Network Working Group P. Saint-Andre
Internet-Draft J. Miller
Expires: March 7, 2004 Jabber Software Foundation
September 7, 2003
XMPP Core
draft-ietf-xmpp-core-18
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes the core features of the Extensible Messaging
and Presence Protocol (XMPP), a protocol for streaming XML [1]
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 . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Discussion Venue . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Intellectual Property Notice . . . . . . . . . . . . . . . . 6
2. Generalized Architecture . . . . . . . . . . . . . . . . . . 7
2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Server . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Client . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Gateway . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5 Network . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Addressing Scheme . . . . . . . . . . . . . . . . . . . . . 9
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Domain Identifier . . . . . . . . . . . . . . . . . . . . . 9
3.3 Node Identifier . . . . . . . . . . . . . . . . . . . . . . 10
3.4 Resource Identifier . . . . . . . . . . . . . . . . . . . . 10
4. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2 Stream Attributes . . . . . . . . . . . . . . . . . . . . . 12
4.2.1 Version Support . . . . . . . . . . . . . . . . . . . . . . 13
4.3 Namespace Declarations . . . . . . . . . . . . . . . . . . . 14
4.4 Stream Features . . . . . . . . . . . . . . . . . . . . . . 14
4.5 Stream Encryption and Authentication . . . . . . . . . . . . 15
4.6 Stream Errors . . . . . . . . . . . . . . . . . . . . . . . 15
4.6.1 Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.6.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.6.3 Defined Conditions . . . . . . . . . . . . . . . . . . . . . 16
4.6.4 Application-Specific Conditions . . . . . . . . . . . . . . 18
4.7 Simple Streams Example . . . . . . . . . . . . . . . . . . . 19
5. Stream Encryption . . . . . . . . . . . . . . . . . . . . . 21
5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2 Narrative . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.3 Client-to-Server Example . . . . . . . . . . . . . . . . . . 24
5.4 Server-to-Server Example . . . . . . . . . . . . . . . . . . 25
6. Stream Authentication . . . . . . . . . . . . . . . . . . . 28
6.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 28
6.2 Narrative . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.3 SASL Errors . . . . . . . . . . . . . . . . . . . . . . . . 31
6.4 SASL Definition . . . . . . . . . . . . . . . . . . . . . . 32
6.5 Client-to-Server Example . . . . . . . . . . . . . . . . . . 32
6.6 Server-to-Server Example . . . . . . . . . . . . . . . . . . 35
7. Server Dialback . . . . . . . . . . . . . . . . . . . . . . 39
7.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 39
7.2 Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 41
8. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . 45
8.1 Common Attributes . . . . . . . . . . . . . . . . . . . . . 45
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8.1.1 to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
8.1.2 from . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
8.1.3 id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
8.1.4 type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
8.1.5 xml:lang . . . . . . . . . . . . . . . . . . . . . . . . . . 46
8.2 Basic Semantics . . . . . . . . . . . . . . . . . . . . . . 47
8.2.1 Message Semantics . . . . . . . . . . . . . . . . . . . . . 47
8.2.2 Presence Semantics . . . . . . . . . . . . . . . . . . . . . 47
8.2.3 IQ Semantics . . . . . . . . . . . . . . . . . . . . . . . . 47
8.3 Stanza Errors . . . . . . . . . . . . . . . . . . . . . . . 49
8.3.1 Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
8.3.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
8.3.3 Defined Conditions . . . . . . . . . . . . . . . . . . . . . 51
8.3.4 Application-Specific Conditions . . . . . . . . . . . . . . 52
9. XML Usage within XMPP . . . . . . . . . . . . . . . . . . . 54
9.1 Restrictions . . . . . . . . . . . . . . . . . . . . . . . . 54
9.2 XML Namespace Names and Prefixes . . . . . . . . . . . . . . 54
9.2.1 Stream Namespace . . . . . . . . . . . . . . . . . . . . . . 54
9.2.2 Default Namespace . . . . . . . . . . . . . . . . . . . . . 55
9.2.3 Dialback Namespace . . . . . . . . . . . . . . . . . . . . . 55
9.3 Validation . . . . . . . . . . . . . . . . . . . . . . . . . 56
9.4 Inclusion of Text Declaration . . . . . . . . . . . . . . . 56
9.5 Character Encoding . . . . . . . . . . . . . . . . . . . . . 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 Nodeprep Profile of Stringprep . . . . . . . . . . . . . . . 58
10.6 Resourceprep Profile of Stringprep . . . . . . . . . . . . . 58
10.7 Existing Registrations . . . . . . . . . . . . . . . . . . . 59
11. Internationalization Considerations . . . . . . . . . . . . 60
12. Security Considerations . . . . . . . . . . . . . . . . . . 61
12.1 High Security . . . . . . . . . . . . . . . . . . . . . . . 61
12.2 Client-to-Server Communications . . . . . . . . . . . . . . 61
12.3 Server-to-Server Communications . . . . . . . . . . . . . . 62
12.4 Order of Layers . . . . . . . . . . . . . . . . . . . . . . 63
12.5 Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . 63
12.6 Mandatory to Implement Technologies . . . . . . . . . . . . 63
12.7 Stringprep Profiles . . . . . . . . . . . . . . . . . . . . 63
13. Server Rules for Handling XML Stanzas . . . . . . . . . . . 65
13.1 No 'to' Address . . . . . . . . . . . . . . . . . . . . . . 65
13.2 Foreign Domain . . . . . . . . . . . . . . . . . . . . . . . 65
13.3 Subdomain . . . . . . . . . . . . . . . . . . . . . . . . . 66
13.4 Bare Domain or Specific Resource . . . . . . . . . . . . . . 66
13.5 Node in Same Domain . . . . . . . . . . . . . . . . . . . . 66
14. Compliance Requirements . . . . . . . . . . . . . . . . . . 68
14.1 Servers . . . . . . . . . . . . . . . . . . . . . . . . . . 68
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14.2 Clients . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Normative References . . . . . . . . . . . . . . . . . . . . 70
Informative References . . . . . . . . . . . . . . . . . . . 72
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 72
A. Nodeprep . . . . . . . . . . . . . . . . . . . . . . . . . . 73
A.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 73
A.2 Character Repertoire . . . . . . . . . . . . . . . . . . . . 73
A.3 Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . 73
A.4 Normalization . . . . . . . . . . . . . . . . . . . . . . . 73
A.5 Prohibited Output . . . . . . . . . . . . . . . . . . . . . 74
A.6 Bidirectional Characters . . . . . . . . . . . . . . . . . . 75
B. Resourceprep . . . . . . . . . . . . . . . . . . . . . . . . 76
B.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 76
B.2 Character Repertoire . . . . . . . . . . . . . . . . . . . . 76
B.3 Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . 76
B.4 Normalization . . . . . . . . . . . . . . . . . . . . . . . 76
B.5 Prohibited Output . . . . . . . . . . . . . . . . . . . . . 77
B.6 Bidirectional Characters . . . . . . . . . . . . . . . . . . 77
C. XML Schemas . . . . . . . . . . . . . . . . . . . . . . . . 78
C.1 Stream namespace . . . . . . . . . . . . . . . . . . . . . . 78
C.2 Stream error namespace . . . . . . . . . . . . . . . . . . . 79
C.3 TLS namespace . . . . . . . . . . . . . . . . . . . . . . . 80
C.4 SASL namespace . . . . . . . . . . . . . . . . . . . . . . . 81
C.5 Dialback namespace . . . . . . . . . . . . . . . . . . . . . 82
C.6 Stanza error namespace . . . . . . . . . . . . . . . . . . . 83
D. Differences Between Jabber and XMPP . . . . . . . . . . . . 85
D.1 Authentication . . . . . . . . . . . . . . . . . . . . . . . 85
D.2 Channel Encryption . . . . . . . . . . . . . . . . . . . . . 85
D.3 JID Processing . . . . . . . . . . . . . . . . . . . . . . . 85
D.4 Error Handling . . . . . . . . . . . . . . . . . . . . . . . 86
D.5 Internationalization . . . . . . . . . . . . . . . . . . . . 86
D.6 Stream Version Attribute . . . . . . . . . . . . . . . . . . 86
E. Revision History . . . . . . . . . . . . . . . . . . . . . . 87
E.1 Changes from draft-ietf-xmpp-core-17 . . . . . . . . . . . . 87
E.2 Changes from draft-ietf-xmpp-core-16 . . . . . . . . . . . . 87
E.3 Changes from draft-ietf-xmpp-core-15 . . . . . . . . . . . . 88
E.4 Changes from draft-ietf-xmpp-core-14 . . . . . . . . . . . . 88
E.5 Changes from draft-ietf-xmpp-core-13 . . . . . . . . . . . . 88
E.6 Changes from draft-ietf-xmpp-core-12 . . . . . . . . . . . . 88
E.7 Changes from draft-ietf-xmpp-core-11 . . . . . . . . . . . . 89
E.8 Changes from draft-ietf-xmpp-core-10 . . . . . . . . . . . . 89
E.9 Changes from draft-ietf-xmpp-core-09 . . . . . . . . . . . . 89
E.10 Changes from draft-ietf-xmpp-core-08 . . . . . . . . . . . . 89
E.11 Changes from draft-ietf-xmpp-core-07 . . . . . . . . . . . . 90
E.12 Changes from draft-ietf-xmpp-core-06 . . . . . . . . . . . . 90
E.13 Changes from draft-ietf-xmpp-core-05 . . . . . . . . . . . . 90
E.14 Changes from draft-ietf-xmpp-core-04 . . . . . . . . . . . . 90
E.15 Changes from draft-ietf-xmpp-core-03 . . . . . . . . . . . . 91
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E.16 Changes from draft-ietf-xmpp-core-02 . . . . . . . . . . . . 91
E.17 Changes from draft-ietf-xmpp-core-01 . . . . . . . . . . . . 91
E.18 Changes from draft-ietf-xmpp-core-00 . . . . . . . . . . . . 91
E.19 Changes from draft-miller-xmpp-core-02 . . . . . . . . . . . 92
Intellectual Property and Copyright Statements . . . . . . . 94
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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 [20] 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 (Section 4) to and from authorized clients, servers, and
other entities) and to route appropriately-addressed XML stanzas
(Section 8) among such entities over XML streams. Most XMPP-compliant
servers also assume responsibility for the storage of data that is
used by clients (e.g., contact lists for users of XMPP-based instant
messaging and presence applications); in this case, the XML data is
processed directly by the server itself on behalf of the client and
is not routed to another entity. Compliant server implementations
MUST ensure in-order processing of XML stanzas between any two
entities.
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2.3 Client
Most clients connect directly to a server over a TCP socket and use
XMPP to take full advantage of the functionality provided by a server
and any associated services. Although there is no necessary coupling
of an XML stream to a TCP socket (e.g., a client COULD connect via
HTTP [21] polling or some other mechanism), this specification
defines a binding of 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 Addressing Scheme (Section 3). The port registered with the
Internet Assigned Numbers Authority (IANA) [5] for connections
between a client and a server is 5222 (see IANA Considerations
(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 IANA
Considerations (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 [22]. 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 and presence 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 (Appendix A) 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 when it provides the information necessary to complete
stream authentication (Section 6). 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 (Appendix B) 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), while
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
(the "initial stream" and the "response stream"), 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. An XML element sent for the purpose of stream
encryption, stream authentication, or server dialback is not
considered to be an XML stanza.
Consider the example of a client's session with a server. In order to
connect to a server, a client MUST initiate an XML stream by sending
an opening <stream> tag to the server, optionally preceded by a text
declaration specifying the XML version supported and the character
encoding (see also Character Encoding (Section 9.5)). 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
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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 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
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entity replies to the initiating entity; however, if a 'to'
attribute is included, it SHOULD be silently ignored by the
initiating entity.
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 -- The presence of the version attribute set to a value of
"1.0" signals support for the stream features defined in this
specification. Detailed rules regarding generation and handling of
this attribute are defined below.
We can summarize as follows:
| initiating to receiving | receiving to initiating
------------------------------------------------------------
to | hostname of receiver | silently ignored
from | silently ignored | hostname of receiver
id | silently ignored | session key
version | signals XMPP 1.0 support | signals XMPP 1.0 support
4.2.1 Version Support
The following rules apply to the generation and handling of the
'version' attribute:
1. If the initiating entity complies with the XML streams protocol
defined herein (including Stream Encryption (Section 5), Stream
Authentication (Section 6), and Stream Errors (Section 4.6)), it
MUST include the '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".
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2. If the initiating entity includes the 'version' attribute set to
a value of "1.0" in its stream header and the receiving entity
supports XMPP 1.0, the receiving entity MUST reciprocate by
including the 'version' attribute set to a value of "1.0" in its
stream header response.
3. If the initiating entity does not include the 'version' attribute
in its stream header, the receiving entity still SHOULD include
the 'version' attribute set to a value of "1.0" in its stream
header response.
4. If the initiating entity includes the 'version' attribute set to
a value other than "1.0", the receiving entity SHOULD include the
'version' attribute set to a value of "1.0" in its stream header
response, but MAY at its discretion generate an
<unsupported-version/> stream error and terminate the XML stream
and underlying TCP connection.
5. If the receiving entity includes the 'version' attribute set to a
value other than "1.0" in its stream header response, the
initiating entity SHOULD generate an <unsupported-version/>
stream error and terminate the XML stream and underlying TCP
connection.
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 [10]). For detailed
information regarding the stream namespace and default namespace, see
Namespace Names and Prefixes (Section 9.2).
4.4 Stream Features
If the initiating entity includes the '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
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 Stream Authentication (Section 6) below). If an
entity does not understand or support some features, it SHOULD
silently ignore them.
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4.5 Stream Encryption and Authentication
XML streams in XMPP 1.0 SHOULD be encrypted as defined under Stream
Encryption (Section 5) and MUST be authenticated as defined under
Stream Authentication (Section 6). If the initiating entity attempts
to send an XML stanza before the stream is authenticated, the
receiving entity SHOULD return a <not-authorized/> stream error to
the initiating entity and then terminate both the XML stream and the
underlying TCP connection.
4.6 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.
4.6.1 Rules
The following rules apply to stream-level errors:
o It is assumed that all stream-level errors are unrecoverable;
therefore, if an error occurs at the level of the stream, the
entity that detects the error MUST send a stream error to the
other entity, 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.
4.6.2 Syntax
The syntax for stream errors is as follows:
<stream:error>
<defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<text xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
OPTIONAL descriptive text
</text>
[OPTIONAL application-specific condition element]
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</stream:error>
The <error/> element:
o MUST contain a child element corresponding to one of the defined
stanza error conditions defined below; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-streamstreams'
namespace
o MAY contain a <text/> child containing CDATA that describes the
error in more detail; this element MUST be qualified by the
'urn:ietf:params:xml:ns:xmpp-streams' namespace and SHOULD possess
an 'xml:lang' attribute
o MAY contain a child element for an application-specific error
condition; this element MUST be qualified by an
application-defined namespace, and its structure is defined by
that namespace
The <text/> element is OPTIONAL. If included, it SHOULD be used only
to provide descriptive or diagnostic information that supplements the
meaning of a defined condition or application-specific condition. It
SHOULD NOT be interpreted programmatically by an application. It
SHOULD NOT be used as the error message presented to user, but MAY be
shown in addition to the error message associated with the included
condition element (or elements).
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 [23].
4.6.3 Defined Conditions
The following stream-level error conditions are defined:
o <bad-format/> -- the entity has sent XML that cannot be processed;
this error may be used rather than more specific XML-related
errors such as <bad-namespace-prefix/>, <invalid-xml/>,
<restricted-xml/>, <unsupported-encoding/>, and
<xml-not-well-formed/>, although the more specific errors are
preferred.
o <bad-namespace-prefix/> -- the entity has sent a namespace prefix
that is unsupported, or has sent no namespace prefix on an element
that requires such a prefix (see the XML Namespace Names and
Prefixes (Section 9.2) section of this document).
o <conflict/> -- the server is closing the active stream for this
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entity because a new stream has been initiated that conflicts with
the existing stream.
o <connection-timeout/> -- the entity has not generated any traffic
over the stream for some period of time (configurable according to
a local service policy).
o <host-gone/> -- the value of the 'to' attribute provided by the
initiating entity in the stream header corresponds to a hostname
that is no longer hosted by the server.
o <host-unknown/> -- the value of the 'to' attribute provided by the
initiating entity in the stream header does not correspond to a
hostname that is hosted by the server.
o <improper-addressing/> -- a stanza sent between two servers lacks
a 'to' or 'from' attribute (or the attribute has no value).
o <internal-server-error/> -- the server has experienced a
misconfiguration or an otherwise-undefined internal error that
prevents it from servicing the stream.
o <invalid-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"
(see the XML Namespace Names and Prefixes (Section 9.2) section of
this document).
o <invalid-xml/> -- the entity has sent invalid XML over the stream
to a server that performs validation (see the Validation (Section
9.3) section of this document).
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 <policy-violation/> -- the entity has violated some local service
policy; the server MAY choose to specify the policy in the <text/>
element.
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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-constrained and
is unable to service the stream.
o <restricted-xml/> -- the entity has attempted to send a comment,
processing instruction, DTD, entity reference, or unescaped
character (see the Restrictions (Section 9.1) section of this
document).
o <see-other-host/> -- the server will not provide service to the
initiating entity but is redirecting traffic to another host; the
server SHOULD specify the alternate hostname or IP address in the
<text/> element.
o <system-shutdown/> -- the server is being shut down and all active
streams are being closed.
o <undefined-condition/> -- the error condition is not one of those
defined by the other conditions in this list; this error condition
SHOULD be used only in conjunction with an application-specific
condition.
o <unsupported-encoding/> -- the initiating entity has encoded the
stream in an encoding that is not supported by the server (see the
Character Encoding (Section 9.5) section of this document).
o <unsupported-stanza-type/> -- the initiating entity has sent a
first-level child of the stream that is not supported by the
server.
o <unsupported-version/> -- the value of the 'version' attribute
provided by the initiating entity in the stream header specifies a
version of XMPP that is not supported by the server; the server
MAY specify the version(s) it supports in the <text/> element.
o <xml-not-well-formed/> -- the initiating entity has sent XML that
is not well-formed as defined by the XML specification [1].
4.6.4 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
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contain two or three child elements:
<stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<text xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
Some special application diagnostic information!
</text>
<escape-your-data xmlns='application-ns'/>
</stream:error>
</stream:stream>
4.7 Simple Streams Example
The following is a stream-based session of a client on a server
(where the "C" lines are sent from the client to the server, and the
"S" lines are sent from the server to the client):
A basic session:
C: <?xml version='1.0'?>
<stream:stream
to='example.com'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
S: <?xml version='1.0'?>
<stream:stream
from='example.com'
id='id_123456789'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
... authentication ...
C: <message from='juliet@example.com'
to='romeo@example.net'
xml:lang='en'>
C: <body>Art thou not Romeo, and a Montague?</body>
C: </message>
S: <message from='romeo@example.net'
to='juliet@example.com'
xml:lang='en'>
S: <body>Neither, fair saint, if either thee dislike.</body>
S: </message>
C: </stream:stream>
S: </stream:stream>
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A stream gone bad:
C: <?xml version='1.0'?>
<stream:stream
to='example.com'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
S: <?xml version='1.0'?>
<stream:stream
from='example.com'
id='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) [11] protocol, along with a "STARTTLS"
extension that is modelled after similar extensions for the IMAP
[24], POP3 [25], and ACAP [26] protocols as described in RFC 2595
[27]. 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 [23].
An administrator of a given domain MAY require the use of TLS for
client-to-server communications, server-to-server communications, or
both. Servers SHOULD use TLS between two domains for the purpose of
securing server-to-server communications. See Mandatory to Implement
Technologies (Section 12.6) regarding mechanisms that MUST be
supported.
The following rules apply:
1. An initiating entity that complies with this specification MUST
include the 'version' attribute set to a value of "1.0" in the
initiating stream header.
2. If the TLS negotiation occurs between two servers,
communications MUST NOT proceed until the DNS hostnames asserted
by the servers have been resolved (see Server-to-Server
Communications (Section 12.3)).
3. When a receiving entity that complies with this specification
receives an initiating stream header that includes the '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.
4. If the initiating entity chooses to use TLS for stream
encryption, TLS negotiation MUST be completed before proceeding
to SASL negotiation.
5. The receiving entity MUST consider the TLS negotiation to have
begun immediately after sending the closing ">" of the <proceed/
> 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.
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6. 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 [12]. 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 human (e.g., an end user or server administrator) for
approval; if presented, the receiver MUST deliver the entire
certificate chain to the human, 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.
7. If the TLS negotiation is successful, the receiving entity MUST
discard any knowledge obtained from the initiating entity before
TLS takes effect.
8. If the TLS negotiation is successful, the initiating entity MUST
discard any knowledge obtained from the receiving entity before
TLS takes effect.
9. 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.
10. If the TLS negotiation is successful, the initiating entity
SHOULD continue with SASL negotiation.
11. 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 the '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
the '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 [11].
7. If the TLS negotiation is unsuccessful, the receiving entity MUST
terminate the TCP connection. If the TLS negotiation is
successful, the initiating entity MUST initiate a new stream by
sending an opening XML stream header to the receiving entity.
8. Upon receiving the new stream header from the initiating entity,
the receiving entity MUST respond by sending a new XML stream
header to the initiating entity along with the 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 IANA Considerations (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'/>
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'/>
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</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 Stream Authentication (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 IANA Considerations (Section 10)).
Step 1: Server1 initiates stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
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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.
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'
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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 Stream Authentication (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)
[13]. 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 [13] (the XMPP-specific namespace name is
'urn:ietf:params:xml:ns:xmpp-sasl', which adheres to the format
defined in The IETF XML Registry [23]). Finally, see Mandatory to
Implement Technologies (Section 12.6) regarding mechanisms that MUST
be supported.
The following rules apply:
1. If the SASL negotiation occurs between two servers,
communications MUST NOT proceed until the DNS hostnames asserted
by the servers have been resolved (see Server-to-Server
Communications (Section 12.3)).
2. 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.
3. If the initiating entity is capable of authenticating via SASL,
it MUST include the 'version' attribute set to a value of "1.0"
in the initiating stream header.
4. 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 the 'version' attribute set to a
value of "1.0").
5. Upon successful SASL negotiation that involves negotiation of a
security layer, the receiving entity MUST discard any knowledge
obtained from the initiating entity which was not obtained from
the SASL negotiation itself.
6. Upon successful SASL negotiation that involves negotiation of a
security layer, the initiating entity MUST discard any knowledge
obtained from the receiving entity which was not obtained from
the SASL negotiation itself.
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7. The initiating entity MUST provide an authzid during SASL
negotiation. 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. The initiating
entity MAY process the authzid-value in accordance with the rules
defined in Addressing Scheme (Section 3) before providing it to
the receiving entity, but is NOT REQUIRED to do so; however, the
receiving entity MUST verify that the authzid-value provided by
the initiating entity conforms to the rules defined therein.
8. Any character data contained within the XML elements used during
SASL negotiation MUST be encoded using base64.
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
the 'version' attribute in the opening XML stream header sent to
the receiving entity, with the value set to "1.0".
2. After sending an XML stream header in 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 needs to 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
(refer to RFC 2222 [13]).
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.
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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).
6. If necessary, the receiving entity sends more challenges and the
initiating entity sends more responses.
This series of challenge/response pairs continues until one of three
things happens:
1. The initiating entity aborts the handshake by sending an <abort/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
namespace to the receiving entity. Upon receiving the <abort/>
element, the receiving entity MUST terminate the TCP connection.
2. The receiving entity reports failure of the handshake by sending
a <failure/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
entity (the particular cause of failure SHOULD be communicated in
an appropriate child element of the <failure/> element as defined
under SASL Errors (Section 6.3)). Immediately after sending the
<failure/> element, the receiving entity MUST terminate both the
XML stream and the underlying TCP connection.
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"). Upon receiving
the <success/> element, the initiating entity MUST initiate a new
stream by sending an opening XML stream header to the receiving
entity (it is not necessary to send a closing </stream:stream>
element first, since the receiving entity and initiating entity
MUST consider the original stream to be closed upon sending or
receiving the <success/> element). Upon receiving the new stream
header from the initiating entity, the receiving entity MUST
respond by sending a new XML stream header to the initiating
entity, along with any remaining available features (but NOT
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including the STARTTLS feature or any authentication mechanisms)
or an empty features element (to signify that no additional
features are available); note that any such additional features
are not defined herein, and MUST be defined by the relevant
extension to XMPP.
6.3 SASL Errors
The following SASL-related error conditions are defined:
o <aborted/> -- The receiving entity acknowledges an <abort/>
element sent by the initiating entity; sent in response to the
<abort/> element.
o <bad-protocol/> -- The data provided by the initiating entity
could not be processed, e.g. because does not adhere to the
protocol for the requested mechanism; sent in response to the
<response/> element.
o <encryption-required/> -- The mechanism chosen by the initiating
entity may be used only if the stream is already encrypted; sent
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; sent in response to a <response/> element.
o <invalid-mechanism/> -- The initiating entity did not provide a
mechanism or requested a mechanism that is not supported by the
receiving entity; sent in response to the <auth/> 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; sent in
response to a <response/> element.
o <mechanism-too-weak/> -- The mechanism requested by the initiating
entity is weaker than server policy permits for that initiating
entity; sent in response to the <response/> element.
o <not-authorized/> -- The authentication failed because the
initiating entity did not provide valid credentials (this includes
the case of an unknown username); sent in response to a <response/
> element.
o <temporary-auth-failure/> -- The authentication failed because of
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a temporary error condition within the receiving entity; sent in
response to an <auth/> element or <response/> element.
6.4 SASL Definition
Section 4 of the SASL specification [13] 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.) Upon successful
negotiation, both sides consider the original XML stream closed
and new <stream> headers are sent by both entities.
security layer negotiation: The security layer takes effect
immediately after sending the closing ">" character of the
<success/> element for the server, and immediately after receiving
the closing ">" character of the <success/> element for the client
(this element is 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 IANA Considerations (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>DIGEST-MD5</mechanism>
<mechanism>PLAIN</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'>
cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi
xxb3A9ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz
</challenge>
The decoded challenge is:
realm="cataclysm.cx",nonce="OA6MG9tEQGm2hh",\
qop="auth",charset=utf-8,algorithm=md5-sess
Step 5 (alt): Server returns error to client:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism-too-weak/>
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</failure>
</stream:stream>
Step 6: Client responds to the challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9InJvYiIscmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik
9BNk1HOXRFUUdtMmhoIixjbm9uY2U9Ik9BNk1IWGg2VnFUclJrIixuYz0w
MDAwMDAwMSxxb3A9YXV0aCxkaWdlc3QtdXJpPSJ4bXBwL2NhdGFjbHlzbS
5jeCIscmVzcG9uc2U9ZDM4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxNDNh
ZjcsY2hhcnNldD11dGYtOCxhdXRoemlkPSJyb2JAY2F0YWNseXNtLmN4L2
15UmVzb3VyY2Ui
</response>
The decoded response is:
username="rob",realm="cataclysm.cx",\
nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\
nc=00000001,qop=auth,digest-uri="xmpp/cataclysm.cx",\
response=d388dad90d4bbd760a152321f2143af7,charset=utf-8,\
authzid="rob@cataclysm.cx/myResource"
Step 7: Server sends another challenge to client:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
</challenge>
The decoded challenge is:
rspauth=ea40f60335c427b5527b84dbabcdfffd
Step 7 (alt): Server returns error to client:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<invalid-realm/>
</failure>
</stream:stream>
Step 8: Client responds to the challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9: Server informs client of successful authentication:
<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
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Step 9 (alt): Server informs client of failed authentication:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
</stream:stream>
Step 10: Client initiates a new stream to server:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='capulet.com'
version='1.0'>
Step 11: Server responds by sending a stream header to client along
with any additional features (or an empty features element):
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='12345678'
from='capulet.com'
version='1.0'>
<stream:features/>
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 IANA Considerations (Section 10)).
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'
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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
Step 5 (alt): Server2 returns error to Server1:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<encryption-required/>
</failure>
</stream:stream>
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:
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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>
</stream:stream>
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'/>
Step 9 (alt): Server2 informs Server1 of failed authentication:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
</stream:stream>
Step 10: Server1 initiates a new stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
to='montague.net'
version='1.0'>
Step 11: Server2 responds by sending a stream header to Server1 along
with any additional features (or an empty features element):
<stream:stream
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xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
from='montague.net'
id='12345678'
version='1.0'>
<stream:features/>
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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 Server-to-Server
Communications (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.
Because dialback depends on the Domain Name System, inter-domain
communications MUST NOT proceed until the DNS hostnames asserted by
the servers have been resolved (see Server-to-Server Communications
(Section 12.3)).
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:
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 |
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| <---------------------- |
| |
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:
<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'
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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
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
MUST maintain the existing connection until it validates whether
the new connection is legitimate; additionally, Receiving Server
MAY choose to generate a <not-authorized/> stream error
condition for the new connection and then terminate both the XML
stream and the underlying TCP connection related to the new
request.
5. 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'
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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 a stream error 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
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
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<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
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
<improper-addressing/> stream error condition and terminate both the
XML stream and the underlying TCP connection. Furthermore, a server
MUST verify that the 'from' attribute of stanzas received from the
other server includes a validated domain for the stream; if a stanza
does not meet this restriction, the server that receives the stanza
MUST generate 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
Once XML streams in both directions have been authenticated and (if
desired) encrypted, XML stanzas can be sent over the streams. Three
kinds of XML stanza are defined for the 'jabber:client' and
'jabber:server' namespaces: <message/>, <presence/>, and <iq/>. In
addition, there are five common attributes for these kinds of stanza.
These common attributes and the basic semantics of the three stanza
kinds are defined herein; more detailed information regarding the
syntax of XML stanzas in relation to instant messaging and presence
applications is provided in XMPP IM [20].
8.1 Common Attributes
The following five attributes are common to message, presence, and IQ
stanzas:
8.1.1 to
The 'to' attribute specifies the JID of the intended recipient for
the stanza.
In the 'jabber:client' namespace, a stanza SHOULD possess a 'to'
attribute, although a stanza sent from a client to a server for
handling by that server (e.g., presence sent to the server for
broadcasting to other entities) SHOULD NOT possess a 'to' attribute.
In the 'jabber:server' namespace, a stanza MUST possess a 'to'
attribute; if a server receives a stanza that does not meet this
restriction, it MUST generate an <improper-addressing/> stream error
condition and terminate both the XML stream and the underlying TCP
connection with the offending server.
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.
8.1.2 from
The 'from' attribute specifies the JID of the sender.
In the 'jabber:client' namespace, a client MUST NOT include a 'from'
attribute on the stanzas it sends to a server; if a server receives
an XML 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. When a client sends an XML stanza
within the context of an authenticated stream, the server MUST stamp
the stanza with the full JID (<user@domain/resource>) of the
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connected resource that generated the stanza as defined by the
authzid provided in the SASL negotiation. If a client attempts to
send an XML stanza before the stream is authenticated, the server
SHOULD return a <not-authorized/> stream error to the client and then
terminate both the XML stream and the underlying TCP connection.
In the 'jabber:server' namespace, a stanza MUST possess a 'from'
attribute; if a server receives a stanza that does not meet this
restriction, it MUST generate an <improper-addressing/> stream error
condition. Furthermore, the domain identifier portion of the JID
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.
8.1.3 id
The optional 'id' attribute MAY be used by a sending entity for
internal tracking of stanzas that it sends and receives (especially
for tracking the request-response interaction inherent in the use of
IQ stanzas). The value of the 'id' attribute is NOT REQUIRED to be
unique either globally, within a domain, or within a stream. The
semantics of IQ stanzas impose additional restrictions; see the IQ
Semantics (Section 8.2.3) section of this document.
8.1.4 type
The 'type' attribute specifies detailed information about the purpose
or context of the message, presence, or IQ stanza. The particular
allowable values for the 'type' attribute vary depending on whether
the stanza is a message, presence, or IQ; the values for message and
presence stanzas are specific to instant messaging and presence
applications and therefore are defined in XMPP IM [20], whereas the
values for IQ stanzas specify the role of an IQ stanza in a
structured request-response "conversation" and thus are described
under IQ Semantics (Section 8.2.3) below. The only 'type' value
common to all three stanzas is "error"; see Stanza Errors (Section
8.3).
8.1.5 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 [14], "internationalization is for humans").
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The value of the 'xml:lang' attribute specifies the default language
of any such XML character data, which MAY be overridden by the
'xml:lang' attribute of a specific child element. The value of the
attribute MUST be an NMTOKEN and MUST conform to the format defined
in RFC 3066 [15].
8.2 Basic Semantics
8.2.1 Message Semantics
The <message/> stanza kind can be seen as a "push" mechanism whereby
one entity pushes information to another entity, similar to the
communications that occur in a system such as email. All message
stanzas SHOULD possess a 'to' attribute that specifies the intended
recipient of the message; upon receiving such a stanza, a server
SHOULD route or deliver it to the intended recipient (see Server
Rules for Handling XML Stanzas (Section 13) for general routing and
delivery rules related to XML stanzas).
8.2.2 Presence Semantics
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. In general, a publishing entity SHOULD
send a presence stanza with no 'to' attribute, in which case the
server to which the entity is connected SHOULD broadcast or multiplex
that stanza to all subscribing entities. However, a publishing entity
MAY also send a presence stanza with a 'to' attribute, in which case
the server SHOULD route or deliver that stanza to the intended
recipient. See Server Rules for Handling XML Stanzas (Section 13) for
general routing and delivery rules related to XML stanzas, and XMPP
IM [20] for presence-specific rules in the context of an instant
messaging and presence application.
8.2.3 IQ Semantics
Info/Query, or IQ, is a request-response mechanism, similar in some
ways to HTTP [21]. The semantics of IQ enable an entity to make a
request of, and receive a response from, another entity. The data
content of the request and response is defined by the namespace
declaration of a direct child element of the IQ element, and the
interaction is tracked by the requesting entity through use of the
'id' attribute. Thus IQ interactions follow a common pattern of
structured data exchange such as get/result or set/result (although
an error may be returned in response to a request if appropriate):
Requesting Responding
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Entity Entity
---------- ----------
| |
| <iq type='get' id='1'> |
| ------------------------> |
| |
| <iq type='result' id='1'> |
| <------------------------ |
| |
| <iq type='set' id='2'> |
| ------------------------> |
| |
| <iq type='error' id='2'> |
| <------------------------ |
| |
In order to enforce these semantics, the following rules apply:
1. The 'id' attribute is REQUIRED for IQ stanzas.
2. The 'type' attribute is REQUIRED for IQ stanzas. The value SHOULD
be one of the following (all other values SHOULD be ignored):
* get -- The stanza is a request for information or
requirements.
* set -- The stanza provides required data, sets new values, or
replaces existing values.
* result -- The stanza is a response to a successful get or set
request.
* error -- An error has occurred regarding processing or
delivery of a previously-sent get or set (see Stanza Errors
(Section 8.3)).
3. An entity that receives an IQ request of type "get" or "set" MUST
reply with an IQ response of type "result" or "error" (which
response MUST preserve the 'id' attribute of the request).
4. An entity that receives a stanza of type "result" or "error" MUST
NOT respond to the stanza by sending a further IQ response of
type "result" or "error"; however, as shown above, the requesting
entity MAY send another request (e.g., an IQ of type "set" in
order to provide required information discovered through a get/
result pair).
5. An IQ stanza of type "get" or "set" MUST contain one and only one
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child element (properly-namespaced as defined in XMPP IM [20])
that specifies the semantics of the particular request or
response.
6. An IQ stanza of type "result" MUST include zero or one child
elements.
7. An IQ stanza of type "error" SHOULD include the child element
contained in the associated "get" or "set" and MUST include an
<error/> child; for details, see Stanza Errors (Section 8.3).
8.3 Stanza Errors
Stanza-related errors are handled in a manner similar to stream
errors (Section 4.6), except that hints are also provided to the
receiving application regarding actions to take in response to the
error.
8.3.1 Rules
The following rules apply to stanza-related errors:
o The receiving or processing entity that detects an error condition
in relation to a stanza MUST return to the sending entity a stanza
of the same kind (message, presence, or IQ) whose 'type' attribute
is set to a value of "error".
o The entity that generates a stanza of type "error" SHOULD (but is
NOT REQUIRED to) include the original XML sent so that the sender
can inspect and if necessary correct the XML before attempting to
resend.
o A stanza whose 'type' attribute has a value of "error" MUST
contain an <error/> child element.
o An <error/> child MUST NOT be included if the 'type' attribute has
a value other than "error" (or if there is no 'type' attribute).
o An entity that receives 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.
8.3.2 Syntax
The syntax for stanza-related errors is as follows:
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<stanza-name to='sender' type='error'>
[RECOMMENDED to include sender XML here]
<error type='error-type'>
<defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
OPTIONAL descriptive text
</text>
[OPTIONAL application-specific condition element]
</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:
o MUST contain a child element corresponding to one of the defined
stanza error conditions defined below; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace
o MAY contain a <text/> child containing CDATA that describes the
error in more detail; this element MUST be qualified by the
'urn:ietf:params:xml:ns:xmpp-stanzas' namespace and SHOULD possess
an 'xml:lang' attribute
o MAY contain a child element for an application-specific error
condition; this element MUST be qualified by an
application-defined namespace, and its structure is defined by
that namespace
The <text/> element is OPTIONAL. If included, it SHOULD be used only
to provide descriptive or diagnostic information that supplements the
meaning of a defined condition or application-specific condition. It
SHOULD NOT be interpreted programmatically by an application. It
SHOULD NOT be used as the error message presented to user, but MAY be
shown in addition to the error message associated with the included
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condition element (or elements).
Note: the XML namespace name 'urn:ietf:params:xml:ns:xmpp-stanzas'
that qualifies the descriptive element adheres to the format defined
in The IETF XML Registry [23].
8.3.3 Defined Conditions
The following stanza-related error conditions are defined for use in
stanza errors.
o <bad-request/> -- the sender has sent XML that is malformed or
that cannot be processed (e.g., a client-generated stanza includes
a 'from' address, or an IQ stanza includes an unrecognized value
of the 'type' attribute); the associated error type SHOULD be
"modify".
o <conflict/> -- access cannot be granted because an existing
resource or session exists with the same name or address; the
associated error type SHOULD be "cancel".
o <feature-not-implemented/> -- the feature requested is not
implemented by the recipient or server and therefore cannot be
processed; the associated error type SHOULD be "cancel".
o <forbidden/> -- the requesting entity does not possess the
required permissions to perform the action; the associated error
type SHOULD be "auth".
o <internal-server-error/> -- the server could not process the
stanza because of a misconfiguration or an otherwise-undefined
internal server error; the associated error type SHOULD be "wait".
o <item-not-found/> -- the addressed JID or item requested cannot be
found; the associated error type SHOULD be "cancel".
o <jid-malformed/> -- the value of the 'to' attribute in the
sender's stanza does not adhere to the syntax defined in
Addressing Scheme (Section 3); the associated error type SHOULD be
"modify".
o <not-allowed/> -- the recipient does not allow any entity to
perform the action; the associated error type SHOULD be "cancel".
o <payment-required/> -- the user is not authorized to access the
requested service because payment is required; the associated
error type SHOULD be "auth".
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o <recipient-unavailable/> -- the specific recipient requested is
currently unavailable; the associated error type SHOULD be "wait".
o <registration-required/> -- the user is not authorized to access
the requested service because registration is required; the
associated error type SHOULD be "auth".
o <remote-server-not-found/> -- a remote server or service specified
as part or all of the JID of the intended recipient does not
exist; the associated error type SHOULD be "cancel".
o <remote-server-timeout/> -- a remote server or service specified
as part or all of the JID of the intended recipient could not be
contacted within a reasonable amount of time; the associated error
type SHOULD be "wait".
o <resource-constraint/> -- the server is resource-constrained and
is unable to service the request; the associated error type SHOULD
be "wait".
o <service-unavailable/> -- the service requested is currently
unavailable on the server; the associated error type SHOULD be
"cancel".
o <subscription-required/> -- the user is not authorized to access
the requested service because a subscription is required; the
associated error type SHOULD be "auth".
o <undefined-condition/> -- the error condition is not one of those
defined by the other conditions in this list; any error type may
be associated with this condition, and it SHOULD be used only in
conjunction with an application-specific condition.
o <unexpected-request/> -- the recipient understood the request but
was not expecting it at this time (e.g., the request was out of
order); the associated error type SHOULD be "wait".
8.3.4 Application-Specific Conditions
As noted, an application MAY provide application-specific stanza
error information by including a properly-namespaced child in the
error element. The application-specific element SHOULD supplement or
further qualify a defined element. Thus the <error/> element will
contain two or three child elements:
<iq type='error' id='some-id'>
<error type='modify'>
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<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<too-many-parameters xmlns='application-ns'/>
</error>
</iq>
<message type='error' id='another-id'>
<error type='modify'>
<undefined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<text xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
Some special application diagnostic information!
</text>
<special-application-condition xmlns='application-ns'/>
</error>
</message>
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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 needs to 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)
o character data or attribute values containing unescaped characters
that map to the predefined entities (Section 4.6); such characters
MUST be escaped
With regard to XML processing, if an XMPP implementation receives
such restricted XML data, it MUST ignore the data.
9.2 XML Namespace Names and Prefixes
XML Namespaces [10] 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. Rules for XML namespace names and prefixes
are defined below.
9.2.1 Stream Namespace
A stream namespace declaration is REQUIRED in both XML stream
headers. 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. An implementation
SHOULD generate only the 'stream:' prefix for these elements, and for
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historical reasons MAY accept only the 'stream:' prefix.
9.2.2 Default Namespace
A default namespace declaration 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 by the prefix of the stream
namespace or the dialback namespace).
A server implementation MUST support the following two default
namespaces (for historical reasons, some implementations MAY support
only these two default namespaces):
o jabber:client -- this default namespace is declared when the
stream is used for communications between a client and a server
o jabber:server -- this default namespace is declared when the
stream is used for communications between two servers
A client implementation MUST support the 'jabber:client' default
namespace, and for historical reasons MAY support only that default
namespace.
An implementation MUST NOT generate namespace prefixes for elements
in the default namespace if the default namespace is 'jabber:client'
or 'jabber:server'. An implementation SHOULD NOT generate namespace
prefixes for elements qualified by content (as opposed to stream)
namespaces other than 'jabber:client' and 'jabber:server'.
Note: the 'jabber:client' and 'jabber:server' namespaces are nearly
identical but are used in different contexts (client-to-server
communications for 'jabber:client' and server-to-server
communications for 'jabber:server'). The only difference between the
two is that the 'to' and 'from' attributes are OPTIONAL on stanzas
sent within 'jabber:client', whereas they are REQUIRED on stanzas
sent within 'jabber:server'. If a compliant implementation accepts a
stream that is qualified by the 'jabber:client' or 'jabber:server'
namespace, it MUST support the common attributes (Section 8.1) and
basic semantics (Section 8.2) of all three core stanza kinds
(message, presence, and IQ).
9.2.3 Dialback Namespace
A dialback namespace declaration is REQUIRED for all elements used in
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server dialback. The name of the dialback namespace MUST be
'jabber:server:dialback'. All elements qualified by this namespace
MUST be prefixed. An implementation SHOULD generate only the 'db:'
prefix for such elements and MAY accept only the 'db:' prefix.
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 (although an implementation MUST NOT
accept XML that is not well-formed). Clients SHOULD NOT rely on the
ability to send data which does not conform to the schemas, and
SHOULD ignore any non-conformant elements or attributes on the
incoming XML stream. Validation of XML streams and stanzas is NOT
REQUIRED or recommended, and schemas are included herein for
descriptive purposes only.
9.4 Inclusion of Text Declaration
Implementations SHOULD send a text declaration before sending a
stream header. Applications MUST follow the rules in the XML
specification [1] regarding the circumstances under which a text
declaration is included.
9.5 Character Encoding
Implementations MUST support the UTF-8 (RFC 2279 [16]) transformation
of Universal Character Set (ISO/IEC 10646-1 [17]) characters, as
required by RFC 2277 [14]. Implementations MUST NOT attempt to use
any other encoding.
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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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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 Nodeprep Profile of Stringprep
The Nodeprep profile of stringprep is defined in Appendix A (Appendix
A) of this document. If and when this document becomes an RFC, the
Nodeprep profile shall be registered in the stringprep profile
registry maintained by the IANA [5].
Name of this profile:
Nodeprep
RFC in which the profile is defined:
This document
Indicator whether or not this is the newest version of the profile:
This is the first version of Nodeprep
10.6 Resourceprep Profile of Stringprep
The Resourceprep profile of stringprep is defined in Appendix B
(Appendix B) of this document. If and when this document becomes an
RFC, the Resourceprep profile shall be registered in the stringprep
profile registry maintained by the IANA [5].
Name of this profile:
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Resourceprep
RFC in which the profile is defined:
This document
Indicator whether or not this is the newest version of the profile:
This is the first version of Resourceprep
10.7 Existing Registrations
The IANA registers "xmpp" as a GSSAPI [18] service name, as defined
under SASL Definition (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
XML streams MUST be encoded in UTF-8 as specified under the Character
Encoding (Section 9.5) section of this document. As speficied under
xml:lang (Section 8.1.5), an XML stanza SHOULD include an 'xml:lang'
attribute if the stanza contains XML character data that is intended
to be presented to a human user. 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 Stream
Encryption (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
Stream Authentication (Section 6)) provides a reliable mechanism for
validating that a client connecting to a server is who it claims to
be.
Client-to-server communications MUST NOT proceed until the DNS
hostname asserted by the server has been resolved. Such resolutions
SHOULD first attempt to resolve the hostname using an SRV [19]
Service of "jabber-client" and Proto of "tcp", resulting in resource
records such as "_jabber-client._tcp.example.com." (the use of the
string "jabber-client" for the service identifier is consistent with
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the existing IANA registration). If the SRV lookup fails, the
fallback is a normal A lookup to determine the IP address, using the
"jabber-client" port of 5222 assigned by the Internet Assigned
Numbers Authority [5].
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.
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 Server Dialback (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.
Inter-domain connections MUST NOT proceed until the DNS hostnames
asserted by the servers have been resolved. Such resolutions MUST
first attempt to resolve the hostname using an SRV [19] Service of
"jabber-server" and Proto of "tcp", resulting in resource records
such as "_jabber-server._tcp.example.com." (the use of the string
"jabber-server" for the service identifier is consistent with the
existing IANA registration; note well that the "jabber-server"
service identifier supersedes the earlier use of a "jabber" service
identifier, since the earlier usage did not conform to RFC 2782
[19]). 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].
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 [28] 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
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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
5222 (client-to-server) or port 5269 (server-to-server), as
registered with the IANA [5] (see IANA Considerations (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 plus SASL EXTERNAL(using the
TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher supporting client-side
certificates)
12.7 Stringprep Profiles
XMPP defines two profiles of stringprep [9]: Nodeprep (Appendix A)
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(for node identifiers) and Resourceprep (Appendix B) (for resource
identifiers).
The Unicode and ISO/IEC 10646 repertoires have many characters that
look similar. In many cases, users of security protocols might do
visual matching, such as when comparing the names of trusted third
parties. Because it is impossible to map similar-looking characters
without a great deal of context such as knowing the fonts used,
stringprep does nothing to map similar-looking characters together
nor to prohibit some characters because they look like others.
A node identifier can be employed as one part of an entity's address
in XMPP. One common usage is as the username of an instant messaging
user; another is as the name of a multi-user chat room; and many
other kinds of entities could use node identifiers as part of their
addresses. The security of such services could be compromised based
on different interpretations of the internationalized node
identifier; for example, a user entering a single internationalized
node identifier could access another user's account information, or a
user could gain access to an otherwise restricted chat room or
service.
A resource identifier can be employed as one part of an entity's
address in XMPP. One common usage is as the name for an instant
messaging user's active session; another is as the nickname of a user
in a multi-user chat room; and many other kinds of entities could use
resource identifiers as part of their addresses. The security of such
services could be compromised based on different interpretations of
the internationalized resource identifier; for example, a user could
attempt to initiate multiple sessions with the same name, or a user
could send a message to someone other than the intended recipient in
a multi-user chat room.
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13. Server Rules for Handling XML Stanzas
Each server implementation will contain its own "delivery tree" for
handling stanzas it receives. Such a tree determines whether a stanza
needs to be routed to another domain, processed internally, or
delivered to a resource associated with a connected node. The
following rules apply:
13.1 No 'to' Address
If the stanza possesses no 'to' attribute, the server SHOULD process
it on behalf of the entity that sent it. Because all stanzas received
from other servers MUST possess a 'to' attribute, this rule applies
only to stanzas received from a registered entity (such as a client)
that is connected to the server. If the server receives a presence
stanza with no 'to' attribute, the server SHOULD broadcast it to the
entities that are subscribed to the sending entity's presence, if
applicable (the semantics of presence broadcast for instant messaging
and presence applications are defined in XMPP IM [20]). If the server
receives an IQ stanza of type "get" or "set" with no 'to' attribute
and it understands the namespace that qualifies the content of the
stanza, it MUST either process the stanza on behalf of sending entity
(where the meaning of "process" is determined by the semantics of the
qualifying namespace) or return an error to the sending entity.
13.2 Foreign Domain
If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute does not match one of the configured hostnames
of the server itself or a subdomain thereof, the server SHOULD route
the stanza to the foreign domain (subject to local service
provisioning and security policies regarding inter-domain
communication). There are two possible cases:
A server-to-server stream already exists between the two domains: The
sender's server routes the stanza to the authoritative server for
the foreign domain over the existing stream
There exists no server-to-server stream between the two domains: The
sender's server first (1) resolves the hostname of the foreign
domain (as defined under Server-to-Server Communications (Section
12.3)), (2) negotiates a server-to-server stream between the two
domains, and (3) routes the stanza to the authoritative server for
the foreign domain over the newly-established stream
If routing to the recipient's server is unsuccessful, the sender's
server MUST return an error to the sender; if the recipient's server
can be contacted but delivery by the recipient's server to the
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recipient is unsuccessful, the recipient's server MUST return an
error to the sender by way of the sender's server.
13.3 Subdomain
If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute matches a subdomain of one of the configured
hostnames of the server itself, the server MAY process the stanza
itself or MAY route the stanza to a specialized service that is
responsible for that subdomain (if any).
13.4 Bare Domain or Specific Resource
If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute matches the hostname of the server itself and
the JID contained in the 'to' attribute is of the form <domain> or
<domain/resource>, the server (or a defined resource thereof) SHOULD
process the stanza as appropriate for the stanza kind. If the stanza
is an IQ stanza and the server understands the namespace that
qualifies the content of the stanza, the server MUST either process
the request according to the semantics of the qualifying namespace or
reply with an IQ of type "error".
13.5 Node in Same Domain
If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute matches the hostname of the server itself and
the JID contained in the 'to' attribute is of the form <node@domain>
or <node@domain/resource>, the server SHOULD deliver the stanza to
the intended recipient of the stanza as represented by the JID
contained in the 'to' attribute. The following rules apply:
1. If the JID contains a resource identifier (i.e., is of the form
<node@domain/resource>) and there is an available resource whose
authzid matches the full JID, the recipient's server SHOULD
deliver the stanza to the session that exactly matches the
resource identifier.
2. If the JID contains a resource identifier and there is no
available resource whose authzid matches the full JID, the
recipient's server SHOULD return to the sender a
<recipient-unavailable/> stanza error.
3. If the JID is of the form <node@domain> and there is at least one
available resource available for the node, the recipient's server
MUST deliver the stanza to at least one of the available
resources, according to application-specific rules (a set of
delivery rules for instant messaging and presence applications is
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defined in XMPP IM [20]).
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14. Compliance Requirements
This section summarizes the specific aspects of the Extensible
Messaging and Presence Protocol that MUST be supported by servers and
clients in order to be considered compliant implementations, as well
as additional protocol aspects that SHOULD be supported. For
compliance purposes, we draw a distinction between core protocols
(which MUST be supported by any server or client, regardless of the
specific application) and instant messaging protocols (which MUST be
supported only by instant messaging and presence applications built
on top of the core protocols). Compliance requirements that apply to
all servers and clients are specified in this section; compliance
requirements for instant messaging servers and clients are specified
in the corresponding section of XMPP IM [20].
14.1 Servers
A server MUST support the following core protocols in order to be
considered compliant:
o Enforcement of the Nodeprep (Appendix A) and Resourceprep
(Appendix B) profiles of stringprep
o XML streams (Section 4) as defined in this document, including
stream encryption (Section 5) using TLS and stream authentication
(Section 6) using SASL
o The basic semantics of the three defined XML stanzas (Section 8)
(i.e., <message/>, <presence/>, and <iq/>)
o Generation (and, where appropriate, handling) of error syntax and
semantics related to streams, TLS, SASL, and XML stanzas
In addition, a server SHOULD support the following core protocol:
o Server dialback (Section 7)
14.2 Clients
A client MUST support the following core protocols in order to be
considered compliant:
o XML streams (Section 4) as defined in this document, including
stream encryption (Section 5) using TLS and stream authentication
(Section 6) using SASL
o The basic semantics of the three defined XML stanzas (Section 8)
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(i.e., <message/>, <presence/>, and <iq/>)
o Handling (and, where appropriate, generation) of error syntax and
semantics related to streams, TLS, SASL, and XML stanzas
In addition, a client SHOULD support the following core protocols:
o Generation of addresses in accordance with the Nodeprep (Appendix
A) and Resourceprep (Appendix B) profiles of stringprep
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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 (IDN)", RFC 3491, March
2003.
[9] Hoffman, P. and M. Blanchet, "Preparation of Internationalized
Strings ("stringprep")", RFC 3454, December 2002.
[10] World Wide Web Consortium, "Namespaces in XML", W3C xml-names,
January 1999, <http://www.w3.org/TR/1999/
REC-xml-names-19990114/>.
[11] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
1999.
[12] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[13] Myers, J., "Simple Authentication and Security Layer (SASL)",
RFC 2222, October 1997.
[14] Alvestrand, H., "IETF Policy on Character Sets and Languages",
BCP 18, RFC 2277, January 1998.
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[15] Alvestrand, H., "Tags for the Identification of Languages", BCP
47, RFC 3066, January 2001.
[16] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
[17] 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.
[18] Linn, J., "Generic Security Service Application Program
Interface, Version 2", RFC 2078, January 1997.
[19] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
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Informative References
[20] Saint-Andre, P. and J. Miller, "XMPP Instant Messaging",
draft-ietf-xmpp-im-17 (work in progress), September 2003.
[21] 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.
[22] 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>.
[23] Mealling, M., "The IANA XML Registry",
draft-mealling-iana-xmlns-registry-05 (work in progress), June
2003.
[24] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 2060, December 1996.
[25] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD
53, RFC 1939, May 1996.
[26] Newman, C. and J. Myers, "ACAP -- Application Configuration
Access Protocol", RFC 2244, November 1997.
[27] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595,
June 1999.
[28] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[29] Jabber Software Foundation, "Jabber Software Foundation",
<http://www.jabber.org/>.
Authors' Addresses
Peter Saint-Andre
Jabber Software Foundation
EMail: stpeter@jabber.org
Jeremie Miller
Jabber Software Foundation
EMail: jeremie@jabber.org
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Appendix A. Nodeprep
A.1 Introduction
This appendix defines the "Nodeprep" profile of stringprep (RFC 3454
[9]). As such, it specifies processing rules that will enable users
to enter internationalized node identifiers in XMPP and have the
highest chance of getting the content of the strings correct. These
processing rules are intended only for XMPP node identifiers (which
are often associated with usernames), and are not intended for
arbitrary text.
This profile defines the following, as required by RFC 3454 [9]:
o The intended applicability of the profile: internationalized node
identifiers within XMPP
o The character repertoire that is the input and output to
stringprep: Unicode 3.2, specified in section 2 of this Appendix
o The mappings used: specified in section 3
o The Unicode normalization used: specified in section 4
o The characters that are prohibited as output: specified in section
5
o Bidirectional character handling: specified in section 6
A.2 Character Repertoire
This profile uses Unicode 3.2 with the list of unassigned code points
being Table A.1, both defined in Appendix A of RFC 3454 [9].
A.3 Mapping
This profile specifies mapping using the following tables from RFC
3454 [9]:
Table B.1
Table B.2
A.4 Normalization
This profile specifies using Unicode normalization form KC, as
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described in RFC 3454 [9].
A.5 Prohibited Output
This profile specifies prohibiting use of the following tables from
RFC 3454 [9].
Table C.1.1
Table C.1.2
Table C.2.1
Table C.2.2
Table C.3
Table C.4
Table C.5
Table C.6
Table C.7
Table C.8
Table C.9
In addition, the following Unicode characters are also prohibited:
#x22 (")
#x26 (&)
#x27 (')
#x2F (/)
#x3A (:)
#x3C (<)
#x3E (>)
#x40 (@)
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A.6 Bidirectional Characters
This profile specifies checking bidirectional strings as described in
section 6 of RFC 3454 [9].
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Appendix B. Resourceprep
B.1 Introduction
This appendix defines the "Resourceprep" profile of stringprep (RFC
3454 [9]). As such, it specifies processing rules that will enable
users to enter internationalized resource identifiers in XMPP and
have the highest chance of getting the content of the strings
correct. These processing rules are intended only for XMPP resource
identifiers (which are often associated with session names), and are
not intended for arbitrary text.
This profile defines the following, as required by RFC 3454 [9]:
o The intended applicability of the profile: internationalized
resource identifiers within XMPP
o The character repertoire that is the input and output to
stringprep: Unicode 3.2, specified in section 2 of this Appendix
o The mappings used: specified in section 3
o The Unicode normalization used: specified in section 4
o The characters that are prohibited as output: specified in section
5
o Bidirectional character handling: specified in section 6
B.2 Character Repertoire
This profile uses Unicode 3.2 with the list of unassigned code points
being Table A.1, both defined in Appendix A of RFC 3454 [9].
B.3 Mapping
This profile specifies mapping using the following tables from RFC
3454 [9]:
Table B.1
B.4 Normalization
This profile specifies using Unicode normalization form KC, as
described in RFC 3454 [9].
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B.5 Prohibited Output
This profile specifies prohibiting use of the following tables from
RFC 3454 [9].
Table C.1.2
Table C.2.1
Table C.2.2
Table C.3
Table C.4
Table C.5
Table C.6
Table C.7
Table C.8
Table C.9
B.6 Bidirectional Characters
This profile specifies checking bidirectional strings as described in
section 6 of RFC 3454 [9].
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Appendix C. XML Schemas
The following XML schemas are descriptive, not normative. For schemas
defining the 'jabber:client' and 'jabber:server' namespaces, refer to
XMPP IM [20].
C.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>
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<xs:element name='error'>
<xs:complexType>
<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>
C.2 Stream error 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='urn:ietf:params:xml:ns:xmpp-streams'
xmlns='urn:ietf:params:xml:ns:xmpp-streams'
elementFormDefault='qualified'>
<xs:import namespace='http://www.w3.org/XML/1998/namespace'
schemaLocation='http://www.w3.org/2001/xml.xsd'/>
<xs:element name='bad-format' type='empty'/>
<xs:element name='bad-namespace-prefix' type='empty'/>
<xs:element name='conflict' type='empty'/>
<xs:element name='connection-timeout' type='empty'/>
<xs:element name='host-gone' type='empty'/>
<xs:element name='host-unknown' type='empty'/>
<xs:element name='improper-addressing' type='empty'/>
<xs:element name='internal-server-error' type='empty'/>
<xs:element name='invalid-id' type='empty'/>
<xs:element name='invalid-namespace' type='empty'/>
<xs:element name='invalid-xml' type='empty'/>
<xs:element name='nonmatching-hosts' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='policy-violation' type='empty'/>
<xs:element name='remote-connection-failed' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='restricted-xml' type='empty'/>
<xs:element name='see-other-host' type='empty'/>
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<xs:element name='system-shutdown' type='empty'/>
<xs:element name='undefined-condition' type='empty'/>
<xs:element name='unsupported-encoding' type='empty'/>
<xs:element name='unsupported-stanza-type' type='empty'/>
<xs:element name='unsupported-version' type='empty'/>
<xs:element name='xml-not-well-formed' type='empty'/>
<xs:element name='text' type='xs:string'>
<xs:complexType>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
C.3 TLS namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-tls'
xmlns='urn:ietf:params:xml:ns:xmpp-tls'
elementFormDefault='qualified'>
<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'>
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<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
C.4 SASL namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-sasl'
xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
elementFormDefault='qualified'>
<xs:element name='mechanisms'>
<xs:complexType>
<xs:sequence>
<xs:element ref='mechanism' maxOccurs='unbounded'/>
</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='aborted'/>
<xs:element ref='bad-protocol'/>
<xs:element ref='encryption-required'/>
<xs:element ref='invalid-authzid'/>
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<xs:element ref='invalid-mechanism'/>
<xs:element ref='invalid-realm'/>
<xs:element ref='mechanism-too-weak'/>
<xs:element ref='not-authorized'/>
<xs:element ref='temporary-auth-failure'/>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:element name='aborted' type='empty'/>
<xs:element name='bad-protocol' type='empty'/>
<xs:element name='encryption-required' type='empty'/>
<xs:element name='invalid-authzid' type='empty'/>
<xs:element name='invalid-mechanism' type='empty'/>
<xs:element name='invalid-realm' type='empty'/>
<xs:element name='mechanism-too-weak' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='temporary-auth-failure' type='empty'/>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
C.5 Dialback namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='jabber:server:dialback'
xmlns='jabber:server:dialback'
elementFormDefault='qualified'>
<xs:element name='result'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs: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'/>
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<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>
C.6 Stanza error 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='urn:ietf:params:xml:ns:xmpp-stanzas'
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
elementFormDefault='qualified'>
<xs:import namespace='http://www.w3.org/XML/1998/namespace'
schemaLocation='http://www.w3.org/2001/xml.xsd'/>
<xs:element name='bad-request' type='empty'/>
<xs:element name='conflict' type='empty'/>
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<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='payment-required' 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='undefined-condition' type='empty'/>
<xs:element name='unexpected-request' type='empty'/>
<xs:element name='text' type='xs:string'>
<xs:complexType>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
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Appendix D. Differences Between Jabber and XMPP
This section is non-normative.
XMPP has been adapted from the protocols originally developed in the
Jabber open-source community, which can be thought of as "XMPP 0.9".
Because there exists a large installed base of Jabber implementations
and deployments, it may be helpful to specify the key differences
between Jabber and XMPP in order to expedite and encourage upgrades
of those implementations and deployments to XMPP. This section
summarizes the core differences, and the corresponding section of
XMPP IM [20] summarizes the differences that relate specifically to
instant messaging and presence applications.
D.1 Authentication
The client-server authentication protocol developed in the Jabber
community uses a basic IQ interaction qualified by the
'jabber:iq:auth' namespace (documentation of this protocol is
contained in "JEP-0078: Non-SASL Authentication", published by the
Jabber Software Foundation [29]). XMPP uses SASL for authentication,
as defined in the Stream Authentication (Section 6) section of this
document.
The Jabber community does not currently possess an authentication
protocol for server-to-server communications, only the Server
Dialback (Section 7) protocol to prevent server spoofing. XMPP
augments Server Dialback with a true server-to-server authentication
protocol, as defined in the Stream Authentication (Section 6) section
of this document.
D.2 Channel Encryption
It is common practice in the Jabber community to use SSL for channel
encryption on ports other than 5222 and 5269 (the convention is to
use ports 5223 and 5270). XMPP uses TLS over the IANA-registered
ports for channel encryption, as defined in the Stream Encryption
(Section 5) section of this document.
D.3 JID Processing
JID processing was somewhat loosely defined by the Jabber community
(documentation of forbidden characters and case handling is contained
in "JEP-0029: Definition of Jabber Identifiers", published by the
Jabber Software Foundation [29]). XMPP defines two stringprep [9]
profiles for JID processing: Nodeprep (Appendix A) and Resourceprep
(Appendix B).
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D.4 Error Handling
Stream-related errors are handled in the Jabber community via simple
CDATA text in a <stream:error/> element. In XMPP, stream-related
errors are handled via an extensible mechanism defined in the Stream
Errors (Section 4.6) section of this document.
Stanza-related errors are handled in the Jabber community via
HTTP-style error codes. In XMPP, stanza-related errors are handled
via an extensible mechanism defined in the Stanza Errors (Section
8.3) section of this document. (Documentation of a mapping between
Jabber and XMPP error handling mechanisms is contained in "JEP-0086:
Legacy Errors", published by the Jabber Software Foundation [29].)
D.5 Internationalization
Although use of UTF-8 has always been standard practice within the
Jabber community, the community did not define mechanisms for
specifying the language of human-readable text provided in CDATA
sections. XMPP specifies the use of the 'xml:lang' attribute in such
contexts, as defined in the xml:lang (Section 8.1.5) section of this
document.
D.6 Stream Version Attribute
The Jabber community does not include a 'version' attribute in stream
headers. XMPP specifies inclusion of that attribute, with a value of
'1.0', as a way to signal support for the stream features
(authentication, encryption, etc.) defined in the Version Support
(Section 4.2.1) section of this document.
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Appendix E. Revision History
Note to RFC Editor: please remove this entire appendix, and the
corresponding entries in the table of contents, prior to publication.
E.1 Changes from draft-ietf-xmpp-core-17
o Specified that UTF-8 is the only allowable encoding.
o Added stream errors for <bad-namespace-prefix/>, <invalid-xml/>,
and <restricted-xml/>, as well as a <bad-format/> error for
generic XML error conditions.
o Folded Nodeprep and Resourceprep profiles into this document.
o Moved most delivery handling rules from XMPP IM to XMPP Core.
o Moved detailed stanza syntax descriptions from XMPP Core to XMPP
IM.
o Moved stanza schemas from XMPP Core to XMPP IM.
E.2 Changes from draft-ietf-xmpp-core-16
o Added <conflict/> and <unsupported-encoding/> stream errors.
o Changed the datatype for the <see-other-host/> and
<unsupported-version/> stream errors from 'xs:string' to 'empty'.
o Further clarified server handling of the basic stanza kinds.
o Further clarified character encoding rules per list discussion.
o Specified meaning of version='1.0' flag in stream headers.
o Added stream closure to SASL failure cases in order to mirror
handling of TLS failures.
o Added section on compliance requirements for server and client
implementations.
o Added non-normative section on differences between Jabber usage
and XMPP specifications.
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E.3 Changes from draft-ietf-xmpp-core-15
o Added <connection-timeout/> and <policy-violation/> stream errors.
o Added <aborted/> SASL error and clarified <bad-protocol/> error.
o Made 'id' required for IQ stanzas.
E.4 Changes from draft-ietf-xmpp-core-14
o Added SRV lookup for client-to-server communications.
o Changed server SRV record to conform to RFC 2782; specifically,
the service identifier was changed from 'jabber' to
'jabber-server'.
E.5 Changes from draft-ietf-xmpp-core-13
o Clarified stream restart after successful TLS and SASL
negotiation.
o Clarified requirement for resolution of DNS hostnames.
o Clarified text regarding namespaces.
o Clarified examples regarding empty <stream:features/> element.
o Added several more SASL error conditions.
o Changed <invalid-xml/> stream error to <improper-addressing/> and
added to schema.
o Made small editorial changes and fixed several schema errors.
E.6 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'.
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o Corrected several small errors in the TLS and SASL sections.
o Corrected several errors in the schemas.
E.7 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.
E.8 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.
E.9 Changes from draft-ietf-xmpp-core-09
o Fixed several dialback error conditions.
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.
E.10 Changes from draft-ietf-xmpp-core-08
o Incorporated list discussion regarding addressing, SASL, TLS, TCP,
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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.
E.11 Changes from draft-ietf-xmpp-core-07
o Made several small editorial changes.
E.12 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 [23].
o Removed note to RFC Editor regarding provisional namespace names.
E.13 Changes from draft-ietf-xmpp-core-05
o Added <invalid-namespace/> as a stream error condition.
o Adjusted security considerations per discussion at IETF 56 and on
list.
E.14 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.
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o Made numerous small editorial changes and clarified text
throughout.
E.15 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.
E.16 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".
E.17 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.
o Made numerous small editorial changes.
E.18 Changes from draft-ietf-xmpp-core-00
o Added information about TLS from list discussion.
o Clarified meaning of "ignore" based on list discussion.
o Clarified information about Universal Character Set data and
character encodings.
o Provided base64-decoded information for examples.
o Fixed several errors in the schemas.
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o Made numerous small editorial fixes.
E.19 Changes from draft-miller-xmpp-core-02
o Brought Streams Authentication section into line with discussion
on list and at IETF 55 meeting.
o Added information about the optional 'xml:lang' attribute per
discussion on list and at IETF 55 meeting.
o Specified that validation is neither required nor recommended, and
that the formal definitions (DTDs and schemas) are included for
descriptive purposes only.
o Specified that the response to an IQ stanza of type "get" or "set"
must be an IQ stanza of type "result" or "error".
o Specified that compliant server implementations must process
stanzas in order.
o Specified that for historical reasons some server implementations
may accept 'stream:' as the only valid namespace prefix on the
root stream element.
o Clarified the difference between 'jabber:client' and
'jabber:server' namespaces, namely, that 'to' and 'from'
attributes are required on all stanzas in the latter but not the
former.
o Fixed typo in Step 9 of the dialback protocol (changed db:result
to db:verify).
o Removed references to TLS pending list discussion.
o Removed the non-normative appendix on OpenPGP usage pending its
inclusion in a separate I-D.
o Simplified the architecture diagram, removed most references to
services, and removed references to the 'jabber:component:*'
namespaces.
o Noted that XMPP activity respects firewall administration
policies.
o Further specified the scope and uniqueness of the 'id' attribute
in all stanza kinds and the <thread/> element in message stanzas.
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o Nomenclature changes: (1) from "chunks" to "stanzas"; (2) from
"host" to "server" and from "node" to "client" (except with regard
to definition of the addressing scheme).
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