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rfc:rfc3920

Network Working Group P. Saint-Andre, Ed. Request for Comments: 3920 Jabber Software Foundation Category: Standards Track October 2004

      Extensible Messaging and Presence Protocol (XMPP): Core

Status of this Memo

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

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

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

Saint-Andre, Ed. Standards Track [Page 1] RFC 3920 XMPP Core October 2004

Table of Contents

 1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.   Generalized Architecture . . . . . . . . . . . . . . . . . .   3
 3.   Addressing Scheme  . . . . . . . . . . . . . . . . . . . . .   5
 4.   XML Streams  . . . . . . . . . . . . . . . . . . . . . . . .   7
 5.   Use of TLS . . . . . . . . . . . . . . . . . . . . . . . . .  19
 6.   Use of SASL  . . . . . . . . . . . . . . . . . . . . . . . .  27
 7.   Resource Binding . . . . . . . . . . . . . . . . . . . . . .  37
 8.   Server Dialback  . . . . . . . . . . . . . . . . . . . . . .  41
 9.   XML Stanzas  . . . . . . . . . . . . . . . . . . . . . . . .  48
 10.  Server Rules for Handling XML Stanzas  . . . . . . . . . . .  58
 11.  XML Usage within XMPP  . . . . . . . . . . . . . . . . . . .  60
 12.  Core Compliance Requirements . . . . . . . . . . . . . . . .  62
 13.  Internationalization Considerations  . . . . . . . . . . . .  64
 14.  Security Considerations  . . . . . . . . . . . . . . . . . .  64
 15.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  69
 16.  References . . . . . . . . . . . . . . . . . . . . . . . . .  71
 A.   Nodeprep . . . . . . . . . . . . . . . . . . . . . . . . . .  75
 B.   Resourceprep . . . . . . . . . . . . . . . . . . . . . . . .  76
 C.   XML Schemas  . . . . . . . . . . . . . . . . . . . . . . . .  78
 D.   Differences Between Core Jabber Protocols and XMPP . . . . .  87
 Contributors. . . . . . . . . . . . . . . . . . . . . . . . . . .  89
 Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . .  89
 Author's Address. . . . . . . . . . . . . . . . . . . . . . . . .  89
 Full Copyright Statement. . . . . . . . . . . . . . . . . . . . .  90

1. Introduction

1.1. Overview

 The Extensible Messaging and Presence Protocol (XMPP) is an open
 Extensible Markup Language [XML] protocol for near-real-time
 messaging, presence, and request-response services.  The basic syntax
 and semantics were developed originally within the Jabber open-source
 community, mainly in 1999.  In 2002, the XMPP WG was chartered with
 developing an adaptation of the Jabber protocol that would be
 suitable as an IETF instant messaging (IM) and presence technology.
 As a result of work by the XMPP WG, the current memo defines the core
 features of XMPP 1.0; the extensions required to provide the instant
 messaging and presence functionality defined in RFC 2779 [IMP-REQS]
 are specified in the Extensible Messaging and Presence Protocol
 (XMPP): Instant Messaging and Presence [XMPP-IM].

Saint-Andre, Ed. Standards Track [Page 2] RFC 3920 XMPP Core October 2004

1.2. Terminology

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

2. Generalized Architecture

2.1. Overview

 Although XMPP is not wedded to any specific network architecture, to
 date it usually has been implemented via a client-server architecture
 wherein a client utilizing XMPP accesses a server over a [TCP]
 connection, and servers also communicate with each other over TCP
 connections.
 The following diagram provides a high-level overview of this
 architecture (where "-" represents communications that use XMPP and
 "=" represents communications that use any other protocol).
 C1----S1---S2---C3
       |
 C2----+--G1===FN1===FC1
 The symbols are as follows:
 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:
 o  to manage connections from or sessions for other entities, in the
    form of XML streams (Section 4) to and from authorized clients,
    servers, and other entities

Saint-Andre, Ed. Standards Track [Page 3] RFC 3920 XMPP Core October 2004

 o  to route appropriately-addressed XML stanzas (Section 9) among
    such entities over XML streams
 Most XMPP-compliant servers also assume responsibility for the
 storage of data that is used by clients (e.g., contact lists for
 users of XMPP-based instant messaging and presence applications); in
 this case, the XML data is processed directly by the server itself on
 behalf of the client and is not routed to another entity.

2.3. Client

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

2.4. Gateway

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

2.5. Network

 Because each server is identified by a network address and because
 server-to-server communications are a straightforward extension of
 the client-to-server protocol, in practice, the system consists of a
 network of servers that inter-communicate.  Thus, for example,
 <juliet@example.com> is able to exchange messages, presence, and
 other information with <romeo@example.net>.  This pattern is familiar
 from messaging protocols (such as [SMTP]) that make use of network
 addressing standards.  Communications between any two servers are
 OPTIONAL.  If enabled, such communications SHOULD occur over XML
 streams that are bound to [TCP] connections.  The RECOMMENDED port
 for connections between servers is 5269, as registered with the IANA
 (see Port Numbers (Section 15.9)).

Saint-Andre, Ed. Standards Track [Page 4] RFC 3920 XMPP Core October 2004

3. Addressing Scheme

3.1. Overview

 An entity is anything that can be considered a network endpoint
 (i.e., an ID on the network) and that can communicate using XMPP.
 All such entities are uniquely addressable in a form that is
 consistent with RFC 2396 [URI].  For historical reasons, the address
 of an XMPP entity is called a Jabber Identifier or JID.  A valid JID
 contains a set of ordered elements formed of a domain identifier,
 node identifier, and resource identifier.
 The syntax for a JID is defined below using the Augmented Backus-Naur
 Form as defined in [ABNF].  (The IPv4address and IPv6address rules
 are defined in Appendix B of [IPv6]; the allowable character
 sequences that conform to the node rule are defined by the Nodeprep
 profile of [STRINGPREP] as documented in Appendix A of this memo; the
 allowable character sequences that conform to the resource rule are
 defined by the Resourceprep profile of [STRINGPREP] as documented in
 Appendix B of this memo; and the sub-domain rule makes reference to
 the concept of an internationalized domain label as described in
 [IDNA].)
    jid             = [ node "@" ] domain [ "/" resource ]
    domain          = fqdn / address-literal
    fqdn            = (sub-domain 1*("." sub-domain))
    sub-domain      = (internationalized domain label)
    address-literal = IPv4address / IPv6address
 All JIDs are based on the foregoing structure.  The most common use
 of this structure is to identify an instant messaging user, the
 server to which the user connects, and the user's connected resource
 (e.g., a specific client) in the form of <user@host/resource>.
 However, node types other than clients are possible; for example, a
 specific chat room offered by a multi-user chat service could be
 addressed as <room@service> (where "room" is the name of the chat
 room and "service" is the hostname of the multi-user chat service)
 and a specific occupant of such a room could be addressed as
 <room@service/nick> (where "nick" is the occupant's room nickname).
 Many other JID types are possible (e.g., <domain/resource> could be a
 server-side script or service).
 Each allowable portion of a JID (node identifier, domain identifier,
 and resource identifier) MUST NOT be more than 1023 bytes in length,
 resulting in a maximum total size (including the '@' and '/'
 separators) of 3071 bytes.

Saint-Andre, Ed. Standards Track [Page 5] RFC 3920 XMPP Core October 2004

3.2. Domain Identifier

 The domain identifier is the primary identifier and is the only
 REQUIRED element of a JID (a mere domain identifier is a valid JID).
 It usually represents the network gateway or "primary" server to
 which other entities connect for XML routing and data management
 capabilities.  However, the entity referenced by a domain identifier
 is not always a server, and may be a service that is addressed as a
 subdomain of a server that provides functionality above and beyond
 the capabilities of a server (e.g., a multi-user chat service, a user
 directory, or a gateway to a foreign messaging system).
 The domain identifier for every server or service that will
 communicate over a network MAY be an IP address but SHOULD be a fully
 qualified domain name (see [DNS]).  A domain identifier MUST be an
 "internationalized domain name" as defined in [IDNA], to which the
 Nameprep [NAMEPREP] profile of stringprep [STRINGPREP] can be applied
 without failing.  Before comparing two domain identifiers, a server
 MUST (and a client SHOULD) first apply the Nameprep profile to the
 labels (as defined in [IDNA]) that make up each identifier.

3.3. Node Identifier

 The node identifier is an optional secondary identifier placed before
 the domain identifier and separated from the latter by the '@'
 character.  It usually represents the entity requesting and using
 network access provided by the server or gateway (i.e., a client),
 although it can also represent other kinds of entities (e.g., a chat
 room associated with a multi-user chat service).  The entity
 represented by a node identifier is addressed within the context of a
 specific domain; within instant messaging and presence applications
 of XMPP, this address is called a "bare JID" and is of the form
 <node@domain>.
 A node identifier MUST be formatted such that the Nodeprep profile of
 [STRINGPREP] can be applied to it without failing.  Before comparing
 two node identifiers, a server MUST (and a client SHOULD) first apply
 the Nodeprep profile to each identifier.

3.4. Resource Identifier

 The resource identifier is an optional tertiary identifier placed
 after the domain identifier and separated from the latter by the '/'
 character.  A resource identifier may modify either a <node@domain>
 or a mere <domain> address.  It usually represents a specific
 session, connection (e.g., a device or location), or object (e.g., a
 participant in a multi-user chat room) belonging to the entity
 associated with a node identifier.  A resource identifier is opaque

Saint-Andre, Ed. Standards Track [Page 6] RFC 3920 XMPP Core October 2004

 to both servers and other clients, and is typically defined by a
 client implementation when it provides the information necessary to
 complete Resource Binding (Section 7) (although it may be generated
 by a server on behalf of a client), after which it is referred to as
 a "connected resource".  An entity MAY maintain multiple connected
 resources simultaneously, with each connected resource differentiated
 by a distinct resource identifier.
 A resource identifier MUST be formatted such that the Resourceprep
 profile of [STRINGPREP] can be applied without failing.  Before
 comparing two resource identifiers, a server MUST (and a client
 SHOULD) first apply the Resourceprep profile to each identifier.

3.5. Determination of Addresses

 After SASL negotiation (Section 6) and, if appropriate, Resource
 Binding (Section 7), the receiving entity for a stream MUST determine
 the initiating entity's JID.
 For server-to-server communications, the initiating entity's JID
 SHOULD be the authorization identity, derived from the authentication
 identity, as defined by the Simple Authentication and Security Layer
 (SASL) specification [SASL], if no authorization identity was
 specified during SASL negotiation (Section 6).
 For client-to-server communications, the "bare JID" (<node@domain>)
 SHOULD be the authorization identity, derived from the authentication
 identity, as defined in [SASL], if no authorization identity was
 specified during SASL negotiation (Section 6); the resource
 identifier portion of the "full JID" (<node@domain/resource>) SHOULD
 be the resource identifier negotiated by the client and server during
 Resource Binding (Section 7).
 The receiving entity MUST ensure that the resulting JID (including
 node identifier, domain identifier, resource identifier, and
 separator characters) conforms to the rules and formats defined
 earlier in this section; to meet this restriction, the receiving
 entity may need to replace the JID sent by the initiating entity with
 the canonicalized JID as determined by the receiving entity.

4. XML Streams

4.1. Overview

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

Saint-Andre, Ed. Standards Track [Page 7] RFC 3920 XMPP Core October 2004

 Definition of XML Stream: An XML stream is a container for the
    exchange of XML elements between any two entities over a network.
    The start of an XML stream is denoted unambiguously by an opening
    XML <stream> tag (with appropriate attributes and namespace
    declarations), while the end of the XML stream is denoted
    unambiguously by a closing XML </stream> tag.  During the life of
    the stream, the entity that initiated it can send an unbounded
    number of XML elements over the stream, either elements used to
    negotiate the stream (e.g., to negotiate Use of TLS (Section 5) or
    use of SASL (Section 6)) or XML stanzas (as defined herein,
    <message/>, <presence/>, or <iq/> elements qualified by the
    default namespace).  The "initial stream" is negotiated from the
    initiating entity (usually a client or server) to the receiving
    entity (usually a server), and can be seen as corresponding to the
    initiating entity's "session" with the receiving entity.  The
    initial stream enables unidirectional communication from the
    initiating entity to the receiving entity; in order to enable
    information exchange from the receiving entity to the initiating
    entity, the receiving entity MUST negotiate a stream in the
    opposite direction (the "response stream").
 Definition of XML Stanza: An XML stanza is a discrete semantic unit
    of structured information that is sent from one entity to another
    over an XML stream.  An XML stanza exists at the direct child
    level of the root <stream/> element and is said to be
    well-balanced if it matches the production [43] content of [XML].
    The start of any XML stanza is denoted unambiguously by the
    element start tag at depth=1 of the XML stream (e.g., <presence>),
    and the end of any XML stanza is denoted unambiguously by the
    corresponding close tag at depth=1 (e.g., </presence>).  An XML
    stanza MAY contain child elements (with accompanying attributes,
    elements, and XML character data) as necessary in order to convey
    the desired information.  The only XML stanzas defined herein are
    the <message/>, <presence/>, and <iq/> elements qualified by the
    default namespace for the stream, as described under XML Stanzas
    (Section 9); an XML element sent for the purpose of Transport
    Layer Security (TLS) negotiation (Section 5), Simple
    Authentication and Security Layer (SASL) negotiation (Section 6),
    or server dialback (Section 8) is not considered to be an XML
    stanza.
 Consider the example of a client's session with a server.  In order
 to connect to a server, a client MUST initiate an XML stream by
 sending an opening <stream> tag to the server, optionally preceded by
 a text declaration specifying the XML version and the character
 encoding supported (see Inclusion of Text Declaration (Section 11.4);
 see also Character Encoding (Section 11.5)).  Subject to local
 policies and service provisioning, the server SHOULD then reply with

Saint-Andre, Ed. Standards Track [Page 8] RFC 3920 XMPP Core October 2004

 a second XML stream back to the client, again optionally preceded by
 a text declaration.  Once the client has completed SASL negotiation
 (Section 6), the client MAY send an unbounded number of XML stanzas
 over the stream to any recipient on the network.  When the client
 desires to close the stream, it simply sends a closing </stream> tag
 to the server (alternatively, the stream may be closed by the
 server), after which both the client and server SHOULD terminate the
 underlying connection (usually a TCP connection) as well.
 Those who are accustomed to thinking of XML in a document-centric
 manner may wish to view a client's session with a server as
 consisting of two open-ended XML documents: one from the client to
 the server and one from the server to the client.  From this
 perspective, the root <stream/> element can be considered the
 document entity for each "document", and the two "documents" are
 built up through the accumulation of XML stanzas sent over the two
 XML streams.  However, this perspective is a convenience only; XMPP
 does not deal in documents but in XML streams and XML stanzas.
 In essence, then, an XML stream acts as an envelope for all the XML
 stanzas sent during a session.  We can represent this in a simplistic
 fashion as follows:
 |--------------------|
 | <stream>           |
 |--------------------|
 | <presence>         |
 |   <show/>          |
 | </presence>        |
 |--------------------|
 | <message to='foo'> |
 |   <body/>          |
 | </message>         |
 |--------------------|
 | <iq to='bar'>      |
 |   <query/>         |
 | </iq>              |
 |--------------------|
 | ...                |
 |--------------------|
 | </stream>          |
 |--------------------|

Saint-Andre, Ed. Standards Track [Page 9] RFC 3920 XMPP Core October 2004

4.2. Binding to TCP

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

4.3. Stream Security

 When negotiating XML streams in XMPP 1.0, TLS SHOULD be used as
 defined under Use of TLS (Section 5) and SASL MUST be used as defined
 under Use of SASL (Section 6).  The "initial stream" (i.e., the
 stream from the initiating entity to the receiving entity) and the
 "response stream" (i.e., the stream from the receiving entity to the
 initiating entity) MUST be secured separately, although security in
 both directions MAY be established via mechanisms that provide mutual
 authentication.  An entity SHOULD NOT attempt to send XML Stanzas
 (Section 9) over the stream before the stream has been authenticated,
 but if it does, then the other entity MUST NOT accept such stanzas
 and SHOULD return a <not-authorized/> stream error and then terminate
 both the XML stream and the underlying TCP connection; note well that
 this applies to XML stanzas only (i.e., <message/>, <presence/>, and
 <iq/> elements scoped by the default namespace) and not to XML
 elements used for stream negotiation (e.g., elements used to
 negotiate Use of TLS (Section 5) or Use of SASL (Section 6)).

4.4. Stream Attributes

 The attributes of the stream element are as follows:
 o  to -- The 'to' attribute SHOULD be used only in the XML stream
    header from the initiating entity to the receiving entity, and
    MUST be set to a hostname serviced by the receiving entity.  There
    SHOULD NOT be a 'to' attribute set in the XML stream header by
    which the receiving entity replies to the initiating entity;
    however, if a 'to' attribute is included, it SHOULD be silently
    ignored by the initiating entity.

Saint-Andre, Ed. Standards Track [Page 10] RFC 3920 XMPP Core October 2004

 o  from -- The 'from' attribute SHOULD be used only in the XML stream
    header from the receiving entity to the initiating entity, and
    MUST be set to a hostname serviced by the receiving entity that is
    granting access to the initiating entity.  There SHOULD NOT be a
    'from' attribute on the XML stream header sent from the initiating
    entity to the receiving entity; however, if a 'from' attribute is
    included, it SHOULD be silently ignored by the receiving entity.
 o  id -- The 'id' attribute SHOULD be used only in the XML stream
    header from the receiving entity to the initiating entity.  This
    attribute is a unique identifier created by the receiving entity
    to function as a session key for the initiating entity's streams
    with the receiving entity, and MUST be unique within the receiving
    application (normally a server).  Note well that the stream ID may
    be security-critical and therefore MUST be both unpredictable and
    nonrepeating (see [RANDOM] for recommendations regarding
    randomness for security purposes).  There SHOULD NOT be an 'id'
    attribute on the XML stream header sent from the initiating entity
    to the receiving entity; however, if an 'id' attribute is
    included, it SHOULD be silently ignored by the receiving entity.
 o  xml:lang -- An 'xml:lang' attribute (as defined in Section 2.12 of
    [XML]) SHOULD be included by the initiating entity on the header
    for the initial stream to specify the default language of any
    human-readable XML character data it sends over that stream.  If
    the attribute is included, the receiving entity SHOULD remember
    that value as the default for both the initial stream and the
    response stream; if the attribute is not included, the receiving
    entity SHOULD use a configurable default value for both streams,
    which it MUST communicate in the header for the response stream.
    For all stanzas sent over the initial stream, if the initiating
    entity does not include an 'xml:lang' attribute, the receiving
    entity SHOULD apply the default value; if the initiating entity
    does include an 'xml:lang' attribute, the receiving entity MUST
    NOT modify or delete it (see also xml:lang (Section 9.1.5)).  The
    value of the 'xml:lang' attribute MUST be an NMTOKEN (as defined
    in Section 2.3 of [XML]) and MUST conform to the format defined in
    RFC 3066 [LANGTAGS].
 o  version -- The presence of the version attribute set to a value of
    at least "1.0" signals support for the stream-related protocols
    (including stream features) defined in this specification.
    Detailed rules regarding the generation and handling of this
    attribute are defined below.

Saint-Andre, Ed. Standards Track [Page 11] RFC 3920 XMPP Core October 2004

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

4.4.1. Version Support

 The version of XMPP specified herein is "1.0"; in particular, this
 encapsulates the stream-related protocols (Use of TLS (Section 5),
 Use of SASL (Section 6), and Stream Errors (Section 4.7)), as well as
 the semantics of the three defined XML stanza types (<message/>,
 <presence/>, and <iq/>).  The numbering scheme for XMPP versions is
 "<major>.<minor>".  The major and minor numbers MUST be treated as
 separate integers and each number MAY be incremented higher than a
 single digit.  Thus, "XMPP 2.4" would be a lower version than "XMPP
 2.13", which in turn would be lower than "XMPP 12.3".  Leading zeros
 (e.g., "XMPP 6.01") MUST be ignored by recipients and MUST NOT be
 sent.
 The major version number should be incremented only if the stream and
 stanza formats or required actions have changed so dramatically that
 an older version entity would not be able to interoperate with a
 newer version entity if it simply ignored the elements and attributes
 it did not understand and took the actions specified in the older
 specification.  The minor version number indicates new capabilities,
 and MUST be ignored by an entity with a smaller minor version number,
 but used for informational purposes by the entity with the larger
 minor version number.  For example, a minor version number might
 indicate the ability to process a newly defined value of the 'type'
 attribute for message, presence, or IQ stanzas; the entity with the
 larger minor version number would simply note that its correspondent
 would not be able to understand that value of the 'type' attribute
 and therefore would not send it.
 The following rules apply to the generation and handling of the
 'version' attribute within stream headers by implementations:
 1.  The initiating entity MUST set the value of the 'version'
     attribute on the initial stream header to the highest version
     number it supports (e.g., if the highest version number it
     supports is that defined in this specification, it MUST set the
     value to "1.0").

Saint-Andre, Ed. Standards Track [Page 12] RFC 3920 XMPP Core October 2004

 2.  The receiving entity MUST set the value of the 'version'
     attribute on the response stream header to either the value
     supplied by the initiating entity or the highest version number
     supported by the receiving entity, whichever is lower.  The
     receiving entity MUST perform a numeric comparison on the major
     and minor version numbers, not a string match on
     "<major>.<minor>".
 3.  If the version number included in the response stream header is
     at least one major version lower than the version number included
     in the initial stream header and newer version entities cannot
     interoperate with older version entities as described above, the
     initiating entity SHOULD generate an <unsupported-version/>
     stream error and terminate the XML stream and underlying TCP
     connection.
 4.  If either entity receives a stream header with no 'version'
     attribute, the entity MUST consider the version supported by the
     other entity to be "0.0" and SHOULD NOT include a 'version'
     attribute in the stream header it sends in reply.

4.5. Namespace Declarations

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

4.6. Stream Features

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

Saint-Andre, Ed. Standards Track [Page 13] RFC 3920 XMPP Core October 2004

4.7. Stream Errors

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

4.7.1. Rules

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

4.7.2. Syntax

 The syntax for stream errors is as follows:
 <stream:error>
   <defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   <text xmlns='urn:ietf:params:xml:ns:xmpp-streams'
         xml:lang='langcode'>
     OPTIONAL descriptive text
   </text>
   [OPTIONAL application-specific condition element]
 </stream:error>
 The <error/> element:
 o  MUST contain a child element corresponding to one of the defined
    stanza error conditions defined below; this element MUST be
    qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace

Saint-Andre, Ed. Standards Track [Page 14] RFC 3920 XMPP Core October 2004

 o  MAY contain a <text/> child containing XML character data that
    describes the error in more detail; this element MUST be qualified
    by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace and SHOULD
    possess an 'xml:lang' attribute specifying the natural language of
    the XML character data
 o  MAY contain a child element for an application-specific error
    condition; this element MUST be qualified by an
    application-defined namespace, and its structure is defined by
    that namespace
 The <text/> element is OPTIONAL.  If included, it SHOULD be used only
 to provide descriptive or diagnostic information that supplements the
 meaning of a defined condition or application-specific condition.  It
 SHOULD NOT be interpreted programmatically by an application.  It
 SHOULD NOT be used as the error message presented to a user, but MAY
 be shown in addition to the error message associated with the
 included condition element (or elements).

4.7.3. Defined Conditions

 The following stream-level error conditions are defined:
 o  <bad-format/> -- the entity has sent XML that cannot be processed;
    this error MAY be used instead of the more specific XML-related
    errors, such as <bad-namespace-prefix/>, <invalid-xml/>,
    <restricted-xml/>, <unsupported-encoding/>, and
    <xml-not-well-formed/>, although the more specific errors are
    preferred.
 o  <bad-namespace-prefix/> -- the entity has sent a namespace prefix
    that is unsupported, or has sent no namespace prefix on an element
    that requires such a prefix (see XML Namespace Names and Prefixes
    (Section 11.2)).
 o  <conflict/> -- the server is closing the active stream for this
    entity because a new stream has been initiated that conflicts with
    the existing stream.
 o  <connection-timeout/> -- the entity has not generated any traffic
    over the stream for some period of time (configurable according to
    a local service policy).
 o  <host-gone/> -- the value of the 'to' attribute provided by the
    initiating entity in the stream header corresponds to a hostname
    that is no longer hosted by the server.

Saint-Andre, Ed. Standards Track [Page 15] RFC 3920 XMPP Core October 2004

 o  <host-unknown/> -- the value of the 'to' attribute provided by the
    initiating entity in the stream header does not correspond to a
    hostname that is hosted by the server.
 o  <improper-addressing/> -- a stanza sent between two servers lacks
    a 'to' or 'from' attribute (or the attribute has no value).
 o  <internal-server-error/> -- the server has experienced a
    misconfiguration or an otherwise-undefined internal error that
    prevents it from servicing the stream.
 o  <invalid-from/> -- the JID or hostname provided in a 'from'
    address does not match an authorized JID or validated domain
    negotiated between servers via SASL or dialback, or between a
    client and a server via authentication and resource binding.
 o  <invalid-id/> -- the stream ID or dialback ID is invalid or does
    not match an ID previously provided.
 o  <invalid-namespace/> -- the streams namespace name is something
    other than "http://etherx.jabber.org/streams" or the dialback
    namespace name is something other than "jabber:server:dialback"
    (see XML Namespace Names and Prefixes (Section 11.2)).
 o  <invalid-xml/> -- the entity has sent invalid XML over the stream
    to a server that performs validation (see Validation (Section
    11.3)).
 o  <not-authorized/> -- the entity has attempted to send data before
    the stream has been authenticated, or otherwise is not authorized
    to perform an action related to stream negotiation; the receiving
    entity MUST NOT process the offending stanza before sending the
    stream error.
 o  <policy-violation/> -- the entity has violated some local service
    policy; the server MAY choose to specify the policy in the <text/>
    element or an application-specific condition element.
 o  <remote-connection-failed/> -- the server is unable to properly
    connect to a remote entity that is required for authentication or
    authorization.
 o  <resource-constraint/> -- the server lacks the system resources
    necessary to service the stream.

Saint-Andre, Ed. Standards Track [Page 16] RFC 3920 XMPP Core October 2004

 o  <restricted-xml/> -- the entity has attempted to send restricted
    XML features such as a comment, processing instruction, DTD,
    entity reference, or unescaped character (see Restrictions
    (Section 11.1)).
 o  <see-other-host/> -- the server will not provide service to the
    initiating entity but is redirecting traffic to another host; the
    server SHOULD specify the alternate hostname or IP address (which
    MUST be a valid domain identifier) as the XML character data of
    the <see-other-host/> element.
 o  <system-shutdown/> -- the server is being shut down and all active
    streams are being closed.
 o  <undefined-condition/> -- the error condition is not one of those
    defined by the other conditions in this list; this error condition
    SHOULD be used only in conjunction with an application-specific
    condition.
 o  <unsupported-encoding/> -- the initiating entity has encoded the
    stream in an encoding that is not supported by the server (see
    Character Encoding (Section 11.5)).
 o  <unsupported-stanza-type/> -- the initiating entity has sent a
    first-level child of the stream that is not supported by the
    server.
 o  <unsupported-version/> -- the value of the 'version' attribute
    provided by the initiating entity in the stream header specifies a
    version of XMPP that is not supported by the server; the server
    MAY specify the version(s) it supports in the <text/> element.
 o  <xml-not-well-formed/> -- the initiating entity has sent XML that
    is not well-formed as defined by [XML].

4.7.4. Application-Specific Conditions

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

Saint-Andre, Ed. Standards Track [Page 17] RFC 3920 XMPP Core October 2004

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

4.8. Simplified Stream Examples

 This section contains two simplified examples of a stream-based
 "session" of a client on a server (where the "C" lines are sent from
 the client to the server, and the "S" lines are sent from the server
 to the client); these examples are included for the purpose of
 illustrating the concepts introduced thus far.
 A basic "session":
 C: <?xml version='1.0'?>
    <stream:stream
        to='example.com'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'
        version='1.0'>
 S: <?xml version='1.0'?>
    <stream:stream
        from='example.com'
        id='someid'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'
        version='1.0'>
 ...  encryption, authentication, and resource binding ...
 C:   <message from='juliet@example.com'
               to='romeo@example.net'
               xml:lang='en'>
 C:     <body>Art thou not Romeo, and a Montague?</body>
 C:   </message>
 S:   <message from='romeo@example.net'
               to='juliet@example.com'
               xml:lang='en'>
 S:     <body>Neither, fair saint, if either thee dislike.</body>
 S:   </message>
 C: </stream:stream>
 S: </stream:stream>

Saint-Andre, Ed. Standards Track [Page 18] RFC 3920 XMPP Core October 2004

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

5. Use of TLS

5.1. Overview

 XMPP includes a method for securing the stream from tampering and
 eavesdropping.  This channel encryption method makes use of the
 Transport Layer Security (TLS) protocol [TLS], along with a
 "STARTTLS" extension that is modelled after similar extensions for
 the IMAP [IMAP], POP3 [POP3], and ACAP [ACAP] protocols as described
 in RFC 2595 [USINGTLS].  The namespace name for the STARTTLS
 extension is 'urn:ietf:params:xml:ns:xmpp-tls'.
 An administrator of a given domain MAY require the use of TLS for
 client-to-server communications, server-to-server communications, or
 both.  Clients SHOULD use TLS to secure the streams prior to
 attempting the completion of SASL negotiation (Section 6), and
 servers SHOULD use TLS between two domains for the purpose of
 securing server-to-server communications.

Saint-Andre, Ed. Standards Track [Page 19] RFC 3920 XMPP Core October 2004

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

Saint-Andre, Ed. Standards Track [Page 20] RFC 3920 XMPP Core October 2004

     "im.example.com", the certificate MUST be checked as
     "example.com".  If a JID for any kind of XMPP entity (e.g.,
     client or server) is represented in a certificate, it MUST be
     represented as a UTF8String within an otherName entity inside the
     subjectAltName, using the [ASN.1] Object Identifier
     "id-on-xmppAddr" specified in Section 5.1.1 of this document.
 9.  If the TLS negotiation is successful, the receiving entity MUST
     discard any knowledge obtained in an insecure manner from the
     initiating entity before TLS takes effect.
 10. If the TLS negotiation is successful, the initiating entity MUST
     discard any knowledge obtained in an insecure manner from the
     receiving entity before TLS takes effect.
 11. If the TLS negotiation is successful, the receiving entity MUST
     NOT offer the STARTTLS extension to the initiating entity along
     with the other stream features that are offered when the stream
     is restarted.
 12. If the TLS negotiation is successful, the initiating entity MUST
     continue with SASL negotiation.
 13. If the TLS negotiation results in failure, the receiving entity
     MUST terminate both the XML stream and the underlying TCP
     connection.
 14. See Mandatory-to-Implement Technologies (Section 14.7) regarding
     mechanisms that MUST be supported.

5.1.1. ASN.1 Object Identifier for XMPP Address

 The [ASN.1] Object Identifier "id-on-xmppAddr" described above is
 defined as follows:
 id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
         dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
 id-on  OBJECT IDENTIFIER ::= { id-pkix 8 }  -- other name forms
 id-on-xmppAddr  OBJECT IDENTIFIER ::= { id-on 5 }
 XmppAddr ::= UTF8String
 This Object Identifier MAY also be represented in the dotted display
 format as "1.3.6.1.5.5.7.8.5".

Saint-Andre, Ed. Standards Track [Page 21] RFC 3920 XMPP Core October 2004

5.2. Narrative

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

Saint-Andre, Ed. Standards Track [Page 22] RFC 3920 XMPP Core October 2004

 8.  Upon receiving the new stream header from the initiating entity,
     the receiving entity MUST respond by sending a new XML stream
     header to the initiating entity along with the available features
     (but not including the STARTTLS feature).

5.3. Client-to-Server Example

 The following example shows the data flow for a client securing a
 stream using STARTTLS (note: the alternate steps shown below are
 provided to illustrate the protocol for failure cases; they are not
 exhaustive and would not necessarily be triggered by the data sent in
 the example).
 Step 1: Client initiates stream to server:
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     to='example.com'
     version='1.0'>
 Step 2: Server responds by sending a stream tag to client:
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     id='c2s_123'
     from='example.com'
     version='1.0'>
 Step 3: Server sends the STARTTLS extension to client along with
 authentication mechanisms and any other stream features:
 <stream:features>
   <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
     <required/>
   </starttls>
   <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <mechanism>DIGEST-MD5</mechanism>
     <mechanism>PLAIN</mechanism>
   </mechanisms>
 </stream:features>
 Step 4: Client sends the STARTTLS command to server:
 <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Saint-Andre, Ed. Standards Track [Page 23] RFC 3920 XMPP Core October 2004

 Step 5: Server informs client that it is allowed to proceed:
 <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
 Step 5 (alt): Server informs client that TLS negotiation has failed
 and closes both stream and TCP connection:
 <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
 </stream:stream>
 Step 6: Client and server attempt to complete TLS negotiation over
 the existing TCP connection.
 Step 7: If TLS negotiation is successful, client initiates a new
 stream to server:
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     to='example.com'
     version='1.0'>
 Step 7 (alt): If TLS negotiation is unsuccessful, server closes TCP
 connection.
 Step 8: Server responds by sending a stream header to client along
 with any available stream features:
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     from='example.com'
     id='c2s_234'
     version='1.0'>
 <stream:features>
   <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <mechanism>DIGEST-MD5</mechanism>
     <mechanism>PLAIN</mechanism>
     <mechanism>EXTERNAL</mechanism>
   </mechanisms>
 </stream:features>
 Step 9: Client continues with SASL negotiation (Section 6).

Saint-Andre, Ed. Standards Track [Page 24] RFC 3920 XMPP Core October 2004

5.4. Server-to-Server Example

 The following example shows the data flow for two servers securing a
 stream using STARTTLS (note: the alternate steps shown below are
 provided to illustrate the protocol for failure cases; they are not
 exhaustive and would not necessarily be triggered by the data sent in
 the example).
 Step 1: Server1 initiates stream to Server2:
 <stream:stream
     xmlns='jabber:server'
     xmlns:stream='http://etherx.jabber.org/streams'
     to='example.com'
     version='1.0'>
 Step 2: Server2 responds by sending a stream tag to Server1:
 <stream:stream
     xmlns='jabber:server'
     xmlns:stream='http://etherx.jabber.org/streams'
     from='example.com'
     id='s2s_123'
     version='1.0'>
 Step 3: Server2 sends the STARTTLS extension to Server1 along with
 authentication mechanisms and any other stream features:
 <stream:features>
   <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
     <required/>
   </starttls>
   <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <mechanism>DIGEST-MD5</mechanism>
     <mechanism>KERBEROS_V4</mechanism>
   </mechanisms>
 </stream:features>
 Step 4: Server1 sends the STARTTLS command to Server2:
 <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
 Step 5: Server2 informs Server1 that it is allowed to proceed:
 <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Saint-Andre, Ed. Standards Track [Page 25] RFC 3920 XMPP Core October 2004

 Step 5 (alt): Server2 informs Server1 that TLS negotiation has failed
 and closes stream:
 <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
 </stream:stream>
 Step 6: Server1 and Server2 attempt to complete TLS negotiation via
 TCP.
 Step 7: If TLS negotiation is successful, Server1 initiates a new
 stream to Server2:
 <stream:stream
     xmlns='jabber:server'
     xmlns:stream='http://etherx.jabber.org/streams'
     to='example.com'
     version='1.0'>
 Step 7 (alt): If TLS negotiation is unsuccessful, Server2 closes TCP
 connection.
 Step 8: Server2 responds by sending a stream header to Server1 along
 with any available stream features:
 <stream:stream
     xmlns='jabber:server'
     xmlns:stream='http://etherx.jabber.org/streams'
     from='example.com'
     id='s2s_234'
     version='1.0'>
 <stream:features>
   <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <mechanism>DIGEST-MD5</mechanism>
     <mechanism>KERBEROS_V4</mechanism>
     <mechanism>EXTERNAL</mechanism>
   </mechanisms>
 </stream:features>
 Step 9: Server1 continues with SASL negotiation (Section 6).

Saint-Andre, Ed. Standards Track [Page 26] RFC 3920 XMPP Core October 2004

6. Use of SASL

6.1. Overview

 XMPP includes a method for authenticating a stream by means of an
 XMPP-specific profile of the Simple Authentication and Security Layer
 (SASL) protocol [SASL].  SASL provides a generalized method for
 adding authentication support to connection-based protocols, and XMPP
 uses a generic XML namespace profile for SASL that conforms to the
 profiling requirements of [SASL].
 The following rules apply:
 1.  If the SASL negotiation occurs between two servers,
     communications MUST NOT proceed until the Domain Name System
     (DNS) hostnames asserted by the servers have been resolved (see
     Server-to-Server Communications (Section 14.4)).
 2.  If the initiating entity is capable of SASL negotiation, it MUST
     include the 'version' attribute set to a value of at least "1.0"
     in the initial stream header.
 3.  If the receiving entity is capable of SASL negotiation, it MUST
     advertise one or more authentication mechanisms within a
     <mechanisms/> element qualified by the
     'urn:ietf:params:xml:ns:xmpp-sasl' namespace in reply to the
     opening stream tag received from the initiating entity (if the
     opening stream tag included the 'version' attribute set to a
     value of at least "1.0").
 4.  During SASL negotiation, an entity MUST NOT send any white space
     characters (matching production [3] content of [XML]) within the
     root stream element as separators between elements (any white
     space characters shown in the SASL examples below are included
     for the sake of readability only); this prohibition helps to
     ensure proper security layer byte precision.
 5.  Any XML character data contained within the XML elements used
     during SASL negotiation MUST be encoded using base64, where the
     encoding adheres to the definition in Section 3 of RFC 3548
     [BASE64].
 6.  If provision of a "simple username" is supported by the selected
     SASL mechanism (e.g., this is supported by the DIGEST-MD5 and
     CRAM-MD5 mechanisms but not by the EXTERNAL and GSSAPI
     mechanisms), during authentication the initiating entity SHOULD
     provide as the simple username its sending domain (IP address or
     fully qualified domain name as contained in a domain identifier)

Saint-Andre, Ed. Standards Track [Page 27] RFC 3920 XMPP Core October 2004

     in the case of server-to-server communications or its registered
     account name (user or node name as contained in an XMPP node
     identifier) in the case of client-to-server communications.
 7.  If the initiating entity wishes to act on behalf of another
     entity and the selected SASL mechanism supports transmission of
     an authorization identity, the initiating entity MUST provide an
     authorization identity during SASL negotiation.  If the
     initiating entity does not wish to act on behalf of another
     entity, it MUST NOT provide an authorization identity.  As
     specified in [SASL], the initiating entity MUST NOT provide an
     authorization identity unless the authorization identity is
     different from the default authorization identity derived from
     the authentication identity as described in [SASL].  If provided,
     the value of the authorization identity MUST be of the form
     <domain> (i.e., a domain identifier only) for servers and of the
     form <node@domain> (i.e., node identifier and domain identifier)
     for clients.
 8.  Upon successful SASL negotiation that involves negotiation of a
     security layer, the receiving entity MUST discard any knowledge
     obtained from the initiating entity which was not obtained from
     the SASL negotiation itself.
 9.  Upon successful SASL negotiation that involves negotiation of a
     security layer, the initiating entity MUST discard any knowledge
     obtained from the receiving entity which was not obtained from
     the SASL negotiation itself.
 10. See Mandatory-to-Implement Technologies (Section 14.7) regarding
     mechanisms that MUST be supported.

6.2. Narrative

 When an initiating entity authenticates with a receiving entity using
 SASL, the steps involved are as follows:
 1.  The initiating entity requests SASL authentication by including
     the 'version' attribute in the opening XML stream header sent to
     the receiving entity, with the value set to "1.0".
 2.  After sending an XML stream header in reply, the receiving entity
     advertises a list of available SASL authentication mechanisms;
     each of these is a <mechanism/> element included as a child
     within a <mechanisms/> container element qualified by the
     'urn:ietf:params:xml:ns:xmpp-sasl' namespace, which in turn is a
     child of a <features/> element in the streams namespace.  If Use
     of TLS (Section 5) needs to be established before a particular

Saint-Andre, Ed. Standards Track [Page 28] RFC 3920 XMPP Core October 2004

     authentication mechanism may be used, the receiving entity MUST
     NOT provide that mechanism in the list of available SASL
     authentication mechanisms prior to TLS negotiation.  If the
     initiating entity presents a valid certificate during prior TLS
     negotiation, the receiving entity SHOULD offer the SASL EXTERNAL
     mechanism to the initiating entity during SASL negotiation (refer
     to [SASL]), although the EXTERNAL mechanism MAY be offered under
     other circumstances as well.
 3.  The initiating entity selects a mechanism by sending an <auth/>
     element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
     namespace to the receiving entity and including an appropriate
     value for the 'mechanism' attribute.  This element MAY contain
     XML character data (in SASL terminology, the "initial response")
     if the mechanism supports or requires it; if the initiating
     entity needs to send a zero-length initial response, it MUST
     transmit the response as a single equals sign ("="), which
     indicates that the response is present but contains no data.
 4.  If necessary, the receiving entity challenges the initiating
     entity by sending a <challenge/> element qualified by the
     'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
     entity; this element MAY contain XML character data (which MUST
     be computed in accordance with the definition of the SASL
     mechanism chosen by the initiating entity).
 5.  The initiating entity responds to the challenge by sending a
     <response/> element qualified by the
     'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the receiving
     entity; this element MAY contain XML character data (which MUST
     be computed in accordance with the definition of the SASL
     mechanism chosen by the initiating entity).
 6.  If necessary, the receiving entity sends more challenges and the
     initiating entity sends more responses.
 This series of challenge/response pairs continues until one of three
 things happens:
 1.  The initiating entity aborts the handshake by sending an <abort/>
     element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
     namespace to the receiving entity.  Upon receiving an <abort/>
     element, the receiving entity SHOULD allow a configurable but
     reasonable number of retries (at least 2), after which it MUST
     terminate the TCP connection; this enables the initiating entity
     (e.g., an end-user client) to tolerate incorrectly-provided
     credentials (e.g., a mistyped password) without being forced to
     reconnect.

Saint-Andre, Ed. Standards Track [Page 29] RFC 3920 XMPP Core October 2004

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

6.3. SASL Definition

 The profiling requirements of [SASL] require that the following
 information be supplied by a protocol definition:
 service name: "xmpp"
 initiation sequence: After the initiating entity provides an opening
    XML stream header and the receiving entity replies in kind, the
    receiving entity provides a list of acceptable authentication
    methods.  The initiating entity chooses one method from the list
    and sends it to the receiving entity as the value of the
    'mechanism' attribute possessed by an <auth/> element, optionally
    including an initial response to avoid a round trip.

Saint-Andre, Ed. Standards Track [Page 30] RFC 3920 XMPP Core October 2004

 exchange sequence: Challenges and responses are carried through the
    exchange of <challenge/> elements from receiving entity to
    initiating entity and <response/> elements from initiating entity
    to receiving entity.  The receiving entity reports failure by
    sending a <failure/> element and success by sending a <success/>
    element; the initiating entity aborts the exchange by sending an
    <abort/> element.  Upon successful negotiation, both sides
    consider the original XML stream to be closed and new stream
    headers are sent by both entities.
 security layer negotiation: The security layer takes effect
    immediately after sending the closing ">" character of the
    <success/> element for the receiving entity, and immediately after
    receiving the closing ">" character of the <success/> element for
    the initiating entity.  The order of layers is first [TCP], then
    [TLS], then [SASL], then XMPP.
 use of the authorization identity: The authorization identity may be
    used by xmpp to denote the non-default <node@domain> of a client
    or the sending <domain> of a server.

6.4. SASL Errors

 The following SASL-related error conditions are defined:
 o  <aborted/> -- The receiving entity acknowledges an <abort/>
    element sent by the initiating entity; sent in reply to the
    <abort/> element.
 o  <incorrect-encoding/> -- The data provided by the initiating
    entity could not be processed because the [BASE64] encoding is
    incorrect (e.g., because the encoding does not adhere to the
    definition in Section 3 of [BASE64]); sent in reply to a
    <response/> element or an <auth/> element with initial response
    data.
 o  <invalid-authzid/> -- The authzid provided by the initiating
    entity is invalid, either because it is incorrectly formatted or
    because the initiating entity does not have permissions to
    authorize that ID; sent in reply to a <response/> element or an
    <auth/> element with initial response data.
 o  <invalid-mechanism/> -- The initiating entity did not provide a
    mechanism or requested a mechanism that is not supported by the
    receiving entity; sent in reply to an <auth/> element.

Saint-Andre, Ed. Standards Track [Page 31] RFC 3920 XMPP Core October 2004

 o  <mechanism-too-weak/> -- The mechanism requested by the initiating
    entity is weaker than server policy permits for that initiating
    entity; sent in reply to a <response/> element or an <auth/>
    element with initial response data.
 o  <not-authorized/> -- The authentication failed because the
    initiating entity did not provide valid credentials (this includes
    but is not limited to the case of an unknown username); sent in
    reply to a <response/> element or an <auth/> element with initial
    response data.
 o  <temporary-auth-failure/> -- The authentication failed because of
    a temporary error condition within the receiving entity; sent in
    reply to an <auth/> element or <response/> element.

6.5. Client-to-Server Example

 The following example shows the data flow for a client authenticating
 with a server using SASL, normally after successful TLS negotiation
 (note: the alternate steps shown below are provided to illustrate the
 protocol for failure cases; they are not exhaustive and would not
 necessarily be triggered by the data sent in the example).
 Step 1: Client initiates stream to server:
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     to='example.com'
     version='1.0'>
 Step 2: Server responds with a stream tag sent to client:
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     id='c2s_234'
     from='example.com'
     version='1.0'>
 Step 3: Server informs client of available authentication mechanisms:
 <stream:features>
   <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <mechanism>DIGEST-MD5</mechanism>
     <mechanism>PLAIN</mechanism>
   </mechanisms>
 </stream:features>

Saint-Andre, Ed. Standards Track [Page 32] RFC 3920 XMPP Core October 2004

 Step 4: Client selects an authentication mechanism:
 <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
       mechanism='DIGEST-MD5'/>
 Step 5: Server sends a [BASE64] encoded challenge to client:
 <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
 cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9ImF1dGgi
 LGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
 </challenge>
 The decoded challenge is:
 realm="somerealm",nonce="OA6MG9tEQGm2hh",\
 qop="auth",charset=utf-8,algorithm=md5-sess
 Step 5 (alt): Server returns error to client:
 <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   <incorrect-encoding/>
 </failure>
 </stream:stream>
 Step 6: Client sends a [BASE64] encoded response to the challenge:
 <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
 dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
 T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
 MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
 LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
 YXJzZXQ9dXRmLTgK
 </response>
 The decoded response is:
 username="somenode",realm="somerealm",\
 nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\
 nc=00000001,qop=auth,digest-uri="xmpp/example.com",\
 response=d388dad90d4bbd760a152321f2143af7,charset=utf-8
 Step 7: Server sends another [BASE64] encoded challenge to client:
 <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
 cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
 </challenge>

Saint-Andre, Ed. Standards Track [Page 33] RFC 3920 XMPP Core October 2004

 The decoded challenge is:
 rspauth=ea40f60335c427b5527b84dbabcdfffd
 Step 7 (alt): Server returns error to client:
 <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   <temporary-auth-failure/>
 </failure>
 </stream:stream>
 Step 8: Client responds to the challenge:
 <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
 Step 9: Server informs client of successful authentication:
 <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
 Step 9 (alt): Server informs client of failed authentication:
 <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   <temporary-auth-failure/>
 </failure>
 </stream:stream>
 Step 10: Client initiates a new stream to server:
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     to='example.com'
     version='1.0'>
 Step 11: Server responds by sending a stream header to client along
 with any additional features (or an empty features element):
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     id='c2s_345'
     from='example.com'
     version='1.0'>
 <stream:features>
   <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
   <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
 </stream:features>

Saint-Andre, Ed. Standards Track [Page 34] RFC 3920 XMPP Core October 2004

6.6. Server-to-Server Example

 The following example shows the data flow for a server authenticating
 with another server using SASL, normally after successful TLS
 negotiation (note: the alternate steps shown below are provided to
 illustrate the protocol for failure cases; they are not exhaustive
 and would not necessarily be triggered by the data sent in the
 example).
 Step 1: Server1 initiates stream to Server2:
 <stream:stream
     xmlns='jabber:server'
     xmlns:stream='http://etherx.jabber.org/streams'
     to='example.com'
     version='1.0'>
 Step 2: Server2 responds with a stream tag sent to Server1:
 <stream:stream
     xmlns='jabber:server'
     xmlns:stream='http://etherx.jabber.org/streams'
     from='example.com'
     id='s2s_234'
     version='1.0'>
 Step 3: Server2 informs Server1 of available authentication
 mechanisms:
 <stream:features>
   <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <mechanism>DIGEST-MD5</mechanism>
     <mechanism>KERBEROS_V4</mechanism>
   </mechanisms>
 </stream:features>
 Step 4: Server1 selects an authentication mechanism:
 <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
       mechanism='DIGEST-MD5'/>
 Step 5: Server2 sends a [BASE64] encoded challenge to Server1:
 <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
 cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9
 ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz
 </challenge>

Saint-Andre, Ed. Standards Track [Page 35] RFC 3920 XMPP Core October 2004

 The decoded challenge is:
 realm="somerealm",nonce="OA6MG9tEQGm2hh",\
 qop="auth",charset=utf-8,algorithm=md5-sess
 Step 5 (alt): Server2 returns error to Server1:
 <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   <incorrect-encoding/>
 </failure>
 </stream:stream>
 Step 6: Server1 sends a [BASE64] encoded response to the challenge:
 <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
 dXNlcm5hbWU9ImV4YW1wbGUub3JnIixyZWFsbT0ic29tZXJlYWxtIixub25j
 ZT0iT0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5j
 PTAwMDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5v
 cmciLHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3
 LGNoYXJzZXQ9dXRmLTgK
 </response>
 The decoded response is:
 username="example.org",realm="somerealm",\
 nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\
 nc=00000001,qop=auth,digest-uri="xmpp/example.org",\
 response=d388dad90d4bbd760a152321f2143af7,charset=utf-8
 Step 7: Server2 sends another [BASE64] encoded challenge to Server1:
 <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
 cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
 </challenge>
 The decoded challenge is:
 rspauth=ea40f60335c427b5527b84dbabcdfffd
 Step 7 (alt): Server2 returns error to Server1:
 <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   <invalid-authzid/>
 </failure>
 </stream:stream>

Saint-Andre, Ed. Standards Track [Page 36] RFC 3920 XMPP Core October 2004

 Step 8: Server1 responds to the challenge:
 <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
 Step 8 (alt): Server1 aborts negotiation:
 <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
 Step 9: Server2 informs Server1 of successful authentication:
 <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
 Step 9 (alt): Server2 informs Server1 of failed authentication:
 <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
   <aborted/>
 </failure>
 </stream:stream>
 Step 10: Server1 initiates a new stream to Server2:
 <stream:stream
     xmlns='jabber:server'
     xmlns:stream='http://etherx.jabber.org/streams'
     to='example.com'
     version='1.0'>
 Step 11: Server2 responds by sending a stream header to Server1 along
 with any additional features (or an empty features element):
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     from='example.com'
     id='s2s_345'
     version='1.0'>
 <stream:features/>

7. Resource Binding

 After SASL negotiation (Section 6) with the receiving entity, the
 initiating entity MAY want or need to bind a specific resource to
 that stream.  In general this applies only to clients: in order to
 conform to the addressing format (Section 3) and stanza delivery
 rules (Section 10) specified herein, there MUST be a resource
 identifier associated with the <node@domain> of the client (which is

Saint-Andre, Ed. Standards Track [Page 37] RFC 3920 XMPP Core October 2004

 either generated by the server or provided by the client
 application); this ensures that the address for use over that stream
 is a "full JID" of the form <node@domain/resource>.
 Upon receiving a success indication within the SASL negotiation, the
 client MUST send a new stream header to the server, to which the
 server MUST respond with a stream header as well as a list of
 available stream features.  Specifically, if the server requires the
 client to bind a resource to the stream after successful SASL
 negotiation, it MUST include an empty <bind/> element qualified by
 the 'urn:ietf:params:xml:ns:xmpp-bind' namespace in the stream
 features list it presents to the client upon sending the header for
 the response stream sent after successful SASL negotiation (but not
 before):
 Server advertises resource binding feature to client:
 <stream:stream
     xmlns='jabber:client'
     xmlns:stream='http://etherx.jabber.org/streams'
     id='c2s_345'
     from='example.com'
     version='1.0'>
 <stream:features>
   <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
 </stream:features>
 Upon being so informed that resource binding is required, the client
 MUST bind a resource to the stream by sending to the server an IQ
 stanza of type "set" (see IQ Semantics (Section 9.2.3)) containing
 data qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace.
 If the client wishes to allow the server to generate the resource
 identifier on its behalf, it sends an IQ stanza of type "set" that
 contains an empty <bind/> element:
 Client asks server to bind a resource:
 <iq type='set' id='bind_1'>
   <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
 </iq>
 A server that supports resource binding MUST be able to generate a
 resource identifier on behalf of a client.  A resource identifier
 generated by the server MUST be unique for that <node@domain>.

Saint-Andre, Ed. Standards Track [Page 38] RFC 3920 XMPP Core October 2004

 If the client wishes to specify the resource identifier, it sends an
 IQ stanza of type "set" that contains the desired resource identifier
 as the XML character data of a <resource/> element that is a child of
 the <bind/> element:
 Client binds a resource:
 <iq type='set' id='bind_2'>
   <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
     <resource>someresource</resource>
   </bind>
 </iq>
 Once the server has generated a resource identifier for the client or
 accepted the resource identifier provided by the client, it MUST
 return an IQ stanza of type "result" to the client, which MUST
 include a <jid/> child element that specifies the full JID for the
 connected resource as determined by the server:
 Server informs client of successful resource binding:
 <iq type='result' id='bind_2'>
   <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
     <jid>somenode@example.com/someresource</jid>
   </bind>
 </iq>
 A server SHOULD accept the resource identifier provided by the
 client, but MAY override it with a resource identifier that the
 server generates; in this case, the server SHOULD NOT return a stanza
 error (e.g., <forbidden/>) to the client but instead SHOULD
 communicate the generated resource identifier to the client in the IQ
 result as shown above.
 When a client supplies a resource identifier, the following stanza
 error conditions are possible (see Stanza Errors (Section 9.3)):
 o  The provided resource identifier cannot be processed by the server
    in accordance with Resourceprep (Appendix B).
 o  The client is not allowed to bind a resource to the stream (e.g.,
    because the node or user has reached a limit on the number of
    connected resources allowed).
 o  The provided resource identifier is already in use but the server
    does not allow binding of multiple connected resources with the
    same identifier.

Saint-Andre, Ed. Standards Track [Page 39] RFC 3920 XMPP Core October 2004

 The protocol for these error conditions is shown below.
 Resource identifier cannot be processed:
 <iq type='error' id='bind_2'>
   <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
     <resource>someresource</resource>
   </bind>
   <error type='modify'>
     <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>
 Client is not allowed to bind a resource:
 <iq type='error' id='bind_2'>
   <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
     <resource>someresource</resource>
   </bind>
   <error type='cancel'>
     <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>
 Resource identifier is in use:
 <iq type='error' id='bind_2'>
   <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
     <resource>someresource</resource>
   </bind>
   <error type='cancel'>
     <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
   </error>
 </iq>
 If, before completing the resource binding step, the client attempts
 to send an XML stanza other than an IQ stanza with a <bind/> child
 qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace, the
 server MUST NOT process the stanza and SHOULD return a
 <not-authorized/> stanza error to the client.

Saint-Andre, Ed. Standards Track [Page 40] RFC 3920 XMPP Core October 2004

8. Server Dialback

8.1. Overview

 The Jabber protocols from which XMPP was adapted include a "server
 dialback" method for protecting against domain spoofing, thus making
 it more difficult to spoof XML stanzas.  Server dialback is not a
 security mechanism, and results in weak verification of server
 identities only (see Server-to-Server Communications (Section 14.4)
 regarding this method's security characteristics).  Domains requiring
 robust security SHOULD use TLS and SASL; see Server-to-Server
 Communications (Section 14.4) for details.  If SASL is used for
 server-to-server authentication, dialback SHOULD NOT be used since it
 is unnecessary.  Documentation of dialback is included mainly for the
 sake of backward-compatibility with existing implementations and
 deployments.
 The server dialback method is made possible by the existence of the
 Domain Name System (DNS), since one server can (normally) discover
 the authoritative server for a given domain.  Because dialback
 depends on DNS, inter-domain communications MUST NOT proceed until
 the Domain Name System (DNS) hostnames asserted by the servers have
 been resolved (see Server-to-Server Communications (Section 14.4)).
 Server dialback is uni-directional, and results in (weak)
 verification of identities for one stream in one direction.  Because
 server dialback is not an authentication mechanism, mutual
 authentication is not possible via dialback.  Therefore, server
 dialback MUST be completed in each direction in order to enable
 bi-directional communications between two domains.
 The method for generating and verifying the keys used in server
 dialback MUST take into account the hostnames being used, the stream
 ID generated by the receiving server, and a secret known by the
 authoritative server's network.  The stream ID is security-critical
 in server dialback and therefore MUST be both unpredictable and
 non-repeating (see [RANDOM] for recommendations regarding randomness
 for security purposes).
 Any error that occurs during dialback negotiation MUST be considered
 a stream error, resulting in termination of the stream and of the
 underlying TCP connection.  The possible error conditions are
 specified in the protocol description below.
 The following terminology applies:
 o  Originating Server -- the server that is attempting to establish a
    connection between two domains.

Saint-Andre, Ed. Standards Track [Page 41] RFC 3920 XMPP Core October 2004

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

8.2. Order of Events

 The following is a brief summary of the order of events in dialback:
 1.  The Originating Server establishes a connection to the Receiving
     Server.
 2.  The Originating Server sends a 'key' value over the connection to
     the Receiving Server.
 3.  The Receiving Server establishes a connection to the
     Authoritative Server.
 4.  The Receiving Server sends the same 'key' value to the
     Authoritative Server.
 5.  The Authoritative Server replies that key is valid or invalid.
 6.  The Receiving Server informs the Originating Server whether it is
     authenticated or not.

Saint-Andre, Ed. Standards Track [Page 42] RFC 3920 XMPP Core October 2004

 We can represent this flow of events graphically as follows:
 Originating               Receiving
   Server                    Server
 -----------               ---------
     |                         |
     |   establish connection  |
     | ----------------------> |
     |                         |
     |   send stream header    |
     | ----------------------> |
     |                         |
     |   send stream header    |
     | <---------------------- |
     |                         |                   Authoritative
     |   send dialback key     |                       Server
     | ----------------------> |                   -------------
     |                         |                         |
                               |   establish connection  |
                               | ----------------------> |
                               |                         |
                               |   send stream header    |
                               | ----------------------> |
                               |                         |
                               |   send stream header    |
                               | <---------------------- |
                               |                         |
                               |   send verify request   |
                               | ----------------------> |
                               |                         |
                               |   send verify response  |
                               | <---------------------- |
                               |
     |  report dialback result |
     | <---------------------- |
     |                         |

8.3. Protocol

 The detailed protocol interaction between the servers is as follows:
 1.  The Originating Server establishes TCP connection to the
     Receiving Server.

Saint-Andre, Ed. Standards Track [Page 43] RFC 3920 XMPP Core October 2004

 2.  The Originating Server sends a stream header to the Receiving
     Server:
 <stream:stream
     xmlns:stream='http://etherx.jabber.org/streams'
     xmlns='jabber:server'
     xmlns:db='jabber:server:dialback'>
 Note: The 'to' and 'from' attributes are OPTIONAL on the root stream
 element.  The inclusion of the xmlns:db namespace declaration with
 the name shown indicates to the Receiving Server that the Originating
 Server supports dialback.  If the namespace name is incorrect, then
 the Receiving Server MUST generate an <invalid-namespace/> stream
 error condition and terminate both the XML stream and the underlying
 TCP connection.
 3.  The Receiving Server SHOULD send a stream header back to the
     Originating Server, including a unique ID for this interaction:
 <stream:stream
     xmlns:stream='http://etherx.jabber.org/streams'
     xmlns='jabber:server'
     xmlns:db='jabber:server:dialback'
     id='457F9224A0...'>
 Note: The 'to' and 'from' attributes are OPTIONAL on the root stream
 element.  If the namespace name is incorrect, then the Originating
 Server MUST generate an <invalid-namespace/> stream error condition
 and terminate both the XML stream and the underlying TCP connection.
 Note well that the Receiving Server SHOULD reply but MAY silently
 terminate the XML stream and underlying TCP connection depending on
 security policies in place; however, if the Receiving Server desires
 to proceed, it MUST send a stream header back to the Originating
 Server.
 4.  The Originating Server sends a dialback key to the Receiving
     Server:
 <db:result
     to='Receiving Server'
     from='Originating Server'>
   98AF014EDC0...
 </db:result>
 Note: This key is not examined by the Receiving Server, since the
 Receiving Server does not keep information about the Originating
 Server between sessions.  The key generated by the Originating Server
 MUST be based in part on the value of the ID provided by the

Saint-Andre, Ed. Standards Track [Page 44] RFC 3920 XMPP Core October 2004

 Receiving Server in the previous step, and in part on a secret shared
 by the Originating Server and Authoritative Server.  If the value of
 the 'to' address does not match a hostname recognized by the
 Receiving Server, then the Receiving Server MUST generate a
 <host-unknown/> stream error condition and terminate both the XML
 stream and the underlying TCP connection.  If the value of the 'from'
 address matches a domain with which the Receiving Server already has
 an established connection, then the Receiving Server MUST maintain
 the existing connection until it validates whether the new connection
 is legitimate; additionally, the Receiving Server MAY choose to
 generate a <not-authorized/> stream error condition for the new
 connection and then terminate both the XML stream and the underlying
 TCP connection related to the new request.
 5.  The Receiving Server establishes a TCP connection back to the
     domain name asserted by the Originating Server, as a result of
     which it connects to the Authoritative Server.  (Note: As an
     optimization, an implementation MAY reuse an existing connection
     here.)
 6.  The Receiving Server sends the Authoritative Server a stream
     header:
 <stream:stream
     xmlns:stream='http://etherx.jabber.org/streams'
     xmlns='jabber:server'
     xmlns:db='jabber:server:dialback'>
 Note: The 'to' and 'from' attributes are OPTIONAL on the root stream
 element.  If the namespace name is incorrect, then the Authoritative
 Server MUST generate an <invalid-namespace/> stream error condition
 and terminate both the XML stream and the underlying TCP connection.
 7.  The Authoritative Server sends the Receiving Server a stream
     header:
 <stream:stream
     xmlns:stream='http://etherx.jabber.org/streams'
     xmlns='jabber:server'
     xmlns:db='jabber:server:dialback'
     id='1251A342B...'>
 Note: If the namespace name is incorrect, then the Receiving Server
 MUST generate an <invalid-namespace/> stream error condition and
 terminate both the XML stream and the underlying TCP connection
 between it and the Authoritative Server.  If a stream error occurs
 between the Receiving Server and the Authoritative Server, then the
 Receiving Server MUST generate a <remote-connection-failed/> stream

Saint-Andre, Ed. Standards Track [Page 45] RFC 3920 XMPP Core October 2004

 error condition and terminate both the XML stream and the underlying
 TCP connection between it and the Originating Server.
 8.  The Receiving Server sends the Authoritative Server a request for
     verification of a key:
 <db:verify
     from='Receiving Server'
     to='Originating Server'
     id='457F9224A0...'>
   98AF014EDC0...
 </db:verify>
 Note: Passed here are the hostnames, the original identifier from the
 Receiving Server's stream header to the Originating Server in Step 3,
 and the key that the Originating Server sent to the Receiving Server
 in Step 4.  Based on this information, as well as shared secret
 information within the Authoritative Server's network, the key is
 verified.  Any verifiable method MAY be used to generate the key.  If
 the value of the 'to' address does not match a hostname recognized by
 the Authoritative Server, then the Authoritative Server MUST generate
 a <host-unknown/> stream error condition and terminate both the XML
 stream and the underlying TCP connection.  If the value of the 'from'
 address does not match the hostname represented by the Receiving
 Server when opening the TCP connection (or any validated domain
 thereof, such as a validated subdomain of the Receiving Server's
 hostname or another validated domain hosted by the Receiving Server),
 then the Authoritative Server MUST generate an <invalid-from/> stream
 error condition and terminate both the XML stream and the underlying
 TCP connection.
 9.  The Authoritative Server verifies whether the key was valid or
     invalid:
 <db:verify
     from='Originating Server'
     to='Receiving Server'
     type='valid'
     id='457F9224A0...'/>
 or
 <db:verify
     from='Originating Server'
     to='Receiving Server'
     type='invalid'
     id='457F9224A0...'/>

Saint-Andre, Ed. Standards Track [Page 46] RFC 3920 XMPP Core October 2004

 Note: If the ID does not match that provided by the Receiving Server
 in Step 3, then the Receiving Server MUST generate an <invalid-id/>
 stream error condition and terminate both the XML stream and the
 underlying TCP connection.  If the value of the 'to' address does not
 match a hostname recognized by the Receiving Server, then the
 Receiving Server MUST generate a <host-unknown/> stream error
 condition and terminate both the XML stream and the underlying TCP
 connection.  If the value of the 'from' address does not match the
 hostname represented by the Originating Server when opening the TCP
 connection (or any validated domain thereof, such as a validated
 subdomain of the Originating Server's hostname or another validated
 domain hosted by the Originating Server), then the Receiving Server
 MUST generate an <invalid-from/> stream error condition and terminate
 both the XML stream and the underlying TCP connection.  After
 returning the verification to the Receiving Server, the Authoritative
 Server SHOULD terminate the stream between them.
 10. The Receiving Server informs the Originating Server of the
     result:
 <db:result
     from='Receiving Server'
     to='Originating Server'
     type='valid'/>
 Note: At this point, the connection has either been validated via a
 type='valid', or reported as invalid.  If the connection is invalid,
 then the Receiving Server MUST terminate both the XML stream and the
 underlying TCP connection.  If the connection is validated, data can
 be sent by the Originating Server and read by the Receiving Server;
 before that, all XML stanzas sent to the Receiving Server SHOULD be
 silently dropped.
 The result of the foregoing is that the Receiving Server has verified
 the identity of the Originating Server, so that the Originating
 Server can send, and the Receiving Server can accept, XML stanzas
 over the "initial stream" (i.e., the stream from the Originating
 Server to the Receiving Server).  In order to verify the identities
 of the entities using the "response stream" (i.e., the stream from
 the Receiving Server to the Originating Server), dialback MUST be
 completed in the opposite direction as well.
 After successful dialback negotiation, the Receiving Server SHOULD
 accept subsequent <db:result/> packets (e.g., validation requests
 sent to a subdomain or other hostname serviced by the Receiving
 Server) from the Originating Server over the existing validated
 connection; this enables "piggybacking" of the original validated
 connection in one direction.

Saint-Andre, Ed. Standards Track [Page 47] RFC 3920 XMPP Core October 2004

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

9. XML Stanzas

 After TLS negotiation (Section 5) if desired, SASL negotiation
 (Section 6), and Resource Binding (Section 7) if necessary, XML
 stanzas can be sent over the streams.  Three kinds of XML stanza are
 defined for the 'jabber:client' and 'jabber:server' namespaces:
 <message/>, <presence/>, and <iq/>.  In addition, there are five
 common attributes for these kinds of stanza.  These common
 attributes, as well as the basic semantics of the three stanza kinds,
 are defined herein; more detailed information regarding the syntax of
 XML stanzas in relation to instant messaging and presence
 applications is provided in [XMPP-IM].

9.1. Common Attributes

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

9.1.1. to

 The 'to' attribute specifies the JID of the intended recipient for
 the stanza.
 In the 'jabber:client' namespace, a stanza SHOULD possess a 'to'
 attribute, although a stanza sent from a client to a server for
 handling by that server (e.g., presence sent to the server for
 broadcasting to other entities) SHOULD NOT possess a 'to' attribute.
 In the 'jabber:server' namespace, a stanza MUST possess a 'to'
 attribute; if a server receives a stanza that does not meet this
 restriction, it MUST generate an <improper-addressing/> stream error
 condition and terminate both the XML stream and the underlying TCP
 connection with the offending server.

Saint-Andre, Ed. Standards Track [Page 48] RFC 3920 XMPP Core October 2004

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

9.1.2. from

 The 'from' attribute specifies the JID of the sender.
 When a server receives an XML stanza within the context of an
 authenticated stream qualified by the 'jabber:client' namespace, it
 MUST do one of the following:
 1.  validate that the value of the 'from' attribute provided by the
     client is that of a connected resource for the associated entity
 2.  add a 'from' address to the stanza whose value is the bare JID
     (<node@domain>) or the full JID (<node@domain/resource>)
     determined by the server for the connected resource that
     generated the stanza (see Determination of Addresses (Section
     3.5))
 If a client attempts to send an XML stanza for which the value of the
 'from' attribute does not match one of the connected resources for
 that entity, the server SHOULD return an <invalid-from/> stream error
 to the client.  If a client attempts to send an XML stanza over a
 stream that is not yet authenticated, the server SHOULD return a
 <not-authorized/> stream error to the client.  If generated, both of
 these conditions MUST result in closure of the stream and termination
 of the underlying TCP connection; this helps to prevent a denial of
 service attack launched from a rogue client.
 When a server generates a stanza from the server itself for delivery
 to a connected client (e.g., in the context of data storage services
 provided by the server on behalf of the client), the stanza MUST
 either (1) not include a 'from' attribute or (2) include a 'from'
 attribute whose value is the account's bare JID (<node@domain>) or
 client's full JID (<node@domain/resource>).  A server MUST NOT send
 to the client a stanza without a 'from' attribute if the stanza was
 not generated by the server itself.  When a client receives a stanza
 that does not include a 'from' attribute, it MUST assume that the
 stanza is from the server to which the client is connected.
 In the 'jabber:server' namespace, a stanza MUST possess a 'from'
 attribute; if a server receives a stanza that does not meet this
 restriction, it MUST generate an <improper-addressing/> stream error
 condition.  Furthermore, the domain identifier portion of the JID

Saint-Andre, Ed. Standards Track [Page 49] RFC 3920 XMPP Core October 2004

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

9.1.3. id

 The optional 'id' attribute MAY be used by a sending entity for
 internal tracking of stanzas that it sends and receives (especially
 for tracking the request-response interaction inherent in the
 semantics of IQ stanzas).  It is OPTIONAL for the value of the 'id'
 attribute to be unique globally, within a domain, or within a stream.
 The semantics of IQ stanzas impose additional restrictions; see IQ
 Semantics (Section 9.2.3).

9.1.4. type

 The 'type' attribute specifies detailed information about the purpose
 or context of the message, presence, or IQ stanza.  The particular
 allowable values for the 'type' attribute vary depending on whether
 the stanza is a message, presence, or IQ; the values for message and
 presence stanzas are specific to instant messaging and presence
 applications and therefore are defined in [XMPP-IM], whereas the
 values for IQ stanzas specify the role of an IQ stanza in a
 structured request-response "conversation" and thus are defined under
 IQ Semantics (Section 9.2.3) below.  The only 'type' value common to
 all three stanzas is "error"; see Stanza Errors (Section 9.3).

9.1.5. xml:lang

 A stanza SHOULD possess an 'xml:lang' attribute (as defined in
 Section 2.12 of [XML]) if the stanza contains XML character data that
 is intended to be presented to a human user (as explained in RFC 2277
 [CHARSET], "internationalization is for humans").  The value of the
 'xml:lang' attribute specifies the default language of any such
 human-readable XML character data, which MAY be overridden by the
 'xml:lang' attribute of a specific child element.  If a stanza does
 not possess an 'xml:lang' attribute, an implementation MUST assume
 that the default language is that specified for the stream as defined
 under Stream Attributes (Section 4.4) above.  The value of the
 'xml:lang' attribute MUST be an NMTOKEN and MUST conform to the
 format defined in RFC 3066 [LANGTAGS].

Saint-Andre, Ed. Standards Track [Page 50] RFC 3920 XMPP Core October 2004

9.2. Basic Semantics

9.2.1. Message Semantics

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

9.2.2. Presence Semantics

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

9.2.3. IQ Semantics

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

Saint-Andre, Ed. Standards Track [Page 51] RFC 3920 XMPP Core October 2004

 Requesting                 Responding
   Entity                     Entity
 ----------                 ----------
     |                           |
     | <iq type='get' id='1'>    |
     | ------------------------> |
     |                           |
     | <iq type='result' id='1'> |
     | <------------------------ |
     |                           |
     | <iq type='set' id='2'>    |
     | ------------------------> |
     |                           |
     | <iq type='error' id='2'>  |
     | <------------------------ |
     |                           |
 In order to enforce these semantics, the following rules apply:
 1.  The 'id' attribute is REQUIRED for IQ stanzas.
 2.  The 'type' attribute is REQUIRED for IQ stanzas.  The value MUST
     be one of the following:
  • get – The stanza is a request for information or

requirements.

  • set – The stanza provides required data, sets new values, or

replaces existing values.

  • result – The stanza is a response to a successful get or set

request.

  • error – An error has occurred regarding processing or

delivery of a previously-sent get or set (see Stanza Errors

        (Section 9.3)).
 3.  An entity that receives an IQ request of type "get" or "set" MUST
     reply with an IQ response of type "result" or "error" (the
     response MUST preserve the 'id' attribute of the request).
 4.  An entity that receives a stanza of type "result" or "error" MUST
     NOT respond to the stanza by sending a further IQ response of
     type "result" or "error"; however, as shown above, the requesting
     entity MAY send another request (e.g., an IQ of type "set" in
     order to provide required information discovered through a
     get/result pair).

Saint-Andre, Ed. Standards Track [Page 52] RFC 3920 XMPP Core October 2004

 5.  An IQ stanza of type "get" or "set" MUST contain one and only one
     child element that specifies the semantics of the particular
     request or response.
 6.  An IQ stanza of type "result" MUST include zero or one child
     elements.
 7.  An IQ stanza of type "error" SHOULD include the child element
     contained in the associated "get" or "set" and MUST include an
     <error/> child; for details, see Stanza Errors (Section 9.3).

9.3. Stanza Errors

 Stanza-related errors are handled in a manner similar to stream
 errors (Section 4.7).  However, unlike stream errors, stanza errors
 are recoverable; therefore error stanzas include hints regarding
 actions that the original sender can take in order to remedy the
 error.

9.3.1. Rules

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

Saint-Andre, Ed. Standards Track [Page 53] RFC 3920 XMPP Core October 2004

9.3.2. Syntax

 The syntax for stanza-related errors is as follows:
 <stanza-kind to='sender' type='error'>
   [RECOMMENDED to include sender XML here]
   <error type='error-type'>
     <defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     <text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
           xml:lang='langcode'>
       OPTIONAL descriptive text
     </text>
     [OPTIONAL application-specific condition element]
   </error>
 </stanza-kind>
 The stanza-kind is one of message, presence, or iq.
 The value of the <error/> element's 'type' attribute MUST be one of
 the following:
 o  cancel -- do not retry (the error is unrecoverable)
 o  continue -- proceed (the condition was only a warning)
 o  modify -- retry after changing the data sent
 o  auth -- retry after providing credentials
 o  wait -- retry after waiting (the error is temporary)
 The <error/> element:
 o  MUST contain a child element corresponding to one of the defined
    stanza error conditions specified below; this element MUST be
    qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace.
 o  MAY contain a <text/> child containing XML character data that
    describes the error in more detail; this element MUST be qualified
    by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace and SHOULD
    possess an 'xml:lang' attribute.
 o  MAY contain a child element for an application-specific error
    condition; this element MUST be qualified by an
    application-defined namespace, and its structure is defined by
    that namespace.
 The <text/> element is OPTIONAL.  If included, it SHOULD be used only
 to provide descriptive or diagnostic information that supplements the
 meaning of a defined condition or application-specific condition.  It
 SHOULD NOT be interpreted programmatically by an application.  It

Saint-Andre, Ed. Standards Track [Page 54] RFC 3920 XMPP Core October 2004

 SHOULD NOT be used as the error message presented to a user, but MAY
 be shown in addition to the error message associated with the
 included condition element (or elements).
 Finally, to maintain backward compatibility, the schema (specified in
 [XMPP-IM]) allows the optional inclusion of a 'code' attribute on the
 <error/> element.

9.3.3. Defined Conditions

 The following conditions are defined for use in stanza errors.
 o  <bad-request/> -- the sender has sent XML that is malformed or
    that cannot be processed (e.g., an IQ stanza that includes an
    unrecognized value of the 'type' attribute); the associated error
    type SHOULD be "modify".
 o  <conflict/> -- access cannot be granted because an existing
    resource or session exists with the same name or address; the
    associated error type SHOULD be "cancel".
 o  <feature-not-implemented/> -- the feature requested is not
    implemented by the recipient or server and therefore cannot be
    processed; the associated error type SHOULD be "cancel".
 o  <forbidden/> -- the requesting entity does not possess the
    required permissions to perform the action; the associated error
    type SHOULD be "auth".
 o  <gone/> -- the recipient or server can no longer be contacted at
    this address (the error stanza MAY contain a new address in the
    XML character data of the <gone/> element); the associated error
    type SHOULD be "modify".
 o  <internal-server-error/> -- the server could not process the
    stanza because of a misconfiguration or an otherwise-undefined
    internal server error; the associated error type SHOULD be "wait".
 o  <item-not-found/> -- the addressed JID or item requested cannot be
    found; the associated error type SHOULD be "cancel".
 o  <jid-malformed/> -- the sending entity has provided or
    communicated an XMPP address (e.g., a value of the 'to' attribute)
    or aspect thereof (e.g., a resource identifier) that does not
    adhere to the syntax defined in Addressing Scheme (Section 3); the
    associated error type SHOULD be "modify".

Saint-Andre, Ed. Standards Track [Page 55] RFC 3920 XMPP Core October 2004

 o  <not-acceptable/> -- the recipient or server understands the
    request but is refusing to process it because it does not meet
    criteria defined by the recipient or server (e.g., a local policy
    regarding acceptable words in messages); the associated error type
    SHOULD be "modify".
 o  <not-allowed/> -- the recipient or server does not allow any
    entity to perform the action; the associated error type SHOULD be
    "cancel".
 o  <not-authorized/> -- the sender must provide proper credentials
    before being allowed to perform the action, or has provided
    improper credentials; the associated error type SHOULD be "auth".
 o  <payment-required/> -- the requesting entity is not authorized to
    access the requested service because payment is required; the
    associated error type SHOULD be "auth".
 o  <recipient-unavailable/> -- the intended recipient is temporarily
    unavailable; the associated error type SHOULD be "wait" (note: an
    application MUST NOT return this error if doing so would provide
    information about the intended recipient's network availability to
    an entity that is not authorized to know such information).
 o  <redirect/> -- the recipient or server is redirecting requests for
    this information to another entity, usually temporarily (the error
    stanza SHOULD contain the alternate address, which MUST be a valid
    JID, in the XML character data of the <redirect/> element); the
    associated error type SHOULD be "modify".
 o  <registration-required/> -- the requesting entity is not
    authorized to access the requested service because registration is
    required; the associated error type SHOULD be "auth".
 o  <remote-server-not-found/> -- a remote server or service specified
    as part or all of the JID of the intended recipient does not
    exist; the associated error type SHOULD be "cancel".
 o  <remote-server-timeout/> -- a remote server or service specified
    as part or all of the JID of the intended recipient (or required
    to fulfill a request) could not be contacted within a reasonable
    amount of time; the associated error type SHOULD be "wait".
 o  <resource-constraint/> -- the server or recipient lacks the system
    resources necessary to service the request; the associated error
    type SHOULD be "wait".

Saint-Andre, Ed. Standards Track [Page 56] RFC 3920 XMPP Core October 2004

 o  <service-unavailable/> -- the server or recipient does not
    currently provide the requested service; the associated error type
    SHOULD be "cancel".
 o  <subscription-required/> -- the requesting entity is not
    authorized to access the requested service because a subscription
    is required; the associated error type SHOULD be "auth".
 o  <undefined-condition/> -- the error condition is not one of those
    defined by the other conditions in this list; any error type may
    be associated with this condition, and it SHOULD be used only in
    conjunction with an application-specific condition.
 o  <unexpected-request/> -- the recipient or server understood the
    request but was not expecting it at this time (e.g., the request
    was out of order); the associated error type SHOULD be "wait".

9.3.4. Application-Specific Conditions

 As noted, an application MAY provide application-specific stanza
 error information by including a properly-namespaced child in the
 error element.  The application-specific element SHOULD supplement or
 further qualify a defined element.  Thus, the <error/> element will
 contain two or three child elements:
 <iq type='error' id='some-id'>
   <error type='modify'>
     <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     <too-many-parameters xmlns='application-ns'/>
   </error>
 </iq>
 <message type='error' id='another-id'>
   <error type='modify'>
     <undefined-condition
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     <text xml:lang='en'
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
       Some special application diagnostic information...
     </text>
     <special-application-condition xmlns='application-ns'/>
   </error>
 </message>

Saint-Andre, Ed. Standards Track [Page 57] RFC 3920 XMPP Core October 2004

10. Server Rules for Handling XML Stanzas

 Compliant server implementations MUST ensure in-order processing of
 XML stanzas between any two entities.
 Beyond the requirement for in-order processing, each server
 implementation will contain its own "delivery tree" for handling
 stanzas it receives.  Such a tree determines whether a stanza needs
 to be routed to another domain, processed internally, or delivered to
 a resource associated with a connected node.  The following rules
 apply:

10.1. No 'to' Address

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

10.2. Foreign Domain

 If the hostname of the domain identifier portion of the JID contained
 in the 'to' attribute does not match one of the configured hostnames
 of the server itself or a subdomain thereof, the server SHOULD route
 the stanza to the foreign domain (subject to local service
 provisioning and security policies regarding inter-domain
 communication).  There are two possible cases:
 A server-to-server stream already exists between the two domains: The
    sender's server routes the stanza to the authoritative server for
    the foreign domain over the existing stream
 There exists no server-to-server stream between the two domains: The
    sender's server (1) resolves the hostname of the foreign domain
    (as defined under Server-to-Server Communications (Section 14.4)),
    (2) negotiates a server-to-server stream between the two domains
    (as defined under Use of TLS (Section 5) and Use of SASL (Section

Saint-Andre, Ed. Standards Track [Page 58] RFC 3920 XMPP Core October 2004

    6)), and (3) routes the stanza to the authoritative server for the
    foreign domain over the newly-established stream
 If routing to the recipient's server is unsuccessful, the sender's
 server MUST return an error to the sender; if the recipient's server
 can be contacted but delivery by the recipient's server to the
 recipient is unsuccessful, the recipient's server MUST return an
 error to the sender by way of the sender's server.

10.3. Subdomain

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

10.4. Mere Domain or Specific Resource

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

10.5. Node in Same Domain

 If the hostname of the domain identifier portion of the JID contained
 in the 'to' attribute matches a configured hostname of the server
 itself and the JID contained in the 'to' attribute is of the form
 <node@domain> or <node@domain/resource>, the server SHOULD deliver
 the stanza to the intended recipient of the stanza as represented by
 the JID contained in the 'to' attribute.  The following rules apply:
 1.  If the JID contains a resource identifier (i.e., is of the form
     <node@domain/resource>) and there exists a connected resource
     that matches the full JID, the recipient's server SHOULD deliver
     the stanza to the stream or session that exactly matches the
     resource identifier.
 2.  If the JID contains a resource identifier and there exists no
     connected resource that matches the full JID, the recipient's
     server SHOULD return a <service-unavailable/> stanza error to the
     sender.

Saint-Andre, Ed. Standards Track [Page 59] RFC 3920 XMPP Core October 2004

 3.  If the JID is of the form <node@domain> and there exists at least
     one connected resource for the node, the recipient's server
     SHOULD deliver the stanza to at least one of the connected
     resources, according to application-specific rules (a set of
     delivery rules for instant messaging and presence applications is
     defined in [XMPP-IM]).

11. XML Usage within XMPP

11.1. Restrictions

 XMPP is a simplified and specialized protocol for streaming XML
 elements in order to exchange structured information in close to real
 time.  Because XMPP does not require the parsing of arbitrary and
 complete XML documents, there is no requirement that XMPP needs to
 support the full feature set of [XML].  In particular, the following
 restrictions apply.
 With regard to XML generation, an XMPP implementation MUST NOT inject
 into an XML stream any of the following:
 o  comments (as defined in Section 2.5 of [XML])
 o  processing instructions (Section 2.6 therein)
 o  internal or external DTD subsets (Section 2.8 therein)
 o  internal or external entity references (Section 4.2 therein) with
    the exception of predefined entities (Section 4.6 therein)
 o  character data or attribute values containing unescaped characters
    that map to the predefined entities (Section 4.6 therein); such
    characters MUST be escaped
 With regard to XML processing, if an XMPP implementation receives
 such restricted XML data, it MUST ignore the data.

11.2. XML Namespace Names and Prefixes

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

Saint-Andre, Ed. Standards Track [Page 60] RFC 3920 XMPP Core October 2004

11.2.1. Streams Namespace

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

11.2.2. Default Namespace

 A default namespace declaration is REQUIRED and is used in all XML
 streams in order to define the allowable first-level children of the
 root stream element.  This namespace declaration MUST be the same for
 the initial stream and the response stream so that both streams are
 qualified consistently.  The default namespace declaration applies to
 the stream and all stanzas sent within a stream (unless explicitly
 qualified by another namespace, or by the prefix of the streams
 namespace or the dialback namespace).
 A server implementation MUST support the following two default
 namespaces (for historical reasons, some implementations MAY support
 only these two default namespaces):
 o  jabber:client -- this default namespace is declared when the
    stream is used for communications between a client and a server
 o  jabber:server -- this default namespace is declared when the
    stream is used for communications between two servers
 A client implementation MUST support the 'jabber:client' default
 namespace, and for historical reasons MAY support only that default
 namespace.
 An implementation MUST NOT generate namespace prefixes for elements
 in the default namespace if the default namespace is 'jabber:client'
 or 'jabber:server'.  An implementation SHOULD NOT generate namespace
 prefixes for elements qualified by content (as opposed to stream)
 namespaces other than 'jabber:client' and 'jabber:server'.
 Note: The 'jabber:client' and 'jabber:server' namespaces are nearly
 identical but are used in different contexts (client-to-server
 communications for 'jabber:client' and server-to-server
 communications for 'jabber:server').  The only difference between the
 two is that the 'to' and 'from' attributes are OPTIONAL on stanzas
 sent within 'jabber:client', whereas they are REQUIRED on stanzas

Saint-Andre, Ed. Standards Track [Page 61] RFC 3920 XMPP Core October 2004

 sent within 'jabber:server'.  If a compliant implementation accepts a
 stream that is qualified by the 'jabber:client' or 'jabber:server'
 namespace, it MUST support the common attributes (Section 9.1) and
 basic semantics (Section 9.2) of all three core stanza kinds
 (message, presence, and IQ).

11.2.3. Dialback Namespace

 A dialback namespace declaration is REQUIRED for all elements used in
 server dialback (Section 8).  The name of the dialback namespace MUST
 be 'jabber:server:dialback'.  All elements qualified by this
 namespace MUST be prefixed.  An implementation SHOULD generate only
 the 'db:' prefix for such elements and MAY accept only the 'db:'
 prefix.

11.3. Validation

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

11.4. Inclusion of Text Declaration

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

11.5. Character Encoding

 Implementations MUST support the UTF-8 (RFC 3629 [UTF-8])
 transformation of Universal Character Set (ISO/IEC 10646-1 [UCS2])
 characters, as required by RFC 2277 [CHARSET].  Implementations MUST
 NOT attempt to use any other encoding.

12. Core Compliance Requirements

 This section summarizes the specific aspects of the Extensible
 Messaging and Presence Protocol that MUST be supported by servers and
 clients in order to be considered compliant implementations, as well
 as additional protocol aspects that SHOULD be supported.  For
 compliance purposes, we draw a distinction between core protocols

Saint-Andre, Ed. Standards Track [Page 62] RFC 3920 XMPP Core October 2004

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

12.1. Servers

 In addition to all defined requirements with regard to security, XML
 usage, and internationalization, a server MUST support the following
 core protocols in order to be considered compliant:
 o  Application of the [NAMEPREP], Nodeprep (Appendix A), and
    Resourceprep (Appendix B) profiles of [STRINGPREP] to addresses
    (including ensuring that domain identifiers are internationalized
    domain names as defined in [IDNA])
 o  XML streams (Section 4), including Use of TLS (Section 5), Use of
    SASL (Section 6), and Resource Binding (Section 7)
 o  The basic semantics of the three defined stanza kinds (i.e.,
    <message/>, <presence/>, and <iq/>) as specified in stanza
    semantics (Section 9.2)
 o  Generation (and, where appropriate, handling) of error syntax and
    semantics related to streams, TLS, SASL, and XML stanzas
 In addition, a server MAY support the following core protocol:
 o  Server dialback (Section 8)

12.2. Clients

 A client MUST support the following core protocols in order to be
 considered compliant:
 o  XML streams (Section 4), including Use of TLS (Section 5), Use of
    SASL (Section 6), and Resource Binding (Section 7)
 o  The basic semantics of the three defined stanza kinds (i.e.,
    <message/>, <presence/>, and <iq/>) as specified in stanza
    semantics (Section 9.2)
 o  Handling (and, where appropriate, generation) of error syntax and
    semantics related to streams, TLS, SASL, and XML stanzas

Saint-Andre, Ed. Standards Track [Page 63] RFC 3920 XMPP Core October 2004

 In addition, a client SHOULD support the following core protocols:
 o  Generation of addresses to which the [NAMEPREP], Nodeprep
    (Appendix A), and Resourceprep (Appendix B) profiles of
    [STRINGPREP] can be applied without failing

13. Internationalization Considerations

 XML streams MUST be encoded in UTF-8 as specified under Character
 Encoding (Section 11.5).  As specified under Stream Attributes
 (Section 4.4), an XML stream SHOULD include an 'xml:lang' attribute
 that is treated as the default language for any XML character data
 sent over the stream that is intended to be presented to a human
 user.  As specified under xml:lang (Section 9.1.5), an XML stanza
 SHOULD include an 'xml:lang' attribute if the stanza contains XML
 character data that is intended to be presented to a human user.  A
 server SHOULD apply the default 'xml:lang' attribute to stanzas it
 routes or delivers on behalf of connected entities, and MUST NOT
 modify or delete 'xml:lang' attributes from stanzas it receives from
 other entities.

14. Security Considerations

14.1. High Security

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

14.2. Certificate Validation

 When an XMPP peer communicates with another peer securely, it MUST
 validate the peer's certificate.  There are three possible cases:
 Case #1: The peer contains an End Entity certificate which appears to
    be certified by a chain of certificates terminating in a trust
    anchor (as described in Section 6.1 of [X509]).

Saint-Andre, Ed. Standards Track [Page 64] RFC 3920 XMPP Core October 2004

 Case #2: The peer certificate is certified by a Certificate Authority
    not known to the validating peer.
 Case #3: The peer certificate is self-signed.
 In Case #1, the validating peer MUST do one of two things:
 1.  Verify the peer certificate according to the rules of [X509].
     The certificate SHOULD then be checked against the expected
     identity of the peer following the rules described in [HTTP-TLS],
     except that a subjectAltName extension of type "xmpp" MUST be
     used as the identity if present.  If one of these checks fails,
     user-oriented clients MUST either notify the user (clients MAY
     give the user the opportunity to continue with the connection in
     any case) or terminate the connection with a bad certificate
     error.  Automated clients SHOULD terminate the connection (with a
     bad certificate error) and log the error to an appropriate audit
     log.  Automated clients MAY provide a configuration setting that
     disables this check, but MUST provide a setting that enables it.
 2.  The peer SHOULD show the certificate to a user for approval,
     including the entire certificate chain.  The peer MUST cache the
     certificate (or some non-forgeable representation such as a
     hash).  In future connections, the peer MUST verify that the same
     certificate was presented and MUST notify the user if it has
     changed.
 In Case #2 and Case #3, implementations SHOULD act as in (2) above.

14.3. Client-to-Server Communications

 A compliant client implementation MUST support both TLS and SASL for
 connections to a server.
 The TLS protocol for encrypting XML streams (defined under Use of TLS
 (Section 5)) provides a reliable mechanism for helping to ensure the
 confidentiality and data integrity of data exchanged between two
 entities.
 The SASL protocol for authenticating XML streams (defined under Use
 of SASL (Section 6)) provides a reliable mechanism for validating
 that a client connecting to a server is who it claims to be.
 Client-to-server communications MUST NOT proceed until the DNS
 hostname asserted by the server has been resolved.  Such resolutions
 SHOULD first attempt to resolve the hostname using an [SRV] Service
 of "xmpp-client" and Proto of "tcp", resulting in resource records
 such as "_xmpp-client._tcp.example.com." (the use of the string

Saint-Andre, Ed. Standards Track [Page 65] RFC 3920 XMPP Core October 2004

 "xmpp-client" for the service identifier is consistent with the IANA
 registration).  If the SRV lookup fails, the fallback is a normal
 IPv4/IPv6 address record resolution to determine the IP address,
 using the "xmpp-client" port of 5222, registered with the IANA.
 The IP address and method of access of clients MUST NOT be made
 public by a server, nor are any connections other than the original
 server connection required.  This helps to protect the client's
 server from direct attack or identification by third parties.

14.4. Server-to-Server Communications

 A compliant server implementation MUST support both TLS and SASL for
 inter-domain communications.  For historical reasons, a compliant
 implementation SHOULD also support Server Dialback (Section 8).
 Because service provisioning is a matter of policy, it is OPTIONAL
 for any given domain to communicate with other domains, and
 server-to-server communications MAY be disabled by the administrator
 of any given deployment.  If a particular domain enables inter-domain
 communications, it SHOULD enable high security.
 Administrators may want to require use of SASL for server-to-server
 communications in order to ensure both authentication and
 confidentiality (e.g., on an organization's private network).
 Compliant implementations SHOULD support SASL for this purpose.
 Inter-domain connections MUST NOT proceed until the DNS hostnames
 asserted by the servers have been resolved.  Such resolutions MUST
 first attempt to resolve the hostname using an [SRV] Service of
 "xmpp-server" and Proto of "tcp", resulting in resource records such
 as "_xmpp-server._tcp.example.com." (the use of the string
 "xmpp-server" for the service identifier is consistent with the IANA
 registration; note well that the "xmpp-server" service identifier
 supersedes the earlier use of a "jabber" service identifier, since
 the earlier usage did not conform to [SRV]; implementations desiring
 to be backward compatible should continue to look for or answer to
 the "jabber" service identifier as well).  If the SRV lookup fails,
 the fallback is a normal IPv4/IPv6 address record resolution to
 determine the IP address, using the "xmpp-server" port 5269,
 registered with the IANA.
 Server dialback helps protect against domain spoofing, thus making it
 more difficult to spoof XML stanzas.  It is not a mechanism for
 authenticating, securing, or encrypting streams between servers as is
 done via SASL and TLS, and results in weak verification of server
 identities only.  Furthermore, it is susceptible to DNS poisoning
 attacks unless DNSSec [DNSSEC] is used, and even if the DNS

Saint-Andre, Ed. Standards Track [Page 66] RFC 3920 XMPP Core October 2004

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

14.5. Order of Layers

 The order of layers in which protocols MUST be stacked is as follows:
 1.  TCP
 2.  TLS
 3.  SASL
 4.  XMPP
 The rationale for this order is that [TCP] is the base connection
 layer used by all of the protocols stacked on top of TCP, [TLS] is
 often provided at the operating system layer, [SASL] is often
 provided at the application layer, and XMPP is the application
 itself.

14.6. Lack of SASL Channel Binding to TLS

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

14.7. Mandatory-to-Implement Technologies

 At a minimum, all implementations MUST support the following
 mechanisms:
 for authentication: the SASL [DIGEST-MD5] mechanism
 for confidentiality: TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA
    cipher)
 for both: TLS plus SASL EXTERNAL(using the
    TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher supporting client-side
    certificates)

Saint-Andre, Ed. Standards Track [Page 67] RFC 3920 XMPP Core October 2004

14.8. Firewalls

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

14.9. Use of base64 in SASL

 Both the client and the server MUST verify any [BASE64] data received
 during SASL negotiation.  An implementation MUST reject (not ignore)
 any characters that are not explicitly allowed by the base64
 alphabet; this helps to guard against creation of a covert channel
 that could be used to "leak" information.  An implementation MUST NOT
 break on invalid input and MUST reject any sequence of base64
 characters containing the pad ('=') character if that character is
 included as something other than the last character of the data
 (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against buffer
 overflow attacks and other attacks on the implementation.  Base 64
 encoding visually hides otherwise easily recognized information, such
 as passwords, but does not provide any computational confidentiality.
 Base 64 encoding MUST follow the definition in Section 3 of RFC 3548
 [BASE64].

14.10. Stringprep Profiles

 XMPP makes use of the [NAMEPREP] profile of [STRINGPREP] for the
 processing of domain identifiers; for security considerations related
 to Nameprep, refer to the appropriate section of [NAMEPREP].
 In addition, XMPP defines two profiles of [STRINGPREP]: Nodeprep
 (Appendix A) for node identifiers and Resourceprep (Appendix B) for
 resource identifiers.
 The Unicode and ISO/IEC 10646 repertoires have many characters that
 look similar.  In many cases, users of security protocols might do
 visual matching, such as when comparing the names of trusted third
 parties.  Because it is impossible to map similar-looking characters
 without a great deal of context, such as knowing the fonts used,
 stringprep does nothing to map similar-looking characters together,
 nor to prohibit some characters because they look like others.
 A node identifier can be employed as one part of an entity's address
 in XMPP.  One common usage is as the username of an instant messaging
 user; another is as the name of a multi-user chat room; many other
 kinds of entities could use node identifiers as part of their

Saint-Andre, Ed. Standards Track [Page 68] RFC 3920 XMPP Core October 2004

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

15. IANA Considerations

15.1. XML Namespace Name for TLS Data

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

15.2. XML Namespace Name for SASL Data

 A URN sub-namespace for SASL-related data in the Extensible Messaging
 and Presence Protocol (XMPP) is defined as follows.  (This namespace
 name adheres to the format defined in [XML-REG].)
 URI: urn:ietf:params:xml:ns:xmpp-sasl
 Specification: RFC 3920
 Description: This is the XML namespace name for SASL-related data in
    the Extensible Messaging and Presence Protocol (XMPP) as defined
    by RFC 3920.
 Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>

Saint-Andre, Ed. Standards Track [Page 69] RFC 3920 XMPP Core October 2004

15.3. XML Namespace Name for Stream Errors

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

15.4. XML Namespace Name for Resource Binding

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

15.5. XML Namespace Name for Stanza Errors

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

15.6. Nodeprep Profile of Stringprep

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

Saint-Andre, Ed. Standards Track [Page 70] RFC 3920 XMPP Core October 2004

 RFC in which the profile is defined:
    RFC 3920
 Indicator whether or not this is the newest version of the profile:
    This is the first version of Nodeprep

15.7. Resourceprep Profile of Stringprep

 The Resourceprep profile of stringprep is defined under Resourceprep
 (Appendix B).  The IANA has registered Resourceprep in the stringprep
 profile registry.
 Name of this profile:
    Resourceprep
 RFC in which the profile is defined:
    RFC 3920
 Indicator whether or not this is the newest version of the profile:
    This is the first version of Resourceprep

15.8. GSSAPI Service Name

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

15.9. Port Numbers

 The IANA has registered "xmpp-client" and "xmpp-server" as keywords
 for [TCP] ports 5222 and 5269 respectively.
 These ports SHOULD be used for client-to-server and server-to-server
 communications respectively, but their use is OPTIONAL.

16. References

16.1. Normative References

 [ABNF]       Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.
 [BASE64]     Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, July 2003.

Saint-Andre, Ed. Standards Track [Page 71] RFC 3920 XMPP Core October 2004

 [CHARSET]    Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.
 [DIGEST-MD5] Leach, P. and C. Newman, "Using Digest Authentication as
              a SASL Mechanism", RFC 2831, May 2000.
 [DNS]        Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.
 [GSS-API]    Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.
 [HTTP-TLS]   Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
 [IDNA]       Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications
              (IDNA)", RFC 3490, March 2003.
 [IPv6]       Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.
 [LANGTAGS]   Alvestrand, H., "Tags for the Identification of
              Languages", BCP 47, RFC 3066, January 2001.
 [NAMEPREP]   Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)", RFC
              3491, March 2003.
 [RANDOM]     Eastlake 3rd, D., Crocker, S., and J. Schiller,
              "Randomness Recommendations for Security", RFC 1750,
              December 1994.
 [SASL]       Myers, J., "Simple Authentication and Security Layer
              (SASL)", RFC 2222, October 1997.
 [SRV]        Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC
              2782, February 2000.
 [STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.
 [TCP]        Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

Saint-Andre, Ed. Standards Track [Page 72] RFC 3920 XMPP Core October 2004

 [TERMS]      Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [TLS]        Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.
 [UCS2]       International Organization for Standardization,
              "Information Technology - Universal Multiple-octet coded
              Character Set (UCS) - Amendment 2: UCS Transformation
              Format 8 (UTF-8)", ISO Standard 10646-1 Addendum 2,
              October 1996.
 [UTF-8]      Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.
 [X509]       Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.
 [XML]        Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler,
              "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C
              REC-xml, October 2000, <http://www.w3.org/TR/REC-xml>.
 [XML-NAMES]  Bray, T., Hollander, D., and A. Layman, "Namespaces in
              XML", W3C REC-xml-names, January 1999,
              <http://www.w3.org/TR/REC-xml-names>.

16.2. Informative References

 [ACAP]       Newman, C. and J. Myers, "ACAP -- Application
              Configuration Access Protocol", RFC 2244, November 1997.
 [ASN.1]      CCITT, "Recommendation X.208: Specification of Abstract
              Syntax Notation One (ASN.1)", 1988.
 [DNSSEC]     Eastlake 3rd, D., "Domain Name System Security
              Extensions", RFC 2535, March 1999.
 [HTTP]       Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
 [IMAP]       Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

Saint-Andre, Ed. Standards Track [Page 73] RFC 3920 XMPP Core October 2004

 [IMP-REQS]   Day, M., Aggarwal, S., Mohr, G., and J. Vincent,
              "Instant Messaging / Presence Protocol Requirements",
              RFC 2779, February 2000.
 [IRC]        Oikarinen, J. and D. Reed, "Internet Relay Chat
              Protocol", RFC 1459, May 1993.
 [JEP-0029]   Kaes, C., "Definition of Jabber Identifiers (JIDs)", JSF
              JEP 0029, October 2003.
 [JEP-0078]   Saint-Andre, P., "Non-SASL Authentication", JSF JEP
              0078, July 2004.
 [JEP-0086]   Norris, R. and P. Saint-Andre, "Error Condition
              Mappings", JSF JEP 0086, February 2004.
 [JSF]        Jabber Software Foundation, "Jabber Software
              Foundation", <http://www.jabber.org/>.
 [POP3]       Myers, J. and M. Rose, "Post Office Protocol - Version
              3", STD 53, RFC 1939, May 1996.
 [SIMPLE]     SIMPLE Working Group, "SIMPLE WG",
              <http://www.ietf.org/html.charters/simple-charter.html>.
 [SMTP]       Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.
 [URI]        Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.
 [USINGTLS]   Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
              2595, June 1999.
 [XML-REG]    Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.
 [XMPP-IM]    Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence", RFC
              3921, October 2004.

Saint-Andre, Ed. Standards Track [Page 74] RFC 3920 XMPP Core October 2004

Appendix A. Nodeprep

A.1. Introduction

 This appendix defines the "Nodeprep" profile of [STRINGPREP].  As
 such, it specifies processing rules that will enable users to enter
 internationalized node identifiers in the Extensible Messaging and
 Presence Protocol (XMPP) and have the highest chance of getting the
 content of the strings correct.  (An XMPP node identifier is the
 optional portion of an XMPP address that precedes a domain identifier
 and the '@' separator; it is often but not exclusively associated
 with an instant messaging username.)  These processing rules are
 intended only for XMPP node identifiers and are not intended for
 arbitrary text or any other aspect of an XMPP address.
 This profile defines the following, as required by [STRINGPREP]:
 o  The intended applicability of the profile: internationalized node
    identifiers within XMPP
 o  The character repertoire that is the input and output to
    stringprep: Unicode 3.2, specified in Section 2 of this Appendix
 o  The mappings used: specified in Section 3
 o  The Unicode normalization used: specified in Section 4
 o  The characters that are prohibited as output: specified in Section
    5
 o  Bidirectional character handling: specified in Section 6

A.2. Character Repertoire

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

A.3. Mapping

 This profile specifies mapping using the following tables from
 [STRINGPREP]:
    Table B.1
    Table B.2

A.4. Normalization

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

Saint-Andre, Ed. Standards Track [Page 75] RFC 3920 XMPP Core October 2004

A.5. Prohibited Output

 This profile specifies the prohibition of using the following tables
 from [STRINGPREP].
    Table C.1.1
    Table C.1.2
    Table C.2.1
    Table C.2.2
    Table C.3
    Table C.4
    Table C.5
    Table C.6
    Table C.7
    Table C.8
    Table C.9
 In addition, the following Unicode characters are also prohibited:
    #x22 (")
    #x26 (&)
    #x27 (')
    #x2F (/)
    #x3A (:)
    #x3C (<)
    #x3E (>)
    #x40 (@)

A.6. Bidirectional Characters

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

Appendix B. Resourceprep

B.1. Introduction

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

Saint-Andre, Ed. Standards Track [Page 76] RFC 3920 XMPP Core October 2004

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

B.2. Character Repertoire

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

B.3. Mapping

 This profile specifies mapping using the following tables from
 [STRINGPREP]:
    Table B.1

B.4. Normalization

 This profile specifies using Unicode normalization form KC, as
 described in [STRINGPREP].

Saint-Andre, Ed. Standards Track [Page 77] RFC 3920 XMPP Core October 2004

B.5. Prohibited Output

 This profile specifies prohibiting use of the following tables from
 [STRINGPREP].
    Table C.1.2
    Table C.2.1
    Table C.2.2
    Table C.3
    Table C.4
    Table C.5
    Table C.6
    Table C.7
    Table C.8
    Table C.9

B.6. Bidirectional Characters

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

Appendix C. XML Schemas

 The following XML schemas are descriptive, not normative.  For
 schemas defining the 'jabber:client' and 'jabber:server' namespaces,
 refer to [XMPP-IM].

C.1. Streams namespace

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='http://etherx.jabber.org/streams'
     xmlns='http://etherx.jabber.org/streams'
     elementFormDefault='unqualified'>
   <xs:element name='stream'>
     <xs:complexType>
       <xs:sequence xmlns:client='jabber:client'
                    xmlns:server='jabber:server'
                    xmlns:db='jabber:server:dialback'>
         <xs:element ref='features' minOccurs='0' maxOccurs='1'/>
         <xs:any namespace='urn:ietf:params:xml:ns:xmpp-tls'
                 minOccurs='0'
                 maxOccurs='unbounded'/>
         <xs:any namespace='urn:ietf:params:xml:ns:xmpp-sasl'
                 minOccurs='0'

Saint-Andre, Ed. Standards Track [Page 78] RFC 3920 XMPP Core October 2004

                 maxOccurs='unbounded'/>
         <xs:choice minOccurs='0' maxOccurs='1'>
           <xs:choice minOccurs='0' maxOccurs='unbounded'>
             <xs:element ref='client:message'/>
             <xs:element ref='client:presence'/>
             <xs:element ref='client:iq'/>
           </xs:choice>
           <xs:choice minOccurs='0' maxOccurs='unbounded'>
             <xs:element ref='server:message'/>
             <xs:element ref='server:presence'/>
             <xs:element ref='server:iq'/>
             <xs:element ref='db:result'/>
             <xs:element ref='db:verify'/>
           </xs:choice>
         </xs:choice>
         <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
       </xs:sequence>
       <xs:attribute name='from' type='xs:string' use='optional'/>
       <xs:attribute name='id' type='xs:NMTOKEN' use='optional'/>
       <xs:attribute name='to' type='xs:string' use='optional'/>
       <xs:attribute name='version' type='xs:decimal' use='optional'/>
       <xs:attribute ref='xml:lang' use='optional'/>
     </xs:complexType>
   </xs:element>
   <xs:element name='features'>
     <xs:complexType>
       <xs:all xmlns:tls='urn:ietf:params:xml:ns:xmpp-tls'
               xmlns:sasl='urn:ietf:params:xml:ns:xmpp-sasl'
               xmlns:bind='urn:ietf:params:xml:ns:xmpp-bind'
               xmlns:sess='urn:ietf:params:xml:ns:xmpp-session'>
         <xs:element ref='tls:starttls' minOccurs='0'/>
         <xs:element ref='sasl:mechanisms' minOccurs='0'/>
         <xs:element ref='bind:bind' minOccurs='0'/>
         <xs:element ref='sess:session' minOccurs='0'/>
       </xs:all>
     </xs:complexType>
   </xs:element>
   <xs:element name='error'>
     <xs:complexType>
       <xs:sequence  xmlns:err='urn:ietf:params:xml:ns:xmpp-streams'>
         <xs:group   ref='err:streamErrorGroup'/>
         <xs:element ref='err:text'
                     minOccurs='0'
                     maxOccurs='1'/>
       </xs:sequence>
     </xs:complexType>

Saint-Andre, Ed. Standards Track [Page 79] RFC 3920 XMPP Core October 2004

   </xs:element>
 </xs:schema>

C.2. Stream error namespace

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='urn:ietf:params:xml:ns:xmpp-streams'
     xmlns='urn:ietf:params:xml:ns:xmpp-streams'
     elementFormDefault='qualified'>
   <xs:element name='bad-format' type='empty'/>
   <xs:element name='bad-namespace-prefix' type='empty'/>
   <xs:element name='conflict' type='empty'/>
   <xs:element name='connection-timeout' type='empty'/>
   <xs:element name='host-gone' type='empty'/>
   <xs:element name='host-unknown' type='empty'/>
   <xs:element name='improper-addressing' type='empty'/>
   <xs:element name='internal-server-error' type='empty'/>
   <xs:element name='invalid-from' type='empty'/>
   <xs:element name='invalid-id' type='empty'/>
   <xs:element name='invalid-namespace' type='empty'/>
   <xs:element name='invalid-xml' type='empty'/>
   <xs:element name='not-authorized' type='empty'/>
   <xs:element name='policy-violation' type='empty'/>
   <xs:element name='remote-connection-failed' type='empty'/>
   <xs:element name='resource-constraint' type='empty'/>
   <xs:element name='restricted-xml' type='empty'/>
   <xs:element name='see-other-host' type='xs:string'/>
   <xs:element name='system-shutdown' type='empty'/>
   <xs:element name='undefined-condition' type='empty'/>
   <xs:element name='unsupported-encoding' type='empty'/>
   <xs:element name='unsupported-stanza-type' type='empty'/>
   <xs:element name='unsupported-version' type='empty'/>
   <xs:element name='xml-not-well-formed' type='empty'/>
   <xs:group name='streamErrorGroup'>
     <xs:choice>
       <xs:element ref='bad-format'/>
       <xs:element ref='bad-namespace-prefix'/>
       <xs:element ref='conflict'/>
       <xs:element ref='connection-timeout'/>
       <xs:element ref='host-gone'/>
       <xs:element ref='host-unknown'/>
       <xs:element ref='improper-addressing'/>

Saint-Andre, Ed. Standards Track [Page 80] RFC 3920 XMPP Core October 2004

       <xs:element ref='internal-server-error'/>
       <xs:element ref='invalid-from'/>
       <xs:element ref='invalid-id'/>
       <xs:element ref='invalid-namespace'/>
       <xs:element ref='invalid-xml'/>
       <xs:element ref='not-authorized'/>
       <xs:element ref='policy-violation'/>
       <xs:element ref='remote-connection-failed'/>
       <xs:element ref='resource-constraint'/>
       <xs:element ref='restricted-xml'/>
       <xs:element ref='see-other-host'/>
       <xs:element ref='system-shutdown'/>
       <xs:element ref='undefined-condition'/>
       <xs:element ref='unsupported-encoding'/>
       <xs:element ref='unsupported-stanza-type'/>
       <xs:element ref='unsupported-version'/>
       <xs:element ref='xml-not-well-formed'/>
     </xs:choice>
   </xs:group>
   <xs:element name='text'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:string'>
           <xs:attribute ref='xml:lang' use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:simpleType name='empty'>
     <xs:restriction base='xs:string'>
       <xs:enumeration value=''/>
     </xs:restriction>
   </xs:simpleType>
 </xs:schema>

C.3. TLS namespace

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='urn:ietf:params:xml:ns:xmpp-tls'
     xmlns='urn:ietf:params:xml:ns:xmpp-tls'
     elementFormDefault='qualified'>

Saint-Andre, Ed. Standards Track [Page 81] RFC 3920 XMPP Core October 2004

   <xs:element name='starttls'>
     <xs:complexType>
       <xs:sequence>
         <xs:element
             name='required'
             minOccurs='0'
             maxOccurs='1'
             type='empty'/>
       </xs:sequence>
     </xs:complexType>
   </xs:element>
   <xs:element name='proceed' type='empty'/>
   <xs:element name='failure' type='empty'/>
   <xs:simpleType name='empty'>
     <xs:restriction base='xs:string'>
       <xs:enumeration value=''/>
     </xs:restriction>
   </xs:simpleType>
 </xs:schema>

C.4. SASL namespace

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='urn:ietf:params:xml:ns:xmpp-sasl'
     xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
     elementFormDefault='qualified'>
   <xs:element name='mechanisms'>
     <xs:complexType>
       <xs:sequence>
         <xs:element name='mechanism'
                     maxOccurs='unbounded'
                     type='xs:string'/>
       </xs:sequence>
     </xs:complexType>
   </xs:element>
   <xs:element name='auth'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='empty'>
           <xs:attribute name='mechanism'

Saint-Andre, Ed. Standards Track [Page 82] RFC 3920 XMPP Core October 2004

                         type='xs:string'
                         use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:element name='challenge' type='xs:string'/>
   <xs:element name='response' type='xs:string'/>
   <xs:element name='abort' type='empty'/>
   <xs:element name='success' type='empty'/>
   <xs:element name='failure'>
     <xs:complexType>
       <xs:choice minOccurs='0'>
         <xs:element name='aborted' type='empty'/>
         <xs:element name='incorrect-encoding' type='empty'/>
         <xs:element name='invalid-authzid' type='empty'/>
         <xs:element name='invalid-mechanism' type='empty'/>
         <xs:element name='mechanism-too-weak' type='empty'/>
         <xs:element name='not-authorized' type='empty'/>
         <xs:element name='temporary-auth-failure' type='empty'/>
       </xs:choice>
     </xs:complexType>
   </xs:element>
   <xs:simpleType name='empty'>
     <xs:restriction base='xs:string'>
       <xs:enumeration value=''/>
     </xs:restriction>
   </xs:simpleType>
 </xs:schema>

C.5. Resource binding namespace

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='urn:ietf:params:xml:ns:xmpp-bind'
     xmlns='urn:ietf:params:xml:ns:xmpp-bind'
     elementFormDefault='qualified'>
   <xs:element name='bind'>
     <xs:complexType>
       <xs:choice minOccurs='0' maxOccurs='1'>
         <xs:element name='resource' type='xs:string'/>
         <xs:element name='jid' type='xs:string'/>

Saint-Andre, Ed. Standards Track [Page 83] RFC 3920 XMPP Core October 2004

       </xs:choice>
     </xs:complexType>
   </xs:element>
 </xs:schema>

C.6. Dialback namespace

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='jabber:server:dialback'
     xmlns='jabber:server:dialback'
     elementFormDefault='qualified'>
   <xs:element name='result'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:token'>
           <xs:attribute name='from' type='xs:string' use='required'/>
           <xs:attribute name='to' type='xs:string' use='required'/>
           <xs:attribute name='type' use='optional'>
             <xs:simpleType>
               <xs:restriction base='xs:NCName'>
                 <xs:enumeration value='invalid'/>
                 <xs:enumeration value='valid'/>
               </xs:restriction>
             </xs:simpleType>
           </xs:attribute>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:element name='verify'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:token'>
           <xs:attribute name='from' type='xs:string' use='required'/>
           <xs:attribute name='id' type='xs:NMTOKEN' use='required'/>
           <xs:attribute name='to' type='xs:string' use='required'/>
           <xs:attribute name='type' use='optional'>
             <xs:simpleType>
               <xs:restriction base='xs:NCName'>
                 <xs:enumeration value='invalid'/>
                 <xs:enumeration value='valid'/>
               </xs:restriction>

Saint-Andre, Ed. Standards Track [Page 84] RFC 3920 XMPP Core October 2004

             </xs:simpleType>
           </xs:attribute>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
 </xs:schema>

C.7. Stanza error namespace

 <?xml version='1.0' encoding='UTF-8'?>
 <xs:schema
     xmlns:xs='http://www.w3.org/2001/XMLSchema'
     targetNamespace='urn:ietf:params:xml:ns:xmpp-stanzas'
     xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
     elementFormDefault='qualified'>
   <xs:element name='bad-request' type='empty'/>
   <xs:element name='conflict' type='empty'/>
   <xs:element name='feature-not-implemented' type='empty'/>
   <xs:element name='forbidden' type='empty'/>
   <xs:element name='gone' type='xs:string'/>
   <xs:element name='internal-server-error' type='empty'/>
   <xs:element name='item-not-found' type='empty'/>
   <xs:element name='jid-malformed' type='empty'/>
   <xs:element name='not-acceptable' type='empty'/>
   <xs:element name='not-allowed' type='empty'/>
   <xs:element name='payment-required' type='empty'/>
   <xs:element name='recipient-unavailable' type='empty'/>
   <xs:element name='redirect' type='xs:string'/>
   <xs:element name='registration-required' type='empty'/>
   <xs:element name='remote-server-not-found' type='empty'/>
   <xs:element name='remote-server-timeout' type='empty'/>
   <xs:element name='resource-constraint' type='empty'/>
   <xs:element name='service-unavailable' type='empty'/>
   <xs:element name='subscription-required' type='empty'/>
   <xs:element name='undefined-condition' type='empty'/>
   <xs:element name='unexpected-request' type='empty'/>
   <xs:group name='stanzaErrorGroup'>
     <xs:choice>
       <xs:element ref='bad-request'/>
       <xs:element ref='conflict'/>
       <xs:element ref='feature-not-implemented'/>
       <xs:element ref='forbidden'/>
       <xs:element ref='gone'/>

Saint-Andre, Ed. Standards Track [Page 85] RFC 3920 XMPP Core October 2004

       <xs:element ref='internal-server-error'/>
       <xs:element ref='item-not-found'/>
       <xs:element ref='jid-malformed'/>
       <xs:element ref='not-acceptable'/>
       <xs:element ref='not-allowed'/>
       <xs:element ref='payment-required'/>
       <xs:element ref='recipient-unavailable'/>
       <xs:element ref='redirect'/>
       <xs:element ref='registration-required'/>
       <xs:element ref='remote-server-not-found'/>
       <xs:element ref='remote-server-timeout'/>
       <xs:element ref='resource-constraint'/>
       <xs:element ref='service-unavailable'/>
       <xs:element ref='subscription-required'/>
       <xs:element ref='undefined-condition'/>
       <xs:element ref='unexpected-request'/>
     </xs:choice>
   </xs:group>
   <xs:element name='text'>
     <xs:complexType>
       <xs:simpleContent>
         <xs:extension base='xs:string'>
           <xs:attribute ref='xml:lang' use='optional'/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
   </xs:element>
   <xs:simpleType name='empty'>
     <xs:restriction base='xs:string'>
       <xs:enumeration value=''/>
     </xs:restriction>
   </xs:simpleType>
 </xs:schema>

Saint-Andre, Ed. Standards Track [Page 86] RFC 3920 XMPP Core October 2004

Appendix D. Differences Between Core Jabber Protocols and XMPP

 This section is non-normative.
 XMPP has been adapted from the protocols originally developed in the
 Jabber open-source community, which can be thought of as "XMPP 0.9".
 Because there exists a large installed base of Jabber implementations
 and deployments, it may be helpful to specify the key differences
 between the relevant Jabber protocols and XMPP in order to expedite
 and encourage upgrades of those implementations and deployments to
 XMPP.  This section summarizes the core differences, while the
 corresponding section of [XMPP-IM] summarizes the differences that
 relate specifically to instant messaging and presence applications.

D.1. Channel Encryption

 It was common practice in the Jabber community to use SSL for channel
 encryption on ports other than 5222 and 5269 (the convention is to
 use ports 5223 and 5270).  XMPP uses TLS over the IANA-registered
 ports for channel encryption, as defined under Use of TLS (Section 5)
 herein.

D.2. Authentication

 The client-server authentication protocol developed in the Jabber
 community used a basic IQ interaction qualified by the
 'jabber:iq:auth' namespace (documentation of this protocol is
 contained in [JEP-0078], published by the Jabber Software Foundation
 [JSF]).  XMPP uses SASL for authentication, as defined under Use of
 SASL (Section 6) herein.
 The Jabber community did not develop an authentication protocol for
 server-to-server communications, only the Server Dialback (Section 8)
 protocol to prevent server spoofing.  XMPP supersedes Server Dialback
 with a true server-to-server authentication protocol, as defined
 under Use of SASL (Section 6) herein.

D.3. Resource Binding

 Resource binding in the Jabber community was handled via the
 'jabber:iq:auth' namespace (which was also used for client
 authentication with a server).  XMPP defines a dedicated namespace
 for resource binding as well as the ability for a server to generate
 a resource identifier on behalf of a client, as defined under
 Resource Binding (Section 7).

Saint-Andre, Ed. Standards Track [Page 87] RFC 3920 XMPP Core October 2004

D.4. JID Processing

 JID processing was somewhat loosely defined by the Jabber community
 (documentation of forbidden characters and case handling is contained
 in [JEP-0029], published by the Jabber Software Foundation [JSF]).
 XMPP specifies the use of [NAMEPREP] for domain identifiers and
 supplements Nameprep with two additional [STRINGPREP] profiles for
 JID processing: Nodeprep (Appendix A) for node identifiers and
 Resourceprep (Appendix B) for resource identifiers.

D.5. Error Handling

 Stream-related errors were handled in the Jabber community via XML
 character data text in a <stream:error/> element.  In XMPP,
 stream-related errors are handled via an extensible mechanism defined
 under Stream Errors (Section 4.7) herein.
 Stanza-related errors were handled in the Jabber community via
 HTTP-style error codes.  In XMPP, stanza-related errors are handled
 via an extensible mechanism defined under Stanza Errors (Section 9.3)
 herein.  (Documentation of a mapping between Jabber and XMPP error
 handling mechanisms is contained in [JEP-0086], published by the
 Jabber Software Foundation [JSF].)

D.6. Internationalization

 Although use of UTF-8 has always been standard practice within the
 Jabber community, the community did not define mechanisms for
 specifying the language of human-readable text provided in XML
 character data.  XMPP specifies the use of the 'xml:lang' attribute
 in such contexts, as defined under Stream Attributes (Section 4.4)
 and xml:lang (Section 9.1.5) herein.

D.7. Stream Version Attribute

 The Jabber community did not include a 'version' attribute in stream
 headers.  XMPP specifies inclusion of that attribute as a way to
 signal support for the stream features (authentication, encryption,
 etc.) defined under Version Support (Section 4.4.1) herein.

Saint-Andre, Ed. Standards Track [Page 88] RFC 3920 XMPP Core October 2004

Contributors

 Most of the core aspects of the Extensible Messaging and Presence
 Protocol were developed originally within the Jabber open-source
 community in 1999.  This community was founded by Jeremie Miller, who
 released source code for the initial version of the jabber server in
 January 1999.  Major early contributors to the base protocol also
 included Ryan Eatmon, Peter Millard, Thomas Muldowney, and Dave
 Smith.  Work by the XMPP Working Group has concentrated especially on
 security and internationalization; in these areas, protocols for the
 use of TLS and SASL were originally contributed by Rob Norris, and
 stringprep profiles were originally contributed by Joe Hildebrand.
 The error code syntax was suggested by Lisa Dusseault.

Acknowledgements

 Thanks are due to a number of individuals in addition to the
 contributors listed.  Although it is difficult to provide a complete
 list, the following individuals were particularly helpful in defining
 the protocols or in commenting on the specifications in this memo:
 Thomas Charron, Richard Dobson, Sam Hartman, Schuyler Heath, Jonathan
 Hogg, Cullen Jennings, Craig Kaes, Jacek Konieczny, Alexey Melnikov,
 Keith Minkler, Julian Missig, Pete Resnick, Marshall Rose, Alexey
 Shchepin, Jean-Louis Seguineau, Iain Shigeoka, Greg Troxel, and David
 Waite.  Thanks also to members of the XMPP Working Group and the IETF
 community for comments and feedback provided throughout the life of
 this memo.

Author's Address

 Peter Saint-Andre (editor)
 Jabber Software Foundation
 EMail: stpeter@jabber.org

Saint-Andre, Ed. Standards Track [Page 89] RFC 3920 XMPP Core October 2004

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Saint-Andre, Ed. Standards Track [Page 90]

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