Network Working Group                                     P. Saint-Andre
Internet-Draft                                                 J. Miller
Expires: August 27, 31, 2003                      Jabber Software Foundation
                                                       February 26,
                                                           March 2, 2003

                               XMPP Core
                        draft-ietf-xmpp-core-04
                        draft-ietf-xmpp-core-05

Status of this Memo

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

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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
   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.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4  5
   1.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  4  5
   1.2   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  4  5
   1.3   Discussion Venue . . . . . . . . . . . . . . . . . . . . . .  4  5
   1.4   Intellectual Property Notice . . . . . . . . . . . . . . . .  4  5
   2.    Generalized Architecture . . . . . . . . . . . . . . . . . .  5  6
   2.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  5  6
   2.2   Server . . . . . . . . . . . . . . . . . . . . . . . . . . .  5  6
   2.3   Client . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.4   Gateway  . . . . . . . . . . . . . . . . . . . . . . . . . .  6  7
   2.5   Network  . . . . . . . . . . . . . . . . . . . . . . . . . .  6  7
   3.    Addressing Scheme  . . . . . . . . . . . . . . . . . . . . .  7  8
   3.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  7  8
   3.2   Domain Identifier  . . . . . . . . . . . . . . . . . . . . .  7  8
   3.3   Node Identifier  . . . . . . . . . . . . . . . . . . . . . .  7  8
   3.4   Resource Identifier  . . . . . . . . . . . . . . . . . . . .  8  9
   4.    XML Streams  . . . . . . . . . . . . . . . . . . . . . . . .  9 10
   4.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  9 10
   4.2   Restrictions . . . . . . . . . . . . . . . . . . . . . . . . 10 11
   4.3   Stream Attributes  . . . . . . . . . . . . . . . . . . . . . 10 11
   4.4   Namespace Declarations . . . . . . . . . . . . . . . . . . . 11 12
   4.5   Stream Features  . . . . . . . . . . . . . . . . . . . . . . 12 13
   4.6   Stream Errors  . . . . . . . . . . . . . . . . . . . . . . . 12 13
   4.6.1 Rules  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.6.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.6.3 Conditions . . . . . . . . . . . . . . . . . . . . . . . . . 15
   4.6.4 Extensibility  . . . . . . . . . . . . . . . . . . . . . . . 16
   4.7   Simple Streams Example . . . . . . . . . . . . . . . . . . . 14 16
   5.    Stream Encryption  . . . . . . . . . . . . . . . . . . . . . 16 18
   5.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 16 18
   5.2   Narrative  . . . . . . . . . . . . . . . . . . . . . . . . . 17 19
   5.3   Client-Server Protocol .   Client-to-Server Example . . . . . . . . . . . . . . . . . . 17 19
   5.4   Certificate-Based Authentication   Server-to-Server Example . . . . . . . . . . . . . . 19 . . . . 21
   6.    Stream Authentication  . . . . . . . . . . . . . . . . . . . 20 24
   6.1   SASL Authentication  . . . . . . . . . . . . . . . . . . . . 20 24
   6.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 20 24
   6.1.2 Narrative  . . . . . . . . . . . . . . . . . . . . . . . . . 21 25
   6.1.3 SASL Definition  . . . . . . . . . . . . . . . . . . . . . . 22 26
   6.1.4 Client-Server Protocol Client-to-Server Example . . . . . . . . . . . . . . . . . . 27
   6.1.5 Server-to-Server Example . . . . . . . . . . . . . . . . . . 23 29
   6.2   Dialback Authentication  . . . . . . . . . . . . . . . . . . 25 31
   6.2.1 Dialback Protocol  . . . . . . . . . . . . . . . . . . . . . 27 34
   7.    XML Stanzas  . . . . . . . . . . . . . . . . . . . . . . . . 31 37
   7.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 31 37
   7.2   Common Attributes  . . . . . . . . . . . . . . . . . . . . . 31 37
   7.2.1 to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 37
   7.2.2 from . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 37
   7.2.3 id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 38
   7.2.4 type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 38
   7.2.5 xml:lang . . . . . . . . . . . . . . . . . . . . . . . . . . 32 38
   7.3   Message Stanzas  . . . . . . . . . . . . . . . . . . . . . . 32 38
   7.3.1 Types of Message . . . . . . . . . . . . . . . . . . . . . . 32 39
   7.3.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 33 39
   7.4   Presence Stanzas . . . . . . . . . . . . . . . . . . . . . . 34 40
   7.4.1 Types of Presence  . . . . . . . . . . . . . . . . . . . . . 34 41
   7.4.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 35 41
   7.5   IQ Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 36 42
   7.5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 36 43
   7.5.2 Types of IQ  . . . . . . . . . . . . . . . . . . . . . . . . 37 43
   7.5.3 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 38 44
   7.6   Extended Namespaces  . . . . . . . . . . . . . . . . . . . . 38
   8.    XML Usage within XMPP 44
   7.7   Stanza Errors  . . . . . . . . . . . . . . . . . . . 40
   8.1   Namespaces . . . . 45
   7.7.1 Rules  . . . . . . . . . . . . . . . . . . . . . 40
   8.2   Validation . . . . . . 45
   7.7.2 Syntax . . . . . . . . . . . . . . . . . . . 40
   8.3   Character Encodings . . . . . . . . 45
   7.7.3 Conditions . . . . . . . . . . . . 40
   8.4   Inclusion of Text Declaration . . . . . . . . . . . . . 46
   7.7.4 Extensibility  . . 40
   9.    IANA Considerations . . . . . . . . . . . . . . . . . . . . 41 . 47
   8.    XML Usage within XMPP  . . . . . . . . . . . . . . . . . . . 48
   8.1   Namespaces . . . . . . . . . . . . . . . . . . . . . . . . . 48
   8.2   Validation . . . . . . . . . . . . . . . . . . . . . . . . . 48
   8.3   Character Encodings  . . . . . . . . . . . . . . . . . . . . 48
   8.4   Inclusion of Text Declaration  . . . . . . . . . . . . . . . 48
   9.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 49
   10.   Internationalization Considerations  . . . . . . . . . . . . 42 50
   11.   Security Considerations  . . . . . . . . . . . . . . . . . . 43 51
   11.1  Client-to-Server Communications  . . . . . . . . . . . . . . 43 51
   11.2  Server-to-Server Communications  . . . . . . . . . . . . . . 43 51
   11.3  Firewalls  . . . . . . . . . . . . . . . . . . . . . . . . . 43 51
   11.4  Minimum Security Mechanisms  . . . . . . . . . . . . . . . . 43 51
         References . . . . . . . . . . . . . . . . . . . . . . . . . 45 53
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 47 55
   A.    Standard Error Codes    XML Schemas  . . . . . . . . . . . . . . . . . . . . 48
   B.    XML Schemas . . . . 56
   A.1   Streams namespace  . . . . . . . . . . . . . . . . . . . . 51
   B.1   streams . 56
   A.2   TLS namespace  . . . . . . . . . . . . . . . . . . . . . 51
   B.2 . . 57
   A.3   SASL namespace . . . . . . . . . . . . . . . . . . . . . . . 52
   B.3 57
   A.4   Dialback namespace . . . . . . . . . . . . . . . . . . . . . 52
   B.4   jabber:client 58
   A.5   Client namespace . . . . . . . . . . . . . . . . . . 54
   B.5   jabber:server . . . . 59
   A.6   Server namespace . . . . . . . . . . . . . . . . . . 56
   C.    Revision History . . . . 62
   A.7   Stream error namespace . . . . . . . . . . . . . . . . . . 60
   C.1   Changes from draft-ietf-xmpp-core-03 . 65
   A.8   Stanza error namespace . . . . . . . . . . . 60
   C.2   Changes from draft-ietf-xmpp-core-02 . . . . . . . . 66
   B.    Provisional Namespace Names  . . . . 60
   C.3   Changes from draft-ietf-xmpp-core-01 . . . . . . . . . . . . 60
   C.4   Changes from draft-ietf-xmpp-core-00 68
   C.    Revision History . . . . . . . . . . . . 60
   C.5 . . . . . . . . . . 69
   C.1   Changes from draft-miller-xmpp-core-02 draft-ietf-xmpp-core-04 . . . . . . . . . . . 61
         Full Copyright Statement . 69
   C.2   Changes from draft-ietf-xmpp-core-03 . . . . . . . . . . . . 69
   C.3   Changes from draft-ietf-xmpp-core-02 . . . . . 63

1. Introduction

1.1 Overview

   The Extensible Messaging and Presence . . . . . . . 69
   C.4   Changes from draft-ietf-xmpp-core-01 . . . . . . . . . . . . 69
   C.5   Changes from draft-ietf-xmpp-core-00 . . . . . . . . . . . . 70
   C.6   Changes from draft-miller-xmpp-core-02 . . . . . . . . . . . 70
         Intellectual Property and Copyright Statements . . . . . . . 72

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
   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]
   (since June 2001) and
   now the XMPP WG. WG (since November 2002). 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.

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
   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.

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 registered with 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 SIMPLE, 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.
   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 Upon opening a TCP socket on the IANA-
   assigned
   IANA-registered port 5269 and to negotiate 5269, there are two methods for negotiating a
   connection using the Dialback
   Protocol between any two servers: server dialback (Section 6.2) defined in this document. and
   SASL authentication (Section 6.1).

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

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, length
   and is subject MUST conform 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
   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 within IM users applications of XMPP 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 tertiary identifier. It usually
   represents a specific session, connection (e.g., a device or
   location), or object (e.g., a participant in a multi-user chat room)
   belonging to the entity associated with a node identifier. 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].

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 and XML stanzas:

   Definition of structured information that are referred to as "XML
   stanzas".  (Note: in this overview we use XML stream: An XML stream is a container for the example
      exchange of
   communications XML elements between any two entities over a client and server; however network.
      An XML streams are
   more generalized and may be used for communications from server to
   server and stream is negotiated from service to server as well.)

   In order to connect an initiating entity (usually a
      client or server) to a server, receiving entity (usually a client must initiate an server),
      normally over a TCP socket. An XML stream
   by sending an opening <stream> tag corresponds to the server, optionally preceded
   by a text declaration specifying
      initiating entity's session with the receiving entity. The start
      of the XML version supported stream is denoted unambiguously by an opening XML
      <stream> tag with appropriate attributes 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.
      declarations. The server SHOULD then reply with a second end of the XML stream back to the client, again optionally preceded by is denoted unambiguously
      be a text
   declaration.

   Within the context closing XML </stream> tag.

   Definition of an XML stream, a sender stanza: An XML stanza is able to send a discrete semantic unit
      of structured information to any recipient.
   This unit of structured information that is a well-balanced sent from one entity to another
      over an XML stream. An XML stanza,
   such as a message, presence, or IQ stanza (a stanza exists at the direct child level
      of an XML
   document the root <stream/> element and 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 unambiguously by the
      element start tag at depth=1 (e.g., <presence>), and the end of
      any XML stanza is
   unambiguously denoted unambiguously by the corresponding close
      tag at depth=1 (e.g., </presence>).  Each An 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 information.

   Consider the example of a client's request session with a server. In order to
   connect to a server, a client must initiate an XML stream by sending a closing </stream>
   an opening <stream> tag to the
   server (a session may also be closed server, optionally preceded by the server).

   Thus a client's session text
   declaration specifying the XML version supported and the character
   encoding. The server SHOULD then reply with a server can be seen as two open-ended second XML documents that are built up through stream back
   to the accumulation of client, again optionally preceded by a text declaration. Once
   the client has authenticated with the server (see Section 6), the
   client MAY send an unlimited number of XML stanzas sent over the course of stream to
   any recipient on the network. When the client desired to close the
   stream, it simply sends a closing </stream> tag to the server
   (alternatively, the session may be closed by the server).

   Thus a client's session (one 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 two XML streams (i.e., 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. each
   document. 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:

   |-------------------|
   | <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"): follows:

   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 XMPP address 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 XMPP address 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 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, attribute and the receiving entity
      supports XMPP 1.0, the receiving entity MUST reciprocate by
      including the attribute in its response (if the
      receiving entity supports XMPP 1.0). response.

   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 the use of a
   <stream:stream/> element as the stream root. The name of the stream
   namespace MUST be "http://etherx.jabber.org/streams".

   Since XML streams function as containers for any XML stanzas sent
   asynchronously between network endpoints.  It endpoints, it should be possible to
   scope an XML stream with any default namespace declaration, i.e., declaration (i.e., it
   should be possible to send any properly-namespaced XML stanza over an
   XML stream.  A stream). At a minimum, 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 flag 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
   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.

4.6.1 Rules

   The following rules apply to stream-level errors:

   o  It is assumed that all stream-level errors are unrecoverable;
      therefore, if an error occurs at the level of the stream, the
      entity that detects the error MUST send a stream error to the
      other entity 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>

   o  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 In 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 case, if the initiating entity provides an
      unknown host in the 'to' attribute (or provides no 'to' attribute
      at all), the server SHOULD provide the server's authoritative
      hostname in the 'from' attribute of the stream header.

4.6.2 Syntax

   The following codes are defined syntax for stream-level errors: stream errors is as follows:

   <stream:error class='error-class'>
     <stream-condition xmlns='urn:ietf:rfc:xmppcore-rfc-number:streams'>
       <descriptive-element-name/>
     </stream-condition>
   </stream:error>

   The value of the 'class' attribute must be one of the following:

   o  302 - Redirect  address -- the condition relates to the JID or domain to which the
      stream was addressed

   o  400 - Bad  format -- the condition relates to XML format or structure

   o  404 - Unknown Host  redirect -- the condition relates to a host redirection

   o  410 - Gone  server -- the condition relates to the internal state of the
      server

   The <stream-condition/> element MUST contain a child element that
   specifies a particular stream-level error condition, as defined in
   the next section. (Note: the XML namespace name
   'urn:ietf:rfc:xmppcore-rfc-number:streams' that scopes the
   <stream-condition/> element adheres to the format defined in RFC 2648
   [16].)

4.6.3 Conditions

   The following stream-level error conditions are defined:

   o  500 - Internal Server Error  <host-gone/> -- the value of the 'to' attribute provided by the
      initiating entity in the stream header corresponds to a hostname
      that is no longer hosted by the server; the associated class is
      "address".

   o  505 - Version Not Supported

   If  <host-unknown/> -- the value of the 'to' attribute provided by the
      initiating entity in the stream header does not correspond to a
      hostname that is hosted by the server; the associated class is
      "address".

   o  <internal-server-error/> -- the server has experienced a
      misconfiguration or an otherwise-undefined internal server error
      that prevents it from servicing the stream; the associated class
      is "server".

   o  <resource-constraint/> -- the server is resource-contrained and is
      unable to service the stream; the associated class is 302 ("Redirect"), "server".

   o  <see-other-host/> -- the server will not provide service to the
      initiating entity but is redirecting traffic to another host; this
      element SHOULD include contain CDATA specifying the alternate hostname or
      IP address to which the initiating entity may MAY attempt to connect.

4.7 Simple Streams Example

   The following connect;
      the associated class is a stream-based session of a client on a server
   (where "redirect".

   o  <system-shutdown/> -- the "C" lines server is being shut down and all active
      streams are sent from the client to being closed; the server, and associated class is "server".

   o  <unsupported-stanza-type/> -- the initiating entity has sent a
      first-level child of the stream that is not supported by the
      server; the associated class is "format".

   o  <unsupported-version/> -- the value of the 'version' attribute
      provided by the initiating entity in the stream header specifies a
      version of XMPP that is not supported by the server; this element
      MAY contain CDATA specifying the XMPP version(s) supported by the
      server; the associated class is "format".

   o  <xml-not-well-formed/> -- the initiating entity has sent XML that
      is not well-formed as defined by the XML specification [1]; the
      associated class is "format".

4.6.4 Extensibility

   If desired, an XMPP application MAY provide custom error information;
   this MUST be contained in a properly-namespaced child of the
   <stream-condition/> element (i.e., the namespace name MUST NOT be one
   of the namespace names defined herein).

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'
          to='domain'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          version='1.0'>
   S: <?xml version='1.0'?>
      <stream:stream
          from='server'
          from='domain'
          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:

   C: <?xml version='1.0'?>
      <stream:stream
          to='server'
          to='domain'
          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'
          from='domain'
          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'/> class='client'>
        <stream-condition xmlns='urn:ietf:rfc:xmppcore-rfc-number:streams'>
         <xml-not-well-formed/>
        </stream-condition>
      </stream:error>
   S: </stream:stream>

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.

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

   7.

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

   8.

   7.  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.

   8.  Whether the TLS negotiation results in success or failure, the
       initiating entity SHOULD continue with SASL negotiation.

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

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

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

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

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

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

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

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

5.3 Client-Server Protocol Client-to-Server Example

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

   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: failed
   (client SHOULD continue with stream authentication (Section 6)):

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

   Step 6: Client and server complete TLS negotiation via TCP.  When
   finished, the client

   Step 7: 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: 8: 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>

   Step 9: Client SHOULD continue with stream authentication (Section
   6).

5.4 Certificate-Based Authentication

   If the client presents a valid client certificate during the TLS
   negotiation, the Server-to-Server Example

   By bilateral agreement, server administrators MAY offer the SASL EXTERNAL mechanism choose to use TLS
   between two domains for the
   client during stream authentication.  (see RFC 2222 [16]).  If purpose of securing server-to-server
   communications.

   The following example shows the
   client selects this mechanism data flow for authentication, two servers securing a
   stream using STARTTLS.

   Step 1: Server1 initiates stream to Server2:

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       version='1.0'>
   Step 2: Server2 responds by sending a stream tag to Server1:

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

   Step 3: Server2 sends the STARTTLS extension to Server1 along with
   authentication
   credentials shall be taken from 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>KERBEROS_V4</mechanism>
     </mechanisms>
   </stream:features>

   Step 4: Server1 sends the presented certificate. STARTTLS command to Server2:

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

   Step 5: Server2 informs Server1 to proceed:

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

   Step 5 (alt): Server2 informs Server1 that TLS negotiation has failed
   (Server1 SHOULD continue with stream authentication (Section 6)):

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

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

   Step 7: Server1 initiates a new stream to Server2:

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       version='1.0'>
   Step 8: Server2 responds by sending a stream header to Server1 along
   with any remaining negotiatiable stream features:

   <stream:stream
       xmlns='jabber:server'
       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>KERBEROS_V4</mechanism>
       <mechanism>EXTERNAL</mechanism>
     </mechanisms>
   </stream:features>

   Step 9: Server1 SHOULD continue with stream authentication (Section
   6).

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]. [22]. When there is no existing trust relationship
   between the two entities, such servers, some level of trust MAY be established based on
   existing trust in DNS; the authentication method used when two such
   entities are servers in this case 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 If SASL is used for
   server-to-server authentication, the servers MUST NOT use dialback.
   Both SASL authentication and dialback 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
   connection-based protocols. XMPP uses a generic XML namespace profile
   for SASL that conforms to section 4 ("Profiling Requirements") of RFC
   2222
   [16] [22] (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.

   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 an initiating entity authenticates with a server, receiving entity, the
   steps involved are as follows:

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

   2.  After sending an XML stream header in response, the server receiving
       entity 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 receiving entity MUST NOT provide that mechanism
       in the list of available SASL authentication methods. If the
       initiating entity presents a valid initiating entity certificate
       during TLS negotiation, the receiving entity MAY offer the SASL
       EXTERNAL mechanism to the initiating entity during stream
       authentication (see RFC 2222 [22]).

   3.  The client initiating entity selects a mechanism by sending an <auth/>
       element to the server; receiving entity; this element MAY optionally
       contain character data (in SASL terminology the "initial
       response") if the mechanism supports or requires it. If the
       initiating entity selects the EXTERNAL mechanism for
       authentication, the authentication credentials shall be taken
       from the certificate presented during TLS negotiation.

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

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

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

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

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

   2.  The server receiving entity reports failure of the handshake by sending
       a <failure/> element to the client. initiating entity. 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 receiving entity reports success of the handshake by sending
       a <success/> element to the client; initiating entity; 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] [22] 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.

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

6.1.4 Client-Server Protocol Client-to-Server Example

   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'/>

   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
   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'/>

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

   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

6.1.5 Server-to-Server Example

   By bilateral agreement, server administrators MAY choose to use SASL
   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 domains for 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 server-to-server
   communications.

   The following example shows the streams between servers, only data flow 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 authenticating
   with another server which is connecting to it is authorized using SASL.

   Step 1: Server1 initiates stream to represent Server2:

   <stream:stream
       xmlns='jabber:server'
       xmlns:stream='http://etherx.jabber.org/streams'
       version='1.0'>
   Step 2: Server2 responds with a
   given server on the Jabber network.  All DNS hostname resolutions
   MUST first resolve the hostname using an SRV [23] record stream tag sent to Server1:

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

   Step 3: Server2 informs Server1 of
   _jabber._tcp.server.  If the SRV lookup fails, the fallback is available authentication
   mechanisms:

   <stream:features>
     <mechanisms xmlns='http://www.iana.org/assignments/sasl-mechanisms'>
       <mechanism>DIGEST-MD5</mechanism>
       <mechanism>KERBEROS_V4</mechanism>
     </mechanisms>
   </stream:features>

   Step 4: Server1 selects an authentication mechanism ("initial
   response"):

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

   Step 5: Server2 sends a
   normal A lookup base64-encoded challenge to Server1:

   <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: Server1 responds to determine the IP address, using challenge:

   <response xmlns='http://www.iana.org/assignments/sasl-mechanisms'>
       cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixjbm9uY2U9Ik9B
       Nk1IWGg2VnFUclJrIixuYz0wMDAwMDAwMSxxb3A9YXV0aCxkaWdlc3QtdXJpPSJ4bXBwL2Nh
       dGFjbHlzbS5jeCIscmVzcG9uc2U9ZDM4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcs
       Y2hhcnNldD11dGYtOAo=
   </response>

   The decoded response is:

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

   Step 7: Server2 sends another challenge to Server1:

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

   The decoded challenge is:

   rspauth=ea40f60335c427b5527b84dbabcdfffd

   Step 8: Server1 responds to the jabber-server
   port challenge:

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

   Step 9: Server2 informs Server1 of 5269 assigned successful authentication:

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

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

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

   Step 10: Server1 initiates a new stream to Server2:

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

   Step 11: Server2 responds by sending a stream header to Server1, with
   the Internet Assigned Numbers Authority [7].

   Note that the stream already authenticated (not followed by further stream
   features):

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

6.2 Dialback Authentication

   XMPP includes a protocol-level method for generating and verifying the keys used in
   the dialback protocol MUST take into account 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. Domains requiring
   more robust security SHOULD use TLS or SASL as defined above.

   Server 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
   [24] 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].

   Note: 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   |
       | ----------------------> |
       |                         |
       |   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):

   <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:

   <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:

   <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.

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.
   Three XML stanzas stanza types are the three core data elements defined for XMPP communications: the 'jabber:client' and
   'jabber:server' namespaces: <message/>, <presence/>, and <iq/>.  These elements are sent as
   direct (depth=1) children of

   In essence, the root <stream/> element and are
   scoped by <message/> stanza type can be seen as a "push"
   mechanism whereby one of entity pushes information to another entity,
   similar to the default namespaces identified in Section 4.4. communications that occur in a system such as email.
   The <presence/> element can be seen as a basic broadcast or
   "publish-subscribe" mechanism, whereby multiple entities receive
   information (in this case, presence information) about an entity to
   which they have subscribed. The <iq/> element can be seen as a
   "request-response" mechanism similar to HTTP, whereby two entities
   can engage in a structured conversation using 'get' or 'set' requests
   and 'result' or 'error' responses.

   The syntax for these stanza types is defined below.

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. attribute and MAY return an error
   to the sender. In addition, a server MUST stamp stanzas received from
   a client with the user@domain/
   resource 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 (or a subdomain thereof) as communicated in the dialback SASL
   negotiation (or a subdomain
   thereof). or dialback negotiation.

7.2.3 id

   The optional 'id' attribute MAY be used to track stanzas sent and
   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]. [23].

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
      conversation.

   o  error

   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 If the message stanza is of type "error", it MUST include an
   <error/> child; for details, see Section 7.7. If the 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 --

7.3.2.1 Body

   The <body/> element contains 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 --

7.3.2.2 Subject

   The subject <subject/> element specifies the topic 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.

   o  thread -- A

7.3.2.3 Thread

   The <thread/> element contains 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 Only one <thread/> element MAY 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 included in 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,
   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 method for generating thread IDs SHOULD be as follows:

   1.  concatenate 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 sender's full JID (user@host/resource) with the standard error codes (Appendix A), MAY possess an
   'xml:lang' attribute, and MAY also contain PCDATA corresponding to
       recipient's full JID

   2.  concatenate these JID strings with a
   natural-language description of full ISO-8601 timestamp
       including year, month, day, hours, minutes, seconds, and UTC
       offset in the error.  An <error/> child MUST
   NOT be included if following format: yyyy-mm-dd-Thh:mm:ss-hh:mm

   3.  hash the stanza type is anything other than "error".
   An entity that receives a message stanza of type 'error' MUST NOT
   respond resulting string according to the stanza by sending a further message stanza of type
   'error'; this helps SHA1 algorithm

   4.  convert the hexidecimal SHA1 output to prevent looping. all lowercase

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) text) and to communicate that status to
   other entities.  They Presence stanzas 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 possess 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 a lack of
   the sender, availability, a
   request to manage a subscription to another entity's 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 If the presence stanza is of type "error", it MUST include
   an <error/> child; for details, see Section 7.7. If the 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

7.4.2.1 Show

   The <show/> element specifies 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 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):

      *

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

      *

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

      *

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

      *

   o  dnd --

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

7.4.2.2 Status

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

   o  status -- An optional natural-language description <status/> element specifies a natural-language
   description of availability status.  Normally It is 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

7.4.2.3 Priority

   The optional <priority/> element specifying specifies 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 For information regarding the presence stanza is
   use of type "error", it MUST include an <error/
   > child, which priority values 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). routing within IM applications, see
   XMPP IM [3].

7.5 IQ Stanzas

7.5.1 Overview

   Info/Query, or IQ, is a request-response mechanism, similar in some
   ways to HTTP [24]. [25]. IQ stanzas in the 'jabber:client' or
   '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.

   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.
   child; for details, see Section 7.7.

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
   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.

8. XML Usage within XMPP

8.1 Namespaces

   XML Namespaces [15] are used within all XMPP-compliant XML to create
   strict boundaries If an entity
   receives an IQ stanza in a namespace it does not understand, the
   entity SHOULD return an IQ stanza of data ownership.  The basic function type "error" with an error
   condition of
   namespaces is <feature-not-implemented/>.

7.7 Stanza Errors

   As defined below, stanza-related errors are handled in a manner
   similar to separate different vocabularies stream errors (Section 4.6).

7.7.1 Rules

   The following rules apply to stanza-related errors:

   o  A stanza of XML elements that
   are structurally mixed together.  Ensuring type "error" MUST contain an <error/> child element.

   o  The receiving or processing entity that XMPP-compliant XML is
   namespace-aware enables any XML returns an error to be structurally mixed the
      sending entity SHOULD include the original XML sent along with any
   data the
      <error/> element within XMPP.

   Additionally, XMPP is more strict about namespace prefixes than and its children so that the
   XML namespace specification requires.

8.2 Validation

   A server is not responsible for validating sender can inspect
      and if necessary correct the XML elements forwarded
   to before re-sending.

   o  An entity that receives a client;  an implementation MAY choose to provide only validated
   data elements but is message stanza of type 'error' MUST NOT REQUIRED
      respond to do so.  Clients SHOULD NOT rely
   on the ability to send data which does not conform stanza by sending a further message stanza of type
      'error'; this helps to prevent looping.

   o  An <error/> child MUST NOT be included if the schemas,
   and SHOULD ignore any non-conformant elements or attributes on the
   incoming XML stream.  Validation of stanza type is
      something other than "error".

7.7.2 Syntax

   The syntax for stanza-related errors is as follows:

   <stanza-name to='sender' type='error'>
     [include sender XML streams and stanzas here]
     <error class='error-class'>
       <stanza-condition xmlns='urn:ietf:rfc:xmppcore-rfc-number:stanzas'>
         <descriptive-element-name/>
       </stanza-condition>
     </error>
   </stanza-name>

   The stanza name is NOT
   REQUIRED one of message, presence, 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. iq.

   The
   encodings value of the transmitted and received streams are independent.
   Software MAY select either UTF-8 'class' attribute must be one of the following:

   o  access -- the condition relates to access rights, permissions, or UTF-16 for
      authorization

   o  address -- the transmitted
   stream, and SHOULD deduce condition relates to the encoding of JID or domain to which the received stream as
   described
      stanza was addressed

   o  app -- the condition is particular to an application and is
      specified in a namespace other than
      'urn:ietf:rfc:xmppcore-rfc-number:stanzas'

   o  format -- the condition relates to XML specification [1].  For historical reasons,
   existing implementations MAY support UTF-8 only.

8.4 Inclusion format or structure

   o  recipient -- the condition relates to the state or capabilities of Text Declaration

   An application MAY send a text declaration.  Applications MUST follow
      the rules in recipient (which may be the XML specification [1] regarding server)

   o  server -- the circumstances
   under which a text declaration is included.

9. IANA Considerations condition relates to the internal state of the
      server

   The IANA registers "xmpp" as <stanza-condition/> element MUST contain a GSSAPI [29] service name, child element that
   specifies a particular stanza-related error condition, as specified defined in Section 6.1.3.

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

10. Internationalization Considerations

   Usage of next section. (Note: the 'xml:lang' attribute is described above.  If a client
   includes an 'xml:lang' attribute in a stanza, XML namespace name
   'urn:ietf:rfc:xmppcore-rfc-number:stanzas' that scopes the server MUST NOT
   modify or delete it.

11. Security Considerations

11.1 Client-to-Server Communications

   The TLS protocol for encrypting XML streams provides a reliable
   mechanism for helping
   <stanza-condition/> element adheres to ensure the privacy and data integrity of
   data exchanged between two entities. format defined in RFC 2648
   [16].)

7.7.3 Conditions

   The SASL protocol for authenticating following stanza-related error conditions are defined:

   o  <bad-request/> -- the sender has sent 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 malformed or
      cannot be made
   available by processed (e.g., a server, nor are any connections other than client-generated stanza includes a
      'from' address, or an IQ stanza includes an unrecognized value of
      the
   original server connection required.  This helps protect 'type' attribute); the client's
   server from direct attack or identification associated class is "format".

   o  <feature-not-implemented/> -- the feature requested is not
      implemented by third parties.

   End-to-end encryption of message bodies the recipient or server and presence status
   information MAY therefore cannot be effected through use of
      processed; the methods defined in
   End-to-End Object Encryption in XMPP [28].

11.2 Server-to-Server Communications

   It associated class is OPTIONAL for any given "recipient".

   o  <forbidden/> -- the stanza is understood but the action is
      forbidden; the associated class is "access".

   o  <internal-server-error/> -- the server to communicate with other
   servers, and server-to-server communications MAY be disabled by could not process the
   administrator
      stanza because of any given deployment.

   If two servers would like to enable communications between
   themselves, they MUST form a relationship misconfiguration or an otherwise-undefined
      internal server error; the associated class is "server".

   o  <jid-malformed/> -- the value of trust at some level,
   either based on trust the 'to' attribute 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
      sender's stanza does not have a pre-existing relationship, they MUST
   use adhere to the Dialback Protocol syntax defined in
      Addressing (Section 6.2), which provides a reliable
   mechanism for preventing 3); the spoofing associated class is "address".

   o  <jid-not-found/> -- the value of servers.

11.3 Firewalls

   Communications using XMPP occur over TCP sockets the 'to' attribute in the
      sender's stanza does not correspond to a Jabber ID on port 5222
   (client-to-server) the user's
      server or port 5269 (server-to-server), as registered
   with a remote server; the IANA [7].  Use of these well-known ports allows
   administrators associated class is "address".

   o  <not-allowed/> -- the action is not permitted when attempted by
      the sender; the associated class is "access".

   o  <recipient-unavailable/> -- the specific recipient requested is
      currently unavailable; the associated class is "recipient".

   o  <registration-required/> -- the user is not authorized to easily enable or disable XMPP activity through
   existing and commonly-deployed firewalls.

11.4 Minimum Security Mechanisms

   Although access
      the requested service provisioning because registration is required; the
      associated class is "access".

   o  <remote-server-not-found/> -- a policy matter, at remote server or service specified
      as part or all of the JID of the intended recipient does not
      exist; the associated class is "address".

   o  <remote-server-timeout/> -- a minimum, remote server or service specified
      as part or all
   implementations MUST support of the following mechanisms:

   for authentication: JID of the SASL DIGEST-MD5 mechanism

   for confidentiality: TLS (using intended recipient could not be
      contacted within a reasonable amount of time; the TLS_RSA_WITH_3DES_EDE_CBC_SHA
      cipher)

   for both: TLS (using associated class
      is "server".

   o  <service-unavailable/> -- the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher
      supporting client-side certificates)

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
         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 service requested is currently
      unavailable on the Identification server; the associated class is "server".

7.7.4 Extensibility

   If desired, an XMPP application MAY provide custom error information;
   this MUST be contained in a properly-namespaced child of Languages", BCP
         47, RFC 3066, January 2001.

   [23]  Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
         location
   <stanza-condition/> element (i.e., the namespace name MUST NOT be one
   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 namespace names defined herein).

8. XML Usage within XMPP

8.1 Namespaces

   XML Namespaces [15] are used within all XMPP-compliant XML to create
   strict boundaries of ISO 10646", RFC
         2279, January 1998.

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

   [27]  International Organization XML streams
   and stanzas is NOT REQUIRED or recommended, and schemas are included
   herein for Standardization, "Information
         Technology - descriptive purposes only.

8.3 Character Encodings

   Software implementing XML streams MUST support the UTF-8 (RFC 2279
   [26]) and UTF-16 (RFC 2781 [27]) transformations of Universal Multiple-octet coded
   Character Set (UCS)
         - Amendment 2: UCS Transformation Format 8 (UTF-8)", ISO
         Standard (ISO/IEC 10646-1 Addendum 2, October 1996.

   [28]  Saint-Andre, P. [28]) characters. Software MUST NOT
   attempt to use any other encoding for transmitted data. The encodings
   of the transmitted and J. Hildebrand, "End-to-End Object
         Encryption 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 XMPP (draft-ietf-xmpp-e2e-00, work 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 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

Appendix A. Standard Error Codes

   A standard error element the XML specification [1] regarding the circumstances
   under which a text declaration is used included.

9. IANA Considerations

   The IANA registers "xmpp" as a GSSAPI [30] service name, as specified
   in Section 6.1.3.

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

10. Internationalization Considerations

   Usage of XML stanzas
   within the "jabber:client" or "jabber:server" namespace.  This
   element 'xml:lang' attribute is described above. If a child of the failed stanza and MUST include a 'code'
   attribute corresponding to client
   includes an appropriate error condition.

   In general the standard error codes were "borrowed" from those used
   in HTTP [24] early 'xml:lang' attribute in a stanza, the development of XMPP within the Jabber
   community. server MUST NOT
   modify or delete it.

11. Security Considerations

11.1 Client-to-Server Communications

   The first digit of the error code defines TLS protocol for encrypting XML streams (defined under Section 5)
   provides a reliable mechanism for helping to ensure the class privacy and
   data integrity of
   response.  The last data exchanged between 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 - entities.

   The action was successfully received, understood,
      and accepted [not currently used within XMPP]

   o  3xx: Redirection - Further action must be taken in order SASL protocol for authenticating XML streams (defined under
   Section 6.1) provides a reliable mechanism for validating that a
   client connecting 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 server is who it claims to fulfill an apparently valid request be.

   The individual values of the numeric status/error codes defined for
   XMPP, IP address and an example set method of corresponding textual descriptions, are
   presented below.  The textual descriptions listed here access of clients MUST NOT be made
   available by a server, nor are only
   recommendations -- they 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 replaced by local equivalents without
   affecting effected through use of the protocol.

   o  302 (Redirect) - Code 302 methods defined in
   End-to-End Object Encryption in XMPP [29].

11.2 Server-to-Server Communications

   It is used when a OPTIONAL for any given 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 communicate with other
   servers, and server-to-server communications MAY be understood disabled by the recipient.  This might be
      generated when an entity sends non-well-formed XML
   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 when based on a
      message stanza does not pre-existing trust
   relationship (e.g., through exchange of certificates). If two servers
   have a 'to' attribute.

   o  401 (Unauthorized) - Code 401 is used to inform clients that pre-existing trust relationship, 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 MAY use SASL
   Authentication (Section 6.1) for future
      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 purpose of authenticating each
   other. If they do not have a sender that no
      recipient was found matching pre-existing relationship, they MUST use
   the JID to Dialback Protocol (Section 6.2), which an XML stanza was
      sent, e.g., if provides a sender has attempted reliable
   mechanism for preventing the spoofing of servers.

11.3 Firewalls

   Communications using XMPP normally 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 send easily enable or disable XMPP activity through
   existing and commonly-deployed firewalls.

11.4 Minimum Security Mechanisms

   Although service provisioning is a message to policy matter, at a JID
      that does not exist.  (Note: if minimum, all
   implementations MUST support the server of following mechanisms:

   for authentication: the intended
      recipient cannot be reached, an error code from SASL DIGEST-MD5 mechanism

   for confidentiality: TLS (using the 500 series
      must be sent.)

   o  405 (Not Allowed) - Code 405 is used when TLS_RSA_WITH_3DES_EDE_CBC_SHA
      cipher)

   for both: TLS (using the action requested is
      not allowed TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher
      supporting client-side certificates)

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-05, work in progress)", March 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 the JID identified by the 'from' address, e.g., if
      a client attempts to set the time or version Internationalized Domain Names (draft-ietf-idn-nameprep-11,
         work in progress)", June 2002.

   [12]  Hoffman, P. and M. Blanchet, "Preparation of a server.

   o  406 (Not Acceptable) - Code 406 is used when an XML stanza is Internationalized
         Strings ("stringprep")", RFC 3454, December 2002.

   [13]  Saint-Andre, P. and J. Hildebrand, "Nodeprep: A Stringprep
         Profile for
      some reason not acceptable to a server or other entity.  This
      might be generated when, Node Identifiers in XMPP
         (draft-ietf-xmpp-nodeprep-01, work in progress)", February
         2003.

   [14]  Saint-Andre, P. and J. Hildebrand, "Resourceprep: A Stringprep
         Profile 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 Resource Identifiers 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) 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]  Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
         August 1999.

   [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 - 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) Version
         4rev1", RFC 2060, December 1996.

   [19]  Myers, J. and M. Rose, "Post Office Protocol - Code 501 is used when the recipient does
      not support the functionality being requested by a sender, e.g.,
      if a user attempts to register 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 a server that does not allow
      registration.

   o  502 (Remote Server Error) - Code 502 is used when delivery of an
      XML stanza fails because IMAP, POP3 and ACAP", RFC 2595,
         June 1999.

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

   [23]  Alvestrand, H., "Tags for the Identification of an inability to reach Languages", BCP
         47, RFC 3066, January 2001.

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

   [25]  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.

   [26]  Yergeau, F., "UTF-8, a remote server timeout, e.g., if transformation format of ISO 10646", RFC
         2279, January 1998.

   [27]  Hoffman, P. and F. Yergeau, "UTF-16, an incorrect hostname is
      specified.

   o  505 (Version Not Supported) encoding of ISO 10646",
         RFC 2781, February 2000.

   [28]  International Organization for Standardization, "Information
         Technology - Code 505 is used when a server does
      not support the Universal Multiple-octet coded Character Set (UCS)
         - Amendment 2: UCS Transformation Format 8 (UTF-8)", ISO
         Standard 10646-1 Addendum 2, October 1996.

   [29]  Saint-Andre, P. and J. Hildebrand, "End-to-End Object
         Encryption in XMPP version requested by an entity that initiates
      a stream to the server. (draft-ietf-xmpp-e2e-00, work in progress)",
         February 2003.

   [30]  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

Appendix B. A. XML Schemas

   The following XML schemas are descriptive, not normative.

B.1 streams

A.1 Streams namespace

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

   <xs:schema
       xmlns:xsd='http://www.w3.org/2001/XMLSchema'
       xmlns:xs='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'/> maxOccurs='1'/>
         <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'/> name='features'>
       <xs:complexType>
         <xs:any
             namespace='##other'
             minOccurs='0'
             maxOccurs='unbounded'/>
       </xs:complexType>
     </xs:element>

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

   </xs:schema>

B.2

A.2 TLS namespace

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

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='http://www.ietf.org/rfc/rfc2595.txt'
       xmlns='http://www.ietf.org/rfc/rfc2595.txt'
       elementFormDefault='qualified'>

     <xs:element name='starttls'/>
     <xs:element name='proceed'/>
     <xs:element name='failure'/>

   </xs:schema>

A.3 SASL namespace
   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xsd='http://www.w3.org/2001/XMLSchema'
       xmlns:xs='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> ref='mechanism' maxOccurs='unbounded'>
       </xs:complexType>
     </xs:element>

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

     <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

A.4 Dialback namespace

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

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

     <xs:element name='result'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs: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>

B.4 jabber:client

A.5 Client namespace

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

   <xs:schema
       xmlns:xsd='http://www.w3.org/2001/XMLSchema'
       xmlns:xs='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'/>
       </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'>
       <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' name='class' use='required'/>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='access'/>
               <xs:enumeration value='address'/>
               <xs:enumeration value='app'/>
               <xs:enumeration value='format'/>
               <xs:enumeration value='recipient'/>
               <xs:enumeration value='server'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>

B.5 jabber:server

A.6 Server namespace

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

   <xs:schema
       xmlns:xsd='http://www.w3.org/2001/XMLSchema'
       xmlns:xs='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'/>

     <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>

     <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' name='class' use='required'/>
         <xs:attribute name='xml:lang' type='xs:NMTOKEN' use='optional'/>
           <xs:simpleType>
             <xs:restriction base='xs:NCName'>
               <xs:enumeration value='access'/>
               <xs:enumeration value='address'/>
               <xs:enumeration value='app'/>
               <xs:enumeration value='format'/>
               <xs:enumeration value='recipient'/>
               <xs:enumeration value='server'/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
       </xs:complexType>
     </xs:element>

   </xs:schema>

A.7 Stream error namespace

   <?xml version='1.0' encoding='UTF-8'?>
   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:rfc:xmppcore-rfc-number:streams'
       xmlns='urn:ietf:rfc:xmppcore-rfc-number:streams'
       elementFormDefault='qualified'>

     <xs:element name='stream-condition'>
       <xs:complexType>
         <xs:any
             namespace='##other'
             minOccurs='0'
             maxOccurs='1'/>
         <xs:choice maxOccurs='1'>
           <xs:element ref='host-gone'/>
           <xs:element ref='host-unknown'/>
           <xs:element ref='internal-server-error'/>
           <xs:element ref='resource-constraint'/>
           <xs:element ref='see-other-host'/>
           <xs:element ref='system-shutdown'/>
           <xs:element ref='unsupported-stanza-type'/>
           <xs:element ref='unsupported-version'/>
           <xs:element ref='xml-not-well-formed'/>
         </xs:choice>
       </xs:complexType>

     <xs:element name='host-gone' type='xs:string'/>
     <xs:element name='host-unknown' type='xs:string'/>
     <xs:element name='internal-server-error' type='xs:string'/>
     <xs:element name='resource-constraint' type='xs:string'/>
     <xs:element name='see-other-host' type='xs:string'/>
     <xs:element name='system-shutdown' type='xs:string'/>
     <xs:element name='unsupported-stanza-type' type='xs:string'/>
     <xs:element name='unsupported-version' type='xs:string'/>
     <xs:element name='xml-not-well-formed' type='xs:string'/>

   </xs:schema>

A.8 Stanza error namespace

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

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:rfc:xmppcore-rfc-number:stanzas'
       xmlns='urn:ietf:rfc:xmppcore-rfc-number:stanzas'
       elementFormDefault='qualified'>
     <xs:element name='stanza-condition'>
       <xs:complexType>
         <xs:any
             namespace='##other'
             minOccurs='0'
             maxOccurs='1'/>
         <xs:choice maxOccurs='1'>
           <xs:element ref='bad-request'/>
           <xs:element ref='feature-not-implemented'/>
           <xs:element ref='forbidden'/>
           <xs:element ref='internal-server-error'/>
           <xs:element ref='jid-malformed'/>
           <xs:element ref='jid-not-found'/>
           <xs:element ref='not-allowed'/>
           <xs:element ref='recipient-unavailable'/>
           <xs:element ref='registration-required'/>
           <xs:element ref='remote-server-not-found'/>
           <xs:element ref='remove-server-timeout'/>
           <xs:element ref='service-unavailable'/>
         </xs:choice>
       </xs:complexType>
     </xs:element>

     <xs:element name='bad-request' type='xs:string'/>
     <xs:element name='feature-not-implemented' type='xs:string'/>
     <xs:element name='forbidden' type='xs:string'/>
     <xs:element name='internal-server-error' type='xs:string'/>
     <xs:element name='jid-malformed' type='xs:string'/>
     <xs:element name='jid-not-found' type='xs:string'/>
     <xs:element name='not-allowed' type='xs:string'/>
     <xs:element name='recipient-unavailable' type='xs:string'/>
     <xs:element name='registration-required' type='xs:string'/>
     <xs:element name='remote-server-not-found' type='xs:string'/>
     <xs:element name='remote-server-timeout' type='xs:string'/>
     <xs:element name='service-unavailable' type='xs:string'/>

   </xs:schema>

Appendix B. Provisional Namespace Names

   Note to RFC editor: prior to publication, the string
   'xmppcore-rfc-number' must be replaced in all instances by the RFC
   number assigned to this draft. (In addition, please remove this
   appendix, and the corresponding entry in the table of contents, prior
   to publication.)

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-04

   o  Added server-to-server examples for TLS and SASL.

   o  Changed error syntax, rules, and examples based on list
      discussion.

   o  Added schemas for the TLS, stream error, and stanza error
      namespaces.

   o  Added note to RFC editor regarding provisional namespace names.

   o  Made numerous small editorial changes and clarified text
      throughout.

C.2 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

C.3 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

C.4 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

C.5 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.

   o  Made numerous small editorial fixes.

C.5

C.6 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.

   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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