guoxing_jin

XMPP

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.

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.

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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].

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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

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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., vs. ) 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,
is able to exchange messages, presence, and
other information with . 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)).

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3. Addressing Scheme

3.1. Overview

An entity is anything that can be considered a network endpoint
(i.e., an ID on the network) and that can communicate using XMPP.
All such entities are uniquely addressable in a form that is
consistent with RFC 2396 [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 .
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 (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
(where "nick" is the occupant's room nickname).
Many other JID types are possible (e.g., 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.

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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
.

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
or a mere 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

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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" ()
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" () 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:

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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 tag (with appropriate attributes and namespace
declarations), while the end of the XML stream is denoted
unambiguously by a closing XML 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,
, , or 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 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., ),
and the end of any XML stanza is denoted unambiguously by the
corresponding close tag at depth=1 (e.g., ). 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 , , and 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 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

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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 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 element can be considered the
document entity for each "document", and the two "documents" are
built up through the accumulation of XML stanzas sent over the two
XML streams. However, this perspective is a convenience only; XMPP
does not deal in documents but in XML streams and XML stanzas.

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

|--------------------|
| |
|--------------------|
| |
| |
| |
|--------------------|
| |
| |
| |
|--------------------|
| |
| |
| |
|--------------------|
| ... |
|--------------------|
| |
|--------------------|

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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 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., , , and
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.

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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.

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We can summarize as follows:

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 (,
, and ). The numbering scheme for XMPP versions is
".". 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").

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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
".".

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
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 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.

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4.7. Stream Errors

The root stream element MAY contain an 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 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 tag, include
the element as a child of the stream element, send the
closing 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:

OPTIONAL descriptive text

[OPTIONAL application-specific condition element]

The 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

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o MAY contain a 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 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 -- the entity has sent XML that cannot be processed;
this error MAY be used instead of the more specific XML-related
errors, such as , ,
, , and
, although the more specific errors are
preferred.

o -- 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 -- the server is closing the active stream for this
entity because a new stream has been initiated that conflicts with
the existing stream.

o -- the entity has not generated any traffic
over the stream for some period of time (configurable according to
a local service policy).

o -- 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.

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o -- 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 -- a stanza sent between two servers lacks
a 'to' or 'from' attribute (or the attribute has no value).

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

o -- 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 -- the stream ID or dialback ID is invalid or does
not match an ID previously provided.

o -- 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 -- the entity has sent invalid XML over the stream
to a server that performs validation (see Validation (Section
11.3)).

o -- 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 -- the entity has violated some local service
policy; the server MAY choose to specify the policy in the
element or an application-specific condition element.

o -- the server is unable to properly
connect to a remote entity that is required for authentication or
authorization.

o -- the server lacks the system resources
necessary to service the stream.

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o -- 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 -- 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 element.

o -- the server is being shut down and all active
streams are being closed.

o -- 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 -- the initiating entity has encoded the
stream in an encoding that is not supported by the server (see
Character Encoding (Section 11.5)).

o -- the initiating entity has sent a
first-level child of the stream that is not supported by the
server.

o -- 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 element.

o -- 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 element will
contain two or three child elements:

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Some special application diagnostic information!

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":

... encryption, authentication, and resource binding ...
C:
C: Art thou not Romeo, and a Montague?
C:
S:
S: Neither, fair saint, if either thee dislike.
S:
C:
S:

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A "session" gone bad:

... encryption, authentication, and resource binding ...
C:
Bad XML, no closing body tag!

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.

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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 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
element. The initiating entity MUST consider the TLS
negotiation to have begun immediately after receiving the closing
">" character of the 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

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"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".

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5.2. Narrative

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

1. The initiating entity opens a TCP connection and initiates the
stream by sending the opening XML stream header to the receiving
entity, including the 'version' attribute set to a value of at
least "1.0".

2. The receiving entity responds by opening a TCP connection and
sending an XML stream header to the initiating entity, including
the 'version' attribute set to a value of at least "1.0".

3. The receiving entity offers the STARTTLS extension to the
initiating entity by including it with the list of other
supported stream features (if TLS is required for interaction
with the receiving entity, it SHOULD signal that fact by
including a element as a child of the
element).

4. The initiating entity issues the STARTTLS command (i.e., a
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 element
or a 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 tag first, since the
receiving entity and initiating entity MUST consider the original
stream to be closed upon successful TLS negotiation).

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

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:

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

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

DIGEST-MD5
PLAIN

Step 4: Client sends the STARTTLS command to server:

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

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

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:

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:

DIGEST-MD5
PLAIN
EXTERNAL

Step 9: Client continues with SASL negotiation (Section 6).

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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:

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

Step 3: Server2 sends the STARTTLS extension to Server1 along with
authentication mechanisms and any other stream features:

DIGEST-MD5
KERBEROS_V4

Step 4: Server1 sends the STARTTLS command to Server2:

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

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

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:

DIGEST-MD5
KERBEROS_V4
EXTERNAL

Step 9: Server1 continues with SASL negotiation (Section 6).

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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
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)

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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
(i.e., a domain identifier only) for servers and of the
form (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 element included as a child
within a container element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace, which in turn is a
child of a element in the streams namespace. If Use
of TLS (Section 5) needs to be established before a particular

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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
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 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
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
element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
namespace to the receiving entity. Upon receiving an
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.

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2. The receiving entity reports failure of the handshake by sending
a 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 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 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 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 tag first, since the
receiving entity and initiating entity MUST consider the original
stream to be closed upon sending or receiving the
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 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 element, optionally
including an initial response to avoid a round trip.

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exchange sequence: Challenges and responses are carried through the
exchange of elements from receiving entity to
initiating entity and elements from initiating entity
to receiving entity. The receiving entity reports failure by
sending a element and success by sending a
element; the initiating entity aborts the exchange by sending an
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
element for the receiving entity, and immediately after
receiving the closing ">" character of the 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 of a client
or the sending of a server.

6.4. SASL Errors

The following SASL-related error conditions are defined:

o -- The receiving entity acknowledges an
element sent by the initiating entity; sent in reply to the
element.

o -- 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
element or an element with initial response
data.

o -- 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 element or an
element with initial response data.

o -- 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 element.

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o -- The mechanism requested by the initiating
entity is weaker than server policy permits for that initiating
entity; sent in reply to a element or an
element with initial response data.

o -- 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 element or an element with initial
response data.

o -- The authentication failed because of
a temporary error condition within the receiving entity; sent in
reply to an element or 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:

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

Step 3: Server informs client of available authentication mechanisms:

DIGEST-MD5
PLAIN

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Step 4: Client selects an authentication mechanism:

Step 5: Server sends a [BASE64] encoded challenge to client:

cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9ImF1dGgi
LGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==

The decoded challenge is:

realm="somerealm",nonce="OA6MG9tEQGm2hh",/
qop="auth",charset=utf-8,algorithm=md5-sess

Step 5 (alt): Server returns error to client:

Step 6: Client sends a [BASE64] encoded response to the challenge:

dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
YXJzZXQ9dXRmLTgK

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:

cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=

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The decoded challenge is:

rspauth=ea40f60335c427b5527b84dbabcdfffd

Step 7 (alt): Server returns error to client:

Step 8: Client responds to the challenge:

Step 9: Server informs client of successful authentication:

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

Step 10: Client initiates a new stream to server:

Step 11: Server responds by sending a stream header to client along
with any additional features (or an empty features element):

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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:

Step 2: Server2 responds with a stream tag sent to Server1:

Step 3: Server2 informs Server1 of available authentication
mechanisms:

DIGEST-MD5
KERBEROS_V4

Step 4: Server1 selects an authentication mechanism:

Step 5: Server2 sends a [BASE64] encoded challenge to Server1:

cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9
ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz

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The decoded challenge is:

realm="somerealm",nonce="OA6MG9tEQGm2hh",/
qop="auth",charset=utf-8,algorithm=md5-sess

Step 5 (alt): Server2 returns error to Server1:

Step 6: Server1 sends a [BASE64] encoded response to the challenge:

dXNlcm5hbWU9ImV4YW1wbGUub3JnIixyZWFsbT0ic29tZXJlYWxtIixub25j
ZT0iT0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5j
PTAwMDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5v
cmciLHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3
LGNoYXJzZXQ9dXRmLTgK

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:

cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=

The decoded challenge is:

rspauth=ea40f60335c427b5527b84dbabcdfffd

Step 7 (alt): Server2 returns error to Server1:

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Step 8: Server1 responds to the challenge:

Step 8 (alt): Server1 aborts negotiation:

Step 9: Server2 informs Server1 of successful authentication:

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

Step 10: Server1 initiates a new stream to Server2:

Step 11: Server2 responds by sending a stream header to Server1 along
with any additional features (or an empty features element):

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 of the client (which is

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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 .

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 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:

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 element:

Client asks server to bind a resource:

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 .

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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 element that is a child of
the element:

Client binds a resource:

someresource

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 child element that specifies the full JID for the
connected resource as determined by the server:

Server informs client of successful resource binding:

[email protected]/someresource

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., ) 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.

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The protocol for these error conditions is shown below.

Resource identifier cannot be processed:

someresource

Client is not allowed to bind a resource:

someresource

Resource identifier is in use:

someresource

If, before completing the resource binding step, the client attempts
to send an XML stanza other than an IQ stanza with a child
qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace, the
server MUST NOT process the stanza and SHOULD return a
stanza error to the client.

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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.

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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.

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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.

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2. The Originating Server sends a stream header to the Receiving
Server:

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 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:

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 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:

98AF014EDC0...

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

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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
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 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:

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 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:

Note: If the namespace name is incorrect, then the Receiving Server
MUST generate an 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 stream

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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:

98AF014EDC0...

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 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 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:

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Note: If the ID does not match that provided by the Receiving Server
in Step 3, then the Receiving Server MUST generate an
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 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 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:

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 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.

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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
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 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:
, , and . 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 stream error
condition and terminate both the XML stream and the underlying TCP
connection with the offending server.

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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
() or the full JID ()
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 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
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 () or
client's full JID (). 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 stream error
condition. Furthermore, the domain identifier portion of the JID

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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
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].

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9.2. Basic Semantics

9.2.1. Message Semantics

The 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 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):

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Requesting Responding
Entity Entity
---------- ----------
| |
| |
| ------------------------> |
| |
| |
| <------------------------ |
| |
| |
| ------------------------> |
| |
| |
| <------------------------ |
| |

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).

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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
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 child element.

o An 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.

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9.3.2. Syntax

The syntax for stanza-related errors is as follows:

[RECOMMENDED to include sender XML here]

OPTIONAL descriptive text

[OPTIONAL application-specific condition element]

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

The value of the 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 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 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.

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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
element.

9.3.3. Defined Conditions

The following conditions are defined for use in stanza errors.

o -- 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 -- 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 -- the feature requested is not
implemented by the recipient or server and therefore cannot be
processed; the associated error type SHOULD be "cancel".

o -- the requesting entity does not possess the
required permissions to perform the action; the associated error
type SHOULD be "auth".

o -- 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 element); the associated error
type SHOULD be "modify".

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

o -- 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".

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o -- 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 -- the recipient or server does not allow any
entity to perform the action; the associated error type SHOULD be
"cancel".

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

o -- 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 -- 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 element); the
associated error type SHOULD be "modify".

o -- the requesting entity is not
authorized to access the requested service because registration is
required; the associated error type SHOULD be "auth".

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

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

o -- the requesting entity is not
authorized to access the requested service because a subscription
is required; the associated error type SHOULD be "auth".

o -- 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 -- 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 element will
contain two or three child elements:

Some special application diagnostic information...

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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

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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
or , 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
or , 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
) 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 stanza error to the
sender.

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3. If the JID is of the form 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.

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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
element and its and 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

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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

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(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.,
, , and ) 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.,
, , and ) 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

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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]).

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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

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"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

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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)

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14.8. Firewalls

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

14.9. Use of base64 in SASL

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

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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,

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,

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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,

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,

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,

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

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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, .

[XML-NAMES] Bray, T., Hollander, D., and A. Layman, "Namespaces in
XML", W3C REC-xml-names, January 1999,
.

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.

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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", .

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

[SIMPLE] SIMPLE Working Group, "SIMPLE WG",
.

[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].

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A.5. Prohibited Output

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

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

In addition, the following Unicode characters are also prohibited:

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

A.6. Bidirectional Characters

This profile specifies 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].

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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

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C.2. Stream error namespace

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C.3. TLS namespace

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RFC 3920 XMPP Core October 2004

C.4. SASL namespace

C.5. Resource binding namespace

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C.6. Dialback namespace

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C.7. Stanza error namespace

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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).

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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 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: [email protected]

Saint-Andre, Ed. Standards Track [Page 89]

RFC 3920 XMPP Core October 2004

Full Copyright Statement

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/S HE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
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on the IETF's procedures with respect to rights in IETF Documents can
be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
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such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
[email protected].

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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

XMPP

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