Network Working Group P. Saint-Andre, Ed. Internet-Draft XMPP Standards Foundation Obsoletes: 3920 (if approved) July 17, 2007 Intended status: Standards Track Expires: January 18, 2008 Extensible Messaging and Presence Protocol (XMPP): Core draft-saintandre-rfc3920bis-03 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 January 18, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). 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 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 Saint-Andre Expires January 18, 2008 [Page 1] Internet-Draft XMPP Core July 2007 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 entities. This document also specifies the format for XMPP addresses, which are fully internationalizable. This document obsoletes RFC 3920. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2. Functional Summary . . . . . . . . . . . . . . . . . . . 5 1.3. Conventions . . . . . . . . . . . . . . . . . . . . . . 7 2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3. Client . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.4. Network . . . . . . . . . . . . . . . . . . . . . . . . 9 3. Addresses . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2. Domain Identifier . . . . . . . . . . . . . . . . . . . 10 3.3. Node Identifier . . . . . . . . . . . . . . . . . . . . 11 3.4. Resource Identifier . . . . . . . . . . . . . . . . . . 11 3.5. Determination of Addresses . . . . . . . . . . . . . . . 12 4. TCP Binding . . . . . . . . . . . . . . . . . . . . . . . . . 12 5. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 13 5.2. Stream Security . . . . . . . . . . . . . . . . . . . . 15 5.3. Stream Attributes . . . . . . . . . . . . . . . . . . . 16 5.4. Namespace Declarations . . . . . . . . . . . . . . . . . 22 5.5. Stream Features . . . . . . . . . . . . . . . . . . . . 22 5.6. Closing Streams . . . . . . . . . . . . . . . . . . . . 23 5.7. Reconnection . . . . . . . . . . . . . . . . . . . . . . 24 5.8. Stream Errors . . . . . . . . . . . . . . . . . . . . . 24 5.9. Simplified Stream Examples . . . . . . . . . . . . . . . 41 6. STARTTLS Negotiation . . . . . . . . . . . . . . . . . . . . 43 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 43 6.2. Rules . . . . . . . . . . . . . . . . . . . . . . . . . 44 6.3. Process . . . . . . . . . . . . . . . . . . . . . . . . 44 6.4. Representation of JIDs in Certificates . . . . . . . . . 48 7. SASL Negotiation . . . . . . . . . . . . . . . . . . . . . . 49 7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 49 7.2. Rules . . . . . . . . . . . . . . . . . . . . . . . . . 49 7.3. Process . . . . . . . . . . . . . . . . . . . . . . . . 51 7.4. SASL Definition . . . . . . . . . . . . . . . . . . . . 56 Saint-Andre Expires January 18, 2008 [Page 2] Internet-Draft XMPP Core July 2007 7.5. SASL Errors . . . . . . . . . . . . . . . . . . . . . . 57 8. Resource Binding . . . . . . . . . . . . . . . . . . . . . . 60 8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 60 8.2. Advertising Support . . . . . . . . . . . . . . . . . . 60 8.3. Server-Generated Resource Identifier . . . . . . . . . . 61 8.4. Client-Generated Resource Identifier . . . . . . . . . . 62 8.5. Binding Multiple Resources . . . . . . . . . . . . . . . 64 9. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 67 9.1. Common Attributes . . . . . . . . . . . . . . . . . . . 67 9.2. Basic Semantics . . . . . . . . . . . . . . . . . . . . 71 9.3. Stanza Errors . . . . . . . . . . . . . . . . . . . . . 73 9.4. Extended Content . . . . . . . . . . . . . . . . . . . . 88 10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 89 10.1. Client-to-Server . . . . . . . . . . . . . . . . . . . . 89 10.2. Server-to-Server Examples . . . . . . . . . . . . . . . 95 11. Server Rules for Processing XML Stanzas . . . . . . . . . . . 99 11.1. No 'to' Address . . . . . . . . . . . . . . . . . . . . 99 11.2. Local Domain . . . . . . . . . . . . . . . . . . . . . . 100 11.3. Resource at Local Domain . . . . . . . . . . . . . . . . 100 11.4. Node at Local Domain . . . . . . . . . . . . . . . . . . 101 11.5. Foreign Domain . . . . . . . . . . . . . . . . . . . . . 101 12. XML Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 102 12.1. Restrictions . . . . . . . . . . . . . . . . . . . . . . 102 12.2. XML Namespace Names and Prefixes . . . . . . . . . . . . 103 12.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 105 12.4. Inclusion of Text Declaration . . . . . . . . . . . . . 106 12.5. Character Encoding . . . . . . . . . . . . . . . . . . . 106 12.6. White Space . . . . . . . . . . . . . . . . . . . . . . 106 13. Compliance Requirements . . . . . . . . . . . . . . . . . . . 106 13.1. Servers . . . . . . . . . . . . . . . . . . . . . . . . 107 13.2. Clients . . . . . . . . . . . . . . . . . . . . . . . . 107 14. Internationalization Considerations . . . . . . . . . . . . . 107 15. Security Considerations . . . . . . . . . . . . . . . . . . . 108 15.1. High Security . . . . . . . . . . . . . . . . . . . . . 108 15.2. Certificate Validation . . . . . . . . . . . . . . . . . 108 15.3. Client-to-Server Communication . . . . . . . . . . . . . 109 15.4. Server-to-Server Communication . . . . . . . . . . . . . 110 15.5. Order of Layers . . . . . . . . . . . . . . . . . . . . 110 15.6. Lack of SASL Channel Binding to TLS . . . . . . . . . . 110 15.7. Mandatory-to-Implement Technologies . . . . . . . . . . 111 15.8. Firewalls . . . . . . . . . . . . . . . . . . . . . . . 111 15.9. Use of base64 in SASL . . . . . . . . . . . . . . . . . 111 15.10. Stringprep Profiles . . . . . . . . . . . . . . . . . . 111 15.11. Address Spoofing . . . . . . . . . . . . . . . . . . . . 112 15.12. Denial of Service . . . . . . . . . . . . . . . . . . . 114 15.13. Presence Leaks . . . . . . . . . . . . . . . . . . . . . 115 15.14. Directory Harvesting . . . . . . . . . . . . . . . . . . 116 16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 116 Saint-Andre Expires January 18, 2008 [Page 3] Internet-Draft XMPP Core July 2007 16.1. XML Namespace Name for TLS Data . . . . . . . . . . . . 116 16.2. XML Namespace Name for SASL Data . . . . . . . . . . . . 116 16.3. XML Namespace Name for Stream Errors . . . . . . . . . . 116 16.4. XML Namespace Name for Resource Binding . . . . . . . . 117 16.5. XML Namespace Name for Stanza Errors . . . . . . . . . . 117 16.6. Nodeprep Profile of Stringprep . . . . . . . . . . . . . 117 16.7. Resourceprep Profile of Stringprep . . . . . . . . . . . 118 16.8. GSSAPI Service Name . . . . . . . . . . . . . . . . . . 118 16.9. Port Numbers . . . . . . . . . . . . . . . . . . . . . . 118 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 118 17.1. Normative References . . . . . . . . . . . . . . . . . . 118 17.2. Informative References . . . . . . . . . . . . . . . . . 120 Appendix A. Nodeprep . . . . . . . . . . . . . . . . . . . . . . 124 A.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 124 A.2. Character Repertoire . . . . . . . . . . . . . . . . . . 124 A.3. Mapping . . . . . . . . . . . . . . . . . . . . . . . . 124 A.4. Normalization . . . . . . . . . . . . . . . . . . . . . 124 A.5. Prohibited Output . . . . . . . . . . . . . . . . . . . 125 A.6. Bidirectional Characters . . . . . . . . . . . . . . . . 125 Appendix B. Resourceprep . . . . . . . . . . . . . . . . . . . . 125 B.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 125 B.2. Character Repertoire . . . . . . . . . . . . . . . . . . 126 B.3. Mapping . . . . . . . . . . . . . . . . . . . . . . . . 126 B.4. Normalization . . . . . . . . . . . . . . . . . . . . . 126 B.5. Prohibited Output . . . . . . . . . . . . . . . . . . . 126 B.6. Bidirectional Characters . . . . . . . . . . . . . . . . 127 Appendix C. XML Schemas . . . . . . . . . . . . . . . . . . . . 127 C.1. Streams namespace . . . . . . . . . . . . . . . . . . . 127 C.2. Stream error namespace . . . . . . . . . . . . . . . . . 128 C.3. TLS namespace . . . . . . . . . . . . . . . . . . . . . 131 C.4. SASL namespace . . . . . . . . . . . . . . . . . . . . . 131 C.5. Resource binding namespace . . . . . . . . . . . . . . . 133 C.6. Stanza error namespace . . . . . . . . . . . . . . . . . 135 Appendix D. Contact Addresses . . . . . . . . . . . . . . . . . 136 Appendix E. Account Provisioning . . . . . . . . . . . . . . . . 137 Appendix F. Differences From RFC 3920 . . . . . . . . . . . . . 137 Appendix G. Copying Conditions . . . . . . . . . . . . . . . . . 138 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 139 Intellectual Property and Copyright Statements . . . . . . . . . 140 Saint-Andre Expires January 18, 2008 [Page 4] Internet-Draft XMPP Core July 2007 1. Introduction 1.1. Overview The Extensible Messaging and Presence Protocol (XMPP) is an Extensible Markup Language [XML] technology for near-real-time messaging, presence, and request-response services. The basic syntax and semantics 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] was published in October 2004. As a result of extensive implementation and deployment experience with XMPP since that time, as well as more formal interoperability testing, 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 and the Simple Authentication and Security Layer) o Definition of several additional error conditions o Addition of TLS plus SASL 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]. 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. Saint-Andre Expires January 18, 2008 [Page 5] Internet-Draft XMPP Core July 2007 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 extensions (e.g., [XMPP-IM] defines extensions for basic instant messaging and presence functionality, and various specifications produced in the XMPP Standards Foundation's XEP series 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 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 Saint-Andre Expires January 18, 2008 [Page 6] Internet-Draft XMPP Core July 2007 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 2. Architecture 2.1. Overview XMPP assumes a client-server architecture, wherein a client utilizing XMPP accesses a server (normally over a [TCP] connection) and servers can also communicate with each other over TCP connections. A simplified architectural diagram for a typical deployment is shown here, where the entities have the following significance: o romeo@example.net -- an XMPP user. o example.net -- an XMPP server. o example.com -- an XMPP server. o juliet@example.com -- an XMPP user. example.net -------------------- example.com | | | | romeo@example.net juliet@example.com Saint-Andre Expires January 18, 2008 [Page 7] Internet-Draft XMPP Core July 2007 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]. 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, with each client 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). Saint-Andre Expires January 18, 2008 [Page 8] Internet-Draft XMPP Core July 2007 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]. 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 3513 resource = 1*(resourcepoint) ; a "resourcepoint" is a UTF-8 encoded Unicode ; code point that satisfies the Resourceprep ; profile of stringprep All JIDs are based on the foregoing structure. One common use of this structure is to identify a messaging and presence account, the Saint-Andre Expires January 18, 2008 [Page 9] Internet-Draft XMPP Core July 2007 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 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 host 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]) but MAY be either an IPv4 or IPv6 address or a text label (commonly called an "unqualified hostname") that is resolvable on a local network. 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]). Saint-Andre Expires January 18, 2008 [Page 10] Internet-Draft XMPP Core July 2007 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].) 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. 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. A resource identifier has no semantic meaning and is opaque to both servers and clients. A resource Saint-Andre Expires January 18, 2008 [Page 11] Internet-Draft XMPP Core July 2007 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, derived from the authentication identity (as defined by [SASL]), if no authorization identity was specified during SASL negotiation (Section 7). For client-to-server communication, the bare JID () SHOULD be the authorization identity, derived from the authentication identity (as defined by [SASL]), 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). 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 As XMPP is defined herein, an initiating entity (client or server) MUST open a TCP connection at the receiving entity (server) before it negotiates XML streams with the receiving entity. However, prior to 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 Saint-Andre Expires January 18, 2008 [Page 12] Internet-Draft XMPP Core July 2007 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.example.com." or "_xmpp-server._tcp.example.com."; the IP address and port at which the initiating entity attempts to connect to the receiving entity shall be those specified in the SRV lookup result. 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]. TCP connections are handled differently in client-to-server communication and server-to-server ommunication. Specifically: o 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). o 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 stanzas from the second server to the first server. Note: 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 Saint-Andre Expires January 18, 2008 [Page 13] Internet-Draft XMPP Core July 2007 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 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). Definition of XML Stanza: An XML STANZA is a discrete semantic unit of structured information that is sent from one entity to another over an XML stream, and is the basic unit of meaning in XMPP. An XML stanza exists at the direct child level of the root 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 an opening tag to the server, optionally preceded by a text declaration specifying the XML version and the character encoding supported (see Section 12.4 and Section 12.5). Subject to Saint-Andre Expires January 18, 2008 [Page 14] Internet-Draft XMPP Core July 2007 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 to any. 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. +--------------------+ | | |--------------------| | | | | | | |--------------------| | | | | | | |--------------------| | | | | | | |--------------------| | [ ... ] | |--------------------| | | +--------------------+ 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. Saint-Andre Expires January 18, 2008 [Page 15] Internet-Draft XMPP Core July 2007 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. 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 XML stream header sent from the initiating entity to the receiving entity 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 XML stream header sent from the initiating entity to the receiving entity and (if included) MUST be set to a hostname serviced by the initiating entity. Saint-Andre Expires January 18, 2008 [Page 16] Internet-Draft XMPP Core July 2007 P: In both client-to-server and server-to-server communications, the 'from' attribute MUST be included in the XML stream header by which the receiving entity responds to the initiating entity and MUST be set to a hostname serviced by the receiving entity that is granting access to the initiating entity. 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 XML stream header sent from the initiating entity to the receiving entity 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 XML stream header by which it replies to the client and (if included) MUST set the 'to' attribute to the bare JID Saint-Andre Expires January 18, 2008 [Page 17] Internet-Draft XMPP Core July 2007 specified in the 'from' attribute of the XML stream header sent from the initiating entity to the receiving entity. 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 XML stream header by which it replies to the initiating entity and (if included) MUST set the 'to' attribute to the hostname specified in the 'from' attribute of the XML stream header sent from the initiating entity to the receiving entity. 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 on the XML stream header sent from the initiating entity to the receiving entity; however, if an 'id' attribute is included, it SHOULD be silently ignored by the receiving entity. C: The 'id' attribute SHOULD be used only in the XML stream header from the receiving entity to the initiating entity. 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 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 by the initiating entity on the header for the initial stream 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 header for the response stream. Saint-Andre Expires January 18, 2008 [Page 19] Internet-Draft XMPP Core July 2007 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. The version of XMPP specified herein is "1.0"; in particular, this encapsulates the stream-related protocols (TLS negotiation (Section 6), SASL negotiation (Section 7), and stream errors (Section 5.8)), as well as the 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 Saint-Andre Expires January 18, 2008 [Page 20] Internet-Draft XMPP Core July 2007 a smaller minor version number, but 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 on 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 on the response stream header to either the value supplied by the initiating entity or the highest version number supported by the receiving entity, whichever is lower. The receiving entity MUST perform a numeric comparison on the major and minor version numbers, not a string match on ".". 3. If the version number included in the response stream header is at least one major version lower than the version number included in the initial stream header and newer version entities cannot interoperate with older version entities as described, the initiating entity SHOULD 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 stream header it sends in reply. 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 | +----------+--------------------------+-------------------------+ Saint-Andre Expires January 18, 2008 [Page 21] Internet-Draft XMPP Core July 2007 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 header for the response stream 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). 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 that 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. Saint-Andre Expires January 18, 2008 [Page 22] Internet-Draft XMPP Core July 2007 If a feature must be negotiated before the initiating entity may proceed, that feature SHOULD include a child element. 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 include an empty element after sending a stream header to the initiating entity. S: S: 5.6. Closing Streams At any time after XML streams have been negotiated between two entities, either entity MAY close its stream to the other entity (even in the absence of a stream error) by sending a closing stream tag: C: The entity that sends the closing stream tag SHOULD wait for the other entity 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 entity 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 entity, it MUST terminate the underlying TCP connection. Note: There is one TCP connection for client-to-server streams, but Saint-Andre Expires January 18, 2008 [Page 23] Internet-Draft XMPP Core July 2007 there are two TCP connections for server-to-server streams. 5.7. Reconnection It can happen that an XMPP server goes offline while servicing connections from clients and from other servers. Because the number of such connections can be quite large, the reconnection algorithm employed by entities that seek to reconnect can have a significant impact on software and network performance. The following guidelines are RECOMMENDED: o The time to live (TTL) specified in Domain Name System records 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. 5.8. 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 (typically a server rather than a client) if it perceives that a stream-level error has occurred. 5.8.1. Rules The following rules apply to stream-level errors. 5.8.1.1. Stream Errors Are Unrecoverable Stream-level errors are unrecoverable. Therefore, if an error occurs at the level of the stream, the entity that detects the error MUST send a stream error to the other entity, send a closing tag, and terminate the underlying TCP connection. C: S: Saint-Andre Expires January 18, 2008 [Page 24] Internet-Draft XMPP Core July 2007 5.8.1.2. 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 terminate the underlying TCP connection. C: S: 5.8.1.3. 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 January 18, 2008 [Page 25] Internet-Draft XMPP Core July 2007 C: S: 5.8.2. Syntax The syntax for stream errors is as follows, where "defined-condition" is a placeholder for one of the conditions defined under Section 5.8.3. [ descriptive text ] [application-specific condition element] The element: o MUST contain a child element corresponding to one of the defined stream error conditions (Section 5.8.3); 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 January 18, 2008 [Page 26] Internet-Draft XMPP Core July 2007 o MAY contain a child element for an application-specific error condition; this element MUST be qualified by an application- defined namespace, and its structure is defined by that namespace (see Section 5.8.4). The 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). 5.8.3. Defined Stream Error Conditions The following stream-level error conditions are defined. 5.8.3.1. bad-format The entity has sent XML that cannot be processed. C: S: This error MAY be used instead of the more specific XML-related errors, such as , , , , and . However, the more specific errors are preferred. 5.8.3.2. bad-namespace-prefix The entity has sent a namespace prefix that is unsupported, or has sent no namespace prefix on an element that requires such a prefix (see Section 12.2). Saint-Andre Expires January 18, 2008 [Page 27] Internet-Draft XMPP Core July 2007 C: S: 5.8.3.3. conflict The server is either (1) closing the existing stream for this entity because a new stream has been initiated that conflicts with the existing stream (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 for the same IP address). Saint-Andre Expires January 18, 2008 [Page 28] Internet-Draft XMPP Core July 2007 C: S: 5.8.3.4. connection-timeout The entity has not generated any traffic over the stream for some period of time (configurable according to a local service policy) and therefore the connection is being dropped. S: 5.8.3.5. host-gone The value of the 'to' attribute provided by the initiating entity in the stream header corresponds to a hostname that is no longer hosted by the server. Saint-Andre Expires January 18, 2008 [Page 29] Internet-Draft XMPP Core July 2007 P: S: 5.8.3.6. host-unknown The value of the 'to' attribute provided by the initiating entity in the stream header does not correspond to a hostname that is hosted by the server. Saint-Andre Expires January 18, 2008 [Page 30] Internet-Draft XMPP Core July 2007 P: S: 5.8.3.7. improper-addressing A stanza sent between two servers lacks a 'to' or 'from' attribute (or the attribute has no value). P: Wherefore art thou? S: 5.8.3.8. internal-server-error The server has experienced a misconfiguration or an otherwise- undefined internal error that prevents it from servicing the stream. S: Saint-Andre Expires January 18, 2008 [Page 31] Internet-Draft XMPP Core July 2007 5.8.3.9. 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. P: Neither, fair saint, if either thee dislike. S: 5.8.3.10. invalid-id The stream ID or server dialback ID is invalid or does not match an ID previously provided (the following example is from server dialback; see [XEP-0220]). P: S: 5.8.3.11. invalid-namespace The streams namespace name is something other than "http://etherx.jabber.org/streams" (see Section 12.2). Saint-Andre Expires January 18, 2008 [Page 32] Internet-Draft XMPP Core July 2007 C: S: 5.8.3.12. invalid-xml The entity has sent invalid XML over the stream to a server that performs validation (see Section 12.3). (In the following example, a peer attempts to send an IQ stanza of type "subscribe" but there is no such value for the 'type' attribute.) P: S: 5.8.3.13. not-authorized The entity has attempted to send XML stanzas before the stream has been authenticated, or otherwise is not authorized to perform an action related to stream negotiation; the receiving entity MUST NOT Saint-Andre Expires January 18, 2008 [Page 33] Internet-Draft XMPP Core July 2007 process the offending stanza before sending the stream error. (In the following example, a client attempts to send XML stanzas before authenticating with the server.) C: S: Wherefore art thou? S: 5.8.3.14. policy-violation The entity has violated some local service policy (e.g., the stanza exceeds a configured size limit); the server MAY choose to specify the policy in the element or an application-specific condition element. C: [ ... the-emacs-manual ... ] S: Saint-Andre Expires January 18, 2008 [Page 34] Internet-Draft XMPP Core July 2007 S: 5.8.3.15. remote-connection-failed The server is unable to properly connect to a remote entity that is required for authentication or authorization. C: S: 5.8.3.16. resource-constraint The server lacks the system resources necessary to service the stream. Saint-Andre Expires January 18, 2008 [Page 35] Internet-Draft XMPP Core July 2007 C: S: 5.8.3.17. 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). C: S: 5.8.3.18. see-other-host The server will not provide service to the initiating entity but is redirecting traffic to another host; the XML character data of the element returned by the server SHOULD specify the 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 Saint-Andre Expires January 18, 2008 [Page 36] Internet-Draft XMPP Core July 2007 connect to that domain identifier at the standard XMPP ports (5222 for client-to-server connections and 5269 for server-to-server connections). C: S: xmpp.example.com:9090 5.8.3.19. system-shutdown The server is being shut down and all active streams are being closed. S: 5.8.3.20. undefined-condition The error condition is not one of those defined by the other conditions in this list; this error condition SHOULD be used only in conjunction with an application-specific condition. Saint-Andre Expires January 18, 2008 [Page 37] Internet-Draft XMPP Core July 2007 S: 5.8.3.21. unsupported-encoding The initiating entity has encoded the stream in an encoding that is not supported by the server (see Section 12.5). C: S: 5.8.3.22. unsupported-stanza-type The initiating entity has sent a first-level child of the stream that is not supported by the server. Saint-Andre Expires January 18, 2008 [Page 38] Internet-Draft XMPP Core July 2007 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: 5.8.3.23. unsupported-version The value of the 'version' attribute provided by the initiating entity in the stream header specifies a version of XMPP that is not supported by the server; the server MAY specify the version(s) it supports in the element. Saint-Andre Expires January 18, 2008 [Page 39] Internet-Draft XMPP Core July 2007 C: S: 5.8.3.24. xml-not-well-formed -- the initiating entity has sent XML that is not well-formed as defined by [XML]. C: S: 5.8.4. Application-Specific Conditions As noted, an application MAY provide application-specific stream error information by including a properly-namespaced child in the error element. The application-specific element SHOULD supplement or further qualify a defined element. Thus the element will contain two or three child elements: Saint-Andre Expires January 18, 2008 [Page 40] Internet-Draft XMPP Core July 2007 C: My keyboard layout is: QWERTYUIOP{}| ASDFGHJKL:" ZXCVBNM<>? S: Some special application diagnostic information! 5.9. Simplified Stream Examples This section contains two simplified examples of a stream-based connection of a client on a server (where the "C" lines are sent from the client to the server, and the "S" lines are sent from the server to the client); these examples are included for the purpose of illustrating the concepts introduced thus far. Saint-Andre Expires January 18, 2008 [Page 41] Internet-Draft XMPP Core July 2007 A basic connection: C: [ ... encryption, authentication, and resource binding ... ] C: Art thou not Romeo, and a Montague? S: Neither, fair saint, if either thee dislike. C: S: Saint-Andre Expires January 18, 2008 [Page 42] Internet-Draft XMPP Core July 2007 A connection gone bad: C: S: [ ... encryption, authentication, and resource binding ... ] C: Bad XML, no closing body tag! S: More detailed examples are provided under Section 10. 6. STARTTLS Negotiation 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 protocol (see [TLS]), specifically a "STARTTLS" extension that is modelled after similar extensions for the [IMAP], [POP3], and [ACAP] protocols as described in [USINGTLS]. Saint-Andre Expires January 18, 2008 [Page 43] Internet-Draft XMPP Core July 2007 The XML namespace name for the STARTTLS extension is 'urn:ietf:params:xml:ns:xmpp-tls'. Support for STARTTLS is REQUIRED in XMPP client and server implementations. An administrator of a given deployment may require the use of TLS for client-to-server communication, server-to-server communication, or both. A deployed client should use TLS to secure its stream with a server prior to attempting the completion of SASL negotiation (Section 7), and deployed servers should use TLS between two domains for the purpose of securing server-to-server communication. 6.2. Rules 6.2.1. Data Formatting 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 for the sake of readability only); this prohibition helps to ensure proper security layer byte precision. 6.2.2. Order of Negotiation If the initiating entity chooses to use TLS, STARTTLS negotiation MUST be completed before proceeding to SASL negotiation (Section 7); this order of negotiation is required to help safeguard authentication information sent during SASL negotiation, as well as to make it possible to base the use of the SASL EXTERNAL mechanism on a certificate (or other credentials) provided during prior TLS negotiation. 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. Saint-Andre Expires January 18, 2008 [Page 44] Internet-Draft XMPP Core July 2007 I: The receiving entity MUST send a response stream header to the initiating entity over the TCP connection opened by the initiating entity (for client-to-server streams) or over a new TCP connection (for server-to-server streams); 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: 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 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 Saint-Andre Expires January 18, 2008 [Page 45] Internet-Draft XMPP Core July 2007 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. 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 proceed case occurs, the receiving entity MUST return a element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace. R: Note: 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. 6.3.3. TLS Negotiation The entities MUST now attempt to complete TLS negotiation over the TCP connection by following the process defined in [TLS]. Note: The entities MUST NOT send any further XML data until the TLS negotiation has either failed or succeeded. If the receiving entity presents a certificate during TLS negotiation, the initiating entity MUST validate the certificate in Saint-Andre Expires January 18, 2008 [Page 46] Internet-Draft XMPP Core July 2007 order to determine if the TLS negotiation shall succeed (see Section 15.2 regarding certificate validation procedures). Specifically, the certificate MUST be checked against the hostname as provided by the initiating entity (e.g., a user), not the hostname as resolved via the Domain Name System; e.g., if the user specifies a hostname of "example.net" but a [DNS-SRV] lookup returns "xmpp.example.net", the certificate MUST be checked as "example.net". See Section 6.4 for information about the representation of XMPP addresses in certificates. Note: See Section 15.7 regarding ciphers that MUST be supported for TLS; naturally, other ciphers MAY be supported as well. 6.3.4. Failure If the TLS negotiation results in failure, the receiving entity MUST terminate the TCP connection. Note: It is not necessary to 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. 6.3.5. Success If the TLS negotiation is successful, then the entities MUST proceed as follows. The receiving entity MUST discard any knowledge obtained in an insecure manner from the initiating entity before TLS took effect. The initiating entity MUST discard any knowledge obtained in an insecure manner from the receiving entity before TLS took effect. The initiating entity MUST send a new stream header to the receiving entity over the secured TCP connection. I: Note: It is not necessary to 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 Saint-Andre Expires January 18, 2008 [Page 47] Internet-Draft XMPP Core July 2007 success of the TLS negotiation. The receiving entity MUST respond with a response stream header. R: EXTERNAL DIGEST-MD5 PLAIN 6.4. Representation of JIDs in Certificates TLS negotiation is commonly based on a digital certificate presented by the receiving entity (or in the case of mutual authentication both the receiving entity and the initiating entity). If a JID for an XMPP user account is represented in a certificate, it MUST be represented as a UTF8String within an otherName entity inside the subjectAltName, using the [ASN.1] Object Identifier "id-on- xmppAddr" specified in this section. If a JID for an XMPP server is represented in a certificate, it SHOULD be represented as a UTF8String within an otherName entity inside the subjectAltName, using the [ASN.1] Object Identifier "id- on-xmppAddr" specified in this section. However, in addition to or instead of the "id-on-xmppAddr" representation, it MAY be represented as a subjectAltName extension of type dNSName; this dNSName MAY contain the wildcard character '*', which applies only to the left- most domain name component or component fragment and is considered to match any single component or component fragment (e.g., *.example.com matches foo.example.com but not bar.foo.example.com, and im*.example.net matches im1.example.net and im2.example.net but not Saint-Andre Expires January 18, 2008 [Page 48] Internet-Draft XMPP Core July 2007 chat.example.net). The [ASN.1] Object Identifier "id-on-xmppAddr" is defined as follows. id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) } id-on OBJECT IDENTIFIER ::= { id-pkix 8 } -- other name forms id-on-xmppAddr OBJECT IDENTIFIER ::= { id-on 5 } XmppAddr ::= UTF8String This Object Identifier MAY be represented in dotted display format (i.e., "1.3.6.1.5.5.7.8.5") or in the Uniform Resource Name notation specified in [URN-OID] (i.e., "urn:oid:1.3.6.1.5.5.7.8.5"). Thus for example the JID "juliet@example.com" as included in a certificate could be formatted in any of the following three ways: o subjectAltName=otherName:id-on-xmppAddr;UTF8:juliet@example.com o subjectAltName=otherName:1.3.6.1.5.5.7.8.5;UTF8:juliet@example.com o subjectAltName=otherName:urn:oid:1.3.6.1.5.5.7.8.5;UTF8:juliet@ example.com 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 is REQUIRED in XMPP client and server implementations. 7.2. Rules 7.2.1. Data Formatting The following formatting rules apply to the data sent during SASL negotiation: Saint-Andre Expires January 18, 2008 [Page 49] Internet-Draft XMPP Core July 2007 1. 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. 2. Any XML character data contained within the XML elements MUST be encoded using base64, where the encoding adheres to the definition in Section 4 of [BASE64] and where the padding bits are set to zero. 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 which was not obtained from the SASL negotiation itself. Upon successful SASL negotiation that involves negotiation of a security layer, the receiving entity MUST discard any knowledge obtained from the initiating entity which was not obtained from the SASL negotiation itself. 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 If provision of a "simple username" is supported by the selected SASL mechanism (e.g., this is supported by the DIGEST-MD5 and CRAM-MD5 mechanisms but not by the EXTERNAL and GSSAPI mechanisms), during authentication the initiating entity SHOULD provide as the simple username its sending domain (IP address or fully qualified domain name as contained in an XMPP domain identifier) in the case of server-to-server communication or its registered account name (user or node name as contained in an XMPP node identifier) in the case of client-to-server communication. In either case, the initiating entity MUST ensure that the username adheres to the [NAMEPREP] or Nodeprep (Appendix A) profile of [STRINGPREP] (as appropriate) before sending it to the receiving entity. 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 Saint-Andre Expires January 18, 2008 [Page 50] Internet-Draft XMPP Core July 2007 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. 7.3. 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. R: EXTERNAL DIGEST-MD5 PLAIN Note: If the initiating entity presents a valid certificate during prior TLS negotiation, the receiving entity SHOULD offer the SASL EXTERNAL mechanism to the initiating entity during SASL negotiation (refer to [SASL]), although 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, it SHOULD signal that fact by including a element as a child of the element. R: EXTERNAL DIGEST-MD5 PLAIN Note: The receiving entity MAY include an application-specific child element inside the element in order to provide information that may be needed by the initiating in order to complete Saint-Andre Expires January 18, 2008 [Page 52] Internet-Draft XMPP Core July 2007 successful SASL negotiation using one or more of the offered mechanisms; however, the syntax and semantics of any such element are out of scope for this specification. 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 ("="), which indicates that the response is present but contains no data. I: = 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). R: cmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLHFvcD0i YXV0aCIsY2hhcnNldD11dGYtOCxhbGdvcml0aG09bWQ1LXNlc3MK The decoded challenge is: realm="example.com",nonce="OA6MG9tEQGm2hh", qop="auth",charset=utf-8,algorithm=md5-sess Note: If the receiving entity does not specify a 'realm' value, the initiating entity MUST default it to the domain identifier portion of the receiving entity's JID. 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). Saint-Andre Expires January 18, 2008 [Page 53] Internet-Draft XMPP Core July 2007 I: dXNlcm5hbWU9Imp1bGlldCIscmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2 TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAwMDAwMDAx LHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20iLHJlc3BvbnNl PWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNoYXJzZXQ9dXRmLTgK The decoded response is: username="juliet",realm="example.com", nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk", nc=00000001,qop=auth,digest-uri="xmpp/example.com", response=d388dad90d4bbd760a152321f2143af7,charset=utf-8 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. I: Upon receiving an element, the receiving entity MUST return an element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace. R: The receiving entity SHOULD allow a configurable but reasonable number of retries (at least 2 and no more than 5); this enables the initiating entity (e.g., an end-user client) to tolerate incorrectly- provided credentials (e.g., a mistyped password) without being forced to reconnect. If the initiating entity exceeds the number of retries, the receiving entity MUST terminate the TCP connection. Saint-Andre Expires January 18, 2008 [Page 54] Internet-Draft XMPP Core July 2007 7.3.5. Failure The receiving entity reports failure of the handshake by sending a element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace (the particular cause of failure SHOULD be communicated in an appropriate child element of the element as defined under Section 7.5). R: If the failure case occurs, the receiving entity SHOULD allow a configurable but reasonable number of retries (at least 2 and no more than 5); this enables the initiating entity (e.g., an end-user client) to tolerate incorrectly-provided credentials (e.g., a mistyped password) without being forced to reconnect. If the initiating entity exceeds the number of retries, the receiving entity MUST terminate the TCP connection. 7.3.6. Success The receiving entity reports success of the handshake by sending a element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY contain XML character data (in SASL terminology, "additional data with success") if required by the chosen SASL mechanism; if the receiving entity needs to send additional data of zero length, it MUST transmit the data as a single equals sign ("="). R: cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo= The decoded value for subsequent authentication is: rspauth=ea40f60335c427b5527b84dbabcdfffd Upon receiving the element, the initiating entity MUST initiate a new stream over the existing TCP connection by sending an initial stream header to the receiving entity. Saint-Andre Expires January 18, 2008 [Page 55] Internet-Draft XMPP Core July 2007 I: tag before sending the initial stream header, since the receiving entity and initiating entity MUST consider the original stream to be closed upon sending or receiving the element. Upon receiving the initial stream header from the initiating entity, the receiving entity MUST respond by sending a new XML stream header to the initiating entity. R: The receiving entity MUST also send stream features, containing any further available features or containing no features (via an empty element); any such additional features not defined herein MUST be defined by the relevant extension to XMPP. R: 7.4. SASL Definition The profiling requirements of [SASL] require that the following information be supplied by a protocol definition: service name: "xmpp" initiation sequence: After the initiating entity provides an opening XML stream header and the receiving entity replies in kind, the receiving entity provides a list of acceptable authentication methods. The initiating entity chooses one method from the list and sends it to the receiving entity as the value of the Saint-Andre Expires January 18, 2008 [Page 56] Internet-Draft XMPP Core July 2007 'mechanism' attribute possessed by an element, optionally including an initial response to avoid a round trip. exchange sequence: Challenges and responses are carried through the exchange of elements from receiving entity to initiating entity and elements from initiating entity to receiving entity. The receiving entity reports failure by sending a element and success by sending a element; the initiating entity aborts the exchange by sending an element. Upon successful negotiation, both sides consider the original XML stream to be closed and new stream headers are sent by both entities. security layer negotiation: The security layer takes effect immediately after sending the closing '>' character of the element for the receiving entity, and immediately after receiving the closing '>' character of the element for the initiating entity. The order of layers is first [TCP], then [TLS], then [SASL], then XMPP. use of the authorization identity: The authorization identity may be used by XMPP to denote the non-default of a client or the sending of a server; an empty string is equivalent to an absent authorization identity. 7.5. SASL Errors The following SASL-related error conditions are defined. 7.5.1. aborted The receiving entity acknowledges an element sent by the initiating entity; sent in reply to the element. I: R: 7.5.2. incorrect-encoding The data provided by the initiating entity could not be processed because the [BASE64] encoding is incorrect (e.g., because the encoding does not adhere to the definition in Section 4 of [BASE64]); sent in reply to a element or an element with initial response data. Saint-Andre Expires January 18, 2008 [Page 57] Internet-Draft XMPP Core July 2007 I: [ ... ] R: 7.5.3. invalid-authzid The authzid provided by the initiating entity is invalid, either because it is incorrectly formatted or because the initiating entity does not have permissions to authorize that ID; sent in reply to a element or an element with initial response data. I: [ ... ] R: 7.5.4. invalid-mechanism The initiating entity did not provide a mechanism or requested a mechanism that is not supported by the receiving entity; sent in reply to an element. I: R: 7.5.5. malformed-request The request is malformed (e.g., the element includes an initial response but the mechanism does not allow that); sent in reply to an , , , or element. I: [ ... ] R: Saint-Andre Expires January 18, 2008 [Page 58] Internet-Draft XMPP Core July 2007 7.5.6. mechanism-too-weak The mechanism requested by the initiating entity is weaker than server policy permits for that initiating entity; sent in reply to an element (with or without initial response data) or a element. I: R: 7.5.7. not-authorized The authentication failed because the initiating entity did not provide proper credentials; sent in reply to a element or an element with initial response data. I: [ ... ] R: Note: This error condition includes but is not limited to the case of incorrect credentials or an unknown username. In order to discourage directory harvest attacks, no differentiation is made between incorrect credentials and an unknown username. 7.5.8. temporary-auth-failure The authentication failed because of a temporary error condition within the receiving entity, and the initiating entity should try again later; sent in reply to an element or element. I: [ ... ] R: Saint-Andre Expires January 18, 2008 [Page 59] Internet-Draft XMPP Core July 2007 8. Resource Binding 8.1. Overview After a client authenticates with a server, it MUST bind a specific resource to the stream so that the server can properly address the client (see Section 3), i.e., there MUST be an XMPP resource identifier associated with the bare JID () of the client with the result that the address for use over that stream is a full JID of the form . This ensures that the server can deliver XML stanzas to and receive XML stanzas from the client (see Section 11). After binding a resource to the stream, the client is referred to as a connected resource. If, before completing the resource binding step, the client attempts to send an outbound XML stanza (i.e., a stanza not directed to the server itself or to the client's own account), the server MUST NOT process the stanza and SHOULD return a stream error to the client. Support for resource binding is REQUIRED in XMPP client and server implementations. 8.2. Advertising Support Upon receiving a success indication within the SASL negotiation, the client MUST send a new stream header to the server, to which the server MUST respond with a stream header as well as a list of available stream features. Specifically, for client-to-server streams the server MUST include a element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace in the stream features it presents to the client; this element SHOULD include an empty element to explicitly indicate that resource binding must be completed at this stage of the stream negotiation process. (Note: The server SHOULD NOT include the stream feature until after successful SASL negotiation.) Saint-Andre Expires January 18, 2008 [Page 60] Internet-Draft XMPP Core July 2007 S: S: Upon being so informed that resource binding is required, the client MUST bind a resource to the stream as described in the following sections. 8.3. Server-Generated Resource Identifier A server that supports resource binding MUST be able to generate an XMPP resource identifier on behalf of a client. The resource identifier generated by the server MUST at a minimum be unique among the connected resources for that and SHOULD be random since the resource identifier may be security-critical. It is RECOMMENDED that the server-generated resource identifier be a Universally Unique Identifier (UUID), for which the format specified in [UUID] is RECOMMENDED. It is RECOMMENDED for the client to ask its server to generate an appropriate resource identifier on its behalf, rather than generating a resource on its own and requesting that the server accept the client-generated resource identifer. 8.3.1. Success Case A client requests a server-generated resource identifier by sending an IQ stanza of type "set" (see Section 9.2.3) containing an empty element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace. C: Once the server has generated an XMPP resource identifier for the client, it MUST return an IQ stanza of type "result" to the client, Saint-Andre Expires January 18, 2008 [Page 61] Internet-Draft XMPP Core July 2007 which MUST include a child element that specifies the full JID for the connected resource as determined by the server. S: juliet@example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb 8.3.2. Error Case It is possible that the client is not allowed to bind a resource to the stream (e.g., because the node or user has reached a limit on the number of connected resources allowed). In this case, the server MUST return a stanza error to the client. S: 8.4. Client-Generated Resource Identifier A client MAY attempt to specify the resource identifier on its own rather than asking the server to generate a resource identifier on its behalf. 8.4.1. Success Case A client asks its server to accept a client-generated resource identifier by sending an IQ stanza of type "set" containing a element with a child element containing non-zero-length XML character data. C: balcony The server MAY accept the resource identifier provided by the client, in which case it returns an IQ stanza of type "result" to the client, including a child element that specifies the full JID for the connected resource. Saint-Andre Expires January 18, 2008 [Page 62] Internet-Draft XMPP Core July 2007 S: juliet@example.com/balcony However, the server MAY instead override the client-generated resource identifier and generate a resource identifier on behalf of the client, as shown in the previous section. 8.4.2. Error Cases When a client attempts to set its own XMPP resource identifier during resource binding, the following stanza error conditions are possible: o The client is not allowed to bind a resource to the stream (e.g., because the node or user has reached a limit on the number of connected resources allowed). o The provided resource identifier cannot be processed by the server, e.g. because it is not in accordance with the Resourceprep (Appendix B) profile of [STRINGPREP]). o The provided resource identifier is already in use but the server does not allow binding of multiple connected resources with the same identifier. 8.4.2.1. Not Allowed If the client is not allowed to bind a resource to the stream, the server MUST return a error. S: 8.4.2.2. Bad Request If the provided resource identifier cannot be processed by the server, the server SHOULD return a error (but MAY instead apply the Resourceprep (Appendix B) profile of [STRINGPREP] or otherwise process the resource identifier so that it is in conformance). S: Saint-Andre Expires January 18, 2008 [Page 63] Internet-Draft XMPP Core July 2007 8.4.2.3. Conflict If there is already a connected resource of the same name, the server MUST do one of the following: 1. Not accept the resource identifier provided by the client but instead override it with an XMPP resource identifier that the server generates. 2. Terminate the current resource and allow the newly-requested resource. 3. Disallow the newly-requested resource and maintain the current resource. Which of these the server does is up to the implementation, although it is RECOMMENDED to implement case #1. In case #2, the server MUST send a stream error to the current resource, terminate the XML stream and underlying TCP connection for the current resource, and return an IQ stanza of type "result" (indicating success) to the newly-requested resource. In case #3, the server MUST send a stanza error to the newly-requested resource but maintain the XML stream for that connection so that the newly-requested resource has an opportunity to negotiate a non-conflicting resource identifier before sending another request for resource binding. 8.5. Binding Multiple Resources A server MAY support binding of multiple resources to the same stream. This functionality is desirable in certain environments (e.g., for devices that are unable to open more than one TCP connection or when a machine runs a local XMPP client daemon that is used by multiple applications). 8.5.1. Support If a server supports binding of multiple resources to a stream, it MUST enable a client to unbind resources. A server that supports unbinding MUST also support binding of multiple resources. Thus a client can discover whether a server supports binding of multiple resources by determining if the server advertises a stream feature of , as follows. Saint-Andre Expires January 18, 2008 [Page 64] Internet-Draft XMPP Core July 2007 S: 8.5.2. Binding an Additional Resource A connected client binds an additional resource by following the protocol for binding of the original resource, i.e., by sending an IQ stanza of type "set" containing a element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace (either empty to request server generation of the resource identifier or containing a element with XML character data to request client generation of the resource identifier). 8.5.3. Unbinding a Resource 8.5.3.1. Success Case A client unbinds a resource by sending an IQ stanza of type "set" containing an element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace, which in turn contains a child element of whose XML character data specifies the resource to be unbound: C: someresource If no error occurs, the server MUST unbind the resource and no longer accept stanzas whose 'from' address specifies the full JID associated with that resource. S: When a client unbinds the only resource associated with the stream, the server SHOULD close the stream and terminate the TCP connection. S: S: Saint-Andre Expires January 18, 2008 [Page 65] Internet-Draft XMPP Core July 2007 8.5.3.2. Error Cases 8.5.3.2.1. Unbind Not Supported If the server does not understand the element, it MUST return a stanza error, which SHOULD be . S: 8.5.3.2.2. No Such Resource If there is no such resource for that stream, the server MUST return an error of . S: 8.5.4. From Addresses When a client binds multiple resources to the same stream, proper management of 'from' addresses is imperative. In particular, a client MUST specify a 'from' address on every stanza it sends over a stream to which it has bound multiple resources, where the 'from' address is the full JID () associated with the relevant resource. If a client does not specify a 'from' address on a stanza it sends over a stream to which it has bound multiple resources, the server MUST return the stanza to the client with an stanza error. C: Wherefore art thou? S: Wherefore art thou? Saint-Andre Expires January 18, 2008 [Page 66] Internet-Draft XMPP Core July 2007 Naturally, the rules regarding validation of asserted 'from' addresses still apply (see Section 11). 9. XML Stanzas After a client has connected to a server or two servers have connected to each other, either party can send XML stanzas over the negotiated stream. Three kinds of XML stanza are defined for the 'jabber:client' and 'jabber:server' namespaces: , , and . In addition, there are five common attributes for these stanza types. These common attributes, as well as the basic semantics of the three stanza types, are defined herein; more detailed information regarding the syntax of XML stanzas for instant messaging and presence applications is provided in [XMPP-IM], and for other applications in the relevant XMPP extension specifications. An XML stanza is the basic unit of meaning in XMPP. A server MUST NOT process a partial stanza and a server MUST NOT attach meaning to the transmission timing of any child element within a stanza. Support for the XML stanza syntax and semantics defined herein is REQUIRED in XMPP client and server implementations. 9.1. Common Attributes The following five attributes are common to message, presence, and IQ stanzas. 9.1.1. to The 'to' attribute specifies the JID of the intended recipient for the stanza. Art thou not Romeo, and a Montague? For information about server processing of inbound and outbound XML stanzas based on the nature of the 'to' address, refer to Section 11. 9.1.1.1. Client-to-Server Streams The following rules apply to the 'to' attribute in the context of XML streams qualified by the 'jabber:client' namespace (i.e., client-to- server streams). Saint-Andre Expires January 18, 2008 [Page 67] Internet-Draft XMPP Core July 2007 1. A stanza with a specific intended recipient MUST possess a 'to' attribute. 2. A stanza sent from a client to a server for direct processing by the server (e.g., presence sent to the server for broadcasting to other entities) SHOULD NOT possess a 'to' attribute. 9.1.1.2. Server-to-Server Streams The following rules apply to the 'to' attribute in the context of XML streams qualified by the 'jabber:server' namespace (i.e., server-to- server streams). 1. A stanza MUST possess a 'to' attribute; if a server receives a stanza that does not meet this restriction, it MUST generate an stream error and terminate both the XML stream and the underlying TCP connection with the offending server. 9.1.2. from The 'from' attribute specifies the JID of the sender. Art thou not Romeo, and a Montague? 9.1.2.1. Client-to-Server Streams The following rules apply to the 'from' attribute in the context of XML streams qualified by the 'jabber:client' namespace (i.e., client- to-server streams). 1. When the server receives an XML stanza from a client and the stanza does not include a 'from' attribute, the server MUST add a 'from' attribute to the stanza, where the value of the 'from' attribute is the full JID () determined by the server for the connected resource that generated the stanza (see Section 3.5), or the bare JID () in the case of subscription-related presence stanzas (see [XMPP-IM]). 2. When the server receives an XML stanza from a client and the stanza includes a 'from' attribute, the server MUST either (a) validate that the value of the 'from' attribute provided by the client is that of a connected resource for the associated entity or (b) override the provided 'from' attribute by adding a 'from' attribute as specified under Rule #1. Saint-Andre Expires January 18, 2008 [Page 68] Internet-Draft XMPP Core July 2007 3. When the server generates a stanza from the server for delivery to the client on behalf of the account of the connected client (e.g., in the context of data storage services provided by the server on behalf of the client), the stanza MUST either (a) not include a 'from' attribute or (b) include a 'from' attribute whose value is the account's bare JID (). 4. When the server generates a stanza from the server itself for delivery to the client, the stanza MUST include a 'from' attribute whose value is the mere domain () of the server. 5. A server MUST NOT send to the client a stanza without a 'from' attribute if the stanza was not generated by the server (e.g., if it was generated by another client or another server); therefore, when a client receives a stanza that does not include a 'from' attribute, it MUST assume that the stanza is from the server to which the client is connected. 9.1.2.2. Server-to-Server Streams The following rules apply to the 'from' attribute in the context of XML streams qualified by the 'jabber:server' namespace (i.e., server- to-server streams). 1. A stanza MUST possess a 'from' attribute; if a server receives a stanza that does not meet this restriction, it MUST generate an stream error and terminate the underlying TCP connection. 2. The domain identifier portion of the JID contained in the 'from' attribute MUST match the hostname of the sending server (or any validated domain thereof, such as a validated local domain hosted by the sending server) as communicated in the SASL negotiation (see Section 7), server dialback (see [XEP-0220], or similar means; if a server receives a stanza that does not meet this restriction, it MUST generate an stream error and terminate the underlying TCP connection. Enforcement of these rules helps to prevent a denial of service attack launched from a rogue server. 9.1.3. id The 'id' attribute MAY be used by a sending entity for internal tracking of stanzas that it sends and receives (especially for tracking the request-response interaction inherent in the semantics of IQ stanzas). It is OPTIONAL for the value of the 'id' attribute to be unique globally, within a domain, or within a stream. The semantics of IQ stanzas impose additional restrictions; see Section 9.2.3. Saint-Andre Expires January 18, 2008 [Page 69] Internet-Draft XMPP Core July 2007 9.1.4. type The 'type' attribute specifies detailed information about the purpose or context of the message, presence, or IQ stanza. The particular allowable values for the 'type' attribute vary depending on whether the stanza is a message, presence, or IQ stanza. The defined values for message and presence stanzas are specific to instant messaging and presence applications and therefore are specified in [XMPP-IM], whereas the values for IQ stanzas specify the role of an IQ stanza in a structured request-response exchange and thus are specified under Section 9.2.3. The only 'type' value common to all three stanzas is "error"; see Section 9.3. 9.1.5. xml:lang A stanza SHOULD possess an 'xml:lang' attribute (as defined in Section 2.12 of [XML]) if the stanza contains XML character data that is intended to be presented to a human user (as explained in [CHARSET], "internationalization is for humans"). The value of the 'xml:lang' attribute specifies the default language of any such human-readable XML character data. dnd Wooing Juliet The value of the 'xml:lang' attribute MAY be overridden by the 'xml: lang' attribute of a specific child element. dnd Wooing Juliet Dvořím se Julii dnd Wooing Juliet S: dnd Wooing Juliet If an inbound stanza received does not possess an 'xml:lang' attribute, an implementation MUST assume that the default language is that specified for the stream as defined under Section 5.3. The value of the 'xml:lang' attribute MUST conform to the NMTOKEN datatype (as defined in Section 2.3 of [XML]) and MUST conform to the format defined in [LANGTAGS]. A server MUST NOT modify or delete 'xml:lang' attributes on stanzas it receives from other entities. 9.2. Basic Semantics 9.2.1. Message Semantics The stanza can be seen as a "push" mechanism whereby one entity pushes information to another entity, similar to the communications that occur in a system such as email. All message stanzas SHOULD possess a 'to' attribute that specifies the intended recipient of the message; upon receiving such a stanza, a server SHOULD route or deliver it to the intended recipient (see Section 11 for general routing and delivery rules related to XML stanzas). 9.2.2. Presence Semantics The stanza can be seen as a specialized broadcast or "publish-subscribe" mechanism, whereby multiple entities receive information about an entity to which they have subscribed (in this case, network availability information). In general, a publishing entity (client) SHOULD send a presence stanza with no 'to' attribute, in which case the server to which the entity is connected SHOULD broadcast or multiplex that stanza to all subscribing entities. However, a publishing entity MAY also send a presence stanza with a 'to' attribute, in which case the server SHOULD route or deliver that stanza to the intended recipient. See Section 11 for general routing and delivery rules related to XML stanzas, and [XMPP-IM] for rules Saint-Andre Expires January 18, 2008 [Page 71] Internet-Draft XMPP Core July 2007 specific to presence applications. 9.2.3. IQ Semantics Info/Query, or IQ, is a request-response mechanism, similar in some ways to [HTTP]. The semantics of IQ enable an entity to make a request of, and receive a response from, another entity. The data content of the request and response is defined by the schema or other structural definition associated with the XML namespace that qualifies the direct child element of the IQ element (see Section 9.4), and the interaction is tracked by the requesting entity through use of the 'id' attribute. Thus, IQ interactions follow a common pattern of structured data exchange such as get/result or set/ result (although an error may be returned in reply to a request if appropriate): Requesting Responding Entity Entity ---------- ---------- | | | | | [ ... payload ... ] | | | | -------------------------> | | | | | | [ ... payload ... ] | | | | <------------------------- | | | | | | [ ... payload ... ] | | | | -------------------------> | | | | | | [ ... condition ... ] | | | | <------------------------- | | | In order to enforce these semantics, the following rules apply: 1. The 'id' attribute is REQUIRED for IQ stanzas. 2. The 'type' attribute is REQUIRED for IQ stanzas. The value MUST be one of the following: Saint-Andre Expires January 18, 2008 [Page 72] Internet-Draft XMPP Core July 2007 * get -- The stanza is a request for information or requirements. * set -- The stanza provides required data, sets new values, or replaces existing values. * result -- The stanza is a response to a successful get or set request. * error -- An error has occurred regarding processing or delivery of a previously-sent get or set (see Section 9.3). 3. An entity that receives an IQ request of type "get" or "set" MUST reply with an IQ response of type "result" or "error". The response MUST preserve the 'id' attribute of the request. 4. An entity that receives a stanza of type "result" or "error" MUST NOT respond to the stanza by sending a further IQ response of type "result" or "error"; however, the requesting entity MAY send another request (e.g., an IQ of type "set" in order to provide required information discovered through a get/result pair). 5. An IQ stanza of type "get" or "set" MUST contain one and only one child element that specifies the semantics of the particular request. 6. An IQ stanza of type "result" MUST include zero or one child elements. 7. An IQ stanza of type "error" MAY include the child element contained in the associated "get" or "set" and MUST include an child; for details, see Section 9.3. 9.3. Stanza Errors Stanza-related errors are handled in a manner similar to stream errors (Section 5.8). Unlike stream errors, stanza errors are recoverable; therefore they do not result in termination of the XML stream and underlying TCP connection. Instead, the entity that discovers the error condition returns an ERROR STANZA to the sender, i.e., a stanza of the same kind (message, presence, or IQ) whose 'type' attribute is set to a value of "error" and which contains an child element that specifies the error condition. The specified error condition provides a hint regarding actions that the sender can take to remedy the error. 9.3.1. Rules The following rules apply to stanza errors: 1. The receiving or processing entity that detects an error condition in relation to a stanza SHOULD return an error stanza (and MUST do so for IQ stanzas). 2. The entity that generates an error stanza MAY include the original XML sent so that the sender can inspect and, if necessary, correct the XML before attempting to resend. Saint-Andre Expires January 18, 2008 [Page 73] Internet-Draft XMPP Core July 2007 3. An error stanza MUST contain an child element. 4. An child MUST NOT be included if the 'type' attribute has a value other than "error" (or if there is no 'type' attribute). 5. An entity that receives an error stanza MUST NOT respond to the stanza with a further error stanza; this helps to prevent looping. 9.3.2. Syntax The syntax for stanza-related errors is: [OPTIONAL to include sender XML here] [ OPTIONAL descriptive text ] [OPTIONAL application-specific condition element] The "stanza-kind" is one of message, presence, or iq. The value of the element's 'type' attribute MUST be one of the following: o cancel -- do not retry (the error cannot be remedied) o continue -- proceed (the condition was only a warning) o modify -- retry after changing the data sent o auth -- retry after providing credentials o wait -- retry after waiting (the error is temporary) The element: o MUST contain a child element corresponding to one of the defined stanza error conditions specified; this element MUST be qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' 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-stanzas' namespace and SHOULD possess an 'xml:lang' attribute specifying the natural language of the XML character data. o MAY contain a child element for an application-specific error condition; this element MUST be qualified by an application- defined namespace, and its structure is defined by that namespace. Saint-Andre Expires January 18, 2008 [Page 74] Internet-Draft XMPP Core July 2007 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 user, but MAY be shown in addition to the error message associated with the included condition element (or elements). Finally, to maintain backward compatibility, the schema for the default namespace (specified in [XMPP-IM]) allows the optional inclusion of a 'code' attribute on the element; for details, see [XEP-0086]. 9.3.3. Defined Conditions The following conditions are defined for use in stanza errors. 9.3.3.1. bad-request The sender has sent a stanza containing XML that does not conform to the appropriate schema or that cannot be processed (e.g., an IQ stanza that includes an unrecognized value of the 'type' attribute); the associated error type SHOULD be "modify". C: S: 9.3.3.2. conflict Access cannot be granted because an existing resource exists with the same name or address; the associated error type SHOULD be "cancel". Saint-Andre Expires January 18, 2008 [Page 75] Internet-Draft XMPP Core July 2007 C: balcony S: 9.3.3.3. feature-not-implemented The feature represented in the XML stanza is not implemented by the recipient or server and therefore the stanza cannot be processed; the associated error type SHOULD be "cancel" or "modify". C: E: 9.3.3.4. forbidden The requesting entity does not possess the required permissions to perform the action; the associated error type SHOULD be "auth". Saint-Andre Expires January 18, 2008 [Page 76] Internet-Draft XMPP Core July 2007 C: E: 9.3.3.5. gone The recipient or server can no longer be contacted at this address (the error stanza MAY contain a new address in the XML character data of the element); the associated error type SHOULD be "cancel" or "modify". C: E: conference.example.com 9.3.3.6. internal-server-error -- the server could not process the stanza because of a misconfiguration or an otherwise-undefined internal server error; the associated error type SHOULD be "wait" or "cancel". Saint-Andre Expires January 18, 2008 [Page 77] Internet-Draft XMPP Core July 2007 C: E: 9.3.3.7. item-not-found The addressed JID or item requested cannot be found; the associated error type SHOULD be "cancel" or "modify". C: someresource S: An application MUST NOT return this error if doing so would provide information about the intended recipient's network availability to an entity that is not authorized to know such information; instead it SHOULD return a error. 9.3.3.8. jid-malformed The sending entity has provided or communicated an XMPP address (e.g., a value of the 'to' attribute) or aspect thereof (e.g., an XMPP resource identifier) that does not adhere to the syntax defined under Section 3; the associated error type SHOULD be "modify". Saint-Andre Expires January 18, 2008 [Page 78] Internet-Draft XMPP Core July 2007 C: E: 9.3.3.9. not-acceptable The recipient or server understands the request but is refusing to process it because it does not meet criteria defined by the recipient or server (e.g., a local policy regarding stanza size limits or acceptable words in messages); the associated error type SHOULD be "modify". C: [ ... the-emacs-manual ... ] S: 9.3.3.10. not-allowed The recipient or server does not allow any entity to perform the action (e.g., sending to entities at a blacklisted domain); the associated error type SHOULD be "cancel". Saint-Andre Expires January 18, 2008 [Page 79] Internet-Draft XMPP Core July 2007 C: E: 9.3.3.11. not-authorized The sender must provide proper credentials before being allowed to perform the action, or has provided improper credentials; the associated error type SHOULD be "auth". C: E: 9.3.3.12. not-modified The item requested has not changed since it was last requested; the associated error type SHOULD be "continue". Saint-Andre Expires January 18, 2008 [Page 80] Internet-Draft XMPP Core July 2007 C:
some-long-opaque-string
S:
some-long-opaque-string
9.3.3.13. payment-required The requesting entity is not authorized to access the requested service because payment is required; the associated error type SHOULD be "auth". C: E: Saint-Andre Expires January 18, 2008 [Page 81] Internet-Draft XMPP Core July 2007 9.3.3.14. recipient-unavailable The intended recipient is temporarily unavailable; the associated error type SHOULD be "wait". C: E: An application MUST NOT return this error if doing so would provide information about the intended recipient's network availability to an entity that is not authorized to know such information; instead it SHOULD return a error. 9.3.3.15. redirect The recipient or server is redirecting requests for this information to another entity, typically in a temporary fashion (the error stanza SHOULD contain the alternate address, which SHOULD be a valid JID, in the XML character data of the element); the associated error type SHOULD be "modify". C: E: characters@conference.example.org Saint-Andre Expires January 18, 2008 [Page 82] Internet-Draft XMPP Core July 2007 9.3.3.16. registration-required The requesting entity is not authorized to access the requested service because prior registration is required; the associated error type SHOULD be "auth". C: E: 9.3.3.17. remote-server-not-found A remote server or service specified as part or all of the JID of the intended recipient does not exist; the associated error type SHOULD be "cancel". C: E: Saint-Andre Expires January 18, 2008 [Page 83] Internet-Draft XMPP Core July 2007 9.3.3.18. remote-server-timeout A remote server or service specified as part or all of the JID of the intended recipient (or required to fulfill a request) could not be contacted within a reasonable amount of time; the associated error type SHOULD be "wait". C: E: 9.3.3.19. resource-constraint The server or recipient lacks the system resources necessary to service the request; the associated error type SHOULD be "wait". C: E: Saint-Andre Expires January 18, 2008 [Page 84] Internet-Draft XMPP Core July 2007 9.3.3.20. service-unavailable The server or recipient does not currently provide the requested service; the associated error type SHOULD be "cancel". C: Hello? S: An application SHOULD return a error instead of or if sending one of the latter errors would provide information about the intended recipient's network availability to an entity that is not authorized to know such information. 9.3.3.21. subscription-required The requesting entity is not authorized to access the requested service because a subscription is required; the associated error type SHOULD be "auth". C: help E: Saint-Andre Expires January 18, 2008 [Page 85] Internet-Draft XMPP Core July 2007 9.3.3.22. undefined-condition The error condition is not one of those defined by the other conditions in this list; any error type may be associated with this condition, and it SHOULD be used only in conjunction with an application-specific condition. C: My lord, dispatch; read o'er these articles. S: 9.3.3.23. unexpected-request The recipient or server understood the request but was not expecting it at this time (e.g., the request was out of order); the associated error type SHOULD be "wait" or "modify". Saint-Andre Expires January 18, 2008 [Page 86] Internet-Draft XMPP Core July 2007 C: E: 9.3.3.24. unknown-sender The stanza 'from' address specified by a connected client is not valid for the stream (e.g., the stanza does not include a 'from' address when multiple resources are bound to the stream); the associated error type SHOULD be "modify". C: Wherefore art thou? S: Wherefore art thou? 9.3.4. Application-Specific Conditions As noted, an application MAY provide application-specific stanza error information by including a properly-namespaced child in the error element. The application-specific element SHOULD supplement or further qualify a defined element. Thus, the element will Saint-Andre Expires January 18, 2008 [Page 87] Internet-Draft XMPP Core July 2007 contain two or three child elements: [ ... application-specific information ... ] 9.4. Extended Content While the message, presence, and IQ stanzas provide basic semantics for messaging, availability, and request-response interactions, XMPP uses XML namespaces (see [XML-NAMES] to extend the basic stanza syntax for the purpose of providing additional functionality. Thus a message or presence stanza MAY contain one or more optional child elements specifying content that extends the meaning of the message (e.g., an XHTML-formatted version of the message body as described in [XEP-0071]), and an IQ stanza of type "get" or "set" MUST contain one such child element. This child element MAY have any name and MUST possess an 'xmlns' namespace declaration (other than "jabber:client", "jabber:server", or "http://etherx.jabber.org/streams") that defines all data contained within the child element. Such a child element is said to be EXTENDED CONTENT and its namespace name is said to be an EXTENDED NAMESPACE. Support for any given extended namespace is OPTIONAL on the part of any implementation. If an entity does not understand such a namespace, the entity's expected behavior depends on whether the entity is (1) the recipient or (2) an entity that is routing the stanza to the recipient: Saint-Andre Expires January 18, 2008 [Page 88] Internet-Draft XMPP Core July 2007 Recipient: If a recipient receives a stanza that contains a child element it does not understand, it SHOULD silently ignore that particular XML data, i.e., it SHOULD not process it or present it to a user or associated application (if any). In particular: * If an entity receives a message or presence stanza that contains XML data qualified by a namespace it does not understand, the portion of the stanza that qualified by the unknown namespace SHOULD be ignored. * If an entity receives a message stanza whose only child element is qualified by a namespace it does not understand, it MUST ignore the entire stanza. * If an entity receives an IQ stanza of type "get" or "set" containing a child element qualified by a namespace it does not understand, the entity SHOULD return an IQ stanza of type "error" with an error condition of . Router: If a routing entity (typically a server) handles a stanza that contains a child element it does not understand, it SHOULD ignore the associated XML data by routing or delivering it untouched to the recipient. 10. Examples 10.1. Client-to-Server The following examples show the XMPP data flow for a client negotiating an XML stream with a server, exchanging XML stanzas, and closing the negotiated stream. The server is "example.com", the server requires use of TLS, the client authenticates via the SASL DIGEST-MD5 mechanism as "juliet@example.com", and the client binds a server-generated resource to the stream. It is assumed that before sending the initial stream header, the client has already resolved an SRV record of _xmpp-client._tcp.example.com and has opened a TCP connection to the advertised port at the resolved IP address. Note: The alternate steps shown are provided only to illustrate the protocol for failure cases; they are not exhaustive and would not necessarily be triggered by the data sent in the examples. Saint-Andre Expires January 18, 2008 [Page 89] Internet-Draft XMPP Core July 2007 10.1.1. TLS Step 1: Client initiates stream to server: C: Step 2: Server responds by sending a stream header to client: S: Step 4: Client sends STARTTLS command to server: C: Step 5: Server informs client that it is allowed to proceed: S: Step 5 (alt): Server informs client that TLS negotiation has failed and closes both XML stream and TCP connection: S: S: Step 6: Client and server attempt to complete TLS negotiation over Saint-Andre Expires January 18, 2008 [Page 90] Internet-Draft XMPP Core July 2007 the existing TCP connection (see [TLS] for details). Step 7: If TLS negotiation is successful, client initiates a new stream to server: C: Step 7 (alt): If TLS negotiation is unsuccessful, server closes TCP connection. 10.1.2. SASL Step 8: Server responds by sending a stream header to client along with any available stream features: S: DIGEST-MD5 PLAIN Step 9: Client selects an authentication mechanism, in this case [DIGEST-MD5] with an empty authorization identity ("="): C: = Saint-Andre Expires January 18, 2008 [Page 91] Internet-Draft XMPP Core July 2007 Step 10: Server sends a [BASE64] encoded challenge to client: S: cmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLHFvcD0i YXV0aCIsY2hhcnNldD11dGYtOCxhbGdvcml0aG09bWQ1LXNlc3MK The decoded challenge is: realm="example.com",nonce="OA6MG9tEQGm2hh", qop="auth",charset=utf-8,algorithm=md5-sess Note: When the server sends a DIGEST-MD5 challenge to the client, the qop list must be quoted since it is a list rather than a single item (even if there is only one item in the list); however, when the client sends its response to the server, the qop must not be quoted since it is a single item rather than a list. Step 10 (alt): Server returns error to client: S: S: Step 11: Client sends a [BASE64] encoded response to the challenge: C: dXNlcm5hbWU9Imp1bGlldCIscmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2 TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAwMDAwMDAx LHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20iLHJlc3BvbnNl PWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNoYXJzZXQ9dXRmLTgK The decoded response is: username="juliet",realm="example.com", nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk", nc=00000001,qop=auth,digest-uri="xmpp/example.com", response=d388dad90d4bbd760a152321f2143af7,charset=utf-8 Step 12: Server informs client of success and includes [BASE64] encoded value for subsequent authentication: S: cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo= Saint-Andre Expires January 18, 2008 [Page 92] Internet-Draft XMPP Core July 2007 The decoded value for subsequent authentication is: rspauth=ea40f60335c427b5527b84dbabcdfffd Step 12 (alt): Server returns error to client: S: Step 13: Client initiates a new stream to server: C: S: Upon being so informed that resource binding is required, the client MUST bind a resource to the stream; here we assume that the client asks the server to generate a resource identifier on its behalf. Saint-Andre Expires January 18, 2008 [Page 93] Internet-Draft XMPP Core July 2007 Step 15: Client binds a resource: C: Step 16: Server generates resource identifier and informs client of successful resource binding: S: juliet@example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb 10.1.4. Stanza Exchange Now the client is allowed to send XML stanzas over the negotiated stream. C: Art thou not Romeo, and a Montague? If necessary, sender's server negotiates XML streams with intended recipient's server (see Section 10.2). The intended recipient replies and the message is delivered to the client. E: Neither, fair saint, if either thee dislike. The client may send and receive an unbounded number of subsequent XML stanzas over the stream. Saint-Andre Expires January 18, 2008 [Page 94] Internet-Draft XMPP Core July 2007 10.1.5. Close Desiring to send no further messages, the client closes the stream. C: Consistent with the recommended stream closing handshake, server closes stream as well: S: Client now terminates the underlying TCP connection. 10.2. Server-to-Server Examples The following examples show the data flow for a server negotiating an XML stream with another server, exchanging XML stanzas, and closing the negotiated stream. The initiating server ("Server1") is example.com; the receiving server ("Server2") is example.net and it requires use of TLS; example.com presents a certificate and authenticates via the SASL EXTERNAL mechanism. It is assumed that before sending the initial stream header, Server1 has already resolved an SRV record of _xmpp-client._tcp.example.net and has opened a TCP connection to the advertised port at the resolved IP address. Note: The alternate steps shown are provided only to illustrate the protocol for failure cases; they are not exhaustive and would not necessarily be triggered by the data sent in the examples. 10.2.1. TLS Step 1: Server1 initiates stream to Server2: S1: Saint-Andre Expires January 18, 2008 [Page 95] Internet-Draft XMPP Core July 2007 Step 2: Server2 responds by sending a stream tag to Server1: S2: Step 3: Server2 sends stream features to Server1: S2: Step 4: Server1 sends the STARTTLS command to Server2: S1: Step 5: Server2 informs Server1 that it is allowed to proceed: S2: Step 5 (alt): Server2 informs Server1 that TLS negotiation has failed and closes stream: S2: S2: Step 6: Server1 and Server2 attempt to complete TLS negotiation via TCP. Step 7: If TLS negotiation is successful, Server1 initiates a new stream to Server2: S1: Step 7 (alt): If TLS negotiation is unsuccessful, Server2 closes TCP connection. Saint-Andre Expires January 18, 2008 [Page 96] Internet-Draft XMPP Core July 2007 10.2.2. SASL Step 8: Server2 responds by sending a stream header to Server1 along with available stream features (including a preference for the SASL EXTERNAL mechanism): S2: S2: EXTERNAL DIGEST-MD5 Step 9: Server1 selects the EXTERNAL mechanism, in this case with an authorization identity encoded according to [BASE64]: S1: ZXhhbXBsZS5jb20K The decoded authorization identity is "example.com". Step 10: Server2 determines that the authorization identity provided by Server1 matches the information in the presented certificate and therefore returns success: S2: Step 11 (alt): Server2 informs Server1 of failed authentication: S2: S2: Saint-Andre Expires January 18, 2008 [Page 97] Internet-Draft XMPP Core July 2007 Step 12: Server1 initiates a new stream to Server2: S1: Step 13: Server2 responds by sending a stream header to Server1 along with any additional features (or, in this case, an empty features element): S2: S2: 10.2.3. Stanza Exchange Now Server1 is allowed to send XML stanzas to Server2 over the negotiated stream; here we assume that the transferred stanzas are those shown earlier for client-to-server communication. Server1 sends XML stanza to Server2: S1: Art thou not Romeo, and a Montague? The intended recipient replies and the message is delivered from Server2 to Server1. Server2 sends XML stanza to Server1: S2: Neither, fair saint, if either thee dislike. Saint-Andre Expires January 18, 2008 [Page 98] Internet-Draft XMPP Core July 2007 10.2.4. Close Desiring to send no further messages, Server1 closes the stream. (In practice, the stream would most likely remain open for some time, since Server1 and Server2 do not immediately know if the stream will be needed for further communication.) S1: Consistent with the recommended stream closing handshake, Server2 closes stream as well: S2: Server1 now terminates the underlying TCP connection. 11. Server Rules for Processing XML Stanzas An XMPP server MUST ensure in-order processing of XML stanzas between any two entities. This includes stanzas sent by a client to its server for direct processing by the server (e.g., in-order processing of a roster get and initial presence as decribed in [XMPP-IM]). Beyond the requirement for in-order processing, each server implementation will contain its own logic for processing stanzas it receives. Such logic determines whether the server needs to ROUTE a given stanza to another domain, DELIVER it to a local entity (typically a connected client associated with a local account), or HANDLE it directly within the server itself. The following rules apply. Note: Particular XMPP applications MAY specify delivery rules that modify or supplement the following rules; for example, a set of delivery rules for instant messaging and presence applications is defined in [XMPP-IM]. 11.1. No 'to' Address 11.1.1. Overview If the stanza possesses no 'to' attribute, the server SHOULD handle it directly on behalf of the entity that sent it. Because all stanzas received from other servers MUST possess a 'to' attribute, this rule applies only to stanzas received from a local entity (such as a client) that is connected to the server. Saint-Andre Expires January 18, 2008 [Page 99] Internet-Draft XMPP Core July 2007 11.1.2. Message If the server receives a message stanza with no 'to' attribute, it SHOULD handle it directly, which may include returning an error to the sending entity. 11.1.3. Presence If the server receives a presence stanza with no 'to' attribute, it SHOULD broadcast it to the entities that are subscribed to the sending entity's presence, if applicable (the semantics of presence broadcast for presence applications are defined in [XMPP-IM]). 11.1.4. IQ If the server receives an IQ stanza of type "get" or "set" with no 'to' attribute, it MUST do the following: 1. If it understands the namespace that qualifies the content of the stanza, it MUST either handle the stanza directly on behalf of sending entity (where the meaning of "handle" is determined by the semantics of the qualifying namespace) or return an appropriate error to the sending entity. 2. If it does not understand the namespace that qualifies the content of the stanza, it MUST return an error to the sending entity, which SHOULD be . 11.2. Local Domain If the hostname of the domain identifier portion of the JID contained in the 'to' attribute matches one of the configured hostnames of the server itself, the server MUST do one of the following: 1. Handle the stanza itself. 2. Route the stanza to a specialized service that is responsible for that local domain. 3. Return an error to the sender if the service providing the local domain is not available. 11.3. Resource at Local Domain If the hostname of the domain identifier portion of the JID contained in the 'to' attribute matches a configured hostname of the server itself and the JID contained in the 'to' attribute is of the form , the server (or a defined resource thereof) MUST either handle the stanza as appropriate for the stanza kind or return an error stanza to the sender. Saint-Andre Expires January 18, 2008 [Page 100] Internet-Draft XMPP Core July 2007 11.4. Node at Local Domain If the hostname of the domain identifier portion of the JID contained in the 'to' attribute matches a configured hostname of the server itself and the JID contained in the 'to' attribute is a bare JID () or full JID (), the server SHOULD deliver the stanza to the intended recipient of the stanza as represented by the JID contained in the 'to' attribute. The following rules apply: 1. If the JID contains an XMPP resource identifier (i.e., is of the form ) and there exists a connected resource that exactly matches the full JID, the recipient's server SHOULD deliver the stanza to that connection. 2. If the JID contains an XMPP resource identifier and there exists no connected resource that exactly matches the full JID, the recipient's server SHOULD return a stanza error to the sender. 3. If the JID is of the form and there exists at least one connected resource for the node, the recipient's server SHOULD deliver the stanza to at least one of the connected resources if the stanza is a message or presence stanza and SHOULD handle it directly on behalf of the node if the stanza is an IQ stanza. 11.5. Foreign Domain 11.5.1. Overview If the hostname of the domain identifier portion of the JID contained in the 'to' attribute does not match one of the configured hostnames of the server itself, the server SHOULD attempt to route the stanza to the foreign domain (subject to local service provisioning and security policies regarding inter-domain communication, since such communication is optional for any given deployment). There are two possible cases. 11.5.2. Existing Stream If a server-to-server stream already exists between the two domains, the sender's server shall attempt to route the stanza to the authoritative server for the foreign domain over the existing stream. 11.5.3. No Existing Stream If there exists no server-to-server stream between the two domains, the sender's server shall proceed as follows: Saint-Andre Expires January 18, 2008 [Page 101] Internet-Draft XMPP Core July 2007 1. Resolve the hostname of the foreign domain (as defined under Section 15.4). 2. Negotiate a server-to-server stream between the two domains (as defined under Section 6 and Section 7). 3. Route the stanza to the authoritative server for the foreign domain over the newly-established stream. 11.5.4. Error Handling If routing to the intended recipient's server is unsuccessful, the sender's server MUST return an error to the sender, which SHOULD be if resolution of the foreign domain is unsuccessful and if resolution succeeds but streams cannot be negotiated. If stream negotiation with the intended recipient's server is successful but the foreign server cannot deliver the stanza to the recipient, the foreign server shall return an error to the sender by way of the sender's server. 12. XML Usage 12.1. Restrictions XMPP is a simplified and specialized protocol for streaming XML elements in order to exchange structured information in close to real time. Because XMPP does not require the parsing of arbitrary and complete XML documents, there is no requirement that XMPP needs to support the full feature set of [XML]. In particular, the following features of XML are prohibited in XMPP: o comments (as defined in Section 2.5 of [XML]) o processing instructions (Section 2.6 therein) o internal or external DTD subsets (Section 2.8 therein) o internal or external entity references (Section 4.2 therein) with the exception of predefined entities (Section 4.6 therein) o character data or attribute values containing unescaped characters that map to the predefined entities (Section 4.6 therein); such characters MUST be escaped An XMPP implementation MUST behave as follow with regard to these features: 1. An XMPP implementation MUST NOT inject characters matching such features into an XML stream. Saint-Andre Expires January 18, 2008 [Page 102] Internet-Draft XMPP Core July 2007 2. If an XMPP implementation receives characters matching such features over an XML stream, it MUST return a stream error, which SHOULD be but MAY be . 12.2. XML Namespace Names and Prefixes XML namespaces (see [XML-NAMES]) are used within all XMPP-compliant XML to create strict boundaries of data ownership. The basic function of namespaces is to separate different vocabularies of XML elements that are structurally mixed together. Ensuring that XMPP- compliant XML is namespace-aware enables any allowable XML to be structurally mixed with any data element within XMPP. Rules for XML namespace names and prefixes are defined in the following subsections. 12.2.1. Streams Namespace A streams namespace declaration is REQUIRED in all XML stream headers and the name of the streams namespace MUST be 'http://etherx.jabber.org/streams'. If this rule is violated, the entity that receives the offending stream header MUST return a stream error to the sending entity, which SHOULD be but MAY be . The element names of the element and its and children MUST be qualified by the streams namespace prefix in all instances. If this rule is violated, the entity that receives the offending element MUST return a stream error to the sending entity, which SHOULD be . An implementation SHOULD generate only the 'stream:' prefix for these elements, and for historical reasons MAY accept only the 'stream:' prefix. If an entity receives a stream header with a streams namespace prefix it does not accept, it MUST return a stream error to the sending entity, which SHOULD be but MAY be . 12.2.2. Default Namespace A default namespace declaration is REQUIRED and is used in all XML streams in order to define the allowable first-level children of the root stream element. This namespace declaration MUST be the same for the initial stream and the response stream so that both streams are qualified consistently. The default namespace declaration applies to the stream and all stanzas sent within a stream (unless explicitly qualified by another namespace or by the prefix of the streams namespace). Saint-Andre Expires January 18, 2008 [Page 103] Internet-Draft XMPP Core July 2007 A server implementation MUST support the following two default namespaces (for historical reasons, an implementation MAY support only these two default namespaces): o jabber:client -- this default namespace is declared when the stream is used for communication between a client and a server o jabber:server -- this default namespace is declared when the stream is used for communication between two servers A client implementation MUST support the 'jabber:client' default namespace, and for historical reasons MAY support only that default namespace. If an implementation accepts a stream that is qualified by the 'jabber:client' or 'jabber:server' namespace, it MUST support the common attributes (Section 9.1) and basic semantics (Section 9.2) of all three core stanza types (message, presence, and IQ). An implementation MUST NOT generate namespace prefixes for elements qualified by the default namespace if the default namespace is 'jabber:client' or 'jabber:server'. Note: The 'jabber:client' and 'jabber:server' namespaces are nearly identical but are used in different contexts (client-to-server communication for 'jabber:client' and server-to-server communication for 'jabber:server'). The only difference between the two is that the 'to' and 'from' attributes are OPTIONAL on stanzas sent over XML streams qualified by the 'jabber:client' namespace, whereas they are REQUIRED on stanzas sent over XML streams qualified by the 'jabber: server' namespace. 12.2.3. Extended Namespaces An EXTENDED NAMESPACE is an XML namespace (other than the default namespace) that is used to qualify XML data contained within an XML stanza. For example, in the following stanza, the extended namespace is 'jabber:iq:roster': An XML stanza MAY contain XML data qualified by more than one extended namespace, either at the direct child level of the stanza (for presence and message stanzas) or in any mix of levels (for all stanzas). Saint-Andre Expires January 18, 2008 [Page 104] Internet-Draft XMPP Core July 2007 sha1-hash-of-image Hello?

Hello?

some-long-opaque-string
An implementation SHOULD NOT generate namespace prefixes for elements qualified by content (as opposed to stream) namespaces other than 'jabber:client' and 'jabber:server'. However, if included, the namespace declarations for those prefixes MUST be included on the stanza root or a child thereof, not at the level of the stream element (this helps to ensure that any such namespace declaration is routed and delivered with the stanza, instead of assumed from the stream). 12.3. Validation A server is not responsible for ensuring that XML data delivered to a client or routed to another server is valid as the term "valid" is defined in Section 2.8 of [XML]. An implementation MAY choose to provide only validated data, but such behavior is OPTIONAL. A client SHOULD NOT rely on the ability to send data that does not conform to the schemas, and SHOULD ignore any non-conformant elements or attributes on the incoming XML stream. Saint-Andre Expires January 18, 2008 [Page 105] Internet-Draft XMPP Core July 2007 Note: The terms "valid" and "well-formed" are distinct in XML. All XMPP data MUST be well-formed as the term "well-formed" is defined in Section 2.1 of [XML]. 12.4. Inclusion of Text Declaration Implementations SHOULD send a text declaration before sending a stream header. Applications MUST follow the rules in [XML] regarding the circumstances under which a text declaration is included. 12.5. Character Encoding Implementations MUST support the [UTF-8] transformation of Universal Character Set ([UCS2]) characters, as required by [CHARSET]. Implementations MUST NOT attempt to use any other encoding. If an implementation detects that an entity has attempted to send XML data with an encoding other than UTF-8, it MUST return a stream error, which SHOULD be . 12.6. White Space Except where explicitly disallowed (i.e., during TLS negotiation (Section 6) and SASL negotiation (Section 7)), either entity MAY send white space characters (matching production [3] content of [XML]) within the root stream element as separators between XML stanzas or between any other first-level elements sent over the stream; one common use for sending such white space characters is to check the viability of the underlying TCP connection after a period of inactivity. 13. Compliance Requirements This section summarizes the specific aspects of the Extensible Messaging and Presence Protocol that MUST be supported by servers and clients in order to be considered compliant implementations, as well as additional protocol aspects that SHOULD be supported. For compliance purposes, we draw a distinction between core protocols (which MUST be supported by any server or client, regardless of the specific application) and instant messaging and presence protocols (which MUST be supported only by instant messaging and presence applications built on top of the core protocols). Compliance requirements that apply to all servers and clients are specified in this section; compliance requirements for instant messaging and presence applications are specified in the corresponding section of [XMPP-IM]. Saint-Andre Expires January 18, 2008 [Page 106] Internet-Draft XMPP Core July 2007 13.1. Servers In addition to all defined requirements with regard to security, XML usage, and internationalization, a server MUST support the following core protocols in order to be considered compliant: o Conformance with [IDNA] for domain identifiers, the Nodeprep (Appendix A) profile of [STRINGPREP] for node identifiers, and the Resourceprep (Appendix B) profile of [STRINGPREP] for resource identifiers, as well as enforcement thereof for clients that authenticate with the server. o XML streams (Section 5), including TLS negotiation (Section 6), SASL negotiation (Section 7), and Resource Binding (Section 8) o The basic semantics of the three defined stanza types (i.e., , , and ) as specified in stanza semantics (Section 9.2) o Generation (and, where appropriate, handling) of error syntax and semantics related to streams, TLS, SASL, and XML stanzas For backward compatibility with the large deployed base of XMPP servers, server developers are advised to implement the server dialback protocol first specified in [RFC3920] and now documented in [XEP-0220], since that protocol is widely used for weak identity verification of peer servers in the absence of domain certificates. 13.2. Clients A client MUST support the following core protocols in order to be considered compliant: o XML streams (Section 5), including TLS negotiation (Section 6), SASL negotiation (Section 7), and Resource Binding (Section 8) o The basic semantics of the three defined stanza types (i.e., , , and ) as specified in Section 9.2 o Handling (and, where appropriate, generation) of error syntax and semantics related to streams, TLS, SASL, and XML stanzas In addition, a client SHOULD support the following core protocols: o Conformance with [IDNA] for domain identifiers, the Nodeprep (Appendix A) profile of [STRINGPREP] for node identifiers, and the Resourceprep (Appendix B) profile of [STRINGPREP] for resource identifiers. 14. Internationalization Considerations XML streams MUST be encoded in UTF-8 as specified under Section 12.5. Saint-Andre Expires January 18, 2008 [Page 107] Internet-Draft XMPP Core July 2007 As specified under Section 5.3, an XML stream SHOULD include an 'xml: lang' attribute specifying the default language for any XML character data sent over the stream that is intended to be presented to a human user. As specified under Section 9.1.5, an XML stanza SHOULD include an 'xml:lang' attribute if the stanza contains XML character data that is intended to be presented to a human user. A server SHOULD apply the default 'xml:lang' attribute to stanzas it routes or delivers on behalf of connected entities, and MUST NOT modify or delete 'xml:lang' attributes on stanzas it receives from other entities. 15. Security Considerations 15.1. High Security For the purposes of XMPP communication (client-to-server and server- to-server), the term "high security" refers to the use of security technologies that provide both mutual authentication and integrity- checking; in particular, when using certificate-based authentication to provide high security, a chain-of-trust SHOULD be established out- of-band, although a shared certificate authority signing certificates could allow a previously unknown certificate to establish trust in- band. See Section 15.2 regarding certificate validation procedures. Implementations MUST support high security. Service provisioning should use high security, subject to local security policies. 15.2. Certificate Validation When an XMPP peer communicates with another peer securely, it MUST validate the peer's certificate. There are three possible cases: Case #1: The peer contains an End Entity certificate that appears to be certified by a chain of certificates terminating in a trust anchor (as described in Section 6.1 of [X509]). Case #2: The peer certificate is certified by a Certificate Authority not known to the validating peer. Case #3: The peer certificate is self-signed. In Case #1, the validating peer MUST do one of two things: 1. Verify the peer certificate according to the rules of [X509]. The certificate SHOULD then be checked against the expected identity of the peer following the rules described in [HTTP-TLS], except that if present an [ASN.1] Object Identifier of "id-on- xmppAddr" (represented as a UTF8String in an otherName entity inside the subjectAltName) MUST be used as the identity. If one of these checks fails, user-oriented clients MUST either notify Saint-Andre Expires January 18, 2008 [Page 108] Internet-Draft XMPP Core July 2007 the user (clients MAY give the user the opportunity to continue with the connection anyway) or terminate the connection with a bad certificate error. Automated clients SHOULD terminate the connection (with a bad certificate error) and log the error to an appropriate audit log. Automated clients MAY provide a configuration setting that disables this check, but MUST provide a setting that enables it. 2. The peer SHOULD show the certificate to a user for approval, including the entire certificate chain. The peer MUST cache the certificate (or some non-forgeable representation such as a hash). In future connections, the peer MUST verify that the same certificate was presented and MUST notify the user if it has changed. In Case #2 and Case #3, implementations SHOULD act as in Rule #2 for Case #1. 15.3. Client-to-Server Communication A compliant client implementation MUST support both TLS and SASL for connections to a server. The TLS protocol for encrypting XML streams (defined under Section 6) provides a reliable mechanism for helping to ensure the confidentiality and data integrity of data exchanged between two entities. The SASL protocol for authenticating XML streams (defined under Section 7) provides a reliable mechanism for validating that a client connecting to a server is who it claims to be. Client-to-server communication MUST NOT proceed until the DNS hostname asserted by the server has been resolved as specified under Section 4. If there is a mismatch between the hostname to which a client attempted to connect (e.g., "example.net") and the hostname to which the client actually connects (e.g., "xmpp.example.net"), the client MUST warn a human user about the mismatch and the human user MUST approve the connection before the client proceeds; however, the client MAY also allow the user to add the presented hostname to a configured set of accepted hostnames in order to expedite future connections. The IP address and method of access of clients MUST NOT be made public by a server, nor are any connections other than the original server connection required. This helps to protect the client's server from direct attack or identification by third parties. Saint-Andre Expires January 18, 2008 [Page 109] Internet-Draft XMPP Core July 2007 15.4. Server-to-Server Communication A compliant server implementation MUST support both TLS and SASL for inter-domain communication. Because service provisioning is a matter of policy, it is optional for any given domain to communicate with other domains, and server- to-server communication may be disabled by the administrator of any given deployment. If a particular domain enables inter-domain communication, it should enable high security. Administrators may want to require use of SASL for server-to-server communication in order to ensure both authentication and confidentiality (e.g., on an organization's private network). Compliant implementations SHOULD support SASL for this purpose. Server-to-server communication MUST NOT proceed until the DNS hostnames asserted by both servers have been resolved as specified under Section 4. 15.5. Order of Layers The order of layers in which protocols MUST be stacked is: 1. TCP 2. TLS 3. SASL 4. XMPP The rationale for this order is that [TCP] is the base connection layer used by all of the protocols stacked on top of TCP, [TLS] is often provided at the operating system layer, [SASL] is often provided at the application layer, and XMPP is the application itself. 15.6. Lack of SASL Channel Binding to TLS The SASL framework does not provide a mechanism to bind SASL authentication to a security layer providing confidentiality and integrity protection that was negotiated at a lower layer. This lack of a "channel binding" prevents SASL from being able to verify that the source and destination end points to which the lower layer's security is bound are equivalent to the end points that SASL is authenticating. If the end points are not identical, the lower layer's security cannot be trusted to protect data transmitted between the SASL authenticated entities. In such a situation, a SASL security layer SHOULD be negotiated that effectively ignores the presence of the lower layer security. Saint-Andre Expires January 18, 2008 [Page 110] Internet-Draft XMPP Core July 2007 15.7. Mandatory-to-Implement Technologies At a minimum, all implementations MUST support the following mechanisms: for authentication: the SASL [DIGEST-MD5] mechanism for confidentiality: TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher) for both: TLS plus SASL PLAIN for client-to-server connections and TLS plus SASL EXTERNAL for server-to-server connections (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher supporting peer certificates) Naturally, implementations MAY support other ciphers with TLS and MAY support other SASL mechanisms. 15.8. Firewalls Communication using XMPP normally occurs over [TCP] connections on port 5222 (client-to-server) or port 5269 (server-to-server), as registered with the IANA (see Section 16). Use of these well-known ports allows administrators to easily enable or disable XMPP activity through existing and commonly-deployed firewalls. 15.9. Use of base64 in SASL Both the client and the server MUST verify any [BASE64] data received during SASL negotiation (Section 7). An implementation MUST reject (not ignore) any characters that are not explicitly allowed by the base64 alphabet; this helps to guard against creation of a covert channel that could be used to "leak" information. An implementation MUST NOT break on invalid input and MUST reject any sequence of base64 characters containing the pad ('=') character if that character is included as something other than the last character of the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against buffer overflow attacks and other attacks on the implementation. Base 64 encoding visually hides otherwise easily recognized information, such as passwords, but does not provide any computational confidentiality. Base 64 encoding MUST follow the definition in Section 4 of [BASE64] and padding bits MUST be set to zero. 15.10. Stringprep Profiles XMPP makes use of the [NAMEPREP] profile of [STRINGPREP] for processing of domain identifiers; for security considerations related to Nameprep, refer to the appropriate section of [NAMEPREP]. In addition, XMPP defines two profiles of [STRINGPREP]: Nodeprep Saint-Andre Expires January 18, 2008 [Page 111] Internet-Draft XMPP Core July 2007 (Appendix A) for node identifiers and Resourceprep (Appendix B) for resource identifiers. The Unicode and ISO/IEC 10646 repertoires have many characters that look similar. In many cases, users of security protocols might do visual matching, such as when comparing the names of trusted third parties. Because it is impossible to map similar-looking characters without a great deal of context, such as knowing the fonts used, stringprep does nothing to map similar-looking characters together, nor to prohibit some characters because they look like others. A node identifier can be employed as one part of an entity's address in XMPP. One common usage is as the username of an instant messaging user; another is as the name of a multi-user conference room; many other kinds of entities could use node identifiers as part of their addresses. The security of such services could be compromised based on different interpretations of the internationalized node identifier; for example, a user entering a single internationalized node identifier could access another user's account information, or a user could gain access to a hidden or otherwise restricted chat room or service. A resource identifier can be employed as one part of an entity's address in XMPP. One common usage is as the name for an instant messaging user's connected resource; another is as the nickname of a user in a multi-user conference room; many other kinds of entities could use resource identifiers as part of their addresses. The security of such services could be compromised based on different interpretations of the internationalized resource identifier; for example, a user could attempt to initiate multiple connections with the same name, or a user could send a message to someone other than the intended recipient in a multi-user conference room. 15.11. Address Spoofing As discussed in [XEP-0165], there are two forms of address spoofing: forging and mimicking. 15.11.1. Address Forging In the context of XMPP technologies, address forging occurs when an entity is able to generate an XML stanza whose 'from' address does not correspond to the account credentials with which the entity authenticated onto the network (or an authorization identity provided during SASL negotiation (Section 7). For example, address forging occurs if an entity that authenticated as "juliet@example.com" is able to send XML stanzas from "nurse@example.com" or "romeo@example.net". Saint-Andre Expires January 18, 2008 [Page 112] Internet-Draft XMPP Core July 2007 Address forging is difficult in XMPP systems, given the requirement for sending servers to stamp 'from' addresses and for receiving servers to verify sending domains via server-to-server authentication. However, address forging is not impossible, since a rogue server could forge JIDs at the sending domain by ignoring the stamping requirement. A rogue server could even forge JIDs at other domains by means of a DNS poisoning attack if [DNSSEC] is not used. This specification does not define methods for discovering or counteracting such rogue servers. 15.11.2. Address Mimicking Address mimicking occus when an entity provides legitimate authentication credentials for and sends XML stanzas from an account whose JID appears to a human user to be the same as another JID. For example, in some XMPP clients the address "paypa1@example.org" (spelled with the number one as the final character of the node identifier) may appear to be the same as "paypal@example.org (spelled with the lower-case version of the letter "L"), especially on casual visual inspection; this phenomenon is sometimes called "typejacking". A more sophisticated example of address mimicking might involve the use of characters from outside the US-ASCII range, such as the Cherokee characters U+13DA U+13A2 U+13B5 U+13AC U+13A2 U+13AC U+13D2 instead of the US-ASCII characters "STPETER". In some examples of address mimicking, it is unlikely that the average user could tell the difference between the real JID and the fake JID. (Naturally, there is no way to distinguish with full certainty which is the fake JID and which is the real JID; in some communication contexts, the JID with Cherokee characters may be the real JID and the JID with US-ASCII characters may thus appear to be the fake JID.) Because JIDs can contain almost any Unicode character, it may be relatively easy to mimic some JIDs in XMPP systems. The possibility of address mimicking introduces security vulnerabilities of the kind that have also plagued the World Wide Web, specifically the phenomenon known as phishing. Mimicked addresses that involve characters from only one character set or from the character set typically employed by a particular user are not easy to combat (e.g., the simple typejacking attack previously described, which relies on a surface similarity between the characters "1" and "l" in some presentations). However, mimicked addresses that involve characters from more than one character set, or from a character set not typically employed by a particular user, can be mitigated somewhat through intelligent presentation. In particular, every human user of an XMPP technology presumably has a preferred language (or, in some cases, a small set of preferred languages), which an XMPP application SHOULD gather either explicitly Saint-Andre Expires January 18, 2008 [Page 113] Internet-Draft XMPP Core July 2007 from the user or implicitly via the operating system of the user's device. Furthermore, every language has a range (or a small set of ranges) of characters normally used to represent that language in textual form. Therefore, an XMPP application SHOULD warn the user when presenting a JID that uses characters outside the normal range of the user's preferred language(s). This recommendation is not intended to discourage communication across language communities; instead, it recognizes the existence of such language communities and encourages due caution when presenting unfamiliar character sets to human users. For more detailed recommendations regarding prevention of address mimicking in XMPP systems, refer to [XEP-0165]. 15.12. Denial of Service [DOS] defines denial of service as follows: A Denial-of-Service (DoS) attack is an attack in which one or more machines target a victim and attempt to prevent the victim from doing useful work. The victim can be a network server, client or router, a network link or an entire network, an individual Internet user or a company doing business using the Internet, an Internet Service Provider (ISP), country, or any combination of or variant on these. [XEP-0205] provides a detailed discussion of potential denial of service attacks against XMPP systems and best practices for preventing such attacks. The recommendations include: 1. A server implementation SHOULD enable a server administrator to limit the number of TCP connections that it will accept from a given IP address at any one time. If an entity attempts to connect but the maximum number of TCP connections has been reached, the receiving server MUST NOT allow the new connection to proceed. 2. A server implementation SHOULD enable a server administrator to limit the number of TCP connection attempts that it will accept from a given IP address in a given time period. (While it is possible to limit the number of connections at the TCP layer rather than at the XMPP application layer, care must be taken in doing so since limits at the TCP layer might result in an inability to access non-XMPP services.) If an entity attempts to connect but the maximum number of connections has been reached, the receiving server MUST NOT allow the new connection to proceed. Saint-Andre Expires January 18, 2008 [Page 114] Internet-Draft XMPP Core July 2007 3. A server MUST NOT process XML stanzas from clients that have not yet provided appropriate authentication credentials and MUST NOT process XML stanzas from peer servers whose identity it has not either authenticated via SASL. 4. A server implementation SHOULD enable a server administrator to limit the number of connected resources it will allow an account to bind at any one time. If a client attempts to bind a resource but it has already reached the configured number of allowable resources, the receiving server MUST return a stanza error. 5. A server implementation SHOULD enable a server administrator to limit the size of stanzas it will accept from a connected client or peer server. If a connected resource or peer server sends a stanza that violates the upper limit, the receiving server SHOULD NOT process the stanza and instead SHOULD return a stanza error. Alternatively (e.g., if the sender has sent an egregiously large stanza), the server MAY instead return a stream error. 6. A server implementation SHOULD enable a server administrator to limit the number of XML stanzas that a connected client may send to distinct recipients within a given time period. If a connected client sends too many stanzas to distinct recipients in a given time period, the receiving server SHOULD NOT process the stanza and instead SHOULD return an stanza error. 7. A server implementation SHOULD enable a server administrator to limit the amount of bandwidth it will allow a connected client or peer server to use in a given time period. 8. A server implementation SHOULD enable a server administrator to limit the types of stanzas (based on the extended content "payload") that it will allow a connected resource or peer server send over an active connection. Such limits and restrictions are a matter of deployment policy. For more detailed recommendations regarding denial of service attacks in XMPP systems, refer to [XEP-0205]. 15.13. Presence Leaks One of the core aspects of XMPP is presence, i.e., ubiquitous information about the network availability of XMPP entities. Although presence is discussed more fully in [XMPP-IM], it is important to note that an XMPP server MUST NOT disclose an entity's presence to entities that are not authorized to know that information (such a disclosure is called a "presence leak"). In particular at the core XMPP level, real-time addressing and network availability is associated with a specific connected resource; therefore, any disclosure of a connected resource's full JID comprises a presence Saint-Andre Expires January 18, 2008 [Page 115] Internet-Draft XMPP Core July 2007 leak. To help prevent such a presence leak, a server MUST NOT return different stanza errors if a potential attacker sends XML stanzas to the entity's bare JID () or full JID (). 15.14. Directory Harvesting To help prevent directory harvesting attacks, server MUST NOT return different stanza errors if a potential attacker sends XML stanzas to an existing entity or a nonexistent entity. The stanza error returned in both cases SHOULD be . 16. IANA Considerations The following sections update the registrations provided in [RFC3920]. 16.1. XML Namespace Name for TLS Data A URN sub-namespace for TLS-related data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].) URI: urn:ietf:params:xml:ns:xmpp-tls Specification: XXXX Description: This is the XML namespace name for TLS-related data in the Extensible Messaging and Presence Protocol (XMPP) as defined by XXXX. Registrant Contact: IETF, XMPP Working Group, 16.2. XML Namespace Name for SASL Data A URN sub-namespace for SASL-related data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].) URI: urn:ietf:params:xml:ns:xmpp-sasl Specification: XXXX Description: This is the XML namespace name for SASL-related data in the Extensible Messaging and Presence Protocol (XMPP) as defined by XXXX. Registrant Contact: IETF, XMPP Working Group, 16.3. XML Namespace Name for Stream Errors A URN sub-namespace for stream-related error data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This Saint-Andre Expires January 18, 2008 [Page 116] Internet-Draft XMPP Core July 2007 namespace name adheres to the format defined in [XML-REG].) URI: urn:ietf:params:xml:ns:xmpp-streams Specification: XXXX Description: This is the XML namespace name for stream-related error data in the Extensible Messaging and Presence Protocol (XMPP) as defined by XXXX. Registrant Contact: IETF, XMPP Working Group, 16.4. XML Namespace Name for Resource Binding A URN sub-namespace for resource binding in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].) URI: urn:ietf:params:xml:ns:xmpp-bind Specification: XXXX Description: This is the XML namespace name for resource binding in the Extensible Messaging and Presence Protocol (XMPP) as defined by XXXX. Registrant Contact: IETF, XMPP Working Group, 16.5. XML Namespace Name for Stanza Errors A URN sub-namespace for stanza-related error data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].) URI: urn:ietf:params:xml:ns:xmpp-stanzas Specification: XXXX Description: This is the XML namespace name for stanza-related error data in the Extensible Messaging and Presence Protocol (XMPP) as defined by XXXX. Registrant Contact: IETF, XMPP Working Group, 16.6. Nodeprep Profile of Stringprep The Nodeprep profile of stringprep is defined under Nodeprep (Appendix A). The IANA has registered Nodeprep in the stringprep profile registry. Name of this profile: Nodeprep RFC in which the profile is defined: Saint-Andre Expires January 18, 2008 [Page 117] Internet-Draft XMPP Core July 2007 XXXX Indicator whether or not this is the newest version of the profile: This is the first version of Nodeprep 16.7. Resourceprep Profile of Stringprep The Resourceprep profile of stringprep is defined under Resourceprep (Appendix B). The IANA has registered Resourceprep in the stringprep profile registry. Name of this profile: Resourceprep RFC in which the profile is defined: XXXX Indicator whether or not this is the newest version of the profile: This is the first version of Resourceprep 16.8. GSSAPI Service Name The IANA has registered "xmpp" as a GSSAPI [GSS-API] service name, as defined under Section 7.4. 16.9. Port Numbers The IANA has registered "xmpp-client" and "xmpp-server" as keywords for [TCP] ports 5222 and 5269 respectively. These ports SHOULD be used for client-to-server and server-to-server communications respectively, but their use is OPTIONAL. 17. References 17.1. Normative References [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006. Saint-Andre Expires January 18, 2008 [Page 118] Internet-Draft XMPP Core July 2007 [CHARSET] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998. [DIGEST-MD5] Leach, P. and C. Newman, "Using Digest Authentication as a SASL Mechanism", RFC 2831, May 2000. [DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [DNS] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [IDNA] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003. [IPv6] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. [LANGTAGS] Phillips, A. and M. Davis, "Tags for Identifying Languages", BCP 47, RFC 4646, September 2006. [NAMEPREP] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)", RFC 3491, March 2003. [RANDOM] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [SASL] Melnikov, A. and K. Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, June 2006. [STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002. [TCP] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [TERMS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security Saint-Andre Expires January 18, 2008 [Page 119] Internet-Draft XMPP Core July 2007 (TLS) Protocol Version 1.1", RFC 4346, April 2006. [UCS2] International Organization for Standardization, "Information Technology - Universal Multiple-octet coded Character Set (UCS) - Amendment 2: UCS Transformation Format 8 (UTF-8)", ISO Standard 10646-1 Addendum 2, October 1996. [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [UUID] Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, July 2005. [X509] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002. [XML] Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F., and T. Bray, "Extensible Markup Language (XML) 1.0 (Fourth Edition)", World Wide Web Consortium Recommendation REC- xml-20060816, August 2006, . [XML-NAMES] Bray, T., Hollander, D., and A. Layman, "Namespaces in XML", W3C REC-xml-names, January 1999, . 17.2. Informative References [ACAP] Newman, C. and J. Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, November 1997. [ASN.1] CCITT, "Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1)", 1988. [DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [DNS-TXT] Rosenbaum, R., "Using the Domain Name System To Store Arbitrary String Attributes", RFC 1464, May 1993. [DOS] Handley, M., Rescorla, E., and IAB, "Internet Denial-of- Service Considerations", RFC 4732, December 2006. Saint-Andre Expires January 18, 2008 [Page 120] Internet-Draft XMPP Core July 2007 [GSS-API] Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, January 2000. [HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [HTTP-TLS] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. [IMP-REQS] Day, M., Aggarwal, S., and J. Vincent, "Instant Messaging / Presence Protocol Requirements", RFC 2779, February 2000. [IRI] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005. [LINKLOCAL] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic Configuration of IPv4 Link-Local Addresses", RFC 3927, May 2005. [MAILBOXES] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND FUNCTIONS", RFC 2142, May 1997. [POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996. [PUNYCODE] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003. [RFC3920] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 3920, October 2004. [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [STD13] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Saint-Andre Expires January 18, 2008 [Page 121] Internet-Draft XMPP Core July 2007 Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [URN-OID] Mealling, M., "A URN Namespace of Object Identifiers", RFC 3061, February 2001. [USINGTLS] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, June 1999. [XEP-0045] Saint-Andre, P., "Multi-User Chat", XSF XEP 0045, April 2007. [XEP-0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish- Subscribe", XSF XEP 0060, September 2006. [XEP-0071] Saint-Andre, P., "XHTML-IM", XSF XEP 0071, March 2007. [XEP-0077] Saint-Andre, P., "In-Band Registration", XSF XEP 0077, January 2006. [XEP-0086] Norris, R. and P. Saint-Andre, "Error Condition Mappings", XSF XEP 0086, February 2004. [XEP-0124] Paterson, I., Smith, D., and P. Saint-Andre, "Bidirectional-streams Over Synchronous HTTP (BOSH)", XSF XEP 0124, February 2007. [XEP-0156] Hildebrand, J. and P. Saint-Andre, "Discovering Alternative XMPP Connection Methods", XSF XEP 0156, January 2007. [XEP-0157] Saint-Andre, P. and J. Konieczny, "Contact Addresses for XMPP Services", XSF XEP 0157, January 2007. [XEP-0165] Saint-Andre, P., "Best Practices to Prevent JID Mimicking", XSF XEP 0165, December 2006. [XEP-0174] Saint-Andre Expires January 18, 2008 [Page 122] Internet-Draft XMPP Core July 2007 Saint-Andre, P., "Link-Local Messaging", XSF XEP 0174, March 2007. [XEP-0175] Saint-Andre, P., "Best Practices for Use of SASL ANONYMOUS", XSF XEP 0175, September 2006. [XEP-0178] Saint-Andre, P. and P. Millard, "Best Practices for Use of SASL EXTERNAL with Certificates", XSF XEP 0178, February 2007. [XEP-0205] Saint-Andre, P., "Best Practices to Discourage Denial of Service Attacks", XSF XEP 0205, January 2007. [XEP-0206] Paterson, I., "XMPP Over BOSH", XSF XEP 0206, February 2007. [XEP-0220] Saint-Andre, P. and J. Miller, "Server Dialback", XSF XEP 0220, June 2007. [XML-REG] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January 2004. [XML-SCHEMA] Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech, "XML Schema Part 1: Structures Second Edition", World Wide Web Consortium Recommendation REC-xmlschema-1-20041028, October 2004, . [XMPP-IM] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence", draft-saintandre-rfc3921bis-03 (work in progress), July 2007. [XMPP-URI] Saint-Andre, P., "Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP)", draft-saintandre-rfc4622bis-01 (work in progress), June 2007. Saint-Andre Expires January 18, 2008 [Page 123] Internet-Draft XMPP Core July 2007 Appendix A. Nodeprep A.1. Introduction This appendix defines the "Nodeprep" profile of [STRINGPREP]. As such, it specifies processing rules that will enable users to enter internationalized node identifiers in the Extensible Messaging and Presence Protocol (XMPP) and have the highest chance of getting the content of the strings correct. (An XMPP node identifier is the optional portion of an XMPP address that precedes an XMPP domain identifier and the '@' separator; it is often but not exclusively associated with an instant messaging username.) These processing rules are intended only for XMPP node identifiers and are not intended for arbitrary text or any other aspect of an XMPP address. This profile defines the following, as required by [STRINGPREP]: o The intended applicability of the profile: internationalized node identifiers within XMPP o The character repertoire that is the input and output to stringprep: Unicode 3.2, specified in Section 2 of this Appendix o The mappings used: specified in Section 3 o The Unicode normalization used: specified in Section 4 o The characters that are prohibited as output: specified in Section 5 o Bidirectional character handling: specified in Section 6 A.2. Character Repertoire This profile uses Unicode 3.2 with the list of unassigned code points being Table A.1, both defined in Appendix A of [STRINGPREP]. A.3. Mapping This profile specifies mapping using the following tables from [STRINGPREP]: Table B.1 Table B.2 A.4. Normalization This profile specifies the use of Unicode normalization form KC, as described in [STRINGPREP]. Saint-Andre Expires January 18, 2008 [Page 124] Internet-Draft XMPP Core July 2007 A.5. Prohibited Output This profile specifies the prohibition of using the following tables from [STRINGPREP]. Table C.1.1 Table C.1.2 Table C.2.1 Table C.2.2 Table C.3 Table C.4 Table C.5 Table C.6 Table C.7 Table C.8 Table C.9 In addition, the following Unicode characters are also prohibited: #x22 (") #x26 (&) #x27 (') #x2F (/) #x3A (:) #x3C (<) #x3E (>) #x40 (@) A.6. Bidirectional Characters This profile specifies checking bidirectional strings, as described in Section 6 of [STRINGPREP]. Appendix B. Resourceprep B.1. Introduction This appendix defines the "Resourceprep" profile of [STRINGPREP]. As such, it specifies processing rules that will enable users to enter internationalized resource identifiers in the Extensible Messaging and Presence Protocol (XMPP) and have the highest chance of getting the content of the strings correct. (An XMPP resource identifier is the optional portion of an XMPP address that follows an XMPP domain identifier and the '/' separator.) These processing rules are intended only for XMPP resource identifiers and are not intended for arbitrary text or any other aspect of an XMPP address. Saint-Andre Expires January 18, 2008 [Page 125] Internet-Draft XMPP Core July 2007 This profile defines the following, as required by [STRINGPREP]: o The intended applicability of the profile: internationalized resource identifiers within XMPP o The character repertoire that is the input and output to stringprep: Unicode 3.2, specified in Section 2 of this Appendix o The mappings used: specified in Section 3 o The Unicode normalization used: specified in Section 4 o The characters that are prohibited as output: specified in Section 5 o Bidirectional character handling: specified in Section 6 B.2. Character Repertoire This profile uses Unicode 3.2 with the list of unassigned code points being Table A.1, both defined in Appendix A of [STRINGPREP]. B.3. Mapping This profile specifies mapping using the following tables from [STRINGPREP]: Table B.1 B.4. Normalization This profile specifies the use of Unicode normalization form KC, as described in [STRINGPREP]. B.5. Prohibited Output This profile specifies the prohibition of using the following tables from [STRINGPREP]. Table C.1.2 Table C.2.1 Table C.2.2 Table C.3 Table C.4 Table C.5 Table C.6 Table C.7 Table C.8 Table C.9 Saint-Andre Expires January 18, 2008 [Page 126] Internet-Draft XMPP Core July 2007 B.6. Bidirectional Characters This profile specifies checking bidirectional strings, as described in Section 6 of [STRINGPREP]. Appendix C. XML Schemas Because validation of XML streams and stanzas is optional, the following XML schemas are provided for descriptive purposes only. These schemas are not normative. The following schemas formally define various XML namespaces used in the core XMPP protocols, in conformance with [XML-SCHEMA]. For schemas defining the 'jabber:client' and 'jabber:server' namespaces, refer to [XMPP-IM]. C.1. Streams namespace Saint-Andre Expires January 18, 2008 [Page 127] Internet-Draft XMPP Core July 2007 C.2. Stream error namespace Saint-Andre Expires January 18, 2008 [Page 128] Internet-Draft XMPP Core July 2007 Saint-Andre Expires January 18, 2008 [Page 129] Internet-Draft XMPP Core July 2007 Saint-Andre Expires January 18, 2008 [Page 130] Internet-Draft XMPP Core July 2007 C.3. TLS namespace C.4. SASL namespace Saint-Andre Expires January 18, 2008 [Page 131] Internet-Draft XMPP Core July 2007 Saint-Andre Expires January 18, 2008 [Page 132] Internet-Draft XMPP Core July 2007 C.5. Resource binding namespace Saint-Andre Expires January 18, 2008 [Page 133] Internet-Draft XMPP Core July 2007 C.6. Stanza error namespace Saint-Andre Expires January 18, 2008 [Page 134] Internet-Draft XMPP Core July 2007 Saint-Andre Expires January 18, 2008 [Page 135] Internet-Draft XMPP Core July 2007 Appendix D. Contact Addresses Consistent with [MAILBOXES], an organization that offers an XMPP service should provide an Internet mailbox of "XMPP" for inquiries related to that service, where the host portion of the resulting mailto URI should be the organization's domain, not necessarily the domain of the XMPP service itself (e.g., the XMPP service might be offered at xmpp.example.net but the Internet mailbox should be ). In addition, the XMPP service should provide a way to discover the XMPP contact address(es) of the service administrator(s), as specified in [XEP-0157]. Saint-Andre Expires January 18, 2008 [Page 136] Internet-Draft XMPP Core July 2007 Appendix E. Account Provisioning Account provisioning is out of scope for this specification. Possible methods for account provisioning include account creation by a server administrator and in-band account registration using the 'jabber:iq:register' namespace as documented in [XEP-0077]. Appendix F. Differences From RFC 3920 Based on consensus derived from implementation and deployment experience as well as formal interoperability testing, the following substantive modifications were made from RFC 3920. o Corrected the ABNF syntax for JIDs to prevent zero-length node identifiers, domain identifiers, and resource identifiers. o Corrected the nameprep processing rules to require use of the UseSTD3ASCIIRules flag. o Encouraged use of the 'from' and 'to' attributes on stream headers. o More fully specified stream closing handshake. o Specified recommended stream reconnection algorithm. o Specified return of stream error in response to receipt of prohibited XML features. o Specified that SASL mechanisms must be sent both before and after negotiation of SASL security layers. o Specified that TLS plus SASL PLAIN is a mandatory-to-implement technology for client-to-server connections, since implementation of SASL EXTERNAL is uncommon in XMPP clients, in part because underlying security features such as X.509 certificates are not yet widely deployed. o Added the SASL error condition to handle an error case discussed in RFC 4422. o More fully specified binding of multiple resources to the same stream. o Added the stanza error condition to provide appropriate handling of stanzas when multiple resources are bound to the same stream. o Added the stanza error condition to enable potential ETags usage. o Removed historical documentation for the server dialback protocol from this specification to a separate specification In addition, numerous changes of an editorial nature were made in order to more fully specify and clearly explain the protocols. Saint-Andre Expires January 18, 2008 [Page 137] Internet-Draft XMPP Core July 2007 Appendix G. Copying Conditions The Contributor grants third parties the irrevocable right to copy, use and distribute the Contribution, with or without modification, in any medium, without royalty, provided that, unless separate permission is granted, redistributed modified works: 1. do not contain misleading author, version, name of work, or endorsement information, and 2. do not claim endorsement of the modified work by the Contributor, or any organization the Contributor belongs to, the Internet Engineering Task Force (IETF), Internet Research Task Force (IRTF), Internet Engineering Steering Group (IESG), Internet Architecture Board (IAB), Internet Assigned Numbers Authority (IANA), Internet Society (ISOC), Request For Comments (RFC) Editor, or any combination or variation of such terms (including without limitation the IETF "4 diamonds" logo), or any terms that are confusingly similar thereto, and 3. remove any claims of status as an Internet Standard, including without limitation removing the RFC boilerplate. The IETF suggests that any citation or excerpt of unmodified text reference the RFC or other document from which the text is derived. Index B Bare JID 11 C Connected Resource 11 D Domain Identifier 10 E Entity 9 Error Stanza 73 Extended Content 88 F Full JID 11 I Initial Stream 14 IQ Stanza 72 Saint-Andre Expires January 18, 2008 [Page 138] Internet-Draft XMPP Core July 2007 J Jabber Identifier 9 M Message Stanza 71 N Node Identifier 11 P Presence Stanza 71 R Resource Identifier 11 Response Stream 14 X XML Stanza 14 XML Stream 14 Author's Address Peter Saint-Andre (editor) XMPP Standards Foundation Email: stpeter@jabber.org URI: xmpp:stpeter@jabber.org Saint-Andre Expires January 18, 2008 [Page 139] Internet-Draft XMPP Core July 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Saint-Andre Expires January 18, 2008 [Page 140]