Network Working Group P. Saint-Andre, Ed. Internet-Draft XMPP Standards Foundation Obsoletes: 3920 (if approved) June 2, 2008 Intended status: Standards Track Expires: December 4, 2008 Extensible Messaging and Presence Protocol (XMPP): Core draft-saintandre-rfc3920bis-05 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 4, 2008. Abstract This document defines the core features of the Extensible Messaging and Presence Protocol (XMPP), a technology for streaming Extensible Markup Language (XML) elements in order to exchange structured information in close to real time between any two or more network- aware entities. XMPP provides a generalized, extensible framework for incrementally exchanging XML data, upon which a variety of applications can be built. The framework includes methods for stream setup and teardown, channel encryption, authentication of a client to a server and of one server to another server, and primitives for push-style messages, publication of network availability information ("presence"), and request-response interactions between any two XMPP Saint-Andre Expires December 4, 2008 [Page 1] Internet-Draft XMPP Core June 2008 entities. This document also specifies the format for XMPP addresses, which are fully internationalizable. This document obsoletes RFC 3920. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 9 1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2. Functional Summary . . . . . . . . . . . . . . . . . . . 10 1.3. Conventions . . . . . . . . . . . . . . . . . . . . . . 11 1.4. Discussion Venue . . . . . . . . . . . . . . . . . . . . 11 2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 11 2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3. Client . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.4. Network . . . . . . . . . . . . . . . . . . . . . . . . 13 3. Addresses . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2. Domain Identifier . . . . . . . . . . . . . . . . . . . 14 3.3. Node Identifier . . . . . . . . . . . . . . . . . . . . 16 3.4. Resource Identifier . . . . . . . . . . . . . . . . . . 17 3.5. Determination of Addresses . . . . . . . . . . . . . . . 17 4. TCP Binding . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.2. Hostname Resolution . . . . . . . . . . . . . . . . . . 18 4.3. Client-to-Server Communications . . . . . . . . . . . . 19 4.4. Server-to-Server Communications . . . . . . . . . . . . 19 4.5. Reconnection . . . . . . . . . . . . . . . . . . . . . . 19 4.6. Other Bindings . . . . . . . . . . . . . . . . . . . . . 20 5. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20 5.2. Stream Security . . . . . . . . . . . . . . . . . . . . 22 5.3. Stream Attributes . . . . . . . . . . . . . . . . . . . 23 5.3.1. from . . . . . . . . . . . . . . . . . . . . . . . . 23 5.3.2. to . . . . . . . . . . . . . . . . . . . . . . . . . 24 5.3.3. id . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.3.4. xml:lang . . . . . . . . . . . . . . . . . . . . . . 26 5.3.5. version . . . . . . . . . . . . . . . . . . . . . . 26 5.3.6. Summary . . . . . . . . . . . . . . . . . . . . . . 28 5.4. Namespace Declarations . . . . . . . . . . . . . . . . . 28 5.5. Stream Features . . . . . . . . . . . . . . . . . . . . 28 5.6. Closing Streams . . . . . . . . . . . . . . . . . . . . 30 5.6.1. With Stream Error . . . . . . . . . . . . . . . . . 30 5.6.2. Without Stream Error . . . . . . . . . . . . . . . . 30 5.6.3. Handling of Idle Streams . . . . . . . . . . . . . . 31 5.7. Stream Errors . . . . . . . . . . . . . . . . . . . . . 31 Saint-Andre Expires December 4, 2008 [Page 2] Internet-Draft XMPP Core June 2008 5.7.1. Rules . . . . . . . . . . . . . . . . . . . . . . . 31 5.7.1.1. Stream Errors Are Unrecoverable . . . . . . . . . 31 5.7.1.2. Stream Errors Can Occur During Setup . . . . . . 32 5.7.1.3. Stream Errors When the Host is Unspecified . . . 32 5.7.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 33 5.7.3. Defined Stream Error Conditions . . . . . . . . . . 34 5.7.3.1. bad-format . . . . . . . . . . . . . . . . . . . 34 5.7.3.2. bad-namespace-prefix . . . . . . . . . . . . . . 34 5.7.3.3. conflict . . . . . . . . . . . . . . . . . . . . 35 5.7.3.4. connection-timeout . . . . . . . . . . . . . . . 36 5.7.3.5. host-gone . . . . . . . . . . . . . . . . . . . . 36 5.7.3.6. host-unknown . . . . . . . . . . . . . . . . . . 37 5.7.3.7. improper-addressing . . . . . . . . . . . . . . . 38 5.7.3.8. internal-server-error . . . . . . . . . . . . . . 38 5.7.3.9. invalid-from . . . . . . . . . . . . . . . . . . 39 5.7.3.10. invalid-id . . . . . . . . . . . . . . . . . . . 39 5.7.3.11. invalid-namespace . . . . . . . . . . . . . . . . 40 5.7.3.12. invalid-xml . . . . . . . . . . . . . . . . . . . 40 5.7.3.13. not-authorized . . . . . . . . . . . . . . . . . 41 5.7.3.14. policy-violation . . . . . . . . . . . . . . . . 42 5.7.3.15. remote-connection-failed . . . . . . . . . . . . 43 5.7.3.16. resource-constraint . . . . . . . . . . . . . . . 43 5.7.3.17. restricted-xml . . . . . . . . . . . . . . . . . 44 5.7.3.18. see-other-host . . . . . . . . . . . . . . . . . 44 5.7.3.19. system-shutdown . . . . . . . . . . . . . . . . . 45 5.7.3.20. undefined-condition . . . . . . . . . . . . . . . 46 5.7.3.21. unsupported-encoding . . . . . . . . . . . . . . 46 5.7.3.22. unsupported-stanza-type . . . . . . . . . . . . . 47 5.7.3.23. unsupported-version . . . . . . . . . . . . . . . 47 5.7.3.24. xml-not-well-formed . . . . . . . . . . . . . . . 48 5.7.4. Application-Specific Conditions . . . . . . . . . . 49 5.8. Simplified Stream Examples . . . . . . . . . . . . . . . 49 6. STARTTLS Negotiation . . . . . . . . . . . . . . . . . . . . 51 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 52 6.2. Rules . . . . . . . . . . . . . . . . . . . . . . . . . 52 6.2.1. Mechanism Preferences . . . . . . . . . . . . . . . 52 6.2.2. Data Formatting . . . . . . . . . . . . . . . . . . 52 6.2.3. Order of Negotiation . . . . . . . . . . . . . . . . 52 6.3. Process . . . . . . . . . . . . . . . . . . . . . . . . 53 6.3.1. Exchange of Stream Headers and Stream Features . . . 53 6.3.2. Initiation of STARTTLS Negotiation . . . . . . . . . 54 6.3.2.1. STARTTLS Command . . . . . . . . . . . . . . . . 54 6.3.2.2. Failure Case . . . . . . . . . . . . . . . . . . 54 6.3.2.3. Proceed Case . . . . . . . . . . . . . . . . . . 55 6.3.3. TLS Negotiation . . . . . . . . . . . . . . . . . . 55 6.3.3.1. Rules . . . . . . . . . . . . . . . . . . . . . . 55 6.3.3.2. TLS Failure . . . . . . . . . . . . . . . . . . . 55 6.3.3.3. TLS Success . . . . . . . . . . . . . . . . . . . 56 Saint-Andre Expires December 4, 2008 [Page 3] Internet-Draft XMPP Core June 2008 7. SASL Negotiation . . . . . . . . . . . . . . . . . . . . . . 57 7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 57 7.2. Rules . . . . . . . . . . . . . . . . . . . . . . . . . 57 7.2.1. Data Formatting . . . . . . . . . . . . . . . . . . 57 7.2.2. Security Layers . . . . . . . . . . . . . . . . . . 57 7.2.3. Simple Usernames . . . . . . . . . . . . . . . . . . 58 7.2.4. Authorization Identities . . . . . . . . . . . . . . 58 7.2.5. Round Trips . . . . . . . . . . . . . . . . . . . . 58 7.3. Process . . . . . . . . . . . . . . . . . . . . . . . . 59 7.3.1. Exchange of Stream Headers and Stream Features . . . 59 7.3.2. Initiation . . . . . . . . . . . . . . . . . . . . . 61 7.3.3. Challenge-Response Sequence . . . . . . . . . . . . 61 7.3.4. Abort . . . . . . . . . . . . . . . . . . . . . . . 61 7.3.5. Failure . . . . . . . . . . . . . . . . . . . . . . 62 7.3.6. Success . . . . . . . . . . . . . . . . . . . . . . 63 7.4. SASL Definition . . . . . . . . . . . . . . . . . . . . 64 7.5. SASL Errors . . . . . . . . . . . . . . . . . . . . . . 64 7.5.1. aborted . . . . . . . . . . . . . . . . . . . . . . 65 7.5.2. account-disabled . . . . . . . . . . . . . . . . . . 65 7.5.3. credentials-expired . . . . . . . . . . . . . . . . 65 7.5.4. encryption-required . . . . . . . . . . . . . . . . 66 7.5.5. incorrect-encoding . . . . . . . . . . . . . . . . . 66 7.5.6. invalid-authzid . . . . . . . . . . . . . . . . . . 66 7.5.7. invalid-mechanism . . . . . . . . . . . . . . . . . 67 7.5.8. malformed-request . . . . . . . . . . . . . . . . . 67 7.5.9. mechanism-too-weak . . . . . . . . . . . . . . . . . 67 7.5.10. not-authorized . . . . . . . . . . . . . . . . . . . 68 7.5.11. temporary-auth-failure . . . . . . . . . . . . . . . 68 7.5.12. transition-needed . . . . . . . . . . . . . . . . . 68 8. Resource Binding . . . . . . . . . . . . . . . . . . . . . . 69 8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 69 8.2. Advertising Support . . . . . . . . . . . . . . . . . . 69 8.3. Generation of Resource Identifiers . . . . . . . . . . . 70 8.4. Server-Generated Resource Identifier . . . . . . . . . . 71 8.4.1. Success Case . . . . . . . . . . . . . . . . . . . . 71 8.4.2. Error Case . . . . . . . . . . . . . . . . . . . . . 71 8.5. Client-Generated Resource Identifier . . . . . . . . . . 72 8.5.1. Success Case . . . . . . . . . . . . . . . . . . . . 72 8.5.2. Error Cases . . . . . . . . . . . . . . . . . . . . 72 8.5.2.1. Not Allowed . . . . . . . . . . . . . . . . . . . 73 8.5.2.2. Bad Request . . . . . . . . . . . . . . . . . . . 73 8.5.2.3. Conflict . . . . . . . . . . . . . . . . . . . . 73 8.6. Binding Multiple Resources . . . . . . . . . . . . . . . 74 8.6.1. Support . . . . . . . . . . . . . . . . . . . . . . 74 8.6.2. Binding an Additional Resource . . . . . . . . . . . 74 8.6.3. Unbinding a Resource . . . . . . . . . . . . . . . . 75 8.6.3.1. Success Case . . . . . . . . . . . . . . . . . . 75 8.6.3.2. Error Cases . . . . . . . . . . . . . . . . . . . 75 Saint-Andre Expires December 4, 2008 [Page 4] Internet-Draft XMPP Core June 2008 8.6.4. From Addresses . . . . . . . . . . . . . . . . . . . 76 9. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 76 9.1. Common Attributes . . . . . . . . . . . . . . . . . . . 77 9.1.1. to . . . . . . . . . . . . . . . . . . . . . . . . . 77 9.1.1.1. Client-to-Server Streams . . . . . . . . . . . . 77 9.1.1.2. Server-to-Server Streams . . . . . . . . . . . . 77 9.1.2. from . . . . . . . . . . . . . . . . . . . . . . . . 78 9.1.2.1. Client-to-Server Streams . . . . . . . . . . . . 78 9.1.2.2. Server-to-Server Streams . . . . . . . . . . . . 79 9.1.3. id . . . . . . . . . . . . . . . . . . . . . . . . . 79 9.1.4. type . . . . . . . . . . . . . . . . . . . . . . . . 79 9.1.5. xml:lang . . . . . . . . . . . . . . . . . . . . . . 80 9.2. Basic Semantics . . . . . . . . . . . . . . . . . . . . 81 9.2.1. Message Semantics . . . . . . . . . . . . . . . . . 81 9.2.2. Presence Semantics . . . . . . . . . . . . . . . . . 81 9.2.3. IQ Semantics . . . . . . . . . . . . . . . . . . . . 81 9.3. Stanza Errors . . . . . . . . . . . . . . . . . . . . . 83 9.3.1. Rules . . . . . . . . . . . . . . . . . . . . . . . 83 9.3.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 83 9.3.3. Defined Conditions . . . . . . . . . . . . . . . . . 85 9.3.3.1. bad-request . . . . . . . . . . . . . . . . . . . 85 9.3.3.2. conflict . . . . . . . . . . . . . . . . . . . . 85 9.3.3.3. feature-not-implemented . . . . . . . . . . . . . 86 9.3.3.4. forbidden . . . . . . . . . . . . . . . . . . . . 86 9.3.3.5. gone . . . . . . . . . . . . . . . . . . . . . . 87 9.3.3.6. internal-server-error . . . . . . . . . . . . . . 87 9.3.3.7. item-not-found . . . . . . . . . . . . . . . . . 88 9.3.3.8. jid-malformed . . . . . . . . . . . . . . . . . . 88 9.3.3.9. not-acceptable . . . . . . . . . . . . . . . . . 89 9.3.3.10. not-allowed . . . . . . . . . . . . . . . . . . . 89 9.3.3.11. not-authorized . . . . . . . . . . . . . . . . . 89 9.3.3.12. not-modified . . . . . . . . . . . . . . . . . . 90 9.3.3.13. payment-required . . . . . . . . . . . . . . . . 91 9.3.3.14. recipient-unavailable . . . . . . . . . . . . . . 91 9.3.3.15. redirect . . . . . . . . . . . . . . . . . . . . 92 9.3.3.16. registration-required . . . . . . . . . . . . . . 92 9.3.3.17. remote-server-not-found . . . . . . . . . . . . . 93 9.3.3.18. remote-server-timeout . . . . . . . . . . . . . . 93 9.3.3.19. resource-constraint . . . . . . . . . . . . . . . 93 9.3.3.20. service-unavailable . . . . . . . . . . . . . . . 94 9.3.3.21. subscription-required . . . . . . . . . . . . . . 94 9.3.3.22. undefined-condition . . . . . . . . . . . . . . . 95 9.3.3.23. unexpected-request . . . . . . . . . . . . . . . 96 9.3.3.24. unknown-sender . . . . . . . . . . . . . . . . . 97 9.3.4. Application-Specific Conditions . . . . . . . . . . 97 9.4. Extended Content . . . . . . . . . . . . . . . . . . . . 98 10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 99 10.1. Client-to-Server . . . . . . . . . . . . . . . . . . . . 99 Saint-Andre Expires December 4, 2008 [Page 5] Internet-Draft XMPP Core June 2008 10.1.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . 100 10.1.2. SASL . . . . . . . . . . . . . . . . . . . . . . . . 101 10.1.3. Resource Binding . . . . . . . . . . . . . . . . . . 102 10.1.4. Stanza Exchange . . . . . . . . . . . . . . . . . . 103 10.1.5. Close . . . . . . . . . . . . . . . . . . . . . . . 104 10.2. Server-to-Server Examples . . . . . . . . . . . . . . . 104 10.2.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . 104 10.2.2. SASL . . . . . . . . . . . . . . . . . . . . . . . . 106 10.2.3. Stanza Exchange . . . . . . . . . . . . . . . . . . 107 10.2.4. Close . . . . . . . . . . . . . . . . . . . . . . . 108 11. Server Rules for Processing XML Stanzas . . . . . . . . . . . 108 11.1. No 'to' Address . . . . . . . . . . . . . . . . . . . . 108 11.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . 108 11.1.2. Message . . . . . . . . . . . . . . . . . . . . . . 109 11.1.3. Presence . . . . . . . . . . . . . . . . . . . . . . 109 11.1.4. IQ . . . . . . . . . . . . . . . . . . . . . . . . . 109 11.2. Local Domain . . . . . . . . . . . . . . . . . . . . . . 109 11.2.1. Mere Domain . . . . . . . . . . . . . . . . . . . . 110 11.2.2. Resource at Domain . . . . . . . . . . . . . . . . . 110 11.2.3. Node at Domain . . . . . . . . . . . . . . . . . . . 110 11.3. Foreign Domain . . . . . . . . . . . . . . . . . . . . . 110 11.3.1. Existing Stream . . . . . . . . . . . . . . . . . . 111 11.3.2. No Existing Stream . . . . . . . . . . . . . . . . . 111 11.3.3. Error Handling . . . . . . . . . . . . . . . . . . . 111 12. XML Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 111 12.1. Restrictions . . . . . . . . . . . . . . . . . . . . . . 111 12.2. XML Namespace Names and Prefixes . . . . . . . . . . . . 112 12.2.1. Streams Namespace . . . . . . . . . . . . . . . . . 112 12.2.2. Default Namespace . . . . . . . . . . . . . . . . . 113 12.2.3. Extended Namespaces . . . . . . . . . . . . . . . . 114 12.3. Well-Formedness . . . . . . . . . . . . . . . . . . . . 115 12.4. Validation . . . . . . . . . . . . . . . . . . . . . . . 115 12.5. Inclusion of Text Declaration . . . . . . . . . . . . . 115 12.6. Character Encoding . . . . . . . . . . . . . . . . . . . 116 12.7. White Space . . . . . . . . . . . . . . . . . . . . . . 116 13. Compliance Requirements . . . . . . . . . . . . . . . . . . . 116 13.1. Servers . . . . . . . . . . . . . . . . . . . . . . . . 116 13.2. Clients . . . . . . . . . . . . . . . . . . . . . . . . 117 14. Internationalization Considerations . . . . . . . . . . . . . 117 15. Security Considerations . . . . . . . . . . . . . . . . . . . 118 15.1. High Security . . . . . . . . . . . . . . . . . . . . . 118 15.2. Certificates . . . . . . . . . . . . . . . . . . . . . . 118 15.2.1. Certificate Generation . . . . . . . . . . . . . . . 118 15.2.1.1. Server Certificates . . . . . . . . . . . . . . . 118 15.2.1.2. Client Certificates . . . . . . . . . . . . . . . 120 15.2.1.3. ASN.1 Object Identifier . . . . . . . . . . . . . 121 15.2.2. Certificate Validation . . . . . . . . . . . . . . . 121 15.2.2.1. Server Certificates . . . . . . . . . . . . . . . 121 Saint-Andre Expires December 4, 2008 [Page 6] Internet-Draft XMPP Core June 2008 15.2.2.2. Client Certificates . . . . . . . . . . . . . . . 123 15.3. Client-to-Server Communication . . . . . . . . . . . . . 124 15.4. Server-to-Server Communication . . . . . . . . . . . . . 124 15.5. Order of Layers . . . . . . . . . . . . . . . . . . . . 125 15.6. Lack of SASL Channel Binding to TLS . . . . . . . . . . 125 15.7. Mandatory-to-Implement Technologies . . . . . . . . . . 126 15.8. Firewalls . . . . . . . . . . . . . . . . . . . . . . . 126 15.9. Use of base64 in SASL . . . . . . . . . . . . . . . . . 126 15.10. Stringprep Profiles . . . . . . . . . . . . . . . . . . 127 15.11. Address Spoofing . . . . . . . . . . . . . . . . . . . . 127 15.11.1. Address Forging . . . . . . . . . . . . . . . . . . 128 15.11.2. Address Mimicking . . . . . . . . . . . . . . . . . 128 15.12. Denial of Service . . . . . . . . . . . . . . . . . . . 129 15.13. Presence Leaks . . . . . . . . . . . . . . . . . . . . . 131 15.14. Directory Harvesting . . . . . . . . . . . . . . . . . . 131 16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 131 16.1. XML Namespace Name for TLS Data . . . . . . . . . . . . 131 16.2. XML Namespace Name for SASL Data . . . . . . . . . . . . 131 16.3. XML Namespace Name for Stream Errors . . . . . . . . . . 132 16.4. XML Namespace Name for Resource Binding . . . . . . . . 132 16.5. XML Namespace Name for Stanza Errors . . . . . . . . . . 132 16.6. Nodeprep Profile of Stringprep . . . . . . . . . . . . . 133 16.7. Resourceprep Profile of Stringprep . . . . . . . . . . . 133 16.8. GSSAPI Service Name . . . . . . . . . . . . . . . . . . 133 16.9. Port Numbers . . . . . . . . . . . . . . . . . . . . . . 133 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 134 17.1. Normative References . . . . . . . . . . . . . . . . . . 134 17.2. Informative References . . . . . . . . . . . . . . . . . 136 Appendix A. Nodeprep . . . . . . . . . . . . . . . . . . . . . . 139 A.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 139 A.2. Character Repertoire . . . . . . . . . . . . . . . . . . 140 A.3. Mapping . . . . . . . . . . . . . . . . . . . . . . . . 140 A.4. Normalization . . . . . . . . . . . . . . . . . . . . . 140 A.5. Prohibited Output . . . . . . . . . . . . . . . . . . . 140 A.6. Bidirectional Characters . . . . . . . . . . . . . . . . 141 Appendix B. Resourceprep . . . . . . . . . . . . . . . . . . . . 141 B.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 141 B.2. Character Repertoire . . . . . . . . . . . . . . . . . . 142 B.3. Mapping . . . . . . . . . . . . . . . . . . . . . . . . 142 B.4. Normalization . . . . . . . . . . . . . . . . . . . . . 142 B.5. Prohibited Output . . . . . . . . . . . . . . . . . . . 142 B.6. Bidirectional Characters . . . . . . . . . . . . . . . . 142 Appendix C. XML Schemas . . . . . . . . . . . . . . . . . . . . 143 C.1. Streams namespace . . . . . . . . . . . . . . . . . . . 143 C.2. Stream error namespace . . . . . . . . . . . . . . . . . 144 C.3. STARTTLS namespace . . . . . . . . . . . . . . . . . . . 147 C.4. SASL namespace . . . . . . . . . . . . . . . . . . . . . 147 C.5. Resource binding namespace . . . . . . . . . . . . . . . 149 Saint-Andre Expires December 4, 2008 [Page 7] Internet-Draft XMPP Core June 2008 C.6. Stanza error namespace . . . . . . . . . . . . . . . . . 151 Appendix D. Contact Addresses . . . . . . . . . . . . . . . . . 152 Appendix E. Account Provisioning . . . . . . . . . . . . . . . . 153 Appendix F. Differences From RFC 3920 . . . . . . . . . . . . . 153 Appendix G. Copying Conditions . . . . . . . . . . . . . . . . . 154 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 155 Intellectual Property and Copyright Statements . . . . . . . . . 156 Saint-Andre Expires December 4, 2008 [Page 8] Internet-Draft XMPP Core June 2008 1. Introduction 1.1. Overview The Extensible Messaging and Presence Protocol (XMPP) is an application profile of the Extensible Markup Language [XML] for streaming XML data in close to real time between any two (or more) network-aware entities. XMPP is typically used to exchange messages, share presence information, and engage in structured request-response interactions. The basic syntax and semantics of XMPP were developed originally within the Jabber open-source community, mainly in 1999. In late 2002, the XMPP Working Group was chartered with developing an adaptation of the core Jabber protocol that would be suitable as an IETF instant messaging (IM) and presence technology. As a result of work by the XMPP WG, [RFC3920] and [RFC3921] were published in October 2004, representing the most complete definition of XMPP at that time. As a result of extensive implementation and deployment experience with XMPP since 2004, as well as more formal interoperability testing carried out under the auspices of the XMPP Standards Foundation (XSF), this document reflects consensus from the XMPP developer community regarding XMPP's core XML streaming technology. In particular, this document incorporates the following backward- compatible changes from RFC 3920: o Corrections and errata o Additional examples throughout o Clarifications and more complete specification of matters that were underspecified o Modifications to reflect updated technologies for which XMPP is a using protocol, e.g., Transport Layer Security (TLS) and the Simple Authentication and Security Layer (SASL) o Definition of several additional stream, stanza, and SASL error conditions o Addition of TLS plus the SASL PLAIN mechanism [PLAIN] as a mandatory-to-implement technology o Definition of optional support for multiple resources over the same connection o Removal of historical documentation for the server dialback protocol from this specification to a separate specification Therefore, this document defines the core features of XMPP 1.0 and obsoletes RFC 3920. Note: The XMPP extensions required to provide the basic instant messaging and presence functionality defined in [IMP-REQS] are specified in [XMPP-IM]. Saint-Andre Expires December 4, 2008 [Page 9] Internet-Draft XMPP Core June 2008 1.2. Functional Summary This non-normative section provides a developer-friendly, functional summary of XMPP; refer to the sections that follow for a normative definition of XMPP. The purpose of XMPP is to enable the exchange of relatively small pieces of structured data (called "XML stanzas") over a network between any two (or more) entities. XMPP is implemented using a client-server architecture, wherein a client must connect to a server in order to gain access to the network and thus be allowed to exchange XML stanzas with other entities. The process whereby a client connects to a server, exchanges XML stanzas, and ends the connection is: 1. Determine the hostname and port at which to connect 2. Open a TCP connection 3. Open an XML stream 4. Complete TLS negotiation for channel encryption (recommended) 5. Complete SASL negotiation for authentication 6. Bind a resource to the stream 7. Exchange an unbounded number of XML stanzas with other entities on the network 8. Close the XML stream 9. Close the TCP connection In the sections following discussion of XMPP architecture and XMPP addresses, this document specifies how clients connect to servers and specifies the basic semantics of XML stanzas. However, this document does not define the "payloads" of the XML stanzas that might be exchanged once a connection is successfully established; instead, definition of such semantics is provided by XMPP extensionsl. For example, [XMPP-IM] defines extensions for basic instant messaging and presence functionality. In addition, various specifications produced in the XSF's XEP series [XEP-0001] define extensions for a wide range of more advanced functionality. Within the client-server architecture used by XMPP, one server may optionally connect to another server to enable inter-domain or inter- server communication. For this to happen, the two servers must negotiate a connection between themselves and then exchange XML stanzas; the process for doing so is: 1. Determine the hostname and port at which to connect 2. Open a TCP connection 3. Open an XML stream Saint-Andre Expires December 4, 2008 [Page 10] Internet-Draft XMPP Core June 2008 4. Complete TLS negotiation for channel encryption (recommended) 5. Complete SASL negotiation for authentication 6. Exchange an unbounded number of XML stanzas both directly for the servers and indirectly on behalf of entities associated with each server (e.g., connected clients) 7. Close the XML stream 8. Close the TCP connection Note: Depending on local service policies, a service may wish to use the older server dialback protocol to provide weak identity verification in cases where SASL negotiation would not result in strong authentication (e.g., because the certificate presented by the peer service during TLS negotiation is self-signed and thus provides only weak identity); for details, see [XEP-0220]. 1.3. Conventions The following keywords are to be interpreted as described in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL". In examples, lines have been wrapped for improved readability, "[...]" means elision, and the following prepended strings are used: o C: = client o E: = any XMPP entity o I: = initiating entity o P: = peer server o R: = receiving entity o S: = server o S1: = server1 o S2: = server2 1.4. Discussion Venue The editor welcomes discussion and comments related to the topics presented in this document. The preferred forum is the mailing list, for which archives and subscription information are available at <>. 2. Architecture 2.1. Overview XMPP assumes a client-server architecture, wherein a client utilizing XMPP accesses a server (normally over a [TCP] connection) and servers Saint-Andre Expires December 4, 2008 [Page 11] Internet-Draft XMPP Core June 2008 can also communicate with each other over TCP connections. A simplified architectural diagram for a typical deployment is shown here, where the entities have the following significance: o romeo@example.net -- an XMPP user. o example.net -- an XMPP server. o im.example.com -- an XMPP server. o juliet@im.example.com -- an XMPP user. example.net ---------------- im.example.com | | | | romeo@example.net juliet@im.example.com Note: Architectures that employ the syntax of XML stanzas (Section 9) but that establish peer-to-peer connections directly between clients using technologies based on [LINKLOCAL] have been deployed, but such architectures are not XMPP and are best described as "XMPP-like"; for details, see [XEP-0174]. 2.2. Server A SERVER is an entity whose primary responsibilities are to: o Manage XML streams (Section 5) with local clients and deliver XML stanzas (Section 9) to those clients over the negotiated XML streams. o Subject to local service policies on server-to-server communication, manage XML streams (Section 5) with foreign servers and route XML stanzas (Section 9) to those servers over the negotiated XML streams. Depending on the application, the secondary responsibilities of an XMPP server may include: o Storing XML data that is used by clients (e.g., contact lists for users of XMPP-based instant messaging and presence applications); in this case, the relevant XML stanza is handled directly by the server itself on behalf of the client and is not routed to a foreign server or delivered to a local entity. o Hosting local services that also use XMPP as the basis for communication but that provide additional functionality beyond that defined in this document or in [XMPP-IM]; examples include multi-user conferencing services as specified in [XEP-0045] and publish-subscribe services as specified in [XEP-0060]. Saint-Andre Expires December 4, 2008 [Page 12] Internet-Draft XMPP Core June 2008 2.3. Client A CLIENT is an entity that establiishes an XML stream with a server by authenticating using the credentials of a local account and that then completes resource binding (Section 8) in order to enable delivery of XML stanzas via the server to the client. A client then uses XMPP to communicate with its server, other clients, and any other accessible entities on a network. Multiple clients may connect simultaneously to a server on behalf of a local account, where each client is differentiated by the resource identifier portion of an XMPP address (e.g., vs. ), as defined under Section 3 and Section 8. The RECOMMENDED port for TCP connections between a client and a server is 5222, as registered with the IANA (see Section 16.9). 2.4. Network Because each server is identified by a network address and because server-to-server communication is a straightforward extension of the client-to-server protocol, in practice the system consists of a network of servers that inter-communicate. Thus, for example, is able to exchange messages, presence, and other information with . This pattern is familiar from messaging protocols (such as [SMTP]) that make use of network addressing standards. Communication between any two servers is OPTIONAL. If enabled, such communication SHOULD occur over XML streams that are bound to [TCP] connections. The RECOMMENDED port for TCP connections between servers is 5269, as registered with the IANA (see Section 16.9). 3. Addresses 3.1. Overview An ENTITY is anything that is network-addressable and that can communicate using XMPP. For historical reasons, the native address of an XMPP entity is called a JABBER IDENTIFIER or JID. A valid JID contains a set of ordered elements formed of an XMPP domain identifier, node identifier, and resource identifier. The syntax for a JID is defined as follows using the Augmented Backus-Naur Form as specified in [ABNF]. Saint-Andre Expires December 4, 2008 [Page 13] Internet-Draft XMPP Core June 2008 jid = [ node "@" ] domain [ "/" resource ] node = 1*(nodepoint) ; a "nodepoint" is a UTF-8 encoded Unicode code ; point that satisfies the Nodeprep profile of ; stringprep domain = fqdn / address-literal / idnlabel fqdn = (idnlabel 1*("." idnlabel)) ; an "idnlabel" is an internationalized label ; as described in RFC 3490 address-literal = IPv4address / IPv6address ; the "IPv4address" and "IPv6address" rules are ; defined in Appendix B of RFC 2373 resource = 1*(resourcepoint) ; a "resourcepoint" is a UTF-8 encoded Unicode ; code point that satisfies the Resourceprep ; profile of stringprep Note: The "IPv4address" and "IPv6address" rules are indeed provided in [RFC2373] and were removed from [IPv6], which supersedes RFC 2373. All JIDs are based on the foregoing structure. One common use of this structure is to identify a messaging and presence account, the server that hosts the account, and a connected resource (e.g., a specific device) in the form of . However, node types other than clients are possible; for example, a specific chat room offered by a multi-user conference service (see [XEP-0045]) could be addressed as (where "room" is the name of the chat room and "service" is the hostname of the multi-user conference service) and a specific occupant of such a room could be addressed as (where "nick" is the occupant's room nickname). Many other JID types are possible (e.g., could be a server-side script or service). Each allowable portion of a JID (node identifier, domain identifier, and resource identifier) MUST NOT be more than 1023 bytes in length, resulting in a maximum total size (including the '@' and '/' separators) of 3071 bytes. Note: While the format of a JID is consistent with [URI], an entity's address on an XMPP network MUST be a JID (without a URI scheme) and not a [URI] or [IRI] as specified in [XMPP-URI]; the latter specification is provided only for use by non-XMPP applications. 3.2. Domain Identifier The DOMAIN IDENTIFIER portion of a JID is that portion after the '@' character (if any) and before the '/' character (if any); it is the primary identifier and is the only REQUIRED element of a JID (a mere Saint-Andre Expires December 4, 2008 [Page 14] Internet-Draft XMPP Core June 2008 domain identifier is a valid JID). Typically a domain identifier identifies the "home" server to which clients connect for XML routing and data management functionality. (Note: A single server may service multiple domain identifiers, i.e., multiple local domains.) However, it is not necessary for an XMPP domain identifier to identify an entity that provides core XMPP server functionality (e.g., a domain identifier may identity an entity such as a multi- user conference service, a publish-subscribe service, or a user directory). The domain identifier for every server or service that will communicate over a network SHOULD be a fully qualified domain name (see [DNS]); while the domain identifier MAY be either an Internet Protocol (IPv4 or IPv6) address or a text label (commonly called an "unqualified hostname") that is resolvable on a local network, domain identifiers that are IP addresses may not be acceptable to other services for the sake of interdomain communication and domain identifiers that are text labels MUST NOT be used on public networks. If the domain identifier includes a final character considered to be a label separator (dot) by [IDNA] or [STD13], this character MUST be stripped from the domain identifier before the JID of which it is a part is used for the purpose of routing an XML stanza, comparing against another JID, or constructing an [XMPP-URI]; in particular, the character should be stripped before any other canonicalization steps are taken (such as application of the [NAMEPREP] profile of [STRINGPREP] or completion of the ToASCII operation as described in [IDNA]). A domain identifier MUST be an "internationalized domain name" as defined in [IDNA], that is, "a domain name in which every label is an internationalized label". When preparing a text label (consisting of a sequence of Unicode code points) for representation as an internationalized label in the process of constructing an XMPP domain identifier or comparing two XMPP domain identifiers, an application MUST ensure that for each text label it is possible to apply without failing the ToASCII operation specified in [IDNA] with the UseSTD3ASCIIRules flag set (thus forbidding ASCII code points other than letters, digits, and hyphens). If the ToASCII operation can be applied without failing, then the label is an internationalized label. An internationalized domain name (and therefore an XMPP domain identifier) is constructed from its constituent internationalized labels by following the rules specified in [IDNA]. (Note: The ToASCII operation includes application of the [NAMEPREP] profile of [STRINGPREP] and encoding using the algorithm specified in [PUNYCODE]; for details, see [IDNA].) Saint-Andre Expires December 4, 2008 [Page 15] Internet-Draft XMPP Core June 2008 3.3. Node Identifier The NODE IDENTIFIER portion of a JID is an optional secondary identifier placed before the domain identifier and separated from the latter by the '@' character. Typically a node identifier uniquely identifies the entity requesting and using network access provided by a server (i.e., a local account), although it can also represent other kinds of entities (e.g., a chat room associated with a multi- user conference service). The entity represented by an XMPP node identifier is addressed within the context of a specific domain. When the domain is an XMPP server and the entity is a local account on the server, the resulting address (of the form ) is called a BARE JID. A node identifier MUST be formatted such that the Nodeprep profile of [STRINGPREP] can be applied without failing (see Appendix A). Before comparing two node identifiers, an application MUST first apply the Nodeprep profile to each identifier. Note: Because the additional characters prohibited by Nodeprep (see Appendix A) are prohibited after normalization, an implementation should not enable a human user to input any Unicode code point whose decomposition includes those characters; such code points include but are not necessarily limited to the following (refer to [UNICODE] for further information). o 2100 (ACCOUNT OF) o 2101 (ADDRESSED TO THE SUBJECT) o 2105 (CARE OF) o 2106 (CADA UNA) o 226E (NOT LESS-THAN) o 226F (NOT GREATER-THAN) o 2A74 (DOUBLE COLON EQUAL) o FE13 (SMALL COLON) o FE60 (SMALL AMPERSAND) o FE64 (SMALL LESS-THAN SIGN) o FE65 (SMALL GREATER-THAN SIGN) o FE6B (SMALL COMMERCIAL AT) o FF02 (FULLWIDTH QUOTATION MARK) o FF06 (FULLWIDTH AMPERSAND) o FF07 (FULLWIDTH APOSTROPHE) o FF0F (FULLWIDTH SOLIDUS) o FF1A (FULLWIDTH COLON) o FF1C (FULLWIDTH LESS-THAN SIGN) o FF1E (FULLWIDTH GREATER-THAN SIGN) o FF20 (FULLWIDTH COMMERCIAL AT) Saint-Andre Expires December 4, 2008 [Page 16] Internet-Draft XMPP Core June 2008 3.4. Resource Identifier The RESOURCE IDENTIFIER portion of a JID is an optional tertiary identifier placed after the domain identifier and separated from the latter by the '/' character. A resource identifier may modify either a address or a mere address. Typically a resource identifier uniquely identifies a specific connection (e.g., a device or location) or object (e.g., a participant in a multi-user conference room) belonging to the entity associated with an XMPP node identifier at a local domain. XMPP entities SHOULD consider resource identifiers to be opaque strings and SHOULD NOT impute meaning to any given resource identifier. A resource identifier is negotiated between a client and a server during resource binding (Section 8), after which the entity is referred to as a CONNECTED RESOURCE and its address (of the form ) is referred to as a FULL JID. An entity MAY maintain multiple connected resources simultaneously, with each connected resource differentiated by a distinct resource identifier. A resource identifier MUST be formatted such that the Resourceprep profile of [STRINGPREP] can be applied without failing (see Appendix B). Before comparing two resource identifiers, an application MUST first apply the Resourceprep profile to each identifier. 3.5. Determination of Addresses After SASL negotiation (Section 7) and, if appropriate, resource binding (Section 8), the receiving entity for a stream MUST determine the initiating entity's JID. For server-to-server communication, the initiating entity's JID SHOULD be the authorization identity (as defined by [SASL]), either (1) as directly communicated by the initiating entity during SASL negotiation (Section 7) or (2) as derived from the authentication identity if no authorization identity was specified during SASL negotiation (Section 7). For client-to-server communication, the client's bare JID () SHOULD be the authorization identity (as defined by [SASL]), either (1) as directly communicated by the initiating entity during SASL negotiation (Section 7) or (2) as derived from the authentication identity if no authorization identity was specified during SASL negotiation (Section 7). The resource identifier portion of the full JID () SHOULD be the resource identifier negotiated by the client and server during resource binding (Section 8). Saint-Andre Expires December 4, 2008 [Page 17] Internet-Draft XMPP Core June 2008 The receiving entity MUST ensure that the resulting JID (including node identifier, domain identifier, resource identifier, and separator characters) conforms to the rules and formats defined earlier in this section; to meet this restriction, the receiving entity may need to replace the JID sent by the initiating entity with the canonicalized JID as determined by the receiving entity. 4. TCP Binding 4.1. Scope As XMPP is defined in this specification, an initiating entity (client or server) MUST open a Transmission Control Protocol [TCP] connection at the receiving entity (server) before it negotiates XML streams with the receiving entity. The rules specified in the following sections apply to the TCP binding. 4.2. Hostname Resolution Before opening the TCP connection, the initiating entity first MUST resolve the Domain Name System (DNS) hostname associated with the receiving entity and determine the appropriate TCP port for communication with the receiving entity. The process is: 1. Attempt to resolve the hostname using a [DNS-SRV] Service of "xmpp-client" (for client-to-server connections) or "xmpp-server" (for server-to-server connections) and Proto of "tcp", resulting in resource records such as "_xmpp-client._tcp.xmpp.example.net." or "_xmpp-server._tcp.im.example.com.". The result of the SRV lookup will be one or more combinations of a port and hostname; the initiating entity MUST resolve one of the hostnames in order to determine an IP address at which to connect. 2. If the SRV lookup fails, the fallback SHOULD be a normal IPv4 or [IPv6] address record resolution to determine the IP address, where the port used is the "xmpp-client" port of 5222 for client- to-server connections or the "xmpp-server" port 5269 for server- to-server connections. 3. For client-to-server connections, the fallback MAY be a [DNS-TXT] lookup for alternative connection methods, for example as described in [XEP-0156]. Note: Many XMPP servers are implemented in such a way that they can host additional services (byond those defined in this specification and [XMPP-IM]) at hostnames that are subdomains of the hostname of the main XMPP service (e.g., conference.example.net for a [XEP-0045] service associated with the example.net XMPP service) or subdomains of the first-level domain of the underlying host (e.g., Saint-Andre Expires December 4, 2008 [Page 18] Internet-Draft XMPP Core June 2008 muc.example.com for a [XEP-0045] service associated with the im.example.com XMPP service). If an entity from a remote domain wishes to use such additional services, it would generate an appropriate XML stanza and the remote domain itself would attempt to resolve the service's hostname via an SRV lookup on resource records such as "_xmpp-server._tcp.conference.example.net." or "_xmpp- server._tcp.muc.example.com.". Therefore if a service wishes to enable entities from remote domains to acess these additional services it should advertise the appropriate "_xmpp-server" SRV records in addition to the "_xmpp-server" record for its main XMPP service. 4.3. Client-to-Server Communications Because a client is subordinate to a server and therefore a client authenticates to the server but the server does not authenticate to the client, it is necessary to have only one TCP connection between client and server. Thus the server MUST allow the client to share a single TCP connection for XML stanzas sent from client to server and from server to client (i.e., the inital stream and response stream as specified under Section 5). 4.4. Server-to-Server Communications Because two servers are peers and therefore each peer must authenticate with the other, the servers MUST use two TCP connections: one for XML stanzas sent from the first server to the second server and another (initiated by the second server) for XML stanzas from the second server to the first server. This rule applies only to XML stanzas (Section 9). Therefore during STARTTLS negotiation (Section 6) and SASL negotiation (Section 7) the servers would use one TCP connection, but after stream setup that TCP connection would be used only for the initiating server to send XML stanzas to the receiving server. In order for the receiving server to send XML stanzas to the initiating server, the receiving server would need to reverse the roles and negotiate an XML stream from the receiving server to the initiating server. 4.5. Reconnection It can happen that an XMPP server goes offline while servicing TCP connections from local clients and from other servers. Because the number of such connections can be quite large, the reconnection algorithm employed by entities that seek to reconnect can have a significant impact on software and network performance. The following guidelines are RECOMMENDED: Saint-Andre Expires December 4, 2008 [Page 19] Internet-Draft XMPP Core June 2008 o The time to live (TTL) specified in Domain Name System records SHOULD be honored, even if DNS results are cached; if the TTL has not expired, an entity that seeks to reconnect SHOULD NOT re- resolve the server hostname before reconnecting. o The time that expires before an entity first seeks to reconnect SHOULD be randomized (e.g., so that all clients do not attempt to reconnect 30 seconds after being disconnected). o If the first reconnection attempt does not succeed, an entity SHOULD back off exponentially on the time between subsequent reconnection attempts. 4.6. Other Bindings There is no necessary coupling of an XML stream to a TCP connection. For example, two entities could connect to each other via another transport, such as [HTTP] as specified in [XEP-0124] and [XEP-0206]. However, this specification defines a binding of XMPP to TCP only. 5. XML Streams 5.1. Overview Two fundamental concepts make possible the rapid, asynchronous exchange of relatively small payloads of structured information between presence-aware entities: XML streams and XML stanzas. These terms are defined as follows. Definition of XML Stream: An XML STREAM is a container for the exchange of XML elements between any two entities over a network. The start of an XML stream is denoted unambiguously by an opening STREAM HEADER (i.e., an XML tag with appropriate attributes and namespace declarations), while the end of the XML stream is denoted unambiguously by a closing XML tag. During the life of the stream, the entity that initiated it can send an unbounded number of XML elements over the stream, either elements used to negotiate the stream (e.g., to complete TLS negotiation (Section 6) or SASL negotiation (Section 7)) or XML stanzas. The INITIAL STREAM is negotiated from the initiating entity (typically a client or server) to the receiving entity (typically a server), and can be seen as corresponding to the initiating entity's "connection" or "session" with the receiving entity. The initial stream enables unidirectional communication from the initiating entity to the receiving entity; in order to enable information exchange from the receiving entity to the initiating entity, the receiving entity MUST negotiate a stream in the opposite direction (the RESPONSE STREAM). Saint-Andre Expires December 4, 2008 [Page 20] Internet-Draft XMPP Core June 2008 Definition of XML Stanza: An XML STANZA is a discrete semantic unit of structured information that is sent from one entity to another over an XML stream. An XML stanza is the basic unit of meaning in XMPP. An XML stanza exists at the direct child level of the root element and is said to be well-balanced if it matches the production [43] content of [XML]. The start of any XML stanza is denoted unambiguously by the element start tag at depth=1 of the XML stream (e.g., ), and the end of any XML stanza is denoted unambiguously by the corresponding close tag at depth=1 (e.g., ); a server MUST NOT process a partial stanza and MUST NOT attach meaning to the transmission timing of any part of a stanza (before receipt of the close tag). The only XML stanzas defined herein are the , , and elements qualified by the default namespace for the stream, as described under Section 9; an XML element sent for the purpose of TLS negotiation (Section 6) or SASL negotiation (Section 7) is not considered to be an XML stanza. An XML stanza MAY contain child elements (with accompanying attributes, elements, and XML character data) as necessary in order to convey the desired information, which MAY be qualified by any XML namespace (see [XML-NAMES] as well as Section 9.4 herein). Consider the example of a client's connection to a server. In order to connect to a server, a client MUST initiate an XML stream by sending a stream header to the server, optionally preceded by a text declaration specifying the XML version and the character encoding supported (see Section 12.5 and Section 12.6). Subject to local policies and service provisioning, the server SHOULD then reply with a second XML stream back to the client, again optionally preceded by a text declaration. Once the client has completed SASL negotiation (Section 7) and resource binding (Section 8), the client MAY send an unbounded number of XML stanzas over the stream. When the client desires to close the stream, it simply sends a closing tag to the server (see Section 5.6). In essence, then, an XML stream acts as an envelope for all the XML stanzas sent during a connection. We can represent this in a simplistic fashion as follows. Saint-Andre Expires December 4, 2008 [Page 21] Internet-Draft XMPP Core June 2008 +--------------------+ | | |--------------------| | | | | | | |--------------------| | | | | | | |--------------------| | | | | | | |--------------------| | | | | | | |--------------------| | [ ... ] | |--------------------| | | +--------------------+ Note: Those who are accustomed to thinking of XML in a document- centric manner may wish to view a client's connection to a server as consisting of two open-ended XML documents: one from the client to the server and one from the server to the client. From this perspective, the root element can be considered the document entity for each "document", and the two "documents" are built up through the accumulation of XML stanzas sent over the two XML streams. However, this perspective is a convenience only; XMPP does not deal in documents but in XML streams and XML stanzas. 5.2. Stream Security For the purpose of stream security, both Transport Layer Security (see Section 6) and the Simple Authentication and Security Layer (see Section 7) are mandatory to implement. When negotiating XML streams in XMPP 1.0, TLS SHOULD be used as defined under Section 6 and SASL MUST be used as defined under Section 7. The initial stream and the response stream MUST be secured separately, although security in both directions MAY be established via mechanisms that provide mutual authentication. The initiating entity SHOULD NOT attempt to send XML stanzas (Section 9) over the stream before the stream has been authenticated. Saint-Andre Expires December 4, 2008 [Page 22] Internet-Draft XMPP Core June 2008 However, if it does attempt to do so, the receiving entity MUST NOT accept such stanzas and MUST return a stream error and then terminate both the XML stream and the underlying TCP connection. Note: This applies to XML stanzas only (i.e., , , and elements qualified by the default namespace) and not to XML elements used for stream negotiation (e.g., elements used to complete TLS negotiation (Section 6) or SASL negotiation (Section 7)). 5.3. Stream Attributes The attributes of the root element are as follows. 5.3.1. from In client-to-server communication, the 'from' attribute SHOULD be included in the initial stream header and (if included) MUST be set to the account name (i.e., bare JID = ) of the entity controlling the client. C: In server-to-server communication, the 'from' attribute SHOULD be included in the initial stream header and (if included) MUST be set to a hostname serviced by the initiating entity. P: In both client-to-server and server-to-server communications, the 'from' attribute MUST be included in the response stream header and MUST be set to a hostname serviced by the receiving entity that is granting access to the initiating entity. Saint-Andre Expires December 4, 2008 [Page 23] Internet-Draft XMPP Core June 2008 S: Note: Each entity MUST verify the identity of the other entity before exchanging XML stanzas with it (see Section 15.3 and Section 15.4). 5.3.2. to In both client-to-server and server-to-server communications, the 'to' attribute SHOULD be included in the initial stream header and (if included) MUST be set to a hostname serviced by the receiving entity. C: In client-to-server communication, if the client included a 'from' address in the initial stream header then the server SHOULD include a 'to' attribute in the response stream header and (if included) MUST set the 'to' attribute to the bare JID specified in the 'from' attribute of the initial stream header. S: In server-to-server communication, if the initiating entity included a 'from' address in the initial stream header then the receiving entity SHOULD include a 'to' attribute in the response stream header Saint-Andre Expires December 4, 2008 [Page 24] Internet-Draft XMPP Core June 2008 and (if included) MUST set the 'to' attribute to the hostname specified in the 'from' attribute of the initial stream header. S: Note: Each entity MUST verify the identity of the other entity before exchanging XML stanzas with it (see Section 15.3 and Section 15.4). 5.3.3. id There SHOULD NOT be an 'id' attribute in the initial stream header; however, if an 'id' attribute is included, it SHOULD be silently ignored by the receiving entity. C: The 'id' attribute MUST be included in the response XML stream header. This attribute is a unique identifier created by the receiving entity to function as a identifier for the initiating entity's two streams with the receiving entity, and MUST be unique within the receiving application (normally a server). S: Note: The stream ID may be security-critical and therefore MUST be Saint-Andre Expires December 4, 2008 [Page 25] Internet-Draft XMPP Core June 2008 both unpredictable and nonrepeating (see [RANDOM] for recommendations regarding randomness for security purposes). 5.3.4. xml:lang An 'xml:lang' attribute (as defined in Section 2.12 of [XML]) SHOULD be included in the initial stream header to specify the default language of any human-readable XML character data it sends over that stream. C: If the attribute is included, the receiving entity SHOULD remember that value as the default for both the initial stream and the response stream; if the attribute is not included, the receiving entity SHOULD use a configurable default value for both streams, which it MUST communicate in the response stream header. S: For all stanzas sent over the initial stream, if the initiating entity does not include an 'xml:lang' attribute, the receiving entity SHOULD apply the default value; if the initiating entity does include an 'xml:lang' attribute, the receiving entity MUST NOT modify or delete it (see also Section 9.1.5). The value of the 'xml:lang' attribute MUST conform to the NMTOKEN datatype (as defined in Section 2.3 of [XML]) and MUST conform to the format defined in [LANGTAGS]. 5.3.5. version The presence of the version attribute set to a value of at least "1.0" signals support for the stream-related protocols (including stream features) defined in this specification. Saint-Andre Expires December 4, 2008 [Page 26] Internet-Draft XMPP Core June 2008 The version of XMPP specified herein is "1.0"; in particular, XMPP 1.0 encapsulates the stream-related protocols (TLS negotiation (Section 6), SASL negotiation (Section 7), and stream errors (Section 5.7)), as well as the basic semantics of the three defined XML stanza types (, , and ). The numbering scheme for XMPP versions is ".". The major and minor numbers MUST be treated as separate integers and each number MAY be incremented higher than a single digit. Thus, "XMPP 2.4" would be a lower version than "XMPP 2.13", which in turn would be lower than "XMPP 12.3". Leading zeros (e.g., "XMPP 6.01") MUST be ignored by recipients and MUST NOT be sent. The major version number should be incremented only if the stream and stanza formats or required actions have changed so dramatically that an older version entity would not be able to interoperate with a newer version entity if it simply ignored the elements and attributes it did not understand and took the actions specified in the older specification. The minor version number should be incremented only if significant new capabilities have been added to the core protocol (e.g., a newly defined value of the 'type' attribute for message, presence, or IQ stanzas). The minor version number MUST be ignored by an entity with a smaller minor version number, but MAY be used for informational purposes by the entity with the larger minor version number (e.g., the entity with the larger minor version number would simply note that its correspondent would not be able to understand that value of the 'type' attribute and therefore would not send it). The following rules apply to the generation and handling of the 'version' attribute within stream headers: 1. The initiating entity MUST set the value of the 'version' attribute in the initial stream header to the highest version number it supports (e.g., if the highest version number it supports is that defined in this specification, it MUST set the value to "1.0"). 2. The receiving entity MUST set the value of the 'version' attribute in the response stream header to either the value supplied by the initiating entity or the highest version number supported by the receiving entity, whichever is lower. The receiving entity MUST perform a numeric comparison on the major and minor version numbers, not a string match on ".". 3. If the version number included in the response stream header is at least one major version lower than the version number included in the initial stream header and newer version entities cannot Saint-Andre Expires December 4, 2008 [Page 27] Internet-Draft XMPP Core June 2008 interoperate with older version entities as described, the initiating entity SHOULD generate an stream error and terminate the XML stream and underlying TCP connection. 4. If either entity receives a stream header with no 'version' attribute, the entity MUST consider the version supported by the other entity to be "0.9" and SHOULD NOT include a 'version' attribute in the response stream header. 5.3.6. Summary We can summarize the attributes of the root element as follows. +----------+--------------------------+-------------------------+ | | initiating to receiving | receiving to initiating | +----------+--------------------------+-------------------------+ | to | JID of receiver | JID of initiator | | from | JID of initiator | JID of receiver | | id | silently ignored | stream identifier | | xml:lang | default language | default language | | version | XMPP 1.0+ supported | XMPP 1.0+ supported | +----------+--------------------------+-------------------------+ Note: The attributes of the root element are not prepended by a 'stream:' prefix because, in accordance with Section 5.3 of [XML-NAMES], the default namespace does not apply to attribute names. 5.4. Namespace Declarations The stream element MUST possess both a streams namespace declaration and a default namespace declaration (as "namespace declaration" is defined in [XML-NAMES]). For detailed information regarding the streams namespace and default namespace, see Section 12.2. 5.5. Stream Features If the initiating entity includes the 'version' attribute set to a value of at least "1.0" in the initial stream header, after sending the response stream header the receiving entity MUST send a child element (prefixed by the streams namespace prefix) to the initiating entity in order to announce any stream-level features that can be negotiated (or capabilities that otherwise need to be advertised). Saint-Andre Expires December 4, 2008 [Page 28] Internet-Draft XMPP Core June 2008 S: S: Stream features are used mainly to advertise TLS negotiation (Section 6), SASL negotiation (Section 7), and resource binding (Section 8); however, stream features also can be used to advertise features associated with various XMPP extensions. If an entity does not understand or support a feature, it SHOULD silently ignore the associated feature. If one or more security features (e.g., TLS and SASL) need to be successfully negotiated before a non-security-related feature (e.g., resource binding) can be offered, the non-security-related feature SHOULD NOT be included in the stream features that are advertised before the relevant security features have been negotiated. If a feature must be negotiated before the initiating entity may proceed, that feature SHOULD include a child element and the receiving entity SHOULD NOT advertize any other stream features until the required feature has been negotiated. The order of child elements contained in any given element is not significant. After completing negotiation of any stream feature (even stream features that do not require a stream restart), the receiving entity MUST send an updated list of stream features to the initiating entity. However, if there are no features to be advertised (e.g., in the stream reset initiated after successful SASL negotiation for a server-to-server connection, or after resource binding for a client- to-server stream) then the receiving entity MUST send an empty element. Saint-Andre Expires December 4, 2008 [Page 29] Internet-Draft XMPP Core June 2008 S: S: 5.6. Closing Streams An XML stream between two entities can be closed because a stream error has occurred or in some cases in the absence of an error. Where possible, it is preferable to trigger a stream close only because a stream error has occurred. 5.6.1. With Stream Error If a stream error has occurred, the entity that detects the error MUST close the stream as described under Section 5.7.1. 5.6.2. Without Stream Error At any time after XML streams have been negotiated between two entities, either entity MAY close its stream to the other party in the absence of a stream error by sending a closing stream tag: P: The entity that sends the closing stream tag SHOULD wait for the other party to also close its stream: S: However, the entity that sends the first closing stream tag MAY consider both streams to be void if the other party does not send its closing stream tag within a reasonable amount of time (where the definition of "reasonable" is left up to the implementation or deployment). After an entity sends a closing stream tag, it MUST NOT send further data over that stream. After the entity that sent the first closing stream tag receives a reciprocal closing stream tag from the other party (or if it considers the stream to be void after a reasonable amount of time), Saint-Andre Expires December 4, 2008 [Page 30] Internet-Draft XMPP Core June 2008 it MUST terminate the underlying TCP connection or connections. 5.6.3. Handling of Idle Streams An XML stream can become idle, i.e., neither entity has sent XMPP traffic over the stream for some period of time (usually at least several minutes). A server MAY close an idle stream with a local client or remote server. The idle timeout period is a matter of implementation and local service policy; however, it is RECOMMENDED to be liberal in accepting idle streams, since experience has shown that doing so improves the reliability of communications over XMPP networks. In particular, it is typically more efficient to maintain a stream between two servers than it is to aggressively timeout such a stream, especially with regard to synchronization of presence information as described in [XMPP-IM], so it is RECOMMENDED to maintain such a stream since experience has shown that server-to- server streams are cyclical and typically need to be re-established every 24 to 48 hours if they are timed out. An XML stream can appear idle at the XMPP level because the undelying TCP connection has become idle (e.g., a client's network connection has been lost). The typical method for detecting an idle TCP connection is to send a white space character over the TCP connection between XML stanzas, which is allowed for XML streams as described under Section 12.7. The time between such "whitespace pings" is a matter of implementation and local service policy; however, it is RECOMMENDED that these pings be sent not more than once every 60 seconds. 5.7. Stream Errors The root stream element MAY contain an child element that is prefixed by the streams namespace prefix. The error child shall be sent by a compliant entity if it perceives that a stream-level error has occurred. 5.7.1. Rules The following rules apply to stream-level errors. 5.7.1.1. Stream Errors Are Unrecoverable Stream-level errors are unrecoverable. Therefore, if an error occurs at the level of the stream, the entity that detects the error MUST send a stream error to the other entity, send a closing tag, and immediately terminate the underlying TCP connection. Saint-Andre Expires December 4, 2008 [Page 31] Internet-Draft XMPP Core June 2008 C: S: 5.7.1.2. Stream Errors Can Occur During Setup If the error occurs while the stream is being set up, the receiving entity MUST still send the opening tag, include the element as a child of the stream element, send the closing tag, and immediately terminate the underlying TCP connection. C: S: 5.7.1.3. Stream Errors When the Host is Unspecified If the initiating entity provides no 'to' attribute or provides an unknown host in the 'to' attribute and the error occurs during stream setup, the receiving entity SHOULD provide its authoritative hostname in the 'from' attribute of the stream header sent before termination. Saint-Andre Expires December 4, 2008 [Page 32] Internet-Draft XMPP Core June 2008 C: S: 5.7.2. Syntax The syntax for stream errors is as follows, where "defined-condition" is a placeholder for one of the conditions defined under Section 5.7.3. [ [ ... descriptive text ... ] ] [application-specific condition element] The element: o MUST contain a child element corresponding to one of the defined stream error conditions (Section 5.7.3); this element MUST be qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace. o MAY contain a child element containing XML character data that describes the error in more detail; this element MUST be qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace and SHOULD possess an 'xml:lang' attribute specifying the natural language of the XML character data. Saint-Andre Expires December 4, 2008 [Page 33] Internet-Draft XMPP Core June 2008 o MAY contain a child element for an application-specific error condition; this element MUST be qualified by an application- defined namespace, and its structure is defined by that namespace (see Section 5.7.4). The element is OPTIONAL. If included, it SHOULD be used only to provide descriptive or diagnostic information that supplements the meaning of a defined condition or application-specific condition. It SHOULD NOT be interpreted programmatically by an application. It SHOULD NOT be used as the error message presented to a human user, but MAY be shown in addition to the error message associated with the included condition element or elements. 5.7.3. Defined Stream Error Conditions The following stream-level error conditions are defined. 5.7.3.1. bad-format The entity has sent XML that cannot be processed. (In the following example, the client sends an XMPP message that is not well-formed XML.) C: No closing body tag! S: This error MAY be used instead of the more specific XML-related errors, such as , , , , and . However, the more specific errors are preferred. 5.7.3.2. bad-namespace-prefix The entity has sent a namespace prefix that is unsupported, or has sent no namespace prefix on an element that requires such a prefix (see Section 12.2). (In the following example, the client specifies a namespace prefix of "foobar" for the XML streams namespace.) Saint-Andre Expires December 4, 2008 [Page 34] Internet-Draft XMPP Core June 2008 C: S: 5.7.3.3. conflict The server is either (1) closing the existing stream for this entity because a new stream has been initiated that conflicts with the existing stream, or (2) is refusing a new stream for this entity because allowing the new stream would conflict with an existing stream (e.g., because the server allows only a certain number of connections from the same IP address). Saint-Andre Expires December 4, 2008 [Page 35] Internet-Draft XMPP Core June 2008 C: S: 5.7.3.4. connection-timeout The entity has not generated any traffic over the stream for some period of time (configurable according to a local service policy) and therefore the connection is being dropped. P: 5.7.3.5. host-gone The value of the 'to' attribute provided in the initial stream header corresponds to a hostname that is no longer hosted by the receiving entity. (In the following example, the peer specifies a 'to' address of "foo.im.example.com" when connecting to the "im.example.com" server, but the server no longer hosts a service at that address.) Saint-Andre Expires December 4, 2008 [Page 36] Internet-Draft XMPP Core June 2008 P: S: 5.7.3.6. host-unknown The value of the 'to' attribute provided in the initial stream header does not correspond to a hostname that is hosted by the receiving entity. (In the following example, the peer specifies a 'to' address of "example.org" when connecting to the "im.example.com" server, but the server knows nothing of that address.) Saint-Andre Expires December 4, 2008 [Page 37] Internet-Draft XMPP Core June 2008 P: S: 5.7.3.7. improper-addressing A stanza sent between two servers lacks a 'to' or 'from' attribute (or the attribute has no value). (In the following example, the peer sends a stanza without a 'to' address.) P: Wherefore art thou? S: 5.7.3.8. internal-server-error The server has experienced a misconfiguration or an otherwise- undefined internal error that prevents it from servicing the stream. Saint-Andre Expires December 4, 2008 [Page 38] Internet-Draft XMPP Core June 2008 S: 5.7.3.9. invalid-from The JID or hostname provided in a 'from' address does not match an authorized JID or validated domain negotiated between servers via SASL, or between a client and a server via authentication and resource binding. (In the following example, a peer that has authenticated only as "example.net" attempts to send a stanza from an address at "example.org".) P: Neither, fair saint, if either thee dislike. S: 5.7.3.10. invalid-id The stream ID or server dialback ID is invalid or does not match an ID previously provided. (In the following example, the server dialback ID is invalid; see [XEP-0220].) P: S: Saint-Andre Expires December 4, 2008 [Page 39] Internet-Draft XMPP Core June 2008 5.7.3.11. invalid-namespace The streams namespace name is something other than "http://etherx.jabber.org/streams" (see Section 12.2). (In the following example, the client specifies a streams namespace of 'http://wrong.namespace.example.org/' instead of the correct namespace of "http://etherx.jabber.org/streams".) C: S: 5.7.3.12. invalid-xml The entity has sent invalid XML over the stream to a server that performs validation (see Section 12.4). (In the following example, the peer attempts to send an IQ stanza of type "subscribe" but there is no such value for the 'type' attribute.) Saint-Andre Expires December 4, 2008 [Page 40] Internet-Draft XMPP Core June 2008 P: S: 5.7.3.13. not-authorized The entity has attempted to send XML stanzas before the stream has been authenticated, or otherwise is not authorized to perform an action related to stream negotiation; the receiving entity MUST NOT process the offending stanza before sending the stream error. (In the following example, the client attempts to send XML stanzas before authenticating with the server.) Saint-Andre Expires December 4, 2008 [Page 41] Internet-Draft XMPP Core June 2008 C: S: Wherefore art thou? S: 5.7.3.14. policy-violation The entity has violated some local service policy (e.g., the stanza exceeds a configured size limit); the server MAY choose to specify the policy in the element or an application-specific condition element. (In the following example, the client sends an XMPP message that is too large according to the server's local service policy.) C: [ ... the-emacs-manual ... ] S: S: Saint-Andre Expires December 4, 2008 [Page 42] Internet-Draft XMPP Core June 2008 5.7.3.15. remote-connection-failed The server is unable to properly connect to a remote entity that is required for authentication or authorization. C: S: 5.7.3.16. resource-constraint The server lacks the system resources necessary to service the stream. Saint-Andre Expires December 4, 2008 [Page 43] Internet-Draft XMPP Core June 2008 C: S: 5.7.3.17. restricted-xml The entity has attempted to send restricted XML features such as a comment, processing instruction, DTD, entity reference, or unescaped character (see Section 12.1). (In the following example, the client sends an XMPP message containing an XML comment.) C: This message has no subject. S: 5.7.3.18. see-other-host The server will not provide service to the initiating entity but is redirecting traffic to another host; the XML character data of the element returned by the server SHOULD specify the Saint-Andre Expires December 4, 2008 [Page 44] Internet-Draft XMPP Core June 2008 alternate hostname or IP address at which to connect, which SHOULD be a valid domain identifier but may also include a port number; if no port is specified, the initiating entity SHOULD perform a [DNS-SRV] lookup on the provided domain identifier but MAY assume that it can connect to that domain identifier at the standard XMPP ports (i.e., 5222 for client-to-server connections and 5269 for server-to-server connections). C: S: im.example.com:9090 5.7.3.19. system-shutdown The server is being shut down and all active streams are being closed. S: Saint-Andre Expires December 4, 2008 [Page 45] Internet-Draft XMPP Core June 2008 5.7.3.20. undefined-condition The error condition is not one of those defined by the other conditions in this list; this error condition SHOULD be used only in conjunction with an application-specific condition. S: 5.7.3.21. unsupported-encoding The initiating entity has encoded the stream in an encoding that is not supported by the server (see Section 12.6) or has otherwise improperly encoded the stream (e.g., by violating the rules of the UTF-8 encoding). (In the following example, the client attempts to encode data using UTF-16 instead of UTF-8.) C: S: Saint-Andre Expires December 4, 2008 [Page 46] Internet-Draft XMPP Core June 2008 5.7.3.22. unsupported-stanza-type The initiating entity has sent a first-level child of the stream that is not supported by the server or consistent with the default namespace. (In the following example, the client attempts to send an XML stanza of when the default namespace is "jabber:client".) C: Soliloquy To be, or not to be: that is the question: Whether 'tis nobler in the mind to suffer The slings and arrows of outrageous fortune, Or to take arms against a sea of troubles, And by opposing end them? tag:denmark.lit,2003:entry-32397 2003-12-13T18:30:02Z 2003-12-13T18:30:02Z S: 5.7.3.23. unsupported-version The value of the 'version' attribute provided by the initiating entity in the stream header specifies a version of XMPP that is not supported by the server; the server MAY specify the version(s) it supports in the element. (In the following example, the client specifies an XMPP version of "11.0" but the server supports only version "1.0" and "1.1".) Saint-Andre Expires December 4, 2008 [Page 47] Internet-Draft XMPP Core June 2008 C: S: 1.0, 1.1 5.7.3.24. xml-not-well-formed The initiating entity has sent XML that violates the well-formedness rules of [XML] or [XML-NAMES]. (In the following example, the client sends an XMPP message that is not well-formed XML.) C: No closing body tag! S: Saint-Andre Expires December 4, 2008 [Page 48] Internet-Draft XMPP Core June 2008 5.7.4. Application-Specific Conditions As noted, an application MAY provide application-specific stream error information by including a properly-namespaced child in the error element. The application-specific element SHOULD supplement or further qualify a defined element. Thus the element will contain two or three child elements: C: My keyboard layout is: QWERTYUIOP{}| ASDFGHJKL:" ZXCVBNM<>? S: Some special application diagnostic information! 5.8. Simplified Stream Examples This section contains two simplified examples of a stream-based connection of a client on a server; these examples are included for the purpose of illustrating the concepts introduced thus far. Saint-Andre Expires December 4, 2008 [Page 49] Internet-Draft XMPP Core June 2008 A basic connection: C: [ ... channel encryption ... ] [ ... authentication ... ] [ ... resource binding ... ] C: Art thou not Romeo, and a Montague? S: Neither, fair saint, if either thee dislike. C: S: Saint-Andre Expires December 4, 2008 [Page 50] Internet-Draft XMPP Core June 2008 A connection gone bad: C: S: [ ... channel encryption ... ] [ ... authentication ... ] [ ... resource binding ... ] C: No closing body tag! S: More detailed examples are provided under Section 10. 6. STARTTLS Negotiation Saint-Andre Expires December 4, 2008 [Page 51] Internet-Draft XMPP Core June 2008 6.1. Overview XMPP includes a method for securing the stream from tampering and eavesdropping. This channel encryption method makes use of the Transport Layer Security [TLS] protocol, specifically a "STARTTLS" extension that is modelled after similar extensions for the [IMAP], [POP3], and [ACAP] protocols as described in [USINGTLS]. The XML namespace name for the STARTTLS extension is 'urn:ietf:params:xml:ns:xmpp-tls'. Support for STARTTLS is REQUIRED in XMPP client and server implementations. An administrator of a given deployment may require the use of TLS for client-to-server communication, server-to-server communication, or both. A deployed client should use TLS to secure its stream with a server prior to attempting the completion of SASL negotiation (Section 7), and deployed servers should use TLS between two domains for the purpose of securing server-to-server communication. 6.2. Rules 6.2.1. 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, where the list is ordered by the perceived strength of the mechanisms. A server MUST offer and a client MUST try SASL mechanisms in the order of their perceived strength. For example, if the server offers the ordered list "PLAIN DIGEST-MD5 GSSAPI" or "DIGEST-MD5 GSSAPI PLAIN" but the client's ordered list is "GSSAPI DIGEST-MD5", the client shall try GSSAPI first and then DIGEST-MD5 but shall never try PLAIN (since PLAIN is not on its list). 6.2.2. Data Formatting The entities MUST NOT send any white space characters (matching production [3] content of [XML]) within the root stream element as separators between elements (any white space characters shown in the STARTTLS examples provided in this document are included only for the sake of readability); this prohibition helps to ensure proper security layer byte precision. 6.2.3. Order of Negotiation If the initiating entity chooses to use TLS, STARTTLS negotiation MUST be completed before proceeding to SASL negotiation (Section 7); this order of negotiation is required to help safeguard authentication information sent during SASL negotiation, as well as Saint-Andre Expires December 4, 2008 [Page 52] Internet-Draft XMPP Core June 2008 to make it possible to base the use of the SASL EXTERNAL mechanism on a certificate (or other credentials) provided during prior TLS negotiation. 6.3. Process 6.3.1. Exchange of Stream Headers and Stream Features The initiating entity resolves the hostname of the receiving entity as specified under Section 4, opens a TCP connection to the advertised port at the resolved IP address, and sends an initial stream header to the receiving entity; if the initiating entity is capable of STARTTLS negotiation, it MUST include the 'version' attribute set to a value of at least "1.0" in the initial stream header. I: The receiving entity MUST send a response stream header to the initiating entity over the TCP connection opened by the initiating entity; if the receiving entity is capable of STARTTLS negotiation, it MUST include the 'version' attribute set to a value of at least "1.0" in the response stream header. R: element (qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace) to indicate that the receiving entity supports STARTTLS negotiation. R: Saint-Andre Expires December 4, 2008 [Page 53] Internet-Draft XMPP Core June 2008 If the receiving entity requires the use of STARTTLS, it SHOULD include an empty element as a child of the element. R: 6.3.2. Initiation of STARTTLS Negotiation 6.3.2.1. STARTTLS Command In order to begin the STARTTLS negotiation, the initiating entity issues the STARTTLS command (i.e., a element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct the receiving entity that it wishes to begin a STARTTLS negotiation to secure the stream. I: The receiving entity MUST reply with either a element (proceed case) or a element (failure case) qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace. 6.3.2.2. Failure Case If the failure case occurs, the receiving entity MUST return a element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace, terminate the XML stream, and terminate the underlying TCP connection. Causes for the failure case include but are not limited to: 1. The initiating entity has sent a malformed STARTTLS command. 2. The receiving entity does not offer STARTTLS negotiation either temporarily or permanently. 3. The receiving entity cannot complete STARTTLS negotiation because of an internal error. R: R: If the failure case occurs, the initiating entity MAY attempt to reconnect as explained under Section 4.5. Saint-Andre Expires December 4, 2008 [Page 54] Internet-Draft XMPP Core June 2008 6.3.2.3. Proceed Case If the proceed case occurs, the receiving entity MUST return a element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace. R: The receiving entity MUST consider the TLS negotiation to have begun immediately after sending the closing '>' character of the element to the initiating entity. The initiating entity MUST consider the TLS negotiation to have begun immediately after receiving the closing '>' character of the element from the receiving entity. The entities now proceed to TLS negotiation as explained in the next section. 6.3.3. TLS Negotiation 6.3.3.1. Rules In order to complete TLS negotiation over the TCP connection, the entities MUST follow the process defined in [TLS]. The following rules apply: 1. The entities MUST NOT send any further XML data until the TLS negotiation has either failed or succeeded. 2. If the receiving entity presents a certificate during TLS negotiation, the initiating entity MUST validate the certificate in order to determine if the TLS negotiation shall succeed; see Section 15.2.2 regarding certificate validation procedures. Note: See Section 15.7 regarding ciphers that MUST be supported for TLS; naturally, other ciphers MAY be supported as well. 6.3.3.2. TLS Failure If the TLS negotiation results in failure, the receiving entity MUST terminate the TCP connection. The receiving entity MUST NOT send a closing tag before terminating the TCP connection, since the receiving entity and initiating entity MUST consider the original stream to be closed upon failure of the TLS negotiation. Saint-Andre Expires December 4, 2008 [Page 55] Internet-Draft XMPP Core June 2008 6.3.3.3. TLS Success If the TLS negotiation is successful, then the entities MUST proceed as follows. 1. The receiving entity MUST discard any knowledge obtained in an insecure manner from the initiating entity before TLS took effect. 2. The initiating entity MUST discard any knowledge obtained in an insecure manner from the receiving entity before TLS took effect. 3. The initiating entity MUST send a new initial stream header to the receiving entity over the secured TCP connection. I: Note: The initiating entity MUST NOT send a closing tag before sending the initial stream header, since the receiving entity and initiating entity MUST consider the original stream to be closed upon success of the TLS negotiation. 4. The receiving entity MUST respond with a response stream header. R: EXTERNAL PLAIN Saint-Andre Expires December 4, 2008 [Page 56] Internet-Draft XMPP Core June 2008 7. SASL Negotiation 7.1. Overview XMPP includes a method for authenticating a stream by means of an XMPP-specific profile of the Simple Authentication and Security Layer protocol (see [SASL]). SASL provides a generalized method for adding authentication support to connection-based protocols, and XMPP uses an XML namespace profile of SASL that conforms to the profiling requirements of [SASL]. Support for SASL negotiation is REQUIRED in XMPP client and server implementations. 7.2. Rules 7.2.1. Data Formatting The following data formattting rules apply to the SASL negotiation: 1. As formally specified in the XML schema for the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace under Appendix C.4, the receiving entity MAY include one or more application-specific child elements inside the element to provide information that may be needed by the initiating entity in order to complete successful SASL negotiation using one or more of the offered mechanisms; however, the syntax and semantics of all such elements are out of scope for this specification. 2. The entities MUST NOT send any white space characters (matching production [3] content of [XML]) within the root stream element as separators between elements (any white space characters shown in the SASL examples provided in this document are included for the sake of readability only); this prohibition helps to ensure proper security layer byte precision. 3. Any XML character data contained within the XML elements MUST be encoded using base64, where the encoding adheres to the definition in Section 4 of [BASE64] and where the padding bits are set to zero. 7.2.2. Security Layers Upon successful SASL negotiation that involves negotiation of a security layer, the initiating entity MUST discard any knowledge obtained from the receiving entity that was not obtained via the SASL negotiation. Upon successful SASL negotiation that involves negotiation of a security layer, the receiving entity MUST discard any knowledge Saint-Andre Expires December 4, 2008 [Page 57] Internet-Draft XMPP Core June 2008 obtained from the initiating entity that was not obtained via the SASL negotiation. The receiving entity SHOULD also include an updated list of SASL mechanisms with the stream features so that the initiating entity is able to detect any changes to the list of mechanisms supported by the receiving entity. 7.2.3. Simple Usernames Provision of a "simple username" may be supported by the selected SASL mechanism (e.g., this is supported by the DIGEST-MD5 and CRAM- MD5 mechanisms but not by the EXTERNAL and GSSAPI mechanisms). The simple username provided during authentication SHOULD be as follows: Client-to-server communication: The initiating entity's registered account name, i.e., a user name or node name as described under Section 3.3. The simple username MUST adhere to the Nodeprep (Appendix A) profile of [STRINGPREP]. Server-to-server communication: The initiating entity's sending domain, i.e., IP address or fully qualified domain name as contained in an XMPP domain identifier. The simple username MUST adhere to the [NAMEPREP] profile of [STRINGPREP]. 7.2.4. Authorization Identities If the initiating entity wishes to act on behalf of another entity and the selected SASL mechanism supports transmission of an authorization identity, the initiating entity MUST provide an authorization identity during SASL negotiation. If the initiating entity does not wish to act on behalf of another entity, it MUST NOT provide an authorization identity. As specified in [SASL], the initiating entity MUST NOT provide an authorization identity unless the authorization identity is different from the default authorization identity derived from the authentication identity. If provided, the value of the authorization identity MUST be of the form (i.e., an XMPP domain identifier only) for servers and of the form (i.e., node identifier and domain identifier) for clients. 7.2.5. Round Trips [SASL] specifies that a using protocol such as XMPP can define two methods by which the protocol can save round trips where allowed for the SASL mechanism: 1. When the SASL client (the XMPP "initiating entity") requests an authentication exchange, it can include "initial response" data with its request. In XMPP this is done by including the initial response as the XML character data of the element. Saint-Andre Expires December 4, 2008 [Page 58] Internet-Draft XMPP Core June 2008 2. At the end of the authentication exchange, the SASL server (the XMPP "receiving entity") can include "additional data with success". In XMPP this is done by including the additional data as the XML character data of the element. For the sake of protocol efficiency, it is RECOMMENDED for XMPP clients and servers to use these methods, however they MUST support the less efficient modes as well. 7.3. Process The process for SASL negotiation is as follows. 7.3.1. Exchange of Stream Headers and Stream Features If SASL negotiation follows successful STARTTLS negotation (Section 6), then the SASL negotiation occurs over the existing stream. If not, the initiating entity resolves the hostname of the receiving entity as specified under Section 4, opens a TCP connection to the advertised port at the resolved IP address, and sends an initial stream header to the receiving entity; if the initiating entity is capable of STARTTLS negotiation, it MUST include the 'version' attribute set to a value of at least "1.0" in the initial stream header. I: The receiving entity MUST send a response stream header to the initiating entity; if the receiving entity is capable of SASL negotiation, it MUST include the 'version' attribute set to a value of at least "1.0" in the response stream header. R: element (qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace) that contains one child element for each authentication mechanism the receiving entity offers to the initiating entity. The order of elements in the XML indicates the preference order of the SASL mechanisms according to the receiving entity, however the initiating entity MUST maintain its own preference order independent of the preference order of the receiving entity. R: EXTERNAL PLAIN Note: If during prior TLS negotiation the initiating entity presented a certificate that is acceptable to the receiving entity for purposes of strong identity verification in accordance with local service policies, the receiving entity SHOULD offer the SASL EXTERNAL mechanism to the initiating entity during SASL negotiation (refer to [SASL]) and SHOULD prefer that mechanism. However, the EXTERNAL mechanism MAY be offered under other circumstances as well. Note: If TLS negotiation (Section 6) needs to be completed before a particular authentication mechanism may be used, the receiving entity MUST NOT provide that mechanism in the list of available SASL authentication mechanisms prior to TLS negotiation. Note: See Section 15.7 regarding mechanisms that MUST be supported; naturally, other SASL mechanisms MAY be supported as well (best practices for the use of several SASL mechanisms in the context of XMPP are described in [XEP-0175] and [XEP-0178]). If successful SASL negotiation is required for interaction with the receiving entity, the receiving entity SHOULD signal that fact by including a element as a child of the element. R: EXTERNAL PLAIN Saint-Andre Expires December 4, 2008 [Page 60] Internet-Draft XMPP Core June 2008 7.3.2. Initiation In order to begin the SASL negotiation, the initiating entity sends an element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace and includes an appropriate value for the 'mechanism' attribute. This element MAY contain XML character data (in SASL terminology, the "initial response") if the mechanism supports or requires it; if the initiating entity needs to send a zero-length initial response, it MUST transmit the response as a single equals sign character ("="), which indicates that the response is present but contains no data. I: R0m30R0cks 7.3.3. Challenge-Response Sequence If necessary, the receiving entity challenges the initiating entity by sending a element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY contain XML character data (which MUST be generated in accordance with the definition of the SASL mechanism chosen by the initiating entity). The initiating entity responds to the challenge by sending a element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY contain XML character data (which MUST be generated in accordance with the definition of the SASL mechanism chosen by the initiating entity). If necessary, the receiving entity sends more challenges and the initiating entity sends more responses. This series of challenge/response pairs continues until one of three things happens: o The initiating entity aborts the handshake. o The receiving entity reports failure of the handshake. o The receiving entity reports success of the handshake. These scenarios are described in the following sections. 7.3.4. Abort The initiating entity aborts the handshake by sending an element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace. Saint-Andre Expires December 4, 2008 [Page 61] Internet-Draft XMPP Core June 2008 I: Upon receiving an element, the receiving entity MUST return a element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace and containing an child element. R: