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XMPP Working GroupP. Saint-Andre, Ed.
Internet-DraftXSF
Expires: July 30, 2007January 26, 2007


Extensible Messaging and Presence Protocol (XMPP): Core
draft-saintandre-rfc3920bis-01

Status of this Memo

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Copyright Notice

Copyright © The IETF Trust (2007).

Abstract

This memo defines the core features of the Extensible Messaging and Presence Protocol (XMPP), a technology for streaming Extensible Markup Language (XML) elements in order to exchange structured information in close to real time between any two network-aware entities. XMPP provides a generalized, extensible framework for incrementally exchanging XML data, upon which a variety of applications can be built. The framework includes methods for stream setup and teardown, channel encryption, authentication of a client to a server and of one server to another server, and primitives for push-style messages, publication of presence and availability information, and request-response interactions between any two XMPP entities. This document also specifies the format for XMPP addresses, which are fully internationalizable.

This document obsoletes RFC 3920.



Table of Contents

1.  Introduction
    1.1.  Overview
    1.2.  Functional Summary
    1.3.  Conventions
2.  Architecture
    2.1.  Overview
    2.2.  Server
    2.3.  Client
    2.4.  Network
3.  Addresses
    3.1.  Overview
    3.2.  Domain Identifier
    3.3.  Node Identifier
    3.4.  Resource Identifier
    3.5.  Determination of Addresses
4.  TCP Binding
5.  XML Streams
    5.1.  Overview
    5.2.  Stream Security
    5.3.  Stream Attributes
    5.4.  Namespace Declarations
    5.5.  Stream Features
    5.6.  Closing Streams
    5.7.  Reconnection
    5.8.  Stream Errors
    5.9.  Simplified Stream Examples
6.  TLS Negotiation
    6.1.  Overview
    6.2.  Narrative
7.  SASL Negotiation
    7.1.  Overview
    7.2.  Narrative
    7.3.  SASL Definition
    7.4.  SASL Errors
8.  Resource Binding
    8.1.  Binding Multiple Resources
9.  XML Stanzas
    9.1.  Common Attributes
    9.2.  Basic Semantics
    9.3.  Stanza Errors
    9.4.  Extended Namespaces
10.  Examples
    10.1.  Client-to-Server
    10.2.  Server-to-Server Examples
11.  Server Rules for Handling XML Stanzas
    11.1.  No 'to' Address
    11.2.  Foreign Domain
    11.3.  Subdomain
    11.4.  Mere Domain or Specific Resource
    11.5.  Node in Same Domain
12.  XML Usage
    12.1.  Restrictions
    12.2.  XML Namespace Names and Prefixes
    12.3.  Validation
    12.4.  Inclusion of Text Declaration
    12.5.  Character Encoding
    12.6.  White Space
13.  Compliance Requirements
    13.1.  Servers
    13.2.  Clients
14.  Internationalization Considerations
15.  Security Considerations
    15.1.  High Security
    15.2.  Certificate Validation
    15.3.  Client-to-Server Communications
    15.4.  Server-to-Server Communications
    15.5.  Order of Layers
    15.6.  Lack of SASL Channel Binding to TLS
    15.7.  Mandatory-to-Implement Technologies
    15.8.  Firewalls
    15.9.  Use of base64 in SASL
    15.10.  Stringprep Profiles
16.  IANA Considerations
    16.1.  XML Namespace Name for TLS Data
    16.2.  XML Namespace Name for SASL Data
    16.3.  XML Namespace Name for Stream Errors
    16.4.  XML Namespace Name for Resource Binding
    16.5.  XML Namespace Name for Stanza Errors
    16.6.  Nodeprep Profile of Stringprep
    16.7.  Resourceprep Profile of Stringprep
    16.8.  GSSAPI Service Name
    16.9.  Port Numbers
17.  References
    17.1.  Normative References
    17.2.  Informative References
Appendix A.  Nodeprep
    A.1.  Introduction
    A.2.  Character Repertoire
    A.3.  Mapping
    A.4.  Normalization
    A.5.  Prohibited Output
    A.6.  Bidirectional Characters
Appendix B.  Resourceprep
    B.1.  Introduction
    B.2.  Character Repertoire
    B.3.  Mapping
    B.4.  Normalization
    B.5.  Prohibited Output
    B.6.  Bidirectional Characters
Appendix C.  Server Dialback
    C.1.  Overview
    C.2.  Order of Events
    C.3.  Protocol
    C.4.  Reuse of Negotiated Connections
    C.5.  Dialback Key Generation
    C.6.  Advertisement
Appendix D.  XML Schemas
    D.1.  Streams namespace
    D.2.  Stream error namespace
    D.3.  TLS namespace
    D.4.  SASL namespace
    D.5.  Resource binding namespace
    D.6.  Dialback namespace
    D.7.  Server dialback stream feature namespace
    D.8.  Stanza error namespace
Appendix E.  Contact Addresses
Appendix F.  Differences From RFC 3920
§  Author's Address
§  Intellectual Property and Copyright Statements




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1.  Introduction



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1.1.  Overview

The Extensible Messaging and Presence Protocol (XMPP) is an Extensible Markup Language XML (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.) [XML] technology for near-real-time messaging, presence, and request-response services. The basic syntax and semantics were developed originally within the Jabber open-source community, mainly in 1999. In 2002, the XMPP WG was chartered with developing an adaptation of the core Jabber protocol that would be suitable as an IETF instant messaging (IM) and presence technology. As a result of work by the XMPP WG as well as implementation experience and interoperability testing completed since the publication of RFC 3920, this document defines the core features of XMPP 1.0; the extensions required to provide the instant messaging and presence functionality defined in [IMP‑REQS] (Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” February 2000.) are specified in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.).

This document obsoletes RFC 3920.



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1.2.  Functional Summary

The purpose of XMPP is to enable the exchange of relatively small pieces of structured data (called "XML stanzas") over a network between any two (or more) entities. XMPP is implemented using a client-server architecture, wherein a client must connect to a server in order to gain access to the network and thus be allowed to exchange XML stanzas with other entities. The process whereby a client connects to a server, exchanges XML stanzas, and ends the connection is as follows:

  1. Determine the hostname and port at which to connect
  2. Open a TCP connection
  3. Open an XML stream
  4. Complete TLS negotiation for channel encryption (RECOMMENDED)
  5. Complete SASL negotiation for authentication
  6. Bind a resource to the stream
  7. Exchange XML stanzas with other entities on the network
  8. Close the stream when further communications are not needed or desired
  9. Close the TCP connection.

In the sections following discussion of XMPP architecture and XMPP addresses, this document specifies how clients connect to servers and specifies the basic semantics of XML stanzas, but does not define the "payloads" of the XML stanzas that might be exchanged once a connection is successfully established; instead, definition of such semantics is provided by various XMPP extensions (e.g., [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.) for basic instant messaging and presence applications).

Within the client-server architecture used by XMPP, one server may optionally connect to another server to enable inter-domain or inter-server communication. For this to happen, the two servers must negotiate a connection between themselves and then exchange XML stanzas; the process for doing so is as follows:

  1. Determine the hostname and port at which to connect
  2. Open a TCP connection
  3. Open an XML stream
  4. Complete TLS negotiation for channel encryption (RECOMMENDED)
  5. Complete SASL negotiation for authentication
  6. Exchange XML stanzas both directly for the servers and indirectly on behalf of entities associated with each server (e.g., connected clients)
  7. Close the stream when further communications are not needed or desired
  8. Close the TCP connection.

Note: Depending on local policies, a service may wish to use server dialback to provide weak verification in cases where SASL negotiation would not result in strong authentication (e.g., because the certificate presented by the peer service during TLS negotiation is self-signed and thus provides only weak identity); for details, see Appendix C (Server Dialback).



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1.3.  Conventions

The following keywords are to be interpreted as described in [TERMS] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.): "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

In examples, lines have been wrapped for improved readability.



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2.  Architecture



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2.1.  Overview

XMPP assumes a client-server architecture, wherein a client utilizing XMPP accesses a server (normally over a [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.) connection) and servers can also communicate with each other over TCP connections. Architectures that use the syntax of XML stanzas (XML Stanzas) but that establish peer-to-peer connections directly between clients using technologies based on [LINKLOCAL] (Cheshire, S., Aboba, B., and E. Guttman, “Dynamic Configuration of IPv4 Link-Local Addresses,” May 2005.) have been deployed, but such architectures are not XMPP and are best described as "XMPP-like"; for details, see [XEP‑0174] (Saint-Andre, P., “Link-Local Messaging,” December 2006.).

An architectural diagram for a typical deployment is shown here, where the entities have the following significance:

  example.net----------------------example.com
     |                                |
     |                                |
romeo@example.net               juliet@example.com



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2.2.  Server

A server acts as an intelligent abstraction layer for XMPP communications. Its primary responsibilities are:

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 instant messaging and presence 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.



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2.3.  Client

Most clients connect directly to a server over a [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.) connection and use XMPP to take full advantage of the functionality provided by a server and any associated services. Multiple resources (e.g., devices or locations) MAY connect simultaneously to a server on behalf of each authorized client, with each resource differentiated by the resource identifier of an XMPP address (e.g., <node@domain/home> vs. <node@domain/work>) as defined under Addresses (Addresses) and Resource Binding (Resource Binding). The RECOMMENDED port for connections between a client and a server is 5222, as registered with the IANA (see Port Numbers (Port Numbers)).



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2.4.  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, for example, <juliet@example.com> is able to exchange messages, presence, and other information with <romeo@example.net>. This pattern is familiar from messaging protocols (such as [SMTP] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.)) that make use of network addressing standards. Communications between any two servers are OPTIONAL. If enabled, such communications SHOULD occur over XML streams that are bound to [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.) connections. The RECOMMENDED port for connections between servers is 5269, as registered with the IANA (see Port Numbers (Port Numbers)).



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3.  Addresses



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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 on the network. For historical reasons, the address of an XMPP entity is called a Jabber Identifier or JID. A valid JID contains a set of ordered elements formed of a domain identifier, node identifier, and resource identifier.

The syntax for a JID is defined as follows using the Augmented Backus-Naur Form as defined in [ABNF] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” October 2005.).

   jid             = [ node "@" ] domain [ "/" resource ]
   node            = 1*(nodepoint)
                     ; a "nodepoint" is a UTF-8 encoded Unicode code
                     ; point that satisfies the Nodeprep profile of
                     ; stringprep
   domain          = fqdn / address-literal / idnlabel
   fqdn            = (idnlabel 1*("." idnlabel))
                     ; an "idnlabel" is an internationalized domain
                     ; label as described in RFC 3490
   address-literal = IPv4address / IPv6address
                     ; the "IPv4address" and "IPv6address" rules are
                     ; defined in Appendix B of RFC 3513
   resource        = 1*(resourcepoint)
                     ; a "resourcepoint" is a UTF-8 encoded Unicode
                     ; code point that satisfies the Resourceprep
                     ; profile of stringprep

All JIDs are based on the foregoing structure. One common use of this structure is to identify a messaging and presence account, the server that hosts the account, and a connected resource (e.g., a specific device) in the form of <node@domain/resource>. However, node types other than clients are possible; for example, a specific chat room offered by a multi-user chat service (see [XEP‑0045] (Saint-Andre, P., “Multi-User Chat,” September 2006.)) could be addressed as <room@service> (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 <room@service/nick> (where "nick" is the occupant's room nickname). Many other JID types are possible (e.g., <domain/resource> could be a server-side script or service).

Each allowable portion of a JID (node identifier, domain identifier, and resource identifier) MUST NOT be more than 1023 bytes in length, resulting in a maximum total size (including the '@' and '/' separators) of 3071 bytes.

Note: While the format of a JID is consistent with [URI] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), an entity's address on an XMPP network MUST be a JID (without a URI scheme) and not a [URI] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) or [IRI] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) as specified in [XMPP‑URI] (Saint-Andre, P., “Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP),” August 2006.); the latter specification is provided only for use by non-XMPP applications.



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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 or "home" 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 that provides functionality above and beyond the capabilities of a server (e.g., a multi-user chat service or a user directory).

The domain identifier for every server or service that will communicate over a network MAY be an IP address but SHOULD be a fully qualified domain name (see [DNS] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.)). A domain identifier MUST be an "internationalized domain name" as defined in [IDNA] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.), to which the [NAMEPREP] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.) profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) can be applied without failing. Before comparing two domain identifiers, a server MUST (and a client SHOULD) first apply the Nameprep profile to the labels (as defined in [IDNA] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.)) that make up each identifier. Note: When applying the Nameprep profile, the UseSTD3ASCIIRules flag MUST be set to true.



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3.3.  Node Identifier

The NODE IDENTIFIER is an optional secondary identifier placed before the domain identifier and separated from the latter by the '@' character. It usually represents the entity requesting and using network access provided by a server (i.e., a client), although it can also represent other kinds of entities (e.g., a 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 instant messaging and presence applications of XMPP, this address is called a "bare JID" and is of the form <node@domain>.

A node identifier MUST be formatted such that the Nodeprep profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) can be applied without failing (see Appendix A (Nodeprep)). Before comparing two node identifiers, a server MUST (and a client SHOULD) first apply the Nodeprep profile to each identifier.



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3.4.  Resource Identifier

The RESOURCE IDENTIFIER is an optional tertiary identifier placed after the domain identifier and separated from the latter by the '/' character. A resource identifier may modify either a <node@domain> address or a mere <domain> address. It usually represents a specific 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 when it provides the information necessary to complete Resource Binding (Resource Binding) (although it may be generated by a server on behalf of a client), after which the entity is referred to as a "connected resource" and its address is referrred to as a "full JID" (<node@domain/resource>). An entity MAY maintain multiple connected resources simultaneously, with each connected resource differentiated by a distinct resource identifier.

A resource identifier MUST be formatted such that the Resourceprep profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) can be applied without failing (see Appendix B (Resourceprep)). Before comparing two resource identifiers, a server MUST (and a client SHOULD) first apply the Resourceprep profile to each identifier.



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3.5.  Determination of Addresses

After SASL negotiation (SASL Negotiation) and, if appropriate, Resource Binding (Resource Binding), the receiving entity for a stream MUST determine the initiating entity's JID.

For server-to-server communications, the initiating entity's JID SHOULD be the authorization identity, derived from the authentication identity, as defined by [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.), if no authorization identity was specified during SASL negotiation (SASL Negotiation).

For client-to-server communications, the "bare JID" (<node@domain>) SHOULD be the authorization identity, derived from the authentication identity, as defined in [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.), if no authorization identity was specified during SASL negotiation (SASL Negotiation); the resource identifier portion of the "full JID" (<node@domain/resource>) SHOULD be the resource identifier negotiated by the client and server during Resource Binding (Resource Binding).

The receiving entity MUST ensure that the resulting JID (including node identifier, domain identifier, resource identifier, and separator characters) conforms to the rules and formats defined earlier in this section; to meet this restriction, the receiving entity may need to replace the JID sent by the initiating entity with the canonicalized JID as determined by the receiving entity.



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4.  TCP Binding

Although there is no necessary coupling of an XML stream to a [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.) connection (e.g., two entities could connect to each other via another transport, e.g. [HTTP] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) as specified in [XEP‑0124] (Paterson, I., Smith, D., and P. Saint-Andre, “HTTP Binding,” April 2006.)), this specification defines a binding of XMPP to TCP only.

Therefore, as XMPP is defined herein, an initiating entity (client or server) MUST open a TCP connection at the receiving entity (server) before it negotiates XML streams with the receiving entity. However, prior to opening the TCP connection the initiating entity first MUST resolve the Domain Name System (DNS) hostname associated with the receiving entity and determine the appropriate TCP port for communications with the receiving entity. The process is as follows:

  1. Attempt to resolve the hostname using a [DNS‑SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) Service of "xmpp-client" (for client-to-server connections) or "xmpp-server" (for server-to-server connections) and Proto of "tcp", resulting in resource records such as "_xmpp-client._tcp.example.com." or "_xmpp-server._tcp.example.com."; the IP address and port at which the initiating entity attempts to connect to the receiving entity shall be those specified in the SRV lookup result.
  2. If the SRV lookup fails, the fallback SHOULD be a normal IPv4 or [IPv6] (Hinden, R. and S. Deering, “Internet Protocol Version 6 (IPv6) Addressing Architecture,” April 2003.) address record resolution to determine the IP address, where the port used is the "xmpp-client" port of 5222 for client-to-server connectionsn or the "xmpp-server" port 5269 for client-to-server connections.
  3. However, the fallback MAY be a DNS TXT lookup (see [DNS‑TXT] (Rosenbaum, R., “Using the Domain Name System To Store Arbitrary String Attributes,” May 1993.)) for alternative connection methods, for example as described in [XEP‑0156] (Hildebrand, J. and P. Saint-Andre, “A DNS TXT Resource Record Format for XMPP Connection Methods,” May 2005.).

TCP connections are handled differently in client-to-server communications and server-to-server communications. Specifically:



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5.  XML Streams



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5.1.  Overview

Two fundamental concepts make possible the rapid, asynchronous exchange of relatively small payloads of structured information between presence-aware entities: XML streams and XML stanzas. These terms are defined as follows:

Definition of XML Stream:
An XML STREAM is a container for the exchange of XML elements between any two entities over a network. The start of an XML stream is denoted unambiguously by an opening XML <stream> tag (with appropriate attributes and namespace declarations), while the end of the XML stream is denoted unambiguously by a closing XML </stream> tag. During the life of the stream, the entity that initiated it can send an unbounded number of XML elements over the stream, either elements used to negotiate the stream (e.g., to complete TLS negotiation (TLS Negotiation) or SASL negotiation (SASL Negotiation)) or XML stanzas. The "initial stream" is negotiated from the initiating entity (usually a client or server) to the receiving entity (usually a server), and can be seen as corresponding to the initiating entity's "connection" with the receiving entity. The initial stream enables unidirectional communication from the initiating entity to the receiving entity; in order to enable information exchange from the receiving entity to the initiating entity, the receiving entity MUST negotiate a stream in the opposite direction (the "response stream").
Definition of XML Stanza:
An XML STANZA is a discrete semantic unit of structured information that is sent from one entity to another over an XML stream, and is the basic unit of meaning in XMPP. An XML stanza exists at the direct child level of the root <stream/> element and is said to be well-balanced if it matches the production [43] content of [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.). The start of any XML stanza is denoted unambiguously by the element start tag at depth=1 of the XML stream (e.g., <presence>), and the end of any XML stanza is denoted unambiguously by the corresponding close tag at depth=1 (e.g., </presence>); a server MUST NOT process, deliver, or route a partial stanza and MUST NOT attach meaning to the transmission timing of any part of a stanza (before receipt of the close tag). The only XML stanzas defined herein are the <message/>, <presence/>, and <iq/> elements qualified by the default namespace for the stream, as described under XML Stanzas (XML Stanzas); an XML element sent for the purpose of TLS negotiation (TLS Negotiation), SASL negotiation (SASL Negotiation), or server dialback (Server Dialback) is not considered to be an XML stanza. An XML stanza MAY contain child elements (with accompanying attributes, elements, and XML character data) as necessary in order to convey the desired information, which MAY be qualified by any XML namespace (see [XML‑NAMES] (Bray, T., Hollander, D., and A. Layman, “Namespaces in XML,” January 1999.)).

Consider the example of a client's connection to a server. In order to connect to a server, a client MUST initiate an XML stream by sending an opening <stream> tag to the server, optionally preceded by a text declaration specifying the XML version and the character encoding supported (see Inclusion of Text Declaration (Inclusion of Text Declaration) and Character Encoding (Character Encoding)). Subject to local policies and service provisioning, the server SHOULD then reply with a second XML stream back to the client, again optionally preceded by a text declaration. Once the client has completed SASL negotiation (SASL Negotiation), the client MAY send an unbounded 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 </stream> tag to the server; for details, see Section 5.6 (Closing Streams).

Those who are accustomed to thinking of XML in a document-centric manner may wish to view a client's connection to a server as consisting of two open-ended XML documents: one from the client to the server and one from the server to the client. From this perspective, the root <stream/> element can be considered the document entity for each "document", and the two "documents" are built up through the accumulation of XML stanzas sent over the two XML streams. However, this perspective is a convenience only; XMPP does not deal in documents but in XML streams and XML stanzas.

In essence, then, an XML stream acts as an envelope for all the XML stanzas sent during a connection. We can represent this in a simplistic fashion as follows:

|--------------------|
| <stream>           |
|--------------------|
| <presence>         |
|   <show/>          |
| </presence>        |
|--------------------|
| <message to='foo'> |
|   <body/>          |
| </message>         |
|--------------------|
| <iq to='bar'>      |
|   <query/>         |
| </iq>              |
|--------------------|
| ...                |
|--------------------|
| </stream>          |
|--------------------|


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5.2.  Stream Security

When negotiating XML streams in XMPP 1.0, TLS SHOULD be used as defined under TLS negotiation (TLS Negotiation) and SASL MUST be used as defined under SASL negotiation (SASL Negotiation). The initial stream and the response stream MUST be secured separately, although security in both directions MAY be established via mechanisms that provide mutual authentication. An entity SHOULD NOT attempt to send XML Stanzas (XML Stanzas) over the stream before the stream has been authenticated, but if it does, then the other entity MUST NOT accept such stanzas and SHOULD return a <not-authorized/> stream error and then terminate both the XML stream and the underlying TCP connection; note well that this applies to XML stanzas only (i.e., <message/>, <presence/>, and <iq/> elements qualified by the default namespace) and not to XML elements used for stream negotiation (e.g., elements used to complete TLS negotiation (TLS Negotiation) or SASL negotiation (SASL Negotiation)).



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5.3.  Stream Attributes

The attributes of the stream element are as follows:

We can summarize as follows:

+----------+--------------------------+-------------------------+
|          | initiating to receiving  | receiving to initiating |
+----------+--------------------------+-------------------------+
| to       | JID of receiver          | JID of initiator        |
| from     | JID of initiator         | JID of receiver         |
| id       | silently ignored         | stream identifier       |
| xml:lang | default language         | default language        |
| version  | XMPP 1.0 supported       | XMPP 1.0 supported      |
+----------+--------------------------+-------------------------+

Note: The attributes of the root <stream/> element are not prepended by a 'stream:' prefix because, in accordance with Section 5.3 of XML namespaces specification (Bray, T., Hollander, D., and A. Layman, “Namespaces in XML,” January 1999.) [XML‑NAMES], the default namespace does not apply to attribute names.



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5.3.1.  Version Support

The version of XMPP specified herein is "1.0"; in particular, this encapsulates the stream-related protocols (TLS negotiation (TLS Negotiation), SASL negotiation (SASL Negotiation), and Stream Errors (Stream Errors)), as well as the semantics of the three defined XML stanza types (<message/>, <presence/>, and <iq/>). The numbering scheme for XMPP versions is "<major>.<minor>". The major and minor numbers MUST be treated as separate integers and each number MAY be incremented higher than a single digit. Thus, "XMPP 2.4" would be a lower version than "XMPP 2.13", which in turn would be lower than "XMPP 12.3". Leading zeros (e.g., "XMPP 6.01") MUST be ignored by recipients and MUST NOT be sent.

The major version number should be incremented only if the stream and stanza formats or required actions have changed so dramatically that an older version entity would not be able to interoperate with a newer version entity if it simply ignored the elements and attributes it did not understand and took the actions specified in the older specification. The minor version number indicates new capabilities, and MUST be ignored by an entity with a smaller minor version number, but used for informational purposes by the entity with the larger minor version number. For example, a minor version number might indicate the ability to process a newly defined value of the 'type' attribute for message, presence, or IQ stanzas; the entity with the larger minor version number would simply note that its correspondent would not be able to understand that value of the 'type' attribute and therefore would not send it.

The following rules apply to the generation and handling of the 'version' attribute within stream headers by implementations:

  1. The initiating entity MUST set the value of the 'version' attribute on the initial stream header to the highest version number it supports (e.g., if the highest version number it supports is that defined in this specification, it MUST set the value to "1.0").
  2. The receiving entity MUST set the value of the 'version' attribute on the response stream header to either the value supplied by the initiating entity or the highest version number supported by the receiving entity, whichever is lower. The receiving entity MUST perform a numeric comparison on the major and minor version numbers, not a string match on "<major>.<minor>".
  3. If the version number included in the response stream header is at least one major version lower than the version number included in the initial stream header and newer version entities cannot interoperate with older version entities as described above, the initiating entity SHOULD generate an <unsupported-version/> stream error and terminate the XML stream and underlying TCP connection.
  4. If either entity receives a stream header with no 'version' attribute, the entity MUST consider the version supported by the other entity to be "0.9" and SHOULD NOT include a 'version' attribute in the stream header it sends in reply.


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5.4.  Namespace Declarations

The stream element MUST possess both a streams namespace declaration and a default namespace declaration (as "namespace declaration" is defined in the [XML‑NAMES] (Bray, T., Hollander, D., and A. Layman, “Namespaces in XML,” January 1999.)). For detailed information regarding the streams namespace and default namespace, see Namespace Names and Prefixes (XML Namespace Names and Prefixes).



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5.5.  Stream Features

If the initiating entity includes the 'version' attribute set to a value of at least "1.0" in the initial stream header, the receiving entity MUST send a <features/> child element (prefixed by the streams namespace prefix) to the initiating entity in order to announce any stream-level features that can be negotiated (or capabilities that otherwise need to be advertised). Currently, this is used only to advertise TLS negotiation (TLS Negotiation), SASL negotiation (SASL Negotiation), resource binding (Resource Binding), and server dialback (Server Dialback) as defined herein; however, the stream features functionality can be used to advertise other negotiable features as well. If an entity does not understand or support some features, it SHOULD silently ignore them. If one or more security features (e.g., TLS and SASL) need to be successfully negotiated before a non-security-related feature (e.g., Resource Binding) can be offered, the non-security-related feature SHOULD NOT be included in the stream features that are advertised before the relevant security features have been negotiated. If a feature must be negotiated before the initiating entity may proceed, that feature SHOULD include a <required/> child element.



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5.6.  Closing Streams

At any time after XML streams have been negotiated between two entities, either entity MAY close its stream to the other entity (even in the absence of a stream error) by sending a closing stream tag:

</stream:stream>

The entity that sends the closing stream tag SHOULD wait for the other entity to also close its stream:

</stream:stream>

However, the entity that sends the first closing stream tag MAY consider both streams to be void if the other entity does not send its closing stream tag within a reasonable amount of time (where the definition of "reasonable" is up to the implementation or deployment).

After an entity sends a closing stream tag, it MUST NOT send further data over that stream.

After the entity that sent the first closing stream tag receives a reciprocal closing stream tag from the other entity, it MUST terminate the underlying TCP connection.



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5.7.  Reconnection

It can happen that an XMPP server goes offline while servicing connections from clients and from other servers. Because the number of such connections can be quite large, the reconnection algorithm employed by entities that seek to reconnect can have a significant impact on software and network performance. The following guidelines are RECOMMENDED:



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5.8.  Stream Errors

The root stream element MAY contain an <error/> child element that is prefixed by the streams namespace prefix. The error child MUST be sent by a compliant entity (usually a server rather than a client) if it perceives that a stream-level error has occurred.



 TOC 

5.8.1.  Rules

The following rules apply to stream-level errors:



 TOC 

5.8.2.  Syntax

The syntax for stream errors is as follows:

<stream:error>
  <defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
  [<text xmlns='urn:ietf:params:xml:ns:xmpp-streams'
        xml:lang='langcode'>
    OPTIONAL descriptive text
  </text>]
  [OPTIONAL application-specific condition element]
</stream:error>

The <error/> element:

The <text/> element is OPTIONAL. If included, it SHOULD be used only to provide descriptive or diagnostic information that supplements the meaning of a defined condition or application-specific condition. It SHOULD NOT be interpreted programmatically by an application. It SHOULD NOT be used as the error message presented to a user, but MAY be shown in addition to the error message associated with the included condition element (or elements).



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5.8.3.  Defined Conditions

The following stream-level error conditions are defined:



 TOC 

5.8.4.  Application-Specific Conditions

As noted, an application MAY provide application-specific stream error information by including a properly-namespaced child in the error element. The application-specific element SHOULD supplement or further qualify a defined element. Thus the <error/> element will contain two or three child elements:

<stream:error>
  <xml-not-well-formed
      xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
  <text xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
    Some special application diagnostic information!
  </text>
  <escape-your-data xmlns='application-ns'/>
</stream:error>
</stream:stream>


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5.9.  Simplified Stream Examples

This section contains two simplified examples of a stream-based connection of a client on a server (where the "C" lines are sent from the client to the server, and the "S" lines are sent from the server to the client); these examples are included for the purpose of illustrating the concepts introduced thus far.

A basic connection:

C: <?xml version='1.0'?>
   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       xml:lang='en'
       from='juliet@example.com'
       to='example.com'
       version='1.0'>
S: <?xml version='1.0'?>
   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       xml:lang='en'
       from='example.com'
       id='someid'
       to='juliet@example.com'
       version='1.0'>
...  encryption, authentication, and resource binding ...
C:   <message from='juliet@example.com/balcony'
              to='romeo@example.net'
              xml:lang='en'>
C:     <body>Art thou not Romeo, and a Montague?</body>
C:   </message>
S:   <message from='romeo@example.net/orchard'
              to='juliet@example.com/balcony'
              xml:lang='en'>
S:     <body>Neither, fair saint, if either thee dislike.</body>
S:   </message>
C: </stream:stream>
S: </stream:stream>

A connection gone bad:

C: <?xml version='1.0'?>
   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       xml:lang='en'
       from='juliet@example.com'
       to='example.com'
       version='1.0'>
S: <?xml version='1.0'?>
   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       xml:lang='en'
       from='example.com'
       id='someid'
       to='juliet@example.com'
       version='1.0'>
...  encryption, authentication, and resource binding ...
C: <message xml:lang='en'>
     <body>Bad XML, no closing body tag!
   </message>
S: <stream:error>
    <xml-not-well-formed
        xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
S: </stream:stream>

More detailed examples are provided under Section 10 (Examples).



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6.  TLS Negotiation



 TOC 

6.1.  Overview

XMPP includes a method for securing the stream from tampering and eavesdropping. This channel encryption method makes use of the Transport Layer Security (TLS) protocol (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.) [TLS], along with a "STARTTLS" extension that is modelled after similar extensions for the [IMAP] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.), [POP3] (Myers, J. and M. Rose, “Post Office Protocol - Version 3,” May 1996.), and [ACAP] (Newman, C. and J. Myers, “ACAP -- Application Configuration Access Protocol,” November 1997.) protocols as described in [USINGTLS] (Newman, C., “Using TLS with IMAP, POP3 and ACAP,” June 1999.). The namespace name for the STARTTLS extension is 'urn:ietf:params:xml:ns:xmpp-tls'.

An administrator of a given domain MAY require the use of TLS for client-to-server communications, server-to-server communications, or both. Clients SHOULD use TLS to secure the streams prior to attempting the completion of SASL negotiation (SASL Negotiation), and servers SHOULD use TLS between two domains for the purpose of securing server-to-server communications.

The following rules apply:

  1. An initiating entity that complies with this specification MUST include the 'version' attribute set to a value of "1.0" in the initial stream header.
  2. If the TLS negotiation occurs between two servers, communications MUST NOT proceed until the Domain Name System (DNS) hostnames asserted by the servers have been resolved (see Server-to-Server Communications (Server-to-Server Communications)).
  3. When a receiving entity that complies with this specification receives an initial stream header that includes the 'version' attribute set to a value of at least "1.0", after sending a stream header in reply (including the version flag), it MUST include a <starttls/> element (qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace) along with the list of other stream features it supports.
  4. If the initiating entity chooses to use TLS, TLS negotiation MUST be completed before proceeding to SASL negotiation; this order of negotiation is required to help safeguard authentication information sent during SASL negotiation, as well as to make it possible to base the use of the SASL EXTERNAL mechanism on a certificate provided during prior TLS negotiation.
  5. During TLS negotiation, an entity MUST NOT send any white space characters (matching production [3] content of [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.)) within the root stream element as separators between elements (any white space characters shown in the TLS examples that follow are included for the sake of readability only); this prohibition helps to ensure proper security layer byte precision.
  6. The receiving entity MUST consider the TLS negotiation to have begun immediately after sending the closing ">" character of the <proceed/> element to the initiating entity. The initiating entity MUST consider the TLS negotiation to have begun immediately after receiving the closing ">" character of the <proceed/> element from the receiving entity.
  7. The initiating entity MUST validate the certificate presented by the receiving entity; see Certificate Validation (Certificate Validation) regarding certificate validation procedures.
  8. Certificates MUST be checked against the hostname as provided by the initiating entity (e.g., a user), not the hostname as resolved via the Domain Name System; e.g., if the user specifies a hostname of "example.net" but a [DNS‑SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) lookup returned "im.example.net", the certificate MUST be checked as "example.net". If a JID for an XMPP client (e.g., an end user account) is represented in a certificate, it MUST be represented as a UTF8String within an otherName entity inside the subjectAltName, using the [ASN.1] (CCITT, “Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1),” 1988.) Object Identifier "id-on-xmppAddr" specified in Section 6.1.1 (ASN.1 Object Identifier for XMPP Address) of this document. If a JID for an XMPP server is represented in a certificate, it SHOULD be represented as a UTF8String within an otherName entity inside the subjectAltName, using the [ASN.1] (CCITT, “Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1),” 1988.) Object Identifier "id-on-xmppAddr" specified in Section 6.1.1 (ASN.1 Object Identifier for XMPP Address) of this document; however, the JID for an XMPP server MAY also or instead be represented as a subjectAltName extension of type dNSName, where the dNSName may contain the wildcard character '*', which applies only to the left-most domain name component or component fragment and is considered to match any single component or component fragment (e.g., *.example.com matches foo.example.com but not bar.foo.example.com, and im*.example.net matches im1.example.net and im2.example.net but not chat.example.net).
  9. If the TLS negotiation is successful, the initiating entity MUST send a new stream header to the receiving entity.
  10. If the TLS negotiation is successful, the receiving entity MUST discard any knowledge obtained in an insecure manner from the initiating entity before TLS takes effect.
  11. If the TLS negotiation is successful, the initiating entity MUST discard any knowledge obtained in an insecure manner from the receiving entity before TLS takes effect.
  12. 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 after the new stream header is received and responded to.
  13. If the TLS negotiation is successful, the initiating entity MUST continue with SASL negotiation.
  14. If the TLS negotiation results in failure, the receiving entity MUST terminate both the XML stream and the underlying TCP connection.
  15. See Mandatory-to-Implement Technologies (Mandatory-to-Implement Technologies) regarding mechanisms that MUST be supported.


 TOC 

6.1.1.  ASN.1 Object Identifier for XMPP Address

The [ASN.1] (CCITT, “Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1),” 1988.) Object Identifier "id-on-xmppAddr" described above is defined as follows:

id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
        dod(6) internet(1) security(5) mechanisms(5) pkix(7) }

id-on  OBJECT IDENTIFIER ::= { id-pkix 8 }  -- other name forms

id-on-xmppAddr  OBJECT IDENTIFIER ::= { id-on 5 }

XmppAddr ::= UTF8String

This Object Identifier MAY also be represented in dotted display format (i.e., "1.3.6.1.5.5.7.8.5") or in the Uniform Resource Name notation specified in [URN‑OID] (Mealling, M., “A URN Namespace of Object Identifiers,” February 2001.) (i.e., "urn:oid:1.3.6.1.5.5.7.8.5").

Thus for example the JID "example.com" as included in a certificate might be formatted as "subjectAltName=otherName:1.3.6.1.5.5.7.8.5;UTF8:example.com".



 TOC 

6.2.  Narrative

When an initiating entity secures a stream with a receiving entity using TLS, the steps involved are as follows:

  1. The initiating entity opens a TCP connection and initiates the stream by sending the opening XML stream header to the receiving entity, including the 'version' attribute set to a value of at least "1.0".
  2. The receiving entity responds by opening a TCP connection and sending an XML stream header to the initiating entity, including the 'version' attribute set to a value of at least "1.0".
  3. The receiving entity offers the STARTTLS extension to the initiating entity by including it with the list of other supported stream features (if successful TLS negotiation is required for interaction with the receiving entity, it SHOULD signal that fact by including a <required/> element as a child of the <starttls/> element); the receiving entity SHOULD also include a list of supported SASL mechanisms in the stream features.
  4. The initiating entity issues the STARTTLS command (i.e., a <starttls/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct the receiving entity that it wishes to begin a TLS negotiation to secure the stream.
  5. The receiving entity MUST reply with either a <proceed/> element or a <failure/> element qualified 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 (failure cases include when the initiating entity sends a malformed STARTTLS command, when the receiving entity does not offer TLS negotiation either temporarily or permanently, and when the receiving entity cannot complete TLS negotiation because of an internal error). If the proceed case occurs, the entities MUST attempt to complete the TLS negotiation over the TCP connection and MUST NOT send any further XML data until the TLS negotiation is complete.
  6. The initiating entity and receiving entity attempt to complete a TLS negotiation in accordance with [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.).
  7. If the TLS negotiation is unsuccessful, the receiving entity MUST terminate the TCP connection. If the TLS negotiation is successful, the initiating entity MUST initiate a new stream by sending an opening XML stream header to the receiving entity (it is not necessary to send a closing </stream> tag first, since the receiving entity and initiating entity MUST consider the original stream to be closed upon successful TLS negotiation).
  8. Upon receiving the new stream header from the initiating entity, the receiving entity MUST respond by sending a new XML stream header to the initiating entity along with the available features (but not including the STARTTLS feature) and SHOULD include an updated list of SASL mechanisms so that the initiating entity can detect any changes to the list of SASL mechanisms supported by the receiving entity.

Examples of TLS negotiation are provided under Section 10 (Examples).



 TOC 

7.  SASL Negotiation



 TOC 

7.1.  Overview

XMPP includes a method for authenticating a stream by means of an XMPP-specific profile of the Simple Authentication and Security Layer protocol (see [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.)). SASL provides a generalized method for adding authentication support to connection-based protocols, and XMPP uses a generic XML namespace profile for SASL that conforms to the profiling requirements of [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.).

The following rules apply:

  1. If the SASL negotiation occurs between two servers, communications MUST NOT proceed until the Domain Name System (DNS) hostnames asserted by the servers have been resolved (see Server-to-Server Communications (Server-to-Server Communications)).
  2. If the initiating entity is capable of SASL negotiation, it MUST include the 'version' attribute set to a value of at least "1.0" in the initial stream header.
  3. If the receiving entity is capable of SASL negotiation, it MUST advertise one or more authentication mechanisms within a <mechanisms/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace in reply to the opening stream tag received from the initiating entity (if the opening stream tag included the 'version' attribute set to a value of at least "1.0").
  4. During SASL negotiation, an entity MUST NOT send any white space characters (matching production [3] content of [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.)) within the root stream element as separators between elements (any white space characters shown in the SASL examples that follow are included for the sake of readability only); this prohibition helps to ensure proper security layer byte precision.
  5. Any XML character data contained within the XML elements used during SASL negotiation MUST be encoded using base64, where the encoding adheres to the definition in Section 3 of RFC 3548 (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” July 2003.) [BASE64].
  6. If the receiving entity does not include a 'realm' value, the initiating entity must default it to the domain identifier portion of the receiving entity's JID.
  7. If provision of a "simple username" is supported by the selected SASL mechanism (e.g., this is supported by the DIGEST-MD5 and CRAM-MD5 mechanisms but not by the EXTERNAL and GSSAPI mechanisms), during authentication the initiating entity SHOULD provide as the simple username its sending domain (IP address or fully qualified domain name as contained in a domain identifier) in the case of server-to-server communications or its registered account name (user or node name as contained in an XMPP node identifier) in the case of client-to-server communications. In either case, the initiating entity MUST ensure that the username adheres to the [NAMEPREP] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.) or Nodeprep (Nodeprep) profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) (as appropriate) before sending it to the receiving entity. (Note: Account provisioning is out of scope for this specification; possible methods for account provisioning include account creation by a server administrator and in-band account registration using the 'jabber:iq:register' namespace as documented in [XEP‑0077] (Saint-Andre, P., “In-Band Registration,” January 2006.).)
  8. If the initiating entity wishes to act on behalf of another entity and the selected SASL mechanism supports transmission of an authorization identity, the initiating entity MUST provide an authorization identity during SASL negotiation. If the initiating entity does not wish to act on behalf of another entity, it MUST NOT provide an authorization identity. As specified in [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.), the initiating entity MUST NOT provide an authorization identity unless the authorization identity is different from the default authorization identity derived from the authentication identity. If provided, the value of the authorization identity MUST be of the form <domain> (i.e., a domain identifier only) for servers and of the form <node@domain> (i.e., node identifier and domain identifier) for clients.
  9. If the SASL negotiation is successful, the initiating entity MUST send a new stream header to the receiving entity.
  10. Upon successful SASL negotiation that involves negotiation of a security layer, the receiving entity MUST discard any knowledge obtained from the initiating entity which was not obtained from the SASL negotiation itself; the receiving entity SHOULD also send new stream features (including an updated list of SASL mechanisms) so that the initiating entity can detect any changes to the list of mechanisms supported by the receiving entity.
  11. Upon successful SASL negotiation that involves negotiation of a security layer, the initiating entity MUST discard any knowledge obtained from the receiving entity which was not obtained from the SASL negotiation itself.
  12. See Mandatory-to-Implement Technologies (Mandatory-to-Implement Technologies) regarding mechanisms that MUST be supported; naturally, other SASL mechanisms MAY be supported as well (best practices for the use of several SASL mechanisms in the context of XMPP are described in [XEP‑0175] (Saint-Andre, P., “Best Practices for Use of SASL ANONYMOUS,” September 2006.) and [XEP‑0178] (Saint-Andre, P. and P. Millard, “Best Practices for Use of SASL EXTERNAL,” January 2007.)).


 TOC 

7.2.  Narrative

When an initiating entity authenticates with a receiving entity using SASL, the steps involved are as follows:

  1. The initiating entity requests SASL authentication by including the '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 reply, the receiving entity advertises a list of available SASL authentication mechanisms as stream features; each of these is a <mechanism/> element included as a child within a <mechanisms/> container element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace, which in turn is a child of a <features/> element in the streams namespace. If TLS negotiation (TLS Negotiation) needs to be completed before a particular authentication mechanism may be used, the receiving entity MUST NOT provide that mechanism in the list of available SASL authentication mechanisms prior to TLS negotiation. If the initiating entity presents a valid certificate during prior TLS negotiation, the receiving entity SHOULD offer the SASL EXTERNAL mechanism to the initiating entity during SASL negotiation (refer to [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.)), although the EXTERNAL mechanism MAY be offered under other circumstances as well. If successful SASL negotiation is required for interaction with the receiving entity, it SHOULD signal that fact by including a <required/> element as a child of the <mechanisms/> element.
  3. The initiating entity selects a mechanism by sending an <auth/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the receiving entity and including an appropriate value for the 'mechanism' attribute. This element MAY contain XML character data (in SASL terminology, the "initial response") if the mechanism supports or requires it; if the initiating entity needs to send a zero-length initial response, it MUST transmit the response as a single equals sign ("="), which indicates that the response is present but contains no data.
  4. If necessary, the receiving entity challenges the initiating entity by sending to the initiating entity a <challenge/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY contain XML character data (which MUST be computed in accordance with the definition of the SASL mechanism chosen by the initiating entity).
  5. The initiating entity responds to the challenge by sending to the receiving entity a <response/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY contain XML character data (which MUST be computed in accordance with the definition of 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 <abort/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the receiving entity. Upon receiving an <abort/> element, the receiving entity SHOULD allow a configurable but reasonable number of retries (at least 2), after which it MUST terminate the TCP connection; this enables the initiating entity (e.g., an end-user client) to tolerate incorrectly-provided credentials (e.g., a mistyped password) without being forced to reconnect.
  2. The receiving entity reports failure of the handshake by sending a <failure/> element qualified 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 <failure/> element as defined under SASL Errors (SASL Errors)). If the failure case occurs, the receiving entity SHOULD allow a configurable but reasonable number of retries (at least 2), after which it MUST terminate the TCP connection; this enables the initiating entity (e.g., an end-user client) to tolerate incorrectly-provided credentials (e.g., a mistyped password) without being forced to reconnect.
  3. The receiving entity reports success of the handshake by sending a <success/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating entity; this element MAY contain XML character data (in SASL terminology, "additional data with success") if required by the chosen SASL mechanism; if the receiving entity needs to send additional data of zero length, it MUST transmit the data as a single equals sign ("="). Upon receiving the <success/> element, the initiating entity MUST initiate a new stream by sending an opening XML stream header to the receiving entity (it is not necessary to send a closing </stream> tag first, since the receiving entity and initiating entity MUST consider the original stream to be closed upon sending or receiving the <success/> element). Upon receiving the new stream header from the initiating entity, the receiving entity MUST respond by sending a new XML stream header to the initiating entity, along with any available features or an empty <features/> element (to signify that no additional features are available); any such additional features not defined herein MUST be defined by the relevant extension to XMPP. As noted, if SASL negotiation involved establishment of a security layer, the receiving entity SHOULD send an updated list of SASL mechanisms so that the initiating entity can detect any changes to the list of mechanisms supported by the receiving entity.


 TOC 

7.3.  SASL Definition

The profiling requirements of [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) require that the following information be supplied by a protocol definition:

service name:
"xmpp"
initiation sequence:
After the initiating entity provides an opening XML stream header and the receiving entity replies in kind, the receiving entity provides a list of acceptable authentication methods. The initiating entity chooses one method from the list and sends it to the receiving entity as the value of the 'mechanism' attribute possessed by an <auth/> element, optionally including an initial response to avoid a round trip.
exchange sequence:
Challenges and responses are carried through the exchange of <challenge/> elements from receiving entity to initiating entity and <response/> elements from initiating entity to receiving entity. The receiving entity reports failure by sending a <failure/> element and success by sending a <success/> element; the initiating entity aborts the exchange by sending an <abort/> element. Upon successful negotiation, both sides consider the original XML stream to be closed and new stream headers are sent by both entities.
security layer negotiation:
The security layer takes effect immediately after sending the closing ">" character of the <success/> element for the receiving entity, and immediately after receiving the closing ">" character of the <success/> element for the initiating entity. The order of layers is first [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.), then [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.), then [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.), then XMPP.
use of the authorization identity:
The authorization identity may be used by xmpp to denote the non-default <node@domain> of a client or the sending <domain> of a server; an empty string is equivalent to an absent authorization identity.


 TOC 

7.4.  SASL Errors

The following SASL-related error conditions are defined:

Examples of SASL negotiation are provided under Section 10 (Examples).



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8.  Resource Binding

After a client authenticates with a server, it MUST bind a specific resource to the stream so that the server can properly address the client (see addresses (Addresses)) and route XML stanzas to and from the client (see stanza delivery rules (Server Rules for Handling XML Stanzas)). That is, there MUST be a resource identifier associated with the "bare JID" (<node@domain>) of the client; this ensures that the address for use over that stream is a "full JID" of the form <node@domain/resource>. After binding a resource to the stream, the client is referred to as a CONNECTED RESOURCE.

Upon receiving a success indication within the SASL negotiation, the client MUST send a new stream header to the server, to which the server MUST respond with a stream header as well as a list of available stream features. Specifically, if the server requires the client to bind a resource to the stream after successful SASL negotiation, it MUST include a <bind/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace in the stream features list it presents to the client upon sending the header for the response stream sent after successful SASL negotiation (but not before); this <bind/> element SHOULD include an empty <required/> element as well.

Server advertises resource binding feature to client:

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    xml:lang='en'
    from='example.com'
    id='c2s_345'
    to='juliet@example.com'
    version='1.0'>
<stream:features>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <required/>
  </bind>
</stream:features>

Upon being so informed that resource binding is required, the client MUST bind a resource to the stream by sending to the server an IQ stanza of type "set" (see IQ Semantics (IQ Semantics)) containing data qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace.

If the client wishes to allow the server to generate the resource identifier on its behalf, it sends an IQ stanza of type "set" that contains an empty <bind/> element.

Client asks server to bind a resource:

<iq type='set' id='bind_1'>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</iq>

A server that supports resource binding MUST be able to generate a resource identifier on behalf of a client. A resource identifier generated by the server MUST be currently unique for that <node@domain>.

If the client wishes to specify the resource identifier, it MUST send an IQ stanza of type "set" that contains the desired resource identifier as the non-zero-length XML character data of a <resource/> element that is a child of the <bind/> element.

Client binds a resource:

<iq type='set' id='bind_2'>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <resource>balcony</resource>
  </bind>
</iq>

Once the server has generated a resource identifier for the client or accepted the resource identifier provided by the client, it MUST return an IQ stanza of type "result" to the client, which MUST include a <jid/> child element that specifies the full JID for the connected resource as determined by the server.

Server informs client of successful resource binding:

<iq type='result' id='bind_2'>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <jid>juliet@example.com/balcony</jid>
  </bind>
</iq>

A server SHOULD accept the resource identifier provided by the client, but MAY override it with a resource identifier that the server generates; in this case, the server SHOULD NOT return a stanza error (e.g., <forbidden/>) to the client but instead SHOULD communicate the generated resource identifier to the client in the IQ result as shown above.

When a client supplies a resource identifier, the following stanza error conditions are possible (see Stanza Errors (Stanza Errors)):

The protocol for these error conditions is as follows.

Resource identifier cannot be processed:

<iq type='error' id='bind_2'>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <resource>someresource</resource>
  </bind>
  <error type='modify'>
    <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

Client is not allowed to bind a resource:

<iq type='error' id='bind_2'>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <resource>someresource</resource>
  </bind>
  <error type='cancel'>
    <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

If there is already a connected resource of the same name, the server MUST do one of the following:

  1. Not accept the resource identifier provided by the client but instead override it with a resource identifier that the server generates.
  2. Terminate the current resource and allow the newly-requested resource.
  3. Disallow the newly-requested resource and maintain the current resource.

Which of these the server does is up to the implementation, although it is RECOMMENDED to implement case #1. In case #2, the server MUST send a <conflict/> stream error to the current resource, terminate the XML stream and underlying TCP connection for the current resource, and return a IQ stanza of type "result" (indicating success) to the newly-requested resource. In case #3, the server MUST either (a) return a server-generated resource name or (b) send a <conflict/> stanza error to the newly-requested resource but maintain the XML stream for that connection so that the newly-requested resource has an opportunity to negotiate a non-conflicting resource identifier before sending another request for resource binding.

Resource identifier is in use:

<iq type='error' id='bind_2'>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <resource>someresource</resource>
  </bind>
  <error type='cancel'>
    <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

If, before completing the resource binding step, the client attempts to send an outbound XML stanza (i.e., a stanza not directed to the server itself or to the client's own account), the server MUST NOT process the stanza and SHOULD return a <not-authorized/> stanza error to the client.



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8.1.  Binding Multiple Resources

A server MAY support binding of multiple resources to the same stream. This functionality is desirable in certain environments (e.g., for devices that are unable to open more than one TCP connection or when a machine runs an XMPP client daemon that is used by multiple applications). If a server supports binding of multiple resources to a stream, it MUST enable a client to unbind resources. This shall be completed by sending an IQ-set with a child element of <unbind/> qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace, which in turn has a child element of <resource/> whose XML character data specifies the resource to be unbound:

<iq type='set' id='unbind_1'>
  <unbind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <resource>someresource</resource>
  </unbind>
</iq>

If the server does not understand the <unbind/> element, it MUST return an error of <bad-request/>. Otherwise, if there is no such resource for that stream, the server MUST return an error of <item-not-found/>. When the client unbinds the only resource associated with the stream, the server SHOULD close the stream and terminate the TCP connection.

A server SHOULD advertise its support for the 'urn:ietf:params:xml:ns:xmpp-bind' namespace by returning an appropriate stream feature as follows:

<stream:features>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
  <unbind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>

When a client binds multiple resources to the same stream, proper management of 'from' addresses is imperative. In particular, a client MUST specify a 'from' address on every stanza it sends over a stream to which it has bound multiple resources, where the 'from' address is the full JID (<node@domain.tld/resource>) associated with the relevant resource. If a client does not specify a 'from' address on a stanza it sends over a stream to which it has bound multiple resources (or if it specifies as the 'from' address a full JID other than one of the bound resources), the server MUST return the stanza to the client with an <unknown-sender/> stanza error.

Naturally, the rules regarding validation of asserted 'from' addresses still apply (see Section 11 (Server Rules for Handling XML Stanzas)).



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9.  XML Stanzas

After a client has connected to a server or two servers have connected to each other, either party can send XML stanzas over the negotiated stream. Three kinds of XML stanza are defined for the 'jabber:client' and 'jabber:server' namespaces: <message/>, <presence/>, and <iq/>. In addition, there are five common attributes for these kinds of stanza. These common attributes, as well as the basic semantics of the three stanza kinds, are defined herein; more detailed information regarding the syntax of XML stanzas for instant messaging and presence applications is provided in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.), and for other applications in the relevant XMPP extension specifications.

An XML stanza is the basic unit of meaning in XMPP. A server MUST NOT process, deliver, or route a partial stanza and a server MUST NOT attach meaning to the transmission timing of any child element within a stanza.



 TOC 

9.1.  Common Attributes

The following five attributes are common to message, presence, and IQ stanzas:



 TOC 

9.1.1.  to

The 'to' attribute specifies the JID of the intended recipient for the stanza.

In the 'jabber:client' namespace, a stanza with a specific intended recipient MUST possess a 'to' attribute, whereas a stanza sent from a client to a server for direct processing by that server (e.g., presence sent to the server for broadcasting to other entities) SHOULD NOT possess 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 <improper-addressing/> stream error condition and terminate both the XML stream and the underlying TCP connection with the offending server.

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, setting the 'from' attribute of the error stanza to the value provided in the 'to' attribute of the offending stanza.



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9.1.2.  from

The 'from' attribute specifies the JID of the sender.

When a server receives an XML stanza within the context of an authenticated stream qualified by the 'jabber:client' namespace, it MUST do one of the following:

  1. validate that the value of the 'from' attribute provided by the client is that of a connected resource for the associated entity
  2. add a 'from' address to the stanza whose value is the full JID (<node@domain/resource>) determined by the server for the connected resource that generated the stanza (see Determination of Addresses (Determination of Addresses)), or the bare JID (<node@domain>) in the case of subscription-related presence stanzas (see [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.) for details)

If a client attempts to send an XML stanza for which the value of the 'from' attribute does not exactly match one of the connected resources for that entity, the server SHOULD return an <invalid-from/> stream error to the client. If a client attempts to send an XML stanza over a stream that is not yet authenticated, the server SHOULD return a <not-authorized/> stream error to the client. If generated, both of these conditions MUST result in closure of the stream and termination of the underlying TCP connection; this helps to prevent a denial of service attack launched from a rogue client.

When a server generates a stanza from the server itself for delivery to a connected client (e.g., in the context of data storage services provided by the server on behalf of the client), the stanza MUST either (1) not include a 'from' attribute or (2) include a 'from' attribute whose value is the account's bare JID (<node@domain>) or connected resource's full JID (<node@domain/resource>). A server MUST NOT send to the client a stanza without a 'from' attribute if the stanza was not generated by the server itself. When a client receives a stanza that does not include a 'from' attribute, it MUST assume that the stanza is from the server to which the client is connected.

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 <improper-addressing/> 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 thereof, such as a validated subdomain of the sending server's hostname or another validated domain hosted by the sending server) as communicated in the SASL negotiation or dialback negotiation; if a server receives a stanza that does not meet this restriction, it MUST generate an <invalid-from/> stream error condition. Both of these conditions MUST result in closure of the stream and termination of the underlying TCP connection; this helps to prevent a denial of service attack launched from a rogue server.



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9.1.3.  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 semantics of IQ stanzas). It is OPTIONAL for the value of the 'id' attribute to be unique globally, within a domain, or within a stream. The semantics of IQ stanzas impose additional restrictions; see IQ Semantics (IQ Semantics).



 TOC 

9.1.4.  type

The 'type' attribute specifies detailed information about the purpose or context of the message, presence, or IQ stanza. The particular allowable values for the 'type' attribute vary depending on whether the stanza is a message, presence, or IQ; the values for message and presence stanzas are specific to instant messaging and presence applications and therefore are defined in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.), whereas the values for IQ stanzas specify the role of an IQ stanza in a structured request-response "conversation" and thus are defined under IQ Semantics (IQ Semantics) below. The only 'type' value common to all three stanzas is "error"; see Stanza Errors (Stanza Errors).



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9.1.5.  xml:lang

A stanza SHOULD possess an 'xml:lang' attribute (as defined in Section 2.12 of [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.)) if the stanza contains XML character data that is intended to be presented to a human user (as explained in [CHARSET] (Alvestrand, H., “IETF Policy on Character Sets and Languages,” January 1998.), "internationalization is for humans"). The value of the 'xml:lang' attribute specifies the default language of any such human-readable XML character data, which MAY be overridden by the 'xml:lang' attribute of a specific child element. If a stanza does not possess an 'xml:lang' attribute, an implementation MUST assume that the default language is that specified for the stream as defined under Stream Attributes (Stream Attributes) above. The value of the 'xml:lang' attribute MUST be an NMTOKEN and MUST conform to the format defined in [LANGTAGS] (Alvestrand, H., “Tags for the Identification of Languages,” January 2001.).



 TOC 

9.2.  Basic Semantics



 TOC 

9.2.1.  Message Semantics

The <message/> stanza kind can be seen as a "push" mechanism whereby one entity pushes information to another entity, similar to the communications that occur in a system such as email. All message stanzas SHOULD possess a 'to' attribute that specifies the intended recipient of the message; upon receiving such a stanza, a server SHOULD route or deliver it to the intended recipient (see Server Rules for Handling XML Stanzas (Server Rules for Handling XML Stanzas) for general routing and delivery rules related to XML stanzas).



 TOC 

9.2.2.  Presence Semantics

The <presence/> element can be seen as a specialized broadcast or "publish-subscribe" mechanism, whereby multiple entities receive information about an entity to which they have subscribed (in this case, network availability information). In general, a publishing entity SHOULD send a presence stanza with no 'to' attribute, in which case the server to which the entity is connected SHOULD broadcast or multiplex that stanza to all subscribing entities. However, a publishing entity MAY also send a presence stanza with a 'to' attribute, in which case the server SHOULD route or deliver that stanza to the intended recipient. See Server Rules for Handling XML Stanzas (Server Rules for Handling XML Stanzas) for general routing and delivery rules related to XML stanzas, and [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.) for rules specific to presence applications.



 TOC 

9.2.3.  IQ Semantics

Info/Query, or IQ, is a request-response mechanism, similar in some ways to [HTTP] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.). The semantics of IQ enable an entity to make a request of, and receive a response from, another entity. The data content of the request and response is defined by the schema or other structural definition associated with the XML namespace that qualifies the direct child element of the IQ element (see extended namespaces (Extended Namespaces)), and the interaction is tracked by the requesting entity through use of the 'id' attribute. Thus, IQ interactions follow a common pattern of structured data exchange such as get/result or set/result (although an error may be returned in reply to a request if appropriate):

Requesting                 Responding
  Entity                     Entity
----------                 ----------
    |                           |
    | <iq type='get' id='1'>    |
    | ------------------------> |
    |                           |
    | <iq type='result' id='1'> |
    | <------------------------ |
    |                           |
    | <iq type='set' id='2'>    |
    | ------------------------> |
    |                           |
    | <iq type='error' id='2'>  |
    | <------------------------ |
    |                           |

In order to enforce these semantics, the following rules apply:

  1. The 'id' attribute is REQUIRED for IQ stanzas.
  2. The 'type' attribute is REQUIRED for IQ stanzas. The value MUST be one of the following:
  3. An entity that receives an IQ request of type "get" or "set" MUST reply with an IQ response of type "result" or "error" (the response MUST preserve the 'id' attribute of the request).
  4. An entity that receives a stanza of type "result" or "error" MUST NOT respond to the stanza by sending a further IQ response of type "result" or "error"; however, 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).
  5. An IQ stanza of type "get" or "set" MUST contain one and only one child element that specifies the semantics of the particular request or response.
  6. An IQ stanza of type "result" MUST include zero or one child elements.
  7. An IQ stanza of type "error" SHOULD include the child element contained in the associated "get" or "set" and MUST include an <error/> child; for details, see Stanza Errors (Stanza Errors).


 TOC 

9.3.  Stanza Errors

Stanza-related errors are handled in a manner similar to stream errors (Stream Errors). However, unlike stream errors, stanza errors are recoverable; therefore error stanzas include hints regarding actions that the original sender can take in order to remedy the error.



 TOC 

9.3.1.  Rules

The following rules apply to stanza-related errors:



 TOC 

9.3.2.  Syntax

The syntax for stanza-related errors is as follows:

<stanza-kind to='sender' type='error'>
  [RECOMMENDED to include sender XML here]
  <error type='error-type'>
    <defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    [<text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
          xml:lang='langcode'>
      OPTIONAL descriptive text
    </text>]
    [OPTIONAL application-specific condition element]
  </error>
</stanza-kind>

The "stanza-kind" is one of message, presence, or iq.

The value of the <error/> element's 'type' attribute MUST be one of the following:

The <error/> element:

The <text/> element is OPTIONAL. If included, it SHOULD be used only to provide descriptive or diagnostic information that supplements the meaning of a defined condition or application-specific condition. It SHOULD NOT be interpreted programmatically by an application. It SHOULD NOT be used as the error message presented to a user, but MAY be shown in addition to the error message associated with the included condition element (or elements).

Finally, to maintain backward compatibility, the schema (specified in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.)) allows the optional inclusion of a 'code' attribute on the <error/> element; for details, see [XEP‑0086] (Norris, R. and P. Saint-Andre, “Error Condition Mappings,” February 2004.).



 TOC 

9.3.3.  Defined Conditions

The following conditions are defined for use in stanza errors.



 TOC 

9.3.4.  Application-Specific Conditions

As noted, an application MAY provide application-specific stanza error information by including a properly-namespaced child in the error element. The application-specific element SHOULD supplement or further qualify a defined element. Thus, the <error/> element will contain two or three child elements:

<iq type='error' id='some-id'>
  <error type='modify'>
    <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <too-many-parameters xmlns='application-ns'/>
  </error>
</iq>
<message type='error' id='another-id'>
  <error type='modify'>
    <undefined-condition
          xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <text xml:lang='en'
          xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
      Some special application diagnostic information...
    </text>
    <special-application-condition xmlns='application-ns'/>
  </error>
</message>


 TOC 

9.4.  Extended Namespaces

While the message, presence, and IQ stanza kinds provide basic semantics for messaging, availability, and request-response interactions, XMPP uses XML namespaces to extend the stanzas for the purpose of providing additional functionality. Thus a message or presence stanza MAY contain one or more optional child elements specifying content that extends the meaning of the message (e.g., an XHTML-formatted version of the message body as described in [XEP‑0071] (Saint-Andre, P., “XHTML-IM,” January 2006.)), and an IQ stanza MAY contain one such child element. This child element MAY have any name and MUST possess an 'xmlns' namespace declaration (other than "jabber:client", "jabber:server", or "http://etherx.jabber.org/streams") that defines all data contained within the child element. Such a child element is said to be defined by an EXTENDED NAMESPACE.

Support for any given extended namespace is OPTIONAL on the part of any implementation. If an entity does not understand such a namespace, the entity's expected behavior depends on whether the entity is (1) the recipient or (2) an entity that is routing the stanza to the recipient:

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 qualified by a 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 whose only child element is qualified by a namespace it does not understand, it MUST ignore the entire stanza.
  • If an entity receives an IQ stanza of type "get" or "set" containing a child element qualified by a namespace it does not understand, the entity SHOULD return an IQ stanza of type "error" with an error condition of <service-unavailable/>.
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.


 TOC 

10.  Examples



 TOC 

10.1.  Client-to-Server

The following examples show the data flow for a client negotiating an XML stream with a server, exchanging XML stanzas, and closing the negotiated stream. The server is "example.com", the server requires use of TLS, the client authenticates via the SASL DIGEST-MD5 mechanism as "juliet@example.com", and the client binds the resource "balcony" to the stream. (Note: The alternate steps shown below are provided to illustrate the protocol for failure cases; they are not exhaustive and would not necessarily be triggered by the data sent in the examples.)

Step 1: Client initiates stream to server:

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    xml:lang='en'
    from='juliet@example.com'
    to='example.com'
    version='1.0'>

Step 2: Server responds by sending a stream header to client:

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    xml:lang='en'
    from='example.com'
    id='c2s_123'
    to='juliet@example.com'
    version='1.0'>

Step 3: Server sends stream features to client (STARTTLS extension and authentication mechanisms):

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
</stream:features>

Step 4: Client sends STARTTLS command to server:

<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Step 5: Server informs client that it is allowed to proceed:

<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Step 5 (alt): Server informs client that TLS negotiation has failed and closes both XML stream and TCP connection:

<failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
</stream:stream>

Step 6: Client and server attempt to complete TLS negotiation over the existing TCP connection (see [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.) for details).

Step 7: If TLS negotiation is successful, client initiates a new stream to server:

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    xml:lang='en'
    from='juliet@example.com'
    to='example.com'
    version='1.0'>

Step 7 (alt): If TLS negotiation is unsuccessful, server closes TCP connection.

Step 8: Server responds by sending a stream header to client along with any available stream features (notice that the server now shows a different set of SASL mechanisms; here the server accepts the SASL PLAIN mechanism once the stream has been secured via TLS):

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    xml:lang='en'
    from='example.com'
    id='c2s_234'
    to='juliet@example.com'
    version='1.0'>
<stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>DIGEST-MD5</mechanism>
    <mechanism>PLAIN</mechanism>
    <required/>
  </mechanisms>
</stream:features>

Step 9: Client selects an authentication mechanism, in this case [DIGEST‑MD5] (Leach, P. and C. Newman, “Using Digest Authentication as a SASL Mechanism,” May 2000.) with an empty authorization identity ("="):

<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
      mechanism='DIGEST-MD5'>=</auth>

Step 10: Server sends a [BASE64] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” July 2003.) encoded challenge to client:

<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLHFvcD0i
YXV0aCIsY2hhcnNldD11dGYtOCxhbGdvcml0aG09bWQ1LXNlc3MK
</challenge>

The decoded challenge is:

realm="example.com",nonce="OA6MG9tEQGm2hh",
qop="auth",charset=utf-8,algorithm=md5-sess

Note: When the server sends a DIGEST-MD5 challenge to the client, the qop list must be quoted since it is a list rather than a single item (even if there is only one item in the list); however, when the client sends its response to the server (see below), the qop must not be quoted since it is a single item rather than a list.

Step 10 (alt): Server returns error to client:

<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
  <temporary-auth-failure/>
</failure>
</stream:stream>

Step 11: Client sends a [BASE64] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” July 2003.) encoded response to the challenge:

<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9Imp1bGlldCIscmVhbG09ImV4YW1wbGUuY29tIixub25jZT0iT0E2
TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAwMDAwMDAx
LHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20iLHJlc3BvbnNl
PWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNoYXJzZXQ9dXRmLTgK
</response>

The decoded response is:

username="juliet",realm="example.com",
nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",
nc=00000001,qop=auth,digest-uri="xmpp/example.com",
response=d388dad90d4bbd760a152321f2143af7,charset=utf-8

Step 12: Server informs client of success and includes [BASE64] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” July 2003.) encoded value for subsequent authentication:

<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
</success>

The decoded value for subsequent authentication is:

rspauth=ea40f60335c427b5527b84dbabcdfffd

Step 12 (alt): Server returns error to client:

<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
  <temporary-auth-failure/>
</failure>
</stream:stream>

Step 13: Client initiates a new stream to server:

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    xml:lang='en'
    from='juliet@example.com'
    to='example.com'
    version='1.0'>

Step 14: Server responds by sending a stream header to client along with supported features (in this case resource binding):

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    xml:lang='en'
    from='example.com'
    id='c2s_345'
    to='juliet@example.com'
    version='1.0'>
<stream:features>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <required/>
  </bind>
</stream:features>

Upon being so informed that resource binding is required, the client MUST bind a resource to the stream; here we assume that the client binds a resource called "balcony".

Step 15: Client binds a resource:

<iq type='set' id='bind_1'>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <resource>balcony</resource>
  </bind>
</iq>

Step 16: Server informs client of successful resource binding:

<iq type='result'
    to='juliet@example.com/balcony'
    id='bind_1'>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <jid>juliet@example.com/balcony</jid>
  </bind>
</iq>

Now the client is allowed to send XML stanzas over the negotiated stream.

Client sends XML stanza to other entity:

<message from='juliet@example.com/balcony'
         to='romeo@example.net'
         xml:lang='en'>
    <body>Art thou not Romeo, and a Montague?</body>
</message>

If necessary, sender's server negotiates XML streams with intended recipient's server (see Server-to-Server Examples (Server-to-Server Examples)).

The intended recipient replies and the message is delivered to the client.

Client receives XML stanza from other entity:

<message from='romeo@example.net/orchard'
         to='juliet@example.com/balcony'
         xml:lang='en'>
  <body>Neither, fair saint, if either thee dislike.</body>
</message>

Desiring to send no further messages, the client closes the stream.

Client closes the stream:

</stream:stream>

Consistent with the recommended stream closing handshake, server closes stream as well:

Server closes the stream:

</stream:stream>

Client now terminates the underlying TCP connection.



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10.2.  Server-to-Server Examples

The following examples show the data flow for a server negotiating an XML stream with another server, exchanging XML stanzas, and closing the negotiated stream. The initiating server ("Server1") is "example.com"; the receiving server ("Server2") is example.net and it requires use of TLS; example.com presents a certificate and authenticates via the SASL EXTERNAL mechanism. (Note: The alternate steps shown below are provided to illustrate the protocol for failure cases; they are not exhaustive and would not necessarily be triggered by the data sent in the examples.)

Step 1: Server1 initiates stream to Server2:

<stream:stream
    xmlns='jabber:server'
    xmlns:stream='http://etherx.jabber.org/streams'
    from='example.com'
    to='example.net'
    version='1.0'>

Step 2: Server2 responds by sending a stream tag to Server1:

<stream:stream
    xmlns='jabber:server'
    xmlns:stream='http://etherx.jabber.org/streams'
    from='example.net'
    id='s2s_123'
    to='example.com'
    version='1.0'>

Step 3: Server2 sends stream features to Server1 (STARTTLS extension and authentication mechanisms):

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
</stream:features>

Step 4: Server1 sends the STARTTLS command to Server2:

<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Step 5: Server2 informs Server1 that it is allowed to proceed:

<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Step 5 (alt): Server2 informs Server1 that TLS negotiation has failed and closes stream:

<failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
</stream: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:

<stream:stream
    xmlns='jabber:server'
    xmlns:stream='http://etherx.jabber.org/streams'
    from='example.com'
    to='example.net'
    version='1.0'>

Step 7 (alt): If TLS negotiation is unsuccessful, Server2 closes TCP connection.

Step 8: Server2 responds by sending a stream header to Server1 along with available stream features (notice that Server2 now prefers the SASL EXTERNAL mechanism):

<stream:stream
    xmlns='jabber:server'
    xmlns:stream='http://etherx.jabber.org/streams'
    from='example.net'
    id='s2s_234'
    to='example.com'
    version='1.0'>
<stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>EXTERNAL</mechanism>
    <mechanism>DIGEST-MD5</mechanism>
    <required/>
  </mechanisms>
</stream:features>

Step 9: Server1 selects the EXTERNAL mechanism, in this case with an authorization identity encoded according to [BASE64] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” July 2003.):

<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
      mechanism='EXTERNAL'/>ZXhhbXBsZS5jb20K</auth>

The decoded authorization identity is "example.com".

Step 10: Server2 determines that the authorization identity provided by Server1 matches the valid id-xmppAddr-on or Common Name in the presented certificate and therefore returns success:

<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

Step 11 (alt): Server2 informs Server1 of failed authentication:

<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
  <not-authorized/>
</failure>
</stream:stream>

Step 12: Server1 initiates a new stream to Server2:

<stream:stream
    xmlns='jabber:server'
    xmlns:stream='http://etherx.jabber.org/streams'
    from='example.com'
    to='example.net'
    version='1.0'>

Step 13: Server2 responds by sending a stream header to Server1 along with any additional features (or, in this case, an empty features element):

<stream:stream
    xmlns='jabber:server'
    xmlns:stream='http://etherx.jabber.org/streams'
    from='example.net'
    id='s2s_345'
    to='example.com'
    version='1.0'>
<stream:features/>

Now Server1 is allowed to send XML stanzas to Server2 over the negotiated stream; here we assume that the transferred stanzas are those shown earlier for client-to-server communications.

Server1 sends XML stanza to Server2:

<message from='juliet@example.com/balcony'
         to='romeo@example.net'
         xml:lang='en'>
    <body>Art thou not Romeo, and a Montague?</body>
</message>

The intended recipient replies and the message is delivered from Server2 to Server1.

Server2 sends XML stanza to Server1:

<message from='romeo@example.net/orchard'
         to='juliet@example.com/balcony'
         xml:lang='en'>
  <body>Neither, fair saint, if either thee dislike.</body>
</message>

Desiring to send no further messages, Server1 closes the stream. (In practice, the stream would most likely remain open for some time, since Server1 and Server2 do not immediately know if the stream will be needed for further communications.)

Server1 closes the stream:

</stream:stream>

Consistent with the recommended stream closing handshake, Server2 closes stream as well:

Server2 closes the stream:

</stream:stream>

Server1 now terminates the underlying TCP connection.



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11.  Server Rules for Handling XML Stanzas

Compliant server implementations MUST ensure in-order processing of XML stanzas between any two entities. This includes stanzas sent by a client to its server for direct processing by the server.

Beyond the requirement for in-order processing, each server implementation will contain its own "delivery tree" for handling stanzas it receives. Such a tree determines whether a stanza needs to be routed to another domain, processed direct, or delivered to a resource associated with a connected node. The following rules apply.



 TOC 

11.1.  No 'to' Address

If the stanza possesses no 'to' attribute, the server SHOULD process it directly on behalf of the entity that sent it. Because all stanzas received from other servers MUST possess a 'to' attribute, this rule applies only to stanzas received from a registered entity (such as a client) that is connected to the server. If the server receives a presence stanza with no 'to' attribute, the server SHOULD broadcast it to the entities that are subscribed to the sending entity's presence, if applicable (the semantics of presence broadcast for presence applications are defined in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.)). If the server receives an IQ stanza of type "get" or "set" with no 'to' attribute and it understands the namespace that qualifies the content of the stanza, it MUST either process the stanza directly on behalf of sending entity (where the meaning of "process" is determined by the semantics of the qualifying namespace) or return an error to the sending entity.



 TOC 

11.2.  Foreign Domain

If the hostname of the domain identifier portion of the JID contained in the 'to' attribute does not match one of the configured hostnames of the server itself or a configured subdomain thereof, the server SHOULD route the stanza to the foreign domain (subject to local service provisioning and security policies regarding inter-domain communication, since such communication is OPTIONAL). There are two possible cases:

A server-to-server stream already exists between the two domains:
The sender's server routes the stanza to the authoritative server for the foreign domain over the existing stream
There exists no server-to-server stream between the two domains:
The sender's server (1) resolves the hostname of the foreign domain (as defined under Server-to-Server Communications (Server-to-Server Communications)), (2) negotiates a server-to-server stream between the two domains (as defined under TLS negotiation (TLS Negotiation) and SASL negotiation (SASL Negotiation)), and (3) routes the stanza to the authoritative server for the foreign domain over the newly-established stream

If routing to the recipient's server is unsuccessful, the sender's server MUST return an error to the sender; if the recipient's server can be contacted but delivery by the recipient's server to the recipient is unsuccessful, the recipient's server MUST return an error to the sender by way of the sender's server.



 TOC 

11.3.  Subdomain

If the hostname of the domain identifier portion of the JID contained in the 'to' attribute matches a subdomain of one of the configured hostnames of the server itself, the server MUST either process the stanza itself or route the stanza to a specialized service that is responsible for that subdomain (if the subdomain is configured), or return an error to the sender (if the subdomain is not configured).



 TOC 

11.4.  Mere Domain or Specific Resource

If the hostname of the domain identifier portion of the JID contained in the 'to' attribute matches a configured hostname of the server itself and the JID contained in the 'to' attribute is of the form <domain> or <domain/resource>, the server (or a defined resource thereof) MUST either process the stanza as appropriate for the stanza kind or return an error stanza to the sender.



 TOC 

11.5.  Node in Same Domain

If the hostname of the domain identifier portion of the JID contained in the 'to' attribute matches a configured hostname of the server itself and the JID contained in the 'to' attribute is of the form <node@domain> or <node@domain/resource>, the server SHOULD deliver the stanza to the intended recipient of the stanza as represented by the JID contained in the 'to' attribute. The following rules apply:

  1. If the JID contains a resource identifier (i.e., is of the form <node@domain/resource>) and there exists a connected resource that exactly matches the full JID, the recipient's server SHOULD deliver the stanza to the stream or connection that exactly matches the resource identifier.
  2. If the JID contains a resource identifier and there exists no connected resource that exactly matches the full JID, the recipient's server SHOULD return a <service-unavailable/> stanza error to the sender.
  3. If the JID is of the form <node@domain> and there exists at least one connected resource for the node, the recipient's server SHOULD deliver the stanza to at least one of the connected resources, according to application-specific rules.

Particular XMPP applications MAY specify delivery rules that modify or supplement the foregoing rules; for example, a set of delivery rules for instant messaging and presence applications is defined in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.).



 TOC 

12.  XML Usage



 TOC 

12.1.  Restrictions

XMPP is a simplified and specialized protocol for streaming XML elements in order to exchange structured information in close to real time. Because XMPP does not require the parsing of arbitrary and complete XML documents, there is no requirement that XMPP needs to support the full feature set of [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.). 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:

With regard to XML processing, if an XMPP implementation receives such restricted XML data, it MUST return a <restricted-xml/> stream error.



 TOC 

12.2.  XML Namespace Names and Prefixes

XML namespaces (see [XML‑NAMES] (Bray, T., Hollander, D., and A. Layman, “Namespaces in XML,” January 1999.)) are used within all XMPP-compliant XML to create strict boundaries of data ownership. The basic function of namespaces is to separate different vocabularies of XML elements that are structurally mixed together. Ensuring that XMPP-compliant XML is namespace-aware enables any allowable XML to be structurally mixed with any data element within XMPP. Rules for XML namespace names and prefixes are defined in the following subsections.



 TOC 

12.2.1.  Streams Namespace

A streams namespace declaration is REQUIRED in all XML stream headers. The name of the streams namespace MUST be 'http://etherx.jabber.org/streams'. The element names of the <stream/> element and its <features/> and <error/> children MUST be qualified by the streams namespace prefix in all instances. An implementation SHOULD generate only the 'stream:' prefix for these elements, and for historical reasons MAY accept only the 'stream:' prefix.



 TOC 

12.2.2.  Default Namespace

A default namespace declaration is REQUIRED and is used in all XML streams in order to define the allowable first-level children of the root stream element. This namespace declaration MUST be the same for the initial stream and the response stream so that both streams are qualified consistently. The default namespace declaration applies to the stream and all stanzas sent within a stream (unless explicitly qualified by another namespace, or by the prefix of the streams namespace or the dialback namespace).

A server implementation MUST support the following two default namespaces (for historical reasons, some implementations MAY support only these two default namespaces):

A client implementation MUST support the 'jabber:client' default namespace, and for historical reasons MAY support only that default namespace.

An implementation MUST NOT generate namespace prefixes for elements qualified by the default namespace if the default namespace is 'jabber:client' or 'jabber:server'. An implementation SHOULD NOT generate namespace prefixes for elements qualified by content (as opposed to stream) namespaces other than 'jabber:client' and 'jabber:server'.

Note: 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 qualified by the 'jabber:client' or 'jabber:server' namespace, it MUST support the common attributes (Common Attributes) and basic semantics (Basic Semantics) of all three core stanza kinds (message, presence, and IQ).



 TOC 

12.2.3.  Dialback Namespace

A dialback namespace declaration is REQUIRED for all elements used in server dialback (Server Dialback). The name of the dialback namespace MUST be 'jabber:server:dialback'. All elements qualified by this namespace MUST be prefixed. An implementation SHOULD generate only the 'db:' prefix for such elements and MAY accept only the 'db:' prefix.



 TOC 

12.3.  Validation

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 this is OPTIONAL (although an implementation MUST NOT accept XML that is not well-formed). 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 OPTIONAL, and schemas are included herein for descriptive purposes only.



 TOC 

12.4.  Inclusion of Text Declaration

Implementations SHOULD send a text declaration before sending a stream header. Applications MUST follow the rules in [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.) regarding the circumstances under which a text declaration is included.



 TOC 

12.5.  Character Encoding

Implementations MUST support the [UTF‑8] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) transformation of Universal Character Set ([UCS2] (International Organization for Standardization, “Information Technology - Universal Multiple-octet coded Character Set (UCS) - Amendment 2: UCS Transformation Format 8 (UTF-8),” October 1996.)) characters, as required by [CHARSET] (Alvestrand, H., “IETF Policy on Character Sets and Languages,” January 1998.). Implementations MUST NOT attempt to use any other encoding.



 TOC 

12.6.  White Space

Except where explicitly disallowed (i.e., during TLS negotiation (TLS Negotiation) and SASL negotiation (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) [SASL]), either entity MAY send white space characters (matching production [3] content of [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” October 2000.)) within the root stream element as separators between XML stanzas or between any other first-level elements sent over the stream; one common use for sending such white space characters is to check the viability of the underlying TCP connection after a period of inactivity.



 TOC 

13.  Compliance Requirements

This section summarizes the specific aspects of the Extensible Messaging and Presence Protocol that MUST be supported by servers and clients in order to be considered compliant implementations, as well as additional protocol aspects that SHOULD be supported. For compliance purposes, we draw a distinction between core protocols (which MUST be supported by any server or client, regardless of the specific application) and instant messaging and presence protocols (which MUST be supported only by instant messaging and presence applications built on top of the core protocols). Compliance requirements that apply to all servers and clients are specified in this section; compliance requirements for instant messaging and presence applications are specified in the corresponding section of [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.).



 TOC 

13.1.  Servers

In addition to all defined requirements with regard to security, XML usage, and internationalization, a server MUST support the following core protocols in order to be considered compliant:

In addition, for historical reasons a server SHOULD support the following core protocol:



 TOC 

13.2.  Clients

A client MUST support the following core protocols in order to be considered compliant:

In addition, a client SHOULD support the following core protocols:



 TOC 

14.  Internationalization Considerations

XML streams MUST be encoded in UTF-8 as specified under Character Encoding (Character Encoding). As specified under Stream Attributes (Stream Attributes), an XML stream SHOULD include an 'xml:lang' attribute specifying the default language for any XML character data sent over the stream that is intended to be presented to a human user. As specified under xml:lang (xml:lang), an XML stanza SHOULD include an 'xml:lang' attribute if the stanza contains XML character data that is intended to be presented to a human user. A server SHOULD apply the default 'xml:lang' attribute to stanzas it routes or delivers on behalf of connected entities, and MUST NOT modify or delete 'xml:lang' attributes stanzas it receives from other entities.



 TOC 

15.  Security Considerations



 TOC 

15.1.  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. See Section 15.2 (Certificate Validation) below regarding certificate validation procedures.

Implementations MUST support high security. Service provisioning SHOULD use high security, subject to local security policies.



 TOC 

15.2.  Certificate Validation

When an XMPP peer communicates with another peer securely, it MUST validate the peer's certificate. There are three possible cases:

Case #1:
The peer contains an End Entity certificate which appears to be certified by a chain of certificates terminating in a trust anchor (as described in Section 6.1 of [X509] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.)).
Case #2:
The peer certificate is certified by a Certificate Authority not known to the validating peer.
Case #3:
The peer certificate is self-signed.

In Case #1, the validating peer MUST do one of two things:

  1. Verify the peer certificate according to the rules of [X509] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.). The certificate SHOULD then be checked against the expected identity of the peer following the rules described in [HTTP‑TLS] (Rescorla, E., “HTTP Over TLS,” May 2000.), except that if present an [ASN.1] (CCITT, “Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1),” 1988.) Object Identifier of "id-on-xmppAddr" (represented as a UTF8String in an otherName entity inside the subjectAltName) MUST be used as the identity. If one of these checks fails, user-oriented clients MUST either notify the user (clients MAY give the user the opportunity to continue with the connection in any case) or terminate the connection with a bad certificate error. Automated clients SHOULD terminate the connection (with a bad certificate error) and log the error to an appropriate audit log. Automated clients MAY provide a configuration setting that disables this check, but MUST provide a setting that enables it.
  2. The peer SHOULD show the certificate to a user for approval, including the entire certificate chain. The peer MUST cache the certificate (or some non-forgeable representation such as a hash). In future connections, the peer MUST verify that the same certificate was presented and MUST notify the user if it has changed.

In Case #2 and Case #3, implementations SHOULD act as in (2) above.



 TOC 

15.3.  Client-to-Server Communications

A compliant client implementation MUST support both TLS and SASL for connections to a server.

The TLS protocol for encrypting XML streams (defined under TLS negotiation (TLS Negotiation)) 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 SASL negotiation (SASL Negotiation)) provides a reliable mechanism for validating that a client connecting to a server is who it claims to be.

Client-to-server communications MUST NOT proceed until the DNS hostname asserted by the server has been resolved as specified under TCP Binding (TCP Binding). If there is a mismatch between the hostname to which a client attempted to connect (e.g., "example.net") and the hostname to which the client actually connects (e.g., "im.example.net"), the client MUST warn a human user about the mismatch and the human user MUST approve the connection before the client proceeds; however, the client MAY allow the user to add the presented hostname to a configured set of accepted hostnames in order to expedite future connections.

The IP address and method of access of clients MUST NOT be made public by a server, nor are any connections other than the original server connection required. This helps to protect the client's server from direct attack or identification by third parties.



 TOC 

15.4.  Server-to-Server Communications

A compliant server implementation MUST support both TLS and SASL for inter-domain communications. For historical reasons, a compliant implementation SHOULD also support Server Dialback (Server Dialback).

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.

Administrators may want to require use of SASL for server-to-server communications in order to ensure both authentication and confidentiality (e.g., on an organization's private network). Compliant implementations SHOULD support SASL for this purpose.

Server-to-server communications MUST NOT proceed until the DNS hostnames asserted by both servers have been resolved as specified under TCP Binding (TCP Binding).

Server dialback helps protect against domain spoofing, thus making it more difficult to spoof XML stanzas. It is not a mechanism for authenticating, securing, or encrypting streams between servers as is done via SASL and TLS, and results in weak verification of server identities only. Furthermore, it is susceptible to DNS poisoning attacks unless [DNSSEC] (Eastlake, D., “Domain Name System Security Extensions,” March 1999.) is used, and even if the DNS information is accurate, dialback cannot protect from attacks where the attacker is capable of hijacking the IP address of the remote domain. Domains requiring robust security SHOULD use TLS and SASL. If SASL is used for server-to-server authentication, dialback SHOULD NOT be used since it is unnecessary.



 TOC 

15.5.  Order of Layers

The order of layers in which protocols MUST be stacked is as follows:

  1. TCP
  2. TLS
  3. SASL
  4. XMPP

The rationale for this order is that [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.) is the base connection layer used by all of the protocols stacked on top of TCP, [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.) is often provided at the operating system layer, [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) is often provided at the application layer, and XMPP is the application itself.



 TOC 

15.6.  Lack of SASL Channel Binding to TLS

The SASL framework does not provide a mechanism to bind SASL authentication to a security layer providing confidentiality and integrity protection that was negotiated at a lower layer. This lack of a "channel binding" prevents SASL from being able to verify that the source and destination end points to which the lower layer's security is bound are equivalent to the end points that SASL is authenticating. If the end points are not identical, the lower layer's security cannot be trusted to protect data transmitted between the SASL authenticated entities. In such a situation, a SASL security layer should be negotiated that effectively ignores the presence of the lower layer security.



 TOC 

15.7.  Mandatory-to-Implement Technologies

At a minimum, all implementations MUST support the following mechanisms:

for authentication:
the SASL [DIGEST‑MD5] (Leach, P. and C. Newman, “Using Digest Authentication as a SASL Mechanism,” May 2000.) mechanism
for confidentiality:
TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher)
for both:
TLS plus SASL PLAIN for client-to-server connections and TLS plus SASL EXTERNAL for server-to-server connections (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher supporting peer certificates)

Naturally, implementations MAY support other ciphers with TLS and MAY support other SASL mechanisms.



 TOC 

15.8.  Firewalls

Communications using XMPP normally occur over [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.) connections on port 5222 (client-to-server) or port 5269 (server-to-server), as registered with the IANA (see IANA Considerations (IANA Considerations)). Use of these well-known ports allows administrators to easily enable or disable XMPP activity through existing and commonly-deployed firewalls.



 TOC 

15.9.  Use of base64 in SASL

Both the client and the server MUST verify any [BASE64] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” July 2003.) data received during SASL negotiation. An implementation MUST reject (not ignore) any characters that are not explicitly allowed by the base64 alphabet; this helps to guard against creation of a covert channel that could be used to "leak" information. An implementation MUST NOT break on invalid input and MUST reject any sequence of base64 characters containing the pad ('=') character if that character is included as something other than the last character of the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against buffer overflow attacks and other attacks on the implementation. Base 64 encoding visually hides otherwise easily recognized information, such as passwords, but does not provide any computational confidentiality. Base 64 encoding MUST follow the definition in Section 3 of [BASE64] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” July 2003.).



 TOC 

15.10.  Stringprep Profiles

XMPP makes use of the [NAMEPREP] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.) profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.) for processing of domain identifiers; for security considerations related to Nameprep, refer to the appropriate section of [NAMEPREP] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.).

In addition, XMPP defines two profiles of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.): Nodeprep (Nodeprep) for node identifiers and Resourceprep (Resourceprep) for resource identifiers.

The Unicode and ISO/IEC 10646 repertoires have many characters that look similar. In many cases, users of security protocols might do visual matching, such as when comparing the names of trusted third parties. Because it is impossible to map similar-looking characters without a great deal of context, such as knowing the fonts used, stringprep does nothing to map similar-looking characters together, nor to prohibit some characters because they look like others.

A node identifier can be employed as one part of an entity's address in XMPP. One common usage is as the username of an instant messaging user; another is as the name of a multi-user chat room; many other kinds of entities could use node identifiers as part of their addresses. The security of such services could be compromised based on different interpretations of the internationalized node identifier; for example, a user entering a single internationalized node identifier could access another user's account information, or a user could gain access to an otherwise restricted chat room or service.

A resource identifier can be employed as one part of an entity's address in XMPP. One common usage is as the name for an instant messaging user's connected resource; another is as the nickname of a user in a multi-user chat room; many other kinds of entities could use resource identifiers as part of their addresses. The security of such services could be compromised based on different interpretations of the internationalized resource identifier; for example, a user could attempt to initiate multiple connections with the same name, or a user could send a message to someone other than the intended recipient in a multi-user chat room.



 TOC 

16.  IANA Considerations



 TOC 

16.1.  XML Namespace Name for TLS Data

A URN sub-namespace for TLS-related data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in The IETF XML Registry (Mealling, M., “The IETF XML Registry,” January 2004.) [XML‑REG].)

URI:
urn:ietf:params:xml:ns:xmpp-tls
Specification:
RFC 3920
Description:
This is the XML namespace name for TLS-related data in the Extensible Messaging and Presence Protocol (XMPP) as defined by RFC 3920.
Registrant Contact:
IETF, XMPP Working Group, <xmppwg@xmpp.org>


 TOC 

16.2.  XML Namespace Name for SASL Data

A URN sub-namespace for SASL-related data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML‑REG] (Mealling, M., “The IETF XML Registry,” January 2004.).)

URI:
urn:ietf:params:xml:ns:xmpp-sasl
Specification:
RFC 3920
Description:
This is the XML namespace name for SASL-related data in the Extensible Messaging and Presence Protocol (XMPP) as defined by RFC 3920.
Registrant Contact:
IETF, XMPP Working Group, <xmppwg@xmpp.org>


 TOC 

16.3.  XML Namespace Name for Stream Errors

A URN sub-namespace for stream-related error data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML‑REG] (Mealling, M., “The IETF XML Registry,” January 2004.).)

URI:
urn:ietf:params:xml:ns:xmpp-streams
Specification:
RFC 3920
Description:
This is the XML namespace name for stream-related error data in the Extensible Messaging and Presence Protocol (XMPP) as defined by RFC 3920.
Registrant Contact:
IETF, XMPP Working Group, <xmppwg@xmpp.org>


 TOC 

16.4.  XML Namespace Name for Resource Binding

A URN sub-namespace for resource binding in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML‑REG] (Mealling, M., “The IETF XML Registry,” January 2004.).)

URI:
urn:ietf:params:xml:ns:xmpp-bind
Specification:
RFC 3920
Description:
This is the XML namespace name for resource binding in the Extensible Messaging and Presence Protocol (XMPP) as defined by RFC 3920.
Registrant Contact:
IETF, XMPP Working Group, <xmppwg@xmpp.org>


 TOC 

16.5.  XML Namespace Name for Stanza Errors

A URN sub-namespace for stanza-related error data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML‑REG] (Mealling, M., “The IETF XML Registry,” January 2004.).)

URI:
urn:ietf:params:xml:ns:xmpp-stanzas
Specification:
RFC 3920
Description:
This is the XML namespace name for stanza-related error data in the Extensible Messaging and Presence Protocol (XMPP) as defined by RFC 3920.
Registrant Contact:
IETF, XMPP Working Group, <xmppwg@xmpp.org>


 TOC 

16.6.  Nodeprep Profile of Stringprep

The Nodeprep profile of stringprep is defined under Nodeprep (Nodeprep). The IANA has registered Nodeprep in the stringprep profile registry.

Name of this profile:

Nodeprep

RFC in which the profile is defined:

RFC 3920

Indicator whether or not this is the newest version of the profile:

This is the first version of Nodeprep


 TOC 

16.7.  Resourceprep Profile of Stringprep

The Resourceprep profile of stringprep is defined under Resourceprep (Resourceprep). The IANA has registered Resourceprep in the stringprep profile registry.

Name of this profile:

Resourceprep

RFC in which the profile is defined:

RFC 3920

Indicator whether or not this is the newest version of the profile:

This is the first version of Resourceprep


 TOC 

16.8.  GSSAPI Service Name

The IANA has registered "xmpp" as a GSSAPI (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) [GSS‑API] service name, as defined under SASL Definition (SASL Definition).



 TOC 

16.9.  Port Numbers

The IANA has registered "xmpp-client" and "xmpp-server" as keywords for [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.) ports 5222 and 5269 respectively.

These ports SHOULD be used for client-to-server and server-to-server communications respectively, but their use is OPTIONAL.



 TOC 

17.  References



 TOC 

17.1. Normative References

[ABNF] Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” RFC 4234, October 2005.
[BASE64] Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” RFC 3548, July 2003.
[CHARSET] Alvestrand, H., “IETF Policy on Character Sets and Languages,” BCP 18, RFC 2277, January 1998 (TXT, HTML, XML).
[DIGEST-MD5] Leach, P. and C. Newman, “Using Digest Authentication as a SASL Mechanism,” RFC 2831, May 2000.
[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” RFC 2782, February 2000.
[DNS] Mockapetris, P., “Domain names - implementation and specification,” STD 13, RFC 1035, November 1987.
[GSS-API] Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” RFC 2743, January 2000.
[HMAC] National Institute of Standards and Technology, “The Keyed-Hash Message Authentication Code (HMAC),” FIPS PUB 198, March 2002.
[HTTP-TLS] Rescorla, E., “HTTP Over TLS,” RFC 2818, May 2000.
[IDNA] Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” RFC 3490, March 2003.
[IPv6] Hinden, R. and S. Deering, “Internet Protocol Version 6 (IPv6) Addressing Architecture,” RFC 3513, April 2003.
[LANGTAGS] Alvestrand, H., “Tags for the Identification of Languages,” BCP 47, RFC 3066, January 2001.
[NAMEPREP] Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” RFC 3491, March 2003.
[RANDOM] Eastlake, D., Crocker, S., and J. Schiller, “Randomness Recommendations for Security,” RFC 1750, December 1994.
[SASL] Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” RFC 4422, June 2006.
[SHA] National Institute of Standards and Technology, “Secure Hash Standard,” FIPS PUB 180-2, August 2002.
[STRINGPREP] Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” RFC 3454, December 2002.
[TCP] Postel, J., “Transmission Control Protocol,” STD 7, RFC 793, September 1981.
[TERMS] Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[TLS] Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” RFC 4346, April 2006.
[UCS2] International Organization for Standardization, “Information Technology - Universal Multiple-octet coded Character Set (UCS) - Amendment 2: UCS Transformation Format 8 (UTF-8),” ISO Standard 10646-1 Addendum 2, October 1996.
[UTF-8] Yergeau, F., “UTF-8, a transformation format of ISO 10646,” STD 63, RFC 3629, November 2003.
[X509] Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” RFC 3280, April 2002.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” W3C REC-xml, October 2000.
[XML-NAMES] Bray, T., Hollander, D., and A. Layman, “Namespaces in XML,” W3C REC-xml-names, January 1999.


 TOC 

17.2. Informative References

[ACAP] Newman, C. and J. Myers, “ACAP -- Application Configuration Access Protocol,” RFC 2244, November 1997.
[ASN.1] CCITT, “Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1),” 1988.
[DNSSEC] Eastlake, D., “Domain Name System Security Extensions,” RFC 2535, March 1999.
[DNS-TXT] Rosenbaum, R., “Using the Domain Name System To Store Arbitrary String Attributes,” RFC 1464, May 1993.
[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” RFC 2616, June 1999 (TXT, PS, PDF, HTML, XML).
[IMAP] Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” RFC 3501, March 2003.
[IMP-REQS] Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” RFC 2779, February 2000.
[IRI] Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” RFC 3987, January 2005.
[LINKLOCAL] Cheshire, S., Aboba, B., and E. Guttman, “Dynamic Configuration of IPv4 Link-Local Addresses,” RFC 3927, May 2005.
[MAILBOXES] Crocker, D., “MAILBOX NAMES FOR COMMON SERVICES, ROLES AND FUNCTIONS,” RFC 2142, May 1997 (TXT, HTML, XML).
[POP3] Myers, J. and M. Rose, “Post Office Protocol - Version 3,” STD 53, RFC 1939, May 1996.
[SMTP] Klensin, J., “Simple Mail Transfer Protocol,” RFC 2821, April 2001.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” STD 66, RFC 3986, January 2005 (TXT, HTML, XML).
[URN-OID] Mealling, M., “A URN Namespace of Object Identifiers,” RFC 3061, February 2001.
[USINGTLS] Newman, C., “Using TLS with IMAP, POP3 and ACAP,” RFC 2595, June 1999.
[XEP-0045] Saint-Andre, P., “Multi-User Chat,” XSF XEP 0045, September 2006.
[XEP-0071] Saint-Andre, P., “XHTML-IM,” XSF XEP 0071, January 2006.
[XEP-0077] Saint-Andre, P., “In-Band Registration,” XSF XEP 0077, January 2006.
[XEP-0086] Norris, R. and P. Saint-Andre, “Error Condition Mappings,” XSF XEP 0086, February 2004.
[XEP-0124] Paterson, I., Smith, D., and P. Saint-Andre, “HTTP Binding,” XSF XEP 0124, April 2006.
[XEP-0156] Hildebrand, J. and P. Saint-Andre, “A DNS TXT Resource Record Format for XMPP Connection Methods,” XSF XEP 0156, May 2005.
[XEP-0157] Saint-Andre, P. and J. Konieczny, “Contact Addresses for XMPP Services,” XSF XEP 0157, January 2007.
[XEP-0174] Saint-Andre, P., “Link-Local Messaging,” XSF XEP 0174, December 2006.
[XEP-0175] Saint-Andre, P., “Best Practices for Use of SASL ANONYMOUS,” XSF XEP 0175, September 2006.
[XEP-0178] Saint-Andre, P. and P. Millard, “Best Practices for Use of SASL EXTERNAL,” XSF XEP 0178, January 2007.
[XML-REG] Mealling, M., “The IETF XML Registry,” BCP 81, RFC 3688, January 2004.
[XMPP-IM] Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” RFC 3921, October 2004.
[XMPP-URI] Saint-Andre, P., “Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP),” RFC 4622, August 2006.


 TOC 

Appendix A.  Nodeprep



 TOC 

A.1.  Introduction

This appendix defines the "Nodeprep" profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.). As such, it specifies processing rules that will enable users to enter internationalized node identifiers in the Extensible Messaging and Presence Protocol (XMPP) and have the highest chance of getting the content of the strings correct. (An XMPP node identifier is the optional portion of an XMPP address that precedes a domain identifier and the '@' separator; it is often but not exclusively associated with an instant messaging username.) These processing rules are intended only for XMPP node identifiers and are not intended for arbitrary text or any other aspect of an XMPP address.

This profile defines the following, as required by [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.):



 TOC 

A.2.  Character Repertoire

This profile uses Unicode 3.2 with the list of unassigned code points being Table A.1, both defined in Appendix A of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).



 TOC 

A.3.  Mapping

This profile specifies mapping using the following tables from [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.):

Table B.1
Table B.2


 TOC 

A.4.  Normalization

This profile specifies the use of Unicode normalization form KC, as described in [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).



 TOC 

A.5.  Prohibited Output

This profile specifies the prohibition of using the following tables from [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).

Table C.1.1
Table C.1.2
Table C.2.1
Table C.2.2
Table C.3
Table C.4
Table C.5
Table C.6
Table C.7
Table C.8
Table C.9

In addition, the following Unicode characters are also prohibited:

#x22 (")
#x26 (&)
#x27 (')
#x2F (/)
#x3A (:)
#x3C (<)
#x3E (>)
#x40 (@)


 TOC 

A.6.  Bidirectional Characters

This profile specifies checking bidirectional strings, as described in Section 6 of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).



 TOC 

Appendix B.  Resourceprep



 TOC 

B.1.  Introduction

This appendix defines the "Resourceprep" profile of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.). As such, it specifies processing rules that will enable users to enter internationalized resource identifiers in the Extensible Messaging and Presence Protocol (XMPP) and have the highest chance of getting the content of the strings correct. (An XMPP resource identifier is the optional portion of an XMPP address that follows a domain identifier and the '/' separator.) These processing rules are intended only for XMPP resource identifiers and are not intended for arbitrary text or any other aspect of an XMPP address.

This profile defines the following, as required by [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.):



 TOC 

B.2.  Character Repertoire

This profile uses Unicode 3.2 with the list of unassigned code points being Table A.1, both defined in Appendix A of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).



 TOC 

B.3.  Mapping

This profile specifies mapping using the following tables from [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.):

Table B.1


 TOC 

B.4.  Normalization

This profile specifies the use of Unicode normalization form KC, as described in [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).



 TOC 

B.5.  Prohibited Output

This profile specifies the prohibition of using the following tables from [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).

Table C.1.2
Table C.2.1
Table C.2.2
Table C.3
Table C.4
Table C.5
Table C.6
Table C.7
Table C.8
Table C.9


 TOC 

B.6.  Bidirectional Characters

This profile specifies checking bidirectional strings, as described in Section 6 of [STRINGPREP] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.).



 TOC 

Appendix C.  Server Dialback



 TOC 

C.1.  Overview

Server dialback is a reverse DNS lookup method whose results are communicated over XML streams, thus making it more difficult to spoof XMPP server domains and XML stanzas sent over XML streams between servers. Server dialback is not a security mechanism, and results only in weak verification of server identities (see Server-to-Server Communications (Server-to-Server Communications) regarding this method's security characteristics). Domains requiring robust security SHOULD use TLS and SASL; see Server-to-Server Communications (Server-to-Server Communications) for details. If SASL is used for server-to-server authentication, dialback SHOULD NOT be used since it is unnecessary. Documentation of dialback is included mainly for the sake of backward-compatibility with existing implementations and deployments. However, depending on local policies, a service may wish to use dialback to provide weak identity verification in cases where SASL negotiation would not result in strong authentication (e.g., because the certificate presented by the peer service during TLS negotiation is self-signed and thus provides even weaker identity verification than DNS).

The server dialback method is made possible by the existence of the Domain Name System (DNS), since one server can (normally) discover the authoritative server for a given domain. Because dialback depends on DNS, inter-domain communications MUST NOT proceed until the Domain Name System (DNS) hostnames asserted by the servers have been resolved (see Server-to-Server Communications (Server-to-Server Communications)).

Server dialback is uni-directional, and results in weak identity verification for one stream in one direction. Because server dialback is not an authentication mechanism, mutual authentication is not possible via dialback. Therefore, server dialback MUST be completed in each direction in order to enable bi-directional communications between two domains.

The method for generating and verifying the keys used in server dialback MUST take into account the hostnames being used, the stream ID generated by the receiving server, and a secret known by the authoritative server's network; see Appendix C.5 (Dialback Key Generation) for the recommended algorithm.

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:



 TOC 

C.2.  Order of Events

The following is a brief summary of the order of events in dialback:

  1. The Originating Server establishes a connection to the Receiving Server.
  2. The Originating Server sends a 'key' value over the connection to the Receiving Server.
  3. The Receiving Server establishes a connection to the Authoritative Server.
  4. The Receiving Server sends the same 'key' value to the Authoritative Server.
  5. The Authoritative Server replies that key is valid or invalid.
  6. The Receiving Server informs the 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    |
    | ----------------------> |
    |                         |
    |   send stream header    |
    | <---------------------- |
    |                         |                   Authoritative
    |   send dialback key     |                       Server
    | ----------------------> |                   -------------
    |                         |                         |
                              |   establish connection  |
                              | ----------------------> |
                              |                         |
                              |   send stream header    |
                              | ----------------------> |
                              |                         |
                              |   send stream header    |
                              | <---------------------- |
                              |                         |
                              |   send verify request   |
                              | ----------------------> |
                              |                         |
                              |   send verify response  |
                              | <---------------------- |
                              |
    |  report dialback result |
    | <---------------------- |
    |                         |


 TOC 

C.3.  Protocol

This section describes the detailed protocol interaction between the Originating Server, the Receiving Server, and the Authoritative Server.

This section uses the following domain names, IP addresses, stream IDs, and shared secret in the examples:

To assist the reader, the following conventions are used to clarify the flow of packets:

The flow of events is as follows:

  1. The Originating Server (asserted to be "example.org") performs a DNS lookup for the Receiving Server (in accordance with the procedure described under Section 4 (TCP Binding)) and establishes a TCP connection to the Receiving Server at the IP address and port discovered during the DNS lookup (here assumed to be "192.0.2.0" and "5269").
  2. The Originating Server sends a stream header to the Receiving Server:
    O2R: <stream:stream
              xmlns='jabber:server'
              xmlns:db='jabber:server:dialback'
              xmlns:stream='http://etherx.jabber.org/streams'
              from='example.org'
              to='xmpp.example.com'>
    

    Note: The inclusion of the xmlns:db namespace declaration with the name shown indicates to the Receiving Server that the Originating Server supports server dialback. If any of the namespace names provided by the Originating Server is incorrect, then the Receiving Server MUST generate an <invalid-namespace/> stream error condition and terminate both the XML stream and the underlying TCP connection. If the value of the 'to' address provided by the Originating Server does not match a hostname serviced by the Receiving Server, then the Receiving Server MUST generate a <host-unknown/> stream error condition and terminate both the XML stream and the underlying TCP connection.

  3. The Receiving Server SHOULD send a stream header back to the Originating Server over the same TCP connection, including a unique ID for this interaction:
    R2O: <stream:stream
             xmlns='jabber:server'
             xmlns:db='jabber:server:dialback'
             xmlns:stream='http://etherx.jabber.org/streams'
             from='xmpp.example.com'
             id='D60000229F'
             to='example.org'>
    

    Note: The Receiving Server SHOULD reply but MAY silently terminate the XML stream and underlying TCP connection depending on local security policies; however, if the Receiving Server desires to proceed, it MUST send a stream header back to the Originating Server. If any of the namespace names provided by the Receiving Server is incorrect, then the Originating Server MUST generate an <invalid-namespace/> stream error condition and terminate both the XML stream and the underlying TCP connection. If the value of the 'to' address provided by the Receiving Server does not match a hostname serviced by the Originating Server, then the Originating Server MUST generate a <host-unknown/> stream error condition and terminate both the XML stream and the underlying TCP connection.

  4. The Receiving Server SHOULD also send stream features to the Originating Server, including the dialback feature as described under Appendix C.6 (Advertisement):
    R2O: <stream:features>
           <dialback xmlns='urn:xmpp:features:dialback'>
             <required/>
           </dialback>
         </stream:features>
    
  5. The Originating Server MUST then send a dialback key to the Receiving Server:
    O2R: <db:result
             to='xmpp.example.com'
             from='example.org'>
           37c69b1cf07a3f67c04a5ef5902fa5114f2c76fe4a2686482ba5b89323075643
         </db:result>
    

    If the value of the 'to' address provided by the Originating Server does not match a hostname serviced by the Receiving Server, then the Receiving Server MUST generate a <host-unknown/> stream error condition and terminate both the XML stream and the underlying TCP connection. The key generated by the Originating Server MUST be based in part on the value of the ID provided by the Receiving Server in the previous step (here "D60000229F"), and in part on a secret shared by the Originating Server and the Authoritative Server (here "s3cr3tf0rd14lb4ck"). Any verifiable method MAY be used to generate the key; however, the method specified under Appendix C.5 (Dialback Key Generation) is RECOMMENDED. The key is not examined by the Receiving Server, since the key is validated by the Authoritative Server.

  6. The Receiving Server performs a DNS lookup for the Authoritative Server (in accordance with the procedure described under Section 4 (TCP Binding)) and establishes a TCP connection to the Authoritative Server at the IP address and port discovered during the DNS lookup (here assumed to be "192.0.2.1" and "5269").
  7. The Receiving Server sends a stream header to the Authoritative Server:
    R2A: <stream:stream
             xmlns='jabber:server'
             xmlns:db='jabber:server:dialback'
             xmlns:stream='http://etherx.jabber.org/streams'
             from='xmpp.example.com'
             to='example.org'>
    

    Note: If the namespace name is incorrect, then the Authoritative Server MUST generate an <invalid-namespace/> stream error condition and terminate both the XML stream and the underlying TCP connection. If the value of the 'to' address provided by the Receiving Server does not match a hostname serviced by the Authoritative Server, then the Authoritative Server MUST generate a <host-unknown/> stream error condition and terminate both the XML stream and the underlying TCP connection.

  8. The Authoritative Server sends the Receiving Server a stream header:
    A2R: <stream:stream
             xmlns='jabber:server'
             xmlns:db='jabber:server:dialback'
             xmlns:stream='http://etherx.jabber.org/streams'
             from='example.org'
             id='F92200006D'
             to='xmpp.example.com'>
    

    Note: If any of the namespace names provided by the Authoritative Server is incorrect, then the Receiving Server MUST generate an <invalid-namespace/> stream error condition and terminate both the XML stream and the underlying TCP connection between it and the Authoritative Server. If the value of the 'to' address provided by the Authoritative Server does not match a hostname serviced by the Receiving Server, then the Receiving Server MUST generate a <host-unknown/> stream error condition and terminate both the XML stream and the underlying TCP connection. If a stream error occurs between the Receiving Server and the Authoritative Server, then the Receiving Server MUST not only terminate the XML stream and the underlying TCP connection between it and the Authoritative Server but also terminate the XML stream and the underlying TCP connection between it and the Originating Server, generating a <remote-connection-failed/> stream error for the latter stream.

  9. The Receiving Server sends the Authoritative Server a request for verification of a key:
    R2A: <db:verify
             from='xmpp.example.com'
             to='example.org'
             id='D60000229F'>
           37c69b1cf07a3f67c04a5ef5902fa5114f2c76fe4a2686482ba5b89323075643
         </db:verify>
    

    Note: Passed here are the hostnames of the Receiving Server ('from') and the Originating Server ('to'), the original identifier from the Receiving Server's stream header to the Originating Server in Step 3, and the key that the Originating Server sent to the Receiving Server in Step 5. Based on this information, as well as shared secret information within the Authoritative Server's network, the Authoritative Server determines whether the key is valid or invalid. If the value of the 'to' address does not match a hostname serviced by the Authoritative Server, then the Authoritative Server MUST generate a <host-unknown/> 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 sent by the Receiving Server in the 'from' address of the stream header it sent to the Authoritative Server in Step 7, then the Authoritative Server MUST generate an <invalid-from/> stream error condition and terminate both the XML stream and the underlying TCP connection.

  10. The Authoritative Server determines whether the key was valid or invalid and informs the Receiving Server of its determination:
    A2R: <db:verify
             from='example.org'
             to='xmpp.example.com'
             id='D60000229F'
             type='valid'/>
    
    or
    A2R: <db:verify
             from='example.org'
             to='xmpp.example.com'
             id='D60000229F'
             type='invalid'/>
    

    Note: If the ID does not match that provided by the Receiving Server in Step 3, then the Receiving Server MUST generate an <invalid-id/> 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 serviced by the Receiving Server, then the Receiving Server MUST generate a <host-unknown/> 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 the Originating Server in the 'from' address of the stream header it sent to the Receiving Server in Step 2, then the Receiving Server MUST generate an <invalid-from/> stream error condition and terminate both the XML stream and the underlying TCP connection. After returning the verification to the Receiving Server, the Authoritative Server SHOULD terminate the stream between them and the underlying TCP connection.

  11. The Receiving Server informs the Originating Server of the result:
    R2O: <db:result
             from='xmpp.example.com'
             to='example.org'
             id='D60000229F'
             type='valid'/>
    

    Note: At this point, the connection from the Originating Server to the Receiving Server has either been validated or reported as invalid. If the connection is invalid, then the Receiving Server MUST terminate both the XML stream and the underlying TCP connection between itself and the Originating Server. If the connection is valid, the Receiving Server has verified the identity of the Originating Server; as a result, the Originating Server may now send XML stanzas to the Receiving Server over the validated connection (i.e., over the "initial stream" from the Originating Server to the Receiving Server).

Until the initial stream has been validated, the Receiving Server MUST silently drop all XML stanzas sent by the Originating Server to the Receiving Server.

After successful dialback negotiation, the Receiving Server MUST verify that all XML stanzas received from the Originating Server include a 'from' attribute and a 'to' attribute; if a stanza does not meet this restriction, the Receiving Server MUST generate an <improper-addressing/> stream error condition and terminate both the XML stream and the underlying TCP connection. Furthermore, the Receiving Server MUST verify that the 'from' attribute of stanzas received from the Originating Server includes a domain name that has been validated for the stream; if a stanza does not meet this restriction, the Receiving Server MUST generate an <invalid-from/> 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.

As mentioned, server dialback results in weak identity verification in one direction only (in the foregoing text, verification of the Originating Server by the Receiving Server). In order to proceed with bi-directional communication so that the Receiving Server may sent XML stanzas to the Originating Server, the Receiving Server MUST now also initiate a dialback negotiation with the Originating Server.



 TOC 

C.4.  Reuse of Negotiated Connections

After the Receiving Server has validated the connection from the Originating Server, the Originating Server may wish to reuse that connection for validation of additional domains. One common motivation for such reuse is the existence of additional services associated with the Originating Server but hosted at subdomains of the Originating Server (the use of subdomains helps to ensure proper routing of XML stanzas to the hosted services). For example, the "example.org" XMPP server may host a groupchat service at "chat.example.org". In order to accept XML stanzas from rooms at "chat.example.org" intended for addresses at "xmpp.example.com", the "xmpp.example.com" will need to validate the "chat.example.org" domain (just as it already did for the "example.org" domain). Thus the "example.org" server would now initiate a dialback negotiation with "xmpp.example.com" but specify the Originating Server as "chat.example.org". Several optimizations are possible:

These optimizations effectively enable "piggybacking" of the previously negotiated connections.



 TOC 

C.5.  Dialback Key Generation

As mentioned, the dialback key is generated based on four different pieces of information:

The stream ID is security-critical in server dialback and therefore MUST be both unpredictable and non-repeating (see [RANDOM] (Eastlake, D., Crocker, S., and J. Schiller, “Randomness Recommendations for Security,” December 1994.) for recommendations regarding randomness for security purposes).

It is RECOMMENDED for the dialback key to be the hexadecimal representation of a Keyed-Hash Message Authentication Code (see [HMAC] (National Institute of Standards and Technology, “The Keyed-Hash Message Authentication Code (HMAC),” March 2002.)) that uses the SHA-256 algorithm (see [SHA] (National Institute of Standards and Technology, “Secure Hash Standard,” August 2002.)), as follows:

HMAC-SHA256
(
  SHA256(secret),
  {'Receiving Server' 'Originating Server' 'Stream ID'}
)

The shared secret SHOULD either be set up in a configuration option for each host or process within the Authoritative Server's network or generated as a random string when starting each host or process. The secret's length SHOULD be at least 128 bits or 16 characters long.

Consider the following scenario:

The resulting dialback key would be:

HMAC-SHA256
(
  SHA256(secret),
  {'Receiving Server' 'Originating Server' 'Stream ID'}
)

that is,

HMAC-SHA256
(SHA256('s3cr3tf0rd14lb4ck'),
{'xmpp.example.net',' ','example.org',' ','D60000229F'})

that is,

HMAC-SHA256
('a7136eb1f46c9ef18c5e78c36ca257067c69b3d518285f0b18a96c33beae9acc',
{'xmpp.example.com example.org D60000229F'})

that is,

37c69b1cf07a3f67c04a5ef5902fa5114f2c76fe4a2686482ba5b89323075643


 TOC 

C.6.  Advertisement

Support for the server dialback protocol can be indicated in two ways:

  1. By inclusion of the server dialback feature in a given set of stream features.
  2. By inclusion of the dialback namespace declaration in the stream header.

The former method is preferred, but the latter method is also specified herein for the purpose of backwards-compatibility with older "XMPP 0.9" deployments.

The server dialback stream feature is advertised by including in any given set of stream features a <dialback/> element qualified by the 'urn:xmpp:features:dialback' namespace; the <dialback/> element MAY also include an empty <required/> element, indicating that the entity sending the stream features requires dialback to be negotiated for the stream.

Server2 informs Server1 that it supports (and requires) server dialback:

<stream:features>
  <dialback xmlns='urn:xmpp:features:dialback'>
    <required/>
  </dialback>
</stream:features>

As mentioned, support for the server dialback protocol can also be advertised by including the dialback namespace declaration in a stream header.

<stream:stream
    xmlns='jabber:server'
    xmlns:db='jabber:server:dialback'
    xmlns:stream='http://etherx.jabber.org/streams'
    from='example.com'
    to='example.net'>

No matter which method is used, a service SHOULD advertise support for server dialback only at a point in the stream negotiation when it will accept negotiation of server dialback for that stream. For example, if a service wishes to be backwards-compatible with older "XMPP 0.9" deployments, it SHOULD include the server dialback namespace declaration in the initial stream header it sends to other servers (so that "XMPP 0.9" servers can proceed with dialback in the absence of TLS and SASL authentication). However, in the midst of stream negotiation with an XMPP 1.0 or higher server, a service SHOULD advertise the dialback stream feature only at a point when negotiation of server dialback is allowed in accordance with local service policies (e.g., after TLS negotiation in the case when a self-signed certificate was presented by the Originating Server and local service policies stipulate that it is preferable to weakly identify the Originating Server via server dialback rather than depend on the self-signed certificate for identity verification).



 TOC 

Appendix D.  XML Schemas

The following XML schemas are descriptive, not normative. For schemas defining the 'jabber:client' and 'jabber:server' namespaces, refer to [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.).



 TOC 

D.1.  Streams namespace

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='http://etherx.jabber.org/streams'
    xmlns='http://etherx.jabber.org/streams'
    elementFormDefault='unqualified'>

  <xs:import namespace='jabber:client'/>
  <xs:import namespace='jabber:server'/>
  <xs:import namespace='jabber:server:dialback'/>

  <xs:element name='stream'>
    <xs:complexType>
      <xs:sequence xmlns:client='jabber:client'
                   xmlns:server='jabber:server'
                   xmlns:db='jabber:server:dialback'>
        <xs:element ref='features' minOccurs='0' maxOccurs='1'/>
        <xs:any namespace='urn:ietf:params:xml:ns:xmpp-tls'
                minOccurs='0'
                maxOccurs='unbounded'/>
        <xs:any namespace='urn:ietf:params:xml:ns:xmpp-sasl'
                minOccurs='0'
                maxOccurs='unbounded'/>
        <xs:choice minOccurs='0' maxOccurs='1'>
          <xs:choice minOccurs='0' maxOccurs='unbounded'>
            <xs:element ref='client:message'/>
            <xs:element ref='client:presence'/>
            <xs:element ref='client:iq'/>
          </xs:choice>
          <xs:choice minOccurs='0' maxOccurs='unbounded'>
            <xs:element ref='server:message'/>
            <xs:element ref='server:presence'/>
            <xs:element ref='server:iq'/>
            <xs:element ref='db:result'/>
            <xs:element ref='db:verify'/>
          </xs:choice>
        </xs:choice>
        <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
      </xs:sequence>
      <xs:attribute name='from' type='xs:string' use='optional'/>
      <xs:attribute name='id' type='xs:NMTOKEN' use='optional'/>
      <xs:attribute name='to' type='xs:string' use='optional'/>
      <xs:attribute name='version' type='xs:decimal' use='optional'/>
      <xs:attribute ref='xml:lang' use='optional'/>
    </xs:complexType>
  </xs:element>

  <xs:element name='features'>
    <xs:complexType>
      <xs:any namespace='##other'/>
    </xs:complexType>
  </xs:element>

  <xs:element name='error'>
    <xs:complexType>
      <xs:sequence  xmlns:err='urn:ietf:params:xml:ns:xmpp-streams'>
        <xs:group   ref='err:streamErrorGroup'/>
        <xs:element ref='err:text'
                    minOccurs='0'
                    maxOccurs='1'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>


 TOC 

D.2.  Stream error namespace

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:ietf:params:xml:ns:xmpp-streams'
    xmlns='urn:ietf:params:xml:ns:xmpp-streams'
    elementFormDefault='qualified'>

  <xs:element name='bad-format' type='empty'/>
  <xs:element name='bad-namespace-prefix' type='empty'/>
  <xs:element name='conflict' type='empty'/>
  <xs:element name='connection-timeout' type='empty'/>
  <xs:element name='host-gone' type='empty'/>
  <xs:element name='host-unknown' type='empty'/>
  <xs:element name='improper-addressing' type='empty'/>
  <xs:element name='internal-server-error' type='empty'/>
  <xs:element name='invalid-from' type='empty'/>
  <xs:element name='invalid-id' type='empty'/>
  <xs:element name='invalid-namespace' type='empty'/>
  <xs:element name='invalid-xml' type='empty'/>
  <xs:element name='not-authorized' type='empty'/>
  <xs:element name='policy-violation' type='empty'/>
  <xs:element name='remote-connection-failed' type='empty'/>
  <xs:element name='resource-constraint' type='empty'/>
  <xs:element name='restricted-xml' type='empty'/>
  <xs:element name='see-other-host' type='xs:string'/>
  <xs:element name='system-shutdown' type='empty'/>
  <xs:element name='undefined-condition' type='empty'/>
  <xs:element name='unsupported-encoding' type='empty'/>
  <xs:element name='unsupported-stanza-type' type='empty'/>
  <xs:element name='unsupported-version' type='empty'/>
  <xs:element name='xml-not-well-formed' type='empty'/>

  <xs:group name='streamErrorGroup'>
    <xs:choice>
      <xs:element ref='bad-format'/>
      <xs:element ref='bad-namespace-prefix'/>
      <xs:element ref='conflict'/>
      <xs:element ref='connection-timeout'/>
      <xs:element ref='host-gone'/>
      <xs:element ref='host-unknown'/>
      <xs:element ref='improper-addressing'/>
      <xs:element ref='internal-server-error'/>
      <xs:element ref='invalid-from'/>
      <xs:element ref='invalid-id'/>
      <xs:element ref='invalid-namespace'/>
      <xs:element ref='invalid-xml'/>
      <xs:element ref='not-authorized'/>
      <xs:element ref='policy-violation'/>
      <xs:element ref='remote-connection-failed'/>
      <xs:element ref='resource-constraint'/>
      <xs:element ref='restricted-xml'/>
      <xs:element ref='see-other-host'/>
      <xs:element ref='system-shutdown'/>
      <xs:element ref='undefined-condition'/>
      <xs:element ref='unsupported-encoding'/>
      <xs:element ref='unsupported-stanza-type'/>
      <xs:element ref='unsupported-version'/>
      <xs:element ref='xml-not-well-formed'/>
    </xs:choice>
  </xs:group>

  <xs:element name='text'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:string'>
          <xs:attribute ref='xml:lang' use='optional'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>


 TOC 

D.3.  TLS namespace

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:ietf:params:xml:ns:xmpp-tls'
    xmlns='urn:ietf:params:xml:ns:xmpp-tls'
    elementFormDefault='qualified'>

  <xs:element name='starttls'>
    <xs:complexType>
      <xs:sequence>
        <xs:element name='required'
                    minOccurs='0'
                    type='empty'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:element name='proceed' type='empty'/>
  <xs:element name='failure' type='empty'/>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>


 TOC 

D.4.  SASL namespace

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:ietf:params:xml:ns:xmpp-sasl'
    xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
    elementFormDefault='qualified'>

  <xs:element name='mechanisms'>
    <xs:complexType>
      <xs:sequence>
        <xs:element name='mechanism'
                    maxOccurs='unbounded'
                    type='xs:string'/>
        <xs:element name='required'
                    minOccurs='0'
                    type='empty'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:element name='auth'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:string'>
          <xs:attribute name='mechanism'
                        type='xs:string'
                        use='optional'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <xs:element name='challenge' type='xs:string'/>
  <xs:element name='response' type='xs:string'/>
  <xs:element name='abort' type='empty'/>
  <xs:element name='success' type='xs:string'/>

  <xs:element name='failure'>
    <xs:complexType>
      <xs:choice minOccurs='0'>
        <xs:element name='aborted' type='empty'/>
        <xs:element name='incorrect-encoding' type='empty'/>
        <xs:element name='invalid-authzid' type='empty'/>
        <xs:element name='invalid-mechanism' type='empty'/>
        <xs:element name='malformed-request' type='empty'/>
        <xs:element name='mechanism-too-weak' type='empty'/>
        <xs:element name='not-authorized' type='empty'/>
        <xs:element name='temporary-auth-failure' type='empty'/>
      </xs:choice>
    </xs:complexType>
  </xs:element>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>


 TOC 

D.5.  Resource binding namespace

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:ietf:params:xml:ns:xmpp-bind'
    xmlns='urn:ietf:params:xml:ns:xmpp-bind'
    elementFormDefault='qualified'>

  <xs:element name='bind'>
    <xs:complexType>
      <xs:sequence>
        <xs:choice minOccurs='0' maxOccurs='1'>
          <xs:element name='resource' type='resourceType'/>
          <xs:element name='jid' type='fullJIDType'/>
        </xs:choice>
        <xs:element name='required'
                    minOccurs='0'
                    maxOccurs='1'
                    type='empty'/>
        </xs:choice>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:element name='unbind'>
    <xs:complexType>
      <xs:sequence minOccurs='0'>
        <xs:element name='resource' type='resourceType'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:simpleType name='resourceType'>
    <xs:restriction base='xs:string'>
      <xs:minLength value='1'/>
      <xs:maxLength value='1023'/>
    </xs:restriction>
  </xs:simpleType>

  <xs:simpleType name='fullJIDType'>
    <xs:restriction base='xs:string'>
      <xs:minLength value='8'/>
      <xs:maxLength value='3071'/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>


 TOC 

D.6.  Dialback namespace

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='jabber:server:dialback'
    xmlns='jabber:server:dialback'
    elementFormDefault='qualified'>

  <xs:element name='result'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:token'>
          <xs:attribute name='from' type='xs:string' use='required'/>
          <xs:attribute name='to' type='xs:string' use='required'/>
          <xs:attribute name='type' use='optional'>
            <xs:simpleType>
              <xs:restriction base='xs:NCName'>
                <xs:enumeration value='invalid'/>
                <xs:enumeration value='valid'/>
              </xs:restriction>
            </xs:simpleType>
          </xs:attribute>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <xs:element name='verify'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:token'>
          <xs:attribute name='from' type='xs:string' use='required'/>
          <xs:attribute name='id' type='xs:NMTOKEN' use='required'/>
          <xs:attribute name='to' type='xs:string' use='required'/>
          <xs:attribute name='type' use='optional'>
            <xs:simpleType>
              <xs:restriction base='xs:NCName'>
                <xs:enumeration value='invalid'/>
                <xs:enumeration value='valid'/>
              </xs:restriction>
            </xs:simpleType>
          </xs:attribute>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

</xs:schema>


 TOC 

D.7.  Server dialback stream feature namespace

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:features:dialback'
    xmlns='urn:xmpp:features:dialback'
    elementFormDefault='qualified'>

  <xs:element name='dialback'>
    <xs:complexType>
      <xs:sequence>
        <xs:element name='required'
                    minOccurs='0'
                    maxOccurs='1'
                    type='empty'/>
      </xs:sequence>
    </xs:complexType>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>


 TOC 

D.8.  Stanza error namespace

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:ietf:params:xml:ns:xmpp-stanzas'
    xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
    elementFormDefault='qualified'>

  <xs:element name='bad-request' type='empty'/>
  <xs:element name='conflict' type='empty'/>
  <xs:element name='feature-not-implemented' type='empty'/>
  <xs:element name='forbidden' type='empty'/>
  <xs:element name='gone' type='xs:string'/>
  <xs:element name='internal-server-error' type='empty'/>
  <xs:element name='item-not-found' type='empty'/>
  <xs:element name='jid-malformed' type='empty'/>
  <xs:element name='not-acceptable' type='empty'/>
  <xs:element name='not-allowed' type='empty'/>
  <xs:element name='not-modified' type='empty'/>
  <xs:element name='payment-required' type='empty'/>
  <xs:element name='recipient-unavailable' type='empty'/>
  <xs:element name='redirect' type='xs:string'/>
  <xs:element name='registration-required' type='empty'/>
  <xs:element name='remote-server-not-found' type='empty'/>
  <xs:element name='remote-server-timeout' type='empty'/>
  <xs:element name='resource-constraint' type='empty'/>
  <xs:element name='service-unavailable' type='empty'/>
  <xs:element name='subscription-required' type='empty'/>
  <xs:element name='undefined-condition' type='empty'/>
  <xs:element name='unexpected-request' type='empty'/>

  <xs:group name='stanzaErrorGroup'>
    <xs:choice>
      <xs:element ref='bad-request'/>
      <xs:element ref='conflict'/>
      <xs:element ref='feature-not-implemented'/>
      <xs:element ref='forbidden'/>
      <xs:element ref='gone'/>
      <xs:element ref='internal-server-error'/>
      <xs:element ref='item-not-found'/>
      <xs:element ref='jid-malformed'/>
      <xs:element ref='not-acceptable'/>
      <xs:element ref='not-allowed'/>
      <xs:element ref='not-modified'/>
      <xs:element ref='payment-required'/>
      <xs:element ref='recipient-unavailable'/>
      <xs:element ref='redirect'/>
      <xs:element ref='registration-required'/>
      <xs:element ref='remote-server-not-found'/>
      <xs:element ref='remote-server-timeout'/>
      <xs:element ref='resource-constraint'/>
      <xs:element ref='service-unavailable'/>
      <xs:element ref='subscription-required'/>
      <xs:element ref='undefined-condition'/>
      <xs:element ref='unexpected-request'/>
    </xs:choice>
  </xs:group>

  <xs:element name='text'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:string'>
          <xs:attribute ref='xml:lang' use='optional'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>


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Appendix E.  Contact Addresses

Consistent with [MAILBOXES] (Crocker, D., “MAILBOX NAMES FOR COMMON SERVICES, ROLES AND FUNCTIONS,” May 1997.), an organization that offers an XMPP service SHOULD provide an Internet mailbox of "XMPP" for inquiries related to that service, where the host portion of the resulting mailto URI SHOULD be the organization's domain, not necessarily the domain of the XMPP service itself (e.g., the XMPP service might be offered at im.example.net but the Internet mailbox should be <xmpp@example.net>).

In addition, the XMPP service SHOULD provide a way to discover the XMPP contact address(es) of the service administrator(s), as specified in [XEP‑0157] (Saint-Andre, P. and J. Konieczny, “Contact Addresses for XMPP Services,” January 2007.).



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Appendix F.  Differences From RFC 3920

Based on consensus derived from implementation and deployment experience as well as formal interoperability testing, the following modifications were made from RFC 3920. In addition, several other changes were made to more clearly specify and explain the protocols.



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Author's Address

  Peter Saint-Andre (editor)
  XMPP Standards Foundation
Email:  stpeter@jabber.org
URI:  xmpp:stpeter@jabber.org


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Full Copyright Statement

Intellectual Property

Acknowledgment