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Versions: (RFC 3920) 00 01 02 03 04 05 06 07 08 09

XMPP Working Group                                   P. Saint-Andre, Ed.
Internet-Draft                                                       JSF
Intended status: Informational                          October 13, 2006
Expires: April 16, 2007


        Extensible Messaging and Presence Protocol (XMPP): Core
                     draft-saintandre-rfc3920bis-00

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   This Internet-Draft will expire on April 16, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This memo defines the core features of the Extensible Messaging and
   Presence Protocol (XMPP), a technology for streaming Extensible
   Markup Language (XML) elements in order to exchange structured
   information in close to real time between any two network-aware
   entities.  XMPP provides a generalized, extensible framework for
   incrementally exchanging XML data, upon which a variety of
   applications can be built.  The framework includes methods for stream
   setup and teardown, channel encryption, authentication of a client to



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   a server and of one server to another server, and primitives for
   push-style messages, publication of presence and availability
   information, and request-response interactions between any two XMPP
   entities.  This document also specifies the format for XMPP
   addresses, which are fully internationalizable.

   This document obsoletes RFC 3920.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Addresses . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  XML Streams . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  TLS Negotiation . . . . . . . . . . . . . . . . . . . . . . .  22
   6.  SASL Negotiation  . . . . . . . . . . . . . . . . . . . . . .  26
   7.  Resource Binding  . . . . . . . . . . . . . . . . . . . . . .  31
   8.  XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . .  36
   9.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  46
   10. Server Rules for Handling XML Stanzas . . . . . . . . . . . .  55
   11. XML Usage . . . . . . . . . . . . . . . . . . . . . . . . . .  58
   12. Compliance Requirements . . . . . . . . . . . . . . . . . . .  60
   13. Internationalization Considerations . . . . . . . . . . . . .  62
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  62
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  67
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  69
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  69
     16.2.  Informative References . . . . . . . . . . . . . . . . .  71
   Appendix A.  Nodeprep . . . . . . . . . . . . . . . . . . . . . .  73
   Appendix B.  Resourceprep . . . . . . . . . . . . . . . . . . . .  75
   Appendix C.  Server Dialback  . . . . . . . . . . . . . . . . . .  77
   Appendix D.  XML Schemas  . . . . . . . . . . . . . . . . . . . .  86
   Appendix E.  Contact Addresses  . . . . . . . . . . . . . . . . .  98
   Appendix F.  Differences From RFC 3920  . . . . . . . . . . . . .  98
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  99
   Intellectual Property and Copyright Statements  . . . . . . . . . 100














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1.  Introduction

1.1.  Overview

   The Extensible Messaging and Presence Protocol (XMPP) is an
   Extensible Markup Language XML [XML] technology for near-real-time
   messaging, presence, and request-response services.  The basic syntax
   and semantics were developed originally within the Jabber open-source
   community, mainly in 1999.  In 2002, the XMPP WG was chartered with
   developing an adaptation of the core Jabber protocol that would be
   suitable as an IETF instant messaging (IM) and presence technology.
   As a result of work by the XMPP WG as well as implementation
   experience and interoperability testing completed since the
   publication of RFC 3920, this document defines the core features of
   XMPP 1.0; the extensions required to provide the instant messaging
   and presence functionality defined in [IMP-REQS] are specified in
   [XMPP-IM].

   This document obsoletes RFC 3920.

1.2.  Functional Summary

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

   1.  Open an XML stream
   2.  Complete TLS negotiation for channel encryption (RECOMMENDED)
   3.  Complete SASL negotiation for authentication
   4.  Bind a resource to the stream
   5.  Exchange XML stanzas with other entities on the network
   6.  Close the stream when further communications are not needed or
       desired

   This document specifies how clients connect to servers and specifies
   the basic semantics of XML stanzas, but does not define the
   "payloads" of the XML stanzas that might be exchanged once a
   connection is successfully established; instead, definition of such
   semantics is provided by various XMPP extensions (e.g., [XMPP-IM] for
   basic instant messaging and presence applications).

   Furthermore, in the client-server architecture used by XMPP, one
   server may optionally connect to another server to enable inter-



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   domain or inter-server communication.  For this to happen, the two
   servers must negotiate a connection between themselves and then
   exchange XML stanzas; the process for doing so is as follows:

   1.  Open an XML stream
   2.  Complete TLS negotiation for channel encryption (RECOMMENDED)
   3.  Complete SASL negotiation for authentication
   4.  Exchange XML stanzas both directly for the servers and indirectly
       on behalf of entities associated with each server (e.g.,
       connected clients)
   5.  Close the stream when further communications are not needed or
       desired

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

1.3.  Conventions

   The following keywords are to be interpreted as described in [TERMS]:
   "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD",
   "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

   In examples, lines have been wrapped for improved readability.


2.  Architecture

2.1.  Overview

   XMPP assumes a client-server architecture, wherein a client utilizing
   XMPP accesses a server (normally over a [TCP] connection) and servers
   can also communicate with each other over TCP connections.
   Architectures that use the syntax of XML stanzas (Section 8) but that
   establish peer-to-peer connections directly between clients using
   technologies based on [LINKLOCAL] have been deployed, but such
   architectures are not XMPP and are best described as "XMPP-like"; for
   details, see [XEP-0174].

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

   o  romeo@example.net -- an XMPP user.





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   o  example.net -- an XMPP server.
   o  juliet@example.com -- an XMPP user.
   o  example.com -- an XMPP server.

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


2.2.  Server

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

   o  to manage connections from or sessions for other entities, in the
      form of XML streams (Section 4) to and from authorized clients,
      servers, and other entities
   o  to route appropriately-addressed XML stanzas (Section 8) among
      such entities over XML streams

   Most XMPP-compliant servers also assume responsibility for the
   storage of data that is used by clients (e.g., contact lists for
   users of XMPP-based instant messaging and presence applications); in
   this case, the XML data is processed directly by the server itself on
   behalf of the client and is not routed to another entity.

2.3.  Client

   Most clients connect directly to a server over a [TCP] connection and
   use XMPP to take full advantage of the functionality provided by a
   server and any associated services.  Multiple resources (e.g.,
   devices or locations) MAY connect simultaneously to a server on
   behalf of each authorized client, with each resource differentiated
   by the resource identifier of an XMPP address (e.g.,
   <node@domain/home> vs. <node@domain/work>) as defined under Addresses
   (Section 3) and Resource Binding (Section 7).  The RECOMMENDED port
   for connections between a client and a server is 5222, as registered
   with the IANA (see Port Numbers (Section 15.9)).

2.4.  Network

   Because each server is identified by a network address and because
   server-to-server communications are a straightforward extension of
   the client-to-server protocol, in practice, the system consists of a
   network of servers that inter-communicate.  Thus, for example,
   <juliet@example.com> is able to exchange messages, presence, and
   other information with <romeo@example.net>.  This pattern is familiar



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   from messaging protocols (such as [SMTP]) that make use of network
   addressing standards.  Communications between any two servers are
   OPTIONAL.  If enabled, such communications SHOULD occur over XML
   streams that are bound to [TCP] connections.  The RECOMMENDED port
   for connections between servers is 5269, as registered with the IANA
   (see Port Numbers (Section 15.9)).


3.  Addresses

3.1.  Overview

   An entity is anything that can be considered a network endpoint
   (i.e., an ID on the network) and that can communicate using XMPP.
   All such entities are uniquely addressable on the network.  For
   historical reasons, the address of an XMPP entity is called a Jabber
   Identifier or JID.  A valid JID contains a set of ordered elements
   formed of a domain identifier, node identifier, and resource
   identifier.

   The syntax for a JID is defined below using the Augmented Backus-Naur
   Form as defined in [ABNF].

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

   All JIDs are based on the foregoing structure.  One common use of
   this structure is to identify a messaging and presence account, the
   server that hosts the account, and a connected resource (e.g., a
   specific device) in the form of <node@domain/resource>.  However,
   node types other than clients are possible; for example, a specific
   chat room offered by a multi-user chat service (see [XEP-0045]) could
   be addressed as <room@service> (where "room" is the name of the chat
   room and "service" is the hostname of the multi-user chat service)



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   and a specific occupant of such a room could be addressed as
   <room@service/nick> (where "nick" is the occupant's room nickname).
   Many other JID types are possible (e.g., <domain/resource> could be a
   server-side script or service).

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

   Note: While the format of a JID is consistent with [URI], an entity's
   address on an XMPP network MUST be a JID (without a URI scheme) and
   not a [URI] or [IRI] as specified in [XMPP-URI]; the latter
   specification is provided only for use by non-XMPP applications.

3.2.  Domain Identifier

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

   The domain identifier for every server or service that will
   communicate over a network MAY be an IP address but SHOULD be a fully
   qualified domain name (see [DNS]).  A domain identifier MUST be an
   "internationalized domain name" as defined in [IDNA], to which the
   [NAMEPREP] profile of [STRINGPREP] can be applied without failing.
   Before comparing two domain identifiers, a server MUST (and a client
   SHOULD) first apply the Nameprep profile to the labels (as defined in
   [IDNA]) that make up each identifier.  Note: Both the
   UseSTD3ASCIIRules and AllowUnassigned flags MUST be set to true.

3.3.  Node Identifier

   The node identifier is an optional secondary identifier placed before
   the domain identifier and separated from the latter by the '@'
   character.  It usually represents the entity requesting and using
   network access provided by a server (i.e., a client), although it can
   also represent other kinds of entities (e.g., a chat room associated
   with a multi-user chat service).  The entity represented by a node
   identifier is addressed within the context of a specific domain;
   within instant messaging and presence applications of XMPP, this
   address is called a "bare JID" and is of the form <node@domain>.




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   A node identifier MUST be formatted such that the Nodeprep profile of
   [STRINGPREP] can be applied without failing (see Appendix A).  Before
   comparing two node identifiers, a server MUST (and a client SHOULD)
   first apply the Nodeprep profile to each identifier.

3.4.  Resource Identifier

   The resource identifier is an optional tertiary identifier placed
   after the domain identifier and separated from the latter by the '/'
   character.  A resource identifier may modify either a <node@domain>
   address or a mere <domain> address.  It usually represents a specific
   session, connection (e.g., a device or location), or object (e.g., a
   participant in a multi-user chat room) belonging to the entity
   associated with a node identifier.  A resource identifier is opaque
   to both servers and other clients, and is typically defined by a
   client implementation when it provides the information necessary to
   complete Resource Binding (Section 7) (although it may be generated
   by a server on behalf of a client), after which the entity is
   referred to as a "connected resource" and its address is referrred to
   as a "full JID" (<node@domain/resource>).  An entity MAY maintain
   multiple connected resources simultaneously, with each connected
   resource differentiated by a distinct resource identifier.

   A resource identifier MUST be formatted such that the Resourceprep
   profile of [STRINGPREP] can be applied without failing (see
   Appendix B).  Before comparing two resource identifiers, a server
   MUST (and a client SHOULD) first apply the Resourceprep profile to
   each identifier.

3.5.  Determination of Addresses

   After SASL negotiation (Section 6) and, if appropriate, Resource
   Binding (Section 7), the receiving entity for a stream MUST determine
   the initiating entity's JID.

   For server-to-server communications, the initiating entity's JID
   SHOULD be the authorization identity, derived from the authentication
   identity, as defined by [SASL], if no authorization identity was
   specified during SASL negotiation (Section 6).

   For client-to-server communications, the "bare JID" (<node@domain>)
   SHOULD be the authorization identity, derived from the authentication
   identity, as defined in [SASL], if no authorization identity was
   specified during SASL negotiation (Section 6); the resource
   identifier portion of the "full JID" (<node@domain/resource>) SHOULD
   be the resource identifier negotiated by the client and server during
   Resource Binding (Section 7).




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


4.  XML Streams

4.1.  Overview

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

   Definition of XML Stream:  An XML stream is a container for the
      exchange of XML elements between any two entities over a network.
      The start of an XML stream is denoted unambiguously by an opening
      XML <stream> tag (with appropriate attributes and namespace
      declarations), while the end of the XML stream is denoted
      unambiguously by a closing XML </stream> tag.  During the life of
      the stream, the entity that initiated it can send an unbounded
      number of XML elements over the stream, either elements used to
      negotiate the stream (e.g., to complete TLS negotiation
      (Section 5) or SASL negotiation (Section 6)) or XML stanzas.  The
      "initial stream" is negotiated from the initiating entity (usually
      a client or server) to the receiving entity (usually a server),
      and can be seen as corresponding to the initiating entity's
      "session" with the receiving entity.  The initial stream enables
      unidirectional communication from the initiating entity to the
      receiving entity; in order to enable information exchange from the
      receiving entity to the initiating entity, the receiving entity
      MUST negotiate a stream in the opposite direction (the "response
      stream").
   Definition of XML Stanza:  An XML stanza is a discrete semantic unit
      of structured information that is sent from one entity to another
      over an XML stream, and is the basic unit of meaning in XMPP.  An
      XML stanza exists at the direct child level of the root <stream/>
      element and is said to be well-balanced if it matches the
      production [43] content of [XML].  The start of any XML stanza is
      denoted unambiguously by the element start tag at depth=1 of the
      XML stream (e.g., <presence>), and the end of any XML stanza is
      denoted unambiguously by the corresponding close tag at depth=1
      (e.g., </presence>); a server MUST NOT process, deliver, or route
      a partial stanza and MUST NOT attach meaning to the transmission
      timing of any part of a stanza (before receipt of the close tag).



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      The only XML stanzas defined herein are the <message/>,
      <presence/>, and <iq/> elements qualified by the default namespace
      for the stream, as described under XML Stanzas (Section 8); an XML
      element sent for the purpose of TLS negotiation (Section 5), SASL
      negotiation (Section 6), or server dialback (Appendix C) is not
      considered to be an XML stanza.  An XML stanza MAY contain child
      elements (with accompanying attributes, elements, and XML
      character data) as necessary in order to convey the desired
      information, which MAY be qualified by any XML namespace (see
      [XML-NAMES]).

   Consider the example of a client's session with a server.  In order
   to connect to a server, a client MUST initiate an XML stream by
   sending an opening <stream> tag to the server, optionally preceded by
   a text declaration specifying the XML version and the character
   encoding supported (see Inclusion of Text Declaration (Section 11.4)
   and Character Encoding (Section 11.5)).  Subject to local policies
   and service provisioning, the server SHOULD then reply with a second
   XML stream back to the client, again optionally preceded by a text
   declaration.  Once the client has completed SASL negotiation
   (Section 6), the client MAY send an unbounded number of XML stanzas
   over the stream to any recipient on the network.  When the client
   desires to close the stream, it simply sends a closing </stream> tag
   to the server; for details, see Section 4.7.

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

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













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

4.2.  Binding to TCP

   Although there is no necessary coupling of an XML stream to a [TCP]
   connection (e.g., two entities could connect to each other via
   another mechanism such as [HTTP], as specified in [XEP-0124]), this
   specification defines a binding of XMPP to TCP only.  In the context
   of client-to-server communications, a server MUST allow a client to
   share a single TCP connection for XML stanzas sent from client to
   server and from server to client (i.e., the inital stream and
   response stream).  In the context of server-to-server communications,
   a server MUST use one TCP connection for XML stanzas sent from the
   server to the peer and another TCP connection (initiated by the peer)
   for stanzas from the peer to the server, for a total of two TCP
   connections.

4.3.  Stream Security

   When negotiating XML streams in XMPP 1.0, TLS SHOULD be used as
   defined under TLS negotiation (Section 5) and SASL MUST be used as
   defined under SASL negotiation (Section 6).  The initial stream and
   the response stream MUST be secured separately, although security in
   both directions MAY be established via mechanisms that provide mutual
   authentication.  An entity SHOULD NOT attempt to send XML Stanzas
   (Section 8) over the stream before the stream has been authenticated,
   but if it does, then the other entity MUST NOT accept such stanzas
   and SHOULD return a <not-authorized/> stream error and then terminate
   both the XML stream and the underlying TCP connection; note well that
   this applies to XML stanzas only (i.e., <message/>, <presence/>, and



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   <iq/> elements qualified by the default namespace) and not to XML
   elements used for stream negotiation (e.g., elements used to complete
   TLS negotiation (Section 5) or SASL negotiation (Section 6)).

4.4.  Stream Attributes

   The attributes of the stream element are as follows:

   o  from -- In client-to-server communications, the 'from' attribute
      SHOULD be included in the XML stream header sent from the
      initiating entity to the receiving entity and (if included) MUST
      be set to the account name (i.e., "bare JID" = <node@domain>) of
      the entity controlling the client.  In server-to-server
      communications, the 'from' attribute SHOULD be included in the XML
      stream header sent from the initiating entity to the receiving
      entity and (if included) MUST be set to a hostname serviced by the
      initiating entity.  In both client-to-server and server-to-server
      communications, the 'from' attribute MUST be included in the XML
      stream header by which the receiving entity responds to the
      initiating entity and MUST be set to a hostname serviced by the
      receiving entity that is granting access to the initiating entity.
      Note: Each entity MUST verify the identity of the other entity
      before exchanging XML stanzas with it (see the Client-to-Server
      Communications (Section 14.3) and Server-to-Server Communications
      (Section 14.4) sections of this document for details).
   o  to -- In both client-to-server and server-to-server
      communications, the 'to' attribute SHOULD be included in the XML
      stream header sent from the initiating entity to the receiving
      entity and (if included) MUST be set to a hostname serviced by the
      receiving entity.  In client-to-server communications, if the
      client included a 'from' address in the initial stream header then
      the server SHOULD include a 'to' attribute in the XML stream
      header by which it replies to the client and (if included) MUST
      set the 'to' attribute to the bare JID specified in the 'from'
      attribute of the XML stream header sent from the initiating entity
      to the receiving entity.  In server-to-server communications, if
      the initiating entity included a 'from' address in the initial
      stream header then the receiving entity SHOULD include a 'to'
      attribute in the XML stream header by which it replies to the
      initiating entity and (if included) MUST set the 'to' attribute to
      the hostname specified in the 'from' attribute of the XML stream
      header sent from the initiating entity to the receiving entity.
      Note: Each entity MUST verify the identity of the other entity
      before exchanging XML stanzas with it (see the Client-to-Server
      Communications (Section 14.3) and Server-to-Server Communications
      (Section 14.4) sections of this document for details).





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   o  id -- The 'id' attribute SHOULD be used only in the XML stream
      header from the receiving entity to the initiating entity.  This
      attribute is a unique identifier created by the receiving entity
      to function as a session key for the initiating entity's streams
      with the receiving entity, and MUST be unique within the receiving
      application (normally a server).  Note well that the stream ID may
      be security-critical and therefore MUST be both unpredictable and
      nonrepeating (see [RANDOM] for recommendations regarding
      randomness for security purposes).  There SHOULD NOT be an 'id'
      attribute on the XML stream header sent from the initiating entity
      to the receiving entity; however, if an 'id' attribute is
      included, it SHOULD be silently ignored by the receiving entity.
   o  xml:lang -- An 'xml:lang' attribute (as defined in Section 2.12 of
      [XML]) SHOULD be included by the initiating entity on the header
      for the initial stream to specify the default language of any
      human-readable XML character data it sends over that stream.  If
      the attribute is included, the receiving entity SHOULD remember
      that value as the default for both the initial stream and the
      response stream; if the attribute is not included, the receiving
      entity SHOULD use a configurable default value for both streams,
      which it MUST communicate in the header for the response stream.
      For all stanzas sent over the initial stream, if the initiating
      entity does not include an 'xml:lang' attribute, the receiving
      entity SHOULD apply the default value; if the initiating entity
      does include an 'xml:lang' attribute, the receiving entity MUST
      NOT modify or delete it (see also xml:lang (Section 8.1.5)).  The
      value of the 'xml:lang' attribute MUST be an NMTOKEN (as defined
      in Section 2.3 of [XML]) and MUST conform to the format defined in
      [LANGTAGS].
   o  version -- The presence of the version attribute set to a value of
      at least "1.0" signals support for the stream-related protocols
      (including stream features) defined in this specification.
      Detailed rules regarding the generation and handling of this
      attribute are defined below.

   We can summarize as follows:

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

   Note: The attributes of the root <stream/> element are not prepended



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   by a 'stream:' prefix because, in accordance with Section 5.3 of XML
   namespaces specification [XML-NAMES], the default namespace does not
   apply to attribute names.

4.4.1.  Version Support

   The version of XMPP specified herein is "1.0"; in particular, this
   encapsulates the stream-related protocols (TLS negotiation
   (Section 5), SASL negotiation (Section 6), and Stream Errors
   (Section 4.9)), as well as the semantics of the three defined XML
   stanza types (<message/>, <presence/>, and <iq/>).  The numbering
   scheme for XMPP versions is "<major>.<minor>".  The major and minor
   numbers MUST be treated as separate integers and each number MAY be
   incremented higher than a single digit.  Thus, "XMPP 2.4" would be a
   lower version than "XMPP 2.13", which in turn would be lower than
   "XMPP 12.3".  Leading zeros (e.g., "XMPP 6.01") MUST be ignored by
   recipients and MUST NOT be sent.

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

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

   1.  The initiating entity MUST set the value of the 'version'
       attribute on the initial stream header to the highest version
       number it supports (e.g., if the highest version number it
       supports is that defined in this specification, it MUST set the
       value to "1.0").
   2.  The receiving entity MUST set the value of the 'version'
       attribute on the response stream header to either the value
       supplied by the initiating entity or the highest version number
       supported by the receiving entity, whichever is lower.  The
       receiving entity MUST perform a numeric comparison on the major
       and minor version numbers, not a string match on
       "<major>.<minor>".



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   3.  If the version number included in the response stream header is
       at least one major version lower than the version number included
       in the initial stream header and newer version entities cannot
       interoperate with older version entities as described above, the
       initiating entity SHOULD generate an <unsupported-version/>
       stream error and terminate the XML stream and underlying TCP
       connection.
   4.  If either entity receives a stream header with no 'version'
       attribute, the entity MUST consider the version supported by the
       other entity to be "0.9" and SHOULD NOT include a 'version'
       attribute in the stream header it sends in reply.

4.5.  Namespace Declarations

   The stream element MUST possess both a streams namespace declaration
   and a default namespace declaration (as "namespace declaration" is
   defined in the [XML-NAMES]).  For detailed information regarding the
   streams namespace and default namespace, see Namespace Names and
   Prefixes (Section 11.2).

4.6.  Stream Features

   If the initiating entity includes the 'version' attribute set to a
   value of at least "1.0" in the initial stream header, the receiving
   entity MUST send a <features/> child element (prefixed by the streams
   namespace prefix) to the initiating entity in order to announce any
   stream-level features that can be negotiated (or capabilities that
   otherwise need to be advertised).  Currently, this is used only to
   advertise TLS negotiation (Section 5), SASL negotiation (Section 6),
   resource binding (Section 7), and server dialback (Appendix C) as
   defined herein; however, the stream features functionality can be
   used to advertise other negotiable features as well.  If an entity
   does not understand or support some features, it SHOULD silently
   ignore them.  If one or more security features (e.g., TLS and SASL)
   need to be successfully negotiated before a non-security-related
   feature (e.g., Resource Binding) can be offered, the non-security-
   related feature SHOULD NOT be included in the stream features that
   are advertised before the relevant security features have been
   negotiated.  If a feature must be negotiated before the initiating
   entity may proceed, that feature SHOULD include a <required/> child
   element.

4.7.  Closing Streams

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



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   </stream:stream>

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

   </stream:stream>

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

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

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

4.8.  Reconnection

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

   o  The time to live (TTL) specified in Domain Name System records
      SHOULD be honored, even if DNS results are cached; if the TTL has
      not expired, an entity that seeks to reconnect SHOULD NOT re-
      resolve DNS before reconnecting.
   o  The time that expires before an entity first seeks to reconnect
      SHOULD be randomized (e.g., so that all clients do not attempt to
      reconnect 30 seconds after being disconnected).
   o  If the first reconnection attempt does not succeed, an entity
      SHOULD back off exponentially on the time between subsequent
      reconnection attempts.

4.9.  Stream Errors

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





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4.9.1.  Rules

   The following rules apply to stream-level errors:

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

4.9.2.  Syntax

   The syntax for stream errors is as follows:

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

   The <error/> element:

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

   The <text/> element is OPTIONAL.  If included, it SHOULD be used only
   to provide descriptive or diagnostic information that supplements the



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   meaning of a defined condition or application-specific condition.  It
   SHOULD NOT be interpreted programmatically by an application.  It
   SHOULD NOT be used as the error message presented to a user, but MAY
   be shown in addition to the error message associated with the
   included condition element (or elements).

4.9.3.  Defined Conditions

   The following stream-level error conditions are defined:

   o  <bad-format/> -- the entity has sent XML that cannot be processed;
      this error MAY be used instead of the more specific XML-related
      errors, such as <bad-namespace-prefix/>, <invalid-xml/>,
      <restricted-xml/>, <unsupported-encoding/>, and <xml-not-well-
      formed/>, although the more specific errors are preferred.
   o  <bad-namespace-prefix/> -- the entity has sent a namespace prefix
      that is unsupported, or has sent no namespace prefix on an element
      that requires such a prefix (see XML Namespace Names and Prefixes
      (Section 11.2)).
   o  <conflict/> -- the server is closing the active stream for this
      entity because a new stream has been initiated that conflicts with
      the existing stream.
   o  <connection-timeout/> -- the entity has not generated any traffic
      over the stream for some period of time (configurable according to
      a local service policy).
   o  <host-gone/> -- the value of the 'to' attribute provided by the
      initiating entity in the stream header corresponds to a hostname
      that is no longer hosted by the server.
   o  <host-unknown/> -- the value of the 'to' attribute provided by the
      initiating entity in the stream header does not correspond to a
      hostname that is hosted by the server.
   o  <improper-addressing/> -- a stanza sent between two servers lacks
      a 'to' or 'from' attribute (or the attribute has no value).
   o  <internal-server-error/> -- the server has experienced a
      misconfiguration or an otherwise-undefined internal error that
      prevents it from servicing the stream.
   o  <invalid-from/> -- the JID or hostname provided in a 'from'
      address does not match an authorized JID or validated domain
      negotiated between servers via SASL or dialback, or between a
      client and a server via authentication and resource binding.
   o  <invalid-id/> -- the stream ID or dialback ID is invalid or does
      not match an ID previously provided.
   o  <invalid-namespace/> -- the streams namespace name is something
      other than "http://etherx.jabber.org/streams" or the dialback
      namespace name is something other than "jabber:server:dialback"
      (see XML Namespace Names and Prefixes (Section 11.2)).





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   o  <invalid-xml/> -- the entity has sent invalid XML over the stream
      to a server that performs validation (see Validation
      (Section 11.3)).
   o  <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.
   o  <policy-violation/> -- the entity has violated some local service
      policy (e.g., the entity is on a provisioned blacklist); the
      server MAY choose to specify the policy in the <text/> element or
      an application-specific condition element.
   o  <remote-connection-failed/> -- the server is unable to properly
      connect to a remote entity that is required for authentication or
      authorization.
   o  <resource-constraint/> -- the server lacks the system resources
      necessary to service the stream.
   o  <restricted-xml/> -- the entity has attempted to send restricted
      XML features such as a comment, processing instruction, DTD,
      entity reference, or unescaped character (see Restrictions
      (Section 11.1)).
   o  <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 domain identifier but
      may also include a port number; if no port is specified, the
      initiating entity SHOULD perform a [DNS-SRV] lookup on the
      provided domain identifier but MAY assume that it can connect to
      that domain identifier at the standard XMPP ports (5222 for
      client-to-server connections and 5269 for server-to-server
      connections).
   o  <system-shutdown/> -- the server is being shut down and all active
      streams are being closed.
   o  <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.
   o  <unsupported-encoding/> -- the initiating entity has encoded the
      stream in an encoding that is not supported by the server (see
      Character Encoding (Section 11.5)).
   o  <unsupported-stanza-type/> -- the initiating entity has sent a
      first-level child of the stream that is not supported by the
      server.
   o  <unsupported-version/> -- the value of the 'version' attribute
      provided by the initiating entity in the stream header specifies a
      version of XMPP that is not supported by the server; the server
      MAY specify the version(s) it supports in the <text/> element.



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   o  <xml-not-well-formed/> -- the initiating entity has sent XML that
      is not well-formed as defined by [XML].

4.9.4.  Application-Specific Conditions

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

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

4.10.  Simplified Stream Examples

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























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   A basic "session":

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



















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   A "session" gone bad:

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

   More detailed examples are provided under Section 9.


5.  TLS Negotiation

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) protocol [TLS], along with a
   "STARTTLS" extension that is modelled after similar extensions for
   the [IMAP], [POP3], and [ACAP] protocols as described in [USINGTLS].
   The namespace name for the STARTTLS extension is
   'urn:ietf:params:xml:ns:xmpp-tls'.

   An administrator of a given domain MAY require the use of TLS for
   client-to-server communications, server-to-server communications, or
   both.  Clients SHOULD use TLS to secure the streams prior to
   attempting the completion of SASL negotiation (Section 6), and



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   servers SHOULD use TLS between two domains for the purpose of
   securing server-to-server communications.

   The following rules apply:

   1.   An initiating entity that complies with this specification MUST
        include the 'version' attribute set to a value of "1.0" in the
        initial stream header.
   2.   If the TLS negotiation occurs between two servers,
        communications MUST NOT proceed until the Domain Name System
        (DNS) hostnames asserted by the servers have been resolved (see
        Server-to-Server Communications (Section 14.4)).
   3.   When a receiving entity that complies with this specification
        receives an initial stream header that includes the 'version'
        attribute set to a value of at least "1.0", after sending a
        stream header in reply (including the version flag), it MUST
        include a <starttls/> element (qualified by the
        'urn:ietf:params:xml:ns:xmpp-tls' namespace) along with the list
        of other stream features it supports.
   4.   If the initiating entity chooses to use TLS, TLS negotiation
        MUST be completed before proceeding to SASL negotiation; this
        order of negotiation is required to help safeguard
        authentication information sent during SASL negotiation, as well
        as to make it possible to base the use of the SASL EXTERNAL
        mechanism on a certificate provided during prior TLS
        negotiation.
   5.   During TLS negotiation, an entity MUST NOT send any white space
        characters (matching production [3] content of [XML]) within the
        root stream element as separators between elements (any white
        space characters shown in the TLS examples below are included
        for the sake of readability only); this prohibition helps to
        ensure proper security layer byte precision.
   6.   The receiving entity MUST consider the TLS negotiation to have
        begun immediately after sending the closing ">" character of the
        <proceed/> element to the initiating entity.  The initiating
        entity MUST consider the TLS negotiation to have begun
        immediately after receiving the closing ">" character of the
        <proceed/> element from the receiving entity.
   7.   The initiating entity MUST validate the certificate presented by
        the receiving entity; see Certificate Validation (Section 14.2)
        regarding certificate validation procedures.
   8.   Certificates MUST be checked against the hostname as provided by
        the initiating entity (e.g., a user), not the hostname as
        resolved via the Domain Name System; e.g., if the user specifies
        a hostname of "example.com" but a [DNS-SRV] lookup returned
        "im.example.com", the certificate MUST be checked as
        "example.com".  If a JID for any kind of XMPP entity (e.g.,
        client or server) is represented in a certificate, it MUST at a



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        minimum be represented as a UTF8String within an otherName
        entity inside the subjectAltName, using the [ASN.1] Object
        Identifier "id-on-xmppAddr" specified in Section 5.1.1 of this
        document; however, subject to local service policy the JID for
        an XMPP server or service MAY also be represented as a Common
        Name.
   9.   If the TLS negotiation is successful, the initiating entity MUST
        send a new stream header to the receiving entity.
   10.  If the TLS negotiation is successful, the receiving entity MUST
        discard any knowledge obtained in an insecure manner from the
        initiating entity before TLS takes effect.
   11.  If the TLS negotiation is successful, the initiating entity MUST
        discard any knowledge obtained in an insecure manner from the
        receiving entity before TLS takes effect.
   12.  If the TLS negotiation is successful, the receiving entity MUST
        NOT offer the STARTTLS extension to the initiating entity along
        with the other stream features that are offered after the new
        stream header is received and responded to.
   13.  If the TLS negotiation is successful, the initiating entity MUST
        continue with SASL negotiation.
   14.  If the TLS negotiation results in failure, the receiving entity
        MUST terminate both the XML stream and the underlying TCP
        connection.
   15.  See Mandatory-to-Implement Technologies (Section 14.7) regarding
        mechanisms that MUST be supported.

5.1.1.  ASN.1 Object Identifier for XMPP Address

   The [ASN.1] Object Identifier "id-on-xmppAddr" described above is
   defined as follows:

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

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

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

   XmppAddr ::= UTF8String

   This Object Identifier MAY also be represented in dotted display
   format (i.e., "1.3.6.1.5.5.7.8.5") or in the Uniform Resource Name
   notation specified in [URN-OID] (i.e., "urn:oid:1.3.6.1.5.5.7.8.5").

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




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5.2.  Narrative

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

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





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   8.  Upon receiving the new stream header from the initiating entity,
       the receiving entity MUST respond by sending a new XML stream
       header to the initiating entity along with the available features
       (but not including the STARTTLS feature) and SHOULD include an
       updated list of SASL mechanisms so that the initiating entity can
       detect any changes to the list of SASL mechanisms supported by
       the receiving entity.

   Examples of TLS negotiation are provided under Section 9.


6.  SASL Negotiation

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]).  SASL provides a generalized method for adding
   authentication support to connection-based protocols, and XMPP uses a
   generic XML namespace profile for SASL that conforms to the profiling
   requirements of [SASL].

   The following rules apply:

   1.   If the SASL negotiation occurs between two servers,
        communications MUST NOT proceed until the Domain Name System
        (DNS) hostnames asserted by the servers have been resolved (see
        Server-to-Server Communications (Section 14.4)).
   2.   If the initiating entity is capable of SASL negotiation, it MUST
        include the 'version' attribute set to a value of at least "1.0"
        in the initial stream header.
   3.   If the receiving entity is capable of SASL negotiation, it MUST
        advertise one or more authentication mechanisms within a
        <mechanisms/> element qualified by the
        'urn:ietf:params:xml:ns:xmpp-sasl' namespace in reply to the
        opening stream tag received from the initiating entity (if the
        opening stream tag included the 'version' attribute set to a
        value of at least "1.0").
   4.   During SASL negotiation, an entity MUST NOT send any white space
        characters (matching production [3] content of [XML]) within the
        root stream element as separators between elements (any white
        space characters shown in the SASL examples below are included
        for the sake of readability only); this prohibition helps to
        ensure proper security layer byte precision.
   5.   Any XML character data contained within the XML elements used
        during SASL negotiation MUST be encoded using base64, where the
        encoding adheres to the definition in Section 3 of RFC 3548
        [BASE64].



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   6.   If the receiving entity does not include a 'realm' value, the
        initiating entity must default it to the domain identifier
        portion of the receiving entity's JID.
   7.   If provision of a "simple username" is supported by the selected
        SASL mechanism (e.g., this is supported by the DIGEST-MD5 and
        CRAM-MD5 mechanisms but not by the EXTERNAL and GSSAPI
        mechanisms), during authentication the initiating entity SHOULD
        provide as the simple username its sending domain (IP address or
        fully qualified domain name as contained in a domain identifier)
        in the case of server-to-server communications or its registered
        account name (user or node name as contained in an XMPP node
        identifier) in the case of client-to-server communications.  In
        either case, the initiating entity MUST ensure that the username
        adheres to the [NAMEPREP] or Nodeprep (Appendix A) profile of
        [STRINGPREP] (as appropriate) before sending it to the receiving
        entity.  (Note: Account provisioning is out of scope for this
        specification; possible methods for account provisioning include
        account creation by a server administrator and in-band account
        registration using the 'jabber:iq:register' namespace as
        documented in [XEP-0077].)
   8.   If the initiating entity wishes to act on behalf of another
        entity and the selected SASL mechanism supports transmission of
        an authorization identity, the initiating entity MUST provide an
        authorization identity during SASL negotiation.  If the
        initiating entity does not wish to act on behalf of another
        entity, it MUST NOT provide an authorization identity.  As
        specified in [SASL], the initiating entity MUST NOT provide an
        authorization identity unless the authorization identity is
        different from the default authorization identity derived from
        the authentication identity.  If provided, the value of the
        authorization identity MUST be of the form <domain> (i.e., a
        domain identifier only) for servers and of the form
        <node@domain> (i.e., node identifier and domain identifier) for
        clients.
   9.   If the SASL negotiation is successful, the initiating entity
        MUST send a new stream header to the receiving entity.
   10.  Upon successful SASL negotiation that involves negotiation of a
        security layer, the receiving entity MUST discard any knowledge
        obtained from the initiating entity which was not obtained from
        the SASL negotiation itself; the receiving entity SHOULD also
        send new stream features (including an updated list of SASL
        mechanisms) so that the initiating entity can detect any changes
        to the list of mechanisms supported by the receiving entity.
   11.  Upon successful SASL negotiation that involves negotiation of a
        security layer, the initiating entity MUST discard any knowledge
        obtained from the receiving entity which was not obtained from
        the SASL negotiation itself.




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   12.  See Mandatory-to-Implement Technologies (Section 14.7) regarding
        mechanisms that MUST be supported; naturally, other SASL
        mechanisms MAY be supported as well.

6.2.  Narrative

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

   1.  The initiating entity requests SASL authentication by including
       the 'version' attribute in the opening XML stream header sent to
       the receiving entity, with the value set to "1.0".
   2.  After sending an XML stream header in reply, the receiving entity
       advertises a list of available SASL authentication mechanisms as
       stream features; each of these is a <mechanism/> element included
       as a child within a <mechanisms/> container element qualified by
       the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace, which in turn
       is a child of a <features/> element in the streams namespace.  If
       TLS negotiation (Section 5) needs to be completed before a
       particular authentication mechanism may be used, the receiving
       entity MUST NOT provide that mechanism in the list of available
       SASL authentication mechanisms prior to TLS negotiation.  If the
       initiating entity presents a valid certificate during prior TLS
       negotiation, the receiving entity SHOULD offer the SASL EXTERNAL
       mechanism to the initiating entity during SASL negotiation (refer
       to [SASL]), although the EXTERNAL mechanism MAY be offered under
       other circumstances as well.  If successful SASL negotiation is
       required for interaction with the receiving entity, it SHOULD
       signal that fact by including a <required/> element as a child of
       the <mechanisms/> element.
   3.  The initiating entity selects a mechanism by sending an <auth/>
       element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
       namespace to the receiving entity and including an appropriate
       value for the 'mechanism' attribute.  This element MAY contain
       XML character data (in SASL terminology, the "initial response")
       if the mechanism supports or requires it; if the initiating
       entity needs to send a zero-length initial response, it MUST
       transmit the response as a single equals sign ("="), which
       indicates that the response is present but contains no data.
   4.  If necessary, the receiving entity challenges the initiating
       entity by sending to the initiating entity a <challenge/> element
       qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace;
       this element MAY contain XML character data (which MUST be
       computed in accordance with the definition of the SASL mechanism
       chosen by the initiating entity).
   5.  The initiating entity responds to the challenge by sending to the
       receiving entity a <response/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY



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       contain XML character data (which MUST be computed in accordance
       with the definition of the SASL mechanism chosen by the
       initiating entity).
   6.  If necessary, the receiving entity sends more challenges and the
       initiating entity sends more responses.

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

   1.  The initiating entity aborts the handshake by sending an <abort/>
       element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
       namespace to the receiving entity.  Upon receiving an <abort/>
       element, the receiving entity SHOULD allow a configurable but
       reasonable number of retries (at least 2), after which it MUST
       terminate the TCP connection; this enables the initiating entity
       (e.g., an end-user client) to tolerate incorrectly-provided
       credentials (e.g., a mistyped password) without being forced to
       reconnect.
   2.  The receiving entity reports failure of the handshake by sending
       a <failure/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
       entity (the particular cause of failure SHOULD be communicated in
       an appropriate child element of the <failure/> element as defined
       under SASL Errors (Section 6.4)).  If the failure case occurs,
       the receiving entity SHOULD allow a configurable but reasonable
       number of retries (at least 2), after which it MUST terminate the
       TCP connection; this enables the initiating entity (e.g., an end-
       user client) to tolerate incorrectly-provided credentials (e.g.,
       a mistyped password) without being forced to reconnect.
   3.  The receiving entity reports success of the handshake by sending
       a <success/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
       entity; this element MAY contain XML character data (in SASL
       terminology, "additional data with success") if required by the
       chosen SASL mechanism; if the receiving entity needs to send
       additional data of zero length, it MUST transmit the data as a
       single equals sign ("=").  Upon receiving the <success/> element,
       the initiating entity MUST initiate a new stream by sending an
       opening XML stream header to the receiving entity (it is not
       necessary to send a closing </stream> tag first, since the
       receiving entity and initiating entity MUST consider the original
       stream to be closed upon sending or receiving the <success/>
       element).  Upon receiving the new stream header from the
       initiating entity, the receiving entity MUST respond by sending a
       new XML stream header to the initiating entity, along with any
       available features or an empty <features/> element (to signify
       that no additional features are available); any such additional
       features not defined herein MUST be defined by the relevant



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       extension to XMPP.  As noted, if SASL negotiation involved
       establishment of a security layer, the receiving entity SHOULD
       send an updated list of SASL mechanisms so that the initiating
       entity can detect any changes to the list of mechanisms supported
       by the receiving entity.

6.3.  SASL Definition

   The profiling requirements of [SASL] require that the following
   information be supplied by a protocol definition:

   service name:  "xmpp"
   initiation sequence:  After the initiating entity provides an opening
      XML stream header and the receiving entity replies in kind, the
      receiving entity provides a list of acceptable authentication
      methods.  The initiating entity chooses one method from the list
      and sends it to the receiving entity as the value of the
      '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], then
      [TLS], then [SASL], then XMPP.
   use of the authorization identity:  The authorization identity may be
      used by xmpp to denote the non-default <node@domain> of a client
      or the sending <domain> of a server; an empty string is equivalent
      to an absent authorization identity.

6.4.  SASL Errors

   The following SASL-related error conditions are defined:

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





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   o  <incorrect-encoding/> -- The data provided by the initiating
      entity could not be processed because the [BASE64] encoding is
      incorrect (e.g., because the encoding does not adhere to the
      definition in Section 3 of [BASE64]); sent in reply to a
      <response/> element or an <auth/> element with initial response
      data.
   o  <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.
   o  <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.
   o  <malformed-request/> -- The challenge or response is malformed
      (e.g., the <auth/> element includes an initial response but the
      mechanism does not allow that); sent in reply to an <abort/>,
      <auth/>, <challenge/>, or <response/> element.
   o  <mechanism-too-weak/> -- The mechanism requested by the initiating
      entity is weaker than server policy permits for that initiating
      entity; sent in reply to a <response/> element or an <auth/>
      element with initial response data.
   o  <not-authorized/> -- The authentication failed because the
      initiating entity did not provide proper credentials (this
      includes but is not limited to the case of an unknown username,
      and no differentiation is made between an unknown username and
      incorrect credentials); sent in reply to a <response/> element or
      an <auth/> element with initial response data.
   o  <temporary-auth-failure/> -- The authentication failed because of
      a temporary error condition within the receiving entity, and the
      initiating entity should try again later; sent in reply to an
      <auth/> element or <response/> element.

   Examples of SASL negotiation are provided under Section 9.


7.  Resource Binding

   After a client authenticates with a server, it MUST bind a specific
   resource to the stream so that the server can properly address the
   client (see addresses (Section 3)) and route XML stanzas to and from
   the client (see stanza delivery rules (Section 10)).  That is, there
   MUST be a resource identifier associated with the "bare JID"
   (<node@domain>) of the client; this ensures that the address for use
   over that stream is a "full JID" of the form <node@domain/resource>.

   Upon receiving a success indication within the SASL negotiation, the
   client MUST send a new stream header to the server, to which the



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

   Server advertises resource binding feature to client:

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

   Upon being so informed that resource binding is required, the client
   MUST bind a resource to the stream by sending to the server an IQ
   stanza of type "set" (see IQ Semantics (Section 8.2.3)) containing
   data qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace.

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

   Client asks server to bind a resource:

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

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

   If the client wishes to specify the resource identifier, it MUST send
   an IQ stanza of type "set" that contains the desired resource



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   identifier as the non-zero-length XML character data of a <resource/>
   element that is a child of the <bind/> element.

   Client binds a resource:

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

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

   Server informs client of successful resource binding:

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

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

   When a client supplies a resource identifier, the following stanza
   error conditions are possible (see Stanza Errors (Section 8.3)):

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

   The protocol for these error conditions is shown below.





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   Resource identifier cannot be processed:

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

   Client is not allowed to bind a resource:

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

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

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

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







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   Resource identifier is in use:

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

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

7.1.  Binding Multiple Resources

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

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

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

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





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

   When a client binds multiple resources to a stream, proper management
   of 'from' addresses is imperative.  The following rules apply:

   1.  A client SHOULD specify a 'from' address on every stanza.
   2.  If a client does not specify a 'from' address, the server MUST
       stamp a 'from' address, which SHOULD be the "default" resource as
       specified below.

   The "default" resource SHOULD be the oldest resource bound to the
   stream; normally that will be the initial resource, but it may be a
   resource bound later (i.e., if subsequently the initial resource has
   been unbound).

   Naturally, the rules regarding validation of asserted 'from'
   addresses still apply (see Section 10).


8.  XML Stanzas

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

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

8.1.  Common Attributes

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






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8.1.1.  to

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

   In the 'jabber:client' namespace, a stanza with a specific intended
   recipient MUST possess a 'to' attribute, whereas a stanza sent from a
   client to a server for direct processing by that server (e.g.,
   presence sent to the server for broadcasting to other entities)
   SHOULD NOT possess a 'to' attribute.

   In the 'jabber:server' namespace, a stanza MUST possess a 'to'
   attribute; if a server receives a stanza that does not meet this
   restriction, it MUST generate an <improper-addressing/> stream error
   condition and terminate both the XML stream and the underlying TCP
   connection with the offending server.

   If the value of the 'to' attribute is invalid or cannot be contacted,
   the entity discovering that fact (usually the sender's or recipient's
   server) MUST return an appropriate error to the sender, setting the
   'from' attribute of the error stanza to the value provided in the
   'to' attribute of the offending stanza.

8.1.2.  from

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

   When a server receives an XML stanza within the context of an
   authenticated stream qualified by the 'jabber:client' namespace, it
   MUST do one of the following:
   1.  validate that the value of the 'from' attribute provided by the
       client is that of a connected resource for the associated entity
   2.  add a 'from' address to the stanza whose value is the full JID
       (<node@domain/resource>) determined by the server for the
       connected resource that generated the stanza (see Determination
       of Addresses (Section 3.5)), or the bare JID (<node@domain>) in
       the case of subscription-related presence stanzas (see [XMPP-IM]
       for details)

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



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

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

8.1.3.  id

   The optional 'id' attribute MAY be used by a sending entity for
   internal tracking of stanzas that it sends and receives (especially
   for tracking the request-response interaction inherent in the
   semantics of IQ stanzas).  It is OPTIONAL for the value of the 'id'
   attribute to be unique globally, within a domain, or within a stream.
   The semantics of IQ stanzas impose additional restrictions; see IQ
   Semantics (Section 8.2.3).

8.1.4.  type

   The 'type' attribute specifies detailed information about the purpose
   or context of the message, presence, or IQ stanza.  The particular
   allowable values for the 'type' attribute vary depending on whether
   the stanza is a message, presence, or IQ; the values for message and
   presence stanzas are specific to instant messaging and presence
   applications and therefore are defined in [XMPP-IM], whereas the
   values for IQ stanzas specify the role of an IQ stanza in a
   structured request-response "conversation" and thus are defined under
   IQ Semantics (Section 8.2.3) below.  The only 'type' value common to



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   all three stanzas is "error"; see Stanza Errors (Section 8.3).

8.1.5.  xml:lang

   A stanza SHOULD possess an 'xml:lang' attribute (as defined in
   Section 2.12 of [XML]) if the stanza contains XML character data that
   is intended to be presented to a human user (as explained in
   [CHARSET], "internationalization is for humans").  The value of the
   'xml:lang' attribute specifies the default language of any such
   human-readable XML character data, which MAY be overridden by the
   'xml:lang' attribute of a specific child element.  If a stanza does
   not possess an 'xml:lang' attribute, an implementation MUST assume
   that the default language is that specified for the stream as defined
   under Stream Attributes (Section 4.4) above.  The value of the 'xml:
   lang' attribute MUST be an NMTOKEN and MUST conform to the format
   defined in [LANGTAGS].

8.2.  Basic Semantics

8.2.1.  Message Semantics

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

8.2.2.  Presence Semantics

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






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8.2.3.  IQ Semantics

   Info/Query, or IQ, is a request-response mechanism, similar in some
   ways to [HTTP].  The semantics of IQ enable an entity to make a
   request of, and receive a response from, another entity.  The data
   content of the request and response is defined by the namespace
   declaration of a direct child element of the IQ element (see extended
   namespaces (Section 8.4)), and the interaction is tracked by the
   requesting entity through use of the 'id' attribute.  Thus, IQ
   interactions follow a common pattern of structured data exchange such
   as get/result or set/result (although an error may be returned in
   reply to a request if appropriate):

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

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

   1.  The 'id' attribute is REQUIRED for IQ stanzas.
   2.  The 'type' attribute is REQUIRED for IQ stanzas.  The value MUST
       be one of the following:
       *  get -- The stanza is a request for information or
          requirements.
       *  set -- The stanza provides required data, sets new values, or
          replaces existing values.
       *  result -- The stanza is a response to a successful get or set
          request.
       *  error -- An error has occurred regarding processing or
          delivery of a previously-sent get or set (see Stanza Errors
          (Section 8.3)).
   3.  An entity that receives an IQ request of type "get" or "set" MUST
       reply with an IQ response of type "result" or "error" (the
       response MUST preserve the 'id' attribute of the request).




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   4.  An entity that receives a stanza of type "result" or "error" MUST
       NOT respond to the stanza by sending a further IQ response of
       type "result" or "error"; however, as shown above, the requesting
       entity MAY send another request (e.g., an IQ of type "set" in
       order to provide required information discovered through a get/
       result pair).
   5.  An IQ stanza of type "get" or "set" MUST contain one and only one
       child element that specifies the semantics of the particular
       request or response.
   6.  An IQ stanza of type "result" MUST include zero or one child
       elements.
   7.  An IQ stanza of type "error" SHOULD include the child element
       contained in the associated "get" or "set" and MUST include an
       <error/> child; for details, see Stanza Errors (Section 8.3).

8.3.  Stanza Errors

   Stanza-related errors are handled in a manner similar to stream
   errors (Section 4.9).  However, unlike stream errors, stanza errors
   are recoverable; therefore error stanzas include hints regarding
   actions that the original sender can take in order to remedy the
   error.

8.3.1.  Rules

   The following rules apply to stanza-related errors:

   o  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), where such an "error stanza" is a stanza of
      the same kind (message, presence, or IQ) whose 'type' attribute is
      set to a value of "error".
   o  The entity that generates an error stanza SHOULD include the
      original XML sent so that the sender can inspect and, if
      necessary, correct the XML before attempting to resend.
   o  An error stanza MUST contain an <error/> child element.
   o  An <error/> child MUST NOT be included if the 'type' attribute has
      a value other than "error" (or if there is no 'type' attribute).
   o  An entity that receives an error stanza MUST NOT respond to the
      stanza with a further error stanza; this helps to prevent looping.

8.3.2.  Syntax

   The syntax for stanza-related errors is as follows:







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

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

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

   o  cancel -- do not retry (the error is unrecoverable)
   o  continue -- proceed (the condition was only a warning)
   o  modify -- retry after changing the data sent
   o  auth -- retry after providing credentials
   o  wait -- retry after waiting (the error is temporary)

   The <error/> element:

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

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

   Finally, to maintain backward compatibility, the schema (specified in
   [XMPP-IM]) allows the optional inclusion of a 'code' attribute on the
   <error/> element; for details, see [XEP-0086].




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8.3.3.  Defined Conditions

   The following conditions are defined for use in stanza errors.

   o  <bad-request/> -- the sender has sent XML that is malformed or
      that cannot be processed (e.g., an IQ stanza that includes an
      unrecognized value of the 'type' attribute); the associated error
      type SHOULD be "modify".
   o  <conflict/> -- access cannot be granted because an existing
      resource or session exists with the same name or address; the
      associated error type SHOULD be "cancel".
   o  <feature-not-implemented/> -- the feature requested is not
      implemented by the recipient or server and therefore cannot be
      processed; the associated error type SHOULD be "cancel" or
      "modify".
   o  <forbidden/> -- the requesting entity does not possess the
      required permissions to perform the action; the associated error
      type SHOULD be "auth".
   o  <gone/> -- the recipient or server can no longer be contacted at
      this address (the error stanza MAY contain a new address in the
      XML character data of the <gone/> element); the associated error
      type SHOULD be "cancel" or "modify".
   o  <internal-server-error/> -- the server could not process the
      stanza because of a misconfiguration or an otherwise-undefined
      internal server error; the associated error type SHOULD be "wait".
   o  <item-not-found/> -- the addressed JID or item requested cannot be
      found; the associated error type SHOULD be "cancel".
   o  <jid-malformed/> -- the sending entity has provided or
      communicated an XMPP address (e.g., a value of the 'to' attribute)
      or aspect thereof (e.g., a resource identifier) that does not
      adhere to the syntax defined under Addresses (Section 3); the
      associated error type SHOULD be "modify".
   o  <not-acceptable/> -- the recipient or server understands the
      request but is refusing to process it because it does not meet
      criteria defined by the recipient or server (e.g., a local policy
      regarding stanza size limits or acceptable words in messages); the
      associated error type SHOULD be "modify".
   o  <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".
   o  <not-authorized/> -- the sender must provide proper credentials
      before being allowed to perform the action, or has provided
      improper credentials; the associated error type SHOULD be "auth".
   o  <not-modified/> -- the item requested has not changed since it was
      last requested; the associated error type SHOULD be "continue".
   o  <payment-required/> -- the requesting entity is not authorized to
      access the requested service because payment is required; the
      associated error type SHOULD be "auth".



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   o  <recipient-unavailable/> -- the intended recipient is temporarily
      unavailable; the associated error type SHOULD be "wait" (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).
   o  <redirect/> -- the recipient or server is redirecting requests for
      this information to another entity, usually temporarily (the error
      stanza SHOULD contain the alternate address, which MUST be a valid
      JID, in the XML character data of the <redirect/> element); the
      associated error type SHOULD be "modify".
   o  <registration-required/> -- the requesting entity is not
      authorized to access the requested service because prior
      registration is required; the associated error type SHOULD be
      "auth".
   o  <remote-server-not-found/> -- a remote server or service specified
      as part or all of the JID of the intended recipient does not
      exist; the associated error type SHOULD be "cancel".
   o  <remote-server-timeout/> -- a remote server or service specified
      as part or all of the JID of the intended recipient (or required
      to fulfill a request) could not be contacted within a reasonable
      amount of time; the associated error type SHOULD be "wait".
   o  <resource-constraint/> -- the server or recipient lacks the system
      resources necessary to service the request; the associated error
      type SHOULD be "wait".
   o  <service-unavailable/> -- the server or recipient does not
      currently provide the requested service; the associated error type
      SHOULD be "cancel".
   o  <subscription-required/> -- the requesting entity is not
      authorized to access the requested service because a subscription
      is required; the associated error type SHOULD be "auth".
   o  <undefined-condition/> -- the error condition is not one of those
      defined by the other conditions in this list; any error type may
      be associated with this condition, and it SHOULD be used only in
      conjunction with an application-specific condition.
   o  <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".

8.3.4.  Application-Specific Conditions

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





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   <iq type='error' id='some-id'>
     <error type='modify'>
       <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       <too-many-parameters xmlns='application-ns'/>
     </error>
   </iq>


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

8.4.  Extended Namespaces

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

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

   Recipient:  If a recipient receives a stanza that contains a child
      element it does not understand, it SHOULD ignore that specific XML
      data, i.e., it SHOULD not process it or present it to a user or
      associated application (if any).  In particular:
      *  If an entity receives a message or presence stanza that
         contains XML data qualified by a namespace it does not
         understand, the portion of the stanza that is in the unknown



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         namespace SHOULD be ignored.
      *  If an entity receives a message stanza whose only child element
         is qualified by a namespace it does not understand, it MUST
         ignore the entire stanza.
      *  If an entity receives an IQ stanza of type "get" or "set"
         containing a child element qualified by a namespace it does not
         understand, the entity SHOULD return an IQ stanza of type
         "error" with an error condition of <service-unavailable/>.
   Router:  If a routing entity (usually a server) handles a stanza that
      contains a child element it does not understand, it SHOULD ignore
      the associated XML data by passing it on untouched to the
      recipient.


9.  Examples

9.1.  Client-to-Server

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

   Step 1: Client initiates stream to server:

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














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

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

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

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

   Step 4: Client sends STARTTLS command to server:

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

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

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

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

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

   Step 6: Client and server attempt to complete TLS negotiation over
   the existing TCP connection (see [TLS] for details).












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

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

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

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

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

   Step 9: Client selects an authentication mechanism, in this case
   [DIGEST-MD5]:

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

   Step 10: Server sends a [BASE64] encoded challenge to client:

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

   The decoded challenge is:



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

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

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

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

   Step 11: Client sends a [BASE64] encoded response to the challenge:

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

   The decoded response is:

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

   Step 12: Server informs client of success and includes [BASE64]
   encoded value for subsequent authentication:

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

   The decoded value for subsequent authentication is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd









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

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

   Step 13: Client initiates a new stream to server:

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

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

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

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

   Step 15: Client binds a resource:

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






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   Step 16: Server informs client of successful resource binding:

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

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

   Client sends XML stanza to other entity:

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

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

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

   Client receives XML stanza from other entity:

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

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

   Client closes the stream:

   </stream:stream>

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

   Server closes the stream:

   </stream:stream>




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   Client now terminates the underlying TCP connection and the session
   is over.

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
   "example.com"; the receiving server ("Server2") is example.net and it
   requires use of TLS; example.com presents a certificate and
   authenticates via the SASL EXTERNAL mechanism.  (Note: The alternate
   steps shown below are provided to illustrate the protocol for failure
   cases; they are not exhaustive and would not necessarily be triggered
   by the data sent in the examples.)

   Step 1: Server1 initiates stream to Server2:

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

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

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

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

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





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   Step 4: Server1 sends the STARTTLS command to Server2:

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

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

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

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

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

   Step 6: Server1 and Server2 attempt to complete TLS negotiation via
   TCP.

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

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

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

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

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




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   Step 9: Server1 selects the EXTERNAL mechanism and includes a
   [BASE64] authorization identity:

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

   The decoded authorization identity is "example.com".

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

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

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

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

   Step 12: Server1 initiates a new stream to Server2:

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

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

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

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





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   Server1 sends XML stanza to Server2:

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

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

   Server2 sends XML stanza to Server1:

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

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

   Server1 closes the stream:

   </stream:stream>

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

   Server2 closes the stream:

   </stream:stream>

   Server1 now terminates the underlying TCP connection and the session
   is over.


10.  Server Rules for Handling XML Stanzas

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

   Beyond the requirement for in-order processing, each server
   implementation will contain its own "delivery tree" for handling
   stanzas it receives.  Such a tree determines whether a stanza needs



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   to be routed to another domain, processed direct, or delivered to a
   resource associated with a connected node.  The following rules
   apply.

10.1.  No 'to' Address

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

10.2.  Foreign Domain

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

   A server-to-server stream already exists between the two domains:
      The sender's server routes the stanza to the authoritative server
      for the foreign domain over the existing stream
   There exists no server-to-server stream between the two domains:  The
      sender's server (1) resolves the hostname of the foreign domain
      (as defined under Server-to-Server Communications (Section 14.4)),
      (2) negotiates a server-to-server stream between the two domains
      (as defined under TLS negotiation (Section 5) and SASL negotiation
      (Section 6)), and (3) routes the stanza to the authoritative
      server for the foreign domain over the newly-established stream

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



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10.3.  Subdomain

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

10.4.  Mere Domain or Specific Resource

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

10.5.  Node in Same Domain

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

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

   Particular XMPP applications MAY specify delivery rules that modify
   or supplement the foregoing rules; for example, a set of delivery
   rules for instant messaging and presence applications is defined in
   [XMPP-IM].






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11.  XML Usage

11.1.  Restrictions

   XMPP is a simplified and specialized protocol for streaming XML
   elements in order to exchange structured information in close to real
   time.  Because XMPP does not require the parsing of arbitrary and
   complete XML documents, there is no requirement that XMPP needs to
   support the full feature set of [XML].  In particular, the following
   restrictions apply.

   With regard to XML generation, an XMPP implementation MUST NOT inject
   into an XML stream any of the following:

   o  comments (as defined in Section 2.5 of [XML])
   o  processing instructions (Section 2.6 therein)
   o  internal or external DTD subsets (Section 2.8 therein)
   o  internal or external entity references (Section 4.2 therein) with
      the exception of predefined entities (Section 4.6 therein)
   o  character data or attribute values containing unescaped characters
      that map to the predefined entities (Section 4.6 therein); such
      characters MUST be escaped

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

11.2.  XML Namespace Names and Prefixes

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

11.2.1.  Streams Namespace

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



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11.2.2.  Default Namespace

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

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

   o  jabber:client -- this default namespace is declared when the
      stream is used for communications between a client and a server
   o  jabber:server -- this default namespace is declared when the
      stream is used for communications between two servers

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

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

   Note: The 'jabber:client' and 'jabber:server' namespaces are nearly
   identical but are used in different contexts (client-to-server
   communications for 'jabber:client' and server-to-server
   communications for 'jabber:server').  The only difference between the
   two is that the 'to' and 'from' attributes are OPTIONAL on stanzas
   sent within 'jabber:client', whereas they are REQUIRED on stanzas
   sent within 'jabber:server'.  If a compliant implementation accepts a
   stream that is qualified by the 'jabber:client' or 'jabber:server'
   namespace, it MUST support the common attributes (Section 8.1) and
   basic semantics (Section 8.2) of all three core stanza kinds
   (message, presence, and IQ).

11.2.3.  Dialback Namespace

   A dialback namespace declaration is REQUIRED for all elements used in
   server dialback (Appendix C).  The name of the dialback namespace
   MUST be 'jabber:server:dialback'.  All elements qualified by this



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   namespace MUST be prefixed.  An implementation SHOULD generate only
   the 'db:' prefix for such elements and MAY accept only the 'db:'
   prefix.

11.3.  Validation

   A server is not responsible for validating the XML elements forwarded
   to a client or another server; an implementation MAY choose to
   provide only validated data elements but this is OPTIONAL (although
   an implementation MUST NOT accept XML that is not well-formed).
   Clients SHOULD NOT rely on the ability to send data which does not
   conform to the schemas, and SHOULD ignore any non-conformant elements
   or attributes on the incoming XML stream.  Validation of XML streams
   and stanzas is OPTIONAL, and schemas are included herein for
   descriptive purposes only.

11.4.  Inclusion of Text Declaration

   Implementations SHOULD send a text declaration before sending a
   stream header.  Applications MUST follow the rules in [XML] regarding
   the circumstances under which a text declaration is included.

11.5.  Character Encoding

   Implementations MUST support the [UTF-8] transformation of Universal
   Character Set ([UCS2]) characters, as required by [CHARSET].
   Implementations MUST NOT attempt to use any other encoding.

11.6.  White Space

   Except where explicitly disallowed (i.e., during TLS negotiation
   (Section 5) and SASL negotiation [SASL]), either entity MAY send
   white space characters (matching production [3] content of [XML])
   within the root stream element as separators between XML stanzas or
   between any other elements sent over the stream.  Any meaning
   attached to such white space characters by the entity that receives
   them is out of scope for this document.


12.  Compliance Requirements

   This section summarizes the specific aspects of the Extensible
   Messaging and Presence Protocol that MUST be supported by servers and
   clients in order to be considered compliant implementations, as well
   as additional protocol aspects that SHOULD be supported.  For
   compliance purposes, we draw a distinction between core protocols
   (which MUST be supported by any server or client, regardless of the
   specific application) and instant messaging and presence protocols



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   (which MUST be supported only by instant messaging and presence
   applications built on top of the core protocols).  Compliance
   requirements that apply to all servers and clients are specified in
   this section; compliance requirements for instant messaging and
   presence applications are specified in the corresponding section of
   [XMPP-IM].

12.1.  Servers

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

   o  Application of the [NAMEPREP], Nodeprep (Appendix A), and
      Resourceprep (Appendix B) profiles of [STRINGPREP] to addresses
      (including ensuring that domain identifiers are internationalized
      domain names as defined in [IDNA])
   o  XML streams (Section 4), including TLS negotiation (Section 5),
      SASL negotiation (Section 6), and Resource Binding (Section 7)
   o  The basic semantics of the three defined stanza kinds (i.e.,
      <message/>, <presence/>, and <iq/>) as specified in stanza
      semantics (Section 8.2)
   o  Generation (and, where appropriate, handling) of error syntax and
      semantics related to streams, TLS, SASL, and XML stanzas

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

   o  Server dialback (Appendix C)

12.2.  Clients

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

   o  XML streams (Section 4), including TLS negotiation (Section 5),
      SASL negotiation (Section 6), and Resource Binding (Section 7)
   o  The basic semantics of the three defined stanza kinds (i.e.,
      <message/>, <presence/>, and <iq/>) as specified in stanza
      semantics (Section 8.2)
   o  Handling (and, where appropriate, generation) of error syntax and
      semantics related to streams, TLS, SASL, and XML stanzas

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

   o  Generation of addresses to which the [NAMEPREP], Nodeprep
      (Appendix A), and Resourceprep (Appendix B) profiles of
      [STRINGPREP] can be applied without failing



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13.  Internationalization Considerations

   XML streams MUST be encoded in UTF-8 as specified under Character
   Encoding (Section 11.5).  As specified under Stream Attributes
   (Section 4.4), an XML stream SHOULD include an 'xml:lang' attribute
   specifying the default language for any XML character data sent over
   the stream that is intended to be presented to a human user.  As
   specified under xml:lang (Section 8.1.5), an XML stanza SHOULD
   include an 'xml:lang' attribute if the stanza contains XML character
   data that is intended to be presented to a human user.  A server
   SHOULD apply the default 'xml:lang' attribute to stanzas it routes or
   delivers on behalf of connected entities, and MUST NOT modify or
   delete 'xml:lang' attributes stanzas it receives from other entities.


14.  Security Considerations

14.1.  High Security

   For the purposes of XMPP communications (client-to-server and server-
   to-server), the term "high security" refers to the use of security
   technologies that provide both mutual authentication and integrity-
   checking; in particular, when using certificate-based authentication
   to provide high security, a chain-of-trust SHOULD be established out-
   of-band, although a shared certificate authority signing certificates
   could allow a previously unknown certificate to establish trust in-
   band.  See Section 14.2 below regarding certificate validation
   procedures.

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

14.2.  Certificate Validation

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

   Case #1:  The peer contains an End Entity certificate which appears
      to be certified by a chain of certificates terminating in a trust
      anchor (as described in Section 6.1 of [X509]).
   Case #2:  The peer certificate is certified by a Certificate
      Authority not known to the validating peer.
   Case #3:  The peer certificate is self-signed.

   In Case #1, the validating peer MUST do one of two things:
   1.  Verify the peer certificate according to the rules of [X509].
       The certificate SHOULD then be checked against the expected
       identity of the peer following the rules described in [HTTP-TLS],



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       except that a subjectAltName extension of type "xmpp" MUST be
       used as the identity if present.  If one of these checks fails,
       user-oriented clients MUST either notify the user (clients MAY
       give the user the opportunity to continue with the connection in
       any case) or terminate the connection with a bad certificate
       error.  Automated clients SHOULD terminate the connection (with a
       bad certificate error) and log the error to an appropriate audit
       log.  Automated clients MAY provide a configuration setting that
       disables this check, but MUST provide a setting that enables it.
   2.  The peer SHOULD show the certificate to a user for approval,
       including the entire certificate chain.  The peer MUST cache the
       certificate (or some non-forgeable representation such as a
       hash).  In future connections, the peer MUST verify that the same
       certificate was presented and MUST notify the user if it has
       changed.

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

14.3.  Client-to-Server Communications

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

   The TLS protocol for encrypting XML streams (defined under TLS
   negotiation (Section 5)) provides a reliable mechanism for helping to
   ensure the confidentiality and data integrity of data exchanged
   between two entities.

   The SASL protocol for authenticating XML streams (defined under SASL
   negotiation (Section 6)) provides a reliable mechanism for validating
   that a client connecting to a server is who it claims to be.

   Client-to-server communications MUST NOT proceed until the DNS
   hostname asserted by the server has been resolved.  Such resolutions
   SHOULD first attempt to resolve the hostname using a [DNS-SRV]
   Service of "xmpp-client" and Proto of "tcp", resulting in resource
   records such as "_xmpp-client._tcp.example.com." (the use of the
   string "xmpp-client" for the service identifier is consistent with
   the IANA registration).  If the SRV lookup fails, the fallback SHOULD
   be a normal IPv4 or [IPv6] address record resolution to determine the
   IP address, using the "xmpp-client" port 5222 that is registered with
   the IANA; hoever, the fallback MAY be a DNS TXT lookup (see
   [DNS-TXT]) for alternative connection methods, for example as
   described in [XEP-0156].  If there is a mismatch between the hostname
   to which the client attempted to connect (e.g., "example.org") and
   the hostname to which the client actually connects (e.g.,
   "theconnector.example.org"), the client MUST warn a human user about
   the mismatch and the human user MUST approve the connection before



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   the client proceeds; however, the client MAY allow the user to add
   the presented hostname to a configured set of accepted hostnames in
   order to expedite future connections.

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

14.4.  Server-to-Server Communications

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

   Because service provisioning is a matter of policy, it is OPTIONAL
   for any given domain to communicate with other domains, and server-
   to-server communications MAY be disabled by the administrator of any
   given deployment.  If a particular domain enables inter-domain
   communications, it SHOULD enable high security.

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

   Inter-domain connections MUST NOT proceed until the DNS hostnames
   asserted by the servers have been resolved.  Such resolutions MUST
   first attempt to resolve the hostname using a [DNS-SRV] Service of
   "xmpp-server" and Proto of "tcp", resulting in resource records such
   as "_xmpp-server._tcp.example.com." (the use of the string "xmpp-
   server" for the service identifier is consistent with the IANA
   registration; note well that the "xmpp-server" service identifier
   supersedes the earlier use of a "jabber" service identifier, since
   the earlier usage did not conform to [DNS-SRV]; implementations
   desiring to be backward compatible should continue to look for or
   answer to the "jabber" service identifier as well).  If the SRV
   lookup fails, the fallback is a normal IPv4/IPv6 address record
   resolution to determine the IP address, using the "xmpp-server" port
   5269, registered with the IANA.

   Server dialback helps protect against domain spoofing, thus making it
   more difficult to spoof XML stanzas.  It is not a mechanism for
   authenticating, securing, or encrypting streams between servers as is
   done via SASL and TLS, and results in weak verification of server
   identities only.  Furthermore, it is susceptible to DNS poisoning
   attacks unless [DNSSEC] is used, and even if the DNS information is
   accurate, dialback cannot protect from attacks where the attacker is



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   capable of hijacking the IP address of the remote domain.  Domains
   requiring robust security SHOULD use TLS and SASL.  If SASL is used
   for server-to-server authentication, dialback SHOULD NOT be used
   since it is unnecessary.

14.5.  Order of Layers

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

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

   The rationale for this order is that [TCP] is the base connection
   layer used by all of the protocols stacked on top of TCP, [TLS] is
   often provided at the operating system layer, [SASL] is often
   provided at the application layer, and XMPP is the application
   itself.

14.6.  Lack of SASL Channel Binding to TLS

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

14.7.  Mandatory-to-Implement Technologies

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

   for authentication:  the SASL [DIGEST-MD5] mechanism
   for confidentiality:  TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA
      cipher)
   for both:  TLS plus SASL PLAIN for client-to-server connections and
      TLS plus SASL EXTERNAL for server-to-server connections (using the
      TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher supporting peer certificates)

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



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14.8.  Firewalls

   Communications using XMPP normally occur over [TCP] connections on
   port 5222 (client-to-server) or port 5269 (server-to-server), as
   registered with the IANA (see IANA Considerations (Section 15)).  Use
   of these well-known ports allows administrators to easily enable or
   disable XMPP activity through existing and commonly-deployed
   firewalls.

14.9.  Use of base64 in SASL

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

14.10.  Stringprep Profiles

   XMPP makes use of the [NAMEPREP] profile of [STRINGPREP] for
   processing of domain identifiers; for security considerations related
   to Nameprep, refer to the appropriate section of [NAMEPREP].

   In addition, XMPP defines two profiles of [STRINGPREP]: Nodeprep
   (Appendix A) for node identifiers and Resourceprep (Appendix B) for
   resource identifiers.

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

   A node identifier can be employed as one part of an entity's address
   in XMPP.  One common usage is as the username of an instant messaging
   user; another is as the name of a multi-user chat room; many other
   kinds of entities could use node identifiers as part of their
   addresses.  The security of such services could be compromised based



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   on different interpretations of the internationalized node
   identifier; for example, a user entering a single internationalized
   node identifier could access another user's account information, or a
   user could gain access to an otherwise restricted chat room or
   service.

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


15.  IANA Considerations

15.1.  XML Namespace Name for TLS Data

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

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

15.2.  XML Namespace Name for SASL Data

   A URN sub-namespace for SASL-related data in the Extensible Messaging
   and Presence Protocol (XMPP) is defined as follows.  (This namespace
   name adheres to the format defined in [XML-REG].)

   URI:  urn:ietf:params:xml:ns:xmpp-sasl
   Specification:  RFC 3920
   Description:  This is the XML namespace name for SASL-related data in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by RFC 3920.






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   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@jabber.org>

15.3.  XML Namespace Name for Stream Errors

   A URN sub-namespace for stream-related error data in the Extensible
   Messaging and Presence Protocol (XMPP) is defined as follows.  (This
   namespace name adheres to the format defined in [XML-REG].)

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

15.4.  XML Namespace Name for Resource Binding

   A URN sub-namespace for resource binding in the Extensible Messaging
   and Presence Protocol (XMPP) is defined as follows.  (This namespace
   name adheres to the format defined in [XML-REG].)

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

15.5.  XML Namespace Name for Stanza Errors

   A URN sub-namespace for stanza-related error data in the Extensible
   Messaging and Presence Protocol (XMPP) is defined as follows.  (This
   namespace name adheres to the format defined in [XML-REG].)

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

15.6.  Nodeprep Profile of Stringprep

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

   Name of this profile:



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      Nodeprep

   RFC in which the profile is defined:

      RFC 3920

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

      This is the first version of Nodeprep

15.7.  Resourceprep Profile of Stringprep

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

   Name of this profile:

      Resourceprep

   RFC in which the profile is defined:

      RFC 3920

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

      This is the first version of Resourceprep

15.8.  GSSAPI Service Name

   The IANA has registered "xmpp" as a GSSAPI [GSS-API] service name, as
   defined under SASL Definition (Section 6.3).

15.9.  Port Numbers

   The IANA has registered "xmpp-client" and "xmpp-server" as keywords
   for [TCP] ports 5222 and 5269 respectively.

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


16.  References

16.1.  Normative References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.



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   [BASE64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, July 2003.

   [CHARSET]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.

   [DIGEST-MD5]
              Leach, P. and C. Newman, "Using Digest Authentication as a
              SASL Mechanism", RFC 2831, May 2000.

   [DNS-SRV]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [GSS-API]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [HMAC]     National Institute of Standards and Technology, "The
              Keyed-Hash Message Authentication Code (HMAC)", FIPS PUB
              198, March 2002, <http://csrc.nist.gov/publications/fips/
              fips198/fips-198a.pdf>.

   [HTTP-TLS]
              Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [IDNA]     Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [IPv6]     Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.

   [LANGTAGS]
              Alvestrand, H., "Tags for the Identification of
              Languages", BCP 47, RFC 3066, January 2001.

   [NAMEPREP]
              Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)",
              RFC 3491, March 2003.

   [RANDOM]   Eastlake, D., Crocker, S., and J. Schiller, "Randomness
              Recommendations for Security", RFC 1750, December 1994.

   [SASL]     Melnikov, A. and K. Zeilenga, "Simple Authentication and



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              Security Layer (SASL)", RFC 4422, June 2006.

   [SHA]      National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-2, August 2002, <http://
              csrc.nist.gov/publications/fips/fips180-2/
              fips180-2withchangenotice.pdf>.

   [STRINGPREP]
              Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [TCP]      Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [TERMS]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [UCS2]     International Organization for Standardization,
              "Information Technology - Universal Multiple-octet coded
              Character Set (UCS) - Amendment 2: UCS Transformation
              Format 8 (UTF-8)", ISO Standard 10646-1 Addendum 2,
              October 1996.

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [X509]     Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler,
              "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C REC-
              xml, October 2000, <http://www.w3.org/TR/REC-xml>.

   [XML-NAMES]
              Bray, T., Hollander, D., and A. Layman, "Namespaces in
              XML", W3C REC-xml-names, January 1999,
              <http://www.w3.org/TR/REC-xml-names>.

16.2.  Informative References

   [ACAP]     Newman, C. and J. Myers, "ACAP -- Application
              Configuration Access Protocol", RFC 2244, November 1997.



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   [ASN.1]    CCITT, "Recommendation X.208: Specification of Abstract
              Syntax Notation One (ASN.1)", 1988.

   [DNSSEC]   Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [DNS-TXT]  Rosenbaum, R., "Using the Domain Name System To Store
              Arbitrary String Attributes", RFC 1464, May 1993.

   [HTTP]     Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [IMP-REQS]
              Day, M., Aggarwal, S., and J. Vincent, "Instant Messaging
              / Presence Protocol Requirements", RFC 2779,
              February 2000.

   [IRI]      Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [LINKLOCAL]
              Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              May 2005.

   [MAILBOXES]
              Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
              FUNCTIONS", RFC 2142, May 1997.

   [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [URN-OID]  Mealling, M., "A URN Namespace of Object Identifiers",
              RFC 3061, February 2001.

   [USINGTLS]
              Newman, C., "Using TLS with IMAP, POP3 and ACAP",



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              RFC 2595, June 1999.

   [XEP-0045]
              Saint-Andre, P., "Multi-User Chat", JSF XEP 0045,
              September 2006.

   [XEP-0071]
              Saint-Andre, P., "XHTML-IM", JSF XEP 0071, January 2006.

   [XEP-0077]
              Saint-Andre, P., "In-Band Registration", JSF XEP 0077,
              January 2006.

   [XEP-0086]
              Norris, R. and P. Saint-Andre, "Error Condition Mappings",
              JSF XEP 0086, February 2004.

   [XEP-0124]
              Paterson, I., Smith, D., and P. Saint-Andre, "HTTP
              Binding", JSF XEP 0124, April 2006.

   [XEP-0156]
              Hildebrand, J. and P. Saint-Andre, "A DNS TXT Resource
              Record Format for XMPP Connection Methods", JSF XEP 0156,
              May 2005.

   [XEP-0174]
              Saint-Andre, P., "Link-Local Messaging", JSF XEP 0174,
              July 2006.

   [XML-REG]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [XMPP-IM]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence",
              RFC 3921, October 2004.

   [XMPP-URI]
              Saint-Andre, P., "Internationalized Resource Identifiers
              (IRIs) and Uniform Resource Identifiers (URIs) for the
              Extensible Messaging and Presence Protocol (XMPP)",
              RFC 4622, August 2006.


Appendix A.  Nodeprep






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A.1.  Introduction

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

   This profile defines the following, as required by [STRINGPREP]:

   o  The intended applicability of the profile: internationalized node
      identifiers within XMPP
   o  The character repertoire that is the input and output to
      stringprep: Unicode 3.2, specified in Section 2 of this Appendix
   o  The mappings used: specified in Section 3
   o  The Unicode normalization used: specified in Section 4
   o  The characters that are prohibited as output: specified in Section
      5
   o  Bidirectional character handling: specified in Section 6

   Note: Both the UseSTD3ASCIIRules and AllowUnassigned flags MUST be
   set to true.

A.2.  Character Repertoire

   This profile uses Unicode 3.2 with the list of unassigned code points
   being Table A.1, both defined in Appendix A of [STRINGPREP].

A.3.  Mapping

   This profile specifies mapping using the following tables from
   [STRINGPREP]:

      Table B.1
      Table B.2

A.4.  Normalization

   This profile specifies the use of Unicode normalization form KC, as
   described in [STRINGPREP].






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A.5.  Prohibited Output

   This profile specifies the prohibition of using the following tables
   from [STRINGPREP].

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

   In addition, the following Unicode characters are also prohibited:

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

A.6.  Bidirectional Characters

   This profile specifies checking bidirectional strings, as described
   in Section 6 of [STRINGPREP].


Appendix B.  Resourceprep

B.1.  Introduction

   This appendix defines the "Resourceprep" profile of [STRINGPREP].  As
   such, it specifies processing rules that will enable users to enter
   internationalized resource identifiers in the Extensible Messaging
   and Presence Protocol (XMPP) and have the highest chance of getting
   the content of the strings correct.  (An XMPP resource identifier is
   the optional portion of an XMPP address that follows a domain
   identifier and the '/' separator; it is often but not exclusively
   associated with an instant messaging session name.)  These processing
   rules are intended only for XMPP resource identifiers and are not
   intended for arbitrary text or any other aspect of an XMPP address.



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   This profile defines the following, as required by [STRINGPREP]:

   o  The intended applicability of the profile: internationalized
      resource identifiers within XMPP
   o  The character repertoire that is the input and output to
      stringprep: Unicode 3.2, specified in Section 2 of this Appendix
   o  The mappings used: specified in Section 3
   o  The Unicode normalization used: specified in Section 4
   o  The characters that are prohibited as output: specified in Section
      5
   o  Bidirectional character handling: specified in Section 6

   Note: Both the UseSTD3ASCIIRules and AllowUnassigned flags MUST be
   set to true.

B.2.  Character Repertoire

   This profile uses Unicode 3.2 with the list of unassigned code points
   being Table A.1, both defined in Appendix A of [STRINGPREP].

B.3.  Mapping

   This profile specifies mapping using the following tables from
   [STRINGPREP]:

      Table B.1

B.4.  Normalization

   This profile specifies the use of Unicode normalization form KC, as
   described in [STRINGPREP].

B.5.  Prohibited Output

   This profile specifies the prohibition of using the following tables
   from [STRINGPREP].

      Table C.1.2
      Table C.2.1
      Table C.2.2
      Table C.3
      Table C.4
      Table C.5
      Table C.6







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      Table C.7
      Table C.8
      Table C.9

B.6.  Bidirectional Characters

   This profile specifies checking bidirectional strings, as described
   in Section 6 of [STRINGPREP].


Appendix C.  Server Dialback

C.1.  Overview

   The Jabber protocols from which XMPP was adapted include a "server
   dialback" method for protecting against domain spoofing, thus making
   it more difficult to spoof XML stanzas.  Server dialback is not a
   security mechanism, and results in weak verification of server
   identities only (see Server-to-Server Communications (Section 14.4)
   regarding this method's security characteristics).  Domains requiring
   robust security SHOULD use TLS and SASL; see Server-to-Server
   Communications (Section 14.4) for details.  If SASL is used for
   server-to-server authentication, dialback SHOULD NOT be used since it
   is unnecessary.  Documentation of dialback is included mainly for the
   sake of backward-compatibility with existing implementations and
   deployments.  However, depending on local policies, a service may
   wish to use dialback to provide weak verification in cases where SASL
   negotiation would not result in strong authentication (e.g., because
   the certificate presented by the peer service during TLS negotiation
   is self-signed and thus provides only weak identity).

   The server dialback method is made possible by the existence of the
   Domain Name System (DNS), since one server can (normally) discover
   the authoritative server for a given domain.  Because dialback
   depends on DNS, inter-domain communications MUST NOT proceed until
   the Domain Name System (DNS) hostnames asserted by the servers have
   been resolved (see Server-to-Server Communications (Section 14.4)).

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

   The method for generating and verifying the keys used in server
   dialback MUST take into account the hostnames being used, the stream
   ID generated by the receiving server, and a secret known by the



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   authoritative server's network; see Appendix C.4 for the recommended
   algorithm.

   Any error that occurs during dialback negotiation MUST be considered
   a stream error, resulting in termination of the stream and of the
   underlying TCP connection.  The possible error conditions are
   specified in the protocol description below.

   The following terminology applies:

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

C.2.  Order of Events

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

   1.  The Originating Server establishes a connection to the Receiving
       Server.
   2.  The Originating Server sends a 'key' value over the connection to
       the Receiving Server.
   3.  The Receiving Server establishes a connection to the
       Authoritative Server.
   4.  The Receiving Server sends the same 'key' value to the
       Authoritative Server.
   5.  The Authoritative Server replies that key is valid or invalid.
   6.  The Receiving Server informs the Originating Server whether it is
       authenticated or not.

   We can represent this flow of events graphically as follows:














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   Originating               Receiving
     Server                    Server
   -----------               ---------
       |                         |
       |   establish connection  |
       | ----------------------> |
       |                         |
       |   send stream header    |
       | ----------------------> |
       |                         |
       |   send stream header    |
       | <---------------------- |
       |                         |                   Authoritative
       |   send dialback key     |                       Server
       | ----------------------> |                   -------------
       |                         |                         |
                                 |   establish connection  |
                                 | ----------------------> |
                                 |                         |
                                 |   send stream header    |
                                 | ----------------------> |
                                 |                         |
                                 |   send stream header    |
                                 | <---------------------- |
                                 |                         |
                                 |   send verify request   |
                                 | ----------------------> |
                                 |                         |
                                 |   send verify response  |
                                 | <---------------------- |
                                 |
       |  report dialback result |
       | <---------------------- |
       |                         |

C.3.  Protocol

   The detailed protocol interaction between the servers is as follows:

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








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   <stream:stream
       xmlns='jabber:server'
       xmlns:db='jabber:server:dialback'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='Originating Server'
       to='Receiving Server'>

        Note: The inclusion of the xmlns:db namespace declaration with
        the name shown indicates to the Receiving Server that the
        Originating Server supports dialback.  If the namespace name is
        incorrect, then the Receiving Server MUST generate an <invalid-
        namespace/> stream error condition and terminate both the XML
        stream and the underlying TCP connection.
   3.   The Receiving Server SHOULD send a stream header back to the
        Originating Server, including a unique ID for this interaction:

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

        If the namespace name is incorrect, then the Originating Server
        MUST generate an <invalid-namespace/> stream error condition and
        terminate both the XML stream and the underlying TCP connection.
        Note well that the Receiving Server SHOULD reply but MAY
        silently terminate the XML stream and underlying TCP connection
        depending on security policies in place; however, if the
        Receiving Server desires to proceed, it MUST send a stream
        header back to the Originating Server.
   4.   The Receiving Server sends stream features to the Originating
        Server, including the dialback feature as described under
        Appendix C.5 (RECOMMENDED):

   <stream:features>
     <dialback xmlns='http://jabber.org/features/dialback'>
       <required/>
     </dialback>
   </stream:features>

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







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

        Note: This key is not examined by the Receiving Server, since
        the Receiving Server does not keep information about the
        Originating Server between sessions.  The key generated by the
        Originating Server MUST be based in part on the value of the ID
        provided by the Receiving Server in the previous step, and in
        part on a secret shared by the Originating Server and the
        Authoritative Server.  If the value of the 'to' address does not
        match a hostname recognized by the Receiving Server, then the
        Receiving Server MUST generate a <host-unknown/> stream error
        condition and terminate both the XML stream and the underlying
        TCP connection.  If the value of the 'from' address matches a
        domain with which the Receiving Server already has an
        established connection, then the Receiving Server MUST maintain
        the existing connection until it validates whether the new
        connection is legitimate; additionally, the Receiving Server MAY
        choose to generate a <not-authorized/> stream error condition
        for the new connection and then terminate both the XML stream
        and the underlying TCP connection related to the new request.
   6.   The Receiving Server establishes a TCP connection back to the
        domain name asserted by the Originating Server, as a result of
        which it connects to the Authoritative Server.  (Note: As an
        optimization, an implementation MAY reuse an existing TCP
        connection here.)
   7.   The Receiving Server sends the Authoritative Server a stream
        header:

   <stream:stream
       xmlns='jabber:server'
       xmlns:db='jabber:server:dialback'
       xmlns:stream='http://etherx.jabber.org/streams'
       from='Receiving Server'
       to='Authoritative Server'>

        Note: If the namespace name is incorrect, then the Authoritative
        Server MUST generate an <invalid-namespace/> stream error
        condition and terminate both the XML stream and the underlying
        TCP connection.
   8.   The Authoritative Server sends the Receiving Server a stream
        header:






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

        Note: If the namespace name is incorrect, then the Receiving
        Server MUST generate an <invalid-namespace/> stream error
        condition and terminate both the XML stream and the underlying
        TCP connection between it and the Authoritative Server.  If a
        stream error occurs between the Receiving Server and the
        Authoritative Server, then the Receiving Server MUST generate a
        <remote-connection-failed/> stream error condition and terminate
        both the XML stream and the underlying TCP connection between it
        and the Originating Server.
   9.   The Receiving Server sends the Authoritative Server a request
        for verification of a key:

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

        Note: Passed here are the hostnames, the original identifier
        from the Receiving Server's stream header to the Originating
        Server in Step 3, and the key that the Originating Server sent
        to the Receiving Server in Step 4.  Based on this information,
        as well as shared secret information within the Authoritative
        Server's network, the key is verified.  Any verifiable method
        MAY be used to generate the key; however, the method specified
        in Appendix C.4 is RECOMMENDED.  If the value of the 'to'
        address does not match a hostname recognized by the
        Authoritative Server, then the Authoritative Server MUST
        generate a <host-unknown/> stream error condition and terminate
        both the XML stream and the underlying TCP connection.  If the
        value of the 'from' address does not match the hostname
        represented by the Receiving Server when opening the TCP
        connection (or any validated domain thereof, such as a validated
        subdomain of the Receiving Server's hostname or another
        validated domain hosted by the Receiving Server), then the
        Authoritative Server MUST generate an <invalid-from/> stream
        error condition and terminate both the XML stream and the
        underlying TCP connection.




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

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

        or

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

        Note: If the ID does not match that provided by the Receiving
        Server in Step 3, then the Receiving Server MUST generate an
        <invalid-id/> stream error condition and terminate both the XML
        stream and the underlying TCP connection.  If the value of the
        'to' address does not match a hostname recognized by the
        Receiving Server, then the Receiving Server MUST generate a
        <host-unknown/> stream error condition and terminate both the
        XML stream and the underlying TCP connection.  If the value of
        the 'from' address does not match the hostname represented by
        the Originating Server when opening the TCP connection (or any
        validated domain thereof, such as a validated subdomain of the
        Originating Server's hostname or another validated domain hosted
        by the Originating Server), then the Receiving Server MUST
        generate an <invalid-from/> stream error condition and terminate
        both the XML stream and the underlying TCP connection.  After
        returning the verification to the Receiving Server, the
        Authoritative Server SHOULD terminate the stream between them.
   11.  The Receiving Server informs the Originating Server of the
        result:

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

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



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        to the Receiving Server SHOULD be silently dropped.

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

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

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

C.4.  Dialback Key Generation

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

   o  the hostname of the Originating Server
   o  the hostname of the Receiving Server
   o  the Stream ID
   o  a shared secret known by the Authoritative Server's network

   The stream ID is security-critical in server dialback and therefore
   MUST be both unpredictable and non-repeating (see [RANDOM] for
   recommendations regarding randomness for security purposes).

   It is RECOMMENDED for the dialback key to be the hexadecimal
   representation of a Keyed-Hash Message Authentication Code (see
   [HMAC]) that uses the SHA-256 algorithm (see [SHA]), as follows:



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

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

   Consider the following scenario:

   o  The Originating Server is "example.com"
   o  The Receiving Server is "example.net"
   o  The Stream ID is "D60000229F"
   o  The secret is "s3cr3tf0rd14lb4ck"

   The resulting dialback key would be:

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

   that is,

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

   that is,

   HMAC-SHA256
   ('s3cr3tf0rd14lb4ck',{'example.netexample.comD60000229F'})

   that is,

   HMAC-SHA256
   ('s3cr3tf0rd14lb4ckexample.netexample.comD60000229F'})

   that is,

   4184830b1e166e2e072fb2f51817d1df052ff52796bc6df1a61beea3aa113fe5

C.5.  Advertisement

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

   1.  By inclusion of the server dialback feature in a given set of
       stream features.





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   2.  By inclusion of the dialback namespace declaration in the stream
       header.

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

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

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

   <stream:features>
     <dialback xmlns='http://jabber.org/features/dialback'>
       <required/>
     </dialback>
   </stream:features>

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

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

   No matter which method is used, a service SHOULD advertise support
   for server dialback only at a point in the stream negotiation when it
   will accept server dialback negotiations.  For example, if a service
   wishes to be backwards-compatible with older "XMPP 0.9" deployments,
   it should include the server dialback namespace declaration in the
   initial stream header it sends to other servers; whereas in the midst
   of negotiation with an XMPP 1.0 or higher server (e.g., after TLS
   negotiation), it should advertise the dialback stream feature.


Appendix D.  XML Schemas

   The following XML schemas are descriptive, not normative.  For
   schemas defining the 'jabber:client' and 'jabber:server' namespaces,



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   refer to [XMPP-IM].

D.1.  Streams namespace

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

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

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

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



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         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

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

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

   </xs:schema>

D.2.  Stream error namespace

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

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

     <xs:element name='bad-format' type='empty'/>
     <xs:element name='bad-namespace-prefix' type='empty'/>
     <xs:element name='conflict' type='empty'/>
     <xs:element name='connection-timeout' type='empty'/>
     <xs:element name='host-gone' type='empty'/>
     <xs:element name='host-unknown' type='empty'/>
     <xs:element name='improper-addressing' type='empty'/>
     <xs:element name='internal-server-error' type='empty'/>
     <xs:element name='invalid-from' type='empty'/>
     <xs:element name='invalid-id' type='empty'/>
     <xs:element name='invalid-namespace' type='empty'/>
     <xs:element name='invalid-xml' type='empty'/>
     <xs:element name='not-authorized' type='empty'/>
     <xs:element name='policy-violation' type='empty'/>
     <xs:element name='remote-connection-failed' type='empty'/>



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     <xs:element name='resource-constraint' type='empty'/>
     <xs:element name='restricted-xml' type='empty'/>
     <xs:element name='see-other-host' type='xs:string'/>
     <xs:element name='system-shutdown' type='empty'/>
     <xs:element name='undefined-condition' type='empty'/>
     <xs:element name='unsupported-encoding' type='empty'/>
     <xs:element name='unsupported-stanza-type' type='empty'/>
     <xs:element name='unsupported-version' type='empty'/>
     <xs:element name='xml-not-well-formed' type='empty'/>

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

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



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

   </xs:schema>

D.3.  TLS namespace

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

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

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

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

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

   </xs:schema>

D.4.  SASL namespace

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

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



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

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

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

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

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



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       </xs:restriction>
     </xs:simpleType>

   </xs:schema>

D.5.  Resource binding namespace













































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

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

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

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

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

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

   </xs:schema>




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D.6.  Dialback namespace


















































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

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

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

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

   </xs:schema>



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D.7.  Server dialback stream feature namespace

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

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

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

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

   </xs:schema>

D.8.  Stanza error namespace

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

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

     <xs:element name='bad-request' type='empty'/>
     <xs:element name='conflict' type='empty'/>
     <xs:element name='feature-not-implemented' type='empty'/>
     <xs:element name='forbidden' type='empty'/>
     <xs:element name='gone' type='xs:string'/>
     <xs:element name='internal-server-error' type='empty'/>
     <xs:element name='item-not-found' type='empty'/>
     <xs:element name='jid-malformed' type='empty'/>
     <xs:element name='not-acceptable' type='empty'/>
     <xs:element name='not-allowed' type='empty'/>



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     <xs:element name='not-modified' type='empty'/>
     <xs:element name='payment-required' type='empty'/>
     <xs:element name='recipient-unavailable' type='empty'/>
     <xs:element name='redirect' type='xs:string'/>
     <xs:element name='registration-required' type='empty'/>
     <xs:element name='remote-server-not-found' type='empty'/>
     <xs:element name='remote-server-timeout' type='empty'/>
     <xs:element name='resource-constraint' type='empty'/>
     <xs:element name='service-unavailable' type='empty'/>
     <xs:element name='subscription-required' type='empty'/>
     <xs:element name='undefined-condition' type='empty'/>
     <xs:element name='unexpected-request' type='empty'/>

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

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



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     </xs:element>

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

   </xs:schema>


Appendix E.  Contact Addresses

   Consistent with [MAILBOXES], an organization that offers an XMPP
   service SHOULD provide an Internet mailbox of "XMPP" for inquiries
   related to that service, where the host portion of the resulting
   mailto URI SHOULD be the organization's domain, not necessarily the
   domain of the XMPP service itself (e.g., the XMPP service might be
   offered at jabber.example.com but the Internet mailbox should be
   <xmpp@example.com>).

   In addition, the mere domain of the XMPP service SHOULD be used as an
   alias for the server administrators when routing <message/> stanzas,
   such that an XMPP message addressed to the service's domain name
   (e.g., "jabber.example.com") shall be delivered to the JIDs of the
   server administrators (this does not apply to <iq/> or <presence/>
   stanzas).


Appendix F.  Differences From RFC 3920

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

   o  Corrected the ABNF syntax for JIDs to prevent zero-length node
      identifiers, domain identifiers, and resource identifiers.
   o  Corrected the stringprep processing rules to require use of the
      UseSTD3ASCIIRules and AllowUnassigned flags.
   o  Encouraged use of the 'from' and 'to' attributes stream headers.
   o  More clearly specified stream closing handshake.
   o  Specified recommended stream reconnection algorithm.
   o  Specified return of <restricted-xml/> stream error in response to
      receipt of restricted XML.
   o  Specified that SASL mechanisms must be sent both before and after
      negotiation of TLS and of SASL security layers.




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   o  Specified that TLS plus SASL PLAIN is a mandatory-to-implement
      technology for client-to-server connections, since implementation
      of SASL EXTERNAL is uncommon in XMPP clients, in part because
      underlying security features such as X.509 certificates are not
      yet widely deployed.
   o  Added the <malformed-request/> SASL error condition to handle an
      error case discussed in RFC 4422.
   o  More clearly specified binding of multiple resources to the same
      stream.
   o  Added the <not-modified/> stanza error condition to enable
      potential ETags usage.
   o  Added section on advertisement of server dialback support,
      including server dialback stream feature.
   o  Recommended use of HMAC-SHA256 for generation of server dialback
      key.


Author's Address

   Peter Saint-Andre (editor)
   Jabber Software Foundation

   Email: stpeter@jabber.org
   URI:   xmpp:stpeter@jabber.org



























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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
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   Administrative Support Activity (IASA).





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