TOC 
Network Working GroupP. Saint-Andre
Internet-DraftCisco Systems, Inc.
Obsoletes: 3920 (if approved)July 7, 2010
Intended status: Standards Track 
Expires: January 8, 2011 


Extensible Messaging and Presence Protocol (XMPP): Core
draft-ietf-xmpp-3920bis-10

Abstract

The Extensible Messaging and Presence Protocol (XMPP) is an application profile of the Extensible Markup Language (XML) that enables the near-real-time exchange of structured yet extensible data between any two or more network entities. This document defines XMPP's core protocol methods: setup and teardown of XML streams, channel encryption, authentication, error handling, and communication primitives for messaging, network availability ("presence"), and request-response interactions.

Status of this Memo

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This Internet-Draft will expire on January 8, 2011.

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Table of Contents

1.  Introduction
    1.1.  Overview
    1.2.  History
    1.3.  Functional Summary
    1.4.  Terminology
    1.5.  Acknowledgements
    1.6.  Discussion Venue
2.  Architecture
    2.1.  Global Addresses
    2.2.  Presence
    2.3.  Persistent Streams
    2.4.  Structured Data
    2.5.  Distributed Network of Clients and Servers
3.  TCP Binding
    3.1.  Scope
    3.2.  Hostname Resolution
        3.2.1.  Preferred Process: SRV Lookup
        3.2.2.  Fallback Processes
        3.2.3.  When Not to Use SRV
        3.2.4.  Use of SRV Records with Add-On Services
    3.3.  Directionality
    3.4.  Reconnection
    3.5.  Reliability
4.  XML Streams
    4.1.  Overview
    4.2.  Stream Negotiation
        4.2.1.  Overview
        4.2.2.  Stream Features Format
        4.2.3.  Restarts
        4.2.4.  Resending Features
        4.2.5.  Completion of Stream Negotiation
        4.2.6.  Determination of Addresses
        4.2.7.  Flow Chart
    4.3.  Closing a Stream
    4.4.  Handling of Silent Peers
        4.4.1.  Idle Peer
        4.4.2.  Broken Stream
        4.4.3.  Dead Connection
        4.4.4.  Use of Checking Methods
    4.5.  Stream Attributes
        4.5.1.  from
        4.5.2.  to
        4.5.3.  id
        4.5.4.  xml:lang
        4.5.5.  version
        4.5.6.  Summary of Stream Attributes
    4.6.  Namespaces
        4.6.1.  Streams Namespace
        4.6.2.  Default Namespace
        4.6.3.  Other Namespaces
        4.6.4.  Namespace Declarations and Prefixes
        4.6.5.  Mandatory-to-Implement Default Namespaces
    4.7.  Stream Errors
        4.7.1.  Rules
            4.7.1.1.  Stream Errors Are Unrecoverable
            4.7.1.2.  Stream Errors Can Occur During Setup
            4.7.1.3.  Stream Errors When the Host is Unspecified or Unknown
            4.7.1.4.  Where Stream Errors Are Sent
        4.7.2.  Syntax
        4.7.3.  Defined Stream Error Conditions
            4.7.3.1.  bad-format
            4.7.3.2.  bad-namespace-prefix
            4.7.3.3.  conflict
            4.7.3.4.  connection-timeout
            4.7.3.5.  host-gone
            4.7.3.6.  host-unknown
            4.7.3.7.  improper-addressing
            4.7.3.8.  internal-server-error
            4.7.3.9.  invalid-from
            4.7.3.10.  invalid-namespace
            4.7.3.11.  invalid-xml
            4.7.3.12.  not-authorized
            4.7.3.13.  policy-violation
            4.7.3.14.  remote-connection-failed
            4.7.3.15.  reset
            4.7.3.16.  resource-constraint
            4.7.3.17.  restricted-xml
            4.7.3.18.  see-other-host
            4.7.3.19.  system-shutdown
            4.7.3.20.  undefined-condition
            4.7.3.21.  unsupported-encoding
            4.7.3.22.  unsupported-feature
            4.7.3.23.  unsupported-stanza-type
            4.7.3.24.  unsupported-version
            4.7.3.25.  xml-not-well-formed
        4.7.4.  Application-Specific Conditions
    4.8.  Simplified Stream Examples
5.  STARTTLS Negotiation
    5.1.  Overview
    5.2.  Stream Negotiation Rules
        5.2.1.  Mandatory-to-Negotiate
        5.2.2.  Restart
        5.2.3.  Data Formatting
        5.2.4.  Order of TLS and SASL Negotiations
        5.2.5.  TLS Renegotiation
        5.2.6.  TLS Extensions
    5.3.  Process
        5.3.1.  Exchange of Stream Headers and Stream Features
        5.3.2.  Initiation of STARTTLS Negotiation
            5.3.2.1.  STARTTLS Command
            5.3.2.2.  Failure Case
            5.3.2.3.  Proceed Case
        5.3.3.  TLS Negotiation
            5.3.3.1.  Rules
            5.3.3.2.  TLS Failure
            5.3.3.3.  TLS Success
6.  SASL Negotiation
    6.1.  Overview
    6.2.  Stream Negotiation Rules
        6.2.1.  Mandatory-to-Negotiate
        6.2.2.  Restart
        6.2.3.  Mechanism Preferences
        6.2.4.  Mechanism Offers
        6.2.5.  Data Formatting
        6.2.6.  Security Layers
        6.2.7.  Simple Username
        6.2.8.  Authorization Identity
        6.2.9.  Realms
        6.2.10.  Round Trips
    6.3.  Process
        6.3.1.  Exchange of Stream Headers and Stream Features
        6.3.2.  Initiation
        6.3.3.  Challenge-Response Sequence
        6.3.4.  Abort
        6.3.5.  Failure
        6.3.6.  Success
    6.4.  SASL Errors
        6.4.1.  aborted
        6.4.2.  account-disabled
        6.4.3.  credentials-expired
        6.4.4.  encryption-required
        6.4.5.  incorrect-encoding
        6.4.6.  invalid-authzid
        6.4.7.  invalid-mechanism
        6.4.8.  malformed-request
        6.4.9.  mechanism-too-weak
        6.4.10.  not-authorized
        6.4.11.  temporary-auth-failure
        6.4.12.  transition-needed
    6.5.  SASL Definition
7.  Resource Binding
    7.1.  Overview
    7.2.  Stream Negotiation Rules
        7.2.1.  Mandatory-to-Negotiate
        7.2.2.  Restart
    7.3.  Advertising Support
    7.4.  Generation of Resource Identifiers
    7.5.  Server-Generated Resource Identifier
        7.5.1.  Success Case
        7.5.2.  Error Cases
            7.5.2.1.  Resource Constraint
            7.5.2.2.  Not Allowed
    7.6.  Client-Submitted Resource Identifier
        7.6.1.  Success Case
        7.6.2.  Error Cases
            7.6.2.1.  Bad Request
            7.6.2.2.  Conflict
        7.6.3.  Retries
8.  XML Stanzas
    8.1.  Common Attributes
        8.1.1.  to
            8.1.1.1.  Client-to-Server Streams
            8.1.1.2.  Server-to-Server Streams
        8.1.2.  from
            8.1.2.1.  Client-to-Server Streams
            8.1.2.2.  Server-to-Server Streams
        8.1.3.  id
        8.1.4.  type
        8.1.5.  xml:lang
    8.2.  Basic Semantics
        8.2.1.  Message Semantics
        8.2.2.  Presence Semantics
        8.2.3.  IQ Semantics
    8.3.  Stanza Errors
        8.3.1.  Rules
        8.3.2.  Syntax
        8.3.3.  Defined Conditions
            8.3.3.1.  bad-request
            8.3.3.2.  conflict
            8.3.3.3.  feature-not-implemented
            8.3.3.4.  forbidden
            8.3.3.5.  gone
            8.3.3.6.  internal-server-error
            8.3.3.7.  item-not-found
            8.3.3.8.  jid-malformed
            8.3.3.9.  not-acceptable
            8.3.3.10.  not-allowed
            8.3.3.11.  not-authorized
            8.3.3.12.  payment-required
            8.3.3.13.  policy-violation
            8.3.3.14.  recipient-unavailable
            8.3.3.15.  redirect
            8.3.3.16.  registration-required
            8.3.3.17.  remote-server-not-found
            8.3.3.18.  remote-server-timeout
            8.3.3.19.  resource-constraint
            8.3.3.20.  service-unavailable
            8.3.3.21.  subscription-required
            8.3.3.22.  undefined-condition
            8.3.3.23.  unexpected-request
        8.3.4.  Application-Specific Conditions
    8.4.  Extended Content
9.  Examples
    9.1.  Client-to-Server Examples
        9.1.1.  TLS
        9.1.2.  SASL
        9.1.3.  Resource Binding
        9.1.4.  Stanza Exchange
        9.1.5.  Close
    9.2.  Server-to-Server Examples
        9.2.1.  TLS
        9.2.2.  SASL
        9.2.3.  Stanza Exchange
        9.2.4.  Close
10.  Server Rules for Processing XML Stanzas
    10.1.  No 'to' Address
        10.1.1.  Overview
        10.1.2.  Message
        10.1.3.  Presence
        10.1.4.  IQ
    10.2.  Remote Domain
        10.2.1.  Existing Stream
        10.2.2.  No Existing Stream
        10.2.3.  Error Handling
    10.3.  Local Domain
        10.3.1.  Mere Domain
        10.3.2.  Domain with Resource
        10.3.3.  Localpart at Domain
            10.3.3.1.  No Such User
            10.3.3.2.  Bare JID
            10.3.3.3.  Full JID
11.  XML Usage
    11.1.  Restrictions
    11.2.  XML Namespace Names and Prefixes
    11.3.  Well-Formedness
    11.4.  Validation
    11.5.  Inclusion of XML Declaration
    11.6.  Character Encoding
    11.7.  Whitespace
    11.8.  XML Versions
12.  Internationalization Considerations
13.  Security Considerations
    13.1.  Overview
    13.2.  Threat Model
    13.3.  Order of Layers
    13.4.  Confidentiality and Integrity
    13.5.  Peer Entity Authentication
    13.6.  Strong Security
    13.7.  Certificates
        13.7.1.  Certificate Generation
            13.7.1.1.  General Considerations
            13.7.1.2.  Server Certificates
            13.7.1.3.  Client Certificates
            13.7.1.4.  XmppAddr Identifier Type
        13.7.2.  Certificate Validation
            13.7.2.1.  Server Certificates
            13.7.2.2.  Client Certificates
            13.7.2.3.  Checking of Certificates in Long-Lived Streams
            13.7.2.4.  Use of Certificates in XMPP Extensions
    13.8.  Mandatory-to-Implement Technologies
    13.9.  Technology Reuse
        13.9.1.  Use of base64 in SASL
        13.9.2.  Use of DNS
        13.9.3.  Use of Hash Functions
        13.9.4.  Use of SASL
        13.9.5.  Use of TLS
        13.9.6.  Use of UTF-8
        13.9.7.  Use of XML
    13.10.  Information Leaks
        13.10.1.  IP Addresses
        13.10.2.  Presence Information
    13.11.  Directory Harvesting
    13.12.  Denial of Service
    13.13.  Firewalls
    13.14.  Interdomain Federation
    13.15.  Non-Repudiation
14.  IANA Considerations
    14.1.  XML Namespace Name for TLS Data
    14.2.  XML Namespace Name for SASL Data
    14.3.  XML Namespace Name for Stream Errors
    14.4.  XML Namespace Name for Resource Binding
    14.5.  XML Namespace Name for Stanza Errors
    14.6.  GSSAPI Service Name
    14.7.  Port Numbers
15.  Conformance Requirements
16.  References
    16.1.  Normative References
    16.2.  Informative References
Appendix A.  XML Schemas
    A.1.  Streams Namespace
    A.2.  Stream Error Namespace
    A.3.  STARTTLS Namespace
    A.4.  SASL Namespace
    A.5.  Resource Binding Namespace
    A.6.  Stanza Error Namespace
Appendix B.  Contact Addresses
Appendix C.  Account Provisioning
Appendix D.  Differences from RFC 3920
Appendix E.  Copying Conditions
§  Author's Address




 TOC 

1.  Introduction



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1.1.  Overview

The Extensible Messaging and Presence Protocol (XMPP) is an application profile of the Extensible Markup Language [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.) that enables the near-real-time exchange of structured yet extensible data between any two or more network entities. This document defines XMPP's core protocol methods: setup and teardown of XML streams, channel encryption, authentication, error handling, and communication primitives for messaging, network availability ("presence"), and request-response interactions.



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1.2.  History

The basic syntax and semantics of XMPP were developed originally within the Jabber open-source community, mainly in 1999. In late 2002, the XMPP Working Group was chartered with developing an adaptation of the core Jabber protocol that would be suitable as an IETF instant messaging (IM) and presence technology in accordance with [IMP‑REQS] (Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” February 2000.). In October 2004, [RFC3920] (Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.) and [RFC3921] (Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” October 2004.) were published, representing the most complete definition of XMPP at that time.

Since 2004 the Internet community has gained extensive implementation and deployment experience with XMPP, including formal interoperability testing carried out under the auspices of the XMPP Standards Foundation (XSF). This document incorporates comprehensive feedback from software developers and service providers, including a number of backward-compatible modifications summarized under Appendix D (Differences from RFC 3920). As a result, this document reflects the rough consensus of the Internet community regarding the core features of XMPP 1.0, thus obsoleting RFC 3920.



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1.3.  Functional Summary

This non-normative section provides a developer-friendly, functional summary of XMPP; refer to the sections that follow for a normative definition of XMPP.

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

  1. Determine the hostname and port at which to connect
  2. Open a Transmission Control Protocol [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.) connection
  3. Open an XML stream over TCP
  4. Negotiate Transport Layer Security [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) for channel encryption (recommended)
  5. Authenticate using a Simple Authentication and Security Layer [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) mechanism
  6. Bind a resource to the stream
  7. Exchange an unbounded number of XML stanzas with other entities on the network
  8. Close the XML stream
  9. Close the TCP connection

Within XMPP, one server can optionally connect to another server to enable inter-domain or inter-server communication. For this to happen, the two servers need to negotiate a connection between themselves and then exchange XML stanzas; the process for doing so is:

  1. Determine the hostname and port at which to connect
  2. Open a TCP connection
  3. Open an XML stream
  4. Negotiate TLS for channel encryption (recommended)
  5. Authenticate using a Simple Authentication and Security Layer [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) mechanism *
  6. Exchange an unbounded number of XML stanzas both directly for the servers and indirectly on behalf of entities associated with each server (e.g., connected clients)
  7. Close the XML stream
  8. Close the TCP connection

* Implementation Note: At the time of writing, most deployed server will use the older server dialback protocol to provide weak identity verification instead of using SASL to provide strong authentication, especially in cases where SASL negotiation would not result in strong authentication anyway (e.g., because TLS negotiation was not mandated by the peer server, or because the certificate presented by the peer server during TLS negotiation is self-signed and has not been previously accepted); for details, see [XEP‑0220] (Miller, J., Saint-Andre, P., and P. Hancke, “Server Dialback,” March 2010.).

This document specifies how clients connect to servers and specifies the basic semantics of XML stanzas. However, this document does not define the "payloads" of the XML stanzas that might be exchanged once a connection is successfully established; instead, those payloads are defined by various XMPP extensions. For example, [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.) defines extensions for basic instant messaging and presence functionality. In addition, various specifications produced in the XSF's XEP series [XEP‑0001] (Saint-Andre, P., “XMPP Extension Protocols,” January 2008.) define extensions for a wide range of applications.



 TOC 

1.4.  Terminology

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

Certain security-related terms are to be understood in the sense defined in [SEC‑TERMS] (Shirey, R., “Internet Security Glossary, Version 2,” August 2007.); such terms include, but are not limited to, "assurance", "attack", "authentication", "authorization", "certificate", "certification authority", "certification path", "confidentiality", "credential", "downgrade", "encryption", "fingerprint", "hash value", "identity", "integrity", "signature", "security perimeter", "self-signed certificate", "sign", "spoof", "tamper", "trust", "trust anchor", "trust chain", "validate", "verify". Other security-related terms (for example, "denial of service") are to be understood in the sense defined in the referenced specifications.

The term "whitespace" is used to refer to any character that matches production [3] content of [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.), i.e., any instance of SP, HT, CR, or LF.

We define the following terms with regard to XML stanzas or parts thereof:

deliver:
for a server, to pass the data to a connected client
ignore:
for a client or server, to discard the data without acting upon it, presenting it a human user, or returning an error to the sender
route:
for a server, to pass the data to a remote server for subsequent delivery

In examples, lines have been wrapped for improved readability, "[...]" means elision, and the following prepended strings are used (these prepended strings are not to be sent over the wire):

In adherence to the "XML Notation" used in [IRI] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) to represent characters that cannot be rendered in ASCII-only documents, some examples in this document use the form "&#x...." as a notational device to represent [UNICODE] (The Unicode Consortium, “The Unicode Standard, Version 3.2.0,” 2000.) code points (e.g., the string "ř" stands for the Unicode character LATIN SMALL LETTER R WITH CARON).

In adherence to the convention used in [URI] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) to represent Uniform Resource Indentifiers, XMPP addresses in running text are enclosed between '<' and '>' (despite the fact that natively they are not URIs).



 TOC 

1.5.  Acknowledgements

The editor of this document finds it impossible to appropriately acknowledge the many individuals who have provided comments regarding the protocols defined in this specification. However, thanks are due to those who have provided implementation feedback, bug reports, requests for clarification, and suggestions for improvement since the publication of the RFC this document supersedes. The editor has endeavored to address all such feedback, but is solely responsible for any remaining errors and ambiguities.



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1.6.  Discussion Venue

The document editor and the broader XMPP developer community welcome discussion and comments related to the topics presented in this document. The primary and preferred venue is the <xmpp@ietf.org> mailing list, for which archives and subscription information are available at https://www.ietf.org/mailman/listinfo/xmpp. Related discussions often occur on the <standards@xmpp.org> mailing list, for which archives and subscription information are available at http://mail.jabber.org/mailman/listinfo/standards.



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2.  Architecture

XMPP provides a technology for the asynchronous, end-to-end exchange of structured data by means of direct, persistent XML streams among a distributed network of globally-addressable, presence-aware clients and servers. Because this architectural style involves ubiquitous knowledge of network availability and a conceptually unlimited number of concurrent information transactions in the context of a given client-to-server or server-to-server session, we label it "Availability for Concurrent Transactions" (ACT) to distinguish it from the "Representational State Transfer" [REST] (Fielding, R., “Architectural Styles and the Design of Network-based Software Architectures,” .) architectural style familiar from the World Wide Web. Although the architecture of XMPP is similar in important ways to that of email (see [EMAIL‑ARCH] (Crocker, D., “Internet Mail Architecture,” July 2009.)), it introduces several modifications to facilitate communication in close to real time. The salient features of this ACTive architectural style are as follows.



 TOC 

2.1.  Global Addresses

As with email, XMPP uses globally-unique addresses (based on the Domain Name System) in order to route and deliver messages over the network. All XMPP entities are addressable on the network, most particularly clients and servers but also various additional services that can be accessed by clients and servers. In general, server addresses are of the form <domain.tld> (e.g., <im.example.com>), accounts hosted at a server are of the form <localpart@domainpart> (e.g., <juliet@im.example.com>), and a particular connected device or resource that is currently authorized for interaction on behalf of an account is of the form <localpart@domainpart/resource> (e.g., <juliet@im.example.com/balcony>). For historical reasons, XMPP addresses are often called Jabber IDs or JIDs. Because the formal specification of the XMPP address format depends on internationalization technologies that are in flux at the time of writing, the format is defined in [XMPP‑ADDR] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Address Format,” July 2010.) instead of this document.



 TOC 

2.2.  Presence

XMPP includes the ability for an entity to advertise its network availability or "presence" to other entities. Such availability for communication is signalled end-to-end via dedicated communication primitives in XMPP (the <presence/> stanza). Although knowledge of network availability is not strictly necessary for the exchange of XMPP messages, it facilitates real-time interaction because the originator of a message can know before initiating communication that the intended recipient is online and available. End-to-end presence is defined in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.).



 TOC 

2.3.  Persistent Streams

Availability for communication is also built into a point-to-point "hop" through the use of persistent XML streams over long-lived TCP connections. These "always-on" client-to-server or server-to-server streams enable each party to push data to the other party at any time for immediate routing or delivery. XML streams are defined under Section 4 (XML Streams).



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2.4.  Structured Data

The basic protocol data unit in XMPP is not an XML stream (which simply provides the transport for point-to-point communication) but an XML "stanza", which is essentially a fragment of XML that is sent over a stream. The root element of a stanza includes routing attributes (such as "from" and "to" addresses) and the child elements of the stanza contain a payload for delivery to the intended recipient. XML stanzas are defined under Section 8 (XML Stanzas).



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2.5.  Distributed Network of Clients and Servers

In practice, XMPP consists of a network of clients and servers that inter-communicate (however, communication between any two given deployed servers is strictly OPTIONAL). Thus, for example, the user <juliet@im.example.com> associated with the server <im.example.com> might be able to exchange messages, presence, and other structured data with the user <romeo@example.net> associated with the server <example.net>. This pattern is familiar from messaging protocols that make use of global addresses, such as the email network (see [SMTP] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.) and [EMAIL‑ARCH] (Crocker, D., “Internet Mail Architecture,” July 2009.)). As a result, end-to-end communication in XMPP is logically peer-to-peer but physically client-to-server-to-server-to-client, as illustrated in the following diagram.

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

Informational Note: Architectures that employ XML streams (XML Streams) and XML stanzas (XML Stanzas) but that establish peer-to-peer connections directly between clients using technologies based on [LINKLOCAL] (Cheshire, S., Aboba, B., and E. Guttman, “Dynamic Configuration of IPv4 Link-Local Addresses,” May 2005.) have been deployed, but such architectures are not defined in this specification and are best described as "XMPP-like"; for details, see [XEP‑0174] (Saint-Andre, P., “Link-Local Messaging,” November 2008.). In addition, XML streams can be established end-to-end over any reliable transport, including extensions to XMPP itself; however, such methods are out of scope for this specification.

The following paragraphs describe the responsibilities of clients and servers on the network.

A CLIENT is an entity that establishes an XML stream with a server by authenticating using the credentials of a local account and that then completes resource binding (Resource Binding) in order to enable delivery of XML stanzas between the server and the client over the negotiated stream. The client then uses XMPP to communicate with its server, other clients, and any other entities on the network, where the server is responsible for delivering stanzas to local entities or routing them to remote entities. Multiple clients can connect simultaneously to a server on behalf of the same local account, where each client is differentiated by the resourcepart of an XMPP address (e.g., <juliet@im.example.com/balcony> vs. <juliet@im.example.com/chamber>), as defined under [XMPP‑ADDR] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Address Format,” July 2010.) and Section 7 (Resource Binding).

A SERVER is an entity whose primary responsibilities are to:

Depending on the application, the secondary responsibilities of an XMPP server can include:



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3.  TCP Binding



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3.1.  Scope

As XMPP is defined in this specification, an initiating entity (client or server) MUST open a Transmission Control Protocol [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.) connection to the receiving entity (server) before it negotiates XML streams with the receiving entity. The parties then maintain that TCP connection for as long as the XML streams are in use. The rules specified in the following sections apply to the TCP binding.

Informational Note: There is no necessary coupling of XML streams to TCP, and other transports are possible. For example, two entities could connect to each other by means of [HTTP] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) as specified in [XEP‑0124] (Paterson, I., Smith, D., and P. Saint-Andre, “Bidirectional-streams Over Synchronous HTTP (BOSH),” April 2009.) and [XEP‑0206] (Paterson, I., “XMPP Over BOSH,” October 2008.). However, this specification defines only a binding of XMPP to TCP.



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3.2.  Hostname Resolution

Because XML streams are sent over TCP, the initiating entity needs to determine the IPv4 or IPv6 address (and port) of the receiving entity's "origin domain" before it can attempt to connect to the XMPP network.



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3.2.1.  Preferred Process: SRV Lookup

The preferred process for hostname resolution is to use [DNS‑SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) records as follows:

  1. The initiating entity constructs a DNS SRV query whose inputs are:

  2. The resulting is a query such as "_xmpp-client._tcp.example.net." or "_xmpp-server._tcp.im.example.com.".

  3. If a response is received, it will contain one or more combinations of a port and hostname, each of which is weighted and prioritized as described in [DNS‑SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.). However, if the result of the SRV lookup is a single resource record with a Target of ".", i.e. the root domain, then the initiating entity MUST abort SRV processing at this point (but SHOULD attempt the fallback process described in the next section).

  4. The initiating entity chooses at least one of the returned hostnames to resolve (following the rules in [DNS‑SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.)), which it does by using a DNS "A" or "AAAA" lookup on the hostname; this will result in an IPv4 or IPv6 address.

  5. The initiating entity uses the IP address(es) from the first successfully resolved hostname (with the corresponding port number returned by the SRV lookup) as the connection address for the receiving entity.

  6. If the initiating entity fails to connect using that IP address but the "A" or "AAAA" lookup returned more than one IP address, then the initiating entity uses the next resolved IP address for that hostname as the connection address.

  7. If the initiating entity fails to connect using all resolved IP addresses for a given hostname, then it repeats the process of resolution and connection for the next hostname returned by the SRV lookup.

  8. If the initiating entity fails to connect using any hostname returned by the SRV lookup, then it can either abort the connection attempt or use the fallback process described in the next section.



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3.2.2.  Fallback Processes

The fallback process SHOULD be a normal "A" or "AAAA" address record resolution to determine the IPv4 or IPv6 address of the origin domain, where the port used is the "xmpp-client" port of 5222 for client-to-server connections or the "xmpp-server" port 5269 for server-to-server connections.

For client-to-server connections, the fallback MAY be a [DNS‑TXT] (Rosenbaum, R., “Using the Domain Name System To Store Arbitrary String Attributes,” May 1993.) lookup for alternative connection methods, for example as described in [XEP‑0156] (Hildebrand, J. and P. Saint-Andre, “Discovering Alternative XMPP Connection Methods,” June 2007.).



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3.2.3.  When Not to Use SRV

If the initiating entity has been explicitly configured to associate a particular hostname (and potentially port) with the origin domain of the receiving entity (say, to "hardcode" an association from an origin domain of example.net to a configured hostname of webcm.example.com:80), the initiating entity SHOULD use the configured name instead of performing the preferred SRV resolution process on the origin name.



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3.2.4.  Use of SRV Records with Add-On Services

Many XMPP servers are implemented in such a way that they can host add-on services (beyond those defined in this specification and [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.)) at hostnames that typically are subdomains of the hostname of the main XMPP service (e.g., conference.example.net for a [XEP‑0045] (Saint-Andre, P., “Multi-User Chat,” July 2007.) service associated with the example.net XMPP service) or subdomains of the first-level domain of the underlying host (e.g., muc.example.com for a [XEP‑0045] (Saint-Andre, P., “Multi-User Chat,” July 2007.) service associated with the im.example.com XMPP service). If an entity from a remote domain wishes to use such add-on services, it would generate an appropriate XML stanza and the remote domain itself would attempt to resolve the service's hostname via an SRV lookup on resource records such as "_xmpp-server._tcp.conference.example.net." or "_xmpp-server._tcp.muc.example.com.". Therefore if a service wishes to enable entities from remote domains to access these add-on services, it needs to advertise the appropriate "_xmpp-server" SRV records in addition to the "_xmpp-server" record for its main XMPP service. The same fallback methods apply in case SRV records are not available.



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3.3.  Directionality

An XML Stream is either bidirectional, allowing stanzas to be sent in either direction at any time, or unidirectional, allowing stanzas to be send only in one, fixed, direction.

Clients establish a single, bidirectional link to a server for a session, allowing XML stanzas to be sent from client to server and server to client. Servers, for historical reasons, establish two unidirectional connections between each other, one in each direction. A future extension might allow servers to establish bidirectional links given suitable feature negotiation; however, definition of such a feature is out of scope for this document.

The directionality of a stream applies only to XML stanzas (XML Stanzas). Therefore during STARTTLS negotiation (STARTTLS Negotiation) and SASL negotiation (SASL Negotiation) two servers would use one TCP connection, but after stream setup that TCP connection would be used only for the initiating server to send XML stanzas to the receiving server. In order for the receiving server to send XML stanzas to the initiating server, the receiving server would need to reverse the roles and negotiate an XML stream from the receiving server to the initiating server.



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3.4.  Reconnection

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



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3.5.  Reliability

The use of long-lived TCP connections in XMPP implies that the sending of XML stanzas over XML streams can be unreliable, since the parties to a long-lived TCP connection might not discover a connectivity disruption in a timely manner. At the XMPP application layer, long connectivity disruptions can result in undelivered stanzas. Although the core XMPP technology defined in this specification does not contain features to overcome this lack of reliability, there exist XMPP extensions for doing so (e.g., [XEP‑0198] (Karneges, J., Hildebrand, J., Saint-Andre, P., and F. Forno, “Stream Management,” June 2009.)).



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4.  XML Streams



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4.1.  Overview

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

Definition of XML Stream:
An XML STREAM is a container for the exchange of XML elements between any two entities over a network. The start of an XML stream is denoted unambiguously by an opening STREAM HEADER (i.e., an XML <stream> tag with appropriate attributes and namespace declarations), while the end of the XML stream is denoted unambiguously by a closing XML </stream> tag. During the life of the stream, the entity that initiated it can send an unbounded number of XML elements over the stream, either elements used to negotiate the stream (e.g., to complete TLS negotiation (STARTTLS Negotiation) or SASL negotiation (SASL Negotiation)) or XML stanzas. The INITIAL STREAM is negotiated from the initiating entity (typically a client or server) to the receiving entity (typically a server), and can be seen as corresponding to the initiating entity's "connection to" or "session with" the receiving entity. The initial stream enables unidirectional communication from the initiating entity to the receiving entity; in order to enable exchange of stanzas 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 the basic unit of meaning in XMPP. Only a first-level <message/>, <presence/>, or <iq/> element qualified by the default namespace is an XML stanza. By contrast, a first-level XML element sent for any other purpose is not an XML stanza (stream errors, stream features, TLS-related elements, SASL-related elements, etc.). An XML stanza typically contains one or more child elements (with accompanying attributes, elements, and XML character data) as necessary in order to convey the desired information, which MAY be qualified by any XML namespace (see [XML‑NAMES] (Thompson, H., Hollander, D., Layman, A., Bray, T., and R. Tobin, “Namespaces in XML 1.0 (Third Edition),” December 2009.) as well as Section 8.4 (Extended Content) in this specification).

Consider the example of a client's connection to a server. The client initiates an XML stream by sending a stream header to the server, optionally preceded by a text declaration specifying the XML version and the character encoding supported (see Section 11.5 (Inclusion of XML Declaration) and Section 11.6 (Character Encoding)). Subject to local policies and service provisioning, the server then replies with a second XML stream back to the client, again optionally preceded by a text declaration. Once the client has completed SASL negotiation (SASL Negotiation) and resource binding (Resource Binding), the client can send an unbounded number of XML stanzas over the stream. When the client desires to close the stream, it simply sends a closing </stream> tag to the server as further described under Section 4.3 (Closing a Stream).

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

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

Those who are accustomed to thinking of XML in a document-centric manner might find the following analogies useful:

However, these analogies are merely that, because XMPP does not deal in documents and fragments but in streams and stanzas.

The remainder of this section defines the following aspects of XML streams:



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4.2.  Stream Negotiation



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4.2.1.  Overview

Because the receiving entity for a stream acts as a gatekeeper to the domains it services, it imposes certain conditions for connecting as a client or as a peer server. At a minimum, the initiating entity needs to authenticate with the receiving entity before it is allowed to send stanzas to the receiving entity, typically using SASL as described under Section 6 (SASL Negotiation). However, the receiving entity can consider conditions other than authentication to be mandatory, such as encryption using TLS as described under Section 5 (STARTTLS Negotiation). The receiving entity informs the initiating entity about such conditions by communicating STREAM FEATURES: the set of particular protocol interactions that are mandatory for the initiating entity to complete before the receiving entity will accept XML stanzas from the initiating entity (e.g., authentication), as well as any protocol interactions that are voluntary but that might improve the handling of an XML stream (e.g., establishment of application-layer compression as described in [XEP‑0138] (Hildebrand, J. and P. Saint-Andre, “Stream Compression,” May 2009.)).

The existence of conditions for connecting implies that streams need to be negotiated. The order of layers (TCP, then TLS, then SASL, then XMPP; see Section 13.3 (Order of Layers)) implies that stream negotiation is a multi-stage process. Further structure is imposed by two factors: (1) a given stream feature might be offered only to certain entities or only after certain other features have been negotiated (e.g., resource binding is offered only after SASL authentication), and (2) stream features can be either mandatory-to-negotiate or voluntary-to-negotiate. Finally, for security reasons the parties to a stream need to discard knowledge that they gained during the negotiation process after successfully completing the protocol interactions defined for certain features (e.g., TLS in all cases and SASL in the case when a security layer might be established, as defined in the specification for the relevant SASL mechanism); this is done by flushing the old stream context and exchanging new stream headers over the existing TCP connection.



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4.2.2.  Stream Features Format

If the initiating entity includes the 'version' attribute set to a value of at least "1.0" in the initial stream header, after sending the response stream header the receiving entity MUST send a <features/> child element (prefixed by the streams namespace prefix) to the initiating entity in order to announce any conditions for continuation of the stream negotiation process. Each condition takes the form of a child element of the <features/> element, qualified by a namespace that is different from the streams namespace and the default namespace. The <features/> element can contain one child, contain multiple children, or be empty.

Implementation Note: The order of child elements contained in any given <features/> element is not significant.

If a particular stream feature is or can be mandatory-to-negotiate, the definition of that feature needs to do one of the following:

  1. Declare that the feature is always mandatory-to-negotiate (e.g., this is true of resource binding for XMPP clients); or

  2. Specify a way for the receiving entity to flag the feature as mandatory-to-negotiate for this interaction (e.g., this is done for TLS by including an empty <required/> element in the advertisement for that stream feature); it is RECOMMENDED that stream feature definitions for mandatory-to-negotiate features do so by including an empty <required/> element as is done for TLS.

Informational Note: Because there is no generic format for indicating that a feature is mandatory-to-negotiate, it is possible that a feature which is not understood by the initiating entity might be considered mandatory-to-negotiate by the receiving entity, resulting in failure of the stream negotiation process. Although such an outcome would be undesirable, the working group deemed it rare enough that a generic format was not needed.

For security reasons, certain stream features necessitate the initiating entity to send a new initial stream header upon successful negotiation of the feature (e.g., TLS in all cases and SASL in the case when a security layer might be established). If this is true of a given stream feature, the definition of that feature needs to declare that a stream restart is expected after negotiation of the feature.

A <features/> element that contains at least one mandatory-to-negotiate feature indicates that the stream negotiation is not complete and that the initiating entity MUST negotiate further features.

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

A <features/> element MAY contain more than one mandatory feature. This means that the initiating entity can choose among the mandatory features. For example, perhaps a future technology will perform roughly the same function as TLS, so the receiving entity might advertise support for both TLS and the future technology.

A <features/> element that contains both mandatory and voluntary features indicates that the negotiation is not complete but that the initiating entity MAY complete the voluntary feature(s) before it attempts to negotiate the mandatory feature(s).

R: <stream:features>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
     <compression xmlns='http://jabber.org/features/compress'>
       <method>zlib</method>
       <method>lzw</method>
     </compression>
   </stream:features>

A <features/> element that contains only voluntary features indicates that the stream negotiation is complete and that the initiating entity is cleared to send XML stanzas, but that the initiating entity MAY negotiate further features if desired.

R: <stream:features>
     <compression xmlns='http://jabber.org/features/compress'>
       <method>zlib</method>
       <method>lzw</method>
     </compression>
   </stream:features>

An empty <features/> element indicates that the stream negotiation is complete and that the initiating entity is cleared to send XML stanzas.

R: <stream:features/>


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4.2.3.  Restarts

On successful negotiation of a feature that necessitates a stream restart, both parties MUST consider the previous stream to be replaced but MUST NOT terminate the underlying TCP connection; instead, the parties MUST reuse the existing connection, which might be in a new state (e.g., encrypted as a result of TLS negotiation). The initiating entity then MUST send a new initial stream header, which SHOULD be preceded by an XML declaration as described under Section 11.5 (Inclusion of XML Declaration). When the receiving entity receives the new initial stream header, it MUST generate a new stream ID (instead of re-using the old stream ID) before sending a new response stream header (which SHOULD be preceded by an XML declaration as described under Section 11.5 (Inclusion of XML Declaration)).



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4.2.4.  Resending Features

The receiving entity MUST send an updated list of stream features to the initiating entity after a stream restart, and MAY do so after completing negotiation of a stream feature that does not require a stream restart. The list of updated features MAY be empty if there are no further features to be advertised or MAY include any combination of features.



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4.2.5.  Completion of Stream Negotiation

The receiving entity indicates completion of the stream negotiation process by sending to the initiating entity either an empty <features/> element or a <features/> element that contains only voluntary features. After doing so, the receiving entity MAY send an empty <features/> element (e.g., after negotiation of such voluntary features) but MUST NOT send additional stream features to the initiating entity (if the receiving entity has new features to offer, preferably limited to mandatory-to-negotiate or security-critical features, it can simply close the stream using a <reset/> stream error and then advertise the new features when the initiating entity reconnects, preferably closing existing streams in a staggered way so that not all of the initiating entities reconnect at once). Once stream negotiation is complete, the initiating entity is cleared to send XML stanzas over the stream for as long as the stream is maintained by both parties.

Informational Note: Resource binding as specified under Section 7 (Resource Binding) is an historical exception to the foregoing rule, since it is mandatory-to-negotiate for clients but uses XML stanzas for negotiation purposes.

The initiating entity MUST NOT attempt to send XML stanzas (XML Stanzas) to entities other than itself (i.e., the client's connected resource or any other authenticated resource of the client's account) or the server to which it is connected until stream negotiation has been completed. However, if it does attempt to do so, the receiving entity MUST NOT accept such stanzas and MUST return a <not-authorized/> stream error. This rule applies to XML stanzas only (i.e., <message/>, <presence/>, and <iq/> elements qualified by the default namespace) and not to XML elements used for stream negotiation (e.g., elements used to complete TLS negotiation (STARTTLS Negotiation) or SASL negotiation (SASL Negotiation)).



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4.2.6.  Determination of Addresses

After the parties to an XML stream have completed the appropriate aspects of stream negotiation (typically SASL negotiation (SASL Negotiation) and, for client-to-server streams, resource binding (Resource Binding)) the receiving entity for a stream MUST determine the initiating entity's JID.

For client-to-server communication, the client's bare JID (<localpart@domainpart>) MUST be the authorization identity (as defined by [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.)), either (1) as directly communicated by the client during SASL negotiation (SASL Negotiation) or (2) as derived by the server from the authentication identity if no authorization identity was specified during SASL negotiation (SASL Negotiation). The resourcepart of the full JID (<localpart@domainpart/resource>) MUST be the resource negotiated by the client and server during resource binding (Resource Binding). A client MUST NOT attempt to guess at its JID but instead MUST consider its JID to be whatever the server returns to it during resource binding.

For server-to-server communication, the initiating server's JID MUST be the authentication identity communicated by the initiating entity during SASL negotiation (SASL Negotiation); in the absence of SASL negotiation (e.g., when the older server dialback protocol is used as specified in [XEP‑0220] (Miller, J., Saint-Andre, P., and P. Hancke, “Server Dialback,” March 2010.)), the receiving server MAY consider the authentication identity to be the 'from' address on the initial stream header sent after TLS negotiation (information contained in stream headers sent before successful TLS negotation can be modified in transit by a third party). Note that authorization identities are not allowed during SASL negotiation for server-to-server streams; for details, see under Section 6.2.8 (Authorization Identity).

The receiving entity MUST ensure that the resulting JID (including localpart, domainpart, resourcepart, and separator characters) conforms to the canonical format for XMPP addresses defined in [XMPP‑ADDR] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Address Format,” July 2010.); to meet this restriction, the receiving entity MAY replace the JID sent by the initiating entity with the canonicalized JID as determined by the receiving entity.



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4.2.7.  Flow Chart

We summarize the foregoing rules in the following non-normative flow chart for the stream negotiation process, presented from the perspective of the initiating entity.

                     +------------+
                     |  open TCP  |
                     | connection |
                     +------------+
                           |
                           v
                    +---------------+
                    | send initial  |<-------------------------+
                    | stream header |                          ^
                    +---------------+                          |
                           |                                   |
                           v                                   |
                   +------------------+                        |
                   | receive response |                        |
                   | stream header    |                        |
                   +------------------+                        |
                           |                                   |
                           v                                   |
                    +----------------+                         |
                    | receive stream |                         |
+------------------>| features       |                         |
^                   +----------------+                         |
|                          |                                   |
|                          v                                   |
|       +<-----------------+                                   |
|       |                                                      |
|    {empty?} ----> {all voluntary?} ----> {some mandatory?}   |
|       |      no          |          no         |             |
|       | yes              | yes                 | yes         |
|       |                  v                     v             |
|       |           +---------------+    +----------------+    |
|       |           | MAY negotiate |    | MUST negotiate |    |
|       |           | any or none   |    | one feature    |    |
|       |           +---------------+    +----------------+    |
|       |                  |                     |             |
|       v                  v                     |             |
|   +----------+      +-----------+              |             |
|   | process  |<-----| negotiate |              |             |
|   | complete |  no  | a feature |              |             |
|   +----------+      +-----------+              |             |
|                          |                     |             |
|                     yes  |                     |             |
|                          v                     v             |
|                          +--------->+<---------+             |
|                                     |                        |
|                                     v                        |
+<-------------------------- {restart mandatory?} ------------>+
               no                                     yes



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4.3.  Closing a Stream

An XML stream between two entities can be closed at any time, either because a specific stream error has occurred or in the absence of an error (e.g., when a client simply ends its session).

A stream is closed by sending a closing </stream> tag.

S: </stream:stream>

The entity that sends the closing stream tag SHOULD behave as follows:

  1. Wait for the other party to also close its stream before terminating the underlying TCP connection (this gives the other party an opportunity to finish transmitting any data in the opposite direction before the TCP connection is terminated).

  2. Refrain from initiating the sending of further data over that stream but continue to process data sent by the other entity (and, if necessary, react to such data).

  3. Consider both streams to be void if the other party does not send its closing stream tag within a reasonable amount of time (where the definition of "reasonable" is a matter of implementation or deployment).

  4. After receiving a reciprocal closing stream tag from the other party or waiting a reasonable amount of time with no response, MUST terminate the underlying TCP connection.



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4.4.  Handling of Silent Peers

When an entity that is a party to a stream has not received any XMPP traffic from its stream peer for some period of time, the peer might appear to be silent. There are several reasons why this might happen:

  1. The peer is idle and simply has not sent any XMPP traffic over the XML stream.

  2. The XML stream is broken despite the fact that the underlying TCP connection is alive.

  3. The underlying TCP connection is dead.

These three conditions are best handled separately, as described in the following sections.



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4.4.1.  Idle Peer

If the stream peer is idle at the XMPP application layer, it is appropriate for the entity to simply close the stream using the handshake described under Section 4.3 (Closing a Stream). The timeout period is a matter of implementation and local service policy; however, it is RECOMMENDED to be liberal in accepting idle peers, since experience has shown that doing so improves the reliability of communication over XMPP networks. In particular, it is typically more efficient to maintain a stream between two servers than it is to aggressively timeout such a stream, especially with regard to synchronization of presence information as described in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.); therefore it is RECOMMENDED to maintain such a stream since experience has shown that server-to-server streams are cyclical and typically need to be re-established fairly frequently (e.g. at least every 24 hours) if they are timed out because the peer is idle.



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4.4.2.  Broken Stream

In this case, the peer never responds to XMPP traffic that the entity sends, whether normal stanzas or specialized stream-checking traffic such as the application-level pings defined in [XEP‑0199] (Saint-Andre, P., “XMPP Ping,” June 2009.) or the more comprehensive stream management protocol defined in [XEP‑0198] (Karneges, J., Hildebrand, J., Saint-Andre, P., and F. Forno, “Stream Management,” June 2009.). It is appropriate for the entity to terminate a broken stream using the <connection-timeout/> stream error described under Section 4.7.3.4 (connection-timeout).



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4.4.3.  Dead Connection

If the underlying TCP connection is dead, stream-level checks (e.g., [XEP‑0199] (Saint-Andre, P., “XMPP Ping,” June 2009.) and [XEP‑0198] (Karneges, J., Hildebrand, J., Saint-Andre, P., and F. Forno, “Stream Management,” June 2009.)) are ineffective. Therefore it is unnecessary to close the stream with or without an error, and it is appropriate instead to simply close the TCP connection.

One common method for checking the TCP connection is to send a space character (U+0020) between XML stanzas, which is allowed for XML streams as described under Section 11.7 (Whitespace); the sending of such a space character is properly called a WHITESPACE KEEPALIVE (the term "whitespace ping" is often used, despite the fact that it is not a ping since no "pong" is possible).



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4.4.4.  Use of Checking Methods

Implementers are advised to support whichever stream-checking and connection-checking methods they deem appropriate, but to carefully weigh the network impact of such methods against the benefits of discovering broken streams and dead TCP connections in a timely manner. The length of time between the use of any particular check is very much a matter of local service policy and depends strongly on the network environment and usage scenarios of a given deployment and connection type; at the time of writing, it is RECOMMENDED that any such check be performed not more than once every 5 minutes and that, ideally, such checks will be initiated by clients rather than servers. Those who implement XMPP software and deploy XMPP services are encouraged to seek additional advice regarding appropriate timing of stream-checking and connection-checking methods, particularly when power-constrained devices are being used (e.g., in mobile environments).



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4.5.  Stream Attributes

The attributes of the root <stream/> element are defined in the following sections.

Security Note: Until and unless the confidentiality and integrity of a stream header is ensured via Transport Layer Security as described under Section 5 (STARTTLS Negotiation), the attributes provided in a stream header could be tampered with by an attacker.

Implementation Note: The attributes of the root <stream/> element are not prepended by a namespace prefix because, as explained in [XML‑NAMES] (Thompson, H., Hollander, D., Layman, A., Bray, T., and R. Tobin, “Namespaces in XML 1.0 (Third Edition),” December 2009.), "[d]efault namespace declarations do not apply directly to attribute names; the interpretation of unprefixed attributes is determined by the element on which they appear."



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4.5.1.  from

The 'from' attribute communicates an XMPP identity of the entity sending the stream element.

For initial stream headers in client-to-server communication, if the client knows the XMPP identity of the principal controlling the client (typically an account name of the form <localpart@domainpart>), then it SHOULD include the 'from' attribute and set its value to that identity once the stream is in a state in which it is willing to perform authentication, e.g. once TLS has been negotiated. However, because the client might not know the XMPP identity of the principal controlling the entity (e.g., because the XMPP identity is assigned at a level other than the XMPP application layer, as in the General Security Service Application Program Interface [GSS‑API] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.)), inclusion of the 'from' address is OPTIONAL.

Security Note: Including the XMPP identity before the stream is protected via TLS can expose that identity to eavesdroppers.

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

For initial stream headers in server-to-server communication, a server MUST include the 'from' attribute and MUST set the value to one of its own hostnames (because the initiating entity might have more than one XMPP identity, e.g., in the case of a server that provides virtual hosting, it will need to choose an identity that is associated with this stream).

I: <?xml version='1.0'?>
   <stream:stream
       from='example.net'
       to='im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'>

For response stream headers in both client-to-server and server-to-server communication, the receiving entity MUST include the 'from' attribute and MUST set the value to one of the receiving entity's hostnames (which MAY be a hostname other than that specified in the 'to' attribute of the initial stream header; see Section 4.7.1.3 (Stream Errors When the Host is Unspecified or Unknown) and Section 4.7.3.6 (host-unknown)).

R: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>

Whether or not the 'from' attribute is included, each entity MUST verify the identity of the other entity before exchanging XML stanzas with it, as described under Section 13.5 (Peer Entity Authentication).

Interoperability Note: It is possible that implementations based on [RFC3920] (Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.) will not include the 'from' address on stream headers; an entity SHOULD be liberal in accepting such stream headers.



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4.5.2.  to

For initial stream headers in both client-to-server and server-to-server communication, the initiating entity MUST include the 'to' attribute and MUST set its value to a hostname that the initiating entity knows or expects the receiving entity to service. (The same information can be provided in other ways, such as a server name indication during TLS negotiation as described in [TLS‑EXT] (3rd, D., “Transport Layer Security (TLS) Extensions: Extension Definitions,” June 2010.).)

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

For response stream headers in client-to-server communication, if the client included a 'from' attribute in the initial stream header then the server MUST include a 'to' attribute in the response stream header and MUST set its value to the bare JID specified in the 'from' attribute of the initial stream header. If the client did not include a 'from' attribute in the initial stream header then the server MUST NOT include a 'to' attribute in the response stream header.

R: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>

For response stream headers in server-to-server communication, the receiving entity MUST include a 'to' attribute in the response stream header and MUST set its value to the hostname specified in the 'from' attribute of the initial stream header.

R: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
       to='example.net'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'>

Whether or not the 'to' attribute is included, each entity MUST verify the identity of the other entity before exchanging XML stanzas with it, as described under Section 13.5 (Peer Entity Authentication).

Interoperability Note: It is possible that implementations based on [RFC3920] (Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.) will not include the 'to' address on stream headers; an entity SHOULD be liberal in accepting such stream headers.



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4.5.3.  id

The 'id' attribute communicates a unique identifier for the stream, called a STREAM ID. The stream ID MUST be generated by the receiving entity when it sends a response stream header and MUST BE unique within the receiving application (normally a server).

Security Note: The stream ID MUST be both unpredictable and non-repeating because it can be security-critical (see [RANDOM] (Eastlake, D., Schiller, J., and S. Crocker, “Randomness Requirements for Security,” June 2005.) for recommendations regarding randomness for security purposes).

For initial stream headers, the initiating entity MUST NOT include the 'id' attribute; however, if the 'id' attribute is included, the receiving entity MUST ignore it.

For response stream headers, the receiving entity MUST include the 'id' attribute.

R: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>


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4.5.4.  xml:lang

The 'xml:lang' attribute communicates an entity's preferred or default language for any human-readable XML character data to be sent over the stream (an XML stanza can also possess an 'xml:lang' attribute, as discussed under Section 8.1.5 (xml:lang)). The syntax of this attribute is defined in Section 2.12 of [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.); in particular, the value of the 'xml:lang' attribute MUST conform to the NMTOKEN datatype (as defined in Section 2.3 of [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.)) and MUST conform to the language identifier format defined in [LANGTAGS] (Phillips, A. and M. Davis, “Tags for Identifying Languages,” September 2009.).

For initial stream headers, the initiating entity SHOULD include the 'xml:lang' attribute.

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

For response stream headers, the receiving entity MUST include the 'xml:lang' attribute. The following rules apply:

R: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>

If the initiating entity included the 'xml:lang' attribute in its initial stream header, the receiving entity SHOULD remember that value as the default xml:lang for all stanzas sent by the initiating entity over the current stream. As described under Section 8.1.5 (xml:lang), the initiating entity MAY include the 'xml:lang' attribute in any XML stanzas it sends over the stream. If the initiating entity does not include the 'xml:lang' attribute in any such stanza, the receiving entity SHOULD add the 'xml:lang' attribute to the stanza, where the value of the attribute MUST be the identifier for the language preferred by the initiating entity (even if the receiving entity does not support that language for human-readable XML character data it generates and sends to the initiating entity, such as in stream or stanza errors). If the initiating entity includes the 'xml:lang' attribute in any such stanza, the receiving entity MUST NOT modify or delete it.



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4.5.5.  version

The inclusion of the version attribute set to a value of at least "1.0" signals support for the stream-related protocols defined in this specification, including TLS negotiation (STARTTLS Negotiation), SASL negotiation (SASL Negotiation), stream features (Stream Features Format), and stream errors (Stream Errors).

The version of XMPP specified in this specification is "1.0"; in particular, XMPP 1.0 encapsulates the stream-related protocols as well as the basic 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 will be incremented only if the stream and stanza formats or obligatory 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 defined in the older specification.

The minor version number will be incremented only if significant new capabilities have been added to the core protocol (e.g., a newly defined value of the 'type' attribute for message, presence, or IQ stanzas). The minor version number MUST be ignored by an entity with a smaller minor version number, but MAY be used for informational purposes by the entity with the larger minor version number (e.g., the entity with the larger minor version number would simply note that its correspondent would not be able to understand that value of the 'type' attribute and therefore would not send it).

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

  1. The initiating entity MUST set the value of the 'version' attribute in the initial stream header to the highest version number it supports (e.g., if the highest version number it supports is that defined in this specification, it MUST set the value to "1.0").

  2. The receiving entity MUST set the value of the 'version' attribute in the response stream header to either the value supplied by the initiating entity or the highest version number supported by the receiving entity, whichever is lower. The receiving entity MUST perform a numeric comparison on the major and minor version numbers, not a string match on "<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, the initiating entity SHOULD generate an <unsupported-version/> stream error.

  4. If either entity receives a stream header with no 'version' attribute, the entity MUST consider the version supported by the other entity to be "0.9" and SHOULD NOT include a 'version' attribute in the response stream header.



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4.5.6.  Summary of Stream Attributes

The following table summarizes the attributes of the root <stream/> element.

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


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4.6.  Namespaces

Readers are referred to the specification of XML namespaces [XML‑NAMES] (Thompson, H., Hollander, D., Layman, A., Bray, T., and R. Tobin, “Namespaces in XML 1.0 (Third Edition),” December 2009.) for a full understanding of the concepts used in this section.



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4.6.1.  Streams Namespace

The root <stream/> element ("stream header") MUST be qualified by the namespace 'http://etherx.jabber.org/streams' (the "streams namespace"). If this rule is violated, the entity that receives the offending stream header MUST return a stream error to the sending entity, which SHOULD be <invalid-namespace/> (although some existing implementations send <bad-format/> instead).



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4.6.2.  Default Namespace

An entity MAY declare a default namespace for data sent over the stream. If so, (1) the default namespace MUST be other than the streams namespace, and (2) the default namespace MUST be the same for the initial stream and the response stream so that both streams are qualified consistently. The default namespace applies to all first-level child elements sent over the stream unless explicitly qualified by another namespace.

Alternatively (i.e., instead of declaring a default namespace), an entity MAY explicitly qualify the namespace for each first-level child element of the stream, using so-called "prefix-free canonicalization".

When a default namespace is declared, in rough outline a stream will look something like the following.

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

When a default namespace is not declared and so-called "prefix-free canonicalization" is used instead, in rough outline a stream will look something like the following.

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

Historically, most XMPP implementations have used the default namespace style rather than the prefix-free canonicalization style for stream headers; however, both styles are acceptable since they are semantically equivalent.



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4.6.3.  Other Namespaces

Either party to a stream MAY send data qualified by namespaces other than the default namespace (if declared) and the streams namespace. For example, this is how data related to TLS negotiation and SASL negotiation are exchanged, as well as XMPP extensions such as server dialback [XEP‑0220] (Miller, J., Saint-Andre, P., and P. Hancke, “Server Dialback,” March 2010.) and stream management [XEP‑0198] (Karneges, J., Hildebrand, J., Saint-Andre, P., and F. Forno, “Stream Management,” June 2009.).



 TOC 

4.6.4.  Namespace Declarations and Prefixes

Because the default namespace is other than the streams namespace, if a default namespace is declared then the following statements are true:

  1. The stream header needs to contain a namespace declaration for both the default namespace and the streams namespace.

  2. The streams namespace declaration needs to include a namespace prefix for the streams namespace.

Interoperability Note: For historical reasons, an implementation MAY accept only the prefix 'stream' for the streams namespace (resulting in prefixed names such as <stream:stream> and <stream:features>). If an entity receives a stream header with a streams namespace prefix it does not accept, it MUST return a stream error to the sending entity, which SHOULD be <bad-namespace-prefix/> (although some existing implementations send <bad-format/> instead).

By definition, the namespace declaration for the default namespace will not include a namespace prefix (e.g., "xmlns='jabber:client'"). Furthermore, an implementation MUST NOT generate namespace prefixes for elements qualified by the default namespace if the default namespace is 'jabber:client' or 'jabber:server'.

Namespaces declared in a stream header MUST apply only to that stream (e.g., the 'jabber:server:dialback' namespace used in server dialback [XEP‑0220] (Miller, J., Saint-Andre, P., and P. Hancke, “Server Dialback,” March 2010.)). In particular, because XML stanzas intended for routing or delivery over streams with other entities will lose the namespace context declared in the header of the stream in which those stanzas originated, namespaces for extended content within such stanzas MUST NOT be declared in that stream header (see also Section 8.4 (Extended Content)). If either party to a stream declares such namespaces, the other party to the stream SHOULD close the stream with a stream error of <invalid-namespace/>. In any case, an entity MUST ensure that such namespaces are properly declared (according to this section) when routing or delivering stanzas originating from such a stream over streams with other entities.



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4.6.5.  Mandatory-to-Implement Default Namespaces

XMPP as defined in this specification uses two default namespaces: 'jabber:client' and 'jabber:server'. These namespaces are nearly identical but are used in different contexts (client-to-server communication for 'jabber:client' and server-to-server communication for 'jabber:server'). The only difference between the two is that the 'to' and 'from' attributes are OPTIONAL on stanzas sent over XML streams qualified by the 'jabber:client' namespace, whereas they are REQUIRED on stanzas sent over XML streams qualified by the 'jabber:server' namespace. Support for these default namespaces implies support for the common attributes (Common Attributes) and basic semantics (Basic Semantics) of all three core stanza types (message, presence, and IQ).

An implementation MAY support default namespaces other than 'jabber:client' or 'jabber:server'. However, because such namespaces would define applications other than XMPP, they are to be defined in separate specifications.

An implementation MAY refuse to support any other default namespaces. If an entity receives a first-level child element qualified by a default namespace it does not support, it MUST return an <invalid-namespace/> stream error.

Client implementations MUST support the 'jabber:client' default namespace.

Server implementations MUST support both the 'jabber:client' default namespace (when the stream is used for communication between a client and a server) and the 'jabber:server' default namespace (when the stream is used for communication between two servers).

Implementation Note: Because a client sends stanzas over a stream whose default namespace is 'jabber:client', if the server to which the client is connected needs to route a client-generated stanza to another server then it MUST "re-scope" the stanza so that its default namespace is 'jabber:server' (i.e., it MUST NOT send a stanza qualified by the 'jabber:client' namespace over a stream whose default namespace is 'jabber:server'). Similarly, a routing server MUST "re-scope" a stanza received over a server-to-server stream (whose default namespace is 'jabber:server') so that the stanza is qualified by the 'jabber:client' namespace before sending it over a client-to-server stream (whose default namespace is 'jabber:client').



 TOC 

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 SHALL be sent by a compliant entity if it perceives that a stream-level error has occurred.



 TOC 

4.7.1.  Rules

The following rules apply to stream-level errors.



 TOC 

4.7.1.1.  Stream Errors Are Unrecoverable

Stream-level errors are unrecoverable. Therefore, if an error occurs at the level of the stream, the entity that detects the error MUST send an <error/> element with an appropriate child element that specifies the error condition and immediately close the stream as described under Section 4.3 (Closing a Stream).

C: <message><body>No closing tag!</message>

S: <stream:error>
     <xml-not-well-formed
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>

The entity that generates the stream error then shall close the stream as explained under Section 4.3 (Closing a Stream).

C: </stream:stream>


 TOC 

4.7.1.2.  Stream Errors Can Occur During Setup

If the error is triggered by the initial stream header, the receiving entity MUST still send the opening <stream> tag, include the <error/> element as a child of the stream element, and send the closing </stream> tag (preferably all at the same time).

C: <?xml version='1.0'?>
   <stream:stream
       from='juliet@im.example.com'
       to='im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://wrong.namespace.example.org/'>

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>
   <stream:error>
     <invalid-namespace
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.1.3.  Stream Errors When the Host is Unspecified or Unknown

If the initiating entity provides no 'to' attribute or provides an unknown host in the 'to' attribute and the error occurs during stream setup, the value of the 'from' attribute returned by the receiving entity in the stream header sent before closing the stream MUST be either an authoritative hostname for the receiving entity or the empty string.

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>
   <stream:error>
     <host-unknown
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.1.4.  Where Stream Errors Are Sent

When two TCP connections are used between the initiating entity and the receiving entity (one in each direction) rather than using a single bidirectional connection, the following rules apply:



 TOC 

4.7.2.  Syntax

The syntax for stream errors is as follows, where "defined-condition" is a placeholder for one of the conditions defined under Section 4.7.3 (Defined Stream Error Conditions) and XML data shown within the square brackets '[' and ']' is OPTIONAL.

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

The <error/> element:

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



 TOC 

4.7.3.  Defined Stream Error Conditions

The following stream-level error conditions are defined.



 TOC 

4.7.3.1.  bad-format

The entity has sent XML that cannot be processed.

(In the following example, the client sends an XMPP message that is not well-formed XML, which alternatively might trigger an <xml-not-well-formed/> stream error.)

C: <message>
     <body>No closing tag!
   </message>

S: <stream:error>
     <bad-format
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>

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/>. However, the more specific errors are RECOMMENDED.



 TOC 

4.7.3.2.  bad-namespace-prefix

The entity has sent a namespace prefix that is unsupported, or has sent no namespace prefix on an element that needs such a prefix (see Section 11.2 (XML Namespace Names and Prefixes)).

(In the following example, the client specifies a namespace prefix of "foobar" for the XML streams namespace.)

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>
   <stream:error>
     <bad-namespace-prefix
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.3.  conflict

The server either (1) is closing the existing stream for this entity because a new stream has been initiated that conflicts with the existing stream, or (2) is refusing a new stream for this entity because allowing the new stream would conflict with an existing stream (e.g., because the server allows only a certain number of connections from the same IP address).

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>
   <stream:error>
     <conflict
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.4.  connection-timeout

One party is closing the stream because it has reason to believe that the other party has permanently lost the ability to communicate over the stream. The lack of ability to communicate can be discovered using various methods, such as whitespace keepalives as specified under Section 4.3 (Closing a Stream), XMPP-level pings as defined in [XEP‑0199] (Saint-Andre, P., “XMPP Ping,” June 2009.), and XMPP stream management as defined in [XEP‑0198] (Karneges, J., Hildebrand, J., Saint-Andre, P., and F. Forno, “Stream Management,” June 2009.).

P: <stream:error>
     <connection-timeout
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>

Interoperability Note: RFC 3920 specified that the <connection-timeout/> stream error is to be used if the peer has not generated any traffic over the stream for some period of time. That behavior is no longer recommended; instead, the error SHOULD be used only if the connected client or peer server has not responded to data sent over the stream.



 TOC 

4.7.3.5.  host-gone

The value of the 'to' attribute provided in the initial stream header corresponds to a hostname that is no longer serviced by the receiving entity.

(In the following example, the peer specifies a 'to' address of "foo.im.example.com" when connecting to the "im.example.com" server, but the server no longer hosts a service at that address.)

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
       to='example.net'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'>
   <stream:error>
     <host-gone
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.6.  host-unknown

The value of the 'to' attribute provided in the initial stream header does not correspond to a hostname that is serviced by the receiving entity.

(In the following example, the peer specifies a 'to' address of "example.org" when connecting to the "im.example.com" server, but the server knows nothing of that address.)

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
       to='example.net'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'>
   <stream:error>
     <host-unknown
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.7.  improper-addressing

A stanza sent between two servers lacks a 'to' or 'from' attribute, the 'from' or 'to' attribute has no value, or the value is not a valid XMPP address.

(In the following example, the peer sends a stanza without a 'to' address over a server-to-server stream.)

P: <message from='juliet@im.example.com'>
     <body>Wherefore art thou?</body>
   </message>

S: <stream:error>
     <improper-addressing
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.8.  internal-server-error

The server has experienced a misconfiguration or an otherwise-undefined internal error that prevents it from servicing the stream.

S: <stream:error>
     <internal-server-error
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.9.  invalid-from

The JID or hostname provided in a 'from' address is not a valid JID or does not match an authorized JID or validated domain as negotiated between servers via SASL or server dialback, or as negotiated between a client and a server via authentication and resource binding.

(In the following example, a peer that has authenticated only as "example.net" attempts to send a stanza from an address at "example.org".)

P: <message from='romeo@example.org' to='juliet@im.example.com'>
     <body>Neither, fair saint, if either thee dislike.</body>
   </message>

S: <stream:error>
     <invalid-from
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.10.  invalid-namespace

The streams namespace name is something other than "http://etherx.jabber.org/streams" (see Section 11.2 (XML Namespace Names and Prefixes)) or the default namespace is not supported (e.g., something other than "jabber:client" or "jabber:server").

(In the following example, the client specifies a namespace of 'http://wrong.namespace.example.org/' for the stream.)

C: <?xml version='1.0'?>
   <stream:stream
       from='juliet@im.example.com'
       to='im.example.com'
       version='1.0'
       xmlns='jabber:client'
       xmlns:stream='http://wrong.namespace.example.org/'>

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>
   <stream:error>
     <invalid-namespace
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.11.  invalid-xml

The entity has sent invalid XML over the stream to a server that performs validation (see Section 11.4 (Validation)).

(In the following example, the peer attempts to send an IQ stanza of type "subscribe" but the XML schema defines no such value for the 'type' attribute.)

P: <iq from='example.net'
       id='l3b1vs75'
       to='im.example.com'
       type='subscribe'>
     <ping xmlns='urn:xmpp:ping'/>
   </iq>

S: <stream:error>
     <invalid-xml
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.12.  not-authorized

The entity has attempted to send XML stanzas before the stream has been authenticated, or otherwise is not authorized to perform an action related to stream negotiation; the receiving entity MUST NOT process the offending stanza before sending the stream error.

(In the following example, the client attempts to send XML stanzas before authenticating with the server.)

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'

C: <message to='romeo@example.net'>
     <body>Wherefore art thou?</body>
   </message>

S: <stream:error>
     <not-authorized
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.13.  policy-violation

The entity has violated some local service policy (e.g., the stanza exceeds a configured size limit); the server MAY choose to specify the policy in the <text/> element or in an application-specific condition element.

(In the following example, the client sends an XMPP message that is too large according to the server's local service policy.)

C: <message to='juliet@im.example.com' id='foo'>
     <body>[ ... the-emacs-manual ... ]</body>
   </message>

S: <stream:error>
     <policy-violation
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
     <stanza-too-big xmlns='urn:xmpp:errors'/>
   </stream:error>

S: </stream:stream>


 TOC 

4.7.3.14.  remote-connection-failed

The server is unable to properly connect to a remote entity that is needed for authentication or authorization, such as a remote authentication database or (in server dialback) the authoritative server.

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>
   <stream:error>
     <remote-connection-failed
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.15.  reset

The server is closing the stream because it has new (typically security-critical) features to offer or because it needs to reset the stream for some other reason (e.g., because the certificates used to establish a secure context for the stream have expired or have been revoked during the life of the stream).


S: <stream:error>
     <reset
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.16.  resource-constraint

The server lacks the system resources necessary to service the stream.

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>
   <stream:error>
     <resource-constraint
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.17.  restricted-xml

The entity has attempted to send restricted XML features such as a comment, processing instruction, DTD subset, or XML entity reference (see Section 11.1 (Restrictions)).

(In the following example, the client sends an XMPP message containing an XML comment.)

C: <message to='juliet@im.example.com'>
     <!--<subject/>-->
     <body>This message has no subject.</body>
   </message>

S: <stream:error>
     <restricted-xml
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.18.  see-other-host

The server will not provide service to the initiating entity but is redirecting traffic to another host; the XML character data of the <see-other-host/> element returned by the server SHOULD specify the alternate hostname or IP address at which to connect, which SHOULD be a valid domainpart but MAY also include a port number. When it receives a see-other-host stream error, the initiating entity SHOULD cleanly handle the disconnection and then reconnect to the host specified in the <see-other-host/> element; if no port is specified, the initiating entity SHOULD perform a [DNS‑SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) lookup on the provided domainpart but MAY assume that it can connect to that domainpart at the standard XMPP ports (i.e., 5222 for client-to-server connections and 5269 for server-to-server connections).

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>
   <stream:error>
     <see-other-host
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
       [2001:41D0:1:A49b::1]:9222
     </see-other-host>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.19.  system-shutdown

The server is being shut down and all active streams are being closed.

S: <stream:error>
     <system-shutdown
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.20.  undefined-condition

The error condition is not one of those defined by the other conditions in this list; this error condition SHOULD be used only in conjunction with an application-specific condition.

S: <stream:error>
     <undefined-condition
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
     <app-error xmlns='http://example.com/ns'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.21.  unsupported-encoding

The initiating entity has encoded the stream in an encoding that is not supported by the server (see Section 11.6 (Character Encoding)) or has otherwise improperly encoded the stream (e.g., by violating the rules of the [UTF‑8] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) encoding).

(In the following example, the client attempts to encode data using UTF-16 instead of UTF-8.)

C: <?xml version='1.0' encoding='UTF-16'?>
   <stream:stream
       from='juliet@im.example.com'
       to='im.example.com'
       version='1.0'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
   <stream:error>
     <unsupported-encoding
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.22.  unsupported-feature

The receiving entity has advertised a mandatory stream feature that the initiating entity does not support, and has offered no other mandatory feature alongside the unsupported feature.

(In the following example, the receiving entity requires negotiation of an example feature but the initiating entity does not support the feature.)

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

I: <stream:error>
     <unsupported-feature
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.23.  unsupported-stanza-type

The initiating entity has sent a first-level child of the stream that is not supported by the server, either because the receiving entity does not understand the namespace or because receiving entity does not understand the element name for the applicable namespace (which might be the default namespace).

(In the following example, the client attempts to send a first-level child element of <pubsub/> qualified by the 'jabber:client' namespace, but the schema for that namespace defines no such element.)

C: <pubsub xmlns='jabber:client'>
     <publish node='princely_musings'>
       <item id='ae890ac52d0df67ed7cfdf51b644e901'>
         <entry xmlns='http://www.w3.org/2005/Atom'>
           <title>Soliloquy</title>
           <summary>
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?
           </summary>
           <link rel='alternate' type='text/html'
                 href='http://denmark.example/2003/12/13/atom03'/>
           <id>tag:denmark.example,2003:entry-32397</id>
           <published>2003-12-13T18:30:02Z</published>
           <updated>2003-12-13T18:30:02Z</updated>
         </entry>
       </item>
     </publish>
   </pubsub>

S: <stream:error>
     <unsupported-stanza-type
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.24.  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.

C: <?xml version='1.0'?>
   <stream:stream
       from='juliet@im.example.com'
       to='im.example.com'
       version='11.0'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
   <stream:error>
     <unsupported-version
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

4.7.3.25.  xml-not-well-formed

The initiating entity has sent XML that violates the well-formedness rules of [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.) or [XML‑NAMES] (Thompson, H., Hollander, D., Layman, A., Bray, T., and R. Tobin, “Namespaces in XML 1.0 (Third Edition),” December 2009.).

(In the following example, the client sends an XMPP message that is not namespace-well-formed.)

C: <message>
     <foo:body>What is this foo?</foo:body>
   </message>

S: <stream:error>
     <xml-not-well-formed
         xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>


 TOC 

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.

C: <message>
     <body>
       My keyboard layout is:

       QWERTYUIOP{}|
       ASDFGHJKL:"
       ZXCVBNM<>?
     </body>
   </message>

S: <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='http://example.com/ns'/>
   </stream:error>
   </stream:stream>


 TOC 

4.8.  Simplified Stream Examples

This section contains two simplified examples of a stream-based connection between a client and a server; these examples are included for the purpose of illustrating the concepts introduced thus far.

A basic connection:

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>

[ ... channel encryption ... ]

[ ... authentication ... ]

[ ... resource binding ... ]

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

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

C: </stream:stream>

S: </stream:stream>

A connection gone bad:

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

S: <?xml version='1.0'?>
   <stream:stream
       from='im.example.com'
       id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>

[ ... channel encryption ... ]

[ ... authentication ... ]

[ ... resource binding ... ]

C:   <message from='juliet@im.example.com/balcony'
              to='romeo@example.net'
              xml:lang='en'>
       <body>No closing tag!
     </message>

S: <stream:error>
    <xml-not-well-formed
        xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>

More detailed examples are provided under Section 9 (Examples).



 TOC 

5.  STARTTLS Negotiation



 TOC 

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] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) protocol, specifically a "STARTTLS" extension that is modelled after similar extensions for the [IMAP] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.), [POP3] (Myers, J. and M. Rose, “Post Office Protocol - Version 3,” May 1996.), and [ACAP] (Newman, C. and J. Myers, “ACAP -- Application Configuration Access Protocol,” November 1997.) protocols as described in [USINGTLS] (Newman, C., “Using TLS with IMAP, POP3 and ACAP,” June 1999.). The XML namespace name for the STARTTLS extension is 'urn:ietf:params:xml:ns:xmpp-tls'.

Support for STARTTLS is REQUIRED in XMPP client and server implementations. An administrator of a given deployment MAY specify that TLS is obligatory for client-to-server communication, server-to-server communication, or both. An initiating entity SHOULD use TLS to secure its stream with the receiving entity before proceeding with SASL authentication.



 TOC 

5.2.  Stream Negotiation Rules



 TOC 

5.2.1.  Mandatory-to-Negotiate

If the receiving entity advertises only the STARTTLS feature or if the receiving entity includes the <required/> child element as explained under Section 5.3.1 (Exchange of Stream Headers and Stream Features), the parties MUST consider TLS as mandatory-to-negotiate. If TLS is mandatory-to-negotiate, the receiving entity SHOULD NOT advertise support for any stream feature except STARTTLS during the initial stage of the stream negotiation process, because further stream features might depend on prior negotiation of TLS given the order of layers in XMPP (e.g., the particular SASL mechanisms offered by the receiving entity will likely depend on whether TLS has been negotiated).



 TOC 

5.2.2.  Restart

After TLS negotiation, the parties MUST restart the stream.



 TOC 

5.2.3.  Data Formatting

During STARTTLS negotiation, the entities MUST NOT send any whitespace as separators between XML elements (i.e., from the last character of the <starttls/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace at depth=1 of the stream as sent by the initiating entity, until the last character of the <proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace at depth=1 of the stream as sent by the receiving entity). This prohibition helps to ensure proper security layer byte precision. Any such whitespace shown in the STARTTLS examples provided in this document is included only for the sake of readability.



 TOC 

5.2.4.  Order of TLS and SASL Negotiations

If the initiating entity chooses to use TLS, STARTTLS negotiation MUST be completed before proceeding to SASL negotiation (SASL Negotiation); this order of negotiation is necessary to help safeguard authentication information sent during SASL negotiation, as well as to make it possible to base the use of the SASL EXTERNAL mechanism on a certificate (or other credentials) provided during prior TLS negotiation.



 TOC 

5.2.5.  TLS Renegotiation

The TLS protocol allows either party in a TLS-protected channel to initiate a new handshake that establishes new cryptographic parameters (see [TLS‑NEG] (Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, “Transport Layer Security (TLS) Renegotiation Indication Extension,” February 2010.)). The cases most commonly mentioned are:

  1. Refreshing encryption keys.

  2. Wrapping the TLS sequence number as explained in Section 6.1 of [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.).

  3. Protecting client credentials by completing server authentication first and then completing client authentication over the protected channel.

Because it is relatively inexpensive to establish streams in XMPP, for the first two cases it is preferable to use an XMPP stream reset (as described under Section 4.7.3.15 (reset)) instead of performing TLS renegotiation.

The third case has improved security characteristics when the TLS client (which might be an XMPP server) presents credentials to the TLS server. If communicating such credentials to an unauthenticated server might leak private information, it can be appropriate to complete TLS negotiation for the purpose of server authentication and then attempt TLS renegotiation for the purpose of client authentication with the TLS server.

However, the third case is sufficiently rare that XMPP entities SHOULD NOT blindly attempt TLS renegotiation.

If an entity that does not support TLS renegotiation detects a renegotiation attempt, then it MUST immediately close the underlying TCP connection without returning a stream error (since the violation has occurred at the TLS layer, not the XMPP layer; see Section 13.3 (Order of Layers)).

If an entity that supports TLS renegotiation detects a TLS renegotiation attempt that does not use the TLS Renegotiation Extension [TLS‑NEG] (Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, “Transport Layer Security (TLS) Renegotiation Indication Extension,” February 2010.), then it MUST immediately close the underlying TCP connection without returning a stream error (since the violation has occurred at the TLS layer, not the XMPP layer; see Section 13.3 (Order of Layers)).

Support for TLS renegotiation is strictly OPTIONAL. However, implementations that support TLS renegotiation MUST implement and use the TLS Renegotiation Extension [TLS‑NEG] (Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, “Transport Layer Security (TLS) Renegotiation Indication Extension,” February 2010.).



 TOC 

5.2.6.  TLS Extensions

Either party to a stream MAY include any TLS extension during the TLS negotiation itself. This is a matter for the TLS layer, not the XMPP layer.



 TOC 

5.3.  Process



 TOC 

5.3.1.  Exchange of Stream Headers and Stream Features

The initiating entity resolves the hostname of the receiving entity as specified under Section 3 (TCP Binding), opens a TCP connection to the advertised port at the resolved IP address, and sends an initial stream header to the receiving entity; if the initiating entity is capable of STARTTLS negotiation, it MUST include the 'version' attribute set to a value of at least "1.0" in the initial stream header.

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

The receiving entity MUST send a response stream header to the initiating entity over the TCP connection opened by the initiating entity; if the receiving entity is capable of STARTTLS negotiation, it MUST include the 'version' attribute set to a value of at least "1.0" in the response stream header.

R: <stream:stream
     from='im.example.com'
     id='t7AMCin9zjMNwQKDnplntZPIDEI='
     to='juliet@im.example.com'
     version='1.0'
     xml:lang='en'
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'

The receiving entity then MUST send stream features to the initiating entity. If the receiving entity supports TLS, the stream features MUST include an advertisement for support of STARTTLS negotiation, i.e., a <starttls/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace.

If the receiving entity considers STARTTLS negotiation to be mandatory, the <starttls/> element SHOULD contain an empty <required/> child element.

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


 TOC 

5.3.2.  Initiation of STARTTLS Negotiation



 TOC 

5.3.2.1.  STARTTLS Command

In order to begin the STARTTLS negotiation, 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 STARTTLS negotiation to secure the stream.

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

The receiving entity MUST reply with either a <proceed/> element (proceed case) or a <failure/> element (failure case) qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace.



 TOC 

5.3.2.2.  Failure Case

If the failure case occurs, the receiving entity MUST return a <failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace and close the XML stream.

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

R: </stream:stream>

Causes for the failure case include but are not limited to:

  1. The initiating entity has sent a malformed STARTTLS command.

  2. The receiving entity did not offer the STARTTLS feature in its stream features.

  3. The receiving entity cannot complete STARTTLS negotiation because of an internal error.

Informational Note: STARTTLS failure is not triggered by TLS errors such as bad_certificate or handshake_failure, which are generated and handled during the TLS negotiation itself as described in [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.).

If the failure case occurs, the initiating entity MAY attempt to reconnect as explained under Section 3.4 (Reconnection).



 TOC 

5.3.2.3.  Proceed Case

If the proceed case occurs, the receiving entity MUST return a <proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls' namespace.

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

The receiving entity MUST consider the TLS negotiation to have begun immediately after sending the closing '>' character of the <proceed/> element to the initiating entity. The initiating entity MUST consider the TLS negotiation to have begun immediately after receiving the closing '>' character of the <proceed/> element from the receiving entity.

The entities now proceed to TLS negotiation as explained in the next section.



 TOC 

5.3.3.  TLS Negotiation



 TOC 

5.3.3.1.  Rules

In order to complete TLS negotiation over the TCP connection, the entities MUST follow the process defined in [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.).

The following rules apply:

  1. The entities MUST NOT send any further XML data until the TLS negotiation is complete.

  2. When using any of the mandatory-to-implement cipher suites specified under Section 13.8 (Mandatory-to-Implement Technologies), the receiving entity MUST present a certificate.

  3. So that mutual authentication will be possible, the receiving entity SHOULD send a certificate request to the initiating entity and the initiating entity SHOULD send a certificate (if available) to the receiving entity.

  4. The initiating entity MUST validate the certificate to determine if the TLS negotiation will succeed; see Section 13.7.2 (Certificate Validation) regarding certificate validation procedures.

  5. The receiving entity SHOULD choose which certificate to present based on the 'to' attribute of the initial stream header.

  6. Following successful TLS negotiation, all further data transmitted by either party MUST be encrypted.

Security Note: See Section 13.8 (Mandatory-to-Implement Technologies) regarding ciphers that MUST be supported for TLS; naturally, other ciphers MAY be supported as well.



 TOC 

5.3.3.2.  TLS Failure

If the TLS negotiation results in failure, the receiving entity MUST terminate the TCP connection.

The receiving entity MUST NOT send a closing </stream> tag before terminating the TCP connection, since the receiving entity and initiating entity MUST consider the original stream to be replaced upon failure of the TLS negotiation.

The initiating entity MAY attempt to reconnect as explained under Section 3.4 (Reconnection), with or without attempting TLS negotiation (in accordance with local service policy, user-configured prefernces, etc.).



 TOC 

5.3.3.3.  TLS Success

If the TLS negotiation is successful, then the entities MUST proceed as follows.

  1. The initiating entity MUST discard any information transmitted in layers above TCP that it obtained from the receiving entity in an insecure manner before TLS took effect (e.g., the receiving entity's 'from' address or the stream ID and stream features received from the receiving entity).

  2. The receiving entity MUST discard any information transmitted in layers above TCP that it obtained from the initiating entity in an insecure manner before TLS took effect (e.g., the initiating entity's from address).

  3. The initiating entity MUST send a new initial stream header to the receiving entity over the encrypted connection.
    I: <stream:stream
         from='juliet@im.example.com'
         to='im.example.com'
         version='1.0'
         xml:lang='en'
         xmlns='jabber:client'
         xmlns:stream='http://etherx.jabber.org/streams'>
    

    Implementation Note: The initiating entity MUST NOT send a closing </stream> tag before sending the new initial stream header, since the receiving entity and initiating entity MUST consider the original stream to be replaced upon success of the TLS negotiation.



  4. The receiving entity MUST respond with a new response stream header over the encrypted connection (for which it MUST generate a new stream ID instead of re-using the old stream ID).
    R: <stream:stream
         from='im.example.com'
         id='vgKi/bkYME8OAj4rlXMkpucAqe4='
         to='juliet@im.example.com'
         version='1.0'
         xml:lang='en'
         xmlns='jabber:client'
         xmlns:stream='http://etherx.jabber.org/streams'
    


  5. The receiving entity also MUST send stream features to the initiating entity, which MUST NOT include the STARTTLS feature but which SHOULD include the SASL stream feature as described under Section 6 (SASL Negotiation).
    R: <stream:features>
         <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
           <mechanism>EXTERNAL</mechanism>
           <mechanism>PLAIN</mechanism>
         </mechanisms>
       </stream:features>
    



 TOC 

6.  SASL Negotiation



 TOC 

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

Support for SASL negotiation is REQUIRED in XMPP client and server implementations.



 TOC 

6.2.  Stream Negotiation Rules



 TOC 

6.2.1.  Mandatory-to-Negotiate

The parties to a stream MUST consider SASL as mandatory-to-negotiate.



 TOC 

6.2.2.  Restart

After SASL negotiation, the parties MUST restart the stream.



 TOC 

6.2.3.  Mechanism Preferences

Any entity that will act as a SASL client or a SASL server MUST maintain an ordered list of its preferred SASL mechanisms according to the client or server, where the list is ordered according to local policy or user configuration (which SHOULD be in order of perceived strength to enable the strongest authentication possible). A server MUST offer and a client MUST try SASL mechanisms in preference order. For example, if the server offers the ordered list "PLAIN SCRAM-SHA-1 GSSAPI" or "SCRAM-SHA-1 GSSAPI PLAIN" but the client's ordered list is "GSSAPI SCRAM-SHA-1", the client MUST try GSSAPI first and then SCRAM-SHA-1 but MUST never try PLAIN (since PLAIN is not on its list).



 TOC 

6.2.4.  Mechanism Offers

If the receiving entity considers TLS negotiation (STARTTLS Negotiation) to be mandatory before use of a particular SASL authentication mechanism will be acceptable, the receiving entity MUST NOT advertise that mechanism in its list of available SASL authentication mechanisms prior to successful TLS negotiation.

If during prior TLS negotiation the initiating entity presented a certificate that is acceptable to the receiving entity for purposes of strong identity verification in accordance with local service policies (e.g., because the receiving entity expects that the initiating entity will be able to authenticate and authorize as the identity provided in an unexpired, unrevoked certificate that is anchored to a root trusted by the receiving entity), then the receiving entity MUST offer the SASL EXTERNAL mechanism to the initiating entity during SASL negotiation (refer to [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.)) and SHOULD prefer that mechanism to enable mutual authentication. However, the EXTERNAL mechanism MAY be offered under other circumstances as well.

See Section 13.8 (Mandatory-to-Implement Technologies) regarding mechanisms that MUST be supported; naturally, other SASL mechanisms MAY be supported as well. Best practices for the use of SASL in the context of XMPP are described in [XEP‑0175] (Saint-Andre, P., “Best Practices for Use of SASL ANONYMOUS,” November 2007.) for the ANONYMOUS mechanism and in [XEP‑0178] (Saint-Andre, P. and P. Millard, “Best Practices for Use of SASL EXTERNAL with Certificates,” February 2007.) for the EXTERNAL mechanism.



 TOC 

6.2.5.  Data Formatting

The following data formatting rules apply to the SASL negotiation:

  1. During SASL negotiation, the entities MUST NOT send any whitespace as separators between XML elements (i.e., from the last character of the <auth/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace at depth=1 of the stream as sent by the initiating entity, until the last character of the <success/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace at depth=1 of the stream as sent by the receiving entity). This prohibition helps to ensure proper security layer byte precision. Any such whitespace shown in the SASL examples provided in this document is included only for the sake of readability.

  2. Any XML character data contained within the XML elements MUST be encoded using base64, where the encoding adheres to the definition in Section 4 of [BASE64] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) and where the padding bits are set to zero.

  3. As formally specified in the XML schema for the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace under Appendix A.4 (SASL Namespace), the receiving entity MAY include one or more application-specific child elements inside the <mechanisms/> element to provide information that might be needed by the initiating entity in order to complete successful SASL negotiation using one or more of the offered mechanisms; however, the syntax and semantics of all such elements are out of scope for this specification.



 TOC 

6.2.6.  Security Layers

Upon successful SASL negotiation that involves negotiation of a security layer, both the initiating entity and the receiving MUST discard any application-layer state (i.e, state from the XMPP layer, excluding state from the TLS negotiation or SASL negotiation).



 TOC 

6.2.7.  Simple Username

Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify that the authentication identity used in the context of such mechanisms is a "simple username" combined with a password (see Section 2 of [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.)). The exact form of the simple username in any particular mechanism or deployment thereof is a local matter, and a simple username does not necessarily map to an application identifier such as a JID or JID component (e.g., a localpart). However, in the absence of local information provided by the server, an XMPP client SHOULD assume that the authentication identity for such a SASL mechanism is a username/password combination, where the simple username is the localpart of the user's JID.



 TOC 

6.2.8.  Authorization Identity

An authorization identity is an optional identity specified by the initiating entity, which is typically used by an administrator to perform some management task on behalf of another user. If the initiating entity wishes to act on behalf of another entity and the selected SASL mechanism supports transmission of an authorization identity, the initiating entity MUST provide an authorization identity during SASL negotiation. If the initiating entity does not wish to act on behalf of another entity, it MUST NOT provide an authorization identity. As specified in [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.), the initiating entity MUST NOT provide an authorization identity unless the authorization identity is different from the default authorization identity derived from the authentication identity.

Authorization identities are allowed only for client-to-server streams. Because a server-to-server stream is valid only for the particular domain pair authenticated during stream negotiation, authorization of any additional domains for the stream needs to be completed by means of protocol extensions above the core XMPP stream layer. Therefore in the case of server-to-server communication the initiating server MUST NOT attempt to negotiate an authorization identity during SASL negotiation and the receiving server MUST NOT accept an authorization identity from the initiating server.

In the case of client-to-server communication, the value of an authorization identity MUST be a bare JID of the form <localpart@domainpart> (i.e., localpart and domainpart).

Informational Note: The authorization identity communicated during SASL negotiation is used to determine the canonical address for the initiating client or server according to the receiving server, as described under Section 4.2.6 (Determination of Addresses).



 TOC 

6.2.9.  Realms

The receiving entity MAY include a realm when negotiating certain SASL mechanisms. If the receiving entity does not communicate a realm, the initiating entity MUST NOT assume that any realm exists. The realm MUST be used only for the purpose of authentication; in particular, an initiating entity MUST NOT attempt to derive an XMPP hostname from the realm information provided by the receiving entity.



 TOC 

6.2.10.  Round Trips

[SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) specifies that a using protocol such as XMPP can define two methods by which the protocol can save round trips where allowed for the SASL mechanism:

  1. When the SASL client (the XMPP "initiating entity") requests an authentication exchange, it can include "initial response" data with its request if appropriate for the SASL mechanism in use. In XMPP this is done by including the initial response as the XML character data of the <auth/> element.

  2. At the end of the authentication exchange, the SASL server (the XMPP "receiving entity") can include "additional data with success" if appropriate for the SASL mechanism in use. In XMPP this is done by including the additional data as the XML character data of the <success/> element.

For the sake of protocol efficiency, it is REQUIRED for clients and servers to support these methods and RECOMMENDED to use them; however clients and servers MUST support the less efficient modes as well.



 TOC 

6.3.  Process

The process for SASL negotiation is as follows.



 TOC 

6.3.1.  Exchange of Stream Headers and Stream Features

If SASL negotiation follows successful STARTTLS negotiation (STARTTLS Negotiation), then the SASL negotiation occurs over the encrypted stream that has already been negotiated. If not, the initiating entity resolves the hostname of the receiving entity as specified under Section 3 (TCP Binding), opens a TCP connection to the advertised port at the resolved IP address, and sends an initial stream header to the receiving entity; if the initiating entity is capable of SASL negotiation, it MUST include the 'version' attribute set to a value of at least "1.0" in the initial stream header.

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

The receiving entity MUST send a response stream header to the initiating entity (for which it MUST generate a new stream ID instead of re-using the old stream ID); if the receiving entity is capable of SASL negotiation, it MUST include the 'version' attribute set to a value of at least "1.0" in the response stream header.

R: <stream:stream
     from='im.example.com'
     id='vgKi/bkYME8OAj4rlXMkpucAqe4='
     to='juliet@im.example.com'
     version='1.0'
     xml:lang='en'
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'

The receiving entity also MUST send stream features to the initiating entity. If the receiving entity supports SASL, the stream features SHOULD include an advertisement for support of SASL negotiation, i.e., a <mechanisms/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace; typically the only case in which support for SASL negotiation would not be advertised here is before STARTTLS negotiation when TLS is required.

The <mechanisms/> element MUST contain one <mechanism/> child element for each authentication mechanism the receiving entity offers to the initiating entity. The order of <mechanism/> elements in the XML indicates the preference order of the SASL mechanisms according to the receiving entity; however the initiating entity MUST maintain its own preference order independent of the preference order of the receiving entity.

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


 TOC 

6.3.2.  Initiation

In order to begin the SASL negotiation, the initiating entity sends an <auth/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace and includes an appropriate value for the 'mechanism' attribute, thus starting the handshake for that particular authentication mechanism. This element MAY contain XML character data (in SASL terminology, the "initial response") if the mechanism supports or requires it; if the initiating entity needs to send a zero-length initial response, it MUST transmit the response as a single equals sign character ("="), which indicates that the response is present but contains no data.

I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
         mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

If the initiating entity subsequently sends another <auth/> element (even if the ongoing authentication handshake has not yet completed), the server SHOULD discard the ongoing handshake and begin a new handshake for the subsequently requested SASL mechanism.



 TOC 

6.3.3.  Challenge-Response Sequence

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; this element MAY contain XML character data (which MUST be generated in accordance with the definition of the SASL mechanism chosen by the initiating entity).

The initiating entity responds to the challenge by sending a <response/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY contain XML character data (which MUST be generated in accordance with the definition of the SASL mechanism chosen by the initiating entity).

If necessary, the receiving entity sends more challenges and the initiating entity sends more responses.

This series of challenge/response pairs continues until one of three things happens:

These scenarios are described in the following sections.



 TOC 

6.3.4.  Abort

The initiating entity aborts the handshake for this authentication mechanism by sending an <abort/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace.

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

Upon receiving an <abort/> element, the receiving entity MUST return a <failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace and containing an <aborted/> child element.

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


 TOC 

6.3.5.  Failure

The receiving entity reports failure of the handshake for this authentication mechanism by sending a <failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace (the particular cause of failure MUST be communicated in an appropriate child element of the <failure/> element as defined under Section 6.4 (SASL Errors)).

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

Where appropriate for the chosen SASL mechanism, the receiving entity SHOULD allow a configurable but reasonable number of retries (at least 2 and no more than 5); this enables the initiating entity (e.g., an end-user client) to tolerate incorrectly-provided credentials (e.g., a mistyped password) without being forced to reconnect.

If the initiating entity attempts a reasonable number of retries with the same SASL mechanism and all attempts fail, it MAY fall back to the next mechanism in its ordered list by sending a new <auth/> request to the receiving entity, this starting a new handshake for that authentication mechanism. If all handshakes fail and there are no remaining mechanisms in the initiating entity's list of supported and acceptable mechanisms, the initiating entity SHOULD simply close the stream.

If the initiating entity exceeds the number of retries, the receiving entity MUST return a stream error, which SHOULD be <policy-violation/> (although some existing implementations send <not-authorized/> instead).



 TOC 

6.3.6.  Success

The receiving entity reports success of the handshake by sending a <success/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY contain XML character data (in SASL terminology, "additional data with success") if the chosen SASL mechanism supports or requires it; if the receiving entity needs to send additional data of zero length, it MUST transmit the data as a single equals sign character ("=").

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

Informational Note: The authorization identity communicated during SASL negotiation is used to determine the canonical address for the initiating client according to the receiving server, as described under Section 4.2.6 (Determination of Addresses).

Upon receiving the <success/> element, the initiating entity MUST initiate a new stream over the existing TCP connection by sending a new initial stream header to the receiving entity.

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

Implementation Note: The initiating entity MUST NOT send a closing </stream> tag before sending the new initial stream header, since the receiving entity and initiating entity MUST consider the original stream to be replaced upon sending or receiving the <success/> element.

Upon receiving the new initial stream header from the initiating entity, the receiving entity MUST respond by sending a new response stream header to the initiating entity (for which it MUST generate a new stream ID instead of re-using the old stream ID).

R: <stream:stream
     from='im.example.com'
     id='gPybzaOzBmaADgxKXu9UClbprp0='
     to='juliet@im.example.com'
     version='1.0'
     xml:lang='en'
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'>

The receiving entity MUST also send stream features, containing any further available features or containing no features (via an empty <features/> element).

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


 TOC 

6.4.  SASL Errors

The syntax of SASL errors is as follows, where "defined-condition" is one of the SASL-related error conditions defined in the following sections and XML data shown within the square brackets '[' and ']' is OPTIONAL.

<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
  <defined-condition/>
  [<text xml:lang='langcode'>
      OPTIONAL descriptive text
  </text>]
</failure>

Inclusion of a defined condition is REQUIRED.

Inclusion of the <text/> element is OPTIONAL, and can be used to provide application-specific information about the error condition, which information MAY be displayed to a human but only as a supplement to the defined condition.



 TOC 

6.4.1.  aborted

The receiving entity acknowledges an <abort/> element sent by the initiating entity; sent in reply to the <abort/> element.

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

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


 TOC 

6.4.2.  account-disabled

The account of the initiating entity has been temporarily disabled; sent in reply to an <auth/> element (with or without initial response data) or a <response/> element.

I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
         mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <account-disabled/>
     <text xml:lang='en'>Call 212-555-1212 for assistance.</text>
   </failure>


 TOC 

6.4.3.  credentials-expired

The authentication failed because the initiating entity provided credentials that have expired; sent in reply to a <response/> element or an <auth/> element with initial response data.

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

R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <credentials-expired/>
   </failure>


 TOC 

6.4.4.  encryption-required

The mechanism requested by the initiating entity cannot be used unless the underlying stream is encrypted; sent in reply to an <auth/> element (with or without initial response data).

I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
         mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <encryption-required/>
   </failure>


 TOC 

6.4.5.  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,” October 2006.) encoding is incorrect (e.g., because the encoding does not adhere to the definition in Section 4 of [BASE64] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.)); sent in reply to a <response/> element or an <auth/> element with initial response data.

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

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


 TOC 

6.4.6.  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.

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

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


 TOC 

6.4.7.  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.

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

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


 TOC 

6.4.8.  malformed-request

The request is malformed (e.g., the <auth/> element includes initial response data but the mechanism does not allow that, or the data sent violates the syntax for the specified SASL mechanism); sent in reply to an <abort/>, <auth/>, <challenge/>, or <response/> element.

(In the following example, the XML character data of the <auth/> element contains more than 255 UTF-8-encoded Unicode characters and therefore violates the "token" production for the SASL ANONYMOUS mechanism as specified in [ANONYMOUS] (Zeilenga, K., “Anonymous Simple Authentication and Security Layer (SASL) Mechanism,” June 2006.).)

I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
         mechanism='ANONYMOUS'>[ ... some-long-token ... ]</auth>

R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <malformed-request/>
   </failure>


 TOC 

6.4.9.  mechanism-too-weak

The mechanism requested by the initiating entity is weaker than server policy permits for that initiating entity; sent in reply to an <auth/> element (with or without initial response data).

I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
         mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <mechanism-too-weak/>
   </failure>


 TOC 

6.4.10.  not-authorized

The authentication failed because the initiating entity did not provide proper credentials or the receiving entity has detected an attack but wishes to disclose as little information as possible to the attacker; sent in reply to a <response/> element or an <auth/> element with initial response data.

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

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

Security Note: This error condition includes but is not limited to the case of incorrect credentials or a nonexistent username. In order to discourage directory harvest attacks, no differentiation is made between incorrect credentials and a nonexistent username.



 TOC 

6.4.11.  temporary-auth-failure

The authentication failed because of a temporary error condition within the receiving entity, and it is advisable for the initiating entity to try again later; sent in reply to an <auth/> element or a <response/> element.

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

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


 TOC 

6.4.12.  transition-needed

The authentication failed because the mechanism cannot be used until the initiating entity provides (for one time only) a plaintext password so that the receiving entity can build a hashed password for use in future authentication attempts; sent in reply to an <auth/> element with or without initial response data.

I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
         mechanism='CRAM-MD5'>[ ... ]</auth>

R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <transition-needed/>
   </failure>

Security Note: An XMPP client MUST treat a <transition-needed/> SASL error with extreme caution, SHOULD NOT provide a plaintext password over an XML stream that is not encrypted via Transport Layer Security, and MUST warn a human user before allowing the user to provide a plaintext password over an unencrypted connection. Even so, the attacker could be located on the server, attempting to capture the plaintext password.



 TOC 

6.5.  SASL Definition

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

service name:
"xmpp"
initiation sequence:
After the initiating entity provides an opening XML stream header and the receiving entity replies in kind, the receiving entity provides a list of acceptable authentication methods. The initiating entity chooses one method from the list and sends it to the receiving entity as the value of the 'mechanism' attribute possessed by an <auth/> element, optionally including an initial response to avoid a round trip.
exchange sequence:
Challenges and responses are carried through the exchange of <challenge/> elements from receiving entity to initiating entity and <response/> elements from initiating entity to receiving entity. The receiving entity reports failure by sending a <failure/> element and success by sending a <success/> element; the initiating entity aborts the exchange by sending an <abort/> element. Upon successful negotiation, both sides consider the original XML stream to be closed and new stream headers are sent by both entities.
security layer negotiation:
The security layer takes effect immediately after sending the closing '>' character of the <success/> element for the receiving entity, and immediately after receiving the closing '>' character of the <success/> element for the initiating entity. The order of layers is first [TCP] (Postel, J., “Transmission Control Protocol,” September 1981.), then [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.), then [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.), then XMPP.
use of the authorization identity:
The authorization identity can be used in XMPP to denote the non-default <localpart@domainpart> of a client; an empty string is equivalent to an absent authorization identity.


 TOC 

7.  Resource Binding



 TOC 

7.1.  Overview

After a client authenticates with a server, it MUST bind a specific resource to the stream so that the server can properly address the client. That is, there MUST be an XMPP resource associated with the bare JID (<localpart@domainpart>) of the client, so that the address for use over that stream is a full JID of the form <localpart@domainpart/resource> (including the resourcepart). This ensures that the server can deliver XML stanzas to and receive XML stanzas from the client in relation to entities other than the server itself or the client's account, as explained under Section 10 (Server Rules for Processing XML Stanzas) (the client could exchange stanzas with the server itself or the client's account before binding a resource since the full JID is needed only for addressing outside the context of the stream negotiated between the client and the server, but this is not commonly done).

After a client has bound a resource to the stream, it is referred to as a CONNECTED RESOURCE. A server SHOULD allow an entity to maintain multiple connected resources simultaneously, where each connected resource is associated with a distinct XML stream and differentiated from the other connected resources by a distinct resourcepart.

Security Note: A server SHOULD enable the administrator of an XMPP service to limit the number of connected resources in order to prevent certain denial of service attacks as described under Section 13.12 (Denial of Service).

If, before completing the resource binding step, the client attempts to send an XML stanza to an entity other than the server itself or the client's account, the server MUST NOT process the stanza and MUST return a <not-authorized/> stream error to the client.

Support for resource binding is REQUIRED in XMPP client and server implementations.



 TOC 

7.2.  Stream Negotiation Rules



 TOC 

7.2.1.  Mandatory-to-Negotiate

The parties to a stream MUST consider resource binding as mandatory-to-negotiate.



 TOC 

7.2.2.  Restart

After resource binding, the parties MUST NOT restart the stream.



 TOC 

7.3.  Advertising Support

Upon sending a new response stream header to the client after successful SASL negotiation, the server MUST include a <bind/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace in the stream features it presents to the client.

The server MUST NOT include the resource binding stream feature until after the client has authenticated, typically by means of successful SASL negotiation.

S: <stream:stream
       from='im.example.com'
       id='gPybzaOzBmaADgxKXu9UClbprp0='
       to='juliet@im.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>

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

Upon being informed that resource binding is mandatory, the client MUST bind a resource to the stream as described in the following sections.



 TOC 

7.4.  Generation of Resource Identifiers

A resourcepart MUST at a minimum be unique among the connected resources for that <localpart@domainpart>. Enforcement of this policy is the responsibility of the server.

Security Note: A resourcepart can be security-critical. For example, if a malicious entity can guess a client's resourcepart then it might be able to determine if the client (and therefore the controlling principal) is online or offline, thus resulting in a presence leak as described under Section 13.10.2 (Presence Information). To prevent that possibility, a client can either (1) generate a random resourcepart on its own or (2) ask the server to generate a resourcepart on its behalf, which MUST be random (see [RANDOM] (Eastlake, D., Schiller, J., and S. Crocker, “Randomness Requirements for Security,” June 2005.)). One method for ensuring that the resourcepart is random is to generate a Universally Unique Identifier (UUID) as specified in [UUID] (Leach, P., Mealling, M., and R. Salz, “A Universally Unique IDentifier (UUID) URN Namespace,” July 2005.).



 TOC 

7.5.  Server-Generated Resource Identifier

A server MUST be able to generate an XMPP resourcepart on behalf of a client.



 TOC 

7.5.1.  Success Case

A client requests a server-generated resourcepart by sending an IQ stanza of type "set" (see Section 8.2.3 (IQ Semantics)) containing an empty <bind/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace.

C: <iq id='tn281v37' type='set'>
    <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
   </iq>

Once the server has generated an XMPP resourcepart for 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.

S: <iq id='tn281v37' type='result'>
    <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
      <jid>
        juliet@im.example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb
      </jid>
    </bind>
   </iq>


 TOC 

7.5.2.  Error Cases

When a client asks the server to generate a resourcepart during resource binding, the following stanza error conditions are defined (and others not specified here are possible; see under Section 8.3 (Stanza Errors)):



 TOC 

7.5.2.1.  Resource Constraint

If the account has reached a limit on the number of simultaneous connected resources allowed, the server MUST return a <resource-constraint/> stanza error.

S: <iq id='wy2xa82b4' type='error'>
     <error type='wait'>
       <resource-constraint
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>


 TOC 

7.5.2.2.  Not Allowed

If the client is otherwise not allowed to bind a resource to the stream, the server MUST return a <not-allowed/> stanza error.

S: <iq id='wy2xa82b4' type='error'>
     <error type='cancel'>
       <not-allowed
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>


 TOC 

7.6.  Client-Submitted Resource Identifier

Instead of asking the server to generate a resourcepart on its behalf, a client MAY attempt to submit a resourcepart that it has generated or that the controlling user has provided.



 TOC 

7.6.1.  Success Case

A client asks its server to accept a client-submitted resourcepart by sending an IQ stanza of type "set" containing a <bind/> element with a child <resource/> element containing non-zero-length XML character data.

C: <iq id='wy2xa82b4' type='set'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>balcony</resource>
     </bind>
   </iq>

The server SHOULD accept the client-submitted resourcepart. It does so by returning an IQ stanza of type "result" to the client, including a <jid/> child element that specifies the full JID for the connected resource and contains without modification the client-submitted text.

S: <iq id='wy2xa82b4' type='result'>
    <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
      <jid>juliet@im.example.com/balcony</jid>
    </bind>
   </iq>

Alternatively, in accordance with local service policies the server MAY refuse the client-submitted resourcepart and override it with a resourcepart that the server generates.

S: <iq id='wy2xa82b4' type='result'>
    <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
      <jid>
   juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
      </jid>
    </bind>
   </iq>


 TOC 

7.6.2.  Error Cases

When a client attempts to submit its own XMPP resourcepart during resource binding, the following stanza error conditions are defined in addition to those described under Section 7.5.2 (Error Cases) (and others not specified here are possible; see under Section 8.3 (Stanza Errors)):



 TOC 

7.6.2.1.  Bad Request

If the provided resourcepart cannot be processed by the server (e.g. because it is of zero length or because it is not in accordance with the Resourceprep profile of stringprep specified in [XMPP‑ADDR] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Address Format,” July 2010.)), the server MAY return a <bad-request/> stanza error (but SHOULD instead apply the Resourceprep profile or otherwise process the resourcepart so that it is in conformance).

S: <iq id='wy2xa82b4' type='error'>
     <error type='modify'>
       <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>


 TOC 

7.6.2.2.  Conflict

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

  1. Not accept the resourcepart provided by the client but instead override it with a resourcepart that the server generates.

  2. Disallow the newly-requested resource and maintain the current resource.

  3. Terminate the current resource and allow the newly-requested resource.

Which of these the server does is up to the implementation, although it is RECOMMENDED to implement case #1.

S: <iq id='wy2xa82b4' type='result'>
    <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
      <jid>
   juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
      </jid>
    </bind>
   </iq>

In case #2, the server MUST send a <conflict/> stanza error to the newly-requested resource but maintain the XML stream for that connection so that the newly-requested resource has an opportunity to negotiate a non-conflicting resourcepart before sending another request for resource binding.

S: <iq id='wy2xa82b4' type='error'>
     <error type='modify'>
       <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

In case #3, the server MUST send a <conflict/> stream error to the current resource and return an IQ stanza of type "result" (indicating success) to the newly-requested resource.

S: <iq id='wy2xa82b4' type='result'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <jid>
         juliet@im.example.com/balcony
       </jid>
     </bind>
   </iq>


 TOC 

7.6.3.  Retries

If an error occurs when a client submits a resourcepart, the server SHOULD allow a configurable but reasonable number of retries (at least 5 and no more than 10); this enables the client to tolerate incorrectly-provided resourceparts (e.g., bad data formats or duplicate text strings) without being forced to reconnect.

After the client has reached the retry limit, the server MUST return a <policy-violation/> stream error to the client.



 TOC 

8.  XML Stanzas

After a client and a server (or two servers) have completed stream negotiation, either party can send XML stanzas. 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 stanza types. These common attributes, as well as the basic semantics of the three stanza types, are defined in this specification; more detailed information regarding the syntax of XML stanzas for instant messaging and presence applications is provided in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.), and for other applications in the relevant XMPP extension specifications.

Support for the XML stanza syntax and semantics defined in this specification is REQUIRED in XMPP client and server implementations.

Security Note: A server MUST NOT process a partial stanza and MUST NOT attach meaning to the transmission timing of any part of a stanza (before receipt of the close tag).



 TOC 

8.1.  Common Attributes

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



 TOC 

8.1.1.  to

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

<message to='romeo@example.net'>
  <body>Art thou not Romeo, and a Montague?</body>
</message>

For information about server processing of inbound and outbound XML stanzas based on the 'to' address, refer to Section 10 (Server Rules for Processing XML Stanzas).



 TOC 

8.1.1.1.  Client-to-Server Streams

The following rules apply to inclusion of the 'to' attribute in the context of XML streams qualified by the 'jabber:client' namespace (i.e., client-to-server streams).

  1. A stanza with a specific intended recipient MUST possess a 'to' attribute whose value is an XMPP address.

  2. A stanza sent from a client to a server for direct processing by the server as described in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.) for rosters (e.g., presence sent to the server for broadcasting to other entities) MUST NOT possess a 'to' attribute.



 TOC 

8.1.1.2.  Server-to-Server Streams

The following rules apply to inclusion of the 'to' attribute in the context of XML streams qualified by the 'jabber:server' namespace (i.e., server-to-server streams).

  1. A stanza MUST possess a 'to' attribute whose value is an XMPP address; if a server receives a stanza that does not meet this restriction, it MUST generate an <improper-addressing/> stream error.

  2. The domainpart of the JID in the 'to' attribute MUST match a hostname serviced by the receiving server; if a server receives a stanza that does not meet this restriction, it MUST generate a <host-unknown/> or <host-gone/> stream error.



 TOC 

8.1.2.  from

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

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


 TOC 

8.1.2.1.  Client-to-Server Streams

The following rules apply to the 'from' attribute in the context of XML streams qualified by the 'jabber:client' namespace (i.e., client-to-server streams).

  1. When the server receives an XML stanza from a client, the server MUST add a 'from' attribute to the stanza or override the 'from' attribute specified by the client, where the value of the 'from' attribute is the full JID (<localpart@domainpart/resource>) determined by the server for the connected resource that generated the stanza (see Section 4.2.6 (Determination of Addresses)), or the bare JID (<localpart@domainpart>) in the case of subscription-related presence stanzas (see [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.)).

  2. When the server generates a stanza from the server itself for delivery to the client, the stanza MUST include a 'from' attribute whose value is the bare JID (i.e., <domain>) of the server as agreed upon during stream negotiation (e.g., based on the 'to' attribute of the initial stream header).

  3. When the server generates a stanza from the server for delivery to the client on behalf of the account of the connected client (e.g., in the context of data storage services provided by the server on behalf of the client), the stanza MUST either (a) not include a 'from' attribute or (b) include a 'from' attribute whose value is the account's bare JID (<localpart@domainpart>).

  4. A server MUST NOT send to the client a stanza without a 'from' attribute if the stanza was not generated by the server (e.g., if it was generated by another client or another server); therefore, when a client receives a stanza that does not include a 'from' attribute, it MUST assume that the stanza is from the user's account on the server.



 TOC 

8.1.2.2.  Server-to-Server Streams

The following rules apply to the 'from' attribute in the context of XML streams qualified by the 'jabber:server' namespace (i.e., server-to-server streams).

  1. A stanza MUST possess a 'from' attribute whose value is an XMPP address; if a server receives a stanza that does not meet this restriction, it MUST generate an <improper-addressing/> stream error.

  2. The domainpart of the JID contained in the 'from' attribute MUST match the hostname of the sending server (or any validated domain thereof) as communicated via SASL negotiation (see Section 6 (SASL Negotiation)), server dialback (see [XEP‑0220] (Miller, J., Saint-Andre, P., and P. Hancke, “Server Dialback,” March 2010.), or similar means; if a server receives a stanza that does not meet this restriction, it MUST generate an <invalid-from/> stream error.

Enforcement of these rules helps to prevent certain denial of service attacks as described under Section 13.12 (Denial of Service).



 TOC 

8.1.3.  id

The 'id' attribute is used by the entity that generates a stanza ("the originating entity") to track any response or error stanza that it might receive in relation to the generated stanza from another entity (such as an intermediate server or the intended recipient).

It is up to the originating entity whether the value of the 'id' attribute will be unique only within its current stream or unique globally.

For <message/> and <presence/> stanzas, it is RECOMMENDED for the originating entity to include an 'id' attribute; for <iq/> stanzas, it is REQUIRED.

If the generated stanza includes an 'id' attribute then it is REQUIRED for the response or error stanza to also include an 'id' attribute, where the value of the 'id' attribute MUST match that of the generated stanza.

The semantics of IQ stanzas impose additional restrictions; see Section 8.2.3 (IQ Semantics).



 TOC 

8.1.4.  type

The 'type' attribute specifies the purpose or context of the message, presence, or IQ stanza. The particular allowable values for the 'type' attribute vary depending on whether the stanza is a message, presence, or IQ stanza. The defined values for message and presence stanzas are specific to instant messaging and presence applications and therefore are defined in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.), whereas the values for IQ stanzas specify the role of an IQ stanza in a structured request-response exchange and therefore are specified under Section 8.2.3 (IQ Semantics). The only 'type' value common to all three stanzas is "error"; see Section 8.3 (Stanza Errors).



 TOC 

8.1.5.  xml:lang

A stanza SHOULD possess an 'xml:lang' attribute (as defined in Section 2.12 of [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.)) if the stanza contains XML character data that is intended to be presented to a human user (as explained in [CHARSETS] (Alvestrand, H., “IETF Policy on Character Sets and Languages,” January 1998.), "internationalization is for humans"). The value of the 'xml:lang' attribute specifies the default language of any such human-readable XML character data.

<presence from='romeo@example.net/orchard' xml:lang='en'>
  <show>dnd</show>
  <status>Wooing Juliet</status>
</presence>

The value of the 'xml:lang' attribute MAY be overridden by the 'xml:lang' attribute of a specific child element.

<presence from='romeo@example.net/orchard' xml:lang='en'>
  <show>dnd</show>
  <status>Wooing Juliet</status>
  <status xml:lang='cs'>Dvo&#x0159;&#x00ED;m se Julii</status>
</presence

If an outbound stanza generated by a client does not possess an 'xml:lang' attribute, the client's server SHOULD add an 'xml:lang' attribute whose value is that specified for the stream as defined under Section 4.5.4 (xml:lang).

C: <presence from='romeo@example.net/orchard'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
   </presence>

S: <presence from='romeo@example.net/orchard'
             to='juliet@im.example.com'
             xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
   </presence>

If an inbound stanza received by a client or server does not possess an 'xml:lang' attribute, an implementation MUST assume that the default language is that specified for the stream as defined under Section 4.5.4 (xml:lang).

The value of the 'xml:lang' attribute MUST conform to the NMTOKEN datatype (as defined in Section 2.3 of [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.)) and MUST conform to the format defined in [LANGTAGS] (Phillips, A. and M. Davis, “Tags for Identifying Languages,” September 2009.).

A server MUST NOT modify or delete 'xml:lang' attributes on stanzas it receives from other entities.



 TOC 

8.2.  Basic Semantics



 TOC 

8.2.1.  Message Semantics

The <message/> stanza can be seen as a "push" mechanism whereby one entity pushes information to another entity, similar to the communications that occur in a system such as email. All message stanzas SHOULD possess a 'to' attribute that specifies the intended recipient of the message; upon receiving such a stanza, a server SHOULD route or deliver it to the intended recipient (see Section 10 (Server Rules for Processing XML Stanzas) for general routing and delivery rules related to XML stanzas).



 TOC 

8.2.2.  Presence Semantics

The <presence/> stanza can be seen as a specialized broadcast or "publish-subscribe" mechanism, whereby multiple entities receive information (in this case, network availability information) about an entity to which they have subscribed. In general, a publishing entity (client) SHOULD send a presence stanza with no 'to' attribute, in which case the server to which the entity is connected SHOULD broadcast that stanza to all subscribed entities. However, a publishing entity MAY also send a presence stanza with a 'to' attribute, in which case the server SHOULD route or deliver that stanza to the intended recipient. See Section 10 (Server Rules for Processing XML Stanzas) for general routing and delivery rules related to XML stanzas, and [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.) for rules specific to presence applications.



 TOC 

8.2.3.  IQ Semantics

Info/Query, or IQ, is a request-response mechanism, similar in some ways to the Hypertext Transfer Protocol [HTTP] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.). The semantics of IQ enable an entity to make a request of, and receive a response from, another entity. The data content of the request and response is defined by the schema or other structural definition associated with the XML namespace that qualifies the direct child element of the IQ element (see Section 8.4 (Extended Content)), and the interaction is tracked by the requesting entity through use of the 'id' attribute. Thus, IQ interactions follow a common pattern of structured data exchange such as get/result or set/result (although an error can be returned in reply to a request if appropriate):

Requesting                  Responding
  Entity                      Entity
----------                  ----------
    |                            |
    | <iq id='1' type='get'>     |
    |   [ ... payload ... ]      |
    | </iq>                      |
    | -------------------------> |
    |                            |
    | <iq id='1' type='result'>  |
    |   [ ... payload ... ]      |
    | </iq>                      |
    | <------------------------- |
    |                            |
    | <iq id='2' type='set'>     |
    |   [ ... payload ... ]      |
    | </iq>                      |
    | -------------------------> |
    |                            |
    | <iq id='2' type='error'>   |
    |   [ ... condition ... ]    |
    | </iq>                      |
    | <------------------------- |
    |                            |

To enforce these semantics, the following rules apply:

  1. The 'id' attribute is REQUIRED for IQ stanzas.

  2. The 'type' attribute is REQUIRED for IQ stanzas. The value MUST be one of the following (if the value is other than one of the following strings, the recipient or an intermediate router MUST return a stanza error of <bad-request/>):

  3. An entity that receives an IQ request of type "get" or "set" MUST reply with an IQ response of type "result" or "error". The response MUST preserve the 'id' attribute of the request (or be empty if the generated stanza did not include an 'id' attribute).

  4. An entity that receives a stanza of type "result" or "error" MUST NOT respond to the stanza by sending a further IQ response of type "result" or "error"; however, the requesting entity MAY send another request (e.g., an IQ of type "set" to provide obligatory information discovered through a get/result pair).

  5. An IQ stanza of type "get" or "set" MUST contain exactly one child element, which specifies the semantics of the particular request.

  6. An IQ stanza of type "result" MUST include zero or one child elements.

  7. An IQ stanza of type "error" MAY include the child element contained in the associated "get" or "set" and MUST include an <error/> child; for details, see Section 8.3 (Stanza Errors).


 TOC 

8.3.  Stanza Errors

Stanza-related errors are handled in a manner similar to stream errors (Stream Errors). Unlike stream errors, stanza errors are recoverable; therefore they do not result in termination of the XML stream and underlying TCP connection. Instead, the entity that discovers the error condition returns an ERROR STANZA, which is a stanza that:



 TOC 

8.3.1.  Rules

The following rules apply to stanza errors:

  1. The receiving or processing entity that detects an error condition in relation to a stanza SHOULD return an error stanza (and MUST do so for IQ stanzas).

  2. The error stanza MUST simply swap the 'from' and 'to' addresses from the generated stanza.

  3. If the generated stanza was <message/> or <presence/> and included an 'id' attribute then it is REQUIRED for the error stanza to also include an 'id' attribute. If the generated stanza was <iq/> then the error stanza MUST include an 'id' attribute. In all cases, the value of the 'id' attribute MUST match that of the generated stanza (or be empty if the generated stanza did not include an 'id' attribute).

  4. An error stanza MUST contain an <error/> child element.

  5. The entity that returns an error stanza MAY pass along its JID to the sender of the generated stanza (e.g., for diagnostic or tracking purposes) through the addition of a 'by' attribute to the <error/> child element.

  6. The entity that returns an error stanza MAY include the original XML sent so that the sender can inspect and, if necessary, correct the XML before attempting to resend (however, this is a courtesy only and the originating entity MUST NOT depend on receiving the original payload).

  7. An <error/> child MUST NOT be included if the 'type' attribute has a value other than "error" (or if there is no 'type' attribute).

  8. An entity that receives an error stanza MUST NOT respond to the stanza with a further error stanza; this helps to prevent looping.


 TOC 

8.3.2.  Syntax

The syntax for stanza-related errors is as follows, where XML data shown within the square brackets '[' and ']' is OPTIONAL, 'intended-recipient' is the JID of the entity to which the original stanza was addressed, and 'sender' is the JID of the originating entity.

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

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

The "error-type" MUST be one of the following:

The "defined-condition" MUST correspond to one of the stanza error conditions defined under Section 8.3.3 (Defined Conditions).

The <error/> element:

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

Interoperability Note: The syntax defined in [RFC3920] (Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.) included a legacy 'code' attribute, whose semantics have been replaced by the defined condition elements; information about mapping defined condition elements to values of the legacy 'code' attribute can be found in [XEP‑0086] (Norris, R. and P. Saint-Andre, “Error Condition Mappings,” February 2004.).



 TOC 

8.3.3.  Defined Conditions

The following conditions are defined for use in stanza errors.



 TOC 

8.3.3.1.  bad-request

The sender has sent a stanza containing XML that does not conform to the appropriate schema or that cannot be processed (e.g., an IQ stanza that includes an unrecognized value of the 'type' attribute, or an element that is qualified by a recognized namespace but that violates the defined syntax for the element); the associated error type SHOULD be "modify".

C: <iq from='juliet@im.example.com/balcony'
       id='zj3v142b'
       to='im.example.com'
       type='subscribe'>
     <ping xmlns='urn:xmpp:ping'/>
   </iq>

S: <iq from='im.example.com'
       id='zj3v142b'
       to='juliet@im.example.com/balcony'
       type='error'>
     <error type='modify'>
       <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>


 TOC 

8.3.3.2.  conflict

Access cannot be granted because an existing resource exists with the same name or address; the associated error type SHOULD be "cancel".

C: <iq id='wy2xa82b4' type='set'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>balcony</resource>
     </bind>
   </iq>

S: <iq id='wy2xa82b4' type='error'>
     <error type='cancel'>
       <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>


 TOC 

8.3.3.3.  feature-not-implemented

The feature represented in the XML stanza is not implemented by the intended recipient or an intermediate server and therefore the stanza cannot be processed (e.g., the entity understands the namespace but does not recognize the element name); the associated error type SHOULD be "cancel" or "modify".

C: <iq from='juliet@im.example.com/balcony'
       id='9u2bax16'
       to='pubsub.example.com'
       type='get'>
     <pubsub xmlns='http://jabber.org/protocol/pubsub'>
       <subscriptions/>
     </pubsub>
   </iq>

E: <iq from='pubsub.example.com
       id='9u2bax16'
       to='juliet@im.example.com/balcony'
       type='error'>
     <error type='cancel'>
       <feature-not-implemented
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       <unsupported
           xmlns='http://jabber.org/protocol/pubsub#errors'
           feature='retrieve-subscriptions'/>
     </error>
   </iq>


 TOC 

8.3.3.4.  forbidden

The requesting entity does not possess the necessary permissions to perform the action; the associated error type SHOULD be "auth".

C: <presence
       from='juliet@im.example.com/balcony'
       id='y2bs71v4'
       to='characters@muc.example.com/JulieC'>
     <x xmlns='http://jabber.org/protocol/muc'/>
   </presence>

E: <presence
       from='characters@muc.example.com/JulieC'
       id='y2bs71v4'
       to='juliet@im.example.com/balcony'
       type='error'>
     <error type='auth'>
       <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </presence>


 TOC 

8.3.3.5.  gone

The recipient or server can no longer be contacted at this address, typically on a permanent basis (as opposed to the <redirect/> error condition, which is used for temporary addressing failures); the associated error type SHOULD be "cancel" and the error stanza SHOULD include a new address (if available) as the XML character data of the <gone/> element (which MUST be a Uniform Resource Identifier [URI] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) or Internationalized Resource Identifier [IRI] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) at which the entity can be contacted, typically an XMPP IRI as specified in [XMPP‑URI] (Saint-Andre, P., “Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP),” February 2008.)).

C: <message
       from='juliet@im.example.com/churchyard'
       id='sj2b371v'
       to='romeo@example.net'
       type='chat'>
     <body>Thy lips are warm.</body>
   </message>

S: <message
       from='romeo@example.net'
       id='sj2b371v'
       to='juliet@im.example.com/churchyard'
       type='error'>
     <error by='example.net'
            type='cancel'>
       <gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
         xmpp:romeo@afterlife.example.net
       </gone>
     </error>
   </message>


 TOC 

8.3.3.6.  internal-server-error

The server could not process the stanza because of a misconfiguration or an otherwise-undefined internal server error; the associated error type SHOULD be "cancel".

C: <presence
       from='juliet@im.example.com/balcony'
       id='y2bs71v4'
       to='characters@muc.example.com/JulieC'>
     <x xmlns='http://jabber.org/protocol/muc'/>
   </presence>

E: <presence
       from='characters@muc.example.com/JulieC'
       id='y2bs71v4'
       to='juliet@im.example.com/balcony'
       type='error'>
     <error type='wait'>
       <internal-server-error
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </presence>


 TOC 

8.3.3.7.  item-not-found

The addressed JID or item requested cannot be found; the associated error type SHOULD be "cancel".

C: <presence from='userfoo@example.com/bar'
             id='pwb2n78i'
             to='nosuchroom@conference.example.org/foo'/>

S: <presence from='nosuchroom@conference.example.org/foo'
             id='pwb2n78i'
             to='userfoo@example.com/bar'
             type='error'>
     <error type='cancel'>
       <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

Security Note: An application MUST NOT return this error if doing so would provide information about the intended recipient's network availability to an entity that is not authorized to know such information; instead it MUST return a <service-unavailable/> stanza error.



 TOC 

8.3.3.8.  jid-malformed

The sending entity has provided (e.g., during resource binding) or communicated (e.g., in the 'to' address of a stanza) an XMPP address or aspect thereof that does not adhere to the syntax defined in [XMPP‑ADDR] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Address Format,” July 2010.); the associated error type SHOULD be "modify".

C: <presence
       from='juliet@im.example.com/balcony'
       id='y2bs71v4'
       to='ch@r@cters@muc.example.com/JulieC'>
     <x xmlns='http://jabber.org/protocol/muc'/>
   </presence>

E: <presence
       from='ch@r@cters@muc.example.com/JulieC'
       id='y2bs71v4'
       to='juliet@im.example.com/balcony'
       type='error'>
     <error by='muc.example.com'
            type='modify'>
       <jid-malformed
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </presence>

Implementation Note: Enforcement of the format for XMPP localparts is primarily the responsibility of the service at which the associated account or entity is located (e.g., the example.com service is responsible for returning <jid-malformed/> errors related to all JIDs of the form <localpart@example.com>), whereas enforcement of the format for XMPP domainparts is primarily the responsibility of the service that seeks to route a stanza to the service identified by that domainpart (e.g., the example.org service is responsible for returning <jid-malformed/> errors related to stanzas that users of that service have to tried send to JIDs of the form <localpart@example.com>). However, any entity that detects a malformed JID MAY return this error.



 TOC 

8.3.3.9.  not-acceptable

The recipient or server understands the request but cannot process it because the request does not meet criteria defined by the recipient or server (e.g., a request to subscribe to information that does not simultaneously include configuration parameters needed by the recipient); the associated error type SHOULD be "modify".

C: <message to='juliet@im.example.com' id='yt2vs71m'>
     <body>[ ... the-emacs-manual ... ]</body>
   </message>

S: <message from='juliet@im.example.com' id='yt2vs71m'>
     <error type='modify'>
       <not-acceptable
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </message>


 TOC 

8.3.3.10.  not-allowed

The recipient or server does not allow any entity to perform the action (e.g., sending to entities at a blacklisted domain); the associated error type SHOULD be "cancel".

C: <presence
       from='juliet@im.example.com/balcony'
       id='y2bs71v4'
       to='characters@muc.example.com/JulieC'>
     <x xmlns='http://jabber.org/protocol/muc'/>
   </presence>

E: <presence
       from='characters@muc.example.com/JulieC'
       id='y2bs71v4'
       to='juliet@im.example.com/balcony'
       type='error'>
     <error type='cancel'>
       <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </presence>


 TOC 

8.3.3.11.  not-authorized

The sender needs to provide credentials before being allowed to perform the action, or has provided improper credentials; the associated error type SHOULD be "auth".

C: <presence
       from='juliet@im.example.com/balcony'
       id='y2bs71v4'
       to='characters@muc.example.com/JulieC'>
     <x xmlns='http://jabber.org/protocol/muc'/>
   </presence>

E: <presence
       from='characters@muc.example.com/JulieC'
       id='y2bs71v4'
       to='juliet@im.example.com/balcony'>
     <error type='auth'>
       <not-authorized xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </presence>


 TOC 

8.3.3.12.  payment-required

The requesting entity is not authorized to access the requested service because payment is necessary; the associated error type SHOULD be "auth".

C: <iq from='romeo@example.net/foo'
       id='7isf2v4'
       to='pubsub.example.com'
       type='get'>
     <pubsub xmlns='http://jabber.org/protocol/pubsub'>
       <items node='my_musings'/>
     </pubsub>
   </iq>

E: <iq from='pubsub.example.com'
       id='7isf2v4'
       to='romeo@example.net/foo'
       type='error'>
     <error type='auth'>
       <payment-required
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>


 TOC 

8.3.3.13.  policy-violation

The entity has violated some local service policy (e.g., a message contains words that are prohibited by the service); the server MAY choose to specify the policy in the <text/> element or in an application-specific condition element; the associated error type SHOULD be "modify" or "wait" depending on the policy being violated.

(In the following example, the client sends an XMPP message that is too large according to the server's local service policy.)

C: <message from='romeo@example.net/foo'
            to='bill@im.example.com'
            id='vq71f4nb'>
     <body>%#&@^!!!</body>
   </message>

S: <message from='bill@im.example.com'
            id='vq71f4nb'
            to='romeo@example.net/foo'>
     <error by='example.net' type='cancel'>
       <policy-violation
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </message>


 TOC 

8.3.3.14.  recipient-unavailable

The intended recipient is temporarily unavailable, undergoing maintenance, etc.; the associated error type SHOULD be "wait".

C: <presence
       from='juliet@im.example.com/balcony'
       id='y2bs71v4'
       to='characters@muc.example.com/JulieC'>
     <x xmlns='http://jabber.org/protocol/muc'/>
   </presence>

E: <presence
       from='characters@muc.example.com/JulieC'
       id='y2bs71v4'
       to='juliet@im.example.com/balcony'>
     <error type='wait'>
       <recipient-unavailable
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </presence>

Security Note: An application MUST NOT return this error if doing so would provide information about the intended recipient's network availability to an entity that is not authorized to know such information; instead it MUST return a <service-unavailable/> stanza error.



 TOC 

8.3.3.15.  redirect

The recipient or server is redirecting requests for this information to another entity, typically in a temporary fashion (as opposed to the <gone/> error condition, which is used for permanent addressing failures); the associated error type SHOULD be "modify" and the error stanza SHOULD contain the alternate address in the XML character data of the <redirect/> element (which MUST be a URI or IRI at which the entity can be contacted, typically an XMPP IRI as specified in [XMPP‑URI] (Saint-Andre, P., “Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP),” February 2008.)).

C: <presence
       from='juliet@im.example.com/balcony'
       id='y2bs71v4'
       to='characters@muc.example.com/JulieC'>
     <x xmlns='http://jabber.org/protocol/muc'/>
   </presence>

E: <presence
       from='characters@muc.example.com/JulieC'
       id='y2bs71v4'
       to='juliet@im.example.com/balcony'
       type='error'>
     <error type='modify'>
       <redirect xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
         xmpp:characters@conference.example.org
       </redirect>
     </error>
   </presence>


 TOC 

8.3.3.16.  registration-required

The requesting entity is not authorized to access the requested service because prior registration is necessary; the associated error type SHOULD be "auth".

C: <presence
       from='juliet@im.example.com/balcony'
       id='y2bs71v4'
       to='characters@muc.example.com/JulieC'>
     <x xmlns='http://jabber.org/protocol/muc'/>
   </presence>

E: <presence
       from='characters@muc.example.com/JulieC'
       id='y2bs71v4'
       to='juliet@im.example.com/balcony'>
     <error type='auth'>
       <registration-required
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </presence>


 TOC 

8.3.3.17.  remote-server-not-found

A remote server or service specified as part or all of the JID of the intended recipient does not exist or cannot be resolved (e.g., there is no _xmpp-server._tcp DNS SRV record, the A or AAAA fallback resolution fails, or A/AAAA lookup succeeds but there is no response on the IANA-registered port 5269); the associated error type SHOULD be "cancel".

C: <message
       from='romeo@example.net/home'
       id='ud7n1f4h'
       to='bar@example.org'
       type='chat'>
    <body>yt?</body>
   </message>

E: <message
       from='bar@example.org'
       id='ud7n1f4h'
       to='romeo@example.net/home'
       type='error'>
     <error type='cancel'>
       <remote-server-not-found
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </message>


 TOC 

8.3.3.18.  remote-server-timeout

A remote server or service specified as part or all of the JID of the intended recipient (or needed to fulfill a request) was resolved but communications could not be established within a reasonable amount of time (e.g., an XML stream cannot be established at the resolved IP address and port, or an XML stream can be established but stream negotiation fails because of problems with TLS, SASL, server dialback, etc.); the associated error type SHOULD be "wait".

C: <message
       from='romeo@example.net/home'
       id='ud7n1f4h'
       to='bar@example.org'
       type='chat'>
    <body>yt?</body>
   </message>

E: <message
       from='bar@example.org'
       id='ud7n1f4h'
       to='romeo@example.net/home'
       type='error'>
     <error type='wait'>
       <remote-server-timeout
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </message>


 TOC 

8.3.3.19.  resource-constraint

The server or recipient is busy or lacks the system resources necessary to service the request; the associated error type SHOULD be "wait".

C: <iq from='romeo@example.net/foo'
       id='kj4vz31m'
       to='pubsub.example.com'
       type='get'>
     <pubsub xmlns='http://jabber.org/protocol/pubsub'>
       <items node='my_musings'/>
     </pubsub>
   </iq>

E: <iq from='pubsub.example.com'
       id='kj4vz31m'
       to='romeo@example.net/foo'
       type='error'>
     <error type='wait'>
       <resource-constraint
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>


 TOC 

8.3.3.20.  service-unavailable

The server or recipient does not currently provide the requested service; the associated error type SHOULD be "cancel".

C: <message from='romeo@example.net/foo'
            to='juliet@im.example.com'>
     <body>Hello?</body>
   </message>

S: <message from='juliet@im.example.com/foo'
            to='romeo@example.net'>
     <error type='cancel'>
       <service-unavailable
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </message>

Security Note: An application MUST return a <service-unavailable/> stanza error instead of <item-not-found/> or <recipient-unavailable/> if sending one of the latter errors would provide information about the intended recipient's network availability to an entity that is not authorized to know such information.



 TOC 

8.3.3.21.  subscription-required

The requesting entity is not authorized to access the requested service because a prior subscription is necessary; the associated error type SHOULD be "auth".

C: <message
       from='romeo@example.net/orchard'
       id='pa73b4n7'
       to='playwright@shakespeare.example.com'
       type='chat'>
     <subject>ACT II, SCENE II</subject>
     <body>help, I forgot my lines!</body>
   </message>

E: <message
       from='playwright@shakespeare.example.com'
       id='pa73b4n7'
       to='romeo@example.net/orchard'
       type='error'>
     <error type='auth'>
       <subscription-required
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </message>


 TOC 

8.3.3.22.  undefined-condition

The error condition is not one of those defined by the other conditions in this list; any error type can be associated with this condition, and it SHOULD be used only in conjunction with an application-specific condition.

C: <message
       from='northumberland@shakespeare.example'
       id='richard2-4.1.247'
       to='kingrichard@royalty.england.example'>
     <body>My lord, dispatch; read o'er these articles.</body>
     <amp xmlns='http://jabber.org/protocol/amp'>
       <rule action='notify'
             condition='deliver'
             value='stored'/>
     </amp>

S: <message from='example.org'
            id='amp1'
            to='northumberland@example.net/field'
            type='error'>
     <amp xmlns='http://jabber.org/protocol/amp'
          from='kingrichard@example.org'
          status='error'
          to='northumberland@example.net/field'>
       <rule action='error'
             condition='deliver'
             value='stored'/>
     </amp>
     <error type='modify'>
       <undefined-condition
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       <failed-rules xmlns='http://jabber.org/protocol/amp#errors'>
         <rule action='error'
               condition='deliver'
               value='stored'/>
       </failed-rules>
     </error>
   </message>


 TOC 

8.3.3.23.  unexpected-request

The recipient or server understood the request but was not expecting it at this time (e.g., the request was out of order); the associated error type SHOULD be "wait" or "modify".

C: <iq from='romeo@example.net/foo'
       id='o6hsv25z'
       to='pubsub.example.com'
       type='set'>
     <pubsub xmlns='http://jabber.org/protocol/pubsub'>
        <unsubscribe
            node='my_musings'
            jid='romeo@example.net'/>
     </pubsub>
   </iq>

E: <iq from='pubsub.example.com'
       id='o6hsv25z'
       to='romeo@example.net/foo'
       type='error'>
     <error type='cancel'>
       <unexpected-request
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       <not-subscribed
           xmlns='http://jabber.org/protocol/pubsub#errors'/>
     </error>
   </iq>


 TOC 

8.3.4.  Application-Specific Conditions

As noted, an application MAY provide application-specific stanza error information by including a properly-namespaced child within the error element. Typically, the application-specific element supplements or further qualifies a defined element. Thus, the <error/> element will contain two or three child elements.

<iq id='ixc3v1b9' type='error'>
  <error type='modify'>
    <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <too-many-parameters xmlns='http://example.com/ns'/>
  </error>
</iq>
<message type='error' id='7h3baci9'>
  <error type='modify'>
    <undefined-condition
          xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <text xml:lang='en'
          xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
      [ ... application-specific information ... ]
    </text>
    <too-many-parameters xmlns='http://example.com/ns'/>
  </error>
</message>

An entity that receives an application-specific error condition it does not understand MUST ignore that condition but appropriately process the rest of the error stanza.



 TOC 

8.4.  Extended Content

Although the message, presence, and IQ stanzas provide basic semantics for messaging, availability, and request-response interactions, XMPP uses XML namespaces (see [XML‑NAMES] (Thompson, H., Hollander, D., Layman, A., Bray, T., and R. Tobin, “Namespaces in XML 1.0 (Third Edition),” December 2009.)) to extend the basic stanza syntax for the purpose of providing additional functionality.

A message or presence stanza MAY contain one or more optional child elements specifying content that extends the meaning of the message (e.g., an XHTML-formatted version of the message body as described in [XEP‑0071] (Saint-Andre, P., “XHTML-IM,” September 2008.)), and an IQ stanza of type "get" or "set" MUST contain one such child element. Such a child element MAY have any name and MUST possess a namespace declaration (other than "jabber:client", "jabber:server", or "http://etherx.jabber.org/streams") that defines the data contained within the child element. Such a child element is called an EXTENSION ELEMENT. An extension element can be included either at the direct child level of the stanza or in any mix of levels.

Similarly, EXTENSION ATTRIBUTES are allowed. That is: a stanza itself (i.e., the <iq/>, <message/>, and <presence/> elements qualified by the "jabber:client" or "jabber:server" namespace declared as the default namespace for the stream) and any child element of such a stanza (whether a child element qualifed by the default namespace or an extension element) MAY also include one or more attributes that are qualified by XML namespaces that are different from the default namespace or the reserved "xml" prefix (including the "empty namespace" if the attribute is not prefixed).

Interoperability Note: For the sake of backward compatibility and maximum interoperability, an entity that generates a stanza SHOULD NOT include such attributes in the stanza itself or in child elements of the stanza that are qualified by the default namespace (e.g., the message <body/> element).

An extension element or extension attribute is said to be EXTENDED CONTENT and the namespace name for such an element or attribute is said to be an EXTENDED NAMESPACE.

Informational Note: Although extended namespaces for XMPP are commonly defined by the XMPP Standards Foundation (XSF) and by the IETF, no specification or IETF standards action is required to define extended namespaces, and any individual or organization is free to define XMPP extensions.

To illustrate these concepts, several examples follow.

The following stanza contains one direct child element whose extended namespace is 'jabber:iq:roster':

<iq from='juliet@capulet.com/balcony'
    id='h83vxa4c'
    type='get'>
 <query xmlns='jabber:iq:roster'/>
</iq>

The following stanza contains two direct child elements with two different extended namespaces.

<presence from='juliet@capulet.com/balcony'>
  <c xmlns='http://jabber.org/protocol/caps'
     hash='sha-1'
     node='http://code.google.com/p/exodus'
     ver='QgayPKawpkPSDYmwT/WM94uAlu0='/>
  <x xmlns='vcard-temp:x:update'>
    <photo>sha1-hash-of-image</photo>
  </x>
</presence>

The following stanza contains two child elements, one of which is qualified by the default namespace and one of which is qualified by an extended namespace; the extension element in turn contains a child element that is qualified by a different extended namespace.

<message to='juliet@capulet.com'>
  <body>Hello?</body>
  <html xmlns='http://jabber.org/protocol/xhtml-im'>
    <body xmlns='http://www.w3.org/1999/xhtml'>
      <p style='font-weight:bold'>Hello?</t>
    </body>
  </html>
</message>

It is conventional in the XMPP community for implementations to not generate namespace prefixes for elements that are qualified by extended namespaces (outside the XMPP community, this convention is sometimes called "prefix-free canonicalization"). However, if an implementation generates such namespace prefixes then it MUST include the namespace declaration in the stanza itself or a child element of the stanza, not in the stream header (see Section 4.6.3 (Other Namespaces)).

Routing entities (typically servers) SHOULD try to maintain prefixes when serializing XML stanzas for processing, but receiving entities MUST NOT depend on the prefix strings to have any particular value.

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

If a recipient receives a stanza that contains an element or attribute it does not understand, it MUST NOT attempt to process that XML data and instead MUST proceed as follows.

If a server handles a stanza that is intended for delivery to another entity and that contains a child element it does not understand, it MUST route the stanza unmodified to a remote server or deliver the stanza unmodified to a connected client associated with a local recipient.



 TOC 

9.  Examples

The examples in this section further illustrate the protocols defined in this specification. The examples are not exhaustive and not normative. The alternate steps shown illustrate the handling of failure cases but would not necessarily be triggered by the data sent in the examples.



 TOC 

9.1.  Client-to-Server Examples

The following examples show the XMPP data flow for a client negotiating an XML stream with a server, exchanging XML stanzas, and closing the negotiated stream. The server is "im.example.com", the server requires use of TLS, the client authenticates via the SASL SCRAM-SHA-1 mechanism as <juliet@im.example.com", and the client binds a client-submitted resource to the stream. It is assumed that before sending the initial stream header, the client has already resolved an SRV record of _xmpp-client._tcp.im.example.com and has opened a TCP connection to the advertised port at the resolved IP address.



 TOC 

9.1.1.  TLS

Step 1: Client initiates stream to server:

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

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

S: <stream:stream
     from='im.example.com'
     id='t7AMCin9zjMNwQKDnplntZPIDEI='
     to='juliet@im.example.com'
     version='1.0'
     xml:lang='en'
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'

Step 3: Server sends stream features to client (only the STARTTLS extension at this point):

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

Step 4: Client sends STARTTLS command to server:

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

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

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

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

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

S: </stream:stream>

Step 6: Client and server attempt to complete TLS negotiation over the existing TCP connection (see [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) for details).

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

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

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



 TOC 

9.1.2.  SASL

Step 8: Server responds by sending a stream header to client along with any available stream features:

S: <stream:stream
     from='im.example.com'
     id='vgKi/bkYME8OAj4rlXMkpucAqe4='
     to='juliet@im.example.com'
     version='1.0'
     xml:lang='en'
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'

S: <stream:features>
     <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
       <mechanism>SCRAM-SHA-1-PLUS</mechanism>
       <mechanism>SCRAM-SHA-1</mechanism>
       <mechanism>PLAIN</mechanism>
     </mechanisms>
   </stream:features>

Step 9: Client selects an authentication mechanism, in this case SCRAM-SHA-1, including initial response data:

C: <auth xmlns="urn:ietf:params:xml:ns:xmpp-sasl"
         mechanism="SCRAM-SHA-1">
     biwsbj1qdWxpZXQscj1vTXNUQUF3QUFBQU1BQUFBTlAwVEFBQUFBQUJQVTBBQQ==
   </auth>

The decoded base64 data is "n,,n=juliet,r=oMsTAAwAAAAMAAAANP0TAAAAAABPU0AA".

Step 10: Server sends a challenge:

S: <challenge xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
     cj1vTXNUQUF3QUFBQU1BQUFBTlAwVEFBQUFBQUJQVTBBQWUxMjQ2OTViLTY5Y
     TktNGRlNi05YzMwLWI1MWIzODA4YzU5ZSxzPU5qaGtZVE0wTURndE5HWTBaaT
     AwTmpkbUxUa3hNbVV0TkRsbU5UTm1ORE5rTURNeixpPTQwOTY=
   </challenge>

The decoded base64 data is "r=oMsTAAwAAAAMAAAANP0TAAAAAABPU0AAe124695b-69a9-4de6-9c30-b51b3808c59e,s=NjhkYTM0MDgtNGY0Zi00NjdmLTkxMmUtNDlmNTNmNDNkMDMz,i=4096" (line breaks not included in actual data).

Step 11: Client sends a response:

C: <response xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
     Yz1iaXdzLHI9b01zVEFBd0FBQUFNQUFBQU5QMFRBQUFBQUFCUFUwQUFlMTI0N
     jk1Yi02OWE5LTRkZTYtOWMzMC1iNTFiMzgwOGM1OWUscD1VQTU3dE0vU3ZwQV
     RCa0gyRlhzMFdEWHZKWXc9
   </response>

The decoded base64 data is "c=biws, r=oMsTAAwAAAAMAAAANP0TAAAAAABPU0AAe124695b-69a9-4de6-9c30-b51b3808c59e, p=UA57tM/SvpATBkH2FXs0WDXvJYw=" (line breaks not included in actual data).

Step 12: Server informs client of success, including additional data with success:

S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     dj1wTk5ERlZFUXh1WHhDb1NFaVc4R0VaKzFSU289
   </success

The decoded base64 data is "v=pNNDFVEQxuXxCoSEiW8GEZ+1RSo=".

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

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

Step 13: Client initiates a new stream to server:

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


 TOC 

9.1.3.  Resource Binding

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

S: <stream:stream
     from='im.example.com'
     id='gPybzaOzBmaADgxKXu9UClbprp0='
     to='juliet@im.example.com'
     version='1.0'
     xml:lang='en'
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'>

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

Upon being informed that resource binding is mandatory, the client needs to bind a resource to the stream; here we assume that the client submits a human-readable text string.

Step 15: Client binds a resource:

C: <iq id='yhc13a95' type='set'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <resource>balcony</resource>
     </bind>
   </iq>

Step 16: Server accepts submitted resourcepart and informs client of successful resource binding:

S: <iq id='yhc13a95' type='result'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
       <jid>
         juliet@im.example.com/balcony
       </jid>
     </bind>
   </iq>

Step 16 (alt): Server returns error to client:

S: <iq id='yhc13a95' type='error'>
     <error type='cancel'>
       <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>


 TOC 

9.1.4.  Stanza Exchange

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

C: <message from='juliet@im.example.com/balcony'
            id='ju2ba41c'
            to='romeo@example.net'
            type='chat'
            xml:lang='en'>
     <body>Art thou not Romeo, and a Montague?</body>
   </message>

If necessary, sender's server negotiates XML streams with intended recipient's server (see Section 9.2 (Server-to-Server Examples)).

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

E: <message from='romeo@example.net/orchard'
            id='ju2ba41c'
            to='juliet@im.example.com/balcony'
            type='chat'
            xml:lang='en'>
     <body>Neither, fair saint, if either thee dislike.</body>
   </message>

The client can subsequently send and receive an unbounded number of subsequent XML stanzas over the stream.



 TOC 

9.1.5.  Close

Desiring to send no further messages, the client closes the stream but waits for incoming data from the server.

C: </stream:stream>

Consistent with Section 4.3 (Closing a Stream), the server might send additional data and then closes the stream as well.

S: </stream:stream>

The client now terminates the underlying TCP connection.



 TOC 

9.2.  Server-to-Server Examples

The following examples show the data flow for a server negotiating an XML stream with another server, exchanging XML stanzas, and closing the negotiated stream. The initiating server ("Server1") is im.example.com; the receiving server ("Server2") is example.net and it requires use of TLS; im.example.com presents a certificate and authenticates via the SASL EXTERNAL mechanism. It is assumed that before sending the initial stream header, Server1 has already resolved an SRV record of _xmpp-server._tcp.example.net and has opened a TCP connection to the advertised port at the resolved IP address. Note how Server1 uses a default namespace ("jabber:server") and prefixes for stream-related elements, whereas Server2 uses prefix-free canonicalization.



 TOC 

9.2.1.  TLS

Step 1: Server1 initiates stream to Server2:

S1: <stream:stream
      from='im.example.com'
      to='example.net'
      version='1.0'
      xmlns='jabber:server'
      xmlns:stream='http://etherx.jabber.org/streams'>

Step 2: Server2 responds by sending a response stream header to Server1:

S2: <stream
      from='example.net'
      id='hTiXkW+ih9k2SqdGkk/AZi0OJ/Q='
      to='im.example.com'
      version='1.0'
      xmlns='http://etherx.jabber.org/streams'>

Step 3: Server2 sends stream features to Server1 (only the STARTTLS extension at this point):

S2: <features xmlns='http://etherx.jabber.org/streams'>
      <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
        <required/>
      </starttls>
    </features>

Step 4: Server1 sends the STARTTLS command to Server2:

S1: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

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

S2: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

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

S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

S2: </stream>

Step 6: Server1 and Server2 attempt to complete TLS negotiation via TCP (see [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) for details).

Step 7: If TLS negotiation is successful, Server1 initiates a new stream to Server2:

S1: <stream:stream
      from='im.example.com'
      to='example.net'
      version='1.0'
      xmlns='jabber:server'
      xmlns:stream='http://etherx.jabber.org/streams'>

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



 TOC 

9.2.2.  SASL

Step 8: Server2 sends a response stream header to Server1 along with available stream features (including a preference for the SASL EXTERNAL mechanism):

S2: <stream
      from='example.net'
      id='RChdjlgj/TIBcbT9Keu31zDihH4='
      to='im.example.com'
      version='1.0'
      xmlns='http://etherx.jabber.org/streams'>

S2: <features xmlns='http://etherx.jabber.org/streams'>
      <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <mechanism>EXTERNAL</mechanism>
      </mechanisms>
    </features>

Step 9: Server1 selects the EXTERNAL mechanism (including an empty response of "="):

S1: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
          mechanism='EXTERNAL'/>=</auth>

Step 10: Server2 returns success:

S2: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

Step 10 (alt): Server2 informs Server1 of failed authentication:

S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
      <not-authorized/>
    </failure>

S2: </stream>

Step 11: Server1 initiates a new stream to Server2:

S1: <stream:stream
      from='im.example.com'
      to='example.net'
      version='1.0'
      xmlns='jabber:server'
      xmlns:stream='http://etherx.jabber.org/streams'>

Step 12: Server2 responds by sending a stream header to Server1 along with any additional features (or, in this case, an empty features element):

S2: <stream
      from='example.net'
      id='MbbV2FeojySpUIP6J91qaa+TWHM='
      to='im.example.com'
      version='1.0'
      xmlns='http://etherx.jabber.org/streams'>

S2: <features xmlns='http://etherx.jabber.org/streams'/>


 TOC 

9.2.3.  Stanza Exchange

Now Server1 is allowed to send XML stanzas to Server2 over the negotiated stream from im.example.com to example.net; here we assume that the transferred stanzas are those shown earlier for client-to-server communication, albeit over a server-to-server stream qualified by the 'jabber:server' namespace.

Server1 sends XML stanza to Server2:

S1: <message from='juliet@im.example.com/balcony'
             id='ju2ba41c'
             to='romeo@example.net'
             type='chat'
             xml:lang='en'>
    <body>Art thou not Romeo, and a Montague?</body>
   </message>


 TOC 

9.2.4.  Close

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

S1: </stream:stream>

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

S2: </stream>

Server1 now terminates the underlying TCP connection.



 TOC 

10.  Server Rules for Processing XML Stanzas

An XMPP server MUST ensure in-order processing of XML stanzas between any two entities. This includes stanzas sent by a client to its server for direct processing by the server (e.g., in-order processing of a roster get and initial presence as described in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.)).

Beyond the requirement for in-order processing, each server implementation will contain its own logic for processing stanzas it receives. Such logic determines whether the server needs to ROUTE a given stanza to another domain, DELIVER it to a local entity (typically a connected client associated with a local account), or HANDLE it directly within the server itself. The following rules apply.

Implementation Note: Particular XMPP applications MAY specify delivery rules that modify or supplement the following rules; for example, a set of delivery rules for instant messaging and presence applications is defined in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.).



 TOC 

10.1.  No 'to' Address



 TOC 

10.1.1.  Overview

If the stanza possesses no 'to' attribute, the server MUST handle it directly on behalf of the entity that sent it, where the meaning of "handle it directly" depends on whether the stanza is message, presence, or IQ. Because all stanzas received from other servers MUST possess a 'to' attribute, this rule applies only to stanzas received from a local entity (such as a client) that is connected to the server.



 TOC 

10.1.2.  Message

If the server receives a message stanza with no 'to' attribute, it MUST treat the message as if the 'to' address were the bare JID <localpart@domainpart> of the sending entity.



 TOC 

10.1.3.  Presence

If the server receives a presence stanza with no 'to' attribute, it MUST broadcast it to the entities that are subscribed to the sending entity's presence, if applicable ([XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.) defines the semantics of such broadcasting for presence applications).



 TOC 

10.1.4.  IQ

If the server receives an IQ stanza with no 'to' attribute, it MUST process the stanza on behalf of the account from which received the stanza, as follows:

  1. If the IQ stanza is of type "get" or "set" and the server understands the namespace that qualifies the payload, the server MUST handle the stanza on behalf of the sending entity or return an appropriate error to the sending entity. Although the meaning of "handle" is determined by the semantics of the qualifying namespace, in general the server will respond to the IQ stanza of type "get" or "set" by returning an appropriate IQ stanza of type "result" or "error", responding as if the server were the bare JID of the sending entity. As an example, if the sending entity sends an IQ stanza of type "get" where the payload is qualified by the 'jabber:iq:roster' namespace (as described in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.)), then the server will return the roster associated with the sending entity's bare JID to the particular resource of the sending entity that requested the roster.

  2. If the IQ stanza is of type "get" or "set" and the server does not understand the namespace that qualifies the payload, the server MUST return an error to the sending entity, which MUST be <service-unavailable/>.

  3. If the IQ stanza is of type "error" or "result", the server MUST handle the error or result in accordance with the payload of the associated IQ stanza or type "get" of "set" (if there is no such associated stanza, the server MUST ignore the error or result stanza).



 TOC 

10.2.  Remote Domain

If the domainpart of the JID contained in the 'to' attribute does not match one of the configured hostnames of the server, the server SHOULD attempt to route the stanza to the remote domain (subject to local service provisioning and security policies regarding inter-domain communication, since such communication is optional for any given deployment). As described in the following sections, there are two possible cases.

Security Note: These rules apply only client-to-server streams. As described under Section 8.1.1.2 (Server-to-Server Streams), a server MUST NOT accept a stanza over a server-to-server stream if the domainpart of the JID in the 'to' attribute does not match a hostname serviced by the receiving server.



 TOC 

10.2.1.  Existing Stream

If a server-to-server stream already exists between the two domains, the sender's server will attempt to route the stanza to the authoritative server for the remote domain over the existing stream.



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10.2.2.  No Existing Stream

If there exists no server-to-server stream between the two domains, the sender's server will proceed as follows:

  1. Resolve the hostname of the remote domain, as described under Section 13.9.2 (Use of DNS)).

  2. Negotiate a server-to-server stream between the two domains (as defined under Section 5 (STARTTLS Negotiation) and Section 6 (SASL Negotiation)).

  3. Route the stanza to the authoritative server for the remote domain over the newly-established stream.



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10.2.3.  Error Handling

If routing of a stanza to the intended recipient's server is unsuccessful, the sender's server MUST return an error to the sender. If resolution of the remote domain is unsuccessful, the stanza error MUST be <remote-server-not-found/>. If resolution succeeds but streams cannot be negotiated, the stanza error MUST be <remote-server-timeout/>.

If stream negotiation with the intended recipient's server is successful but the remote server cannot deliver the stanza to the recipient, the remote server MUST return an appropriate error to the sender by way of the sender's server.



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10.3.  Local Domain

If the hostname of the domainpart of the JID contained in the 'to' attribute matches one of the configured hostnames of the server, the server MUST first determine if the hostname is serviced by the server itself or by a specialized local service. If the latter, the server MUST route the stanza to that service. If the former, the server MUST proceed as follows.



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10.3.1.  Mere Domain

If the JID contained in the 'to' attribute is of the form <domain>, then the server MUST either handle the stanza as appropriate for the stanza kind or return an error stanza to the sender.



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10.3.2.  Domain with Resource

If the JID contained in the 'to' attribute is of the form <domain/resource>, then the server MUST either handle the stanza as appropriate for the stanza kind or return an error stanza to the sender.



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10.3.3.  Localpart at Domain

For addresses of this type, more detailed rules in the context of instant messaging and presence applications are provided in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.).



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10.3.3.1.  No Such User

If there is no local account associated with the <localpart@domainpart>, how the stanza is processed depends on the stanza type.



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10.3.3.2.  Bare JID

If the JID contained in the 'to' attribute is of the form <localpart@domainpart>, how the stanza is processed depends on the stanza type.



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10.3.3.3.  Full JID

If the JID contained in the 'to' attribute is of the form <localpart@domainpart/resource> and there is no connected resource that exactly matches the full JID, the stanza SHOULD be processed as if the JID were of the form <localpart@domainpart>.

If the JID contained in the 'to' attribute is of the form <localpart@domainpart/resource> and there is a connected resource that exactly matches the full JID, the server SHOULD deliver the stanza to that connected resource.



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11.  XML Usage



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11.1.  Restrictions

XMPP defines a class of data objects called XML streams as well as the behavior of computer programs that process XML streams. XMPP is an application profile or restricted form of the Extensible Markup Language [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.), and a complete XML stream (including start and end stream tags) is a conforming XML document.

However, XMPP does not deal with XML documents but with XML streams. Because XMPP does not require the parsing of arbitrary and complete XML documents, there is no requirement that XMPP needs to support the full feature set of [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.). In particular, the following features of XML are prohibited in XMPP:

An XMPP implementation MUST behave as follows with regard to these features:

  1. An XMPP implementation MUST NOT inject characters matching such features into an XML stream.

  2. If an XMPP implementation receives characters matching such features over an XML stream, it MUST return a stream error, which SHOULD be <restricted-xml/> (although some existing implementations send <bad-format/> instead).



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11.2.  XML Namespace Names and Prefixes

XML namespaces (see [XML‑NAMES] (Thompson, H., Hollander, D., Layman, A., Bray, T., and R. Tobin, “Namespaces in XML 1.0 (Third Edition),” December 2009.)) are used within XMPP streams to create strict boundaries of data ownership. The basic function of namespaces is to separate different vocabularies of XML elements that are structurally mixed together. Ensuring that XMPP streams are namespace-aware enables any allowable XML to be structurally mixed with any data element within XMPP. XMPP-specific rules for XML namespace names and prefixes are defined under Section 4.6 (Namespaces) for XML streams and Section 8.4 (Extended Content) for XML stanzas.



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11.3.  Well-Formedness

There are two varieties of well-formedness:

The following rules apply.

An XMPP entity MUST NOT generate data that is not XML-well-formed. An XMPP entity MUST NOT accept data that is not XML-well-formed; instead it MUST return an <xml-not-well-formed/> stream error and close the stream over which the data was received.

An XMPP entity MUST NOT generate data that is not namespace-well-formed. An XMPP server MUST NOT route or deliver data that is not namespace-well-formed, and MUST return a stanza error of <not-acceptable/> or a stream error of <xml-not-well-formed/> in response to the receipt of such data.

Interoperability Note: Because these restrictions were underspecified in [RFC3920] (Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.), it is possible that implementations based on that specification will send data that does not comply with these restrictions.



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11.4.  Validation

A server is not responsible for ensuring that XML data delivered to a client or routed to another server is valid, in accordance with the definition of "valid" provided in Section 2.8 of [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.). An implementation MAY choose to accept or provide only data that has been explicitly validated against the schemas provided in this document, but such behavior is OPTIONAL. A client SHOULD NOT rely on the ability to send data that does not conform to the schemas, and SHOULD ignore any non-conformant elements or attributes on the incoming XML stream.

Informational Note: The terms "valid" and "well-formed" are distinct in XML.



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11.5.  Inclusion of XML Declaration

Before sending a stream header, an implementation SHOULD send an XML declaration (matching production [23] content of [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.)). Applications MUST follow the rules provided in [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.) regarding the format of the XML declaration and the circumstances under which the XML declaration is included.



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11.6.  Character Encoding

Implementations MUST support the UTF-8 transformation of Universal Character Set [UCS2] (International Organization for Standardization, “Information Technology - Universal Multiple-octet coded Character Set (UCS) - Amendment 2: UCS Transformation Format 8 (UTF-8),” October 1996.) characters, as needed for conformance with [CHARSETS] (Alvestrand, H., “IETF Policy on Character Sets and Languages,” January 1998.) and as defined in [UTF‑8] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.). Implementations MUST NOT attempt to use any other encoding. If one party to an XML stream detects that the other party has attempted to send XML data with an encoding other than UTF-8, it MUST return a stream error, which SHOULD be <unsupported-encoding/> (although some existing implementations send <bad-format/> instead).

Implementation Note: Because it is mandatory for an XMPP implementation to support all and only the UTF-8 encoding and because UTF-8 always has the same byte order, an implementation MUST NOT send a byte order mark ("BOM") at the beginning of the data stream. If an entity receives the Unicode character U+FEFF anywhere in an XML stream (including as the first character of the stream), it MUST interpret that character as a zero width no-break space, not as a byte order mark.



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11.7.  Whitespace

Except where explicitly disallowed (e.g., during TLS negotiation (STARTTLS Negotiation) and SASL negotiation (SASL Negotiation)), either entity MAY send whitespace as separators between XML stanzas or between any other first-level elements sent over the stream. One common use for sending such whitespace is explained under Section 4.3 (Closing a Stream).



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11.8.  XML Versions

XMPP is an application profile of XML 1.0. A future version of XMPP might be defined in terms of higher versions of XML, but this specification defines XMPP only in terms of XML 1.0.



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12.  Internationalization Considerations

As specified under Section 11.6 (Character Encoding), XML streams MUST be encoded in UTF-8.

As specified under Section 4.5 (Stream Attributes), an XML stream SHOULD include an 'xml:lang' attribute specifying the default language for any XML character data that is intended to be presented to a human user. As specified under Section 8.1.5 (xml:lang), an XML stanza SHOULD include an 'xml:lang' attribute if the stanza contains XML character data that is intended to be presented to a human user. A server SHOULD apply the default 'xml:lang' attribute to stanzas it routes or delivers on behalf of connected entities, and MUST NOT modify or delete 'xml:lang' attributes on stanzas it receives from other entities.

Internationalization of XMPP addresses is specified in [XMPP‑ADDR] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Address Format,” July 2010.).



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13.  Security Considerations



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13.1.  Overview

XMPP technologies are typically deployed using a decentralized client-server architecture. As a result, several paths are possible when two XMPP entities need to communicate:

  1. Both entities are servers. In this case, the entities can establish a direct server-to-server stream between themselves.

  2. One entity is a server and the other entity is a client whose account is hosted on that server. In this case, the entities can establish a direct client-to-server stream between themselves.

  3. Both entities are clients whose accounts are hosted on the same server. In this case, the entities cannot establish a direct stream between themselves, but there is only one intermediate entity between them, whose policies they might understand and in which they might have some level of trust (e.g., the server might require the use of Transport Layer Security for all client connections).

  4. Both entities are clients but their accounts are hosted on different servers. In this case, the entities cannot establish a direct stream between themselves and there are two intermediate entities between them; each client might have some trust in the server that hosts its account but might know nothing about the policies of the server to which the other client connects.

This specification covers only the security of a direct XML stream between two servers or between a client and a server (cases #1 and #2), where each stream can be considered a single "hop" along a communication path. The goal of security for a multi-hop path (cases #3 and #4), although very desirable, is out of scope for this specification.

In accordance with [SEC‑GUIDE] (Rescorla, E. and B. Korver, “Guidelines for Writing RFC Text on Security Considerations,” July 2003.), this specification covers communication security (confidentiality, data integrity, and peer entity authentication), non-repudiation, and systems security (unauthorized usage, inappropriate usage, and denial of service). We also discuss common security issues such as information leaks, firewalls, and directory harvesting, as well as best practices related to the re-use of technologies such as base64, DNS, cryptographic hash functions, SASL, TLS, UTF-8, and XML.



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13.2.  Threat Model

The threat model for XMPP is in essence the standard "Internet Threat Model" described in [SEC‑GUIDE] (Rescorla, E. and B. Korver, “Guidelines for Writing RFC Text on Security Considerations,” July 2003.). Attackers are assumed to be interested in and capable of launching the following attacks against unprotected XMPP systems:

Where appropriate, the following sections describe methods for protecting against these threats.



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13.3.  Order of Layers

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

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

This order has important security characteristics, as described throughout these security considerations.



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13.4.  Confidentiality and Integrity

The use of Transport Layer Security (TLS) with non-null cipher suites provides a reliable mechanism for the ensuring the confidentiality and integrity of data exchanged between a client and a server or between two servers. Therefore TLS helps to protect against eavesdropping, password sniffing, man-in-the-middle attacks, and stanza replays, insertion, deletion, and modification over an XML stream. XMPP clients and servers MUST support TLS as defined under Section 5 (STARTTLS Negotiation).

Security Note: The confidentiality and integrity of a stream can be ensured by methods other than TLS, e.g. by means of a SASL mechanism that involves negotiation of a security layer.



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13.5.  Peer Entity Authentication

The use of the Simple Authentication and Security Layer (SASL) for authentication provides a reliable mechanism for peer entity authentication. Therefore SASL helps to protect against user spoofing, unauthorized usage, and man-in-the middle attacks. XMPP clients and servers MUST support SASL as defined under Section 6 (SASL Negotiation).



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13.6.  Strong Security

[STRONGSEC] (Schiller, J., “Strong Security Requirements for Internet Engineering Task Force Standard Protocols,” August 2002.) defines "strong security" and its importance to communication over the Internet. For the purpose of XMPP communication over client-to-server and server-to-server streams, the term "strong security" refers to the use of security technologies that provide both mutual authentication and integrity checking (e.g., a combination of TLS encryption and SASL authentication using appropriate SASL mechanisms). In particular, when using certificate-based authentication to provide strong security, a trust chain SHOULD be established out-of-band, although a shared certification authority signing certificates could allow a previously unknown certificate to establish trust in-band. See the next section regarding certificate validation procedures.

Implementations MUST support strong security. Service provisioning SHOULD use strong security.

The initial stream and the response stream MUST be secured separately, although security in both directions MAY be established via mechanisms that provide mutual authentication.



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13.7.  Certificates

Channel encryption of an XML stream using Transport Layer Security as described under Section 5 (STARTTLS Negotiation), and in some cases also authentication as described under Section 6 (SASL Negotiation), is commonly based on a digital certificate presented by the receiving entity (or, in the case of mutual authentication, both the receiving entity and the initiating entity). This section describes best practices regarding the generation of digital certificates to be presented by XMPP entities and the verification of digital certificates presented by XMPP entities.

In general, the following sections rely on and extend the rules and guidelines provided in [X509] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.) and [TLS‑CERTS] (Saint-Andre, P. and J. Hodges, “Representation and Verification of Application Server Identity in Certificates Used with Transport Layer Security (TLS),” June 2010.). The reader is referred to those specifications for a detailed understanding of certificates and their use in TLS.



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13.7.1.  Certificate Generation



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13.7.1.1.  General Considerations

The following rules apply to public key certificates that are issued to XMPP entities:

  1. The certificate MUST conform to [X509] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.).

  2. The certificate MUST NOT contain a basicConstraints extension with the cA boolean set to TRUE.

  3. The subject field MUST NOT be null.

  4. The hash algorithm for the signature SHOULD be SHA-256 as defined by [X509‑ALGO] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.).

  5. The certificate SHOULD include an Authority Information Access (AIA) extension that specifies the address of an Online Certificate Status Protocol [OCSP] (Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, “X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP,” June 1999.) responder.

The following rules apply to issuers of XMPP certificates:

  1. The certificate MUST conform to [X509] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.).

  2. The certificate MUST contain a keyUsage extension with the digitalSignature bit set.

  3. The subject field MUST NOT be null.

  4. The hash algorithm for the signature SHOULD be SHA-256 as defined by [X509‑ALGO] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.).

  5. For issuers of public key certificates, the issuer's certificate MUST contain a basicConstraints extension with the cA boolean set to TRUE.

  6. For issuers of access certificates, the issuer's certificate MUST NOT contain a basicConstraints extension with the cA boolean set to TRUE.



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13.7.1.2.  Server Certificates



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13.7.1.2.1.  Rules

In a digital certificate to be presented by an XMPP server (i.e., a SERVER CERTIFICATE), it is RECOMMENDED for the certificate to include one or more JIDs (i.e., domainparts) associated with domains serviced at the server. The rules and guidelines defined in [TLS‑CERTS] (Saint-Andre, P. and J. Hodges, “Representation and Verification of Application Server Identity in Certificates Used with Transport Layer Security (TLS),” June 2010.) apply to XMPP server certificates. The following preference order of identifiers is RECOMMENDED, where the identifier types are as described in [TLS‑CERTS] (Saint-Andre, P. and J. Hodges, “Representation and Verification of Application Server Identity in Certificates Used with Transport Layer Security (TLS),” June 2010.) unless otherwise noted.

  1. SRV-ID (see also [X509‑SRV] (Santesson, S., “Internet X.509 Public Key Infrastructure Subject Alternative Name for Expression of Service Name,” August 2007.))
  2. DNS-ID
  3. XmppAddr as specified under Section 13.7.1.4 (XmppAddr Identifier Type)
  4. CN-ID

XMPP client and server software implementations MUST be able to validate both the SRV-ID and DNS-ID identifier types. Certification authorities that issue XMPP-specific certificates MUST support the DNS-ID identifier type and SHOULD support the SRV-ID identifier type. Service providers SHOULD request and prefer certificates that include an SRV-ID identifier type. Support for the XmppAddr and CN-ID identifier types is encouraged for the sake of backward-compatibility.



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13.7.1.2.2.  Examples

For our first (relatively simple) example, consider a company called "Example Products, Inc." It hosts an XMPP service at "im.example.com" (i.e., user addresses at the service are of the form "user@im.example.com"), and SRV lookups for the xmpp-client and xmpp-server services at "im.example.com" yield one machine, called "x.example.com", as follows:

_xmpp-client._tcp.im.example.com. 400 IN SRV 20 0 5222 x.example.com
_xmpp-server._tcp.im.example.com. 400 IN SRV 20 0 5269 x.example.com

The certificate presented by x.example.com contains the following representations:

For our second (more complex) example, consider an ISP called "Example Internet Services". It hosts an XMPP service at "example.net" (i.e., user addresses at the service are of the form "user@example.net"), but SRV lookups for the xmpp-client and xmpp-server services at "example.net" yield two machines ("x1.example.net" and "x2.example.net"), as follows:

_xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x1.example.net.
_xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x2.example.net.
_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x1.example.net.
_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x2.example.net.

Example Internet Services also hosts chatrooms at chat.example.net, and provides an xmpp-server SRV record for that service as well (thus enabling entity from remote domains to access that service). It also might provide other such services in the future, so it wishes to represent a wildcard in its certificate to handle such growth.

The certificate presented by either x1.example.net or x2.example.net contains the following representations:



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13.7.1.3.  Client Certificates

In a digital certificate to be presented by an XMPP client controlled by a human user (i.e., a CLIENT CERTIFICATE), it is RECOMMENDED for the certificate to include one or more JIDs associated with an XMPP user. If included, a JID MUST be represented as an XmppAddr as specified under Section 13.7.1.4 (XmppAddr Identifier Type).



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13.7.1.4.  XmppAddr Identifier Type

The XmppAddr identifier type is 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 below.

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

As an alternative to the "id-on-xmppAddr" notation, this Object Identifier MAY be represented in dotted display format (i.e., "1.3.6.1.5.5.7.8.5") or in the Uniform Resource Name notation specified in [URN‑OID] (Mealling, M., “A URN Namespace of Object Identifiers,” February 2001.) (i.e., "urn:oid:1.3.6.1.5.5.7.8.5").

Thus for example the JID <juliet@im.example.com" as included in a certificate could be formatted in any of the following three ways:

id-on-xmppAddr:
subjectAltName=otherName:id-on-xmppAddr;UTF8:juliet@im.example.com
dotted display format:
subjectAltName=otherName:1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com
URN notation:
subjectAltName=otherName:urn:oid:1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com

Use of the "id-on-xmppAddr" format is RECOMMENDED in the generation of certificates, but all three formats MUST be supported for the purpose of certificate validation.

The "id-on-xmppAddr" object identifier MAY be used on conjuction with the extended key usage extension specified in Section 4.2.1.12 of [X509] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.) in order to explicitly define and limit the intended use of a certificate to the XMPP network.



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13.7.2.  Certificate Validation

When an XMPP entity is presented with a server certificate or client certificate by a peer for the purpose of encryption or authentication of XML streams as described under Section 5 (STARTTLS Negotiation) and Section 6 (SASL Negotiation), the entity MUST attempt to validate the certificate to determine if the certificate will be considered a TRUSTED CERTIFICATE, i.e., a certificate that is acceptable for encryption and/or authentication in accordance with the XMPP entity's local service policies or configured settings.

For both server certificates and client certificates, the validating entity MUST attempt to verify the integrity of the certificate, MUST attempt to verify that the certificate has been properly signed by the issuing Certificate Authority, MUST attempt to validate the full certification path, and MUST support certificate revocation messages. An implementation MUST enable a human user to view information about the certification path.

If these validation attempts fail, either entity MAY choose to unilaterally terminate the session.

The following sections describe certificate validation rules for server-to-server and client-to-server streams.



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13.7.2.1.  Server Certificates

For server certificates, the rules and guidelines defined in [TLS‑CERTS] (Saint-Andre, P. and J. Hodges, “Representation and Verification of Application Server Identity in Certificates Used with Transport Layer Security (TLS),” June 2010.) apply. The following preference order of identifiers is RECOMMENDED, where the identifier types are as described in [TLS‑CERTS] (Saint-Andre, P. and J. Hodges, “Representation and Verification of Application Server Identity in Certificates Used with Transport Layer Security (TLS),” June 2010.) unless otherwise noted.

  1. SRV-ID (see also [X509‑SRV] (Santesson, S., “Internet X.509 Public Key Infrastructure Subject Alternative Name for Expression of Service Name,” August 2007.))
  2. DNS-ID
  3. XmppAddr as specified under Section 13.7.1.4 (XmppAddr Identifier Type)
  4. CN-ID



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13.7.2.2.  Client Certificates

When an XMPP server validates a certificate presented by a client, there are three possible cases, as discussed in the following sections.



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13.7.2.2.1.  Case #1

If the client certificate appears to be certified by a certification path terminating in a trust anchor (as described in Section 6.1 of [X509] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.)), the server MUST check the certificate for any instances of the XmppAddr as described under Section 13.7.1.4 (XmppAddr Identifier Type). There are three possible sub-cases:

Sub-Case #1:
The server finds one XmppAddr for which the domainpart of the represented JID matches one of the configured hostnames of the server; the server SHOULD use this represented JID as the validated identity of the client.
Sub-Case #2:
The server finds more than one XmppAddr for which the domainpart of the represented JID matches one of the configured hostnames of the server; the server SHOULD use one of these represented JIDs as the validated identity of the client, choosing among them according to local service policies or based on the 'to' address of the initial stream header.
Sub-Case #3:
The server finds no XmppAddrs, or finds at least one XmppAddr but the domainpart of the represented JID does not match one of the configured hostnames of the server; the server MUST NOT use the represented JID (if any) as the validated identity of the client but instead MUST either validate the identity of the client using other means.



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13.7.2.2.2.  Case #2

If the client certificate is certified by a Certificate Authority not known to the server, the server MUST proceed as under Case #1, Sub-Case #3.



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13.7.2.2.3.  Case #3

If the client certificate is self-signed, the server MUST proceed as under Case #1, Sub-Case #3.



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13.7.2.3.  Checking of Certificates in Long-Lived Streams

Because XMPP uses long-lived XML streams, it is possible that a certificate presented during stream negotiation might expire or be revoked while the stream is still live (this is especially relevant in the context of server-to-server streams). Therefore, each party to a long-lived stream SHOULD:

  1. Cache the expiration date of the certificate presented by the other party and any certificates on which that certificate depends (such as a root or intermediate certificate for a certification authority), and close the stream when any such certificate expires.

  2. Periodically query the Online Certificate Status Protocol [OCSP] (Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, “X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP,” June 1999.) responder listed in the Authority Information Access (AIA) extension of the certificate presented by the other party and any certificates on which that certificate depends (such as a root or intermediate certificate for a certification authority), and close the stream if any such certificate has been revoked.

If the identity presented by the initiating entity changes between the old stream and the new stream, then the receiving entity MUST perform a full re-authentication on the new stream.



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13.7.2.4.  Use of Certificates in XMPP Extensions

Certificates MAY be used in extensions to XMPP for the purpose of application-layer encryption or authentication above the level of XML streams (e.g., for end-to-end encryption). Such extensions will define their own certificate handling rules, which at a minimum SHOULD be consistent with the rules defined in this specification but MAY specify additional rules.



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13.8.  Mandatory-to-Implement Technologies

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

for authentication only:
the SASL Salted Challenge Response mechanism [SCRAM] (Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, “Salted Challenge Response (SCRAM) SASL and GSS-API Mechanism,” February 2010.), in particular the SCRAM-SHA-1 variant (REQUIRED) and SCRAM-SHA-1-PLUS variant (RECOMMENDED if channel binding is possible); although the SASL PLAIN mechanism [PLAIN] (Zeilenga, K., “The PLAIN Simple Authentication and Security Layer (SASL) Mechanism,” August 2006.) can also be used for authentication only, such usage is strongly discouraged
for confidentiality only:
TLS (using the TLS_RSA_WITH_AES_128_CBC_SHA cipher)
for both confidentiality and authentication with passwords:
TLS plus SCRAM-SHA-1 (REQUIRED) or SCRAM-SHA-1-PLUS (RECOMMENDED); alternatively, TLS plus SASL PLAIN (but see further security considerations under Section 13.9.4 (Use of SASL))
for both confidentiality and authentication without passwords:
TLS plus the SASL EXTERNAL mechanism (see Appendix A of [SASL] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.)) using the TLS_RSA_WITH_AES_128_CBC_SHA cipher supporting peer certificates (clients SHOULD support this, and servers MUST)

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

Interoperability Note: The use of the SCRAM-SHA-1 or SASL-SCRAM-SHA-1-PLUS mechanism replaces the SASL DIGEST-MD5 mechanism as XMPP's mandatory-to-implement password-based method for authentication only, and the use of TLS plus either of those SCRAM variants or TLS plus PLAIN replaces TLS plus DIGEST-MD5. For backward-compatibility with existing deployed infrastructure, implementations are encouraged to continue supporting the DIGEST-MD5 mechanism as specified in [DIGEST‑MD5] (Leach, P. and C. Newman, “Using Digest Authentication as a SASL Mechanism,” May 2000.), however there are known interoperability issues with DIGEST-MD5 that make it impractical in the long term. The use of the SCRAM-SHA-1 and SCRAM-SHA-1-PLUS mechanisms is strongly preferred over the SASL PLAIN mechanism because of their superior security properties, and PLAIN is intended to be a fallback only for implementations that do not yet support SCRAM. For important security considerations related to these SASL mechanisms, see Section 13.9.4 (Use of SASL) and also refer to [SCRAM] (Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, “Salted Challenge Response (SCRAM) SASL and GSS-API Mechanism,” February 2010.) and [PLAIN] (Zeilenga, K., “The PLAIN Simple Authentication and Security Layer (SASL) Mechanism,” August 2006.).



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13.9.  Technology Reuse



 TOC 

13.9.1.  Use of base64 in SASL

Both the client and the server MUST verify any base64 data received during SASL negotiation (SASL Negotiation). An implementation MUST reject (not ignore) any characters that are not explicitly allowed by the base64 alphabet; this helps to guard against creation of a covert channel that could be used to "leak" information.

An implementation MUST NOT break on invalid input and MUST reject any sequence of base64 characters containing the pad ('=') character if that character is included as something other than the last character of the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against buffer overflow attacks and other attacks on the implementation.

While base 64 encoding visually hides otherwise easily recognized information (such as passwords), it does not provide any computational confidentiality.

All uses of base 64 encoding MUST follow the definition in Section 4 of [BASE64] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) and padding bits MUST be set to zero.



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13.9.2.  Use of DNS

XMPP typically relies on the Domain Name System (specifically [DNS‑SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) records) to resolve a fully qualified domain name to an IP address before a client connects to a server or before a peer server connects to another server. Before attempting to negotiate an XML stream, the initiating entity MUST NOT proceed until it has resolved the DNS domain name of the peer server as specified under Section 3 (TCP Binding) (although it is not necessary to resolve the DNS domain name before each connection attempt, because DNS resolution results can be temporarily cached in accordance with time-to-live values). However, in the absence of a secure DNS option (e.g., as provided by [DNSSEC] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.)), a malicious attacker with access to the DNS server data, or able to cause spoofed answers to be cached in a recursive resolver, can potentially cause the initiating entity to connect to any XMPP server chosen by the attacker. Deployment and validation of server certificates helps to prevent such attacks.



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13.9.3.  Use of Hash Functions

XMPP itself does not directly mandate the use of any particular hash function. However, technologies on which XMPP depends (e.g., TLS and particular SASL mechanisms), as well as various XMPP extensions, might make use of hash functions. Those who implement XMPP technologies or who develop XMPP extensions are advised to closely monitor the state of the art regarding attacks against cryptographic hashes in Internet protocols as they relate to XMPP. For helpful guidance, refer to [HASHES] (Hoffman, P. and B. Schneier, “Attacks on Cryptographic Hashes in Internet Protocols,” November 2005.).



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13.9.4.  Use of SASL

Because the initiating entity chooses an acceptable SASL mechanism from the list presented by the receiving entity, the initiating entity depends on the receiving entity's list for authentication. This dependency introduces the possibility of a downgrade attack if an attacker can gain control of the channel and therefore present a weak list of mechanisms. To help prevent this attack, the parties SHOULD protect the channel using TLS before attempting SASL negotiation.

If the initiating entity (typically a client) is willing to use SASL PLAIN over TLS to authenticate to an XMPP server, it MUST verify the server certificate. If the server has not provided any certificate, or if certificate validation fails (as described under Section 13.7.2 (Certificate Validation)), the initiating entity MUST NOT attempt to authenticate using the SASL PLAIN mechanism. After a successful TLS negotiation, the initiating entity MUST check its understanding of the server hostname against the server's identity as presented in the TLS Certificate message, in order to prevent man-in-the-middle attacks. If the match fails, the client MUST NOT attempt to authenticate using the SASL PLAIN mechanism. Server identity matching MUST follow the rules specified in [TLS‑CERTS] (Saint-Andre, P. and J. Hodges, “Representation and Verification of Application Server Identity in Certificates Used with Transport Layer Security (TLS),” June 2010.).

The SASL framework itself does not provide a method for binding SASL authentication to a security layer providing confidentiality and integrity protection that was negotiated at a lower layer. Such a binding is known as a "channel binding" (see [CHANNEL] (Williams, N., “On the Use of Channel Bindings to Secure Channels,” November 2007.)). Some SASL mechanisms provide channel bindings, which in the case of XMPP would typically be a binding to TLS (see [CHANNEL‑TLS] (Altman, J., Williams, N., and L. Zhu, “Channel Bindings for TLS,” March 2010.)). If a SASL mechanism provides a channel binding (e.g., this is true of [SCRAM] (Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, “Salted Challenge Response (SCRAM) SASL and GSS-API Mechanism,” February 2010.)), then XMPP entities using that mechanism SHOULD prefer the channel binding variant (e.g., preferring "SCRAM-SHA-1-PLUS" over "SCRAM-SHA-1"). If a SASL mechanism does not provide a channel binding, then the mechanism cannot provide a way to verify that the source and destination end points to which the lower layer's security is bound are equivalent to the end points that SASL is authenticating; furthermore, if the end points are not identical, then the lower layer's security cannot be trusted to protect data transmitted between the SASL-authenticated entities. In such a situation, a SASL security layer SHOULD be negotiated that effectively ignores the presence of the lower-layer security.

Most XMPP servers authenticate account connections by means of passwords. It is well-known that most human users choose relatively weak passwords. Although service provisioning is out of scope for this document, XMPP servers that allow password-based authentication SHOULD enforce minimal criteria for password strength to help prevent dictionary attacks.

Some SASL mechanisms (e.g., [ANONYMOUS] (Zeilenga, K., “Anonymous Simple Authentication and Security Layer (SASL) Mechanism,” June 2006.)) do not provide strong peer entity authentication of the client to the server. Service administrators are advised to enable such mechanisms with caution. Best practices for the use of the SASL ANONYMOUS mechanism in XMPP are described in [XEP‑0175] (Saint-Andre, P., “Best Practices for Use of SASL ANONYMOUS,” November 2007.).



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13.9.5.  Use of TLS

Implementations of TLS typically support multiple versions of the Transport Layer Security protocol as well as the older Secure Sockets Layer (SSL) protocol. Because of known security vulnerabilities, XMPP servers and clients MUST NOT request, offer, or use SSL 2.0. See Appendix E.2 of [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) for further details.



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13.9.6.  Use of UTF-8

The use of UTF-8 makes it possible to transport non-ASCII characters, and thus enables character "spoofing" scenarios, in which a displayed value appears to be something other than it is. Furthermore, there are known attack scenarios related to the decoding of UTF-8 data. On both of these points, refer to [UTF‑8] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) for more information.



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13.9.7.  Use of XML

Because XMPP is an application profile of the Extensible Markup Language [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.), many of the security considerations described in [XML‑MEDIA] (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.) and [XML‑GUIDE] (Hollenbeck, S., Rose, M., and L. Masinter, “Guidelines for the Use of Extensible Markup Language (XML) within IETF Protocols,” January 2003.) also apply to XMPP. Several aspects of XMPP mitigate the risks described there, such as the prohibitions specified under Section 11.1 (Restrictions) and the lack of external references to style sheets or transformations, but these mitigating factors are by no means comprehensive.



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13.10.  Information Leaks



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13.10.1.  IP Addresses

A client's IP address and method of access MUST NOT be made public by a server.



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13.10.2.  Presence Information

One of the core aspects of XMPP is presence: information about the network availability of an XMPP entity (i.e., whether the entity is currently online or offline). A PRESENCE LEAK occurs when an entity's network availability is inadvertently and involuntarily revealed to a second entity that is not authorized to know the first entity's network availability.

Although presence is discussed more fully in [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.), it is important to note that an XMPP server MUST NOT leak presence. In particular at the core XMPP level, real-time addressing and network availability is associated with a specific connected resource; therefore, any disclosure of a connected resource's full JID comprises a presence leak. To help prevent such a presence leak, a server MUST NOT return different stanza errors if a potential attacker sends XML stanzas to the entity's bare JID (<localpart@domainpart>) or full JID (<localpart@domainpart/resource>).



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13.11.  Directory Harvesting

If a server generates an error stanza in response to receiving a stanza for a user account that does not exist, using the <service-unavailable/> stanza error condition can help protect against directory harvesting attacks, since this is the same error condition that is returned if, for instance, the namespace of an IQ child element is not understood, or if offline message storage or message forwarding is not enabled for a domain. However, subtle differences in the exact XML of error stanzas, as well as in the timing with which such errors are returned, can enable an attacker to determine the network presence of a user when more advanced blocking technologies are not used (see for instance [XEP‑0016] (Millard, P. and P. Saint-Andre, “Privacy Lists,” February 2007.) and [XEP‑0191] (Saint-Andre, P., “Simple Communications Blocking,” February 2007.)). Therefore, a server that exercises a higher level of caution might not return any error at all in response to certain kinds of received stanzas, so that a non-existent user appears to behave like a user that has no interest in conversing with the sender.



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13.12.  Denial of Service

[DOS] (Handley, M., Rescorla, E., and IAB, “Internet Denial-of-Service Considerations,” December 2006.) defines denial of service as follows:

A Denial-of-Service (DoS) attack is an attack in which one or more machines target a victim and attempt to prevent the victim from doing useful work. The victim can be a network server, client or router, a network link or an entire network, an individual Internet user or a company doing business using the Internet, an Internet Service Provider (ISP), country, or any combination of or variant on these.

Some considerations discussed in this document help to prevent denial of service attacks (e.g., the mandate that a server MUST NOT process XML stanzas from clients that have not yet provided appropriate authentication credentials and MUST NOT process XML stanzas from peer servers whose identity it has not either authenticated via SASL or weakly verified via server dialback).

In addition, [XEP‑0205] (Saint-Andre, P., “Best Practices to Discourage Denial of Service Attacks,” January 2009.) provides a detailed discussion of potential denial of service attacks against XMPP systems and best practices for preventing such attacks. The recommendations include:

  1. A server implementation SHOULD enable a server administrator to limit the number of TCP connections that it will accept from a given IP address at any one time. If an entity attempts to connect but the maximum number of TCP connections has been reached, the receiving server MUST NOT allow the new connection to proceed.

  2. A server implementation SHOULD enable a server administrator to limit the number of TCP connection attempts that it will accept from a given IP address in a given time period. If an entity attempts to connect but the maximum number of connection attempts has been reached, the receiving server MUST NOT allow the new connection to proceed.

  3. A server implementation SHOULD enable a server administrator to limit the number of connected resources it will allow an account to bind at any one time. If a client attempts to bind a resource but it has already reached the configured number of allowable resources, the receiving server MUST return a <resource-constraint/> stanza error.

  4. A server implementation SHOULD enable a server administrator to limit the size of stanzas it will accept from a connected client or peer server (where "size" is inclusive of all XML markup as defined in Section 2.4 of [XML] (Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.), from the opening "<" character of the stanza to the closing ">" character). An entity's maximum stanza size MUST NOT be smaller than 10000 bytes. If a connected resource or peer server sends a stanza that violates the upper limit, the receiving server MUST either return a <policy-violation/> stanza error (thus allowing the sender to recover) or close the stream with a <policy-violation/> stream error.

  5. A server implementation SHOULD enable a server administrator to limit the number of XML stanzas that a connected client is allowed to send to distinct recipients within a given time period. If a connected client sends too many stanzas to distinct recipients in a given time period, the receiving server SHOULD NOT process the stanza and instead SHOULD return a <policy-violation/> stanza error.

  6. A server implementation SHOULD enable a server administrator to limit the amount of bandwidth it will allow a connected client or peer server to use in a given time period.

  7. A server implementation MAY enable a server administrator to limit the types of stanzas (based on the extended content "payload") that it will allow a connected resource or peer server send over an active connection. Such limits and restrictions are a matter of deployment policy.

  8. A server implementation MAY refuse to route or deliver any stanza that it considers to be abusive, with or without returning an error to the sender.

For more detailed recommendations regarding denial of service attacks in XMPP systems, refer to [XEP‑0205] (Saint-Andre, P., “Best Practices to Discourage Denial of Service Attacks,” January 2009.).



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13.13.  Firewalls

Although DNS SRV records can instruct connecting entities to use TCP ports other than 5222 (client-to-server) and 5269 (server-to-server), communication using XMPP typically occurs over those ports, which are registered with the IANA (see Section 14 (IANA Considerations)). Use of these well-known ports allows administrators to easily enable or disable XMPP activity through existing and commonly-deployed firewalls.



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13.14.  Interdomain Federation

The term "federation" is commonly used to describe communication between two servers.

Because service provisioning is a matter of policy, it is OPTIONAL for any given server to support federation. If a particular server enables federation, it SHOULD enable strong security as previously described to ensure both authentication and confidentiality; compliant implementations SHOULD support TLS and SASL for this purpose.

Before RFC 3920 defined TLS plus SASL EXTERNAL with certificates for encryption and authentication of server-to-server streams, the only method for weak identity verification of a peer server was server dialback as defined in [XEP‑0220] (Miller, J., Saint-Andre, P., and P. Hancke, “Server Dialback,” March 2010.). Even when [DNSSEC] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.) is used, dialback provides only weak identity verification and provides no confidentiality or integrity. At the time of writing, server dialback is still the most widely-used technique for some level of assurance over server-to-server streams. This reality introduces the possibility of a downgrade attack from TLS + SASL EXTERNAL to dialback if an attacker can gain control of the channel and therefore convince the initiating server that the receiving server does not support TLS or does not have an appropriate certificate. To help prevent this attack, the parties SHOULD protect the channel using TLS before proceeding, even if the presented certificates are self-signed or otherwise untrusted.



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13.15.  Non-Repudiation

Systems that provide both peer entity authentication and data integrity have the potential to enable an entity to prove to a third party that another entity intended to send particular data. Although XMPP systems can provide both peer entity authentication and data integrity, XMPP was never designed to provide non-repudiation.



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14.  IANA Considerations

The following sections update the registrations provided in [RFC3920] (Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.). This section is to be interpreted according to [IANA‑GUIDE] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).



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14.1.  XML Namespace Name for TLS Data

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

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


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14.2.  XML Namespace Name for SASL Data

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

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


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14.3.  XML Namespace Name for Stream Errors

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

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


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14.4.  XML Namespace Name for Resource Binding

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

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


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14.5.  XML Namespace Name for Stanza Errors

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

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


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14.6.  GSSAPI Service Name

The IANA has registered "xmpp" as a [GSS‑API] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) service name, as defined under Section 6.5 (SASL Definition).



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14.7.  Port Numbers

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

These ports SHOULD be used for client-to-server and server-to-server communications respectively, but other ports MAY be used.



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15.  Conformance Requirements

This section describes a protocol feature set that summarizes the conformance requirements of this specification. This feature set is appropriate for use in software certification, interoperability testing, and implementation reports. For each feature, this section provides the following information:

The feature set specified here attempts to adhere to the concepts and formats proposed by Larry Masinter within the IETF's NEWTRK Working Group in 2005, as captured in [INTEROP] (Masinter, L., “Formalizing IETF Interoperability Reporting,” October 2005.). Although this feature set is more detailed than called for by [REPORTS] (Dusseault, L. and R. Sparks, “Guidance on Interoperation and Implementation Reports for Advancement to Draft Standard,” September 2009.), it provides a suitable basis for the generation of implementation reports to be submitted in support of advancing this specification from Proposed Standard to Draft Standard in accordance with [PROCESS] (Bradner, S., “The Internet Standards Process -- Revision 3,” October 1996.).

Feature:
bind-gen
Description:
Generate a random resource on demand.
Section:
Section 7.5 (Server-Generated Resource Identifier)
Roles:
Client N/A, Server MUST.

Feature:
bind-mtn
Description:
Consider resource binding as mandatory-to-negotiate.
Section:
Section 7.2.1 (Mandatory-to-Negotiate)
Roles:
Client MUST, Server MUST.

Feature:
bind-restart
Description:
Do not restart the stream after negotiation of resource binding.
Section:
Section 7.2.2 (Restart)
Roles:
Client MUST, Server MUST.

Feature:
bind-support
Description:
Support binding of client resources to an authenticated stream.
Section:
Section 7 (Resource Binding)
Roles:
Client MUST, Server MUST.

Feature:
sasl-errors
Description:
Support SASL errors during the negotiation process.
Section:
Section 6.4 (SASL Errors)
Roles:
Client MUST, Server MUST.

Feature:
sasl-mtn
Description:
Consider SASL as mandatory-to-negotiate.
Section:
Section 6.2.1 (Mandatory-to-Negotiate)
Roles:
Client MUST, Server MUST.

Feature:
sasl-restart
Description:
Initiate or handle a stream restart after SASL negotiation.
Section:
Section 6.2.2 (Restart)
Roles:
Client MUST, Server MUST.

Feature:
sasl-support
Description:
Support the Simple Authentication and Security Layer for stream authentication.
Section:
Section 6 (SASL Negotiation)
Roles:
Client MUST, Server MUST.

Feature:
sasl-whitespace
Description:
Ensure that no whitespace is sent between XML elements during SASL negotiation.
Section:
Section 6.2.5 (Data Formatting)
Roles:
Client MUST, Server MUST.

Feature:
security-mti-auth-plain
Description:
Support the SASL PLAIN mechanism for authentication only.
Section:
Section 13.8 (Mandatory-to-Implement Technologies)
Roles:
Client MUST, Server MUST.

Feature:
security-mti-auth-scram
Description:
Support the SASL Salted Challenge Response mechanism for authentication only.
Section:
Section 13.8 (Mandatory-to-Implement Technologies)
Roles:
Client MUST, Server MUST.

Feature:
security-mti-both-external
Description:
Support TLS with SASL EXTERNAL for confidentiality and authentication.
Section:
Section 13.8 (Mandatory-to-Implement Technologies)
Roles:
Client SHOULD, Server MUST.

Feature:
security-mti-both-plain
Description:
Support TLS with SASL PLAIN for confidentiality and authentication.
Section:
Section 13.8 (Mandatory-to-Implement Technologies)
Roles:
Client MUST, Server MUST.

Feature:
security-mti-both-scram
Description:
Support TLS with SASL SCRAM for confidentiality and authentication.
Section:
Section 13.8 (Mandatory-to-Implement Technologies)
Roles:
Client MUST, Server MUST.

Feature:
security-mti-confidentiality
Description:
Support TLS using the TLS_RSA_WITH_AES_128_CBC_SHA cipher for confidentiality only.
Section:
Section 13.8 (Mandatory-to-Implement Technologies)
Roles:
Client MUST, Server MUST.

Feature:
stanza-attribute-from
Description:
Support the common 'from' attribute for all stanza kinds.
Section:
Section 8.1.1 (to)
Roles:
Client MUST, Server MUST.

Feature:
stanza-attribute-from-validate
Description:
Validate the 'from' address of all stanzas received from connected clients or peer servers.
Section:
Section 8.1.2 (from)
Roles:
Client N/A, Server MUST.

Feature:
stanza-attribute-id
Description:
Support the common 'id' attribute for all stanza kinds.
Section:
Section 8.1.3 (id)
Roles:
Client MUST, Server MUST.

Feature:
stanza-attribute-to
Description:
Support the common 'to' attribute for all stanza kinds.
Section:
Section 8.1.1 (to)
Roles:
Client MUST, Server MUST.

Feature:
stanza-attribute-to-validate
Description:
Ensure that all stanzas received from peer servers include a 'to' address.
Section:
Section 8.1.1 (to)
Roles:
Client N/A, Server MUST.

Feature:
stanza-attribute-type
Description:
Support the common 'type' attribute for all stanza kinds.
Section:
Section 8.1.4 (type)
Roles:
Client MUST, Server MUST.

Feature:
stanza-attribute-xmllang
Description:
Support the common 'xml:lang' attribute for all stanza kinds.
Section:
Section 8.1.5 (xml:lang)
Roles:
Client MUST, Server MUST.

Feature:
stanza-error
Description:
Generate and handle stanzas of type "error" for all stanza kinds.
Section:
Section 8.3 (Stanza Errors)
Roles:
Client MUST, Server MUST.

Feature:
stanza-error-child
Description:
Ensure that stanzas of type "error" include an <error/> child element.
Section:
Section 8.3 (Stanza Errors)
Roles:
Client MUST, Server MUST.

Feature:
stanza-error-id
Description:
Ensure that stanzas of type "error" preserve the 'id' provided in the triggering stanza.
Section:
Section 8.3 (Stanza Errors)
Roles:
Client MUST, Server MUST.

Feature:
stanza-error-reply
Description:
Do not reply to a stanza of type "error" with another stanza of type "error".
Section:
Section 8.3 (Stanza Errors)
Roles:
Client MUST, Server MUST.

Feature:
stanza-extension
Description:
Correctly process XML data qualified by an unsupported XML namespace, where "correctly process" means to ignore that portion of the stanza in the case of a message or presence stanza and return an error in the case of an IQ stanza (for the intended recipient), and to route or deliver the stanza (for a routing entity such as a server).
Section:
Section 8.4 (Extended Content)
Roles:
Client MUST, Server MUST.

Feature:
stanza-iq-child
Description:
Include exactly one child element in an <iq/> stanza of type "get" or "set", zero or one child elements in an <iq/> stanza of type "result", and one or two child elements in an <iq/> stanza of type "error".
Section:
Section 8.2.3 (IQ Semantics)
Roles:
Client MUST, Server MUST.

Feature:
stanza-iq-id
Description:
Ensure that all <iq/> stanzas include an 'id' attribute.
Section:
Section 8.2.3 (IQ Semantics)
Roles:
Client MUST, Server MUST.

Feature:
stanza-iq-reply
Description:
Reply to an <iq/> stanza of type "get" or "set" with an <iq/> stanza of type "result" or "error".
Section:
Section 8.2.3 (IQ Semantics)
Roles:
Client MUST, Server MUST.

Feature:
stanza-iq-type
Description:
Ensure that all <iq/> stanzas include a 'type' attribute whose value is "get", "set", "result", or "error".
Section:
Section 8.2.3 (IQ Semantics)
Roles:
Client MUST, Server MUST.

Feature:
stanza-kind-iq
Description:
Support the <iq/> stanza.
Section:
Section 8.2.3 (IQ Semantics)
Roles:
Client MUST, Server MUST.

Feature:
stanza-kind-message
Description:
Support the <message/> stanza.
Section:
Section 8.2.1 (Message Semantics)
Roles:
Client MUST, Server MUST.

Feature:
stanza-kind-presence
Description:
Support the <presence/> stanza.
Section:
Section 8.2.2 (Presence Semantics)
Roles:
Client MUST, Server MUST.

Feature:
stream-attribute-initial-from
Description:
Include a 'from' attribute in the initial stream header.
Section:
Section 4.5.1 (from)
Roles:
Client SHOULD, Server SHOULD.

Feature:
stream-attribute-initial-lang
Description:
Include an 'xml:lang' attribute in the initial stream header.
Section:
Section 4.5.4 (xml:lang)
Roles:
Client SHOULD, Server SHOULD.

Feature:
stream-attribute-initial-to
Description:
Include a 'to' attribute in the initial stream header.
Section:
Section 4.5.2 (to)
Roles:
Client SHOULD, Server SHOULD.

Feature:
stream-attribute-response-from
Description:
Include a 'from' attribute in the response stream header.
Section:
Section 4.5.1 (from)
Roles:
Client N/A, Server MUST.

Feature:
stream-attribute-response-id
Description:
Include an 'id' attribute in the response stream header.
Section:
Section 4.5.3 (id)
Roles:
Client N/A, Server MUST.

Feature:
stream-attribute-response-id-unique
Description:
Ensure that the 'id' attribute in the response stream header is unique within the context of the receiving entity.
Section:
Section 4.5.3 (id)
Roles:
Client N/A, Server MUST.

Feature:
stream-attribute-response-to
Description:
Include a 'to' attribute in the response stream header.
Section:
Section 4.5.2 (to)
Roles:
Client N/A, Server SHOULD.

Feature:
stream-error-generate
Description:
Generate a stream error (followed by a closing stream tag and termination of the TCP connection) upon detecting a stream-related error condition.
Section:
Section 4.7 (Stream Errors)
Roles:
Client MUST, Server MUST.

Feature:
stream-hostname-resolution
Description:
Resolve hostnames before opening a TCP connection.
Section:
Section 3.2 (Hostname Resolution)
Roles:
Client MUST, Server MUST.

Feature:
stream-negotiation-complete
Description:
Do not consider the stream negotiation process to be complete until the receiving entity sends a stream features advertisement that is empty or that contains only voluntary-to-negotiate features.
Section:
Section 4.2.5 (Completion of Stream Negotiation)
Roles:
Client MUST, Server MUST.

Feature:
stream-negotiation-features
Description:
Send stream features after sending a response stream header.
Section:
Section 4.2.2 (Stream Features Format)
Roles:
Client N/A, Server MUST.

Feature:
stream-negotiation-restart
Description:
Consider the previous stream to be replaced upon negotiation of a stream feature that necessitates a stream restart, and send or receive a new initial stream header after negotiation of such a stream feature.
Section:
Section 4.2.3 (Restarts)
Roles:
Client MUST, Server MUST.

Feature:
stream-reconnect
Description:
Reconnect with exponential backoff if a TCP connection is terminated unexpectedly.
Section:
Section 3.4 (Reconnection)
Roles:
Client MUST, Server MUST.

Feature:
stream-tcp-binding
Description:
Bind an XML stream to a TCP connection.
Section:
Section 3 (TCP Binding)
Roles:
Client MUST, Server MUST.

Feature:
tls-certs
Description:
Check the identity specified in a certificate that is presented during TLS negotiation.
Section:
Section 13.7.2 (Certificate Validation)
Roles:
Client MUST, Server MUST.

Feature:
tls-mtn
Description:
Consider TLS as mandatory-to-negotiate if STARTTLS is the only feature advertised or if the STARTTLS feature includes an empty <required/> element.
Section:
Section 5.2.1 (Mandatory-to-Negotiate)
Roles:
Client MUST, Server MUST.

Feature:
tls-restart
Description:
Initiate or handle a stream restart after TLS negotiation.
Section:
Section 5.2.2 (Restart)
Roles:
Client MUST, Server MUST.

Feature:
tls-support
Description:
Support Transport Layer Security for stream encryption.
Section:
Section 5 (STARTTLS Negotiation)
Roles:
Client MUST, Server MUST.

Feature:
tls-whitespace
Description:
Ensure that no whitespace is sent between XML elements during TLS negotiation.
Section:
Section 5.2.3 (Data Formatting)
Roles:
Client MUST, Server MUST.

Feature:
xml-namespace-default
Description:
Ensure that there is a default namespace for the stream (other than the streams namespace).
Section:
Section 4.6.2 (Default Namespace)
Roles:
Client MUST, Server MUST.

Feature:
xml-namespace-default-client
Description:
Support 'jabber:client' as a default namespace.
Section:
Section 4.6.2 (Default Namespace)
Roles:
Client MUST, Server MUST.

Feature:
xml-namespace-default-server
Description:
Support 'jabber:server' as a default namespace.
Section:
Section 4.6.2 (Default Namespace)
Roles:
Client N/A, Server MUST.

Feature:
xml-namespace-streams-declaration
Description:
Ensure that there is a namespace declaration for the 'http://etherx.jabber.org/streams' namespace.
Section:
Section 4.6.1 (Streams Namespace)
Roles:
Client MUST, Server MUST.

Feature:
xml-namespace-streams-prefix
Description:
Ensure that all elements qualified by the 'http://etherx.jabber.org/streams' namespace are prefixed by the prefix defined in the namespace declaration.
Section:
Section 4.6.1 (Streams Namespace)
Roles:
Client MUST, Server MUST.

Feature:
xml-restriction-comment
Description:
Do not generate or accept XML comments.
Section:
Section 11.1 (Restrictions)
Roles:
Client MUST, Server MUST.

Feature:
xml-restriction-dtd
Description:
Do not generate or accept internal or external DTD subsets.
Section:
Section 11.1 (Restrictions)
Roles:
Client MUST, Server MUST.

Feature:
xml-restriction-pi
Description:
Do not generate or accept XML processing instructions.
Section:
Section 11.1 (Restrictions)
Roles:
Client MUST, Server MUST.

Feature:
xml-restriction-ref
Description:
Do not generate or accept internal or external entity references with the exception of the predefined entities.
Section:
Section 11.1 (Restrictions)
Roles:
Client MUST, Server MUST.

Feature:
xml-wellformed-xml
Description:
Do not generate or accept data that is not XML-well-formed.
Section:
Section 11.3 (Well-Formedness)
Roles:
Client MUST, Server MUST.

Feature:
xml-wellformed-ns
Description:
Do not generate or accept data that is not namespace-well-formed.
Section:
Section 11.3 (Well-Formedness)
Roles:
Client MUST, Server MUST.



 TOC 

16.  References



 TOC 

16.1. Normative References

[BASE64] Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” RFC 4648, October 2006 (TXT).
[CHARSETS] Alvestrand, H., “IETF Policy on Character Sets and Languages,” BCP 18, RFC 2277, January 1998 (TXT, HTML, XML).
[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” RFC 2782, February 2000 (TXT).
[KEYWORDS] Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[LANGTAGS] Phillips, A. and M. Davis, “Tags for Identifying Languages,” BCP 47, RFC 5646, September 2009 (TXT).
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, “X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP,” RFC 2560, June 1999 (TXT).
[PLAIN] Zeilenga, K., “The PLAIN Simple Authentication and Security Layer (SASL) Mechanism,” RFC 4616, August 2006 (TXT).
[RANDOM] Eastlake, D., Schiller, J., and S. Crocker, “Randomness Requirements for Security,” BCP 106, RFC 4086, June 2005 (TXT).
[SASL] Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” RFC 4422, June 2006 (TXT).
[SCRAM] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, “Salted Challenge Response (SCRAM) SASL and GSS-API Mechanism,” draft-ietf-sasl-scram-11 (work in progress), February 2010 (TXT).
[TCP] Postel, J., “Transmission Control Protocol,” STD 7, RFC 793, September 1981 (TXT).
[TLS] Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” RFC 5246, August 2008 (TXT).
[TLS-CERTS] Saint-Andre, P. and J. Hodges, “Representation and Verification of Application Server Identity in Certificates Used with Transport Layer Security (TLS),” draft-saintandre-tls-server-id-check-07 (work in progress), June 2010 (TXT).
[UCS2] International Organization for Standardization, “Information Technology - Universal Multiple-octet coded Character Set (UCS) - Amendment 2: UCS Transformation Format 8 (UTF-8),” ISO Standard 10646-1 Addendum 2, October 1996.
[UNICODE] The Unicode Consortium, “The Unicode Standard, Version 3.2.0,” 2000.

The Unicode Standard, Version 3.2.0 is defined by The Unicode Standard, Version 3.0 (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5), as amended by the Unicode Standard Annex #27: Unicode 3.1 (http://www.unicode.org/reports/tr27/) and by the Unicode Standard Annex #28: Unicode 3.2 (http://www.unicode.org/reports/tr28/).

[UTF-8] Yergeau, F., “UTF-8, a transformation format of ISO 10646,” STD 63, RFC 3629, November 2003 (TXT).
[UUID] Leach, P., Mealling, M., and R. Salz, “A Universally Unique IDentifier (UUID) URN Namespace,” RFC 4122, July 2005 (TXT, HTML, XML).
[URI] Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” STD 66, RFC 3986, January 2005 (TXT, HTML, XML).
[X509] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” RFC 5280, May 2008 (TXT).
[X509-ALGO] Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” RFC 3447, February 2003 (TXT).
[X509-SRV] Santesson, S., “Internet X.509 Public Key Infrastructure Subject Alternative Name for Expression of Service Name,” RFC 4985, August 2007 (TXT).
[XML] Maler, E., Yergeau, F., Sperberg-McQueen, C., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” World Wide Web Consortium Recommendation REC-xml-20081126, November 2008 (HTML).
[XML-GUIDE] Hollenbeck, S., Rose, M., and L. Masinter, “Guidelines for the Use of Extensible Markup Language (XML) within IETF Protocols,” BCP 70, RFC 3470, January 2003 (TXT, HTML, XML).
[XML-MEDIA] Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” RFC 3023, January 2001 (TXT).
[XML-NAMES] Thompson, H., Hollander, D., Layman, A., Bray, T., and R. Tobin, “Namespaces in XML 1.0 (Third Edition),” World Wide Web Consortium Recommendation REC-xml-names-20091208, December 2009 (HTML).
[XMPP-ADDR] Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Address Format,” draft-ietf-xmpp-address-01 (work in progress), July 2010 (TXT).


 TOC 

16.2. Informative References

[ACAP] Newman, C. and J. Myers, “ACAP -- Application Configuration Access Protocol,” RFC 2244, November 1997 (TXT).
[ANONYMOUS] Zeilenga, K., “Anonymous Simple Authentication and Security Layer (SASL) Mechanism,” RFC 4505, June 2006 (TXT).
[ASN.1] CCITT, “Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1),” 1988.
[CHANNEL] Williams, N., “On the Use of Channel Bindings to Secure Channels,” RFC 5056, November 2007 (TXT).
[CHANNEL-TLS] Altman, J., Williams, N., and L. Zhu, “Channel Bindings for TLS,” draft-altman-tls-channel-bindings-10 (work in progress), March 2010 (TXT).
[DIGEST-MD5] Leach, P. and C. Newman, “Using Digest Authentication as a SASL Mechanism,” RFC 2831, May 2000 (TXT).
[DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” RFC 4033, March 2005 (TXT).
[DNS-TXT] Rosenbaum, R., “Using the Domain Name System To Store Arbitrary String Attributes,” RFC 1464, May 1993 (TXT).
[DOS] Handley, M., Rescorla, E., and IAB, “Internet Denial-of-Service Considerations,” RFC 4732, December 2006 (TXT).
[EMAIL-ARCH] Crocker, D., “Internet Mail Architecture,” RFC 5598, July 2009 (TXT).
[ETHERNET] “Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 3: Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications",” IEEE Standard 802.3, September 1998.
[GSS-API] Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” RFC 2743, January 2000 (TXT).
[HASHES] Hoffman, P. and B. Schneier, “Attacks on Cryptographic Hashes in Internet Protocols,” RFC 4270, November 2005 (TXT).
[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” RFC 2616, June 1999 (TXT, PS, PDF, HTML, XML).
[IANA-GUIDE] Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” BCP 26, RFC 5226, May 2008 (TXT).
[IMAP] Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” RFC 3501, March 2003 (TXT).
[IMP-REQS] Day, M., Aggarwal, S., and J. Vincent, “Instant Messaging / Presence Protocol Requirements,” RFC 2779, February 2000 (TXT).
[INTEROP] Masinter, L., “Formalizing IETF Interoperability Reporting,” draft-ietf-newtrk-interop-reports-00 (work in progress), October 2005 (TXT).
[IRI] Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” RFC 3987, January 2005 (TXT).
[LINKLOCAL] Cheshire, S., Aboba, B., and E. Guttman, “Dynamic Configuration of IPv4 Link-Local Addresses,” RFC 3927, May 2005 (TXT).
[MAILBOXES] Crocker, D., “MAILBOX NAMES FOR COMMON SERVICES, ROLES AND FUNCTIONS,” RFC 2142, May 1997 (TXT, HTML, XML).
[POP3] Myers, J. and M. Rose, “Post Office Protocol - Version 3,” STD 53, RFC 1939, May 1996 (TXT).
[PROCESS] Bradner, S., “The Internet Standards Process -- Revision 3,” BCP 9, RFC 2026, October 1996 (TXT).
[REPORTS] Dusseault, L. and R. Sparks, “Guidance on Interoperation and Implementation Reports for Advancement to Draft Standard,” BCP 9, RFC 5657, September 2009 (TXT).
[REST] Fielding, R., “Architectural Styles and the Design of Network-based Software Architectures,”  2000.
[RFC3920] Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Core,” RFC 3920, October 2004 (TXT, HTML, XML).
[RFC3921] Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” RFC 3921, October 2004 (TXT, HTML, XML).
[SMTP] Klensin, J., “Simple Mail Transfer Protocol,” RFC 5321, October 2008 (TXT).
[SEC-GUIDE] Rescorla, E. and B. Korver, “Guidelines for Writing RFC Text on Security Considerations,” BCP 72, RFC 3552, July 2003 (TXT).
[SEC-TERMS] Shirey, R., “Internet Security Glossary, Version 2,” RFC 4949, August 2007 (TXT).
[STRONGSEC] Schiller, J., “Strong Security Requirements for Internet Engineering Task Force Standard Protocols,” BCP 61, RFC 3365, August 2002 (TXT).
[TLS-EXT] 3rd, D., “Transport Layer Security (TLS) Extensions: Extension Definitions,” draft-ietf-tls-rfc4366-bis-09 (work in progress), June 2010 (TXT).
[TLS-NEG] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, “Transport Layer Security (TLS) Renegotiation Indication Extension,” RFC 5746, February 2010 (TXT).
[URN-OID] Mealling, M., “A URN Namespace of Object Identifiers,” RFC 3061, February 2001 (TXT).
[USINGTLS] Newman, C., “Using TLS with IMAP, POP3 and ACAP,” RFC 2595, June 1999 (TXT).
[XEP-0001] Saint-Andre, P., “XMPP Extension Protocols,” XSF XEP 0001, January 2008.
[XEP-0016] Millard, P. and P. Saint-Andre, “Privacy Lists,” XSF XEP 0016, February 2007.
[XEP-0045] Saint-Andre, P., “Multi-User Chat,” XSF XEP 0045, July 2007.
[XEP-0060] Millard, P., Saint-Andre, P., and R. Meijer, “Publish-Subscribe,” XSF XEP 0060, September 2008.
[XEP-0071] Saint-Andre, P., “XHTML-IM,” XSF XEP 0071, September 2008.
[XEP-0077] Saint-Andre, P., “In-Band Registration,” XSF XEP 0077, January 2006.
[XEP-0086] Norris, R. and P. Saint-Andre, “Error Condition Mappings,” XSF XEP 0086, February 2004.
[XEP-0124] Paterson, I., Smith, D., and P. Saint-Andre, “Bidirectional-streams Over Synchronous HTTP (BOSH),” XSF XEP 0124, April 2009.
[XEP-0138] Hildebrand, J. and P. Saint-Andre, “Stream Compression,” XSF XEP 0138, May 2009.
[XEP-0156] Hildebrand, J. and P. Saint-Andre, “Discovering Alternative XMPP Connection Methods,” XSF XEP 0156, June 2007.
[XEP-0174] Saint-Andre, P., “Link-Local Messaging,” XSF XEP 0174, November 2008.
[XEP-0175] Saint-Andre, P., “Best Practices for Use of SASL ANONYMOUS,” XSF XEP 0175, November 2007.
[XEP-0178] Saint-Andre, P. and P. Millard, “Best Practices for Use of SASL EXTERNAL with Certificates,” XSF XEP 0178, February 2007.
[XEP-0191] Saint-Andre, P., “Simple Communications Blocking,” XSF XEP 0191, February 2007.
[XEP-0198] Karneges, J., Hildebrand, J., Saint-Andre, P., and F. Forno, “Stream Management,” XSF XEP 0198, June 2009.
[XEP-0199] Saint-Andre, P., “XMPP Ping,” XSF XEP 0199, June 2009.
[XEP-0205] Saint-Andre, P., “Best Practices to Discourage Denial of Service Attacks,” XSF XEP 0205, January 2009.
[XEP-0206] Paterson, I., “XMPP Over BOSH,” XSF XEP 0206, October 2008.
[XEP-0220] Miller, J., Saint-Andre, P., and P. Hancke, “Server Dialback,” XSF XEP 0220, March 2010.
[XML-FRAG] Grosso, P. and D. Veillard, “XML Fragment Interchange,” World Wide Web Consortium CR CR-xml-fragment-20010212, February 2001 (HTML).
[XML-REG] Mealling, M., “The IETF XML Registry,” BCP 81, RFC 3688, January 2004 (TXT).
[XML-SCHEMA] Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech, “XML Schema Part 1: Structures Second Edition,” World Wide Web Consortium Recommendation REC-xmlschema-1-20041028, October 2004 (HTML).
[XMPP-IM] Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” draft-ietf-xmpp-3921bis-07 (work in progress), July 2010 (TXT).
[XMPP-URI] Saint-Andre, P., “Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP),” RFC 5122, February 2008 (TXT).


 TOC 

Appendix A.  XML Schemas

Because validation of XML streams and stanzas is optional, the following XML schemas are provided for descriptive purposes only. These schemas are not normative.

The following schemas formally define various XML namespaces used in the core XMPP protocols, in conformance with [XML‑SCHEMA] (Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech, “XML Schema Part 1: Structures Second Edition,” October 2004.). For schemas defining the 'jabber:client' and 'jabber:server' namespaces, refer to [XMPP‑IM] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence,” July 2010.).



 TOC 

A.1.  Streams Namespace

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

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

  <xs:import namespace='jabber:client'/>
  <xs:import namespace='jabber:server'/>
  <xs:import namespace='urn:ietf:params:xml:ns:xmpp-sasl'/>
  <xs:import namespace='urn:ietf:params:xml:ns:xmpp-streams'/>
  <xs:import namespace='urn:ietf:params:xml:ns:xmpp-tls'/>

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

  <xs:element name='features'>
    <xs:complexType>
      <xs:any namespace='##other'
              minOccurs='0'
              maxOccurs='unbounded'/>
    </xs:complexType>
  </xs:element>

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

</xs:schema>


 TOC 

A.2.  Stream Error Namespace

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

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

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

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

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

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

</xs:schema>


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A.3.  STARTTLS Namespace

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

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

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

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

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

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

</xs:schema>


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A.4.  SASL Namespace

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

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

  <xs:element name='mechanisms'>
    <xs:complexType>
      <xs:sequence>
        <xs:element name='mechanism'
                    minOccurs='1'
                    maxOccurs='unbounded'
                    type='xs:NMTOKEN'/>
        <xs:any namespace='##other'
                minOccurs='0'
                minOccurs='unbounded'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:element name='abort' type='empty'/>

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

  <xs:element name='challenge' type='xs:string'/>

  <xs:element name='response' type='xs:string'/>

  <xs:element name='success' type='xs:string'/>

  <xs:element name='failure'>
    <xs:complexType>
      <xs:sequence>
        <xs:choice minOccurs='0'>
          <xs:element name='aborted' type='empty'/>
          <xs:element name='account-disabled' type='empty'/>
          <xs:element name='credentials-expired' type='empty'/>
          <xs:element name='encryption-required' type='empty'/>
          <xs:element name='incorrect-encoding' type='empty'/>
          <xs:element name='invalid-authzid' type='empty'/>
          <xs:element name='invalid-mechanism' type='empty'/>
          <xs:element name='malformed-request' type='empty'/>
          <xs:element name='mechanism-too-weak' type='empty'/>
          <xs:element name='not-authorized' type='empty'/>
          <xs:element name='temporary-auth-failure' type='empty'/>
          <xs:element name='transition-needed' type='empty'/>
        </xs:choice>
        <xs:element ref='text' minOccurs='0' maxOccurs='1'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

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

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

</xs:schema>


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A.5.  Resource Binding Namespace

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

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

  <xs:element name='bind'>
    <xs:complexType>
      <xs:choice>
        <xs:element name='resource' type='resourceType'/>
        <xs:element name='jid' type='fullJIDType'/>
      </xs:choice>
    </xs:complexType>
  </xs:element>

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

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

</xs:schema>


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A.6.  Stanza Error Namespace

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

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

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

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

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

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

</xs:schema>


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Appendix B.  Contact Addresses

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



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Appendix C.  Account Provisioning

Account provisioning is out of scope for this specification. Possible methods for account provisioning include account creation by a server administrator and in-band account registration using the 'jabber:iq:register' namespace as documented in [XEP‑0077] (Saint-Andre, P., “In-Band Registration,” January 2006.). An XMPP server implementation or administrative function MUST ensure that any JID assigned during account provisioning (including localpart, domainpart, resourcepart, and separator characters) conforms to the canonical format for XMPP addresses defined in [XMPP‑ADDR] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Address Format,” July 2010.).



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Appendix D.  Differences from RFC 3920

Based on consensus derived from implementation and deployment experience as well as formal interoperability testing, the following substantive modifications were made from RFC 3920.

In addition, numerous changes of an editorial nature were made in order to more fully specify and clearly explain XMPP.



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Appendix E.  Copying Conditions

Regarding this entire document or any portion of it, the author makes no guarantees and is not responsible for any damage resulting from its use. The author grants irrevocable permission to anyone to use, modify, and distribute it in any way that does not diminish the rights of anyone else to use, modify, and distribute it, provided that redistributed derivative works do not contain misleading author or version information. Derivative works need not be licensed under similar terms.



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Author's Address

  Peter Saint-Andre
  Cisco Systems, Inc.
  1899 Wyknoop Street, Suite 600
  Denver, CO 80202
  USA
Phone:  +1-303-308-3282
Email:  psaintan@cisco.com