Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2004).

Abstract

This memo defines the core features of the Extensible Messaging and Presence Protocol (XMPP), a protocol for streaming Extensible Markup Language (XML) elements in order to exchange structured information in close to real time between any two network endpoints. While XMPP provides a generalized, extensible framework for exchanging XML data, it is used mainly for the purpose of building instant messaging and presence applications that meet the requirements of RFC 2779.

Table of Contents

1.  Introduction
    1.1  Overview
    1.2  Terminology
2.  Generalized Architecture
    2.1  Overview
    2.2  Server
    2.3  Client
    2.4  Gateway
    2.5  Network
3.  Addressing Scheme
    3.1  Overview
    3.2  Domain Identifier
    3.3  Node Identifier
    3.4  Resource Identifier
    3.5  Determination of Addresses
4.  XML Streams
    4.1  Overview
    4.2  Binding to TCP
    4.3  Stream Security
    4.4  Stream Attributes
    4.5  Namespace Declarations
    4.6  Stream Features
    4.7  Stream Errors
    4.8  Simplified Stream Examples
5.  Use of TLS
    5.1  Overview
    5.2  Narrative
    5.3  Client-to-Server Example
    5.4  Server-to-Server Example
6.  Use of SASL
    6.1  Overview
    6.2  Narrative
    6.3  SASL Definition
    6.4  SASL Errors
    6.5  Client-to-Server Example
    6.6  Server-to-Server Example
7.  Resource Binding
8.  Server Dialback
    8.1  Overview
    8.2  Order of Events
    8.3  Protocol
9.  XML Stanzas
    9.1  Common Attributes
    9.2  Basic Semantics
    9.3  Stanza Errors
10.  Server Rules for Handling XML Stanzas
    10.1  No 'to' Address
    10.2  Foreign Domain
    10.3  Subdomain
    10.4  Mere Domain or Specific Resource
    10.5  Node in Same Domain
11.  XML Usage within XMPP
    11.1  Restrictions
    11.2  XML Namespace Names and Prefixes
    11.3  Validation
    11.4  Inclusion of Text Declaration
    11.5  Character Encoding
12.  Core Compliance Requirements
    12.1  Servers
    12.2  Clients
13.  Internationalization Considerations
14.  Security Considerations
    14.1  High Security
    14.2  Certificate Validation
    14.3  Client-to-Server Communications
    14.4  Server-to-Server Communications
    14.5  Order of Layers
    14.6  Lack of SASL Channel Binding to TLS
    14.7  Mandatory-to-Implement Technologies
    14.8  Firewalls
    14.9  Use of base64 in SASL
    14.10  Stringprep Profiles
15.  IANA Considerations
    15.1  XML Namespace Name for TLS Data
    15.2  XML Namespace Name for SASL Data
    15.3  XML Namespace Name for Stream Errors
    15.4  XML Namespace Name for Resource Binding
    15.5  XML Namespace Name for Stanza Errors
    15.6  Nodeprep Profile of Stringprep
    15.7  Resourceprep Profile of Stringprep
    15.8  GSSAPI Service Name
    15.9  Port Numbers
16.  References
    16.1  Normative References
    16.2  Informative References
§  Author's Address
A.  Nodeprep
    A.1  Introduction
    A.2  Character Repertoire
    A.3  Mapping
    A.4  Normalization
    A.5  Prohibited Output
    A.6  Bidirectional Characters
B.  Resourceprep
    B.1  Introduction
    B.2  Character Repertoire
    B.3  Mapping
    B.4  Normalization
    B.5  Prohibited Output
    B.6  Bidirectional Characters
C.  XML Schemas
    C.1  Streams namespace
    C.2  Stream error namespace
    C.3  TLS namespace
    C.4  SASL namespace
    C.5  Resource binding namespace
    C.6  Dialback namespace
    C.7  Stanza error namespace
D.  Differences Between Core Jabber Protocols and XMPP
    D.1  Channel Encryption
    D.2  Authentication
    D.3  Resource Binding
    D.4  JID Processing
    D.5  Error Handling
    D.6  Internationalization
    D.7  Stream Version Attribute
E.  Contributors
F.  Acknowledgements
§  Intellectual Property and Copyright Statements

1. Introduction

1.1 Overview

The Extensible Messaging and Presence Protocol (XMPP) is an open Extensible Markup Language XMLBray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler, Extensible Markup Language (XML) 1.0 (2nd ed), October 2000.[XML] protocol for near-real-time messaging, presence, and request-response services. The basic syntax and semantics were developed originally within the Jabber open-source community, mainly in 1999. In 2002, the XMPP WG was chartered with developing an adaptation of the Jabber protocol that would be suitable as an IETF instant messaging (IM) and presence technology. As a result of work by the XMPP WG, the current memo defines the core features of XMPP 1.0; the extensions required to provide the instant messaging and presence functionality defined in RFC 2779Day, M., Aggarwal, S. and J. Vincent, Instant Messaging / Presence Protocol Requirements, February 2000.[IMP-REQS] are specified in Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, October 2004..

1.2 Terminology

The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119Bradner, S., Key words for use in RFCs to Indicate Requirement Levels, March 1997.[TERMS].

2. Generalized Architecture

2.1 Overview

Although XMPP is not wedded to any specific network architecture, to date it usually has been implemented via a client-server architecture wherein a client utilizing XMPP accesses a server over a [TCP]Postel, J., Transmission Control Protocol, September 1981. connection, and servers also communicate with each other over TCP connections.

The following diagram provides a high-level overview of this architecture (where "-" represents communications that use XMPP and "=" represents communications that use any other protocol).

C1----S1---S2---C3
      |
C2----+--G1===FN1===FC1

The symbols are as follows:

• C1, C2, C3 = XMPP clients
• S1, S2 = XMPP servers
• G1 = A gateway that translates between XMPP and the protocol(s) used on a foreign (non-XMPP) messaging network
• FN1 = A foreign messaging network
• FC1 = A client on a foreign messaging network

2.2 Server

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

• to manage connections from or sessions for other entities, in the form of XML streamsXML Streams to and from authorized clients, servers, and other entities
• to route appropriately-addressed XML stanzasXML Stanzas among such entities over XML streams

Most XMPP-compliant servers also assume responsibility for the storage of data that is used by clients (e.g., contact lists for users of XMPP-based 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.

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 Addressing SchemeAddressing Scheme. The RECOMMENDED port for connections between a client and a server is 5222, as registered with the IANA (see Port NumbersPort Numbers).

2.4 Gateway

A gateway is a special-purpose server-side service whose primary function is to translate XMPP into the protocol used by a foreign (non-XMPP) messaging system, as well as to translate the return data back into XMPP. Examples are gateways to email (see [SMTP]Klensin, J., Simple Mail Transfer Protocol, April 2001.), Internet Relay Chat (see [IRC]Oikarinen, J. and D. Reed, Internet Relay Chat Protocol, May 1993.), SIMPLE (see [SIMPLE]SIMPLE Working Group, SIMPLE WG, .), Short Message Service (SMS), and legacy instant messaging services such as AIM, ICQ, MSN Messenger, and Yahoo! Instant Messenger. Communications between gateways and servers, and between gateways and the foreign messaging system, are not defined in this document.

2.5 Network

Because each server is identified by a network address and because server-to-server communications are a straightforward extension of the client-to-server protocol, in practice, the system consists of a network of servers that inter-communicate. Thus, 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 NumbersPort Numbers).

3. Addressing Scheme

3.1 Overview

An entity is anything that can be considered a network endpoint (i.e., an ID on the network) and that can communicate using XMPP. All such entities are uniquely addressable in a form that is consistent with RFC 2396 [URI]Berners-Lee, T., Fielding, R. and L. Masinter, Uniform Resource Identifiers (URI): Generic Syntax, August 1998.. 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 below using the Augmented Backus-Naur Form as defined in [ABNF]Crocker, D. and P. Overell, Augmented BNF for Syntax Specifications: ABNF, November 1997.. (The IPv4address and IPv6address rules are defined in Appendix B of [IPv6]Hinden, R. and S. Deering, Internet Protocol Version 6 (IPv6) Addressing Architecture, April 2003.; the allowable character sequences that conform to the node rule are defined by the Nodeprep profile of [STRINGPREP]Hoffman, P. and M. Blanchet, Preparation of Internationalized Strings ("stringprep"), December 2002. as documented in Appendix A of this memo; the allowable character sequences that conform to the resource rule are defined by the Resourceprep profile of [STRINGPREP]Hoffman, P. and M. Blanchet, Preparation of Internationalized Strings ("stringprep"), December 2002. as documented in Appendix B of this memo; and the sub-domain rule makes reference to the concept of an internationalized domain label as described in [IDNA]Faltstrom, P., Hoffman, P. and A. Costello, Internationalizing Domain Names in Applications (IDNA), March 2003..)

   jid             = [ node "@" ] domain [ "/" resource ]
   domain          = fqdn / address-literal
   fqdn            = (sub-domain 1*("." sub-domain))
   sub-domain      = (internationalized domain label)
   address-literal = IPv4address / IPv6address

All JIDs are based on the foregoing structure. The most common use of this structure is to identify an instant messaging user, the server to which the user connects, and the user's connected resource (e.g., a specific client) in the form of <user@host/resource>. However, node types other than clients are possible; for example, a specific chat room offered by a multi-user chat service could be addressed as <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.

3.2 Domain Identifier

The domain identifier is the primary identifier and is the only REQUIRED element of a JID (a mere domain identifier is a valid JID). It usually represents the network gateway or "primary" server to which other entities connect for XML routing and data management capabilities. However, the entity referenced by a domain identifier is not always a server, and may be a service that is addressed as a subdomain of a server that provides functionality above and beyond the capabilities of a server (e.g., a multi-user chat service, a user directory, or a gateway to a foreign messaging system).

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 NameprepHoffman, P. and M. Blanchet, Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN), March 2003.[NAMEPREP] profile of stringprepHoffman, P. and M. Blanchet, Preparation of Internationalized Strings ("stringprep"), December 2002.[STRINGPREP] 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.

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 the server or gateway (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. Before comparing two node identifiers, a server MUST (and a client SHOULD) first apply the Nodeprep profile to each identifier.

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> or a mere <domain> address. It usually represents a specific session, connection (e.g., a device or location), or object (e.g., a participant in a multi-user chat room) belonging to the entity associated with a node identifier. A resource identifier is opaque to both servers and other clients, and is typically defined by a client implementation when it provides the information necessary to complete Resource BindingResource Binding (although it may be generated by a server on behalf of a client), after which it is referred to as a "connected 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. Before comparing two resource identifiers, a server MUST (and a client SHOULD) first apply the Resourceprep profile to each identifier.

3.5 Determination of Addresses

After SASL negotiationUse of SASL and, if appropriate, Resource BindingResource 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 the Simple Authentication and Security Layer (SASL) specificationMyers, J., Simple Authentication and Security Layer (SASL), October 1997.[SASL], if no authorization identity was specified during SASL negotiationUse of SASL.

For client-to-server communications, the "bare JID" (<node@domain>) SHOULD be the authorization identity, derived from the authentication identity, as defined in [SASL]Myers, J., Simple Authentication and Security Layer (SASL), October 1997., if no authorization identity was specified during SASL negotiationUse of SASL; the resource identifier portion of the "full JID" (<node@domain/resource>) SHOULD be the resource identifier negotiated by the client and server during Resource BindingResource 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.

4. XML Streams

4.1 Overview

Two fundamental concepts make possible the rapid, asynchronous exchange of relatively small payloads of structured information between presence-aware entities: XML streams and XML stanzas. These terms 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 negotiate Use of TLSUse of TLS or Use of SASLUse of SASL) or XML stanzas (as defined herein, <message/>, <presence/>, or <iq/> elements qualified by the default namespace). 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 "session" with the receiving entity. The initial stream enables unidirectional communication from the initiating entity to the receiving entity; in order to enable information exchange from the receiving entity to the initiating entity, the receiving entity MUST negotiate a stream in the opposite direction (the "response stream").

Definition of XML Stanza:

An XML stanza is a discrete semantic unit of structured information that is sent from one entity to another over an XML stream. 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>). An XML stanza MAY contain child elements (with accompanying attributes, elements, and XML character data) as necessary in order to convey the desired information. 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 StanzasXML Stanzas; an XML element sent for the purpose of Transport Layer Security (TLS) negotiationUse of TLS, Simple Authentication and Security Layer (SASL) negotiationUse of SASL, or server dialbackServer Dialback is not considered to be an XML stanza.

Consider the example of a client's session with a server. In order to connect to a server, a client MUST initiate an XML stream by sending an opening <stream> tag to the server, optionally preceded by a text declaration specifying the XML version and the character encoding supported (see Inclusion of Text DeclarationInclusion of Text Declaration; see also Character EncodingCharacter 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 negotiationUse of SASL, 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 (alternatively, the stream may be closed by the server), after which both the client and server SHOULD terminate the underlying connection (usually a TCP connection) as well.

Those who are accustomed to thinking of XML in a document-centric manner may wish to view a client's session with a server as consisting of two open-ended XML documents: one from the client to the server and one from the server to the client. From this perspective, the root <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 session. 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>          |
|--------------------|

4.2 Binding to TCP

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 mechanism such as polling over [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.), this specification defines a binding of XMPP to TCP only. In the context of client-to-server communications, a server MUST allow a client to share a single TCP connection for XML stanzas sent from client to server and from server to client. In the context of server-to-server communications, a server MUST use one TCP connection for XML stanzas sent from the server to the peer and another TCP connection (initiated by the peer) for stanzas from the peer to the server, for a total of two TCP connections.

4.3 Stream Security

When negotiating XML streams in XMPP 1.0, TLS SHOULD be used as defined under Use of TLSUse of TLS and SASL MUST be used as defined under Use of SASLUse of SASL. The "initial stream" (i.e., the stream from the initiating entity to the receiving entity) and the "response stream" (i.e., the stream from the receiving entity to the initiating entity) 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 StanzasXML 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 scoped by the default namespace) and not to XML elements used for stream negotiation (e.g., elements used to negotiate Use of TLSUse of TLS or Use of SASLUse of SASL).

4.4 Stream Attributes

The attributes of the stream element are as follows:

• to -- The 'to' attribute SHOULD be used only in the XML stream header from the initiating entity to the receiving entity, and MUST be set to a hostname serviced by the receiving entity. There SHOULD NOT be a 'to' attribute set in the XML stream header by which the receiving entity replies to the initiating entity; however, if a 'to' attribute is included, it SHOULD be silently ignored by the initiating entity.
• from -- The 'from' attribute SHOULD be used only in the XML stream header from the receiving entity to the initiating entity, and MUST be set to a hostname serviced by the receiving entity that is granting access to the initiating entity. There SHOULD NOT be a 'from' attribute on the XML stream header sent from the initiating entity to the receiving entity; however, if a 'from' attribute is included, it SHOULD be silently ignored by the receiving entity.
• id -- The 'id' attribute SHOULD be used only in the XML stream header from the receiving entity to the initiating entity. This attribute is a unique identifier created by the receiving entity to function as a session key for the initiating entity's streams with the receiving entity, and MUST be unique within the receiving application (normally a server). Note well that the stream ID may be security-critical and therefore MUST be both unpredictable and nonrepeating (see [RANDOM]Eastlake, D., Crocker, S. and J. Schiller, Randomness Recommendations for Security, December 1994. for recommendations regarding randomness for security purposes). There SHOULD NOT be an 'id' attribute on the XML stream header sent from the initiating entity to the receiving entity; however, if an 'id' attribute is included, it SHOULD be silently ignored by the receiving entity.
• xml:lang -- 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.) SHOULD be included by the initiating entity on the header for the initial stream to specify the default language of any human-readable XML character data it sends over that stream. If the attribute is included, the receiving entity SHOULD remember that value as the default for both the initial stream and the response stream; if the attribute is not included, the receiving entity SHOULD use a configurable default value for both streams, which it MUST communicate in the header for the response stream. For all stanzas sent over the initial stream, if the initiating entity does not include an 'xml:lang' attribute, the receiving entity SHOULD apply the default value; if the initiating entity does include an 'xml:lang' attribute, the receiving entity MUST NOT modify or delete it (see also xml:langxml:lang). The value of the 'xml:lang' attribute MUST be an NMTOKEN (as defined in Section 2.3 of [XML]Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler, Extensible Markup Language (XML) 1.0 (2nd ed), October 2000.) and MUST conform to the format defined in RFC 3066Alvestrand, H., Tags for the Identification of Languages, January 2001.[LANGTAGS].
• version -- The presence of the version attribute set to a value of at least "1.0" signals support for the stream-related protocols (including stream features) defined in this specification. Detailed rules regarding the generation and handling of this attribute are defined below.

We can summarize as follows:

         |  initiating to receiving  |  receiving to initiating
---------+---------------------------+-----------------------
to       |  hostname of receiver     |  silently ignored
from     |  silently ignored         |  hostname of receiver
id       |  silently ignored         |  session key
xml:lang |  default language         |  default language
version  |  signals XMPP 1.0 support |  signals XMPP 1.0 support

4.4.1 Version Support

The version of XMPP specified herein is "1.0"; in particular, this encapsulates the stream-related protocols (Use of TLSUse of TLS, Use of SASLUse of SASL, and Stream ErrorsStream 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 "0.0" and SHOULD NOT include a 'version' attribute in the stream header it sends in reply.

4.5 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 namespaces specificationBray, T., Hollander, D. and A. Layman, Namespaces in XML, January 1999.[XML-NAMES]). For detailed information regarding the streams namespace and default namespace, see Namespace Names and PrefixesXML Namespace Names and Prefixes.

4.6 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 Use of TLSUse of TLS, Use of SASLUse of SASL, and Resource BindingResource Binding as defined herein, and for Session Establishment as defined in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, October 2004.; however, the stream features functionality could be used to advertise other negotiable features in the future. 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.

4.7 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.

4.7.1 Rules

The following rules apply to stream-level errors:

• It is assumed that all stream-level errors are unrecoverable; therefore, if an error occurs at the level of the stream, the entity that detects the error MUST send a stream error to the other entity, send a closing </stream> tag, and terminate the underlying TCP connection.
• If the error occurs while the stream is being set up, the receiving entity MUST still send the opening <stream> tag, include the <error/> element as a child of the stream element, send the closing </stream> tag, and terminate the underlying TCP connection. In this case, if the initiating entity provides an unknown host in the 'to' attribute (or provides no 'to' attribute at all), the server SHOULD provide the server's authoritative hostname in the 'from' attribute of the stream header sent before termination.

4.7.2 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:

• MUST contain a child element corresponding to one of the defined stanza error conditions defined below; this element MUST be qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace
• MAY contain a <text/> child containing XML character data that describes the error in more detail; this element MUST be qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace and SHOULD possess an 'xml:lang' attribute specifying the natural language of the XML character data
• MAY contain a child element for an application-specific error condition; this element MUST be qualified by an application-defined namespace, and its structure is defined by that namespace

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).

4.7.3 Defined Conditions

The following stream-level error conditions are defined:

• <bad-format/> -- the entity has sent XML that cannot be processed; this error MAY be used instead of the more specific XML-related errors, such as <bad-namespace-prefix/>, <invalid-xml/>, <restricted-xml/>, <unsupported-encoding/>, and <xml-not-well-formed/>, although the more specific errors are preferred.
• <bad-namespace-prefix/> -- the entity has sent a namespace prefix that is unsupported, or has sent no namespace prefix on an element that requires such a prefix (see XML Namespace Names and PrefixesXML Namespace Names and Prefixes).
• <conflict/> -- the server is closing the active stream for this entity because a new stream has been initiated that conflicts with the existing stream.
• <connection-timeout/> -- the entity has not generated any traffic over the stream for some period of time (configurable according to a local service policy).
• <host-gone/> -- the value of the 'to' attribute provided by the initiating entity in the stream header corresponds to a hostname that is no longer hosted by the server.
• <host-unknown/> -- the value of the 'to' attribute provided by the initiating entity in the stream header does not correspond to a hostname that is hosted by the server.
• <improper-addressing/> -- a stanza sent between two servers lacks a 'to' or 'from' attribute (or the attribute has no value).
• <internal-server-error/> -- the server has experienced a misconfiguration or an otherwise-undefined internal error that prevents it from servicing the stream.
• <invalid-from/> -- the JID or hostname provided in a 'from' address does not match an authorized JID or validated domain negotiated between servers via SASL or dialback, or between a client and a server via authentication and resource binding.
• <invalid-id/> -- the stream ID or dialback ID is invalid or does not match an ID previously provided.
• <invalid-namespace/> -- the streams namespace name is something other than "http://etherx.jabber.org/streams" or the dialback namespace name is something other than "jabber:server:dialback" (see XML Namespace Names and PrefixesXML Namespace Names and Prefixes).
• <invalid-xml/> -- the entity has sent invalid XML over the stream to a server that performs validation (see ValidationValidation).
• <not-authorized/> -- the entity has attempted to send data before the stream has been authenticated, or otherwise is not authorized to perform an action related to stream negotiation; the receiving entity MUST NOT process the offending stanza before sending the stream error.
• <policy-violation/> -- the entity has violated some local service policy; the server MAY choose to specify the policy in the <text/> element or an application-specific condition element.
• <remote-connection-failed/> -- the server is unable to properly connect to a remote entity that is required for authentication or authorization.
• <resource-constraint/> -- the server lacks the system resources necessary to service the stream.
• <restricted-xml/> -- the entity has attempted to send restricted XML features such as a comment, processing instruction, DTD, entity reference, or unescaped character (see RestrictionsRestrictions).
• <see-other-host/> -- the server will not provide service to the initiating entity but is redirecting traffic to another host; the server SHOULD specify the alternate hostname or IP address (which MUST be a valid domain identifier) as the XML character data of the <see-other-host/> element.
• <system-shutdown/> -- the server is being shut down and all active streams are being closed.
• <undefined-condition/> -- the error condition is not one of those defined by the other conditions in this list; this error condition SHOULD be used only in conjunction with an application-specific condition.
• <unsupported-encoding/> -- the initiating entity has encoded the stream in an encoding that is not supported by the server (see Character EncodingCharacter Encoding).
• <unsupported-stanza-type/> -- the initiating entity has sent a first-level child of the stream that is not supported by the server.
• <unsupported-version/> -- the value of the 'version' attribute provided by the initiating entity in the stream header specifies a version of XMPP that is not supported by the server; the server MAY specify the version(s) it supports in the <text/> element.
• <xml-not-well-formed/> -- the initiating entity has sent XML that is not well-formed as defined by [XML]Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler, Extensible Markup Language (XML) 1.0 (2nd ed), October 2000..

4.7.4 Application-Specific Conditions

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

4.8 Simplified Stream Examples

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

A basic "session":

C: <?xml version='1.0'?>
   <stream:stream
       to='example.com'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       version='1.0'>
S: <?xml version='1.0'?>
   <stream:stream
       from='example.com'
       id='someid'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       version='1.0'>
...  encryption, authentication, and resource binding ...
C:   <message from='juliet@example.com'
              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'
              to='juliet@example.com'
              xml:lang='en'>
S:     <body>Neither, fair saint, if either thee dislike.</body>
S:   </message>
C: </stream:stream>
S: </stream:stream>

A "session" gone bad:

C: <?xml version='1.0'?>
   <stream:stream
       to='example.com'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       version='1.0'>
S: <?xml version='1.0'?>
   <stream:stream
       from='example.com'
       id='someid'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       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>

5. Use of TLS

5.1 Overview

XMPP includes a method for securing the stream from tampering and eavesdropping. This channel encryption method makes use of the Transport Layer Security (TLS) protocolDierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and P. Kocher, The TLS Protocol Version 1.0, January 1999.[TLS], along with a "STARTTLS" extension that is modelled after similar extensions for the IMAPCrispin, M., INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1, March 2003.[IMAP], POP3Myers, J. and M. Rose, Post Office Protocol - Version 3, May 1996.[POP3], and ACAPNewman, C. and J. Myers, ACAP -- Application Configuration Access Protocol, November 1997.[ACAP] protocols as described in RFC 2595Newman, C., Using TLS with IMAP, POP3 and ACAP, June 1999.[USINGTLS]. 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 negotiationUse of SASL, 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 CommunicationsServer-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 below 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. 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 ValidationCertificate 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.com" but a DNS SRVGulbrandsen, A., Vixie, P. and L. Esibov, A DNS RR for specifying the location of services (DNS SRV), February 2000.[SRV] lookup returned "im.example.com", the certificate MUST be checked as "example.com". If a JID for any kind of XMPP entity (e.g., client or server) 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 5.1.1ASN.1 Object Identifier for XMPP Address of this document.
9. 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.
10. 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.
11. If the TLS negotiation is successful, the receiving entity MUST NOT offer the STARTTLS extension to the initiating entity along with the other stream features that are offered when the stream is restarted.
12. If the TLS negotiation is successful, the initiating entity MUST continue with SASL negotiation.
13. If the TLS negotiation results in failure, the receiving entity MUST terminate both the XML stream and the underlying TCP connection.
14. See Mandatory-to-Implement TechnologiesMandatory-to-Implement Technologies regarding mechanisms that MUST be supported.

5.1.1 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 as "1.3.6.1.5.5.7.8.5".

5.2 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 TLS 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).
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. 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., Allen, C., Treese, W., Karlton, P., Freier, A. and P. Kocher, The TLS Protocol Version 1.0, January 1999..
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).

5.3 Client-to-Server Example

The following example shows the data flow for a client securing a stream using STARTTLS (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 example).

Step 1: Client initiates stream to server:

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

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

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    id='c2s_123'
    from='example.com'
    version='1.0'>

Step 3: Server sends the STARTTLS extension to client along with authentication mechanisms and any other stream features:

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>DIGEST-MD5</mechanism>
    <mechanism>PLAIN</mechanism>
  </mechanisms>
</stream:features>

Step 4: Client sends the 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 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.

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'
    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:

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

Step 9: Client continues with SASL negotiationUse of SASL.

5.4 Server-to-Server Example

The following example shows the data flow for two servers securing a stream using STARTTLS (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 example).

Step 1: Server1 initiates stream to Server2:

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

Step 3: Server2 sends the STARTTLS extension to Server1 along with authentication mechanisms and any other stream features:

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>DIGEST-MD5</mechanism>
    <mechanism>KERBEROS_V4</mechanism>
  </mechanisms>
</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'
    to='example.com'
    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 any available stream features:

<stream:stream
    xmlns='jabber:server'
    xmlns:stream='http://etherx.jabber.org/streams'
    from='example.com'
    id='s2s_234'
    version='1.0'>
<stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>DIGEST-MD5</mechanism>
    <mechanism>KERBEROS_V4</mechanism>
    <mechanism>EXTERNAL</mechanism>
  </mechanisms>
</stream:features>

Step 9: Server1 continues with SASL negotiationUse of SASL.

6. Use of SASL

6.1 Overview

XMPP includes a method for authenticating a stream by means of an XMPP-specific profile of the Simple Authentication and Security Layer (SASL) protocolMyers, J., Simple Authentication and Security Layer (SASL), October 1997.[SASL]. 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]Myers, J., Simple Authentication and Security Layer (SASL), October 1997..

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 CommunicationsServer-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 below 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 3548Josefsson, S., The Base16, Base32, and Base64 Data Encodings, July 2003.[BASE64].
6. 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.
7. 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]Myers, J., Simple Authentication and Security Layer (SASL), October 1997., 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, as described in [SASL]Myers, J., Simple Authentication and Security Layer (SASL), October 1997.. 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.
8. 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.
9. 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.
10. See Mandatory-to-Implement TechnologiesMandatory-to-Implement Technologies regarding mechanisms that MUST be supported.

6.2 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; 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 Use of TLSUse of TLS needs to be established before a particular authentication mechanism may be used, the receiving entity MUST NOT provide that mechanism in the list of available SASL authentication 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]Myers, J., Simple Authentication and Security Layer (SASL), October 1997.), although the EXTERNAL mechanism MAY be offered under other circumstances as well.
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 a <challenge/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating entity; 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 a <response/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the receiving entity; 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 ErrorsSASL 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. 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 (but not including the STARTTLS and SASL 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.

6.3 SASL Definition

The profiling requirements of [SASL]Myers, J., Simple Authentication and Security Layer (SASL), October 1997. 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., Allen, C., Treese, W., Karlton, P., Freier, A. and P. Kocher, The TLS Protocol Version 1.0, January 1999., then [SASL]Myers, J., Simple Authentication and Security Layer (SASL), October 1997., 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.

6.4 SASL Errors

The following SASL-related error conditions are defined:

• <aborted/> -- The receiving entity acknowledges an <abort/> element sent by the initiating entity; sent in reply to the <abort/> element.
• <incorrect-encoding/> -- The data provided by the initiating entity could not be processed because the [BASE64]Josefsson, S., The Base16, Base32, and Base64 Data Encodings, July 2003. encoding is incorrect (e.g., because the encoding does not adhere to the definition in Section 3 of [BASE64]Josefsson, S., The Base16, Base32, and Base64 Data Encodings, July 2003.); sent in reply to a <response/> element or an <auth/> element with initial response data.
• <invalid-authzid/> -- The authzid provided by the initiating entity is invalid, either because it is incorrectly formatted or because the initiating entity does not have permissions to authorize that ID; sent in reply to a <response/> element or an <auth/> element with initial response data.
• <invalid-mechanism/> -- The initiating entity did not provide a mechanism or requested a mechanism that is not supported by the receiving entity; sent in reply to an <auth/> element.
• <mechanism-too-weak/> -- The mechanism requested by the initiating entity is weaker than server policy permits for that initiating entity; sent in reply to a <response/> element or an <auth/> element with initial response data.
• <not-authorized/> -- The authentication failed because the initiating entity did not provide valid credentials (this includes but is not limited to the case of an unknown username); sent in reply to a <response/> element or an <auth/> element with initial response data.
• <temporary-auth-failure/> -- The authentication failed because of a temporary error condition within the receiving entity; sent in reply to an <auth/> element or <response/> element.

6.5 Client-to-Server Example

The following example shows the data flow for a client authenticating with a server using SASL, normally after successful TLS negotiation (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 example).

Step 1: Client initiates stream to server:

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

Step 2: Server responds with a stream tag sent to client:

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    id='c2s_234'
    from='example.com'
    version='1.0'>

Step 3: Server informs client of available authentication mechanisms:

<stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>DIGEST-MD5</mechanism>
    <mechanism>PLAIN</mechanism>
  </mechanisms>
</stream:features>

Step 4: Client selects an authentication mechanism:

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

Step 5: 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'>
cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9ImF1dGgi
LGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
</challenge>

The decoded challenge is:

realm="somerealm",nonce="OA6MG9tEQGm2hh",\
qop="auth",charset=utf-8,algorithm=md5-sess

Step 5 (alt): Server returns error to client:

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

Step 6: 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'>
dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
YXJzZXQ9dXRmLTgK
</response>

The decoded response is:

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

Step 7: Server sends another [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'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
</challenge>

The decoded challenge is:

rspauth=ea40f60335c427b5527b84dbabcdfffd

Step 7 (alt): Server returns error to client:

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

Step 8: Client responds to the challenge:

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

Step 9: Server informs client of successful authentication:

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

Step 9 (alt): Server informs client of failed authentication:

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

Step 10: Client initiates a new stream to server:

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

Step 11: Server responds by sending a stream header to client along with any additional features (or an empty features element):

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

6.6 Server-to-Server Example

The following example shows the data flow for a server authenticating with another server using SASL, normally after successful TLS negotiation (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 example).

Step 1: Server1 initiates stream to Server2:

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

Step 2: Server2 responds with a stream tag sent to Server1:

<stream:stream
    xmlns='jabber:server'
    xmlns:stream='http://etherx.jabber.org/streams'
    from='example.com'
    id='s2s_234'
    version='1.0'>

Step 3: Server2 informs Server1 of available authentication mechanisms:

<stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>DIGEST-MD5</mechanism>
    <mechanism>KERBEROS_V4</mechanism>
  </mechanisms>
</stream:features>

Step 4: Server1 selects an authentication mechanism:

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

Step 5: Server2 sends a [BASE64]Josefsson, S., The Base16, Base32, and Base64 Data Encodings, July 2003. encoded challenge to Server1:

<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9
ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz
</challenge>

The decoded challenge is:

realm="somerealm",nonce="OA6MG9tEQGm2hh",\
qop="auth",charset=utf-8,algorithm=md5-sess

Step 5 (alt): Server2 returns error to Server1:

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

Step 6: Server1 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'>
dXNlcm5hbWU9ImV4YW1wbGUub3JnIixyZWFsbT0ic29tZXJlYWxtIixub25j
ZT0iT0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5j
PTAwMDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5v
cmciLHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3
LGNoYXJzZXQ9dXRmLTgK
</response>

The decoded response is:

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

Step 7: Server2 sends another [BASE64]Josefsson, S., The Base16, Base32, and Base64 Data Encodings, July 2003. encoded challenge to Server1:

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

The decoded challenge is:

rspauth=ea40f60335c427b5527b84dbabcdfffd

Step 7 (alt): Server2 returns error to Server1:

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

Step 8: Server1 responds to the challenge:

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

Step 8 (alt): Server1 aborts negotiation:

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

Step 9: Server2 informs Server1 of successful authentication:

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

Step 9 (alt): Server2 informs Server1 of failed authentication:

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

Step 10: Server1 initiates a new stream to Server2:

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

Step 11: Server2 responds by sending a stream header to Server1 along with any additional features (or an empty features element):

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

7. Resource Binding

After SASL negotiationUse of SASL with the receiving entity, the initiating entity MAY want or need to bind a specific resource to that stream. In general this applies only to clients: in order to conform to the addressing formatAddressing Scheme and stanza delivery rulesServer Rules for Handling XML Stanzas specified herein, there MUST be a resource identifier associated with the <node@domain> of the client (which is either generated by the server or provided by the client application); this ensures that the address for use over that stream is a "full JID" of the form <node@domain/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 an empty <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):

Server advertises resource binding feature to client:

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    id='c2s_345'
    from='example.com'
    version='1.0'>
<stream:features>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-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 SemanticsIQ 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 unique for that <node@domain>.

If the client wishes to specify the resource identifier, it sends an IQ stanza of type "set" that contains the desired resource identifier as the 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>someresource</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>somenode@example.com/someresource</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 ErrorsStanza Errors):

• The provided resource identifier cannot be processed by the server in accordance with ResourceprepResourceprep.
• The client is not allowed to bind a resource to the stream (e.g., because the node or user has reached a limit on the number of connected resources allowed).
• The provided resource identifier is already in use but the server does not allow binding of multiple connected resources with the same identifier.

The protocol for these error conditions is shown below.

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>

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 XML stanza other than an IQ stanza with a <bind/> child qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace, the server MUST NOT process the stanza and SHOULD return a <not-authorized/> stanza error to the client.

8. Server Dialback

8.1 Overview

The Jabber protocols from which XMPP was adapted include a "server dialback" method for protecting against domain spoofing, thus making it more difficult to spoof XML stanzas. Server dialback is not a security mechanism, and results in weak verification of server identities only (see Server-to-Server CommunicationsServer-to-Server Communications regarding this method's security characteristics). Domains requiring robust security SHOULD use TLS and SASL; see Server-to-Server CommunicationsServer-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.

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 CommunicationsServer-to-Server Communications).

Server dialback is uni-directional, and results in (weak) verification of identities 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. 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).

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:

• Originating Server -- the server that is attempting to establish a connection between two domains.
• Receiving Server -- the server that is trying to authenticate that the Originating Server represents the domain which it claims to be.
• Authoritative Server -- the server that answers to the DNS hostname asserted by the Originating Server; for basic environments this will be the Originating Server, but it could be a separate machine in the Originating Server's network.

8.2 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 |
    | <---------------------- |
    |                         |

8.3 Protocol

The detailed protocol interaction between the servers is as follows:

1. The Originating Server establishes TCP connection to the Receiving Server.
2. The Originating Server sends a stream header to the Receiving Server:

   <stream:stream
       xmlns:stream='http://etherx.jabber.org/streams'
       xmlns='jabber:server'
       xmlns:db='jabber:server:dialback'>
   

   Note: The 'to' and 'from' attributes are OPTIONAL on the root stream element. The inclusion of the xmlns:db namespace declaration with the name shown indicates to the Receiving Server that the Originating Server supports dialback. If the namespace name 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.

3. The Receiving Server SHOULD send a stream header back to the Originating Server, including a unique ID for this interaction:

   <stream:stream
       xmlns:stream='http://etherx.jabber.org/streams'
       xmlns='jabber:server'
       xmlns:db='jabber:server:dialback'
       id='457F9224A0...'>
   

   Note: The 'to' and 'from' attributes are OPTIONAL on the root stream element. If the namespace name 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. Note well that the Receiving Server SHOULD reply but MAY silently terminate the XML stream and underlying TCP connection depending on security policies in place; however, if the Receiving Server desires to proceed, it MUST send a stream header back to the Originating Server.

4. The Originating Server sends a dialback key to the Receiving Server:

   <db:result
       to='Receiving Server'
       from='Originating Server'>
     98AF014EDC0...
   </db:result>
   

   Note: This key is not examined by the Receiving Server, since the Receiving Server does not keep information about the Originating Server between sessions. 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, and in part on a secret shared by the Originating Server and the Authoritative Server. If the value of the 'to' address does not match a hostname recognized 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 matches a domain with which the Receiving Server already has an established connection, then the Receiving Server MUST maintain the existing connection until it validates whether the new connection is legitimate; additionally, the Receiving Server MAY choose to generate a <not-authorized/> stream error condition for the new connection and then terminate both the XML stream and the underlying TCP connection related to the new request.

5. The Receiving Server establishes a TCP connection back to the domain name asserted by the Originating Server, as a result of which it connects to the Authoritative Server. (Note: As an optimization, an implementation MAY reuse an existing connection here.)
6. The Receiving Server sends the Authoritative Server a stream header:

   <stream:stream
       xmlns:stream='http://etherx.jabber.org/streams'
       xmlns='jabber:server'
       xmlns:db='jabber:server:dialback'>
   

   Note: The 'to' and 'from' attributes are OPTIONAL on the root stream element. 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.

7. The Authoritative Server sends the Receiving Server a stream header:

   <stream:stream
       xmlns:stream='http://etherx.jabber.org/streams'
       xmlns='jabber:server'
       xmlns:db='jabber:server:dialback'
       id='1251A342B...'>
   

   Note: If the namespace name 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 a stream error occurs between the Receiving Server and the Authoritative Server, then the Receiving Server MUST generate a <remote-connection-failed/> stream error condition and terminate both the XML stream and the underlying TCP connection between it and the Originating Server.

8. The Receiving Server sends the Authoritative Server a request for verification of a key:

   <db:verify
       from='Receiving Server'
       to='Originating Server'
       id='457F9224A0...'>
     98AF014EDC0...
   </db:verify>
   

   Note: Passed here are the hostnames, 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 4. Based on this information, as well as shared secret information within the Authoritative Server's network, the key is verified. Any verifiable method MAY be used to generate the key. If the value of the 'to' address does not match a hostname recognized by 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 represented by the Receiving Server when opening the TCP connection (or any validated domain thereof, such as a validated subdomain of the Receiving Server's hostname or another validated domain hosted by the Receiving Server), then the Authoritative Server MUST generate an <invalid-from/> stream error condition and terminate both the XML stream and the underlying TCP connection.

9. The Authoritative Server verifies whether the key was valid or invalid:

   <db:verify
       from='Originating Server'
       to='Receiving Server'
       type='valid'
       id='457F9224A0...'/>
   

   or

   <db:verify
       from='Originating Server'
       to='Receiving Server'
       type='invalid'
       id='457F9224A0...'/>
   

   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 recognized 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 when opening the TCP connection (or any validated domain thereof, such as a validated subdomain of the Originating Server's hostname or another validated domain hosted by the Originating Server), 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.

10. The Receiving Server informs the Originating Server of the result:

    <db:result
        from='Receiving Server'
        to='Originating Server'
        type='valid'/>
    

    Note: At this point, the connection has either been validated via a type='valid', or reported as invalid. If the connection is invalid, then the Receiving Server MUST terminate both the XML stream and the underlying TCP connection. If the connection is validated, data can be sent by the Originating Server and read by the Receiving Server; before that, all XML stanzas sent to the Receiving Server SHOULD be silently dropped.

The result of the foregoing is that the Receiving Server has verified the identity of the Originating Server, so that the Originating Server can send, and the Receiving Server can accept, XML stanzas over the "initial stream" (i.e., the stream from the Originating Server to the Receiving Server). In order to verify the identities of the entities using the "response stream" (i.e., the stream from the Receiving Server to the Originating Server), dialback MUST be completed in the opposite direction as well.

After successful dialback negotiation, the Receiving Server SHOULD accept subsequent <db:result/> packets (e.g., validation requests sent to a subdomain or other hostname serviced by the Receiving Server) from the Originating Server over the existing validated connection; this enables "piggybacking" of the original validated connection in one direction.

Even if dialback negotiation is successful, a server MUST verify that all XML stanzas received from the other server include a 'from' attribute and a 'to' attribute; if a stanza does not meet this restriction, the server that receives the stanza MUST generate an <improper-addressing/> stream error condition and terminate both the XML stream and the underlying TCP connection. Furthermore, a server MUST verify that the 'from' attribute of stanzas received from the other server includes a validated domain for the stream; if a stanza does not meet this restriction, the server that receives the stanza MUST generate 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.

9. XML Stanzas

After TLS negotiationUse of TLS if desired, SASL negotiationUse of SASL, and Resource BindingResource Binding if necessary, XML stanzas can be sent over the streams. 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 in relation to 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..

9.1 Common Attributes

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

9.1.1 to

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

In the 'jabber:client' namespace, a stanza SHOULD possess a 'to' attribute, although a stanza sent from a client to a server for handling by that server (e.g., presence sent to the server for broadcasting to other entities) 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.

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 bare JID (<node@domain>) or the full JID (<node@domain/resource>) determined by the server for the connected resource that generated the stanza (see Determination of AddressesDetermination of Addresses)

If a client attempts to send an XML stanza for which the value of the 'from' attribute does not 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 s