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Versions: (draft-miller-xmpp-core) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 RFC 3920

Network Working Group                                     P. Saint-Andre
Internet-Draft                                                 J. Miller
Expires: August 27, 2003                      Jabber Software Foundation
                                                       February 26, 2003


                               XMPP Core
                        draft-ietf-xmpp-core-04

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at http://
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 27, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document describes the core features of the Extensible Messaging
   and Presence Protocol (XMPP), a protocol for streaming XML in near-
   real-time that is used mainly for the purpose of instant messaging
   (IM) and presence by the servers, clients, and other applications
   that comprise the Jabber network.









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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.2   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.3   Discussion Venue . . . . . . . . . . . . . . . . . . . . . .  4
   1.4   Intellectual Property Notice . . . . . . . . . . . . . . . .  4
   2.    Generalized Architecture . . . . . . . . . . . . . . . . . .  5
   2.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.2   Server . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.3   Client . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.4   Gateway  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.5   Network  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.    Addressing Scheme  . . . . . . . . . . . . . . . . . . . . .  7
   3.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.2   Domain Identifier  . . . . . . . . . . . . . . . . . . . . .  7
   3.3   Node Identifier  . . . . . . . . . . . . . . . . . . . . . .  7
   3.4   Resource Identifier  . . . . . . . . . . . . . . . . . . . .  8
   4.    XML Streams  . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.2   Restrictions . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.3   Stream Attributes  . . . . . . . . . . . . . . . . . . . . . 10
   4.4   Namespace Declarations . . . . . . . . . . . . . . . . . . . 11
   4.5   Stream Features  . . . . . . . . . . . . . . . . . . . . . . 12
   4.6   Stream Errors  . . . . . . . . . . . . . . . . . . . . . . . 12
   4.7   Simple Streams Example . . . . . . . . . . . . . . . . . . . 14
   5.    Stream Encryption  . . . . . . . . . . . . . . . . . . . . . 16
   5.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   5.2   Narrative  . . . . . . . . . . . . . . . . . . . . . . . . . 17
   5.3   Client-Server Protocol . . . . . . . . . . . . . . . . . . . 17
   5.4   Certificate-Based Authentication . . . . . . . . . . . . . . 19
   6.    Stream Authentication  . . . . . . . . . . . . . . . . . . . 20
   6.1   SASL Authentication  . . . . . . . . . . . . . . . . . . . . 20
   6.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   6.1.2 Narrative  . . . . . . . . . . . . . . . . . . . . . . . . . 21
   6.1.3 SASL Definition  . . . . . . . . . . . . . . . . . . . . . . 22
   6.1.4 Client-Server Protocol . . . . . . . . . . . . . . . . . . . 23
   6.2   Dialback Authentication  . . . . . . . . . . . . . . . . . . 25
   6.2.1 Dialback Protocol  . . . . . . . . . . . . . . . . . . . . . 27
   7.    XML Stanzas  . . . . . . . . . . . . . . . . . . . . . . . . 31
   7.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 31
   7.2   Common Attributes  . . . . . . . . . . . . . . . . . . . . . 31
   7.2.1 to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
   7.2.2 from . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
   7.2.3 id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
   7.2.4 type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
   7.2.5 xml:lang . . . . . . . . . . . . . . . . . . . . . . . . . . 32
   7.3   Message Stanzas  . . . . . . . . . . . . . . . . . . . . . . 32



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   7.3.1 Types of Message . . . . . . . . . . . . . . . . . . . . . . 32
   7.3.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 33
   7.4   Presence Stanzas . . . . . . . . . . . . . . . . . . . . . . 34
   7.4.1 Types of Presence  . . . . . . . . . . . . . . . . . . . . . 34
   7.4.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   7.5   IQ Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 36
   7.5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 36
   7.5.2 Types of IQ  . . . . . . . . . . . . . . . . . . . . . . . . 37
   7.5.3 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 38
   7.6   Extended Namespaces  . . . . . . . . . . . . . . . . . . . . 38
   8.    XML Usage within XMPP  . . . . . . . . . . . . . . . . . . . 40
   8.1   Namespaces . . . . . . . . . . . . . . . . . . . . . . . . . 40
   8.2   Validation . . . . . . . . . . . . . . . . . . . . . . . . . 40
   8.3   Character Encodings  . . . . . . . . . . . . . . . . . . . . 40
   8.4   Inclusion of Text Declaration  . . . . . . . . . . . . . . . 40
   9.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 41
   10.   Internationalization Considerations  . . . . . . . . . . . . 42
   11.   Security Considerations  . . . . . . . . . . . . . . . . . . 43
   11.1  Client-to-Server Communications  . . . . . . . . . . . . . . 43
   11.2  Server-to-Server Communications  . . . . . . . . . . . . . . 43
   11.3  Firewalls  . . . . . . . . . . . . . . . . . . . . . . . . . 43
   11.4  Minimum Security Mechanisms  . . . . . . . . . . . . . . . . 43
         References . . . . . . . . . . . . . . . . . . . . . . . . . 45
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 47
   A.    Standard Error Codes . . . . . . . . . . . . . . . . . . . . 48
   B.    XML Schemas  . . . . . . . . . . . . . . . . . . . . . . . . 51
   B.1   streams namespace  . . . . . . . . . . . . . . . . . . . . . 51
   B.2   SASL namespace . . . . . . . . . . . . . . . . . . . . . . . 52
   B.3   Dialback namespace . . . . . . . . . . . . . . . . . . . . . 52
   B.4   jabber:client namespace  . . . . . . . . . . . . . . . . . . 54
   B.5   jabber:server namespace  . . . . . . . . . . . . . . . . . . 56
   C.    Revision History . . . . . . . . . . . . . . . . . . . . . . 60
   C.1   Changes from draft-ietf-xmpp-core-03 . . . . . . . . . . . . 60
   C.2   Changes from draft-ietf-xmpp-core-02 . . . . . . . . . . . . 60
   C.3   Changes from draft-ietf-xmpp-core-01 . . . . . . . . . . . . 60
   C.4   Changes from draft-ietf-xmpp-core-00 . . . . . . . . . . . . 60
   C.5   Changes from draft-miller-xmpp-core-02 . . . . . . . . . . . 61
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 63













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1. Introduction

1.1 Overview

   The Extensible Messaging and Presence Protocol (XMPP) is an open XML
   [1] protocol for near-real-time messaging, presence, and request-
   response services.  The protocol was developed originally within the
   Jabber community starting in 1998, and since 2001 has continued to
   evolve under the auspices of the Jabber Software Foundation [2] and
   now the XMPP WG.  The current document defines the core features of
   XMPP; XMPP IM [3] defines the extensions necessary to provide the
   instant messaging (IM) and presence functionality defined in RFC 2779
   [4].

1.2 Terminology

   The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [5].

1.3 Discussion Venue

   The authors welcome discussion and comments related to the topics
   presented in this document.  The preferred forum is the
   <xmppwg@jabber.org> mailing list, for which archives and subscription
   information are available at <http://www.jabber.org/cgi-bin/mailman/
   listinfo/xmppwg/>.

1.4 Intellectual Property Notice

   This document is in full compliance with all provisions of Section 10
   of RFC 2026.  Parts of this specification use the term "jabber" for
   identifying namespaces and other protocol syntax.  Jabber[tm] is a
   registered trademark of Jabber, Inc.  Jabber, Inc.  grants permission
   to the IETF for use of the Jabber trademark in association with this
   specification and its successors, if any.














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2. Generalized Architecture

2.1 Overview

   Although XMPP is not wedded to any specific network architecture, to
   this point it has usually been implemented via a typical client-
   server architecture, wherein a client utilizing XMPP accesses a
   server over a TCP [6] socket.

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

   C1 -  S1 - S2 - C3
        /  \
   C2 -     G1 = FN1 = FC1

   The symbols are as follows:

   o  C1, C2, C3 -- XMPP clients

   o  S1, S2 -- XMPP servers

   o  G1 -- A gateway that translates between XMPP and the protocol(s)
      used on a foreign (non-XMPP) messaging network

   o  FN1 -- A foreign messaging network

   o  FC1 -- A client on a foreign messaging network


2.2 Server

   A server acts as an intelligent abstraction layer for XMPP
   communications.  Its primary responsibilities are to manage
   connections from or sessions for other entities (in the form of XML
   streams to and from authorized clients, servers, and other entities)
   and to route appropriately-addressed XML data "stanzas" among such
   entities over XML streams.  Most XMPP-compliant servers also assume
   responsibility for the storage of data that is used by clients (e.g.,
   contact lists for users of XMPP-based IM applications); in this case,
   the XML data is processed directly by the server itself on behalf of
   the client and is not routed to another entity.  Compliant server
   implementations MUST ensure in-order processing of XML stanzas
   received from connected clients, servers, and services.






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2.3 Client

   Most clients connect directly to a server over a TCP socket and use
   XMPP to take full advantage of the functionality provided by a server
   and any associated services, although it must be noted that there is
   no necessary coupling of an XML stream to a TCP socket (e.g., a
   client COULD connect via HTTP polling or some other mechanism).
   Multiple resources (e.g., devices or locations) MAY connect
   simultaneously to a server on behalf of each authorized client, with
   each resource connecting over a discrete TCP socket and
   differentiated by the resource identifier of a JID (Section 3) (e.g.,
   user@domain/home vs.  user@domain/work).  The port assigned by the
   IANA [7] for connections between a Jabber client and a Jabber server
   is 5222.  For further details about client-to-server communications
   expressly for the purpose of instant messaging and presence, refer to
   XMPP IM [3].

2.4 Gateway

   A gateway is a special-purpose server-side service whose primary
   function is to translate XMPP into the protocol(s) of another
   messaging system, as well as to translate the return data back into
   XMPP.  Examples are gateways to Internet Relay Chat (IRC), Short
   Message Service (SMS), SMTP, and foreign instant messaging networks
   such as Yahoo!, MSN, ICQ, and AIM.  Communications between gateways
   and servers, and between gateways and the foreign messaging system,
   are not defined in this document.

2.5 Network

   Because each server is identified by a network address (typically a
   DNS hostname) and because server-to-server communications are a
   straightforward extension of the client-to-server protocol, in
   practice the system consists of a network of servers that inter-
   communicate.  Thus user-a@domain1 is able to exchange messages,
   presence, and other information with user-b@domain2.  This pattern is
   familiar from messaging protocols (such as SMTP) that make use of
   network addressing standards.  The usual method for providing a
   connection between two servers is to open a TCP socket on the IANA-
   assigned port 5269 and to negotiate a connection using the Dialback
   Protocol (Section 6.2) defined in this document.










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3. Addressing Scheme

3.1 Overview

   An entity is anything that can be considered a network endpoint
   (i.e., an ID on the network) and that can communicate using XMPP.
   All such entities are uniquely addressable in a form that is
   consistent with RFC 2396 [8].  In particular, a valid Jabber
   Identifier (JID) contains a set of ordered elements formed of a
   domain identifier, node identifier, and resource identifier in the
   following format: [node@]domain[/resource].

   All JIDs are based on the foregoing structure.  The most common use
   of this structure is to identify an IM user, the server to which the
   user connects, and the user's active session or connection (e.g., a
   specific client) in the form of user@domain/resource.  However, node
   types other than clients are possible; for example, a specific chat
   room offered by a multi-user chat service could be addressed as
   room@service (where "room" is the name of the chat room and "service"
   is the hostname of the multi-user chat service) and a specific
   occupant of such a room could be addressed as room@service/nick
   (where "nick" is the occupant's room nickname).

3.2 Domain Identifier

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

   The domain identifier for every server or service that will
   communicate over a network SHOULD resolve to a Fully Qualified Domain
   Name.  A domain identifier MUST conform to RFC 952 [9] and RFC 1123
   [10].  A domain identifier MUST be no more than 1023 bytes in length,
   and is subject to comparison in accordance with the rules defined in
   the nameprep [11] profile of stringprep [12].

3.3 Node Identifier

   The node identifier is an optional secondary identifier.  It usually
   represents the entity requesting and using network access provided by
   the server or gateway (i.e., a client), although it can also
   represent other kinds of entities (e.g., a multi-user chat room



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   associated with a multi-user chat service).  The entity represented
   by a node identifier is addressed within the context of a specific
   domain; in the context of IM users this address is called a "bare
   JID" and is of the form <user@domain>.

   A node identifier MUST be no more than 1023 bytes in length and MUST
   conform to the nodeprep [13] profile of stringprep [12].

3.4 Resource Identifier

   The resource identifer is an optional third identifier.  It
   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.  An entity
   may maintain multiple resources simultaneously.

   A resource identifier MUST be no more than 1023 bytes in length and
   MUST conform to the resourceprep [14] profile of stringprep [12].

































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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, as a result,
   discrete units of structured information that are referred to as "XML
   stanzas".  (Note: in this overview we use the example of
   communications between a client and server; however XML streams are
   more generalized and may be used for communications from server to
   server and from service to server as well.)

   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 supported and the
   character encoding.  A compliant entity SHOULD accept any namespace
   prefix on the <stream/> element; however, for historical reasons some
   entities MAY accept only a 'stream' prefix, resulting in use of a
   <stream:stream/> element.  The server SHOULD then reply with a second
   XML stream back to the client, again optionally preceded by a text
   declaration.

   Within the context of an XML stream, a sender is able to send a
   discrete semantic unit of structured information to any recipient.
   This unit of structured information is a well-balanced XML stanza,
   such as a message, presence, or IQ stanza (a stanza of an XML
   document is said to be well-balanced if it matches production [43]
   content of the XML specification [1]).  These stanzas exist at the
   direct child level of the root <stream/> element.  The start of any
   XML stanza is unambiguously denoted by the element start tag at
   depth=1 (e.g., <presence>), and the end of any XML stanza is
   unambiguously denoted by the corresponding close tag at depth=1
   (e.g., </presence>).  Each XML stanza MAY contain child elements or
   CDATA sections as necessary in order to convey the desired
   information from the sender to the recipient.  The session is closed
   at the client's request by sending a closing </stream> tag to the
   server (a session may also be closed by the server).

   Thus a client's session with a server can be seen as two open-ended
   XML documents that are built up through the accumulation of the XML
   stanzas sent over the course of the session (one from the client to
   the server and one from the server to the client), and the root
   <stream/> element can be considered the document entity for those
   streams.  In essence, then, an XML stream acts as an envelope for all
   the XML stanzas sent during a session.  We can represent this
   graphically as follows:




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


4.2 Restrictions

   XML streams are used to transport a subset of XML.  Specifically, XML
   streams SHOULD NOT contain processing instructions, predefined
   entities (as defined in Section 4.6 of the XML specification [1]),
   comments, or DTDs.  Any such XML data SHOULD be ignored by a
   compliant implementation.

4.3 Stream Attributes

   The attributes of the stream element are as follows (we now
   generalize the endpoints by using the terms "initiating entity" and
   "receiving entity"):

   o  to -- The 'to' attribute SHOULD be used only in the XML stream
      from the initiating entity to the receiving entity, and MUST be
      set to the JID of the receiving entity.  There SHOULD be no 'to'
      attribute set in the XML stream by which the receiving entity
      replies to the initiating entity; however, if a 'to' attribute is
      included, it SHOULD be ignored by the initiating entity.

   o  from -- The 'from' attribute SHOULD be used only in the XML stream
      from the receiving entity to the initiating entity, and MUST be
      set to the JID of the receiving entity granting access to the
      initiating entity.  There SHOULD be no 'from' attribute on the XML
      stream sent from the initiating entity to the receiving entity;
      however, if a 'from' attribute is included, it SHOULD be ignored



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      by the receiving entity.

   o  id -- The 'id' attribute SHOULD be used only in the XML stream
      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 session
      with the receiving entity.  There SHOULD be no 'id' attribute on
      the XML stream sent from the initiating entity to the receiving
      entity; however, if an 'id' attribute is included, it SHOULD be
      ignored by the receiving entity.

   o  version -- The 'version' attribute MAY be used in the XML stream
      from the initiating entity to the receiving entity in order signal
      compliance with the protocol defined herein; this is done by
      setting the value of the attribute to "1.0".  If the initiating
      entity includes the version attribute, the receiving entity MUST
      reciprocate by including the attribute in its response (if the
      receiving entity supports XMPP 1.0).

   We can summarize these values as follows:

           |  initiating to receiving  |  receiving to initiating
   ------------------------------------------------------------
   to      |  JID of receiver          |  ignored
   from    |  ignored                  |  JID of receiver
   id      |  ignored                  |  session key
   version |  signals XMPP 1.0 support |  signals XMPP 1.0 support


4.4 Namespace Declarations

   The stream element MAY also contain namespace declarations as defined
   in the XML namespaces specification [15].

   A default namespace declaration ('xmlns') is REQUIRED and is used in
   both XML streams in order to scope the allowable first-level children
   of the root stream element for both streams.  This namespace
   declaration MUST be the same for the initiating stream and the
   responding stream so that both streams are scoped consistently.  The
   default namespace declaration applies to the stream and all stanzas
   sent within a stream.

   A stream namespace declaration (e.g., 'xmlns:stream') is REQUIRED in
   both XML streams.  A compliant entity SHOULD accept any namespace
   prefix on the <stream/> element; however, for historical reasons some
   entities MAY accept only a 'stream' prefix, resulting in use of a
   <stream:stream/> element as the stream root.  The name of the stream
   namespace MUST be "http://etherx.jabber.org/streams".



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   XML streams function as containers for any XML stanzas sent
   asynchronously between network endpoints.  It should be possible to
   scope an XML stream with any default namespace declaration, i.e., it
   should be possible to send any properly-namespaced XML stanza over an
   XML stream.  A compliant implementation MUST support the following
   two namespaces (for historical reasons, existing implementations MAY
   support only these two default namespaces):

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

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

   The jabber:client and jabber:server namespaces are nearly identical
   but are used in different contexts (client-to-server communications
   for jabber:client and server-to-server communications for
   jabber:server).  The only difference between the two is that the 'to'
   and 'from' attributes are OPTIONAL on stanzas sent within
   jabber:client, whereas they are REQUIRED on stanzas sent within
   jabber:server.  If a compliant implementation accepts a stream that
   is scoped by the 'jabber:client' or 'jabber:server' namespace, it
   MUST support all three core stanza types (message, presence, and IQ)
   as described herein and defined in the schema.

4.5 Stream Features

   The root stream element MAY contain a features child element (e.g.,
   <stream:features/> if the stream namespace prefix is 'stream').  This
   is used to communicate generic stream-level capabilities including
   stream-level features that can be negotiated as the streams are set
   up.  If the initiating entity sends a "version='1.0'" attribute in
   its initiating stream element, the receiving entity MUST send a
   features child element to the initiating entity if there are any
   capabilities that need to be advertised or features that can be
   negotiated for the stream.  Currently this is used for SASL and TLS
   negotiation only, but it could be used for other negotiable features
   in the future (usage is defined under Stream Encryption (Section 5)
   and Stream Authentication (Section 6) below).  If an entity does not
   understand or support some features, it SHOULD ignore them.

4.6 Stream Errors

   The root stream element MAY contain an error child element (e.g.,
   <stream:error/> if the stream namespace prefix is 'stream').  The
   error child MUST be sent by a Jabber entity (usually a server rather
   than a client) if it perceives that a stream-level error has
   occurred.  Examples of error conditions include the sending of



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   invalid XML, the shutdown of a server, an internal server error such
   as the shutdown of a session manager, and inclusion of an unsupported
   version number in the initiating stream header.  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 and then send a closing
   </stream> tag.  Specifically, XML of the following form is sent
   within the context of an existing stream (the error element MUST
   possess the 'code' attribute):

   <stream:stream ...>
   ...
   <stream:error code='400'/>
   </stream:stream>

   If the error occurs while the stream is being set up, the receiving
   entity MUST still send the opening and closing stream tags and
   include the error element as a child of the stream element.  The
   following example illustrates this principle (where the "C" lines are
   sent from the client to the server, and the "S" lines are sent from
   the server to the client):

   C: <stream:stream
          to='somedomain'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='bad.version'>
   S: <stream:stream
          from='somedomain'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   S: <stream:error code='505'/>
   S: </stream:stream>

   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.

   The following codes are defined for stream-level errors:

   o  302 - Redirect

   o  400 - Bad XML

   o  404 - Unknown Host




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   o  410 - Gone

   o  500 - Internal Server Error

   o  505 - Version Not Supported

   If the error is 302 ("Redirect"), the server SHOULD include CDATA
   specifying the alternate hostname or IP address to which the
   initiating entity may attempt to connect.

4.7 Simple Streams Example

   The following is a stream-based session of a client on a server
   (where the "C" lines are sent from the client to the server, and the
   "S" lines are sent from the server to the client):

   A basic session:

   C: <?xml version='1.0'?>
      <stream:stream
          to='server'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   S: <?xml version='1.0'?>
      <stream:stream
          from='server'
          id='id_123456789'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   ... authentication ...
   C:   <message from='alex@graham-bell' to='watson@graham-bell'>
   C:     <body>Watson come here, I want you!</body>
   C:   </message>
   S:   <message from='watson@graham-bell' to='alex@graham-bell'>
   S:     <body>I'm on my way!</body>
   S:   </message>
   C: </stream:stream>
   S: </stream:stream>

   These are in actuality a sending stream and a receiving stream, which
   can be viewed a-chronologically as two XML documents:








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   C: <?xml version='1.0'?>
      <stream:stream
          to='server'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   C:   <message from='alex@graham-bell' to='watson@graham-bell'>
   C:     <body>Watson come here, I want you!</body>
   C:   </message>
   C: </stream:stream>

   S: <?xml version='1.0'?>
      <stream:stream
          from='server'
          id='id_123456789'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   S:   <message from='watson@graham-bell' to='alex@graham-bell'>
   S:     <body>I'm on my way!</body>
   S:   </message>
   S: </stream:stream>

   A session gone bad:

   C: <?xml version='1.0'?>
      <stream:stream
          to='server'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   S: <?xml version='1.0'?>
      <stream:stream
          from='server'
          id='id_123456789'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   C: <message><body>Bad XML, no closing body tag!</message>
   S: <stream:error code='400'/>
   S: </stream:stream>










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5. Stream Encryption

5.1 Overview

   XMPP includes a method for securing the stream from tampering and
   eavesdropping.  This channel encryption method makes use of the
   Transport Layer Security (TLS) [17] protocol, along with a "STARTTLS"
   extension that is modelled on similar extensions for the IMAP [18],
   POP3 [19], and ACAP [20] protocols as described in RFC 2595 [21].
   The namespace identifier for the STARTTLS extension is 'http://
   www.ietf.org/rfc/rfc2595.txt'.  TLS may be used between any
   initiating entity and any receiving entity (e.g., a stream from a
   client to a server or from one server to another).

   The following rules MUST be observed:

   1.  If the initiating entity is capable of using the STARTTLS
       extension, it MUST include the "version='1.0'" flag in the
       initiating stream header.

   2.  If the receiving entity is capable of using the STARTTLS
       extension, it MUST send the <starttls/> element in the defined
       namespace along with the list of features that it sends in
       response to the opening stream tag received from the initiating
       entity.

   3.  If the initiating entity chooses to use TLS for stream
       encryption, TLS negotiation MUST be completed before proceeding
       to authenticate the stream using SASL.

   4.  If TLS is used for stream encryption, the receiving entity MUST
       close the stream whether the TLS negotiation results in success
       or failure.

   5.  If the TLS negotiation is successful, TLS takes effect
       immediately following the closing ">" character of the <starttls/
       > element for the client and immediately following the closing
       ">" character of the <proceed> element for the server.  A new
       stream MUST then be initiated by the initiating entity.

   6.  If the TLS negotiation is successful, the receiving entity MUST
       discard any knowledge obtained from the initiating entity before
       TLS takes effect.

   7.  If the TLS negotiation is successful, the initiating entity MUST
       discard any knowledge obtained from the receiving entity before
       TLS takes effect.




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   8.  If the TLS negotiation is successful, the receiving entity MUST
       NOT offer the STARTTLS extension to the initiating entity along
       with the other stream features that are offered when the stream
       is restarted.

   9.  If TLS is used for stream encryption, SASL MUST NOT be used for
       anything but stream authentication (i.e., a security layer MUST
       NOT be negotiated using SASL).  Conversely, if a security layer
       is to be negotiated via SASL, TLS MUST NOT be used.


5.2 Narrative

   When a client secures a stream with a server, the steps involved are
   as follows:

   1.  The client opens a TCP connection and initiates the stream by
       sending the opening XML stream header to the server, including
       the "version='1.0'" flag.

   2.  The server responds by opening a TCP connection and sending an
       XML stream header to the client.

   3.  The server offers the STARTTLS extension to the client by sending
       it along with the list of supported stream features.

   4.  The client issues the STARTTLS command to instruct the server
       that it wishes to begin a TLS negotiation to secure the stream.

   5.  The server MUST reply with either an empty <proceed/> element or
       an empty <failure/> element, but keep the underlying TCP
       connection open.

   6.  The client begins a TLS negotiation in accordance with RFC 2246
       [17].  Upon completion of the negotiation, the client initiates a
       new stream by sending a new opening XML stream header to the
       server.

   7.  The server responds by sending an XML stream header to the client
       along with the remaining available features (but NOT including
       the STARTTLS element).


5.3 Client-Server Protocol

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




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   Step 1: Client initiates stream to server:

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

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

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       version='1.0'>

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

   <stream:features>
     <starttls xmlns='http://www.ietf.org/rfc/rfc2595.txt'/>
     <mechanisms xmlns='http://www.iana.org/assignments/sasl-mechanisms'>
       <mechanism>DIGEST-MD5</mechanism>
       <mechanism>PLAIN</mechanism>
     </mechanisms>
   </stream:features>

   Step 4: Client sends the STARTTLS command to the server:

   <starttls xmlns='http://www.ietf.org/rfc/rfc2595.txt'/>

   Step 5: Server informs client to proceed:

   <proceed xmlns='http://www.ietf.org/rfc/rfc2595.txt'/>

   Step 5 (alt): Server informs client that TLS negotiation has faile
   has failedd:

   <failure xmlns='http://www.ietf.org/rfc/rfc2595.txt'/>












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   Step 6: Client and server complete TLS negotiation via TCP.  When
   finished, the client initiates a new stream to the server:

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

   Step 7: Server responds by sending a stream header to the client
   along with any remaining negotiatiable stream features:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       version='1.0'>
   <stream:features>
     <mechanisms xmlns='http://www.iana.org/assignments/sasl-mechanisms'>
       <mechanism>DIGEST-MD5</mechanism>
       <mechanism>PLAIN</mechanism>
       <mechanism>EXTERNAL</mechanism>
     </mechanisms>
   </stream:features>


5.4 Certificate-Based Authentication

   If the client presents a valid client certificate during the TLS
   negotiation, the server MAY offer the SASL EXTERNAL mechanism to the
   client during stream authentication.  (see RFC 2222 [16]).  If the
   client selects this mechanism for authentication, the authentication
   credentials shall be taken from the presented certificate.


















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6. Stream Authentication

   XMPP includes two methods for enforcing authentication at the level
   of XML streams.  When one entity is already known to another (i.e.,
   there is an existing trust relationship between the entities such as
   that established when a user registers with a server or an
   administrator configures a server to trust another server), the
   preferred method for authenticating streams between the two entities
   uses an XMPP adaptation of the Simple Authentication and Security
   Layer (SASL) [16].  When there is no existing trust relationship
   between the two entities, such trust MAY be established based on
   existing trust in DNS; the authentication method used when two such
   entities are servers is the server dialback protocol that is native
   to XMPP (no such ad-hoc method is defined between a client and a
   server).  Both of these methods are described in this section.

   Stream authentication is REQUIRED for all direct communications
   between two entities; if an entity sends a stanza to an
   unauthenticated stream, the receiving entity SHOULD silently drop the
   stanza and MUST NOT process it.

6.1 SASL Authentication

6.1.1 Overview

   The Simple Authentication and Security Layer (SASL) provides a
   generalized method for adding authentication support to connection-
   based protocols.  XMPP uses a generic XML namespace profile for SASL
   that conforms to section 4 ("Profiling Requirements") of RFC 2222
   [16] (the namespace identifier for this protocol is 'http://
   www.iana.org/assignments/sasl-mechanisms').

   The following rules MUST be observed:

   1.  If TLS is used for stream encryption, SASL MUST NOT be used for
       anything but stream authentication (i.e., a security layer MUST
       NOT be negotiated using SASL).  Conversely, if a security layer
       is to be negotiated via SASL, TLS MUST NOT be used.

   2.  If the initiating entity is capable of authenticating via SASL,
       it  it MUST include the "version='1.0'" flag in the initiating
       stream header.

   3.  If the receiving entity is capable of accepting authentications
       via SASL, it MUST send one or more authentication mechanisms
       within a <mechanisms/> element in response to the opening stream
       tag received from the initiating entity.




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   4.  If the SASL negotiation involves negotiation of a security layer,
       the receiving entity MUST discard any knowledge obtained from the
       initiating entity which was not obtained from the SASL
       negotiation itself.

   5.  If the SASL negotiation involves negotiation of a security layer,
       the initiating entity MUST discard any knowledge obtained from
       the receiving entity which was not obtained from the SASL
       negotiation itself.


6.1.2 Narrative

   When a client authenticates with a server, the steps involved are as
   follows:

   1.  The client requests SASL authentication by including a 'version'
       attribute in the opening XML stream header sent to the server,
       with the value set to "1.0".

   2.  After sending an XML stream header in response, the server sends
       a list of available SASL authentication mechanisms, each of which
       is a <mechanism/> element included as a child within a
       <mechanisms/> container element that is sent as a child of the
       first-level <features/> element.  If channel encryption must be
       established before a particular authentication mechanism may be
       used, the server MUST NOT provide that mechanism in the list of
       available SASL authentication methods.

   3.  The client selects a mechanism by sending an <auth/> element to
       the server; this element MAY optionally contain character data
       (in SASL terminology the "initial response") if the mechanism
       supports or requires it.

   4.  If necessary, the server challenges the client by sending a
       <challenge/> element to the client; this element MAY optionally
       contain character data.

   5.  The client responds to the challenge by sending a <response/>
       element to the server; this element MAY optionally contain
       character data.

   6.  If necessary, the server sends more challenges and the client
       sends more responses.

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




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   1.  The client aborts the handshake by sending an <abort/> element to
       the server.

   2.  The server reports failure of the handshake by sending a
       <failure/> element to the client.  The particular cause of
       failure optionally may be communicated in the 'code' attribute of
       the <failure/> element, and may be any one of 432 (password
       transition is needed), 534 (authentication mechanism is too
       weak), or 454 (temporary authentication failure).

   3.  The server reports success of the handshake by sending a
       <success/> element to the client; this element MAY optionally
       contain character data (in SASL terminology "additional data with
       success").

   Any character data contained within these elements MUST be encoded
   using base64.

6.1.3 SASL Definition

   Section 4 of the SASL specification [16] requires that the following
   information be supplied by a protocol definition:

   service name: "xmpp"

   initiation sequence: After the initiating entity provides an opening
      XML stream header and the receiving entity replies in kind, the
      receiving entity provides a list of acceptable authentication
      methods.  The initiating entity chooses one method from the list
      and sends it to the receiving entity as the value of the
      'mechanism' attribute possesed 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.

   security layer negotiation: If a security layer is negotiated, both
      sides consider the original stream closed and new <stream/>
      headers are sent by both entities.  The security layer takes
      effect immediately following the ">" character of the empty
      <response/> element for the client and immediately following the
      closing ">" character of the <succeed/> element for the server.




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   use of the authorization identity: The authorization identity, if
      present, is unused by xmpp.


6.1.4 Client-Server Protocol

   The following example shows the data flow for a client authenticating
   with a server using SASL.

   Step 1: Client initiates stream to server:

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

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

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       from='domain'
       version='1.0'>

   Step 3: Server informs client of available authentication mechanisms:

   <stream:features>
     <mechanisms xmlns='http://www.iana.org/assignments/sasl-mechanisms'>
       <mechanism>DIGEST-MD5</mechanism>
       <mechanism>PLAIN</mechanism>
     </mechanisms>
   </stream:features>

   Step 4: Client selects an authentication mechanism ("initial
   response"):

   <auth
       xmlns='http://www.iana.org/assignments/sasl-mechanisms'
       mechanism='DIGEST-MD5'/>










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   Step 5: Server sends a base64-encoded challenge to the client:

   <challenge xmlns='http://www.iana.org/assignments/sasl-mechanisms'>
       cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi
       xxb3A9ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz
   </challenge>

   The decoded challenge is:

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

   Step 6: Client responds to the challenge:

   <response xmlns='http://www.iana.org/assignments/sasl-mechanisms'>
       dXNlcm5hbWU9InJvYiIscmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik
       9BNk1HOXRFUUdtMmhoIixcIGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5j
       PTAwMDAwMDAxLHFvcD1hdXRoLFwgZGlnZXN0LXVyaT0ieG1wcC9jYXRhY2
       x5c20uY3giLFwgcmVzcG9uc2U9ZDM4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZ
       jIxNDNhZjcsY2hhcnNldD11dGYtOA==
   </response>

   The decoded response is:

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

   Step 7: Server sends another challenge to the client:

   <challenge xmlns='http://www.iana.org/assignments/sasl-mechanisms'>
       cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
   </challenge>

   The decoded challenge is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd

   Step 8: Client responds to the challenge:

   <response xmlns='http://www.iana.org/assignments/sasl-mechanisms'/>









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   Step 9: Server informs client of successful authentication:

   <success xmlns='http://www.iana.org/assignments/sasl-mechanisms'/>

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

   <failure code='454'
       xmlns='http://www.iana.org/assignments/sasl-mechanisms'/>

   Step 10: Client initiates a new stream to the server:

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

   Step 11: Server responds by sending a stream header to the client,
   with the stream already authenticated (not followed by further stream
   features):

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='12345678'
       from='domain'
       version='1.0'>


6.2 Dialback Authentication

   XMPP includes a protocol-level method for verifying that a connection
   between two servers can be trusted (at least as much as the DNS can
   be trusted).  The method is called dialback and is used only within
   XML streams that are declared under the "jabber:server" namespace.

   The purpose of the dialback protocol is to make server spoofing more
   difficult, and thus to make it more difficult to forge XML stanzas.
   Dialback is not intended as a mechanism for securing or encrypting
   the streams between servers, only for helping to prevent the spoofing
   of a server and the sending of false data from it.  Dialback is made
   possible by the existence of DNS, since one server can verify that
   another server which is connecting to it is authorized to represent a
   given server on the Jabber network.  All DNS hostname resolutions
   MUST first resolve the hostname using an SRV [23] record of
   _jabber._tcp.server.  If the SRV lookup fails, the fallback is a
   normal A lookup to determine the IP address, using the jabber-server
   port of 5269 assigned by the Internet Assigned Numbers Authority [7].



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   Note that the method for generating and verifying the keys used in
   the dialback protocol MUST take into account the hostnames being
   used, along with a secret known only by the receiving server and the
   random ID generated for the stream.  Generating unique but verifiable
   keys is important to prevent common man-in-the-middle attacks and
   server spoofing.

   In the description that follows we use the following terminology:

   o  Originating Server -- the server that is attempting to establish a
      connection between the two servers

   o  Receiving Server -- the server that is trying to authenticate that
      Originating Server represents the Jabber server which it claims to
      be

   o  Authoritative Server -- the server that is given when a DNS lookup
      is performed on the name that Originating Server initially gave;
      for basic environments this will be Originating Server, but it
      could be a separate machine in Originating Server's network

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

   1.  Originating Server establishes a connection to Receiving Server.

   2.  Originating Server sends a 'key' value over the connection to
       Receiving Server.

   3.  Receiving Server establishes a connection to Authoritative
       Server.

   4.  Receiving Server sends the same 'key' value to Authoritative
       Server.

   5.  Authoritative Server replies that key is valid or invalid.

   6.  Receiving Server tells Originating Server whether it is
       authenticated or not.

   We can represent this flow of events graphically as follows:

   Originating               Receiving
      Server                     Server
   -----------               ---------
       |                         |
       |  establish connection   |
       | ----------------------> |
       |                         |



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       |   send stream header    |
       | ----------------------> |
       |                         |
       |  establish connection   |
       | <---------------------- |
       |                         |
       |   send stream header    |
       | <---------------------- |
       |                         |                   Authoritative
       |   send dialback key     |                       Server
       | ----------------------> |                   -------------
       |                         |                         |
                                 |  establish connection   |
                                 | ----------------------> |
                                 |                         |
                                 |   send stream header    |
                                 | ----------------------> |
                                 |                         |
                                 |  establish connection   |
                                 | <---------------------- |
                                 |                         |
                                 |   send stream header    |
                                 | <---------------------- |
                                 |                         |
                                 |   send dialback key     |
                                 | ----------------------> |
                                 |                         |
                                 |  validate dialback key  |
                                 | <---------------------- |
                                 |
       |  report dialback result |
       | <---------------------- |
       |                         |


6.2.1 Dialback Protocol

   The traffic sent between the servers is as follows:

   1.   Originating Server establishes TCP connection to Receiving
        Server

   2.   Originating Server sends a stream header to Receiving Server
        (the 'to' and 'from' attributes are NOT REQUIRED on the root
        stream element):






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

        Note: the value of the xmlns:db namespace declaration indicates
        to Receiving Server that Originating Server supports dialback.

   3.   Receiving Server sends a stream header back to Originating
        Server (the 'to' and 'from' attributes are NOT REQUIRED on the
        root stream element):

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

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

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

        Note: this key is not examined by Receiving Server, since
        Receiving Server does not keep information about Originating
        Server between sessions.

   5.   Receiving Server now establishes a connection back to
        Originating Server, getting Authoritative Server.

   6.   Receiving Server sends Authoritative Server a stream header (the
        'to' and 'from' attributes are NOT REQUIRED on the root stream
        element):

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

   7.   Authoritative Server sends Receiving Server a stream header:








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

   8.   Receiving Server sends Authoritative Server a stanza indicating
        it wants Authoritative Server to verify 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 Receiving Server's stream header to Originating Server in
        step 2, and the key Originating Server gave Receiving Server in
        step 3.  Based on this information and shared secret information
        within the 'Originating Server' network, the key is verified.
        Any verifiable method can be used to generate the key.

   9.   Authoritative Server sends a stanza back to Receiving Server
        verifying 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...'/>

   10.  Receiving Server informs Originating Server of the result:










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   <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.  Once the connection is
        validated, data can be sent by Originating Server and read by
        Receiving Server; before that, all data stanzas sent to
        Receiving Server SHOULD be dropped.  As a final guard against
        domain spoofing, Receiving Server MUST verify that all XML
        stanzas received from Originating Server include a 'from'
        attribute and that the value of that attribute includes the
        validated domain.  In addition, all XML stanzas MUST include a
        'to' attribute.




































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7. XML Stanzas

7.1 Overview

   Once the XML streams in each direction have been authenticated and
   (if desired) encrypted, XML stanzas can be sent over the streams.
   XML stanzas are the three core data elements for XMPP communications:
   <message/>, <presence/>, and <iq/>.  These elements are sent as
   direct (depth=1) children of the root <stream/> element and are
   scoped by one of the default namespaces identified in Section 4.4.

7.2 Common Attributes

   Five attributes are common to message, presence, and IQ stanzas.
   These are defined below.

7.2.1 to

   The 'to' attribute specifies the JID of the intended recipient for
   the stanza.  In the 'jabber:client' namespace, a stanza SHOULD
   possess a 'to' attribute, although a stanza sent from a client to a
   server for handling by that server (e.g., presence sent to the server
   for broadcasting to other entities) MAY legitimately lack a 'to'
   attribute.  In the 'jabber:server' namespace, a stanza MUST possess a
   'to' attribute.

7.2.2 from

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

   In the 'jabber:client' namespace, a client MUST NOT include a 'from'
   attribute on the stanzas it sends to a server; if a server receives a
   stanza from a client and the stanza possesses a 'from' attribute, it
   MUST ignore the value of the 'from' attribute.  In addition, a server
   MUST stamp stanzas received from a client with the user@domain/
   resource (full JID) of the connected resource that generated the
   stanza.

   In the 'jabber:server' namespace, a stanza MUST possess a 'from'
   attribute.  In particular, a server MUST include a 'from' attribute
   on stanzas it routes to other servers.  The domain identifier of the
   JID contained in the 'from' attribute MUST match the hostname of the
   server as communicated in the dialback negotiation (or a subdomain
   thereof).

7.2.3 id

   The optional 'id' attribute MAY be used to track stanzas sent and



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   received.  The 'id' attribute is generated by the sender.  An 'id'
   attribute included in an IQ request of type "get" or "set" SHOULD be
   returned to the sender in any IQ response of type "result" or "error"
   generated by the recipient of the request.  A recipient of a message
   or presence stanza MAY return that 'id' in any replies, but is NOT
   REQUIRED to do so.

   The value of the 'id' attribute is not intended to be unique --
   globally, within a domain, or within a stream.  It is generated by a
   sender only for internal tracking of information within the sending
   application.

7.2.4 type

   The 'type' attribute specifies detailed information about the purpose
   or context of the message, presence, or IQ stanza.  The particular
   allowable values for the 'type' attribute vary depending on whether
   the stanza is a message, presence, or IQ, and thus are specified in
   the following sections.

7.2.5 xml:lang

   Any message or presence stanza MAY possess an 'xml:lang' attribute
   specifying the default language of any CDATA sections of the stanza
   or its child elements.  An IQ stanza SHOULD NOT possess an 'xml:lang'
   attribute, since it is merely a vessel for data in other namespaces
   and does not itself contain children that have CDATA.  The value of
   the 'xml:lang' attribute MUST be an NMTOKEN and MUST conform to the
   format defined in RFC 3066 [22].

7.3 Message Stanzas

   Message stanzas in the 'jabber:client' or 'jabber:server' namespace
   are used to "push" information to another entity.  Common uses in the
   context of instant messaging include single messages, messages sent
   in the context of a chat conversation, messages sent in the context
   of a multi-user chat room, headlines, and errors.  These messages
   types are identified more fully below.

7.3.1 Types of Message

   The 'type' attribute of a message stanza is OPTIONAL; if included, it
   specifies the conversational context of the message.  The sending of
   a message stanza without a 'type' attribute signals that the message
   stanza is a single message.  However, the 'type' attribute MAY also
   have one of the following values:

   o  chat -- The message is sent in the context of a one-to-one chat



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      conversation.

   o  groupchat -- The message is sent in the context of a multi-user
      chat environment.

   o  headline -- The message is generated by an automated service that
      delivers content (news, sports, market information, etc.).

   o  error - A message returned to a sender specifying an error
      associated with a previous message sent by the sender (for a full
      list of error messages, see error codes (Appendix A)).

   For detailed information about the meaning of these message types,
   refer to XMPP IM [3].

7.3.2 Children

   As described under extended namespaces (Section 7.6), a message
   stanza MAY contain any properly-namespaced child element as long as
   the namespace name is not "jabber:client", "jabber:server", or
   "http://etherx.jabber.org/streams", and as long as the element name
   does not match that of one of the core data elements, stream
   elements, or defined children thereof.

   In accordance with the default namespace declaration, by default a
   message stanza is in the 'jabber:client' or 'jabber:server'
   namespace, which defines certain allowable children of message
   stanzas.  Specifically, if a message stanza has no 'type' attribute
   or has a 'type' attribute with a value of "chat", "groupchat", or
   "headline", it MAY contain any of the following child elements
   without an explicit namespace declaration:

   o  body -- The textual contents of the message; normally included but
      NOT REQUIRED.  The <body/> element MUST NOT possess any
      attributes, with the exception of the 'xml:lang' attribute.
      Multiple instances of the <body/> element MAY be included but only
      if each instance possesses an 'xml:lang' attribute with a distinct
      language value.  The <body> element MUST NOT contain mixed
      content.

   o  subject -- The subject of the message.  The <subject/> element
      MUST NOT possess any attributes, with the exception of the
      'xml:lang' attribute.  Multiple instances of the <subject/>
      element MAY be included for the purpose of providing alternate
      versions of the same subject, but only if each instance possesses
      an 'xml:lang' attribute with a distinct language value.  The
      <subject> element MUST NOT contain mixed content.




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   o  thread -- A random string that is generated by the sender and that
      SHOULD be copied back in replies; it is used for tracking a
      conversation thread (sometimes referred to as an "IM session")
      between two entities.  If used, it MUST be unique to that
      conversation thread within the stream and MUST be consistent
      throughout that conversation.  The use of the <thread/> element is
      optional and is not used to identify individual messages, only
      conversations.  The method for generating thread IDs SHOULD be as
      follows: (1) concatenate the sender's full JID (user@host/
      resource) with the recipient's full JID; (2) concatenate these JID
      strings with a full ISO-8601 timestamp including year, month, day,
      hours, minutes, seconds, and UTC offset in the following format:
      yyyy-mm-dd-Thh:mm:ss-hh:mm; (3) hash the resulting string
      according to the SHA1 algorithm; (4) convert the hexidecimal SHA1
      output to all lowercase.  Only one <thread/> element MAY be
      included in a message stanza, and it MUST NOT possess any
      attributes.  The <thread/> element MUST be treated as an opaque
      string by entities; no semantic meaning may be derived from it,
      and only exact, case-insensitve comparisons be made against it.
      The <thread> element MUST NOT contain mixed content.

   If the message stanza is of type "error", it MUST include an <error/>
   child, which in turn MUST possess a 'code' attribute corresponding to
   one of the standard error codes (Appendix A), MAY possess an
   'xml:lang' attribute, and MAY also contain PCDATA corresponding to a
   natural-language description of the error.  An <error/> child MUST
   NOT be included if the stanza type is anything other than "error".
   An entity that receives a message stanza of type 'error' MUST NOT
   respond to the stanza by sending a further message stanza of type
   'error'; this helps to prevent looping.

7.4 Presence Stanzas

   Presence stanzas are used in the 'jabber:client' or 'jabber:server'
   namespace to express an entity's current availability status (offline
   or online, along with various sub-states of the latter and optional
   user-defined descriptive tex and optional user-defined descriptive
   textt) and to communicate that status to other entities.  They are
   also used to negotiate and manage subscriptions to the presence of
   other entities.

7.4.1 Types of Presence

   The 'type' attribute of a presence stanza is optional.  A presence
   stanza that does not have a 'type' attribute is used to signal to the
   server that the sender is online and available for communication.  If
   included, the 'type' attribute specifies the availability state of
   the sender, a request to manage a subscription to another entity's



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   presence, a request for another entity's current presence, or an
   error related to a previously-sent presence stanza.  The 'type'
   attribute MAY have one of the following values:

   o  unavailable -- Signals that the entity is no longer available for
      communication.

   o  subscribe -- The sender wishes to subscribe to the recipient's
      presence.

   o  subscribed -- The sender has allowed the recipient to receive
      their presence.

   o  unsubscribe -- A notification that an entity is unsubscribing from
      another entity's presence.

   o  unsubscribed -- The subscription request has been denied or a
      previously-granted subscription has been cancelled.

   o  probe -- A request for an entity's current presence.  In general
      SHOULD NOT be sent by a client.

   o  error -- An error has occurred regarding processing or delivery of
      a previously-sent presence stanza.

   Information about the subscription model used within XMPP can be
   found in XMPP IM [3].

7.4.2 Children

   As described under extended namespaces (Section 7.6), a presence
   stanza MAY contain any properly-namespaced child element as long as
   the namespace name is not "jabber:client", "jabber:server", or
   "http://etherx.jabber.org/streams", and as long as the element name
   does not match that of one of the core data elements, stream
   elements, or defined children thereof.

   In accordance with the default namespace declaration, by default a
   presence stanza is in the 'jabber:client' or 'jabber:server'
   namespace, which defines certain allowable children of presence
   stanzas.  Specifically, if a presence stanza possesses no 'type'
   attribute, it MAY contain any of the following child elements (note
   that the <status/> child MAY be sent in a presence stanza of type
   "unavailable" or, for historical reasons, "subscribe"):

   o  show -- Describes the availability status of an entity or specific
      resource.  Only one <show/> element MAY be included in a presence
      stanza, and it MUST NOT possess any attributes.  The value SHOULD



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      be one of the following (values other than these four MAY be
      ignored; additional availability types could be defined through a
      properly-namespaced child element of the presence stanza):

      *  away -- The entity or resource is temporarily away.

      *  chat -- The entity or resource is actively interested in
         chatting.

      *  xa -- The entity or resource is away for an extended period (xa
         = "eXtended Away").

      *  dnd -- The entity or resource is busy (dnd = "Do Not Disturb").

   o  status -- An optional natural-language description of availability
      status.  Normally used in conjunction with the show element to
      provide a detailed description of an availability state (e.g., "In
      a meeting").  The <status/> element MUST NOT possess any
      attributes, with the exception of the 'xml:lang' attribute.
      Multiple instances of the <status/> element MAY be included but
      only if each instance possesses an 'xml:lang' attribute with a
      distinct language value.

   o  priority -- An optional element specifying the priority level of
      the connected resource.  The value may be any integer between -128
      to 127.  Only one <priority/> element MAY be included in a
      presence stanza, and it MUST NOT possess any attributes.

   If the presence stanza is of type "error", it MUST include an <error/
   > child, which in turn MUST possess a 'code' attribute corresponding
   to one of the standard error codes (Appendix A) and MAY contain
   PCDATA corresponding to a natural-language description of the error.
   An <error/> child MUST NOT be included if the stanza type is anything
   other than "error".  An entity that receives a presence stanza of
   type 'error' MUST NOT respond to the stanza by sending a further
   presence stanza of type 'error'; this helps to prevent looping.

   As described under extended namespaces (Section 7.6), a presence
   stanza MAY also contain any properly-namespaced child element (other
   than the core data elements, stream elements, or defined children
   thereof).

7.5 IQ Stanzas

7.5.1 Overview

   Info/Query, or IQ, is a request-response mechanism, similar in some
   ways to HTTP [24].  IQ stanzas in the 'jabber:client' or



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   'jabber:server' namespace enable an entity to make a request of, and
   receive a response from, another entity.  The data content of the
   request and response is defined by the namespace declaration of a
   direct child element of the IQ element, and the interaction is
   tracked by the requesting entity through use of the 'id' attribute,
   which responding entities SHOULD return in any response.

   Most IQ interactions follow a common pattern of structured data
   exchange such as get/result or set/result (although an error may be
   returned in response to a request if appropriate):

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

   An entity that receives an IQ request of type 'get' or 'set' MUST
   reply with an IQ response of type 'result' or 'error' (which response
   SHOULD preserve the 'id' attribute of the request).  An entity that
   receives a stanza of type 'result' or 'error' MUST NOT respond to the
   stanza by sending a further IQ response of type 'result' or 'error';
   however, as shown above, the requesting entity MAY send another
   request (e.g., an IQ of type 'set' in order to provide required
   information discovered through a get/result pair).

7.5.2 Types of IQ

   The 'type' attribute of an IQ stanza is REQUIRED.  The 'type'
   attribute specifies a distinct step within a request-response
   interaction.  The value SHOULD be one of the following (all other
   values MAY be ignored):

   o  get -- The stanza is a request for information.

   o  set -- The stanza provides required data, sets new values, or
      replaces existing values.



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   o  result -- The stanza is a response to a successful get or set
      request.

   o  error -- An error has occurred regarding processing or delivery of
      a previously-sent get or set.


7.5.3 Children

   As described under extended namespaces (Section 7.6), an IQ stanza
   MAY contain any properly-namespaced child element as long as the
   namespace name is not "jabber:client", "jabber"server", or "http://
   etherx.jabber.org/streams", and as long as the element name does not
   match that of one of the core data elements, stream elements, or
   defined children thereof.  However, an IQ stanza contains no children
   in the 'jabber:client' or 'jabber:server' namespace since it is a
   vessel for XML in another namespace.

   If the IQ stanza is of type "error", it MUST include an <error/>
   child, which in turn MUST possess a 'code' attribute corresponding to
   one of the standard error codes (Appendix A) and MAY contain PCDATA
   corresponding to a natural-language description of the error.  An
   <error/> child MUST NOT be included if the stanza type is anything
   other than "error".  An entity that receives an IQ stanza of type
   'error' MUST NOT respond to the stanza by sending a further IQ stanza
   of type 'error'; this helps to prevent looping.

7.6 Extended Namespaces

   While the core data elements in the "jabber:client" or
   "jabber:server" namespace (along with their attributes and child
   elements) provide a basic level of functionality for messaging and
   presence, XMPP uses XML namespaces to extend the core data elements
   for the purpose of providing additional functionality.  Thus a
   message, presence, or IQ stanza MAY house one or more optional child
   elements containing content that extends the meaning of the message
   (e.g., an encrypted form of the message body).  This child element
   MAY be any element (other than the core data elements, stream
   elements, or defined children thereof).  The child element MUST
   possess an 'xmlns' namespace declaration (other than the stream
   namespace and the default namespace) 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, it MUST ignore the associated XML data (if the stanza is
   being routed on to another entity, ignore means "pass it on
   untouched").  If an entity receives an IQ stanza in a namespace it



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   does not understand, the entity SHOULD return an IQ stanza of type
   "error" with an error element of code 501 (Not Implemented).  If an
   entity receives a message or presence stanza that contains XML data
   in an extended namespace it does not understand, the portion of the
   stanza that is in the unknown namespace SHOULD be ignored.  If an
   entity receives a message stanza without a <body/> element but
   containing only a child element bound by a namespace it does not
   understand, it MUST ignore the entire stanza.











































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8. XML Usage within XMPP

8.1 Namespaces

   XML Namespaces [15] are used within all XMPP-compliant XML to create
   strict boundaries of data ownership.  The basic function of
   namespaces is to separate different vocabularies of XML elements that
   are structurally mixed together.  Ensuring that XMPP-compliant XML is
   namespace-aware enables any XML to be structurally mixed with any
   data element within XMPP.

   Additionally, XMPP is more strict about namespace prefixes than the
   XML namespace specification requires.

8.2 Validation

   A server is not responsible for validating the XML elements forwarded
   to a client;  an implementation MAY choose to provide only validated
   data elements but is NOT REQUIRED to do so.  Clients SHOULD NOT rely
   on the ability to send data which does not conform to the schemas,
   and SHOULD ignore any non-conformant elements or attributes on the
   incoming XML stream.  Validation of XML streams and stanzas is NOT
   REQUIRED or recommended, and schemas are included herein for
   descriptive purposes only.

8.3 Character Encodings

   Software implementing XML streams MUST support the UTF-8 (RFC 2279
   [25]) and UTF-16 (RFC 2781 [26]) transformations of Universal
   Character Set (ISO/IEC 10646-1 [27]) characters.  Software MUST NOT
   attempt to use any other encoding for transmitted data.  The
   encodings of the transmitted and received streams are independent.
   Software MAY select either UTF-8 or UTF-16 for the transmitted
   stream, and SHOULD deduce the encoding of the received stream as
   described in the XML specification [1].  For historical reasons,
   existing implementations MAY support UTF-8 only.

8.4 Inclusion of Text Declaration

   An application MAY send a text declaration.  Applications MUST follow
   the rules in the XML specification [1] regarding the circumstances
   under which a text declaration is included.









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9. IANA Considerations

   The IANA registers "xmpp" as a GSSAPI [29] service name, as specified
   in Section 6.1.3.

   Additionally, the IANA registers "jabber-client" and "jabber-server"
   as keywords for TCP ports 5222 and 5269 respectively.












































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10. Internationalization Considerations

   Usage of the 'xml:lang' attribute is described above.  If a client
   includes an 'xml:lang' attribute in a stanza, the server MUST NOT
   modify or delete it.














































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11. Security Considerations

11.1 Client-to-Server Communications

   The TLS protocol for encrypting XML streams provides a reliable
   mechanism for helping to ensure the privacy and data integrity of
   data exchanged between two entities.

   The SASL protocol for authenticating XML streams (defined under
   Section 6.1 above) provides a reliable mechanism for validating that
   a client connecting to a server is who it claims to be.

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

   End-to-end encryption of message bodies and presence status
   information MAY be effected through use of the methods defined in
   End-to-End Object Encryption in XMPP [28].

11.2 Server-to-Server Communications

   It is OPTIONAL for any given server to communicate with other
   servers, and server-to-server communications MAY be disabled by the
   administrator of any given deployment.

   If two servers would like to enable communications between
   themselves, they MUST form a relationship of trust at some level,
   either based on trust in DNS or based on a pre-existing trust
   relationship (e.g., through exchange of certificates).  If two
   servers have a pre-existing trust relationship, they MAY use SASL
   Authentication (Section 6.1) for the purpose of authenticating each
   other.  If they do not have a pre-existing relationship, they MUST
   use the Dialback Protocol (Section 6.2), which provides a reliable
   mechanism for preventing the spoofing of servers.

11.3 Firewalls

   Communications using XMPP occur over TCP sockets on port 5222
   (client-to-server) or port 5269 (server-to-server), as registered
   with the IANA [7].  Use of these well-known ports allows
   administrators to easily enable or disable XMPP activity through
   existing and commonly-deployed firewalls.

11.4 Minimum Security Mechanisms

   Although service provisioning is a policy matter, at a minimum, all



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   implementations MUST support the following mechanisms:

   for authentication: the SASL DIGEST-MD5 mechanism

   for confidentiality: TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA
      cipher)

   for both: TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher
      supporting client-side certificates)










































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References

   [1]   World Wide Web Consortium, "Extensible Markup Language (XML)
         1.0 (Second Edition)", W3C xml, October 2000, <http://
         www.w3.org/TR/2000/REC-xml-20001006>.

   [2]   Jabber Software Foundation, "Jabber Software Foundation",
         August 2001, <http://www.jabber.org/>.

   [3]   Saint-Andre, P. and J. Miller, "XMPP Instant Messaging (draft-
         ietf-xmpp-im-04, work in progress)", February 2003.

   [4]   Day, M., Aggarwal, S., Mohr, G. and J. Vincent, "A Model for
         Presence and Instant Messaging", RFC 2779, February 2000,
         <http://www.ietf.org/rfc/rfc2779.txt>.

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

   [6]   University of Southern California, "Transmission Control
         Protocol", RFC 793, September 1981, <http://www.ietf.org/rfc/
         rfc0793.txt>.

   [7]   Internet Assigned Numbers Authority, "Internet Assigned Numbers
         Authority", January 1998, <http://www.iana.org/>.

   [8]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 2396, August
         1998, <http://www.ietf.org/rfc/rfc2396.txt>.

   [9]   Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet host
         table specification", RFC 952, October 1985.

   [10]  Braden, R., "Requirements for Internet Hosts - Application and
         Support", STD 3, RFC 1123, October 1989.

   [11]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile
         for Internationalized Domain Names (draft-ietf-idn-nameprep-11,
         work in progress)", June 2002.

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

   [13]  Saint-Andre, P. and J. Hildebrand, "Nodeprep: A Stringprep
         Profile for Node Identifiers in XMPP (draft-ietf-xmpp-nodeprep-
         01, work in progress)", February 2003.

   [14]  Saint-Andre, P. and J. Hildebrand, "Resourceprep: A Stringprep



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         Profile for Resource Identifiers in XMPP (draft-ietf-xmpp-
         resourceprep-01, work in progress)", February 2003.

   [15]  World Wide Web Consortium, "Namespaces in XML", W3C xml-names,
         January 1999, <http://www.w3.org/TR/1999/REC-xml-names-
         19990114/>.

   [16]  Myers, J., "Simple Authentication and Security Layer (SASL)",
         RFC 2222, October 1997.

   [17]  Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
         P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
         1999.

   [18]  Crispin, M., "Internet Message Access Protocol - Version
         4rev1", RFC 2060, December 1996.

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

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

   [21]  Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595,
         June 1999.

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

   [23]  Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
         location of services (DNS SRV)", RFC 2052, October 1996.

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

   [25]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
         2279, January 1998.

   [26]  Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO 10646",
         RFC 2781, February 2000.

   [27]  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.

   [28]  Saint-Andre, P. and J. Hildebrand, "End-to-End Object



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         Encryption in XMPP (draft-ietf-xmpp-e2e-00, work in progress)",
         February 2003.

   [29]  Linn, J., "Generic Security Service Application Program
         Interface, Version 2", RFC 2078, January 1997.


Authors' Addresses

   Peter Saint-Andre
   Jabber Software Foundation

   EMail: stpeter@jabber.org
   URI:   http://www.jabber.org/people/stpeter.php


   Jeremie Miller
   Jabber Software Foundation

   EMail: jeremie@jabber.org
   URI:   http://www.jabber.org/people/jer.php






























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Appendix A. Standard Error Codes

   A standard error element is used for failed processing of XML stanzas
   within the "jabber:client" or "jabber:server" namespace.  This
   element is a child of the failed stanza and MUST include a 'code'
   attribute corresponding to an appropriate error condition.

   In general the standard error codes were "borrowed" from those used
   in HTTP [24] early in the development of XMPP within the Jabber
   community.  The first digit of the error code defines the class of
   response.  The last two digits do not have any categorization role.
   There are five possible values for the first digit:

   o  1xx: Informational - Request received, continuing process [not
      currently used within XMPP]

   o  2xx: Success - The action was successfully received, understood,
      and accepted [not currently used within XMPP]

   o  3xx: Redirection - Further action must be taken in order to
      complete the request

   o  4xx: Sender Error - The sender's request contains bad syntax or
      cannot be fulfilled

   o  5xx: Receiver Error - The receiving or routing entity (often but
      not always a server) failed to fulfill an apparently valid request

   The individual values of the numeric status/error codes defined for
   XMPP, and an example set of corresponding textual descriptions, are
   presented below.  The textual descriptions listed here are only
   recommendations -- they MAY be replaced by local equivalents without
   affecting the protocol.

   o  302 (Redirect) - Code 302 is used when a server needs to redirect
      stream initiation requests to another hostname or IP address.

   o  400 (Bad Request) - Code 400 is used to inform a sender that a
      request could not be understood by the recipient.  This might be
      generated when an entity sends non-well-formed XML or when a
      message stanza does not have a 'to' attribute.

   o  401 (Unauthorized) - Code 401 is used to inform clients that they
      have provided incorrect authorization information, e.g., an
      incorrect password or unknown username when attempting to
      authenticate with a service.

   o  402 (Payment Required) - Code 402 is being reserved for future



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      use.

   o  403 (Forbidden) - Code 403 is used to inform an entity that its
      request was understood but that the recipient is refusing to
      fulfill it, e.g., if a user attempts to set information associated
      with another user.

   o  404 (Not Found) - Code 404 is used to inform a sender that no
      recipient was found matching the JID to which an XML stanza was
      sent, e.g., if a sender has attempted to send a message to a JID
      that does not exist.  (Note: if the server of the intended
      recipient cannot be reached, an error code from the 500 series
      must be sent.)

   o  405 (Not Allowed) - Code 405 is used when the action requested is
      not allowed for the JID identified by the 'from' address, e.g., if
      a client attempts to set the time or version of a server.

   o  406 (Not Acceptable) - Code 406 is used when an XML stanza is for
      some reason not acceptable to a server or other entity.  This
      might be generated when, for example, a user attempts to register
      with a service using an empty password.

   o  407 (Registration Required) - Code 407 is used when a message or
      request is sent to a service that requires prior registration,
      e.g., if a user attempts to send a message through a gateway to a
      foreign messaging system without having first registered with that
      gateway.

   o  408 (Request Timeout) - Code 408 is returned when a recipient does
      not produce a response within the time that the sender was
      prepared to wait.

   o  409 (Conflict) - Code 409 is returned when a request cannot be
      fulfilled because of an inherent conflict (e.g., because a client
      attempts to authorize a resouce name that is already in use).

   o  410 (Gone) - Code 410 is returned by a server when the hostname
      requested by an entity initiating a stream request is no longer
      provided by a server.

   o  500 (Internal Server Error) - Code 500 is used when a server or
      service encounters an unexpected condition which prevents it from
      handling a stream initiation request or an XML stanza from a
      sender.

   o  501 (Not Implemented) - Code 501 is used when the recipient does
      not support the functionality being requested by a sender, e.g.,



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      if a user attempts to register with a server that does not allow
      registration.

   o  502 (Remote Server Error) - Code 502 is used when delivery of an
      XML stanza fails because of an inability to reach the intended
      remote server or service, e.g., because a remote server's hostname
      could not be resolved.

   o  503 (Service Unavailable) - Code 503 is used when a sender
      requests a service that a recipient is temporarily unable to
      offer.

   o  504 (Remote Server Timeout) - Code 504 is used when attempts to
      contact a remote server timeout, e.g., if an incorrect hostname is
      specified.

   o  505 (Version Not Supported) - Code 505 is used when a server does
      not support the XMPP version requested by an entity that initiates
      a stream to the server.
































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Appendix B. XML Schemas

   The following XML schemas are descriptive, not normative.

B.1 streams namespace

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

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

     <xs:element name='stream'>
       <xs:complexType>
         <xs:element ref='features' minOccurs='0' maxOccurs='unbounded'/>
         <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
         <xs:choice>
           <xs:any
                namespace='jabber:client'
                maxOccurs='1'/>
           <xs:any
                namespace='jabber:server'
                maxOccurs='1'/>
         </xs:choice>
         <xs:attribute name='to' type='xs:string' use='optional'/>
         <xs:attribute name='from' type='xs:string' use='optional'/>
         <xs:attribute name='id' type='xs:ID' use='optional'/>
         <xs:attribute name='version' type='xs:decimal' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'/>
       <xs:complexType>
         <xs:attribute name='code' type='xs:string' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:nonNegativeInteger'>
               <xs:enumeration value='302'/>
               <xs:enumeration value='400'/>
               <xs:enumeration value='404'/>
               <xs:enumeration value='410'/>
               <xs:enumeration value='500'/>
               <xs:enumeration value='505'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>



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     </xs:element>

   </xs:schema>


B.2 SASL namespace

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

   <xs:schema
       xmlns:xsd='http://www.w3.org/2001/XMLSchema'
       targetNamespace='http://www.iana.org/assignments/sasl-mechanisms'
       xmlns='http://www.iana.org/assignments/sasl-mechanisms'
       elementFormDefault='qualified'>

     <xs:element name='mechanisms'>
       <xs:complexType>
         <xs:sequence minOccurs='0' maxOccurs='unbounded'>
           <xs:element ref='mechanism'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <xs:element name='mechanism'/>

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

     <xs:element name='challenge' type='xs:string'/>
     <xs:element name='response' type='xs:string'/>
     <xs:element name='abort'/>
     <xs:element name='success'/>
     <xs:element name='failure'>
       <xs:complexType>
         <xs:attribute name='code' type='xs:string' use='optional'/>
       </xs:complexType>
     </xs:element>

   </xs:schema>


B.3 Dialback namespace

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




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   <xs:schema
       xmlns:xsd='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:string'>
             <xs:attribute name='from' type='xs:string' use='required'/>
             <xs:attribute name='to' type='xs:string' use='required'/>
             <xs:attribute name='type' type='xs:string' 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:string'>
             <xs:attribute name='from' type='xs:string' use='required'/>
             <xs:attribute name='to' type='xs:string' use='required'/>
             <xs:attribute name='id' type='xs:string' use='required'/>
             <xs:attribute name='type' type='xs:string' 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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B.4 jabber:client namespace

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

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

     <xs:element name='message'>
        <xs:complexType>
           <xs:choice maxOccurs='unbounded'>
              <xs:element ref='body' minOccurs='0' maxOccurs='unbounded'/>
              <xs:element ref='subject' minOccurs='0' maxOccurs='unbounded'/>
              <xs:element ref='thread' minOccurs='0' maxOccurs='1'/>
              <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
              <xs:any
                  namespace='##other'
                  minOccurs='0'
                  maxOccurs='unbounded'/>
           </xs:choice>
           <xs:attribute name='to' type='xs:string' use='optional'/>
           <xs:attribute name='from' type='xs:string' use='optional'/>
           <xs:attribute name='id' type='xs:ID' use='optional'/>
           <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
           <xs:attribute name='type' use='optional'>
             <xs:simpleType>
               <xs:restriction base='xs:NCName'>
                 <xs:enumeration value='chat'/>
                 <xs:enumeration value='groupchat'/>
                 <xs:enumeration value='headline'/>
                 <xs:enumeration value='error'/>
               </xs:restriction>
             </xs:simpleType>
           </xs:attribute>
        </xs:complexType>
     </xs:element>

     <xs:element name='body' type='xs:string'>
       <xs:complexType>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='subject' type='xs:string'>
       <xs:complexType>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>



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       </xs:complexType>
     </xs:element>

     <xs:element name='thread' type='xs:string'/>

     <xs:element name='presence'>
       <xs:complexType>
         <xs:choice maxOccurs='unbounded'>
           <xs:element ref='show' minOccurs='0' maxOccurs='1'/>
           <xs:element ref='status' minOccurs='0' maxOccurs='unbounded'/>
           <xs:element ref='priority' minOccurs='0' maxOccurs='1'/>
           <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
           <xs:any
               namespace='##other'
               minOccurs='0'
               maxOccurs='unbounded'/>
         </xs:choice>
         <xs:attribute name='to' type='xs:string' use='optional'/>
         <xs:attribute name='from' type='xs:string' use='optional'/>
         <xs:attribute name='id' type='xs:ID' use='optional'/>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
         <xs:attribute name='type' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='subscribe'/>
               <xs:enumeration value='subscribed'/>
               <xs:enumeration value='unsubscribe'/>
               <xs:enumeration value='unsubscribed'/>
               <xs:enumeration value='unavailable'/>
               <xs:enumeration value='error'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

     <xs:element name='show'>
       <xs:simpleType>
         <xs:restriction base='xs:NCName'>
           <xs:enumeration value='away'/>
           <xs:enumeration value='chat'/>
           <xs:enumeration value='xa'/>
           <xs:enumeration value='dnd'/>
         </xs:restriction>
       </xs:simpleType>
     </xs:element>

     <xs:element name='status' type='xs:string'>



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       <xs:complexType>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='priority' type='xs:byte'/>

     <xs:element name='iq'>
       <xs:complexType>
         <xs:choice maxOccurs='unbounded'>
           <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
           <xs:any
               namespace='##other'
               minOccurs='0'
               maxOccurs='unbounded'/>
         </xs:choice>
         <xs:attribute name='to' type='xs:string' use='optional'/>
         <xs:attribute name='from' type='xs:string' use='optional'/>
         <xs:attribute name='id' type='xs:ID' use='optional'/>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='get'/>
               <xs:enumeration value='set'/>
               <xs:enumeration value='result'/>
               <xs:enumeration value='error'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:attribute
             name='code'
             type='xs:nonNegativeInteger'
             use='required'/>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
       </xs:complexType>
     </xs:element>

   </xs:schema>


B.5 jabber:server namespace

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



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

     <xs:element name='message'>
        <xs:complexType>
           <xs:choice maxOccurs='unbounded'>
              <xs:element ref='body' minOccurs='0' maxOccurs='unbounded'/>
              <xs:element ref='subject' minOccurs='0' maxOccurs='unbounded'/>
              <xs:element ref='thread' minOccurs='0' maxOccurs='1'/>
              <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
              <xs:any
                  namespace='##other'
                  minOccurs='0'
                  maxOccurs='unbounded'/>
           </xs:choice>
           <xs:attribute name='to' type='xs:string' use='required'/>
           <xs:attribute name='from' type='xs:string' use='required'/>
           <xs:attribute name='id' type='xs:ID' use='optional'/>
           <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
           <xs:attribute name='type' use='optional'>
             <xs:simpleType>
               <xs:restriction base='xs:NCName'>
                 <xs:enumeration value='chat'/>
                 <xs:enumeration value='groupchat'/>
                 <xs:enumeration value='headline'/>
                 <xs:enumeration value='error'/>
               </xs:restriction>
             </xs:simpleType>
           </xs:attribute>
        </xs:complexType>
     </xs:element>

     <xs:element name='body' type='xs:string'>
       <xs:complexType>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='subject' type='xs:string'>
       <xs:complexType>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='thread' type='xs:string'/>



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     <xs:element name='presence'>
       <xs:complexType>
         <xs:choice maxOccurs='unbounded'>
           <xs:element ref='show' minOccurs='0' maxOccurs='1'/>
           <xs:element ref='status' minOccurs='0' maxOccurs='unbounded'/>
           <xs:element ref='priority' minOccurs='0' maxOccurs='1'/>
           <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
           <xs:any
               namespace='##other'
               minOccurs='0'
               maxOccurs='unbounded'/>
         </xs:choice>
         <xs:attribute name='to' type='xs:string' use='required'/>
         <xs:attribute name='from' type='xs:string' use='required'/>
         <xs:attribute name='id' type='xs:ID' use='optional'/>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
         <xs:attribute name='type' use='optional'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='subscribe'/>
               <xs:enumeration value='subscribed'/>
               <xs:enumeration value='unsubscribe'/>
               <xs:enumeration value='unsubscribed'/>
               <xs:enumeration value='unavailable'/>
               <xs:enumeration value='error'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

     <xs:element name='show'>
       <xs:simpleType>
         <xs:restriction base='xs:NCName'>
           <xs:enumeration value='away'/>
           <xs:enumeration value='chat'/>
           <xs:enumeration value='xa'/>
           <xs:enumeration value='dnd'/>
         </xs:restriction>
       </xs:simpleType>
     </xs:element>

     <xs:element name='status' type='xs:string'>
       <xs:complexType>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
       </xs:complexType>
     </xs:element>




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     <xs:element name='priority' type='xs:byte'/>

     <xs:element name='iq'>
       <xs:complexType>
         <xs:choice maxOccurs='unbounded'>
           <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
           <xs:any
               namespace='##other'
               minOccurs='0'
               maxOccurs='unbounded'/>
         </xs:choice>
         <xs:attribute name='to' type='xs:string' use='required'/>
         <xs:attribute name='from' type='xs:string' use='required'/>
         <xs:attribute name='id' type='xs:ID' use='optional'/>
         <xs:attribute name='type' use='required'>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='get'/>
               <xs:enumeration value='set'/>
               <xs:enumeration value='result'/>
               <xs:enumeration value='error'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:attribute
             name='code'
             type='xs:nonNegativeInteger'
             use='required'/>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
       </xs:complexType>
     </xs:element>

   </xs:schema>













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Appendix C. Revision History

   Note to RFC editor: please remove this entire appendix, and the
   corresponding entries in the table of contents, prior to publication.

C.1 Changes from draft-ietf-xmpp-core-03

   o  Clarified rules and procedures for TLS and SASL.

   o  Amplified stream error code syntax per list discussion.

   o  Made numerous small editorial changes.


C.2 Changes from draft-ietf-xmpp-core-02

   o  Added dialback schema.

   o  Removed all DTDs since schemas provide more complete definitions.

   o  Added stream error codes.

   o  Clarified error code "philosophy".


C.3 Changes from draft-ietf-xmpp-core-01

   o  Updated the addressing restrictions per list discussion and added
      references to the new nodeprep and resourceprep profiles.

   o  Corrected error in Stream Authentication regarding "version='1.0'"
      flag.

   o  Made numerous small editorial changes.


C.4 Changes from draft-ietf-xmpp-core-00

   o  Added information about TLS from list discussion.

   o  Clarified meaning of "ignore" based on list discussion.

   o  Clarified information about Universal Character Set data and
      character encodings.

   o  Provided base64-decoded information for examples.

   o  Fixed several errors in the schemas.



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   o  Made numerous small editorial fixes.


C.5 Changes from draft-miller-xmpp-core-02

   o  Brought Streams Authentication section into line with discussion
      on list and at IETF 55 meeting.

   o  Added information about the optional 'xml:lang' attribute per
      discussion on list and at IETF 55 meeting.

   o  Specified that validation is neither required nor recommended, and
      that the formal definitions (DTDs and schemas) are included for
      descriptive purposes only.

   o  Specified that the response to an IQ stanza of type 'get' or 'set'
      must be an IQ stanza of type 'result' or 'error'.

   o  Specified that compliant server implementations must process
      stanzas in order.

   o  Specified that for historical reasons some server implementations
      may accept 'stream:' as the only valid namespace prefix on the
      root stream element.

   o  Clarified the difference between 'jabber:client' and
      'jabber:server' namespaces, namely, that 'to' and 'from'
      attributes are required on all stanzas in the latter but not the
      former.

   o  Fixed typo in Step 9 of the dialback protocol (changed db:result
      to db:verify).

   o  Removed references to TLS pending list discussion.

   o  Removed the non-normative appendix on OpenPGP usage pending its
      inclusion in a separate I-D.

   o  Simplified the architecture diagram, removed most references to
      services, and removed references to the 'jabber:component:*'
      namespaces.

   o  Noted that XMPP activity respects firewall administration
      policies.

   o  Further specified the scope and uniqueness of the 'id' attribute
      in all stanza types and the <thread/> element in message stanzas.




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   o  Nomenclature changes: (1) from "chunks" to "stanzas"; (2) from
      "host" to "server" and from "node" to "client" (except with regard
      to definition of the addressing scheme).
















































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Full Copyright Statement

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Acknowledgement

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