MUST
stanza-iq-type🏷️ Ensure that all<iq/>stanzas include a 'type' attribute whose value is "get", "set", "result", or "error".Section: Section 8.2.3·Roles: Client MUST, Server MUST
stanza-kind-iq🪄 Support the<iq/>stanza.Section: Section 8.2.3·Roles: Client MUST, Server MUST
stanza-kind-message💬 Support the<message/>stanza.Section: Section 8.2.1·Roles: Client MUST, Server MUST
stanza-kind-presence🟢 Support the<presence/>stanza.Section: Section 8.2.2·Roles: Client MUST, Server MUST
stream-attribute-initial-from🚀 Include a 'from' attribute in the initial stream header.Section: Section 4.7.1·Roles: Client SHOULD, Server MUST
stream-attribute-initial-lang🌷 Include an 'xml:lang' attribute in the initial stream header.Section: Section 4.7.4·Roles: Client SHOULD, Server SHOULD
stream-attribute-initial-to🎯 Include a 'to' attribute in the initial stream header.Section: Section 4.7.2·Roles: Client MUST, Server MUST
stream-attribute-response-from🌤️ Include a 'from' attribute in the response stream header.Section: Section 4.7.1·Roles: Client N/A, Server MUST
stream-attribute-response-id🪪 Include an 'id' attribute in the response stream header.Section: Section 4.7.3·Roles: Client N/A, Server MUST
stream-attribute-response-id-unique🦄 Ensure that the 'id' attribute in the response stream header is unique within the context of the receiving entity.Section: Section 4.7.3·Roles: Client N/A, Server MUST
stream-attribute-response-to📨 Include a 'to' attribute in the response stream header.Section: Section 4.7.2·Roles: Client N/A, Server SHOULD
stream-error-generate🚨 Generate a stream error (followed by a closing stream tag and termination of the TCP connection) upon detecting a stream-related error condition.Section: Section 4.9·Roles: Client MUST, Server MUST
stream-fqdn-resolution🗺️ Resolve FQDNs before opening a TCP connection to the receiving entity.Section: Section 3.2·Roles: Client MUST, Server MUST
stream-negotiation-complete🏁 Do not consider the stream negotiation process to be complete until the receiving entity sends a stream features advertisement that is empty or that contains only voluntary-to-negotiate features.Section: Section 4.3.5·Roles: Client MUST, Server MUST
stream-negotiation-features📋 Send stream features after sending a response stream header.Section: Section 4.3.2·Roles: Client N/A, Server MUST
stream-negotiation-restart🔄 Consider the previous stream to be replaced upon negotiation of a stream feature that necessitates a stream restart, and send or receive a new initial stream header after negotiation of such a stream feature.Section: Section 4.3.3·Roles: Client MUST, Server MUST
stream-reconnect⏳ Reconnect with exponential backoff if a TCP connection is terminated unexpectedly.Section: Section 3.3·Roles: Client MUST, Server MUST
stream-tcp-binding🔗 Bind an XML stream to a TCP connection.Section: Section 3·Roles: Client MUST, Server MUST
tls-certs📜 Check the identity specified in a certificate that is presented during TLS negotiation.Section: Section 13.7.2·Roles: Client MUST, Server MUST
tls-mtn🔒 Consider TLS as mandatory-to-negotiate if STARTTLS is the only feature advertised, or if the STARTTLS feature advertisement includes an empty<required/>element.Section: Section 5.3.1·Roles: Client MUST, Server MUST
tls-restart🔄 Initiate or handle a stream restart after TLS negotiation.Section: Section 5.3.2·Roles: Client MUST, Server MUST
tls-support🔐 Support Transport Layer Security for stream encryption.Section: Section 5·Roles: Client MUST, Server MUST
tls-correlate🪞 When validating a certificate presented by a stream peer during TLS negotiation, correlate the validated identity with the 'from' address (if any) in the stream header that peer sent you.Section: Section 13.7.2·Roles: Client SHOULD, Server SHOULD
xml-namespace-content-client🎀 Support 'jabber:client' as a content namespace.Section: Section 4.8.2·Roles: Client MUST, Server MUST
Feature: xml-namespace-content-server
Description: Support 'jabber:server' as a content namespace.
Section: Section 4.8.2
Roles: Client N/A, Server MUST.
Feature: xml-namespace-streams-declaration
Description: Ensure that there is a namespace declaration for the
'http://etherx.jabber.org/streams' namespace.
Section: Section 4.8.1
Roles: Client MUST, Server MUST.
Feature: xml-namespace-streams-prefix
Description: Ensure that all elements qualified by the
'http://etherx.jabber.org/streams' namespace are prefixed by the
prefix (if any) defined in the namespace declaration.
Section: Section 4.8.1
Roles: Client MUST, Server MUST.
Feature: xml-restriction-comment
Description: Do not generate or accept XML comments.
Section: Section 11.1
Roles: Client MUST, Server MUST.
Feature: xml-restriction-dtd
Description: Do not generate or accept internal or external DTD
subsets.
Section: Section 11.1
Roles: Client MUST, Server MUST.
Feature: xml-restriction-pi
Description: Do not generate or accept XML processing instructions.
Section: Section 11.1
Roles: Client MUST, Server MUST.
Feature: xml-restriction-ref
Description: Do not generate or accept internal or external entity
references with the exception of the predefined entities.
Section: Section 11.1
Roles: Client MUST, Server MUST.
Feature: xml-wellformed-xml
Description: Do not generate or accept data that is not XML-well-
formed.
Section: Section 11.3
Roles: Client MUST, Server MUST.
Feature: xml-wellformed-ns
Description: Do not generate or accept data that is not namespace-
well-formed.
Section: Section 11.3
Roles: Client MUST, Server MUST.16. References ✨
Okay, bestie, this is the reading list 📚🎀 Everything the spec leans on is listed right here, with all its details, so nothing gets lost.
16.1. Normative References 💜
These are the ones that are basically load-bearing, so no skipping! 🫶
- [BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006.
- [CHANNEL] Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, November 2007.
- [CHANNEL-TLS] Altman, J., Williams, N., and L. Zhu, "Channel Bindings for TLS", RFC 5929, July 2010.
- [CHARSETS] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998.
- [DNS-CONCEPTS] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.
- [DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.
- [IPv6-ADDR] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6 Address Text Representation", RFC 5952, August 2010.
- [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
- [LANGMATCH] Phillips, A. and M. Davis, "Matching of Language Tags", BCP 47, RFC 4647, September 2006.
- [LANGTAGS] Phillips, A. and M. Davis, "Tags for Identifying Languages", BCP 47, RFC 5646, September 2009.
- [OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 2560, June 1999.
- [PKIX] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008.
- [PKIX-ALGO] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003.
- [PKIX-SRV] Santesson, S., "Internet X.509 Public Key Infrastructure Subject Alternative Name for Expression of Service Name", RFC 4985, August 2007.
- [PLAIN] Zeilenga, K., "The PLAIN Simple Authentication and Security Layer (SASL) Mechanism", RFC 4616, August 2006.
- [RANDOM] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005.
- [SASL] Melnikov, A. and K. Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, June 2006.
- [SCRAM] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, "Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.
- [STRONGSEC] Schiller, J., "Strong Security Requirements for Internet Engineering Task Force Standard Protocols", BCP 61, RFC 3365, August 2002.
- [TCP] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981.
- [TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
- [TLS-CERTS] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, March 2011.
- [TLS-NEG] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, "Transport Layer Security (TLS) Renegotiation Indication Extension", RFC 5746, February 2010.
- [TLS-SSL2] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer (SSL) Version 2.0", RFC 6176, March 2011.
- [UCS2] International Organization for Standardization, "Information Technology - Universal Multiple-octet coded Character Set (UCS) - Amendment 2: UCS Transformation Format 8 (UTF-8)", ISO Standard 10646-1 Addendum 2, October 1996.
- [UNICODE] The Unicode Consortium, "The Unicode Standard, Version 6.0", 2010,
<http://www.unicode.org/versions/Unicode6.0.0/>. - [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.
- [URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
- [X509] International Telecommunications Union, "Information technology - Open Systems Interconnection - The Directory: Public-key and attribute certificate frameworks", ITU-T Recommendation X.509, ISO Standard 9594-8, March 2000.
- [XML] Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, "Extensible Markup Language (XML) 1.0 (Fifth Edition)", World Wide Web Consortium Recommendation REC-xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126>. - [XML-GUIDE] Hollenbeck, S., Rose, M., and L. Masinter, "Guidelines for the Use of Extensible Markup Language (XML) within IETF Protocols", BCP 70, RFC 3470, January 2003.
- [XML-MEDIA] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types", RFC 3023, January 2001.
- [XML-NAMES] Thompson, H., Hollander, D., Layman, A., Bray, T., and R. Tobin, "Namespaces in XML 1.0 (Third Edition)", World Wide Web Consortium Recommendation REC-xml-names-20091208, December 2009,
<http://www.w3.org/TR/2009/REC-xml-names-20091208>. - [XMPP-ADDR] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Address Format", RFC 6122, March 2011.
- [XMPP-IM] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence", RFC 6121, March 2011.
16.2. Informative References ✨
okay bestie, this is the reading list 📚 these ones aren't required to get the core stuff, but they're the best backstage passes if you wanna go deeper 🫶
- [AAA] Housley, R. and B. Aboba, "Guidance for Authentication, Authorization, and Accounting (AAA) Key Management", BCP 132, RFC 4962, July 2007. 🔐
- [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. 🧩
- [ACAP] Newman, C. and J. Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, November 1997.
- [ANONYMOUS] Zeilenga, K., "Anonymous Simple Authentication and Security Layer (SASL) Mechanism", RFC 4505, June 2006. 🫥
- [ASN.1] CCITT, "Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1)", 1988.
- [DIGEST-MD5] Leach, P. and C. Newman, "Using Digest Authentication as a SASL Mechanism", RFC 2831, May 2000.
- [DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. 🛡️
- [DNS-TXT] Rosenbaum, R., "Using the Domain Name System To Store Arbitrary String Attributes", RFC 1464, May 1993.
- [DOS] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-Service Considerations", RFC 4732, December 2006. 🚨
- [E2E-REQS] Saint-Andre, P., "Requirements for End-to-End Encryption in the Extensible Messaging and Presence Protocol (XMPP)", Work in Progress, March 2010.
- [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, July 2009. 📬
- [ETHERNET] "Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 3: Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications", IEEE Standard 802.3, September 1998. 🔌
- [GSS-API] Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, January 2000.
- [HASHES] Hoffman, P. and B. Schneier, "Attacks on Cryptographic Hashes in Internet Protocols", RFC 4270, November 2005.
- [HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 🌐
- [IANA-GUIDE] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
- [IANA-PORTS] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Transport Protocol Port Number and Service Name Registry", Work in Progress, February 2011.
- [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. 📥
- [IMP-REQS] Day, M., Aggarwal, S., and J. Vincent, "Instant Messaging / Presence Protocol Requirements", RFC 2779, February 2000. 💬
- [INTEROP] Masinter, L., "Formalizing IETF Interoperability Reporting", Work in Progress, October 2005.
- [IRC] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810, April 2000. 🗨️
- [IRI] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005.
- [LDAP] Zeilenga, K., "Lightweight Directory Access Protocol (LDAP): Technical Specification Road Map", RFC 4510, June 2006. 📇
- [LINKLOCAL] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic Configuration of IPv4 Link-Local Addresses", RFC 3927, May 2005.
- [MAILBOXES] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND FUNCTIONS", RFC 2142, May 1997.
- [POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996. 📮
- [PROCESS] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996.
- [REPORTS] Dusseault, L. and R. Sparks, "Guidance on Interoperation and Implementation Reports for Advancement to Draft Standard", BCP 9, RFC 5657, September 2009.
- [REST] Fielding, R., "Architectural Styles and the Design of Network-based Software Architectures", 2000. 🏛️
- [RFC3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 3920, October 2004. 💖
- [RFC3921] Saint-Andre, P., Ed., "Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence", RFC 3921, October 2004. ✨
- [SASLPREP] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, February 2005.
- [SEC-TERMS] Shirey, R., "Internet Security Glossary, Version 2", RFC 4949, August 2007. 📖
- [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. 📨
- [SEC-GUIDE] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, July 2003. 🔒
- [TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, January 2011. 🎀
- [TLS-RESUME] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, January 2008.
- [URN-OID] Mealling, M., "A URN Namespace of Object Identifiers", RFC 3061, February 2001.
- [USINGTLS] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, June 1999.
- [UUID] Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, July 2005. 🫧
- [XEP-0001] Saint-Andre, P., "XMPP Extension Protocols", XSF XEP 0001, March 2010.
- [XEP-0016] Millard, P. and P. Saint-Andre, "Privacy Lists", XSF XEP 0016, February 2007. 🙈
- [XEP-0045] Saint-Andre, P., "Multi-User Chat", XSF XEP 0045, July 2007. 👯♀️
- [XEP-0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish-Subscribe", XSF XEP 0060, July 2010.
- [XEP-0071] Saint-Andre, P., "XHTML-IM", XSF XEP 0071, September 2008.
- [XEP-0077] Saint-Andre, P., "In-Band Registration", XSF XEP 0077, September 2009.
- [XEP-0086] Norris, R. and P. Saint-Andre, "Error Condition Mappings", XSF XEP 0086, February 2004.
- [XEP-0100] Saint-Andre, P. and D. Smith, "Gateway Interaction", XSF XEP 0100, October 2005.
- [XEP-0114] Saint-Andre, P., "Jabber Component Protocol", XSF XEP 0114, March 2005.
- [XEP-0124] Paterson, I., Smith, D., and P. Saint-Andre, "Bidirectional-streams Over Synchronous HTTP (BOSH)", XSF XEP 0124, July 2010.
- [XEP-0138] Hildebrand, J. and P. Saint-Andre, "Stream Compression", XSF XEP 0138, May 2009. 🗜️
- [XEP-0156] Hildebrand, J. and P. Saint-Andre, "Discovering Alternative XMPP Connection Methods", XSF XEP 0156, June 2007.
- [XEP-0160] Saint-Andre, P., "Best Practices for Handling Offline Messages", XSF XEP 0160, January 2006. 📭
- [XEP-0174] Saint-Andre, P., "Link-Local Messaging", XSF XEP 0174, November 2008.
- [XEP-0175] Saint-Andre, P., "Best Practices for Use of SASL ANONYMOUS", XSF XEP 0175, September 2009.
- [XEP-0178] Saint-Andre, P. and P. Millard, "Best Practices for Use of SASL EXTERNAL with Certificates", XSF XEP 0178, February 2007.
- [XEP-0191] Saint-Andre, P., "Simple Communications Blocking", XSF XEP 0191, February 2007. 🚫
- [XEP-0198] Karneges, J., Hildebrand, J., Saint-Andre, P., Forno, F., Cridland, D., and M. Wild, "Stream Management", XSF XEP 0198, February 2011.
- [XEP-0199] Saint-Andre, P., "XMPP Ping", XSF XEP 0199, June 2009. 🏓
- [XEP-0205] Saint-Andre, P., "Best Practices to Discourage Denial of Service Attacks", XSF XEP 0205, January 2009.
- [XEP-0206] Paterson, I. and P. Saint-Andre, "XMPP Over BOSH", XSF XEP 0206, July 2010.
- [XEP-0220] Miller, J., Saint-Andre, P., and P. Hancke, "Server Dialback", XSF XEP 0220, March 2010.
- [XEP-0225] Saint-Andre, P., "Component Connections", XSF XEP 0225, October 2008.
- [XEP-0233] Miller, M., Saint-Andre, P., and J. Hildebrand, "Domain-Based Service Names in XMPP SASL Negotiation", XSF XEP 0233, June 2010.
- [XEP-0288] Hancke, P. and D. Cridland, "Bidirectional Server-to-Server Connections", XSF XEP 0288, October 2010. 🔁
- [XML-FRAG] Grosso, P. and D. Veillard, "XML Fragment Interchange", World Wide Web Consortium CR CR-xml-fragment-20010212, February 2001,
<http://www.w3.org/TR/2001/CR-xml-fragment-20010212>. 🧩 - [XML-REG] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January 2004.
- [XML-SCHEMA] Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech, "XML Schema Part 1: Structures Second Edition", World Wide Web Consortium Recommendation REC-xmlschema-1-20041028, October 2004,
<http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>. 📐 - [XMPP-URI] Saint-Andre, P., "Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP)", RFC 5122, February 2008. 🌸
Appendix A. XML Schemas 🎀
okay, the schemas are coming up next and they're giving formal definitions vibes 💅
The following schemas formally define various namespaces used in this document, in conformance with [XML-SCHEMA]. Because validation of XML streams and stanzas is optional, these schemas are not normative and are provided for descriptive purposes only. 🫶
A.1. Stream Namespace 💖
okay so this one is the schema for the stream namespace, the main container everything else lives inside 🫶
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='http://etherx.jabber.org/streams'
xmlns='http://etherx.jabber.org/streams'
elementFormDefault='unqualified'>
<xs:import namespace='jabber:client'/>
<xs:import namespace='jabber:server'/>
<xs:import namespace='urn:ietf:params:xml:ns:xmpp-sasl'/>
<xs:import namespace='urn:ietf:params:xml:ns:xmpp-streams'/>
<xs:import namespace='urn:ietf:params:xml:ns:xmpp-tls'/>
<xs:element name='stream'>
<xs:complexType>
<xs:sequence xmlns:client='jabber:client'
xmlns:server='jabber:server'>
<xs:element ref='features'
minOccurs='0'
maxOccurs='1'/>
<xs:any namespace='urn:ietf:params:xml:ns:xmpp-tls'
minOccurs='0'
maxOccurs='1'/>
<xs:any namespace='urn:ietf:params:xml:ns:xmpp-sasl'
minOccurs='0'
maxOccurs='1'/>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'
processContents='lax'/>
<xs:choice minOccurs='0' maxOccurs='1'>
<xs:choice minOccurs='0' maxOccurs='unbounded'>
<xs:element ref='client:message'/>
<xs:element ref='client:presence'/>
<xs:element ref='client:iq'/>
</xs:choice>
<xs:choice minOccurs='0' maxOccurs='unbounded'>
<xs:element ref='server:message'/>
<xs:element ref='server:presence'/>
<xs:element ref='server:iq'/>
</xs:choice>
</xs:choice>
<xs:element ref='error' minOccurs='0' maxOccurs='1'/>
</xs:sequence>
<xs:attribute name='from' type='xs:string' use='optional'/>
<xs:attribute name='id' type='xs:string' use='optional'/>
<xs:attribute name='to' type='xs:string' use='optional'/>
<xs:attribute name='version' type='xs:decimal' use='optional'/>
<xs:attribute ref='xml:lang' use='optional'/>
<xs:anyAttribute namespace='##other' processContents='lax'/>
</xs:complexType>
</xs:element>
<xs:element name='features'>
<xs:complexType>
<xs:sequence>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'
processContents='lax'/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='error'>
<xs:complexType>
<xs:sequence xmlns:err='urn:ietf:params:xml:ns:xmpp-streams'>
<xs:group ref='err:streamErrorGroup'/>
<xs:element ref='err:text'
minOccurs='0'
maxOccurs='1'/>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='1'
processContents='lax'/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>A.2. Stream Error Namespace 🚨
this one holds all the stream error names, the drama queen list of everything that can go wrong 💅
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-streams'
xmlns='urn:ietf:params:xml:ns:xmpp-streams'
elementFormDefault='qualified'>
<xs:element name='bad-format' type='empty'/>
<xs:element name='bad-namespace-prefix' type='empty'/>
<xs:element name='conflict' type='empty'/>
<xs:element name='connection-timeout' type='empty'/>
<xs:element name='host-gone' type='empty'/>
<xs:element name='host-unknown' type='empty'/>
<xs:element name='improper-addressing' type='empty'/>
<xs:element name='internal-server-error' type='empty'/>
<xs:element name='invalid-from' type='empty'/>
<xs:element name='invalid-id' type='empty'/>
<xs:element name='invalid-namespace' type='empty'/>
<xs:element name='invalid-xml' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='not-well-formed' type='empty'/>
<xs:element name='policy-violation' type='empty'/>
<xs:element name='remote-connection-failed' type='empty'/>
<xs:element name='reset' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='restricted-xml' type='empty'/>
<xs:element name='see-other-host' type='xs:string'/>
<xs:element name='system-shutdown' type='empty'/>
<xs:element name='undefined-condition' type='empty'/>
<xs:element name='unsupported-encoding' type='empty'/>
<xs:element name='unsupported-stanza-type' type='empty'/>
<xs:element name='unsupported-version' type='empty'/>
<xs:group name='streamErrorGroup'>
<xs:choice>
<xs:element ref='bad-format'/>
<xs:element ref='bad-namespace-prefix'/>
<xs:element ref='conflict'/>
<xs:element ref='connection-timeout'/>
<xs:element ref='host-gone'/>
<xs:element ref='host-unknown'/>
<xs:element ref='improper-addressing'/>
<xs:element ref='internal-server-error'/>
<xs:element ref='invalid-from'/>
<xs:element ref='invalid-id'/>
<xs:element ref='invalid-namespace'/>
<xs:element ref='invalid-xml'/>
<xs:element ref='not-authorized'/>
<xs:element ref='not-well-formed'/>
<xs:element ref='policy-violation'/>
<xs:element ref='remote-connection-failed'/>
<xs:element ref='reset'/>
<xs:element ref='resource-constraint'/>
<xs:element ref='restricted-xml'/>
<xs:element ref='see-other-host'/>
<xs:element ref='system-shutdown'/>
<xs:element ref='undefined-condition'/>
<xs:element ref='unsupported-encoding'/>
<xs:element ref='unsupported-stanza-type'/>
<xs:element ref='unsupported-version'/>
</xs:choice>
</xs:group>
<xs:element name='text'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>A.3. STARTTLS Namespace 🔐
this is the STARTTLS schema, the little handshake bestie that gets the encryption party started ✨
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-tls'
xmlns='urn:ietf:params:xml:ns:xmpp-tls'
elementFormDefault='qualified'>
<xs:element name='starttls'>
<xs:complexType>
<xs:choice minOccurs='0' maxOccurs='1'>
<xs:element name='required' type='empty'/>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:element name='proceed' type='empty'/>
<xs:element name='failure' type='empty'/>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>A.4. SASL Namespace 🦋
last one in this batch, the SASL schema, where the auth mechanisms and their little failure reasons all live 🌈
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-sasl'
xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
elementFormDefault='qualified'>
<xs:element name='mechanisms'>
<xs:complexType>
<xs:sequence>
<xs:element name='mechanism'
minOccurs='1'
maxOccurs='unbounded'
type='xs:NMTOKEN'/>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'
processContents='lax'/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='abort' type='empty'/>
<xs:element name='auth'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute name='mechanism'
type='xs:NMTOKEN'
use='required'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='challenge' type='xs:string'/>
<xs:element name='response' type='xs:string'/>
<xs:element name='success' type='xs:string'/>
<xs:element name='failure'>
<xs:complexType>
<xs:sequence>
<xs:choice minOccurs='0'>
<xs:element name='aborted' type='empty'/>
<xs:element name='account-disabled' type='empty'/>
<xs:element name='credentials-expired' type='empty'/>
<xs:element name='encryption-required' type='empty'/>
<xs:element name='incorrect-encoding' type='empty'/>
<xs:element name='invalid-authzid' type='empty'/>
<xs:element name='invalid-mechanism' type='empty'/>
<xs:element name='malformed-request' type='empty'/>
<xs:element name='mechanism-too-weak' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='temporary-auth-failure' type='empty'/>
</xs:choice>
<xs:element ref='text' minOccurs='0' maxOccurs='1'/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='text'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>A.5. Client Namespace ✨
okay so this one is the schema for the jabber:client namespace 🎀 it's where the stanzas people actually send from their clients get defined: message, presence, iq, plus the bits they're made of like body, subject, thread, show, status, priority, and error. it imports the stanza errors namespace too, so the error stuff lines up 💅
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='jabber:client'
xmlns='jabber:client'
elementFormDefault='qualified'>
<xs:import
namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<xs:element name='message'>
<xs:complexType>
<xs:sequence>
<xs:choice minOccurs='0' maxOccurs='unbounded'>
<xs:element ref='subject'/>
<xs:element ref='body'/>
<xs:element ref='thread'/>
</xs:choice>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'
processContents='lax'/>
<xs:element ref='error'
minOccurs='0'/>
</xs:sequence>
<xs:attribute name='from'
type='xs:string'
use='optional'/>
<xs:attribute name='id'
type='xs:NMTOKEN'
use='optional'/>
<xs:attribute name='to'
type='xs:string'
use='optional'/>
<xs:attribute name='type'
use='optional'
default='normal'>
<xs:simpleType>
<xs:restriction base='xs:NMTOKEN'>
<xs:enumeration value='chat'/>
<xs:enumeration value='error'/>
<xs:enumeration value='groupchat'/>
<xs:enumeration value='headline'/>
<xs:enumeration value='normal'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='body'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='subject'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='thread'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:NMTOKEN'>
<xs:attribute name='parent'
type='xs:NMTOKEN'
use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='presence'>
<xs:complexType>
<xs:sequence>
<xs:choice minOccurs='0' maxOccurs='unbounded'>
<xs:element ref='show'/>
<xs:element ref='status'/>
<xs:element ref='priority'/>
</xs:choice>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'
processContents='lax'/>
<xs:element ref='error'
minOccurs='0'/>
</xs:sequence>
<xs:attribute name='from'
type='xs:string'
use='optional'/>
<xs:attribute name='id'
type='xs:NMTOKEN'
use='optional'/>
<xs:attribute name='to'
type='xs:string'
use='optional'/>
<xs:attribute name='type' use='optional'>
<xs:simpleType>
<xs:restriction base='xs:NMTOKEN'>
<xs:enumeration value='error'/>
<xs:enumeration value='probe'/>
<xs:enumeration value='subscribe'/>
<xs:enumeration value='subscribed'/>
<xs:enumeration value='unavailable'/>
<xs:enumeration value='unsubscribe'/>
<xs:enumeration value='unsubscribed'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='show'>
<xs:simpleType>
<xs:restriction base='xs:NMTOKEN'>
<xs:enumeration value='away'/>
<xs:enumeration value='chat'/>
<xs:enumeration value='dnd'/>
<xs:enumeration value='xa'/>
</xs:restriction>
</xs:simpleType>
</xs:element>
<xs:element name='status'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='string1024'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='string1024'>
<xs:restriction base='xs:string'>
<xs:minLength value='1'/>
<xs:maxLength value='1024'/>
</xs:restriction>
</xs:simpleType>
<xs:element name='priority' type='xs:byte'/>
<xs:element name='iq'>
<xs:complexType>
<xs:sequence>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='1'
processContents='lax'/>
<xs:element ref='error'
minOccurs='0'/>
</xs:sequence>
<xs:attribute name='from'
type='xs:string'
use='optional'/>
<xs:attribute name='id'
type='xs:NMTOKEN'
use='required'/>
<xs:attribute name='to'
type='xs:string'
use='optional'/>
<xs:attribute name='type' use='required'>
<xs:simpleType>
<xs:restriction base='xs:NMTOKEN'>
<xs:enumeration value='error'/>
<xs:enumeration value='get'/>
<xs:enumeration value='result'/>
<xs:enumeration value='set'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='error'>
<xs:complexType>
<xs:sequence xmlns:err='urn:ietf:params:xml:ns:xmpp-stanzas'>
<xs:group ref='err:stanzaErrorGroup'/>
<xs:element ref='err:text'
minOccurs='0'/>
</xs:sequence>
<xs:attribute name='by'
type='xs:string'
use='optional'/>
<xs:attribute name='type' use='required'>
<xs:simpleType>
<xs:restriction base='xs:NMTOKEN'>
<xs:enumeration value='auth'/>
<xs:enumeration value='cancel'/>
<xs:enumeration value='continue'/>
<xs:enumeration value='modify'/>
<xs:enumeration value='wait'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
</xs:schema>A.6. Server Namespace 🌐
okay so this is the schema for the server namespace, jabber:server 💅 it pulls in the stanza error namespace and defines message, presence, and iq for server-to-server traffic 🫶
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='jabber:server'
xmlns='jabber:server'
elementFormDefault='qualified'>
<xs:import
namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<xs:element name='message'>
<xs:complexType>
<xs:sequence>
<xs:choice minOccurs='0' maxOccurs='unbounded'>
<xs:element ref='subject'/>
<xs:element ref='body'/>
<xs:element ref='thread'/>
</xs:choice>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'
processContents='lax'/>
<xs:element ref='error'
minOccurs='0'/>
</xs:sequence>
<xs:attribute name='from'
type='xs:string'
use='required'/>
<xs:attribute name='id'
type='xs:NMTOKEN'
use='optional'/>
<xs:attribute name='to'
type='xs:string'
use='required'/>
<xs:attribute name='type'
use='optional'
default='normal'>
<xs:simpleType>
<xs:restriction base='xs:NMTOKEN'>
<xs:enumeration value='chat'/>
<xs:enumeration value='error'/>
<xs:enumeration value='groupchat'/>
<xs:enumeration value='headline'/>
<xs:enumeration value='normal'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='body'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='subject'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='thread'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:NMTOKEN'>
<xs:attribute name='parent'
type='xs:NMTOKEN'
use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='subject'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:NMTOKEN'>
<xs:attribute name='parent'
type='xs:NMTOKEN'
use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='presence'>
<xs:complexType>
<xs:sequence>
<xs:choice minOccurs='0' maxOccurs='unbounded'>
<xs:element ref='show'/>
<xs:element ref='status'/>
<xs:element ref='priority'/>
</xs:choice>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'
processContents='lax'/>
<xs:element ref='error'
minOccurs='0'/>
</xs:sequence>
<xs:attribute name='from'
type='xs:string'
use='required'/>
<xs:attribute name='id'
type='xs:NMTOKEN'
use='optional'/>
<xs:attribute name='to'
type='xs:string'
use='required'/>
<xs:attribute name='type' use='optional'>
<xs:simpleType>
<xs:restriction base='xs:NMTOKEN'>
<xs:enumeration value='error'/>
<xs:enumeration value='probe'/>
<xs:enumeration value='subscribe'/>
<xs:enumeration value='subscribed'/>
<xs:enumeration value='unavailable'/>
<xs:enumeration value='unsubscribe'/>
<xs:enumeration value='unsubscribed'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='show'>
<xs:simpleType>
<xs:restriction base='xs:NMTOKEN'>
<xs:enumeration value='away'/>
<xs:enumeration value='chat'/>
<xs:enumeration value='dnd'/>
<xs:enumeration value='xa'/>
</xs:restriction>
</xs:simpleType>
</xs:element>
<xs:element name='status'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='string1024'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='string1024'>
<xs:restriction base='xs:string'>
<xs:minLength value='1'/>
<xs:maxLength value='1024'/>
</xs:restriction>
</xs:simpleType>
<xs:element name='priority' type='xs:byte' default='0'/>
<xs:element name='iq'>
<xs:complexType>
<xs:sequence>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='1'
processContents='lax'/>
<xs:element ref='error'
minOccurs='0'/>
</xs:sequence>
<xs:attribute name='from'
type='xs:string'
use='required'/>
<xs:attribute name='id'
type='xs:NMTOKEN'
use='required'/>
<xs:attribute name='to'
type='xs:string'
use='required'/>
<xs:attribute name='type' use='required'>
<xs:simpleType>
<xs:restriction base='xs:NMTOKEN'>
<xs:enumeration value='error'/>
<xs:enumeration value='get'/>
<xs:enumeration value='result'/>
<xs:enumeration value='set'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='error'>
<xs:complexType>
<xs:sequence xmlns:err='urn:ietf:params:xml:ns:xmpp-stanzas'>
<xs:group ref='err:stanzaErrorGroup'/>
<xs:element ref='err:text'
minOccurs='0'/>
</xs:sequence>
<xs:attribute name='by'
type='xs:string'
use='optional'/>
<xs:attribute name='type' use='required'>
<xs:simpleType>
<xs:restriction base='xs:NMTOKEN'>
<xs:enumeration value='auth'/>
<xs:enumeration value='cancel'/>
<xs:enumeration value='continue'/>
<xs:enumeration value='modify'/>
<xs:enumeration value='wait'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
</xs:schema>A.7. Resource Binding Namespace 🎀
next up is the resource binding namespace, urn:ietf:params:xml:ns:xmpp-bind 💖 it's giving a bind element with either a resource or a jid inside 🦋
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-bind'
xmlns='urn:ietf:params:xml:ns:xmpp-bind'
elementFormDefault='qualified'>
<xs:element name='bind'>
<xs:complexType>
<xs:choice>
<xs:element name='resource' type='resourceType'/>
<xs:element name='jid' type='fullJIDType'/>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:simpleType name='fullJIDType'>
<xs:restriction base='xs:string'>
<xs:minLength value='8'/>
<xs:maxLength value='3071'/>
</xs:restriction>
</xs:simpleType>
<xs:simpleType name='resourceType'>
<xs:restriction base='xs:string'>
<xs:minLength value='1'/>
<xs:maxLength value='1023'/>
</xs:restriction>
</xs:simpleType>
</xs:schema>A.8. Stanza Error Namespace ✨
last one in this part: the stanza error namespace, urn:ietf:params:xml:ns:xmpp-stanzas 🔥 this is where all the error conditions live, from bad-request to unexpected-request, bundled up in the stanzaErrorGroup 👑
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-stanzas'
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
elementFormDefault='qualified'>
<xs:element name='bad-request' type='empty'/>
<xs:element name='conflict' type='empty'/>
<xs:element name='feature-not-implemented' type='empty'/>
<xs:element name='forbidden' type='empty'/>
<xs:element name='gone' type='xs:string'/>
<xs:element name='internal-server-error' type='empty'/>
<xs:element name='item-not-found' type='empty'/>
<xs:element name='jid-malformed' type='empty'/>
<xs:element name='not-acceptable' type='empty'/>
<xs:element name='not-allowed' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='policy-violation' type='empty'/>
<xs:element name='recipient-unavailable' type='empty'/>
<xs:element name='redirect' type='xs:string'/>
<xs:element name='registration-required' type='empty'/>
<xs:element name='remote-server-not-found' type='empty'/>
<xs:element name='remote-server-timeout' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='service-unavailable' type='empty'/>
<xs:element name='subscription-required' type='empty'/>
<xs:element name='undefined-condition' type='empty'/>
<xs:element name='unexpected-request' type='empty'/>
<xs:group name='stanzaErrorGroup'>
<xs:choice>
<xs:element ref='bad-request'/>
<xs:element ref='conflict'/>
<xs:element ref='feature-not-implemented'/>
<xs:element ref='forbidden'/>
<xs:element ref='gone'/>
<xs:element ref='internal-server-error'/>
<xs:element ref='item-not-found'/>
<xs:element ref='jid-malformed'/>
<xs:element ref='not-acceptable'/>
<xs:element ref='not-authorized'/>
<xs:element ref='not-allowed'/>
<xs:element ref='policy-violation'/>
<xs:element ref='recipient-unavailable'/>
<xs:element ref='redirect'/>
<xs:element ref='registration-required'/>
<xs:element ref='remote-server-not-found'/>
<xs:element ref='remote-server-timeout'/>
<xs:element ref='resource-constraint'/>
<xs:element ref='service-unavailable'/>
<xs:element ref='subscription-required'/>
<xs:element ref='undefined-condition'/>
<xs:element ref='unexpected-request'/>
</xs:choice>
</xs:group>
<xs:element name='text'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>Appendix B. Contact Addresses 📬
Okay so, consistent with [MAILBOXES] 💌, organizations that offer XMPP services are encouraged to set up an Internet mailbox of "XMPP" for inquiries about that service. The host part of the resulting mailto URI should be the organization's domain, not the domain of the XMPP service itself. For example, the XMPP service might live at im.example.com, but the Internet mailbox would be <xmpp@example.com> 🎀
Appendix C. Account Provisioning 🧚
Account provisioning is out of scope for this spec, bestie 🙈 Possible methods include account creation by a server administrator, or in-band account registration using the jabber:iq:register namespace as documented in [XEP-0077]. Either way, an XMPP server implementation or administrative function MUST make sure that any JID assigned during account provisioning (including localpart, domainpart, resourcepart, and separator characters) conforms to the canonical format for XMPP addresses defined in [XMPP-ADDR] 💅
Appendix D. Differences from RFC 3920 🔥
Based on consensus from implementation and deployment experience, plus formal interoperability testing, the following substantive modifications were made from RFC 3920 (on top of numerous editorial changes) 💖
- Moved specification of the XMPP address format to a separate document.
- Recommended or mandated use of the
'from'and'to'attributes on stream headers. - More fully specified the stream closing handshake.
- Specified the recommended stream reconnection algorithm.
- Changed the name of the
<xml-not-well-formed/>stream error condition to<not-well-formed/>for compliance with the XML specification. - Removed the unnecessary and unused
<invalid-id/>stream error (see RFC 3920 for historical documentation). - Specified return of the
<restricted-xml/>stream error in response to receipt of prohibited XML features. - More completely specified the format and handling of the
<see-other-host/>stream error, including consistency with RFC 3986 and RFC 5952 with regard to IPv6 addresses (e.g., enclosing the IPv6 address in square brackets'['and']') 🌸 - Specified that the SASL SCRAM mechanism is a mandatory-to-implement technology for client-to-server streams.
- Specified that TLS plus the SASL PLAIN mechanism is a mandatory-to-implement technology for client-to-server streams.
- Specified that support for the SASL EXTERNAL mechanism is required for servers but only recommended for clients (since end-user X.509 certificates are difficult to obtain and not yet widely deployed).
- Removed the hard two-connection rule for server-to-server streams.
- More clearly specified the certificate profile for both public key certificates and issuer certificates.
- Added the
<reset/>stream error (Section 4.9.3.16) condition to handle expired/revoked certificates or the addition of security-critical features to an existing stream. - Added the
<account-disabled/>,<credentials-expired/>,<encryption-required/>, and<malformed-request/>SASL error conditions to handle error flows mistakenly left out of RFC 3920 or discussed in RFC 4422 but not in RFC 2222 🫶 - Removed the unused
<payment-required/>stanza error. - Removed the unnecessary requirement for escaping of characters that map to certain predefined entities, since they do not need to be escaped in XML.
- Clarified the process of DNS SRV lookups and fallbacks.
- Clarified the handling of SASL security layers.
- Clarified that a SASL simple user name is the localpart, not the bare JID.
- Clarified the stream negotiation process and associated flow chart.
- Clarified the handling of stream features.
- Added a
'by'attribute to the<error/>element for stanza errors so that the entity that has detected the error can include its JID for diagnostic or tracking purposes. - Clarified the handling of data that violates the well-formedness definitions for XML 1.0 and XML namespaces.
- Specified the security considerations in more detail, especially with regard to presence leaks and denial-of-service attacks.
- Moved documentation of the Server Dialback protocol from this specification to a separate specification maintained by the XMPP Standards Foundation.
Appendix E. Acknowledgements 🌈
This document is an update to, and derived from, RFC 3920. It would have been totally impossible without the work of the contributors and commenters acknowledged there ✨
Hundreds of people have sent implementation feedback, bug reports, requests for clarification, and suggestions for improvement since RFC 3920 was published 🙌 The document editor did his best to address all of it, but he is solely responsible for any remaining errors and ambiguities 😭
Special thanks are due to Kevin Smith, Matthew Wild, Dave Cridland, Philipp Hancke, Waqas Hussain, Florian Zeitz, Ben Campbell, Jehan Pages, Paul Aurich, Justin Karneges, Kurt Zeilenga, Simon Josefsson, Ralph Meijer, Curtis King, and others for their comments during Working Group Last Call 👑
Thanks also to Yaron Sheffer and Elwyn Davies for their reviews on behalf of the Security Directorate and the General Area Review Team, respectively 🦋
The Working Group chairs were Ben Campbell and Joe Hildebrand. The responsible Area Director was Gonzalo Camarillo 💜
Author's Address 💌
Peter Saint-Andre
Cisco
1899 Wyknoop Street, Suite 600
Denver, CO 80202
USA
Phone: +1-303-308-3282
EMail: psaintan@cisco.com📦 where this page comes from
- ietf: rfc6120 2011-03
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms