Network Working Group P. Saint-Andre
Internet-Draft J. Miller
Expires: December 3, 2003 Jabber Software Foundation
June 04, 2003
XMPP Core
draft-ietf-xmpp-core-13
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on December 3, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes the core features of the Extensible Messaging
and Presence Protocol (XMPP), a protocol for streaming XML elements
in order to exchange messages and presence information in close to
real time. While XMPP provides a generalized, extensible framework
for transporting structured information, 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 . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Discussion Venue . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Intellectual Property Notice . . . . . . . . . . . . . . . . 5
2. Generalized Architecture . . . . . . . . . . . . . . . . . . 6
2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Server . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Client . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4 Gateway . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5 Network . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Addressing Scheme . . . . . . . . . . . . . . . . . . . . . 8
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Domain Identifier . . . . . . . . . . . . . . . . . . . . . 8
3.3 Node Identifier . . . . . . . . . . . . . . . . . . . . . . 9
3.4 Resource Identifier . . . . . . . . . . . . . . . . . . . . 9
4. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2 Stream Attributes . . . . . . . . . . . . . . . . . . . . . 11
4.3 Namespace Declarations . . . . . . . . . . . . . . . . . . . 12
4.3.1 Stream Namespace Declaration . . . . . . . . . . . . . . . . 12
4.3.2 Default Namespace Declaration . . . . . . . . . . . . . . . 13
4.4 Stream Features . . . . . . . . . . . . . . . . . . . . . . 13
4.5 Stream Errors . . . . . . . . . . . . . . . . . . . . . . . 14
4.5.1 Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.5.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.5.3 Defined Conditions . . . . . . . . . . . . . . . . . . . . . 15
4.5.4 Application-Specific Conditions . . . . . . . . . . . . . . 16
4.6 Simple Streams Example . . . . . . . . . . . . . . . . . . . 16
5. Stream Encryption . . . . . . . . . . . . . . . . . . . . . 19
5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2 Narrative . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.3 Client-to-Server Example . . . . . . . . . . . . . . . . . . 22
5.4 Server-to-Server Example . . . . . . . . . . . . . . . . . . 23
6. Stream Authentication . . . . . . . . . . . . . . . . . . . 26
6.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.2 Narrative . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.3 SASL Errors . . . . . . . . . . . . . . . . . . . . . . . . 28
6.4 SASL Definition . . . . . . . . . . . . . . . . . . . . . . 29
6.5 Client-to-Server Example . . . . . . . . . . . . . . . . . . 29
6.6 Server-to-Server Example . . . . . . . . . . . . . . . . . . 32
7. Server Dialback . . . . . . . . . . . . . . . . . . . . . . 36
7.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 36
7.2 Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 38
8. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . 43
8.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 43
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8.2 Common Attributes . . . . . . . . . . . . . . . . . . . . . 43
8.2.1 to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
8.2.2 from . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
8.2.3 id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
8.2.4 type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
8.2.5 xml:lang . . . . . . . . . . . . . . . . . . . . . . . . . . 44
8.3 Message Stanzas . . . . . . . . . . . . . . . . . . . . . . 45
8.3.1 Types of Message . . . . . . . . . . . . . . . . . . . . . . 45
8.3.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 45
8.4 Presence Stanzas . . . . . . . . . . . . . . . . . . . . . . 46
8.4.1 Types of Presence . . . . . . . . . . . . . . . . . . . . . 47
8.4.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 47
8.5 IQ Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 49
8.5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 49
8.5.2 Types of IQ . . . . . . . . . . . . . . . . . . . . . . . . 50
8.5.3 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 50
8.6 Extended Namespaces . . . . . . . . . . . . . . . . . . . . 50
8.7 Stanza Errors . . . . . . . . . . . . . . . . . . . . . . . 51
8.7.1 Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
8.7.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
8.7.3 Defined Conditions . . . . . . . . . . . . . . . . . . . . . 53
8.7.4 Application-Specific Conditions . . . . . . . . . . . . . . 54
9. XML Usage within XMPP . . . . . . . . . . . . . . . . . . . 55
9.1 Restrictions . . . . . . . . . . . . . . . . . . . . . . . . 55
9.2 Namespaces . . . . . . . . . . . . . . . . . . . . . . . . . 55
9.3 Validation . . . . . . . . . . . . . . . . . . . . . . . . . 55
9.4 Character Encodings . . . . . . . . . . . . . . . . . . . . 56
9.5 Inclusion of Text Declaration . . . . . . . . . . . . . . . 56
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 57
10.1 XML Namespace Name for TLS Data . . . . . . . . . . . . . . 57
10.2 XML Namespace Name for SASL Data . . . . . . . . . . . . . . 57
10.3 XML Namespace Name for Stream Errors . . . . . . . . . . . . 57
10.4 XML Namespace Name for Stanza Errors . . . . . . . . . . . . 58
10.5 Existing Registrations . . . . . . . . . . . . . . . . . . . 58
11. Internationalization Considerations . . . . . . . . . . . . 59
12. Security Considerations . . . . . . . . . . . . . . . . . . 60
12.1 High Security . . . . . . . . . . . . . . . . . . . . . . . 60
12.2 Client-to-Server Communications . . . . . . . . . . . . . . 60
12.3 Server-to-Server Communications . . . . . . . . . . . . . . 61
12.4 Order of Layers . . . . . . . . . . . . . . . . . . . . . . 61
12.5 Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . 61
12.6 Mandatory to Implement Technologies . . . . . . . . . . . . 62
Normative References . . . . . . . . . . . . . . . . . . . . 63
Informative References . . . . . . . . . . . . . . . . . . . 65
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 65
A. XML Schemas . . . . . . . . . . . . . . . . . . . . . . . . 67
A.1 Stream namespace . . . . . . . . . . . . . . . . . . . . . . 67
A.2 Stream error namespace . . . . . . . . . . . . . . . . . . . 69
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A.3 TLS namespace . . . . . . . . . . . . . . . . . . . . . . . 70
A.4 SASL namespace . . . . . . . . . . . . . . . . . . . . . . . 70
A.5 Dialback namespace . . . . . . . . . . . . . . . . . . . . . 71
A.6 Client namespace . . . . . . . . . . . . . . . . . . . . . . 73
A.7 Server namespace . . . . . . . . . . . . . . . . . . . . . . 77
A.8 Stanza error namespace . . . . . . . . . . . . . . . . . . . 81
B. Revision History . . . . . . . . . . . . . . . . . . . . . . 82
B.1 Changes from draft-ietf-xmpp-core-12 . . . . . . . . . . . . 82
B.2 Changes from draft-ietf-xmpp-core-11 . . . . . . . . . . . . 82
B.3 Changes from draft-ietf-xmpp-core-10 . . . . . . . . . . . . 82
B.4 Changes from draft-ietf-xmpp-core-09 . . . . . . . . . . . . 82
B.5 Changes from draft-ietf-xmpp-core-08 . . . . . . . . . . . . 83
B.6 Changes from draft-ietf-xmpp-core-07 . . . . . . . . . . . . 83
B.7 Changes from draft-ietf-xmpp-core-06 . . . . . . . . . . . . 83
B.8 Changes from draft-ietf-xmpp-core-05 . . . . . . . . . . . . 83
B.9 Changes from draft-ietf-xmpp-core-04 . . . . . . . . . . . . 84
B.10 Changes from draft-ietf-xmpp-core-03 . . . . . . . . . . . . 84
B.11 Changes from draft-ietf-xmpp-core-02 . . . . . . . . . . . . 84
B.12 Changes from draft-ietf-xmpp-core-01 . . . . . . . . . . . . 84
B.13 Changes from draft-ietf-xmpp-core-00 . . . . . . . . . . . . 85
B.14 Changes from draft-miller-xmpp-core-02 . . . . . . . . . . . 85
Intellectual Property and Copyright Statements . . . . . . . 87
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1. Introduction
1.1 Overview
The Extensible Messaging and Presence Protocol (XMPP) is an open XML
[1] protocol for near-real-time messaging, presence, and
request-response services. The 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 document defines the core features of
XMPP; XMPP IM [23] defines the extensions required to provide the
instant messaging and presence functionality defined in RFC 2779 [2].
1.2 Terminology
The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [3].
1.3 Discussion Venue
The authors welcome discussion and comments related to the topics
presented in this document. The preferred forum is the
<xmppwg@jabber.org> mailing list, for which archives and subscription
information are available at <http://www.jabber.org/cgi-bin/mailman/
listinfo/xmppwg/>.
1.4 Intellectual Property Notice
This document is in full compliance with all provisions of Section 10
of RFC 2026. Parts of this specification use the term "jabber" for
identifying namespaces and other protocol syntax. Jabber[tm] is a
registered trademark of Jabber, Inc. Jabber, Inc. grants permission
to the IETF for use of the Jabber trademark in association with this
specification and its successors, if any.
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2. Generalized Architecture
2.1 Overview
Although XMPP is not wedded to any specific network architecture, to
this point it usually has been implemented via a typical
client-server architecture, wherein a client utilizing XMPP accesses
a server over a TCP [4] socket.
The following diagram provides a high-level overview of this
architecture (where "-" represents communications that use XMPP and
"=" represents communications that use any other protocol).
C1 - S1 - S2 - C3
/ \
C2 - G1 = FN1 = FC1
The symbols are as follows:
o C1, C2, C3 -- XMPP clients
o S1, S2 -- XMPP servers
o G1 -- A gateway that translates between XMPP and the protocol(s)
used on a foreign (non-XMPP) messaging network
o FN1 -- A foreign messaging network
o FC1 -- A client on a foreign messaging network
2.2 Server
A server acts as an intelligent abstraction layer for XMPP
communications. Its primary responsibilities are to manage
connections from or sessions for other entities (in the form of XML
streams to and from authorized clients, servers, and other entities)
and to route appropriately-addressed XML data "stanzas" among such
entities over XML streams. Most XMPP-compliant servers also assume
responsibility for the storage of data that is used by clients (e.g.,
contact lists for users of XMPP-based instant messaging
applications); in this case, the XML data is processed directly by
the server itself on behalf of the client and is not routed to
another entity. Compliant server implementations MUST ensure in-order
processing of XML stanzas between any two entities.
2.3 Client
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Most clients connect directly to a server over a TCP socket and use
XMPP to take full advantage of the functionality provided by a server
and any associated services. Although there is no necessary coupling
of an XML stream to a TCP socket (e.g., a client COULD connect via
HTTP polling or some other mechanism), this specification defines a
binding for XMPP to TCP only. Multiple resources (e.g., devices or
locations) MAY connect simultaneously to a server on behalf of each
authorized client, with each resource connecting over a discrete TCP
socket and differentiated by the resource identifier of a JID (e.g.,
<user@domain/home> vs. <user@domain/work>) as defined under Section
3. The port registered with the IANA for connections between a client
and a server is 5222 (see Section 10).
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 Internet Relay Chat (IRC),
Short Message Service (SMS), SMTP, and legacy instant messaging
networks 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 user-a@domain1 is
able to exchange messages, presence, and other information with
user-b@domain2. This pattern is familiar from messaging protocols
(such as SMTP) that make use of network addressing standards.
Communications between any two servers are OPTIONAL; if enabled, such
communications occur over XML streams that are normally bound to TCP
sockets, using port 5269 as registered with the IANA (see Section
10).
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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 [24]. For historical reasons, the address of such an
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 in the following format:
[node@]domain[/resource]. Each allowable portion of a JID (node
identifier, domain identifier, and resource identifier) may be up to
1023 bytes in length, resulting in a maximum total size (including
the '@' and '/' separators) of 3071 bytes.
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 active session or
connection (e.g., a specific client) in the form of <user@domain/
resource>. However, node types other than clients are possible; for
example, a specific chat room offered by a multi-user chat service
could be addressed as <room@service> (where "room" is the name of the
chat room and "service" is the hostname of the multi-user chat
service) and a specific occupant of such a room could be addressed as
<room@service/nick> (where "nick" is the occupant's room nickname).
Many other JID types are possible (e.g., <domain/resource> could be a
server-side script or service).
3.2 Domain Identifier
The domain identifier is the primary identifier and is the only
REQUIRED element of a JID (a mere domain identifier is a valid JID).
It usually represents the network gateway or "primary" server to
which other entities connect for XML routing and data management
capabilities. However, the entity referenced by a domain identifier
is not always a server, and may be a service that is addressed as a
subdomain of a server and that provides functionality above and
beyond the capabilities of a server (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 SHOULD resolve to a Fully Qualified Domain
Name. A domain identifier MUST conform to RFC 952 [6] and RFC 1123
[7]. In addition, a domain identifier MUST be no more than 1023 bytes
in length and MUST conform to the nameprep [8] profile of stringprep
[9].
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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 applications of XMPP this
address is called a "bare JID" and is of the form <node@domain>.
A node identifier MUST be no more than 1023 bytes in length and MUST
conform to the nodeprep [10] profile of stringprep [9].
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 <user@domain> or
mere <domain> address. It usually represents a specific session,
connection (e.g., a device or location), or object (e.g., a
participant in a multi-user chat room) belonging to the entity
associated with a node identifier. A resource identifier is opaque to
both servers and other clients, and is typically defined by a client
implementation as the authzid value provided during stream
authentication. An entity may maintain multiple resources
simultaneously.
A resource identifier MUST be no more than 1023 bytes in length and
MUST conform to the resourceprep [11] profile of stringprep [9].
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4. XML Streams
4.1 Overview
Two fundamental concepts make possible the rapid, asynchronous
exchange of relatively small payloads of structured information
between presence-aware entities: XML streams and XML stanzas. These
terms may be defined as follows:
Definition of XML Stream: An XML stream is a container for the
exchange of XML elements between any two entities over a network.
An XML stream is negotiated from an initiating entity (usually a
client or server) to a receiving entity (usually a server),
normally over a TCP socket, and corresponds to the initiating
entity's "session" with the receiving entity. The start of the XML
stream is denoted unambiguously by an opening XML <stream> tag
with appropriate attributes and namespace declarations, and the
end of the XML stream is denoted unambiguously by a closing XML </
stream> tag. An XML stream is unidirectional; in order to enable
bidirectional information exchange, the initiating entity and
receiving entity must negotiate one stream in each direction,
normally over the same TCP connection.
Definition of XML Stanza: An XML stanza is a discrete semantic unit
of structured information that is sent from one entity to another
over an XML stream. An XML stanza exists at the direct child level
of the root <stream/> element and is said to be well-balanced if
it matches production [43] content of the XML specification [1]).
The start of any XML stanza is denoted unambiguously by the
element start tag at depth=1 of the XML stream (e.g., <presence>),
and the end of any XML stanza is denoted unambiguously by the
corresponding close tag at depth=1 (e.g., </presence>). An XML
stanza MAY contain child elements (with accompanying attributes,
elements, and CDATA) as necessary in order to convey the desired
information.
Consider the example of a client's session with a server. In order to
connect to a server, a client must initiate an XML stream by sending
an opening <stream> tag to the server, optionally preceded by a text
declaration specifying the XML version supported and the character
encoding (see also Section 9.4). The server SHOULD then reply with a
second XML stream back to the client, again optionally preceded by a
text declaration. Once the client has authenticated with the server
(see Section 6), the client MAY send an unlimited number of XML
stanzas over the stream to any recipient on the network. When the
client desires to close the stream, it simply sends a closing </
stream> tag to the server (alternatively, the stream may be closed by
the server), after which both the client and server SHOULD close the
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underlying TCP connection as well.
Those who are accustomed to thinking of XML in a document-centric
manner may wish to view a client's session with a server as
consisting of two open-ended XML documents: one from the client to
the server and one from the server to the client. From this
perspective, the root <stream/> element can be considered the
document entity for each "document", and the two "documents" are
built up through the accumulation of XML stanzas sent over the two
XML streams. However, this perspective is a convenience only, and
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 graphically as
follows:
|--------------------|
| <stream> |
|--------------------|
| <presence> |
| <show/> |
| </presence> |
|--------------------|
| <message to='foo'> |
| <body/> |
| </message> |
|--------------------|
| <iq to='bar'> |
| <query/> |
| </iq> |
|--------------------|
| ... |
|--------------------|
| </stream> |
|--------------------|
4.2 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 the JID of the receiving entity. There SHOULD be no
'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 the JID of the receiving entity granting access to
the initiating entity. There SHOULD be no '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). There SHOULD be no '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 version -- If the initiating entity complies with the protocol
defined herein, it MUST include a 'version' attribute in the XML
stream header it sends to the receiving entity, and it MUST set
the value of the 'version' attribute to "1.0". If the initiating
entity includes the version attribute and the receiving entity
supports XMPP 1.0, the receiving entity MUST reciprocate by
including the attribute in its response.
We can summarize these values as follows:
| initiating to receiving | receiving to initiating
------------------------------------------------------------
to | hostname of receiver | silently ignored
from | silently ignored | hostname of receiver
id | silently ignored | session key
version | signals XMPP 1.0 support | signals XMPP 1.0 support
4.3 Namespace Declarations
The stream element MUST possess both a stream namespace declaration
and a default namespace declaration (as "namespace declaration" is
defined in the XML namespaces specification [12]).
4.3.1 Stream Namespace Declaration
A stream namespace declaration (e.g., 'xmlns:stream') is REQUIRED in
both XML streams. A compliant entity SHOULD accept any namespace
prefix on the <stream/> element; however, for historical reasons some
entities MAY accept only a 'stream' prefix, resulting in the use of a
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<stream:stream/> element as the stream root. The name of the stream
namespace MUST be "http://etherx.jabber.org/streams". The element
names of the <stream/> element and its <features/> and <error/>
children MUST be qualified by the stream namespace prefix in all
instances.
4.3.2 Default Namespace Declaration
A default namespace declaration ('xmlns') is REQUIRED and is used in
both 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 initiating stream and the responding 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 the
stream namespace prefix).
Since XML streams function as containers for any XML stanzas sent
asynchronously between network endpoints, it should be possible to
qualify an XML stream with any default namespace declaration. At a
minimum, a compliant implementation MUST support the following two
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
The 'jabber:client' and 'jabber:server' namespaces are nearly
identical but are used in different contexts (client-to-server
communications for 'jabber:client' and server-to-server
communications for 'jabber:server'). The only difference between the
two is that the 'to' and 'from' attributes are OPTIONAL on stanzas
sent within 'jabber:client', whereas they are REQUIRED on stanzas
sent within 'jabber:server'. If a compliant implementation accepts a
stream that is qualified by the 'jabber:client' or 'jabber:server'
namespace, it MUST support all three core stanza types (message,
presence, and IQ) as described herein and defined in the schema.
4.4 Stream Features
If the initiating entity includes a 'version' attribute set to a
value of "1.0" in its initiating stream element, the receiving entity
MUST send a <features/> child element (prefixed by the stream
namespace prefix) to the initiating entity in order to announce any
stream-level features that can be negotiated (or capabilities that
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otherwise need to be advertised). Currently this is used for TLS and
SASL negotiation only, but it could be used for other negotiable
features in the future (usage is defined under Stream Encryption
(Section 5) and Section 6 below). If an entity does not understand or
support some features, it SHOULD silently ignore them.
4.5 Stream Errors
The root stream element MAY contain an <error/> child element that is
prefixed by the stream 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.
4.5.1 Rules
The following rules apply to stream-level errors:
o It is assumed that all stream-level errors are unrecoverable;
therefore, if an error occurs at the level of the stream, the
entity that detects the error MUST send a stream error to the
other entity, send a closing </stream> tag, and terminate the
underlying TCP connection.
o If the error occurs while the stream is being set up, the
receiving entity MUST still send the opening <stream> tag, include
the <error/> element as a child of the stream element, send the
closing </stream> tag, and terminate the underlying TCP
connection. In this case, if the initiating entity provides an
unknown host in the 'to' attribute (or provides no 'to' attribute
at all), the server SHOULD provide the server's authoritative
hostname in the 'from' attribute of the stream header sent before
termination.
4.5.2 Syntax
The syntax for stream errors is as follows:
<stream:error>
<descriptive-element-name
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
The error element MUST contain a child element that specifies a
particular stream-level error condition, as defined in the next
section. (Note: the XML namespace name
'urn:ietf:params:xml:ns:xmpp-streams' that qualifies the descriptive
element adheres to the format defined in The IETF XML Registry [25].)
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4.5.3 Defined Conditions
The following stream-level error conditions are defined:
o <host-gone/> -- the value of the 'to' attribute provided by the
initiating entity in the stream header corresponds to a hostname
that is no longer hosted by the server.
o <host-unknown/> -- the value of the 'to' attribute provided by the
initiating entity in the stream header does not correspond to a
hostname that is hosted by the server.
o <internal-server-error/> -- the server has experienced a
misconfiguration or an otherwise-undefined internal error that
prevents it from servicing the stream.
o <invalid-id/> -- the stream ID or dialback ID is invalid or does
not match an ID previously provided.
o <invalid-namespace/> -- the stream namespace name is something
other than "http://etherx.jabber.org/streams" or the dialback
namespace name is something other than "jabber:server:dialback".
o <nonmatching-hosts/> -- the hostname provided in a 'from' address
does not match the hostname (or other validated domain) negotiated
via SASL or dialback.
o <not-authorized/> -- the entity has attempted to send data before
authenticating, or otherwise is not authorized to perform an
action related to stream negotiation; the receiving entity SHOULD
silently drop the offending stanza and MUST NOT process it before
sending the stream error.
o <remote-connection-failed/> -- the server is unable to properly
connect to a remote resource that is required for authentication
or authorization.
o <resource-constraint/> -- the server is resource-contrained and is
unable to service the stream.
o <see-other-host/> -- the server will not provide service to the
initiating entity but is redirecting traffic to another host; this
element SHOULD contain CDATA specifying the alternate hostname or
IP address to which the initiating entity MAY attempt to connect.
o <system-shutdown/> -- the server is being shut down and all active
streams are being closed.
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o <unsupported-stanza-type/> -- the initiating entity has sent a
first-level child of the stream that is not supported by the
server.
o <unsupported-version/> -- the value of the 'version' attribute
provided by the initiating entity in the stream header specifies a
version of XMPP that is not supported by the server; this element
MAY contain CDATA specifying the XMPP version(s) supported by the
server.
o <xml-not-well-formed/> -- the initiating entity has sent XML that
is not well-formed as defined by the XML specification [1].
4.5.4 Application-Specific Conditions
As noted, an application MAY provide application-specific stream
error information by including a properly-namespaced child in the
error element. The application-specific element SHOULD supplement or
further qualify a defined element. Thus the <error/> element will
contain two child elements, for example:
<stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<escape-your-data xmlns='application-ns'/>
</stream:error>
</stream:stream>
4.6 Simple Streams Example
The following is a stream-based session of a client on a server
(where the "C" lines are sent from the client to the server, and the
"S" lines are sent from the server to the client):
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A basic session:
C: <?xml version='1.0'?>
<stream:stream
to='shakespeare.lit'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
S: <?xml version='1.0'?>
<stream:stream
from='shakespeare.lit'
id='id_123456789'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
... authentication ...
C: <message from='juliet@shakespeare.lit'
to='romeo@shakespeare.lit'
xml:lang='en'>
C: <body>Art thou not Romeo, and a Montague?</body>
C: </message>
S: <message from='romeo@shakespeare.lit'
to='juliet@shakespeare.lit'
xml:lang='en'>
S: <body>Neither, fair saint, if either thee dislike.</body>
S: </message>
C: </stream:stream>
S: </stream:stream>
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A stream gone bad:
C: <?xml version='1.0'?>
<stream:stream
to='shakespeare.lit'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
S: <?xml version='1.0'?>
<stream:stream
from='shakespeare.lit'
id='id_123456789'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
... authentication ...
C: <message xml:lang='en'>
<body>Bad XML, no closing body tag!
</message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
S: </stream:stream>
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5. Stream Encryption
5.1 Overview
XMPP includes a method for securing the stream from tampering and
eavesdropping. This channel encryption method makes use of the
Transport Layer Security (TLS) [13] protocol, along with a "STARTTLS"
extension that is modelled after similar extensions for the IMAP
[26], POP3 [27], and ACAP [28] protocols as described in RFC 2595
[29]. The namespace name for the STARTTLS extension is
'urn:ietf:params:xml:ns:xmpp-tls', which adheres to the format
defined in The IETF XML Registry [25].
An administrator of a given domain MAY require the use of TLS for
either or both client-to-server communications and server-to-server
communications. Servers SHOULD use TLS betweeen two domains for the
purpose of securing server-to-server communications. When the remote
domain is already known, the server can verify the credentials of the
known domain by comparing known keys or certificates. When the remote
domain is not recognized, it may still be possible to verify a
certificate if it is signed by a common trusted authority. Even if
there is no way to verify certificates (e.g., an unknown domain with
a self-signed certificate, or a certificate signed by an unrecognized
authority), if the servers choose to communicate despite the lack of
verified credentials, TLS still SHOULD be used to provide channel
encryption. Finally, see Section 12.6 regarding mechanisms that MUST
be supported.
The following rules apply:
1. An initiating entity that complies with this specification MUST
include a 'version' attribute set to a value of "1.0" in the
initiating stream header.
2. When a receiving entity that complies with this specification
receives an initiating stream header that includes a 'version'
attribute set to a value of "1.0", after sending a stream header
in reply (including the version flag) it MUST include a
<starttls/> element (qualified by the
'urn:ietf:params:xml:ns:xmpp-tls' namespace) along with the list
of other stream features it supports.
3. If the initiating entity chooses to use TLS for stream
encryption, TLS negotiation MUST be completed before proceeding
to SASL negotiation.
4. The receiving entity MUST consider the TLS negotiation to have
begun immediately after sending the closing ">" of the <proceed/
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> element. The initiating entity MUST consider the TLS
negotiation to have begun immediately after receiving the
closing ">" of the <proceed/> element from the receiving entity.
5. The initiating entity MUST validate the certificate presented by
the receiving entity; there are two cases:
Case 1 -- The initiating entity has been configured with a set
of trusted root certificates: Normal certificate validation
processing is appropriate, and SHOULD follow the rules
defined for HTTP over TLS [14]. The trusted roots may be
either a well-known public set or a manually configured Root
CA (e.g., an organization's own Certificate Authority or a
self-signed Root CA for the service as described under High
Security (Section 12.1)). This case is RECOMMENDED.
Case 2 -- The initiating entity has been configured with the
receiving entity's self-signed service certificate: Simple
comparison of public keys is appropriate. This case is NOT
RECOMMENDED (see High Security (Section 12.1) for details).
If the above methods fail, the certificate SHOULD be presented
to a user for approval; if presented, the receiver MUST deliver
the entire certificate chain to the user, who SHOULD be given
the option to store the Root CA certificate (not the service or
End Entity certificate) and to not be queried again regarding
acceptance of the certificate for some reasonable period of
time.
6. If the TLS negotiation is successful, the receiving entity MUST
discard any knowledge obtained from the initiating entity before
TLS takes effect.
7. If the TLS negotiation is successful, the initiating entity MUST
discard any knowledge obtained from the receiving entity before
TLS takes effect.
8. If the TLS negotiation is successful, the receiving entity MUST
NOT offer the STARTTLS extension to the initiating entity along
with the other stream features that are offered when the stream
is restarted.
9. If the TLS negotiation is successful, the initiating entity
SHOULD continue with SASL negotiation.
10. If the TLS negotiation results in failure, the receiving entity
MUST terminate both the XML stream and the underlying TCP
connection.
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5.2 Narrative
When an initiating entity secures a stream with a receiving entity,
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 a 'version' attribute set to a value of "1.0".
2. The receiving entity responds by opening a TCP connection and
sending an XML stream header to the initiating entity, including
a 'version' attribute set to a value of "1.0".
3. The receiving entity offers the STARTTLS extension to the
initiating entity by including it with the list of other
supported stream features (if TLS is required for interaction
with the receiving entity, it SHOULD signal that fact by
including a <required/> element as a child of the <starttls/>
element).
4. The initiating entity issues the STARTTLS command (i.e., a
<starttls/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct the
receiving entity that it wishes to begin a TLS negotiation to
secure the stream.
5. The receiving entity MUST reply with either a <proceed/> element
or a <failure/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-tls' namespace. If the failure case
occurs, the receiving entity MUST terminate both the XML stream
and the underlying TCP connection. If the proceed case occurs,
the receiving entity MUST ignore any further XML data sent over
the XML stream but keep the underlying TCP connection open for
the purpose of completing the TLS negotiation.
6. The initiating entity and receiving entity attempt to complete a
TLS negotiation in accordance with RFC 2246 [13].
7. If the TLS negotiation is unsuccessful, the receiving entity MUST
terminate the TCP connection. If the TLS negotiation is
successful, the initiating entity SHOULD initiate a new stream by
sending an opening XML stream header to the receiving entity.
8. Upon receiving the new stream header from the initiating entity,
the receiving entity SHOULD respond by sending a new XML stream
header to the initiating entity along with the remaining
available features (but NOT including the STARTTLS feature).
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5.3 Client-to-Server Example
The following example shows the data flow for a client securing a
stream using STARTTLS (the IANA-registered port 5222 SHOULD be used;
see Section 10).
Step 1: Client initiates stream to server:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='capulet.com'
version='1.0'>
Step 2: Server responds by sending a stream tag to client:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='12345678'
from='capulet.com'
version='1.0'>
Step 3: Server sends the STARTTLS extension to client along with
authentication mechanisms and any other stream features:
<stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>
Step 4: Client sends the STARTTLS command to server:
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5: Server informs client to proceed:
<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
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Step 5 (alt): Server informs client that TLS negotiation has failed
and closes both stream and TCP connection:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
</stream:stream>
Step 6: Client and server attempt to complete TLS negotiation over
the existing TCP connection.
Step 7: If TLS negotiation is successful, client initiates a new
stream to server:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='capulet.com'
version='1.0'>
Step 7 (alt): If TLS negotiation is unsuccessful, server MUST close
TCP connection.
Step 8: Server responds by sending a stream header to client along
with any remaining negotiable stream features:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
from='capulet.com'
id='12345678'
version='1.0'>
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>PLAIN</mechanism>
<mechanism>EXTERNAL</mechanism>
</mechanisms>
</stream:features>
Step 9: Client SHOULD continue with Section 6.
5.4 Server-to-Server Example
The following example shows the data flow for two servers securing a
stream using STARTTLS (the IANA-registered port 5269 SHOULD be used;
see Section 10).
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Step 1: Server1 initiates stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
to='montague.net'
version='1.0'>
Step 2: Server2 responds by sending a stream tag to Server1:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
from='montague.net'
id='12345678'
version='1.0'>
Step 3: Server2 sends the STARTTLS extension to Server1 along with
authentication mechanisms and any other stream features:
<stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
<required/>
</starttls>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>KERBEROS_V4</mechanism>
</mechanisms>
</stream:features>
Step 4: Server1 sends the STARTTLS command to Server2:
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5: Server2 informs Server1 to proceed:
<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5 (alt): Server2 informs Server1 that TLS negotiation has failed
and closes stream:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
</stream:stream>
Step 6: Server1 and Server2 attempt to complete TLS negotiation via
TCP.
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Step 7: If TLS negotiation is successful, Server1 initiates a new
stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
to='montague.net'
version='1.0'>
Step 7 (alt): If TLS negotiation is unsuccessful, server MUST close
TCP connection.
Step 8: Server2 responds by sending a stream header to Server1 along
with any remaining negotiable stream features:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
from='montague.net'
id='12345678'
version='1.0'>
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>KERBEROS_V4</mechanism>
<mechanism>EXTERNAL</mechanism>
</mechanisms>
</stream:features>
Step 9: Server1 SHOULD continue with Section 6.
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6. Stream Authentication
6.1 Overview
XMPP includes a method for authenticating a stream using an XMPP
adaptation of the Simple Authentication and Security Layer (SASL)
[15]. 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 section 4 ("Profiling
Requirements") of RFC 2222 [15] (the XMPP-specific namespace name is
'urn:ietf:params:xml:ns:xmpp-sasl', which adheres to the format
defined in The IETF XML Registry [25]). Finally, see Section 12.6
regarding mechanisms that MUST be supported.
The following rules apply:
1. If TLS is used for stream encryption, SASL SHOULD NOT be used for
anything except stream authentication (i.e., a security layer
SHOULD NOT be negotiated using SASL). Conversely, if a security
layer is to be negotiated via SASL, TLS SHOULD NOT be used.
2. If the initiating entity is capable of authenticating via SASL,
it MUST include a 'version' attribute set to a value of "1.0" in
the initiating stream header.
3. If the receiving entity is capable of negotiating authentication
via SASL, it MUST send one or more authentication mechanisms
within a <mechanisms/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace in response to the
opening stream tag received from the initiating entity (if the
opening stream tag included a 'version' attribute set to a value
of "1.0").
4. If the SASL negotiation involves negotiation of a security layer,
the receiving entity MUST discard any knowledge obtained from the
initiating entity which was not obtained from the SASL
negotiation itself.
5. If the SASL negotiation involves negotiation of a security layer,
the initiating entity MUST discard any knowledge obtained from
the receiving entity which was not obtained from the SASL
negotiation itself.
6. The SASL negotiation MUST include an authzid, and the
authzid-value MUST be a valid JID of the form <domain> (i.e., a
domain identifier only) for servers and of the form <user@domain/
resource> (i.e., node identifier, domain identifier, and resource
identifier) for clients.
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6.2 Narrative
When an initiating entity authenticates with a receiving entity, the
steps involved are as follows:
1. The initiating entity requests SASL authentication by including a
'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 response, the receiving
entity sends a list of available SASL authentication mechanisms;
each of these is a <mechanism/> element included as a child
within a <mechanisms/> container element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace, which in turn is a
child of a <features/> element in the streams namespace. If
channel encryption must be established before a particular
authentication mechanism may be used, the receiving entity MUST
NOT provide that mechanism in the list of available SASL
authentication methods prior to channel encryption. If the
initiating entity presents a valid certificate during prior TLS
negotiation, the receiving entity MAY offer the SASL EXTERNAL
mechanism to the initiating entity during stream authentication
(see RFC 2222 [15]).
3. The initiating entity selects a mechanism by sending an <auth/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
namespace to the receiving entity and including an appropriate
value for the 'mechanism' attribute; this element MAY optionally
contain character data (in SASL terminology the "initial
response") if the mechanism supports or requires it. If the
initiating entity selects the EXTERNAL mechanism for
authentication, the authentication credentials shall be taken
from the certificate presented during prior TLS negotiation.
4. If necessary, the receiving entity challenges the initiating
entity by sending a <challenge/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
entity; this element MAY optionally contain character data (which
MUST be computed in accordance with the SASL mechanism chosen by
the initiating entity).
5. The initiating entity responds to the challenge by sending a
<response/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the receiving
entity; this element MAY optionally contain character data (which
MUST be computed in accordance with the SASL mechanism chosen by
the initiating entity).
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6. If necessary, the receiving entity sends more challenges and the
initiating entity sends more responses.
This series of challenge/response pairs continues until one of three
things happens:
1. The initiating entity aborts the handshake by sending an <abort/>
element to the receiving entity.
2. The receiving entity reports failure of the handshake by sending
a <failure/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
entity. The particular cause of failure SHOULD be communicated in
an appropriate child element of the <failure/> element as defined
under Section 6.3.
3. The receiving entity reports success of the handshake by sending
a <success/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
entity; this element MAY optionally contain character data (in
SASL terminology "additional data with success").
Any character data contained within these elements MUST be encoded
using base64.
6.3 SASL Errors
The following SASL-related error conditions are defined:
o <encryption-required/> -- The mechanism chosen by the initiating
entity may be used only if the stream is already encrypted;
provided in response to the <auth/> element.
o <invalid-authzid/> -- The authzid provided by the initiating
entity is invalid, either because it is incorrectly formatted or
because the initiating entity does not have permissions to
authorize that ID; provided in response to a <response/> element.
o <invalid-realm/> -- The realm provided by the initiating entity
(in mechanisms that support the concept of a realm) does not match
one of the hostnames served by the receiving entity; provided in
response to a <response/> element.
o <mechanism-too-weak/> -- The mechanism chosen by the initiating
entity is weaker than server policy permits for that initiating
entity; provided in response to the <auth/> element.
o <temporary-auth-failure/> -- The authentication failed because of
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a temporary error condition within the receiving entity; provided
in response to an <auth/> element or <response/> element.
6.4 SASL Definition
Section 4 of the SASL specification [15] requires that the following
information be supplied by a protocol definition:
service name: "xmpp"
initiation sequence: After the initiating entity provides an opening
XML stream header and the receiving entity replies in kind, the
receiving entity provides a list of acceptable authentication
methods. The initiating entity chooses one method from the list
and sends it to the receiving entity as the value of the
'mechanism' attribute possessed by an <auth/> element, optionally
including an initial response to avoid a round trip.
exchange sequence: Challenges and responses are carried through the
exchange of <challenge/> elements from receiving entity to
initiating entity and <response/> elements from initiating entity
to receiving entity. The receiving entity reports failure by
sending a <failure/> element and success by sending a <success/>
element; the initiating entity aborts the exchange by sending an
<abort/> element. (All of these elements are qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace.)
security layer negotiation: If a security layer is negotiated, both
sides consider the original stream closed and new <stream> headers
are sent by both entities. The security layer takes effect
immediately following the ">" character of the <response/> element
for the client and immediately following the closing ">" character
of the <succeed/> element for the server. (Both of these elements
are qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
namespace.)
use of the authorization identity: The authorization identity is used
by xmpp to denote the "full JID" (<user@domain/resource>) of a
client or the sending domain of a server.
6.5 Client-to-Server Example
The following example shows the data flow for a client authenticating
with a server using SASL (the IANA-registered port 5222 SHOULD be
used; see Section 10).
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Step 1: Client initiates stream to server:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='capulet.com'
version='1.0'>
Step 2: Server responds with a stream tag sent to client:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='12345678'
from='capulet.com'
version='1.0'>
Step 3: Server informs client of available authentication mechanisms:
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>PLAIN</mechanism>
<mechanism>OTP</mechanism>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>CRAM-MD5</mechanism>
<mechanism>ANONYMOUS</mechanism>
</mechanisms>
</stream:features>
Step 4: Client selects an authentication mechanism:
<auth
xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'/>
Step 5: Server sends a base64-encoded challenge to client:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
bm9uY2U9Imd4MFAwMVJSVm9PZkpWcklKNmFOa0ZsaW5oOW5NL1VwOWRhel
ZURlRxMWM9IixyZWFsbT0iY2FwdWxldC5jb20iLHFvcD0iYXV0aCxhdXRo
LWludCxhdXRoLWNvbmYiLGNpcGhlcj0icmM0LTQwLHJjNC01NixyYzQsZG
VzLDNkZXMiLG1heGJ1Zj0yMDQ4LGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGht
PW1kNS1zZXNz
</challenge>
The decoded challenge is:
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nonce="gx0P01RRVoOfJVrIJ6aNkFlinh9nM/Up9dazVTFTq1c=",\
realm="capulet.com",qop="auth,auth-int,auth-conf",\
cipher="rc4-40,rc4-56,rc4,des,3des",maxbuf=2048,\
charset=utf-8,algorithm=md5-sess
Step 5 (alt): Server returns error to client:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism-too-weak/>
</failure>
Step 6: Client responds to the challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9Imp1bGlldCIscmVhbG09ImNhcHVsZXQuY29tIixhdXRoem
lkPSJqdWxpZXRAY2FwdWxldC5jb20vYmFsY29ueSIsbm9uY2U9Imd4MFAw
MVJSVm9PZkpWcklKNmFOa0ZsaW5oOW5NL1VwOWRhelZURlRxMWM9Iixjbm
9uY2U9ImJkaTdVL0R1aGQ5N2ZmWHBTOW1OaGhLM2NpWTBGR085eHVxSWxh
cEJLNG89IixuYz0wMDAwMDAwMSxxb3A9YXV0aC1jb25mLGNpcGhlcj0icm
M0IixtYXhidWY9MjA0OCxkaWdlc3QtdXJpPSJ4bXBwL2NhcHVsZXQuY29t
IixyZXNwb25zZT04ZTBhYzYwMTA4MzNiMTIyNGZjYzQ3NDJhZmJkYjM1Mg
==
</response>
The decoded response is:
username="juliet",realm="capulet.com",\
authzid="juliet@capulet.com/balcony",\
nonce="gx0P01RRVoOfJVrIJ6aNkFlinh9nM/Up9dazVTFTq1c=",\
cnonce="bdi7U/Duhd97ffXpS9mNhhK3ciY0FGO9xuqIlapBK4o=",\
nc=00000001,qop=auth-conf,cipher="rc4",maxbuf=2048,\
digest-uri="xmpp/capulet.com",\
response=8e0ac6010833b1224fcc4742afbdb352
Step 7: Server sends another challenge to client:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1mMTBjYjIzNTYwY2NmZTVjNDQxNzQ4OGEzZWRkZjc3NA==
</challenge>
The decoded challenge is:
rspauth=f10cb23560ccfe5c4417488a3eddf774
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Step 7 (alt): Server returns error to client:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<invalid-realm/>
</failure>
Step 8: Client responds to the challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9: Server informs client of successful authentication:
<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9 (alt): Server informs client of failed authentication:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
Step 10: Client initiates a new stream to server:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='capulet.com'
version='1.0'>
Step 11: Server responds by sending a stream header to client, with
the stream already authenticated (not followed by further stream
features):
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='12345678'
from='capulet.com'
version='1.0'>
6.6 Server-to-Server Example
The following example shows the data flow for a server authenticating
with another server using SASL (the IANA-registered port 5269 SHOULD
be used; see Section 10).
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Step 1: Server1 initiates stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
to='montague.net'
version='1.0'>
Step 2: Server2 responds with a stream tag sent to Server1:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
from='montague.net'
id='12345678'
version='1.0'>
Step 3: Server2 informs Server1 of available authentication
mechanisms:
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>KERBEROS_V4</mechanism>
</mechanisms>
</stream:features>
Step 4: Server1 selects an authentication mechanism:
<auth
xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'/>
Step 5: Server2 sends a base64-encoded challenge to Server1:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi
xxb3A9ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz
</challenge>
The decoded challenge is:
realm="cataclysm.cx",nonce="OA6MG9tEQGm2hh",\
qop="auth",charset=utf-8,algorithm=md5-sess
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Step 5 (alt): Server2 returns error to Server1:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<encryption-required/>
</failure>
Step 6: Server1 responds to the challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi
xjbm9uY2U9Ik9BNk1IWGg2VnFUclJrIixuYz0wMDAwMDAwMSxxb3A9YXV0
aCxkaWdlc3QtdXJpPSJ4bXBwL2NhdGFjbHlzbS5jeCIscmVzcG9uc2U9ZD
M4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcsY2hhcnNldD11dGYt
OAo=
</response>
The decoded response is:
realm="cataclysm.cx",nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\
nc=00000001,qop=auth,digest-uri="xmpp/cataclysm.cx",\
response=d388dad90d4bbd760a152321f2143af7,charset=utf-8
Step 7: Server2 sends another challenge to Server1:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
</challenge>
The decoded challenge is:
rspauth=ea40f60335c427b5527b84dbabcdfffd
Step 5 (alt): Server2 returns error to Server1:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<invalid-authzid/>
</failure>
Step 8: Server1 responds to the challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9: Server2 informs Server1 of successful authentication:
<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
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Step 9 (alt): Server2 informs Server1 of failed authentication:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
Step 10: Server1 initiates a new stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
to='montague.net'
version='1.0'>
Step 11: Server2 responds by sending a stream header to Server1, with
the stream already authenticated (not followed by further stream
features):
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
from='montague.net'
id='12345678'
version='1.0'>
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7. Server Dialback
7.1 Overview
The Jabber protocol from which XMPP was adapted includes a "server
dialback" method for protecting against domain spoofing, thus making
it more difficult to spoof XML stanzas (see Section 12.3 regarding
this method's security characteristics). Server dialback also makes
it easier to deploy systems in which outbound messages and inbound
messages are handled by different machines for the same domain. The
server dialback method is made possible by the existence of DNS,
since one server can (normally) discover the authoritative server for
a given domain. All DNS hostname resolutions MUST first resolve the
hostname using an SRV [18] record of _jabber._tcp.server (the use of
the string "jabber" for SRV purposes is historical). If the SRV
lookup fails, the fallback is a normal A lookup to determine the IP
address, using the jabber-server port of 5269 assigned by the
Internet Assigned Numbers Authority [5].
The method for generating and verifying the keys used in server
dialback MUST take into account the hostnames being used, the random
ID generated for the stream, and a secret known by the authoritative
server's network.
Any error that occurs during dialback negotiation MUST be considered
a stream error, resulting in termination of the stream and of the
underlying TCP connection. The possible error conditions are
specified in the protocol description below.
The following terminology applies:
o Originating Server -- the server that is attempting to establish a
connection between two domains.
o Receiving Server -- the server that is trying to authenticate that
Originating Server represents the domain which it claims to be.
o Authoritative Server -- the server that answers to the DNS
hostname asserted by Originating Server; for basic environments
this will be Originating Server, but it could be a separate
machine in Originating Server's network.
The following is a brief summary of the order of events in dialback:
1. Originating Server establishes a connection to Receiving Server.
2. Originating Server sends a 'key' value over the connection to
Receiving Server.
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3. Receiving Server establishes a connection to Authoritative
Server.
4. Receiving Server sends the same 'key' value to Authoritative
Server.
5. Authoritative Server replies that key is valid or invalid.
6. Receiving Server informs Originating Server whether it is
authenticated or not.
We can represent this flow of events graphically as follows:
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Originating Receiving
Server Server
----------- ---------
| |
| establish connection |
| ----------------------> |
| |
| send stream header |
| ----------------------> |
| |
| send stream header |
| <---------------------- |
| | Authoritative
| send dialback key | Server
| ----------------------> | -------------
| | |
| establish connection |
| ----------------------> |
| |
| send stream header |
| ----------------------> |
| |
| establish connection |
| <---------------------- |
| |
| send stream header |
| <---------------------- |
| |
| send dialback key |
| ----------------------> |
| |
| validate dialback key |
| <---------------------- |
|
| report dialback result |
| <---------------------- |
| |
7.2 Protocol
The interaction between the servers is as follows:
1. Originating Server establishes TCP connection to Receiving
Server.
2. Originating Server sends a stream header to Receiving Server:
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<stream:stream
xmlns:stream='http://etherx.jabber.org/streams'
xmlns='jabber:server'
xmlns:db='jabber:server:dialback'>
Note: the 'to' and 'from' attributes are NOT REQUIRED on the
root stream element. The inclusion of the xmlns:db namespace
declaration with the name shown indicates to Receiving Server
that Originating Server supports dialback. If the namespace name
is incorrect, then Receiving Server MUST generate an
<invalid-namespace/> stream error condition and terminate both
the XML stream and the underlying TCP connection.
3. Receiving Server SHOULD send a stream header back to Originating
Server, including a unique ID for this interaction:
<stream:stream
xmlns:stream='http://etherx.jabber.org/streams'
xmlns='jabber:server'
xmlns:db='jabber:server:dialback'
id='457F9224A0...'>
Note: The 'to' and 'from' attributes are NOT REQUIRED on the
root stream element. If the namespace name is incorrect, then
Originating Server MUST generate an <invalid-namespace/> stream
error condition and terminate both the XML stream and the
underlying TCP connection. Note well that Receiving Server is
NOT REQUIRED to reply and MAY silently terminate the XML stream
and underlying TCP connection depending on security policies in
place.
4. Originating Server sends a dialback key to Receiving Server:
<db:result
to='Receiving Server'
from='Originating Server'>
98AF014EDC0...
</db:result>
Note: this key is not examined by Receiving Server, since
Receiving Server does not keep information about Originating
Server between sessions. The key generated by Originating Server
must be based in part on the value of the ID provided by
Receiving Server in the previous step, and in part on a secret
shared by Originating Server and Authoritative Server. If the
value of the 'to' address does not match a hostname recognized
by Receiving Server, then Receiving Server MUST generate a
<host-unknown/> stream error condition and terminate both the
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XML stream and the underlying TCP connection. If the value of
the 'from' address matches a domain with which Receiving Server
already has an established connection, then Receiving Server
SHOULD generate a <not-authorized/> stream error condition and
terminate both the XML stream and the underlying TCP connection.
5. Receiving Server establishes a TCP connection back to the domain
name asserted by Originating Server, as a result of which it
connects to Authoritative Server. (Note: as an optimization, an
implementation MAY reuse an existing trusted connection here
rather than opening a new TCP connection.)
6. Receiving Server sends Authoritative Server a stream header:
<stream:stream
xmlns:stream='http://etherx.jabber.org/streams'
xmlns='jabber:server'
xmlns:db='jabber:server:dialback'>
Note: the 'to' and 'from' attributes are NOT REQUIRED on the
root stream element. If the namespace name is incorrect, then
Authoritative Server MUST generate an <invalid-namespace/>
stream error condition and terminate both the XML stream and the
underlying TCP connection.
7. Authoritative Server sends Receiving Server a stream header:
<stream:stream
xmlns:stream='http://etherx.jabber.org/streams'
xmlns='jabber:server'
xmlns:db='jabber:server:dialback'
id='1251A342B...'>
Note: if the namespace name is incorrect, then Receiving Server
MUST generate an <invalid-namespace/> stream error condition and
terminate both the XML stream and the underlying TCP connection
between it and Authoritative Server. If the ID does not match
that provided by Receiving Server in Step 3, then Receiving
Server MUST generate an <invalid-id/> stream error condition and
terminate both the XML stream and the underlying TCP connection
between it and Authoritative Server. If either of the foregoing
stream errors occurs between Receiving Server and Authoritative
Server, then Receiving Server MUST generate a
<remote-connection-failed/> stream error condition and terminate
both the XML stream and the underlying TCP connection between it
and Originating Server.
8. Receiving Server sends Authoritative Server a stanza requesting
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that Authoritative Server verify a key:
<db:verify
from='Receiving Server'
to='Originating Server'
id='457F9224A0...'>
98AF014EDC0...
</db:verify>
Note: passed here are the hostnames, the original identifier
from Receiving Server's stream header to Originating Server in
Step 3, and the key that Originating Server sent to 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 Authoritative Server, then
Authoritative Server MUST generate a <host-unknown/> stream
error condition and terminate both the XML stream and the
underlying TCP connection. If the value of the 'from' address
does not match the hostname represented by Receiving Server when
opening the TCP connection (or any validated domain), then
Authoritative Server MUST generate a <nonmatching-hosts/> stream
error condition and terminate both the XML stream and the
underlying TCP connection.
9. Authoritative Server sends a stanza back to Receiving Server
verifying whether the key was valid or invalid:
<db:verify
from='Originating Server'
to='Receiving Server'
type='valid'
id='457F9224A0...'/>
or
<db:verify
from='Originating Server'
to='Receiving Server'
type='invalid'
id='457F9224A0...'/>
Note: if the ID does not match that provided by Receiving Server
in Step 3, then Receiving Server MUST generate an <invalid-id/>
stream error condition and terminate both the XML stream and the
underlying TCP connection. If the value of the 'to' address does
not match a hostname recognized by Receiving Server, then
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Receiving Server MUST generate a <host-unknown/> stream error
condition and terminate both the XML stream and the underlying
TCP connection. If the value of the 'from' address does not
match the hostname represented by Originating Server when
opening the TCP connection (or any validated domain), then
Receiving Server MUST generate a <nonmatching-hosts/> stream
error condition and terminate both the XML stream and the
underlying TCP connection.
10. Receiving Server informs Originating Server of the result:
<db:result
from='Receiving Server'
to='Originating Server'
type='valid'/>
Note: At this point the connection has either been validated via
a type='valid', or reported as invalid. If the connection is
invalid, then Receiving Server MUST terminate both the XML
stream and the underlying TCP connection. If the connection is
validated, data can be sent by Originating Server and read by
Receiving Server; before that, all data stanzas sent to
Receiving Server SHOULD be silently dropped.
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
<invalid-xml/> 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 a <nonmatching-hosts/> 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.
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8. XML Stanzas
8.1 Overview
Once XML streams in both directions have been authenticated and (if
desired) encrypted, XML stanzas can be sent over the streams. Three
XML stanza types are defined for the 'jabber:client' and
'jabber:server' namespaces: <message/>, <presence/>, and <iq/>.
In essence, the <message/> stanza type 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.
The <presence/> element can be seen as a basic broadcast or
"publish-subscribe" mechanism, whereby multiple entities receive
information (in this case, presence information) about an entity to
which they have subscribed. The <iq/> element can be seen as a
"request-response" mechanism similar to HTTP, whereby two entities
can engage in a structured conversation using 'get' or 'set' requests
and 'result' or 'error' responses.
8.2 Common Attributes
The following five attributes are common to message, presence, and IQ
stanzas:
8.2.1 to
The 'to' attribute specifies the JID of the intended recipient for
the stanza.
In the 'jabber:client' namespace, a stanza SHOULD possess a 'to'
attribute, although a stanza sent from a client to a server for
handling by that server (e.g., presence sent to the server for
broadcasting to other entities) 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 <invalid-xml/> stream error
condition and terminate both the XML stream and the underlying TCP
connection with the offending server.
If the value of the 'to' attribute is invalid or cannot be contacted,
the entity discovering that fact (usually the sender's or recipient's
server) MUST return an appropriate error to the sender.
8.2.2 from
The 'from' attribute specifies the JID of the sender.
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In the 'jabber:client' namespace, a client MUST NOT include a 'from'
attribute on the stanzas it sends to a server; if a server receives a
stanza from a client and the stanza possesses a 'from' attribute, it
MUST ignore the value of the 'from' attribute and MAY return an error
to the sender. In addition, a server MUST stamp stanzas received from
a client with the full JID (<user@domain/resource>) of the connected
resource that generated the stanza as defined by the authzid provided
in the SASL negotiation.
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 <invalid-xml/> stream error
condition. Furthermore, the domain identifier portion of the JID
contained in the 'from' attribute MUST match the hostname of the
sending server (or any validated domain) as communicated in the SASL
negotiation or dialback negotiation; if a server receives a stanza
that does not meet this restriction, it MUST generate a
<nonmatching-hosts/> stream error condition. Both of these conditions
MUST result in closing of the stream and termination of the
underlying TCP connection.
8.2.3 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 use of
IQ stanzas). If the stanza sent by the sending entity is an IQ stanza
of type "get" or "set", the receiving entity MUST include an 'id'
attribute with the same value in any replies of type "result" or
"error". The value of the 'id' attribute is NOT REQUIRED to be unique
either globally, within a domain, or within a stream.
8.2.4 type
The 'type' attribute specifies detailed information about the purpose
or context of the message, presence, or IQ stanza. The particular
allowable values for the 'type' attribute vary depending on whether
the stanza is a message, presence, or IQ, and thus are defined in the
following sections.
8.2.5 xml:lang
A stanza SHOULD possess an 'xml:lang' attribute (as defined in
Section 2.12 of the XML specification [1]) if the stanza contains XML
character data that is intended to be presented to a human user (as
explained in RFC 2277 [31], "internationalization is for humans").
The value of the 'xml:lang' attribute specifies the default language
of any such XML character data, which MAY be overridden by the
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'xml:lang' attribute of a specific child element. The attribute MUST
be an NMTOKEN and MUST conform to the format defined in RFC 3066
[17].
8.3 Message Stanzas
Message stanzas in the 'jabber:client' or 'jabber:server' namespace
are used to "push" information to another entity. Common uses in the
context of instant messaging include single messages, messages sent
in the context of a chat conversation, messages sent in the context
of a multi-user chat room, headlines, and errors.
8.3.1 Types of Message
The 'type' attribute of a message stanza is RECOMMENDED; if included,
it specifies the conversational context of the message. The 'type'
attribute SHOULD have one of the following values:
o chat
o error
o groupchat
o headline
o normal
A message stanza with a different value, or without a 'type'
attribute, SHOULD be handled as if the 'type' were "normal".
For information regarding the meaning of these message types in the
context of XMPP-based instant messaging and presence applications,
refer to XMPP IM [23].
8.3.2 Children
As described under extended namespaces (Section 8.6), a message
stanza MAY contain any properly-namespaced child element.
In accordance with the default namespace declaration, by default a
message stanza is in the 'jabber:client' or 'jabber:server'
namespace, which defines certain allowable children of message
stanzas. If the message stanza is of type "error", it MUST include an
<error/> child; for details, see Section 8.7. Otherwise, the message
stanza MAY contain any of the following child elements without an
explicit namespace declaration:
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1. <subject/>
2. <body/>
3. <thread/>
8.3.2.1 Subject
The <subject/> element specifies the topic of the message. The
<subject/> element SHOULD NOT possess any attributes, with the
exception of the 'xml:lang' attribute. Multiple instances of the
<subject/> element MAY be included for the purpose of providing
alternate versions of the same subject, but only if each instance
possesses an 'xml:lang' attribute with a distinct language value. The
<subject/> element MUST NOT contain mixed content.
8.3.2.2 Body
The <body/> element contains the textual contents of the message;
this child element is normally included but NOT REQUIRED. The <body/>
element SHOULD NOT possess any attributes, with the exception of the
'xml:lang' attribute. Multiple instances of the <body/> element MAY
be included but only if each instance possesses an 'xml:lang'
attribute with a distinct language value. The <body/> element MUST
NOT contain mixed content.
8.3.2.3 Thread
The <thread/> element contains a string that is generated by the
sender and that SHOULD be copied back in replies; it is used for
tracking a conversation thread (sometimes referred to as an "instant
messaging session") between two entities. If used, it MUST be unique
to that conversation thread within the stream and MUST be consistent
throughout that conversation (a client that receives a message from
the same full JID but with a different thread ID MUST assume that the
message in question exists outside the context of the existing
conversation thread). The use of the <thread/> element is OPTIONAL
and is not used to identify individual messages, only conversations.
Only one <thread/> element MAY be included in a message stanza, and
it MUST NOT possess any attributes. The <thread/> element MUST be
treated as an opaque string by entities; no semantic meaning may be
derived from it, and only exact comparisons may be made against it.
The <thread/> element MUST NOT contain mixed content.
8.4 Presence Stanzas
Presence stanzas are used in the 'jabber:client' or 'jabber:server'
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namespace to express an entity's current availability status (offline
or online, along with various sub-states of the latter and optional
user-defined descriptive text) and to communicate that status to
other entities. Presence stanzas are also used to negotiate and
manage subscriptions to the presence of other entities.
8.4.1 Types of Presence
The 'type' attribute of a presence stanza is OPTIONAL. A presence
stanza that does not possess a 'type' attribute is used to signal to
the server that the sender is online and available for communication.
If included, the 'type' attribute specifies a lack of availability, a
request to manage a subscription to another entity's presence, a
request for another entity's current presence, or an error related to
a previously-sent presence stanza. The 'type' attribute MAY have one
of the following values:
o unavailable -- Signals that the entity is no longer available for
communication.
o subscribe -- The sender wishes to subscribe to the recipient's
presence.
o subscribed -- The sender has allowed the recipient to receive
their presence.
o unsubscribe -- A notification that an entity is unsubscribing from
another entity's presence.
o unsubscribed -- The subscription request has been denied or a
previously-granted subscription has been cancelled.
o probe -- A request for an entity's current presence. In general
SHOULD NOT be sent by a client.
o error -- An error has occurred regarding processing or delivery of
a previously-sent presence stanza.
For information regarding presence semantics and the subscription
model used in the context of XMPP-based instant messaging and
presence applications, refer to XMPP IM [23].
8.4.2 Children
As described under extended namespaces (Section 8.6), a presence
stanza MAY contain any properly-namespaced child element.
In accordance with the default namespace declaration, by default a
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presence stanza is in the 'jabber:client' or 'jabber:server'
namespace, which defines certain allowable children of presence
stanzas. If the presence stanza is of type "error", it MUST include
an <error/> child; for details, see Section 8.7. If the presence
stanza possesses no 'type' attribute, it MAY contain any of the
following child elements (note that the <status/> child MAY be sent
in a presence stanza of type "unavailable" or, for historical
reasons, "subscribe"):
1. <show/>
2. <status/>
3. <priority/>
8.4.2.1 Show
The OPTIONAL <show/> element specifies the particular availability
status of an entity or specific resource (if a <show/> element is not
provided, default availability is assumed). Only one <show/> element
MAY be included in a presence stanza, and it SHOULD NOT possess any
attributes. The CDATA value SHOULD be one of the following (values
other than these four SHOULD be ignored; additional availability
types could be defined through a properly-namespaced child element of
the presence stanza):
o away
o chat
o xa
o dnd
For information regarding the meaning of these values in the context
of XMPP-based instant messaging and presence applications, refer to
XMPP IM [23].
8.4.2.2 Status
The OPTIONAL <status/> element contains a natural-language
description of availability status. It is normally used in
conjunction with the show element to provide a detailed description
of an availability state (e.g., "In a meeting"). The <status/>
element SHOULD NOT possess any attributes, with the exception of the
'xml:lang' attribute. Multiple instances of the <status/> element MAY
be included but only if each instance possesses an 'xml:lang'
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attribute with a distinct language value.
8.4.2.3 Priority
The OPTIONAL <priority/> element specifies the priority level of the
connected resource. The value may be any integer between -128 and
127. Only one <priority/> element MAY be included in a presence
stanza, and it MUST NOT possess any attributes. If no priority is
provided, a server SHOULD the priority to be zero. For information
regarding the semantics of priority values in stanza routing within
instant messaging applications, see XMPP IM [23].
8.5 IQ Stanzas
8.5.1 Overview
Info/Query, or IQ, is a request-response mechanism, similar in some
ways to HTTP [32]. IQ stanzas in the 'jabber:client' or
'jabber:server' namespace enable an entity to make a request of, and
receive a response from, another entity. The data content of the
request and response is defined by the namespace declaration of a
direct child element of the IQ element, and the interaction is
tracked by the requesting entity through use of the 'id' attribute.
Most IQ interactions follow a common pattern of structured data
exchange such as get/result or set/result (although an error may be
returned in response to a request if appropriate):
Requesting Responding
Entity Entity
---------- ----------
| |
| <iq type='get' id='1'> |
| ------------------------> |
| |
| <iq type='result' id='1'> |
| <------------------------ |
| |
| <iq type='set' id='2'> |
| ------------------------> |
| |
| <iq type='error' id='2'> |
| <------------------------ |
| |
An entity that receives an IQ request of type "get" or "set" MUST
reply with an IQ response of type "result" or "error" (which response
MUST preserve the 'id' attribute of the request). An entity that
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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).
8.5.2 Types of IQ
The 'type' attribute of an IQ stanza is REQUIRED. The 'type'
attribute specifies a distinct step within a request-response
interaction. The value SHOULD be one of the following (all other
values SHOULD be ignored):
o get -- The stanza is a request for information or requirements.
o set -- The stanza provides required data, sets new values, or
replaces existing values.
o result -- The stanza is a response to a successful get or set
request.
o error -- An error has occurred regarding processing or delivery of
a previously-sent get or set.
8.5.3 Children
As described under extended namespaces (Section 8.6), an IQ stanza
MAY contain any properly-namespaced child element. Note that an IQ
stanza contains no children in the 'jabber:client' or 'jabber:server'
namespace since it is a vessel for XML in another namespace.
An IQ stanza of type "get" or "set" MUST include one and only one
child element. An IQ stanza of type "result" MUST include zero or one
child elements. An IQ stanza of type "error" SHOULD include the child
element contained in the associated "get" or "set" and MUST include
an <error/> child; for details, see Section 8.7.
8.6 Extended Namespaces
While the core data elements in the "jabber:client" or
"jabber:server" namespace (along with their attributes and child
elements) provide a basic level of functionality for messaging and
presence, XMPP uses XML namespaces to extend the core data elements
for the purpose of providing additional functionality. Thus a
message, presence, or IQ stanza MAY house one or more optional child
elements containing content that extends the meaning of the message
(e.g., an XHTML-formatted version of the message body). This child
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element MAY have any name and MUST possess an 'xmlns' namespace
declaration (other than "jabber:client", "jabber:server", or "http://
etherx.jabber.org/streams") that defines all data contained within
the child element.
Support for any given extended namespace is OPTIONAL on the part of
any implementation. If an entity does not understand such a
namespace, the entity's expected behavior depends on whether the
entity is (1) the recipient or (2) an entity that is routing the
stanza to the recipient:
Recipient: If a recipient receives a stanza that contains a child
element it does not understand, it SHOULD ignore that specific XML
data, i.e., it SHOULD not process it or present it to a user or
associated application (if any). In particular:
* If an entity receives a message or presence stanza that
contains XML data qualified by a namespace it does not
understand, the portion of the stanza that is in the unknown
namespace SHOULD be ignored.
* If an entity receives a message stanza containing only a child
element qualified by a namespace it does not understand, it
MUST ignore the entire stanza.
* If an entity receives an IQ stanza of type "get" or "set"
containing a child element qualified by a namespace it does not
understand, the entity SHOULD return an IQ stanza of type
"error" with an error condition of <feature-not-implemented/>.
Router: If a routing entity (usually a server) handles a stanza that
contains a child element it does not understand, it SHOULD ignore
the associated XML data by passing it on untouched to the
recipient.
8.7 Stanza Errors
Stanza-related errors are handled in a manner similar to stream
errors (Section 4.5), except that hints are also provided to the
receiving application regarding actions to take in reponse to the
error.
8.7.1 Rules
The following rules apply to stanza-related errors:
o A stanza whose 'type' attribute has a value of "error" MUST
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contain an <error/> child element.
o The receiving or processing entity that returns an error to the
sending entity SHOULD include the original XML sent so that the
sender can inspect and if necessary correct the XML before
re-sending.
o An entity that receives a stanza whose 'type' attribute has a
value of "error" MUST NOT respond to the stanza with a further
stanza of type "error"; this helps to prevent looping.
o An <error/> child MUST NOT be included if the 'type' attribute has
a value other than "error" (or if there is no 'type' attribute).
8.7.2 Syntax
The syntax for stanza-related errors is as follows:
<stanza-name to='sender' type='error'>
[include sender XML here]
<error type='error-type'>
<descriptive-element-name
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</stanza-name>
The stanza-name is one of message, presence, or iq.
The value of the <error/> element's 'type' attribute MUST be one of
the following:
o cancel -- do not retry (the error is unrecoverable)
o continue -- proceed (the condition was only a warning)
o modify -- retry after changing the data sent
o auth -- retry after providing credentials
o wait -- retry after waiting (the error is temporary)
The <error/> element MUST contain a child element that specifies a
particular stanza-related error condition. This descriptive element
SHOULD be one of those defined in the next section. However, an
application MAY supplement or further qualify a defined descriptive
element with an application-specific element, resulting in the
inclusion of two descriptive elements; in this case, the
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application-specific element MUST further explain or qualify the
defined element.
8.7.3 Defined Conditions
The following stanza-related error conditions are defined for use in
stanza errors (the namespace name for these elements is
'urn:ietf:params:xml:ns:xmpp-stanzas', which adheres to the format
defined in The IETF XML Registry [25]).
o <bad-request/> -- the sender has sent XML that is malformed or
that cannot be processed (e.g., a client-generated stanza includes
a 'from' address, or an IQ stanza includes an unrecognized value
of the 'type' attribute); the associated error type is "modify".
o <conflict/> -- access cannot be granted because an existing
resource or session exists with the same name or address; the
associated error type is "cancel".
o <feature-not-implemented/> -- the feature requested is not
implemented by the recipient or server and therefore cannot be
processed; the associated error type is "cancel".
o <forbidden/> -- the requesting entity does not possess the
required permissions to perform the action; the associated error
type is "auth".
o <internal-server-error/> -- the server could not process the
stanza because of a misconfiguration or an otherwise-undefined
internal server error; the associated error type is "wait".
o <item-not-found/> -- the addressed JID or item requested cannot be
found; the associated error type is "cancel".
o <jid-malformed/> -- the value of the 'to' attribute in the
sender's stanza does not adhere to the syntax defined in
Addressing (Section 3); the associated error type is "modify".
o <not-allowed/> -- the recipient does not allow any entity to
perform the action; the associated error type is "cancel".
o <recipient-unavailable/> -- the specific recipient requested is
currently unavailable; the associated error type is "wait".
o <registration-required/> -- the user is not authorized to access
the requested service because registration is required; the
associated error type is "auth".
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o <remote-server-not-found/> -- a remote server or service specified
as part or all of the JID of the intended recipient does not
exist; the associated error type is "cancel".
o <remote-server-timeout/> -- a remote server or service specified
as part or all of the JID of the intended recipient could not be
contacted within a reasonable amount of time; the associated error
type is "wait".
o <resource-constraint/> -- the server is resource-contrained and is
unable to service the request; the associated error type is
"wait".
o <service-unavailable/> -- the service requested is currently
unavailable on the server; the associated error type is "cancel".
o <subscription-required/> -- the user is not authorized to access
the requested service because a subscription is required; the
associated error type is "auth".
o <unexpected-request/> -- the recipient understood the request but
was not expecting it at this time (e.g., the request was out of
order); the associated error type is "wait".
8.7.4 Application-Specific Conditions
As noted, an application MAY provide application-specific stanza
error information by including a properly-namespaced child in the
error element. The application-specific element SHOULD supplement or
further qualify a defined element. Thus the <error/> element will
contain two child elements, for example:
<iq type='error' id='some-id'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<too-many-parameters xmlns='application-ns'/>
</error>
</iq>
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9. XML Usage within XMPP
9.1 Restrictions
XMPP is a simplified and specialized protocol for streaming XML
elements in order to exchange messages and presence 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 must support the full XML specification [1]. 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 the XML specification [1])
o processing instructions (Section 2.6)
o internal or external DTD subsets (Section 2.8)
o internal or external entity references (Section 4.2) with the
exception of predefined entities (Section 4.6)
With regard to XML processing, if an XMPP implementation receives
such restricted XML data, it MUST ignore the data.
9.2 Namespaces
XML Namespaces [12] are used within all XMPP-compliant XML to create
strict boundaries of data ownership. The basic function of namespaces
is to separate different vocabularies of XML elements that are
structurally mixed together. Ensuring that XMPP-compliant XML is
namespace-aware enables any XML to be structurally mixed with any
data element within XMPP.
Additionally, an XMPP implementation MAY be more strict about
namespace prefixes than the XML namespace specification requires.
9.3 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 is NOT REQUIRED to do so. Clients SHOULD NOT rely on the ability
to send data which does not conform to the schemas, and SHOULD ignore
any non-conformant elements or attributes on the incoming XML stream.
Validation of XML streams and stanzas is NOT REQUIRED or recommended,
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and schemas are included herein for descriptive purposes only.
9.4 Character Encodings
Software implementing XML streams MUST support the UTF-8 (RFC 2279
[19]) and UTF-16 (RFC 2781 [20]) transformations of Universal
Character Set (ISO/IEC 10646-1 [21]) characters. Software MUST NOT
attempt to use any other encoding for transmitted data. The encodings
of the transmitted and received streams are independent. Software MAY
select either UTF-8 or UTF-16 for the transmitted stream, and SHOULD
deduce the encoding of the received stream as described in the XML
specification [1]. For historical reasons, existing implementations
MAY support UTF-8 only.
9.5 Inclusion of Text Declaration
An application MAY send a text declaration. Applications MUST follow
the rules in the XML specification [1] regarding the circumstances
under which a text declaration is included.
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10. IANA Considerations
10.1 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.
URI: urn:ietf:params:xml:ns:xmpp-tls
Specification: [RFCXXXX]
Description: This is the XML namespace name for TLS-related data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by [RFCXXXX].
Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
10.2 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.
URI: urn:ietf:params:xml:ns:xmpp-sasl
Specification: [RFCXXXX]
Description: This is the XML namespace name for SASL-related data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by [RFCXXXX].
Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
10.3 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.
URI: urn:ietf:params:xml:ns:xmpp-streams
Specification: [RFCXXXX]
Description: This is the XML namespace name for stream-related error
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by [RFCXXXX].
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Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
10.4 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.
URI: urn:ietf:params:xml:ns:xmpp-stanzas
Specification: [RFCXXXX]
Description: This is the XML namespace name for stanza-related error
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by [RFCXXXX].
Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
10.5 Existing Registrations
The IANA registers "xmpp" as a GSSAPI [22] service name, as specified
in Section 6.4.
Additionally, the IANA registers "jabber-client" and "jabber-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 NOT REQUIRED. The use
of the string "jabber" in these keywords is historical.
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11. Internationalization Considerations
Each XML stanza SHOULD include an 'xml:lang' attribute. Servers MUST
NOT modify or delete 'xml:lang' attributes from stanzas they receive
from other entities.
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12. Security Considerations
12.1 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.
Standalone, self-signed service certificates SHOULD NOT be used;
rather, an entity that wishes to generate a self-signed service
certificate SHOULD first generate a self-signed Root CA certificate
and then generate a signed service certificate. Entities that
communicate with the service SHOULD be configured with the Root CA
certificate rather than the service certificate; this avoids problems
associated with simple comparison of service certificates. If a
self-signed service certificate is used, an entity SHOULD NOT trust
it if it is changed to another self-signed certificate or a
certificate signed by an unrecognized authority.
Implementations MUST support high security. Service provisioning
SHOULD use high security, subject to local security policies.
12.2 Client-to-Server Communications
A compliant implementation MUST support both TLS and SASL for
connections to a server.
The TLS protocol for encrypting XML streams (defined under 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
Section 6) provides a reliable mechanism for validating that a client
connecting to a server is who it claims to be.
The IP address and method of access of clients MUST NOT be made
available by a server, nor are any connections other than the
original server connection required. This helps to protect the
client's server from direct attack or identification by third
parties.
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12.3 Server-to-Server Communications
A compliant implementation MUST support both TLS and SASL for
inter-domain communications. For historical reasons, a compliant
implementation SHOULD also support Section 7.
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.
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. Furthermore, it is susceptible to DNS
poisoning attacks unless DNSSec [30] is used, and even if the DNS
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.
12.4 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.
12.5 Firewalls
Communications using XMPP normally occur over TCP sockets on port
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5222 (client-to-server) or port 5269 (server-to-server), as
registered with the IANA [5] (see Section 10). Use of these
well-known ports allows administrators to easily enable or disable
XMPP activity through existing and commonly-deployed firewalls.
12.6 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 (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher
supporting client-side certificates)
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Normative References
[1] World Wide Web Consortium, "Extensible Markup Language (XML)
1.0 (Second Edition)", W3C xml, October 2000, <http://
www.w3.org/TR/2000/REC-xml-20001006>.
[2] Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging /
Presence Protocol Requirements", RFC 2779, February 2000.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] University of Southern California, "Transmission Control
Protocol", RFC 793, September 1981, <http://www.ietf.org/rfc/
rfc0793.txt>.
[5] Internet Assigned Numbers Authority, "Internet Assigned Numbers
Authority", January 1998, <http://www.iana.org/>.
[6] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet host
table specification", RFC 952, October 1985.
[7] Braden, R., "Requirements for Internet Hosts - Application and
Support", STD 3, RFC 1123, October 1989.
[8] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile
for Internationalized Domain Names (draft-ietf-idn-nameprep-11,
work in progress)", June 2002.
[9] Hoffman, P. and M. Blanchet, "Preparation of Internationalized
Strings ("stringprep")", RFC 3454, December 2002.
[10] Saint-Andre, P. and J. Hildebrand, "Nodeprep: A Stringprep
Profile for Node Identifiers in XMPP",
draft-ietf-xmpp-nodeprep-03 (work in progress), June 2003.
[11] Saint-Andre, P. and J. Hildebrand, "Resourceprep: A Stringprep
Profile for Resource Identifiers in XMPP",
draft-ietf-xmpp-resourceprep-03 (work in progress), June 2003.
[12] World Wide Web Consortium, "Namespaces in XML", W3C xml-names,
January 1999, <http://www.w3.org/TR/1999/
REC-xml-names-19990114/>.
[13] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
1999.
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[14] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[15] Myers, J., "Simple Authentication and Security Layer (SASL)",
RFC 2222, October 1997.
[16] Leach, P. and C. Newman, "Using Digest Authentication as a SASL
Mechanism", RFC 2831, May 2000.
[17] Alvestrand, H., "Tags for the Identification of Languages", BCP
47, RFC 3066, January 2001.
[18] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
location of services (DNS SRV)", RFC 2052, October 1996.
[19] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
[20] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO 10646",
RFC 2781, February 2000.
[21] 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.
[22] Linn, J., "Generic Security Service Application Program
Interface, Version 2", RFC 2078, January 1997.
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Informative References
[23] Saint-Andre, P. and J. Miller, "XMPP Instant Messaging",
draft-ietf-xmpp-im-12 (work in progress), June 2003.
[24] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998, <http://www.ietf.org/rfc/rfc2396.txt>.
[25] Mealling, M., "The IANA XML Registry",
draft-mealling-iana-xmlns-registry-04 (work in progress), June
2002.
[26] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 2060, December 1996.
[27] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD
53, RFC 1939, May 1996.
[28] Newman, C. and J. Myers, "ACAP -- Application Configuration
Access Protocol", RFC 2244, November 1997.
[29] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595,
June 1999.
[30] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[31] Alvestrand, H., "IETF Policy on Character Sets and Languages",
BCP 18, RFC 2277, January 1998.
[32] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
Authors' Addresses
Peter Saint-Andre
Jabber Software Foundation
EMail: stpeter@jabber.org
URI: http://www.jabber.org/people/stpeter.php
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Jeremie Miller
Jabber Software Foundation
EMail: jeremie@jabber.org
URI: http://www.jabber.org/people/jer.php
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Appendix A. XML Schemas
The following XML schemas are descriptive, not normative.
A.1 Stream namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='http://etherx.jabber.org/streams'
xmlns='http://etherx.jabber.org/streams'
elementFormDefault='unqualified'>
<xs:element name='stream'>
<xs:complexType>
<xs:sequence>
<xs:element ref='features' minOccurs='0' maxOccurs='1'/>
<xs:choice minOccurs='0' maxOccurs='1'>
<xs:any namespace='jabber:client'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:any namespace='jabber:server'
minOccurs='0'
maxOccurs='unbounded'/>
</xs:choice>
<xs:element ref='error' minOccurs='0' maxOccurs='1'/>
</xs:sequence>
<xs:attribute name='to' type='xs:string' use='optional'/>
<xs:attribute name='from' type='xs:string' use='optional'/>
<xs:attribute name='id' type='xs:NMTOKEN' use='optional'/>
<xs:attribute name='version' type='xs:decimal' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='features'>
<xs:complexType>
<xs:sequence>
<xs:any
namespace='##other'
minOccurs='0'
maxOccurs='unbounded'/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='error'>
<xs:complexType>
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<xs:sequence>
<xs:any namespace='urn:ietf:params:xml:ns:xmpp-streams'
maxOccurs='1'/>
<xs:any
namespace='##other'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
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A.2 Stream error namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-streams'
xmlns='urn:ietf:params:xml:ns:xmpp-streams'
elementFormDefault='qualified'>
<xs:element name='host-gone' type='empty'/>
<xs:element name='host-unknown' type='empty'/>
<xs:element name='internal-server-error' type='empty'/>
<xs:element name='invalid-id' type='empty'/>
<xs:element name='invalid-namespace' type='empty'/>
<xs:element name='nonmatching-hosts' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='remote-connection-failed' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='see-other-host' type='xs:string'/>
<xs:element name='system-shutdown' type='empty'/>
<xs:element name='unsupported-stanza-type' type='empty'/>
<xs:element name='unsupported-version' type='xs:string'/>
<xs:element name='xml-not-well-formed' type='empty'/>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
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A.3 TLS namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-tls'
xmlns='urn:ietf:params:xml:ns:xmpp-tls'
elementFormDefault='qualified'>
<xs:element name='starttls'>
<xs:complexType>
<xs:sequence>
<xs:element
ref='required'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='required' type='empty'/>
<xs:element name='proceed' type='empty'/>
<xs:element name='failure' type='empty'/>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
A.4 SASL namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-sasl'
xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
elementFormDefault='qualified'>
<xs:element name='mechanisms'>
<xs:complexType>
<xs:sequence>
<xs:element ref='mechanism' maxOccurs='unbounded'/>
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</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='mechanism' type='xs:string'/>
<xs:element name='auth'>
<xs:complexType>
<xs:attribute name='mechanism'
type='xs:NMTOKEN'
use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='challenge' type='xs:NMTOKEN'/>
<xs:element name='response' type='xs:NMTOKEN'/>
<xs:element name='abort' type='empty'/>
<xs:element name='success' type='empty'/>
<xs:element name='failure'>
<xs:complexType>
<xs:choice maxOccurs='1'>
<xs:element ref='encryption-required'/>
<xs:element ref='invalid-authzid'/>
<xs:element ref='invalid-realm'/>
<xs:element ref='mechanism-too-weak'/>
<xs:element ref='temporary-auth-failure'/>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:element name='encryption-required' type='empty'/>
<xs:element name='invalid-authzid' type='empty'/>
<xs:element name='invalid-realm' type='empty'/>
<xs:element name='mechanism-too-weak' type='empty'/>
<xs:element name='temporary-auth-failure' type='empty'/>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
A.5 Dialback namespace
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<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='jabber:server:dialback'
xmlns='jabber:server:dialback'
elementFormDefault='qualified'>
<xs:element name='result'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:NMTOKEN'>
<xs:attribute name='from' type='xs:string' use='required'/>
<xs:attribute name='to' type='xs:string' use='required'/>
<xs:attribute name='type' use='optional'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='invalid'/>
<xs:enumeration value='valid'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='verify'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:NMTOKEN'>
<xs:attribute name='from' type='xs:string' use='required'/>
<xs:attribute name='to' type='xs:string' use='required'/>
<xs:attribute name='id' type='xs:NMTOKEN' use='required'/>
<xs:attribute name='type' use='optional'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='invalid'/>
<xs:enumeration value='valid'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:schema>
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A.6 Client namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
xmlns:xml='http://www.w3.org/XML/1998/namespace'
targetNamespace='jabber:client'
xmlns='jabber:client'
elementFormDefault='qualified'>
<xs:import namespace='http://www.w3.org/XML/1998/namespace'
schemaLocation='http://www.w3.org/2001/xml.xsd'/>
<xs:element name='message'>
<xs:complexType>
<xs:sequence>
<xs:element ref='subject'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:element ref='body'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:element ref='thread'
minOccurs='0'
maxOccurs='1'/>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:element ref='error'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
<xs:attribute name='to'
type='xs:string'
use='optional'/>
<xs:attribute name='from'
type='xs:string'
use='optional'/>
<xs:attribute name='id'
type='xs:NMTOKEN'
use='optional'/>
<xs:attribute ref='xml:lang'/>
<xs:attribute name='type' use='optional'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='chat'/>
<xs:enumeration value='error'/>
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<xs:enumeration value='groupchat'/>
<xs:enumeration value='headline'/>
<xs:enumeration value='normal'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
<xs:element name='body' type='xs:string'>
<xs:complexType>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='subject' type='xs:string'>
<xs:complexType>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='thread' type='xs:NMTOKEN'/>
<xs:element name='presence'>
<xs:complexType>
<xs:sequence>
<xs:element ref='show'
minOccurs='0'
maxOccurs='1'/>
<xs:element ref='status'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:element ref='priority'
minOccurs='0'
maxOccurs='1'/>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:element ref='error'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
<xs:attribute name='to'
type='xs:string'
use='optional'/>
<xs:attribute name='from'
type='xs:string'
use='optional'/>
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<xs:attribute name='id'
type='xs:NMTOKEN'
use='optional'/>
<xs:attribute ref='xml:lang'/>
<xs:attribute name='type' use='optional'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='subscribe'/>
<xs:enumeration value='subscribed'/>
<xs:enumeration value='unsubscribe'/>
<xs:enumeration value='unsubscribed'/>
<xs:enumeration value='unavailable'/>
<xs:enumeration value='error'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
<xs:element name='show'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='away'/>
<xs:enumeration value='chat'/>
<xs:enumeration value='xa'/>
<xs:enumeration value='dnd'/>
</xs:restriction>
</xs:simpleType>
</xs:element>
<xs:element name='status' type='xs:string'>
<xs:complexType>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='priority' type='xs:byte'/>
<xs:element name='iq'>
<xs:complexType>
<xs:sequence>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='1'/>
<xs:element ref='error'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
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<xs:attribute name='to'
type='xs:string'
use='optional'/>
<xs:attribute name='from'
type='xs:string'
use='optional'/>
<xs:attribute name='id'
type='xs:NMTOKEN'
use='optional'/>
<xs:attribute ref='xml:lang'/>
<xs:attribute name='type' use='required'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='get'/>
<xs:enumeration value='set'/>
<xs:enumeration value='result'/>
<xs:enumeration value='error'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
<xs:element name='error'>
<xs:complexType>
<xs:sequence>
<xs:any namespace='urn:ietf:params:xml:ns:xmpp-stanzas'
maxOccurs='1'/>
<xs:any
namespace='##other'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
<xs:attribute name='type' use='required'/>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='cancel'/>
<xs:enumeration value='continue'/>
<xs:enumeration value='modify'/>
<xs:enumeration value='auth'/>
<xs:enumeration value='wait'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
</xs:schema>
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A.7 Server namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
xmlns:xml='http://www.w3.org/XML/1998/namespace'
targetNamespace='jabber:server'
xmlns='jabber:server'
elementFormDefault='qualified'>
<xs:import namespace='http://www.w3.org/XML/1998/namespace'
schemaLocation='http://www.w3.org/2001/xml.xsd'/>
<xs:element name='message'>
<xs:complexType>
<xs:sequence>
<xs:element ref='subject'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:element ref='body'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:element ref='thread'
minOccurs='0'
maxOccurs='1'/>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:element ref='error'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
<xs:attribute name='to'
type='xs:string'
use='required'/>
<xs:attribute name='from'
type='xs:string'
use='required'/>
<xs:attribute name='id'
type='xs:NMTOKEN'
use='optional'/>
<xs:attribute ref='xml:lang'/>
<xs:attribute name='type' use='optional'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='chat'/>
<xs:enumeration value='error'/>
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<xs:enumeration value='groupchat'/>
<xs:enumeration value='headline'/>
<xs:enumeration value='normal'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
<xs:element name='body' type='xs:string'>
<xs:complexType>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='subject' type='xs:string'>
<xs:complexType>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='thread' type='xs:NMTOKEN'/>
<xs:element name='presence'>
<xs:complexType>
<xs:sequence>
<xs:element ref='show'
minOccurs='0'
maxOccurs='1'/>
<xs:element ref='status'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:element ref='priority'
minOccurs='0'
maxOccurs='1'/>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='unbounded'/>
<xs:element ref='error'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
<xs:attribute name='to'
type='xs:string'
use='required'/>
<xs:attribute name='from'
type='xs:string'
use='required'/>
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<xs:attribute name='id'
type='xs:NMTOKEN'
use='optional'/>
<xs:attribute ref='xml:lang'/>
<xs:attribute name='type' use='optional'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='subscribe'/>
<xs:enumeration value='subscribed'/>
<xs:enumeration value='unsubscribe'/>
<xs:enumeration value='unsubscribed'/>
<xs:enumeration value='unavailable'/>
<xs:enumeration value='error'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
<xs:element name='show'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='away'/>
<xs:enumeration value='chat'/>
<xs:enumeration value='xa'/>
<xs:enumeration value='dnd'/>
</xs:restriction>
</xs:simpleType>
</xs:element>
<xs:element name='status' type='xs:string'>
<xs:complexType>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='priority' type='xs:byte'/>
<xs:element name='iq'>
<xs:complexType>
<xs:sequence>
<xs:any namespace='##other'
minOccurs='0'
maxOccurs='1'/>
<xs:element ref='error'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
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<xs:attribute name='to'
type='xs:string'
use='required'/>
<xs:attribute name='from'
type='xs:string'
use='required'/>
<xs:attribute name='id'
type='xs:NMTOKEN'
use='optional'/>
<xs:attribute ref='xml:lang'/>
<xs:attribute name='type' use='required'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='get'/>
<xs:enumeration value='set'/>
<xs:enumeration value='result'/>
<xs:enumeration value='error'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
<xs:element name='error'>
<xs:complexType>
<xs:sequence>
<xs:any namespace='urn:ietf:params:xml:ns:xmpp-stanzas'
maxOccurs='1'/>
<xs:any
namespace='##other'
minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
<xs:attribute name='type' use='required'/>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='cancel'/>
<xs:enumeration value='continue'/>
<xs:enumeration value='modify'/>
<xs:enumeration value='auth'/>
<xs:enumeration value='wait'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
</xs:schema>
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A.8 Stanza error namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-stanzas'
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
elementFormDefault='qualified'>
<xs:element name='bad-request' type='empty'/>
<xs:element name='conflict' type='empty'/>
<xs:element name='feature-not-implemented' type='empty'/>
<xs:element name='forbidden' type='empty'/>
<xs:element name='internal-server-error' type='empty'/>
<xs:element name='item-not-found' type='empty'/>
<xs:element name='jid-malformed' type='empty'/>
<xs:element name='not-allowed' type='empty'/>
<xs:element name='recipient-unavailable' type='empty'/>
<xs:element name='registration-required' type='empty'/>
<xs:element name='remote-server-not-found' type='empty'/>
<xs:element name='remote-server-timeout' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='service-unavailable' type='empty'/>
<xs:element name='subscription-required' type='empty'/>
<xs:element name='unexpected-request' type='empty'/>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
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Appendix B. Revision History
Note to RFC Editor: please remove this entire appendix, and the
corresponding entries in the table of contents, prior to publication.
B.1 Changes from draft-ietf-xmpp-core-12
o Moved server dialback to a separate section; clarified its
security characteristics and its role in the protocol.
o Adjusted error handling syntax and semantics per list discussion.
o Further clarified length of node identifiers and total length of
JIDs.
o Documented message type='normal'.
o Corrected several small errors in the TLS and SASL sections.
o Corrected several errors in the schemas.
B.2 Changes from draft-ietf-xmpp-core-11
o Corrected several small errors in the TLS and SASL sections.
o Made small editorial changes and fixed several schema errors.
B.3 Changes from draft-ietf-xmpp-core-10
o Adjusted TLS content regarding certificate validation process.
o Specified that stanza error extensions for specific applications
are to be properly namespaced children of the relevant descriptive
element.
o Clarified rules for inclusion of the 'id' attribute.
o Specified that the 'xml:lang' attribute SHOULD be included (per
list discussion).
o Made small editorial changes and fixed several schema errors.
B.4 Changes from draft-ietf-xmpp-core-09
o Fixed several dialback error conditions.
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o Cleaned up rules regarding TLS and certificate processing based on
off-list feedback.
o Changed <stream-condition/> and <stanza-condition/> elements to
<condition/>.
o Added or modified several stream and stanza error conditions.
o Specified only one child allowed for IQ, or two if type="error".
o Fixed several errors in the schemas.
B.5 Changes from draft-ietf-xmpp-core-08
o Incorporated list discussion regarding addressing, SASL, TLS, TCP,
dialback, namespaces, extensibility, and the meaning of 'ignore'
for routers and recipients.
o Specified dialback error conditions.
o Made small editorial changes to address RFC Editor requirements.
B.6 Changes from draft-ietf-xmpp-core-07
o Made several small editorial changes.
B.7 Changes from draft-ietf-xmpp-core-06
o Added text regarding certificate validation in TLS negotiation per
list discussion.
o Clarified nature of XML restrictions per discussion with W3C, and
moved XML Restrictions subsection under "XML Usage within XMPP".
o Further clarified that XML streams are unidirectional.
o Changed stream error and stanza error namespace names to conform
to the format defined in The IETF XML Registry [25].
o Removed note to RFC Editor regarding provisional namespace names.
B.8 Changes from draft-ietf-xmpp-core-05
o Added <invalid-namespace/> as a stream error condition.
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o Adjusted security considerations per discussion at IETF 56 and on
list.
B.9 Changes from draft-ietf-xmpp-core-04
o Added server-to-server examples for TLS and SASL.
o Changed error syntax, rules, and examples based on list
discussion.
o Added schemas for the TLS, stream error, and stanza error
namespaces.
o Added note to RFC Editor regarding provisional namespace names.
o Made numerous small editorial changes and clarified text
throughout.
B.10 Changes from draft-ietf-xmpp-core-03
o Clarified rules and procedures for TLS and SASL.
o Amplified stream error code syntax per list discussion.
o Made numerous small editorial changes.
B.11 Changes from draft-ietf-xmpp-core-02
o Added dialback schema.
o Removed all DTDs since schemas provide more complete definitions.
o Added stream error codes.
o Clarified error code "philosophy".
B.12 Changes from draft-ietf-xmpp-core-01
o Updated the addressing restrictions per list discussion and added
references to the new nodeprep and resourceprep profiles.
o Corrected error in Stream Authentication regarding 'version'
attribute.
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o Made numerous small editorial changes.
B.13 Changes from draft-ietf-xmpp-core-00
o Added information about TLS from list discussion.
o Clarified meaning of "ignore" based on list discussion.
o Clarified information about Universal Character Set data and
character encodings.
o Provided base64-decoded information for examples.
o Fixed several errors in the schemas.
o Made numerous small editorial fixes.
B.14 Changes from draft-miller-xmpp-core-02
o Brought Streams Authentication section into line with discussion
on list and at IETF 55 meeting.
o Added information about the optional 'xml:lang' attribute per
discussion on list and at IETF 55 meeting.
o Specified that validation is neither required nor recommended, and
that the formal definitions (DTDs and schemas) are included for
descriptive purposes only.
o Specified that the response to an IQ stanza of type "get" or "set"
must be an IQ stanza of type "result" or "error".
o Specified that compliant server implementations must process
stanzas in order.
o Specified that for historical reasons some server implementations
may accept 'stream:' as the only valid namespace prefix on the
root stream element.
o Clarified the difference between 'jabber:client' and
'jabber:server' namespaces, namely, that 'to' and 'from'
attributes are required on all stanzas in the latter but not the
former.
o Fixed typo in Step 9 of the dialback protocol (changed db:result
to db:verify).
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o Removed references to TLS pending list discussion.
o Removed the non-normative appendix on OpenPGP usage pending its
inclusion in a separate I-D.
o Simplified the architecture diagram, removed most references to
services, and removed references to the 'jabber:component:*'
namespaces.
o Noted that XMPP activity respects firewall administration
policies.
o Further specified the scope and uniqueness of the 'id' attribute
in all stanza types and the <thread/> element in message stanzas.
o Nomenclature changes: (1) from "chunks" to "stanzas"; (2) from
"host" to "server" and from "node" to "client" (except with regard
to definition of the addressing scheme).
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