Network Working Group P. Saint-Andre Internet-Draft J. Miller Expires: November 2, 2003 Jabber Software Foundation May 04, 2003 XMPP Core draft-ietf-xmpp-core-12 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 November 2, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document describes the core features of the Extensible Messaging and Presence Protocol (XMPP), a protocol for streaming XML elements in order to exchange messages and presence information in close to real time. XMPP is used mainly for the purpose of building instant messaging (IM) and presence applications, such as the servers and clients that comprise the Jabber network. Saint-Andre & Miller Expires November 2, 2003 [Page 1] Internet-Draft XMPP Core May 2003 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 Discussion Venue . . . . . . . . . . . . . . . . . . . . . . 5 1.4 Intellectual Property Notice . . . . . . . . . . . . . . . . 5 2. Generalized Architecture . . . . . . . . . . . . . . . . . . 6 2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2 Server . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3 Client . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.4 Gateway . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.5 Network . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Addressing Scheme . . . . . . . . . . . . . . . . . . . . . 8 3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2 Domain Identifier . . . . . . . . . . . . . . . . . . . . . 8 3.3 Node Identifier . . . . . . . . . . . . . . . . . . . . . . 8 3.4 Resource Identifier . . . . . . . . . . . . . . . . . . . . 9 4. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . 10 4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.2 Stream Attributes . . . . . . . . . . . . . . . . . . . . . 11 4.3 Namespace Declarations . . . . . . . . . . . . . . . . . . . 12 4.3.1 Stream Namespace Declaration . . . . . . . . . . . . . . . . 12 4.3.2 Default Namespace Declaration . . . . . . . . . . . . . . . 13 4.4 Stream Features . . . . . . . . . . . . . . . . . . . . . . 13 4.5 Stream Errors . . . . . . . . . . . . . . . . . . . . . . . 14 4.5.1 Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.5.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.5.3 Conditions . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.5.4 Extensibility . . . . . . . . . . . . . . . . . . . . . . . 16 4.6 Simple Streams Example . . . . . . . . . . . . . . . . . . . 16 5. Stream Encryption . . . . . . . . . . . . . . . . . . . . . 19 5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.2 Narrative . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.3 Client-to-Server Example . . . . . . . . . . . . . . . . . . 21 5.4 Server-to-Server Example . . . . . . . . . . . . . . . . . . 23 6. Stream Authentication . . . . . . . . . . . . . . . . . . . 26 6.1 SASL Authentication . . . . . . . . . . . . . . . . . . . . 26 6.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 26 6.1.2 Narrative . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.1.3 SASL Definition . . . . . . . . . . . . . . . . . . . . . . 29 6.1.4 Client-to-Server Example . . . . . . . . . . . . . . . . . . 30 6.1.5 Server-to-Server Example . . . . . . . . . . . . . . . . . . 32 6.2 Dialback Authentication . . . . . . . . . . . . . . . . . . 35 6.2.1 Dialback Protocol . . . . . . . . . . . . . . . . . . . . . 37 7. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . 42 7.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 42 7.2 Common Attributes . . . . . . . . . . . . . . . . . . . . . 42 Saint-Andre & Miller Expires November 2, 2003 [Page 2] Internet-Draft XMPP Core May 2003 7.2.1 to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 7.2.2 from . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 7.2.3 id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 7.2.4 type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 7.2.5 xml:lang . . . . . . . . . . . . . . . . . . . . . . . . . . 43 7.3 Message Stanzas . . . . . . . . . . . . . . . . . . . . . . 44 7.3.1 Types of Message . . . . . . . . . . . . . . . . . . . . . . 44 7.3.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 44 7.4 Presence Stanzas . . . . . . . . . . . . . . . . . . . . . . 46 7.4.1 Types of Presence . . . . . . . . . . . . . . . . . . . . . 46 7.4.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 46 7.5 IQ Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 48 7.5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 48 7.5.2 Types of IQ . . . . . . . . . . . . . . . . . . . . . . . . 48 7.5.3 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 49 7.6 Extended Namespaces . . . . . . . . . . . . . . . . . . . . 49 7.7 Stanza Errors . . . . . . . . . . . . . . . . . . . . . . . 50 7.7.1 Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 7.7.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 7.7.3 Conditions . . . . . . . . . . . . . . . . . . . . . . . . . 51 7.7.4 Extensibility . . . . . . . . . . . . . . . . . . . . . . . 52 8. XML Usage within XMPP . . . . . . . . . . . . . . . . . . . 54 8.1 Restrictions . . . . . . . . . . . . . . . . . . . . . . . . 54 8.2 Namespaces . . . . . . . . . . . . . . . . . . . . . . . . . 54 8.3 Validation . . . . . . . . . . . . . . . . . . . . . . . . . 54 8.4 Character Encodings . . . . . . . . . . . . . . . . . . . . 55 8.5 Inclusion of Text Declaration . . . . . . . . . . . . . . . 55 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 56 9.1 XML Namespace Name for TLS Data . . . . . . . . . . . . . . 56 9.2 XML Namespace Name for SASL Data . . . . . . . . . . . . . . 56 9.3 XML Namespace Name for Stream Errors . . . . . . . . . . . . 56 9.4 XML Namespace Name for Stanza Errors . . . . . . . . . . . . 57 9.5 Existing Registrations . . . . . . . . . . . . . . . . . . . 57 10. Internationalization Considerations . . . . . . . . . . . . 58 11. Security Considerations . . . . . . . . . . . . . . . . . . 59 11.1 High Security . . . . . . . . . . . . . . . . . . . . . . . 59 11.2 Client-to-Server Communications . . . . . . . . . . . . . . 59 11.3 Server-to-Server Communications . . . . . . . . . . . . . . 60 11.4 Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . 60 11.5 Mandatory to Implement Technologies . . . . . . . . . . . . 60 Normative References . . . . . . . . . . . . . . . . . . . . 61 Informative References . . . . . . . . . . . . . . . . . . . 63 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 63 A. XML Schemas . . . . . . . . . . . . . . . . . . . . . . . . 65 A.1 Streams namespace . . . . . . . . . . . . . . . . . . . . . 65 A.2 TLS namespace . . . . . . . . . . . . . . . . . . . . . . . 66 A.3 SASL namespace . . . . . . . . . . . . . . . . . . . . . . . 66 A.4 Dialback namespace . . . . . . . . . . . . . . . . . . . . . 67 Saint-Andre & Miller Expires November 2, 2003 [Page 3] Internet-Draft XMPP Core May 2003 A.5 Client namespace . . . . . . . . . . . . . . . . . . . . . . 68 A.6 Server namespace . . . . . . . . . . . . . . . . . . . . . . 72 A.7 Stream error namespace . . . . . . . . . . . . . . . . . . . 76 A.8 Stanza error namespace . . . . . . . . . . . . . . . . . . . 77 B. Revision History . . . . . . . . . . . . . . . . . . . . . . 79 B.1 Changes from draft-ietf-xmpp-core-09 . . . . . . . . . . . . 79 B.2 Changes from draft-ietf-xmpp-core-09 . . . . . . . . . . . . 79 B.3 Changes from draft-ietf-xmpp-core-08 . . . . . . . . . . . . 79 B.4 Changes from draft-ietf-xmpp-core-07 . . . . . . . . . . . . 80 B.5 Changes from draft-ietf-xmpp-core-06 . . . . . . . . . . . . 80 B.6 Changes from draft-ietf-xmpp-core-05 . . . . . . . . . . . . 80 B.7 Changes from draft-ietf-xmpp-core-04 . . . . . . . . . . . . 80 B.8 Changes from draft-ietf-xmpp-core-03 . . . . . . . . . . . . 80 B.9 Changes from draft-ietf-xmpp-core-02 . . . . . . . . . . . . 81 B.10 Changes from draft-ietf-xmpp-core-01 . . . . . . . . . . . . 81 B.11 Changes from draft-ietf-xmpp-core-00 . . . . . . . . . . . . 81 B.12 Changes from draft-miller-xmpp-core-02 . . . . . . . . . . . 81 Intellectual Property and Copyright Statements . . . . . . . 83 Saint-Andre & Miller Expires November 2, 2003 [Page 4] Internet-Draft XMPP Core May 2003 1. Introduction 1.1 Overview The Extensible Messaging and Presence Protocol (XMPP) is an open XML [1] protocol 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 2002, the XMPP WG was chartered with developing an adaptation of the Jabber protocol that would be suitable as an IETF instant messaging and presence technology. As a result of work by the XMPP WG, the current document defines the core features of XMPP; XMPP IM [23] defines the extensions required to provide the instant messaging (IM) and presence functionality defined in RFC 2779 [2]. 1.2 Terminology The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [3]. 1.3 Discussion Venue The authors welcome discussion and comments related to the topics presented in this document. The preferred forum is the mailing list, for which archives and subscription information are available at . 1.4 Intellectual Property Notice This document is in full compliance with all provisions of Section 10 of RFC 2026. Parts of this specification use the term "jabber" for identifying namespaces and other protocol syntax. Jabber[tm] is a registered trademark of Jabber, Inc. Jabber, Inc. grants permission to the IETF for use of the Jabber trademark in association with this specification and its successors, if any. Saint-Andre & Miller Expires November 2, 2003 [Page 5] Internet-Draft XMPP Core May 2003 2. Generalized Architecture 2.1 Overview Although XMPP is not wedded to any specific network architecture, to this point it usually has been implemented via a typical client-server architecture, wherein a client utilizing XMPP accesses a server over a TCP [4] socket. The following diagram provides a high-level overview of this architecture (where "-" represents communications that use XMPP and "=" represents communications that use any other protocol). C1 - S1 - S2 - C3 / \ C2 - G1 = FN1 = FC1 The symbols are as follows: o C1, C2, C3 -- XMPP clients o S1, S2 -- XMPP servers o G1 -- A gateway that translates between XMPP and the protocol(s) used on a foreign (non-XMPP) messaging network o FN1 -- A foreign messaging network o FC1 -- A client on a foreign messaging network 2.2 Server A server acts as an intelligent abstraction layer for XMPP communications. Its primary responsibilities are to manage connections from or sessions for other entities (in the form of XML streams to and from authorized clients, servers, and other entities) and to route appropriately-addressed XML data "stanzas" among such entities over XML streams. Most XMPP-compliant servers also assume responsibility for the storage of data that is used by clients (e.g., contact lists for users of XMPP-based IM applications); in this case, the XML data is processed directly by the server itself on behalf of the client and is not routed to another entity. Compliant server implementations MUST ensure in-order processing of XML stanzas between any two entities. 2.3 Client Saint-Andre & Miller Expires November 2, 2003 [Page 6] Internet-Draft XMPP Core May 2003 Most clients connect directly to a server over a TCP socket and use XMPP to take full advantage of the functionality provided by a server and any associated services. Although there is no necessary coupling of an XML stream to a TCP socket (e.g., a client COULD connect via HTTP polling or some other mechanism), this specification defines a binding for XMPP to TCP only. Multiple resources (e.g., devices or locations) MAY connect simultaneously to a server on behalf of each authorized client, with each resource connecting over a discrete TCP socket and differentiated by the resource identifier of a Jabber Identifier (e.g., user@domain/home vs. user@domain/work) as defined under Section 3. The port registered with the IANA [5] for connections between a Jabber client and a Jabber server is 5222. 2.4 Gateway A gateway is a special-purpose server-side service whose primary function is to translate XMPP into the protocol used by a foreign (non-XMPP) messaging system, as well as to translate the return data back into XMPP. Examples are gateways to Internet Relay Chat (IRC), Short Message Service (SMS), SMTP, and legacy instant messaging networks such as AIM, ICQ, MSN Messenger, and Yahoo! Instant Messenger. Communications between gateways and servers, and between gateways and the foreign messaging system, are not defined in this document. 2.5 Network Because each server is identified by a network address and because server-to-server communications are a straightforward extension of the client-to-server protocol, in practice the system consists of a network of servers that inter-communicate. Thus user-a@domain1 is able to exchange messages, presence, and other information with user-b@domain2. This pattern is familiar from messaging protocols (such as SMTP) that make use of network addressing standards. There are two methods for negotiating a connection between any two servers: primarily SASL authentication (Section 6.1) and secondarily server dialback (Section 6.2). Saint-Andre & Miller Expires November 2, 2003 [Page 7] Internet-Draft XMPP Core May 2003 3. Addressing Scheme 3.1 Overview An entity is anything that can be considered a network endpoint (i.e., an ID on the network) and that can communicate using XMPP. All such entities are uniquely addressable in a form that is consistent with RFC 2396 [24]. In particular, a valid Jabber Identifier (JID) contains a set of ordered elements formed of a domain identifier, node identifier, and resource identifier in the following format: [node@]domain[/resource]. All JIDs are based on the foregoing structure. The most common use of this structure is to identify an IM user, the server to which the user connects, and the user's active session or connection (e.g., a specific client) in the form of . However, node types other than clients are possible; for example, a specific chat room offered by a multi-user chat service could be addressed as (where "room" is the name of the chat room and "service" is the hostname of the multi-user chat 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). 3.2 Domain Identifier The domain identifier is the primary identifier and is the only REQUIRED element of a JID (a mere domain identifier is a valid JID). It usually represents the network gateway or "primary" server to which other entities connect for XML routing and data management capabilities. However, the entity referenced by a domain identifier is not always a server, and may be a service that is addressed as a subdomain of a server and that provides functionality above and beyond the capabilities of a server (a multi-user chat service, a user directory, a gateway to a foreign messaging system, etc.). The domain identifier for every server or service that will communicate over a network SHOULD resolve to a Fully Qualified Domain Name. A domain identifier MUST conform to RFC 952 [6] and RFC 1123 [7]. A domain identifier MUST be no more than 1023 bytes in length and MUST conform to the nameprep [8] profile of stringprep [9]. 3.3 Node Identifier The node identifier is an optional secondary identifier. It usually represents the entity requesting and using network access provided by the server or gateway (i.e., a client), although it can also Saint-Andre & Miller Expires November 2, 2003 [Page 8] Internet-Draft XMPP Core May 2003 represent other kinds of entities (e.g., a multi-user chat room associated with a multi-user chat service). The entity represented by a node identifier is addressed within the context of a specific domain; within IM applications of XMPP this address is called a "bare JID" and is of the form . A node identifier MUST be no more than 1023 bytes in length and MUST conform to the nodeprep [10] profile of stringprep [9]. 3.4 Resource Identifier The resource identifier is an optional tertiary identifier, which may modify either a or mere address. It usually represents a specific session, connection (e.g., a device or location), or object (e.g., a participant in a multi-user chat room) belonging to the entity associated with a node identifier. A resource identifier is opaque to both servers and other clients, and is typically defined by a client implementation as the authzid value provided during stream authentication. An entity may maintain multiple resources simultaneously. A resource identifier MUST be no more than 1023 bytes in length and MUST conform to the resourceprep [11] profile of stringprep [9]. Saint-Andre & Miller Expires November 2, 2003 [Page 9] Internet-Draft XMPP Core May 2003 4. XML Streams 4.1 Overview Two fundamental concepts make possible the rapid, asynchronous exchange of relatively small payloads of structured information between presence-aware entities: XML streams and XML stanzas. These terms may be 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. An XML stream is negotiated from an initiating entity (usually a client or server) to a receiving entity (usually a server), normally over a TCP socket, and corresponds to the initiating entity's "session" with the receiving entity. The start of the XML stream is denoted unambiguously by an opening XML tag with appropriate attributes and namespace declarations, and the end of the XML stream is denoted unambiguously by a closing XML tag. An XML stream is unidirectional; in order to enable bidirectional information exchange, the initiating entity and receiving entity must negotiate one stream in each direction, normally over the same TCP connection. Definition of XML Stanza: An XML stanza is a discrete semantic unit of structured information that is sent from one entity to another over an XML stream. An XML stanza exists at the direct child level of the root element and is said to be well-balanced if it matches production [43] content of the XML specification [1]). 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., ). An XML stanza MAY contain child elements (with accompanying attributes, elements, and CDATA) as necessary in order to convey the desired information. Consider the example of a client's session with 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 supported and the character encoding (see also Section 8.4). 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 authenticated with the server (see Section 6), the client MAY send an unlimited number of XML stanzas over the stream to any recipient on the network. When the client desires to close the stream, it simply sends a closing tag to the server (alternatively, the stream may be closed by the server), after which both the client and server SHOULD close the Saint-Andre & Miller Expires November 2, 2003 [Page 10] Internet-Draft XMPP Core May 2003 underlying TCP connection as well. Those who are accustomed to thinking of XML in a document-centric manner may wish to view a client's session with 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, and XMPP does not deal in documents but in XML streams and XML stanzas. In essence, then, an XML stream acts as an envelope for all the XML stanzas sent during a session. We can represent this graphically as follows: |--------------------| | | |--------------------| | | | | | | |--------------------| | | | | | | |--------------------| | | | | | | |--------------------| | ... | |--------------------| | | |--------------------| 4.2 Stream Attributes The attributes of the stream element are as follows: o to -- The 'to' attribute SHOULD be used only in the XML stream header from the initiating entity to the receiving entity, and MUST be set to the XMPP address of the receiving entity. There SHOULD be no 'to' attribute set in the XML stream header by which the receiving entity replies to the initiating entity; however, if a 'to' attribute is included, it SHOULD be silently ignored by the initiating entity. Saint-Andre & Miller Expires November 2, 2003 [Page 11] Internet-Draft XMPP Core May 2003 o from -- The 'from' attribute SHOULD be used only in the XML stream header from the receiving entity to the initiating entity, and MUST be set to the XMPP address of the receiving entity granting access to the initiating entity. There SHOULD be no 'from' attribute on the XML stream header sent from the initiating entity to the receiving entity; however, if a 'from' attribute is included, it SHOULD be silently ignored by the receiving entity. o id -- 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 session key for the initiating entity's streams with the receiving entity. There SHOULD be no '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. o version -- If the initiating entity complies with the protocol defined herein, it MUST include a 'version' attribute in the XML stream header it sends to the receiving entity, and it MUST set the value of the 'version' attribute to "1.0". If the initiating entity includes the version attribute and the receiving entity supports XMPP 1.0, the receiving entity MUST reciprocate by including the attribute in its response. We can summarize these values as follows: | initiating to receiving | receiving to initiating ------------------------------------------------------------ to | hostname of receiver | silently ignored from | silently ignored | hostname of receiver id | silently ignored | session key version | signals XMPP 1.0 support | signals XMPP 1.0 support 4.3 Namespace Declarations The stream element MUST possess both a stream namespace declaration and a default namespace declaration (as "namespace declaration" is defined in the XML namespaces specification [12]). 4.3.1 Stream Namespace Declaration A stream namespace declaration (e.g., 'xmlns:stream') is REQUIRED in both XML streams. A compliant entity SHOULD accept any namespace prefix on the element; however, for historical reasons some entities MAY accept only a 'stream' prefix, resulting in the use of a element as the stream root. The name of the stream Saint-Andre & Miller Expires November 2, 2003 [Page 12] Internet-Draft XMPP Core May 2003 namespace MUST be "http://etherx.jabber.org/streams". 4.3.2 Default Namespace Declaration A default namespace declaration ('xmlns') is REQUIRED and is used in both XML streams in order to define the allowable first-level children of the root stream element for both streams. This namespace declaration MUST be the same for the initiating stream and the responding stream so that both streams are scoped consistently. The default namespace declaration applies to the stream and all stanzas sent within a stream (unless explicitly scoped by another namespace). Since XML streams function as containers for any XML stanzas sent asynchronously between network endpoints, it should be possible to scope an XML stream with any default namespace declaration. At a minimum, a compliant implementation MUST support the following two namespaces (for historical reasons, some implementations MAY support only these two default namespaces): o jabber:client -- this default namespace is declared when the stream is used for communications between a client and a server o jabber:server -- this default namespace is declared when the stream is used for communications between two servers The 'jabber:client' and 'jabber:server' namespaces are nearly identical but are used in different contexts (client-to-server communications for 'jabber:client' and server-to-server communications for 'jabber:server'). The only difference between the two is that the 'to' and 'from' attributes are OPTIONAL on stanzas sent within 'jabber:client', whereas they are REQUIRED on stanzas sent within 'jabber:server'. If a compliant implementation accepts a stream that is scoped by the 'jabber:client' or 'jabber:server' namespace, it MUST support all three core stanza types (message, presence, and IQ) as described herein and defined in the schema. 4.4 Stream Features If the initiating entity sends a "version='1.0'" flag in its initiating stream element, the receiving entity MUST send a features child element to the initiating entity in order to announce any stream-level features that can be negotiated (or capabilities that otherwise need to be advertised). Currently this is used for SASL and TLS negotiation only, but it could be used for other negotiable features in the future (usage is defined under Stream Encryption (Section 5) and Stream Authentication (Section 6) below). If an entity does not understand or support some features, it SHOULD silently ignore them. Saint-Andre & Miller Expires November 2, 2003 [Page 13] Internet-Draft XMPP Core May 2003 4.5 Stream Errors The root stream element MAY contain an error child element (e.g., if the stream namespace prefix is 'stream'). The error child MUST be sent by a compliant entity (usually a server rather than a client) if it perceives that a stream-level error has occurred. 4.5.1 Rules The following rules apply to stream-level errors: o It is assumed that all 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 close the underlying TCP connection. o If the error occurs while the stream is being set up, the receiving entity MUST still send the opening and closing stream tags and include the error element as a child of the stream element. In this case, if the initiating entity provides an unknown host in the 'to' attribute (or provides no 'to' attribute at all), the server SHOULD provide the server's authoritative hostname in the 'from' attribute of the stream header sent before termination. 4.5.2 Syntax The syntax for stream errors is as follows: The value of the 'class' attribute must be one of the following: o address -- the condition relates to the JID or domain to which the stream was addressed o format -- the condition relates to XML format or structure o redirect -- the condition relates to a host redirection o server -- the condition relates to the internal state of the Saint-Andre & Miller Expires November 2, 2003 [Page 14] Internet-Draft XMPP Core May 2003 server The element MUST contain a child element that specifies a particular stream-level error condition, as defined in the next section. (Note: the XML namespace name 'urn:ietf:params:xml:ns:xmpp-streams' that scopes the element adheres to the format defined in The IETF XML Registry [25].) 4.5.3 Conditions The following stream-level error conditions are defined: o -- 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; the associated class is "address". o -- 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; the associated class is "address". o -- the server has experienced a misconfiguration or an otherwise-undefined internal error that prevents it from servicing the stream; the associated class is "server". o -- the stream ID or dialback ID is invalid or does not match an ID previously provided; the associated class is "format". o -- the stream namespace name is something other than "http://etherx.jabber.org/streams" or the dialback namespace name is something other than "jabber:server:dialback"; the associated class is "format". o -- the hostname provided in a 'from' address does not match the hostname (or any validated domain) negotiated via SASL or dialback; the associated class is "address". o -- the entity does not possess sufficient privileges to perform the desired action; the associated class is "access". o -- the server is unable to properly connect to a remote resource that is required for authentication or authorization; the associated class is "server". Saint-Andre & Miller Expires November 2, 2003 [Page 15] Internet-Draft XMPP Core May 2003 o -- the server is resource-contrained and is unable to service the stream; the associated class is "server". o -- the server will not provide service to the initiating entity but is redirecting traffic to another host; this element SHOULD contain CDATA specifying the alternate hostname or IP address to which the initiating entity MAY attempt to connect; the associated class is "redirect". o -- the server is being shut down and all active streams are being closed; the associated class is "server". o -- the initiating entity has sent a first-level child of the stream that is not supported by the server; the associated class is "format". o -- 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; this element MAY contain CDATA specifying the XMPP version(s) supported by the server; the associated class is "format". o -- the initiating entity has sent XML that is not well-formed as defined by the XML specification [1]; the associated class is "format". 4.5.4 Extensibility If desired, an XMPP application MAY provide custom error information; this MUST be contained in a properly-namespaced child of the element (i.e., the namespace name MUST NOT be one of the namespace names defined herein). 4.6 Simple Streams Example The following is a stream-based session 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): Saint-Andre & Miller Expires November 2, 2003 [Page 16] Internet-Draft XMPP Core May 2003 A basic session: C: S: ... authentication ... C: C: Art thou not Romeo, and a Montague? C: S: S: Neither, fair saint, if either thee dislike. S: C: S: Saint-Andre & Miller Expires November 2, 2003 [Page 17] Internet-Draft XMPP Core May 2003 A session gone bad: C: S: ... authentication ... C: Bad XML, no closing body tag! S: S: Saint-Andre & Miller Expires November 2, 2003 [Page 18] Internet-Draft XMPP Core May 2003 5. Stream Encryption 5.1 Overview XMPP includes a method for securing the stream from tampering and eavesdropping. This channel encryption method makes use of the Transport Layer Security (TLS) [13] protocol, along with a "STARTTLS" extension that is modelled after similar extensions for the IMAP [26], POP3 [27], and ACAP [28] protocols as described in RFC 2595 [29]. The namespace name for the STARTTLS extension is 'urn:ietf:params:xml:ns:xmpp-tls', which adheres to the format defined in The IETF XML Registry [25].) TLS SHOULD be used between any initiating entity and any receiving entity (e.g., a stream from a client to a server or from one server to another). An administrator of a given domain MAY require use of TLS for either or both client-to-server communications and server-to-server communications. Servers SHOULD use TLS betweeen two domains for the purpose of securing server-to-server communications. When the remote domain is already known, the server can verify the credentials of the known domain by comparing known keys or certificates. When the remote domain is not recognized, it may still be possible to verify a certificate if it is signed by a common trusted authority. Even if there is no way to verify certificates (e.g., an unknown domain with a self-signed certificate, or a certificate signed by an unrecognized authority), if the servers choose to communicate despite the lack of verified credentials, TLS still SHOULD be used to provide channel encryption. The following business rules apply: 1. An initiating entity that complies with this specification MUST include the "version='1.0'" flag in the initiating stream header. 2. When a receiving entity that complies with this specification receives an initiating stream header that includes the "version='1.0'" flag, after sending a stream header in reply (including the version flag) it MUST include a element scoped by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace with the list of other stream features it supports. 3. If the initiating entity chooses to use TLS for stream encryption, TLS negotiation MUST be completed before proceeding to SASL negotiation. 4. The initiating entity MUST validate the certificate presented by the receiving entity: Saint-Andre & Miller Expires November 2, 2003 [Page 19] Internet-Draft XMPP Core May 2003 Case 1 -- The initiating entity has been configured with a set of trusted root certificates: Normal certificate validation processing is appropriate, and SHOULD follow the rules defined for HTTP over TLS [14]. The trusted roots may be either a well-known public set or a manually configured Root CA (e.g., an organization's own Certificate Authority or a self-signed Root CA for the service as described under High Security (Section 11.1)). This case is RECOMMENDED. Case 2 -- The initiating entity has been configured with the receiving entity's self-signed service certificate: Simple comparison of public keys is appropriate. This case is NOT RECOMMENDED (see High Security (Section 11.1) for details). If the above methods fail, the certificate SHOULD be presented to a user for approval; if presented, the receiver MUST deliver the entire certificate chain to the user, who SHOULD be given the option to store the Root CA certificate (not the service or End Entity certificate) and to not be queried again regarding acceptance of the certificate for some reasonable period of time. 5. If the TLS negotiation is successful, the receiving entity MUST discard any knowledge obtained from the initiating entity before TLS takes effect. 6. If the TLS negotiation is successful, the initiating entity MUST discard any knowledge obtained from the receiving entity before TLS takes effect. 7. If the TLS negotiation is successful, the receiving entity MUST NOT offer the STARTTLS extension to the initiating entity along with the other stream features that are offered when the stream is restarted. 8. If the TLS negotiation results in success, the initiating entity SHOULD continue with SASL negotiation. 9. If the TLS negotiation results in failure, the receiving entity MUST terminate both the XML stream and the underlying TCP connection. 5.2 Narrative When an initiating entity secures a stream with a receiving entity, the steps involved are as follows: 1. The initiating entity opens a TCP connection and initiates the Saint-Andre & Miller Expires November 2, 2003 [Page 20] Internet-Draft XMPP Core May 2003 stream by sending the opening XML stream header to the receiving entity, including the "version='1.0'" flag. 2. The receiving entity responds by opening a TCP connection and sending an XML stream header to the initiating entity, including the "version='1.0'" flag. 3. The receiving entity offers the STARTTLS extension to the initiating entity by including it with the list of other supported stream features (if TLS is required for interaction with the receiving entity, it SHOULD signal that fact by including a element as a child of the element). 4. The initiating entity issues the STARTTLS command to instruct the receiving entity that it wishes to begin a TLS negotiation to secure the stream. 5. The receiving entity MUST reply with either a element or a element scoped by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace. If the failure case occurs, the receiving entity MUST terminate both the XML stream and the underlying TCP connection. If the proceed case occurs, the receiving entity MUST ignore any further XML data sent over the XML stream but keep the underlying TCP connection open for the purpose of completing the TLS negotiation. 6. The initiating entity and receiving entity attempt to complete a TLS negotiation in accordance with RFC 2246 [13]. 7. If the TLS negotiation is successful, the initiating entity SHOULD initiate a new stream by sending an opening XML stream header to the receiving entity. If the TLS negotiation is unsuccessful, the receiving entity MUST terminate the TCP connection. 8. Upon receiving the new stream header from the initiating entity, the receiving entity SHOULD respond by sending a new XML stream header to the initiating entity along with the remaining available features (but NOT including the STARTTLS element). 5.3 Client-to-Server Example The following example shows the data flow for a client securing a stream using STARTTLS (the IANA registers port 5222 for client-to-server communications using XMPP/Jabber, but another port MAY be used). Saint-Andre & Miller Expires November 2, 2003 [Page 21] Internet-Draft XMPP Core May 2003 Step 1: Client initiates stream to server: Step 2: Server responds by sending a stream tag to the client: Step 3: Server sends the STARTTLS extension to the client along with authentication mechanisms and any other stream features: DIGEST-MD5 PLAIN Step 4: Client sends the STARTTLS command to the server: Step 5: Server informs client to proceed: Step 5 (alt): Server informs client that TLS negotiation has failed and closes both stream and TCP connection: Step 6: Client and server attempt to complete TLS negotiation over the existing TCP connection. Saint-Andre & Miller Expires November 2, 2003 [Page 22] Internet-Draft XMPP Core May 2003 Step 7: If TLS negotiation is successful, client initiates a new stream to the server: Step 7 (alt): If TLS negotiation is unsuccessful, server MUST close TCP connection. Step 8: Server responds by sending a stream header to the client along with any remaining negotiable stream features: DIGEST-MD5 PLAIN EXTERNAL Step 9: Client SHOULD continue with stream authentication (Section 6). 5.4 Server-to-Server Example The following example shows the data flow for two servers securing a stream using STARTTLS (the IANA registers port 5269 for server-to-server communications using XMPP/Jabber, but another port MAY be used). Step 1: Server1 initiates stream to Server2: Saint-Andre & Miller Expires November 2, 2003 [Page 23] Internet-Draft XMPP Core May 2003 Step 2: Server2 responds by sending a stream tag to Server1: Step 3: Server2 sends the STARTTLS extension to Server1 along with authentication mechanisms and any other stream features: DIGEST-MD5 KERBEROS_V4 Step 4: Server1 sends the STARTTLS command to Server2: Step 5: Server2 informs Server1 to proceed: Step 5 (alt): Server2 informs Server1 that TLS negotiation has failed and closes stream: 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: Step 7 (alt): If TLS negotiation is unsuccessful, server MUST close TCP connection. Saint-Andre & Miller Expires November 2, 2003 [Page 24] Internet-Draft XMPP Core May 2003 Step 8: Server2 responds by sending a stream header to Server1 along with any remaining negotiable stream features: DIGEST-MD5 KERBEROS_V4 EXTERNAL Step 9: Server1 SHOULD continue with stream authentication (Section 6). Saint-Andre & Miller Expires November 2, 2003 [Page 25] Internet-Draft XMPP Core May 2003 6. Stream Authentication XMPP includes two methods for enforcing authentication at the level of XML streams. The secure and preferred method for authenticating streams between two entities uses an XMPP adaptation of the Simple Authentication and Security Layer (SASL) [15]. If SASL negotiation is not possible, some level of trust MAY be established between servers based on existing trust in DNS; the authentication method used in this case is the server dialback protocol that is native to XMPP (no such ad-hoc method is defined between a client and a server). If SASL is used for server-to-server authentication, the servers MUST NOT use dialback. For further information about the relative merits of these two methods, consult Security Considerations (Section 11). Stream authentication is REQUIRED for all direct communications between two entities; if an entity sends a stanza to an unauthenticated stream, the receiving entity SHOULD silently drop the stanza, MUST NOT process it, and MAY terminate both the XML stream and the underlying TCP connection. 6.1 SASL Authentication 6.1.1 Overview The Simple Authentication and Security Layer (SASL) provides a generalized method for adding authentication support to connection-based protocols. XMPP uses a generic XML namespace profile for SASL that conforms to section 4 ("Profiling Requirements") of RFC 2222 [15] (the XMPP-specific namespace name is 'urn:ietf:params:xml:ns:xmpp-sasl'), which adheres to the format defined in The IETF XML Registry [25].) The following business rules apply: 1. If TLS is used for stream encryption, SASL SHOULD NOT be used for anything except stream authentication (i.e., a security layer SHOULD NOT be negotiated using SASL). Conversely, if a security layer is to be negotiated via SASL, TLS SHOULD NOT be used. 2. If the initiating entity is capable of authenticating via SASL, it MUST include the "version='1.0'" flag in the initiating stream header. 3. If the receiving entity is capable of accepting authentications via SASL, it MUST send one or more authentication mechanisms within a element scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace in response to the opening stream tag received from the initiating entity. Saint-Andre & Miller Expires November 2, 2003 [Page 26] Internet-Draft XMPP Core May 2003 4. If the SASL negotiation 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. 5. If the SASL negotiation 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. The following syntax rules apply: 1. The initial challenge MUST include a realm, nonce, qop, charset, and algorithm. 2. The inital response for client-to-server negotiation MUST include all and only a username, realm, nonce, cnonce, nc, qop, digest-uri, response, charset, and authzid. 3. The inital response for server-to-server negotiation MUST include all and only a realm, nonce, cnonce, nc, qop, digest-uri, response, and charset. 4. The realm-value MUST be no more than 1023 bytes in length and MUST conform to the nameprep [8] profile of stringprep [9]. 5. The username-value MUST be no more than 1023 bytes in length and MUST conform to the nodeprep [10] profile of stringprep [9]. 6. The response-value MUST be computed in accordance with the relevant SASL mechanism as defined by the appropriate RFC (e.g., RFC 2831 [16] for digest authentication). 7. The authzid-value MUST be a Jabber Identifier of the form , i.e., it MUST include a node identifier, domain identifier, and resource identifier. 6.1.2 Narrative When an initiating entity authenticates with a receiving entity, the steps involved are as follows: 1. The initiating entity requests SASL authentication by including a 'version' attribute in the opening XML stream header sent to the receiving entity, with the value set to "1.0". 2. After sending an XML stream header in response, the receiving Saint-Andre & Miller Expires November 2, 2003 [Page 27] Internet-Draft XMPP Core May 2003 entity sends a list of available SASL authentication mechanisms; each of these is a element included as a child within a container element scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace, which in turn is a child of a element in the streams namespace. If channel encryption must be established before a particular authentication mechanism may be used, the receiving entity MUST NOT provide that mechanism in the list of available SASL authentication methods prior to channel encryption. If the initiating entity presents a valid initiating entity certificate during prior TLS negotiation, the receiving entity MAY offer the SASL EXTERNAL mechanism to the initiating entity during stream authentication (see RFC 2222 [15]). 3. The initiating entity selects a mechanism by sending an element scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the receiving entity and including the appropriate value for the 'mechanism' attribute; this element MAY optionally contain character data (in SASL terminology the "initial response") if the mechanism supports or requires it. If the initiating entity selects the EXTERNAL mechanism for authentication, the authentication credentials shall be taken from the certificate presented during prior TLS negotiation. 4. If necessary, the receiving entity challenges the initiating entity by sending a element scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating entity; this element MAY optionally contain character data (which MUST be computed in accordance with the SASL mechanism chosen by the initiating entity). 5. The initiating entity responds to the challenge by sending a element scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the receiving entity; this element MAY optionally contain character data (which MUST be computed in accordance with the SASL mechanism chosen by the initiating entity). 6. 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: 1. The initiating entity aborts the handshake by sending an element to the receiving entity. 2. The receiving entity reports failure of the handshake by sending Saint-Andre & Miller Expires November 2, 2003 [Page 28] Internet-Draft XMPP Core May 2003 a element scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating entity. The particular cause of failure SHOULD be communicated in an appropriate child element of the element. The following conditions are defined: * * * * * 3. The receiving entity reports success of the handshake by sending a element scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating entity; this element MAY optionally contain character data (in SASL terminology "additional data with success"). Any character data contained within these elements MUST be encoded using base64. 6.1.3 SASL Definition Section 4 of the SASL specification [15] requires 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 '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. (All of these elements are scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace.) Saint-Andre & Miller Expires November 2, 2003 [Page 29] Internet-Draft XMPP Core May 2003 security layer negotiation: If a security layer is negotiated, both sides consider the original stream closed and new headers are sent by both entities. The security layer takes effect immediately following the ">" character of the element for the client and immediately following the closing ">" character of the element for the server. (Both of these elements are scoped by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace.) use of the authorization identity: The authorization identity is used by xmpp only in negotiation between a client and a server, and denotes the "full JID" (user@domain/resource) requested by the user or application associated with the client. 6.1.4 Client-to-Server Example The following example shows the data flow for a client authenticating with a server using SASL (the IANA registers port 5222 for client-to-server communications using XMPP/Jabber, but another port MAY be used). Step 1: Client initiates stream to server: Step 2: Server responds with a stream tag sent to the client: Step 3: Server informs client of available authentication mechanisms: DIGEST-MD5 PLAIN Saint-Andre & Miller Expires November 2, 2003 [Page 30] Internet-Draft XMPP Core May 2003 Step 4: Client selects an authentication mechanism: Step 5: Server sends a base64-encoded challenge to the client: cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi xxb3A9ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz The decoded challenge is: realm="cataclysm.cx",nonce="OA6MG9tEQGm2hh",\ qop="auth",charset=utf-8,algorithm=md5-sess Step 6: Client responds to the challenge: dXNlcm5hbWU9InJvYiIscmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik 9BNk1HOXRFUUdtMmhoIixjbm9uY2U9Ik9BNk1IWGg2VnFUclJrIixuYz0w MDAwMDAwMSxxb3A9YXV0aCxkaWdlc3QtdXJpPSJ4bXBwL2NhdGFjbHlzbS 5jeCIscmVzcG9uc2U9ZDM4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxNDNh ZjcsY2hhcnNldD11dGYtOCxhdXRoemlkPSJyb2JAY2F0YWNseXNtLmN4L2 15UmVzb3VyY2Ui The decoded response is: username="rob",realm="cataclysm.cx",\ nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\ nc=00000001,qop=auth,digest-uri="xmpp/cataclysm.cx",\ response=d388dad90d4bbd760a152321f2143af7,charset=utf-8,\ authzid="rob@cataclysm.cx/myResource" Step 7: Server sends another challenge to the client: cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA== The decoded challenge is: rspauth=ea40f60335c427b5527b84dbabcdfffd Saint-Andre & Miller Expires November 2, 2003 [Page 31] Internet-Draft XMPP Core May 2003 Step 8: Client responds to the challenge: Step 9: Server informs client of successful authentication: Step 9 (alt): Server informs client of failed authentication: Step 10: Client initiates a new stream to the server: Step 11: Server responds by sending a stream header to the client, with the stream already authenticated (not followed by further stream features): 6.1.5 Server-to-Server Example The following example shows the data flow for a server authenticating with another server using SASL (the IANA registers port 5269 for server-to-server communications using XMPP/Jabber, but another port MAY be used). Step 1: Server1 initiates stream to Server2: Saint-Andre & Miller Expires November 2, 2003 [Page 32] Internet-Draft XMPP Core May 2003 Step 2: Server2 responds with a stream tag sent to Server1: Step 3: Server2 informs Server1 of available authentication mechanisms: DIGEST-MD5 KERBEROS_V4 Step 4: Server1 selects an authentication mechanism: Step 5: Server2 sends a base64-encoded challenge to Server1: cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi xxb3A9ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz The decoded challenge is: realm="cataclysm.cx",nonce="OA6MG9tEQGm2hh",\ qop="auth",charset=utf-8,algorithm=md5-sess Step 6: Server1 responds to the challenge: cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi xjbm9uY2U9Ik9BNk1IWGg2VnFUclJrIixuYz0wMDAwMDAwMSxxb3A9YXV0 aCxkaWdlc3QtdXJpPSJ4bXBwL2NhdGFjbHlzbS5jeCIscmVzcG9uc2U9ZD M4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcsY2hhcnNldD11dGYt OAo= The decoded response is: Saint-Andre & Miller Expires November 2, 2003 [Page 33] Internet-Draft XMPP Core May 2003 realm="cataclysm.cx",nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\ nc=00000001,qop=auth,digest-uri="xmpp/cataclysm.cx",\ response=d388dad90d4bbd760a152321f2143af7,charset=utf-8 Step 7: Server2 sends another challenge to Server1: cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA== The decoded challenge is: rspauth=ea40f60335c427b5527b84dbabcdfffd Step 8: Server1 responds to the challenge: Step 9: Server2 informs Server1 of successful authentication: Step 9 (alt): Server2 informs Server1 of failed authentication: Step 10: Server1 initiates a new stream to Server2: Step 11: Server2 responds by sending a stream header to Server1, with the stream already authenticated (not followed by further stream features): Saint-Andre & Miller Expires November 2, 2003 [Page 34] Internet-Draft XMPP Core May 2003 6.2 Dialback Authentication XMPP includes a protocol-level method for verifying that a connection between two servers can be trusted as much as the DNS can be trusted. The method is called dialback and is used only within XML streams that are declared under the "jabber:server" namespace. The purpose of the dialback protocol is to make server spoofing more difficult, and thus to make it more difficult to forge XML stanzas. Dialback is decidedly not intended as a mechanism for securing or encrypting the streams between servers as is done via SASL and TLS, only for helping to prevent the spoofing of a server and the sending of false data from it. In particular, dialback authentication is susceptible to DNS poisoning attacks unless DNSSec [30] is used. Furthermore, even if the DNS information is accurate, dialback authentication cannot protect from attacks where the attacker is capable of hijacking the IP address of the remote domain. Domains requiring more robust security SHOULD use TLS and SASL as defined above. Server dialback is made possible by the existence of DNS, since one server can verify that another server which is connecting to it is authorized to represent a given hostname. All DNS hostname resolutions MUST first resolve the hostname using an SRV [18] record of _jabber._tcp.server. If the SRV lookup fails, the fallback is a normal A lookup to determine the IP address, using the jabber-server port of 5269 assigned by the Internet Assigned Numbers Authority [5]. The method for generating and verifying the keys used in the dialback protocol MUST take into account the hostnames being used, the random ID generated for the stream, and a secret known by the authoritative server's network. Generating unique but verifiable keys is important to prevent common man-in-the-middle attacks and server spoofing. Any error that occurs during dialback negotiation MUST be considered a stream error, resulting in termination of the stream and of the underlying TCP connection. The possible error conditions are specified in the protocol description below. The following terminology applies: o Originating Server -- the server that is attempting to establish a connection between two domains. o Receiving Server -- the server that is trying to authenticate that Originating Server represents the domain which it claims to be. o Authoritative Server -- the server that answers to the DNS Saint-Andre & Miller Expires November 2, 2003 [Page 35] Internet-Draft XMPP Core May 2003 hostname asserted by Originating Server; for basic environments this will be Originating Server, but it could be a separate machine in Originating Server's network. The following is a brief summary of the order of events in dialback: 1. Originating Server establishes a connection to Receiving Server. 2. Originating Server sends a 'key' value over the connection to Receiving Server. 3. Receiving Server establishes a connection to Authoritative Server. 4. Receiving Server sends the same 'key' value to Authoritative Server. 5. Authoritative Server replies that key is valid or invalid. 6. Receiving Server informs Originating Server whether it is authenticated or not. We can represent this flow of events graphically as follows: Originating Receiving Server Server ----------- --------- | | | establish connection | | ----------------------> | | | | send stream header | | ----------------------> | | | | establish connection | | <---------------------- | | | | send stream header | | <---------------------- | | | Authoritative | send dialback key | Server | ----------------------> | ------------- | | | | establish connection | | ----------------------> | | | | send stream header | | ----------------------> | Saint-Andre & Miller Expires November 2, 2003 [Page 36] Internet-Draft XMPP Core May 2003 | | | establish connection | | <---------------------- | | | | send stream header | | <---------------------- | | | | send dialback key | | ----------------------> | | | | validate dialback key | | <---------------------- | | | report dialback result | | <---------------------- | | | 6.2.1 Dialback Protocol The interaction between the servers is as follows: 1. Originating Server establishes TCP connection to Receiving Server. 2. Originating Server sends a stream header to Receiving Server: Note: the 'to' and 'from' attributes are NOT REQUIRED on the root stream element. The inclusion of the xmlns:db namespace declaration with the name shown indicates to Receiving Server that Originating Server supports dialback. If the namespace name is incorrect, then Receiving Server MUST generate an stream error condition and terminate both the XML stream and the underlying TCP connection. 3. Receiving Server SHOULD send a stream header back to Originating Server, including a unique ID for this interaction: Saint-Andre & Miller Expires November 2, 2003 [Page 37] Internet-Draft XMPP Core May 2003 Note: The 'to' and 'from' attributes are NOT REQUIRED on the root stream element. If the namespace name is incorrect, then Originating Server MUST generate an stream error condition and terminate both the XML stream and the underlying TCP connection. Note well that Receiving Server is NOT REQUIRED to reply and MAY silently terminate the XML stream and underlying TCP connection depending on security policies in place. 4. Originating Server sends a dialback key to Receiving Server: 98AF014EDC0... Note: this key is not examined by Receiving Server, since Receiving Server does not keep information about Originating Server between sessions. The key generated by Originating Server must be based in part on the value of the ID provided by Receiving Server in the previous step, and in part on a secret shared by Originating Server and Authoritative Server. If the value of the 'to' address does not match a hostname recognized by Receiving Server, then Receiving Server MUST generate a stream error condition and terminate both the XML stream and the underlying TCP connection. If the value of the 'from' address matches a domain with which Receiving Server already has an established connection, then Receiving Server SHOULD generate a stream error condition and terminate both the XML stream and the underlying TCP connection. 5. Receiving Server establishes a TCP connection back to the domain name asserted by Originating Server, as a result of which it connects to Authoritative Server. (Note: as an optimization, an implementation MAY reuse an existing trusted connection here rather than opening a new TCP connection.) 6. Receiving Server sends Authoritative Server a stream header: Saint-Andre & Miller Expires November 2, 2003 [Page 38] Internet-Draft XMPP Core May 2003 Note: the 'to' and 'from' attributes are NOT REQUIRED on the root stream element. If the namespace name is incorrect, then Authoritative Server MUST generate an stream error condition and terminate both the XML stream and the underlying TCP connection. 7. Authoritative Server sends Receiving Server a stream header: Note: if the namespace name is incorrect, then Receiving Server MUST generate an stream error condition and terminate both the XML stream and the underlying TCP connection between it and Authoritative Server. If the ID does not match that provided by Receiving Server in Step 3, then Receiving Server MUST generate an stream error condition and terminate both the XML stream and the underlying TCP connection between it and Authoritative Server. If either of the foregoing stream errors occurs between Receiving Server and Authoritative Server, then Receiving Server MUST generate a stream error condition and terminate both the XML stream and the underlying TCP connection between it and Originating Server. 8. Receiving Server sends Authoritative Server a stanza requesting that Authoritative Server verify a key: 98AF014EDC0... Note: passed here are the hostnames, the original identifier from Receiving Server's stream header to Originating Server in Step 3, and the key that Originating Server sent to Receiving Server in Step 4. Based on this information as well as shared secret information within the Authoritative Server's network, Saint-Andre & Miller Expires November 2, 2003 [Page 39] Internet-Draft XMPP Core May 2003 the key is verified. Any verifiable method MAY be used to generate the key. If the value of the 'to' address does not match a hostname recognized by Authoritative Server, then Authoritative Server MUST generate a stream error condition and terminate both the XML stream and the underlying TCP connection. If the value of the 'from' address does not match the hostname represented by Receiving Server when opening the TCP connection (or any validated domain), then Authoritative Server MUST generate a stream error condition and terminate both the XML stream and the underlying TCP connection. 9. Authoritative Server sends a stanza back to Receiving Server verifying whether the key was valid or invalid: or Note: if the ID does not match that provided by Receiving Server in Step 3, then Receiving Server MUST generate an stream error condition and terminate both the XML stream and the underlying TCP connection. If the value of the 'to' address does not match a hostname recognized by Receiving Server, then Receiving Server MUST generate a stream error condition and terminate both the XML stream and the underlying TCP connection. If the value of the 'from' address does not match the hostname represented by Originating Server when opening the TCP connection (or any validated domain), then Receiving Server MUST generate a stream error condition and terminate both the XML stream and the underlying TCP connection. 10. Receiving Server informs Originating Server of the result: Saint-Andre & Miller Expires November 2, 2003 [Page 40] Internet-Draft XMPP Core May 2003 Note: At this point the connection has either been validated via a type='valid', or reported as invalid. If the connection is invalid, then Receiving Server MUST terminate both the XML stream and the underlying TCP connection. If the connection is validated, data can be sent by Originating Server and read by Receiving Server; before that, all data stanzas sent to Receiving Server SHOULD be silently dropped. Even if dialback negotiation is successful, a server MUST verify that all XML stanzas received from the other server include a 'from' attribute and a 'to' attribute; if a stanza does not meet this restriction, the server that receives the stanza MUST generate an stream error condition and terminate both the XML stream and the underlying TCP connection. Furthermore, a server MUST verify that the 'from' attribute of stanzas received from the other server includes a validated domain for the stream; if a stanza does not meet this restriction, the server that receives the stanza MUST generate a stream error condition and terminate both the XML stream and the underlying TCP connection. Both of these checks help to prevent spoofing related to particular stanzas. Saint-Andre & Miller Expires November 2, 2003 [Page 41] Internet-Draft XMPP Core May 2003 7. XML Stanzas 7.1 Overview Once the XML streams in each direction have been authenticated and (if desired) encrypted, XML stanzas can be sent over the streams. Three XML stanza types are defined for the 'jabber:client' and 'jabber:server' namespaces: , , and . In essence, the stanza type 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. The element can be seen as a basic broadcast or "publish-subscribe" mechanism, whereby multiple entities receive information (in this case, presence information) about an entity to which they have subscribed. The element can be seen as a "request-response" mechanism similar to HTTP, whereby two entities can engage in a structured conversation using 'get' or 'set' requests and 'result' or 'error' responses. 7.2 Common Attributes The following five attributes are common to message, presence, and IQ stanzas: 7.2.1 to The 'to' attribute specifies the JID of the intended recipient for the stanza. In the 'jabber:client' namespace, a stanza SHOULD possess a 'to' attribute, although a stanza sent from a client to a server for handling by that server (e.g., presence sent to the server for broadcasting to other entities) MAY legitimately lack a 'to' attribute. In the 'jabber:server' namespace, 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 condition and terminate both the XML stream and the underlying TCP connection. If the value of the 'to' attribute is invalid or cannot be contacted, the entity discovering that fact (usually the sender's or recipient's server) MUST return an appropriate error to the sender. 7.2.2 from Saint-Andre & Miller Expires November 2, 2003 [Page 42] Internet-Draft XMPP Core May 2003 The 'from' attribute specifies the JID of the sender. In the 'jabber:client' namespace, a client MUST NOT include a 'from' attribute on the stanzas it sends to a server; if a server receives a stanza from a client and the stanza possesses a 'from' attribute, it MUST ignore the value of the 'from' attribute and MAY return an error to the sender. In addition, a server MUST stamp stanzas received from a client with the user@domain/resource (full JID) of the connected resource that generated the stanza as defined by the authzid provided in the SASL negotiation. In the 'jabber:server' namespace, 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 condition. Furthermore, the domain identifier portion of the JID contained in the 'from' attribute MUST match the hostname of the sending server (or any validated domain) as communicated in the SASL negotiation or dialback negotiation; if a server receives a stanza that does not meet this restriction, it MUST generate a stream error condition. Both of these conditions MUST result in closing of the stream and termination of the underlying TCP connection. 7.2.3 id The optional '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 use of IQ stanzas). If the stanza sent by the sending entity is an IQ stanza of type "get" or "set", the receiving entity MUST include an 'id' attribute with the same value in any replies of type "result" or "error". The value of the 'id' attribute is NOT REQUIRED to be unique either globally, within a domain, or within a stream. 7.2.4 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, and thus are defined in the following sections. 7.2.5 xml:lang A stanza SHOULD possess an 'xml:lang' attribute (as defined in Section 2.12 of the XML specification [1]) specifying the default language of any XML character data contained in the stanza or its child elements. The value of the 'xml:lang' attribute MUST be an Saint-Andre & Miller Expires November 2, 2003 [Page 43] Internet-Draft XMPP Core May 2003 NMTOKEN and MUST conform to the format defined in RFC 3066 [17]. 7.3 Message Stanzas Message stanzas in the 'jabber:client' or 'jabber:server' namespace are used to "push" information to another entity. Common uses in the context of instant messaging include single messages, messages sent in the context of a chat conversation, messages sent in the context of a multi-user chat room, headlines, and errors. These messages types are identified more fully below. 7.3.1 Types of Message The 'type' attribute of a message stanza is OPTIONAL; if included, it specifies the conversational context of the message. The sending of a message stanza without a 'type' attribute signals that the message stanza is a single message. However, the 'type' attribute MAY also have one of the following values: o chat o error o groupchat o headline For information about the meaning of these message types, refer to XMPP IM [23]. 7.3.2 Children As described under extended namespaces (Section 7.6), a message stanza MAY contain any properly-namespaced child element. In accordance with the default namespace declaration, by default a message stanza is in the 'jabber:client' or 'jabber:server' namespace, which defines certain allowable children of message stanzas. If the message stanza is of type "error", it MUST include an child; for details, see Section 7.7. If the message stanza has no 'type' attribute or has a 'type' attribute with a value of "chat", "groupchat", or "headline", it MAY contain any of the following child elements without an explicit namespace declaration: 7.3.2.1 Subject The element specifies the topic of the message. The element SHOULD NOT possess any attributes, with the Saint-Andre & Miller Expires November 2, 2003 [Page 44] Internet-Draft XMPP Core May 2003 exception of the 'xml:lang' attribute. Multiple instances of the element MAY be included for the purpose of providing alternate versions of the same subject, but only if each instance possesses an 'xml:lang' attribute with a distinct language value. The element MUST NOT contain mixed content. 7.3.2.2 Body The element contains the textual contents of the message; this child element is normally included but NOT REQUIRED. The element SHOULD NOT possess any attributes, with the exception of the 'xml:lang' attribute. Multiple instances of the element MAY be included but only if each instance possesses an 'xml:lang' attribute with a distinct language value. The element MUST NOT contain mixed content. 7.3.2.3 Thread The element contains a random string that is generated by the sender and that SHOULD be copied back in replies; it is used for tracking a conversation thread (sometimes referred to as an "IM session") between two entities. If used, it MUST be unique to that conversation thread within the stream and MUST be consistent throughout that conversation. The use of the element is optional and is not used to identify individual messages, only conversations. Only one element MAY be included in a message stanza, and it MUST NOT possess any attributes. The element MUST be treated as an opaque string by entities; no semantic meaning may be derived from it, and only exact, case-insensitve comparisons may be made against it. The element MUST NOT contain mixed content. The method for generating thread IDs SHOULD be as follows: 1. concatenate the sender's full JID (user@domain/resource) with the recipient's full JID 2. concatenate these JID strings with a full ISO-8601 timestamp including year, month, day, hours, minutes, seconds, and UTC offset in the following format: YYYY-MM-DD-Thh:mm:ssTZD (where TZD is either "Z" for UTC or "[+|-]hh:mm" for an offset from UTC) 3. hash the resulting string according to the SHA1 algorithm 4. convert the hexidecimal SHA1 output to all lowercase Saint-Andre & Miller Expires November 2, 2003 [Page 45] Internet-Draft XMPP Core May 2003 7.4 Presence Stanzas Presence stanzas are used in the 'jabber:client' or 'jabber:server' namespace to express an entity's current availability status (offline or online, along with various sub-states of the latter and optional user-defined descriptive text) and to communicate that status to other entities. Presence stanzas are also used to negotiate and manage subscriptions to the presence of other entities. 7.4.1 Types of Presence The 'type' attribute of a presence stanza is optional. A presence stanza that does not possess a 'type' attribute is used to signal to the server that the sender is online and available for communication. If included, the 'type' attribute specifies a lack of availability, a request to manage a subscription to another entity's presence, a request for another entity's current presence, or an error related to a previously-sent presence stanza. The 'type' attribute MAY have one of the following values: o unavailable -- Signals that the entity is no longer available for communication. o subscribe -- The sender wishes to subscribe to the recipient's presence. o subscribed -- The sender has allowed the recipient to receive their presence. o unsubscribe -- A notification that an entity is unsubscribing from another entity's presence. o unsubscribed -- The subscription request has been denied or a previously-granted subscription has been cancelled. o probe -- A request for an entity's current presence. In general SHOULD NOT be sent by a client. o error -- An error has occurred regarding processing or delivery of a previously-sent presence stanza. Detailed information about presence semantics and about the subscription model used within XMPP can be found in XMPP IM [23]. 7.4.2 Children As described under extended namespaces (Section 7.6), a presence stanza MAY contain any properly-namespaced child element. Saint-Andre & Miller Expires November 2, 2003 [Page 46] Internet-Draft XMPP Core May 2003 In accordance with the default namespace declaration, by default a presence stanza is in the 'jabber:client' or 'jabber:server' namespace, which defines certain allowable children of presence stanzas. If the presence stanza is of type "error", it MUST include an child; for details, see Section 7.7. If the presence stanza possesses no 'type' attribute, it MAY contain any of the following child elements (note that the child MAY be sent in a presence stanza of type "unavailable" or, for historical reasons, "subscribe"): 7.4.2.1 Show The optional element specifies a particular availability status of an entity or specific resource (if a element is not provided, default availability is assumed). Only one element MAY be included in a presence stanza, and it SHOULD NOT possess any attributes. The CDATA value SHOULD be one of the following (values other than these four SHOULD be ignored; additional availability types could be defined through a properly-namespaced child element of the presence stanza): o away o chat o xa o dnd For information about the meaning of these values, refer to XMPP IM [23]. 7.4.2.2 Status The optional element contains a natural-language description of availability status. It is normally used in conjunction with the show element to provide a detailed description of an availability state (e.g., "In a meeting"). The element SHOULD NOT possess any attributes, with the exception of the 'xml:lang' attribute. Multiple instances of the element MAY be included but only if each instance possesses an 'xml:lang' attribute with a distinct language value. 7.4.2.3 Priority The optional element specifies the priority level of the connected resource. The value may be any integer between -128 and 127. Only one element MAY be included in a presence Saint-Andre & Miller Expires November 2, 2003 [Page 47] Internet-Draft XMPP Core May 2003 stanza, and it MUST NOT possess any attributes. For information regarding the semantics of priority values in stanza routing within IM applications, see XMPP IM [23]. 7.5 IQ Stanzas 7.5.1 Overview Info/Query, or IQ, is a request-response mechanism, similar in some ways to HTTP [31]. IQ stanzas in the 'jabber:client' or 'jabber:server' namespace 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 namespace declaration of a direct child element of the IQ element, and the interaction is tracked by the requesting entity through use of the 'id' attribute. Most IQ interactions follow a common pattern of structured data exchange such as get/result or set/result (although an error may be returned in response to a request if appropriate): Requesting Responding Entity Entity ---------- ---------- | | | | | ------------------------> | | | | | | <------------------------ | | | | | | ------------------------> | | | | | | <------------------------ | | | An entity that receives an IQ request of type "get" or "set" MUST reply with an IQ response of type "result" or "error" (which response MUST preserve the 'id' attribute of the request). 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, as shown above, 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). 7.5.2 Types of IQ Saint-Andre & Miller Expires November 2, 2003 [Page 48] Internet-Draft XMPP Core May 2003 The 'type' attribute of an IQ stanza is REQUIRED. The 'type' attribute specifies a distinct step within a request-response interaction. The value SHOULD be one of the following (all other values SHOULD be ignored): o get -- The stanza is a request for information or requirements. o set -- The stanza provides required data, sets new values, or replaces existing values. o result -- The stanza is a response to a successful get or set request. o error -- An error has occurred regarding processing or delivery of a previously-sent get or set. 7.5.3 Children As described under extended namespaces (Section 7.6), an IQ stanza MAY contain any properly-namespaced child element. Note that an IQ stanza contains no children in the 'jabber:client' or 'jabber:server' namespace since it is a vessel for XML in another namespace. An IQ stanza of type "get" or "set" MUST include one and only one child element. An IQ stanza of type "error" SHOULD include the child element contained in the associated "set" or "get" and MUST include an child; for details, see Section 7.7. 7.6 Extended Namespaces While the core data elements in the "jabber:client" or "jabber:server" namespace (along with their attributes and child elements) provide a basic level of functionality for messaging and presence, XMPP uses XML namespaces to extend the core data elements for the purpose of providing additional functionality. Thus a message, presence, or IQ stanza MAY house one or more optional child elements containing content that extends the meaning of the message (e.g., an encrypted form of the message body). 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. 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 Saint-Andre & Miller Expires November 2, 2003 [Page 49] Internet-Draft XMPP Core May 2003 stanza to the recipient. In particular: Recipient: If a recipient receives a stanza that contains a child element it does not understand, it SHOULD ignore that specific 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 in an extended namespace it does not understand, the portion of the stanza that is in the unknown namespace SHOULD be ignored. * If an entity receives a message stanza without a element but containing only a child element bound by a namespace it does not understand, it MUST ignore the entire stanza. * If an entity receives an IQ stanza in 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 (usually a server) handles a stanza that contains a child element it does not understand, it SHOULD ignore the associated XML data by passing it on untouched to the recipient. 7.7 Stanza Errors As defined herein, stanza-related errors are handled in a manner similar to stream errors (Section 4.5). 7.7.1 Rules The following rules apply to stanza-related errors: o A stanza whose 'type' attribute has a value of "error" MUST contain an child element. o The receiving or processing entity that returns an error to the sending entity SHOULD include the original XML sent along with the element and its children so that the sender can inspect and if necessary correct the XML before re-sending. o An entity that receives a stanza whose 'type' attribute has a value of "error" MUST NOT respond to the stanza with a further stanza of type "error"; this helps to prevent looping. Saint-Andre & Miller Expires November 2, 2003 [Page 50] Internet-Draft XMPP Core May 2003 o An child MUST NOT be included if the 'type' attribute has a value other than "error". 7.7.2 Syntax The syntax for stanza-related errors is as follows: [include sender XML here] The stanza-name is one of message, presence, or iq. The value of the 'class' attribute MUST be one of the following: o access -- the condition relates to access rights, permissions, or authorization o address -- the condition relates to the JID or domain to which the stanza was addressed o format -- the condition relates to XML format or structure o recipient -- the condition relates to the state or capabilities of the recipient (which may be the server) o server -- the condition relates to the internal state of the server The element MUST contain a child element that specifies a particular stanza-related error condition, and its namespace name MUST be 'urn:ietf:params:xml:ns:xmpp-stanzas', which adheres to the format defined in The IETF XML Registry [25]. 7.7.3 Conditions The following stanza-related error conditions are defined: o -- the sender has sent XML that is malformed or cannot be processed (e.g., a client-generated stanza includes a 'from' address, or an IQ stanza includes an unrecognized value of the 'type' attribute); the associated class is "format". Saint-Andre & Miller Expires November 2, 2003 [Page 51] Internet-Draft XMPP Core May 2003 o -- access cannot be granted because an existing resource or session exists with the same name or address; the associated class is "access". o -- the feature requested is not implemented by the recipient or server and therefore cannot be processed; the associated class is "recipient". o -- the stanza is understood but the action is forbidden; the associated class is "access". o -- the server could not process the stanza because of a misconfiguration or an otherwise-undefined internal server error; the associated class is "server". o -- the addressed JID or item requested cannot be found; the associated class is "address". o -- the value of the 'to' attribute in the sender's stanza does not adhere to the syntax defined in Addressing (Section 3); the associated class is "address". o -- the action is not permitted when attempted by the sender; the associated class is "access". o -- the specific recipient requested is currently unavailable; the associated class is "recipient". o -- the user is not authorized to access the requested service because registration is required; the associated class is "access". o -- a remote server or service specified as part or all of the JID of the intended recipient does not exist; the associated class is "address". o -- a remote server or service specified as part or all of the JID of the intended recipient could not be contacted within a reasonable amount of time; the associated class is "server". o -- the service requested is currently unavailable on the server; the associated class is "server". 7.7.4 Extensibility If desired, an XMPP application MAY provide custom error information Saint-Andre & Miller Expires November 2, 2003 [Page 52] Internet-Draft XMPP Core May 2003 by including a properly-namespaced child of the appropriate descriptive element name, for example: Saint-Andre & Miller Expires November 2, 2003 [Page 53] Internet-Draft XMPP Core May 2003 8. XML Usage within XMPP 8.1 Restrictions XMPP is a simplified and specialized protocol for streaming XML elements in order to exchange messages and presence 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 must support the full XML specification [1]. In particular, the following restrictions apply: With regard to XML generation, an XMPP implementation MUST NOT inject into an XML stream any of the following: o comments (as defined in Section 2.5 of the XML specification [1]) o processing instructions (Section 2.6) o internal or external DTD subsets (Section 2.8) o internal or external entity references (Section 4.2) with the exception of predefined entities (Section 4.6) With regard to XML processing, if an XMPP implementation receives such restricted XML data, it MUST ignore the data. 8.2 Namespaces XML Namespaces [12] 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 XML to be structurally mixed with any data element within XMPP. Additionally, an XMPP implementation MAY be more strict about namespace prefixes than the XML namespace specification requires. 8.3 Validation Except as noted with regard to 'to' and 'from' addresses for stanzas within the 'jabber:server' namespace, a server is not responsible for validating the XML elements forwarded to a client or another server; an implementation MAY choose to provide only validated data elements but is NOT REQUIRED to do so. Clients SHOULD NOT rely on the ability to send data which does not conform to the schemas, and SHOULD ignore any non-conformant elements or attributes on the incoming XML stream. Validation of XML streams and stanzas is NOT REQUIRED or recommended, Saint-Andre & Miller Expires November 2, 2003 [Page 54] Internet-Draft XMPP Core May 2003 and schemas are included herein for descriptive purposes only. 8.4 Character Encodings Software implementing XML streams MUST support the UTF-8 (RFC 2279 [19]) and UTF-16 (RFC 2781 [20]) transformations of Universal Character Set (ISO/IEC 10646-1 [21]) characters. Software MUST NOT attempt to use any other encoding for transmitted data. The encodings of the transmitted and received streams are independent. Software MAY select either UTF-8 or UTF-16 for the transmitted stream, and SHOULD deduce the encoding of the received stream as described in the XML specification [1]. For historical reasons, existing implementations MAY support UTF-8 only. 8.5 Inclusion of Text Declaration An application MAY send a text declaration. Applications MUST follow the rules in the XML specification [1] regarding the circumstances under which a text declaration is included. Saint-Andre & Miller Expires November 2, 2003 [Page 55] Internet-Draft XMPP Core May 2003 9. IANA Considerations 9.1 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. URI: urn:ietf:params:xml:ns:xmpp-tls Specification: [RFCXXXX] Description: This is the XML namespace name for TLS-related data in the Extensible Messaging and Presence Protocol (XMPP) as defined by [RFCXXXX]. Registrant Contact: IETF, XMPP Working Group, 9.2 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. URI: urn:ietf:params:xml:ns:xmpp-sasl Specification: [RFCXXXX] Description: This is the XML namespace name for SASL-related data in the Extensible Messaging and Presence Protocol (XMPP) as defined by [RFCXXXX]. Registrant Contact: IETF, XMPP Working Group, 9.3 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. URI: urn:ietf:params:xml:ns:xmpp-streams Specification: [RFCXXXX] Description: This is the XML namespace name for stream-related error data in the Extensible Messaging and Presence Protocol (XMPP) as defined by [RFCXXXX]. Saint-Andre & Miller Expires November 2, 2003 [Page 56] Internet-Draft XMPP Core May 2003 Registrant Contact: IETF, XMPP Working Group, 9.4 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. URI: urn:ietf:params:xml:ns:xmpp-stanzas Specification: [RFCXXXX] Description: This is the XML namespace name for stanza-related error data in the Extensible Messaging and Presence Protocol (XMPP) as defined by [RFCXXXX]. Registrant Contact: IETF, XMPP Working Group, 9.5 Existing Registrations The IANA registers "xmpp" as a GSSAPI [22] service name, as specified in Section 6.1.3. Additionally, the IANA registers "jabber-client" and "jabber-server" as keywords for TCP ports 5222 and 5269 respectively. Saint-Andre & Miller Expires November 2, 2003 [Page 57] Internet-Draft XMPP Core May 2003 10. Internationalization Considerations Each XML stanza SHOULD include the 'xml:lang' attribute as described above. Servers MUST NOT modify or delete 'xml:lang' attributes from stanzas they receive from other entities. Saint-Andre & Miller Expires November 2, 2003 [Page 58] Internet-Draft XMPP Core May 2003 11. Security Considerations 11.1 High Security For the purposes of XMPP communications (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. Standalone, self-signed service certificates SHOULD NOT be used; rather, an entity that wishes to generate a self-signed service certificate SHOULD first generate a self-signed Root CA certificate and then generate a signed service certificate. Entities that communicate with the service SHOULD be configured with the Root CA certificate rather than the service certificate; this avoids problems associated with simple comparison of service certificates. If a self-signed service certificate is used, an entity SHOULD NOT trust it if it is changed to another self-signed certificate or a certificate signed by an unrecognized authority. Implementations MUST support high security. Service provisioning SHOULD use high security, subject to local security policies. 11.2 Client-to-Server Communications The TLS protocol for encrypting XML streams (defined under Section 5) 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 6.1) provides a reliable mechanism for validating that a client connecting to a server is who it claims to be. The IP address and method of access of clients MUST NOT be made available 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. End-to-end encryption of message bodies and presence status information MAY be effected through use of the methods defined in End-to-End Object Encryption in XMPP [32]. Saint-Andre & Miller Expires November 2, 2003 [Page 59] Internet-Draft XMPP Core May 2003 11.3 Server-to-Server Communications A compliant implementation MUST support both TLS and SASL for inter-domain communications. For historical reasons, a compliant implementation SHOULD also support the lower-security Dialback Protocol (Section 6.2) as a fallback mechanism that helps to prevent the spoofing of domains. Because service provisioning is a matter of policy, it is OPTIONAL for any given domain to communicate with other domains, and server-to-server communications MAY be disabled by the administrator of any given deployment. If a particular domain enables inter-domain communications, it SHOULD enable high security. In the absence of high security, a domain MAY use server dialback for inter-domain communications. Administrators may want to require use of SASL for server-to-server communications in order to ensure authentication and confidentiality (e.g., on an organization's private network). Compliant implementations SHOULD support SASL for this purpose. 11.4 Firewalls Communications using XMPP normally occur over TCP sockets on port 5222 (client-to-server) or port 5269 (server-to-server), as registered with the IANA [5]. Use of these well-known ports allows administrators to easily enable or disable XMPP activity through existing and commonly-deployed firewalls. 11.5 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 (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher supporting client-side certificates) Saint-Andre & Miller Expires November 2, 2003 [Page 60] Internet-Draft XMPP Core May 2003 Normative References [1] World Wide Web Consortium, "Extensible Markup Language (XML) 1.0 (Second Edition)", W3C xml, October 2000, . [2] Day, M., Aggarwal, S., Mohr, G. and J. Vincent, "A Model for Presence and Instant Messaging", RFC 2779, February 2000, . [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [4] University of Southern California, "Transmission Control Protocol", RFC 793, September 1981, . [5] Internet Assigned Numbers Authority, "Internet Assigned Numbers Authority", January 1998, . [6] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet host table specification", RFC 952, October 1985. [7] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989. [8] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile for Internationalized Domain Names (draft-ietf-idn-nameprep-11, work in progress)", June 2002. [9] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002. [10] Saint-Andre, P. and J. Hildebrand, "Nodeprep: A Stringprep Profile for Node Identifiers in XMPP (draft-ietf-xmpp-nodeprep-02, work in progress)", April 2003. [11] Saint-Andre, P. and J. Hildebrand, "Resourceprep: A Stringprep Profile for Resource Identifiers in XMPP (draft-ietf-xmpp-resourceprep-02, work in progress)", April 2003. [12] World Wide Web Consortium, "Namespaces in XML", W3C xml-names, January 1999, . [13] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January Saint-Andre & Miller Expires November 2, 2003 [Page 61] Internet-Draft XMPP Core May 2003 1999. [14] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [15] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC 2222, October 1997. [16] Leach, P. and C. Newman, "Using Digest Authentication as a SASL Mechanism", RFC 2831, May 2000. [17] Alvestrand, H., "Tags for the Identification of Languages", BCP 47, RFC 3066, January 2001. [18] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2052, October 1996. [19] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. [20] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO 10646", RFC 2781, February 2000. [21] 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. [22] Linn, J., "Generic Security Service Application Program Interface, Version 2", RFC 2078, January 1997. Saint-Andre & Miller Expires November 2, 2003 [Page 62] Internet-Draft XMPP Core May 2003 Informative References [23] Saint-Andre, P. and J. Miller, "XMPP Instant Messaging (draft-ietf-xmpp-im-11, work in progress)", May 2003. [24] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998, . [25] Mealling, M., "The IANA XML Registry", draft-mealling-iana-xmlns-registry-04 (work in progress), June 2002. [26] Crispin, M., "Internet Message Access Protocol - Version 4rev1", RFC 2060, December 1996. [27] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996. [28] Newman, C. and J. Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, November 1997. [29] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, June 1999. [30] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. [31] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [32] Saint-Andre, P. and J. Hildebrand, "End-to-End Object Encryption in XMPP (draft-ietf-xmpp-e2e-02, work in progress)", April 2003. Authors' Addresses Peter Saint-Andre Jabber Software Foundation EMail: stpeter@jabber.org URI: http://www.jabber.org/people/stpeter.php Saint-Andre & Miller Expires November 2, 2003 [Page 63] Internet-Draft XMPP Core May 2003 Jeremie Miller Jabber Software Foundation EMail: jeremie@jabber.org URI: http://www.jabber.org/people/jer.php Saint-Andre & Miller Expires November 2, 2003 [Page 64] Internet-Draft XMPP Core May 2003 Appendix A. XML Schemas The following XML schemas are descriptive, not normative. A.1 Streams namespace Saint-Andre & Miller Expires November 2, 2003 [Page 65] Internet-Draft XMPP Core May 2003 A.2 TLS namespace A.3 SASL namespace Saint-Andre & Miller Expires November 2, 2003 [Page 66] Internet-Draft XMPP Core May 2003 A.4 Dialback namespace Saint-Andre & Miller Expires November 2, 2003 [Page 67] Internet-Draft XMPP Core May 2003 A.5 Client namespace Saint-Andre & Miller Expires November 2, 2003 [Page 69] Internet-Draft XMPP Core May 2003 Saint-Andre & Miller Expires November 2, 2003 [Page 70] Internet-Draft XMPP Core May 2003 Saint-Andre & Miller Expires November 2, 2003 [Page 71] Internet-Draft XMPP Core May 2003 A.6 Server namespace Saint-Andre & Miller Expires November 2, 2003 [Page 72] Internet-Draft XMPP Core May 2003 Saint-Andre & Miller Expires November 2, 2003 [Page 74] Internet-Draft XMPP Core May 2003 Saint-Andre & Miller Expires November 2, 2003 [Page 75] Internet-Draft XMPP Core May 2003 A.7 Stream error namespace Saint-Andre & Miller Expires November 2, 2003 [Page 76] Internet-Draft XMPP Core May 2003 A.8 Stanza error namespace Saint-Andre & Miller Expires November 2, 2003 [Page 77] Internet-Draft XMPP Core May 2003 Saint-Andre & Miller Expires November 2, 2003 [Page 78] Internet-Draft XMPP Core May 2003 Appendix B. Revision History Note to RFC Editor: please remove this entire appendix, and the corresponding entries in the table of contents, prior to publication. B.1 Changes from draft-ietf-xmpp-core-09 o Adjusted TLS content regarding certificate validation process. o Specified that stanza error extensions for specific applications are to be properly namespaced children of the relevant descriptive element. o Clarified rules for inclusion of the 'id' attribute. o Specified that the 'xml:lang' attribute SHOULD be included (per list discussion). o Made small editorial changes and fixed several schema errors. B.2 Changes from draft-ietf-xmpp-core-09 o Fixed several dialback error conditions. o Cleaned up business rules regarding TLS and certificate processing based on off-list feedback. o Changed and elements to . o Added or modified several stream and stanza error conditions. o Specified only one child allowed for IQ, or two if type="error". o Fixed several errors in the schemas. B.3 Changes from draft-ietf-xmpp-core-08 o Incorporated list discussion regarding addressing, SASL, TLS, TCP, dialback, namespaces, extensibility, and the meaning of 'ignore' for routers and recipients. o Specified dialback error conditions. o Made small editorial changes to address RFC Editor requirements. Saint-Andre & Miller Expires November 2, 2003 [Page 79] Internet-Draft XMPP Core May 2003 B.4 Changes from draft-ietf-xmpp-core-07 o Made several small editorial changes. B.5 Changes from draft-ietf-xmpp-core-06 o Added text regarding certificate validation in TLS negotiation per list discussion. o Clarified nature of XML restrictions per discussion with W3C, and moved XML Restrictions subsection under "XML Usage within XMPP". o Further clarified that XML streams are unidirectional. o Changed stream error and stanza error namespace names to conform to the format defined in The IETF XML Registry [25]. o Removed note to RFC Editor regarding provisional namespace names. B.6 Changes from draft-ietf-xmpp-core-05 o Added as a stream error condition. o Adjusted security considerations per discussion at IETF 56 and on list. B.7 Changes from draft-ietf-xmpp-core-04 o Added server-to-server examples for TLS and SASL. o Changed error syntax, rules, and examples based on list discussion. o Added schemas for the TLS, stream error, and stanza error namespaces. o Added note to RFC Editor regarding provisional namespace names. o Made numerous small editorial changes and clarified text throughout. B.8 Changes from draft-ietf-xmpp-core-03 o Clarified rules and procedures for TLS and SASL. Saint-Andre & Miller Expires November 2, 2003 [Page 80] Internet-Draft XMPP Core May 2003 o Amplified stream error code syntax per list discussion. o Made numerous small editorial changes. B.9 Changes from draft-ietf-xmpp-core-02 o Added dialback schema. o Removed all DTDs since schemas provide more complete definitions. o Added stream error codes. o Clarified error code "philosophy". B.10 Changes from draft-ietf-xmpp-core-01 o Updated the addressing restrictions per list discussion and added references to the new nodeprep and resourceprep profiles. o Corrected error in Stream Authentication regarding "version='1.0'" flag. o Made numerous small editorial changes. B.11 Changes from draft-ietf-xmpp-core-00 o Added information about TLS from list discussion. o Clarified meaning of "ignore" based on list discussion. o Clarified information about Universal Character Set data and character encodings. o Provided base64-decoded information for examples. o Fixed several errors in the schemas. o Made numerous small editorial fixes. B.12 Changes from draft-miller-xmpp-core-02 o Brought Streams Authentication section into line with discussion on list and at IETF 55 meeting. Saint-Andre & Miller Expires November 2, 2003 [Page 81] Internet-Draft XMPP Core May 2003 o Added information about the optional 'xml:lang' attribute per discussion on list and at IETF 55 meeting. o Specified that validation is neither required nor recommended, and that the formal definitions (DTDs and schemas) are included for descriptive purposes only. o Specified that the response to an IQ stanza of type "get" or "set" must be an IQ stanza of type "result" or "error". o Specified that compliant server implementations must process stanzas in order. o Specified that for historical reasons some server implementations may accept 'stream:' as the only valid namespace prefix on the root stream element. o Clarified the difference between 'jabber:client' and 'jabber:server' namespaces, namely, that 'to' and 'from' attributes are required on all stanzas in the latter but not the former. o Fixed typo in Step 9 of the dialback protocol (changed db:result to db:verify). o Removed references to TLS pending list discussion. o Removed the non-normative appendix on OpenPGP usage pending its inclusion in a separate I-D. o Simplified the architecture diagram, removed most references to services, and removed references to the 'jabber:component:*' namespaces. o Noted that XMPP activity respects firewall administration policies. o Further specified the scope and uniqueness of the 'id' attribute in all stanza types and the element in message stanzas. o Nomenclature changes: (1) from "chunks" to "stanzas"; (2) from "host" to "server" and from "node" to "client" (except with regard to definition of the addressing scheme). Saint-Andre & Miller Expires November 2, 2003 [Page 82] Internet-Draft XMPP Core May 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication 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 implementors or users of this specification can be obtained from the IETF Secretariat. 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